In a 2009 JACS editorial (10.1021/ja9038104), Thomas E Mallouk and Peidong Yang wrote:
Although the use of colloidal particles of metals and semiconductors as pigments dates back many centuries, and although the recipe for stable 6 nm diameter particles of gold (“Ruby gold”) was famously devised by Faraday in 1857, the unique properties of nanomaterials and their promise for applications in biochemistry, cell biology, and medicine have only recently been appreciated. Prior to the 1990s, the principal role of inorganic colloids in biological research was as high-contrast stains for electron microscopy. A paradigm shift occurred in 1996, when Mirkin, Alivisatos, and co-workers coupled metal nanoparticles to DNA. Their experiments demonstrated not only that DNA could be used for the organization of nanostructures, as had been suggested in earlier experiments by Seeman, but also that nanoparticles were highly sensitive spectroscopic reporters for the base-pairing of DNA
This is a commonly held view and there is no doubt that the 1996 paper is an important milestone.
Yet, beyond electron microscopy, gold nanoparticles had been introduced as a diagnostic tool based on a color change 84 years before Mirkin, Alivisatos and co-workers paper. In 1912, Carl Friedrich August Lange introduced gold nanoparticles to detect diseases . Writing a few years later in the Journal of Experimental Pathology , John Cruickshank, MD, writes:
It occurred to Lange to examine syphilitic and normal sera by this method, and later to apply the reaction to spinal fluids, as the amount of globulin and albumen was known to vary in different pathological conditions of the central nervous system. Lange found, however, that certain spinal fluids, in addition to exhibiting protective effect on gold colloid, had also unexpected precipitating properties. The spinal fluids of cases of dementia paralytica in particular showed this precipitating property, and as a result of the examination of a series of cases Lange recommended the test for the diagnosis of this disease.
For several decades, the Lange test based on gold nanoparticle color change was used in clinics as reported in numerous papers. It also motivated the synthesis of suitable nanoparticles, e.g. “The Preparation and Standardization of Colloidal Gold for the Lange Test” in 1931 by Jocelyn Patterson.
 Lange, C. Die Ausflockung kolloidalen Groldes durch Cerebrospinalflussigkeit bei luetischen Affecktionen des Zentraluerxensystem,^’ Zeitschr. f. Chemotherap., 1912, 1, 44
 Cruickshank, J. Br J Exp Pathol. Apr 1920; 1(2): 71–88
 Br J Exp Pathol. Jun 1931; 12(3): 143–146. PMCID: PMC2048186