Environmental Engineering Reference
In-Depth Information
The DO sensors are polarographically recessed cathode Au electrodes. Thus,
for DO microelectrodes, a recess was created at the tip of each microelectrode
(Fig. 6.4c). To create the recess, the glass core and Ti exposed by beveling were
etched using HF-based etchant for 5 min. The beveling and etching steps permit
precise control of the recess opening size and depth. The exposed Au was etched in
a 1:4:40 (m/m/v) mixture of I
2
, KI, and H
2
O for 3 min to relocate the Au sensing
area inside the formed recess. Microelectrodes were cleaned ultrasonically in DI
water after each etching step. For both sensors, tip diameters were on the order of
1-2
μ
m.
Since Au exposed to the solution enables ORP measurements, both structures
in Fig. 6.5b and c could be used as an ORP sensor. The recessed structure in
Fig. 6.5c not only can measure ORP but also DO. However, the beveled ORP sensor
in Fig. 6.5b has the glass core structure and thus is more robust than the recessed
ORP microelectrode in Fig. 6.5c for penetrating samples.
For phosphate sensors, cobalt (Co) was electrodeposited on the exposed gold tips
(Fig. 6.5d). Two cobalt plates, 3
5 cm, were used as anodes. Plates were cleaned
in 20% HCl for 15 min to remove any cobalt oxide layer. An electrolyte solution
was prepared by dissolving 33 g CoSO
4
and 3 g H
3
BO
3
in 100 mL of water. The
MEA was placed between the cobalt plates at 1 cm distance from each. Cobalt
was electrodeposited to ~0.2
×
μ
m thickness on gold tips using a current density of
10 mA/cm
2
for about 2 min.
The needle-type multi-analyte MEAs integrating ORP & DO & phosphate were
successfully fabricated, as illustrated in Fig. 6.6. This new MEA sensor consisted
of four 2-cm long probes at 900
m center-to-center spacing packaged on a PCB
carrier. The single MEA sensor contained DO, ORP, and phosphate sensors within
the array structure.
μ
Fig. 6.5
Tip structures of the multi-analyte MEA sensor: (
a
) Parylene C insulating layer covering
the entire microelectrode, (
b
) beveled and exposed glass and gold cathode, (
c
) glass and Au etching
for the recessed oxygen sensor, and (
d
) cobalt electroplating for the phosphate sensor
