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The potential dependent structure change of a self-assembled monolayer of 11-ferrocenyl-1-undecanethiol (FcC11SH) on a gold electrode surface during the redox reaction of the terminal ferrocene group was investigated in 0.1 M HClO4 solution by electrochemical in situ Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS). A number of bands were observed in the 3200−1200 cm-1 region with p-polarization measurement, but no band was observed by s-polarization measurement when the potential was kept more negative than +1.2 V. The intensity of these bands corresponded well to the degree of oxidation of the terminal ferrocene group in the monolayer. The potential dependent IRRAS behavior can be explained by considering an orientation change of the monolayer induced by the redox reaction of the terminal ferrocene moiety in the monolayer.

The potential dependent structure change and mass transport at an 11-ferrocenyl-1-undecanethiol (FcC 11 SH) self-assembled monolayer on a gold electrode with various coverages were investigated by simultaneous Fourier transform infrared reÑection absorption spectroscopy (IRRAS) and electrochemical quartz crystal microbalance (EQCM) measurements in 0.1 M solution. As soon as the terminal ferrocene group was HClO 4 oxidized to a ferricenium cation (Fc`), a number of upward and downward IRRAS bands were observed and the surface mass was increased. The intensity of the IRRAS bands corresponded well with the mass change. This behavior is attributed to the redox reaction induced orientation change in the monolayer and FcC 11 SH ion pair formation between the Fc`cation and the perchlorate anion in solution. The monolayer FcC 11 SH with a lower coverage was less stable and was decomposed at a less positive potential than that with a high coverage. Models are proposed to explain the coverage dependent behavior of the monolayer FcC 11 SH induced by the redox reaction of the terminal ferrocene moiety.

The self-assembling process of ferrocenylundecanethiol on a gold surface has been measured in situ using the electrochemical quartz crystal microbalance (EQCM). Initial adsorption was fast and was followed by the slow adsorption whose rate was 2 orders of magnitude smaller. The number of adsorbed thiol molecules, estimated from the frequency change, suggests the formation of the monomolecular layer of the thiol. This number agrees well with that calculated from the electrochemical charge associated with oxidationtreduction of the monolayer in 1 M HClOd. The EQCM was also used to follow the mass change during the oxidationtreduction of the ferrocenylundecanethiol monolayer. It was confirmed that anions are incorporated into the self-assembled layer upon oxidation of the ferrocene group. IntroductionControlled modification of electrode surfaces with monolayer film of organic molecules has attracted much attention because of the relevance to the construction of molecular devices. The modified electrode is also usefui for the understanding of electron transfer mechanism at electrode/electrolyte interfaces at a molecular level. Oriented monolayer assembly can be created by both the Langmuir-Blodgett and the self-assembling methods using various alkane derivatives and substrates.' The selfassembled monolayers of alkane derivatives with sulfurcontaining head groups on gold substrates have been widely examined recently, since the binding between S atoms and Au surface is strong and the S-anchored monolayers thus formed are in well-oriented ~tructure.~-~ Structure and orientation of these monolayer assemblies have been examined by infrared reflectance spectroscopy? ellipsometry? X-ray photoelectron spectroscopy,lO etc. Although these techniques give useful information, these studies have been conducted only under ex situ conditions. To understand the structure of the assembled monolayer and electron-transfer mechanism, it is essential to conduct quantitative in situ monitoring of mass transport both at an assembling process and at an electron transfer process so that we can quantify the number of adsorbed molecules and the movement of ion and solvent accompanying an electron transfer process. A recently developed electrochemical quartz crystal microbalance (EQCM)ll is one of the best techniques to obtain this quantitative information. So far, the EQCM has been used in only a few works as far as self-assembled monolayers are concerned. Buttry and his co-workers have examined mass transport during the redox of self-assembled layers of viologen derivatives12

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.

This paper reports a miniaturized immunosensor designed to determine a trace level cardiac marker, B-type natriuretic peptide (BNP), using a microfluidic device combined with a portable surface plasmon resonance (SPR) sensor system. Sample BNP solution was introduced into the microchannel after an immunoreaction with acetylcholine esterase-labeled antibody (conjugate), and only unbound conjugate was trapped on the BNP-immobilized surface in the flow channel. Then, the thiol compound generated by the enzymatic reaction with the trapped conjugate was accumulated on a gold thin film located downstream in the microchannel to monitor the real-time SPR angle shift. We achieved a detectable concentration range of 5 pg/mL−100 ng/mL by monitoring the SPR angle shift, which covers the required detection range for the BNP concentrations found in blood. This success resulted from the use of a T-shaped microfluidic device structure, which prevents the sample solution from flowing over the gold film used for SPR detection. We were able to measure trace levels of BNP peptide (15 fg) within 30 min since the procedure with our immunosensor is simpler than a multistep immunoassay through the simultaneous use of a labeled enzymatic reaction and the real-time monitoring of enzymatic product accumulation in the microfluidic device. We employed the procedure to detect serum BNP by using spiked samples in human serum and achieved satisfactory recovery for heat-treated samples to denature the esterase in the serum before the immunoreaction.

This paper describes the characterization, electrochemical properties, and applications of carbon films prepared by the electron cyclotron resonance (ECR) sputtering method. The ECR-sputtered carbon film was deposited within several minutes at room temperature. The optimized sputtering conditions significantly change the film structure, which includes many more sp3 bonds (sp3/sp2 = 0.702) than previously reported film (sp3/sp2 = 0.274)1 with an extremely flat surface (0.7 A). The ECR-sputtered carbon films exhibit excellent electrochemical properties. For example, they have nearly the same potential window in the positive direction as that of high-quality, boron-doped diamond (moderately doped, 10(19)-10(20) boron atoms/cm3)2 and an even wider potential window in the negative direction with a low background current, high stability, and suppression of fouling by electroactive species without pretreatment. The electron-transfer rates at ECR-sputtered carbon films are similar to those of glassy carbon (GC) for Ru(NH3)(6)(2+)/(3+) and Fe(CN)(6)(3-)/(4-), whereas they are much slower than those of GC for Fe2+/3+, dopamine oxidation, and O2 reduction due to weak interactions between electroactive species and the ECR-sputtered carbon film surface. Such a response can be attributed to the ultraflat surface and low surface O/C ratios of ECR-sputtered carbon films. ECR-sputtered carbon film is advantageous for measuring biochemicals with high oxidation potentials because of its wide potential window and high stability. Highly reproducible and well-defined cyclic voltammograms were obtained for histamine and azide ions with a peak potential at 1.25 and 1.12 V vs Ag/AgCl, respectively. The film is very stable for continuous voltammetry measurements in 10 microM bisphenol A, which usually fouls the electrode surface with oxidation products.

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