The following fabrication protocol outlines a general method for incorporating a planar multiridge ATR crystal with the light coupling into any microfluidic device design (Figure ), includes the fabrication of two main components, the ATR sensing layer (Figure a), and the microchannel feature layer (Figure ). This is a different approach than used previous to integrate planar detection elements at the base of the channel. The fabrication of the ATR sensing layer starts by placing the crystal face-down (groove-side up) on a thin polyethylene (PE) film (Plastic wrap, Selection, Canada), which is supported by a glass substrate and set at the bottom of a petri dish (Figure ). Then, a Teflon plug with a tapered edge of 20 o was pressed against the ridge side. This angle matched the light path angle from the ATR spectroscopy mapping aperture accessory (ASMAA). To improve the reproducibility and accuracy of the optical access port, a jig was machined that aligns the ATR crystal and the Teflon plug and applies even pressure such that their contact was uniform, leaving no spaces for the pre-cured PDMS to penetrate between them (Figure ). Then a mixture of liquid PDMS and cross linker (Sylgard184, Dow corning, Canada) at a 10:1 ratio was poured on top of the crystal (Figure (ii)) until the level was approximately 1 mm higher than the backside and cured at 70 °C for 4 h. After the PDMS solidified, the Teflon plug was removed, aided by its tapered edge. In some cases, a small meniscus extended along the tapered edge of the plug and remained after solidification (Figure (iii)). The result is a sensing layer with an optical access port with sloped walls to accommodate incoming light from the spectrometer source without causing ATR ridge side. The opposite side features a completely planar PDMS/ATR surface for bonding to the microfluidic feature side with the ATR being aligned with the characterisation component. The microfluidic feature layer was created with the use of a photoresist mould, which was created using a lamination method as described elsewhere(Figure (i)) . The same PDMS and cross-linker mixture was poured on top of mould cured at 70 °C (Figure (ii)). After removal of the microchannel feature layer from the mould, inlet and outlet access holes were punched (Figure (iii)). Finally, the microchannel layer and the ATR sensing layer were aligned and bonded after exposure to plasma gas (PDC-001, Harrick Plasma, USA) (Figure ). Alignment was achieved by either cross hair marks or the edges of channel in the microchannel feature layer that matched the ATR crystal corners.