ABSTRACT:We have developed a novel approach to the rapid visual detection of Cu 2+ in natural samples based on the copper-mediated leaching of gold nanorods (GNRs). In the presence of hexadecyltrimethylammonium bromide, which can reduce the redox potential of Au(I)/ Au, the GNRs are catalytically etched by Cu 2+ preferentially along the longitudinal direction. And as a result, the localized surface plasmon resonance extinction peak shifts to short wavelength, accompanied by a color change from blue to red. The leaching mechanism has been carefully discussed in a series of control experiments. Under optimal conditions, this sensor exhibits good sensitivity (LOD = 0.5 nM). Most importantly, the approach is highlighted by its high selectivity for and tolerance of interference, which enables the sensor to detect Cu 2+ directly in a complex matrix, especially in seawater. Moreover, such a nanoparticle-based sensor is also successfully applied to test paper for the visual detection of Cu 2+ .

A simple colorimetric method with high sensitivity and selectivity was developed for sensing of nitrite as low as 4.0 mM by naked eyes, which is based on etching of gold nanorods accompanied by shape changes in aspect ratios (length/width) and a visible color change from bluish green to red and then to colorless with the increase of nitrite.As type A inorganic contaminants in drinking water, nitrite and nitrate have proven to be of great threat to human health and may result in diseases like methemoglobinemia, esophageal cancer, etc. 1-3Many methods, such as ion chromatography, 4 capillary electrophoresis, 5 chemiluminescence 6 and fluorescence spectrum, 7 have been developed for the detection of NO 2 À and NO 3 À . However, all these methods have poor applicability to field tests. As the largest developing country, China is encountering a great drinking water crisis resulting from the worsening quality of source water. The situation is even worse in its rural regions where people mainly rely on simply prepared tap water or even untreated surface water, shallow groundwater, cellar water, etc. The concentrations of NO 2 À and NO 3 À in most of such water may have reached dangerous level [8][9][10] and have barely been monitored due to the lack of funds, equipments and qualified persons. It is urgent to develop a simple test method that can be quickly handled by local inhabitants. Colorimetric assays have proven to be effective for the inspection of NO 2 À in food and wastewater. [11][12][13] It is unfortunate that photoabsorption coefficients of employed organic dyes are rather low, which withhold further applications of most colorimetric assays in the quality assurance of drinking water. Benefiting from high photoabsorption coefficients, nano-materials have been widely applied in colorimetric assays for the detection of various targets, As shown in Scheme 1, the solution of GNRs (length/diameter ratio about 1.3 : 1) appeared bluish green owing to the intense longitudinal surface plasmon resonance (SPR) absorption of GNRs around 630 nm. The addition of 10 mM NO 2 À to the colloidal solution caused a color change from bluish green to red within 20 minutes. More NO 2 À (40 mM) caused the solution to be almost colorless. The progressive color change corresponds to the partial to complete dissolution of GNRs.To confirm the dissolution of GNRs caused by NO 2 À , the absorption spectra of GNRs after incubation in NO 2 À solutions (pH z 0) at different intervals were examined. As shown in Fig. 1, the absorption spectrum of GNRs (curve a) exhibited strong SPR absorption at bands of 530 and 630 nm corresponding to the

We have described a simple and low-cost visual method for on-site detection of hydrogen sulfide (H2S) in air based on the antiaggregation of gold nanoparticles (AuNPs). The bubbling of H2S into a weak alkaline buffer solution leads to the formation of HS-, which can stabilize the AuNPs and ensure the AuNPs maintain their red color even in a Tris buffer solution containing 80 mM NaCl with the presence of Tween 80. The stabilization of the AuNPs is attributed to the adsorption of negatively charged S2- on the AuNPs surface. In contrast, without the bubbling of H2S, AuNPs aggregate and change color from red to blue. Under optimal conditions, the proposed method exhibits excellent visual sensitivity with a naked-eye detectable limit of 0.5 ppm (v/v), making the on-site detection of H2S possible. This method also possesses good selectivity toward H2S over other gases by using a simple SO2 removal device. The successful determination of the concentrations of H2S in local air indicates the potential application of this cost-effective method.

a b s t r a c tBased on the catalytic etching of gold nanoparticles (AuNPs), a label-free colorimetric probe was developed for the detection of Cu 2+ in aqueous solutions. AuNPs were first stabilized by hexadecyltrimethylammonium bromide in NH 3 -NH 4 Cl (0.6 M/0.

Based on inducing the aggregation of gold nanoparticles (AuNPs), a simple colorimetric method with high sensitivity and selectivity was developed for the sensing of thiocyanate (SCN À ) in aqueous solutions. Citrate-capped AuNPs were prepared following a classic method and Tween 20 was subsequently added as a stabilizer. With the addition of SCN À , citrate ions on AuNPs surfaces were replaced due to the high affinity between SCN À and Au. As a result, Tween 20 molecules adsorbed on the AuNPs surfaces were separated and the AuNPs aggregated. The process was accompanied by a visible color change from red to blue within 5 min. The sensing of SCN À can therefore be easily achieved by a UV-vis spectrophotometer or even by the naked eye. The potential effects of relevant experimental conditions, including concentration of Tween 20, pH, incubation temperature and time, were evaluated to optimize the method. Under optimized conditions, this method yields excellent sensitivity (LOD ¼ 0.2 mM or 11.6 ppb) and selectivity toward SCN À . Our attempt may provide a costeffective, rapid and simple solution to the inspection of SCN À ions in saliva and environmental aqueous samples.

A fluorescence assay for the highly sensitive and selective detection of Hg 2+ using a gold nanoparticle (AuNP)-based probe was proposed.The assay was based on the formation of Hg-Au alloys, which accelerated the oxidization of o-phenylenediamine by dissolved oxygen to produce 2,3-diaminophenazine, a fluorescent product.As a bioaccumulative and highly toxic heavy metal, mercury causes serious human health problems even at very low concentration. In this study, we found that the formed Hg-Au alloys could also accelerate the oxidization of o-phenylenediamine by dissolved oxygen to produce a uorescent product. We try to use this phenomenon to develop a uorescent method for sensing of Hg 2+ . Compared with Huang's and Chang's work, the method avoided the addition of extra oxidant, making the sensing system simpler. Scheme 1 illustrates the sensing mechanism. In the absence of polyethylene glycol (PEG) stabilized AuNPs, the oxidization of Scheme 1 Schematic illustration for the fluorescent sensing of mercury(II) using AuNPs.

In this work, a sensitive colorimetric method for determination of copper (II) ) with a linear range from 7 to 50 nM. Furthermore, the costeffective method allows rapid and simple determination of the content of copper in shellfish samples.

As a sensitive and selective analytical technique, gold nanoparticles-based colorimetric sensing was characterized by its simplicity and cost-effectiveness. Specific methods have been extensively developed for different targets in diverse samples. In this study, a label-free method for sensing Co 2+ in aqueous solutions was described. The target was achieved by the induced aggregation of thiosulfate (S 2 O 3 2À ) stabilized gold nanoparticles (AuNPs) in the presence of ethylenediamine (en). Co 2+ first reacted with en and formed complexes of Co(en) 3 2+ in aqueous solutions, which was followed by the oxidation of Co(en) 3 2+ to Co(en) 3 3+ by dissolved oxygen. Co(en) 3 3+ then attacked S 2 O 3 2À ligands adsorbed on the AuNPs' surfaces, forming positively charged (en) 2 CoS 2 O 3 + on the AuNPs' surfaces, which reduced the surface charges of AuNPs and induced the aggregation of AuNPs. The process was accompanied by a red-shift in the adsorption spectrum and a visible colour change from wine red to blue. Potential effects of relevant experimental conditions, including pH, concentrations of S 2 O 3 2À and en, and incubation time were evaluated for optimization of the method. The proposed method is sensitive (LOD ¼ 0.0 4 mM or 2.36 ppb) and selective (by at least 100-fold over other metal ions except for Cu 2+ ) toward Co 2+ with a linear range from 0.1 to 0.7 mM. The cost-effective method allows rapid and simple determination of the concentrations of Co 2+ ions in drinking water.