A fluorescence chemosensor, 2-hydroxy-1-naphthaldehyde azine (HNA) was designed and synthesized for sequential detection of Cu²⁺ and biothiols. It was found that HNA can specifically bind to Cu²⁺ with 1:1 stoichiometry, accompanied with a dramatic fluorescence quenching and a remarkable bathochromic-shift of the absorbance peak in HEPES buffer. The generated HNA-Cu²⁺ ensemble displayed a “turn-on” fluorescent response specific for biothiols (Hcy, Cys and GSH) based on the displacement approach, giving a remarkable recovery of fluorescence and UV-Vis spectra. The detection limits of HNA-Cu²⁺ to Hcy, Cys and GSH were estimated to be 1.5 μM, 1.0 μM and 0.8 μM, respectively, suggesting that HNA-Cu²⁺ is sensitive enough for the determination of thiols in biological systems. The biocompatibility of HNA towards A549 human lung carcinoma cell, was evaluated by an MTT assay. The capability of HNA-Cu²⁺ to detect biothiols in live A549 cells was then demonstrated by a microscopy fluorescence imaging assay.

We report the development of a new colorimetric probe (L-ol) for investigations of the redox process regulated by hypochlorous acid (HOCl) and glutathione (GSH). The HOCl/GSH redox-switching cycle process was investigated in detail by UV-vis absorption spectroscopy, colorimetric analysis assay and high-resolution mass spectrometry (HRMS). The switchable absorbance responses were attributed to the HOCl-induced oxidation of the p-methoxyphenol unit to the benzoquinone derivative (L-one) and sequential reduction of L-one to hydroquinone (L-ol’) by GSH. In phosphate-buffered saline (PBS) buffer, the absorbance of L-ol at 619 nm underwent a remarkable bathochromic-shift, accompanied by a color change from pale yellow to blue in the presence of HOCl. With further addition of GSH, the absorbance of L-one exclusively recovered to the original level. Meanwhile, the blue-colored solution returned to the naive pale yellow color in the presence of GSH. The detection limits for HOCl and GSH were calculated to be 6.3 and 96 nM according to the IUPAC criteria. Furthermore, L-ol-loaded chromatography plates have been prepared and successfully applied to visualize and quantitatively analyze HOCl in several natural waters.

The development of effective bioanalytical methods for the visualization of hypochlorous acid (HOCl) in situ in rheumatoid arthritis (RA) directly contributes to better understanding the roles of HOCl in this disease. In this work, a new quinoline-based fluorescence probe (HQ) has been developed for the detection and visualization of a HOCl-mediated inflammatory response in a RA model. HQ possesses a donor–π–acceptor (D–π–A) structure that was designed by conjugating p-hydroxybenzaldehyde (electron donor) and 1-ethyl-4-methylquinolinium iodide (electron acceptor) through a CC double bond. In the presence of HOCl, oxidation of phenol to benzoquinone led to the red-shift (93 nm) of the adsorption and intense quenching of the fluorescence emission. The proposed response reaction mechanism was verified by high performance liquid chromatography (HPLC) and high-resolution mass spectroscopy (HRMS) titration analysis. The remarkable color changes of the HQ solution from pale yellow to pink enabled the application of HQ-stained chromatography plates for the “naked-eye” detection of HOCl in real-world water samples. HQ featured high selectivity and sensitivity (6.5 nM), fast response time (<25 s) to HOCl, reliability at different pH (3.0 to 11.5) and low cytotoxicity. HQ's application in biological systems was then demonstrated by the monitoring of HOCl-mediated treatment response to RA. This work thus provided a new tool for the detection and imaging of HOCl in inflammatory disorders.

A new Gadolinium(III)–coumarin complex, DO3A-Gd-CA, was designed and prepared as a dual-modal probe for simultaneous fluorescence and relaxivity responses to fluoride ions (F⁻) in aqueous media and mice. DO3A-Gd-CA was designed by using Gd(III) center as an MRI signal output unit and fluoride binding site, and the 4-(diethylamino)-coumarin-3-carboxylic acid (CA) as a fluorescence reporter. Upon the addition of fluoride ions to the solution of DO3A-Gd-CA, the liberation of the coordinated CA ligand led to a 5.7-fold fluorescence enhancement and a 75% increase in the longitudinal relaxivity (r₁). The fluorescent detection limit for fluoride ions was determined to be 8 μM based on a 3σ/slope. The desirable features of the proposed DO3A-Gd-CA, such as high sensitivity and specificity, reliability at physiological pH and low cytotoxicity enable its application in visualization of fluoride ion in mice. The successful in vivo imaging indicates that DO3A-Gd-CA could be potentially used in biomedical diagnosis fields.

Quick and effective detection of biothiols in biological fluids has gained increasing attention due to its vital biological functions. In this paper, a novel reversible fluorescence chemosensor (L-Cu²⁺) based on a benzocoumarin-Cu²⁺ ensemble has been developed for the detection of biothiols (Cys, Hcy and GSH) in human urine. The chemosensing ensemble (L-Cu²⁺) contains a 2:1 stoichiometry structure between fluorescent ligand L and paramagnetic Cu²⁺. L was found to exclusively bond with Cu²⁺ ions accompanied with a dramatic fluorescence quenching maximum at 443 nm and an increase of an absorbance band centered at 378 nm. Then, the in situ generated fluorescence sluggish ensemble, L-Cu²⁺, was successfully used as a chemosensor for the detection of biothiols with a fluorescence “OFF-ON” response modality. Upon the addition of biothiols, the decomplexation of L-Cu²⁺ led to the liberation of the fluorescent ligand, L, resulting in the recovery of fluorescence and absorbance spectra. Studies revealed that L-Cu²⁺ possesses simple synthesis, excellent stability, high sensitivity, reliability at a broad pH range and desired renewability (at least 5 times). The practical application of L-Cu²⁺ was then demonstrated by the detection of biothiols in human urine sample.