Nanoparticles exist everywhere, the vast majority of them being naturally formed. In wellwaters sampled from a sandstone aquifer in Birmingham, we  have estimated there to be around 1011 particles /L, most <100 nm. In unpolluted sandstone ground-waters, this work found most particles to be of silica, though other elements also present included Al, Fe, and Ti. Humic acids, fulvic acids, exudates from bacteria, and viruses are also present.
However, industrial manufacture has widened the range of NP types very considerably. mNPs are presently used in skin care products such as sun blocking creams (TiO2, ZnO, SiO2), self-cleaning glass (TiO2), self-cleaning refrigerators (Ag), anti-odour impregnation in clothes (Ag), car components (CNT), medicines (Au, Fe oxides), plastics (CNTs, Fe oxides), biotechnology ('quantum dots', CdSe), solar panels (TiO2), fuel additives (Ce oxides), paper (TiO2), paints (TiO2, Au), some food additives (TiO2), electronics (Au), fuel cells (CNTs), and sports equipment (C60) [4, 5]. Morphologies are also diverse, from carbon nanotubes (CNTs), ribbons, and wires, to acicular crystals, to spheres composed of one metal coated by a second. The types of particle available are growing, and the total number possible is vast.
The main reason that NPs are of interest is that they have chemical behaviours very different from the same material in larger particle size. This is partly caused by their large surface areas: a 1 cm cube of SiO2 would produce 1015 cubes of side 100 nm, increasing the surface area from 6 x 10-4 m2 to 60 m2! - at 1 nm, the surface area would be 6,000 m2. The fraction of total atoms present on the particle surface increases approximately exponentially as size decreases, reaching ~50% by about 3 nm , thus greatly increasing reactivity. Also, at these sizes, quantum effects become important and various properties are altered relative to dissolved and bulk phases, including those associated with magnetic, optical, electronic state, and catalytic reactions. This results in different types and extents of interaction with dissolved species and biological materials. It is these 'new' properties that industry is exploiting with such élan.
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