Sol–gel’s intriguing chemistry, coupled with the ease with which these materials can be made and processed, has attracted the interest of chemists, engineers, physicists, and materials scientists. any novel chemical sensors composite materials is blossoming (1). and solid-state electrochemCompared with commonly used orical devices used today owe ganic polymers, the sol–gel process their existence to the combined veraffords enormous flexibility in terms satility and flexibility of electro- and of the types of materials that can be sol–gel chemistry. Materials and diagprepared, the surfaces they can be nostics research performed at the informed on, and their ion-exchange tersection of these two fields has proproperties. Their fundamentally induced a plethora of new materials and applications triguing chemistry and the ease with which and improved our understanding of the micro- these materials can be made, modified, and proscopic properties of complex cessed have attracted the inhost structures. terest of chemists, engineers, Maryanne M. Collinson Today, the use of sol–gel physicists, and materials scichemistry to prepare inorentists (2–7 ). Annette R. Howells ganic and organic–inorganic From an “analytical” point Kansas State University
M
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of view, the sol–gel process provides a relatively simple way to encapsulate reagents in a stable host matrix (2–7 ). Moreover, sol–gel-derived glasses used as host materials provide better optical transparency, stability, and permeability than many organic polymers. Proteins and enzymes entrapped in silica gel have been used in numerous biological sensing applications, and sol–gel materials doped with organic and organometallic compounds have been utilized as sensors for gases, metals, ions, and pH. Analytical chemists are also studying sol–gel derived materials at the microscopic level to characterize and understand their properties so that new materials and composites with faster responsiveness and even greater sensitivity, selectivity, and efficiency can be produced. Electroanalytical chemistry has played an important role in the development and characterization of sol–gel-derived materials and devices (7 ). Electrons crossing the electrode–gel interface can be determined with good sensitivity
by measuring the current and can be related to species concentration or used to evaluate material conductivity, dopant diffusivity, and reactivity. We will discuss the coupling of electrochemistry and sol–gel processing, offer examples of how sol–gel chemistry can be used to fabricate materials for electroanalytical applications, and demonstrate how electrochemical techniques can be used to characterize the molecular-scale properties of these complex solids.
Sol–gel chemistry Sol–gel technology provides a relatively straightforward way to fabricate glasslike or ceramic materials via the hydrolysis and condensation of suitable metal alkoxides (1). The most popular starting precursors for the fabrication of silica-based materials are tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS). These reagents can be hydrolyzed (Equation 1) and condensed (Equations 2 and/or 3) under relatively mild conditions (room temperature and pressure), as illustrated in the N O V E M B E R 1 , 2 0 0 0 / A N A LY T I C A L C H E M I S T R Y
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material is prepared and dried (1–7 ). From the perspective of materials proH2O the sol–gel approach affords many cessing, R-Si(OR’)3 + Si(OR’)4 ROH advantages. Most notably, materials in variFilm ous configurations can be easily produced. R = CH3, C6H5, A silica sol can be spin-cast or dip-coated CH2CH2CH2SH onto an electrode surface, glass slide, siliCH2CH2CH2NH2 Fiber con wafer, or optical fiber to form a thin film, or it can be poured into a polystyrene cuvette, vial, or glass tube to form a block R monolith (gel, xerogel) (Figure 1a). AlterO O O O natively, monodisperse powders or fibers Si Si O O O Si O Particles R can be fabricated using carefully chosen Si O Si R OH Si Sol sol–gel processing conditions (1–7 ). Thin R O O O R OH O Si films have been frequently used to prepare O Si Si O Si O OR O chemical sensors because of their relatively OH HO Si O O short pathlengths for diffusion, which imSi O O R prove sensor response times and recovery O rates. Bulk monoliths have been frequently Organically modified silicate Monolith used in spectroscopic investigations because of their longer optical pathlengths, whereas high surface area powders are useful for catalysis applications. The sol–gel approach also affords numerous advantages FIGURE 1. Sol–gel derived materials are easy to prepare. from a chemical perspective. Organosilicon precursors of the (a) The sol can be cast or coated onto a substrate to form a thin film or poured general formula R–Si(OR´)3 (R is the desired reagent or funcinto a container to form a monolith. Fibers or particles can be produced from a judiciously prepared sol. (b) Specific reagents can be introduced in a stable host tional group and R´ is CH3 or C2H5), can be hydrolyzed and matrix by physically doping the reagent into the sol prior to gelation or by using co-condensed with TMOS or TEOS to fabricate organorganosilicon precursors. Likewise, the polarity of the host can be modified by ic–inorganic hybrid materials with tailor-made polarity, refrachydrolyzing and condensing organosilicon precursors with a tetrafunctional silicon alkoxide (TMOS). tive index, and flexibility (Figure 1b) (4, 5). Alternatively, they can be used to chemically bond specific reagents or functionalities to the silica framework for developing leak-free chemical following simplified reaction sequence for TMOS (1). sensors, stationary phases for chromatography, and nonlinear devices (4, 5). Because of the relatively mild polymerization Si(OCH3)4 + nH2O ?Si(OCH3)42n(OH)n + nCH3OH (1) conditions used, reagents can also be physically trapped in the (2) ;Si–OH + HO–Si ;?;Si–O–Si ;+ H2O host matrix by doping them into the silica sol before gelation (3) ;Si–OCH3 + HO–Si ;?;Si–O–Si ;+ CH3OH (2–6). It is now well established that molecular species, when encapsulated in wet gels, retain many of the chemical properIn a typical procedure, TMOS is mixed with water in a mutual ties that they exhibit in solution and are accessible to external solvent (methanol) and a catalyst [acid (HCl), base (NH3), or reagents (2–6). nucleophile (F–)] is added. During sol–gel formation, the viscosity of the solution gradually increases as the sol (colloidal Composites for electroanalytical suspension of small particles) becomes interconnected to form applications a rigid, porous structure—the gel. Gelation can take place on a Organic–inorganic hybrid materials. Organically modified timescale ranging from seconds to months depending on the silicates can be used as composite materials for electroanalytical processing conditions (Si:H2O ratio, type and concentration of and bioanalytical applications. For example, ion-exchange and catalyst, alkoxide precursors, etc.). permselective films can be prepared by hydrolyzing and coDuring drying, alcohol and water evaporate from the pores, condensing organosilicon derivatives with acidic or basic sites, causing the matrix to shrink. Xerogels, or fully dried gels, are such as 3-aminopropyltrimethoxysilane (silane-NH3+) or N-[3significantly less porous than their hydrated counterparts. To (trimethoxysilyl)propyl]ethylenediamine triacetic acid (silanemaintain porosity and pore structure, the gels can be supercrit- COO–) with TMOS or another precursor, and coating the reically dried to form an aerogel (1). The surface areas of these sultant sol on electrode surfaces (8). These materials can be materials often exceed 1000 m2/g (1). The physical properties used to prevent unwanted substances from reaching the underof the resultant structure, such as average pore size, pore size lying surface and preconcentrate an analyte for electrochemical distribution, pore shape, and surface area, strongly depend on detection or trace electroanalysis (Figure 2). the sol–gel process parameters and the method at which the Organosilicon derivatives that contain electroactive func-
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tionalities (Figure 3) can be used to introduce redox-active reagents into the inorganic framework (7, 9–16). The immobilized redox molecules can act as an electron shuttle to transfer the electron back and forth between the electrode surface and an encapsulated enzyme, for example. Alternatively, they can be used to study the microscopic properties of the host as it gels and dries. Compared with physically doping the reagent into the sol, the covalent attachment of the reagent to the silica matrix often affords greater stability because leaching (or loss of reagent due to diffusion out of the matrix into solution) is lessened. Slow loss of the electron transfer mediator, for example, can severely affect the overall usefulness of an amperometric sensor because the signal level will change over time and the solution will become contaminated. Polymer–silica composites. Rather than using particular organosilicon derivatives, which must often be synthesized and can be tricky to work with, it is possible to introduce ion-exchange, redox, and conducting polymers into the silica sol before gelation to impart specific properties to the matrix (17–19). By mixing polymers with inorganic hosts, materials with physical and chemical properties superior to either the polymer or the host alone can be produced for a variety of applications. For example, ionomers such as Nafion, poly(vinylsulfonic acid), poly(dimethyldiallylammonium chloride), or poly(styrenesulfonate) can be trapped in a silica sol and used to prepare hybrid polymer–silica ion-exchange films for electrochemical and spectroelectrochemical sensing applications (17 ). An illustration of a cation-exchange polymer–silicacoated electrode is shown in Figure 2a. Alternatively, redox polymers, such as an osmium bipyridine derivative, can be incorporated into the silica framework (18). The fixed redox sites can act as electron transfer mediators to shuttle the electron between an entrapped enzyme and the electrode surface. Carbon–silica composites. Many electrochemical investigations use metal, carbon, and/or doped semiconductor electrodes as the indicators or working electrodes. In recent work, a new class of electrodes was prepared by adding carbon powder to a silica sol and packing the resultant slurry into a glass tube or other suitable container (Figure 2b) (20). In these composites, the carbon powder imparts conductivity to the silica matrix via a percolation mechanism. The surface of the electrode can be easily renewed with mechanical polishing. These electrodes are somewhat analogous to carbon paste electrodes, which use mineral oil or composites prepared using Teflon, Kel-F, or epoxy as the matrix. Carbon–silica composites have good stability, are amenable to modification, and can be used in a variety of different electroanalytical applications (20). For example, when the silica sol is prepared from organosilicon precursors that contain a nonpolar functional group, such as methyl- or phenyltrimethoxysilane, the composite electrode exhibits a hydrophobic surface before and after polishing. Because only the outermost surface is wetted by water, these electrodes can be used as indicator electrodes for amperometric sensing, biosensing, and chromatographic applications (20, 21). In contrast, when the silica sol is prepared from TMOS or an organosilicon precursor that
contains a hydrophilic functional group, such as 2-cyanoethyltriethoxysilane, porous, high-surface-area electrodes can be prepared for electrocatalysis or potentiometric applications (20). Gold–silica composites. Composite electrodes for electrochemical applications can also be prepared using gold particles rather than carbon as the conducting media. For example, nanosized gold particles can be directly synthesized in an amine-modified silica sol (22). The amine functionality coordinates to the gold nanoparticles after formation, thus preventing aggregation. Analogous to carbon–silica composite electrodes, micrometer-sized gold powder can be dispersed into a silica sol, and the resultant slurry can be packed into a cavity and allowed to dry (23). The voltammetric characteristics of the gold–silica composite electrodes are similar to those of conventional gold electrodes, but the composites are advantageous because they can be easily modified (doped with various reagents) and used in a variety of applications (23).
Ion, gas, and neutral species sensors Potentiometric sensors. One method for preparing potentiometric sensors for K+, Cl–, and Na+ involves the incorporation of specific ionophores (tridodecylmethylammonium chloride) or neutral carrier agents (crown ethers) into a silica sol (24–26). These carrier agents can be introduced into the silica framework by physical doping into a hybrid sol or via covalent bonding through alkoxysilylated derivatives. The resultant sol can be poured on a flat surface and then cut into membranes to mount on electrodes, or it can be spin-cast on the surface of an ionsensitive field-effect transistor. These materials have good selectivity and biocompatibility, particularly relative to commonly used poly(vinyl chloride) membranes (26). Electrogenerated chemiluminescence (ECL). ECL-based sol–gel sensors have been described for oxalate and various aliphatic tertiary amines (27, 28). In these studies, the luminescence produced upon the reaction of gel-entrapped ruthenium(II) tris(bipyridine) (Ru(bpy)32+) with a strong reductant is related to the concentration of the analyte in solution. A simplified reaction scheme is Ru(bpy)32+ ?Ru(bpy)33+ + e2 Ru(bpy)33+ + reducing agent ?[Ru(bpy)32+]* [Ru(bpy)32+]* ?Ru(bpy)32+ + hn (610 nm)
(4) (5) (6)
Because the reagents are generated electrochemically rather than through photoexcitation, this technique provides two distinct advantages over fluorescence—a simplified optical system and little background emission or scattered light. Ru(bpy)32+ can be immobilized on the electrode surface in different ways. The simplest method involves physically doping Ru(bpy)32+ into a silica sol and then spin-casting the doped sol on the electrode surface. This method is not ideal, however, because Ru(bpy)32+ diffuses out of the matrix upon oxidation, leading to a continual decrease in the luminescence. More viable approaches for the entrapment of Ru(bpy)32+ involve overcoating procedures or polymer–silica composites (27, 28).
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In one method, a Ru(bpy)32+-modified chitosan complex is prepared and cast on the surface of a platinum disk electrode. A silica sol is then cast on top of the chitosan-coated electrode and dried (27). Another method involves preparing Nafion–silica composite films on glassy carbon electrodes (28). Ru(bpy)32+ is immobilized into the film by ion exchange after immersing the Nafion–silica-modified electrode in a dilute solution of Ru(bpy)32+. The composite materials are more sensitive than pure Nafion films and have lower detection limits for the determination of oxalate and tripropylamine (28). Solid-state amperometric gas sensors. Amperometric sensors for gas-phase analytes such as ammonia, carbon dioxide, hydrogen peroxide, and oxygen have been prepared by spin-coating the sol (vanadium oxide or silica) on the surface of an interdigitated microelectrode array (29–31). Linear sweep voltammetry, cyclic voltammetry (CV), and differential pulse voltammetry have been used to characterize the oxidation of gas-phase analytes at the solid-state sensor. Flow injection amperometry at a constant applied potential has also been used to evaluate sensor performance and stability. Another approach for the fabrication of a gas-phase oxygen sensor with applica-
FIGURE 2. Common configurations used in electroanalytical applications and investigations. (a) The silica sol can be doped with ion-exchange polymers and cast as a thin film on an electrode surface and used to selectively uptake cations for electroanalysis. (b) Carbon powder can be added to the silica sol and the resultant mixture packed in a glass tube to construct an indicator electrode. (c) The silica sol can be doped with a receptor (crown ether) and cast as a thin film on an electrode surface and used in the electrochemical sensing of the ion of interest. (d) In the sandwich configuration, a silica sol is first cast on an electrode surface followed by enzyme and then a second layer of the sol. (e) Alternatively, the electrode can be placed into a doped silica sol prior to gelation to study the microscopic properties of the silicate matrix by measuring (e) current, (f) luminescence, or (g) impedance.
tions to fuel cells is based on carbon–silica composite electrodes (32). An inert metal (palladium or platinum) or an organometallic (cobalt porphyrin) catalyst modifier is incorporated in the composite, and CV is used to evaluate the electrocatalytic reduction of oxygen at the catalyst-modified electrodes compared with blank electrodes (32).
Bioanalytical applications The entrapment of biomolecules in a silica sol–gel matrix and their use in chemical sensing applications have blossomed during the past decade (3). Research has shown that proteins and enzymes can be entrapped in a random orientation in sol–gelderived glasses while still maintaining their native properties and reactivities (3). In contrast to entrapped small molecules, such as indicator dyes, essentially no leaching or loss of the biomolecule occurs, an advantage that extends the lifetime and usability of these devices. Much of the interest in bioentrapment stems from the fact that many enzymatic reactions that occur in solution can be accomplished in the pores of the silica host (3). In some cases, the sol–gel-derived matrix can actually stabilize the biomolecule and protect it from denaturation under extreme conditions, such as exposure to high temperature, high pH, and organic solvents. Successfully encapsulating biomolecules requires more stringent control over sol–gel processing conditions than that required for the simple doping of indicator dyes (3). Conventional sol–gel procedures employed in electroanalytical methods often use relatively high concentrations of alcohol and/or strong acids, which cause many proteins and enzymes to become partially or fully denatured. As a result, the silica sols are usually prepared without adding alcohol and often with a high water-to-silicon ratio. Before adding the protein, the silica sol is usually chilled, and buffer is added to increase the pH of the sol (3).
Composites (a)
Electrochemical sensing
(b)
C+
(c)
(d) Thin film
C+ C+
Electrode C+
Glass tube
Electrode Electrode
X
C+
Sol–gel film
C+
E n z y m e
Silica–polymer composite C+ = Electroactive cation
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Sandwich configuration
X
GOx Of all the enzymes and proteins encapsulated in sol–gel-derived glasses, glucose oxidase (GOx) has been the most popular, because it is inexpensive, stable, well characterized, and medically important. This enzyme catalyzes the oxidation of glucose with oxygen to form gluconic acid and hydrogen peroxide. Electrochemical methods have been frequently used to monitor enzyme activity and quantify glucose in solution (7, 33). Sol–gel-derived amperometric biosensors have been prepared in a variety of ways in several different configurations to measure glucose (Figure 2d). Sandwich, microfabricated, and screen-printable approaches have been demonstrated, as has the use of silica–polymer, silica–gold, and silica–carbon composite electrodes (18, 22, 23, 33–37). In the screen-printing approach, the “ink” is prepared by mixing graphite powder (or palladium–graphite powder), the enzyme, and a binder into a silica sol (37 ). The electrodes were then prepared with standard screen-printing protocols but with low-temperature curing. Microfabricated miniaturized electrode sensors have many applications in clinical and biomedical areas, particularly those that involve constrained environments or small sample sizes (36). In the sandwich configuration, multiple layers are coated on an electrode surface and the enzyme (Figure 2d). Typically, the silica sol is cast on an electrode surface, then the GOx is deposited, and the second sol–gel layer is cast on top (34, 35). Conventional sol–gel processing procedures can be used to prepare the sol without denaturing the enzyme, because the enzyme is never added to the sol. The top silica layer can be used as a “protection” layer to stabilize and prevent the enzyme from dissolving in solution. In addition, the response time of the sensor can be greatly reduced by controlling the thickness of the top silica layer, and electron transfer mediators can also be trapped in the bottom film to minimize or eliminate leaching. The electrochemical oxidation of hydrogen peroxide pro-
duced by the enzymatic reaction has been frequently used to assess the reactivity of GOx and serves as the basis for chemical sensing. CV has also been a useful tool for qualitatively assessing the presence of hydrogen peroxide. The relatively high overvoltage that is required to oxidize hydrogen peroxide, however, makes the sensor more susceptible to interferences such as ascorbic acid and urate, which are commonly present in biological samples. Electrochemical mediators (both freely diffusing and immobilized) have also been frequently used to reduce the dependence of the signal on molecular oxygen (which can be important in biological tissue, where the oxygen concentration can be low) and the operating potential (7, 33). Ferrocene, ferrocene derivatives, tetrathiafulvalene, and osmium bipyridine derivatives have been successfully used as electrochemical mediators in sol–gel-derived biosensing schemes for determining glucose (7, 33). In the simplest method, the mediator is doped into the sol along with the enzyme before it is deposited on an electrode surface. Alternatively, the mediator can be doped into the sol and cast on the electrode surface, which could be followed by adding GOx and a thin layer of sol (Figure 2d). The one major disadvantage of utilizing freely diffusing mediators is that they are often small enough to readily diffuse from the pores of the matrix upon prolonged use. Using organically modified silanes (R–Si(OR´)3, in which R is an electron transfer mediator) or redox polymers can circumvent this problem (9). Another route that has been taken involves directly wiring the electron transfer mediator (ferrocene) to the enzyme and then doping the enzyme–mediator complex into the silica sol (38).
Electrochemical characterization Electrochemical methods, including CV, chronoamperometry (CA), ac impedance, and ECL, have also been used to probe
Material characterization (e)
(f)
V
(g) V
i
Electrodes
Sol–gel Gel containing Ru(bpy) 32+
Sol–gel
M R O
i → f(D, C, n)
Impedance → Rb
i → f(D, C) hn
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the physical and chemical properties of sol–gel-derived materials (7, 39–47 ). In these experiments, the gel matrix essentially serves as a solid electrolyte, providing a medium for the movement of redox species and ions. Typically, an electroactive species is incorporated into the gel matrix by physically doping it into the sol or by using organosilicon precursors (Figure 3). The doped silica sol can be spin-cast on the surface of an electrode and then inserted into an electrolyte solution (42–44), or the working and auxiliary/reference electrodes can be incorporated directly into the monolith (39–41, 46, 47) (Figure 2c). Information about the gel structure, the mobility of the entrapped species, and the extent of intermolecular interactions can be obtained by monitoring the electrochemical response of the entrapped species during the gelation, aging, and drying processes. The bulk resistance (conductivity) of undoped silica gels has been measured by ac impedance (45). These experiments were performed by pouring the silica sol into a polystyrene cuvette containing two opposite walls coated with gold and measuring the cell impedance as a function of time (Figures 2e–g) (45). In ac impedance, the response of the cell to small oscillating perturbations in applied potential is measured and then modeled by an equivalent circuit. Information that can be readily obtained from such measurements includes bulk resistance, charge transfer resistance, and capacitance across the electrode interface which shed light on gel microstructure and bulk conductivity. CV and CA have been used to characterize the diffusion coefficient D of the redox probes trapped in hydrated sol–gel-derived materials (39–41). The rate at which an entrapped reagent diffuses within the solid matrix, as well as into and out of the
matrix, is an important factor to consider when designing materials to be used in chemical sensor or solid-state devices. Variations in measured values of D for different redox probes can provide additional insight into the extent of surface interactions between the entrapped reagents and the walls of the silica host. In these investigations, the Faradaic current that flows when a redox species undergoes electron transfer at the electrode– gel interface provides a direct measure of the apparent diffusion coefficient of the electroactive species (39–41). Relative changes in the diffusion of the entrapped molecules during the sol–gel transformation can be readily followed by measuring the current required to oxidize or reduce the electroactive reagent (39–41). In these investigations, both large macroscopic electrodes and ultramicroelectrodes (radius ~10–50 µm) have been used (39–41). Ultramicroelectrodes have many advantages over larger electrodes for determining D in sol–gel-derived solids. Their small size minimizes the cracking of the gel around the electrodes upon drying. Their reduced double-layer capacitance enables high sweep rates to be used, allowing the gel–electrode interface to be probed. Because electrochemical probing of the diffusion coefficient of redox molecules trapped in dried gels is limited by the mechanical cracking of the gel, it is especially important to minimize cracking and verify the integrity of the gel–electrode interface (39). Using ultramicroelectrodes also provides a relatively simple means for calculating D without prior knowledge of solution concentration, which is important because the concentration of the entrapped reagent increases as the gel dries due to solvent evaporation and subsequent gel shrinkage. In this procedure, the chronoamperometric response obtained at the ultramicrodisk electrode is normalized by the steady-state limitingcurrent, and D is obtained by fitting the experimental data to an equation (39). Most of these studies have shown that al-
FIGURE 3. Examples of electroactive organosilicon precursors.
Ferrocene derivatives (Refs. 11–13)
Polymerizable derivatives (Refs. 7, 16) N–(CH2)3Si(OCH3)3
Fe
N+
(CH3O)3Si
S
Si(OCH3)3
(CH2)n—Si(OCH3)3
Viologen derivative (Ref. 14)
n = 3.8
2+ (CH3O)3Si—(CH2)3
Si(OCH3)3
N
N
(CH2)3—Si(OCH3)3
Fe Si(OCH3)3
Redox dyes (Refs. 10, 15) N
O CH2 Fe
(CH3O)3Si NH–(CH2)3Si(OCH3)3
N
N N
N S+ Br-
N
Si(OCH3)3
(CH3O)3Si Si(OCH3)3
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Ru
N N
N
NH(CH2)3Si(OEt)3 NH(CH2)3Si(OEt)3
though the gel is macroscopically rigid, it is microscopically fluid, and many reagents diffuse in the hydrated monolith at rates similar to that measured in solution (39–41). The electrochemistry inside these hydrated, partially dried gels is similar to that observed in organic hydrogels. ECL also provides a way to characterize the diffusion and reactivity of redox probes trapped in silica gel solids. The ECL of Ru(bpy)32+ trapped in gels prepared from TMOS, organic– inorganic hybrids, and silica–polymer composites has been examined (46–48). The magnitude of the luminescence produced when electrogenerated precursors diffuse together and react in the gel can provide an indirect measure of the mobility of entrapped reagents (46–48). Geometric restrictions and the presence of surface interactions between the walls of the silica host and entrapped species can reduce the diffusivity and hence the ECL magnitude relative to that observed in solution under a similar set of conditions (46, 47 ). In fact, large differences have been observed in the relative ECL obtained for the Ru(bpy) 32+/C2O 422 and Ru(bpy) 32+/aliphatic amine systems trapped in a silica gel. This indicates the differences in the mobility and reactivity of the immobilized reagents (46, 47 ). The modification of the surface of the gels with specific functionalities (via the use of organosilicon precursors) leads to ECL enhancement through changes in polarity, porosity, and intermolecular surface interactions.
Where do we go from here? Given the ability of electrochemistry and sol–gel chemistry to be easily integrated into small spaces, the future will likely see a push toward miniaturization with applications in environmental and in vivo analysis. Electroanalytical chemistry will also likely continue to play an equally important role in evaluating how the interfacial structure influences the translational mobility and reactivity of gel-entrapped reagents so that materials with improved properties can be designed and fabricated. Support of the author’s work in this field by the National Science Foundation (CHE) and the Office of Naval Research is gratefully acknowledged. Maryanne M. Collinson is associate professor and Annette R. Howells was a postdoctoral research associate at Kansas State University. Collinson’s research interests include electroanalytical chemistry, sol–gel chemistry, microelectrodes, electrochemiluminescence, and microporous/mesoporous materials. Howells’ research interests include electrocatalysis and photochemistry. Address correspondence about this article to Collinson at Department of Chemistry, Kansas State University, Manhattan, KS 66506-3701 (
[email protected]).
References (1) (2) (3) (4) (5) (6)
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