Sensors and Actuators B 238 (2017) 281–291 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical jo ur nal home page: www.elsevier.com/locate/snb Effect of GPTMS functionalization on the improvement of the pH-sensitive methyl red photostability Maria Rosaria Plutino a , Emanuela Guido b , Claudio Colleoni b , Giuseppe Rosace b,∗ a Institute for the Study of Nanostructured Materials, ISMN—CNR, O.U. Palermo, c/o Department of Chibiofaram, University of Messina, Viale F. Stagno d’Alcontres 31, Vill. S. Agata, 98166 Messina, Italy b Department of Engineering and Applied Sciences, University of Bergamo, Viale Marconi 5, 20244 Dalmine, Bg, Italy a r t i c l e i n f o Article history: Received 16 May 2016 Received in revised form 26 June 2016 Accepted 12 July 2016 Available online 14 July 2016 Keywords: Sol–gel 3-Glycidoxypropyltrimethoxysilane Methyl red Hydrogen bonding pH-sensor a b s t r a c t The photostability of silylated Methyl Red (MR-GPTMS) dyestuff at 360 nm and under Visible light was investigated in water solution, in comparison to that of pure Methyl Red (MR). Both the dyestuffs were found relatively stable under visible light, while substantial decomposition, with different reaction times, occurs under UV irradiation, also on dependence of the pH value. In order to elucidate the mechanism of the color fading, a detailed spectroscopic study of the chemical structure of both molecules (i.e. MR and MR-GPTMS) was performed by means of UV–Vis, IR and NMR techniques, together with the employ- ment of semi-empirical calculations. In the case of the functionalized MR-GPTMS dyestuff, an interesting behavior was found at acidic pH in comparison to the free MR: the presence of the bound GPTMS produces a decrease of the dyestuff decomposition rate of one order of magnitude. As a matter of fact, the function- alized azo-dye retains its pH-sensing behavior, while it improved significantly the sol-gel functionalized MR-GPTMS dye photostability. The rates of decolorization were found to fit a pseudo-first-order kinetic model. The enhanced stability is due to the covalent bond between the carboxyl group of azo dye and the opened epoxy group of GPTMS. This functionalization increase the polarity of silylated Methyl Red in the region of the azo group and consequently the surrounding electron density making it less susceptible to cleavage by hydroxyl radicals attack. The analysis of the photolyzed products using Fourier Transform Infrared Spectroscopy (FTIR) further confirms that the ultraviolet degradation of both dyestuffs proceeded by following the same reaction pathway. © 2016 Published by Elsevier B.V. 1. Introduction The development of novel treatments of textile fabric surfaces has attracted considerable attention across many scientific dis- ciplines with interesting ranging from fundamental research to practical applications [1]. In recent years, Hybrid Organic-Inorganic Materials (HOIM) have fascinated the scientific community for fun- damental research and technological applications thanks to their novel and well-defined structure together with their unique opto- electronic properties. In particular, the incorporation of organic and biological molecules into nano-porous silica sol–gel hosts can open new research perspectives. Aside from being biocompatible, the porous inorganic silica matrix has demonstrated to enhance the thermal stability of coated polymers [2] and the thermodynamic ∗ Corresponding author. E-mail address: giuseppe.rosace@unibg.it (G. Rosace). stability of encapsulated guest molecules, mostly credited to the effect of cage confinement on molecular motions. Thus, it is well- known that organic dyes gain an exceptional photostability when entrapped inside silica sol–gel matrixes [3]. Since the beginning in 1984 [4], the sol–gel method has allowed the encapsulation of many types of molecules in oxide gels [5–8] and the optical proper- ties of these systems have been studied [9,10]. This encapsulation is usually accomplished using either doping or grafting processes to give organo-functional sol–gel materials. Doping technique, involv- ing the physical entrapment of a reagent inside the substrate, is simple and applicable to many organic compounds, but the pore size must be carefully tailored, as dopant leaching is often a prob- lem. On the other side, grafting technique, involving a reagent anchoring through covalent bonding, yield highly reproducible, sta- ble products, but the overall process can be quite laborious. The silyl reagents need to contain a −Si(OR) 3 group, and this condition limits the selection of reagents suitable for the grafting processes. The porous nature of sol–gels allows for the delivery of analytes to http://dx.doi.org/10.1016/j.snb.2016.07.050 0925-4005/© 2016 Published by Elsevier B.V.