Characterization and preparation of a gold surface covalently functionalized with single-walled carbon nanotubes for DNA sensing

Deoxyribonucleic acid (DNA) sensing can be applied to disease diagnosis, drug discovery and environmental testing. Optical methods are the most used techniques for DNA detection, but they are expensive and time consuming. Instead, electrochemical techniques offer sensitivity, selectivity, low cost and low power consumption. However, there is a lack of suitable substrates that do not use signal amplifiers. A gold surface modified with self-assembled 11-amino-1-undecanethiol (AUT) monolayer was created for the covalent immobilization of single-walled carbon nanotubes (SWNTs). This substrate was further attached with a single-stranded deoxyribonucleic acid (ssDNA) for being used as a template for the electrochemical DNA hybridization sensing. SWNTs were used due to their desirable properties like high conductivity, stability, tensile strength, and biocompatibility. Additionally, the covalent attachment of SWNTs has demonstrated to show a better electrochemical response with signal amplification, density control and stable attachment. To demonstrate the step by step modification of the surface we used: X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, ellipsometry, and scanning electron microscopy (SEM).;The electrochemical DNA hybridization capability of the electrode modified with SWNTs and ssDNA was demonstrated using two methods. Firstly, we used methylene blue (MB) as intercalator, which shows to have higher affinity to ssDNA compared to dsDNA, allowing us to demonstrate the covalent attachment of ssDNA on the SWNTs modified surface. The hybridization of the electrode modified with SWNTs and ssDNA was demonstrated by a decrease in MB response. The second method used was electrochemical impedance spectroscopy. We were capable of detecting the DNA hybridization process by changes in the charge transfer resistance, RCT. In general, we observed an increase in RCT with DNA hybridization due to repulsion between the negative charge of the DNA phosphates groups and the negative charge of [Fe(CN) 6]3-/4- in solution. This method demonstrated to be label-free with the capability to detect the DNA hybridization process. By using both electrochemical methods we were capable to demonstrate a linear range between 100 through 1000 nM. Also, the electrode modified with SWNTs and ssDNA demonstrated to be stable, reusable, and a selective electrochemical DNA sensor.