X-ray absorption by materials plays an important role in a large number of applications ranging from therapy to detection to imaging. Upon absorption of X-rays, energy is released from the absorbing material and deposited in the surrounding material, which is then converted to chemical or other forms of energy. It is possible to expand the application potential of X-rays by manipulating the host materials to control the absorption of X-rays and geometry of energy deposition. Therefore, investigating the mechanism of X-ray absorption and energy deposition is essential to finding the optimal nanostructures that can create maximal nanoscale (1 to a few hundred nanometers) energy deposition from the absorption of hard X-ray radiation.
Such energy deposition can be studied by following the events experienced by the electrons and photons released from materials as the result of absorption of primary X-ray photons. In most cases the majority of energy released from the absorbing material is carried away in photoelectrons. The released photoelectrons (and in a few cases Auger electrons) are often energetic enough (>5 keV in kinetic energy) to travel micrometers to tens of micrometers in the surrounding media such as water. We call this the remote effect or Type 1 Physical Enhancement (T1PE) because this dimension is much greater than the nanoscale dimensions mentioned above. Most of the recent reports of employing nanomaterials to enhance the effect of X-rays have cited this mechanism as the basis for designing their experiments and interpreting their results.1-3 A general rule of thumb is that adding one weight percent (1 wt %) of gold nanomaterials dissolved in water may generate up to 140% T1PE compared with background water. Such an enhancement can cause a similar amount of increase in the production of reactive oxygen species (ROS) such as hydroxyl radicals (.OH), as long as there are no other side effects such as scavenging of radicals by the introduced nanomaterials. However, contrary to what is commonly believed by researchers working in this field, this mechanism may not play a major role in many applications because it is difficult to dissolve 1 wt % of gold nanoparticles into water without introducing significant amounts of scavengers.
In addition to the energetic photoelectrons, there are low-energy (<5 keV) photoelectrons, Auger and secondary electrons generated upon absorption of X-rays. These electrons, although carrying only a fraction of the total energy released to the surrounding, can generate greater densities of energy deposition around the absorbing materials due to the much shorter distances (a few to a few hundred nanometers) traveled by these low-energy electrons in water. Because the penetration depth of these electrons in water is of the order of nanometers, it is possible to achieve higher energy deposition densities if the geometry and/or composite of nanomaterials are so arranged as to increase the overlap of trajectories of these low-energy electrons. This type of energy deposition is referred to as nanoscale energy deposition or Type 2 Physical Enhancement (T2PE), and this concept is the basis for geometry enhanced nanoscale energy deposition. In addition to giving rise to geometry enhancement, using nanomaterials has several other advantages. First, due to the high density of atoms in nanomaterials, they can be much more absorbing to X-rays than molecular complexes within a volume of nanometer dimension. Second, nanomaterials have large surface-to-volume ratios, which favor the escape of low-energy electrons, making them much more preferred than micrometer sized or bulk materials; the latter two can significantly attenuate the low-energy electrons. Third, it is possible to synthesize and assemble nanostructures to maximize energy deposition, thus making geometry enhancement more attainable practically.
To date no reports exist in the area of using nanostructures to maximize nanoscale energy deposition from the absorbed X-rays. Nonetheless, a few studies have dealt with geometry-insensitive enhancement of the effect of X-rays.4-8 In those studies, various chemical or biological methods such as DNA strand breaks1, 4, 9, cell and tissue damage2, and production of fluorescent molecules10 have been used to probe the enhancement of X-ray radiation in bulk media or near the surface of nanoparticles. One such study discussed nanoscale energy deposition or T2PE due to X-ray absorbing spherical gold nanoparticles.4 Although no complex geometries were used in that study, the results implied that it is possible to achieve high local T2PE using small amounts of nanomaterials (much less than 1 wt %).
We present here the results of studying how the geometry of nanostructures affects energy deposition and how nanostructures can be used to manipulate energy deposition on the nanometer scale. Such energy deposition may lead to increased amounts of chemical and biological reactions in water.