Towards point-of-care tests: Comparative study of different antibody densities for plasmonic surfaces to achieve custom IL-18 biosensors

This work focuses on molecular recognition layer (MRL) characteristics because, in carbodiimide-mediated receptor immobilization on carboxyl-terminated self-assembled monolayers, the antibody concentration is crucial, as it can modulate surface crowding, probe orientation, and the fraction of binding-competent sites. More specifically, how changes in antibody concentration during the functionalization process influence the resulting MRL and whether these features translate into measurable differences in surface plasmon resonance (SPR) biosensor performance of disposable SPR-plastic optical fiber (POF) biosensors for IL-18 detection were investigated. With this aim, gold nanofilms on planar substrates and SPR plastic optical fiber (POF) platforms were functionalized and studied for IL-18 detection. Using planar chips, MRLs were first characterized by fluorescence microscopy and time-of-flight secondary ion mass spectrometry (ToF-SIMS), which indicated a marked dependence of the exposed antibody domains on the loading conditions. Finally, the impact on binding sensitivity for IL-18 detection was assessed using SPR-POF probes as a proof-of-concept. Two SPR-POF biosensors, obtained using different antibody concentrations, are evaluated for selectivity and sensitivity. Both produced negligible responses to interferents and quantified samples in simulated diluted serum. However, different dose-response curves changing the antibody concentration are achieved, showing that at low antibody concentration delivered substantially enhanced low-concentration sensitivity (0.25 nm/pM vs 0.053 nm/pM) and a lower detection limit (1.06 pM vs 10.8 pM), with an operating range shifted toward lower concentrations (1.06-25 pM vs 10.8-250 pM). Overall, this work links surface-chemistry evidence to functional SPR outcomes, showing that, for the disposable IL-18 SPR-POF biosensors, Ab concentration during carbodiimide coupling is a practical parameter to tune sensing performance, with the potential to reduce the cost per chip and improve SPR-biosensor performance simultaneously, characteristics useful for point-of-care tests.