Nanoparticles

Magnetic nanoparticles (MNPs) have been the focus of an increasing amount of the recent literature, which has chronicled research into both the fabrication and applications of MNPs. The explosion of research in this area is driven by the extensive technological applications of MNPs which includes single-bit elements in high-density magnetic data storage arrays, magneto-optical switches, and novel photoluminescent materials. In biomedicine, MNPs serve as contrast enhancement agents for Magnetic Resonance Imaging, selective probes for bimolecular interactions, and cell sorters. Nanoparticles of magnetic metals are also finding applications as catalysts, nucleators for the growth of high-aspectratio nanomaterials, and toxic waste remediation. Methodologies for the synthesis of MNPs are being developed by scientists working in fields spanning Biology, Chemistry, and Materials Science. In the last decade, these efforts have provided access to nanoscale magnetic materials ranging from inorganic metal clusters to custom-built Single Molecule Magnets. The goal of this chapter is to provide broad overviews of both the applications of MNPs and the synthetic methodologies used in their production.1,2

Semiconductors are key components of devices used everyday, including computers, light emitting diodes, sensors, etc. Semiconductors are a unique class of materials in that they can assume characteristic properties of both metals and insulators, depending on conditions that determine the electronic nature of the valence and conduction bands. In the ground state, the valence band is completely filled and separated from the conduction band by a narrow band gap (E_g). When sufficient energy is applied to a semiconductor, it becomes conducting by excitation of electrons from the valence band into the conduction band. This excitation process leaves holes in the valence band, and thus creates “electron-hole pairs” (EHPs). When these EHPs are in intimate contact (i.e., the electrons and holes have not dissociated) they are termed “excitons.” In the presence of an external electric field, the electron and the hole will migrate (in opposite directions) in the conduction and valence bands, respectively (Figure 2.1).

Crystals consist of a periodic alternation of specific repeating molecules. The individual repeating molecules have quantized electronic structures while crystals have continuous electronic band structures that result from the overlap and combination of molecular orbitals of repeating molecules. Therefore, isolated molecules exhibit quantum mechanical properties, while the chemical and physical properties of bulk crystals obey the laws of classical mechanics. However, when the crystal size decreases into the nano-scale regime (1 ∼ 100 nm), the electronic band of the crystals starts to be quantized and the resulting nanocrystals behave as an intermediate between molecules and crystals.2‐5 These nanocrystals exhibit novel properties which differ from both molecular and bulk properties. For example, CdSe semiconductor crystals on the 10 nm scale, the characteristic red luminescence is no longer observed but the luminescence can be continuously tuned from red to blue by varying the crystal size. The melting temperature of nanocrystals simultaneously decreases when the nanocrystal size is reduced.6,7 In the nanoscopic world, crystal properties are highly dependent on the size, shape, and surface state of the crystals.

Modified metal and semiconductor nanoparticles have become the subject of an extremely active field of chemical research in the last few years. This widespread interest is understandable considering the proliferation of simple chemical methods for the preparation of metal and semiconductor particles with sizes in the range 1–100 nm, 1 which is of great interest in emerging nanotechnologies. The properties of metal and semiconductor nanoparticles in this size range differ from those of the corresponding bulk materials and approach the molecular limit.2 For the smallest nanoparticles, the distribution of electronic states departs from the usual band structure and discrete states will appear at the band edges. Electrons may undergo quantum confinement and these systems may exhibit unique electronic, magnetic and optical properties associated with the socalled quantum dots. Furthermore, as the size of the nanoparticle decreases, a larger fraction of the constituent atoms are located on its surface. This is particularly important when dealing with metals with applications in catalysis, because of the high cost associated with these materials.

Catalysis plays a vital role in providing fuels, fine chemicals, Pharmaceuticals, and means for strengthening environmental safeguards. In comparison with many other fields of chemical and materials sciences, catalysis was perhaps one of the first fields to take advantage of nanotechnology. Supported noble metal catalysts with particle sizes down to a few hundred nanometers and zeolite catalysts with pore size of subnanometers, all developed in the 1950s–60s, are widely used in today’s chemical processes. The field of catalysis continuously reinvents itself and become highly interdisciplinary. Many of the recent advances, some of which are discussed in this chapter, are a result of such interdisciplinary developments involving nanotechnology. The nanotechnology-guided design and fabrication of catalysts, enhancement of catalytic activity or selectivity, and reduction in cost of catalysts will have enormous impacts to the chemical industry. The ability to harness the large surface area-tovolume ratios and the unique binding sites of nanoparticles (1–100 nm), especially in heterogeneous catalysis, constitutes a major driving force in fundamental research and practical applications of nanoparticle catalysts. Importantly, deliberate tailoring of nanoparticle size, shape and surface could lead to improved or new catalytic properties. This aspect was indeed inspired by the surprising discovery of high catalytic activity of nanosized gold towards oxidation or reduction of hydrocarbons.1‐4 Gold is traditionally considered catalytically inactive as a practical catalyst, but this property is completely changed when the size dimension is reduced to a few nm.

There is intense interest in the development of photonic crystal materials for applications in optics, optical computing and for the control of electromagnetic radiation propagation.1‐3 The excitement in this area stems from the recent development of a deep understanding of light propagation within materials with periodic optical dielectric constant modulations, and the development of fabrication methods for forming these materials.4

The first examples of nanotechnology, some historians might be inclined to argue, could very well be accredited to some glassblowers from the days of imperial Rome. Those ancient craftsmen were able to embed colloidal metal particles within their glassy works to enhance their lustrous qualities. Although they were most likely unaware of the nanoscopic nature of these inclusions, this did not prevent them from appreciating the enigmatic hues produced upon a change of incident light. One of the most striking examples of such Roman glasses is the famed Lycurgus cup, which dates back to the 4th century A.D. The chalice has a dark greenish tint under reflected lighting, but when illuminated from behind the goblet appears red colors are attributed to the optical responses of colloidal gold particles dispersed throughout the glass. Similar phenomena are also featured in the stainedglass windows of many medieval cathedrals, most often from colloidal particles of coinage metals such as copper and gold (red) or silver (yellow).

Self-assembly of nanoparticles mediated by polymers provides access to stabilized metal and semiconductor nanocomposites as well as allows for the fabrication of new structured nanoscale materials. Several attributes of the individual building blocks such as the size and shape of individual metal clusters, composition of the monolayer, and functional groups on the polymers allows control over the properties of these nanocomposites. Additional control over the structure and morphology of such nanocomposites can be induced by changes in the polymer structure. Such construction of composite materials using the assembly of nanoscopic building blocks or the ‘bottom-up’ approach provides a methodology complementary to ‘top-down’ lithographic methods. The ‘bottom-up’ approach provides access to structures smaller and with greater 3-dimensional control than is possible through sophisticated lithographic techniques such as electron-beam lithography.1 Successful integration of these two approaches is a challenge that needs to be addressed. Such multiscale engineering would allow fabrication of intricate functional devices with atomic level structural control that manifests and spans itself into the macroscopic world and thus could provide a successful integration of the two approaches.

There is much excitement in the study of nanoscale matter with respect to their fundamental properties, organization to form superstructures and applications. The unusual physicochemical and optoelectronic properties of nanoparticles are primarily due to confinement of electrons within particles of dimensions smaller than the bulk electron delocalization length, this process being termed quantum confinement.1‐3 The exotic properties of nanoparticles have been considered in applications such as optoelectronics, 4 catalysis, 5 reprography, 6 single-electron transistors (SETs) and light emitters, 7 non-linear optical devices8 and photoelectrochemical applications.9 Magnetic nanoparticles are being viewed with interest from a fundamental point of view (superparamagnetism in the nanoparticles)10 as well as in applications such as magnetic memory storage devices, 11 magnetic resonance image enhancement12 and magnetic refrigeration.13 The ability to tune the optical absorption/emission properties of semiconductor nanoparticles (the so-called “quantum dots”) by simple variation in nanoparticle size is particularly attractive in the facile band-gap engineering of materials14 and the growth of quantum dot lasers.15 More recently, nanoscale matter has been looked at with interest for potential application in nanocomputers, synthesis of advanced materials, energy storage devices, electronic and optical displays, chemical and biosensors as well as biomedical devices.16 It is expected that some of the more immediate applications of nanoparticles will be in medical diagnosis and therapeutics. Exciting examples include detection of genetic disorders using gold nanoparticles, 17,18 color-coded fluorescent labeling of cells using semiconductor quantum dots19,20 and cell transfection for gene therapy and drug delivery.21

Over the last few decades, nanometer sized particles have been extensively exploited in conjunction with macromolecular species in the development of analytical tools for biological purposes; this trend did not stop through the years. Actually the growing interest and knowledge on nanoscale materials has positively increased the tools available in cell biology, analytical biochemistry, cytohistochemistry and immunocytochemistry. Historically, the major role was played by bioconjugate latex nanospheres1 and gold colloids, 2,3 while more recently low and high nuclearity gold clusters have been exploited. An emerging literature concerns group II-VI and III-V semiconductor nanocrystals bioconjugates, having unique and appealing optical properties. In this overview latex bioconjugates will not be discussed; gold colloids, gold clusters and semiconductor bioconjugates will be described in some detail, with particular attention to gold colloids and clusters.