Disposable Paper-Based Analytical Device for Visual Speciation Analysis of Ag(I) and Silver Nanoparticles (AgNPs)

A disposable, instrument-free, height readout paper-based analytical device (HR-PAD) based on a paper strip inkjet-printed with CdTe quantum dots (QDs) was developed for the sensitive speciation analysis of Ag⁺ and silver nanoparticles (AgNPs). When the paper strip is immersed into a sample solution, capillary action draws it through the surface and any Ag⁺ in the solution quenches the fluorescence of CdTe QDs via a cation exchange reaction between Ag⁺ and the CdTe QDs, with the height of the quenched band being proportional to the concentration of Ag⁺. In contrast, fluorescence quenching cannot be observed when only AgNPs are present in solution. Thus, the concentration of AgNPs can be obtained by subtracting the Ag⁺ content from the total silver determined by the HR-PAD after digestion with HNO₃. Under optimized conditions, the methodology provides high selectivity, sensitivity, and accuracy for the detection of Ag⁺ or AgNPs in various samples, even at concentrations as low as 0.05 mg L^-1. Precisions of 4.5% and 2.2% RSDs were achieved at concentrations of 1 mg L^-1 and 7 mg L^-1 of Ag⁺, respectively. Compared to conventional methods, this approach is inexpensive and user-friendly and eliminates the need for expensive and sophisticated detection instruments. The practicality of the method was demonstrated via the speciation analysis of AgNPs and Ag⁺ in river water and 12 commercial products with satisfactory results.