A thiol terminated ethylene oxide oligomer and the method of substituting the oligomer onto a gold nanocluster surface using a thiol-exchange reaction to form a charge-neutral, non-ionizable, water soluble, ethylene oxide protected gold nanocluster. The thiol terminated ethylene oxide oligomer has the formula CH.sub.3(OCH.sub.2CH.sub.2).sub.xSH where x varies from one to ten. Also disclosed is the related method of making the thiol terminated ethylene oxide oligomer.