Time-resolved photoluminescence decay characteristics of bovine serum albumin-conjugated semiconductor nanocrystallites

Abstract

We report processing and luminescence decay characteristics of Cd_{1− x} Zn _x S composite nanocrystals (NCs) conjugated with bovine serum albumin (BSA) proteins. Time-resolved study on unconjugate NCs (with dimensions less than the bulk exciton Bohr radius) suggests that in the radiative emission, the fast (τ ₁) and the slow (τ ₂) carrier components are equally competitive for a given stoichiometry. Conversely, bioconjugate NCs advocate that the decay component due to the free exciton recombination is ∼9 times faster than the component due to the surface recombination emission. The observation of two distinct decay parameters is due to the fact that the NCs have experienced photostability by way of binding and protecting NC surface with biomolecules (BSA) as binding agents. The occurrence of two decay constants would help in extracting information with regard to the nature of surface recombination, free-exciton relaxation along with the strength of emission. Furthermore, with the increase in % Zn, slow carrier component gets slower owing to the incorporation of extra surface traps due to Zn/Cd incompatibility while making perfect lattice sites in the NCs. As a result, surface emission intensity gets improved compared to the radiative intensity due to core-state direct transitions. Understanding photoluminescence decay of bioconjugated NCs, on a comparative basis, would find scope for biomolecular labelling, sensing and electrophysiology applications.

Keywords:

• time resolved
• nanocrystals
• bioconjugation

Acknowledgements

The authors would like to thank colleagues for TR-PL measurements and D. Mohanta would like to thank the Indian Academy of Science for sponsoring the work carried out under the teacher fellow scheme during 2006–2007.