Mesoporous silica nanoparticles (MSNs), especially with ordered mesopores, have attracted increasing interest as carriers in, e.g., biomedical, catalytic, and optical applications, due to their desired properties, such as their high specific surface area, well-defined and tunable pore-sizes, transparency, or biocompatibility [1
]. MSNs obtained for the first time in the early 2000s [7
] have been developed towards various dimensions, morphologies and pores sizes. MSNs are commonly synthesized using a sol–gel reaction of silica species in a surfactant templating method [11
]. The Stöber method [12
], involving the hydrolysis of tetraalkyl silicates in a mixture of alcohol and water using ammonia as a catalyst, was also developed for the synthesis of submicrometer mesoporous silica particles originally by Grun et al. [13
]. MSNs with radially aligned mesopores were also later synthesized using a modified Stöber approach [14
]. Such ordered mesopores may be formed thanks to the self-organization of the micellar rods into a hexagonal matrix at proper conditions, which serves as a template for the organization of the silica precursors.
Among the various modifications of MSNs, the structures containing solid cores and mesoporous shells (SCMS) are especially useful for applications requiring recycling of the nanoparticles, serving as carriers or dispersible reactors [15
] as well as for sorption applications [16
]. Such structures provide higher density and mechanical durability [17
] necessary for, e.g., centrifugation or filtration purposes, but also enable the formation of hollow mesoporous particles after selective removal of the cores [18
The fabrication of SCMS silica particles of the desired properties requires careful optimization of the synthetic conditions, which include variation of composition, pH of the reaction mixture, temperature, or solvent selection. While pH was found to be the most influential factor determining the diameters of MSNs [19
], temperature was also used to regulate the size of MSNs [3
], although the trends may differ, likely due to the different mechanisms of their synthesis [1
]. Nevertheless, the synthesis of SCMS silica particles has been mainly realized at room or higher temperatures and the influence of lower temperature on the size and properties of the mesoporous shell has been often neglected [22
]. However, the temperature of the synthesis and concentration of the applied surfactant should significantly influence the properties of the mesoporous phase in the SCMS particles, obtained using micellar template methods, because critical micellar concentration (CMC), Krafft point, as well as the stability of the micelles that depend on those parameters [25
]. While decreasing the temperature might improve the ordering of micellar templates, the Krafft temperature (TK
) limits the applicability of a given surfactant at lower temperatures (solubility of a surfactant is lower than CMC below TK
). This is particularly important for ionic surfactants with long aliphatic chains.
Herein, we present a synthesis of monodisperse SCMS silica submicrometer particles at various temperatures enabling tuning of their sizes and properties. The syntheses at conditions below room temperature have been especially neglected in the literature and here they have been shown to result in particles of improved sorption properties. Model dye molecules were adsorbed in order to test the sorption abilities of the synthesized particles, which can be later used in wastewater treatment or as dispersible photoreactors. The observed correlations were explained mainly in terms of variations of the behavior of surfactant templates that influence, e.g., shell thickness, specific surface area, and the pore sizes of the synthesized SCMS particles.
2. Materials and Methods
Tetraethoxysilane (TEOS, 98%, GC), hexadecyltrimethylammonium bromide (CTAB, 98%), and poly(ethyleneimine) (PEI, branched, Mn ≈ 10,000 g/mol) were purchased from Sigma Aldrich (St. Louis, MS, USA). Rhodamine 6G (Rh6G, 99%) was purchased from Acros Organics (Geel, Belgium). Ammonia solution (30%, p.a.) and ethanol (96%, p.a.) were purchased from Chempur (Piekary Slaskie, Poland). Deionized water was used in all procedures.
N2 sorption studies were conducted at −196 °C using a 3Flex v1.00 (Micromeritics, Norcross, GA, USA) automated gas adsorption system. Prior to the analyses, the samples were degassed under vacuum at 350 °C for 24 h. Fourier-Transform Infrared Spectroscopy (FT-IR) spectra were recorded using a Nicolet iS10 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA USA) with an ATR accessory. UV-VIS spectra were recorded on a Varian Cary 50 (Palo Alto, CA, USA) UV-VIS spectrophotometer. Transmission Electron Microscopy (TEM) measurements were carried out by means of a FEI (Lausanne, Switzerland) Tecnai Osiris microscope with an X-FEG Schottky field emitter operated at 200 kV. Scanning Transmission Electron Microscopy (STEM) imaging was performed using a high-angle annular dark-field (HAADF) detector. Before analysis, the samples were ultrasonically dispersed in ethanol and dropped on a Lacey type copper grid (200 mesh) (Agar Scientific, Stansted, UK). Conductivity measurements were performed using a multifunction computer meter CX-741 (Elmetron, Zabrze, Poland). A 0.01 M KCl solution of known specific conductivity was used to determine the specific conductivities of the measured solutions. Centrifugation was carried out with an MPW-250 (MPW Med. Instruments, Warsaw, Poland). A Spin 150 wafer spinner (APT GmbH, Korbach, Germany) was used for spin-coating.
2.3.1. Synthesis of SCMS Particles
The synthesis of solid core mesoporous shell (SCMS) silica particles was based on the procedure reported in Reference [24
], which was then modified here. In the first step, 100 mL of ethanol, 8 mL of water, and 4 mL of ammonia solution (catalyst) were mixed and kept at a given temperature (8 °C, 15 °C, 22 °C, 45 °C, or 60 °C) in covered beakers. Subsequently, 6 mL of TEOS was added to the mixture that was stirred for 6 h at the same temperature leading to the formation of the suspension of particles (solid cores). A portion of the obtained suspension was centrifuged (5 min, 7900 RCF) and the obtained particles were washed with ethanol and dried at 50 °C overnight. The resulting particles were named, indicating the temperature of the synthesis (e.g., 8 °C-SC). One-hundred milliliters of the suspension prepared in the first step was diluted with 200 mL of water and 30 mL of CTAB surfactant solution in ethanol/H2
O (1:2 v
) was added. The total concentration of CTAB in the reaction mixture was equal to 0.01 M. After 30 min of vigorous stirring, 2.15 mL of TEOS was added and the mixture was stirred overnight at a given temperature (the same as for the formation of cores). The resulting core-shell particles were isolated and purified the same way as the core particles and named to indicate the temperature of the formation (e.g., 8 °C-SCMS). To remove the surfactant template SCMS particles were heated in air at 550 °C for 4 h (heating rate: 2 K/min), resulting in the formation of the calcined samples, SCMS-C. Additionally, for the samples formed at 22 °C, four fractions were collected after 6, 12, 24, and 48 h of mesoporous shell formation.
2.3.2. Rhodamine 6G Adsorption Studies
For the adsorption studies, 7.5 ± 0.05 mg of a given SCMS-C sample was added to 8 mL of Rh6G aqueous solution (c = 5 mg/L, absorbance at 529 nm, A = 0.84) and sonicated for 15 min, and then shaken for 1 h using a multi-vortex v32. Afterwards, the suspension was centrifuged (5 min, 7900 RCF) and the UV-VIS spectrum of the supernatant was measured.
2.3.3. Scanning Electron Microscopy Measurements
Scanning electron microscopy studies were performed using a Phenom Pro microscope (Phenom World, Eindhoven, The Netherlands) working at an operational voltage of 10 kV. Silicon substrate was used for the deposition of the obtained particles. The wafer was purified by immersing it in the “piranha” solution (H2O2/H2SO4 1:3 v/v) for 15 min. This procedure must be carried out with caution, as it is a highly corrosive and oxidative mixture. The wafer was subsequently washed with water and left for 5 min in a PEI aqueous solution (2 g/L) to reverse the surface charge. Afterwards, the coated substrate was washed again with water and the respective suspension of particles was deposited on it by spin-coating (2000 RPM, 1 min). For high-resolution imaging, such prepared dried samples were coated with a nanometric layer of gold. Images of at least 100 particles for each sample with circularity higher than 0.9 were captured at a magnification equal to 40,000× or larger. Diameters of the particles were determined based on the surface areas of the particles from the SEM images using automatic detection offered by Fiji, a Java-based image-processing program developed at the National Institutes of Health.
Monodisperse solid core mesoporous shell all silica particles with radially ordered pores were synthesized using a one-pot surfactant templated method at various temperatures. By varying the temperature in the 8–60 °C range, the diameter of the dense cores and thickness of the mesoporous shells could be easily tuned. The SCMS particles underwent calcination for the removal of cationic surfactant, leaving the mesopores with the average size of 3–4 nm. The BET surface area was also found to significantly depend on the temperature of synthesis, reaching 464 m2/g for 15 °C. It was shown that lowering the temperature below the Krafft point (10 °C), determined for the used CTAB surfactant in the applied solvent mixture, lead to the formation of particles with lower content of surfactant, implying also a smaller specific surface area and weaker sorption abilities as tested using the model cationic dye (rhodamine 6G). However, for temperatures higher than 15 °C the mentioned properties of the particles were also smaller due to lower density or increasing destabilization (larger CMC for higher temperatures) of the micellar templates. Thus, although synthesis of the solid silica cores may be performed using this method at various temperatures, tailoring its diameter (ca. 170–800 nm) for a given application, the best mesoporous shells, in terms of sorption abilities, should be prepared at low temperatures, slightly above the Krafft temperature. The presented low temperature strategy may be extended for the synthesis of other core-shell systems that can be tailored for specific applications such as submicrometer carriers or reactors that require not only high specific surface area but also mechanical robustness for, e.g., facile isolation from suspensions. Moreover, by varying the solvent composition, one may further tune the solubility of a surfactant (Krafft temperature) and proceed with syntheses at even lower temperatures that may result in thicker and better ordered mesoporous shells.