Randomly arrayed G-rich DNA sequence for label-free and real- time assay of enzyme activity

Terminal deoxynucleotidyl transferase (TdT) was purchased from Fermentas Inc. (Vilnius, Lithuania). 2,2'-amino-di(2-ethyl-benzothiazoline sulphonic acid-6) ammonium salt (ABTS2-), deoxyadenosine triphosphate (dATP), deoxythymidine triphosphate (dTTP), deoxyguanosine triphosphate (dGTP), the oligonucleotide, SYBR Green II and 2-(4-morpholino) ethanesulfonic acid (MES) were obtained from Sangon (Shanghai, China). Hemin was purchased from Sigma-Aldrich (St. Louis, MO, USA). Human blood serum samples were obtained from Hunan Provincial People’s Hospital, China. All other chemicals were of analytical reagent grade and used without further purification. Ultrapure water with an electric resistance of 18.3 MΩ was obtained from a Millipore filtration system and used throughout.