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A novel and versatile approach to electrichemically triggering the release of a reagent, β-cyclodextrin (β-CD), selectively to the proximal leaflet of a supported lipid bilayer is described. Selective delivery is achieved by creating a spanning lipid bilayer across a microcavity array and exploiting the irreversible redox disassembly of the host-guest complex formed between thiolated ferrocene (Fc) and β-cyclodextrin (β-CD) in the presence of chloride. Self-assembled monolayers of the ferrocene-alkanethiols were formed regioselectively on the interior surface of highly ordered 2.8 μm cavities while the exterior top surface of the array was blocked with a monolayer of mercaptoethanol. The Fc monolayers were complexed with β-CD or β-CD-conjugated to streptavidin (β-CD-SA). Phospholipid bilayers were then assembled across the array via combined Langmuir-Blodgett/vesicle fusion leading to a spanning bilayer suspended across the aqueous filled microcavities. Upon application of a positive potential, ferrocene is oxidized to ferrocinium cation, disrupting the inclusion complex and leading to the release of the β-CD into the microcavity solution where it diffuses to the lower leaflet of the suspended bilayer. Disassembly of the supramolecular complex within the cavities and binding of the β-CD-SA to a biotinylated bilayer was followed by voltammetry and impedance spectroscopy where it caused a large increase in membrane resistance. For unmodified β-CD, the extraction of cholesterol from a cholesterol containing bilayer was evident in a decrease in the bilayer resistance. For the first time, this direct approach to targeted delivery of a reagent to the proximal layer of a lipid bilayer offers the potential to build models of bidirectional signaling (inside-out vs outside-in) in cell membrane model systems.

Thin films of the adduct formed from the electrostatic association of the metallopolymer, [Ru(bpy)(2)(PVP)(10)](2+), and the Dawson polyoxomolybdate α-[Mo(18)O(54)(SO(4))(2)](4-), POMo, have been formed on ITO electrodes using an alternate immersion approach. The Ru/POMo ratio is 4.5:1, which exceeds the 2:1 ratio expected on the basis of the charges of the Ru(2+) and POMo(4-) building blocks. This behavior arises because of the polymeric character of the cation. In the presence of a substrate that has an abstractable proton such as benzyl alcohol, these ruthenium-sensitized polyoxomolybdate films generate significant photocurrents under visible irradiation. Significantly, increasing the surface coverage of the adduct from 1.4 × 10(-10) to 8.1 × 10(-10) mol cm(-2) does not measurably increase the photocurrent observed. Scan-rate-dependent cyclic voltammetry reveals that the rate of homogeneous charge transport through the film is slow, which most likely results in only a fraction of the film thickness being active for photoelectrocatalysis. The photocurrent increases markedly when the driving force for the oxidation of POMo(5-), created by the photoelectrocatalytic oxidation of benzyl alcohol, is increased. This result is consistent with the dynamics of heterogeneous electron transfer being centrally important to the regeneration of the photoelectrocatalyst. A system in which the surface coverage and applied overpotential are optimized produces a photocurrent density of 190 ± 18 nA cm(-2) under 480 ± 5 nm irradiation.

Water-soluble 4-(dimethylamino) pyridine (DMAP) stabilized gold nanoparticles (DMAP-AuNP) were synthesized by ligand exchange and phase transfer (toluene/water). The DMAP-AuNPs are positively charged with the core diameter of 4 +/- 1 nm. Metallopolymer-gold nanocomposites were prepared by mixing gold nanoparticles and [Ru(bpy)(2)PVP(10)](ClO(4))(2), in water at different mole ratios; bpy is 2,2'-bipyridyl and PVP is poly (4-vinylpyridine). The photoluminescence emission intensity of the metallopolymer decreases with increasing AuNP loading and approximately 57% of the emission intensity is quenched when the Au NP:Ru mole ratio is 14.8 x 10(-2). The rate of homogeneous charge transfer through thin layers of the nanocomposite deposited on glassy carbon electrodes increases with increasing nanoparticle loading. The homogeneous charge transport diffusion coefficient, D(CT), for the composite (AuNP:Ru mole ratio 13.2 x 10(-2)) is (2.8 +/- 0.8) x 10(-11) cm(2) s(-1) and is approximately 3-fold higher than that found for the pure metallopolymer. Significantly, despite the ability of the metal nanoparticles to quench the ruthenium-based emission, the electrochemiluminescence of the nanocomposite with a AuNP:Ru mole ratio of 4.95 x 10(-2) is approximately three times more intense than the parent metallopolymer. This enhancement arises from the increased rate of charge transport that leads to a greater number of excited states per unit time while minimizing the quenching effects. The implications of these findings for the design of electrochemiluminescent sensors are discussed.

The electrochemically induced desorption of Oregon green labeled fibrinogen layers from clean gold surfaces at negative potentials has been probed using capacitance, fluorescence microscopy, and atomic force microscopy. Capacitance measurements on fibrinogen layers indicate that desorption occurs at potentials more negative than -0.8 V and that complete desorption occurs when the electrode is biased at -1.2 V. Significantly, the fluorescence intensity initially increases as the dye labeled protein is electrochemically desorbed due to a decrease in quenching by the gold surface. Following this initial increase, the protein diffuses into solution and the fluorescence intensity decreases over time. More than 90% of the dye labeled fibrinogen is desorbed and diffuses out of the confocal volume in less than 2000 s when the potential is stepped to -1.2 V. AFM before and after application of the desorbing potential confirms removal of the protein. Collection of the desorbed protein in solution reveals a surface coverage of (4.0 +/- 2.3) x 10(-13) mol cm(-2) or an area of occupation of 400 +/- 140 nm(2) per molecule, which indicates that the protein is not extensively spread on the bare gold surface. Significantly, SDS-PAGE analysis indicates that the adsorption-desorption cycle dramatically effects the protein structure, with the electrochemically desorbed fibrinogen showing extensive fragmentation compared to native protein.

The electrochemical and photophysical properties of a luminescent metal center, [Os(bpy)(3)](2+), are significantly modified by encapsulation within a conducting polymer composite film. Cyclic voltammetry reveals that the encapsulation in an inherently conducting polymer, polyaniline (Pani) or polypyrrole (PPy), can dramatically influence the charge-transfer rates between the metal centers. The increased electron transport, most likely mediated through the conducting polymer backbone, significantly enhances the electrochemiluminescence (ECL) efficiency. The increased communication between adjacent metal centers can also result in other interesting properties, such as photoinduced electron-transfer processes. In situ electron spin resonance (ESR) spectroscopy has been used to probe the photo-oxidation of an osmium metal center encapsulated in a PPy composite film. The irradiation of PPy in the presence of the osmium metal center resulted in the photo-oxidation of the Os(2+) to Os(3+) state and the consequent reduction of the PPy polyelectrolyte. The degree of communication between luminescent metal centers allows the composite properties to be tuned for various applications including ECL sensor devices and light-switching and light-harvesting systems.

Monolayers of di-6A,6B-deoxy-6-(4-pyridylmethyl)amino-gamma-cyclodextrin (gamma-CD-(py)2) have been formed on polycrystalline platinum electrodes and investigated using electrochemical and surface-enhanced Raman spectroscopy (SERS). The behavior of self-assembled monolayers of (gamma-CD-(py)2) alone, (gamma-CD-(py)2) backfilled with 1-nonanethiol, and 1-nonanethiol are reported. The potential dependence of the capacitance indicates that the film capacitance is higher for the backfilled CD layers than for 1-nonanethiol layers, most likely due to ion flux through the CD cavity. SERS spectra of the backfilled layer exhibit features associated with both pyridine-functionalized CD and alkane moieties. Investigations using [Fe(CN)6]4- as a solution-phase probe indicate that the backfilled CD-alkane thiol layer exhibits enhanced blocking properties compared to gamma-CD-(py)2 films alone. Complete blocking was achieved by a combination of backfilling and insertion of a high-affinity guest 1-adamantylamine into the cavity. Significantly, an electroactive guest with high affinity for gamma-CD, [Co(biptpy)2]2+, does not exhibit a redox response at the gamma-CD-(py)2 layer but molecular recognition is turned on by backfilling the CD layer with 1-nonanethiol molecules. This switching on of the electrochemical activity suggests that the CD hosts are initially inaccessible but reorientate upon backfilling, exposing the CD opening to solution and permitting a supramolecular host-guest complex to form. The binding of [Co(biptpy)2]2+ to gamma-CD in the backfilled monolayer depends on the bulk concentration of guest and is modeled by the Langmuir isotherm, yielding an association constant for the Co2+ state of 1.45 +/- 0.46 x 105 M-1 and a limiting surface coverage 1.49 +/- 0.25 x 10-11 mol cm-2. The surface coverage of the divalent state is higher than the trivalent state, reflecting the dynamic nature of the inclusion.

Dense monolayers of [Ru(dpp)2Qbpy]2+, where dpp is 4,4'-diphenylphenanthroline and Qbpy is 2,2':4,4' ':4'4' '-quarterpyridyl, have been formed by spontaneous adsorption onto clean platinum microelectrodes. The cyclic voltammetry of these monolayers is nearly ideal, and three redox states are accessible over the potential range of +/-1.3 V. Chronoamperometry conducted on the microsecond time scale has been used to probe the dynamics of heterogeneous electron transfer and indicates that the standard heterogeneous electron-transfer rate constant, k degrees , is approximately 106 s-1. The metal complex emits at approximately 600 nm in fluid and solid solution as well as when bound to a platinum electrode surface within a dense monolayer. In the case of the monolayers, it appears that the excited states are not completely deactivated by radiationless energy transfer to the metal because electronic coupling between the adsorbates and the electrode is weak. The dynamics of lateral electron transfer between the electronically excited Ru2+* and ground-state Ru3+ species has been explored by measuring the luminescence intensity after defined quantities of Ru3+ have been produced electrochemically within the monolayer. The rate of lateral electron transfer is between 8 x 106 and 3 x 108 M-1 s-1, indicating efficient electron transfer between adsorbates in close-packed assemblies. Voltammetry conducted at megavolt per second scan rates has been used to directly probe the redox properties of the electronically excited species.

Thin films of the perchlorate salt of an [Os(N,N'-alkylated-2,2'-biimidazole3)2+/3+-containing polymer have been formed on planar platinum microelectrodes. The electrochemical response associated with the Os2+/3+ couple occurs at -0.19 V. In aqueous perchlorate media at near-neutral pH the voltammetric response is close to that expected for an electrochemically reversible reaction involving a surface-confined reactant. Chronoamperometry conducted on a microsecond time scale indicates that the film and solution resistances are comparable for low concentrations of supporting electrolyte. However, for LiClO4 concentrations greater than 0.4 M, RFilm contributes less than 25% of the overall cell resistance. These results suggest that when the film is dehydrated and the density of redox centers is increased, electron or hole hopping dominates the rate of homogeneous charge transport through the film. The rate of homogeneous charge transport, characterized by D(CT)1/2Ceff, where DCT is the homogeneous charge transport diffusion coefficient and Ceff is the effective concentration of osmium centers within the film, depends weakly on the concentration of LiClO4 as supporting electrolyte decreasing from (8.1 +/- 0.16) x 10(-9) to (4.7 +/- 0.4) x 10(-9) mol cm(-2) s(-1/2) as the perchlorate concentration increases from 0.1 to 1.0 M. These values are about 2 orders of magnitude lower than those of the chemically cross-linked chloride salt of the polymer. The rate of heterogeneous electron transfer is unusually rapid in this system and increases from (5.2 +/- 0.4) x 10(-3) to (7.8 +/- 0.4) x 10(-3) cm s(-1) on going from 0.1 to 0.4 M LiClO4 before becoming independent of the supporting electrolyte concentration at (9.2 +/- 0.6) x 10(-3) cm s(-1) for [LiClO4]or = 0.6 M.

Solid particles of a ruthenium dimer, [(Ru(bpy) 2 ) 2 bpzt - ] 3 + , have been abrasively attached to macroelectrodes and microelectrodes, where bpy is 2,2'-bipyridine and bpzt - is 3,5-bis(pyrazin-2-yl)-1,2,4-triazole. The voltammetry of these solids deposits is unusually ideal in NH 4 PF 6 -containing aqueous solutions, and the response is characterized by semi-infinite linear diffusion for scan rates between approximately 50 and 2000 mV s - 1 . SEM imaging reveals that sparse films of solid particles (1-10 μm in diameter) are efficiently transformed into microcrystals by voltammetric cycling. The charge transport diffusion coefficient, D C T , has been determined by systematically varying the voltammetric scan rate. For reduction of the deposits, D C T increases from 2.4 to 3.6 x 10 - 1 0 cm 2 s - 1 as the NH 4 PF 6 concentration is increased from 0.1 to 2.0 M, while, for oxidation of the deposit, D C T increases from 1.1 to 3.9 x 10 - 1 0 cm 2 s - 1 over the same concentration range. The maximum D C T observed would correspond to an electron self-exchange rate constant of 1.1 x 10 5 M - 1 s - 1 . Despite the smaller electron-transfer distance expected within the solid, this apparent self-exchange rate constant is more than 2 orders of magnitude smaller than that typically found for ruthenium bis-bpy complexes in solution. This observation suggests that ion rather than electron transfer may limit homogeneous charge transport through these solid deposits.

Solid deposits of [Os(OMe-bpy) 3 ](PF 6 ) 2 have been formed on a platinum microelectrode, where OMe-bpy is 4,4'-dimethoxy, 2-2'-dipyridyl. The voltammetry associated with the Os 2 + / 3 + redox processes is nearly ideal where the supporting electrolyte is 0.1 M HClO 4 and suggests that the redox centers within the deposits are solvated. Scanning electron microscopy reveals that prior to voltammetric cycling, the deposits exist as an array of microscopic particles. In contrast, after voltammetric cycling in perchloric acid the deposit becomes microcrystalline in the form of thin plates. Electrocrystallization is not observed when the deposits are cycled in neutral electrolyte suggesting that protonation of the methoxy groups plays an important role in the process. Where the supporting electrolyte is HClO 4 , voltammetry conducted under semi-infinite linear diffusion conditions yields a homogeneous charge transport diffusion coefficient, D C T , of 1.5 ′ 0.1 x 10 - 9 cm 2 s - 1 for 0.1 ≤ [HClO 4 ] ≤ 0.6 M. In contrast, above 0.6 M HClO 4 D C T increases significantly reaching 13 ′ 1 x 10 - 9 cm 2 s - 1 in 1.0 M HClO 4 . Analysis of these data according to the Dahms-Ruff equation yields a self-exchange rate constant of 5.7 x 10 6 M - 1 s - 1 . High scan rate voltammetry, 5 ≤ υ ≤ 100 V s - 1 , has been used to determine the standard heterogeneous electron transfer rate constant, k°, as 1.0 ′ 0.05 x 10 - 4 and 6.1 ′ 0.2 x 10 - 5 cm s - 1 for solid and solution phase, respectively.

Monolayers of 1-amino-2-sulfonic-4-hydroxyanthraquinone, 1,2,4-AQASH, have been formed on mercury electrodes by spontaneous adsorption. Films were deposited from solutions containing either quinone or hydroquinone forms where the bulk concentration, CB, was between 0.1 and 5 μM. These films have been used as model systems to probe the effect of intersite separation and redox composition on hydrogen bonding interactions in two-dimensions. Both resonance Raman spectroscopy and surface coverage data indicate that at the open-circuit potential the adsorbates are coplanar with the electrode surface. For surface coverages, Γ, less than approximately 1.4 × 10-10 mol cm-2 (CB ≤ 1.5 μM) monolayers formed from the quinone exhibit close to ideal voltammetric responses with a peak-to-peak splitting of 17 mV being observed. Also, the full widths at half-maximum are 53 and 63 mV for the cathodic and anodic branches, respectively. Capacitance data reveal that the adsorbates interact only very weakly within fully oxidize...

The production of “molecular wires” that promote fastheterogeneous electron transfer between a remote redoxcenter and a metal surface is important for high-speedmolecular electronics applications, e.g., molecule-basedcomputing. One notable strategy in this area is to useconjugatedratherthanaliphaticbridgestoachievefasterelectron-transferrates.Forexample,conjugatedmoleculesof [1,1′:4′,1′′-terphenyl]-4-methanethiol (TP) have beenembedded in self-assembled monolayers of insulating

Photostability, electrochemistry, and monolayers of [M(bpy)2(trans-1,2-bis(4-pyridyl)ethylene)L](+) (M = Ru, Os; L = Cl, H2O)

Single- and two-component monolayers of anthraquinone-2,7-disulfonic acid (2,7-AQDS) and 1-chloro-4-hydroxy-anthraquinone (1,4-AQClOH) have been formed by spontaneous adsorption from low pH supporting electrolyte solutions (typically 1.0 M HClO4 in 80:20 H2O:DMF) and examined using cyclic voltammetry. The voltammetric response for the single monolayers is well defined and the concentration dependence of the surface coverage is well described by the Frumkin adsorption isotherm over the concentration range 1−30 μM for both 2,7-AQDS and 1,4-AQClOH. The saturation coverages (Γsat) are (6.8 ± 0.32) × 10-11 and (1.9 ± 0.15) × 10-10 mol cm-2, respectively, while the adsorption coefficients (β) are (4.9 ± 0.25) × 104 and (7.5 ± 0.5) × 104 M-1. Both monolayers show stabilizing lateral interactions. However, the 2,7-AQDS adsorbates interact more strongly with a Frumkin interaction parameter (g) of −1.62 ± 0.21 being observed compared to −0.21 ± 0.02 for the 1,4-AQClOH monolayers. Mixed monolayers, formed from simul...

Single component monolayers of anthraquinone-2,6-disulfonic acid (2,6-AQDS) or anthraquinone-1,5-disulfonic acid (1,5-AQDS) have been formed by equilibrium adsorption from aqueous 1.0 M HClO 4 onto mercury microelectrodes. The adsorption thermodynamics follow the Langmuir isotherm over the concentration range from 2 x 10 -8 M to 8 x 10 -7 M. The same limiting surface coverage, Γ s (1.0 ± 0.08 x 10 -10 mol cm -2 ), and energy parameter, β (5.5 ± 0.7 x 10 6 M -1 ), are observed for both anthraquinones. The cyclic voltammetry of these single component monolayers is nearly ideal, and the potential dependence of the redox composition follows the Nernst equation with the expected theoretical slope. Microsecond time scale chronoamperometry has been used to probe both the rate of heterogeneous electron transfer to the adsorbed anthraquinone moieties and their surface coverages. Binary monolayers have been formed by simultaneous adsorption of both anthraquinones. A plot of the differential capacitance versus the applied potential exhibits a capacitance minimum at the potential of zero charge, -0.300 V. The film capacitance is estimated to be 30 ± 5 μF cm -2 . The surface pK a of the sulfonic acid groups has been estimated as 2.9 ± 0.5 by measuring the interfacial capacitance as the solution pH is systematically varied. The formal potentials of 2,6-AQDS and 1,5-AQDS are almost identical. Therefore, binary monolayers containing both species exhibit only a single voltammetric peak. Under these circumstances, traditional electroanalytical techniques cannot be used to determine the surface coverages of the individual species. However, in short time scale potential step experiments, three single exponential current decays are separated on a microsecond time scale. These decays correspond to double layer charging and heterogeneous electron transfer to the 2,6-AQDS and 1,5-AQDS redox centers, respectively. This kinetic separation of the Faradaic responses allows the surface coverages of the individual components within the monolayer to be determined. Despite their identical formal potentials, the concentrations of the two anthraquinones in solution have been determined by combining information about heterogeneous kinetics and adsorption thermodynamics.

For spontaneously adsorbed monolayers of [Os(bpy) 2 Cl(p2p)] + , where p2p is 1,2-bis(4-pyridyl)ethane and bpy is 2,2'-dipyridyl, reduction induces rapid loss of the chloride ligand. Cyclic voltammetry and chronoamperometry in DMF reveal that two sequential, one-electron ligand-based reductions lead to exchange of chloride for solvent at the metal center. High speed chronoamperometry has been used to probe the kinetics ofthis coupled electron-transfer chemical reaction. The heterogeneous electron transfer rate constant for the second electron transfer (k), linking charge states 0 and 1-, has been evaluated using nanosecond time scale chronoamperometry. The rate constant depends exponentially on overpotential for absolute overpotentials less than 0.200 V, and transfer coefficients (α) of 0.500 ± 0.08 are observed. The standard heterogeneous rate constant increases with increasing supporting electrolyte concentration over the range 0.01-0.5 M, and a limiting value of 7.8 ± 0.6 x 10 4 s -1 is observed for high electrolyte concentrations. The structure of the monolayer and the potential distribution at the modified interface have been probed from the electrolyte concentration dependence of the interfacial capacitance. These data suggest that, for some electrolyte concentrations, the electric field decays linearly over at least part of the monolayer thickness,leading to a large interfacial electric field at the reaction site. The kinetics of chloride elimination have been probed by monitoring the charge recovered during monolayer reoxidation as the reducing potential triggering chloride ligand loss is held for various times. These studies revealed that chloride elimination is a first-order process that occurs on a microsecond time scale. The rate of chloride loss (k elim ) increases linearly with an increasingly more negative potential, and k elim also increases linearly with increasing square root of the supporting electrolyte concentration. Temperature-resolved measurements of the elimination rate reveal that not only does the activation energy depend on the field strength but so too does the pre-exponential factor.