Bioconjugates of CdTe Nanowires and Au Nanoparticles:  Plasmon−Exciton Interactions, Luminescence Enhancement, and Collective Effects

Nanoscale superstructures made from CdTe nanowires (NWs) and metal nanoparticles (NPs) are prepared via bioconjugation reactions. Prototypical biomolecules, such as d-biotin and streptavidin pair, were utilized to connect NPs and NWs in solution. It was found that Au NPs form a dense shell around a CdTe NW. The superstructure demonstrated unusual optical effects related to the long-distance interaction of the semiconductor and noble metal nanocolloids. The NW−NP complex showed 5-fold enhancement of luminescence intensity and a blue shift of the emission peak as compared to unconjugated NW. The system was theoretically examined in a simple model of CdTe NW coaxially surrounded by an Au shell, which gives excellent agreement with experimental data. The combination of time-resolved luminescence and calculations demonstrated that increase of fluorescence intensity occurs due to stimulation of photon emission by an electromagnetic field generated around the Au NP. The process is reminiscent of SERS, although displaying smaller enhancement factors. Importantly, the luminescence enhancement should be treated as a collective response of Au−NPs, rather than a result of isolated NP−NW interactions.