Volatile interface of biological oxidant and luminescent CdTequantum dots: implications in nanodiagnostics

In order to explore the feasibility of using quantum dots (QDs) as nanodiagnostic tools in cells under oxidative stress, the interaction of cysteine-functionalized CdTe QDs with peroxynitrite (PN), a powerful biological oxidant, was spectroscopically investigated. The photoluminescence (PL) of CdTe QDs was quenched to the extent of 85% at 86 microM concentration of peroxynitrite. In contrast, the loss of PL intensity was merely 7% for CdS QDs under identical conditions. The quenching pattern shows a positive deviation from Stern-Volmer equation, but obeys the Perrin equation for static quenching. The Perrin quenching constants increased with decreasing temperature. Size dependence was also prominently observed as quenching efficiency increased by 70% on moving from 4.0 nm sized CdTe QDs to 2.8 nm ones. Interestingly, the QDs exhibited a time-dependent auto-recovery of photoluminescence. At 25 and 15 degrees C, the recovery time was found to be 1.25 and 3.5 h, respectively, however, the extent of recovery remained nearly the same, about 22%. Peroxynitrite followed a first order decay kinetics with a half-life of 45.6 min, which got substantially reduced to 2.9 min in the presence of CdTe QDs. The over-all mechanism of the interaction has been explained as a two-pronged attack of peroxynitrite on (i) the cysteine-capping layer and (ii) the CdTe core, resulting in a reversible and irreversible loss of QD-photoluminescence.