Dual-wavelength extinction method for fast sizing of metal nanosphere ensembles.

We propose a simple dual-wavelength extinction (DWE) method to measure the average size of spherical metal nanoparticle (NP) ensembles. Unlike the spectroscopic methods that need to measure the full spectra of scattering and/or extinction to retrieve the NP size, the DWE method can estimate the NP size by measuring the light extinction at only two properly selected wavelengths and thus is useful for fast sizing of metal NP ensembles. The influences of the NP shape deviation and ensemble dispersancy on the measurement accuracy are analyzed and discussed in detail. An empirical correction procedure is established to compensate these influences to further improve accuracy. The feasibility and reliability of the DWE method are corroborated by experimentally measuring several typical gold spherical NP ensembles and comparing the results with those obtained by three other standard methods. The experimental results indicate satisfactory accuracy of the DWE method for measuring gold NPs from 30 to 100 nm by using two measurement wavelengths of 532 and 573 nm. The studies show that the DWE method is efficient, reliable, and easy to implement. It may find wide applications in the metrology of NPs.