A smart molecule, QT490, containing thiosemicarbazide moiety acts as a highly selective turn-on in vitro NO sensor through the unprecedented NO-induced transformation of thiosemicarbazide moiety to 1,3,4-oxadiazole heterocycle with the concomitant release of HSNO, thereby eliminating any interference from various endogenous biomolecules including dehydroascorbic acid, ascorbic acid, etc. The kinetic studies of the reactions between QT490 and NO provide a mechanistic insight into formation of HSNO/RSNO from the reaction between H2S/RSH and NO in the biological system. This novel probe is non-cytotoxic, cell permeable, water-soluble, and appropriate for intracellular cytoplasmic NO sensing with the possibilities of in vivo applications.

In this report, the interaction between a phenanthrene–pyrene-based fluorescent probe (PPI) and bovine serum albumin (BSA), a transport protein, has been explored by steady-state emission spectroscopy, fluorescence anisotropy, far-ultraviolet circular dichroism (CD), time-resolved spectral measurements, and molecular docking simulation study. The blue shift along with emission enhancement indicates the interaction between PPI and BSA. The binding of the probe causes quenching of BSA fluorescence through both static and dynamic quenching mechanisms, revealing a 1:1 interaction, as delineated from Benesi–Hildebrand plot, with a binding constant of ∼10 5 M –1 , which is in excellent agreement with the binding constant extracted from fluorescence anisotropy measurements. The thermodynamic parameters, Δ H °, Δ S °, and Δ G °, as determined from van’t Hoff relationship indicate the predominance of van der Waals/extensive hydrogen-bonding interactions for the binding phenomenon. The molecular docking and site-selective binding studies reveal the predominant binding of PPI in subdomain IIA of BSA. From the fluorescence resonance energy transfer study, the average distance between tryptophan 213 of the BSA donor and the PPI acceptor is found to be 3.04 nm. CD study demonstrates the reduction of α-helical content of BSA protein on binding with PPI, clearly indicating the change of conformation of BSA.

A novel sensor (HL5) recognizes sensitively and selectively trivalent metal ions M3+ (M = Al, Fe and Cr) with prominent enhancement in emission intensities with logic gate circuits and memory devices with living cell imaging application.

We report a novel phenazine-embedded fluorescent probe (2-[2-(pyridin-2-ylmethoxy)-phenyl]-1H-imidazo­[4,5-b]­phenazine, PIP), which upon complexation with Cu­(II)-ion-forming [(PIP)­CuII(Cl)] becomes nonfluorescent but regenerates fluorescence in a selective reaction with NO and HNO over different biologically reactive oxygen and nitrogen (ROS/RNS) species under physiological conditions. The fluorescence intensity of PIP gets quenched due to the formation of the [(PIP)­CuII(Cl)] complex, which regenerates the fluorescence by 67 and 84% upon reaction either with NO or HNO, respectively, in the presence of other biological reducing species. Details of photophysical properties of PIP, [(PIP)­CuII(Cl)], and [(PIP)­CuI] have been studied by density functional theory (DFT) calculations. The recognition efficacy of [(PIP)­CuII(Cl)] for exogenous and endogenous NO and HNO in A549 and RAW 264.7 cells with the flow cytometry application has also been demonstrated successfully.

We introduce herein, a novel copper complex-based fluorescent probe [CuII(DQ468)Cl]+ that exhibits a significant fluorescence turn-on response towards nitroxyl with high selectivity over other biological reactive oxygen, nitrogen and sulfur species, including nitric oxide.