From the lab: A uniform nano world

As of now, we create nanoparticles either by breaking down larger normal sized materials to nano size, or by assembling atoms or molecules in building-block fashion to reach nano sizes.

Pune | Published: March 20, 2016 1:51:15 am

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There are enormous possibilities of nanoscience applications in diverse fields such as health, drug delivery, food processing, water purification, electronics, and many others.

A nanoparticle (or nanopowder or nanocluster or nanocrystal) is a microscopic particle with at least one of its dimensions less than 100 nanometres (nm). One nanometre is a billionth of a metre. Typically, a nanoparticle would be about thousand times smaller than the tip of human hair.

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The reason why nanoparticles become special, and therefore have been attracting so much scientific interest, is the fact that these materials exhibit very different properties when they are reduced to nano-sizes. For example, a nanoparticle of gold does not have golden colour. It exhibits very different colours depending on the size of the particle in the nano-range. So a 4 nm gold particle has a different colour than a 50 nm or a 200 nm gold particle, and none of them is golden. Similarly, white gold, as we see it, melts at about 1100 degrees Celsius, while nanoparticles of gold can melt at 100 degrees Celsius.

At nano-sizes, particles also start exhibiting some quantum properties, like super-paramagnetism. Quantum properties are associated with sub-atomic particles like electrons or neutrons that behave very differently than objects of sizes that we are familiar with. Such special properties open up a variety of possibilities for use of known materials for novel applications.

As of now, we create nanoparticles either by breaking down larger normal sized materials to nano size, or by assembling atoms or molecules in building-block fashion to reach nano sizes. The reductionist approach involves chemical reactions. For example, gold nanoparticles can be synthesised by reacting gold chloride and sodium borohydride.

While both methods of synthesis of nanoparticles work fine, neither of them produces evenly-sized nano particles. Rather, randomly sized particles in the nano-range are achieved. This is called polydisperse preparation.
Scientists for long have been trying to develop methods that produce nanoparticles of a specific size, or to achieve monodispersity. It is not difficult to see why this is important. Let’s look at gold again. A mixture of gold nanoparticles of all sizes will show no particular colour. That means that some properties, colour in this case, would not become evident in a random mixture of all sizes. Since we are interested in using specific properties of these nanoparticles, and since many of these properties are size-dependent, it becomes very important to create nanoparticles of uniform size that exhibit those specific properties.

It is here that we have been able to achieve some success. As mentioned, nanoparticles can be prepared using chemical reactions. These generally lead to polydisperse preparations, especially in case of polymers which have large and complex molecular structures. To achieve monodispersity, the reaction parameters like temperature, rate of reaction, time, concentration of reactants etc need to be very precisely controlled and monitored. But this is very difficult to achieve in normal reactions.

So we decided to confine the reaction volume to minute quantities in order to precisely control the reaction parameters. For our experiment we decided to produce nanoparticles of chitosan, an FDA-cleared biodegradable and biocompatible natural polymer that is already being used as drug-delivery agent, for gene therapy and other processes. We used the reactant, chitosan, as micro-droplets. With such confinement, we were able to control factors like temperature, rate of reaction, time etc. This reaction did indeed produce nanoparticles of uniform size. And unlike normal reactions, which take between 20 and 60 minutes to produce chitosan nanoparticles, we were able to produce the nanoparticles in just 5 seconds.

For our experiment, we designed and fabricated a micro-reactor that can be operated in a continuous flow mode, so that large quantities of monodisperse nanoparticles can be produced in a sequential manner. The cylindrical micro-reactor has an inlet channel and an outlet channel. The reactants are introduced in a continuous flow and nanoparticles are also synthesised continuously. Using this method, we have now been able to produce chitosan nanoparticles of any size between 50 nm and 600 nm within an accuracy error of below 10 per cent.

By- Kishore Paknikar & team

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