Figure 7.

Colorimetric detection of (+)-methamphetamine with an aptamer-based dye-displacement assay. (A) Absorbance spectra of the cyanine dye X-732–91B dissolved in DMSO (left) and aqueous buffer (right) at concentrations of 0–10 μM. The structure of the dye is shown at the top center, and a photograph of solutions containing various concentrations of dye is shown at bottom center. (B) Scheme of the dye-displacement assay using aptamer ML4 and X-732–91B. Target binding displaces the dye from the aptamer into solution, causing the dye to aggregate and inducing a concomitant color change. (C) Calibration curve of this assay in both buffer (black) and 50% saliva (red). (D) Response of the assay to 0–6.4 μM (+)-methamphetamine. (E) Assay cross-reactivity to 50 μM interferents. The red line demarcates 25% cross-reactivity relative to 25 μM (+)-methamphetamine.