Dark field and coherent anti‐stokes raman (Df‐cars) imaging of cell uptake of core‐shell, magnetic‐plasmonic nanoparticles

Magnetic‐plasmonic, Fe3O4‐Au, core‐shell nanoparticles are popular in many applications, most notably in therapeutics and diagnostics, and thus, the imaging of these nanostructures in biological samples is of high importance. These nanostructures are typically imaged in biological material by dark field scatter imaging, which requires an even distribution of nanostructures in the sample and, therefore, high nanoparticle doses, potentially leading to toxicology issues. Herein, we explore the nonlinear optical properties of magnetic nanoparticles coated with various thicknesses of gold using the open aperture z‐scan technique to determine the nonlinear optical properties and moreover, predict the efficacy of the nanostructures in nonlinear imaging. We find that the magnetic nanoparticles coated with gold nanoseeds and thinner gold shells (ca. 4 nm) show the largest nonlinear absorption coefficient β and imaginary part of the third‐order susceptibility Im χ⁽³⁾, suggesting that these nanostructures would be suitable contrast agents. Next, we combine laser dark field microscopy and epi‐detected coherent anti‐Stokes Raman (CARS) microscopy to image the uptake of magnetic‐plasmonic nanoparticles in human pancreatic cancer cells. We show the epi-detected CARS technique is suitable for imaging of the magnetic‐plasmonic nanoparticles without requiring a dense distribution of nanoparticles. This technique achieves superior nanoparticle contrasting over both epi‐detected backscatter imaging and transmission dark field imaging, while also attaining label‐free chemical contrasting of the cell. Lastly, we show the high biocompatibility of the Fe3O4 nanoparticles with ca. 4‐nm thick Au shell at concentrations of 10–100 μg/mL.