A novel two-photon fluorescent probe with long-wavelength emission for monitoring HClO in living cells and tissues

Sulfur-based fluorescent probes for biological analysis: A review

The widespread adoption of small-molecule fluorescence detection methodologies in scientific research and industrial contexts can be ascribed to their inherent merits, including elevated sensitivity, exceptional selectivity, real-time detection capabilities, and non-destructive characteristics. In recent years, there has been a growing focus on small-molecule fluorescent probes engineered with sulfur elements, aiming to detect a diverse array of biologically active species. This review presents a comprehensive survey of sulfur-based fluorescent probes published from 2017 to 2023. The diverse repertoire of recognition sites, including but not limited to N, N-dimethylthiocarbamyl, disulfides, thioether, sulfonyls and sulfoxides, thiourea, thioester, thioacetal and thioketal, sulfhydryl, phenothiazine, thioamide, and others, inherent in these sulfur-based probes markedly amplifies their capacity for detecting a broad spectrum of analytes, such as metal ions, reactive oxygen species, reactive sulfur species, reactive nitrogen species, proteins, and beyond. Owing to the individual disparities in the molecular structures of the probes, analogous recognition units may be employed to discern diverse substrates. Subsequent to this classification, the review provides a concise summary and introduction to the design and biological applications of these probe molecules. Lastly, drawing upon a synthesis of published works, the review engages in a discussion regarding the merits and drawbacks of these fluorescent probes, offering guidance for future endeavors.

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.

A specific dual-locked fluorescence probe to visualize the dynamic changes of lipid droplets and hypochlorous acid in inflammation

Inflammation is a key factor leading to the occurrence and development of many diseases, both lipid droplets (LDs) and hypochlorous acid (HClO/ClO-) are regarded as the important biomarkers of inflammation. Therefore, it is of great significance to develop an efficient single chemical sensor that can simultaneously detect these two biomarkers. To achieve the goal, we developed a dual-locked fluorescence probe (TPA-DNP) by fusing two targets activated reporting system, its implementation was achieved by turning-on the fluorescence of TPA-DNP through LDs and HClO/ClO- simultaneously. In simulated LDs environment, TPA-DNP displayed excellent selectivity to HClO/ClO-, high sensitivity (LOD = 0.527 μM) and strong anti-interference ability. In addition, cell and zebrafish imaging experiments showed that TPA-DNP could be utilized to visualize exogenous/endogenous HClO/ClO- in LDs environment, and could also be used to observe the impact of LDs changes on the HClO/ClO- detection. On the basis, TPA-DNP served as a favorable tool to achieve visualization of inflammatory dynamic changes.

A Simple ICT-Based Fluorescent Probe for HOCl and Bioimaging Applications

Over the past few decades, drug-induced liver damage (DILI) has become a serious public health problem due to drug abuse. Among multifarious reactive oxygen species, mounting evidence attests that ClO^- has been used as a potential biomarker in DILI. In this work, a new "turn-on" fluorescent probe 1 was designed and synthesized by modifying 4'-hydroxybiphenyl-4-carbonitrile (dye 2) with N, N-dimethylthiocarbamate as a response site for detecting ClO^-. Probe 1 displayed a low detection limit (72 nM), fast response time (30 s), wide pH operating range (6-8), great tissue penetration, large Stokes shift (125 nm) and 291-fold fluorescence enhancement at 475 nm in the mapping of ClO^-. Probe 1 could trace amounts of exogenous and endogenous ClO^- with high sensitivity in MCF-7 cells and HeLa cells. Expectantly, the fluoxetine-induced liver injury model is successfully established, and probe 1 has been used for detecting the fluctuation of ClO^- levels in the mouse model of fluoxetine-induced liver injury. All in all, probe 1 with its high specificity, good biological compatibility and liver tissue penetration ability is expected to assist with the early diagnosis of DILI and the clinical screening of various new drugs. We expect that probe 1 could be efficiently used as a powerful molecular tool to predict clinical DILI and explore molecular mechanisms between molecules and disease.

A dual-site fluorescent probe for discriminately detecting low and high concentration of hypochlorite in living cells

Hypochlorite (ClO^-) is an important bioactive molecule of living system which plays essential roles in many physiological and pathological processes. There is no doubt that the biological roles of ClO^- depend highly on the concentration of ClO^-. Unfortunately, the relationship between the concentration of ClO^- and the biological process is unclear. Toward this purpose, in this work we addressed a core challenge for developing a powerful fluorescence tool for monitoring a wide concentration change (0-14 eq.) of ClO^- via two distinct detection manners. The probe displayed fluorescence variation (red to green) upon addition of ClO^- (0-4 eq.) and the color of test medium changed from red to colorless witnessed by the naked eyes. Surprisingly, in the presence of higher concentration of ClO^- (4-14 eq.), the probe displayed another fluorescent signal change from green to blue. After demonstrating the excellent sensing properties of the probe with ClO^- in vitro, it was successfully used to imaging different concentration of ClO^- in living cells. We expected that the probe could act as an exciting chemistry tool for imaging of ClO^- concentration-dependent oxidative stress event in biological system.

Converting commonly-used paper into nano-engineered fluorescent biomass-based platform for rapid ClO- quantitative detection in living cells and water sources

Hypochlorous acid (HClO) and derivative ionic form (ClO^-) are significant components of reactive oxygen species, and thus various diseases are correlatively related to the concentration of ClO^-. Recently, paper-based indicators have been confirmed to be efficient strategy for sensing hazardous and noxious substances. However, most of these materials can only achieve qualitative detection of the substrates. Herein, an extremely simple manufacturing strategy was proposed to convert commonly-used paper into nano-engineered fluorescent biomass-based platform (CMJL-FP) integrated with on-demand self-assembled colorimetric and ratiometric fluorescence sensor (CMJL) for rapid ClO^- quantitative detection in organisms or water sources using smartphones. The CMJL exhibited a highly selective and sensitive ratiometric response to ClO^- at a low detection limit (LOD = 92.6 nM). The associating interactions between the fluorescence nano-particles and micro-nano fibers of CMJL-FP ensure good-stability during ClO^- detection. It has been experimentally demonstrated that CMJL-FP allows one to realize the rapid quantitative detection of ClO^- ions in living cells and large-scale water sources by using color recognition software as part of a simple smartphone. Therefore, integrating the proposed fluorescent paper with smartphones provides an effective, sustainable, cheap and conceptual strategy for quantitative detection of hazardous and noxious substances in organisms and environments.

A time-dependent density functional theory study of a fluorescent probe to detect hydroxyl radicals: Inhibiting the twisted intramolecular charge-transfer process

Due to the relevance to excited-state processes, sensing mechanisms of fluorescent probes were difficult to study directly by experimental methods. This work investigated theoretically the sensing mechanism of a reported bifunctional fluorescent probe to detect intracellular hydroxyl radicals and their environmental viscosity (J. Am. Chem. Soc. 2019, 141, 18301). Calculations were performed at the B3P86/TZVP/SMD level using density functional theory and time-dependent density functional theory. The transition from the ground-state (S₀) to the first singlet excited state (S₁) was calculated to have the largest oscillation strength for the probe. The wavelength that corresponded to the S₀-S₁ vertical excitation energy (427 nm) agreed well with the maximum absorption band at 400 nm in the ultraviolet-visible spectra. Theoretical results showed that the probe had two distinct geometries in the S₀ and S₁ states, respectively. This difference was caused by the different distributions of frontier molecular orbitals that were involved in the S₀-S₁ transition and corresponds to a twisted intramolecular charge transfer. The S₁-state potential energy curve of the probe molecule confirmed that the twisted intramolecular charge transfer could proceed spontaneously with a potential barrier of only 12.20 kJ/mol. This result provided an irradiative approach for the probe molecule to dissipate the S₁-state energy, which explained its fluorescence quenching. In contrast, the hydroxyl oxidation reaction changed frontier molecular orbitals of the probe molecule, which made its S₁ state a local S₁ state with a strong fluorescence emission. Precisely due to the mechanism, the hydroxyl radicals could be detected by changes in the fluorescence signal of the probe molecule.

Synthesis, optical properties, determination and imaging in living cells and bamboo of cinnamaldehyde derivatives

Two Schiff-base fluorescent probes (1 and 2) were directly synthesized from natural cinnamaldehyde, and they were characterized by FT-IR, ¹H and ¹³C NMR, HRMS. Compound 1 had no fluorescence, while compound 2 could emit significant yellow fluorescence in solid and provide green light in solution. Probe 1 could selectively sense ClO^- with a fluorescence enhancement, providing a good linear relationship between the fluoresence intensity and ClO^- concentrations (0-5.5 × 10^-5 mol/L), y = 175.64x-19.399, R² = 0.9937, and the limit of detection (LOD) was 39.4 nM. Probe 2 was sensitive for Cu²⁺ by quenching with two linear relationships at the Cu²⁺ concentrations from 0 to 2.1 × 10^-5 mol/L, LOD = 73.9 nM. Furthermore, live celluar imaging of human astrocytoma U-251 MG cells and human liver cancer cells (Hu-7) had achieved using the 1 + ClO^- and 2, offering clear intracellular fluorescence. Finally, the 1 + ClO^- and 2 could also be used to dye bamboo tissues for a good use. Thus, the cinnamaldehyde derivatives could be further used in the field of celluar and bamboo imaging.

Selective Imaging of HClO in the Liver Tissue In Vivo Using a Near-infrared Hepatocyte-specific Fluorescent Probe

Liver injury is typified by an inflammatory response. Hypochlorous acid (HClO), an important endogenous reactive oxygen species, is regarded as a biomarker associated with liver injury. Near-infrared (NIR) fluorescent probes with the advantage of deep tissue penetrating and low auto-fluorescence interference are more suitable for bioimaging in vivo. Thus, in this work, we designed and synthesized a novel NIR hepatocyte-specific fluorescent probe named NHF. The probe NHF showed fast response (<3 s), large spectral variation, and good selectivity to trace HClO in buffer solution. By employing N-acetylgalactosamine (GalNAc) as the targeting ligand, probe NHF can be actively delivered to the liver tissue of zebrafish and mice. It is important that probe NHF is the first NIR hepatocyte-specific fluorescent probe, which successfully visualized the up-regulation of endogenous HClO in the oxygen-glucose deprivation/reperfusion (OGD/R) model HepG2 cells and dynamically monitored APAP-induced endogenous HClO in the liver tissue of zebrafish and mice.

A near-infrared fluorescent probe for highly specific and ultrasensitive detection of hypochlorite ions in living cells

Hypochlorite (ClO^-) is an important reactive oxygen species (ROS) in organisms. In this work, a fluorescent probe DBTM based on triphenylamine was synthesized successfully and characterized by spectral methods. The designed probe can rapidly respond to ClO^- in just 1 min, followed by the apparent color change from red to yellow. The colorimetric and ratiometric absorbance change of DBTM was attributed to the strong oxidation of ClO^-, which broke the connected double bonds and destroyed the conjugate system. The probe DBTM showed an excellent selectivity towards ClO^- in comparison with other ROS probes. Besides, the DBTM probe exhibited a highly sensitive response to ClO^-, with the detection limits calculated to be 3.3 nM. The probe can be applied in the form of cotton swabs and test strips that could detect ClO^- easily, suggesting its potential use as imaging agents for realistic ClO^- detection. In particular, DBTM exhibited very low background fluorescence in living cells and was able to detect the minor variation of endogenous hypochlorite in L929 cells. Based on these advantages, the probe DBTM could be a good candidate for detecting ClO^- in biological systems.

Rapid and selective visualization of mitochondrial hypochlorite by a red region water-soluble fluorescence probe

Hypochlorite (^-OCl) has long been recognized as an effective microbicidal agent in immune system. Herein, we report the design, preparation and spectral characteristics of a ^-OCl fluorescent probe (FI-Mito). The probe exhibited remarkable fluorescence turn-on signal in the red region upon ^-OCl titration with the detection limit as low as 0.9 nM. FI-Mito displayed specific response for ^-OCl in completely aqueous solution. Meanwhile, the introduction of quaternized pyridine realized mitochondria-targeting ability. FI-Mito was further applied to monitor the generation of endogenous ^-OCl in the mitochondria of macrophage cells and mice. Therefore, it was established that FI-Mito may serve as a useful molecular tool for ^-OCl detection in vivo.

Mitochondria-Targeted Chemosensor to Discriminately and Continuously Visualize HClO and H2S with Multiresponse Fluorescence Signals for In Vitro and In Vivo Bioimaging

Bioactive molecules play a vital role in the process of regulating the redox balance in the intracellular environment, especially in maintaining the function of organelles. To explore the association and function of bioactive molecules in organelles, it is essential to develop a chemosensor tool that uses multiresponse fluorescence signals to distinguish between and track two related bioactive molecules in organelles. However, the development of sensors with multiresponse functions is still a challenging task. Herein, we present a unique and practical single chemosensor (Mito-CTC) that can monitor HClO (as an oxidative substance) and H₂S (as a reductive substance) in mitochondria (organelle targeting) with multiresponse fluorescence signals. The response of the sensor to HClO and H₂S changes from red to green and blue channel emission simultaneously, respectively, thereby providing a specific signal response to reductive/oxidative substances in the mitochondria. Using a single chemosensor, we have realized multichannel bioimaging of the exogenous and endogenous HClO and H₂S in cellular mitochondria. Additionally, the excellent properties of the sensor Mito-CTC can be used to reveal the relationship between HClO and H₂S in mitochondria. Meanwhile, Mito-CTC has been endowed with the ability to image in bacteria and zebrafish attributed to the good permeability and low cytotoxicity. Expectantly, drug-induced liver injury (DILI) caused by fluoxetine (an antidepressant drug) and the degree of drug-induced toxicity to the liver were evaluated using Mito-CTC through discriminating and imaging HClO, indicating that Mito-CTC has the potential function of evaluating the toxicity of the drug to the liver.

Employing a fluorescent and colorimetric picolyl-functionalized rhodamine for the detection of glyphosate pesticide

The ongoing poisoning of agricultural products has pushed the security problem to become an important issue. Among them, exceeding the standard rate of pesticide residues is the main factor influencing the quality and security of agricultural products. Monitoring pesticide residues and developing simple, yet ultrasensitive detection systems for pesticide residues are urgently needed. In this study, we successfully developed a novel rhodamine derivative as fluorescent and colorimetric chemosensor R-G for the rapid, selective and ultrasensitive detection of glyphosate pesticide residue in aqueous solution. Through a Cu²⁺-indicator displacement strategy, glyphosate can displace an indicator (R-G) from a Cu²⁺-indicator complex due to its strong affinity to bind with Cu²⁺ to give a turn-on fluorescence and distinct color change. Moreover, a test strip was also fabricated to achieve a facile detection of glyphosate pesticide. To demonstrate the possibility of practical applications, glyphosate was detected on the surface of cabbage and in a spiked soil sample. The detection limit of 4.1 nM and the response time of 2 min indicate that the method is enough sensitive and rapid to detect the glyphosate residue at or below levels that pose a health risk.

When the strategies for cellular selectivity fail. Challenges and surprises in the design and application of fluorescent benzothiadiazole derivatives for mitochondrial staining

Design, synthesis, molecular architecture and the unexpected behavior of fluorescent benzothiadiazole for selective mitochondrial and plasma membrane staining are investigated.