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Voltammetric and Potentiometric Study of
Cysteine at Cobalt(II) Phthalocyanine Modified Carbon-paste Electrode
S.S. Khaloo, M.K. Amini*,
S. Tangestaninejad, S. Shahrokhian and R. Kia
Chemistry Department, University of Isfahan,
Isfahan 81746-73441, Iran
(Received 10 December 2003, Accepted 5
January 2004)
Cyclic voltammetry
and potentiometry were used to investigate the electrochemical behavior of
cysteine at a chemically modified electrode prepared by incorporating cobalt(II)
phthalocyanine [Co(II)Pc] into carbon paste matrix. The modified electrode
showed high electrocatalytic activity toward cysteine; the overpotential for the
oxidation of cysteine was decreased by more than 100 mV, and the corresponding
peak current increased significantly. The electrocatalytic process was highly
dependent on the pH of the supporting electrolyte. The peak currents decreased
when the pH was raised to 6 and totally disappeared at pH≥ 7, resulting from the
autocatalytic oxidation of cysteine by Co(II)Pc at the electrode surface.
Therefore, at pH values of 6 to 8, the modified electrode was used as a
potentiometric sensor for quantitative measurement of cysteine in the presence
of oxygen in air saturated solutions. In fact, the Co(II)Pc/Co(I)Pc couple acts
as a suitable mediator for indirect oxidation of cysteine by dissolved oxygen at
approximately neutral pH values. Under the optimized conditions, the
potentiometric response of the modified electrode was linear against the
concentration of cysteine in the range of 0.6
mM
to 2 mM. The limit of detection was found to be 0.5
mM.
The potentiometric response time was
£15
s. The electrode showed long term stability; the
standard deviation of the slope obtained after repeated calibration during a
period of two months was 2.8% (n = 7). Application of the electrode in a
recovery experiment for the determination of cysteine added to a synthetic serum
sample is described.
Keywords:
Cysteine, Modified electrodes, Carbon-paste
electrode, Potentiometric sensors, Cobalt(II) phthalocyanine
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