Dynamic pH sensing in micro-fluidic devices using adaptive coatings based on polyaniline
Florea, Larisa and Lahiff, Emer and Diamond, Dermot and Benito-Lopez, Fernando (2011) Dynamic pH sensing in micro-fluidic devices using adaptive coatings based on polyaniline. In: MicroTAS 2011, 2-6 Oct 2011, Seattle, WA, USA.
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In this abstract, we present a micro-fluidic device that has integrated pH optical sensing capabilities based on polyaniline (PAni) suitable for pH detection in continuous flow. The polyaniline coating is covalently attached to the inner wall of the micro-channel using the “grafting from” approach. The optical proprieties of these polyaniline coatings change in response to the pH of the solution that is flushed inside the micro-channel. These unique properties offer the possibility of monitoring pH in continuous flow over a wide pH range over the entire channel length.
In the last few years conducting polymers have been used to prepare optical pH sensors. Through oxidative polymerisation of an appropriate monomer, conducting polymer films with suitable optical properties for pH sensing can be obtained. This technique eliminates the need of using organic dyes as in conventional optical sensors. Among these conducting polymers, polyaniline has received significant attention because of its suitability over a wide pH range [2-4]. By focusing on PAni nanofibres we can dramatically increase the surface area of the material, which manifests in improved response times and sensitivity. Therefore our approach presents a new, simple, and fast photometric method to measure pH using PAni based coatings in micro-channels. The pH measurement can be in continuous flow mode using fiber-optic light guides, or along the entire micro-fluidic system using digital imaging.
The functionalisation of the inner walls of the micro-fluidic channel with PAni nanofibres was achieved. Briefly, immediately after exposure of the PDMS chips to oxygen plasma and sealing to the glass slide/PDMS layer, the activated channels (1000x100μm) were flushed with a 20%wt solution of N-[3-(Trimethoxylsilyl) propyl]aniline in ethanol for 60 min at a flow rate of 0.5 μl/min. The channel was then washed with ethanol and filled with 1M HCl solution containing an equimolar concentration of oxidant (ammonium peroxydisulfate) and aniline. After polymerisation, the channels were washed extensively with water to remove any unattached polyaniline nanofibres. Using this technique, homogeneous PAni coatings were obtained, covalently attached to the internal walls of micro-channels made of PDMS/PDMS or PDMS/glass. The transformation of PAni between its Emeraldine Salt form (ES) to the Emeraldine Base (EB) in response to basic solution passing through the channel is accompanied by significant changes in colour. The UV-VIS spectra of the polyaniline coating shows the potential of these adaptive coatings for pH sensing in micro-fluidic devices. A plot of the absorbance at 580 nm vs. pH shows the excellent performance for detection over the range pH 2-10. Moreover, the rapid dynamics of the coating response across the entire micro- fluidic system implies that it can be used to monitor spatial effects and diffusion processes in channels.
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