## 4 Conclusions and Outlook  
Carbon nanotubes are a material with exciting properties. For potential-controlled chromatography, they are primarily interesting due to their conductivity, inertness and high surface area. Our work demonstrates that CNT-K can be used as 3D packed bed electrode to electrically trigger retention and elution of maleic acid under flow in a chromatographic set up. Nevertheless, the nanotubes are also a challenging material. While our column optimization leads to a homogeneous and stable packed bed, the matrix structure suffers, and the packing loses homogeneity after approximately 300 working hours, as we have learned from chronoamperometry and cyclic voltammetry measurements. Along with the structural degradation, redox reactions take place on the carbon surface and change the electrode properties. Thus, aging causes a loss of consistency and efficiency of the potential-dependent retention process. This work focusses on understanding fundamental dependencies and effects in the PCC process to evaluate the impacts of individual components on the solid-liquid interface. The presence of capacitive and faradaic currents is confirmed; and column characteristics, such as the electric charge of the packing and the specific resistance, are determined at different applied potentials. As far as we know, there have been no similar analyses of the interaction of a multi-walled carbon nanotube solid phase with water in a through-flow cell, which, as we have demonstrated, induces a pH shift of the solution. Although the column resistance in water is high, the adsorption capacity for maleic acid is significantly higher (\(\approx\)40%) than in 10 mM phosphate buffer, where ion hinderance is more relevant. Figure 7: Relative amounts of maleic acid in the adsorption phase (shaded bars) and in the desorption phase (bare bars). Potential-step experiment with 5 g/L maleic acid in DI-water (blue) and 10 mM phosphate buffer (green) (1 mL/min, 50 mL, 216 nm). The adsorption phase was executed by applying different potentials (x-axis); a potential switch to \(-\)800 mV induced the elution phase. The experiment was performed in a column B with a potential switch after 30 mL (experiments repeated twice). In presence of a buffer, the faradaic current is reduced and the pH value stabilized. This knowledge is important for later applications in biotechnology or other life sciences, wherein many target molecules are sensitive to pH oscillations. In this study, we demonstrated that the combination of electrochemistry and chromatography is a powerful strategy to evaluate quality and aging of the nanotube packed bed; to record and control the potential-controlled sorption process; and to estimate column performance. However, parameters such as the influence of the fluid dynamics and the effect of different electrolytes are also highly relevant and should be examined in future work. In addition, we continue to work toward identifying the processing framework for enhanced interaction profiles between charged molecules and MWCNTs. The following are available online at [[http://www.mdpi.com/2076-3417/10/3/1133/s1](http://www.mdpi.com/2076-3417/10/3/1133/s1)]([http://www.mdpi.com/2076-3417/10/3/1133/s1](http://www.mdpi.com/2076-3417/10/3/1133/s1)). Figure S1: Working principle of potential-controlled chromatography. Figure S2: Scheme of the adapted potential-controlled chromatographic (PCC) column. Figure S3: 1 M NaCl tracer to determine the quality of a CNT packing. Table S1: Long-term stability and quality of different particle packings. Figure S4: Minimum length, outer diameter and inner diameter of CNT-K. Figure S5: Particle size distribution of more than 2000 CNT-K agglomerates. Figure S6: Analysis of CNT-K ends. Figure S7: XPS analysis of the CNT functionalization based on the ratios between the surface oxygen/carbon. Figure S: Exemplary raw data of the XPS measurements. Figure S9: Tracer experiment to describe the packing quality at the beginning of the column life and after several experiments.