Facile synthesis and photophysical characterization of luminescent CdTe quantum dots for Forster resonance energy transfer based immunosensing of staphylococcal enterotoxin B.

Aqueous phase synthesis of CdTe quantum dots (QDs) with surface functionalization for bioconjugation remains the best approach for biosensing and bioimaging applications. We present a facile aqueous phase method to prepare CdTe QDs by adjusting precursor and ligand concentrations. CdTe QDs had photoluminescence quantum yield up to ≈33% with a narrow spectral distribution. The powder X-ray diffraction profile elucidated characteristic broad peaks of zinc blende cubic CdTe nanoparticles with 2.5-3 nm average crystalline size having regular spherical morphology as revealed by transmission electron microscopy. Infra-red spectroscopy confirmed disappearance of characteristic absorptions for -SH thiols inferring thiol coordinated CdTe nanoparticles. The effective molar concentration of 1 : 2.5 : 0.5 respectively for Cd(2+)/3-mercaptopropionic acid/HTe(-) at pH 9 ± 0.2 resulted in CdTe quantum dots of 2.2-3.06 nm having band gap in the range 2.74-2.26 eV respectively. Later, QD523 and QD601 were used for monitoring staphylococcal enterotoxin B (SEB; a bacterial superantigen responsible for food poisoning) using Forster resonance energy transfer based two QD fluorescence. QD523 and QD601 were bioconjugated to anti-SEB IgY antibody and SEB respectively according to carbodiimide protocol. The mutual affinity between SEB and anti-SEB antibody was relied upon to obtain efficient energy transfer between respective QDs resulting in fluorescence quenching of QD523 and fluorescence enhancement of QD601. Presence of SEB in the range 1-0.05 µg varied the rate of fluorescence quenching of QD523 , thereby demonstrating efficient use of QDs in the Forster resonance energy transfer based immunosensing method by engineering the QD size.