Surface-enhanced Raman scattering (SERS) is under active investigation in biomedical diagnostics due to its high sensitivity, increased levels of multiplexing, robustness, and ability to perform detection in blood and other biological matrices. SERS has been successfully applied for labeling cells 1 and tissues, 2 for multiplexed biomarker labeling to monitor apoptotic processes, 3 and for real-time monitoring of single live cell signaling processes. 4We have developed SERS tags (Nanoplex biotags, a trademark of Oxonica Inc.), comprised of one or more SERS-active metal nanoparticles (Au) and a sub-monolayer of reporter molecules adsorbed to the metal surface, all encapsulated in a protective and functionalized silica coat ( Figure 1A). 5 Nanoplex biotags have been successfully used in biological applications. 6,7 Here we describe a novel application of Nanoplex biotags for the direct detection of rare cancer cells in whole blood.Malignant cells are shed and circulate in the bloodstream of patients with solid tumors. 8 Since the "seed and soil" theory for circulating tumor cells (CTCs) was hypothesized 9 and confirmed, 10 two major approaches, based on polymerase chain reaction or cytometric methods (such as the CellSearch system), have been established for CTC detection. 8,11 However, high instrument cost and labor-intensive and time-consuming procedures remain a major concern and hamper their use in clinical diagnostics. Taking advantage of the intrinsic properties of the SERS tags, we have developed a novel, homogeneous, no-wash assay platform that overcomes the current assay limitations. We use magnetic beads for CTC capture and Nanoplex biotags for rapid and sensitive detection directly in human whole blood. Scheme 1 illustrates the concept in which magnetic beads, conjugated to an epithelial cell-specific antibody (epithelial cell adhesion molecule, anti-EpCAM), and the SERS tags, conjugated to an anti-her2 antibody (human epidermal growth factor receptor-2), bind to a tumor cell. Since the breast cancer cell is of epithelial origin, the magnetic bead-EpCAM antibody will specifically bind to this tumor cell but not regular circulating blood cells. Since the her2 receptor is highly expressed on the breast cancer cell membrane, the anti-her2-SERS tag will specifically recognize these tumor cells. By adding the magnetic bead-EpCAM and SERS tag-her2 conjugates to a patient's blood sample, circulating breast cancer cells (CTCs) can be detected rapidly and with good sensitivity in the presence of whole blood.In a proof-of-concept experiment, the breast cancer cell line SKBR3, expressing high levels of her2 receptor on the cell surface, 12 was used as a model target. After a 30 min incubation of SKBR3 cells with magnetic bead-EpCAM and SERS-her2 conjugates, small volumes of the reaction mixtures were loaded on a glass slide. Bright-field microscopy imaging showed specific binding of the magnetic beads-EpCAM to the tumor cells ( Figure 1B), and anti-her2
