Exploiting silver nanoplates as colorimetric label in Lateral Flow Immunoassay

The Lateral Flow Immunoassay (LFIA) is one of the most successful paper-based (bio)analytical technique for the on-site detection of target substances, where the sample is added on a standalone device and the result is obtained in a few minutes. This analytical platform has been commonly known for the pregnancy tests and now is taking the footlights for the COVID-19 rapid tests. Typically, colorimetric LFIAs employ spherical gold nanoparticles (s-GNPs) as labels [1]. This choice is mainly due to their simple preparation, strong surface plasmon resonance (SPR) in the visible spectroscopy region (around 520 nanometers), high extinction coefficients, and easy conjugation to antibodies (Ab) that are the most extensively used recognition elements in LFIA [2]. As a consequence, the use of s-GNPs allows to evaluate the assay results just by the naked eye without the use of additional instrumentation. Besides common s-GNPs, during the last decades, the use of other colorimetric labels have been exploited to try to improve the analytical performances of the assay and its versatility. The use of other noble metal nanoparticles that shares some of the desirable peculiarities of s-GNPs can be considered a choice worthy of further study. In this work, different silver nanoplates (AgNPs), previously applied to develop a plasmon sensor for the detection of Hg2+ [3], have been evaluated as an alternative label in LFIA. In order to assess their suitable use, a preliminary study on the passive adsorption of antibodies on the AgNPs as a function of the medium and the amount of Ab used for conjugation with AgNPs was conducted. Both the stability of the sol and the performance of the labeled antibodies in model LFIAs, in terms of signal intensities and analytical sensitivity, were considered.