Photocurrent generation of biohybrid systems based on bacterial reaction centers and graphene electrodes

Csiki R., Drieschner S., Lyuleeva A., Cattani-Scholz A., Stutzmann M., Garrido J.ADiamond and Related Materials89: 286 – 292. 2018. 10.1016/j.diamond.2018.09.005.

The direct conversion of sunlight into chemical energy via photosynthesis is a unique capability of plants and some bacterial species. Aimed at mimicking this energy conversion process, the combination of inorganic substrates and organic photoactive proteins into an artificial biohybrid system is of a great interest for artificial bio-photovoltaic applications. It also allows to better understand charge transfer processes involved in the photosynthetic chain. In this work, single layer graphene (SLG) and multilayer graphene (MLG) electrodes are used as a platform for the immobilization of reaction centers (RCs) from purple bacteria Rhodobacter sphaeroides, a protein complex responsible for the generation of photo-excited charges.

Electrochemical experiments with graphene electrodes and redox molecules reveal fundamental differences in the charge transfer processes for SLG and MLG films. We demonstrate that both graphene-based materials enable the immobilization of RCs without loss of functionality, attested by a photocurrent generation under illumination with IR-light at a wavelength of 870 nm. Furthermore, we report on the dependence of the generated photocurrent on the applied bias voltage, as well as on the presence of charge mediators in the surrounding electrolyte. This work demonstrates that SLG and MLG are a suitable platform for RC immobilization and subsequent photocurrent generation, suggesting a promising potential for graphene-based materials in bio-photovoltaics. © 2018 Elsevier B.V.

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Understanding the bias dependence of low frequency noise in single layer graphene FETs

Mavredakis N., Garcia Cortadella R., Bonaccini Calia A., Garrido J.A., Jiménez D. Nanoscale10 (31): 14947 – 14956. 2018. 10.1039/c8nr04939d.

This letter investigates the bias-dependent low frequency noise of single layer graphene field-effect transistors. Noise measurements have been conducted with electrolyte-gated graphene transistors covering a wide range of gate and drain bias conditions for different channel lengths. A new analytical model that accounts for the propagation of the local noise sources in the channel to the terminal currents and voltages is proposed in this paper to investigate the noise bias dependence. Carrier number and mobility fluctuations are considered as the main causes of low frequency noise and the way these mechanisms contribute to the bias dependence of the noise is analyzed in this work. Typically, normalized low frequency noise in graphene devices has been usually shown to follow an M-shape dependence versus gate voltage with the minimum near the charge neutrality point (CNP).

Our work reveals for the first time the strong correlation between this gate dependence and the residual charge which is relevant in the vicinity of this specific bias point. We discuss how charge inhomogeneity in the graphene channel at higher drain voltages can contribute to low frequency noise; thus, channel regions nearby the source and drain terminals are found to dominate the total noise for gate biases close to the CNP. The excellent agreement between the experimental data and the predictions of the analytical model at all bias conditions confirms that the two fundamental 1/f noise mechanisms, carrier number and mobility fluctuations, must be considered simultaneously to properly understand the low frequency noise in graphene FETs. The proposed analytical compact model can be easily implemented and integrated in circuit simulators, which can be of high importance for graphene based circuits’ design. © The Royal Society of Chemistry.

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Single-layer graphene modulates neuronal communication and augments membrane ion currents

Pampaloni N.P., Lottner M., Giugliano M., Matruglio A., D’Amico F., Prato M., Garrido J.A., Ballerini L., Scaini D. Nature Nanotechnology13(8): 755 – 764. 2018. 10.1038/s41565-018-0163-6.

The use of graphene-based materials to engineer sophisticated biosensing interfaces that can adapt to the central nervous system requires a detailed understanding of how such materials behave in a biological context. Graphene’s peculiar properties can cause various cellular changes, but the underlying mechanisms remain unclear. Here, we show that single-layer graphene increases neuronal firing by altering membrane-associated functions in cultured cells. Graphene tunes the distribution of extracellular ions at the interface with neurons, a key regulator of neuronal excitability. The resulting biophysical changes in the membrane include stronger potassium ion currents, with a shift in the fraction of neuronal firing phenotypes from adapting to tonically firing. By using experimental and theoretical approaches, we hypothesize that the graphene–ion interactions that are maximized when single-layer graphene is deposited on electrically insulating substrates are crucial to these effects. © 2018, The Author(s).

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Lipid Monolayer Formation and Lipid Exchange Monitored by a Graphene Field-Effect Transistor

Blaschke B.M., Böhm P., Drieschner S., Nickel B., Garrido J.A. Langmuir34 (14): 4224 – 4233. 2018. 10.1021/acs.langmuir.8b00162.

Anionic and cationic lipids are key molecules involved in many cellular processes; their distribution in biomembranes is highly asymmetric, and their concentration is well-controlled. Graphene solution-gated field-effect transistors (SGFETs) exhibit high sensitivity toward the presence of surface charges. Here, we establish conditions that allow the observation of the formation of charged lipid layers on solution-gated field-effect transistors in real time. We quantify the electrostatic screening of electrolyte ions and derive a model that explains the influence of charged lipids on the ion sensitivity of graphene SGFETs. The electrostatic model is validated using structural information from X-ray reflectometry measurements, which show that the lipid monolayer forms on graphene. We demonstrate that SGFETs can be used to detect cationic lipids by self-exchange of lipids. Furthermore, SGFETs allow measuring the kinetics of layer formation induced by vesicle fusion or spreading from a reservoir. Because of the high transconductance and low noise of the electrical readout, we can observe characteristic conductance spikes that we attribute to bouncing-off events of lipid aggregates from the SGFET surface, suggesting a great potential of graphene SGFETs to measure the on-off kinetics of small aggregates interacting with supported layers. © 2018 American Chemical Society.

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On the use of Parylene C polymer as substrate for peripheral nerve electrodes

De La Oliva N., Mueller M., Stieglitz T., Navarro X., Del Valle J. Scientific Reports8 (1, 5965) 2018. 10.1038/s41598-018-24502-z. IF: 4.122

Parylene C is a highly flexible polymer used in several biomedical implants. Since previous studies have reported valuable biocompatible and manufacturing characteristics for brain and intraneural implants, we tested its suitability as a substrate for peripheral nerve electrodes. We evaluated 1-year-aged in vitro samples, where no chemical differences were observed and only a slight deviation on Young’s modulus was found. The foreign body reaction (FBR) to longitudinal Parylene C devices implanted in the rat sciatic nerve for 8 months was characterized. After 2 weeks, a capsule was formed around the device, which continued increasing up to 16 and 32 weeks. Histological analyses revealed two cell types implicated in the FBR: macrophages, in contact with the device, and fibroblasts, localized in the outermost zone after 8 weeks. Molecular analysis of implanted nerves comparing Parylene C and polyimide devices revealed a peak of inflammatory cytokines after 1 day of implant, returning to low levels thereafter. Only an increase of CCL2 and CCL3 was found at chronic time-points for both materials. Although no molecular differences in the FBR to both polymers were found, the thick tissue capsule formed around Parylene C puts some concern on its use as a scaffold for intraneural electrodes. © 2018 The Author(s).

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Uniformly coated highly porous graphene/MnO2 foams for flexible asymmetric supercapacitors

Drieschner S., Seckendorff M.V., Corro E.D., Wohlketzetter J., Blaschke B.M., Stutzmann M., Garrido J.A. Nanotechnology29 (22, 225402) 2018. 10.1088/1361-6528/aab4c2.

Supercapacitors are called to play a prominent role in the newly emerging markets of electric vehicles, flexible displays and sensors, and wearable electronics. In order to compete with current battery technology, supercapacitors have to be designed with highly conductive current collectors exhibiting high surface area per unit volume and uniformly coated with pseudocapacitive materials, which is crucial to boost the energy density while maintaining a high power density. Here, we present a versatile technique to prepare thickness-controlled thin-film micro graphene foams (μGFs) with pores in the lower micrometer range grown by chemical vapor deposition which can be used as highly conductive current collectors in flexible supercapacitors.

To fabricate the μGF, we use porous metallic catalytic substrates consisting of nickel/copper alloy synthesized on nickel foil by electrodeposition in an electrolytic solution. Changing the duration of the electrodeposition allows the control of the thickness of the metal foam, and thus of the μGF, ranging from a few micrometers to the millimeter scale. The resulting μGF with a thickness and pores in the micrometer regime exhibits high structural quality which leads to a very low intrinsic resistance of the devices. Transferred onto flexible substrates, we demonstrate a uniform coating of the μGFs with manganese oxide, a pseudocapacitively active material. Considering the porous structure and the thickness of the μGFs, square wave potential pulses are used to ensure uniform coverage by the oxide material boosting the volumetric and areal capacitance to 14 F cm-3 and 0.16 F cm-2. The μGF with a thickness and pores in the micrometer regime in combination with a coating technique tuned to the porosity of the μGF is of great relevance for the development of supercapacitors based on state-of-the-art graphene foams. © 2018 IOP Publishing Ltd.

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