Multiphoton imaging of tumor biomarkers with conjugates of single-domain antibodies and quantum dots.

ABSTRACT

An ideal multiphoton fluorescent nanoprobe should combine a nanocrystal with the largest possible two-photon absorption cross section (TPACS) and the smallest highly specific recognition molecules bound in an oriented manner. CdSe/ZnS quantum dots (QDs) conjugated to 13-kDa single-domain antibodies (sdAbs) derived from camelid IgG or streptavidin have been used as efficient two-photon excitation (TPE) probes for carcinoembryonic antigen (CEA) imaging on normal human appendix and colon carcinoma tissue. The TPACS for some conjugates was higher than 49,000 GM (Goeppert-Mayer units), considerably exceeding that of organic dyes being close to the theoretical value of 50,000 GM calculated for CdSe QDs. The ratio of sdAb-QD emission to the autofluorescence for 800 nm TPE was 40 times higher than that for 457.9 nm one-photon excitation. TPE ensures a clear discrimination of CEA-overexpressing tumor areas from normal tissue. Oriented sdAb-QD conjugates are bright specific labels for detecting low concentrations of antigens using multiphoton microscopy. FROM THE CLINICAL EDITOR This study demonstrates carcinoembryonic antigen imaging on normal human appendix and colon carcinoma tissue utilizing CdSe/ZnS quantum dots conjugated to streptavidin or to 13-kDa single-domain antibodies as efficient two-photon excitation probes.