Application of micromixers - Micromixers Fundamentals Design and Fabrication - page 329

streams have the same flow rate and the same viscosity, the interface of the two streams will be in the
middle of the mixing channel ( Fig. 9.3 (a)). The native fluorescein has its initial low level of intensity,
while the intensity at the interface increases due to diffusion and subsequent binding reaction between
the fluorescein and the protein. The higher the concentration of the protein, the higher the intensity
peak at the interface ( Fig. 9.3 (b)).
Veenstra et al. used a micromixer for the detection of ammonia in aqueous solutions [42] . The
Berthelot reaction was used for the detection. Ammonia in an aqueous solution was converted into
indophenol blue using a two-step reaction. The first step is the chlorination of ammonia to produce
monochloramine NH 2 Cl. In the second step, two phenol molecules bind to form monochloramine,
resulting in indophenol blue. Indophenol blue can be detected with an absorption measurement
because it has a peak at 625 nm in the absorption spectrum. Because the kinetics of formation of
indophenol blue is relatively slow, the micromixer should allow a residence time on the order of 1 min
for a complete conversion of all ammonia molecules in the solution into indophenol blue.

9.2.2 Improving chemical and biochemical analysis
Protein folding is controlled by the solvent composition of a protein solution. The changes in protein
conformation as a response to changes in solvent composition can be measured using time-resolved
nuclear magnetic resonance (NMR) spectroscopy. Time-resolved measurement of reaction kinetics
using nuclear magnetic resonance (NMR) can benefit from the fast mixing time in a micromixer [43] .
The solvent/protein interaction time depends on the mixing length and the flow rates of the mixed
streams. Adjusting these two parameters allows the measurement of NMR spectra at a precise time
instance. Microcoils for NMR can be integrated with the micromixer to facilitate on-chip measurement
( Fig. 9.4 (a)). The integration of an array of microcoils would allow simultaneous measurement of
multiple detection points. Such micromixers with integrated microcoils for NMR spectroscopy can be
used for investigations of reaction intermediates and molecular interactions. Kakuta et al. used
ubiquitin as the test protein. Ubiquitin changes its conformation from native to A-state at low pH and in
40% or higher methanol/water solvents. The micromixer was used for mixing the ubiquitin solution
with the methanol solution. The concentration of A-state increases with better mixing. Because the
micromixer in use was a Y-mixer with parallel lamination (Chapter 2), good mixing was achieved at
low Peclet number or low flow rates. The reported NMR measurement can resolve the changes in
a time scale of seconds.
Another application of micromixers is the labeling reaction of molecules after their separation
using techniques such as capillary electrophoresis (CE). Because fluorescence measurement is
commonly used for detecting these molecules, they must be derivatized with a fluorescent label.
Micromixers can be integrated in a CE chip as postcolumn reactors. Because the reactants and the
products continue to be separated in the mixing channel, micromixers for this purpose should be
efficient enough to allow rapid reaction. Slow reaction may lead to band broadening caused by the
different mobilities of reactants and products. Fluri et al. [44] combine capillary electrophoresis (CE)
separation with a T-shaped intersection for the reaction of amino acids with the labeling reagent
o -phthaldialdehyde (OPA; Fig. 9.4 (b)). Fluid flows in this system were electrokinetically driven.

Fast mixing with a micromixer was used for freeze-quenching technique, which is useful for
trapping meta-stable intermediates during fast chemical or biochemical reactions [45] . The determi-
nation of the molecular properties of these intermediates leads to further understanding of chemical and