Inorganic Nanohybrids: From Strategic

Review Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

pubs.acs.org/CR

Versatile Types of Organic/Inorganic Nanohybrids: From Strategic Design to Biomedical Applications Nana Zhao,† Liemei Yan,† Xiaoyi Zhao,† Xinyan Chen,† Aihua Li,‡ Di Zheng,† Xin Zhou,† Xiaoguang Dai,† and Fu-Jian Xu*,†

Downloaded via UNIV OF KANSAS on January 4, 2019 at 05:19:35 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

State Key Laboratory of Chemical Resource Engineering, Beijing Laboratory of Biomedical Materials, Key Laboratory of Carbon Fiber and Functional Polymers (Beijing University of Chemical Technology), Ministry of Education, Beijing University of Chemical Technology, Beijing, 100029, China ‡ College of Materials Science and Engineering, Institute for Graphene Applied Technology Innovation, Laboratory of Fiber Materials and Modern Textiles, Growing Base for State Key Laboratory, Collaborative Innovation Center for Marine Biomass Fibers Materials and Textiles of Shandong Province, Qingdao University, Qingdao 266071, China ABSTRACT: Organic/inorganic nanohybrids have attracted widespread interests due to their favorable properties and promising applications in biomedical areas. Great efforts have been made to design and fabricate versatile nanohybrids. Among different organic components, diverse polymers offer unique avenues for multifunctional systems with collective properties. This review focuses on the design, properties, and biomedical applications of organic/inorganic nanohybrids fabricated from inorganic nanoparticles and polymers. We begin with a brief introduction to a variety of strategies for the fabrication of functional organic/inorganic nanohybrids. Then the properties and functions of nanohybrids are discussed, including properties from organic and inorganic parts, synergistic properties, morphology-dependent properties, and self-assembly of nanohybrids. After that, current situations of nanohybrids applied for imaging, therapy, and imaging-guided therapy are demonstrated. Finally, we discuss the prospect of organic/inorganic nanohybrids and highlight the challenges and opportunities for the future investigations.

CONTENTS 1. Introduction 2. Construction of Organic/Inorganic Nanohybrids in the Biomedical Field 2.1. Organic Species 2.1.1. Small Molecules 2.1.2. Synthetic Polymers 2.1.3. Biomolecules 2.2. Inorganic Nanoparticles 2.2.1. Gold Nanoparticles 2.2.2. Silica Nanoparticles 2.2.3. Iron Oxide Nanoparticles 2.2.4. Quantum Dots 2.2.5. Other Inorganic Nanoparticles 2.3. Fabrication Strategies of Organic/Inorganic Nanohybrids 2.3.1. “Grafting onto” Strategy 2.3.2. “Grafting from” Strategy 2.3.3. Self-Assembly Strategy 2.3.4. One-Pot Synthesis 2.3.5. Wrapping 3. Property and Function of Organic/Inorganic Nanohybrids 3.1. Organic Parts 3.1.1. Stimuli Responsiveness 3.1.2. Targeting © XXXX American Chemical Society

3.1.3. Guest Molecule Loading 3.1.4. Multifunctions 3.2. Inorganic Parts 3.2.1. Optical Properties and Related Functions 3.2.2. Magnetic Properties and Related Functions 3.2.3. Electrical Properties and Related Functions 3.2.4. Other Properties 3.3. Synergistic Properties of Organic and Inorganic Parts 3.4. Morphology-Dependent Properties of Nanohybrids 3.5. Self-Assembly of Nanohybrids 4. Versatile Biomedical Applications 4.1. Flexible Imaging 4.1.1. Magnetic Resonance Imaging 4.1.2. Fluorescence Imaging 4.1.3. Imaging with Other Modalities 4.1.4. Multimodal Imaging 4.2. Diverse Therapies 4.2.1. Phototherapy

B B B C C D D D E E F J J L P T W Y AB AB AC AE

AE AG AI AI AK AK AL AM AP AT BB BC BC BE BF BG BH BH

Received: June 26, 2018

A

DOI: 10.1021/acs.chemrev.8b00401 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 4.2.2. Chemotherapy 4.2.3. Gene Therapy 4.2.4. Multimodal Therapy 4.3. Imaging-Guided Therapy 4.3.1. Monomodal Imaging-Guided Therapy 4.3.2. Multimodal Imaging-Guided Therapy 5. Conclusions and Perspectives Author Information Corresponding Author ORCID Notes Biographies Acknowledgments Abbreviations References

Review

functions such as stimuli-responsiveness, targeting, guest molecule loading, biodegradability, and controlled release manner.29 The design of specific composition, morphology, and surface property of nanohybrids endows them with tunable properties and paves the way for versatile biomedical applications. To construct feasible nanohybrids, ligand exchange processes are usually prerequisite except that onestep synthesis or wrapping with organic components was carried out. When polymers are introduced, “grafting from”, “grafting onto”, and self-assembly could be employed to fabricate organic/inorganic nanohybrids. Furthermore, the assembled structures of nanohybrids such as vesicles or capsules could enrich the collective properties and potential applications.30 The scope of the current review is to summarize recent progress in versatile types of organic/inorganic nanohybrids for diverse biomedical applications including drug delivery, gene delivery, phototherapy, imaging, and multifunctional systems. We will start from the fabrication of organic/inorganic nanohybrids. The commonly used synthetic methods will be summarized to offer information and guide for the design of nanohybrids. Then, we will emphasize the properties of applied nanohybrids in biomedical areas, as well as the synergetic properties of assembled structures. Thereafter, the bioapplication of nanohybrids is focused on essential principles and mechanisms. Particularly, the current situations of applied nanohybrids in imaging, therapy, and imaging-guided therapy will be divided into categories and discussed, respectively. Various design rationales will be presented to illustrate the concepts of new nanohybrid platforms. The functions of organic and inorganic components as well as their coordination will be further explored. The fabricated multifunctional systems, including multimodal imaging, combined therapy and imaging-guided therapy, will be highlighted to display the advantages of nanohybrids. The review will be concluded by discussing the challenges and outlook of organic/inorganic nanohybrids for biomedical applications.

BJ BJ BK BM BM BR BT BT BT BT BT BT BU BU BW

1. INTRODUCTION Organic/inorganic nanohybrids composed of organic and inorganic components have attracted intense attention in the realm of biorelated systems due to their favorable physicochemical properties.1−7 Fascinating organic/inorganic nanohybrids can be widely found in nature such as mollusk shells and teeth, which are composed of biomacromolecules and inorganic ingredients through nanoscale hybridization.8 Herein, we define organic/inorganic nanohybrids as heteronanoparticles of organic and inorganic components which possess discrete domains. The intriguing nanohybrids not only combine the original functions of organic and inorganic parts, but also are expected to produce new properties due to the synergy of individual components, such as desirable optical, magnetic, and electrical properties.9 The unique feature of hybridization at nanoscale exhibits significantly different characteristics from corresponding bulk materials,10 which is promising in clinical practice.11,12 Enhanced permeation and retention (EPR) effect could help preferential aggregation of the nanoparticles (NPs) in tumor regions.13 Nowadays, with the demanding requirements for personalized therapy,14 great efforts have been made to monitor the response of treatment and the concept of theranostics offers new opportunities.15 In this regard, organic/inorganic nanohybrids integrating polymers, metals, and semiconductors could be promising candidates for the new generation biomaterials. A variety of organic/inorganic nanohybrid nanocarriers were applied to deliver drugs and biomolecules to achieve satisfying therapeutic effects. In addition, imaging and theranostic materials employing organic/inorganic nanohybrids were developed for precise treatment.16,17 Inorganic NPs such as gold,18−20 iron oxide,21 and upconversion NPs22−24 could introduce the functions of computed tomography (CT), magnetic resonance (MR), and fluorescence (FL) imaging. For biorelated applications, surface functionalization with organic stabilizers including small molecules, polymers, and biomolecules is necessary to improve the dispersibility of NPs.25 Moreover, organic components are also required to ensure the biocompatibility and stability of nanohybrids. In order to minimize the side effects of chemotherapy, sitespecific targeting and controlled drug release manner are critical to reach the pathological region without causing damage to normal tissue.26 Strategies based on the distinct tumor microenvironments were put forward for the design and synthesis of theranostic agents.27,28 Generally speaking, the organic parts of organic/inorganic nanohybrids could bring the

2. CONSTRUCTION OF ORGANIC/INORGANIC NANOHYBRIDS IN THE BIOMEDICAL FIELD 2.1. Organic Species

A wide range of organic species offer diverse choices for the construction of nanohybrids, which also comprise the essential part of nanohybrids. The surfaces of inorganic NPs usually need to be functionalized to modulate the interactions between NPs and organisms.31−33 Since the capping ligands on the surface of inorganic NPs after synthesis may be hydrophobic or toxic, which is not suitable for biomedical applications, inorganic materials, or organic ligands could be used to cap the surface for reduced cytotoxicity and enhanced properties.34 Organic species possess more flexibility to tailor functionality and improve dispersibility, stability, and biocompatibility. For example, the surface functionalization with appropriate organic species can impart NPs with efficient cellular uptake and site specificity. The types of organic species attached to the surface of NPs depend on the nature and property of NPs to a large extent.35 Generally speaking, Au NPs could be conjugated with thiol group-terminated ligands, while silica NPs could be functionalized with silane species. The unique feature of layered double hydroxides (LDH) to exchange anions in the interlayer space could be utilized to carry organic anions including biomolecules. Herein, we introduce the mostly used B


Chemical Reviews

Review

Figure 1. Typical organic and inorganic components employed in nanohybrids.

designed to attain various purposes. As shown in Figure 1, linear homopolymers, block copolymers, branched polymers, and dendrimers as well as their derivatives are widely investigated. In biological fluid, protein corona will form on the surface of organic, inorganic, and hybrid NPs, which is influenced by particle size, surface properties, and hydrophobicity.43 Particularly, surface properties affect the composition of the protein corona that correlates closely with cell-NP interactions, which determine the final NP transport, location, and fate in vivo.44,45 From this point of view, organic species of the nanohybrids should be more responsible. It is supposed that there are still apparent differences in cellular uptake depending on surface charge, although the presence of protein corona decreases the internalization of both positively and negatively charged NPs.46 Moreover, the type of disease may also substantially change the protein corona around NPs, resulting in complications to the actual in vivo intracellular trafficking of NPs.46 A better understanding of all these aspects will contribute to the design and preparation of appropriate nanohybrids. Polyethylene glycol (PEG) seems the most commonly used polymer due to its high biocompatibility and hydrophilicity, which could effectively suppress the protein adsorption and improve hemocompatibility of nanohybrids. In addition, PEG is also bioinert, nonimmunogenic, and nontoxic as an additive approved by the Food and Drug Administration (FDA). Hydrophilic PEG was usually used to functionalize NPs to attain good colloidal stability for biomedical usage.47 For example, compared with the as prepared silica NPs with hydroxyl groups and carboxyl-terminated small molecules, PEG functionalized silica NPs are found to exhibit prolonged blood circulation and minimal entrapment in the reticuloendothelial system (RES) organs.48 This rule also applies to other kinds of NPs. PEG-conjugated LDH NPs by introducing phosphonic acid-terminated PEG via electrostatic interaction are demonstrated with improved colloidal and biological stability.49 As a result, PEG-terminated NPs show excellent biodistribution, efficient passive targeting effects to tumor sites, and improved renal clearance when utilized as delivery carriers.50,51 Zwitterionic polymers with both positively and negatively charged groups which demonstrate overall neutral charge could play similar roles as PEG to resist nonspecific protein adsorption.52 Phosphorylcholine-, sulfobetaine-, and carboxybetaine-based zwitterionic polymers all behaved as alternatives to PEG.53−55 It is even demonstrated that zwitterionic Au NPs functionalized with carboxybetainebased polymers possess longer circulation half-life than PEGfunctionalized analogues.56

organic species roughly in small molecules, synthetic polymers, and biomolecules (Figure 1). Biomacromolecules such as polysaccharides are divided into the category of biomolecules. 2.1.1. Small Molecules. To synthesize nanohybrids through surface modification, small organic molecules such as 3-aminopropyl-trimethoxysilane (APTMS), 3-aminopropyltriethoxysilane (APTES), and 3-(trihydroxysilyl)-propylmethylphosphonate carboxyethylsilanetriol are widely used to be anchored onto the surface of silica NPs with alkoxysilane groups. The amino groups of APTES will be utilized as the connection to introduce more functions or impart biomedical gene delivery capability directly via electrostatic interactions.36 The terminal amino group could be employed as a linker to conjugate with biomolecules or polymers with carboxylic groups or N-hydroxysuccinimide (NHS) esters. Small thiolated molecules are extensively used to be attached to the surface of Au or semiconductor NPs through covalent bonds.37−39 Three different kinds of alkanethiols (aminethanethiol, 3-mercaptoethanoic acid, and 2-mercaptoethanol) with different functional groups (amine, carboxyl, and hydroxyl groups) were employed to modify Au NPs with Au−S covalent bonds.40 The influence of resultant Au nanohybrids with different surface properties on human bone marrow-derived mesenchymal stem cells behaviors was then investigated to provide information for future tissue engineering. Some small molecules conjugated with polymers could play the role of reductants. Dimethylaminopropylamine (DAA) was connected to the backbone of hyaluronic acid (HA) via amide bonds and then the amphiphilic block copolymers of DAA conjugated HA-b-PCL (polycaprolactone) could be synthesized to form micelles. After reducing HAuCl4 with the DAA part, gold nanoclusters on the shell of polymeric micelles were produced for therapeutic applications.41 Other kinds of small molecules are conjugated with polymers to bind onto the surface of NPs for certain functions. For example, nitroaromatic molecules could be employed as ultrasensitive fluorescent probes for hypoxia imaging.42 In addition, 2propionic-3-methylmaleic anhydride (CDM) is a typical linker between two polymers to fabricate pH sensitive polymers, taking the advantages of its sensitive cleavage in tumor environment (pH ∼6.8).28 Since in most cases small molecules work with polymers or biomolecules together to achieve versatile biomedical applications, we do not place too much emphasis on them in this review. 2.1.2. Synthetic Polymers. Compared with small molecules, polymeric ligands bring more diversity to nanohybrids since both components and structures could be C


Chemical Reviews

Review

(Figure 1) have demonstrated advantages in biomedicine fields. In this subsection, the synthetic approaches of NPs will be first introduced to present a general idea since preparation is the starting point and basis for their biological applications. We mainly focus on bottom-up techniques, and therefore most physical methods are excluded from the review. Among various chemical approaches, solution phase synthetic methods such as seed-mediated growth methods, reaction mediated by polyols and high temperature synthesis in organic solvents are most widely employed.71 The final morphology of NPs will be tailored by the nucleation and growth steps. Usually capping ligands such as surfactants or polymers are needed to define the growth of crystals in the nanoscale. The interactions between different crystal facets and capping agents will change their relative growth rates, resulting in various shapes and sizes, which will significantly influence the circulation and penetration behaviors of NPs.72−74 The synthetic approaches of major types of inorganic NPs with controlled morphology are listed as follows. 2.2.1. Gold Nanoparticles. Gold nanoparticles (Au NPs) are considered as the most widely utilized metal NPs in nanomedicine nowadays due to their biocompatibility and unique optical properties. Reduction of solvated gold salt is generally carried out with the assistance of surface capping agents to synthesize Au NPs with different morphologies. Meanwhile, the capping agents could help prevent the aggregation of NPs in solution. The size of Au NPs could be controlled by adjusting relative amounts of gold ions, reductants, and capping agents. Spherical Au NPs could be synthesized using citrate or thiols as capping agent in aqueous or organic solution.75,76 Since the development of seedmediated growth procedure by Murphy and El-Sayed,77,78 gold nanorods with different aspect ratios as well as Au NPs with other shapes such as cubes, stars, and octahedrons were successfully prepared. Basically, uniform spherical gold “seeds” around 4 nm will be first prepared through the reaction of chlorauric acid with sodium borohydride. Then with the assistance of structure-directing capping agent cetyltrimethylammonium bromide (CTAB) and a mild reductant ascorbic acid, Au NPs with different shapes could be produced by controlling synthetic parameters in the growth solution. Taking gold nanorods as an example, gold seeds stabilized with CTAB were first prepared, followed by the preferential growth along the [001] direction. The addition of silver and halide ions, impurities in CTAB and alloy formation were all proposed to affect the binding of CTAB on the different facets that finally influence gold nanorods formation.79 Poly(vinylpyrrolidone) (PVP) was also employed as capping agent for the synthesis of Au NPs with different morphologies with ethylene glycol or N,N-dimethylformamide (DMF) as reducing agents.80,81 Besides nanorods, gold nanoshells are also attractive due to similarly strong optical absorption in the desirable nearinfrared (NIR) region for photothermal therapy (PTT) and photoacoustic (PA) imaging.82 Gold nanoshells were usually formed on the surface of silica or other NPs as templates. Typically, small Au NPs were first attached onto the surface of cores through the reduction with tetrakis(hydroxymethyl)phosphonium chloride (THPC), and then gold nanoshells were produced through further reduction of gold.83 Templatebased methods are also employed to synthesize Au NPs with special morphologies. Gold nanocages, nanoframes, or nanospheres were achieved by galvanic replacement reaction

Polymers that are pH-, redox-, temperature-, or enzymeresponsive could be employed to construct the smart nanohybrids adapted to tumor microenvironments. For example, polymers with disulfide bonds will undergo reduction for redox responsiveness while acid-cleavable hydrazone linker and Schiff base are utilized for the construction of pH-sensitive nanohybrids.5 Polymers displaying a low critical solution temperature (LCST) or an upper critical solution temperature (UCST) in aqueous solution could be utilized for temperature responsiveness. Poly(N-isopropylacrylamide) (PNIPAM) is one of the most commonly used thermally responsive polymers with LCST at 32 °C. Cationic polymers such as polyethylenimine (PEI), polyamidoamine (PAMAM), and poly((2-dimethyl amino)ethyl methacrylate) (PDMAEMA) could condense nucleic acids through electrostatic interactions for gene delivery, which are considered as typical nonviral gene carriers.57 The steric effect and electrostatic repulsion from the cationic polymers may contribute to the protection of DNA from degradation by enzymes in the plasma. Due to the rich functional groups carried by polymers, epoxy group-, carboxylic acid group-, hydroxyl group-, aldehyde group-, and primary amine group-containing polymers could immobilize biomolecules for applications in biosensors or delivery systems with specific targeting.26,58,59 2.1.3. Biomolecules. Biomolecules are usually used to introduce the improved biocompatibility, solubility, and especially active target specificity to nanohybrids. Folic acid receptor-directed targeting has been intensively investigated to facilitate cellular uptake of functionalized various NPs.60 Other typical biomolecules such as antibodies,61 aptamers,62 and peptides63 are also widely employed to achieve biorecognition of specific membrane receptors or antigens on the target cells.64 For example, anti-epidermal growth factor receptor (EGFR) antibody could target EGFR with high specificity. Aptamers are able to bind to specific antigens through shape recognition,65 which could be easily conjugated to the surface of amine-functionalized NPs through carbodiimide coupling chemistry for enhanced ability to recognize cancer cells.66 Transferrin is also often used to target cancer cells with abundant expression of transferrin receptors.67 Some kinds of natural polysaccharides including pullulan and HA possess inherent targeting function. Pullulan demonstrates specific affinity to hepatoma cells with asialoglycoprotein receptor (ASGPR) overexpression,68 while HA is able to target tumor cells that overexpress CD44 receptor.69 2.2. Inorganic Nanoparticles

Inorganic NPs with at least one dimension in the range of 1− 100 nm and a variety of morphologies could be synthesized with physical or chemical strategies through “top-down” or “bottom-up” approaches.34 Besides the prerequisites of biocompatibility and nontoxicity, further surface functionalization is crucial to achieve suitable solubility and dispersibility in body fluids for biomedical applications.64,70 Control over the composition, size, shape, and surface engineering endows inorganic NPs with distinct properties for effective diagnostic and therapeutic applications. Compared with organic biomaterials, the additional advantages of inorganic NPs rely on their potential to facilitate imaging and monitor the therapeutic process.26 With the principles applied in vitro and in vivo systems, several major types of inorganic NPs including gold, silica, iron oxide, quantum dots, carbon, and rare earth NPs D


Chemical Reviews

Review

“surface-protected etching” strategy simplifies the procedure of selective etching of core/shell solid nanospheres. Furthermore, inhomogeneous silica NPs synthesized using the Stöber method were found to undergo selective etching of the inner part in the absence of any surface protection, resulting in hollow nanospheres or nanorattles.113,114 The realization of these hollow structures through self-templating approach lies in the fabrication of inhomogeneous silica particles possessing different outmost and inner parts. The facile structure deformation based on selective etching provides a new avenue to hollow NPs to save the template removal processes. Similarly, one-dimensional (1D) silica nanotubes could also be synthesized through the templating method.115,116 Silica nanorods with different aspect ratios were prepared successfully by modulating the concentration of reaction reagents with the assistance of CTAB.36,117 Silica NPs are considered degradable although the degradation may take a long time owing to the dense silicate network.118 The shape of silica NPs was found to influence their biodistribution and clearance.36 In order to accelerate the degradation and elimination, different strategies have been developed to synthesize biodegradable silica NPs.119−123 Encapsulation of small molecular drug or hydroxyapatite in silica NPs makes the composites degradable.119,120 Disulfide (S−S)-bridged silsesquioxane was also incorporated in the silica backbone to realize the redox-responsive degradation of silica NPs.121−123 2.2.3. Iron Oxide Nanoparticles. Iron oxide nanoparticles (such as Fe3O4 NPs) with unique magnetic properties have demonstrated great potential in biomedical areas such as delivery system, therapeutic platform, and magnetic resonance imaging (MRI).15,21,124−126 A variety of synthetic approaches have been proposed to prepare Fe3O4 NPs, including physical, wet chemical preparation, and microbial methods.127−129 Coprecipitation is the simplest synthetic strategy, where Fe3O4 NPs were prepared by the chemical reactions of ferrous and ferric salts (stoichiometric ratio of 1:2) with base in aqueous solution.130 Although the growth of Fe3O4 NPs could be tailored by experimental conditions such as iron strength, acidity, and temperature, the control over the size distribution and stability seems poor. The superiority of this strategy relies on the production on a large scale, which is favorable for potential industrial applications. Hydrothermal synthesis carried out in autoclaves under high temperature and pressure will produce Fe3O4 NPs with improved crystallinity.131 Unique iron oxide nanorings and nanodiscs could achieved by the hydrothermal and following reduction reactions.132,133 Polyol process was also employed for the synthesis of iron oxide, where polyol serves as the solvent, stabilizer and reducing agent.134 Thermal decomposition is considered as one powerful strategy to obtain monodispersed Fe3O4 NPs with high degree of crystallinity.135 Decomposition of iron organic precursor at high temperature around 300 °C occurred in organic solvents with surfactants. The size and morphology of Fe3O4 NPs could be tailored utilizing suitable precursor, solvent, temperature, and capping ligands. For example, nanocubes with the size from 20 to 160 nm could be achieved using thermal decomposition of iron acetylacetonate in benzyl ether in the presence of oleic acid as surface modifier.136 The draw-back of the thermal decomposition approach is also obvious since the synthetic procedure accompanying with high temperature and organic solvent is not environmental friendly.

utilizing silver nanocubes or cobalt nanospheres as templates.84−86 Polystyrene (PS) NPs could also be utilized to obtain hollow gold nanospheres, and the template was removed by dissolution with tetrahydrofuran (THF) or calcination.87 It is noticed that novel cap-like Au NPs could be achieved by a facile solution method employing PbS nanostars as the sacrificial templates, which were etched by hydrochloric acid.88 Similarly, PbS nanooctahedrons were also employed to obtain gold nanocups.89 The unique cavities within gold nanocaps or semishells caused red-shifted optical absorption in NIR region and photothermal effect.90−93 Au NPs are usually considered too inert to be degraded or cleared in vivo. Actually, structural alteration and crystalline reorganization could occur in the presence of thiols in cells.94 Moreover, internalized Au NPs will undergo shape transition while degradation happens on some faces. 2.2.2. Silica Nanoparticles. With the favorable virtues of controlled morphology, porous structure, ease of facile surface functionalization, and biocompatibility, silica NPs stand as ideal candidates for biomedical applications, especially in drug/ gene delivery due to their capability to carry different kinds of cargos. Both mesoporous (with pores ranging from 2 to 50 nm95−98 and nonporous silica NPs99 (with amorphous structures) are attractive in biomedicine fields. The synthetic procedure could be achieved by controlled hydrolysis and condensation of organosilane precursors (such as tetraethyl orthosilicate (TEOS)), while the morphology could be controlled by adjusting the concentration of reaction reagents, the addition of different surfactants or catalysts, as well as the reaction temperature and time. The Stöber method is wellknown to synthesize monodispersed silica NPs with the size from 50 nm to 2 μm.100 Ammonia-catalyzed growth process occurred through surface reaction-limited condensation and mesoporous silica NPs (MSNs) could be synthesized with minor modification of the Stö ber approach by adding surfactants as capping agents. MCM-41 silica NPs with ordered two-dimensional (2D) hexagonal mesopores were achieved using surfactant-mediated Stöber method.101,102 MCM-48 silica NPs with three-dimensional (3D) bicontinuous mesopores could be obtained using similar method in the presence of binary surfactants of CTAB and Pluronic F127.103 Hollow silica NPs could be synthesized employing soft templates such as micelle, vesicle, and microemulsion.104−107 Although soft templating method is powerful, obtained silica NPs demonstrated wide size distribution while aggregation usually happened in solution. Well-defined hollow NPs could also be achieved through hard templating methods and the templates will be removed by either template calcination or solvent extraction.108 It is worth mentioning that selective etching strategies were developed to silica NPs with hollow interiors based on the differences in composition or structure. Typically, multilayer core/shell solid NPs were first fabricated as the templates and hollow nanosperes or nanorattles could be obtained after selective etching of the core or middle layer of original structures. For example, three-layer structured silica NPs were designed, where the core and outermost ingredients were silica condensed from TEOS while the middle part was hybrid siloxane hydrolyzed from the mixture of TEOS and N[3-(trimethoxysilyl) propyl]ethylenediamine.109 After controlled etching of the middle part which is less compact than ordinary silica, nanorattles could be achieved. Silica nanospheres with PVP capping on the surface could turn to hollow/ rattle structures after alkaline treatment.110−112 This kind of E


Chemical Reviews

Review

Figure 2. Construction of organic/inorganic nanohybrids through surface functionalization, wrapping and one-pot synthesis.

including multifunctional imaging and therapeutic systems.15,147−150 Compared with conventional organic dyes, QDs possess long-term photostability, high photoluminescence quantum yields, as well as size-tunable and narrow-band emissions, which make QDs emerge as ideal fluorescent probes to track important biological processes.151 The electronic and optical properties of QDs could be tuned by their composition, size, and shape that could be controlled by the reaction temperature, surfactants, and precursors. High quality CdSe QDs with narrow size distribution and high quantum yields were synthesized using high temperature reaction of organometallic precursors developed by Bawendi et al.152 Thermal decomposition of metal precursors after rapid injection at temperatures higher than 280 °C in the presence of capping agents resulted in homogeneous seeding and growth of QDs. In order to reduce cytotoxicity, metal oxides or other salts were then employed as precursors to obtain QDs.153,154 Passive layer such as ZnS, CdS and CdSe is usually added to the QD core by forming core−shell structure to preserve or increase the photoluminescent (PL) quantum yield.155 Due to the maturity of synthetic methods, Cd-based QDs with potential toxicity predominate in biomedical areas. Cd-free QDs with comparable performances such as copper indium sulfide (CuInS2), indium phosphide (InP), Ag2S and ZnO were also fabricated for safe purpose.150 The synthetic strategies of InP QDs are similar to those of Cd-based QDs employing the high

Fe3O4 NPs could also form one-dimensional nanostructures, which demonstrate unique properties and potential applications.137 Through magnetic dipole-directed assembly process, where an external magnetic field or ultrasonic irradiation is usually applied, 1D nanostructure could be induced.138,139 Many efforts have been made to keep the as-produced structures when the external forces were withdrawn. Nanochain could be retained after the coating of inorganic NPs or polymers on the surface of 1D nanostructure.140−142 Besides, 1D iron oxide nanoworms could be synthesized utilizing coprecipitation protocol with the assistance of dextran, where the hydrogen bonding between hydroxyl groups from dextran and the surface of iron oxide played an important role.143 Interestingly, linear nanochains of iron oxide could also be formed through covalent bonding with liposome.144 The degradation of Fe3O4 NPs is supposed to follow the mechanism related to ferritin and transferrin, which will be dissolved in acidic environment of the lysosome.145 The excess ions released from Fe3O4 NPs will be cleared with the help of transferrin and ferritin by storage and transportation.146 The degradation rate of Fe3O4 NPs in vivo depends on the dosage, the amount of iron storage proteins, and the type of surface coating ligands. 2.2.4. Quantum Dots. Semiconductor quantum dots (QDs) are considered as ideal fluorescent probes due to their intrinsic optical properties, which have attracted considerable interests for their promising bioapplications, F


Chemical Reviews

Review

Table 1. Overview of Construction Approaches of Organic/Inorganic Nanohybrids approach grafting onto

interaction Au−S bonding

Cd−S bonding Mo−S/S-S bonding

Cu−S bonding W−S bonding Bi−S bonding amide bonding

organic component PEG-SH

PEG-LA PEI-SH PEI-LA PS-SH PLGA-SH P(NIPAm-co-AAm)-SH P(OEOMA-co-MEMA)-SH PEO-b-PS-SH PAA-b-PS-SH PGED-LA CS-LA CNC-LA PGEA-LA PGEA-SS-PGEA PDM-SS-PDM RGD-SH BSA-PEO-LA PEG-SH PEG-LA PEI-LA G5 PAMAM-LA BSA PEG-SH PEG-LA PEG-SH PEG-NHS

PEG-COOH PEG-NH2

Schiff base bonding

PAE PEI PNIPAM-NH2 HA TAT peptide CS CXCR4 antibody BSA RGD peptide PEG-NH2

inorganic component

refs

Au nanohexapod, nanorod, nanocage, nanocup, nanosphere, hollow NP, nanoring, nanostar, nanobipyramid mesoporous SiO2@Au core−shell nanorod hollow CuS@Cu2S@Au shell/satellite nanosphere Fe3O4@Au core−shell nanorose Bi2S3−Au heterojunction nanorod Janus Au−Fe3O4 NP Janus Au−Fe3O4@C NP Janus Au-PAA/Ca3(PO4)2 NP Au nanorod, nanosphere Fe3O4@Au nanostar Au nanosphere Janus Au−Fe3O4 NP Au nanorod yolk-shell Fe3O4@Au, core−shell Fe3O4@Au NPs hollow CuS@Cu2S@Au shell/satellite nanosphere Au nanosphere Au nanosphere Au nanorod Au nanosphere Au nanorod, nanosphere Au nanorod Gd2O3:Yb/Er@Au core−shell nanorod Au nanorod Au nanosphere CdSe/CdZnS QD MoS2 nanoflake

190 193 194 195 196 191 192 197 199 198 196 185 201 193 202,203 203 200 204 197 200 206 205 187 207 211

MoS2 nanosheet, flower-like MoS2 nanoflake MoS2 nanoflake MoS2 nanoflake flower-like MoS2 nanosphere, layered MoS2 nanoflower NaYbF4:Er/Gd@SiO2@CuS core/satellite nanosphere WS2 QD, nanosheet Bi2S3 nanorod hollow silica nanosphere Janus Ag-SiO2 NP C@SiO2 core−shell nanosphere Au@SiO2 core−shell nanorod CdTe@ZnS QD C dot yolk-like Fe3O4@Gd2O3 NP C dot nanosized GO MoS2 nanoflake β-NaGdF4:Yb/Tm@NaGdF4 core−shell NP rattle-type SiO2@Au NP rattle-type Fe3O4@CuS NP Janus Au@SiO2 NP Janus Fe3O4−SiO2 NP SiO2 nanosphere NaYF4:Yb/Tm@NaGdF4/Yb nanosphere SWCNT β-NaYF4:Yb/Er NP NaYF4:Yb/Er/Tm@TiO2 NP NaYF4:Yb/Er NP Au@SiO2 nanorod NaGdF4:Yb/Er NP Ag2S QD SiO2 nanosphere

212,213 211 214 215 208 209,210 195 218 219 220 221,222 223 224 225 226−228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245

G

93,180−189


Chemical Reviews

Review

Table 1. continued approach

interaction

refs

dopamine coupling

PEG-dopamine

phosphate adsorption

P(OEGA)-phosphonic acid

dumbbell-like Au−Fe3O4 NP Fe3O4 nanosphere

261 263,264

γ-Fe2O3 NP Fe3O4 NP

265 266

Janus Au−Fe3O4 NP NaYF4:Yb/Tm@NaGdF4 nanosphere CeO2:Yb/Tm nanosphere Janus Au−Fe3O4 NP NaGdF4:Yb/Er@NaGdF4 nanosphere Au nanosphere, nano-octahedron, nanorod, arrow-headed nanorod

196 267 268 196 269 272,273

P(GMA-co-PEGMA) P(PEGMA) PDMAPS P(OEGA)

SiO2 nanosphere, hollow nanosphere, nanorod, chiral nanorod GO nanosheet Fe3O4 nanosphere Fe3O4 nanosphere hollow SiO2 nanosphere Janus SiO2 nanosheet Janus SiO2 nanosheet Fe3O4 nanosphere spherical and peapod-like Fe3O4@SiO2 NP spherical and wormlike CdxZn1−xTe/CdS@SiO2 NP spherical and starlike Au@SiO2 NP Fe3O4 nanosphere Fe3O4 nanosphere layered double hydroxide NP Fe3O4 nanosphere

274 275 276 277 278 279 279 281,282 284 285 92 283 286−288 289 263

P(PEGMA) PS POEGMA PAA P4VP PDEAEMA-co-PPDSM PDEAEMA-b-POEGMA PHPMA-b-PDMAEMA HPG PEI β-CD

Fe3O4 nanosphere Fe3O4 nanosphere NaYF4:Tm/Yb NP SiO2 nanosphere SiO2 nanosphere SiO2 nanosphere SiO2 nanotube SiO2 nanosphere MoS2 nanosheet SWCNT NaYF4@SiO2@mSiO2 core−shell−shell nanosphere

295 299 296 297 298 300 301 302 303 304 311

PEG-Ad PGEA-CD

SiO2 nanosphere, hollow nanosphere SiO2 nanosphere, starlike and spherical hollow nanosphere Au@SiO2−CdTe core−shell nanorod Au nanorod and nanosphere SiO2−WS2 nanosphere SiO2 nanosphere

312,313 315,316 317 197 318 319

silane coupling

ATRP

EGF PEG-silane

P(HBA)-b-P(OEGA)-phosphonic acid PEG-phosphate PEG-DSPE PEG-phosphorylated serine PS-phosphate RGD phosphopeptide PDMAEMA

PMEO2MA-b-PDMAEMA PDEAEMA PNIPAM PGMA

Cu(0)-mediated LRP RAFT

ROP selfassembly

inorganic component SiO2 nanosphere NaYF4:Tm/Yb@NaYbF4@SiO2 nanosphere UCNP, Fe3O4, Cu9S5, and WS2:Cd nanoflake SiO2 nanosphere NaYF4:Yb/Tm@NaGdF4@SiO2 nanosphere CdSeTe@ZnS QD SiO2 nanorod Cu9S5@mesoporous SiO2 nanosphere Janus Au-MnO@SiO2 NP β-NaGdF4:Yb/Er@SiO2−Au25 nanosphere NaGdF4:Yb/Er@NaGdF4:Yb@Mn-SiO2 NP NaGdF4:Yb/Tm@NaGdF4:Yb@NaNdF4:Yb@NaGdF4@SiO2 @TiO2 nanosphere Fe3O4 nanosphere

click linkage

grafting from

organic component sericin protein PEG-N3 PEG-SH PEG-MAL

host−guest interaction

tLyP-1-CD LbA-CD

H

246 249 250 251 252 253 254 255 256 257 258 259


Chemical Reviews

Review


interaction electrostatic interaction

hydrophobic interaction

organic component

one-pot

320

polyamine dendrimer PEG-(PAH/DMMA) PAH, PAA PEI,PAA,P(AA-co-glucose)/ P(AA-co-galactose) PLL

ZnFe2O4@Au core−shell nanosphere C dot hollow MnO2 nanosphere Au NP

321 322 323 324

Prussian blue nanocube Au nanosphere MWCNT SWCNT, nanohorn

325 326 327 329,330

hollow Si/C nanosphere single-walled C@SiO2 core−shell nanotube Janus Au@SiO2 NP Au−Cu9S5 NP WS2:Gd nanoflake NaYbF4: Er/Ce@NaYF4 nanosphere SWCNT Janus Au−Fe2C NP CdTe@ZnS QD NaYF4:Yb, Er nanosphere CaCO3 nanosphere MnOx/Ta4C3 nanosheet C dot-C3N4 nanocomposite oxidized C nanosphere Fe3O4 nanosphere QD MoS2 nanosheet MoS2/Bi2S3 nanosheet MoS2/Fe3O4 nanoflake Fe3O4 nanosphere Fe3O4 NP Mn2+-doped Ca3(PO4)2 nanosphere CaCO3 nanosphere Ca3(PO4)2 nanosphere Ca3(PO4)2 nanosphere CaCO3 nanosphere Au NP Fe3O4 NP Te nanodot Au NP graphene/Fe3O4 Au nanocluster Ag NP Au nanocluster Au NP hollow Ca3(PO4)2 nanobowl Fe2O3, Fe3O4, SnO2, Ag NP Au nanosphere, branch NaYF4:Yb,Er@NaYF4:Yb@NaNdF4:Yb@NaYF4:Yb nanosphere Fe3O4 nanosphere Au nanorod CdSe/ZnS QD ZnFe2O4 nanosphere Fe3O4 NP C dot NaYF4:Yb,Er NP NaGdF4:Yb,Er@NaGdF4 nanoplate Bi2Se3 nanoplate Fe3O4 nanosphere

331 332 333 334 335 336 337,338 339 340 341 342 343 347 348 349 350 351 352 353 354 355 356 357 358.359 360 361 362 363 364 365 366 41 367 368 369−371 372 373 374 375 376 377−379 380 381 382 383 385 386 388 389

TAT-CS PEG-C18PMH

biorecognition

SP PpIX-PEG-RGD Cy3-labeled ssDNA Cell-derived vesicle-biotin

capping

PEG

P(OEGMA-co-MAA) P(OEGA)-b-P(PAEA) PEG-b-P(Glu) PEG-b-PAsp

precipitation

PEG−PAsp-PPhe F127 matrix-alginate POEGMA-b-PDMAEMA-b-PS POEGMA-b-PMAA-b-PS HSA dextran PEI DAA-HA-b-PCL polycat BSA PS-b-P4VP PDA, PAA PAA

cross-linking

PFODBT CS

reduction

selective growth wrapping

MOF polymerization

refs

ZnxS−AgyIn1‑yS2 QD

C18PMH PEG−PL PEG-DSPE

π−π stacking

inorganic component

PEI

PDA PCBMA

I


Chemical Reviews

Review


interaction

assembly

organic component PS CS, PEG PAH sodium alginate, CS, HA HA PEI-CD, PEG-Ad, PAMAM-Ad lipid-PEG mPEG-b-P(DPA-DE)LG PLGA PEG−PLA-PEG P(MEO2MA-co-(HEMA-g-PCL)) F127, PAA PS dextran

inorganic component

refs

Fe3O4@SiO2 nanosphere C dot Au nanocluster Fe3O4@GO nanosphere SWCNT Zn0.4Fe2.6O4 NPs VS2 nanodot Fe3O4 nanosphere Fe3O4 nanosphere Fe3O4 nanosphere, QD Fe3O4 nanosphere Au nanosphere Fe3O4 nanosphere Fe3O4@SiO2 nanosphere, hemishell Fe3O4 nanosphere

temperature solution methods while aqueous CuInS2 QDs could be prepared by hydrothermal method.156 For biomedical applications, QDs should be water-soluble and ligand exchange was necessary for QDs with organic ligands. On the other side, aqueous synthesis of QDs would be more convenient for diverse functionalities.157 Precipitation reaction could be employed with appropriate capping agents to result in CdS, ZnS, CdTe, and PbS QDs. Thiol molecules were powerful and widely used in synthesizing QDs with confined size. Mercapto acids were advantageous in obtaining QDs with satisfying optical properties and stability.158 The aqueous synthesis was also assisted by microwave or hydrothermal procedures to provide homogeneous heat. For example, CdxZn1-xTe/CdS (core/shell) QDs with high FL were synthesized adopting microwave-assisted procedure in water with mercaptopropionic acid (MPA) and glutathione (GSH) as the capping agents.159 Recently, a new type of 2D QDs such as graphene, MoS2, WS2, and MXene appears as the emerging QDs, which are smaller forms of their corresponding 2D layered materials.160 2D QDs could be synthesized using both top-down strategies of exfoliation techniques and bottom-up strategies via pyrolysis, hydrothermal, hydrolysis, or microwave-assisted treatments. 2.2.5. Other Inorganic Nanoparticles. In addition to the aforementioned inorganic NPs, a variety of NPs applied in biomedical areas are also investigated. Upconversion NPs (UCNPs) which could convert low-energy NIR light to high energy ultraviolet/visible (UV/vis) light are intriguing candidates for upconversion luminescence (UCL) imaging. The unique 4f orbitals of rare-earth elements endow them with distinct properties.161,162 UCNPs could be synthesized adopting hydrothermal/solvothermal strategy, coprecipitation method, sol−gel method, or thermal decomposition.22,23,161,163−166 Heterogeneous nanocrystals of UCNPs with programmable morphologies and components were successfully fabricated through controlling the ratio of surface ligands.167 Similar to iron oxide, UCNPs carrying organic ligands such as oleic acid or trioctylphosphine oxide (TOPO) are prepared through thermal decomposition, which is supposed as the most effective strategy to obtain well-defined UCNPs.23 Core−shell structures were also designed with precisely defined concentrations of reactants to produce efficient upconversion emission.168,169

390 391 392 393 394 395 397 398 399 401 400 402 405 406,407 408

Other inorganic NPs such as calcium phosphate, calcium carbonate, apatite, transition metal dichalcogenide, carbon nanomaterials, as well as heteronanoparticles also could be utilized for biomedical applications.170−174 Top-down and bottom-up strategies such as hydro/solvothermal approach, coprecipitation method, mechanical and chemical exfoliations, chemical vapor deposition, laser ablation, as well as hard/soft templating method were usually employed to synthesize these NPs.175 2.3. Fabrication Strategies of Organic/Inorganic Nanohybrids

A variety of strategies have been developed for the construction of organic/inorganic nanohybrids. Overall there are three main kinds of methods, including surface functionalization, one-pot synthesis, and wrapping. Figure 2 provides an overview of various construction strategies. In this subsection, we briefly summarize the construction strategies for organic/inorganic nanohybrids by exploring appropriate methods, which may provide valuable information for the design and synthesis of required nanohybrids with desired functions and performances. Surface functionalization of inorganic NPs is the most widely utilized approach to fabricate nanohybrids, since surface engineering not only gives rise to the combination of organic and inorganic components but also provides a powerful tool to tailor the properties of the nanohybrids. Typically, surface functionalization with polymers is powerful to endow nanohybrids with biocompatible hydrophilic surfaces and functional groups to work with biomolecules such as peptides or genes for targeting or delivery.176 When polymer chains are covalently tethered to a surface or interface by one chain end with considerate grafting density and coating thickness, they could be defined as polymer brushes.177 Polymer brushes on the surface of inorganic NPs could be fabricated via “grafting from” or “grafting onto” approaches. The former method adopts the growth of polymer chains from the initiators anchored on surfaces, which has attracted much attention since it allows control of the architecture of polymer brushes such as grafting density, thickness of coating, and choice of monomers. Particularly, surface-initiated controlled/ living radical polymerizations, including reversible addition− fragmentation chain transfer (RAFT) polymerization,178 atom transfer radical polymerization (ATRP),179 and Cu(0)mediated polymerization, are most frequently employed for J


Chemical Reviews

Review

Figure 3. Construction of nanohybrids through Au−S bonds. Synthetic route of (a) PGED, PGEA, and (b) PDM-SS-PDM functionalized Au NPs, as well as conjugation of CNC and Au (c) through Au−S bonding.

the fabrication of well-defined polymer brushes for their bioapplications. Herein, we introduce the construction of nanohybrids through surface functionalization of inorganic NPs with polymer brushes via “grafting onto”, “grafting from”, and selfassembly strategies, which are categorized relying on the ways

polymers joined the nanohybrids. One-pot synthesis and wrapping taking advantage of noncovalent interactions between organic and inorganic components are also discussed. Some representative examples of these approaches are listed in Table 1. K


Chemical Reviews

Review

Figure 4. Schematic illustrations of (a) synthesis of PAE and (b) construction of PAE/silica nanohybrids. Reproduced with permission from ref 236. Copyright 2016 Wiley-VCH.

2.3.1. “Grafting onto” Strategy. For the integration of NPs and polymers, the easiest way is to graft the required polymers or biomolecules directly onto the surface of assynthesized NPs. In this case, the molecular weight of grafted polymer is fixed. This process could also be considered as chemical substitutions through suitable conjugation strategies, where the grafting density is limited due to steric hindrance. Therefore, there will be a compromise between synthetic convenience and corresponding effectiveness. As discussed in section 2.1, metal−sulfur bonds could be utilized for grafting polymers with thiol groups onto metallic surfaces. The most well-known example is Au-thiol bonding, which constitutes the predominant interaction of Au NPs with thiolated polymers. The most commonly used polymer PEG could be successfully introduced into the nanohybrids by grating thiol-ended PEG (PEG-SH) onto gold surfaces, including Au NPs with various morphologies,93,180−189 as well as core−shell and Janus hetero-NPs,190−196 as shown in Table 1. Lipoic acid (LA) with a dithiolane was conjugated with PEG to form PEG-LA, which can readily bind to gold surfaces through multiple dithiolane rings.197 Other polymers including branched polymers and block copolymers could also be grafted onto gold surfaces through thiol-ended or LA conjugated formulations (such as thiolated PEI (PEI-SH), PS (PS-SH), poly(lactic-co-glycolic acid) (PLGA-SH), poly(Nisopropylacrylamide-co-acrylamide) (P(NIPAM-co-AAM)SH), poly(ethylene oxide)-b-polystyrene (PEO-b-PS-SH), and LA conjugated PEI (PEI-LA)).185,193,196,198−203 Chitosan (CS) could also be grafted onto the surface of Au NPs by the preparation of CS-LA.204 Thiol-ended polymers could be synthesized using RAFT with the subsequent reduction of chain transfer agent (CTA)202,203 and ring opening polymer-

ization (ROP) or ATRP from disulfide-containing initiator followed by the reduction of disulfide bonds.185,193,201 In order to conjugate LA to polymers for Au−S bonding, different precursors and reactions could be utilized to improve the efficacy. Two kinds of cationic polymers, ethylenediamine (ED) and ethanolamine (EA) functionalized poly(glycidyl methacrylate) (PGMA; denoted by PGED and PGEA, respectively), were here demonstrated as the examples. As shown in Figure 3a, LA was linked to PGED through carbodiimide coupling protocol to produce PGED-LA while lipoic acid anhydride (LA-LA) was employed to link dithiolane rings to the side chain of PGEA.200 Notably, cellulose nanocrystals (CNC) could also be modified with LA (CNCLA) to bind Au NPs and nanorods (Figure 3b), taking advantage of the hydroxyl groups on the surface of CNC.197 Disulfide-linked polymers such as PDMAEMA (PDM-SSPDM, Figure 3c) and PGEA (PGEA-SS-PGEA) are also synthesized from ATRP of disulfide-containing initiator to produce nanohybrids through Au−S bonding.205,206 Biomolecules such as thiol-containing arginine-glycine-aspartic acid (RGD) peptides (RGD-SH) could bind to Au NPs by stable Au−S bonds.187 Similarly, thiol-ended and LA-conjugated polymers could be grafted onto various metallic surfaces through Cd−S,207 Cu−S,208 W−S,209,210 and Bi−S195 bonding to fabricate diverse nanohybrids of polymers and MoS2, Bi2S3, and heteronanoparticles. These polymers can also be attached to the surface of MoS2 NPs easily;211−215 however, the binding mechanism is still obscure. So far there are no sufficient experimental supports for the formation of Mo−S bond while disulfide bonds were detected. Mcdonald et al. proved that the thiol group of cysteine was physisorbed on MoS2 instead of covalent or dative bonding.216,217 Although more insights in the specific interaction need further study, conjugation with L


Chemical Reviews

Review

Figure 5. Schematic illustrations of synthesis of (a) phosphonate bearing initiator and P(OEGA) functionalized iron oxide, diblock stealth glycopolymers followed by post modification reaction with mannose/glucose (b) or (c) thiol-epoxy. Reproduced with permission from ref 264. Copyright 2014 The Royal Society of Chemistry.

thiol-containing polymers is indeed favorable to fabricate nanohybrids of MoS2 via “grafting onto” strategy.

Amide linkage is another well-known bond to graft polymers onto the surface of NPs, which forms between amine and M


Chemical Reviews

Review

Figure 6. Schematic illustration of nanohybrids fabricated from linear or cyclic phosphopeptides and UCNPs. The starting green particle stands for oleic acid-modified UCNP. Reproduced with permission from ref 269. Copyright 2016 American Chemical Society.

way, biomaterials with carboxylic groups such as HA and TAT (transactivator of transcription) peptide have been successfully immobilized on the surface of amine-functionalized NPs by EDC-mediated amide coupling.239,240 Amino group-containing biomaterials including CS, CXCR4 antibody, and RGD peptide could also be grafted onto the surface of inorganic NPs with terminal carboxylic groups through amide formation.241−244 Schiff base linkage and click reaction have also been applied for the grafting of polymers and biomolecules onto the surface of NPs. Amino group-terminal PEG and sericin protein were successfully attached to silica NPs functionalized with aldehyde groups through the formation of pH-sensitive Schiff base bonds.245,246 Click chemistry provides a reliable and convenient strategy to fabricate a wide variety of nanohybrids with high efficiency.247,248 Taking advantage of the classical click reaction of Cu(I)-catalyzed azide−alkyne cycloaddition, PEG with terminal azido groups has been readily linked to silica coated UCNP with terminal alkyne groups.249 To be noted, the drawback of potential cytotoxicity introduced by the catalyst should also be taken into consideration. Extensive dialysis is usually required to get rid of copper(I) for further biomedical applications. Therefore, click reactions without the addition of toxic copper catalyst such as thiol−ene and thiolMichael addition could be preferable. For instance, PEG-SH was successfully conjugated onto the surface of oleic acid or oleylamine functionalized inorganic NPs including UCNP, Fe3O4, Cu9S5, and WS2:Cd.250 PEG-maleimide (PEG-MAL) could be anchored to thiol group-terminated silica or silica

carboxyl groups. Carbodiimide chemistry is usually employed for amide formation. Polymers with carboxyl groups are readily grafted onto the surface of amino groups-functionalized NPs. In turn, amino groups-containing polymers will react with carboxyl groups on the surface of NPs to achieve amide bondlinked nanohybrids. 1-Ethyl-3-(3-(dimethylamino)propyl) carbodiimide (EDC), dicyclohexylcarbodiimide (DCC), and NHS are common carboxyl activating agents in coupling reactions to form amide bonds. As mentioned in section 2.1.1, APTES is widely used to obtain amine-functionalized surfaces for the following grafting of polymers. PEG-NHS was grafted onto various APTES-modified NPs, including silica as well as silica-based Janus and core−shell NPs and QDs.218−224 Carboxylic group-terminated PEG (PEG-COOH) was also grafted onto the surface of yolk-like Fe3O4@Gd2O3 NPs in the presence of EDC and NHS.225 Alternatively, amino groupterminated PEG (PEG-NH2) could be grafted onto carboxylic group-terminated NPs, such as carbon dots,226−228 graphene oxide (GO),229 MoS2 nanoflakes,230 UCNP,231 as well as core−shell and Janus heteronanoparticles.232−235 Cationic polymer, poly(β-amino esters) (PAE) synthesized by Michael addition reaction, was grafted onto the surface of APTES modified silica NPs through a two-step amidation reaction (Figure 4).236 3,3′-Dithiodipropionic acid (DTDPA) was employed as the intermediate molecule to form two kinds of amide bonds and introduce disulfide linkers for redoxresponsive gene delivery. Polymers with amino groups (such as PEI) and animated polymers are able to be grafted onto NPs modified with terminal carboxylic groups.237,238 In the same N


Chemical Reviews

Review

Figure 7. Schematic illustration of the synthetic routes of PDMAEMA/Au (a) and PDMAEMA/SiO2 nanohybrids (b). Reproduced with permission from refs 272 and 274. Copyright 2014 and 2015 Elsevier.

coated UCNP by Michael addition.251,252 With the thiol Michael addition, biomolecules such as reduced EGF with free sulfhydryl groups have been conjugated to amine-functionalized CdSeTe/ZnS QDs.253 Moreover, with the help of versatile silane chemistry as discussed in section 2.1.1, silane-containing polymers could also be conjugated onto the silica surfaces through the formation of Si−O bonds besides small molecular silane species. For example, PEG-functionalized NPs could be accomplished by “grafting onto” approach to produce diverse nanohybrids composed of PEG and silica or silica coated inorganic NPs.254−259 By taking use of the coordination bonds between the bidentate catechol and iron oxide surfaces,260 nanohybrids can be fabricated through direct conjugation of dopaminecontaining polymers to iron oxide surfaces. For example, dopamine-conjugated PEG was grafted onto the Fe3O4 NPs or the selective Fe 3 O 4 side of dumbbell-like Au−Fe 3 O 4 NPs.261,262 In addition, several anchoring groups such as phosphate exhibit strong affinity to the iron oxide surfaces and phosphate- or phosphonic acid-terminated polymers such as poly(oligo(ethylene glycol) acrylate) (P(OEGA)) were used to functionalize iron oxide NPs,263,264 which could be synthesized from Cu(0) mediated living radical polymerization (LRP) of oligo(ethylene glycol) acrylate (OEGA) from a phosphonate functional initiator (Figure 5a). Furthermore, diblock glycopolymers P(OEGA)-b-P(sugar) could be synthesized through the chain extension of P(OEGA) with glycidyl

acrylate (GA) for subsequent modification (Figure 5b,c) with sugar moieties. These block copolymers were successfully grafted onto the surface of Fe3O4 NPs. Alternatively, αphosphonic acid, ω-dithiopyridine functionalized P(OEGA) was synthesized via the RAFT polymerization employing a trithiocarbonate RAFT agent bearing dimethyl phosphonate group and then grafted onto the surface of γ-Fe2O3 NPs through the α-chain end of the polymer.265 In this case, P(OEGA)/γ-Fe2O3 nanohybrids could be further functionalized with peptides through the ω-dithiopyridine functionality. In addition, phosphonic acid-terminated block copolymer P(HBA)-b-P(OEGA) (poly(4-O-acryloyl benzaldehyde-b-oligoethylene glycol-acrylate)) prepared via RAFT was grafted onto Fe3O4 surfaces.266 Phosphonated PEG and PS were grafted onto the iron oxide side of Janus Au−Fe3O4 NP through the similar coupling.196 Moreover, by making use of the affinity of phosphate to lanthanide elements, phosphate-containing polymers including PEG-DSPE and PEG-phosphorylated serine were linked to the surface of UCNPs.267,268 Zhang et al. synthesized phosphorylated linear and cyclic RGD peptides, which allow for dramatically enhanced adsorption on the surface of UCNPs to fabricate nanohybrids with target recognition,269 as shown in Figure 6. The starting green particle stands for oleic acidmodified UCNP. Molecular dynamics (MD) simulation revealed that strong binding was driven by the electrostatic interactions between phosphopeptides and surface which may O


Chemical Reviews

Review

Figure 8. Schematic illustration of the synthetic routes of (a) 2-(2-bromoisobutyryloxy)ethyl) phosphonic acid (BiBEP) and (b) PDMAEM/Fe3O4 nanohybrids. Reproduced with permission from ref 276. Copyright 2011 American Chemical Society.

variety of morphologies were readily synthesized for the investigation of morphology effect.273 With the help of silane chemistry, APTES was conjugated to the surface of silica through a siloxane bond to produce terminal amine groups, which reacted with 2-bromoisobytyryl bromide (BIBB) to fabricate ATRP initiator SiO2−Br, as shown in Figure 7b. PDMAEMA could be then grafted from the surface of silica NPs with diverse morphologies, including spheres, hollow spheres, nanorods, and chiral nanorods with different aspect ratios.274 Biocleavable PDMAEMA/GO nanohybrids were developed, and the flexible introduction of disulfide-containing ATRP initiators relies on the activation of the hydroxyl and carboxyl groups on the surface of GO sheets with cystamine in the presence of 1,1′-carbonyldiimidazole (CDI).275 PDMAEMA/Fe3O4 nanohybrids were achieved by exploiting the strong affinity between the phosphonic acid group and iron oxide. As illustrated in Figure 8, 2-(2bromoisobutyryloxy)ethyl) phosphonic acid (BiBEP) was first synthesized to immobilize the initiators for the subsequent ATRP.276 The grafted PDMAEMA brushes could be further transformed to poly(quaternary ammonium) after quaternization for recyclable antibacterial applications due to their magnetic response. Diblock copolymer of PDMAEMA and thermoresponsive poly[2-(2-methoxyethoxy)ethyl methacrylate] (PMEO2MA) were grafted from the surface of Fe3O4 NPs with APTES as the linker to immobilize the ATRP initiator, and finally the as-fabricated nanohybrids realized magnetic field/temperature dual stimulus gene transfection.277 Similarly, pH-responsive polymer poly(2-(diethylamino)-ethyl methacrylate) (PDEAEMA) was grafted from the exterior surface of hollow silica nanosphere, while reduction-cleavable disulfide bonds and light-cleavable o-nitrobenzyl esters were introduced as the linkages to fabricate triple-responsive nanohybrids as drug carriers.278 Interestingly, Yang et al. constructed Janus PNIPAM/SiO2/PDEAEMA nanohybrids by grafting thermal responsive PNIPAM and pH responsive PDEAEMA from the two sides of the Janus silica nanosheets via sequential ATRP processes.279 Janus silica nanosheets were first synthesized by crushing hollow spheres with exterior surface of terminal amine groups and the interior surface of

offer a versatile way to design nanohybrids through phosphorylation tethering. 2.3.2. “Grafting from” Strategy. In contrast to the “grafting onto” strategy, the “grafting from” strategy by means of bottom up approaches, where polymers grow via surfaceinitiated polymerization from the surface of NPs, produce higher grafting densities. Such strategy is considered eminent in the construction of nanohybrids. In this way, the development of various initiators for controlled surfaceinitiated polymerization has enabled the anchor of densely tethered polymer brushes. As a result, the control over versatility and chemical robustness of the polymer brush is accessible while the synthetic flexibility enables facile introduction of functional groups.270,271 In this subsection, we will introduce several typical examples of nanohybrids constructed employing “grafting-from” strategy. Surface-initiated controlled radical polymerization is widely used and convenient to fabricate nanohybrids from polymers with controllable architecture, composition, and molecular weight.176 In particular, surface-initiated ATRP offers a powerful tool, and the immobilization of ATRP initiators onto the surface of NPs is simple and broadly applicable. The appropriate reaction system including monomer, catalyst, and solvent also matters to tether the polymers. To immobilize ATRP initiators on NPs, the surface properties and the binding affinity of NPs will be taken into account. For example, the affinity of gold to thiol or disulfide groups has been intensively utilized in the conjugation of initiators, while the hydroxyl groups on the surface of silica and metal oxides are usually involved. Recently, cationic polymer brushes functionalized inorganic NPs via ATRP were put forward as gene carriers. In order to improve the biocompatibility and dispersity, Zhao et al. applied bovine serum albumin (BSA) to react with αbromoisobutyric acid (BIBA) in the presence of EDC and NHS to fabricate BSA-Br, which could be easily bind to the surface of Au nanorods as initiator. After grafting polycationic PDMAEMA via ATRP, PDMAEMA/Au nanohybrids could be constructed for multifunctional gene carriers.272 The synthetic routes are illustrated in Figure 7a. Taking advantage of this “grafting-from” strategy, PDMAEMA/Au nanohybrids with a P


Chemical Reviews

Review

Figure 9. Schematic illustration of the synthetic routes of PGMA (a) and P(GMA-co-PEGMA) (b) functionalized Fe3O4 via ATRP and the postmodification with folic acid. Reproduced with permission from refs 281 and 283. Copyright 2011 The Royal Society of Chemistry and 2012 American Chemical Society.

presynthesized APTES-BIBB with a triethoxysilane group as the initiator.281,282 The epoxy group of PGMA could be further conjugated with folic acid for efficient folate-receptor targeting.281 Moreover, the epoxy group could also be converted to aldehyde groups with sulfuric acid treatment. Then hydrazide functionalization was performed for the enrichment of glycopeptide.282 Copolymer of PGMA and PEGMA could also be grafted onto the surface of SPIONs, utilizing similar silane-immobilized initiator and ATRP to achieve P(GMA-co-PEGMA)/Fe3O 4 nanohybrids.283 As shown in Figure 9b, click chemistry was then applied to conjugate folic acid after ring-opening reaction of epoxy groups in the PGMA segment for targeting, while the PEGMA segment provided stability, compatibility, and dispersibility.

terminal hydroxyl groups. BIBB was then selectively reacted with amine groups, resulting in the initiators on the initial amine side. After grafting PNIPAM from one side of the nanosheets via ATRP, PDEAEMA was successfully grafted from the other side of the PNIPAM/SiO2 Janus nanohybrids utilizing silane coupling agent. As the most commonly used epoxy polymer, PGMA has attracted extensive attention since versatile functions may be achieved after epoxide ring-opening reactions with different amine species.280 Remarkably, hydroxyl-rich cationic polymers synthesized from PGMA could be used for versatile and multifunctional delivery carriers, including PGEA and PGED, as mentioned in section 2.3.1. As displayed in Figure 9a, PGMA was first grafted from the surface of superparamagnetic iron oxide NPs (SPIONs), taking advantage of the Q


Chemical Reviews

Review

Figure 10. Schematic illustration for the preparation and postmodification processes of QD@SiO2−PGEA (a) and Au@SiO2−PGEA (b) nanohybrids. Reproduced with permission from refs 285 and 92. Copyright 2016 Springer and 2017 Wiley-VCH.

from spherical and starlike hollow silica coated Au nanocaps to produce multifunctional nanohybrids.92 The starlike overall morphology with six symmetrical horns and cap-shaped Au as well as the efficient PGEA contributed to the excellent performance of the nanohybrids. Poly(poly(ethylene glycol) methyl ether methacrylate) (P(PEGMA)), a PEG derivative, was successfully grafted from the surface of Fe3O4 nanospheres via solvent-free ATRP, where 3-chloropropionic acid or a silane coupling agent, (2bromo-2-methyl)propionyloxypropyl triethoxysilane (BPE), was used to anchor initiator.286−288 P(OEGA) was also grafted from iron oxide with the Cu(0)-mediated LRP with the anchor of BiBEP as initiators.263 The authors further carefully compared the P(OEGA)/Fe3O4 nanohybrids synthesized utilizing the “grafting from” and “grafting onto” approaches. The “grafting from” approach was proved to be superior with higher grafting density. Consequently, nanohybrids fabricated

Interestingly, 1D peapod-like NPs were synthesized by incorporating two or three Fe3O4 NPs in a line with silica coating.284 After PGMA was grafted from the silane coupled initiator immobilized on the surface of peapod-like Fe3O4@ SiO2 NPs, ring-opening reaction with excess ethanolamine was carried out for gene delivery. The 1D morphology and magnetic response of the resultant Fe3O4@SiO2−PGEA nanohybrids endow them with enhanced gene transfection. In the same way, spherical or 1D wormlike silica NPs with QDs aligning in the center or center line could be functionalized with PGEA via ATRP to obtain QD@SiO2− PGEA nanohybrids with different shapes (Figure 10a), which could track the process of gene delivery.285 Nanohybrids fabricated from the wormlike QD@SiO2 NPs with an aspect ratio of 3.5 (W2) performed better than both wormlike NPs with smaller aspect ratio of 2 (W1) and spherical NPs (S). As shown in Figure 10b, PGEA was further proved to be grafted R


Chemical Reviews

Review

Figure 11. Schematic illustration of the surface bound bifunctional heteroinitiator and the synthetic routes to functionalize MSNs with PHPMA and PDEAEMA. Reproduced with permission from ref 302. Copyright 2012 Wiley-VCH.

by “grafting from” approach demonstrated better colloidal stability and higher r2/r1 relaxivity ratios than those by “grafting to” approach, which could produce more distinct contrast in T2/T2*-weighted MRI. In addition, hemocompatible nanohybrids of zwitterionic polymer sulfobetaine (poly(3dimethyl(methacryloyloxyethyl) ammonium propanesulfonate) (PDMAPS)) and layered double hydroxide (LDH) were synthesized via ATRP utilizing hydroxyl groups on the surface of LDH.289 It is worth mentioning that, as the nanohybrids fabricated via ATRP are used for biomedical applications, the removal of toxic metal complexes resulting from the reaction medium is of great importance. In the aforementioned examples, copper is the most commonly used transition metal catalyst to conduct ATRP, which possesses a mild toxicity. In addition, various metals including iron, platinum, nickel, ruthenium, and dual metallic catalysts were also employed to mediate ATRP.290,291 Among these catalyst systems, iron-based catalysts are considered least toxic.292,293 Furthermore, ATRP catalysts could be bioinspired and metal-free, which are more environmental friendly.294 However, the fabrication of nanohybrids via noncopper catalysts mediated ATRP is still rarely reported so far. Hopefully, with the development of noncopper mediated process, they could be more applicable to a wide range of monomers and contribute to the “green” construction of versatile nanohybrids. RAFT polymerization is also an important “grafting from” method to construct nanohybrids with a wide range of functionalities in monomer types and mild reaction conditions. The CTAs for nanohybrids are usually fabricated by the immobilization of either presynthesized silane-containing RAFT agents on NPs or RAFT agents on amine-functionalized NPs. For example, P(PEGMA) chains could be grafted from Fe3O4 nanospheres via surface-initiated RAFT before the anchoring of a silane-containing CTA, S-benzyl S′-trimethoxysilylpropyltrithiocarbonate.295 Poly(oligo(ethylene glycol) methacrylate) (POEGMA) chains were grafted from the

surface of UCNP@SiO2 NPs via photoenergy/electron transfer-RAFT polymerization after RAFT agent was anchored on APTES-modified NPs.296 Alternatively, epoxide or benzyl chloride functionalized NPs can also anchor the RAFT agent.297,298 For instance, pH-responsive poly(acrylic acid) (PAA) could be grafted from the surface of 5,6-epoxyhexyltriethoxysilane-modified MSNs via RAFT polymerization of acrylic acid as the smart nanovalve for guest molecules.297 In addition, a distinct advantage of RAFT polymerization is that the as-prepared polymer brushes could be readily reduced to terminal thiol groups, which could react with a variety of metallic surfaces, as mentioned in section 2.3.1. An example is PS brushes grafted from the surface of Fe3O4 NPs via RAFT polymerization with the assistance of APTES as coupling agents to anchor S-1-dodecyl-S-(α,α′-dimethyl-α″-acetic acid) trithiocarbonate as CTA. After the reduction with NaBH4, thiol-terminated PS/Fe3O4 nanohybrids could be used to connect with Au NPs.299 Nanohybrids from copolymers with designed composites and structures could also be accomplished via RAFT polymerization. For example, PDEAEMA-coPPDSM (PPDSM: poly(pyridyldisulfide ethyl methacrylate)) grafted silica NPs were obtained through the anchoring of a trithiol RAFT agent, thioazoline active ester.300 Diblock copolymer PDEAEMA-b-POEGMA coated silica nanotubes were obtained through two-step RAFT polymerization of DEAEMA and OEGMA from RAFT agent functionalized silica nanotubes.301 It is worth mentioning that controllable bifunctionalized MSNs were developed by combining ATRP and RAFT polymerization to tether two heteropolymers.302 In this work, pH sensitive PDEAEMA and hydrophilic PHPMA grew independently from silica surfaces, making use of the bifunctional heteroinitiator, as shown in Figure 11. The green part is designed as the RAFT agent while the red part is employed as the ATRP initiator. The blue part was used to anchor amino groups on the surface of silica. This special design of bifunctionalization from homogeneous silica NPs S


Chemical Reviews

Review

Figure 12. Schematic illustration of the synthetic routes of MoS2-g-HPG through ROP and the post functionalization with rhodamine B. Reproduced with permission from ref 303. Copyright 2017 Elsevier.

provides a new way to regulate the surface properties to result in pH responsive drug release. Surface-initiated ROP had also been employed for the fabrication of nanohybrids. Hyperbranched polyglycerol (HPG) was grafted from the surfaces of MoS2 nanosheets to acquire satisfying solubility and biocompatibility due to the abundant hydroxyl groups.303 The surface of MoS2 was first conjugated with 1-thioglycerol (TG) ligand, which produced enough hydroxyl groups as initiators for anionic ROP, as illustrated in Figure 12. The resultant MoS2-g-HPG nanohybrids could be further conjugated with specific functional molecules such as fluorescent dye rhodamine B (RB). Dendritic structured PEI was also grafted from the surface of amine-functionalized single-walled carbon nanotubes (SWCNTs) by cationic ROP of aziridine for gene delivery.304 The carbon nanotubes were modified with ED in the presence of DCC to result in terminal amino groups in advance. These “grafting from” strategies enrich the construction of nanohybrids with versatile structures and functionalities. 2.3.3. Self-Assembly Strategy. Self-assembly as a spontaneous bottom-up organization process occurs universally in nature, which is the fundamental strategy to build up amazing biological systems.305 Therefore, along with the booming development of biomimetics, self-assembly is regarded as a promising and practical approach for the construction of intelligent nanohybrids.8,306,307 In this subsection, we will introduce the nanohybrids fabricated from the self-assembly of organic and inorganic components as a consequence of specific interactions. Among these interactions, host−guest molecular recognition with inherent selectivity and switchability is commonly used, especially the classic cyclodextrin-adamantane (CD-Ad) assembly system with strong binding affinity arising from their size-match and desired hydrophilic and hydrophobic properties. β-CD is widely employed to construct gatekeepers for MSNs relying on its appropriate molecular size and excellent biocompatibility.308−310 Gan et al. designed uniform UCNP@SiO2@mSiO2 core−shell−shell spherical NPs (one core plus two shells) to conjugate guest Ad through 1O2-sensitive linkers.311 After host−guest interaction with β-CD, the photoresponsive nanohybrids could be achieved to control the release of drug loaded in the outmost mesoporous silica shell with the assistance of NIR light and photosensitizer in the middle solid silica shell, as shown in Figure 13. PEG with Ad at one end and

Figure 13. Schematic illustration of the fabrication and NIR-triggered activation of UCNP@SiO2@mSiO2 core−shell−shell NPs. Reproduced with permission from ref 311. Copyright 2014 Wiley-VCH.

FA at the other end was immobilized on the surface of MSNs terminated with β-CD rings through strong Ad/β-CD complexation in aqueous solution.312 Similarly, PEG shielding delivery system was also realized with the host−guest interaction between 3-(3,4-dihydroxyphenyl) propionic acid (DHPA) functionalized β-CD on the surface of hollow silica nanospheres and Ad conjugated PEG.313 T


Chemical Reviews

Review

Figure 14. Schematic illustration of the preparation of starlike nanohybrids through CD-Ad host−guest interaction. Reproduced with permission from ref 316. Copyright 2015 Wiley-VCH.

functionalized ZnFe2O4@Au core−shell nanospheres.321 An anionic copolymer (PEG-(PAH/DMMA)) of dimethylmaleic acid (DMMA) and PEG functionalized poly(allyamine) was readily complexed with carbon dots by electrostatic interaction, which would undergo charge reversal in mildly acidic tumor extracellular microenvironment.322 Layer-by-layer (LBL) strategy was successfully employed by Liu et al. to fabricate nanohybrids composed of negatively charged MnO2 nanoshells, cationic polymer PAH, and anionic polymer PAA through electrostatic interactions.323 Boyer et al. reported sugar-functional Au NPs employing a consecutive LBL process, where positively charged PEI, negatively charged PAA, P(AAco-glucose), or P(AA-co-galactose) were assembled onto the surface of negatively charged Au NPs.324 Nanohybrids of inorganic NPs and biomolecules are also fabricated by electrostatic interaction. For example, cationic poly-L-lysine (PLL) was coated on Prussian blue nanocubes or citratestabilized Au NPs for facilitated cell internalization.325,326 Dong et al. developed nanohybrid delivery systems through the electrostatic interaction between negatively charged multiwall carbon nanotubes with terminal carboxylic groups and positively charged TAT-CS conjugates.327 Other important assembly examples to construct organic/ inorganic nanohybrids include noncovalent hydrophobic− hydrophobic interactions, π−π interactions, and biorecognition. A PEGylated branched polymer, PEG-grafted poly(maleic anhydride-alt-1-octadecene) (PEG-C18PMH) with the hydrophobic C18 chain, was developed by Dai group for the functionalization of carbon nanotubes by hydrophobic interactions.328 Later, this polymer was adopted to construct nanohybrids of SWCNTs, carbon nanohorns, or hollow Si/C nanospheres as theranostic agents.329−331 The functionalized surface could be further extended to mesoporous silica and organosilica which was premodified with octadecyltrimethoxysilane (C18TMS). Consequently, taking advantage of hydrophobic−hydrophobic interaction, the nanohybrids of PEGC18PMH and silica coated SWCNTs or Janus Au@SiO2

Cationic polymer CD-PGEA developed by Xu et al. by integrating β-CD as cores with PGEA arms was recently proposed to construct a series of organic/inorganic nanohybrids through host−guest interaction.197,314−317 Guest molecules Ad were immobilized on the surface of silica NPs or silica coating layer with the assistance of silane chemistry and carbodiimide coupling protocols.315−317 For example, 3(glycidoxypropyl) triethoxysilane (GPTS) was first attached onto the surface of starlike hollow silica NPs to produce terminal epoxy groups, which were then react with cystamine to introduce disulfide bonds. Then Ad was immobilized through the reaction of amino groups and carboxylic groups of Ad-COOH in the presence of EDC and NHS, as shown in Figure 14. Finally, CD-PGEA could be flexibly introduced through host−guest interaction with the plentiful disulfide bond-linked Ad guests on the surface of silica NPs. Alternatively, Ad conjugated silane coupling agent could be synthesized first and then immobilized on the surfaces of silica NPs for the subsequent assembly with CD-PGEA.315 To functionalize gold surfaces with Ad guest, intermediate AdPEG-LA was designed. While LA could link with gold through Au−S bonds, the other end could introduce CD-PGEA via host−guest interaction.197 A short peptide, CGNKRTR (tLyP1), was also successfully assembled onto the surface of SiO2− WS2 NPs through host−guest interaction of LA modified βCD (LA-CD) on WS2 through W−S bonds and Ad-containing tLyP-1 for tumor homing and deep tissue penetration.318 Lactobionic acid grafted β-CD (LbA-CD) was also complexed with Ad modified MSNs for targeted gatekeeper for specific affinity with the asialoglycoprotein receptor (ASGP-R).319 Electrostatic interaction is another self-assembly strategy to associate organic and inorganic parts for the preparation of nanohybrids. Cationic PEI coating was achieved through electrostatic interaction on the negatively charged surface of ZnxS−AgyIn1−yS2 QDs modified with MPA to condense siRNA.320 Similarly, a cationic polyamine dendrimer was demonstrated to coat 11-mercaptoundecanoic acid (MUA) U


Chemical Reviews

Review

Figure 15. Fabrication of PEGylated Ce6-loaded CaCO3 nanohybrids. (a) Synthetic illustration of Ce6(Mn)@CaCO3−PEG nanohybrids. (b) Scanning electron microscopy (SEM), (c) transmission electron microscopy (TEM), and (d) scanning TEM (STEM) images of Ce6(Mn)@ CaCO3 NPs. Reproduced with permission from ref 342. Copyright 2016 Elsevier.

subsequent functionalization with PEG-DSPE. Herein, Mn chelated Ce6 was loaded into CaCO3 NPs by a gas diffusion reaction (Figure 15b−d). Then 1,2-dioleoyl-sn-glycero-3phosphate (DOPA) was modified on the surface of CaCO3 through the coordination of Ca2+ with the phosphate group in DOPA to produce hydrophobic surfaces. After mixing the DOPA modified NPs with cholesterol, 1,2-dihexadecanoyl-snglycero-3-phosphocholine (DPPC), and PEG-DSPE, a lipid layer was formed to result in the Ce6(Mn)@CaCO3−PEG nanohybrids. Soybean phospholipid (SP) could also be employed for the surface functionalization of MnOx/Ta4C3 nanosheets to produce MnOx/Ta4C3−SP nanohybrids for improved biocompatibility.343 π−π stacking is notable in the surface functionalization of carbon nanomaterials, which works between the sp2 hybridized carbon and aromatic derivatives.344−346 Recently, Zhang et al. design amphipathic polymer composed of photosensitizer protoporphyrin IX, PEG linker, and RGD (PpIX-PEG-RGD),

nanostructures were readily fabricated for cancer combination therapy.332,333 The hydrophobic ligands coating on inorganic NPs during their synthesis provide insertion point for alkyl chains of amphiphilic polymer. In this case, PEG-C18PMH could functionalize hydrophobic Au−Cu 9 S 5 NPs and WS2:Gd3+ nanoflakes through hydrophobic−hydrophobic interaction.334,335 Dai and Hu et al. created a hydrophilic polymer shell of PEG on the surface of UCNPs by anchoring the C18PMH alkyl polymer chains between the hydrophobic oleic acid molecules.336 Phospholipids grafted PEG (PEG− PL) was also utilized for the nanohybrids of SWCNTs through hydrophobic interaction between the two PL alkyl chains and SWCNT sidewall.337,338 PEG-DSPE is another commonly used amphiphilic polymer to transfer hydrophobic NPs to aqueous solutions, such as Janus Au−Fe2C NPs,339 CdTe@ZnS QDs,340 UCNPs,341 and CaCO3 NPs.342 Figure 15a shows the fabrication of photosensitizer chlorin e6 (Ce6) loaded CaCO3 NPs and the V


Chemical Reviews

Review

Figure 16. Fabrication of MoS2−PEG nanohybrids. (a) Synthetic illustration of synthetic route of MoS2−PEG nanohybrids. TEM images of nanohybrids with the diameter of (b) 50, (c) 80, (d) 100, (e) 200, and (f) 300 nm. Reproduced with permission from ref 351. Copyright 2015 Elsevier.

acrylate-co-methacrylic acid) (P(OEGMA-co-MAA)) as stabilizer. The interaction of methacrylic acid segment with Fe3O4 tethered the copolymers on the surface of iron oxide while the oligo(ethylene glycol) part offered water solubility and biocompatibility.354 Three block copolymers of P(OEGA) containing different anchoring groups including phosphonic acid, carboxylic acid, and glycerol groups were investigated to stabilize Fe3O4 NPs by coprecipitation approach.355 Poly(oligoethylene glycol acrylate)-b-poly(phosphonic acid ethyl acrylate) (P(OEGA)-b-P(PAEA)) block copolymers containing phosphonic acid groups demonstrated the highest grafting density among the three kinds of copolymers. Poly(ethylene glycol)-b-poly(glutamic acid) (PEG-b-P(Glu)) could interact with Mn2+, Ca2+, and HPO42− to produce nanohybrids with Mn2+-doped calcium phosphate nanparticles.356 Utilizing the similar concept, poly(ethylene glycol)-b-poly(L-aspartic acid) (PEG-b-PAsp) mediated one-pot synthesis of nanohybrids with calcium carbonate and calcium phosphate, respectively.357−359 In addition, the polymer micelles of poly(ethylene glycol)-b-poly(L-aspartic acid)-b-poly(L-phenylalanine) (PEG−PAsp-PPhe) provide template for the generation of calcium phosphate nanohybrids while calcium carbonate NPs formed in the matrix of cross-linked micelles of diacrylated pluronic F127 in the presence of alginate.360,361 Polymer/Au nanohybrids with tunable morphologies were successfully fabricated through polymerization-induced selfassembly of POEGMA-b-PDMAEMA-b-PS triblock copolymer and in situ Au NPs formation.362 The tertiary aminecontaining block of the polymeric template was exploited to complex gold ions which were subsequently reduced in the presence of NaBH4. In a similar way, self-organized POEGMAb-PMAA-b-PS triblock copolymer aggregates could incorporate Fe3O4 NPs through the complexation of Fe(II)/Fe(III) with carboxylic acid groups and the subsequent in situ coprecipitation.363 Notably, well-defined tellurium nanodots were successfully synthesized within the nanocages of human

which was assembled with carbon dots-decorated carbon nitride NPs (CCN) through π−π stacking between PpIX and C3N4 to achieve nanohybrids (PCCN).347 Biomolecules with aromatic structures such as DNA could also be π−π stacked onto the surface of carbon nanomaterials. Huang et al. constructed nanohybrids of a Cy3-labeled single-stranded DNA (ssDNA) probe and oxidized mesoporous carbon nanospheres for multiple cancer diagnosis.348 Biorecognition based on the specific interaction between biomolecules could also be utilized for the construction of nanohybrids. Zhao and Chen et al. proposed the strategy to realize membrane biotinylation of cell-derived vesicle with DSPE-PEG-biotin. Then, streptavidin-conjugated Fe3O4 NPs and QDs could be assembled on the vesicles through the high affinity between biotin and streptavidin, respectively.349,350 2.3.4. One-Pot Synthesis. One-pot synthesis offers a simple and efficient approach to construct organic/inorganic nanohybrids, where inorganic parts usually formed directly during one step reaction in the presence of organic parts to work as surface capping agents or templates. In most cases, the organic parts do not participate in the reaction and only play the role of capping. For example, PEGlyated MoS2 nanosheets with different sizes could be simply obtained by one-pot solvothermal synthesis, as shown in Figure 16.351 (NH4)2MoS4 served as both Mo and S precursors and PEG could efficiently anchor on the surface of MoS2 nanosheets during the synthetic process. The size of nanohybrids could be adjusted by the selection of solvent and precursor concentration. By introducing Bi ions, MoS2/Bi2S3−PEG nanosheets could be obtained accordingly through the one-step solvothermal method.352 Zhao et al. extended the PEGlaytion of MoS2 to MoS2/Fe3O4 composites by adopting another hydrothermal reaction after MoS2−PEG nanohybrids were obtained through one-pot synthesis.353 PEGylated ultrasmall Fe3O4 NPs were synthesized by onestep coprecipitation using poly(oligo(ethylene glycol) methW


Chemical Reviews

Review

Figure 17. PS-b-P4VP/Au nanohybrids fabricated from selective growth of Au NPs. TEM (a,b) and SEM (c) images of organic NPs with tunable geometry assembled from PS-b-P4VP before (a,c) and after (b) selective growth of Au NPs within P4VP domains. Reproduced with permission from ref 370. Copyright 2015 American Chemical Society.

Figure 18. Fabrication of PEGlyated Janus PAA-Au nanohybrids (a). TEM image of nanohybrids of PAA and spherical Au NPs (b) or Au branches (c). Reproduced with permission from ref 374. Copyright 2016 Wiley-VCH.

works as a deoxidizer for reducing GO and stabilizer to regulate the surface charge of the nanohybrids.366 As mentioned in section 2.1.1, Park et al. designed and synthesized DAA-conjugated amphiphilic HA-b-PCL as the reducing agent to construct nanohybrids of DAA-HA-b-PCL and gold nanoclusters.41 Boyer et al. prepared Ag NPs with three kinds of stabilizers working as both reducing and capping agents: catechin, cross-linking catechin with sodium tetraborate (cat-borax), and a water-soluble oligomer from catechin

serum albumin (HSA) through one-step reduction reaction to obtain bifunctional nanohybrids.364 In some cases, the organic parts in one-pot synthesis will be involved in the reaction of nanohybrids formation. The role of reducing agents is the typical example. Nanohybrids of dextran/Au NPs were simply achieved by boiling an aqueous solution of gold salt and dextran, where dextran serves as both reducing agent and capping agent.365 In the recent report of one-pot synthesis of PEI/graphene/Fe3O4 nanohybrids, PEI X


Chemical Reviews

Review

Figure 19. Fabrication of CS-Au nanohybrids. (a) Schematic illustration of the synthetic routes of CS-Au nanohybrids. (b) Size distribution and (c) TEM images of CS nanospheres, CS-ICG nanospheres, and CS-Au and CS-Au-ICG nanohybrids. Reproduced with permission from ref 378. Copyright 2013 Elsevier.

(polycat).367 BSA was also reported to prepare the BSA/Au nanhybrids with red emissions through the facile and “green” one-step reaction to reduce the gold precursors in situ.368 Yang group developed a facile and versatile strategy to obtain organic/inorganic nanohybrids through selective growth of Au NPs from diverse organic NPs assembled from diblock copolymer. Polystyrene-b-poly(4-vinylpyridine) (PS-bP4VP) was employed for the generation of NPs with tunable geometry through 3D controlled self-assembly via the emulsion solvent-evaporation route.369−371 Au NPs could then in situ grow selectively on the P4VP domains to produce nanohybrids, as shown in Figure 17. Moreover, unique Janus hollow polydopamine (PDA)/mesoporous calcium phosphate nanohybrids were also synthesized through selective deposition of calcium phosphate on the PAA surface of Janus PDA/PAA NPs. The selective growth on the PAA surface was attributed to the coordination between the carboxylic groups and calcium ions.372 2.3.5. Wrapping. Wrapping is another common strategy to construct organic/inorganic nanohybrids through noncovalent interaction between organic and inorganic components. In some cases, organic components usually form NP to encapsulate all or part of inorganic NPs through precipitation or chemical reaction processes. Self-assembled nanostructures of organic parts could also encapsulate the inorganic NPs to produce nanohybrids. Concentric and eccentric core−shell structured nanohybrids of PAA and a variety of inorganic NPs including Fe2O3, Fe3O4, SnO2, and Ag nanospheres were readily fabricated through the addition of isopropyl alcohol.373 This facile and general method could be further extended to construct concentric UCNP@PAA core−shell nanohybrids and Janus Au-PAA nanohybrids where the spherical Au NPs further grew to Au branches.374,375 PEG and lactobionic acid could be then selectively conjugated to the PAA and Au domains for improved stability, biocompatibility, and active

tumor targeting, as shown in Figure 18. The resultant octopustype nanohybrids were proved to provide an intriguing platform for actively targeted synergistic chemo-photothermal cancer therapy. Applying the nanoprecipitation approach, Fe3O4 NPs were readily encapsulated with fluorescent semiconducting polymers poly[2,7-(9,9-dioctylfluorene)-alt4,7-bis (thiophen-2-yl)benzo-2,1,3-thiadiazole] (PFODBT) to afford Janus nanohybrids that possessed optical and magnetic properties for cancer cell labeling and in vivo tracking.376 In order to realize PFODBT wrapping, the copolymer poly(styrene-co-maleic anhydride) (PSMA) was also added in the system to work with the hydrophobic ligands on the surface of Fe3O4 NPs via the hydrophobic interaction between oleic acid and styrene units of PSMA. Besides direct precipitation, additional chemical reaction will help the formation and stabilization of organic network. Jiang group developed a simple nonsolvent-induced counterion complexation method to fabricate nanohybrids of CS matrix and encapsulated gold nanorods.377 During the synthesis, ethylene diamine tetraacetic acid (EDTA) was introduced to work with positively charged CS and gold nanorods stabilized with CTAB through electrostatic interaction. Ethanol was employed as nonsolvent to precipitate the nanohybrids, followed by the cross-linking of CS moiety by glutaraldehyde to encapsulate gold nanorods. The CS matrix could be used to incorporate anticancer drug cisplatin or indocyanine green (ICG) for chemotherapy or photodynamic therapy (PDT), while photothermal gold nanorods could be utilized for PTT.378,379 Figure 19 shows the synthetic procedure, size distribution, and TEM images of CS nanospheres and CS-Au nanohybrids before and after ICG was loaded. Later they extended the system to the nanohybrids of CS and QDs for FL imaging, superparamagnetic Fe3O4, or magnetic ZnFe2O4 NPs for MRI.380−382 Zhou et al. also employed this strategy to synthesize nanohybrids of CS and carbon dots for NIRY


Chemical Reviews

Review

Figure 20. Fabrication of P(MEO2MA-co-(HEMA-g-PCL))/Au nanohybrids through assembly. (a) Schematic illustration of the synthesis of hybrid gold assembly and their potential theranostic application. (b) TEM images and schematic pictures of the assembly process of hybrid gold assemblies. Reproduced with permission from ref 402. Copyright 2015 Wiley-VCH.

enhanced drug release.383 On the other hand, the organic part could also be extended from CS to other polysaccharides such as pullulan and dextran. Unlockable Au nanorods were encapsulated in the cross-linked matrix of aminated pullulan and dextran for the fabrication of nanohybrids.384 Liver celltargeting pullulan/Au nanohybrids with suitable amount of Au NRs were demonstrated impressive photothermal effect. Nanoscale metal−organic frameworks (nMOF) composed of metal ions or clusters and organic ligands through coordination bonds are also accessible to coat or grow on inorganic NPs to achieve nanohybrids. The eccentric and octahedral core−shell UCNP@nMOF nanohybrids were realized through the coating of amino-functionalized octahedral iron carboxylate nMOF shells on the UCNP cores.385 Janus UCNP@nMOF nanohybrids were also successfully achieved through the anisotropic growth of porphyrinic nMOF onto the surface of UCNPs.386 PDA as a highly biocompatible and biodegradable polymer that could simultaneously form an ultrastable coating on the surface of inorganic NPs is also frequently employed to construct organic/ inorganic nanohybrids.387 For example, PDA shell coated Bi2Se3 nanoplates through in situ polymerization of dopamine were simply achieved by dispersing the nanoplates in the alkaline dopamine solution.388 With a similar idea, iron oxide NPs could be encapsulated in zwitterionic nanogels during the polymerization of carboxybetaine methacrylate (CBMA) with a disulfide cross-linker.389 Monodisperse Fe3O4@SiO2@PS core−shell nanohybrids were readily obtained through emulsion polymerization of styrene employing [3-(methacryloyloxy)propyl]trimethoxysilane (MPS) modified Fe3O4@SiO2 NPs as seeds and sodium dodecyl sulfate as surfactants.390 Carbon dots@ PEG-CS nanohybrids were designed and synthesized by a precipitation polymerization approach, where PEG macromonomers, CS, and carbon dots were complexed via hydrogen bonding among the hydroxyl/amino groups of CS, hydroxyl/

carboxylic groups of carbon dots and ether oxygen of PEG macromonomers.391 After PEG macromonomers were polymerized and cross-linked, CS and carbon dots were trapped in situ for the construction of hybrid nanogels. The last representative wrapping strategy to construct organic/inorganic nanohybrids we would like to discuss here is through assembly. The most convenient protocol to wrap NPs via self-assembly is to utilize the electrostatic interaction. Guével et al. reported the nanohybrids of polyelectrolyte PAH and GSH modified gold nanoclusters through the electrostatic interaction between protonated amine groups of PAH and carboxylic groups of GSH.392 Khashab et al. designed nanohybrids of polysaccharides (sodium alginate, CS, and HA), iron oxide, and GO through electrostatic assembly of the oppositely charged polysaccharides.393 Zhu et al. proposed nanohybrids of SWCNTs, which was successfully wrapped in the hydrophobic “caves” of HA NPs.394 The host−guest molecular recognition between Ad and β-CD could also mediate flexible wrapping of NPs. Tseng et al. prepared supramolecular magnetic nanohybrids through the supramolecular synthetic strategy by coassembly of Ad modified Zn0.4Fe2.6O4 superparamagnetic NPs, Ad-grafted polyamidoamine dendrimers (PAMAM), β-CD-grafted PEI, Ad-functionalized PEG.395 In this example, magnetic NPs play the role of cross-linkers for the synthesis of nanohybrids. Similarly, crosslinking of CD-grafted PEI mediated by Ad-functionalized Au NPs and Ad-grafted PEG through multivalent molecular recognition between CD and Ad motifs could produce supramolecular nanohybrids.396 RGD peptide was further assembled onto the surface of the resultant nanohybrids by AdPEG-RGD for targeting. The resultant assemblies were proved to exhibit enhanced photothermal effects for targeted PTT. Moreover, examples with the use of DNA-, peptide-, or polymer-functionalized NPs as cross-linkers are included in section 3.5. Z


Chemical Reviews

Review

Figure 21. Schematic illustration of the (a) fabrication and (b) in vivo delivery process of cascade pH-responsive HMSN-β-CD/Ad-PEG system. Reproduced with permission from ref 313. Copyright 2016 Elsevier.

posed of the backbone polymer poly[2-(2-methoxyethoxy) ethyl methacrylate-co-(2-hydroxyethyl methacrylate)] P(MEO2MA-co-HEMA) and the comb teeth polymer PCL via RAFT and ROP.402 The nanohybrid of P(MEO2MA-co(HEMA-g-PCL)) and Au NPs was then prepared by the in situ formation of Au NPs with the assistance of polymers, as illustrated in Figure 20a. The multimercapto-terminated groups of the polymer and hydrophilic/hydrophobic interactions arising from the comb-like structure motivated the hybrid assemblies. Chainlike structures, small loop aggregations from gold chains, as well as loosely and tightly packed compound micelle were observed as the assembly process took place with dialysis time expansion and water content increasing, as shown in Figure 20b. Emulsion process and phase separation are also considered facile and efficient to realize the wrapping of inorganic NPs via

In addition, polymers could modify and self-assemble inorganic NPs into hybrid micelles or vesicles. For example, Liu et al. presented a kind of theranositic nanohybrids by wrapping VS2 nanodots inside PEG modified lipid micelles, which were converted from VS2 nanosheets in the presence of DOPA and could be degraded through oxidation.397 Fe3O4 NPs could be embedded in methyloxy-poly(ethylene glycol)-bpoly[dopamine-2-(dibutylamino)ethylamine -L-glutamate] (mPEG-b-P(DPA-DE)LG) micelles, PLGA vesicle, or wormlike PEG−PLA-PEG polymer vesicle to form hybrid polymersome.398−400 PLGA vesicle was also used to cowrap Fe3O4 NPs and QDs for multimodal imaging and drug delivery.401 Compared with linear polymers, comb-like polymers may bring more versatile assembled structures for the construction of organic/inorganic nanohybrids. Zhang et al. synthesized a comb-like polymer P(MEO2MA-co-(HEMA-g-PCL)) comAA


Chemical Reviews

Review

Figure 22. Schematic illustration for the preparation (a) and circulation (b) of charge-convertible nanohybrids based on carbon dots. Reproduced with permission from ref 322. Copyright 2016 American Chemical Society.

self-assembly.403,404 For example, hybrid hollow assembly of Fe3O4 NPs wrapped in temperature-sensitive Pluronic F127 and high-polarity PAA polymer shells was prepared by a singlestep emulsion solvent evaporation method. The hydrogen bonding between PAA and the poly(ethylene oxide) (PEO) segments of F127 in the shell stabilized the hollow assembly while Fe3O4 NPs were wrapped due to the affinity of hydrophobic ligand on the surface of Fe3O4 NPs and the PPO block of F127.405 Janus nanohybrids of PS-Fe3O4@SiO2 composed of a PS nanosphere as the core and a silica NP or a half silica shell with Fe3O4 NPs loaded inside were successfully fabricated through the combination of miniemulsion polymerization process and sol−gel reaction.406,407 Moreover, supercooling self-assembly was proposed to arrange glucosemodified Fe3O4 NPs surrounding the dextran cores through hydrogen bonding to construct the core−shell structured organic/inorganic nanohybrids.408

at nanoscale might give rise to new properties, which is intriguing in the design and synthesis of potentially useful nanohybrids. In this regard, the intense understanding of the properties and corresponding functions of nanohybrids could help reveal the relationships between material synthesis, structure, property and performance. Self-assembly of organic/inorganic nanohybrids has also drawn great attention due to the promising properties and functions originating from the ordered arrangement of nanohybrids for diverse applications. In this subsection, we start with organic parts, inorganic parts, and synergetic properties of organic and inorganic parts to discuss the general account of the components of nanohybrids. The morphology and assembled structure dependent properties of nanohybrids are also summarized to give more ideas for the reasonable design and construction to satisfy the ever increasing demands for efficient nanohybrids. All of these concepts and typical examples may offer valuable clues for the fundamental understanding of synthesis, structure, property, and performance of nanohybrids.

3. PROPERTY AND FUNCTION OF ORGANIC/INORGANIC NANOHYBRIDS The property of organic/inorganic nanohybrids is attractive since the collective properties of both organic and inorganic parts are worth expecting, which depends greatly on the composition, size, shape, and surface property of nanohybrids. More importantly, the synergistic interaction of two segments

3.1. Organic Parts

As stated in section 2.1, organic segments such as small molecules, synthetic polymers, and biomolecules play an important role in the stability and biocompatibility of nanohybrids. Besides the representative properties and functions of the organic segments, e.g., extended circulation AB


Chemical Reviews

Review

Figure 23. (a) Schematic representation for the structure of and different mechanisms of triggered drug release and (b) synthetic route of the triple-responsive nanohybrids. Reproduced with permission from ref 278. Copyright 2015 American Chemical Society.

endosomal intracellular pH (∼4.5−6.5). The hydrophilic feature of PDEMEMA in acidic solution in contrast with its hydrophobic property in neutral or alkaline solution is usually utilized as the gatekeeper on silica NPs for pH-responsive drug release.301,302 Cai et al. fabricated tumor microenvironment triggering cascade pH-responsive delivery system through the functionalization of hollow silica NPs with PEG shielding through CD-Ad host−guest interaction.313 In this work, β-CD was grafted onto the surface of hollow mesoporous silica NPs (HMSNs) via boronic acid-catechol ester bonds which will hydrolyze under low endosomal pH (4.5−6.5), while Ad was conjugated with PEG through benzoic-imine bonds which could be cleaved under weak acid tumor microenvironment (pH 6.8), as show in Figure 21. With the rational design, the outer PEG layer guaranteed the prolong blood circulation of the delivery system by suppressing nonspecific protein adsorption and cellular uptake. When the carriers reached tumor site, benzoic-imine bonds broke down to dissociate PEG where the cellular uptake could be facilitated. Afterward, the boronic acid-catechol ester linkers would be hydrolyzed to detach the gatekeeper β-CD and release drugs to induce cell

time by PEGylation and targeting specificity of certain small molecules and biomolecules, organic parts of the nanohybrids usually comprise more than one kind of components. Herein, we introduce several commonly employed properties and functions of the organic parts in nanohybrids including stimuli responsiveness, targeting, guest molecule loading, and multifunctions to present their design rationale and resultant performances. 3.1.1. Stimuli Responsiveness. Smart nanohybrids that respond to the changes in physiological environments are of great significance since they work only in the specific diseased site to minimize the adverse effect of normal region. The flexible organic segments especially polymers allow for smart chemistry designs with facile sensitive groups to construct stimuli-responsive systems.409,410 Various advanced nanohybrids taking advantage of external stimuli such as pH, temperature, light, and redox have been intensively studied and developed through the elaborate design and synthesis of organic parts.1,28 pH-responsiveness is widely exploited due to the characters of tumor with mildly acidic extracellular pH (∼6.0−7.0) and AC


Chemical Reviews

Review

Table 2. Drug Loading Strategies with the Organic Parts of Organic/Inorganic Nanohybrids category

targeting molecule

target

small molecule

folic acid

folate receptor

polysaccharide

HA

CD44 receptor

peptide

pullulan RGD

ASGPR αVβ3 integrin

TAT

nuclear

CGKRK (Cys-Gly-Lys-Arg-Lys) CXCR4 antibody EGF transferrin

p32 protein CXCR4 EGFR transferrin receptor

HIF-1α A9 RNA

cancer stem cell prostate-specific membrane antigen

protein

aptamer

apoptosis. The constructed cascade pH-responsive drug delivery system (HMSN-β-CD/Ad-PEG) was proved to efficiently inhibit tumor growth by delivering drugs to the tumor cells utilizing the tumor microenvironment while keeping the virtues of PEG shielding. The switchable surface charge behavior from negatively charged and corona-free surface to the subsequent positively charged surface in the tumor site was also achieved by the rational design of pHinduced charge switchable dopamine on the surface of manganese oxide NPs.411 The cleavable amide bonds formed through the reaction of 2,3-dimethylmaleic anhydride with the amino groups in dopamine endow the nanohybrids with the feature with prolonged circulation time and promoted cancer cell uptake simultaneously. Besides the cleavage of covalent bonds, electrostatic interaction could also be involved to construct pH-responsive nanohybrids. As stated in section 2.3.3, Zhao et al. constructed charge-convertible carbon dots as tumor extracellular microenvironment-responsive drug carriers through the coating of anionic polymer PEG-(PAH/DMMA) by electrostatic interaction with amino groups functionalized carbon dots.322 Under normal physiological condition, the as-prepared nanohybrids withstand protein adsorption and rapid clearance of immune system benefiting from PEGlyation. After the hydrolysis of PEG-(PAH/DMMA) at the acidic environment of cancer cells (pH 6.8), the resultant positively charged polymer will dissociate from positively charged carbon dots due to electrostatic repulsion, which will enhance tumor cell internalization and facilitate endosome escape, as shown in Figure 22.

connecting ligand

refs

PEG-SH PEG-NH2 PEG-Ad CS G5 PAMAM-NH2 PCL sodium alginate, CS 5β-cholanic acid PAA --PCBMA PEG-SH PEG-DSPE PEG-PpIX PEG-Ad PS-b-PAA PAA MPA phosphate group -APTES CS PEG-MAL APTES dithiothreitol PEG-MAL PGMA PEG-Mn(II) (TCG)7 oligonucleotide

192 225,234,441,385 312 241 603 41 393 394,623 695 239,443,739 384 389 193 341 347 396 425 442 244 269 187 240 327 472 242 253 444 429 574 62

Redox-responsiveness is also realized by the organic part of the nanohybrids, especially in the case to respond to the reducing environment. The introduction of disulfide bonds in nanohybrids is an intensively employed strategy to realize redox responsiveness.409,410 Thermoresponsive polymers such as P(NIPAm-co-AAm), and P(OEOMA-co-MEMA) were successfully applied as the gatekeeper on the surface of inorganic NPs to regulate the drug release or as the shielding shell to expose the targeting molecules.193,201 Moreover, the incorporation of photo-, singlet oxygen-, or enzyme-sensitive polymers or linkers in the organic parts of nanohybrids will bring more possibility and versatility in the resultant properties and functions.311,312,341,412 Dual- and triple-responsiveness were also achieved through the formulation of organic parts. For example, the dual pHand thermoresponsiveness of nanohybrids composed of iron oxide and thermoresponsive polymer (P(DEGMA-co-PEGMAb-[TMSPMA-co-VBA]) was realized by conjugating drug DOX (through the reaction of amine groups with aldehyde groups of the polymer) and tuning the LCST of the polymer at 40 °C.413 To construct pH, redox, and photo triple-responsive nanohybrids, Zhao et al. performed one intelligent design with organic segments (Figure 23a).278 pH responsive polymer PDEAEMA was functionalized on the surface of HMSNs with reduction-cleavable disulfide bonds and light-cleavable onitrobenzyl esters as the linkages (Figure 23b). With the development of rational designs of nanohybrids, the organic parts usually carry more than one component and become complicated. Therefore, in most cases, these kinds of stimuliAD


Chemical Reviews

Review

Table 3. Drug Loading Strategies with the Organic Parts of Organic/Inorganic Nanohybrids

carried by nanohybrids. Besides the porous structure of inorganic parts to entrap payloads through physical interactions, organic parts play an essential role in guest molecule loading. The organic segments could carry drug through physical or chemical interactions including covalent bond, hydrophobic interaction, encapsulation, and electrostatic interaction. Several representative examples of each strategy to load drugs are summarized in Table 3. Anticancer drug DOX which owns amino groups was linked to polymer ligands of the nanohybrids through Schiff base bonds, amide bonds, or hydrazone bonds.194,252,266,418−420 Other drugs could also be loaded with covalent bonds through the rich functional groups of organic segments and versatile linkages. For example, avastin could be linked to the surface of gold nanorods through the reaction with the carboxylic groups of PEG-COOH.182 Retinoic acid and simvastatin utilized in the treatment of Alzheimer’s disease were conjugated to the organic ligands of poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(carboxybetaine) (PCB) segments through ester bonds and diselenide bonds, respectively.421,422 In some cases, drugs were immobilized on the surface of inorganic NPs by forming covalent bonds using small molecules as linkers. For example, a facile delivery system of drug-self-gated MSNs was developed employing pH-sensitive dynamic benzoic-imine covalent bond to conjugate anticancer drug doxorubicin (DOX), as shown in Figure 24a.245 The amino groups of DOX were reacted with benzaldehydefunctionalized MSN to render DOX as both drug and gatekeeper, which will be released at mild acidic tumor region (Figure 24b). Afterward, amino PEG was also conjugated onto the surface of MSN via benzoic-imine bond to reduce nonspecific uptake by RES. The resultant PEG shell will be cleaved at the acidic region to facilitate tumor cellular internalization (Figure 24c). pH-responsive delivery systems of an anti-tuberculosis (TB) drug, isoniazid (INH), based on MSNs were designed by Horwitz et al.423 In order to realize high INH loading, INH is covalently bonded to aldehydefunctionalized MSNs via a hydrazone bond to form a prodrug (MSN−CHO-INH) which could be released in acidic

responsiveness work with other functions and will be discussed in section 3.1.4. 3.1.2. Targeting. Ideal nanohybrids applied in biomedical area including drug delivery and functional imaging should provide efficient accumulation and uptake at the target sites to minimize adverse effects.414 Passive targeting mediated by EPR effect in tumor sites and active targeting with the involvement of biological receptor are both of great interest. With regards to passive targeting, the physical properties such as particle size, shape, and surface charge matter.415 Hydrophilic polymers are considered to produce increased circulation time and accumulation in the targeted region compared with hydrophobic counterparts. Active targeting mainly depends on the design of the organic parts of the nanohybrids. In order to validate the specific recognition and ligand−receptor interactions, prolonged blood circulation time is believed to be critical to help nanohybrids with the extravasation out of the vasculatures and migration into the malignant cells. Biomolecules such as folic acid, antibodies, peptides, and polysaccharides could specifically bind to the receptors overexpressed on the membrane of tumor cells,305 as stated in section 2.1.3. In the design of active targeting nanohybrids, the targeting molecules are usually chemically conjugated or self-assembled to constitute the organic part to fulfill the targeting function.416 For instance, PEG is usually employed to link targeting moiety in the nanohybrids to maximize the cell recognition with longer circulation time.192,225,234 The design rationale for the organic part of targeted delivery system including the selection of disease-specific receptor, targeting ligand, spacer, and cleavable linkers could be found in the review paper on this subject written by Low et al.417 Table 2 summarizes some representative targeting molecules as well as the corresponding targets, and connecting ligands. In most cases of nanohybrids, targeting usually comes with other functions to realize multifunctional platforms, which are discussed in section 3.1.4. 3.1.3. Guest Molecule Loading. To realize versatile imaging or therapy, payloads such as drug, gene, photosensitizer, protein, or a combination of these molecules are AE


Chemical Reviews

Review

Figure 24. Drug-self-gated mesoporous antitumor nanoplatform. (a) The dynamic interaction between DOX and benzaldehyde via pH-sensitive benzoic−imine bond. (b) Schematic illustration of the DOX-self-gated MSNs with pH-responsive drug release property. (c) Dynamically PEGylated and DOX-self-gated MSNs with site-specific drug release and cell uptake at weak acidic tumor tissue/cells. Reproduced with permission from ref 245. Copyright 2017 Wiley-VCH.

of the nMOF domain of UCNF-nMOF heterodimers.386,401 Zheng et al. found that paclitaxel could be absorbed and bounded by dextran, which was encapsulated in the cavity of core/shell structures of magnetic shelled nanohybrids.408 Nie et al. encapsulated DOX in the hollow cavity of nanoscale magneto-vesicles prepared through the assembly of polymertethered Fe3O4 NPs and PS-b-PAA.425 Electrostatic interaction between drugs and organic parts is also utilized as the convenient strategy to carry drugs. For example, positively charged DOX was successfully adsorbed onto the surface of nanohybrids through electrostatic interactions with negatively charged polymer segments.192,357,388,426,427 These strategies are also feasible to load other guest molecules. For example, ICG and Rose Bengal photosensitizers could be loaded in polymer layers through hydrophobic interaction.424,428 Rhodamine B and transferrin were loaded by PGMA/Fe3O4 nanohybrids through the covalent bonds formed with the epoxide groups of PGMA.429 A second-generation photosensitizer, Al(III)

environment (pH 5−6) such as endolysosomal compartment. PEI−PEG was subsequently coated on the surface for improved dispersibility and stability. The as-prepared nanohybrids were avidly ingested by TB-infected human macrophages for markedly improved TB treatment in mice. Hydrophobic drugs could be loaded in the hydrophobic segments of organic ligands, as illustrated in Table 2. Hydrophobic drugs such as kartogenin, Dox, and AB3 (a histone deacetylase inhibitor) were carried in the hydrophobic cavity of β-CD, hydrophobic poly(L-phenylalanine) (PPhe) inner core or poly(4,5-dimethoxy-2-nitrobenzyl methacrylate) (PNBMA) segments of organic parts.223,360,424 DOX could also be loaded in the matrix of HSA through hydrogen bonding-based hydrophobic interaction.388 Encapsulation is another strategy for the organic parts to carry drugs. Anticancer drug busulfan was encapsulated in the PLGA-iron oxide nanohybrids formed through an emulsion-evaporation method, where DOX was encapsulated in the porous structure AF


Chemical Reviews

Review

Figure 25. Design of Fe3O4-RGD-mPEG nanohybrids to reduce nonspecific uptake by normal healthy cells. In the tumor microenvironment, the acid-labile linker is broken to expose RGD to bind to cancer cells. Reproduced with permission from ref 442. Copyright 2017 American Chemical Society.

amphiphiles, as well as polysaccharide-based carriers via polymer grafts, were developed as gene carriers, which offer more opportunities for the construction of nanohybrids for gene loading.52,280,436−438 3.1.4. Multifunctions. The organic parts of nanohybrids could realize multifunctions through integrating different components. For instance, the combination of disulfide bonds and targeted molecules could realize redox-responsive drug delivery with tumor specificity.319,389,439,440 Zhao et al. modified HMSNs with disulfide bonds as the intermediate linker to introduce Ad, which has specific complexation with LbA-CD. The redox-cleavable disulfide bonds render the carriers highly sensitive to the GSH stimuli while LbA-CD works as both gatekeeper for the encapsulated drugs and the targeting ligand toward HepG2 cells, which could help cellular uptake and actively permeation into tumor sites.319 Bräuchle et al. developed multifunctional nanohybrids as one pHresponsive targeted drug delivery system by covalent attachment of pH-sensitive poly(2-vinylpyridine) (P2VP) via EDCmediated amidation reaction on the surface of MSN.441 The protonated state of P2VP rendered the NPs well dispersed in water while hydrophobic shells formed at the deprotonated state. Further anchor of PEG and targeting ligands (folic acid) to P2VP via covalent bonds guarantees multifunctions of the nanohybrids. In a design of nanohybrids composed of Fe3O4 NPs, dimeric RGD, and poly(ethylene glycol) methyl ether (mPEG), dimeric RGD was conjugated onto the surface of Fe3O4 NPs via amide bonds, while mPEG was grafted via an acid labile β-thiopropionate linker to minimize the nonspecific uptake by normal cells.442 After the linker was cleaved at the acidic tumor condition, dimeric RGD hidden in the mPEG stealth would be exposed to cancer cells and realizes specific targeting, as illustrated in Figure 25. PEGylation of nanohybrids is usually adopted in multifunctional systems to greatly

phthalocyanine chloride tetrasulfonic acid (AlPcS4) was deposited by LBL assembly through electrostatic interactions with positively charged PAH.186 Nitric oxide (NO) was delivered by poly(oligoethylene glycol methyl ether methacrylate)-b-poly(vinyl benzyl chloride) (P(OEGMA)-bPVBC)) grafted Au NPs for Pseudomonas aeruginosa biofilm dispersal and cancer cell apoptosis.430 The above-mentioned block copolymer was able to load NO after the introduction of secondary amine groups and their subsequent conversion to diazeniumdiolate NO donor molecules. Moreover, NO could be loaded in PDA/Fe3O4 nanohybrids through the formation of N-diazeniumdiolate functionality via the reaction between NO and secondary amine moieties on PDA.431 A hydrophilic and amino groups-containing polymer (P(OEGMA)-b-P(ABA)) was then grafted onto the PDA/Fe3O4 nanohybrids to impart steric stabilization via Schiff base/Michael addition reaction. The resultant nanohybrids demonstrated better biofilm dispersal and antibacterial activities against P. aeruginosa. The positive nature of organic ligands also endows nanohybrids with the property to tether negatively charged genes through electrostatic interactions. Polycations stand for the most prevalent gene carriers and a variety of PEI-modified inorganic NPs are fabricated to testify the gene delivery capability of the resultant nanohybrids.5,320,432−434 To overcome protein adsorption problems by the positive charge of polycations, Boyer et al. reported poly(oligoethylene glycol) methyl ether acrylate (P(OEG-A)) and poly(dimethylaminoethyl acrylate) (P(DMAEA)) functionalized γ-Fe2O3 NPs via “grafting onto” strategy.435 The inner layer of positively charged P(DMAEA) retains siRNA complexation capability while the external antifouling shell of P(OEG-A) confers biocompatibility and long circulation to the nanohybrids. Moreover, versatile types of polycations, cationic AG


Chemical Reviews

Review

Figure 26. Multifunctional pH-sensitive self-assembled nanohybrids. (a) Design of the ligands for pH-responsive, ligand assisted self-assembly of extremely small iron oxide (ESIONs). (b) pH-responsive structural transformation behavior of the nanohybrids. Reproduced with permission from ref 446. Copyright 2014 American Chemical Society.

reduce the trapping of nanohybrids by RES, so that the nanohybrids were cleared mainly through the kidneys instead of liver or spleen.227 Redox-responsiveness and loading of genes were also achieved through the integration of disulfide bonds and polycations as the organic part.211,236,316 Polysaccharides such as HA not only take the responsibility to load drugs by

assembled capsules or work as gatekeepers, but also ensure tumor cell targeting.393,443 In addition, enzyme-responsive drug release would take place after the degradation of HA by hyaluronidase. Some organic components possess intrinsic properties for cancer therapy. For instance, PDA shell could act as PTT agent and the combination with transferrin and lipophilic triphenylphosphonium cation realized the targeting AH


Chemical Reviews

Review

for cancer cells and mitochondria simultaneously.444 Cationic conjugated polyelectrolyte brushes with both cationic side chains and photosensitizer performance were applied for siRNA delivery and PDT under NIR light irradiation. Moreover, the photoresponsive conjugated polyelectrolyte brush turned from positive to zwitterionic, facilitating the siRNA release.445 The rational design based on abundant reactive groups of organic ligands help realize the multifunctions of nanohybrids. Hyeon et al. reported multifunctional pH-sensitive selfassembled nanohybrids by the controlled synthesis of organic parts.446 The photosensitizer Ce6 and imidazole were conjugated to poly(ethylene glycol)-poly(β-benzyl-L-aspartate) (PEG−PBLA) to impart the functions of FL imaging, PDT and pH sensitivity. PEG−PBLA was then engineered to carry catechol groups as anchors for iron oxide with high affinity through self-assembly (Figure 26a). In addition, 3-phenyl-1propylamine could be added to produce a critical phase transition of nanohybrids by tumor endo/lysosomal pH of ∼5.5 (Figure 26b). Qiu et al. synthesized a series of amphiphilic polyphosphazenes (PNPs) with 2-diethylaminoethyl 4-aminobenzoate (DEAB) as the hydrophobic segment and amino-terminal PEG as the hydrophilic segment.447 In this work, polymersome was formed through modulating the ratio of DEAB and PEG to load anticancer drug DOX, where the DEAB segment could conjugate with Au NPs through tertiary amino groups to improve the stability by increasing packing density of polymer chains.

vitro diagnostics or chemical/biological sensing450,451 When the LSPR peaks of Au NPs are modulated to locate in the NIR region, NIR irradiation with deep tissue penetrations could induce photothermal heating, which could be utilized for PTT or photothermal imaging. At the same time, acoustic signals based on photoacoustic effect will be detected for PA imaging, where fast heat generation causes thermos-elastic expansion and explosive vaporization.18 Moreover, light scattering imaging such as optical coherent tomography (OCT) and two-photon luminescence (TPL) taking advantage of the optical properties of Au NPs are also reported for imaging.18,19 Surface-enhanced Raman scattering (SERS) induced by gold or silver NPs could be adopted for diagnosis or imaging. Since Raman signal is weak, Au NPs-enabled SERS imaging is crucial for signal amplification due to the virtues of Au NPs such as biocompatibility and versatility for surface functionalization.452 Au-based NPs with various morphologies and compositions with tunable LSPR are already available to maximize SERS enhancement for diagnostics.19,450,451 Silica-coated Au nanostars have been developed for SERS imaging in clinical applications with high sensitivity.453 Aggregation of Au NPs could produce stronger Raman enhancement compared with individual NPs due to increased electromagnetic fields. Assembly of anti-EGFR antibodies conjugated Au nanorods produced sharp, highly polarized, and well-resolved Raman signals that differentiate cancer cells from normal cells.454 Nevertheless, it is still exceedingly difficult to extract useful information due to the weakness of Raman signal. The rational design of Au-based NPs is expected to produce amplified signals in the target disease. Au NPs with large X-ray absorption coefficients are also used for imaging contrast agents for computed tomography (CT) imaging. Due to the high radiation absorption, Au nanoclusters were also considered as promising radio-sensitizers to enhance the radiotherapy (RT) efficacy.234,455 In addition to Au nanorods, nanostars, nanoshells, or nanocaps with LSPR features in NIR region,93,180,182,188−190,418 various NPs such as MoS2 nanosheets, Bi2Se3 nanoplates, Te nanodots, SWCNT, VS2 nanosheets, WS2 QDs, Co9Se8 nanoplates, Ti3C2 nanosheets, and FeS2 nanocubes also demonstrate strong absorption in NIR region, which are suitable as the inorganic parts to fabricate versatile nanohybrids for PTT and PA imaging.211,212,351,364,388,397,426,427,456−459 It is worth mentioning that Fe3O4 NPs could also work as photothermal agents.460 Chu et al. reported that all Fe3O4 NPs with spherical, hexagonal, and wire-like shapes demonstrated strong photothermal effect induced by red and NIR laser irradiation, attributable to their comparable absorptions.461 Photothermal effect of individual and clustered Fe3O4 NPs was investigated. Clustered Fe3O4 NPs were found to perform better than individual analogues in both in vitro and in vivo experiments.462 The significant increase in the NIR absorption by clustered Fe3O4 NPs is responsible for the enhanced PTT efficacy. Furthermore, taking advantage of the magnetic property of Fe3O4 NPs, biocompatible carboxymethyl chitosan/Fe3O4 nanohybrids realized MRI-guided PTT with enhanced accumulation in the tumor with the assistance of an external magnetic field.463 Also, a series of nanohybrids from heteronanostructures including AgPd-Au nanobipyramids, hollow-structured satellite CuS@Cu2S@Au, B2S3−Au nanorods, MoS2/Bi2S3 nanocomposites, Janus Au−Cu9S5, and Janus Au−Fe2C NPs with NIR absorption were all successfully applied for PTT.193,195,334,339,352,464 In addition to PTT,

3.2. Inorganic Parts

In this subsection, we discuss the physicochemical properties and functions of the inorganic parts in nanohybrids, which are the bridge to connect the controlled synthesis and potential biomedical applications. The underlying structure−property relationships may contribute to the rational design and prediction of functional nanohybrids. Due to tremendous works on inorganic NPs, a few related reviews on this subject were published.15,34,260,448,449 We herein roughly overview the optical, magnetic, electrical properties, et al. of inorganic NPs and mainly emphasize the typical properties and related functions that inorganic parts bring to the corresponding nanohybrids. 3.2.1. Optical Properties and Related Functions. The optical properties of inorganic NPs refer specifically to the absorption, photoluminescence (PL), and Raman spectroscopy of NPs, which usually correspond to the functions of imaging and phototherapy. When it comes to the optical properties of metallic NPs, the first to mention is usually the well-known localized surface plasmon resonance (LSPR). LSPR effect arises from the resonance oscillations of delocalized conduction electrons at the interface stimulated by the incident light with an appropriate wavelength and the typical inorganic parts are Au NPs. A variety of parameters influence the LSPR absorption peaks including composition, size, shape, dispersibility and surrounding ligands or solvent.18−20 For example, the aspect ratio of gold nanorods as well as the shell thickness of gold nanoshells matter and produce tunable plasmonic absorption in a broad range of wavelengths.19 Furthermore, the assembly or aggregation of Au NPs induces red-shifting in LSPR peak caused by plasmon coupling between NPs. The LSPR absorption band shift and color change caused by specific binding of analytes on the surface of Au NPs or analyte-induced aggregation or dispersion were utilized for in AI


Chemical Reviews

Review

Figure 27. Library of ZnxS−AgyIn1−yS2 QDs with tunable PL properties. (a) TEM images of one of the ZnxS−AgyIn1−yS2 (x = 0, y = 0.2). Inset shows the high resolution image of a single NP. (b) PL spectra of selected ZnxS−AgyIn1−yS2 QDs (a: x = 0.6, y = 0.5; b: x = 0.3, y = 0.4; c: x = 0, y = 0.2; d: x = 0, y = 0.5). (c) PL properties of ZnxS−AgyIn1−yS2 library. (d) Representative fluorescent image of the library of QDs with different compositions. Reproduced with permission from ref 320. Copyright 2012 Wiley-VCH.

soluble Ag2S QDs with a wide range emission from 520 to 1150 nm.244 After the conjugation with tumor-targeting peptide, the nanohybrids could realize efficient cancer imaging. Wan et al. developed intracellular synthesis of NIR fluorescent Ag2S QDs in cultured cancer cells, where abundant GSH worked as surface stabilizer.471 Detectable NIR FL emission of Ag2S QDs with high efficiency and excellent optical stability was observed after the precursors were delivered into cells, which provides new insights for biosynthesis and imaging. Notably, multimodal optical imaging including NIR emission, time-gated imaging and TPL imaging was achieved by cadmium-free CuInSe2/ZnS core−shell QDs with minimal toxicity and satisfying photostability.472 Compared with conventional FL imaging, UCL is intriguing since the upconversion process generates visible or UV light from NIR light. The lower-energy excitation applied results in several virtues such as negligible autofluorescence and photodamage, and deep tissue penetration.23 The optical emission properties of UCNP could be tuned by doping of lanthanide ions, control over size, shape, and surface ligands et al.168 UCL is usually utilized for high contrast bioimaging,239,241,336,473 and sometimes with other imaging modalities to realized multimodal imaging with combined advantages to obtain precise information.208,258,385,474 The other important function of UCL is to exploit the emitted visible or UV light to cooperate with payloads or other components of the nanohybrids for achieving stimuli-responsiveness or PDT.240,243,259,268,341,386,412,475 The synergistic properties

plasmonic inorganic NPs such as Au, W18O49 and Cu2‑xS NPs were reported to sensitize generation of reactive oxygen species and exert intrinsic NIR induced PDT effect for cancer treatment.465−468 As introduced in Section 2.2.4, fluorescent NPs such as QDs demonstrate great potential in biological imaging due to their intrinsic optical properties originating from quantum confinement effects. The potential cytotoxicity of QDs from heavy metal ions greatly hinders their adopting in cellular and in vivo imaging.469 In response to the obstacle, Lee et al. synthesized a library of nontoxic ZnxS−AgyIn1−yS2 QDs with tunable optical (PL) properties employing sonochemical synthetic approach under ultrasound (US) irradiation (Figure 27a,b).320 The resultant QDs exhibited intense emission at room temperature, which is not size-dependent. It is also interesting that the emission could be tuned from 480 nm (blue) to 700 nm (red) by controlling the molar ratios of the precursors (Figure 27c). The chemical composition-dependent tunable FL feature of the QD library was depicted in a 3D heat map to summarize the optical properties including emission wavelength and corresponding PL peak intensities, as illustrated in Figure 27d. The remarkable FL of the QDs inside cells could be utilized for cellular imaging and siRNA delivery tracking. In other cases, polymer, BSA, or silica coating of QDs before the formation of nanohybrids could also minimize cytotoxicity.207,285,380,470 Recently, FL in the NIR region attracted intense attention due to its advantages in in vivo imaging with deep penetration and reduced tissue autofluorescence. Achilefu et al. adopted two orthogonal approaches to successfully develop waterAJ


Chemical Reviews

Review

Figure 28. CCN NPs for enhanced PDT via water splitting. (a) Schematic illustration of the C3N4-mediated water splitting process; (b) UV−vis absorption spectra of C3N4, CCN, and PCCN. Reproduced with permission from ref 347. Copyright 2016 American Chemical Society.

nanocube and centrosymmetric clusters.478 The particle arrangement influenced the dipolar interaction effect and the resultant magnetic hyperthermia performances. Remarkably, the increased heating power of Fe3O4 NPs was achieved by a magnetophotothermal therapeutic approach, taking advantage of the duality of Fe3O4 nanocubes as both magnetic and photothermal mediators.479 With the exposure of Fe3O4 nanocubes to both an AMF and NIR laser irradiation, satisfying heating conversion efficiency with minimal collateral tissue damage could be obtained. Magnetic targeting to the specific region is also considered popular and available targeting techniques. However, other parameters such as size, composition, and surface properties should also be taken into account for efficient magnetic targeting.26 Due to the magnetic property of NPs, severe aggregation has chance to happen and surface functionalization with polymers to form nanohybrids presented a convenient approach to prevent potential aggregation. Meanwhile, more functionalities could be attainable through the formation of nanohybrids from magnetic NPs. NPs containing paramagnetic Fe3+, Gd3+, or Mn2+ are available for NMR as T1 contrast agents. For example, released Mn2+ from the surface of manganese oxide NPs could facilitate MR T1 imaging through water interaction when they are evolved in the lower pH tumor environment.480 Rare earth NPs carrying Cd3+ such as Gd2O3 and NaGdF4 provide T1weighted MRI to nanohybrids.231,258,475,481 The strategic design of the magnetic inorganic parts could also realize T1T2 dual-modal MRI with both spatial and temporal imaging match. Monodisperse Gd2O3−Fe3O4 heteronanoparticles were proved to be satisfying T1-T2 dual-modal contrast agents with enhanced MR signals.482 Magnetic properties of Fe3O4 NPs could also be utilized for magnetic particle imaging (MPI), an emerging imaging modality for molecular imaging to apply time-varying magnetic fields to directly detect magnetic NPs instead of MRI signal. MPI is supposed to possess the advantages of deeper penetration, no radiation, and nearly no background from tissues et al.483,484 Rao et al. developed Janus nanohybrids from Fe3O4 NPs, which are responsible for MPI with high contrast.376 3.2.3. Electrical Properties and Related Functions. Inorganic NPs with exceptional electrical properties such as carbon nanotubes, reduced GO, graphite, gold, and molybdenum disulfide NPs demonstrate great potential in tissue regeneration via electrical stimulation, since electrical stimulus might influence the material-cell interaction and the resultant cell functions.485 Typically, they are employed to construct nanohybrids as scaffolds with high electrical conductivity to regulate the differentiation of stem cells when external

between UCNP and organic parts will be mainly discussed in section 3.3. In addition to the typical optical properties from representative inorganic NPs mentioned above, other functions could also be achieved by exploring the optical properties for potential applications. An example is carbon nitride NPs, which possess tunable band gap and band position. They were used for watering splitting for O2 generation under red light (Figure 28a). As mentioned earlier, CCN composed of C dots and C3N4 was synthesized to produce enhanced red light absorption. As displayed in Figure 28b, CCN had obvious enhanced absorption at the red light region compared with the pristine carbon nitride. The generated O2 with high efficiency at a 630 nm laser could overcome tumor hypoxia and contribute to improved efficiency of PDT.347 3.2.2. Magnetic Properties and Related Functions. Magnetic property of inorganic NPs renders them desirable in biomedical applications since magnetic targeting, MRI and magnetic hyperthermia could be simultaneously realized.126−128 When the size of NPs is reduced below a critical value, superparamagnetism effect occurs to present a net magnetization of zero. When external magnetic field is applied, the superparamagnetic NPs will produce a strong response by orienting themselves in the same direction.26 This magnetic behavior facilitates MRI by increasing the T2 relaxation time of the protons of the surrounding water molecules. In addition, the superparamagnetism allows NPs to absorb and convert the energy of an alternating magnetic field (AMF) into heat, which could be used for hyperthermia therapy. SPIONs are currently the most prevalent and widely studied NPs with magnetic properties. They are used as the contrast agents for T2 MRI, as well as imaging-guided therapy to monitor the therapeutic process and provide feedback. Kang et al. developed multifunctional nanohybrids with magnetic Fe3O4 NPs as inorganic cores to impart MRI and magnetic hyperthermia for precise tumor treatment.476 For the nanohybrids comprising Zn0.4Fe2.6O4 superparamagnetic NPs reported by Tseng et al., the magnetic hyperthermia induced by the high saturation magnetization value was applied to trigger the release of encapsulated drug.395 The local hyperthermia mediated by polymer functionalized Fe3O4 NPs was also found to induce the detachment of Pseudomonas aeruginosa biofilms for improved antimicrobial therapy.477 Moreover, complex magnetic interaction caused by particle arrangements or assembly is supposed to affect the heat generated by Fe3O4 NPs when exposed to AMF. Pallegrino et al. found that anisotropic structures from the controlled grouping of two or three Fe3O4 nanocubes resulted in enhanced heating efficiency compared with both individual AK


Chemical Reviews

Review

Figure 29. Janus UCNP-nMOF nanohybrids for FRET. (a) Schematic structure of Janus UCNP-nMOF nanohybrid. (b) Schematic illustration of the energy-transfer mechanism for the UCNPs. (c) TEM, HRTEM, (d) HAADF-STEM images of UCNPs. (e) TEM image of Janus UCNPnMOF nanohybrids. UCL spectra of UCNPs and Janus nanohybrids upon excitation with (f) 808 and (g) 980 nm NIR laser. Reproduced with permission from ref 386. Copyright 2017 American Chemical Society.

electrical field is applied. The electrical-stimulated process could enhance the proliferation ability and differentiation tendency of stem cells into specific cell types such as neural cells.486,487 Fibronectin-coated Au NPs were proved to influence the differentiation of human embryonic stem cells (hESC) after stimulation with electricity.488 The combination of intracellular Au NPs and extracellular conducting substrates containing Au NPs was found to direct the transformation of human mesenchymal stem cells (hMSC) preferentially to neural cells.489 Electroactive carbon nanotubes were applied to reorient the stem cells and regulate their differentiation toward a cardiomyocyte lineage.490 It is notable that SWCNTs were combined with low-voltage electrical stimulation to improve accumulation and drug delivery through enhanced EPR effect and reversible electroporation.491 Amplified electro-stimulation mediated by SWCNTs was supposed to be responsible for the enhanced antitumor effect via electrochemotherapy. Moreover, electromagnetized PEGylated Au NPs exposed to specific electromagnetic fields were able to facilitate direct lineage reprogramming and restore the function of dopaminergic neurons in Parkinson’s disease.187 3.2.4. Other Properties. In addition to the above optical, magnetic and electrical properties, the inorganic parts of

nanohybrids could also provide other functions due to their distinct properties. The most typical example is MSNs, which possess mesopores with the size ranging from 2 to 50 nm. They could be utilized to carry a variety of active payloads through physical adsorption or chemical reaction.96,492 Similarly, other porous or hollow NPs could also be adopted for guest molecule loading.449,493,494 Meanwhile, they offer the matrix for the intelligent design of gatekeepers for controlled release through surface functionalization with organic ligands.95,308,309 Nonporous silica could also work as drug carriers by encapsulating or conjugating the guest molecules,99,495,496 which could be more convenient since the payloads will be incorporated in the carriers through one-pot approach to minimize the tedious processes. Moreover, these inorganic-payloads nanocomposites could be designed to degrade and release the payloads with stimuli-responsiveness.119,120,315 In other cases, inorganic NPs are excellent payload carriers utilizing chemical reaction or supramolecular interaction. For instance, carbon dots with amino groups on the surface prepared through a thermal pyrolysis method could be conjugated with the carboxylic acid group of cisplatin(IV) prodrug by carbodiimide coupling chemistry for drug AL


Chemical Reviews

Review

Figure 30. Photoregulated drug release via UCNPs. (a,b) Schematic illustration of the NIR-regulated drug release carriers and the photolysis of the prodrug under emission from the UCNPs. (c,d) TEM images of yolk−shell structured UCNPs. (e) Photoregulated release of chlorambucil from carriers controlled by a 980 nm laser. Reproduced with permission from ref 504. Copyright 2014 Wiley-VCH.

loading.322,497 GO with ultrahigh polyaromatic surface area enables the loading of aromatic drugs by π−π stacking and hydrophobic interaction.498,499 More interestingly, inorganic NPs themselves could work as drugs. Hyeon et al. proposed a Pt nanocluster assembly, which would disassemble to release Pt nanoclusters in an acidic intracellular environment. The Pt nanoclusters with large specific surface induced toxic Pt ions release to kill hepatocellular carcinoma (HCC) cells via DNA damage and could overcome cisplatin resistance.500 Inorganic NPs of nanohybrids could also work as gatekeepers to realize the stimuli responsive payload release. For example, peptidemodified WS2 QDs were tethered on the surface of MSNs with pH-responsive bonds to control the release of encapsulated DOX.318 Moreover, the mechanical properties of carbon nanotubes including remarkable flexibility and strength make them ideal candidates for tissue engineering scaffolds.172 Inorganic NPs could also function with interesting chemical properties. Manganese oxide and platinum NPs are able to react with tumor endogenous H2O2 to relieve tumor hypoxia.455,501 Titanium oxide NPs with distinct energy-band structure could realize US-triggered separation of electrons and holes, which have been applied for sonodynamic cancer therapy.502 Sometimes, the inorganic NPs with favorable properties work on the organic parts of nanohybrids. Au NPs could conjugate with the amino groups of organic parts to stabilize the assembly of polymersome.447 The hollow Au−Ag bimetallic nanoshells with the feature of catalytic activity were utilized to

accelerate oxidative processes of the hydroxyl groups of the organic part on the surface.418 These functions will be discussed more in section 3.3. 3.3. Synergistic Properties of Organic and Inorganic Parts

In addition to the combination of properties and functions of organic parts and inorganic parts, nanohybrids are attractive due to the intriguing cooperative properties between the organic and inorganic parts, which could produce remarkable synergistic effect and new functions. In this subsection, we discuss several types of synergetic properties and related functions to reveal the advantages of versatile structures of nanohybrids. UCNPs are ideal candidates to work with organic parts synergistically, taking advantage of their unique UCL emissions of UV or visible light under NIR irradiation as the donors to transfer energy to photosensitizers or PTT agents for phototherapy. When the nanohybrids are designed for PDT, the structure of UCNPs for emission regions, overlap of spectra between donor and acceptor as well as their distance should all be taken into account to realize efficient reactive oxygen species (ROS) production. In a design of multifunctional theranostic platform, GdOF:Ln@SiO2 yolk-like nanocapsules as the inorganic parts induced red emission with NIR irradiation, which excited the organic segment zinc(II)phthalocyanine to produce singlet oxygen (1O2) for PDT.503 In the example of Janus UCNP-nMOF nanohybrids developed by Yan et al., the NIR light harvested by the UCNPs allowed AM


Chemical Reviews

Review

Figure 31. PCCN nanohybrids for enhanced PDT against hypoxic tumor. (a) Structure of PCCN and schematic diagram of 630 nm light-driven water splitting enhanced PDT. (b) CLSM images of PDT-induced hypoxia reversion and intracellular ROS generation. Reproduced with permission from ref 347. Copyright 2016 American Chemical Society.

residual molecules remained in the capsule due to their hydrophobicity. The drug release profile with the on−off pattern of the NIR irradiation revealed the photoregulated release of chlorambucil (Figure 30e). Moreover, Fan et al. developed the nanohybrids of cationic conjugated polyelectrolyte and UCNPs, where upconverted UV irradiation from UCNPs was successfully used to activate conjugated polyelectrolyte for ROS production and cleave the photodegradable group o-nitrobenzyl for siRNA release.445 In another example of activating photosensitizer to generate ROS, blue and UV emissions from the same core−shell UCNPs were employed respectively to break photocleavable linker containing o-nitrobenzyl group to release siRNA.249 Aside from UCNPs, ration engineering of semiconductor QDs could also enable ROS production by energy transfer.340 QD-Zn-porphyrin nanohybrids realized remarkable 1 O 2 production via a dual energy transfer process. One energy transfer process was from QDs to porphyrin and the other was from the encapsulated fluorescent photosensitizer rhodamine 6G, whose maximum emission and the absorption of Znporphyrin overlapped well to facilitate energy transfer. PDT was usually restricted by the hypoxia feature of tumor microenvironment. The organic and inorganic parts could

energy transfer to nMOFs, which was then transferred to surrounding oxygen molecules to produce ROS for PDT. This function was attributed to the spectra overlap and the close proximity of the nMOF and UCNP domains, as shown in Figure 29.386 The Janus UCNP-nMOF nanohybrids were proved to realize dual NIR light (808 and 980 nm) induced tunable energy transfer from UCNPs to nMOFs. In most cases, fluorescence resonance energy transfer (FRET) occurs from UCNPs to photosensitizers such as Rose Bengal and Ce6 to generate ROS.240,243 In other cases, the emission of UCNPs could also be utilized to construct phototrigger-controlled drug-release carriers. NIRinitiated UCNPs-based drug delivery systems have demonstrated great potential in spatial and temporal determination of therapeutic release, which confers advantages over conventional light based systems with enhanced tissue penetration depth.504 A kind of hydrophobic conjugated prodrug comprising of amino-coumarin as trigger and chlorambucil as caged anticancer drug was loaded in yolk−shell structured UCNPs.505 As shown in Figure 30a,b, the UV emission from UCNP cores would cause the photolysis of the conjugated prodrug to release chlorambucil. This design guarantees that the drug release was triggered by NIR irradiation while the AN


Chemical Reviews

Review

Figure 32. Synergistic interaction between SPN and MnO2. (a) Schematic illustration of H2O2-responsive mechanism of SPN-M and (b) detailed mechanism of SPN-M1 for amplified PDT in tumor. (c) Chemical structure of the organic parts. (d) Synthetic routes of SPN-M. (e) TEM and STEM images of SPN and SPN-M. Reproduced with permission from ref 506. Copyright 2018 American Chemical Society.

method formed in situ on the surface of SPNs (SPN-M).506 In this design, the organic SPN core demonstrates high NIR absorption, which could be used for NIR FL imaging and PDT. The inorganic MnO2 nanosheets will react with H2O2 in the acidic tumor microenvironment to generate O2 in hypoxic solid tumor (Figure 32). Thus, the sacrificing inorganic shell acted as the supplier of O2 to promote the PDT efficacy of the organic cores to work synergistically. The organic and inorganic parts of the nanohybrids displayed cooperative properties while maintaining the individual properties of each other. A kind of biocompatible carbon dots@PEG-CS hybrid nanogels worked synergistically in this way. The compositions of the hybrid nanogels include fluorescent carbon dots, thermoresponsive PEG and pHresponsive chitosan.391 CS was used to sense the changes in pH over the physiological range of 5.0−7.4 to regulate the release of DOX loaded in the nanogels. Thermoresponsive PEG network triggered the drug release under hyperthermia induced by the NIR irradiated photothermal carbon dots. In addition, the hyperthermia generated by photothermal inorganic NPs could also cooperate with the organic parts for targeting or drug release. For example, the photothermal effect of hollow structured CuS@Cu2S@Au NPs drove the

cooperate to improve the therapeutic effect of PDT. As discussed earlier, in the example of PCCN nanohybrids consisting of PpIX-PEG-RGD and CCN, CCN could achieve improved water splitting through the doping of carbon dots to decrease the band gap of carbon nitride and enhance red light absorption.347 PCCN could accumulate at the tumor region with the assistance of active RGD targeting and EPR effect. When the 630 nm laser was irradiated, CCN was responsible to split water and generate O2 (Figure 31a). Meanwhile, the photosensitizer PpIX would transmit the energy to the generated O2 to produced 1O2 for efficient PDT. As displayed in Figure 31b, ROS-ID with red FL was applied for hypoxia detection. After irradiation, negligible FL was observed with PCCN-mediated cells in both normoxic and hypoxic environment. In contrast, PpIX and polymer modified carbon nitride (PCN)-treated cells appeared remarkably enhanced red FL in hypoxic environment. Confocal laser scanning microscopy (CLSM) images indicate comparable ROS generation ability of PCCN in both environments, verifying the capability of PCCN to overcome hypoxia through synergistic property between PpIX and CCN. Recently, Pu et al. developed the core−shell nanohybrids of semiconductor polymer NPs (SPNs) and MnO2 nanosheets for NIR PDT via a one-pot straightforward AO


Chemical Reviews

Review

Figure 33. Self-catalytic oxidation of dextran-coated hollow Au−Ag nanoshell. (a) Synthesis of dextran/Au−Ag nanohybrids by galvanic replacement reaction. (b) Au−Ag surface-mediated self-catalytic intraparticle alcohol oxidation of dextran, giving aldehyde and ketone moieties. Reproduced with permission from ref 418. Copyright 2014 American Chemical Society.

3.4. Morphology-Dependent Properties of Nanohybrids

thermal-responsive polymer brushes to shrink and the RGD molecules shielded in the brushes were exposed for targeting.193 In another smart system, Cu2S NPs entrapped in the thermosensitive nanogels under NIR irradiation caused the shrinkage of the nanogels to realize the controllable drug release.507 The hyperthermia of Au NRs was also applied synergistically to open the gatekeeper of CD-PGEA on the surface of silica for drug release.317 Taking advantage of hyperthermia effects of magnetic Fe3O4 NPs upon AMF exposure, the controlled release of the drugs conjugated to the organic ligand by Diels−Alder (DA) reaction could be realized through retro-DA reaction.508 In another example, the drugs conjugated onto the organic segments through Schiff base bonds could be released in dual-stimuli responsive manner to the acidic microenvironment of cancerous cells and magnetic hyperthermia generated by the acicular Fe3O4@SiO2 NPs.509 Other synergistic properties between the organic and inorganic parts of nanohybrids were also exploited for the design of intelligent drug delivery systems. As shown in Figure 33a, hollow dextran/Au−Ag nanohybrids were prepared through galvanic replacement reaction from solid dextran/Ag nanohybrids employing dextran as reductant.418 The hollow Au−Ag alloy with the catalytic activity could oxidize the hydroxyl groups of dextran to carbonyl groups for DOX loading through Schiff base formation (Figure 33b). Besides the drug loading through covalent bonds, the cavity of the inorganic NPs also served as space for drug loading through physical absorption. The temperature elevation originating from Au−Ag alloy also contributed to the release of loaded drugs.

The morphology of NPs is considered to affect their interaction with biological systems to some extent.510−512 In this subsection, we are trying to discuss morphology-dependent properties and functions of nanohybrids, as well as the related biological responses, including cellular uptake, biodistribution, cytotoxicity, and clearance et al. Furthermore, we attempt to reveal the structure−activity relationships between the morphology of nanohybrids and performances. Hopefully, nanohybrids with appropriate morphologies could be readily designed with these clues to meet specific requirements. Herein, morphology mainly refers to the size and shape of nanohybrids. Particle size is an important factor, and NPs with the size of 10−100 nm are considered to be suitable to prevent rapid clearance from body. The cellular uptake efficiencies of both silica and Au NPs are supposed to be size-dependent.513−515 The nanohybrids of PEGylated MSNs and incorporated folic acid with three different sizes of 48, 72, and 100 nm were prepared and compared for cancer-targeted drug delivery in vivo.312 The nanohybrids with the size of 48 nm were proved to be superior and exhibited highest uptake and accumulation in tumor tissues. The particle size of Fe3O4 NPs-based nanohybrids is considered to evidently influence their MRI contrast ability.516,517 The nanohybrids of RGD peptides, mPEG and exceedingly small magnetic Fe3O4 NPs with different sizes below 5 nm (i.e., 1.9, 2.6, 3.3, 3.6, 4.2, 4.8, and 4.9 nm) were synthesized as T1-weighted contrast agents for MRI.442 3.6 nm was found to be the optimal particle size of the nanohybrids with the brightest MR images and highest intensity. This was caused by their highest longitudinal relaxivity (r1) and lowest r2/r1 (r2 represents transversal relaxivity) among all the nanohybrids. Moreover, MRI contrast AP


Chemical Reviews

Review

Figure 34. Schematic illustration of the morphology effect of Au-PDMAEMA nanohybrids on gene transfection and endocytosis. Reproduced with permission from ref 273. Copyright 2015 The Royal Society of Chemistry.

P(VBA)) functionalized Au nanospheres, nanorods, and starlike NPs prepared via an “grafting onto” approach were also investigated for drug delivery and PTT.527 Star-like nanohybrids were proved to be most effective for PTT and drug delivery while the spherical analogues showed the lowest efficiency. It is noticeable that nanohybrids of WNV envelope protein (WNVE) coated Au NPs working as vaccines also demonstrated shape-dependent immune response and WNVEAu nanorods produced highest level of WNVE-specific antibodies.519 The nanohybrids were supposed to enhance the immune response via different cytokine pathways, where rod-shaped nanohybrids activated inflammasome-dependent cytokine secretion while spherical and cubic nanohybrids induced inflammatory cytokine production. Recently, Zhao and Xu et al. designed a series of AuPDMAEMA nanohybrids with different morphologies to investigate the size and shape effects of nanohybrids on gene transfection.273 The shape of the nanohybrids was found to play an important role in cellular uptake and the subsequent gene transfection. As shown in Figure 34, various nanohybrids from Au nanospheres (41 nm), nano-octahedra (43 nm), nanorods with different aspect ratios (39 nm × 10 and 105 nm × 15 nm, respectively), and arrow-headed nanorods (44 nm × 12 nm) were fabricated for gene delivery. The nanohybrids of PDMAEMA and arrow-headed Au nanorods were observed to exhibit the higher gene transfection efficiency followed by the nanohybrids from nanorods, nano-octahedra, and nanospheres, which correlated positively with the corresponding internalization ratios. The nanohybrids with higher aspect ratios (from nanorods and arrow-headed nanorods) were more efficient gene carriers than the nanohybrids from nanospheres or nanooctahedra. Notably, nano-octahedra and arrow-headed nanorods worked better than the nanospheres and ordinary nanorods, respectively. That is to say, nanohybrids with sharp corns might be superior over their counterparts, probably due to the enhanced penetration during cellular internalization. Similarly, SiO2−PDMAEMA nanohybrids also demonstrated shape-dependent properties.274 Among the nanohybrids with different morphologies from silica nanospheres (100 nm), hollow nanospheres (100 nm), nanorods (300 nm × 100 nm), and chiral nanorods with different aspect ratios (300 nm × 100 and 200 nm × 100 nm, respectively), the nanohybrids

effects of manganese-doped Fe3O4 NPs were observed to be highly size-dependent. T1-dominated (5 nm), T2-dominated (12 nm), and T1-T2 (7 nm) dual-modal MRI could be realized by controlling their sizes.518 Compared with size effect, the shape effect of nanohybrids may be more complicated. Ample evidence reveals that the shape of NPs plays an important role in cellular internalization. Generally speaking, 1D nanostructures including nanorods, nanotubes, and nanoworms with larger aspect ratio are supposed to facilitate cellular uptake than ordinary spherical NPs.73,117,519,520 The APTES-SiO2 nanohybrids with larger aspect ratio were reported to have a greater impact on cellular function such as cellular proliferation, adhesion, migration, and apoptosis.117 The internalization pathways, uptake kinetics as well as surface properties were also extensively exploited, which shed light on the understanding the interactions between cells and NPs with different shapes.521−524 Furthermore, the shape of the nanohybrids could also affect their in vivo behaviors including biodistribution, clearance, and tumor tissue penetration.36,74 Therefore, the shape might be a vital property to be investigated for the design of effective nanohybrids. Among these shape-dependency studies, surface chemistry was also important in some cases.525 Xia et al. designed and synthesized nanohybrids of PEG, anti-HER2, or poly(allyamine hydrochloride) (PAAH)-functionalized Au NPs (nanospheres or cubic nanocages) and investigated the effect of shape and surface properties of the nanohybrids on the internalization by SK-BR-3 cells.526 Spherical nanohybrids were preferred when PEG or anti-HER2 was coated, while cubic nanohybrids displayed higher cellular uptake than the spherical counterparts when PAAH was on the surface. Tang et al. discovered that APTES-SiO2 nanohybrids with smaller aspect ratio were easily trapped in the liver while the longer ones mainly distributed in the spleen.36 However, PEGylated silica nanohybrids with both aspect ratios were found to accumulate in the lung. To evaluate the shape effect, the surface property of the nanohybrids usually remained constant. For instance, PEGylated Au nanohexapods, nanorods, and nanocages were compared for photothermal treatment. The nanohybrids from Au nanohexapods displayed the highest cellular uptake, as well as significant blood circulation, tumor accumulation, and photothermal effect.180 Diblock copolymer (P(OEGMA)-bAQ


Chemical Reviews

Review

Figure 35. Starlike and spherical Au@SiO2−PGEA nanohybrids with photothermal Au caps. (a) TEM images of starlike and spherical Au@SiO2 NPs. (b) Photothermal images of C6 tumor-bearing mice with starlike nanohybrids. (c) FL images and flow cytometry analysis plots of C6 cells treated with spherical and starlike Au@SiO2−PGEA/pDNA, where the YOYO-1-labeled pDNA is shown in green and the DAPI-labeled nucleus is shown in blue. Reproduced with permission from ref 92. Copyright 2017 Wiley-VCH.

the basis of the starlike nanohybrids, photothermal Au nanocapes were encapsulated to incorporate the functions of PA imaging and PTT (Figure 35a,b).92 In the same way, the starlike Au@SiO2−PGEA nanohybrids favored cellular uptake (Figure 35c), which contributed to improved performances compared with the spherical Au@SiO2−PGEA counterparts. Recently, virus-, rambutan-, raspberry-, or flower-like silica NPs with rough surfaces were synthesized successfully and observed to facilitate cellular uptake efficacy by enhanced adhesion toward cellular or bacterial surfaces.434,528−530 It is worth mentioning that rambutan-like SiO2−PEI nanohybrids with spiky surfaces facilitated plasmid DNA binding and possessed the best gene transfection performances compared with smooth, raspberry-, or flower-like analogues.434 The 3D nanotopographies of these kinds of rough NPs were reconstructed from a series of tomograms sliced from the center of one NP. As demonstrated in Figure 36, rambutan-like silica NPs (Ram-SNPs) exhibited spike-type subunits on the shell (Figure 36a,d,g,j), while raspberry-like (Ras-SNPs) and flower-like NPs (Flw-SNPs) displayed hemisphere- and bowltype subunits (Figure 36h,k,i,l), respectively. pDNA was supposed to be entangled inside the spiky shells of RamSNPs-PEI nanohybrids, benefiting from the unique nanotopography. The gene transfection performance of Ram-SNPsPEI nanohybrids was observed to be better than Flw-SNPsPEI, Ras-SNPs-PEI, and smooth analogues (S-SNPs-PEI). Compared with Lipofecatmine-2k and smooth S-SNPs-PEI, rough nanohybrids provided pDNA protection capability against DNase I degradation. The spiky surface of Ram-

composed of PDMEAMA and the longer chiral nanorods performed best. 1D nanohybrids were found to exhibit higher gene transfection efficiency than spherical nanohybrids. Moreover, hollow nanohybrids performed better than the solid counterparts, which could be attributable to their higher outside shell surface areas to carry pDNA by electrostatic interaction. These findings confirmed the shape effect on the properties and related functions of the nanohybrids. However, the exact mechanism needs further exploration and detailed works are needed to clarify the interactions between the cells and nanohybrids. Based on the general understanding of morphologydependent property, nanohybrids with optimized shapes were designed and fabricated for diverse functions and enhanced performances. With the concept that 1D nanohybrids facilitate cellular uptake and the subsequent gene transfection, 1D peapod-like magnetic nanohybrids comprising cationic polymer PGEA and Fe3O4 NPs were fabricated for gene delivery and MRI.284 Compared with the spherical counterparts, 1D peapod-like NPs demonstrated evidently improved gene transfection performance. In the same way, 1D wormlike fluorescent nanohybrids comprising PGEA and QDs were proved to be superior gene carriers in contrast with spherical ones.285 Taking advantage of the high endocytosis efficiency of NPs with sharp horns and hollow characteristic, starlike hollow nanohybrids with six symmetrical sharp horns synthesized from starlike silica NPs and CD-PGEA possess better performances in cellular uptake, gene delivery, and antitumor effectiveness than the spherical nanohybrids.316 On AR


Chemical Reviews

Review

Figure 36. Silica NPs with different kinds of rough surfaces for plasmid DNA delivery. (a−c) SEM images, (d−f) TEM images, (g−i) electron tomogram slices, (j−l) reconstructed subunits of Ram-SNPs (a,d,g,j), Ras-SNPs (b,e,h,k), and Flw-SNPs (c,f,i,l). (m) Representative confocal images of green fluorescent protein (GFP) gene expression mediated by Lipofectamine-2000, Ram-SNPs-PEI, Flw-SNPs-PEI, Ras-SNPs-PEI, and S-SNPs-PEI without and with DNase I treatment. Reproduced with permission from ref 434. Copyright 2017 American Chemical Society.

SNPs-PEI contributed to the highest extent of pDAN protection among the three kinds of silica NPs (Figure 36m). Feng et al. proposed the concept of virus-surface mimicking organic/inorganic nanohybrid gene carriers by electrostatically

associating DNA-entrapped PEI nanocomplex (+27 mV) with citrate capped Au NPs (−35.8 mV), where the PEI/pDNA nanocomplex was readily covered by plenty of Au NPs, as shown in Figure 37.531 The unique virus morphology of PEI/ AS


Chemical Reviews

Review

Figure 37. Fabrication of virus-surface-mimicking nanohybrids of DNA-entrapped NP with gold NPs for enhanced gene transfection and NIR photothermal therapy. (a) TEM and (b) SEM image of PEI/pDNA/Au NPs and (c) TEM image of PEI/pDNA. Reproduced with permission from ref 531. Copyright 2015 Wiley-VCH.

most cases, the organic parts of nanohybrids provide opportunity by tailoring the interactions between nanohybrids for the assembly. With the exploration for the new properties compared with the individual nanohybrids, more interesting functions could be achieved. In this subsection, we will discuss the self-assembly of nanohybrids via various strategies with interesting properties for biomedical applications. Physical forces are the most commonly used strategy to motivate the self-assembly of nanohybrids. For example, nanohybrids could be aligned to form ensembles through the hydrophobic interaction of organic parts.30 Self-assembly of nanohybrids with block copolymers or binary polymer brushes as the organic segments could be achieved by controlling the solubility in selective solvents. Nie et al. developed a facile strategy to assemble nanohybrids of PS-b-PEO-grafted Au NPs in the selective solvents of THF and water.533 With the addition of water into a THF solution, the self-assembly process occurred as the hydrophobic PS blocks collapsed to minimize the overall free energy of the system. The resultant assemblies exist in the morphology of unimolecular micelles, small clusters, and hollow vesicles, depending on the size of Au NPs and length of PS blocks. The dissipative particle dynamics (DPD) simulations reveal that the morphological transition of assemblies is determined by the deformability of nanohybrids. The LSPR peak of the assembled nanostructures displayed obvious red-shift compared with that of the individual nanohybrids. Consequently, the enhanced LSPR absorption in the NIR region could be utilized for PTT and PA imaging. Later, Nie et al. discovered that through controlling the grafting density of polymers on Au NPs, chain vesicles composed of linear nanohybrid strings or nonchain vesicles with a uniform distribution of nanohybrids could be obtained.534 More interestingly, chain vesicles displayed a drastic red-shift in the LSPR peak in contrast with the nonchain vesicles, arising from strong plasmonic coupling

pDNA/Au nanocomplexes led to significantly enhanced cellular uptake and 100-fold promotion of transfection efficacy compared with PEI/pDNA alone. The 3D location of gold nanoclusters on the surface of nanocomplexes also produced one distinguished feature for NIR-induced PTT, in contrast with individual Au NPs. Recently, rattle-structured rough nanohybrids comprising PEGA-functionalized silica nanocapsules and in situ-formed Au nanorods cores were proposed for PA/CT dual-modal imaging-guided complementary gene/ chemo/photothermal therapy.532 The priority of the rough nanohybrids over smooth counterparts was well established by comparing both in vitro and in vivo performances. The shape of nanohybrids could also be optimized for imaging. Zhao et al. investigated the morphology effect of surface charge-reversal manganese oxide nanohybrids for MR imaging of tumors.411 They synthesized manganese oxide nanospheres and nanocubes with different sizes and found that nanocubes manifest exceptionally stronger T1 contrast effects than nanospheres with comparable size. The higher surface-tovolume ratio of nanocubes was considered to allow more manganese ions exposed on the surface and accelerate the spin relaxation process of water protons. The {100} facet of nanocubes was proved to further improve T1 relaxivity due to its high metal package density and low surface energy. 3.5. Self-Assembly of Nanohybrids

Self-assembly of nanohybrids attracts intense interest since nanohybrid ensembles in 2D or 3D lattices might present collective properties, including the coupling of optical, magnetic, and electrical properties.71 Herein, organic/inorganic nanohybrids are considered as the building blocks to be assembled into ordered structures for specific requirements. There are several approaches to realize the self-assembly of nanohybrids, including hydrophobic interactions, chemical bonding, electrostatic forces, and host−guest interactions. In AT


Chemical Reviews

Review

Figure 38. Self-assembly of PS-b-PEO/Au nanohybrids into vesicles. (a) Schematic illustration and related UV−vis spectra of the chain vesicles and nonchain vesicles. (b) UV−vis spectra of the nanohybrids and the resultant assemblies at different water concentrations. (c−e) Representative TEM images of the assemblies obtained at 5, 10, and 100 vol % of water in water/THF mixture. Scale bar: 20 nm. (f) Mechanisms of the formation of chain vesicles. Reproduced with permission from ref 534. Copyright 2015 Wiley-VCH.

between adjacent Au NPs with smaller interparticle distance within each string (0.8 nm) than that between Au NPs in nonchain vesicles (9.0 nm), as shown in Figure 38a. This shape-dependent property resulted in high efficiency in PA imaging. As water was added into the solution of nanohybrids in THF to trigger the self-assembly in a stepwise manner, gradual red-shift was observed in the corresponding UV−vis spectra (Figure 38b), which is consistent with the TEM images as the water content increased (Figure 38c−e). The formation of chain vesicles achieved by nanohybrids with low grafting density could be explained by the stepwise assembly with the

formation of string at the beginning due to the repulsion at the center of NP pairs and attraction at both poles (Figure 38f). Biodegradable vesicles were also designed through employing PEG-b-PCL block copolymer-grafted Au NPs as the building blocks.535 With the introduction of the disulfide bond at the terminus, the organic segment PEG-b-PCL allows dense packing of nanohybrids, leading to ultrastrong plasmonic coupling effect and enhanced PA imaging. At the same time, the efficient PTT could induce the collapse of vesicles when the temperature was higher than the melting point of PCL at AU


Chemical Reviews

Review

Figure 39. Self-assembly of PGED/Au or Fe3O4/PGED nanohybrids and SiO2. Schematic illustrations of (a) preparation process of CD-grafted PGED/Au and Fe3O4/PGED nanohybrids, and (b) self-assembly of the nanohybrids and their applications for combined PTT/GT. TEM images of assembled (c,e) SiO2@Fe3O4/PGED, and (d,f) SiO2@Au/PGED structures before (c,d) and after (e,f) laser irradiation (808 nm, 2 W/cm2). Reproduced with permission from ref 541. Copyright 2018 The Royal Society of Chemistry.

around 60 °C, which facilitates the clearance of the dissociated nanohybrids. Film rehydration method is also widely utilized for the selfassembly of nanohybrids to vesicles by rehydrating a film of nanohybrids in water under sonication.536,537 The vesicles by assembling PEO-b-PS/Au nanohybrids employing film dehydration method demonstrated strong absorption in the NIR range and capability of loading photosensitizer Ce6, which allows for multimodal imaging-guided PTT/PDT.538 Magnetovesicles with tunable thickness were achieved by coassembly of PS-b-PEO/Fe3O4 nanohybrids and free PS-b-PAA through modulating the hydrophobic/hydrophilic balance of nanohybrids.425 The multilayer vesicle displayed the highest r2 for MRI due to the high packing density of nanohybrids. After conjugation with RGD peptide, the DOX-loaded vesicle could also be utilized for targeted drug delivery. Emulsion method is also efficient for the self-assembly of nanohybrids. PEG/PLGA-grafted ultrasmall Au nanorods produced vesicles by generating chloroform-in-water emulsion droplets with the assistance of poly(vinyl alcohol) (PVA) and the subsequent evaporation of chloroform.185 Enhanced photothermal conversion as well as PA imaging could be achieved, benefiting from the strong plasmonic coupling of the Au nanorods in the vesicular shells. With the hydrolysis of PLGA, the degraded vesicle facilitated their clearance. The nanoclusters assembled from PEG and poly(methyl meth-

acrylate) (PMMA) coated Fe3O4 NPs by emulsion method were reported to exhibit enhanced T2 relaxivity in MRI.539 It was found that magnetic field inhomogeneity caused by reduced field symmetry contributed to the high-performance MRI. This interpretation may shed light on the design of assembled structures from nanohybrids as promising contrast agents. Furthermore, the above strategies could be extended to the self-assembly of multiple types of NPs to vesicular structures. Reduced GO could be loaded in the vesicle assembled from PEG/PLGA/Au nanohybrids employing a double-emulsion method.499 The resultant hybrid vesicles possess high loading capacity of DOX, since both the cavity of vesicle and reduced RO could be used for DOX loading. Pd−Ir NPs could be encapsulated in the vesicle composed of PEO-b-PS/Au nanohybrids during the film dehydration process.203 Carrying the catalytic properties of Pd−Ir NPs, the as-prepared hybrid vesicles could be used for colorimetric assay of disease biomarkers with substantially high detection sensitivity. Through the coassembly of PS-b-PEO/Au nanohybrids, free PS-b-PAA, and hydrophobic Fe3O4 NPs, hybrid Janus vesicles with controlled morphology were realized by modulating the size and type of the components.202 Hemispherical Janus vesicles demonstrating magneto-plasmonic properties were utilized for magnetic field-enhanced MRI and PA imaging. Moreover, Janus nanohybrids fabricated from Janus NPs give AV


Chemical Reviews

Review

Table 4. Overview of Bioapplications of Organic/Inorganic Nanohybrids. bioapplication

modality

nanohybrid

refs

imaging

MRI

C-Gd(III)-DO3A(1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid) Au-Gd(III)-DO3A Gd2O3-PVP NaGdF4-PVP hollow MnO2@SiO2-F127 MnFe2O4-mPEG ESIONs-PEG Fe3O4-PEG Fe3O4-mPEG-PEI-polyDOPA(poly(L-3,4-dihydroxyphenylalanine)) Fe3O4-alginate-PEG Fe3O4-mPEG Fe3O4-PEG-RGD Fe3O4-P75 Fe3O4-PLGA-alendronic acid Fe3O4-BSA-sialic acid MnFe2O4-herceptin SPION-HPG Fe3O4-PEG-PMMA Fe3O4-dextran-F3 Fe3O4-MMP-PEG-CXCR4 Fe3O4-PMAA-PTTM(thiol functionalized poly(methacrylic acid)) FeCo/C-PEG Fe3O4/Gd2O3-HDA-G2 (1-hexdecanamine-poly(amido amine)) Fe3O4-mPEG-Gd(III)-DTPA Fe3O4-PEG-Mn(II)-HIF-1α Fe3O4@MnO/SiO2-CD133 antibody Au/Pt/Fe3O4-Gd(III)-DOTA(1,4,7,10-tetraazacyclododecane-1,4,7,10tetraaceticacid)-PEG MnxFe3-xO4@SiO2-PNIPAM-Gd(III)-DO3A SPION/MnOx-CS PbS/CdS/ZnS QD-PEG C/ZnS dot-PEG C-PEG-RGD graphene QD-PEG C dot-PVP CdSe/CdZnS QD-PEG PbS QD-J-aggregate ZnCuInSe/ZnS-P(SPP-N3-4VIM)(poly-(methacrylamidosulfobetaine-b-4vinylimidazole))-RGD C dot-PEG NaYF4:Yb/Tm@NaYF4@HMSN-[Ru(dpp)3]2+Cl2 NaYF4:Yb/Er-PEG NaGdF4:Nd-PbS/CdS/ZnS QD-PLGA NaYF4:Gd/Yb/Er nanorod-PAA Zn1.1Ga1.8Ge0.1O4:Cr,Eu-PEG-TAT SiO2-AIEgen-PS-b-PAA-PEG Au-rec1-resilin SWCNT-ICG-PEG-RGD Au@SiO2-PEG CuS-PEG Au-Cu2-xSe-PEG Au-G5-PAMAM Au-G5-PAMAM-PEG Au-G5-PAMAM-FA Au-glucose MWCNT-PEI-FITC-mAbPSCA(prostate stem cell antigen monoclonal antibody) CaCO3-F127-alginate Fe3O4-PMAO(poly(maleic anhydride-alt-1-octadecene))-PEG SPION-PMAO-PEG Si/Mn QD-dextran-64Cu2+-DO3A Bi-PEG

550 551 552 553 554 555 516 557,560,571 558 559 561 562 563 564 565 566 567 539 568 569 570 572 482 573 574 575 576

FL

PA

CT

US MPI

FL/CT

AW

577 578 580 581 582 583 584 585 586 587 588 590 591 592 593 594 595 596 597,598 181 599 600 601 602 603 604 605 361 606 607 608 611


Chemical Reviews

Review

Table 4. continued bioapplication

modality

FL/MR

CT/MR

PA/MR MR/US

PA/US MR/PET FL/MR/CT

FL/MR/MPI

therapy

US/CT/MR/PET PTT

PDT

chemotherapy

nanohybrid

refs

[email protected] Au-PLL-RITC Ag-AIEgen Mn3O4@SiO2-RITC-FA Fe3O4-poly(isobutylene-alt-maleic anhydride)-IR-820 dye SPION-PLA-G3-5MF(fluorescein-5-maleimide-labeled DARPin G3) Fe3O4-osteopontin antibody-Cy5.5 Fe3O4-PEG-ANNA(N-carboxyhexyl derivative of 3-amino-1,2,4-triazole fused 1,8naphthalimide)-Cy5.5-FA chrysotile nanotube/Fe3O4-tetra(4-sulfonatophenyl) porphyrin NaYF4:Yb/Tm/Gd@NaGdF4-PEG-angiopep-2 NaGdF4:Yb/Er@NaGdF4-PEG-osteopontin antibody C dot/Fe-pullulan Fe3O4@Au-PGA-PLL Au-G5-PAMAM-Gd(III)-DOTA-PEG Au-G5-PAMAM-Gd(III)-DOTA-PEG-FA Au/Fe3O4-PEI-PEG Fe3O4/TaOx-PEG Au/Fe3O4-GSH Janus Au/Fe3O4-PEG-PS Au/Fe3O4-PEG-poly-L-histidine HMSN-Gd(III)-DTPA-RGD SPION-poly(butyl cyanoacrylate) SiO2@FePt-SPION-PMAO CuS-PEG 99m Tc-Fe3O4-PEG-RGD 69 Ge-SPION-PAA-PEG C-Fe3O4 QD-poly(γ-glutamic acid) Au nanocluster/Fe3O4-mercaptosuccinic acid Au nanocluster/Gd3+-GSH Fe3O4@Au-PEG Fe3O4-PFODBT Fe3O4-PEG-Cy5.5 68 Ga-SPION-PEG Au nanohexapod, nanorod, nanocage-PEG Au nanobipyramid-PEG AgPd-tipped Au nanobipyramid-PEG@SiO2 Au supramolecular NP-RGD Au@SiO2@Au-PEG Au nanorod-11-mercaptoundecyl-trimethylammonium bromide Au-PDA-PEG Au nanocage-red blood cell membrane E. coli MG1655@Au reduced GO-PEG flower-like MoS2-PEG MoS2 nanoflake-PAA-PEG MoS2 nanosheet-PEG Cu2-xSe-poly(maleic anhydride) Ti3C2 nanosheet-SP Janus Au/Fe3O4-poly(difurfuryl adipate-bismaleimido diphenyl methane)-b-PEO Au/GO sheet-α-synuclein protein HMSN-PEG-FA Ca3(PO4)2-PEG-b-PAsp calcium phosphosilicate-PEG-CD117 antibody Au nanoring-PEG-PAH MOF/Pt-PEG NaYF4:Yb/Tm-graphene QD-tetramethylrhodamine Janus Fe3O4/SiO2-PEG MSN-PEG MSN-sericin MSN-PEG-FA

184 326 612 613 614 615 616 617

AX

618 481 619 620 625 626 627 628 629 630 196 632 633 634 635 636 637 638 621 622 540 194 376 624 639 180 189 464 396 641 642 643 644 645 646 213 230 351 647 459 648 649 218 358 650 186 501 651 235 245 246 312


Chemical Reviews

Review


modality

GT

RT immunotherapy magnetic hyperthermia SDT microwave thermal therapy antibacterial therapy PTT/chemotherapy

PTT/RT PTT/PDT

PTT/GT

nanohybrid

refs

HMSN-PEG HMSN-FITC Janus PS-Fe3O4@SiO2-FA Mn3O4/γ-Fe2O3 nanocage-dextran Pt-octadecylamine-poly(1-(3-aminopropyl)imidazole-Asp)10-SP94 peptide Au-PNP NaYF4:Yb/Tm@NaGdF4:Yb-PEI SWCNT-PEG Si/Ni/Au nanospear-PEI rambutan-, raspberry-, flower-like SiO2-PEI SWCNT-PEI Fe3O4/SiO2 nanocluster-PEI-protein pVIII Au nanosphere, nano-octahedron, nanorod, arrow-headed nanorod-PDMAEMA SiO2 nanosphere, hollow nanosphere, nanorod, chiral nanorod-PDMAEMA Au-PDAMA-HEMA Fe3O4@HMSN-tannic acid Ca3(PO4)2-lipid-PEG-anisamide Ca3(PO4)2/CaCO3-lipid Au nanoScript Au nanocluster-TAT-lipid-PEG Au nanowire-cysteine Au-arginine Janus Au/TiO2-PEG Fe3O4-dextran-TAT GO-PEG-CD16 antibody Fe3O4 nanoring-PEG C/Zn-PEG MoS2 nanoflower-BSA Fe3O4-quaternized PDMAEMA SWCNT-PNIPAM/gelatin hydrogel Au nanorod-BSA-RGD SWCNT-Evans blue-albumin Au nanorod-RGD-exosome CuSiO3 hollow sphere-poly(ε-caprolactone)/poly(D,L-lactic acid) Se@SiO2/CuS-FA hollow Au-DPPC-DSPE-PEG V Na15[MoVI 126Mo28O462H14(H2O)70]·400H2O-PEG Cu39S28-L-cysteine-FA graphene nanosheet@SiO2-PEG-IL-13 peptide Au nanorod@SiO2-PEG-RGD Janus Au-PAA@SiO2-PEG-lactobionic acid MoS2 nanosheet-PEG HMSN@MoS2-transferrin Janus Ag/SiO2-PEG-FA 188 Re-WS2-PEG Au nanocluster-DAA-HA-b-PCL Fe3O4 nanocluster@Au-PEG GO-PEG Au-collagen hydrogel Au nanoechinus-Lipofectamine 2000 W18O49 nanowire-PEG MoOx-PEG CuMo2S3-PVP Au nanorod-PEG MoS2 nanoflake-G5 PAMAM SWCNT-PEI Au-TAT-lipid-PEG oxidized mesoporous C-PEI GO-PEI-PEG MoS2-PEI-PEG reduced GO-PEI-PEG

313 653 406 654 500 447 237 338 432 434 656 657 273 274 658 659 660 661 662−664 665 666 667 669 670 671 132 672 213 276 238 673 674 675 676 677 678 679 680 681 222 374 211,426 682 219 683 41 684 685 686 465 467 687 688 689 214 304 690 691 692 211 693

AY


Chemical Reviews

Review


modality PTT/starvation therapy GT/chemotherapy

PDT/chemotherapy

PDT/GT

PDT/RT

PTT/SDT microwave dynamic/thermal therapy chemotherapy/PDT/PTT

chemotherapy/GT/PTT

imaging-guided therapy

MRI

CDT

CDT/chemotherapy CDT/PDT CDT/PDT/PTT CDT/PTT chemotherapy

GT PDT

PDT/PTT PTT

FL

SDT chemotherapy

chemotherapy/PDT chemotherapy/PTT GT PDT

nanohybrid

refs

Au nanorod-PSS-PDDAC Prussian blue-HA-PEG HMSN-PAE SiO2-PGEA starlike, spherical HMSN-PGEA MSN-PEI MSN-TAT-PAH-galactose-modified trimethyl chitosan MSN/Au nanorod/Fe3O4-pospholipid MOF-tiphenyldicarboxylic acid SiO2@Au-PEG Ca3(PO4)2-HA MSN-PEG mesoporous SiO2 nanorod-BSA-PEG TiO2-zinc phthalocyanine MOF/MnO2-F127 MnO2-HSA NaYF4:Tm/Yb@NaYbF4@SiO2-PEG NaYF4:Tm/Yb-quaternary ammoniation of N-functionalized polyfluorene Au nanoechinus-lipid SnWO4/NaYF4:Yb/Tm@NaGdF4-PEG MOF-PEG LiYF4:Ce@SiO2@ZnO-PEG TiO2@TiO2-x-PEG Mn/ZrMOF-PEG Au-DNA nanomachine Au nanorod@SiO2-PEG-lactobionic acid Au@CuS yolk-shell NP-poly(N-isopropylacrylamide-co-acrylamide) Au nanorod-PEG-dendrimer-dextran hydrogel porous hollow C-PEI-PEG Au nanocage-PEI-PEG-HA amorphous Fe-PVP

694 695 236 315 316 696 697 698 699 700 701 702 703 704 705 706 249 445 707 267 708 709 502 710 711 712 713 182 714 715 721

rFeOx-HMSN-PEG MSN@MnO2-PEG FeS2@FexO nanocube-PEG CuFe2O4@BSA Co-P@PDA Mn-HMSN-PEG Fe3O4@Gd2O3-PEG SPION-PEO-b-PS/PAA-b-PS peapod-like Fe3O4@SiO2-PGEA ZnFe2O4@Au-MUA MnO2-PAH-PEG NaGdF4:Yb/Tm@SiO2@TiO2-FA SPION-PpIX Au/Fe3O4/P nanosheet-PEI Fe3O4-carboxymethyl CS-PEG Fe3O4@Au@MSN-dsDNA Fe3O4-red blood cell membrane Fe3O4@CuS-PEG hollow mesoporous organosilica-MnOx-RGD C dot-PEG-(PAH/DMMA) LiYF4:Yb/Tm@SiO2-CS-BSA-PEG NaYF4:Yb/Tm@SiO2-PEG-RGD NaYF4@SiO2@MSN-CD NaYF4:Yb/Er/Tm-PNBMA-PEG C dot-CS C dot-PEG-CS CdxZn1−xTe/CdS QD@SiO2-PGEA ZnS-AgInS2 QD-PEI CdTe/ZnS QD-PEG

730 719 458 727 728 717 225 425 284 321 716 720 726 725 722 723 724 233 718 322 412 738 311 424 383 391 285 320 340

AZ


Chemical Reviews

Review


modality

nanohybrid

refs

C dot-PEG SiO2@MnO2 nanosheet-PCPDTBT(poly(cyclopentadithiophene-alt-benzothiadiazole))/PS-b-PAA/PEG-b-polypropylene glycol-b-PEG GO-PEG Janus Au@SiO2-bis-pyrene-PEG Janus MnFe2O4-NaYF4:Yb/Er-PAAm-FA NaYF4:Yb/Er@CuS-HA Au/MnO2-BSA CuS-melanin-PEG hollow CuS@Cu2S@Au-PEG-P(OEOMA-co-MEMA) hollow Si/C-PEG hollow mesoporous Ca3(PO4)2-PDA-PEG Au nanorod@MSN-PEI SWCNT-HA MOF@MSN-PEG Au-DNA CuS-TRPV1(transient receptor potential vanilloid subfamily 1) SWCNT-C18PMH-PEG Au nanoring-PEG Janus Au-Ca3(PO4)2-PAA-PEG Au@SiO2-PEG (BiO)2CO3 nanotube-PVP Bi2Se3 nanoplate-PDA-HSA MoS2 nanosheet-CS Au-Cu9S5-C18PMH-PEG Bi2S3-Au nanorod-PEG Au nanocup-PEG WS2 QD-PEG Bi2Se3-D-(+)-glucose β-NaGdF4:Yb/Er@SiO2-Au25-PEG Janus Au-Fe2C-PEG MSN@Au core-shell nanorod-PEG-transferrin Au@SiO2 nanorod-CXCR4 single-walled C@SiO2 core-shell nanotube-PEG MoS2/Fe3O4 nanoflake-PEG MoS2 nanoflake-Gd-BSA Janus Au/Fe3O4-PS-b-PAA/PS-b-PEO Co9Se8-PAA MoS2@Fe3O4 nanoflower-PEI

228 506

Mn-C dot-PEG Au nanorod-PEG-PLGA reduced GO-Au nanorod-PEG

755 185 499

PTT/chemotherapy

yolk-shell Fe3O4@Au-P(NIPAm-co-AAm)

201

PTT

VS2 dot-lipid-PEG

397

PDT/chemotherapy chemotherapy PTT

CaCO3-PEG MSN/Eu/Gd-FA SWCNT-PEG

342 758 330

microwave ablation/ chemotherapy PTT

MoS2 nanosheet/Fe3O4-mPEG-PLGA

751

Fe3O4@Au nanostar-PEI Cu7S4-Au-polysuccinimide-19F-PEG NaYbF4:Er/Gd@SiO2@CuS@SiO2/CuS-PEG

199 760 208

NaGdF4:Yb/Tm@NaGdF4:Yb@NaNdF4:Yb@NaGdF4@SiO2@TiO2-PEG NaYF4:Yb,Tm@NaYF4@TiO2@MnO2 core/shell/sheet-PEG CaF2:Yb/Er/Mn-poly(2-aminoethyl methacrylate) NaGdF4:Yb/Tm@NaGdF4:Yb-ZnFe2O4-PEG

259 753 756 754

PDT/PTT PTT

PA

RT chemotherapy/RT chemotherapy/PTT

GT/PTT PDT/PTT PTT

CT

chemotherapy

chemotherapy/PTT PTT PA/CT

PA/CT/MR PA/FL

PA/MR

PTT/PDT PTT/RT PTT/chemotherapy PTT/PDT PTT PTT/chemotherapy PTT PTT PTT/chemotherapy

PA/FL/MR PA/PET PA/PET/ US PA/PET/ MR PA/MR/ SPECT FL/MR FL/MR/ Raman CT/MR

CT/MR/ UCL

PTT/PDT/ chemotherapy PDT PTT PTT/chemotherapy

PTT/RT PDT

CT/MR/ UCL/PA

chemotherapy PDT/chemotherapy

BA

229 188 737 739 455 744 193 331 743 740 394 745 542 742 329 741 192 234 746 388 456 334 195 92 209 750 257 339 190 242 332 353 759 202 427 752


Chemical Reviews

Review


modality UCL/PA FL/PA FL/MR FL/PA/MR

nanohybrid

refs

PTT/GT PTT

CeO2:Yb/Tm-PEG-RGD Au/GO-PEG-Cy5.5 Au nanorod@MSN-PEG Fe3O4-PEI-RITC Fe3O4-PLGA-PEI-Rhodamine B CaCO3-PDA-PEG Fe3O4-PDA-DNA nanoprobe Fe3O4-PLGA-BSA Fe3O4-PAA-b-PS-PEG starlike Au@SiO2-PGEA Au nanorod-CNC-PEG-PGEA Au nanorod-PGEA MoS2 QD-polyaniline-PEG Fe3O4/Au-PDMAEMA Gd2O3:Yb/Er@Au-PGEA Au@SiO2/CdTe-PGEA MSN-polypyrrole-PEG-transferrin 64 Cu-Fe3O4-melanin

268 761 254 762 764 757 766 399 763 92 197 200 749 205 206 317 765 767

PTT

NaYF4:Yb/Er-polypyrrole-PIMA(poly(isobutylene-alt-maleic anhydride))-PEG

768

PDT/chemotherapy PTT PTT/PDT GT

CT/PA

PDT PTT SDT US ablation PTT/GT

PA/CT/MR PA/CT/FL

PTT/RT PTT/GT PTT/GT

MR/US

PA/US PET/MR/ PA FL/CT

rise to more flexible assembled vesicles. Duan et al. reported double-layered hybrid vesicles assembled from Janus amphiphilic Au−Fe3O4 nanohybrids.196 PS and PEG with different hydrophilicity were grafted onto the Janus Au−Fe3O4 NPs separately. It is interesting that two kinds of inverse vesicles were formed when the amphiphilic property of polymers on the two parts of Janus NPs were changed, where the orientation of Au or Fe3O4 domains in the vesicular shell was arranged. Dual-modal MRI and PA imaging were established, benefiting from the plasmonic coupling and magnetic dipole interaction of the Janus Au−Fe3O4 NPs. These versatile hybrid vesicles open intriguing strategies for the self-assembly of nanohybrids with tunable properties and related functions. In addition to hydrophobic interaction manipulated in selective solvents, film rehydration, and emulsion method, electrostatic attraction and molecular recognition also mediate self-assembly of nanohybrids. These flexible assembly strategies gave rise to the fabrication of multifunctional systems by integrating nanohybrids with various types of components for combined advantages. GSH/Au nanohybrids carrying negatively charged carboxylic groups were assembled with gadolinium ions (Gd3+) through the electrostatic interactions.540 The assembled NPs allow for NIR FL/CT/MR trimodal imaging due to their excellent properties of enhanced FL, high X-ray attenuation, and r1 relaxivity. To construct multifunctional theranostic platform, ternary PDMAEMA/ Fe3O4−Au hierarchical nanocomposites were resulted from electrostatic assembly of PDA/Fe3O4 nanohybrids, disulfidelinked PDMAEMA, and PDMAEMA/Au nanohybrids.205 Consequently, the assembled nanohybrids integrated the properties of each component to realize the functions of PA/ CT/MR trimodal imaging as well as PTT and gene therapy (GT). With the facile supramolecular strategy, CD-grafted PGED/ Au or Fe3O4/PGED nanohybrids were self-assembled with Adfunctionalized SiO2 nanorods through host−guest interaction (Figure 39a-d).541 The DNA condensation ability of the nanohybrids, photothermal effect of Au nanorods or Fe3O4

NPs, as well as the featured chiral SiO2 nanorods could be integrated by the assembly process. Notably, the hyperthermia was not only used for PTT but also induced the dissociation of the assembled structures (Figure 39e,f), which could facilitate DNA release. Self-assembly of DNA-based nanohybrids through complementary base pairing is also intriguing due to the flexible design of DNA sequences for hybridization and the easy introduction of a variety functions from inorganic NPs. Selfassembly of α-CD-based DNA/Au nanohybrids was realized by the aggregation induced by complementary base pairing when α-CD was released in acidic tumor microenvironment.542 As a result, the aggregates possessing NIR absorption could be used for PTT and PA imaging. Aside from Au NPs, DNAtemplated self-assembly could be applied to the system of other inorganic NPs for rational combination of each functionality. Core−satellite DNA/Au-UCNP assemblies were performed for multifunctional theranostics while linear DNA/Fe3O4-QDs assemblies were efficient for magnetic isolation and detection of circulating tumor cells.543,544 Self-assembly of nanohybrids could also be achieved by chemical reactions between the building blocks. To realize light-triggered assembly of PEG/Au nanohybrids, further functionalization was carried out to obtain diazirine-terminated nanohybrids, which aggregated through the cross-linking upon photoirradiation.545 The resultant strong coupling of Au NPs enables the SPR peak to shift to the NIR region. The lightaddressable assembly might be applied to other systems for advanced functions. Peptide/lanthanide nanohybrids were cross-linked to assemblies with the HS-PEG-SH, which could disassemble in response to the reductive tumor microenvironment.546 In addition to imaging functions, the resultant assemblies could carry peptide drugs for safe tumor therapy.

4. VERSATILE BIOMEDICAL APPLICATIONS Organic/inorganic nanohybrids with featured properties as discussed in section 3 give rise to prospective development with great potentials in various aspects of biomedical applications. Due to the overwhelming investigations in this BB


Chemical Reviews

Review

Figure 40. Application of nanohybrids for flexible imaging. The review describes MRI, FL imaging, other modalities, and multimodal imaging mediated by nanohybrids. MR images are reproduced with permission from ref 564. Copyright 2018 The Royal Society of Chemistry. FL image is reproduced with permission from ref 348. Copyright 2015 American Chemistry Society.

area, we mainly focus on the work during the past five years in this subsection. The currently reported biomedical applications mediated by nanohybrids are roughly categorized in three parts including imaging, therapy, and imaging-guided therapy with the intention to provide an overview of the frontier of these applications. Table 4 shows the examples of representative nanohybrids and their corresponding applications in imaging, therapy, and imaging-guided therapy. In each section, we summarize and discuss the recent status of nanohybrids for the corresponding applications. The potential and new opportunities will also be addressed. It is anticipated that the present work could inspire discoveries for the development of new nanohybrids to meet the ever-increasing demand.

sensitivity is usually utilized for soft tissue imaging, while CT imaging with high contrast resolution is good at bone visualization. Herein, we discuss the research work that applied organic/inorganic nanohybrids only for imaging purposes. Typically, nanohybrids with the combination of distinct properties are expected to realize multifunctions. As a result, only a few examples are investigated for single imaging functions. We focus on the mostly studied MRI and FL imaging arising from nanohybrids, as well as other modalities and multimodal imaging, which is summarized in Figure 40. 4.1.1. Magnetic Resonance Imaging. As mentioned above, MRI is a noninvasive and nonionizing modality which allows the reconstruction of atomic nuclear magnetization signal into images with high spatial resolution.547 While MRI presents information that could not be replaced by other imaging techniques, it suffers from low sensitivity, which is caused by the mechanism. MRI attempts to measure the relaxation of water protons, which reflects information of the microenvironments.448 T1 reflects the longitudinal relaxation with positive contrast mechanism, which produces bright signal contrast in T1-weighted MR images. In contrast, T2 represents the transverse relaxation which gives negative contrast and dark signal contrast in T2-weighted images. To obtain sensitive MRI, contrast agents are usually employed by shortening T1 or T2 relaxation time that is expressed by the r1 or r2 relaxivity.548 Inorganic NPs with distinct magnetic properties as discussed in section 3.2.2 are extensively studied as MRI contrast agents. Herein we will introduce some typical examples of organic/

4.1. Flexible Imaging

A variety of imaging modalities are exploited based on NPs such as optical imaging, MRI, positron emission tomography (PET) imaging, US imaging and CT imaging.15 These imaging modalities are expected to provide precise information of anatomical, physiological, and molecular phenomena, which will contribute to early diagnosis of disease and prediction of therapeutic response. For clinical applications, the visualization of physical structures with satisfying contrast is of great significance to the interpretation of data to help physicians make proper decision for the treatment.448 Although each modality possesses prominent advantages, there are also disadvantages accompanying. Therefore, suitable imaging modality needs to be chosen for specific requirements. For example, MRI with high spatial resolution and relatively low BC


Chemical Reviews

Review

Figure 41. Nanoclusters assembled from iron oxides and their application in MRI. (a−e) TEM, high-resolution TEM images, and cartoons of nanoclusters assembled from iron oxide (IO) NPs with different sizes and shapes. r2 values and (f) MR phantom (g,h) of different kinds of nanoclusters. (i) Scheme of T2-weighted MRI of liver tumor by intravenous injection of contrast agents. (j,k) Coronal and axial MR images acquired at preinjection and 1 h post injection (p.i.) of IO cluster C3 or ferumoxytol at a region in the liver (red dotted circle), respectively. Reproduced with permission from ref 539. Copyright 2017 Nature Publishing Group.

inorganic nanohybrids for T1-, T2-, and dual mode T1-T2 weighted MRI. T1 contrast agents are mainly paramagnetic metal chelating complexes, such as Gd3+, Mn2+, and Fe3+-based materials. Gd3+ with seven unpaired electrons is well-known for T1-weighted MRI, which is approved for clinical applications. However, the potential nephrotoxicity of gadolinium chelates is the obstacle for their future usage.549 Nanohybrids constructed by the conjugation of Gd(III) chelates to the surface of NPs could facilitate the accumulation and MR contrast enhancement of the cancer cells and tumors, due to the increased Gd(III) payload.3 For example, biocompatible Au NP-Gd(III) and nanodiamond-Gd(III) nanohybrids were successfully synthesized as promising MR contrast agents to image tumor tissue in vivo with satisfying contrast enhancement.550,551 Notably, the organic ligand of the nanohybrids greatly influences the MR signals since the relaxation of water protons will be changed. Ding et al. manipulated the surface coating of ultrasmall Gd2O3 NPs and found that PVP/Gd2O3 nanohybrids produced the improved r1 compared with oleic acid and CTAB coated Gd2O3 NPs.552 PVP was considered to scroll and wrap Gd2O3 NPs, so that water could pass through while the bilayer hydrophobic chains of oleic acid-CTAB prevented water

protons from the core of the nanohybrids. Yan et al. further exploited the ligand−water interaction to improve relaxivity.553 PAA-NaGdF4 nanohybrids displayed superior relaxivity over PAA- and PEI-based nanohybrids, due to the strong hydrogen bonds between PAA and water. MnO NPs were encapsulated in hollow F127-SiO2 nanohybrids with improved accessibility to the surrounding water, and significantly high r2 was observed when an optimal number of MnO NPs are encapsulated.554 The nanohybrids of mPEG and ultrasmall MnFe2O4 NPs with the sizes smaller than 4 nm also demonstrated high r1 relaxivity due to the Mn2+ dopant.555 Similarly, PEGylated 3 nm-sized Fe3O4 NPs enabled highresolution MRI.516 T2 contrast agents are mostly superparamagnetic NPs such as Fe3O4 NPs.124,548,556 In order to improve contrast efficiency, Fe3O4 NPs are usually functionalized with PEG to obtain biocompatibility and long blood circulation time.557−559 Particularly, the anchoring group of PEG also affects the magnetic properties of Fe3O4 NPs and the resultant performance. Gao et al. designed PEG with diphosphate, hydroxamate, and catechol groups to reveal the effects of anchoring group during the ligand exchange process of Fe3O4 NPs.560 The conjugated structure-bearing PEG could largely improve the BD


Chemical Reviews

Review

T2-weighted MR performance by increasing inhomogeneity of the local magnetic field, which might offer a new avenue to tailor nanohybrids through the design of anchoring groups of the organic segments. Moreover, high T2 relaxivity could be attained by finely tuning the size of Fe3O4 NPs and the chain length of PEG. Bao et al. compared Fe3O4 NPs (with the sizes of 5 and 14 nm) with PEG coating with different lengths and discovered that nanohybrids of 14 nm core and PEG1000 performed best.561 To improve the resolution of diseased tissues, targeting molecules such as peptides are incorporated in the organic parts of nanohybrids for targeted MRI.562−565 Furthermore, other parameters are also disclosed to influence the contrast efficiency, such as the composition and assembly of magnetic NPs. The manipulation of the composition of the magnetic NPs was proved to result in MR signal enhancement. Ultrasensitive MRI for cancer cell detection and tumor visualization was realized by Mn-doped iron oxide nanohybrids through conjugation with tumor-targeting antibody.566 It is interesting that the organic composite could also contribute to the highly sensitive MRI. Kong et al. reported the nanohybrids of Fe3O4 NPs packaged by HPG through emulsification to identify the regions of defective vasculature.567 The high relaxivity of the nanohybrids is supposed to originate from the molecular architecture of HPG, which mimics the polysaccharide, glycogen, to absorb water and reduce diffusivity. As stated in section 3.5, the enhanced field inhomogeneity in the nanoclusters assembled from Fe3O4 NPs produced higher T2 contrast efficiency compared with single NPs due to the interaction between adjacent NPs.539 The size and shape of Fe3O4 NPs, particle distribution and fraction of Fe3O4 NPs with different sizes, as well as the molecular weight of PMMA all play vital roles in the magnetic field coupling effect and the resultant MRI performance, as shown in Figure 41a−h. The accumulation of the assembled nanoclusters was successfully applied to detect the liver tumor with enhanced T2 contrast over clinically used ferumoxytol (Figure 41i−k). The elongated assembly of iron oxide nanohybrids was also found with improved T2 relaxivity for MRI in contrast with the spherical counterparts. The superiority of the elongated assembly arises from its larger surface area, which carried multiple targeting ligands to interact with cell surfaces cooperatively.568 The distance of Fe3O4 NPs encapsulated in the PLGA matrix was tuned and the denser packing of Fe3O4 NPs could enhance r2 relaxivity.564 It is worth mentioning that self-assembly of iron oxide nanohybrids with T2 signal amplification was designed to occur in the presence of matrix metalloproteinase (MMP) enzymes overexpressed in tumors.569 This concept was realized by the functionalization of Fe3O4 NPs with azide or alkyne moieties as well as MMP-specific peptides, which could be cleaved to allow for click conjugation of the iron oxide nanohybrids. Interestingly, nanohybrids could also be employed as T1-T2 dual-modal MR contrast agents through rational design. Different from other dual- or multimodal imaging, T1-T2 dual-modal MRI is performed on one instrument, which combines the advantages of high spatial and temporal resolutions.547 Magnetic NPs including ultrasmall iron oxide and FeCo NPs that intrinsically display both positive and negative contrast effects were utilized to fabricate nanohybrids with biocompatible polymers as T1-T2 dual-modal MR contrast agents.570−572 In other cases, contrast agents for T1- and T2weighted MRI were combined in one nanohybrid for dual-

modal imaging. PEGylated Mn(II)- or Gd(III)-complexed Fe3O4 nanohybrids as well as Gd2O3- or MnO-embeded Fe3O4 nanohybrids were all successfully be employed as enhanced T1T2 dual-modal MR contrast agents.482,573−575 However, magnetic coupling between the T1 and T2 contrast agents is inevitable to lead to quenching of both signals when the two parts are close. Several strategies were applied to insert distance between the T1 and T2 contrast agents. To minimize the undesirable interference, Cheng et al. designed Janus Au−Fe3O4 NP, where a Pt nanocube connects the Au and Fe3O4 part.576 Then the Au domain was complexed with Gd while the Fe 3 O 4 component was functionalized with PEG. The resultant Janus nanohybrids provided optimal contrast on both T1and T2-weighted MRI. More interestingly, smart nanohybrids that are responsive to tumor microenvironment were developed for T1-T2 dual-modal MR with improved contrast. Redox-responsive nanohybrids prepared from Fe3O4@Mn3O4 core−shell NPs were activated in reducing environments by GSH to release Mn2+ ions, where Fe3O4 was exposed to water protons simultaneously. Thermal-responsive dual-modal MR contrast agents were achieved by PNIPAM/MnxFe3‑xO4@ SiO2−Gd(III) nanohybrids. pH-responsive nanohybrids were constructed by encapsulating Fe3O4 NPs and Mn species in CS nanogels to “turn on” MRI in specific environment.577,578 4.1.2. Fluorescence Imaging. Optical FL imaging presents the most intuitive information to eyesight, which is noninvasive and no harmful effect occurs by nonionizing radiation. The limited penetration depth and relatively low spatial resolution are the main obstacles of FL imaging modality. Therefore, the emerging NIR FL imaging including the expanded imaging in the second NIR region (NIR-II, 1000−1700 nm) is intriguing due to the advantage of deep penetration and high spatial resolution.579,580 As introduced in section 3.2.1, the fluorescent properties usually stem from the inorganic parts of nanohybrids. For imaging applications, fluorescent carbon dots or nanospheres are typically functionalized with PEG or PVP to improve the biocompatibility for imaging in vivo.581−584 Furthermore, targeting ligands such as RGD could be further conjugated with PEGylated carbon dots for targeted imaging.582 Since most of the high-quality semiconductor QDs are synthesized from high-temperature organic solution method, the resultant hydrophobic QDs undergo ligand exchange before biomedical applications. The ligand exchange process could be realized through metal−sulfur bonds, chemical reaction or hydrophobic interactions to introduce water-soluble or amphiphilic ligands.585−588 UCL imaging excited by NIR light exhibits improved penetration depth is mainly realized by rare earthcontaining UCNPs.589,590 PEG-phospholipid/UCNP nanohybrids were fabricated to visualize the intracellular pathway in individual living HeLa cells, which benefits from the excellent photostability and low cytotoxicity of the nanohybrids.591 Controllable excitation and emission in the desirable NIR region also could be achieved through suitable selection of the components of the nanohybrids. PLGA encapsulated NaGdF4:Nd3+ NPs and PbS/CdS/ZnS QDs were excited in the first biological window (650−950 nm) to produce emission in the second biological window (1000−1350 nm).592 Surprising emission in the NIR-IIb region (1500− 1700 nm) was obtained by the nanohybrids composed of PAA and NaYF4:Gd/Yb/Er nanorods (Figure 42a,b). The remarkBE


Chemical Reviews

Review

Figure 42. Nanohybrids of PAA and NaYF4:Gd/Yb/Er and the related NIR-II emission. (a) Schematic illustration of nanohybrids for cerebral vessel imaging. (b) TEM images of NaYF4:Gd/Yb/Er nanorods. (c) Emission spectra of NaYF4:Gd/Yb/Er NPs in the visible (left) and NIR-IIb (right) regions under excitation of 980 nm. (d) Time-dependent NIR-IIb brain images of cerebral vessels. Reproduced with permission from ref 593. Copyright 2018 Elsevier.

no adverse effect.594 Moreover, organic dyes including cyanine 3 and quinoline-malononitrile derivative, as well as fluorescent protein rec1-resilin, were involved during the fabrication of nanohybrids to produce FL imaging with high sensitivity.348,595,596 4.1.3. Imaging with Other Modalities. Aside from the MR and FL imaging, nanohybrids were also applied in other

able enhanced downconversion NIR-IIb emission allows for visualization of tumor and cerebral vessel imaging with high spatial and temporal resolution (Figure 42c,d).593 In addition, the nanohybrids composed of penetrating peptide and PEGylated long persistent luminescence NPs (Zn1.1Ga1.8Ge0.1O4:Cr3+,Eu3+) were successfully used to label stem cells with long lasting NIR-persistent luminescence and BF


Chemical Reviews

Review

performance.606 In addition, highly sensitive and safe in vivo detection and quantification of gastrointestinal bleeding could be realized by MPI.607 PET imaging was also achieved by nanohybrids. The biodistribution and clearance of 64Cu2+complexed nanohybrids of dextran coated Si QDs were successfully tracked by PET imaging, which may provide valuable information for the design of desirable nanohybrids.608 4.1.4. Multimodal Imaging. As discussed above, different imaging modalities possess their own virtues and limitations. From this point of view, multimodal imaging with two or more modalities in a single structural unit is highly desirable to provide comprehensive and accurate information with combined advantages of real time, high sensitivity, spatial/ temporal/contrast resolution, and deep penetration.23,124,609 Organic/inorganic nanohybrids could serve as multimodal contrast agents to achieve diverse imaging. In the subsection, we will introduce some typical examples of nanohybrids for multimodal imaging, as illustrated in Figure 40. Optical imaging especially FL imaging is attractive since it provides the highest sensitivity and excellent planar resolution. However, the penetration depth is limited and spatial resolution is relatively low. In this regard, other imaging modalities could work with FL imaging to acquire complementary information. For example, the anatomical information provided by CT could better reconstruct the FL data for more accurate visualization.610 Bismuth NPs carrying the largest Xray attenuation coefficient and FL property are regarded as favorable FL/CT dual-modal imaging probe. Yang et al. observed excitation wavelength-dependent emission of Bi NPs and conducted PEGylated Bi nanohybrids for in vivo imaging with high CT contrast and sensitivity.611 In other cases, CT contrast agents such as Au NPs and organic ligands were conjugated with fluorescent dye such as Cy5.5 or rhodamine B isothiocyanate (RITC) to form the nanohybrids with FL/CT dual-modal imaging.184,326 Cy5.5 -conjugated thrombin specific-cleavable peptides were incorporated with PEGylated Au NPs to produce targeted thrombus dual imaging in vivo by accurate anatomical information with high sensitivity.326 In this example, FL quenching by plasmonic NPs was utilized to design dual imaging probe where FL could be activated in the presence of thrombin. An alternative strategy to overcome the quenching effect proposed by Tang et al. is to exploit the nanohybrids of aggregation-induced emission luminogen (AIEgen) and plasmonic Ag NPs.612 The AIEgen-derived nanohybrids were synergistically prepared by one-step synthesis, where silver ions were reduced by AIEgen accompanying the self-assembly of the AIEgen to result in core−shell structure. The preserved properties of both components realized FL/CT imaging modalities in a single nanohybrid. The combination of FL and MR imaging in one platform allows for excellent 3D spatial resolution and high sensitivity. The nanohybrids for the application of FL/MR dual-modal imaging were typically fabricated from flexible integration of organic fluorescent molecules and Mn3O4 or Fe3O4 NPs as T1 or T2 MRI contrast agents.613−618 Notably, FL/MR dualmodal imaging could be achieved by Gd-doped UCNPs with intrinsic UCL features.481,619 In addition, fluorescent carbon dots and positive MRI contrast agent zerovalent iron NPs were integrated for promising dual-modal imaging.620 The organic parts of these dual modal probes are generally composed of PEG and targeting ligands including peptide, polysaccharide, and protein. The resultant nanohybrids were applied

imaging modalities. As an emerging modality, PA imaging is promising since it is based on acoustic waves from thermal expansion and can overcome the penetration depth limitation of ordinary optical imaging. NPs with high optical absorption cross sections in the visible to NIR region are favorable for PA imaging with satisfying contrast and spatial resolution. As mentioned in Section 3.2.1, Au NPs with specific morphologies, SWCNT, WS2, and CuS et al. could be utilized as PA contrast agents. To improve the optical absorbance, optical dyes such as ICG were bound to PEGylated SWCNTs to produce more than 100-times higher PA contrast in living tissues compared with pristine SWCNTs.597,598 To conquer the loss of absorbance intensity and the resultant reduced sensitivity after the cellular uptake of Au nanorods, silica coating could be applied on Au nanorods to prevent plasmon coupling by steric hindrance. Comenge et al. developed PEG/Au@SiO2 nanohybrids to preserve the appealing optical properties for PA detection of stem cells in vivo with improved sensitivity.181 PEGylated CuS nanohybrids which absorb light in the NIR-II region (1064 nm) were successfully applied for deep tissue PA imaging.599 Furthermore, PEGylated Au−Cu2‑xSe heterodimer nanohybrids with a broad LSPR in NIR-II region were demonstrated as promising PA contrast agents for deep tissue visualization of sentinel lymph node.600 CT imaging is considered to provide higher spatial and density resolution than other modalities. Shi et al. prepared acetylated generation 5 (G5) PAMAM dendrimer-entrapped Au NPs using one-pot synthesis strategy, which demonstrated great potential for CT imaging of cancer cells.601 Later, they performed PEGylation of the acetylated dendrimer to afford higher loading of Au NPs within the dendrimer interior.602 In this regard, the PEGylated nanohybrids exhibited higher attenuation intensity with enhanced biocompatibility. Moreover, the dendrimer could be covalently linked with folic acid to obtain folic acid-functionalized nanohybrids for targeted CT imaging of human lung adenocarcinoma.603 Ropovtzer et al. reported the in vivo CT imaging of exosomes within brain via intranasal administration of glucose-coated Au NPs, which may be employed as promising diagnostic platforms with high sensitivity for various brain disorders.604 US imaging presents the characteristic real-time, noninvasive, low cost, and nonionizing modality, which is widely used in clinical applications.15 Typically, US imaging could be realized by nanobubbles, nanotubes, silica and calcium carbonate NPs. The nanohybrids composed of multiwalled carbon nanotubes (MWCNTs), PEI, and monoclonal antibody were used for targeted US imaging with selectively accumulation in the malignant tumor issues.605 Interestingly, biocompatible F127-alginate/CaCO3 nanohybrids prepared by one-step synthesis were applied as an effective US enhancedcontrast agent.361 CaCO3 NPs loaded in the nanohybrids were in a relatively unstable state to generate bubbling, while enhanced US contrast was observed in acidic environment. The resultant nanohybrids provide a safe and convenient way for US imaging of tumor in vivo. As mentioned in section 3.2.2, MPI emerges as a promising imaging modality with high sensitivity, resolution and contrast, arising from the magnetic properties of the inorganic NPs. Similar to optical and US imaging, MPI is real-time. This feature is superior over MRI, which requires long scan and postprocessing time.15,448 Long-circulating PEGylated Fe3O4 NPs were employed for in vivo MPI with enhanced BG


Chemical Reviews

Review

PFODBT polymer, while the characteristic absorption peaks of both PFODBT and Fe3O4 NPs as well as the red FL emission of PFODBT were inherited (Figure 44c-e). In addition, the MPI contrast ability and intensity of signals of Fe3O4 NPs were not influenced by the PFODBT polymer wrapping (Figure 44f,g). In this regard, the integration of optical property of the polymer and magnetic property of the Fe3O4 NPs allows for multimodal imaging. As a result, effective cancer cell labeling and in vivo tracking were realized. This concept may be further extended to the construction of nanohybrids with other semiconducting polymers possessing PA contrast ability for PA imaging. By extensively exploiting the functions of inorganic and organic parts, nanohybrids are also applied for multifunctional imaging of other modalities such as CT/MR,625−631 PA/ MR,196,632 MR/US,633−635 PA/US,636 MR/PET,637,638 and US/CT/MR/PET639 imaging. These flexible imagings were realized by versatile nanohybrids and applied successfully in a variety of areas including cell tracking, tumor detection, and visualization of the specific disease region. Such multifunctional imaging might provide accurate and comprehensive information for potential applications.

successfully in various aspects of diagnosis, such as breast cancer, vulnerable atherosclerotic plaque, and glioblastoma, demonstrating the great potential of FL/MR dual-modal imaging. In addition to dual modal imaging, FL imaging-based trimodal imaging mediated by nanohybrids has also been investigated to further improve the sensitivity and resolution. FL/MR/CT trimodal imaging mainly stems from the feature properties of the inorganic parts of the nanohybrids while the organic ligands play the role of stabilization or targeting. For example, nitrogen-doped carbon−iron oxide hybrid quantum dots (C−Fe3O4 QDs) stabilized by poly(γ-glutamic acid) were successfully applied for FL/MR/CT trimodal tumor imaging (Figure 43).621 The characteristics of Au nanoclusters of

4.2. Diverse Therapies

While flexible imaging provides sensitive and accurate detection of disease regions for early diagnosis, effective treatment deserves more efforts since curing is the ultimate goal.640 Currently, diverse therapeutic modalities including phototherapy (PTT and PDT), chemotherapy, GT, RT, and multimodal therapy have been widely investigated. Notably, synergistic effect arising from the integration of different therapeutic modalities could yield enhanced treatment efficacy and minimize adverse effects. In this subsection, the application of organic/inorganic nanohybrids in therapy will be introduced with some typical examples. We will begin with nanohybrids for monotherapy, such as PTT, PDT, chemotherapy, GT, and other therapeutic modalities. The characteristics of each modality are illustrated in Figure 45. Subsequently, multimodal therapies will be emphasized to reveal the relationship between the design of nanohybrids and therapeutic effects, including the resultant cooperative effects of different treatments. 4.2.1. Phototherapy. Phototherapy is noninvasive lightinduced treatment, which is basically local treatment and could reduce damage to normal tissues. PTT is one kind of phototherapy that applies heat converted from absorbed light energy for treatment. Typically, PTT is utilized to kill cancer cells via hyperthermia by photothermal conversion agents. Nanohybrids for PTT monotherapy are usually composed of inorganic NPs with strong NIR absorption, such as Au,180,189,396,464,641−645 GO,646 MoS2,213,230,351 Cu2‑XSe,647 and Ti3C2.459 The organic parts are usually PEG to improve water solubility, dispersibility, and biocompatibility. The interesting PDA/Au nanohybrids with hyperbranched internal structures were applied for PTT in the NIR-II window, which possess broadband absorption from 400 to 1350 nm.643 PTT efficacy of the nanohybrids in the NIR-I (808 nm) and NIR-II (1064 nm) spectral window was compared. Improved PTT outcome of NIR-II over NIR-I was observed, which was caused by the higher maximum permission exposure of NIR-II. Compared with conventional PTT, adjuvant PTT provides uniform light energy distribution by transforming laser beam to plane wave. PEGylated gold nanobipyramids were feasible to

Figure 43. Formation process of C−Fe3O4 QDs and their applications for multimodal imaging in tumor-bearing nude mice. Reproduced with permission from ref 621. Copyright 2016 WielyVCH.

fluorescence and X-ray attenuation render them ideal for the components of nanohybrids for multimodal imaging. The integration of Au nanoclusters and Gd3+ or Fe3O4 NPs realized FL/MR/CT trimodal imaging and cancer diagnosis.540,622 HAfunctionalized MSNs loaded with fluorescent dye (FITC, ZW800), Gd3+ and 64Cu radioisotope were synthesized to track in vivo tumor targeted delivery of mesenchymal stem cell (MSC)-based multifunctional platform.623 Moreover, the emerging MPI also could be combined with MRI and FL imaging, where nanohybrids are composed of Fe3O4 and fluorescent dye or semiconducting polymer to achieve trimodal imaging.376,624 A type of Janus nanohybrid obtained through wrapping of Fe3O4 NPs in semiconducting polymers PFODBT (Fe3O4−PFODBT) was considered as the first example of Janus fluorescent MPI-tailored tracers for MPI, MRI, and FL imaging, as shown in Figure 44a,b.376 The DLS results of nanohybrids proved the successful wrapping of BH


Chemical Reviews

Review

Figure 44. (a) Schematic preparation and (b) TEM image of Janus Fe3O4−PFODBT nanohybrids through nanoprecipitation. (c) DLS size of Fe3O4 and Fe3O4−PFODBT in PBS. (d) UV−vis absorption spectra of PFODBT in THF, and Fe3O4 and Fe3O4−PFODBT in PBS. (e) FL spectrum of Fe3O4−PFODBT (excited at 540 nm). (f) Two-dimensional projection MPI scanning of Fe3O4- and Fe3O4−PFODBT with the same amount of Fe, and their corresponding linear scanning MPI spectrum. (g) Plot of MPI signals of Fe3O4−PFODBT versus amounts of Fe. Reproduced with permission from ref 376. Copyright 2018 American Chemical Society.

phthalocyanine chloride tetrasulfonic acid onto the surfaces of Au nanorings for enhanced PDT of breast cancer. This system took advantage of the electrostatic interaction between the photosensitizer and PAH.186 After internalization, the photosensitizer will be released to recover the photosensitivity which was inhibited by plasmonic Au nanorings. Pt NPs decorated on the photosensitizer-loaded MOF were employed for enhanced PDT, benefiting from the catalase-like activity of Pt NPs to produce oxygen by the decomposition of H2O2.501 UCNPs with featured UCL are attractive to realize NIR-triggered PDT.186,651 Targeted molecules were usually linked to the nanohybrids for nuclear or mitochondrial targeting. Targeted molecules were usually linked to the nanohybrids for nuclear or mitochondrial targeting. Notably, nanophotosensitizer TiO2 and molecule-photosensitizer Ce6 were integrated with UCNPs in the nanohybrids to produce multiple ROS with NIR excitation for the first time.186 Interestingly, photosensitizer 5,10,15,20-tetraphenyl-21H,23H-porphine zinc (ZnTPP) was utilized as a photocatalyst to mediate polymerization-induced self-assembly process for the synthesis of polymeric NPs.652 The resultant ZnTPP-loaded NPs could further be activated under visible light to generate singlet oxygen for PDT.

induce adjuvant PTT with little skin damage, which could be applied to inhibit recurrences after breast-conserving surgery.189 In addition, heteronanoparticles are employed for enhanced PTT. Compared with the individual counterparts, Janus nanohybrids of gold and Fe3O4 NPs with thermocleavable amphiphilic diblock copolymer were found to display significantly enhanced photothermal performance, benefiting from the strong collective effects.648 Nanohybrids of αsynuclein protein coated Au NPs on GO sheets not only stably adhered to cell surfaces but also exhibited remarkable photothermal effect due to strong plasmon coupling between Au NPs.649 Compared with PTT, another kind of phototherapy PDT is oxygen dependent where photosensitizer is usually employed. In this regard, improved tumor oxygenation could result in enhanced performance. The cytotoxic ROS generated by the activation of photosensitizer is responsible to kill cancer cells. Appropriate design of nanohybrids is required to carry hydrophobic photosensitizers for PDT with improved efficacy. Inorganic NPs, including silica,218 calcium carbonate,358 and calcium phosphosilicate650 NPs, could serve as carriers to load photosensitizers by encapsulation or conjugation. A LBL strategy was used to incorporate photosensitizer of Al(III) BI


Chemical Reviews

Review

Figure 45. Schematic illustration of characteristics of therapeutic modalities realized by nanobybrids.

4.2.2. Chemotherapy. Nanohybrids could work as carriers to load drugs for chemotherapy. The advantages rely on the versatile design of nanohybrids to realize increased loading capacity, accumulation and controlled release of drugs. The most commonly used inorganic parts are MSNs with large specific areas, which could carry drug through mesopores or hollow cavities.235,245,246,312,313,653 The rational design of the organic parts of nanohybrids could realize responsiveness, targeting or multifunctions for controlled drug release, as discussed in section 3.1. PEGylated Janus Fe3O4−SiO2 NPs were designed for targeting liver cancer therapy.235 The mesoporous characteristic of SiO2 NPs was exploited to encapsulate drugs and distinct magnetic responsiveness of Fe3O4 was utilized for targeting. In another example of Janus PS/Fe3O4@SiO2 nanohybrids, folic acid was conjugated to the surface of PS matrix for tumor cell targeting, while DOX was linked on the surface of silica shell via pH-responsive bonds for responsive drug release.406 In addition, dextran functionalized iron oxide nanocages were proved excellent drug carriers.654 It is interesting that the cage shape facilitates drug efficacy through affecting the drug release point and cellular uptake. Compared with solid spherical carriers, the therapeutic performance was enhanced 3-fold. The assembled nanohybrids comprising Pt nanoclusters, a pH-responsive polymer and hepatocellular carcinoma-targeting peptide were successfully fabricated for targeted chemotherapy500 After the assembly was destroyed in acidic environment, Pt ions released induced damage to DNA and killed hepatoma carcinoma cells. This mechanism could overcome the cisplatin resistance of cancer stem-like cells.

Moreover, as mentioned earlier, organic parts could also carry drugs via various interactions. As discussed earlier, hybrid PNP polymersomes reported by Qiu et al. were applied for high loading of DOX while in situ generated Au NPs in the shell of the polymersomes alleviated the leakage of drug at pH 7.4.447 In relatively acidic tumor microenvironment, drug release would be facilitated by the protonation of the tertiary amino groups in PNP, resulting in enhanced in vivo drug delivery and antitumor efficacy. 4.2.3. Gene Therapy. Gene therapy (GT) provides a promising approach to treat genetic disorders which involves therapeutic delivery of nucleic acid into cells. Since the obstacle of GT is the lack of efficient and safe gene carriers, organic/inorganic nanohybrids hold great potential to work as favorable candidates. Different from other therapeutic modalities, the organic parts are mainly responsible for gene delivery. As introduced in section 2.1.2, polycations with the charging groups such as amino (−NH2) and imino (−NH−) groups could be utilized as gene carriers.57,280,437,655 Among the organic cationic parts of nanohybrids, PEI, the “gold standard” of nonviral gene carrier, is mostly used to compact DNA through electrostatic interaction.237,338,432,434,656,657 Polycations such as PDMAEMA and poly(2-(dimethylamino) ethyl methacrylate-2-hydroxyethyl methacrylate) (DAMA-HEMA) are also grafted from inorganic NPs via surface-initiated ATRP.273,274,658 The inorganic NPs with rigid structure might facilitate cellular uptake, while the morphology of NPs greatly impact gene transfection,273,274,434 as stated in section 3.4. The distinct properties of inorganic NPs add new functions to BJ


Chemical Reviews

Review

protein 9 (Cas9) system.665−667 Zheng and Jiang et al. reported the delivery of Cas9 protein and single guide RNA (sgRNA) for the effective treatment of melanoma, which demonstrated promising application in genome editing.665,668 Aside from the main types of monotherapy discussed above, versatile nanohybrids with tailored structures are also applied in other therapies including radiotherapy,669,670 immunotherapy,671 magnetic hyperthermia,132 sonodynamic cancer therapy,672 microwave thermal therapy,215 and antibacterial therapy.276 For instance, polycat/Ag nanohybrids exhibited superior antimicrobial activity against both Gram negative and Gram positive bacteria. They also showed substantially enhanced antibiofilm activity compared with catechin/Ag or cat-borax/Ag nanohybrids.367 As mentioned in section 2.3.4, polycat working as both capping and reducing agent could help improve antibacterial efficacy by increasing contact between Ag NPs and bacteria cells. Due to the flexible construction of nanohybrids from diverse components, more applications are in the area of multimodal therapy to avoid the insufficiency of monotherapy. 4.2.4. Multimodal Therapy. The combination of different therapeutic modalities for treatment of serious diseases is promising to conquer drug resistance and other obstacles. Numerous bimodal and trimodal therapies employing nanohybrids are all well established. Regarding bimodal therapy, PTT/chemotherapy is the most commonly used combination therapy. Among the reported work, NIR-induced hyperthermia could enhance cell uptake, 238,673,674 and trigger drug release,675−681 to increase the sensitivity of chemotherapy, resulting in higher therapeutic efficacy. Inorganic NPs with optical absorption in the NIR region are usually responsible for PTT, such as Au,222,374,673,678 MoS2,212,426,682 CuS,677 SWCNT,238,674 et al. The organic components of nanohybrids are usually PEG or dextran for biocompatibility, together with peptide, folic acid, or lactobionic acid for targeting. Drugs for chemotherapy were either loaded by the inorganic components such as MSNs or by organic components through encapsulation or conjugation. Alternatively, PTT could also be realized by loading photothermal agents such as ICG. As shown in Figure 47, PEGylated Janus Ag/SiO2 NPs with folic acid were fabricated for targeted liver cancer PTT/chemotherapy.219 ICG was loaded in the silica compartments for PTT, while silver ions released from silver NPs served as antitumor agent for chemotherapy. Under NIR irradiation, both ICG and silver ions could be triggered to release simultaneously, resulting in combination therapy through hyperthermia-activated chemotherapy with synergistic antitumor efficacy in vitro and in vivo (Figure 47c−e). In addition to drugs, heat generated in PTT could also trigger and accelerate the release of photosensitizers and genes. Therefore, similar to combined PTT/chemotherapy, PTT is also supposed to enhance PDT and GT. Furthermore, aside from tumor ablation, high temperature could also speed up intratumoral blood flow for improved oxygenation, favoring PDT and RT consequently.640,683 The photothermal heating mediated by nanohybrids was demonstrated to facilitate PDT by enhancing the release and cellular uptake of photosensitizers.41,684−686 Besides conventional photosensitizers such as Ce6, ICG porphyrin, and verteporfin, inorganic NPs, including Au nanoechinus,465 W18O49 nanowires,467 MoOx,687 and CuMo2S3 NPs688 could also sensitize the formation of ROS. Notably, these inorganic NPs are capable to exert PTT simultaneously under NIR irradiation. After the suitable

realize enhanced gene transfection performances. For example, the superparamagnetism of Fe3O4 endows the magnetic nanocarriers with improved internalization and gene transfection efficiency in the presence of magnetic field.657 Recently, Weiss et al. reported PEI functionalized Au/Ni/Si nanospears, which were magnetically guided to penetrate the cell membrane for targeted gene transfection.432 As shown in Figure 46a,b, the magnetic nanohybrids could move with the

Figure 46. Targeted gene transfection mediated by nanohybrids of PEI functionalized nanospears. (a) Schematic illustration of nanohybrids to deliver enhanced GFP-expression plasmids with the guide of a magnet. (b) Optical images of the process of targeted intracellular delivery and control of trajectory of a single nanospear. (c) FL microscopy image demonstrating GFP expression by a target U87 cell. (d,e) SEM images illustrating a nanospear docking with its target cell. Scale bars are 10 μm unless noted. Reproduced with permission from ref 432. Copyright 2018 American Chemical Society.

guide of a magnet. The successful gene transfection was displayed by enhanced GPF expression (Figure 46c) while the tip of the nanospear (

Inorganic Nanohybrids: From Strategic

Recommend Documents