Herein we describe a new class of multifunctional gold nanoprobes for ultra-sensitive optical detection of reactive oxygen species (ROS) and hyaluronidase (HAdase). The nanoprobes were fabricated by end-immobilizing near-infrared fluorescence (NIRF) dye labeled hyaluronic acid (HA) onto the surface of gold nanoparticles (AuNPs). The nanoprobes effectively induced nanoparticle surface energy transfer (NSET) between NIRF dyes and AuNPs. When the surface immobilized HA was cleaved by ROS and HAdase, strong fluorescence recovery signals were attained with extreme sensitivity. In live animal models of rheumatoid arthritis (RA) and metastatic tumor, local arthritic inflammation and tumor sites were clearly identified upon systemic injection of the nanoprobes. These results suggest that the gold nanoprobes can be exploited not only as in vitro molecular and cellular imaging sensors for ROS and HAdase, but also as in vivo optical imaging agents for detection of local HA degrading diseases such as RA and tumor. (c) 2008 Elsevier Ltd. All rights reserved.