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 Cd2+/3-mercaptopropionic acid/HTe– at pH 9�0.2 resulted in CdTe quantum dots of 2.2–3.06nm
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 1–0.05mg 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.