The Many Ways to Assemble Nanoparticles Using Light · composition of the NPs’ inorganic core and...

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Progress rePort

The Many Ways to Assemble Nanoparticles Using Light

Tong Bian, Zonglin Chu, and Rafal Klajn*

DOI: 10.1002/adma.201905866

modulating or even “turning on” and “off” the above and other properties at will. Over the past two decades, various ways to reversibly assemble NPs have been developed.[28] Depending on the chemical composition of the NPs’ inorganic core and on the organic molecules attached to their surfaces, self-assembly can be con-trolled by metal ions,[29] redox agents,[30,31] oligonucleotides,[32] light,[33,34] magnetic fields,[35–37] and so on.

Among these external stimuli, light is arguably the most attractive. Light is usually a noninvasive stimulus that can be delivered to closed systems, with high spatial and temporal resolution. Further-

more, it can be supplied in the form of different wavelengths, to which different components of the system can poten-tially respond with high specificity.[38–40] Controlling the self-assembly of NPs using light has traditionally been achieved by functionalizing their surfaces with monolayers of photo-switchable molecules. Depending on the functional groups pre-sent in the molecule, the same photoswitchable moiety can be placed on NPs having a variety of inorganic cores. For example, light-responsive gold, silica, and iron oxide NPs can be pre-pared by decorating them with azobenzenes appended with thiol, silane, and catechol “anchors”, respectively.[41] However, functionalizing NPs with light-responsive molecules is not the only method to obtain photoresponsive NPs—quite the oppo-site; the recent several years have seen the development of a variety of creative approaches to control NP self-assembly using light. These approaches include light-induced reversible cova-lent bond formation, protonating and deprotonating NP-bound ligands using small-molecule photoacids and photobases, light-controlled deposition and desorption of photoswitchable mole-cules onto NPs, inducing phase transitions of NP-bound ther-moresponsive polymers by plasmonic NPs, and light-induced electron transfer between the NP core and the ligands. In this progress report, this remarkable diversity of approaches to con-trol the self-assembly of NPs with light is critically reviewed.

2. Nanoparticles Surface-Functionalized with Photoswitchable Molecules

2.1. Azobenzene-Functionalized Nanoparticles

The pioneering example of reversible, light-induced aggrega-tion of NPs dates to 2003, when Tamada and co-workers inves-tigated strategies to improve the efficiency of azobenzene (AB) switching on nanoparticuled gold.[42] Upon exposure to low-intensity (≈1 mW cm−2) ultraviolet (UV) light, trans-AB rapidly

The ability to reversibly assemble nanoparticles using light is both fundamen-tally interesting and important for applications ranging from reversible data storage to controlled drug delivery. Here, the diverse approaches that have so far been developed to control the self-assembly of nanoparticles using light are reviewed and compared. These approaches include functionalizing nanoparticles with monolayers of photoresponsive molecules, placing them in photoresponsive media capable of reversibly protonating the particles under light, and decorating plasmonic nanoparticles with thermoresponsive polymers, to name just a few. The applicability of these methods to larger, micrometer-sized particles is also discussed. Finally, several perspectives on further developments in the field are offered.

Dr. T. Bian, Dr. Z. Chu, Prof. R. KlajnDepartment of Organic ChemistryWeizmann Institute of ScienceRehovot 76100, IsraelE-mail: [email protected]

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201905866.

1. Introduction

Photoswitchable molecules can be reversibly isomerized between two or more states upon exposure to different wave-lengths of light. For example, azobenzenes and spiropyrans—probably the most commonly studied photoswitches—can be converted from a nonpolar to a polar form upon exposure to UV light, with the reverse reaction proceeding under visible light.[1,2] These reversible isomerization processes are often accompanied by a change in the properties, e.g., electrical conductivity,[3–5] color,[6] fluorescence,[7,8] or the ability to bind metal ions.[9,10] However, there is a long way from individual molecules isomerizing in solution to robust, functional light-responsive materials. An important step in fabricating mate-rials based on photoswitchable molecules is their integration within larger entities,[11–13] such as planar surfaces, nanoporous solids,[14,15] polymer chains,[16–19] and inorganic nanoparti-cles (NPs). NPs, in particular, represent an attractive means of support because of the many interesting physicochemical properties that arise in the quantum-size regime.[20] Impor-tantly, many of these properties are strongly dependent on and can be adjusted by the separation between the NPs. In fact, the optical,[21,22] electronic,[23,24] magnetic,[25] and electric field enhancement[26,27] properties of NPs have all been successfully manipulated by tuning the NP–NP distance within the aggre-gates. The ability to control interparticle spacing in a revers-ible fashion is particularly interesting since it can be used for

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isomerizes to the cis isomer (Figure 1a), which is significantly more bulky and thus requires a certain degree of conforma-tional freedom to form.[43] Tamada and co-workers showed that when adsorbed on 5.2 nm NPs as a mixed disulfide (equivalent to a 50% dilution of a thiolated AB with a shorter, “background” thiol), AB photoisomerizes following first-order kinetics with a rate constant identical to that of free AB in solution, indicating no steric hindrance from the nearby molecules on the same NP. The cis isomer of AB is metastable; it spontaneously back-isomerizes to the trans form (typically within several days) in a reaction that can be accelerated (to seconds) with blue light (Figure 1a). Similar to the forward reaction, the rate of the cis → trans back-isomerization of these NPs did not deviate from that of the solution kinetics.

Although these studies were largely motivated by opti-mizing AB switching on NPs, the authors also noted that upon UV irradiation, the NPs precipitated from their solution in toluene.[42] This precipitation can be explained by the polar nature of cis-AB, i) which the nonpolar solvent molecules are unable to solvate efficiently and ii) which can interact with cis-AB residues on other NPs by means of dipole–dipole inter-actions, thus uniting the NPs. Interestingly, efficient photo-isomerization of AB was reported even earlier[44] on smaller, 2.5 nm Au NPs. These experiments, however, were carried out in dichloromethane, which can efficiently solvate both isomers of AB and can screen the dipolar interactions between them; consequently, no aggregation of NPs was observed.

Azobenzenes are ideally suited to guide the reversible self-assembly of NPs (Figure 1b). First, they are structur-ally simple and chemically robust and can therefore readily be synthesized in the form of different derivatives and are compatible with many functional groups commonly used as anchors for various inorganic NPs. Second, as a result of their chemical stability, ABs can be reversibly switched between the trans and the cis isomers for hundreds of cycles without appreciable fatigue. Third, the wavelengths inducing photo-isomerization can be tuned by varying the substitution pattern on the AB core; for example, appending AB with a dimeth-ylamino group at the para position shifts the wavelength required for trans → cis isomerization from the near-UV to the blue region (420 nm).[45] Fourth, the trans → cis conver-sion is typically accompanied by a large increase in the dipole moment (e.g., 1.2 vs 4.9 D for p-alkoxyazobenzenes[46]) and a substantial change in the molecular structure (planar and elongated trans vs more three-dimensional cis), which can be utilized to control self-assembly.

Prolonged UV irradiation of AB-decorated NPs in nonpolar solvents typically leads to unstructured aggregates with increasing sizes, which gradually precipitate from the solution.[42] However, fine-tuning interparticle interactions can result in well-defined, colloidally stable aggregates. For example, the presence of the positively charged surfactant didodecyldimethylammonium bromide (DDAB) during UV irradiation results in the formation of well-defined, spherical aggregates (“supraspheres”; Figure 1c).[34] Owing to its two long alkyl chains, DDAB readily intercalates between NP-bound ligands, endowing NPs and their aggregates with a positive charge. The gradual accumulation of charge with increasing aggregate size suppresses the attachment of additional NPs due

Tong Bian is currently a postdoctoral researcher working in the Klajn group at the Weizmann Institute of Science. He received his B.S. degree in chemistry from Nanjing University (2009) and his Ph.D. degree in physical chemistry from the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences (2014), under

the supervision of Prof. Tierui Zhang. His current research focuses on tailoring the surface chemistry of nanoparticles for controlling their self-assembly and fabricating new stimuli-responsive materials.

Zonglin Chu completed his Ph.D. degree in chemical engineering at Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, in 2011. His postdoctoral research at ETH Zürich and the University of Zürich focused on organic synthesis and super-repellent surfaces, respectively. He is currently working on nanoparticle

assemblies in the Klajn group at the Weizmann Institute. His research interests lie at the interface of organic synthesis, colloid and interface science, smart materials, and functional nanomaterials and surfaces.

Rafal Klajn completed his Ph.D. degree in chemical and biological engineering at Northwestern University in 2009 and joined the Department of Organic Chemistry at the Weizmann Institute of Science, where he is currently an associate professor. The interests of his research group revolve around nanoscale self-

assembly and reactivity as well as new stimuli-responsive nanomaterials.

to Coulombic repulsion, leading to a batch of aggregates with uniform sizes (self-limited growth; Figure 1c, right).

Irrespective of their structure, these aggregates are meta-stable and disintegrate when the exposure to UV is discon-tinued. This disassembly process takes several days in the dark and hours under ambient light conditions, but it occurs rapidly

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(within tens of seconds) upon exposure to visible (blue) light. Interestingly, the aggregation of plasmonic (e.g., Au and Ag) NPs is accompanied by pronounced changes in the color of the solution due to interparticle coupling of localized surface plasmons. For example, wine-red solutions of AB-coated Au NPs can turn either purple or blue, depending on the extent of coupling. Once UV irradiation ceases, the initial red color gradually returns—a finding that led to the development of multicolor rewritable paper[47] (see below).

Importantly, the cis-AB moieties need to be exposed to the nonpolar solution for NP self-assembly to occur. Temps and co-workers prepared a series of 4.0 nm gold NPs cofunction-alized with thiolated ABs and background alkanethiols with increasing chain lengths.[48] It was found that a short AB within a matrix of a long background thiol photoisomerized efficiently, but without promoting NP self-assembly. This result can be understood by i) the inability of cis-AB units on different NPs to come into close contact with each other and/or ii) the efficient solvation of the NPs, whose peripheries are decorated with alkyl chains both before and after irradiation.

In the above systems, self-assembly begins as a result of the colloidal instability of cis-AB-coated NPs in nonpolar environ-ments. In more polar media, the opposite can be expected, i.e., efficient solvation of NPs decorated with cis-AB groups versus attractive interactions between trans-AB-coated NPs resulting in their aggregation. One such system has recently been reported by Zhou and co-workers, who prepared atomically precise metal–organic NPs, each decorated with 24 AB moieties.[49]

These NPs could be suspended in chloroform under UV illumi-nation (i.e., metastable state); in the dark, the system gradually returns to its ground (i.e., assembled) state, within which the NPs interact by π–π stacking interactions.[49]

Another example whereby disassembled NPs correspond to the metastable (i.e., cis) state of the system was reported by Samorì and co-workers, who worked with 25 nm gold NPs deco-rated with a thiolated azobiphenyl.[50] Adding the extra phenyl ring on each side of the AB moiety greatly enhanced the π–π stacking interactions between trans-ABs, and the NPs spontane-ously assembled, even in toluene. However, efficient stacking was not possible between the bent cis groups; consequently, the aggregates disassembled under UV light.[50] Ginger and co-workers developed another system in which NP self-assembly can be induced with blue light.[51] Here, 15 nm Au NPs were functionalized with thiolated oligonucleotides, rendering them water-soluble. These NPs could be coassembled with similarly sized NPs hosting oligonucleotides with a complementary DNA sequence, throughout which four AB pendant groups were incor-porated. Owing to its flat structure, the trans isomer of AB can readily intercalate between base pairs of the double helix. In con-trast, the more bulky cis form disrupts the interaction between the two oligonucleotides, thus inducing DNA dehybridization and disassembly of NPs (the particles could subsequently be reassembled upon exposure to blue light). An attractive feature of this system—despite its complexity and the long times needed to induce assembly (e.g., 2 h of blue light)—is that it operates in water and thus could be interfaced with biological media.

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Figure 1. Light-induced self-assembly of azobenzene-functionalized nanoparticles. a) Reversible, light-induced isomerization between trans and cis isomers of azobenzene (AB). b) Schematic illustration of azobenzene switching on the surface of a nanoparticle (NP). Note that although the photoisomerization yield is not quantitative (sometimes far from it[45]), enough cis-ABs are generated to induce self-assembly in nonpolar solvents. c) Scanning electron microscopy (SEM) images of an individual (left) and multiple (right) spherical aggregates (“supraspheres”) obtained by exposing AB-functionalized 5 nm Au NPs in toluene to UV light. d) Schematic illustration of a write/self-erase cycle in an NP-doped hydrophobic gel (top) and an example of an image created with metastable NP aggregates as the “ink” (bottom). d) Reproduced with permission.[47] Copyright 2009, Wiley-VCH. e) Left: Transmission electron microscopy (TEM) image of a core–shell assembly of large silica and small iron oxide NPs generated using UV light. Right: Schematic illustration of strong magnetic interactions between core–shell assemblies giving rise to elongated structures. Reproduced with permission.[55] Copyright 2012, American Chemical Society.



NPs whose assembly can be controlled by light in a revers-ible fashion can find interesting applications. In one example, AB-coated gold NPs were incorporated within thin films of non-polar poly(methyl methacrylate) (PMMA) gels, staining them intense red.[47] Irradiation of these gels induced photoisomeri-zation and triggered NP self-assembly—but only in those regions exposed to UV, thus giving rise to high-contrast images (e.g., Figure 1d). Similar to NP aggregates, these images were metastable: they gradually (within hours) “self-erased”, after which a new pattern could be created in the same gel. These self-erasing materials have two major advantages compared to rewritable media based on photochromic molecules. First, the color of the gel depends on the degree of aggregation of the NPs and can be tuned from red to purple to blue for Au-based gels and from yellow to orange to red to purple for gels based on AB-functionalized silver NPs. Therefore, a wide spectrum of colors is achievable with a single type of NP “ink” and multi-color images can be created by varying the irradiation dose over different regions of the same film.[47] Second, it is possible to control the lifetimes of the metastable patterns by adjusting the surface concentration of AB: the more the AB, the longer the patterns persist.

In addition to the color of the sample, NP self-assembly can affect the chemical reactivity of other species present in the system. Wei et al. explored the catalytic performance of 5.5 nm gold NPs cofunctionalized with dodecylamine (DDA) and a small amount of thiolated AB. As a weakly bound ligand, DDA does not nullify the catalytic properties of the NPs, which effi-ciently catalyzed the addition of diphenylsilane to p-methoxy-benzaldehyde in toluene.[52] Under UV light, however, the NPs assembled into spherical aggregates (several hundred nm in diameter), thus drastically reducing the accessible catalytic surface area and, consequently, impeding the catalyzed reac-tion by a factor of ≈90. Unfortunately, this system is inher-ently unstable since weakly protected Au NPs rapidly coalesce in the assembled state—indeed, only three assembly–disas-sembly cycles and a total reaction time of <40 min have been reported.[52] This instability issue was addressed by Zhao et al., who promoted chemical reactions in the interstitial spaces between aggregated NPs tightly protected with thiols (rather than on the gold surface).[41] These researchers showed that when UV-irradiated AB-functionalized NPs self-assemble in nonpolar solvents, they can readily “trap” polar molecules present in the solution. After the polar molecules have been trapped inside these “dynamically self-assembling nanoflasks”, their effective molarity increases drastically, thus promoting chemical reactions between them. In one example, the [4 + 4] cyclodimerization of anthracene was accelerated by about two orders of magnitude in the presence of reversibly aggregating Au NPs.[41] In contrast to the weakly protected, DDA-capped particles (see above), these NPs can be assembled and disassembled for more than 100 cycles without appreciable fatigue and can thus be used in a catalytic fashion. One shortcoming of this strategy is that the AB-coated NPs lack the ability to distinguish between different polar molecules, which they trap nonselectively. This shortcoming could be addressed, however, by decorating the NPs with mixed monolayers comprising ABs and specific recogni-tion handles. For example, Au NPs cofunctionalized with ABs and thiols terminated with chiral groups trapped a model chiral

naphthalene with more than 90% ee.[41] This highly enantioselec-tive trapping, combined with increased reactivity under confine-ment, paves the way toward light-controlled asymmetric catalysis.

Further applications emerge upon endowing light-switchable NPs with additional features, such as responsiveness to mag-netic fields or chemical signals.[53] By placing ABs on the sur-faces of superparamagnetic Fe3O4 NPs, Das et al. synthesized particles capable of responding to light and magnetic fields in an orthogonal fashion.[54] These “dual-responsive” NPs, 11 nm in diameter, were too small to assemble in response to an applied magnetic field alone; however, when aggregated into small clusters with the help of UV light, they rapidly assembled into elongated 1D structures. Exposure to visible light induced the cis → trans back-isomerization, which cancelled out the attractive interactions in between the NPs. In other words, the magnetic responsiveness of the particles could be “turned on” and “off” reversibly, using two different wavelengths of light.[54] This unique feature enabled these NPs to be applied as “dynam-ically self-assembling carriers” for transporting nonmagnetic cargo.[55] In one demonstration, a mixture of 90 nm silica and AB-coated, 11 nm Fe3O4 NPs was exposed to UV light, resulting in the formation of a thin magnetic “film” around each silica particle. Under an applied magnetic field, these SiO2@Fe3O4 core–shell assemblies strongly interacted with each other by means of collective dipole–dipole interactions and assembled into larger structures (Figure 1e) that could rapidly be trans-ported over large distances using a magnet. Once at the desired location, the “cargo” could be released by exposure to visible light. Remarkably, these dynamic carriers are capable of trans-porting “cargo” >20 times their own mass.[55]

2.2. Spiropyran-Functionalized Nanoparticles

The above methodology is not limited to azobenzenes: in principle, any molecular switch undergoing light-induced transformation to a more polar metastable state can promote the self-assembly of NPs in nonpolar media. Another example of a photochromic compound that meets this criterion is spiropyran.[56] Upon exposure to UV light, the closed-ring isomer of spiropyran (SP in Figure 2a) isomerizes to an open one (“merocyanine”; MC), with the inverse reaction proceeding under visible light. The stability of the MC isomer is drastically dependent on the polarity of the medium. In nonpolar solvents, MC is highly unstable (with half-lives, τ1/2, on the order of sec-onds) and rapidly back-isomerizes to SP in the dark. In aqueous media, however, MC can be stabilized to the extent that the SP → MC reaction occurs spontaneously, i.e., under these con-ditions, it is the SP isomer that is metastable and present only under visible light illumination.

The τ1/2 values of MC in toluene are as much as four orders of magnitude smaller than those of typical cis-azobenzenes (τ1/2 ≈ 1 day in toluene). To verify whether this large differ-ence in the stability of the metastable forms could translate into the lifetimes of NP aggregates induced by cis-AB versus MC, Kundu and co-workers synthesized differently sized nan-oparticles[57] and nanoclusters[58] decorated with a thiolated spiropyran. Indeed, aggregates generated by briefly (<1 min) irradiating these NPs to UV light (Figure 2b) persisted for

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much shorter periods, with their decomposition in the dark complete within as little as 1 min (Figure 2c; compare with days for AB-coated NPs).[57] It is important to emphasize, how-ever, that for the disassembly to proceed rapidly, the exposure to UV must be short. Contrasting behaviors of aggregates of SP-coated Au NPs were found in experiments in which longer irradiation times were employed.[59] Although the aggregates of these NPs looked identical to those obtained at short UV irradiation times (Figure 2d), once generated, they remained stable in the dark (Figure 2e).[60] Within NP aggregates, the MC moieties can gradually come into contact with one another and form dimers in which they mutually stabilize each other by strong electrostatic and π–π stacking interactions[56] (Figure 2f). Nevertheless, these aggregates could be disassem-bled under visible light, which facilitates the MC → SP back-isomerization. Interestingly, short exposures to visible light

resulted in the controlled disassembly into smaller aggregates, which maintained their sizes in the dark. This strategy could be used to precisely fine-tune the sizes of NP aggregates, as shown in Figure 2e. Interestingly, other studies[61,62] have dem-onstrated that once assembled, MC-functionalized NPs remain stable even under prolonged exposure to visible light, which highlights the strength of the MC–MC interactions.

Spiropyran-decorated gold NPs could be used for switchable surface-enhanced Raman scattering (SERS).[63] Plasmonic NPs’ ability to enhance Raman signals can be further enhanced in the presence of “hot spots”, i.e., nanosized gaps that arise between the NPs as they aggregate. The ability to reversibly form and destroy such hot spots using external stimuli has the potential to give rise to tunable SERS sensors. Huang and co-workers described an interesting strategy to generate very small aggregates composed of only several NPs each

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Figure 2. Light-induced self-assembly of spiropyran-functionalized nanoparticles. a) Reversible, light-induced isomerization between closed and open forms of spiropyran. Note that the latter exists as a hybrid of the zwitterionic and the quinoidal form. b) TEM image of three spherical aggregates of spiropyran-functionalized 5.5 nm gold NPs obtained by a brief exposure to UV light. c) Rapid disassembly of aggregates of spiropyran-functionalized gold NPs at different fractional coverages of the NPs with spiropyran. b,c) Reproduced with permission.[57] Copyright 2016, The Royal Society of Chemistry. d) TEM image of a spherical aggregate of spiropyran-functionalized gold NPs obtained by prolonged (30 min) exposure to UV light. e) Time-dependent changes in the solvodynamic diameter as a function of UV and visible light irradiation. d,e) Reproduced with permission.[60] Copyright 2014, American Chemical Society. f) Schematic illustration of strong lateral interactions between the MC isomers of spiropyran. g) TEM image of small oligomers of spiropyran-coated NPs generated with UV light. h) NP-induced enhancement of Raman spectra of 4-mercaptopyridine (MPy). Black: NP-free spectrum. Red: Raman spectrum of MPy in the presence of spiropyran-functionalized Au NPs prior to UV irradiation. Blue: Raman spectrum of MPy in the presence of spiropyran-functionalized Au NPs after UV irradiation. Green: Raman spectrum of MPy in the presence of spiropyran-functionalized Au NPs after UV, followed by visible light irradiation. g,h) Reproduced with permission.[63] Copyright 2015, American Chemical Society. i) Self-assembly of spiropyran-functionalized Au NPs induced by simultaneous exposure to UV light and Cu2+. Reproduced with permission.[65] Copyright 2011, Wiley-VCH.



(Figure 2g). In their strategy, 25 nm Au NPs were cofunction-alized with a mixture of a hydrophilic poly(ethylene glycol) (PEG) and a hydrophobic, spiropyran-rich brush.[63] Initially, the particles were soluble in a DMF/water mixture; self-assembly was induced upon exposure to UV light. The authors proposed that the small aggregates formed as a result of the competition between attractive MC–MC interactions and steric repulsion between the PEG chains and that the self-assembly process involved reorganization of the polymer brushes on the NPs in order to maximize the number of MC–MC contacts. The formation of NP oligomers entailed the enhancement of Raman signal intensity by up to five times (Figure 2h). One dis-advantage of this system is that long irradiation times—3 h of UV light and as much as 7 h of visible light—were required to complete a single assembly–disassembly cycle.[63]

Possible application of spiropyran-coated NPs as respon-sive sensors in magnetic resonance imaging (MRI) has also been proposed.[64] This and other biorelated applications call for NPs to reversibly assemble in aqueous media. Fortunately, spiropyran-functionalized NPs can exhibit behavior analogous to AB-coated NPs in polar media, where only the hydrophobic (i.e., trans) isomer triggers self-assembly. Louie and co-workers showed that dextran-coated iron oxide NPs partially functional-ized with spiropyran could be solubilized in water only in the presence of the MC form of the switch. These NPs exhibited a spin–spin relaxation time (T2) of ≈37 ms; exposure to visible light (15 min) induced the formation of the SP isomer and NP self-assembly, which shortened T2 by ≈40%.

The two isomers of spiropyran differ greatly in their ability to interact with metal ions. Whereas the SP form typically has no appreciable interactions, MC can form fairly strong complexes with a variety of divalent and trivalent metal ions (in particular, Cu2+), with which it interacts via its phenolic oxygens.[56] Therefore, for these interactions to take place, spiropyran must be “activated” using UV light. Based on these prerequisites, Liu et al. developed an AND logic gate with spiropyran-functional-ized Au NPs as the stimuli-responsive elements.[65] The thiolated spiropyran was used in a small amount (≈5%) within the matrix of a thiol-terminated oligo(ethylene glycol)—therefore, the NPs exhibited high water solubility. Upon the addition of Cu2+, the particles remained free; only upon exposure of this solution to UV light could self-assembly be initiated (Figure 2i). Interest-ingly, these logic gates could be “reset” to their initial state either by irradiation with visible light or by the addition of a com-plexing agent stronger than MC (e.g., EDTA).[65]

3. Assembling Nanoparticles Using Photodimerization Reactions

When irradiated with long-wave (365 nm) UV light, cou-marins undergo a [2+2] cycloaddition reaction, which can be reversed by exposure to UV light of a shorter wavelength (254 nm) (Figure 3a).[66] To investigate the utility of this reaction for assembling NPs, He et al. synthesized 7.6 nm NPs func-tionalized with thiols terminated with coumarin moieties.[67] Exposing a solution of these NPs in THF to 365 nm light for 72 h resulted in the formation of unstructured NP aggregates in high yield. Subsequent irradiation with 254 nm light for 1 h

was sufficient to regenerate the initial solution of free NPs. The extent of aggregation depended in an interesting way on the number of coumarin moieties per NP. With only 11 coumarins (out of ≈850 binding sites) per NP, light-induced aggregation proceeded readily. However, increasing the loading of cou-marin on these NPs gradually diminished the extent to which they assembled, with no aggregation observed above a critical number of coumarins per NP. These results can be understood by the competitive formation of covalent dimers between cou-marins attached to the same NP, which does not entail NP crosslinking.[67]

One significant advantage of the coumarin-based system over those based on photochromic molecules (Section 2) is that it relies on covalent crosslinking, which i) potentially increases the solvent scope and ii) greatly reduces the number of photo-sensitive ligands necessary to induce self-assembly. In fact, NPs aggregated with as little as 1.3% of their surface binding sites occupied by thiolated coumarins.[67] Furthermore, unlike the noncovalent assemblies of AB- or SP-functionalized NPs,

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Figure 3. Light-induced self-assembly of coumarin-functionalized nan-oparticles. a) Reversible, light-induced dimerization of coumarin. b) Amphiphilic diblock copolymer used for controlling the self-assembly of Au NPs using light. c) TEM image of monodisperse Au NPs stabi-lized with the amphiphilic diblock copolymer. d) Schematic illustration of light-induced assembly and disassembly of NPs functionalized with the coumarin-rich polymer. e) Changes in the UV–vis absorption spectra of Au NPs stabilized with the coumarin-rich polymer upon exposure to 365 nm light. f) Reversible changes in the position of NPs’ wavelength of maximum absorption as a function of 365 nm and 254 nm irradiation. c–f) Reproduced with permission.[68] Copyright 2018, National Academy of Sciences.



the covalently crosslinked NPs do not disassemble thermally, making the system truly bistable.

Despite these benefits, the above system suffers from a major drawback: thiolated gold NPs are unstable in the presence of 254 nm light, which induces rapid photooxidation of Au-bond thiols. Consequently, He et al. were only able to achieve four assembly–disassembly cycles.[67] To address this disadvantage, Lin and co-workers synthesized thiol-free, coumarin-decorated Au NPs following a highly elegant nanoreactor approach.[68] In this design, β-cyclodextrin (β-CD) decorated with 21 bromoisobutyryl groups was used to initiate the growth of star-like amphiphilic diblock copolymers via atom-transfer radical polymerization (ATRP). Each of the resulting 21 polymer chains comprised an inner hydrophilic poly(acrylic acid) (PAA) block and an outer hydrophobic coumarin-rich block (Figure 3b). The PAA core of these “unimolecular micelles” had the ability to concen-trate a gold salt, where it was subsequently reduced to afford spherical gold NPs. This method afforded remarkably size-uniform NPs, with a size dispersity of only 3% (Figure 3c). UV (365 nm) irradiation of these NPs in dichloromethane changed the color of the solution from red to blue within 180 min, with the reverse reaction proceeding within 45 min under 254 nm light (Figure 3d,e). The reversible switching of the wavelength of maximum absorption could be toggled between 525 and 584 nm for multiple cycles (Figure 3f). It is important to point out that the [2+2] cycloaddition reaction took place predomi-nantly within polymer shells on the same NPs; nevertheless, enough interparticle crosslinks formed to assemble the NPs in a near-quantitative fashion.[68]

Similar to coumarins, anthracenes undergo a reversible [4 + 4] cycloaddition reaction;[69–71] however, applying this chem-istry to reversibly assemble NPs remains to be demonstrated.[72]

4. (De)protonation of Nanoparticle-Bound Ligands Using Photoacids/Photobases

All of the examples covered thus far are based on NPs surface-functionalized with light-responsive molecules. However, it is also possible to use light to assemble NPs, which, on their own, are not photoresponsive. The past several years have seen the development of several distinct ways to achieve this goal—these examples are covered in Sections 4–6, which follow. We begin with systems comprising mixtures of non-photoswitchable NPs and small-molecule photoacids and photobases.

Kundu et al. reported a pioneering example of controlling the assembly of non-photoswitchable NPs using light.[73] This was achieved by placing pH-responsive NPs in a “photoresponsive medium”—a dilute solution of spiropyran in methanol. As dis-cussed above, spiropyran can be isomerized from its colorless closed-ring isomer (SP) to the metastable, blue-colored open-ring MC isomer using UV light (Figure 2a). Under acidic con-ditions, however, a third, yellow, protonated open-ring form (MCH+; 1 in Figure 4a) prevails. Blue light irradiation of 1 induces a ring-closing reaction with the release of a proton; in the dark, a spontaneous protonation of the closed form, SP, occurs (Figure 4a). In other words, MCH+ is an example of a photoacid—compound which, upon irradiation, decreases the pH of the solution. In recent years, this light-controlled proton

release has been used extensively for transiently protonating species ranging from polymers[74] to molecular switches[75,76] to ion channels[77] to NPs, as discussed below.

Gold nanoparticles functionalized with a single-component monolayer of 11-mercaptoundecanoic acid are insoluble in all common solvents (including polar protic solvents such as water and methanol) because of the multiple hydrogen bonds holding them together. These bonds, however, can be broken by adding a small amount of a strong acid or base. Indeed, by exposing the “photoresponsive medium” to blue light, the solution’s pH decreased to the extent that the NP aggregates disassembled (Figure 4b, top). In the dark, the SP form acted as a base, abstracting the extra protons from the NPs and inducing their assembly (Figure 4b, bottom). The disassembly–reassembly process could be repeated for more than one hun-dred cycles. The high stability of the system was attributed to

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Figure 4. Light-controlled self-assembly of non-photoswitchable NPs via light-induced proton transfer. a) Reversible, light-induced isomeri-zation between open-protonated and closed forms of spiropyran. b) Light-induced disassembly and spontaneous reassembly of gold NPs functionalized with thiolated carboxylic acids in methanol. b) Reproduced with permission.[73] Copyright 2015, Springer Nature. c) Light-induced self-assembly and spontaneous disassembly of the same NPs in water. Reproduced with permission.[78] Copyright 2016, Wiley-VCH.



the use of a noninvasive external stimulus in the form of low-intensity blue light. Another remarkable feature of this system is the quantitative yield of self-assembly, which is in sharp contrast to the method based on NPs surface-functionalized with photochromic molecules, where a certain fraction of NPs remain unassembled. This unique light-responsive system of non-photoswitchable NPs showed potential for developing photoactuated controlled release platforms as well as time-sensitive information storage media.[73]

In the above example, the ground state of the system corresponded to assembled nanoparticles. However, a system comprising unassembled NPs at equilibrium and transient aggregates under light would arguably be more attractive. Such a system was developed simply by transferring the same NPs into a water-based “photoresponsive medium”—an aqueous solution of a water-soluble MCH+ 3 (Figure 4c).[78] In water, NPs with car-boxylic acid end groups are soluble if some of these groups are deprotonated. A transient decrease in pH caused by blue light irradiation (3 min) induced the protonation of enough COO− groups to induce self-assembly. In the dark, COOH groups lost their protons to 4 and the aggregates disassembled within 5 min (Figure 4c). Unfortunately, the SP/MC switch in water under-goes gradual hydrolysis, which decreased the number of cycles that could be performed to 30–40.[78] The hydrolysis can be sup-pressed by working in more acidic solutions,[79,80] whereby, how-ever, the photoinduced pH drop is less pronounced.

Interestingly, irrespective of the medium (water or methanol) and cause of self-assembly (light-induced vs spontaneous, respectively), the aggregates formed at the early stages of self-assembly had linear (1D) structures.[73,78] In a creative study, Ikkala and co-workers demonstrated that these linear NP chains could serve as a memory element in NP-doped hydrogels, pro-viding the gels with the ability to learn, at least in a minimal-istic sense.[81] These researchers worked with 13 nm gold NPs functionalized with lipoic acid (partially deprotonated). When heated above a critical temperature (“unconditioned stimulus”; an analog to food in the classic Pavlov’s dog experiment), an agarose gel containing the NPs and 3 melted (“unconditioned response”, compared with dog’s salivation). Simultaneous irra-diation of the gel with blue and red light (“neutral stimulus”; a bell in the dog experiment) caused no response in the original gel. However, when both of these stimuli (i.e., heat and blue + red light) were delivered simultaneously (i.e., “conditioning” or associative learning), causing the gel to melt and MCH+ to acidify the solution, NP self-assembly into linear chains was initiated. In contrast to individual NPs, these chains feature an intense surface plasmon resonance (SPR) band in the red part of the UV–vis spectrum. When irradiated with light of a wavelength overlapping with their SPR band, plasmonic NPs or their aggregates can convert the absorbed light energy into heat (i.e., the plasmonic photothermal effect), thereby increasing the temperature in the vicinity of the NP surface.[82] Therefore, exposing the “conditioned” gel to what used to be a neutral stimulus (blue + red light) causes it to melt—in a way similar to a bell causing a conditioned dog to salivate. In other words, the gel has “learned” to associate one stimulus (light) with another (heat). Interestingly, the same material enabled the authors to mimic aspects of forgetting and the spontaneous recovery of memory.[81]

Very recently, photoacid 3 has been used to control the assembly state of nanocomposite tectons (NCTs).[83] NCTs comprise an inorganic core (typically gold) functionalized with long polymer brushes modified at the ω position with a spe-cific recognition unit.[84] In one design, NCTs terminated with terpirydyl (Tpy) groups were assembled by adding Zn2+, which can noncovalently crosslink two Tpy groups. The resulting aggregates were stable in the presence of 3; however, exposing the mixture to blue light released protons (MCH+ → SP + H+), which could protonate the Tpy groups, outcompeting the Zn2+ crosslinker and causing the aggregates to disassemble. In the dark, SP sequestered H+ from Tpy, inducing the reassembly of NCTs. The disassembly–reassembly process could be repeated for multiple cycles.[83]

The disadvantage of the above systems, all involving spiro-pyran-derived photoacids, are the relatively long times required for the thermal recovery of the system (i.e., the solution’s pH returning to the preirradiation value). For example, the thermal half-life of the light-induced deprotonated state (4 + H+) could be as long as 70 s.[85] To address this issue and to combine rapid assembly with rapid disassembly, Amdursky and co-workers developed a clever system based on the simultaneous use of a photoacid and a photobase.[86] For the photoacid, they used a sulfonated hydroxypyrene, which undergoes deprotonation under blue (405 nm) light. 6-Methoxyquinoline was used as the photobase; it is capable of capturing protons and raising the pH upon exposure to UV (340 nm) light. When gold NPs (3.8 nm) functionalized with 6-mercaptohexanoic acid were placed in an aqueous solution of this mixture, they could be assem-bled and disassembled with rates (an initial fast component of double exponential fitting) of only 0.3 and 0.4 s, respectively, for many cycles. In a nonaqueous solution (methanol), the same NPs could be reversibly assembled with malachite green as the photobase.[86]

5. Light-Induced Adsorption of Photoswitchable Molecules

5.1. Photoswitchable Host–Guest Inclusion Complexes on Nanoparticle Surfaces

The formation of host–guest inclusion complexes has been widely used for assembling NPs into higher-order architectures.[12,31] Among them, complexes involving photoswitchable guests or photoswitchable hosts are particularly interesting since they enable remote manipulation of supramolecular architec-tures using light.[87] α-CD and β-CD have long been known to form strong host–guest inclusion complexes with the trans isomer of azobenzene (e.g., the association constant in water, Ka ≈ 6000 m−1 for β-CD[88]). Exposing these complexes to UV light results in their dissociation since the more bulky cis-AB can no longer fit in α- or β-CD’s hydrophobic cavity; how-ever, subsequent irradiation with visible light reestablishes the complex (Figure 5a).

Ravoo and co-workers employed these switchable interactions to control the self-assembly of silica NPs using light.[89] To this end, 58 nm SiO2 NPs were derivatized with a densely packed monolayer of β-CDs via the free-radical thiol–ene reaction

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between monothiolated β-CD and SiO2 NPs functionalized with ligands terminated with CHCH2 moieties. Attrac-tive interactions between the resulting NPs were mediated by a small-molecule linker containing two AB groups at the ends—provided that both ABs were in the trans configura-tion (see Figure 5b). The extent of the aggregation of the NPs could be controlled by the concentration of NaCl, which screens the repulsive electrostatic interactions between the negatively charged silica NPs. Exposing the system to UV light resulted in the expulsion of cis-AB from β-CD cavities and, consequently, the disassembly of NP aggregates. Reassembly of NPs in the resulting solution could be realized using blue light (450 nm).[89]

Recently, Fuchter and co-workers developed arylazopyra-zoles[90] (AAPs)—a new family of photoresponsive azo com-pounds that can similarly form light-switchable host–guest inclu-sion complexes with β-CD,[91] but feature several advantages over ABs. These advantages include the increased thermal stability of the cis form and a large band separation between the trans and cis isomers, allowing each to be addressed with high selectivity (with UV and green light, respectively) and enabling a near-quan-titative conversion between the two states. Taking advantage of these improved features, Ravoo and co-workers used a bis-AAP linker to mediate the attractive interactions between 11.5 nm Au NPs decorated with heptathiolated β-CD (Figure 5b).[91] In

the presence of the linker, these NPs could be assembled and disassembled with green and UV light, respectively (Figure 5c), for at least several cycles (Figure 5d). An interesting finding of this study is that threading of β-CD onto trans-AAP could only occur through the phenyl ring—but not the more bulky trimethylpyrazole ring. Consequently, no inclusion complex formation and NP self-assembly took place in the presence of a linker containing terminal trimethylpyrazole groups.[91]

In a follow-up study, the same group demonstrated the ability to reversibly assemble and disassemble NPs—using a similar bis-AAP linker—without the need for the sometimes delete-rious UV light.[92] They worked with 40 nm lanthanide-doped LiYF4 upconversion NPs, which have the ability to absorb and convert two (or more) photons of relatively low energy into one emitted photon with a higher energy. NPs decorated with α-CD and β-CD appended with six or seven COOH groups, respec-tively, could be assembled in the presence of a bis-AAP or a bis-AB linker. When subjected to low-intensity (0.22 W cm−2) NIR laser radiation (980 nm), the NPs emitted enough UV light to induce trans → cis isomerization in the linkers on their surfaces, causing dissociation of the inclusion complexes and hence, disassembly of NP aggregates.[92] Unfortunately, long irradiation times (several hours) were required. Furthermore, the LiYF4-induced photoswitching led to a poor photostationary state comprising only 47% of the cis isomer, which nevertheless was enough to induce the disassembly process.

Most recently, the above concept was extended to a modular system featuring three kinds of NPs: Au, LiYF4, and iron oxide, each decorated with a monolayer of β-CD.[93] Upon the addi-tion of a bis-AAP crosslinker, spherical aggregates containing all three types of NPs were obtained. This interesting multifunc-tional hybrid material exhibited a combination of plasmonic, upconversion, and magnetic properties. Exposure to UV for 3 min disassembled the aggregates into a solution of free NPs, whose reassembly could be induced by subsequent irradiation with green light (15 min). Remarkably, the LiYF4 NPs within these aggregates exhibited a well-defined emission spectrum, with a single emis-sion band centered at 375 nm. In sharp contrast, the same NPs in a dispersed state had a complex spectrum with multiple peaks.[93] Interestingly, emission at ≈375 nm is ideal for inducing the trans → cis conversion of AAP—consequently, exposure to NIR for 2 h induced the disassembly of the three-component aggregates.

The AAP–β-CD chemistry has also been employed for con-trolling the end-to-end assembly of rod-shaped gold NPs.[94] These nanorods are typically synthesized in water with the surfactant cetyltrimethylammonium bromide (CTAB). CTAB binds strongly to the side faces of nanorods and exhibits a weaker affinity to their tips, where a ligand exchange reaction with heptathiolated β-CD proceeds selectively. CTAB, how-ever, can also bind within the cavity of CD, thus suppressing the formation of the desired trans-AAP⊂β-CD complex.[95] To avoid any competition with AAP, a large excess of thiolated tetra(ethylene glycol) was added prior to the self-assembly experiments, thus removing CTAB from the rods’ side faces. Following this two-step ligand exchange protocol, CD-capped nanorods could reversibly be assembled and disassembled in the presence of a bis-AAP linker upon exposure to green and UV light, respectively.[94]

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Figure 5. Light-controlled self-assembly of non-photoswitchable NPs mediated by small-molecule photoresponsive crosslinkers. a) Reversible, light-induced complexation of azobenzene with α-cyclodextrin (α-CD). b) Schematic illustration of light-controlled self-assembly of β-CD-coated gold NPs in the presence of a bis-azo crosslinker. c) UV–vis absorption spectra of β-CD-coated gold NPs in the presence of bis-azo crosslinker 5 following exposure to UV light (red trace) and green light (green trace). d) Reversible changes in the wavelength of maximum absorption (top) and optical density at 600 nm (bottom) as a function of alternating exposure to green and UV light. b–d) Reproduced with permission.[91] Copyright 2016, American Chemical Society.



5.2. Nonselective Adsorption of Photoswitchable Molecules

In the above examples, light-responsive small molecules guided the self-assembly of NPs by forming well-defined host–guest inclusion complexes with NP-bound macrocycles. It is also possible to induce NP assembly via less specific supramolec-ular interactions. Ikkala and co-workers have recently reported the reversible, light-induced hydrophobization of charged NPs in water.[79] They worked with a mixture of positively charged (decorated with tetraalkylammonium groups) gold NPs and photoacid 3. As we saw in Section 4, compound 3 has suc-cessfully been used to protonate ligand-functionalized NPs (inducing their assembly) upon exposure to blue light. In con-trast, Ikkala and co-workers took advantage of the byproduct of the proton release reaction—spiropyran 4 (Figure 4c), which, owing to its negative charge, can interact electrostatically with the positively charged NPs. The light-induced adsorption of 4 effectively neutralized the particles, inducing strong hydro-phobic forces between them (Figure 6a, right) (a tenfold excess

of photoacid 3 with respect to the positively charged ammo-nium group was used). The charge neutralization was followed by ζ-potential measurements, which decreased from +37 mV for dark-adapted NPs to only +4.8 mV under blue light. When the irradiation was discontinued, 4 underwent protonation and desorbed from the NPs, restoring the Coulombic repulsion between them and inducing the disassembly of the NP aggre-gates (Figure 6b). The lifetimes of these transient aggregates ranged from ≈100 min at 15 °C to >1 min at 50 °C. Remark-ably, when the process was carried out in water in the presence of a toluene phase, it entailed a near-quantitative—yet revers-ible—transfer of the NPs from water to the organic phase.[79]

For other photoresponsive compounds, contrasting affinities of two different isomers to NPs can originate from different molecular shapes. This concept is illustrated by the recent work of Qi and co-workers, who investigated interac-tions between CTAB-coated gold nanorods (29 × 58 nm) and a photoswitchable surfactant, 4-phenylazophenoxyacetate (0.8 × 10−3 m) (6 in Figure 6c).[96] On account of its extended, 1D shape, the trans isomer of 6 readily intercalates between the positively charged CTAB molecules, reducing the ζ-potential of the nanorods from +33 mV to 0 mV and inducing their self-assembly (Figure 6c, top left). Differing from the directional, end-to-end interactions described above for CD complexes, no specific orientation of the nanorods was observed in these aggregates, implying that the adsorption of trans-6 takes place unselectively throughout the surface of the particles. UV irra-diation induced switching to the cis isomer, whose interaction with the CTAB layer was much weaker, inducing the release of 6 to the solution, elevating the ζ-potential of the rods to +24 mV, and disassembling the aggregates (Figure 6c, top right). Repeated switching between assembled and disassem-bled states was achieved by exposure to visible and UV light, respectively. This method is also applicable to spherical gold NPs as well as to longer nanorods (28 × 114 nm).

A more interesting behavior was found when the same sur-factant 6 was used at a higher concentration (1.8 × 10−3 m) (Figure 6c, bottom). Here, the negatively charged surfactant not only neutralized the nanorods—it rendered them negatively charged (−16 mV); consequently, the rods continued to form a stable suspension in water. UV irradiation caused a rapid desorp-tion of the resulting cis-6 and surface charge reversion to +20 mV; therefore, no aggregation was observed. Subsequent exposure of this solution to visible light induced a gradual reassociation of trans-6 within the CTAB layer, reverting the charge back to −16 mV. However, because this process was slow, the transition state with ζ-potential ≈0 mV persisted for long enough for the nanorods to assemble (at ≈30 min). Prolonged irradiation of the same solution desorbed more surfactant from the nanorods and further decreased the ζ-potential, eventually causing the NPs to redis-perse (Figure 6c, bottom). These results constitute an intriguing example of a system where continuous exposure to one stimulus (visible light) results in successive assembly and disassembly.[96]

A similar but positively charged azo-surfactant, 4-butyl-4′-(trimethylammoniumbutyloxy)azobenzene, has been used to control the colloidal stability of carbon-based nanoparticles.[97] Here, the trans isomer of azobenzene interacted strongly (by π–π stacking interactions) with carbon nanotubes and NPs of gra-phene oxide (GO) and reduced GO, rendering these materials

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Figure 6. Light-controlled self-assembly of non-photoswitchable NPs induced by small-molecule photoresponsive surfactants. a) Reversible self-assembly of NPs controlled by light-induced conversion of 3 to 4 (see Figure 4c for structural formulas). b) TEM images of gold NPs decorated with tetraalkylammonium groups before (left) and after (right) exposure to blue light in the presence of 3. b) Reproduced with permission.[79] Copyright 2019, The Royal Society of Chemistry. c) Schematic illustration of light-induced self-assembly of CTAB-coated gold nanorods in the pres-ence of photoresponsive surfactant 6 at low (top) and high (bottom) con-centration. Adapted with permission.[96] Copyright 2019, Springer Nature.



water-soluble. However, no such interactions were possible with the nonplanar cis isomer, which induced NP aggregation.

In the final example, NCTs terminated with cyanuric acid (CA) groups were coassembled (in a nonpolar solvent) with NCTs terminated with a supramolecular receptor for CA, the so-called Hamilton wedge (HW). Exposing the resulting mixture to UV light in the presence of the parent spiropyran (SP in Figure 2a) generated a hydrophilic MC isomer, which, when present in large excess (50 eq), could interfere with the hydrogen bonding between CA and HW. Consequently, NCT aggregates were disassembled under UV light. The subsequent coassembly of these two types of NCTs could be induced by exposure to visible light.[83]

6. Phase Transitions of Thermoresponsive Polymers Induced by Plasmonic Nanoparticles

A conceptually different way to control the assembly of NPs using light is based on utilizing thermoresponsive polymers, whose behavior can be exemplified by poly(N-isopropylacrylamide)

(pNIPAm, Figure 7a). At room temperature, pNIPAm is readily hydrated (due to the hydrogen bonding interactions between water molecules and the polymer’s amide groups) and exhibits high water solubility. Upon warming the solution, however, the hydrogen bonds weaken and the relative contribution of hydro-phobic interactions between the isopropyl groups gradually increases. At the “cloud point” corresponding to 32 °C (the so-called lower critical solution temperature, LCST), the polymer undergoes a volume phase transition and rapidly precipi-tates[98] as a result of entropy-driven dehydration[99] (Figure 7a). pNIPAm adsorbed on NPs behaves similarly, causing the pol-ymer-coated NPs to precipitate from the aqueous solution when heated above the polymer’s LCST.[100]

As discussed above (Section 4), plasmonic NPs irradiated at a wavelength overlapping with their SPR band convert the absorbed light into heat. This effect was utilized to induce the self-assembly of NPs by Kumacheva and co-workers, who worked with gold nanorods whose tips were functional-ized with thiol-terminated pNIPAm chains.[101] These rods were 43 nm long and 12 nm wide and exhibited an intense absorption band in the NIR region due to their longitudinal SPR. Exposing aqueous solutions of the nanorods to 800 nm laser radiation (30 W cm−2) for 3 min resulted in the collapse of the polymer chains, which triggered strong van der Waals (vdW) interactions between the tips of the nanorods, affording linear assemblies (Figure 7b). Although analogous assemblies could be obtained by heating the solution above the LCST, the light-induced approach confined the temperature increase to the immediate surroundings of the rods’ surface and no bulk heating was necessary. This process was reversible: when the irradiation was ceased, the collapsed pNIPAm reinflated and the nanorod chains disassembled within several minutes.

Lu and co-workers noted that the photothermal effect of silver NPs can be as much as ten times higher than that of gold and investigated pNIPAm’s phase transition on the sur-faces of Ag–Fe3O4 heterodimeric NPs.[102] Both the plasmonic and the magnetic domain of these heterodimers were coated with pNIPAm, rendering the NPs soluble in water at room temperature. Exposure to low-intensity visible light (via a solar simulator equipped with a 400/700 nm cutoff filter) for 1 h was enough to induce the collapse of pNIPAm chains, thus removing the steric stabilization of the NPs and inducing their self-assembly into spherical aggregates (Figure 7c) (which, unlike free NPs, could be removed with a magnet). A solution of free NPs was recovered within only a few minutes after the solar simulator was turned off.[102] This work demonstrates that equipping pNIPAm-decorated particles with a plasmonic silver domain represents a viable strategy to control their self-assembly behavior using visible light.

More recently, Ding et al. described the ability of pNIPAm-coated gold NPs to act as “actuating nanotransducers”.[103] They worked with large, 60 nm gold NPs sparsely coated with pNIPAm. When irradiated with a green laser (532 nm, 10 W cm−2), the particles produced enough heat to raise the temperature above the LCST, which induced the collapse of the polymer chains, triggering strong interparticle vdW interac-tions. The reaction was carried out in the presence of extra free pNIPAm in solution, which deposited on the aggregating NPs, thickening the polymer shell around them. Nevertheless, the

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Figure 7. Light-induced self-assembly of pNIPAm-coated NPs. a) Left: Structural formula of thermoresponsive polymer poly(N-isopropy-lacrylamide) (pNIPAm). Right: Schematic illustration of a light-induced phase transition of pNIPAm on the surface of a plasmonic NP. b) Light-induced collapse of pNIPAm at the tips of gold nanorods drives their self-assembly into linear structures. The TEM images show free (left) and assembled (right) pNIPAm-coated gold nanorods. b) Reproduced with permission.[101] Copyright 2009, Royal Society of Chemistry. c) Light-induced self-assembly of pNIPAm-coated Ag–Fe3O4 heterodimeric NPs (top) and TEM images of the NPs before and after exposure to visible light (bottom). Reproduced with permission.[102] Copyright 2013, The Royal Society of Chemistry.



interparticle gaps within the resulting aggregates were small (estimated at 4 nm), which, combined with the large sizes of the constituent NPs, gave rise to a dramatic (>200 nm) red-shift of the SPR band from 532 to 745 nm. These aggregates were remarkably uniform in size (about 400 nm in diameter each), which was attributed to a self-limited growth mecha-nism. When the irradiation was discontinued, NP aggregates slowly cooled down. As soon as the temperature dropped below the LCST, the polymer chains rapidly rehydrated and swelled, exerting substantial force (estimated at several nN) on the NPs, pushing them apart. The process was very rapid, as manifested by an abrupt blueshift back to 539 nm within less than 1 s; in fact, Ding et al. described the collapsed pNIPAm chains as “tightly compressed springs” storing large elastic energy that, below the LCST, placed large forces on the neighboring NPs, “explosively” splitting the aggregates into individual parti-cles.[103] The assembly–disassembly cycle and, consequently, the actuation action, could be repeated many times.

7. Light-Induced Chemical Reduction of Nanoparticle-Bound Ligands

As described above (Section 5.1), the self-assembly of NPs can be driven by the formation of host–guest inclusion complexes. Recent years have seen increased interest in supramolecular complexes involving CB[8] as the host.[104] The unique feature of CB[8], or cucurbit[8]uril, is that its hydrophobic cavity is large enough to simultaneously accommodate two guest molecules. Methyl viologen radical cation MV+• has been widely inves-tigated as a guest forming 2:1 complexes with CB[8]. It has been reported[105] that by simultaneously encapsulating two molecules of MV+•, CB[8] enhances the radical’s dimerization constant from ≈200 to 2 × 107 m−1.

When the MV+• groups are attached to different, nonin-teracting NPs, the addition of CB[8] can potentially cross-link these NPs, giving rise to higher-order architectures (Figure 8a). However, the MV+• radical cations are unstable and they typically need to be generated by chemical reduction of the corresponding dication, MV2+. To eliminate the use of chemical reductants (typically Na2S2O4), Qu and co-workers attached MV2+ units to the surfaces of TiO2 NPs via the cat-echol linker.[106] As expected, these NPs could be assembled upon the addition of Na2S2O4 with CB[8]. Importantly, how-ever, assembly could also be achieved using UV (365 nm) light (Figure 8a). Exposing titania to UV gives rise to electron–hole pairs; with isopropanol as the sacrificial hole scavenger, the photogenerated electrons can reduce the MV2+ moieties to MV+•, thus inducing NP self-assembly. Further appeal of this design lies in the fact that disassembly can be induced simply by exposing the system to air, which induces oxidation of the radical cations, (MV+•)2⊂CB[8] → MV2+⊂CB[8] + MV2+, thus removing the crosslinks between the NPs.[107] Therefore, both the formation and the disassembly of NP aggregates can be induced with environmentally friendly inputs (light and air, respectively). The authors also demonstrated the utility of their system for switchable catalysis by showing that in the dis-persed state, the NPs are fivefold more active as photocatalysts for decomposing a model dye (Figure 8b).[106] More recently,

the same chemistry has been used to control self-assembly of MV2+-decorated ZnO NPs.[108]

8. Irreversible Self-Assembly of Nanoparticles

Despite the many benefits of being able to assemble nano-particles in a reversible fashion, some applications (such as in, e.g., sensing, catalysis, and photothermal therapy) do not require that the assembly be reversible. Nevertheless, light has some advantages over other methods in generating irrevers-ible NP assemblies, for example, aggregation can be triggered remotely and with a high spatial and temporal resolution. In this section, we describe different strategies, illustrated by recent examples from the literature, to irreversibly assemble NPs using light.

Azobenzene-terminated ligands adsorbed on many inor-ganic NPs can be photoisomerized between the trans and cis state for many cycles[12] (Figure 1a), driving the reversible self-assembly of these NPs in nonpolar solvents (Section 2.1). Similarly, 13 nm gold NPs decorated with PMMA with AB side groups aggregated upon exposure to UV light in toluene; inter-estingly, however, this process was reported to be irreversible. The high stability of the formed aggregates could be due to the stabilization of cis-AB within the condensed polymer phase and/or strong vdW forces between the NPs induced upon self-assembly.[109]

In Section 4, we discussed examples of the reversible self-assembly of non-photoswitchable NPs in light-responsive media. Li and co-workers designed a light-responsive

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Figure 8. Assembling TiO2 NPs via photoinduced electron transfer. a) TEM images and schematic illustrations of free (left) and assembled (right) TiO2 NPs. Bottom: Structural formulas of a catechol–viologen ligand for TiO2 NPs in the oxidized (left) and reduced (right) form. b) Decolorization curves accompanying photodecomposition of a model dye rhodamine B in the presence of free (red) and aggregated (blue) TiO2 NPs. a,b) Reproduced with permission.[106] Copyright 2015, Wiley-VCH.



medium capable of decreasing the pH irreversibly, thus inducing the formation of permanent NP aggregates.[110] For the photosensitive component of the light-responsive medium, these researchers used a solution of a commercially available, water-soluble photoacid generator, diphenyliodo-nium nitrate. Gold NPs were functionalized with a lipoic acid derivative terminated with boronic acid groups. These parti-cles were stable in an aqueous solution at pH 10 with diphe-nyliodonium nitrate. However, within less than 10 min of UV irradiation, the maximum of the NPs’ SPR band moved from 525 nm all the way up to 675 nm, indicating pronounced NP aggregation. Unfortunately, a large excess of the active com-pound (100 eq with respect to NP-bound boronic acids) had to be used for such SPR shifts to be observed. Interestingly, the self-assembly process began with the formation of 1D chains of NPs, similar to those observed previously in other systems.[73,78,81]

In Section 5.2, we saw how the colloidal stability of NPs can be controlled by reversibly photoswitching surfactants. Santer and co-workers reported an interesting example of an irreversible surfactant desorption, hence NP aggregation.[111] Negatively charged, polydisperse gold NPs prepared by laser ablation were treated with an excess of a positively charged azo-surfactant, 4-butyl-4′-(trimethylammoniumhexyloxy)azobenzene, resulting in a stable colloidal suspension in water. Exposing these particles to UV light for 5 min led to a dramatic change in the UV–vis absorption spectra, with the appearance of a new band centered at ≈600 nm, indica-tive of NP aggregation. This process took place as a result of photoisomerization of the surfactant to the more bulky cis form, which was removed from the NP surface, reducing the ζ-potential to ≈0, and initiating attractive interparticle interactions.[111]

Irreversible aggregation can also occur as a result of light-induced oxidation of thiolate ligands. Bian et al. found that small (≈3 nm), thiolated gold NPs exposed to a high-pressure Hg lamp could be converted into well-defined and uniform-sized vesicles within less than 20 min.[112] When subjected to high-power radiation, thiolate ligands can undergo oxida-tion to sulfonic acids, which greatly decreases their affinity to the NPs, triggering ligand desorption and inducing strong vdW interactions between the inorganic NP cores—indeed, oxygen was essential for the colloidal destabilization of the NPs and the formation of vesicles. The method was success-fully extended to NPs having other compositions, such as Pd, Pt, and CdSe.[112]

Such permanently crosslinked NPs may find important biomedical applications. Cheng et al. synthesized 20 nm gold NPs functionalized with photolabile diazirine moieties.[113] Upon exposure to a 405 nm laser, diazirine groups undergo photo-chemical decomposition to carbenes, which rapidly react with amine groups also present on the NPs. Although the reaction can preferentially occur between ligands adsorbed on the same NP, enough covalent crosslinks were formed between NPs to induce their assembly into permanent NP aggregates. These aggregates exhibited pronounced absorption bands in the near-infrared region and were used to convert the incident NIR light (808 nm) into heat. The aggregates, formed at tumor sites using 405 nm light and subsequently subjected to NIR radiation,

could successfully inhibit tumor growth, suggesting their use in applications in photothermal cancer therapy.[113] More recently, the same group employed another photolabile group to induce a photo-crosslinking reaction.[114] In this design, 23 nm gold NPs were cofunctionalized with ligands terminated with tetra-zole and methacrylate moieties. When exposed to a 405 nm laser, tetrazoles release N2 and undergo addition to the terminal CC bonds to afford pyrazoline cycloadducts, which act as covalent crosslinks between the NPs. This strategy has success-fully tackled the drawback of the earlier design, namely, the low stability of diazirines in water.

9. Extension to Microparticles

Although this progress report focuses on small NPs, most of the strategies described here can also be applied to control the assembly of larger, micrometer-sized particles. In fact, the pioneering example of guiding particle self-assembly using light was reported on 150 nm silica colloids decorated with azobenzene-functionalized calixarenes.[115] Upon exposure to UV light, these particles rapidly precipitated from cyclohexane. Similarly, spiropyran was used to control the self-assembly of 300 nm silica particles long before being applied to small NPs.[116]

Recently, Peng et al. adapted the coumarin-driven reversible bond formation (see Section 3) to reversibly assemble silica colloids.[117,118] When irradiated with UV light (365 nm) in chloro-form, coumarin-functionalized silica particles formed assem-blies of several distinct morphologies, all of which disassembled upon exposure to 254 nm light. The photodimerization reaction was accompanied by an increase in the colloids’ hydrophobic properties. Water droplets deposited on a surface decorated with these colloids exhibited a contact angle of 102°; upon expo-sure to 365 nm light, the angle increased to 163°, indicating the superhydrophobic nature of the surface. Water droplets could freely roll off the irradiated surface, suggesting possible appli-cations in remotely controlled, self-cleaning surfaces.[117,118] Similarly, Kuehne and co-workers demonstrated[119] that the aggregation state of non-photoresponsive microgel particles could be controlled by light in the presence of photoacid 3 (see Section 4), whereas Santer and co-workers showed that micrometer-sized silica colloids could be assembled in the presence of an azobenzene surfactant (dubbed “photosoap”) in solution[120] (compare with Section 5.2).

Unlike small NPs, micrometer-sized beads can be modified asymmetrically by metal sputtering. Introducing asymmetry enables further functionalization of either hemisphere of these “Janus” particles by chemical derivatization. Alternatively, sputtering can be performed on particles uniformly prefunc-tionalizated with monolayers of organic molecules. This latter approach has been adapted by Ren and co-workers to fabricate light-responsive “Janus micromotors”.[121] Initially, 2 µm SiO2 colloids were functionalized with spiropyran groups. In the second step, half of each sphere was coated with a platinum layer (30 nm thick). Pt is an efficient catalyst for the decomposi-tion of hydrogen peroxide into water and oxygen; the production of O2 bubbles can induce autonomous self-propulsion of Pt-coated colloids in solutions containing the H2O2 “fuel”. Ren

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and co-workers have demonstrated the ability to reversibly assemble their Janus motors into small oligomers upon expo-sure to UV light in a DMF–water mixture. The assembled motors moved collectively and exhibited different motion patterns compared to individual particles.[121]

The light-controlled complexation of azobenzene by cyclo-dextrins (see Section 5.1) has been employed on the micro-scale by Wu and co-workers, who asymmetrically coated 1 µm polystyrene particles with thin layers of gold.[122] Metallic patches of decreasing sizes were created by changing the angle of incidence of the evaporated gold. These patches were then functionalized with thiolated β-CD. Addition of a polymer with azobenzene side groups resulted in the formation of trans-AB⊂β-CD inclusion complexes, assembling the Janus particles into clusters. Interestingly, the sizes of the clusters could be controlled by the size of the “sticky” gold patch—the smaller the patch, the smaller the clusters. Alternating the expo-sure to UV and visible light led to reversible dissociation and reformation of the inclusion complexes, and therefore of the aggregates. In other recent notable examples of light-controlled self-assembly of microparticles, Wegner and co-workers demon-strated light-induced narcissistic[123] and social[124] self-sorting of 2 µm polystyrene beads decorated with photoswitchable pro-teins, whereas Fan and co-workers developed dual-responsive colloids by coating magnetic Fe3O4 particles with monolayers of spiropyran molecules.[125]

Colloidal, micrometer-sized particles can also be assembled in other ways that are hardly possible for small NPs. Much attention has been devoted to manipulating particles using optical tweezers;[126] unfortunately, the high laser intensities[127] (on the order of MW cm−2) required for this method make it rather impractical. In one exception, Zheng demonstrated the templated assembly of triangular gold particles with a laser of reduced intensity (≈10 kW cm−2).[128] Gold nanoislands on a glass slide were used as the templates. Upon exposure to a 532 nm laser, these islands produced heat via the plasmonic photothermal effect. The resulting temperature-gradient field produced a thermoelectric effect (i.e., a thermally induced local electric field), which drove the positively charged triangles to the hot regions, assembling them within only 10 s. As soon as the irradiation was discontinued, disassembly commenced. Impor-tantly, whereas the gold nanoislands were necessary to initiate self-assembly, the resulting aggregates produced enough heat on their own to maintain the force balance in the system. Con-sequently, these aggregates could be transported using the laser beam over large distances.[128] However, although this method required light intensities ≈100 times smaller than those used for optical tweezing, the intensity employed was still seven orders of magnitude higher than the those typically required for azobenzene photoswitching.

10. Summary and Outlook

In summary, we have categorized the studies on light-controlled self-assembly of NPs hitherto reported into six distinct approaches: i) functionalizing NPs with monolayers of photoswitchable molecules, ii) covalent bond formation and cleavage using two different wavelengths of light, iii)

(de)protonating NP-bound ligands using small-molecule photo-acids/photobases, iv) light-controlled adsorption/desorption of photoswitchable molecules onto NPs, v) inducing phase tran-sitions of NP-bound thermoresponsive polymers on plasmonic NPs through the photothermal effect, and vi) light-induced electron transfer between the particle’s inorganic core and the NP-bound ligands. Despite this diversity of methods ena-bling the control of NP self-assembly using light, research in this field will continue to thrive, with several interesting direc-tions outlined below. First, relatively little attention has been devoted to light-induced self-assembly of nonspherical NPs; with few exceptions,[94,101,102] all the examples reported thus far are based on nanospheres. In principle, all of the approaches described above are applicable to NPs of other shapes, such as cubes and 2D prisms. Second, the recent years have seen the development of a plethora of new photo chromic systems, some of which have significantly improved switching charac-teristics, such as the greatly increased thermal half-lives of the metastable state[129] and the ability to switch using red[130,131] or NIR[132–134] light. The ability to control NP self-assembly using these and other new molecular switches remains to be investi-gated. Third, a valid question is what is the maximum number of components that can be selectively addressed in a complex system of NPs—each decorated with a different photochromic compound—using different colors of light. The key to such selective control is a good separation of absorption bands of the different light-sensitive components of the system. Even with the relatively broad absorption features of most photochromic compounds, it should be possible to selectively assemble a given NP type in a mixture of at least five components; thus far, selective self-assembly in a system comprising only two has been demonstrated.[45] Fourth, it will be interesting to develop NPs capable of responding simultaneously to light and other external stimuli (such as the recently developed “dual-responsive” NPs assembling in response to light and magnetic fields[54]). In this context, NPs self-assembling in response to chemical fuels[135,136] and light are particularly interesting. Fifth, in virtually all of the examples covered in this review, light-induced self-assembly of NPs was carried out in bulk solution. Different dynamics of self-assembly and morphologies of the assembled structures can be expected for processes carried out in other environments, such as at a liquid–air interface[137,138] and inside confined spaces.[139] Sixth, an important outstanding challenge is to reversibly control the self-assembly of NPs using light in biological environments, which can lead to novel ways of modulating biological functions and treating diseases. The major advances in developing molecular switches responding to long-wavelength light will undoubtedly facilitate achieving this goal. One way in which self-assembling NPs can prove useful is to controllably release biologically active cargo upon disassembly induced by visible or NIR light (compare with ref. [55]). It is particularly intriguing to combine this approach with traditional photo pharmacology,[140,141] where the biological activity of small molecules is modulated with light. Last but not least, additional approaches to control the self-assembly of NPs using light—beyond the six reported so far—are possible. To cite one example, we are currently developing a chemical reac-tion network with NP self-assembly as the final outcome of a sequence of several reactions.

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AcknowledgementsThis work was supported by the European Research Council (Grant No. 820008 to R.K.). Z.C. acknowledges support from the Planning and Budgeting Committee of the Council for Higher Education, the Koshland Foundation, and a McDonald-Leapman grant.

Conflict of InterestThe authors declare no conflict of interest.

Keywordslight, nanoparticles, self-assembly

Received: September 8, 2019Revised: October 7, 2019

Published online: November 11, 2019

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The Many Ways to Assemble Nanoparticles Using Light · composition of the NPs’ inorganic core and...

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