On-chip light sheet illumination for nanoparticle tracking in microfluidic channels

A simple and inexpensive method is presented to efficiently integrate light sheet illumination in a microfluidic chip for dark-field microscopic tracking and sizing of nanoparticles. The basic idea is to insert an optical fiber inside a polydimethylsiloxane (PDMS) elastomer microfluidic chip and use it as a cylindrical lens. The optical fiber is in this case no longer seen as only an optical waveguide but as a ready-made micro-optical component that is inexpensive and easy to source. Upon insertion, the optical fiber stretches the PDMS microchannel walls, which has two effects. The first effect is to tone down the intrinsic ripples in the PDMS that would otherwise create inhomogeneities in the light sheet illumination. The second effect is to remove any obliqueness of the channel wall and constrain it to be strictly perpendicular to the propagation of the illumination, avoiding the formation of a prismatic diopter. Through calculations, numerical simulations and measurements, we show that the optimal configuration consists in creating a slowly converging light sheet so that its axial thickness is almost uniform along the tracked area. The corresponding thickness was estimated at 12 μm, or 10 times the depth of field of the optical system. This leads to an at least six-fold increase in the signal-to-noise ratio compared to the case without the cylindrical lens. This original light-sheet configuration is used to track and size spherical gold nanoparticles with diameters of 80 nm and 50 nm.

3D microfabrication by applying the laser-induced bubble method to the thermoset polymer PDMS using a conventional nanosecond laser

We recently developed a microfabrication technique [microfabrication using laser-induced bubble (microFLIB)] and applied it to polydimethylsiloxane (PDMS), a thermoset polymer. The technique enabled the rapid fabrication of a microchannel on a PDMS substrate and selective metallization of the channel via subsequent plating; however, the technique was limited to surface microfabrication. Therefore, we explored the feasibility of three-dimensional (3D) microFLIB of PDMS using a nanosecond laser. In the experiment, a laser beam was focused inside pre-curing liquid PDMS and was scanned both perpendicular and parallel to the laser-beam axis to generate a 3D line of laser-induced bubbles. In the microFLIB processing, the shape of the created bubbles was retained in the pre-curing PDMS for more than 24 h; thus, the line of bubbles generated by the perpendicular laser scanning successfully produced a 3D hollow transverse microchannel inside the PDMS substrate after subsequent thermal curing. In addition, a through-hole with an aspect ratio greater than ∼200 was easily fabricated in the PDMS substrate by parallel laser scanning. The fabrication of a 3D microfluidic device comprising two open reservoirs in a PDMS substrate was also demonstrated for biochip applications.