There is described a process for labeling an article and/or document for authentication purposes. The article or document (such as BOPP and/or cellulose film) incorporates therein particles having a taggant bound to the surface thereof. The preferred taggant is a DNA strand of at least eight (preferably 25) base pairs long attached to inorganic particles by the addition reaction between an acrylate group and a polar group such as amino or hydroxy. Alternatively silica particles can be modified by reacting first with aminopropyltriethoxy silane and then a reagent selected from: a diacid with a polyethylene glycol spacer group (e.g. polyethylene glycol dicarboxymethyl) succinic anhydride; and/or a diisothiocyanate (e.g. 1,4-phenylene diisothiocyanate (PDITC)) to form functional silica particles capable of reacting directly with a terminal amide and/or hydroxy group of an oligonucleotide such as DNA. The tagged particles can be detected by use of a fluorescent probe which hybridises with the DNA sequence selected tag the article or document.