In recent years, gold nanoparticles (AuNPs) have drawn considerable research attention in the fields of catalysis, drug delivery, imaging, diagnostics, therapy and biosensors due to their unique optical and electronic properties. In this review, we summarized recent advances in the development of AuNP-based colorimetric and fluorescent assays for ions including cations (such as Hg(2+), Cu(2+), Pb(2+), As(3+), Ca(2+), Al(3+), etc) and anions (such as NO(2)(-), CN(-), PF(6)(-), F(-), I(-), oxoanions), and small organic molecules (such as cysteine, homocysteine, trinitrotoluene, melamine and cocaine, ATP, glucose, dopamine and so forth). Many of these species adversely affect human health and the environment. Moreover, we paid particular attention to AuNP-based colorimetric and fluorescent assays in practical applications.

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.

Current techniques for plasmonic immunoassay often require the introduction and additional conjugation of enzyme, and thus cannot accommodate conventional immunoassay platforms. Herein, we develop a plasmonic nanosensor that well accommodates conventional immunoassays and dramatically improves their sensitivity and stability. This plasmonic nanosensor directly employs alkaline phosphatase-triggered click chemistry between azide/alkyne functionalized gold nanoparticles as the readout. This straightforward approach broadens the applicability of nanoparticle-based immunoassays and has great potential for applications in resource-constrained settings.

A resettable logic system based on spiropyran‐modified gold nanoparticles that is capable of AND, OR, and INHIBIT logic operations has been constructed. Several methods can record the output of this process, including the naked eye, UV/Vis spectroscopy, determination of the ζ potential, and dynamic light scattering. These logic gates can also detect copper(II) ions in aqueous media.

This report presents a highly sensitive, rhodamine B-covered gold nanoparticle (RB-AuNP) -based assay with dual readouts (colorimetric and fluorometric) for detecting organophosphorus and carbamate pesticides in complex solutions. The detection mechanism is based on the fact that these pesticides can inhibit the activity of acetylcholinesterase (AChE), thus preventing the generation of thiocholine (which turns the RB-AuNP solutions blue and unquenches the fluorescence of RB simultaneously). The color of the RB-AuNP solution remains red and the fluorescence of RB remains quenched. By use of this dual-readout assay, the lowest detectable concentrations for several kinds of pesticides including carbaryl, diazinon, malathion, and phorate were measured to be 0.1, 0.1, 0.3, and 1 μg/L, respectively, all of which are much lower than the maximum residue limits (MRL) as reported in the European Union pesticides database as well as those from the U.S. Department Agriculture (USDA). This assay allows detection of pesticides in real samples such as agricultural products and river water. The results in detecting pesticide residues collected from food samples via this method agree well with those from high-performance liquid chromatography (HPLC). This simple assay is therefore suitable for sensing pesticides in complex samples, especially in combination with other portable platforms.

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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.

We report a method for the rapid and efficient identification of bacteria making use of five probes having fluorescent characteristics (F-array) and subsequent statistical analysis. Eight kinds of bacteria, including normal and multidrug-resistant bacteria, are differentiated successfully. Our easy-to-perform and time-saving method consists of mixing bacteria and probes, recording fluorescent intensity data by automated flow cytometry, and statistical analysis. No washing steps are required in order to identify the different bacteria simultaneously.