This chapter explains how nanoparticles are produced and provides an overview of the main approaches developed to date, highlighting their advantages and limitations.
Nanoparticle production methods are generally classified into two categories: “top-down” and “bottom-up” approaches.
In the top-down approach, energy is applied to bulk materials to progressively break them down into smaller fragments until nanoparticles are formed.
One of the most widely used methods is milling, which relies on mechanical energy. Various types of mills have been developed to process materials of different hardness, brittleness, and textures. Hard but brittle materials are crushed, soft and ductile materials are abraded, and fibrous materials are torn apart. Water with surfactants is often added to assist in material breakdown and to stabilize the resulting nanoparticles (i.e., to prevent aggregation). This method typically yields particles ranging from ~100 to 1000 nm with a broad size distribution.
Ultrasonic dispersion is a variation of the top-down method. When ultrasound propagates through a liquid, it generates vacuum bubbles that grow and collapse, a process known as cavitation. The resulting shockwaves and high-velocity jets disintegrate suspended materials into nanoparticles. Surfactants again help by stabilizing the particles and modifying the acoustic properties of the medium. A key drawback of this method is the high noise level.
Much smaller nanoparticles can be produced using explosive techniques, where stored chemical energy is rapidly released as heat and pressure. This method yields particles between 5 and 30 nm, with controlled shapes (spheres, rods, plates, dendrites), depending on the type and amount of explosive and the design of the detonation system.
Electrical explosions of conductive materials such as metals can also generate nanoparticles. A high-current pulse (10⁴–10⁶ A/mm²) passed through a thin wire causes it to explode. Particle size is influenced by current density, wire diameter, and ambient gas pressure.
Both explosive methods allow rapid nanoparticle formation. However, they often trap significant amounts of gas (up to 10 wt.%) inside the particles due to the high-energy environment. When heated to ~250–500 °C, these gases are released, making the particles extremely reactive. For example, aluminium nanoparticles begin to burn in a nitrogen atmosphere, forming high value nanosized aluminium nitride.
In the bottom-up approach, nanoparticles are assembled from atoms or molecules generated via evaporation (physical methods) or chemical reactions.
Vacuum evaporation produces highly pure, uncharged particles and is ideal for reactive materials like alkali metals. When evaporation occurs through a jet, the atoms or molecules form a directional flow with minimal collisions. The “free path” between collisions depends on the vacuum level, allowing precise control of particle size (from 2 to 1,000,000 atoms or molecules) and surface characteristics.
| Pressure, mmHg | 10-2 | 10-3 | 10-4 | 10-6 |
| Length of the free path, cm | 0.5 | 5 | 50 | >1,000 |
However, particle concentration in such flows is extremely low, and their lifespan is very short (0.001–0.002 s). A common strategy to increase both, is to deposit them onto a substrate, although this poses risks of deformation, fragmentation, or crater formation. These issues are mitigated via matrix isolation, where nanoparticles are co-deposited with an inert gas onto a substrate cooled below the gas’s melting point (e.g., argon at 77 K). The particles become embedded in a solid matrix, which prevents diffusion and aggregation.
For substances with high decomposition or evaporation temperatures, chemical synthesis is preferred. In some cases, reactions occur in a gas phase: a volatile precursor decomposes, producing atoms that then form nanoparticles. For instance, while tungsten evaporates at 5555 °C, its carbonyl complex W(CO)₆ evaporates at 170 °C and decomposes at ~400 °C to yield tungsten atoms.
Another gas-phase method is chemical vapor deposition (CVD), where vapor-phase precursors react upon contact with a substrate. CVD is widely used to produce nanoparticles and thin films—for example, nanodiamonds from methane, or polysilicon films from silane or trichlorosilane.
Performing chemical reactions in liquid media has major advantages. The reagent concentration in a liquid is ~1000 times higher than in gas, resulting in faster nanoparticle growth, higher productivity, and lower cost. However, liquid-phase synthesis often results in product contamination and side reactions. Surfaces of nanoparticles formed in liquid are always coated with adsorbed species, which, along with high diffusivity, can cause aggregation or coalescence into larger particles. Therefore, stabilizers are commonly used.
Various chemical strategies are employed in solution:
A non-reactive bottom-up technique is antisolvent precipitation, where a dissolved substance is mixed with a second solvent in which it is insoluble, triggering nanoparticle formation. This method is widely applied to bioactive compounds.
Some molecules can spontaneously organize into nanostructures, a process known as self-assembly. Surfactants are a prime example. These molecules contain a polar “head” (hydrophilic) and non-polar “tail” (hydrophobic). At concentrations above a critical threshold, surfactants form micelles, spherical structures with hydrophobic tails inside and hydrophilic heads on the surface. As concentration increases, micelles can transition into rod-like structures or liposomes (closed bilayer spheres). In non-polar solvents, reverse micelles form, where the heads are hidden inside.
Self-assembled nanostructures differ fundamentally from other nanoparticles: micelles are stable, dynamic systems that continuously merge and divide. An important exception is solid lipid nanoparticles, in which micelle cores crystallize upon cooling or solvent evaporation. These solidified structures are more permanent and can be engineered through controlled conditions.
Unlike micelles, conventional nanoparticles are intrinsically unstable, as they tend to minimize surface energy by aggregating or coalescing. As such, special efforts are required to control their size, shape, and structure during synthesis.
In the next chapter, we will explore the stages of nanoparticle growth and discuss methods to control and stabilize the resulting particles.