Zoubida Hammadi

Practical Physics Behind Growing Crystals of Biological Macromolecules

The aim of this review is to provide biocrystallographers who intend to tackle protein-crystallization with theory and practical examples. Crystallization involves two separate processes, nucleation and growth, which are rarely completely unconnected. Here we give theoretical background and concrete examples illustrating protein crystallization. We describe the nucleation of a new phase, solid or liquid, and the growth and transformation of existing crystals obtained by primary or secondary nucleation or by seeding. Above all, we believe that a thorough knowledge of the phase diagram is vital to the selection of starting position and path for any crystallization experiment.

A new approach to gas field ion sources

A new approach to gas field ion sources is described. It is based on a structure made by inserting a field emission tip inside a small diameter tube. The tube supplies gas to the tip from a high-pressure chamber into a high-vacuum chamber where ionization takes place. Comparison of projection electron and ion micrographs shows that ionization results from a field ionization process taking place at the very end of the tip. Emission currents in the 10nA range, for a few kV emission voltages, are obtained with various gases including neon, air and hydrogen. Lifetime experiments with H(2) show stable emission for days.

Chemical‐vapor deposition of metallic copper film in the presence of oxygen

Chemical‐vapor deposition of copper using copper (II) bis(acetylacetonate) is reported. It is shown that at 0.1 Torr and temperatures in the range of 250–350 °C, the deposition occurs only if oxygen is added in the reactor. Auger spectroscopy, x‐ray‐diffraction, and resistance measurements as a function of temperature lead to the conclusion that metallic copper is deposited.

Microelectron gun integrating a point-source cathode

A structure integrating a sharp field emission tip inside a coaxial structure with an overall diameter as small as 60 μm is described. It can emit nA electron current with a minimum kinetic energy of 50 eV. It is demonstrated that this structure behaves like an electron gun and is able to produce a low-energy, divergent and highly coherent electron beam at distances as small as 100 μm from the tip.

Monitoring picoliter sessile microdroplet dynamics shows that size does not matter.

We monitor the dissolution of arrayed picoliter-size sessile microdroplets of the aqueous phase in oil, generated using a recently developed fluidic device. Initial pinning of the microdroplet perimeter leads to a nearly constant contact diameter, thus contraction proceeds via microdroplet (micrometer-diameter) height and contact angle reductions. This confirms that picoliter microdroplets contraction or dissolution due to the selective diffusion of water in oil has comparable dynamics with microliter droplet evaporation in air. We observe a constant microdroplet dissolution rate in different aqueous solutions. The application of this simple model to solvent-diffusion-driven crystallization experiments in confined volumes, for instance, would allow us to determine precisely the concentration in the microdroplet during an experiment and particularly at nucleation.

Local supply of gas in vacuum: Application to a field ion sourcea)

The flow of hydrogen, helium, and nitrogen through a millimeter long and micrometer size annulus capillary from a high pressure chamber to a low pressure chamber is measured in a wide pressure range. The corresponding gas conductance is deduced. Molecular, transition, and viscous regimes are observed. The local supply of gas strongly increases with pressure in the viscous regime up to a regime controlled by capillary exit loss. Based on such a geometry, the gas supply to a field ion source with a coaxial structure is shown to be increased by more than three orders of magnitude compared to a conventional supply.

Nanotechnologies dedicated to nucleation control

This paper highlights the work of our group on the control and the observation of nucleation with techniques using nanotechnologies. This control is performed either by triggering nucleation in time with an external field or by localising it spatially in a microdroplet. Localisation in time using light irradiation induces nucleation by forming radicals; the use of electric field acts locally on the density of the solution. Localisation in space with a microfluidic device produces hundreds of nanovolume crystallisers where concentration and temperature are easily monitored. Thus, accurate statistical studies lead to the nucleation parameters (metastable zone, nucleation rate and polymorphism). Lastly, confinement with a microdroplet generator permits to reach very high supersaturations in fL to pL volumes allowing nucleation of a single crystal per microdroplet. All these methods clearly enhance nucleation in the metastable zone. Finally, they use small quantities of products offering potentialities for the screening of crystallisation conditions and phases (polymorphism).

Water nanoelectrolysis: A simple model

A simple model of water nanoelectrolysis—defined as the nanolocalization at a single point of any electrolysis phenomenon—is presented. It is based on the electron tunneling assisted by the electric field through the thin film of water molecules (∼0.3 nm thick) at the surface of a tip-shaped nanoelectrode (micrometric to nanometric curvature radius at the apex). By applying, e.g., an electric potential V 1 during a finite time t 1 , and then the potential −V 1 during the same time t 1 , we show that there are three distinct regions in the plane (t 1 , V 1): one for the nanolocalization (at the apex of the nanoelectrode) of the electrolysis oxidation reaction, the second one for the nanolocalization of the reduction reaction, and the third one for the nanolocalization of the production of bubbles. These parameters t 1 and V 1 completely control the time at which the electrolysis reaction (of oxidation or reduction) begins, the duration of this reaction, the electrolysis current intensity (i.e., the tunneling current), the number of produced O 2 or H 2 molecules, and the radius of the nanolocalized bubbles. The model is in good agreement with our experiments.