Deagglomeration and functionalisation of detonation nanodiamond with long alkyl chains
Introduction
Diamond nanoparticles have found considerable interest recently due to their potential applications in biological systems, composites and electronic applications [1]. Their apparent biocompatibility [2] and chemical inertness make them an ideal candidate for all kinds of biomedical devices. On the other hand, their surface can be functionalised with a variety of groups in order to incorporate them into polymer matrices, surface coatings and the like [3]. An essential prerequisite for these applications is the control of surface structure and particle size and agglomeration.
So far, different approaches for the surface functionalisation and deagglomeration have been taken. Chiganova produced diamond hydrosols from oxidized detonation diamond [4], whereas Xu and coworkers used surface active compounds for the dispersion of nanodiamond particles [5]. Khabashesku et al. obtained small agglomerates of ∼ 160 nm in size by the fluorination of detonation diamond [6]. These particles formed stable suspensions in THF. Another approach is the mechanical deagglomeration in suspension by stirred media milling [7] or beads assisted sonic disintegration [8].
Covalent surface modification of nanodiamond has been reported by several authors. Reactions with fluorine [6], chlorine [9], ammonia [10] and hydrogen [10], [11] lead to homogenized, surface modified nanodiamond materials with increased reactivity and the option for further functionalisation. Oxidation of the surface can be achieved by air oxidation [12] or treatment with oxidizing mineral acids [13]. We have recently reported on a wet-chemical method for the hydroxylation of the nanodiamond surface [14]. This allowed for the subsequent grafting of different trialkoxysilanes and the further functionalisation by covalent bonding [15]. On the other hand, Nakamura and coworkers as well as Tsubota and coworkers have reported on the covalent grafting of alkyl and aryl moieties by radical reactions [16], [17].
Here we report on the grafting of alkyl chains by an esterification reaction on hydroxylated detonation diamond. This simple and efficient method yields “nanodiamond esters” with alkyl chains with different lengths. The latter should have an impact on the dispersibility of the functionalised nanodiamond in organic solvents.
Section snippets
Chemicals
Detonation nanodiamond was purchased from Gansu Lingyun Corp., Lanzhou, China. All other chemicals have been purchased from Fluka and Sigma Aldrich and used without further purification if not stated otherwise. Solvents were dried according to literature procedures. The acid chlorides were synthesized from the respective carboxylic acids using oxalyl chloride [18].
Instrumentation
The FTIR spectra were measured on a Perkin Elmer Paragon 1000 spectrometer with KBr pellets. Thermogravimetry was carried out on a
Results and discussion
Nanodiamond has been hydroxylated according to a literature procedure. This procedure homogenizes the diamond surface by reducing keto groups to hydroxyl groups [14], [15]. The resulting hydrophilic material with an agglomerate size of several hundred nanometers up to several microns was reacted with carboxylic acid chlorides in a toluene suspension (Scheme 1). It has to be mentioned that the synthesis of acid chlorides with oxalyl chloride ((COCl)2) rather than thionyl chloride (SOCl2) is to
Conclusion
In summary we have modified the surface of hydroxylated detonation nanodiamond with alkyl chains of different length by an esterification reaction of carboxylic acid chlorides with the surface hydroxyl groups. The resulting materials had a surface loading of 0.3–0.4 mmol g− 1 and showed a much better dispersibility in several organic solvents along with a smaller particle size of the remaining agglomerates. This enables the homogeneous incorporation of such materials in non-hydrophilic matrices
Acknowledgements
We gratefully acknowledge the financial support of our research by the Deutsche Forschungsgemeinschaft and the European Commission (EQUIND, Nano4Drugs). A.K. thanks the Fonds der Chemischen Industrie for a Liebig Habilitation Fellowship. We acknowledge I. Jeβ and C. Naether for access to thermogravimetry and A. Puls for assistance with the powder XRD.
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