  Biomedical applications of plasmonic nanoparticle conjugates need control
over their optical properties modulated by surface coating with stabilizing or
targeting molecules often attached to or embedded in the secondary
functionalization shell, such as silica. Although current numerical techniques
can simulate the plasmonic response of such structures, it is desirable in
practice to have analytical models based on simple physical ideas that can be
implemented without considerable computer resources. Here, we present two
efficient analytical methods based on improved electrostatic approximation
(IEA) and modal expansion method (MEM) combined with the dipole equivalence
method (DEM). The last approach avoids additional electromagnetic simulations
and provides a direct bridge between analytical IEA and MEM models for bare
particles and those with multilayer shells. As simple as the original IEA and
MEM, the developed analytical extensions provide accurate extinction and
scattering spectra for coated particles compared to exact calculations by
separation of variable method and COMSOL. The possibility and accuracy of
analytical models are illustrated by extensive simulations for prolate and
oblate gold and silver nanoparticles with a maximal size of up to 200 nm,
aspect ratio from 2 to 6, and 3-30 nm dielectric coating.
