Using Hydrogel to Diversify the Adaptability and Applicability of

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Using Hydrogel to Diversify the Adaptability and Applicability of Functional Nanoparticles: from Nanotech-Flavored Jellies to Artificial Enzymes Chongling Cheng, Chuan Zhang, and Dayang Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00254 • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 18, 2019

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Using Hydrogel to Diversify the Adaptability and Applicability of Functional Nanoparticles: from Nanotech-Flavored Jellies to Artificial Enzymes

Chongling Cheng,† Chuan Zhang*, ‡ and Dayang Wang*,†

† State Key Laboratory for Inorganic Synthesis and Preparative Chemistry and College of Chemistry, Jilin University, Changchun 130012, China ‡ Department of Endocrinology, the Second Hospital of Jilin University, Jilin University, Changchun 130041, China.

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ABSTRACT: The use of hydrogel to accommodate nanoparticles is generally aimed at synergetic integration of the peculiar electronic, photonic, magnetic, mechanical, and chemical properties of the nanoparticles with the stimuli-response of the hydrogels into unprecedented, smart, collective functions. The intrinsic water-borne nature of hydrogels further endorses the significant implications of such nanocomposites in biology and medicine. This article will be an account with accent on how to introduce nanoparticles within hydrogels and to utilize the hydrogels to assist the nanoparticles to adapt themselves into different environments with a large span of polarity ranging from orthodox aqueous media to unorthodox organic ones. The related technological development and the associated fundamental issues will be discussed under the umbrella of enabling nanoparticle/hydrogel composites to emulate the unique catalytic performance of enzymes.

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INTRODUCTION:

Smart

Materials

of

Functional

Nanoparticle/Hydrogel

Composites The philosophic goal of chemistry will be to synthesize new materials, which will ultimately lead to an unprecedented synthetic power to embrace the full spectrum of the structural and functional complexity of biological systems. On the way of this Chemistry’s Odyssey, calling a new material or a group of new materials “smart” is an acceptable rhetorical affection, since any changes in structure and/or function, exhibited by such smart materials, in response to an external stimulus is believed as a preliminary resemblance to the primitive function of life. In this context, hydrogel is a subject of interest and importance, since, as its “hydro-” prefix suggests, it bears an imposingly large content of water immobilized and compartmentalized in its three-dimensional network of physically and/or chemically cross-linked, hydrophilic polymers, which is therefore deemed to be a simpler, abiotic version of the structure of biological systems;1 of many biological counterparts jellyfish is an stunning example of highly dilute hydrogel with more than 95 % of its mesoglea made up of water2. What makes hydrogel more special is the fact that extracellular matrix can be regarded as a hydrogel of the composites of polysaccharides (e.g. hyaluronan) and/or proteins (e.g. collagen and elastin), which are inter-locked via hydrophobic interactions and hydrogen bonds to create the scaffolds of jellified biological media to embed cells.3 Besides its structural and chemical supporting role, the extracellular matrix ensures a specific degree of stiffness and elasticity and, more importantly, its mechanical behavior is sensitive to a variety of physicochemical and biochemical stimuli such as pH, temperature, calcium or sodium ions, and protease, which in turn regulate the contraction, migration, proliferation, differentiation, apoptosis of the cells embedded therein. The stimuli-response of hydrogel and its biotic cousins fundamentally reflects, directly or indirectly, noticeable fluctuation in hydration of their polymeric networks, concomitant with which occur an obvious alteration in the water content of hydrogels and in turn their volume. As a consequence, manifold conventional or emerging hydrophilic

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materials have been utilized to form hydrogels with tailored hydration, which can be further modulated with the aid of incorporation of new functional moieties. These technical successes underpin the applicability of hydrogels in a wide variety of industrial areas, including tissue engineering in medicine, controlled delivery of drugs in pharmaceutics, protein and DNA purification, cell culture and bioassay in molecular biology, and control release of moisture, fertilizers, and pesticides in agriculture, water gel explosives, and many others.1,4 Nowadays a prevailing usage of hydrogel is for controlled incorporation and release of proteins, DNA, or other biological macromolecules.1 Its natural extension will be to load synthetic nanoparticles, since the nanoparticle sizes are generally comparable to those of those biological macromolecules5. When inorganic materials are cut down into a particulate form with at least one dimension in the range of a few to tens of nanometers, their physicochemical properties conventionally understood from the bulk structures, crystalline or amorphous, start to be overshadowed by the properties associated with the distorted structures of the nanoscale particulate surfaces owing to substantial increase in specific surface area.6 When the particle sizes are further reduced below the de Broglie wavelength of the electronic wave function, the energy band structure of the materials turns dependent on the sizes of nanoparticles as a result of geometry constrains of electrons (or excitons) at this ultra-small scale, known as quantum confinement effect.7 Thanks to the quantum confinement effect of nanoparticles and the effect associated with their exceedingly large specific surface area at the nanoscale, we are equipped with new gears to design and manipulate the material properties by the geometrical features of inorganic nanoparticles, namely sizes and shapes,8 which is to some extent reminiscent of sorting out of elements or design of molecules or clusters by atomic weight9. More intriguingly, self-assembly of nanoparticles allow genesis of meta-materials with new functions hardly expected for individual constituent nanoparticles as well as their molecular or atomic constituent components, thus resulting in enrichment of our nanoparticle toolbox by including latex,

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silica particles and other conventional colloidal nanoparticles for material property design at the nanoscale.10 In a nanotechnology-prevailing climate, therefore, the past decades have witnessed explosion in the field of integration of nanoparticles or their ensembles into hydrogels.11

Figure 1. Examples of multifunctional hydrogels in which functional nanoparticles or their ensembles are embedded. (a) Light-regulated temperature-response of the volume phase transition of gold nanorod-coated microgels of poly(N-isoproprylacrylamide).12c (b) Chemical and biochemical response of colloidal crystal – embedded hydrogel for applications in sensing.11a (c) Thermal actuation performance of a cube made up of single walled CNT-embedded hydrogel, which fold in water at 48 °C and unfold at 20 °C.17a (d) Conducting elastomers of GO-based hydrogels for tissue engineering of skeletal muscles.17c

To highlight the merits of marriage of hydrogel with functional nanoparticles, a few examples are shown in Figure 1. On one hand, the peculiar physicochemical function of nanoparticle guests offers additional level of flexibility to tailor the stimuli-responsive behavior of hydrogel hosts.12 For instance, coating of gold nanorods onto temperaturesensitive microgel made of poly(N-isoproprylacrylamide) enables reversible alteration in the volume of composite hydrogels, as large as about 50%, in response to near-infrared 5


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laser irradiation as a result of the local heating effect stemming from the surface plasmon resonance (SPR) of the loaded gold nanorods (Figure 1a).12a On the other hand, the stimuliresponsive behaviors of hydrogel enable fine modulation of the physicochemical properties of functional nanoparticles loaded in the hydrogel13 and particularly the collective properties of their ensembles as a whole.11a For instance, the volume phase transition, i.e. shrinking or swelling, of hydrogel in response to a particular type of chemical or biochemical species will change in the distance between nanoparticle guests. Provided the nanoparticle ensembles are embedded in a highly periodic array in the hydrogel matrices, the inter-particle spacing change shifts the Bragg diffraction light wavelength from the ordered nanoparticle ensembles, known as colloidal crystals, 14 and results in a noticeable (structural) color change, which allows highly sensitive detection of many analytes, such as pH, surfactants, metal ions, proteins, anionic drugs, and ammonia (Figure 1b).11a The discoveries of fullerene, carbon nanotube (CNT), and graphene have been expanded the spectrum of functional nanoparticles;15 especially the recognition of graphene has promoted a strong resurgence of interest in two dimensional nanosheets of either carbon-based organic or inorganic materials. In spite of much speculation of the potential applications of CNT and graphene in electronics, optics, and even catalysis, the technical exploitation of their unique mechanical properties is anticipated to bring out the first commercial success.16 In line with that anticipation, there are a great dealt of endeavors to derive novel actuators from nanocomposites of stimuli-responsive hydrogel with CNT or graphene.17 For instance, loading of signal-walled CNTs into temperature-sensitive hydrogels of polyNIPAM at concentration as tiny as 0.75 mg/mL results in 5 times reduction in the volume phase transition time from ca. 150 s of pristine polyNIPAM hydrogel to ca. 35 s of CNT/polyNIPAM composite hydrogel in response to the surrounding temperature. This has been designed primitive thermal actuators with cube or flower shapes, which show fast and reversible folding or unfolding with warming up the surrounding water to 48 °C or cooling down to 20 °C (Figure 1c).17a When fairly

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hydrophobic graphite is transformed into fairly hydrophilic graphene oxide (GO) through strong oxidation treatment, the GO nanosheets, well dispersed in aqueous media, can be interlocked by for instance tropoelastin – a protein found in the elastic tissues of the human body – into GO-bore biological hydrogel via hydrophobic interactions (Figure 1d)，17c which displays excellent twisting and stretching performance as a result of the synergy of the mechanical properties of both GO and tropoelastin. Taking advantage of the good electrical properties of the GO components, the resulting GO/tropoelastin composite hydrogel has been utilized as conducting glue to connect explanted muscles together, thus leading to significant implications in regenerative medicine. Despite design and manufacture sophistication of various nanoparticle/hydrogel composites with multiplex functions reported in literatures,11-13,17 the hydrogel’s primary role in these innovative composites remains bluntly simple from a viewpoint of colloid and interface science: effective and, in most popular (or desirable) cases, adjustable colloidal stabilization of the nanoparticles in a targeted aqueous milieu. The hydrogel host effectively prevents the spacing distance between the nanoparticle guests far away from the primary minimum, as suggested according to DLVO theory,18 where the irreversible coagulation of neighboring nanoparticle is hardly avoidable. Meanwhile, the stimuliresponse volume phase transition of the hydrogel guest allows fine tuning of the interparticle spacing and thus the inter-particle interaction, thus customizing the collective properties of nanoparticle ensembles.11 It is the fundamental base of almost all technical innovations designed to derive advanced smart materials from nanoparticle/hydrogel composites. This ethos of material design to a small or large extent diverts the research attention from the impact of a hydrogel host on the properties of its individual nanoparticle guests and the potential technological outcomes derived thereof. When functional nanoparticles are embedded in a hydrogel, the volume phase transition of the hydrogel will adjust not only the spacing distance between neighboring nanoparticles, but also the water content in the local surrounding of individual nanoparticles, which is a more 7


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straightforward result of water release from or uptake into the hydrogel. The latter will cause a noticeable change in the surrounding dielectric constant or refractive index of individual nanoparticles embedded in the hydrogel, which accordingly tailors the surface properties of individual nanoparticles. This awareness may pave new pathways to technical exploitation of nanoparticle/hydrogel composites, since most, if not all, peculiar properties of nanoparticles hinge on their surface (distorted) structures and their interactions with the surrounding environment,6-8 for instance the SPR-related properties of noble metal nanoparticles19. Up to date, unfortunately, that remains out of the spotlight of mainstream research on nanoparticle/hydrogel composites; the research field may be flooded with the activities to make use of the peculiar properties of nanoparticles to upgrade the function of hydrogel12,13, 17 or to take advantage of stimuli-responsive behavior of hydrogel to enhance the collective properties of nanoparticle ensembles.11 Taken together, herein we plan to summarize and reappraise some recent activities of fabrication of functional nanoparticle/hydrogel composites from the standpoint of how the hydrogel host can assist individual nanoparticle guests to advance their surface properties in order to broaden and advance the nanoparticle adaptability and applicability in different technical areas. Our particular incentive for that is that the concept of using hydrogel to enhance the properties of functional nanoparticles to a large degree reminds of our understanding of unique enzymatic catalysis20. Responding the presence of targeted reaction substrates in the surrounding environment, enzymes are known to alter their structures, for instance open a channel with specific geometry and hydrophilicity,21 to facilitate the activation of the substrates and thus the catalytic performance of the catalytic centers of the enzymes, which, in most cases, is expected not very efficacious if not being present in the enzymes. Taking into account that, we will specifically encapsulate the recent developments relevant to catalysis, including hydrogel-assisted phase transfer of nanoparticles

into

media

with

different

polarity

and

technical

success

of

nanoparticle/hydrogel composites in catalysis. That will follow an overview of currently

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available, three major strategies designed for incorporation of functional nanoparticles in hydrogel. In Conclusions and Outlook, the implications of nanoparticle/hydrogel composites in enzyme-mimicking catalysis and the technical issues associated thereof will be further discussed.

OVERVIEW: Hydrogel-Assisted Fabrication, Transfer, and Catalytic Applications of Functional Nanoparticles.

1.

Fabrication of Functional Nanoparticle/Hydrogel Composites. Thanks to great progress in synthesis and modification of synthetic or biological

hydrogels and functional nanoparticles, including carbon-based nanostructured materials, in past decades, it is technically attainable to fabricate nanoparticle/hydrogel composites. The technical sophistication is resting on the choice of either functional nanoparticles or hydrogels or both to be precisely defined in terms of their structural and chemical features in order to achieve targeted, technical performance of their nanocomposites as a whole. As a consequence, all the currently available approaches can be simply placed into three categories: (1) synthesis of functional nanoparticles within pre-formed hydrogels; (2) conversion of aqueous dispersions of pre-formed nanoparticles into hydrogels; and (3) incorporation of pre-formed nanoparticles into pre-formed hydrogels. These three methodologies will be discussed, respectively, with typical examples to underline their technical advantages and disadvantages as well as potential applications. Strategy I: Synthesis of functional nanoparticles within pre-formed hydrogels. Hydrogel is a three-dimensional, physically or chemically cross-linked network of hydrophilic polymers, which bears two features beneficial for growth of nanoparticles inside. Owing to its exceedingly large proportion of water (over 90%),1 firstly, hydrogel is a superporous polymeric matrix with hierarchical porosity varying from micropores (2 nm and below) to mesopores (2 nm – 50 nm) and macropores (up to several µms from 50

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nm).22 The former two types of pores act as templates for growth of nanoparticles with maximal sizes arrested in the range where the resulting nanoparticles exhibit perceptible size-dependent physicochemical properties. Secondly, the hydrophilic polymeric network of hydrogel bears abundant polar groups, which enables the hydrogel to effectively take up different ionic precursors for growth of targeted nanoparticles and, furthermore, to effectively lock up the growing nanoparticles inside. This pays a very simple and straightforward pathway, referred to as Strategy I, to incorporate a variety of inorganic nanoparticles in hydrogel.23 The operational ease of Strategy I is further endorsed by little heed paid on control of the colloidal stability of nanoparticles growing in hydrogel, which is usually a tricky issue for synthesis of inorganic particles especially in aqueous media but can be effectively secured by the porous hydrogel network in Strategy I. The obvious technical drawback of Strategy I is its lack of control of the size and shape distribution of the nanoparticles grown in the hydrogel, while the nanoparticles are expected to be sparsely fastened by the polar groups of the hydrophilic polymeric network and the majority of their surfaces is bare and directly exposed to the surrounding environment. That will be a large technical convenience for applications such as adsorption and catalysis where the technical performance is significantly benefited by the exceedingly large specific surface area of functional nanoparticles and the presence of unorthodox structural features on these ultrasmall particles. From the standpoint of synthetic chemistry, the success of Strategy I is underpinned by deliberate selection of different functional polar groups such as sulfate, carboxylate, phosphate, and amine groups into the polymeric network of a hydrogel in order to secure strong binding affinity to ionic precursors,23a which are subsequently converted into targeted nanoparticles of metals, the surfaces of targeted inorganic nanoparticles of variety of metals, metal oxides and metal chalcogenides (Figure 2). Such pre-formed functional hydrogels are to a large extent reminiscent of ion exchange resins (Figure 2a).23a They can be generated either via co-polymerization of multiplex monomer mixtures to create (semi-)

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inter-penetrated23b or hybrized polymeric networks23c or via polymerization of newly designed functional monomers,23a for instance dopamine-based monomers with catechol groups with strong binding affinity to almost all types of metallic ions 23d.

Figure 2. Examples of synthesis of inorganic nanoparticles in functional hydrogels. (a) Conversion of Co2+, Ni2+, Ag+, Fe3+, Fe2+, Ru2+, and Cu2+ ions into the corresponding metallic nanoparticles into hydrogel blocks.23a (b) Synthesis of Ag nanoparticles in the semi-interpenetrated (semi-IPN) networks of hydrogels, in which the networks of polyacrylamide are interpenetrated by composite polysaccharide chains of gum acacia, carboxymethylcellulose, and starch.23b (c) Conversion of Cd2+ ions to CdS nanoparticles in hydrogel blocks of poly(2-acrylamido-2-methyl-propanesulfonic acid), in which dangling sulfate side groups are designed to chelate Cd2+ ions.23e (d) Synthesis of Ag nanoparticles in hydrogel blocks of polymers derived from dopamine-functionalized methacrylamide, in which the catechol side groups are used for adsorption of Ag ions for antimicrobial application.23d

Thanks to effective spatial confinement of the hierarchical porous structures, the inorganic nanoparticles trapped in hydrogel show excellent colloidal stability, for instance no perceptible aggregation; the nanoparticle surfaces are expected to be sparsely stabilized by the functional groups of the surrounding hydrogel network, though. Designed functional hydrogels and their composites with inorganic nanoparticles loaded therein may be able to respond to the change in the surrounding environment, provided the networks of their major constituent hydrophilic polymers are the stimuli-responsive. Nevertheless, one 11


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technical issue associated with such functional hydrogels is that the introduction of functional groups or nanoparticles into the hydrogel network may cause a noticeable alteration in the stimuli-response of the hydrogel. The hydrogels become less sensitive after incorporated with highly polar groups and highly hydrophilic nanoparticles due to strong hydration of these functional guests, while they become more sensitive after modification with weakly polar groups and poorly hydrophilic nanoparticles with weak hydration.23c Strategy II: Conversion of aqueous dispersions of pre-formed nanoparticles into composite hydrogels. The electronic, photonic, magnetic, chemical and even mechanical properties of inorganic nanoparticles is rigorously dependent on the particle sizes and shapes.7,8 Should the aim of fabrication of inorganic nanoparticle/hydrogel composites is to make use of the peculiar properties of the nanoparticle guests and especially the collective behavior of the nanoparticle ensembles in the hydrogel guests, the sizes and shapes of the inorganic nanoparticles must be controlled in an exceedingly narrow range of distribution. Up to date, a variety of wet synthetic strategies have been successfully developed to inorganic nanoparticles with defined but varied sizes and shapes as well as surface chemical features.8, 24 Unfortunately, these synthetic strategies have been hardly adopted into successful growth of monodisperse nanoparticles in hydrogel, which is possibly due to the mass transport and crystallization kinetics in gelled media very different from those in highly fluidic, liquid media. Though, crystal growth in gel is an exotic research theme.25 Taking into account that, the straightforward way to produce composite hydrogels with high-quality inorganic nanoparticles loaded inside will be to gel the aqueous solutions of pre-formed nanoparticles, obtained either via direct synthesis in aqueous media or via phase transfer from organic reaction media to water, which is referred to as Strategy II. Strategy II is expected to be executed via two pathways. One is to introduce gelling agents into aqueous media and the other to tether gelling agents on the surfaces of per-formed nanoparticles; the gelling agents can be initiators, monomers or cross-linkers conventionally used for hydrogel synthesis.

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Figure 3. Examples of gelation of aqueous dispersions of pre-formed functional nanoparticles. (a and b) Conversion of aqueous dispersions of (a) layered double hydroxides (LDHs)26a and (b) cellulose nanocrystals (CNC) into hydrogel via in-situ radical polymerization26b. (c) Gelation of aqueous dispersions of CdSe quantum dots, obtained via phase transfer from organic reaction media by capping the nanoparticles with thioglycolic acid (TGA), by using alginate as gelling agents.26c (d) Schematic depiction of using gelling agents to glue various carbon-based nanostructured materials into composite hydrogels.26d

As compared with Strategy I, Strategy II is a more delicate process and its operational complexity is connected mainly with the technical issues arising from colloidal stabilization of as-prepared nanoparticles during the whole process of Strategy II. When gelling agents are introduced into either their surfaces or their aqueous dispersions or both, these alien additives will disturb the existing balance between various attractive and repulsive interactions exerted on the nanoparticles. In most cases, that disturbing severely deteriorates the colloidal stabilization of the nanoparticles and in turn gives rise to the nanoparticle agglomeration especially at the early stages of exchange of existing ligands on the nanoparticles with gelling agents, addition of gelling agents into the nanoparticle dispersions, and gelation of the nanoparticle dispersion. Some of new additives, for instance non-ionic oligomers or polymers, are expected to render steric repulsion to the nanoparticles to enhance their stability if the nanoparticle dispersions can quickly pass

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through that disturbing early stages, though. Provided one is able to cope with technical issues related to the colloidal stabilization of pre-formed nanoparticles, a variety of monodisperse functional nanoparticles with defined surface chemical features are available to be utilized either as nanoscopic initiators or crosslinkers for synthesis of chemically cross-linked hydrogels or as (specific or nonspecific) complementary counterparts of targeted hydrophilic – synthetic or biological – polymers to form physically cross-linked hydrogels.26 Figure 3 presents four examples to illustrate formation of nanoparticle/hydrogel composites via Strategy II. Naturally occurring layered double hydroxide (LDH) can be readily exfoliated in water into colloidally stable, two-dimensional nanosheets with the surfaces rich in hydroxyl groups and active metallic ions.27 When water-soluble initiator (ammonium persulfate, APS), catalyst (tetramethylethyleneamine, TEMED), and monomer (acrylamide, AM), are added into the aqueous dispersions of the exfoliated LDH nanosheets, the resulting mixture solutions can be converted into polyacrylamide hydrogel via radical polymerization at ambient conditions (Figure 3a).26a Similarly, another type of naturally occurring nanomaterials, cellulose nanocrystals (CNCs) – an emerging one-dimensional materials with unique mechanical behavior28 – can be utilized to produce the hydrogels of polyacrylamide via radical polymerization, which the CNCs may act as cross-linkers by adsorbing radials produced via for instance potassium persulfate (KPS)/sodium bisulfite (SBS) redox reaction or cross-linkers such as N, N’-methylenebisacrylamide (NMBA) (Figure 3b).26b Since monodisperse semiconductor quantum dots with superior physicochemical properties can be produced only by injection of reagents into hot organic reaction media of stabilizing ligands, the incorporation of these high-quality nanoparticles into hydrogel starts with phase transfer of them from organic reaction media into water. Subsequently, the resulting aqueous dispersion of the quantum dots can be readily converted into hydrogel by gelling agents such as alginate consisting of mannuronic acid and gluronic acid units, which are cross-linked by barium ions (Figure 3c).26c Similar to

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hydrophobic quantum dots, as summarized in Figure 3d,26d carbon-based nanomaterials ranging from fullerene to CNT and graphene need to be transfer into aqueous media either via phase transfer with amphiphilic molecules or polymers or direct oxidation treatment of their surfaces, which, in turn, can be utilized to one-, two- and three-dimensional, hierarchical composite hydrogels. Strategy III: Incorporation of pre-formed nanoparticles into pre-formed hydrogels. One of the widely recognized applications of hydrogels is the usage as preferred vehicles for loading, delivery, and release of drugs, nucleic acids, proteins or many other therapeutic or analytical substances in a stimuli-responsive, controlled manner.1 Insofar as the molecular dimension is concerned, biological macromolecules are comparable to inorganic nanoparticles with sizes in the range of 1 – 100 nm.5 This argument has inspired to incorporate pre-formed inorganic nanoparticles into pre-formed hydrogels to fabricate nanoparticle/hydrogel composites, referred to as Strategy III.29 Figure 4 show a typical example to pH-controlled loading and release of pre-formed, hydrophilic nanoparticles into pre-formed hydrogel microparticles.29a Upon being incubated together in water, hydrophilic CdTe quantum dots can be taken into the hydrogel microparticles, made up of copolymers of N-isopropylacrylamide and 4-vinylpyridine (PNIPVP) in response to the surrounding pH being lower than the pKa of the pyridine groups of the PNIPVP network, which is as a result of the strong electrostatic attraction between the positively charged hydrogel network and the negatively charged surfaces of the CdTe quantum dots (Figures 4a,b). On the other hand, when the pH of the surrounding aqueous media is adjusted above the pKa of the pyridine groups of the PNIPVP network, the loaded CdTe quantum dots can release out of the neutralized PNIPVP network due to loss of the electrostatic attraction between them (Figure 4c). In this case, the pH-response of the PNIPVP microparticles manifests its role in both adjustment of the pore sizes of the hydrogel networks and their electrostatic interactions with the stabilizing ligands of CdTe quantum dots during loading and release of the quantum dots. The change in the photoluminescence behavior of CdTe

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quantum dots, incurred during loading in and release from by the hydrogel, can be reasonably ignored. Taking advantage of the broad pore size distribution of hydrogels, CdTe quantum dots with different sizes can be readily incorporated into the PNIPVP microparticles, which is further extended for loading a mixture of differently sized quantum dots into one hydrogel microparticles to fabricate luminescent markers encoded with multiplex emission colors, which are determined by the formulations of the quantum dot mixtures (Figure 4d).

Figure 4. a) Schematic shows that loading of CdTe quantum dots in PVIPVP hydrogel and their controlled released by pH. b) plot of loading amount of 3 nm CdTe quantum dots per PNIPVP hydrogel sphere virus the concentration of CdTe quantum dots added.(Inset: Fluorescence images of PNIPVP spheres loaded with 3 nm CdTe quantum dots at pH 4 after centrifugation at 2000 g for 10 min. c) Plot of the amount of 3 nm CdTe quantum dots released from PNIPVP hydrogel spheres versus time at pH 10 (down triangles) ,pH 11(square), Ph 13(circles) and pH 13 (upon triangles) with 0,05 M NaCl from bottom to de upon. (Inset: Fluorescence images of CdTe-PNIPVP hydrogelspheres at pH after centrifugation at 10000 g for 1 min) d)Fluorescence images of PNIPVP spheres embedded with 2.5 nm CdTe quantum dots (a), 4 nm CdTe quantum dots (f) and mixture of these two quantum dots with varied molar ratios of small to large quantum dots: 5:4 (b), 1:1 (c), 1:2 (d), and 1:3 (e).29a

The advantage of Strategy III is obviously credited to the fact that the properties of 16


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both nanoparticles and hydrogels are well pre-determined by the formulations used for synthesis of the nanoparticles and the hydrogels, respectively. Similar to loading of proteins or nucleic acids, illustrated in Figure 4, the successful confinement of nanoparticles in hydrogels hinges on the size matching between the nanoparticles and the mesopores of the hydrogel hosts, which can be well defined by the formulations used for hydrogel synthesis and, at the same time, deliberately modulated by the hydrogel stimuli-responsive behavior. The confinement effectiveness can be further enhanced by the attractive interactions between the polar groups tethered on the hydrogel networks and the polar groups decorated on the nanoparticle surfaces. From the standpoint of synthetic chemistry, therefore, not the chemical nature of pre-formed nanoparticles but their surface chemical nature is one of the key factors for effective loading of the nanoparticles into pre-formed, stimuli-responsive hydrogel. With the aid of deliberate design of their stabilizing ligands, a variety of functional inorganic nanoparticles of metals, metal chalcogenides, and metal oxides can be reversibly incorporated into pre-formed hydrogel blocks or microparticles by taking advantage of the expansion or shrinkage of the hydrogel networks in response to environmental stimuli such as pH,29a,b temperature,29c and ionic strength29d. As compared with Strategies I and II, Strategy III is expected to cause minimal impact on the physicochemical properties of the nanoparticle guests, including those associated with their surface structural and chemical features, but it gives rise to non-negligible alteration on the stimuli-responsive behavior of the hydrogel hosts. As discussed above, the stimuliresponse of hydrogel may be enhanced or weakened depending of the hydration efficiency of the surfaces of the nanoparticles incorporated therein,23c which needs to be taken into consideration for design of the technical performance of targeted nanoparticle/hydrogel composites obtained via Strategy III. 2.

Using Hydrogel to Disperse Functional Nanoparticles in Disparate Media. The aforementioned three strategies, especially Strategy II, and their ingenious

variants have brought about a great success in fabrication of a diversity of

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nanoparticle/hydrogel composites. If one is overcritical of their applicability, however, all of them are limited to the nanoparticles with excellent surface hydrophilicity, which is widely accepted the prerequisite for the nanoparticles to adapt themselves to the aqueous milieu either confined within a hydrogel or used for the hydrogel synthesis. Any hydrophobic nanoparticles, including CNT, graphene, and other carbon-based nanostructured materials, can be applied to produce the composites with a hydrogel only after phase transfer into aqueous media by means of hydrophilization of their surfaces either via oxidization or coating with amphiphilic molecules or polymers. This appears in good agreement with our common sense about hydrogel that, as its name suggests, is made intrinsically water-borne. However, if one carefully scrutinizes the molecular structures of constituent hydrophilic polymers of any hydrogel, regardless of being of synthetic or naturally occurring nature, they all consist of both polar and apolar moieties implanted either on their sides or on their backbone or both. As a result of this binary molecular nature, hydrophilic polymers are amphiphilic in water as a result of the hydration of the polar moieties substantially more effective than the hydration of the apolar moieties. Once being introduced into water, hydrophilic polymers therefore entails inter-molecular aggregation or intra-molecular entanglement depending on how effectively the hydration of their polar moieties can offset mainly the hydrophobic interactions between their apolar moieties. The amphiphilicity of hydrophilic polymers also show themselves in apolar organic solvents as a result of their apolar moieties more effectively bonded by the apolar organic solvents than their polar moieties. In contrast to water and apolar ones, however, polar organic solvents are of binary molecular nature, namely consisting of both polar and apolar moieties, and in turn amphiphilic, which are therefore expected to readily dissolve hydrophilic polymers thanks to effective solvation of both polar and apolar moieties of the polymers. As such, it should be technically feasible to use polar organic solvents to displace water confined within the network of hydrophilic polymers in a hydrogel, which may be further benefited by the ability of polar organic solvents, especially protic ones, to form hydrogen bonds

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with the polar moieties of the hydrophilic polymers and their miscibility with water. This intuitive rationalization has been translated into successful use of hydrogels as phase transfer vehicles to reversibly direct nanoparticles from aqueous to organic media or vice versa, as described below, which paves a simple way to widen the adaptability of nanoparticles and in turn its applicability.29b, 20, 31

Hydrogel assisted transfer of hydrophilic nanoparticles from aqueous to organic media. When a hydrogel loaded with or without functional hydrophilic nanoparticles are placed into a polar organic solvent, the substitution of this organic solvent for water is expected to readily occur in the nanoparticle-loaded, hydrogel network due to the excellent miscibility between them. The substitution efficiency can be further enhanced, provided the polar organic solvents can effectively form hydrogen bonds with the polar groups tethered on the hydrogel network and the nanoparticle surfaces. After this solvent exchange completed, the hydrophilic nanoparticles, embedded in the hydrogel, are brought in direct contact with a polar organic solvent without aggregation thanks to the spatial confinement of the polymeric networks of the hydrogel hosts. Furthermore, the proposed solvent exchange can also be applied to displace polar organic solvents with apolar ones in pristine or composite hydrogels thanks to the reasonable miscibility between them. During solvent exchange, the structural stability of hydrogels, especially those composed of physically cross-linked polymers, may be expected as a potential technical issue, since the introduction of organic solvents will weaken the hydrophobic interactions between the apolar moieties of the constituent hydrophilic polymer chains of the hydrogels. Fortunately, that issue may be effectively overcome by the enhancement of the polar interactions, especially hydrogen bonding between the polar moieties of the hydrogel network with aqueous media being displaced by organic media.29b, 30

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Figure 5. (a) Schematic illustration of loading of 12 nm Au nanoparticles into PNIPMAA microparticles (left panel) and plot of the plasmon absorption peak position of the loaded versus the refractive index of the dispersion solvents of the composite microparticles, ranging from water to ethanol, THF, DMF, and toluene (right panel). The dispersion of the composite microparticles is implemented via solvent exchange. The inset shows photographs of composite microparticles dispersed in water, ethanol, and toluene.29b (b) Transfer of CalB-loaded agarose hydrogel beads into heptane via solvent exchange (left panel) and the catalytic performance of the composite beads for esterification of 1-octanol and octanoic acid as compared with native CalB (right panel).30a (c) (Left panel) Schematic diagram of transfer of GO-loaded agarose hydrogel beads from water to hexadecane via stepwise solvent exchange using ethanol as intermediate and adsorption of lipophilic dye by the composite hydrogel beads in hexadecane and (right panel) Photographs of the agarose hydrogel blocks loaded without (upper panel) and with GOs (lower panel) taken after they are placed in the NR solutions in hexadecane (left vials), incubated for 2 h (middle vials), and transferred back to pure hexadecane (right vials).30c The NR concentration in hexadecane is 10 mg·L-1. The agarose content is 15 mg·g-1 and the GO content 0.2 mg·g-1 in the hydrogel blocks.

Figure 5a shows that after citrate-stabilized gold nanoparticles are incorporated into hydrogel microparticles of copolymers of N-isopropylacrylamide and methacrylic acid (PNIPMAA), they can be readily brought into various polar organic solvents, followed by transfer into apolar solvents. In response to the solvent exchange in the hydrogel network, the SPR absorption peak position of the gold nanoparticles gradually red-shifts with an increase in refractive index of the surrounding environment from water to ethanol, tetrahydrofuran (THF), and N, N-dimethylformamide (DMF) and eventually toluene.29b 20


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This influence of the environmental refractive index on the SPR behavior of gold nanoparticles can be hardly unraveled, since, on one hand, gold nanoparticles with pristine or ion-stabilized surfaces suffer severe aggregation upon being contact with organic solvents and effective stabilizing ligand coating, on the other hand, makes the gold nanoparticles less sensitive to the surrounding refractive index change. The aforementioned solvent exchange protocol has been also applied to other hydrophilic nanoparticles such as quantum dots (QDs) and enzymes.30a,b Figure 5b shows agarose hydrogel assisted transfer of lipases B from Candida antarctica (CalB) into heptane for catalysis of the esterification of 1-octanol and octanoic acid.

30a

GOs have abundant

functional groups on the surfaces and at the edges, which are able to strongly adsorb ionic or polar contaminants of inorganic or organic nature via hydrogen bonding, electrostatic force and so on for wastewater treatment.65 When agarose hydrogel beads, loaded with GOs inside, are transferred into apolar organic solvent such as hexadecane , the GO guests can effectively and efficiently remove polar organic compounds such as Nile Red in the apolar media, since the GO polar groups remain hardly change in terms of amount and chemical nature during solvent exchange (Figure 5c).30c As a comparison, the adsorption capability of GO powder towards Nile red in hexadecane is about 70 times smaller than that of GO-loaded agarose hydrogels, which is a result of poor dispersibility of hydrophilic GOs in oil.

Hydrogel assisted transfer of hydrophobic nanoparticles from organic to aqueous media. Since successful displacement of one solvent by another in a hydrogel is based on the miscibility between these two solvents and the hydrogel network remains hardly changed during solvent exchange, the solvent exchange strategy is expected reversible. That is, the environment of a hydrogel can be reversibly changed from water to a waterimmiscible organic solvent and then back to water again by using a water-miscible, polar organic solvent as the intermediate. As shown in Figure 6,31 when agarose hydrogel is

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transferred into the dispersions of hydrophobic CdSe QDs, stabilize by octadecylamine, in chloroform, its highly porous interior structure enables the hydrogel to imbibe the quantum dots. After exchange of chloroform with a water-miscible organic solvent such as THF, the agarose hydrogel can be readily dispersed back in water, during which the CdSe QDs, imbibed in the hydrogel network during incubation in chloroform, are effectively confined within the hydrogel due to the incompatibility of the hydrophobic surface coating of the QDs with the surrounding aqueous environment.

Figure 6. (a) Cartoon showing the loading of hydrophobic NPs into hydrogel MPs via solvent exchange. The red spots represent hydrophobic NPs and their hydrophobic surface shells are highlighted by yellow. The large sphere with dashed grids represents a hydrogel MP. (b) Optical photographs of aqueous dispersions of agarose microspheres loaded with octadecylamine-stabilized CdSe quantum dots with diameters of 9.3 nm (left), 5.5 nm (middle), and 5.0 nm (right). Their emission colors are red (left), yellow (middle) and green (right) under UV irradiation.31

This simple solvent exchange protocol enables phase transfer of highly hydrophobic QDs from organic into aqueous media with little change in the QD surface hydrophobic passivation coating, which makes the photoluminescence behavior of the QDs comparable to the original ones in their chloroform dispersions and, at the same time, chemically stable in acidic or alkaline aqueous media. Multiplex functions can be easily introduced into hydrogel by adjusting the formulations of organic dispersions bearing different types of hydrophobic nanoparticles. As such, the solvent exchange-based, hydrogel-assisted phase transfer should have significant implications in nanotechnology, especially those related to 22


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biology and medicine, since more wet chemical strategy are available to produce nanoparticles with much higher quality in organic media than in aqueous media.

Figure 7. Conversion of GOs to rGOs in agarose hydrogel beads (AgarBs) for removal of organic dyes in water.33 (a-c) Images of the aqueous solutions of RhB, within which AgarBs (left vials), GO-AgarBs (middle vials), and rGO-AgarBs (right vials) are placed. The images are taken immediately after addition of the pristine composite AgarBs into the RhB solutions (a), after 8 h incubation under sun light (b), and, under irradiation of UV-light (365 nm) (c), respectively. The initial RhB concentration is 1

g∙mL-1. The mass of

the pristine or composite AgarBs used for RhB adsorption is 1 g. (d) Adsorption kinetic profiles of rGOAgarBs (open circles) and GO-AgarBs (open squares) toward RhB in water, fitted with pseudo-first-order (dashed curves) and pseudo-second-order models (solid curves), respectively. The initial RhB concentration is 5

g∙mL-1. (e) Adsorption isotherms of rGO-AgarBs (open circles) and GO-AgarBs (open squares)

towards RhB in water, fitted by Langmuir (solid curves) and Freundlich models (dashed curves). (f) Plots of the q" values of rGO-AgarBs (circles) and GO-AgarBs (squares) versus the concentration of MgCl2 (solid symbols) and NaCl (open symbols) in water.

Hydrogel assisted formation of hydrophobic nanoparticles in aqueous media. As discussed above, when nanoparticles are incorporated directly in hydrogels, since the highly porous structure of the hydrogel networks ensures sufficient stabilization of the nanoparticles against aggregation in water, which, in turn, enable easy and direct chemical modification the nanoparticle surfaces. In this regard, graphene is an ideal candidate of nanoparticles owing to well-established synthetic chemistry available for its modification.32 For instance, when hydrophilic GOs are loaded in agarose hydrogel beads,

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they can be readily converted into hydrophobic, reduced graphene oxides (rGOs) via NaBH4 reduction, which exhibit highly effective and efficient adsorption towards organic compounds in water (Figure 7).33 The driving force for the rGOs to adsorb the organic compounds with reasonably large solubility in water is the hydrophobic interactions between the adsorbents and the adsorbates, which is fairly long-ranged and reasonably enhanced by the salinity of aqueous media, which cannot be achieved by other intermolecular interactions such as electrostatic or polar interactions conventionally designed for hydrophilic adsorbents for water purification. The intentional introduction of tailored local hydrophobicity in an aqueous nanostructured system is of great interest especially in terms of mimicking the biological functions encountered in biological systems such as enzymes, but it still remains rarely touched in literature.

3.

En route toward Artificial Enzymes. When nanoparticles are incorporated in a hydrogel, as discussed above, the

superporous structure of the hydrogel host enables the nanoparticles to expose their surfaces directly to targeted substances present in the surrounding media. Taking into account that, the use of nanoparticle/hydrogel composites as catalysts should have farreaching consequences.23g,34 The applicability of nanoparticle/hydrogel composites in catalysis can be further endorsed by two important roles played by the hydrogel hosts: (1) to effectively confine the nanoparticle guests, even if their surfaces are barely coated by stabilizers and (2) to enable reactants to easily diffuse and reach to the catalytically active nanoparticle surfaces. Here a number of recent developments in this regard are overviewed in the following three aspects: (a) Immobilization of catalytic nanoparticles in microgels. Nowadays a diversity of hydrogels can be produced in the form of microparticles, known as microgels, which have been not only applied in many tehnical applications such as coating and drug delivery but also utilized as ideal model systems to study the solution-dependent properties of

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hydrogels.1a In this scenario, nanoparticle-loaded microgels will offer an excellent starting point to test how the interplay between nanoparticle guests and hydrogel hosts affects the technical performance of their composites in catalysis. Figure 8a shows synthesis of catalytic Ag nanoparticles inside microgels consisting of inter-locked chitosan and poly(acrylic acid) chains. The resulting composite microgels can efficiently catalyze decomposition of organic dyes such as methyl red and Congo red in 30 mins in the presence of NaBH4.34a The Ag nanoparticle-loaded microgels offer electron relay systems in which the Ag nanoparticles start the catalytic reduction by accepting electrons from electrondonor, BH4- ions and then transferring them to electron-acceptor, dye molecule. From a viewpoint of mass transportation, on the other hand, the porous hydrogel network enables the dye molecules to freely transport and effectively adsorb onto the bare surfaces of the Ag nanoparticles due to strong binding affinity, while the H2 generated in close proximity to the nanoparticle surfaces causes the convection flow to remove the products and in turn keep the nanoparticle surfaces uncovered, thus maintaining the high catalytic reactivity. When catalytic nanoparticles are produced in stimuli-responsive hydrogels, the volume phase transition of the hydrogel hosts in response to external stimuli can be utilized to adjust the access of reactants to the nanoparticle surfaces. For instance noble nanoparticles such as gold and silver are loaded in thermoresponsive poly(N-isopropylarylamide) (polyNIPAM) microgels (Figure 8b)34b or the polyNIPAM shells coated on inert latex particles (Figure 8c),34c the active surfaces of the metal nanoparticle guests are fully accessible to reactants whe the polyNIPAM networks are in the swollen state at temperature below the lower critical solution temperature (LCST) of polyNIPAM. With the environmental temperature increasing above the LCST, however, the catalytic reaction is significantly slowed down, since the shrinkage of the polyNIPAM network inhibits the access of the reactants to the nanoparticle surfaces. Furthermore, loading of catalytic nanoparticles in stimuli-responsive hydrogels may be also conducive to recycling and reuse of the catalytic guests, since the hydrogels are easily removed from the reaction media at

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their collapse stage under a given external stimulus.

Figure 8. (a) Synthesis of Ag nanoparticles in chitosan (CTS) and poly(acrylic acid) (AA) composite hydrogel microparticles with the surfaces cross-linked by Al3+ ions and their catalytic performance in reductive decomposition of methyl red (MB) and Congo red (CB) by using NaNH4 is assessed by monitoring the absorption intensity of the organic dyes in water with the reaction time.34a (b) Fabrication of thermoresponsive microgel of poly(N-isopropylarylamide) (polyNIPAM) with single gold nanoparticles loaded inside and the catalysis performance of the gold nanoparticle cores in the electron-transfer reaction between hexacyanoferrate (III) and borohydride ions can be regulated by the reaction temperature thanks of the

hydrogel

temperature-response.34b

c)

Temperature-dependent

catalytic

performance

of

polystyrene/polyNIPAM, core/shell latex particles with Ag nanoparticles loaded within the polyNIPAM shells in reduction of 4-nitrophenol by NaBH4.34c d) Fabrication of yolk/shell composte particles composed of polyNIPAM hollow microgels loaded with single mobile Au nanoparticle cores. The temperaturedependent catalytic performance of these yolk/shell particles are assessed in reduction of 4-nitrophenol and nitrobenzene by NaBH4, respectively. 34d

Concomitant with the volume phase transition of hydrogel in response to an external stimulus, the local hydrophibicity change within the hydrogel network. For instance, the network of polyNIPAM is reasonably hydrophilic at temperature belove its LCST, while its hydrophibicity may be significantly reduced at temperature above its LCST as a result of dehydration of the polyNIPAM network and formation of hydrogen bonds and hydrophobic interactions between the polyNIPAM segtments. Figure 8d show the 26


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temperature-dependent catalytic performance of polyNIPAM hollow spheres loaded with single gold nanoparticle cores in the reaction of using NaBH4 to 4-nitrophenol and nitrobenzene. The results demonstrate that the gold nanoparticles tend to preferentially catalyze reduction of 4-nitrophenol over nitrobenzene at reaction temperature below the LCST of the polyNIPAM network as a result of the better permeability of hydrophilic 4nitrophenol through the water-swollen polyNIPAM shells. In contrast, gold nanoparticles tend to preferentially catalyze reduction of nitrobenzene over 4-nitrophenol at reaction temperature above the LCST of the polyNIPAM network as a result of the better permeability of hydrophobic nitrobenzene through the dehyrdated polyNIPAM shells . The stimuli-responsive catalysis selective should be unquire for nanoparticle/microgel composites, which should be of great interest especially for fundamental catalysis research. (b) Immobilization of catalytic nanoparticles in bulk hydrogels. In compared with microgels, the growth and loading of functional nanoparticles in bulk hydrogels are technically facile, as the colloidal stabilization is not a technical concern for the bulk hydrogels but it is crucial for the microgels. What makes bulk hydrogels more appealing catalysts relies on the chemical sophistication of design and modification of the polymeric networks of the hydrogel hosts, which will extend or transform their roles from supporting catalytic reactions, such as nanoparticle stabilization and reactant diffusion control, to being actively involved in catalytic reactions. As shown in Figure 9a,35a when polyanilines are used to gel carbon nitride nanosheets with three-dimensional nanostructured hydrogels, the PANIs are used to effectively adsorb organic pollutant molecules from water into the resulting composite hydrogels and, at the same time, form preferred electronic structures at the interfaces with the CNNS to ensure excellent separation between photoinduced electron (moving onto the CNNS LUMO) and photoinduced holes (moving onto the PANI HOMO). That significantly promotes the photocatalytic performance of the PANI/CNNS composite hydrogels in oxidative decomposition of the adsorbed organic pollutants in water. When inorganic nanoparticles are synthesized as primary catalysts in a hydrogel, the

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porous polymer network of the hydrogel hosts may be still able to adsorb secondary catalysts such as enzymes. Figure 9d show a highly sensitive glucose sensor built on the cooperation of platinum nanoparticles and glucose oxidases incorporated in the hydrogel consisting of polyaniline and phytic acid, in which after glucose oxidases catalyze oxidation of glucoses into gluconic acid and hydrogen peroxides newly formed hydrogen peroxides are immediately removed from the proximity to the enzymes as a result of decomposition catalyzed by the platinum nanoparticles.35b This cascaded style of catalysis makes nanoparticle/hydrogel composites largely reminiscent of the catalytic process occurring in enzymes. As shown in Figure 9c,35c highly ordered macroporous hydrogels can be used as carriers for immobilization of multiple enzymes, in wich structural uniformity, small reaction volume, complete penetrating structure, and controllable motion can be integrated in one to improve the efficiency of biocatalysis. As discussed in Figure 1b, furthermore, the light scattering by the ordered porous microstructure of the hydrogel hosts impart peculiar structural colors, which enables in-situ monitoring of the enzymatic catalysis process. Figure 9d shows sophisticated microfluidic celles with the microstructured walls composed of periodically arranged stimuli-response hydrogel pillars of poly(acrylamide-co-acrylic acid) with catalysts such as platium nanoparticles affixed on the tips.35d Upon an aqueous bilayer flows into the microfluidic chanals, chemcial or biochemcial reaction designed in the upper aqeuous layer can take place only when the hydrogel pillars are fully swolled by the lower aqueous layer and poke their calaytic tips into the upper reaction layer to trigger the reaction. This new system can be in principle extended to a variety of stimuli-responsive hydrogels combined with single or mulple kinds of catalysts, enabling multiple reactions taking place in a tailored temporal order via periodic inputs of external stimuli. The on/off catalytic performance, built on the stimuliresponsive, mechanical actuation of the hydrogel pillars, may be expected as a primitive model mimicking the catalytic behavior of enzymes.

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Figure 9. (a) Fabrication of polyaniline (PANI) / carbon nitride nanosheet (CNNS) composite hydrogels and, as compared with individual constituent components, the PANI/CNNS composite shows photocatalytic oxidative decomposition of organic pollutant molecules from water under visible light as a result of effective charge separation at the PANI/CNNS interfaces.35a (b) Fabrication of polyaniline/ phytic acid composite hydrogels loaded with platinum nanoparticles (PtNP) and glucose oxidases (GOx), which are used as highly sensitive glucose thanks to highly efficient glucose oxidation consecutively catalyzed by the PtNP and GOx.35b (c) Development of inverse opal beads of polyacrylamide hydrogel for immobilization multiple enzymes such as horseradish peroxidase and urease for biocatalysis.35c (d) Development of self-regulated mechanochemical adaptively reconfigurable tunable system to reversibly transduce external or internal chemical inputs into user-defined chemical outputs based on the mechanical actuation of stimuli-responsive hydrogel pillars tipped with catalysts or sensing molecules.35d

(c) Immobilization of catalytic nanoparticles in gelled Pickering emulsions. Since catalysis needs the direct contact of reactants with the active surface sites of nanoparticles, it should be beneficial to use hydrogel to transfer hydrophilic nanoparticles with nearly naked surfaces directly into organic media via solvent exchange (Figure 5). Furthermore, the polar segments of the hydrogel network, located near the loaded nanoparticles, will act 29


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as Lewis acid or base sites to effectively adsorb reactants in apolar reaction media, as suggested in Figure 5c, which may be beneficial for the reactants to be activated during the nanoparticle catalysis. This argument is in line with that the outstanding catalytic performance of enzymes is commonly believed to take advantages of the presence of cofactors or co-catalysts near their active centers and the mesoscopic channels with defined hydrophobicity to regulate the access of reactants to the active sites.20, 21 In terms of enzyme mimicking, Pickering emulsions may offer high flexibility and ingenuity in design nanoparticle/hydrogel composites.36 A great variety of colloidal particles have been used as stabilizers to form emulsions, known as Pickering emulions.37 When water-in-oil (w/o) Pickering emulsions are formed, various water-soluble molecules, polymers, nanoparticles, including enzymes, can be readily trapped in the water droplets enclosed by relatively hydrophobic nanoparticles stabilizers, which become more easily to handle after the water droplets are gelled (Figure 10a).36a Immobilization in Pickering emulsion and the following gelation result in little change in the chemical properties of the encapsulated substances. Furthermore, since the nature of aqueous media has little impact on the formation and stabilization of Pickering emulsions, which is determined by the size and surface nature of solid particle emulsifiers, so one can load multiple functional substances into individual Pickering emulsion droplets, for instance enzymes and their cofactors, which is a technical imperative for most of enzymatic catalysis processes. Figure 10b shows that when very vulnerable enzymes – benzaldehyde lyase (BAL) from Pseudomonas fluorescens Biovar I – and all essential cofactors are immobilized in gelled water droplets, emulsified by hydrophobic silica nanoparticles, in methyl tert-butyl ether, the BALs maintain their catalytic activity for catalysis of stereoselective formation of benzoin (Figure 10b)36a.

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Figure 10. (a) Schemes of using the Pickering emulsion for enzyme catalysis in organic media. (b) Chart of the activity of native BAL, BAL immobilized in liquid (BAL + NP) and jellified (BAL + NP + agar) Pickering emulsion droplets stabilized by 140 nm SiO2 NPs. Activity values are included.36a

In addition to the flexibility of immobilization of multiplex functional substances in single Pickering emulsion droplets, the recent development in directing nanoparticles to self-assemble at oil/water interfaces enables us to stabilize emulsions not only conventional chemically inert silica or latex particles, but also functional nanoparticles made of nobel metals and semiconductors and even biological nanoparticles such as virus and yeast. Of most important, different nanoparticles with distant chemical natures can be layer-by-layer assembled at oil/water interfaces to create multifunctional shells.38 On the other hand, the shells of Pickering emsulsions are composed of (randomly) closely-packed particle stabilizers, so the interstices between the particles can be easily adjusted at the nanoscale by the sizes and shapes of the nanoparticle stabilizers, which have been utilized to modulate the transporation of nanoparticles and macromolecules, loaded within the Pickering emulsion droplets, across the emulsion shells.39 The great flexibility in immobilization of multiplex functional nanoparticles in Pickering emulsions and the nanoscale defined openings of their shells will open up promising prospect in design of unprecedented cascade catalytic processes between the catalytic particles such as enzymes within the emulsion droplets and the other catalytic particles such as nobel metal nanoparticles in the emulsion shells and, at the same time, execute directional transport and separation of 31


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reaction substrates and products.

CONCLUSIONS AND OUTLOOK The use of hydrogel to accommodate nanoparticles offers a hotchpotch where polymers, supramolecular, inorganic nanoparticles, biomaterials, biological materials, cells, microbes, and other microorganisms coexist with their own identities well preserved. That all is thanks to the tolerant and inclusive character of hydrogel, stemming from their exceedingly large water content, hierarchical porosity, and abundant polar groups with defined but varied molecular nature. The cooperation between the nanoparticulate guests residing in a hydrogel will bring up unprecedented collective properties, which will be further benefited by control of water uptake and release out of the hydrogel network in response to a specific external stimulus. Hydrogel nanocomposites are therefore the promising candidates to artificial smart systems with at least primitive structural and functional features of sophisticated biological systems for instance extracellular matrix. For that promising but ambitious dream to come true, it requires not only great ingenuity in integration of hydrogels and nanoparticles in one, but also high precision in comprehension and control of their structures and properties, respectively. In this regard, the structural complexity of nanoparticles is well recognized in literature and remains an attractive research topic of being studied extensively at present. In contrast, hydrogel is simply treated as a water-filled skeleton made of interlocked, featureless chains, which is practically sufficient, in most cases, to rationalize the volume phase transition of the hydrogel and its stimuli-response via balance between chain-chain interaction and waterchain interaction. This prevailing model, however, is too rough for hydrogel to be employed as a smart matrix to mimic enzymatic catalysis and other biological processes, in which targeted substances need not only to freely transport cross the hydrogel network but to position themselves in selected domains in the hydrogel network. Taking into account the amphiphilic nature of hydrophilic polymers, a hydrogel ought

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to embrace a tertiary self-organized structure of its constituent hydrophilic polymer chains, which draws an analogy with biomacromolecular entities such as proteins or DNA. The early study of agarose hydrogel in 1970’s has unveiled that the macroporous gel is made of inter-locked nanosheets, which are composed of the double helices of polysaccharide chains.40 Although the macroporous structure of agarose hydrogel is widely used for electrophoresis and purification of proteins or DNA, the underlying tertiary self-organized structures has drawn little research attention. Even when constituent polymer chains are covalently bonded together, their amphiphilic nature may to a small or large degree drive microphase separation within the resulting networks. Hence the structure of a chemically cross-linked hydrogel should be more complicated than a featureless network of randomly knotted coils. This structural complexity may become more pronounced when the hydrogel network shrinks as a result of the hydrophobic attraction overwhelming the hydrationinduced repulsion between or within the hydrophilic polymer chains in response to a given external stimuli. In this scenario, the tertiary structure of a hydrogel will be the critical information for precise rationalization and control of how the hydrogel network interacts with molecular or nanoparticulate substances present the surrounding environment, which will lay the knowledge foundation to derive biomimetic or bioinspired smart materials from hydrogel. Thus, the information of the tertiary structural feature of hydrogel and its impact on the hydrogel performance needs to be on the research radar. To bear in mind the technological sophistication of molecular and supramolecular chemistry, it is doable but challenging to design and tailor the tertiary structure of hydrogels. This deliberate endeavor will be aimed at empowering the new hydrogels with two capabilities: one is to selectively incorporate different molecules or nanoparticles into targeted domains of the hydrogel network and the other to modulate the interactions between constituent polymer chains and their interactions with the surrounding with the aid of external stimuli. Capitalized on these capabilities built in hydrogels, integration of synthetic or naturally occurring nanoparticles will not only be the ingenious process to devise advanced smart materials or systems with

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biomimetic and bioinspired functions, but also offer a splendid platform for chemists, biologists, materials scientist, and electronic engineers to expand their fantasy about medical nanorobotics. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] and [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS. DW is grateful to Professor Helmuth Möhwald for encouraging mentorship, inspiring discussion, and generous support and all colleagues, especially S. Bai, M. Kuang, Z. Mao, and H. Xia for the valuable contributions in order to reach the current stage of understanding about the research field present in this article. CC thanks China Postdoctoral Science Foundation (2018M641768) and China Postdoctoral International Exchange Program for financial support. CZ thanks National Natural Science Foundation of China for a research grant (Grant No. 81471028) and the Health Special Project of Jilin Provincial Department of Finance for a research grant (Grant No. 3D518V233429). REFERENCES: 1. For reviews on hydrogel and the references cited therein, see: (a) Saunders, B. R.; Vincent, B.; Microgel particles as model colloids theory, properties and applications. Advances in Colloid and Interface Science 1999, 80, 1–25; (b) Peppas, N. A.; Huang, Y.; Torres-Lugo, M.; Ward, J. H.; Zhang, J., Physicochemical foundations and structural design of hydrogels in medicine and biology. Annual Review of Biomedical Engineering 2000, 2, 9–29; (c) Hoffman, A. S.; Hydrogels for biomedical applications. Advanced Drug Delivery Reviews 2012, 64, 18–23. 2. Henry, M; The state of water in living systems: from the liquid to the jellyfish. Cellular and Molecular Biology 2005, 51, 677–702. 3. For a review on extracellular matrix, see: Theocharis, A. D.; Skandalis, S. S.; Gialeli, 34


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