Next generation sensor platforms will require significant improvements in sensitivity, specificity, parallelism, chemical stability, and bio-compatibility in order to meet the future needs in various fields. Nanowires are new materials, which have characteristics of low weight with sometimes extraordinary mechanical, electrical, thermal and multifunctional properties.By creating nanostructures; it is possible to control the fundamental properties of materials without changing their chemical composition. In this way the attractive world of low dimensional systems, together with the fabrication of functional nanostructured arrays will play a major role in the new trend of chemical and bio-chemical nanotechnology. Nanostructures can be used for tunable transport of electrons with electronic properties strongly influenced by little perturbations on the surface, for giant surface-to-volume ratio enhancements which are important for chemical/bio-chemical applications, and for generation of well defined molecular patterns on bio-sensor surfaces.
Nanowires are generated by a) self-assembly of small sized structures to form larger structures (“bottom up”) or b) by reduction of large systems down to small size (“top-down”). For biosensing devices, these structures need to offer main advantages over carbon nanotubes (CNTs) which are: 1) Material properties can be controlled by manipulation of synthesis conditions (“dimensions”, “morphology”). 2) Conductivity can be controlled by doping, which allows to fabricate insulating, semiconducting, and metallic wires. 3) The surface of nano-wires can be functionalized to add chemical sensing or bio-sensing properties by use of well established chemistries. Applied materials like Si, SiO 2, gold, glassy carbon, SnO 2, and ZnO 2, do not possess desired chemical stability and reproducibility of biochemical surfaces in electrolyte solutions. Only diamond is known to be outstanding with respect to electrochemical properties. Its electrochemical background current in phosphate buffer is ten times lower than Au and 100 times lower than glassy carbon. Diamond has a wide working window due to large over-potentials for hydrogen and oxygen evolution. A typical window of 3.25 V or greater is normal for high-quality films. Diamond can be n- and p-type doped from insulating to semiconducting to metallic, thereby changing from transparent (optical gap of 5.47 eV) to black.
The surface of diamond shows unique properties as it can be terminated with hydrogen, with oxygen and OH which allows optimizing the electronic properties of the solid/electrolyte interface. Diamond surfaces are hydrophobic in case of H-termination and hydrophilic for O-termination. In addition, diamond is known to be chemically inert, bio-compatible, and shows strongest bonding stability to DNA. Diamond is ultra-hard (50-150 GaP) which is promising with respect to mechanical stability of diamond nano-wires. The realization of diamond nano-wires started already in 1997 by Shiomi, who demonstrated for the formation of porous diamond films by reactive ion etching (RIE) using O2. Later in 2000, nano-structured diamond honeycomb films have been prepared by etching through a porous anodic alumina mask, triggering some activities which are summarized in an article of Shenderova et al. Growth induced formation of nano-scale tubular structures have been reported for the first time in 2003, applying a microwave plasma of hydrogen under a bias potential.
In 2008, Zou et al. reported about the fabrication of nanopilar arrays using self aligned Au nanodots as etching mask in a bias-assisted reactive ion etching, applying a hydrogen/argon plasma. Although these achievements demonstrate that vertically aligned diamond nano-wires can be fabricated by a variety of methods, no applications in electro- or bio-chemistry have up to now been reported. In this paper, we introduce for the first time the fabrication of vertically aligned diamond nano-wires from metallically boron doped single crystalline CVD diamond, by use of diamond nano-particles. A top-down procedure is optimized to fabricate diamond nano-wires where firstly atomically flat diamond is grown by homo-epitaxy of metallically (p-type) doped (100) oriented single crystalline diamond on insulating Ib substrates. Then a self organized etching mask from nano-diamond particles is deposited on the surface with particles of typical 10 nm diameter.Reactive ion etching in O2 /CF 4 gas mixture is applied to form patterns of vertically aligned diamond nano-wires. 4) These wires are functionalized by use of an electrochemical phenyl-linker molecule attachment schema, which preferentially bonds phenyl linker-molecules to tips of wires.
Such functionalized nano-wires are used to bond geometrically controlled oligonucleotide molecules to diamond, thereby combining the outstanding electrochemical properties of diamond as transducer with the advantages coming by dispersed and controlled bonding “like in aqueous solution” of DNA molecules. Sensing properties of this new gene-sensor platform are characterized in detail with respect to sensitivity and chemical stability using cyclic (CV) and differential pulse voltammetry (DPV), and impedance spectroscopy (IMP). Finally, ultra-micro electrode (UME) arrays from diamond will be introduced, which are extremely robust and suitable for applications in high through-put systems for gene sensing in clinical environments.
