SUPPLEMENTARY INFORMATION

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1 SUPPLEMENTARY INFORMATION Study of Heat Transfer Dynamics from Gold Nanorods to the Environment via Time-Resolved Infrared Spectroscopy Son C. Nguyen, Qiao Zhang, Karthish Manthiram, Xingchen Ye, Justin P. Lomont, Charles B. Harris, Horst Weller, A. Paul Alivisatos 1. Preparation and characterization of CTAB capped gold nanorods (GNRs) and silicacoated GNRs colloids for time-resolved IR experiment Preparation of CTAB capped GNRs. CTAB capped GNRs were prepared via a seed-mediated growth method in a binary surfactant mixture of hexadecyltrimethylammonium bromide (CTAB) and sodium oleate (NaOL) in aqueous solution, developed by Christopher Murray and coworkers. 1 The characterization of GNRs are summarized in Table S1. To prepare samples for TRIR experiments, an as-synthesized 1 L batch was centrifuged at 7000 RPM to crash the particles and redispersed in 250 ml of Millipore water, which was repeated twice. In the final cleanning step, the particles were redispersed in 15 ml of Millipore water for use in the flow cell of the TRIR experiment. This solution had an absorbance of 0.77 OD at 800 nm in the flow cell with 70 m spacer. Preparation of silica coated GNRs. First, the CTAB capped GNRs were prepared as described above, except that the final solution was redispersed in 20 ml of Millipore water. The particles were coated with silica using base-catalyzed hydrolysis of tetraethyl orthosilicate (TEOS), causing condensation of silica onto the surface of CTAB capped GNRs. 2 Briefly, 20 ml of colloidal GNRs in the first step was added to a mixture of 1.2 L H 2 O, 0.6 L EtOH and 0.2 L stock CTAB solution under rigorously stirring for 30 minutes. The CTAB stock solution was prepared by adding 8 g CTAB to a solution of 65 ml EtOH and 135 ml H 2 O, which was stirred at 50 o C for 30 minutes to ensure the complete dissolution of CTAB powder. EtOH helps CTAB dissolve easily in the solution. 3 The colloidal GNRs solution has a final volume of 2 L and the longitudinal SPR peak has an absorption of ca. 1.3 OD in a 1 cm path length cell. We found this concentration was diluted enough to avoid multiple particles being coated in single shells but also high enough to avoid excessive volume in this step of the synthesis. Ammonium hydroxide solution (~3%, ~20 ml) was added slowly to adjust the ph to ~ 10. For the thick shells, 1.1 ml TEOS was finally added to the solution, and for the thin shells, 0.66 ml of TEOS was used. The reaction was completed after 24 h under rigorously stirring at room temperature. The product was centrifuged and redispersed in 250 ml Millipore water, which was repeated twice, and the final product was redispersed in to 12 ml colloidal solution for TRIR experiment. Characterization of this thick silica-coated GNRs is summarized in Table S1. The thick and thin 1

2 silica-coated GNRs solutions have absorbance of 0.85 and 1.0 OD, respectively, at 800 nm in the flow cell using 70 m spacer. Determination of CTAB concentration in colloidal sample in TRIR experiment: The supernatant of the colloidal solution was dried and redispersed into D 2 O for IR characterization. The OD stretching vibration of D 2 O in the low frequency region allowed us to detect the CH 3 stretching of CTAB at 2900 cm -1 region to determine the concentration of CTAB, otherwise the OH stretching of H 2 O overlapped strongly with this region. The CTAB concentrations in the colloidal solutions of CTAB capped GNRs and silica-coated GNRs for TRIR experiment are 1.6 and 1.8 mm, respectively. Optical and TEM characterization. Vis/Near-IR spectra were measured with Agilent 8453 UV- VIS spectrometer. TEM images were acquired using a 200 kv Tecnai G2 20 S-TWIN with a Gatan SC200 CCD camera. Determination of molar extinction coefficient. The CTAB capped GNRs and silica-coated GNRs colloid were digested by adding aqua regia solution. The gold concentrations were determined by a Perkin Elmer Inductively Coupled Plasma Optical Emission Optima 7000 DV system. Spherically capped cylinder geometry with dimensions from TEM characterization was used to calculate the average volume and number of gold atoms in the nanorod. Table S1. Characteristics of studied GNRs GNRs in this study CTAB capped GNRs Thick silicacoated GNRs Thin silicacoated GNRs Longitudinal SPR peak (nm) nanorod size (diameter x length, nm) 808 (20 ± 2) x (81 ± 12) 787 (36 ± 4) x (106 ± 9) 794 (26 ± 4) x (93 ± 7) Silica coated thickness (measured at the side x end of the rod, nm) molar extinction at 800 nm ε 800 (M -1 cm -1 ) molar extinction at SPR peak ε (M -1 cm -1 ) NA 1.73 x x (91 ± 5) x (84 ± 6) (35 ± 2) x (28 ± 3) 7.69 x x x x Temperature dependent IR spectra of pure water, colloidal solution of GNRs with thick silica coating We provide more spectral evidence for the linear dependence between the IR absorption of water solvent and the temperature change. A sample of pure water was used as a reference to ensure that our spectral observation on all samples comes from water solvent. The sample of thick silica-coated GNRs colloid was used to evaluate the temperature dependence of water absorption in the solution as well as the silica coating shell. 2

3 Fig. S1. FTIR spectra of pure water, thick silica-coated GNR colloidal solutions, and their temperature dependences. (A) The similarity of these spectra indicates the dominant absorption of water in colloidal solutions. The circled region shows the relatively weak absorption of the combination band of water that makes it possible to do TRIR spectroscopic study of colloidal solutions in flow cell. The spectra are shifted vertically for visualization. (B and D) Broad absorption of the combination band of water in pure water solution and thick silica-coated GNR solution at room temperature in 25 and 70 µm spacer cells, respectively. (C and E) Different spectra in which the B and D spectrum has been subtracted from those collected at elevated temperature, respectively. The inset shows the linear dependence of the temperature change and the absorption area change in the region from 2140 to 2180 cm Ultrafast Near-IR-Pump Mid-IR-Probe Spectroscopy. The ultrafast time-resolved IR experimental setup has been described in detail elsewhere. 4 The setup consists of a Ti:sapphire regenerative amplifier (Spectra Physics, Spitfire) seeded by a Ti:sapphire oscillator (Spectra Physics, Tsunami) to produce a 1 khz train of 100 fs pulses centered at 800 nm with an average pulse power of 1.1 mj. The output of this commercial system is split, and 30% of the output is used as pump pulses with power at sample in range of 2-20 mw ( equivalent to a fluence of mj/(cm 2.pulse) or the laser field intensity of 0.3x10 9-3x10 9 W/(cm 2.pulse)). The other 70% is used to pump a home built two pass BBO based optical parametric amplifier (OPA), 5 the output of which is mixed in a AgGaS 2 crystal to produce mid IR probe pulses tunable from 3.0 to 6.0 μm with a 200 cm -1 spectral width and a ca. 100 fs pulse duration. The 800 nm pump pulses pass through a 25 cm silica rod, which stretches their duration in time to 1 ps, and gives a cross correlation to the mid-ir pulses of 1.1 ps at the sample. The stretched 800 nm pulses are necessary to achieve a high pump fluence while still reducing the artifacts resulting from nonlinear optical effects in the sample cell windows. To reduce the effect of non-uniform 3

4 intensity of the pump (Gaussian profile) on the non-uniform temperature of probe region, the beam is diverged to ~ 1 cm diameter and then passed through an iris to have a 1000 μm diameter at the sample. Scheme 1. Pump-probe geometry and sequences of laser pulses in TRIR experiment A computer controlled translation stage (Newport) allows for variable time delays up to ca. 3 ns between pump and probe pulses. The sample is flowed using a mechanical pump through a stainless steel cell (Harrick Scientific) fitted with 2 mm thick CaF 2 windows separated by 70 μm spacers. The flow rate is controlled to be fast enough to refresh the sample at pump-probe spot after each pair of pump-probe pulses. The pump and probe beams are spatially overlapped at the sample and focused so that the beam diameters are ca and 200 μm respectively. Reference and signal mid-ir beams are sent along a parallel path through a computer controlled spectrograph with entrance slits set at 70 μm (Acton Research Corporation, SpectraPro-150) and detected by a 2 32 element MCT-array IR detector (InfraRed Associates, Inc.) and a highspeed signal acquisition system and data-acquisition software (Infrared Systems Development Corp.) with a resolution of ca. 2.5 cm -1. Collected signals are averaged over laser shots to correct for shot to shot fluctuations. Differences in optical density as small as are observable after 1 s of data collection. All TRIR spectra are reported in mod as collected. We note that the reported OD value needs to be scaled up by a factor of 2 to get the actual (quantitatively accurate) value expected for the magnitude of the temperature rise. This is due to the calibration of our experimental setup. This calibration was used for the spectra of the water solvating electron to calculate the quantum yield of the electron injection later on. Artifact spectral feature at 2090 cm -1 in Fig. 3C. The negative absorption at the 2090 cm -1 region in Fig. 3C is an artifact from our experimental setup. This spectral artifact appeared constantly in all negative and positive delay times beyond the cross-correlation time of the pump and probe pulse. This artifact was also seen with a pure water sample or an empty cell. We found that this artifact came from the scattering of the pump pulse on the surface of the CaF 2 window, creating unwanted signal to the detector. This artifact has no dynamics and happens only at this particular frequency. Thus, we believe this artifact does not affect our study. In the silica coated samples, a longpass filter (cut-off at 1.2 μm) is used to filter out this artifact (see Fig. 3E). Estimated time for heat diffusing out of the probe area. The thermal diffusivity χ of water is nm 2 /ps under the ambient conditions and the mean diffusion length at time t is 2(χt) 1/2. For heat to diffuse only 1 μm outside of the probed area (ca. 200 μm in diameter), it is expected to take about 2 μs, a very long period compared to the timescale of the TRIR experiment. Thus there is no heat diffusing out of the probe beam during the delay time of the TRIR spectra. 4. Reshaping of CTAB capped GNRs and silica-coated GNRs after high power excitation. 4

5 High excitation power can cause transformation of particle shape. 6-8 The shape transformation was verified via optical spectroscopy and electron microscope after each TRIR experiment at each excitation power. The samples started to have noticeable shape transformation after long run (~10 6 pump laser shots) at 8 mw excitation for the CTAB capped GNRs and 20 mw for the silica-coated GNRs. The GNRs have more ill-defined shape and become more round under higher power experiment. As a result, the longitudinal SPR absorption is blue-shifted. Fig. S2. Shape transformation of CTAB capped GNRs under high power excitation. (A) The Vis/Near-IR spectra of colloidal GNRs in a 70 µm spacer cell in TRIR experiment. The spectra show the decrease of the longitudinal SPR peak due the reshape of the GNRs after each long run experiment at each laser power excitation. (B) TEM image of representative GNRs in the colloidal sample before performing TRIR experiment. (C) and (D) TEM images of representative GNRs in the colloidal sample after each TRIR experiment. 5

6 Fig. S3. Silica shell improving thermal stability of the GNRs. (A) The Vis/Near-IR spectra of colloidal silica coated GNRs in a 70µm spacer cell in TRIR experiment. The spectra show the decrease of the longitudinal SPR peak due the reshaping of the GNRs after each long-run experiment at each laser power excitation. (B) TEM image of representative silica-coated GNRs in the colloidal sample before preforming TRIR experiment. (C) and (D) TEM images of representative silica-coated GNRs in the colloidal sample after each TRIR experiment. 5. Calculating the temperature rise of gold nanoparticles The temperature rise of gold nanoparticles were calculated after each excitation pulse. The highest temperatures of these particles were calculated on average by dividing the total photon energy absorbed into the total amount of gold in the laser pump beam spot, using the formula below, 9 T = 21 + T = 21 + I 0 (1 ξ)2 (1 10 A ) C p [Au]σl In which: T ( o C): the average highest temperature of Gold NPs, 21 o C is room temperature 6

7 (1 ξ) 2 =0.938, a factor which accounts for a small reflection loss of the 800 nm pump laser at the front and back surfaces of the CaF 2 window of the sample cell. The reflection loss ξ = ( n 1 n 2 n 1 +n 2 ) 2 is calculated from the refractive index of air (n 2 = 1) and CaF 2 (n 2 = 1.431) at 800 nm. I 0 (J/pulse): energy of 800 nm laser pump A: absorbance of the sample C p (J g -1 K -1 ): heat capacity of gold metal σ (dm 2 ): laser pump spot size l (dm): path length of the sample [Au](g/L): concentration of gold in the sample [Au] = C gold NPs N Au M w In which: N Au : average number of gold atoms in one gold NP M w : atomic weight of gold C gold NPs (M): molar concentration of gold NPs in sample solution Small amount of photon scattering and energy loss due to electron injection are ignored in this calculation. 6. Calculation of temperature rise of local solvent To calculate the average temperature rise of the local solvent at a specific delay time after photoexciting the GNRs, the amount of heat being transferred to solvent and the volume of heated solvent were estimated. The amount of heat transferred to solvent at a specific delay time was estimated from the total photon energy absorbed on the GNR and the experimental time constant of heat transfer. The volume of the heated solvent was estimated from the heat diffusion length in the solvent at a specific delay time. Heat capacity and thermal diffusivity of water were used. For demonstration purpose, we chose the experimental condition of 2 mw laser excitation on CTAB capped GNRs solution, which gave the absorbed photon energy of 7.4x10-15 J/particle and the heat transfer time constant set at 350 ps. The duration of the laser pulse is kept as 1 ps (Fig. S4.A) or stretched to 1 ns (Fig. S4.B) or 10 ns (Fig. S4.C) long. Figure S4 shows the estimated temperature profile at various excitation pulse durations. 7

8 Fig. S4. The profile of laser pulse with various simulated durations, corresponding amount of heat being transferred from the GNR to solvent, diffusion length of the heat from the GNR and average temperature rise of local solvent. The absorbed energy on the GNRs is adopted from the 2 mw experiment with CTAB capped GNR solution and keept unchange across the three simulated laser pulse durations. The adopted heat transfer time constant is 350 ps. Reference 1. Ye, X.; Zheng, C.; Chen, J.; Gao, Y.; Murray, C. B. Using Binary Surfactant Mixtures To Simultaneously Improve the Dimensional Tunability and Monodispersity in the Seeded Growth of Gold Nanorods. Nano Lett. 2013, 13, Gorelikov, I.; Matsuura, N. Single-Step Coating of Mesoporous Silica on Cetyltrimethyl Ammonium Bromide-Capped Nanoparticles. Nano Lett. 2008, 8, Wang, F.; Cheng, S.; Bao, Z.; Wang, J. Anisotropic Overgrowth of Metal Heterostructures Induced by a Site-Selective Silica Coating. Angew. Chem. 2013, 125, Nguyen, S. C.; Lomont, J. P.; Zoerb, M. C.; Hill, A. D.; Schlegel, J. P.; Harris, C. B. Chemistry of the Triplet 14-Electron Complex Fe(CO) 3 in Solution Studied by Ultrafast Time- Resolved IR Spectroscopy. Organometallics 2012, 31, Hamm, P.; Kaindl, R. A.; Stenger, J. Noise Suppression in Femtosecond Mid-infrared Light Sources. Opt. Lett. 2000, 25, Link, S.; El-Sayed, M. A. Shape and Size Dependence of Radiative, Non-radiative and Photothermal Properties of Gold Nanocrystals. Int. Rev. Phys. Chem. 2000, 19, Petrova, H.; Perez Juste, J.; Pastoriza-Santos, I.; Hartland, G. V.; Liz-Marzan, L. M.; Mulvaney, P. On the Temperature Stability of Gold Nanorods: Comparison between Thermal and Ultrafast Laser-induced Heating. Phys. Chem. Chem. Phys. 2006, 8,

9 8. Chen, Y.-S.; Frey, W.; Kim, S.; Homan, K.; Kruizinga, P.; Sokolov, K.; Emelianov, S. Enhanced Thermal Stability of Silica-coated Gold Nanorods for Photoacoustic Imaging and Image-guided Therapy. Opt. Express 2010, 18, Hu, M.; Hartland, G. V. Heat Dissipation for Au Particles in Aqueous Solution: Relaxation Time versus Size. J. Phys. Chem. B 2002, 106,