In this abstract, we report the specific mixing and fluidic behavior of two reacting solutions of HCl and NaOH in a glass/PDMS microchip, using adaptive coatings, covalently attached to the microchannel walls, based on the conductive polymer, polyaniline (PAni). Lab-on-a-chip technology is attracting great interest as the miniaturisation of reaction systems offers practical advantages over classical bench-top chemical synthesis. In particular, rapid mixing of the fluids flowing through a micro-channel is very important for various applications of micro-fluidic systems [1]. In addition, on-chip detection techniques are essential for the continuous monitoring of the mixing behavior of confluent streams. For this purpose many spectroscopic detection methods have been employed: laser-induced fluorescence, confocal fluorescence microscopy, ultraviolet absorption, chemiluminescence. These spectroscopic techniques provide good opportunities for the detection of chemical species and are suitable for studying mixing in micro-fluidic devices. However, these techniques typically require the addition of a dye or pretreatment of a solute species with florescent tags to allow on-chip detection. Consequently, in these approaches one follows the bulk behaviour of an added solute, rather than the solvent/liquid itself.
In this paper we demonstrate the possibility of quantitatively evaluate the mixing process in label and solute free conditions, using chromo-responsive coatings based on polyaniline. Polyaniline is an example of conductive polymer whose optical proprieties change in response to changes in the local environment. Thus PAni has huge potential for sensing applications and has been used extensively as material for optical pH sensors due to its strong pH sensitivity [2]. We were interested in investigating whether coatings based on PAni could be used to study pH gradients in micro-fluidic devices.
The functionalisation of the inner walls of the micro-fluidic channel with PAni nanofibres was achieved using the "grafting from" approach. In this way, homogeneous PAni coatings were obtained on the microchannel surface while maintaining the nanomorphology of PAni. These PAni coatings respond very well to changes in pH as shown by the absorption measurements of the channel coating. To study mixing in this device, colorless hydrochloric acid (10-2M, pH=2) and sodium hydroxide (10-3M, pH=11) solutions were pumped into the two arms of a Y-shaped microchannel, 1000x100μm and 30mm long. The two liquid streams meet at the Y-junction, and have an interaction time defined by the flow rate, which was varied between 0.5-3μl/min. A plot of the mixing point (i.e. the point at which the blue colour disappears relative to the meeting point at the Y- junction) against flow rate presents good linearity showing the utility of this approach for investigating diffusion and mixing processes of solutions in micro-channels and also for obtaining useful results for the optimal design of micro-reactors for chemical synthesis applications. Moreover these coatings can also be employed as indicators in the case of non-reacting fluids offering a new method of studying proton diffusion with and without a chemical reaction [3].
REFERENCES:
1. “Chaotic mixer for microchannels,” A. D. Stroock, S. K. W. Dertinger, A. Ajdari, I. Mezic, H. A. Stone, G. M. Whiteside, Science (Washington, D.C.) 295, 647 (2002). 2. “Optical sensing of pH using thin films of substituted polyanilines,” E. Pringsheim, E. Terpetschnig, O.S. Wolfbeis, Analytica Chimica Acta, 357, 247 (1997).
3. “Rapid proton diffusion in microfluidic devices by means of micro-LIF technique,” K. Shinohara, Y. Sugii, A. Hibara, M. Tokeshi, T. Kitamori, K. Okamoto; Experiments in Fluids 38, 117 (2005)
