Regular ArticleEnhancing the colloidal stability of detonation synthesized diamond particles in aqueous solutions by adsorbing organic mono-, bi- and tridentate molecules
Graphical abstract
Introduction
Nanodiamond (ND, crystal size approx. 5 nm) particles, which is also called ultradispersed diamond (UDD) or ultrafine diamond (UDF), is primarily synthesized via the detonation method, where Trinitrotoluene (TNT), 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX), or Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is utilized as explosive, in industrial quantities [1]. Due to the oxygen-deficient environment and the high-pressure high temperature (HTHP) regime, tiny ND crystals are formed from the soot with a narrow size distribution [2]. These detonation ND crystals are promising candidates for copious applications in material science [3], biology [4] and electronics [5]. Nanocrystalline (NC) diamond thin films out of adsorbed ND are interesting candidates for optoelectronic devices, [6], such as solar cells [7], light-emitting diodes and photodetectors [8], due to the quantum confinement effect, which is especially present at diameters close or lower than the exciton Bohr radius [9]. Applications in the biomedical field include biolabels [10], drug delivery vehicles [11], [12] and effective enterosorbent [13], because the ND particles possess high biocompatibility and are considered non-cytotoxic [14]. ND particles with a special charge can be incorporated into various materials for drug delivery [15]. Furthermore, colloidal ND particles are utilized as seeding agents for the nucleation and subsequent growth of diamond coatings on predominantly hard tools and devices. Here, still open problems are frequently the low seeding density [16], which could be increased by enhanced electrostatic interactions between substrate surface and colloidal ND, and the surface roughness [17], which could be improved by varying ionic strength and ND modification, enhancing also the colloidal stability [18].
For all the aforementioned applications, a stable dispersion of the ND crystals in the utilized liquid as well as a high dispersity are needed. However, as received ND solutions lack the colloidal stability, leading to fast aggregation (coagulation) [13]. In order to apply the ND dispersions, stable dispersion of the ND crystals in the utilized liquid as well as high dispersal are needed. Dynamic light scattering (DLS) is typically used to determine the stability of colloids [19], [20], [21]. DLS gives direct information about hydrodynamic diameters (sizes) of the primary particles and their aggregates, which are generated by collision of the nanoparticles. The colloidal stability can be directly measured by determining the hydrodynamic sizes of the dispersed particles over time or after specific time intervals via DLS. The aggregation and coagulation time of ND particles depends on thermal diffusion of the particles and their interactions by collisions. Thus, the kinetics of coagulation is dependent on kT (thermal diffusion) and typically Coulomb repulsion and van der Waals interactions (see DLVO theory, named after Derjaguin, Landau, Verwey, and Overbeek) [22]. Here, the Coulomb repulsion acts as a repulsive barrier, while once overcome, the van der Waals interactions result in strong connection of the nanoparticles. One method for slow coagulation and therefore generation of stable dispersions is to increase the repulsive barrier to somewhat higher than the thermal energy of the particles, attaining a low coagulation yield due to collision of nanoparticles.
To render flocculating ND dispersions stable, several methods were proposed, which were either based on Coulomb interactions or non-electrostatic nature, like steric hindrance or stabilization by a Lewis acid-base mechanism. For example, Wheeler et al. utilized organic ligands (2-butanone) to stabilize Si nanocrystals, which were previously terminated with chlorine (Cl) [9]. The 2-butanone acts here as an electron donor (Lewis base), stabilizing the nanoparticle, while the Si is in a hypervalent state. Xu et al. suggested the use of surfactants to stabilize the dispersion as well as regulate the zeta potential of the nanoparticles [1]. The stabilizing effect was here ascribed mainly to electrostatic interactions. However, most surfactants lack the biocompatibility needed for biomedical applications [4]. Furthermore, for several applications a specific charge of the nanoparticle is necessary. This surface charge can be obtained by different surface treatments, for example by oxygen plasma, nitrogen plasma or polymer grafting [23]. Though these surface modifications were successful, an industrial application of surface modification as a means to enhance the dispersion stability is out of economic viability unlikely. Therefore, facile and cost-effective methods to change zeta potential/surface charge of the nanoparticle are needed.
We confront the issue of ND colloidal stability by utilizing a simple chemisorption approach. As stabilizing agents, such as carboxylic acids, amino acids, and amines, were used. These stabilizing agents possess negative or positive charges, or are amphoteric in solution, and are therefore capable to vary the zeta potential and the colloidal stability of the ND particles. The stabilizing agents were added to the dispersed ND solution with a concentration, which represents a full surface coverage of the nanoparticle surface. Subsequently, the dispersion stability was assessed by bare eye, DLS and zeta potential measurements. Furthermore, the dependence of the dispersion stability on the pH of the solution media was investigated, which was corroborated with zeta potential measurements. Fourier transform infrared spectroscopy (FTIR) measurements were carried out for the stabilized ND particles to unravel the stabilization mechanism. The here shown facile and fast method for ND stabilization is, by virtue of its cost-efficiency and variable surface charge of the particle, an interesting pathway for applications in surface coating, i.e. for the previously mentioned seeding of the surfaces with ND.
Section snippets
Materials and methods
The diamond particles used in this article are commercially available detonation nanodiamond (ND) particles, which were bought as an aqueous suspension (Plasmachem GmbH, PL-NanoPure, grade G01). The aqueous suspension of the ND was dispersed in DI (deionized) water. The colloidal solution was adjusted to pH values ranging from 2 to 12 by utilizing HCl and NaOH with concentrations ranging from 0.1 M to 0.001 M. After addition of the stabilizing agents, ND particles were added to the mixed
Surface properties of as-received ND particles and its colloidal stability
The surface functional groups of ND-particles can be analyzed by FTIR (see Fig. 1). The FTIR spectra show strong and broad peaks at 3432 cm−1 and 3429 cm−1, which correspond to the absorption of hydroxyl and amino stretching vibration. Furthermore, the asymmetric and symmetric C-H vibrations were observed due to absorption bands at approx. 2959 cm−1 and 2922 cm−1. In addition, carbonyl and carboxyl groups were found at the ND interface due to an absorption peak at 1746 cm−1, while the absorption
Conclusion
Nanodiamond (ND) particles were stabilized with carboxylic acids, amino acids and a primary amine. This method is facile, fast and possesses the virtue of cost-efficiency and variable surface charge of the particle. Citric acid, oxalic acid, and propylamine showed to stabilize the dispersion with the smallest particle size (primary soot particles, dh 30–50 nm), while glutamic acid stabilized the ND particles as primary aggregates. The dispersion stability was found to be dependent on
Acknowledgement
This work was supported by Shenzhen Municipality Project (JCYJ20150630114942259), Shenzhen Municipality Science and Technology Planning Project (JSGG20160229202951528, KQJSCX20160301145319, JCYJ20160122143847150), Scientific Equipment Project of Chinese Academy of Sciences (yz201440), Science and Technology Planning Project of Guangdong Province (Nos. 2014A010105032, 2014A010106016), Natural Science Foundation of Guangdong Province, China (2014A030310482) and Guangdong Innovative and
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Both authors contributed equally to this publication.