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Hybrid sol–gel thin films doped with a pH indicator: effect of organic modification on optical pH response and film surface hydrophilicity

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Abstract

Optical sensors for application in innovative wearable sensing systems such as textile-integrated systems and wireless sensor platforms rely on the development of low-cost multifunctional materials compatible with standard fabrication technologies. We are developing optically responsive pH sensitive sol–gel coatings for integration with a mobile wireless smart tag sensing system. For this application, we have fabricated a range of thin pH sensitive films using bromocresol green (BCG) indicator immobilised in inorganic–organic silica hybrid matrices prepared by a sol–gel method and deposited by spin-coating onto glass substrates. The surface hydrophilicity of the films were varied by using the inorganic sol–gel precursor tetraethoxysilane together with either methyltriethoxysilane, ethyltriethoxysilane, phenyltrimethoxysilane or glycidoxypropyltrimethoxysilane as organically modified sol–gel precursors, co-polymerised in different ratios. Spectral characterisation of the films was performed using visible absorption spectroscopy. The shift in absorption maxima and other spectral changes of the different matrices have been identified, and the apparent pK app values of the immobilised BCG pH indicator determined. The surface wettability properties of the films have been studied by measuring the contact angle of water, formamide and diiodomethane which has allowed the estimation of the surface free energy (SFE) using three different models: Owens–Wendt, Wu and van Oss-Chaudhury-Good. It is shown that the SFE of the hybrid films is directly related to the type and the degree of organic modification, which in turn has a significant effect on the pH response-time of these sensing films.

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Acknowledgments

This work was co-financed by The National Foundation for Science, Higher Education and Technological Development of the Republic of Croatia (NZZ) through the project Distributed wireless sensors for smart chemical and biological detection systems: chemo- and biosensor interface and applications development, with additional support from the Ministry of Science, Education and Sport of the Republic of Croatia (MSES) under science project Grant Number 125-0000000-3221, both of which are gratefully acknowledged. The authors would like to thank Dr Matthew D. Steinberg for helpful discussions and review of the work.

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  1. Faculty of Chemical Engineering and Technology, University of Zagreb, Marulićev trg 19, 10000, Zagreb, Croatia

    Petar Kassal, Robert Šurina, Domagoj Vrsaljko & Ivana Murković Steinberg

  2. Pliva Hrvatska d.o.o., Prilaz Baruna Filipovića 25, 10000, Zagreb, Croatia

    Robert Šurina

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  1. Petar Kassal
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Correspondence to Ivana Murković Steinberg.

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Kassal, P., Šurina, R., Vrsaljko, D. et al. Hybrid sol–gel thin films doped with a pH indicator: effect of organic modification on optical pH response and film surface hydrophilicity. J Sol-Gel Sci Technol 69, 586–595 (2014). https://doi.org/10.1007/s10971-013-3261-9

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