Nanoparticles: Generation, Surface Functionalization, and Ion Sensing
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During the past decade or so, considerable interest has grown in the potential commercial and technological exploitation of nanoscale materials in analytical chemistry. A particular focus of this interest has been the utilization of specific size-dependent electronic, optical, or magnetic properties. Nanoparticles are crystalline clusters (metallic, semiconducting, or insulating) composed of a few hundred to a few thousand atoms and are, characteristically, of nanometer dimension. One consequence of this size is that many of their properties are dominated by their surface rather than by the bulk volume and, indeed, this fact alone can make these structures very amenable to chemical or environmental influence, or “tuning.” Their nanometer diameter is also the same order of magnitude as the de Broglie wavelength of electrons/holes at room temperature and this, then, can lead to a quantization of electronic/hole energy levels, a phenomenon that has led to the term “quantum dots” (QD). The energy-level spacings are related to particle size. Although the term “nanoparticles” has been coined only at a time when advances in microscopy have allowed us to resolve them as such, nanoparticles have been in use for many years; for example, Faraday carried out pioneering work with gold nanoparticles, and some of the intense colors evident in stained glass arise from the presence of nanometer-sized oxide clusters.
Gold nanoparticles received substantial attention during the past decade or so. The potential technical importance of monolayer-protected metal nanoparticles in developing nanoscale optoelectronic devices, (bio)chemical sensors, corrosion-resistant materials, and new catalysts has made them one of the primary targets of highly intensive, nanoparticle-based research activity. To date, for example, both single electron and nonlinear optical devices have been constructed from these materials. In addition to utilizing the optical and electronic properties inherent in metallic structures, the magnetic characteristics of suitable particles have also been of some interest, particularly in consideration of separation technologies. For example, nanoparticles of iron oxide (Fe3O4) can be suitably modified with a biological moiety of interest, e.g., a cell and their “superparamagnetic” properties used in magnetic field modulated manipulations/separation. The possible use of magnetic nanoparticles as contrast agents in magnetic resonance imaging has also been cited. Through suitable surface chemical modification, it should be possible to generate magnetic nanoparticles with a propensity to concentrate in particular tissue (or cellular) regions allowing, e.g., enhanced image contrast generation between diseased or cancerous cells and healthy cells. To date, these have both been shown to be more effective than conventional magnetic resonance imaging (MRI) agents (such as gadolinium complexes) and capable of allowing in vivo cellular tracking.
This report will aim to outline the general properties of metallic and semiconducting nanoparticles, their modification and possible application in high-sensitivity, selective ion sensing. There is no universally adopted definition as to what constitutes a nanoparticle, but we will be concerned here with the properties and utilization of particles < 100 nm in diameter. Although more progress has been made in the functionalization of metallic (notably gold and silver) particles, the properties and potential utilization of semiconducting particles will also be discussed.