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1 DOI: /NCHEM.2303 Light-controlled self-assembly of non-photoresponsive nanoparticles Pintu K. Kundu, 1 Dipak Samanta, 1 Ron Leizrowice, 1,2 Baruch Margulis, 1,3 Hui Zhao, 1 Martin Börner, 1,4 T. Udayabhaskararao, 1 Debasish Manna, 1 and Rafal Klajn*,1 1 Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel. 2 Department of Chemistry, University of Southampton, Southampton SO17 1BJ, United Kingdom. 3 School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel. 4 Institute of Inorganic Chemistry, University of Leipzig, Leipzig 04109, Germany. * rafal.klajn@weizmann.ac.il. NATURE CHEMISTRY 1

2 General information: All chemical reagents were purchased from commercial suppliers and used without purification. Abbreviations used: dodecylamine, DDA; didodecyldimethylammonium bromide, DDAB; tetrabutylammonium borohydride, TBAB; 11-mercaptoundecanoic acid, MUA; 6-mercaptohexanoic acid, MHA. Transmission electron microscopy studies were carried out on a CM120 Super Twin microscope (Philips) operating at 120 kv. As a visible light source, we used a fluorescent bulb (50 W, light intensity ~1.0 mw cm 2 ). UV-Vis spectra were recorded on a UV-3600 UV-Vis-NIR spectrophotometer (Shimadzu). Dynamic light scattering was studied with Zetasizer Nano ZS (Malvern). Synthesis and functionalization of 2.5 nm Au NPs: DDAB stock solution was first prepared by dissolving 833 mg DDAB in 18 ml toluene (with sonication). DDA (450 mg, 2.43 mmol) and HAuCl 4 3H 2 O (50 mg, 125 µmol) were dissolved in 12.5 ml of the stock solution with sonication. Gold (III) was then reduced by fast addition of tetrabutylammonium bromine (125 mg, 486 µmol) in 5 ml of the DDAB stock solution under vigorous stirring, affording a solution of DDA-stabilized 2.5 nm Au NPs. After 1 h, a solution of MUA in toluene (35 mg in 0.5 ml, which corresponds to a large (ca. 7-fold) excess of MUA, calculated assuming that a single thiolate ligand occupies an area of nm 2 on the surface of gold) was added and the mixture was stirred for 15 min. The resulting precipitate was collected with centrifugation and copiously washed with 20 ml of toluene. Finally, MUA-functionalized 2.5 nm Au NPs were dissolved in 17.5 ml of MeOH. Synthesis and functionalization of 11 nm Au NPs: 11 nm Au NPs were prepared via sequential growth of small gold particles (seeds). First, a seed solution was prepared by injecting a solution of 25 mg TBAB in 4 ml of toluene containing 46 mg of DDAB into a solution of 185 mg of DDAB, 105 mg of DDA, and 11 mg of HAuCl 4 3H 2 O (dissolved in this order) in 4 ml of toluene. The resulting mixture was allowed to age for 24 hours. To 1 ml of the seed solution was added a solution containing 23 mg of DDAB, 43 mg of DDA, and 4.5 mg of HAuCl 4 3H 2 O (dissolved in this order) in 1.1 ml of toluene. To the resulting growth solution was added dropwise (within 5 minutes) a solution of 3 µl of N 2 H 4 H 2 O in 0.5 ml of toluene containing 37 mg of DDAB under vigorous stirring. The resulting mixture was allowed to age for 24 hours. To the resulting solution of ~7 nm NPs in 2.6 ml of toluene was added a solution containing 106 mg of DDAB, 198 mg of DDA, and 21 mg of HAuCl 4 3H 2 O (dissolved in this order) in 5.25 ml of toluene. Finally, to the resulting growth solution was added dropwise a solution of 13.7 µl of N 2 H 4 H 2 O in 2 ml of toluene containing 176 mg of DDAB under vigorous stirring. The mixture was allowed to age overnight, resulting in a monodisperse batch of 11 nm Au NPs (c Au = 7.1 mm; concentration in terms of gold atoms). Next, 6 ml solution of a diluted (c Au = 4.8 mm) solution of 11 nm Au NPs was mixed with 5 ml of methanol, and the mixture was left on an orbital shaker (at a slow shaking rate) for about one hour. The resulting black precipitate was collected by decantation and redissolved in 6 ml of pure toluene. Solid MUA (7 mg, which corresponds to a large (ca. 26-fold) excess of the ligand) was added and the mixture was stirred for several hours. The resulting black precipitate was collected with centrifugation and washed NATURE CHEMISTRY 2

3 extensively with toluene (25 ml) to remove excess of unbound MUA. Finally, modified Au NPs were redissolved in 6 ml of MeOH containing 8 µl of a 1.25 M methanolic solution of HCl. Synthesis of SP: A solution of 2,3,3-trimethylindolenine (7.7 g, mmol) and methyl iodide (17.2 g, mmol) in acetonitrile (90 ml) was refluxed for 18 h under an argon atmosphere. The solvent was removed under reduced pressure and the residue was triturated with ether (120 ml), yielding a powder. The solid was filtered off, washed with ether (2 130 ml), and then dried under high vacuum. The resulting solid was then added to an aqueous solution of KOH, stirred for 30 min, and extracted with ether to afford 1,3,3-trimethyl-2-methyleneindoline (7.5 g, 89%). Next, 1,3,3-trimethyl-2-methyleneindoline (4 g, mmol) was dissolved in ethanol (270 ml) and refluxed with salicylaldehyde (3.14 ml, 29.8 mmol) under a nitrogen atmosphere for 15 h. The reaction mixture was then cooled to room temperature and the solvent was removed under reduced pressure. The residue was dissolved in CH 2 Cl 2 and washed with aqueous NaOH. The crude product was then purified by column chromatography on silica using EtOAc-hexane (3:97) as eluent to afford the pure product (5.6 g, 87%) as a crystalline solid. Supplementary Figure 1 1 H NMR (CD 2 Cl 2, 300 MHz) spectrum of SP. NATURE CHEMISTRY 3

4 Supplementary Figure 2 Representative measurements of ζ-potential vs. time. At t = 0, irradiation with blue light was discontinued and the sample was placed in the dark. In general, following kinetics by ζ-potential is challenging because of the significant amount of time that is required to record an averaged ζ-potential vs. intensity plot (hence, we used diluted samples in these experiments to slow down the aggregation process). We established that the minimum time required to record such a single plot was ~20 sec. The figure shows representative plots (out of ca. 100 plots recorded in ten aggregation experiments). For the blue and red traces, ζ = mv and +4.4 mv, respectively. The lowest ζ-potential we measured prior to the onset of self-assembly was 3.5 mv. Supplementary Figure 3 Following reversible (here, we show six cycles) disassembly and assembly of MUA-functionalized 5.5 nm Au NPs (upon exposure to blue light and under ambient conditions) with UV-Vis spectroscopy. NATURE CHEMISTRY 4

5 Supplementary Figure 4 Reversible (here, five cycles shown) disassembly and assembly of MHA-functionalized 5.5 nm Au NPs upon exposure to blue light and under ambient conditions. Supplementary Figure 5 Following the reversible assembly and disassembly of MHA-decorated 5.5 nm Au NPs using DLS. NATURE CHEMISTRY 5

6 Supplementary Figure 6 TEM images of 11 nm Au NPs functionalized with MUA. The images in the top panel show particles from a sample prepared under blue light illumination. The images in the bottom panel show aggregated NPs (the samples were prepared under ambient light conditions). Supplementary Figure 7 TEM images showing quasi-linear assemblies of MUA-functionalized gold NPs. Here, the experiment was performed with an equimolar (in terms of surface-bound MUA ligands) mixture of 5.5 nm and 11 nm NPs. The images shown in the top panel and the two images on the left in the bottom panel are of samples that were collected within seconds after the onset of assembly. The last two images show aggregates formed several minutes after the onset of assembly. NATURE CHEMISTRY 6

7 Synthesis of a model dye (benzoxadiazole derivative): The compound was synthesized based on a previously published procedure (Onoda, M., Uchiyama, S., Santa, T. & Imai, K. A photoinduced electron-transfer reagent for peroxyacetic acid, 4-ethylthioacetylamino-7-phenylsulfonyl-2,1,3- benzoxadiazole, based on the method for predicting the fluorescence quantum yields. Anal. Chem. 74, (2002)). HO H N N O N NO 2 Supplementary Figure 8 1 H NMR (DMSO-d 6, 300 MHz) spectrum of a model dye (NBD). Supplementary Figure 9 Additional photographs of a freestanding piece of a crosslinked PEG gel containing MUA-functionalized 5.5 nm Au NPs and SP. The cut piece shown here has dimensions of about 3 cm 4 cm 1 mm. NATURE CHEMISTRY 7