"Functionalization of Boron-doped diamond surfaces for (bio)molecular electronics."
In this thesis work, we have contributed to the development and optimization of specific surface functionalization methods for the introduction of different organic (bio)molecules on mainly boron-doped diamond (BDD) surfaces. These methods are based on electrochemical and wet-chemical concepts. The functionalized diamond platform was also tested for its possible applications.
In the first part of Chapter 1, a general description on diamond classification and a brief explanation on how to grow boron-doped diamond is given. Then, the surface properties of diamond and the electrochemical properties of boron-doped diamond are discussed. The last part of this chapter focuses on the surface chemistry of diamond.
In Chapter 2, a general introduction to all the characterization techniques used throughout this thesis is given, such as X-ray Photoelectron Spectroscopy (XPS), Ultra-violet Photoelectron Spectroscopy (UPS), Near-Edge X-ray Absorption Fine Structure Spectroscopy (NEXAFS), Cyclic Voltammetry (CV), and Impedance Spectroscopy.
In Chapter 3, we show that N3 dye molecules [cis-bis(isothiocyanato)-bis(2,2'-bipyridyl-4,4'-dicarboxylato)-ruthenium(II)] can be covalently attached onto boron-doped nanocrystalline diamond (B:NCD) thin films through a combination of coupling chemistries, i.e. diazonium, Suzuki and EDC-NHS. Spectroscopic techniques such as XPS, UPS, and NEXAFS were used to verify the covalent bonding of the dye on the B:NCD surface. The spectroscopic results confirmed the presence of a dense N3 chromophore layer. The positions of the frontier orbitals of the dye relative to the band edge of the B:NCD thin film were inferred as well using UPS. Our proof of concept photoelectrochemical measurements showed a strong increase in photocurrent as compared to non-dye-functionalized B:NCD films.
In Chapter 4, donor-acceptor type light-harvesting molecular dyes are covalently attached to B:NCD surfaces via a combination of diazonium electrografting and Suzuki cross-coupling. In order to achieve a high coupling yield for the Suzuki reaction, various catalytic systems were compared with respect to their imposed surface coverage. We found that when combining 2-dicyclohexylphosphino-2’,6’-dimethoxybiphenyl (SPhos) and Pd(0), the diamond coverage improved considerably (by 98%) as compared to the standard tetrakis(triphenylphosphino)palladium(0) (Pd(PPh3)4) catalyst. From UPS measurements, we found that the highest occupied molecular orbital (HOMO) level of the molecular dyes aligns well with the valence band maximum (VBM) of B:NCD. It is concluded that this should favor hole injection from the molecular dyes to the B:NCD during photoexcitation.
In Chapter 5, we report a straightforward protocol for the covalent functionalization of BDD electrodes with either ferrocene or single-stranded deoxyribonucleic acid (ss-DNA). An azide-terminated organic layer was first electrografted on the diamond surface by electrochemical reduction of 4-azidophenyldiazonium chloride. The azidophenyl-modified surface then reacts rapidly and efficiently with molecules bearing a terminal alkyne moiety by means of a Cu(I)-catalyzed alkyne-azide cycloaddition. Covalent attachment of the ferrocene moieties was analyzed by means of XPS and CV, whereas impedance spectroscopy was applied for the characterization of the immobilized DNA.