Iron oxide nanoparticles and silica particles: Only a targeted combination of methods for particle characterization provides a complete and reliable picture of their surfaces.
Source: BAM
The safe use of nanomaterials and their optimum performance depend largely on their surfaces. However, the reliable characterization of nanomaterial surfaces is still challenging. Two new studies by the Federal Institute for Materials Research and Testing (BAM) show that only a targeted combination of modern analytical methods can deliver reliable results. The researchers are thereby laying an important foundation for the safe and sustainable use of nanomaterials.
The surface of nanomaterials largely determines their function, how they interact with their environment, and how safe they are. However, nanomaterial surface chemistry is often complex, sensitive to environmental conditions, and difficult to measure in practice. Single measurement methods often provide incomplete or method-specific results—for example, when surface coatings cannot be clearly distinguished from other substances.
To address this challenge, BAM conducted two studies on the analysis of different types of nanoparticles used in life sciences and consumer products. In these studies, conventional and newly developed measurement methods and combinations of methods were systematically compared.
More detailed analysis of citrate-coated iron oxide nanoparticles
The first study focused on magnetic iron oxide nanoparticles with citrate as a surface ligand. This molecule binds to the particle surface and influences nanoparticle stability and interactions with the environment. Citrate ensures that the particles remain stable in water—an important prerequisite for applications, e.g., in life and environmental sciences, medicine, sensor technology, and materials research.
The researchers found that conventional methods such as thermogravimetry often provide inaccurate values because they cannot reliably distinguish citrate from other substances. Only the combination of modern analytical methods, in particular high-performance liquid chromatography and quantitative nuclear magnetic resonance spectroscopy (qNMR), enabled a significantly more precise, albeit chemo-selective determination of the surface coating. To apply qNMR to magnetic nanomaterials, a special sample preparation method had to be developed beforehand to reliably remove interfering magnetic iron compounds.
The results, obtained in the European metrology projects MetrINo and SMURFnano, were published in the journal Analytical Chemistry. They form an important basis for future measurement standards and applications of highly selective qNMR on challenging samples such as magnetic nanomaterials.
New approaches for determining functional groups on silica nanoparticles
In a second study within the framework of SMURFnano, the team investigated methods for characterizing and quantifying functional groups, ligands, and coatings on nanomaterials. The project is funded by the European metrology program. The focus was on silica nanoparticles whose surfaces are equipped with so-called amino groups. These groups act as small "docking sites." They enable the particles to be bound or modified with other substances—such as dyes, specific recognition structures, or biomolecules. Such functional groups are crucial for the stability, compatibility, and function of the particles. However, until now, there exist no reliable, validated and standardized methods for determining their exact number and chemical composition.
The researchers showed that only a targeted combination of methods – such as optical techniques, electrochemical analyses, qNMR, and X-ray Photoelectron Spectroscopy – provide a complete and reliable picture of the particle surface. This finding is central to quality control, the development of new materials, and the assessment of their safety. The electrochemical techniques used have since been presented in the journal ACS Measurement Science Au.
Basis for future standards
"Our studies show how important it is to combine different measurement methods to evaluate the actual functionality and safety of nanomaterials," explains Ute Resch-Genger, head of the projects at BAM and coordinator of SMURFnano. "Together with the developed and validated measurement methods and reference data, this is a decisive step toward the development of sustainable and high-performance nanomaterials and their safe use."
The work was funded by the European Partnership on Metrology (EPM) and provides an important basis for future standards in nanomaterial analysis, such as ISO TC229 Nanotechnologies.