  This study demonstrates a rapid, wash-free dip-type fiber optic plasmonic
sandwich (DiP) assay capable of zeptomole analyte detection and 6-orders of
wide dynamic range. The DiP assay is realized on a compact U-bent fiber optic
probe surface by dipping the antibody functionalized probe into a mixture of
sample solution and AuNP labeled reagent. U-bent fiberoptic probes with a high
evanescent wave absorbance sensitivity allow detection of the high extinction
gold nanoparticle (AuNP) labels in terms of a drop in the light intensity,
which is measured with the help of a pair of LED and photodetector (PD). This
simple and low-cost DiP assay gave rise to unprecedented detection limit down
to 0.17 zeptomole of human immunoglobulin G (HIgG) in 0.025 ml buffer solution
within an assay time of 25 min. Further, silver enhancement of AuNP labels over
5 min resulted in a limit of quantitation (LoQ) down to 0.17 zeptomole (~100
molecules in 0.025 ml). The DiP assay with high sensitivity and low detection
limits within an assay time of 30 min demonstrated here could facilitate low
cost point-of-care diagnostics as well as high-throughput systems.
