NOVEL NON-CONDUCTING FILMSFOR INTERFERENCE-FREE ELECTROCHEMICAL SENSORS

1. Istituto per lo Studio dei Materiali Nanostrutturati NOVEL NON-CONDUCTING FILMSFOR INTERFERENCE-FREE ELECTROCHEMICAL SENSORS M. BADEA a , A. CURULLI b*, G. PALLESCHI a , S. KACIULIS c , A. MEZZI c a Università di Roma ‘Tor Vergata’, Dipartimento di Scienze e Tecnologie Chimiche, Rome, Italy b CNR ISMN Istituto per lo Studio di Materiali Nanostrutturati Sezione Roma 2, Rome Italy c CNR ISMN Istituto per lo Studio di Materiali Nanostrutturati Sezione di Montelibretti, Rome, Italy Cyclic voltammograms for electropolymerization of 2,6-DHN, AP-EA and electrocopolymerization of 2,6-DHN with AP-EA on Pt electrodes AIM OF WORK Electropolymerised films based on phenol, aniline, vinylindole and pyrrole have been used as an alternative to conventional membranes. The oxidation of different monomers can lead to modified electrodes with peculiar properties, including selectivity and protection of the electrode from passivation. Interest vs modified electrodes was focused on conducting films, but non-conducting films seem to be equivalent to ideal membranes. Because of non-conducting films self-limited growth, the formed polymers are very thin (10-100 nm) and the response of these modified electrodes is very fast. Other advantages are that such films are usually pinholes free and potentially robust. This type of films can be used to immobilise enzymes during or after the electropolymerisation step by covalent attachment. In our work new non-conducting polymers based on different dihydroxynaptalenes (DHN) and 2-(4-aminophenyl)-ethylamine (AP-EA) have been synthesised on the surface of Pt electrodes to assemble fast-response and interference-free amperometric sensors for hydrogen peroxide. The electropolymerisation was performed by cyclic voltammetry. Different scan rates and scan ranges were investigated and selected according to the monomer used. All the sensors obtained were tested for hydrogen peroxide, ascorbic acid and acetaminophen by cyclic voltammetry and amperometry. Studies on reproducibility, interference, response time, buffers, storage and operational time of the sensors have been performed. The chemical composition of obtained films was analysed by X-ray Photoelectron Spectroscopy. 2,6-DHN & AP-EA 2,6 - Dihydroxynaphtalene (2,6-DHN) 2-(4-aminophenyl)-ethylamine (AP-EA) P = ( Ifilm / Ibare ) x 100% • Ifilm = current measured at the electrode covered with the electropolymerized film • Ibare = current measured at the naked electrode • Permeability of various electropolymerized films • to 1mM hydrogen peroxide, 1mM ascorbic acid • and 1mM acetaminophen (E= + 650 mV) Influence of the scan rate used in the film preparation on the permeability to 1 mM ascorbic acid Stability of the response of copolymer/Pt electrode to hydrogen peroxide, ascorbic acid and acetaminophen RESULTS • Five different dihydroxynaphtalenes were tested: 2,3 DHN, 1,6-DHN, 2,6-DHN, 1,5-DHN and 1,2-DHN. For all of these, an oxidation peak was recorded in positive potential range between 250 – 800 mV. The decrease of the oxidation current after the first cycle indicates the passivation of the electrode surface. The polymers formed were transparent and strongly adherent on the surface of the platinum electrode • In our knowledge, the electropolymerisation of a new monomer 2-(4-aminophenyl)-ethylamine (AP-EA), which has a similar structure of tyramine, is reported, for the first time. The resulting film showed a very good rejection of ascorbic acid, a high permeability for hydrogen peroxide but a poor stability in time. • In order to improve the stability of poly(AP-EA), a copolymerisation of AP-EA with 2,6-DHN was studied. Different ratios 2,6-DHN / APAE were studied and the best results in terms of stability and interference rejection were obtained using a mixture 0.5 mM 2,6-DHN and 10 mM AP-EA. • Different parameters (scan rate, range of potential, monomer concentration, number of cycles, pH buffer) for preparation and storage of polymers were optimised. • The permeability for hydrogen peroxide and rejection of ascorbic acid and acetaminophen was tested for all the films by cyclic voltammetry and amperometry (applied potential +650 mV vs Ag/AgCl), in batch and also in flow injection conditions. • The presence of the free amino-groups in the copolymer (2,6-DHN – AP-EA) (see the XPS results) structure permits the covalent attachment of the enzymes via peptide bond formation. Preliminary experiments for covalent immobilisation of hydrogen peroxide producing enzymes were carried out using this copolymer and glucose oxidase, as a model enzyme. XPS results for poly(2,6-DHN), poly(AP-EA) and (2,6-DHN - AP-EA) copolymer CONCLUSIONS • New non-conducting films were electrosynthesised starting from dihydroxynaphtalenes by cyclic voltammetry. Poly(2,6-DHN) showed the best characteristics in terms of interference rejection, permeability for hydrogen peroxide and stability in operational conditions. • A film with free amino groups on its skeleton has been synthesised, from AP-EA , which was for the first time reported. The presence of the free amino-groups on the poly(AP-EA) backbone allowed the covalent attachment of the enzymes via peptide bond formation. • In order to improve the poly(AP-EA) stability, a copolymerisation with 2,6-DHN was performed and optimised. The resulting film was characterised by a very good rejection of interferents, a long life time (more then 60 days) associated with the presence of the free amino groups on its structure. • Preliminary results obtained for the covalent immobilisation of the glucose oxidase were very promising for a interference free biosensor assembling. XP Spectra relevant to N1s level recorded for copolymer • Acknowledgements: The authors thank the European Community ( MCFA-2000-000725) and the CNR Target Project MSTA II for the financial support.