Article pubs.acs.org/Langmuir
Synthesis of Nanospheres-on-Microsphere Silica with Tunable Shell Morphology and Mesoporosity for Improved HPLC Adham Ahmed, Peter Myers, and Haifei Zhang* Department of Chemistry, University of Liverpool, Oxford Street, Liverpool L69 7ZD, United Kingdom S Supporting Information *
ABSTRACT: Core−shell particles have a wide range of applications. Most of the core−shell particles are prepared in two or multiple steps. Core−shell silica microspheres, with solid core and porous shell, have been used as novel packing materials in recent years for highly efficient liquid chromatography separation with relatively low back-pressure. These core− shell silica microspheres are usually prepared by the time-consuming layerby-layer technique. Built on our previous report of one-pot synthesis of core−shell nanospheres-on-microspheres (termed as SOS particles for “spheres-on-spheres”), we describe here a two-stage synthesis for the introduction of shell mesoporosity into SOS particles with tunable shell morphology by co-condensation of tetraethyl orthosilicate (TEOS) with 3mercaptopropyltrimethoxysilane (MPTMS) in the presence of surfactant in the second stage. With MPTMS as the primary precursor at the first stage, some other silica precursors (apart from TEOS) are also employed at the second stage. Expansion of the surfactant-templated mesopores with swelling agents during the reaction and by hydrothermal postsynthesis treatment is then performed to allow the pore sizes (> 6 nm) suitable for separation of small molecules in liquid chromatography. Compared to the standard SOS silica (both the nanospheres and microspheres contain nearly no mesopores), the introduction of mesoporosity into the nanosphere shell increases the separation efficiency of small molecule mixtures by 4 times as judged by the height equivalent plate number, while the separation of protein mixtures is not negatively affected.
1. INTRODUCTION Core−shell particles, either nanoparticles or microparticles, have been extensively investigated and have wide applications in catalysis,1 battery,2 drug delivery,3 biomedical area,4 and chromatography.5 Core−shell particles are usually synthesized by a two-step or multiple-step process. The core particles are synthesized first, and the shells are then formed on the core particles via different methods, depending on the type of core and shell materials and their morphologies.6 While core−shell nanoparticles are mostly investigated,1−4 core−shell microspheres have found unique applications as novel packing materials for chromatography.5 In liquid chromatography or high performance liquid chromatography (HPLC), silica microspheres are the mostly used packing materials. The HPLC performance can be normally improved via the use of small and monodispersed silica microspheres. Currently, sub-2 μm silica microspheres are the state-of-the-art for HPLC. Nanospheres are rarely used for HPLC. It is not that one cannot prepare smaller microspheres or nanospheres but that packing the smaller spheres into a column will considerably increase the column back-pressure which is a huge burden for chromatography instrumentation. Halving the particle size may double the separation performance (in terms of theoretical plate numbers) but can also quadruple the back-pressure (ΔP ∝ 1/d2), which is highly unfavorable.7 © XXXX American Chemical Society
A type of porous shell silica particles, with solid core and porous shell, has been employed as an alternative to the conventional porous silica microspheres.5 In the late 1960s, several core−shell pellicular sorbent particles were commercialized. But these particles had low surface areas in the range of 5−15 m2 g−1, which resulted in very low loading capacities and poor analyte retention.8,9 The new generation of core−shell particles prepared by layer-by-layer (LBL) techniques could offer improved performance due to a high surface area of around 150 m2 g−1 and narrow particle size distribution.10,11 In HPLC, these core−shell particles are also known as fused-core, porous shell, or superficially porous particles. The widely used 2.7 μm core−shell particle has a 1.7 μm core and 0.5 μm shell with 9 nm mesopores in the shell. These particles yield efficiency closer to that of sub-2 μm particles but with a column pressure closer to that of 3 μm particles.12 The advantage with the core−shell particles as packing materials is that the smaller superficial pore volume reduces the volume present for peak broadening from longitudinal diffusion (B term in the van Deemter equation). The short diffusion path length can reduce the contribution of the C term due to the fast mass transfer.13 Received: July 29, 2014 Revised: September 22, 2014
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beneficial for many applications1−5 and particularly for the separation of small molecules in HPLC.7 Therefore, introduction of mesopores into SOS particles is investigated while maintaining the superficial macroporosity. Because surfactanttemplated silica can be formed by co-condensation of MPTMS with TEOS,22−26 we have decided to adopt a two-stage synthesis: (1) in the first stage, MPTMS is used to maintain the SOS morphology, and (2) in the second stage, TEOS (or some other precursors) is added so that surfactant templates can be incorporated to introduce the mesopores into the materials. CTAB present in the reactions is responsible for the surfactanttemplated mesopores (2.1 nm). In order to be suitable for HPLC separation,7 the CTAB-templated mesopores are further expanded with swelling agents during the reaction or via hydrothermal post-treatment. The SOS particles with shell mesoporosity show significantly improved separation of small molecules by HPLC while maintaining the protein separation efficiency.
These particles are commonly prepared by the time-consuming LBL technique due to the repeating deposition and rinsing steps. A fast preparation method would be highly beneficial. A recent study by Dong et al. showed the possibility of improving the traditional LBL assembly into a multilayer-by-multilayer (MLBML) approach, which showed a more efficient deposition of up to 6−7 layers of silica nanoparticles in each coating.14 The resulting porous shell indicated the presence of around 9 nm pores with up to 255 m2 g−1 surface area. Even though the MLBML procedure is more efficient than the current LBL methods, it still requires up to five coatings to achieve the desired morphology to be comparable to current superficially porous particles. The sol−gel synthesis approach has also been employed to produce core−shell silica particles. In an early report, sequential addition of tetraethyl orthosilicate (TEOS) and TEOS/ octadecyltrimethoxysilane (OTMS) in one-pot synthesis was employed to produce solid-core mesoporous shell particles. The condensation of TEOS in the presence of OTMS resulted in the formation of mesoporous silica shell with an average pore diameter of 3.8 nm after removal of the porogen by calcination.15 The two-step synthesis has been more widely used to produce such type of core−shell particles. Monodispersed nonporous silica spheres are commonly prepared first by a Stöber method, which are then used as the core particles suspended in a mixture also containing Si precursors and structure directing agents for the sol−gel synthesis of the shell. The removal of the directing agent from the shell (e.g., by washing or calcination) produces mesoporosity.16−20 These two-step procedures may offer better control on shell thickness and mesopore morphology in the shell. However, these surfactant-templated mesopore are normally in the range of 6 nm) are required for separation of even small molecules.7 Surfactant-templated mesopores can be expanded by introducing a swelling agent into the structure-directing template, either in the sol−gel preparation step or by the F
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Figure 7. N2 sorption results for the calcined pore-expanded CTAB-templated SOS particles by (A, B) one-pot modification with swelling agents TMB and DMDA and (C, D) two-step post-treatment both under hydrothermal conditions with TMB and DMDA. (A, C) Sorption isotherm profiles and (B, D) BJH pore size distributions.
TMB with TEOS, increased hysteresis loops are observed, indicating the formation of larger mesopore (Figure 7A). Indeed, the peak pore sizes are enlarged from 2.8 to 3.9 and 4.2 nm by using DMDA and TMB as swelling agents, respectively (Figure 7B). With the expansion of the pores, both the surface areas and the pore volumes are increased (540 m2 g−1 and 0.53 cm3 g−1 for DMDA; 417 m2 g−1 and 0.64 cm3 g−1 for TMB) compared to the TEOS-only sample. For the post-treatment approach, the as-prepared CTABtemplated t60 min SOS particles were treated with DMDA or TMB under the hydrothermal conditions. After treatment, the SOS morphology remained intact and the nanospheres were still attached to the microsphere surface (Figure S10D). Figure 7C shows the N2 sorption isothermal profiles for the treated samples after calcination. An increasing size of hysteresis loop can be seen for SOS particles post-treated with DMDA and TMB. The pore size distributions in Figure 7D show that the pores are expanded from 2.1 to 4.3 and 6.9 nm by using DMDA and TMB as swelling agents, respectively. With the expansion of the pores, the pore volumes were increased accordingly (0.21 cm3 g−1 of which 0.03 cm3 g−1 for micropore volume for the untreated sample, and the pore volumes of 0.37 cm3 g−1 for DMDA and 0.44 cm3 g−1 for TMB). The BET surface areas were decreased from 476 to 385 m2 g−1 for DMDA and to 337 m2 g−1 for TMB; a similar trend was observed before.37 The micropore volumes were kept at the same level for the treated samples. 3.6. HPLC Evaluation of SOS Particles with Mesoporous Shells. After calcination at 600 °C, the standard SOS particles gave a surface area of around 245 m2 g−1, which was resulted from the micropores (about 1.4 nm) formed due to the loss of organic component during calcination. The superficial macroporosity from the interstitial space of the
hydrothermal post-treatment. Both approaches have been adopted in this study. For the incorporation during the synthesis, the swelling agent could get into the surfactant micelles and enlarge the size of the micelles.33 For the hydrothermal post-treatment, the silica/surfactant composite spheres were treated with the swelling agents in water or in water/ethanol at elevated temperatures.34,35 The flexible silica structure could allow the incorporation of the swelling agent into the surfactant templates. Generally, hydrophobic agents can be used as the swelling agents to expand the pores. These can include aromatic hydrocarbons, long-chain alkanes, alkylamines, and auxiliary alkyl surfactants.33 Both TMB34 and DMDA35 have been widely used. Particularly, the use of TMB could significantly enhance the surfactant-templated mesopores.36,37 TMB and DMDA were therefore chosen as the swelling agents in this study. For the use of swelling agent during the reaction, TEOS and one swelling agent were added in sequential at the reaction time of 60 min. The whole reaction was then subjected to hydrothermal treatment. TEOS only was added as well with the resulting particles as a control sample. SOS particles were produced under all the conditions with the samples of TEOS and TEOS + TMB showing similar SOS morphologies (densely coated nanospheres around 200 nm, Figure S10A−C). The nanosphere growth was strongly affected when DMDA was utilizedlarger nanospheres and some smooth microspheres were formed (Figure S10B). After calcination at 600 °C, the N2 isotherm showed the formation of small hysteresis loop for the sample with TEOS only added, which is reflected by the mesopores around 2.8 nm with BET surface area 370 m2 g−1 and a total pore volume of 0.12 cm3 g−1 (micropore volume 0.061 cm3 g−1 and mesopore volume 0.062 cm3 g−1) (Figure 7A,B). For the samples produced by the addition of DMDA or G
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Figure 8. Chromatograms obtained from the calcined mesoporous SOS column (2.1 mm i.d. × 50 mm L) with 5 μL injection and UV detection at 254 nm at 25 °C. (A) Analysis of the test mixture under the normal phase condition. Mobile phase: heptane:isopropanol (95:5 v/v). Flow rate: 0.5 cm3 min−1. (B) Separation of seven proteins with C8-bonded SOS particles under the reversed phase condition. Mobile phase: the mixture of ACN solution (containing 0.1% TFA) in water (also containing 0.1% TFA) with a gradient change: 0 min: 1% v/v ACN, 0.5 min: 20% v/v ACN, 4 min: 70% v/v ACN. Flow rate: 3 cm3 min−1.
cytochrome c, lysozyme, trypsin, and bovine serum albumin (BSA). The mobile phase was a mixture of 0.1% v/v TFA in acetonitrile (ACN) and water. A three-step gradient profile was used in order to achieve the separation of these proteins in just over 2.5 min (Figure S13)a similar performance as the nonmesoporous SOS particles.21 However, the presence of mesopores did increase the back-pressure (recorded as 251 bar at the end of the gradient). The column efficiency was comparable to the standard SOS particles as the peak capacity was calculated to be 17.4 (SOS peak capacity = 19.1).21 A more complex mixture containing seven proteins including ribonuclease A, insulin, cytochrome c, lysozyme, myoglobin, BSA, and carbonic anhydrase was further examined. The separation of the proteins was performed within 3 min with a back-pressure of 355 bar (Figure 8B), which is the average pressure for a standard narrow bore silica column. Thus, the HPLC test results demonstrate the capability of the SOS particles with shell mesoporosity for improved separation of small molecules and large molecules such as proteins with no loss in performance.
surface nanospheres made this material efficient for large molecule protein separation. The lack of mesoporosity could limit separation efficiency for small molecules.21 Under the normal phase conditions, the optimal column efficiency achieved was 16 270 plates/m based on p-nitroaniline.21 For the SOS particle with expanded mesopores, it is expected that the column efficiency for separation of small molecules would be improved. Here, the calcined TMB post-treated SOS particle with 6.9 nm mesopores were assessed in a normal phase HPLC for separation of toluene, nitrobenzene, and nitroaniline isomers. Figure 8A shows five well-resolved peaks within 6 min, with a back-pressure of 39 bar. The column efficiency was calculated to be 68 000 plates/m based on p-nitroaniline, calculated by retention time and half peak width. This represents an increased efficiency of about 4 times. The results showed that this modified SOS column (retention factor k = 1.56 for the first two peaks) had a higher retention time than the SOS column (k = 0.50) due to the increase in surface area. Peak broadening is expected for totally porous microspheres with 6.9 nm mesopores. However, for the SOS particles with mesoporous shells, the eluents only moved through a shorter distance of the thickness of the shell (the size of nanospheres for the SOS particles, ca. 200 nm), which minimizes band broadening effect. A van Deemter plot was obtained for the nitroaniline isomers (Figure S11). The plate height of the pnitroaniline was only modestly increased with increasing linear velocity. This suggests that the separation may be made faster by increasing flow rates without significant loss of column efficiency. Another normal phase test mixture containing toluene, 2,4-di-tert-butylphenol, o-nitroaniline, and cinnamyl alcohol (TM1 test mixture, from Thermal Fisher Scientific) was also evaluated. Baseline separation with symmetrical peak shapes was achieved using a mobile phase of isopropanol:heptane (85:15 v/v) within 5 min (Figure S12). The theoretical plate number based on cinnamyl alcohol was calculated to be 59 000 plates/m. With the unique core−shell property and the superficial macroporosity, the packed mesoporous SOS particles can also be good stationary phase for separation of large proteins. The mesoporosity within the shell structure would provide very limiting access to proteins; hence, the separation should mainly occur in the shell interstitial space. The silica was modified with C8 ligands and assessed under the reversed phase mode for separation of a mixture of proteins including ribonuclease A,
4. CONCLUSION One-pot two-stage synthesis has been developed to prepare SOS silica particles with tunable surface morphology and importantly shell mesoporosity. MPTMS was the precursor added at the start of the reaction to generate the SOS morphology. Among the precursors added in the second stage, TEOS and TMOS could produce the desired SOS particles. Amount of precursors, addition time, and concentration of surfactant CTAB in the reactions can be varied to tune surface morphology and produce SOS particles with high surface area and large mesoporosity in the nanosphere shells. The mesopores resulted from CTAB templates via the cocondensation reactions of MPTMS and TEOS/TMOS in the second stage. The CTAB-templated mesopores could be enlarged by swelling agents DMDA and TMB via one-pot modification process during the synthesis or hydrothermal post-treatment of the SOS particles containing CTAB templates. The mesopores could be expanded from 2.1 up to 6.9 nm with TMB post-treatment, the pore sizes suitable for liquid chromatographic separation of small molecules. The calcined SOS particles with shell mesopores of 6.9 nm were evaluated for HPLC separation of small molecules under normal phase, which showed a higher efficiency up to 68 000 H
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plates/m (∼4 times higher) based on p-nitroaniline compared to the standard nonmesoporous SOS particles. After bonding with C8 ligands, the particles were further assessed for protein separation without any loss in resolution and peak capacity. This study demonstrated that SOS particles with mesoporous shell could be used for highly efficient HPLC separation for mixtures of both small molecules and biomacromolecules.
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ASSOCIATED CONTENT
S Supporting Information *
More SEM images, N2 sorption data, UV−vis data, TGA data, and HPLC graphs. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel +44 151 7943545; Fax +44 151 7943588; e-mail
[email protected] (H.Z.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for the access to the facilities in the Centre for Materials Discovery at the University of Liverpool.
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REFERENCES
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