Bioimaging of a fluorescence rhodamine-based probe for reversible detection of Hg (II) and its application in real water environment

A ratiometric fluorescent probe with a large Stokes shift for the detection of Hg2+ and its applications in environmental sample and living system analysis

Toxic mercury ions (Hg2+) can cause serious environmental pollution and accumulate in living organisms via the food chain. Therefore, monitoring Hg2+ is crucial in ensuring the safety of ecosystems and organisms. In this work, a novel ratiometric fluorescent probe CMT (5-(4-(diphenylamino)phenyl)-1-(7-hydroxy-coumarin-3-yl)-4-pentene-1,3-dione) based on coumarin was developed for detecting Hg2+, which displayed obvious fluorescence changes, a low detection limit (2.24 × 10-7 M), good selectivity, and a large Stokes shift (255 nm). The CMT probe could detect Hg2+ in real environmental soil and water samples. Furthermore, the CMT probe enabled the naked-eye detection of Hg2+ using test paper experiments. CMT was also applied for fluorescence imaging in living zebrafish and plants. This work provides a highly efficient tool for monitoring Hg2+ in environmental samples and biological systems.

Mercury transformations in algae, plants, and animals: The occurrence, mechanisms, and gaps

Mercury (Hg) is a global pollutant showing potent toxicity to living organisms. The transformations of Hg are critical to global Hg cycling and Hg exposure risks, considering Hg mobilities and toxicities vary depending on Hg speciation. Though currently well understood in ambient environments, Hg transformations are inadequately explored in non-microbial organisms. The primary drivers of in vivo Hg transformations are far from clear, and the impacts of these processes on global Hg cycling and Hg associated health risks are not well understood. This hinders a comprehensive understanding of global Hg cycling and the effective mitigation of Hg exposure risks. Here, we focused on Hg transformations in non-microbial organisms, particularly algae, plants, and animals. The process of Hg oxidation/reduction and methylation/demethylation in organisms were reviewed since these processes are the key transformations between the dominant Hg species, i.e., elemental Hg (Hg0), divalent inorganic Hg (IHgII), and methylmercury (MeHg). By summarizing the current knowledge of Hg transformations in organisms, we proposed the potential yet overlooked drivers of these processes, along with potential challenges that hinder a full understanding of in vivo Hg transformations. Knowledge summarized in this review would help achieve a comprehensive understanding of the fate and toxicity of Hg in organisms, providing a basis for predicting Hg cycles and mitigating human exposure.

An overview on recent advances of reversible fluorescent probes and their biological applications

Due to the simplicity and low detection limit, fluorescent probes are widely used in both analytical sensing and optical imaging. Compared to conventional fluorescent probes, reversibility endows the reversible fluorescent probe outstanding advantages and special properties, making reversible fluorescent probes with capable of quantitative, repetitive or circulatory. Reversible fluorescent probes can also monitor the concentration dynamics of target analytes in real time, such as metal ions, proteins and enzymes, as well as intracellular redox processes, which have been widely applied in various fields. This review summarized the types and excellent properties of reversible fluorescent probes designed and developed in recent years. It also summarized the applications of reversible fluorescent probe in fluorescence imaging, biological testing, monitoring redox cycles, and proposed the remaining challenges and future development directions of the reversible fluorescent probe. This review provided comprehensive overview of reversible fluorescent probe, which may provide valuable references for the design and fabrication of the reversible fluorescent probe.

Highly selective, sensitive and biocompatible rhodamine-based isomers for Al3+ detection: A comparative study

Selective detection of a metal ion with high selectivity is of great importance to understand its existence and its role in many chemical and biological processes. We report here the synthesis, characterization and Al3+ sensing properties of two rhodamine-based isomers, (E)-2-((2-(allyloxy)benzylidene)amino)ethyl)-3',6'-bis(ethylamine)-2',7'-dimethylspiro[isoindoline-1,9'-xanthen]-3-one (L-2-oxy) and (E)-2-((4-(allyloxy)benzylidene)amino)ethyl)-3',6'-bis(ethylamine)-2',7'-dimethylspiro[isoindoline-1,9'-xanthen]-3-one (L-4-oxy). L-2-oxyand L-4-oxy show pink coloration, significant enhancement in absorbance at 530 nm and fluorescence intensity at 553 nm in the presence of Al3+ among several cations. Quantum yield and lifetime of the probes increase in the presence of Al3+. LOD values have been determined as low as ∼1.0 nM for both the isomers. DFT study suggests that the cation induces opening of spirolactam ring resulting in the changes of the rhodamine dyes. Additional reason could be Chelation Enhanced Fluorescence (CHEF) effect due to the subsequent chelation of the metal ion. Between two isomers, L-2-oxy displays better sensing ability towards Al3+ in terms of fluorescence enhancement, limit of detection, lifetime enhancement. Both the probes have been utilized in cell imaging studies using rat skeletal myoblast cell line (L6 cell line).

A Study of Small Molecule-Based Rhodamine-Derived Chemosensors and their Implications in Environmental and Biological Systems from 2012 to 2021: Latest Advancement and Future Prospects

Rhodamine-based chemosensors have sparked considerable interest in recent years due to their remarkable photophysical properties, which include high absorption coefficients, exceptional quantum yields, improved photostability, and significant red shifts. This article presents an overview of the diverse fluorometric, and colorimetric sensors produced from rhodamine, as well as their applications in a wide range of fields. The ability of rhodamine-based chemosensors to detect a wide range of metal ions, including Hg+2, Al3+, Cr3+, Cu2+, Fe3+, Fe2+, Cd2+, Sn4+, Zn2+, and Pb2+, is one of their major advantages. Other applications of these sensors include dual analytes, multianalytes, and relay recognition of dual analytes. Rhodamine-based probes can also detect noble metal ions such as Au3+, Ag+, and Pt2+. They have been used to detect pH, biological species, reactive oxygen and nitrogen species, anions, and nerve agents in addition to metal ions. The probes have been engineered to undergo colorimetric or fluorometric changes upon binding to specific analytes, rendering them highly selective and sensitive by ring-opening via different mechanisms such as Photoinduced Electron Transfer (PET), Chelation Enhanced Fluorescence (CHEF), Intramolecular Charge Transfer (ICT), and Fluorescence Resonance Energy Transfer (FRET). For improved sensing performance, light-harvesting dendritic systems based on rhodamine conjugates has also been explored for enhanced sensing performance. These dendritic arrangements permit the incorporation of numerous rhodamine units, resulting in an improvement in signal amplification and sensitivity. The probes have been utilised extensively for imaging biological samples, including imaging of living cells, and for environmental research. Moreover, they have been combined into logic gates for the construction of molecular computing systems. The usage of rhodamine-based chemosensors has created significant potential in a range of disciplines, including biological and environmental sensing as well as logic gate applications. This study focuses on the work published between 2012 and 2021 and emphasises the enormous research and development potential of these probes.

Highly selective chemosensor for the sensitive detection of Hg2+ in aqueous media and its cell imaging application

The deleterious toxicity of Hg²⁺ on ecological and biological system makes it crucial for the precise monitoring of Hg²⁺. Herein, we prepared a novel "turn-on" chemosensor N'-(4-(methylthio)butan-2-ylidene) rhodamine B hydrazide (denoted as MTRH) by a simple two-step reaction. MTRH exhibited an ultra-low detection limit (LOD) in fluorescence measurement of Hg²⁺ in pure aqueous media, which was estimated to be 1.3 × 10^-9 mol·L^-1. Moreover, the proposed chemosensor holds the ability of visualizing Hg²⁺ by the distinct color change of the solution. The corresponding recognition mechanism was investigated by Job's plots, mass spectrometry and DFT calculation analysis. Importantly, the characteristics such as high sensitivity, low cytotoxicity and good biocompatibility of MTRH exhibited in the application of detecting Hg²⁺ in real water sample and bioimaging of intracellular Hg²⁺ prove that MTRH is a promising tool to evaluate the levels of Hg²⁺ in complex biological systems.

Nanoscale semiconducting polymer dots with rhodamine spirolactam as fluorescent sensor for mercury ions in living systems

Mercury ion (Hg²⁺), as one of the most poisonous heavy metal ions, could seriously damage mental and neurological functions thus causing severe diseases. A fluorescent ratiometric sensor based on semiconducting polymer dots (Pdots) and rhodamine spirolactam derivate was developed for the detection of Hg²⁺. The Pdots were prepared by Poly [(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-diphenylene-vinylene-2-methoxy-5-{2-ethylhexyloxy}-benzene)] (PDDB) with emitting strong green fluorescence. The organic fluorescence dye N-(rhodamine-B) lactam-hydrazine (RhBH), as Hg²⁺-recognizing monomer, was conjugated to the surface of Pdots. Hg²⁺ could specifically trigger ring-opening process of RhBH and thus induce strong Förster resonance energy transfer (FRET) effect, resulting in the green fluorescence decrease of Pdots (energy donor) and red emission derived from the ring-opened RhBH (energy acceptor) increasing. PDDB@RhBH showed a sensitive and reversible response toward Hg²⁺ and had a great performance on resisting interferences from various biological analytes. Additionally, both fluorescent imaging in living cells and zebrafish, and systemic toxicity analysis in rats demonstrated that PDDB@RhBH was a great potential fluorescent sensor for quantitative Hg²⁺ imaging in living systems.

Rhodamine-Based Fluorescent Probe for Highly Selective Determination of Hg2

The determination of mercuric ions (Hg²⁺) in environmental and biological samples has attracted the attention of researchers lately. In the present work, a novel turn-on Hg²⁺ fluorescent probe utilizing a rhodamine derivative had been constructed and prepared. The probe could highly sensitively and selectively sense Hg²⁺. In the presence of excessive Hg²⁺, the probe displayed about 52-fold fluorescence enhancement in 50% H₂O/CH₃CH₂OH (pH, 7.24). In the meantime, the colorless solution of the probe turned pink upon adding Hg²⁺. Upon adding mercuric ions, the probe interacted with Hg²⁺ and formed a 1:1 coordination complex, which had been the basis for recognizing Hg²⁺. The probe displayed reversible dual colorimetric and fluorescence sensing of Hg²⁺ because rhodamine's spirolactam ring opened upon adding Hg²⁺. The analytical performances of the probe for sensing Hg²⁺ were also studied. When the Hg²⁺ concentration was altered in the range of 8.0 × 10^-8 to 1.0 × 10^-5 mol L^-1, the fluorescence intensity showed an excellent linear correlation with Hg²⁺ concentration. A detection limit of 3.0 × 10^-8 mol L^-1 had been achieved. Moreover, Hg²⁺ in the water environment and A549 cells could be successfully sensed by the proposed probe.

Design, synthesis and evaluation of liver-targeting fluorescent probes for detecting mercury ions

Three fluorescent glycosyl-rhodamine probes with good selectivity and sensitivity toward Hg²⁺ were developed. The detection limit of the probes toward Hg²⁺ is as low as 94.6 nM, which can be used to detect trace Hg²⁺ in solution. 1 : 1 stoichiometry was the most possible recognition mode of the probes toward Hg²⁺, and the OFF/ON mechanism of the probes toward Hg²⁺ could be attributed to the closing or opening of the rhodamine spiral structure caused by Hg²⁺. The detection of Hg²⁺ is reversible, which is beneficial for the recycling of probes. Moreover, these low cytotoxic probes can be safely and selectively applied to monitor Hg²⁺ levels in hepatocytes, and the fluorescence response follows a trend of Rho-Gal > Rho-Lac > Rho-Glu in HepG2 cells because the galactose group in Rho-Gal can selectively recognize ASGPR overexpressed on HepG2 cells.

Rapid electrochemical quantification of trace Hg2+ using a hairpin DNA probe and quantum dot modified screen-printed gold electrodes

Rapid, simple, sensitive and specific approaches for mercury(ii) (Hg²⁺) detection are essential for toxicology assessment, environmental protection, food analysis and human health. In this study, a ratiometric hairpin DNA probe based electrochemical biosensor, which relies on hairpin DNA probes conjugated with water-soluble and carboxyl functionalized quaternary Zn–Ag–In–S quantum dot (QD) on screen-printed gold electrodes (SPGE), referred to as the HP-QDs-SPGE electrochemical biosensor in this study, was developed for Hg²⁺ detection. Based on the “turn-off” reaction of a hairpin DNA probe binding with a mismatched target and Hg²⁺ through the formation of T–Hg²⁺–T coordination, the HP-QDs-SPGE electrochemical biosensor can rapidly quantify trace Hg²⁺ with high ultrasensitivity, specificity, repeatability and reproducibility. The conformational change of the hairpin DNA probe caused a significant decrease in electrochemical intensity, which could be used for the quantification of Hg²⁺. The linear dynamic range and high sensitivity of the HP-QDs-SPGE electrochemical biosensor for the detection of Hg²⁺ was studied in vitro, with a broad linear dynamic range of 10 pM to 1 μM and detection limits of 0.11 pM. In particular, this HP-QDs-SPGE electrochemical biosensor showed excellent selectivity toward Hg²⁺ ions in the presence of other metal ions. More importantly, this biosensor has been successfully used to detect Hg²⁺ in deionized water, tap water, groundwater and urine samples with good recovery rate and small relative standard deviations. In summary, the developed HP-QDs-SPGE electrochemical biosensor exhibited promising potential for further applications in on-site analysis.

A ratiometric hairpin DNA probe based electrochemical biosensor, which relies on hairpin DNA probes conjugated with water-soluble and carboxyl functionalized quantum dot on screen-printed gold electrodes, was developed for Hg²⁺ detection.

A simple quinoline-thiophene Schiff base turn-off chemosensor for Hg2+ detection: spectroscopy, sensing properties and applications

A new Schiff base probe (QT) consisting of 8-aminoquinoline (Q) and thiophene-2-carboxaldehyde (T) moieties has been synthesized. QT undergoes chelation-enhanced fluorescence quenching when exposed to Hg²⁺ due to coordination by the sulfur and nitrogen atoms of QT thus forming a facile "turn-off" sensor. The formation of the chelation complex was confirmed by UV-visible absorption and emission spectral measurements, ¹H NMR titration and density functional theory calculations. These studies revealed that the probe exhibits high selectivity and sensitivity towards Hg²⁺ in the presence of other common metal ions. A low detection limit of 23.4 nM was determined and a Job plot confirmed a 2:1 stoichiometry between QT and Hg²⁺. The potential utility of QT as a sensor for Hg²⁺ ions in human HeLa cells was determined by confocal fluorescence microscopy, and its suitability for use in the field with environmental samples was tested with Whatman filter paper strips.

Adsorption mechanism of rhein-coated Fe3O4 as magnetic adsorbent based on low-field NMR

In the present study, a magnetic adsorbent, rhein-coated magnetic Fe₃O₄ nanoparticle (RMNP), for Pb²⁺ and Mg²⁺ had been developed, and adsorption mechanism was studied via low-field NMR. RMNP was characterized by TEM, FTIR, and XRD. RMNP could adsorb and remove Pb²⁺ and Mg²⁺ from water and was successfully applied to remove Pb²⁺ and Mg²⁺ from wastewater, with satisfactory recovery rates and high adsorption capacities. The calculated maximum adsorption capacity for Mg²⁺ and Pb²⁺ was approximately 69.3 and 64.9 mg g^-1 of RMNP, respectively, which was better than some results reported. Low-field NMR results showed that Pb²⁺ or Mg²⁺ enhanced the T₂ relaxation time of RMNP, which suggested that RMNP selectively coordinated with Pb²⁺ or Mg²⁺ and led to the aggregation of RMNP, furthermore removal of Pb²⁺ or Mg²⁺ from water. The standard curves for △T₂-cation concentration exhibited good line correlation. The linear ranges were from 4.2 × 10^-6 to 2.0 × 10^-4 mol L^-1 for Pb²⁺ and from 5.0 × 10^-6 mol L^-1 to 1.0 × 10^-4 mol L^-1 for Mg²⁺, respectively. The limits of detection were 1.4 × 10^-6 mol L^-1 for Pb²⁺ and 2.1 × 10^-6 mol L^-1 for Mg²⁺, respectively. In short, low-field NMR could clearly display the interaction between RMNP and Pb²⁺ or Mg²⁺, even be used to detect Pb²⁺ or Mg²⁺ in suitable condition. Besides, this method could be expanded to study the interaction between other magnetic adsorbents and analytes.

A novel thioctic acid-carbon dots fluorescence sensor for the detection of Hg2+ and thiophanate methyl via S-Hg affinity

Mercury ions and thiophanate methyl (TM), are common contaminants present in the environment and food products. These contaminants cause neurovirulence and carcinogenicity effect on the human body. Herein, thioctic acid-carbon dots (SCDs) was synthesized and applied in a fluorescent "turn-off-on" probe to detect Hg²⁺ and TM. The presence of other common metal ions and pesticides did not affect the response of the developed sensor. Further investigation revealed that the fluorescent "turn-off-on" model were static, wherein the "turn-off" was induced by an electron transfer effect, while the "turn-on" was caused by the formation of TM-Hg complexes. Under optimal conditions, the fluorescence sensor method exhibited limits of detection as low as 33.3 nmol/L and 7.6 nmol/L for Hg²⁺ and TM, respectively. The developed sensor was designed to detect Hg²⁺ and TM in real tap water, grape juice and Citri Reticulatae Pericarpium (CRP) water samples.

DNA-Gold Nanozyme-Modified Paper Device for Enhanced Colorimetric Detection of Mercury Ions

In this work, a paper device consisted of a patterned paper chip, wicking pads, and a base was fabricated. On the paper chip, DNA–gold nanoparticles (DNA–AuNPs) were deposited and Hg²⁺ ions could be adsorbed by the DNA–AuNPs. The formed DNA–AuNP/Hg²⁺ nanozyme could catalyze the tetramethylbenzidine (TMB)–H₂O₂ chromogenic reaction. Due to the wicking pads, a larger volume of Hg²⁺ sample could be applied to the paper device for Hg²⁺ detection and therefore the color response could be enhanced. The paper device achieved a cut-off value of 50 nM by the naked eye for Hg²⁺ under optimized conditions. Moreover, quantitative measurements could be implemented by using a desktop scanner and extracting grayscale values. A linear range of 50–2000 nM Hg²⁺ was obtained with a detection limit of 10 nM. In addition, the paper device could be applied in the detection of environmental water samples with high recoveries ranging from 85.7% to 105.6%. The paper-device-based colorimetric detection was low-cost, simple, and demonstrated high potential in real-sample applications.

Rapid detection of mercury (II) ions and water content by a new rhodamine B-based fluorescent chemosensor

A rhodamine B-based sensor (RS) was designed and synthesized by a combination of the spirolacton rhodamine B (fluorophore) and multidentate chelates (ionophore) with high affinity towards Hg²⁺. In the presence of Hg²⁺, the resulting red-orange fluorescence (under UV light) and naked eye red color of RS are supposed to be used for quantitative and qualitative measurement of Hg²⁺. Further fluorescent titration and analysis demonstrate that RS can selectively detect Hg²⁺ within 1 s with a low limit of detection (LOD) of 16 nM in acetonitrile media, meanwhile, the association constant (K_a) was calculated to be 0.32 × 10⁵ M^-1. More importantly, the resultant complex (RSHg) of RS and Hg²⁺ has also been successfully applied to detect limited water content in acetonitrile solution.

New Pyridine-Bridged Ferrocene-Rhodamine Receptor for the Multifeature Detection of Hg2+ in Water and Living Cells

A challenge in the design of optical and redox-active receptors is how to combine a specific recognition center with an efficient responsive system to facilely achieve multifeature detection in biological and environmental analyses. Herein, a novel ferrocene-rhodamine receptor conjugated with a pyridine bridge was designed and synthesized. This receptor can sensitively sense Hg²⁺ in aqueous media via chromogenic, fluorogenic, and electrochemical multisignal outputs with a low detection limit and fast response time. Moreover, it can be qualified as a fluorescent probe for effectively monitoring Hg²⁺ in living cells. A plausible recognition mode was proposed and rationalized with theoretical calculations.

A rhodamine based biocompatible chemosensor for Al3+, Cr3+ and Fe3+ ions: extraordinary fluorescence enhancement and a precursor for future chemosensors

A rhodamine based chemosensor, 3-(((2-(3',6'-bis(ethylamino)-2',7'-dimethyl-3-oxospiro[isoindoline-1,9'-xanthen]-2-yl)ethyl)imino)methyl)-2-hydroxy-5-methylbenzaldehyde (HL-CHO), has been developed for the detection of Al3+, Cr3+ and Fe3+ ions. The absorbance of HL-CHO at 528 nm increases significantly in HEPES buffer in methanol : water (9 : 1, v/v) (pH 7.4) in the presence of Al3+, Cr3+ and Fe3+ ions with the alteration of solution color from colorless to pink. The fluorescence intensity of the probe at 550 nm enhances by 1465, 588 and 800 fold in the presence of Al3+, Cr3+ and Fe3+ ions, respectively. To the best of our knowledge, this huge increase in fluorescence intensity with Al3+ and Cr3+ has not been observed for other rhodamine based chemosensing systems. The weak fluorescence and no coloration of the probe are due to the existence of a spirolactam ring. The trivalent cations induce the opening of the spirolactam ring and consequently change the color and the fluorescence intensity followed by the 1 : 1 complex formation with HL-CHO which are evident from Job's analysis, ESI mass spectral analysis and elemental analysis. The quantum yield and lifetime of HL-CHO have increased considerably in the presence of the trivalent cations. The high sensitivity of the probe towards all the cations is evident from the nM order of LOD values. This has been used in living cell imaging studies with the human neuroblastoma SH-SY5Y cell line. Having appended -CHO groups for Schiff-base condensation with other amines, HL-CHO could be a potential precursor for future chemosensors.