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Recent trends in analytical applications of organically modified silicate materials
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Recent trends in analytical applications of organically modi¢ed silicate materials Maryanne M. Collinson*
Department of Chemistry, 111 Willard Hall, Manhattan, KS 66506-3701, USA Organically modi¢ed silicate (ORMOSIL ) materials prepared by the sol-gel process have become an attractive ¢eld of study due to the versatility and £exibility associated with this method of preparation. In this review, the development of analytical applications involving ORMOSILs for the speci¢c case where the individual precursors are covalently bound is described. A brief explanation of sol-gel processing is given followed by applications in the areas of chemical sensing, ion-exchange coatings, chromatography, and templated materials. A speci¢c focus of this article will be on material published in the last 5 years. z2002 Elsevier Science B.V. All rights reserved. Keywords: Sol-gel; Organically modi¢ed silicate; Sensors; Chromatography; Templating
1. Introduction The sol-gel process involves the fabrication of glass-like or ceramic materials through the hydrolysis and condensation of suitable metal alkoxides [ 1 ]. For the preparation of silicate materials, tetramethoxysilane (TMOS ) is one of the most popular alkoxides. This reagent can be hydrolyzed and condensed under relatively mild conditions as indicated below, Hydrolysis: Si OCH3 4 nH2 O!Si OCH3 43n OHn nCH3 OH
3 In a typical procedure, TMOS is mixed with water typically in a mutual solvent such as methanol followed by addition of a catalyst ( e.g. hydrochloric acid ). During the sol-gel formation, the viscosity of the solution gradually increases as the sol ( colloidal suspension of small particles, 1^100 nm ) becomes interconnected through polycondensation reactions to form a rigid, porous network ^ the gel [ 1 ]. Depending on the sol-gel processing conditions (Si:H2 O ratio, type and concentration of catalyst, alkoxide precursors,T ), gelation can take place on the time scale of seconds to minutes to days to months. During drying, alcohol and water evaporate from the pores causing the gel to shrink. Xerogels, or fully dried gels, are signi¢cantly less porous than their hydrated counterparts. From a chemical and materials perspective, sol-gel derived materials have many advantages [ 1^4 ], õ
1
Condensation:
õ
2
*Tel.: +1 (785) 532-1468; Fax: +1 (785) 532-6666. E-mail: [email protected]
and / or
Materials in various configurations (films, fibers, monoliths, powders) can be readily made (Fig. 1). Thin films are most often utilized in chemical sensor applications due to the short pathlength for diffusion. Bulk monoliths have been frequently used in spectroscopic measurements due to the longer optical pathlength. High surface area powders are useful in catalysis applications. Reagents can be readily incorporated in a stable host matrix by simply adding them to the sol prior to its gelation (Fig. 1). Alternatively, reagents can be added by copolymerizing TMOS with organoalkoxysilanes. In some cases, the matrix actually stabilizes the entrapped reagent from photodegradation or caustic solution environments.
0165-9936/02/$ ^ see front matter PII: S 0 1 6 5 - 9 9 3 6 ( 0 1 ) 0 0 1 2 5 - X
ß 2002 Elsevier Science B.V. All rights reserved.
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Fig. 1. The immense £exibility of the sol-gel process. õ
The materials are chemically, photochemically, and electrochemically stable. They are also optically transparent, which is a definite plus for optical characterization.
Materials prepared using sol-gel processing can range from the relatively `simple' inorganic glasses described above to the more chemically complex hybrid composites. The blending of inorganic precursors ( e.g. TMOS ) with organoalkoxysilanes can lead to materials with properties better than those prepared alone [ 5^9 ]. These materials termed ORMOSIL ( organically modi¢ed silicates ) can be prepared by mixing organosilicon precursors of the general formula R43x Si(ORP )x , where R represents the desirable reagent or functional group and x is 1^3, with TMOS, or alternatively alone [ 5^9 ]. Speci¢c functional groups that have been used include CH3 , C2 H5 , C6 H5 , ( CH2 )3 NH2 , ( CH2 )3 SH. Relative to the pure inorganic glasses, ORMOSILs possess many advantages including:
õ
õ
õ
õ
Flexibility of the silica gel can be improved thus enabling thick, crack-free films to be prepared and utilized Specific functional groups can be covalently attached to the silicon^oxygen network thus minimizing its loss in solution Reactive functional groups can be introduced in the matrix which can be subsequently used to anchor molecular recognition groups on the matrix Relative to physically entrapped species, higher concentrations of reagents can be incorporated into the matrix.
Several reviews have been written recently on the design and applications of these novel organic^inorganic hybrid materials, particularly from a materials point of view [ 5^9 ]. This review will focus speci¢cally on the preparation and recent ( within 5 years ) analytical applications of the ORMOSILs for the condition where the organic
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modi¢er is covalently bound to the silica^oxygen network. The interested reader is referred to other articles for information about the preparation and applications of inorganic^organic hybrid materials where the organic component is physically embedded into the inorganic network [ 2,3,5 ] or where organic monomers or polymers are introduced into the inorganic networks [ 5^7 ]. This report is not meant to be a comprehensive compilation of recent applications, but is rather meant to provide the interested reader with an overview of the chemistry and the types of applications that can be obtained when inorganic and organosilicon precursors are combined. Recent examples of the inherent usefulness of these composite materials in the design of materials for chemical sensors, ion-exchange coatings, chromatographic applications, and molecular templating are presented.
2. Applications 2.1. Chemical sensors A widely used approach for the development of chemical sensors involves the immobilization of a reagent into a suitable matrix followed by interconnection of this solid to a suitable transducer. Relative to many organic polymers, sol-gel derived matrices as hosts provide better stability, optical transparency, £exibility, and permeability [ 2^ 4,10 ]. Much of the interest in sol-gel materials as host matrices stems from the fact that reagents of various size and charge can be trapped into a stable host. In addition, the matrix is porous enough to allow external analyte species to diffuse into the network and react with the entrapped reagent. The development of sol-gel derived chemical sensors has blossomed since the mid-1980s. Chelating agents, indicator dye molecules, proteins, enzymes, antibodies, crown ethers, cyclodextrins, and zeolites are some of the many different types of receptors that have been incorporated into the solgel matrix [ 2^4,10 ].
2.1.1. Organic modi¢cation ORMOSILs hold much promise in the development of chemical sensors. They can be used to alter material pore size, hydrophobicity, and £exibility or introduce a speci¢c functional group into the matrix as needed to improve material performance ( response time, leaching rate, etc. ). For example,
pH sensitive dyes have been incorporated into the silica host and used as pH sensors. One of the major problems with these materials is leaching ^ the process where an entrapped reagent diffuses out into solution. It has been shown recently that organic^inorganic hybrid materials can signi¢cantly improve the stability of the sensor. MacCraith and coworkers prepared a silicate matrix from methyltrimethoxysilane (MTMOS ) and showed reduced leaching using bromophenol blue as the colorimetric indicator [ 11 ]. Collinson and Makote hydrolyzed and co-condensed tetraethoxysilane (TEOS ) with either MTMOS, phenyltrimethoxysilane (PTMOS ), or isobutyltrimethoxysilane ( BTMOS ) and observed no signi¢cant leaching of the dye molecules bromocresol green and cresol red [ 12 ]. The improved stability of the encapsulated dye is likely in part due to a change in the porosity of the matrix, stabilization of the dye in the hydrophobic matrix and / or speci¢c dye^functional group interactions [ 11,12 ]. Increasing the hydrophobicity of the silica network by the hydrolysis and condensation of TEOS or TMOS with an organoalkoxysilane is also a bene¢t to some applications. For example, MacCraith and coworkers have shown that ORMOSIL ¢lms prepared by hydrolyzing and co-condensing TEOS with methyltriethoxysilane (MTEOS ) or ethyltriethoxysilane are much better suited for dissolved oxygen sensing compared to ¢lms prepared solely from TEOS [ 13 ]. In this work the sensor response is based on the £uorescence quenching of an entrapped ruthenium complex. They attribute the optimal performance of the sensor due to the increased hydrophobicity of the ¢lm which `reduces water solubility in the matrix and causes the partitioning of oxygen out of solution into the gas phase within the sensing ¢lm' [ 13 ]. Organosilicon precursors can also be used to increase the £exibility of the matrix enabling thick crack-free ¢lms to be prepared and cut [ 14 ]. The lower extent of cross-linking also can improve the response time of the sensor. For example, Saavedra and coworkers have fabricated planar waveguide sensors for gaseous iodine from multicomponent mixtures of diphenyldiethoxysilane, MTEOS, and dimethyldiethoxysilane ( DMES ) [ 15 ]. The phenyl groups formed a charge transfer complex with iodine, thus serving as the basis for chemical sensing. DMES and MTEOS were used to obtain optically transparent ¢lms with higher porosity [ 15 ]. In more recent work, ¢lms prepared from DMES,
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MTEOS and TEOS were used in the chemical sensing of water vapor [ 16 ]. Again the ratio of DMES, MTEOS, and TEOS was chosen to prepare ¢lms that are durable and porous enough to give rise to fast response times and recovery rates [ 16 ]. Kimura and coworkers have also prepared ion sensing ¢lms from DMES and TEOS whereby a ratio of 1:3 TEOS:DMES afforded the highest sensitivity [ 17 ].
2.1.2. Immobilized receptors One speci¢c concern associated with simply physically doping a reagent into the matrix is leaching. While the porosity of the network and the mobility of the encapsulated reagent are essential for chemical sensor applications, it can be detrimental to the development of sensors for biological or environmental analysis because the reagent can leach out of the matrix. Leaching is a signi¢cant concern for small reagents such as electron transfer mediators, dye molecules, and complexing agents. One of the most promising features the ORMOSILs offer is the ability to design and fabricate leak-free chemical sensors by utilization of an organoalkoxysilane that contains the reagent of interest ( see Fig. 2 for examples ). This precursor can be combined with TMOS or used alone to fabricate the silicate matrix [ 18^27 ]. Since the reagent is covalently attached to the silicate framework, it cannot leach out of the host matrix. The one disadvantage of this approach is that the organoalkoxysilane must be synthesized. Covalently entrapped reagents have been used as pH [ 21^23 ], oxygen [ 24,25 ] and ion [ 26,27 ] sensors. For example, Avnir and coworkers have compared the spectral response of three methyl red indicators, one of which was functionalized with a silicon alkoxide functionality (2d ). Compared to methyl red, the alkoxide-functionalized dye's pKi shifted from 5.05 to 3.30 due to the blockage of the carboxyl moiety. The covalently bound indicator showed little to no leaching at high acidity [ 22 ]. Wolfbeis and coworkers have fabricated a pH sensor using an amino£uorescein derivative ( 2e ) as the pH sensitive indicator [ 23 ]. In their study, amino£uorescein was mixed with either 3-( trimethoxysilyl )propylisocyanate ( ICPS ) or ( glycidyloxypropyl )trimethoxysilane ( GOPS ) to form the alkoxysilane-modi¢ed dye. After 12 h, TMOS, alcohol, and acid were directly added to the mixture. The materials thus produced did show some leaching suggesting that not all the dye reacted with ICPS or GOPS [ 23 ].
Fluorescent dye molecules have also been used in the sensing of oxygen. MacCraith and coworkers prepared a ruthenium tris( bipyridine ) alkoxysilane ( 2f ) [ 24 ]. The covalently entrapped dye displays an emission band at 610 nm upon excitation, the £uorescence of which is quenched by the presence of oxygen. The sensors showed a good response to both gaseous and dissolved oxygen and are stated to have enhanced performance to leaching in a variety of solvents [ 24 ]. In another example, Rolison and Leventis prepared a silica aerogel monolith with £uorescent 2,7-diazapyrenium moieties ( 2g ) [ 25 ]. Aerogels in contrast to xerogels are much more porous and thus are potentially more suitable for the fabrication of chemical sensors as the response times would be fast. However, leaching is particularly problematic because of the washing steps that are necessary to replace the pore liquid with a pure solvent prior to supercritical £uid drying. As described in this study, covalent entrapment of the dye into the matrix solves this dif¢culty. They were able to prepare monoliths that showed very fast response times ( 6 8 s ) toward gaseous oxygen [ 25 ]. Leak-free ion sensors have also been prepared by incorporating crown ether alkoxysilane derivatives ( 2h ) in the matrix [ 26 ]. These derivatives were combined with TEOS and diethoxydimethylsilane ( DEDMS ) in a 1:3 ratio. The DEDMS was used to impart £exibility to the matrix as membranes with too little DEDMS were too hard for high sensitivity. The modi¢ed electrode had fairly good selectivity towards K , but somewhat slower response times relative to the physically doped reagents [ 26 ].
2.1.3. Composite materials Sol-gel derived composite electrodes have become increasingly useful for the design of amperometric sensors [ 28^32 ]. These electrodes can easily be made by mixing conductive particles ( e.g. carbon [ 28^32 ] or gold [ 33 ]) with an ORMOSIL sol and the resultant mixture packed into a glass tube or spread on a suitable surface. The ORMOSIL sol is generally made by combining MTMOS with methanol, water, and hydrochloric acid. The use of this organoalkoxysilane precursor ensures that only the outermost surface of the composite electrode is wetted. It is possible to dope the metal^ ceramic composites with a wide variety of reagents to be used in chemical sensing [ 28^31 ]. For example, Sampath and Lev have demonstrated the fab-
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Fig. 2. Selected examples of organoalkoxysilanes used to prepare hybrid composites.
rication of a composite electrode prepared by combining carbon with oxidoreductase enzymes ( e.g. glucose oxidase, lactate oxidase, or L-amino acid oxidase ) both with and without palladium. The doped carbon is mixed with the ORMOSIL sol and an electrode prepared. In this study, the carbon powder provides electrical conductivity, the enzyme is used for biocatalysis, and the palladium used for `electrocatalysis of the biochemical reaction product' [ 29 ]. In another recent example, carbon composite electrodes have been used as oxygen sensors [ 31 ]. The electrodes were made by dispersing carbon powder, cobalt porphyrin or
( palladium or platinum ) in the ORMOSIL sol. According to the authors, the carbon powder provides conductivity, the silica network provides high porosity, and the organometallic or inert metal serves as the catalyst modi¢er [ 31 ]. All four catalyst-modi¢ed electrodes were superior to the unmodi¢ed, blank electrode with the platinummodi¢ed carbon composite electrode exhibiting the best performance [ 31 ]. Another approach for the formation of sol-gel derived composite electrodes involves the direct synthesis of gold and other noble metals in ORMOSIL matrices [ 34,35 ]. In this approach, the ORMO-
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SIL sol is made out of N-[ 3-( trimethoxysilyl )propyl )]ethylene diamine ( EDAS ) or 3-aminopropyltrimethoxysilane ( APS ) [ 34,35 ]. For the preparation of Au nanoparticles, AuCl4 3 was mixed with either APS or EDAS. After hydrolysis and condensation was initiated, the gold salt was reduced using sodium borohydride. Spherical metal nanoparticles of a diameter of 4^6 nm were obtained in the sols and resultant gels. The amine functionalities on the ORMOSIL precursor are believed to stabilize the metal salts before reduction and to cap and prevent aggregation of the metal after reduction [ 34 ]. Similar procedures were utilized for the fabrication of platinum, palladium, and silver sols and gels with the exception that different salts were used [ 35 ].
2.2. Ion-exchange coatings Ion-exchange ¢lms have many applications in analytical chemistry. They can be used to prevent an unwanted substance from reaching an underlying surface or they can be used to preconcentrate an analyte for analysis. Relative to commonly used organic polymers, ORMOSILs provide many advantages for the construction of ion-exchange coatings. The inherent £exibility associated with material preparation and processing is one such advantage. The ability to fabricate thin ¢lms, control material porosity and polarity, and incorporate speci¢c ionexchange sites in the ¢lm enables materials with the ideal response, permselectivity, and ion-exchange properties to be developed. Hsueh and Collinson have prepared permselective and ion-exchange ¢lms by hydrolyzing and cocondensing MTMOS with organosilicon precursors that contain NH3 , COO3 , and COOEt functionalities [ 36 ]. The NH2 precursor can be tricky to work with as it is basic and will cause the immediate condensation of TMOS unless acidi¢ed ¢rst. The ORMOSIL sol was cast on a glassy carbon electrode and the ion-exchange properties evaluated using cyclic voltammetry. Potassium ferricyanide, ruthenium hexaammine, dopamine, and methyl viologen served as the redox probes. When placed in neutral buffer solutions, ¢lms prepared using the NH3 silane ion-exchanged potassium ferricyanide and excluded ruthenium hexaammine. In contrast, ¢lms prepared with the COO3 silane ionexchanged ruthenium hexaammine and excluded potassium ferricyanide [ 36 ]. In more recent work, Wei and Collinson have hydrolyzed and co-condensed 3-APS with either PTMOS, MTMOS,
BTMOS, or TMOS in various ratios [ 37 ]. The goal of this work was to prepare materials with different quantities of ion-exchange sites, different hydrophobicity, and variable porosity. The results showed that both the magnitude and rate of ionexchange can be manipulated by changing the organic modi¢er included in the ¢lms. Films prepared from BTMOS or PTMOS showed the fastest ion-exchange re£ecting higher permeability [ 37 ]. In other studies, a silicon alkoxide modi¢ed with a quaternary ammonium functional group has been incorporated in a silica host and used to sense anions [ 38,39 ]. Chau and coworkers have hydrolyzed and co-condensed N-trimethoxysilylpropylN,N,N-trimethyl ammonium chloride with TMOS to prepare anion-exchangeable ORMOSIL powders and ¢lms [ 38 ]. The materials thus produced were able to ion-exchange potassium ferricyanide and exhibited a selectivity for Cl3 over Br3 and I3 . In another example, tetradecyldimethyl( 3-trimethoxysilylpropyl ) ammonium chloride was combined with DEDMS and TEOS and the resultant sol coated on a glass membrane surface [ 39 ]. The resultant anion-selective glass electrode showed a Nernstian response to Cl3 activity in a wide activity range. The anion selectivity essentially obeys the Hofmeister series.
2.3. Molecular templating Template-based approaches have been used to prepare materials that show selectivity for certain analytes in solution [ 40,41 ]. In this procedure, a polymeric network is assembled around a suitable template molecule, which upon removal yields diffusional pathways and / or microcavities with a speci¢c size, shape, and / or chemical functionality in the cross-linked host. These `molecularly designed cavities' show an af¢nity for the template molecule over other structurally related compounds. Organoalkoxysilanes have been shown to be very useful in the preparation of templated sol-gel glasses [ 41^ 45 ]. In one approach, organoalkoxysilanes, chosen for their af¢nity toward a template molecule, are combined with TMOS to form a hybrid composite. Collinson and Makote showed the advantages of this approach for the preparation of ¢lms that were selective toward dopamine [ 42 ]. In this work, PTMOS was combined with MTMOS and TMOS. PTMOS was utilized as the functionalized
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silane due to its hydrophobicity and likely af¢nity for the aromatic functionality on the template. MTMOS was incorporated into the sol to introduce additional hydrophobicity and stability to the resultant materials. Dopamine was added to the hydrolyzed sol and the resultant mixture cast as a thin ¢lm on the surface of a glassy carbon electrode. Dopamine was extracted from the dried ¢lm by soaking it in phosphate buffer, and the af¢nity and selectivity of the dopamine templated ¢lms characterized with cyclic voltammetry. The dopamine templated ¢lms had a relatively fast response time and showed an increased af¢nity for dopamine over related molecules. It is believed that ¢lm porosity, electrostatics, and hydrophobicity play a role in governing diffusion and permeation into the ¢lm [ 42 ]. In another approach, the template can be covalently attached to the inorganic framework. For example, a silicon alkoxide derivatized template can be combined with TMOS to form the crosslinked network [ 43,46 ]. The template can then be removed via thermal decomposition or chemical treatments. This approach has been recently described by Maier and coworkers using borneol, fenchol, or camphor as the template [ 43 ]. In this work, the materials were prepared by condensing TEOS and MTEOS with R-Si(ORP )3 where R is the template ( borneol or fenchol ). The templated molecule was removed thermally or via oxidative treatment. In this work, the authors found that the adsorption selectivity for the templated materials was dependent on the speci¢c pore sizes and not necessarily on an `imprint effect'. In addition to the templating of thin silica ¢lms or monolithic gels ( powders ), surface imprinting routes have also been described. Brinker, Shea and coworkers, for example, have recently described a surface imprinting procedure for the creation of `synthetic receptors' for phosphates and phosphonates [ 44,45 ]. In this study, the organoalkoxysilane precursor, 3-trimethoxysilylpropyl1-guanidinium chloride, was condensed onto the surface of a xerogel host prepared from TEOS in the presence of the template, phenylphosphonic acid. Solid state nuclear magnetic resonance (NMR ) was used to de¢ne the binding sites [ 45 ] and high-performance liquid chromatography was used to evaluate the relative af¢nities of the templated materials to the template and related species as a function of pH and ionic strength [ 44 ]. The phenylphosphonic acid templated materials showed an enhancement in the binding constants
for phosphate and phenylphosphonic acid relative to a randomly functionalized polymer [ 44 ].
2.4. Separations Sol-gel chemistry has found many uses in chromatographic science [ 9,41 ]. The development of new stationary phases, in particular, has been a prime area of interest. Direct bonded stationary phases prepared by chemically attaching a monolayer( s ) directly to a silica support are often time consuming to prepare and have limited sample capacity and in some cases stability. The sol-gel approach is relatively simple. Preparation of columns generally involves ( 1 ) cleaning the support to expose the maximum number of silanol functionalities, ( 2 ) preparation of the hybrid sol, ( 3 ) ¢lling the column with the hybrid sol, and ( 4 ) removal of the sol with pressurized gas followed by drying. A number of research groups, most notably Colon and coworkers and Malik and coworkers, have become interested in utilizing the sol-gel method to prepare chromatographic supports for different applications [ 47^52 ]. In recent work, open tubular columns for liquid chromatography have been prepared using a hybrid sol prepared from n-octyltriethoxysilane ( C8 -TEOS ) and TEOS. The mole ratio of the precursors was varied to adjust the retention and selectivity of the columns. Anthracene derivatives were used as model analytes to evaluate column performance. Plate numbers of 200 000 theoretical plates / m were reported [ 47 ]. In another study, 3APS was used as the precursor to fabricate a hydrolytically stable glass coating for capillary electrophoresis. The magnitude and direction of the electroosmotic £ow were altered by modifying the charge on the column through changes in the pH of the electrolyte buffer solution. These coatings were shown to be ef¢cient in the separation of basic biomolecules [ 48 ]. In a more recent report, submicron-sized organosilica spheres for capillary electrochromatography have been prepared by the copolymerization of C8 -TEOS with TEOS. The number of theoretical plates in a column packed with 450 nm particles varied between 370 000 and 480 000 for 4-pyrene derivatives [ 49 ]. In related studies, the hydrolysis and condensation of the individual silicon precursors as well as the composition of the sol used to coat the chromatographic columns were studied with 29 Si and 1 H NMR [ 53,54 ]. The reaction kinetics of the individual
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precursors in hybrid mixtures can vary signi¢cantly and produce supports with different properties depending on the time at which the sol was used. The primary goal of this research was to identify the precursors present in the sol at the time the column is coated for silica sols prepared from TEOS, C8 TEOS, C8 -TEOS /TEOS [ 53 ], and most recently for C18 -TEOS and TEOS / C18 -TEOS mixtures [ 54 ]. By doing this, it may be feasible to design better coatings with improved ( or optimized ) chromatographic ef¢ciencies. In both studies it was found that the rates of hydrolysis for C8 -TEOS and C18 TEOS were slower than that observed for TEOS. In mixtures of TEOS:C8 -TEOS and TEOS:C18 TEOS their rates increase. The maximum degree of condensation correlated with maximum chromatographic retention for the test compounds [ 53,54 ].
2.5. Conclusion ORMOSILs have a promising future in analytical science. They can be used to change the porosity, hydrophobicity, and £exibility of silicate sol-gel derived glasses. They can also be used to chemically attach a suitable reagent to the inorganic framework. A diversity of different scientists have been attracted to this ¢eld with numerous applications in the areas of chemical sensing and chromatography being reported. The future would continue to see the use of ORMOSILs in analytical applications. In addition, trends toward unraveling the complexities associated with hybrid preparation, the nature of entrapment, leaching of entrapped guests, and the formation of controlled homogeneous and heterogeneous composites will likely be at the forefront of science.
Acknowledgements Support of the author's work in this ¢eld by the National Science Foundation ( CHE ) and the Of¢ce of Naval Research is gratefully acknowledged.
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ognition with Imprinted Polymers, ACS Symposium Series, Vol. 703, p. 314. [ 45 ] D.Y. Sasaki, T.M. Alam, Chem. Mater. 12 ( 2000 ) 1400. [ 46 ] A. Katz, M.E. Davis, Nature 403 ( 2000 ) 286. [ 47 ] Y. Guo, L.A. Colon, Chromatographia 43 ( 1996 ) 477. [ 48 ] Y. Guo, G.A. Imahori, L.A. Colon, J. Chromatogr. 744 ( 1996 ) 17. [ 49 ] K.J. Reynolds, L.A. Colon, J. Liq. Chromatogr. Relat. Technol. 23 ( 2000 ) 161. [ 50 ] D. Wang, S.L. Chong, A. Malik, Anal. Chem. 69 ( 1997 ) 4566. [ 51 ] J.D. Hayes, A. Malik, Anal. Chem. 72 ( 2000 ) 4090. [ 52 ] J.D. Hayes, A. Malik, Anal. Chem. 73 ( 2001 ) 987. [ 53 ] S.A. Rodriguez, L.A. Colon, Chem. Mater. 11 ( 1999 ) 754. [ 54 ] S.A. Rodriguez, L.A. Colon, Appl. Spectroscopy 55 ( 2001 ) 472.
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Recent trends in analytical applications of organically modi¢ed silicate materials Maryanne M. Collinson*
Department of Chemistry, 111 Willard Hall, Manhattan, KS 66506-3701, USA Organically modi¢ed silicate (ORMOSIL ) materials prepared by the sol-gel process have become an attractive ¢eld of study due to the versatility and £exibility associated with this method of preparation. In this review, the development of analytical applications involving ORMOSILs for the speci¢c case where the individual precursors are covalently bound is described. A brief explanation of sol-gel processing is given followed by applications in the areas of chemical sensing, ion-exchange coatings, chromatography, and templated materials. A speci¢c focus of this article will be on material published in the last 5 years. z2002 Elsevier Science B.V. All rights reserved. Keywords: Sol-gel; Organically modi¢ed silicate; Sensors; Chromatography; Templating
1. Introduction The sol-gel process involves the fabrication of glass-like or ceramic materials through the hydrolysis and condensation of suitable metal alkoxides [ 1 ]. For the preparation of silicate materials, tetramethoxysilane (TMOS ) is one of the most popular alkoxides. This reagent can be hydrolyzed and condensed under relatively mild conditions as indicated below, Hydrolysis: Si OCH3 4 nH2 O!Si OCH3 43n OHn nCH3 OH
3 In a typical procedure, TMOS is mixed with water typically in a mutual solvent such as methanol followed by addition of a catalyst ( e.g. hydrochloric acid ). During the sol-gel formation, the viscosity of the solution gradually increases as the sol ( colloidal suspension of small particles, 1^100 nm ) becomes interconnected through polycondensation reactions to form a rigid, porous network ^ the gel [ 1 ]. Depending on the sol-gel processing conditions (Si:H2 O ratio, type and concentration of catalyst, alkoxide precursors,T ), gelation can take place on the time scale of seconds to minutes to days to months. During drying, alcohol and water evaporate from the pores causing the gel to shrink. Xerogels, or fully dried gels, are signi¢cantly less porous than their hydrated counterparts. From a chemical and materials perspective, sol-gel derived materials have many advantages [ 1^4 ], õ
1
Condensation:
õ
2
*Tel.: +1 (785) 532-1468; Fax: +1 (785) 532-6666. E-mail: [email protected]
and / or
Materials in various configurations (films, fibers, monoliths, powders) can be readily made (Fig. 1). Thin films are most often utilized in chemical sensor applications due to the short pathlength for diffusion. Bulk monoliths have been frequently used in spectroscopic measurements due to the longer optical pathlength. High surface area powders are useful in catalysis applications. Reagents can be readily incorporated in a stable host matrix by simply adding them to the sol prior to its gelation (Fig. 1). Alternatively, reagents can be added by copolymerizing TMOS with organoalkoxysilanes. In some cases, the matrix actually stabilizes the entrapped reagent from photodegradation or caustic solution environments.
0165-9936/02/$ ^ see front matter PII: S 0 1 6 5 - 9 9 3 6 ( 0 1 ) 0 0 1 2 5 - X
ß 2002 Elsevier Science B.V. All rights reserved.
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Fig. 1. The immense £exibility of the sol-gel process. õ
The materials are chemically, photochemically, and electrochemically stable. They are also optically transparent, which is a definite plus for optical characterization.
Materials prepared using sol-gel processing can range from the relatively `simple' inorganic glasses described above to the more chemically complex hybrid composites. The blending of inorganic precursors ( e.g. TMOS ) with organoalkoxysilanes can lead to materials with properties better than those prepared alone [ 5^9 ]. These materials termed ORMOSIL ( organically modi¢ed silicates ) can be prepared by mixing organosilicon precursors of the general formula R43x Si(ORP )x , where R represents the desirable reagent or functional group and x is 1^3, with TMOS, or alternatively alone [ 5^9 ]. Speci¢c functional groups that have been used include CH3 , C2 H5 , C6 H5 , ( CH2 )3 NH2 , ( CH2 )3 SH. Relative to the pure inorganic glasses, ORMOSILs possess many advantages including:
õ
õ
õ
õ
Flexibility of the silica gel can be improved thus enabling thick, crack-free films to be prepared and utilized Specific functional groups can be covalently attached to the silicon^oxygen network thus minimizing its loss in solution Reactive functional groups can be introduced in the matrix which can be subsequently used to anchor molecular recognition groups on the matrix Relative to physically entrapped species, higher concentrations of reagents can be incorporated into the matrix.
Several reviews have been written recently on the design and applications of these novel organic^inorganic hybrid materials, particularly from a materials point of view [ 5^9 ]. This review will focus speci¢cally on the preparation and recent ( within 5 years ) analytical applications of the ORMOSILs for the condition where the organic
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modi¢er is covalently bound to the silica^oxygen network. The interested reader is referred to other articles for information about the preparation and applications of inorganic^organic hybrid materials where the organic component is physically embedded into the inorganic network [ 2,3,5 ] or where organic monomers or polymers are introduced into the inorganic networks [ 5^7 ]. This report is not meant to be a comprehensive compilation of recent applications, but is rather meant to provide the interested reader with an overview of the chemistry and the types of applications that can be obtained when inorganic and organosilicon precursors are combined. Recent examples of the inherent usefulness of these composite materials in the design of materials for chemical sensors, ion-exchange coatings, chromatographic applications, and molecular templating are presented.
2. Applications 2.1. Chemical sensors A widely used approach for the development of chemical sensors involves the immobilization of a reagent into a suitable matrix followed by interconnection of this solid to a suitable transducer. Relative to many organic polymers, sol-gel derived matrices as hosts provide better stability, optical transparency, £exibility, and permeability [ 2^ 4,10 ]. Much of the interest in sol-gel materials as host matrices stems from the fact that reagents of various size and charge can be trapped into a stable host. In addition, the matrix is porous enough to allow external analyte species to diffuse into the network and react with the entrapped reagent. The development of sol-gel derived chemical sensors has blossomed since the mid-1980s. Chelating agents, indicator dye molecules, proteins, enzymes, antibodies, crown ethers, cyclodextrins, and zeolites are some of the many different types of receptors that have been incorporated into the solgel matrix [ 2^4,10 ].
2.1.1. Organic modi¢cation ORMOSILs hold much promise in the development of chemical sensors. They can be used to alter material pore size, hydrophobicity, and £exibility or introduce a speci¢c functional group into the matrix as needed to improve material performance ( response time, leaching rate, etc. ). For example,
pH sensitive dyes have been incorporated into the silica host and used as pH sensors. One of the major problems with these materials is leaching ^ the process where an entrapped reagent diffuses out into solution. It has been shown recently that organic^inorganic hybrid materials can signi¢cantly improve the stability of the sensor. MacCraith and coworkers prepared a silicate matrix from methyltrimethoxysilane (MTMOS ) and showed reduced leaching using bromophenol blue as the colorimetric indicator [ 11 ]. Collinson and Makote hydrolyzed and co-condensed tetraethoxysilane (TEOS ) with either MTMOS, phenyltrimethoxysilane (PTMOS ), or isobutyltrimethoxysilane ( BTMOS ) and observed no signi¢cant leaching of the dye molecules bromocresol green and cresol red [ 12 ]. The improved stability of the encapsulated dye is likely in part due to a change in the porosity of the matrix, stabilization of the dye in the hydrophobic matrix and / or speci¢c dye^functional group interactions [ 11,12 ]. Increasing the hydrophobicity of the silica network by the hydrolysis and condensation of TEOS or TMOS with an organoalkoxysilane is also a bene¢t to some applications. For example, MacCraith and coworkers have shown that ORMOSIL ¢lms prepared by hydrolyzing and co-condensing TEOS with methyltriethoxysilane (MTEOS ) or ethyltriethoxysilane are much better suited for dissolved oxygen sensing compared to ¢lms prepared solely from TEOS [ 13 ]. In this work the sensor response is based on the £uorescence quenching of an entrapped ruthenium complex. They attribute the optimal performance of the sensor due to the increased hydrophobicity of the ¢lm which `reduces water solubility in the matrix and causes the partitioning of oxygen out of solution into the gas phase within the sensing ¢lm' [ 13 ]. Organosilicon precursors can also be used to increase the £exibility of the matrix enabling thick crack-free ¢lms to be prepared and cut [ 14 ]. The lower extent of cross-linking also can improve the response time of the sensor. For example, Saavedra and coworkers have fabricated planar waveguide sensors for gaseous iodine from multicomponent mixtures of diphenyldiethoxysilane, MTEOS, and dimethyldiethoxysilane ( DMES ) [ 15 ]. The phenyl groups formed a charge transfer complex with iodine, thus serving as the basis for chemical sensing. DMES and MTEOS were used to obtain optically transparent ¢lms with higher porosity [ 15 ]. In more recent work, ¢lms prepared from DMES,
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MTEOS and TEOS were used in the chemical sensing of water vapor [ 16 ]. Again the ratio of DMES, MTEOS, and TEOS was chosen to prepare ¢lms that are durable and porous enough to give rise to fast response times and recovery rates [ 16 ]. Kimura and coworkers have also prepared ion sensing ¢lms from DMES and TEOS whereby a ratio of 1:3 TEOS:DMES afforded the highest sensitivity [ 17 ].
2.1.2. Immobilized receptors One speci¢c concern associated with simply physically doping a reagent into the matrix is leaching. While the porosity of the network and the mobility of the encapsulated reagent are essential for chemical sensor applications, it can be detrimental to the development of sensors for biological or environmental analysis because the reagent can leach out of the matrix. Leaching is a signi¢cant concern for small reagents such as electron transfer mediators, dye molecules, and complexing agents. One of the most promising features the ORMOSILs offer is the ability to design and fabricate leak-free chemical sensors by utilization of an organoalkoxysilane that contains the reagent of interest ( see Fig. 2 for examples ). This precursor can be combined with TMOS or used alone to fabricate the silicate matrix [ 18^27 ]. Since the reagent is covalently attached to the silicate framework, it cannot leach out of the host matrix. The one disadvantage of this approach is that the organoalkoxysilane must be synthesized. Covalently entrapped reagents have been used as pH [ 21^23 ], oxygen [ 24,25 ] and ion [ 26,27 ] sensors. For example, Avnir and coworkers have compared the spectral response of three methyl red indicators, one of which was functionalized with a silicon alkoxide functionality (2d ). Compared to methyl red, the alkoxide-functionalized dye's pKi shifted from 5.05 to 3.30 due to the blockage of the carboxyl moiety. The covalently bound indicator showed little to no leaching at high acidity [ 22 ]. Wolfbeis and coworkers have fabricated a pH sensor using an amino£uorescein derivative ( 2e ) as the pH sensitive indicator [ 23 ]. In their study, amino£uorescein was mixed with either 3-( trimethoxysilyl )propylisocyanate ( ICPS ) or ( glycidyloxypropyl )trimethoxysilane ( GOPS ) to form the alkoxysilane-modi¢ed dye. After 12 h, TMOS, alcohol, and acid were directly added to the mixture. The materials thus produced did show some leaching suggesting that not all the dye reacted with ICPS or GOPS [ 23 ].
Fluorescent dye molecules have also been used in the sensing of oxygen. MacCraith and coworkers prepared a ruthenium tris( bipyridine ) alkoxysilane ( 2f ) [ 24 ]. The covalently entrapped dye displays an emission band at 610 nm upon excitation, the £uorescence of which is quenched by the presence of oxygen. The sensors showed a good response to both gaseous and dissolved oxygen and are stated to have enhanced performance to leaching in a variety of solvents [ 24 ]. In another example, Rolison and Leventis prepared a silica aerogel monolith with £uorescent 2,7-diazapyrenium moieties ( 2g ) [ 25 ]. Aerogels in contrast to xerogels are much more porous and thus are potentially more suitable for the fabrication of chemical sensors as the response times would be fast. However, leaching is particularly problematic because of the washing steps that are necessary to replace the pore liquid with a pure solvent prior to supercritical £uid drying. As described in this study, covalent entrapment of the dye into the matrix solves this dif¢culty. They were able to prepare monoliths that showed very fast response times ( 6 8 s ) toward gaseous oxygen [ 25 ]. Leak-free ion sensors have also been prepared by incorporating crown ether alkoxysilane derivatives ( 2h ) in the matrix [ 26 ]. These derivatives were combined with TEOS and diethoxydimethylsilane ( DEDMS ) in a 1:3 ratio. The DEDMS was used to impart £exibility to the matrix as membranes with too little DEDMS were too hard for high sensitivity. The modi¢ed electrode had fairly good selectivity towards K , but somewhat slower response times relative to the physically doped reagents [ 26 ].
2.1.3. Composite materials Sol-gel derived composite electrodes have become increasingly useful for the design of amperometric sensors [ 28^32 ]. These electrodes can easily be made by mixing conductive particles ( e.g. carbon [ 28^32 ] or gold [ 33 ]) with an ORMOSIL sol and the resultant mixture packed into a glass tube or spread on a suitable surface. The ORMOSIL sol is generally made by combining MTMOS with methanol, water, and hydrochloric acid. The use of this organoalkoxysilane precursor ensures that only the outermost surface of the composite electrode is wetted. It is possible to dope the metal^ ceramic composites with a wide variety of reagents to be used in chemical sensing [ 28^31 ]. For example, Sampath and Lev have demonstrated the fab-
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Fig. 2. Selected examples of organoalkoxysilanes used to prepare hybrid composites.
rication of a composite electrode prepared by combining carbon with oxidoreductase enzymes ( e.g. glucose oxidase, lactate oxidase, or L-amino acid oxidase ) both with and without palladium. The doped carbon is mixed with the ORMOSIL sol and an electrode prepared. In this study, the carbon powder provides electrical conductivity, the enzyme is used for biocatalysis, and the palladium used for `electrocatalysis of the biochemical reaction product' [ 29 ]. In another recent example, carbon composite electrodes have been used as oxygen sensors [ 31 ]. The electrodes were made by dispersing carbon powder, cobalt porphyrin or
( palladium or platinum ) in the ORMOSIL sol. According to the authors, the carbon powder provides conductivity, the silica network provides high porosity, and the organometallic or inert metal serves as the catalyst modi¢er [ 31 ]. All four catalyst-modi¢ed electrodes were superior to the unmodi¢ed, blank electrode with the platinummodi¢ed carbon composite electrode exhibiting the best performance [ 31 ]. Another approach for the formation of sol-gel derived composite electrodes involves the direct synthesis of gold and other noble metals in ORMOSIL matrices [ 34,35 ]. In this approach, the ORMO-
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SIL sol is made out of N-[ 3-( trimethoxysilyl )propyl )]ethylene diamine ( EDAS ) or 3-aminopropyltrimethoxysilane ( APS ) [ 34,35 ]. For the preparation of Au nanoparticles, AuCl4 3 was mixed with either APS or EDAS. After hydrolysis and condensation was initiated, the gold salt was reduced using sodium borohydride. Spherical metal nanoparticles of a diameter of 4^6 nm were obtained in the sols and resultant gels. The amine functionalities on the ORMOSIL precursor are believed to stabilize the metal salts before reduction and to cap and prevent aggregation of the metal after reduction [ 34 ]. Similar procedures were utilized for the fabrication of platinum, palladium, and silver sols and gels with the exception that different salts were used [ 35 ].
2.2. Ion-exchange coatings Ion-exchange ¢lms have many applications in analytical chemistry. They can be used to prevent an unwanted substance from reaching an underlying surface or they can be used to preconcentrate an analyte for analysis. Relative to commonly used organic polymers, ORMOSILs provide many advantages for the construction of ion-exchange coatings. The inherent £exibility associated with material preparation and processing is one such advantage. The ability to fabricate thin ¢lms, control material porosity and polarity, and incorporate speci¢c ionexchange sites in the ¢lm enables materials with the ideal response, permselectivity, and ion-exchange properties to be developed. Hsueh and Collinson have prepared permselective and ion-exchange ¢lms by hydrolyzing and cocondensing MTMOS with organosilicon precursors that contain NH3 , COO3 , and COOEt functionalities [ 36 ]. The NH2 precursor can be tricky to work with as it is basic and will cause the immediate condensation of TMOS unless acidi¢ed ¢rst. The ORMOSIL sol was cast on a glassy carbon electrode and the ion-exchange properties evaluated using cyclic voltammetry. Potassium ferricyanide, ruthenium hexaammine, dopamine, and methyl viologen served as the redox probes. When placed in neutral buffer solutions, ¢lms prepared using the NH3 silane ion-exchanged potassium ferricyanide and excluded ruthenium hexaammine. In contrast, ¢lms prepared with the COO3 silane ionexchanged ruthenium hexaammine and excluded potassium ferricyanide [ 36 ]. In more recent work, Wei and Collinson have hydrolyzed and co-condensed 3-APS with either PTMOS, MTMOS,
BTMOS, or TMOS in various ratios [ 37 ]. The goal of this work was to prepare materials with different quantities of ion-exchange sites, different hydrophobicity, and variable porosity. The results showed that both the magnitude and rate of ionexchange can be manipulated by changing the organic modi¢er included in the ¢lms. Films prepared from BTMOS or PTMOS showed the fastest ion-exchange re£ecting higher permeability [ 37 ]. In other studies, a silicon alkoxide modi¢ed with a quaternary ammonium functional group has been incorporated in a silica host and used to sense anions [ 38,39 ]. Chau and coworkers have hydrolyzed and co-condensed N-trimethoxysilylpropylN,N,N-trimethyl ammonium chloride with TMOS to prepare anion-exchangeable ORMOSIL powders and ¢lms [ 38 ]. The materials thus produced were able to ion-exchange potassium ferricyanide and exhibited a selectivity for Cl3 over Br3 and I3 . In another example, tetradecyldimethyl( 3-trimethoxysilylpropyl ) ammonium chloride was combined with DEDMS and TEOS and the resultant sol coated on a glass membrane surface [ 39 ]. The resultant anion-selective glass electrode showed a Nernstian response to Cl3 activity in a wide activity range. The anion selectivity essentially obeys the Hofmeister series.
2.3. Molecular templating Template-based approaches have been used to prepare materials that show selectivity for certain analytes in solution [ 40,41 ]. In this procedure, a polymeric network is assembled around a suitable template molecule, which upon removal yields diffusional pathways and / or microcavities with a speci¢c size, shape, and / or chemical functionality in the cross-linked host. These `molecularly designed cavities' show an af¢nity for the template molecule over other structurally related compounds. Organoalkoxysilanes have been shown to be very useful in the preparation of templated sol-gel glasses [ 41^ 45 ]. In one approach, organoalkoxysilanes, chosen for their af¢nity toward a template molecule, are combined with TMOS to form a hybrid composite. Collinson and Makote showed the advantages of this approach for the preparation of ¢lms that were selective toward dopamine [ 42 ]. In this work, PTMOS was combined with MTMOS and TMOS. PTMOS was utilized as the functionalized
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silane due to its hydrophobicity and likely af¢nity for the aromatic functionality on the template. MTMOS was incorporated into the sol to introduce additional hydrophobicity and stability to the resultant materials. Dopamine was added to the hydrolyzed sol and the resultant mixture cast as a thin ¢lm on the surface of a glassy carbon electrode. Dopamine was extracted from the dried ¢lm by soaking it in phosphate buffer, and the af¢nity and selectivity of the dopamine templated ¢lms characterized with cyclic voltammetry. The dopamine templated ¢lms had a relatively fast response time and showed an increased af¢nity for dopamine over related molecules. It is believed that ¢lm porosity, electrostatics, and hydrophobicity play a role in governing diffusion and permeation into the ¢lm [ 42 ]. In another approach, the template can be covalently attached to the inorganic framework. For example, a silicon alkoxide derivatized template can be combined with TMOS to form the crosslinked network [ 43,46 ]. The template can then be removed via thermal decomposition or chemical treatments. This approach has been recently described by Maier and coworkers using borneol, fenchol, or camphor as the template [ 43 ]. In this work, the materials were prepared by condensing TEOS and MTEOS with R-Si(ORP )3 where R is the template ( borneol or fenchol ). The templated molecule was removed thermally or via oxidative treatment. In this work, the authors found that the adsorption selectivity for the templated materials was dependent on the speci¢c pore sizes and not necessarily on an `imprint effect'. In addition to the templating of thin silica ¢lms or monolithic gels ( powders ), surface imprinting routes have also been described. Brinker, Shea and coworkers, for example, have recently described a surface imprinting procedure for the creation of `synthetic receptors' for phosphates and phosphonates [ 44,45 ]. In this study, the organoalkoxysilane precursor, 3-trimethoxysilylpropyl1-guanidinium chloride, was condensed onto the surface of a xerogel host prepared from TEOS in the presence of the template, phenylphosphonic acid. Solid state nuclear magnetic resonance (NMR ) was used to de¢ne the binding sites [ 45 ] and high-performance liquid chromatography was used to evaluate the relative af¢nities of the templated materials to the template and related species as a function of pH and ionic strength [ 44 ]. The phenylphosphonic acid templated materials showed an enhancement in the binding constants
for phosphate and phenylphosphonic acid relative to a randomly functionalized polymer [ 44 ].
2.4. Separations Sol-gel chemistry has found many uses in chromatographic science [ 9,41 ]. The development of new stationary phases, in particular, has been a prime area of interest. Direct bonded stationary phases prepared by chemically attaching a monolayer( s ) directly to a silica support are often time consuming to prepare and have limited sample capacity and in some cases stability. The sol-gel approach is relatively simple. Preparation of columns generally involves ( 1 ) cleaning the support to expose the maximum number of silanol functionalities, ( 2 ) preparation of the hybrid sol, ( 3 ) ¢lling the column with the hybrid sol, and ( 4 ) removal of the sol with pressurized gas followed by drying. A number of research groups, most notably Colon and coworkers and Malik and coworkers, have become interested in utilizing the sol-gel method to prepare chromatographic supports for different applications [ 47^52 ]. In recent work, open tubular columns for liquid chromatography have been prepared using a hybrid sol prepared from n-octyltriethoxysilane ( C8 -TEOS ) and TEOS. The mole ratio of the precursors was varied to adjust the retention and selectivity of the columns. Anthracene derivatives were used as model analytes to evaluate column performance. Plate numbers of 200 000 theoretical plates / m were reported [ 47 ]. In another study, 3APS was used as the precursor to fabricate a hydrolytically stable glass coating for capillary electrophoresis. The magnitude and direction of the electroosmotic £ow were altered by modifying the charge on the column through changes in the pH of the electrolyte buffer solution. These coatings were shown to be ef¢cient in the separation of basic biomolecules [ 48 ]. In a more recent report, submicron-sized organosilica spheres for capillary electrochromatography have been prepared by the copolymerization of C8 -TEOS with TEOS. The number of theoretical plates in a column packed with 450 nm particles varied between 370 000 and 480 000 for 4-pyrene derivatives [ 49 ]. In related studies, the hydrolysis and condensation of the individual silicon precursors as well as the composition of the sol used to coat the chromatographic columns were studied with 29 Si and 1 H NMR [ 53,54 ]. The reaction kinetics of the individual
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precursors in hybrid mixtures can vary signi¢cantly and produce supports with different properties depending on the time at which the sol was used. The primary goal of this research was to identify the precursors present in the sol at the time the column is coated for silica sols prepared from TEOS, C8 TEOS, C8 -TEOS /TEOS [ 53 ], and most recently for C18 -TEOS and TEOS / C18 -TEOS mixtures [ 54 ]. By doing this, it may be feasible to design better coatings with improved ( or optimized ) chromatographic ef¢ciencies. In both studies it was found that the rates of hydrolysis for C8 -TEOS and C18 TEOS were slower than that observed for TEOS. In mixtures of TEOS:C8 -TEOS and TEOS:C18 TEOS their rates increase. The maximum degree of condensation correlated with maximum chromatographic retention for the test compounds [ 53,54 ].
2.5. Conclusion ORMOSILs have a promising future in analytical science. They can be used to change the porosity, hydrophobicity, and £exibility of silicate sol-gel derived glasses. They can also be used to chemically attach a suitable reagent to the inorganic framework. A diversity of different scientists have been attracted to this ¢eld with numerous applications in the areas of chemical sensing and chromatography being reported. The future would continue to see the use of ORMOSILs in analytical applications. In addition, trends toward unraveling the complexities associated with hybrid preparation, the nature of entrapment, leaching of entrapped guests, and the formation of controlled homogeneous and heterogeneous composites will likely be at the forefront of science.
Acknowledgements Support of the author's work in this ¢eld by the National Science Foundation ( CHE ) and the Of¢ce of Naval Research is gratefully acknowledged.
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