Recent advances in isatin-based chemosensors: A comprehensive review.

A comprehensive review presents an illuminating exploration of the vast potential of isatin, an easily accessible organic compound. This review is a valuable resource, offering a concise yet comprehensive account of the recent breakthroughs in isatin applications in medicinal chemistry, fluorescence sensing, and organic synthesis. Moreover, it dives into the exciting advancements in isatin-based chemosensors, demonstrating their remarkable ability to detect and recognize diverse cations and anions with exceptional precision. Researchers and scientists in the fields of sensing and organic chemistry will find this review indispensable for sparking innovation and developing cutting-edge technologies with significant real-world impact.

An activatable azophenyl fluorescent probe for hypoxic fluorescence imaging in living cells.

Cellular hypoxia is a common pathological process in various diseases. Detecting cellular hypoxia is of great scientific significance for early diagnosis of tumors. The hypoxia fluorescence probe analysis method can efficiently and conveniently evaluate the hypoxia status in tumor cells. These probes are covalently linked by hypoxic recognition groups and organic fluorescent molecules. Currently, the fluorescent molecules used in these probes often exhibit the aggregation-caused quenching effect, which is not conducive to fluorescence imaging in water. Herein, an activatable hypoxia fluorescence probe was constructed by covalently linking aggregation-induced emission luminogens to the hypoxic recognition group azobenzene. It does not emit fluorescence in solution and in solid state under light excitation due to the presence of photosensitive azo bonds. It can be cleaved by intracellular azoreductase into fluorescent amino derivatives with aggregation-induced emission characteristic. As the concentration of oxygen in cells decreases, its fluorescence intensity increases, making it suitable for fluorescence imaging to detect hypoxic environment in live cancer cells. This work broadens the molecular design approach for activatable hypoxia fluorescent probes.

A fluorescence probe for imaging lipid droplet and visualization of diabetes-related polarity variations.

Diabetes mellitus is a disorder that affects lipid metabolism. Abnormalities in the lipid droplets (LDs) can lead to disturbances in lipid metabolism, which is a significant feature of diabetic patients. Nevertheless, the correlation between diabetes and the polarity of LDs has received little attention in the scientific literature. In order to detect LDs polarity changes in diabetes illness models, we created a new fluorescence probe LD-DCM. This probe has a stable structure, high selectivity, and minimal cytotoxicity. The probe formed a typical D-π-A molecular configuration with triphenylamine (TPA) and dicyanomethylene-4H-pyran (DCM) as electron donor and acceptor parts. The LD-DCM molecule has an immense solvatochromic effect (λem = 544-624 nm), fluorescence enhancement of around 150 times, and a high sensitivity to polarity changes within the linear range of Δf = 0.28 to 0.32, all due to its distinctive intramolecular charge transfer effect (ICT). In addition, LD-DCM was able to monitor the accumulation of LDs and the reduction of LDs polarity in living cells when stimulated by oleic acid, lipopolysaccharide, and high glucose. More importantly, LD-DCM has also been used effectively to detect polarity differences in organs from diabetic, drug-treated, and normal mice. The results showed that the liver polarity of diabetic mice was lower than that of normal mice, while the liver polarity of drug-treated mice was higher than that of diabetic mice. We believe that LD-DCM has the potential to serve as an efficient instrument for the diagnosis of disorders that are associated with the polarity of LDs.

Cell-resistant wavelength-shifting molecular beacons made of L-DNA and a clickable L-configured uridine.

Wavelength-shifting molecular beacons were prepared from L-DNA. The clickable anchor for the two dyes, Cy3 and Cy5, was 2'-O-propargyl-L-uridine and was synthesized from L-ribose. Four clickable molecular beacons were prepared and double-modified with the azide dyes by a combination of click chemistry on a solid support for Cy3 during DNA synthesis and postsynthetic click chemistry for Cy5 in solution. Cy3 and Cy5 successfully formed a FRET pair in the beacons, and the closed form (red fluorescence) and the open form (green fluorescence) can be distinguished by the two-color fluorescence readout. Two molecular beacons were identified to show the greatest fluorescence contrast in temperature-dependent fluorescence measurements. The stability of the L-configured molecular beacons was demonstrated after several heating and cooling cycles as well as in the cell lysate. In comparison, D-configured molecular beacons showed a rapid decrease of fluorescence contrast in the cell lysate, which is caused by the opening of the beacons, probably due to degradation. This was confirmed in cell experiments using confocal microscopy. The L-configured molecular beacons are potential intracellular thermometers for future applications.

Structure-activity relationships of aniline-based squaraines for distinguishable staining and bright two-photon fluorescence bioimaging in plant cells.

An organelle-selective vision provides insights into the physiological response of plants and crops to environmental stresses in sustainable agriculture ecosystems. Biological applications often require two-photon excited fluorophores with low phototoxicity, high brightness, deep penetration, and tuneable cell entry. We obtained three aniline-based squaraines (SQs) tuned from hydrophobic to hydrophilic characteristics by modifying terminal pendant groups and substituents, and investigated their steady-state absorption and far-red-emitting fluorescence properties. The SQs exhibited two-photon absorption (2PA) ranging from 750 to 870 nm within the first biological spectral window; their structure-property relationships, corresponding to the 2PA cross sections (δ2PA), and structure differences were demonstrated. The maximum δ2PA value was ∼1220 GM at 800 nm for hydrophilic SQ3. Distinct biological staining efficiency and selective SQ bioimaging were evaluated utilizing the onion epidermal cell model. Contrary to the hydrophobic SQ1 results in the onion epidermal cell wall, amphiphilic SQ2 tagged the vacuole and nucleus and SQ3 tagged the vacuole. Distinguishable staining profiles in the roots and leaves were achieved. We believe that this study is the first to demonstrate distinct visualisation efficiency induced by the structure differences of two-photon excited SQs. Our results can help establish the versatile roles of novel near-infrared-emitting SQs in biological applications.

An activated near-infrared mitochondrion-targetable fluorescent probe for rapid detection of NADH.

A novel NIR fluorescent probe based on quinoline-conjugated benzo[cd]indol dual-salt for NADH was developed. This probe swiftly detects and responds sensitively to both endogenous and exogenous NADH alterations, enabling imaging of NADH fluctuations in type II diabetic and AD model cells.

A new biphenol-dipicolylamine based ligand and its dinuclear Zn2+ complex as fluorescent sensors for ibuprofen and ketoprofen in aqueous solution.

In this work, the study of the new ligand 3,3'-bis[N,N-bis(pyridine-2-ylmethyl)aminomethyl]-2,2'-dihydroxybiphenyl (L) is reported, where a central 2,2'-biphenol (BPH) fluorophore was functionalized at 3,3'-positions with two dipicolylamine (DPA) side arms as receptor units. Following the synthesis and full chemical-physical characterization, the acid-base and Zn2+-coordination abilities of L were investigated through a combination of potentiometric, UV-Vis, fluorescence, NMR, XRD and DFT measurements. The optical properties of the ligand turned out to be strongly dependent on the pH, being straightforwardly associated with the protonation state of the BPH moiety, whereas its peculiar design allowed to form stable mono and dinuclear Zn2+ complexes. In the latter species, the presence of two Zn2+ ions coordinatively unsaturated and placed at close distance to each other, prompted us to test their usefulness as metallo-receptors for two environmental pollutants of great relevance, ibuprofen and ketoprofen. Potentiometric and fluorescence investigations evidenced that these important non-steroidal anti-inflammatory drugs (NSAIDs) are effectively coordinated by the metallo-receptors and, of relevance, both the stability and the fluorescence properties of the resulting ternary adducts are markedly affected by the different chemical architectures of the two substrates. This study aims at highlighting the promising perspectives arising from the use of polyamino phenolic ligands as chemosensors for H+/Zn2+ and other additional anionic targets in their metal-complexed forms.

An Allysine-Conjugatable Probe for Fluorogenically Imaging Fibrosis.

Allysine, a pivotal biomarker in fibrogenesis, has prompted the development of various radioactive imaging probes. However, fluorogenic probes targeting allysine remain largely unexplored. Herein, by leveraging the equilibrium between the nonfluorescent spirocyclic and the fluorescent zwitterionic forms of rhodamine-cyanine hybrid fluorophores, we systematically fine-tuned the environmental sensitivity of this equilibrium toward the development of fluorogenic probes for fibrosis. The trick lies in modulating the nucleophilicity of the ortho-carboxyl group, which is terminated with a hydrazide group for allysine conjugation. Probe B2 was developed with this strategy, which featured an N-sulfonyl amide group and exhibited superior fibrosis-to-control imaging contrast. Initially presenting as nonfluorescent spirocyclic aggregates in aqueous solutions, B2 displayed a notable fluorogenic response upon conjugation with protein allysine through its hydrazide group, inducing deaggregation and switching to the fluorescent zwitterionic form. Probe B2 outperformed the traditional Masson stain in imaging contrast, achieving an about 260-2600-fold ratio for fibrosis-to-control detection depending on fibrosis severity. Furthermore, it demonstrated efficacy in evaluating antifibrosis drugs. Our results emphasize the potential of this fluorogenic probe as an alternative to conventional fibrosis detection methods. It emerges as a valuable tool for antifibrosis drug evaluation.

Composite Eu@Cd-CP as a fluorescent probe for the detection of some food additives.

A transition metal coordination polymer (CP), [Cd(Hdpcp)]n (Cd-CP) was prepared based on 3-(2,4-dicarboxyphenyl)-6-carboxypyridine ligand (H3dpcp), and then its composite Eu@Cd-CP was synthesized by the post-modification through loading Eu3+ ions on Cd-CP. Eu@Cd-CP has outstanding fluorescence stability in aqueous solution with a wide range of pH. Furthermore, Eu@Cd-CP can distinguish sodium salicylate (SS) and sodium dehydroacetate (SA) in some food additives by quenching the characteristic fluorescence of Eu3+ ion. Eu@Cd-CP is the first known CP-based fluorescent probe for selective detection of SS and SA. In addition, the fluorescence mechanisms of discerning above analytes by Eu@Cd-CP have been thoroughly evaluated. It has found that synergistic effect of the dynamic process, photoinduced electron transfer (PET) process, energy absorption competition, and formation of Eu-O bonding interactions in sensing SA lead to the fluorescence quenching of Eu@Cd-CP. The fluorescence response mechanism of Eu@Cd-CP with SA is ascribed to the combination of the dynamic process, PET process, and energy absorption competition. A series of portable devices based on Eu@Cd-CP including fluorescence test strips, lamp beads, and composite films were developed to discern SS and SA via visual changes in luminescence color. This composite material can be potentially used as a multifunctional fluorescent probe for practical applications.

An antagonist-based two-photon fluorogenic probe for imaging metabotropic glutamate receptor 5 in neuronal cells.

The expression of metabotropic glutamate receptor 5 (mGluR5) is subject to developmental regulation and undergoes significant changes in neuropsychiatric disorders and diseases. Visualizing mGluR5 by fluorescence imaging is a highly desired innovative technology for biomedical applications. Nevertheless, there are substantial problems with the chemical probes that are presently accessible. In this study, we have successfully developed a two-photon fluorogenic probe, mGlu-5-TP, based on the structure of mGluR5 antagonist 6-methyl-2-(phenylethynyl)pyridine (MPEP). Due to this antagonist-based probe selectively recognizes mGluR5, high expression of mGluR5 on living SH-SY5Y human neuroblastoma cells has been detected during intracellular inflammation triggered by lipopolysaccharides (LPS). Of particular significance, the probe can be employed along with two-photon fluorescence microscopy to enable real-time visualization of the mGluR5 in Aß fiber-treated neuronal cells, thereby establishing a connection to the progression of Alzheimer's disease (AD). These results revealed that the probe can be a valuable imaging tool for studying mGluR5-related diseases in the nervous system.

Micelle-based fluorogenic sensing of trypsin: a sensitive method in pancreatic disease diagnosis.

Protamine-mediated micellar aggregates, featuring an AIE-based fluorescent sensor, facilitate efficient detection of trypsin activity. This method enables the detection of trypsin at exceptionally low concentrations (0.01-0.1 µg mL-1) in urine, demonstrating its potential for early clinical diagnosis of trypsin-related pancreatic diseases.

Development of a ratiometric fluorescent probe for the detection of peroxynitrite.

Peroxynitrite is one of the important reactive oxygen species in the human body and is closely related to the physiological and pathological processes of many diseases. Therefore, the development of probes to detect peroxynitrite is important for diagnostic and pathologic studies of many diseases. In this work, a ratiometric probe was designed using benzopyran as the recognition site, and the sensitivity and selectivity of the probe were tuned by modification of substituents on benzopyran. Upon reaction with peroxynitrite, the color of the solution changes to the naked eye (from blue to yellow), and the fluorescence changes from red to blue. The probe SJ has the advantages of large Stokes shift (237 nm), fast response (≤10 s), wide linear range, good selectivity, low detection line (21.3 nm), and low cytotoxicity. Probe SJ has been successfully used for bioimaging of endogenous and exogenous peroxynitrite.

A near-infrared fluorescent probe for the detection of Cu2+ in Chinese herbal medicine and imaging in living cells.

Copper is one of the common among the heavy metal pollution in Chinese herbal medicine (CHM). So, it is essential to develop rapid and accurate testing method to quantify the Cu2+ content in CHM. Herein, we prepared a coordination-based near-infrared fluorescent probe (NRh6G-FA) by introducing a hemicyanine dye in rhodamine 6G scaffold. NRh6G-FA had a high sensitivity, anti-interference performance, fast response (within 60 s), visualization (from light yellow to green) for Cu2+ and excellent sensing performance for the detection of Cu2+ at low concentrations (LOD = 0.225 µM). The most likely mechanism was verified on the basis of Job's plot, ESI-HRMS and DFT calculations. NRh6G-FA could be successfully applied for the detection and "naked eye" recognition of Cu2+ in CHM samples. Moreover, NRh6G-FA was used to visualize Cu2+ in living MCF-7 cells by confocal fluorescence imaging.

A novel NIR fluorescent probe for enhanced ß-galactosidase detection and tumor imaging in ovarian cancer models.

The advancement of biological imaging techniques critically depends on the development of novel near-infrared (NIR) fluorescent probes. In this study, we introduce a designed NIR fluorescent probe, NRO-ßgal, which exhibits a unique off-on response mechanism to ß-galactosidase (ß-gal). Emitting a fluorescence peak at a wavelength of 670 nm, NRO-ßgal showcases a significant Stokes shift of 85 nm, which is indicative of its efficient energy transfer and minimized background interference. The probe achieves a remarkably low in vitro detection limit of 0.2 U/L and demonstrates a rapid response within 10 min, thereby underscoring its exceptional sensitivity, selectivity, and operational swiftness. Such superior analytical performance broadens the horizon for its application in intricate biological imaging studies. To validate the practical utility of NRO-ßgal in bio-imaging, we employed ovarian cancer cell and mouse models, where the probe's efficacy in accurately delineating tumor cells was examined. The results affirm NRO-ßgal's capability to provide sharp, high-contrast images of tumor regions, thereby significantly enhancing the precision of surgical tumor resection. Furthermore, the probe's potential for real-time monitoring of enzymatic activity in living tissues underscores its utility as a powerful tool for diagnostics in oncology and beyond.

A novel "Turn-Off-On" fluorescent probe for specific sequential detection of Cu2+ and glyphosate and its application in biological imaging.

As common pollutants, Cu2+ and glyphosate pose a serious threat to human health and the ecosystem. Herein, a fluorescent probe (E)-7-(diethylamino)-N'(4-(diethylamino)-2-hydroxybenzyl)-2-oxo-2H chromophore-3-carbazide (DDHC) was designed and synthesised for the sequential recognition of Cu2+ and glyphosate. DDHC has the advantages of a short synthesis path, easy-to-obtain raw materials, good anti-interference ability, and strong stability. The interaction of the DDHC-Cu2+ complexes with glyphosate allows the amino and carboxyl groups in glyphosate molecules to coordinate with Cu2+ strongly, competing for the Cu2+ in the DDHC-Cu2+ complexes and releasing the DDHC, leading to the recovery of fluorescence. The recognition was further validated through Job's plot, HRMS, and DFT calculations. In addition, the successful recovery of Cu2+ and glyphosate in different environmental water samples fully demonstrates the practical application potential of DDHC. Especially, DDHC has low cytotoxicity and can enter zebrafish and HeLa cells, rapidly reacting with Cu2+ and glyphosate in the body, generating visible fluorescence quenching and recovery phenomena, achieving real-time visual monitoring of exogenous Cu2+ and glyphosate in zebrafish and HeLa cells. The targeting and dual selectivity of DDHC greatly enhance its potential application value in the field of detection, providing important theoretical support for studying the fate of multiple pollutants in the environment.

Rational design of peptide-based fluorescent probe for sequential recognitions of Cu(II) ions and glyphosate: Smartphone, test strip, real sample and living cells applications.

A new peptide-based fluorescent probe named DMDH with easy-to-synthesize, excellent stability, good water solubility and large Stokes shift (225 nm) was synthesized for highly selective sequential detections of copper ions (Cu2+) and glyphosate (Glyp). DMDH demonstrated great detection performance towards Cu2+via strong fluorescence quenching, and forming non-fluorescence DMDH-Cu2+ ensemble. As a new promising cascade probe, the fluorescence of DMDH-Cu2+ ensemble was significantly recovered based on displacement approach after glyphosate was added. Interestingly, the limit of detections (LODs) for Cu2+ and glyphosate were 40.6 nM and 10.6 nM, respectively, which were far lower than those recommended by the WHO guidelines for drinking water. More importantly, DMDH was utilized to evaluate Cu2+ and glyphosate content in three real water samples, demonstrating that its effectiveness in water quality monitoring. Additionally, it is worth noting that DMDH was also applied to analyze Cu2+ and glyphosate in living cells in view of significant cells permeability and low cytotoxicity. Moreover, DMDH soaked in filter paper was used to create qualitative test strips and visually identify Cu2+ and glyphosate through significant color changes. Furthermore, smartphone RGB color recognition provided a new method for semi-quantitative testing of Cu2+ and glyphosate in the absence of expensive instruments.

A mitochondrial targeted dual ratiometric near-infrared fluorescent probe based on ICT effect for the detection of SO2 derivative and its bioimaging.

SO2 derivatives play an important role in many metabolic processes, excessive ingestion of them can lead to serious complications of various diseases. In this work, a novel dual ratiometric NIR fluorescent probe XT-CHO based on ICT effect was synthesized for detecting SO2 derivative. In the design of the probe, the α, ß-unsaturated bond formed between benzopyran and coumarin was used as the reaction site for SO2, meanwhile, the extended π-conjugate system promoted maximum emission wavelength of the probe up to 708 nm. Notably, the probe exhibited high selectivity and sensitivity for detecting SO2, the limit of detection reached 2.13 nM and 58.5 nM in fluorescence spectra and UV-Vis absorption spectra, respectively. The reaction mechanism of SO2 and XT-CHO had been verified by 1H NMR, ESI-MS spectra and DFT calculation. Moreover, the probe was successfully applied in detecting endogenous and exogenous SO2 in living cells and proved possessed the mitochondrial targeted ability.

Triphenylphosphine-bonded coumaranone dyes realize dual color imaging of mitochondria and nucleoli.

Probing intracellular organelles with fluorescent dyes offers opportunities to understand the structures and functions of these cellular compartments, which is attracting increasing interests. Normally, the design principle varies for different organelle targets as they possess distinct structural and functional profiles against each other. Therefore, developing a probe with dual intracellular targets is of great challenge. In this work, a new sort of donor-π-bridge-acceptor (D-π-A) type coumaranone dyes (CMO-1/2/3/4) have been prepared. Four fluorescent probes (TPP@CMO-1/2/3/4) were then synthesized by linking these coumaranone dyes with an amphiphilic cation triphenylphosphonium (TPP). Interestingly, both TPP@CMO-1 and TPP@CMO-2 exhibited dual color emission upon targeting to two different organelles, respectively. The green emission is well localized in mitochondria, while, the red emission realizes nucleoli imaging. RNA is the target of TPP@CMOs, which was confirmed by spectroscopic analysis and computational calculation. More importantly, the number and morphology changes of nucleoli under drug stress have been successfully evaluated using TPP@CMO-1.

Fast and easy detection of hypochlorite by a smartphone-based fluorescent turn-on probe: Applications to water samples, zebrafish and plant imaging.

We have developed a fluorescent probe DBT-Cl ((E)-2-(2-(4-(diphenylamino)benzylidene) hydrazinyl)-N,N,N-trimethyl-2-oxoethan-1-aminium chloride) for ClO- with an aggregation-induced emission (AIE) strategy depending on solvent polarity. DBT-Cl possessed a prominent solvatochromic emission property with intramolecular charge transfer (ICT) from the TPA (triphenylamine) to the amide group, which was studied by spectroscopic analysis and DFT calculations. These unique AIE properties of DBT-Cl led to the recognition of ClO- with high fluorescent selectivity. DBT-Cl quickly detected ClO- in less than 1 sec with a fluorescent color change from green to cyan. DBT-Cl had a low detection limit of 9.67 µM to ClO-. Detection mechanism of DBT-Cl toward ClO- was illustrated to be oxidative cleavage of DBT-Cl by 1H NMR titrations, ESI-mass, and DFT calculations. We established the viability for dependable detection of ClO- in actual water samples, as well as zebrafish and plant imaging. In particular, DBT-Cl was capable of easily monitoring ClO- through a smartphone application. Therefore, DBT-Cl assured a promising approach for a fast-responsive and multi-applicable ClO- probe in environmental and living organism systems.

A comparative study of two thienopyrimidine Schiff base probes for sequential monitoring of Ga3+ and Pd2.

Two Schiff base probes (S1 and S2) were prepared and synthesized by incorporating thienopyrimidine into salicylaldehyde or 3-ethoxysalicylaldehyde individually, with the aim of detecting Ga3+ and Pd2+ sequentially. Upon chelation with Ga3+, S1 and S2 exhibited fluorescence enhancement in DMSO/H2O buffer. Both S1-Ga3+ and S2-Ga3+ were quenched by Pd2+. The limit of detection for S1 in response to Ga3+ and Pd2+ was 2.86 × 10-7 and 4.4 × 10-9 M, respectively. For S2, the limit of detection for Ga3+ and Pd2+ was 4.15 × 10-8 and 3.0 × 10-9 M, respectively. Furthermore, the complexation ratios of both S1 and S2 with Ga3+ and Pd2+ were determined to be 1:2 through Job's plots, ESI-MS analysis, and theoretical calculations. Two molecular logic gates were constructed, leveraging the response behaviors of S1 and S2. Moreover, the potential utility of S1 and S2 for monitoring Ga3+ and Pd2+ in domestic water was verified.