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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 Au 80 Ag 20 NPs caused a more prominent color change. Copper ions (Cu2+) can be rapidly reduced by nicotinamide adenine dinucleotide (NADH) under the catalysis of Au 80 Ag 20 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 Au 80 Ag 20 NPs by the excess Cu2+ and different aggregation degrees of Au 80 Ag 20 NPs lead to observable color change. A linear correlation of A 600 /A 505 = 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, Au 80 Ag 20 NPs was replaced by several different composition nanoparticles (Au NPs, Au 70 Ag 30 NPs, Au 50 Ag 50 NPs) to be a chromogenic substrate, and the results suggest the Au 80 Ag 20 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. In the absence of NTR, NADH can reduce Cu2+ to Cu (0) under the catalysis of the Au 80 Ag 20 NPs. When NTR is introduced, NADH is consumed in the catalytic reaction, and Cu2+ that cannot be reduced will cause the aggregation of Au 80 Ag 20 NPs due to the strong affinity between Cu2+ and citrate. [Display omitted] • Our assay is a rapid and simple colorimetric detection for nitroreductase. • Visual detection of nitroreductase can be accompanied due to the Au x Ag y NPs property. • The sensitivity of the assay by the naked eye is comparable to that of the apparatus. [ABSTRACT FROM AUTHOR]

Detecting cancer biomarker levels in body fluids is essential for medical diagnosis. Enzyme-linked immunosorbent assay (ELISA) has been broadly used to detect cancer biomarkers. However, colorimetric ELISA based solely on nanoparticles (NPs) are susceptible to environmental influences, which often results in the detection inaccuracy, being limited in clinical applications. In this regard, the dual-mode approach would add signal diversity to the detection, making the results more reliable. We present colorimetric and photothermal immunosensor that enables direct reading of the color and temperature of the solution. A core-satellite nanoprobe constructed by polydopamine (PDA) as the core and gold seeds as satellites is rationally designed as the signal reporter. When ascorbic acid is present in the solution, PDA can cooperate with ascorbic acid to reduce chloroauric acid and mediate the growth of gold seeds on the PDA surface, inducing a redshift of the localized surface plasmon resonance peak of the nanosensor and the change in photothermal conversion efficiency. The method is further combined with the sandwiched immunoassay to construct an alkaline phosphatase based colorimetric and photothermal ELISA for the highly sensitive and accurate evaluation and detection of prostate-specific antigen (PSA). The linear range was from 0.05 to 100 ng mL−1 with a detection limit of 6.71 pg mL−1 for the colorimetric detection, while the linear range was from 0.5 to 90 ng mL−1 with a detection limit of 0.13 ng mL−1 in the photothermal analysis. The accurate detection of PSA levels in serum samples was well demonstrated with the dual-mode approach. The presented immunoassay allows straightforward, sensitive, and selective readout by color and temperature without advanced instrumentation. Particularly, the LOD was much lower than the threshold in clinical trials for PSA. Therefore, this method has a great prospect in the early diagnosis of cancer biomarkers based on a dual-mode multifunctional platform. [Display omitted] • We developed a dual-mode immunosensor based on gold nanoparticle growth. • The nanoprobe showed color and temperature changes caused by the cancer biomarker. • The assay had high detection sensitivity and accuracy. • The strategy was used to assess prostate-specific antigen levels in serum samples. [ABSTRACT FROM AUTHOR]