Metal Oxide Thin Films Grafted on Silica Gel Surfaces : Recent Advances on the Analytical Application of these Materials

Nos óxidos metálicos MxOy altamente dispersos como monocamadas finas sobre a superfície porosa de SiO2 (denominados como SiO2/MxOy), a ligação covalente Si-O-M formada sobre a superfície SiO2 restringe a mobilidade dos óxidos ancorados resultando em óxidos metálicos coordenativamente insaturados (LAS) em adição aos sítios ácidos de Brφnsted (BAS). Os BAS originam-se dos grupos MOH e SiOH, sendo o último devido aos grupos silanóis não reagidos. Como os óxidos ancorados estão fortemente imobilizados sobre a superficie, eles são também muito estáveis termicamente. O caráter anfotérico de muitos dos óxidos ancorados, permite a imobilização de várias espécies químicas, ácidos ou bases, resultando em uma ampla aplicação destes materiais com superfícies modificadas. Neste trabalho, muitas das recentes aplicações dos óxidos metálicos ancorados à superfície da sílica são descritas, tais como adsorventes seletivos, em processos de pré-concentração, como materiais de recheio para uso em HPLC, suporte para imobilização de enzimas, eletrodos amperométricos, sensores e biossensores.


Introduction
The surface of silica gel contains many different types of silanol groups, ≡SiOH, which can react with reagents such as metal alkoxides or halides.Preparation methods using these reagents in order to obtain active metal oxides, dispersed as monolayer films on the surface of SiO 2 , have been an area of growing interest in recent years.Experimental procedures, aiming to produce monolayers of the coating metal oxides with thermal stability, have been designed [1][2][3][4][5][6][7][8][9][10][11][12][13] .Different from metal oxides supported on solid matrices prepared by impregnation or co-precipitation processes, grafting of the reagent on the solid surface gives rise to oxides whose acidic properties are associated with high thermal and chemical stability, which are not characteristic of the respective oxides in the bulk phase 14 .From the view point of basic research, this procedure is particularly advantageous because formation of a well defined surface species allows a more precise study of the acid-base character of the active sites 15,16 .Although SiO 2 has been considered as a relatively inert material, surface silanol groups or strained siloxane groups, as stated above, can react according to the following equations: (≡Si) 2 O + MX n → ≡SiX + ≡SiOMX n-1 (2)   where MX n is an active metal compound and ≡SiOH stands for the silanol group.
e-mail: gushikem@iqm.unicamp.br Careful hydrolysis of attached metals leads to formation of a bidimensional oxide structure in which the metals are bound to the surface by a ≡SiO-M bond.In this work some features of monolayer attached oxides on a silica gel surface are discussed.
Many of these materials have been designed to satisfy the demands of activity and selectivity in a catalytic system by a thorough control of surface morphology.For this purpose many studies have been carried out, aiming a better understanding of the surface structure of the oxide coated species 4,6,[17][18][19][20][21][22] .Thus, most of the metal oxide coated silica materials, SiO 2 /M x O y , have been prepared for using in catalytic reactions [3][4][5]7,10,11,[23][24][25][26] .
Highly dispersed metal oxides on a porous silica surface are characterized to present coordinatively unsaturated metal oxide (Lewis acid sites, LAS) in addition to the Brφnsted acid sites (BAS), mainly due to the MOH groups.These LAS or BAS can adsorb many molecular species which can be immobilized by covalent bonding or electrostatic interaction.The preparation procedures, properties and applications of these species immobilized on the SiO 2 /M x O y surface, apart from applications in catalytic reactions, are described in this work.We aim to give an overview on new applications of these materials in various fields such as selective adsorbents, enzyme immobilization supports, new packing materials for HPLC columns, carbon paste modified electrodes, electrochemical sensors and biosensors.

Preparation and Characteristics of Oxide Monolayers
Supported active oxides are prepared as mentioned above by a careful hydrolysis of ≡SiOMX n-1 according to the following reaction equations: and It is supposed that the metal oxides form a complete and coherent layer.However in no instance, in the various examples cited in the literature, is the surface of the porous solid entirely coated.For instance, the density of niobium atoms for Nb 2 O 5 coated on a SiO 2 surface 27 , i.e.SiO 2 / Nb 2 O 5 , is 1.7 atoms nm -2 while complete coverage 28 requires 2.2 atoms nm -2 .Since in most cases the porous solid specific surface areas decrease after the coating treatment, it is not difficult to conceive that the finest pores remain untouched or blocked.In order to assure complete surface coverage, the treatment of the solid surface, i.e., the reactions described by equations1-4, can be repeated.However, treatment of the solid by successive coating of the porous surface may result in agglomeration of the particles.

Thermal Stability
The mobility of the oxides on the surface under thermal treatment depends on the particular metal, i.e. of the Si-O-M energy bond.An indication of how mobile the metal particles are on the surface is given by XPS measurements.The results are presented in Table 1.For SiO 2 /Sb 2 O 5 and SiO 2 /Nb 2 O 5 , heat treatments of the samples down to 1373K, in the first case, and 1273K, in the second case, practically do not change the atomic ratio while for SiO 2 /TiO 2 , the atomic ratio decreases from 0.082 down to 0.045 between 673K and 1273K.Mobility of titanium oxide on the surface is therefore considerable if compared to that observed for antimony and niobium oxides.Diffusion of titanium into the matrix or agglomeration into larger particles may be occurring in this case.Bulk phase binding energies of the metals in the oxides are (in eV): Sb 2 O 5 , 3d 3/ 2 = 540.5 29,30 ; Nb 2 O 5 , 3d 3/2 =210.2 and 3d 5/2 = 207.4 31 ; TiO 2 (rutile or anatase), 2p 3/2 = 459.0 32.Comparing these results with those of the metals in the coating oxides, only Sb(V), Nb(V) or Ti(IV) are detected.In every case the bulk phase oxides under heat treatment undergo phase transformations or chemical transformations at higher temperatures.For instance, the Sb 3d 3/2 binding energy of Sb 2 O 5 heat treated at 1373K is the same as that presented by Sb 2 O 3 32 , indicating a reduction of Sb(V) to Sb(III) at this temperature.For SiO 2 / Nb 2 O 5 heat treated at this temperature, no decomposition of Nb(V) was observed 27 .
Bulk amorphous phase Nb 2 O 5 under heat treatment becomes crystalline at ca. 773 K 33 while SiO 2 /Nb 2 O 5 at this temperature is amorphous, as shown in Figure 1(a).The X-ray diffraction patterns are the same upon heating the sample up to 1273K, Figures 1(b) and 1(c), but above this temperature, more precisely at 1573K, Figure 1(e), the material becomes crystalline, with separation of the phases and, according to the diffraction peaks, into H-Nb 2 O 5 and crystalline SiO 2 .An important conclusion taken from this experiment is that Nb 2 O 5 is strongly attached on the silica surface and, thus, only at higher temperatures can the dispersed oxides move and agglomerate to form crystalline particles 27 .For SiO 2 /Al 2 O 3 , 27 Al NMR spectroscopy detected two species for samples heated between 423 and 1023 K, Al oct (SiO 4 ) 6 (~ 10 ppm) and Al tetr (SiO 4 ) 4 (~55 ppm).A third one was observed after higher heating temperature (> 1173 K) due to Al tbp (SiO 4 ) 5 (~ 40 ppm).These observed changes of the 27 Al NMR spectra are due to the phase separation of Al 2 O 3 on the surface 34 .is given by δ= Nf x N/S BET , where Nf is the quantity of metal atoms (in mol g -1 ) in the material, N is Avogadro's number and S BET is the specific surface area (in nm 2 g -1 ).In Table 2 the values of δ and the respective average interatomic distances, l, for some materials are presented.

Acidic and Basic Properties
In most cases the attached oxides retain the acid-base properties presented by the corresponding oxides in bulk phase.However, the nature of the acid-base sites can be very different in the case of the highly dispersed oxides.
Assuming that the oxides uniformly cover the porous surface, the average metal atom density, δ, on the surface The main characteristic of these submonolayer oxides is that the atoms on the surface are separated sufficiently to allow interaction between them and, thus, they are coordinatively unsaturated.For instance, pyrochlore Sb 2 O 5 (crystalline oxide) shows only Brφnsted acid sites 38 while SiO 2 /Sb 2 O 5 (δ= 0.65) also shows, in addition to BAS (py 19b mode at 1546 cm -1 ), Lewis acid sites, LAS (py 8a mode at 1617 cm -1 and 19b at 1456 cm -1 ) 36 .Such differences between the bulk structure and the highly dispersed form of antimony (V) oxide are due to the absence of a crystalline lattice in SiO 2 /Sb 2 O 5 .Interaction of a base with LAS can occur, as shown in the Scheme 1.
Scheme 1. Structure proposed for Sb on the surface of SiO 2 .In this scheme, Sb ions are represented as coordinatively unsaturated and this is responsible for the Lewis acidity.
Generation of acid sites is also possible in aqueous solution on the surface of SiO 2 /M x O y .Many of the oxides have an amphoteric character.At solution pH below the point of zero charge (pzc), they can adsorb negatively charged species according to two possible reactions [39][40][41] : At solution pH above (pzc) they can adsorb positively charged species: where MOH stands for the metal oxide grafted on the silica surface As an example of anionic adsorption, as shown in equation 5 and 6, the interaction of the anionic complex [Fe(CN) 6 ] 4-with SiO 2 /TiO 2 and SiO 2 /ZrO 2 can be cited.The adherence of this species on the substrate surface is predominantly due to an ion pair interaction (equation 6).The infrared active CN stretching mode for K 4 [Fe(CN) 6 ] is observed at 2041 cm -1 (T 1u mode; O h symmetry) and, for the adsorbed species on SiO 2 /TiO 2 or SiO 2 /ZrO 2 surfaces, it is not shifted towards higher frequency as normally expected for M-NC bond formation 42 nor is it split into 2A 1 + E (C 4v symmetry) 43,44 .
It is also possible to graft phosphate groups to the silica gel surface coated with titanium oxide: This new material was used to accumulate positively charged mediators such as Meldola's Blue or methylene blue, by ion exchange with the protons of either the phosphate groups or the hydroxyl groups on the titanium atom [45][46][47][48] .Experimental conditions were optimised to give the highest mediation efficiency together with the best immobilization quality.
The [Co(sepulchrate)] 3+ complex immobilized on the surface of silica gel, SiO 2 /ZrO 2 and SiO 2 /ZrO 2 /phosphate, shows electrochemical responses due to the redox process of the Co(III)/Co(II) couple 49 .Acidity of the immobilized zirconium phosphate is higher than that of other matrices.Therefore, a larger amount of the cobalt complex was found and the redox potential of the Co(III)/Co(II) couple was less affected by changes in the pH values of the solutions.The electrochemical behaviour of the adsorbed complex on the matrices is affected as the pH of the solutions, as well as the nature and concentration of the supporting electrolyte, are changed.

Reaction of the Adsorbed Species
Reactions of the adsorbed species giving rise to a new phase on the surface is also possible.For example, strongly adhered [Fe(CN) 6 ] 4-anion on a SiO 2 /M x O y surface, (MOH 2 + ) 4 [Fe(CN) 6 ] 4-, can react with a series of metal ions Y n+ forming the well known mixed valence complexes, (M + ) (4-n) {Y[Fe(CN) 6 ]} (4-n)-.The same surface species are formed when (MO -) n Y n+ reacts with [Fe(CN) 6 ] 4-in acid solution.The reactions that describe the formation of the complexes are: In equation 9, the adsorbed hexacyanoferrate complex reacts with a metal ion and in the second case, equation 10, (10)   the adsorbed metal ion reacts with the hexacyanoferrate ion at the solid-surface interface, resulting in the same mixed valence complex as in equation 9.
Evidence that supports formation of such structured complexes on the surface is given by the values of the midpoint potential, obtained from cyclic voltammetry measurements by using carbon paste electrodes made with these materials 37,[43][44][45][46][47][48][49][50][51][52][53] .Table 3 contains the midpoint redox potentials, E m [ E m = (E pa + E pc )/2; where E pa and E pc are the anodic and cathodic peak potentials respectively].It is shown that E m depends on the nature of the metal Y in the mixed valence compound, as is observed for bulk phase complexes.Also it is shown that for a given mixed valence complex formed on the surface, E m depends on the nature of the supporting electrolyte.For instance, in 1 mol L -1 NaNO 3 , KNO 3 and NH 4 NO 3 , the E m found (against SCE electrode) 37 , using the SiO 2 /SnO 2 /{Ni[Fe(CN) 6 ]} carbon paste electrode, are: 0.35, 0.46 and 0.47 V, respectively.Since the surface mixed valence complex has a zeolitic type structure, such dependence is explained on the basis of the following mechanism proposed for the redox reaction:  (11)   where in the redox process the supporting electrolyte s + must diffuse into or move out of the surface structured complex.
The hydrated ionic sizes of the ions are (in nm): Li + = 0.47, Na + = 0.36, K + ~ NH 4 + = 0.24 54 .Since the channel diameter of the surface complex is 0.31 nm 55 , ions having nearly the same hydrated radio (NH 4 + and K + ) have the same midpoint potential and for Na + , that presents a larger size, the potential is higher.When using Li + as the supporting electrolyte, no redox peak is observed because it can not pass through the cavity of the surface complex.Similar results have been observed with other modified mixed valence complexes adsorbed on SiO 2 /M x O y 37,43-53 .An interesting example of reactivity of adsorbed species is a property of the adsorbed complex SiO 2 /ZrO 2 / [Ru(edta)(H 2 O)] -(Scheme 2) 56 .The adsorbed coordinated H 2 O is kinetically labile due to hydrogen bonding and can be easily replaced by a ligand L. If the basicity of ligand L changes, we observe that the midpoint potential of the reaction depends on the basicity of the ligand L i.e., the more basic the ligand, the more positive will be the midpoint potential (Table 4 ).In certain cases molecules can be adsorbed onto the substrate surface by a chemical bonding, as is observed for ascorbic acid on SiO 2 /Nb 2 O 5 57 .When dipping the solid into a solution containing ascorbic acid, the solid turns yellow immediately, presumably due to the formation of the surface complex.The adsorbed ascorbic acid can efficiently reduce some metal ions at the solid-solution interface.The reduction yield observed for some metal ions is: Fe(III)= 95.4 ± 2.2%, The material SiO 2 /TiO 2 is very efficient as an adsorbent because it is very porous while the bulk phase is normally non-porous.The active sites ≡TiOH groups in the former case are distributed over a narrower size pores, and thus, they are more available.When SiO 2 /TiO 2 was immersed in H 2 O 2 solution, the coated Ti(IV) oxide exhibited coordination properties.The appearance of an orange color on the solid surface in contact with hydrogen peroxide is a result of the coordination of H 2 O 2 with the Lewis acid site 60 .The maximum sorption capacity, 1.2 molecule nm -2 , was achieved in solution at pH 3. The adsorbed hydrogen peroxide retained its oxidation capacity for several weeks.After the sorption of the H 2 O 2 by SiO 2 /TiO 2 , formation of super oxide radicals was observed, presumably resulting from peroxide decomposition.
When the material SiO 2 /Sb 2 O 5 was immersed in an aqueous Fe 3+ solution, it showed two remarkable changes: an increase of the Sb:Si atomic ratio, due to the enrichment of the amount of Sb(V) content on the matrix surface, and formation of a new phase, FeSbO 4 , as established by the studies of X-ray photoelectron spectroscopy and Mössbauer spectroscopy, respectively 61 .With the intention of understanding the mechanism of formation of the FeSbO 4 phase, two different routes for the preparation of the material were studied: (i) SiO 2 /Sb 2 O 5 was immersed in an aqueous solution of Fe 3+ , followed by thermal treatment at 423 K or (ii) SiO 2 /Sb 2 O 5 was immersed in a solution of Fe(CO) 5 in CCl 4 followed by further hydrolysis and heat treatment at 1073 K 62,63 .In both cases an excess of Sb(V) was found on the surface, in relation to Fe(III).This fact was interpreted as a result of the migration of Sb(V) from the internal surface to the external surface when method (i) was used and as being due to migration of Fe(III) in the opposite direction when the material was obtained by method (ii).

In preconcentration processes and chemical analyses
SiO 2 /MO 2 (M= Ti, Zr) selectively adsorb Cr 2 O 7 2-from an acid solution 64,65 : where MOH stands for hydrated MO 2 grafted on the silica surface.This property has been used to preconcentrate and analyse low concentrations of this ion in water 66 .Adsorption properties of metallic ions, such as cadmium, lead and mercury, on SiO 2 /ZrO 2 /phosphate were investigated by using the batch system 67 .A selective adsorption towards heavy metals with increasing affinity order Cd(II)<Pb(II)<Hg(II) was observed.A conjugated use of SiO 2 /ZrO 2 /phosphate and silica modified with n-propylimidazole (SiIm) has been carried out for the separation of heavy-metals, such as cadmium, lead and mercury 68,69 .
SiO 2 /Al 2 O 3 has been used to adsorb the soluble polyelectrolyte, 3(n-propylpyridinium) chloride silsesquioxane polymer 70 , whose structure is shown in

Support for high-performance liquid chromatography
In high-performance liquid chromatography (HPLC), normally porous SiO 2 covered with organofunctional groups has been used as the stationary phase.However, the use of this modified substrate has limited chemical stability at pH higher than 7.In addition, silanol groups that were not reacted with organic groups or remained untouched after treatment with end capping reagents, may cause tailing or irreversible adsorption of basic solutes during operation of the column.In recent work, toward overcoming this limitation Collins et al. have coated a SiO 2 surface with ZrO 2 or TiO 2 protective layers (monolayer or submonolayer) by a grafting process and, in a further step, treated this material with organofunctional siloxanes such as polymethylsiloxane (PMOS), followed by γ irradiation to promote the cross linking of the organic groups [72][73][74][75][76] .A considerable increase in efficiency and excellent stationary phase stability has been observed when using alkaline mobile phases.Figure 2 shows the performance achieved with SiO 2 /ZrO 2 /PMOS irradiated with γ radiation.Its stability was measured by plotting the relative efficiency against the column volumes of mobile phase using an NH 3 /NH 4 + buffer pH 10 in MeOH in the proportion 30:70 v/v 75 .Column I refers to SiO 2 /PMOS and column II to SiO 2 /ZrO 2 /PMOS.It is observed that above 1000 column volumes the efficiency for SiO 2 /ZrO 2 /PMOS is about 95% and for SiO 2 /PMOS, is about 75%, of the initially measured efficiencies.
The good results obtained by using SiO 2 /MO 2 /PMOS (M=Ti, Zr) have been attributed to the reduction of the acidic properties of silica through titanization or zirconization, as well as to PMOS chain cross linking, resulting in stationary phases that show significant promise as a support when high pH mobile phase is required.

Electrochemical sensors and biosensors
Metal oxides coated on silica gel have been used for immobilization of various electron transfer mediator species.Such modified materials are very attractive because they are the basis for developing various sensors and biosensors.Important characteristics of these materials are high surface area and porosity, very easy procedures for mediator immobilization, preventing leaching from the electrode into the solution, and the mechanical and chemical resistance conferred by the silica matrix.
The interest in silica-modified electrodes with respect to their applications in electroanalysis was recently reviewed by Walcarius 77 .Table 5 presents a short summary of the use of SiO 2 /M x O y -containing electrodes for the development of electrochemical sensors and biosensors.
The following overview describes the main uses found for these materials up to now.

Ascorbic acid sensor
The hexacyanoferrate ion adsorbed on SiO 2 /ZrO 2 has been used to quantitatively determine vitamin C in natural

/ %
and processed juices, as well as in tablets 44 .The hexacyanoferrate anion can mediate the catalytic oxidation of ascorbic acid at lower potential and thus avoid interference from other species normally present in real samples.

Dissolved oxygen sensor
The material SiO 2 /ZrO 2 /[Ru(edta)(H 2 O)] -was used to fabricate an amperometric sensor for determination of dioxygen in natural water 56 .On this electrode surface dissolved O 2 is reduced at -300 mV vs SCE, against -500 mV on a bare electrode surface.The electrode is very selective and the use of a semi permeable membrane, as used in the Clark electrode, is not necessary in the present case.The lower limit of detectable dissolved oxygen is about 0.5 mg L -1 .
Cobalt (II) hematoporphyrin IX or cobalt (II) protoporphyrin IX are adsorbed on SiO 2 /Nb 2 O 5 surface by Nb-OOC-chemical bond formation 58 (Scheme 6).In the presence of dissolved O 2 , a cathodic peak at -0.18 V (vs SCE) is observed.Between pH 4 and 7, the electrode response is not affected, keeping the cathodic potential constant.

NADH sensor
Meldola's blue (MB) immobilized on a SiO 2 /TiO 2 / Phosphate carbon paste electrode 45 is an effective electron mediator for NADH.
Methylene blue was immobilized on a SiO 2 /Nb 2 O 5 surface and a carbon paste electrode of this material was also tested to oxidize NADH 81 .An important characteristic of the electrode is that the redox potential was not affected between pH 4 and 7.

Fructose biosensor
A carbon paste electrode made with SiO 2 /TiO 2 / Phosphate/MB was used as a biosensor for fructose 46 and salicylate 47 .In both cases, the presence of the mediator allowed the application of a low overpotential for the detection of the analytes.The electrode was prepared by incorporating fructose 5-dehydrogenase (FDH).The enzyme FDH catalyses the oxidation of D-fructose to 5keto-D-fructose 82 : D-fructose + aceptor(ox) 5-keto-D-fructose + aceptor(red) (14)   This biosensor was used to determine fructose in sweets and fruit jellies.

Salicylate biosensor
In the case of the biosensor for salicylate 47 , the reaction catalysed by the enzyme salicylate hydroxylase can be represented by the equation: The consumption of NADH or O 2 or the formation of catechol or CO 2 can be measured and are proportional to the salicylate concentration.This biosensor detects the cathecol produced in the enzymatic reaction through its electroxidation on the mediator.

Glucose biosensor
The immobilization of ferrocene carboxylic acid and the enzyme glucose oxidase on the surface of SiO 2 /TiO 2 was reported 83 .The electrode is used to determine glucose in blood serum 83 .The mechanism of the electron transfer was proposed as being electron tunnelling by the flavin of the enzyme to the mediator, and the high current level was assigned to the high surface area of the silica matrix.The immobilized material presented excellent stability, maintaining the same activity for at least six months.

Hydrazine and cysteine sensor
The catalytic properties of metallophthalocyanines immobilized on SiO 2 /TiO 2 have been explored as sensors for the indicated chemical species [78][79][80] .Iron tetra-sulfophthalocyanine immobilized on SiO 2 /TiO 2 shows great stability in acid 78 .In a similar way, nickel tetra-sulfophthalocyanine immobilized on SiO 2 /TiO 2 was used for development of an electrochemical sensor for hydrazine 79 .

Phenol biosensor
Recently, Kubota et al. 84 immobilized horseradish peroxidase on SiO 2 /TiO 2 and used this for biosensor development.The direct electron transfer of hydrogen peroxide reduction by HRP was blocked by immobilizing the enzyme on silica-titanium.In this case, the sensitivity of the biosensor improved significantly, obtaining a device with better characteristics with respect to sensitivity and operational range.The mechanism of electrode response mediated for phenol can be visualized in Scheme 7. The stability of this electrode configuration was lower, compared to that obtained for the peroxidase immobilized directly onto graphite, probably due to its environment.The influence of different additives, like bovine serum albumin (BSA), polyethylenimine (PEI) and DNA, on the sensitivity and operational stability of a phenol biosensor based on peroxidase immobilized on silica-titanium and mixed in carbon was studied 85 .The high sensitivity of the biosensor and the stability obtained by DNA incorporation permitted phenol determination in real samples of industrial effluents and the waters of polluted rivers.The results showed that the biosensor is more sensitive for phenol determination in the different types of samples, in comparison to the spectrometric method.

Affinity biosensor
SiO 2 /TiO 2 was also used as a base for the immobilization of a single-stranded oligonucleotide 86 .Such immobilization proceeds through the reaction of the titanium oxide monolayer (on the silica surface) with the phosphate backbone of the oligonucleotide probe, leaving the nucleobases readily accessible for the hybridisation event.
The use of SiO 2 /TiO 2 results in a homogeneous dispersion of the nucleic acid probe within the interior of the carbon paste composite.The hybridisation response obtained with this format of a DNA biosensor was reproducible.

Conclusions
Metal oxides highly and uniformly dispersed as thin monolayer surfaces on a silica gel surface constitute a class of new materials which provide a relatively inert support possessing high chemical and thermal stabilities.Despite its insulating nature, they can be successfully be used in fabrication of electrochemical devices such as electrochemical sensors.The high sensitivity achieved by using such kinds of materials is mainly due to the porous character of the SiO 2 matrix used to prepare most of the materials described in this work.A very important aspect is that the matrix is rigid and does not swell when immersed in a liquid phase.From a thermodynamic point of view, such behaviour is sometimes desirable, because interface phenomena are more predictable and thus easier to study and more clearly understandable.
Recent reports have shown the potential use of these materials, especially in the construction of a class of highly sensitive electrochemical sensors 56 .Other possibilities are: in enzyme immobilizations, for use in water desalinisation processes, for using these materials as adsorbents of chemical species in non-aqueous solvents, for specific chemical reactions of the immobilized species and as very promising materials for use in HPLC.

Scheme 3 .
Scheme 3. Scheme proposed for binding of ferrocenecarboxylic acid onto the SiO 2 /Nb 2 O 5 surface.(Reproduction from Ref. 59 by permission of Elsevier Science B.V.).

Scheme 4 .Scheme 5 .
Scheme 4. The polymer does not immobilize directly onto a SiO 2 surface because it is highly water soluble.However, SiO 2 /Al 2 O 3 has a monolayer of aluminium oxide which can easily react with the free SiOH groups of the polymer, resulting in a layer of the polyelectrolyte strongly adhered to the surface by formation of the Al-O-Si bond, illustrated in Scheme 5.The SiO 2 /Al 2 O 3 with the polylectrolyte has a very high ion exchange capacity and has been used in adsorption of metal ions from ethanol solutions71 .

Scheme 7 .
Scheme 7. Mechanism of mediated electron transfer at SiO 2 /TiO 2 /HRPmodified carbon paste electrode.M ox and M red are the oxidized and reduced forms of the mediator, respectively.(Reproduction from Ref. 84 by permission of Elsevier Science B.V.).

Table 1 .
X-Ray photoelectron spectroscopy data for SiO 2 /M x O y .

Table 2 .
Calculated surface attached metal densities, δ, and the respective interatomic distances, l, for some materials.

Table 5 .
Applications of modified carbon paste electrodes with modified silica in chemical analysis.