Bottom-up Method

Bottom-up or constructive method is the build-up of material from atom to clusters to nanoparticles. Sol-gel, pyrolysis, biosynthesis, spinning and chemical vapor deposition (CVD) are the most commonly used bottom-up methods for nanoparticle production.

Sol-gel is the most preferred bottom-up method due to its simplicity and versatility as most of the nanoparticles can be synthesized from this method. The sol is a colloidal solution of solids suspended in a liquid phase. The gel is a solid macromolecule submerged in a solvent. This method is a wet-chemical process containing a chemical solution acting as a precursor for an integrated system of discrete particles. Metal oxides and chlorides are the typically used precursors in sol-gel process. The precursor is then dispersed in a host liquid assisted either by mechanical mixing or sonication, and the resultant system contains a liquid and a solid phase. A phase separation is carried out to recover the nanoparticles by various methods such as sedimentation, filtration and centrifugation and the moisture is further removed by drying.

Pyrolysis is the most commonly used process in industries for large scale production of nanoparticle. It involves burning of precursors with flame. The precursor, either liquid or vapor, is fed into the furnace at high pressure through a small hole where it burns. The combustion or by-product gas is then air classified to recover the nanoparticles. Some of the furnaces use laser and plasma instead of flame to produce high temperature for easy evaporation. Pyrolysis provides a simple, high-yield, and continuous process for cost-effective production of nanoparticles.

Biosynthesis is an environmental friendly approach for the synthesis of nanoparticles that are nontoxic and biodegradable. Instead of using chemicals, biosynthesis takes advantage of naturally occurring substances such as bacteria, plant extracts, fungi along with the precursors to produce nanoparticles for bioreduction and capping purposes. The biosynthesized nanoparticles have unique and enhanced properties that find their way in biomedical applications.

Chemical Vapour Deposition (CVD) is the deposition of a thin film of gaseous reactants onto a substrate. The deposition is carried out in a reaction chamber at ambient temperature by combining gas molecules. A chemical reaction occurs when a heated substrate comes in contact with the combined gas. This reaction produces a thin film of product on the substrate surface that is recovered and used. Substrate temperature is the influencing factor in CVD. Advantages of CVD include high product purity, uniformity, and unique robustness. Disadvantages may include the requirement of special equipment and the toxic gaseous by-products.

Reference:
Ealia, S. "A review on the classification, characterisation, synthesis of nanoparticles and their application." IOP Conference Series: Materials Science and Engineering. IOP Publishing, 2017.

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