Microelectrodes for in-vivo determination of pH - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 275

As a secondary reference electrode, the Ag/AgCl electrode is the most common due
to its simplicity, stability, and capability of miniaturization. A conventional Ag/AgCl
reference electrode is a silver wire that is coated with a thin layer of silver chloride
either by electroplating or by dipping the wire in molten silver chloride. The electrode
reaction is as follows

Cl e

Ag (solid) AgCl (solid) (11)

The potential developed is determined by the chloride concentration of the inner solu-
tion, as defi ned by the Nernst equation. As can been seen from the above reaction,
the potential of the electrode remains constant as long as the chloride concentration
remains constant. Potassium chloride is widely used for the inner solution because
it does not generally interfere with pH measurements, and the mobility of the potas-
sium and chloride ions is nearly equal. Thus, it minimizes liquid-junction potentials.
The saturated potassium chloride is mainly used, but lower concentrations such as 1 M
potassium chloride can also be used. When the electrode is placed in a saturated potas-
sium chloride solution, it develops a potential of 199 mV vs the standard hydrogen
electrode.
Silver chloride is slightly soluble (about 6 10 3 mol/L at 25ºC) in strong potas-

sium chloride solutions [54]. It is a common practice to saturate the potassium chloride
with AgCl salt to minimize stripping of the silver chloride off the silver wire [107].
When used in thin fi lm format as a microreference electrode, such AgCl dissolution
will certainly limit its lifetime. The usable lifetime of thin fi lm all-solid-state Ag/AgCl
electrodes was reported to be in only minutes to hours, as opposed to the lifetime in
months or years for conventional electrodes [108]. Dissolution of AgCl in the body,
when used in vivo or as an implantable sensor, was responsible for the toxic reactions
with surrounding tissue [109]. Polymer coatings such as Nafi on and polyurethane have
been used to prolong the lifetime of implanted reference electrodes [110].
Research work that focuses on the development of all-solid-state or microreference
electrodes to match a miniaturized pH measurement system, such as lab-on-a-chip, has
received increased attention in the past decades. Several approaches have been pur-
sued, such as scaling down a macroscopic reference electrode to form a miniature con-
ventional reference electrode [111], using electrolyte loaded polymers or hydrogel to
replace inner electrolyte solution [112, 113], and using solid contact to eliminate inner
reference solution [108, 114, 115]. The fi rst two approaches produced functional refer-
ence electrodes, but their lifetimes were limited by the volume of the inner solution
and the leach rate of the electrolyte from polymer. The outfl ow of the inner solution
through micro salt bridges and the variable leach rate from the gel layer were attributed
to unstable liquid-junction potential in practical applications [80]. The last approach
produced all-solid-state reference electrodes. However, because no constant chloride
concentration was maintained at the Ag/AgCl-solution interface, its potential changed
with the composition of the solution.
Covering the Ag/AgCl reference electrode surface with hydrophobic polymer, such
as a solvent-processible polyurethane (PU), was reported to provide a stable potential