Soft nanoparticles: when polymers meet soap

Original paper: Self-Assembly of Complex Salts of Cationic Surfactants and Anionic− Neutral Block Copolymers. Dispersions with Liquid-Crystalline Internal Structure  

For more than four decades, scientists have been investigating the properties of small objects dispersed in solutions. Some of these objects – produced in laboratories – are the so called soft nanoparticles. The name soft comes from the fact that these particles are partly solid and partly liquid. One of the scientists’ aims is to design nanoparticles that will be used as carriers of medical compounds (like drugs, DNA segments, and enzymes). The nanoparticles’ role will be to protect this cargo from partial degradation through the human body until reaching the specific target cells where the nanoparticles’ structure will break up and the useful compounds will be released. This technology will allow for disease treatments using smaller amounts of drugs, which will mean fewer side effects for the patients.

The effectiveness of this treatment depends on several factors that control the nanoparticles’ properties. It turns out that one important factor is the protocol of preparation, which is the recipe used to make the nanoparticles. Today’s paper by Leticia Vitorazi, Jean-Francois Berret and Watson Loh introduces an alternative method of preparation and shows how the individual chemicals that are chosen for creating the nanoparticles can influence the nanoparticles’ properties.

The formation of the soft nanoparticles is a spontaneous process that takes place in specific mixtures of solutions because of the electrostatic attraction between two different compounds, macromolecules and surfactants. Each of these compounds usually exist as individual molecules in water (or other solvents).

Macromolecules consist of long, chain-like synthetic molecules. They can have either one long chain (called homopolymers) or two long chemically different chains connected together by covalent bonds (called diblock copolymers) (Figure 1).


figure 1.homopolymers
Figure 1. Representation of a homopolymer with a long charged chain and a diblock copolymer with a charged (A) and a non-charged (B) chain. Each sphere represents a repeated unit of the chain.


The other type of compound, surfactants, comprise short molecules with both hydrophilic (water-loving) and hydrophobic (water-hating) parts. Surfactant is what soap is made from, and its name is a shortcut to the term surface active agents. When the amount of surfactant molecules in a water solution exceeds a specific number (above the so called critical micelle concentration), the surfactants clump together and form small spheres (called micelles) with the hydrophobic parts inside the sphere to avoid contact with the water molecules, leaving the hydrophilic parts at the surface to be in contact with the water (Figure 2).


Figure 2 micelles
Figure 2. Representation of surfactant molecules with a hydrophobic tail and hydrophilic head (red spheres) surrounded by oppositely charged small ions (counterions). Above a specific concentration of surfactant in water, the surfactants molecules organize themselves in micelles with the tails inside the micelle to be protected from water and the heads in the micelle surface to be in contact with water.


Mixing macromolecule solutions with surfactants/micelle solutions causes the two compounds to come together because their opposite charges attract, and a new object is created: a nanoparticle, composed of macromolecular chains surrounded by surfactant micelles. If the macromolecule is a diblock, with a second type of non-charged chain attached chemically to the charged chain, the resulting nanoparticles usually have a specific structure with an internal core consisting of surfactant micelles and oppositely charged macromolecule chains, surrounded by an external shell. This shell consists of the macromolecules’ non-charged chains and acts as a non-stick coating that prevents the particles from clumping together (Figure 3).


Figure 3. Nanoparticles scheme
Figure 3. Schematic of soft nanoparticles with an inner core comprised of charged surfactant micelles (red), and oppositely charged polymer chains (yellow) surrounded by an external shell comprised of non-charged polymer chains (green). The red dots represent the surfactant micelles that form inside the nanoparticle core at a cubic order (enlarged scheme). Adapted from Vitorazi and colleagues.


The most popular method of creating the nanoparticles is the direct mixing method of the macromolecules and surfactants. The two compounds are dissolved separately in water (or an appropriate solvent), and then they are mixed together to reach specific amounts of the opposite charges in the solution. The mixtures consist of charged macromolecules and oppositely charged surfactants surrounded by small ions. The resulting nanoparticles are small: between 30 and 50 nm. (For comparison, HIV virus is about 120 nm large and bacteria are about 1000 nm.)

Vitorazi and co-workers used a different method named “complex salts.” The recipe of making the complex salts consists of mixing the macromolecule solutions with an increasing amount of the surfactant solutions until all acidic groups of the macromolecules bind to the hydroxide groups of the surfactants. The next step of the process is the removal of the solvent to create a single-component powder, which is freed from the small ions that surrounded the individual compounds in the solution (Figure 4A). Finally, the powder is dissolved in water at different concentrations and the electrostatic attraction between the surfactant micelles and oppositely charged macromolecules results in the formation of the nanoparticles.


Figure 4. the complex salts vs direct mixing
Figure 4. Representation of a diblock copolymer with charged (yellow parts) and non-charged (green parts) chains surrounded by surfactant micelles (red parts) in the absence of small ions (A) and in the presence of small ions (B).


The researchers used the complex salts method for different macromolecular chain lengths to explore the effect of the macromolecules’ chain lengths on the nanoparticles’ size and core structures, and they compared the results with the direct mixing method that was used in previous years for similar mixtures. They found that if the two sub-chains of the diblock copolymer (A and B in Figure 1) have roughly equal lengths then the nanoparticles were larger, compared to those made through the direct mixing method. As shown in Figure 3, these particles consist of a core with a cubic ordered structure where the surfactant micelles are positioned at specific places in the core. They also found that for unequal sub-chain lengths, the core was disordered and the nanoparticles were smaller. This is an important finding because the ordered structure and the larger size of the nanoparticles can incorporate larger amounts and different types of drugs in the core.

They also studied the effect of the addition of salt or macromolecular solutions in the nanoparticle solutions. The amount of salt strongly affects the nanoparticles’ properties because the added ions in the nanoparticles’ solution weaken the attraction between the macromolecules and the oppositely charged surfactant micelles. Therefore, it is a useful factor to be considered for the biomedical applications in the salty environment of the cell system in our body. Vitorazi and co-workers found that after the addition of salt or macromolecule solutions in the nanoparticles’ solution, the already formed nanoparticles lost their internal cubic structure (meaning that the micelles were now randomly oriented in the core) but the stability of the system was preserved.

To conclude, the method of preparing soft nanoparticles plays an important role in determining their properties and therefore affects their performance as drug carriers. Compared to direct mixing methods, the complex salts formed stable and 3 times larger nanoparticles with a core of cubic internal order. In a charged environment (created by addition of small or macromolecular ions) the final structure lost its cubic structure, but the nanoparticles were still stable, making them important candidates for the future of drug delivery technology.


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