Kinetics of interfacial electron transfer at single-layer graphene electrodes in aqueous and nonaqueous solutions.

We present a catalog of electron transfer mediators for investigating the heterogeneous electron transfer kinetics of large-area, single-layer graphene electrodes. Scanning electrochemical microscopy (SECM) was used to probe the apparent standard electron transfer rate constant of mediators in aqueous solutions and in acetonitrile and dimethylformamide, allowing for studies of graphene electroactivity at different potentials and in both aqueous and nonaqueous media. In aqueous solution, iron(III) ethylenediaminetetraacetic acid, hexacyanoruthenate(II), hexacyanoferrate(II), hexacyanoferrate(III), octacyanomalybdate(IV), cobalt(III) sepulchrate, and hydroxymethylferrocene exhibited quasi-reversible electron transfer behavior. The electron transfer kinetics of hexaammineruthenium(III), methyl viologen, and tris(2,2'-bipyridyl)ruthenium(II) were found to be reversible in these studies. The electron transfer rate constant of hydroxymethylferrocene and ferrocene, in organic media, was similar to that for hydroxymethylferrocene in water, which, although fast, shows clear kinetic complications that we believe are inherent to graphene. A series of viologens, known to be reversible at metal electrodes, exhibited quasi-reversible electron transfer. For [Co(dapa)(2)](2+), where dapa is 2,6-bis[1-(phenylimino)ethyl]pyridine, in dimethylformamide, the oxidation state of the redox pair investigated affected the observed kinetics. Under similar experimental conditions, the Co(I/II) couple exhibited nearly reversible behavior whereas Co(II/III) had finite kinetics. This behavior was ascribed to the large difference in self-exchange rates for these two processes. To demonstrate the utility of using these mediators for examining graphene electrodes, the kinetics of two mediators with quasi-reversible electron transfer behavior, iron ethylenediaminetetraacetic acid and hexacyanoruthenate, were measured in the presence of a redox-active species [Os(bpy)(2)(dipy)Cl]PF(6), where bpy is 2,2'-bipyridine and dipy is 4,4'-trimethylenedipyridine, adsorbed onto the graphene surface. The kinetics of both mediators were enhanced in the presence of one-hundredth of a monolayer of the osmium complex, showing that even small amounts of impurities on the graphene surface are capable of enhancing the observed kinetics.

Quantification of the surface diffusion of tripodal binding motifs on graphene using scanning electrochemical microscopy.

The surface diffusion of a cobalt bis-terpyridine, Co(tpy)(2)-containing tripodal compound (1·2PF(6)), designed to noncovalently adsorb to graphene through three pyrene moieties, has been studied by scanning electrochemical microscopy (SECM) on single-layer graphene (SLG). An initial boundary approach was designed in which picoliter droplets (radii ~15-50 µm) of the tripodal compound were deposited on an SLG electrode, yielding microspots in which a monolayer of the tripodal molecules is initially confined. The time evolution of the electrochemical activity of these spots was detected at the aqueous phosphate buffer/SLG interface by SECM, in both generation/collection (G/C) and feedback modes. The tripodal compound microspots exhibit differential reactivity with respect to the underlying graphene substrate in two different electrochemical processes. For example, during the oxygen reduction reaction, adsorbed 1·2PF(6) tripodal molecules generate more H(2)O(2) than the bare graphene surface. This product was detected with spatial and temporal resolution using the SECM tip. The tripodal compound also mediates the oxidation of a Fe(II) species, generated at the SECM tip, under conditions in which SLG shows slow interfacial charge transfer. In each case, SECM images, obtained at increasing times, show a gradual decrease in the electrochemical response due to radial diffusion of the adsorbed molecules outward from the microspots onto the unfunctionalized areas of the SLG surface. This response was fit to a simple surface diffusion model, which yielded excellent agreement between the two experiments for the effective diffusion coefficients: D(eff) = 1.6 (±0.9) × 10(-9) cm(2)/s and D(eff) = 1.5 (±0.6) × 10(-9) cm(2)/s for G/C and feedback modes, respectively. Control experiments ruled out alternative explanations for the observed behavior, such as deactivation of the Co(II/III) species or of the SLG, and verified that the molecules do not diffuse when confined to obstructed areas. The noncovalent nature of the surface functionalization, together with the surface reactivity and mobility of these molecules, provides a means to couple the superior electronic properties of graphene to compounds with enhanced electrochemical performance, a key step toward developing dynamic electrode surfaces for sensing, electrocatalysis, and electronic applications.

Amphiphilic peptide-polymer conjugates based on the coiled-coil helix bundle.

Amphiphilic peptide-polymer conjugates can lead to hierarchically structured, biomolecular materials. Because the peptide structure determines the size, shape, and intermolecular interactions of these building blocks, systematic understanding of how the peptide structure and functionality are affected upon implementing hydrophobicity is required to direct their assemblies in solution and in the solid state. However, depending on the peptide sequence and native structure, previous studies have shown that the hydrophobic moieties affect peptide structures differently. Here, we present a solution study of amphiphilic peptide-polymer conjugates, where a hydrophobic polymer, polystyrene, is covalently linked to the N-terminus of a coiled-coil helix bundle-forming peptide. The effect of conjugated hydrophobic polymers on the peptide secondary and tertiary structures was examined using two types of model, coiled-coil helix bundles. In particular, the integrity of the binding pocket within the helix bundle upon hydrophobic polymer conjugation was evaluated. Upon attachment of polystyrene to the peptide N-terminus, the coiled-coil helices partially unfolded and functionality within the bundle core was inhibited. These observations are attributed to favorable interactions between hydrophobic residues with the PS block at the peptide-polymer interface that lead to rearrangement of peptide residues and consequently, unfolding of peptide structures. Thus, the hydrophobicity of the covalently linked polymers modifies the conjugates' architecture, size, and shape and may be used to tailor the assembly and disassembly process. Furthermore, the hydrophobicity of the covalently linked polymer needs to be taken into consideration to maintain the built-in functionalities of protein motifs when constructing amphiphilic peptide-polymer conjugates.

New design of helix bundle peptide-polymer conjugates.

We present a new design of peptide-polymer conjugates where a polymer chain is covalently linked to the side chain of a helix bundle-forming peptide. The effect of conjugated polymer chains on the peptide structure was examined using a de novo designed three-helix bundle and a photoactive four-helix bundle. Upon attachment of poly(ethylene glycol) to the exterior of the coiled-coil helix bundle, the peptide secondary structure was stabilized and the tertiary structure, that is, the coiled-coil helix bundle, was retained. When a heme-binding peptide as an example is used, the new peptide-polymer conjugate architecture also preserves the built-in functionalities within the interior of the helix bundle. It is expected that the conjugated polymer chains act to mediate the interactions between the helix bundle and its external environment. Thus, this new peptide-polymer conjugate design strategy may open new avenues to macroscopically assemble the helix bundles and may enable them to function in nonbiological environments.

Single layer graphene as an electrochemical platform.

Over the past decade, there has been a great deal of interest in graphene with regards to its electrochemical behavior. Previous studies have focused on understanding fundamental processes such as charge transfer and molecular transport at the graphene-electrolyte interface as well as on applications of graphene in electronic, optical, and mechanical systems. We present illustrative examples of large area, single layer graphene platforms for applications such as optical and sensing devices as well as microfluidic systems. Three examples of graphene modified with thin polymer films are discussed. We have explored the use of graphene as an electrochemical platform for surface-generated electrogenerated chemiluminescence (ECL) using poly-[Ru(v-bpy)3](2+), where v-bpy is 4-vinyl, 4'-methyl 2,2'-bipyridine, as a model system. We found that while graphene can sustain ECL conditions, there was film degradation during ECL, as demonstrated by a decrease in ECL intensity upon potential cycling even in the presence of a graphene coating ("graphene blanket"). Using poly 3,4-ethylenedioxythiophene (EDOT), we demonstrate a facile method of fabricating electrochromic electrodes from large area graphene. The oxidation of NADH at graphene was catalyzed using an electrodeposited layer of 3,4-dihydroxybenzaldehyde as an effective redox mediator. In addition, we describe the fabrication and characterization of a microfluidic device based on a solution-gated field effect transistor which was able to detect changes of 60 mV per pH unit change in an inverted cell design. On the other hand, a 29 mV shift in the Dirac point per unit pH change was measured with our microfluidic devices, and a ca. 10% FET conductance change was measured when we continuously changed the pH in solution from 6.91 to 7.64 in the microfluidic channel, demonstrating local microfluidic pH sensing (albeit non-Nerstian) in real time.

Reactivity of monolayer chemical vapor deposited graphene imperfections studied using scanning electrochemical microscopy.

Imperfections that disrupt the sp(2) conjugation of graphene can alter its electrical, chemical, and mechanical properties. Here we report on the examination of monolayer chemical vapor deposited graphene imperfections using scanning electrochemical microscopy in the feedback mode. It was found that the sites with a large concentration of defects are approximately 1 order of magnitude more reactive, compared to more pristine graphene surfaces, toward electrochemical reactions. Furthermore, we successfully passivated the activity of graphene defects by carefully controlling the electropolymerization conditions of o-phenylenediamine. With further electropolymerization, a thin film of the polymer was formed, and it was found to be insulating in nature toward heterogeneous electron transfer processes. The use of spatially resolved scanning electrochemical microscopy for detecting the presence and the "healing" of defects on graphene provides a strategy for in situ characterization and control of this attractive surface, enabling optimization of its properties for application in electronics, sensing, and electrocatalysis.

Electrochemistry of individual monolayer graphene sheets.

We report on the fabrication and measurement of devices designed to study the electrochemical behavior of individual monolayer graphene sheets as electrodes. We have examined both mechanically exfoliated and chemical vapor deposited (CVD) graphene. The effective device areas, determined from cyclic voltammetric measurements, show good agreement with the geometric area of the graphene sheets, indicating that the redox reactions occur on clean graphene surfaces. The electron transfer rates of ferrocenemethanol at both types of graphene electrodes were found to be more than 10-fold faster than at the basal plane of bulk graphite, which we ascribe to corrugations in the graphene sheets. We further describe an electrochemical investigation of adsorptive phenomena on graphene surfaces. Our results show that electrochemistry can provide a powerful means of investigating the interactions between molecules and the surfaces of graphene sheets as electrodes.