Study and characterization of diamond surface for biosensoring applications

Diamond is the only wide band gap representative of the elemental semiconductors, with a crystal structure identical to its more common relatives silicon and germanium.
On first glance one might also expect similar surface properties in terms of reconstructions, surface states, and surface band diagrams. In part, this expectation is indeed fulfilled, but diamond also exhibits a number of unusual and potentially very useful surface properties.
Particularly when the surface dangling bonds are saturated by monovalent hydrogen atoms, (donor-like) surface states are removed from the gap, the electron affinity changes sign and becomes negative, and the material becomes susceptible to an unusual type of transfer doping where holes are injected by acceptors located at the surface instead of inside the host lattice. In such a way diamond surface becomes a conductor material and can be used as an electrode to collect electrical signals.
Since diamond electrical and chemical properties are strictly related to its surface termination, a surface nondestructive analysis technique is essential to check its status. Among all the possible surface techniques, XPS (X-ray Photoelectron Spectroscopy) and UPS (UV-ray Photoelectron Spectroscopy) are the most suitable, since their scanning depth is nearly 3 nm and 1.5 nm respectively.
Moreover they give information not only about chemical species present onto diamond surface, but they can also state how these atoms are linked together, thanks to their chemical shift. This particular behavior has made diamond a good candidate for the implementation of biosensors.
An ideal biosensor should combine nature sensitivity and specificity with the advantages of modern microelectronics: diamond is especially attractive because, in addition to having good electrical and chemical properties, it is widely considered to be biocompatible and chemically inert and can be deposited as a robust, thin film on silicon and other microelectronic-compatible substrates. Moreover, if we use a diamond as a substrate for a biosensor, we can also exploit its bulk properties such as extreme mechanical hardness and, the most important, broad optical transparency, from the deep ultraviolet to the far infrared. This makes possible simultaneous recording of electrical and optical signals from living cells, as sensor elements.
Hydrogen terminated diamond surfaces have been used to attach DNA fragments, showing that diamond is a good platform onto which biomolecules, with specific funtional groups, can be linked. The first evidence of selective attachment of mammalian neurons and ordered outgrowth of neurites on patterned diamond surfaces has been recently reported by Specht et al. [16]. However, to our best knowledge, no report about the functional viability of neurons on functionalised diamond surfaces has been published to date.
To explore the possibility of exploiting the above mentioned properties of diamond for the realisation of biosensors it is essential to investigate adhesion and neuronal excitability (i.e., the ability of neurons to generate and propagate trains of electrical impulses) on hydrogenated (HTD) or oxidised (OTD) diamond surfaces.
Thus the study of diamond surface properties, with different terminations, is fundamental if we are interested in biosensoring applications.

My PhD research activity thus will be focused onto three main topics:
- Implementation and characterization of an XPS apparatus, in order to characterize diamond surface properties and to check out its modification due to hydrogenation and/or oxidation processes.
- Study different methods to perform hydrogenation (plasma, molecular or HFCVD) and oxidation (chemical, plasma or UV technique), in order to find out the best way to modify diamond surface, without introducing any sort of contamination and damage.
- Study of neurons adhesion and viability onto diamond, to check their bio-compatibility.