Silicon nanoparticle suspension, and labeling agent for biological substance

Disclosed is a silicon nanoparticle suspension which is applicable in vivo because any fluorinated compound is removed therefrom, and which has excellent brightness. Also disclosed is a labeling agent for a biological substance, which is safe for living bodies and can be produced by using the silicon nanoparticle suspension. The silicon nanoparticle suspension is produced through the steps of: producing a silicon oxide film containing a silicon atom on a base plate; subjecting the silicon oxide film to a heat treatment (annealing); subjecting the resulting silicon oxide film to the treatment with hydrofluoric acid; and removing a fluorine ion from hydrofluoric acid and separating the base plate, wherein the total content of fluorinated compounds in the silicon nanoparticle suspension is 100 ppm or less in terms of a fluorine ion content.