Nanoporous photonic crystals with tailored surface chemistry for ionic copper sensing

We present a study on optical and surface chemistry engineering of nanoporous photonic crystals as sensing platforms for detection of ionic copper. The optical sensing system combines glutaraldehyde-crosslinked double-layered polyethyleneimine (PEI-GA-PEI)-functionalized nanoporous anodic alumina gradient-index filters (NAA-GIFs) with reflection spectroscopy for label-free, selective detection of ionic copper in water matrices. The spectral position of the photonic stopband (PSB) of PEI-GA-PEI-functionalized NAA-GIFs is tuned across the visible-NIR spectral region to assess the impact of this optical parameter on the sensing performance of the system. Spectral shifts in the characteristic PSB (Delta lambda(PSB)) of PEI-GA-PEI-functionalized NAA-GIFs upon exposure to analytical solutions of ionic copper are used as a sensing parameter. Shifts in Delta lambda(PSB) of these photonic crystals are monitored in real-time under dynamic flow conditions. Calibration of the sensing system with analytical solutions of ionic copper from 0.1 to 100 mM shows a dual sensing regime, at low (from 1 to 10 mM) and high (from 10 to 100 mM) concentrations, which is associated with conformational changes of the PEI-GA-PEI functional layer. The binding mechanism of Cu2+ in PEI-GA-PEI-modified NAA-GIFs follows a Freundlich isotherm model. The performance of the PEI-GA-PEI-NAA-GIFs for real-life applications is demonstrated using environmental water. The system shows excellent correlation in both environmental and analytical water solutions. This study provides new opportunities to engineer portable optical sensing systems with tailor-designed features to detect ionic copper for environmental applications.