A facile approach to synthesize SiO2 · Re2O3 (Re = Y, Eu, La, Sm, Tb, Pr) hollow sphere and its application in drug release
© Li et al.; licensee Springer. 2013
Received: 21 June 2013
Accepted: 27 September 2013
Published: 21 October 2013
Multifunctional SiO2 · Re2O3 (Re = Y, Eu, La, Sm, Tb, Pr) hollow spheres (HSs) have been fabricated using an acidic Re3+ ion solution. Under ultraviolet radiation, functional HSs emit different colors of light according to the different rare-earth ions embedded into the shell of SiO2 hollow spheres. The as-prepared hollow capsules were characterized by X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, Brunauer-Emmett-Teller method, scanning electron microscopy, and energy-dispersive spectrometry. Drug loading and release experiments have been carried out using SiO2 · Eu2O3 HSs that acted as drug carriers. The results demonstrate that the multifunctional HSs exhibit a high storage capacity and the ability of retaining drug stability and activity, which indicates that the as-synthesized fluorescent hollow capsules are a potential candidate as drug delivery materials.
Recently, much attention have been focused on the research of hollow SiO2 spheres (HSSs) because of their excellent properties such as thermal stability, large surface areas, low density, low toxicity, and good compatibilities with other materials [1–15]. So, HSSs have attracted intense interest and have been widely applied in a variety of fields, such as catalysis, sensors, chromatography, dyes, inks, photonic crystals, cells, waste removal, shield for enzymes or proteins, delivery vehicle of drugs, and large biomolecular release [16–28]. In general, three approaches are employed to prepare HSSs: template methods [6, 29–31], self-assembly technique, and microemulsions [32, 33]. HSSs [34–36] have been fabricated with the soft template and hard template methods, which involve complicated procedures such as shell formation and core removal. The template-free method has attracted much attention due to its simple and economical characteristic [27, 28]. In 2008, Zhang et al.  developed a self-template method to convert solid silica into hollow spheres (HSs). In the process, silica is dissolved into NaBH4 solution and silicate species deposited on the surface of the silica colloid. The shell formed over the silicate species via Ostwald ripening results in HSSs. An alkalescent environment is an inevitable synthesis condition that is reported in nearly all papers [37–50]; the only exception is that of Chen et al. who used HF as an etching agent . In 2011, Wang’s group reported firstly the synthesis of HSSs in generic acidic media . Unfortunately, the published work was based on the study of only two factors: pH value and Na+ ion, which influence the formation process of HSSs.
Herein, we report a one-step self-driven process to synthesize multifunctional HSSs under an acidic condition with rare-earth ion assistance. Compared with Wang’s report, the synthetic approach of HSSs is simpler. Being synthesized with the assistance of rare-earth ions, the as-prepared HSSs can emit bright fluorescence under ultraviolet radiation, which is convenient to be detected in real time if it is used in biological applications. Typical drug loading and release experiments are carried out using our prepared multifunctional HSSs, SiO2 · Eu2O3 HSs.
All chemicals were of analytical grade and purchased from Jinan Camolai Trading Company (Jinan, China), which were used as received without further purification: tetraethyl orthosilicate (TEOS, 98%), ammonium hydroxide solution (NH3 · H2O, approximately 25% in water), nitric acid (HNO3, 65%), Re2O3, and ethanol (C2H5OH). Rare-earth nitrate solutions [Re(NO3)3 (Re = Y, Eu, La, Sm, Tb, Pr)] with a concentration of 0.04 to 0.08 mol/L were prepared by ourselves.
Synthesis of monodisperse silica spheres
Synthesis of SiO2 · Re2O3 hollow spheres
In a typical synthesis, silica spheres (0.06 g) were added to 20 mL Re(NO3)3 (0.06 mol/L) and stirred for 30 min. The pH of the solution is 4.5 (adjusted with dilute nitric acid). The mixture was transferred into a Teflon-lined stainless autoclave (capacity 25 mL) and heated at 250°C for 12 h. After the products naturally cooled down to room temperature, they were washed with deionized water and separated by centrifugation (4,000 rpm) for three times and then dried at 60°C for 4 h in the air.
Drug storage and release
SiO2 · Eu2O3 HSs (1 g) were added into a 50-mL hexane solution containing 40 mg/mL ibuprofen (IBU). The mixture was sealed and stirred for 24 h. Then the sample was separated by centrifugation and dried at 60°C in the air. The filter was characterized by UV-visible (UV–vis; 264 nm) spectroscopy.
The dry SiO2 · Eu2O3 loaded with IBU (0.1 g) was immersed into 50 mL of simulated body fluid (SBF; pH = 7.4) at 37°C and stirred at the rate of 100 rpm. Three milliliters from the top of the solution was used for release measurement at different intervals, and then 3 mL of fresh SBF is added into the solution to keep the volume unchanged.
Characterization and instruments
The samples were characterized by X-ray diffraction (XRD) with a Philips X’Pert Super diffractometer (Amsterdam, The Netherlands) with graphite-monochromatized Cu Kα radiation (λ = 1.54178 Å) in the 2θ range of 1.5° to 10° and 10° to 80°.
The transmission electron microscopy (TEM) images and electron diffraction (ED) patterns were obtained using a Hitachi 800 transmission electron microscope (Chiyoda-ku, Japan) with an accelerating voltage of 200 kV.
The high-resolution transmission electron microscopy (HRTEM) images were obtained using JEOL-2010 (Akishima-shi, Japan).
The UV–vis absorption spectra of the samples were measured using a UV-1800 ultraviolet–visible spectrophotometer (Shanghai Meipuda Instrument Co., Ltd., Shanghai, China).
The samples used for characterization were ultrasonically dispersed in absolute ethanol for 30 min before the TEM and HRTEM tests.
Results and discussion
Characterization of SiO2 · Eu2O3 HSs
Influencing factors of the synthetic process of SiO2 · Re2O3 (Re = Y, Eu, La, Sm, Tb, Pr) hollow structures
The experiments showed that the pH value of the solution, reaction temperature and time, and different rare-earth ions and concentrations played an outstanding role in the synthesis of SiO2 · Re2O3 hollow structures, which are discussed in detail as follows.
Effect of the pH value of the solution
The pH value of the solution was adjusted with dilute nitric acid. The studied pH range was from 7 to 3 under the following reaction conditions: Re3+ = 0.06 mol/L and T = 250°C. Hollow-structure particles could be obtained under the range of 4 ≤ pH < 5.5, and the optimum pH value was 4.5. No hollow structure products appeared when 6 ≤ pH ≤ 8. No HSSs appeared when 2 < pH < 3. Normally, a few HSSs could have emerged in the product at the conditions of 3 < pH < 4.0 or 5 < pH < 6 if the reaction time was more than 10 h. The detailed results are shown in Additional file 1: Table S1 and Figure S3. It is known that SiO2 is an amphoteric oxide which can dissolve into an acidic or basic solvent. The experiments showed that a weak acid solution was in favor of hollow structure formation. The experiments also revealed that the experimental results were unaffected when using other acids such as HCl or H2SO4 to adjust the pH value of the solution.
Effect of reaction temperature
The temperature of the hydrothermal reaction affected greatly not only the reaction (going or not) but also the reaction rate (slow or fast). Additional file 1: Figure S1 shows the TEM images of the as-prepared samples at different reaction temperatures. No hollow-structure products appeared if the temperature T < 230°C in our experiments. The morphology and size of nanocrystals became difficult to control when the temperature was up to 260°C or higher because the higher the temperature was, the faster the reaction rate was. When T = 255°C, the quality of the obtained SiO2 · Re2O3 HSs was always poor. The experiments verify that the moderate temperature was 250°C.
Effect of Re3+ ion and its concentration
It was reported that Na2SO4 and NaCl were advantageous to HSS formation  and the work matter was Na+ cation, which was in line with our experimental data. Hereby, we investigated the synthesis of HSSs under different rare-earth ions and bivalent cations.
In order to get uniform hollow structures, the optimal concentration of the rare-earth ions was usually kept in the range of 0.04 to 0.08 mol/L. The experimental data and TEM images are depicted in Additional file 1: Table S1 and Figure S2. The concentration less than 0.03 mol/L resulted in poor quality in production, and the concentration greater than 0.08 mol/L always led to products with not all having a hollow structure. The experiments showed that the lower or higher concentration of Re3+ ions was not good for HSS formation and 0.06 mol/L was the optimal concentration.
Although the SiO2 · Re2O3 HSs were obtained based on the rare-earth ion assistance strategy, their quality was quite different under assistance of different kinds of rare-earth ions. By keeping other reaction conditions unchanged such as the pH value of the solution, reaction time, and reaction temperature, the influence of different Re3+ ions (Re = Y, Eu, La, Sm, Tb, Pr) on the structure of the as-prepared products was investigated (see Additional file 1: Table S2 and Figure S4). Additional file 1: Figure S4 clearly shows that the influence sequence of Re3+ was as follows: Eu3 + ≈ Sm3 + > Y3 + > Pr3 + ≈ La3 + > Tb3 +. Nearly all of the as-prepared samples were hollow spheres with good quality under the effect of Eu3+ and Sm3+ existence, and the experiments showed good reproducibility and satisfactory results. With Y3+, Pr3+, and La3+ ions included, all of the products always formed a mixture of HSSs and core/shell structure. Furthermore, all of the samples can be formed into a hollow sphere if the reaction time is prolonged, but the yield of HSSs was lower. Only a small amount of HSs could be obtained with Tb3+ existence. The experiments indicated that changing the reaction time did not work.
We compared the synthesis approach under the same reaction conditions using Mg2+, Ca2+, Mn2+, Zn2+, and Cu2+ instead of Re3+ ions. The results showed that SiO2 · HSs could barely be obtained at the above situations, which indicated that rare-earth ion was an indispensable factor in hollow structure formation.
Experimental data showed that the rare-earth ions were advantageous to HSS formation; however, further study is needed to understand why the effect of different Re3+ ions on the formation of HSSs has a different role.
Effect of reaction time
From the above, our synthesis procedure of HSSs is very simple and effective compared with those previously reported.
Formation mechanism of SiO2 · Re2O3 HSs
The experiments showed that the diameter of SiO2 · Re2O3 HSs was almost the same as that of the template, which indicated that the size of SiO2 · Re2O3 HSs was determined by the SiO2 solid spheres. Therefore, we can control the size of SiO2 · Re2O3 HSs by controlling the diameter of SiO2 solid spheres.
Drug delivery and release
Considering that HSs have numerous mesoporous structures on the surface, they can act as drug loading capsules. IBU, a typical anti-inflammatory drug, is a good example used for drug loading experiments [49, 53]. Herein, IBU was used to study the drug delivery and release behavior of nanostructured HSs.
The SiO2 · Re2O3 HSs were 1 g after loading IBU (see the ‘Methods’ section), and the IBU storage in nanostructured SiO2 · Re2O3 HSs reached 287.8 mg/g, which means that the as-prepared SiO2 · Re2O3 HSs have a high loading capacity.
where R is the total release rate, C i is the IBU concentration in SBF at time i, i is the time of the IBU medium solution taking out from the SBF, and m represents the total mass of the IBU in the HSs.
Figure 7A shows the release behavior of the IBU-loaded SiO2 · Eu2O3 HSs in SBF. Compared with the pure IBU disk release in SBF, the release rate of the IBU-loaded SiO2 · Eu2O3 HSs lasted long. The drug release rate was very fast within 12 h, which showed a nearly linear relationship between drug release rate and release time at the first 12 h. Then the drug release process was very slow and finished after 60 h. The results indicated that the sustained release behavior of the drug carrier was in favor of a durative drug effect.
In order to investigate the properties of the loaded drug, the UV–vis absorption spectra of the IBU hexane solution before and after IBU loading in SiO2 · Eu2O3 HSs were measured. The results are shown in Figure 7B. Curves a, b, and c were the IBU hexane solution before drug loading, the SBF solution after the release of IBU-loaded SiO2 · Eu2O3 HSs for 4 h, and the SBF solution after the release of IBU-loaded SiO2 · Eu2O3 HSs for 70 h, respectively. It was noticed that the shape of the absorption curves was essentially the same, which demonstrated that the property of IBU was not changed in the loading and release processes.
We noticed that the samples still emitted fluorescence after the experiments of drug delivery and release, which indicated that the leftover via the loading and release processes can be tracked and detected.
We have reported an approach of the synthesis of functional SiO2 · Re2O3 HSs using silica spheres, rare-earth ions, and an acidic environment. The size of synthesized hollow capsules can be modulated by controlling the diameter of the silica template. The facile and economical synthesis protocol is valuable and convenient for wide use. Acting as drug-loaded capsules, the SiO2 · Re2O3 HSs demonstrated much excellent properties of high payloads, retained drug activity and stability, and slow drug release rate. Furthermore, real-time detection may be carried out during drug delivery and release with SiO2 · Re2O3 HSs by measuring their fluorescence.
The authors express their thanks to Associate Prof. Rusen Yang (University of Minnesota) for the language polishing.
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