ElectroSpray Ionisation Deposition for biosensor application

The ElectroSpray Ionization (ESI) developed by Fenn et al. [1] allows to bring large organic molecules (proteins, enzymes etc) as intact and isolated units in the gas phase. The technique is based on the use of a low-concentration solution of the molecule of interest flowing in a small capillary held at high voltage (typically a few kV) with respect to a grounded counter electrode placed some 10-15 mm away. On the tip of the emitter, the surface tension of the liquid cannot support the formed charge and therefore the liquid forms the so-called 'Taylor cone' inside which a Coulomb explosion creates a spray of charged droplets. The size of the droplets continue decreasing as the solvent evaporates by releasing a gas of molecular ions. The ESI process usually takes place in the air (Figure 1a) and then the jet can be transported into a vacuum chamber for the analysis. Originally developed for protein studies with mass spectrometry, ESI was then used with other types of systems (polymers, Nanoparticles, bacteria ...) as well as for different applications, as 'soft-landing' deposition. In these cases, the apparatus combines electrostatic transport and filter devices, to select the ions of interest and guide them towards either a spectrometer for the analysis or a surface for the deposition. Such a device is under construction at the CNR-ISM Montelibretti. Presently, it consists of i) a heated capillary to transport the charged species in vacuum and ii) a first differential pumping stage equipped with an octupole ion guide, see Figure 1b. A vacuum chamber housing a quadrupole mass spectrometer (QMS) for mass-over-charge, m/z, selection of the charged species, followed by a quadrupolar deflector to direct them towards a deposition chamber is being designed and will soon be commissioned. All in all, an in-vacuum 'soft-landing' deposition apparatus opens-up the unique possibility to perform m/z selected deposition in UHV condition of large biomolecular species [2], with control over the kinetic energy of the deposited species. On the other hand it is a quite complex and costly set-up, suffering severe limitations due to the low fluency of the m/z selected species. The main long term goal of the project is to increase the intensity of the ESI beam by mean of a customised, home-made, apparatus in order to make 'soft landing' a competitive technique for nanotechnology applications such as the production of biosensors and organic devices.