Ultrasensitive and selective colorimetric and smartphone-based detection of arsenic ions in aqueous solution using alliin-chitosan-AgNPs

In this study, we developed a highly selective and sensitive colorimetric sensor for arsenic [As(iii)] detection using alliin-chitosan-stabilized silver nanoparticles (AC-AgNPs). The AC-AgNPs were synthesized using a complex prepared by mixing aqueous garlic extract containing alliin and chitosan extracted from shrimp. The synthesis of AC-AgNPs was confirmed by UV-vis spectroscopy, which showed a surface plasmon resonance (SPR) band at 403 nm, and TEM analysis revealing spherical nanoparticles with a mean diameter of 7.57 ± 3.52 nm. Upon the addition of As3+ ions, the brownish-coloured solution of AC-AgNPs became colourless. Moreover, the computational study revealed that among all the metal ions, only As3+ was able to form a stable complex with AC-AgNPs, with a binding energy of 8.7 kcal mol-1. The sensor exhibited a linear response to As(iii) concentrations ranging from 0.02 to 1.4 fM, with a detection limit of 0.023 fM. The highest activity was observed at pH 7 and temperature 25 °C. Interference studies demonstrated high selectivity against common metal ions. The study also demonstrated that the concentration of As3+ ions can be estimated by the decrease in red intensity and increase in green intensity in smartphone optical transduction signals. These results indicate the potential of the AC-AgNP-based sensor for reliable and efficient arsenic detection in environmental monitoring.

Advances in Nanomaterials and Colorimetric Detection of Arsenic in Water: Review and Future Perspectives

Arsenic, existing in various chemical forms such as arsenate (As(V)) and arsenite (As(III)), demands serious attention in water and environmental contexts due to its significant health risks. It is classified as "carcinogenic to humans" by the International Agency for Research on Cancer (IARC) and is listed by the World Health Organization (WHO) as one of the top 10 chemicals posing major public health concerns. This widespread contamination results in millions of people globally being exposed to dangerous levels of arsenic, making it a top priority for the WHO. Chronic arsenic toxicity, known as arsenicosis, presents with specific skin lesions like pigmentation and keratosis, along with systemic manifestations including chronic lung diseases, liver issues, vascular problems, hypertension, diabetes mellitus, and cancer, often leading to fatal outcomes. Therefore, it is crucial to explore novel, cost-effective, and reliable methods with rapid response and improved sensitivities (detection limits). Most of the traditional detection techniques often face limitations in terms of complexity, cost, and the need for sophisticated equipment requiring skilled analysts and procedures, which thereby impedes their practical use, particularly in resource-constrained settings. Colorimetric methods leverage colour changes which are observable and quantifiable using simple instrumentation or even visual inspection. This review explores the colorimetric techniques designed to detect arsenite and arsenate in water. It covers recent developments in colorimetric techniques, and advancements in the role of nanomaterials in colorimetric arsenic detection, followed by discussion on current challenges and future prospects. The review emphasizes efforts to improve sensitivity, selectivity, cost, and portability, as well as the role of advanced materials/nanomaterials to boost the performance of colorimetric assays/sensors towards combatting this pervasive global health concern.

Quantifying colors at micrometer scale by colorimetric microscopy (C-Microscopy) approach

The color is the primal property of the objects around us and is direct manifestation of light-matter interactions. The color information is used in many different fields of science, technology and industry to investigate material properties or for identification of concentrations of substances. Usually the color information is used as a global parameter in a macro scale. To quantitatively measure color information in micro scale one needs to use dedicated microscope spectrophotometers or specialized micro-reflectance setups. Here, the Colorimetric Microscopy (C-Microscopy) approach based on digital optical microscopy and a free software is presented. The C-Microscopy approach uses color calibrated image and colorimetric calculations to obtain physically meaningful quantities i.e., dominant wavelength and excitation purity maps at micro level scale. This allows for the discovery of the local color details of samples surfaces. Later, to fully characterize the optical properties, the hyperspectral reflectance data at micro scale (reflectance as a function of wavelength for a each point) are colorimetrically recovered. The C-Microscopy approach was successfully applied to various types of samples i.e., two metamorphic rocks unakite and lapis lazuli, which are mixtures of different minerals; and to the surface of gold 99.999 % pellet, which exhibits different types of surface features. The C-Microscopy approach could be used to quantify the local optical properties changes of various materials at microscale in an accessible way. The approach is freely available as a set of python jupyter notebooks.

M13 bacteriophage as biometric component for orderly assembly of dynamic light scattering immunosensor

The ordered assembly of nanostructure is an effective strategy used to manipulate the hydrodynamic diameter (D_H) of nanoparticles. Herein, a versatile dynamic light scattering (DLS) immunosensing platform is presented to sensitively detect small molecules and biomacromolecules by using the M13 phage as the building module to order the assembly of gold nanoflowers and gold-coated magnetic nanoparticles, respectively. After the directional assembly of M13 phage, the D_H of the probes was significantly increased due to its larger filamentous structure, thus improving the detection sensitivity of the DLS immunosensor. The designed M13 assembled DLS immunosensor with competitive and sandwich formats showed high sensitivities for ochratoxin A and alpha-fetoprotein in real corn and undiluted serum samples, with the detection limits of 1.37 and 57 pg/mL, respectively. These values are approximately 15.8 and 164.9 times lower than those of traditional phage-based enzyme-linked immunosorbent assays. Collectively, this work provides a promising strategy to manipulate the D_H of nanoparticles by highly evolved biomaterials such as engineered M13 phages and opens upon a new direction for developing DLS immunosensors to detect various targets by the fusion expression of special peptide or nanobody on the pIII or pVIII protein of M13 phage.

The Integration of Gold Nanoparticles with Polymerase Chain Reaction for Constructing Colorimetric Sensing Platforms for Detection of Health-Related DNA and Proteins

Polymerase chain reaction (PCR) is the standard tool in genetic information analysis, and the desirable detection merits of PCR have been extended to disease-related protein analysis. Recently, the combination of PCR and gold nanoparticles (AuNPs) to construct colorimetric sensing platforms has received considerable attention due to its high sensitivity, visual detection, capability for on-site detection, and low cost. However, it lacks a related review to summarize and discuss the advances in this area. This perspective gives an overview of established methods based on the combination of PCR and AuNPs for the visual detection of health-related DNA and proteins. Moreover, this work also addresses the future trends and perspectives for PCR-AuNP hybrid biosensors.

A Universal Boronate-Affinity Crosslinking-Amplified Dynamic Light Scattering Immunoassay for Point-of-Care Glycoprotein Detection

Herein, we report a universal boronate-affinity crosslinking-amplified dynamic light scattering (DLS) immunoassay for point-of-care (POC) glycoprotein detection in complex samples. This enhanced DLS immunoassay consists of two elements, i.e., antibody-coated magnetic nanoparticles (MNP@mAb) for target capture and DLS signal transduction, and phenylboronic acid-based boronate-affinity materials as crosslinking amplifiers. Upon the addition of targets, glycoproteins are first captured by MNP@mAb and amplified by target-induced crosslinking stemming from the selective binding between the boronic acid ligand and cis-diol-containing glycoprotein, thereby resulting in a remarkably increased DLS signal in the average nanoparticle size. Benefiting from the multivalent binding and fast boronate-affinity reaction between glycoproteins and crosslinkers, the proposed immunosensing strategy has achieved the ultrasensitive and rapid quantitative assay of glycoproteins at the fM level within 15 min. Overall, this work provides a promising and versatile design strategy for extending the DLS technique to detect glycoproteins even in the field or at POC.

Gold and Silver Nanoparticle-Based Colorimetric Sensors: New Trends and Applications

Gold and Silver nanoparticles (AuNPs and AgNPs) are perfect platforms for developing sensing colorimetric devices thanks to their high surface to volume ratio and distinctive optical properties, particularly sensitive to changes in the surrounding environment. These characteristics ensure high sensitivity in colorimetric devices. Au and Ag nanoparticles can be capped with suitable molecules that can act as specific analyte receptors, so highly selective sensors can be obtained. This review aims to highlight the principal strategies developed during the last decade concerning the preparation of Au and Ag nanoparticle-based colorimetric sensors, with particular attention to environmental and health monitoring applications.

Glutathione-functionalized long-period fiber gratings sensor based on surface plasmon resonance for detection of As3+ ions

Development of simple and accurate methods for the detection of As³⁺is highly desirable and technically important. In this work, a highly sensitive and selective long-period fiber gratings sensor based on surface plasmon resonance was developed for As³⁺detection by designing glutathione-functionalized Au nanoparticles as a signal amplification tag. Based on the chemical interaction between As³⁺and glutathione, the self-assembling glutathione on the surface of the gold film combines selectively with As³⁺, and then anchors the glutathione-functionalized Au nanoparticles, which changes the refractive index of the surrounding environment, resulting in a shift of the transmission spectrum. Results show that the sensor could detect As³⁺with concentrations ranging from 0.02 to 2 ppb. The sensor exhibited excellent specificity for As³⁺against other metal ions, such as Na⁺, Fe³⁺, Mg²⁺, Cu²⁺, Pb²⁺, Ni²⁺, Ba²⁺, and Co³⁺. The fiber sensor was successfully employed to detect As³⁺in pond water samples, demonstrating that it has the potential for As³⁺detection with the advantages of low cost, high sensitivity, and a simple structure.

Recent Advances in Colorimetric Detection of Arsenic Using Metal-Based Nanoparticles

Nowadays, arsenic (III) contamination of drinking water is a global issue. Laboratory and instrument-based techniques are typically used to detect arsenic in water, with an accuracy of 1 ppb. However, such detection methods require a laboratory-based environment, skilled labor, and additional costs for setup. As a result, several metal-based nanoparticles have been studied to prepare a cost-effective and straightforward detector for arsenic (III) ions. Among the developed strategies, colorimetric detection is one of the simplest methods to detect arsenic (III) in water. Several portable digital detection technologies make nanoparticle-based colorimetric detectors useful for on-site arsenic detection. The present review showcases several metal-based nanoparticles that can detect arsenic (III) colorimetrically at a concentration of ~0.12 ppb or lower in water. A literature survey suggests that biomolecule-based metal nanoparticles could serve as low-cost, facile, susceptible, and eco-friendly alternatives for detecting arsenic (III). This review also describes future directions, perspectives and challenges in developing this alternative technology, which will help us reach a new milestone in designing an effective arsenic detector for commercial use.

The rapid diagnosis and effective inhibition of coronavirus using spike antibody attached gold nanoparticles

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the cause of the coronavirus disease that began in 2019 (COVID-19), has been responsible for 1.4 million deaths worldwide as of 13 November 2020. Because at the time of writing no vaccine is yet available, a rapid diagnostic assay is very urgently needed. Herein, we present the development of anti-spike antibody attached gold nanoparticles for the rapid diagnosis of specific COVID-19 viral antigen or virus via a simple colorimetric change observation within a 5 minute time period. For rapid and highly sensitive identification, surface enhanced Raman spectroscopy (SERS) was employed using 4-aminothiophenol as a reporter molecule, which is attached to the gold nanoparticle via an Au-S bond. In the presence of COVID-19 antigen or virus particles, owing to the antigen-antibody interaction, the gold nanoparticles undergo aggregation, changing color from pink to blue, which allows for the determination of the presence of antigen or virus very rapidly by the naked eye, even at concentrations of 1 nanogram (ng) per mL for COVID-19 antigen and 1000 virus particles per mL for SARS-CoV-2 spike protein pseudotyped baculovirus. Importantly, the aggregated gold nanoparticles form "hot spots" to provide very strong SERS signal enhancement from anti-spike antibody and 4-aminothiophenol attached gold nanoparticles via light-matter interactions. Finite-difference time-domain (FDTD) simulation data indicate a 4-orders-of-magnitude Raman enhancement in "hot spot" positions when gold nanoparticles form aggregates. Using a portable Raman analyzer, our reported data demonstrate that our antibody and 4-aminothiophenol attached gold nanoparticle-based SERS probe has the capability to detect COVID-19 antigen even at a concentration of 4 picograms (pg) per mL and virus at a concentration of 18 virus particles per mL within a 5 minute time period. Using HEK293T cells, which express angiotensin-converting enzyme 2 (ACE2), by which SARS-CoV-2 enters human cells, we show that anti-spike antibody attached gold nanoparticles have the capability to inhibit infection by the virus. Our reported data show that antibody attached gold nanoparticles bind to SARS-CoV-2 spike protein, thereby inhibiting the virus from binding to cell receptors, which stops virus infection and spread. It also has the capability to destroy the lipid membrane of the virus.

Chemical sensing with Au and Ag nanoparticles

Noble metal nanoparticles (NPs) are ideal scaffolds for the fabrication of sensing devices because of their high surface-to-volume ratio combined with their unique optical and electrical properties which are extremely sensitive to changes in the environment. Such characteristics guarantee high sensitivity in sensing processes. Metal NPs can be decorated with ad hoc molecular building blocks which can act as receptors of specific analytes. By pursuing this strategy, and by taking full advantage of the specificity of supramolecular recognition events, highly selective sensing devices can be fabricated. Besides, noble metal NPs can also be a pivotal element for the fabrication of chemical nose/tongue sensors to target complex mixtures of analytes. This review highlights the most enlightening strategies developed during the last decade, towards the fabrication of chemical sensors with either optical or electrical readout combining high sensitivity and selectivity, along with fast response and full reversibility, with special attention to approaches that enable efficient environmental and health monitoring.

A colorimetric assay for the determination of trace arsenic based on in-situ formation of AuNPs with synergistic effect of arsine and iodide

In this work, we propose a colorimetric assay for the determination of trace arsenic based on in-situ formation of AuNPs with the synergistic effect of arsine (AsH₃) and iodide. AsH₃, generated by hydride generation of As^III in the sample or standard solution, enters into the HAuCl₄ solution containing polyvinyl alcohol (PVA) and KI, and then reacts rapidly to form AuNPs, resulting in the solution color changing from light yellow to pink. Hydride generation applied here not only produces a strong reducing agent AsH₃, but also effectively reduces matrix interference. The introduction of I^- promotes the reaction by reducing the Au precursor from trivalent state to monovalent state, thus accelerating the formation of AuNPs with AsH₃ and improving the sensitivity for the detection of arsenic. Trace As^III as low as 10 μg L^-1 in 3 mL sample solution can produce the change in color visible to the naked eye. Moreover, the use of the stabilizer PVA and the gaseous strong-reducing agent AsH₃ evenly dispersed in the reaction solution lead to the formation of well-distributed and fine AuNPs of size changing little with the dosage of AsH₃. The whole analysis process only takes 30 min under ambient condition without complicated synthesis and pretreatment. The proposed assay is simple, stable, sensitive and selective, providing a convenient and cost-effective choice for on-site trace arsenic detection in real samples.

Nano-Enabled sensors for detection of arsenic in water

The elevated cases of arsenic contamination reported across the globe have made its early detection and remediation an active area of research. Although, the World Health Organisation has set the maximum provisional value for arsenic in drinking water at 10 parts per billion, yet concentrations as high as 5000 parts per billion are still reported. In human beings, chronic arsenic exposure can culminate into lethal diseases such as cancer. Thus, there is a need for urgent emergence of efficient and reliable detection system. This paper offers an overview of the state-of-art knowledge on current arsenic detection mechanisms. The central agenda of this paper is to develop an understanding into the nano-enabled methods for arsenic detection with an emphasis on strategic fabrication of nanostructures and the modulation of nanomaterial chemistry in order to strengthen the knowledge into novel nano-enabled solutions for arsenic contamination. Towards the end prospects for arsenic detection in water are also prompted.

Long-term continuous and real-time in situ monitoring of Pb(II) toxic contaminants in wastewater using solid-state ion selective membrane (S-ISM) Pb and pH auto-correction assembly

Lead (Pb) contaminants in wastewater have inhibited microbial activities and thus exerted high energy consumption in wastewater treatment plants (WWTPs). Current Pb monitoring has been conducted ex situ and off line, unable to affect real-time proactive control and operation. This study targets the crucial challenge of better and faster Pb monitoring by developing novel mm-sized screen-printed solid-state ion-selective membrane (S-ISM) Pb sensors with low-cost, high accuracy and long-term durability and that enable real-time in situ monitoring of Pb(II) ion contamination down to low concentrations (15 ppb-960 ppb) in wastewater. An innovative pH auto-correction data-driven model was built to overcome the inextricable pH inferences on Pb(II) ISM sensors in wastewater. Electrochemical impedance spectroscopy (EIS) and cyclic voltammograms (CV) analysis showed (3,4-ethylenedioxythiophene, EDOT) deposited onto the mm-sized screen-printed carbon electrodes using electropolymerization effectively alleviated the interferences from dissolved oxygen and improved long-term stability in wastewater. Monte Carlo simulation of the nitrification process predicted that real-time, and high accurate in situ monitoring of Pb(II) in wastewater and swift feedback control could save ∼53 % of energy consumption by alleviating the errors from pH and DO impacts in WWTPs.

Portable smartphone-integrated paper sensors for fluorescence detection of As(III) in groundwater

Arsenic contamination in groundwater is a supreme environmental problem, and levels of this toxic metalloid must be strictly monitored by a portable, sensitive and selective analytical device. Herein, a new system of smartphone-integrated paper sensors with Cu nanoclusters was established for the effective detection of As(III) in groundwater. For the integration system, the fluorescence emissive peak of Cu nanoclusters at 600 nm decreased gradually with increasing As(III) addition. Meanwhile, the fluorescence colour also changed from orange to colourless, and the detection limit was determined as 2.93 nM (0.22 ppb) in a wide detection range. The interfering ions also cannot influence the detection selectivity of As(III). Furthermore, the portable paper sensors based on Cu nanoclusters were fabricated for visual detection of As(III) in groundwater. The quantitative determination of As(III) in natural groundwater has also been accomplished with the aid of a common smartphone. Our work has provided a portable and on-site detection technique toward As(III) in groundwater with high sensitivity and selectivity.

Direct competitive ELISA enhanced by dynamic light scattering for the ultrasensitive detection of aflatoxin B1 in corn samples

Compared with absorbance, scattering-based dynamic light scattering (DLS) signal has higher sensitivity because its light-scattering intensity is very sensitive to changes in size, thereby enhancing the sensitivity. Herein, we first developed a DLS-enhanced direct competitive enzyme-linked immunosorbent assay (DLS-dcELISA) for ultrasensitive detection of aflatoxin B₁ (AFB₁) in corn. By using hydroxyl radical-induced gold nanoparticle (AuNP) aggregation to amplify AuNP scattering signals, the developed DLS-dcELISA exhibited ultrahigh sensitivity for AFB₁. The detection limit was 0.12 pg mL^-1, which was 153- and 385-fold lower than those obtained using plasmonic and colorimetric dcELISA. In addition, the DLS-dcELISA exhibited excellent selectivity, high accuracy, and strong practicality. Overall, this work presented a simple and universal strategy for improving the sensitivity of traditional ELISA platform only by using the sensitive DLS signals. This technique can replace absorbance-based plasmonic or colored signals as immunoassay signal output for enhanced competitive detection of mycotoxins.

Ion-mediated self-assembly of Cys-capped quantum dots for fluorescence detection of As(iii) in water

A sensitive As(iii) ion detection method has been developed based on ion-mediated self-assembly of cysteine (Cys)-capped quantum dots (QDs), and fluorescence self-quenching. A variety of Cys-capped core/shell CdTe/CdS QDs were prepared via hydrothermal methods. Based on the coordination binding between the As(iii) ion and cystine groups anchored on the QDs, addition of As(iii) ions led to self-assembly of the Cys-capped QDs, which was accompanied by fluorescence self-quenching. The fluorescence response was attributed to the exciton energy transfer of the QD aggregates. The ion-mediated fluorescence quenching was further exploited for quantitative determination of As(iii) ions in water. A limit of detection (LOD) of 10 ng L^-1 (3σ method) and a linear range from 14 to 70 ng L^-1 were obtained for the sensing of As(iii) ions. The system was evaluated using a series of interference targets, and demonstrated high selectivity after addition of mask agents. Finally, the proposed method was successfully employed for the detection of As(iii) in a real water sample. The method was sensitive and specific, and shows great promise in quantitative determination of heavy metal ions in lakes and rivers.

Design of Fluorescence-Enhanced Silver Nanoisland Chips for High-Throughput and Rapid Arsenite Assay

High-throughput and rapid arsenite (As(III)) monitoring is an urgent task to deal with the critical threat from As(III) contamination in the environment. In this study, an effective, portable, and sensitive As(III) assay was developed using the plasmonic silver (pAg) chips for As(III) detection. The pAg chips were fabricated by a simple seed-mediated method to grow the silver nanoisland films (Ag-NIFs) with the compact nanoislands and adjustable interisland gaps on the large-sized substrates. With appropriate surface functionalization and optimal chip manufacturing, Cy7.5 fluorescence dye can be immobilized on the surface of Ag-NIFs in the presence of As(III) to output the enhanced fluorescence signals up to 10-fold and improve the detection limit of As(III) less than 10 ppb. According to our results, the high-throughput detection measurements and wide dynamic range over 4 orders of magnitude implied the broad prospects of pAg chips in fluorescence-enhanced assays. The proposed As(III) assay has shown great opportunities for the practical application of ultratrace As(III) monitoring.

Mercury speciation based on mercury-stimulated peroxidase mimetic activity of gold nanoparticles

Mercury speciation is of significant importance in environmental and biological analysis because its toxicity and metabolic behavior in the human body differ among species. Nanomaterial-assisted optical sensors are widely used for mercury ion detection but rarely applied in mercury speciation analysis. In this work, we develop a novel colorimetric sensing strategy for mercury speciation based on mercury-stimulated peroxidase mimetic activity of gold nanoparticles with the assistance of different reductants. In the presence of a weak reductant, only inorganic mercury can be reduced to Hg⁰, whereas both inorganic mercury and organic mercury can be reduced to Hg⁰ in the presence of a strong reductant. Due to the high affinity between Hg and Au, Hg⁰ deposits on the AuNP surface in the form of a Au-Hg amalgam, leading to a remarkable enhancement of peroxidase mimetic activity of gold nanoparticles. On the basis of this effect, inorganic mercury and total mercury can be detected by using 3,3',5,5'-tetramethylbenzidine (TMB) as the substrate. The limits of detection for inorganic mercury and total mercury are 1.9 and 0.9 nM within 5-100 nM, respectively. The selectivity of this sensing system is high due to the specificity of Au-Hg interaction. Its practical applications are further demonstrated by organic mercury analysis in a fish sample and mercury speciation in a human hair sample.

A simple paper-based approach for arsenic determination in water using hydride generation coupled with mercaptosuccinic-acid capped CdTe quantum dots

This research aims to develop a simple paper-based device for arsenic detection in water samples where a hydride generation technique coupled with mercaptosuccinic acid-capped CdTe quantum dots (MSA-CdTe QDs) as a detection probe was applied to the detection system. MSA-CdTe QDs were coated on a paper strip, inserted into the cover cap of a reaction bottle, to react with the developed arsine gas. Fluorescent emission of the QDs was quenched upon the presence of arsenic in solutions, whereby only a small amount of the MSA-CdTe QDs was required. The excitation and emission wavelengths for fluorescent detection were 278.5 nm and 548.5 nm, respectively. The proposed system provided a limit of detection of 0.016 mg L-1 and a limit of quantitation of 0.053 mg L-1, and a detection range of 0.05-30.00 mg L-1. In addition, the tolerance level of the detection approach to interference by other vapor-generated species was successfully improved by placing another paper strip coated with a solution of saturated lead acetate in front of the detection paper strip. This developed approach offered a simple and fast, yet accurate and selective detection of arsenic contaminated in water samples. In addition, the mechanism of fluorescent quenching was also proposed.

Sensitive colorimetric detection of Pb2+ by geometric field amplification and surface plasmon resonance visualization

Pb²⁺ is one of the major environmental pollutants, which can be visually detected by surface plasmon resonance of nanoparticles. Paper based analytical device, as a newly developed microfluidic detection platform, is featured in cost-effective and suitable for on-site analysis. In this paper, a sensitive and portable detection method for Pb²⁺ was proposed, in which Pb²⁺ was electrokinetically stacked on the paper fluidic channel by geometric field amplification effect and visualized online by glutathione-modified silver nanoparticles. Colorimetric quantification of the visualized stacking band was conducted by smart phone camera. To avoid unfavorable influence from pH change on the surface plasmon resonance visualization, field amplification effect was introduced by geometric design of the paper fluidic channel. The enriched Pb²⁺ was clearly visible on the paper substrate, and the stacking band intensity was about four orders of magnitude enhanced, comparing to the intensity without stacking. A linear response to Pb²⁺ was observed in the range of 0.3-7.0 μM (R² = 0.997) with a limit of detection of 86 nM and a limit of quantity of 0.28 μM. The established method was used in the detection of Pb²⁺ from river and lake water samples, and the results were confirmed by atomic absorption spectroscopy method.

Enhanced Thermometric Sensor for Arsenate Analysis Based on Dual Temperature Readout Signaling Strategy

New methods for portable detection of arsenate are still in urgent need. Herein, we explored a simple but sensitive thermometric strategy for arsenate determination without complex instruments and skilled technicians. Cobalt oxyhydroxide (CoOOH) nanoflakes, can ingeniously decompose hydrogen peroxide into oxygen in a sealed reaction vessel, accompanied by marked pressure and significant temperature increase due to the exothermic reaction effect (ΔH = -98.2 kJ/mol). The increased pressure then compelled a certain amount of H₂O overflowing from the drainage device into another vessel, leading to a significant temperature decrease due to the preloaded ammonium nitrate (NH₄NO₃) and its good dissolution endothermic effect (ΔH = 25.4 kJ/mol). In the presence of arsenate, the catalytic activity of CoOOH nanoflakes for H₂O₂ decomposition was inhibited dramatically, resulting in an obvious decrease of the pressure, weighting water and temperature response. The two temperature responses with increasing and decreasing feature were easily measured through a common thermometer, and exhibited an effective signaling amplification via coupling both "signal-on" and "signal-off" temperature readout elements. The obtained dual superimposing temperature readout exhibits a good linear with the concentration of arsenate with a lower detection limit (51 nM, 3.8 ppb). Compared to the inductively coupled plasma mass spectrometry, this enhanced thermometric strategy provides a simple, rapid, convenient, low cost, and portable platform for sensing arsenate in real environmental water.

Interactions between gold, thiol and As(iii) for colorimetric sensing

Arsenite cannot crosslink glutathione-capped gold nanoparticles but a high concentration of arsenite can displace adsorbed glutathione, indicating that any two species from gold, thiol and arsenite can react.

Gold Nanoflower-Enhanced Dynamic Light Scattering Immunosensor for the Ultrasensitive No-Wash Detection of Escherichia coli O157:H7 in Milk

Gold nanoflowers (GNFs) exhibit stronger light scattering ability than gold nanospheres (GNSs) with the same diameter, thereby contributing to enhancing the sensitivity of the scattering-based sensing method. However, the application of GNFs in biosensors based on dynamic light scattering (DLS) has not been yet reported. Herein, we describe for the first time an improved no-wash immunosensor based on dynamic light scattering for the detection of Escherichia coli O157:H7 (E. coli O157:H7) in milk using GNFs for sensitive signal transduction. To achieve this goal, a thiolated amphiphilic carboxyl ligand was introduced to modify the GNF surface and improve solution stability and antibody functionalization. Several key factors that affect the detection sensitivity of our developed GNF_DLS immunosensor were systematically investigated. Under the optimal conditions, our proposed GNF_DLS immunosensor provided an excellent linear detection for E. coli O157:H7 within the range from 6 × 10⁰ to 6 × 10⁴ colony-forming units (CFU)/mL, with a limit of detection of 2.7 CFU/mL. Combined with our previously reported two-step large-volume immunomagnetic separation (IMS) method, the designed GNF_DLS immunosensor can sensitively, selectively, and accurately detect the presence of E. coli O157:H7 in pasteurized milk. The potential of our GNF_DLS method for monitoring the presence of a single bacterial cell in 1 mL of sample solution was also demonstrated. Overall, the developed GNF_DLS immunosensor can be used for the rapid and high-sensitivity determination of pathogenic bacteria and can be extended for the ultrasensitive no-wash detection of other trace analytes.

Core-shell Fe3O4@Au nanocomposite as dual-functional optical probe and potential removal system for arsenic (III) from Water

We report for the very first time, development of a dual functional nanocomposite to perform as an optical probe as well as removal system for As(III) from ground water. Upon suitable thiolation using dithiothreitol (DTT), the Fe₃O₄(core)-Au(shell) nanocomposite (DTT-Fe₃O₄@Au) has been fabricated that can detect As(III) in aqueous solution with significantly low limit of detection and holds potential for selective removal of As(III) from water owing to its magnetic core. Due to high affinity of -SH groups for As(III), the nanoparticles undergo aggregation in the presence of As(III), resulting in a significant decrease in absorbance, yielding the limit of detection (LOD) as 0.86 ppb, which is much lower than the World Health Organisation (WHO) recommended threshold value of 10 ppb. UV-vis spectroscopy in conjunction with dynamic light scattering and electron microscopy techniques have further elaborated the detailed interaction between As(III) and DTT-Fe₃O₄@Au nanocomposite regarding their size dynamics and solution behaviour during the interaction. Moreover, ca.70% removal of As(III) from aqueous solution by DTT-Fe₃O₄@Au has been observed by ICP-MS measurement strengthening the potential of this nanocomposite as a dual-functional probe and filter.

Aggregation induced emission of amino-thiol capped gold nanoparticles (GNPs) through metal-amino-coordination

Au₁₁(SG)₇ gold nanoparticles (GNPs) were synthesized from HAuCl₄ using thiol compounds containing an amino group to serve as both the reducing agent and the ligand. A three-dimensional network structure (…Au-SNH₂→Mⁿ⁺⟵H₂NS-Au…) was formed after the Mⁿ⁺ (Pb²⁺, Cd²⁺, Zn²⁺ and Ag⁺) coordinated the gold nanoparticles through the amino group in the thiol ligand, which promoted aurophilicity (…Au…Au…) and induced GNP aggregation and emission. The differences in coordination between the amino group and metal ions resulted in different emission wavelengths (Pb²⁺, Cd²⁺, Zn²⁺ and Ag⁺: λ_ex = 365 nm˜370 nm, λ_em = 580, 645, 630 and 565 nm). Aggregation induced emission of amino thiol capped GNPs via coordination of Pb²⁺ or Cd²⁺ can be used as a fluorescent sensor of the both metal ions (λ_ex = 365 nm, λ_em = 580/645 nm) and were used for living bioimaging in vivo and in vitro.

Multi-angle dynamic light scattering analysis based on successive updating of the angular weighting

The angular weighting coefficient is key to accurate particle size distribution (PSD) measurement using multiangle dynamic light scattering (MDLS). However, determining the weighting coefficient is affected by the noise in the measured MDLS data. In this paper, a novel successive updating of the angular weighting (AWSU) method is proposed. By using information character weighting, the angular weighting coefficient and the character weighting matrix are updated once with each additional angle. The effect of autocorrelation function (ACF) denoising and information extraction is improved by using information character weighting step-by-step. The results for broad unimodal and closely-spaced bimodal PSDs demonstrate the effectiveness of this method. Compared with another routinely used inversion method, it is found that the increase in PSD information retrieved from MDLS depends not only on increasing the number of angles, but also on the calculation of the angular weightings. An accurate weighting algorithm can reduce the number of scattering angles required. Using the AWSU method for PSD recovery with 4 scattering angles can give better results than the usual method with 6 scattering angles. It is also found that the AWSU method not only improves the accuracy of the weightings, but it also contributes directly to the PSD recovery by reconstructing the ACF with the PSD obtained by the weighted inversion. The combination of these two contributions not only reduces the number of scattering angles needed, but also keeps the PSD recovery results from getting worse because of excessive scattering angles.

Adsorption of Arsenite on Gold Nanoparticles Studied with DNA Oligonucleotide Probes

Gold nanoparticles (AuNPs) have been extensively used for detecting arsenite, As(III). Many methods rely on a DNA aptamer that claimed to bind specifically to inorganic arsenic. In these cases, the focus was on arsenic binding to the aptamer, while the potential interactions between As(III) and the AuNP surface were ignored. Herein, a set of spectroscopic and isothermal titration calorimetry (ITC) experiments were conducted to measure the adsorption of As(III) by AuNPs and its competition with DNA adsorption. With 10 mM As(III), 18% of adsorbed DNA was displaced from AuNPs, while preadsorption of only 20 μM As(III) inhibited DNA adsorption by around 50%. The affinity of As(III) on AuNPs is comparable to Br^- and guanosine. ITC and Raman spectroscopy both indicated that only As(III) can be adsorbed, while As(V) had no measurable interactions with the AuNPs. Based on this understanding, a random DNA sequence was used and a similar colorimetric response in the presence of As(III) was observed. This study confirmed the affinity between As(III) and the gold surface. The As(III)/gold interaction is strong enough to affect DNA adsorption, and care should be taken to interpret the observations based on the color change of AuNPs for the detection of As(III).

Colorimetric Assay Conversion to Highly Sensitive Electrochemical Assay for Bimodal Detection of Arsenate Based on Cobalt Oxyhydroxide Nanozyme via Arsenate Absorption

This study reports a novel and convenient bimodal method for label-free and signal-off detection of arsenate in environmental samples. Cobalt oxyhydroxide (CoOOH) nanoflakes with facile preparation and intrinsic peroxidase-like activity as nanozyme can efficiently catalyze the conversion of chromogenic substrate such as 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) with the presence of H₂O₂ into green-colored oxidation products. CoOOH nanoflakes can specifically bind with arsenate via electrostatic attraction and As-O bond interaction, which gives rise to inhibition of the peroxidase-like activity of CoOOH. Thus, through arsenate specific inhibition of CoOOH nanozyme toward ABTS catalysis, a simple colorimetric method was developed for arsenate detection with a detection limit of 3.72 ppb. Based on the system of CoOOH nanozyme and ABTS substrate, this colorimetric method can be converted into an electrochemical sensor for arsenate assay by the utilization of CoOOH nanoflake-modified electrode. The electrochemical measurement can be realized by chronoamperometry, which showed more sensitive and a lower limit of detection as low as 56.1 ppt. The applicability of this bimodal method was demonstrated by measuring arsenate and total arsenic in different real samples such as natural waters and soil extracted solutions, and the results are of satisfactory accuracy as confirmed by inductively coupled plasma mass spectrometry analysis. The bimodal strategy offers obvious advantages including a label-free step, convenient operation, on-site assay, low cost, and high sensitivity, which is promising for reliable detection of arsenate and total arsenic in environmental samples.

Detection of Gold Nanoparticles Aggregation Using Light Scattering for Molecular Sensing

Gold nanoparticles (AuNPs) have been commonly used in molecular sensing, in the form of observation of the color change from red to blue of the AuNP solution, caused by target-molecule-induced AuNP aggregation. In this work, the changes in absorbance and scattering spectra caused by AuNP aggregation were studied using thrombin-induced AuNP aggregation as a model. We demonstrated for the first time that scattering spectra is more sensitive to the changes owing to AuNP aggregation than absorbance spectra. Moreover, a digital color analysis of darkfield images using dark field microscopy (DFM) facilitated a simple method for detection of AuNPs aggregation without the use of spectroscopic analysis. Furthermore, we demonstrated that DFM is useful for detecting AuNPs aggregation in a colored solution, in which the color change by AuNPs aggregation is not visible.

Ranolazine-Functionalized Copper Nanoparticles as a Colorimetric Sensor for Trace Level Detection of As3

This study involves environmentally friendly synthesis of copper nanoparticles in aqueous medium without inert gas protection, using ranolazine as a capping material. UV-Visible (UV-Vis) spectrometry showed that ranolazine-derived copper nanoparticles (Rano-Cu NPs) demonstrate a localized surface plasmon resonance (LSPR) band at 573 nm with brick-red color under optimized parameters, including pH, reaction time, and concentrations of copper salt, hydrazine hydrate, and ranolazine. The coating of ranolazine on the surface of Cu NPs was studied via Fourier transform infrared (FTIR) spectroscopy. Scanning electron microscopy (SEM) revealed that Rano-Cu NPs consist of spherical particles. X-ray diffraction (XRD) verified that Rano-Cu NPs are crystalline in nature. Atomic force microscopy (AFM) showed that the average size of Rano-Cu NPs was 40 ± 2 nm in the range of 22⁻95 nm. Rano-Cu NPs proved to be highly sensitive as a selective colorimetric sensor for As³⁺ via color change from brick red to dark green, in the linear range of 3.0 × 10^-7 to 8.3 × 10^-6 M, with an R² value of 0.9979. The developed sensor is simple, cost effective, highly sensitive, and extremely selective for As³⁺ detection, showing a low detection limit (LDL) of 1.6 × 10^-8 M. The developed sensor was effectively tested for detection of As³⁺ in some water samples.

Label-free gold nanorod-based plasmonic sensing of arsenic(iii) in contaminated water

An efficient label-free strategy for arsenic(iii) sensing in water through the suppression of iron(iii)-catalyzed oxidative shortening of gold nanorods.

Immobilization of ssDNA on a metal–organic framework derived magnetic porous carbon (MPC) composite as a fluorescent sensing platform for the detection of arsenate ions

In this work, we fabricated a metal–organic framework derived magnetic porous carbon (MPC) and with ssDNA achieved specific and efficient recognition of harmful arsenate ions. The detection limit was achieved at 630 pM.

Rapid and simple detection of Tamiflu-resistant influenza virus: Development of oseltamivir derivative-based lateral flow biosensor for point-of-care (POC) diagnostics

We have developed a novel oseltamivir derivative (oseltamivir hexylthiol; OHT) that exhibits a higher binding affinity for Tamiflu-resistant virus (Tamiflu resistance) than for the wild-type virus (Tamiflu-susceptible virus; WT) as an antibody. First, OHT-modified gold nanoparticles (OHT-GNPs) are used in a simple colorimetric assay as nanoprobes for the Tamiflu-resistant virus. In the presence of Tamiflu-resistant virus, they show a colorimetric change from deep red to purple because of the OHT-GNP aggregation driven by strong interactions between OHT and neuraminidase (NA) on the surface of the Tamiflu-resistance. Moreover, the color gradually turns purple as the concentration of the Tamiflu-resistant virus increases, allowing the determination of the presence of the virus with the naked eye. Furthermore, an OHT-based lateral flow assay (LFA) has been developed as a rapid and easy detection device for Tamiflu resistance. It shows detection specificity for various virus concentrations of Tamiflu-resistant virus even for the mixture of WT and Tamiflu-resistant viruses, where the limit of detection (LOD) is 5 × 10² ~ 10³ PFU per test (=1 × 10⁴ PFU/mL). It has been confirmed that this platform can provide accurate information on whether a virus exhibits Tamiflu resistance, thus supporting the selection of appropriate treatments using point-of-care (POC) diagnostics.

A Colorimetric Probe Based on Functionalized Gold Nanorods for Sensitive and Selective Detection of As(III) Ions

A colorimetric probe for determination of As(III) ions in aqueous solutions on basis of localized surface plasmon resonance (LSPR) was synthesized. The dithiothreitol molecules with two end thiols covalently combined with Au Nanorods (AuNRs) with an aspect ratio of 2.9 by Au-S bond to form dithiothreitol coated Au Nanorods (DTT-AuNRs), acting as colorimetric probe for the determination of As(III) ions. With the adding of As(III) ions, the AuNRs will be aggregated and leading the longitudinal SPR absorption band of DTT-AuNRs decrease due to the As(III) ions can bind with three DTT molecules through an As-S linkage. The potential factors affect the response of DTT-AuNRs to As(III) ions including the concentration of DTT, pH values of DTT-AuNRs, reaction time and NaCl concentration were optimized. Under optimum assay conditions, the DTT-AuNRs colorimetric probe has high sensitivity towards As(III) ions with low detection limit of 38 nM by rules of 3σ/k and excellent linear range of 0.13–10.01 μM. The developed colorimetric probe shows high selectivity for As(III) ions sensing and has applied to determine of As(III) in environmental water samples with quantitative spike-recoveries range from 95.2% to 100.4% with low relative standard deviation of less than 4.4% (n = 3).