Enhancing Single-Nanoparticle Surface-Chemistry by Plasmonic Overheating in an Optical Trap

Abstract

Surface-chemistry of individual, optically trapped plasmonic nanoparticles is modified and accelerated by plasmonic overheating. Depending on the optical trapping power, gold nanorods can exhibit red-shifts of their plasmon resonance (i.e. increasing aspect ratio) under oxidative conditions. In contrast, in bulk exclusively blue shifts (decreasing aspect ratios) are observed. Supported by calculations, we explain this finding by local temperatures in the trap exceeding the boiling point of the solvent which can not be achieved in bulk.Keywords noble metal nanoparticles; plasmon resonance; colloidal chemistry; nano-/microfluidics; gold nanorods; plasmonic heating Miniaturization is a successful approach to faster, more efficient applications in physics and chemistry. In chemistry, the reduction of reaction volumes is expected to lead to highthroughput, portable analytical and sensing applications, and fundamental insights into single-molecule chemical reactions. [1][2][3] Lab-on-a-chip applications frequently employ microor nanofluidic structures. [4][5][6][7] However, these approaches do not change the outcome of a chemical reaction. Optical trapping on the other hand, being compatible with micro-or nanofluidic systems, is a well-established technique that has been widely applied to singlemolecule force and optical spectroscopy, non-invasive manipulation, and chemical reactions monitoring . [8][9][10][11][12][13] More recently, optical trapping has been applied to plasmonic noble metal nanoparticles. [14][15][16] Due to their localized surface plasmon resonances, noble metal nanoparticles provide strongly enhanced and highly localized electromagnetic fields close to their surface. 17 Their optical and field-enhancing properties allow for manipulating and enhancing fluorescence, Raman scattering, charge and energy transfer, and local temperature. [18][19][20][21] The surface plasmon resonances can be tuned through controlling size, shape and material of the nanoparticles via advanced colloidal chemistry. 22 Gold nanorods for instance, display two different plasmon modes: one transversal, in which the electrons oscillate perpendicular to the long axis of the rod, and the other red shifted longitudinal, in which the electrons oscillate along the long axis of the nanorod. 23 Europe PMC Funders Author Manuscripts sensitively depends on the nanorod's aspect ratio: the higher the aspect ratio, the more red shifted the longitudinal plasmon resonance.Here we apply the plasmonic heating effect of a single gold nanoparticle in an optical trap to both accelerate and modify chemical reactions on the surface of the particle. We show that individual optically trapped plasmonic particles modify and accelerate surface-chemistry by plasmonic overheating to yield different and faster results than the corresponding bulk reactions. Individual gold nanorods under oxidative conditions can display a red-shift of their localized surface plasmon resonances corresponding to an increasing aspect ratio, while in bulk exclusively bl...

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