Fig. 4.

UV–vis absorption spectra of Au NRs coated with Ag nanoshell of different thickness resulting from adding the different amounts of silver nitrate. The absorption spectra of various concentrations of Cr(VI): a from 10 μM to 60 μM in AuNR@Ag-1 (Au NRs coated with 350 μL of 0.01 M AgNO₃), b from 5 μM to 35 μM in AuNR@Ag-2 (Au NRs coated with 250 μL of 0.01 M AgNO₃), c from 2 μM to 12 μM in AuNR@Ag-3 (Au NRs coated with 150 μL of 0.01 M AgNO₃) (Insets: photograph of color change of the AuNR@Ag colloid solutions upon the addition of Cr(VI) with different concentrations). d A graph of three distinctive measurement ranges of three nanoparticles with different Ag shell thicknesses (a) AuNR@Ag-1 (b) AuNR@Ag-2 and (c) AuNR@Ag-3. Data of less than 1% change rate were excluded