Synthesis of Mesoporous Silica Nanoparticles with Tunable Shape and Size

QIU Ke-xin(仇可新)，HE Chuang-long(何創(chuàng)龍) ，F(xiàn)ENG Wei(馮 煒)，WANG Wei-zhong(王偉忠)，ZHOU Xiao-jun(周小軍)，YIN Zhi-qi(尹郅祺)，CHEN Liang(陳 良)，MO Xiu-mei(莫秀梅)

1 State Key Laboratory for Modification of Chemical Fibers and Polymer Materials，Donghua University，Shanghai 201620，China

2 College of Chemistry，Chemical Engineering and Biotechnology，Donghua University，Shanghai 201620，China

3 College of Materials Science and Engineering，Donghua University，Shanghai 201620，China

Introduction

Recently， the development of mesoporous silica nanoparticles (MSNs)as drug or gene delivery system has got considerable concern，due to their facile functionalization and excellent biocompatibility［1］.However， the structural and morphological properties of MSNs have greatly affected their in vivo biological behaviors.For example，Vallhov et al.［2］investigated the size effect of MSNs on the immune response of human monocytedrived dendritic cells.He et al.［3］compared the cytotoxicity of spherical MSNs with different diameters on human breast cancer cells.Therefore，synthesis of MSNs with well-defined shape and size has recently attracted much more attention in the biomedical field.

Surfactant-templating method has been proposed as an effective method to synthesize the amorphous MSNs［4］.The main procedures include the silica source hydrolysis，combination with surfactant，formation of a liquid-crystalline mesophase，and transformation of mesophases，which are influenced by pH value of the reaction system.Yang et al.［5］demonstrated the micrometer-sized MSNs with different shapes and surface patterns could be obtained under low acidic condition.Cai et al.［6］prepared MSNs with an average size of 110 nm under a basic condition.Nevertheless，few studies have reported the synthesis of MSNs under the neutral reaction condition.Moreover，there are many factors influencing the synthesis of MSNs，such as the templating agent［7］，the reaction temperature［8］，and the relative addition rate of silica source.However，there are few reports about how to control the shape and particle size of MSNs under the neutral condition.He et al.［8］selected nonionic-cationic composite surfactants of cetyltrimethylammonium bromide (CTAB) and Brij-56 as structure-directing agent to synthesize size-controlled MSNs under a neutral condition.Nevertheless，it is still a challenge to synthesize MSNs with tunable size and shape without Brij-56.

Herein，we discussed the influence of reaction temperature and the concentration of templating agent (CTAB)on the size，shape，and dispersivity of MSNs under a neutral condition.

1 Experimental

1.1 Materials

Tetraethylorthosilicate (TEOS)and CTAB were purchased from Sigma-Aldrich (Shanghai) Trading Co.， Ltd.(Shanghai，China).Ethanol and concentrated hydrochloric acid were obtained from Sinopharm Chemical Reagents Co.，Ltd.(Shanghai，China).All the reagents were used without further purification.

1.2 Preparation of MSNs

Two synthesis variables were used to synthesize MSNs.The detailed synthesis conditions are shown in Table 1.Briefly，CTAB was dissolved in phosphate-buffered saline (PBS，pH 7.0)at the desired temperature.Then TEOS was added dropwise to the mixed solution under vigorous stirring and the mixture was aged for 20 h.The as-synthesized samples were filtered，washed with double-distilled water and ethanol，and then dried at room temperature.The template (CTAB) was finally removed from MSNs by the acidic extraction method.

Table 1 The experimental parameters

1.3 Characterization

The shape and surface morphology of resulting MSNs was observed using field emission scanning electron microscope(FESEM，Hitachi S-4800，Japan).The mesoporous structure of MSN was determined by transmission electron microscopy(TEM，Hitachi H-800，Japan).Prior to measurements，MSNs were suspended in ethanol solution，and the suspension was then dropped on the silicon chip and air-dried.The powder diffraction patterns were recorded by small angle X-ray diffraction (SAXRD，RIGAKU D/Max-2550 PC，Japan).The size distribution and average particle size were measured using Particle Size ＆ Zeta Potential Analyzer (Nano ZS，Malvern Instruments Ltd.， UK).Nitrogen adsorption-desorption isotherms were measured with a Micromeritics Tristar II analyzer(Micromeritics，USA).Average pore size distributions of MSNs were determined from the desorption branches of isotherms by the Barrett-Joyner-Halenda (BJH)method and the specific surface area was calculated according to the Brunauer-Emmett-Teller (BET)method.

2 Results and Discussion

2.1 Effect of the temperature

The effects of the reaction temperature (25，60，80，and 95 ℃)were compared under the same CTAB concentration (10 mmol·L-1).Figures 1 (A)and (B)show the FESEM and TEM images of the samples synthesized at different temperatures.It can be found that the particle size of prepared MSNs was varied with the temperature increasing.Meanwhile，the shape became more round;mesopores (porous channels)and pore structure of MSNs became clearer and more ordered，which is in agreement with the small-angle X-ray diffraction(SAXRD)result.As depicted in Fig.1 (D)，the diffraction peaks at 2θ = 1.8° become narrower and stronger as the temperature increases.Figure 1 (C)indicates that the particle size of MSNs first increases from 55.0 nm to 120.5 nm and then decreases to 77.5 nm，but finally increases to 120 nm as the temperature increases.

The nitrogen adsorption-desorption isotherms and pore size distribution data of MSNs are showed in Fig.2，which exhibites the characteristic of mesoporous materials［9］.The specific surface area decreases as the reaction temperature increases from 25 ℃to 95 ℃.Moreover，it can be seen that their average pore size approximately focuses on 3 nm from the inset of Fig.2.

Therefore，the MSNs with larger size and uniform shape can be achieved at high reaction temperature of 95 ℃，which may be caused by the fact that the high reaction temperature facilitates the hydrolysis of TEOS and the growth of MSNs.

Fig.1 FESEM images (A)，TEM images(B)，SAXRD (C)，and particle size distribution (D)of MSNs synthesized at different reaction temperatures:(a)25 ℃;(b)60 ℃;(c)80 ℃;(d)95 ℃

Fig.2 Nitrogen adsorption-desorption isotherms (inset is pore size distribution) of MSNs synthesized at different reaction temperatures:(a)25 ℃;(b)60 ℃;(c)80 ℃;(d)95 ℃

2.2 Effect of template (CTAB)

The influence of the concentration of CTAB on the pore structure，particle size，and the dispersivity of MSNs was studied at the same reaction temperature (95 ℃).As indicated in Figs.3 (A)and (B)，the dispersivity of MSNs becomes poorer with CTAB concentration increasing.Figure 3 (C)indicates that the particle size of MSNs has a slight decrease with the increasing of CTAB concentration.This phenomenon can be explained that the increase of CTAB will cause more mesoporous silica nuclei formed，therefore the particle size becomes smaller.However，the SAXRD peaks in Fig.4 (A)are narrower and stronger as the CTAB concentration increases，suggesting that MSNs synthesized at higher CTAB concentration have higher order channel structure.In addition， MSNs synthesized at CTAB concentrations of 10，30，and 50 mmol/L have the similar specific surface area around 200 m2·g-1(Fig.4 (B))and narrow pore size distribution.It can be inferred that the increase of CTAB within the measured concentrations doesn't exert obvious effect on the specific surface area.Therefore，the concentration of CTAB may affect the dispersivity and the particle size of MSNs，and MSNs with good dispersivity can be synthesized at CTAB concentration of 10 mmol/L.

Fig.3 FESEM images (A)，TEM images (B)，and particle size distribution (C)of samples synthesized at different CTAB concentrations:(a)10 mmol·L -1;(b)30 mmol·L -1;(c)50 mmol·L -1

Fig.4 SAXRD (A)and nitrogen adsorption-desorption isotherms (inset is pore size distribution)(B)of samples synthesized at different CTAB concentrations:(a)10 mmol·L -1;(b)30 mmol·L -1;(c)50 mmol·L -1

3 Conclusions

MSNs with tunable shape and size were successfully synthesized under the neutral reaction condition.The particle size and shape of MSNs can be tuned by changing reaction temperature and CTAB.In addition，the reduction of the CTAB concentration can improve the dispersivity of MSNs.Therefore，the particle size and shape of MSNs can be tuned by using the optimal synthesis conditions for specific biomedical applications.

［1］He Q J，Gao Y，Zhang L X，et al.A pH-Responsive Mesoporous Silica Nanoparticles-Based Multi-drug Delivery System for Overcoming Multi-drug Resistance［J］.Biomaterials，2011，32(30):7711-7720.

［2］Vallhov H，Gabrielsson S，Stromme M，et al.Mesoporous Silica Particles Induce Size Dependent Effects on Human Dendritic Cells［J］.Nano Letters，2007，7(12):3576-3582.

［3］He Q J，Zhang Z W，Gao Y.Intracellular Localization and Cytotoxicity of Spherical Mesoporous Silica Nano- and Microparticles［J］.Small，2009，5(23):2722-2729.

［4］Lin Y S，Haynes C L.Impacts of Mesoporous Silica Nanoparticle Size，Pore Ordering，and Pore Integrity on Hemolytic Activity［J］.Journal of the American Chemical Society，2010，132(13):4834-4842.

［5］Yang H，Coombs N，Ozin G A.Morphogenesis of Shapes and Surface Patterns in Mesoporous Silica［J］.Nature，1997，386:692-695.

［6］Cai Q，Luo Z S，Pang W Q，et al.Dilute Solution Routes to Various Controllable Morphologies of MCM-41 Silica with a Basic Medium［J］.Chemistry of Materials，2001，13(2):258-263.

［7］He Q J，Shi J L，Chen F，et al.An Anticancer Drug Delivery System Based on Surfactant-Templated Mesoporous Silica Nanoparticles［J］.Biomaterials，2010，31(12):3335-3346.

［8］He Q J，Cui X Z，Cui F M，et al.Size-Controlled Synthesis of Monodispersed Mesoporous Silica Nano-spheres under a Neutral Condition［J］.Microporous and Mesoporous Materials，2009，

117(3):609-616.

［9］Yang Y J，Tao X，Hou Q，et al.Fluorescent Mesoporous Silica Nanotubes Incorporating CdS Quantum Dots for Controlled Release of Ibuprofen［J］.Acta Biomaterialia，2009，5(9):3488-3496.