Environmental Engineering Reference
In-Depth Information
Microelectrodes can be fabricated in a number of ways [31, 32], most commonly
by pulling a glass micropipette, inserting a metal wire (such as Pt or Au), and then
filling with a low-melting-point alloy. Alternatively, a metal wire can be inserted
into a glass micropipette first and then the metal/glass assembly pulled under heat to
simultaneously decrease the wire diameter and tightly seal the metal within the glass
capillary. Fine Au or Pt wires can also be sealed into glass by inserting them into a
glass capillary and melting it around the wire. Such “conventional” microelectrodes
have been used to investigate microscale distribution of oxygen consumption [21,
36-38], photosynthesis [39], sulfate reduction [25, 26, 40, 41], and nitrification and
de-nitrification [39, 42].
Although these microelectrode fabrication methods are well-established, a num-
ber of inherent disadvantages still exist, such as low success rate, poor reproducibil-
ity, fragility, and difficulty in making a multi-sensor device [43-45]. Further, these
microelectrode sensors are susceptible to electrical interference and have to be oper-
ated in specialized laboratories inside a Faraday cage [42-44]. Therefore, a need
for robust, sensitive, and easy-to-fabricate sensors for
in situ
measurements still
remains.
Microelectromechanical systems (MEMS) miniaturization technologies offer
many advantages for fabrication and integration of sensor components [46]. These
include reduced costs due to batch fabrication, increased integration, and potentially
reduced power consumption due to smaller size. The use of MEMS fabrication tech-
niques can also reduce complexity and increase reproducibility of the fabrication
process. The most important advantage of using MEMS fabrication, however, is the
increase in sensor reliability due to redundancy and better process control.
Many researchers that apply MEMS technologies to solving sensor problems
have focused on microfluidic lab-on-a-chip (LOC) systems [47-49]. Such systems
typically contain microfluidic channels for sample collection, preparation, or trans-
port with planar sensing areas for specific target analytes. Others have developed
ion selective sensors based on field-effect transistors (or ISFETs) [50]. These sen-
sors also are based on planar electrodes, often integrated with a microfluidic system.
Nevertheless, they key drawback of such systems is of course that samples still must
be extracted from the site of interest, which often is not acceptable. In order to per-
form
in situ
measurements, 3-D microelectrode sensors are needed to be capable of
penetrating directly into samples such as soil pores or biofilms.
MEMS technologies have been used to develop penetrating 3-D microelectrode
sensors for neuroscience applications. Fofonoff et al. [51] combined wire elec-
trical discharge machining (EDM) with a chemical etching process to fabricate
titanium microelectrode arrays for neural activity recording in mice. Several struc-
tures such as a 10
10 rectangular shape and a honeycomb pattern containing
1141 electrodes were demonstrated with a device that was 1 mm long and 80
×
m
wide. Motta and Judy [52] developed neural microprobes using a 3-D continu-
ous electroplating process which yielded 22 mm long microprobes. A 3-D flexible
microprobe array was designed by Takeuchi et al. [53] using polyimide deposited on
a 250
μ
m thick silicon substrate. Deep reactive ion etching (DRIE) and XeF
2
etch-
ing processes removed the silicon substrate to achieve flexible polyimide probes.
μ
