A nanomolar detection of mercury(II) ion by a chemodosimetric rhodamine-based sensor in an aqueous medium: Potential applications in real water samples and as paper strips

Development of a novel sensory material for rapid detection of mercury ions in various water sources: Solution and solid-state analysis

A xanthene propane nitrile-based sensor material was successfully prepared, and an attempt towards the preparation of polymer bead form was made for the sensitive or selective detection of mercury ions (Hg2+) in water. The sensor material in solution as well as in polymeric form showed amazing selectivity over other added metal ions with a naked eye color change, UV visible spectral and fluorescence spectral change, and a rapid and excellent color change from colorless to purple. The 1H NMR study exposed the probable binding site of the probe with the added mercury ion. In this study, the imine nitrogen and the C = O interact with the mercury ion, resulting in the ring opening of lactam with a vivid color change. The EDTA test was done to verify the reversible behavior of the probe and confirmed its reversibility by UV-visible and fluorescence spectral studies. The polymer bead made using this probe can be used as a tool for monitoring mercury ions in real time in different sources of water samples. The sensor molecule itself senses the mercury ion in its solid state by simple grinding and changes its color from pale yellow to deep purple. The sensor color change response is very rapid towards mercury detection, which is confirmed by the prepared test strip.

A Prompt Study on Recent Advances in the Development Of Colorimetric and Fluorescent Chemosensors for "Nanomolar Detection" of Biologically Important Analytes

Fluorescent and colorimetric chemosensors for selective detection of various biologically important analytes have been widely applied in different areas such as biology, physiology, pharmacology, and environmental sciences. The research area based on fluorescent chemosensors has been in existence for about 150 years with the development of large number of fluorescent chemosensors for selective detection of cations as metal ions, anions, reactive species, neutral molecules and different gases etc. Despite the progress made in this field, several problems and challenges still exist. The most important part of sensing is limit of detection (LOD) which is the lowest concentration that can be measured (detected) with statistical significance by means of a given analytical procedure. Although there are so many reports available for detection of millimolar to micromolar range but the development of chemosensors for the detection of analytes in nanomolar range is still a challenging task. Therefore, in our current review we have focused the history and a general overview of the development in the research of fluorescent sensors for selective detection of various analytes at nanomolar level only. The basic principles involved in the design of chemosensors for specific analytes, binding mode, photophysical properties and various directions are also covered here. Summary of physiochemical properties, mechanistic view and type of different chemosensors has been demonstrated concisely in the tabular forms.

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.

NIR squaraine dyes for dual colorimetric and fluorescent determination of Fe3+, Cu2+, and Hg2+ ions

Four benzoindolenine-based squaraine dyes (SQs), which have the advantages of intense visible and near-infrared (NIR) absorption and emission (λ_abs/max 663-695 nm, λ_em/max 686-730 nm) were synthesized and characterized by UV-vis absorption, fluorescent emission spectrophotometry, FTIR, NMR and HRMS analysis. Among them, BBSQ showed excellent performance, which exhibited high selectivity to Fe³⁺, Cu²⁺, and Hg²⁺ in acetonitrile solution even in the presence of other competitive metal ions, accompanied by obvious color change easily detected by the naked eye. The detection limit was 14.17 μM for Fe³⁺ and 6.06 μM for Cu²⁺. Most importantly, the response mechanism of BBSQ to Fe³⁺, Cu²⁺, and Hg²⁺ involves the coordination of BBSQ and metal ions through the O atom on the central squarate ring, N atom, and olefin π bond of BBSQ and has been demonstrated by Job's plot, FTIR, and ¹H NMR titration analyses. Furthermore, BBSQ was applied successfully to detect Fe³⁺, Cu²⁺, and Hg²⁺ in thin-layer chromatography (TLC) plates with good precision and is quite promising for the quantitative detection of Fe³⁺ and Cu²⁺ ions in water samples.

A Simple and Effective Visual Fluorescent Sensing Paper-Based Chip for the Ultrasensitive Detection of Mercury Ions in Environmental Water

Traces of mercury ions in environmental water can harm humans and animals. Paper-based visual detection methods have been widely developed for the rapid detection of mercury ions; however, existing methods are not sensitive enough to be used in real environments. Here, we developed a novel, simple and effective visual fluorescent sensing paper-based chip for the ultrasensitive detection of mercury ions in environmental water. CdTe-quantum-dots-modified silica nanospheres were firmly absorbed by and anchored to the fiber interspaces on the paper's surface to effectively avoid the unevenness caused by liquid evaporation. The fluorescence of quantum dots emitted at 525 nm can be selectively and efficiently quenched with mercury ions, and the ultrasensitive visual fluorescence sensing results attained using this principle can be captured using a smartphone camera. This method has a detection limit of 2.83 µg/L and a fast response time (90 s). We successfully achieved the trace spiking detection of seawater (from three regions), lake water, river water and tap water with recoveries in the range of 96.8-105.4% using this method. This method is effective, low-cost, user-friendly and has good prospects for commercial application. Additionally, the work is expected to be utilized in the automated big data collection of large numbers of environmental samples.

A Recent Update on Rhodamine Dye Based Sensor Molecules: A Review

Herein we have discussed such important modified rhodamine compounds which have been used as chemosensors for the last 7-8 years. This review covered some chemosensors for the detection of metal ions like Al(III), Cu(II), Hg(II), Co(II), Fe(III), Au(III), Cr(III), and some anion like CN-. The selectivity, sensitivity, photophysical properties (i.e., UV-Vis spectral studies, fluorescence studies giving special emphasis to absorption wavelength in UV-Vis spectra and excitation and emission wavelength in fluorescence spectra), binding affinity, the limit of detection, and the application of those chemosensors are described clearly. Here we have also discussed some functionalized rhodamine-based chemosensors that emit in the near-infrared region (NIR) and can target lysosomes and detect lysosomal pH. Their versatile applicability in the medicinal ground is also delineated. We have focused on the photophysical properties of spirolactam rhodamine photoswitches and applications in single-molecule localization microscopy and volumetric 3D light photoactivable dye displays. The real-time detection of radical intermediates has also been exemplified.

Vinyl-functionalized polyphenolic-carbon dot-based fluorometric turn-off-on biosensor for the dual detection of mercury and cysteine and their in vivo sensing in zebrafish larvae

The fluorometric turn-off-on biosensor was developed for the ultra-sensitive detection of mercury (Hg²⁺) and cysteine (Cys) utilizing the highly fluorescent carbon dots (CDs). Herein, the sophisticated low-temperature reflux-mediated reaction was adopted using precursors namely citric acid (CA) and polyphenolic kaempferol (KMP) by using dimethylformamide (DMF) as a solvent. The resulting CDs (i.e., CKCDs) were in the highly negative charged groups (-OH) presented with a bright-orange fluorescence. These CKCDs were functionalized with 4-vinylaniline (4-VA) by employing EDC/NHS coupling reaction, which switched its photoluminescence (PL) towards the strong-blue colored emission and termed as V-CKCDs. The functionalized V-CKCDs can be capable enough to detect mercury via the strong electrostatic interactions between positively charged Hg²⁺ cations and negatively charged anions (-OH groups). Hence, an adequate fluorescence quenching was observed in V-CKCDs with the lowest concentrations of Hg²⁺ around 0.5 μM. Significantly, after adding the complex of V-CKCDs-Hg²⁺ to the Cys, the fluorescence enhancement was observed. This might be attributed from the strong interactions between Hg²⁺ in the fluorescence sensing system and thiol (-SH) moieties from the Cys. The developed V-CKCDs are highly sensitive for detecting Hg²⁺ and Cys, which showed detection limits of 10.6 and 42. 48 nM, respectively. Also, the in vivo studies were investigated in zebrafish larvae using V-CKCDs for the detection of Hg²⁺ and Cys. The V-CKCDs were investigated in the real water samples and human serum to detect Hg²⁺ and Cys, respectively.

Asymmetric Nanochannel Network-Based Bipolar Ionic Diode for Enhanced Heavy Metal Ion Detection

A higher rectification degree in ionic diodes is required to achieve better performance in applications. Nonetheless, the active geometrical change that is critical for inducing electrical potential asymmetry is difficult to realize in typical ionic diodes because of the intrinsic limitation of the fabrication method. Here, we propose a nanochannel-network-based bipolar diode with a high rectification degree of ∼1600─the highest value realized until now, to the best of our knowledge. Such a high rectification is obtained based on the synergetic effect of the bipolar surface charge and the optimization of the microchannel through experimental studies and multiphysics numerical simulations. It induces ion concentrations at the heterogeneous junction based on the accumulation effect under the forward potential bias. In particular, this proposed molecular concentration occurs in the ohmic region without vortex and instability that is inevitable at the conventional nano-electrokinetic concentration. Combining this accumulation with the horizontally aligned configuration of the nanochannel network membrane (NCNM), a highly sensitive and quantitative mercury ion (Hg²⁺) sensor based on a fluorescent signal is fabricated that allows direct measurement using a general fluorescent microscope. The detection limit of Hg²⁺ is 10 pM, which is ∼10 times lower than the best detection limit realized so far (∼100 pM) in fluorescent dye-based detection. This demonstrates the potential of asymmetric NCNM for high-performance ion transport in applications such as energy conversion, based on its design and material flexibility.

Statistical optimization of silver nanoparticle synthesis by green tea extract and its efficacy on colorimetric detection of mercury from industrial waste water

For the optimization of silver nanoparticle production, a central composite design was used with three parameters: AgNO₃ concentration, green tea extract concentration, and temperature at three different levels. The size of the synthesized silver nanoparticle, its UV absorbance, zeta potential, and polydispersity index were set as the response parameters. Silver nanoparticles obtained in the optimization process were characterized and its efficacy on colorimetric detection of mercury was evaluated. The response variables were significant for the factors analyzed, and each variable had a significant model (P < 0.05). The ideal conditions were: 1 mM AgNO₃, 0.5% green tea extract, and 80 °C temperature. To analyze the produced AgNPs under certain ideal conditions, Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used. The UV-visible spectra of AgNPs revealed an absorption maxima at 424 nm. The XRD pattern reveals a significant diffraction peak at 38.25°, 44.26°, 64.43°, and 77.49°, which corresponds to the (111), (200), (220), and (311) planes of polycrystalline face-centered cubic (fcc) silver, respectively. The TEM and SEM analyses confirmed that the particles were spherical, and dynamic light scattering study determined the average diameter of AgNPs to be 77.4 nm. The AgNPs have a zeta potential of -62.6 mV, as determined by the zeta sizer analysis. The AgNPs detects mercury at a micromolar concentration. Furthermore, the environmentally friendly generated AgNPs were used to detect mercury in a colorimetric method that was effectively employed for analytical detection of Hg²⁺ ions in an aqueous environment for the purpose of practical application.

Simple easy to make xanthene based optical probe for solid and liquid state Hg2+ ion detection

An easy to make xanthene based optical probe synthesized, precise recognition towards mercury ion been achieved by the probe RP and can detect Hg²⁺ effectively in both for solid and liquid state with a vivid color change. The other tested ion showed no interference, visual and instrumental methods confirms the probe selectivity. Stoichiometry (1:1) confirmed by job's plot, plausible binding of Hg²⁺ ion with the probe confirmed by mass and NMR studies. Test strip prepared for the prompt onsite detection in aqueous medium with outstanding color variation in daylight.

Paper-Based Analytical Devices for Colorimetric and Luminescent Detection of Mercury in Waters: An Overview

Lab-on-paper technologies, also known as paper-based analytical devices (PADs), have received increasing attention in the last years, and nowadays, their use has spread to virtually every application area, i.e., medical diagnostic, food safety, environmental monitoring, etc. Advantages inherent to on-field detection, which include avoiding sampling, sample preparation and conventional instrumentation in central labs, are undoubtedly driving many developments in this area. Heavy metals represent an important group of environmental pollutants that require strict controls due to the threat they pose to ecosystems and human health. In this overview, the development of PADs for Hg monitoring, which is considered the most toxic metal in the environment, is addressed. The main emphasis is placed on recognition elements (i.e., organic chromophores/fluorophores, plasmonic nanoparticles, inorganic quantum dots, carbon quantum dots, metal nanoclusters, etc.) employed to provide suitable selectivity and sensitivity. The performance of both microfluidic paper-based analytical devices and paper-based sensors using signal readout by colorimetry and luminescence will be discussed.

A simple organic probe for ratiometric fluorescent detection of Zn(II), Cd(II) and Hg(II) ions in aqueous solution via varying emission colours to distinguish one another

A bis (thiosemicarbazone) based probe has been synthesized and structurally characterized. The probe exhibits good selectivity towards Zn(II), Cd(II) and Hg(II) ions in an aqueous solution containing 95% water with ratiometric fluorescence changes. The modes of coordination of the probe with these metal ions and binding properties have been examined using different spectral techniques. The binding constants, determined using fluorescence titration data, are found to be 9.8 × 10³, 1.39 × 10⁵ and 2.03 × 10¹³ M^-1, respectively for Zn(II), Cd(II) and Hg(II) complexes. The high sensitivity of the probe has been demonstrated by the very low limit of detection i.e. 5.1, 3.4 and 0.51 μM for Zn(II), Cd(II) and Hg(II) ions, respectively. Different coordination mode of these metal ions with the probe has resulted in varying intra-ligand fluorescence (λ_em nm, Zn(II): 488, Cd(II): 470 and Hg(II): 578) among these metal complexes.

A novel rhodamine-based optical probe for mercury(II) ion in aqueous medium: A nanomolar detection, wide pH range and real water sample application

In present work, we designed and synthesized new chemosensor RPy, containing the rhodamine and 2,6-pyridinedicarboxaldehyde functionality, for the selective detection of mercury (II) (Hg²⁺) ion in aqueous DMSO solvents. The RPy acts as "turn ON" probe for Hg²⁺ ion with high selectivity and sensitivity over the series of other competing metal ions based on colorimetric and fluorimetric techniques. Due to the incorporation of two rhodamine moieties enhance the chelation sites for mercury binding, which reflects in the lowering of the detection limit up to 26 nM. The Job plot method confirms the 1:2 stoichiometric interactions between the RPy and Hg²⁺ ion. The formation of the chelation complex between RPy and Hg²⁺ ion with spirolactam ring opening was thoroughly investigated by absorption, emission, ¹H NMR, and mass analysis. The detection of Hg²⁺ ion by RPy is retained at broad pH range 4-9. Further, the probe RPy is successfully explored to measure the contamination of Hg²⁺ ion in the real water samples using spike and recovery method.

Cp*Co(iii)-catalyzed annulation of azines by C–H/N–N bond activation for the synthesis of isoquinolines

An atom economic and external oxidant free methodology has been developed for the synthesis of isoquinolines by Cp*Co(iii) catalyzed annulation of azines via C–H/N–N activation.