Khlebtsov Boris N, Burov Andrey M, Zarkov Sergey V, Khlebtsov Nikolai G
Institute of Biochemistry and Physiology of Plants and Microorganisms, Saratov Scientific Centre of the Russian Academy of Sciences, 13 Prospekt Entuziastov, Saratov 410049, Russia.
Saratov State University, 83 Ulitsa Astrakhanskaya, Saratov 410012, Russia.
Phys Chem Chem Phys. 2023 Nov 22;25(45):30903-30913. doi: 10.1039/d3cp04541b.
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 580-1020 nm at a constant concentration of the particles. The particles were functionalized with Cy7.5 and NBT, and the dependence of the intensity at 940 cm (Cy7.5) and 1343 cm (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 red-shifted, 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.
电磁理论预测，对于最大表面增强拉曼散射（SERS）增强因子（EF）而言，局域表面等离子体共振（LPR）波长的最佳值是激光波长与拉曼波长之和的一半。对于小拉曼频移，理论增强因子与局部场强的四次方成正比。然而，实验数据往往与这些理论结论不一致，这使得选择用于获得最大SERS信号的最佳等离子体共振这一问题仍未得到解决。在此，我们展示了分别模拟一维、二维和三维等离子体纳米结构的金纳米棒（AuNRs）、金纳米三角形（AuNTs）和金纳米星（AuNSTs）的实验数据。通过化学蚀刻在580 - 1020 nm范围内对粒子浓度恒定的情况下对LPR波长进行了调谐。用Cy7.5和NBT对粒子进行功能化处理，并针对633 nm和785 nm的激光波长研究了940 cm（Cy7.5）和1343 cm（NBT）处的强度对LPR波长的依赖性。通过对粒子表面及其随机取向处激光波长和拉曼波长的局部场强乘积求平均来计算电磁SERS增强因子。计算得到的SERS等离子体谱相对于激光波长发生了红移。对于785 nm激发，计算得到的增强因子比633 nm激发时高五到七倍。使用AuNR@Cy7.5和AuNT@Cy7.5时，实验得到的SERS比用NBT功能化粒子时强35倍，但使用AuNST@Cy7.5和AuNST@NBT时，SERS响应相似。在所有测试的纳米粒子中，与激光波长相比，785 nm激发后的SERS等离子体谱略有蓝移，这可能归因于内滤效应。633 nm激发后，SERS谱发生红移，这与电磁理论一致。在所有情况下，等离子体增强因子谱与计算得到的相比都显著展宽，并且不遵循四次方定律。
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