Uniform magnetite particles stabilized by citrate groups were successfully synthesized by a modified high‐temperature solvothermal reaction. Cell imaging reveals that the water‐dispersible particles can readily penetrate into cells without destroying them, indicating an excellent biocompatibility. A high enrichment capacity of the magnetite particles for separation of trace peptides is observed.

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Fluorescence targeted imaging in vivo has proven useful in tumor recognition and drug delivery. In the process of in vivo imaging, however, a high autofluorescence background could mask the signals from the fluorescent probes. Herein, a high contrast upconversion luminescence (UCL) imaging protocol was developed for targeted imaging of tumors based on RGD-labeled upconversion nanophosphors (UCNPs) as luminescent labels. Confocal Z-scan imaging of tissue slices revealed that UCL imaging showed no autofluorescence signal even at high penetration depth (approximately 600 microm). More importantly, region of interest (ROI) analysis of the UCL signal in vivo showed that UCL imaging achieved a high signal-to-noise ratio (approximately 24) between the tumor and the background. These results demonstrate that the UCL imaging technique appears particularly suited for applications in tracking and labeling components of complex biological systems.

Strong autofluorescence from living tissues, and the scattering and absorption of short-wavelength light in living tissues, significantly reduce sensitivity of in vivo fluorescence imaging. These issues can tackled by using imaging probes that emit in the near-infrared (NIR) wavelength range. Here we describe self-luminescing NIR-emitting nanoparticles employing an energy transfer relay that integrates bioluminescence resonance energy transfer (BRET) and fluorescence resonance energy transfer (FRET), enabling in vivo NIR imaging without external light excitation. Nanoparticles were 30-40 nm in diameter, contained no toxic metals, exhibited long circulation time and high serum stability, and produced strong NIR emission. Using these nanoparticles, we successfully imaged lymphatic networks and vasculature of xenografted tumors in living mice. The self-luminescing feature provided excellent tumor-to-background ratio (>100) for imaging very small tumors (2-3 mm in diameter). Our results demonstrate that these new nanoparticles are well suited to in vivo imaging applications such as lymph node mapping and cancer imaging.