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Water‐soluble and highly stable N,S‐doped CQDs (N,S‐CQDs) were synthesized using a low‐cost strategy with citric acid and thiosemicarbazide in one step for use as a fluorescent nanosensor. The achieved N,S‐CQDs produced strong emission at 446 nm upon excitation at 370 nm and a high quantum yield of 58.5%. The quenching effect on the prepared N,S‐CQDs was utilized for determination of trimetazidine (TMZ) spectrofluorimetrically over a wide linear range 0.04–0.5 μM (0.0106–0.133 μg ml−1) and a low limit of detection of 0.01 μM (0.002 μg ml−1). Furthermore, CDs were used as a simple and rapid fluorescent probe to determine TMZ in its pharmaceutical formulations as well as in human plasma. The method was tested in compliance with International Council for Harmonisation guidelines. The results obtained were compared statistically with those given for a reported method showing no significant variation regards accuracy and precision.

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The binding of small molecular drugs with human serum albumin (HSA) has a crucial influence on their pharmacokinetics. The binding interaction between the antihypertensive eplerenone (EPL) and HSA was investigated using multi-spectroscopic techniques for the first time. These techniques include ultraviolet-visible (UV-vis) spectroscopy, Fourier-transform infrared (FTIR), native fluorescence spectroscopy, synchronous fluorescence spectroscopy and molecular docking approach. The fluorescence spectroscopic study showed that EPL quenched HSA inherent fluorescence.The mechanism for quenching of HSA by EPL has been determined to be static in nature and confirmed by UV absorption and fluorescence spectroscopy. The modified Stern-Volmer equation was used to estimate the binding constant (K b ) as well as the number of bindings (n). The results indicated that the binding occurs at a single site (K b = 2.238 Â 10 3 L mol À1 at 298 K). The enthalpy and entropy changes (ΔH and ΔS) were 58.061 and 0.258 K J mol À1 , respectively, illustrating that the principal intermolecular interactions stabilizing the EPL-HSA system are hydrophobic forces.Synchronous fluorescence spectroscopy revealed that EPL binding to HSA occurred around the tyrosine (Tyr) residue and this agreed with the molecular docking study.The Förster resonance energy transfer (FRET) analysis confirmed the static quenching mechanism. The esterase enzyme activity of HSA was also evaluated showing its decrease in the presence of EPL. Furthermore, docking analysis and sitespecific markers experiment revealed that EPL binds with HSA at subdomain IB (site III).

Since the small molecular drugs pharmacodynamics and pharmacokinetics could be affected by human serum albumin (HSA) transport, we used spectroscopic and molecular docking studies to investigate the interaction between HSA and the angina-prevention drug trimetazidine (TMZ). As shown by synchronous fluorescence spectroscopy, this interaction affects the microenvironment confirmation around tyrosine residues. The site-competitive experiments showed that TMZ had an affinity toward subdomain III A (site II) of HSA. The enthalpy and entropy changes (ΔH and ΔS), which were 37.75 and 0.197 K J mol-1, respectively, showed that the predominant intermolecular interactions stabilizing the TMZ-HSA combination are hydrophobic forces. According to FTIR research, the interaction between HSA and TMZ caused polypeptide carbonyl-hydrogen bonds to rearrange. Additionally, the HSA esterase enzyme activity was assessed, which revealed a decrease in activity with TMZ. Docking analysis confirmed the site-competitive experiments and thermodynamic results. Also, it showed that TMZ has strong affinity for binding to HSA and inserted into HSA subdomain III A (site II) with score of -12.65. This study demonstrated that TMZ interacted with HSA, and the structure and function of HSA were influenced by TMZ. This study could aid in understanding the pharmacokinetics of TMZ and provide basic data for safe use.

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