sorption-based optical signal. The general biocompatibility of gold, coupled with extensive prior medical applications of gold colloid, suggests a more straightforward regulatory path toward ultimate clinical use than for many other nanomaterials currently under development.

For more than 50 years, cancer was the second leading cause of death in the United States, accounting for more than 25 percent of deaths in the population. However, in the past two years, death from cancer has exceeded deaths from heart attacks, and cancer has become the primary cause of deaths in the United States. Early detection is recognized as a highly effective approach to reducing the morbidity and mortality associated with cancer. When diagnosed at an early stage when the cancer is still localized and risk for metastasis is low, most cancers are highly treatable and prognoses are favorable. However, if cancer is not diagnosed until metastasis to distant sites has already occurred, five-year survival is poor for a wide variety of organ sites (Table 1) (American Cancer Society, 2006). Thus, there is a significant clinical need for novel methods of early detection and treatment with improved sensitivity, specificity, and cost effectiveness.

In recent years, a number of groups have demonstrated that photonics-based technologies can be valuable in addressing this need. Optical technologies promise high-resolution, noninvasive functional imaging of tissue at competitive costs. However, in many cases, these technologies are limited by the inherently weak optical signals of endogenous chromophores and the subtle spectral differences between normal and diseased tissue.

In the past several years, there has been increasing interest in combining emerging optical technologies with novel exogenous contrast agents designed to probe the molecular-specific signatures of cancer to improve the detection limits and clinical effectiveness of optical imaging. For instance, Sokolov et al. (2003) recently demonstrated the use of gold colloid conjugated to antibodies to the epidermal growth factor receptor as a scattering contrast agent for biomolecular