Plasmonic flow cytometry by immunolabeled nanorods.

Fluorescence-based flow cytometry measures multiple cellular characteristics, including levels of receptor expression, by assessing the fluorescence intensity from a population of cells whose cell surface receptors are bound by a fluorescently labeled antibody or ligand for that receptor. Functionalized noble metal nanoparticles provide a complementary method of receptor labeling based on plasmonics for population analysis by flow cytometry. The potential benefits of using plasmonic nanoparticles to label cell surface receptors in flow cytometry include scattering intensity from a single particle that is equivalent to fluorescence intensity of 10⁵ fluorescein molecules, biocompatibility and low cytotoxicity, and nonquenching optical properties. The large spectral tunability of nanorods also provides convenient access to plasmonic markers with peak surface plasmon resonances ranging from 600 to 2,200 nm, unlike gold nanosphere markers that are limited to visible wavelengths. Gold nanorod-based plasmonic flow cytometry is demonstrated herein by comparing the scattering of cells bound to anti-epidermal growth factor receptor (EGFR)-conjugated nanorods to the emission of cells bound to anti-EGFR-conjugated fluorescent labels. EGFR-expressing cells exhibited a statistically significant six-fold increase in scattering when labeled with anti-EGFR-conjugated nanorods compared with labeling with IgG1-conjugated nanorods. Large scattering intensities were observed despite using a 1,000-fold lower concentration of nanorod-conjugated antibody relative to the fluorescently labeled antibody.