Sensitivity and selectivity limits of multiplex nanospr biosensor assays

Gold nanorods (GNRs) of different aspect ratios were fabricated through seed mediated growth; partial and full functionalization procedures were developed to attach antibodies to the GNRs and yield Gold Nanorods Molecular Probes (GNrMPs). Multiplex sensing was achieved by the distinct response of the plasmon spectra of GNrMPs to binding events of up to three targets. A mathematical model formulated adequately described the ligand binding response of GNrMPs and concentrations of multiple targets were determined from experimental data. The GNrMP sensors were found to be highly specific and sensitive and the dynamic response was found to be in the range between 10-9 M and 10-6 M. Comparison of the experimental data with the theoretical model yielded an affinity constant K_a=1.32x10⁷ M ^-1which was in agreement with the IgG-antiIgG binding affinity reported in the literature. The limit of detection (LOD) of GNrMPs was found to be in the low nano-molar range, and is a function of the binding affinity: for a higher probe-target affinity pair the LOD can be expected to reach femto molar levels. This technique can play a key role in developing tunable sensors for sensitive and precise monitoring of biological interactions.