SPECTRAL EXTINCTION OF COLLOIDAL GOLD AND ITS BIOSPECIFIC CONJUGATES

We have prepared gold sols with mean particle diameters d(m) in the ra nge 4 to 50 nm and measured their extinction spectra and size dependen ces of the extinction peak position lambda(max) and value E(max). The measured increasing function lambda(max)(d(m)) displays a pronounced b end 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 par ticle size and axial ratio polydispersity are calculated using Mie's t heory, 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)) depen dences are inconsistent with calculations based on Mie's theory and th e bulk gold optical constants. The most generalized model including th e size dependence of the imaginary part of the dielectric permeability and the size and shape polydispersity gives good agreement with exper imental extinction spectra for 5-, 10-, 24-, and 40-nm sols, as well a s with the measured functions lambda(max)(d(m)) and E(max)(d(m)). Base d on electron-microscopic and spectral data, calibration curves are ob tained for efficient spectrophotometric control over the particle size and for estimation of the amount of restorer essential for the prepar ation of particles of a given size. A simplest two-layer spherical mod el is employed to elucidate the basic changes in sol spectra after con jugation with specific biomacromolecules and to draw some conclusions about the conjugate shell structure. (C) 1996 Academic Press, Inc.