However, these methods are usually time-consuming, complicated, and expensive. Photochromism, on the contrary, has proved to be a convenient method for metal ions detection, due to its high sensitivity and efficiency.
Photochromism involves the reversible interconversion of a chemical species between two isomers induced by light which results in a change in UVeVIS spectra.
Families of photochromic compounds commonly used in polymeric system include azobenzenes, spiropyrans, spirooxazines, diarylethenes and fulgides .
Among these compounds, spiropyrans have been extensively studed due to their quick response to external stimuli.
Spiropyrans consist of two orthogonal aromatic rings which are connected by a carbon atom of sp3 hybridization.
Upon the irradiation of ultraviolet light, the colorless, nonpolar and hydrophobic spiropyran (SP) undergoes the cleavage of CeO bond, forming the colorful, polar and hydrophilic merocyanine (MC).
The merocyanine can complex with metal cations via the phenolate oxygen atom as shown in Scheme 1 .
Increasing attentions have been focused on the design of photochromic probes for divalent metal ions, such as Hg2þ Cu2þ , Ni2þ , and Co2þ.