Optical and Electronic Phenomena in Sol-Gel Glasses and Modern Application
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An aerogel is a gel that has been dried by removing the solvent at temperatures above the critical point thus eliminating normal capillary pressures which tend to cause the gel to shrink or fracture. Thus, it is 27 assumed that in an aerogel the structure is much closer to that in the wet state. This feature of aerogels will be used in Chapter 5 to elucidate structural changes during aging.
Silica sols typically have particle sizes in the 5-lO0nm range and under certain conditions can persist for long periods of time without loss of stability. Silica sols lose their colloidal stability when particles aggregate. This aggregation can be through gelation, coagulation, flocculation or coacervation. Iler l1er79 defines these terms as follows:. Gelling - Gelling is a process in which the particles are linked together in branched chains that fill the whole volume of sol so that there is no increase in the concentration of silica in any macroscopic region in the medium.
Instead, the overall medium becomes viscous and then is solidified by a coherent network of particles that, by capillary action, retains the liquid. Coagulation - Coagulation is the process in which particles come together into relatively close-packed clumps in which the silica is more concentrated than in the original sol so the coagulum settles as a relatively dense precipitate. Industrial precipitated silicas are powders formed when the ultimate silica particles are coagulated as loose aggregates in the aqueous medium, recovered, washed, and dried.
A simple way to differentiate between a precipitate and a gel is that a precipitate encloses only a part of the liquid in which it is formed. Flocculation - Flocculated particles are linked together by bridges of the flocculating agent that are sufficiently long so that the aggregated structure remains open and voluminous. It is apparent that these differences will be noted mainly in dilute sols containing only a few percent of silica.
In concentrated mixtures one can distinguish a gel, which is rigid, but not between a coagulate and a flocculate. Coacerivation - A fourth type of aggregation is coacervation, in which the silica particles are surrounded by an adsorbed layer of material that makes the particles less hydrophilic, but does not form bridges between particles. The particles aggregate as a concentrated liquid phase immiscible with the aqueous phase.
Optical And Electronic Phenomena In Sol Gel Glasses And Modern Application by Renata Reisfeld
As the size and concentration of the colloidal particles increase, the sol either gels into a solid mass or the colloids aggregate and precipitate out of solution. The entire process is mediated by the aqueous environment, the most important aspect of which seems to be the pH. The condensation reaction between two silanol groups at first glance appears to resemble the condensation type polymerization that occurs with organic polymers.
Investigations by Carmen as far back as however, indicate that silicic acid polymerizes first into discrete colloidal particles which in turn aggregate into either chains and networks or larger particles depending on the conditions Car4O. Hler summarizes the process in three steps: 1. Polymerization of monosilicic acid to form primary particles. Growth of particles.
Linking of particles into chains and networks which are linked to form larger particles which extend throughout the liquid, thickening it into a gel. Under some conditions, particularly at high pH, the primary particles form larger colloidal aggregates which precipitate out. This occurs for example in the production of monodisperse silica spheres using the Stober process Stob A solution of ammonium hydroxide in a water ethanol solution pH is first prepared after which TEOS is added.
After a few minutes the solution begins to turn cloudy as the particles grow large enough to scatter light. The combination promotes rapid condensation which encourages the formation of more compact particulates over linear or network-like chains of small primary particles. The ultimate sphere size is controlled by the concentrations of ammonia and water and in some cases temperature.
Using TEOS as the primary silica source, spheres up to 1. With other alkoxides such as tetrapentyl orthosilicate, spheres up to 2j im have been reported Bog Larger spheres begin to lose their uniformity as radius of curvature increases altering the driving force for growth. Rapid particle formation is enhanced through the Ostwald ripening mechanism where larger particles grow at the expense of smaller, more soluble particles with higher surface energy.
This is especially true at higher pH due to the greater overall solubility of silica in this environment. A similar effect is produced by neutral pH solutions Of NH4F where the fluoride ion acts in much the same way as the OH-- in promoting condensation. LaMer and Dinegar explained the growth of monodisperse particles by a nucleation and growth model LaM5O. According to this model, a critical concentration of silicic acid is produced by the rapid hydrolysis and condensation. A burst of nucleation is then produced which rapidly reduces the concentration below Cn. Thus nucleation occurs almost as a discrete step.
After the concentration decreases below Cn further nucleation is unlikely and growth of the existing nuclei proceeds uniformly until an equilibrium is reached. It is this single burst of nucleation which confers monodispersity to the colloid. This is consistent with what we know about the catalytic effect of OH-- on the condensation reaction between two silanol groups.
Spherical silica particles made by the Stober process and allowed to slowly settle over a one year period to form an opaline material. Top - a fracture surface. Bottom - surface of opaline compact showing the layered hexagonal close-packed array of spheres. Advantages of using these precursors include higher purity and better control over colloid formation and growth. The chemical reactions of these reagents in aqueous environments fall into two primary categories, hydrolysis and condensation.
Likewise, depending on the concentrations and conditions, any of these reactions can and do proceed to an appreciable extent in the reverse direction, a process called re-esterification. Reaction rates are governed not only by the reactant concentrations and reaction conditions but also by inductive and steric effects of the alkoxy groups. In hydrolysis the silicon alkoxide is separated from the alcoholic ligands to form a supersaturated solution of silicic acid. Hydrolysis occurs by the 33 nucleophilic attack of water on the silicon atom in the silicon alkoxide as demonstrated by isotopic studies using 1"0 Kha The most common catalysts are mineral acids or ammonium hydroxide, but organic acids, mineral bases, fluorides, hydrogen peroxide and many other compounds have been discovered that catalyze the reaction.
In general, acid catalysis gives a more favorable hydrolysis rate while base catalysis is more favorable for condensation. Steric and inductive factors of the silicon ligands play a strong role in hydrolysis. As the R group on the alkoxide grows larger, the rate constant for hydrolysis decreases. This is particularly true for branched groups. TMOS, with its small methoxy group ;zO. An acid catalyzed solution of TMOS with an excess of water is substantially hydrolyzed within several minutes with a rapid rise in temperature whereas complete hydrolysis of a similar TEOS solution may take hours or even days.
Consequently, the rate of gelation of TMOS sols is normally considered to be condensation limited while those of TEOS and higher alkoxides is typically limited by the hydrolysis of the precursors. See Figure 2. The rapid hydrolysis of TMOS makes it hazardous, particularly to the eyes, lungs and mucous membranes. In addition to its higher vapor pressure, TMOS hydrolyzes on contact with moist surfaces forming silica and toxic methanol.
TEOS although still hazardous reacts more slowly and produces less toxic ethanol. In either case, it is 34 z z O 0. The reaction order of hydrolysis is second order with respect to water and first order with respect to silicate. This is indicative of a bimolecular nucleophilic displacement reaction SN2 involving a pentacoordinate or higher transition state. Molecular orbital MO calculations by Davis and Burgraaf showed a pentacoordinate intermediate with unusual stability Dav This petacoordinate intermediate stretches the silicon oxygen bonds making the alkoxides better leaving groups.
In acid catalyzed hydrolysis the alkoxide group is first protonated followed by nucleophilic attack of water on the silicon. Protonation of the alkoxy group withdraws electron density from the silicon making it more subject to nucleophilic attack by water. Inductive effects are also important in hydrolysis. Any electron providing group substituted on the silicon will, under acidic conditions, increase the reaction rate. Electron withdrawing substituents decrease the electron density in the silicon center making it less subject to nucleophilic attack.
Under basic conditions, the mechanism is still SN2 but with the hydroxyl anion acting as the nucleophile. Under base hydrolysis, however, electron withdrawing groups increase reactivity by making the silicon center a better electrophile. This explains why hydrolysis is more favored by acid catalysts and also why the siloxane bond is more vulnerable under basic conditions.
Note that under acid conditions one would expect a decrease in reaction rate for each successive hydrolysis electron withdrawing while the opposite holds for basic conditions. As might be deduced, the concentration of water is a key factor, a large excess promoting more rapid hydrolysis. SN2 Hydrolysis of tetramethoxysilane. Relative acidity of common ligands attached to oligomneric species of silica.
It is important to note that both the hydrolysis and alcoholic condensation rates for TMOS are significantly greater than the corresponding rates for TEOS even though the -GEt substituents of TEOS would be expected to provide greater electron density. It is apparent therefore, that the steric effects are more important in determining the reaction kinetics. The former produces a water molecule while the latter results in the respective alcohol. As indicated earlier, the same catalysts employed to increase hydrolysis are also effective for increasing the condensation rate, namely most acids, bases and fluoride.
Using this standard, Figure 2. It has been shown that below a pH of about 2 the polymerization rate is proportional to [H'] while above this pH it is proportional to [Off ] Bi9O. However the O- concentration seems to have the more significant effect. The condensation rate is maximized near neutral pH where significant concentrations of both protonated and deprotonated silanols exist.
At higher pH, base catalyzed condensation has been noted to result in more condensed structures leading to particle formation or, in the case where gelation can be induced, coarser, more particulate gels. Several reasons have been suggested for this behavior. At higher pH silica becomes more soluble as lower order oligimers rather than silicic acid. Thus the basic building blocks of the gel may be larger to begin with. Iler 39 suggested that under conditions where depolymerization is favored, restructuring occurs more readily resulting in the formation of highly condensed colloidal particles 11er It is in this high pH range where depolymerization begins to occur at an appreciable rate leading to reconstruction into more condensed species.
Siloxane bonds impart a greater acidity to an adjacent silanol than do other silanols. In other words, polysilicic acid is a stronger acid than monosilicic acid. Consequently, monomers react preferentially with higher polymerized species yielding condensed structures. Finally, larger colloidal particles are stabilized by surface charge which increases with both pH and with particle size resulting in more stable particulate sols at higher pHs.
Due to the large zeta potential at higher pHs and the formation of stable colloids, gelation is less likely as pH increases above 8 or 9. Hence large colloids or aggregates tend to form at high p The Stober process is a good example of this. In MCA, growth occurs by the addition of monomners to higher molecular weight clusters. In CCA growth occurs through the addition of cluster to cluster.
Reaction limited conditions indicate that the condensation rate Is low enough with respect to diffusion for the cluster or monomer to contact many sites before reacting at the most favorable one. According to Brinker, reaction-limited conditions apply to most silicate synthesis schemes Bri MCA requires a continuous source of monomers, which as we have seen above react preferentially with higher molecular weight species in high pH conditions. Because such growth occurs monomer by monomer, growth sites are all accessible and a dense compact product is produced.
Consequently, growth must occur between clusters. Due to steric considerations such as the screening of the interior of clusters, highly ramified structures are formed characterized by a mass fractal dimension of Dz2 Porod slopez This structure is most often observed in gels produced by acid catalyzed conditions.
In contrast with the basic sols, 29 Si NM shows acidic sols depleted in silica monomers very early in the polymerization process. Fluoride has a particularly large catalytic effect on silica reactions even at low concentrations. Slightly smaller than the hydroxyl ion, the fluoride ion is even more highly nucleophilic and can be used in an acidic low 0H- environment to promote rapid condensation 1sr Thus the addition of a small amount of fluoride under acid conditions catalyzes both hydrolysis and condensation, resulting in rapid gel formation at low pH and rapid aggregation at high pHs.
Fluoride catalysis is considered in greater depth in Chapter 4. The rate of re-esterification is only 3. This results in acid catalyzed gels containing residual alkoxide groups even though the hydrolysis is nearly complete. Low R ratios and high alcohol content also promote re-esterifi cation via LeChatelier's principle.
It is likely that the first step of re-esterification under acidic conditions involves the protonation of the silanol group while in basic condition it involves the deprotonation of an alcohol group. Due to the ease with the silanol is protonated under acidic conditions, this reaction tends to occur more readily under acid catalyzed conditions.
Re-esterification of the gel 41 surface is often observed upon drying gels in excess alcohol or after a solvent exchange where water is substantially removed prior to the alcohol. Even in the case where hydrolysis is rapid, homogenization of the solution is due in part to the production of the respective alcohol. Therefore, the presence of a cosolvent must always be considered. The rapid homogenization of the solution promotes faster hydrolysis by allowing greater contact between the reactants.
However, alcohol in excess of that needed for complete mixing slows down the condensation reaction by diluting the reactants and by competing for the active silanol sites re-esterification. This is seen most clearly in the acid catalyzed condensation limited sols such as those employed in Chapters 5 and 6.
Clusters grow by polymerization or the aggregation of particles followed by cluster-cluster linkages which eventually lead to the formation of a giant spanning cluster which fills the container. This imparts rigidity to the entire mass such that the sol gel no longer pours when it is tipped. At the gel point, the reaction is by no means completed as there remain many unreacted species and unconnected clusters which continue to react, further increasing the rigidity and stiffness to the mass.
Theoretically, the sol is gelled when the last link is formed creating the spanning network. However, the wide variation in the rates of gelation under different conditions make it difficult to determine an exact point of gelation. In this model, bonds are randomly formed between adjacent nodes on an infinite Cayley tree or Bethe lattice.
The Flory- Stockmeyer model successfully predicts the emergence of an infinite cluster at some critical point but when applied to silicon alkoxide derived sols fails to account for cyclical species which are very prevalent in acid catalyzed reactions. This deficiency in turn, leads to the prediction of a fractal dimension of 4 resulting in a divergent density for large clusters Bri Percolation theory, as developed by Stauffer et.
Structural properties in the vicinity of the gel point agree much more closely with that predicted by percolation theory than classical FS theory. Brinker and Scherer describe in greater detail the application of these two theories to gelation in their book Sol Gel Science Bri-9O. Gelation can be viewed as a process by which connectivity throughout the sol increases through the linking of larger and larger clusters until one giant "spanning" extends across the entire vessel. The determination of the gel point is an important aspect of sol gel chemistry. As previously mentioned the gel point Is often used as a measure of condensation kinetics and represents a Critical point in the overall structural evolution of the gel.
Consequently, many attempts have been made to define the gel point through rheological means See for example, Sacks and Sheu Sac As the sol nears the gel point viscosity increases rapidly and the gel transforms to elastic behavior. Viscoelastic definitions of the gelation point tend to fall short in practical terms due in no small part to the wide variation in the rates of gelation of different systems from a few seconds to several months and the difficulty in measuring the viscosity of a gel near the gelation point.
The gel point can be most simply defined as the point at which the meniscus of a sol in a container no longer remains horizontal when the container is tilted. This definition is well suited to sols which gel within a reasonably short period of time e. Polycondensation continues until both free reactive species and reactive sites on the gel network are depleted Zer86, 0rc Kelts et al used Si29 NNM to demonstrate that a substantial number of Q2 species remain at gelation and decrease with time.
There is a concurrent increase in Q3 and Q4 species during aging. Where Qn represents a Si atom bonded through n siloxane bonds to other Si atoms. Thus reactive terminal silanols that are in close proximity reorient and react with one another to form additional network connections Kel As these reactions continue, the network begins to shrink resulting in the expulsion of liquid from the pores, a process called syneresis. The shrinkage of the gel, in turn, brings additional terminal groups into proximity which then react to perpetuate the shrinkage until the network stiffens sufficiently to resist further syneresis.
The rate and extent of shrinkage depends on several factors including the density and strength of the original network, the concentration of remaining reactive groups, temperature, the chemical environment, pore structure and even the size of the gel. The rate of syneresis decreases with time as the gel stiffens and the remaining reactive groups are depleted. Presumably this is because the uncharged nature of the silica surface at the IEP 44 results in a slow condensation rate Bri9O. This points toward condensation as being the major driving force in syneresis.
It has long been known that the rate of shrinkage during syneresis is slower for large gels than small ones. Scherer proposed that as a gel shrinks it creates a compressive load that forces liquid out of the pores. Due to the short diffusion distances, small gels offer less resistance to the escape of the liquid whereas large gels require a steep pressure gradient to force the liquid through the much longer network to the surface Sch Syneresis can therefore be viewed as a competition between those forces promoting shrinkage condensation and consolidation and those resisting shrinkage the stiffness of the network, the pressure required to force out the liquid, and depletion of reactive species.
Although there are numerous factors, Brinker concludes that, in general, the rate of shrinkage decreases primarily due to the stiffening of the network rather than the depletion of reactive groups Br19O. Concurrent with continued polycondensation and syneresis, coarsening or "Ostwald ripening" occurs during aging. This irreversible process occurs through the preferential dissolution of high energy convex surfaces followed by deposition in regions of negative curvature.
Thus necks begin to form between primary particles and small pores are filled in at the expense of larger pores. Although coarsening can retard syneresis through the stiffening of the network, it normally does not have a large effect as it is slow relative to those processes contributing to gel shrinkage. Over time however, and particularly at elevated temperatures, coarsening has a marked effect on the pore structure of the gel.
As one might expect, coarsening of the gel is dramatically affected by the solubility of silica and is most pronounced at high pH- and at higher temperatures. Alkaline conditions during aging typically decreases the specific surface area dramatically and slightly increases the pore volume. The average pore radius hydraulic pore radius increases markedly, primarily due to the lower surface area Liu HT or stogaids can also promote aging with similar results She HYI has long been known to dissolve silica through the effectiveness of the FP ion in breaking siloxane bonds.
Thus coarsening takes place across the spectrum of pH. Though often overlooked, aging continues during the drying process and can dramatically change the pore structure due to the effects of long drying times, high temperatures and changes in solvent composition. In the production of large silica gel monoliths this is particularly important as drying times must be extended to produce crack free pieces.
Note also that as a gel dries the solvent composition changes as the more volatile components normally the alcohols evaporate first. Thus the solubility of silica increases due to both the higher temperature and a greater percentage of water. This is because the capillary stresses induced by the emptying of the pore network generally result in cracking of the gel unless the conditions and rate of drying are very carefully controlled. Scherer divides the drying process into three stages, the constant rate stage, first falling rate period and second falling rate period Sch These three stages are illustrated in Figure 2.
As the name implies, the evaporation per unit area is relatively independent of time. The 46 00 Evaporation 0. As the liquid evaporates menisci begin to develop at the surface. The radius of curvature is defined as negative when the center of curvature is outside of the liquid. The tension in the liquid is balanced by a corresponding compressive force on the gel network which contracts to prevent the exposure of the solid phase.
Thus the liquid-vapor interface remains at the surface of the gel and the rate of liquid loss is substantially constant as the gel shrinks in response to fluid evaporation. The total volume of liquid lost is equal to the decrease in the volume of the gel. As the gel contracts compressive forces eventually build up until the body becomes stiff enough to prevent further shrinkage. This is the end of the constant rate period and the beginning of the first falling rate period, which is characterized by the pores beginning to empty from the surface inward.
The latter term is most descriptive in that it is at this point that cracking is most likely to occur. Schematic of the first falling rate period. Gel begins to turn opaque as the drying front penetrates the gel. Visually, the onset of the first falling rate period is often recognizable by the appearance of cauliflower-shaped opaque regions which form in areas where isolated pockets of pores remain filled with liquid longer than their surroundings. These opaque areas usually but not always progress from the exterior of the gel to the interior until the entire gel is opaque.
During the first falling rate period the rate of evaporation decreases as liquid must find its way from the interior of the gel to thle surface. The surfaces of the pores that have emptied retain a contiguous film of liquid, a condition referred to as funicular. See Figure 2, There are two possible mechanisms by which liquid transport occurs, diffusion of vapor from the remaining filled pores through the empty network to the surface, and fluid flow along the funicular surface.
According to Brinker Bri9O , it is generally conceded that the latter mechanism is responsible for the bulk of liquid transfer during this stage of drying. Evaporation occurs at thle outer surface due to the lower relative humidity. This creates a gradient in capillary stress from the interior to the exterior surface where the tension of the liquid is the greatest. Liquid therefore flows along the funicular layer according to Darcy's law in which the flux of liquid is proportional to the gradient in pressure in the liquid. Funicular and pendular states in a porous body. Saturated Pendular 51 the force per unit area of the liquid Pa , 7iL is the viscosity of the liquid Pa-sec , and D is the permeability and has units of area in.
It is interesting to note that the properties of the funicular layer normally water have been shown to be significantly different than those of the bulk solvent Isr This is another reason why large gels take longer to dry. The liquid nearest the exterior of the body eventually begins to enter the pendular state, that is; where the continuity of the surface layer is disrupted forming isolated pockets of liquid. Since the liquid can no longer flow along a continuous path to the surface, it can only migrate to the surface through vapor diffusion.
In very small pores this diffusion is of the Knudsen type where the water molecules collide more frequently with the walls of the pores than with each other Kau Knudsen diffusion is also associated with a reduction in thermal conductivity of the gel. This combined with a low bulk density is what makes aerogels such excellent insulating materials. Exacerbating the reduction in diffusion rate is the strong interaction of the polar water molecules with the highly hydroxylated surface of the pore wall Suz The slow diffusion of vapor through the pore network causes an abrupt decrease in the drying rate which is indicative of the beginning of the second falling rate period.
One of the reasons gels are difficult to dry is that the permeability of the gels is usually very low because of the small pore size , so rapid drying rates produce very steep pressure gradients.
The high rate of evaporation and hence higher tension of the liquid near the outer surface makes that portion of the network shrink faster than the interior. The steeper VP, the greater the difference in shrinkage rate between the exterior and interior, and the more likely the gel is to fracture Bri9O. For the larger pore gels in this study, the greatest propensity for cracking occurs at or just after the critical point or sometimes during the first falling rate period. It is at this point that the pores begin to empty.
Logically, the maximum pressure differential possible is that between an empty pore and a filled pore at the critical point. Here the liquid is under maximum tension and the empty funicular pore under none. Therefore as the pores empty the boundary between the empty funicular and filled pores creates a stress maximum which can result in the failure of the gel.
This stress maximum transits the gel as it empties. In larger pore gels the boundary can be observed by watching the progression of the opaque stage as it travels through the gel. Chapter 6 shows that the rate at which the drying front transits the gel also plays a role in the failure of gels. Water can be retained in silica either in the bulk or on the 53 surface as silanol groups or as physically adsorbed water. The latter tends to be restricted to the surface of the silica and is generally removed by heating in the vicinity of '0C. Chemisorbed water in the form of surface silanol groups is more persistent and some remains even at high stabilization temperatures.
Silanol groups exist in several configurations depending on their location and coordination with other silanols. They can exist as: single silanols Q3 on silica tetrahedra with three siloxane Si-O-Si bonds. The surface silanol groups are the main features of a hydrated silica surface and are responsible for the hydrophilic nature of most silica gels and the colloidal properties of silica powders.
See figure 2. The relative abundance of surface silanol groups depends primarily the degree of hydroxylation of the silica surface. Zhuravlev measured the surface silanol concentrations of a variety of different types of amorphous silica using deuterium exchange and determined the average silanol concentration ctOH on fully hydroxylated surfaces to be 4.
Measurements by other authors are in substantial agreement with this number. The narrow range of experimental values suggests that this figure is relatively independent of type and history of the amorphous silica. Different types of silanol groups on the surface of hydroxylated silica. Concentration of silanols on the surface of various forms of silica Zhur As the silica surface is heated, the water content decreases steadily with increasing temperature.
Physically adsorbed water is removed at temperatures between and 'C at which point the silica surface consists only of silanol groups and surface siloxane bonds. With increasing temperature vincinal silanol groups condense to form strained siloxane bonds. At ''C the vincinal silanols are gone and the surface consists of single and geminal silanols and strained siloxane bonds. Bergna estimates the ratio of single to geminal silanol groups at this point to be Berg Strained siloxane bonds are primarily made up of three membered ring structures and are rapidly rehydroxylated upon exposure to water Bri Thus, up to this point, the dehydroxylation of the surface is considered to be reversible.
Above 'C the strained siloxane bonds begin to relax to form stable siloxane groups Hen9O. Stable siloxane surfaces can also be rehydroxylated albeit much more slowly due to the much higher activation energy required to break the unstrained siloxane bond. The hydrolysis rate of an unstrained four membered ring is approximately times slower than that of a three membered ring Bri9O.
As silanols react to form siloxane bonds, the surface becomes progressively more hydrophobic in character. The purpose is to prevent the porous body from readsorbing water or other atmospheric contaminants which can degrade its performance or in the case of monoliths result in structural failure. As the name implies, thermal stabilization consists of 57 heating the silica gel to continue the removal of surface silanol groups. As previously mentioned heat treatments below about 'C result in surfaces that are easily rehydroxylated.
Above 'C structural relaxation and the increasing elimination of isolated silanol groups produces a surface that becomes progressively less hydrophilic and progressively more difficult to rehydroxylate. Zhuravlev Zhur94 summarized this process in terms of the fraction of available sites occupied by silanols as follows: 1. The temperature threshold corresponding to the removal of physically adsorbed water from the hydroxylated SiO2 surface lies at 0 C. This equates to an average value of the silanol number cL of 4. This is a physiochemical constant independent of the type of the amorphous silica used, the method of preparing it, and the structural characteristics.
The kinetic order of the thermal desorption of water in the range 1 'C requires the pair wise reaction of hydroxyl groups to form siloxane bonds. The degree of coverage with silanol groups, OOH , in this temperature range varies from 1. Over the range 1. Rehydroxylation, involving the breaking of the siloxane bridges and the formation of new silanol groups, is a rapid non-activated or weakly activated process. In order for the condensation reaction to take place protons must migrate to other active sites resulting in the evolution of H20 molecules due to the chance interaction of pairs of OH groups.
Rehydroxylation of such a surface is a slow and strongly activated process. In addition to surface OH groups, there may exist OH groups inside the silica skeleton and inside fine micropores that have diameters comparable to that of the water molecule. The concentration of such intraskeletal OH groups depends on the method of preparation and subsequent treatment of silica. Chemical modification of the silica surface by replacing silanol groups with more hydrophobic species is another method of stabilization. For example, fluoride catalyzed gels having a substantial number of silanols substituted by fluoride are less hydrophilic and more stable to atmospheric moisture.
Brinker has modified internal pore surfaces by reacting gels with methylated alkoxides to produce hydrophobic surfaces Bri Densification is brought about by heating the silica to a temperature where viscous flow occurs and the porosity is eliminated to form dense silica glass. As heating continues between approximately 'C - 1 'C, three processes take place, condensation of slianols with the elimination of water, structural relaxation, and viscous sintering.
The first two result in skeletal densification the structural density of the solid phase of the porous gel and surface modification. The third results in pore closure and consolidation. Condensation reactions and the elimination of water continue throughout the process as isolated silanols come into proximity with each other and sufficient activation energy is provided.
This occurs only slowly as it is postulated that it requires the diffusion of protons along strained siloxane bonds until an adjacent pair of silanols is formed at which point they condense to form water 11er As the process continues it becomes progressively more difficult for isolated silanols to cover the difflisional distances necessary to react with one another. Additionally, the water must diffuse out of the pore network before it is eliminated entirely. During this process it can recombine with the surface to form silanols.
This is one reason why it is impossible to completely eliminate water from the glass by thermal treatments alone. Structural relaxation begins at about - 'C and continues throughout the sintering process. The degree of skeletal densification that takes place at intermediate temperatures depends on the starting structural density which can vary from 1. Elimination of water and relaxation drive the skeletal density toward the equilibrium value of 2. At about 'C viscous sintering commences and continues up to 1 'C, the glass transition temperature for silica.
The process of viscous sintering is driven by the reduction of interfacial energy Sch Material moves by viscous flow to reduce the solid vapor interface; pores shrink and are eventually eliminated. The viscosity of the glass is not constant in most gels but increases as the water content decreases increasing structural density. Therefore as sintering occurs there is a corresponding increase in viscosity as water is liberated. Therefore, the rate at which sintering occurs is dependent on the pore size, surface area of the gel, and the viscosity; which in turn, depends on the temperature and the amount of water remaining.
Acid catalyzed gels characterized by small pore sizes, high surface areas, low skeletal densities and high water content, begin to sinter in the vicinity of 'C. Coarser particulate gels with high structural density, large pore sizes and low surface area and less water content may not sinter substantially until 1 0C. A rapid heating rate can actually promote densification at lower temperatures due to the lower viscosity effected by a higher water content.
This is particularly true in gels with high surface areas and consequently high percentages of chimisorbed water. Condensation between silanol groups on two hydrolyzed TEOS molecules a and b and between a silanol group and an adjacent alkoxy group c and d result in the production of free H2O and ethanol respectively. Referring to Fig. The central atom of the substrate e. This nucleophilic addition has important implications when considering the role of catalysts in the sol-gel process .
The variation in charge causes instability in the hypervalent substrate molecule that is immediately rectified by the transfer of a proton from the nucleophile H2O to the ester bond of an opposing alkoxy group. Cleavage of the single r-bond between the opposing alkoxy i. The fact that both reactions occur simultaneously defines hydrolysis of alkoxides as SN2 reactions. Conversely, condensation does not proceed by this same mechanism. As Fig. This mechanism is therefore an SN1 reaction and can occur as either dehydration or dealco-holation .
For the former to occur, two HO- groups must take part in the formation of an Si-O-Si bond, whereas the latter results from the direct transfer of proton to the leaving group on a neighboring substrate. Evident from these reactions, a decrease in pH can promote hydrolysis through protonation of the leaving groups therefore reducing the stability of the ligand in question. Alternatively, higher pH will induce the deprotonation of -OH groups and therefore favors condensation . This relationship means that higher and lower pH is also able to promote condensation and hydrolysis, respectively.
In the sol-gel route synthesis, a stepwise reaction scheme has been undertaken to control the ratio of hydrolysis to condensation rates . In general, the rate of hydrolysis is fast compared to that of condensation in strong acidic conditions. Therefore, a well-ordered hexagonal arrangement of mesopores a pore structure that is commonly formed in sol-gel silica materials is formed at low pH in acidic conditions. Meanwhile, in neutral or basic conditions ranging from pH 7 to pH 9, the rate of condensation is faster than that of hydrolysis, and eventually.
It is hence an interesting attempt to synthesize ordered mesoporous materials by a two-step sol-gel route at a lower acidic pH followed by a higher pH. Up to now, however, a two-step sol-gel reaction has not been applied for the fabrication of ordered mesoporous structure, because the reaction mechanism is not yet fully understood with most well-ordered silica structures that have been achieved under strong acidic conditions. Expectedly, reaction kinetics reaches a minimum at the isoelectric point . Thus the promotion or inhibition of alkoxy polymerization can be tailored to the substrate of interest by choosing a pH that provides the optimal set of reaction kinetics.
Several other silane oligomers apart from TEOS can be used to produce a similar result [5,] with the prerequisite being a set of functionalised groups capable of taking part in the hydrolysis and condensation reactions. In addition to silicate, phosphate-based materials may be used for the same purpose [26,46], as can various transition metal elements [5,40,43,] as shown in Table 2. Network modifiers such as magnesium and calcium can be introduced to the network in salt form  or as alkoxides.
However, the higher electronegativity of transition metal species in comparison to silicone and phosphorous can cause issues where condensation reactions proceed with an unfavorable bias away from the desired network composition or the end particulate structure [44,49]. Fortunately, the versatility of the sol-gel process has allowed for phenomena such as steric hindrance, chelation and selective catalysis to be exploited and enable viable production of a far greater range of materials than is possible with traditional synthesis methods alone. These benefits have led to the development of a series of methods that exploit differing chemistries to enable efficient and effective production of specifically-tailored bioactive materials.
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Like silicon, phosphorous and vanadium precursors can be used as network-formers within the sol-gel process [44,46,50,51]. However, the r-p double bond common to the latter two substrates reduces the expected coordination number and further produces repulsion between the oxygen atoms coordinated under adjacent r bonds. This difference in network connectivity results in a more relaxed network structure when compared to silica based networks , as silicate is able to share all 4 oxygen atoms with neighboring cationic groups .
As with conventional melt-derived materials, solubility can be controlled by the combination of network modifiers, as can the release of active agents or tailoring of other physical properties of the material in question. Nandy et al. The approach of these authors was to reduce the catalytic. Like silanes, the addition of further organic groups on the amine reduces its capacity to act as a Lewis base. However, the steric effects of bound organic groups also prevent the nucleophilic base from entering a reactive field with the substrate; hence, sterically hindering catalytic activity.
More directly, the concept that steric properties can influence nucleophilic reactions is well known in a number of fields . As a result, the Tolman cone angle has been proposed as a means by which the reactivity of a complex can be predicted , although this is not to say that such a measure can be applied to the sol-gel process as whole. As noted above, a number of factors are able to influence the process with respect to the steric properties of the ultimate leaving groups and represent only one aspect of the system.
The solvent itself also plays an important role in determining the rate of gelation reactions [37,45,54,57]. This solvation effect can occur in two ways: through viscosity and hydration effects. For example, NiO2 particle size was successfully directed by Talebian and Kheiri , who by employing a series of solvents with increasing viscosity, were able to increase the particle size in a controlled manner.
However, the solvent was also shown to alter the NiO2 crystalline structure, which serves to highlight the need for experimental confirmation as, with such a variety of avenues available to exploit, comes as set of variable that must also be controlled. The effects of viscosity, or more precisely the dielectric constant, have also been recently described by Han et al. These authors made use of non-contingent esterification between the solvent an alcohol and sulfuric acid to provide water for polymerization of the titanium-based network.
In doing so, results were confounded through the porogenic nature of water itself. However, what was demonstrated clearly was the influence of the solvent on the physical structure of the particles produced. A more definitive explanation was put forward by Nabavi et al. By altering the solvent species from that of the alkoxide itself, the substrate can become coordinated with a mixture of alkoxy groups.
Undoubtedly, this would affect polymerization in a way that is dependent on specific combination of coordinated groups present on network-forming substrates throughout the solution. Colloidal solutions can be defined as solutions containing discrete particles that do not settle, but remain suspended for several years, unless induced to do otherwise .
Depending on the size of the colloid particles, which usually range from 1 nm to 1 im , those with a density greater than that of the solvent can be maintained in suspension through the effects of Brownian motion alone; however, factors specific to the surface chemistry of the colloids themselves also make a significant contribution. Work within this area first began in the s and was initially followed by efforts that aimed to control both particle size in absolute terms and the uniformity of the sols that could be produced.
Today, methods of producing uniform particle sizes are further understood [36,48] with colloidal sol-gel methods remaining an active area of research. This product offers a range of potential applications; the most pertinent to biomedical application is its ability to form condensed silica networks upon drying. This property results from condensation of -OH groups present on adjacent silica particles. The initial Si-O-Si bridges that are formed can be further strengthened by passive deposition of silica on the initial bridge as a result of the equilibrium silicic acid Si OH 4 and the silica making up the mass of the colloids.
It is important to note the delineation between this process and Ostwald ripening; the latter being influenced by a propensity to increase the thermodynamic stability of the system and thus showing a preference to enlarge the size of discrete particles. Furthermore, colloidal sol-gel methods are not limited to silica or silica-based systems . The applicability of the colloidal methods is based on two key aspects of the process: stabilisation of the colloidal particles within the sol and coalescence or flocculation to form the gel.
With particles that possess the same electrostatic charge, colloidal suspensions are maintained by the f-potential, which in turn reflects the magnitude of the electrical double layer present on charged particles in solution. As noted above, the removal of the solvent is one method by which the aggregation of colloidal particles can be achieved.
Alternatively, altering the pH, salinity or temperature can induce depeptisation,. Despite the potential of colloidal methods to provide much thicker, more structurally resistant films and deposits than methods that rely on de novo synthesis, this route has seen the least number of applications within biomedical research.
This is not to say that the methods have fallen out of fashion completely. These authors were able to show an enhanced preservation of protein conformation in comparison to direct synthesis from alkoxide precursors. The gelation rate correlated well with the structural stability of the encapsulated proteins and served to demonstrate an ability to circumvent conditions that would otherwise damage sensitive molecules.
For certain applications however, the colloidal sol-gel method does not provide the degree of protection required. For example, living cells are extremely sensitive to their immediate environment. Aside from pH, temperature, nutrient availability, and specific growth factors, salinity alone can rupture cellular walls resulting in immediate death of the organism. Colloidal methods offer a further benefit over alkoxide based systems in that the majority of the network is already present in the sol.
Adaptations such as the introduction of osmoprotectants can therefore be applied without significantly interfering with the integrity of inorganic capsule itself . In aqueous solution, metal ions are coordinated by a hydration shell, the nature of which depends both on the valence of the specific metal in question and the pH of the solution .
The unique combination of the polar solvent coordination and the ability of hydrated metal cations to act as Lewis acids enable the processes of olation and oxolation to take place. This ultimately results in the formation of polymeric oxides in solution . For a detailed description of the chemistry evolved in the polymerization of hydrated metal ions the reader is referred to Livage et al.
This results in the release of a proton into the aqueous medium and is followed by a subsequent deprotonation event to from a resultant M-O-M covalent bond. From this brief description alone, the influence of pH on the process can also be deduced; an increase in pH favoring olation with lower pH inhibiting the process. As noted above however, the polymerization process seldom proceeds smoothly throughout aqueous media.
Localized shifts in the immediate vicinity of the complex undergoing polymerization leading to the formation of heterogeneous ceramic aggregates, as opposed to an even dispersal that can be produced when sol-gel methods are applied [35,36]. For biomedical applications, sporadic synthesis is by no means ideal to the end function of the material i. Undoubtedly, materials composed of metal oxides exhibit a wide range of desirable properties [5,31,32,36,61] and as a result, a series of methods have been developed based on chelation of metal precursors in order to control the natural polymerization processes.
Essentially, metal chelation solgel methods employ strong chelating agents such as citric acid or EDTA as a means of controlling the formation of the highly reactive hydrated complexes [36,59]. Although discussed here in terms of chelated inorganic precursors, chelation itself is not limited to inorganic processes. Such methods can also be applied to modify the polycondensation of metal alkoxides whereby the rate of reaction is reduced following the replacement of alkoxide leaving groups with a chelating ligand in more stable conformation .
In further discerning metal chelation methods from the alkoxide sol-gel route, the underlying principle is that polycondensation of the metal itself occurs through hydration processes described above. The mesoporous nature of the material was achieved with use of a surfactant system involving TEA and cetyl-trimethylammonium bromide CTAB ; however, the incorporation of Fe III within the material itself was made possible through the chelation of the metal with the TEA aspect of the system.
Furthermore, the viability of synthesis within a phosphate based network offers a pathway for the design of biomaterials that completely degradation in aqueous media such as biological fluids. Clearly, adjustments to these procedures would require careful planning of the factors that influence the process ab initio. Nevertheless, TEA is able to form chelation complexes with a wide range of transition metal elements  therefore offering a plausible route for further biologically relevant substitutions within the network.
The use of epoxides as gelation agents provides another useful synthesis pathway. Although not strictly a chelation-based method, parallels can be observed between this approach and those more typical chelation methods that aim to prevent natural polymerization processes until required.
In a natural extension from metal chelates methods are the polymeric sol-gel methods. Essentially, these methods involved the chelation of reactive inorganic gel-forming agents within an organic polymer network although, depending on the material to be produced, chelation is secondary to the stabilisation . In more broader terms, gel-forming agents are maintained in a state of dispersion throughout the solution, thereby preventing the precipitation of aggregates within in the sol .
This method does however require subsequent heat treatment to remove the organic polymer following the formation of the inorganic gel. Recently, Yang et al. The nanoparticle exhibited a superior degree of porosity when compared with particles that were synthesized without the presence of the polymer network. Whilst not a biomedical application, the ability to control the textural aspects that influence functionality was clearly demonstrated.
Chemical properties, such as the biomimetic molar ratios of apatites, can also be achieved with polymer assisted stabilisation  due to the homogeneous elemental distribution of the gel network. For example, Valliant et al. Their results showed a homogenous distribution of calcium in the silica network, and the larger polymers also appeared to show reduced degradation rates.
Here, a biocompatible polymer was purposefully chosen so as to remain within the network and thus avoid processing complications encountered during thermal treatment. Inorganic networks can also be formed in situ through the polymerization of organic precursors . Originally patented by Pechini , this method involved the formation of a three-dimensional 3D polyester network as a result of the reaction between ethylene glycol and citric acid; the citric acid acting as a chelation agent due an available bi-dentate binding mechanism followed by an esterification with ethylene glycol Fig.
The organic network can then be removed as with ex situ polymers described above. This route offers a robust and effective means of synthesizing materials that disperse poorly in viscous solutions or that would otherwise form reaction products prior to assembly into the required form. This is especially useful for the calcium phosphates CaPs which have a tendency to form a diverse range of minerals when present at low concentration in aqueous solution . This family.
Esterification condensation of ethylene glycol and citrate in the presence of a cationic ligand calcium. Effective production of both biocompatible ceramic-mineral composites  and apatites with predefined stoichiometry has been achieved [65,71] with polymer-assisted sol-gel methods. Such research may also have broader implications than in solving issues associated with biocompat-ibility as, with the advent of controlled deposition of inorganic mineral layers, abiotic hard tissue regeneration may also be within reach. Silica-based sol-gel materials have been the subject of intense interest for the last three decades [19,32,43].
However, research on sol-gel processing of these materials is dated back to as early as mid-eighteenth century by Ebelmen and Graham [8,11,48]. These authors noticed that a solution of polysilicic or silicon alkoxide, such as TEOS is able to hydrolyze under acidic conditions and in the presence of H2O.
This yielded SiO2 in the form of a "glass-like" material. Later works were able to obtain a clear film by spreading a colloidal solution of salicylic acid on a spinning substrate . The process of reacting from the liquid allows these materials to be drawn into fibers, micro or nano-sized spheres, or as a thin film on a substrate [1,73,74]. In addition, because of the low processing temperature, there is a possibility to entrap most organic and biomolecules in the sol-gel network during the formation of the gel matrix [6,75]. Biomolecular encapsulation within sol-gel derived silica matrices was first introduced by Braun et al.
Following that, Reetz et al. Other follow-up? During the last few decades, silica-based materials have supplied successful solutions for soft and hard tissue regeneration . These materials are highly biocompatible and the positive biological effects of their reaction products, make them an interesting group of materials for tissue regeneration .
Silica-based bioactive glasses BGs were first synthesized via a sol-gel technique by Li et al. Pereira et al. Silica-based sol-gel glasses exhibit many of the properties associated with an ideal material for tissue regeneration, such as high surface area and a porous structure, in terms of overall porosity and pore size that promote cell-material interactions and cell invasion .
Research on these glasses showed that the porous structure of these glasses brings higher surface area that exhibits higher tissue bonding rates . Another study by Greenspan et al. A-E Porous structure of bioactive glass scaffolds created by sol-gel methods with different techniques, and F micro-computed tomography image of typical scaffold and human trabecular bone . This hierarchical pore structure of the scaffold is beneficial for stimulating interaction with cells as it mimics the hierarchical structure of many natural tissues e.
As a result, these scaffolds degrade and convert faster to HA than those of melt-derived glass with the same composition. However, these sol-gel-derived scaffolds have a relatively low compressive strength MPa , and consequently, they are primarily suitable for applications focused on low load-bearing orthopaedic sites. The discovery of mesoporous silica nanoparticles MSNs in was quickly recognized as a breakthrough that could lead to a variety of important applications .
In general, solution synthesis is carried out under a basic condition, which is similar to the traditional Stober method for preparation of silica particles. In order to match the negatively charged silica surface, cationic surfactants are normally used.
Thus, the hydrolysis and condensation rates of the silica sources e. The effect of pH is critical as it affects nucleation and growth. For example, Lu et al. As the pH decreased from On the other hand, Qiao et al. These previous reports hint that the minimum particle size can be obtained at around pH , indicating that the condensation rates, rather than the hydrolysis rate, of silica precursors highly affect the final particle sizes. In fact, Chiang et al. There have been very few trials to prepare MSNs under acidic conditions [99,].
One of the advantages of acidic conditions is the capability of using block copolymers, which have a templating. Since the condensation reaction solidifies the silica network, one can expect that a too rapid condensation reaction will create a cross-linked network faster than the mesostruc-ture organizes.
This explanation is based on a formation mechanism where the particle formation, silica condensation, and ordering of the pore system are considered as separate processes. Thus, in order to obtain an ordered material the rates of the different processes must be properly adjusted relative to each other . However, compared with basic synthetic systems, it is difficult to obtain MSNs with uniform particle size less than nm and with spherical morphologies, but the large surface area of the pores allows the MSNs to be filled with different biomolecules for biomedical applications . Phosphate-based materials are inorganic polymers, based upon the tetrahedral phosphate anion which is linked to form a 3D network .
Despite many structural similarities between silica and phosphate-based materials, they exhibit very different chemical behaviors in solution . As explained, the sol-gel chemistry of silica-based materials has been extensively studied, but very little research has been carried out on the sol-gel synthesis of phosphate-based materials and much fundamental work remains to be done.
The novelty of phosphate-based materials in biomedical applications is related to their solubility and controllable dissolution rate in aqueous media compare to silica-based sol-gel materials . Researchers have reported that these materials can be applied as a third generation of biomaterials that are bioresorbable [,]. This property of resorption allows for a variety of new biomedical applications, such as temporary implants that can be replaced with natural tissue as the body repairs a wound or DDSs that allow local and sustained release of macromolecules at the implant site [,].
Moreover, the breakdown components are monovalent and divalent ions and oligophosphates, which can be easily metabolised in the body . Studies on phosphate-based melt-derived glasses have found that the degradation rate can be easily altered via addition of different modifier oxides such as CaO, TiO2, and Na2O, thus allowing one to tailor the persistent of the biomaterial, which is an important parameter in many tissue engineering and drug delivery applications .
The idea of using phosphate-based sol-gel derived glasses for biomedical applications was first introduced by Knowles , and a few years later Carta et al. Following studies by the same group confirmed that the structure of sol-gel synthesized glasses is comparable to the melt-derived glasses with the same composition, and similar bioactivity can be expected for a variety of biomedical applications .
Pickup et al. Later, these authors showed the potential drug delivery application of these glasses with the subsequent release of drug molecules into an aqueous solution . Biocompatibility and degradation rate of phosphate-based melt-derived glasses has also been extensively studied, but is outside the scope of this review. The interested reader might see the review by Abou Neel et al. However, pertinent to the sol-gel process, the degradation rate of the ternary P2O5-CaO-Na2O glass system was found to be too high of the order of hours for many tissue engineering applications, so oxide elements like titanium oxides are often introduced to reduce the degradation rates [,].
Moreover, the challenges associated with tailoring the morphology of melt-derived glasses for biomedical applications can be overcome through further development of the sol-gel route. One point to note is that mesoporous phosphate-based materials have yet to be synthesized by the sol-gel route, and it is likely that this is due to a very small processing window between the loss of organics from the structure and the softening temperature at which the glass collapses. This remains a significantly challenging area for future efforts.
Considerable efforts have been spent in the development of iron oxide nanoparticles, particularly those with magnetic properties, i. MNPs possessing the appropriate physico-chemical and tailored surface properties have been extensively investigated for drug delivery, magnetic resonance imaging MRI , tissue engineering and repair, biosensing, biochemical separations, and bioanalysis . Due to their biocompatibility and biodegradabil-ity these materials have received considerable attention in the medical and pharmaceutical fields [,].
The range of applications speaks to the potential value of efficient synthesis methods and, as with the previous examples given, the sol-gel route is properly applicable. Sol-gel is a facile and convenient way to synthesize iron oxides from aqueous iron-based salt solutions. Addition of a base under inert atmosphere prevents the formation of undesired oxides or changes in the oxidation state of the metal cation . However, as alluded to above, MNPs are unstable under ambient conditions, and are easily oxidized to maghemite or dissolved in an acidic medium.
However, both pH sensitivity and the propensity for oxidation to maghemite can be exploited through typical sol-gel means. In this instance, transformation is achieved by dispersing magnetite a mineral possessing iron in both ferric and ferrous oxidation states in acidic medium, then addition of ferric iron nitrate. The maghemite particles obtained are then chemically stable in alkaline and acidic medium . However, even if the magnetite particles are converted into maghemite after their initial formation, further experimental challenges in the synthesis of Fe3O4 by sol-gel lies in control of the particle size and thus achieving a narrow or uniform particle size distribution.
Since the blocking temperature depends on particle size, a wide particle size distribution will result in a wide range of blocking temperatures and therefore non-ideal magnetic behavior for many applications. At present, particles prepared by sol-gel unfortunately tend to be rather polydisperse .
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However, it is well known that a short burst of nucleation and subsequent slow controlled growth are crucial to produce monodisperse particles . Controlling these processes is therefore the key in the production of monodisperse iron oxide MNPs. As one of the most common materials used in biomedical research, methods for the preparation and extended uses of titanium dioxide TiO2 have been the subject of interest for many years.
Specifically biomedical applications of TiO2 have motivated strong interest owing to its unique photocat-alytic properties, excellent biocompatibility, high chemical stability, and low toxicity . Advances in nanoscale science suggest that some of the current problems associated with alternative materials could be resolved or at least improved through applying TiO2.
For example, the TiO2 content of phosphate-based glasses produced by the sol-gel route can be far higher than is possible by conventional melt-quench techniques . Titanium itself naturally forms in four main phases: rutile, anatase, brookite, and monoclinic TiO2. The relative stability of the four titania phases depends on particle size, with rutile being the thermodynamically stable form in bulk titanium but anatase being the most stable phase at sizes below 14 nm .
Brookite and monoclinic TiO2 are metastable forms that are not commonly observed in minerals and are difficult to synthesize in pure form. It therefore stands to reason that the sol-gel approach would be able to improve the range of titanium-based materials that can be effectively produced and would thus enable exploitation of their respective beneficial properties . In early examples, particle sizes obtained via the sol-gel method were several hundreds of nanometers. For example, Matijevic et al. This step was followed by aging for long periods of time days or weeks. Similar uniform micrometer-size particles were prepared by Barringer and Bowen  through the fast hydrolysis of titanium ethoxide or isopropoxide precursors.
The most important step in their preparation was the promotion of homogeneous. However, porous and nonporous monodisperse spheres were reported more recently by a similar procedure but with addition of polymers to promote the pore formation . Yet more recently, titania particle size was controlled to achieve smaller size regimes. Morales et al. These authors investigated the consequences of adding several acids and bases as additives during the synthesis and found some contradictory results.
Use of HCl at mild acidic pH resulted in the isolation of crystalline particles containing all three phases of titania. On the other hand, use of oxalic acid or ammonium hydroxide caused the formation of mainly amorphous titania phase. In another report, in the presence of 0. From this brief history, a clear development in the sol-gel approach to the production of TiO2 NPs can be seen. One of the main problems faced in sol-gel chemistry is proper control of the hydrolysis and condensation rates when using titanium precursors. As described above, the metal chelating method can be an attractive way to overcome this issue by modifying the chemical activity of the precursors with complexing ligands that reduce the rate of or prevent hydrolysis.
Scolan and Sanchez  used acety-lacetone as such a ligand and conducted the hydrolysis in the presence ofp-toluenesulfonic acid to obtain crystalline, dispersible anatase NPs nm whereas Khanna et al. Jiu et al. In another variation of the classical sol-gel method, in the early s Sugimoto and Sakata  developed the so-called "gel-sol" method.
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Originally applied to the preparation of a-Fe2O3 particles , it is based on the preparation of a metal hydroxide gel that is then aged to obtain a sol in which colloidal particles are dispersed. By this technique, colloidal dispersions of titania particles can also be produced by a combination of the sol-gel methods and an anodic alumina membrane AAM template. TiO2 nanorods have also been successfully synthesized by dipping porous AAMs into a boiled TiO2 sol followed by drying and heating processes .
By electrophoretic deposition of TiO2 colloidal suspensions into the pores of an AAM, ordered TiO2 nanowire arrays can be obtained . Porous TiO2 based films have been obtained by the sol-gel method using tetrabutoxytitanium and polyethylene glycol PEG as the precursor and template, respectively .
The morphology of porous TiO2 was shown to be dependent on the amount of water, type of solvent, complexing agent s , and the concentration and molecular weight of the template. Metal oxide NPs, including ZnO, are versatile platforms for biomedical applications and therapeutic intervention, especially in the treatment and imaging of cancers. There is an urgent need to develop new classes of anticancer agents, and recent studies demonstrate that ZnO NPs hold considerable promise.
ZnO NPs have also been shown to exhibit strong protein adsorption properties, which can be exploited to modulate their cytotoxicity, metabolism or other cellular responses . The sol-gel method proposed by Spanhel and Anderson  in is commonly used to obtain ZnO nanometer-sized particles to prepare nanoparticulate films with strong visible luminescent properties, which are useful for optical and imaging methods .
The favorable optical properties of NPs obtained by the sol-gel method have also become a common topic of research as reflected in numerous scientific publications . Benhebal et al. Yue et al. High-filling, uniform, ordered ZnO nan-otubes have been successfully prepared by sol-gel method into ultrathin anodised aluminum oxide. Integrating the ultrathin anodised aluminum oxide membranes with the sol-gel technique may help to fabricate high-quality 1D nanomaterials and to extend its application as a template for nanostructures growth. Sol-gel routes are regarded as the optimal method for modifying ZnO quantum dots QDs , because the synthetic reactions proceed near room temperature and do not harm the structure of the nanoma-terial .