A portable acoustic biosensing platform combined with paper-based capillary fluidics for the rapid detection of antibodies in serum.

We report the development of a sensitive biosensing platform based on a shear-horizontal surface acoustic wave (SH-SAW) device and paper fluidics, with the potential to be used outside centralized laboratory settings. Systematic research on the biorecognition surface, blocking agent, fluidics and measuring unit allowed us to transform a laboratory-based method into a field-deployable device. As a proof-of-concept, the platform was used for the detection of SARS-CoV-2 anti-spike antibodies on a surface-immobilized spike protein, tested in both simulated and human blood serum samples. A poly-L-lysine (PLL) layer was selected as a biocompatible surface for spike protein immobilization; the polymer layer can be easily removed through gentle mechanical rubbing, allowing regeneration and multiple uses of the sensing device. This surface, combined with novel paper-based capillary fluidics, enabled real-time monitoring of spike antibody binding via acoustic wave phase measurements in the range of 1-100 nM antibodies in 1% v/v serum. Further acoustic wave amplitude amplification and a tenfold improvement in the detection limit (0.1 nM) were achieved by the use of gold nanoparticles conjugated with a secondary antibody. This optimized assay was successfully evaluated in a small pilot clinical study of 20 patient samples. Our new SH-SAW immunosensor exhibited sensitivity and specificity comparable to commercial systems with standard fluidics and instrumentation; importantly, its limit of detection is better than the clinically relevant value of ∼11 RU mL-1. This portable, low-cost platform, combining a pocket-size network analyzer with disposable paper fluidics and a regenerable sensing surface, offers a promising solution for quantitative antibody detection near or at the point-of-care.