Molecular imaging of epidermal growth factor receptor in live cells with refractive index sensitivity using dark-field microspectroscopy and immunotargeted nanoparticles.

We present a study using plasmonic nanoparticles (NPs) to image epidermal growth factor receptor (EGFR) in live cells. Through detailed analysis of the NP scattering spectra, we determine the intracellular refractive index (RI) within attoliter volumes inside of the living cells. Molecular imaging is demonstrated using anti-EGFR labeled gold nanospheres delivered to cancer cells that overexpress EGFR, with targeted binding confirmed by appropriate control experiments. RI determination is achieved by measurement of the bound NPs' scattering spectra, acquired using a precision dark-field microspectroscopy system and through careful characterization of the NP properties throughout the immuno-labeling procedure. To demonstrate the effect of receptor-mediated uptake, the data are compared to similar spectral measurements using antibody-free NPs, taken up by the cells through nonspecific mechanisms. In these experiments, NP aggregation introduces interparticle effects in the scattering spectra, suggesting that EGFR-mediated internalization of NPs may provide an advantage for maintaining NP isolation upon uptake. The results of this study show the potential utility of dark-field microspectroscopy and labeled NPs for live cell imaging. By demonstrating RI sensitivity over nanometer length scales, this study also presents a potential new avenue for assessing the structure and dynamics of live cells.