Graphene Ionic Liquid Research Articles

Revealing capacitive behaviors of graphene in ionic liquids using molecular dynamics simulation

Abstract This work reveals the capacitive behaviors of graphene nanomaterials in typical ionic liquids of [BMI+] [BF4 −] using molecular dynamics simulations. It is found that the charging process induces a structural transition from disordered to partially ordered and then ordered multilayer structures of [BMI+][BF4 −] electrolyte, yielding a multiple layer-by-layer packing structure of BMI+ cation and BF4 − anion. Especially, a counterion-adsorption-dominated Helmholtz layer is revealed at a high charge density of 16 μC/cm2, which is primarily responsible for the superior capacitance results. Besides, the differential capacitance of [BMI+][BF4 −] exhibits a symmetric convexity-shaped curve, showing a maximal capacitance value of 4.83 μF/cm2. The as-obtain simulation results help to understand the microscopic charge storage mechanisms of graphene in ionic liquids.

Effect of the external electric fields on the polarization and vibrational spectrum of 1-ethyl-3-methylimidazolium bis(trifluormethylsulfonyl)imide on graphene surface

Ionic liquids, emerging as innovative solvents for energy harvesting, necessitate a comprehensive understanding of their structural characteristics at the graphene-ionic liquid interface. This work delves into the effects of external electric fields (EEFs) on the polarization and vibrational spectrum of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim][NTf2]) on a graphene surface, employing density functional theory calculations. The analysis reveals that the geometrical configuration and stability of functional groups within the graphene-[Emim][NTf2] system are influenced by both the direction and intensity of the applied EEF. Notably, the threshold EEF strength required for dissociation varies based on the structural configuration of the anion. The electronic energy, dipole moment, and polarization rate under varying EEF conditions have been quantitatively assessed, providing insights into their EEF-dependent behaviors. Utilizing the independent gradient model approach, the inter-ion interactions, such as hydrogen bonding and van der Waals forces, under the influence of EEFs were analyzed, and how these interactions evolve with changes in the direction and intensity of the EEFs were also explored. Furthermore, the vibrational spectrum of [Emim][NTf2] on the graphene surface, spanning a range of 10–3500 cm−1, was meticulously calculated to systematically investigate the impact of the EEF on the vibrational spectra of the graphene-[Emim][NTf2] system. A key finding of this work is the distinct shifts in the vibrational peaks of the cis and trans isomers of [NTf2]− on the graphene surface, which can be attributed to differences in their exchange energies. Specifically, the exchange energy for the cis isomer of [NTf2]− is measured at 107.43 kJ∙mol−1, in contrast to 98.51 kJ∙mol−1 for the trans isomer.

A novel and intelligent molecularly imprinted enzymatic biosensor for biosensing of human serum albumin in the presence of gamma-globulin, and glucose as uncalibrated interference

Human serum albumin (HSA) is not electroactive, because the electroactive amino acids have been buried in its structure. Therefore, direct electrochemical determination of the HSA as the most abundant protein in human plasma cannot be performed. In this work, a novel electrochemical biosensor has been developed based on synthesis of dual templates molecularly imprinted polymers (DTMIPs) having trypsin (TRP) and HSA as template molecules onto the surface of a glassy carbon electrode (GCE) modified with graphene-ionic liquid (Gr-IL). By incubation of the biosensor with TRP, and HSA, tryptic hydrolysis of the HSA is occurred which breaks the HSA down into free amino acids. The HSA involves five electroactive amino acids including cysteine, tryptophan, tyrosine, methionine and histidine whose differential normal pulse voltammetric (DNPV) responses are overlapped and generated a single peak. In order to increase the sensitivity of the DTMIP/Gr-IL/GCE for determination of the HSA, hydrodynamic DNPV (HDNPV) data were generated, and used for analytical purposes. The second-order HDNPV data were generated at different pulse amplitudes and used to develop four second-order calibration models by multivariate curve resolution-alternating least squares (MCR-ALS), parallel factor analysis2 (PARAFAC2), multi-way partial least squares/residual bilinearization (N-PLS/RBL), and unfolded partial least squares/residual bilinearization (U-PLS/RBL) to select the best procedure for determination of the HSA in the presence of gamma-globulin and glucose as uncalibrated interferences. The results confirmed the best performance for the biosensor assisted by MCR-ALS for ultrasensitive and selective determination of the HSA in both synthetic and real matrices which was comparable with HPLC-UV as a reference method.

Ionic Liquid–Graphene Interface: Effect of Anions on the Fermi Level

Since energy conversion and storage processes take place at the electrolyte–electrode interface, it is important to develop experimental and theoretical procedures to understand the interfacial nanostructure in graphene-based electrochemical storage devices where ionic liquids (ILs) are used as electrolytes. In this contribution, the impact of the anions of imidazolium-based ILs on the IL–graphene interface as well as on the electronic structure of graphene is investigated. Raman spectroscopy unveils that 1-butyl-3-methylimidazolium ILs having smaller anions induce n-type doping, while ILs with larger anions have a negligible effect on the doping. Molecular modeling simulations reveal that changes in the electrostatic potential at the IL–graphene interface are responsible for the n-type doping.

Co-operation of electrochemistry and chemometrics to develop a novel electrochemical aptasensor based on generation of first- and second-order data for selective and sensitive determination of the prostate specific antigen biomarker

In this work, a novel electrochemical aptasensor was fabricated based on modification of glassy carbon electrode with prostate specific antigen (PSA) binding DNA aptamer (5′-HS-(CH2)6-TTTTTA ATT AAA GCT CGC CAT CAA ATA GCTTT-3′)/gold nanoparticles/electrochemically reduced fullerene-C60/multi-walled carbon nanotubes- ionic liquid-graphene (APT/Au NPs/C60/MWCNTs-IL-Gr/GCE). By incubation of the aptasensor with the PSA and binding of the PSAs with the APTs, steric hindrance at the aptasensor was increased therefore, the voltammetric response of the aptasensor immersed in the electrochemical probe solution was decreased. Three electrochemical methods including differential pulse voltammetry (DPV), square wave voltammetry (SWV) and amperometry were used to assist the aptasensor for determination of the PSA, and the best one was chosen for the analysis of real matrices. Second-order DPV data, first-order SWV and amperometric data were generated and used to assist the aptasensor. Second-order DPV data were modeled by PARAFAC2 and MCR-ALS while SWV and amperometric data were modeled based on classical univariate calibration methods to build the calibration models. After evaluation of the performances of different calibration models in the analysis of synthetic samples, the second-order calibration based on DPV data modeled by MCR-ALS was chosen as the best calibration model to assist the aptasensor for the analysis of serum samples as real matrices. Fortunately, the aptasensor assisted by MCR-ALS had a very good performance for the determination of the PSA in serum samples which was comparable with a reference method.

Double layer in ionic liquids: Temperature effect and bilayer model

This work describes the effect of potential and temperature on the graphene–ionic liquid (EMImBF4) interfacial structure and properties with the focus on a novel phenomenon of ionic saturation. We apply classical molecular dynamics simulations to reproduce well-known phenomena of overscreening, monolayer formation, and temperature-induced smearing of the interfacial structure. Using quantum density functional theory calculations, we show how quantum capacitance dampens the influence of temperature and improves the agreement with the experimental data. Using a bilayer model, we study characteristic features of capacitance–potential dependence and relate them to the changes in interfacial structure. These insights are of fundamental and practical importance for the application of similar interfaces in electrochemical energy storage and transformation devices such as capacitors and actuators.

Bioinspired ionic liquid-graphene based smart membranes with electrical tunable channels for gas separation

Fast response and adjustable selectivity have always been the main goals pursued by the manufacture of smart membranes. Herein, we demonstrate smart membrane materials composed of ionic liquid (IL), PVDF, and graphene. The infusion of IL aims to increase the β crystal phase of PVDF, which is also assisted by the presence of graphene in the system. β-PVDF increased from 48.9% to 79.8% when the ionic liquid content [BMIM][BF4] increases from 2 to 10 wt.%, which makes the membrane more electro responsive. The optimum membrane of PG-BF4 10 wt.%, upon supply of 3 V DC electricity, exhibited CO2/N2 selectivity value of about 129.03 and CO2 permeability of 193.55 Barrer, exceeding the 2008 Robeson upper bound. Positron Annihilation Lifetime Spectroscopy (PALS) data and Density Function Theory (DFT) simulations helped to realize the mechanism of selective transportation of CO2 gas through the membrane both theoretically and experimentally. PALS results indicated that the free volume of PVDF increased; indicating that CO2 molecules can easily pass through the system. DFT simulation revealed that the CO2 molecules show more attraction and lower adsorption energy under electric stimuli, confirming that CO2 molecules adsorb and diffuse quickly through the system than O2 or N2. Overall, tuning the free volume and molecular interactions within the membrane channels provide a rational molecular sieving approach to improve membrane selectivity.

Electrocatalytic determination of levodopa in presence of cabergoline using carbon paste electrode modified with graphene quantum dots/2-chlorobenzoyl ferrocene/ionic liquid

The electrochemical sensor was fabricated for the simultaneous determination of levodopa and cabergoline using carbon paste electrode (CPE) modified with graphene quantum dots (GQD), 2-chlorobenzoyl ferrocene (2CBF) and ionic liquid (IL). Then, the electrochemical behavior of levodopa alone and simultaneously with cabergoline at the surface of GQDs/2CBF/IL/CPE was investigated in phosphate buffer solution (PBS). Under optimal PBS, pH=7 condition, oxidation peak current has been found proportional to levodopa concentration in the range between 0.07 μM and 500.0 μM, with the limit of detection (LOD) of 0.02 μM (S/N=3). Outputs showed that at GQDs/2CBF/IL/CPE surface, the levodopa and cabergoline oxidation peaks are separated by the potential difference of 200 mV. In addition, it was found that this modified electrode possesses acceptable sensitivity, selectivity, stability and repeatability. All these properties were sufficient to allow simultaneous detection of levodopa and cabergoline in real samples at the surface of GQDs/2CBF/IL/CPE. This was supported by the successful application of this electro­chemical sensor electrode for the determination of levodopa and cabergoline in urine, serum, and cabergoline tablets.

Understanding the Photoelectrochemistry of Covalently Grafted Ionic Liquid–Graphene Nanosheets toward NADH Detection

A completely unprecedented photoelectrochemical detection of β-nicotinamide adenine dinucleotide (NADH), using a covalently grafted imidazolium ionic liquid (IL) on reduced graphene oxide (RGO) nan...

Molecular dynamics studies on the exfoliation of graphene in room temperature ionic liquids

The theoretical principle of choosing suitable room temperature ionic liquids (RTILs) for liquid phase exfoliation of graphene sheet remains a challenge. A simple thermodynamic model is proposed to show the vital role of the solvation energy as an indication of solvent quality for spontaneous exfoliation. The solvation energy of ten ILs surrounding a single graphene sheet has been determined by molecular dynamics (MD) simulations. It is found to be strongly correlated with surface excess, sheet deformation and surface tension. From the MD simulations [BMIM][TF2N] (-580.86 mJ.m−2), [BPY][ TF2N] (-565.19 mJ.m−2) and [BMP][TF2N] (-551.77 mJ.m−2) shows stronger affinity with graphene surfaces as compared to other ILs. On the basis of the established mechanism, one is able to predict and screen suitable IL for spontaneous exfoliation of graphene.

Sensitive detection of carcinoembryonic antigen based on a low-potential-triggered electrochemiluminescence of tris(2,2′-bipyridine)ruthenium(II) with oxalate as coreactant

Recently, tris(2,2′-bipyridine)ruthenium(ii) (Ru(bpy)32+) has been widely used in electrochemiluminescence (ECL) biosensors. However, its ECL has to be triggered at high positive or negative voltage, leading to a series of undesired side effects containing electrical damage toward biomolecules and hydrogen/oxygen evolution reaction. In this work, a new low-potential-triggered Ru(bpy)32+ ECL system was reported using oxalate as coreactant with the aid of platinum nanoparticles (Pt NPs) electrodeposited on the surface of the graphene-ionic liquid composite. When cyclic scanning in a potential range from 0.3 to −0.2 V, the ECL signal appeared at onset potential of 0.05 V (vs. Ag/AgCl), and reached the maximum at about −0.15 V (vs. Ag/AgCl), which were quite lower than that in the typical Ru(bpy)32+-oxalate (C2O42−) ECL system. The possible mechanism was discussed in detail. Meanwhile, the low-potential-triggered ECL signals were strong and stable, which were further exploited to detect the carcinoembryonic antigen (CEA). The constructed ECL sensor exhibited a good linearity between the ECL signals and the logarithm of the concentrations of CEA in a range from 0.1 pg mL−1 to 100 ng mL−1 with the detection limit of 0.01 pg mL−1. In addition, the sensor showed a promising potential in clinical detection.

Sensitive Detection of Sulfide Ion Based on Fluorescent Ionic Liquid-Graphene Quantum Dots Nanocomposite.

Sulfide ions (S2−) that are widely distributed in biological and industrial fields are extremely toxic and pose great harms to both ecological environment and human health. However, fluorescent sensors toward S2− ions commonly use S2−-recovered fluorescence of fluorophore that is first quenched mainly by metal ions. Fluorescent probe which enables direct, selective, and sensitive detection of S2− ion is highly desirable. Herein, we demonstrate one-step preparation of fluorescent ionic liquid–graphene quantum dots (IL-GQDs) nanocomposite, which can act as a fluorescent probe for direct and sensitive detection of S2− ion. The IL-GQDs nanocomposite is easily synthesized via facile molecular fusion of carbon precursor and in situ surface modification of GQDs by IL under hydrothermal condition. The as-prepared IL-GQDs nanocomposite has uniform and ultrasmall size, high crystallinity, and bright green fluorescence (absolute photoluminescence quantum yield of 18.2%). S2− ions can strongly and selectively quench the fluorescence of IL-GQDs because of the anion exchange ability of IL. With IL-GQDs nanocomposite being fluorescent probe, direct and sensitive detection of S2− is realized with a linear detection range of 100nM–10μM and 10μM–0.2mM (limit of detection or LOD of 23nM). Detection of S2− ions in environmental river water is also achieved.

Synergistic Behavior of Graphene and Ionic Liquid as Bio-Based Lubricant Additive

The constant utilization of petroleum-based products has prompted concerns about the environment, hence a replacement for these products must be explored. Biolubricants are a suitable replacement for petroleum-based lubricants as they provide better lubricity. Biolubricant performance can be improved by the addition of graphene. However, there are reports that graphene is unable to form a stable suspension for a long period. This study used a graphene-ionic liquid additive combination to stabilize the dispersion in a biolubricant. Graphene and ionic liquid were dispersed into the biolubricant via a magnetic stirrer. The samples were tested using a high frequency reciprocating rig. The cast iron sample was then further observed using various techniques to determine the lubricating mechanism of the lubricant. Different dispersion stability of graphene was observed for different biolubricants, which can be improved with ionic liquids. All ionic liquid samples maintained an absorbance value of three for one month. The utilization of ionic liquid was also able to decrease the frictional performance by 33%. Further study showed that by using the ionic liquid alone, the frictional could only reduce the friction coefficient by 13% and graphene could only reduce the friction by 7%. A smooth worn surface scar can be seen on the graphene-IL sample compared to the prominent corrosive spot on the IL samples and abrasive scars on graphene samples. This indicates synergistic behavior between the two additives. It was found that the ionic liquid does not only improve the dispersion stability, but also plays a role in forming the tribolayer.

Ionic Liquid-Graphene Oxide for Strengthening Microwave Curing Epoxy Composites

The challenge for fully exerting the excellent nature of graphene oxide (GO) within polymer composites was to realize its uniform dispersion and strong interfacial bonding in the polymer matrix thr...

Tuning the hybrid borophene−/graphene-ionic liquid interface: Effect of metal cations on the electronic and photonic properties

A density functional theory study of the effect of the adsorption of a mixture of potassium salt and an imidazolium-based ionic liquid on the electronic and optical properties of graphene and borophene is performed. This addition leads to a downward shift of the Dirac cone for both structures due to the electron transferred from potassium to the graphene and borophene sheets. The transferred charge to the wall and the binding energy of potassium are bigger than in the case of the adsorption of potassium on pristine graphene. The Bader analysis shows that the presence of potassium makes the charge distributions in graphene and the layers of boron atoms in borophene more homogenous and the electronic distribution in the sheets is consequently less affected by the electromagnetic interaction with the ions in the mixture. Moreover, the results show that the optical properties are altered by the metallic cation, especially at low frequencies where ionic liquid-enhanced 2D plasmons are registered due to the doping of 2D sheets by electrons transferred from the potassium atoms leading to near zero values of the parallel components of the dielectric function and refractive index.

Investigation of the Ionic Liquid Graphene Electric Double Layer in Supercapacitors Using Constant Potential Simulations

In this work, we investigate the effect of the cation structure on the structure and dynamics of the electrode–electrolyte interface using molecular dynamics simulations. A constant potential method is used to capture the behaviour of 1-ethyl-3-methylimidazolium bis (trifluoromethane)sulfonimide ([Cmim][NTf]) and butyltrimethylammonium bis(trifluoromethane) sulfonimide ([N][NTf]) ionic liquids at varying potential differences applied across the supercapacitor. We find that the details of the structure in the electric double layer and the dynamics differ significantly, yet the charge profile and capacitance do not vary greatly. For the systems considered, charging results in the rearrangement and reorientation of ions within ∼1 nm of the electrode rather than the diffusion of ions to/from the bulk region. This occurs on timescales of (10 ns) for the ionic liquids considered, and depends on the viscosity of the fluid.

An ultrasensitive label-free electrochemical immunosensor based on 3D porous chitosan–graphene–ionic liquid–ferrocene nanocomposite cryogel decorated with gold nanoparticles for prostate-specific antigen

A highly sensitive and selective label-free electrochemical immunosensor was successfully fabricated for measuring prostate-specific antigen (PSA). A composite of chitosan, graphene, ionic liquid and ferrocene (CS-GR–IL–Fc) was drop casted onto a screen-printed carbon electrode (SPCE) and frozen to create a layer of 3D porous cryogel (CS-GR-IL-Fc cry) which was decorated with gold nanoparticles (AuNPs). The biocompatibility and porosity of the cryogel increased the surface area available for AuNPs loading via amino groups and the population of anti-PSA, immobilized on the AuNPs via chemisorption, could be increased. The CS-GR-IL-Fc cry displayed excellent conductivity, enhancing electron transfer and amplifying the current signal. Differential pulse voltammetry was employed to determine PSA by measuring the reduction in the Fc oxidation peak current in response to the formation of PSA/anti-PSA immunocomplex. Under the optimized incubation time and electrolyte pH, the developed immunosensor displayed excellent analytical performances, including a wide linear range at concentrations from 1.0 × 10−7 to 1.0 × 10−1 ng mL−1, with a very low limit of detection of 4.8 × 10−8 ng mL−1 and good reproducibility (relative standard deviation of <4.6%, n = 6), stability (90% sensitivity within 20 days), repeatability (12 cycles of binding–rebinding, the sensitivity > 90%) and selectivity. The results obtained from the device for the determination of PSA in human serum were consistent with results from the enzyme-linked immunosorbent assay (P > 0.05), and indicated the promising potential of the proposed immunosensor in clinical diagnosis.

Graphene–Ionic Liquid Interfacial Potential Drop from Density Functional Theory-Based Molecular Dynamics Simulations

Ionic liquids (IL) are promising electrolytes for electrochemical applications due to their remarkable stability and high charge density. Molecular dynamics simulations are essential for better understanding the complex phenomena occurring at the electrode-IL interface. In this work, we have studied the interface between graphene and 1-ethyl-3-methyl-imidazolium tetrafluoroborate IL, using density functional theory-based molecular dynamics simulations at variable surface charge densities. We have disassembled the electrical double layer potential drop into two main components: one involving atomic charges and the other - dipoles. The latter component arises due to the electronic polarisation of the surface and is related to concepts hotly debated in the literature, such as the Thomas-Fermi screening length, effective surface charge plane, and quantum capacitance.

Electrochemical determination of rutin based on molecularly imprinted poly (ionic liquid) with ionic liquid-graphene as a sensitive element

A novel electrochemical sensor for rutin determination based on molecularly imprinted poly (ionic liquid) (MIPIL)/ionic liquid-graphene (IL-GR) modified electrode was developed. MIPIL was synthesized via free radical polymerization using rutin as the template, 1-allyl-3-ethyl imidazolium bromide ([AEIm]Br) as the functional monomer and 1,4-butanediyl-3,3′-bis-l-vinylimidazolium dibromide ([V2C4(mim)2]Br2) as the cross-linker. The composite of IL-GR as an electrode sensitive element was prepared using carboxymethyl-3-methylimidazolium chloride ([HO2CMMIm]Cl) by one-step ultrasound method. Fourier transform infrared spectroscopy and scanning electron microscope were used to characterize IL-GR and MIPIL. The increased surface area and conductivity of IL-GR/GCE improved the sensitivity of rutin sensor. Under the optimum condition, good linearity for rutin analysis by the MIPIL-based sensor was obtained from 0.03 to 1 μM. A low limit of detection for rutin was 0.01 μM (S/N = 3). The MIPIL-based sensor demonstrated superiority on DPV response compared to the imprinted sensors based on traditional cross-linkers. The proposed sensor presented good selectivity for rutin and successfully applied for rutin determination in tablets with RSD of 3.05 % and the average recovery range was 98.3 %.

Differential capacitance of ionic liquid interface with graphene: The effects of correlation and finite size of ions

We use a mean field model for ionic liquids, which takes into account both the ion correlation and the finite ion size effects, in order to calculate the differential capacitance of the ionic liquid interface with single-layer graphene. Besides choosing ion packing fractions that give rise to the camel-shaped and bell-shaped capacitances of the diffuse layer in ionic liquids, we also explore small packing fractions in the regime of “dilute electrolytes”, as well as different packing fractions for different ion species in asymmetric ionic liquids. We find that the main effect of a graphene electrode arises due to a V-shaped minimum in its quantum capacitance at the point of zero charge (PZC), which is a manifestation of the Dirac cone structure of graphene's π electron bands. As a result, the total capacitance of a graphene–ionic liquid interface exhibits a camel-shaped dependence on the total applied potential, even for large ion packing fractions and finite ion correlation lengths. While the minimum at the PZC in the total capacitance is “inherited” from graphene's quantum capacitance, the two peaks that occur for applied potentials ∼±1 V are sensitive to the presence of the ion correlation and a Stern layer, which both tend to reduce the height and flatten the peaks in the camel-shaped total capacitance. Finally, we have found that the largest fraction of the applied potential goes to charging the graphene electrode.