Iron oxide exhibits fascinating physical properties especially in the nanometer range, not only from the standpoint of basic science, but also for a variety of engineering, particularly biomedical applications. For instance, Fe3O4 behaves as superparamagnetic as the particle size is reduced to a few nanometers in the single-domain region depending on the type of the material. The superparamagnetism is an important property for biomedical applications such as magnetic hyperthermia therapy of cancer. In this review article, we report on some of the most recent experimental and theoretical studies on magnetic heating mechanisms under an alternating (AC) magnetic field. The heating mechanisms are interpreted based on Néel and Brownian relaxations, and hysteresis loss. We also report on the recently discovered photoluminescence of Fe3O4 and explain the emission mechanisms in terms of the electronic band structures. Both optical and magnetic properties are correlated to the materials parameters of particle size, distribution, and physical confinement. By adjusting these parameters, both optical and magnetic properties are optimized. An important motivation to study iron oxide is due to its high potential in biomedical applications. Iron oxide nanoparticles can be used for MRI/optical multimodal imaging as well as the therapeutic mediator in cancer treatment. Both magnetic hyperthermia and photothermal effect has been utilized to kill cancer cells and inhibit tumor growth. Once the iron oxide nanoparticles are up taken by the tumor with sufficient concentration, greater localization provides enhanced effects over disseminated delivery while simultaneously requiring less therapeutic mass to elicit an equal response. Multi-modality provides highly beneficial co-localization. For magnetite (Fe3O4) nanoparticles the co-localization of diagnostics and therapeutics is achieved through magnetic based imaging and local hyperthermia generation through magnetic field or photon application. Here, Fe3O4 nanoparticles are shown to provide excellent conjugation bases for entrapment of therapeutic molecules, fluorescent agents, and targeting ligands; enhancement of solid tumor treatment is achieved through co-application of local hyperthermia with chemotherapeutic agents.

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Photoluminescence (PL) of Fe3O4 nanoparticle was observed from the visible to near-infrared (NIR) range by laser irradiation at 407 nm. PL spectra of ∼10 nm diameter Fe3O4 nanoparticles organized in different spatial configuration, showed characteristic emissions with a major peak near 560 nm, and two weak peaks near 690 nm and 840 nm. Different band gap energies were determined for these Fe3O4 nanoparticle samples corresponding to, respectively, the electron band structures of the octahedral site (2.2 eV) and the tetrahedral site (0.9 eV). Photothermal effect of Fe3O4 nanoparticles was found to be associated with the photoluminescence emissions in the NIR range. Also discussed is the mechanism responsible for the photothermal effect of Fe3O4 nanoparticles in medical therapy.

A significant energy loss results from the poor thermal insulations of the commercial and public buildings. Windows diffuse a large fraction of building heating and cooling energy to the external environment, representing an annual impact of 4.1 quadrillion British thermal unit of primary energy in the US. The current technology for efficient windows relies upon the double-pane insulated glass unit with an insulating gas in between. A key challenge is to reduce thermal conductivity of the windows without relying on insulating materials. The photothermal effect can be possibly utilized for particular functionalities that can collect solar energy for reducing heat loss. The insulation efficiency is quantified through the U-factor, defined as the ratio of the heat flux (H) per unit area through the pane to the difference (ΔT) between the window interior surface and exterior temperatures. Upon solar irradiation, single-panes can "self-heat" via the photothermal effect from the nanoparticle coatings. This can effectively reduce ΔT for enhanced thermal insulation. In this study, the photothermal effect on Fe 3 O 4 nanoparticles stimulated by solar light was investigated for nanoparticles in solutions and as thin films for energy-efficient windows. The Fe 3 O 4 nanoparticles were surface-functionalized with different polymers to modulate colloidal stability and for the investigation of the photothermal effect. The photothermal heating efficiencies of Fe 3 O 4 with different surface coatings were found to be much greater under the white-light irradiation than near infrared (NIR) in both aqueous suspension and as thin films. The mechanism for the photothermal effect of Fe 3 O 4 was identified in terms of its band structure. Both Urbach energy and band gap were obtained based on absorption spectra of various Fe 3 O 4 nanoparticles. The Urbach "tail" was found consistent with nanoparticle surface defect structures, while the band gap (~3.1eV) corresponded to the electronic transitions in the octahedral site of Fe 3 O 4. We also discuss the absorption-based photonic physics responsible for the much-enhanced photothermal heating by white-light as compared with NIR. Based on the photothermal heating, the U-factors were obtained with the nanoparticle coatings that show promise in producing energy efficient windows.

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