PhD Defence Laura Folkertsma-Hendriks

Laura Folkertsma-Hendriks is a PhD student in the MESA+ research group BIOS, the Lab-on-a-chip group. Her promoter is Albert van den Berg.

Electrochemical sensing using micro- and nanostructured poly(ferrocenylsilane)s

In this thesis, we look for ways to use the polymer poly(ferrocenylsilane) in sensor applications. Poly(ferrocenylsilane)s (PFS) contain ferrocene groups, resulting in redox activity. In the polymer’s backbone, the ferrocene groups are alternated with silicon atoms. The two remaining side groups of the silicon atom can be freely varied and used to introduce additional functionalities in the polymer.

A poly-cationic polymer is obtained by introducing a vinylimidazole group, resulting in PFS – Vim+. Drying a mix of PFS-Vim with polyacrylic acid (PAA) results in a partially phase-separated layer. We have visualised this using electron microscopy (SEM) and X‑ray scattering (SAXS). When the dried layer is exposed to ammonia, a porous membrane is formed. The size of the pores in the membrane can be influenced by oxidation and reduction of the PFS. We investigated fully oxidised and fully reduced membranes using electron microscopy (SEM) and X-rays (SAXS). Furthermore, we developed an electrochemical cell which al- lows us to carry out in-situ SAXS measurements: we can perform SAXS measurements while electrochemically changing the oxidation state of the layer. Using this cell, we cannot only observe the two extreme membrane states, but also the transition between them. Finally, we found that each redox state of the membrane has a unique impedance spectrum.

Subsequently, we attempted to use the porous PFS membrane to fabricate a sensor. The applicability of the porous membrane was tested for ascorbic acid, Fe3+, hydrogen peroxide and enzymatic sensors. We obtained some encouraging results, but the stability of the membrane, the slow response and the limited conductibility complicate matters. There is a lot left to be desired in the reproducibility and reliability of sensors based on a porous PFS layer.

Alternatively, in an attempt to construct a photonic sensor, we tested three types of PFS for use in two-photon-lithography. All three variants were suitable, but a number of obstacles had to be overwon. These obstacles—which will generally be encountered when developing a new resist for two-photon-lithography—and possible solutions were addressed in detail.

Additionally, we discovered that the technology used for fabricating porous membranes can also be used to make porous microparticles. The first step herein is the fabrication of PFS/PAA micro particles. We man- aged to do this by means of a simple microfluidic device: a T-junction that forms droplets of polymer solution in PDMS oil. Because the polymer solvent mixes with the PDMS oil, it slowly vacates the droplets, leaving behind a PFS/PAA particle. After a drying step they can be exposed to ammonia and porous PFS/PAA particles are obtained.

Finally, a reference-electrode-free pH and conductivity sensor based on indium-tin-oxide (ITO) electrodes was developed. During an attempt to make a pH and glucose sensor using polymer brushes on ITO, the response of ITO turned out to be dominant over the response of the polymer brushes. Moreover, the impedance was found to be a very good indicator of the pH and the conductivity of a solution. The impedance value at a low frequency (10 Hz) mainly depends on the double-layer capacity of the electrode, resulting in a pH sensor, while the value at a high frequency (300 kHz) depends on the resistance of the electrolyte and thus gives a measure of the solution’s conductivity.