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This invention provides a class of nanoreporter particles stabilized by surface-bound polyampholytes. Polyampholytes are ionic polymers containing both anionic and cationic groups, unlike polyelectrolytes which contain either one or the other. This diversity of ionizable groups allows polyampholytes to maintain some degree of charge across a wide range of salinity and pH. When dispersed in water, the like-charged polyampholyte-coated nanoreporters electrostatically repel one another and resist aggregation, thereby stabilizing the suspension.

In tests using both synthetic and natural reservoir brines, the novel nanoparticles described in this invention have achieved long-term colloidal stability at salinities as high as 120,000 mg/dm3 and temperatures as high as 90 ℃ -- roughly twice the stability limit previously achieved.