The 2002 Chemistry Nobel prize winner J. B. Fenn wrote in an article in Science 246, 64 (1998) that “electrospray ionization has recently emerged as a powerful technique for producing intact ions in vacuo from large and complex species in solution…this technique makes the power and elegance of mass spectrometric analysis applicable to the large and fragile polar molecules that play such vital roles in biological systems”. In recent years our group has been working towards extending electrospray ionization to the deposition of large and fragile molecules under ultra-high vacuum conditions (UHV) for the equally elegant techniques of electron spectroscopy and scanning tunneling microscopy (STM).

Our group has been working on two different implementations of electrospray: atmospheric and in-vacuum. With the latter we were able to deposit carbon nanotubes directly from a suspension in ethanol onto a native silicon oxide surface for ex-situ analysis by AFM. The resulting Nanotechnology article can be found here: Nanotechnology 18, 035707 (2007). In-vacuum electrospray has the advantage of a dramatic reduction in gas-load on the system (the spray chamber is held at a pressure of 10^-3 mbar, six orders less than atmosphere). The other characteristic feature of in-vacuum electrospray is the lack of molecular desolvation. This is due to a reduction in the number of collisions that the charged liquid droplets undergo, which would otherwise help to induce solvent evaporation and droplet fission. As a result the molecules tend to impinge on the surface cocooned in a nanoscale liquid droplet…great for soft-landing carbon nanotubes, but not really an appropriate way to deposit individual molecules.

Ambient electrospray ionization, on the other hand, forms the basis for virtually all electrospray mass-spectrometry applications and has revolutionized the study of large biomolecules through the successful generation of desolvated multiply charged molecular ions (hence the Nobel prize). In our lab we have taken the essential features of this technique to develop a portable electrospray deposition system capable of depositing molecules onto surfaces under UHV conditions. Our current record is a pressure of 5 x 10^-8 mbar for the deposition of a biological protoporphyrin molecule deposited from a toluene/methanol solution at the UK synchrotron radiation source in Daresbury.

The application of UHV techniques to non-volatile molecules

In many cases, ex-situ measurements can tell us all we need to know about a particular novel surface or architecture based on non-volatile molecules. But more often than not, there will always remain a question which can only be addressed by investigating a model system at the single molecule level or with high-resolution electron spectroscopy. A prime example of this is a dye-sensitised solar cell. The efficiency, voltage and current can be easily measured for the macroscopic device, but a full understanding of how the electrons are injected from the dye into the conduction band of the oxide surface can only be gained from a combination of sophisticated electron spectroscopies. Complex dye molecules and a vast library of functional and biological molecules are thermally labile and cannot be sublimed under UHV conditions...electrospray is perhaps the most promising route to studying these systems with UHV techniques.

Recent electrospray results from our group

Many of our recent experiments are currently being prepared for publication so the details will have to wait. Some recent highlights, however, include: an electron spectroscopy study of a famous dye molecule deposited in UHV on a titanium dioxide surface at the MAX-lab Swedish synchrotron, the  UHV-STM study of a well-known fullerene on a gold surface, AFM studies of UHV-deposited polymers, and the electron spectroscopy of an important biological protoporphyrin on a titanium dioxide surface studied at the SRS UK synchrotron.