Supporting Information for: Graphene oxide/gold nanorod nanocomposite for stable surface enhanced Raman spectroscopy

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1 Supporting Information for: Graphene oxide/gold nanorod nanocomposite for stable surface enhanced Raman spectroscopy Pilar G. Vianna, Daniel Grasseschi, Greice K. B. Costa,, Isabel C. S. Carvalho, Sergio H. Domingues, Jake Fontana, Christiano J. S. de Matos* MackGraphe Graphene and Nanomaterials Research Center, Mackenzie Presbyterian University, São Paulo , Brazil. Department of Physics, Pontifícia Universidade Católica do Rio de Janeiro, Rio de Janeiro , Brazil. Photonic and Instrumentation Laboratory, UFRJ, Rio de Janeiro , Brazil. Naval Research Laboratory, 4555 Overlook Ave. SW, Washington D.C , United States of America. * cjsdematos@mackenzie.br (10 Pages, 9 Figures, 2 Tables). S1

2 1. Experimental AuNR Rayleigh spectra for the end-to-end configuration In the experimental dark-field optical images, the same Rayleigh spectral profile was observed with different center positions, corresponding to points with different colors in the obtained images (see Figure S1). These points are attributed to end-to-end AuNR dimers because of their resemblance with spectra obtained by Funston et al 1. for the same dimer type, and due to their higher intensities and red shift relative to the other observed profiles (both of which, simulations confirm, are characteristics associated to end-to-end dimers). The spectral shifts observed in Figure S1, with basically the same spectral profile and intensity, may be a consequence of variations in the AuNR size and interparticle gaps. A statistical analysis of the different spectral profiles, attributed to the different dimer configurations and to isolated AuNRs, is shown in Figure S2. End-to-end dimers are found to be the most frequent scatterers in the dark-field image. Figure S1: AuNR Rayleigh spectra for the end-to-end dimer configuration. The solid vertical lines refer to the considered wavelength used for the reconstruction of the RGB images. Thus, the yellow particles have higher intensity of the green component (549nm) of the image. The red particles have higher intensity of the red component (645nm), and the orange one shows contributions of booth components. The blue component (450nm) contribution is equal for the three types of aggregates. S2

3 Figure S2: Statistical analysis of the dark field images. A is the hyperspectral dark-field image of the AuNR sample. B, C, D and E are the decomposed images for the counting of the four aggregation geometries. For the statistical analysis, the three spectra shown in Figure S1 were considered to correspond to the same (end-to-end) configuration. The spectra corresponding to the other configurations are those shown in Figure 1 of the main text. 2. Raw dark-field spectra for AuNR, GO, and GO/AuNR samples In the dark-field images, significant scattering from borders, folds and smaller aggregates of GO flakes is observed as an intense yellowish light (Figure 1e). Figure S3 shows a corresponding scattering spectrum. A noisy and broadband signal is observed. To avoid this GO intense scattering from saturating the camera and preventing the identification of the AuNR scattering spots, the microscope lamp intensity was reduced by 50% from the AuNRs only to the GO/AuNRs measurements, which naturally lowers the scattering intensities, thus increasing the noise in the normalized graph comparison (Figures 1c and 1f). Figure S4 compares spectra from the AuNR and GO/AuNR samples without normalization. It is possible to observe that the scattering intensity from the latter is approximately half the intensity of the former, which roughly matches the lamp intensity 50% reduction. As a consequence, the scattering strengths, and therefore the EM-SERS enhancement factors, are found to be about the same for both cases. S3

4 Figure S3: Graphene oxide scattering spectrum. Figure S4: Raw (not normalized) scattering spectra corresponding to the end-to-end AuNR dimer, in the AuNR and GO/AuNR samples. S4

5 3. Assignment of the CTAB SERS spectrum Table S1 refers to the tentative assignment of the SERS blinking bands appearing in pure AuNR samples to that of pure CTAB, both measured as a powder, using a 633- nm laser line, and from a reference available in the literature 2. Table S1: AuNR sample CTAB blinking peaks; Pure CTAB Raman peaks measured as a powder, at 633 nm; Raman peaks from reference 2; and tentative Raman mode assignment also from reference 2. Frequency (cm -1 ) AuNR Blinking Pure CTAB Reference 2 Tentative Assignment ,4 ν(au-br) 3, τ(ch 3) + δ(ccc) τ(ch 3) + ρ(ch 3) δ(ccc) + δ(cnc) δ(ccc) + δ(cnc) δ(ccc) + δ(cnc) δ(ccc) + δ(cnc) δ(ccc) + δ(cnc) ρ(ch 2) ρ(ch 2) ρ(ch 2) + t(ch 2) ρ(ch 3) + ν(cc) ν(cn) + ρ(ch 3) ν(cn) + ν(cc) ν(cn) + ν(cc) + ρ(ch 3) ν(cc) ν(cc) ν(cc) + ρ(ch 3) ν(cc) + δ(ccc) + ρ(ch 3) ѡ(ch 3) + ρ(ch 3) + ѡ(ch 2) ρ(ch 3) + ρ(ch 2) ѡ(ch 2) + ν(cn) + δ(cnc) ѡ(ch 2) t(ch 2) ѡ(ch 2) δ(ch 3) δ S(CH 2) δ S(CH 3) ν stretching, δ bending, δ S scissoring, t twisting, ѡ wagging, τ torsion and ρ rocking modes. S5

6 4. Experimental GO/AuNR nanocomposite Raman spectrum with different laser-line excitations Figure S5: GO/AuNR nanocomposite Raman spectra at the 488-nm (0.394mW; blue), 532-nm (0.419mW; green) and 633-nm (0.605mW; red) laser lines. The spectra were acquired with 1-s integration time and 10 accumulations at the same sample position. Note that the Au-Br peak (below 250 cm -1 ) only appears for the 633-nm laser line due to the SERS effect. 5. Blinking coefficient of variation versus power Figure S4 shows the coefficient of variation (CV), calculated as described in Methods, as a function of the excitation laser power (633-nm laser line) for the AuNR (red) and GO/AuNR (blue) samples. Each shown data points correspond to the CV average over 9 measurements, with the error bars corresponding to the obtained standard deviation. For the GO/AuNR sample, only data containing clear Au-Br Raman mode were considered, as a sign of the rods existence at the analyzed point. As mentioned in the main manuscript text, the data shows no clear CV trend with incident laser power. S6

7 Figure S6: Coefficient of variation as a function of the incident laser power for the AuNR (red) and GO/AuNR (blue) samples. 6. Assignment of RH640 bands and theoretical spectrum calculated by DFT at different excitation wavelengths. Table S2 shows the tentative assignment of the RH640 experimental SERS spectra, from its comparison with the theoretical resonant Raman spectra calculated by DFT. Figure S5 shows a comparison between the DFT-calculated RH640 Raman spectra with resonant (633 nm) and non-resonant (1064 nm) laser excitation. Table S2: GO/AuNR/RH640 sample and RH640 Raman mode assignment. Frequency (cm -1 ) GO/AuNR/RH640 RH640 Tentative Assignment DFT λ=633nm ,4 ν(au-br) 3, Skeletal vibration Furyl ring breath δ ip(ccc) (benzene) + ρ(ch 2) ν(ccc) (xanthene) + δ ip(cnc) t(ch 2) + δ ip(ccc) (xanthene) Benzene ring breath + ν(c-oh) t(ch 2) t(ch 2) + ν sy(c-c) ν(c=c) (benzene) + δ ip(cch) + ν sy(c-o) (Xanthene) δ(coh) + ν(c-c) (carboxyl) + t(ch 2) ρ(ch 2) + Xanthene ring breath ρ(ch 2) S7

8 δ ass(ch 2) δ sy(ch 2) Ν syc-c + δ np(c-o-c) (Xanthene ring) ν sy(c-c) (benzene ring) ν ass(c-c) + δ ip (C-O-C) (Xanthene ring) ν(c=o) ν stretching, ν sy symmetrical stretching, δ bending, δ ass asymmetrical bending, δ ip in-plane bending, δ S scissoring, t twisting, ѡ wagging, τ torsion and ρ rocking modes. Figure S7: Theoretical Raman spectra of RH640 calculated by DFT using two different excitation wavelengths: 1064 nm (black) and 633 nm (red). S8

9 7. Rhodamine 640 normal modes calculated by DFT Figure S8: Rhodamine 640 normal modes calculated by DFT with an excitation wavelength of 633 nm. The blue arrows represent the atom s displacement vector and the yellow arrows indicate the induced transition dipole moment, respectively. S9

10 8. Time series for different concentrations of RH640 on the nanocomposite Figures S9a and S9b show the SERS spectrum time series for GO/AuNR/RH640 samples with RH640 concentrations of 10-8 and M, respectively. Blinking is not observed in none of these cases, indicating stable SERS. A gradual decay in the signal intensity of the Rhodamine bands is, however, observed and can be attributed to photobleaching, which, is expected to become more evident at lower dye concentrations. Figure S9: SERS spectrum time series for Rhodamine 640 on the GO/AuNR nanocomposite with (a) 10-8 M (b) M Rhodamine concentrations. References: (1) Funston, A. M.; Novo, C.; Davis, T. J.; Mulvaney, P.; Funston, A. M.; Novo, C.; Davis, T. J.; Mulvaney, P. Plasmon Coupling of Gold Nanorods at Short Distances and in Different Geometries. Nano Lett. 2009, No. 9, (2) Gökce, H.; Bahçeli, S. The Molecular Structures, Vibrational Spectroscopies ( FT IR and Raman) and Quantum Chemical Calculations of N Alkyltrimethylammonium Bromides 1. Opt. Spectrosc. 2013, 115, (3) Boca, S. C.; Astilean, S. Detoxification of Gold Nanorods by Conjugation with Thiolated Poly(ethylene Glycol) and Their Assessment as SERS-Active Carriers of Raman Tags. Nanotechnology 2010, 21, (4) Eftekhari, F.; Lee, A.; Kumacheva, E.; Helmy, a S. Examining Metal Nanoparticle Surface Chemistry Using Hollow-Core, Photonic-Crystal, Fiber-Assisted SERS. Opt Lett 2012, 37, S10