Recent Advances in Photoswitchable Fluorescent and Colorimetric Probes

In recent years, significant advancements have been made in the research of photoswitchable probes. These probes undergo reversible structural and electronic changes upon light exposure, thus exhibiting vast potential in molecular detection, biological imaging, material science, and information storage. Through precisely engineered molecular structures, the photoswitchable probes can toggle between "on" and "off" states at specific wavelengths, enabling highly sensitive and selective detection of targeted analytes. This review systematically presents photoswitchable fluorescent and colorimetric probes built on various molecular photoswitches, primarily focusing on the types involving photoswitching in their detection and/or signal response processes. It begins with an analysis of various molecular photoswitches, including their photophysical properties, photoisomerization and photochromic mechanisms, and fundamental design concepts for constructing photoswitchable probes. The article then elaborates on the applications of these probes in detecting diverse targets, including cations, anions, small molecules, and biomacromolecules. Finally, it offers perspectives on the current state and future development of photoswitchable probes. This review aims to provide a clear introduction for researchers in the field and guidance for the design and application of new, efficient fluorescent and colorimetric probes.

Selective chiroptical sensing of D/L-cysteine

A chromophoric bifunctional probe design that allows selective chiroptical sensing of cysteine in aqueous solution is introduced. The common need for chiral HPLC separation is eliminated which expedites and simplifies the sample analysis while reducing solvent waste. Screening of the reaction between six phenacyl bromides and the enantiomers of cysteine showed that cyclization to an unsaturated thiomorpholine scaffold coincides with characteristic UV and CD effects, in particular when the reagent carries a proximate auxochromic nitro group. The UV changes and CD inductions were successfully used for determination of the absolute configuration, enantiomeric composition and total concentration of 18 test samples. This assay is highly selective for free cysteine while other amino acids, cysteine derived small peptides and biothiols do not interfere with the chiroptical signal generation.

Rational design of a dual-reactive probe for imaging the biogenesis of both H2S and GSH from L-Cys rather than D-Cys in live cells

Biothiols and their interconversion are involved in cellular redox homestasis as well as many physiological processes. Here, a dual-reactive dual-quenching fluorescent probe was rationally developed based on thiolysis reactions of...

Thiolysis of CBD arylethers for development of highly GSH-selective fluorescent probes

Thiolysis of 7-cyanobenzoxadiazole (CBD) arylether was investigated for development of GSH-selective fluorescent probes for the first time. The results demonstrate that CBD-based probes have tunable reactivities and appropriate dissociation constants for GSH, and are highly GSH-selective and suitable for bioimaging.

NBD-based synthetic probes for sensing small molecules and proteins: design, sensing mechanisms and biological applications

Compounds with a nitrobenzoxadiazole (NBD) skeleton exhibit prominent useful properties including environmental sensitivity, high reactivity toward amines and biothiols (including H2S) accompanied by distinct colorimetric and fluorescent changes, fluorescence-quenching ability, and small size, all of which facilitate biomolecular sensing and self-assembly. Amines are important biological nucleophiles, and the unique activity of NBD ethers with amines has allowed for site-specific protein labelling and for the detection of enzyme activities. Both H2S and biothiols are involved in a wide range of physiological processes in mammals, and misregulation of these small molecules is associated with numerous diseases including cancers. In this review, we focus on NBD-based synthetic probes as advanced chemical tools for biomolecular sensing. Specifically, we discuss the sensing mechanisms and selectivity of the probes, the design strategies for multi-reactable multi-quenching probes, and the associated biological applications of these important constructs. We also highlight self-assembled NBD-based probes and outline future directions for NBD-based chemosensors. We hope that this comprehensive review will facilitate the development of future probes for investigating and understanding different biological processes and aid the development of potential theranostic agents.

A turn-on homodimer fluorescent probe based on homo-FRET for the sensing of biothiols in lysosome: a trial of a new turn-on strategy

Fluorescence resonance energy transfer (FRET) is often applied to construct fluorescent probes for acquiring high selectivity and sensitivity. According to the FRET theory, a homodimer composed of two identical fluorophores with a small Stokes shift has only weak fluorescence due to homo-FRET between fluorophores, and the fluorescence could be recovered after the destruction of the homodimer. In this study, we designed and synthesized a homodimer fluorescent probe, namely 1,3,5,7-tetramethyl-8-(4'-phenylthiophenol)-boron difluoride-dipyrrole methane dimer (D-TMSPB), based on this turn-on strategy. In D-TMSPB, the disulfide moiety was selected as the response moiety of biothiols, and BODIPY fluorophore was chosen as both donor and acceptor in FRET due to the ultra-small Stokes shifts and obvious overlap of its excitation/emission peak. D-TMSPB exhibited only weak fluorescence. After selective reaction with biothiols, FRET was destroyed and the derivative exhibited strong fluorescence at 514 nm with the limit of detection of about 0.15 μM for GSH. Notably, the derivative of biothiols shows remarkable fluorescence only in acidic conditions, which accords with the internal environment of lysosome. Thus, D-TMSPB was applied to image the biothiols of lysosome in living cells. The turn-on fluorescence of D-TMSPB indicated that homo-FRET is a practical strategy to design turn-on fluorescent probes, particularly for the sensing mechanism based on leaving groups.

A porous organic polymer nanosphere-based fluorescent biosensing platform for simultaneous detection of multiplexed DNA via electrostatic attraction and π–π stacking interactions

One key challenge in oligonucleotide sequence sensing is to achieve multiplexed DNA detection in one sensor. Herein, a simple and efficient fluorescent biosensing platform is constructed to simultaneously detect multiplexed DNA depending on porous organic polymer (POP) nanospheres. The developed sensor is based on the concept that the POP nanospheres can efficiently quench the fluorescence emission of dye-labeled single-stranded DNA (ssDNA). Fluorescence quenching is achieved by the non-covalent assembly of multiple probes on the surface of POP nanospheres through electrostatic attraction and π–π stacking interactions, in which the electrostatic attraction plays a more critical role than π–π stacking. The formed dsDNA could be released off the surface of POP via hybridizing with the target DNA. Consequently, the target DNA can be quickly detected by fluorescence recovery. The biosensor could sensitively and specifically identify three target DNAs in the range of 0.1 to 36 nM, and the lowest detection limits are 50 pM, 100 pM, and 50 pM, respectively. It is noteworthy that the proposed platform is successfully applied to detect DNA in human serum. We perceive that the proposed sensing system represents a simple and sensitive strategy towards simultaneous and multiplexed assays for DNA monitoring and early clinical diagnosis.

This communication reports a simple and efficient fluorescent biosensing platform to simultaneously detect multiplexed DNA depending on porous organic polymer (POP) nanospheres by electrostatic attraction and π–π stacking interaction.

Investigation of thiolysis of 4-substituted SBD derivatives and rational design of a GSH-selective fluorescent probe

In order to evaluate 7-sulfonamide benzoxadiazole (SBD) derivatives for the development of fluorescent probes, herein we investigated the thiolysis reactivity and selectivity of a series of SBD compounds with different atoms (N/O/S/Se) at the 4-position. Both SBD-amine and SBD-ether are stable toward biothiols in buffer (pH 7.4), while SBD-selenoether can react efficiently with biothiols GSH/Hcy, Cys, and H2S to produce SBD-SG/S-Hcy, SBD-NH-Cys, and SBD-SH, respectively, with three different sets of spectral signals. Therefore, the SBD-selenoether compounds should be useful platforms for the differentiation of these biothiols. Though SBD-alkylthioether shows much lower reactivity than SBD-selenoether, SBD-arylthioether is a tunable motif and structural modifications at the aryl moiety enable the rate of thiol-mediated thiolysis to be modified. To this end, an ER-targeted GSH-selective fluorescent probe 7 was rationally designed via thiolysis of SBD-arylthioether. Compared with control probe SBD-Cl, probe 7 exhibits improved GSH selectivity and better biocompatibility. In total, this study highlights that the modification at the 4-position of SBD is an efficient strategy for the development of new fluorescent probes with tunable reactivity and selectivity.