The use of polymer composites offers the possibility of mass production. However, their main limitation is that the polymer usually coats the surface of the CNTs insulating them from the active electrochemical species. Furthermore, the surface of the CNT is not available for robust anchoring of the appropriate biomolecules used as specific receptors. In this context, our work has overcome these limitations by applying a mild oxygen plasma to the polystyrene–CNT composite, which partially removes the polystyrene layer. Thus, the CNT surface is released to interact with the electrochemical species and anchor the appropriate biomolecules. We show that the electrochemical signal is increased by a factor of 25 upon 5 minutes of plasma. The advantage of our method lies in its full compatibility with the microelectronic fabrication processes and therefore can be applied to wafer scale to batch production.
We have also studied the importance of the biomolecule immobilization strategy to obtain accurate and reproducible signals. Using a model analyte rabbit IgG–antirabbit IgG we have demonstrated how the signal is enhanced by a factor of 130 when the biomolecules are covalently attached to the CNTs, using a peptide bond, compared to physical adsorption. We are now studying the sensitivity of our device and preliminary results show detection limits of the order of nanograms per milliliter, which are comparable to techniques such as ELISA for this model analyte.
Our future investigations will consist of applying these sensors to clinically relevant assays as well as the integration of the sensors within a microfluidic platform. The work presented in our paper represents a step forward in the fabrication of lab-on-a-chip devices based on CNTs.