Critical Analysis of the Absorption/Adsorption of Small Molecules by Polydimethylsiloxane in Microfluidic Analytical Devices

Polydimethylsiloxane (PDMS) devices are ubiquitously used in microfluidics. However, PDMS is hydrophobic and highly porous, which potentially leads to the absorption of small hydrophobic molecules thus altering the concentration of analytes and affecting result interpretation. Here, we evaluated the effect of solute/solvent pairings, concentration, and residence time on the absorption of small molecules by PDMS. Critically, it was at lower concentrations that absorption by PDMS became more significant, with the relative concentration of rhodamine-B at 20 μM remaining around 90% when compared to just 10% at 1 μM. Several PDMS modifications were tested, with oxygen plasma oxidation followed by immersion in Pluronic® F127 appearing to be the most stable, with the water contact angle remaining under 50° for 7 days. A surface treatment using paraffin, though leading to increased hydrophobicity, reduced the absorption of Nile red when compared to unmodified PDMS. A sol-gel treatment with tetraethoxysilane hindered the diffusion of rhodamine-B into the elastomer. However, despite the general decrease in contact angle, none of the modifications tested led to a significant reduction in the absorption of small hydrophobic molecules. Overall, this study indicated that a careful case-by-case evaluation is required to determine if absorption by PDMS can alter experimental outcomes. Ultimately, where it is deemed to be problematic, alternative materials should be considered for device fabrication