All HPLC-purified oligonucleotide sequences were synthesized by Sangon Biotech (Shanghai, China). The nucleotide sequences are given in Table . The buffer composition used in this study are listed in Table . (USA). Horseradish peroxidase (HRP) and 3,3′,5,5′-tetramethylbenzidine (TMB) were purchased from Sigma-Aldrich (Sweden). All the other chemicals (analytical reagents) were obtained from commercial sources and were used without additional purification.