Nanomaterials and nanoparticles: sources and toxicity.

This review is presented as a common foundation for scientists interested in nanoparticles, their origin,activity, and biological toxicity. It is written with the goal of rationalizing and informing public health concerns related to this sometimes-strange new science of "nano," while raising awareness of nanomaterials' toxicity among scientists and manufacturers handling them.We show that humans have always been exposed to tiny particles via dust storms, volcanic ash, and other natural processes, and that our bodily systems are well adapted to protect us from these potentially harmful intruders. There ticuloendothelial system, in particular, actively neutralizes and eliminates foreign matter in the body,including viruses and nonbiological particles. Particles originating from human activities have existed for millennia, e.g., smoke from combustion and lint from garments, but the recent development of industry and combustion-based engine transportation has profoundly increased an thropogenic particulate pollution. Significantly, technological advancement has also changed the character of particulate pollution, increasing the proportion of nanometer-sized particles--"nanoparticles"--and expanding the variety of chemical compositions. Recent epidemiological studies have shown a strong correlation between particulate air pollution levels, respiratory and cardiovascular diseases, various cancers, and mortality. Adverse effects of nanoparticles on human health depend on individual factors such as genetics and existing disease, as well as exposure, and nanoparticle chemistry, size, shape,agglomeration state, and electromagnetic properties. Animal and human studies show that inhaled nanoparticles are less efficiently removed than larger particles by the macrophage clearance mechanisms in the lungs, causing lung damage, and that nanoparticles can translocate through the circulatory, lymphatic, and nervous systems to many tissues and organs, including the brain. The key to understanding the toxicity of nanoparticles is that their minute size, smaller than cells and cellular organelles, allows them to penetrate these basic biological structures, disrupting their normal function.Examples of toxic effects include tissue inflammation, and altered cellular redox balance toward oxidation, causing abnormal function or cell death. The manipulation of matter at the scale of atoms,"nanotechnology," is creating many new materials with characteristics not always easily predicted from current knowledge. Within the nearly limitless diversity of these materials, some happen to be toxic to biological systems, others are relatively benign, while others confer health benefits. Some of these materials have desirable characteristics for industrial applications, as nanostructured materials often exhibit beneficial properties, from UV absorbance in sunscreen to oil-less lubrication of motors.A rational science-based approach is needed to minimize harm caused by these materials, while supporting continued study and appropriate industrial development. As current knowledge of the toxicology of "bulk" materials may not suffice in reliably predicting toxic forms of nanoparticles,ongoing and expanded study of "nanotoxicity" will be necessary. For nanotechnologies with clearly associated health risks, intelligent design of materials and devices is needed to derive the benefits of these new technologies while limiting adverse health impacts. Human exposure to toxic nanoparticles can be reduced through identifying creation-exposure pathways of toxins, a study that may someday soon unravel the mysteries of diseases such as Parkinson's and Alzheimer's. Reduction in fossil fuel combustion would have a large impact on global human exposure to nanoparticles, as would limiting deforestation and desertification.While nanotoxicity is a relatively new concept to science, this review reveals the result of life's long history of evolution in the presence of nanoparticles, and how the human body, in particular, has adapted to defend itself against nanoparticulate intruders.

Oxidation of evaporated porous silicon rugate filters.

Rugate filters are thin-film optical interference coatings with sinusoidal variation of the refractive index. Several of these filters were fabricated with glancing angle deposition, which exploits atomic competition during growth to create nanoporous materials with controllable effective refractive index. This method enables the fabrication of devices with almost arbitrary refractive index profiles varying between the ambient, 1.0, and the index of the film material, in this case silicon with an index of 4.0 (at 600 nm). As these filters are inherently porous, oxidation of the silicon can occur throughout the device layer, and here we study the intentional oxidation of silicon filters by high-temperature reaction with gaseous oxygen. We find that a significant portion of the silicon filter oxidizes in approximately 10 min when heated to 600 degrees C-650 degrees C in an oxygen environment; oxidation then continues slowly over several hours. The presence of water vapor has little apparent effect on the oxidation reaction, and attempts to oxidize with ozone at room temperature were unsuccessful. As silicon filters oxidize to become silica, spectral blueshifts and increased short-wavelength transmittance are observed. Measured and calculated transmittance spectra generally agree, although the lack of absorption and dispersion in the theoretical model limits detailed comparison.

Biaxial thin-film coated-plate polarizing beam splitters.

We present a design for a biaxial thin-film coated-plate polarizing beam splitter that transmits the p-polarized component of a beam of light without change of direction and reflects the s-polarized component. The beam splitter has a periodic structure and is planned for fabrication by serial bideposition in mutually orthogonal planes. Recent experimental data for form-birefringent silicon is used to establish the feasibility of the design for a beam splitter to be used at 1310 nm and at an angle of 45 degrees in air.

Control of power law scaling in the growth of silicon nanocolumn pseudo-regular arrays deposited by glancing angle deposition.

Nanocolumn pseudo-regular arrays of silicon with controlled aspect ratio and porosity are fabricated by electron-beam evaporation using the glancing angle deposition (GLAD) method with vapour impinging at oblique incidence onto rapidly rotating substrates. The width W at positions y along the height of one individual column scales with y following a power law dependence W approximately y(p). We demonstrate that the scaling exponent value, p, can be modified from 0.6 to 0.3 by varying the vapour incidence angle from 75 degrees to a glancing 89 degrees from the substrate normal. This exponent is an important morphological factor for thin films, as it determines the morphological correlation length, nanocolumn profile, size, and spacing. The nanocolumn mean diameter can be varied between 12 and 40 nm, while the intercolumnar spacing can be adjusted between 37 and 85 nm via changing the incidence angle. The growth mechanism and film morphology are explored in detail.

Enhanced birefringence in vacuum evaporated silicon thin films.

We report an experimental study of enhanced optical birefringence in silicon thin films on glass substrates. Form anisotropy is introduced as an atomic-scale morphological structure through dynamic control of growth geometry. The resulting birefringence is large compared with naturally anisotropic crystals and is comparable to two-dimensional photonic crystals. The films are fabricated with serial bideposition onto a substrate held at a fixed tilt angle relative to the impinging vapor. Films were analyzed by spectroscopic ellipsometry and scanning electron microscopy, the latter clearly revealing form anisotropy in a morphology of bunched columns perpendicular to the deposition plane with dimensions of hundreds of nanometers and smaller. The observed linear birefringence varies with wavelength and tilt angle, with a maximum of 0.4 at a 630-nm wavelength and 0.25 at 1500 nm.

Birefringent omnidirectional reflector.

Anisotropic optical coatings offer unique polarizing properties, unmatched by conventional isotropic devices. Here we demonstrate the fabrication of a birefringent omnidirectional reflector, a type of photonic crystal, which exhibits complete reflection of radiation at 1.1 microm for all incidence angles and polarizations. The thin-film device was deposited from electron-beam evaporated silicon, with refractive-index variation arising from atomic-scale porosity created with glancing-angle deposition. Birefringence was found to enhance the performance of the device compared with its isotropic counterpart by enlarging the photonic bandgap region of omnidirectional reflection.

Vacuum evaporated porous silicon photonic interference filters.

Porous materials with nanometer-scale structure are important in a wide variety of applications including electronics, photonics, biomedicine, and chemistry. Recent interest focuses on understanding and controlling the properties of these materials. Here we demonstrate porous silicon interference filters, deposited in vacuum with a technique that enables continuous variation of the refractive index between that of bulk silicon and that of the ambient (n approximately 3.5 to 1). Nanometer-scale oscillations in porosity were introduced with glancing angle deposition, a technique that combines oblique deposition onto a flat substrate of glass or silicon in a high vacuum with computer control of substrate tilt and rotation. Complex refractive index profiles were achieved including apodized filters, with Gaussian amplitude modulations of a sinusoidal index variation, as well as filters with index matching antireflection regions. A novel quintic antireflection coating is demonstrated where the refractive index is smoothly decreased to that of the ambient, reducing reflection over a broad range of the infrared spectrum. Optical transmission characterstics of the filters were accurately predicted with effective medium modeling coupled with a calibration performed with spectroscopic ellipsometry.