SERS Biotags (SBTs) for the Quantitative Ratiometric Discrimination between Noncancerous and Cancerous Prostate Cells
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SERS Biotags (SBTs) for the Quantitative Ratiometric Discrimination between Noncancerous and Cancerous Prostate Cells Alessia Pallaoro1, Gary Braun1, Martin Moskovits1 1 Chemistry and Biochemistry, University of California Santa Barbara, Santa Barbara, CA, United States. ABSTRACT We report on a multiplexed, ratiometric method that can confidently distinguish between cancerous and non-cancerous epithelial prostate cells in vitro, as demonstrated by a double blind experiment. The technique is based on bright SERRS biotags (SBTs) infused with unique Raman reporter molecules, and carrying cell-specific peptides. Two sets of SERRS biotags were used. One targets the neuropilin-1 (NRP) receptors of cancer cells through the RPARPAR peptide. The other functions as a positive control (PC) and binds to both non-cancerous and cancer cells through the HIV derived TAT peptide. Averaging the SERRS signal over a given cell yielded an NRP/PC ratio from which a robust quantitative measure of the overexpression of the NRP-1 by the cancer cell line was extracted. The use of a local, on-cell reference produces quantitative, statistically robust measures of overexpression independent of such sources of uncertainty such as variations in the location of the focal plane, the local cell concentration and turbidity. INTRODUCTION Early and rapid identification of malignant cells is a central goal in cancer research (ideally free-flowing in bio fluids such as urine [1] or blood [2,3]). Normal Raman signals from cells and their infrared (FTIR) absorbance patterns have been previously used to differentiate between cancer and normal cells [4,5], while surface-enhanced Raman spectroscopy (SERS) has been used as an alternative immunohistochemistry tool [6,7] for the detection of biomarkers in biological fluids or in vivo [8], and for cancer detection from blood [9]. However, Raman and FTIR signals measured directly from cells are typically much weaker than those that are measurable with bright labels. SERS biotags (SBTs) can be routinely synthesized [10] and simultaneously excited with a single, very low intensity laser source, making the determination of the relative contribution of the individual SBTs to the overall spectrum tractable. Importantly for biomedical applications, SERS employs tissue-penetrating lasers in the red to near-infrared range resulting in low autofluorescence. Here, we describe a multiplexed, ratiometric method that can confidently distinguish between cancerous and noncancerous epithelial prostate cells in vitro based on neuropilin-1 (NRP-1) overexpression, as demonstrated by a double blind experiment. SERS biotags are made from polymer-encapsulated silver nanoparticle dimers infused with unique Raman reporter molecules, and carry peptides as recognition moieties. Two sets of SBTs were used: One targets the NRP-1 receptors of cancer cells and the other functions as a positive control (PC). Point-by-point 2D Raman ratio maps were constructed with subcellular resolution from cells simultaneously incubated with the two sets of S
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