Aqueous Synthesis of Protein-Encapsulated ZnSe Quantum Dots and Physical Significance of Semiconductor-Induced Cu Ion Sensing.

In view of their promising bio-applicability, we have synthesized water-soluble bovine serum albumin (BSA)-encapsulated ZnSe quantum dots (QDs) with visible emission with longer average luminescence lifetimes of approximately 125 ns at ambient conditions. BSA-ZnSe QDs are shown to be efficient selective copper ion probes in the presence of physiologically important metal ions through luminescence quenching with a high Stern-Volmer constant (3.3×10  m ). The mechanism of sensing has been explained in terms of electron transfer processes and the apparent rate of electron transfer (K ) from ZnSe QDs to Cu has been calculated to be 2.8×10  s . It is demonstrated that the negative conduction band potential plays a major role in the feasibility of the electron transfer process, which is reflected in the higher efficacy of ZnSe QDs in sensing copper(II) ions over other group II-VI quantum dots, namely, CdSe, ZnS, or CdS. The results observed with cysteine-capped QDs are almost identical to those with BSA-encapsulated QDs and this presumably negates the possible reason of Cu ion induced quenching ascribed to its binding with surface groups or replacement of metal sites as proposed by several groups previously.