Supercritical carbon dioxide versus toluene as reaction media in silica functionalisation: Synthesis and characterisation of bonded aminopropyl silica intermediate.

This research reports supercritical carbon dioxide versus toluene as reaction media in silica functionalisation for use in liquid chromatography. Bonded aminopropyl silica (APS) intermediates were prepared when porous silica particles (Exsil-pure, 3µm) were reacted with 3-aminopropyltriethoxysilane (3-APTES) or N,N-dimethylaminopropyltrimethoxysilane (DMAPTMS) using supercritical carbon dioxide (sc-CO2) and toluene as reaction media. Covalent bonding to silica was confirmed using elemental microanalysis (CHN), thermogravimetric analysis (TGA), zeta potential (ξ), diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, scanning electron microscopy (SEM) and solid-state nuclear magnetic resonance (CP/MAS NMR) spectroscopy. The results demonstrate that under sc-CO2 conditions of 100°C/414bar in a substantial reduced time of 3h, the surface coverage of APS (evaluated from%C obtained from elemental analysis) prepared with APTES (%C: 8.03, 5.26µmol/m-2) or DMAPTES (%C: 5.12, 4.58µmol/m2) is somewhat higher when compared to organic based reactions under reflux in toluene at a temperature of 110°C in 24h with APTES (%C: 7.33, 4.71µmol/m2) and DMAPTMS (%C: 4.93, 4.38µmol/m2). Zeta potential measurements revealed a change in electrostatic surface charge from negative values for bare Exsil-pure silica to positive for functionalised APS materials indicating successful immobilization of the aminosilane onto the surface of silica.

Synthesis and characterisation of non-bonded 1.7µm thin-shell (TS1.7-100nm) silica particles for the rapid separation and analysis of uric acid and creatinine in human urine by hydrophilic interaction chromatography.

A rapid chromatographic method for the simultaneous determination of uric acid (UA) and creatinine (Cr) in human urine is described, using a non-bonded 1.7µm thin-shell (TS1.7-100nm) silica particle prepared by the seeded-growth approach. The new shell particle was characterised by scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS) and BET analysis. TEM reveals 1.5µm solid core and 100nm shell thickness. DLS shows polydispersity index <0.2. Expanded pore size of 88Å and specific surface area of 78m2/g were determined by BET. Chromatographic results demonstrate that UA, Cr and hypoxanthine (Hyp, as internal standard) can be separated in less than 1min on the in-house packed TS1.7-100 column (4.6 ID x 100mm), using chromatographic conditions with mobile phase 70% acetonitrile, 10mM ammonium acetate buffer, pH 6.78, flow rate 1.25ml/min and UV detection at 254nm. A linear relationship between the ratio of the peak area of the standard UA and Cr to that of the internal standard (Hyp) and the concentration of standards was obtained for both UA and Cr with the square of the correlation coefficients, R2=0.998 for both renal biomarkers. The calculated detection limits were 0.03µg/ml and 0.05µg/ml for UA and Cr respectively. Urine samples tested were found to contain UA and Cr in the concentration range of 782-1206µg/ml and 535-862µg/ml respectively. The recovery ranges for spiked urine standards were 85.7-93.2% for UA and 91.9-94.6% for Cr and the relative standard deviations (RSD) for both biomarkers were 3.05% and 0.88% respectively. The developed rapid HILIC method can have application in determining the concentrations of UA and Cr for early prediction in patients with developing disease conditions, including acute kidney injury (AKI).

Sub-2-µm seeded growth mesoporous thin shell particles for high-performance liquid chromatography: Synthesis, functionalisation and characterisation.

Nanometer control over the porous shell thickness of sub-2-µm-shell particles is investigated. Three seeded growth mesoporous thin shell particles for HPLC were prepared, with 0.05µm (or 50nm) porous shell layers: particle sizes 1.5µm (solid core diameters 1.4µm), 1.7µm (solid core diameter 1.6µm), 1.9µm (solid core diameter 1.8µm) and compared with a fourth 1.7µm particle (solid core diameter 1.4µm) surrounded by 0.15µm (or 150nm) porous shell thickness. The thin shell particles were functionalised using a mono-functional octadecyldimethylchlorosilane ligand (C20H43SiCl) under optimised reflux conditions and packed in-house in narrow bore columns (2.1 I.D.×50mm) denoted as TS1.5-50-C18, TS1.7-50-C18, and TS1.9-50-C18 respectively. The synthesised thin shell particles and bonded materials were comprehensively characterised using scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS), zeta potential, BET analysis, elemental analysis (CHN), thermogravimetric analysis (TGA) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Experimental data from inverse size exclusion chromatography (ISEC) was used to measure external, internal and total column porosities. Five probe analytes (uracil, naphthalene, acetophenone, benzene and toluene) were chosen for the chromatographic performance analysis of these columns. Column evaluation and measurements of height equivalent to a theoretical plate (HETP) data were performed on naphthalene using 55% acetonitile in water. The retention coefficients for the thin shell particles (TS1.9-50-C18, TS1.7-50-C18, TS1.5-50-C18) were in the range 1.26-1.35 and 5.6 for the core-shell particle (EiS1.7-150-C18). The minimum reduced plate heights range from 3.89 to 4.26 for the thin shell particles and 2.03 for the core-shell particle.

Spectroscopic and chromatographic characterisation of a pentafluorophenylpropyl silica phase end-capped in supercritical carbon dioxide as a reaction solvent.

This research uses solid-state nuclear magnetic resonance (NMR) spectroscopy to characterise the nature and amount of different surface species, and chromatography to evaluate phase properties of a pentafluorophenylpropyl (PFPP) bonded silica phase prepared and end-capped using supercritical carbon dioxide (sc-CO2) as a reaction solvent. Under sc-CO2 reaction conditions (at temperature of 100 °C and pressure of 414 bar), a PFPP silica phase was prepared using 3-[(pentafluorophenyl)propyldimethylchlorosilane] within 1h. The bonded PFPP phase was subsequently end-capped with bis-N,O-trimethylsilylacetamide (BSA), hexamethyldisilazane (HMDS) and trimethylchlorosilane (TMCS) within 1h under the same sc-CO2 reaction conditions (100 °C/4141 bar). Elemental microanalysis, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) were used to provide support data to solid-state NMR and chromatographic evaluation. Results revealed a surface coverage of 2.2 µmol/m(2) for the non-end-capped PFPP silica phase while the PFPP phase end-capped with BSA gave a higher surface coverage (3.9 µmol/m(2)) compared to HMDS (2.9 µmol/m(2)) and TMCS (2.8 µmol/m(2)). (29)Si CP/MAS NMR analysis of the PFPP end-capped with BSA shows a significant decrease in the amount of Q(3) (free silanols) and Q(4) (siloxane groups) species, coupled with the absence of the most reactive Q(2) (geminal silanols) in addition to increased amount of a single resonance peak centred at +13 ppm (MH) corresponding to -Si-O-*Si-CH3 bond. (13)C CP/MAS NMR shows the resonance corresponding to the propyl linkage (CH3CH2CH2-) and methyl groups (Si(CH3)n) confirming successful silanisation and endcapping reactions in sc-CO2. Chromatographic evaluation of the BSA end-capped PFPP phase with Neue text mixture revealed improved chromatographic separation as evidenced in the enhanced retention of hydrophobic markers and decreased retention for basic solutes. Moreover, chromatography revealed a change in column selectivity for the BSA end-capped PFPP phase with dipropylphthalate eluting before naphthalene, indicating decreased silanol groups and increased hydrophobicity. The extend of BSA end-capping as measured by the increase in column efficiency (67,260 N/m vs. 60,480 N/m) on a 2.1 i.d.×50 mm column, methylene group selectivity (α(CH(2)) = 2.27 vs. 2.14) and decreased silanophilic interactions (S=3.7 vs. 4.10) indicate that the increase in carbon loading (3.9 µmol/m(2) vs. 2.2 µmol/m(2)) and improvement in chromatography in good peak shape and symmetry is attributed to end-capping with trimethylsilyl groups.

Synthesis, characterisation and chromatographic evaluation of pentafluorophenyl and phenyl bonded silica phases prepared using supercritical carbon dioxide as a reaction solvent.

Pentafluorophenyl and phenyl silica stationary phases offer alternative selectivity compared to alkyl bonded C18 and C8 stationary phases, through other interactions such as π-π interactions, dipole-dipole and hydrogen bond interactions. Pentafluorophenyl and phenyl silica bonded stationary phases were efficiently prepared in sc-CO2 specifically pentafluorophenyl propyl (PFPP), pentafluorophenyl (PFP), phenyl propyl (PP) and phenyl (P) silica stationary phases. The bonded phases were characterised by elemental analysis, thermogravimetric analysis (TGA), BET, and by solid-state NMR spectroscopy. Chromatographic performance of the supercritical fluid generated phases was also investigated using the Neue test. The authors present results which demonstrate that pentafluorophenyl and phenyl stationary phases can be prepared successfully under supercritical conditions of 100 °C, 414 bar in a reaction time of 1h with surface coverage comparable to traditional organic solvent based methods. Chromatographic results reveal that the pentafluorophenyl propyl (PFPP) phase provides superior separation performance for Neue test solutes despite having a lower ligand density (C: 5.67%, 2.2 µmol/m²) compared to the phenyl propyl (PP) analogue having the highest ligand density (C: 6.67%, 2.5 µmol/m²). The difference chromatographic performance is attributed to the polarity of the CF bond in PFPP phase. Moreover, as the alkyl chain length decreases, the hydrophobic interaction also decreases, and the PFPP phase (with a propyl linkage) provides better separation compared to the PFP phase.

Preparation and characterization of bonded silica hydride intermediate from triethoxysilane and dimethylmethoxysilane using supercritical carbon dioxide and dioxane as reaction medium.

This research examines bonding methodology, surface coverage and silanol conversion efficiencies on the preparation of silica hydride (SiH) intermediate from triethoxysilane (TES) and dimethylmethoxysilane (DMMS) using sc-CO(2) and dioxane as reaction solvent. Under sc-CO(2) reaction conditions (at temperature and pressure of 100 °C, 414 bar, respectively and 3h reaction time), the surface coverages of SiH (evaluated from %C obtained from elemental analysis) prepared with DMMS (3.39 µmol/m(2)) and TES (4.46 µmol/m(2)) increased by 2- and 4-folds respectively, when compared to reaction performed in dioxane (2.66 µmol/m(2), SiH, DMMS and 0.69 µmol/m(2), SiH, TES). The relatively higher surface coverage of SiH from TES over DMMS generated in sc-CO(2) is due to the inherent trialkoxy moiety of the TES that favours siloxane crosslinkage, forming polymeric surface attachments to yield a higher ligand density than the monomeric DMMS ligand. A conversion efficiency of ∼84.4% of SiH prepared from TES in sc-CO(2) estimated from (29)Si CP/MAS NMR analysis is comparable to TES silanization in dioxane or toluene. Moreover, silica hydride (SiH) conversion efficiency of ca. 42.4% achieved for the hydride intermediate prepared from DMMS in sc-CO(2) is more superior to 33.3% efficiency obtained in dioxane. The differences in conversion efficiencies is attributed to the ability of sc-CO(2) being able to access silica pores that are inaccessible in organic solvents. Bonded silica hydride from TES, DMMS prepared in sc-CO(2) were characterized using elemental analysis, thermogravimetric analysis (TGA), BET surface area, Fourier transform infrared (FI-IR) and solid state NMR spectroscopy. Silica hydride technology/chemical functionalization of silica in sc-CO(2) avoid extended purification steps (i.e. filtration and washing), generation of waste organic solvent and the need of costly or energy consuming drying processing with improved modification efficiency.

Synthesis and characterisation of bonded mercaptopropyl silica intermediate stationary phases prepared using multifunctional alkoxysilanes in supercritical carbon dioxide as a reaction solvent.

This research employed (29)Si and (13)C Cross-Polarisation/Magic Angle Spinning (CP/MAS) NMR spectroscopy to characterise the nature and amount of surface species of di-and trifunctional mercaptopropylsilane (MPS) bonded silica using supercritical carbon dioxide (sc-CO(2)) as a reaction solvent without additives (co-solvent) or catalysts. The MPS stationary phases were prepared within 1h at a temperature and pressure of 70°C and 414 bar, respectively. Complementary analysis including elemental analysis, thermogravimetric analysis (TGA), DRIFT spectroscopy and BET surface area measurements were employed to characterise the bonded MPS intermediate stationary phases in support of data obtained from solid-state NMR analysis. The results revealed that modification of silica with a trimethoxymercaptopropylsilane (MPTMS) results in ligand surface coverage that is larger than when dimethoxymethylmercaptopropysilane (MPDMMS) is employed as a silanisation reagent. This observation is attributed to greater reactivity and cross-linkage of trifunctional silane. Reaction in sc-CO(2) in comparison to reflux in organic solvents, is rapid, reducing product recovery procedures.