Signal amplification in a surface acoustic wave (SAW) immunosensor is demonstrated by increasing mass and/or viscoelasticity on a delay line sensing surface with gold nanoparticles (AuNPs) for monitoring of airborne house dust mite (HDM) allergens. Streptavidin conjugated AuNPs with size of 20 nm are affinity-bound to biotinylated detection antibodies (dAb) that are already on the sensor surface together with capture antibody (cAb) and group 1 HDM allergens of Dermatophagoides farinae (Der f 1) as a result of a sandwich assay. This immunocomplex except for immobilized cAb on the sensor surface is then removed by pH change for regeneration of the surface. Surface modification of the SAW immunosensor with a self-assembled monolayer of pH resistant protein (ORLA85 protein) allows to repeat a process including immunoassay and surface regeneration for a repetitive measurement of Der f 1. It is revealed that capping a surface of AuNP with a monolayer of (11-mercaptoundecyl)tri(ethylene glycol) (3PEG-thiol) reduces non-specific binding to the sensor surface, and aggregation among AuNPs. The sensor output is amplified by a factor of 3 with respect to that obtained without AuNP when a molar concentration of AuNPs is about 3-fold higher than that of dAb. A limit of detection for Der f 1 is 2.5 ng/mL (equivalent to 100 pM), which is about 2.5-fold improvement from the sandwich assay without AuNPs. Also highly reproducible sensor outputs along with efficient regeneration through successive measurement demonstrates the capability of repetitive measurement of Der f 1. These results suggest that use of AuNPs under an optimum condition in the SAW immunosensor is expected to realize a sensitive and repetitive measurement of HDM allergens, which is useful for airborne HDM monitoring system.
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