Iontronic monitoring and regulation of colloidal particles and electrochemical reactions inside nanostructures
Authors:   , Zhang Zhu 
Published: October 2023

In this thesis, we have introduced new optical microscopy methods for investigating the movement of colloid particles (CPs) in electrolyte solutions and for operando optical investigations of the electrochemical adsorption of ions at the solid-liquid interface.

First, in Chapter 2 Chapter 3, we experimentally investigated the electro-osmotic aggregation of colloidal particles at the opening of a micrometer-sized silica channel in the presence of a salt gradient.
Under the application of a DC electric field, the aggregation of CPs at the microchannel opening occurred, which led to the clogging of the microchannel.
We have formulated a recipe for continuous CPs transport by applying a square-waveform electric potential with an appropriately tuned duty cycle.

We have shown by numerical calculations that this is predominantly caused by the competition between CPs advection in the osmotically induced fluid flow throughout the channel, and phoresis in the electric field.

Through the CPs transport phenomena in the electroosmotic pump, we have found that the ions at the solid-liquid interface significantly affect the electro-osmotic flow in micro-/nanochannels and the electro-phoretic flow of CPs under an electric field.
The response of CPs to an electric field is mainly influenced by the properties of the Electric Double Layer (EDL).

Using numerical modeling, we have proven the ability of the Iontronics Microscopy to probe the ions charging/discharging around nano-objects.
To demonstrate it experimentally, we have constructed an Iontronics Microscope to investigate electric-double-layer charging/discharging processes and electrochemical reactions inside nanoholes and around edges.

In Chapter 4, we have introduced a numerical model for computing local ion concentration changes that are the origin of optical contrast for Electric-double-Layer-Modulation microscopy.
The numerical results from nano-objects with different surface charges and geometry dimensions demonstrated that the EDL-modulation contrast is sensitive to both local curvature and local surface charge.

Next, in Chapter 5, we have constructed an experimental apparatus Iontronics Microscope based on EDL-modulation microscopy for monitoring EDL charging/discharging and electrochemical reactions around nano-objects.
We have presented the working conditions and properties of the apparatus, and demonstrated the sample stage stability and detection sensitivity of the apparatus, as required for the experiments of monitoring electrochemical reactions.

In Chapter 6, we have investigated the dependence of the EDL-modulation optical contrast of the nanoholes on the amplitude and frequency of the potential modulation and on the bulk ion concentration.
The results show that the Iontronics Microscopy method implemented with the lock-in detection technique has a higher sensitivity than the EDL-modulation microscopy that works on DC scanning and slow averaging.

While scanning electric potential through the electrochemical reactivity with small potential modulation, by monitoring the modulated scattering light variations of the nanoholes, we found a reduction of the optical signal of nanoholes at the potential window where the redox reactions of Fe(MeOH)2 happens.

As a final subject, in Chapter 7, we have demonstrated that Iontronics Microscopy can be used for monitoring electrochemical reactions in 2D dimensions.
We have presented the 2D images of ion concentration variations around the tungsten microelectrode during EDL charging/discharging and electrochemical reactions.