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The application of microfabrication technologies in the field
of (electro)chemical microreactor design has led to an explosion of new
potential (electro)analytical devices. In this section we briefly examine
the development of microelectrochemical reactors which exploit
hydrodynamic conditions to create new devices for the investigation of
electrolysis reactions and analytical sensing.
Construction of microelectrochemical reactors
The basic principles of microelectrochemical reactor design and
fabrication are identical to those discussed in the above microreactor
sections (link). Which ever microfabrication approach is employed
typically the end product generated consists of a single (or set) of
channels which are typically about 10 - 100 microns in depth, with a
cell width in the range 10-500 microns. A schematic of a typical single
channel microelectrochemical reactor is depicted below.
The cell is constructed from two components: (i) a
microfabricated channel and (ii) a glass cover slip on to which
gold/silver or platinium electrodes are
evaporated. These two components are bonded together to create a
rectangular duct through which reagent solutions may be pumped and
voltammetric measurements performed. The image below shows
the microfabricated inlets for a cell which permits three different
reagents to be introduced into a reactor simultaneously. In this example
the cell has been fabricated using a glass substrate
(link) etched using a dilute HF solution.
Schematic of a microelectrochemical
reactor and an image of a three inlet microreactor
device
Photolithography (link) can be used to create a template for the
fabrication of the microelectrodes. In this case a glass cover slip can
be spin coated with a thin layer of photoresist. Irradiation of
specific regions of the cover plate with UV light then acts to develop
the photoresit which can then be washed away, leaving bear the regions
on the glass where the electrodes will be formed. The cover plate is then
placed in an evaporator and metal is deposited onto the plate. The final
stage involves the removal of the remaining photo resist leaving a set
of electrodes which can then be sealed onto the substrate containing
the microchannel. The figure below shows a typical set of gold
electrodes which have been fabricated using this approach.
Image of a set of microelectrodes fabricated on a glass
cover slip.
Hydrodynamic voltammetry in microreactors
Once complete electroactive reagent solution is pumped through the cell
and voltammetric measurements performed in an analogous manner to those
discussed in the hydrodynamic voltammetry section.
The figure below
reveals two hydrodynamic voltammograms recorded at different volume
flow rates within a microelectrochemical reactor for a reagent solution
containing a electroactive material undergoing a transport limited one
electron oxidation at the working electrode.
Linear sweep voltammograms recorded in a
microelectrochemical
reactor
The response observed is similar to that seen in the larger scale channel
electrode described earlier. Indeed the variation of the transport
limited current as a function of the velocity of solution through
the cell also shows the cube root relationship observed for
macroscopic channel cells. However, it should be noted that the
full analysis of the current variation does require more complex 3
dimension mass transport modelling due to the fluid flow properties within
the cells.
At this stage these devices have yet to be fully exploited for
mechanitic analysis, however, the reactors should offer some
significnat benefits over the traditional techniques such as the
rotating disc electrode etc. These benefits should include
access to more rapid chemical and electrochemically reactive
processes (due to the higher rates of mass transport within
the cells) and as a result of the microfabrication approach
a vast range of potential cell designs will be available offering the
opportunity to 'tailor' make specific configurations for a particular
chemical investigation.
Multiphase microelectrochemical reactors
The application of microreactor technology has not been restricted to the
analysis of processess occurring in single solvent phases. Recently
electrochmeical devices have been developed to permit the analysis of
reactions occurring between reagents flowing in immiscible liquids. The
figure below shows a schematic of a typical reactor designed for two phase
flow measurements
Microelectrochemical reactor designed for two phase flow
measurements
In this case a two inlet arrangement is employed with the immisicible
solvent streams (eg water/dichloroethane) introduced into the device via
separate inlets. At some predetermined point the two streams converge and
reagents may then transfer between the two phases (eg phase
transfer catalysis). Electrode sensors are placed within the separate
streams to permit analysis of the reagents in each solvent and to detect
the products of any reactions occurring. In the figures below an image
recorded of a multiphase flow measurement is shown for a 'U tube'
environment, (a blue dye has been added to the organic phase to
aid visualisation) and on the right a straight cell constructed with
mircroelectrode sensors in each stream.
Images of devices used for immiscible two phase flow
investigations
This microreactor approach also permits more complex multiphase flow
conditions to be examined. For example in the images below the approach
has been extended to a three phase environment, where in this case three
solvent streams containing different chemical reagents may be brought
together at a predetermined point to permit chemical
reaction. The figure on the left below shows the
three inlet arrangement and on the right an image taken about 2.5 cms
from the inlet region. It is apparent that a chemical
reaction has occurred in this case an interfacial electron transfer has
created the coloured product in the central stream.
Images of devices used for immiscible three phase flow
investigations
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