Shuo Kang

Single-molecule detection in electrochemical nanogap devices

Promotion date: March 28.

Promotor: Prof.dr. Serge Lemay

This project was aimed at realizing electrochemical single-molecule detection in water. A microfabricated nanogap device with significantly enhanced properties based on redox cycling, was developed.

The nanogaps were fabricated with optical lithography and standard microfabrication techniques. They consist of two electrodes with a length in the range of 10 to 100 µm and a width of several microns, embedded in the ceiling and floor of a nanofluidic channel with a height of ~50 nm.

Redox molecules enter the detection region defined by the two electrodes through entrance holes, located at the two ends of the channel. With the electrodes biased at high over-potentials, the molecules are oxidized and reduced repeatedly, generating an amplified current.

Based on a self-aligned fabrication scheme - that minimized the gap size and dead volume of the device redox cycling efficiency was improved, and electrochemical detection in aqueous solution with single-molecule resolution was achieved. This is the first reported electrochemical detection of single-molecules in water solutions in a nanofluidic device. Also in the first study ever, mass transport of individual redox molecules, using an electrochemical method, was demonstrated.

Was your PhD project application driven?

Yes, very much so. Single molecule detection has become a hot topic in science in recent years, which enables the understanding of molecular mechanisms and low-concentration detection. Up till now single-molecule studies are mainly based on optical and mechanical techniques.

In this project we aimed at developing an electrochemical single-molecule technique which has the inherent advantage that the output signal is electrical that the transducers could conveniently be integrated with integrated circuits. Imagine an iPhone-like point-of-care device opening up innovative techniques for home-based early diagnostics that are accessible to a vast audience.

Our group is collaborating with a semiconductor company to explore the possibilities for building up an integrated system for health care. But of course, first we need to understand some fundamental problems to be able to make use of the system.

So, your work was both theoretical and experimental?

I made lots of effort in the cleanroom to fabricate the devices, bringing the electrodes close together and reducing the device dead volume to amplify the signal level, which is the main challenge for all the single-molecule studies. After many trials we found a solution to fabricate the experimental platform which enabled the single-molecule detection in water. Like in many cases, the experimental value is of comparing it with the theoretical value. We need to reconsider the calculations and involve more factors that could influence the signal, and try to verify them.

Were your results published?

An article about electrochemical single-molecule detection in water was accepted in ASC Nano in 2013. Based on this robust technique, we managed to characterize the mass transportation of single molecules in nanochannels, based on statistics of Brownian motion. The summarized results will be submitted to the Journal of the American Chemical Society.

How did you value working at Mesa+?

People within Mesa+ are willing to share their knowledge and expertise. For example, when we had problems with processing, we always got quick and helpful replies once we sent emails or went to knock on our colleague’s door.

What, in your opinion, is important for Mesa+ to stay successful as a renowned institution?

Some real important focus points are there, I believe, in nanofluidics, labs-on-a-chip and biophysics. Attracting talented researchers and broadening research topics will strengthen Mesa+ on an international stage.