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 Prompt Study on Recent Advances in the Development Of Colorimetric and Fluorescent Chemosensors for "Nanomolar Detection" of Biologically Important Analytes

Fluorescent and colorimetric chemosensors for selective detection of various biologically important analytes have been widely applied in different areas such as biology, physiology, pharmacology, and environmental sciences. The research area based on fluorescent chemosensors has been in existence for about 150 years with the development of large number of fluorescent chemosensors for selective detection of cations as metal ions, anions, reactive species, neutral molecules and different gases etc. Despite the progress made in this field, several problems and challenges still exist. The most important part of sensing is limit of detection (LOD) which is the lowest concentration that can be measured (detected) with statistical significance by means of a given analytical procedure. Although there are so many reports available for detection of millimolar to micromolar range but the development of chemosensors for the detection of analytes in nanomolar range is still a challenging task. Therefore, in our current review we have focused the history and a general overview of the development in the research of fluorescent sensors for selective detection of various analytes at nanomolar level only. The basic principles involved in the design of chemosensors for specific analytes, binding mode, photophysical properties and various directions are also covered here. Summary of physiochemical properties, mechanistic view and type of different chemosensors has been demonstrated concisely in the tabular forms.

Detection of aqueous and gaseous hydrogen sulfide with lanthanide-macrocycle binary complexes

Two novel, discrete lanthanide-macrocycle binary complexes for the detection of hydrogen sulfide are reported. The hydrogen sulfide sensing mechanism utilises the copper sequestration at a secondary binding site, with resulting bimetallic lanthanide(III)/copper(II) complexes (Ln = Eu3+ and Tb3+) exhibiting high selectivity, good sensitivity and excellent reversibility for aqueous hydrogen sulfide. The inclusion of the DO2A macrocycle and 4-(2-pyridyl)-1,2,3-triazole dipicolinic acid ligand, results in a complex with good solubility and stability. The europium(III) complex also displayed a low limit of detection (665 ppb) with a response time of 30 seconds with gaseous hydrogen sulfide. The improved water solubility and stability over a previous complex results in these sensors having the potential for use in environmental monitoring and biological studies for various functional settings.

The design strategies and biological applications of probes for the gaseous signaling molecule hydrogen sulfide

H₂S, the smallest and simplest biological thiol in living systems, is the third member of the family of signaling mediators. H₂S participates in the regulation of a series of complex physiological and pathological functions in the body, making it a critical fulcrum that balances health and disease in human physiology. Small-molecule fluorescent probes have been proven to possess the unique advantages of high temporal and spatial resolution, good biocompatibility and high sensitivity, and thus their use is a powerful approach for monitoring the level and dynamics of H₂S in living cells and organisms and better understanding its basic cellular functions. The field of small-molecule fluorescent probes for monitoring the complex biological activities of H₂S in vivo has been thriving in recent years. Herein, we systematically summarize the latest developments in the field of fluorescent probes for the detection of H₂S, illustrate their biological applications according to the classification of target-responsive sites, and emphasize the development direction and challenges of H₂S-responsive fluorescent probes, hoping to give implications of researchers on fluorescent probes for future research.

Real time sensor for Fe3+, Al3+, Cu2+ & PPi through quadruple mechanistic pathways using a novel dipodal quinoline-based molecular probe

A quinoline-based small molecular probe, H₂L was designed, synthesized and characterized by different spectroscopic methods. It was utilized as a multi-responsive probe for the detection of Fe³⁺, Al³⁺, Cu²⁺ and PPi. It showed very selective instant turn-on fluorimetric response towards Fe³⁺and Al³⁺ with a detection limit in nanomolar range. Solutions of H₂L containing Fe³⁺ or Al³⁺ could sequentially sense PPi by a turn-off mechanism. Also, H₂L could determine the presence of Cu²⁺ very selectively among a series of other metal ions by a sharp change in colour. Detection of Cu²⁺ through colorimetry was further investigated by systematic UV-Vis studies and the potential of H₂L to act as a potential colorimetric sensor for Cu²⁺ was suitably established. Filter-paper strip experiments were conducted to demonstrate the practical utility of the proposed sensor. Potential applications of H₂L as a sensor for pH in the acidic range has also been explored.

Arousing Electrochemiluminescence Out of Non-Electroluminescent Monomers within Covalent Organic Frameworks

Covalent organic frameworks (COFs) with stable long-range ordered arrangements are promising materials for organic optoelectronics. However, their electrochemiluminescence (ECL) from non-ECL active monomers has not been realized. Here, we report a design strategy for ECL-emitting COF family. The donors and acceptors co-crystallized and stacked into the highly aligned array of olefin-linked COFs, so that electrons can be transported freely. By this means, a tunable ECL is activated from non-ECL molecules with the maximum efficiency of 32.1% in water with the dissolved oxygen as an inner coreactant, and no additional noxious co-reactant is needed any more. Quantum chemistry calculations further demonstrate that this design reduces the COFs' band gaps and the overlap of electrons and holes in the excited state for better photoelectric properties and stronger ECL signals. This work exploits a basis to envisage the broad application potential of ECL-COFs for various biosensors and light-emitting display.

A selective polypyrrole-based sub-ppm impedimetric sensor for the detection of dissolved hydrogen sulfide and ammonia in a mixture

An impedance-transducer sensor was developed for in situ detection of hydrogen sulfide (H₂S) and ammonia (NH₃) in aqueous media. Using cyclic voltammetry (CV), polypyrrole (PPy) was deposited on the surface of the microfabricated interdigitated gold electrode. Due to the proton acid doping effect of H₂S on PPy and ionic conduction of the film, the sensor showed a decreasing impedance response to H₂S unlike other reducing chemicals, i.e., ammonia (NH₃). The recorded faradaic data was then associated with an equivalent circuit and compared with that of NH₃ to examine the selectivity of the sensor. An electrochemical impedance spectroscopy (EIS) analysis was applied to the mixture of H₂S and NH₃ prepared at different ratios for the concentrations ranging from 2 ppm to 20 ppm (below 2-ppm, no response was observed due to the formation of NH₄HS, not sensible with PPy). The principal component analysis (PCA) was used to train a real-time prediction model for both classification (for the type of the analyte) and regression (the concentration of the analyte). The results showed the high performance of the sensor in determining individual analytes while the model was able to accurately predict the amount of H₂S and NH₃ in the mixture.

Pyridinyl Conjugate of UiO-66-NH2 as Chemosensor for the Sequential Detection of Iron and Pyrophosphate Ion in Aqueous Media

A new chemosensor UiO-66-N-Py (Py = 2-methinepyridine, N = imine nitrogen) based on isoreticular UiO-66 (University of Oslo) Metal–Organic Framework (MOF) containing 2-methinepyridine functionalized organic linker was solvothermally synthesized and characterized. This UiO-66-N-Py was very selective and sensitive for detecting the Fe3+ ion and sequential detection of the pyrophosphate (PPi) anion. The limits of detection for the Fe3+ ion and PPi were calculated to be 10 ppb (0.19 μM) and 50 ppb (0.3 μM), respectively. The quenching constant Ksv for Fe3+ and the binding constant for PPi were 1.4 × 105 M−1 and 1.7 × 105 M−1, respectively. The functionalization of UiO-66-NH2 with 2-methinepyridine enhanced its fluorescence emission properties and introduced more binding sites for the analytes. We additionally studied the interaction of the sensor and the analytes with Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). This chemosensor also demonstrated a regenerative emission property without loss in the detection ability for six consecutive cycles.

Design and synthesis of two new terbium and europium complex-based luminescent probes for the selective detection of zinc ions

Zinc plays a key role in many physiological processes and has implications for the environment. Consequently, detection of chelatable zinc ion (Zn²⁺ ) has attracted widespread interest from the research community. Lanthanide-based luminescent probes offer particular advantages, such as high water solubility, long luminescence lifetimes and a large Stokes' shift, over common organic dye-based fluorescent sensors. Here, we report the synthesis of terbium and europium complex-based probes, Tb-1 and Eu-1, for sensitive and selective detection of Zn²⁺ in water. These probes featured the incorporation of bis(2-pyridylmethyl)]amine (DPA) receptor for Zn²⁺ chelation and the 1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane (DO3A) ring to chelate lanthanide (Ln³⁺ ). Tb-1 and Eu-1 displayed high selectivity for Zn²⁺ ions over a wide range of competing ions, with limits of detection of 0.50 ± 0.1 μM and 1.5 ± 0.01 μM, respectively. Density functional theory simulations were in good agreement with experimental observations, displaying high Zn²⁺ selectivity compared with most competing ions. In the competing ions experiments, the luminescence response of Tb-1 and Eu-1 was moderately quenched by some ions such as Cu²⁺ , this was linked to the comparable binding abilities of these ions for the receptor of the probe.

A highly efficient red-emitting luminescent paper-based chemosensor for hydrogen sulfide

The first discrete bimetallic europium(iii)/copper(ii) complex for the fast, sensitive and selective luminescent detection of both aqueous and gaseous hydrogen sulfide has been developed. The chemosensor displayed an impressive response time of 30 seconds and a low theoretical limit of detection (100 ppb) for gaseous hydrogen sulfide.

Time-Resolved Terbium-Based Probe for the Detection of Zinc(II) Ions: Investigation of the Formation of a Luminescent Ternary Complex

Because of their unique photochemical and photophysical properties, luminescent lanthanide-based complexes have long captivated chemists. In recent years, the number of reports of luminescent lanthanide complex-based probes for monitoring of biological and environmental processes has dramatically increased, namely, because of their selectivity for particular analytes, lower limits of detection, and the fact that they allow monitoring of analytes in real time. Lanthanide-based probes need to be paired with an appropriate antenna/sensitizer to allow maximum energy transfer, with the antenna typically covalently attached to the stable lanthanide chelate. We have recently investigated "dark" lanthanide-based probes where the sensitizer is not covalently linked to the lanthanide chelate. Herein we report the use of a luminescent lanthanide-based probe system for the detection of Zn²⁺ ions based on the formation of a ternary complex between a "dark" terbium complex, lumazine, and Zn²⁺. The terbium(III)-based probe incorporates a 1,4,7,10-tetraazacyclododecane-1,4,7,10-triacetic acid macrocyclic chelator covalently attached to a cyclen moiety, which is the Zn²⁺ ion binding group. In the presence of Zn²⁺ ions and lumazine (a strongly UV-absorbing sensitizer), a 1:1:1 ternary complex forms. The resulting complex is highly luminescent and selective for Zn²⁺ ions over other cations of environmental significance. Furthermore, with a limit of detection of 1.2 μM, this probe can detect the level of chronic zinc(II) concentrations denoted by the U.S. Environmental Protection Agency.

A terbium(iii) luminescent ATCUN-based peptide sensor for selective and reversible detection of copper(ii) in biological media

Detection of copper(ii) in biological media via time-delayed luminescence by a selective and reversible terbium(iii)-luminescent peptide sensor with pM affinity.

Superb-selective chemodosimetric signaling of sulfide in the absence and in the presence of CT-DNA and imaging in living cells by a plant alkaloid berberine analogue

New berberine analogue (BER-S), as a colorimetric probe in the absence of DNA and turn-on fluorometric probe in the presence of DNA towards S²⁻ detection is reported.

Qualitative colorimetric analysis of a Ir(iii)/Eu(iii) dyad in the presence of chemical warfare agents and simulants on a paper matrix

The addition of G- and V-series organophosphorus chemical warfare agents and simulants to a paper-based assay of a dual-luminescent Ir(iii)/Eu(iii) dyad generated different emissive responses between the classes and compound types. The emission responses are complex and based not only on altering the balance between red Eu(iii)-based and blue Ir(iii)-based luminescent components, but also incorporate other factors such as analyte volatility, concentration and UV absorption. The extent of this emission colour change was analysed colorimetrically and related to the change in RGB output over time.

A dual emissive Ir(iii)/Eu(iii) dyad in a paper-based luminescence assay for liquid chemical warfare agents demonstrated clear visual responses.

Dual-Emitting Eu(III)-Cu(II) Heterometallic-Organic Framework: Simultaneous, Selective, and Sensitive Detection of Hydrogen Sulfide and Ascorbic Acid in a Wide Range

As important biomolecules, the deficiency or maladjustment of hydrogen sulfide (H₂S) or ascorbic acid (AA) is associated with the symptoms of the same disease (e.g., cardiovascular disease or cancer). There is an urgent need to develop a fluorescent probe capable of distinguishing between H₂S and AA simultaneously. Here, we report the syntheses, structure, and property of the first dual-detection fluorescent probe which can differentiate H₂S or/and AA in aqueous media. Accordingly, a novel [EuCu(pydc)₂(ox)_0.5(H₂O)₃·1.5H₂O]_{2 n} (1, H₂pydc = 2,3-pyridinedicarboxylic acid and ox = oxalic acid) for selective and sensitive detection of H₂S and AA in a wide range has been constructed (H₂S: [130 nM, +∞); AA: [55 nM, +∞)), exhibiting excellent catalytic activity comparable to horseradish peroxidase. In addition, the highly efficient detection in human serum sample also proves the potential application in medical diagnosis. Meanwhile, a combinatorial logic gate (AND(INH)-OR) based on activated 1 has also been constructed. Furthermore, this approach for simultaneous H₂S and AA detection suggests that the current work will expand the potential application of metal-organic frameworks for dual or multiple detections in biomedical fields.

Application of lanthanide luminescence in probing enzyme activity

Enzymes play critical roles in the regulation of cellular function and are implicated in numerous disease conditions. Reliable and practicable assays are required to study enzyme activity, to facilitate the discovery of inhibitors and activators of enzymes related to disease. In recent years, a variety of enzyme assays have been devised that utilise luminescent lanthanide(iii) complexes, taking advantage of their high detection sensitivities, long luminescence lifetimes, and line-like emission spectra that permit ratiometric and time-resolved analyses. In this Feature article, we focus on recent progress in the development of enzyme activity assays based on lanthanide(iii) luminescence, covering a variety of strategies including Ln(iii)-labelled antibodies and proteins, Ln(iii) ion encapsulation within defined peptide sequences, reactivity-based Ln(iii) probes, and discrete Ln(iii) complexes. Emerging approaches for monitoring enzyme activity are discussed, including the use of anion responsive lanthanide(iii) complexes, capable of molecular recognition and luminescence signalling of polyphosphate anions.

An ‘‘off–on–off’’ sensor for sequential detection of Cu2+ and hydrogen sulfide based on a naphthalimide–rhodamine B derivative and its application in dual-channel cell imaging

A novel colorimetric and fluorometric sensor with unique dual-channel emission to sequentially detect Cu²⁺ and hydrogen sulfide (H₂S) was synthesized from naphthalimide–rhodamine B through the PET and FRET mechanism. The sensor showed a selective “off–on” fluorescence response with a 120-fold increase toward Cu²⁺, and its limits of detection were 0.26 μM and 0.17 μM for UV-vis and fluorescence measurements, respectively. In addition, 1–Cu²⁺ was an efficient “on–off” sensor to detect H₂S with detection limits of 0.40 μM (UV-vis measurement) and 0.23 μM (fluorescence measurement), respectively. Furthermore, the sensor can also be used for biological imaging of intracellular staining in living cells. Therefore, the sensor should be highly promising for the detection of low level Cu²⁺ and H₂S with great potential in many practical applications.

A novel colorimetric and fluorometric sensor with unique dual-channel emission to sequentially detect Cu²⁺ and hydrogen sulfide (H₂S) was synthesized from naphthalimide–rhodamine B through the PET and FRET mechanism.

A new fluorescent probe for quick and highly selective detection of hydrogen sulfide and its application in living cells

Hydrogen sulfide (H₂S) is one of the endogenous regulators of many physiological processes.

Weak interactions cause selective cocrystal formation of lanthanide nitrates and tetra-2-pyridinylpyrazine

Two series of cocrystals Ln(NO₃)₅·H₂TPPZ are synthesized enlightening the lanthanide contraction effect and weak hydrogen bonding interactions in determining the selective cocrystallization, stability and energy transfer for luminescence.

Interaction between Diethyldithiocarbamate and Cu(II) on Gold in Non-Cyanide Wastewater

A surface-enhanced Raman scattering (SERS) detection method for environmental copper ions (Cu2+) was developed according to the vibrational spectral change of diethyldithiocarbamate (DDTC) on gold nanoparticles (AuNPs). The ultraviolet-visible (UV-Vis) absorption spectra indicated that DDTC formed a complex with Cu2+, showing a prominent peak at ~450 nm. We found Raman spectral changes in DDTC from ~1490 cm-1 to ~1504 cm-1 on AuNPs at a high concentration of Cu2+ above 1 μM. The other ions of Zn2+, Pb2+, Ni2+, NH₄⁺, Mn2+, Mg2+, K⁺, Hg2+, Fe2+, Fe3+, Cr3+, Co2+, Cd2+, and Ca2+ did not produce such spectral changes, even after they reacted with DDTC. The electroplating industrial wastewater samples were tested under the interference of highly concentrated ions of Fe3+, Ni2+, and Zn2+. The Raman spectroscopy-based quantification of Cu2+ ions was able to be achieved for the wastewater after treatment with alkaline chlorination, whereas the cyanide-containing water did not show any spectral changes, due to the complexation of the cyanide with the Cu2+ ions. A micromolar range detection limit of Cu2+ ions could be achieved by analyzing the Raman spectra of DDTC in the cyanide-removed water.

Sequential detection of Fe3+/2+ and pyrophosphate by a colorimetric chemosensor in a near-perfect aqueous solution

A colorimetric chemosensor was developed for Fe^3+/2+ and pyrophosphate with low detection limit and practical application for Fe³⁺ in water samples.

2D carboxylate-bridged LnIII coordination polymers: displaying slow magnetic relaxation and luminescence properties in the detection of Fe3+, Cr2O72− and nitrobenzene

Carboxylate-bridged Ln-CPs were obtained. 1-Dy shows slow magnetic relaxation. 1-Eu (or 1-Tb) exhibits multifunctional luminescence properties i.e., detecting Fe³⁺, Cr₂O₇²⁻ and nitrobenzene.