An ultra-sensitive and ratiometric fluorescent probe based on the DTBET process for Hg2+ detection and imaging applications

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.

Recent advances in AIEgen-based chemosensors for small molecule detection, with a focus on ion sensing

Since the aggregation-based emission (AIE) phenomenon emerged in 2001, numerous chemical designs have been built around the AIE concept, displaying its utility for diverse applications, including optics, electronics, energy, and biosciences. The present review critically evaluates the broad applicability of AIEgen-based chemical models towards sensing small analytes and the structural design strategies adjusting the mode of action reported since the last decade. Various AIEgen models have been discussed, providing qualitative and quantitative estimation of cationic metal ions and anionic species, as well as biomolecular, cellular, and organelle-specific probes. A systematic overview of the reported structural design and the underlying working mode will pave the way for designing and developing the next generation of AIEgens for specific applications.

Self-indicating polymers: a pathway to intelligent materials

Self-indicating polymers have emerged as a promising class of smart materials that possess the unique ability to undergo detectable variations in their physical or chemical properties in response to various stimuli. This article presents an overview of the most important mechanisms through which these materials exhibit self-indication, including aggregation, phase transition, covalent and non-covalent bond cleavage, isomerization, charge transfer, and energy transfer. Aggregation is a prevalent mechanism observed in self-indicating polymers, where changes in the degree of molecular organization result in variations in optical or electrical properties. Phase transition-induced self-indication relies on the transformation between different phases, such as liquid-to-solid or crystalline-to-amorphous transitions, leading to observable changes in color or conductivity. Covalent bond cleavage-based self-indicating polymers undergo controlled degradation or fragmentation upon exposure to specific triggers, resulting in noticeable variations in their structural or mechanical properties. Isomerization is another crucial mechanism exploited in self-indicating polymers, where the reversible transformation between the different isomeric forms induces detectable changes in fluorescence or absorption spectra. Charge transfer-based self-indicating polymers rely on the modulation of electron or hole transfer within the polymer backbone, manifesting as changes in electrical conductivity or redox properties. Energy transfer is an essential mechanism utilized by certain self-indicating polymers, where energy transfer between chromophores or fluorophores leads to variations in the emission characteristics. Furthermore, this review article highlights the diverse range of applications for self-indicating polymers. These materials find particular use in sensing and monitoring applications, where their responsive nature enables them to act as sensors for specific analytes, environmental parameters, or mechanical stress. Self-indicating polymers have also been used in the development of smart materials, including stimuli-responsive coatings, drug delivery systems, food sensors, wearable devices, and molecular switches. The unique combination of tunable properties and responsiveness makes self-indicating polymers highly promising for future advancements in the fields of biotechnology, materials science, and electronics.

A naphthalimide functionalized fluoran with AIE effect for ratiometric sensing Hg2+ and cell imaging application

The pollution caused by mercury ions (Hg²⁺) poses a potential threat to public health. Therefore, monitoring Hg²⁺ concentration in the environment is necessary and significant. In this work, a naphthalimide functionalized fluoran dye NAF has been prepared, which shows a new red-shift in emission at 550 nm with the maximum intensity in a mixture of water-CH₃CN (v/v = 7/3) due to aggregating induced emission (AIE) effect. Meanwhile, NAF can be employed as a Hg²⁺ ions sensor, which displays a selective and sensitive response to Hg²⁺ ions by the reduced fluorescence of naphthalimide fluorophore and increased fluorescence of fluoran group, respectively, showing ratiometric fluorescence signal changes with more than 65-fold emission intensity ratio increase and naked eyes visible color change. In addition, the response time is fast (within 1 min) and the sensing can be conducted in a wide pH range (4.0-9.0). Moreover, the detection limit has been evaluated to be 5.5 nM. The sensing mechanism may be attributed to the formation of a π-extended conjugated system due to the Hg²⁺ ions-induced conversion of spironolactone to the ring-opened form, partially accompanied by the fluorescence resonance energy transfer (FRET) process. Significantly, NAF exhibits suitable cytotoxicity to living HeLa cells, which allows it to be utilized for ratiometric imaging of Hg²⁺ ions assisted by confocal fluorescence imaging.

A quinoline-rhodamine hybrid probe for ratiometricly sensing of Hg2+ in water and cell imaging application

To develop efficient tools for monitoring toxicant Hg²⁺ in aqueous solution attracts great attention because the abnormal distribution of Hg²⁺ in environment poses great threat to human health. We here report the preparation of a novel quinoline-rhodamine hybrid fluorescent probe P7RHg for ratiometricly sensing of Hg²⁺ in water, with a spirolactam-thiosemicarbazide reaction group. Upon treatment by Hg²⁺, the ratio of fluorescence intensity (F₆₀₀/F₄₆₀) exhibits nearly 90-fold enhancement, presenting two well-resolved emission peaks (140 nm). Meanwhile, the specific Hg²⁺-induced desulfurization provides probe P7RHg an excellent selectivity to Hg²⁺, with a detection limit of 8.6 nM. Moreover, the low cytotoxicity allows P7RHg to be employed for tracing Hg²⁺ in living cells by confocal fluorescence imaging.

Fibrous aggregates: Amplifying aggregation-induced emission to boost health protection

Environmental monitoring and personal protection are critical for preventing and for protecting human health during all infectious disease outbreaks (including COVID-19). Fluorescent probes combining sensing, imaging and therapy functions, could not only afford direct visualizing existence of biotargets and monitoring their dynamic information, but also provide therapeutic functions for killing various bacteria or viruses. Luminogens with aggregation-induced emission (AIE) could be well suited for above requirements because of their typical photophysical properties and therapeutic functions. Integration of these molecules with fibers or textiles is of great interest for developing flexible devices and wearable systems. In this review, we mainly focus on how fibers and AIEgens to be combined for health protection based on the latest advances in biosensing and bioprotection. We first discuss the construction of fibrous sensors for visualization of biomolecules. Next recent advances in therapeutic fabrics for individual protection are introduced. Finally, the current challenges and future opportunities for "AIE + Fiber" in sensing and therapeutic applications are presented.

A rhodamine–TPE scaffold-based fluorescent probe for visualizing phosgene with a portable smartphone via test TLC strips

A rhodamine–TPE scaffold-based ratiometric signal readout probe was developed for sensitive, rapid, and in situ phosgene determination.

Detection of Mercury Ion with High Sensitivity and Selectivity Using a DNA/Graphene Oxide Hybrid Immobilized on Glass Slides

Excessive mercury ions (Hg²⁺) cause great pollution to soil/water and pose a major threat to human health. The high sensitivity and high selectivity in the Hg²⁺ detection demonstrated herein are significant for the research areas of analytical chemistry, chemical biology, physical chemistry, drug discovery, and clinical diagnosis. In this study, a series of simple, low-cost, and highly sensitive biochips based on a graphene oxide (GO)/DNA hybrid was developed. Hg²⁺ is detected with high sensitivity and selectivity by GO/DNA hybrid biochips immobilized on glass slides. The performance of the biosensors can be improved by introducing more phosphorothioate sites and complementary bases. The best limit of detection of the biochips is 0.38 nM with selectivity of over 10:1. This sensor was also used for Hg²⁺ detection in Dendrobium. The results show this biochip is promising for Hg²⁺ detection.

Modulation of Aggregation-Induced Emission by Excitation Energy Transfer: Design and Application

Excitation energy transfer (EET) as a fundamental photophysical process is well-explored for developing functional materials with tunable photophysical properties. Compared to traditional fluorophores, aggregation-induced emission luminogens (AIEgens) exhibit unique advantages for building EET systems, especially serving as energy donors, due to their outstanding photophysical properties such as bright fluorescence in aggregation state, broad absorption and emission spectra, large Stokes shift, and high photobleaching resistance. In addition, the photophysical properties of AIEgens can be modulated by energy transfer for improved luminescence performance. Therefore, a variety of EET systems based on AIEgens have been constructed and their applications in different areas have been explored. In this review, we summarize recent progress in the design strategy of AIE-based energy transfer systems for light-harvesting, fluorescent probes and theranostic systems, with an emphasis on design strategies to achieve desirable properties. The limitations, challenges and future opportunities of AIE-EET systems are briefly outlined. Design strategies and applications (light-harvesting, fluorescent probe and theranostics) of AIEgen-based excitation energy systems are discussed in this review.

Tetraphenylethene-based fluorescent probe with aggregation-induced emission behavior for Hg2+ detection and its application

A tetraphenylethene (TPE) derivative was designed and synthesized upon conjugation with bis(thiophen-2-ylmethyl) amine (BTA) containing a mercury-binding moiety and further characterized by using Nuclear magnetic resonance (NMR), LC-MS, UV-Vis, and fluorescence spectroscopic methods. The resulting TPE-BTA exhibited comprehensive aggregation-induced emission while expressing a high quantum yield and emission intensity at 70% water fraction. The probe exhibited a good photochromic effect with a Stokes shift of 178 nm, and the emission intensity at 550 nm increased considerably with the color turning from dark green to bright green under a UV lamp upon the addition of 5 μM Hg²⁺. The lowest-energy conformation of the probe showed that two thiophene rings were perpendicular to the phenyl ring, while two BTA molecules were situated in a staggered form to each other. The sulfur and nitrogen atoms present in TPE-BTA were coordinated to the Hg²⁺ ion, and these binding sites were confirmed by the NMR parameters, X-ray photoelectron spectroscopy signals, and structural calculations. The binding of Hg²⁺ to TPE-BTA was believed to restrict the intramolecular motion of TPE-BTA, thus inducing it to shine brighter according to the unique aggregation-induced emission effect. The concentration of Hg²⁺ was determined based on the enhancement of the emission intensity, and the present probe showed an extremely high sensitivity with a limit of detection of 10.5 nM. Furthermore, TPE-BTA enabled selective detection of Hg²⁺ even in the presence of a 1000-fold excess of other interfering metal ions. The proposed method was successfully employed to determine Hg²⁺ in living HeLa cells and real water samples.

A dual thiourea-appended perylenebisimide "turn-on" fluorescent chemosensor with high selectivity and sensitivity for Hg2+ in living cells

Sensing heavy metal ions particularly for the most toxic Hg²⁺ is a long-term pursuit for chemists because of its obvious and extreme harmfulness to both the environment and human health. Herein, a novel 'turn-on' perylenebisimide-thiourea fluorescent probe PBI-BTB is achieved for rapid detection of Hg²⁺ in a DMSO/H₂O (5/1, v/v) solution through a typical Hg²⁺-promoting desulfurization reaction, which has been investigated through Job's plot titration, FT-IR, ¹H NMR and HRMS analysis. A remarkable fluorescence emission enhancement at 540 and 580 nm is observed in the presence of Hg²⁺, which is visible to the naked eye with high selectivity and sensitivity. Moreover, probe PBI-BTB combined strong anti-interference recognition with short response time (< 1 min). The rapid fluorescence response with low limit of detection (0.35 μM) in a wide pH range of 3.0-11.0 makes PBI-BTB a promising candidate for detection of Hg²⁺ without any buffer system. Furthermore, the practicability of probe PBI-BTB upon the Hg²⁺ recognition in human liver cancer cells (HepG-2) has been studied through fluorescent live cell imaging which reveals the probe's low toxicity to organism as well as the favorable cell permeability of PBI-BTB for detecting Hg²⁺ in biological systems.

Recent Progress in Small-Molecule Fluorescent Probes for Detecting Mercury Ions

Mercury is a highly toxic and non-essential element that is found in every corner of the globe. The small amount of mercury produced by various pathways eventually enters freshwater and marine ecosystems, circulating through the food chain (especially fish) and causing various environmental problems in aspects including plants, animals, and human. There are several traditional quantitative methods developed for mercury ions (II) analysis in water samples. However, due to the complexity of the detection process, high cost and strong technical expertise, it is difficult to detect mercury ions in real-time. Therefore, in recent years, a large number of researchers have developed small-molecule fluorescent probes for Hg ions detection. Fluorimetry has the advantages of convenient detection, short response time, high sensitivity and good selectivity. This review summarized the small-molecule fluorescent probes for mercuric ion detection developed in recent years according to the chemical structural classification, compared their performances and elaborated the mechanism. We hope that the review will help the researches for the designs of metal ions fluorescent probes and their applications with certain reference value.

AND logic gate based fluorescence probe for simultaneous detection of peroxynitrite and hypochlorous acid

Hypochlorous acid (HOCl) and peroxynitrite (ONOO^-) are two of the most important reactive species and associated with various diseases in various physiological and pathological processes. Nonetheless, many of their roles are still vague due to the shortage of methods for simultaneously detecting HOCl and ONOO^-. Herein, three simple yet useful fluorogenic probes, LG-1, LG-2 and LG-3, have been fabricated with facile synthesis route and used to monitor the coexistence of HOCl and ONOO^- as AND-based logic gate fluorescent probe firstly. LG-1 and LG-2, which consists of 1,3-oxathiolane group and boronate group respectively, were designed to verify the capacity of monitoring HOCl and ONOO^- without interference from each other. The result showed that these two groups are perfect reaction sites of detecting HOCl and ONOO^- respectively via specific analyte-induced reactions. Hence, LG-3, which is attached by these two groups to suppress the fluorophore core, can response to HOCl and ONOO^- simultaneously without mutual interference and generate the significant time-dependent fluorescence enhancement. By investigating the absorption and fluorescence properties of LG-3 towards HOCl and ONOO^- individually and collectively, the result confirmed clearly that LG-3 has the capacity of monitoring the coexistence of HOCl and ONOO^-, which could act as a two-input AND-based logic gate fluorescent probe.

Coumarin Thiourea-Based Fluorescent Turn-on Hg2+ Probe That Can Be Utilized in a Broad pH Range 1-11

A novel coumarin-thiourea conjugate was synthesized facilely. It served as a fluorescent turn-on chemosensor for selective detection of Hg²⁺ ion over other common competitive metal ions including Li⁺, Na⁺, K⁺, Ag⁺, Cu²⁺, Fe²⁺, Zn²⁺, Co²⁺, Ni²⁺, Mn²⁺, Sr²⁺, Ca²⁺, Mg²⁺, Al³⁺, Cr³⁺ and Fe³⁺ ions based on the Hg²⁺-promoted desulfurization and cyclization reactions. Addition of Hg²⁺ ion to the sensor solution in 2:8 EtOH/H₂O induced a hypsochromic shift of the UV-Vis absorption band from 360 nm to 340 nm accompanying distinct enhancement in the absorption intensity while addition of other metal ions failed to bring about substantial change in the absorption spectra. Addition of Hg²⁺ to the sensor solution also caused marked increase in the fluorescence emission intensity and most common competitive metal ions did not interfere with the selective sensing of Hg²⁺ ion by the sensor. The detection limit of Hg²⁺ ion by the probe was calculated to be 1.46 × 10^-7 M and the probe could be utilized for selective detection of Hg²⁺ ion by fluorescence turn-on mode over a broad pH range of 1-11.

New colorimetric and fluorometric chemosensor for selective Hg2+ sensing in a near-perfect aqueous solution and bio-imaging

We report a new 7-nitrobenzo-2-oxa-1, 3-diazolyl (NBD)-based chemosensor containing a piperazine derivative, NBDP, for detection of mercury ions in almost 100% aqueous medium. The chemosensor shows sensing exclusively toward Hg²⁺ with a switch-on fluorescence response at 543 nm, which could be attributed to the blocking of PET (photo-induced electron transfer) process upon complexation with mercury ions. The molar ratio of Hg(Ⅱ) to NBDP in the complex is 1:1 based on the Job's plot and HRMS studies. Optimized configurations of NBDP and NBDP-Hg²⁺ complexes were simulated by means of DFT calculations. The reversible fluorescence response with low detection limit (19.2 nM) in the pH range of 6.0-7.5 renders NBDP a promising candidate for Hg²⁺ detection in neutral aqueous environments. For the practical application of the chemosensor, test strips were successfully fabricated for rapid detection of Hg²⁺ ions. Moreover, the utility of NBDP showing the mercury recognition in Human liver cancer cells (SMMC-7721) and zebrafish as well as in live tissues of Arabidopsis thaliana has been demonstrated as monitored by fluorescence imaging.

Tunable multi-color luminescence from a self-assembled cyanostilbene and cucurbit[7]uril in aqueous media

A tunable multicolor luminescent supramolecular system was designed in aqueous media employing the self-assembly of a cationic amphiphilic cyanostilbene and the host–guest chemistry of cucurbit[7]uril.

Recent Advances in AIEgens for Metal Ion Biosensing and Bioimaging

Metal ions play important roles in biological system. Approaches capable of selective and sensitive detection of metal ions in living biosystems provide in situ information and have attracted remarkable research attentions. Among these, fluorescence probes with aggregation-induced emission (AIE) behavior offer unique properties. A variety of AIE fluorogens (AIEgens) have been developed in the past decades for tracing metal ions. This review highlights recent advances (since 2015) in AIE-based sensors for detecting metal ions in biological systems. Major concerns will be devoted to the design principles, sensing performance, and bioimaging applications.

Autocatalytic DNA circuit for Hg2+ detection with high sensitivity and selectivity based on exonuclease III and G-quadruplex DNAzyme

Utilizing G-quadruplex as the signal report probe, an ultrasensitive and label-free autocatalytic DNA circuit for Hg²⁺ detection on the basis of exonuclease III (Exo III)-assisted cascade signal amplification has been proposed. In the absence of Hg²⁺, the hairpin A and the DNA1 cannot hybridize due to the thymine-thymine (T-T) mismatches. Therefore, hairpin probes with the 3'-protruding terminus can be resistant to Exo III digestion, preventing the G-rich sequence to be released. In the presence of Hg²⁺, the combination of the DNA1 with the 3' end-extruding hairpin A via T-Hg²⁺-T coordination chemistry triggers the digestion reaction of Exo III, leading to the release of the DNA1 and the sequence with domains c, d, and e. Both of the DNA1 and the sequence with domains c, d, and e can combine with other hairpin probes and activate another round of the cleavage reaction. The produced G-rich sequence can form G-quadruplex structure by binding with N-Methyl mesoporphyrin IX (NMM). The biosensor exhibits excellent selectivity and high sensitivity for Hg²⁺. The linear range of this biosensor is from 10 fM to 100 nM, and the linear equation can be expressed as: F₆₁₀ = 1.3 × 10⁵ Lg C + 7.40 × 10⁴ (R² = 0.998), in which F₆₁₀ is the fluorescence intensity at 610 nm, C represents the Hg²⁺ concentrations, and Lg is the logarithm of 10. The detection limit is 10 fM. The biosensor is robust and can be applied to the detection of Hg²⁺ in water samples. By substituting the target-recognition elements, this sensing system can also be used for the detection of other metal ions.

A water-soluble AIE-gen for organic-solvent-free detection and wash-free imaging of Al3+ ions and subsequent sensing of F− ions and DNA tracking

A water-soluble TPE-based AIEgen (TPE-diBuS) was developed for organic-solvent-free detection of Al³⁺ ions and its wash-free cell imaging. The TPE-diBuS-Al ensemble was used for the detection of F⁻ ions and DNA tracking.

An ultra-sensitive ratiometric fluorescent probe for hypochlorous acid detection by the synergistic effect of AIE and TBET and its application of detecting exogenous/endogenous HOCl in living cells

TR-OClexhibits ultra-high sensitivity towards HOCl with a 7000-fold enhancement in the fluorescence ratio (I₅₈₉/I₄₇₇) and a detection limit of 1.29 nM, which is one of the highest recorded so far.