Chemo- and biosensing applications of spiropyran and its derivatives - A review

Spiropyrans are a class of photochromic compounds that feature two main isoforms, a closed spiropyran (SP) and an open merocyanine (MC), which are inter-switchable with UV and visible light. The double bond containing MC isomer has several resonance forms with different optical spectra. Consequently, the overall spectral characteristic of the spiropyran solution depends on the relative contribution of each resonance form. In addition, the reversible changes of the absorbance or fluorescence spectra of MC are dictated by the environment. By utilizing these properties, spiropyran and its derivatives have been employed to monitor a variety of molecules and ions that can alter the resonance forms of MC. In this review, we provide a comprehensive discussion on a range of species that can be detected by spiropyran including metal ions, anions, acids, solvents, and even biomolecules, the major analytical detection mechanisms involved, and the novelty of such techniques. This study establishes spiropyran as a versatile platform for the detection of both organic and inorganic target molecules. [Display omitted] •Two isoforms of spiropyran exhibit starkly different physical properties.•Reversible isomerization can be induced by multiple stimuli.•Properties of spiropyran can be tuned by introducing various functional groups.•Spiropyran is able to detect a broad range of organic and inorganic molecules.•Emerging applications include core-material for chemosensors and biosensors.