## 3 Results  
### Selectivity and Sensitivity Investigation of MA-Capped Au NPs to Aqueous Cr(VI)  
The molecules used for Au NPs modification to effect colorimetric assays for heavy metal ions [7; 32; 55; 56; 57] all have carboxylic acid groups. Examples include 5,5-dithiobis(2-nitrobenzoic acid), [55] 1,4-dithiothreitol [56] and 5-thio-(2-nitrobenzoic acid) [57] can be used as the capping reagent for Cr(III) or Cr(VI) sensing. We used maleic acid (MA), another organic acid containing two carboxylic groups, was then used as the capping reagent for the functionalization of Au NPs. By incubating MA with citrate-prepared Au NPs for 2 h under rotation, MA-capped Au NPs could be obtained; MA at high concentration displaces the citrate. We compared the absorption spectra and aggregation response for Au NPs before and after MA functionalization, in the presence of a range of metal ions. See Figure 2A-C. While the citrate-bearing Au NPs solution showed poor selectivity to the metal ions, MA-treated Au NPs exhibited a highly selective response to Cr(VI) over other metal ions tested. As can be seen in Figure 2C, Ba\({}^{2+}\), Ca\({}^{2+}\), Cd\({}^{2+}\), Co\({}^{2+}\), Cu\({}^{2+}\), Cr(III), Mg\({}^{2+}\), Mn\({}^{2+}\), Ni\({}^{2+}\), Pb\({}^{2+}\), Zn\({}^{2+}\), and Al\({}^{3+}\) can not cause a significant effect on the color of MA-capped Au NPs solution, while an obvious color change could occur in the presence of Cr(VI) at a concentration of 1.0 \(\mu\)g L\({}^{-1}\). We further recorded the UV-Vis spectra response of MA-capped Au NPs in the presence of different metal ions, which could further display the excellent selectivity of our probe for aqueous Cr(VI) (Figure 2D,E). Our probe selectivity was found to be working well as well as the methods described by [PERSON] et al., [PERSON] et al., and [PERSON] et al. [6, 7, 10]. Moreover, TEM images of MA-capped Au NPs in the presence and absence of Cr(VI) further confirmed their aggregation phenomenon induced by Cr(VI) (Figure 2F,G).  
The selectivity of our probe to Cr(VI) could be ascribed to the hypothesis that Cr(VI), existing in the form of Cr\({}_{2}\)O\({}_{7}\)\({}^{2-}\), could coordinate with the carboxyl group from maleic acid through hydrogen bond in an energetically favorable manner. In addition, this discrimination between Cr(VI) and Cr(III) could be ascribed to surface oxidation reaction between maleic acid and Cr\({}_{2}\)O\({}_{7}\)\({}^{2-}\) because of the strong oxidization ability of Cr\({}_{2}\)O\({}_{7}\)\({}^{2-}\). Figure 3A,B show the evolution of color and absorbance spectra after adding 10 \(\upmu\)L of different concentrations of aqueous Cr(VI) into MA-capped Au NPs. A change in color already discernable by the naked eye is evident already at a concentration as low as 0.4 \(\upmu\)L\({}^{-1}\) by the naked eye. According to Figure 3B, the absorption intensity at 520 nm declines and the  
Figure 2: UV-Vis absorption spectra of citrate- and maleic acid-capped Au NPs (**A**). Optical responses of citrate-capped (**B**) and MA-capped (**C**) Au NPs to various metal ions, all the concentrations of metal ions are 1.0 \(\upmu\)L\({}^{-1}\). (**D**) UV-Vis spectra of MA-capped Au NPs in the presence of various metal ions and the corresponding A 650/A525 responses for those metal ions (**E**). (**F**,**G**) are the TEM images of MA-capped Au NPs in the absence and presence of 10 \(\upmu\)L\({}^{-1}\)Cr(VI), respectively. Both of the two-scale bars are 200 nm.  
intensity at 650 nm grows as the concentration of Cr(VI) increases. Figure 3C shows plots of the absorption intensity ratio A650/A520 against the concentration of aqueous Cr(VI). Below about 2 ug L\({}^{-1}\), the limit beyond which the response reaches saturation, the plot is linear, described by the regression line (y = 0.40089 + 0.31557x) with a 0.94045 correlation coefficient (inset).