Quantifying surface groups on aminated silica nanoparticles of different size, surface chemistry, and porosity with solution NMR, XPS, optical assays, and potentiometric titration

We assessed the quantification of surface amino functional groups (FGs) for a large set of commercial and
custom-made aminated silica nanoparticles (SiO2 NPs) with sizes of 20–100 nm, prepared with different
sol–gel routes, different amounts of surface amino FGs, and different porosity with four methods
providing different, yet connected measurands in a bilateral study of two laboratories, BAM and NRC,
with the overall aim to develop standardizable measurements for surface FG quantification. Special
emphasis was dedicated to traceable quantitative magnetic resonance spectroscopy (qNMR) performed
with dissolved SiO2 NPs. For the cost efficient and automatable screening of the amount of surface
amino FGs done in a first step of this study, the optical fluorescamine assay and a potentiometric
titration method were utilized by one partner, i.e., BAM, yielding the amount of primary amino FGs
accessible for the reaction with a dye precursor and the total amount of (de)protonatable FGs. These
measurements, which give estimates of the minimum and maximum number of surface amino FGs, laid
the basis for quantifying the amount of amino silane molecules with chemo-selective qNMR with
stepwise fine-tuned workflows, involving centrifugation, drying, weighting, dissolution, measurement,
and data evaluation steps jointly performed by BAM and NRC. Data comparability and relative standard
deviations (RSDs) obtained by both labs were used as quality measures for method optimization and as
prerequisites to identify method-inherent limitations to be later considered for standardized
measurement protocols. Additionally, the nitrogen (N) to silicon (Si) ratio in the near-surface region of
the SiO2 NPs was determined by both labs using X-ray photoelectron spectroscopy (XPS), a well
established surface sensitive analytical method increasingly utilized for microparticles and nano-objects
which is currently also in the focus of international standardization activities. Overall, our results
underline the importance of multi-method characterization studies for quantifying FGs on NMs involving
at least two expert laboratories for effectively identifying sources of uncertainty, validating analytical
methods, and deriving NM structure–property relationships.