Naked Eye Immunosensing of Food Biotoxins Using Gold Nanoparticle-Antibody Bioconjugates

Colorimetric immunoassays using gold nanoparticles (AuNP) form a special class of assays where AuNP act as a transducer to monitor binding events between an antigen and an antibody. Indeed, AuNP display unique optical properties that can been exploited in various ways to develop biosensors. One of the most striking properties of colloidal AuNP (and more generally of noble metal nanomaterials) is their extremely high extinction coefficient in the visible range of the spectrum owing to the localized surface plasmon resonance (LSPR) phenomenon. This feature makes AuNP detectable down to very low concentrations by absorption spectroscopy or even by the naked eye. Herein we took advantage of the high detectability of AuNP to design a solid-phase, sandwich-type, colorimetric immunosensor aiming at the detection of staphylococcal enterotoxin A (SEA). A test zone comprised of a polyclonal anti-SEA antibody was created at the surface of amino-functionalized glass slides via high affinity binding to covalently immobilized Protein A. The same antibody was conjugated to 13 nm diameter AuNP to afford the nanoimmunoprobe. After the glass slides were successively exposed to SEA and AuNP-antibody bioconjugate, a distinct red spot appeared at the detection zone from as low as 1 ng SEA in buffer. Quantification of SEA in the 10–500 ng/mL range was established using a benchtop UV–visible spectrometer by integration of the LSPR band centered at 530 nm. Eventually, this biosensor was applied to the detection of SEA in milk with a limit of detection of 1.5 ng/mL.