Metrologia aplicada a nanomateriais: estudo comparativo de técnicas e métodos

Abstract

Nanomaterial manipulation has increased scientifically and commercially, and for both, the reliability of measurement is essential. Measurements at the nanoscale must be comparable and reliable whenever the measurement is made.In this work we discuss several parameters that may influence the non conductive nanoparticles (NP) size measurements by analysing polystyrene samples with certified sizes (102 ± 3)nm using transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM) and dynamic light scattering (DLS) techniques. It was shown that by an adequate manipulation of their parameters and regarding their inherent limitation (e.g. hydrodynamic diameter for DLS) all techniques allow finding values compatible with the certified ones, according to one sample Students t-test at a 95% confidence level. The TEM technique presented the best results in terms of repeatability and bias to the certified value. The sample preparation is the major source of error of this technique. Measurements by SEM did not present repeatability over time and showed the highest bias to the certified value, according to Dunnets test at a 95% confidence level. AFM results presented the highest standard deviation, despite its high precision associated with the Z-axis. It was found that both the software for data treatment and the type of flattening procedure were shown to influence the particle size measurement. The results obtained from the DLS technique proved to be sensitive to several operating parameters (such as dilution and the use of dispersant) and showed a large standard deviation. Furthermore, comparison of results of NPs sizes obtained by DLS with those obtained by microscopy techniques should be performed carefully, since the technique measures hydrodynamic diameter and provides intensity distribution (not by number). In addition, the errors associated with different methods of sample preparation on silicon and mica substrates for measurement in AFM were investigated. It was shown that the dilution of the polymeric nanoparticle suspension was enough to achieve good dispersion on the mica substrate. For silicon substrate the sample preparation was significantly improved by treating the substrate with glow discharge.In summary, the identification of the deviations in nanoparticles size, obtained from each technique, and the parameters that contribute to these deviations constitute a metrological study that allows a better understanding of the size measurement errors and contribute to increasing their accuracy.