Pillar arrays of tethered polyvinyltetrazole on silicon as a visualization platform for sensing of lead ions

Using click reactions, we have grafted N-propargyl polyacrylonitrile (PAN) to a dot matrix presenting azido-terminated surfaces on a silicon substrate to generate pillar arrays of two-dimensional photonic crystals (2DPCs). The tethered PAN 2DPCs were then subjected to a cyano-to-tetrazole conversion reaction to generate tethered polyvinyltetrazole (PVT) 2DPCs. Over 88% of the cyano groups on the PAN were converted to tetrazolyl groups, determined according to the change in average molecular weight. The tetrazolyl groups exhibited optimal Pb2+ absorption at pH 5.3. Each Pb2+ ion was chelated to two or four tetrazolyl groups. In addition, the adsorption of Pb2+ to the tetrazolyl groups was highly selective in solutions also containing Na+, K+, Ca2+, and Mg2+ ions. The absorption of Na+, K+, Ca2+, Mg2+, Fe3+, or Zn2+ ions resulted in insignificant changes in the effective refractive index of the tethered PVT 2DPC; in contrast, absorbing Pb2+ ions led to a dramatic change in effective refractive index, due to the high refractive index of Pb2+ ions, resulting in a color change from green to dark red, observable to the naked eye along an incident angle of 50-60°. The tethered PVT 2DPC-based assay exhibited high sensitivity and selectivity in an experimentally simple setup for Pb2+ sensing. The color change of the 2DPC should be applicable in the development of other diffraction-based sensing technologies.