Nanochemistry concerns the synthesis, characterization, and application of materials with dimensions in the 1-100 nm range. At this scale, materials exhibit properties distinct from both isolated atoms and bulk solids due to two key factors: the high surface-to-volume ratio and quantum confinement effects. As particle size decreases, the fraction of atoms on the surface increases dramatically, making surface energy and surface chemistry dominant. For a 10 nm particle, approximately 15% of atoms are on the surface; for a 2 nm particle, this exceeds 60%. This has profound implications for catalysis, sensing, and reactivity.
Quantum Confinement
Quantum confinement occurs when the dimensions of a material become comparable to or smaller than the exciton Bohr radius, restricting the movement of electrons and holes. The most important consequence is that the band gap (E_g) increases with decreasing particle size, shifting optical absorption to higher energies (shorter wavelengths). This size-dependent band gap is described by the Brus equation: E_g(R) = E_g(bulk) + h²/(8R²)(1/m_e* + 1/m_h*) - 1.8e²/(4πεε₀R), where R is the particle radius, and m_e* and m_h* are effective masses of the electron and hole. The first additional term represents the quantum localization energy, while the second accounts for the Coulomb attraction between the electron and hole (exciton binding energy).
Synthesis Strategies
Nanomaterials are synthesized by two complementary approaches. Top-down methods start with bulk material and reduce its dimensions: lithography (photolithography, electron-beam lithography) patterns nanoscale features on surfaces; ball milling mechanically grinds materials into nanoparticles; and laser ablation produces nanoparticles by vaporizing a target. Bottom-up methods build nanostructures from atomic or molecular precursors: sol-gel synthesis involves hydrolysis and condensation of metal alkoxides to form oxide networks; chemical vapor deposition (CVD) decomposes volatile precursors on a heated substrate; and colloidal synthesis using hot-injection methods produces monodisperse nanoparticles by rapidly nucleating all particles followed by controlled growth (Ostwald ripening).
Metal Nanoparticles
Gold nanoparticles (AuNPs) are perhaps the most studied metal nanostructures due to their intense colors arising from surface plasmon resonance (SPR). SPR occurs when conduction electrons in the nanoparticle oscillate collectively in resonance with incident light, producing strong absorption and scattering in the visible region. The SPR wavelength depends on particle size, shape, and local dielectric environment: spherical AuNPs absorb at ~520 nm (red), while nanorods exhibit two plasmon bands (transverse and longitudinal) with the longitudinal band tunable into the near-infrared. This tunability enables applications in colorimetric sensing (e.g., pregnancy tests), photothermal therapy, and surface-enhanced Raman spectroscopy (SERS). Silver nanoparticles have even stronger SPR and are widely used as antibacterial agents in coatings, textiles, and wound dressings.
Quantum Dots and Carbon Nanomaterials
Quantum dots (QDs) are semiconductor nanocrystals that exhibit size-tunable photoluminescence. Cadmium selenide (CdSe) QDs are the most investigated, typically passivated with a zinc sulfide (ZnS) shell to improve quantum yield and photostability. The emission wavelength is tuned from blue to red by increasing particle size (2-10 nm). QDs have narrow, symmetric emission spectra and broad absorption, making them superior to organic dyes for multiplexed imaging and bio-labeling. Carbon nanomaterials represent another major class. Fullerenes (C₆₀, C₇₀) are zero-dimensional carbon cages; carbon nanotubes (CNTs) are one-dimensional rolled graphene sheets (single-walled or multi-walled); and graphene is a two-dimensional sheet of sp²-hybridized carbon with exceptional electrical, thermal, and mechanical properties.
Characterization Techniques
Characterizing nanomaterials requires specialized techniques capable of resolving sub-nanometer features. Transmission electron microscopy (TEM) uses a focused electron beam to image particles down to atomic resolution, providing information on size, shape, and crystallinity. Scanning electron microscopy (SEM) images surfaces with nanometer resolution by detecting secondary electrons. Atomic force microscopy (AFM) uses a sharp tip to map surface topography with sub-nanometer vertical resolution, applicable to both conducting and insulating samples. Dynamic light scattering (DLS) measures particle size in solution by analyzing fluctuations in scattered light intensity due to Brownian motion. X-ray diffraction (XRD) confirms crystallinity and measures crystallite size via the Scherrer equation.
Applications of Nanomaterials
Nanotechnology has transformative applications across diverse fields. In medicine, nanoparticles serve as drug delivery vehicles that improve bioavailability, enable targeted delivery (via surface functionalization with antibodies or ligands), and provide controlled release. Nanocarriers such as liposomes, polymeric nanoparticles, and mesoporous silica nanoparticles are in clinical use or advanced trials. In catalysis, nanoparticles offer high surface area and size-dependent activity; gold nanoparticles, inert as bulk material, become excellent catalysts for CO oxidation and other reactions below 5 nm. In sensing, the extreme sensitivity of plasmonic nanoparticles to local refractive index changes enables label-free detection of biomolecules at attomolar concentrations. Environmental applications include nanofiltration membranes, photocatalytic degradation of pollutants using TiO₂ nanoparticles, and nanosensors for heavy metal detection.
