Microfluidics for Biosensor Chips - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 142

7.3.2 Microfluidic Mixer to Improve PPR Detection

Microluidic-assisted PPR assays described in Section 3.1 employ
devices of immobilized noble metal particles on substrates.
Although the dimension of a micro-channel in delivering sample
solutions is only hundreds of microns, large targets must still expend
considerable diffusion time to contact with the immobilized probes
on the nanoparticles to accomplish binding.
Reynolds number scaling with both luid speed and container
dimension is only 10 or smaller in the sub-millimeter low channel
when the luid speed is approximately 1 mm/sec. Under this small
Reynolds number, luids in microchannels are irrotational and
dificult for creating any turbulence in mixing solutes.
Non-linear electro-osmotic lows have been used to generate
micro-vortices in microluidic devices. 21,22 Non-linear electro-
osmosis can occur at the charged surface of conducting substrates.
Due to the conducting properties, external electric ield lines can
penetrate the substrate surface. Solution ions in the bulk are driven
to migrate along the electric ield lines. However, the electrical
double layer (Debye layer) on the charged surface is ion-selective to
prevent the co-ions of the surface charge from reaching the surface.
The co-linearity between electric iled lines and ion migration paths
is no longer valid to break down the irrotationality of the luid.
Additionally the depletion of co-ions at the regime near the substrate
surface creates one extended electrical double layer, one or two
orders of magnitude thicker than the original layer. When the counter
ions accumulated in the extended layer are driven to migrate by the
external ield, these ions drag surrounding luid to generate electro-
osmotic low (EOF) of which the speed is 10-100 times faster than
typical EOF generated at the charge surface of an insulated substrate
(such as a fused silica capillary wall). Ion migration speed and Debye
layer thickness are both proportional to the external ield strength;
therefore, the EOF speed ascends quadratically with the ield
strength. High speed (~1 cm/sec) micro-vortices are observed near
the surface of conducting ion-exchange resin under even moderate
DC ield 100 V/cm. When these resin granules move back and forth,
the localized micro-vortices travel throughout the mixing cell to
advect solutes chaotically, which accomplish mixing eficiently (100
times faster than diffusion mixing). Although the metal electrode
is conducting, a high frequency AC ield (~100 kHz) must be used
to avoid polarization and allow the counter ion accumulation by