Spectroscopic Analysis of Metal Ion Binding in Spiropyran Containing Copolymer Thin Films

In this article, we describe the synthesis and characterization of a series of spiropyran containing copolymers that were used as colorimetric sensors for a series of divalent metal ions. The composition of spiropyran contained in the polymer backbone was varied from 10−100 mol % to investigate the influence of free volume and sterics on the photochromic response. Fourier transform-infrared (FT-IR) spectroscopy was used to characterize the photoinduced conversion, as well as the merocyanine−metal ion (MC−M2+) interaction. FT-IR spectra were analyzed using chemometric methods to elucidate the chemical binding environment between MC and M2+ and to selectively identify different metal ions bound to MC. By means of UV−vis absorption spectroscopy, we also demonstrate that each metal ion gives rise to a unique colorimetric response that is dependent on the amount of spiropyran comonomer contained in the polymer backbone and that by increasing the concentration of chromophore in the copolymer, the selectivity between different metal ions decreases. With the use of chemometric methods, UV−vis spectra can be analyzed to quantitatively identify metal ions in a concentration range from 1 μM to 100 mM.