Methane-Induced SPR Shifts of Cryptophane A-Doped Polymer Layers

Abstract

The macromolecular cage cryptophane A can host methane molecules in its cavity and is thus suited as methane-sensing molecules for sensors based on refractive index (RI) sensitive analytical methods such as surface plasmon resonance (SPR). In this work, we assessed the methane response of different cryptophane A-doped polymer membranes by using a custom-made, angular-based SPR device with an RI resolution of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.4\cdot 10\,\,^{\mathrm{ -5}}$ </tex-math></inline-formula> . We show that cryptophane A needs to be dissolved in doped polymer coatings for successful methane detection. For membranes made of poly(methylmethacrylate) (PMMA), polycarbonate (PC), polystyrene (PS), and polymethylpentene (PMP), cryptophane A could be dissolved, and the solution could not be achieved with polydimethylsiloxane (PDMS) polymers, irrespective of the solvent used. Signal responses and response times toward a methane atmosphere were quantified for different layer thicknesses and cryptophane A contents. The best results were observed for thin but highly doped polymer layers, which could be achieved best for PS layers.