Optical sensing of urinary melatonin with molecularly imprinted poly(ethylene-co-vinyl alcohol) coated zinc oxide nanorod arrays

► Aligned ZnO nanorods were selectively hydrothermally grown on acetate-seeded spots.
► Melatonin was imprinted into EVAL which was coated onto the ZnO nanorod arrays.
► The ZnO nanorods increased the surface area and constituted an optical sensing element.
► The ZnO fluorescence decreases when targets bind to the imprinted EVAL film.
► A limit of detection of ∼pg/mL of urinal melatonin was achieved with 2 μL sample.

In this work, aligned zinc oxide (ZnO) nanorods were selectively hydrothermally grown on acetate-seeded spots on a gold substrate; the nanorods had an average length and diameter of 1.7 μm and 240 nm, respectively. Melatonin was imprinted into poly(ethylene-co-vinyl alcohol), EVAL, which was coated onto ZnO nanorod arrays. The ZnO nanorods not only increased the surface area for sensing target molecules, but also constituted an optical sensing element, as the ZnO fluorescence decreases when targets bind to the imprinted EVAL film; the fluorescence decrease, as a function of melatonin concentration, is well fit by a Langmuir adsorption isotherm. Poly(ethylene-co-vinyl alcohol) with 44 mol% ethylene showed the best imprinting effectiveness (ratio of the fluorescence decrease on binding melatonin to imprinted vs. non-imprinted EVAL-coated ZnO nanorod arrays) among the several compositions studied. In real urine analysis, the MIP films responded linearly to added (exogenous) melatonin, even in the presence of many possible interfering compounds in urine. This demonstrates the feasibility of using these MIPs as part of a total urinalysis MIP system.