Nanoparticles & Properties

The nanoscale spans 1–100 nm. At this range, structures are larger than individual atoms or small molecules (which are ~0.1–1 nm) but smaller than bulk materials. The prefix 'nano' derives from the Greek for 'dwarf'; 1 nm = 1 × 10⁻⁹ m. Nanoparticles can be zero-dimensional (quantum dots), one-dimensional (nanowires, nanotubes), or two-dimensional (graphene sheets). Their properties depend critically on both their size and shape.

For a spherical nanoparticle of radius r, surface area = 4πr² and volume = (4/3)πr³, giving SA:V = 3/r. Halving the radius doubles the SA:V ratio. For a 1 nm radius gold nanoparticle, SA:V ≈ 3 × 10⁹ m⁻¹, whereas for a 1 mm radius gold sphere, SA:V ≈ 3000 m⁻¹ — a million-fold difference. As a result, a 10 nm gold nanoparticle has roughly 30% of its atoms on the surface, compared to far less than 1% for a bulk grain. These surface atoms have incomplete coordination shells, making them highly reactive and fundamentally changing chemical and physical behaviour.

Bulk gold appears yellow, but colloidal gold nanoparticles (~20 nm) appear deep red. This is explained by localised surface plasmon resonance (LSPR): the free electrons in the nanoparticle oscillate collectively in resonance with incident light at a specific frequency. For gold, this resonance falls in the green-blue region (~520 nm), so green light is absorbed and red light is transmitted or scattered. As particle size changes, the resonance frequency shifts, producing colours from red through purple. This quantum-scale optical behaviour is entirely absent in bulk gold.

Nanoparticles have significantly lower melting points than their bulk equivalents. For gold, the bulk melting point is 1064 °C, but 2 nm gold nanoparticles melt at approximately 300 °C. This occurs because surface atoms are less strongly bound (fewer nearest neighbours) and require less thermal energy to overcome cohesive forces. The Gibbs–Thomson equation describes this: ΔT_m = T_m(bulk) − T_m(nano) ∝ 1/r. This property is exploited in nanoparticle-based solders and conductive inks.

The enhanced SA:V ratio means nanoparticle catalysts provide vastly more surface-active sites per gram of material. Platinum nanoparticles (~3 nm) used in catalytic converters can decompose carbon monoxide: \ce{2CO + O2 -> 2CO2} at lower temperatures and with far less platinum than bulk catalysts. Gold, which is inert as a bulk metal, becomes an excellent catalyst as nanoparticles for reactions like CO oxidation at room temperature. Iron oxide nanoparticles (\ce{Fe3O4}) catalyse degradation of organic pollutants via Fenton-like reactions.

Traditional sunscreens used micron-sized \ce{TiO2} or \ce{ZnO} particles, which scatter visible light and leave a white residue on skin. Nanoparticle formulations (20–50 nm) are small enough not to scatter visible light (wavelength 400–700 nm), so they are transparent, yet they still absorb UV radiation (280–400 nm) effectively. \ce{TiO2} absorbs UVB (280–315 nm) and \ce{ZnO} absorbs across UVA (315–400 nm) and UVB. The nanoparticles are usually coated with silica (\ce{SiO2}) or alumina (\ce{Al2O3}) to prevent photocatalytic degradation of skin lipids, since bare \ce{TiO2} generates reactive oxygen species (\ce{O2^{.-}}, \ce{HO^.}) under UV illumination.

The same properties that make nanoparticles useful — high reactivity, small size, ability to penetrate membranes — raise legitimate safety concerns. Nanoparticles can potentially cross biological membranes, including the blood–brain barrier, and their increased reactivity may cause oxidative stress in cells. Silver nanoparticles (\ce{Ag} NPs), used in antibacterial textiles, release \ce{Ag+} ions that are toxic to aquatic organisms. Regulatory bodies such as the European Chemicals Agency (ECHA) require nanomaterials to be assessed separately from their bulk equivalents under REACH legislation. This remains an active research area.

• SA — Surface Area (m²): The total outer surface area of the spherical nanoparticle, equal to 4πr².
• V — Volume (m³): The total volume of the spherical nanoparticle, equal to (4/3)πr³.
• r — Radius (m (or nm)): The radius of the nanoparticle. Decreasing r by a factor of 10 increases SA:V by a factor of 10.

Problem. A chemist prepares two spherical gold nanoparticles: Particle A has a radius of 5 nm and Particle B has a radius of 50 nm. (a) Calculate the SA:V ratio for each particle. (b) By what factor is Particle A's SA:V ratio greater than Particle B's? (c) Suggest why Particle A would be a more effective catalyst.

Answer. Particle A SA:V = 6.0 × 10⁸ m⁻¹; Particle B SA:V = 6.0 × 10⁷ m⁻¹; Particle A has a ratio 10 times greater, giving more surface active sites and greater catalytic activity.

• Adsorption and Surface Area — Understanding adsorption on solid surfaces is essential context for nanoparticle catalysis — the high SA:V ratio of nanoparticles maximises the number of adsorption sites available for reactant molecules.
• Heterogeneous Catalysis — Nanoparticle catalysts are a prime example of heterogeneous catalysis, where the catalyst exists in a different phase from the reactants. Concepts of activation energy and reaction rate connect directly to nanoscale surface chemistry.
• Quantum Mechanics and Atomic Structure — The unique optical properties of nanoparticles (e.g. surface plasmon resonance, quantum confinement in quantum dots) arise from quantum mechanical effects that only emerge when particle dimensions approach the de Broglie wavelength of electrons.
• Metallic Bonding — The free electron model of metallic bonding explains bulk metal properties. Nanoparticles deviate from bulk behaviour precisely because their small size disrupts the continuous band structure assumed in classical metallic bonding models.
• Factors Affecting Reaction Rate — Surface area is a key factor affecting reaction rate. Nanoparticles dramatically demonstrate this principle: the same mass of gold catalyst as nanoparticles versus bulk metal can increase reaction rates by many orders of magnitude.