Electrochemical Performance of Angstrom Level Flat Sputtered Carbon Film Consisting of sp² and sp³ Mixed Bonds

Abstract

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 A, which is much flatter than that of nanocrystalline diamond film. The potential limit of ECR sputtered carbon (current limit < +/-500 muA/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.

Local Imaging of an Electrochemical Active/Inactive Region on a Conductive Carbon Surface by Using Scanning Electrochemical Microscopy

“…We have already reported the surface and electrochemical properties of these films, [15][16][17][18][19] and their various advantages as regards SECM measurements based on our concept, namely a homogeneous structure over the entire region, sufficient conductivity for electrochemical measurements (>20 S/cm) and an angstrom-level flat surface. 15,16 A photoresist TSMR-C90LB (Tokyo Ohka Kogyo Co., Ltd., Kawasaki, Japan) was spin-coated on the ECR nano-carbon substrate. The pattern was exposed with a mask aligner PLA-501F (Canon Inc., Tokyo, Japan), developed in NMD-W developer (Tokyo Ohka Kogyo Co., Ltd.), rinsed in ultra-pure water, and then dried in a nitrogen gas flow.…”

“…3(C). Since the original ECR nano-carbon film had an ultraflat surface, 15 these etched regions could result from slight etching caused by the short oxygen plasma treatment. However, the etching depth was very shallow at only 1.0 nm, as shown in Fig.…”

“…In addition, the average flatness (Ra) values obtained from a 500 ¥ 500 nm area in an AFM image for oxygen rich and poor regions were 0.7 and 0.8 Å, respectively. Since the Ra for the original ECR nano-carbon is 0.7 Å, 15,19 our plasma process introduced oxygen-containing groups on the surface while maintaining the surface roughness. This 1.0 nm height step and ultraflat surface did not affect the current signal obtained at a tip with a sub ~10 mm radius, since sample steps about one thousand times smaller than this could not be detected with the SECM technique, as shown by the approach curve.…”

“…We used the surface of a very flat carbon substrate, 15 whose bulk conductivity was almost identical in its substrate, to obtain images attributed solely to surface functional groups. We masked the carbon surface by patterning it with a photoresist, and selectively introduced oxygen functional groups by using a short oxygen plasma treatment in a micrometer-order region.…”

Newly Developed Chemical Probes and Nano-Devices for Cellular Analysis

“…8, and a window almost equivalent to that of boron-doped microcrystalline diamond. 20,21 The ECR nano-carbon films showed an excellent stability when they were used to measure various biochemicals and environmentally important compounds. 22 We measured small oligonucleotides (5′-GAGA-3′, 5 -15 μg/ml) in 1.0 M pH 5.0 acetate buffer by backgroundsubtracted DPV (differential pulse voltammetry) with GC (glassy carbon) and ECR nano-carbon film electrodes.…”