Microelectrodes for in-vivo determination of pH - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 276

Liquid-junction membrane

Pt PPt

substrate

hydrogel layer

Si x N y

SiO x

Ag/AgCl

FIGURE 10.5 A schematic diagram of an all-solid-state polymer membrane-based Ag/AgCl reference
electrode formed on silicon substrate. Platinum trace lines and silver layer were deposited on 4 inch silicon
wafers with a pre-coated 1.2 µm of thermal oxide using a lift-off process. PECVD Si3N4 was used as a top
insulation layer and contacts were opened using a reactive ion etch. The exposed silver electrodes were chlo-
rinated with 0.1 M FeCl3 for 5 min. A drop of hydrogel ( 3 µL) was dispensed and dried on the Ag/AgCl
electrode to form an internal electrolyte layer, and 3-5 µL of the CA/PU membrane cocktail was deposited
to cover the hydrogel layer. (Reproduced from [113], with permission from Elsevier.)

comparable to a solid-state inner reference electrode for about 2 weeks [112]. By
introducing hydrophilic cellulose acetate (CA) to the PU to form a CA/PU membrane,
the reference electrode was reported to reach a 5-month operational time [74]. Ha
et al. [113] have demonstrated a mass produced solid-state miniaturized Ag/AgCl ref-
erence electrode using the CA/PU membrane (30/70 wt.%) as a polymeric junction, and
an electrolyte loaded hydrogel layer as internal solution. The electrodes were micro-
fabricated on ceramic or silicon chips. A schematic diagram of one such electrode on
Si wafer is shown in Fig. 10.5. The hydrogel layer contains 3 M KCl with a 6 wt.%
water soluble polymer PVP. The microporous membrane provided micro-channels
for the hydration and diffusion of the inner electrolyte. The electrode provided a stable
reference potential for about 25 mins for Si chip-based electrodes, while this stabil-
ity was held for about 90 mins for a ceramic chip-based electrode after a hydration
time of 100-200 s. After these stable times, the electrode potential drifted rapidly at
a rate of 90 mV/h. The authors attributed the shorter lifetime for Si chip-

based electrodes to their thinner membranes and smaller internal electrolyte volumes,
as compared to those based on ceramic chips. Another possible cause for such a short
lifetime, although not discussed in their paper [113], might be due to partial membrane
adhesion failure. Poorer adhesion of membrane on smooth Si chips compared to adhe-
sion on rough ceramic chips may contribute to the shorter lifetime of Si chip-based
electrodes.
Such electrodes should be suffi cient as a reference electrode for short-term usage or
as a disposable electrode. However, the requirement of a pre-hydration time may limit
its applications for fast measurements, such as POCT (the point-of-care testing), due to
its slow response time. In fact, the lack of long-term stable microreference electrodes
will continue to hamper the development of integrated pH sensing systems.

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