HETEROGENEOUS NANOSTRUCTURES FOR BIOMEDICAL DIAGNOSTICS - Bio-Inspired and Nanoscale Integrated Computing - page 448

conductance change. (Web site information is available from http://www.lsbu.
ac.uk/water/explan4.html.) Several approaches are available to increase the
sensitivity for conductivity detection. First, suppress the background conductance
by using a suppressor column inserted after the separation column [38]. Second,
increase the sample plug concentration by using sample preconcentration techni-
ques (Dionex Corp., Sunnyvale, California), the use of concentrator columns in
ion chromatography, 1994. Third, reduce sample injection volume and system
dead volume. Fourth, reduce the sensor cell constant K cell which is especially
achievable using MEMS technology [39]. The state-of-the-art conductivity detec-
tion has a detection limit of 100 ppb (

10 nM) for unsuppressed detection and

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10 ppb ( 1 nM) for suppressed detection using a 50 m L sample injection for

separation [38].
The synergism of micro- and nanocomponents for cardiovascular diagnostics
includes (1) integration of sensors with the biological activities of vascular
endothelium and circulating oxidized LDL from patients enrolled in the existing
clinical trials at the Atherosclerosis Research Unit, and (2) a potential for
investigation of in vivo animal models. Clinically, direct measurement of shear
stress is important to predict in-stent restenosis and bypass graft occlusion. The
MEMS LC as a lab-on-a-chip platform minimizes sample volume, providing an
efficacious venue to detect clinical markers such as the levels of circulating
oxidized LDL for early detection of unstable angina and prevention of acute
coronary syndromes.

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16.5. INTEGRATED MICRO- AND NANOSCALE COMPONENTS
BIOFUEL CELLS

Fundamental to numerous biosensing applications are the biochemical redox
reactions via micro electrodes and nanoscale molecules such as enzymes. The last
decades have attested intense investigation in surface chemistry to optimize
electron transport from the enzymatic reactions to the electrodes as a basis of
biofuel cells. By converting chemical energy to electrical energy, biofuel cells have
brought about the possibility to generate power for the micro implantable devices.
Biofuel cells consist of an anode and a cathode where the transfer of electron
between the electrodes generates electrical power (Fig. 16.20). Miniaturization of
biofuel cells has been materialized with microelectrodes and nanoscale particles.
In an ideal microenvironment, the cell voltage is generated by virtue of the
difference in the potentials between the anode, where fuel such as glucose is
converted to gluconolactone, and cathode, where oxygen reacts with proton to
form water. However, efficiency of power generation hinges on the kinetic electron
transfer across the electrode interfaces, ohmic resistances of the electrode, and
rates of electron formation by the redox reactions. Also important to the rate of
cell current are the surface area of electrodes and proton transfer across the
permeable membrane that separate the catholyte and anolyte compartments of the
biofuel cells [41].