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

A novel red-to-near-infrared AIE fluorescent probe for detection of Hg2+ with large Stokes shift in plant and living cells

Due to the highly toxic nature of mercury ions to living organisms, accurately detecting Hg2+ in water samples and biological systems is of great significance. In this study, we designed and synthesized a novel red-to-near-infrared Aggregation-Induced Emission (AIE) fluorescent probe (named as DS) based Fluorene derivatives on specifically for Hg2+ detection. Probe DS can visually identify Hg2+ through an red-to-near-infrared fluorescence enhancement change, characterized by a large Stokes shift (130 nm) and AIE feature. This probe offers a fast response, high selectivity and sensitivity. The Hg2+-induced deprotection reaction of the thioketal mechanism was thoroughly investigated using nuclear magnetic resonance spectroscopy (NMR), mass spectrometry (MS) and density functional theory (DFT) calculation. Additionly, dynamic light scattering (DLS) results indicated that the aggregation states changes of the molecular play a crucial role in the AIE fluorescence response of probe DS toward Hg2+. The red-to-near-infrared response with AIE feature not only avoids the interference of auto-fluorescence signals in complex environments, but also reduces the fluorescence quenching caused by probe molecular aggregation. This makes probe DS highly suitable for high-quality imaging detection of Hg2+ in aqueous environments. Furthermore, probe DS demonstrates the capability for visual fluorescence detection of Hg2+ concentrations in water sample, plant roots and living cells.

Ultrasensitive detection of trace Hg(Ⅱ) in acidic conditions using DMABR loaded on sepiolite: Function, application and mechanism studies

Fast and real-time detection of trace Hg(Ⅱ) by fluorescent probes under acidic conditions is urgently required due to the high toxicity and accessibility to creatures and human being. However, fluorescent probes for Hg(Ⅱ) detection in environmental samples are rarely reported due to the protonation potential of acidic mercury sources. In this study, the SD probe was developed by 5-(p-dimethylaminobenzylidene) rhodanine (DMABR) loaded on sepiolite by hydrothermal treatment, and showed excellent Hg(Ⅱ) detection performances for mercury sources at pH 4-10 due to buffering ability of the hyperconjugated lactam rings. Sepiolite functioned as the support skeleton to decrease intermolecular transition, and thus increased the sensitivity. At pH 4, the SD probe showed high selectivity and sensitivity for Hg(Ⅱ) among various species, with low LOD and binding constant of 4.78 × 10-9 M and 1.34 × 106 M-1, respectively. Through DFT calculations, MAS 1H NMR and 2D-COS analysis, the detection mechanism was demonstrated as SN1 substitution of the spontaneous leaving H on amino groups in the transient state during tautomeric equilibrium, rather than the expected high-affinity sulphydryl. Additionally, the SD probe exhibited promising potential in quantifying water-soluble and bioavailable Hg(Ⅱ) in acidic polluted soil and water samples. Moreover, real-time detection was realized by paper-based strips.

Thiourea Functionalised Receptor for Selective Detection of Mercury Ions and its Application in Serum Sample

A thiourea functionalised fluorescent probe 1-phenyl-3-(pyridin-4-yl)thiourea was synthesized and utilised as a fluorescent turn-on chemosensor for the selective recognition of Hg2+ ion over competitive metal ions including Na+, Mn2+, Li+, Cr2+, Ni2+, Ca2+, Cd2+, Mg2+, K+, Co2+, Cu2+, Zn2+, Al3+ and Fe2+ ions based on the inter-molecular charge transfer (ICT). Intriguingly, the receptor demonstrated unique sensing capabilities for Hg2+ in DMSO: H2O (10:90, v/v). The addition of Hg2+ ions to the sensor resulted in a blue shift in the absorption intensity and also enhancement in fluorescence intensity at 435 nm. Fluorescence emission intensity increased linearly with Hg2+ concentration ranging from 0 to 80 µL. The detection limit and binding constant were determined as 0.134 × 10-6 M and 1.733 × 107 M-1, respectively. The sensing behavior of Hg2+ was further examined using DLS, SEM and FTIR. The probe could detect Hg2+ ions across a wide pH range. Furthermore, the receptor L demonstrated good sensing performance for Hg2+ in bovine serum albumin and actual water samples.

Two-mode sensing strategies based on tunable cobalt metal organic framework active sites to detect Hg2

Heavy metal pollution poses a major threat to human health, and developing a user-deliverable heavy metal detection strategy remains a major challenge. In this work, two-mode Hg2+ sensing platforms based on the tunable cobalt metal-organic framework (Co-MOF) active site strategy are constructed, including a colorimetric, and an electrochemical assay using a personal glucose meter (PGM) as the terminal device. Specifically, thymine (T), a single, adaptable nucleotide, is chosen to replace typical T-rich DNA aptamers. The catalytic sites of Co-MOF are tuned competitively by the specific binding of T-Hg2+-T, and different signal output platforms are developed based on the different enzyme-like activities of Co-MOF. DFT calculations are utilized to analyze the interaction mechanism between T and Co-MOF with defect structure. Notably, the two-mode sensing platforms exhibit outstanding detection performance, with LOD values as low as 0.5 nM (colorimetric) and 3.69 nM (PGM), respectively, superior to recently reported nanozyme-based Hg2+ sensors. In real samples of tap water and lake water, this approach demonstrates an effective recovery rate and outstanding selectivity. Surprisingly, the method is potentially versatile and, by exchanging out T-Hg2+-T, can also detect Ag+. This simple, portable, and user-friendly Hg2+ detection approach shows plenty of promise for application in the future.

Fine-tuning bromide AIE probes for Hg2+ detection in mitochondria with wash-free staining

Mercury ions (Hg2+) primarily target mitochondria in the cells. Therefore, the development of novel probes that specifically target mitochondria in the presence of Hg2+ is of immense importance. Most previously reported probes that utilize the softness of S, Te, O, and/or N atoms for Hg2+ binding often face problems such as fluorescence quenching and off-target signals. In this study, bromide-hydrocarbon pyridinium salts were designed to target the mitochondria and chelate Hg2+ via Hg-Br coordination bonds. As a prototype, four aggregation-induced emission (AIE) fluorogens, namely TPP-Br, TPP-Cl, R1, and R2, with a similar D-π-A structure but slight differences in their halogen substituents, were designed. Among them, only TPP-Br achieved the highly selective and sensitive detection of Hg2+ by triggering its AIE properties, resulting in remarkable emission enhancement (80-fold), colorimetry, and the Tyndall effect. TPP-Br exhibited high selectivity and sensitivity to Hg2+ with a detection limit of 0.35 μM, rapid response time (<10 s), and large Stokes shift of 185 nm. Their interaction modes were studied using a combination of 1H nuclear magnetic resonance spectroscopy, scanning electron microscopy, fluorescent lifetime decay, and theoretical calculations. TPP-Br exhibited a low emission background in cells, whereas in the presence of Hg2+, mitochondria were lit up with wash-free staining. This study provides a powerful tool for accurately diagnosing mercury poisoning-related diseases in mitochondria.

Essential Rule Derived from Thermodynamics and Kinetics Studies of Benzopyran Compounds

Compounds with benzopyran as the core structure play an important role in the total synthesis of antioxidants, drugs, and natural products. Herein, the thermodynamic data of benzopyran compounds and their intermediates were measured and calculated by combining thermodynamics with kinetics. The mechanism of reactions between four benzopyran compounds and organic hydride acceptors was proven to be a one-step hydride transfer. The thermodynamic properties of these compounds and their corresponding intermediates were elucidated. The rationality and accuracy of the electrochemical measurement method were proved. Furthermore, the essential rule of unique structures being present between the C-H bond and para-substituent constants on the benzene ring, as shown in previous studies, was investigated. A simultaneous correlation between thermodynamics and kinetics was found for the hydride transfer reaction, in which the reaction site is connected with the substituent through the benzene ring, a double bond, or a N atom. The likely reason for the correlation between thermodynamic and kinetic is that the benzene ring, double bond, or N atom have the role of transferring the electronic effect. This finding can be applied to the calculation of the activation energy of hydride self-exchange reactions, the prediction of kinetic isotope effects, and explorations of selective reduction processes of hydride transfer in such organic hydride compounds.

Photoelectrochemical Sensor with a Z-Scheme Fe2O3/CdS Heterostructure for Sensitive Detection of Mercury Ions

Mercury (Hg2+) is a highly toxic element and can seriously affect human health. This work proposed a photoelectrochemical (PEC) sensor with a Z-scheme Fe2O3/CdS heterostructure and two thymine-rich DNA strands (DNA-1 and Au@DNA-2) for sensitive detection of Hg2+. The light excitation of the Fe2O3/CdS composite accelerated the electron transfer among Fe2O3, CdS, and the electrode to produce a stable photocurrent response. Upon the recognition of Hg2+ to thymine bases (T) in two DNA strands to form a stable T-Hg2+-T biomimetic structure, the photocurrent response increased with the increasing concentration of Hg2+ due to the opening of electronic transmission channels from Au nanoparticles to Fe2O3/CdS nanocomposite. Under the optimal conditions screened by the Box-Behnken experiments, the proposed PEC sensor showed excellent analytical performance for Hg2+ detection with high sensitivity, a detection limit of 0.20 pM at a signal-to-noise ratio of 3, high selectivity, a detectable concentration range of 1 pM-100 nM, and acceptable stability. The good recovery and low relative standard deviation for the analysis of Hg2+ in lake and tap water samples demonstrated the potential application of the designed Z-scheme Fe2O3/CdS heterostructure in the PEC detection of heavy metal ions.

A multifunctional fluorescent probe based on Schiff base with AIE and ESIPT characteristics for effective detections of Pb2+, Ag+ and Fe3

In this work, three Schiff-based fluorescent probes with aggregation-induced emission (AIE) and excited intramolecular proton transfer (ESIPT) characters were synthesized by grafting 2-aminobenzothiazole group onto 4-substituted salicylaldehydes. More important, a rare tri-responsive fluorescent probe (SN-Cl) was developed by purposeful variation of substituents in the molecule. It could selectively identify Pb2+, Ag+ and Fe3+ in different solvent systems or with the help of masking agent and show complete fluorescence enhancement without interference of other ions. Meanwhile, the other two probes (SN-ON and SN-N) could only recognize Pb2+ in DMSO/Tris-HCl buffer (3: 7, v/v, pH = 7.4). According to Job's plot, density functional theory (DFT) calculations and NMR analysis, coordination between SN-Cl and Pb2+/Ag+/Fe3+ was determined. The LOD values for three ions were as low as 0.059 μM, 0.012 μM and 8.92 μM, respectively. Ideally, SN-Cl showed satisfactory performance in real water samples detection and test paper experiments for three ions. Also, SN-Cl could be used as an excellent imaging agent for Fe3+ in HeLa cells. Therefore, SN-Cl has the ability to be a "single fluorescent probe for three targets".

Recent Development in Fluorescent Probes for the Detection of Hg2+ Ions

Mercury, a highly toxic heavy metal, poses significant environmental and health risks, necessitating the development of effective and responsive techniques for its detection. Organic chromophores, particularly small molecules, have emerged as promising materials for sensing Hg²⁺ ions due to their high selectivity, sensitivity, and ease of synthesis. In this review article, we provide a systematic overview of recent advancements in the field of fluorescent chemosensors for Hg²⁺ ions detection, including rhodamine derivatives, Schiff bases, coumarin derivatives, naphthalene derivatives, BODIPY, BOPHY, naphthalimide, pyrene, dicyanoisophorone, bromophenol, benzothiazole flavonol, carbonitrile, pyrazole, quinoline, resorufin, hemicyanine, monothiosquaraine, cyanine, pyrimidine, peptide, and quantum/carbon dots probes. We discuss their detection capabilities, sensing mechanisms, limits of detection, as well as the strategies and approaches employed in their design. By focusing on recent studies conducted between 2022 and 2023, this review article offers valuable insights into the performance and advancements in the field of fluorescent chemosensors for Hg²⁺ ions detection.

Discriminating normal and inflammatory mice models by viscosity changes with a two-photon fluorescent probe

Studies have found that the intracellular viscosity changes have close relationship with many diseases, therefore design and synthesis of fluorescent probe for testing intracellular viscosity is of great significance to the development of clinical. Herein, we developed a new two-photon near infrared probe (HCT) for viscosity imaging to discriminate normal and inflammatory models. Experimental results displayed that HCT has great sensitivity for the detection of viscosity, and based on the excellent performance of its photostability and lower cytotoxicity, HCT was successfully utilized for single-photon/ two-photon fluorescence imaging of the viscosity in living cells. More importantly, we employ HCT to further showcase in living tissues. Additionally, HCT could be used to discriminate between normal and inflamed mice, heralding its practical application in biomedical aspects.

Metal-Enhanced Hg2+-Responsive Fluorescent Nanoprobes: From Morphological Design to Application to Natural Waters

Metal-enhanced fluorescence (MEF) is a powerful tool in the design of sensitive chemical sensors by improving brightness and photostability of target-responsive fluorophores. Compounding these advantages with the modest hardware requirements of fluorescence sensing compared to that of centralized elemental analysis instruments, thus expanding the use of MEF to the detection of low-level inorganic pollutants, is a compelling aspiration. Among the latter, monitoring mercury in the environment, where some of its species disseminate through the food chain and, in time, to humans, has elicited a broad research effort toward the development of Hg²⁺-responsive fluorescent sensors. Herein, a Hg²⁺-sensitive MEF-enabled probe was conceived by grafting a Hg²⁺-responsive fluorescein derivative to concentric Ag@SiO₂ NPs, where the metallic core enhances fluorescence emission of molecular probes embedded in a surrounding silica shell. Time-resolved fluorescence measurements showed that the fluorophore's excited-state lifetime decreases from 3.9 ns in a solid, coreless silica sphere to 0.4 ns in the core-shell nanoprobe, granting the dye a better resistance to photobleaching. The Ag-core system showed a sizable improvement in the limit of detection at 2 nM (0.4 ppb) compared to 50 nM (10 ppb) in silica-only colloids, and its effectiveness for natural water analysis was demonstrated. Overall, the reported nanoarchitecture hints at the potential of MEF for heavy metal detection by fluorescence detection.

A Novel Diarylethene-rhodamine Unit Based Chemosensor for Fluorimetric and Colorimetric Detection of Hg2

A novel fluorimetric and colorimetric chemosensor (1O) was synthesized with diarylethene-rhodamine unit and characterized by ESI-MS, ¹H NMR, and ¹³C NMR. The chemosensor can selectively recognize extremely low concentrations of Hg²⁺ over a variety of metal ions with remarkable colorimetric and fluorescent responses. The colorimetric and fluorescent changes were ascribed the reaction between 1O and Hg²⁺ destructed the rhodamine hydrazide into open-ring form which was proved by mass spectrometry and nuclear magnetic titration analyses. The detection limits of the UV absorption and fluorescence methods for Hg²⁺ were found to be 0.708 μM and 24.6 nM, respectively. Moreover, the chemosensor exhibited excellent photochromism and outstanding fatigue resistance property under alternating UV and visible light irradiation. The application potential of the chemosensor was demonstrated with the qualitative detection of Hg²⁺ in real water samples.