Nanoparticle-nanoparticle interactions in biological media by atomic force microscopy

Abstract

Particle-particle interactions in physiological media are important determinants for nanoparticle fate and transport. Herein, such interactions are assessed by a novel atomic force microscopy (AFM)-based platform. Industry-relevant CeO2, Fe2O3, and SiO2 nanoparticles of various diameters were made by the flame spray pyrolysis (FSP)-based Harvard Versatile Engineering Nanomaterials Generation System (Harvard VENGES). The nanoparticles were fully characterized structurally and morphologically, and their properties in water and biological media were also assessed. The nanoparticles were attached on AFM tips and deposited on Si substrates to measure particle-particle interactions. The corresponding force was measured in air, water, and biological media that are widely used in toxicological studies. The presented AFM-based approach can be used to assess the agglomeration potential of nanoparticles in physiological fluids. The agglomeration potential of CeO2 nanoparticles in water and RPMI 1640 (Roswell Park Memorial Institute formulation 1640) was inversely proportional to their primary particle (PP) diameter, but for Fe2O3 nanoparticles, that potential is independent of PP diameter in these media. Moreover, in RPMI+10% Fetal Bovine Serum (FBS), the corona thickness and dispersibility of the CeO2 are independent of PP diameter, while for Fe2O3, the corona thickness and dispersibility were inversely proportional to PP diameter. The present method can be combined with dynamic light scattering (DLS), proteomics, and computer simulations to understand the nanobio interactions, with emphasis on the agglomeration potential of nanoparticles and their transport in physiological media.

Figure 1

AFM apparatus and typical atomic force curves: (a) Illustration showcasing the direct force measurement apparatus. (b) In the case where the nanoparticles agglomerate there is strong adhesion force indicated by the force required to pull the tip away from the surface. (c) In the case that the protein corona is present there is a strong repulsion between the particles starting the moment the two coronas are brought in contact until they are fully compressed. The pictorial indicates the physical meaning of the RLT and its relation to the protein corona thickness.

Figure 2

The process of producing the substrates and the nanoparticle attachment on the AFM tips. (a) The substrates are produced by direct deposition of the particles by FSP system. (b) The attachment of the nanoparticles on the tip. i) The tips are coated with the creation of a fine droplet on the edge of a fine capillary. ii) The tip is brought in contact with the created droplet and is dunked several times. iii) A micro sized droplet is formed at the edge of the tip. iv) The droplet is left to dry to create a small particle aggregate.

Figure 3

Structural characterization of the synthesized materials. XRD patterns of (a) cerium oxide and (b) iron oxide. TEM images of (c) CeO₂(S), (d) CeO₂(M) and (e) CeO₂(L) (f) Fe₂O₃(S), (g) Fe₂O₃(M), (h) Fe₂O₃(L) where L, M & S correspond to the large (50–100 nm), medium (10–20 nm) an small (5–10 nm) sized ranges. The scale bar is the same for all images.

Figure 4

The AFM topography of the substrates and the respective SEM images The AFM topography of (a) SiO₂ (b) Fe₂O₃(L) and (c) CeO₂(L). The respective SEM images of (d) SiO₂ (e) Fe₂O₃(L) and (f) CeO₂(L).

Figure 5

SEM images of the AFM tips with (a) SiO₂ (b) Fe₂O₃(L) and (c) CeO₂(L) attached. A freshly made AFM tip with the Fe₂O₃(L) nanoparticles attached (d) unused, (e) after it was used in air to estimate the spring constant and (f) after it has been used in RPMI and RPMI+10%FBS.

Figure 6

Representation of the averaged force curves. The AFM force measured for the CeO₂ in (a) air, (b) water and (c) RPMI+10%FBS. Similarly the AFM force measured for the Fe₂O₃ in (d) air, (e) water and (f) RPMI+10%FBS.

Figure 7

Collective representation of the AFM results. (a) The adhesion force between the CeO₂ nanoparticles and (b) Fe₂O₃ nanoparticles. (c) Similarly the RLT for the CeO₂ nanoparticles and (d) Fe₂O₃ nanoparticles. (e) The repulsive layer thickness as a function of the primary particle size. (f) The relation between the hydrodynamic diameter and the RLT for all the particles. The XRD measured PP size is indicated in the graph.