Biosensing Properties of Diamond and Carbon Nanotubes Wei Choong Poh and Kian Ping Loh* Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 Wei De Zhang and Sudhiranjan Triparthy Institute of Material Research and Engineering, 3 Research Link, Singapore 11760 Jian-Shan Ye and Fwu-Shan Sheu Department of Biological Science, National University of Singapore, 14 Science Drive 4, Singapore 117543 Received April 9, 2004 The biochemical properties of boron-doped diamond (BDD), carbon nanofiber, fullerene, and multiwalled carbon nanotube (MWCNT) electrodes have been investigated comparatively. Physiochemical factors which affect the biosensing properties such as surface hydrophobicities, effective surface area, and intrinsic material properties are studied. Voltammetric responses of the as-grown thin film electrode and surface- modified electrode to biomolecules such as L-ascorbic acid (L-AA), dopamine (DA), and uric acid are examined. As-grown MWCNT electrodes exhibit selective voltammetric responses to the different biomolecules and faster electron-transfer kinetics compared to BDD. The selective response is due to the considerably lower anodic potential of L-AA on MWCNT (-48 mV vs Ag|AgCl compared to 575 mV on BDD). This electrocatalytic response can be replicated on a nonselective carbon nanofiber electrode by coating it with gold nanoparticles. BDD has no intrinsic selective response to L-AA, and surface modification by anodic polarization is necessary for resolving L-AA and DA. 1. Introduction Boron-doped diamond (BDD) and multiwalled carbon nanotube (MWCNT) electrodes have unique electronic and structural properties that are useful for integrated bio- sensing and signal processing systems. BDD is attractive due to its wide electrochemical potential window and chemical inertness. 1 MWCNT is special due to its array of conducting wiring networks that can provide a high- surface matrix for the entrapment of biomolecules and the mediation of electrocommunication between the biomolecule and the substrate. 2 Currently, there are two camps of researchers who focus their research efforts in extolling the virtues of either one of these two materials for electroanalysis and biosensing. 1-8 It is timely to perform a comparative study of BDD and MWCNT in terms of their selectivity and sensitivity for biosensing applications. These performance issues depend funda- mentally on the structural and electronic properties of the substrate material, as these impact on the speed of electron transfer between the enzyme active site and the electrochemical transducer. Other important require- ments include resistance to fouling by the oxidation products, long-term stability, and biocompatibility. The unique structure of MWCNT allows the entrapment of small proteins both in the inner channel and on the outer wall by hydrophobic or electrostatic interactions. In contrast, BDD provides a two-dimensional, structurally compact surface for the attachment of biomolecules. Recently, Takahashi and co-workers reported that the diamond substrate provides the highest density of DNA chip per unit area and exhibits high stability and reusability for preservation of gene samples. 8 In this work, we compare the biosensing properties of diamond, MWCNT array electrodes, carbon nanofibers, and fullerenes for the voltammetric detection of biomol- ecules that are natural interferents in the human body. Low levels of dopamine (DA) have been found in patients with Parkinson’s disease. A major problem encountered in voltammetric detection is the coexistence of interfering compounds such as ascorbic acid (L-AA) and uric acid (UA). 9,10 Generally, voltammetric differentiation of these biomolecules is not possible on bare metal or carbon electrodes due to the overlap of oxidation voltages for these species. Major efforts in biosensor research essentially involve elaborate surface modification steps to impart perm-selection or electrostatic selection on these electrodes toward DA, L-AA, and UA. 9-12 The research effort here is to understand the physiochemical properties of diamond and MWCNT which influence their selective response to the three biomolecules. * Corresponding author. E-mail: chmlohkp@nus.edu.sg (K. P. Loh). (1) Granger, M. C. Anal. Chem. 2000, 72, 3793-3804. (2) Popa, E.; Notsu, H.; Miwa, T.; Tyrk, D. A.; Fujishima, A. Electrochem. Solid-State Lett. 1999, 2 (1), 49. (3) Lau, C. H.; Grehan, K. J.; Compton, R. G.; Foord, J. S.; Marden, F. Diamond Relat. 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