**Figure 3.**

*(a) The custom-designed cradle for smartphone-based spectrometer. Immersion probe for absorbance measurement is given in (b) and overall design is illustrated in (c). (d) The spectral images obtained from smartphone spectrometer.*

of blank solution (left) and 5-ppm BPA solution (right) are given in **Figure 3d** to demonstrate color variation can be detected with the spectrometer system.
