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Tphysicsletters/6879/10/1490/9586tpl/Surface-Enhanced Raman Scattering from Au Nanorods, Nanotriangles, and Nanostars with Tuned Plasmon Resonances
Tuesday, October 3, 2023 at 11:45:00 AM UTC
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Surface-Enhanced Raman Scattering from Au Nanorods, Nanotriangles, and Nanostars with Tuned Plasmon Resonances
Boris N. Khlebtsov,1
Andrey M. Burov,1 Sergey V. Zarkov,1
Nikolai G. Khlebtsov,1,2,*
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1 Institute of Biochemistry and Physiology of Plants and Microorganisms, "Saratov Scientific
Centre of the Russian Academy of Sciences," 13 Prospekt Entuziastov, Saratov 410049, Russia
2 Saratov State University, 83 Ulitsa Astrakhanskaya, Saratov 410012, Russia
Acknowledgement
This research was supported by the Russian Science Foundation (project no. № 23-24-00062). The work on computer simulations was funded by the Ministry of Science and Higher Education of the Russian Federation as a state assignment for the Saratov Scientific Centre of the Russian Academy of Sciences (research topic no. 121032300310-8). We thank A.A. Merdalimova for SERS measurements with LabRam HR Evolution spectrometer.
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Abstract
Electromagnetic theory predicts that the optimal value of the localized plasmon resonance (LPR) wavelength for the maximal SERS enhancement factor (EF) is half the sum of the laser and Raman wavelengths. For small Raman shifts, the theoretical EF scales as the fourth power of the local field. However, experimental data often disagree with these theoretical conclusions, leaving the question of choosing the optimal plasmon resonance for the maximal SERS signal unresolved. Here, we present experimental data for gold nanorods (AuNRs), gold nanotriangles (AuNTs), and gold nanostars (AuNSTs) simulating 1D, 2D, and 3D plasmonic nanostructures, respectively. The LPR wavelengths were tuned by chemical etching within 550 1050 nm at constant number concentrations of the particles. The particles were functionalized with Cy7.5 and NBT, and the dependence of the intensity at 940 cm-1 (Cy7.5) and 1343 cm-1 (NBT) on the LPR wavelength was examined for laser wavelengths of 633 nm and 785 nm. The electromagnetic SERS EFs were calculated by averaging the product of the local field intensities at the laser and Raman wavelengths over the particle surface and their random orientations. The calculated SERS plasmonic profiles were redshifted compared to the laser wavelength. For 785- nm excitation, the calculated EFs were five to seven times higher than those for 633-nm excitation. With AuNR@Cy7.5 and AuNT@ Cy7.5, the experimental SERS was 35-fold stronger than it was with NBT-functionalized particles, but with AuNST@Cy7.5 and AuNST@NBT, the SERS responses were similar. With all nanoparticles tested, the SERS plasmonic profiles after 785 nm excitation were slightly blue-shifted, as compared with the laser wavelength, possibly owing to the inner filter effect. After 633-nm excitation, the SERS profiles were redshifted, in agreement with EM theory. In all cases, the plasmonic EF profiles were much broadened compared to the calculated ones and did not follow the four-power law.
Introduction
Strong local field enhancement near plasmonic nanoparticles is the primary physical mechanism behind the surface-enhanced Raman scattering (SERS).1,2 Recent advances in wetchemical technologies3 provide great possibilities for precise tuning of the localized plasmonresonance (LPR) to any desired wavelength from blue (~400 nm) to NIR.4 These synthesis advances raise the following important question:5 Which excitation wavelength maximizes the SERS response for a given LPR nanostructure? There are two possible approaches to answer. The first one is the so-called plasmon-sampled SER excitation spectroscopy (PS-SERES) in which a set of substrates such as nanosphere-lithography (NL) Ag prisms,6 Au nanostar (AuNST)7 or Au nanorod (AuNR) colloids8 with different LPRs is excited by a fixed laser wavelength to find the sample with the maximal SERS response. The second approach is the wavelength-scanned SER excitation spectroscopy (WS-SERES) in which light with variable wavelengths excites a particular Ag-island film,9, 10 Ag-NL,11 or vertically aligned AuNRs12 with a specific LPR spectral profile. This method seems straightforward but is more technically challenging because of the limited laser source with variable wavelength. Nevertheless, the multiple-laser-wavelength excitation of AuNRs functionalized with different dyes allows for SERS bioimaging of cells within the 514-1064 nm spectral band.13Although it is usually believed that excitation at the LPR wavelength gives maximal SERS response,14 15 there is experimental evidence for optimal excitation wavelengths that are slightly blue shifted, as compared with the LPR.16 What is more, Weitz et al.9 proposed a phenomenological relation to approximate the relative SERS intensity, from which the optimal LPR resonance wavelength was found17 to be half of the excitation and Raman scattering wavelengths ( )/2 LPR ex RS (the absorption spectrum is assumed to have the Lorentzian shape). This rule has been confirmed in experiments with Ag island film substrates and, more precisely, with Ag-NL substrates in PS-SERES,6 WS-SERES,11 and WS-SERRES18 experiments.
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Conclusion
In this work, we synthesized, characterized by spectroscopy and TEM methods, and studied the dependence of the SERS electromagnetic amplification factor on the plasmon resonance wavelength of three types of colloidal nanoparticles (AuNRs, AuNTs, AuNSTs) functionalized by two types of Raman molecules and excited by two laser wavelengths of 785 and 633 nm. Comparing the simulated extinction spectra of AuNRs with the T-matrix and COMSOL methods, we showed that the theoretical dependence of the LPR wavelength on the aspect ratio does not agree with the TEM-derived data if the 3-nm CTAB layer on the nanorod surface is not included in simulations. Using the chemical etching method, we synthesized 13 AuNR samples with LPR wavelengths from 1017 to 580 nm, functionalized them with Cy7.5 and NBT molecules, and measured their SERS spectra upon excitation with 633 nm and 785 nm lasers. For comparison with the measurement data, the plasmon profiles of the SERS enhancement factor EF were calculated by averaging the product of the field intensities of the local field at the laser and Raman wavelengths over the surface of particles and their random orientations. Qualitatively, the simulation results agree with the measurements. In particular, the theoretical maximum EF for AuNR@Cy7.5 particles is two times greater than the maximum for AuNR@NBT. Together with a higher Raman cross section of Cy7.5 (compared to NBT), this explains the experimental 35-fold difference between SERS EFs for AuNR@Cy7.5 and AuNR@NBT particles. The main difference between measurements and simulations is that the experimental maximum EF upon excitation with a 785 nm laser is observed at a wavelength LPR less than the laser wavelength, while in theory, it is vice versa. Finally, the width of the experimental plasmon gain SERS profile is much broader than the calculated one. For gold nanotriangles, efficient methods have been developed for averaging the integral optical cross sections over random particle orientations. The results of measurement and analysis of the SERS spectra for AuNT@Cy7.5 and AuNT@NBT particles upon excitation by 633 nm and 785 nm lasers are largely similar to the conclusions obtained for nanorods. In the case of gold nanostars, the main difference from the results for previous experimental models is a small difference between the maximum EF values for AuNT@Cy7.5 and AuNT@NBT particles. The rest of the conclusions apply to these particles as well. In summary, our experimental and theoretical data provide new insight into the physical mechanisms behind the plasmonic enhancement of Raman signals by nanoparticles with different morphologies.
References
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Abstract
Introduction
Conclusion
References
