Rapid visual detection for nitroreductase based on the copper ions-induced and NADH-mediated aggregation of gold-silver alloy nanoparticles

Hypoxia refers to the lack of oxygen supply to cells or tissues. The overexpression of nitroreductase has been shown to be closely related to the degree of hypoxia, which leads to the level of nitroreductase (NTR) being used as an indicator of hypoxia. We reported a facile visual detection of NTR based on the aggregation of gold and silver alloy nanoparticles. Compared with gold nanoparticles (AuNPs), the aggregation behavior of Au₈₀Ag₂₀ NPs caused a more prominent color change. Copper ions (Cu²⁺) can be rapidly reduced by nicotinamide adenine dinucleotide (NADH) under the catalysis of Au₈₀Ag₂₀ NPs. But NADH is consumed as an electron donor during the catalytic reduction reaction of p-nitrophenol (pNP) by NTR. A decrease of NADH amount results in the aggregation of Au₈₀Ag₂₀ NPs by the excess Cu²⁺ and different aggregation degrees of Au₈₀Ag₂₀ NPs lead to observable color change. A linear correlation of A₆₀₀/A₅₀₅ = 0.0285 [NTR]+0.361 (R² = 0.980) was obtained with a limit of detection (LOD) of 0.23 μg/mL for UV-vis spectrophotometer. For visual detection, the values of R/B against the concentration of NTR obtains a calibration curve of R/B = -0.031 [NTR]+ 1.54 (R² = 0.985) with a LOD of 0.76 μg/mL, which is of the same order of magnitude as the UV-vis spectrophotometer analysis. As a comparison, Au₈₀Ag₂₀ NPs was replaced by several different composition nanoparticles (Au NPs, Au₇₀Ag₃₀ NPs, Au₅₀Ag₅₀ NPs) to be a chromogenic substrate, and the results suggest the Au₈₀Ag₂₀ NPs is the most sensitive substrate in our assay. Selectivity tests showed that the detection system did not respond to other common substances, and the reaction mechanism was verified by inhibitor research. Finally, the assay was used on the human serum samples with spiking NTR, and the recovery rates of this assay with UV-vis spectrophotometer were basically consistent with RGB analysis.

An electrochemiluminescence energy resonance transfer system for highly sensitive detection of brombuterol

In this work, an electrochemiluminescence resonance energy transfer (ECL-RET) system was established based on the modified graphite phase carbon nitride to detect brombuterol residues in food. The ultrasonic-assisted acidification exfoliation modification improved the conductivity and specific surface area of the graphite phase carbon nitride (g-C₃N₄). In addition, the carboxylated g-C₃N₄ nanosheets as ECL donors and the Au-Ag alloy nanoparticles as ECL acceptors could respectively directly carry antigen and antibody. Therefore, the trouble of introducing additional bridge molecules was avoided. A competitive immunoassay strategy was used for the detection of brombuterol, where brombuterol in the sample would compete with the coating antigen for the limited binding sites on antibody. The proposed ECL immunosensor for brombuterol detection exhibited high sensitivity with a wide linear range from 0.001 ng mL^-1 to 1000 ng mL^-1 and a low detection limit at 0.31 pg mL^-1. This work adopts a very simple way to design the sensor without losing its sensitivity, bringing convenience to its possible future applications.

Protein-protected gold/silver alloy nanoclusters in metal-enhanced singlet oxygen generation and their correlation with photoluminescence

Photoluminescent noble metal nanoclusters (NCs, core size <2 nm) have recently emerged as a new type of photosensitizers advantageous over conventional photosensitizers due to their high singlet oxygen (¹O₂) generation efficiency, excellent photostability and water solubility, as well as good biocompatibility for photodynamic therapy and bioimaging. However, no correlation has been established between the intrinsic ¹O₂ generation and photoluminescence properties of metal NCs with their size, composition, and concentration, which is important to customize the molecule-like properties of NCs for different applications. Herein, we report a systematic study to uncover the rational design of bimetallic NCs with controllable ¹O₂ generation efficiency by tuning their compositions through spontaneous galvanic displacement reaction. A series of ultrasmall gold/silver alloy nanoclusters (AuAgNCs) were synthesized by reacting bovine serum albumin (BSA) protein-protected Ag₁₃NCs (13 Ag atoms/cluster) with varying concentrations of gold precursor at room temperature. It was found that the ¹O₂ generation efficiency of the resultant BSA-protected AuAgNCs were inversely correlated to their photoluminescence intensity. Interestingly, plasmonic gold nanoparticles (>10 nm) were also formed simultaneously by photobleaching of the BSA-AuAgNCs, leading to significant metal enhancement effect to the ¹O₂ generation rate much higher (~45 times) than that of the monometallic BSA-Ag₁₃NC. This versatile two-for-one strategy to develop next generation metal-enhanced bimetallic NC photosensitizers in one pot opens up new opportunities in designing advanced hybrid nanomaterials with complementary and/or enhanced functionalities.