A Photoelectrochemical Sensor for the Detection of Hypochlorous Acid with a Phenothiazine-Based Photosensitizer

This paper presents the development of a photoelectrochemical sensor for hypochlorous acid (HOCl) detection, employing a phenothiazine-based organic photosensitizer (Dye-PZ). The designed probe, Dye-PZ, follows a D-π-A structure with phenothiazine as the electron-donating group and a cyano-substituted pyridine unit as the electron-accepting group. A specific reaction of the phenothiazine sulfur atom with HOCl enables selective recognition. The covalent immobilization of Dye-PZ onto a titanium dioxide nanorod-coated fluorine-doped tin oxide electrode (FTO/TiO2) using bromo-silane coupling agent (BrPTMS) resulted in the fabrication of the photoanode FTO/TiO2/BrPTMS/Dye-PZ. The photoanode exhibited a significant photoresponse under visible-light irradiation, with a subsequent reduction in photocurrent upon reaction with HOCl. The oxidation of the phenothiazine sulfur atom to a sulfoxide diminished the internal charge transfer (ICT) effect. Leveraging this principle, the successful photoelectrochemical sensing of HOCl was achieved. The sensor showed high stability, excellent reproducibility, and selective sensitivity for HOCl detection. Our study provides a novel approach for the development of efficient photoelectrochemical sensors based on organic photosensitizers, with promising applications in water quality monitoring and biosensing.

Recent development of small-molecule fluorescent probes based on phenothiazine and its derivates

Fluorescence probes, as analytical tools with the ability to perform rapid and sensitive detection of target analytes, have made outstanding contributions to environmental analysis and bioassays. Considering the expanding developments in these areas, fluorophores play a key role in the de-sign of fluorescence probes. Compared to classical fluorophores, phenothiazines with elec-tron-rich characteristics have been widely applied to construct electron donor-acceptor dyes, which exhibit outstanding performance in both fluorimetric and colorimetric analysis. In addition, these probes also exhibit the pronounced ability in both solution and solid-state, achieving portable detection for environmental analysis. In this review, we summarize recent advances in the performance of phenothiazine-based fluorescent probes for detecting various analytes, especially in cations, anions, ROS/RSS, enzyme and other small molecules. The general design rules, response mechanisms and practical applications of the probes are analyzed, followed by a discussion of exiting challenges and future research perspectives. It is hoped that this review will provide a few strategies for the development of phenothiazine-based fluorescent probes.

A phenothiazine coumarin based ratiometric fluorescent probe for real-time detection of lysosomal hypochlorite in living cell and zebra fish

Hypochlorite (ClO^-), a type of reactive oxygen species (ROS), plays an essential role in complex biological systems. Real-time detection of the content and distribution of ClO^- in cells or subcellular organelle is critically essential. In this paper, a lysosomal-targeted fluorescent probe, Cou-Lyso, was constructed for real-time detection of ClO^- in a ratiometric manner, achieving high sensitivity with a low detection limit (0.58 μM). Upon reaction with ClO^-, this probe was subjected to a significant fluorescence change from red emission (λ_maxem = 610 nm) to green emission (λ_maxem = 535 nm) with the ratio of I_{535 nm}/I_{610 nm} displaying a 76-fold enhancement from 0.04 to 3.03. The confocal imaging experiments for Cou-Lyso showed that this probe could detect ClO^- in living cell and zebra fish. This probe has been successfully applied to stain lysosome and image lysosomal ClO^- based on co-localization imaging experiments.

Novel near-infrared fluorescent probe with a large Stokes shift for sensing hypochlorous acid in mitochondria

Hypochlorous acid (HOCl) plays a crucial role in various physiological and pathological processes. However, it is still a challenge to design a xanthene-based near-infrared (NIR) fluorescent probe with a large Stokes shift for sensing HOCl. In this work, a novel mitochondria-targeted fluorescent probe, MXS, with a large Stokes shift based on a xanthene-hemicyanine dyad structure, has been successfully designed and synthesized for the specific detection of HOCl. Gratifyingly, the peak-to-peak Stokes shift of MXS was found to be 130 nm, which was obviously larger than those of conventional rhodamine dyes and most reported xanthene-based hypochlorous acid probes. As expected, MXS exhibited high selectivity, high sensitivity, and fast response time (30 s) for the detection of HOCl via a specific HOCl-promoted intramolecular charge transfer process. The detection limit of MXS for HOCl is calculated to be as low as 72 nM, enabling its use within the physiological concentration range of HOCl (5-25 μM). Importantly, MXS is able to permeate cell membranes and accumulate in the mitochondria, which is convenient for monitoring the variation of hypochlorous acid concentration in the mitochondria of living cells.

A lysosome-targetable and ratiometric fluorescent probe for hypochlorous acid in living cells based on a 1,8-naphthalimide derivative

Hypochlorous acid (HClO) is an important reactive oxygen species (ROS) and plays a significant role in living organisms. Highly selective and lysosome-targetable probes for sensing hypochlorous acid are rare. In this article, we designed and prepared a new lysosome-targeting and ratiometric fluorescent probe for monitoring the levels of hypochlorous acid. 4-Aminonaphthalimide was chosen as the fluorescent group and (2-aminoethyl) thiourea group was used as a specific recognition group for HClO. A morpholine unit was employed as a lysosome-targeting group. In the absence of HClO the probe underwent intramolecular charge transfer (ICT) and showed a green emission. When excess HClO is present, the ICT process was interrupted which caused a 57 nm blue-shift of fluorescence emission from 533 nm to 476 nm. The ratiometric fluorescent probe showed outstanding selectivity toward HClO over other various bioactive species. Furthermore, the ratiometric fluorescent probe exhibited rapid response time and ability of working in a wide pH range. The linear response of I_476nm/I_533nm toward HClO was obtained in a concentration range of HClO from 1.0 × 10^-6 to 1.0 × 10^-4 mol·L^-1. The detection limit was estimated to be 8.0 × 10^-7 mol·L^-1 for HClO. Moreover, the probe showed a perfect lysosome-targeting ability, and has been successfully used to the confocal imaging of HClO in lysosomes of HepG2 cells with little cell toxicity. All of such good properties illustrated that it could be applied to determine HClO at lysosomes in living cells.

A mitochondria-targeted near-infrared fluorescent probe with a large Stokes shift for real-time detection of hypochlorous acid

A real-time mitochondria-targeted near-infrared fluorescent probeLhas been synthesized with large Stokes shifts, and high selectivity and sensitivity.

A fast-responsive two-photon fluorescent probe for monitoring endogenous HClO with a large turn-on signal and its application in zebrafish imaging

Fast-responsive two-photon fluorescent probe NS-ClO for imaging endogenous HClO in vivo with a large turn-on signal (about 860 times) and Stokes shift (about 90 nm), fast response (about 1 min) and good selectivity.

A simple sensor based on 1,8-naphthalimide with large Stokes shift for detection of hypochlorous acid in living cells

Probe 1shows excellent selectivity to HOCl among different ions including common ROS, high sensitivity, high fluorescence quantum yield (Φ= 0.93), low detection limit (0.237 μM) and successfully used for the detection of HOCl in cells.

Dicyanoisophorone-Based Near-Infrared-Emission Fluorescent Probe for Detecting NAD(P)H in Living Cells and in Vivo

NADH and NADPH are ubiquitous coenzymes in all living cells that play vital roles in numerous redox reactions in cellular energy metabolism. To accurately detect the distribution and dynamic changes of NAD(P)H under physiological conditions is essential for understanding their biological functions and pathological roles. In this work, we developed a near-infrared (NIR)-emission fluorescent small-molecule probe (DCI-MQ) composed of a dicyanoisophorone chromophore conjugated to a quinolinium moiety for in vivo NAD(P)H detection. DCI-MQ has the advantages of high water solubility, a rapid response, extraordinary selectivity, great sensitivity (a detection limit of 12 nM), low cytotoxicity, and NIR emission (660 nm) in response to NAD(P)H. Moreover, the probe DCI-MQ was successfully applied for the detection and imaging of endogenous NAD(P)H in both living cells and tumor-bearing mice, which provides an effective tool for the study of NAD(P)H-related physiological and pathological processes.

Recent advances of dicyano-based materials in biology and medicine

We highlight the development of dicyano-based fluorescent materials in biology and medicine.

A rhodamine-based fast and selective fluorescent probe for monitoring exogenous and endogenous nitric oxide in live cells

A sensitive and selective fluorescent probe for fast detection of nitric oxide was synthesized by grafting a NO-trapper o-phenylenediamine onto a rhodamine fluorophore.