Iodide-Mediated Rapid and Sensitive Surface Etching of Gold Nanostars for Biosensing

A self-assembled 3D nanoflowers based nano-ELISA platform for the sensitive detection of pyridaben

Biomimetic methods are invariably employed to synthesize hybrid organic-inorganic multilevel structure nanoflowers with self-assembly processes in aqueous solutions, which is an ideal way to meet the challenges of immobilizing antibodies or enzymes in nanomaterial based enzyme-linked immunosorbent assay (nano-ELISA). In this study, we developed protein-inorganic hybrid 3D nanoflowers composed of bovine serum albumin (BSA), horseradish peroxidase (HRP)-conjugated goat anti-mouse IgG (IgG-HRP) and copper(Ⅱ) phosphate (BSA-(IgG-HRP)-Cu3(PO4)2) using a self-assembly biomimetic method. The preparation process avoided the use of any organic solvent and protein immobilization did not require covalent modifications. Additionally, the unique hierarchical structure enhances the thermal and storage stability of HRP. The BSA-(IgG-HRP)-Cu3(PO4)2 hybrid 3D nanoflower was then applied to a nano-ELISA platform for pyridaben detection, achieving a 50% inhibition concentration of 3.90 ng mL-1. The nano-ELISA achieved excellent accuracy for pyridaben detection. Such a novel BSA-(IgG-HRP)-Cu3(PO4)2 hybrid 3D nanoflower provide an excellent reagent for small molecule immunoassay.

Concave Gold Nanocubes Exhibit Growth-Etching Behavior: Unexpected Morphological Transformations

Chemical equilibrium stands as a fundamental principle governing the dynamics of chemical systems. However, it may become intricate when it refers to nanomaterials because of their unique properties. Here, we invesitigated concave gold nanocubes (CGNs) subjected to an akaline Au3+/H2O2 solution, which exhibit both etching and growth in a monotonic solution. When CGNs were subjected to an increasingly alkaline Au3+/H2O2 solution, their dimensions increased from 107 to 199 nm and then decreased to 125 nm. Transmission electron microscopy (TEM) demonstrated that their morphology undergoes intricate alternations from concave to mutibranch and finally to concave again. Real-time ultraviolet-visible spectroscopy and time-dependent TEM also demonstrated reduction first and then oxidation in one solution. Among the nanomaterials, the obtained carpenterworm-like gold nanoparticles revealed the best catalytic performance in p-nitrophenol reduction by NaBH4, with a chemical rate that continues to increase until the reaction reaches completion. Growth leading to atomic dislocation, distortion, and exposure on nanoparticles and the redox of H2O2 plausibly account for the further etching due to the Ostwald ripening effect. Our study may spur more interest in the tuning of the properties, engineering, investigation, and design of new kinds of nanomaterials.

Real-Time Imaging of Interfacial Copper(II) Ion-Initiated Selective Etching in the Core Region of Single Cuprous Oxide-Bismoclite Core-Shell Microcrystals

The core-shell microstructures are attracting much interest, most notably for their superior performance compared with their pure counterparts because of the interfacial effect. Comprehensively understanding the mechanism of the interfacial effect is critical but still elusive. Here, we report real-time dark-field optical microscopy (DFM) imaging of the selective etching in the core region of single cuprous oxide-bismoclite (Cu2O@BiOCl) core-shell microcrystals by I-. In situ DFM observations reveal that the reaction activity of Cu2O is significantly improved after coating the BiOCl shell layer, and the I- diffuses through the BiOCl shell and approaches the interface region, followed by etching the Cu2O core. During the etching process, two distinct reaction pathways, such as interfacial Cu2+-driven redox etching and confinement-governed dissolution, are identified. The interfacial Cu2+ is generated due to the coordination number difference at the core-shell interface. Moreover, according to the in situ DFM single-crystal imaging results, the ensemble adsorption capacity improvement for I- is also demonstrated in Cu2O@BiOCl core-shell microcrystals. These findings provide deep insights into the interfacial effect of core-shell microcrystals and establish a bridge between microscopic imaging and macroscopic practical application.

Seeded Growth of Plasmonic Nanostructures in Deformable Polymer Confinement

Plasmonic nanostructures exhibit optical properties highly related to their morphologies, enabling diverse applications in various areas such as biosensing, bioimaging, chemical detection, cancer therapy, and solar energy conversion. The expansive uses of these nanostructures necessitate robust and versatile synthesis methods suitable for large-scale production. Here, we introduce our recent efforts in developing a new strategy for controlling the seeded growth of plasmonic metal nanostructures, employing deformable polymer capsules to regulate the growth kinetics and the resulting particle morphology. Employing sol-gel-derived resorcinol-formaldehyde (RF) resin as a typical capsule material, we highlight its advanced features, including mechanical deformability and molecular permeability, that can be manipulated by tuning the capsule thickness and cross-linking degree. These features enable highly controllable confined seeded growth of plasmonic nanostructures. We reveal the significant role of the Ostwald ripening process of the seeds and the capsule structures in determining the morphological evolution of the plasmonic nanostructures. Moreover, we highlight some distinctive plasmonic nanostructures resulting from this unique synthesis strategy and their intriguing functionalities in various potential applications. Our discussion concludes with potential research directions to advance the development of the deformable polymer-confined seeded growth strategy into a general and robust synthesis platform for creating cutting-edge functional plasmonic nanostructures.

Hydrogen Bonding-Induced Aggregation of Chiral Functionalized AuNS@Ag NPs for Photothermal Enantioanalysis

Toward the challenges of signaling transduction amplified in enantioselective recognition, we herein devised an innovative strategy for highly selective recognition of amino acids and their derivatives, leveraging photothermal effects. In this approach, bifunctional l-ascorbic acid is employed to reduce silver ions in situ on Au nanostars. Simultaneously, its oxidate (l-dehydroascorbic acid) is bonded to the silver shell as a chiral selector to prepare chiral nanoparticles (C-AuNS@Ag NPs) with the ability to recognize stereoisomers and sensitively modulate the photothermal effect. l-Dehydroascorbic acid can selectively capture one of the enantiomers of the two forms through hydrogen bonding and drive aggregation of the nanoparticles, which sharply enhances the photothermal effect. Consequently, the two forms of the system exhibit a significant temperature difference, which enables the discrimination and quantification of enantiomers. Our strategy verifies that six chiral amino acids and their derivatives can be discriminated with enantioselective response values of up to 79. Additionally, the chiral recognition mechanism was revealed through density functional theory (DFT) calculations, providing a paradigm shift in the development of enantiomeric recognition strategies.

A point-of-care solid-phase colorimetric sensor based on the enzyme-induced metallization for ALP detection

Alkaline phosphatase (ALP) is a crucial biomarker for clinical diagnosis, which is closely related to the physiological homeostasis regulation process of human body. And the abnormal level of ALP is associated with numerous diseases, such as liver dysfunction, bone diseases, diabetes, and so on. In order to meet the demand of personalized healthcare, it is particularly important to develop a miniaturized point-of-care testing (POCT) device for ALP detection. Herein, a portable solid-phase colorimetric sensor based on enzyme-induced metallization signal amplification strategy was constructed for ALP detection. The AuNPs modified on the glass slides acted as crystal seeds, allowing Ag+ in the solution to be reduced and deposited on the surface of AuNPs, which further formed the gold core and silver shell (Au@Ag) complex and generated visual signals. The visual signals were recorded by a smartphone and quantified using open-source ImageJ software. Under the optimal conditions, the proposed method exhibited a good linear relationship from 2.0 to 16.0 pM, and the detection limit was as low as 0.9 pM. In addition, it was further successfully applied for ALP detection in non-transparent and complex samples (milk, different types of cells). A sensitive, low cost, rapid and convenient solid-phase sensor was developed for ALP detection, which was expected to provide a promising strategy for POCT devices.

Photothermal hydrogel-integrated paper-based point-of-care platform for visible distance-readout of glucose

BACKGROUND

The accurate detection of glucose and cholesterol plays a pivotal role in disease diagnosis and home care. To this end, biochemical analyzers have become extensively utilized tools for measuring disease biomarkers. Nonetheless, their poor portability and high cost have restricted their accessibility, limiting their use to laboratory settings and hindering the adoption of point-of-care testing (POCT). In contrast, the emergence of portable and affordable paper-based testing platform has revolutionized diagnostic testing by providing distance signals, enhancing intuitiveness and visual accessibility. Consequently, these platforms have become increasingly suitable for POCT.

RESULTS

We have developed a POCT platform that integrated AuNS@Ag, stimulus-responsive hydrogel and test strips, enabling visual distance reading of glucose. The silver-coated AuNS and enzyme were encapsulated within a temperature-responsive N-isopropylacrylamide-acrylamide (NIPAM-AcAm) hydrogel to act as target recognition and reaction units respectively. Glucose can diffuse freely within the hydrogel porous matrix, thereby instigating enzyme-catalyzed reaction that induce alterations in the photothermal effect of the system. This dynamic process ensures efficient and responsive modulation of the system's photothermal properties. By ingeniously capturing distance signals induced by the photothermal effect-mediated water release the visualization and quantification of target substances are achieved, with a linear range spanning from 0 to 30 mM. The consistency between distance-based POCT platform and commercial blood glucose meter demonstrates that the platform provides a portable, affordable and reliable method for visual reading biomarkers.

SIGNIFICANCE

The proposed strategy enables direct, visual quantitative analysis of the target without the need for additional analytical instruments. Particularly, this method holds significant promise as an efficient platform for cholesterol and other disease markers measurement.

Selective detection of fast and thermal neutrons in mixed-radiation fields using tungsten-silica and gold-iodine-silica nanoparticles and their boron-loaded aqueous dispersions

Tungsten-silica and gold-iodine-silica nanoparticles and their boron-loaded aqueous dispersions were used to selectively detect fast and thermal neutrons in mixed-radiation fields generated by a cyclotron on the order of mSv at a neutron flux of 1.0 ×106(neutron/sec∙cm2). The photo-image intensity, fluorescence spectra, absorption spectra, and XRD of their aqueous dispersions were measured immediately and eighteen days after irradiation. The immediate measurements of photo-image intensity and fluorescence spectral area ratios for gold-iodine-silica nanoparticle aqueous dispersions indicated the dose dependence of photo-image intensity and fluorescence spectral area ratios. Measurements of the relative fluorescence and absorption spectral areas of gold-iodine-silica nanoparticle aqueous dispersions 18 days after irradiation also showed similar dose dependences. The precipitates of gold-iodine-silica nanoparticles showed a linear relationship between the XRD peak ratio and the dose with a correlation coefficient of 0.9. The photo-image intensities, fluorescence spectral area, absorption spectral area, and XRD peak ratios were found to be affected by fast and thermal neutrons. Simple methods of fluorescence, absorption, and XRD measurements are proposed for the selective detection of fast and thermal neutrons in mixed-radiation fields.

TMB+-mediated etching of urchin-like gold nanostructures for colorimetric sensing

The morphology-dependent localized surface plasmon resonance of gold nanostructures has been widely utilized for designing sensors. One method relies on the color change of gold nanoparticles upon etching. In previous work, TMB2+oxidized from 3,3',5,5'-tetramethylbenzidine (TMB) was found to etch gold nanorods (AuNRs), leading to a spectrum of different colors. However, the preparation of TMB2+needs the addition of a strong acid and other harsh conditions. Herein, a new colorimetric biosensing platform was developed using urchin-like gold nanoparticles (AuNUs). Compared with AuNRs, the etching of AuNUs can happen under mild conditions by TMB+at pH 6, protecting enzymes and proteins from denaturation. The role of CTAB surfactant was dissected, and its bromide ions were found to be involved in the etching process. Based on these observations, a one-step colorimetric detection of H2O2was realized by using horseradish peroxidase and H2O2to oxidize TMB. Within 30 min, this system achieved a detection limit of 80 nM H2O2. This work offered fundamental insights into the etching of anisotropic gold nanostructures and optimized the etching conditions. These advancements hold promise for broader applications in biosensing and analytical chemistry.

Quantitative Galactose Colorimetric Competitive Assay Based on Galactose Dehydrogenase and Plasmonic Gold Nanostars

We describe a competitive colorimetric assay that enables rapid and sensitive detection of galactose and reduced nicotinamide adenine dinucleotide (NADH) via colorimetric readouts and demonstrate its usefulness for monitoring NAD+-driven enzymatic reactions. We present a sensitive plasmonic sensing approach for assessing galactose concentration and the presence of NADH using galactose dehydrogenase-immobilized gold nanostars (AuNS-PVP-GalDH). The AuNS-PVP-GalDH assay remains turquoise blue in the absence of galactose and NADH; however, as galactose and NADH concentrations grow, the reaction well color changes to a characteristic red color in the presence of an alkaline environment and a metal ion catalyst (detection solution). As a result, when galactose is sensed in the presence of H2O2, the colored response of the AuNS-PVP-GalDH assay transforms from turquoise blue to light pink, and then to wine red in a concentration-dependent manner discernible to the human eye. This competitive AuNS-PVP-GalDH assay could be a viable analytical tool for rapid and convenient galactose quantification in resource-limited areas.

Depletion force optimization for high-purity gold nanotriangles prepared using different growth methods

A homogeneous structural distribution in metal nanoparticle is commonly required for their application, and despite high-yield growth techniques, unavoidable structural heterogeneity remains a concern in metal nanoparticle synthesis. Gold nanotriangles (AuNTs) were synthesized using seed-mediated and seedless growth methods. Recent advancements in high-yield synthesis processes have enabled easy handling of AuNTs, which exhibit unique localized surface plasmon resonance characteristics due to their anisotropic triangular form. The flocculation and subsequent precipitation technique was used to purify AuNTs of different sizes synthesized using seed-mediated and seedless growth methods. The optimal conditions for obtaining high-purity AuNTs were explored by introducing a high concentration of cetyltrimethylammonium chloride. Additionally, the depletion force necessary for achieving high-purity AuNTs was calculated to reveal variations in the required depletion forces for AuNTs synthesized using different growth techniques. The alternations in the size distribution of AuNTs during the flocculation step were tracked using dynamic light scattering, and the surface charge of AuNTs synthesized through different growth methods was evaluated by ζ-potential. The high purity of the AuNTs produced using the seedless growth method required a larger depletion force than the seed-mediated grown AuNTs. The difference in the required depletion forces results from the difference in the electrostatic forces caused by the different growth methods.

Three-dimensional building of anisotropic gold nanoparticles under confinement in submicron capsules

The low collision rate and contact time of gold nanoparticles (NPs) in solution afford a low welding probability, which hinders their welding structure, orientation, and dimension. Encapsulated anisotropic NPs, gold nanotriangles (AuNTs), were successfully assembled into a three-dimensional structure inside a permeable silica nanocapsule under light illumination to generate localized surface plasmon resonance (LSPR). AuNTs were trapped in the permeable silica nanocapsules and diffused in the nanospace because of copolymer release, which increased the contact probability of AuNTs and promoted the three-dimensional building of AuNTs. Electron energy loss mapping simulations revealed that the obtained three-dimensional AuNT structure exhibited spatially separated multiple LSPR modes with different energies of incident light, which are photophysically attractive beyond the facet-selective chemical growth of NPs, and postmodification for anchoring substances with site-selective attachment to the obtained structure will be applicable to expand the sensing design and class of substances for sensing.

Dual Functional Ultrasensitive Point-of-Care Clinical Diagnosis Using Metal-Organic Frameworks-Based Immunobeads

Sepsis is an acute systemic infectious syndrome with high fatality. Fast and accurate diagnosis, monitoring, and medication of sepsis are essential. We exploited the fluorescent metal-AIEgen frameworks (MAFs) and demonstrated the dual functions of protein detection and bacteria identification: (i) ultrasensitive point-of-care (POC) detection of sepsis biomarkers (100 times enhanced sensitivity); (ii) rapid POC identification of Gram-negative/positive bacteria (selective aggregation within 20 min). Fluorescent lateral flow immunoassays (LFAs) are convenient and inexpensive for POC tests. MAFs possess a large surface area, excellent photostability, high quantum yield (∼80%), and multiple active sites serving as protein binding domains for ultrasensitive detection of sepsis biomarkers (IL-6/PCT) on LFAs. The limit of detection (LOD) for IL-6/PCT is 0.252/0.333 pg/mL. Rapid appraisal of infectious bacteria is vital to guide the use of medicines. The dual-functional fluorescent MAFs have great potential in POC tests for the clinical diagnosis of bacterial infections.

Overview on the Development of Alkaline-Phosphatase-Linked Optical Immunoassays

The drive to achieve ultrasensitive target detection with exceptional efficiency and accuracy requires the advancement of immunoassays. Optical immunoassays have demonstrated significant potential in clinical diagnosis, food safety, environmental protection, and other fields. Through the innovative and feasible combination of enzyme catalysis and optical immunoassays, notable progress has been made in enhancing analytical performances. Among the kinds of reporter enzymes, alkaline phosphatase (ALP) stands out due to its high catalytic activity, elevated turnover number, and broad substrate specificity, rendering it an excellent candidate for the development of various immunoassays. This review provides a systematic evaluation of the advancements in optical immunoassays by employing ALP as the signal label, encompassing fluorescence, colorimetry, chemiluminescence, and surface-enhanced Raman scattering. Particular emphasis is placed on the fundamental signal amplification strategies employed in ALP-linked immunoassays. Furthermore, this work briefly discusses the proposed solutions and challenges that need to be addressed to further enhance the performances of ALP-linked immunoassays.

Chromogenic Mechanisms of Colorimetric Sensors Based on Gold Nanoparticles

The colorimetric signal readout method is widely used in visualized analyses for its advantages, including visualization of test results, simple and fast operations, low detection cost and fast response time. Gold nanoparticles (Au NPs), which not only exhibit enzyme-like activity but also have the advantages of tunable localized surface plasmon resonance (LSPR), high stability, good biocompatibility and easily modified properties, provide excellent platforms for the construction of colorimetric sensors. They are widely used in environmental monitoring, biomedicine, the food industry and other fields. This review focuses on the chromogenic mechanisms of colorimetric sensors based on Au NPs adopting two different sensing strategies and summarizes significant advances in Au NP-based colorimetric sensing with enzyme-like activity and tunable LSPR characteristics. In addition, the sensing strategies based on the LSPR properties of Au NPs are classified into four modulation methods: aggregation, surface modification, deposition and etching, and the current status of visual detection of various analytes is discussed. Finally, the review further discusses the limitations of current Au NP-based detection strategies and the promising prospects of Au NPs as colorimetric sensors, guiding the design of novel colorimetric sensors.

High-Yield Gold Nanohydrangeas on Three-Dimensional Carbon Nanotube Foams for Surface-Enhanced Raman Scattering Sensors

Recently, carbon nanomaterial-supported plasmonic nanocrystals used as high-performance surface-enhanced Raman scattering (SERS) substrates have attracted increasing attention due to their ultra-high sensitivity of detection. However, most of the work focuses on the design of 2-D planar substrates with traditional plasmonic structures, such as nanoparticles, nanorods, nanowires, and so forth. Here, we report a novel strategy for the preparation of high-yield Au nanohydrangeas on three-dimensional porous polydopamine (PDA)/polyvinyl alcohol (PVA)/carbon nanotube (CNT) foams. The structures and growth mechanisms of these specific Au nanocrystals are systematically investigated. PDA plays the role of both a reducing agent as well as an anchoring site for Au nanohydrangeas' growth. We also show that the ratio of surfactant KBr to the gold precursor (HAuCl4) is key to obtain these structures in a manner of high production. Moreover, the substrate of the CNT foam-Au nanohydrangea hybrid can be employed as SERS sensors and can detect the analytes down to 10-9 M.

Plasmonic colorimetric immunosensor based on Poly-HRP and AuNS etching for tri-modal readout of small molecule

It was urgent to improve the intuitive, portable, sensitive and multi-modal detection method for small molecules. In this study, a tri-modal readout of plasmonic colorimetric immunosensor (PCIS) for small molecule (zearalenone, ZEN, as an example) had been established based on the Poly-HRP amplification and gold nanostars (AuNS) etching. The immobilized Poly-HRP from the competitive immunoassay was used to catalyze iodide (I^-) into iodine (I₂), which could prevent the AuNS etching by I^-. With the increasing of ZEN, the AuNS etching was enhanced, and the localized surface plasmon resonance (LSPR) peak of AuNS showed stronger blue shift, which resulted in the color changing from deep blue (no-etching) to blue violet (half-etching) and finally to shiny red (all-etching). The results of PCIS could be selectively obtained by the tri-modal readout: (1) naked eye (LOD of 0.10 ng/mL), (2) smartphone (LOD of 0.07 ng/mL) and (3) UV-spectrum (LOD of 0.04 ng/mL). The proposed PCIS had performed well in the sensitivity, specificity, accuracy and reliability. In addition, the harmless reagents were used in the overall process to further guarantee the environmental friendliness. Therefore, the PCIS might provide a novel and green avenue for the tri-modal readout of ZEN via the intuitive naked eye, portable smartphone and accurate UV-spectrum, which hold great potential for small molecule monitoring.

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.

Iodide-Mediated Etching of Gold Nanostar for the Multicolor Visual Detection of Hydrogen Peroxide

A multicolor visual method for the detection of hydrogen peroxide (H₂O₂) was reported based on the iodide-mediated surface etching of gold nanostar (AuNS). First, AuNS was prepared by a seed-mediated method in a HEPES buffer. AuNS shows two different LSPR absorbance bands at 736 nm and 550 nm, respectively. Multicolor was generated by iodide-mediated surface etching of AuNS in the presence of H₂O₂. Under the optimized conditions, the absorption peak Δλ had a good linear relationship with the concentration of H₂O₂ with a linear range from 0.67~66.67 μmol L^-1, and the detection limit is 0.44 μmol L^-1. It can be used to detect residual H₂O₂ in tap water samples. This method offered a promising visual method for point-of-care testing of H₂O₂-related biomarkers.

Dual-modular immunosensor for bongkrekic acid detection using specific monoclonal antibody

Bongkrekic acid (BA) is a mitochondrial toxin that causes high mortality but is often mistakenly categorized as other food poisonings. The immunoassay of BA is still challenging since the specific antibody is unavailable. In this work, a monoclonal antibody specific to BA was first generated and a dual-modular immunosensor for on-site and laboratory detection was established. The antibody showed good affinity (Kd=0.33 μM) and sensitivity (IC₅₀ =17.9 ng/mL in ELISA) with negligible cross-reactivity with common mycotoxins. In dual-modular conditions, fluorescence assay (FA) was conducted based on the inner filter effect of carbon dots (CDs) and oxidized 3,3',5,5'-tetramethylbenzidine (TMB), while the colorimetric assay (CA) was conducted using TMB²⁺-mediated rapid surface etching of gold nanostars (Au NSs). The proposed immunosensor showed good sensitivity and reproducibility to BA in food samples, with a limit of detection lower than 10 ng/mL and recovery ranging from 80.0% to 103.6%, which was in good consistence with that of standard LC-MS/MS. Overall, the proposed immunosensor is an ideal tool for screening BA contaminants in food with good sensitivity and high effectivity.

Ultrasensitive iodide detection in biofluids based on hot electron-induced reduction of p-Nitrothiophenol on Au@Ag core-shell nanoparticles

Surveillance of iodine intake is important because either inadequate or excessive amount of iodine may lead to thyroid malfunctions. Herein, we report a method for fast iodide quantification based on a plasmonic hot electron-driven chemical reaction, which occurs on Au@Ag core-shell nanoparticles (NPs) coated with p-nitrothiophenol (PNTP) molecules. Upon resonant light illumination, hot electron-hole pairs are generated in the NPs. The hot holes capture iodide ions (I^-) and form AgI which decomposes under light; while the hot electrons are shifted to the electron orbital (LUMO) of PNTP and trigger its reduction to p-aminothiophenol (PATP). By measuring characteristic surface-enhanced Raman spectroscopic (SERS) peaks of PNTP and PATP, the concentration of I^- in water can be quantitatively determined, with a linear response in the 0.5-20 μM range and a detection limit of 0.30 μM. The Au@Ag nanosensor was then applied for I^- detection in various biofluids including urine, serum and saliva, exhibiting superior detection sensitivity and high selectivity. This sensing assay requires a small sample volume of ∼10 μL and completes the entire detection process in ∼2 min, and therefore holds significant potential for application in point-of-care settings.

Methyl orange as a novel colorimetric iodide indicator with in situ generation of H2O2 by etching uncoated Ag-Ti3C2 nanohybrids

Iodine intake remains a major public health concern, as both iodine excess and deficiency are related to adverse effects on health. Therefore, developing simple and economical methods to detect I^- is still in great demand. Herein, we constructed a visual I^- sensing platform based on the uncoated Ag-Ti₃C₂ nanohybrids using methyl orange (MO) as a colorimetric indicator. Plasmonic nanostructures are frequently employed in colorimetric analysis, but uncoated Ag nanoparticles (NPs) are unstable because their surface energies are usually high. Considering that Ag NPs can be etched by I^- via forming Ag-I bond, we introduce Ag-Ti₃C₂ nanohybrids because uncoated Ag NPs with immaculate surfaces are more conducive to binding with I^- and being etched. Dissolved O₂ molecules adsorbed on Ti³⁺ of Ti₃C₂ MXenes enable the in situ generation of H₂O₂ by iodine-etching of uncoated Ag-Ti₃C₂ nanohybrids. ∙OH radicals promote the degradation of MO through a self-driven Fenton-like process, exhibiting the color variation from orange to transparent. Under optimal conditions, the absorbance of MO at 465 nm decreases linearly with the concentration of I^- in the range of 0.5-300 μM, with a limit of detection as low as 0.31 μM. This work opens the feasibility of iodine-etching of Ag in developing novel probes for facile colorimetric determination of I^-.

Enzymatic Electrochemical/Fluorescent Nanobiosensor for Detection of Small Chemicals

The detection of small molecules has attracted enormous interest in various fields, including the chemical, biological, and healthcare fields. In order to achieve such detection with high accuracy, up to now, various types of biosensors have been developed. Among those biosensors, enzymatic biosensors have shown excellent sensing performances via their highly specific enzymatic reactions with small chemical molecules. As techniques used to implement the sensing function of such enzymatic biosensors, electrochemical and fluorescence techniques have been mostly used for the detection of small molecules because of their advantages. In addition, through the incorporation of nanotechnologies, the detection property of each technique-based enzymatic nanobiosensors can be improved to measure harmful or important small molecules accurately. This review provides interdisciplinary information related to developing enzymatic nanobiosensors for small molecule detection, such as widely used enzymes, target small molecules, and electrochemical/fluorescence techniques. We expect that this review will provide a broad perspective and well-organized roadmap to develop novel electrochemical and fluorescent enzymatic nanobiosensors.

Quinine-Fabricated Surface-Enhanced Raman Spectroscopy Chiral Sensing Platform Enables Simultaneous Enantioselective Discrimination and Identification of Aliphatic Amino Acids

Due to low optical activity and structural simplicity, synchronous chiral discrimination and identification of aliphatic amino acids (AAs) are still challenging yet demanding. Herein, we developed a novel surface-enhanced Raman spectroscopy (SERS)-based chiral discrimination-sensing platform for aliphatic AAs, in which l- and d-enantiomers are able to discriminately bind with quinine to generate distinct differences in the SERS vibrational modes. Meanwhile, the plasmonic sub-nanometer gaps supported by the rigid quinine enable the maximization of SERS signal enhancement to reveal feeble signals, allowing for simultaneously acquiring the structural specificity and enantioselectivity of aliphatic amino acid enantiomers in a single SERS spectrum. Different kinds of chiral aliphatic AAs were successfully identified by using this sensing platform, demonstrating its potential and practicality in recognizing chiral aliphatic molecules.

Alkaline Phosphatase-Mediated Bioetching of CoOOH/BiVO4 for Signal-On Organic Photoelectrochemical Transistor Bioanalysis

Organic photoelectrochemical transistor (OPECT) bioanalytics has recently appeared as a promising route for biological measurements, which has major implications in both next-generation photoelectrochemical (PEC) bioanalysis and futuristic biorelated implementations. Via biological dissociation of materials, bioetching is a useful technique for bio-manufacturing and bioanalysis. The intersection of these two domains is expected to be a possible way to achieve innovative OPECT bioanalytics. Herein, we validate such a possibility, which is exemplified by alkaline phosphatase (ALP)-mediated bioetching of a CoOOH/BiVO₄ gate for a signal-on OPECT immunoassay of human immunoglobulin G (HIgG) as the model target. Specifically, target-dependent bioetching of the upper CoOOH layer could result into an enhanced electrolyte contact and light accessibility to BiVO₄, leading to the modulated response of the polymeric poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) channel that could be monitored by the channel current. The introduced biosensor achieves sensitive detection of HIgG with high selectivity and sensitivity. This work features bioetching-enabled high-efficacy OPECT bioanalysis and is anticipated to serve as a generic protocol, considering the diverse bioetching routes.

Gold Nanoparticles-Based Colorimetric Assays for Environmental Monitoring and Food Safety Evaluation

Recent years have witnessed an exponential increase in the research on gold nanoparticles (AuNPs)-based colorimetric sensors to revolutionize point-of-use sensing devices. Hence, this review is compiled focused on current progress in the design and performance parameters of AuNPs-based sensors. The review begins with the characteristics of AuNPs, followed by a brief explanation of synthesis and functionalization methods. Then, the mechanisms of AuNPs-based sensors are comprehensively explained in two broad categories based on the surface plasmon resonance (SPR) characteristics of AuNPs and their peroxidase-like catalytic properties (nanozyme). SPR-based colorimetric sensors further categorize into aggregation, anti-aggregation, etching, growth-mediated, and accumulation-based methods depending on their sensing mechanisms. On the other hand, peroxidase activity-based colorimetric sensors are divided into two methods based on the expression or inhibition of peroxidase-like activity. Next, the analytes in environmental and food samples are classified as inorganic, organic, and biological pollutants, and recent progress in detection of these analytes are reviewed in detail. Finally, conclusions are provided, and future directions are highlighted. Improving the sensitivity, reproducibility, multiplexing capabilities, and cost-effectiveness for colorimetric detection of various analytes in environment and food matrices will have significant impact on fast testing of hazardous substances, hence reducing the pollution load in environment as well as rendering food contamination to ensure food safety.

Monocrystalline Labeling Enables Stable Plasmonic Enhancement for Isolation-Free Extracellular Vesicle Analysis

Sensitive detection of extracellular vesicles (EVs) as emerging biomarkers has shown great promises for disease diagnosis. Plasmonic metal nanostructures conjugated with molecules that bind specific biomarker targets are widely used for EVs sensing but involve tradeoffs between particle-size-dependent signal intensity and conjugation efficiency. One solution to this problem would be to induce nucleation on nanoparticles that have successfully bound a target biomarker to permit in situ nanoparticle growth for signal amplification, but approaches that are evaluated to date require harsh conditions or lack nucleation specificity, prohibiting their effective use with most biological specimens. This study describes a one-step in situ strategy to induce monocrystalline copper shell growth on gold nanorod probes without decreasing signal by disrupting probe-target interactions or lipid bilayer integrity to enable EV biomarker detections. This approach increases the detected nanoparticle signal about two orders of magnitude after a 10 min copper nanoshell growth reaction. This has significant implications for improved disease detection, as indicated by the ability of a novel immunoassay using this approach to detect low abundance EVs carrying a pathogen-derived biomarker, after their direct capture from serum, to facilitate the diagnosis of tuberculosis cases in a diagnostically challenging pediatric cohort.

Optical Biosensor for Ochratoxin A Detection in Grains Using an Enzyme-Mediated Click Reaction and Polystyrene Nanoparticles

Herein, we develop an optical biosensor for highly sensitive and facile detection of ochratoxin A (OTA) using an enzyme-mediated click reaction for signal amplification and polystyrene nanoparticles (PNPs) for signal readout. Alkaline phosphatase was employed to hydrolyze the ascorbic acid-phosphate to generate ascorbic acid, which reduces Cu(II) to Cu(I). Cu(I) can catalyze the click reaction between alkyne-functionalized magnetic beads and azide-functionalized PNPs to form complexes, while unbound PNPs acted as the signal probe. This strategy utilized the high efficiency of click chemistry and the inherent optical absorption properties of PNPs, which effectively improved the sensitivity of conventional immunoassays and simplified the procedures using magnetic separation technology. This optical biosensor enabled OTA detection in a linear range of 0.1 to 50 ng/mL with a detection limit of 54 pg/mL. Moreover, it has been successfully challenged with OTA detection in maize samples, revealing its potential as a promising tool for mycotoxin screening.

Expanding the Scope of Nanobiocatalysis and Nanosensing: Applications of Nanomaterial Constructs

The synergistic interaction between advanced biotechnology and nanotechnology has allowed the development of innovative nanomaterials. Those nanomaterials can conveniently act as supports for enzymes to be employed as nanobiocatalysts and nanosensing constructs. These systems generate a great capacity to improve the biocatalytic potential of enzymes by improving their stability, efficiency, and product yield, as well as facilitating their purification and reuse for various bioprocessing operating cycles. The different specific physicochemical characteristics and the supramolecular nature of the nanocarriers obtained from different economical and abundant sources have allowed the continuous development of functional nanostructures for different industries such as food and agriculture. The remarkable biotechnological potential of nanobiocatalysts and nanosensors has generated applied research and use in different areas such as biofuels, medical diagnosis, medical therapies, environmental bioremediation, and the food industry. The objective of this work is to present the different manufacturing strategies of nanomaterials with various advantages in biocatalysis and nanosensing of various compounds in the industry, providing great benefits to society and the environment.

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.

A CRISPR-Cas12a-powered magnetic relaxation switching biosensor for the sensitive detection of Salmonella

Magnetic relaxation switching (MRS) biosensors are attractive in the field of food safety owing to their simplicity and high signal-to-noise ratio. But they are less in sensitivity and stability caused by the insufficient crosslinking or non-specific binding of magnetic nanoparticles (MNPs) with targets. To address this problem, the CRISPR-Cas12a system was introduced into an MRS biosensor for the first time, to precisely control the binding of two types of MNPs with sizes of 130 nm (MNP₁₃₀) and 30 nm (MNP₃₀), for the sensitive detection of Salmonella. Delicately, the biosensor was designed based on the different magnetic properties of the two sizes of MNPs. The target Salmonella activated the collateral cleavage activity of the CRISPR-Cas12a system, which inhibited the binding of the two sizes of MNPs, resulting in an increase of unbound MNP₃₀. After separating MNP₁₃₀-MNP₃₀ complexes and MNP₁₃₀ from MNP₃₀, the free MNP₃₀ left in solution acted as transverse relaxation time (T₂) signal reporters for Salmonella detection. Under optimized conditions, the CRISPR-MRS biosensor presented a limit of detection of 1.3 × 10² CFU mL^-1 for Salmonella, which is lower than most MRS biosensor analogues. It also showed satisfactory specificity and performed well in spiked chicken meat samples. This biosensing strategy not only extends the reach of the CRISPR-Cas12a system in biosensors but also offers an alternative for pathogen detection with satisfactory sensitivity.

Embedding Proteins within Spatially Controlled Hierarchical Nanoarchitectures for Ultrasensitive Immunoassay

Modulating the precise self-assembly of functional biomacromolecules is a critical challenge in biotechnology. Herein, functional biomacromolecule-assembled hierarchical hybrid nanoarchitectures in a spatially controlled fashion are synthesized, achieving the biorecognition behavior and signal amplification in the immunoassay simultaneously. Biomacromolecules with sequential assembly on the scaffold through the biomineralization process show significantly enhanced stability, bioactivity, and utilization efficiency, allowing tuning of their functions by modifying their size and composition. The hierarchically hybrid nanoarchitectures show great potential in construction of ultrasensitive immunoassay platforms, achieving a three order-of-magnitude increase in sensitivity. Notably, the well-designed HRP@Ab₂ nanoarchitectures allow for optical immunoassays with a detection range from picogram mL^-1 to microgram mL^-1 on demand, providing great promise for quantitative analysis of both low-abundance and high-residue targets for biomedical applications.

Iodide etching for one-step quantitative assay of the number of DNA molecules capped on gold nanoparticles

Developing a direct method to easily quantify the number of DNA capped on gold nanoparticles (GNPs) is of great significance. Herein, we found that the high concentration of iodine ion (I^-) can not only replace the ligands on the surface of GNPs but can also completely etch the particles by virtue of its strong reducibility. According to this finding, a mild, cost-effective, environment-friendly, and non-toxic strategy was constructed to directly and accurately estimate the amount of DNA coupled on GNPs. Due to nanometal surface energy transfer (NSET) that happened between the DNA-FAM donor and the GNPs receptor, the fluorescence was quenched; after incubating with the etching reagent 6 M I^-, the recuperative fluorescence was detected directly. This method can easily estimate the number of DNA attached on the GNPs surface by one step. In a nutshell, it is a smart strategy to apply iodide etching for DNA quantification on the surface of GNPs, which breaks through the drawbacks of traditional DNA quantification strategies such as pollution, being expensive and even dangerous. This strategy takes a solid step forward for the refinement and optimization of DNA quantification and can also be more effective in detecting the number of other molecules capped on the GNPs surface, indicating that the iodide etching method is greatly helpful in bio-detection assays and nanoparticle-based therapeutics.

Click Chemistry-Mediated Particle Counting Sensing via Cu(II)-Polyglutamic Acid Coordination Chemistry and Enzymatic Reaction

An electrical resistance-based particle counter (ERPC) with simple operation and high resolution has proved to be a promising biosensing toolkit, whereas amplification-free ERPC biosensors are incapable of analyzing trace small molecules due to their relatively low sensitivity. In this work, click chemistry-mediated particle counting sensing of small-molecule hazards in food samples with high sensitivity was developed. In this strategy, unbound alkyne-functionalized polystyrene microspheres were collected by magnetic separation from the copper-ion-mediated click reaction between alkyne-functionalized polystyrene microspheres and azido-functionalized magnetic beads, which could be used as signal probes for the readout. This click chemistry-mediated ERPC biosensor converts the detection of targets to the quantification of copper ions or ascorbic acid by performing competitive immunoassay-based coordination chemistry and enzymatic reaction, respectively. The sensitivity of the ERPC biosensor has been improved by an order of magnitude due to the signal amplification effects of click chemistry, coordination adsorption, and enzyme catalysis. Furthermore, because of the efficient separation and enrichment of immunomagnetic beads and the robustness of click chemistry, the interference from food matrixes and immunoassay is effectively reduced, and thus, our strategy is exceedingly suitable for detecting trace targets in complex samples.

Noble Metal Nanoparticle Biosensors: From Fundamental Studies toward Point-of-Care Diagnostics

Noble metal nanoparticles (NMNPs) have become firmly established as effective agents to detect various biomolecules with extremely high sensitivity. This ability stems from the collective oscillation of free electrons and extremely large electric field enhancement under exposure to light, leading to various light-matter interactions such as localized surface plasmon resonance (LSPR) and surface-enhanced Raman scattering. A remarkable feature of NMNPs is their customizability by mechanisms such as particle etching, growth, and aggregation/dispersion, yielding distinct color changes and excellent opportunities for colorimetric biosensing in user-friendly assays and devices. They are readily functionalized with a large variety of capping agents and biomolecules, with resultant bioconjugates often possessing excellent biocompatibility, which can be used to quantitatively detect analytes from physiological fluids. Furthermore, they can possess excellent catalytic properties that can achieve significant signal amplification through mechanisms such as the catalytic transformation of colorless substrates to colored reporters. The various excellent attributes of NMNP biosensors have put them in the spotlight for developing high-performance in vitro diagnostic (IVD) devices that are particularly well-suited to mitigate the societal threat that infectious diseases pose. This threat continues to dominate the global health care landscape, claiming millions of lives annually. NMNP IVDs possess the potential to sensitively detect infections even at very early stages with affordable and field-deployable devices, which will be key to strengthening infectious disease management. This has been the major focal point of current research, with a view to new avenues for early multiplexed detection of infectious diseases with portable devices such as smartphones, especially in resource-limited settings.In this Account, we provide an overview of our original inspiration and efforts in NMNP-based assay development, together with some more sophisticated IVD assays by ourselves and many others. Our work in the area has led to our recent efforts in developing IVDs for high-profile infectious diseases, including Ebola and HIV. We emphasize that integration with digital platforms represents an opportunity to establish and efficiently manage widespread testing, tracking, epidemiological intelligence, and data sharing backed by community participation. We highlight how digital technologies can address the limitations of conventional diagnostic technologies at the point of care (POC) and how they may be used to abate and contain the spread of infectious diseases. Finally, we focus on more recent integrations of noble metal nanoparticles with Raman spectroscopy for accurate, noninvasive POC diagnostics with improved sensitivity and specificity.

Monovalent Antigen-Induced Aggregation (MAA) Biosensors Using Immunomagnetic Beads in Both Sample Separation and Signal Generation for Label-Free Detection of Enrofloxacin

Exploring new functions of nanomaterials can help facilitate the development of biosensors for the detection of antibiotics. Herein, a new detection modality based on monovalent antigen-induced aggregation (MAA) of immunomagnetic beads (IMBs) was proposed for rapid and label-free detection of enrofloxacin (ENR), which endowed IMBs with the abilities of both sample separation and signal generation. In the presence of ENR, the initially well-dispersed IMBs were aggregated and the degree of aggregation was in a concentration-dependent manner. After exploring the mechanism underlying IMB aggregation and investigating the key parameters affecting it, a label-free biosensing platform was developed for rapid and sensitive detection of ENR. Based on the significant differences in the magnetic separation speed and size between the aggregated and well-dispersed IMBs, two methods were proposed for quantitatively determining ENR, i.e., measuring the turbidity of the IMB supernatant after magnetic separation for a given time and visualizing and calculating the grayscale value of the aggregated IMBs trapped on the surface of a nitrocellulose membrane. A three-dimensional (3D)-printed syringe was designed and fabricated for automatic filtration of IMBs. This immunosensor allowed for sensitive detection of ENR in less than 15 min without any labels. It exhibited a satisfactory limit of detection of 0.79 ng mL^-1 and showed the feasibility for ENR detection of spiked chicken meat with recovery rates ranging from 74.8 to 98.3%. The MAA immunosensor can act as a promising tool to detect trace levels of ENR and has the potential to be applied to complex food samples.

Detection of monoamine oxidase B using dark-field light scattering imaging and colorimetry

Detection of MAO-B using dark-field light scattering imaging and colorimetry based on localized surface plasmon resonance induced by silver deposited gold nanostars.

Plasmonic biosensor for the highly sensitive detection of microRNA-21 via the chemical etching of gold nanorods under a dark-field microscope

MicroRNAs involved in tumor-related tissues at abnormal expression level present tremendous potential in the early diagnosis of cancers. However, their intrinsic shortcomings, for instance, low abundance and high sequence homology, make it challengeable to quantify them with high sensitivity and selectivity. Herein, a highly sensitive platform with great specificity was developed for microRNA-21 based on the produced-I₂ triggered chemical etching of gold nanorods to a smaller size, resulting in a significant blue shift and a great intensity decrease in the localized surface plasmon resonance (LSPR) scattering. The synergism of strand displacement and enzymatic reaction enabled the proposed strategy with a high sensitivity and selectivity toward microRNA-21 in a dynamic range from 0.1 to 10,000 pM and a low limit of detection of 71.22 fM (3σ/k) by dark-field microscope. Additionally, the remarkable discrimination of single nucleotide difference suggested the superior selectivity towards microRNA-21, which presented a satisfactory recovery in human serum samples. The proposed plasmon platform could also serve as a universal and sensitive detection of cancer biomarkers, presenting the amusing application prospects in the early diagnosis of various cancers by adapting the corresponding nucleic acid sequences.

Iodide-Mediated Rapid and Sensitive Surface Etching of Gold Nanostars for Biosensing

Iodide-mediated surface etching can tailor the surface plasmon resonance of gold nanostars through etching of the high-energy facets of the nanoparticle protrusions in a rapid and sensitive way. By exploring the underlying mechanisms of this etching and the key parameters influencing it (such as iodide, oxygen, pH, and temperature), we show its potential in a sensitive biosensing system. Horseradish peroxidase-catalyzed oxidation of iodide enables control of the etching of gold nanostars to spherical gold nanoparticles, where the resulting spectral shift in the surface plasmon resonance yields a distinct color change of the solution. We further develop this enzyme-modulated surface etching of gold nanostars into a versatile platform for plasmonic immunoassays, where a high sensitivity is possible by signal amplification via magnetic beads and click chemistry.