Label-Free DNA Sensor on Nanoporous Silicon-Polypyrrole Chip for Monitoring Salmonella Species

Abstract

Label-free DNA sensors based on nanoporous silicon (nPS) substrate were fabricated and electrochemically characterized. A low resistivity (0.01-0.02 Omega ldr cm) p-type silicon wafer (100 orientation) was electrochemically anodized in an ethanoic hydrofluoric acid (HF) solution containing ethanol to construct the nPS layer with pore diameter of about 10 nm. This nano structure is compatible with organic polymeric conductors. Polypyrrole (PPy) film was directly electropolymerized on the nPS substrate without pre-deposition of any metallic thin-film underlayer. The rough surface of the nPS layer was favorable for strong adsorption of the PPy film. The intrinsic negative charge of the DNA backbone was exploited to adsorb 26 base pairs of probe DNA (pDNA) into the PPy film by applying positive bias forming the nPS/PPy+pDNA layer. DNA from salmonella enterica serovar enteritidis (tDNA) was extracted using standard protocol and, subsequently, amplified by polymerase chain reaction (PCR). Salmonella species are classified as bioterrorism threat agents by the centers for disease control and prevention (CDC). Results from the scanning electron microscopy (SEM) image of the cross section of the nPS/PPy multilayered film shows successful direct electropolymerization of PPy on the nPS substrate. Results also show that the tDNA concentration is inversely related to the peak current (i <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> ) at 0.2 V versus Ag/AgCl. The plot of i <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> versus incubation time showed that current density (J) decreases by 29 muA ldr cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> per hour. The sensitivity obtained from the plot of i <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> versus tDNA concentration is -166.6 muA ldr cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ldr muM <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Current density decreases with increasing incubation time and tDNA concentration. These results demonstrate that the nPS substrate with PPy+pDNA+tDNA film has been successfully developed for a label-free DNA sensor in rapidly and specifically detecting select and threat agents.