New portable smartphone-based PDMS microfluidic kit for the simultaneous colorimetric detection of arsenic and mercury

A smartphone-based microfluidic platform was developed for point-of-care (POC) detection using surface plasmon resonance (SPR) of gold nanoparticles (GNPs). The simultaneous colorimetric detection of trace arsenic and mercury ions (As³⁺ and Hg²⁺) was performed using a new image processing application (app). To achieve this goal, a microfluidic kit was fabricated using a polydimethylsiloxane (PDMS) substrate with the configuration of two separated sensing regions for the quantitative measurement of the color changes in GNPs to blue/gray. To fabricate the microfluidic kit, a Plexiglas mold was cut using a laser based on the model obtained from AutoCAD and Comsol outputs. The colorimetric signals originated from the formation of nanoparticle aggregates through the interaction of GNPs with dithiothreitol – 10,12-pentacosadiynoic acid (DTT-PCDA) and lysine (Lys) in the presence of As³⁺ and Hg²⁺ ions. This assembly exhibited the advantages of simplicity, low cost, and high portability along with a low volume of reagents and multiplex detection. Heavy Metals Detector (HMD), as a new app for the RGB reader, was programmed for an Android smartphone to quantify colorimetric analyses. Compared with traditional image processing, this app provided significant improvements in sensitivity, time of analysis, and simplicity because the color intensity is measured through a new normalization equation by converting RGB to an Integer system. As a simple, real-time, and portable analytical kit, the fabricated sensor could detect low concentrations of As³⁺ (710 to 1278 μg L⁻¹) and Hg²⁺ (10.77 to 53.86 μg L⁻¹) ions in water samples at ambient conditions.

A smartphone-based microfluidic platform was developed for point-of-care (POC) detection using surface plasmon resonance (SPR) of gold nanoparticles (GNPs).     

Sulfhydryl functionalized carbon quantum dots as a turn-off fluorescent probe for sensitive detection of Hg2+

Mercury ion (Hg²⁺) is one of the most toxic heavy metal ions and lowering the detection limit of Hg²⁺ is always a challenge in analytical chemistry and environmental analysis. In this work, sulfhydryl functionalized carbon quantum dots (HS-CQDs) were synthesized through a one-pot hydrothermal method. The obtained HS-CQDs were able to detect mercury ions Hg²⁺ rapidly and sensitively through fluorescence quenching, which may be ascribed to the formation of nonfluorescent ground-state complexes and electron transfer reaction between HS-CQDs and Hg²⁺. A modification of the HS-CQD surface by –SH was confirmed using Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The HS-CQDs sensing system obtained a good linear relationship over a Hg²⁺ concentration ranging from 0.45 μM to 2.1 μM with a detection limit of 12 nM. Delightfully, the sensor has been successfully used to detect Hg²⁺ in real samples with satisfactory results. This means that the sensor has the potential to be used for testing actual samples.

Schematic presentation of the synthesis of HS-CQDs and the application as a “turn-off” fluorescent probe for Hg²⁺ detection.     

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.     

Gram-scale synthesis of nitrogen doped graphene quantum dots for sensitive detection of mercury ions and l-cysteine

Sensitive and reliable detection of mercury ions (Hg²⁺) and l-cysteine (l-Cys) is of great significance for toxicology assessment, environmental protection, food analysis and human health. Herein, we present gram-scale synthesis of nitrogen doped graphene quantum dots (N-GQDs) for sensitive detection of Hg²⁺ and l-Cys. The N-GQDs are one-step synthesized using bottom-up molecular fusion in a hydrothermal process with gram-scale yield at a single run. N-GQDs exhibit good structural characteristics including uniform size (∼2.1 nm), high crystallinity, and single-layered graphene thickness. Successful doping of N atom enables bright blue fluorescence (absolute photoluminescence quantum yield of 24.8%) and provides unique selectivity towards Hg²⁺. Based on the fluorescence quenching by Hg²⁺ (turn-off mode), N-GQDs are able to serve as an effective fluorescent probe for sensitive detection of Hg²⁺ with low limit of detection (19 nM). As l-Cys could recover the fluorescence of N-GQDs quenched by Hg²⁺, fluorescent detection of l-Cys is also demonstrated using turn-off-on mode.

One-step and gram-scale synthesis of nitrogen doped graphene quantum dots is realized for their sensitive detection of Hg²⁺ and l-Cys.     

A near-infrared fluorescent probe with an improved Stokes shift achieved by tuning the donor–acceptor–donor character of the rhodamine skeleton and its applications

In this paper, we report a novel near-infrared (NIR) mitochondrion-targeted fluorescent probe, RQS, with an improved Stokes shift (96 nm) for the specific detection of mitochondrial mercury ion (Hg²⁺) because mitochondrion is one of the main targeted organelles of Hg²⁺. For the preparation of the probe, a novel asymmetrical fluorescent xanthene dye RQ was first synthesized by tuning the donor–acceptor–donor (D–A–D) character of the rhodamine skeleton, and then the probe RQS was constructed by the mechanism of mercury-promoted ring-opening reaction. As expected, RQS could be used for the specific detection of Hg²⁺ with high selectivity, high sensitivity, and a detection limit down to the nanomolar range (2 nM). Importantly, RQS is capable of specifically distributing in mitochondria, and thus detect Hg²⁺ in real-time and provided a potential tool for studying the cytotoxic mechanisms of Hg²⁺.

A novel mitochondrion-targeting Hg²⁺ probe, RQS, with NIR emission (680 nm) and a large Stokes shift (96 nm) was developed by tuning the D–A–D character of the rhodamine skeleton.     

Plasmonic nanoprobes based on the shape transition of Au/Ag core–shell nanorods to dumbbells for sensitive Hg-ion detection

We report a sensitive and selective localized surface plasmon resonance (LSPR) nanoprobe for the detection of mercuric ions (Hg²⁺) using gold/silver core–shell nanorods as an optical nanosubstrate. Sulfide can quickly react with silver atoms to generate Ag₂S at room temperature in the presence of oxygen. The transformation from Ag shell to Ag₂S on the nanorod surface results in its LSPR absorption band shifting to a longer wavelength, which is attributed to their different refractive indices. Interestingly, the morphology also changed from a rod-like to dumbbell shape. However, in the presence of Hg²⁺, this morphology transformation is inhibited because the sulfide reacts with free Hg²⁺ prior to the Ag atoms. The amount of Ag₂S reduced with the increasing concentration of Hg²⁺, and the absorption band shift was also decreased. According to this “rod-like to dumbbell or not” shape change, a sensitive and selective LSPR nanoprobe was established, assisted by UV-Vis absorption spectroscopy. The detection limit of this probe for Hg²⁺ was as low as 13 nM. The efficiency of this probe in complex samples was evaluated by the detection of Hg²⁺ in spiked water samples.

Sensitive plasmonic nanoprobes for the sensitive detection of mercury ions based on a “rod-like to dumbbell or not” morphology transition of the Au/Ag core–shell hybrid nanorods.     

Colorimetric screening of elevated urinary mercury levels by a novel Hg2+-selective probe of resorufin phosphinothioate

Urinary mercury levels are the most reliable indicators of mercury exposure but identifying them requires complex techniques and heavy instruments. In this research, we reported a simple and convenient urinary mercury analysis method using a readily available office scanner. Probe MP-1 synthesized by the reaction of resorufin and dimethylthiophosphinoyl chloride revealed Hg²⁺-selective chromogenic and fluorescent signaling behavior. Signaling was realized through Hg²⁺-induced deprotection of the phosphinothioate protecting group in the resorufin-based probe MP-1 to yield the parent fluorochrome. A pronounced colorimetric response of color change from light yellow to pink alongside a turn-on type fluorescence enhancement was perceived exclusively toward Hg²⁺ ions over other metal ions and anions. The colorimetry provided a more advantageous ratiometric approach than the simple fluorometric analysis exhibiting an off–on type response, with a detection limit of 12 nM (2.4 ppb). The Hg²⁺ signaling of the MP-1 probe was not disturbed by the presence of coexisting metal ions and anions. The sensitive and convenient diagnosis of clinically important neurological symptoms and fatal inorganic mercury levels in urine was successfully demonstrated using a standard office scanner.

A Hg²⁺ selective signaling probe, resorufin phosphinothioate, for the colorimetric diagnosis of clinically elevated mercury levels in urine samples using an office scanner was developed.     

A glass nanopore ionic sensor for surface charge analysis

Surface charge-based nanopore characterization techniques unfold unique properties and provide a powerful platform for a variety of sensing applications. In this paper, we have proposed a nanoconfined inner wall surface charge characterization method with glass nanopores. The glass nanopores were functionalized with DNA aptamers that were designed for mercury (Hg²⁺) ion immobilization by forming thymine–Hg²⁺–thymine structures. The surface charge of the nanopores was modulated by surface chemistry and Hg²⁺ ion concentrations and analysed by combining zeta potential measurements on glass slides and the ionic current rectification ratio of the nanopores. Also, 1 pM Hg²⁺ ions could be detected by the nanopores.

Surface charge-based nanopore characterization techniques unfold unique properties and provide a powerful platform for a variety of sensing applications.     

A facile one-pot green synthesis of gold nanoparticle-graphene-PEDOT:PSS nanocomposite for selective electrochemical detection of dopamine

A facile one-pot and green method was developed to prepare a nanocomposite of gold nanoparticle (AuNP), graphene (GP) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). Graphene was first electro-exfoliated in a polystyrene sulfonate solution, followed by a one-step simultaneous in situ formation of gold nanoparticle and PEDOT. The as-synthesized aqueous dispersion of AuNP-GP-PEDOT:PSS was thereafter used to modify the glassy carbon electrode (GCE). For the first time, the quaternary composite between AuNP, GP, PEDOT and PSS was used for selective determination of dopamine (DA) and uric acid (UA) in the presence of ascorbic acid (AA). In comparison to a bare GCE, the nanocomposite electrode shows considerably higher electrocatalytic activities toward the oxidation of DA and UA due to a synergistic effect between AuNP, GP, PEDOT and PSS. Using differential pulse voltammetry (DPV), selective determination of DA and UA in the presence of AA could be achieved with a peak potential separation of 110 mV between DA and UA. The sensor exhibits wide linear responses for DA and UA in the ranges of 1 nM to 300 μM and 10 μM to 1 mM with detection limits (S/N = 3) of 100 pM and 10 μM, respectively. Furthermore, the proposed sensor was also successfully used to determine DA in a real pharmaceutical injection sample as well as DA and UA in human serum with satisfactory recovery results.

A facile one-pot green synthesis of gold nanoparticle-graphene-PEDOT:PSS nanocomposite was successfully demonstrated.     

On-demand radiosynthesis of N-succinimidyl-4-[18F]fluorobenzoate ([18F]SFB) on an electrowetting-on-dielectric microfluidic chip for 18F-labeling of protein

An all-electronic, droplet-based batch microfluidic device, operated using the electrowetting on dielectric (EWOD) mechanism was developed for on-demand synthesis of N-succinimidyl-4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB), the most commonly used ¹⁸F-prosthetic group for biomolecule labeling. In order to facilitate the development of peptides, and proteins as new diagnostic and therapeutic agents, we have diversified the compact EWOD microfluidic platform to perform the three-step radiosynthesis of [¹⁸F]SFB starting from the no carrier added [¹⁸F]fluoride ion. In this report, we established an optimal microliter droplet reaction condition to obtain reliable yields and synthesized [¹⁸F]SFB with sufficient radioactivity for subsequent conjugation to the anti-PSCA cys-diabody (A2cDb) and for small animal imaging. The three-step, one-pot radiosynthesis of [¹⁸F]SFB radiochemistry was adapted to a batch microfluidic platform with a reaction droplet sandwiched between two parallel plates of an EWOD chip, and optimized. Specifically, the ratio of precursor to base, droplet volume, reagent concentration, reaction time, and evaporation time were found be to be critical parameters. [¹⁸F]SFB was successfully synthesized on the EWOD chip in 39 ± 7% (n = 4) radiochemical yield in a total synthesis time of ∼120 min ([¹⁸F]fluoride activation, [¹⁸F]fluorination, hydrolysis, and coupling reaction, HPLC purification, drying and reformulation). The reformulation and stabilization step for [¹⁸F]SFB was important to obtain a high protein labeling efficiency of 33.1 ± 12.5% (n = 3). A small-animal immunoPET pilot study demonstrated that the [¹⁸F]SFB-PSCA diabody conjugate showed specific uptake in the PSCA-positive human prostate cancer xenograft. The successful development of a compact footprint of the EWOD radiosynthesizer has the potential to empower biologists to produce PET probes of interest themselves in a standard laboratory.

An all-electronic, droplet-based batch microfluidic device, operated using the electrowetting on dielectric (EWOD) mechanism was developed for on-demand synthesis of acommonly used ¹⁸F-prosthetic group for biomolecule labeling.     

Ion specific effects on the immobilisation of charged gold nanoparticles on metal surfaces

Since the pioneering work of F. Hofmeister, Arch. Exp. Pathol. Pharmakol., 1888, 24, 247, ion specific effects have been steadily reported in the context of colloidal or protein stabilisation in electrolyte solutions. Although the observed effects are omnipresent in chemistry and biology, their origin is still under ferocious discussion. Here, we report on ion specific effects affecting the self-assembly of amine and carboxylic acid functionalised gold nanoparticles on metal surfaces as well as in electrolyte solution as a function of the monovalent cations Li⁺, Na⁺, K⁺ and Cs⁺. Mercaptooctanoic acid and 1,8-amine-octanethiol functionalised gold nanoparticles were adsorbed on structured AuPd/Pt substrates under addition of the respective chloride salts. Furthermore, the influence of the same salts on the salt induced aggregation of these AuNP was investigated. Our results demonstrate that the assembly processes on the metal surface as well as in electrolyte solution are influenced by the addition of different cations. We attribute the observed effects to ion pairing of the functional end groups with the added cations. With these findings we introduce a new parameter to control the self-assembly of 2D AuNP arrays on solid supports or of 3D AuNP networks in solution, which could be of relevance for the fabrication of new tailor-made functional materials or for biomedical applications.

Different monovalent cations influence the immobilisation of carboxylic acid and amine terminated gold nanoparticles.     

Sensitive distance-based paper-based quantification of mercury ions using carbon nanodots and heating-based preconcentration

This article reports the first fluorescent distance-based paper device coupled with an evaporating preconcentration system for determining trace mercury ions (Hg²⁺) in water. The fluorescent nitrogen-doped carbon dots (NCDs) were synthesized by a one-step microwave method using citric acid and ethylenediamine. The fluorescence turn-off of the NCDs in the presence of Hg²⁺ was visualized with a common black light, and the distance of the quenched fluorescence correlated to Hg²⁺ concentration. The optimal conditions for pH, NCD concentration, sample volume, and reaction time were investigated. Heating preconcentration was used to improve the detection limits of the fluorescent distance-based paper device by a factor of 100. Under the optimal conditions, the naked eye limit of detection (LOD) was 5 μg L⁻¹ Hg²⁺. This LOD is sufficient for monitoring drinking water where the maximum allowable mercury level is 6 μg L⁻¹ as established by the World Health Organization (WHO). The fluorescent distance-based paper device was successfully applied for Hg²⁺ quantification in water samples without interference from other cations. The proposed method provides several advantages over atomic absorption spectroscopy including ease of use, inexpensive material and fabrication, and portability. In addition, the devices are simple to fabricate and have a long shelf-life (>5 months).

This article reports the first fluorescent distance-based paper device coupled with an evaporating preconcentration system for determining trace mercury ions (Hg²⁺) in water.     

Highly sensitive and selective colorimetric detection of dual metal ions (Hg2+ and Sn2+) in water: an eco-friendly approach

Application of an alliin-based precursor for the synthesis of silver nanoparticles (Ag NPs) which is an emerging, reliable and rapid sensor of heavy metal ion contaminants in water is reported here. The Ag NPs were characterized by using UV-visible spectroscopy, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy analysis techniques. The Ag NPs simultaneously and selectively detect Hg²⁺ and Sn²⁺ ions from aqueous solution. The sensitivity and selectivity of the prepared Ag NPs towards other representative transition-metal ions, alkali metal ions and alkaline earth metal ions were also studied. For more precise evidence, a density functional theory study was carried out to understand the possible mechanism and interaction in the detection of Hg²⁺ and Sn²⁺ by Ag NPs. The limits of detection for Hg²⁺ and Sn²⁺ ions were found as 15.7 nM and 11.25 nM, respectively. This assay indicates the possible use of garlic extract-synthesized Ag NPs for sensing Hg²⁺ and Sn²⁺ in aqueous solution very significantly. So, the simple, green, eco-friendly and easy method to detect the dual metal ions may further lead to a potential sensor of heavy metal ion contaminants in water of industrial importance.

Synthesis of Ag nanoparticles using Allin based garlic extract for highly sensitive and selective detection of metal ions Hg²⁺ and Sn²⁺ in water. The limit of detection (LoD) for Hg²⁺ and Sn²⁺ ions were found as 15.7 nM and 11.25 nM respectively.     

Interaction effects on a gold nanoparticle-based colorimetric assay for antioxidant capacity evaluation of polyphenols

A gold nanoparticle (AuNP)-based colorimetric assays have been applied to evaluate the total antioxidant capacities (TAC) of various food samples, while the potential endogenous polyphenol interaction effects on the AuNP assays are less explored. In this work, 12 representative polyphenols were strategically selected, and their corresponding AuNPs were synthesized and characterized by UV-Vis spectroscopy and transmission electron microscopy, which conformed the formation of AuNPs. Then, the TAC of the polyphenols, alone and in their equimolar binary or ternary combinations, were estimated and their synergistic effects were evaluated. In addition, the matrix effects of endogenous components from 6 kinds of foods on the recovery of the TAC estimation of gallic acid and rutin were evaluated. Markedly stronger interaction effects and matrix effects were observed in the AuNP assay than in conventionally well-established methods, indicating that the validation and application of the AuNP assay for TAC evaluation should be fully studied and explored.

Interaction effects of endogenous polyphenols remarkably influenced the performance of gold nanoparticle-based colorimetric assay for antioxidant capacity evaluation.     

A gold nanoparticle-single-chain fragment variable antibody as an immunoprobe for rapid detection of morphine by dipstick

Gold nanoparticle (AuNP)-based optical assays are of significant interest since the molecular phenomenon can be examined easily with change in the color of AuNPs. Herein, we report the development of a dipstick using a AuNP-labeled single-chain fragment variable (scFv) antibody for the detection of morphine. The scFv antibodies for morphine were developed using phage display-based antibody library. Immunoglobulin variable regions of heavy (V_H)- and light (V_L)-chain genes were connected via a glycine–serine linker isolated from murine immune repertoire and cloned into the expression vector pIT2. The scFv was produced in Escherichia coli HB2151, yielding a functional protein with a molecular weight of approximately 32 kDa. The morphine scFv was labeled with gold nanoparticles and used as an optical immunoprobe in a dipstick. The competitive dipstick assay characterized the ability of the scFv antibody to recognize free morphine. The detection range was 1–1000 ng mL⁻¹ with a limit of detection (LOD) of 5 ng mL⁻¹ under optimal conditions, and the IC₅₀ value was 14 ng mL⁻¹ for morphine. The developed optical dipstick kit of scFv antibody was capable of specifically binding to free morphine and its analogs in a solution in less than 5 min and could be useful for on-site screening of a real sample in blood, urine, and saliva.

Dipstick device developed on the principle of lateral flow using gold nanoparticles for analysis of morphine in urine by morphine/scFv/immunoprobe.     

New sensing platform of poly(ester-urethane)urea doped with gold nanoparticles for rapid detection of mercury ions in fish tissue

A new electrochemical sensor has been fabricated based on the in situ synthesis of poly(ester-urethane) urea (PUU) doped with gold nanoparticles (AuNPs), and the obtained composite materials (PUU/AuNPs) were used as a new sensing platform for highly sensitive and selective detection of mercury(II) ions in fish tissue. PUU was synthesized and fully characterized by XRD, TGA, DSC, and FTIR to analyze the chemical structure, thermal stability, and morphological properties. As a polymeric structure, the PUU consists of urethane and urea groups that possess pronounced binding abilities to Hg²⁺ ions. SEM-EDX was carried out to confirm this kind of interaction. Using ferricyanide as the redox probe, PUU alone exhibited weak electrochemical signals due to its low electrical conductivity. Therefore, a new series of nanocomposites of PUU with different nanostructured materials were applied, and their electrochemical performances were evaluated. Among these materials, the PUU/AuNP-modified electrode showed high voltammetric signals towards Hg²⁺. Consequently, the parameters affecting the performance of the assay, such as electrode composition, scan rate, and sensing time, as well as the effect of electrolyte and pH were studied and optimized. The sensor showed a linear range of 5 ng mL⁻¹ to 155 ng mL⁻¹ with the regression coefficient R² = 0.986, while the calculated values of the limit of detection (LOD) and limit of quantification (LOQ) were 0.235 ng mL⁻¹ and 0.710 ng mL⁻¹, respectively. In terms of cross reactivity testing, the sensor exhibited a high selectivity against heavy metals which are commonly determined in seafood (Cd²⁺, Pb²⁺, As³⁺, Cr³⁺, Mg²⁺, and Cu²⁺). For real applications, total Hg²⁺ ions in fish tissue were determined with very high recovery and no prior complicated treatments.

A new electrochemical sensor based on poly(ester-urethane) urea doped with gold nanoparticles (PUU/AuNPs) for highly sensitive and selective detection of mercury(ii) ions in fish tissue.     

A dual colorimetric probe for rapid environmental monitoring of Hg2+ and As3+ using gold nanoparticles functionalized with d-penicillamine

A highly sensitive and selective colorimetric assay for the dual detection of Hg²⁺ and As³⁺ using gold nanoparticles (AuNPs) conjugated with d-penicillamine (DPL) was developed. When Hg²⁺ and As³⁺ ions coordinate with AuNP-bound DPLs, the interparticle distance decreases, inducing aggregation; this results in a significant color change from wine red to dark midnight blue. The Hg4f and As3d signals in the X-ray photoelectron spectra of Hg²⁺ (As³⁺)-DPL-AuNPs presented binding energies indicative of Hg²⁺–N(O) and As³⁺–N(O) bonds, and the molecular fragment observed in time-of-flight secondary ion mass spectra confirmed that Hg²⁺ and As³⁺ coordinated with two oxygen and two nitrogen atoms in DPL. The detection of Hg²⁺ and As³⁺ can be accomplished by observing the color change with the naked eye or by photometric methods, and this was optimized to provide optimal probe sensitivity. The assay method can be applied for environmental monitoring by first selectively quantifying Hg²⁺ in water samples at pH 6, then estimating the As³⁺ concentration at pH 4.5. The efficiency of the DPL-AuNP probe was evaluated for the sequential quantification of Hg²⁺ and As³⁺ in tap, pond, waste, and river water samples, and absorbance ratios (A₇₃₀/A₅₂₅) were correlated with Hg²⁺ and As³⁺ concentrations in the linear range of 0–1.4 μM. The limits of detection in water samples were found to be 0.5 and 0.7 nM for Hg²⁺ and As³⁺, respectively. This novel probe can be utilized for the dual determination of Hg²⁺ and As³⁺, even in the presence of interfering substances in environmental samples.

A highly sensitive and selective colorimetric assay for the dual detection of Hg²⁺ and As³⁺ using gold nanoparticles (AuNPs) conjugated with d-penicillamine (DPL) was developed.     

A ratiometric and colorimetric probe for detecting Hg2+ based on naphthalimide–rhodamine and its staining function in cell imaging

In this work, a rhodamine derivative was developed as a colorimetric and ratiometric fluorescent probe for Hg²⁺. It exhibited a highly sensitive fluorescence response toward Hg²⁺. Importantly, studies revealed that the probe could be used for ratiometric detection of Hg²⁺, with a low detection limit of 0.679 μM. The mechanism of Hg²⁺ detection using compound 1 was confirmed by ESI-MS, ¹H NMR, and HPLC. Upon the addition of Hg²⁺, the rhodamine receptor was induced to be in the ring-opening form via an Hg²⁺-promoted hydrolysis of rhodamine hydrazide to rhodamine acid. In addition to Hg²⁺ detection, the naphthalimide–rhodamine compound was proven to be effective in cell imaging.

A new probe based on naphthalimide–rhodamine was applied in recognition of Hg²⁺ by a FRET mechanism.     

1,3,4-Thiadiazol derivative functionalized-Fe3O4@SiO2 nanocomposites as a fluorescent probe for detection of Hg2+ in water samples

5-Amino-1,3,4-thiadiazole-2-thiol was used to synthesize a novel fluorescent functionalizing group on a Fe₃O₄@SiO₂ magnetic nanocomposite surface for detection of heavy metal ions in water samples. The prepared probe was characterized by using X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, and a vibrating sample magnetometer. Among various tested ions, the new nanocomposite responded to Hg²⁺ ions with an intense fluorescence “turn-off”. The limit of detection of the probe shows that it is sensitive to the minimum Hg²⁺ concentration of 48.7 nM. Theoretical calculations were done for estimating binding energies of the three possible bonding modes and the visualized molecular orbitals were presented.

VBYT-Fe₃O₄@SiO₂ fluorescent probe was designed for sensitive detection of mercury in water samples.     

Ultrasmall aqueous starch-capped CuS quantum dots with tunable localized surface plasmon resonance and composition for the selective and sensitive detection of mercury(ii) ions

Ultrasmall starch-capped CuS quantum dots (QDs) with controllable size were chemically fabricated in an aqueous medium. The phase of the CuS QDs was confirmed via X-ray diffraction (XRD), whereas the characteristic localized surface plasmon resonance (LSPR) peak in the near-infrared (NIR) region was measured using UV-Vis spectroscopy. Transmission electron microscopy and high bandgap analysis confirmed the formation of ultrasmall CuS QDs in the size range of 4–8 nm. CuS QDs have been used for the selective and sensitive detection of Hg²⁺ ions through colorimetric and spectroscopic techniques. The selective sensing of Hg²⁺ ions from various metal ions was detected via a remarkable change in color, damping in LSPR intensity, significant change in the Fourier-transform infrared spectra and X-ray photoelectron spectroscopic measurements. The mechanism of interaction between the CuS QDs and Hg²⁺ ions has been deeply explored in terms of the role played by the starch and the reorganization of sulfide and disulfide bonds to facilitate the access of Hg²⁺ ions into the CuS lattice. Finally, an intermediate Cu_2−xHg_xS nanostructure resulted in the leaching of Cu⁺ ions into the solution, which were further recovered and reused for the formation of fluorescent Cu₂S nanoparticles. Thus, the entire process of synthesis, sensing and reuse paves the way for sustainable nanotechnology.

Colorimetric response of CuS QDs with mercury ions.