Organic–Inorganic Mesoporous Silica Nanotube Hybrid Anodic Alumina Membranes for Ultrafine Filtration of Noble Metal Nanoparticles

Novel metal nanoparticles (NPs), such as gold (Au), platinum (Pt), palladium (Pd), and silver (Ag) have gained increased attention due to their distinctive physical and chemical properties as well as effective applications in catalysis, imaging, gene expression, chemical sensing, biolabeling, disease diagnostics, disease therapeutics, nanoscale electronics, and photonics. The physical and chemical properties as well as potential applications of these NPs are strongly dependent on their shapes, sizes, and structural geometries. However, conventional synthesis methods produce novel metal NPs with broad size distributions and uncontrollable precise morphologies. Therefore, it is necessary to develop a simple, effective, and fast method for separating NPs with different sizes and shapes.


Platinum nanoparticles
Recent interest on Pt materials focused on its nanostructures included of NPs, among others in order to improve their cost effectiveness and potential applications.Different morphologies of nanostructured Pt, such as nanocages, nanorods and NPs, have also been successfully fabricated (Onizawa et al., 2009, Petroski et al., 1998).The properties of Pt NPs are dependent on their shapes, sizes, and crystal structures, especially the crystallographic planes exposed on the surfaces of the NPs, thus the development of the facile method for the large-scale synthesis of Pt NPs with the ability to control the size, shape, or morphology is a crucial issue (Tsung et al., 2009).Recent research about the design of novel Pt and Pt-based nanomaterials with unique properties have been greatly intensified, because of their potential for new applications and improvement of current ones.The synthesis and control of the morphologies of Pt NPs have been extensively investigated for various applications, including catalytic applications in automobile exhaust purification, fuel-cell technology, and those for the petrochemical and fine chemical industries.To date, various methods have been developed for the synthesis of Pt NPs, including the sol−gel method, electrodeposition, electroless deposition and physical approaches, as well as the hydrothermal and solvothermal techniques (Chen A. et al., 2011).Among these, the polyol method is among the most popular pathways for synthesizing Pt NPs.This method has also been applied to synthesize metallic powders of Ru, Rh, Sn, Re, W, Pt, Au, Fe-Cu, Co-Cu, Ni-Cu, Fe, Co, Ni, Cu, Pd, and Ag (Kurihara et al., 1995, Siekkinen et al., 2006).For instance, the Xia group has introduced the process for the reduction of a Pt(II) or Pt(IV) precursor in the presence of a stabilizing polymer; the group achieved this by reducing agents, such as alcohols, sodium borohydride, or hydrogen gas (Herricks et al., 2004).In the polyol synthesis of Pt NPs, the type and amount of the surface capping-agents strongly affect the size and shape of the synthesized NPs.Polyvinylpyrrolidone is the most widely used capping agent in the synthesis of noble metal NPs.El-Sayed and coworkers controlled the shapes and sizes of the synthesis of Pt NPs by changing the concentration ratios of the capping polymer material and the platinum cations used in the reductive synthesis of colloidal particles in the solution (Ahmadi et al., 1996).The obtained products involved an abundance of different shapes, such as tetrahedal, polyhedal, irregular prismatic and cubic, and a wide particle size distribution ranging from 4 to 14 nm.Meanwhile, Miyazaki et al. synthesized the Pt NPs using polyvinylpyrrolidone, poly(N-isopropylacrylamide), and sodium polyacrylate as capping agents (Miyazaki et al., 2003).The NPs they produced had different shapes, such as hexagonal, square and triangular, depending on the capping agents used.Other methods, such as soft template (Attard et al., 1997), hard templates (Fukuoka et al., 2002) and electrochemical deposition (Tian et al., 2007), have been applied in order to control the morphologies of Pt nanocrystals.The catalytic-assisted synthesis is also an effective method for controlling the morphologies of Pt NPs.For instance, Xia and coworkers reported the synthesis of quasioctahedral shapes by simply reducing the H 2 PtCl 6 precursor with PVP in aqueous solutions containing a trace amount of FeCl 3 (Lim et al., 2008).Another method for the synthesis of Pt NPs in an organic medium involves the Schiffrin method, in which aqueous PtCl 6 2− ions are transferred to non-polar organic solvents by phase-transfer molecules, such as tetra-alkyl-ammonium salts reduced by borohydride treatment and capped with stabilizing alkyl isocyanide molecules (Horswell et al., 1999).The prepared particles have a well-defined crystalline structure and diameters ranging from 1 to 3 nm.Others methods, such as radiofrequency, sputtering, reverse micelles, electron beam www.intechopen.comOrganic-Inorganic Mesoporous Silica Nanotube Hybrid Anodic Alumina Membranes for Ultrafine Filtration of Noble Metal Nanoparticles 133 lithography and chemical vapor deposition techniques, have also been developed for the synthesis of Pt NPs (Welch  Compton, 2006).In general, Pt NPs can be controlled using various synthesis methods with different conditions despite the shapes and sizes; thus, the final products still produced NPs of different shapes or sizes (Fig. 2).Fig. 2. TEM images of Pt NPs of different shapes and sizes (reprinted from Herricks et al., 2004)

Palladium nanoparticles
Pd is an important novel metal because of its unique physical and chemical properties and great potential applications in various fields; it can be used as a catalyst, for biological labeling, controlled drug delivery as well as electronic and optical devices (Nieto et al., 2008, Bianchini et al., 2009).However, their optical and catalytic properties and potential applications are dependent on their sizes, shapes, and morphologies.Therefore, the development of facile synthetic methods for the formation of Pd NPs with well-controlled size and shape is a very important topic that has been gaining attention in recent years (Horinouchi et al., 2006).In particular, the fabrications of noble metal NPs (including Pd) are generally conducted using wet chemical methods, such as the polyol process.A variety of Pd NPs with different shapes and sizes have been synthesized using a modified polyol reduction process in the presence of ethylene glycol and stabilizer (Xiong et al., 2005).For instance, Nemamcha et al. reported the synthesis of Pd NPs through the sonochemical reduction of Pd (II) nitrate in aqueous solution (Nemamcha et al., 2006).They prepared the starting solutions by adding different concentrations of Pd(II) nitrate in ethylene glycol and PPV.Afterwards, the resulting mixtures were irradiated with ultrasonic 50 kHz waves in a glass vessel for hours to obtain Pd NPs with particle sizes ranging from 3 to 6 nm, depending on the synthesis conditions.Roy et al. reported the characteristics of Pd NPs obtained by a one-pot reduction of Pd chloride in aqueous solution using citric acid in the presence of steric stabilizer polyvinylalcohol (Roy et al., 2006).The diameters of the Pd NPs can be controlled in a range of ~8-53 nm, using different synthesis conditions, such as reflux time, concentrations of metal ion, reducing agent, and capping polymer.The solution and seed-mediated growth methods have also been reported on the size and shape evolution of metal nanostructures.For instance, Kim et al. reported

on the synthesis of monodisperse Pd
NPs by thermal decomposition of a Pd−surfactant complex (Kim SW. et al., 2003).They controlled the particle size of Pd NPs within 3.5-7 nm by varying the concentration of the stabilizing surfactant.Niu et al. introduced the shape-controlled synthesis of Pd nanocrystals via the seed-mediated method with CTAB, potassium iodide, and ascorbic acid as the surfactant additive and reductant, respectively (Niu et al., 2010).Several types of Pd nanocrystals, such as rhombic dodecahedral, cubic and octahedral have been prepared by manipulating potassium iodide concentration and the reaction temperature.However, synthesizing Pd NPs with fine and narrow particle size distribution is still a challenge.Pd NPs of different shapes and sizes fabricated by wet chemical route are shown in Fig. 3. Fig. 3. TEM images and size distributions of Pd NPs of different morphologies (reprinted from Xia et al., 2009)

Silver nanoparticles
Ag NPs and their related nanostructures have been investigated extensively, because of their unique properties and great potential applications in oplasmonics, anti-bacterial materials, sensing, and spectroscopy.For instance, the anti-bacterial activity and good conductivity of Ag have been known since the ancient times and are now widely investigated for real applications (Kosmala et al., 1011).In addition, Ag NPs exhibit the surface plasmon resonance effect and strong bacterial resistance to antibiotics, making them ideal for biotechnological applications (Shrivastava et al., 2009).Ag NPs can be synthesized using various methods, such as chemical reduction (Pal et al., 2007), electrochemical (Santos et al., 2002), γ-radiation (Choi et al., 2005), laser ablation (Amendola et al., 2007), solvothermal (Yang et al., 2007), hydrothermal (Shen et al., 2011), photochemical (Henglein, 1998), and sonochemical (Salkar et al., 1999).Generally, different shapes and sizes of Ag NPs can be fabricated by wet chemical routes just simply varying the synthetic conditions.For instance, Dong et al. reported (Dong et al., 2010).They realized that the formation of the triangular nanoprisms is dependent on the molar ratios of sodium borohydride and trisodium citrate used in the reactions, in which a balance between the precursor contributed to the formation of the small spherical particles.The transformation of the spherical NPs is critical for the synthesis of the triangular nanoprisms.The TEM images of obtained silver NPs are shown in Fig. 4, and as can be seen, the Ag NPs have different shapes and sizes depending on the reduction time.
In summary, almost all aforementioned methods used the reducing agents to reduce the Ag salts to form Ag NPs with different shapes and sizes by carefully controlling the reducing environment and stabilizer concentration during synthesis.However, none of the above methods appear suitable for general utility.Most methods offer an acceptably narrow size distribution; however, they do not offer much variability in the size that can be produced.The preparation of Ag NPs with homogenous particle size is highly important in increasing their potential applications.

Optical properties of novel metal NPs
Noble metal NPs exhibit diverse and unique characteristics, including catalytic, electronic, and optical properties.Among other, their optical properties are most interesting because of the interactions of electromagnetic field from the light and NPs, resulting different colors of NPs.For instant, the color of Au colloidal solution can be changed from red to light blue by varying the size and shapes of NPs (Jones et al., 2011).The abnormal behaviors have been attributed to the interaction between the electromagnetic radiation and the conduction band electrons (free electron cloud) of NPs (Fig. 5).
When the oscillating electric field from incident light interacts with the delocalized electrons of a metal, the free electron cloud is perturbed and displaced from the metal framework, thus generating a dipole in the NP.However, the Coulombic attraction from the positively charged nuclei of the metal pulls the electron cloud back to its initial position.Due to the large difference in mass, the heavy nuclei remain in a fixed position, whereas the lighter electrons experience motion.With the electric field component of incident light acting as a sinusoidal driving force and Coulombic attraction acting as a restoring force, all components of a harmonic oscillator are present.Consequently, resonant conditions can be achieved when light is coupled in phase to the natural frequency of the plasmon oscillation.At these resonant conditions, the metal structures absorb the maximum amount of incident electromagnetic radiation, thereby causing the greatest amount of charge displacement.The oscillation wavelength depends strongly on the surrounding medium, size and shape of NP, which lead to different surface plasmon bands of NP solutions and different colors, as shown in Fig. 6.The separation of different molecules, ions, and particles from their mixture is a critical issue in the fields of nanoscience, nanotechnology, and especially in pharmacy and biology (Han et al., 2008).In recent years, several separation methods have been developed, including membrane filtration, size-exclusion chromatography, and electrophoresis.Membrane-based separations are becoming increasingly relevant for a number of applications due to their low energy requirements, potentially low fabrication cost, and steady-state operation (Jang et al., 2011).The membrane filtrations have been applied for the separation of gas phases, liquid-liquid phases, submicron particles from water, organic mixture, and hydrocabon mixture (Gin et al., 1011).They are very effective in separating molecules of different sizes, solids from liquids, and/or large size particles from small sizes by imperatively imposing the mixture solution through the proper porous membranes (Soria, 1995, Asatekin et al., 2011).Synthetic membranes have been developed and used in many separation processes, from industrial-scale ones (e.g., separating atmospheric gases for medical and industrial use and removing salt from seawater) to smaller-scale processes in chemical synthesis and purification, in which the membranes work by forming a barrier between two phases (e.g., salt water and fresh water) that restrict the movement of some molecules while allowing others to pass through.Diverse types of membrane filtration have been investigated and manufactured for the separation of gases, molecules, and particles of different sizes, permeabilities, and hydrophobicities.The membranes are classified as either dense (nonporous) or porous, depending on how the molecules (filtrates) move across the barrier.The porous membranes are further divided into asymmetric and symmetric.Those membranes have been classified based on their porous characteristics (Table 1).

Membrane barrier structure
Trans-membrane gradient

Electrodialysis (ED) Electrodialysis
Table 1.Classification of membranes and membrane process for separation via passive transport (Ulbricht, 2006) The membranes are classified as non-porous (dense), microporous (d p 2 nm), mesoporous (d p =2-50 nm), and macroporous (d p =50-500 nm) based on the barrier structure.These membranes are also classified if they are intended for gas separation (GS), reverse osmosis (RO), nano-filtration (NF), ultra-filtration (UF), and micro-filtration (MF) based on the trans-membrane gradient.The separation based on the gradient of concentration is classified into pressure evaporation and dialysis.However, the classifications are relative and associated.For instance, the pore sizes of RO, NF, UF, and MF fall within the ranges of www.intechopen.comNoble Metals 138 10 -4 -10 -3 , 10 -3 -10 -2 , 10 -2 -10 -1 and 10 -1 -10 1 m, respectively (Wagner, 2001).The GS is used for air separation or natural gas purification, such as H 2 , He, O 2 , N 2 , CO 2 , C 2 H 6 , C 3 H 8 , and CH 4 .The RO membrane is applied in liquid/liquid separation, where the water is the only material passing through the membrane.RO is effectively used to purify water, especially in successful applications for desalination of sea and brackish water, waste treatment, and various separations in chemical, food, pharmaceutical, and other industries; all dissolved and suspended material are rejected through the membrane.The NF rejects only ions with more than one negative charge, such as sulfate or phosphate, while passing single charged ions.NF also rejects uncharged, dissolved materials, and positively charged ions according to the size and shape of the molecule in question.UF is a process, in which the high molecular weight component (e.g., protein and suspended solids) are rejected, whereas all low molecular weight components pass through the membrane freely.Mono-and disaccharides, salts, amino acids, organics, inorganicacids, or sodium hydroxide are not rejected by UF.The MF is a process where in only suspended solids are rejected, whereas proteins pass the membrane freely.
The membrane filtration can be classified as organic (e.g., polyethylene terephthalate, cellulose acetate, polyvinylidenedifluoride) and inorganic (e.g., carbon, alumina, silica) types based on the materials.In addition, they can be classified as having flat and hollow fiber forms based on the geometries.For instance, the commercial ordered porous organic polymeric membrances are track-etched polycarbonate and polyethylene terephthalate, whereas the commercial available inorganic membranes are glass, metal, zirconia, zeolite, and alumina (Robeson, 2008, Choi et al., 2009).Figure 7 shows the SEM images of hollow fiber poly(vinylidenefluoride), (a) hollow fiber carbon, (b) photos of commercial track etched polycarbonate, and the (c) high-ordered porous anodic alumina membranes (d).Organic-Inorganic Mesoporous Silica Nanotube Hybrid Anodic Alumina Membranes for Ultrafine Filtration of Noble Metal Nanoparticles 139 The membrane can be designed with the most appropriate geometry, size, and characteristics, depending on the purpose of application.Polymeric membrane filtrations are more popular than their inorganic counterparts, because they have the advantage of easy handling and better functionality (high selectivity).They are also suited for large-scale, low-cost fabrication processes, and can be designed with different structures (e.g., hollow, porous fibers or porous films).However, they suffer from several limitations such as low permeance, high temperature instability, as well as swelling and decomposition of organic solvents.On the other hand, inorganic membranes, such as porous anodic alumina, zeolite and porous silica possess high permeability, tunable selectivity, and high thermal and chemical resistance (Hsieh, 1991).The potential application of inorganic membranes is widely recognized since the production of high-quality porous ceramic membranes for large-scale industrial usage have been realized.Some inorganic membranes, such as mica, glass and alumina, are commercially available nowadays.Despite the extensive development and investigation of inorganic membranes for the separation of liquids and gases, reports on porous membranes for NP separation are rare, especially those for separating noble metals that are less than 10 nm in particle size.

Ultrafiltration and nanofiltration membranes
For the separation of NPs that are less than 10 nm in size, membranes should have proper porous structures, with a homogenous pore size distribution.Ultrafiltration and nanofiltration membranes are actually ideal candidates for separating NPs and large molecules, such as proteins, because they have pore sizes in the range of 1-100 nm.Ultrafiltration and nanofiltration membranes have been extensively used in industry and research for purifying and concentrating macromolecular solutions (Iglesias et al., 2009).They are effective in separating proteins and NPs due to the simplicity of the process and their tunable proper pore sizes.Ultrafiltration and nanofiltration membranes also enable the continuous separation of a large volume of feed, in which the feed stream is applied using a relative high pressure that forces the mixture against the porous membrane.The solutes with high molecular weight and large particles are retained, while those with low molecular weight and small particles pass through the membrane.Using ultrafiltration membranes, it is easy to separate the large and small particles as well as the large and small molecules through the size cut-off sieving processes.Polymer ultrafiltration membranes used for separation by a variety of food, biological, and pharmaceutical systems, as well as by the water purification and treatment industries, have been welldeveloped and are commercially available.However, polymeric membranes are not effective in separating NPs, because of their low porous density.In addition, they exhibit lower selectivity, poor thermal and chemical stability than inorganic counterparts.Inorganic membranes have high thermal chemical stabilities, and some inorganic membranes, such as glass, metal, alumina, zirconia, zeolite, and carbon membranes are commercially available.Despite that, these membranes vary greatly in pore size, support material, and configuration; in addition, they are not efficient in separating noble metal NPs.Furthermore, the design of a proper anisotropic porous inorganic membrane with higher flux, better selectivity, thermal stability, and chemical stability for the separation of large molecules and NPs remains a challenge.The fabrication design of the hierarchal mesoporous filter membranes with nano-space pores and of chemically robust and mechanically stable NTs can solve those problems.

Filtration flow methods
During membrane filtration, flow directions or filtration methods have a great impact on separation performances.Fig. 8 illustrates two types of flow filtration, namely, tangential (cross) flow (A) and dead-end flow (B).In the tangential flow filtration shown in Fig. 8(A), the feed stream is applied tangential to the surface of the porous membrane (i.e., crossing the pore channels of the membrane).This feeding flow method minimizes membrane fouling, maintains a high filtration rate and ensures higher product recovery, because the sample remains safely in solution separation and the retentate stays on the surface of the membrane.To be usable for tangential flow filtration, the membranes are designed as tubular or hollow fiber membranes, as shown in Figs.7(A)-(B).In the deadend filtration shown in Fig. 8(B), the feed is applied perpendicularly to the surface of the membrane (i.e., feed stream flows through the pore channels of the membrane).This flow method enables rapid separation, because the feed is forced against the membrane surface.To be usable for dead-end flow filtration, the membranes are designed as flat sheets, as shown in Figs.7(C)-(D).However, flat sheet membranes require the product stream to spread across the entire surface of the individual sheets prior to recirculation.This non-uniform flow path causes build-up and product loss in the "membrane corners" that see slower flow rates.In addition, dead-end filtration results in a build-up of product on the membrane surface; this may damage the product and cause lower recovery, thereby "fouling" the membrane.

Design of HOM silica NTs membranes for nanofiltration
Silica NTs have been investigated extensively since their discovery in 1995 (Nakamura . Matsui, 1995).Thereafter, enormous efforts have been devoted to identify their synthesis, fundamental properties, and applications.To date, researchers have been able to fabricate nanofibers, NTs, nanowires, and nanorods of high alignment and with the ability to control the porous structures by combining the use of soft and hard templates.Substantial efforts have particularly been devoted to the fabrication of 1D or two-dimensional (2D) hexagonally ordered or stacked doughnut (circular) silica mesostructures inside the columnar pores of the AAM nanochannels using template-guided syntheses.The synthesis pathway is the so-called dual template method has been widely used to fabricate a range of 1D and 2D mesostructures of silica, carbon, polymer, and metals.However, controlling the mesopore geometry and dimension into the AAM nanochannels has generally required labor-intensive or complicated design strategies that involve aging time, temperature, pH, and humidity.Table 2 provides a brief summary of the fabrication of hierarchical mesoporous structures inside the 1D nanochannels synthesized in AAM templates.A feasible control over the engineering of 2D hexagonal and 3D mesopore cubic Im3m cage structures in the entire silica NTs that are wellaligned with AAM nanochannels has also been recently reported (see Table 2).
In general, dual template pathways have been applied in synthesizing silica NTs of porous structures (see table 2).This synthetic pathway uses the AAM pore channels as the hard template to maintain the tubular structure of the silica, whereas the self-assembly of the liquid crystal or surfactant is used as the soft template to control the porous structure of the silica.This general synthetic pathway can be extended by varying synthetic conditions, including the composition and type of surfactants for synthesizing various mesoporous structures of silica NTs.The use of dual template for the synthesis of highly ordered mesoporous silica NTs has some advantages, including the ability to control the porous structure of silica NTs, homogenous pore size distribution, and the enhancement of the mechanical stability of membrane.However, to improve the mesoporous membranes synthesized using AAM as hard templates, the following chemical and mechanical stability problems must be addressed: (1) synthesis of robust, tubular structures into the entire AAM nanochannels; the (2) formation of unconnected pores and irregular shapes due to the detachment of mesoporous silica NTs from the AAM walls; (3) creation of air gaps within the shrinking mesoporous silica upon high-temperature calcination of soft templates; and (4) low-term chemical stability of the membrane within the separation reuse-cycles (El-Safty et al., 2010, 2011).Normally, the manufacturing defects in mesoporous silica NT hybrid AAM limit the long-term storage stability of NTs inside AAM (even for a month) and reduce the potential of NT filtration systems to control the filtration rates and diffusivity of NPs.Therefore, the fabrication design of chemically robust and mechanically stable NTs as well as hierarchal mesoporous filter membranes with nano-space pores remains a key challenge in the field of materials science.
The development of a new strategy leading to synthetically constructed mesostructures with tunable pore sizes throughout the 1D nanochannels is highly desirable for the size-exclusion separation of macromolecules and NPs.As a more detailed example, a method for the fabrication of hierarchal mesofilters based on the nanolinker approach templated inside the nanochannels of AAM has been successfully developed in recent years.In this development method, impermeable layers coated with silica NT hybrid AAM have been used to create chemically robust and mechanically stable designs, as shown in Fig. 9.

Mesoporous
[b] Not used for any applications.The synthesis of densely-engineered mesofilters has been achieved using the following steps.First, the 2D hexagonal and 3D cubic Im3m mesoporous silica-NTs were synthesized inside the AAM or TMAC-AAM linkers using a direct templating method with cationic or nonionic surfactants.A key advantage of this approach is the capability to control the building of mesopores throughout the AAM nanochannels.Notably, the use of the direct templating method can lead to a more thermodynamically stable phase with hexagonal and 3D cubic Im3m structures inside the AAM nanolinkers.In addition, layered surface chemistry has been achieved by coating the inner mesopores inside the silica NTs with hydrophobic trimethylsilyl (TMS) groups.The hydrophobic TMS coating in mesoporous NTs can lead to the treatment of the tortuous-pore openings that form during the removal of the soft templates.In this approach, the organic monolayers (TMS) are densely packed and are highly stable under thermal conditions.This mesofilter membrane design is the key to broadening the nanofiltration applications for bio-molecule and NP separation.

Fabrication of HOM silica NTs using direct-templating method
The formation of hierarchically ordered silica NTs can be achieved by templating microemulsion liquid crystal phases with different surfactants inside AAM to control the mesoporous structures of the NTs.A typical synthesis of highly ordered mesoporous silica NTs on AAM templates includes the following: (i) the fabrication of a microemulsion precursor, and (ii) penetration of the microemulsion precursor in the pore channels of the AAM template, followed by (iii) calcinations at high temperature or extraction to remove the surfactants.
The schematic diagram of a system used for the synthesis of highly ordered silica NTs and the nanofiltration separation of noble metal NPs is shown in Fig. 10.When used to fabricate the HOM silica NT membrane, the pristine AAM is settled in the system, after which the precursor solution is dropped to fill in its pore channels with the assistance of slight vacuum pumping.After drying and calcinations, the HOM silica NTs were obtained inside the pore channels of AAM.However, when used for NPs separation, the HOM silica NT membrane is settled in a filtering system instead of a pristine AAM; then, the solution of NPs is dropped into the filtering funnel.Using vacuum pumping, the solution is separated through the membrane.In this regard, the NPs may go through the membrane with a smaller particle size than the pore diameter of NTs, whereas those with a larger particle size are retained in the filtering tunnel.The separated size of NPs is controlled by the pore size of silica NTs; in turn, the pore size and pore structure of silica NTs can be controlled by varying the surfactants and compositions.

Synthesis of HOM silicas NTs using Brij-type (C x EO y ) templates
Typically, Brij-type surfactants were dissolved in a mixture of ethanol and acidic aqueous solution (H 2 O/HCl).The resultant mixture was stirred for 30 min under refluxing conditions.Then, tetraethyl orthosilicate (TEOS) was added dropwise to the mixture, with continuous stirring at 30 °C for 2.5 h.The precursor solution was added dropwise into the AAM.Moderate aspiration was applied using an aspirator (Ulvac, Model MD A-15),

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Organic-Inorganic Mesoporous Silica Nanotube Hybrid Anodic Alumina Membranes for Ultrafine Filtration of Noble Metal Nanoparticles 145 leading to the penetration of the precursor solution into the columnar alumina pores.The resultant surfactant-silica hybrid membrane was then dried at room temperature for 1 h.The surfactant was removed by calcination at 600 °C for 2 h under air, using a Yamato FO 310 muffle furnace with a temperature rate of 2 °C/min (El-Safty et al., 2010).

Synthesis of HOM silica NTs using CTAB templates
Briefly, the microemulsion phase domains containing TMOS, cationic surfactants (DDAB, CTAB), decane, HCl-H 2 O, and EtOH at their molar compositions of 1:0.165:2.08:8.284and 1:0.209:2.08:8.284for DDAB and CTAB systems, respectively, were obtained by mixing thoroughly for 30 min.The penetration of the microemulsion domains into the AAM was done by means of a gentle vacuuming pressure of 0.04 MPa.The silica NTs were allowed to stand in a sealed container at 45 °C for 10 h to complete the drying process.The AAM embedded microemulsion phases were calcined at 600 °C in air for 3 h to remove the organic compounds and form the mesoporous silica NTs (El-Safty et al., 2010).

Synthesis of HOM silicas NTs using F108 templates
Quaternary microemulsion liquid crystalline phases of triblock copolymer F108/C 12alkane/TMOS/H 2 O-HCl were used to facilitate the template-guided synthesis of 3D mesocage surfactant/silica phases inside AAM.In this direct synthesis, the precursor solution containing TMOS, F108, dodecane, H 2 O-HCl, and ethanol at a respective molar composition ratio of 1:0.00732:0.313:2.59:16.56 was applied dropwise into the AAM nanochannels.Penetration of the precursor solution into the membrane was achieved by means of vacuum at a starting pressure of 0.04 MPa, thus allowing control of the silica-NTs inside the membrane pores.The addition of C12-alkane led to the formation of 3D cubic Im3m structures with large cage pores and uniform open entrances.The hybrid organicinorganic membrane was allowed to stand in a sealed container at 45 °C for 10 h to complete the drying process.The organic moieties were removed by calcination at 600 °C under air (El-Safty et al., 2010, 2011).

Synthesis of HOM silicas NTs using F127 templates
The quaternary microemulsion liquid crystalline phases of triblock copolymer F127/C 12alkane/TMOS/H 2 O-HCl (1.4 g of F127, 0.7 g of dodecane, 2 g of TMOS, 1.25 g of H 2 O-HCl, and 10 g of ethanol) were used to facilitate template-guided synthesis of 3D mesocage surfactant/silica phases.The precursor solution was applied dropwise into the TMAC-AAM nanochannels (TMCS=trimethylcholrosilane)).Penetration of the precursor solution into the membrane was achieved by means of vacuum at a pressure of ≤ 0.04 MPa.It is important to note that the use of the quaternary microemulsion liquid crystalline phase could lead to a more thermodynamically stable phase with a high curvature of cubic Im3m geometry.Furthermore, the addition of C 12 -alkane led to the formation of 3D cubic Im3m structures with large cage pores and uniform open entrances.The hybrid organic-inorganic membrane was allowed to stand in a sealed container at 45 °C for 10 h to complete the drying process.Next, the hybrid organic-inorganic membrane was washed thoroughly with ethanol/acidified H 2 O for several cycles in a Soxhlet apparatus to remove the F127 surfactant template; this was then dried at 60 °C.
In general, the quaternary microemulsion liquid crystalline phases of triblock copolymers were used as a template-guided synthesis of cubic Im3m mesocage pores inside the NTs.The 3D mesopore cubic Im3m cage array with uniform entrances (5 nm) and large cavities (17.3 nm) inside AAM allowed the development of size-exclusion nano-filter membranes (El-Safty et al., 2010, 2011).

Characteristics of HOM silica NTs membranes
The study of vertically-aligned nanochannels and the coexistence of circular or columnar mesopores and mesostructured geometries inside the AAM channels has gained considerable research attention in recent years.Such architectures inside the AAM channels are very effective for the size-exclusion separation of macromolecules and NPs.In this section, the characteristics of fabricated HOM silica NTs inside nanopores of AAM are investigated using various tools of analyses, such as nitrogen adsorption/desorption isotherm, small-angle X-ray scattering (SAXS), field emission scanning electron microscopy (FE-SEM), and high-resolution transmission electron microscopy (HRTEM).Details about the characterization method can be found in references (Hoa  El-Safty, 2011).
The advanced SAXS technique is one of the most powerful methods in investigating the mesoporous structure of HOM silica NTs.As reported in recent publications, silica NTs prepared using different surfactants and compositions have different mesophase structures, such as hexagonal and cubic Im3m or disordered mesostructures (El-Safty et al., 2010, 2011).
As an example, Fig. 11   Nitrogen adsorption/desorption isotherm was used for characterizing the mesoporous materials, including porous silica NTs fabricated inside the AAM pore channels.From the N 2 adsorption isotherm data, various parameters, such as the porous structure, pore size, pore volume and specific surface area, can be estimated.For instance, the mesoporous silica NT membranes have a large surface area (S BET  100-310 m 2 /g), as shown in Fig. 11(B).The average pore size of HOM silica NTs synthesized using F127 surfactant is about 12 nm.The NTs are stable and retain their mesoporous structures after removal of surfactant, functionalization with the TMS group and use for separation.

Separation of NPs using HOM silica NTs membranes
The performance of the mesofilter membranes synthesized using a template-guided synthesis for the separation of noble metal NPs, such as Ag, Au, and Pt with different shapes and sizes was evaluated.To synthesize these NPs, two-phase redox reactions were used to control the fabrication of the organic solution-phase Ag NPs with a size distribution ranging from 1 to 16 nm.Initially, 0.081 g of sodium laureate (SL) was dissolved in 5 ml of water.This SL solution was added dropwise to the AgNO 3 aqueous solution (AgNO 3 :0.091g in 5 ml of water), yielding white precipitates of Ag-SL salt.Collected Ag-SL salt was rinsed with water 10 times and then washed with a small amount of methanol thrice.An organic Ag-SL solution was formed by adding 20 ml of toluene to the methanol (5 ml) solution of Ag-SL salt (0.09 g).Sodium tetrahydroborate solution (Aldrich, 0.0063 g in 10 ml of water) was then added dropwise into the organic Ag-SL solution with vigorous stirring.The toluene phase immediately became yellow.After 10 min of stirring, a yellow toluene phase was collected and stored at 4 °C (Mekawy et al., 2011).
The NPs of gold and platinum were synthesized as well, and the details are presented below.A mixture of 40 mg AuCl, and 1 g oleylamine was dissolved in 10 ml chloroform and stirred for 2h at 60 °C.To this mixture composition, 20 ml acetone was added to precipitate the Au NPs.The precipitated Au NPs were then collected and thoroughly washed with deionized water and ethanol by means of a centrifuge.The Au NPs were dried in air at 45 °C.The synthesis of the PtNPs with octadecylamine followed the same procedures; however, the Pt compound dipotassium hexachloroplatinum (K 2 PtCl 6 ,) was used as the Pt source.The Pt or AuNPs were dispersed in 2 ml of chloroform for subsequent separation.
The schematic design for the size-exclusive separation system of wide distribution sizes of NPs based on TMS-immobilized silica hybrid AAM for producing ultrafine, uniformlyspherical NPs is shown in Fig. 14(A).Typically, the rigid mesoporous silica NTs inside AAM channels having a pore diameter of ~4 nm were used as ultrafine filtration systems (nanofiltration) for separating noble metal NPs fabricated in a wide-range of sizes and spherical/pyramidal morphologies.The grafting of hydrophobic trimethylsilyl (TMS) groups onto the inner pores of the silica NTs walls is very important in creating size-and shape-exclusive separation systems, because the presence of the TMS groups allows the silica NTs to retain their inner pore structure without distortion during the pressurized separation assays that could cause tortuous-pore membranes.Fig. 14 shows the sizeexclusive porous silica NT membranes, a photo of an NT membrane after calcination (B) and a top-view HRTEM image (C) of a highly ordered porous silica NT used for NP separation.

Separation performance of NPs using HOM silica NTs membranes
The separation performance of NPs can be evaluated through quantitative analysis, thus validating the efficiency of the mesoporous TMS-silica hybrid membrane as a size-exclusive separator of NPs.The silica NT membrane is not only effective in the separation of Ag NPs, it also performs excellently in the separation of other NPs, including of Au, Pt, CdS, and ZnS.The HR-TEM images of the NPs before filtration show irregularly-shaped NPs with a wide distribution of sizes, as shown in Figs.17(A), (C), and (E).In contrast, the HR-TEM images taken after filtration, which occurred on a second timescale, show uniformly spherical and regularlyshaped NPs less than 4.5 nm in diameter, indicating the ultra-fine filtration of NPs.Particles size distributions of NPs before and after separation are shown in the insets, respectively.

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Organic-Inorganic Mesoporous Silica Nanotube Hybrid Anodic Alumina Membranes for Ultrafine Filtration of Noble Metal Nanoparticles 151 The size-exclusion cut-off (<4.5 nm) of the membrane filters agrees well with the NLDFT pore-size distribution of the TMS-silica NT.During the filtration process, small-sized particles easily go through the pore channels, whereas the large particles are retained on the surface of membrane.Because the NTs have nanopores with uniform sizes, thus HOM silica NT membranes are effective for the NPs separation.Despite that, after using several times, the particles would stack and block the pore channels, leading to insufficient separation.Therefore, the cleaning processes are required for the reuse of the HOM silica NT membranes.For the cleaning processes, the stacked membranes were settled upside down in the nanofiltration system (Fig. 10).Thereafter, the distilled water and ethanol were filtrated through membrane with the assistance of slight vacuum pumping.

Conclusion and outlook
The development design of hierarchically ordered TMS-silica NTs inside 1D columnar channels has enhanced the potential ability and functionality of the application of membranes in size-exclusive separation systems of particles and small molecules.To design a robust nanofilter, cationic surfactants are used as soft templates for predicting ordered assemblies of surfactant/silica composites through strong interactions within AAM pockets.This approach forms densely-packed NTs in the entirety of the AAM channels.These NTs are coated with layers of organic moieties to create a powerful technique for ultrafine filtration.The nanofilter membranes can then be used effectively to separate several NPs.
The developed membranes are effective in separating novel NPs with particle sizes of less than 4.5 nm.This precise size-cutoff separation is very important in investigating and understanding the quantum size effects in nanomaterials.Physicists have predicted and confirmed that metal NPs in the diameter range of 1-10 nm can display electronic structures, reflecting the electronic band structure of the NPs, as well as exhibit abnormal optical properties.The smaller spherical NPs exhibit a single and sharp localized surface plasmon resonance absorption band in the visible region, while larger spheres exhibit increased scattering and have peaks that broaden significantly and shift towards longer wavelengths.Additionally, the membranes exhibit excellent size-cutoff separation of NPs and help enhance the dispersal of the NPs, with the aggregated NPs becoming monodispersed NPs when forced to go through the membrane.The enhancement of dispersal and precise size-cutoff separation of NPs can increase the potential applications of noble metal NPs in biological, optoelectronic and sensing areas, where their optical properties are strongly influenced by interparticle interactions, shapes, and sizes.
Furthermore, multiple strategies are being pursued for the development of nanofilter membranes that fulfill all requirements of high-performance molecular transport and the size-exclusion selectivity of biomolecules and small particles.However, research on the science and engineering of such membranes is needed to combine all of these attributes into practical systems.Advances in nanofabrication methods have opened up exciting new avenues to overcome the limitations of conventional porous materials; however, practical, scalable fabrication processes should also be created.Indeed, there is still a demand for controlled fabrication of mesoscale pores inside metals or metal oxide NTs or NW hybrid AAM.The formation of mesoporous structures inside NTs or NWs can open exciting new applications, including electronics, optics, gas and bio-sensors, and nano-fluidic devices.

Fig. 1 .
Fig. 1.TEM images of Au NPs with different shapes and sizes synthesized by different pathways (reprinted from Grzelczak et al., 2008).

Fig. 4 .
Fig. 4. TEM images of Ag NPs obtained through different time duration reductions of silver nitrate by sodium borohydride (reprinted fromDong et al., 2010)

Fig. 10 .
Fig.10.Schematic diagram of a nanofiltration system used for the synthesis of highly ordered silica NT membrane and the NP filtration(El-Safty et al., 2010, 2011) (A)  shows the SAXS patterns of porous silica NTs fabricated inside the AAM pore channels using triblock copolymers of Pluronic F127 as a template.The silica NTs exhibit the cubic Im3m mesostructure.The inset of Fig.11(A) is the structural parameters of fabricated HOM silica NTs characterized by SAXS.

Fig. 11 .
Fig. 11.SAXS (A) and N 2 isotherms (B) of mesofilter membranes based on mesoporous cubic Im3m cage silica NT hybrid AAM before (a) and after (b) modification with TMS groups and after removal of the blocked proteins from the AAM surfaces and pores during the second use of the mesofilters (c) (reprinted from El-Safty, 2011).
FE-SEM images shown in Figs.12(A)-(B) were recorded with partial etching of the alumina matrix with 8% H 3 PO 4 of the silica NT hybrid AAM membrane.SEM micrographs show evidence of the formation of silica NTs inside the 200 nm diameter nanochannels of AAM.After complete etching of the alumina matrix, SEM micrographs reveal that the silica NTs fabricated are well aligned within the AAM pores (Figs.12 (C)-(D)).Silica NTs with regular and continuous alignment along the perpendicular axis are also observed (Fig.12(E)) even after the TMS coating inside the pores (Fig.12(F)).The results, in general, also indicate that the open pore silica NTs fabricated using structure directing surfactants fill the diameter (~200 nm) and most of the length (~60 μm) of the AAM pores (Fig.12(E)).The NT alignment observed in Fig.12(F) may have been attained within the inclusive TMS immobilization process inside AAM.

Fig. 12 .
Fig. 12. FE-SEM cross section [(A), (B)] silica NT hybrid AAM membrane after partial etching of the alumina matrix.FE-SEM cross section [(C), (D)] and plan-view (E) images of vertically aligned silica NTs.FE-SEM plan-view image (F) recorded with TMS-silica NTs.(reprinted from El-Safty et al., 2010).Meanwhile, the HR-TEM images of silica and TMS-silica NTs after the removal of the alumina matrix are shown in Fig.13.The HR-TEM images clearly reveal large domain sizes of ordered hexagonal mesoporous networks running along the 1D AAM nanochannels.However, the HR-TEM profiles show evidence of the successful formation of hollow mesostructures with a predominantly columnar orientation.The most prominent feature in

Fig. 14 .
Fig. 14. (A) Schematic design for the size-exclusive separation system, (B) photo of a NT membrane used for NPs separation, and (C) a top-view HRTEM image of a highly ordered porous silica NT.
Figs.15 (A)-(C) show the adsorption spectrum and color change of NPs before and after separation, respectively.As revealed in Fig.15(A), 20% of the original amount of Ag NPs can be separated in monodispersed and uniform Ag NPs within minutes.However, large NPs with size >4.5 nm are caught in the pores of NTs or on the surface of the hybrid AAM.The color of the membrane changed after the separation process due to the blockage of large NPs, as shown in Fig.15 (A), inset.The membrane also exhibited effective separation of other NPs, such as gold (Fig.15(B)) and CdS (Fig.15(C)).Generally, the amount of blocked NPs depends on the shapes and sizes of NPs in the feed solution.Spherical ones with particles smaller than the pore size of the NT membrane are filtered through the membrane, whereas non-spherical ones with large particles are caught in the pores of NTs or on the surface of the membrane.

Fig. 15 .
Fig. 15.Adsorption spectrum and color change of NPs before and after separation: (A) Silver NPs; (B) Au NPs; (C) CdS NPs (reprinted fromEl-Safty et al., 2010, 2011) Further evaluation of the separation performances of NPs using HRTEM images is shown in Fig.16.The HRTEM image of Ag NPs before separation (Fig.16(A)) shows that the NPs are spherical and irregularly-shaped particles with a wide particle size distribution (Fig.16(D)).After the simple separation process, the TEM analysis shows uniform spherical and regularly shaped particles (Figs.16(B) and (C)) less than 4.5 nm in diameter, indicating the ultra-fine separation of Ag NPs by size and shape (see the histograms, Figs.16 (E) and (F)).