Making surface-functionalized nanoscale candidate reference and test materials for interlaboratory comparisons involving multi-method studies.

The reliable characterization of engineered nanomaterials (NMs) and nano-enabled advanced materials (NAMs) requires validated analytical methods for measuring key properties such as size, size distribution, morphology, chemical composition, and surface chemistry. Establishing such methods and verifying instrument performance and measurement competence requires (certified) reference materials (CRMs/RMs), representative test materials (RTMs), reference data (RD), and interlaboratory comparisons (ILCs). Although several nanoscale size standards exist, no RMs/RTMs with well-defined surface chemistry are available, despite the critical role of surface functional groups (FGs), ligands, and coatings in NM functionality and safety. Here, we report the development and characterization of two surface-modified nanoscale candidate RTMs, SiO2-50-NH2 and SiO2-80-NH2, comprising nonporous silica nanoparticles with total amino FG amounts of 205.7 ± 4.8 and 103.2 ± 3.0 nmol mg⁻1, respectively. Material selection, bottling, homogeneity, and stability were assessed using a previously established multi-method characterization strategy. Analytical techniques included dynamic light scattering (DLS), nanoparticle tracking analysis (NTA), zeta potential measurements, electron microscopy (EM), an automated optical Fluram assay, and quantitative nuclear magnetic resonance spectroscopy (qNMR) for determining derivatizable and total amino FGs. The results highlight the importance of combining complementary analytical methods rather than relying on a single technique. Additionally, a set of aminated silica NMs with varying amino FG densities were used to calibrate X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS), enabling amino FG quantification and method correlation. This characterization concept represents a first step towards establishing RMs/RTMs for NM surface chemistry measurements and ILCs assessing suitable analytical methods and their performance.