Mechanism of mercury detection based on interaction of single-strand DNA and hybridized DNA with gold nanoparticles

Mechanisms of interaction of single-strand DNA and hybridized DNA on gold nanoparticles in the presence of Hg²⁺ was studied in this work. Recently the detection of Hg²⁺ using unmodified gold nanoparticles (AuNPs) combined with DNA is becoming a promising technique with the advantages of simplicity, cost-effectiveness and high sensitivity. However, few studies focused on the interaction of ssDNA and hybridized DNA on AuNPs to date. In the present work, we compared the interactions of different DNA probes on AuNPs using both absorption and fluorescence detection. It was found that there were only small partial dsDNA dissociated from the surface of AuNPs after hybridization in the presence of Hg²⁺. Moreover, we found that the aggregated AuNPs/DNA system tended to be dispersed again with increasing Hg²⁺ concentration up to 250 μM. Based on these results, the mechanisms of mercury detection based on interaction between DNA-conjugated gold nanoparticles were investigated. Positively charged dsDNA could bind to the surface of AuNPs and dominate the electrostatic interactions and consequently aggregation of the AuNPs/DNA system.     

FRET‐based Fluorescent and Colorimetric Probe for Selective Detection of Hg(II) and Cu(II) with Dual‐mode

A dual‐function fluorescence resonance energy transfer (FRET)‐based fluorescent and colorimetric probe was rationally fabricated from an energy donor coumarin moiety and an energy acceptor rhodamine moiety linked by a thiohydrazide arm for selective detection of Hg²⁺ and Cu²⁺. Two distinct mechanisms were used for the selective detection. Results revealed that probe 1 showed high fluorescent selectivity towards Hg²⁺ and evident colorimetric selectivity for Cu²⁺, which was suitable for ‘naked‐eye’ detection.     

A colorimetric and surface-enhanced Raman scattering dual-signal sensor for Hg based on Bismuthiol II-capped gold nanoparticles

The addition of Bismuthiol II to the gold nanoparticles (AuNPs) solution led to the aggregation of AuNPs with a color change from red to blue. As a result, hot spots were formed and strong surface-enhanced Raman scattering (SERS) signal of Bismuthiol II was observed. However, the Bismuthiol II-induced aggregation of AuNPs could be reversed by Hg²⁺ in the system, accompanied by a remarkable color change from blue to red. As evidenced by UV–vis and SERS spectroscopy, the variation in absorption band and SERS intensity was strongly dependent on the concentration of Hg²⁺, suggesting a colorimetric and SERS dual-signal sensor for Hg²⁺. The sensor had a high sensitivity, low detection limits of 2 nM and 30 nM could be achieved by UV–vis spectroscopy and by SERS spectroscopy, respectively. Other environmentally relevant metal ions did not interfere with the detection of Hg²⁺. The method was successfully applied to detect Hg²⁺ in water samples. It was simple, rapid and cost-effective without any modifying or labeling procedure.     

A novel colorimetric aptasensor for detection of chloramphenicol based on lanthanum ion–assisted gold nanoparticle aggregation and smartphone imaging

A label-free, rapid response colorimetric aptasensor for sensitive detection of chloramphenicol (CAP) was proposed, which was based on the strategy of ssDNA-modified gold nanoparticle (AuNP) aggregation assisted by lanthanum (La³⁺) ions. The AuNPs generated a color change that could be monitored in the red, green, and blue and analyzed by the smartphone imaging app. La³⁺, as a trigger agent, strongly combined with the phosphate groups of the surface of ssDNA-AuNPs probe, which helps create AuNP aggregation and the color change of AuNPs from red to blue. On the contrary, when mixing with CAP, the aptamer (Apt) bound to CAP to form a rigid structure of the Apt-CAP complex, and La³⁺ attached to the phosphate groups of the complex, which prevented the aptamer from binding to the surface of the AuNPs. As a result, the color of the AuNPs changed to violet-red. Finally, UV-vis absorption spectroscopy and the smartphone imaging app were employed to determine CAP with a lower detection limit of 7.65 nM and 5.88 nM, respectively. The proposed strategy featuring high selectivity and strong anti-interference ability for detection of CAP in practical samples was achieved. It is worth mentioning that the simple and portable colorimetric aptasensor will be used for facilitating on-site detection of food samples.

     

Characterization of a Hg2+-Selective Fluorescent Probe Based on Rhodamine B and Its Imaging in Living Cells

A small organic molecule P was synthesized and characterized as a fluorometric and colorimetric dual-modal probe for Hg²⁺. The sensing characteristics of the proposed probe for Hg²⁺ were studied in detail. A fluorescent enhancing property at 583 nm (>30 fold) accompanied with a visible colorimetric change, from colorless to pink, was observed with the addition of Hg²⁺ to P in an ethanol-water solution (8:2, v/v, 20 mM HEPES, pH 7.0), which would be helpful to fabricate Hg²⁺-selective probes with “naked-eye” and fluorescent detection. Meanwhile, cellular experimental results demonstrated its low cytotoxicity and good biocompatibility, and the application of P for imaging of Hg²⁺ in living cells was satisfactory.     

Label-free colorimetric sensor for mercury(II) and DNA on the basis of mercury(II) switched-on the oxidase-mimicking activity of silver nanoclusters

In this paper, a novel colorimetric biosensor for Hg²⁺ and DNA molecules is presented based on Hg²⁺ stimulated oxidase-like activity of bovine serum albumin protected silver clusters (BSA-Ag NCs). Under mild conditions, Hg²⁺ activated BSA-Ag NCs to show high catalytic activity toward the oxidation of 3,3′,5, 5′-tetramethylbenzidine (TMB) using ambient dissolved oxygen as an oxidant. The oxidase-like activity of BSA-Ag NCs was “switched-on” selectively in the presence of Hg²⁺, which permitted a novel and facile colorimetric sensor for Hg²⁺. As low as 25 nmol L⁻¹ Hg²⁺ could be detected with a linear range from 80 nmol L⁻¹ to 50 mmol L⁻¹. In addition, the sensing strategy was also employed to detect DNA molecules. Hg²⁺ is known to bind very strongly and specifically with two DNA thymine bases (T) to form thymine–Hg²⁺–thymine (T–Hg²⁺–T) base pairs. The hairpin-structure was disrupted and Hg²⁺ ions were released after hybridization with the DNA target. By coupling the Hg²⁺ switched-on the oxidase-mimicking activity of BSA-Ag NCs, we developed a novel label-free strategy for facile and fast colorimetric detection of DNA molecules. More important, target DNA can be detected as low as 10 nmol L⁻¹ with a linear range from 30 to 225 nmol L⁻¹. Compared with other methods, this method presents several advantages such as the independence of hydrogen peroxide, high sensitivity and good selectivity, avoiding any modification or immobilization of DNA, which holds a great potential of metal NCs for clinical application in biosensing and biotechnology.     

Emulsification-based dispersive liquid microextraction and HPLC determination of carbazole-based explosives

We have developed a simple method for the highly selective colorimetric detection of dissolved mercury(II) ions via direct formation of gold nanoparticles (AuNPs). The dithia-diaza ligand 2-[3-(2-amino-ethylsulfanyl)-propylsulfanyl]-ethylamine (AEPE) was used as a stabilizer to protect AuNPs from aggregation and to impart highly selective recognition of Hg(II) ion over other metal ions. A solution of Au(III) ion is directly reduced by sodium borohydride in the presence of AEPE and the detergent Triton X-100. This results in the formation of AEPE-AuNPs and a red coloration of the solution. On the other hand, in the presence of Hg(II), the solution turns blue within a few seconds after the addition of borohydride. This can be detected spectrophotometrically or even visually. The method was successfully applied to quantify Hg(II) levels in water sample, with a minimum detectable concentration as low as 35?nM.

Selective ratiometric detection of Hg in pure water using a phenoxazinium-based probe

A ratiometric, near-infrared, and fully water-soluble probe, a phenoxazinium-based chemosensor bearing an anilino thiaazacrown, was successfully synthesized and characterized. The use of this probe for the selective ratiometric detection of Hg²⁺ in pure water is reported. The probe shows good selectivity for Hg²⁺, and a large blue shift (75 nm) of the complex’s absorption maximum was observed.     

Direct colorimetric visualization of mercury (Hg) based on the formation of gold nanoparticles

It is critical to be able to detect and quantify Hg²⁺ ions under aqueous conditions with high sensitivity and selectivity. The technique presented herein provides a direct way for simple colorimetric visualization of Hg²⁺ ions in aqueous solution, based on the formation of gold nanoparticles through the Hg²⁺ catalyzed HAuCl₄/NH₂OH reaction. The outstanding selectivity and sensitivity result from the well-known amalgamation process that occurs between mercury and gold. The entire procedure takes less than 20 min. The limit of detection (2 ppb) shows excellent potential for monitoring ultralow levels of mercury in water samples.     

A Naphthalimide-Based Fluorescent Probe for the Detection and Imaging of Mercury Ions in Living Cells

The selective and efficient monitoring of mercury (Hg²⁺) contamination found in the environment and ecosystem has been carried out. Thus, a new 1,8-naphthalimide-based fluorescent probe NADP for the detection of Hg²⁺ based on a fluorescence enhancement strategy has been designed and synthesized. The NADP probe can detect Hg²⁺ with high selectivity and sensitivity and a low detection limit of 13 nm . The detection mechanism was based on a Hg²⁺-triggered deprotection reaction, resulting in a dramatic change in fluorescence from colorless to green at physiological pH. Most importantly, biological investigation has shown that the NADP probe can be successfully applied to the monitoring of Hg²⁺ in living cells and zebrafish with low cytotoxicity.     

The exquisite integration of ESIPT,PET and AIE for constructing fluorescent probe for Hg(II) detection and poisoning

Excessive mercury ions (Hg²⁺) in the environment can accumulate in human body along with the food chain to cause serious physiological reactions. The fluorescence probes were considered as convenient tool with great potential for Hg²⁺ detection. Most existing probes suffer from aggregation-induced quenching (ACQ) effects and insufficient sensitivity. Herein, a novel type of fluorophore was developed by combining the aggregation-induced emission (AIE) and excited state intramolecular proton transfer (ESIPT) characteristics. Subsequently, a phenyl thioformate group with photoinduced electron transfer (PET) effect was connected to give an efficient "turn-on" probe (HTM), which exhibited good selectivity toward Hg²⁺, short response time (30 min), coupled with extremely low detection limit (LOD = 1.68 nmol/L). In addition, HTM was used successfully in real samples, cells and drug evaluation, underlying the superiority of HTM to detect Hg²⁺ in practical applications.     

A Rational Designed Dual‐channel Chemosensor for Mercury Ions Based on Hydrolysis of Schiff Base

A bilateral Schiff base is reported for the colorimetric and fluorometric dual‐channel sensing of Hg²⁺ ions by taking advantage of the hydrolysis of carbon‐nitrogen double bond, altering an ICT state mechanism and then Hg²⁺ ions coordinating with amino moieties of 1,5‐DAN and leading to the aggregation of 1,5‐DAN. Meanwhile, it formed a stable neutral complex of amino‐Hg‐amino. In addition, test strips based on L were fabricated, which also exhibited a good selectivity to Hg²⁺ as in solution. This work provides a novel approach for the selective recognition of mercury ions. Notably, the color changes are very significant and all the recognition processes can be observed by the naked eyes. We believe the test strips can act as a convenient and efficient Hg²⁺ test kit.     

Label-free aptamer-based colorimetric detection of mercury ions in aqueous media using unmodified gold nanoparticles as colorimetric probe

We report a simple and sensitive aptamer-based colorimetric detection of mercury ions (Hg²⁺) using unmodified gold nanoparticles as colorimetric probe. It is based on the fact that bare gold nanoparticles interact differently with short single-strand DNA and double-stranded DNA. The anti-Hg²⁺ aptamer is rich in thymine (T) and readily forms T–Hg²⁺–T configuration in the presence of Hg²⁺. By measuring color change or adsorption ratio, the bare gold nanoparticles can effectively differentiate the Hg²⁺-induced conformational change of the aptamer in the presence of a given salt with high concentration. The assay shows a linear response toward Hg²⁺ concentration through a five-decade range of 1 × 10⁻⁴ mol L⁻¹ to 1 × 10⁻⁹ mol L⁻¹. Even with the naked eye, we could identify micromolar Hg²⁺ concentrations within minutes. By using the spectrometric method, the detection limit was improved to the nanomolar range (0.6 nM). The assay shows excellent selectivity for Hg²⁺ over other metal cations including K⁺, Ba²⁺, Ni²⁺, Pb²⁺, Cu²⁺, Cd²⁺, Mg²⁺, Ca²⁺, Zn²⁺, Al³⁺, and Fe³⁺. The major advantages of this Hg²⁺ assay are its water-solubility, simplicity, low cost, visual colorimetry, and high sensitivity. This method provides a potentially useful tool for the Hg²⁺ detection.     

A novel electrochemical method to detect mercury (II) ions

A novel and sensitive electrochemical method for determination of mercury (II) ions (Hg²⁺) based on the formation of thymine–Hg²⁺–thymine complexes and gold nanoparticle-mediated signal amplification is reported. Two 5′ end thiolated complementary oligonucleotides containing six strategically placed thymine–thymine mistakes were introduced in this work. One of the two oligonucleotides was immobilized on a gold electrode and the other one on gold nanoparticles (AuNPs). Due to six thymine–thymine mistakes the two oligonucleotides were not able to be hybridized, so AuNPs could not be immobilized onto the electrode surface after the electrode was immersed in the DNA–AuNPs solution. However, if Hg²⁺ existed, T–Hg²⁺–T complexes could be formed and AuNPs could be immobilized onto the electrode surface. Meanwhile, large numbers of [Ru(NH₃)₆]³⁺ molecules as electrochemical species could be localized onto the electrode surface. The Hg²⁺ detection limit of this assay could be as low as 10 nM, which is the US Environmental Protection Agency (EPA) limit of Hg²⁺ for drinkable water. This method is proven to be simple, convenient, high sensitive and selective.     

Hg2+-mediated aggregation of gold nanoparticles for colorimetric screening of biothiols

In this work, we report a colorimetric assay for the screening of biothiols including glutathione (GSH), cysteine (Cys), and homocysteine (Hcys) based on Hg(2+)-mediated aggregation of gold nanoparticles (AuNPs). Hg(2+) can induce aggregation of thiol-containing naphthalimide (1) capped AuNPs due to the cross-linking interactions from the resulting "thymine-Hg(2+)-thymine" (T-Hg(2+)-T) analogous structure. When Hg(2+) is firstly treated with biothiols, followed by mixing with 1-capped AuNPs suspension, AuNPs undergo a transformation from an aggregation to a dispersion state depending on the concentration of biothiols. This anti-aggregation or re-dispersion of AuNPs is due to the higher affinity of Hg(2+) for biothiols relative to compound 1. The corresponding color variation in the process of anti-aggregation of AuNPs can be used for the quantitative screening of biothiols through UV-vis spectroscopy or by the naked eye. Under optimized conditions, a good linear relationship in the range of 0.025-2.28 μM is obtained for GSH, 0.035-1.53 μM for Cys, and 0.040-2.20 μM for Hcys. The detection limits of this assay for GSH, Cys, and Hcys are 17, 9, and 18 nM, respectively. This colorimetric assay exhibits a high selectivity and sensitivity with tunable dynamic range. The proposed method has been successfully used in the determination of total biothiol content in human urine samples.     

Colorimetric determination of Hg2+ via thiosemicarbazide-to-oxadiazole transformation of a coumarin-benzopyrylium dye

A dual signaling reaction-based probe derived from a hybrid coumarin-benzopyrylium dye was developed for Hg²⁺ signaling. The probe exhibited selective colorimetric Hg²⁺ signaling via the Hg²⁺-induced thiosemicarbazide-oxadiazole transformation. Interference from Cu²⁺ ions was successfully circumvented using a citrate buffer as a masking agent. The detection limit for Hg²⁺ ions was found to be 1.88?×?10^–7 M. Practical application to Hg²⁺ detection in simulated wastewater was realized using a typical smartphone as a stand-alone data capture and data processing device.     

Electrocatalytic assay of mercury(II) ions using a bifunctional oligonucleotide signal probe

Engineered nucleic acid probes containing recognition and signaling functions find growing interest in biosensor design. In this paper, we developed a novel electrochemical biosensor for sensitive and selective detecting of Hg²⁺ based on a bifunctional oligonucleotide signal probe combining a mercury-specific sequence and a G-quadruplex (G4) sequence. For constructing the electrochemical Hg²⁺ biosensor, a thiolated, mercury-specific oligonucleotide capture probe was first immobilized on gold electrode surface. In the presence of Hg²⁺, a bifunctional oligonucleotide signal probe was hybridized with the immobilized capture probe through thymine–mercury(II)–thymine interaction-mediated surface hybridization. The further interaction between G4 sequence of the signal probe and hemin generated a G4–hemin complex, which catalyzed the electrochemical reduction of hydrogen peroxide, producing amplified readout signals for Hg²⁺ interaction events. This electrochemical Hg²⁺ biosensor was highly sensitive and selective to Hg²⁺ in the concentration of 1.0 nM to 1 μM with a detection limit of 0.5 nM. The new design of bifunctional oligonucleotide signal probes also provides a potential alternative for developing simple and effective electrochemical biosensors capable of detecting other metal ions specific to natural or artificial bases.     

Surface reaction strategy for Raman probing trace cadmium ion

Prevalent contamination of water by cadmium ion (Cd²⁺) brings environmental risk, which threatening to human health including renal dysfunction, reduced lung capacity, and some cancers. It calls for rapid, sensitive and selective protocol to analyze Cd²⁺ on field. In this work, specific Raman probe is rationally designed by gold nanoparticles (AuNPs) functionalized with tripeptide glutathione (GSH) and Rhodamine 6G (R6G) (denoted as R6G/GSH/AuNPs), which is explored for indirectly determining trace Cd²⁺ in river water. Based on chelating interaction between GSH and Cd²⁺ to form tetrahedral Cd(SG)_4, which trends easy detachment from R6G/GSH/AuNPs. With increasing Cd²⁺ concentration, the Raman probe without protection of GSH happens due aggregation and Raman signal of R6G increases correspondingly. The R6G/GSH/AuNPs-based Raman detection of Cd²⁺ in river water shows rapidness, excellent sensitivity, selectivity and good repeatability. Limit of detection of 10 ppb with a widely linear range of 0.5–20 ppm could be achieved. It could be perspective that such Raman probe could be extended to be used for on-site monitoring trace Cd²⁺ in river water system.     

Selective colorimetric sensing of cysteine in aqueous solutions using silver nanoparticles in the presence of Cr³+

We here in report an extensive study on the development of a highly facile, selective and sensitive colorimetric probe for cysteine detection using silver nanoparticles (Ag NPs). The efficacy of the process relies upon the surface plasmon resonance properties of Ag NPs and the interaction of Ag-cysteine complex with chromium ions (Cr³⁺) in a ratio of 2:1. In the presence of Cr³⁺, cysteine was able to induce the aggregation of Ag NPs thereby resulting in a change in yellow colour of the Ag colloid to purple. The reported probe has a limit of detection down to 1 nM which is to the best of our knowledge the lowest ever reported for the colorimetric detection of cysteine. Furthermore, a remarkable feature of this method is that it involves a simple technique exhibiting high selectivity to cysteine over other tested amino acids.