We propose porous silicon nanoparticles (PSi NPs) with natural oxide coating as biocompatible and bioresorbable contrast agents for magnetic resonant imaging (MRI). A strong shortening of the transversal proton relaxation time (T2) was observed for aqueous suspensions of PSi NPs, whereas the longitudinal relaxation time (T1) changed moderately. The longitudinal and transversal relaxivities are estimated to be 0.03 and 0.4 l/(g·s), respectively, which are promising for biomedical studies. The proton relaxation is suggested to undergo via the magnetic dipole-dipole interaction with Si dangling bonds on surfaces of PSi NPs. MRI experiments with phantoms have revealed the remarkable contrasting properties of PSi NPs for medical diagnostics.

Porous and nonporous silicon (Si) nanoparticles (NPs) prepared by ball-milling of electrochemically etched porous Si layers and crystalline Si wafers were studied as potential agents for enhancement of the proton spin relaxation in aqueous media. While nonporous Si NPs did not significantly influence the spin relaxation, the porous ones resulted in strong shortening of the transverse relaxation times. In order to investigate an effect of the electron spin density in porous Si NPs on the proton spin relaxation, we use thermal annealing of the NPs in vacuum or in air. The transverse relaxation rate of about 0.5 l/(g s) was achieved for microporous Si NPs, which were thermally annealing in vacuum to obtain the electron spin density of the order of 1017 g−1. The transverse relaxation rate was found to be almost proportional to the concentration of porous Si NPs in the range from 0.1 to 20 g/l. The obtained results are discussed in view of possible biomedical applications of Si NPs as contrast agents for magnetic resonance imaging.

Silicon (Si) nanoparticles (NPs) with small (10 À3 -10 À1 at%) content of iron oxide (Fe 2 O 3 ) are prepared by plasma-assisted ablative synthesis. Powders of the prepared Si-iron oxide (SIO) NPs are investigated by means of the transmission electron microscopy, Raman spectroscopy, electron paramagnetic resonance, and magnetic susceptibility measurements. Aqueous suspensions of the NPs are studied by using dynamic light scattering and nuclear magnetic resonance technique. The longitudinal and transverse relaxation times of protons in aqueous suspensions of the NPs are found to be dependent on the iron content. The stronger decrease of the proton relaxation is detected for the samples with higher iron content. Magnetic resonance imaging (MRI) experiments show that SIO NPs have properties of the MRI contrast agent and it is confirmed by in vivo experiments with cancer tumor. Aqueous suspensions of SIO NPs are explored as sensitizers of electromagnetic radio frequency hyperthermia and the highest heating rate is observed for the NPs with smaller hydrodynamic size (%50 nm). The obtained results indicate possible ways for applications of SIO NPs in the MRI diagnostics and mild therapy of cancer.

Crystalline silicon (Si) nanoparticles (NPs) doped with iron (Fe) in the range from 0.02 to 2.5 at.% were prepared by plasma-ablative synthesis and were investigated by means of the transmission electron microscopy, X-ray diffraction (XRD), dynamic light scattering (DLS), infrared spectroscopy and nuclear magnetic resonance relaxometry. While the nanocrystal size in Si:Fe NPs did not depend significantly on Fe content, the hydrodynamic diameter of NPs in aqueous suspensions increases from 50 to 180[Formula: see text]nm. Both the transverse and longitudinal proton relaxation time were found to decrease in the prepared suspensions of Si:Fe NPs. Maximal shortening of the transverse relaxion was observed for Si:Fe NPs with 0.2 at.% of Fe and the relaxation rate was almost linearly proportional to the NP concentration. Both these findings and in vivo tests indicate that Si:Fe NPs are promising for biomedical applications in magnetic resonance imaging (MRI) and therapy of cancer.

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