Journal of Electroanalytical Chemistry 447 (1998) 63 – 69 In situ AFM/STM characterization of porphyrin electrode films for electrochemical detection of neurotransmitters B. Duong, R. Arechabaleta, N.J. Tao * Department of Physics, Florida International Uniersity, Miami, FL 33199, USA Received 9 May 1997; received in revised form 4 August 1997 Abstract We have studied the adsorption and electropolymerization of protoporphyrin IX (PP), Fe(III)-protoporphyrin IX (Fe-PP) and Zn(II)-protoporphyrin IX (Zn-PP) on highly oriented pyrolytic graphite (HOPG) with in situ atomic force microscopy (AFM) and scanning tunneling microscopy (STM) and examined the performance of the porphyrin-coated electrodes for electrochemical detection of the neurotransmitter, dopamine. AFM and STM reveal that the porphyrins adsorb spontaneously onto HOPG and self-assemble into ordered monolayers with the individual molecules lying flat on the surface. On increasing the electrode potential to the oxidation potentials, islands, corresponding to electrooxidative polymerization, begin to form on the monolayers. The islands grow in size and number and form a continuous film that provides a convenient way to modify the graphite electrode. The porphyrin-coated electrodes can suppress the unwanted oxidation of ascorbic acid by 100-fold without sacrificing much of the electrode response time and sensitivity for detecting dopamine. © 1998 Elsevier Science S.A. All rights reserved. Keywords: AFM/STM; Graphite; Porphyrin; Polymerization; Biosensors; Neurotransmitters 1. Introduction Metalloporphyrins are of great interest both because of their critical roles in many biological processes and their electrocatalytic properties for reducing O 2 [1]. These molecules have been studied by a variety of electrochemical and spectroscopic methods on mercury [2–4], graphite [5–9] and glassy carbon [10] electrodes. Luttrull et al. have observed that individual porphyrin molecules randomly distribute on gold electrodes using ex situ STM [11]. Our recent in situ atomic force microscopy (AFM) and scanning tunneling microscopy (STM) study has directly observed that the molecules adsorb onto graphite electrodes and self-assemble into ordered monolayers [12]. In addition to film formation by spontaneous adsorption, Macor and Spiro have shown that the metallo-protoporphyrin can be elec- trooxidatively polymerized on Pt electrodes and can form stable films up to 1000 layers thick [13]. The electropolymerization of iron protoporphyrin IX (Fe- PP) on graphite and gold electrodes has been studied recently by Snyder and White with cyclic voltammetry, ex situ STM and quartz crystal microbalance tech- niques [14]. In this work, we have carried out an in situ AFM/STM study of the electropolymerization of Fe- PP, zinc protoporphyrin IX (Zn-PP) and protopor- phyrin IX (PP) in real time, and investigated the performance of the porphyrin-coated electrodes for electrochemical detection of dopamine. 2. Experimental For electropolymerization of the corresponding por- phyrins on graphite electrodes, 0.05 mM Fe-PP (Ko- dak), Zn-PP (Aldrich) and PP (Aldrich) in 0.05 M Na 2 B 4 O 7 , with pH adjusted to 10 by NaOH, were used. * Corresponding author. Tel.: +1 305 3482605; fax: +1 305 3483053; e-mail: taon@fiu.edu 0022-0728/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0022-0728(97)00463-4