Supporting Information:

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1 Supporting Information: Sequential Actuation of Shape-Memory Polymers through Wavelength-Selective Photothermal Heating of Gold Nanospheres and Nanorods Sumeet R. Mishra and Joseph B. Tracy*, Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States * S-1

2 Experimental Section Chemicals. Cetyltrimethylammonium bromide (CTAB, Sigma-Aldrich, 99%, H6269), KBr (Alfa Aesar, ACS, 99% min), AgNO3 (Alfa Aesar, %), HAuCl4 xh2o (Alfa Aesar, %, where x was estimated as 3), deionized water (Ricca, ACS Reagent grade, ASTM Type I, ASTM Type II), ascorbic acid (AA, J.T. Baker, 99.5%), NaBH4 (Sigma-Aldrich, 99%, ), oleylamine (Acros, 97% primary amine), poly(ethylene glycol)-monomethyl ether thiol (PEG-SH, Sigma-Aldrich, MW 2000), and anhydrous toluene (EMD, DriSolv) were used for synthesizing Au nanospheres (GNSs) and Au nanorods (GNRs). For GNS and GNR purification, storage, and composite preparation, methanol (Macron, ChromAR), tetrahydrofuran (THF, EMD, OmniSolv, Non-UV), and toluene (Macron, ChromAR) were used as obtained. Gold Nanosphere Synthesis. GNSs were synthesized according to an established method mg of HAuCl4 xh2o was added to a mixture of 0.6 ml of toluene and 0.5 ml of oleylamine to prepare the Au precursor solution. 24 ml of toluene and 1.6 ml of oleylamine were mixed in a separate 3-neck, round-bottomed flask fitted with a condenser at room temperature and heated under ambient atmosphere to 110 C. Once boiling commenced, indicated by vapors of toluene condensing on the walls of the condenser and flask, the Au precursor solution was quickly injected by syringe. After observing the solution change from bright orange to colorless within a few seconds, the solution was held at 110 C for 2 hours, after which it appeared a dark red wine color, indicating formation of GNSs. An aliquot of the product was purified by adding excess methanol (~1.5 times the volume of the aliquot) and centrifuging (IEC Centra MP4, 816 rotor) at 4500 rpm (2880 g) for 2 minutes. The sedimented GNSs were redispersed in THF. Gold Nanorod Synthesis. GNRs were synthesized on a 1000 ml scale according to an established seeded-growth method. 2 Two different solutions were prepared, a seed solution and a growth solution. The seed solution, which contains small Au nanoparticles, was added to the growth solution, in which they grew into GNRs. The growth solution was prepared as follows: 36.3 g of CTAB was dissolved in 450 ml of deionized water in a 1000 ml media bottle. Complete dissolution was achieved by gently heating using a heat gun and keeping the bottle immersed in a water bath at 30 C for at least 1 hour. 10 ml of aqueous KBr solution (112 mg/ml) was added to the CTAB solution, followed by 10 ml of aqueous AgNO3 (3.255 mg/ml). In a separate container, ml of aqueous HAuCl4 xh2o (20.0 mg/ml) was added to 500 ml of deionized water. The resulting dilute Au precursor solution was slowly added to the growth solution, while swirling the mixture in the bottle by hand. 15 ml of AA solution (12.4 mg/ml) was then quickly added to the growth solution, and the bottle was gently swirled by hand, until the growth solution turned colorless. For preparation of the seed solution, g of CTAB was dissolved in 8 ml of deionized water ml of aqueous KBr (112 mg/ml) and ml of deionized water were then added, followed by adding 1 ml of HAuCl4 solution (2 mg/ml). 0.6 ml of freshly prepared, ice-cold aqueous NaBH4 (0.376 mg/ml) was then added while stirring vigorously. The solution immediately turned dark brown, indicating formation of small Au nanoparticles used to seed growth of GNRs. Stirring was stopped after 3 minutes, and the seed solution was allowed to stand for an additional 2 minutes ml of the seed solution was then added to the growth solution and gently shaken to completely disperse the Au nanoparticle seeds. The growth solution was then allowed to stand in the water bath at 30 C for 60 minutes without stirring, over which growth of the GNRs started, indicated by the solution becoming a pale pink color. In a secondary growth step, 30 ml of AA solution (2.773 mg/ml) was then continuously added using a syringe pump at S-2

3 a rate of 173 µl/min for a total injection time of 173 minutes. The growth solution gradually turned dark red, indicating formation of larger GNRs. The products were purified by centrifugation (Thermo Scientific Sorvall Legend X1R with Fiberlite F15-6x100y rotor) at rpm (15777 g) for 15 minutes. For characterization of the nanoparticles prior to dispersion in the polymer using a JEOL 2000FX transmission electron microscope, small volumes of GNSs and GNRs were further diluted in toluene and water, respectively. Drops of each dilute solution were cast onto Cu TEM grids coated with ultrathin carbon and Formvar support films. The nanoparticle sizes were manually measured using ImageJ. The average dimensions of the GNRs were 81.0 nm 24.0 nm, and the average diameter of the GNSs was 14.1 nm. To facilitate dispersion in the polymer, 1 ml of the purified GNR solution (3.6 mg/ml) was diluted with deionized water to a total volume of 5 ml and PEGylated by adding 2.85 mg of PEG-SH and stirring for 24 hours. The PEGlyated GNRs were purified through two rounds of centrifugation (IEC Centra MP4, 854 rotor) at rpm (8418 g) for 5 minutes. After the first centrifugation cycle, the sedimented, PEGylated GNRs were dispersed in 5 ml of THF. The final product was also dispersed in THF. Composite Preparation. GNSs or PEGlyated GNRs were dispersed in 5 ml of THF to prepare films with concentrations of 3 wt% GNSs or 0.5 wt% PEGylated GNRs in Diaplex (SMP Technologies, MM5520), a thermoplastic polyurethane (TPU), through solvent casting. These loadings are based on assuming complete reduction of HAuCl4 during the synthesis of GNSs and GNRs and no losses during purification and PEGylation of GNRs. 0.2 g of Diaplex in the form of pellets was added to each dispersion of GNSs or PEGylated GNRs while stirring with a magnetic stir bar to create homogenous dispersions of GNSs or PEGylated GNRs and Diaplex in THF. The Diaplex pellets completely dissolved within 30 minutes, and the mixture because much more viscous. For each sample, the mixture was then poured into a polytetrafluoroethylene (PTFE) mold with lateral dimensions of 102 mm 25 mm and covered with Al foil with three pinholes for controlled evaporation of the solvent. After allowing 6 hours for the THF to evaporate, the Al foil was removed, and the composite polymer film was pulled out of the PTFE mold using a pair of forceps. Using a mold composed of PTFE prevents the film from getting stuck to the mold. The composite films had a thickness of 100 µm and were stored at room temperature. The GNS-TPU and GNR-TPU composite films shown in Figure 2 were attached to Al sample mounts using carbon tape and imaged using a FEI Verios 460L field-emission scanning electron microscope with a concentric backscatter detector with an accelerating voltage of 5 kv and a bias voltage of 500 V. Photothermal Heating. Two types of commercially available LEDs were assembled into aluminum housings, an Osram Golden Dragon SFH 4750 with peak emission at 860 nm and three Luxeon Rebel LXML-PM LEDs mounted on the same board with peak emission at 530 nm. These are referred to as the 860 nm and 530 nm LEDs, respectively. Filters were added to the LEDs as a further precaution to minimize any potential spectral overlap. A nm bandpass filter (Thorlabs, FGB39) was taped to the housing for the 530 nm LED, and a 780 nm longpass filter (Thorlabs, FGL780) was taped to the housing for the 860 nm LED. Shape recovery of the samples was triggered by wavelength-selective photothermal heating, where the samples were exposed to one of the LEDs or sequentially to both LEDs. Wavelength-Controlled Stage. A composite device was constructed from a piece of folded Al foil (35 mm 12 mm) with two accordion-shaped strips of GNR-TPU and GNS-TPU, each with dimensions of 42 mm 17 mm 100 µm. These strips served as legs to push the Al foil top up and were cut from the same films described earlier. Each strip was folded by hand into an S-3

4 accordion shape at room temperature and attached using a small piece of double-sided tape to the rectangular Al foil stage on one end and to a glass-slide base on the other end. 530 nm and 860 nm LEDs were positioned to the sides of the GNS-TPU and GNR-TPU legs, respectively. Sequential extension of the accordion legs and lifting and tilting of the Al foil top were investigated by sequentially switching on and off the 530 nm and 860 nm LEDs. S-4

5 Scanning Electron Microscopy of GNS-TPU and GNR-TPU Thin Films (a) GNS-TPU (b) GNS-TPU (c) GNR-TPU (d) GNR-TPU Figure S1. Scanning electron micrographs of (a,b) GNS-TPU and (c,d) GNR-TPU thin films at (a,c) low magnification with a common scale bar and (b,d) high magnification with a common scale bar. References (1) Hiramatsu, H.; Osterloh, F. E. A Simple Large-Scale Synthesis of Nearly Monodisperse Gold and Silver Nanoparticles with Adjustable Sizes and with Exchangeable Surfactants. Chem. Mater. 2004, 16, (2) Kozek, K. A.; Kozek, K. M.; Wu, W.-C.; Mishra, S. R.; Tracy, J. B. Large-Scale Synthesis of Gold Nanorods through Continuous Secondary Growth. Chem. Mater. 2013, 25, S-5