The synergistic effect enhanced chemical etching of gold nanorods for the rapid and sensitive detection of biomarks

A portable hydrogel kit based on Au@GM88A/I combined with mobile phone for polychromatic semi-quantitative and quantitative sensing analysis

The development of an affordable, portable, and instrument-free colorimetric biosensor holds significant importance for routine monitoring and clinical diagnosis. To overcome the limitations that traditional monochromatic colorimetric kits struggle to distinguish subtle color changes with the naked eye, we designed and constructed a portable hydrogel kit for polychromatic semi-quantitative and quantitative sensing analysis. When the actual samples and I- were introduced into a gelatin hydrogel encapsulated with MIL-88A(Fe), Au NRs and oxidase (Au@GM88A/I), a noticeable color change occurred. Additionally, a mathematic model between Hue and multicolor signal was set up for the first time by mobile phone photo technology, successfully applied to the glucose detection in serum. The visual detection had a wide concentration range of 0.02-0.80 mM with a limit of detection down to 0.02 mM. Above all, hydrogel kit prepared with gelatin as a carrier addressed the issues of uneven color and slow response rate commonly seen in gels like sodium alginate and agarose. This improvement would be beneficial for enhancing the accuracy of color captured by mobile phone assisted hydrogel kits, making it a valuable tool for biomarker analysis.

Platinum nanoparticles confined in metal-organic frameworks as excellent peroxidase-like nanozymes for detection of uric acid

High levels of uric acid (UA) in humans can cause a range of diseases, and traditional assays that rely on uric acid enzymes to break down uric acid are limited by the inherent deficiencies of natural enzymes. Fortunately, the rapid development of nanozymes in recent years is expected to solve the above-mentioned problems. Hence, we used a host-guest strategy to synthesize a platinum nanoparticle confined in a metal-organic framework (Pt NPs@ZIF) that can sensitively detect UA levels in human serum. Unlike previously reported free radical-catalyzed oxidation systems, its unique electron transfer mechanism confers excellent peroxidase-like activity to Pt NPs@ZIF. In addition, UA can selectively inhibit the chromogenic reaction of TMB, thus reducing the absorbance of the system. Therefore, using the peroxidase-like activity of Pt NPs@ZIF and using TMB as a chromogenic substrate, UA can be detected directly without relying on natural enzymes. The results showed a relatively wide detection range (10-1000 μM) and a low detection limit (0.2 μM). Satisfactory results were also obtained for UA in human serum. This study with simple operation and rapid detection offers a promising method for efficiently detecting UA in serum.

Highly sensitive visual colorimetric sensor for xanthine oxidase detection by using MnO2-nanosheet-modified gold nanoparticles

In this study, a highly sensitive colorimetric assay has been constructed for the determination of xanthine oxidase (XOD) activity by the GNP@MnO₂ core-shell nanoparticles as probe. In the presence of XOD, xanthine can be oxidized to produce H₂O₂, which makes the MnO₂ shell fallen off. With the single particle detection (SPD) based dark field microscopy (DFM), the scattering color of GNP@MnO₂ NP probe shows obvious change before and after etching process. At the single particle level, noticeable color change of the single probe can be easily detected in the existence of trace XOD. This SPD-based colorimetric strategy displays broad linear dynamic range (0.02-4 mU/mL) and low detection limit of 7.82 μU/mL, which is more sensitive than the results from ensemble sample measurement. In addition, we tested the inhibitory effect of quercetin on the activity of XOD and obtained good inhibition effect. As a consequence, this SPD-based colorimetric strategy provides new perception for the ultrasensitive detection of molecules in complex system.

The restructure of Au@Ag nanorods for cell imaging with dark-field microscope

The facile and noninjurious image of cells with high resolution and low toxicity is essential since imaging can offer rich and direct information and insights into metabolic activities, clinical diagnosis, drug delivery and cancer therapy. In this contribution, a smart imaging probe was employed as a contrast agent for dark-field cell imaging. Au core/Ag shell nanorods (Au@Ag NRs) that characterized by X-ray diffraction and X-ray photoelectron spectroscopy, formed Au@Ag@AgI NRs when exposed to iodine, which greatly enhanced the light scattering of nanorods. Herein, the silver shell acted as the response element for iodine as well as the protective agent for Au core. When conjugated with folate, the nanorods can be used to image human cervical cancer cells (HeLa cells) under a dark-field microscope. Nanorods were demonstrated with excellent tumor cellular uptake ability without obvious cytotoxicity, making them ideal candidates in biosensing and bioimaging applications.

A novel light-controlled colorimetric detection assay for nitroreductase based on p-aminophenol-catalyzed and NADH-mediated synthesis of silver nanoparticles

A novel and efficient light-controlled colorimetric assay for the quantification and detection of nitroreductase (NTR) was constructed based on p-aminophenol (pAP)-catalyzed and nicotinamide adenine dinucleotide (NADH)-mediated generation of AgNPs. Due to the hydrolysis of p-nitrophenol by NTR in the presence of NADH, the hydrolysis product can be used as a catalyst to catalyze the reduction of Ag+ by NADH under the light. As the concentration of NTR increases, the value of absorbance at ca. 400 nm (A400) decreases and the color of the solution turns from brown to bright yellow. A linear correlation was obtained between A400 and the NTR concentration in the range from 1-50 μg mL-1 and the limit of detection (LOD) is 0.27 μg mL-1. The detection system does not respond to other common biological molecules due to the specificity of enzymes and the effect of the nitroreductase inhibitor on the NTR activity was also tested. Finally, we applied the assay to determine NTR in human serum samples by spiking different concentrations of NTR with a recovery of 85.2%-92.5%.

Size-modulated optical property of gold nanorods for sensitive and colorimetric detection of thiourea in fruit juice

As an important sulfur compound, thiourea (TU) has caused great concern because of its wide application as well as its serious toxicity and hazard to the environment. Thus, it is necessary to develop a sensitive and selective method for TU analysis. In this work, gold nanorods (AuNRs) acted as an optical probe to realize the sensitive and colorimetric detection of TU. In HCl medium, Fe³⁺ at low concentration was difficult to oxide Au⁰ to form Au⁺ because of the high redox potential or the positive Gibbs free energy change. However, this process was possible when TU was present since the association constant between Au⁺ and TU is great enough to bind with TU to form a stable complex to further promote the etching of AuNRs, resulting in the lower aspect ratio of AuNRs with the blue shift and intensity decrease in extinction spectra, accompanied by the divisive colors of AuNRs solution or colorful dark-field light scattering imaging of single AuNR. The blue-shift of AuNRs longitudinal plasmon resonance absorption (LPRA) band was proportional to the concentration of TU in the range of 1-250 nM and the limit of detection (3σ/k) was as low as 0.4 nM. In addition, the colorimetric method was proven with high selectivity in the presence of potential interfering compounds, which was successfully applied to the detection of TU in fruit juice samples. This proposed colorimetric method provides a simple, sensitive yet selective measurement tool for TU sensing, which may offer new opportunities in the development of colorimetric sensors for food safety in the future.

Smartphone colorimetric assay of acid phosphatase based on a controlled iodine-mediated etching of gold nanorods

A simple but efficient colorimetric assay was developed for the detection and quantification of acid phosphatase (ACP) using a smartphone. This strategy is based on target-controlled iodine-mediated etching of gold nanorods (AuNRs). Due to effective hydrolysis of the substrate pyrophosphate (PPi) by ACP, chelated Cu²⁺ with PPi was released, which promoted the redox reaction with an iodide ion (I^-), leading to the formation of I₃^-. As the etching agent of AuNRs, I₃^- caused a blueshift of the localized surface plasmon resonance peak and, more importantly, an observable color change. The vivid colors were recorded with a smartphone camera and directly analyzed using an image-processing app. On the basis of the direct correlation between ACP concentration and the etching degree of AuNRs as well as color change, this smartphone nanocolorimetry technique showed a good linear response toward ACP over the range of 0-15.0 U/L, with a detection limit of 0.97 U/L. Using the standard addition method, the practical applicability of the proposed smartphone-based assay was successfully demonstrated by determining ACP in human serum samples, with results consistent with those obtained by UV-Vis spectrophotometry.