Lab on fiber nano-cavity integrated with charge responsive microgels for biosensing

The development of photonic-based point of care (POC) devices requires the use of compact integrated bio- sensing platforms enabling reliable detection performances in small volumes. The sensing system should be designed to overcome limitations of benchtop technologies in terms of portability and the easy-of-use. The Lab- on-Fiber (LOF) technology is continuously trying to develop miniaturized, lightweight, and light-coupled optr- odes with unprecedented detection performances associated with portable, plug-and-play and user-friendly features. The sensing platforms, yet successfully developed in planar configurations, are being reimagined and suitably scaled down to be directly integrated onto optical fibers. In the wake of this ‘chip-to-fiber translation’ vision, we have realized a micro-sized optical cavity made of smart materials based on synthetic microgels and integrated on the top of an optical fiber. As a specific case study, the optrode has been here tested within a ‘general purpose’ detection assay for biochemical applications. Specifically, we detected nanomolar concentra- tions of homopolyarginines and positively charged linear polymers that are frequently exploited as surplus in indirect binding assays. The high degree of miniaturization of our probe enables a significant improvement in the minimum detectable concentration (at least 1 nM) compared to those achieved with similar devices having planar configurations. The probe sensitivity and overall performance are strongly influenced by the molecular weight and number of positive charges of the polymer. Overall, our findings represent a step forward in establishing LOF optrodes as sensing units for optical POC, further enlarging their applicability for the detection of a broad range of relevant biomarkers.