Spectral extinction of colloidal gold and its biospecific conjugates

We have prepared gold sols with mean particle diameters d(m) in the range 4 to 50 nm and measured their extinction spectra and size dependences of the extinction peak position lambda(max) and value E(max). The measured increasing function lambda(max)(d(m)) displays a pronounced bend near the particle diameter d(m) similar to 10 nm, where the value of lambda(max) sharply decreases with reduction in the particle size. To explain these findings, the extinction spectra of sols with the particle size and axial ratio polydispersity are calculated using Mie's theory, the T-matrix method, and various experimental sets of the bulk gold optical constants modified with regard to size-limiting effects. It is shown that the measured lambda(max)(d(m)) and E(max)(d(m)) dependences are inconsistent with calculations based on Mie's theory and the bulk gold optical constants. The most generalized model including the size dependence of the imaginary part of the dielectric permeability and the size and shape polydispersity gives good agreement with experimental extinction spectra for 5-, 10-, 24-, and 40-nm sols, as well as with the measured functions lambda(max)(d(m)) and E(max)(d(m)). Based on electron-microscopic and spectral data, calibration curves are obtained for efficient spectrophotometric control over the particle size and for estimation of the amount of restorer essential for the preparation of particles of a given size. A simplest two-layer spherical model is employed to elucidate the basic changes in sol spectra after conjugation with specific biomacromolecules and to draw some conclusions about the conjugate shell structure. (C) 1996 Academic Press, Inc.