Prototyping of microfluidic systems with integrated ... - DTU Nanotech

2.4 Optical measurements in microfluidics 19
In practice, to determine the concentration of a given substance in
a sample, different pre-treatment steps like filtering, aliquoting, and
chemical reactions are made, so that one is certain that the readout
from the instrument really stems from the substance, and that it is not
altered by the presence of other substances.
Furthermore, one needs a reference measurement - typically a sample
of the solvent (e.g. water or ethanol) alone, and one or several with
known concentrations of the substance.
In microfluidic systems the reference can be solved in the same ways
as when using macroscopic instruments. Either the same optical cuvette
can measure the references and the samples sequentially, or the system
can be parallelised, so that multiple structures measure on the sample
and references simultaneously.
An elegant, commercially available example of applied optical spectroscopy
is the Radiometer NPT7 blood gas analyzer [15]. Several values,
like pH and pCO2, are measured on a desktop sized device using singleuse
polymer cartridges with integrated optical cuvettes. By making one
side of the cuvettes flexible, several hundred optical measurements in
the infrared part of the spectrum can be made with different absorption
lengths, meaning that the optical loss from the system can be separated
from the loss stemming from the sample. In this manner, no reference
measurements or calibration are needed.
2.4.1 Planar microfluidics
Most microfluidic systems today are two-dimensional or 2.5-dimensional,
which roughly means that the structure can be manufactured from
one side using e.g. clean room processing or micro milling. If more
complicated three-dimensional structures are needed, they are usually
fabricated by stacking layers of 2.5-dimensional structures. For many
applications, the use of systems with a single, fixed channel depth for
the entire system, is sufficient.
Several methods for integrating optical detection in microfluidics
exist. The microfluidic system can either be inserted in a macroscopic
setup, where a light source shines light through a cuvette perpendicular
to the microfluidic substrate. The light is then detected by e.g. a
spectrometer on the other side. Using this approach, the length of
the optical cuvette is defined by the thickness of the substrate, and is
therefore relatively short – typically in the order of 1 mm. By coupling
light into the plane of the substrate, the optical path can be prolonged