Here we report a “blue-to-red” colorimetric method for determination of mercury ions (Hg2+) and silver ions (Ag+) based on stabilization of gold nanoparticles (AuNPs) by redox formed metal coating in the presence of ascorbic acid (AA). AuNPs were first stabilized by Tween 20 in phosphate buffer solution with high ionic strength. In a target ion-free system, the addition of N-acetyl-l-cysteine resulted in the aggregation of Tween 20 stabilized AuNPs for mercapto ligand self-assembled on the surface of AuNPs, which induced the AuNPs to be unstable. This would lead to a color change from red to blue. By contrast, in an aqueous solution with Hg2+ or Ag+, the ions could be reduced with the aid of AA to form Hg–Au alloy or Ag coating on the surface of AuNPs. This metal coating blocked mercapto ligand assembly and AuNPs kept monodispersed after addition of N-acetyl-l-cysteine, exhibiting a red color. Therefore, taking advantage of this mechanism, a “blue-to-red” colorimetric sensing strategy could be established for Hg2+ and Ag+ detection. Compare with the commonly reported aggregation-based method (‘red-to-blue’), the color change from blue to red seems more eye-sensitive, especial in low concentration of target. Moreover, selective analysis of Hg2+ and Ag+ was simply achieved by the redox nature of target ions and the application of classic ion masking agents, avoiding the design and selection of ion chelating moieties and complicated gold surface modification procedure. This method could selectively detect Hg2+ and Ag+ as low as 5 nM and 10 nM in pure water with a linear range of 5 × 10–7 to 1 × 10–5 M for Hg2+ and 1 × 10–6 to 8 × 10–6 M for Ag+, respectively. It was successfully applied to determination of Hg2+ and Ag+ in tap water and drinking water.

A highly sensitive surface-enhanced Raman scattering (SERS) platform for the selective trace analysis of As3+ ions was reported based on glutathione (GSH)/4-mercaptopyridine (4-MPY)-modified silver nanoparticles (AgNPs). Here, GSH conjugated on the surface of AgNPs for specifical binding with As3+ ions in aqueous solution through As–O linkage and 4-MPY was used as a Raman reporter. When As3+ ions were added to the system, the binding of As3+ with GSH resulted in the aggregation of AgNPs, and excellent Raman signal of 4-MPY reporters was obtained which can reflect the concentration of As3+ indirectly. Under optimal assay conditions, the limit of detection (LOD) was estimated to be as low as 0.76 ppb, which is lower than the WHO defined limit (10 ppb), and an excellent linear range of 4–300 ppb was obtained. The practical application had been carried out for determination of As3+ in real water samples.

A novel nanomaterial/ionophore-modified glassy carbon electrode for anodic stripping analysis of lead (Pb 2+ ) is described. Nanosized hydroxyapatite (NHAP) with width of 20-25 nm and length of 50-100 nm has been prepared and used to improve the sensitivity for detection of Pb 2+ because it provides unique threedimensional network structure and has strong adsorption ability toward Pb 2+ . An ionophore, usually used in ion-selective electrodes, is utilized here for its excellent selectivity toward Pb 2+ . Nafion, a cationexchange polymer, is employed as the conductive matrix in which NHAP and the ionophore can be tightly attached to the electrode surface. Such a designed NHAP/ionophore/Nafion-modified electrode shows remarkably improved sensitivity and selectivity to Pb 2+ . The electrode has a linear range of 5.0 nM to 0.8 µM with a 10 min accumulation time at open-circuit potential. The sensitivity and detection limit of the proposed sensor are 13 µA/µM and 1.0 nM, respectively. Interference from other heavy metal ions such as Cd 2+ , Cu 2+ , and Hg 2+ associated with lead analysis can be effectively diminished. The practical application of the proposed sensor has been carried out for determination of trace levels of Pb 2+ in real water samples.

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A novel platform for effective "turn-on" fluorescence sensing of lead ions (Pb 2+ ) in aqueous solution was developed based on gold nanoparticle (AuNP)-functionalized graphene. The AuNP-functionalized graphene exhibited minimal background fluorescence because of the extraordinarily high quenching ability of AuNPs. Interestingly, the AuNP-functionalized graphene underwent fluorescence restoration as well as significant enhancement upon adding Pb 2+ , which was attributed to the fact that Pb 2+ could accelerate the leaching rate of the AuNPs on graphene surfaces in the presence of both thiosulfate (S 2 O 3 2− ) and 2-mercaptoethanol (2-ME). Consequently, this could be utilized as the basis for selective detection of Pb 2+. With the optimum conditions chosen, the relative fluorescence intensity showed good linearity versus logarithm concentration of Pb 2+ in the range of 50−1000 nM (R = 0.9982), and a detection limit of 10 nM. High selectivity over common coexistent metal ions was also demonstrated. The practical application had been carried out for determination of Pb 2+ in tap water and mineral water samples. The Pb 2+ -specific "turn-on" fluorescence sensor, based on Pb 2+ accelerated leaching of AuNPs on the surface of graphene, provided new opportunities for highly sensitive and selective Pb 2+ detection in aqueous media.

A surface-enhanced Raman scattering (SERS) strategy based on 4-mercaptopyridine (MPY)-modified gold nanoparticles (AuNPs) was developed for the rapid and sensitive detection of melamine in milk powder. The SERS measurement of melamine strongly relied on the "hotspot" effect, in which AuNPs immediately aggregated upon the addition of melamine, leading to significantly enhanced Raman intensity of the reporter molecule MPY and a color change for the solution from red to blue-gray. The limit of detection based on a signal to noise of 3 (S/N=3) was found to be as low as 0.1 ppb of melamine, with an excellent linearity of 0.5-100 ppb, demonstrating a higher sensitivity and a wider quantitation range than direct SERS sensing methods based on enhanced substrate. An impressive specificity for melamine detection over various common metal ions and excipients in dairy products, even at concentrations of 100-fold higher than melamine, was achieved. Good recoveries of 88.5% and 111.7% were obtained from milk samples spiked to 20 and 100 ppb levels, respectively. The proposed method is potentially applicable for the rapid in situ determination of melamine in complex matrices.

A simple and sensitive method for the colorimetric detection of mercury ions (Hg 2+ ) has been proposed by using anti-aggregation of gold nanoparticles (AuNPs) based on the co-ordination between thymine and mercury ions. The thymine can bind to the AuNPs through Au-N bonds and induce aggregation of AuNPs. In the presence of Hg 2+ , the thymine was released from the surface of AuNPs via the formation of a thymine-Hg 2+ coordination complex, leading to the dispersion of AuNPs. The detection reagent can be simply prepared by mixing thymine with citrate-capped AuNPs. This method is not only costeffective, but also avoids complicated surface modifications and tedious separation processes.

An approach for mercury ions (Hg 2+ ) sensing based on the Hg 2+ -induced aggregation of thymine (T)-SH-functionalized gold nanoparticles (AuNPs) has been reported. The T-SH ligands that we synthesized can easily be coupled to the surface of AuNPs through the Au-S bond and can recognize Hg 2+ with high selectivity by forming a T-Hg-T complex with strong affinity. For the T-SHfunctionalized AuNPs (T-S-AuNPs) sensor, upon addition of Hg 2+ , the formation of the T-Hg-T complex induces aggregation of T-S-AuNPs and results in a significant change of color and UV-Vis absorption spectra. Thus, our method can be used for the rapid, easy and reliable screening of Hg 2+ in aqueous solution, with high sensitivity (2.8 nM) and selectivity over competing analytes. The developed method is successfully applied to the sensing of Hg 2+ in real environmental samples.