We report that proteins in physiological buffers spontaneously attach to polyethylene glycol coated fluorescent quantum dots (QDs) and strongly inhibit the gradual loss of QD fluorescence. This interaction does not strongly depend on the nature or solubility of the proteins tested (amyloid beta, barstar, cytochrome c, bovine serum albumin and lysozyme). Fluorescence correlation spectroscopy shows that each QD acquires a 'corona' of similar to 100 protein molecules, which confers protection against ionic attacks in aqueous solutions. This effect would be important for any biological application, and also offers a potential in vitro assay for soluble proteins. (C) 2007 Elsevier B.V. All rights reserved.