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Here, we hypothesized that a water-soluble photoacid, indi-cated in Figure 1 as MCH + , could be used to reversibly tune the strength of interparticle interactions in aqueous environments. The design of our system is illustrated in Figure 1 , bottom panel. Initially, NPs functionalized with a ω -mercaptocarboxylic acid are readily soluble in an aqueous solution of MCH + owing to partial deprotonation of the COOH groups. Visible-light-induced release [ 25 ] of H + causes protonation of the terminal COO – moieties, thereby inducing NP assembly. The resulting aggregates, however, are metastable—they disassemble when irradiation is discontinued, and the assembly–disassembly cycle could be repeated. This approach would enable us to not only switch to aqueous environments, but it would offer a system complementary to, and arguably more attractive than the one described before, [ 22 ] in that the metastable state of the system would be the assembled one.

MCH + was fi rst described by Liao and co-workers in 2011. [ 26 ] Following the original report, several creative applications of this water-soluble “photoacid” have been reported. Apraha-mian and co-workers demonstrated that MCH + could be used to reversibly operate a hydrazone-based switch, which other-wise relied on consecutive injection of an acid and a base to the system. [ 27 ] Eelkema, van Esch et al. achieved spatial control over gelation by using MCH + as an acid catalyst for the formation of hydrogels. [ 28 ] Liao and co-workers utilized the light-induced pH changes to inactivate multidrug-resistant bacteria. [ 29 ] Thanks to MCH + , the above and other diverse functions, [ 30–32 ] which previ-ously relied on acid inputs, can now be realized by means of light. However, coupling of the light-induced proton release to nanoparticle assembly in water has yet to be demonstrated.

In our initial attempt to address this defi ciency, we worked with 5.8 nm gold NPs functionalized with a monolayer of 11-mercaptoundecanoic acid (MUA) (5.8 nm refers to the dia-meter of the inorganic core). These NPs were readily soluble in water provided that a suffi cient fraction of the COOH groups was deprotonated (which we induced by adding ≈4 equivalents of tetramethylammonium hydroxide (TMA + OH – ) for each surface-bound COOH; see the Experimental Section). The excellent solu-bility of these NPs can be attributed to i) the effi cient solvation of the terminal COO – moieties by water molecules, and ii) electro-static repulsion between the negatively charged NPs. Conse-quently, acidifying the NP solution (induced by introducing a small amount of HCl) led to NP aggregation, which could be reversed by adding a base. However, we found that the amount of H + released during the ⎯ →⎯ ++ +MCH SP HVis reaction was insuffi cient to induce the MUA-coated particles to fl occulate (we tested a variety of initial pH values), even in the presence of a saturated solution of MCH + . Hence, we decided to work with NPs functionalized with another ω -mercaptocarboxylic acid, namely, MHA (6-mercaptohexanoic acid), which, because of its shorter length, would facilitate the attractive van der Waals (vdW) interactions between the NP cores. Indeed, aggregation

Aqueous Light-Controlled Self-Assembly of Nanoparticles

Dipak Samanta and Rafal Klajn *

Dr. D. Samanta, Dr. R. Klajn Department of Organic Chemistry Weizmann Institute of Science Rehovot 76100 , Israel E-mail: rafal.klajn@weizmann.ac.il

DOI: 10.1002/adom.201600364

The ability to reversibly tune the properties of materials using external stimuli is an important concept in contemporary materials science as it serves as a route toward the develop-ment of new stimuli-responsive materials. [ 1–4 ] In that respect, self-assembly of inorganic nanoparticles (NPs)—whose proper-ties often depend on the degree of aggregation—have attracted considerable attention. [ 5,6 ] In particular, signifi cant efforts have been devoted to NPs reversibly assembling in response to light, [ 7–10 ] as this stimulus can be delivered remotely, with a high spatial and temporal precision, and in the form of different wavelengths, which can potentially induce different processes.

An attractive way to render NPs responsive to light is based on decorating their surfaces with monolayers of the photo-switchable trans- azobenzene molecules. [ 11–16 ] The resulting NPs are readily soluble in nonpolar solvents; however, exposure to UV light results in azobenzene isomerizing to the more polar cis- isomer, which induces attractive interparticle inter actions. [ 17 ] The resulting NP aggregates are metastable, and they disinte-grate in the dark within hours to days. This reversible, light-controlled aggregation of NPs has been used to dynamically control the occlusion of polar colloids by the light-sensitive NPs, [ 18 ] the optical properties of NP-doped gels, [ 19 ] and the kinetics of chemical reactions occurring in the system. [ 20,21 ] Despite the diversity of these functions, however, the practical potential of azobenzene-functionalized NPs has been greatly limited by the fact that the above self-assembly scheme can only operate in hydrophobic solvents.

Recently, we reported on a novel methodology to assemble NPs using light, which does not require that the NPs be func-tionalized with light-responsive ligands. [ 22 ] In this approach, a protonated merocyanine exposed to visible light undergoes a ring-closing reaction, thereby releasing H + ions, which solubi-lize otherwise insoluble NP aggregates ( Figure 1 , top panel). When light was turned off, the closed-ring isomer of the switch acted as a base, re-capturing H + and inducing the NPs to re-assemble. The appeal of this strategy lies in the facts that the sometimes challenging syntheses of photoresponsive NPs are not required and, more importantly, the photoresponsive mole-cules not confi ned to the surfaces of metallic NPs often exhibit greatly improved switching performances. Despite these sig-nifi cant advantages, this strategy is limited to organic solvents such as methanol, and the grand challenge—namely, to achieve reversible, light-induced self-assembly of NPs in water—has remained largely [ 23,24 ] unmet.

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of MHA-functionalized NPs stabilized with TMA + OH – could be induced by exposing the solutions to visible light in the pres-ence of MCH + —unfortunately, the process was irreversible in that once aggregated, the NPs could not be redispersed. After screening solutions having different ionic strengths and pH values, we identifi ed 52 × 10 −3 M NaCl in the presence of a dilute (3.5 × 10 −3 M ) pH 6 phosphate buffer and ≈0.14 × 10 −3 M MCH + (i.e., ≈10 eq. with respect to NP-bound COOH groups) as the ideal photoresponsive medium in which MHA-coated NPs could be reversibly assembled.

Figure 2 a (red traces) shows typical light absorption spectra of the optimized solution in the dark or under ambient laboratory light. The intense absorption below ≈500 nm is due to the pres-ence of MCH + . When the solution was exposed to a visible light source (we used a regular desk lamp equipped with a 50 W fl u-orescent bulb), we observed a rapid NP assembly, as manifested by the red shift of the NPs’ localized surface plasmon resonance (SPR) band accompanied by an increase in the overall absorp-tion in the low-energy part of the spectrum (Figure 2 a, purple traces). The process was induced by the ≈420–460 nm part of the visible spectrum (i.e., blue light), which we concluded by

exposing our solutions to various light-emitting diodes. As soon as the exposure to light was discontinued, disassembly of the NP aggregates commenced. Interestingly, the assembly–dis-assembly sequence could be repeated at least 30–40 cycles (of which the initial ten are shown in Figure 2 b), and the system showed no deterioration in performance 24 h after prepara-tion. However, prolonged storage resulted in the hydrolysis of MCH + , in agreement with previous reports. [ 27,33,34 ]

Dynamic light scattering (DLS) proved to be another con-venient way to study the reversible assembly of NPs. The dashed lines in Figure 2 c represent distributions of hydrody-namic diameters after exposing the solutions to visible light for 1 min; the peak is focused at 70–90 nm. Following 5 min in the dark, the peak shifted and was in the 12–16 nm range, typical for small non-interacting NPs (Figure 2 c, solid lines). We also followed the reversible aggregation by transmission electron microscopy (TEM) (Figure 2 e), which allowed us to determine

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Figure 1. Light-controlled self-assembly of non-photoresponsive nano-particles. Top: Light-induced disassembly of MUA-coated NPs in meth-anol. [ 22 ] The particles spontaneously reassemble in the dark. Bottom: Light-induced assembly of MHA-coated NPs in water. Here, the transient NP aggregates spontaneously disassemble in the dark.

Figure 2. Reversible, light-controlled self-assembly of MHA-coated 5.8 nm gold nanoparticles. a) Absorption spectra of 5.8 nm Au NPs in an aqueous solution of MCH + recorded in the dark (red traces) and immedi-ately following exposure to visible light (purple traces). Repeated spectra correspond to consecutive assembly–disassembly cycles. b) Changes in the absorbance at 800 nm caused by alternating exposure to visible light (purple markers) and dark incubation (red markers). c) DLS profi les of the NPs in the dark (solid traces) and immediately following exposure to visible light (dashed traces). d) Changes in the hydrodynamic diameters caused by alternating exposure to visible light (purple markers) and dark incubation (red markers).e) Typical TEM images of dark-adapted NPs (left) and a sample exposed to visible light (right).

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that three minutes of visible light irradiation were suffi cient to quantitatively assemble the NPs (i.e., virtually no free NPs were observed at that point), whereas the disassembly step was com-plete within fi ve minutes in the dark.

Importantly, our strategy could also be applied for differ-ently sized NPs. Figure 3 shows the behavior of 10 nm Au NPs functionalized with MHA. Similar to the smaller NPs, the 10 nm particles placed in the photoresponsive medium reversibly aggregated when exposed to a source of visible light (Figure 3 a–e). However, the larger NPs were inferior with respect to reversibility: typically, only 7–8 cycles could be per-formed with full reversibility, after which the dark-adapted solu-tions contained NP aggregates of increasing sizes.

We were also interested in investigating the kinetics of assembly and disassembly as a function of NP size ( Figure 4 ). Since both processes proceeded too quickly to be

studied by DLS measurements (which typically require at least 10 s to accurately measure particle size distributions), we decided to follow the absorbance at 800 nm ( A 800 ), which is proportional to the extent of NP aggregation. In our experi-ments, we exposed solutions of MHA-coated NPs in the photo-responsive medium to visible light for short periods of time (i.e., not long enough to complete the assembly process), after which changes in A 800 were followed—this strategy allowed us to study both the assembly and disassembly in a single experi-ment. Typical experimental data are shown in Figure 4 a, where the red and the blue markers correspond to 5.8 nm and 10 nm NPs, respectively, present at the same surface concentration of MHA and placed in a solution containing identical concentra-tions of MCH + and other (inorganic) solutes (i.e., the only dif-ference is the NP size). Interestingly, we repeatedly observed that the larger NPs both assembled and disassembled more rapidly than the smaller ones (see the normalized data in Figure 4 b). The faster assembly of the 10 nm NPs can easily be understood considering their i) smaller curvature, hence decreased distances between the terminal COO – moieties, [ 35,36 ] which facilitates the capture of H + released from MCH + , and ii) larger size, and consequently stronger vdW interactions. The fact that once aggregated, the larger NPs also disassem-bled faster is a more intriguing fi nding. We hypothesize that this observation can be explained by taking into consideration the larger curvature of (and therefore increased conformational freedom [ 37,38 ] of MHA on) the smaller NPs. In other words, the terminal COO – groups on 5.8 nm NPs may be able to adjust their mutual orientations in order to form the COO – ···HOOC bridges more effi ciently.

Encouraged by these results, we examined the possibility of selectively assembling large vs. small MHA-coated NPs sharing the same photoresponsive medium. To this end, we prepared a solution containing 5.8 nm and 10 nm MHA-coated Au NPs, mixed in such a ratio that the total numbers of MHA ligands bound to both NPs were the same. Despite signifi cant efforts, however, we did not observe pronounced differences in the assembly kinetics of the two types of NPs in solutions in which they coexisted. Representative TEM images taken at dif-ferent times of exposure to visible light are shown in Figure 4 d; whereas the unassembled NPs observed at short irradiation times were mostly the small ones, and the cores of the aggre-gates contained primarily the large particles, the effect was rather minor. Similarly, no selective dissolution of the larger NPs from the mixed NP aggregates was found; as Figure 4 e shows, residual aggregates found after increasing periods of dark incu-bation comprised similar numbers of the large and the small NPs. These results could be rationalized by the rapid exchange of H + between the different NPs.

In conclusion, we developed a novel nanoscale self-assembly system allowing for light-controlled, reversible assembly of nanoparticles in aqueous media. The key component of the system is a water-soluble photoacid, which, upon exposure to visible light, transiently decreases the pH of the solution, thereby inducing the formation of dynamic NP aggregates. In the dark, rapid disassembly takes place. As such, our system represents a unique example of light-induced, dissipative self-assembly [ 39–43 ] in water. We also uncovered a peculiar size-dependent behavior of NPs by showing that larger particles

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Figure 3. Reversible, light-controlled self-assembly of MHA-coated 10 nm gold nanoparticles. a) Absorption spectra of 10 nm Au NPs in an aqueous solution of MCH + recorded in the dark (red traces) and immediately following exposure to visible light (purple traces). Repeated spectra correspond to consecutive assembly–disassembly cycles. b) Changes in the absorbance at 800 nm caused by alter-nating exposure to visible light (purple markers) and dark incubation (red markers). c) DLS profi les of the NPs in the dark (solid traces) and immediately following exposure to visible light (dashed traces). d) Changes in the hydrodynamic diameters caused by alternating expo-sure to visible light (purple markers) and dark incubation (red markers).e) Typical TEM images of dark-adapted NPs (left) and a sample exposed to visible light (right).

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exhibit increased kinetics of not only light-induced assembly, but also of spontaneous disassembly. Future work will focus on coupling the aqueous NP self-assembly to various biological molecules and processes.

Experimental Section Synthesis of 5.8 nm Gold Nanoparticles : Didodecyldimethylammonium

bromide (DDAB) stock solution was fi rst prepared by dissolving DDAB (833 mg; 1.80 mmol) in toluene (18 mL) (with sonication). HAuCl 4 ·3H 2 O (50 mg; 125 µmol) and dodecylamine (DDA) (450 mg; 2.43 mmol) were added to 12.5 mL of the stock solution and sonicated until completely dissolved. Gold(III) was then reduced by rapidly adding tetrabutylammonium borohydride (TBAB) (125 mg; 486 µmol) in DDAB stock solution (5 mL) under vigorous stirring at room temperature, and stirring was continued for an additional 30 min. A solution of ≈2.6 nm NPs (“seeds”) prepared this way was aged for 24 h. A growth solution was prepared by adding to 50 mL of pure toluene the following reagents, in the following order: 1) DDAB (1.00 g), 2) DDA (1.85 g), 3) HAuCl 4 ·3H 2 O (200 mg), and 4) aged seed solution (7 mL). Finally, 131 µL of N 2 H 4 ·H 2 O dissolved in 20 mL of the DDAB stock solution was added dropwise to the growth solution under vigorous stirring, and the resulting mixture was stirred overnight. The obtained NPs had a diameter 5.75 ± 0.52 nm.

Synthesis of 10 nm Gold Nanoparticles : A growth solution was fi rst prepared by adding to 5.25 mL of pure toluene the following reagents, in the following order: 1) DDAB (106 mg), 2) DDA (198 mg),3) HAuCl 4 ·3H 2 O (21 mg), and 4) 2.6 mL of a toluene solution of 5.8 nm Au NPs (see above). 13.7 µL of N 2 H 4 ·H 2 O dissolved in 2.0 mL toluene containing 176 mg DDAB was then added dropwise to the growth solution under vigorous stirring, and the resulting mixture was stirred overnight. The obtained NPs had a diameter 10.07 ± 0.67 nm.

Functionalization of Gold Nanoparticles with 6-Mercaptohexanoic Acid : Prior to functionalization with MHA, as-prepared Au NPs (2.0 mL) were purifi ed from an excess of surfactants (DDA + DDAB) by mixing with one volume of methanol, decantation (after the NPs have sedimented; ≈1 h) (without removing the solvent to dryness), and redissolution in pure toluene. MHA dissolved in a small volume of toluene was added (we used eightfold excess for 5.8 nm NPs and 10-fold excess for 10 nm NPs; eight- or 10-fold excess refers to the number of the MHA molecules with respect to the number of the binding sites on Au, calculated assuming that a single thiolate moiety occupies an area of 0.214 nm 2 on the surface of Au [ 44 ] ) and the solution was shaken (on an orbital shaker) for 8 h. The resulting precipitate was collected, washed extensively with pure toluene to remove excess of unbound MHA, and fi nally dissolved in 2.0 mL of water containing 4 eq. of tetramethylammonium hydroxide with respect to the surface-bound MHA.

Light-Controlled Self-Assembly of MHA-Functionalized Nanoparticles : The photoresponsive medium was fi rst prepared by adding to 750 µL of a saturated aqueous solution of MCH + ( c ≈ 0.2 × 10 −3 M ) 38 µL of a phosphate buffer (14.7 × 10 −3 M KH 2 PO 4 + 81 × 10 −3 M Na 2 HPO 4 + 1.369 M

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Figure 4. Studying the effect of the nanoparticle size. a) Dynamic changes in the absorbance at 800 nm ( A 800 ) as a manifestation of NP assembly/disas-sembly. In these experiments, MHA-coated 5.8 nm (red) and 10 nm (blue) were fi rst exposed to visible light for 45 s, after which A 800 was monitored. 5.8 nm and 10 nm NPs were used at the same surface concentration of MHA. b) Data shown in (a) after normalization. c) Schematic representation of the NP size-dependent differences in NP (dis)assembly. d) TEM images of NP aggregates prepared by exposing a mixture of 5.8 nm and 10 nm NPs to visible light for increasing time intervals. e) Following the disassembly of transient NP aggregates by TEM.

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NaCl + 27 × 10 −3 M KC l), 20 µL of 0.1 M aqueous HCl, and 162 µL of distilled water. 50–60 µL of the nanoparticle solution prepared as described above was then added, and NP self-assembly was induced by exposing the resulting solution to visible light. Disassembly of the resulting NP aggregates commenced in the dark.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by the European Research Council (grant no. 336080) and by the Rothschild Caesarea Foundation. The authors gratefully acknowledge Dr. T. Udayabhaskararao for his assistance with nanoparticle synthesis.

Received: May 15, 2016 Revised: May 29, 2016

Published online: June 23, 2016

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