New elastomeric polymethylsiloxane membranes bearing cationic exchanging sites for anionic dyestuffs sensors

• Materials made of cationic cross-linked silicone-based polymers are reported.
• The materials are applied as anion-exchanging membranes of chemical sensors.
• Various cationic groups provide selectivity to the detection of anionic dyes.

Novel anion exchanging materials based on functional polymethylsiloxane elastomers have been prepared and used as sensitive membranes of electrochemical sensors made of SiO2/Si heterostructures coated with thin films of functional polymer. Their synthesis and their use in electrolyte/insulator/semiconductor (EIS) electrochemical devices are reported. The anion exchanging materials are cationic cross-linked polymers based on either poly(methylhydrosiloxane) (PMHS) or poly(methylhydrosiloxane-co-dimethylsiloxane) copolymer (PMHS-co-PDMS 50/50). Their synthesis has been carried out in two stages. A brominated derivative was firstly prepared by hydrosilylation reaction of undecenyl bromide with SiH bonds of various polymethylsiloxane in the presence of Karstedt’s catalyst. The kinetics of the hydrosilylation reaction was investigated. The second stage is based on the formation of quaternary ammonium (+N(CH2CH3)3), pyridinium (+NC5H5) and phosphonium (+P(C6H5)3) groups by quaternarization reactions to brominated polymers. Full characterizations of the materials by IR, liquid 1H and 13C NMR and solid 29Si NMR spectroscopy are given. The sensitivity of EIS devices to different anionic dyestuffs species have been assessed for Acid Blue 25 (AB25), Acid Blue 74 (AB74) and Acid Yellow 99 (AY99) in aqueous solution. The shifts of flat band potential and the variations of capacitance in inversion mode were extracted from the impedance measurements as a function of the concentration of anionic dyestuffs species in solution. The electrical parameters of the EIS devices came from a non-specific adsorption of organic dyes and anion exchange phenomena that showed specificity with respect to the type of anion exchanging group. The membranes bearing pyridinium or phosphonium groups gave a Nernstian response towards anionic dyestuffs AB25 and AY99.

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