Correlation of AFM/SEM/EDS Images to Discriminate Several Nanoparticle Populations Mixed in Cosmetics
A. Delvallée,1 * M. Oulalite,1 *
alexandra.delvallee@
lne.fr
Abstract: This paper presents a proof of concept for the discrimination of several nanoparticle populations mixed in consumer products. The methodology proposes correlation of AFM, SEM, and EDS data to obtain structural and chemical information on each particle in a mixed population. To this end, emphasis is placed on sample preparation, imaging specifications for each instrument, and data correlation with adapted software.
Keywords: AFM, SEM, EDS, nanoparticle, correlated measurements
Introduction Multiple techniques are currently available for observing
particles at the nanoscale. Scanning electron microscopy (SEM), atomic force microscopy (AFM), and energy dispersive X-ray spectroscopy (EDS) are some that are commonly employed in industry and academic laboratories. Nevertheless, even if each technique is very efficient in obtaining one particular type of information, extracting further information may require a correlative combination of these. In France, regulatory authorities require producers and suppliers of products involving nanoparticles to clearly determine the size and size distribution of each nanoparticle population [1]. Tis step remains a challenge in the case of nanoparticle mixtures. In most cases, it is possible to discriminate particles in a
mixture with only SEM because particles are different in size and/or shape (Figure 1, leſt). Unfortunately, it is also possible to be confronted with a mixture where particles are chemically different but very similar in size and shape as shown on the right in Figure 1. In this particular case, the correlation of AFM/SEM/EDS
measurements provides a solution for discriminating the two populations. Te SEM has a resolution of around 1 nm in the XY plane, but a single micrograph produces no quantitative information in the Z direction. AFM tip shape convolution results in reduced lateral resolution (x,y) of nano-objects, while measurements along the vertical axis (Z) are well-resolved. EDS mapping can be used to collect additional information to chemically classify the particles. But correlation of data col- lected by the three techniques is only possible if the data pro- vided by the different instruments can be loaded, processed, and correlated with one unique soſtware program.
Materials and Methods Materials. In this paper, which presents a feasibility study, and ZnO nanoparticles were selected for their easily recog-
Fe2
nizable shapes and for their well-separated X-ray peaks (Zn Lα, 1.012 keV; Fe Lα, 0.705 keV) that can be easily detected at a very low voltage (typically 3 keV) with an appropriate EDS detector, thus protecting the sample from contamination and charging effects.
O3 46 doi:10.1017/S1551929521000638 Two suspensions of Fe2O3 and ZnO powders commonly
used in cosmetics were prepared by dispersion in ultra-pure water. In most cases, this kind of preparation (powder directly in water) leads to agglomeration/aggregation of the particles in the solution. To avoid agglomeration and correctly disperse the particles in the solution, an ultrasonic gun and bath were used. Te sonicator generates vibrations amplified and transmitted to the solution by the probe, producing micrometric bubbles. Tese bubbles are subject to great external pressure, which results in bursting. Te dissipate energy breaks the weak bonds in the agglomerates in the suspension. To perform measurements on the same object using dif-
ferent equipment, a specific substrate must be used to facilitate localization. Te system used is described in [2]. It consists of a silicon chip on which crosses and labels are lithographed. Te substrate is robust, can be easily handled, and has low rough- ness, which is ideal for observation of nanoparticles by AFM. Moreover, it is conductive enough to avoid the low-voltage charging effects familiar to SEM and EDS users. Deposition of nanoparticles. Te deposition step must
also avoid agglomeration as much as possible. First, the particles must adhere when they are in contact with the substrate. Indeed, if the particles have an electrical charge of the same sign as the substrate, the repulsive electrostatic forces prevent the particles from adhering. Tus, either the particle charge or the substrate charge must be controlled. Here, the substrate charge was modified. Several solutions exist and could be used for this, such as adding polymer chains to the substrate [3–5], but at the Laboratoire National de Métrologie et D’essais (LNE) a glow discharge treatment with a specific gas atmosphere (ELMO™, Cordouan Technologies) is used [6,7]. Depending on the gas used, this system can make the substrate hydrophilic or hydrophobic and positively or negatively charged. In our study, an electrical discharge was produced in an amylamine atmosphere to obtain a hydrophobic positively charged substrate. As the particles tend to agglomerate during static deposi-
tion [8,9] leading to the well-known “coffee-ring effect” [10], a strategy must be deployed to avoid this phenomenon. Tus, a spin-coating deposition method was used [11]. With this tech- nique, the droplet is spread out on the entire chip surface to achieve a homogeneous deposit of the particles at a surface concentration similar to that found in the suspension. Te deposit is performed in two steps:
• First, a droplet (7.5 μL) is spread on the substrate at low rotation speed (between 300 rpm and 1200 rpm). During this step, the particles undergo a Brownian-type
www.microscopy-today.com • 2021 May L. Crouzier,1 N. Bouzakher Ghomrasni,1 N. Feltin,1 S. Ducourtieux,1 A. Viot,2 N. Lambeng,1 and C. Jamet2
1Laboratoire National de Métrologie et D’essais (LNE), 29 avenue Roger Hennequin, 78197 Trappes Cedex, France 2Digital Surf, 16 rue Lavoisier, 25000 Besançon, France
W. Amor,1
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