Using Ethanol for synthesis of nano-materials and porous materials

By Adrian Dingle

The simple alcohol ethanol, C₂H₅OH, is a ubiquitous solvent. As a relatively non-toxic, water-miscible, and renewable resource, its versatility makes it an attractive choice for many industrial, commercial, and domestic situations. One such modern use is in the production of various types of nanoparticles.

What are nanoparticles?

Before a discussion of ethanol’s role in nanoparticle production, we should define what we mean by a nanoparticle. Nanoparticles are ultrafine pieces of matter that have diameters between 1 and 100 nanometers (nm) in size, i.e., are between 1 x 10^-9 m and 1 x 10^-7 m across. To get an idea of the scale we are talking about here, single atoms are usually in the range of 0.1 to 0.5 nm or 1 x 10^-10 m to 5 x 10^-10 m in diameter.

Such tiny particles cannot be seen with normal optical microscopes, and they can only be viewed using electron microscopes. These almost impossible to imagine particles are at the center of what we more generally call nanotechnology, and they offer exciting new possibilities in areas such as health and medicine.

What makes nanoparticles special?

The extremely tiny nature of nanoparticles gives them some extraordinary properties that are quite unpredictable when compared to the larger versions of the same materials. At such small sizes, the surface area of the particles becomes very large when compared to their volume. This is in direct contrast to the same materials in bulk sizes where the surface area is much smaller compared to the total volume. In addition, at the nanoparticle level, we start to observe quantum effects. Quantum effects occur when and where the properties of the materials are dominated by the sub-atomic particles like electrons and quantum mechanics, rather than the classical, Newtonian physics that exist on the macroscale. Several metals (copper and gold for example) have large differences observed in some physical properties such as malleability, ductility, and melting point when one compares the bulk and nanoparticle arrangements of the same atoms.

Copper is a much harder material at the nanoscale when compared to the bulk.

Use of ethanol in the production of nanoparticles

In 2017 a United States patent was issued for, “The Synthesis of nanoparticles using ethanol” to Brookhaven Science Associates LLC of Upton, NY, and inventor Jia Xu Wang. The patent describes the role of ethanol as a solvent, and as a reducing agent in the creation of noble-metal* nanoparticles.

*Noble metals are metals that are resistant to corrosion and reaction such as gold, platinum and palladium among others.

Such metals are desirable for use as nano-catalysts as part of the process of making hydrogen fuel-cells. The production of such catalysts for commercial use must meet several criteria. There needs to be a uniformity of particle size – that’s to say, a narrow distribution of particle size. The particles need to be durable to allow for repeated use. Finally, the method of production needs to be economically viable. Prior to the use of ethanol in nanoparticle production, expensive and potentially harmful chemicals were used. The alternative use of ethanol makes the new process one that the inventor claims is “green.” The patent also reports both very high yields of more than 98%, and when starting with metal salts and using ethanol, the need for only simple rinsing rather than more extensive cleaning of the nanoparticles at the end of the process.

The role of ethanol in the patented process is two-fold. Firstly, it can act as a simple solvent, and secondly and perhaps more unusually, it can act as a reducing agent by giving electrons away to convert the positive metal ions in the salts into the metal atoms themselves.

Oxidation of ethanol, i.e., its role as a reducing agent

Ethanol can be oxidized (i.e., act as a reducing agent) in a two-step process. The first step involves its conversion to ethanal (CH₃CHO), an aldehyde, and can be summarized by the equation below where [O] represents a generic oxidizing agent.

• C₂H₅OH + [O]  CH₃CHO + H₂O (step 1)

The second step in the process involves the oxidation of the ethanol product from step 1, summarized by the equation,

• CH₃CHO + [O]  CH₃COOH (step 2)

According to the patent, the specificity of the reducing power of ethanol can be carefully fine-tuned by the use of alkali solutions, and by controlling the temperature. Such control is highly desirable, and is highlighted in one particular case during the production of ruthenium (Ru) nanoparticles.

Ruthenium nanoparticle production

Ru³⁺ salts can be reduced to Ru²⁺ by utilizing the first step in the ethanol oxidation. According to the authors, this yields a stable ethanolic solution that can be stored at room temperature for long periods. The next stage is the addition of an aqueous alkali, and this provokes the second stage of ethanol’s oxidation, converting the ethanal to ethanoic acid (CH₃COOH) and in the process reducing Ru²⁺ to Ru. The process is said to produce, “Ru nucleation in a very uniform manner, and thus results in a narrow particle size distribution”. A highly desirable outcome.

Mesoporous Silica Nanoparticles (MSNs)

Meso-(or medium)-porous silica nanoparticles are porous nanoparticles with diameters of between 2 and 50 nm. One of the main uses of MSNs is in drug delivery. The pores allow the drug to be first introduced to the nanoparticles that act as carriers, and those carriers allow cells to take up the drug. Additionally, MSNs can be used as biosensors, carrying dyes into cells to aid observation – which would otherwise be impossible since the cells would normally be impenetrable to the dye.

The role of ethanol in the synthesis of silica nanoparticles

Ethanol is used as a co-solvent alongside other solvents such as larger alcohols like propanol, butanol, and even longer chain compounds such as dodecanol. By varying the composition of the solvent mixture, both the size of the nanoparticles produced, and the speed of the reaction can be controlled. Ethanol has at least one other important role in the production of MSNs. It is commonly used as a washing agent to remove unwanted molecules from the nanoparticles at the end of the production process.

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