Au@Ag core/shell nanoparticles as colorimetric probes for cyanide sensing

Colorimetric sensing for translational applications: from colorants to mechanisms

Colorimetric sensing offers instant reporting via visible signals. Versus labor-intensive and instrument-dependent detection methods, colorimetric sensors present advantages including short acquisition time, high throughput screening, low cost, portability, and a user-friendly approach. These advantages have driven substantial growth in colorimetric sensors, particularly in point-of-care (POC) diagnostics. Rapid progress in nanotechnology, materials science, microfluidics technology, biomarker discovery, digital technology, and signal pattern analysis has led to a variety of colorimetric reagents and detection mechanisms, which are fundamental to advance colorimetric sensing applications. This review first summarizes the basic components (e.g., color reagents, recognition interactions, and sampling procedures) in the design of a colorimetric sensing system. It then presents the rationale design and typical examples of POC devices, e.g., lateral flow devices, microfluidic paper-based analytical devices, and wearable sensing devices. Two highlighted colorimetric formats are discussed: combinational and activatable systems based on the sensor-array and lock-and-key mechanisms, respectively. Case discussions in colorimetric assays are organized by the analyte identities. Finally, the review presents challenges and perspectives for the design and development of colorimetric detection schemes as well as applications. The goal of this review is to provide a foundational resource for developing colorimetric systems and underscoring the colorants and mechanisms that facilitate the continuing evolution of POC sensors.

Fluorescent Carbon Dots: Aggregation-Induced Emission Enhancement, Application as Probe for CN- and Cr2O7-2, Sensing Strips and Bio-imaging Agent

Fluorescent carbon dots (Trp-CDs) were prepared using tryptophan as precursor and were characterized on the basis of elemental analysis, powder-XRD, IR, Raman spectroscopy, 13C-NMR, UV-Vis, fluorescence and TEM. Trp-CDs exhibit poor fluorescence in 100% water but showed strong Aggregation Induced Emission (AIE) in ethanol and higher alcohols. The anion sensing study of Trp-CD revealed that it selectively detects CN- and Cr2O7-2 and from fluorescence quenching titration study, quenching constant, LOD and range of detection were evaluated. The emission life-time of Trp-CD before and after addition of CN- and Cr2O7-2 were measured, the decay curve before addition of anion was best fitted with a bi-exponential function with life-time of τ1 2.79 ns (10.74%) and τ2 18.93 ns (89.26%). The mechanistic study revealed that for CN-, the fluorescence quenching is due to its interaction with protons attached to surface functional groups and for Cr2O7-2, it is due to inner filter effect (IFE). Sensing strips were prepared by coating Trp-CDs onto various solid surfaces including agarose films and were used for detection of CN- and Cr2O7-. Trp-CD was found to be nontoxic and biocompatible and used as staining agent for Artemia and Bacteria (Bacillus Subtilis, Pseudomonas) and detection of CN- and Cr2O7-.

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.

Point-of-Care Detection of Antioxidant in Agarose-Based Test Strip through Antietching of Au@Ag Nanostars

Antioxidants are crucial for human health, and the detection of antioxidants can provide valuable information for disease diagnosis and health management. In this work, we report a plasmonic sensing approach for the determination of antioxidants based on their antietching capacity toward plasmonic nanoparticles. The Ag shell of core-shell Au@Ag nanostars can be etched by chloroauric acid (HAuCl₄), whereas antioxidants can interact with HAuCl₄, which prevents the surface etching of Au@Ag nanostars. We modulate the thickness of the Ag shell and morphology of the nanostructures, showing that the core-shell nanostars with the smallest thickness of Ag shell have the best etching sensitivity. Owing to the extraordinary surface plasmon resonance (SPR) property of Au@Ag nanostars, the antietching effect of antioxidants can induce a significant change in both the SPR spectrum and the color of solution, facilitating both the quantitative detection and naked-eye readout. This antietching strategy enables the determination of antioxidants such as cystine and gallic acid with a linear range of 0.1-10 μM. The core-shell Au@Ag nanostars are further immobilized in agarose gels to fabricate test strips, which can display different color changes in the presence of HAuCl₄ from 0 to 1000 μM. The agarose-based test strip is also capable of detecting antioxidants in real samples, which allows naked-eye readout and quantitative detection by a smartphone.

RPA-CRISPR/Cas12a Combined with Rolling Circle Amplification-Enriched DNAzyme: A Homogeneous Photothermal Sensing Strategy for Plant Pathogens

Alternaria is an endemic fungus associated with brown spot disease, which is one of the most serious citrus diseases. In addition, the mycotoxins metabolized by Alternaria threaten human health seriously. Herein, a novel homogeneous and portable qualitative photothermal method based on recombinase polymerase amplification (RPA), CRISPR/Cas12a, and rolling circle amplification (RCA) for the detection of Alternaria is described. Using RCA primers as substrates for CRISPR/Cas12a trans-cleavage, the two systems, RPA-CRISPR/Cas12a and RCA-enriched G-quadruplex/hemin DNAzyme, are intelligently combined. Target DNA at fg/μL levels can be detected with high specificity. Additionally, the practicability of the proposed method is demonstrated by analyzing cultured Alternaria from different fruit and vegetable samples, as well as citrus fruit samples collected in the field. Furthermore, the implementation of this method does not require any sophisticated equipment and complicated washing steps. Therefore, it has great potential to screen Alternaria in poor laboratories.

Cysteamine-decorated gold nanoparticles for plasmon-based colorimetric on-site sensors for detecting cyanide ions using the smart-phone color ratio and for catalytic reduction of 4-nitrophenol

In this paper, we have reported the cyanide ions (CN^-) sensing in environmental water samples using cysteamine-capped gold nanoparticles (Cyst-AuNPs) by spectrophotometric, colorimetric, and smartphone-based RGB color detection. The surface plasmon resonance shift at around 525 nm for the Cyst-AuNPs could be used to detect quantitatively the amounts of CN^- with concomitant alteration of their color from wine red to purple visualized by the naked eye. For the first time, the Cyst-AuNPs-based visual sensing of CN^- was performed using smartphone-based detection with its detection limit of 159 × 10^-9 M, ten times lower than that of the highest tolerance level (2 × 10^-6 M) permitted by the world health organization. The Cyst-AuNPs displayed excellent specificity for detecting the concentration of 30 × 10^-6 M even amid the presence of other interfering inorganic anions with their concentrations about five times higher than it. Environmental real water samples were used to arrange the three different CN^- concentrations for plasmon-based colorimetric detection and smartphone-based method. Additionally, the catalytic performance of Cyst-AuNPs was demonstrated for the fast catalytic conversion of hazardous 4-nitrophenol (selected environmental contaminant) to the analogous amino aromatic compounds. A chemical kinetic study showed the conversion rate to be estimated as 1.65 × 10^-2 s^-1. Cyst-AuNPs can find an application in colorimetric sensing of CN^- while being able to be utilized as a catalytic nanomaterial for ecological remedies associated with health care.

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.

An Overview on Coinage Metal Nanocluster-Based Luminescent Biosensors via Etching Chemistry

The findings from the synthetic mechanism of metal nanoclusters yield the etching chemistry based on coinage metal nanoclusters. The utilization of such chemistry as a tool that can alter the optical properties of metal nanoclusters has inspired the development of a series of emerging luminescent biosensors. Compared with other sensors, the luminescent biosensors have the advantages of being more sensitive, saving time and saving cost. We reviewed topics on the luminescent sensors based on the etching of emissive coinage metal nanoclusters. The molecules possessing varied etching ability towards metal nanoclusters were categorized with discussions of corresponding etching mechanisms. The understanding of etching mechanisms favored the discussions of how to use etching methods to detecting biochemical molecules. The emerging luminescent biosensors via etching chemistry also provided challenges and new opportunities for analytical chemistry and sensors.

Waterproof Cellulose-Based Substrates for In-Drop Plasmonic Colorimetric Sensing of Volatiles: Application to Acid-Labile Sulfide Determination in Waters

The present work reports on the assessment of widely available waterproof cellulose-based substrates for the development of sensitive in-drop plasmonic sensing approaches. The applicability of three inexpensive substrates, namely, Whatman 1PS, polyethylene-coated filter paper, and tracing paper, as holders for microvolumes of colloidal solutions was evaluated. Waterproof cellulose-based substrates demonstrated to be highly convenient platforms for analytical purposes, as they enabled in situ generation of volatiles and syringeless drop exposure unlike conventional single-drop microextraction approaches and can behave as sample compartments for smartphone-based colorimetric sensing in an integrated way. Remarkably, large drop volumes (≥20 μL) of colloidal solutions can be employed for enrichment processes when using Whatman 1PS as holder. In addition, the stability and potential applicability of spherical, rod-shaped, and core-shell metallic NPs onto waterproof cellulose-based substrates was evaluated. In particular, Au@AgNPs showed potential for the colorimetric detection of in situ generated H₂S, I₂, and Br₂, whereas AuNRs hold promise for I₂, Br₂, and Hg⁰ colorimetric sensing. As a proof of concept, a smartphone-based colorimetric assay for determination of acid-labile sulfide in environmental water samples was developed with the proposed approach taking advantage of the ability of Au@AgNPs for H₂S sensing. The assay showed a limit of detection of 0.46 μM and a repeatability of 4.4% (N = 8), yielding satisfactory recoveries (91-107%) when applied to the analysis of environmental waters.

Microwave-assisted synthesis of nitrogen-doped carbon dots using prickly pear as the carbon source and its application as a highly selective sensor for Cr(VI) and as a patterning agent

Preparation of water-dispersible carbon dots from inexpensive natural carbon precursors and its application for purposes such as sensing, bio-imaging and patterning agents is showing growing interest in recent years. In this study, we have reported the preparation of nitrogen-doped carbon dots (N-CDs) using prickly pear as the carbon source and m-xylylenediamine as the nitrogen source using a one-step microwave-assisted synthetic process. The N-CDs prepared were characterized on the basis of elemental analysis, XPS, powder-XRD, FT-IR, Raman, TEM, UV-vis and fluorescence spectroscopy. Doping of nitrogen in the N-CDs made them highly fluorescent and the study on their ion-recognition property revealed that they detect highly toxic Cr(VI) with high selectivity and sensitivity (LOD, 0.04 μM) and without interference from the other ions used in this study. By immobilizing these N-CDs onto filter paper, sensor strips were prepared for on-site monitoring/field applications and they were successfully used for the detection of Cr(VI) in water. Detailed spectral analysis revealed that the mechanism of Cr(VI) sensing involved a phenomenon called the "inner filter effect" and analysis of the fluorescence lifetime data suggested the "static quenching" of fluorescence intensity. These N-CDs were used to prepare fluorescent carbon ink and were successfully used as patterning agents.

Rosmarinic Acid-Capped Silver Nanoparticles for Colorimetric Detection of CN- and Redox-Modulated Surface Reaction-Aided Detection of Cr(VI) in Water

Rosmarinic acid-capped silver nanoparticles (Ro-AgNPs) were prepared and applied as a probe for selective colorimetric detection of cyanide (CN-) and chromium(VI) [Cr(VI)] under different conditions in aqueous media. The carbon atom of CN- interacts with the AgNPs, and the carbon atom donates electrons from the HOMO to the vacant orbitals of the coordinatively unsaturated surface atom (Ag0). After donating electrons, CN- attached onto the surface of the nanoparticles becomes very reactive and interacts with dissolved oxygen and generates reactive oxygen species (ROS) such as superoxide (O2 -), singlet oxygen (1O2), and so forth. In this process, Ag0 oxidizes to Ag+ and combines with CN- forming water-insoluble AgCN, and the ROS (O2 -) formed reacts with Ag/Ag+ to form Ag2O. The oxidation of Ag0 to Ag+ resulted in dissolution of AgNPs, which causes disappearance of the surface plasmon resonance band and color change from yellow to colorless. For detection of Cr(VI), ascorbic acid and CN- were added first; the ascorbic acid replaced the rosmarinic acid and then reduced the added Cr(VI) to Cr(III), and, in this process, ascorbic acid was oxidized to dehydroascorbic acid, which moved away from the nanoparticles' surface. CN- then interacted with the surface Ag0 atom, got activated, and interacted with dissolved oxygen forming Ag+ and ROS, which then followed the same process as described for CN- to form AgCN and Ag2O with a color change. The limits of detection were found to be 0.01 and 0.03 μM for CN- and Cr(VI), respectively. The material was also used for sensing CN- and Cr(VI) in real samples, and the results obtained were satisfactory. For field application, agarose-based strips were prepared by immobilizing the nanoparticles onto the agarose film and successfully used for the detection of CN- and Cr(VI) in water.

A CH-Controlled Colorimetric Probe Based on Anthracene Carboximide for Near-Infrared Cyanide Detection

A chemical sensor that can induce near-infrared red-shifted response represents a promising strategy for the design and development of anion probes. In this work, novel CH-controlled colorimetric probe 3 based on anthracene carboximide was developed for near-infrared detection of cyanide. Probe 3 consisted of CHCN binding site to anthracene carboximide fluorophore, and showed a significant visual change from yellow-green (535 nm) to deep violet (825 nm) with a larger redshift (≈ 290 nm) and fluorescence quenching at 480 nm and 520 nm upon interacting with cyanide. Job curves determined 1:1 binding stoichiometry of probe 3 with cyanide. Additonally, probe 3 detected cyanide ion conveniently in aqueous solution and could be reused after trifluoroacetic acid treatment. Colorimetric test paper was used to detect cyanide in aqueous solutions. The C-H deprotonation sensing mechanism was confirmed by ¹H NMR titration. The near-infrared detection of cyanide by CH-controlled probes was founded for the first time.

Bimetallic Nanocrystals: Structure, Controllable Synthesis and Applications in Catalysis, Energy and Sensing

In recent years, bimetallic nanocrystals have attracted great interest from many researchers. Bimetallic nanocrystals are expected to exhibit improved physical and chemical properties due to the synergistic effect between the two metals, not just a combination of two monometallic properties. More importantly, the properties of bimetallic nanocrystals are significantly affected by their morphology, structure, and atomic arrangement. Reasonable regulation of these parameters of nanocrystals can effectively control their properties and enhance their practicality in a given application. This review summarizes some recent research progress in the controlled synthesis of shape, composition and structure, as well as some important applications of bimetallic nanocrystals. We first give a brief introduction to the development of bimetals, followed by the architectural diversity of bimetallic nanocrystals. The most commonly used and typical synthesis methods are also summarized, and the possible morphologies under different conditions are also discussed. Finally, we discuss the composition-dependent and shape-dependent properties of bimetals in terms of highlighting applications such as catalysis, energy conversion, gas sensing and bio-detection applications.

Visual Recognition and Detection of Clindamycin by Au@Ag Core-Shell Nanoparticles

The work described a new colorimetric sensor for the quantitative detection of clindamycin based on Au@Ag core-shell nanoparticles (Au@Ag NPs). The obtained Au@Ag NPs were characterized by transmission electron microscopy (TEM) and ultraviolet and visible spectrophotometry (UV-vis). When Au@Ag NPs were added to a clindamycin solution, it can be observed that the color immediately changed from bright yellow to gray-blue and the absorption spectrum also changed, realizing the visual detection of clindamycin. Under optimal conditions, the absorption ratio (A ₅₄₆/A ₄₀₀) of the UV-vis spectra increased linearly with the concentration of clindamycin ranging from 6.25 × 10^-7 to 7.50 × 10^-6 mol/L (R ² = 0.9945), with a limit of detection (LOD) of 2.00 × 10^-7 mol/L and good recovery of 100.0-102.0% (relative standard deviation (RSD) < 2%). The detection process was convenient without complicated instruments. Compared with other analytes, the Au@Ag NPs detection system has good selectivity for clindamycin. In addition, the Au@Ag NPs colorimetric sensor was successfully used to determine clindamycin in human urine samples. This study provides a simple, rapid, intuitive, and low-cost visualization analysis method of clindamycin, which was helpful for the visualization detection of other targets.

Convenient detection of H2S based on the photothermal effect of Au@Ag nanocubes using a handheld thermometer as readout

Hydrogen sulfide (H₂S), as a hazardous gas, is often found around dump areas. Long term exposure can cause harm to health, it is highly necessary to develop some simple and sensitive methods for on-site H₂S detection. Herein, a convenient photothermal assay has been designed for the quantitation of H₂S using a handheld thermometer as readout. Au@Ag nanocubes (Au@Ag NCs), a core-shell nanocomposite with strong light absorption at ∼450 nm, was chosen as a novel photothermal agent in this study. Under the laser irradiation at 450 nm, the Au@Ag NCs show a strong photothermal effect, and a significant temperature enhancement can be measured by the thermometer easily. The presence of H₂S can lead to the deposition of sulfur onto Au@Ag NCs, altering the localized surface plasmon resonance absorption, size, surface composition, and morphology of Au@Ag NCs and hence leading to the reduction of photothermal effect. The change of the temperature has a linear relationship with the H₂S concentration in the range of 0.5-80.0 μM with a detection limit of 0.35 μM. By combining with simple sample purification procedures, the developed method has been applied to detect H₂S in garbage odor gas with satisfactory results.

A naphthalimide-linked new pyridylazo phenol derivative for selective sensing of cyanide ions (CN-) in sol-gel medium

Napthalimide-linked pyridylazo derivatives 1 and 2 have been designed and synthesized. Compound 1 acts as a gelator in DMF-H₂O (1 : 1, v/v). The brown gel is photostable and shows good viscoelastic properties. The value of G' is almost 10 times higher than that of G'' over the entire range of frequencies at a constant strain of 1%. The SEM image shows the presence of densely stacked flakes. In comparison, compound 2, devoid of free phenolic -OH, does not show gelation properties under identical conditions. However, the brown gel of 1 shows selective sensing of CN^- ions over a series of anions involving phase change through the deprotonation mechanism. While the brown gel of 1 is selectively ruptured in the presence of CN^- to the sol, compound 1 in solution shows measurable UV-vis and emission changes in the presence of CN^- over the other anions and validates the visual sensing of CN^-. In the test-kit application, the yellow paper strip turned into pinkish-red upon contact with CN^-.

Manipulating Bimetallic Nanostructures With Tunable Localized Surface Plasmon Resonance and Their Applications for Sensing

Metal nanocrystals with well-controlled shape and unique localized surface plasmon resonance (LSPR) properties have attracted tremendous attention in both fundamental studies and applications. Compared with monometallic counterparts, bimetallic nanocrystals endow scientists with more opportunities to precisely tailor their LSPR and thus achieve excellent performances for various purposes. The aim of this mini review is to present the recent process in manipulating bimetallic nanostructures with tunable LSPR and their applications for sensing. We first highlight several significant strategies in controlling the elemental ratio and spatial arrangement of bimetallic nanocrystals, followed by discussing on the relationship between their composition/morphology and LSPR properties. We then focus on the plasmonic sensors based on the LSPR peak shift, which can be well-controlled by seed-mediated growth and selective etching. This review provides insights of understanding the “rules” involving in the formation of bimetallic nanocrystals with different structures and desired LSPR properties, and also forecasts the development directions of plasmonic sensors in the future.

Bimetallic gold-silver nanoparticles mediate bacterial killing by disrupting the actin cytoskeleton MreB

The actin cytoskeleton is required for the maintenance of the cell shape and viability of bacteria. It remains unknown to which extent nanoparticles (NPs) can orchestrate the mechanical instability by disrupting the cytoskeletal network in bacterial cells. Our work demonstrates that Au-Ag NPs disrupt the bacterial actin cytoskeleton specifically, fluidize the inner membrane and lead to killing of bacterial cells. In this study, we have tried to emphasize on the key parameters important for NP-cell interactions and found that the shape, specific elemental surface localization and enhanced electrostatic interaction developed due to the acquired partial positive charge by silver atoms in the aggregated NPs are some of the major factors contributing towards better NP interactions and subsequent cell death. In vivo studies in bacterial cells showed that the NPs exerted a mild perturbation of the membrane potential. However, its most striking effect was on the actin cytoskeleton MreB resulting in morphological changes in the bacterial cell shape from rods to predominantly spheres. Exposure to NPs resulted in the delocalization of MreB patches from the membrane but not the tubulin homologue FtsZ. Concomitant with the redistribution of MreB localization, a dramatic increase of membrane fluid regions was observed. Our studies reveal for the first time that Au-Ag NPs can mediate bacterial killing and disrupt the actin cytoskeletal functions in bacteria.

Colorimetric and visual detection of cyanide ions based on the morphological transformation of gold nanobipyramids into gold nanoparticles

In this study, we developed a facile, rapid, selective and sensitive colorimetric method for the detection of cyanide ions (CN⁻) by using gold nanobipyramids (Au NBPs).

A recyclable colorimetric probe: in situ fabrication of highly stable HPEI–AuNPs for selective Ag+ detection

A highly stable and recyclable AuNP-based colorimetric sensor was prepared by an in situ reduction reaction and can detect Ag⁺ with high selectivity.

Multicomponent Plasmonic Nanoparticles: From Heterostructured Nanoparticles to Colloidal Composite Nanostructures

Plasmonic nanostructures possessing unique and versatile optoelectronic properties have been vastly investigated over the past decade. However, the full potential of plasmonic nanostructure has not yet been fully exploited, particularly with single-component homogeneous structures with monotonic properties, and the addition of new components for making multicomponent nanoparticles may lead to new-yet-unexpected or improved properties. Here we define the term "multi-component nanoparticles" as hybrid structures composed of two or more condensed nanoscale domains with distinctive material compositions, shapes, or sizes. We reviewed and discussed the designing principles and synthetic strategies to efficiently combine multiple components to form hybrid nanoparticles with a new or improved plasmonic functionality. In particular, it has been quite challenging to precisely synthesize widely diverse multicomponent plasmonic structures, limiting realization of the full potential of plasmonic heterostructures. To address this challenge, several synthetic approaches have been reported to form a variety of different multicomponent plasmonic nanoparticles, mainly based on heterogeneous nucleation, atomic replacements, adsorption on supports, and biomolecule-mediated assemblies. In addition, the unique and synergistic features of multicomponent plasmonic nanoparticles, such as combination of pristine material properties, finely tuned plasmon resonance and coupling, enhanced light-matter interactions, geometry-induced polarization, and plasmon-induced energy and charge transfer across the heterointerface, were reported. In this review, we comprehensively summarize the latest advances on state-of-art synthetic strategies, unique properties, and promising applications of multicomponent plasmonic nanoparticles. These plasmonic nanoparticles including heterostructured nanoparticles and composite nanostructures are prepared by direct synthesis and physical force- or biomolecule-mediated assembly, which hold tremendous potential for plasmon-mediated energy transfer, magnetic plasmonics, metamolecules, and nanobiotechnology.

Homogeneous and high-density gold unit implanted optical labels for robust and sensitive point-of-care drug detection

Controllable integration of gold building blocks into mesoscopic architecture produces improved optical signals with preferable stability for biological sensing. Here, we developed novel optical labels with homogeneous and high-density implanted hydrophobic gold nanoparticles (AuNPs) throughout three-dimensional silica scaffolds. The dendritic silica supports with an extra-large pore size and highly accessible central-radial channels were employed as metal-affinity templates, for anchoring with AuNPs directly from the organic phase. The nano-assemblies exhibited a high unit loading capacity while maintaining the intrinsic optical characteristics of AuNPs. After phase transfer by the alkylsilane intermediate layer and exterior silica shell encapsulation, the nanocomposites revealed an amplified plasmonic absorption signal, excellent colloidal/optical stability and convenient surface functionalization. By integrating the silica labels into the lateral flow immunoassay strip for signal enhancement, the sensitive point-of-care detection of methamphetamine in urine was established. The limit of detection achieved 0.026 ng mL-1, with a detection range from 0.023 to 375 ng mL-1 in a 10 min assay, allows both visual and on-site quantitative analysis. Encouragingly, the potential interfering drugs in the sample matrix showed a negligible influence on the results, validating the superior specificity of the current immunoassay. The newly developed gold-implanted optical labels show prospects for point-of-care testing in a complex biological matrix with the desirable stability and signal amplification.

Distance-Based Paper Device Combined with Headspace Extraction for Determination of Cyanide

We report for the first time a distance-based paper device based on gold/silver core shell nanoparticles (Au@Ag NPs) for a simple, inexpensive, instrument-free, and portable determination of cyanide by the naked eye. Au@Ag NPs immobilized on a paper channel were etched by cyanide ions so that a yellow color band length of Au@Ag NPs is proportional to a decrease in the cyanide concentration. Quantification is achieved by measuring color length, thus eliminating the need to differentiate hues and intensities by the user, and the processing data of each imaging device. Moreover, the paper-based headspace extraction was combined with the distance-based paper device to improve the sensitivity. The enrichment factor was found to be 30-fold and the linearity was found in the range 0.05–1 mg L⁻¹. The naked eye detection limit was 10 μg L⁻¹ where the World Health Organization (WHO) have regulated the maximum level of cyanide in drinking water as 70 μg L⁻¹. Our proposed device also showed no interference from common cations and anions presenting in seawater and waste water including thiocyanate, chloride. Finally, our device has been successfully applied to determine cyanide ions in seawater, drinking water, tap water and wastewater providing satisfactory precision and accuracy.

Hydrogen Peroxide-Assisted Ultrasonic Synthesis of BCNO QDs for Anthrax Biomarker Detection

A facile 3% hydrogen peroxide-assisted ultrasonic synthetic strategy is demonstrated to successfully synthesize fluorescent boron carbon oxynitride quantum dots (BCNO QDs). The obtained BCNO QDs exhibit intense blue fluorescence and favorable biocompatibility and water solubility. The quantum yield of the BCNO QDs is 19.9%. Owing to the absorbance energy-transfer emission effect, an efficient ratiometric fluorescence biosensor is developed for anthrax biomarker detection based on the BCNO QD-ethylenediaminetetraacetic acid disodium salt-Eu³⁺ complex. Under optimal conditions, the detection limit of the anthrax biomarker is 0.5 nM. Furthermore, the sensitivity of the system was evaluated by Bacillus subtilis spores and with the detection limit as low as 1.95 × 10⁶ spores. On combining a smartphone with the home-made BCNO QD test paper, the lowest recorded visual detection limit of 1.0 μM anthrax biomarker was achieved using a portable UV lamp. The fast response speed, excellent sensitivity, and selectivity of the approach show potential applications in clinical analysis.

Toward ultrasensitive and fast colorimetric detection of indoor formaldehyde across the visible region using cetyltrimethylammonium chloride-capped bone-shaped gold nanorods as “chromophores”

A colorimetric method for detecting formaldehyde was developed by coupling bone-shaped gold nanorods (AuNRs) with silver mirror reaction, which enables low detection limit, wide linear range and high visual resolution.

Development of a New Colorimetric Chemosensor for Selective Determination of Urinary and Vegetable Oxalate Concentration Through an Indicator Displacement Assay (IDA) in Aqueous Media

The paper proposes a method that exhibits operational simplicity for the indirect spectrophotometric determination of oxalate ion. We developed Reactive Blue 4 (RB4) as a sensor by complexation with copper ion as a simple, inexpensive yet selective colorimetric chemosensing ensemble for the recognition of oxalate over other available competitive analytes via indicator displacement assay (IDA) in both solution (aqueous medium) and solid state (paper-based experiment). The addition of oxalate to RB4-Cu²⁺ complex changed the colour from sky blue to dark blue due to the regeneration of RB4 by the chelation of oxalate as the competitive analyte with Cu²⁺. The absorbance band increase is linear with oxalate concentration from 1.76 to 49.4 µmol/L with a detection limit of 0.62 µmol/L. This measurement mode did not show any influence of interferences (available anions and ascorbic acid). This approach eliminated the need for the separation stages, enzymatic multiple-step reactions, sample preparation, organic solvent mixture, chemical modifications and equipment developed to a high degree of complexity. The oxalate determination gave results in different real samples such as urine, mushroom and spinach, which demonstrated the applicability of the existing method. Furthermore, this colorimetric system can serve as IMPLICATION molecular logic gate using Cu²⁺ and oxalate (C₂O₄^2–) as inputs and UV-Vis absorbance signal as the output with potential monitoring applications.

Optical assays based on colloidal inorganic nanoparticles

Colloidal inorganic nanoparticles have wide applications in the detection of analytes and in biological assays. A large number of these assays rely on the ability of gold nanoparticles (AuNPs, in the 20 nm diameter size range) to undergo a color change from red to blue upon aggregation. AuNP assays can be based on cross-linking, non-cross linking or unmodified charge-based aggregation. Nucleic acid-based probes, monoclonal antibodies, and molecular-affinity agents can be attached by covalent or non-covalent means. Surface plasmon resonance and SERS techniques can be utilized. Silver NPs also have attractive optical properties (higher extinction coefficient). Combinations of AuNPs and AgNPs in nanocomposites can have additional advantages. Magnetic NPs and ZnO, TiO2 and ZnS as well as insulator NPs including SiO2 can be employed in colorimetric assays, and some can act as peroxidase mimics in catalytic applications. This review covers the synthesis and stabilization of inorganic NPs and their diverse applications in colorimetric and optical assays for analytes related to environmental contamination (metal ions and pesticides), and for early diagnosis and monitoring of diseases, using medically important biomarkers.

Construction of a Nano Biosensor for Cyanide Anion Detection and Its Application in Environmental and Biological Systems

We have developed a Ag@Au core-shell nanoparticle (NP)/iridium(III) complex-based sensing platform for the sensitive luminescence "turn-on" sensing of cyanide ions, an acutely toxic pollutant. The assay is based on the quenching effect of Ag@Au NPs on the emission of complex 1, but luminescence is restored after the addition of cyanide anions due to their ability to dissolve the Au shell. Our sensing platform exhibited a high sensitivity toward cyanide anions with a detection limit of 0.036 μM, and also showed high selectivity for cyanide over 10-fold excess amounts of other anions. The sensing platform was also successfully applied to monitor cyanide anions in drinking water and in living cells.

Sunlight Induced Preparation of Functionalized Gold Nanoparticles as Recyclable Colorimetric Dual Sensor for Aluminum and Fluoride in Water

A sunlight induced simple green route has been developed for the synthesis of polyacrylate functionalized gold nanoparticles (PAA-AuNPs), in which poly(acrylic acid) functions as a reducing as well as stabilizing agent. This material has been characterized on the basis of spectroscopic and microscopic studies; it exhibited selective colorimetric detection of Al³⁺ in aqueous media, and the Al³⁺ induced aggregated PAA-AuNPs exhibited detection of F^- with sharp color change and high selectivity and sensitivity out of a large number of metal ions and anions tested. The mechanistic study revealed that, for Al³⁺, the color change is due to a shift of the SPR band because of the Al³⁺ induced aggregation of PAA-AuNPs, whereas for F^-, the reverse color change (blue to red) with return of the SPR band to its original position is due to dispersion of aggregated PAA-AuNPs, as F^- removes Al³⁺ from the aggregated species by complex formation. Only concentration-dependent fluoride ion can prevent Al³⁺ from aggregating PAA-AuNPs. The method is successfully used for the detection of F^- in water collected from various sources by the spiking method, in toothpastes of different brands by the direct method. The solid Al³⁺-PAA-AuNPs were isolated, adsorbed on ZIF@8 (zeolitic imidazolate framework) and on a cotton strip, and applied as solid sensing material for detection of F^- in aqueous media.

Polyethyleneimine-mediated seed growth approach for synthesis of silver-shell silica-core nanocomposites and their application as a versatile SERS platform

PEI-mediated seed growth approach for synthesis of high performance silver-shell silica-core nanocomposites.

Visual and optical detection of hypochlorite in water samples based on etching of gold/silver alloy nanoparticles

In this work, we have described the cost-effective, simple, selective and sensitive approach for the detection of hypochlorite (ClO⁻) using gold/silver alloy nanoparticles (Au/Ag alloy NPs) as a colorimetric probe.

A halogen-free synthesis of gold nanoparticles using gold(III) oxide

Gold nanoparticles are one of the most used nanomaterials. They are usually synthesized by the reduction of gold(III) chloride. However, the presence of halide ions in the reaction mixture is not always welcome. In some cases, these ions have detrimental influence on the morphology and structure of resulting nanoparticles. Here, we present a simple and halogen-free procedure to prepare gold nanoparticles by reduction of gold(III) oxide in neat oleylamine. The method provides the particles with an average size below 10 nm and dispersity of tens of percent. The process of nanoparticle formation was monitored using UV-Vis spectroscopy. The structure and chemical composition of the nanoparticles was determined by SEM, XPS and EDX. We also proposed the mechanism of reduction of gold(III) oxide based on MS, IR and NMR data. Importantly, the synthetic protocol is general and applicable for the preparation of other coinage metal nanoparticles from the corresponding metal oxides. For instance, we demonstrated that the absence of halogen enables efficient alloying of metals when preparing gold-silver bimetallic nanoparticles.

A non-enzymatic sensor based on Au@Ag nanoparticles with good stability for sensitive detection of H2O2

Synthesis of Au@Ag NPs by a seed-mediated growth procedure and fabrication of a non-enzymatic H₂O₂ sensor.

Polarization state-based refractive index sensing with plasmonic nanostructures

Spectral-based methods are often used for label-free biosensing. However, practical implementations with plasmonic nanostructures suffer from a broad line width caused by strong radiative and nonradiative losses, and the sensing performance characterized by figure of merit is poor for these spectral-based methods. This study provides a polarization state-based method using plasmonic nanostructures to improve the sensing performance. Instead of the intensity spectrum, the polarization state of the transmitted field is monitored to analyze variations of the surrounding medium. The polarization state of incidence is strongly modified due to the excitation of surface plasmons, and the ellipticity of the transmitted field changes dramatically around plasmon resonances. Sharp resonances with line widths down to sub-nanometer are achieved by plotting the spectra of the reciprocal of ellipticity. Therefore, the sensing performance can be significantly improved, and a theoretical value of the figure of merit exceeding 1700 is achieved by using the polarization state-based sensing approach.

Highly selective and sensitive nanoprobes for cyanide based on gold nanoclusters with red fluorescence emission

We report a novel and environmentally friendly fluorescent probe for detecting the cyanide ion (CN(-)) using L-amino acid oxidase (LAAOx)-protected Au nanoclusters (LAAOx@AuNCs) with red emission. The fluorescence-based sensing behaviour of LAAOx@AuNCs towards anions was investigated in buffered aqueous media. Among the anions studied, CN(-) was found to effectively quench the fluorescence emission of AuNCs based on CN(-) induced Au core decomposition. Excellent sensitivity and selectivity toward the detection of CN(-) in aqueous solution were observed. The CN(-) detection limit was determined to be approximately 180 nM, which is 15 times lower than the maximum level (2700 nM) of CN(-) in drinking water permitted by the World Health Organization (WHO). A linear relationship between the fluorescence intensity and CN(-) concentration was observed in two ranges of CN(-) concentration, including 3.2 × 10(-6) to 3.4 × 10(-5) mol L(-1) and 3.81 × 10(-5) to 1.04 × 10(-4) mol L(-1). The high sensitivity and selectivity to CN(-) among the 17 types of anions make the AuNCs good candidates for use in fluorescent nanoprobes of CN(-).