All-solid-state pH Sensor Based on Conducting Polymer W ...

592 Functionalized and Sensing Materials
may be not suitable.
Experiment
Electrodes fabrication. For the electrodeposition of PPy and silver layers, a GW Instek
Power Supply (Model GPC 3030D, Taiwan) was used with a one compartment cell. Stainless steel
rods (Grade 430) with a diameter of 0.078 cm 2 fitted into Teflon tubes were used as working and
auxiliary electrode. The electrodes were mechanically polished with 100 and 1,200 fine grade
sandpapers and finally with alumina, rinsed with distilled water and cleaned in an ultrasonic bath
before each synthesis.
To prepare the <strong>solid</strong>-<strong>state</strong> silver pseudo-reference electrode, the silver layer was deposited
onto the stainless steel working electrode by applying a constant potential of 1.5 V for 30 min in a
1% AgNO 3 solution. The silver-coated electrode was rinsed with distilled water and dried in a
desiccator overnight.
To prepare the PPy based <strong>pH</strong> electrode, the PPy thin film was electropolymerization in a
solution containing 50 mM pyrrole and a mixture of dopant, oxalic acid and hydroquinone
monosulfonate (HQS) [3]. The ratio of oxalic acid and HQS was maintained at 1:1. A constant
potential of 1.5 V was applied for 10 min, The PPy coated electrode was then rinsed with distilled
water, dried in desiccator for at least 4 h and conditioned in 50 mM HQS for at least 1 h before
testing.
Potentiometric study. <strong>All</strong> potential measurements were conducted using a 2700 digital
multimeter/data acquisition system (Keithley, USA). Potentials of the silver electrode in buffer
solution with different <strong>pH</strong> values were determined using a commercial Ag/AgCl electrode as a
reference electrode. Standard <strong>pH</strong> buffer solutions were prepared with mixture of citric acid, boric
acid and Na 3 PO 4 .10H 2 O. The actual <strong>pH</strong> of the buffer solutions was determined by a commercial
glass electrode connected to a 510 <strong>pH</strong> meter (Eutech, US).
Results and Discussion
One of the several requirements for good reference electrodes is that their potentials should
be independence of the composition of the analyzed ions. The potentials of silver reference
electrode versus the Ag/AgCl reference electrode was determined in different <strong>pH</strong> buffer solutions
and shown in Fig.1. No significant change in the potential is observed over a <strong>pH</strong> range of 2 to 10.
This indicates that the performance of the silver reference electrode and commercial Ag/AgCl
reference electrode are consistent in all aspect. The potential of the silver electrode, however,
decreased rapidly when immersed in the solution having <strong>pH</strong> > 10 due to oxide formation.
Therefore, the silver reference electrode is possible to use as a reference electrode in <strong>pH</strong>
measurement over the range of 2 to 10.
The stability of the reference electrode is an important factor in any electroanalytical
procedure, since any variation can affect the response of the working electrode. In this regard, the
stability of the silver reference electrode was investigated by repeatedly performing the same
experiment with the silver reference electrode after storage in a desiccator for a period of time and
presented in Fig. 2. It was found that the potential changed upon the storage time. The potential
change of 0.03 V after one day storage suggests that the silver reference electrode may be
performed as a single-use reference electrode. Nevertheless, the production cost for the silver
reference electrode is relatively low due to the use of inexpensive supporting substrate, compared
with that for commercial Ag/AgCl reference electrode, it is well suited to mass production.