Abstract
We present magnetic FePt nanoparticles with a hydrophilic, inert, and biocompatible silico-tungsten oxide shell. The particles can be functionalized, optically detected, and optically manipulated. To show the functionalization the fluorescent dye NOPS was bound to the FePt core-shell nanoparticles with propyl-triethoxy-silane linkers and fluorescence of the labeled particles were observed in ethanol (EtOH). In aqueous dispersion the NOPS fluorescence is quenched making them invisible using 1-photon excitation. However, we observe bright luminescence of labeled and even unlabeled magnetic core-shell nanoparticles with multi-photon excitation. Luminescence can be detected in the near ultraviolet and the full visible spectral range by near infrared multi-photon excitation. For optical manipulation, we were able to drag clusters of particles, and maybe also single particles, by a focused laser beam that acts as optical tweezers by inducing an electric dipole in the insulated metal nanoparticles. In a first application, we show that the luminescence of the core-shell nanoparticles is bright enough for in vivo multi-photon imaging in the mouse neocortex down to cortical layer 5.
Highlights
Since the development of colloid chemical routes for nanoparticle synthesis, metallic nanoparticles greatly raised expectations in view of their biomedical applications
One suitable compound that can be employed for coating nanoparticles are polyoxometalates, which are inorganic cluster molecules synthesized in aqueous solution
Polyoxometalates offer a high degree of thermodynamic stability [3] and have been shown to cover the surface of gold nanoparticles in water via electrostatic interaction [4,5]
Summary
Since the development of colloid chemical routes for nanoparticle synthesis, metallic nanoparticles greatly raised expectations in view of their biomedical applications. Polyoxometalates offer a high degree of thermodynamic stability [3] and have been shown to cover the surface of gold nanoparticles in water via electrostatic interaction [4,5] They coat magnetic FePt nanoparticles during synthesis in non-polar organic solutions when modified to match the required solubility. Multi-photon microscopy allows sectioning, and reduces photo-toxicity These advantages make multi-photon microscopy in combination with genetic encoded or synthetic functional fluorescent dyes one of the most important imaging techniques with optical resolution in biology, and in basic neuroscience [8]. Fluorescent dyes and noble metal surfaces and gold nanoparticles can be excited by multi-photon absorption to emit light. The nanoparticles used in this paper are ferromagnetic as will be shown elsewhere
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