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
