Spectroscopic studies on the thermodynamics of L-cysteine capped CdSe/CdS quantum dots--BSA interactions

CdSe/CdS quantum dots (QDs) capped with L-cysteine can provide an effective platform for the interactions with bovine serum albumin (BSA). In this study, absorption and fluorescence (FL) spectroscopy were used to study the binding reactions of QDs with BSA, respectively. The binding constant (≈10(4) M(-1)) from FL quenching method matches well with that determined from the absorption spectral changes. The modified Stern-Volmer quenching constant (5.23 × 10(4), 5.22 × 10(4), and 4.90 × 10(4) M(-1)) and the binding sites (≈1) at different temperatures (304 K, 309 K, and 314 K) and corresponding thermodynamic parameters were calculated (∆G < 0, ∆H < 0, and ∆S < 0). The results show the quenching constant is inversely correlated with temperature. It indicates the quenching mechanism is the static quenching in nature rather than dynamic quenching. The negative values of free energy (∆G < 0) suggest that the binding process is spontaneous, ∆H < 0 and ∆S < 0 suggest that the binding of QDs to BSA is enthalpy-driven. The enthalpy and entropy changes for the formation of ground state complex depend on the capping agent of QDs and the protein types. Furthermore, the reaction forces were discussed between QDs and BSA, and the results show hydrogen bonds and van der Waals interactions play a major role in the binding reaction.