Interaction between human serum albumin and toxic free InP/ZnS QDs using multi-spectroscopic study: An excellent alternate to heavy metal based QDs

Abstract

Heavy metal free and non-toxic semiconductor nanocrystals are the most interested segment of the current bio-nanotechnology. In the present paper, structural changes of human serum albumin (HSA) upon its interaction with InP/ZnS QDs coated with amphilic poly ethylene glycol was probed by employing multi-spectroscopic tools. Steady state and time resolved fluorescence studies implied formation of non-fluorescent complex between HSA and QDs resulting out of static quenching. Interestingly, temperature dependent fluorescence quantitative studies show that at higher temperatures these bioconjugates are unstable and static quenching mechanism turns towards dynamic quenching as QDs diffuses through excited state of HSA. Binding and synchronous fluorescence spectroscopic analysis show that QDs binds predominantly to Trp-214 residue in the HSA subdomain IIA with hydrophobic forces. Hence, observed blue shift of about 5 nm in PL spectra of HSA is the clear sign of conformational deformation as the Trp-214 residue gradually enters into hydrophobic microenvironment during its conjugation with QDs. Energy transfer efficiency (ET), transfer rate (KT) and critical transfer distance (R0) were all calculated for HSA and PEG-InP/ZnS QDs system from Förster resonance energy transfer (FRET) studies. Gibb's free energy, enthalpy and entropy obtained from temperature dependent fluorescence studies implied, HSA-InP/ZnS QDs bioconjugation is spontaneous, endothermic, entropy driven and is in accordance with the second law of thermodynamics. Hill coefficient (n ≅ 1) and binding affinities (kb) confirmed strong binding of HSA to QDs surface through ‘non-cooperative interactions’. Also, experimental result of FTIR spectra confirmed the adsorption of HSA onto the surfaces of PEG-InP/ZnS QDs. In addition these results proves that greener, heavy metal free and non-toxic semiconductor nanoparticle (PEG-InP/ZnS QDs) can potentially replace heavy metal and toxic QDs in biophysical fields like conformational studies of proteins.