Elsevier

Thin Solid Films

Volume 518, Issue 8, 1 February 2010, Pages 2234-2240
Thin Solid Films

Organically modified silicate thin films doped with colourimetric pH indicators methyl red and bromocresol green as pH responsive sol–gel hybrid materials

https://doi.org/10.1016/j.tsf.2009.07.158Get rights and content

Abstract

The colourimetric pH indicators methyl red and bromocresol green have been immobilised in organic–inorganic silica hybrid matrices prepared by a sol–gel method. The optically transparent sol–gel thin films containing encapsualated dye molecules were deposited onto glass substrates using spin-coating and dip-coating techniques. The organic–inorganic hybrid materials were obtained by co-polymerisation of tetraethoxysilane together with glycidoxypropyltrimethoxysilane as an organically modified sol–gel precursor. Matrix polarity and morphology were varied by using solutions containing different ratios of sol–gel precursors and surfactants. Spectral characterisation of the films was performed using ultra-violet and visible absorption spectroscopy, and the spectral changes in different matrices were identified. The effects of the sol–gel mixtures composition on the mechanical stability and homogeneity of the films, and the colourimetric response to solutions of differing pH were studied. The apparent pKapp values of the immobilised pH indicators were also determined in the different matrices.

Introduction

Sol–gel glasses have been widely used as solid matrices for the immobilisation of chemical and biochemical reagents used in chemical sensors and biosensors, and in particular for optical sensors [1], [2], [3]. Sol–gel technology is especially convenient in the development of optical pH sensors, since it allows low-temperature preparation of optically transparent porous thin films with tuneable properties. In addition, preparation of sol–gel sensing layers in the form of thin films using spin-coating or dip-coating techniques offers simple and reproducible deposition methods, which are compatible with standard industrial thin film fabrication facilities. Such low-cost, high quality optical films produced from a single mixture of chemicals (one-pot synthesis), are particularly suited to high volume manufacturing and have been proposed for innovative applications such as smart food packaging [4], [5], [6], and integrated optoelectronic devices such as complementary metal-oxide-semiconductor based luminescence multi-sensors [7]. We recently demonstrated the applicability of a pH sensitive sol–gel thin film as a chemically sensitive interface in an integrated optoelectronic wireless chemical sensor based on the radio-frequency identification smart card format [8].

The most frequently used sol–gel precursors for preparation of inorganic silicate glass matrices in optical sensing applications are tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS). The introduction of organic functional groups into inorganic precursors and their co-polymerisation with TEOS or TMOS results in inorganic/organic hybrid materials, or OrmoSils (Organically modified Silicates), containing non-hydrolysable Si–C bonds. This allows the introduction of organic groups directly bonded to the polymer-like silica network [9]. OrmoSils bring a number of attractive features to inorganic sol–gel materials, ranging from improved chemical and physical properties, versatile and relatively basic chemistry that allows for immobilisation of reagents via covalent bonding using OrmoSil functional groups, biocompatibility, wettability, varying degrees of lipophilicity etc., to improved mechanical properties that include easy shaping and patterning, better adhesion to the substrate, and crack-free surfaces [10], [11]. The numerous applications of OrmoSils, including molecular entrapment, use as chemical sensors and biosensors, protective coatings, catalytic and scavenger functions, and photochemical applications have been recently reviewed [12].

In addition to using different organically modified precursors, film properties can be tuned with surfactants known to have an effect on the film morphology and porosity as well as on the chemical activity of encapsulated reagents [13], [14]. Optimisation of the glass composition and the right choice of processing parameters are vital for obtaining films of high quality that will retain the required sensing performance characteristics.

In this work, we have studied the effects of epoxy-modification of silicate matrices containing entrapped pH colourimetric indicators as functional inorganic–organic hybrid materials, which are potentially very attractive for the preparation of optical pH sensors and pH-based biosensors. The epoxy-modification of inorganic silica matrix (precursor TEOS) was introduced using glycidoxypropyltrimethoxysilane (GLYMO) as a sol–gel precursor known to bring a number of attractive features to hybrid inorganic–organic composite materials, including increased wettability, improved scratch resistance, corrosion resistance and flexibility [15], [16], [17], [18]. In addition to improving the material properties, GLYMO can also be used as a chemically active precursor for covalent attachment of bioactive molecules, such as enzymes or other proteins, via coupling with aminoacid residues [19], as well as for the covalent attachement of pH indicator molecules for optical sensors [20].

The aim of this work was to prepare a range of thin hybrid films containing entrapped pH indicators methyl red (MR) and bromocresol green (BCG) and to study the effects of epoxy-modification of the silica thin films on their physical and mechanical properties and their pH response. The chemical structures of the precursors and indicators used are shown in Fig. 1. It is known that the entrapment of indicators into hybrid sol–gel matrices considerably affects their protonation equlibria and thus changes their pKa values. The shift of pKa values is often related to the chemical structure of the indicator and the nature of the sol–gel precursors used. The indicators MR and BCG were specifically chosen for this study since they belong to different families of pH indicator dyes (MR is an azo-dye and BCG a triphenylmethane dye), but at the same time their pKa values in aqueous solutions are very similar: 4.9 and 4.7, respectively.

In order to obtain matrices with suitable properties for use in sensing applications, including appropriate surface polarity, mechanical stability, porosity, thickness, and uniformity, the sol–gel precursors were mixed in various ratios with indicators and the resulting sol–gel mixtures were cast onto glass substrates. The effects of film processing parameters and the effects of organic modification of the silica matrix on the ultra-violet and visible (UV–Vis) spectral properties and analytical performance of the entrapped indicators were determined. The potential for further development of these films as optical sensors and biosensors is discussed.

Section snippets

Reagents

The sol–gel precursors TEOS and GLYMO and cetyltrimethyl ammonium chloride (CTAC) were obtained from Sigma Aldrich. Indicators MR and BCG, ethanol (w = 96%) HCl (conc.), NaOH and buffer compounds were obtained from Kemika (Croatia). Buffer solutions were prepared for different pH ranges as follows: 100 mM citrate buffer (pH range 1–4), 100 mM acetate buffer (pH range 4–5) and 67 mM phosphate buffer (pH range 5–9).

Thin film preparation and film pre-treatment

Over forty different multi-component sols were subsequently prepared by varying the

Optimisation of the film composition and deposition methods

Good quality thin films that remain optically active must have adequate mechanical stability and adhesion to the glass substrate, optical transparency and compositional and structural uniformity. In addition to these parameters, when used as sensing layers in optical sensor applications, thin films must also demonstrate a fast and reversible response to the target analyte, and efficient entrapment of active reagents in the matrix i.e., exhibit little or no leaching. It is known that composite

Conclusions

Sol–gel hybrid materials containing GLYMO as an organic, epoxy-modified precursor and doped with MR and BCG indicators have been used for the preparation of pH sensitive thin films. Organic modification of the sol–gel mixtures was found to delay the onset of gelation, and also to affect the pH response of the final films, such that pH sensitivity was either lost altogether, or response times significantly increased together with a considerable shift in the pKa value of the indicator. It was

Acknowledgements

This work was partly financed by the Ministry of Science, Education and Sport of the Republic of Croatia (MSES) under science project grant number 125-0000000-3221, and by Erasmus Technology LLP and 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’, which are

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    1

    Current address: GlaxoSmithKline Research Centre Zagreb Ltd., Prilaz Baruna Filipovica 29, HR-10000 Zagreb, Croatia.

    2

    Current address: Pliva d.d., Prilaz Baruna Filipovića 25, HR-10000 Zagreb, Croatia.

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