Simplifying Electron Diffraction Pattern Identification of Mixed-Material Nanoparticles
Jacopo Samson,1 Patrick C. Nahirney,2 Charles Michael Drain,1 and Irene Piscopo3 * 1Department of Chemistry, Hunter College, 695 Park Avenue, New York, NY 10065 2Division of Medical Sciences, University of Victoria, Victoria, BC V8W 2Y2, Canada 3EM Consulting, 57 Soundview Drive, Huntington, NY 11743
*
irene.piscopo@gmail.com
Introduction Metallic and non-metallic nanoparticles (NPs), ranging
in size from 1–200 nm, have unique functional properties that differ from their bulk materials and their component atoms or molecules [1]. Tese unique properties have driven the demand for nano-sized materials and new methods to synthesize NPs, which are used in drug delivery systems [2], bio-imaging agents [3], catalysts [4], photonics, and optical devices [5]. Inorganic NPs can be synthesized with a variety of methods that impart size,
shape, and other structural properties. Cobalt-based
NPs, for instance, display unique size and shape-dependent magnetic properties [6], while the band gap, UV blocking properties and stability of zinc oxide (ZnO) NPs enable new applications in products ranging from cosmetics [7] to solar cell power [8]. Approaches
to NP synthesis include solvothermal,
biological, and other templates [9], as well as ligands to seed NP growth and molding strategies [10]. Our approach for synthesizing metal NPs involves using toroidal
topologies
of plasmid DNA as sacrificial molds and varying conditions to fabricate size-tunable gold, nickel, and cobalt NPs [9]. Plasmid DNA provides a relatively inexpensive monodispersed template that can be engineered to form in a range of sizes and exploits the well-established high affinity for metal cations. Tis strategy is generally a greener approach to NP synthesis because the solvent is water and the template is biodegradable. We have characterized these NPs by atomic force
microscopy (AMF) and transmission electron microscopy (TEM). For example, a pcDNA3.1 (+) plasmid can be used as a sacrificial mold to yield disc-shaped gold and nickel NPs in the range of 28 ± 3 nm × 8 ± 1 nm and 52 ± 5 nm × 13 ± 1 nm, respectively. Columnar-shaped ZnO NPs were synthesized using a pH gradient and imaged to reveal a bimodal distribution in the range of 70 ± 10 nm × 50 ± 10 nm and 135 ± 15 nm × 80 ± 10 nm. In order to confirm the nature of these NPs, which were composed of both metals and non-metallic materials, we compared their electron microdiffraction (µD) patterns to known standards [11–12]. Tere are two methods for obtaining electron diffraction
(ED) patterns [13]. Te selected area diffraction (SAD) method uses an aperture to select the area producing the ED pattern, while µD and convergent beam electron diffraction (CBED) techniques use the beam to select the area producing the pattern. Te minimum area that can be selected on a 100 kV TEM by the SAD method is 1 µ [12]. Because µD uses the beam to select the area, the minimum size in the TEM mode is limited by the electron source. Te sharp diffracted beams of µD, as opposed to the discs of CBED, are produced by using a small (20–30 µ) second condenser aperture [14]. Because the
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size of the NPs under examination was less than 200 nm, µD was the method of choice. Microdiffraction (µD) is a reliable method of verifying the
identity of individual NPs when there is not enough sample for powder X-ray diffraction (XRD) analysis. In the use of plasmid molds, the resulting materials could be the starting metal ion salts, the metal oxides, the target metal NPs, or combinations of these (for example, nickel metal, NiO, Ni2O3, NiX2). Similar analytical criteria are needed for the formation of inorganic materials such as ZnO and TiO2. Morphology alone cannot differentiate these NPs because the metals in the NPs sometimes exist in more than one oxidation state. In other cases, similar morphologies proved to be two different materials. Identification of the NPs necessitated the indexing of individual diffraction patterns, a very time consuming and tedious procedure. To simplify the identification of materials, when one has
an idea what the material might be (that is, NiO or Ni2O3) and standards with which to compare them, we present two easily applied and straightforward methods for comparing electron diffraction (ED) patterns. Identifying total unknowns will still require indexing individual diffraction patterns. Te example shown in Figure 1 illustrates that this technique can be applied to inclusions in tissue samples as well as to particulate materials.
Materials and Methods Materials. Me3PAuCl and Co(II)Cl2 . 6H2O were pur-
chased from Sigma Aldrich, and Ni(II)Cl2 . 6H2O was purchased from Fisher Scientific. Te pcDNA3.1(+) plasmid was obtained from Invitrogen, amplified with Qiagen kit to a mother stock suspension of 1 mg/mL and diluted when mixed with the cationic-containing solutions. Te 12 mM stock solutions of metal chlorides were prepared in nanopure water. Te gold solution was prepared by adding an equal portion of deionized water to 100 mL of a 24 mM stock solution of Me3PAuCl dissolved in acetone. Zn(NO3)2 . 6H2O (Sigma Aldrich) was dissolved in deionized water at a concentration of 50 mM. Tris (Sigma Aldrich) was prepared in deionized water at a concen- tration of 100 mM. Tris-EDTA (TE) buffer (10 mM Tris, 1 mM EDTA, pH 8) was included in the Qiagen kit. Instrumentation. Samples for TEM observation and µD
were dispersed onto carbon-coated copper grids (Electron Microscopy Sciences). Te samples were imaged and the µD patterns collected at 120 kV using a Tecnai G2 Biotwin (FEI). All images and µD patterns were collected with an AMT 2K CCD camera. We obtained µD patterns of known standards and
then, under the same conditions (kV and camera length), we obtained µD patterns of the unknowns. For both methods to
doi:10.1017/S1551929511001179
www.microscopy-today.com • 2011 September
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