Optimization of poly(dimethylsiloxane) hollow prisms for optical sensing.

A new generation of simple, robust and compact microfluidic systems with optical readout is presented. The devices consist of hollow prisms fabricated by soft-lithography, together with microlenses and self-aligned channels for fibre optic positioning, conferring the system with a high degree of monolithic integration. Its working principle is based on the absorption of the working wavelength (lambda = 460 nm) by the different substances that can fill the hollow prisms. By modifying the volumes and geometries, optimization of the presented systems has been achieved. Results show how the limit of detection (LOD) for fluorescein and methylorange diluted in phosphate buffer can be significantly lowered, by increasing the size of the prism or increasing the total deviation angle. For our investigations we used concentrations for which the Beer-Lambert law is fulfilled and the measurements showed a LOD in the microM range for both species. Finally, since the change in the fractions of the methylorange as a function of the pH causes a variation of the total absorption of the solution, the hollow prisms have also been used for pH measurements.