Using XPS for the Characterization of ‘Click’ Surface Chemistry

The characterization of a conductive polymer device is performed using the Thermo Scientific K-Alpha XPS. The conductive polymer surface acts as a precursor in the ‘click’ chemistry process. New data processing techniques allow a reduction in acquisition time and achieving very good data quality.

An essential task in material science is the functionalizing of polymer surfaces. The design and manufacture of economical and disposable devices that can be fine-tuned to specific clinical applications, is one growth prospect in this field.

Cancer immunotherapy is one such kind of application where it is possible to use chemically modified microelectrodes for capturing, modifying and analyzing white blood cells before their introduction into the patient.

Conducting polymers such as PEDOT, 1 poly(3,4- ethylenedioxylthiophene), are appealing microelectrode materials because of easy processing, low cost and good cell compatibility. Click chemistry is used for modification of the device surfaces, joining tinier units together for generation of active surfaces in mild conditions with high selectivity (Figure 1). Here the required clickable handle on the PEDOT material has been made by the addition of an azide (triple nitrogen) group to the monomer at the time of synthesis.

The chemical shifts determined in XPS analysis can be used for evaluating the integrity of the starting material, the success of the ‘click’ process, and the device patterning. To enable shorter acquisition times, new XPS data processing algorithms have been refined that can reduce the chance of damage to the sensitive surface material at the same time retaining chemical state information. These techniques also enable enhancements in the spatial resolution of an acquired XPS map.

Experimental Procedure

For surface characterization, two separate experiments were performed. The first experiment was for studying the PEDOT polymer changes over time caused due to X-ray exposure. The X-ray spot size that was deployed was 400µm.

In the second experiment, the polymer surface was mapped using a 30µm X-ray spot. In order to produce a map with the Thermo Scientific K- Alpha, the scanning of the sample stage is performed under the X-ray spot. K-alpha’s 128-channel detector enables spectra collection using the rapid snapshot acquisition mode.

The X-ray exposure is kept to a minimum and complete spectral regions can be accumulated in seconds, however still retaining the chemical state information. For the analysis, the K- Alpha’s turn-key charge compensation system was used. Using this system, it is possible to analyze insulating samples with the same ease as conductors.

Components and principle of ‘click’ chemistry

Figure 1. Components and principle of ‘click’ chemistry

Experimental Results

XPS offers a number of benefits for analyzing these types of samples – chemical state information, surface distribution and quantitative elemental information. The sample damage resulting because of extended X-ray radiation at the time of the analysis and the tiny structures are the challenges faced in this method.

Bringing down the acquisition time and maintaining the chemical and spatial state information can be performed using deconvolution methods, which have been combined in the Thermo Scientific Avantage data system.

Figure 2 shows N1s XPS spectra from a large sample area. Figure 2a shows two peaks, which are due to the distinct oxidation states of the N atoms in the PEDOT-azide polymer. The peaks resulting because of the N+ species and the N- species (Figure 2a insert) lie in the ratio 1:2.

The azide group begins degrading under X-ray exposure causing the N3 species (Figure 2b). This issue can be overcome and device characterization can be done using K-Alpha’s rapid snapshot acquisition mode and the deconvolution methods.

Degradation of PEDOT-azide can be seen in N1s spectra

Figure 2. Degradation of PEDOT-azide can be seen in N1s spectra

The chemical maps of the micropatterned surface prior to and subsequent to deconvolution of the data are seen in Figure 3.The manner in which deconvolution enhances image resolution is shown by the line scans across the map. The dashed lines and the solid lines are from the map prior to and subsequent to deconvolution respectively.

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

Figure 3. Image deconvolution improves the data acquisition time without compromising the data quality

Conclusions

The deconvolution routines have considerable benefits for XPS analysis of polymer systems. By the use of higher pass energies, reductions in acquisition time are possible, causing a lower exposure of sensitive samples to the X-ray source.

It is highly convenient to extract information from the data because of improved signal-to-noise ratios. Image deconvolution overcomes limitations caused because of the size of the X-ray spot, thus improving 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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