Facile colorimetric detection of Hg2+ based on anti-aggregation of silver nanoparticles

Recent advances and trends in innovative biosensor-based devices for heavy metal ion detection in food

Low-cost, reliable, and efficient biosensors are crucial in detecting residual heavy metal ions (HMIs) in food products. At present, based on distance-induced localized surface plasmon resonance of noble metal nanoparticles, enzyme-mimetic reaction of nanozymes, and chelation reaction of metal chelators, the constructed optical sensors have attracted wide attention in HMIs detection. Besides, based on the enrichment and signal amplification strategy of nanomaterials on HMIs and the construction of electrochemical aptamer sensing platforms, the developed electrochemical biosensors have overcome the plague of low sensitivity, poor selectivity, and the inability of multiplexed detection in the optical strategy. Moreover, along with an in-depth discussion of these different types of biosensors, a detailed overview of the design and application of innovative devices based on these sensing principles was provided, including microfluidic systems, hydrogel-based platforms, and test strip technologies. Finally, the challenges that hinder commercial application have also been mentioned. Overall, this review aims to establish a theoretical foundation for developing accurate and reliable sensing technologies and devices for HMIs, thereby promoting the widespread application of biosensors in the detection of HMIs in food.

Alginic acid-functionalized silver nanoparticles: A rapid monitoring tool for detecting the technology-critical element tellurium

The increasing global demand for tellurium, driven by its critical role in alloys, photovoltaic devices, and electronics, has raised concerns about its environmental pollution and neurotoxicity. In response, the potential of alginic acid (AA), a renewable, low-cost, and sustainable biopolymer, was explored for the biosynthesis of ultra-small silver nanoparticles (AgNPs) and their application in the detection of tellurium (Te(IV)). The effect of key synthesis parameters on desired physicochemical properties and yield of AgNPs was established to ensure high specificity and sensitivity towards Te(IV). The purified AgNPs with AA surface ligands were utilized to demonstrate a ratiometric absorbance sensor that exhibits excellent linearity and nanomolar-level affinity. This approach achieved a high correlation coefficient of ∼ 0.982, with a low detection limit of about 22 nM. Further investigations into the effect of pH, ionic strength, and organic molecules were conducted to elucidate detection performance and molecular understanding. The detection mechanism relies on the coordination between Te(IV) ions and the carboxylate groups of AA, which initiates aggregation-induced plasmon coupling in adjacent AgNPs. The capability of this analytical method to monitor Te(IV) in real-world water samples features its rapidity, user-friendliness, and suitability for point-of-care monitoring, making it a promising alternative to more complex techniques.

Recent Advances in the Application of Bionanosensors for the Analysis of Heavy Metals in Aquatic Environments

Heavy-metal ions (HMIs) as a pollutant, if not properly processed, used, and disposed of, will not only have an influence on the ecological environment but also pose significant health hazards to humans, making them a primary factor that endangers human health and harms the environment. Heavy metals come from a variety of sources, the most common of which are agriculture, industry, and sewerage. As a result, there is an urgent demand for portable, low-cost, and effective analytical tools. Bionanosensors have been rapidly developed in recent years due to their advantages of speed, mobility, and high sensitivity. To accomplish effective HMI pollution control, it is important not only to precisely pinpoint the source and content of pollution but also to perform real-time and speedy in situ detection of its composition. This study summarizes heavy-metal-ion (HMI) sensing research advances over the last five years (2019-2023), describing and analyzing major examples of electrochemical and optical bionanosensors for Hg2+, Cu2+, Pb2+, Cd2+, Cr6+, and Zn2+.

Plasmonic nanoparticle's anti-aggregation application in sensor development for water and wastewater analysis

Colorimetric sensors have emerged as a powerful tool in the detection of water pollutants. Plasmonic nanoparticles use localized surface plasmon resonance (LSPR)-based colorimetric sensing. LSPR-based sensing can be accomplished through different strategies such as etching, growth, aggregation, and anti-aggregation. Based on these strategies, various sensors have been developed. This review focuses on the newly developed anti-aggregation-based strategy of plasmonic nanoparticles. Sensors based on this strategy have attracted increasing interest because of their exciting properties of high sensitivity, selectivity, and applicability. This review highlights LSPR-based anti-aggregation sensors, their classification, and role of plasmonic nanoparticles in these sensors for the detection of water pollutants. The anti-aggregation based sensing of major water pollutants such as heavy metal ions, anions, and small organic molecules has been summarized herein. This review also provides some personal insights into current challenges associated with anti-aggregation strategy of LSPR-based colorimetric sensors and proposes future research directions.

Optimization of a silver-nanoprism conjugated with 3,3',5,5'-tetramethylbenzidine towards easy-to-make colorimetric analysis of acetaldehyde: a new platform towards rapid analysis of carcinogenic agents and environmental technology

Acetaldehyde acts as an important mediator in the metabolism of plants and animals; however, its abnormal level can cause problems in biological processes. Although acetaldehyde is found naturally in many organisms, exposure to high concentrations can have effects on the eyes, respiratory system, etc. Due to the importance of detecting acetaldehyde in environmental samples and biofluids, determination of its concentration is highly demanded. There are some reports showing exposure to high concentrations of acetaldehyde for a long time can increase the risk of cancer by reacting with DNA. In this work, we presented a novel colorimetric method for rapid and sensitive detection of acetaldehyde with high reproducibility using different AgNPs with various morphologies. The redox reaction between AgNPs, 3,3',5,5'-tetramethylbenzidine (TMB) solution, and analytes endows a color change in 15 minutes that is detectable by the naked eye. UV spectrophotometry was further used for quantitative analysis. An iron mold with a hexagonal pattern and liquid paraffin were also used to prepare the paper-based microfluidic substrate, as a low cost, accessible, and rapid detection tool. Different types of AgNPs showed different lower limits of quantification (LLOQ). The AgNPs-Cit and AgNPrs could identify acetaldehyde with linear range of 10^-7 to 10 M and an LLOQ of 10^-7 M. The AgNWs showed the best color change activity with a linear range 10^-5 to 10 M and the lowest diagnostic limit is 10^-5 M. Finally, analysis of human biofluids as real samples were successfully performed using this system.

A Real-Time Detection Method of Hg2+ in Drinking Water via Portable Biosensor: Using a Smartphone as a Low-Cost Micro-Spectrometer to Read the Colorimetric Signals

This paper reported a real-time detection strategy for Hg²⁺ inspired by the visible spectrophotometer that used a smartphone as a low-cost micro-spectrometer. In combination with the smartphone's camera and optical accessories, the phone's built-in software can process the received light band image and then read out the spectral data in real time. The sensor was also used to detect gold nanoparticles with an LOD of 0.14 μM, which are widely used in colorimetric biosensors. Ultimately, a gold nanoparticles-glutathione (AuNPs-GSH) conjugate was used as a probe to detect Hg²⁺ in water with an LOD of 1.2 nM and was applied successfully to natural mineral water, pure water, tap water, and river water samples.

Carbon dot-functionalized macroporous adsorption resin for bifunctional ultra-sensitive detection and fast removal of iron(III) ions

Heavy metal pollution has spread around the world with the development of industry, posing a major threat to human health. It is urgent to design and fabricate bifunctional materials for detection and adsorption of heavy metal ions. Herein, poly(glycidyl methacrylate-co-ethylene glycol dimethacrylate) microspheres, a kind of common macroporous adsorption resin (MAR), were employed as the matrix, and carbon dots (CDs) with excellent optical properties were grafted onto the surface of MAR by surface-initiated atom transfer radical polymerization (SI-ATRP) and photo-initiated "thiol-yne" click chemistry. The synthesized MAR@poly(PA)@CD could produce fluorescence quenching with Fe³⁺. A simple fluorescence spectrometric method for detection of Fe³⁺ was established. The fluorescence intensity of MAR@poly(PA)@CD decreased linearly with the concentration of Fe³⁺ in the range of 0-70 nmol L^-1, with a limit of detection (LOD) of 6.6 nmol L^-1, which had the potential for trace detection. In addition, after SI-ATRP modification, many adsorption sites were generated on the surface of MAR, and the adsorption capacity for Fe³⁺ was 23.8 mg g^-1. Isothermal and kinetic adsorption experiments were more consistent with the Langmuir model (r = 0.9992) and pseudo-second-order model (r = 0.9902), indicating that the adsorption was monolayer adsorption and chemical adsorption, respectively. MAR@poly(PA)@CD with dual functions of detecting and adsorbing Fe³⁺ was successfully prepared, showing great application prospects in the environmental field.

Plasmonic Nano Silver: An Efficient Colorimetric Sensor for the Selective Detection of Hg2+ Ions in Real Samples

Environmental pollution caused by heavy metal ions has become a major health problem across the world. In this study, a selective colorimetric sensor based on starch functionalized silver nanoparticles (St-Ag NPs) for rapid detection of Hg2+ in real samples was developed. The environmentally friendly green approach was utilized to synthesize starch functionalized silver nanoparticles (St-AgNPs). A multi-technique approach involving UV-Vis absorption spectroscopy, Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and scanning electron microscope (SEM) was used for the characterization of St-Ag NPs. These starch functionalized AgNPs were tested for the detection of heavy metals at 25 °C. The screening process revealed clear changes in the AgNPs color and absorption intensity only in the presence of Hg2+ due to the redox reaction between Ag0 and Hg2+. The color and absorption intensity of nanoparticles remain unchanged in the presence of all the other tested metals ion. The proposed method has strong selectivity and sensitivity to Hg2+ ions, with a detection limit of 1 ppm revealed by UV-visible spectrophotometry. The proposed procedure was found to be successful for the detection of Hg2+ in real samples of tap water.

Novel Carboxylic Acid-Capped Silver Nanoparticles as Antimicrobial and Colorimetric Sensing Agents

The present work reports the synthesis, characterization, and antimicrobial activities of adipic acid-capped silver nanoparticles (AgNPs@AA) and their utilization for selective detection of Hg²⁺ ions in an aqueous solution. The AgNPs were synthesized by the reduction of Ag⁺ ions with NaBH₄ followed by capping with adipic acid. Characterization of as-synthesized AgNPs@AA was carried out by different techniques, including UV–Visible spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Dynamic Light Scattering (DLS), and zeta potential (ZP). In the UV–Vis absorption spectrum, the characteristic absorption band for AgNPs was observed at 404 nm. The hydrodynamic size of as-synthesized AgNPs was found to be 30 ± 5.0 nm. ZP values (−35.5 ± 2.4 mV) showed that NPs possessed a negative charge due to carboxylate ions and were electrostatically stabilized. The AgNPs show potential antimicrobial activity against clinically isolated pathogens. These AgNPs were found to be selectively interacting with Hg²⁺ in an aqueous solution at various concentrations. A calibration curve was constructed by plotting concentration as abscissa and absorbance ratio (A_Control − A_Hg/A_Control) as ordinate. The linear range and limit of detection (LOD) of Hg²⁺ were 0.6–1.6 μM and 0.12 μM, respectively. A rapid response time of 4 min was found for the detection of Hg²⁺ by the nano-probe. The effect of pH and temperature on the detection of Hg²⁺ was also investigated. The nano-probe was successfully applied for the detection of Hg²⁺ from tap and river water

Highly efficient peroxidase-like activity of a metal–oxide-incorporated CeO2–MIL(Fe) metal–organic framework and its application in the colorimetric detection of melamine and mercury ions via induced hydrogen and covalent bonds

The illegal addition of melamine to dairy products and the contamination of water with Hg2+ are serious threats to human health. This necessitates the search for new and efficient probe for detection of melamine in foods and Hg2+ in water samples.

Smartphone-Based Device for Colorimetric Detection of MicroRNA Biomarkers Using Nanoparticle-Based Assay

The detection of microRNAs (miRNAs) is emerging as a clinically important tool for the non-invasive detection of a wide variety of diseases ranging from cancers and cardiovascular illnesses to infectious diseases. Over the years, miRNA detection schemes have become accessible to clinicians, but they still require sophisticated and bulky laboratory equipment and trained personnel to operate. The exceptional computing ability and ease of use of modern smartphones coupled with fieldable optical detection technologies can provide a useful and portable alternative to these laboratory systems. Herein, we present the development of a smartphone-based device called Krometriks, which is capable of simple and rapid colorimetric detection of microRNA (miRNAs) using a nanoparticle-based assay. The device consists of a smartphone, a 3D printed accessory, and a custom-built dedicated mobile app. We illustrate the utility of Krometriks for the detection of an important miRNA disease biomarker, miR-21, using a nanoplasmonics-based assay developed by our group. We show that Krometriks can detect miRNA down to nanomolar concentrations with detection results comparable to a laboratory-based benchtop spectrophotometer. With slight changes to the accessory design, Krometriks can be made compatible with different types of smartphone models and specifications. Thus, the Krometriks device offers a practical colorimetric platform that has the potential to provide accessible and affordable miRNA diagnostics for point-of-care and field applications in low-resource settings.

DNA-templated fluorescent silver nanoclusters on-off switch for specific and sensitive determination of organic mercury in seafood

Organic mercury including methyl-mercury and ethyl-mercury (CH₃Hg⁺ and C₂H₅Hg⁺) has high toxicity and bio-accumulation, and thus is easy to generate bio-amplification in food chain. Hence, the specific detection of organic mercury has great significance for objectively assessing the health risk of mercury in seafood. We herein designed an aptamer (A_S-T7), which consists of a silver nanoclusters (AgNCs) scaffold sequence (A_S) and a T-rich sequence (A_T7), for simultaneously synthetizing DNA-templated AgNCs and recognizing organic mercury, and further developed a label-free fluorescent method for the sensitive and specific determination of organic mercury (CH₃Hg⁺ and C₂H₅Hg⁺ total concentration) by using DNA-templated AgNCs as signal. Without organic mercury, Ag⁺ in the mixture of aptamer and Ag⁺ was bond on A_S of aptamer to form A_S-templated AgNCs after reduction, and thus emitted strong fluorescence. Whereas, in the presence of organic mercury, CH₃Hg⁺/C₂H₅Hg⁺ was bond on A_T7 of aptamer to generate photoinduced electron transfer (PET) between CH₃Hg⁺/C₂H₅Hg⁺ and A_S-templated AgNCs, and thus results in fluorescence quenching of A_S-templated AgNCs. The fluorescent method could be used to rapidly detect organic mercury with a detection limit of 5.0 nM (i.e. 1.01 ng Hg/g), which meets the U.S. EPA standard of 0.3 mg/kg (wet). The method was successfully used to detect organic mercury in water and fish muscle with a recovery of 96%-104% and an inter-days RSD (n = 5) < 7%. The success of the study promised a reliable method for rapid and specific detection of organic mercury in environmental and biological samples.

A Facile Colorimetric Sensor for 6-Mercaptopurine Based on Silver Nanoparticles

A facile colorimetric method was developed for detecting 6-mercaptopurine (6-MP) using silver nanoparticles (AgNPs). The addition of 6-MP to AgNPs led to the aggregation of AgNPs with a color change from yellow to brown. The ratio between the absorbance at 394 and 530 nm (A394/A530) was used for a quantitative analysis of 6-MP. A linear range of 0 - 0.5 μM was obtained with a detection limit of 10 nM. The developed method is cost-effective and simple.

Rapid and ultrasensitive surface-enhanced Raman spectroscopy detection of mercury ions with gold film supported organometallic nanobelts

Rapid, ultrasensitive and reliable detection of mercury ions (Hg²⁺) by surface enhanced Raman spectroscopy (SERS) is of importance, but is restricted by the extremely low Raman cross section of the Hg²⁺. Here, we report a facile methodology that can realize such detection based on the organometallic Cu(CH₄N₂S)Cl · 0.5H₂O nanobelts and SERS. In the assay, Hg²⁺ react with the nanobelts coated on a SERS active gold nanoparticle (NP) film to form ultrafine HgS NPs in situ. Subsequently, solid HgS is SERS determined to mirror the presence of Hg²⁺. Importantly, such detection is rapid and ultrasensitive. Within 10 min, limit of detection (LoD) of ppt level can be realized. The high detection efficiency is attributed to the superhydrophilicity, rich micropores and ultrathin nature of the organometallic nanobelts besides the strong SERS effect of Au NP film. In addition, this detection is highly resistant to various metal ions (Cu²⁺, Fe³⁺, Bi³⁺, Cr³⁺, Na⁺, Ni²⁺, Cd²⁺, etc) and is highly reliable in actual water (lake and tap water). Finally, influences of some substrate parameters and detection conditions on the test results are revealed. The optimal thickness of the gold NP film is about 80 nm, and the optimal wavelength of excitation light is about 633 nm. A small amount of Cu(CH₄N₂S)Cl · 0.5H₂O nanobelts or a large volume of Hg²⁺ contaminated solution contributes to low LoDs. We believe that this work provides a rapid and sensitive detection for Hg²⁺.

Paper-based sensors for the application of biological compound detection

This chapter describes the importance of PADs for biomarker detection. The screening of disease markers and other biomolecules that related to health conditions have play important roles for an indication of the risk from infections and other diseases. Paper-based analytical devices (PADs) is an excellent option for applications of biomarker detection because it contains all advantages which arise from the paper material. Moreover, the uncomplicated techniques including electrochemistry and colorimetry can be easily applied on PADs for the analytical detection. The detection method can be categorized into three main topics: enzymatic methods, immunoassays, and DNA sensors. Following the main context, other interesting applications also present in this chapter.

Bifunctionalized Silver Nanoparticles as Hg2+ Plasmonic Sensor in Water: Synthesis, Characterizations, and Ecosafety

In this work, hydrophilic silver nanoparticles (AgNPs), bifunctionalized with citrate (Cit) and L-cysteine (L-cys), were synthesized. The typical local surface plasmon resonance (LSPR) at λ _max = 400 nm together with Dynamic Light Scattering (DLS) measurements (<2R_H> = 8 ± 1 nm) and TEM studies (Ø = 5 ± 2 nm) confirmed the system nanodimension and the stability in water. Molecular and electronic structures of AgNPs were investigated by FTIR, SR-XPS, and NEXAFS techniques. We tested the system as plasmonic sensor in water with 16 different metal ions, finding sensitivity to Hg²⁺ in the range 1–10 ppm. After this first screening, the molecular and electronic structure of the AgNPs-Hg²⁺ conjugated system was deeply investigated by SR-XPS. Moreover, in view of AgNPs application as sensors in real water systems, environmental safety assessment (ecosafety) was performed by using standardized ecotoxicity bioassay as algal growth inhibition tests (OECD 201, ISO 10253:2006), coupled with determination of Ag⁺ release from the nanoparticles in fresh and marine aqueous exposure media, by means of ICP-MS. These latest studies confirmed low toxicity and low Ag⁺ release. Therefore, these ecosafe AgNPs demonstrate a great potential in selective detection of environmental Hg²⁺, which may attract a great interest for several biological research fields.

Cytosine-rich ssDNA-templated fluorescent silver and copper/silver nanoclusters: optical properties and sensitive detection for mercury(II)

DNA-templated silver nanoclusters (DNA-Ag NCs) with cytosine (C)-rich sequences (four or more segments of consecutive (2-5) C-bases) were synthesized. They display green and/or orange/red emissions under different excitation wavelengths. There is indication that more consecutive C-bases lead to longer emission wavelengths. The ratio of the red and green emissions of the DNA-Ag NCs depends on whether the NCs were synthesized under acidic or basic conditions. We also prepared the DNA copper/silver nanoclusters (DNA-Cu/Ag NCs) which can be synthesized in shorter time and display higher stability. The DNA-Cu/Ag NCs, under 470 nm photoexcitation, always display green emission, with a peak at 550 nm, irrespective of whether being prepared under acidic or basic conditions. The fluorescence of the Cu/Ag NCs is selectively quenched by Hg(II) ions which can be quantified by this nanoprobe with a detection limit as low as 2.4 nM. The quenching mechanism was studied by Stern-Volmer plots and lifetime studies which revealed that both static and dynamic quenching are operative. Graphical abstract Schematic presentation of fluorescent DNA-Ag nanoclusters (NCs) exhibiting red and green emission under different pH values, and green emissive DNA-Cu/Ag NCs for sensitive and rapid detection of Hg²⁺.

Progress in rapid optical assays for heavy metal ions based on the use of nanoparticles and receptor molecules

This review (with 230 refs.) covers recent progress in rapid optical assays for heavy metals (primarily lead and mercury as the most relevant) based on the use of nanoparticles and receptor molecules. An introduction surveys the importance, regulatory demands (such as maximum permissible concentrations) and potential and limitations of various existing methods. This is followed by a general discussion on the use of nanoparticles in optical assays of heavy metals (including properties, basic mechanisms of signal generation). The next sections cover methods for the functionalization of nanoparticles with (a) sulfur-containing compounds (used for modification of nanoparticles or added to the reaction medium), (b) nitrogen-containing compounds (such as amino acids, polypeptides, and heterocyclic molecules), and (c) oxygen-containing species (such as hydroxy and carbonyl compounds). This is continued by a specific description of specific assays based on the use of aptamers as receptors, on the use of deoxyribozymes as synthetic reaction catalysts, of G-quadruplex aptamers, of aptamers in logic gate-type of assays of linear (unstructured) aptamers ("hairpins"), and on the use of aptamers in lateral flow assays. A next section covers assays based on the employment of antibodies as receptors (used in the immunoassay development). The properties of various nanoparticles and their applicability in optical assays are also discussed in some detail. Final sections discuss the selectivity of assays, potential interferences by other cations, methods for their elimination, and also matrix effects and approaches for sample pretreatment. A concluding section discusses current challenges and future trends. Analysis based on enzyme inhibition assay is not treated here but enzyme-like action of some receptor molecules such as DNAzymes is discussed. Graphical abstract Schematic presentation of main principles of application of various nanoparticles with receptor molecules (S-, N-, O-containing, heterocyclic compounds, proteins, antibody, aptamers) for heavy metals ions detection. The included methods cover optical assays with description of mechanisms of interactions and signal generation.

Plasmonic spectral determination of Hg(II) based on surface etching of Au-Ag core-shell triangular nanoplates: From spectrum peak to dip

In this work, we develop a simple and selective sensing method for the detection of mercury ions based on surface plasmon resonance (SPR) spectrum change of Au-Ag core-shell triangular nanoplates. When the concentration of mercury is increased, the etching-induced change of particle size and shape also leads to the decrease of the absorption peak at the fixed wavelength, until a spectrum dip takes place. This spectral change of "peak-to-dip" greatly enlarges the detection range of mercury ions, which could be fine tuned by changing the initial thickness of the Ag coating. Under optimal conditions, the decrease of the logarithmic absorption intensity has a good linear response with the concentration of mercury ions increasing from 10 to 1000 μM, and the limit of detection (LOD) is 0.88 μM. Interference studies and real samples test indicate that, this new sensing method has a good selection for mercury ions and can be practically used in lake water. This work shows the surface etching-induced SPR shift can also leads to the intensity change with "peak-to-dip" fashion, which greatly enlarge the concentration range of the detection and could be widely applied in the spectroscopy sensing based on SPR.

A novel and ultrasensitive yellow to taupe brown colorimetric sensing and removal method for Hg(ii) based on the thermosensitive poly(N-isopropyl acrylamide) stabilized silver nanoparticles

AgNP/PNIPAm was developed as a dual-functional colorimetric probe and removal system for Hg²⁺.

Functionalized Silver Nano-Sensor for Colorimetric Detection of Hg2+ Ions: Facile Synthesis and Docking Studies

In the present study, we describe the facile synthesis of silver nanoparticles (AgNPs) and their nanostructures functionalized with 2-aminopyrimidine-4,6-diol (APD-AgNPs) for Hg²⁺ ion detection. The promising colorimetric response of APD-AgNPs to detect Hg²⁺ ions was visible with naked eyes and spectroscopic changes were examined by using a UV-Visible spectrophotometer. The aggregation of APD-AgNPs upon addition of Hg²⁺ ions was due to the chelation effect of the functionalized nanostructures and results in a color change from pale brown to deep yellow color. The probing sensitivity was observed within five minutes with a detection limit of about 0.35 µM/L. The TEM images of APD-AgNPs showed polydispersed morphologies with hexagonal, heptagonal and spherical nanostructures with an average size between 10 to 40 nm. Furthermore, the sensing behavior of APD-AgNPs towards Hg²⁺ ions detection was investigated using docking and interaction studies.

Colorimetric Detection of Mercury(II) Ion in Aqueous Solution Using Silver Nanoparticles

Novel green-chemistry synthesis of silver nanoparticles (AgNPs) is introduced as a low-cost, rapid and easy-to-use analytical method for mercury ion detection. Aqueous fruit extract of water apple (Syzygium aqueum) was used for the first time as bioreductant to synthesize stable AgNPs. The prepared AgNPs have a yellowish-brown color with a surface plasmon resonance peak at 420 nm. The addition of Hg(II) ions then changes the AgNPs color to colorless. The color change was in proportion to the concentration of Hg(II) ions. The presence of other metal ions in the system was also evaluated. The proposed method shows good selectivity and sensitivity towards Hg(II) ions. Using UV-visible spectrophotometry, the detection limit of the developed method was 8.5 × 10^-7 M. The proposed method has been successfully applied for determination of Hg(II) ions in tap and lake water samples with precision better than 5%.

Aptamer immobilization on amino-functionalized metal-organic frameworks: an ultrasensitive platform for the electrochemical diagnostic of Escherichia coli O157:H7

Herein, we report the development of an electrochemical biosensor for Escherichia coli O157:H7 diagnostic based on amino-functionalized metal-organic frameworks (MOFs) as a new generation of organic-inorganic hybrid nanocomposites. The electrical and morphological properties of MOFs were enhanced by interweaving each isolated MOF crystal with polyaniline (PANI). Subsequent attachment of the amine-modified aptamer to the polyanilinated MOFs was accomplished using glutaraldehyde (GA) as a cross-linking agent. The prepared biocompatible platform was carefully characterized by means of field-emission scanning electron microscopy (FESEM), energy-dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR), and X-ray powder diffraction (XRD) techniques. The biosensor fabrication and its electrochemical characterizations were monitored by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques. Differential pulse voltammetry (DPV) was applied to monitoring and quantitation of the interaction between the aptamer and E. coli O157:H7 using methylene blue (MB) as an electrochemical indicator. Changes in the reduction peak current of MB in the presence of E. coli O157:H7 was recorded as an analytical signal and indicated a relationship with the logarithm of the E. coli O157:H7 concentration in the range of 2.1 × 10¹ to 2.1 × 10⁷ CFU mL^-1 with a LOQ of 21 CFU mL^-1 and LOD of 2 CFU mL^-1. The electrochemical aptasensor displayed good recovery values for the detection of E. coli O157:H7 in environmental real samples and also could act as a smart device to investigate the effects of antibacterial agents against E. coli O157:H7.

A wavelength-modulated localized surface plasmon resonance (LSPR) optical fiber sensor for sensitive detection of mercury(II) ion by gold nanoparticles-DNA conjugates

The study presented herein investigated an easy preparation, high performance, wavelength-modulated LSPR optical fiber chemosensor coated by gold nanospheres(AuNS) for Hg²⁺ detection based on thymine-Hg²⁺-thymine base pair mismatches and the coupled plasmonic resonance effect.Utilizing electrostatic self-assembly method, the high density and dispersivity monolayer AuNS coated LSPR fiber sensor had the near field refractive index sensitivity up to 2016 nm/RIU. The single-strand probe DNA served as a binding element for free AuNS labelled-target DNA conjugates was attached to the monolayer AuNS by Au-S bond. In the present of Hg²⁺, the coupled plasmonic resonance band between monolayer AuNS and free AuNS was produced by thymine-Hg²⁺-thymine structure and leaded to red-shift of LSPR peak. Under the optimal conditions, the enlarged red-shift in peak of LSPR spectroscopy was linearly with the concentration of Hg²⁺ in the range from 1.0 × 10^-9 to 5.0 × 10^-8 M with the coefficient of 0.976. The limit of detection was 0.7 nM(S/N = 3). The specificity of the sensor was proved high by evaluating the response to other heavy metal ions. The proposed fiber sensor provided a label-free, miniature, low-cost approach for the Hg²⁺ detection and had potential in real environmental evaluations.

Influence of ligand chemistry on silver nanoparticles for colorimetric detection of Cr3+ and Hg2+ ions

In this work, we describe the role of ligand chemistry on the surfaces of silver nanoparticles (Ag NPs) for tuning their analytical applications. The citrate and melamine (MA) molecules were used as ligands for the surface modification of Ag NPs. The addition of Cr³⁺ ion in citrate-Ag NPs (Cit-Ag NPs) and of Hg²⁺ ion in melamine-Ag NPs (MA-Ag NPs) cause Ag NPs aggregation, and are accompanied by a color change and a red-shift. The resulting distinctly visual readouts are favorable for colorimetric detection of Cr³⁺ and Hg²⁺ ions. Under optimal conditions, the linear ranges are observed in the concentration ranges of 1.0-50.0 and of 10.0-100.0 μM, and with detection limit of 0.52 and 1.80 μM for Cr³⁺ and Hg²⁺ ions. The simultaneous detection of Cr³⁺ and Hg²⁺ ion is driven by the changing the ligand chemistry on the surfaces of Ag NPs that allows to tune their specific interactions with target analytes. Finally, the functionalized Ag NPs were successfully applied to detect Cr³⁺ and Hg²⁺ ions in water samples with satisfactory recoveries.

Thiamine-functionalized silver nanoparticles for the highly selective and sensitive colorimetric detection of Hg2+ ions

Hg²⁺ contamination is a serious threat to the environment; hence, the development of methods for its trace level detection is urgently required.