A highly sensitive and selective resonance scattering spectral assay for potassium ion based on aptamer and nanosilver aggregation reactions

The 5 nm nanosilver was prepared by the sodium borohydride procedure, using sodium citrate as a stabilizing reagent. The nanosilver particle was combined with the aptamer for K⁺ to form aptamer-nanosilver complex that was stabile in pH 7.0 Na₂HPO₄-NaH₂PO₄ buffer solutions and in the presence of high concentration of NaCl. In 85 °C water bath, K⁺ interacted with the aptamer to form very stable G-quadruplex that cannot stabilize the nanosilver particle. The uncombined nanosilver particles are aggregated to big particles in high concentration of NaCl, that led to the resonance scattering (RS) intensity at 470 nm increased greatly. Under the selected conditions, the increased RS intensity (ΔI) is linear to K⁺ concentration in the range of 0.060-3350 μmol/L, with a regression equation of ΔI = 0.177C + 0.55, a correlation coefficient of 0.9964, and a detection limit of 0.006 μmol/L K⁺. The aptamer-nanosilver RS assay has been applied to determination of K⁺ in serum and rain water, with satisfactory results.