Fabrication of two-dimensional photonic crystals of tethered polyvinyltetrazole on silicon surfaces for visualization in Cu2+ ion sensing

Abstract

We grafted polyacrylonitrile (PAN) from a flat silicon substrate via atom transfer radical polymerization (ATRP) as a bottom layer. Fabrication processes, involving very-large-scale integration (VLSI) and reactive ion etching (RIE), are sequentially used to generate 800 nm-diameter hole array of tethered PAN layer as 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. Fabricating hole array could be regarded as air filling within the PVT layer, resulted in a significant decrease of effective refractive index of the PVT film. In addition, the adsorption of Cu2+ ions to the tetrazolyl groups was highly selective in solutions also containing Na+, K+, Ca2+, and Mg2+ ions. After absorption of Na+, K+, Ca2+, Mg2+, Fe3+, and Zn2+ ions, the tethered PVT 2DPC displayed insignificant changes in its effective refractive index; in contrast, upon absorbing Cu2+ ions, a dramatic change in effective refractive index occurred, resulting in a color change from red to green, observable to the naked eye along an incident angle of 30–40°. The holes dimension within the array of tethered PVT shrunk after absorption of Cu2+ ions leading to a predominated blue-shift in the diffractive peak position. The tethered PVT 2DPC-based assay exhibited high sensitivity and selectivity in an experimentally simple setup for visualization in Cu2+ ion sensing.