A Water-Soluble, Two-Photon Probe for Imaging Endogenous Hypochlorous Acid in Live Tissue

Ex Tenebris Lux: Illuminating Reactive Oxygen and Nitrogen Species with Small Molecule Probes

Reactive oxygen and nitrogen species are small reactive molecules derived from elements in the air─oxygen and nitrogen. They are produced in biological systems to mediate fundamental aspects of cellular signaling but must be very tightly balanced to prevent indiscriminate damage to biological molecules. Small molecule probes can transmute the specific nature of each reactive oxygen and nitrogen species into an observable luminescent signal (or even an acoustic wave) to offer sensitive and selective imaging in living cells and whole animals. This review focuses specifically on small molecule probes for superoxide, hydrogen peroxide, hypochlorite, nitric oxide, and peroxynitrite that provide a luminescent or photoacoustic signal. Important background information on general photophysical phenomena, common probe designs, mechanisms, and imaging modalities will be provided, and then, probes for each analyte will be thoroughly evaluated. A discussion of the successes of the field will be presented, followed by recommendations for improvement and a future outlook of emerging trends. Our objectives are to provide an informative, useful, and thorough field guide to small molecule probes for reactive oxygen and nitrogen species as well as important context to compare the ecosystem of chemistries and molecular scaffolds that has manifested within the field.

ICT-modulated NIR water-soluble fluorescent probe with large Stokes shift for selective detection of cysteine in living cells and zebrafish

The fluorescent probes with good water-solubility, long-wavelength emission and large Stokes shift are greatly desirable for in vivo detection. Herein, we designed a novel 1,8-naphthalimide-based near-infrared (NIR) optical and fluorescent probe (NTC) for sensing cysteine (Cys). Using acrylate as the recognition site, the probe demonstrated high selectivity and sensitivity for Cys with a low detection limit (0.093 μM) in aqueous buffer solution due to the excellent water-solubility. Upon the reaction with Cys, the recovery of intramolecular charge transfer (ICT) in the probe led to about 40-fold fluorescence enhancement. Furthermore, the reaction result was investigated by ¹H NMR, and HRMS analyses, and the sensing mechanism was validated by quantum calculations. Finally, NTC was applied to image exogenous and endogenous Cys in HeLa cells and zebrafish selectively, implying that the probe possessed great potential application in biological fluorescence sensing.

A water-soluble BODIPY-based fluorescent probe for rapid and selective detection of hypochlorous acid in living cells

We designed and synthesized 4,4-di-(4'-methylmercaptophenoxy)-8-(N-methylpyridinium-2-yl)-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (probe 1) as a water-soluble BODIPY derivative for rapid and selective detection of hypochlorous acid. The pyridinium-2-yl linked at the meso position of BODIPY core was used to maintain highly fluorescent nature and to increase water solubility. Methylmercaptophenoxy was selected as responsive site installed on the boron atom (to replace the fluorine atom) and induced the photoinduced electron-transfer (PeT) effect to quench the fluorescence of BODIPY. The probe exhibited 83.9 μg mL^-1 solubility in PBS (10 mM, pH 7.4), and possessed very low fluorescence (Φ_f = 0.0013). Upon addition of HClO, the probe could display a distinct response in 1 min and generate marked fluorescence enhancement by 100-fold due to the oxidation of thioether into sulfoxide to terminate PeT process. A limit of detection of 53 nM was calculated for HClO in the linear response range from 0 μM to 10 μM, and the probe was successfully applied to image HClO in living cells.

An o-hydroxyl aldehyde structure based naphthalimide derivative: Reversible photochromic properties and its application in ClO- detection in living cells

A bifunctional organic compound 2-butyl-6-hydroxy-1,3-dioxo-2,3-dihydro-1H-benzo[de] isoquinoline-5-carbaldehyde (BHC) with photochromic properties in solid state and probe detection for ClO^- in complete water solution was synthesized and fully characterized. A 'white-yellow-white' reversible photochromic behavior could be observed when alternating UV/vis light irradiation on the solid BHC powder. Good fatigue resistance and adjustable bleaching rate were shown when heating conditions changes. In addition, BHC displayed a high selectivity and low detection limit (1.16 × 10^-8 M) for ClO^-. The photoluminescent fluorescence "on-off" recognition result can be easily identified and BHC has been tested for safely imaging living cells and detecting hypochlorite anion in vitro and vivo. A better water solubility of BHC effectively reduces damage caused by organic solvent in cell imaging progress.

Red-emissive azabenzanthrone derivatives for photodynamic therapy irradiated with ultralow light power density and two-photon imaging

Photodynamic therapy has proved to be an effective strategy for cancer therapy, and advanced photosensitizers for image-guided photodynamic therapy require biocompatibility, intense absorption, high ROS generation efficiency, phototoxicity, low irradiation power density and efficient emission. In this work, four red emissive azabenzanthrone derivatives have been designed and synthesized, which generally exhibit efficient aggregated state emission. Through structural optimization, 3-diphenylamino-11-azabenzanthrone was found to show satisfactory photo-induced ROS generation and high emission efficiency in the aggregated state. Under the irradiation of a white LED lamp with an ultralow power density of 1.67 mW cm-2, this compound demonstrates significant photo-induced cytotoxicity toward HeLa cells. Moreover, deep tissue penetration can be realized by two-photon imaging of mouse brain vessels with these azabenzanthrone derivatives at vertical depths of up to 280 μm, attributed to the large emission wavelength and efficient emission.

Water solubility is essential for fluorescent probes to image hypochlorous acid in live cells

Our findings, for the first time, highlight the importance of water-solubility of fluorescent probes for HClO imaging.