Dielectric Impedance Spectroscopy in Flexible Polymer Microchip: Towards the Non Contact Biosensors

In recent years, development of miniaturization technology was increasing due to well known advantages and the crucial need for technology to get the micrometer scale measurements and to improve sensitivity of detection methods [1]. Among the various microsystems developed for “laboratory on chip”, non contact microelectrodes in microsystems is a very interesting method for detection because of the absence of direct contact between the electrodes and the solution microchannel [2].

The present work describes new approaches for non contact impedance device which permit to reach a best understanding of this specific transduction. Measurements were carried out through a Polyethylene Terephthalate (PET) microchannel photoablated having a trapezoidal cross-section shape, depth of 45 µm, width of 100 µm and a length of 1.4 cm. The distance separation between both microchannels is 120 µm center to center. The microband of PET is carried out in the following way: microchannels are coated with gold nanoparticules with an average size of 19 nm a standard deviation of 2 nm. The detection is achieved using hydride carbon ink/gold nanoparticules electrodes, thermally laminated by a polyethylene terephthalate/polyethylene terephthalate (PET/PET) layer with thickness of 35 µm at 135°C and a pressure 2 bar. The distance separation in the PET band between the two planar microelectrodes and the main microchannel is equal to 5 µm and the detection surface area per microelectrode is 88 µm x 100µm [3].

Firstly, the study was carried out by dielectric impedance spectroscopy measurement in alternative mode (ac) through the two embedded microelectrodes with the empty microchannel at frequency range from 1 MHz to 1 Hz and 0.1 V amplitude. Then, experiments were performed in the same conditions with streaming electrolyte in the microchannel in order to have the electrical behaviour of the overall microsystem and to extract the specific microchannel response. A procedure for eliminating the contribution of the surrounding polymer on the global impedance response has been established. After correction, the impedance diagrams exhibit a loop from very high frequencies to medium frequencies (1MHz to 100 Hz) and a capacitive behavior at low frequencies. This operation allows a clear observation of solution microchannel impedance such as, resistance, capacitance and diffusion phenomenon in microchannel. As a consequence, the diameter of the loop is inversely proportional to the microchannel conductivity when the streaming electrolyte concentration increases. We also found that the capacitive behaviour at the interface PET / microchannel is clearly influenced by conductivity and the flow rate.

Secondly, the modeling of the non contact transduction is based on a system of partial differential equations representing the phenomena occurring in the microsystem. Solving these equations, using the methods of finite element resolutions with Comsol Multiphysics, allow get back to the values of current and potential in the microsystem. The model validation is obtained by comparison with experimental results. Study on dielectric behavior of the PET by impedance and modeling show predominance of an interfacial contribution on PET resistivity variation. This latter observation underlines that resistivity cannot be neglected, because the overall measurement in the microsystem will also depend on PET resistivity change. From the proposed model, which is also depends on interfacial impedance, microchannel conductivity and PET impedance have to be taken into account for a better understanding of global experimental data at fixed frequencies.

Finally, the bioanalytical application concerns experiments conducted at a fixed frequency (200 Hz) in the interfacial impedance region to monitor the proteins adsorption onto the polymer surface. This will be illustrated with results obtained for protein adsorption monitoring  into the microchannel. Protein need not to be labelled, as in optical biosensor, even if they need to be attached to the polymer surface coupled with the microelectrodes when a biomolecular interaction occurs. Modeling the microchip interface using an appropriate equivalent circuit permits to extract the value of the interfacial capacitance for ultralow protein concentration. The promising results obtained with this methodology make it a competing method in comparison with other transductions for biosensors developments.

[1] J. Gamby, J.-P Abid, H. H. Girault, J. Am. Chem. Soc., 2005, 127(38), 13300.

[2] J. Gamby, J.-P Abid, B. Tribollet, H. H. Girault, Small, 2008, 4, 802.

[3] M. Kechadi, J. Gamby, L. Chaal, H. H. Girault, B. Saidani, B. Tribollet, Electrochim. Acta., 2013, 105, 7.