Polyaniline/Gold Composites for Electrochemical and Optical Biosensing

Development of robust biosensors for on-line/in-line detection would enable continuous monitoring and control of process parameters in pharmaceutical manufacturing and new approaches for medical diagnostics and bioterrorism security. Key attributes of such biosensors are facile fabrication, reproducibility, high sensitivity and selectivity. Electrochemical (EC) detection provides a simple transduction mechanism, converting oxidation and reduction of the target analyte or electrochemical tag into an electrical signal. The equipment necessary for EC detection can be made small, portable and also combined with the electrode array on a single chip using lithographic techniques. Important additional information regarding the identity of an analyte can be obtained from optical techniques, specifically, surface enhanced Raman spectroscopy (SERS). Therefore, we have worked to develop a bi-modal approach, utilizing simultaneous EC and SERS techniques, to enhance a sensor’s figures of merit and its reliability in critical applications. When used in tandem, this bi-modal EC-SERS approach can provide information about electrochemical activity and molecule-specific vibrational signatures.

It is also interesting to note that while EC methods measure an averaged property of the entire surface, SERS can measure the vibrational modes of molecules on a small segment on the surface. Thus, together these methods can provide site-specific and averaged information simultaneously and in real-time. In biosensing applications, the ability to uncouple the behavior of a few molecules, or even a single molecule, from the ensemble average can provide much needed insight into fundamental biochemical processes.

An EC-SERS surface must have sufficient conductivity to act as an electrode, selectively bind the target analyte and have the hot-spots required for enhancement of the Raman signal. Our surface is based on electrochemically deposited polyaniline (PANI) which is subsequently used as a template for controlled gold nanoparticle (Au-NP) growth leading to PANI/Au-NP composites with tunable electrochemical and SERS properties. PANI was chosen because of its rich EC behavior and its ability to spontaneously bind and reduce potassium chloroaurate to metallic gold. The facile deposition procedure is well-suited for applications in biosensing via the EC-SERS bi-modal technique.

The film deposition and ongoing optimization of EC-SERS responses are carried out with an in-house fabricated electrode array and flow-cell for precise control over solution composition and electrical potential. The electrode array consists of 16 individually addressable platinum disc electrodes with diameter of 1 mm. We discuss the efficient examination of a complex, cross-correlated parameter space for these bi-modal biosensor devices, which includes; film thickness, morphology, and nanoparticle size and density.

The fabrication procedure is illustrated in figure 1. First, PANI is grown on platinum electrodes by oxidative electropolymerization of aniline from a diluted HBF₄, HCl or a similar acidic solution. The choice of acid, particularly the anion, plays a large role on the morphology and electrochemical properties of the grown film. The electrochemical method used for deposition; i.e. cyclic voltammetry, constant current, or constant potential, as well as scan rate, potential range, and total charge, are also important for tuning the characteristics of the grown film. Most importantly, these parameters affect the subsequent growth of Au-NPs in the film.

The growth of Au-NPs (step 2) is initiated spontaneously by exposing the PANI electrode to chloroaurate while controlling the electrode potential. In addition to the previously mentioned deposition parameters, the immersion time in the Au salt solution and its concentration are key to controlling the size and density of the NPs. In addition to electrochemical measurements the PANI/Au-NPs are analyzed using optical microscopy, SEM and XPS. The Au-NPs are distributed mostly on the surface of the polymer where they are open to solution and available for further functionalization, for instance by thiol groups, to be used for biomolecule immobilization in step 3.