Electrochemical performance of angstrom level flat sputtered carbon film consisting of sp² and sp³ mixed bonds

We have developed ultra-flat carbon film electrodes with a wide potential window and a low capacitive current by the electron cyclotron resonance (ECR) sputtering method. The film consists of sp² and sp³ bonds (sp³/sp² ratio = 0.702) and is sufficiently conductive for electrochemical measurements without doping. The film has average roughness of 0.7 Å, which is much flatter than that of nanocrystalline diamond film. The potential limit of ECR sputtered carbon (current limit < ±500 μA/cm²) in the positive direction is 2.0 V vs Ag/AgCl, which is slightly lower than that of boron-doped diamond (2.1 V) and much wider than that of a glassy carbon (GC) electrode (1.7 V). In contrast, a much wider potential window can be obtained in the negative direction. The capacitive current is also much lower than that of a GC electrode due to the ultra-flat surface and the low number of oxygen-containing groups at the film surface. ECR sputtered carbon film can be used to measure each base of oligonucleotides by electrochemical oxidation without any pretreatment. The ultra-flat surface and low surface oxygen concentration suppress fouling with electroactive species, such as oligonucleotides, NADH, and bisphenol A.