Report
DesigningInterfacesat the Molecular Level
T
he creation of organized thin films (OTFs) of organic compounds has developed into an increasingly important research area at the frontier of analytical chemistry (1-4). This importance stems from the critical role of liquid-solid and gas-solid interfaces in a host of emerging transduction mechanisms and from the relatively high degree of structural definition afforded by OTFs. In the case of liquid-solid interfaces, modification of an electrode surface can transform a comparatively nonselective electron-transfer process into one with an enhanced specificity based on the ionic charge (5, 6), hydrophobicity or hydrophilicity (7), size ( 2, 8-11), and identity (12) of an electroactive species. In the case of gas-solid interfaces, OTFs provide opportunities for creating interfaces that have well-defined composition, thickness, spatial orientation, and packing density and can be used to gain control of specificity and the extent of interactions between an analyte and a modi-
C h u a n - J i a n Zhong and Marc D. Porter Iowa State University 0003-2700/95/0367-709A/$09.00/0 © 1995 American Chemical Society
Organized thin films can be used to create interfaces for manipulating reactions between analytes and modified surfaces fied surface. These same attributes impart OTFs with many of the characteristics of model interfaces that can be used to explore the fundamental issues related to the predictive design of interfaces with target performance specifications. In this Report we present an overview of the development and application of OTFs to analytical chemistry. We will discuss the origins and preparative methodologies of Langmuir-Blodgett (LB) and spontaneously adsorbed (SA) films, the two most used routes toward the creation of OTFs. We will also examine the struc-
tural attributes of OTFs that can be used to clarify the issues related to transduction mechanisms that are important to analytical chemistry and to designing interfacial architectures with advanced functions. We highlight a few of the many uses of OTFs, beginning with fundamental studies of interfacial processes, followed by emerging applications to chemical analysis. Reviews of applications of OTFs in other areas provide additional fundamental and technological information (1-4). Preparative methods The underlying phenomenon that led to the creation of LB films was discovered many decades before the phenomenon of SA films (2). The first scientific experiments with LB films examined the calming effect of an oil dispersed on water and were reported more than 200 years ago by Benjamin Franklin. It was Langmuir, however, who spearheaded the early systematic investigations of organic films at the air-water interface. Blodgett subsequently developed techniques for the transfer of these films to solid surfaces. These pioneering studies focused primarily on the preparation and transfer of
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densely packed structures of long-chain fatty acids (e.g., barium stéarate and cadmium arachidate). Advances since then have expanded the scope of transfer modes and organizational formats and addressed issues related to compression and transfer mechanisms, the role of the substrate, the construction of mixedcomponent films, and film stability. The fabrication of LB films (2) relies on the formation and transfer of a single layer of amphiphilic molecules spread at the air-water interface, the surface area of which is slowly shrunk to yield a film with a defined molecular packing density. Both vertical and horizontal lifting techniques can then be used to transfer the preorganized layer onto a smooth substrate. The vertical transfer procedure, shown in Figure la, is used to prepare both hydrophobic and hydrophilic OTFs. The horizontal transfer method, in which the substrate "touches" the top of a preorganized layer, is used primarily for the construction of hydrophilic OTFs. In both cases, multilayer structures are formed by repetition of the transfer process. The origins of SA films can be traced largely to the classic wettability studies of Zisman (2). Today, there are three general preparative strategies, all of which rely on the immersion of an appropriate substrate into a dilute solution of an adsorbate precursor. The first strategy is based on the chemisorption of long-chain carboxylic acids at metals with native oxides (e.g., copper, aluminum, silver, and chromium) (2). These systems constitute the platforms used in the noted wettability studies. The second strategy uses organosilane-based monolayers linked to hydroxylated surfaces. The advantages of this technique were first recognized by Sagiv as a route to the construction of oleophobic monolayers with a controllable molecular organization (2). This approach has recently been extended to the formation of multilayer structures by Mallouk (1) by using metal phosphonates as bridges between layers. The third strategy, which has gained a strong foothold in many research areas (2, 3) besides analytical chemistry, is based on organosulfur-derived monolayers chemisorbed at gold (2, 13, 14), a few other metals (3), and semiconductors. The immense interest in this adsorbate-
substrate combination, which takes advantage of the high preferential affinity of gold for sulfur-based ligands, was triggered largely by the report of Nuzzo and Allara on the formation of functionalized disulfides at gold substrates (2). As depicted in Figure lb, the formation of SAfilmsis initialized by the strong affinity of the head group of the adsorbate for the substrate. For example, the chemisorption of long-chain carboxylic acids at metal oxides proceeds through the formation of the corresponding metal carboxylate salt (2). On the other hand, silanes (e.g., RSiX3, R2SLX2, or R3SiX, where R is an alkyl chain and X is a chloro or alkoxy group) are tethered to a hydroxylated surface by a Si-0 bond and can polymerize into 2-D cross-linked networks using RSiXo precursors (1,2). Sulfur-containing compounds (thiols, disulfides, and sulfides) chemisorb at gold as the corresponding thiolate. In these situations, cleavage of S-H, S-S, and S-C bonds is central to the formation process (14). In all cases, the packing of the OTF results from the cohesive interactions between the alkyl chains and is influenced by chain length, end group, solvent, immersion time, substrate morphology, temperature, and several other factors (15). The vast interest in applying OTFs has been prompted by the ease of preparing interfaces with well-defined composition, thickness, and spatial orientation in comparison, for example, to the morphological and compositional heterogeneity at the surface of a polymeric film. The LB and SA approaches to the construction of OTFs have relative strengths and weaknesses that should be considered within the context of the proposed application. The LB technique provides precise control of surface concentration and can be used to modify most smooth surfaces. Preparative flexibility, however, is limited to structures derived from amphiphilic molecules. Durability may also be an issue when structural stability is dominated by the cohesive interactions between neighboring chains. The SA preparation method is more straightforward; it yields a surface structure that, through changes in the end group, can be used to modify the composition of surfaces regardless of size and shape. The types of samples address-
7 1 0 A Analytical Chemistry, December 1, 1995
(a) a> 2
S
Air
ί
3 •Λ
Aqueous phase (b) Liquid
Substrate
End group Organic chain Head group
Figure 1 . Idealized depictions of film formation. (a) Vertical mode LB film transfer of a precompressed amphiphilic layer of molecules at an air-water interface to a solid substrate. (b) Formation of an SA monolayer at a substrate from a solution containing adsorbate precursors.
able are nevertheless defined by the specificity of the interactions between the head group and the substrate. In addition, the ability to control the surface composition by co-assembly depositions is more empirical in nature because of nonideal thermodynamic correlations between surface and solution composition (16). In both types of preparations, the existence of phase-segregated domains and the importance of molecular cooperativity remain significant but unresolved issues. Models and applications
Electroanalytical chemistry was one of the first areas to take advantage of the structural definition of OTFs, which have been used as models for probing fundamental issues of heterogeneous electron transfer (17-27) and electrical double-layer theories (13,19,20) and as platforms for creating selective barriers to electron transfer (9-12). There are three distinct but potentially overlapping pathways for electron transfer
of a solution-based redox species at insu lating OTFs: permeation through the film, discharge at structural defects, and elec tron tunneling. Because defects (e.g., pin holes, adsorbate vacancies, and gauche kinks) represent weaknesses in barrier properties that may degrade perfor mance, many early explorations focused on assessing the structural integrity of OTFs (13, 21). It was found, for exam ple, that the increase in the length of the polymethylene chain in alkanethiolate monolayers at gold effectively increased the "dielectric thickness" of the OTF (13). In one instance, this system appeared to be effective as a barrier in the delineation of key parameters in Marcus theory (e.g., tunneling coefficient and reorganization energy) using solution-based redox cou ples (18). In contrast, the packing limita tion imposed by the larger relative size of the siloxy head group relative to the alkyl chain yields a more disordered chain structure that proved less effective as a barrier for such evaluations (21). Another approach to a critical assess ment of issues relevant to heterogeneous electron-transfer kinetics involves the use of multilayer systems (1) and the use of redox species pendant to thiolate-based monolayers (17, 23-27). This strategy pins down the movement of the redox species, precluding possible complica tions from the electron transfer of a solu tion-based species at defects. Further processing by dilution of the redox sites within an inert methyl-terminated chain structure eliminates contributions from lateral electron-transfer processes. By comparison, studies of OTFs at the airwater interface have proven effective in delineating phenomena involved in lateral electron-transfer processes (22). These and related strategies could serve as via ble frameworks for probing the role of through-space or through-bond mecha nisms and the importance of donoracceptor separation distance, tempera ture, electrolyte, and solvent. Systems based on OTFs have also been used as starting points for unraveling the details of how the microscopic environ ment surrounding a redox site influ ences reactivity (3,23-27). The goal is to understand the predictive translation of reactivity from the 3-D nature of gases and liquids to the lower dimensionality of in
terfaces. In one series of studies, coassembled monolayers were fabricated from a mixture of ferrocenyl- and methylterminated alkanethiols (22). Variations in the immediate environment around the ferrocenyl moiety, through changes in the chain length of the methyl-terminated component, markedly altered the thermo dynamics of the redox process. Thus the apparent reduction potential of a "buried" ferrocenyl moiety was shifted nearly 0.3 V positive of that for the same moiety ex posed to the electrolytic solution. This is attributed to the increased difficulty in the generation and solvation of the posi tive charge created through the oxidation of the ferrocenyl group in a strongly hydrocarbon-like environment. In a related study (26), an electrochem ical quartz crystal microbalance investiga tion probed the importance of the solva tion and identity of the counterions on the
electron propagation pathways can be im parted to the surface structure and whether the resulting materials can func tion as wires or other components in mo lecular-based electronic devices. Some of the more notable pathways have used LB or SA films with pendant charge-transfer complexes, such as tetrathiafulvalene and tetracyanoquinodimethane, metallophthalocyanines and metalloporphyrins (2, 28), and rigid rod oligoimides (29). In most of these cases, as well as in efforts to fabricate 2-D networks of conducting polymers (e.g., polypyrrole [30]), π-π in teractions play an important role in the "stacking" of structures with directed conduction pathways. Opportunities for advancing the design of electrocatalysts by OTFs have also been explored. Single- and multilayer films from an amphiphilic derivative of a Ni(II)containing cyclam were constructed by LB methods at a glassy carbon electrode for the electrocatalytic reduction of CO^ (31). The potency of the solution form of the catalyst was successfully translated to the surface-bound species. Similarly, vari ous Co (II) porphyrins were derivatized with alkanethiol appendages and anchored at gold by SA methods to probe both the stability and orientational aspects (32) of the efficiency for the two-electron catalytic reduction of dioxygen to hydrogen per oxide. These findings will guide the syn thetic design and formation of improved catalysts by OTFs of cofacial porphyrins in which the four-electron reduction of di thermodynamics of the electrochemical transformation of monolayers formed from oxygen has been demonstrated using adsorbed layers on carbon electrodes. a viologen species functionalized on one end with an alkyl chain and on the other Approaches to gain control over the with a thiol-containing appendage. System specificity of reactions have also been pur atic changes in the length of the chains at sued to enhance chemical discrimination both ends of the viologen species revealed in electrochemically based transduction that the smaller the separation distance mechanisms. Strategies range from con between the electrolytic solution and re trol based on surface hydrophobicity to dox site, the easier the solvation. The the exploitation of molecular recogni larger solvation stabilized the dicationic tion phenomena. Design issues center on form of the redox species, driving the the incorporation of a discrete set of one-electron generation of the viologen physical and chemical properties into the monocation to more negative applied po interfacial structure while minimizing the tentials. consequences of defects. Approaches for architectures that have The interfacial properties of OTFs are pathways for vectorial electron and charge dominated by the chemical identity of the conduction constitute another area in end group (1, 2). Thus methyl-termi which the structural features of OTFs are nated OTFs are strongly hydrophobic, and valuable (28-30). At issue are whether carboxylic acid-terminated OTFs are an anisotropic orientation of charge or strongly hydrophilic. These properties can
Electroanalytical chemistry was one of thefirstareas to take advantage ofOTFs.
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be used to manipulate the selectivity of electroanalytical measurements. In one ad aptation, the selectivity of an amperometric detector for LC was modified using methyl-terminated alkanethiolates at gold (7). Because the hydrophobicity of this sys tem increases with the length of the alkyl chain, the preferential permeation of hydro phobic analytes such as chlorpromazine and promethazine through the longer chain structures provided a basis for discrimina tion against hydrophilic analytes such as ferrocyanide and hydrogen peroxide. Concepts using electrostatics consti tute a complementary route to electroana lytical selectivity by OTFs (5, 6). For in stance, the charge generated by the ioniza tion of the carboxylic acid terminus of alkanethiolates at gold can be used to dif ferentiate between ascorbic acid and do pamine (6). At physiological pH, dopa mine is positively charged, whereas ascor bic acid, which is present at much higher concentrations in neurological fluids, is negatively charged. Thus electrostatics diminish the electrooxidation of ascorbic acid and enhance that of dopamine. This approach is similar to that used with elec trodes coated with much thicker films of the anionic polymer Nation (3). How ever, the use of a single molecular layer as the surface modifier should improve re sponse times and detection limits, enhanc ing performance for in vivo applications. The complexation of analytes at OTFs with chelating end groups is yet another route for improving electroanalytical selec tivity. In one particularly intriguing exam ple (5), an indirect signal amplification process was used to enhance detection. This process derives from the creation of access channels to the electrode surface that result from the complexation of an analyte such as Ca(II) with the end groups at lipid multilayer membranes (e.g., didodecyl phosphate) prepared by LB meth ods. These channels appear to form be cause of a combined complexationelectrostatic-induced conformational change in the structure of the mem brane. Channel formation, and hence the presence of an analyte, is detected indi rectly by the voltammetric response of a redox species such as ferrocyanide. When the turnover of the redox species is greater than the amount of analyte bound to the lipid membrane, the observed cur
rent represents an amplified analytical sig nal. Structural derivatizations using OTFs have also had an impact in protein electro chemistry (25, 33, 34). One of the earli est ventures in using SAfilmswas the en hancement of electron-transfer rates for cytochrome c at the interface formed by the chemisorption of bis (4-pyridyl) disul fide at gold (33). By comparison, the
voltammetric response of cytochrome c at uncoated gold is irreversible. The poor re versibility is attributed to an unfolding of the protein upon adsorption at uncoated gold and the improved reversibility to the adsorption of cytochrome c in its native state at this and other compositional vari ants of SA-modified electrodes (25). Studies of other forms of interfacial re activity have used the structural defini-
Microporous nylon membrane
(a) Gold coating
Capture antibody
(b)
Ε Ρ
E< A
*V
p;
E
£
re;
Ρ
Ε
£•
Θ"
Figure 2. An electrochemical enzyme immunoassay. (a) The capture antihuman chorionic gonadotropin (hCG) antibody immobilized by the derivatization of an SA-formed monolayer of thioctic acid on a gold coating supported on a microporous membrane, (b) The capturing process of the immobilized antibody toward a protein Ρ (hCG) in the sample in the presence of an enzyme-labeled conjugate antibody E. (c) The enzy matic reaction upon substrate diffusion from the back side of the membrane through pores and electrochemical detection of the resulting product, aminophenol. (Adapted from Reference 38.)
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tion of OTFs. Many of these investigations have worked toward assessing interfacial acid-base reactivity, with the goal of ad dressing issues related to more compli cated processes. Although it is clear that immobilization can induce large changes in acid-base reactivity (e.g., decreases in effective acid dissociation constants as large as ΙΟ3 ), the relative contributions of interfacial dielectric effects and intermolecular interactions have yet to be untan gled. Efforts that model the double-layer structure (19, 20), coupled with system atic correlations between structure and re activity (23-27), promise to place such issues on firmer experimental and theoret ical grounds. The development of transduction path ways for optical, piezoelectric, and other forms of chemical sensors have taken sev eral creative pathways using OTFs (1). For example, combining the molecular in formation that can be obtained from Ra man spectroscopy with the unique interfa cial properties of OTFs poses a particu larly attractive avenue of exploration. To this end, a partition-based interface was created to detect aromatic compounds by surface-enhanced Raman spectroscopy (SERS) (35). This approach used a SAformed monolayer of octadecanethiolate at a roughened silver surface, the resulting structure of which reversibly extracted ar omatic organic compounds (e.g., o-, m-, and />-xylene) from aqueous solutions. The response of the sensor was internally cal ibrated against a vibrational mode of the alkanethiolate monolayer, a scheme that begins to address a long-standing barrier to Raman spectroscopic measurements in sensor applications. The eventual viabil ity of this approach, which should have a clear impact on environmental monitoring needs, is strongly tied to the successful development of field-deployable hardware. The surfaces of piezoelectric and other novel forms of sensor devices have been modified with OTFs (1). As in the electro chemical studies, differences in hydrophobicity-hydrophilicity, donor-acceptor, and complexation properties have been manipulated through changes in the end group to affect differences in the sorption of gaseous analytes (1, 36), Although the development of interfacial structures with a high degree of specificity remains an obstacle, approaches based on the design
of sensor arrays and multivariant data anal ysis are tractable pathways toward imple mentation (1). Advanced applications Interest in improving specificity has driven the design of OTFs with more complex architectures, leading to new analysis con cepts and variations of existing schemes. OTFs provide a platform from which an enormous number of schemes can be de vised to exploit a complex superposition of chemical and physical interactions for af fecting selectivity. In part, advances have focused on protocols for the synthesis of OTFs by adding sulfur- and silane-containing appendages to target moieties and by subsequent derivatization of the re sulting layer. In the latter case, the carboxylic acid terminus of a long-chain alkanethiolate at
function" interfacial materials. In an early study, an amperometric sensor for glu cose was constructed through the coimmobilization of the enzyme glucose oxi dase (GOx) and a redox mediator 2-aminoethylferrocene (AF). The linkages to gold were formed through a SA layer from 4-aminothiophenol that was subse quently activated with gluteraldehyde for cross-linking with the amine functional groups on both GOx and AF. Thus the two key components for an amperometric assay of glucose were affixed to the elec trode surface. Equations 1-3 summarize the cooperative reaction pathway. The current for the regeneration of the oxi dized form of AF (Equation 3) serves as the analytical signal (39). GOx(oxidized) + glucose -» GOx (reduced) + gluconolactone
(1)
GOx(reduced) + AF(oxidized) -> GOx(oxidized) + AF(reduced)
(2)
OTFs are emerging AF(reduced) -> AF(oxidized) + e~ (3) In another particularly intriguing as an important scheme, an integrated structure based on the stepwise coupling of a capture anti factor in the body to a thioctic acid-derivatized, microporous gold membrane was used to de miniaturization of velop an enzyme immunoassay for pro teins (Figure 2) (38). The concept the analytical exploits the binding of an analyte protein Ρ that was previously incubated with an laboratory. enzyme-labeled conjugate antibody Ε to gold can be transformed to its acid chlo ride by treatment with thionyl chloride (37). This reaction is more effective when performed in a dry gaseous environment and reduces the possibility that the start ing material will reform. Derivatizations have also taken advantage of the many coupling agents (e.g., l-ethyl-3-(3-dimethylaminopropyl) carbodiimide [38] ) used in biochemical labeling. Each of these processes can be used to build a more complex interfacial structure through ester and amide linkages. The relative ease of subsequent derivatization is a key factor that has led to the immense interest in SA-based OTFs. Armed with these synthetic tools, re searchers have created a variety of in creasingly complex transducer interfaces. In the detection of biomolecules, efforts have often entailed the fabrication of "dual
the capture antibody. Adding the enzyme substrate then generates an electroactive product by the immobilized enzyme that is amperometrically detected at the gold membrane. The performance of this system was competitive with that of existing methods and, more importantly, could be applied to assays of whole blood without loss of performance. By using a related pathway, SA films of a small syn thetic peptide modified with a long alkanethiol appendage were tested as in terfaces for the specific recognition of pro teins (40). Detection by surface plasmon resonance spectroscopy revealed both the high specificity and the high affinity of an antibody-antigen reaction, but with a more preferential reversibility. The possible advantages of the recogni tion properties found in solution-based guest-host chemistry has attracted the at tention of several research groups. Routes
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using both SA and LB methods for the organization of immobilized supramolecular molecules such as cyclodextrins (4, 8) and recesorcin[4]- arènes (36) have been explored. Two key features inherent in such architectures are that the "receptor" site must be oriented to facilitate accessibility by the analyte, which is required for efficient binding, and the layer must be effectively devoid of defects, which is critical to minimize interference signals from processes that may occur at structural defects. Recognition by receptor sites provides opportunities for devising detection strategies for a wide range of analytes. Ionic selectivity has been achieved by the construction of a two-component monolayer formed from octadecanethiol as the "sealing" component and 2,2'-thiobis(ethylacetoacetate) as the "ion receptor" component (12). The tetradentate ligand binds divalent cations such as Cu(II) but not trivalent cations such as Fe (III), which do not complex with the ligand in the monolayer. A monolayer of the sulfide alone failed to form a layer sufficiently compact to discriminate against the electrochemical reduction of metal ions other than Cu(II), a finding attributed to imperfections in the interfacial structure. Codeposition with the long-chain component resulted in the successful formation of a layer with ion-selective sites embedded within an effectively defect-free, inert matrix. These issues have also been addressed in efforts to translate the selectivities of other types of LB- and SA-derived receptor structures to electrochemical detection formats (1, 5, 9, 12). The use of receptor-type surfaces to enhance selectivity for piezoelectric sensors has also been pursued. Supramolecular structures have been formed from recesorcin [4] arènes (36), whereby longchain dialkyl sulfides were appended to cause immobilization at gold-coated quartz crystal microbalances. An impressive level of discrimination was found in responses upon the exposure of gaseous analytes (e.g., tetrachloroethylene and trichloroethylene) to surfaces modified with the receptor species in comparison to a series of control structures. Although it is likely that the response reflects the collective uptake of analytes at both receptor and defect sites, these re-
sults, along with those from parallel investigations with SA films of cyclodextrins (9), will undoubtedly serve as a springboard for more intensive studies. In view of the recent evidence for S-C bond cleavage in the formation of monolayers of related systems (14), the descriptions of the structures in the tetrachloroethylene and trichloroethylene examples using sulfidebased precursors may need refinement to assess the contributions of receptor and defect sites to the analytical response. Another attractive route toward control through size and shape selectivity is deliberately incorporating molecularly sized channels within an impermeable OTF support structure (2). The process, as shown in Figure 3, is based on the co-assembly of a template species and a skeleton species at the surface (8,10,11). The function of the template is to imprint a molecularly sized footprint within the 2-D structure of the skeleton. Thus, removing the template creates sites within the framework of the skeleton that have a defined size and shape. Many of the first explorations of this concept focused on evaluating the discrimination capabilities of "skeletonized monolayers" that used long-chain alkyl silanes as impermeable skeletons and cyaninc dyes, porphyrin macrocycles, and other large-sized moieties as templates (2, 8). When SERS was used for detection, imprints formed from zinc tetraphenylporphyrin recognized several structurally similar compounds, including the free base and metallated versions of the template, tetrapyridylporphyrin and protoporphyrin IX (8). The templated structure impressively rejected slightly larger species such as magnesium phthalocyanine and bacteriochlorophyll «. Novel separation techniques based on this concept, using both monolayer and multilayer formats, have recently been described (1). Electroanalytical chemistry has taken an interesting parallel to the above approach for improving specificity. The use of LB techniques, along with ubiquinone as a template within an impermeable octadecanethiol-octadecanol skeleton, yielded a molecularly sized access channel to an underlying electrode (11). At low ubiquinone surface concentrations (< lCr15 mol/cm 2 ), the voltammetric response to the redox probe RU(NH : ,)H + exhibits a shape diagnostic of a random distribu-
714 A Analytical Chemistry, December 1, 1995
Template component
+
Skeleton component
Removal
Refill
Recognition
Recognition
Recognition Analytes
OTF Substrate Figure 3. Pathways for the preparation of "skeletonized" monolayers from OTFs.
tion of ~ 1 nm access channels. Similar types of integrated structures have been constructed by using SA methods (8) in which templates were defined by coassembly from a solution containing shortchain template precursors such as thiophenol and a long-chain skeleton precursor such as octadecanethiol. The use of OTFs has also had an important impact on LC stationary phases (41). Traditionally, the construction of chemically modified stationary phases has relied on the reaction of silanes at the hydroxylated surfaces of silica to form an interface of functional groups tethered through Si-O-Si linkages. These stationary phases, however, are hydrolytically unstable under extremely acidic and basic elution conditions. Although several approaches have been successfully designed to minimize this complication, problems related to separation efficiency, reproducibility, and synthetic complexities have limited their usage (41). Recently, a strategy exploiting the formation of a horizontally polymerized 2-D network of a mixed alkylsilane OTF was devised that promises to overcome these complications (41). The procedure entails
the reaction of trichlorosilanes of two dif ferent alkyl chain lengths (i.e., octadecyltrichlorosilane and methyltrichlorosilane) at the surface of silica particles primed with a single adsorbed layer of water. There are several interrelated attributes of this strategy. First, the 2-D cross-linking of both components of the OTF effectively blocks access to the underlying siloxy link ages. Second, steric barriers to the forma tion of a densely packed structure, which is critical to hydrolytic stability, are greatly reduced by the presence of the second, much shorter, chain component. Third, the loosely packed chain structure provides an effective free volume for the partition ing of analytes at the surface of the station ary phase. The combined weight of these structural features significantly improves hydrolytic stability and separation reproduc ibility. Prospectus Limitations to realizing the full potential of OTFs are caused by complications from structural defects. To date, most attempts to overcome the deleterious effects of structural defects have at best been mar ginally successful. We believe this situa tion mandates a reexamination of the fac tors giving rise to defects by considering film formation and/or transfer mecha nisms. A second obstacle is inherent in the continued development of synthetic protocols for the integration of more com plex chemical functions into the surface structure for analyte selectivity. These more complex functions range from ex panding size and shape selectivity of the template structures to adding advanced chemical or optical gating mechanisms to extending receptor-based processes. Success in extending receptor-based pro cesses depends on construction of a dura ble interfacial structure that predictively translates reactivity. OTFs are also emerging as an impor tant factor in the trend toward "miniatur ization of the analytical laboratory" be cause of their high-resolution spatial pat terning (3, 42). Recent and impending breakthroughs in reducing the key com ponents for bioassays and electrochemi cal, piezoelectric, and chromatographic hardware will undoubtedly take advantage of the many features of OTFs. Electrochemically based manipulations of such
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