Optical biosensing using particle diffusometry on thermoplastic microfluidic chips bonded using direct and indirect chip bonding methods

Thermoplastics offer a scalable and cost-effective platform for fabricating microfluidic devices for point-of-care (POC) diagnostics. Among them, cyclic olefin polymer (COP) stands out due to its exceptional optical clarity, making it suitable for fluorescence-based biosensing. However, bonding COP layers without compromising mechanical strength or imaging quality remains a challenge. This study systematically evaluates four bonding techniques: thermal bonding, solvent bonding, UV-curable adhesives, and pressure-sensitive adhesives (PSA), using standardized 180-degree peel tests to quantify bond strength. PSA bonding exhibited the highest adhesion (0.2–0.8 N/mm), while thermal and solvent bonds were notably weaker (<0.05 N/mm). Plasma treatment improved bond strength and uniformity across most methods. We demonstrate the utility of these bonding techniques by fabricating multilayer microfluidic chips compatible with particle diffusometry (PD), an optical biosensing method that detects Vibrio cholerae DNA via changes in nanoparticle motion. Chips were assessed for both mechanical robustness and image intensity suitability for smartphone-based PD measurements. This work provides practical design criteria for selecting prototyping-compatible bonding strategies in the development of low-cost, optically clear microfluidic diagnostic platforms.