Rational development of an ESIPT-based fluorescent probe with large Stokes shift for imaging of hydrogen sulfide in live cells

Highly selective dual-signal readout H2S probe: Applications in monitoring of water samples, food spoilage and live cell imaging

We present a new probe (DN) for colorimetric and fluorimetric detecting hydrogen sulfide (H2S) on the basis of the H2S-triggered thiolysis of 4-nitro-2,1,3-benzoxadiazoyl (NBD) ether. In H2O/DMSO medium (9/1, v/v, pH 7.4), the aryl ether bond in DN was cleaved by H2S via nucleophilic aromatic substitution to yield a phenolate product with lighting up red fluorescence accompanied with a distinguished color transformation from yellow to pink. Probe DN features near-infrared emission (660 nm), large Stokes Shift (168 nm), good sensitivity (LOD: 15.1 nM) and relative fast response time (5 min) for specific sensing of H2S. Conveniently, a paper strips-based test kit was developed for tracking and assessing meat freshness by monitoring the release of H2S. What is more, DN is demonstrated to image both exogenous and endogenous H2S in live cells with relative short analysis time (30-60 min), providing a new research tool for exploring the relationship between H2S and clinical diseases.

Synthesis and Excited State Proton Transfer (ESPT) Studies of 2-(6-Substitutedbenzo[d]Thiazol-2-Yl)Naphthalen-1-Ol Derivatives

This study reports the rapid intramolecular proton transfer studies upon photo excitation of 2-(benzo[d]thiazol-2-yl)naphthalene-1-ol derivatives, yielding tautomer emission with large Stokes shift. Employing photophysical studies, density functional theory (DFT) and, time-dependent density functional theory (TD-DFT) methods, we scrutinize excited state intramolecular proton transfer (ESIPT) modulation over varying solvent polarities. Analysis of UV-Visible and fluorescence spectra, alongside exploration of hydrogen bond dynamics, reveals solvation effects on the excited state proton transfer process. Theoretical vibrational spectra confirm enhanced hydrogen bond strength in the excited state which is sensitive to the solvent polarity. Energy profile curves and the scatter graph depict impact of solvent polarity on ESIPT. Additionally, molecular interactions and X-ray diffraction studies of the title compound 4a are presented.

An AIE probe for simultaneous monitoring of endogenous and exogenous hypochlorite and Zn2+ at dual channels in living cells

Zn2+ and ClO- may be associated with a variety of pathologies, and their simultaneous measurement is crucial for disease diagnosis and environmental protection. In this work, we synthesized an independent AIE probe, HNTE, through a one step reaction. The probe HNTE displayed a distinctive fluorescence color change from deep yellow to blue for ClO- and to green for Zn2+. More importantly, the probe HNTE could simultaneously detect endogenous and exogenous ClO- and Zn2+ in living cells colorimetrically via the blue and green channels.

Recent advances in meat freshness "magnifier": fluorescence sensing

To address the serious waste of meat resources and food safety problems caused by the decrease in meat freshness due to the action of microorganisms and enzymes, a low-cost, time-saving and high-efficiency freshness monitoring method is urgently needed. Fluorescence sensing could act as a "magnifier" for meat freshness monitoring due to its ability to sense characteristic signal produced by meat spoilage. Here, the magnification mechanism of meat freshness via sensing the water activity, adenosine triphosphate, hydrogen ion, total volatile basic nitrogen, hydrogen sulfide, bioamines was comprehensively analyzed. The existing "magnifier" forms including paper chips, films, labels, arrays, probes, and hydrogels as well as the application in livestock, poultry and aquatic meat freshness monitoring were reviewed. Future research directions involving innovation of principles, visualization and quantification capabilities for various meats freshness were provided. By critically evaluating the potential and limitations, efficient and reliable meat freshness monitoring strategies wish to be developed for the post-epidemic era.

Influence of atomic electronegativity on ESIPT behaviour for the BTDI and its derivatives: Theoretical exploration

In this work, we theoretically explored the influence of atomic electronegativity on excited-state intramolecular proton transfer (ESIPT) behavior among novel fluorescent probes BTDI and its derivatives (BODI and BSeDI). A thorough examination of the optimized structural parameters and infrared vibrational spectra reveals an enhancement in intramolecular hydrogen bonding within BTDI and its derivatives upon light excitation. This finding is further reinforced by topological analysis and interaction region indicator scatter plots, which underscores the sensitivity of atomic electronegativity to variations in hydrogen bonding strength. With regards to absorption and fluorescence spectra, the decrease in atomic electronegativity leads to a pronounced redshift, primarily attributed to the narrowing of the energy gap. Additionally, an analysis of potential energy curves and the exploration of intrinsic reaction coordinate paths based on transition state structures afford a deeper understanding of the mechanism underlying ESIPT and being modulated through the manipulation of atomic electronegativity. We anticipate that this work on atomic electronegativity regulating ESIPT behavior will serve as a catalyst for novel fluorescent probes in the future, offering fresh perspectives and insights.

Ratiometric peptide-based fluorescent probe with large Stokes shift for detection of Hg2+ and S2- and its applications in cells imaging and smartphone-assisted recognition

In this work, a new ratiometric fluorescent probe DKA was synthesized based on the double sides of lysine backbone conjugated with alanine and dansyl groups. DKA exhibited fluorescence ratiometric response for Hg2+ with high sensitivity (13.4 nM), specific selectivity (only Hg2+), strong anti-interference ability (no interference), fast recognition (within 60 s) and wide pH range (5-10). The stoichiometry of binding of DKA and Hg2+ was determined to be 1:1 via Job's plot, ESI-HRMS and 1HNMR titration analysis. Subsequently, the in situ formation of DKA-Hg2+ complex was used for highly selective detection of S2- as a novel fluorescence "on-off" probe, and the lowest detection limit for S2- was 12.9 nM. In addition, DKA possessed excellent cells permeation and low toxicity, and fluorescence imaging of Hg2+ and S2- was performed in living Hacat cells. Most importantly, the digital imaging using a smartphone color recognition APP indicated that DKA could semi-quantitatively and visually detected Hg2+ and S2- without expensive equipment.

An Aggregation-Induced Fluorescence Probe for Detection H2S and Its Application in Cell Imaging

Monitoring hydrogen sulfide (H2S) in living organisms is very important because H2S acts as a regulator in many physiological and pathological processes. Upregulation of endogenous H2S concentration has been shown to be closely related to the occurrence and development of tumors, atherosclerosis, neurodegenerative diseases and diabetes. Herin, a novel fluorescent probe HND with aggregation-induced emission was designed. Impressively, HND exhibited a high selectivity, fast response (1 min) and low detection limit (0.61 μM) for H2S in PBS buffer (10 mM, pH = 7.42). Moreover, the reaction mechanism between HND and H2S was conducted by Job's plot, HR-MS, and DFT. In particular, HND was successfully employed to detect H2S in HeLa cells.

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.

Photochemical Mechanisms of Hydroxyquinoline Benzimidazole: Insights from Electronic Structure Calculations and Nonadiabatic Dynamics Simulations

Excited-state intramolecular double proton transfer (ESIDPT) has received much attention because of its widespread existence in the life reactions of living organisms, and materials with this property are significant for their special luminescent properties. In this work, the complete active space self-consistent field (CASSCF) and OM2/multireference configuration interaction (OM2/MRCI) methods have been employed to study the static electronic structure calculations of the photochemistry and the possibility of ESIDPT process of hydroxyquinoline benzimidazole (HQB) molecule, along with the nonadiabatic dynamics simulations. The computational results show that the HQB molecule is relaxed to the S1-ENOL minimum after being excited to the Franck-Condon point in the S1 state. Subsequently, during the nonadiabatic deactivation process, the OH···N proton transfer and the twisting of benzimidazole occur before arriving at the single proton transfer conical intersection S1S0-KETO. Finally, the system can either return to the initial ground-state structure S0-ENOL or to the single proton transfer ground-state structure S0-KETO, both of which have almost the same probability. The dynamics simulations also show that no double proton transfer occurs. The excited-state lifetime of HQB is fitted to 1.1 ps, and only 64% of the dynamic trajectories return to the ground state within the 2.0 ps simulation time. We hope the detailed reaction mechanism of the HQB molecule will provide new insights into similar systems.

A human serum albumin-binding-based fluorescent probe for monitoring hydrogen sulfide and bioimaging

In this work, a fluorescent probe, TPABF-HS, was developed for detecting hydrogen sulfide (H2S) using a human serum albumin (HSA)-binding-based approach for amplifying the fluorescence signal and extending the linear correlation range. Compared to the most recent probes for H2S, the most interesting feature of the detection system developed herein was the especially wide linear range (0-1000 μM (0-100 eq.)), which covered the physiological and pathological levels of H2S. TPABF-HS could be used in applications high sensitivity and selectivity with an LOD value of 0.42 μM. Further, site-competition experiments and molecular docking simulation experiments indicated that signal amplification was realized by the binding of the TPABF fluorophore to the naproxen-binding site of HSA. Moreover, the extension of the measurement span could allow for applications in living cells and Caenorhabditis elegans for imaging both exogenous and endogenous H2S. This work brings new information to the strategy of signal processing by exploiting fluorescent probes.

ESIPT-based probes for cations, anions and neutral species: recent progress, multidisciplinary applications and future perspectives

Fluorescent and colourimetric probes for small analytes (cations, anions and neutral molecules) have drawn significant attention in recent years. These probes interact with analytes and induce spectral change due to the variations in the photo-physical properties of the fluorophore/chromophore used. Among several photo-physical mechanisms, ESIPT (excited state intramolecular proton transfer) based probes are more advantageous due to their photo-physical properties viz. solvent polarity effect, large spectral shift with multi-channel fluorescence, high quantum yield etc. In recent years, ESIPT-based probes have shown several promising applications, especially monitoring small analytes in biological samples, smartphone app-assisted heavy metal detection in environmental samples, inkless writing, anti-counterfeiting applications etc. Therefore, this review is dedicated to recently reported ESIPT-based probes for small analytes. We have highlighted the organic units responsible for the ESIPT mechanism, their photo-physical parameters, selectivity and sensitivity properties and recent advances in their applications.

A selective fluorescent probe for hydrogen sulfide from a series of flavone derivatives and intracellular imaging

In this work, through the orthogonal design of two fluorophores and two recognition groups, a series of fluorescent probes were developed from the flavone derivatives for hydrogen sulfide (H₂S). The probe FlaN-DN stood out from the primarily screening on the selectivity and response intensities. It could respond to H₂S with both the chromogenic and fluorescent signals. Among the recent reported probes for the H₂S detection, FlaN-DN indicated the most highlighted advantages including the rapid response (within 200 s) and the high response multiplication (over 100 folds). FlaN-DN was sensitive to the pH condition, thus could be applied to distinguish the cancer micro-environment. Moreover, FlaN-DN suggested practical capabilities including a wide linear range (0-400 μM), a relatively high sensitivity (limit of detection 0.13 μM), and high selectivity towards H₂S. As a low cytotoxic probe, FlaN-DN achieved the imaging in living HeLa cells. FlaN-DN could detect the endogenous generation H₂S and visualize the dose-dependent responses to the exogenous H₂S level. This work provided a typical case of natural-sourced derivatives as functional implements, which might inspire the future investigations.

An ESIPT-based ratiometric fluorescent probe for detecting H2O2 in water environment and biosystems

The outbreak of the COVID-19 has resulted in a great increase in the use of H₂O₂ disinfectant, which is listed as one of the commonly used disinfectants for COVID-19 by the U.S. Environmental Protection Agency. However, excessive use of H₂O₂ disinfectant can threaten human health and damage the water environment. Therefore, it's of great importance to detect H₂O₂ in aquatic environments and biological systems. Herein, we proposed a novel ESIPT ratio fluorescent probe (named probe 1) for detecting H₂O₂ in water environment and biosystems. Probe 1 emits blue fluorescence as the introduction of the phenylboronic acid disrupts the ESIPT process. After reacting with H₂O₂, the phenylboronic acid is oxidatively removed, and the ESIPT process is restored, which makes the fluorescence emission wavelength red-shifted. Probe 1 exhibited a short response time, high sensitivity, and a large Stokes shift to H₂O₂. Importantly, it has been successfully used to detect H₂O₂ not only in actual water samples, but also endogenous and exogenous H₂O₂ in living cells. The characteristics of probe 1 have a wide range of applications in environmental and biological systems.

A sensitive and ratiometric fluorescent probe for imaging cytosolic H2S generation

CouMa responded to H2S within 3 minutes ratiometrically, based on an indol–coumarin fluorophore. The positively-charged probe accumulated in cytosol, and imaged NO-relevant H2S generation increment and depression in cytosol of living cells.