Single particle
inductively coupled plasma–mass spectrometry
(spICP-MS) is an emerging technique capable of simultaneously measuring
nanoparticle size and number concentration of metal-containing nanoparticles
(NPs) at environmental levels. single particle ICP-MS will become
an established measurement method once the metrological quality of
the measurement results it produces have been proven incontrovertibly.
This Article presents the first validation of spICP-MS capabilities
for measuring mean NP size and number size distribution of gold nanoparticles
(AuNPs). The validation is achieved by (i) calibration based on the consensus value for particle size
derived from six different sizing techniques applied to National Institute
of Standards and Technology (NIST) Reference Material (RM) 8013; (ii)
comparison with high-resolution scanning electron microscopy (HR-SEM)
used as a reference method, which is linked to the International System
of Units (SI) through a calibration standard characterized by the
NIST metrological atomic force microscope; and (iii) evaluation of
the uncertainty associated with the measurement of the mean particle
size to enable comparison of the spICP-MS and HR-SEM methods. After
establishing HR-SEM and spICP-MS measurement protocols, both methods
were used to characterize commercial AuNP suspensions of three different
sizes (30, 60, and 100 nm) with four different coatings and surface
charge at pH 7. Single particle ICP-MS measurements (corroborated
by HR-SEM) revealed the existence of two distinct subpopulations of
particles in the number size distributions for four of the 60 nm commercial
suspensions, a fact that was not apparent in the measurement results
supplied by the vendor using transmission electron microscopy. This
finding illustrates the utility of spICP-MS for routine characterization
of commercial AuNP suspensions regardless of size or coating.
A new methodology for determining the radial elastic modulus of a one-dimensional nanostructure laid on a substrate has been developed. The methodology consists of the combination of contact resonance atomic force microscopy (AFM) with finite element analysis, and we illustrate it for the case of faceted AlN nanotubes with triangular cross-sections. By making precision measurements of the resonance frequencies of the AFM cantilever-probe first in air and then in contact with the AlN nanotubes, we determine the contact stiffness at different locations on the nanotubes, i.e. on edges, inner surfaces, and outer facets. From the contact stiffness we have extracted the indentation modulus and found that this modulus depends strongly on the apex angle of the nanotube, varying from 250 to 400 GPa for indentation on the edges of the nanotubes investigated.
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