SENSING THE FLOW: ADAPTIVE COATINGS BASED ON POLYANILINE FOR DIRECT OBSERVATION OF MIXING PROCESSES

IN MICRO-FLUIDIC SYSTEMS Larisa Florea1, Emer Lahiff1, Dermot Diamond1 and Fernando Benito-Lopez1* 1CLARITY: The Centre for Sensor Web Technologies, National Centre for Sensor Research,

Dublin City University, Dublin, Ireland ABSTRACT

Micro-fluidic devices are ready to influence the future of the process industry. Therefore the understanding and proper evaluation of the flow and mixing behavior at microscale becomes a very important issue. In this article, we report the spe-cific mixing and fluidic behavior of two reacting solutions of HCl and NaOH in a glass/PDMS micro-chip, using adaptive coatings, covalently attached to the micro-channel walls, based on the conductive polymer, polyaniline (PAni). KEYWORDS: Polyaniline, Mixing Process, Adaptive Coatings, Optical Sensor, Micro-fluidics

INTRODUCTION

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 [2,3], confocal fluorescence microscopy [4], ultraviolet absorption [5,6], chemiluminescence [7,8]. 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.

THEORY

In the last decade there has been an exponential increase in micro-fluidic applications due to high surface-to-volume ra-tios and compactness of microscale devices, which makes them attractive alternatives to conventional systems. The continu-ing growing trends of micro-fluidic highlights the importance to understand the mechanism and fundamental differences in-volved in fluid flow and mixing at microscale. In the present article, adaptive coatings based on polyaniline are employed to study the mixing of two reactive fluids (HCl and NaOH) based on the fact that the absorbance spectrum (350-850nm) of polyaniline is highly pH dependent. In acidic pH polyaniline can be found in its doped conductive state, emeraldine salt (ES). The ES may be converted to the corresponding emeraldine base (EB) by treatment with an alkali solution (Figure 1). Imine sites of the EB forms are easily protonated, with a striking insulator–conductor transition, induced due to the appear-ance of polarons in the lattice, while the number of p-electrons remains constant. As a consequence, new optical properties appear in the ES [9]. Thus PAni has huge potential for sensing applications and has extensively been used as material for op-tical pH sensors due to its strong pH sensitivity [10-13] This propriety indicates that coatings based on polyaniline could be used to study mixing in micro-fluidic devices when using solutions of different pHs.

Figure 1: Protonated (doped) polyaniline emeraldine salt (ES) is deprotonated (dedoped) by treatment with an alkali to pol-yaniline emeraldine base (EB). EXPERIMENTAL Micro-chip fabrication

A PDMS layer is formed by pouring PDMS onto a master mold. Following curing, the PDMS layer is peeled from the master mold. The surfaces of the two pieces - PDMS and glass were cleaned and oxygen plasma oxidased. Following this, the two pieces of PDMS and glass were immediately brought into conformal contact to form an irreversible seal.

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Micro-channel functionalisation process Immediately after exposure of the PDMS chips to oxygen plasma and sealing to the glass slide, the activated channels

where 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 washed with ethanol and then heated in an oven at 80 °C for 60 min. After the chip was cooled to room temperature, the channel was filled with a freshly prepared solution of 1M HCl that containing the oxidant (ammonium peroxydisulfate) and aniline. After a few minutes the polymerisation started inside the micro-channel. The chip was left on the bench over night. The next day the channels were extensively washed with water to remove any polyaniline nanofibres that were not attached to the inner walls. The procedure used for functionalisation is briefly described in Figure 2-left. RESULTS AND DISCUSSION

Using the technique described homogeneous PAni coatings were obtained on the micro-channel surface while maintain-ing the nanomorphology of PAni. These PAni coatings respond very well to changes in pH as shown by the absorption meas-urements of the channel coating, Figure 2-right. 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 micro-channel, 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 (Figure 3) showing the utility of this approach for investigating diffusion and mixing processes of solutions in micro-channels.

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Figure 2: Scheme describing the polyaniline nanofibres functionalisation procedure of the micro-channel (left); Photo of the micro-channel when the 2 solutions (HCl sol. 10-2 M and NaOH sol. 10-3 M) had a flow rate of 10 µl/min each, accompanied by the absorbance spectra of the coating of the two arms of a Y-shaped micro-channel (right).

Figure 3: Photo of the micro-channel when the 2 different solutions (HCl sol. 10-2 M and NaOH sol. 10-3 M) are passed through the channel from left to right at different flow rates. The photo indicates that the mixing point changes according to the flow rate(left); A plot of the mixing point (reported from the Y-junction point) against flow rate (right).

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CONCLUSION In this paper, we report the specific mixing and fluidic behavior of two reacting solutions of HCl and NaOH in a

glass/PDMS microchip, using adaptive coatings based on the conductive polymer, polyaniline (PAni). This approach can be used for investigating diffusion and mixing processes of solutions in micro-channels and also for obtaining use-ful 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 [14]. ACKNOWLEDGEMENTS

The project has been carried out with the support of the Irish Research Council for Science, Engineering and Technology (IRCSET) - Embark Initiative and Science Foundation Ireland under grant 07/CE/I1147. REFERENCES [1] A. D. Stroock, S. K. W. Dertinger, A. Ajdari, I. Mezic, H. A. Stone, G. M. Whiteside, “Chaotic mixer for micro-

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CONTACT **F. Benito-Lopez, tel: +353-1-7007603; fernando.lopez@dcu.ie

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