Microfluidics for Biosensor Chips - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 143

diffusion to create an electrical double layer, because ion polarization
at the electrode surface can completely screen the ield line. The
utility of a high frequency AC ield across the print electrodes has
demonstrated eficient mixing in microluidic devices. 23
Non-linear electro-osmosis can occur not only on the surface
with permanent charges but also at the dielectric structure corner
tips, accumulating induced charges by the external ield. Similar to
the mixing device using print electrodes, a high frequency AC ield is
required to sustain the electrical double layer in generating EOF. 24
The ield application electrodes can be placed outside the mixing
cell channel to avoid contamination due to chemical reactions on
the electrode. Several relief-like strip structures with the thickness
of 30 microns are fabricated on the FO-PPR sensing channel. When
an external AC ield (100 kHz, 50 V/cm) is applied, intense micro-
vortices are generated at the corners of each strip. 25 The external
ield heating slightly raises the solution temperature to circulate
the bulk low via convection. When the circulation low couples
with the micro-vortices, eficient mixing is accomplished via chaotic
solute advection to reduce the PPR sensing response time of
streptavidin docking 10-fold shorter. The sensing of a virus extracted
from orchid leaves is accelerated three times in this PPR sensing cell
using induced ac EOF mixing.
In addition, slender structures fabricated in microchannels have
been used to split a luid into several narrow streams, traveling
along paths of different lengths prior to merging together. 26 Such
passive splitting and merging processes can drive solutes to mix via
chaotic advection actions. Tilted grooves patterned on the bottom of
microchannel, inducing secondary lows when luid passes through
the channel, are able to accomplish similar mixing processes.
Alternatively, these grooves can be crafted with femto-second laser
ablation on the cladding layer of optical ibers suspended in the low
cell to facilitate PPR detection. 27

7.4 Integration of Biosensing Systems

Using Noble Metal Nanoparticles with
Mass Spectrometer

Like other bio-sensing systems using micro-columns, such as
micro-liquid chromatography and CE, 28,29 the most straightforward