The fabrication and voltammetric characteristics of carbon paste electrode modified by ssDNA/ stearic acid are described here, where ssDNA denotes single stranded DNA. A modified carbon paste electrode (SA/CPE) was prepared by mixing carbon powder with stearic acid at 80 ℃, the surface of which has an abundance of —COOH. Stearic acid was used as a binder and a chemical modification agent as well. After the activation of the SA/CPE with water-soluble carbodiimide (EDC) and N-hydroxysuccinimide (NHS), ssDNA was covalently bound to the surface of the electrode to fabricate an ssDNA modified electrode (ssDNA/SA/CPE). The hybridization of ssDNA on ssDNA/SA/CPE with the complementary stranded ssDNA (cDNA) was conducted in phosphate buffer solution and a dsDNA modified electrode was obtained. Cyclic voltammograms of methylene blue [3,7-bis(dimethylamino)phenothiazin-5-ium chloride, MB] at SA/CPE, ssDNA/SA/CPE and dsDNA/SA/CPE in B-R buffer solution were recorded, respectively. The i_pa and the i_pc of MB at ssDNA/SA/CPE were 2.9 and 2.7 times of those at SA/CPE, respectively. The E_f of MB at ssDNA/SA/CPE was shifted negatively by 8 mV from that at SA/CPE. The i_pa and the i_pc of MB at dsDNA/SA/CPE were 2.0 and 1.8 times of those at SA/CPE, respectively. The E_f of MB at dsDNA/SA/CPE was shifted negatively by 18 mV from that at SA/CPE. After the denaturation of dsDNA on dsDNA/SA/CPE by washing with 0.5 mol/L NaOH, the voltammetric parameters for MB at the denaturized DNA modified CPE became almost the same as those at ssDNA/SA/CPE. The i_pc of MB at ssDNA/SA/CPE was directly proportional to the quantity of ssDNA immobilized on SA/CPE in the range from 1.0×10^－7 to 5.0×10^－6 g with a detection limit of 2.0×10^－9 g (S/N＝3). Such an inexpensive and sensitive DNA modified CPE may be used to fabricate electrochemical biosensor for determining the products of the genetically modified plants.

Key words: DNA, stearic acid, carbon paste electrode, methylene blue, voltammetry