Fabrication and characterizations of arbitrary-shaped silver nanoparticles for surface-enhanced fluorescence microscopy

Special shape noble metal nanoparticles at small dimensions are of great interest in nanoscience and technology. Here in this work, arbitrary-shaped silver nanoparticles in the dimension of ~6 nm were synthesized and characterized. Crystal structure, optical characteristics, and electromagnetic (EM) near-field distributions were carried out in details so that such nanoparticles can be realized as plasmon-active tags. UV-vis absorption measurements confirmed a distinct peak at 385 nm in addition to two shoulder peaks at 350 and 450 nm. Optical band gap of as-synthesized nanoparticles was estimated to be 2.87 eV. Further insight of crystal growth and d-spacing was carried out using HRTEM and SAAD pattern. As-synthesized arbitrary-shaped silver nanoparticles were found to have luminescence emission at 428 nm with full width at half maximum of 103.56 nm while excited at 316 nm. It was observed that the fluorescence of R6G got enhanced several orders in presence of such small arbitrary-shaped silver nanoparticles. Surface-enhanced fluorescence (SEF) is known to be a function of SEF-active tags’ whereabouts and EM near-field distributions. In this context, a model was designed and simulated by finite difference time domain analysis to understand and correlate the EM near-field distributions. EM near-field distributions at three different planes, x-, y-, and z-planes, were extracted for three different incident polarizations. Two elemental components, spheroid and cone of same dimensions and parameters, were simulated separately as reference. EM near-field distributions for such arbitrary-shaped silver nanoparticles under different circumstances were demonstrated, and plausible scenarios were elucidated in view of SEF enhancements of SEF-active tags.