Activity-Based Fluorescent Probes for Hydrogen Sulfide and Related Reactive Sulfur Species

Hydrogen sulfide (H2S) is not only a well-established toxic gas but also an important small molecule bioregulator in all kingdoms of life. In contemporary biology, H2S is often classified as a "gasotransmitter," meaning that it is an endogenously produced membrane permeable gas that carries out essential cellular processes. Fluorescent probes for H2S and related reactive sulfur species (RSS) detection provide an important cornerstone for investigating the multifaceted roles of these important small molecules in complex biological systems. A now common approach to develop such tools is to develop "activity-based probes" that couple a specific H2S-mediated chemical reaction to a fluorescent output. This Review covers the different types of such probes and also highlights the chemical mechanisms by which each probe type is activated by specific RSS. Common examples include reduction of oxidized nitrogen motifs, disulfide exchange, electrophilic reactions, metal precipitation, and metal coordination. In addition, we also outline complementary activity-based probes for imaging reductant-labile and sulfane sulfur species, including persulfides and polysulfides. For probes highlighted in this Review, we focus on small molecule systems with demonstrated compatibility in cellular systems or related applications. Building from breadth of reported activity-based strategies and application, we also highlight key unmet challenges and future opportunities for advancing activity-based probes for H2S and related RSS.

A highly selective and sensitive ESIPT-based coumarin–triazole polymer for the ratiometric detection of Hg2+

A reversible ESIPT based system for the detection of Hg²⁺ was developed. The system exhibited better properties compared to that of recently developed ratiometric fluorescent systems.

Benzothiazole integrated into a cryptand for ESIPT-based selective chemosensor for Zn2+ ions

A novel 2(2′-hydroxyphenyl) benzothiazole-based cryptand (L) exhibits high fluorescence intensity in the presence of Zn²⁺ ions by stopping the excited state intramolecular proton transfer (ESIPT) process with a detection limit of 0.20 μM.

A lysosome targetable fluorescent probe for palladium species detection base on an ESIPT phthalimide derivative

A novel lysosome-targetable phthalimide fluorescent probe was designed for detecting palladium based on ESIPT for signal transduction. The fluorescent probe conjugating with allylcarbamate displayed weak fluorescent due to the ESIPT process hinder by allylcarbamate. But with the addition of palladium, the ESIPT emission was recovery though the palladium-catalyzed deallylation reaction and the fluorescence intensity exhibited 40-fold enhancement at 511 nm. In addition, the probe showed excellent selectivity, high sensitivity, fast responds and low limit detection for palladium with a larger Stoke-shift. Moreover, the targetable probe was also successfully applied for detecting palladium in lysosomes of living cells. Hence, the probe though ESIPT modulation is a promising for monitoring palladium in practical samples.

An ESIPT based naphthalimide chemosensor for visualizing endogenous ONOO− in living cells

An ESIPT based naphthalimide chemosensor with high sensitivity and selectivity for visualizing endogenous ONOO⁻ in living cells was developed.

A new simple phthalimide-based fluorescent probe for highly selective cysteine and bioimaging for living cells

A new turn-on phthalimide fluorescent probe has designed and synthesized for sensing cysteine (Cys) based on excited state intramolecular proton transfer (ESIPT) process. It is consisted of a 3-hydroxyphthalimide derivative moiety as the fluorophore and an acrylic ester group as a recognition receptor. The acrylic ester acts as an ESIPT blocking agent. Upon addition of cystein, intermolecular nucleophilic attack of cysteine on acrylic ester releases the fluorescent 3-hydroxyphthalimide derivative, thereby enabling the ESIPT process and leading to enhancement of fluorescence. The probe displays high sensitivity, excellent selectivity and with large Stokes shift toward cysteine. The linear interval range of the fluorescence titration ranged from 0 to 1.0×10^-5M and detection limit is low (6×10^-8M). In addition, the probe could be used for bio-imaging in living cells.

A selective fluorescence probe based on benzothiazole for the detection of Cr3+

A novel benzothiazole-functionalized Schiff-base derivative

Discrimination of tabun mimic diethyl cyanophosphonate from sarin mimic diethyl chlorophosphate via Zn(ii)-triggered photoinduced electron transfer-decoupled excited state intramolecular proton transfer processes

PET-coupled ESIPT platform and its Zn²⁺ complex are used for the discrimination of the nerve agent mimics DCNP and DCP.

A 2-(2'-hydroxyphenyl)benzothiazole (HBT)-quinoline conjugate: a highly specific fluorescent probe for Hg(2+) based on ESIPT and its application in bioimaging

A benzothiazole derived chemosensor L has been designed based on the excited-state intramolecular proton transfer (ESIPT) mechanism to afford a fluorescence turn-on response specifically in the presence of Hg(2+) ions over a host of biologically relevant metal ions as well as toxic heavy metal ions. The chemosensor exhibits high sensitivity with the detection limit down to 0.11 μM. The metal binding is supported by (1)H NMR titrations, ESI-MS spectral analysis and substantiated by theoretical calculations using the density functional theory. The probe shows cell membrane permeability and efficiency for the detection of Hg(2+) in HeLa cells.

A PET-based fluorometric chemosensor for the determination of mercury(II) and pH, and hydrolysis reaction-based colorimetric detection of hydrogen sulfide

A simple fluorescent chemosensor 1 for the detection of Hg(2+) and pH was developed by a combination of 2-aminoethyl piperazine and 4-chloro-7-nitrobenz-2-oxa-1,3-diazole. The sensor 1 showed OFF-ON behavior for different colors of fluorescence in the presence of Hg(2+) and under acidic conditions, respectively, in a near-perfect aqueous solution. The turn-on fluorescence caused by inhibition of photoinduced electron transfer was explained by theoretical calculations. 1 could be used to quantify Hg(2+) in water samples, and its in vitro studies with HeLa cells showed fluorescence in the presence of Hg(2+). In addition, 1 could selectively detect S(2-) by changing its color from orange to pink in a near-perfect aqueous solution. Moreover, 1 could be used as a practical, visible test kit for S(2-).

An ESIPT-based fluorescent probe for sensitive detection of hydrazine in aqueous solution

A fluorescent probe for sensitive detection of hydrazine based on an ESIPT mechanism and a substitution-cyclization-elimination cascade was developed. After the addition of hydrazine, an approximately 50-fold enhancement in fluorescence intensity at 465 nm was observed and the subsequent decrease at 375 nm was observed in 10 min with a detection limit of 0.147 μM. We also detected hydrazine in HeLa cells successfully.

A two-photon ratiometric ESIPT probe for the discrimination of different palladium species and its application in bioimaging

A two-photon ratiometric ESIPT probe for the discrimination of different palladium species which could be used for bioimaging.

A thiosemicarbazone based chemo and fluorogenic sensor for Zn2+with CHEF and ESIPT behaviour: computational studies and cell imaging application

A novel diformyl-p-cresol (DFC)–thiosemicarbazide (TS) – based turn-on dual sensor that enables to determine, for the first time, the stepwise formation constants.

A highly selective colorimetric chemosensor for cyanide and sulfide in aqueous solution: experimental and theoretical studies

A selective chemosensor was developed for the colorimetric detection of CN⁻ and S²⁻ in aqueous solution.

A novel 2-(2′-aminophenyl)benzothiazole derivative displays ESIPT and permits selective detection of Zn2+ ions: experimental and theoretical studies

Synthesis of a novel 2-(2′-aminophenyl)benzothiazole based probe (1) and demonstration of excited state intramolecular proton transfer (ESIPT) with a large Stokes shift (∼246 nm) are presented.

A concentration dependent auto-relay-recognition by the same analyte: a dual fluorescence switch-on by hydrogen sulfide via Michael addition followed by reduction and staining for bio-activity

H₂S is shown, for the first time, to play an extraordinary dual role due to its nucleophilicity and reducing property with a single chemosensor.

Single excited state intramolecular proton transfer (ESIPT) chemodosimeter based on rhodol for both Hg2+ and OCl−: ratiometric detection with live-cell imaging

We have synthesised a ratiometric fluorescence chemodosimeter, STBR, for the detection of Hg²⁺ and OCl⁻ in one platform over other cations, anions and oxidants in aqueous solution.

Engineering a FRET strategy to achieve a ratiometric two-photon fluorescence response with a large emission shift and its application to fluorescence imaging

Two-photon excitation (TPE) probe-based fluorescence imaging has become one of the most attractive diagnostic techniques to investigate biomolecules and biological events in live cells and tissues. At the current stage most of the TPE-based sensing is reflected by fluorescence intensity changes. Nevertheless the mere altering of intensity could be facilely affected by ambient conditions. On the other hand, TPE probes based on an intramolecular charge transfer (ICT) strategy could solve this problem to some extent with a morphology change-induced emission shift. However their applications are yet constrained due to the inherent limitation of ICT, e.g. the high degree of overlap of two emissions bands and shifts of the TPE maxima. To achieve the desired TPE-based sensing and to circumvent the problems stated above, we adapted a Förster resonance energy transfer (FRET) strategy to develop small molecule ratiometric TPE fluorescent probes. Our FRET-based ratiometric TPE fluorescent probe displays a remarkable emission shift (up to 125 nm) with two well-resolved emission bands. Hence the ratio of these two emission bands could enable the measurement of fluorescence changes more accurately, thus further improving imaging in live cells and deep tissues. To the best of our knowledge, the current reported probe has the largest emission shift among all the small molecule ratiometric TPE fluorescent probes while the maximum TPE wavelength remains unchanged. This work has provided a FRET approach to fabricate novel small molecule ratiometric TPE fluorescent probes that improve imaging in deep tissues.

A novel FRET-based ratiometric fluorescent probe for highly sensitive detection of hydrogen sulfide

A novel ratiometric and colorimetric probeH2S-CRfor the quantitative detection of H₂S was designed and synthesized based on a H₂S induced Michael addition–cyclization cascade reaction and the FRET modulated fluorescence off-on response.

A remarkable ratiometric fluorescent chemodosimeter for very rapid detection of hydrogen sulfide in the vapour phase and living cells

A ratiometric fluorescent probe having a fast response and a large Stokes shift detects SH⁻ both in solid and vapour phases and this probe is used for fluorescence imaging of SH⁻ in living cells.

Neighbouring group participation of thiol through aldehyde group assisted thiolysis of active ether: ratiometric and vapor phase fast detection of hydrogen sulfide in mixed aqueous media

Ratiometric and fast detection of H₂S via NGP of thiol rather than direct nucleophilic attack to cleave active ether linkage.

An ESIPT-based fluorescent probe for highly selective and ratiometric detection of mercury(ii) in solution and in cells

A highly selective and sensitive ratiometric fluorescent probe for mercury(ii) has been developed. It has been demonstrated that the probe is able to detect Hg²⁺ level in water samples and living cell.

Pot-in-pot reactions: a simple and green approach to efficient organic synthesis

A simple, flux controlled, technique to circumvent the tedium and wastage in organic synthesis is review. Pot-in-pot reactions, like matryoshka dolls, houses one reaction pot inside another.

Coumarin–benzothiazole–chlorambucil (Cou–Benz–Cbl) conjugate: an ESIPT based pH sensitive photoresponsive drug delivery system

We have developed an ESIPT based drug delivery system, Cou–Benz–Cbl conjugate, with pH sensitive fluorescence properties and photocontrolled release of the anticancer drug chlorambucil.