Pyrene-Appended Boronic Acids on Graphene Foam Electrodes Provide Quantum Capacitance-Based Molecular Sensors for Lactate.

Molecular recognition and sensing can be coupled to interfacial capacitance changes on graphene foam surfaces linked to double layer effects and coupled to enhanced quantum capacitance. 3D graphene foam film electrodes (Gii-Sens; thickness approximately 40 µm; roughness factor approximately 100) immersed in aqueous buffer media exhibit an order of magnitude jump in electrochemical capacitance upon adsorption of a charged molecular receptor based on pyrene-appended boronic acids (here, 4-borono-1-(pyren-2-ylmethyl)pyridin-1-ium bromide, or abbreviated T1). This pyrene-appended pyridinium boronic acid receptor is employed here as a molecular receptor for lactate. In the presence of lactate and at pH 4.0 (after pH optimization), the electrochemical capacitance (determined by impedance spectroscopy) doubles again. Lactic acid binding is expressed with a Hillian binding constant (Klactate = 75 mol-1 dm3 and α = 0.8 in aqueous buffer, Klactate = 460 mol-1 dm3 and α = 0.8 in artificial sweat, and Klactate = 340 mol-1 dm3 and α = 0.65 in human serum). The result is a selective molecular probe response for lactic acid with LoD = 1.3, 1.4, and 1.8 mM in aqueous buffer media (pH 4.0), in artificial sweat (adjusted to pH 4.7), and in human serum (pH adjusted to 4.0), respectively. The role of the pyrene-appended boronic acid is discussed based on the double layer structure and quantum capacitance changes. In the future, this new type of molecular capacitance sensor could provide selective enzyme-free analysis without analyte consumption for a wider range of analytes and complex environments.

Non-covalent π-π functionalized Gii-senseⓇ graphene foam for interleukin 10 impedimetric detection.

Monitoring Interleukin 10 (IL-10) is essential for understanding the vast responses of T-cells in cancer, autoimmunity, and internal homeostasis after physical stress. However, current diagnostic methods are complex and more focused on medical screening rather than point-of-care monitoring. Biosensors based on graphene's conductivity and flexibility are attractive to offer simple single-use and reduced handling. However, oxidation of its carbon lattice to develop functional moieties for biomolecule immobilization cuts down its electronic conductivity potential. In this work, the authors present a microfluidic lab-on-chip device for simple impedimetric monitoring of IL-10 based on graphene foam (GF) flexible electrodes. Graphene's structure was maintained by employing π-π non-covalent functionalization with pyrene carboxylic acid (PCA). Impedimetric measurements could be performed in low ionic strength phosphate-buffered saline (LI-PBS). The PCA-antibody modification showed to endure the incubation, measurement, and washing processes performed in the microfluidic device. Electrode modification and measurements were characterized by, electrochemical impedance spectroscopy (EIS), contact angle, and scanning electron microscopy. From the contact angle results, we found that the wettability of the graphene surface increased gradually after each modification step. Detection measurements performed in the 3D-printed microfluidic device showed a linear response between 10 fg/mL to 100 fg/mL with a limit of detection (LOD) of 7.89 fg/mL in artificial saliva. With these features, the device was used to quantify IL-10 samples by the standard addition method for 10 fg and 50 fg with recoveries between 82% and 99%. Specificity was evaluated towards interleukin 6, TNF-⍺ and bovine serum albumin.

Polymer indicator displacement assay: electrochemical glucose monitoring based on boronic acid receptors and graphene foam competitively binding with poly-nordihydroguaiaretic acid.

The concept of a reversible polymer displacement sensor mechanism for electrochemical glucose monitoring is demonstrated. A pyrene-derivatised boronic acid chemo-receptor for glucose is adsorbed onto a graphene foam electrode. Spontaneous oxidative polymerisation of nordihydroguaiaretic acid (NHG) onto the graphene foam electrode leads to a redox active film (poly-NHG) covalently attached to the boronic acid receptors. Oxidation of poly-NHG frees the boronic acid receptors to interact with glucose from the solution phase, which is detected due to competitive binding when reduced poly-NHG re-binds to the boronic acid functional groups. The sensor shows the anticipated boronic acid selectivity of fructose > glucose. The ratio of charges under the voltammetric peaks for poly-NHG unbound and bound is employed for glucose sensing with an approximately linear analytical range from 1 to 50 mM glucose in aqueous pH 7 buffer. The new methodology is shown to give apparent saccharide - boronic acid binding constants and to work in human serum. Therefore, in the future it could be developed further for glucose monitoring.

New class of metal bound molecular switches involving H-tautomerism.

A potential end-point in the miniaturization of electronic devices lies in the field of molecular electronics, where molecules perform the function of single components. To date, hydrogen tautomerism in unimolecular switches has been restricted to the central macrocycle of porphyrin-type molecules. The present work reveals how H-tautomerism is the mechanism for switching in substituted quinone derivatives, a novel class of molecules with a different chemical structure. We hence reveal that the previous restrictions applying to tautomeric molecular switches bound to a surface are not valid in general. The activation energy of switching in a prototypical quinone derivative is determined using inelastic electron tunneling. Through computational modeling, we show that the mechanism underlying this process is tautomerization of protons belonging to two amino groups. This switching property is retained upon functionalization by the addition of side groups, meaning that the switch can be chemically modified to fit specific applications.

Electrodeposition of palladium onto a pyridine-terminated self-assembled monolayer.

The electrodeposition of Pd onto self-assembled monolayers (SAMs) of 3-(4-pyridine-4-ylphenyl)propane-1-thiol on Au(111) has been investigated by scanning tunneling microscopy. Two schemes are compared: One involves an established two-step procedure where Pd(2+) ions are first coordinated to the pyridine moieties and subsequently reduced in Pd(2+)-free electrolyte. The second deposition routine involves electroreduction in an electrolyte containing low concentration of Pd(2+) which merges both steps and, thus, significantly simplifies metal deposition onto pyridine-terminated SAMs. Both strategies produce identical Pd nanoparticles (NPs) which exhibit a narrow size distribution and an apparent STM height of ∼2.4 nm. The observation of a Coulomb blockade and easy displacement of the nanoparticles in STM experiments evidence deposition on top of the SAM. The NPs are concluded to be essentially spherical. Growth of the NPs is found to be self-limiting since repeating the complexation-deposition cycle increases the density of the nanoparticles rather than their size but only close to full coverage. At high concentration of the Pd(2+) electrolyte, deposition on top of the SAM is impeded by a competitive mushroom-type growth.

Size-selective carbon nanoclusters as precursors to the growth of epitaxial graphene.

The nucleation and growth mechanisms of graphene on Rh(111) via temperature-programmed growth of C(2)H(4) are studied by scanning tunneling microscopy and spectroscopy, and by density functional theory calculations. By combining our experimental and first-principles approaches, we show that carbon nanoislands form in the initial stages of graphene growth, possessing an exclusive size of seven honeycomb carbon units (hereafter labeled as 7C(6)). These clusters adopt a domelike hexagonal shape indicating that bonding to the substrate is localized on the peripheral C atoms. Smoluchowski ripening is identified as the dominant mechanism leading to the formation of graphene, with the size-selective carbon islands as precursors. Control experiments and calculations, whereby coronene molecules, the hydrogenated analogues of 7C(6), are deposited on Rh(111), provide an unambiguous structural and chemical identification of the 7C(6) building blocks.

Coupling epitaxy, chemical bonding, and work function at the local scale in transition metal-supported graphene.

Resonance tunneling spectroscopy and density functional theory calculations are employed to explore local variations in the electronic surface potential of a single graphene layer grown on Rh(111). A work function modulation of 220 meV is experimentally measured, indicating that the chemical bonding strength varies significantly across the supercell of the Moiré pattern formed when graphene is bonded to Rh(111). In combination with high-resolution images, which provide precise knowledge of the local atomic registry at the carbon-metal interface, we identify experimentally, and confirm theoretically, the atomic configuration of maximum chemical bonding to the substrate. Our observations are at odds with reported trends for other transition metal substrates. We explain why this is the case by considering the various factors that contribute to the bonding at the graphene/metal interface.