Quantification and stability of surface amine groups on silica nanoparticles using solution NMR ,

F. Kunc, V. Balhara, A. Brinkmann, Y. Sun, D. M. Leek, and L. J. Johnston*,
Anal. Chem. 90, 13322-13330, (2018).
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Surface chemistry is a critical factor for determining the behavior of a nanomaterial when incorporated in composites, devices and biomedical products and is also important for nanotoxicology studies. We have developed an optimized protocol for dissolution of aminated silicas and determined functional group content by quantitative 1H NMR (qNMR) of the released amines. A number of variables were optimized for the dissolution protocol, including base concentration, mass of silica, time, temperature and method of sample agitation, in order to achieve adequate NMR signals for quantification. The protocol was tested using nanoparticles from a single commercial supplier with sizes ranging from 20 to 120 nm and functionalized with a 3-aminopropyl group. Interestingly the batch to batch variability for some sizes of these aminated silicas was as high as 50%. Amine content measured by a ninhydrin colorimetric assay was typically ~20% lower than that measured by qNMR, consistent with measurement of only ninhydrin-reagent accessible amines. The dissolution/qNMR protocol was compatible with aminated silicas from other commercial suppliers and in these cases an even larger variability in surface coverage was observed. Silica nanoparticles with longer chain amines and variable amine loadings were synthesized to demonstrate the ability to quantify amines with more complex structures and to assess the limit of quantification for the dissolution/qNMR method. Finally the stability of the aminated nanoparticles was examined. Loss of 3-aminopropyl groups occurred in water at room temperature and was significantly more rapid at higher temperatures. Amine loss increased with increasing surface coverage and was slower for long chain amines, consistent with studies of amine stability on planar silica. Overall, this work highlights the importance of developing methods for quantifying surface functionalization, particularly given the variability in surface coverage for commercial samples, and for ensuring that the amine group is stable under its use conditions.