Influence of carbon chain length on the synthesis and yield of fatty amine-coated iron-platinum nanoparticles

Abstract

Iron oxide nanoparticles are among the most widely used and characterized magnetic nanoparticles. However, metal alloys such as superparamagnetic iron-platinum particles (SIPPs), which have better magnetic properties, are receiving increased attention. Scalable techniques to routinely synthesize SIPPs in bulk need further study. Here, we focus on the role played by the fatty amine ligand in the formation of the bimetallic FePt nanocrystal. More specifically, we compare the effect of varying lengths of fatty amine ligands on the shape, structure, uniformity, composition, and magnetic properties of the SIPPs. We synthesized SIPPs by employing a 'green' thermal decomposition reaction using fatty amine ligands containing 12 to 18 carbons in length. Greater fatty amine chain length increased the polydispersity, particle concentration, iron concentration, and the stability of the SIPPs. Additionally, longer reflux times increased the diameter of the particles, but decreased the iron concentration, suggesting that shorter reaction times are preferable. Fourier transform infrared spectroscopy of the SIPPs indicates that the ligands are successfully bound to the FePt cores through the amine group. Superconducting quantum interference device magnetometry measurements suggest that all of the SIPPs were superparamagnetic at room temperature and that SIPPs synthesized using tetradecylamine had the highest saturation magnetization. Our findings indicate that the octadecylamine ligand, which is currently used for the routine synthesis of SIPPs, may not be optimal. Overall, we found that using tetradecylamine and a 30-min reflux reaction resulted in optimal particles with the highest degree of monodispersity, iron content, stability, and saturation magnetization.

Pacs: 81.07.-b; 75.75.Fk; 61.46.Df.

Keywords: Cancer biology; Iron-Platinum nanoparticles; Magnetic nanoparticles; Nanoparticle characterization; Nanoparticle synthesis; SIPPs.

Figure 1

TEM images of SIPPs. TEM images of SIPPs synthesized using ODA (A,B), HDA (C,D), TDA (E,F), and DDA (G,H). SIPPs were allowed to reflux for either 30 min (left column) or 60 min (right column). Scale bars are 50 nm.

Figure 2

FTIR spectrographs of SIPPs and fatty amines. FTIR spectrographs of SIPPs synthesized using ODA (top left), HDA (top right), TDA (bottom left), and DDA (bottom right). Please refer to the text for more details.

Figure 3

TGA thermograms of SIPPs and fatty amines. TGA thermograms of the SIPPs synthesized using ODA (A), HDA (B), TDA (C), and DDA (D). Dotted line = ligand only, black line = 30-min reflux, and gray line = 60-min reflux. The weight percent of ligands and naked alloy, as well as quantification of the number of bound ligands, is listed in Table 1.

Figure 4

DSC curves of SIPPs and fatty amines. DSC curves for the SIPPs synthesized using ODA (A), HDA (B), TDA (C), and DDA (D). Dotted line = ligand only, black line = 30-min reflux, and gray line = 60-min reflux.

Figure 5

Plot of DSC peak at approximately 600°C versus chain length. Plot of the derivative weight percent per degrees Celsius for the iron oxide peak (approximately 580°C to 650°C) versus chain length. Diamond = solid line = 30-min reflux (R² = 0.731). Square = dashed line = 60-min reflux (R² = 0.996).

Figure 6

Stability of SIPPs. Suspensions of SIPPs synthesized using ODA (A), HDA (B), TDA (C), and DDA (D) and allowed to reflux for either 30 or 60 min (left and right vials, respectively). Images were taken 1 week post-synthesis.

Figure 7

Magnetic characteristics of SIPPs. Aliquots (100 μL) of ODA-SIPPs (A, B), HDA-SIPPs (C, D), TDA-SIPPs (E, F), and DDA-SIPPs (G, H) were dried on Qtips® and measured using SQUID magnetometry. Hysteresis curves (M vs. H) are shown for SIPPs synthesized using either a 30-min (A, C, E, G) or 60-min (B, D, F, H) reflux time. The negative slope seen at high field is due to a diamagnetic contribution for the organic molecules (solvent and ligands). Insets show the ZFC (dashed line) and FC (solid line) curves for each of the SIPPs.