Multi-purpose capillary-integrated optical sensors based on spiropyran

Optical chemical sensors for liquid phase monitoring (overwhelmingly focused on water-based samples) often employ a dye or indicator that is immobilised onto a solid support material. However, this strategy presents two main disadvantages: firstly, the immobilisation process may lead to losses in dye sensitivity and secondly, the stability of the sensor is affected by dye leaching into the sample solution over time, making long-term use impractical. Therefore, effective optical-chemical sensors require new materials that are able to overcome all these problems. In this context, inclusion of photocromic molecules in solid matrices is of particular interest for the development of new approaches for such opto-sensing. Spiropyrans offer new routes for the fabrication of multifunctional materials since it is possible to take advantage of their photo-reversible interconversion between the two thermodynamically stable states of the molecule: a spiropyran (SP) form, and a merocyanine (MC) form, which have dramatically different charge, polarity and molecular conformations. It is well known that the open-chain merocyanine isomers of spiropyrans derivatives present negative solvatochromism, meaning that their absorption bands undergo a hypsochromic (blue) shift in solvents of increasing polarity, property that made them used as solvatochromic dyes. Another interesting property of spiropyran is their sensitive towards pH. By adding acid, the opened MC is protonated to form the protonated merocyanine (MCH+) form. A metal ion-binding center can also be formed by a spatial rearrangement of opened MC form, thus spiropyrans present a high interest for photo-reversible metal ion- complexation. Spiropyran derivatives are therefore sensitive towards many external agents, thus making them compounds of choice in the next generation sensors. In this context, here we present the functionalisation of the inner walls of micro-capillaries with polymeric coatings based on a spiropyran derivative and their successful use as capillary integrated optical sensors for a variety of target analytical species (divalent metal ions, solvents of different polarities, H+). The polymeric brushes approach offers a nanostructured to microstructured responsive coating insuring small diffusion paths and fast response times towards the target species. Moreover, this sensing behavior can be switched ON-OFF using light of appropriate wavelengths