Materials
Metal nanoparticles, gold and plasmonics
A block of gold is yellow and shiny. A suspension of gold particles a few tens of nanometres across is a deep wine red. That change of colour, known to glassmakers for centuries and explained only in the modern era, is the entry point to one of nanoscience's most active subjects.
What a nanoparticle is
The Wikipedia article on nanoparticles defines a nanoparticle as a particle of matter 1 to 100 nanometres in diameter, noting that metal particles smaller than about a nanometre are usually called atom clusters instead. It explains two consequences of that size. First, such particles are much smaller than the wavelengths of visible light, so they cannot be seen with ordinary optical microscopes and their suspensions can be transparent. Second, because an atom is only a fraction of a nanometre across, a large share of a nanoparticle's material lies within a few atomic diameters of its surface, which makes its properties differ markedly from those of the bulk metal.
Gold, from stained glass to the Royal Institution
The Wikipedia article on colloidal gold describes it as a suspension of gold nanoparticles in a fluid, usually water, coloured wine red for spherical particles smaller than 100 nm and blue-purple for larger spheres or rod-shaped particles. It notes that colloidal gold has been used since ancient times to stain glass, giving the deep reds seen in some old windows and vessels.
Scientific study began with Michael Faraday. The same article records that in 1856, in a basement laboratory of the Royal Institution, Faraday accidentally produced a ruby-red solution while mounting gold leaf on microscope slides; that he went on to investigate its optical properties; and that in 1857 he prepared the first pure sample of what he called divided gold by reducing a solution of gold chloride with phosphorus. The article adds that samples Faraday made are still optically active, and that he recognised the colour was due to the very small size of the gold particles, at a time when several chemists suspected a gold–tin compound instead.
Plasmons: electrons that ring like a bell
The modern explanation of the colour involves plasmons, collective oscillations of the free electrons in a metal. The Wikipedia article on surface plasmon resonance describes two related phenomena. In a thin metal film, light arriving at a particular angle can excite electrons that travel along the surface, and because the angle depends sensitively on the refractive index at the boundary, the effect can detect molecules binding to the surface; the article notes that biosensors have been built on this principle. In a metal nanoparticle, the equivalent effect is the localised surface plasmon resonance: a collective oscillation of the particle's electrons driven by light, with an enhanced electric field concentrated at the particle that decays rapidly away from it.
For gold and silver particles, that resonance falls in the visible part of the spectrum. The particle strongly absorbs and scatters light near the resonance wavelength, and what passes through carries the complementary colour. Change the size or shape of the particle and the resonance moves, so the colour changes. The colloidal gold article notes, for example, that rod-shaped particles have two absorption peaks, one for oscillations across the rod and one along it.
Size, shape and surface
The colloidal gold article stresses that the properties of gold nanoparticles, and so their possible uses, depend strongly on their size and shape. That is why so much of the metal-nanoparticle literature is about synthesis: making particles of uniform size, growing rods, cubes, shells and stars, and controlling the molecules bound to their surfaces. Surface molecules matter as much as the metal core. They keep particles from clumping, decide whether particles disperse in water or oil, and provide handles for attaching other molecules. The article lists research interest in gold nanoparticles across electron microscopy, electronics, materials science and biomedicine, all resting on their optical, electronic and molecular-recognition properties.
Reading metal-nanoparticle papers
Papers in this area usually report an absorption spectrum, electron micrographs and a size distribution. A few checks are useful. Is the reported size from electron microscopy, which measures the metal core, or from light scattering, which includes the surface layer and any surrounding water? Does the spectrum match what the images show, or does a broad shoulder suggest aggregation? And for claims about sensing or enhancement, how large is the effect compared with a proper control, and was it measured on one sample or many?
Gold particles also feature in biological research, where their size affects how they behave in living systems; a review on that question is discussed on the nanomedicine page. For the semiconductor counterpart to size-dependent colour, see quantum dots; for the broader reasons small things differ from large ones, see what nanotechnology is.