The assay uses a colorimetric detection principle (see ESI†).
In the presence of glucose and oxygen, glucose oxidase (GOx)
catalizes the oxidation of glucose to glucono delta-lactone and
hydrogen peroxide, which is used by HRP to catalyze the oxidation
of colorless iodide to brown iodine36 monitored in the
measurement.
The final LOC functionality tested here in ULOC configuration
is the integration of micromixers. Mixing is a common
preparatory stage in chemical analyses, and accordingly, an
important function for LOCs that must be supported by any
prospective fabrication method. Microfluidic dimensions favor
laminar flow regimes, where mixing occurs as a slow diffusional
process, and different designs exist to favor turbulence
and better mixing performance.37 Chaotic mixers12,37,38 are one
possibility to improve performance, and have been demonstrated
on PDMS-LOC created on 3D printed templates.16
Here a different approach is investigated. The Reynolds number
in a channel is proportional to the channel height,38 and
thus a simple geometric feature, such as a 1 mm through hole,
at the join between flows can improve the mixing efficiency in a
very compact way. Additionally, the through hole allows to incorporate
glass micro beads to favor turbulent regimes, and simultaneously
the ULOC design helps to introduce the beads from
the device backside, thus avoiding problematic bead spillover
that can compromise the front side sealing. In this case, the
device was first sealed from the front side and then loaded with
beads from the backside, and finally taped from the backside.
To assess the mixing performance, the devices were flowed
with fluorescein and rhodamine B aqueous solutions, delivered
with two syringe pumps at 30 μl min−1
.