The cortisol aptamer used in this study is 5'-CTCTCGGGACGACGCCCGCATGTTCCATGGATAGTCTTGACTAGTCGTCCC-3' has a dissociation constant (K d ) of 0.0063 µM. Aptamers were made from a lyophilized powder in TE buffer (10 mM Tris, 1 mM EDTA, pH 8). Aptamer solutions were heated to 95 • C (Stuart Scientific, UK) to unfold for 5 min, followed by 5 min on ice to refold. The aptamers were stored at room temperature.