The Interference of Ascorbic Acid in Sensitive Detection of Dopamine by a Non- Oxidative Sensing Approach Rishi R. Parajuli, Shah R. Ali, Yufeng Ma, Yetunde Balogun, Huixin He* Department of Chemistry, Rutgers University, Newark, NJ, 07102, USA * huixinhe@andromeda.rutgers.edu ABSTRACT Determination of Dopamine electrochemically by direct oxidation is difficult due to the co-oxidation of ascorbic acid present in high concentration in biological fluids in the same potential window. In this report, we present a non oxidative approach to electrochemically detect dopamine with high selectivity and sensitivity using our newly developed poly (anilineboronic acid ) composite with single walled carbon nanotubes. The mechanism of interference by ascorbic acid was studied and the results showed that ascorbic acid interacted with poly (anilineboronic acid) both chemically and electrocatalytically due to its multi- functional diol motif. An association constant between ascorbic acid and phenylboronic acid was determined to quantitate the strength of the interaction between the two molecules. By depositing a layer of permselective ionomer Nafion on top of the composite, the sensor can selectively detect dopamine in pM~ nm range. The high sensitivity along with the improved selectivity of this approach might have potential application toward molecular diagnosis of Parkinson's disease. Keywords: Polyaniline; carbon nanotubes; ascorbic acid; interference; neurotransmitter, dopamine 1 INTRODUCTION Dopamine (DA) is an important neurotransmitter in the mammalian central nervous system and its deficit results in brain disorders such as Parkinson’s disease (PD) and Schizophrenia [1]. As Parkinson’s disease is characterized by the severe depletion of DA, from the in vivo dopamine pool, the ability to sensitively and selectively measure the concentration of neurotransmitter dopamine could potentially be used in the molecular diagnosis of PD. Tremendous effort has been made to develop sensors to detect dopamine in vivo. However, determination of DA electrochemically by direct oxidation method is difficult due to the presence of ascorbic acid (AA) present in biological fluids. It can be oxidized at the same potential window. Furthermore, all the electrochemical techniques, suffered from another common problem, i.e. the oxidation products of dopamine could react with ascorbic acid in the sample, which results in severe interference of the detection. As the concentration of AA is relatively higher than that of DA in biological samples (4-5 orders of magnitude higher than DA), high sensitivity and selectivity are equally important for in vivo measurement of DA. In our recent work, we reported a non-oxidative approach to detect dopamine electrochemically in the presence of AA with high selectivity and sensitivity [2]. In this report, we report the mechanism of AA interference on the composite sensor by studying both the chemical and electrochemical interactions between the two species. For the first time the association constant between ascorbic acid and boronic acid groups was experimentally determined to quantitatively describe the interaction between two species. 2 EXPERIMENTAL SECTION 2.1 Materials Purified single-walled carbon nanotubes were purchased from Carbon Nanotechnologies, Inc. Houston, TX. Single- stranded DNA with sequence d (T)30 was purchased from Integrated DNA Technologies, Inc. Coralville, IA. 3- Aminophenylboronic acid hemisulfate salt, 3- hydroxytyramine hydrochloride (dopamine), L-ascorbic acid, potassium fluoride, potassium dihydrogen phosphate, potassium hydrogen phosphate, sodium phosphate, magnesium chloride, sodium chloride, Alizarin Red S, disodium EDTA, Nafion® perfluorinated ion exchange resin (5 wt.% in mixture of lower aliphatic alcohols and H2O), and all other chemicals were of analytical grade purity and were used as received from Aldrich Chemicals Inc., Milwaukee, WI. All solutions were prepared using nanopure water (18.2 M) (Barnstead). The bundled single-walled carbon nanotubes were dispersed into water using the method described by Zheng et.al[3]. The self- doped polyaniline/carbon nanotube composite was fabricated on the gold electrode surface by following the procedure described in our recent work [2]. AA solutions were prepared immediately before use in the containers protected from light by aluminum foil in order to avoid photoinduced oxidation. 2.2 Electrochemical Measurements All electrochemical measurements and electrochemical polymerization of 3-aminophenylboronic acid and electrochemical characterization of the resulting films were carried out by following the method described in our pervious work [3] at a CH Instrument 750 series electrochemical station. All the potentials quoted in this work are in terms of the Ag/AgCl scale. NSTI-Nanotech 2007, www.nsti.org, ISBN 1420061836 Vol. 2, 2007 523