Nanochemistry

Nanoparticles are between 1 nm and 100 nm in size (1 nm = 1 \times 10^{-9} m). Compare them with fine particles (PM_{2.5}, 100 to 2500 nm) and coarse particles such as dust (PM_{10}, 2500 to 10 000 nm).

Their special properties come from their surface area to volume ratio. Imagine a cube of side 1 cm: surface area 6 cm^2, volume 1 cm^3, ratio 6. Shrink the side to 1 mm and the ratio becomes 60. Every time the side is divided by 10, the ratio goes up 10 times. So far more of a nanoparticle's atoms sit on the outside.

Uses:

• silver nanoparticles kill bacteria, so they are added to wound dressings;
• titanium dioxide and zinc oxide nanoparticles in sun creams block UV light but look clear on the skin;
• nanoparticles make good catalysts because of their large surface area;
• carbon nanotubes are strong and light, used in some sports equipment.

Risks: because they are so small they may be breathed in deeply or pass into cells, and their effects on health and the environment are not fully known.

Size and surface. For a sphere of radius r, surface area A = 4\pi r^2 and volume V = \tfrac{4}{3}\pi r^3, so

Halving the radius doubles the ratio. A gold particle 2 nm across has roughly half its atoms on the surface, whereas in a 1 mm grain the fraction is negligible. Surface atoms have fewer neighbours, so they are higher in energy and more reactive. This is why finely divided metals are used as heterogeneous catalysts: platinum, palladium and rhodium in catalytic converters are spread as nanoparticles over an alumina support to get the most active surface from as little precious metal as possible.

Carbon nanostructures. Fullerenes such as C_{60} are hollow cages of 60 carbon atoms in pentagons and hexagons. Nanotubes are rolled-up graphene sheets: each carbon is bonded to three others, leaving delocalised electrons that let them conduct, while the network of strong covalent bonds gives a very high tensile strength. Graphene, a single layer of graphite, is one atom thick.

Colour from size. Bulk gold reflects yellow light, but gold nanoparticles absorb green light strongly because their conduction electrons oscillate together (a surface plasmon), so a suspension looks red. Ruby glass and the Roman Lycurgus Cup owe their colours to this effect. Semiconductor quantum dots change emission colour with size: smaller dots emit bluer light.

Risks. Properties that make nanoparticles useful, such as high reactivity and ease of entering cells, are also why their toxicity is studied separately from the bulk material. Inhaled particles can reach deep into the lungs, and some generate reactive oxygen species. Regulation now treats nanoforms of a chemical as needing their own safety data.

Making nanoparticles

Bottom-up routes build particles from atoms; top-down routes (milling, lithography) carve them from bulk. A classic bottom-up synthesis is the Turkevich method: HAuCl_4 is reduced by sodium citrate in boiling water, and citrate also caps the particles, giving about 15 to 20 nm gold spheres. In the Brust–Schiffrin method, AuCl_4^- is phase-transferred into toluene, reduced with NaBH_4 and capped by alkanethiols, giving 1 to 5 nm particles that can be isolated as solids.

Size control follows the LaMer model: a rapid burst of nucleation once the monomer concentration passes a critical supersaturation, then growth with no new nuclei, giving a narrow size distribution. Hot injection exploits this: cold precursors are injected into a hot coordinating solvent, nucleation is instantaneous, and the temperature drop stops further nucleation. Later, Ostwald ripening broadens the distribution because small particles, with higher surface energy, dissolve and redeposit on large ones.

Supported catalysts

For catalysis, metal particles are fixed on oxide or carbon supports. Impregnation of a support with a metal salt followed by calcination and reduction is simple but gives broad size distributions, and small particles sinter on heating because of their high surface energy. Sol immobilisation gives better control: gold (or Au–Pd) colloids are first made in solution with a stabilising polymer such as PVA or PVP and NaBH_4, then deposited onto, for example, TiO_2. The rate of a catalysed reaction, in the absence of diffusion limits, scales with exposed metal area, but activity also depends on which crystal faces are exposed and on metal–support interactions; gold becomes active for CO oxidation only below a few nanometres.

Quantum confinement

When a semiconductor crystal is smaller than its exciton Bohr radius (about 5 nm for CdSe), the band gap widens. The Brus model gives approximately

a particle-in-a-sphere confinement term that grows as 1/r^2, minus an electron–hole Coulomb term. So CdSe dots shift from red to blue emission as they shrink, even though bulk CdSe absorbs throughout the visible region.

Characterisation

TEM gives size and shape directly; dynamic light scattering gives a hydrodynamic diameter including the ligand shell; UV–vis tracks plasmon bands and quantum dot absorption edges; powder XRD peak widths give crystallite size through the Scherrer equation.

The 2023 Nobel Prize in Chemistry (Ekimov, Brus and Bawendi) recognised quantum dots, whose story shows how nanochemistry moves from curiosity to product: hot-injection synthesis gave monodisperse dots, core–shell designs (CdSe/ZnS) passivated surface traps to push quantum yields towards unity, and cadmium-free InP dots now dominate QD display films.

Current research themes:

Atomic precision. Thiolate-protected gold clusters such as Au_{25}(SR)_{18} and Au_{102}(SR)_{44} have been solved by single-crystal X-ray diffraction, giving molecule-like nanoparticles whose catalytic and optical properties can be tied to exact structures. Single-atom catalysts push dispersion to its limit, though proving that isolated atoms, not small clusters, are the active sites under reaction conditions is a recurring argument.

Surface chemistry. The ligand shell often controls behaviour more than the core. Ligand exchange, the covalent-bond classification of X-, L- and Z-type ligands on quantum dot surfaces, and NMR studies of binding dynamics are used to design stable, bright, processable particles. For perovskite nanocrystals (CsPbBr_3), surface instability is the main obstacle to devices.

Mechanism and automation. In situ liquid-cell TEM and synchrotron scattering show that many particles form by non-classical pathways (prenucleation clusters, particle attachment), not LaMer kinetics. Flow reactors with real-time spectroscopy and machine-learning optimisation now map synthesis conditions far faster than batch work.

Biology and safety. Lipid nanoparticles delivered the mRNA COVID-19 vaccines. Yet a widely cited meta-analysis found that a median of under 1% of an injected nanoparticle dose reaches a solid tumour, prompting a rethink of passive targeting through leaky vasculature. The protein corona that forms in blood changes particle identity, and nanotoxicology still lacks standard dose metrics (mass, number or surface area).