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Characterization of gold nanoparticles by means of X-ray photoelectron spectroscopy (XPS)

23.12.2021

Gold nanoparticles (AuNPs) have attracted an increasing interest in medicine within the past few years, especially due to their good biocompatibility [1–3]. They are used both in radiation therapy and in diagnostics to increase the effectiveness of radiation. Such a dose‑enhancing  effect could already be detected in cells enriched with gold both for photon and proton radiation. It is based on higher interaction cross sections of the primary radiation that leads to the emission of a high number of low-energy secondary electrons. However, due to the electrons' very limited range, a local dose enhancement is hard to measure and is currently determined by means of Monte Carlo simulations [4–6]. These simulations, however, significantly differ from one another in terms of the local dose enhancements, which can be attributed to differences in the energy spectra used for the simulation [6]. In order to minimize the existing uncertainty in the electron emission spectra, they should be determined experimentally. For this purpose, gold nanoparticles must be deposited onto a suitable sample holder and subsequently characterized in terms of spatial distribution (for example via scanning and transmission electron microscopy). To determine the chemical structure of the AuNPs, the nanoparticles were analyzed using X-ray photoelectron spectroscopy (XPS) at a beamtime in the working group of Dr. Heiko Wende of the Faculty of Physics at the University of Duisburg-Essen.

The gold nanoparticles were deposited on a copper target holder coated with carbon foil using drop‑casting and spin‑coating techniques. XPS measurements were performed using an Mg Kα X‑ray anode. Peak shifts due to charge accumulation were corrected using the C 1s level at 284.8 eV as an internal standard as per the NIST Standard Reference Database [7]. The peaks were subsequently fitted after a Shirley‑type background subtraction using a Gaussian‑Lorentzian peak profile (70 % Gaussian and 30 % Lorentz).

The gold 4f peak of the examined AuNPs in Figure 1 indicates that part of the gold was present in oxidation state +I during the AuNPs' synthesis from a tetrachloridogoldic acid solution, but the majority was reduced to gold (0).

gold 4f peak (diagram)
Figure 1: Gold 4f peak of the examined gold nanoparticles after a Shirley-type background subtraction using a Gaussian-Lorentzian peak function.

The carbon 1s peak in Figure 2 shows C‑C as well as C‑OH, C‑O‑C and C‑O bonds, which can be attributed to the coating made of polyethylen glycol‑11 mercaptoundecanoic acid and the carbon foil. However, carbon contamination can also have an impact on the peak. A thin layer of carbon contamination collects on all samples prepared under ambient air conditions. At the same time, the packaging material of the samples and the exposure of the samples with residual gases in the vacuum chamber can cause further contamination. This also applies to the oxygen 1s peak in Figure 3. It can be attributed to the coating as well as contamination caused by oxidation processes or water. Later sputtering of the sample with argon ions shows that the intensity of the oxygen peak is reduced and that of the carbon peak is increased. The sputtering not only removes contamination, but also a significant part of the coating.

carbon 1s peak (diagram)
Figure 2: Carbon 1s peak of the examined samples after a Shirley-type background subtraction using a Gaussian-Lorentzian peak function.
oxygen 1s peak (diagram)
Figure 3: Oxygen 1s peak of the examined samples after a Shirley-type background subtraction using a Gaussian-Lorentzian peak function.

The examined AuNPs and their coating could be characterized in terms of their chemical composition using XPS . The AuNPs were not completely reduced to Au(0) during synthesis. Additional carbon and oxygen contamination could be detected on the coating and on the carbon foil, which needs to be taken into account in further experiments, because this has an impact on the measured electron emission spectra. The AuNPs characterized in this project were used for initial experiments at the synchrotron radiation facility DESY and for determining the electron emission spectra for proton radiation.

References

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Contact

Opens local program for sending emailP. Hepperle, Department 6.3, Working Group 6.36