Diamond materials produced by different methods have different physical and chemical properties. Producers of diamond materials often use special additives (silicates, fatty acids, amines, etc.) in order to modify product properties for particular applications (prevent caking, improve suspendability, adjust mechanical properties). However, for the use in chromatography diamond based phases often require homogeneous surfaces. Hence, physical and chemical surface modification is an important and necessary step in preparation of diamond adsorbents. There are three main objectives from surface modification:
• The removal of undesirable additives from the surface
• The homogenisation of the diamond surface (to-date a standard approach is not available)
• The introduction of new selectivity to the diamond surface towards specific chromatographic applications.
Aside from the above, surface functionalisation of such diamond based materials provides an opportunity to control the stability of diamond suspensions in various solvents, improves interfacial adhesion of ND within composite matrices, and influences the electronic properties of ND crystallites [67]. Modification techniques applied to diamond particles can involve physical and chemical modification (or both), the latter of which may involve either covalent or non-covalent surface functionalisation (see Fig. 1.5).Physical modification does not typically aim to change the surface properties of diamond phases, but normally is used to modify the mechanical and structural properties of the material, such as particle aggregation, particle size distribution, sintering of nanodiamond, etc.
18 Fig. 1.5. Modification of diamond materials for use in chromatography and solid-phase extraction.
[4,5,14,20,21,25,29-34,51,59,64,68-74]
Diamond
HPHT DND Purification [5,21,51] CVD [32,64] B-doped CVD [33] MSDN (sintering) [14] SDND (disaggregation) [59]Hydrogenation with LiAlH4, H2
[4,20,25,32,68,72,74] Oxidation O2, air, O3 [4,32,68] Chlorination Cl2, CCl4 [20,25,72] Amination with NH3, NaCN/LiAlH4 [20,68,70,72] Grafting of alkyls (C8, C18) [32,71] Embedding in polymers [29,30,31,34] Hydrosols for entrapment
19 As mentioned previously, obtaining stable disaggregated ND suspensions is an important step prior to further modification, and narrow particle size distribution is required to improve column packing.
A simple and effective method for the disaggregation of DND agglomerates and reducing particle size is ultrasonication [75]. This not only allows a reduction in average particle size of DND aggregates, but also can change the surface chemistry and ζ. This shift
in ζ is related to the appearance of new hydroxyl groups at the surfaces during ultrasonication of DND. The size distribution of ND agglomerates can be controlled by adjusting the ultrasound intensity. It has been demonstrated that ultrasonically treated particles can conserve suspension stability and particle size distribution for a period of up to 150 days [76].
In the case of increasing diamond agglomerate sizes, sintering within the thermodynamic boundaries of diamond stability can be done [14]. As mentioned previously, sintering can improve the morphology of agglomerates and lead to materials with high porosity and homogeneous surfaces.
In terms of chemical modification of diamond surfaces, the simplest route is liquid phase acid treatment [55,77]. Oxidative treatment with HNO3 or HClO4 can be used both for removal of on-diamond carbon and metal impurities and for creating additional oxygen containing functional groups (carbonyls, carboxyls, hydroxyls, etc.) [74]. Simultaneously the presence of non-diamond carbon and impurities of metal oxides can be reduced [51]. Oxidised diamond can be used in cation-exchange chromatography [26].
Heating of diamond in various controlled atmospheres, or within the presence of a plasma, is another efficient way for chemical modification of the diamond surface [78,79]. Oxidation of diamond in the air starts at 430–650 °C, depending on the type of diamond, and increases the concentration of carboxyl groups at the surface of diamond and its cation- exchange capacity. Alternatively, heating of diamond at 800–900 °C in the presence of H2 can be used for hydrophobisation and surface homogenation [71]. Chlorination at 400–600 °C has been shown to be effective in the introduction of C-Cl bonds onto diamond surfaces [56]. Somewhat similar results can also be achieved via plasma treatment. Both H2 and Ar plasmas have been applied to remove functionalities from ND surfaces and increase particle hydrophobicity [32], whilst treatment with O2 plasma resulted in more hydrophilic particles with higher content of carbonyl and carboxyl groups [80].
The non-covalent functionalisation of diamond surfaces can be achieved through adsorption of specific molecules. Adsorption modification has been frequently used to stabilise ND suspensions [81,82], or as a step for the preparation of ND composites [30,32].
20 For example, simple addition of surfactants, such as sodium oleate [83] or AlCl3 [81], can increase the absolute value of the ND surface ζ and electrostatically stabilise ND suspensions in non-polar solvents and water, respectively [83].
The surface modification of diamond particles with inorganic material layers is a less common approach, but has been demonstrated with materials such as SiO2 or SnO2 [84]. In this case the diamond particle simply acts as a stable, inert and heat dissipating core, while the attached shell, with its different adsorption properties, could be used for the construction of the stationary phase for UHPLC as reported recently by Waters [10].
Covalent modification of the surface of diamond is a very significant topic, as this approach can bring a selectivity desired for specific chromatographic applications. Since diamond surfaces are normally covered by weakly acidic carboxylic groups, using various amines has become the most common way of attaching different moieties to the diamond surface. For example, long-chain aliphatic amines have been used for preparation of amide coatings with a reduced surface charge and enhanced hydrophobicity [31]. The use of poly(allylamine) coated NDs is reported for solid-phase extraction (SPE) and as intermediate for the preparation of various composite adsorbents functionalised with octadecyl-, octyl- and heptafluorodecyl- groups [30,66,85,86]. Another useful approach includes conversion of carboxyl groups at diamond surface into primary hydroxyls by reaction with LiAlH4 and subsequent treatment either with alkyl isocyanates, chloroanhydrides or other reactive substances to introduce a desired functional group [87].
Another useful approach to the covalent functionalisation of diamond surfaces involves radical reactions using benzoyl peroxide to attach carboxylic or alkyl groups [88]. This method is less common than using nitrogen containing substances; however, it can potentially lead to the formation of more stable bonds between diamond and the attached functional group, as compared to amide or diazo bonds.
The use of radical initiator di-tert-amyl peroxide for mono- and multi-layer coating of hydrogen terminated diamond particles of 70 µm size is reported by Linford et al. [89]. The prepared adsorbent is used for SPE of pesticide cyanazine. Lately this research group used similar surface chemistry for diamond surface initiation and encapsulation of particles into poly(styrene-divinylbenzene) layer [90,91]. The prepared adsorbent and its sulfonated analogue are tested for SPE of 1-naphtylamine.
Overall, in practice, multistep modification is usually required for the preparation of diamond based adsorbents with desirable surface characteristics. Theoretically, presence of
21 reactive carboxyl-, hydroxyl and hydrogen groups allows attachment of all types of functional groups normally used in chromatographic stationary phases.