Scanning Electron Microscopy and Surface Enhanced Raman Spectroscopy Correlation Studies of Functionalized Composite Org
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Scanning Electron Microscopy and Surface Enhanced Raman Spectroscopy Correlation Studies of Functionalized Composite Organic-Inorganic SERS Nanoparticles on Cancer Cells Ai Leen Koh 1 and Robert Sinclair 1 1 Materials Science and Engineering Department, Stanford University, Stanford CA 94305, USA ABSTRACT Composite Organic-Inorganic Nanoparticles (COINs) are a novel type of surfaceenhanced Raman (SER) scattering nanoparticle formed by aggregating inorganic silver particles in the presence of a chosen organic molecule with a distinct Raman fingerprint. Binding between antibody-functionalized COINs and cells is detected primarily using Raman spectroscopy, which measures spectral shifts of the excitation light due to inelastic scattering. It has been suggested that the amount of antibody-conjugated COINs binding on cells will vary according to the antigen-expression levels in cells and will lead to changes in measured SERS intensities. COINs functionalized with antibodies CD54 and CD8 were conjugated to U937 and SupT1 cancer cells and investigated in this study. SERS intensity measurements were obtained from each of the four sample variants and normalized against control samples comprising nonantibody-functionalized COINs with cells. The amount of COINs binding on cells was determined using scanning electron microscopy (SEM) and correlated with the SER spectroscopy intensity. Although we found a positive correlation between the number of COINs binding to cells and their respective SERS intensity, this relationship is not one-to-one, nor does it appear to be linear. We demonstrated that SEM imaging and SER spectroscopy can complement each other to provide information about COINs binding onto cancer cells. INTRODUCTION Applications of nanoparticles for early cancer detection involve functionalizing them with antibodies to target specific biological analytes such as antigens in cancer cells. Binding between nanoparticles and cells is detected by physical sensing principles such as magnetism and Raman scattering [1-7]. It is also important to assess if and where the nanoparticles are binding to the cells and direct microscopy can play a significant role in this regard because it can resolve objects to the (sub)nanometer level and offers the ability to directly image nanoparticles and their attachment to cells. This paper represents an extension of our work in [8], where we described the use of the backscattered electron (BSE) detector in the scanning electron microscope (SEM) to effectively locate CD54-antibody-functionalized composite organic-inorganic nanoparticles (COINs) on U937 leukemia cells. In this present work, COINs were functionalized with different antibodies and conjugated to U937 and SupT1 leukemia cells. We go a step further to quantify the amount of COINs on cells using the SEM data and correlate them to their SER spectra. To our best knowledge, this is the first time such a study has been performed. COINs [9-11] are a novel type of surface-enhanced Raman scattering (SERS) nanoclusters formed by aggregating
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