## 4 Conclusions  
In this work, an MA-capped Au NPs based colorimetric method was developed for the rapid detection of aqueous Cr(VI) with high sensitivity and selectivity. The response of our probe to aqueous Cr(VI) could be simply observed with a color change from wine-red to gray, and the limit of detection could reach as low as 0.4 \(\upmu\)g L\({}^{-1}\) by the naked eye. Moreover, with the help of a smartphone equipped with a color scan application software, Cr(VI) could be quantified on-site in a simple, portable, reliable, and rapid manner and detection liner range from 0.2 to 1.0 \(\upmu\)g L\({}^{-1}\) with a detection limit as low as 0.1 \(\upmu\)g L\({}^{-1}\). The coefficient variations obtained by the proposed method is comparable to that of conventional UV-Vis spectrometer, demonstrating the potential of the proposed method in heavy metal ion detection on site in resource-constrained countries. In the future, it is believed that assays in decentralized studies could be dramatically improved if smartphone were to be integrated with more powerful software for data collection. In addition, the proposed method reported here could potentially be adapted for the detection of any environmental pollutants as long as coordination reagents against it were available, therefore making smartphone-based method a versatile tool for environmental monitoring. Furthermore, this methodology is not confined to colorimetric assay, fluorescence-based assay combined with smartphone seems to be more suitable for on-site detection in real world from the perspective of detection sensitivity, which has been already intensely reported with high accuracy and sensitivity in the past decade.  
Conceptualization, A.M. and C.Y.; methodology, A.M., X.L. ([PERSON]) and C.L.; software, C.Y. and X.L. ([PERSON]); validation, A.M., C.Y. and H.B.; formal analysis, A.M., X.L. ([PERSON]) and C.L.; investigation, A.M., X.L. ([PERSON]), C.L., C.Y., X.L. ([PERSON]) and H.B.; data curation, A.M., X.L. ([PERSON]), C.L. and C.Y.; writing--original draft preparation, A.M. and C.Y.; writing--review and editing, A.M. and H.B.; visualization, A.M. and C.Y.; supervision, C.Y.; project administration, C.Y. and H.B.; funding acquisition, C.Y. and H.B. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Sample b**} & \multicolumn{3}{c}{**The Concentration of Cr(VI) (\(\upmu\)g L\({}^{-1}\))**} \\ \cline{2-4}  & **AAS Method** & **UV-Vis Method/CV** & **Smartphone/CV** \\ \hline South lake & 1.01 \(\pm\) 0.03 & 0.95 \(\pm\) 0.06/12.43\% & 1.08 \(\pm\) 0.04/14.48\% \\ Yezhi lake & 0.91 \(\pm\) 0.04 & 0.96 \(\pm\) 0.02/13.67\% & 1.06 \(\pm\) 0.03/14.52\% \\ Tangxun lake & 1.04 \(\pm\) 0.04 & 1.00 \(\pm\) 0.03/12.28\% & 0.88 \(\pm\) 0.02/13.88\% \\ \hline \hline \end{tabular}  
* Values shown were the calculated mean Cr(VI) concentration for each sample and were determined from three replicates. b all the sample were collected from the local lakes in Hongshan District of Wuhan city, China.  
\end{table}
Table 1: Recovery test of Cr(VI) in a real water sample collected from three local lakes by the smartphone-based method and its coefficient variation comparison with conventional UV-Vis spectrometer a.