The roles of templates consisting of amino acids in the synthesis and application of gold nanoclusters

Gold nanoclusters (AuNCs) with low toxicity, high photostability, and facile synthesis have attracted great attention. The ligand is of great significance in stabilizing AuNCs and regulating their properties. Ligands consisting of amino acids (proteins and peptides) are an ideal template for synthesizing applicative AuNCs due to their inherent bioactivity, biocompatibility, and accessibility. In this review, we summarize the correlation of the template consisting of amino acids with the properties of AuNCs by analyzing different peptide sequences. The selection of amino acids can regulate the fluorescence excitation/emission and intensity, size, cell uptake, and light absorption. By analyzing the role played by AuNCs stabilized by proteins and peptides in the application, universal rules and detailed performances of sensors, antibacterial agents, therapeutic reagents, and light absorbers are reviewed. This review can guide the template design and application of AuNCs when selecting proteins and peptides as ligands.

Challenges and Opportunities of Using Fluorescent Metal Nanocluster-Based Colorimetric Assays in Medicine

Development of rapid colorimetric methods based on novel optical-active metal nanomaterials has provided methods for the detection of ions, biomarkers, cancers, etc. Fluorescent metal nanoclusters (FMNCs) have gained a lot of attention due to their unique physical, chemical, and optical properties providing numerous applications from rapid and sensitive detection to cellular imaging. However, because of very small color changes, their colorimetric applications for developing rapid tests based on the naked eye or simple UV-vis absorption spectrophotometry are still limited. FMNCs with peroxidase-like activity have significant potential in a wide variety of applications, especially for point-of-care diagnostics. In this review, the effect of using various capping agents and metals for the preparation of nanoclusters in their colorimetric sensing properties is explored, and the synthesis and detection mechanisms and the recent advances in their application for ultrasensitive chemical and biological analysis regarding human health are highlighted. Finally, the challenges that remain as well as the future perspectives are briefly discussed. Overcoming these limitations will allow us to expand the nanocluster's application for colorimetric diagnostic purposes in medical practice.

A novel label-free dual-mode aptasensor based on the mutual regulation of silver nanoclusters and MoSe2 nanosheets for reliable detection of ampicillin

Current methods for the rapid detection of trace antibiotics in the environment remains problems of low accuracy and false negative or false positive, making the development of fast, and accurate, and reliable methods for antibiotic testing a major challenge that needs to be addressed. Herein, we developed a novel label-free colorimetric and fluorescent dual-mode aptasensor assembled by the strong interaction of layered MoSe₂ nanosheets (MoSe₂ NSs) with ampicillin (AMP) aptamer functionalized silver nanoclusters (Apt-AgNCs) that specifically bind AMP to allow the sensitive and selective detection of AMP. Apt-AgNCs could be adsorbed on the surface of MoSe₂ NSs via van der Waals force to form a nanocomposite, Apt-AgNCs/MoSe₂ NSs. Interestingly, Apt-AgNCs/MoSe₂ NSs act together to construct dual mode aptasensor through modulation of the intrinsic peroxidase activity of MoSe₂ NSs and the fluorescence of Apt-AgNCs. In the presence of AMP, Apt-AgNCs could specifically bind AMP, triggering desorption from the MoSe₂ NSs surface, leading to a decrease in the peroxidase activity of the system with the recovery in Apt-AgNCs fluorescence. The dual-signal aptasensor exhibited good linear colorimetric and fluorescence responses in the AMP concentration ranges of 0.115-2.00 μM and 6-100 nM, respectively. Furthermore, the aptasensor was successfully measured AMP levels in commercially-bought milk and lake water with satisfactory results. Unlike single-signal aptasensors, the constructed dual-signal aptasensor could not only improve the detection precision, but also reduce the false positive or false negative results. These promising results suggest that the dual-readout strategy as demonstrated is general mode for the detection of other antibiotics or compounds using various aptamers functionalized AgNCs in concert with MoSe₂ NSs.

Cascade reaction biosensor based on gold nanocluster decorated iron-cobalt oxide nanosheets as a superior peroxidase mimic for dual-mode detection of α-glucosidase and its inhibitor

Herein, we have synthesized a novel kind of gold nanoclusters decorated iron-cobalt oxide nanosheets (His-AuNCs@FeCo-ONSs) assembled by electrostatic interaction, which possessed both outstanding peroxidase-like activity and fluorescence property. Taking advantage of our bifunctional hybrid nanozyme and enzyme cascade reactions, a sensitive dual-mode (colorimetric/fluorescent) detection method for α-glucosidase was constructed. The detection limits for α-glucosidase were 2.2 U/L and 3.3 U/L in fluorometric and colorimetric mode, respectively. This method not only provides high sensitivity, but also can correct itself to improve the accuracy of analysis due to the dual-response signals. Furthermore, it was employed for α-glucosidase determination in real samples and screening of α-glucosidase inhibitors.

Gold Nanozymes: Smart Hybrids with Outstanding Applications

Nanozymes are nanostructured artificial enzymes that have attracted great attention among researchers because of their ability to mimic relevant biological reactions carried out by their natural counterparts, but with the capability to overcome natural enzymes’ drawbacks such as low thermostability or narrow substrate scope. The promising enzyme-like properties of these systems make nanozymes excellent candidates for innovative solutions in different scientific fields such as analytical chemistry, catalysis or medicine. Thus, nanozymes with different type of activities are of special interest owing to their versatility since they can reproduce several biological reactions according to the substrates and the environmental conditions. In this context, gold-based nanozymes are a representative example of multifunctional structures that can perform a great number of enzyme-like activities. In addition, the combination of gold-based materials with structures of organic and inorganic chemical nature yields even more powerful hybrid nanozymes, which enhance their activity by providing improved features. This review will carry out a deep insight into gold-based nanozymes, revisiting not only the different type of biological enzymatic reactions that can be achieved with these kinds of systems, but also structural features of some of the most relevant hybrid gold-based nanozymes described in the literature. This literature review will also provide a representative picture of the potential of these structures to solve future technological challenges.

Colorimetric assay for tetracyclines based on europium ion-induced aggregation of gold nanoparticles

Oxytetracycline-capped gold nanoparticles (OTC-Au NPs) were prepared using sodium borohydride as the reductant and OTC as the capping agent, respectively. The prepared OTC-Au NPs with a size of 6 nm have a maximum surface plasma resonance (SPR) absorption located at 514 nm. The OTC on the surface of Au NPs still can coordinate with Eu³⁺ ions. Due to the property that OTC has multivalent binding sites with Eu³⁺ ions, Eu³⁺ ions can induce the aggregation of OTC-Au NPs. Based on the Eu³⁺ ions-aggregated OTC-Au NPs, a simple aptamer-free colorimetric sensing method for TCs was developed. Briefly, free TCs compete with OTC on the surface of Au NPs resulting in the change of OTC-Au NPs from an aggregation state to a dispersed state. The whole process takes only 5 min, and as low as 20 nM OTC, 14 nM tetracycline (TC), and 20 nM doxycycline (DC) could be sensitively detected, respectively. The proposed method was also featured as good repeatability and specificity, and was applied to the detection of OTC in lake water with satisfactory recovery.

Development of enzyme-free single-step immunoassays for glycocholic acid based on palladium nanoparticle-mediated signal generation

Palladium nanoparticles (PdNPs) are composed mainly of inert noble metals, and their outstanding properties have attracted wide attention. PdNPs are not only capable of mimicking the oxidase-like characteristics of natural bio-enzymes, but they also present a clear black band in the test zone. In this work, the synthesized PdNPs promoted a transformation of colorless tetramethylbenzidine (TMB) to a blue oxidation product of TMB, providing a K_m value of 0.09 mM for TMB, and revealing the good catalytic performance of the synthesized PdNPs. For both signal generation and amplification, PdNPs effectively replaced natural bio-enzymes as a new labeling tag. Thus, the PdNP-based enzyme-free single-step immunoassays were successfully developed for efficient and sensitive detection of glycocholic acid (GCA). Under optimal conditions, a noticeable linear relationship was identified by the enzyme-linked immunosorbent assay (ELISA) over a range of 8-2390 ng/mL, while the visual limit of detection (vLOD) in the constructed lateral flow immunoassay (LFA) was 10 ng/mL for GCA. The recovery rate in spiked urine samples obtained by the ELISA ranged from 84.2 to 117.9%, which was consistent with the results in LFA. The present work demonstrates the potential of PdNPs as labeling matrices in enzyme-free single-step immunoassays.

A Review on Metal- and Metal Oxide-Based Nanozymes: Properties, Mechanisms, and Applications

Since the ferromagnetic (Fe3O4) nanoparticles were firstly reported to exert enzyme-like activity in 2007, extensive research progress in nanozymes has been made with deep investigation of diverse nanozymes and rapid development of related nanotechnologies. As promising alternatives for natural enzymes, nanozymes have broadened the way toward clinical medicine, food safety, environmental monitoring, and chemical production. The past decade has witnessed the rapid development of metal- and metal oxide-based nanozymes owing to their remarkable physicochemical properties in parallel with low cost, high stability, and easy storage. It is widely known that the deep study of catalytic activities and mechanism sheds significant influence on the applications of nanozymes. This review digs into the characteristics and intrinsic properties of metal- and metal oxide-based nanozymes, especially emphasizing their catalytic mechanism and recent applications in biological analysis, relieving inflammation, antibacterial, and cancer therapy. We also conclude the present challenges and provide insights into the future research of nanozymes constituted of metal and metal oxide nanomaterials.

Gold nanoclusters: An ultrasmall platform for multifaceted applications

Gold nanoclusters (Au NCs) with a core size below 2 nm form an exciting class of functional nano-materials with characteristic physical and chemical properties. The properties of Au NCs are more prominent and extremely different from their bulk counterparts. The synthesis of Au NCs is generally assisted by template or ligand, which impart excellent cluster stability and high quantum yield. The tunable and sensitive physicochemical properties of Au NCs open horizons for their advanced applications in various interdisciplinary fields. In this review, we briefly summarize the solution phase synthesis and origin of the characteristic properties of Au NCs. A vast review of recent research work introducing biosensors based on Au NCs has been presented along with their specifications and detection limits. This review also highlights recent progress in the use of Au NCs as bio-imaging probe, enzyme mimic, temperature sensing probe and catalysts. A speculation on present challenges and certain future prospects have also been provided to enlighten the path for advancement of multifaceted applications of Au NCs.

A ratiometric fluorescent probe based on sulfur quantum dots and calcium ion for sensitive and visual detection of doxycycline in food

The residue of doxycycline in food can cause harm to human. Therefore, the detection of doxycycline residue is necessary. Herein, a ratiometric fluorescent probe was designed based on sulfur quantum dots (S dots) and Ca²⁺. Due to static quenching and inter filter effect between doxycycline and S dots, doxycycline quenched fluorescence of S dots at 450 nm. Meanwhile, doxycycline and Ca²⁺ formed fluorescent complex through coordination to produce new peak at 520 nm. The ratio of fluorescence intensity (F₅₂₀/F₄₅₀) and doxycycline concentration showed good linear relationship with detection limit of 0.19 μM. The fluorescence color of S dots/Ca²⁺ changed from blue to light green with increasing doxycycline concentration, which was applied for visual semi-quantitative detection of doxycycline. Moreover, the method was used for detecting doxycycline in milk and fish samples with recoveries in the range of 91%-110%. The method showed good application potential in detection of doxycycline in food samples.