Physical and gas-phase routes to nanoparticles

Six physical and gas-phase processes for making or coating particles: laser ablation in liquid and its plasma-to-colloid sequence, the temperature zones of flame spray pyrolysis, a magnetron gas-aggregation cluster source with a mass filter, spray drying of a droplet into a glassy microparticle, and the trimethylaluminium–water cycle of atomic layer deposition. Each caption quotes the cited papers.

1. Laser ablation in liquid: A pulsed laser is aimed perpendicular to a bulk metal target submerged in liquid, from the top or from the side (Subhan 2022); the colloids are ligand-free (Khairani 2023; Molina-Prados 2026). 2. Plasma to colloid: Absorption makes superheated plasma at 4000–6000 K that expands with a shockwave and is quenched and confined by the liquid; a cavitation bubble expands and collapses, and as it collapses the nanoparticles are released into the liquid as a colloid (Subhan 2022). Production is typically in the mg/h range, with work toward g/h (Khairani 2023). 3. Flame spray pyrolysis: A precursor solution is atomised into fine droplets and ignited with an auxiliary flame; near the capillary tip the flame core reaches 3500–2500 °C, the precursor vaporises and primary particles nucleate; they coagulate and sinter at 1700–600 °C, stick into agglomerates below 600 °C and bind into aggregates at 400–200 °C; a pilot plant makes several kg/h (Dimitriou 2023). One step and continuous, it also makes core–shell particles and nanorods (Teoh 2010). 4. Gas aggregation source: A magnetron in a tubular vacuum chamber ignites a plasma and supplies the material; supersaturated metal vapour condenses by homogeneous nucleation and a co-axial gas flow carries the particles through an orifice into another chamber; a shorter aggregation zone keeps the particles smaller, and they reach the substrate at subsonic speed, landing softly (Choukourov 2017). Clusters are size-selected with a lateral time-of-flight mass filter (Tankard 2024; Lu 2020). 5. Spray drying: A liquid formulation is atomised into microdroplets that meet a dry, heated gas stream; the droplets dry rapidly and their polysaccharides form glasses that encapsulate the vaccine; the powders held less than 1 % moisture with glass transitions near 80 °C (Randolph 2025). 6. Atomic layer deposition: ALD of alumina is a two-part, self-limiting cycle: surface hydroxyls react with trimethylaluminium vapour, then with water vapour; each cycle deposits about 2.3 Å of Al₂O₃ and regenerates the hydroxyls; in a fluidised bed, tens to thousands of cycles give 2–500 nm (Randolph 2025). On TiO₂ (P25) powder, ALD SiO₂ grows as conformal sub-nanometre layers and Pt as 1–2 nm islands (Benz 2020).

Starting materials: A bulk metal target in a liquid; a precursor solution in an organic solvent; a sputter target; a liquid formulation; trimethylaluminium and water vapour. Product: Ligand-free colloids, oxide nanoparticles and aggregates, size-selected clusters, glassy microparticles and conformal alumina coatings.

Subhan, Mourad & Al-Douri, “Influence of laser process parameters, liquid medium, and external field on the synthesis of colloidal metal nanoparticles using pulsed laser ablation in liquid: a review”, Nanomaterials 2022 (doi:10.3390/nano12132144); Khairani et al., “Green nanoparticle synthesis at scale: a perspective on overcoming the limits of pulsed laser ablation in liquids for high-throughput production”, Phys. Chem. Chem. Phys. 2023 (doi:10.1039/d3cp01214j); Dimitriou et al., “Advanced flame spray pyrolysis (FSP) technologies for engineering multifunctional nanostructures and nanodevices”, Nanomaterials 2023 (doi:10.3390/nano13233006); Teoh, Amal & Mädler, “Flame spray pyrolysis: an enabling technology for nanoparticles design and fabrication”, Nanoscale 2010 (doi:10.1039/c0nr00017e); Choukourov et al., “Advances and challenges in the field of plasma polymer nanoparticles”, Beilstein J. Nanotechnol. 2017 (doi:10.3762/bjnano.8.200); Tankard et al., “Stable mass-selected AuTiOx nanoparticles for CO oxidation”, Phys. Chem. Chem. Phys. 2024 (doi:10.1039/d4cp00211c); Randolph et al., “Superior immune responses from thermostable, single-administration rabies vaccines prepared using atomic layer deposition”, J. Pharm. Sci. 2025 (doi:10.1016/j.xphs.2025.103936); Benz et al., “Synthesis of a rationally designed multi-component photocatalyst Pt:SiO₂:TiO₂(P25) with improved activity for dye degradation by atomic layer deposition”, Nanomaterials 2020 (doi:10.3390/nano10081496); Molina-Prados et al., “Beam shaping techniques for pulsed laser ablation in liquids: unlocking tunable control of nanoparticle synthesis in liquids”, Beilstein J. Nanotechnol. 2026 (doi:10.3762/bjnano.17.22); Vahl et al., “Single target sputter deposition of alloy nanoparticles with adjustable composition via a gas aggregation cluster source”, Nanotechnology 2017 (doi:10.1088/1361-6528/aa66ef); Lu et al., “Beam generation and structural optimization of size-selected Au923 clusters”, Nanoscale Adv. 2020 (doi:10.1039/d0na00304b); Witeof et al., “Atomic-layer deposition processes applied to phage λ and a phage-like particle platform yield thermostable, single-shot vaccines”, J. Pharm. Sci. 2022 (doi:10.1016/j.xphs.2022.01.013).

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