Characterization of ‘Click’ Surface Chemistry with XPS

In materials science, polymer surface functionalization is an essential task. A developing area in this field is the design and manufacture of cost-effective and disposable devices, which can be customized based on particular clinical applications such as cancer immunotherapy. In cancer immunotherapy, chemically modified microelectrodes are used to capture, adjust, and examine white blood cells before they are reintroduced to a patient.

Conducting polymers, e.g. PEDOT,1 poly(3,4-ethylenedioxylthiophene), are as popular as microelectrode material because they are less expensive, easy to process, and possess high cell compatibility. The surfaces of the device are altered by ‘click’ chemistry, which allows smaller units to be interlinked, to create active surfaces with high selectivity in mild conditions (Figure 1). During synthesis, the addition of an azide (triple nitrogen) group to the monomer creates the essential ‘clickable’ handle on the PEDOT material.

Components and principle of ‘click’ chemistry

Figure 1. Components and principle of ‘click’ chemistry

The chemical shifts detected in XPS analysis enables the patterning of the device, success of the ‘click’ process, and the efficiency of the starting material to all be examined. New XPS data processing algorithms have been improved to reduce the acquisition times without compromising the chemical state information, and minimizing the chances of damaging the sensitive surface material. An acquired XPS map’s spatial resolution can also be improved with these methods. This article discusses the application of the Thermo Scientific K-Alpha X-ray Photoelectron Spectrometer (Figure 1) in the analysis of a conductive polymer device, where the conducting polymer surface acts as a precursor in a ‘click’ chemistry process.

The Thermo Scientific K-Alpha XPS system

Figure 2. The Thermo Scientific K-Alpha XPS system

Experimental

Two experiments were carried out to characterize the surface. The first experiment focused on studying the changes caused by X-ray exposure on the PEDOT polymer. The X-ray spot measured 400 µm. The second experiment involved using a 30 µm X-ray spot to map the polymer surface. Scanning the sample stage takes place under the X-ray spot to create a map using the Thermo Scientific K-Alpha system.

The 128-channel detector in the K-Alpha spectrometer allows the collection of spectra using a fast snapshot acquisition mode. This minimizes the X-ray exposure and enables all of the spectral regions to be collected in seconds, without affecting the quality of the chemical state information. The analysis used the turn-key charge compensation system of the K-Alpha. This system helps to examine insulating samples in the same simple manner as conductors.

Results

For the analysis of the aforementioned type of samples, XPS provides a number of benefits, including surface distribution, chemical state information, and quantitative elemental information. The challenges include tiny structures and sample damage caused by extended X-ray radiation periods. Minimizing the acquisition times, and maintaining information regarding the spatial and chemical state can be addressed with deconvolution methods, which have been included in the Thermo Scientific Avantage data system.

Figure 3 displays N1s XPS spectra from a large area of the sample. The appearance of two peaks on Figure 3a is due to the varied oxidation states of the N atoms in the PEDOT-azide polymer. The peaks form because the N+ species and the N- species (Figure 3a insert) are available in the ratio of 1:2. Under X-ray exposure the azide group begins to degrade, and this results in the N3 species (Figure 3b). The deconvolution methods and the K-Alpha spectrometer’s rapid snapshot acquisition mode can help to resolve this issue and allow the characterization of the device.

Degradation of PEDOT-azide can be seen in N1s spectra

Figure 3. Degradation of PEDOT-azide can be seen in N1s spectra

Figure 4 highlights the chemical maps of the micropatterned surface before and after data deconvolution. The linescans across the maps explain how deconvolution enhances the image resolution. The solid lines are from the deconvoluted map, and the dashed lines from the map before deconvolution.

Image deconvolution improves the data acquisition time without compromising the data quality

Figure 4. Image deconvolution improves the data acquisition time without compromising the data quality

Conclusion

The deconvolution process provides a number of benefits for XPS analysis of polymer systems. Higher pass energies help to reduce the acquisition time, and eventually minimize the duration of exposure of sensitive samples to the X-ray source. Enhanced signal-to-noise ratios allow easy extraction of details from the data. Limitations, due to the size of the X-ray spot, are overcome using image deconvolution, resulting in enhanced XPS mapping abilities.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

For more information on this source, please visit Thermo Fisher Scientific – Materials & Structural Analysis.

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