Studying Charge Transfer Size Dependence Between Semiconductor Quantum Dots and Quantized Metal Nanoparticles
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Semiconductor luminescent quantum dots (QDs) have attracted much research attention due to their unique size-dependent optical properties and their photostability. Previous studies in our group showed that QDs can be involved in fluorescence resonance energy transfer (FRET) where they act as donors that transfer energy to nearby fluorophores. On the nanoscale, discrete energy levels arise that significantly influence the inherent properties of metal nanoparticles. Specifically, metal nanoparticles can be synthesized that display discrete energy levels and thus are able to accept energy when brought in close proximity to nearby donors. The purpose of this study was to understand the size effects and the dependence of the QDs to transferring energy. We hypothesized that metal nanoparticles could store the energy received from luminescent quantum dots (QDs) and transfer the energy as needed, thus inducing QD fluorescence quenching. This hypothesis was tested by synthesizing well-defined and monodisperse CdSe QDs and palladium (Pd) metal nanoparticles with controlled size. We studied the donor-acceptor interactions between the two particles by measuring the fluorescence changes of the QDs when brought in close contact with Pd nanoparticles. We found that Pd nanoparticles induced strong fluorescence quenching of the CdSe QDs. The amount of fluorescence quenching was found to be dependent on the size of the QDs; the smaller the QD the stronger the quenching efficiency, relative to QDs of larger size. Stern-Volmer plots were used to determine the relationship between the QD concentration and the quenching efficiency. It was found that the quenching constant decreased linearly with an increase in Pd nanoparticle core density. This study revealed the non-overlap integral between the emission spectrum of the CdSe QDs and the extinction spectrum of the Pd nanoparticles. Since Pd nanoparticles do not display a surface plasmon resonance and thus do not have an UV-visible absorption spectrum, it was challenging to use the FRET model to explain the fluorescence quenching process. However, nanoparticle surface energy transfer (NSET) mechanism was useful in understanding the factors governing the fluorescence quenching. These studies provide insight into the properties of metal nanoparticles and their role in charge transfer processes.