Comparative Study of Plasmonic Properties of Cysteine-Functionalized Gold and Silver Nanoparticle Aggregates
The absorptance spectra of gold and silver nanoparticle (NP) aqueous dispersions were measured by UV–visible spectroscopy and computed numerically by finite element method. Both NPs were functionalized by l-cysteine amino acid (Cys) in order to develop aggregate-based localized surface plasmon resonance biosensors. Absorptance spectra measured at an analogous pH value of ∼4.9 were compared, where Au-Cys conjugates have moderately split spectra with two commensurate maxima, while Ag-Cys conjugates exhibit the most pronounced secondary peak according to the highest degree of aggregation. The purpose of our theoretical study was to determine the simplest linear chain-like and wavy aggregate geometries, which result in maxima matching the measured peaks. The aggregates were characterized by N number and d diameter of NPs, g gap between the NPs, and t thickness of the l-cysteine covering. By tuning the angle of incidence and E-field oscillation direction in p-polarized light with respect to the aggregates, the contribution of longitudinal and transversal modes was varied. The comparison of measurements and computations revealed that spectra measured on bioconjugate dispersions include effects of numerous aggregates with various geometries, illuminated from different directions and are influenced by inter-aggregate coupling. Inspecting the normalized E-field distribution surrounding the aggregates, it was shown that fundamentally different multipolar modes can be identified at primary and secondary absorptance maxima, due to coupled plasmonic resonances on NPs.
KeywordsNanoparticle aggregates LSPR spectra Coupled plasmonic resonances Array effect
- 13.Csapó E, Patakfalvi R, HornokV TL, Sipos Á, Szalai A, Csete M, Dékány I (2012) Plasmonic properties of biofunctionalized silver nanoparticles at different pH. Colloids Surf B 98:43–49Google Scholar
- 15.Szalai A, Sipos Á, Csapó E, Hornok V, Tóth L, Csete M, Dékány I (2011) Numerical investigation of the plasmonic properties of bare and cysteine-functionalized silver nanoparticles. Plasmonics: Metallic Nanostructures and Their Optical Properties IX, edited by Mark I. Stockman, Proceedings of the SPIE, 8096:80963BGoogle Scholar
- 16.Palik ED (1998) Handbook of optical constants of solids. Academic, San DiegoGoogle Scholar