Compact hybrid (gold nanodendrite-quantum dots) assembly: plasmon enhanced fluorescence-based platform for small molecule sensing in solution.

In this study, we have presented a novel plasmon enhanced fluorescence (PEF) system for label-free sensing of small molecules in bulk solution. The amine-terminated gold nanodendrite (AuND) and carboxyl-terminated QDs directly assemble each other by amine-carboxyl attraction. Without any spacer layers, PEF can be increased by 4 times during the formation of the compact hybrid (AuND-QDs) assembly. Both experiment and finite-difference time domain calculation results indicate that the distinct solution-PEF effect is ascribed to two reasons: (1) The used AuNDs simultaneously possess four features in morphology and topology, well-defined superstructure, sharp tips and edges, moderately elongated subunits, and smaller size. (2) The hybrid (AuND-QDs) assembly has a very compact structure. So, the fluorescence is well enhanced by the effective increase of excitation and radiative decay rates with the decrease of scattering effect. The (AuND-QDs) assembly is then employed for sensing of trinitrotoluene (TNT), one of the highly explosive and environmentally detrimental substances, in bulk solution. The sensing principle is that the analytes can react with primary amines on the AuND surface and form Meisenheimer complexes, which break the preformed assemblies and result in the fluorescence recovery of the QDs. The linear range is 0-8.8 nM with 0.05 nM detection limit. The present quasi-picomole level sensitivity is one of the best results for fluorescent TNT sensing. The developed method is successfully applied to TNT sensing in real environmental samples, indicating the practical potential.