Diatomite is a fossil material made of amorphous porous silica. In this work, polyethylene glycol (PEG)-modified diatomite NPs (PEG-DNPs) are decorated with gold NPs (AuNPs) by one-pot liquid-phase synthesis. Nanocomplexes (PEG-DNPs@ AuNPs), with an average size of about 450 nm, are characterized by dynamic light scattering, electron microscopy, nitrogen adsorption/desorption analysis, UV–vis and photoluminescence spectroscopies. Preliminary studies on the use of the nanocomplex in nanomedicine are also presented. Tests performed incubating PEG-DNPs@ AuNPs in physiological conditions reveal a good stability of material. Cellular uptake of labeled PEG-DNPs@ AuNPs is investigated by confocal microscopy after incubation with human cervix epithelioid carcinoma (HeLa) cells up to 48 h: an efficient cytoplasmic localization is observed. In vitro cytotoxicity of nanocomplexes with a …