Low-field magnetic circular dichroism in silver and gold colloidal nanoparticles of different sizes, shapes, and aggregation states

Abstract

Considerable magneto-optical activity has been observed in aqueous solutions of colloidal noble metal nanoparticles (Au and Ag, 2–50 nm in diameter) in a magnetic field as low as 0.5 T parallel to the propagation of the incident light exciting localized surface plasmons in the nanoparticles. The magnetic circular dichroism (MCD) spectra show pronounced Zeeman splitting in the plasmon absorption bands. The observed magneto-optical effects is due to the enhancement of the magnetic Lorentz force for localized surface plasmons in resonantly excited strongly polarizable Ag and Au nanoparticles. The magnitude and the spectral position of the MCD signal depend on the contribution of scattering and absorption components in the extinction spectra of nanoparticles. Addition of pyridine into the colloidal solution of silver nanoparticles causes aggregation of nanoparticles and the appearance of a characteristic intense long-wavelength band in the extinction spectrum. The MCD spectrum also shows signals from short- and long-wavelength components. The possible method for MCD biosensing based on controlled aggregation of plasmonic nanoparticles in the presence of analyte followed by differential MCD detection in the long-wavelength region is discussed.