NOVEL NON-CONDUCTING FILMSFOR NOVEL NON-CONDUCTING FILMSFOR INTERFERENCE-FREE ELECTROCHEMICAL SENSORSINTERFERENCE-FREE ELECTROCHEMICAL SENSORS

M. BADEA M. BADEA a a , A. CURULLI , A. CURULLI b*b*, G. PALLESCHI , G. PALLESCHI a a , S. KACIULIS , S. KACIULIS c c , A. MEZZI , A. MEZZI cc

a a Università di Roma ‘Tor Vergata’, Dipartimento di Scienze e Tecnologie Chimiche, Rome, ItalyUniversità di Roma ‘Tor Vergata’, Dipartimento di Scienze e Tecnologie Chimiche, Rome, Italyb b CNR ISMN Istituto per lo Studio di Materiali Nanostrutturati Sezione Roma 2, Rome ItalyCNR ISMN Istituto per lo Studio di Materiali Nanostrutturati Sezione Roma 2, Rome Italy

c c CNR ISMN Istituto per lo Studio di Materiali Nanostrutturati Sezione di Montelibretti, Rome, ItalyCNR ISMN Istituto per lo Studio di Materiali Nanostrutturati Sezione di Montelibretti, Rome, Italy

OH

HO

2,6 - Dihydroxynaphtalene

(2,6-DHN)

CH 2 CH 2 NH 2

H2 N

2-(4-aminophenyl)-ethylamine

(AP-EA)2,6-DHN & AP-EA

Cyclic voltammograms for electropolymerization of 2,6-DHN, AP-EA and electrocopolymerization of 2,6-DHN with AP-EA on Pt electrodes

Permeability of various electropolymerized films to 1mM hydrogen peroxide, 1mM ascorbic acid

and 1mM acetaminophen (E= + 650 mV)

P = ( Ifilm / Ibare ) x 100%

Ifilm = current measured at the electrode covered with the electropolymerized filmIbare = current measured at the naked electrode

XP Spectra relevant to N1s level recorded for copolymer

XPS results for poly(2,6-DHN), poly(AP-EA)and (2,6-DHN - AP-EA) copolymer

0 20 40 60 80 1000

5

10

15

20

25

Pas

corb

ic a

cid

(%)

scan rate (mV/sec)

2,6-DHN (1mM) AP-EA (100 mM) copolymer 2,6-DHN (0.5 mM) + AP-EA (10 mM)

0 20 40 60 80 1000

5

10

15

20

25

Pas

corb

ic a

cid

(%)

scan rate (mV/sec)

2,6-DHN (1mM) AP-EA (100 mM) copolymer 2,6-DHN (0.5 mM) + AP-EA (10 mM)

Influence of the scan rate used in the film preparation on the permeability

to 1 mM ascorbic acidMonomer Conc.(mM)

Scan rate(mV/min)

Range(mV)

No. ofcycles

Pascorbic acid

(%)Pacetaminophen

(%)P hydrogen peroxide

(%)

2,3-DHN 1.0 5 0 - 1200 20 4.1 94.3 97.1

1,6-DHN 1.0 20 -500 - 1200 20 0.65 32.3 94.8

2,6-DHN 0.5 20 0 - 1200 30 0.62 5.4 89.3

1,5-DHN 1.0 20 0 - 1000 20 11.5 17.4 95.8

1,2-DHN 1.0 20 -200 - 1200 20 62.9 87.0 97.5

2-(4AP)-EA 100 50 300 - 1300 20 0.78 7.80 95.6

2,6-DHN+

AP-EA

0.5+10

10 100 - 1150 20 0 5.9 93.3

BE Poly(2,6-DHN) Poly(AP-EA)Copolymer(2,6-DHN

with AP-EA)Atom Type

(eV) (%) (%) (%)

aromatic 285.0 58.1 56.0 58.7

C C-OH 286.4 15.2 15.5 15.6

H2N-C 399.0 - 1.8 2.7

N H2N-Ph 400.2 - 4.5 -

O (A) C-OH 533.6 12.2 - -

O (B) 532.5 9.2 17.9 18.7

impurities 102.3 5.3 4.4 4.2

0 10 20 30 40 50

0,0

0,2

0,4

0,6

0,8

1,0

curr

ent (

A)

days

1 mM ascorbic acid 1 mM acetaminophen 0.1 mM hydrogen peroxide

0 10 20 30 40 50

0,0

0,2

0,4

0,6

0,8

1,0

curr

ent (

A)

days

1 mM ascorbic acid 1 mM acetaminophen 0.1 mM hydrogen peroxide

Stability of the response of copolymer/Pt electrode to hydrogen peroxide, ascorbic acid

and acetaminophen

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.

AIM OF WORK

CONCLUSIONS

RESULTSFive 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.

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.

Acknowledgements: The authors thank the European Community ( MCFA-2000-000725) and the CNR Target Project MSTA II for the financial support.

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