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Understanding the performance of graphene devices in contact with highly concentrated aqueous electrolytes is key to integrating graphene into next-generation devices operating in sea water environments, biosensors, and high-density energy production/storage units. Despite significant efforts toward interpreting the structure of the electrochemical double layer at high concentrations, the interface between graphene-based materials and concentrated aqueous solutions has remained vaguely described. In this study, we demonstrate the use of graphene-based chemiresistors as a technique to indirectly quantify the experimental screening length of concentrated electrolytes that could clarify the interpretation of electrochemical measurements conducted at low ionic strength. We report a breakdown of the Debye–Hückel theory in the proximity of graphene surfaces at lower concentrations (10–50 mM) than previously reported for other systems, depending on cation size, dissolved oxygen concentration, and degree of graphene defectivity. [ABSTRACT FROM AUTHOR]

Graphene/silicon Schottky diodes have recently been of heightened interest due to their high sensitivity towards surface chemical modifications. Here we demonstrate the operation of a graphene/n-type silicon Schottky junction in aqueous media, quantifying the impacts of the ionic strength, oxidation-reduction potential, and pH of an aqueous solution on diode characteristics. The roles of the electrical double layer and the surface functional groups of graphene in determining the graphene/silicon junction response to changes in the aqueous environment were investigated. The application of this diode sensor design was demonstrated in an aqueous environment by the introduction of functional groups to the graphene surface via non-covalent functionalization (here with 1-aminopyrene). Free chlorine - a common disinfectant for drinking water - is used to illustrate the sensing capabilities of this new platform, demonstrating up to 80% change in series resistance (4% change in Schottky barrier height) of the functionalized device upon exposure to 1 ppm free chlorine, while the unfunctionalized device only shows a 17% response. The results achieved in this study will open up new opportunities for highly sensitive detection of (bio)analytes using graphene/silicon diode devices in aqueous environments, beyond their conventional gas sensing applications. [Display omitted] [ABSTRACT FROM AUTHOR]

In this study, we demonstrate that a highly pH-sensitive substrate could be fabricated by controlling the type and defect density of graphene derivatives. Nanomaterials from single-layer graphene resembling a defect-free structure to few-layer graphene and graphene oxide with high defect density were used to demonstrate the pH-sensing mechanisms of graphene. We show the presence of three competing mechanisms of pH sensitivity, including the availability of functional groups, the electrochemical double layer, and the ion trapping that determines the overall pH response. The graphene surface was selectively functionalized with hydroxyl, amine, and carboxyl groups to understand the role and density of the graphene pH-sensitive functional groups. Later, we establish the development of highly pH-sensitive graphene oxide by controlling its defect density. This research opens a new avenue for integrating micro–nano-sized pH sensors based on graphene derivatives into next-generation sensing platforms. [ABSTRACT FROM AUTHOR]

Polycarbonate (PC) is widely used as an important engineering polymer in numerous applications and diverse fields, ranging from optical components to building construction. Three-dimensional (3D) graphene-based material has growing interest owing to its excellent properties, impacting research fields in energy storage, biomedical, environmental applications, and more. Although laser treatment-based generation of graphene and graphitization has been demonstrated in various polymers, they have not so far been shown in PC. Here, we report that under the right conditions, industrially important material such as PC can also transformed into graphene and its properties are quite distinct from graphene generated in other organic polymers such as polyimide (PI). A 3D porous hierarchical graphene/graphitic composite structure is formed in PC due to laser treatment and it exhibited the highest electrical conductivity with a sheet resistance down to 0.8 Ω sq−1 among all the laser induced graphitic (LIG) material generated using laser treatment. The unique hierarchical structure in the 3D graphene layer produces a robust and durable superhydrophobicity (contact angle at 170.6° ± 8.8°) in the ambient environment. We demonstrate integration of gold nanoparticles into the graphene layer using the same single step laser treatment for its application as a glucose sensor, and also demonstrate its other applications for direct integration of contact electrode of a chemiresistive sensor, a temperature sensor, and as an electrical heater on insulating materials. Since PC is such an important and widely used industrial plastic, these integration methods are of broad applicability to instrument components made of PC directly. [Display omitted] [ABSTRACT FROM AUTHOR]

• Chemiresistive sensors can be fabricated from percolation networks of few-layer graphene (FLG) flakes. • Functionalization with suitable ligands can achieve selective sensor response to Ag+ ions in the 3 ppb to 1 ppm range. • Sensors are robust and reusable, can be reset at pH3 due to a shift in the complexation equilibrium. • The sensor response was tested in an environmental sample (river water) and found to correlate well with ICP-MS data. Silver is used as a water disinfectant in hospital settings as well as in purifiers for potable water. Although there are no strict regulations on the concentration of silver in water, adverse effects such as argyria and respiratory tract irritation have been correlated to excess silver consumption. Based on this, the levels of silver in water are recommended to be maintained below 100 ppb to ensure safety for human consumption. In this work, we present a silver sensor for use in aqueous media that utilizes bathocuproine, a silver selective chromophore, adsorbed onto few-layer graphene (FLG) flake networks for the chemiresistive detection of silver. Complexation of silver to bathocuproine modulates the conductivity of the FLG film, which can be probed by applying a small voltage bias. The decrease in resistance of the film correlates with the concentration of silver in solution between 3 ppb and 1 ppm. Exposing the sensor to a lower pH resets the sensor, allowing it to be reused and reset multiple times. This sensor demonstrates a new pathway to chemiresistive cation sensing using known selective complexing agents adsorbed onto graphitic thin films. This concept can be expanded to the detection of other relevant analytes in domestic, industrial and environmental water sources. [ABSTRACT FROM AUTHOR]