Covalent Functionalization Of Boron Doped Diamond Electrodes For Biosensor Applications

Diamond display some of the most extreme physical properties even though its practical use in science and engineering has been restricted due its scarcity and expense.With the recent developments of techniques for the deposition of thin films of diamond on various substrates,it is now possible to explore these superior properties in various exciting applications.Diamond,owing to its combination of specific physical,chemical and mechanical properties such as high thermal conductivity,high hardness,large band gap,optical transparency over a wide wavelength region(from UV to IR),stability against chemical reagents,high mechanical stability,corrosion resistance and biocompatibility has been regarded as one of the most promising industrial materials in various fields.Diamond display a very large band-gap(5.45eV),but can be made conducting by doping with certain elements.On basis of all above properties, diamond is a particularly attractive substrate for robust chemical and biochemical modification for sensor applications.In this thesis,we have contributed to the development of easy,controllable and specific surface functionalization methods for the introduction of different functional groups on the diamond surface.These methods are based on chemical,photochemical, and electrochemical concepts.The initial phase of the study includes the oxidation of hydrogen-terminated boron-doped diamond(BDD) surfaces using three different approaches,and the resulting surfaces were characterized by X-ray photoelectron spectroscopy(XPS) and Mott-Schottky analysis.Chemical coupling of 3-aminopropyltriethoxysilane(APTES) and trifluoroacetic acid(CF₃COOH) to the oxidized surfaces were used to verify the difference in terms of grafting density.These organic molecules were investigated for their specific reactivity with hydroxyl groups.It is concluded that diamond interfaces exhibiting better grafting efficiency also show a more positive fiat band position.This behaviour suggests strongly that a positive shift of the fiat band potential is related to the formation of a higher density of C-OH bonds rather than C-O-C groups.Based on this result,the electrochemical oxidation is one of the preferred methods,forming the highest amount of C-OH groups without graphitizing the diamond interface,as observed in the case of oxygen plasma treatment.However,for APTES modified BDD surface,photochemical oxidation for short times(5 to 15min) results in high N/O ratios together with an increase in electron transfer kinetics(for 15 min treatment) and is an alternative for undoped diamond samples.In the second part of my work,"click chemistry" was used for the first time to covalently attach acetylene-bearing molecules(ferrocene,thiophene as well as cyclophane) to azide-terminated BDD surfaces.The azide termination was obtained through an esterification reaction between 4-azidobenzoic acid and the terminal hydroxyl groups of oxidized BDD surfaces.The resulting surfaces were characterized using X-ray photoelectron spectroscopy(XPS),water contact angle and electrochemical measurements.As a result,1) the attachment of an electroactive ferrocene moiety on azide terminated BDD surface was achieved in high selectivity and yield;2) the applicability of azide-alkyne[3+2]cycloaddition was successfully demonstrated with ethynyl thiophene and further electrochemical polymerization of terminal "thiophene" units with thiophene monomers in solution led to the formation of a polythiophene film covalently linked to the BDD surface;3) alkyne-functionalized cyclophane can undergo click chemistry to conveniently attach these units onto a preformed azide-functionalized BDD surface.Another aspect regarding the reactivity of the oxidized BDD surfaces concerns its chemical coupling with an ionic liquid(1-(Methylcarboxylcacid)-3-octylimidazoliumbis (tri fluoromethyl sulfonyl) imide) through an esterification reaction.The resulting surface was characterized and confirmed by X-ray photoelectron spectroscopy,water contact angle and electrochemical measurements.Anion exchange of(CF₃SO₃)₂N^-with BF₄^-,NO₃^- and PF₆^- of the IL modified BDD surface was investigated subsequently.Contact angles of IL modified BDD varied alternatively in the process of anion exchange reactions between(CF₃SO₃)₂N^- and BF₄^-,NO₃^- and PF₆^-, suggesting successful anion exchange.Finally,a versatile strategy for chemical halogenation of hydrogenated boron-doped diamond(H-BDD) surfaces was proposed.Brominated and chlorinated boron-doped diamond electrodes were prepared in a controlled way through a radical substitution reaction using N-halogenosuccinimide(N-chloro or N-bromosuccinimide) in the presence of benzoyl peroxide.This versatile strategy was developed for chemically functionalizing hydrogenated boron-doped diamond(H-BDD) surfaces in a manner that stabilizes the underlying diamond against oxidation and allows subsequent chemical or electrochemical functionalization of the surface.The chemical reactivity of the halogenated BDD surfaces was confirmed by exposing the brominated BDD surface to a Grignard reagent.Furthermore,an azide termination was obtained through a nucleophilic substitution reaction of the brominated BDD surface with sodium azide.Ferrocene was linked with the azide-terminated BDD surface using click chemistry.The resulting surfaces were characterized and confirmed by X-ray photoelectron spectroscopy,water contact angle and electrochemical measurements.