Published: August 09, 2011 r2011 American Chemical Society 7438 dx.doi.org/10.1021/ac201595k | Anal. Chem. 2011, 83, 7438–7443 ARTICLE pubs.acs.org/ac Integrated All-Diamond Ultramicroelectrode Arrays: Optimization of Faradaic and Capacitive Currents Waldemar Smirnov, Nianjun Yang,* Ren  e Hoffmann, Jakob Hees, Harald Obloh, Wolfgang M€ uller-Sebert, and Christoph E. Nebel Fraunhofer Institute for Applied Solid State Physics (IAF), Freiburg 79108, Germany b S Supporting Information M icroelectrode arrays (MEAs) and ultramicroelectrode ar- rays (UMEAs) have been paid extensive attention dur- ing past decades. They amplify the signal of individual (ultra)- microelectrodes but do not lose the beneficial characteristics of individual (ultra)microelectrodes, such as a reduced ohmic resis- tance, an enhanced mass transport, decreased charging currents, and decreased deleterious effects of solution resistance. 1 To improve the sensor performance with respect to the sensitivity, detection limit, lifetime, and reproducibility, the selection of appropriate and optimized material for the fabrication of MEAs and UMEAs is known to be critical. MEAs and UMEAs from heavily boron-doped diamond have been fabricated since they combine the unique characteristics of (ultra)microelectrodes and the electrochemical features of boron- doped diamond. It is known that heavily boron-doped diamond applied as working electrode shows low background currents, wide potential windows, long chemical stability, and a rich sur- face chemistry. 3 Moreover, diamond surfaces can be terminated electrochemically with hydrogen or oxygen. 4 The first diamond MEAs were fabricated in 2002 by Fujishima and co-workers using structured silicon substrates as templates. 5 Later, patterned silicon nitrite 6 and “as-grown” diamond with randomly microstruc- tured topology 7 were used for the fabrication of MEAs and UMEAs. They had different shape, spacing, and number of electrodes. In 2005, Compton and co-workers 8 realized for the first time all-diamond UMEAs. These arrays have been utilized for the detection of metal ions, 6,8,9 neuronal activity measurements, 5,7 and implant application. 10 However, none of these diamond-based MEAs or UMEAs are market-available. To achieve sensitive detection of analytes in solutions with low detection limits by using MEAs or UMEAs, one has to optimize the ratio of faradaic current (signal, S) and capacitive current (background, B). The largest S/B ratio can be obtained when maximum faradaic current is achieved with minimum capacitive current. 1 The capacitive and faradaic currents are known to be affected by the scan rates applied, the spacing between micro- electrodes, and the concentration of supporting electrolyte. For diamond MEAs and UMEAs, they are also affected by the surface terminations and boron-doping densities of the transducer. Our goal was to make diamond UMEAs market-available and also to explore their highly sensitive and reproducible sensing applications. In this paper, we demonstrate the batch production of diamond UMEAs using standard photolithography techniques. For their sensing applications, we first optimized faradaic and capacitive currents on diamond UMEAs with respect to the effect of scan rates, the concentration of supporting electrolyte, surface termination, and boron-doping density of microelectrodes. The sensing performance of diamond UMEAs was then tested using dopamine as the target compound. After the optimization of faradaic and capacitive currents, a 50À100 times lower detection Received: June 22, 2011 Accepted: August 9, 2011 ABSTRACT: Integrated all-diamond ultramicroelectrode arrays (UMEAs) were fabricated using standard photolithography pro- cesses. The array consisted of typically 45 ultramicroelectrodes with a diameter of 10 μm and with a center-to-center spacing of 60 μm. The quasi-reference and counter electrodes were made from conductive diamond and were integrated on a 5 Â 5 mm 2 chip. On the UMEA, a high ratio of faradaic current to capacitive current was achieved on heavily boron-doped and hydrogen-terminated dia- mond surfaces at slow scan rates and in high concentration of supporting electrolyte. A sensitive and reproducible detection of dopamine was achieved on hydrogen-terminated diamond UMEA at slow scan rates. The detection limit of dopamine in the presence of ascorbic acid was 1.0 nM, which is 50À100 times lower than that obtained on the macrosized boron-doped diamond electrodes. This array is promising for sensitive and reproducible detection of analytes in solutions with low detection limits.