Plasmonic Absorption in Antigen-Induced Aggregated Gold Nanoparticles: Toward a Figure of Merit for Optical Nanosensors

Gold nanoparticles (Au NPs) have been extensively used for colorimetric and optical detection of various analytes, exploiting the difference in optical properties when the NPs are dispersed or aggregated. The properties of Au NPs-based optical sensors depend strongly on the particle size, spacing, number, and disposition in the aggregate, which explains the different performances reported so far for this class of sensors. Here, we investigated the optical response of a model Au NP immunosensor and the correlation of its plasmonic absorption with the analyte-dependent aggregation state, supported by transmission electron microscopy and numerical calculations. The antigen–antibody system used for this study is the C-reactive protein (CRP)/anti-CRP couple, well-established and of great applicative interest. The results provide several insights into the evaluation of the extent and type of antigen-induced aggregation for receptor-conjugated Au NPs and point toward the identification of a figure of merit of great utility in the development of particle aggregates with the optimal structure for desirable nanosensor response.