Towards Atom Probe Tomography of Hybrid Organic-Inorganic Nanoparticles

Abstract

Surface functionalization of inorganic nanoparticles is widely used to modify material properties for a variety of applications including medical therapies and diagnostics. While inorganic nanoparticles may be characterized via electron microscopy and X-ray scattering, study of hybrid nanomaterials and, in particular, buried interfaces is very challenging. Here we discuss the application of atom probe tomography (APT) to imaging organic-inorganic interfaces of biological nanoparticles, ferritin, as a model system. Ferritin is a structurally well-characterized protein, critical across biology to store some 4500 iron atoms and transport iron as mineralized Fe (III) [1]. This 24-subunit 12 nm protein has a spherical 8 nm core into which Fe (II) enters and is subsequently oxidized.APT is uniquely capable of providing the necessary chemical and structural insight at the atomic scale by directly probing the location and chemical identity of the atoms within a small sample of material. APT consists of a field-ion microscope coupled to a time-of-flight mass spectrometer, providing chemical and three-dimensional positional information of ions successively field evaporated from a sharp sample. The advent of ultraviolet laser pulsing has widely enlarged the scope of APT to include a range of insulating materials, including hybrid organic-inorganic materials [3]. APT is specifically adept at characterizing nanomaterials with internal organic-inorganic interfaces, due to its elemental sensitivity and spatial accuracy. We recently demonstrated this capability in biological minerals in the magnetite cusp of the chiton tooth [4] and vertebrate bone and dentin [5].We report here on the characterization of native equine spleen ferritin using APT. Greene et al. recently used APT to analyse ferritin as a model for characterizing biological materials using APT. In this work, researchers used a solidified organic/salt matrix, which contained numerous ferritin complexes [6]. These APT tips of dense ferritin/salt deposits were able to undergo APT, providing a proof of concept through mass spectrometry results. However, they were unable to characterize individual proteins and their experiments suffered from drastic tip bending, preventing meaningful reconstructions [6]. To image individual ferritin protein complexes, we deposited a dilute solution of native equine spleen ferritin onto a silicon post coated with Au-Pd (Figure 1) [7]. Further encapsulation of the adsorbed molecules with Au-Pd and subsequent focused ion beam milling allowed for the fabrication of APT sample tips. This method allowed for a small number of ferritin complexes to be characterized. This work not only allows for resolution and characterization of ferritin, but also shows our ability to use APT to characterize discrete nanoparticles.Atom probe tomography characterizes ferritin as spherical accumulations of iron and oxygen surrounded by carbon and nitrogen. Creating an isoconcentration surface we can identify the iron-rich core within the ferritin protein (Figure ...

Encapsulation method for atom probe tomography analysis of nanoparticles

“…Characterization of these NPs is important because the nanoscale microstructure has a great effect on the properties of such materials. There are a few notable examples of successes using APT to analyze particle systems such as catalysts [8][9][10][11][12][13], organic structures [14], or meteoritic nano-diamonds [15]. In some of these recent studies, one of the most promising preparation strategies used was that of encapsulation, whereby the discontinuous material is turned into a semi-continuous structure conducive to standard FIB preparation by embedding it in a matrix.…”

Atom Probe Tomographic Mapping Directly Reveals the Atomic Distribution of Phosphorus in Resin Embedded Ferritin

“…From this information, a unique 3-dimensional reconstruction can be formed revealing the atomic scale composition along with impurity distributions up to a part-per-million sensitivity and subnanometer spatial resolution 6 7 . APT continues to make major contributions in the materials science of structural 8 9 and electronic materials 8 10 , and recently has been demonstrated in geological 11 , bio-mineralogical 12 13 14 , soft organic 15 16 17 and bio-organic 4 18 19 materials. However the regular application of APT to soft biological materials is lacking in large part due to difficulties in specimen preparation.…”

“…The ferritin protein molecule has been the subject of previous Field Ion Microscopy (FIM) based analyses 20 21 and more recent APT-based analyses 18 19 because the iron-rich ferrihydrite core (hydrated iron (III) oxide) can provide an iron signature in the mass spectrum, the detection of which would confirm field evaporation of the protein. Additionally, through an accurate atom-by-atom reconstruction, the mineral core can also serve as a fiducial marker from which the relative composition of the organic protein shell can be tomographically mapped.…”

Modern Focused-Ion-Beam-Based Site-Specific Specimen Preparation for Atom Probe Tomography

“…They are generally incompatible with analysis by APT because the NPs are either too small to be individually transplanted and sharpened into an APT specimen, or because empty, voided regions contained within groups of NPs, or other open-space structures, are distributed in such a way that an APT tip cannot be made without being compromised by these structural defects (Larson et al, 2015). A few specimen preparation strategies have been reported in the literature (Folcke et al, 2012; Greene et al, 2012; Gordon et al, 2013; Felfer et al, 2014, 2015; Heck et al, 2014; Larson et al, 2015; Perea et al, 2016), but we will focus on two general strategies in this review.…”

“…For organic molecules, and suitably functionalized NPs, there are prospects for embedding them in a suitable matrix from solution (Greene et al, 2012; Gordon et al, 2013; Perea et al, 2016). Recently, Perea et al (2016) were able to embed ferritin within an organic polymer resin lacking nitrogen.…”