We provide a highly sensitive and selective assay to detect Hg 2+ in aqueous solutions using gold nanoparticles modified with quaternary ammonium group-terminated thiols at room temperature. The mechanism is the abstraction of thiols by Hg 2+ that led to the aggregation of nanoparticles. With the assistance of solar light irradiation, the detection limit can be as low as 30 nM, which satisfies the guideline concentration of Hg 2+ in drinking water set by the WHO. In addition, the dynamic range of detection is wide (3 × 10 -8 -1 × 10 -2 M). This range, to our best knowledge, is the widest one that has been reported so far in gold nanoparticle (AuNP)-based assays for Hg 2+ .We report a simple method to detect Hg 2+ in aqueous media by quaternary ammonium group-capped gold nanoparticles (QA-AuNPs). Hg 2+ poses severe threats to both human health and the environment. 1 Long-term exposure to high levels of Hg 2+ -based toxins leads to serious and permanent damage of the central nervous system and other organs. 2 Many of the settings required for such assays lack advanced resources, such as electricity. Highly sensitive and selective assays for Hg 2+ , without resorting to advanced instruments are urgently needed. Researchers have published a number of methods for detecting Hg 2+ , based on chemical sensors using small organic molecules, 3 thin films, 4,5 electrochemistry methods, 6,7 polymeric materials, 8 oligonucleotides, 9,10 proteins, 11 inductively coupled plasma-atomic emission spectrometry, 12 and atomic absorption spectroscopy. 13 Most of these methods, however, have limitations with respect to sensitivity and selectivity or require complex instrumentation or at least electricity. In particular, methods that require no sophisticated starting materials and allow visual readout might be very useful for detecting Hg 2+ in resource-poor settings.AuNPs are increasingly employed for a wide spectrum of biological and biomedical applications. [14][15][16][17][18] Colorimetric assays based on AuNPs have attracted increasing consideration on account of their unique and size-dependent optical and electronic properties. Recently, DNA-functionalized AuNPs have been widely used as colorimetric sensors for a variety of targets, including metallic ions. [19][20][21][22][23] The thymine (T) bases in DNA sequences endow DNA-AuNP assays excellent selectivity for Hg 2+ that can interact with T-T mismatches to form T-Hg 2+ -T complexes. However, most DNA-AuNPs assays rely on accurate control of the detection conditions, such as temperature. In addition, DNA can be costly and difficult to handle.

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An eye for color: Antibodies modified by CuO nanoparticles (NPs) are subjected to immunoreaction with the release of CuII, which can be detected by click chemistry. The Cu acts as a catalyst that induces aggregation of Au NPs functionalized with azide and alkyne groups, which can be seen as a color change (see picture). The detection of HIV in blood serum of infected patients is demonstrated.

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