Investigations of novel artificial diamond-based composite substrates for separation science and analytical chemistry

The development of new technologies, such as ultra-high-performance liquid chromatography is an incentive for the development of advanced, pressure- and temperature-resistant adsorbents to achieve faster and more efficient separations. Carbon-based materials for separation science have been studied intensively during the last few decades. Diamonds unique properties such as chemical inertness, mechanical, thermal and hydrolytic stability, excellent thermal conductivity with minimal thermal expansion and intriguing adsorption properties make them one of the most potentially useful carbon-based adsorbents. There are currently three commercially available types of diamond: detonation nanodiamond (DND), high-pressure high-temperature (HPHT) diamond, and chemical vapour deposition (CVD) diamond. The highest surface area among these is attributed to DND, but the particles produced by this method are 5‚Äö-10 nm, which is too small for high-performance liquid chromatography (HPLC) applications (micron and submicron-sized particles are used). Diamond produced by CVD is mostly used as a thin film coating, and the cost of production is significantly higher than that of HPHT and DND diamonds. The size distribution of HPHT diamond can be controlled and ranges from nm to mm, however, the shape of HPHT diamond is irregular, and the surface area is limited. Therefore, the goal of this project was to investigate the HPHT diamond surface, modify it to adjust selectivity and increase its surface area by forming composites with other carbon materials. To set a foundation for this study, a detailed review (Chapter 2) of the diverse array of recently developed synthetic diamond-containing composite adsorbents and their applications in solid-phase extraction and HPLC was composed. A focus was given to the most commonly used synthetic diamond types ‚Äö- DND, HPHT, and CVD diamonds. The applications of these synthetic diamonds are highly diverse, and have been achieved via various physico-chemical modifications including covalent and non-covalent bonds, entrapping into polymer matrices and attaching selective molecular fragments for particular analytes. The developed HPLC stationary phases have been used in the vast majority of chromatography modes, indicating the tunability of the surface. The preferred forms and applications of different types of diamonds are usually governed by their surface and bulk properties, and how this facilitates or limits their use in different solid phase extraction (SPE) and HPLC based applications is discussed. The adsorption and chromatographic properties of HPHT diamond (1‚Äö-2 ˜í¬¿m and 4‚Äö-6 ˜í¬¿m fractions) were investigated, with a particular focus on their cation-exchange capacity (Chapter 3). Adsorbents were prepared by several methods: wet oxidation of the surface of HPHT with H2O2/H2SO4 or HNO3/H2SO4 mixtures; oxidation in air at 700¬¨‚àûC; or a four-step reduction using LiAlH4 and n-butyl lithium. The zeta potential profiles as a function of pH and ion-exchange capacity were measured for the prepared adsorbents as well as isotherms of adsorption. The results of potentiometric titration and zeta potential measurements showed that all diamond samples had cation-exchange properties in the pH range from 1 to 12 due to the presence of carboxylic groups upon the diamond surface. Surface concentrations of carboxylic groups in the prepared adsorbents varied from 0.65 to 2.2 nm\\(^{‚Äöv†v¿2}\\). The retention of metal cations increased for the studied adsorbents in the order reduced < bare < oxidised HPHT diamonds. The ion-exchange selectivity of oxidised HPHT diamond was studied towards alkali, alkaline-earth and transition metal cations. The elution order Li\\(^+\\) < Na\\(^+\\) < NH\\(_4\\)\\(^+<\\) K\\(^+\\)< Mg\\(^{2+}\\) < Mn\\(^{2+}\\) < Cd\\(^{2+}\\) < Zn\\(^{2+}\\) < Ni\\(^{2+}\\) < CO\\(^{2+}\\) < Cu\\(^{2+}\\) < Cr\\(^{3+}\\) in 20 mM nitric acid was observed and corresponded to that known for carboxylic cation-exchange resins. For the next step of this research, HPHT diamond hydrogenated in a flow of pure hydrogen gas at high temperatures was studied and compared to oxidised HPHT diamond (Chapter 4). X-ray photoelectron spectroscopy confirmed a significant reduction of oxygen-containing groups on the diamond surface at 850¬¨‚àûC. The chromatographic retention mechanisms specific to this new hydrogenated diamond were compared with the oxidised diamond by packing chromatographic columns and evaluating retention factors for analytes with different polarities. The composition of the mobile phase differed in pH and in the organic solvent content. The results revealed a mixed-mode retention mechanism which included hydrophobic interactions, anion-exchange, and reversed-phase interactions. Adsorbent stability was confirmed by increasing the temperature of the mobile phase to 75¬¨‚àûC with no ill effects. To increase the effectiveness of the HPHT diamond adsorbent for surface area expansion, a HPHT diamond-rGO composite material was prepared and investigated. To enable the efficient synthesis of hydrogenated HPHT diamond and graphene oxide (GO), GO reduction was investigated by monitoring the hydrazine concentration in the GO suspension (Chapter 5). In order to do so, a new ion chromatography method was developed for the monitoring of hydrazine. The method is based on the ion chromatographic separation of hydrazine (from excess ammonia) and its selective determination by electrochemical detection. The developed analytical protocol overcame the significant practical challenges of atmospheric hydrazine oxidation and minimised the matrix interference in both separation and detection which results from the excess of ammonium with respect to hydrazine (up to 5.8 ‚àöv= 10\\(^4\\) times) in GO reduction experiments. Chromatographic separations were achieved using a high capacity IonPac CS16 cation-exchange column with a 30 mM methanesulfonic acid (MSA) eluent, and an analysis time of less than 20 min. The detection of hydrazine as the hydrazinium ion using an electrochemical detector was linear between 10 ˜í¬¿M and 4 mM, with LOD and LOQ values of 3 ˜í¬¿M and 10 ˜í¬¿M, respectively. Standard additions confirmed 103 ¬¨¬± 0.8% recovery. The developed method was successfully used to determine the point of complete GO reduction with hydrazine. Reaction curves for GO reduction generated using the method were compared to results from Fourier-transform infrared spectroscopy and Raman spectroscopy to verify the utility of the approach. The hypothesis that hydrogenated diamond should have a high affinity towards sufficiently reduced GO formed the basis for the development of a method for the synthesis of HPHT diamond-GO composite material (Chapter 6). The suggested technique comprised in-situ GO reduction in the presence of diamond particles. Synthesis optimisation was attempted by ultra-sonification, hydrazine concentration and substance ratio variation and assessed via UV/vis spectrometry and scanning electron microscopy (SEM). The clustering of diamond-rGO particles resulted in uneven size distribution rendering the composite unsuitable for application as a HPLC adsorbent. Subsequently, preliminary investigations were conducted for alternate applications in analytical chemistry, with a focus on use as an electrode. Electrodes composed of HPHT diamond-rGO and boron-doped diamond (BDD)-rGO were created via three methods (deposited, drop cast and packed) and compared to a commercial glassy carbon electrode (GCE). Electrodes were tested for the electrochemical determination of ascorbic acid and dopamine by cyclic voltammetry. Results showed an increase in oxidation current shift in diamond electrodes compared to GCE, with HPHT diamond outperforming BDD. Oxidation potentials varied between electrode construction methods, with lower peak potential voltage readings recorded for drop cast electrodes. Overall results indicate that the reaction process on diamond-rGO modified glassy carbon is more efficient than that on standard commercial GCE. This research advanced the knowledge of oxidised and hydrogenated HPHT diamond surfaces via liquid chromatography. A new ion chromatography method was developed in an attempt to increase the effectiveness of the HPHT diamond adsorbent via GO attachment with a high surface area. The synthesised HPHT diamond-rGO composite was deemed unsuitable for liquid chromatography but proved to be suitable for electrochemical applications. Hence, this thesis shows the usefulness of synthetic diamond in many areas of separation science and electrochemistry.