Colorimetric detection of mercury ion (Hg2+) based on DNA oligonucleotides and unmodified gold nanoparticles sensing system with a tunable detection range

Photoluminescent Nanocellulosic Film for Selective Hg2+ Ion Detection

We developed a highly sensitive solid-state sensor for mercury detection by stabilizing red-sub-nanometric fluorescent gold nanoclusters (AuNC, 0.9 ± 0.1 nm diameter) with bovine serum albumin in a matrix composed of cellulose nanofibrils (CNF) (BSA-AuNC/CNF). The main morphological and optical features of the system were investigated via atomic force/transmission electron microscopy and UV-Vis/fluorescence spectroscopy. The hybrid film (off-white and highly transparent) showed strong photoluminescene under UV irradiation. The latter is assigned to the AuNC, which also increase the ductility of the emitting film, which was demonstrated for high sensitivity Hg2+ detection. When used as a sensor system, following AuNC printing on CNF hybrid films, a limit of detection <10 nM was confirmed. What is more, nanocellulose films have a high pore structure and selective separation properties, showcasing a wide range of potential applications in many fields such as water treatment and oil-water separation.

DFT analysis on the adsorption of melamine in Ga12-N12/P12 nanocages: solvent effects, SERS analysis, reactivity properties

Due to its negative effects on people, melamine contamination in food products are detected and filtered. Amongst several sensory schemes for the screening of melamine poisoning, one of the most promising techniques is the use of nanomaterial based sensing for real time applicability in industries. In the current work, we have looked into the way melamine binds to Ga12-N12/P12 nanocages. Surface-enhanced Raman scattering (SERS), a successful spectroscopic technique is used to monitor melamine. Density functional theoretical (DFT) computations were used to study the sensing properties of melamine (Me) with Ga12-N12/P12 nanocages. Reactivity and Mulliken charge analyses show charge transfer from melamine to nanocage. Me-Ga12-N12 and Me-Ga12P12 clusters have adsorption energies of -47.54 and -33.12 kcal/mol, respectively. All nanocage-Me systems have a significant increase in polarizability. The electron densities revealed non-covalent interactions in the adsorbed systems. All adsorption energies in aqueous media are negative, indicating an attractive and exothermic reaction, with maximum value in water for Me-Ga12N12 and in acetone for Me-Ga12P12. Evidence of SERS is observed due to the enhancement of different vibrational modes.Communicated by Ramaswamy H. Sarma.

Green-emitting functionalized silicon nanoparticles as an "off-on" fluorescence bio-probe for the sensitive and selective detection of mercury (II) and 3-mercaptopropionic acid

Herein, we developed a class of functionalized silicon nanoparticles (F-SiNPs) bio-probes named thiol-conjugated F-SiNPs. They combine excellent biocompatibility with small dimensions (<10 nm) and biological usefulness with sustained and robust fluorescence (3.32% photoluminescent quantum yield). Identifying 3-Mercaptopropionic acid (3-MPA), which lowers the quantity of gamma-aminobutyric acid in the brain, and mercury (Hg²⁺) was a crucially important step since their excessive levels are a sign of several disorders. Using F-SiNPs as a fluorescent bio-probe, we provided an "off-on" technique for sensitively and selectively determining Hg²⁺ and 3-MPA in this study. The 3-(2-aminoethylamino) propyl (dimethoxymethylsilane) and basic fuchsin as precursors were hydrothermally treated to produce the F-SiNPs exhibiting green fluorescence. Our results suggest that Hg²⁺ reduced the fluorescence of F-SiNPs because of strong ionic interactions and metal-ligand binding among many thiols and carboxyl groupings at the surface of Hg²⁺ and F-SiNPs. Additionally, the resultants demonstrated that after being quenched by Hg²⁺, the produced F-SiNPs led to the distinctive "off-on" response to 3-MPA. Moreover, the method could detect Hg²⁺ and 3-MPA with limits of detection of 0.065 μM and 0.017 μM, respectively. The technique employed is quick, easy, affordable, and environmentally friendly. The sensing platform has successfully determined Hg²⁺ and 3-MPA in urine, water, and human serum samples.

How to Use Localized Surface Plasmon for Monitoring the Adsorption of Thiol Molecules on Gold Nanoparticles?

The functionalization of spherical gold nanoparticles (AuNPs) in solution with thiol molecules is essential for further developing their applications. AuNPs exhibit a clear localized surface plasmon resonance (LSPR) at 520 nm in water for 20 nm size nanoparticles, which is extremely sensitive to the local surface chemistry. In this study, we revisit the use of UV-visible spectroscopy for monitoring the LSPR peak and investigate the progressive reaction of thiol molecules on 22 nm gold nanoparticles. FTIR spectroscopy and TEM are used for confirming the nature of ligands and the nanoparticle diameter. Two thiols are studied: 11-mercaptoundecanoic acid (MUDA) and 16-mercaptohexadecanoic acid (MHDA). Surface saturation is detected after adding 20 nmol of thiols into 1.3 × 10⁻³ nmol of AuNPs, corresponding approximately to 15,000 molecules per AuNPs (which is equivalent to 10.0 molecules per nm²). Saturation corresponds to an LSPR shift of 2.7 nm and 3.9 nm for MUDA and MHDA, respectively. This LSPR shift is analyzed with an easy-to-use analytical model that accurately predicts the wavelength shift. The case of dodecanehtiol (DDT) where the LSPR shift is 15.6 nm is also quickly commented. An insight into the kinetics of the functionalization is obtained by monitoring the reaction for a low thiol concentration, and the reaction appears to be completed in less than one hour.

Colorimetric and turn-on fluorescence determination of mercury (II) by using carbon dots and gold nanoparticles

A colorimetric and turn-on fluorometric assay with high sensitivity and selectivity is described for the optical detection of mercury (II) ions (Hg²⁺), based on carbon dots with -SH (SN-CDs) and gold nanoparticles (AuNPs). On addition of Hg²⁺, the color of the system (SN-CDs/AuNPs) changes from red to blue. A new absorption peak appears at 700 nm, and its absorbance increases with the concentration of Hg²⁺, while at 530 nm, the absorbance of AuNPs decreases. Taking the ratio of absorbance at 700 and 530 nm as a signal, a colorimetric method with linear detection range of 0.5-4.0 μM was established for the determination of Hg²⁺. Meanwhile, citrate ions on the surface of AuNPs can reduce Hg²⁺ to Hg⁰, and through the strong affinity of Hg⁰ and gold, gold-mercury alloys were formed to occupy the surface of AuNPs, so that the SN-CDs were re-free and the fluorescence of SN-CDs was restored. Consequently, a fluorometric method was founded in the linear detection range from 0.5 to 15.0 μM of mercury (II). This dual-mode (colorimetric and turn-on fluorometric) method was applied successfully for determination of Hg²⁺ in real water samples.

On-site, rapid and visual method for nanomolar Hg2+ detection based on the thymine–Hg2+–thymine triggered “double” aggregation of Au nanoparticles enhancing the Tyndall effect

This work describes a new nanosensor for the simple, rapid, portable, colorimetric analysis of mercury(ii) (Hg²⁺) ions by combining the sensitive Tyndall effect (TE) of colloidal Au nanoparticles (AuNPs) with specific thymine–Hg²⁺–thymine (T–Hg²⁺–T) coordination chemistry for the first time. For the TE-inspired assay (TEA), in the presence of Hg²⁺ in a sample, the analyte can selectively mediate the hybridization of three types of flexible single-stranded DNAs (ssDNAs) to form stable rigid double-stranded DNAs (dsDNAs) via the T–Hg²⁺–T ligand interaction. Subsequent self-assembly of the dsDNAs with terminal thiol groups on the AuNPs' surfaces led to their “double” aggregation in addition to the lack of sufficient ssDNAs as the stabilizing molecules in a high-salt solution, resulting in a remarkably enhanced TE signal that positively relied on the Hg²⁺ level. The results demonstrated that such a TEA method enabled rapid naked-eye qualitative analysis of 625 nM Hg²⁺ within 10 min with an inexpensive laser pointer pen as an inexpensive handheld light source to generate the TE response. Making use of a smartphone for portable TE readout could further quantitatively detect the Hg²⁺ ions in a linear concentration range from 156 to 2500 nM with a limit of detection as low as 25 nM. Moreover, the developed equipment-free nanosensor was also used to analyze the Hg²⁺ ions in real samples including tap water, drinking water, and pond water, the obtained recoveries were within the range of 93.68 to 108.71%. To the best of our knowledge, this is the first report of using the AuNPs and functional nucleic acids to design a TE-based biosensor for the analysis of highly toxic heavy metal ions.

A new equipment-free colorimetric nanosensor was initially developed for quantitative point-of-need detection of nanomolar Hg²⁺ ions based on the enhancement in Tyndall effect of Au nanoparticles via their target-triggered “double” aggregation.

Development of luminol-based chemiluminescence approach for ultrasensitive sensing of Hg(II) using povidone-I2 protected gold nanoparticles as an efficient coreactant

In this work, we fabricated gold nanoparticles (AuNPs) capped with both polyvinyl pyrrolidone (PVP) and iodine (I₂) to act as efficient chemiluminescent coreactants for luminol. AuNPs synthesis was based on the direct chemical reduction of Au³⁺ with NaBH₄ in the presence of PVP-I₂ complex. The successful synthesis of PVP-I₂@AuNPs was confirmed with scanning electron microscopy (SEM) and UV-vis spectrophotometry. Chemiluminescence (CL) intensity of luminol was greatly enhanced, upon its chemical reaction with chemisorbed I₂ on AuNPs surfaces owing to the excellent catalytic activity of AuNPs. The PVP-I₂@AuNPs/luminol CL sensing system was successfully applied for determination of Hg²⁺ ions and the results displayed linearity in a wide range from 0.5 to 2000 nM and an ultrasensitive response to 1.0 nM Hg²⁺. The detection limit of Hg²⁺ ions was 0.1 nM, which was 100 times lower than the limit value (10 nM) defined by the U.S. Environmental Protection Agency in drinkable water. This ultrasensitive luminogenic system for Hg²⁺ detection also exhibited excellent selectivity among 13 types of metals, suggesting that the luminol/PVP-I₂@AuNPs system is a promising sensor for real-time detection of Hg²⁺. Graphical abstract.

A review on nanostructure-based mercury (II) detection and monitoring focusing on aptamer and oligonucleotide biosensors

Heavy metal ion pollution is a severe problem in environmental protection and especially in human health due to their bioaccumulation in organisms. Mercury (II) (Hg²⁺), even at low concentrations, can lead to DNA damage and give permanent harm to the central nervous system by easily passing through biological membranes. Therefore, sensitive detection and monitoring of Hg²⁺ is of particular interest with significant specificity. In this review, aptamer-based strategies in combination with nanostructures as well as several other strategies to solve addressed problems in sensor development for Hg²⁺ are discussed in detail. In particular, the analytical performance of different aptamer and oligonucleotide-based strategies using different signal improvement approaches based on nanoparticles were compared within each strategy and in between. Although quite a number of the suggested methodologies analyzed in this review fulfills the standard requirements, further development is still needed on real sample analysis and analytical performance parameters.

Ultrasensitive colorimetric aptasensor for Hg2+ detection using Exo-III assisted target recycling amplification and unmodified AuNPs as indicators

Facile and ultrasensitive detection of Hg²⁺ in water environment remains challenging. Exonuclease III (Exo-III)-assisted target recycling is one of the most popular amplification strategies. Although the magnesium (II) ions are widely acting as cofactors of Exo-III, we recognized that Mg²⁺ cofactors would strongly disturb the charge distribution on citrate-stablized gold nanoparticles (in the general sense, unmodified AuNPs) surface, thus generate false positive colorimetric signals. To address this issue, we first put forward the view that the cobalt (II) ions can function as the Exo-III cofactor and successfully construct a novel label-free colorimetric aptasensor for facile and ultrasensitive detection of Hg²⁺ using Hg²⁺-triggered Exo-III-assisted signal amplification and unmodified AuNPs as indicators. A hairpin-looped DNA probe was rationally designed with thymine-rich recognition termini and specifically recognized trace Hg²⁺ by a stable T-Hg²⁺-T structure. A blue-to-red color change of AuNPs with the addition of Hg²⁺ provided the quantitative detection of Hg²⁺ with a limit of detection of 0.2 nM and a linear working range from 0.5 nM to 5.0 nM. The whole testing time for one assay was approximately 40 min. Real water samples, even containing Hg²⁺ at 1 nM, could be determined by the aptasensor with recovery rates from 97% to 103%.

Utilization of doped GQDs for ultrasensitive detection of catastrophic melamine: A new SERS platform

The detection and filtration of melamine in food products has become an emergence due to its harmful effect on humans. In present work, we have investigated the binding mechanism of melamine over carboxyl group edge-functionalized graphene quantum dots doped with oxygen and sulphur atoms (O-GQD and S-GQD). In order to monitor melamine, surface enhanced Raman scattering (SERS) is adopted which is an effective vibrational spectroscopic approach. Electronic and vibrational properties were analysed by means of well adapted density functional theory (DFT). The calculated adsorption energy of melamine over O-GQD and S-GQD is -1.18 and -0.15 eV respectively. The characteristic peak of melamine at 688 cm^-1 is in good agreement with previously reported experimental work and enhances by 348.4% in SERS spectra of Mel-O-GQD and 48% in SERS spectra of Mel-S-GQD. We have calculated the chemical enhancement factor (EF) for melamine over O-GQD and S-GQD and found the enhancement of 4.51 and 1.48 which is greater than melamine‑silver complexes. Our theoretical studies on SERS of melamine over O-GQD and S-GQD suggest that oxygen is a better candidate for SERS. Our work demonstrates that the graphene quantum dots are remarkable platforms for the detection of melamine.

An Au@NH2-MIL-125(Ti)-based multifunctional platform for colorimetric detections of biomolecules and Hg2+

Au@NH₂-MIL-125(Ti) was fabricated and explored as a multifunctional platform for sensitive colorimetric detections of biomolecules and Hg²⁺.

Ion-induced PCR strategy for mercury detection

Mercury contamination is one of the most serious environmental problems. It can cause serious effects on the human health, such as case damage in the brain, nervous system, immune system, and kidney failure. Therefore, development of an accurate, sensitive, and simple operational detection method for mercury is very necessary. Herein, we report a new strategy for mercury ion detection based on commonly used PCR technique. High selectivity and sensitivity were achieved by the formation of the thymine-Hg-thymine (T-Hg-T) unnatural base pair at the 3'-end of PCR primers. The detection results of PCR amplification in presence of mercury ion could be reported either by using agarose gel analysis or through real-time fluorometric dye tracing for different detection purposes. To our knowledge, this study represents the first application of PCR based technique to the detection of metal ions.

Optical Sensor Assistant with Voltage Enrichment for Ultrasensitive Detection of Mercury Ions

Voltage enrichment-assisted surface-enhanced resonance Raman scattering (SERRS) was employed for monitoring mercury(II) ions based on a nanoporous gold (NPG)/aptamer hybrid sensor. The hybrid sensor based on the coordination chemistry of thymine-Hg²⁺-thymine interaction and an applied voltage that pushed mercury ions toward the NPG film improved both the speed and sensitivity of Hg²⁺ ion detection. The detection limit can reach 0.1 pM, even in the aqueous solution containing 13 metal ions, and the sensor also possesses a fine reproducibility and stability, promising great potential in real-time sensing applications.

Multistage dynamics of Hg2+-DNA interactions: a single-molecule study

The metal ion-DNA interaction is key to biochemical processes and has applications in areas such as metal ion sensors and DNA nanomachines. For example, the formation of the T-Hg2+-T structure has been used in technologies such as DNA-based mercuric ion sensors. Though the interaction is widely used for practical purposes, the underlying mechanism has not been fully understood. In the present study, we used magnetic tweezers to explore the interactions between λ-DNA and two metal ions, Hg2+ and Cd2+, at the single-molecule level. Both metal ions caused considerable DNA conformational changes. The resulting DNA compaction dynamics were related to the ion concentration and the exerted force. The increase in the ion concentration promoted DNA compaction, whereas exerting greater forces inhibited this process. Application of a high force generated two-stage dynamics of the Hg2+-DNA interaction. However, at a sufficiently high Hg2+ concentration, a lower force led to a three-stage process. In contrast, the curves of the binding of Cd2+ ions to DNA had a stepwise pattern. Both the AFM scanning results and the single-molecule measurements confirmed that Hg2+ influences the DNA conformation in a more pronounced manner than Cd2+. The multistage Hg2+-DNA interaction was considered to be a result of the different binding mechanisms, including the mismatched base-pair formation. A model was then proposed to explain the peculiar dynamics.

Fluorescent Sensors for the Detection of Heavy Metal Ions in Aqueous Media

Due to the risks that water contamination implies for human health and environmental protection, monitoring the quality of water is a major concern of the present era. Therefore, in recent years several efforts have been dedicated to the development of fast, sensitive, and selective sensors for the detection of heavy metal ions. In particular, fluorescent sensors have gained in popularity due to their interesting features, such as high specificity, sensitivity, and reversibility. Thus, this review is devoted to the recent advances in fluorescent sensors for the monitoring of these contaminants, and special focus is placed on those devices based on fluorescent aptasensors, quantum dots, and organic dyes.

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 phenolic acid based colourimetric 'naked-eye' chemosensor for the rapid detection of Cu(II) ions

The crucial role of chemosensor for the immediate recognition of environment pollutant motivates the researchers to develop variety of sensing protocols. Of various chemosensory protocols, the colour change observed by the naked eye is considered to be a conceivable and on-site way to indicate the presence of an analyte. We herein report a colourimetric and commercially available absorption probe, sinapic acid (SA) that is completely ready to use for "on-site" visual determination of copper ions. The molecule, SA is well-known phenolic acid, often utilized for its antibacterial activity. In this work, for the first time, we are exploring its ability to work as an efficient Cu²⁺ sensor. This sensor molecule selectively detected Cu²⁺ ions by changing its colour from colourless to pink within detection limit of 64.5nM, which is much lower than other reported sensor molecules and the suggested limit by World Health Organization (WHO) and U. S. Environmental Protection Agency (EPA) guidelines. The sensing mechanism was investigated through UV-vis and ¹H NMR titration along with ESI-MS spectroscopy and further confirmed by DFT computational studies. Studies revealed the participation of hydroxyl group (OH) and methoxy group (OMe) of SA in complexation with Cu²⁺. The binding stoichiometry of SA to Cu²⁺ was found to be 1:2 through Job's plot and ESI-MS analysis. Importantly, paper strips of SA were prepared which could be used for a rapid "on-site" determination of Cu²⁺ containing samples.

AuNS@Ag core-shell nanocubes grafted with rhodamine for concurrent metal-enhanced fluorescence and surfaced enhanced Raman determination of mercury ions

Mercury ion (Hg²⁺) is a highly hazardous and widespread pollutant with bio-accumulative properties. Although the existing Hg²⁺ detection methods have high sensitivity and reliability, whereas there have few reports concerning bimodal detection for Hg²⁺ with one sensor. Toward this goal, a novel sensor based on rhodamine derivatives (RhD) grafted AuNS@Ag core-shell nanocubes (CSN) has been synthesized and shown the bimodal detection capabilities with metal enhanced fluorescence (MEF) and surface enhanced Raman scattering (SERS) for Hg²⁺. Herein, resultant CSN acts as the signal enhancing material; RhD was modified on the outside of the shell to ensure the signal sensitive of the CSN-RhD hybrids. In this work, we investigate the size- and shape-dependent SERS activity of plasmonic CSN comprised of AuNS as cores and Ag cuboids as shells. The SERS activity of CSN with spherical core was found to increase with the increasing thickness of the Ag cubic shell. Sequel, under an optimized condition, a display of strong MEF and SERS signals of the resulting mixtures with increasing of Hg²⁺ concentrations was observed. The proposed bimodal sensor showed excellent performances for Hg²⁺ along with wide linear range of 0.001-1000 ppm and 0.01-1000 ppm as well as the relatively low detection limit of 0.94 and 5.16 ppb for MEF and SERS assays, respectively. Furthermore, the ability of the sensor to detect Hg²⁺was also confirmed in adulterated milk samples.

Novel ratiometric surface-enhanced raman spectroscopy aptasensor for sensitive and reproducible sensing of Hg2

It is important to precisely monitor mercury (II) ions (Hg²⁺) for environment protection and human health monitoring. Although many strategies have been developed in the past decades, there still remains a challenge for developing an ultrasensitive, simple and reliable approach to detect Hg²⁺. Herein, we report a ratiometric surface-enhanced Raman scattering (SERS) aptasensor by employing aptamer-modified Au@Ag core-shell nanoparticles (Au@Ag NPs) as highly functional sensing probes, allowing for ultrasensitive detection of Hg²⁺. In principle, the thiolated 5'-Cy3 labeled aptamer probe (Cy3-aptamer) is firstly immobilized on the SERS substrate surface and then hybridizes with the 5'-Rox labeled complementary DNA (cDNA) to form a rigid double-stranded DNA (dsDNA), in which the Cy3 and Rox Raman labels are used to produce the ratiometric Raman signals. In the presence of Hg²⁺, the aptamer DNA turns into the thymine (T)-Hg²⁺-T mediated hairpin structure, leading to the dissociation of dsDNA. As a result, the Rox labels are away from the Au@Ag NP SERS substrate while Cy3 labels are close to it. Therefore, the intensity of SERS signal from Cy3 labels increases while that from Rox labels decreases. The ratio between the Raman intensities of Cy3 labels and Rox labels is linear with Hg²⁺ concentrations in the range from 0.001 to 1.0nM, and the limit of detection is estimated to be 0.4pM. The proposed strategy provides a new rapid, simple and reliable approach for sensitive detection of Hg²⁺ and may create a universal methodology for developing analogous aptasensors for a wide range of other analytes determination.

Colorimetric method for the detection of melamine using in-situ formed silver nanoparticles via tannic acid

Melamine toxicity has recently attracted worldwide attention as it causes renal failure and the death of humans and animals. Therefore, developing a simple, fast and sensitive method for the routine detection of melamine is the need of the hour. Herein, we have developed a selective colorimetric method for the detection of melamine in milk samples based upon in-situ formation of silver nanoparticles (AgNPs) via tannic acid. The AgNPs thus formed were characterized by UV-Visible spectrophotometer, transmission electron microscope (TEM), zetasizer and dynamic light scattering (DLS). The AgNPs were used to detect melamine under in vitro condition and in raw milk spiked with melamine. Under optimal conditions, melamine could be selectively detected in vitro within the concentration range of 0.05-1.4μM with a limit of detection (LOD) of 0.01μM, which is lower than the strictest melamine safety requirement of 1ppm. In spiked raw milk, the recovery percentage range was 99.5-106.5% for liquid milk and 98.5-105.5% for powdered milk. The present method shows extreme selectivity with no significant interference with other substances like urea, glucose, glycine, ascorbic acid etc. This assay method does not utilize organic cosolvents, enzymatic reactions, light sensitive dye molecules and sophisticated instrumentation, thereby overcoming some of the limitations of the other conventional methods.

Luminescent turn-on detection of Hg(II) via the quenching of an iridium(III) complex by Hg(II)-mediated silver nanoparticles

A novel luminescent turn-on detection method for Hg(II) was developed. The method was based on the silver nanoparticle (AgNP)-mediated quenching of Ir(III) complex 1. The addition of Hg(II) ions causes the luminescence of complex 1 to be recovered due to the oxidation of AgNPs by Hg(II) ions to form Ag(I) and Ag/Hg amalgam. The luminescence intensity of 1 increased in accord with an increased Hg(II) concentration ranging from 0 nM to 180 nM, with the detection limit of 5 nM. This approach offers an innovative method for the quantification of Hg(II).

Utilization of unmodified gold nanoparticles for label-free detection of mercury (II): Insight into rational design of mercury-specific oligonucleotides

Colorimetric detection of mercury (II) with the use of DNA oligonucleotides and unmodified gold nanoparticles (AuNPs) as indicators has been extensively studied. This study provides in-depth insights into the rational design of mercury-specific oligonucleotides (MSO) in the biosensing system. The leftover bases of MSO, as a result of the formation of T-Hg²⁺-T base pairs, can adsorb on the AuNPs and hinder their aggregation at concentrations of salt. This phenomenon was directly verified by the changes in particle sizes characterized by dynamic light scattering for the first time. Based on these findings, we proposed a rational design for the MSO with approximately 20-fold improvement in detection sensitivity. The detection limit of the proposed assay decreased to 15nM with a linear working range from 50nM to 300nM for Hg²⁺. The cross-reactivity against eight other metal ions was negligible compared with the response to Hg²⁺. Considering the diverse applications of AuNPs with oligonucleotides, this study can serve as a good reference and provides important implications in sensing and DNA-directed nanoparticle assembly.

Silver nanoclusters stabilized with denatured fish sperm DNA and the application on trace mercury ions detection

In this study, fluorescent silver nanoclusters (Ag NCs) were synthesized using denatured fish sperm DNA as the template. In contrast to other methods, this method did not use artificial DNA as the template. After their reaction with denatured fish sperm DNA, Ag⁺ ions were reduced by NaBH₄ to form Ag NCs. The Ag NCs showed a strong fluorescence emission at 650 nm when excited at 585 nm. The fluorescence intensity increased fourfold at pH 3.78, controlled with Britton-Robinson buffer solution. The fluorescence of the Ag NCs was quenched in the presence of trace mercury ions (Hg²⁺ ) in a weakly acidic medium and nitrogen atmosphere. The extent of the fluorescence quenching of Ag NCs strongly depends on the Hg²⁺ ion concentration over a linear range from 2.0 nmol L^-1 to 3.0 μmol L^-1 . The detection limit (3σ/k) for Hg²⁺ was 0.7 nmol L^-1 . Thus, a sensitive and rapid method was developed for the detection of Hg²⁺ ions.

Simultaneous bioremediation and biodetection of mercury ion through surface display of carboxylesterase E2 from Pseudomonas aeruginosa PA1

Mercury is a toxic heavy metal and presents significant threats to organisms and natural ecosystems. Recently, the mercury remediation as well as its detection by environmental-friendly biotechnology has received increasing attention. In this study, carboxylesterase E2 from mercury-resistant strain Pseudomonas aeruginosa PA1 has been successfully displayed on the outer membrane of Escherichia coli Top10 bacteria to simultaneously adsorb and detect mercury ion (Hg(2+)). The transmission electron microscopy analysis shows that Hg(2+) can be absorbed by carboxylesterase E2 and accumulated on the outer membrane of surface-displayed E. coli bacteria. The adsorption of Hg(2+) followed a physicochemical, equilibrated and saturatable mechanism, which well fits the traditional Langmuir adsorption model. The surface-displayed system can be regenerated through regulating pH values. As its activity can be inhibited by Hg(2+), carboxylesterase E2 has been used to detect the concentration of Hg(2+) in water samples. The developed surface display system will be of great potential in the simultaneous bioremediation and biodetection of environmental mercury pollution.

A simple and selective colorimetric mercury (II) sensing system based on chitosan stabilized gold nanoparticles and 2,6-pyridinedicarboxylic acid

The development of simple and cost-effective methods for the detection and treatment of Hg²⁺ in the environment is an important area of research due to the serious health risk that Hg²⁺ poses to humans. Colorimetric sensing based on the induced aggregation of nanoparticles is of great interest since it offers a low cost, simple, and relatively rapid procedure, making it perfect for on-site analysis. Herein we report the development of a simple colorimetric sensor for the selective detection and estimation of mercury ions in water, based on chitosan stabilized gold nanoparticles (AuNPs) and 2,6-pyridinedicarboxylic acid (PDA). In the presence of Hg²⁺, PDA induces the aggregation of AuNPs, causing the solution to change colors varying from red to blue, depending on the concentration of Hg²⁺. The formation of aggregated AuNPs in the presence of Hg²⁺ was confirmed using transmission electron microscopy (TEM) and UV-Vis spectroscopy. The method exhibits linearity in the range of 300nM to 5μM and shows excellent selectivity towards Hg²⁺ among seventeen different metal ions and was successfully applied for the detection of Hg²⁺ in spiked river water samples. The developed technique is simple and superior to the existing techniques in that it allows detection of Hg²⁺ using the naked eye and simple and rapid colorimetric analysis, which eliminates the need for sophisticated instruments and sample preparation methods.

A facile and green strategy for preparing newly-designed 3D graphene/gold film and its application in highly efficient electrochemical mercury assay

In this work, we report a facile and green strategy for in situ and one step preparation of a novel 3D graphene/gold (G/Au) film. Triggering with unique driving force from hydrothermal growth, a 3D interlaced graphene framework with hierarchically porous structures was directly attached on a gold substrate pretreated with a self-assembled monolayer. Simultaneously, highly dispersive Au nanoparticles with tunable morphologies were anchored on the framework utilizing generated graphene as an endogenous reductant. Newly-designed 3D G/Au film possessed excellent properties of significantly large specific area, good electrical conductivity, high structure stability and substrate binding strength, etc. As a paradigm, an electrochemical Hg²⁺ biosensor was constructed on 3D G/Au film, in which an exonuclease III-assisted target recycling was introduced. Impressively, an ultralow detection limit of 50 aM (S/N=3), a wide linear range from 0.1 fM to 0.1μM, a high selectivity and a good reliability for Hg²⁺ assay in real water and serum samples were realized using prepared biosensor. It is highly envisioned that this work opens the door towards simply fabricating varying types of 3D graphene based hybrid films, and such G/Au film will have widespread applications in electroanalysis and electrocatalysis.

An enzyme-free colorimetric assay using hybridization chain reaction amplification and split aptamers

A novel unmodified gold nanoparticle (AuNP)-based colorimetric assay was demonstrated using split aptamers and the hybridization chain reaction (HCR) amplification strategy. Here, the aptamer was divided into a structure-switching hairpin probe (DNA probe H1 (or H1')) and a single-stranded probe (DNA probe H2 (or H2')). In the presence of the target, DNA probe H1 (or H1') could specifically capture the target with the assistance of DNA probe H2 (or H2') to form a stable complex. Subsequently, the hairpin structure of DNA probe H1 (or H1') was changed, and then a chain reaction of hybridization events between two other hairpin probes (H3 and H4) propagated, resulting in the formation of nicked double-helices. Since it was difficult for such nicked double-helices to inhibit salt-induced AuNP aggregation, a red-to-blue color change was observed. With the elegant amplification effect of HCR, this assay showed a low detection limit (15 nM for Hg(2+) and 1 μM for adenosine), which was lower than or at least comparable to previous AuNP-based methods. The novel strategy not only eliminated the requirements of enzymatic reactions, separation processes, chemical modifications, and sophisticated instruments, but also could be used for other targets only by simply changing the DNA probe sequences.

Highly Sensitive Detection of Melamine Using a One-Step Sample Treatment Combined with a Portable Ag Nanostructure Array SERS Sensor

There is an urgent need for rapid and reliable methods able to detect melamine in animal feed. In this study, a quick, simple, and sensitive method for the determination of melamine content in animal feed was developed using surface-enhanced Raman spectroscopy on fabricated Ag nanorod (AgNR) array substrates with a one-step sample extraction procedure. The AgNR array substrates washed by HNO₃ solvent (10⁻⁷ M) and methanol and showed the good stability within 6 months. The Raman shift at △ν = 682 cm⁻¹ was used as the characteristic melamine peak in the calculations. Sufficient linearity was obtained in the 2–200 μg·g⁻¹ range (R² = 0.926). The limits of detection and quantification were 0.9 and 2 μg·g⁻¹, respectively. The recovery rates were 89.7–93.3%, with coefficients of variation below 2.02%. The method showed good accuracy compared with the tradition GC-MS analysis. This new protocol only need 2 min to fininsh the detection which could be developed for rapid onsite screening of melamine contamination in quality control and market surveillance applications.

Colorimetric detection based on localised surface plasmon resonance of gold nanoparticles: Merits, inherent shortcomings and future prospects

Localised surface plasmon resonance (LSPR) of gold nanoparticles (AuNPs) has been exploited for two decades in analytical science and has proven to be a powerful tool for the detection of various kinds of substances including small molecules, ions, macro biomolecules and microbes. Detection can be performed by visual colour change observations, photometry or resonance light scattering. A wide range of applications have been studied in the areas of environmental, pharmaceutical and biological analysis and clinical diagnosis. In this article, some fundamental aspects and important applications involving LSPR of AuNPs are reviewed. Several inherent shortcomings of these techniques and possible strategies to circumvent them are discussed.

A novel aptasensor based on single-molecule force spectroscopy for highly sensitive detection of mercury ions

We have developed a novel aptasensor based on single-molecule force spectroscopy (SMFS) capable of detecting mercury ions (Hg(2+)) with sub-nM sensitivity. The single-strand (ss) DNA aptamer used in this work is rich in thymine (T) and readily forms T-Hg(2+)-T complexes in the presence of Hg(2+). The aptamer was conjugated to an atomic force microscope (AFM) probe, and the adhesion force between the probe and a flat graphite surface was measured by single-molecule force spectroscopy (SMFS). The presence of Hg(2+) ions above a concentration threshold corresponding to the affinity constant of the ions for the aptamer (about 5 × 10(9) M(-1)) could be easily detected by a change of the measured adhesion force. With our chosen aptamer, we could reach an Hg(2+) detection limit of 100 pM, which is well below the maximum allowable level of Hg(2+) in drinking water. In addition, this aptasensor presents a very high selectivity for Hg(2+) over other metal cations, such as K(+), Ca(2+), Zn(2+), Fe(2+), and Cd(2+). Furthermore, the effects of the ionic strength and loading rate on the Hg(2+) detection were evaluated. Its simplicity, reproducibility, high selectivity and sensitivity make our SMFS-based aptasensor advantageous with respect to other current Hg(2+) sensing methods. It is expected that our strategy can be exploited for monitoring the pollution of water environments and the safety of potentially contaminated food.