Ultrafast Detection of Small Molecules Using a U-Shaped Transmission Terminal and Straight Plasmonic Sensing Region Fiber Probe Biokinetic Platform: A Case Study of Label-Free Hcy Detection via Indirect Competitive Immunoassay

Immunoassays face significant challenges, including the precise measurement of small molecules, rapid testing, and background interference. This study constructs a biokinetic sensing platform using a late-model plasmonic fiber probe that incorporates an indirect competitive immunoassay system on its surface for the rapid detection of target small molecules. Our plasmonic fiber probe transforms the conventional online transmission-type fiber surface plasmon resonance (SPR) into a probing structure with a U-shaped fiber termination, preserving the high performance of a straight fiber SPR sensing region and enabling direct sample detection via an insertable probe. Taking the detection of homocysteine (Hcy), a key indicator of cardiovascular diseases, as an example, our fiber probe has achieved rapid immunoassay for small molecules within 10 s while eliminating background interference by measuring target molecular binding rates. The probe sensitively identifies concentration gradients based on binding rates, even in serum, demonstrating high selectivity. The Hcy detection range is 0 to 100 μM, covering low to high abnormal concentrations typically observed in humans, with a detection limit of 2.23 nM. This fiber-optic kinetic method provides rapid, accurate Hcy testing; and multichannel networked potential for various small molecules, macromolecules, and genes detection.