Army-funded research identified a new chemistry approach that could remove micropollutants from the environment. Micropollutants are biological or chemical contaminants that make their way into ground and surface waters in trace quantities.
Using a pioneering imaging technique, Cornell University researchers obtained a high-resolution snapshot of how ligands — molecules that bind to other molecules or metals — interact with the surface of nanoparticles. In doing so, they made an unexpected discovery; by varying the concentration of an individual ligand, they could control the shape of the particle it attached to. This approach could result in developing chemical sensors that are sensitive at a very low level to a specific chemical in the environment.
The research studied interactions of ligands and gained new understanding of the strength, or affinity, of ligand adsorption as well as how multiple ligands cooperate, or don’t, with each other. When the molecule adsorbs on the surface of a nanoscale material, it also protects the surface and makes it more stable. This can be utilized to control how nanoscale particles grow and become their eventual shape.
Understanding how ligands interact with the surface of nanoparticles has been a challenge to study. Adsorbed ligands are difficult to identify because there are other molecules in the mix and nanoparticle surfaces are uneven and multifaceted, which means they require incredibly high spatial resolution to be scrutinized.
A nanoparticle’s size and surface structures, or facets, are intrinsically tied to the particle’s potential applications. The larger the particle, the more atoms fit inside it, while smaller particles have less available space internally but a greater surface volume ratio for atoms to sit atop, where they can be utilized for processes such as catalysis and adsorption. The different types of structures the atoms and molecules form on these surface facets are directly correlated with the particle’s shape.
Scientists have used several imaging methods to survey these particles but until now, they haven’t been able to obtain nanometer resolution to explore the nooks and crannies of the multiple surface facets and quantify the affinity, or strength, of a ligand’s adsorption. The team was able to do that by employing a method called COMPetition Enabled Imaging Technique with Super-Resolution (COMPEITS).
The process works by introducing a molecule that reacts with the particle surface and generates a fluorescent reaction. A nonfluorescent molecule is then sent to bind to the surface, where its reaction competes with the fluorescent signal. The resulting decrease in fluorescence, essentially creating a negative image, can then be measured and mapped with super high resolution.
Using COMPEITS on a gold nanoparticle, the team was able to quantify the strength of ligand adsorption and they discovered ligand behavior can be very diverse. At some sites, they cooperate to help each other adsorb but at other sites, they can impair each other’s efforts. The researchers also discovered that sometimes this positive and negative cooperativity exists at the same site. In addition, the researchers learned that the surface density of adsorbed ligands can determine which facet is dominant. This crossover inspired the team to vary the concentrations of individual ligands as a way to tune the shape of the particle itself.
For example, one way to remove micropollutants, such as pesticides, from the environment is to adsorb micro-portions on the surface of some adsorbent particle. After it is adsorbed on the surface of the particle, if the particle is a catalyst, it can catalyze the destruction of the micropollutants.