Highly sensitive photometric determination of cyanide based on selective etching of gold nanorods

A Review of Nanotechnology-Enabled Fluorescent Chemosensors for Environmental Toxic Ion Detection

This review examines the utilization of nanotechnology-based chemosensors for identifying environmental toxic ions. Over recent decades, the creation of nanoscale materials for applications in chemical sensing, biomedical, and biological analyses has emerged as a promising avenue. Nanomaterials play a vital role in improving the sensitivity and selectivity of chemosensors, thereby making them effective tools for monitoring and evaluating environmental contamination. This is due to their highly adjustable size- and shape-dependent chemical and physical properties. Nanomaterials possess distinct surface chemistry, thermal stability, high surface area, and large pore volume per unit mass, which can be harnessed for sensor development. The discussion encompasses different types of nanomaterials utilized in chemosensor design, LOD, their sensing mechanisms, and their efficacy in detecting specific toxic ions. Furthermore, the review explores the progress made, obstacles faced, and future prospects in this rapidly evolving field, highlighting the potential contributions of nanotechnology to the creation of robust sensing platforms for environmental monitoring.

Nanoparticles Based Sensors for Cyanide Ion Sensing, Basic Principle, Mechanism and Applications

Rapidly detecting potentially toxic ions such as cyanide is paramount to maintaining a sustainable and environmentally friendly ecosystem for living organisms. In recent years, molecular sensors have been developed to detect cyanide ions, which provide a naked-eye or fluorometric response, making them an ideal choice for cyanide sensing. Nanosensors, on the other hand, have become increasingly popular over the last two decades due water solubility, quick reaction times, environmental friendliness, and straightforward synthesis. Researchers have designed many nanosensors and successfully utilized them for the detection of cyanide ions in various environmental samples. The majority of these sensors use gold and silver-based nanosensors because cyanide ions have a high affinity for these metals ions and coordinate through covalent bonds. These metal nanoparticles are typically combined or coated with fluorescent materials, which quench their fluorescence. However, adding cyanide ions etches out the metal nanoparticles, restoring their fluorescence/color. This principle has been followed by most nanosensors used for cyanide ion sensing. In this review, different nanosensors and their sensing mechanisms are discussed in relation to cyanide ions. The primary purpose is to compare the sensing abilities of these sensors, mainly their sensitivity, advantages, application and to find out research gaps for future work. In this review paper, the development made in nanosensors in the last thirteen years (2010-2023) was discussed and the nanosensors for cyanide ions were compared with molecular sensors while the nanosensors with the excellent limit of detection were highlighted.

Plasmonic nanoparticle etching-based optical sensors: current status and future prospects

Plasmonic nanoparticles are an emerging tool for developing label-free multicolorimetric sensors for biosensing and chemosensing applications. The color absorbed by nanoparticles from visible light is influenced by their size, shape, orientation, and interparticle distance. Differently sized and shaped gold and silver nanoparticles exhibit a wide range of colors, aiding in the development of label-free sensors. Etching is the process of oxidizing nanoparticles, which alters their aspect ratio, shape, plasmonic peak, and outward appearance. It is typically used to create sensitive sensing platforms. Through etching, analytes could be detected in a simple, sensitive, and selective manner. The multicolor readout of nanoparticle etching-based multicolorimetric sensors can overcome the limitations of conventional colorimetric assays and improve the accuracy of visual inspection. This review discusses different approaches for target sensing using nanoparticle etching strategies like direct etching, enzyme-mediated etching, chemical reaction-driven etching, and anti-etching-based sensors and their mechanisms. In the future, etching strategies could be modified into portable sensing devices to detect a variety of analytes, which will aid in the development of on-time, in situ, and point-of-care sensing systems.

Bottom-Up Then Top-Down Synthesis of Gold Nanostructures Using Mesoporous Silica-Coated Gold Nanorods

Gold nanostructures were synthesized by etching away gold from heat-treated mesoporous silica-coated gold nanorods (AuNR@mSiO2), providing an example of top-down modification of nanostructures made using bottom-up methodology. Twelve different types of nanostructures were made using this bottom-up-then-top-down synthesis (BUTTONS), of which the etching of the same starting nanomaterial of AuNR@mSiO2 was found to be controlled by how AuNR@mSiO2 were heat treated, the etchant concentration, and etching time. When the heat treatment occurred in smooth moving solutions in round-bottomed flasks, red-shifted longitudinal surface plasmon resonance (LSPR) was observed, on the order of 10-30 min, indicating increased aspect ratios of the gold nanostructures inside the mesoporous silica shells. When the heat treatment occurred in turbulent solutions in scintillation vials, a blue shift of the LSPR was obtained within a few minutes or less, resulting from reduced aspect ratios of the rods in the shells. The influence of the shape of the glassware, which may impact the flow patterns of the solution, on the heat treatment was investigated. One possible explanation is that the flow patterns affect the location of opened pores in the mesoporous shells, with the smooth flow of solution mainly removing CTAB surfactants from the pores along the cylindrical body of mSiO2, therefore increasing the aspect ratios after etching, and the turbulent solutions removing more surfactants from the pores of the two ends or tips of the silica shells, hence decreasing the aspect ratios after etching. These new stable gold nanostructures in silica shells, bare and without surfactant protection, may possess unique chemical properties and capabilities. Catalysis using heat-treated nanomaterials was studied as an example of potential applications of these nanostructures.

Novel H2S sensing mechanism derived from the formation of oligomeric sulfide capping the surface of gold nanourchins

A gold nanourchin (AuNU) probe with a novel sensing mechanism for monitoring H2S was developed as a feasible colorimetric sensor. In this study, AuNUs that are selectively responsive to H2S were fabricated in the presence of trisodium citrate and 1,4-hydroquinone using a seed-mediated approach. Upon exposure of the AuNU solution to H2S, the hydrosulfide ions (HS-) in the solution are converted into oligomeric sulfides by 1,4-hydroquinone used as a reducing agent during the synthesis of AuNUs. The oligomeric sulfides formed in the AuNU solution upon the addition of H2S were found to coat the surface of the AuNUs, introducing a blue shift in absorption accompanied by a color change in the solution from sky blue to light green. This colorimetric alteration by the capping of oligomeric sulfides on the surface of AuNUs is unique compared to well-known color change mechanisms, such as aggregation, etching, or growth of nanoparticles. The novel H2S sensing mechanism of the AuNUs was characterized using UV-Vis spectroscopy, high-resolution transmission microscopy, X-ray photoelectron spectroscopy, surface-enhanced Raman spectroscopy, secondary ion mass spectroscopy, liquid chromatography-tandem mass spectrometry, and atom probe tomography. H2S was reliably monitored with two calibration curves comprising two sections with different slopes according to the low (0.3-15 μM) and high (15.0-300 μM) concentration range using the optimized AuNU probe, and a detection limit of 0.29 μM was obtained in tap water.

Fast and Facile Etching of Gold Nanorods by N-Halosuccinimides: Toward Multicolorimetric Identification and Quantification of 20 Natural Amino Acids

Gold nanorods (AuNRs) have recently become fascinating chromophores in the field of colorimetric sensing because of their eye-catching rainbow colors along with the high dimensionality of their optical profile. The etching of AuNRs using an analyte-sensitive oxidizing agent is particularly an attractive tool not only for adjusting their plasmonic behavior through altering their aspect ratio but also for correlating the observed signal with the identity and concentration of the analyte. However, the deployment of this strategy in the field of sensing has been seriously hindered by various factors ranging from slow etching kinetics and the need for nonambient temperatures to low degrees of controllability along with the high toxicity of the etchants. To resolve these challenges, the present study aims to introduce the outstanding potentials of two inexpensive mild oxidants comprising N-bromosuccinimide (NBS) and N-chlorosuccinimide (NCS) in the highly fast and controllable etching of AuNRs at room temperature. By controlling the concentration of the etchant and the pH of the medium, the longitudinal and transversal peaks could be well adjusted with nanometer precision. In an attempt to elucidate the etching mechanism, the effects of various parameters including the etchant concentration and pH, as well as the kinetics of the etching process were thoroughly investigated. After all, the capability of NBS in decarboxylating the amino acids was further exploited in the design of an all-inclusive multicolorimetric sensor array based on the etching of AuNRs for the sensitive quantification and highly accurate discrimination of all 20 amino acids in the micromolar range. To this end, the acquired data set was analyzed by two machine learning techniques including partial least-squares regression (PLSR) and linear discriminant analysis (LDA). The versatility of N-halosuccinimide reactions with various categories of organic compounds underlies ample opportunities for the design of diverse multicolorimetric sensors, further glamorizing the prospect of this approach.

The Role of Water Content of Deep Eutectic Solvent Ethaline in the Anodic Process of Gold Electrode

Traditional coupling of ligands for gold wet etching makes large-scale applications problematic. Deep eutectic solvents (DESs) are a new class of environment-friendly solvents, which could possibly overcome the shortcomings. In this work, the effect of water content on the Au anodic process in DES ethaline was investigated by combining linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). Meanwhile, we employed atomic force microscopy (AFM) to image the evolution of the surface morphology of the Au electrode during its dissolution and passivation process. The obtained AFM data help to explain the observations about the effect of water content on the Au anodic process from the microscopic perspective. High water contents make the occurrence of anodic dissolution of gold at higher potential, but enhances the rate of the electron transfer and gold dissolution. AFM results reveal the occurrence of massive exfoliation, which confirms that the gold dissolution reaction is more violent in ethaline with higher water contents. In addition, AFM results illustrate that the passive film and its average roughness could be tailored by changing the water content of ethaline.

Optical Sensing of Toxic Cyanide Anions Using Noble Metal Nanomaterials

Water toxicity, one of the major concerns for ecosystems and the health of humanity, is usually attributed to inorganic anions-induced contamination. Particularly, cyanide ions are considered one of the most harmful elements required to be monitored in water. The need for cyanide sensing and monitoring has tempted the development of sensing technologies without highly sophisticated instruments or highly skilled operations for the objective of in-situ monitoring. Recent decades have witnessed the growth of noble metal nanomaterials-based sensors for detecting cyanide ions quantitatively as nanoscience and nanotechnologies advance to allow nanoscale-inherent physicochemical properties to be exploited for sensing performance. Particularly, noble metal nanostructure e-based optical sensors have permitted cyanide ions of nanomolar levels, or even lower, to be detectable. This capability lends itself to analytical application in the quantitative detection of harmful elements in environmental water samples. This review covers the noble metal nanomaterials-based sensors for cyanide ions detection developed in a variety of approaches, such as those based on colorimetry, fluorescence, Rayleigh scattering (RS), and surface-enhanced Raman scattering (SERS). Additionally, major challenges associated with these nano-platforms are also addressed, while future perspectives are given with directions towards resolving these issues.

Progress and Challenges of Point-of-Need Photonic Biosensors for the Diagnosis of COVID-19 Infections and Immunity

The new coronavirus disease, COVID-19, caused by SARS-CoV-2, continues to affect the world and after more than two years of the pandemic, approximately half a billion people are reported to have been infected. Due to its high contagiousness, our life has changed dramatically, with consequences that remain to be seen. To prevent the transmission of the virus, it is crucial to diagnose COVID-19 accurately, such that the infected cases can be rapidly identified and managed. Currently, the gold standard of testing is polymerase chain reaction (PCR), which provides the highest accuracy. However, the reliance on centralized rapid testing modalities throughout the COVID-19 pandemic has made access to timely diagnosis inconsistent and inefficient. Recent advancements in photonic biosensors with respect to cost-effectiveness, analytical performance, and portability have shown the potential for such platforms to enable the delivery of preventative and diagnostic care beyond clinics and into point-of-need (PON) settings. Herein, we review photonic technologies that have become commercially relevant throughout the COVID-19 pandemic, as well as emerging research in the field of photonic biosensors, shedding light on prospective technologies for responding to future health outbreaks. Therefore, in this article, we provide a review of recent progress and challenges of photonic biosensors that are developed for the testing of COVID-19, consisting of their working fundamentals and implementation for COVID-19 testing in practice with emphasis on the challenges that are faced in different development stages towards commercialization. In addition, we also present the characteristics of a biosensor both from technical and clinical perspectives. We present an estimate of the impact of testing on disease burden (in terms of Disability-Adjusted Life Years (DALYs), Quality Adjusted Life Years (QALYs), and Quality-Adjusted Life Days (QALDs)) and how improvements in cost can lower the economic impact and lead to reduced or averted DALYs. While COVID19 is the main focus of these technologies, similar concepts and approaches can be used and developed for future outbreaks of other infectious diseases.

Nitrogen-rich silicon quantum dots: facile synthesis and application as a fluorescent "on-off-on" probe for sensitive detection of Hg2+ and cyanide ions

The sensitive and reliable detection of Hg²⁺ and CN^- as harsh environmental contaminants are of great importance. In view of this, a novel "on-off-on" fluorescent probe based on nitrogen-rich silicon quantum dots (NR-SiQDs) has been designed for sensitive detecting Hg²⁺ and CN^- ions in aqueous media. NR-SiQDs were synthesized by a facile, one-step, and environment friendly procedure in the presence of 3-aminopropyl trimethoxysilane (APTMS) and ascorbic acid (AA) as precursors, with L-asparagine as a nitrogen source for surface modification. The NR-SiQDs exhibited strong fluorescence emission at 450 nm with 42.34% quantum yield, satisfactory salt tolerance, and superior photo- and pH-stability. The fluorescence emission was effectively quenched by Hg²⁺ (turn off) due to the formation of a non-fluorescent stable NR-SiQDs/Hg²⁺ complex while after the addition of cyanide ions (CN^- ), Hg²⁺ ions can be leached from the surface of the NR-SiQDs and the fluorescence emission intensity of the quenched NR-SiQDs fully recovered (turn on) due to the formation of highly stable [Hg (CN)₄ ]^2- species. After optimizing the response conditions, the obtained limits of detection were found to be 53 nM and 0.46 μM for Hg²⁺ and CN^- , respectively. Finally, the NR-SiQDs based fluorescence probe was utilized to detect Hg²⁺ and CN^- ions in water samples and satisfactory results were obtained, suggesting its potential application for environmental monitoring.

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.

Precisely tuning the longitudinal localized surface plasmon resonance of gold nanorods via additive-regulated overgrowth

An additive-regulated overgrowth strategy for preparing gold nanorods with precise longitudinal localized surface plasmon resonance is proposed.

CdSe quantum dots-sensitized chemiluminescence system and quenching effect of gold nanoclusters for cyanide detection

An efficient chemiluminescence resonance energy transfer (CRET) induced chemiluminescence (CL) system was developed for the sensitive determination of cyanide ion (CN^-) in environmental and biological samples. The selected CL reaction was hydrogen peroxide (H₂O₂)-bicarbonate (HCO₃^-) system with an ultra-weak emission at about 470 nm. It was found that glutathione-stabilized CdSe quantum dots (CdSe QDs) superbly increase the obtained CL intensity. The high performance CRET between the CL emitters and CdSe QDs with a broad absorption was mainly responsible for the observed improving effect. The absorption spectrum of QDs completely overlaps with the CL emission wavelength of H₂O₂-HCO₃^- system. Besides, CdSe QDs could also catalyze the CL reaction of H₂O₂-HCO₃^-, efficiently. On the other hand, it was observed that the gold nanoclusters (Au NCs) could prohibit the CRET system and turn off the CL emission. This diminishing effect can be useful for the analytical application. Herein, it was successfully exploited for the selective recognition of CN^-, using its leaching effect on Au NCs. After efficient dissolution of NCs, the CRET to CdSe QDs restored and the CL emission was again turned on. This strategy resulted in a high sensitive and reliable measurement of CN^- in the concentration range of 2-225 nM, with a detection limit of 0.46 nM.

Graphene nanosheets modified with curcumin-decorated manganese dioxide for ultrasensitive potentiometric sensing of mercury(II), fluoride and cyanide

A glassy carbon electrode (GCE) was modified by electropolymerization of curcumin on MnO₂-Gr nanosheets to obtain a detection method for Hg(II) and for the anions fluoride and cyanide. The complexation by curcumin can be monitored by potentiometry. The results revealed a cathodic shift for the simultaneous detection of fluoride and cyanide and an anodic shift for the mercury(II) sensing, with peak potentials of -0.24, 0.12 and 0.82 V, respectively (vs. Ag/AgCl). The modified GCE is fairly selective, reproducible and repeatable. The detection limits are 19.2 nM for Hg(II), 17.2 nM for fluoride, and 28.3 nM for cyanide (LOD, S/N = 3). The method was successfully applied to the analysis of spiked samples of tap water, river water and petrochemical refinery wastewater. Graphical abstract Schematic of an electrochemical curcumin-MnO₂-graphene nanosheet platform for the simultaneous assay of fluoride, cyanide and mercury(II) in the ppb concentration range in various natural and wastewater samples.

Colorimetric and visual detection of mercury(II) based on the suppression of the interaction of dithiothreitol with agar-stabilized silver-coated gold nanoparticles

A colorimetric and visual method is described for the determination of mercury(II) ion. A gel consisting of agar-stabilized silver-coated gold nanoparticles (Au@Ag NPs) was prepared. The reaction with dithiothreitol (DTT) via thiol-Ag chemistry results in an orange to purple color change of the gel. However, in the presence of Hg(II), the reaction of DTT with the silver shells is suppressed due to the strong thiophilicity of Hg(II). The color of the gel changes from purple to red to orange in the presence of increasing concentrations of Hg(II). The Au@Ag NPs therefore are a viable optical probe for Hg(II) which can be detected in concentration as low as 78 nM via dual-wavelength ratiometric absorbance (A₃₉₀/A₅₂₀), and at 1 μM levels with bare eyes. The use of agar as a support is mandatory to prevent the aggregation of the NPs and also improves selectivity. The method was applied to the analysis of spiked samples, and recoveries ranged between 96.3 and 104%. The assay is easy, inexpensive, and in our perception represents an attractive tool for on-site visual detection of Hg(II). Graphical abstract Schematic of the assay. With increasing concentrations of Hg(II), the oxidative etching of silver shells caused by dithiothreitol (DTT) is gradually inhibited, and the color of agar-stabilized Au@Ag NP gel varies from purple to red, and finally to orange. This can be used for visual detection of Hg(II).

Plasmon-Based Colorimetric Nanosensors for Ultrasensitive Molecular Diagnostics

Colorimetric detection of target analytes with high specificity and sensitivity is of fundamental importance to clinical and personalized point-of-care diagnostics. Because of their extraordinary optical properties, plasmonic nanomaterials have been introduced into colorimetric sensing systems, which provide significantly improved sensitivity in various biosensing applications. Here we review the recent progress on these plasmonic nanoparticles-based colorimetric nanosensors for ultrasensitive molecular diagnostics. According to their different colorimetric signal generation mechanisms, these plasmonic nanosensors are classified into two categories: (1) interparticle distance-dependent colorimetric assay based on target-induced forming cross-linking assembly/aggregate of plasmonic nanoparticles; and (2) size/morphology-dependent colorimetric assay by target-controlled growth/etching of the plasmonic nanoparticles. The sensing fundamentals and cutting-edge applications will be provided for each of them, particularly focusing on signal generation and/or amplification mechanisms that realize ultrasensitive molecular detection. Finally, we also discuss the challenge and give our future perspective in this emerging field.