Insights into the binding of photothermal therapeutic agent bismuth sulfide nanorods with human serum albumin

The biomedical application of bismuth sulfide (Bi₂S₃) nanorods as a nanomedicine for the photothermal therapy of tumors prompted this study into their interactions with human serum albumin (HSA) to understand their pharmacokinetics. Bi₂S₃ nanorods with orthorhombic crystalline structure were synthesized using a simple microwave irradiation method from bismuth nitrate, sodium sulfide, and starch in an aqueous medium. The synthesized Bi₂S₃ nanorods and their formation mechanism were well-characterized by powder X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and energy-dispersive X-ray (EDX). The interactions of the Bi₂S₃ nanorods with HSA was investigated by absorption spectroscopy, fluorescence spectroscopy and circular dichroism spectroscopy. The absorption and time-resolved fluorescence spectroscopy studies confirmed that the Bi₂S₃ nanorods interacted with HSA through a static mechanism. The steady-state and synchronous fluorescence spectral studies showed that the single binding site is near the tryptophan moiety (Trp-214) in HSA. The moderate binding constant determined from the steady-state fluorescence study suggested the possibility of effective transportation of Bi₂S₃ nanorods inside the body. These results could be very helpful for understanding the mechanisms and pathways responsible for the uptake, distribution and catabolism of Bi₂S₃ nanorods in multiple tissues of the human body.