Surface gradients under electrochemical control

Gradients are systems in which the physicochemical properties of a solution and/or surface change gradually in space and/or time. They are used for a myriad of technological and biological applications, for example for high-throughput screening, or for the investigation of biological systems. The development of methods for the fabrication of solution and surface gradients with desired characteristics, such as length and time scales, is still a fundamental challenge. The work described in this thesis aims at the development and application of electrochemical methods for the fabrication of covalent and non-covalent surface gradients on the micron scale. In the first part of this thesis (Chapter 3-5), an electrochemical system to generate solution and surface gradients on the micron scale has been developed. Via electrochemical reactions, solution gradients in pH or a catalyst (Cu(I)) have been generated. These gradients have been used to study the chemical reactivity of two surface reactions in a highly parallel manner. By optimization of the reaction parameters, the click reaction has been used to tune the density and steepness of surface chemical gradients fabricated on the micron scale. Furthermore, pH gradients have been characterized in real-time at the solid/liquid interface using a pH-sensitive fluorescent platform. In the second part of this thesis (Chapter 6-8), several non-covalent surface gradients have been fabricated on the micron scale. An electrochemical method has been demonstrated that resulted in non-covalent gradients of a charged dye-modified lipid using unprecedentedly low potentials. Furthermore, non-covalent, supramolecular surface gradients of a multivalent guest (Ad2-rhodamine) on the micron scale have been fabricated. Dynamic self-assembly has been shown, under control by a dissipative electrochemical process using (electrical) fuel. We envision that the gradient fabrication methods reported in this thesis, combined with other methods, will be developed in the future in the direction of dynamic gradients, the properties of which can be controlled at will in space and time. When combining dynamic gradients both in solution and on a surface, such a system could be used for the investigation of unique, dynamic systems, in areas such as dynamic cell behavior or cancer. Overall, this work has paved the way for new methods for making and controlling dynamic electrochemical gradients, which may have a bright future.