We propose a highly sensitive immunosensor based on the Localized Surface Plasmon Resonance (LSPR) for 17β-estradiol (E2) quantification in water. E2 molecules are recognized by polyclonal antibodies immobilized onto gold nanoparticles (AuNPs) and act as linkers that cause nanoparticles aggregation. This leads to the change in the optical properties of the solution visible even by naked eyes. The aggregates were characterized by Dynamic Light Scattering (DLS) and Scanning Transmission Electron Microscopy (STEM) that provided an accurate assessment of the inter-particle distance. The finite-difference time-domain (FDTD) method applied to a Mie problem like workspace allowed us to describe the optical behaviour of the AuNP aggregates with excellent agreement between the experimental and numerical results. The limit of detection (LOD), without any preconcentration step, is 3 pg/mL (11 pM), whereas the detection range extends over five decades up to 105 pg/mL. The proposed E2 immunosensor was tested in tap water, where no significant cross-reaction signal was detected by similar molecules (testosterone, progesterone, estrone and estriol). The device described here represents a significant improvement of low E2 levels determination in terms of affordability, time and measuring simplicity, making it suitable for environmental applications
