The Case of Inorganic Submicrometer Capsules - American Chemical

Invited Feature Article pubs.acs.org/Langmuir

Enhancing the Exploitation of Functional Nanomaterials through Spatial Confinement: The Case of Inorganic Submicrometer Capsules Belén Vaz,† Verónica Salgueiriño,‡ Moisés Pérez-Lorenzo,† and Miguel A. Correa-Duarte*,† †

Departments of Organic Chemistry and Physical Chemistry, Biomedical Research Center (CINBIO), and Institute of Biomedical Research of Ourense-Pontevedra-Vigo (IBI) and ‡Departamento de Física Aplicada, Universidade de Vigo, 36310 Vigo, Spain ABSTRACT: Hollow inorganic nanostructures have attracted much interest in the last few years due to their many applications in different areas of science and technology. In this Feature Article, we overview part of our current work concerning the collective use of plasmonic and magnetic nanoparticles located in voided nanostructures and explore the more specific operational issues that should be taken into account in the design of inorganic nanocapsules. Along these lines, we focus our attention on the applications of silica-based submicrometer capsules aiming to stress the importance of creating nanocavities in order to further exploit the great potential of these functional nanomaterials. Additionally, we will examine some of the recent research on this topic and try to establish a perspective for future developments in this area.

1. INTRODUCTION Hollow inorganic nanostructures have seen a considerable burst of interest in the last few years due to their many applications in different areas.1 In this regard, voided nanoarchitectures have been used as catalysts,2,3 energy- and gas-storage materials,4,5 drug-delivery carriers,6,7 sensors,8,9 antibacterial products,10,11 contrast agents for imaging,12,13 and environmental remedies.14,15 Thus, many high-quality review papers on the design, synthesis, and applications of hollow nanostructured systems have been brought to light in recent times.16,17 In this Feature Article, we discuss the current trends as well as future perspectives toward the use of hollow inorganic nanostructures. In these lines, we will place particular emphasis on those cases where the existence of a voided nanospace allows the encapsulation and further reinforcement of the applications of the functional nanomaterials hosted therein. In a strict sense, one of the main goals pursued when creating isolated void spaces is the use of the formed nanocavity to accommodate a variety of molecules or larger structures to preserve and/or optimize their functional properties when immersed in a bulk medium. However, endowing these hosts with new features that cannot be achieved if directly exposed to the surrounding environment is also of critical importance. In any case, there is increasing interest in modulating the interaction of the confined materials with their surroundings. Hence, regulating the transport phenomena occurring between the void space and the bulk must be taken into account in the design of these confined locations. 1.1. Capsule Configuration. When it comes to encapsulating metal nanoparticles in a hollow cavity, the main purpose is not only to provide protection for these functional cores so that their properties are kept unaltered or even improved but also to offer a homogeneous environment for chemical or physical processes mediated by these materials and © XXXX American Chemical Society

to afford storage capacity for cargo that may interact with the nanoparticles with further objectives. In this regard, there are two main configurations when dealing with hollow nanostructures: nanorattles and reverse bumpy balls (Figure 1).2

Figure 1. Illustration depicting the main configurations for hollow nanostructures: yolk−shell and reverse bumpy ball architectures. Reprinted with permission from ref 2. Copyright 2013 Wiley.

Nanorattles, usually also referred to as yolk−shell nanoparticles, are single-metal core-containing structures in a core@void@ shell configuration. These structures have attracted increasing attention in the last few years, and excellent reviews in relation to their preparation, properties, and applications have been published.18−20 The alternative architecture is that in which numerous metal cores remain attached to the inner walls of the shell (either supported or partially embedded) following a void@cores@shell pattern.2 Placing metal nanoparticles on the internal surface of the capsule may provide additional benefits Received: January 9, 2015 Revised: March 3, 2015

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DOI: 10.1021/acs.langmuir.5b00098 Langmuir XXXX, XXX, XXX−XXX

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Invited Feature Article

with respect to yolk−shell nanoparticles. First, reverse bumpy balls can offer a greater quantity of catalytically active sites per nanoreactor and hence can lead to superior performance. It is also important to realize that compared to rattlelike systems, reverse bumpy balls provide intimate contact between the shell and the metal cores. This strong interaction may allow us to exploit the well-known synergy between metal oxide supports and noble metal nanoparticles. This is especially attractive when dealing with photocatalytic reactions. A good example is the combination of porous titania and gold nanoparticles.21 In this case, Au nanoparticles act as electron sinks to enhance the charge separation upon photon absorption, hence improving the quantum yield of superoxide radicals, which leads to an improved photocatalytic activity. This synergistic effect has important implications in the design of new solar cells, the generation of hydrogen and oxygen through water splitting, and the degradation of inorganic, organic, and biological compounds present in the environment. It is apparent that the thickness, porosity, and surface functionalization of the silica shell will play essential roles when it comes to applications. Different synthesis strategies can be followed for achieving good control over these features. Among them, Stöber, microemulsion and supramolecular templating methods are the most common procedures. All of these approaches are based on a sol−gel process in which the shell growth is carried out by an initial hydrolysis of the silica precursor followed by the polycondensation of the silanol groups. At this point, it is worth underlining that the modification of amorphous silica materials by the introduction of organic functionalities constitutes an important focus for applied research. This approach is based on the use of silica growth-directing agents such as surfactants or block copolymers and exploits the electrostatic interactions between the silicate species and those agents to form mesoporous nanostructures. As the silanization reaction progresses, the charge density between the inorganic and organic species influences the arrangement of the structure-directing reagents, determining the final interface and consequently governing the assembly process and the final properties of the shell.22 The organic components are eventually removed by calcination or solvent extraction to form the corresponding mesopores. In the end, this well-developed pore structure with large defined windows improves the accessibility of different chemical species to the nanoparticle-containing cavity. The engineering and structural control of silica-based nanocapsules has been recently reviewed in detail.23 The incorporation of multiple metal nanoparticles into the space delimited by the silica shell has also been described by our group.24,25 1.2. Collective Behavior. Another advantage of using reverse bumpy ball configurations is the possibility of benefitting from the collective behavior of the nanoparticles attached to the inner walls of the capsules. It is well known that noble nanoparticles can strongly absorb visible light because of the so-called surface plasmon resonance (SPR) effect. When assembled in a close-packed structure, individual plasmon oscillations can couple via a near-field interaction with the neighboring particles, giving rise to hybridized plasmon resonance modes delocalized over the entire structure.26 As a result of this coupling, a red shift and broadening of the plasma resonance in the optical spectrum is attained. This enhanced light absorption turns metal nanoparticles into promising nanosources of heat because they can be remotely controlled upon external illumination (Figure 2).27,28

Figure 2. SEM image of a small plasmonic nanoparticle network (PNN) deposited on an ITO/glass substrate (center), two-photon luminescence (left), and temperature images of the same PNN excited at 730 nm (right). Reprinted with permission from ref 28. Copyright 2012 American Chemical Society.

Another kind of collective behavior that can be exploited when dealing with reverse bumpy balls is that in which exchange and dipolar interactions between magnetic nanoparticles take place.29 Both types of interactions improve the magnetic properties of hollow nanostructures in terms of saturation magnetization, magnetic susceptibility, and coercivity. These parameters help to position magnetic capsules precisely and trigger a heat delivery process when exposed to an alternating magnetic field with a suitable frequency and amplitude.30 This capacity to release heat depends on both the magnetic susceptibility and coercivity, which in turn depend on the magnetic anisotropy. In this regard, magnetic nanoparticles and nanostructures show different contributions to their total effective magnetic anisotropy. Besides magnetocrystalline and shape anisotropies, there are other anisotropies that can comprise options to tune this effective anisotropy energy and, consequently, the final magnetic collective behavior. Thus, magnetic nanoparticles in direct contact, as in the present case, will share a particular interface which can imply an important contribution to anisotropy as the one related to the exchange bias interaction.31 Indeed, the direct interaction between ferri- and antiferromagnetic transitionmetal oxides in core−shell particles was demonstrated to depend on the type of interface.32 Lee et al. have also reported exchange-coupled magnetic nanoparticles of two ferrimagnetic materials as a new means of modulating their magnetism, which resulted in a significant enhancement of magnetic heat induction.33 Beside direct interfaces, magnetic hybrid nanostructures with nanoparticles of materials with different magnetic order but grouped in direct contact also became good candidates to study this effect.34 Plasmonic- or magnetic-based heating is especially interesting when dealing with reverse bumpy ball architectures. In this case, the shell of the capsule defines a volume wherein a chemical reaction may be performed. This shell, commonly based on silica, plays several roles in the process. Thus, it prevents nanoparticles from sintering while it acts as a thermal insulator avoiding excessive alterations in the temperature of the bulk solution as a result of the absorption and subsequent thermal dissipation of light energy by the particles. In addition, the permeability of the shell allows the reactants to diffuse into the empty space and the products to diffuse out of the nanocapsule. In this context, it is clear that thermally induced processes are a priority objective. The generation of high temperatures by photoheat conversion in the presence of noble metal nanoparticle assemblies or instead by the application of an alternating magnetic field to transition-metal oxide nanoparticles enables molecules to overcome high potential energy barriers and thus activate different chemical transformations B


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(e.g., Figure 3).35−37 Likewise, speeding up reactions or physical phenomena by exploiting their Arrhenius behavior can also be of great value.

functional materials contained in its void space promotes the implementation of capsules in a wide range of applications. Thus, the modification of the external surface, pore size and orientation, shell thickness, or internal functionalization of the membrane can be used to regulate the interactions between the bulk medium and the contents of the cavity. Accordingly, silica constitutes a particularly attractive material given its colloidal stability, chemical versatility, and biocompatibility, especially when it comes to biomedical applications.23 Obviously, the nature and properties of the confined material define the technological capabilities of the hollow nanostructure. This is especially relevant when dealing with nanoparticles based on noble metals or oxides of other transition elements. In these cases, a careful engineering of the nanostructure can be employed in order to tailor their size, shape, and surface chemistry. Localized SPR when it comes to plasmonic entities and superparamagnetic or blocked behavior in the case of magnetic nanoparticles are also among these tunable parameters. Thus, these functional nanomaterials not only allow the development of nanoarchitectures with a specific purpose but also may endow the capsules with multiple capabilities that can be exploited simultaneously. Furthermore, a precise choice of the properties of the confined materials can also be used to tune the external stimulus to which they respond. Thus, remotely triggerable capsules can be designed and adapted depending on the objective they are intended to address. In any case, it is clear that combining the strengths of silica and novel functional nanomaterials in capsular configurations will propel a vast array of applications in the near future.

Figure 3. Effect of laser-mediated particle heating on the thermal curing of PDMS. (a) SEM images of the PDMS structures formed with a 2 mW laser for an increasing exposure time of between 1 and 16 s, (b) side-view SEM images of the PDMS structures for a constant exposure time of 8 s and an increasing laser power of between 5 and 25 mW (images are taken at a 60° angle), and (c) dependence of the PDMS shell thickness on the heating time for a constant laser power of 2 mW. (d) Simulation of the heat distribution of an 82 nm Au nanoparticle irradiated with a laser beam at a laser power density of 378 kW/cm2 (∼2 mW). Adapted with permission from ref 37. Copyright 2013 American Chemical Society.

2. APPLICATIONS The following section presents some of the recent research concerning the use of noble metal nanoparticles located in voided nanostructures and explores the more specific operational issues that should be taken into account in the design of inorganic nanocapsules. 2.1. Drug Delivery. The use of hollow nanostructures as drug vehicles illustrates the importance of regulating the transport phenomena occurring between the nanocavity and the bulk medium. In this case, the ability of capsular systems to load, transport, shield, and deliver drugs makes these architectures a key tool in improving the treatment of many diseases from cancer to genetic disorders. Surface modification constitutes a critical factor in this matter. A suitable definition of the external surface properties can prevent the organism from recognizing the capsules as a foreign material avoiding to the action of the immune system against them. Likewise, a proper functionalization of the outer shell may enable the delivery of the capsules to particular locations within the body carrying out the target cell identification.39,40 Silica-based nanocapsules constitute a remarkable example of the immense potential of hollow inorganic nanostructures in biomedicine.23 These systems have been widely exploited to host drugs exhibiting poor solubility or little stability in biological fluids. This is largely due their chemical and colloidal stability, which lead to an improved circulation time in the body. Likewise, their reduced size allows them to achieve a high degree of permeation at the target site through the so-called “enhanced permeability and retention” effect.41 These properties together with their biocompatibility make these capsular containers excellent candidates for biomedical applications. An additional benefit of using silica stems from the extensive

In line with the above, it should also be noted that the electromagnetic field generated by the excitation of the SPR frequency upon external illumination can also be used to perform in situ surface-enhanced Raman spectroscopy (SERS). As known, the near-field coupling between neighboring noble metal nanoparticles results in an enhanced field in their junction that strongly promotes the Raman scattering of the molecules in such locations.38 Therefore, the events occurring in the nanocavity may be simultaneously activated and monitored by this technique.24 Evidently, the possibility of performing an ultrasensitive realtime analytical detection in hollow nanocavities has further implications in areas such as biology and modern medicine. In this respect, protecting the key elements of the sensor in a confined space has many advantages when developing biosensors. Thus, the shell material may endow biocompatibility and target specificity to the nanocapsules, minimizing at the same time unwanted side effects on the organism and preventing sensor degradation. Likewise, the tunable permeability of the membrane can avoid potential interferences stemming from the interaction of third-party biological species with the optical materials. Furthermore, the presence of a void nanospace enables easy diffusion into/out of the shell of the molecules involved.8 1.3. Preliminary Conclusions. Having reached this point, it is clear that a precise tailoring of the shell properties and the C


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knowledge about its colloid chemistry. This great deal of understanding allows a precise modification of both pores and the surface area, which in the end leads to precise control of the storage and release capabilities of silica-based nanocapsules. In this respect, one should take into account that such capabilities are somehow opposing. Thus, chemical modifications of silica may induce changes in its electrostatic and/or hydrophobic interactions with the drug, hence leading to a higher loading capacity but also to a less efficient release. In the same way, weaker interactions may promote a burst release which can be pharmacologically dangerous and economically inefficient. In any case, an appropriate compromise between both factors must be found. Going one step beyond the role of hollow nanocapsules as release-retardant excipients, it is important to highlight the importance of approaches that allow the implementation of remotely triggerable drug-delivery systems based on these structures.42 In this regard, the inclusion of optically or magnetically stimuli-responsive nanoparticles within these nanocarriers may provide pulsatile release profiles mimicking the natural release of bioactive molecules in the body. An ondemand control of dose magnitude and timing should ultimately lead to formulations with an enhanced therapeutic effect and reduced toxicity. 2.2. Nanocatalysis. The application of hollow inorganic nanostructures in the field of catalysis has also aroused growing interest in the last few years. In this respect, the encapsulation ability of these architectures establishes them as excellent reactors because they provide a confined environment where reactions can take place. The mechanisms through which chemical transformations proceed are conceptually similar to that operating in living cells in which biochemical processes take place within compartments that are separate from the cytosol. Examples of this strong analogy have been recently reported in the literature.43 Synthesizing enzyme-loaded hollow silica nanospheres, Chang et al. have performed the intracellular catalyzed conversion of a relatively nontoxic prodrug precursor into an active drug for targeting cancer cells. In this case, the porous silica shell protects the encapsulated enzymes against proteolysis, attenuates the immunological response, and allows the release of the cytotoxin (Figure 4). In this way, the pores of the silica shell allow an easy in/outward access of the small molecules involved while keeping the enzymes in the void space and preventing unwanted protein−protein interactions. The obtained results demonstrate the increasingly closer relationship and potential feedback between drug delivery and catalytic applications when it comes to hollow nanostructures. In line with the above, the implementation of synthesis strategies for incorporating metal nanocatalysts within capsular systems has also propelled the application of these architectures in a vast array of transformations, ranging from simple chemical probes to the synthesis of complex molecules.44 These hollow nanoreactors are aimed at preventing metal nanoparticles from sintering even under harsh reaction conditions. Additionally, they are intended to make more efficient the recovery and reuse of the catalysts during the recycling process by endowing the metal nanoparticles with an isolated environment.45 Thus, the surrounding shell protects the nanocatalyst from particles located in neighboring capsules as well as other molecules present in the bulk solution.2 The modulation of catalytic reactivity for product selectivity represents one of the main concerns for chemists when dealing with competing reactions or seeking the preferential formation

Figure 4. Hollow silica nanospheres (HSN) with large interior spaces and permeable silica shells are suitable for loading enzymes in the cavity to carry out intracellular biocatalysis. TEM images of horseradish peroxidase (HRP)-encapsulated silica nanospheres. (a) HRP@HSN, (b) HRP@SSN (solid silica nanospheres), and (c) HRP@HSN stained with uranyl acetate (UA). UA staining showed enhanced electron density in the cavity of HSN. Scale bars are 50 nm. Adapted with permission from ref 43. Copyright 2014 American Chemical Society.

of one of the possible stereoisomers for a given process. In this respect, the inclusion of noble metal nanoparticles into voided structures may constitute a solution to these challenging tasks. Thus, the tunable features of the shell can be used to regulate the transport pathways of the reactants to the catalytically active surface and hence how the molecules involved approach it (Figure 5).46 This way of inducing selectivity is analogous to that resorted to in the case of homogeneous catalysis where

Figure 5. (a) Catalytic cycle of the Suzuki cross-coupling reaction in the presence of Pd nanoparticles, (b) TEM images of Pd@meso-SiO2 nanoreactors, (c) schematic view of the Suzuki reaction between phenylboronic acid and iodobenzene, and (d) schematic view of the Suzuki reaction between 3-biphenylboronic acid and iodobenzene inhibited by the diffusion barrier. Adapted with permission from ref 46. Copyright 2012 American Chemical Society. D


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activate a Diels−Alder cycloaddition by SPR-induced photothermal heating (Figure 6). With this aim, equimolar amounts

selectivity is often achieved by exploiting the influence of the steric effects exerted by ligands coordinated to the metal active center. In this context, it is worth noting that the homogeneous approach is still considered to be the most powerful resource for modulating catalyzed processes. Therefore, greater efforts should be devoted to implementing synthetic strategies to precisely tailor the shell porosity (size, orientation, and internal functionalization) so as to benefit fully from the potential inherent in heterogeneous nanocatalysts. At this point, one should also consider that some of the benefits attained by using hollow nanoreactors can be equally extended to their core−shell counterparts. Therefore, further emphasis should be placed on demonstrating the advantages of conducting chemical processes in void nanospaces compared to those carried out in solid systems. Inorganic porous capsules display rather low coefficients of thermal expansion and refractive indexes, which allows an effective use of heat and light sources and hence lead to an improved catalytic performance in numerous chemical reactions. In addition, the interstitial cavities of capsules are of critical importance in enhancing the efficiency of many metal nanoparticle-catalyzed organic transformations. This is particularly relevant when dealing with reactions in the diffusioncontrolled regime since a bulk homogeneous environment can be mimicked. Furthermore, the cavity of metal nanoparticlecontaining reactors may also be used for the in situ preparation of other nanostructures through seed-mediated growth.25 This is especially appealing given that the synthesized materials may appear in a surfactant-free state, and hence a high activity should be expected from them in terms of catalytic efficiency. In this regard, it is important to point out the compromise between the protection of metal nanoparticles (either against agglomeration or leaching) and free access for the reactants to the nanoparticle surface. Therefore, synthetic methodologies based on the formation of confined spaces can constitute an ideal way to create synergies bridging the gap between the activity and stability of metal nanoparticles in the field of catalysis. When it comes to speeding up chemical reactions, the inclusion of nanoparticles in hollow nanoreactors also has important implications due to the unique optical properties of these functional cores. Upon plasmonic excitation by irradiation, noble metal nanoparticles dissipate the excess energy through electron−phonon interactions, yielding localized heating in the surroundings of the nanomaterial. Because reaction rates are commonly very dependent on temperature, a global increase in the reaction rate can be promoted through plasmonic heating. The first example of an organic transformation promoted by this effect was reported in 2010.47 In this case, the Au nanoparticle-mediated thermal activation by irradiation with a 532 nm laser afforded a 100% splitting of dicumyl peroxide within 75 laser pulses. Considering the Arrhenius equation and the experimental reaction kinetics studied, the authors estimated the local temperature to be over 500 °C. Since then, plasmonic photothermal conversion has been employed to activate or simply accelerate numerous chemical processes.35,36 Our group has recently demonstrated the versatility of hollow nanoreactors for performing the simultaneous photothermal activation and optical monitoring of a chemical reaction.24 In this work, a mesoporous silica-based hollow nanoreactor decorated with gold nanoparticles attached to the inner wall of the cavity provided an efficient platform to locally

Figure 6. Schematic cross-sectional view of the plasmonic nanoreactor where reactants and products diffuse through the mesoporous silica shell and NIR-laser irradiation promotes the chemical reaction allowing a simultaneous in situ SERS monitoring of the process. Reprinted with permission from ref 24. Copyright 2013 American Chemical Society.

of 2,4-hexadienol (as 1,3-diene) and maleic anhydride (as dienophile) were added to a reaction vessel containing a colloidal dispersion of capsules at room temperature. Upon illumination, reactants in close proximity to the hot nanometallic surface could easily overcome the activation energy for the reaction, boosting the formation of the cycloadduct (4isobenzofuran-carboxylic acid) with an ∼300% increase in the reaction conversion compared to that observed in the absence of irradiation. The reaction proceeded with a minor alteration of the bulk solution temperature due to the insulating properties of the outer silica shell. Furthermore, concurrently with the thermal effect induced by the excitation of the localized SPR inside the reactors, an enhancement of the electromagnetic field was promoted, which could be further exploited for the in situ SERS monitoring of the process. Thus, and to put it another way, the same laser source that was used to promote the chemical reaction was also employed to monitor its progress over time. Because of the surface selectivity and sensitivity of SERS as well as the ease of data acquisition under ambient conditions, the conjunction of porous nanoconfined spaces and SERS spectroscopy represents a unique tool for in situ and real time analysis of chemical transformations, especially adequate to biological systems. Taking a step further, the natural evolution of plasmonicmediated heating would consist of the fabrication of nanoreactors capable of harvesting solar energy and catalyzing chemical reactions in an efficient manner. Alone these lines, Yan et al. have studied the effect of both plasmonic photocatalysis and plasmonic photothermal conversion induced by the illumination of the adequate laser for plasmon excitation.48 Differently from previous catalytic materials based on single-component nanocrystals, the authors focused their study on Au−Pd nanostructures in order to efficiently integrate a strong light-absorbing plasmonic material consisting of Au nanorods and high catalytically active components as Pd nanoparticles. These Au nanorods allow for the synthetic tuning of longitudinal plasmon wavelengths,49 granting the coverage of the entire solar spectrum, while nanosized Pd catalyzes the selected model reaction of Suzuki cross-coupling in a variety of substrates. The results obtained showed a larger contribution of plasmonic photocatalysis when the laser E


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attained functionalities can be preserved due to the notable stability achieved by the metal cores when located in these confined environments. As a result, the fabrication of many different complex nanocomposites has come about in the last few years. Aiming to fabricate advanced functional materials based on this concept, our group has recently developed a strategy to encapsulate highly active Pt nanoparticles (Figure 7).25 Given

irradiation was at the localized plasmon resonance wavelength. This is attributed to the direct electron transfer from the Pd to the adsorbed molecules, fueled by the plasmonic excitationinduced hot electrons, conducted by the gold nanorods. The authors demonstrate the catalytic performance of the designed Au−Pd nanostructures in the harvesting of light energy for chemical reactions, using a mixture of Au−Pd nanostructures with different plasmonic properties to obtain a colloidal lightresponsive solution over the visible to near-infrared region. In the presence of this mixture, the Suzuki reactions were completed within 2 h under sunlight, whereas almost no reactions occurred in the dark. Unfortunately, together with the important results referring to the efficient utilization of light energy, an extensive leaching of Pd was detected, directly affecting the recyclability of the catalytic system. An improved design, by using confined spaces holding analogous Au−Pd nanostructures, could overcome this limitation and further extend the use of integrated metal nanocatalysts. As mentioned, capsular nanostructures are capable of stabilizing nanoparticles through an efficient immobilization on their inner walls. In this way, the shell also blocks possible interactions between particles located in neighboring capsules, which is of particular importance to the improvement of the recycling process. As an example of confined catalysis performed by the combination of different metals, El-Sayed et al. have recently reported the design of a nanorattle-like system consisting of a hollow palladium shell hosting a solid gold nanoparticle in its interior.50 Following the same principle described above, the internal gold nanosphere was capable of generating photothermal heat upon plasmonic excitation during the catalytic reaction. As a consequence, an effect of the plasmon excitation on the catalytic efficiency of the Pd was observed. Thus, a 3fold enhancement of the observed rate constant for the reduction reaction of 4-nitrophenol by borohydride was attained during exposure to a moderate light intensity of 5 W/cm2 and wavelength in the range of 530−700 nm. The catalytic effect was shown to be dependent on the relative size of the gold nanosphere with respect to the hollow palladium nanocage. The authors have also demonstrated the importance of gold confinement for the catalytic performance of the rattle because no enhancement of the model reaction was detected when the gold nanoparticles were located outside the palladium shell. In line with the above results, it seems evident that performing chemical reactions in voided nanostructures containing noble metal nanoparticles may offer numerous advantages. Thus, the inclusion of plasmonic nanoparticles can be used in order to thermally activate different organic transformations. Moreover, a properly configured nanoreactor can allow for in situ and real-time monitoring of the chemical or physical events taking place in the confined nanospace. Furthermore, new features and functionalities to this already advanced architecture can be attained by adding catalytic nanoparticles inside the capsules. Ultimately, a broad range of possibilities will arise from adapting these nanoreactors within an each specific context. 2.3. Nanofabrication. In the recent past, the search for strategies to develop metal nanoparticles with precise size and shape control has propelled the use of hollow inorganic nanostructures in different synthetic methodologies. As mentioned, the encapsulation abilities of these architectures establish them as excellent reactors where the preparation of novel nanomaterials can be performed. In the same way, the

Figure 7. Synthesis of Ni/NiO magnetic nanostructures in the cavity of a Pt-seeded nanoreactor. (a) Hybrid particles composed of dendritic Pt nanoparticles deposited onto polystyrene colloidal templates. (b) Capsules obtained after silica coating and polymer dissolution. (c) Formation of Ni-based nanomaterials inside the silica capsule by means of the Pt nanoparticle-catalyzed Ni2+ reduction by hydrazine. Reprinted with permission from ref 25. Copyright 2012 Wiley.

their outstanding catalytic properties, Pt nanoparticles can act as a seed or nucleation point upon which the growth of additional materials may be initiated. In this case, the deposition of dendritic Pt nanoparticles onto polystyrene beads (PS) followed by a directed silica coating was performed so as to afford the corresponding PS@Pt d NPs@SiO 2 composites. Thereafter, the dissolution of the sacrificial template to yield the final void@PtdNPs@SiO2 nanostructure was carried out. Thus, a hollow architecture where the Ptd nanoparticles remain attached to the inner wall of the silica capsule was obtained. At this point, the couple ferri/ ferrocyanide was employed as a redox probe in order to assess the catalytic activity of the encapsulated Ptd nanoparticles in a NaBH4 aqueous solution. In this case, a remarkable reduction of the activation energy compared to that of the noncatalyzed reaction was attained (from 30 to 12.3 ± 0.8 kJ mol−1). It should be noted that this evaluation was additionally performed before the emptying process, achieving a similar value for the energy barrier (12.6 ± 1.2 kJmol−1). In view of these results, it is clear that the catalytic activity stemming from the specific morphology of the Pt nanoparticles is fully preserved along the fabrication of the capsule. Taking advantage of such catalytic efficiency, we explored the formation of additional nanomaterials inside the nanoreactor following a seed-mediated approach. Accordingly, metallic nickel deposition onto the preformed Pt nanoparticles was conducted using nickelhydrazine complexes as a precursor. In this case, the confined F


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Figure 8. (a) Cross-sectional image of the interior of a hollow mesoporous silica capsule with inner gold nanoparticles and XEDS mappings showing the elemental distribution of gold (red) and silica (green). (b) SERS spectra of the capsules as a function of NO concentration. Spectra of paminobenzenethiol, ABT (blue), and hydroxybenzenethiol, HBT (red), are shown for comparison. Thin traces in between represent the resulting spectra with growing concentrations of NO. (c) Optical images and intracellular NO formation over time (obtained through the I1583/(I1583 + I1548) relation) for three different samples upon NO induction with oxygen peroxide (H2O2). A control sample without the presence of H2O2 is also shown for comparison. Representative normalized SERS spectra obtained at different times are shown. The SERS dashed (blue) and dotted (red) spectra represent the reference vibrational patterns for ABT and HBT, respectively. Adapted with permission from ref 8. Copyright 2013 Wiley.

this case, a Fe3O4@silica composite with surrounding satellite nanocrystals of Au and Pd was selected as the starting point. After the reductive dissolution of the iron oxide core, a nanorattle-like structure was generated. This yolk−shell nanoparticle consisted of a 21 ± 2 nm cavity with a tiny encaged Au/Pd-heterodimer nanocrystal of approx. 2 nm. This configuration was demonstrated to be rather effective in catalyzing the reductive growth of Ni nanocrystals but also alternative materials such as Ni/Co-based alloys. Likewise, the synthesized Ni nanocrystals exhibited active performance and good recyclability in catalyzing hydrogen generation from NH3BH3 and the chemoselective reduction of nitroarenes in aqueous solutions at room temperature. All of the above results seem to extend the applicability of hollow inorganic nanostructures hosting single or multiple metal cores in their cavities. Thus, the metal nanoparticles initially introduced into the void may either play a primary role in the functionality of the hollow nanostructure or alternatively may act as a resort to activate or promote the formation of new materials with different functionalities. In any case, the application of a seed-mediated approach in voided architectures affords precise spatial control over nanoparticle growth. Ultimately, this high degree of control allows us to exploit the many benefits arising from preventing and/or modulating the direct contact of the metal nanoparticles with the bulk solution. 2.4. Sensing. In order to further explore the functionalities of hollow nanostructures toward detection, we have reported their application in designing efficient SERS-encoded platforms amenable to their use as tags in multiplexing and highthroughput screenings.9 Taking advantage of the collective plasmon properties of the metallic nanoparticles located on the inner wall of these nanocapsules, we envisioned the

Pt nanoparticles promote the decomposition of free hydrazine, giving rise to negatively charged particles that reduce the nickelhydrazine complexes to afford metallic nickel.51,52 This step is followed by an autocatalytic nickel reduction. In the end, this strategy leads to the selective incorporation of metallic Ni in the interior of the capsules. The tunable magnetic properties of these Ni-based composites allow their facile external manipulation by a magnetic field. In addition, the incorporation of this metal into the capsule widens the panel of chemical transformations to catalyze. This approach may pave the way for the synthesis of hollow nanostructures incorporating additional functionalities inside them, which would otherwise be challenging. Through an analogous approach, Wang et al. have developed a method for the preparation of bimetallic core−shell nanostructures with high catalytic activity through so-called “ship-in-a-bottle” growth.53 In this case, their methodology consists of the use of small Au nanoparticles preintroduced in a hollow mesoporous silica microsphere. These Au nanoparticles act as seeds for the confined growth of Au nanorods with synthetically tunable longitudinal plasmon wavelengths. At this point, the resulting Au nanocrystals can be coated with either Pt or Pd affording bimetallic core−shell nanoparticles. The catalytic performance of this complex nanostructure was assessed through the oxidation of o-phenylene-diamine by hydrogen peroxide to form 2,3-diaminophenazine. The obtained results showed a significantly enhanced reaction rate by the bimetallic Pt/Au catalyst. More importantly, superior stability was noted compared to that of the corresponding unencapsulated nanostructures. Most recently, another example of a seed-engineered strategy was adopted for the preparation of Ni nanocrystals by using hollow nanoreactors with Au/Pt seeds in their inner cavity.54 In G


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problems of aggregation are avoided, preserving stable readouts of the SERS signal over time. This feature, together with the constant biodistribution over time and good signal-to-noise ratio, makes these hollow plasmonic structures suitable for quantitative localized sensing. Zhang et al. have also recently reported the preparation of Au@SnO2 yolk−shell nanospheres and their use as a new sensing material for CO gas detection.56 Thus, the detector was constructed by coating a ceramic tube with these nanoparticles, previously equipped with a pair of gold electrodes. An additional Ni−Cr heating wire was incorporated as a resistor into the design to provide the required operational temperature. The readout of the system was based on measurements of the export voltage of the sensor, showing a correlation of the responses with the increase in CO concentrations (5−100 ppm). In comparison to the pure SnO2 capsules, the performance of Au@SnO2 structures showed improved sensing properties, such as a lower operating temperature, lower detection limit, faster response, and better selectivity. This improvement could be attributed to a synergetic effect of Au and SnO2 to provide active surfaces for CO oxidation.

incorporation of differentiated aromatic compounds on the metallic surface and their use as SERS molecular probes. Thus, after synthesizing a gold-seeded silica nanocapsule, an extensive growth of Au nanoparticles in the void nanospace was crucial to the enhancement of the Raman signal. Then, three aromatic thiols (benzenethiol, 4-nitrobenzenethiol, and 4-hydroxybenzenethiol) were chemisorbed onto the gold surfaces, giving rise to the corresponding spectral fingerprints. Importantly, the shielded nature of these SERS tags prevents their leaching in the further manipulation of the sample, especially in washings and centrifugation steps. An additional advantage of these materials is the versatility of their outer surface for subsequent functionalization, as, for example, the conjugation to antibodies. When this functionalization was correlated to a specific SERS code, this strategy has allowed the nanocapsule recognition of a specific antigen and direct read out by SERS spectroscopy. The accurate identification of the active particles and the short time required for the deconvolution of images (less than 1 min) demonstrate the high potential of these systems for highthroughput biomedical screening applications. Similarly, Lee et al. have recently reported the effective development of SERS-nanoprobes for in vivo targeting.55 With the aim to provide new tools for multiplex bioimaging and in vivo detection, the authors have developed NIR-sensitive nanocapsules, where endogenous tissue absorption coefficients are much lower in comparison to those for UV and visible light. By preparing plasmonic Au/Ag hollow-shelled assemblies via galvanic replacement, the extinction bands of the Au/Ag nanoparticles were tuned to the NIR optical window. These optical receptors were deposited onto silica nanospheres, and the yielded assemblies were protected with silica. The signals from NIR SERS dots were evaluated in animal tissues, using 4fluorobenzenethiol as a labeling compound. These probes were detectable from an 8 mm depth and the SERS signal was strong enough for each probe particle to be recognized. Additionally, three kinds of NIR SERS dots, also injected deep into live animal tissues, produced strong SERS signals, demonstrating their potential for in vivo multiplex detection in the same area. On the basis of SERS spectroscopy and hollow nanocomposites, we have recently developed a new design for intracellular sensors for the quantitative detection of small analytes, such as nitric oxide (Figure 8).8 These sensors comprise a mesoporous silica coating with an inner gold island film functionalized with a chemoreceptor for NO. The shell allows the exchange of the lysosomal solution in and out of the capsule by diffusion. Additionally, it provides stability, biocompatibility, and heat-insulating properties and blocks the interaction of the plasmonic material with proteins, enzymes, and other macromolecules. The molecular probe selected, p-aminobenzenethiol (ABT), covalently binds to gold, and the amino group undergoes a diazotization reaction with equimolar concentrations of NO at acidic pH. Because of illumination with NIR light, strong heat is generated, and the diazonium salt formed spontaneously degrades to the corresponding phenol. The changes in SERS spectra caused by this chemical transformation allow the monitoring of NO by correlating the relative intensity of the two characteristic ring stretching bands from the initial ABT and the final phenol. Intracellular experiments were performed in 3T3 embryonic fibroblasts. Upon internalization, the nanoparticles accumulate inside the lysosomes, where the acidic pH (approx. 4) favors the diazotization reaction. Because of the protected design of the plasmonic material inside the hollow nanostructures, crucial

3. PERSPECTIVES In this section, we will discuss some of the recent trends and future perspectives toward the use of hollow inorganic nanostructures in different areas of science. In that respect, we will place particular emphasis on those cases where the existence of a voided nanospace may allow us to encapsulate and further reinforce the applications of the functional materials located therein. Recently, nanosized supramolecular spheres obtained from the assembly of bifunctional building blocks have been reported to turn inactive AuCl complexes into efficient catalysts.57 This is attributed to the high local concentration of metal complex achieved within these structures, which in the end leads to the necessary preorganization of the catalysts. Thus, whereas the overall metal complex concentration remains on the conventional micro to millimolar level, a local concentration ranging from 0.05 to 1.1 M can be attained in the spheres. In light of these results, one may infer that a similar approach could be exploited when dealing with inorganic hollow nanostructures. Thus, silica nanocapsules could be engineered for the accumulation of catalysts, giving rise to high local concentrations as in the classical colloidal approach.58 In this way, cooperative effects aroused from the high degree of contact between the metal complexes could be addressed. This strategy would also allow us to overcome intrinsic limitations regarding solubility and the catalyst/substrate ratio. In addition, the efficacy of the catalytic materials could be further adjusted by making use of the benefits of the silica nanocapsules on the grounds of thermal and chemical stability as well as shell porosity. Further research efforts should also be devoted to the use of the inner cavity of the capsule for performing simultaneous and complementary functions. As previously mentioned, the attachment of plasmonic nanoparticles to the inner walls of a silica capsule may be employed in order to exploit the interactions between neighboring particles. Along these lines, the coupling of plasmonic structures is ideally suited to concentrating light on the nanoscale either for heat generation or sensing purposes. In the first case, the ability of reverse bumpy balls to act as remotely controllable nanosources of heat makes them excellent candidates for carrying out chemical H


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reactions in temperature-sensitive media such as living cells. In the latter case, the functionalization of plasmonic metal nanoparticles with molecules endowed with sensing capabilities for specific analytes may allow the use of SERS for the ultralow detection of different species given the remarkable enhancement of the Raman signal under these conditions. At this point, it is easy to perceive that both approaches can be combined in such a way that for a given process the thermal activation and the monitoring of the chemical or physical events taking place upon external illumination could be carried out simultaneously. In addition, the encapsulation of catalytic nanoparticles may be exploited in the field of organic reactivity to carry out one-pot procedures involving two or more functionally distinct and noninterfering catalysts or precatalysts. Likewise, the inclusion of such nanoparticles in reverse bumpy ball configurations may be further employed for carrying out size-sieving catalysis by taking advantage of the easily tailored pore structure. A great potential of these nanocapsules is also envisioned in the field of biomedicine. In this area, the development of novel intracellular applications based on the sensing and/or catalytic properties of the hosted materials is particularly relevant. The implementation of architectures for on-demand drug delivery that allow a remote, repeatable, and precise switching of the drug flux through noninvasive stimuli is also of critical importance. In view of all of the above, the incorporation of nanomaterials with different properties could further expand the array of opportunities provided by nanoparticle-containing hollow nanostructures. Thus, the combination of magnetic, semiconductor, catalytic, and plasmonic nanoparticles into the same cavity may offer a wide variety of scientific and technological possibilities. Along these lines, further improvements in the design and synthesis of these nanostructures are expected in the near future.

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Verónica Salgueiriño received her Ph.D. degree in chemistry from Universidade de Vigo (Spain) in 2003. Then, she worked as a postdoctoral researcher at Universität Duisburg-Essen (Germany), Arizona State University, and Universidade de Santiago de Compostela (Spain) until joining the Department of Applied Physics at Universidade de Vigo. Her current research (http://webs.uvigo.es/ magneticmaterials) focuses on the development and physical and chemical characterization of new magnetic nanoparticles and nanocomposites, with particular interest in interfaces between transitionmetal oxides.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

Moisés Pérez-Lorenzo obtained his Ph.D. in chemistry from Universidade de Vigo (Spain) in 2004. After postdoctoral stays at the University of California at Santa Cruz and Universidade de Santiago de Compostela (Spain), he joined the Department of Physical Chemistry of Universidade de Vigo. His current research interest focuses on the design of plasmonic nanoreactors and their catalytic applications in different organic transformations.

Biographies

Belén Vaz obtained her Ph.D. in chemistry from Universidade de Vigo (Spain) in 2004. After a postdoctoral stay at the Max-Planck-Institut für Molekulare Physiologie (Germany), she joined the Department of Organic Chemistry of Universidade de Vigo. Among her current research interests are the incorporation of small molecules into nanostructured materials for the design of SERS-based sensors as well as the development of plasmonic nanoreactors.

Miguel A. Correa-Duarte received his Ph.D. degree in chemistry at Universidade de Vigo (Spain) in 2002. After that, he held postdoctoral I


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(14) Chen, B.; Zhu, Z.; Liu, S.; Hong, J.; Ma, J.; Qiu, Y.; Chen, J. Facile Hydrothermal Synthesis of Nanostructured Hollow Iron− Cerium Alkoxides and Their Superior Arsenic Adsorption Performance. ACS Appl. Mater. Interfaces 2014, 6, 14016−14025. (15) Dui, J.; Zhu, G.; Zhou, S. Facile and Economical Synthesis of Large Hollow Ferrites and Their Applications in Adsorption for as(V) and Cr(VI). ACS Appl. Mater. Interfaces 2013, 5, 10081−10089. (16) Hu, J.; Chen, M.; Fang, X.; Wu, L. Fabrication and Application of Inorganic Hollow Spheres. Chem. Soc. Rev. 2011, 40, 5472−5491. (17) Liu, J.; Liu, F.; Gao, K.; Wu, J.; Xue, D. Recent Developments in the Chemical Synthesis of Inorganic Porous Capsules. J. Mater. Chem. 2009, 19, 6073−6084. (18) Priebe, M.; Fromm, K. M. Nanorattles or Yolk−Shell Nanoparticles−What Are They, How Are They Made, and What Are They Good For? Chem.Eur. J. 2015, n/a−n/a. (19) Li, G.; Tang, Z. Noble Metal Nanoparticle@Metal Oxide Core/ Yolk-Shell Nanostructures as Catalysts: Recent Progress and Perspective. Nanoscale 2014, 6, 3995−4011. (20) Liu, J.; Yang, H. Q.; Kleitz, F.; Chen, Z. G.; Yang, T.; Strounina, E.; Lu, G. Q.; Qiao, S. Z. Yolk−Shell Hybrid Materials with a Periodic Mesoporous Organosilica Shell: Ideal Nanoreactors for Selective Alcohol Oxidation. Adv. Funct. Mater. 2012, 22, 591−599. (21) Wang, X.; Caruso, R. A. Enhancing Photocatalytic Activity of Titania Materials by Using Porous Structures and the Addition of Gold Nanoparticles. J. Mater. Chem. 2011, 21, 20−28. (22) Wan, Y.; Zhao, D. On the Controllable Soft-Templating Approach to Mesoporous Silicates. Chem. Rev. 2007, 107, 2821−2860. (23) Zhang, Y.; Hsu, B. Y. W.; Ren, C.; Li, X.; Wang, J. Silica-Based Nanocapsules: Synthesis, Structure Control and Biomedical Applications. Chem. Soc. Rev. 2015, 44, 315−335. (24) Vázquez-Vázquez, C.; Vaz, B.; Giannini, V.; Pérez-Lorenzo, M.; Alvarez-Puebla, R. A.; Correa-Duarte, M. A. Nanoreactors for Simultaneous Remote Thermal Activation and Optical Monitoring of Chemical Reactions. J. Am. Chem. Soc. 2013, 135, 13616−13619. (25) Sanlés-Sobrido, M.; Pérez-Lorenzo, M.; Rodríguez-González, B.; Salgueiriño, V.; Correa-Duarte, M. A. Highly Active Nanoreactors: Nanomaterial Encapsulation Based on Confined Catalysis. Angew. Chem., Int. Ed. 2012, 51, 3877−3882. (26) Jain, P. K.; El-Sayed, M. A. Plasmonic Coupling in Noble Metal Nanostructures. Chem. Phys. Lett. 2010, 487, 153−164. (27) Baffou, G.; Quidant, R. Thermo-Plasmonics: Using Metallic Nanostructures as Nano-Sources of Heat. Laser Photon. Rev. 2013, 7, 171−187. (28) Sanchot, A.; Baffou, G.; Marty, R.; Arbouet, A.; Quidant, R.; Girard, C.; Dujardin, E. Plasmonic Nanoparticle Networks for Light and Heat Concentration. ACS Nano 2012, 6, 3434−3440. (29) Dormann, J. L.; Fiorani, D.; Tronc, E. Magnetic Relaxation in Fine-Particle Systems. Advances in Chemical Physics; John Wiley & Sons, 1997; pp 283−494. (30) Hergt, R.; Dutz, S. Magnetic Particle Hyperthermia−biophysical Limitations of a Visionary Tumour Therapy. J. Magn. Magn. Mater. 2007, 311, 187−192. (31) Nogués, J.; Schuller, I. K. Exchange Bias. J. Magn. Magn. Mater. 1999, 192, 203−232. (32) Fontaíña Troitiño, N.; Rivas-Murias, B.; Rodríguez-González, B.; Salgueiriño, V. Exchange Bias Effect in CoO@Fe3O4 Core−Shell Octahedron-Shaped Nanoparticles. Chem. Mater. 2014, 26, 5566− 5575. (33) Lee, J.-H.; Jang, J.; Choi, J.; Moon, S. H.; Noh, S.; Kim, J.; Kim, J.-G.; Kim, I.-S.; Park, K. I.; Cheon, J. Exchange-Coupled Magnetic Nanoparticles for Efficient Heat Induction. Nat. Nanotechnol. 2011, 6, 418−422. (34) Otero-Lorenzo, R.; Weber, M. C.; Thomas, P. A.; Kreisel, J.; Salgueiriño, V. Interplay of Chemical Structure and Magnetic Order Coupling at the Interface between Cr2O3 and Fe3O4 in Hybrid Nanocomposites. Phys. Chem. Chem. Phys. 2014, 16, 22337−22342. (35) Baffou, G.; Quidant, R. Nanoplasmonics for Chemistry. Chem. Soc. Rev. 2014, 43, 3898−3907.

positions at the Center of Advanced European Studies and Research, CAESAR (Germany), and Arizona State University. He is currently an associate professor in the Department of Physical Chemistry of Universidade de Vigo, where he leads TeamNanoTech (www. teamnanotech.com), a recently established research group engaged in the design and synthesis of nanomaterials toward the development of novel applications as well as their implementation in devices. His current interests include carbon nanotube functionalization, core−shell nanoparticles, and multifunctional nanocomposites.

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ACKNOWLEDGMENTS This work was funded by Xunta de Galicia (INBIOMEDFEDER “unha maneira de facer Europa” and EM2014/035), Fundación Ramón Areces, Fundación Tatiana Pérez de Guzmán el Bueno, and the European Union Seventh Framework Program (FP7/REGPOT-2012-2013.1) under grant agreement no. 316265, BIOCAPS.

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The Case of Inorganic Submicrometer Capsules - American Chemical

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