arXiv subjects

Nikolai G. Khlebtsov

Publications and source records attributed to Nikolai G. Khlebtsov.

5 recordsLinked to original sources

arXiv2026

Gold Bipyramids as a Promising Alternative to Gold Nanorods for Analytical and Biomedical Applications

Pentagonal gold bipyramids with dimensions of 75x25 nm and a longitudinal plasmon resonance (PR) at 753 nm are synthesized. For comparison, gold nanorods with a diameter of 20 nm, lengths ranging from 95 to 50 nm, and longitudinal PR from 945 to 644 nm were synthesized by chemical etching. The samples were characterized by UV-vis spectroscopy and transmission electron microscopy (TEM). It is shown that the absorption spectral quality factor of the bipyramids is significantly higher than that of the nanorods. To compare the nanoparticles as platforms for surface-enhanced Raman scattering (SERS), their surface was functionalized with thiolated nitrobenzene molecules (NBT). It is demonstrated that the SERS enhancement factor for the bipyramids is approximately three times higher than that for the nanorods. The red shift of the bipyramids' PR after functionalization with NBT molecules is also about three times greater than for nanorods with the same PR. This agrees with the theoretical estimate of the bipyramids' PR shift being more sensitive to variations in the refractive index of the external medium or the dielectric shell thickness than that of gold nanospheres and nanorods. The high efficiency of the bipyramids as thermosensitizers for converting laser radiation into heat in photothermal therapy is experimentally and theoretically demonstrated. Effective photothermal killing of E. coli was shown upon irradiation with a laser at the plasmon resonance wavelength using nanobipyramids or nanorods.

physics.optics↗

arXiv2025

Analytical models for coated plasmonic particles: effects of shape and size-corrected dielectric function

Recently, modal expansion methods (MEMs) were developed to accurately predict the extinction and scattering spectra of bare and coated plasmonic particles. However, it remains uncertain whether the accuracy of analytical models is preserved when incorporating size-corrected dielectric functions. We compare numerical (COMSOL) and analytical extinction and scattering spectra for various particle shapes - rods, disks, prisms, bicones, and bipyramids - using both bulk and size-corrected dielectric functions. Size correction primarily causes broadening and a reduction in the plasmonic peaks. Still, the accuracy of analytical models is maintained for all shapes except sharp bicones, where MEM fails for both bulk and size-corrected dielectric functions. This failure arises from the strong localization of the plasmonic field near the sharp bicone tips. However, for realistic bicone models with nanometer-scale tip curvature, MEM performs well.

physics.optics↗

arXiv2025

Universal analytical modeling of coated plasmonic particles

From a structural point of view, plasmonic nanoparticles are always at least two-layer structures with a dielectric layer of stabilizing, targeting, fluorescent, Raman, or other functional molecules. To optimize the optical properties of such bioconjugates, one needs efficient analytical models based on simple physical ideas and with reasonable accuracy comparable to rigorous numerical methods requiring significant computer resources. Recently, we suggested an analytical approach based on a combination of the modal expansion method (MEM) and the dipole equivalence method (DEM). Here, we extend the MEM+DEM approach for particles of various shapes and orientations. To illustrate the possibilities of our method, we calculate extinction and scattering spectra of gold and silver nanorods, nanodiscs, triangle nanoprisms, bicones, and bipyramids with a dielectric coating thickness from 0 to 100 nm. For the main plasmonic peaks, we found excellent agreement between our analytical method and numerical simulations performed with COMSOL, except for some disagreement between MEM and COMSOL solutions for bicones with small rounding radii. To illustrate the application of our method, we prepared 12 AuNR samples using a chemical etching with fine tuning of the LPR over the 1020-580 nm range. By including the CTAB shell in the simulation model, we achieved excellent agreement between the calculated and measured dependence of the LPR peak position on the AuNR aspect ratio. In summary, our method uses the MEM parameters of the original metal particles and does not require additional numerical calculations to build analytical models for bilayered or multilayered conjugates. Due to the simplicity and reasonable accuracy analytical models, they can help apply machine learning to predict various plasmonic responses such as light absorption, scattering, SERS, and metal-enhanced fluorescence.

physics.optics↗

arXiv2025

Analytical modeling of coated plasmonic particles

Biomedical applications of plasmonic nanoparticle conjugates need control over their optical properties modulated by surface coating with stabilizing or targeting molecules often attached to or embedded in the secondary functionalization shell, such as silica. Although current numerical techniques can simulate the plasmonic response of such structures, it is desirable in practice to have analytical models based on simple physical ideas that can be implemented without considerable computer resources. Here, we present two efficient analytical methods based on improved electrostatic approximation (IEA) and modal expansion method (MEM) combined with the dipole equivalence method (DEM). The last approach avoids additional electromagnetic simulations and provides a direct bridge between analytical IEA and MEM models for bare particles and those with multilayer shells. As simple as the original IEA and MEM, the developed analytical extensions provide accurate extinction and scattering spectra for coated particles compared to exact calculations by separation of variable method and COMSOL. The possibility and accuracy of analytical models are illustrated by extensive simulations for prolate and oblate gold and silver nanoparticles with a maximal size of up to 200 nm, aspect ratio from 2 to 6, and 3-30 nm dielectric coating.

physics.optics↗

arXiv2023

Surface-Enhanced Raman Scattering from Au Nanorods, Nanotriangles, and Nanostars with Tuned Plasmon Resonances

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). 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.

physics.optics↗