Journal of Solid State Electrochemistry

, Volume 12, Issue 10, pp 1205–1218

Different strategies for functionalization of diamond surfaces

Authors

    • Laboratoire d’Electrochimie et de Physicochimie des Matériaux et des Interfaces (LEPMI)CNRS-INPG-UJF
  • Rabah Boukherroub
    • Insstitut de Recherche Interdisciplinaire (IRI), FRE CNRS 2963Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN), UMR CNRS-8520, Cité Scientifique
Review Paper

DOI: 10.1007/s10008-007-0473-3

Cite this article as:
Szunerits, S. & Boukherroub, R. J Solid State Electrochem (2008) 12: 1205. doi:10.1007/s10008-007-0473-3

Abstract

Functionalization of diamond surfaces holds considerable promise from both fundamental and applied research aspects. This review summarizes briefly the state of the art of chemical, photochemical and electrochemical strategies for the grafting of different organic functionalities on diamond. Depending on the sought-after application and the desired physical property of diamond, halogenated, aminated, carboxylated and oxidized diamond surfaces have been proposed. After a brief introduction, the review is primarily divided into two parts, presenting chemical functionalisation strategies used on oxygen-terminated diamond, followed by methods used for the formation of C–C, C–X and C–N bonds on hydrogen-terminated diamond.

Keywords

Hydrogen-terminated diamondOxidized diamondSurface functionalizationBiomolecule immobilizationCharacterization

Introduction

Diamond, a well-known gemstone, is famous among scientists for its combination of exceptional physical and mechanical properties such as high thermal conductivity, chemical inertness, optical transparency (from UV to IR), high mechanical stability and corrosion resistance. Due to its indirect optical band gap of 5.47 eV, diamond belongs to the group of wide band gap semiconductors. It exhibits an insulating character in the undoped state and a semimetallic to metallic behavior upon doping. Its superior properties have triggered many research activities on device-oriented applications, and diamond is, nowadays, used in electronic industry as field emitters. Other applications based on diamond include chemical and biological sensors, DNA and protein chips, and electrodes for electrocatalytic reactions.

These developments would not have been possible without experimental and theoretical efforts towards obtaining a fundamental understanding of the synthesis of diamond. Optimization of the growth conditions of synthetic diamond during recent years allowed the controlled deposition of electronic-grade quality with atomically flat surfaces. For a long time, diamond surface was considered as chemically inert to most reagents and its chemical modification difficult, in contrast to many other semiconductor surfaces. There are however several important exceptions to the generally low reactivity of diamond. First, diamond surfaces can undergo a phase transformation to graphite [13], and the graphite is subsequently used as the basis for additional surface reactions. This may not be particularly useful when relying on properties specific to diamond. Second, a diamond surface can be oxidized. Yet, the most obvious oxidative method, heating diamond in the presence of O2, forms graphite [4].

In the last 15 years, progress has been made in the development of easy, controllable and specific surface modification methods for the introduction of different functional groups on the diamond surface. These methods are based on chemical, photochemical, and in the case of doped diamond, electrochemical concepts. Depending on the sought-after application and the desired physical property of diamond, halogenated, aminated, carboxylated and oxidized diamond surfaces have been prepared. While the interest in fluorinated diamond films comes, for example, from its improved friction properties and water repulsion behavior comparable to poly(tetrafluoroethylene) [5], aminated diamond surfaces have allowed successfully grafting of oligonucleotides, peptides, proteins and gold nanoparticles onto diamond surface [610].

Because of the increased interest in the surface chemistry of diamond, this review is devoted to discuss, in more detail, some of the approaches employed by different groups to functionalize hydrogenated and oxidized diamond surfaces.

Fabrication of diamond substrates

Polycrystalline diamond films

The use of diamond has been facilitated since the beginning of the 1980s through the discovery and development of processes for large-scale production. It was discovered that polycrystalline diamond films (Fig. 1a) can be formed by chemical vapor deposition (CVD) techniques, which involve a chemical reaction occurring above a solid surface in the gas phase, causing deposition of the diamond onto that surface [11]. CVD is nowadays well established and well described in a number of review articles [12, 13]. In short, CVD techniques require, for producing diamond films, a means of activating gas-phase carbon-containing precursor molecules. The activation of gas phase molecules can be achieved by three different means: (1) thermally (e.g., hot filament), (2) via plasma activation (D.C., R.F., or microwave), (3) through the use of a combustion flame (oxyacetylene or plasma torches). While each method differs in its details, they all share common features during the growth process: temperatures in the range of T = 1,000–1,400 K, and the gas precursor is diluted in an excess of hydrogen (typical CH4 mixing ratio ~1–2 vol.%).
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Fig. 1

SEM images of (a) polycrystalline diamond, (b) nanocrystalline diamond and (c) ultrananocrystalline diamond thin films

The fact that diamond films can be prepared by CVD techniques is linked to the presence of hydrogen atoms, which are generated as a result of the gas being activated either thermally or via electron bombardment. The hydrogen atoms are believed to play a number of crucial roles in the CVD process: they undergo hydrogen abstraction reactions with stable gas-phase hydrocarbon molecules, producing highly-reactive carbon-containing radical species. This is important, as stable hydrocarbon molecules do not react to cause diamond growth. These reactive radicals, especially methyl radicals, can diffuse to the substrate surface and react, forming the C–C bond necessary to propagate the diamond lattice. Even though atomic hydrogen etches diamond under typical CVD conditions, the rate of diamond growth exceeds its etch rate. For other forms of carbon (graphite, glassy carbon, etc.) the converse is true, and this is therefore believed to be the basis for the preferential deposition of diamond rather than graphite. Nonetheless, the morphology and quality of the resulting diamond films depends on the precise growth conditions. One of the challenges faced by the researchers in CVD diamond technology was to increase the growth rates to economically viable rates (such as 1 mm h−1) without compromising film quality. High growth rates could be only achieved at the expense of a corresponding loss of film quality, where high film quality implies that the ratio of sp3 (diamond) to sp2-bonded (graphite) carbon in the sample is high, the C–C vs C–H bond content is enhanced and that the film is crystalline. Progress has been made using microwave deposition reactors, as the deposition rate has been found to scale approximately linearly with applied microwave power.

Nanocrystalline and ultrananocrystalline diamond films and powders

The use of CVD techniques for diamond film formation is often hampered by the polycrystalline nature of the films. The numerous grain boundaries and crystal defects in polycrystalline diamond reduce electron and hole motilities and degrade the electronic performance of diamond. The diamond film morphology depends on the reactant gases, their mixing ratios and the substrate temperature. At low partial methane pressure, highly crystalline diamond films are obtained. The crystalline morphology disappears with increasing pressure, forming diamond films with amorphous structure resembling more disordered graphite containing small clusters of diamond nanocrystals. Growing at specific conditions between these two extremes can yield high quality (mostly sp3) nanocrystalline and ultrananocrystalline diamond films, which possess a much smoother surface and enhanced electronic properties (Fig. 1b,c). A number of synthetic methods for the preparation of nanocrystalline diamond (ND) and ultrananocrystalline (UNCD) diamond in the form of films and powders have been developed [1419]. UNCD film consists of diamond grains of predominantly 3–5 nm in size in an amount of no less than 1010 atoms cm−3 with abrupt grain boundaries, consisting of a mixture of sp3- and sp2-bonded carbons. Thin UNCD films, synthesized using microwave plasma chemical vapor deposition with argon-rich Ar/CH4 plasmas, exhibit similar physical, mechanical and electrochemical properties as microcrystalline diamond [2022]. UNCD can be made into very highly n-type, room-temperature conductive via the addition of nitrogen gas to the plasma, yielding films with room temperature conductivities as high as 250 Ω−1 cm−1[23]. Microwave-plasma-enhanced CVD was used also to form single-crystalline diamond with atomically smooth surfaces [2426].

Physical characteristics of hydrogen-terminated diamond

Although diamond is composed of only carbon atoms, the diamond surface is stabilized by bonding to elements rather than carbon, as grown diamond films can be considered as being hydrogen terminated (H-terminated). Undoped H-terminated diamond shows insulating behavior in vacuum but p-type surface conductivity when exposed to air [27]. This contrasts to oxygenated diamond (O-terminated), which is, in its natural state, electrical, insulating in vacuum and in air. It is certain that the conductivity is related to hydrogen content. Several controversial models have been proposed to explain this induced p-type surface conductivity (which is still not well understood) such as (1) surface band bending due to valence-band electron transfer into an adsorbate layer (transfer doping model) [2830], (2) hydrogen-introduced surface acceptors [31] and (3) deep-level passivation by hydrogen [32].

The high electrical resistivity (typically > 108 Ω cm) of undoped diamond can be lowered through doping, replacing the semiconductor behavior by a semimetallic or metallic one depending on the doping level. Boron doping resulting in a p-type semiconductor can be achieved during the synthesis of diamond by addition of diborane gas [33]. While p-type diamond films are already available with good quality, the question of finding a shallow n-type dopant for diamond is still open [3436]. Heavily boron-doped diamond (BDD) films (NA > 3 × 1020 B cm−2) have gained rapidly interest since its first report as a novel electrode. Through the study of the electrolysis of aqueous H2SO4 solutions, the large potential window of water stability has been pointed out [37]. The wide electrochemical potential window of O-terminated diamond together with small background current in aqueous media helped the development of a large range of electroanalytical applications of BDD interfaces [38]. The fluorinated diamond (F-terminated) displays, so far, the widest range of potentials (5 V polarization window) for an electrode material in aqueous solution, being limited only by the formation of free hydrogen [E0 (H+/H2)= −2.3 VSHE] and hydroxyl [E0 (OH,H+/H2O)= 2.74 VSHE] radicals [39, 40]. These electrodes were formed through radio-frequency-based plasma fluorination [41]. It was found that three-quarters of the sites on (111) surface could be fluorinated. On the other hand, on the (100) surface, less than half of the surface sites are fluorinated and low-energy electron diffraction showed no ordered surface structure [42]. This is consistent with the theoretical estimation of Harrison and Belton, who showed appreciable steric hindrance of C–F on diamond (100) and much less on (111) [43].

The way diamond surface is stabilized, H-terminated or O-terminated, has additional physical and chemical effects. Besides the differences in electrical conductivity, it includes the change of wetting properties: hydrophobic for a H-terminated and hydrophilic for an O-terminated diamond film, and the difference in electron affinity: negative electron affinity for H-terminated and positive electron affinity for O-terminated diamond. These differences in electron affinity are mainly caused by the surface dipole difference between C–H and C–O bonds [44] and has important consequences for the kind of surface chemistry that can be successfully performed on diamond films. There might be still additional advantages of diamond electrodes over conventional electrodes which are not yet realized.

Cyclic voltammetry (CV) on doped diamond has proven to be a powerful method to distinguish between the surface states of diamond with the same doping level [45, 46]. This difference is possible, as the heterogeneous electron transfer reaction of several redox mediators at the BDD interface is mediator dependent. Broadly, one can distinguish between two types of redox mediator: outer sphere (e.g., \({\text{Ru(NH}}_{{\text{3}}} )^{{3 + /2^{ + } }} \), IrCl62−/3−, Ru(bpy)33+/2+) and inner sphere [Fe(CN)63−/4−, Fe3+/2+] electron transfer mediators. The characteristics of outer sphere electron transfer mediators are that the electron kinetics for this type of reaction are relatively insensitive to factors such as surface termination and microstructure, but very sensitive to the density of electronic states near the formal potential. Apparent rate constants between 0.01 and 0.2 cm s−1 are commonly found for high-quality diamond electrodes. In the case of an inner-sphere electron transfer, the electrode kinetics is highly sensitive to the diamond termination as the reaction appears to proceed through a specific surface site. In the case of H-terminated and O-terminated diamond, the apparent rate constants are two orders of magnitude apart [47].

Oxidation of H-terminated diamond surfaces

Freshly synthesized H-terminated diamond interfaces can be left without special precautions on the bench for several months. In spite of the high stability of diamond, oxidation of the hydrogenated surface is relatively easy to perform. The resulting oxidized surface is so stable that it can be recovered only by hydrogen plasma treatment at elevated temperatures. Diverse procedures have been reported for the oxidation of H-terminated surfaces including thermal [4850], plasma [51, 52] and electrochemical techniques [5356, 65], as well as the use of singlet oxygen [53], irradiation with vacuum ultraviolet light (VUV, λ = 172 nm, t = 3 h in the presence of O2 and H2O) [57] and ozone treatment [5860]. No real attempt has been made in the literature to compare these different approaches adequately, and no recommendation can be found regarding the best approach. Foord et al. have studied in more detail the difference in efficiency of the electrochemical activation and the oxygen low temperature plasma treatment. They found that oxygen plasma is probably the most efficient method for the oxidation of diamond surfaces [54]. However, the reported electrochemical approaches vary significantly (Table 1):
Table 1

Conditions for electrochemical oxidation of H-terminated diamond

Solution

Potential

Conditions

Reference

1 M HClO4

Potentiostatic at +3 V/SCE

20 min

[60]

Phosphate buffer

Cycling between 0 and 2.5 V/SCE

7 scans

[53]

3 M H2SO4

Cycling between −0.5 and −2.5 V/RHE

Several scans

[61]

0.5 M H2SO4

Potentiostatic at +3 V/SCE

30 min

[62]

0.1 M KOH

Potentiostatic at 2.6 V/Ag/AgCl

60 min

[63]

0.2 M NaSO4

Potentiostatic at 2.2 V/SCE

3 min

[52, 64]

Independent of the activation method used, the presence of oxygen on the diamond surface has a significant influence on the chemical reactivity [55, 66], electrical conductivity [67, 68], field emission [69, 70] and Schottky barrier heights [71].

Figure 2 shows typical high resolution X-ray photoelectron spectra of a hydrogen-terminated and a photochemically oxidized boron-doped (BDD) polycrystalline diamond surface [58]. The XPS spectrum of the C 1s of a hydrogenated BDD sample displays a main peak at ~285 eV. The peak is unsymmetrical. The presence of a shoulder at higher binding energy (~286 eV) is most likely due to the presence of amorphous carbon at the grain boundaries. After photochemical oxidation, the XPS spectrum displays three different features: a peak at 285 eV due to C 1s from the bulk with a shoulder at 286  due to the surface C–O features further seen at 289 eV . Based on the diamond structure, it is expected that the sp3 C–H bonds on the (111) facets will be terminated with hydroxyl groups, while the CH2 bonds on the (100) facets will be transformed to carbonyl and ether functional groups. The signals from surface hydroxyl and ether groups are undistinguishable (~286 eV; 72, 73]). The peak at 289 eV results most likely from a contribution of carbonyl groups on the surface.
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Fig. 2

High resolution XPS spectra of C 1s of hydrogenated and photochemically oxidized boron-doped diamond surfaces

Chemical functionalization of oxygen-terminated diamond surfaces

Silanization

As silane reagents show no reactivity towards carbonyl or ether groups, they can be used to underline the presence of hydroxyl groups on oxidized diamond. Notsu et al. reacted electrochemically and oxygen plasma-oxidized boron-doped diamond surfaces with 3-aminopropyltriethoxysilane (APTES) and characterized the silanized interface using XPS and contact angle measurements [56, 73]. The authors have observed that the water contact angle decreased after surface oxidation (from 67.0 ° to 25.8°) and increased again to some extent after APTES treatment (43.4 °). The presence of a N 1s peak in the XPS spectrum of the modified surface indicates further the success of the silanization reaction. On the other hand, the presence of surface hydroxyl groups on photochemically oxidized diamond surfaces was confirmed by the chemical reaction of the oxidized BDD surface with perfluorodecyl trichlorosilane [CF3–(CF2)7–CH2–CH2–SiCl3] at room temperature (Fig. 3; [72]).
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Fig. 3

Schematic illustration of the silanization reaction of oxidized diamond surface with perfluorodecyl trichlorosilane

XPS analysis of the resulting surface showed the presence of peaks due to F 1s (687 eV), Si 2p (99 eV) and Si 2s (151 eV), consistent with the incorporation of an organic layer on the diamond surface. Furthermore, contact angle measurements agree well with a change of the wetting properties of the surface before and after silanization. Indeed, oxygen-terminated BDD surface displays a hydrophilic character with a water contact angle of 36°, while the perfluorodecyl-modified BDD is hydrophobic with a contact angle of 109°.

The barrier properties of oxidized and silanized diamond surfaces was investigated using cyclic voltammetry. Figure 4 shows the different electrochemical behaviors for H-BDD, oxidized and silanized BDD in the presence of Fe(CN)64−/3−. The electron transfer kinetic is inhibited for the oxidized surface with an apparent rate constant of k0app= 0.0005 cm s−1 (H-BDD: k0app= 0.012 cm s−1). The perfluorodecyl terminated diamond blocks effectively the surface, and a k0app= 8 × 10−8 cm s−1 was determined through modeling [72].
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Fig. 4

Experimental (bold lines) and simulated (DigiSim 3.03, dotted lines) cyclic voltammetric iE curves for hydrogenated, photochemically oxidized and silanized diamond electrode: solution: Fe(CN)64− (10 mM) in KCl (0.1 M)/water, scan rate = 0.1 V s−1, geometric area =0.283 cm2

Recently, we also showed that N-(3-trimethoxysilylpropyl)pyrrole (TMPP) can be linked to oxidized BDD [74]. The functional silane layer allowed localized polymer formation to be achieved on the TMPP-modified BDD interface using the direct mode of a scanning electrochemical microscope (SECM) and an electrochemical scanning near-field optical microscope (E-SNOM). Depending on the method used, polypyrrole dots with diameters in the range of 1–250 μm are electrogenerated.

Esterification

The hydroxyl groups of oxidized BDD were successfully tethered to organic molecules through an esterification reaction. Pyrene alkylcarboxylic acid, [75] biotin [53], and more recently, 3-benzoylbenzoic acid were covalently linked to oxidized diamond (Fig. 5; [76]).
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Fig. 5

Schematic illustration of the esterification reaction of hydroxylated diamond

To reveal the biotin grafting, the authors exploited the strong affinity of biotin to fluorescently labeled streptavidin. On the other hand, the photoelectrochemical behavior of a pyrene-terminated BDD surface was studied, and cathodic photocurrents have been measured in oxygen-saturated electrolyte.

The benzophenone terminated BDD surface was successfully used for photochemical immobilization of DNA, peptides and proteins. Benzophenone is as an efficient photoactivable group, stable under ambient light and protic solvents [77, 78]. Green fluorescence protein (GFP) was linked to benzophenone-modified BDD by exposing the surface to blue light (λ = 350 nm) for 30 min. Figure 6 shows the fluorescence image of the covalently linked GFP on a patterned BDD surface.
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Fig. 6

Fluorescence images of photochemically linked GFP (t = 30 min, λ = 350 nm, P = 5 mWcm−2) on a patterned BDD surface. The bright lines are coated with GFP molecules, while the dark squares correspond to oxidized regions

Zirconium phosphate chemistry has recently been adapted to oxidized diamond (Fig. 7; [79]). Carboxylic acid layers can be bound to the diamond surface by coordination to zirconium phosphate functionalities. Pyrene was linked in this manner to BDD, and a surface coverage based on geometric surface area of 5 × 10−11 mol cm−2 was determined.
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Fig. 7

Zirconium phosphate chemistry on oxidized diamond surface

Functionalisation of hydrogen-terminated diamond surfaces through C–C bond formation

Reduction of diazonium salts

One method available for the formation of a strong C–C bond between diamond and an organic molecule was first reported by McCreery and Swain [80] and consists of the electrochemical reduction of diazonium salts (Fig. 8; [17, 8089]). Different diazonium salts are commercially available, but they can also easily be prepared in one step from a wide range of aromatic amines. The techniques offer the advantage of the availability of a wide range of functional groups (–COOH, X, NO2, etc.) that can be introduced onto the surface in one step.
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Fig. 8

Attachment of organic layers by reduction of aryl diazonium salts

The method, intensively used for other carbon-based materials [90100], is quite simple and can be performed in acetonitrile (CH3CN + 0.1 M Bu4BF4) and acidic aqueous solutions (0.1 M H2SO4) through electrochemical reduction of diazonium salts. Covalent bonds between the diamond and the diazonium salt can, in addition, be formed through a spontaneous binding (in the absence of external bias) of the diazonium salt in the presence of 1% sodium dodecyl sulfate (SDS; [88], Shul, et al. 2007, submitted for publication). We recently showed that solvent-free functionalization of hydrogen-terminated boron-doped diamond surfaces with aryldiazonium salts using ionic liquids (1-butyl-3-methylimidazolium hexafluorophosphate or 1-butyl-3-methylimidazolium methyl sulphate) can be achieved in high yield (Shul, et al. 2007, submitted for publication). The electrons necessary for the reduction of the diazonium salt are, in these cases, most likely provided by the diamond itself [100]. This purely chemical grafting is attractive, as it does not require electrochemical equipment or doped diamond interfaces. The authors have also evidence from ellipsometric measurements that the chemical approach results more readily in monolayers rather than multilayers observed under electrochemical conditions. AFM measurements have evidenced the formation of multilayers using the electrochemical route, and the formation of a real monolayer was only reported on pyrolyzed photoresist [91]. Phenyl radical species are formed next to nitrogen during the grafting process, which react directly with the diamond surface. The formation of multilayers is due to an attack of the first grafted aryl group by another aryl radical (Fig. 9).
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Fig. 9

Multilayer formation using diazonium salt reduction [99]

The principal interest in diazonium-based diamond modification is that the diamond-tethered functional groups are used for the covalent linking of biomolecules. This is due to the reported biocompatible and bio-inert character of diamond electrodes making them promising platforms for biosensing.

The most intensively studied diazonium salt derivative is the nitrophenyl salt. The key to biomolecular functionalization lies in the ability to selectively reduce nitro groups to primary amines to which DNA and other biomolecules can chemically be linked. In aqueous solution, an irreversible multi-electron and multi-proton reaction of the nitro group to amine (–NH2) or hydroxyaminophenyl (–NHOH) groups takes place [101].
$$\begin{aligned}& R - NO_2 + e^ - \to R - NO_2^ - \\& R - NO_2 + 4H^ + + 4e^ - \to R - NHOH + H_2 O \\& R - NHOH + 2H^ + + 2e^ - \to R - NH_2 + H_2 O \\ \end{aligned} $$
The reduction of the nitro group proceeds at potentials between −0.8 and 1.2 V and is mainly completed in the first cycle of the reduction step. Figure 10a shows the electrochemical reduction of a 4-nitroazobenzene (NAB)-modified polycrystalline BDD in aqueous solution to 4-aminoazobenzene-terminated BDD. XPS survey spectra (Fig. 10) recorded after five cycles show that all the –NO2 groups have been converted into –NH2 bonds. Indeed, after modification of the BDD surface with NAB molecule (Fig. 10b), additional peaks at 531, 401 and 407 eV due to O 1s, N 1s from –N=N and NO2, respectively, were observed. The absence of a peak at ~402 eV from N 1s of the diazonium molecule is consistent with a covalent linking of the NAB molecules on the surface and not physisorption. The O 1s results most likely from the –NO2 groups, but may also be due to partial oxidation and/or contamination of the surface. The XPS survey spectrum of the NAB-terminated BDD surface after electrochemical reduction (Fig. 10c) shows the disappearance of the component at 407 eV, in agreement with the reduction of –NO2 to –NH2 groups. However, the electrochemical reduction of the terminal nitro to NH2 groups led only to a small decrease of the intensity of O 1s signal. This confirms that the O 1s peak observed in the XPS survey originates also from surface oxidation and/or contamination.
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Fig. 10

a Cyclic voltammograms recorded in an aqueous (90%water/10%EtOH) 0.1 M KCl solution of a H-BDD modified with 4-nitrophenyl diazonium salt (1 h in SDS/water), scan rate = 50 mV s−1; XPS survey spectra of the BDD before (b) and after electrochemical reduction of NO2 groups (c). The insets shows the high resolution XPS spectra of N 1s before (b) and after NO2 reduction (c; Shul, et al. 2007, submitted for publication)

Integration of the cathodic peak using equation 1 (Q is the electrical charge passed during the electrochemical reduction of the nitro group, F is the Faraday constant, A the area of the electrode and n the number of electrons required for reducing a single nitro group) allowed determining the surface density:
$$\Gamma = \frac{Q}{{nFA}}$$
(1)

A surface density of 5.4 × 10−10 mol cm−2 was found in this particular case (Shul, et al. 2007, submitted for publication). Reported surface densities vary between 1.3 × 10−10 to 7.5 × 10−10 mol cm−2 [17, 82, 85, 89, 102, Shul, et al. 2007, submitted for publication]. A close-packed nitrophenyl layer on flat surfaces is reported to yield a density of 12 × 10−10 mol cm−2 [98].

The covalent linking of enzymes (glucose oxidase and tyrosinase) was achieved on the amine-terminated diamond surfaces. Carlisle et al. [87] reacted the amine terminal groups on the BDD with succinic anhydride and N-hydroxysulfosuccinimide to link glucose oxidase (Fig. 11). Glucose was detected through the oxidation of biocatalytically formed H2O2. An amperometric biosensor for phenol, p-cresol and 4-chlorophenol was developed by Zhin et al. [89]. It is based on covalent immobilization of tyrosinase on amine-terminated BDD via carbodiimide coupling by forming an amide bond between the carboxylic acid of the tyrosine and the NH2 of the BDD. They reported that 90% of the enzyme activity was retained for 5 weeks when storing the sensor at 4 °C in PBS.
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Fig. 11

Covalent immobilization of glucose oxidase on amine-terminated BDD surface

On the other hand, DNA immobilization was achieved by immersing the amine-terminated diamond into a heterobifunctional cross-linker (sulfo-succinimidyl 4-(N-maleimidomethyl) cyclohexan-1-carboxylate) to which thiol-modified DNA was linked (Fig. 12) [84, 85, 88]. Experiments on single-crystalline diamond showed that the DNA chains are tilted by 31° with a DNA density of 6 × 1012 molecules cm−2. The results are not affected by variation of the buffer salinity [84]. The structural and mechanical properties of a DNA double helix bonded to NH2-terminated single crystalline diamond surfaces, prepared via photochemical or electrochemical routes, were evaluated using contact mode AFM. It was shown that DNA molecules can be irreversibly removed from the diamond surface by contact mode AFM with forces N > 45 nN and >76 nN on photochemically and electrochemically functionalized diamonds, respectively [86].
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Fig. 12

Covalent linking of DNA using a heterobifunctional cross-linker (sulfo-succinimidyl 4-(N-maleimidomethyl) cyclohexan-1-carboxylate) to which thiol-modified DNA was linked

Covalent linking of anthraquinone diazonium salt to BDD through electrochemical reduction was recently shown by Foord [81]. In basic solution, the quinone-catalyzed reduction of oxygen was seen at −0.8 V/SCE.

Photochemical reaction with functional alkenes

H-terminated polycrystalline films exhibit a number of usual properties, including the ability to emit electrons from the valence band directly into a vacuum when illuminated with 254 nm light [103]. Ultraviolet irradiation of hydrogen-terminated diamond covered with liquid films of an appropriate alkene is an additional approach to produce functionalized diamond (Fig. 13). This scheme has been first proposed by Hamers et al. [102, 104108] and is now used by different groups [24, 25, 109112]. UV irradiation of hydrogen-terminated diamond surface in the presence of a liquid film of appropriate alkenes (e.g., 12-amino-dec-1-ene protected with a trifluoroacetic acid group, perfluorodecene, trifluoroethyl ester of ω-undecenoic acid, etc.) results in the formation of an organic layer covalently grafted to the surface through C–C bonds. While the stability of the formed monolayer has been proven [113115], many questions remain about how the reaction proceeds mechanistically and how the organic film is organized on the surface. Hamers et al. carried out mechanistic and structural studies on photochemical functionalized diamond surfaces [116, 117]. They demonstrated that the modification process is not controlled by grain boundaries and that the functionalization is a surface-mediated reaction initiated by the photoejection of electrons from the diamond surface into the liquid phase [116].
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Fig. 13

Photochemical linking of organic molecules to H-terminated diamond

Figure 13 displays the strategy used to incorporate terminal –NH2 functional groups onto a diamond surface. To provide chemically reactive amine groups, the trifluoroacetamide protecting group was chemically removed, and DNA was linked to the amine-terminated diamond surfaces using a heterobifunctional cross-linker as discussed above. Comparing the DNA-modified diamond with a silicon surface modified in the similar way showed no loss of DNA over 15 hybridization/denaturation cycles on diamond and a loss of 1.8% on silicon [118]. The direct photochemical coupling using undecylenic acid was recently found to form a higher density of carboxylic acid groups on the diamond compared to trifluoroethylundecenoate [112]. Figure 14 shows the fluorescence image of fluorescein isothiocyanate (FITC)-labeled DNA immobilized on acid-modified diamond and subjected to three denaturation–rehybridization cycles.
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Fig. 14

Fluorescence images of patterned FITC-labeled DNA immobilized on diamond which has been biofunctionalized with undecylenic acid and subjected to repeated denaturation/rehybridization cycles (three cycles; reprint with permission from [112])

While the photochemical derivatization allows the introduction of several functional groups on the surface, the reaction time required is rather long (in the range of 12–15 h), and most of organic molecules absorb at the used wavelength. Nevertheless, glycol compounds could be linked to diamond allowing the study of nonspecific binding of proteins [109]. Electrochemical impedance spectroscopy (EIS) was used to characterize the modified interfaces and showed that the monolayer strongly inhibits electron transfer, but the detection of biological binding events in real time, as a consequence of the field effect induced in the diamond by the charged DNA, is possible [119]. EIS measurements, together with cyclic voltammetry, were performed on horseradish-peroxide-modified nanocrystalline diamond films [111]. The proximity of the haem groups to the diamond surface allowed direct electron transfer between them.

Functionalisation of hydrogen-terminated diamond surfaces through other bond formation

C–X bonds

A practical method for surface modification of diamond is the direct reaction with radical species in the gas phase. The radical moieties known to react directly with diamond include hydrogen [120], fluorine [121] and chlorine atoms [122]. The diamond surface is unreactive to the corresponding molecular species H2, F2 and Cl2 [123]. This implies that reaction conditions necessary to generate atomic species are required. The conditions are vigorous and corrosive (e.g., Cl2/400–500°C; F2/470°; [42, 124]). Fluorinated diamond surfaces are stable in air and water at room temperature and have been electrochemically characterized [40, 125]. The chlorinated diamond interfaces forms hydroxylates at room temperature in contact with water vapor [126].

Milder conditions to halogenate diamond surfaces are employed when halogen atoms are formed in a photochemical reaction. The photodissociation of chlorine gas is rather well known [127]. Miller and Brown [128] prepared chlorinated polycrystalline diamond films, single crystals [123] and diamond powders [123] through the irradiation of the samples at λ  = 245 nm up to 24 h using a pressure Hg-arc lamp in the presence of Cl2 gas [129].

A different photochemical approach has been used by Nakamura et al. to fluorinate H-terminated diamond surfaces [130, 131]. It is based on the photolysis of perfluoroazooctane. During this process, perfluorooctyl radicals abstract a hydrogen atom from the diamond surface. The resulting surface carbon radical reacts with another perfluorooctyl radical to yield a fluorinated surface. A similar approach has been used by Russels et al. [132], who introduced perfluorobutyl groups at low temperature on a (110)-oriented single crystal diamond surface by photolysis of a solution of perfluorobutyl iodide (C4F9I) using UV irradiation with a 200-W Hg-arc lamp.

There are few reports on halogenation in the liquid phase. Ikeda et al. [133] attempted chlorination of H-terminated diamond by treatment with sulfuryl chloride (SO2Cl2) at T = 50 °C under argon atmosphere using 2,2-azobiisobutyronitrile (AIBN) as a radical initiator. The resulting chlorinated diamonds were further used for the immobilization of thymidine molecules [134]. Thymidine is a nucleoside consisting of a base combined with a sugar group in a glycosidic linkage. The DNA used was a double-stranded segment of a human phenylketonuria gene, labeled with fluorescein isothiocyanate molecules. The successful DNA binding was confirmed using confocal fluorescence microscopy. Furthermore, by comparison with transmission images, no preferential binding to certain grain boundaries was observed.

C–N bonds

The chemical reactivity of the chlorinated surface was exploited for the preparation of amine-terminated diamond. While a thermal treatment up to 450 °C in ammonia did not allow surface amination, the reaction of ammonia with chlorinated diamond was markedly different when photochemical conditions were used [123]. The chlorinated sample could be aminated by irradiation of the chlorinated diamond films in NH3(g) at room temperature in vacuum for 3–24 h [123] or through the reaction with quaternary pyridinium salt (Fig. 15; [135]).
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Fig. 15

Preparation of an amine-terminated BDD surface in a two-step procedure, consisting on the photochemical irradiation of a hydrogenated surface with chlorine followed with ammonia gas

Reports on the direct amination of H-terminated diamond in the gas phase have only been reported recently. A radio-frequency plasma was used to promote the direct chemical reaction between the diamond surface and vaporized N-(6-aminohexyl)aminopropyl trimethoxysilane [136]. The relative surface density of primary amines groups (–NH2) attached to the diamond surface was detected by photoluminescence, using fluorescamine in acetone spray as a fluorescence marker. On the other hand, the direct amination of hydrogen-terminated diamond surface using UV irradiation (λ = 254 nm) in an ammonia gas environment was reported recently by the group of Kawarada [6, 137]. The authors have used photolithography to pattern the resulting surface and to immobilize DNA molecules in a controlled fashion. A similar approach was reported by Zhi et al. [9], where an amine functionalized boron-doped diamond surface was prepared by UV irradiation (λ = 254 nm, 6 h) of a hydrogen-terminated surface in the presence of allylamine. It is believed that the vinyl groups of allylamine react photochemically with the C–H surface bonds to yield an amine-terminated monolayer covalently bonded to the surface through C–C bonds.

More recently, our group has demonstrated that the use of a cold NH3 plasma treatment of hydrogenated diamond substrate generates surface terminal amino groups [7]. The aminated diamond surface was further investigated for its ability to bind gold nanoparticles [138] and for the formation of a peptide array [7]. As seen in Fig. 16a, homogeneous and well-distributed gold nanoparticles (Au NPs) were obtained by simple exposure of the amine-terminated surface to an aqueous solution of gold colloid. The Au NPs modified NH2-diamond shows interesting catalytic behavior towards oxygen reduction in basic medium (Fig. 16b), where a significant positive shift of the oxygen reduction peak and an increase in the peak current density upon loading with gold NP is observed.
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Fig. 16

a SEM image of polycrystalline aminated BDD exposed to a gold colloid solution, b Cyclic voltammograms of NH2–BDD (full line) and NH2–BDD/Au NP (dotted line) in aqueous solution of 0.1 M NaOH, scan rate: 100 mV s−1

The linking of glyoxylyl peptides to diamond was based on the formation of a semicarbazide termination on amine-terminated boron-doped diamond (Fig. 17a). Two different peptides (peptide 1: FLAG-NH2 and peptide 2: FLAG-COCHO) were printed in a microarray format on semicarbazide-modified BDD. The fluorescent images (Fig. 17b), obtained after incubation in the presence of antibody anti-peptide FLAG labeled with tetramethylrhodamine shows that the fluorescence observed from peptide 2 is more important than with peptide 1. This is consistent with specific covalent ligation of peptide 2 with the semicarbazide surface and the non-covalent interaction of peptide 2 with the semicarbazide surface.
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Fig. 17

a Formation of peptide array through site-specific α-oxo semicarbazone ligation on aminated diamond, b fluorescence image, top (peptide 1: FLAG–NH2), bottom (peptide 2: FLAG–COCHO)

C–O bonds

Tsubota et al. [139, 140] investigated the modification of H-terminated diamond using alkyl-radicals. Among various radical initiators, benzoylperoxide was found to be the most effective in abstracting hydrogen on the surface (Fig. 18). Due to their low dissociation energy, the O–O bond in these peroxides breaks easily, generating radicals [140148]. When benzoyl peroxide is thus heated up to temperatures of about T = 60–80 °C, it decomposes into benzoyl radicals. In the presence of an H-terminated diamond surface in toluene (under argon atmosphere, at T = 348 K for t = 2h), the benzoyl radicals formed can abstract a hydrogen atom on the diamond surface and bind to it. Further studies using benzoyl peroxide showed that the radical reaction is highly solvent dependent. While the radical substitution reaction works well in THF and DMF, the reaction in toluene depends on the reaction conditions [142, 143]. The same group has demonstrated that aliphatic and aromatic carboxylic acids can be tethered to diamond surface through C–O bond formation by carrying out the thermal decomposition of benzoyl peroxide radical initiator in the presence of aliphatic and aromatic carboxylic acids [145, 146, 148]. In this case, R–COO° radicals are formed and bind to the diamond surface. The final chemical state of the diamond surface is determined by the kinetics of the reaction. The number of hydrogen atoms displaced on the diamond surface depends, however, strongly on the acid used. In the case of the large pyrene-carboxylic acid, rather low abstraction fraction was observed.
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Fig. 18

Liquid phase modification of hydrogenated diamond surfaces using a radical initiator

Conclusion

Even though diamond surfaces are chemically inert, photochemical, electrochemical and chemical approaches have shown their strength in tethering functional groups to this interface. The opportunities for diamond are wide open. To make it competitive with silicon technology, the core advantages of diamond, including the chemical stability, the low electrochemical background current and its wide potential window will have to be optimized. The control of the surface chemistry of diamond will play a crucial role in this perspective. While these interfaces are nowadays used as analytical devices in different areas, the development of new surface chemistries on diamond has somehow stagnated over the years. Two approaches to functionalize diamond interfaces have been mainly used: photochemical linking of functional alkene molecules and diazonium-based approaches. The difficulty in controlling the formation of a monolayer using diazonium-based chemistry and the immobilization of long functionalized alkene chains hindering largely electron transfer opens the search for new functionalization pathways. Some interesting approaches have been recently proposed through the direct amination of H-terminated diamond. The possibility to pattern the surface makes diamond an adapted using photopatterning to introduce functional groups inside the patterned regions makes diamond and adapted platform for the generation of microarrays. Indeed, the future application of diamond will be in high-throughput systems and biotechnologies. Whether diamond will be the material of choice for biosensing applications will depend on the effort put into the control of its surface.

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© Springer-Verlag 2007