A spiropyran-based colorimetric probe for quantitative analysis of cyanide ions in cassava leaves

Abstract A synthetic spiropyran derivative (2-(8-(hydroxymethyl)-3′,3′-dimethyl-6-nitrospiro [chromene-2,2′-indolin]-1′-yl)ethanol; SP) was spectroscopically evaluated as a highly selective probe and applied for colorimetric detection of cyanide ion (CN−) in cassava leaves. Upon addition of CN− into a pink solution of SP in THF/H2O (4.5:5.5 v/v, 0.5 mM HEPES at pH 9), a vivid yellow solution of the [MC-CN]− adduct was clearly observed by the naked eye. The spectral change in absorbance of the mixture between SP and CN− was monitored by UV–visible spectroscopy. The adduct formation through nucleophilic addition was confirmed by 1H NMR experiments. Additionally, the results from Job's plot suggested that SP and CN− formed the adduct at a 1:1 stoichiometric ratio with the binding constant of 8 × 104 M−1. The detection limit was determined to be 0.52 μM which is lower than the maximum permissible level of CN− in drinking water (1.9 μM) as approved by WHO. Moreover, the analysis of CN− in cassava leaves using our synthetic SP was demonstrated by both qualitative and quantitative approaches. The average CN− concentration in cassava leaves determined by the SP probe was 204.2 ppm which is comparable to the value obtained from the standard Chloramine-T/Pyridine-barbituric acid colorimetric method (203.6 ppm). The viable paper-test strips of SP were also developed for CN− colorimetric detection.