Fabrication and electroanalytical characterization of label-free DNA sensor based on direct electropolymerization of pyrrole on p-type porous silicon substrates

Label-free DNA sensors based on porous silicon (PS) substrate were fabricated and electrochemically characterized. p-type silicon wafer was electrochemically anodized in an ethanolic hydrofluoric (HF) solution to construct a PS layer on which polypyrrole (PPy) film was directly electropolymerized. To achieve direct electropolymerization of PPy on PS substrate without pre-deposition of any metallic thin-film underlayer, a low resistivity wafer (0.01–0.02 Ω cm) was used. The rough surface of the PS layer allowed for a strong adsorption of the PPy film. Intrinsic negative charge of the DNA backbone was exploited to electrostatically adsorb 26 base pairs of probe DNA (pDNA) into the PPy film by applying positive bias. The pDNA was designed to hybridize with the target DNA (tDNA) which is the insertion element (Iel) gene of Salmonella enterica serovar Enteritidis. Dependence of peak current (i _p ) around 0.2 V vs Ag/AgCl on tDNA concentration and incubation time were shown from the cyclic voltammograms of PS/PPy + pDNA + tDNA substrates in a 0.01 M potassium perchlorate solution. Plot of i _p vs incubation time showed a reduction in current density (J) by ca. 29 μA cm⁻² every hour. Sensitivity obtained from a plot of i _p vs tDNA concentration was −166.6 μA cm⁻² μM⁻¹. Scanning electron microscopy (SEM) image of the cross-section of a PS/PPy + pDNA + tDNA multilayered film showed successful direct electropolymerization of PPy for a nano-PS DNA biosensor.