Rapid testing, generally refers to the paper-based diagnostic platform known as “lateral flow assay” (LFA), has emerged as a critical asset to the containment of coronavirus disease 2019 (COVID-19) around the world. LFA technology stands out amongst peer platforms due to its cost-effective design, user-friendly interface, and low sample-to-readout times. This article aims to introduce its design, use, and practicality for the purpose of diagnosing SARS-CoV-2 infection. A connection is made from the normal COVID-19 immune response to the design and efficacy of rapid testing. Interference in test results is a challenge shared by most diagnostic platforms and can be rooted in various underlying issues. The current knowledge and situation about interference in rapid COVID-19 tests due to variant strains as well as vaccination are discussed. The cost and societal impact are reviewed as they play important roles in determining how to properly implement public testing practices. Perspectives on improving the performance, especially detection sensitivity, of LFA for COVID-19 are provided. Graphical abstract

Palladium (Pd) nanoparticles are of particular interest to various fundamental studies and emerging areas of technology. The properties of Pd nanoparticles have a strong dependence on particle size. Nevertheless, it has been challenging to synthesize uniform Pd nanoparticles with controllable sizes and in relatively large quantities. Herein, we demonstrate a simple yet robust one-pot synthesis for the preparation of single-crystal Pd nanoparticles with controllable sizes in the range 2−14 nm. The synthesis is simply performed by mixing polyvinylpyrrolidone (PVP), Na 2 PdCl 4 , and ethylene glycol in a glass vial that is placed in an oil bath with stirring. The sizes could be conveniently and tightly controlled by adjusting the amount of PVP or Na 2 PdCl 4 /PVP. The final products are highly uniform in terms of size and shape. Notably, the strategy of size control was successfully extended to Pt and Rh nanoparticles. The synthesis could be scaled up to allow for the production of gram-level quantities of Pd nanoparticles in a short period of time. The uniform Pd nanoparticles with controllable sizes are believed to find important use in different areas, such as fundamental nanoresearch, catalysis, and biomedicine.

In this work, we report a peroxidase mimic composed of iridium (Ir) nanoparticles with an average size of only 1.1 nm that are coupled to a tungsten suboxide (WO2.72) nanorod as support. Such a peroxidase mimic exhibited an area-specific catalytic efficiency as high as 1.5 × 104 s–1 nm–2, outperforming most existing peroxidase mimics of noble metals. As proof-of-concept demonstrations, the peroxidase mimic was applied to colorimetric immunoassays of carcinoembryonic antigen (a cancer biomarker) and aflatoxin B1 (a potent carcinogen), which provided substantially enhanced detection sensitivities compared to conventional enzyme-based assays.

Over the past few decades, peroxidase mimics made of platinum-group metal nanoparticles (PGM NPs) have been actively developed and applied to various biosensing platforms. Nevertheless, there is a lack of a comprehensive study that compares the peroxidase-like activities of PGM NPs and their performance in biosensing. Here, we report a systematic study of PGM NPs as peroxidase mimics, including Pd, Pt, Rh, and Ir NPs. NPs of these elements were uniformly synthesized and their nanoscale features were probed to ensure a consistent size, shape, and chemical ligand on the surface. Our measurements indicate that the Ir NP is the most active one with a catalytic constant as high as 6.27 × 105 s–1, followed by Pt, Rh, and Pd NPs. The binding affinities of NPs to peroxidase substrates during catalysis were also quantitively analyzed and compared. Using enzyme-linked immunosorbent assay as a model biosensing platform, the performance of PGM NPs in detecting carcinoembryonic antigen (a cancer biomarker) was evaluated. The results showed that the detection sensitivity was correlated to the catalytic activity of PGM NPs, where Ir NPs achieved the highest sensitivity with a limit of detection at the level of low picogram per milliliter.