Noble metal nanoparticles (NPs) possess unique plasmonic properties, enabling them to serve as sub-diffraction light sources and nano-antennae for a wide range of applications. Here we report the specific interaction of single Ag NPs with single EGFP molecules and a high dependence of their interaction upon localized-surface-plasmon-resonance (LSPR) spectra of single Ag NPs and EGFP. The LSPR spectra of single red Ag NPs show a stunning 60 nm blue-shift during their incubation with EGFP, whereas they remain unchanged during their incubation with bovine serum albumin (BSA). Interestingly, the peak wavelengths of the LSPR spectra of green and blue Ag NPs remain essentially unchanged during their incubation with either EGFP or BSA. These interesting findings suggest that plasmon-resonance-energy-transfer (PRET) from single Ag NPs to EGFP might follow a two-photon excitation mechanism to excite EGFP and the fluorescence of the excited EGFP might couple with the plasmon of single NPs leading to a blue-shift of the red NPs. These distinctive phenomena are only observed by real-time single NP spectroscopic measurements. This study offers exciting new opportunities to design new sensing and imaging tools with high specificity and sensitivity to study longrange molecular interactions and dynamic events in single live cells, and to probe the underlying molecular mechanisms of PRET.
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