Charged Graphene Research Articles

Graphene Nanoribbon Bending (Nanotubes): Interaction Force between QDs and Graphene

Carbon materials in different shapes—such as fullerene molecules (0D), nanotubes and graphene nanoribbons (1D), graphene sheets (2D), and nanodiamonds (3D)—each have distinct electrical and optical properties. All graphene-based nanostructures are expected to exhibit extraordinary electronic, thermal, and mechanical properties. Moreover, they are therefore promising candidates for a wide range of nanoscience and nanotechnology applications. In this work, we theoretically studied and analyzed how an array of quantum dots affects a charged graphene plate. To that end, the array of quantum dots was embedded on the graphene plate. Then, considering the interaction between QDs and graphene nanoribbons, we transformed the charged plate of a graphene capacitor into a nanotube using the bipolar-induced interaction and the application of an external electromagnetic field. In this work, the dimensions of the graphene plate were 40 nm × 3100 nm. The bending process of a charged graphene plate is controlled by the induced force due to the applied electromagnetic field and the electric field induced by the quantum dots. Finally, using the predetermined frequency and amplitude of the electromagnetic field, the graphene nanoribbon was converted into a graphene nanotube. Since the electrical and optical properties of nanotubes are different from those of graphene plates, this achievement has many practical potential applications in the electro-optical industry.

Molecular dynamics of preferential adsorption in mixed alkali-halide electrolytes at graphene electrodes.

Understanding the microscopic behavior of aqueous electrolyte solutions in contact with graphene and related carbon surfaces is important in electrochemical technologies, such as capacitive deionization or supercapacitors. In this work, we focus on preferential adsorption of ions in mixed alkali-halide electrolytes containing different fractions of Li+/Na+ or Li+/K+ and/or Na+/K+ cations with Cl- anions dissolved in water. We performed molecular dynamics simulations of the solutions in contact with both neutral and positively and negatively charged graphene surfaces under ambient conditions, using the effectively polarizable force field. The simulations show that large ions are often intuitively attracted to oppositely charged electrodes. In contrast, the adsorption behavior of small ions tends to be counterintuitive. In mixed-cation solutions, one of the cations always supports the adsorption of the other cation, while the other cation weakens the adsorption of the first cation. In mixed-cation solutions containing large and small cations simultaneously, adsorption of the larger cations varies dramatically with the electrode charge in an intuitive way, while adsorption of the smaller cations changes oppositely, i.e., in a counterintuitive way. For (Li/K)Cl mixed-cation solutions, these effects allow the control of Li+ adsorption by varying the electrode charge, whereas, for LiCl single-salt solutions, Li+ adsorption is nearly independent of the electrode charge. We rationalize this cation-cation lever effect as a result of a competition between three driving forces: (i) direct graphene-ion interactions, (ii) the strong tendency of the solutions to saturate the network of non-covalent intermolecular bonds, and (iii) the tendency to suppress local charge accumulation in any region larger than typical interparticle distances. We analyze the driving forces in detail using a general method for intermolecular bonding based on spatial distribution functions and different contributions to the total charge density profiles. The analysis helps to predict whether an ion is more affected by each of the three driving forces, depending on the strength of the ion solvation shells and the compatibility between the contributions of the charge density profiles due to the ion and water molecules. This approach is general and can also be applied to other solutions under different thermodynamic conditions.

Polystyrene sulfonate coated activated graphene aerogel for boosting desalination performance using capacitive deionization

Capacitive deionization (CDI) has proven to be a cost-effective and environmentally friendly desalination process. However, the high salt adsorption capacity is hampered by the poor surface wettability of the electrode materials and a lack of charge in the surface electrode. Despite its unique chemical and physical properties, pure reduced graphene oxide is still far from the expected potential desalination performance. In this work, graphene aerogel (GA) and activated graphene aerogel (AGA) with an ultrahigh specific surface area were successfully synthesized. The AGA was functionalized using a charged polystyrene sulfonate (PSS) and utilized as a negative electrode for the asymmetric CDI configuration cell. The synthesized charged PSS-activated graphene aerogel (PSS-AGA) achieved outstanding hydrophilic behavior, a high specific surface area (1401.53 m2/g), and a typical specific capacitance of 178F/g, furthermore excellent capacitance retention of 97.2 % over 5000 cycles. The salt adsorption capacity of the fabricated electrodes is evaluated by conducting a single-pass mode (SP-Mode) experiment under various conditions. The fabricated PSS-AGA//AGA asymmetric cell configuration delivers an exceptional salt adsorption capacity of 26.33 mg/g, excellent charge efficiency of 88 % with low energy consumption at 1000 mg/L, and full regeneration with long-term stability.

Charge carrier modulation in graphene on ferroelectric single-crystal substrates

Charge carrier modulation of graphene using the ferroelectricity of a nearby dielectric can be useful for controlling the electronic properties of graphene. However, when graphene is located on ferroelectric oxides, their electrical coupling frequently shows abnormal behaviors, such as anti-hysteresis, in field-effect transistor operation. From the systematic examination of graphene on a ferroelectric oxide single-crystal [Pb(Mg1/3Nb2/3)O3]1-x–[PbTiO3]x (PMNPT) substrate, we observed that the ferroelectric modulation of graphene was significantly influenced by the ambipolar nature of graphene and ferroelectric-assisted charge trapping with carrier-type dependency. For graphene/hexagonal-BN on the PMNPT, the Coulomb interaction between charges in interfacial traps and ferroelectric polarization seems to decouple the graphene conductance from the polarization field and induce only the charge trap effect on device performance. Consequently, the asymmetric surface charge compensation of ferroelectric oxide by the ambipolar graphene channel determines the detailed coupling process between the charge carrier of graphene and ferroelectric polarization, resulting in direct ferroelectric coupling or indirect anti-hysteretic coupling.

One-Component Multichannel Sensor Array for Rapid Identification of Bacteria.

Bacterial infections routinely cause serious problems to public health. To mitigate the impact of bacterial infections, sensing systems are urgently required for the detection and subsequent epidemiological control of pathogenic organisms. Most conventional approaches are time-consuming and highly instrument- and professional operator-dependent. Here, we developed a novel one-component multichannel array constructed with complex systems made from three modified polyethyleneimine as well as negatively charged graphene oxide, which provided an information-rich multimode response to successfully identify 10 bacteria within minutes via electrostatic interactions and hydrophobic interactions. Furthermore, the concentration of bacteria (from OD600 = 0.025 to 1) and the ratio of mixed bacteria were successfully achieved with our smart sensing system. Our designed sensor array also exhibited huge potential in biological samples, such as in urine (OD600 = 0.125, 94% accuracy). The way to construct a sensor array with minimal sensor element with abundant signal outputs tremendously saves cost and time, providing a powerful tool for the diagnosis and assessment of bacterial infections in the clinic.

Synthesis and Characterization of Hybrid Nanocomposites of Gold Nanostars Encapsulated on Graphene Oxide as Active SERS Nanotags

Hybrid nanocomposites of gold nanostars and graphene oxide exhibit intriguing optical and electrical properties derived from the sharp-tip effect of nanostars and the high conductivity of honeycomb structure from C-sp2 hybridization of graphene-based materials. This work presents a facile way to wrap graphene oxide nano-colloids surrounding the as-prepared gold nanostars via the assistance of linker molecules, 4-aminothiophenol, as effective nanotags. Upon tightly binding to the surface of gold nanostars via gold- thiol interactions, 4-aminothiophenol can modify the surface of gold nanostars positively charged originated from amine groups and thus facilitate the self-assembly of negatively charged graphene oxide to wrap around the surface of gold nanostars. The SEM, TEM images and the shift in the Uv-Vis spectra of the resulting gold nanostars- graphene oxide nanocomposites reveal the successful encapsulation. Besides, the enhanced Raman signal of 4-aminothiophenol highlights the potential of the obtained gold nanostars-graphene oxide hybrids as effective nanotags for applications in surface enhanced Raman scattering sensing and bio-imaging.

Function of Graphene Oxide as the "Nanoquencher" for Hg2+ Detection Using an Exonuclease I-Assisted Biosensor.

Graphene oxide is well known for its excellent fluorescence quenching ability. In this study, positively charged graphene oxide (pGO25000) was developed as a fluorescence quencher that is water-soluble and synthesized by grafting polyetherimide onto graphene oxide nanosheets by a carbodiimide reaction. Compared to graphene oxide, the fluorescence quenching ability of pGO25000 is significantly improved by the increase in the affinity between pGO25000 and the DNA strand, which is introduced by the additional electrostatic interaction. The FAM-labeled single-stranded DNA probe can be almost completely quenched at concentrations of pGO25000 as low as 0.1 μg/mL. A simple and novel FAM-labeled single-stranded DNA sensor was designed for Hg2+ detection to take advantage of exonuclease I-triggered single-stranded DNA hydrolysis, and pGO25000 acted as a fluorescence quencher. The FAM-labeled single-stranded DNA probe is present as a hairpin structure by the formation of T–Hg2+–T when Hg2+ is present, and no fluorescence is observed. It is digested by exonuclease I without Hg2+, and fluorescence is recovered. The fluorescence intensity of the proposed biosensor was positively correlated with the Hg2+ concentration in the range of 0–250 nM (R2 = 0.9955), with a seasonable limit of detection (3σ) cal. 3.93 nM. It was successfully applied to real samples of pond water for Hg2+ detection, obtaining a recovery rate from 99.6% to 101.1%.

Topologically Tuned Obliquity of Klein-Tunnelling Charged Currents Through Graphene Electrostatically-Confined p - n Junctions

Problem of control over Klein-tunnelling states from electrostatically-confined graphene p - n junctions has been discussed. The lack of quasi-bound states, being the states with a finite life time, in a pseudo-Dirac-fermion model for the graphene quantum dot (GQD) is theoretically predicted as inapplicability of the so-called "resonance condition" leading to an inconsistent linear system corresponding to matching conditions. Within a pseudo-Dirac-Weyl fermion model GQD, the graphene charge carriers are topologically nontrivial and can be confined by a staircase-type potential due to competition between Zak curvature and centrifugal-force actions. The predicted topological effects elucidate experimentally observed resonances created by electron beam and laser pulse in crystalline arrays of single-walled carbon nanotubes as the Klein-tunnelling resonant states in the p - n graphene junctions. We present a robust approach to fabricate stable graphene p - n junctions by fine-tuning the topological effects.

Graphene oxide constructed nano Newton's cradle for ultrafast and highly selective CO2 transport

The emergence of Newton's cradle is promising for fast substance transport, which attracts tremendous attentions for industrial applications. Herein, an ultrafast and highly selective nano Newton's cradle for CO2 transport is constructed through the assembly of negatively charged graphene oxide (GO) nanosheets and positively charged zinc hydroxide nanostrands (ZHNs), followed with impregnation of ionic liquid (IL) within the nanochannels. The GO laminates provide robust structure for whole system. The incorporation of IL endows the membrane with sieving ability to different gases while ZHNs play a key role to accelerate selective CO2 adsorption, thus achieving exceptional CO2 separation performance. The resulted nano Newton's cradle demonstrates that a huge CO2 permeance enhancement of more than 53 times as that of the GO/IL membrane without ZHNs, superhigh CO2/light gases selectivities (35 for CO2/H2, 194 for CO2/CH4 and 331 for CO2/N2), out-stripping the Robeson upper bound of traditional polymeric membranes, as well as long-term operation stability. This innovative strategy not only provides an outstanding candidate for carbon capture, but also opens a new avenue to design separation membranes with high performance for various applications.

Graphene field-effect transistors array for detection of liquid conductivities in the physiological range through novel time-multiplexed impedance measurements

In medical applications, graphene field-effect transistors (GFETs) have been employed as sensors for the detection of biological biomarkers and other compounds due to graphene's high sensitivity towards immobilized molecules at its surface. Commonly, the resistivity of a graphene channel is measured under direct drain-source current (DC) and gate voltage sweep. However, like other materials, the electrical response of graphene can also be studied in the form of impedance measurements using alternating drain-source current (AC), which alters the distribution and migration of ionic species across the graphene channel. In this study, an array with 12 non-functionalized GFETs is used for pioneering electrical measurements of prepared liquid solutions in the DC and AC regimes of stimulation. In particular, the transistors were characterized under saline solutions with conductivity levels ranging from 84 μS/cm to 1413 μS/cm. We first report the observation of a larger variation on the graphene's charge neutrality point in gate sweep for the AC regime with varying conductivity levels (ΔVGS = −0.00033∗ΔS) as compared to DC (ΔVGS = −0.00023∗ΔS), which can potentially be explored as a novel way of detecting electrical changes on graphene induced by physiological solutions, chemical analytes, immobilized substances, or cell substrates, when direct current cannot penetrate and influence these structures.

Ultrathin Super-barrier film via 100 % surface coating coverage of Self-assembled graphene oxide sheets

Although graphene-based materials have been projected to be impervious even to Nature's smallest molecule, He, so far graphene-based barriers have exhibited a rather high water vapor transmission rate (WVTR) of 10−1 – 10−2 g/m2·day. This disparity stems from the abundant nano- and micron-scale defects in graphene basal planes, which typically function as shortcuts or highways for water diffusion through the tortuous layers of graphene-based sheets. Here WVTR of 4.1 × 10−5 g/m2·day and lag time of 1 month (730 h) are demonstrated in a tortuously nanostructured thin film solely consisting of reduced graphene oxide (rGO) sheets, approaching the WVTR toward the theoretical impermeability. Effective design principles proven for this ultrathin super-barrier are (i) tortuous nanostructure realized by random vertical order of the oppositely charged graphene oxide (GO) layers, (ii) 100 % surface coverage, (iii) greater than 35 μm lateral width of the building block GO, (iv) suppressed defect density during production of GO, (v) surface hydrophobization, and (vi) interlayer distance narrowing down to 0.36 nm. These design principles consistently follow one simple rule: blockage of all possible water diffusion pathways. Also, it is proven here that such impermeability can be actualized by scalable layer-by-layer assembly process, which can shed a light on mass-produced super-barriers for flexible electronics in the near future.

Water desalination by charged multilayer graphene membrane: A molecular dynamics simulation

Graphene, due to its unique excellent properties, is proposed as a developing method with high efficiency compared to classical water desalination methods. In this regard, charging the membrane is considered a promising and effective approach to enhance the performance of the graphene membrane. In this research, by using molecular dynamics simulations, the water desalination by charged multilayer graphene is evaluated and the influence of electric charge amount and geometric parameters, including the pore diameter and the interlayer distance, are investigated. According to the results, the multilayer nanoporous graphene with 16.35 Å pore diameter, in which the electric charge is distributed with the proper amount and particular arrangement on the carbon atoms on the periphery of the pores, shows a significant enhancement of 59.4% in the water flow rate and improves the salt rejection efficiency from 67.27% to the desired value of 94.54%, compared to the uncharged multilayer graphene with 12.6 Å pore diameter. Moreover, it is demonstrated that increasing the interlayer distance of the membrane diminishes the effect of electric charge on improving the water flux.

Flocculating-filtration-processed mesoporous structure in laminar ion-selective membrane for osmosis energy conversion and desalination

Ion-selective membrane based technologies, such as electrodialysis and reverse electrodialysis technology, show great potential in addressing challenge of the water-energy nexus. However, the ion-selective membrane usually suffers from the trade-off between ionic selectivity and ion transport capacity. Here, we prove that mesoporous structure in lamellar ion-selective membrane can increase the transmembrane ionic transport with no loss of ionic selectivity. The mesoporous structure is constructed via adjusting the pH of graphene oxide/ionic polymer solution and self-assembly of flocculating solution. Negatively or positively charged graphene oxide based membrane with a thickness of 4 μm was used in a three-compartment electrochemical cell for osmosis energy conversion and desalination. An output power density of 5.87 W m−2 is achieved by mixing artificial seawater (0.5 M NaCl) and river water (0.01 M NaCl). The conductivity of seawater can quickly decrease from 12.3 mS cm−1 to 36 μS cm−1 in typical electrodialysis process, showing the excellent desalination ability. This work proves a new strategy of enhancing transmembrane ionic transport in ion-selective membrane and provides new opportunities for 2D nanomaterials.

Molecular dynamics of the interfacial solution structure of alkali-halide electrolytes at graphene electrodes

The interactions of graphene electrodes with aqueous solutions of electrolytes play important roles in many technologies such as capacitive deionisation. Particularly important is the surface adsorption of ions due to the electric potential of the electrode. In this paper, we have studied structural changes in several prototypical aqueous solutions of electrolytes (NaCl, KCl, and LiCl) in contact with graphene induced by its either positive or negative electric charge, under ambient conditions. We have carried out molecular-dynamics simulations using the most accurate interaction models available. We have analysed the solution structure using an advanced analysis of the intermolecular bonding, and also standard properties such as density and charge density profiles, electrostatic potential, and screening functions. Our results show that the graphene charge has nearly no translational effect on water molecules, whereas it significantly changes their orientations, and the effect on ions’ distributions differ from solution to solution. Larger ions, whose hydration shells are weaker, are affected directly in an intuitive fashion, i.e., cations are attracted by negatively charged graphene and vice versa, whereas effects on smaller ions may vary and may be even counterintuitive, e.g., the number of chlorine anions in aqueous KCl in contact with negatively charged graphene is greater when compared to neutral graphene. The surplus of chlorine anions adsorbed on a positively charged electrode strengthens the structure of water and counterintuitively rotates the water molecules in the second layer pointing their electric dipoles preferentially to the electrode. The surplus of cations due to a negatively charged electrode is accompanied by a weakening of the water structure in the case of larger ions, whereas in the case of the lithium cation the structure is stronger due to the direct effects of the graphene charge on water molecules. Regardless of the graphene charge, the total number of intermolecular bonds connected with a single water molecule is nearly independent of the distance from the graphene surface and the same applies to the number of intermolecular bonds connected with a single ion, which means that whenever a particle loses an intermolecular bond it nearly always forms a new bond as a compensation.

Trivalent ion overcharging on electrified graphene

The structure of the electrical double layer (EDL) formed near graphene in aqueous environments strongly impacts its performance for a plethora of applications, including capacitive deionization. In particular, adsorption and organization of multivalent counterions near the graphene interface can promote nonclassical behaviors of EDL including overcharging followed by co-ion adsorption. In this paper, we characterize the EDL formed near an electrified graphene interface in dilute aqueous YCl3 solution using in situ high resolution x-ray reflectivity (also known as crystal truncation rod) and resonant anomalous x-ray reflectivity (RAXR). These interface-specific techniques reveal the electron density profiles with molecular-scale resolution. We find that yttrium ions (Y3+) readily adsorb to the negatively charged graphene surface to form an extended ion profile. This ion distribution resembles a classical diffuse layer but with a significantly high ion coverage, i.e., 1 Y3+ per 11.4 ± 1.6 Å2, compared to the value calculated from the capacitance measured by cyclic voltammetry (1 Y3+ per ∼240 Å2). Such overcharging can be explained by co-adsorption of chloride that effectively screens the excess positive charge. The adsorbed Y3+ profile also shows a molecular-scale gap (⩾5 Å) from the top graphene surfaces, which is attributed to the presence of intervening water molecules between the adsorbents and adsorbates as well as the lack of inner-sphere surface complexation on chemically inert graphene. We also demonstrate controlled adsorption by varying the applied potential and reveal consistent Y3+ ion position with respect to the surface and increasing cation coverage with increasing the magnitude of the negative potential. This is the first experimental description of a model graphene-aqueous system with controlled potential and provides important insights into the application of graphene-based systems for enhanced and selective ion separations.

Machine Learning-Assisted Pattern Recognition of Amyloid Beta Aggregates with Fluorescent Conjugated Polymers and Graphite Oxide Electrostatic Complexes.

Five fluorescent positively charged poly(para-aryleneethynylene) (P1-P5) were designed to construct electrostatic complexes C1-C5 with negatively charged graphene oxide (GO). The fluorescence of conjugated polymers was quenched by the quencher GO. Three electrostatic complexes were enough to distinguish between 12 proteins with 100% accuracy. Furthermore, using these sensor arrays, we could identify the levels of Aβ40 and Aβ42 aggregates (monomers, oligomers, and fibrils) via employing machine learning algorithms, making it an attractive strategy for early diagnosis of Alzheimer's disease.

Magnetic Ordering in Ultrasmall Potassium Ferrite Nanoparticles Grown on Graphene Nanoflakes.

Magnetic nanoparticles are central to the development of efficient hyperthermia treatments, magnetic drug carriers, and multimodal contrast agents. While the magnetic properties of small crystalline iron oxide nanoparticles are well understood, the superparamagnetic size limit constitutes a significant barrier for further size reduction. Iron (oxy)hydroxide phases, albeit very common in the natural world, are far less studied, generally due to their poor crystallinity. Templating ultrasmall nanoparticles on substrates such as graphene is a promising method to prevent aggregation, typically an issue for both material characterization and applications. We generate ultrasmall nanoparticles, directly on the carbon framework by the reaction of a graphenide potassium solution, charged graphene flakes, with iron(II) salts. After mild water oxidation, the obtained composite material consists of ultrasmall potassium ferrite nanoparticles bound to the graphene nanoflakes. Magnetic properties as evidenced by magnetometry and X-ray magnetic circular dichroism, with open magnetic hysteresis loops near room temperature, are widely different from classical ultrasmall superparamagnetic iron oxide nanoparticles. The large value obtained for the effective magnetic anisotropy energy density Keff accounts for the presence of magnetic ordering at rather high temperatures. The synthesis of ultrasmall potassium ferrite nanoparticles under such mild conditions is remarkable given the harsh conditions used for the classical syntheses of bulk potassium ferrites. Moreover, the potassium incorporation in the crystal lattice occurs in the presence of potassium cations under mild conditions. A transfer of this method to related reactions would be of great interest, which underlines the synthetic value of this study. These findings also give another view on the previously reported electrocatalytic properties of these nanocomposite materials, especially for the sought-after oxygen reduction/evolution reaction. Finally, their longitudinal and transverse proton NMR relaxivities when dispersed in water were assessed at 37 °C under a magnetic field of 1.41 T, allowing potential applications in biological imaging.

Generalized dynamical mean-field theory of two-sublattice systems with long-range interactions and its application to study charge and spin correlations in graphene

We investigate magnetic and charge correlations in graphene by using the formulation of extended dynamical mean-field theory (E-DMFT) for two-sublattice systems. First, we map the average non-local interaction onto the effective static interaction between different sublattices, which is treated together with the local interaction within an effective "two-orbital" local model. The remaining part of the non-local interaction is considered by introducing an effective retarded interaction within the E-DMFT approach. The non-local susceptibilities in charge and spin channel are further evaluated in the ladder approximation. We verify the applicability of the proposed method to describe the effect of uniformly screened long-range Coulomb potential $\propto 1/r$, as well as screened realistic long-range electron interaction [T. O. Wehling et al., Phys. Rev. Lett. 106, 236805 (2011)] in graphene. We show that the developed approach describes a competition of semimetal, spin density wave (SDW), and charge-density-wave (CDW) correlations. The obtained phase diagram is in a good agreement with recent results of functional renormalization group (fRG) for finite large graphene nanoflakes and scaling analysis of quantum Monte Carlo data on finite clusters. Similarly to the previously obtained results within the fRG approach, the realistic screening of Coulomb interaction by $\sigma$ bands causes moderate (strong) enhancement of critical long-range interaction strength, needed for the SDW (CDW) instability, compared to the results for the uniformly screened Coulomb potential.

Flexible Electrochemical Sensor With Graphene and Gold Nanoparticles to Detect Dopamine and Uric Acid

Electrochemical sensor is a promising platform in diagnosing the disorders of nervous and metabolic systems for its feasibility of detecting dopamine (DA) and uric acid (UA). Nevertheless, the reported electrochemical sensors still suffer from unfavorable selectivity and the high limit of detection (LOD). To address these problems, this paper introduced a heterogeneous electrode interface to improve the sensor’s selectivity with differently charged regions. The electrode was prepared on a flexible polyolefin substrate and modified with negatively charged graphene (GP) and electroneutral gold nanoparticles (Au NPs). Experimental results show that the optimal electrode with five layers GP and Au NPs (GP5AuNPs5) was capable of detecting DA and UA in the ascorbic acid (AA)-coexisted solution with minimum interference. The GP5AuNPs5 sensor manifests low LODs ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3 {\sigma } /\text{S}$ </tex-math></inline-formula> ) of 10 nM and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.47 {\mu } \text{M}$ </tex-math></inline-formula> for DA and UA, respectively, confirming its feasibility to detect the lowest concentration of the human body. Moreover, the introduction of the Au NPs makes a better separation for current peaks, which might be from the existence of a concentration gradient by electrostatic force difference and an enhanced diffusion from Au NPs. The high-performance sensor with well-separated current peaks is meaningful for detecting DA and UA electrochemically.

High Loading Capacity and Wear Resistance of Graphene Oxide/Organic Molecule Assembled Multilayer Film.

Taking advantage of the strong charge interactions between negatively charged graphene oxide (GO) sheets and positively charged poly(diallyldimethylammonium chloride) (PDDA), self-assembled multilayer films of (GO/PDDA)n were created on hydroxylated silicon substrates by alternating electrostatic adsorption of GO and PDDA. The formation and structure of the films were analyzed by means of water contact angle measurement, thickness measurement, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Meanwhile, tribological behaviors in micro- and macro- scale were investigated by AFM and a ball-on-plate tribometer, respectively. The results showed that (GO/PDDA)n multilayer films exhibited excellent friction-reducing and anti-wear abilities in both micro- and macro-scale, which was ascribed to the special structure in (GO/PDDA)n multilayer films, namely, a well-stacked GO–GO layered structure and an elastic 3D crystal stack in whole. Such a film structure is suitable for design molecular lubricants for MEMS and other microdevices.