Quantitative Analysis of Cr(III) and Cr(VI) Using Gold Nanoparticles with UV-Vis Spectrometry and Smartphone Colorimetric-Sensing

Analysis and speciation of chromium ions are important due to the contrasting nature of essential Cr(III) and toxic Cr(VI) ions. Here, the first simultaneous determination of Cr(III) and Cr(VI) ions is introduced using gold nanoparticles (AuNPs) as a colorimetric agent and subsequent detection using an application attached to a smartphone, in addition to the UV-vis spectrometer. A small amount of AuNPs (1 mM) was used as a result of its excellent selectivity and sensitivity toward Cr(III) and Cr(VI) ions. The color change of AuNPs was monitored using a smartphone and validated by a UV-vis spectrometer. The peak absorbance of AuNPs' surface plasmon resonance (520 nm) was red-shifted to 630 nm when Cr(III) was introduced, while it was blue-shifted to 370 nm when Cr(VI) was added to the system. The absorbance shifts of the system produce a color change that is linearly proportional to the Cr concentration. At pH 5.0, a linear relationship of Cr(III) occurred at an absorbance ratio of 630/520 nm in the concentration range of 0.2-1.0 ppm (R-2 = 0.9911), while those of Cr(VI) at an absorbance of 370 nm in the concentration range of 0.05-1.0 ppm (R-2 = 0.9909). The method's detection limit of UV-Vis spectrometer was found to be 0.051 ppm (0.98 uM) and 0.035 ppm (0.67 uM) for Cr(III) and Cr(VI), respectively. The detection limit of smartphone-digital image colorimetry compared to UV-vis spectrometer was better at about 22%. Finally, the proposed method was tested to analyze Cr concentration in actual samples (drinking and tap water) that give satisfactory accuracy (< 2.0%) and precision (< 5.0%).