Enzymatic construction of single-nucleobase redox-labelled oligo-nucleotides was developed either based on polymerase incorporation of a single modified nucleoside triphosphate (dNTP) followed by primer extension (PEX) with natural dNTPs or based on PEX with a biotinylated one-nucleotide overhang template, magneto-separation and the second PEX with a full-length template.