Supplementary information
Synthesis of gold nanoparticles
Gold NPs were synthesized in an aqueous solution by the reduction of chloroauric acid (HAuCl 4) with
trisodium citrate (Na3C6H5O7). 4 ml of 5 mM chloroauric acid (Aldrich) was added to 100 ml of deionized (DI)
water. The solution was rapidly heated under constant stirring to boil whereupon 5.5 ml of 25 mM trisodium citrate
with a molar ratio of ca. 7:1(Na3C6H5O7:HAuCl4) was added to the solution. Gold nanoparticles were also
synthesized by addition of 5.5ml of chitosan (0.1% solution prepared in 1% acetic acid) for the reduction of
chloroauric acid. For capping with poly (diallyl-dimethylammonium)-chloride (PDDA), 0.28 ml of PDDA (35 wt. %
in water) was added to the solution and stirred for 30 minutes.
Synthesis of silica nanoparticles
Silica nanoparticles (NP’s) were synthesized by the stober method that involves the hydrolysis and condensation of
tetraethyl-orthosilicate TEOS (Si(OC2H5)4) in ethanol. In brief, 100ml of ethanol was mixed with 40 ml of deionized
(DI) water and 18ml of TEOS (99.99%) which was continuously stirred for 30 minutes at controlled temperature of
35°C. Another solution was prepared by adding 14ml of ammonia (25%) to 14ml of DI water which was then added
drop wise to the TEOS mixture under constant stirring. The appearance of a gray color was an indication that silica
NPs were formed. Upon the completion of the reaction the mixture was centrifuged at 4,000 rpm for 20 minutes
following which the supernatant was recovered. An equal volume of DI water was added to this solution and heated
at 90°C to evaporate any remnant ethanol in the aqueous solution. For capping with PDDA, 0.558 ml of PDDA
solution (35 wt. % in water) was added to the silica nanoparticles colloid and stirred for 30 minutes. For capping
with chitosan, 11 ml of chitosan solution (0.1% solution prepared in 1% acetic acid) was added to silica
nanoparticles colloid.
Fig. 1 a) Transmission electron micrograph (TEM) of gold NP’s (inset) with particle size distribution showing
average particle size = 20 nm (b) TEM of silica NP’s (inset) with particle size distribution showing average particle
size = 30 nm
Fig. 2 Optical absorption spectra of the colloidal suspension of PDDA capped silica
nanoparticles ~ 30 nm (solid) and citrate reduced gold nanoparticles ~ 20 nm (dashed)