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Erik van Dijk (promotion date: 11 February 2005)

Single nanoparticles: ultra fast en ultra sensitive detection

Promotion date: 11 February 2005

Eric van Dijk

Research into single particles on a nano scale and single molecules has been going on for some time by studying their fluorescence. Unfortunately, there are only few molecules than fluoresce, this seriously limits the range of systems that can be studied. Also fluorescence is comparatively slow -it takes about a nanosecond-, whereas there are a lot of interesting processes that occur on a much faster time scale. So fluorescence is not the way to study these very fast processes. I concentrated on finding another way to study nano particles as well as using fluorescence in a different way so that it does tell something about these fast processes.

What was your thesis about?

Research into single particles on a nano scale and single molecules has been going on for some time by studying their fluorescence. Unfortunately, there are only few molecules than fluoresce, this seriously limits the range of systems that can be studied. Also fluorescence is comparatively slow -it takes about a nanosecond-, whereas there are a lot of interesting processes that occur on a much faster time scale. So fluorescence is not the way to study these very fast processes. I concentrated on finding another way to study nano particles as well as using fluorescence in a different way so that it does tell something about these fast processes. In the first part of my thesis I developed, the Single Molecule Pump Probe (SM2P) method, which allows ultra fast processes in single molecules to be studied while still detecting fluorescence. In the second part of my thesis I concentrated on developing a new detection method, the Heterodyne Interference Polarisation Scanning Optical Microscope (HIPSOM), that does not rely on the detection of fluorescence. With the HIPSOM I was able to detect gold particles with a diameter down to 2 nanometer.

What are these fast processes in molecules and why would you want to study them?

An example of one of these ultra fast processes taking place in molecules is the transfer of energy from one molecule to the other. Such energy transfer plays a key role in nature in for instance light harvesting complexes that collect the sun’s energy in plants. Studying these processes on an individual molecule level can lead to a better understanding why certain (chemically identical) molecules perform better than others.. The problem is that these transfer processes take place on a Pico second (10-12 s) time scale. Conventional single molecule detection can only measure the spontaneously emitted fluorescence occurring on a nanosecond (10-9 s) time scale. The SM2P method I developed allowed these processes to be directly studied on the single molecular level.

Is this new technology?

Bulk pump-probe techniques have been extensively used since the seventies of the last century to study ultra fast processes in molecular systems. However, all those experiments only report on the average response of a large number of molecules. We for the first time measured such ultra fast processes on single molecules and found that there are large variations from molecule to molecule.

Any thought of future applications?

If you understand these fast processes better you could think of mimicking light harvesting complexes that you find in plants and bacteria. These structures absorb the light and very efficiently transfer their energy to one spot where a reaction takes place (photosynthesis). To be able to imitate and synthesize this very efficient natural process would be progress indeed. It is clear that the functioning of these complexes depends critically on there conformation. This conformation varies from molecule to molecule; in order to elucidate this relation one has to study the individual molecules.

Do you now know why one molecule behaves differently from the other?

Yes, since we not only looked at these ultra fast times, but also at the spectra of the emitted light we can correlate the different parameters and in that we gain a deeper understanding of the nature of the studied processes.

And measuring such small gold particles is also new?

Yes, the optical detection of small non-fluorescent particles has received much attention in recent years. Since only few molecules fluoresce and those that do fluoresce will stop fluorescing after being illuminated for a while, a technique that does not rely on fluorescence is of great interest to study for instance in biological processes. Furthermore, the interaction between single gold particles and fluorescent molecules is very interesting. Our newly developed technique might lead to a better understanding of these subtle interactions.

Are you going to continue in this line of research?

I will stay till the summer at the Optical Techniques group to finish some experiments and write a number of articles. After that I will look for a job, most probably in a company.

For the summary of the thesis, click here.