smartPCF: Smartphone-integrated tapered optofluidic photonic crystal fiber-based fluorescence biosensor

Optofluidic photonic crystal fibers (PCFs) offer strong potential for optical biosensing due to their periodic air-hole microstructure, which allows the incorporation of biological liquid samples along the fiber length in close proximity to the core. This enhances the light-analyte interaction, thereby enabling highly sensitive biomolecular detection. However, most of the optofluidic PCF-based biosensing systems remain confined to laboratory environments because of the requirement for complex optical alignment, sensitive light coupling and bulky external imaging modules, limiting their translation to portable diagnostic platforms. In this work, we present a compact optofluidic PCF fluorescence biosensing platform, smartPCF that integrates a tapered suspended core PCF (SuC PCF) with a simplified optical configuration and smartphone-based imaging and computation. The tapered SuC PCF improves measurement reliability by providing robust and easy light coupling while enhancing optical confinement and fluorescence. The smartphone camera and light emitting diode (LED) flashlight provide real-time visualization and illumination for precise optical alignment. Following the alignment, fluorescence excitation is achieved using a compact laser diode, and emission from the fiber core is collected through the same optical path in a backscattering configuration. A custom-developed mobile application enables automated image processing, fluorescence quantification, and on-device evaluation of sample concentration. The modular magnetic fiber holder and shared optical architecture enable rapid installation and removal of the PCF without realignment of the primary imaging module, supporting plug-and-play operation. As a proof-of-concept, using smartPCF, we demonstrate the detection of interleukin-4 (IL-4) protein, an inflammatory marker over a concentration range of 0.05 nM to 50 nM.