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.