Elemental Mapping of Au Nanowires Inside a Protein Microtubule by Low-Voltage STEM EDS

Achieving material contrast for beam-sensitive, light-element structures (e.g., protein microtubules) is challenging at high acceleration voltages (80 – 300 kV) and can entail additional efforts compared to lower-voltage approaches.

The Delong LVEM 25E electron microscope in combination with the Bruker XFlash ^® 7T30 detector for energy dispersive spectroscopy (EDS) is a suitable solution, as it can be operated at low acceleration voltages (e.g., 15 kV) while providing good spatial resolution and analytical capabilities.

Gold nanowires were grown inside protein microtubules as a controlled method of fabricating high-aspect-ratio conductive nanostructures for nanoscale electronic and photonic devices. Elemental mapping of a protein microtubule confirms the presence of 20 nm diameter gold nanowires embedded within the structure (Fig. 1). Drift correction provided in the ESPRIT software allows proper compensation of the sample drift during the map acquisition. Spectral analysis of selected regions proves the presence of Au.

Quantitative elemental mapping (QMap) was performed with the standardless Cliff-Lorimer quantification method. The quantified map indicates a homogeneous Au distribution along the nanowires, suggesting that gold was properly confined within the microtubules during the nanowire growth process (Fig. 2).

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Fig. 1. Bright-field STEM micrograph of Au nanowires in a protein microtubule acquired with a Delong LVEM 25E electron microscope at 15 kV acceleration voltage (left figure), corresponding elemental map of C and Au (middle figure). Local Au enrichment is confirmed by extracting the spectrum from region 1 (yellow spectrum) and comparing it to the one from region 2 (red spectrum) (right figure).

Fig. 2. Quantified map (QMap in false-color) showing gold concentration within a protein microtubule.

Sample courtesy of Dr. Foram Joshi, Technische Universität Dresden (TUD)