Abstract
Carbon micro- and nanoelectrodes fabricated by carbon microelectromechanical systems (carbon MEMS) are increasingly used in various biosensors and supercapacitor applications. Surface modification of as-produced carbon electrodes with oxygen functional groups is sometimes necessary for biofunctionalization or to improve electrochemical properties. However, conventional surface treatment methods have a limited ability for selective targeting of parts of a surface area for surface modification without using complex photoresist masks. Here, we report microplasma direct writing as a simple, low-cost, and low-power technique for site-selective plasma patterning of carbon MEMS electrodes with oxygen functionalities. In microplasma direct writing, a high-voltage source generates a microplasma discharge between a microelectrode tip and a target surface held at atmospheric pressure. In our setup, water vapor acts as an ionic precursor for the carboxylation and hydroxylation of carbon surface atoms. Plasma direct writing increases the oxygen content of an SU-8-derived pyrolytic carbon surface from ~3 to 27% while reducing the carbon-to-oxygen ratio from 35 to 2.75. Specifically, a microplasma treatment increases the number of carbonyl, carboxylic, and hydroxyl functional groups with the largest increase observed for carboxylic functionalities. Furthermore, water microplasma direct writing improves the hydrophilicity and the electrochemical performance of carbon electrodes with a contact-angle change from ~90° to ~20°, a reduction in the anodic peak to cathodic peak separation from 0.5 V to 0.17 V, and a 5-fold increase in specific capacitance from 8.82 mF∙cm−2 to 46.64 mF∙cm−2. The plasma direct-writing technology provides an efficient and easy-to-implement method for the selective surface functionalization of carbon MEMS electrodes for electrochemical and biosensor applications.
Highlights
Carbon microelectromechanical system techniques enable micro- and nanofabrication of three-dimensional carbon electrodes[1]
Plasma direct-writing setup As shown in Fig. 1, our homebuilt experimental plasma direct-writing setup consists of a filamentary plasma discharge generated from a tungsten microelectrode tip, a water vapor supply to the plasma streamer, and a 3-axis motion control platform
The carbon electrode is a 10-μm-thick microfilm fabricated from patterned SU-8 on silicon wafer through the carbon MEMS process
Summary
MEMS) techniques enable micro- and nanofabrication of three-dimensional carbon electrodes[1]. In a typical carbon MEMS process, patterned polymer photoresists are pyrolyzed to obtain carbon microstructures. The possible oxygen-containing functional groups on a carbon surface include epoxyl, carboxyl, carbonyl, phenol, quinone, and lactone groups in various C=O, O–C=O, and C–OH bond arrangements. Electrochemical treatment, strong acid treatment, vacuum UV treatment, and oxygen plasma treatment are currently the most established techniques to increase oxygen functionalities on a pyrolyzed carbon surface[3,7,9]. Hirabayashi et al investigated different techniques for carboxyl group functionalization of pyrolyzed carbon and found that oxygen plasma surface treatment is more reliable and less damaging to a carbon electrode while producing more oxygen functional groups in significantly less time than acid treatment[3]
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