Plasmon-resonant nanoparticles with optical scattering in the near infrared (NIR) are valuable contrast agents for biophotonic imaging and may be detected at the single-particle limit against a dark background, but their contrast is often limited in environments with high noise. Here we consider gyromagnetic imaging as a dynamic mode of optical contrast, using gold nanostars with superparamagnetic cores. The nanostars exhibit polarization-sensitive NIR scattering, and can produce a frequency-modulated signal in response to a rotating magnetic field gradient. This periodic "twinkling" can be converted into Fourier-domain images with a dramatic reduction in background. We demonstrate gyromagnetic imaging of nanostars inside of tumor cells, using broadband excitation: while their time-domain signals are obscured by incoherent scattering, their Fourierdomain signals can be clearly resolved in less than a second. The gyromagnetically active nanostars do not cause a loss in viability, and can even have a mild stimulatory effect on cell growth.

Plasmon-resonant gold nanostars (NSTs) with magnetic cores were synthesized by a multistep sequence from superparamagnetic Fe3O4 nanoparticles (NPs), and evaluated as optical contrast agents under magnetomotive (MM) imaging conditions. Core–shell Fe3O4@Au NPs were prepared in nonpolar organic solvents with nanometer control over shell thickness, and with good epitaxy to the Fe3O4 surface. Anisotropic growth was performed in micellar solutions of cetyltrimethylammonium bromide (CTAB) under mildly reducing conditions, resulting in NSTs with physical features similar to those produced from colloidal gold seeds. NSTs could be produced below 100 nm from tip to tip but seed size had a significant impact on growth habit, with larger seed particles producing submicron-sized “morning stars.” Both NSTs and aggregated core–shell NPs are responsive to in-plane magnetic field gradients and can provide enhanced near-infrared (NIR) contrast under MM conditions, but do so by different mechanisms. NSTs can modulate polarized NIR scattering with minimal translational motion, giving the appearance of a periodic but stationary “blinking,” whereas core–shell NP aggregates require lateral displacement for signal modulation. The polarization-sensitive MM imaging modality offers the dual advantage of enhanced signal quality and reduced background signal, and can be applied toward the detection of magnetomotive NSTs in heterogenous biological samples, as illustrated by their detection inside of granular cells such as macrophages.

Plasmon-resonant nanostars (NSTs) provide excellent contrast enhancement for photoacoustic tomography. The high photoacoustic sensitivity of NSTs at near-infrared wavelengths enable their in vivo detection in rat sentinel lymph nodes and vessels, with direct application toward lymphangiography.

Murine RAW 264.7 cells were exposed to spheroidal core–shell Fe3O4@Au nanoparticles (SCS-NPs, ca. 34 nm) or nanostars (NSTs, ca. 100 nm) in the presence of bovine serum albumin, with variable effects observed after macrophagocytosis. Uptake of SCS-NPs caused macrophages to adopt a rounded, amoeboid form, accompanied by an increase in surface detachment. In contrast, the uptake of multibranched NSTs did not induce gross changes in macrophage shape or adhesion, but correlated instead with cell enlargement and signatures of macrophage activation such as TNF-α and ROS. MTT assays indicate a low cytotoxic response to either SCS-NPs or NSTs despite differences in macrophage behavior. These observations show that differences in NP size and shape are sufficient to produce diverse responses in macrophages following uptake.

Creation of structural complexity by simple experimental control will be an attractive approach for the preparation of nanomaterials, as a classical bottom-up method is supplemented by a more efficient and more direct artificial engineering method. In this study, structural manipulation of MCM-41 type mesoporous silica is investigated by generating and imbedding hard magnetic CoFe2O4 nanoparticles into mesoporous silica. Depending on the heating rate and target temperature, mesoporous silica undergoes a transformation in shape to form hollow silica, framed silica with interior voids, or melted silica with intact mesostructures. Magnetism is governed by the major CoFe2O4 phase, and it is affected by antiferromagnetic hematite (α-Fe2O3) and olivine-type cobalt silicate (Co2SiO4), as seen in its paramagnetic behavior at the annealing temperature of 430 °C. The early formation of Co2SiO4 than what is usually observed implies the effect of the partial substitution of Fe in the sites of Co. Under slow heating (2.5 °C min(-1)) mesostructures are preserved, but with significantly smaller mesopores (d100 = 1.5 nm). In addition, nonstoichiometric CoxFe1-xO with metal vacancies at 600 °C, and spinel Co3O4 at 700 °C accompany major CoFe2O4. The amorphous nature of silica matrix is thought to contribute significantly to these structurally diverse and rich phases, enabled by off-stoichiometry between Si and O, and accelerated by the diffusion of metal cations into SiO4 polyhedra at an elevated temperature.