Chapter 8: Nanoparticle host interactions in natural systems
Udo Becker, Martin Reich and Subhashis Biswas
Nanoscale phenomena dominate many of the important processes near the surface of the Earth. Therefore these phenomena are of special importance to the environment and human health. As nanoscale processes are intrinsically molecular, there is an immediate synergy between the study of nanoscale particles in natural systems and the disciplines of mineralogy, chemistry, physics and materials science.
Since nanoparticulates have unique properties as isolated entities (e.g. Gilbert et al., 2004), it is tempting to focus on the nanoparticle in isolation. However, for nanoparticles in the environment in particular, it is important to analyse their properties in relation to their immediate atomic-scale environment, e.g. the nanoparticle-host interface. Often, certain types of nanoparticles are associated with specific host phases such as noble metal nanoparticles in sulphides, carbonate nuclei on organic templates, arsenic sulphides on Fe sulphides or oxides, or atmospheric nanoparticles on or in dust particles. In many cases, the nanoparticle structure, stability, chemistry, charge, electronic and magnetic properties, and finally, reactivity, are based on these interface properties. Thus, it is essential to develop an understanding of these interface properties at the atomic level. In order to capture the specific details of these interface properties, it is necessary to apply a combination of approaches in nanoscale characterization. These methods include a wide variety of microbeam and spectroscopic techniques, the synthesis of nanoparticles under controlled conditions (e.g. utilizing ion implantation), the study of surface and interface clusters, and the use of quantum-mechanical and molecular-dynamics simulations to understand the observed processes. In addition, as a number of structural and electronic properties of nanoparticles and their interfaces are still difficult to determine experimentally, molecular simulations can further our understanding of nanoscale phenomena.
At the theory level, it is extremely important to treat the nanoparticle and interface properties at the quantum-mechanical level (Fig. 1). Only quantum mechanics, in contrast to empirical force-field methods, allow one to capture the uniqueness of physicochemical properties at the nanoscale without the influence of force fields that are derived from bulk properties. Empirical force-fields provide some insight into structural properties; however, information about the electronic and magnetic properties of these systems, both of which have an influence on the stability, the chemistry, and the reactivity at the interface, can only be deduced from ab initio calculations. If the model system cannot be treated quantum mechanically, because the number of atoms exceeds the capabilities of quantum-mechanical calculations, force-field calculations and molecular dynamics (MD) can be applied if thoroughly tested in comparison to quantum-mechanical calculations. Thus, it is an important task to also develop good empirical force fields, using the quantum-mechanical information, which capture atom-atom interactions within the nanoparticle (NP), the host, and at the interface. With this information, MD calculations can help to model the size-dependent dynamics of NPs, especially when incorporated into a solid or liquid host.
When characterizing nanoparticles, it is crucial to apply appropriate experimental techniques that capture the structure, the chemistry, and, as a consequence, the reactivity that occurs at these interfaces at the atomic level. As an example, a major challenge to the development of a fundamental understanding of transport and retardation mechanisms of trace metal contaminants (<10 ppm) is their identification and characterization at the nanoscale. Atomic-scale techniques, such as conventional transmission electron microscopy (TEM), although powerful, are limited by the extremely small amounts of material that can be examined. However, recent advances in electron microscopy provide a number of new analytical techniques that expand its application in environmental studies, particularly those concerning heavy metals on airborne particulates or water-borne colloids. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM, Utsunomiya & Ewing, 2003), STEM-energy-dispersive X-ray spectrometry (EDX), and energy-filtered TEM (EFTEM, Moore et al., 2001) can be used effectively to identify and characterize nanoparticles.
An intrinsic problem with studying nanoparticulates is that one has to find them in order to be able to analyze them. This challenge applies to observing a single nucleus as a precursor of biomineralization as well as to finding a gold cluster in a sulphide host mineral or to detect a metal adsorbate on a sulphide or oxide surface. Significant progress has been made in recent years in finding natural nanoparticles. One prominent example is TEM observations of “invisible gold” in sulphidic host minerals (Palenik et al., 2004; Reich et al., 2005). Nevertheless, it can be challenging to find nanoparticles of different sizes in different host phases to perform systematic studies of their physicochemical properties as a function of size, temperature, host composition and structure, to name a few parameters. In order to attack this challenge, ion-beam implantation techniques were recently developed to create nanoparticles of gold in a variety of minerals and then to study their properties in situ under well controlled conditions (e.g. particle growth as a function of temperature using a heating stage and TEM). Furthermore, the deposition of atomic-scale metal clusters on surfaces is a good model for the adsorption of these metals on these minerals from hydrothermal solutions as a precursor of their incorporation into the bulk structure.
The initial stages of biomineralization are another example for nanoparticles that form an interface with another matrix, in this case an organic template. In a natural system, the organic template could for example be the exopolymer of the cell wall of a coccolithophore and the mineral nanoparticle would be the nucleus of a calcium carbonate. Such an interface can be modelled experimentally by observing the initial nucleus formation on functionalized gels and on organic crystals with and without the presence of organic growth inhibitors/modifiers. In order to perform these experiments, the use of surface-sensitive probes such as atomic force microscopy (AFM) with a fluid cell or the use of optical microscopy in combination with Langmuir-Blodgett films can be helpful. The application of molecular-scale calculations allows us to observe transition states and early stages of mineral cluster formation that are too short-lived to be observed experimentally. Furthermore, these calculations give us an atomic picture of the interaction between functional groups of the organic matrix with specific atoms on the surface of the mineral. Simultaneously, information on the most important thermodynamic interface property is obtained, which is the interface energy.
In this chapter, we have chosen to describe a number of projects that cover a variety of geological and environmental phenomena such as ore formation and mineral exploitation, potentially hazardous atmospheric particles, and biomineralization. Furthermore, we briefly describe the synthesis of nanoparticles in specific matrices using ion-beam techniques as means to generate nanoparticles in the laboratory as a natural analogue. More importantly, our goal is to describe the specific behaviour of nanoparticles as a function of the interactions with their respective host. Thus, we start with some properties of isolated nanoparticles, nanoparticles in a liquid environment, nanoparticles in contact with one- and two-dimensional interfaces, and finally, nanoparticles in a three-dimensional host environment.
These systems have in common that their natural or environmental occurrence can be studied and that corresponding synthetic model systems can be created for better analysis. As we learn to create and modify model systems, using both ion-beam techniques and computational models, that mimic their environmental counterpart as closely as possible, we will understand the processes much better that lead to natural nanoparticle formation. We will gain further insight in their structure, chemistry, electronic behaviour, stability, and reactivity in the environment. With this knowledge, significant progress can be made to understand ore formation as a function of host mineral chemistry, which will lead to more environmentally friendly extraction methods. The processes of biomineralization can be studied, which has applications in medical sciences (bone and teeth growth), the formation of biominerals in nature and for the development of optimized biocomposites that have a number of applications in materials science and in medicine. Another application is atmospheric nanoparticles that are of environmental importance.
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