Understanding the Role of the Lateral Dimensional Property of Graphene Oxide on Its Interactions with Renal Cells

Renal excretion is expected to be the major route for the elimination of biomedically applied nanoparticles from the body. Hence, understanding the nanomedicine–kidney interaction is crucially required, but it is still far from being understood. Herein, we explored the lateral dimension- (~70 nm and ~300 nm), dose- (1, 5, and 15 mg/kg in vivo and 0.1~250 μg/mL in vitro), and time-dependent (48 h and 7 d in vivo) deposition and injury of PEGylated graphene oxide sheets (GOs) in the kidney after i.v. injection in mice. We specially investigated the cytotoxic effects on three typical kidney cell types with which GO renal excretion is related: human renal glomerular endothelial cells (HRGECs) and human podocytes, and human proximal tubular epithelial cells (HK-2). By using in vivo fluorescence imaging and in situ Raman imaging and spectroscopic analysis, we revealed that GOs could gradually be eliminated from the kidneys, where the glomeruli and renal tubules are their target deposition sites, but only the high dose of GO injection induced obvious renal histological and ultrastructural changes. We showed that the high-dose GO-induced cytotoxicity included a cell viability decrease and cellular apoptosis increase. GO uptake by renal cells triggered cellular membrane damage (intracellular LDH release) and increased levels of oxidative stress (ROS level elevation and a decrease in the balance of the GSH/GSSG ratio) accompanied by a mitochondrial membrane potential decrease and up-regulation of the expression of pro-inflammatory cytokines TNF-α and IL-18, resulting in cellular apoptosis. GO treatments activated Keap1/Nrf2 signaling; however, the antioxidant function of Nrf2 could be inhibited by apoptotic engagement. GO-induced cytotoxicity was demonstrated to be associated with oxidative stress and an inflammation reaction. Generally, the l-GOs presented more pronounced cytotoxicity and more severe cellular injury than s-GOs did, demonstrating lateral size-dependent toxicity to the renal cells. More importantly, GO-induced cytotoxicity was independent of renal cell type. The results suggest that the dosage of GOs in biomedical applications should be considered and that more attention should be paid to the ability of a high dose of GO to cause renal deposition and potential nephrotoxicity.     

The Toxicity of Graphene Oxides: Dependence on the Oxidative Methods Used

Graphene, a class of two‐dimensional carbon nanomaterial, has attracted extensive interest in recent years, with a significant amount of research focusing on graphene oxides (GOs). They have been primed as potential candidates for biomedical applications such as cell labeling and drug delivery, thus the toxicity and behavior of graphene oxides in biological systems are fundamental issues that need urgent attention. The production of GO is generally achieved through a top‐down route, which includes the usage of concentrated H₂SO₄ along with: 1) concentrated nitric acid and KClO₃ oxidant (Hoffmann); 2) fuming nitric acid and KClO₃ oxidant (Staudenmaier); 3) concentrated phosphoric acid with KMnO₄ (Tour); or 4) sodium nitrate for in‐situ production of nitric acid in the presence of KMnO₄ (Hummers). It has been widely assumed that the properties of these four GOs produced by using the above different methods are roughly similar, so the methods have been used interchangeably. However, several studies have reported that the toxicity of graphene‐related nanomaterials in biological systems may be influenced by their physiochemical properties, such as surface functional groups and structural defects. In addition, considering how GOs are increasingly used in the field of biomedicine, we are interested to see how the oxygen content/functional groups of GOs can impact their toxicological profiles. Since in‐vitro testing is a common first step in assessing the health risks related with engineered nanomaterials, the cytotoxicity of the GOs prepared by the four different oxidative treatments was investigated by measuring the mitochondrial activity in adherent lung epithelial cells (A549) by using commercially available viability assays. The dose–response data was generated by using two assays, the methylthiazolyldiphenyl‐tetrazolium bromide (MTT) assay and the water‐soluble tetrazolium salt (WST‐8). From the viability data, it is evident that there is a strong dose‐dependent cytotoxic response resulting from the four GO nanomaterials tested after a 24 h exposure, and it is suggested that there is a correlation between the amounts of oxygen content/functional groups of GOs with their toxicological behavior towards the A549 cells.     

Graphite Oxides: Effects of Permanganate and Chlorate Oxidants on the Oxygen Composition

Research on graphene materials has refocused on graphite oxides (GOs) in recent years. The fabrication of GO is commonly accomplished by using concentrated sulfuric acid in conjunction with: a) fuming nitric acid and KClO₃ oxidant (Staudenmaier); b) concentrated nitric acid and KClO₃ oxidant (Hofmann); c) sodium nitrate for in situ production of nitric acid in the presence of KMnO₄ (Hummers); or d) concentrated phosphoric acid with KMnO₄ (Tour). These methods have been used interchangeably in the graphene community, since the properties of GOs produced by these different methods were assumed as almost similar. In light of the wide applicability of GOs in nanotechnology applications, in which presence of certain oxygen functional groups are specifically important, the qualities and functionalities of the GOs produced by using these four different methods, side‐by‐side, was investigated. The structural characterizations of the GOs would be probed by using high resolution X‐ray photoelectron spectroscopy, nuclear magnetic resonance, Fourier transform infrared spectroscopy, and Raman spectroscopy. Further electrochemical applicability would be evaluated by using electrochemical impedance spectroscopy and cyclic voltammetry techniques. Our analyses highlighted that the oxidation methods based on permanganate oxidant (Hummers and Tour methods) gave GOs with lower heterogeneous electron‐transfer rates and a higher amount of carbonyl and carboxyl functionalities compared with when using chlorate oxidant (Staudenmaier and Hofmann methods). These observations indicated large disparities between the GOs obtained from different oxidation methods. Such insights would provide fundamental knowledge for fine tuning GO for future applications.     

Surface Modification of Graphene Oxides by Plasma Techniques and Their Application for Environmental Pollution Cleanup

Graphene oxides (GOs) have come under intense multidisciplinary study because of their unique physicochemical properties and possible applications. The large amount of oxygen‐containing functional groups on GOs leads to a high sorption capacity for the removal of various kinds of organic and inorganic pollutants from aqueous solutions in environmental pollution cleanup. However, the lack of selectivity results in difficulty in the selective removal of target pollutants from aqueous solutions in the presence of other coexisting pollutants. Herein, the surface grafting of GOs with special oxygen‐containing functional groups using low‐temperature plasma techniques and the application of the surface‐modified GOs for the efficient removal of organic and inorganic pollutants in environmental pollution are reviewed. This paper gives an account of our research on the application of GO‐based nanomaterials in environmental pollution cleanup, including: (1) the synthesis and surface grafting of functional groups on GOs, summarizing various types of low‐temperature plasma techniques for the synthesis of graphene/GOs; and (2) the application of graphene/GOs and their composites for the efficient removal of organic and inorganic pollutants from aqueous solutions, including the interaction mechanism according to recently published results.     

Hydrogel‐Assisted Transfer of Graphene Oxides into Nonpolar Organic Media for Oil Decontamination

In this work, graphene oxide (GO)‐loaded agarose hydrogel was transferred into oil such as hexadecane via stepwise solvent exchange with no chemical modification of the GO hydrophilic surface and the agarose network. After transfer, the GOs, loaded in the agarose network, could effectively and efficiently adsorb lipophilic dyes in oil via hydrogen bonding between the polar groups of the GOs and the dyes. The maximum adsorption capacity was 355.9 mg g^?1 for Nile red for instance, which is substantially larger than that of pristine agarose hydrogel and hydrophilic GO powder. The dye concentration for effective adsorption can be as low as 0.5 ppm. Thus, the present work demonstrates the promising potential of using hydrophilic adsorbents for efficient removal of polar impurities from oil.     

Enhanced adsorption performance of the graphene oxide with metallic ion impurities by elution

The graphene oxides (GOs) with various content of metallic ions impurities were prepared, and the adsorption performance of the GO before and after elution was evaluated. The prepared GOs were characterized by X‐ray diffraction, Fourier transform infrared spectroscopy, inductively coupled plasma and atomic force microscopy. The results indicated that the metallic ion impurities hardly affected the interlayer distance, microstructure and thickness of the prepared GOs. The adsorption isotherm and adsorption kinetic results showed that the metallic ions adsorbed on the GO surface had a negative influence on both the adsorption capacity and rate. After eluted by HNO₃ or HCl, most of the metallic ions adsorbed on the GO‐91 surface were ion‐exchanged by the protons of the acid eluents, and the purified GO showed enhanced equilibrium capacities and improved adsorption rate. The elution efficiency of HCl was better than that of HNO₃, and the adsorption capacity and rate of the GO eluted by HCl approximately reached to those of the GO prepared from the graphite with high purity. It indicated that HCl could efficiently remove the metallic ions adsorbed on the GO surface. Copyright © 2017 John Wiley & Sons, Ltd.     

Tailoring thermal conductivity of bulk graphene oxide by tuning the oxidation degree

The thermal conductivity of graphene oxides can be tailored by tuning oxidation degree due to the introduction of atomic- and nano-scale phonon scattering centers.     

Synthesis and Th(IV) sorption characteristics of functionalised graphene oxide

A functionalised graphene oxide (FGO) adsorbent was prepared via the γ-radiation-induced grafting of epichlorohydrin (ECH) onto graphene oxide (GO). X-ray photoelectron spectroscopy revealed that ECH was successfully introduced to the GO surface. The grafting yield of ECH increased with an increase of the irradiation dose and with a decrease of the irradiation dose rate. The sorption kinetics of Th(IV) on GO and FGO followed the pseudo-second-order model and the sorption isotherms can be described by the Langmuir model. The maximum sorption capacities of GO and FGO for Th(IV) are approximately 5.80 × 10^?4 and 2.88 × 10^?5 mol/L, respectively. GOs is considered as a kind of materials with high radiation resistance and large sorption capacities, ranking it has high potential industrial applications even under strong radiation environment. In addition, the amount of the oxygen-containing functional groups C=O and O=C–O in FGO decrease with the increase of the irradiation dose, which suggests that C=O and O=C–O contribute more than C–O to the sorption of Th(IV) onto GO and FGO.     

Global optimization of clusters using electronic structure methods

Over the past decade, there has been a significant growth in the development and application of methods for performing global optimization (GO) of cluster and nanoparticle structures using first‐principles electronic structure methods coupled to sophisticated search algorithms. This has in part been driven by the desire to avoid the use of empirical potentials (EPs), especially in cases where no reliable potentials exist to guide the search toward reasonable regions of configuration space. This has been facilitated by improvements in the reliability of the search algorithms, increased efficiency of the electronic structure methods, and the development of faster, multiprocessor high‐performance computing architectures. In this review, we give a brief overview of GO algorithms, though concentrating mainly on genetic algorithm and basin hopping techniques, first in combination with EPs. The major part of the review then deals with details of the implementation and application of these search methods to allow exploration for global minimum cluster structures directly using electronic structure methods and, in particular, density functional theory. Example applications are presented, ranging from isolated monometallic and bimetallic clusters to molecular clusters and ligated and surface supported metal clusters. Finally, some possible future developments are highlighted. © 2013 Wiley Periodicals, Inc.     

Building up polymer architectures on graphene oxide sheet surfaces through sequential atom transfer radical polymerization

The reactivity between the active species of atom transfer radical addition and the unsaturated groups of graphene oxides (GOs) has been demonstrated in this work. The reaction and the sequential surface‐initiated atom transfer radical polymerization provide a convenient approach to anchor various polymer chains and to buildup various polymer architectures, such as linear polymer, V‐shape block polymer, multibonded polymer layer, and hierarchical brush‐on‐layer polymer, on GO sheet surfaces. The chemical structures and morphology of the polymer‐modified GOs have been characterized with Fourier transform infrared spectroscopy, Raman spectroscopy, X‐ray photoelectron spectroscopy, and atomic force microscopy. After organomodification, the GOs exhibit a good dispersion ability in organic solvent over 80 days, amphiphilic characteristics, and temperature‐responsive properties. Reduction of the GOs has been performed to result in graphene‐like materials showing certain extent of electron conductivities. An effective approach to synthesize GO/polymer hybrid materials possessing various polymer architectures and attractive properties has been developed. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1588–1596     

Orthogonal Activation of RNA‐Cleaving DNAzymes in Live Cells by Reactive Oxygen Species

RNA‐cleaving DNAzymes are useful tools for intracellular metal‐ion sensing and gene regulation. Incorporating stimuli‐responsive modifications into these DNAzymes enables their activities to be spatiotemporally and chemically controlled for more precise applications. Despite the successful development of many caged DNAzymes for light‐induced activation, DNAzymes that can be intracellularly activated by chemical inputs of biological importance, such as reactive oxygen species (ROS), are still scarce. ROS like hydrogen peroxide (H₂O₂) and hypochlorite (HClO) are critical mediators of oxidative stress‐related cell signaling and dysregulation including activation of immune system as well as progression of diseases and aging. Herein, we report ROS‐activable DNAzymes by introducing phenylboronate and phosphorothioate modifications to the Zn²⁺‐dependent 8–17 DNAzyme. These ROS‐activable DNAzymes were orthogonally activated by H₂O₂ and HClO inside live human and mouse cells.     

Synthesis of polydopamine‐coated graphene–polymer nanocomposites via RAFT polymerization

Graphene‐polymer nanocomposites have significant potential in many applications such as photovoltaic devices, fuel cells, and sensors. Functionalization of graphene is an essential step in the synthesis of uniformly distributed graphene‐polymer nanocomposites, but often results in structural defects in the graphitic sp² carbon framework. To address this issue, we synthesized graphene oxide (GO) by oxidative exfoliation of graphite and then reduced it into graphene via self‐polymerization of dopamine (DA). The simultaneous reduction of GO into graphene, and polymerization and coating of polydopamine (PDA) on the reduced graphene oxide (RGO) surface were confirmed with XRD, UV–Vis, XPS, Raman, TGA, and FTIR. The degree of reduction of GO increased with increasing DA/GO ratio from 1/4 to 4/1 and/or with increasing temperature from room temperature to 60 °C. A RAFT agent, 2‐(dodecylthiocarbonothioylthio)?2‐methylpropionic acid, was linked onto the surface of the PDA/RGO, with a higher equivalence of RAFT agent in the reaction leading to a higher concentration of RAFT sites on the surface. Graphene‐poly(methyl methacrylate), graphene‐poly(tert‐butyl acrylate), and graphene‐poly(N‐isopropylacrylamide) nanocomposites were synthesized via RAFT polymerization, showing their characteristic solubility in several different solvents. This novel synthetic route was found facile and can be readily used for the rational design of graphene‐polymer nanocomposites, promoting their applications. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3941–3949     

Highly Hydrogenated Graphene through Microwave Exfoliation of Graphite Oxide in Hydrogen Plasma: Towards Electrochemical Applications

Hydrogenated graphenes exhibit a variety of properties with potential applications in devices, ranging from a tunable band gap to fluorescence, ferromagnetism, and the storage of hydrogen. We utilize a one‐step microwave‐irradiation process in hydrogen plasma to create highly hydrogenated graphene from graphite oxides. The procedure serves the dual purposes of deoxygenation and concurrent hydrogenation of the carbon backbone. The effectiveness of the hydrogenation process is investigated on three different graphite oxides (GOs), which are synthesized by using the Staudenmaier, Hofmann, and Hummers methods. A systematic characterization of our hydrogenated graphenes is performed using UV/Vis spectroscopy, SEM, AFM, Raman spectroscopy, FTIR spectroscopy, X‐ray photoelectron spectroscopy (XPS), combustible elemental analysis, and electrical conductivity measurements. The highest hydrogenation extent is observed in hydrogenated graphene produced from the Hummers‐method GO, with a hydrogen content of 19 atomic % in the final product. In terms of the removal of oxygen groups, microwave exfoliation yields graphenes with very similar oxygen contents despite differences in their parent GOs. In addition, we examine the prospective application of hydrogenated graphenes as electrochemical transducers through a cyclic voltammetry (CV) study. The highly hydrogenated graphenes exhibit fast heterogeneous electron‐transfer rates, suggestive of their suitability for electrochemical applications in electrodes, supercapacitors, batteries, and sensors.     

Polyaniline decorated graphene oxide on sulfonated poly(ether ether ketone) membrane for direct methanol fuel cells application

In direct methanol fuel cells (DMFC), methanol crossover is a major issue which has reduced the performance of polymer electrolyte membrane (PEM) for energy generation. In this study, graphene oxide (GO) and conductive polyaniline decorated GO (PANI-GO) were used as additives in fabrication of sulfonated poly(ether ether ketone) (SPEEK) nanocomposite PEM membrane to reduce methanol crossover. PANI-GO was synthesized by in situ polymerization method and the formation of PANI coated GO nanostructures was confirmed by surface morphology and crystallinity analysis. The membrane morphology and topography analysis confirmed that GO and PANI-GO were well dispersed on the surface of SPEEK membrane. 0.1 wt% PANI-GO modified SPEEK nanocomposite membrane exhibited the highest water uptake and ion exchange capacity of 40% and 1.74 meq g^?1, respectively. The oxidative stability of the nanocomposite membranes also improved. Lower methanol permeability of 4.33 × 10^?7 cm^?2S^?1 was noticed for 0.1 wt% PANI-GO modified SPEEK membrane. PANI-GO modified SPEEK membrane enhanced the proton conductivity, which was due to the existence of acidic and hydrophilic group present in PANI and GO. PANI-GO modified SPEEK membrane held higher selectivity of 1.94 × 10⁴ S cm^?3 s^?1. Overall, these studies revealed that PANI-GO modified SPEEK membrane is a potential material for DMFC applications.     

The Anthocyanins,Oenin and Callistephin,Protect RPE Cells Against Oxidative Stress

The retinal pigment epithelium (RPE) is a highly metabolic layer of postmitotic cells lining Bruch's membrane in the retina. While these cells contain endogenous photosensitizers that mediate blue light‐induced damage, it has also been shown that blue light exposure damages mitochondrial DNA in RPE cells resulting in mitochondrial dysfunction and unregulated generation of reactive oxygen species (ROS). As RPE cells are postmitotic, it is imperative to decrease oxidative stress to these cells and preserve function. Dietary plant‐derived antioxidants such as anthocyanins offer a simple and accessible solution for decreasing oxidative stress. The anthocyanins malvidin‐3‐O‐glucoside (oenin) and pelargonidin‐3‐O‐glucoside (callistephin) were tested for their ability and efficacy in decreasing ROS generation and preserving mitochondrial redox activity in blue light‐irradiated ARPE‐19 cells. A significant decrease in intracellular ROS with concurrent increase in mitochondrial redox activity was observed for tested concentrations of oenin, while callistephin was beneficial to stressed cells at higher concentrations. These findings suggest anthocyanins are effective antioxidants in blue light‐stressed RPE cells in vitro. Additionally, oxidation products of these anthocyanins were examined using LC/MS and findings suggest the possibility of multiple oxidation sites for these compounds.     

Neolignans from the Fruits of Magnolia obovata Inhibit NO Production and Have Neuroprotective Effects

One new sesquineolignan, obovatalignan A ( 1 ), and one new neolignan, obovatalignan B ( 2 ), were isolated from the Magnolia obovata fruits. Their chemical structure, including absolute configuration, was determined based on various spectroscopic methods, such as HR‐EI‐MS, 1D‐NMR (¹H, ¹³C, DEPT), 2D‐NMR (gCOSY, gHSQC, gHMBC, NOESY), and CD spectroscopy. The compounds were evaluated for protective effects against glutamate‐induced oxidative stress in HT22‐immortalized hippocampal cells and inhibitory activity against NO production in LPS‐induced RAW 264.7 cells. Compounds 1 and 2 exhibited protective effects against glutamate‐induced oxidative stress with EC₅₀ values of 18.1 ± 1.23 and 7.10 ± 0.78 μm , respectively, as well as inhibitory effects on NO production with IC₅₀ values of > 30.0 and 8.22 ± 2.01 μm , respectively.     

A new poly(l-lactide)-grafted graphite oxide composite: Facile synthesis, electrical properties and crystallization behaviors

A simple method was adopted to prepare poly(l-lactide)-grafted graphite oxide (PLLA-g-GO) by ring opening polymerization of l-lactide in the presence of graphite oxide (GO) with hydroxyl groups. GO was firstly treated with tolylene-2,4-diisocyanate (TDI) to create an anchor site on GO, and then reacted with 1,4-butanediol (BD) to afford functional hydroxyl groups grafted onto the surface of GO. So that, the dispersity of GO in the organic solution was enhanced. According to the thermogravimetric analysis (TGA), the organic composition of GO treated with TDI and BD (GO-TDI-OH) was estimated to be about 13 wt%. Also, using TGA, the composition of GO in the PLLA-g-GOs could be estimated. The hydroxyl groups on the GO surface acted as initiators for the ROP of l-lactide. Further, they also played as a vital role in controlling the molecular weight of the PLLA. The synthesized PLLA-g-GOs were characterized by the FTIR, ¹HNMR and UV/Vis spectroscopies. The dispersion states of GO in the PLLA-g-GOs were investigated by wide angle x-ray diffraction patterns. According to differential scanning calorimeter study, it was found that GO platelets have nucleating effect on the crystallization of PLLA in the PLLA-g-GO. Additionally, the incorporation of GO improved the electrical conductivity of PLLA, indicating that GOs is a good conducting-modifiers for polymers.     

In Situ Synthesis and Nonvolatile Rewritable‐Memory Effect of Polyaniline‐Functionalized Graphene Oxide

A new polyaniline (PANI)‐functionalized graphene oxide (GO‐PANI) was prepared by using an in situ oxidative graft polymerization of aniline on the surface of GO. Its highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), ionization potential (IP), and electron affinity (EA) values experimentally estimated by the onset of the redox potentials were ?5.33, ?3.57, 5.59, and 3.83 eV, respectively. A bistable electrical‐switching effect was observed in electronic device with the GO‐PANI film sandwiched between the indium tin oxide (ITO) and Al electrodes. This device exhibited two accessible conductivity states, that is, the low‐conductivity (OFF) state and the high‐conductivity (ON) state, and can be switched to the ON state under a negative electrical sweep, and can also be reset to the initial OFF state by a reverse (positive) electrical sweep. The ON state is nonvolatile and can withstand a constant voltage stress of ?1 V for 3 h and 10⁸ read cycles at ?1 V under ambient conditions. The nonvolatile nature of the ON state and the ability to write, read, and erase the electrical states, fulfill the functionality of a rewritable memory. An ON/OFF current ratio of more than 10⁴ at ?1 V achieved in this memory device is high enough to promise a low misreading rate through the precise control of the ON and OFF states. The mechanism associated with the memory effects was elucidated from molecular simulation results.     

The Endogenous Tryptophan‐derived Photoproduct 6‐formylindolo[3,2‐b]carbazole (FICZ) is a Nanomolar Photosensitizer that Can be Harnessed for the Photodynamic Elimination of Skin Cancer Cells in Vitro and in Vivo

UV‐chromophores contained in human skin may act as endogenous sensitizers of photooxidative stress and can be employed therapeutically for the photodynamic elimination of malignant cells. Here, we report that 6‐formylindolo[3,2‐b]carbazole (FICZ), a tryptophan‐derived photoproduct and endogenous aryl hydrocarbon receptor agonist, displays activity as a nanomolar sensitizer of photooxidative stress, causing the photodynamic elimination of human melanoma and nonmelanoma skin cancer cells in vitro and in vivo. FICZ is an efficient UVA/Visible photosensitizer having absorbance maximum at 390 nm (ε = 9180 L mol^?1 cm^?1), and fluorescence and singlet oxygen quantum yields of 0.15 and 0.5, respectively, in methanol. In a panel of cultured human squamous cell carcinoma and melanoma skin cancer cells (SCC‐25, HaCaT‐ras II‐4, A375, G361, LOX), photodynamic induction of cell death was elicited by the combined action of solar simulated UVA (6.6 J cm^?2) and FICZ (≥10 nm ), preceded by the induction of oxidative stress as substantiated by MitoSOX Red fluorescence microscopy, comet detection of Fpg‐sensitive oxidative genomic lesions and upregulated stress response gene expression (HMOX1, HSPA1A, HSPA6). In SKH1 “high‐risk” mouse skin, an experimental FICZ/UVA photodynamic treatment regimen blocked the progression of UV‐induced tumorigenesis suggesting feasibility of harnessing FICZ for the photooxidative elimination of malignant cells in vivo.     

Electron Transfer to Decorated Graphene Oxide Particles

Graphene oxides (GOs) are popular catalyst supports for precious metals in nanoparticle form. The hydrogen oxidation reaction (HOR) and the hydrogen evolution reaction (HER) on individual GO platelets decorated with Pd nanoparticles (Pd/GOs) were investigated. The results suggest that the catalytic activity is confined to the zone physically close to the point of electrical contact between platelet and electrode with just a fraction of the platelet active.