Raman based nanoparticle labels for bioassays

4-Mercaptobenzoic acid coated gold nanoparticles were used as Raman based labels in a standard immunoassay for mouse IgG. The Au nanoparticles used had a diameter of 51 ± 2 nm and were synthesised by the citrate reduction method. Dynamic light scattering and TEM image analysis were used to characterise the nanoparticles. The SERS spectra was recorded on an EZRaman desk-top spectrometer operating with a 670 nm laser, 22 mW output and a 0.30 NA focusing lens. For the immunoassay results the Raman signal intensity was recorded at 1071 cm^-1. This gave a linear correlation to the mouse IgG concentration of slope 1.5Exp¹³ counts per mole, giving an estimated detection limit of 0.1 femtomoles.