Ultralight, multifunctional 3D nitrogen-doped graphene aerogel - Part 2

I. PHYSICAL PROPERTIES

• 10 mg of NGA was able to support 5000 fold its own weight
• The NGA was also fire-resistant
• the NGA was electrically conductive with a high conductivity of 262 S/m

 - the NGA is an ideal candidate for highly efficient separation/extraction of specific substances, such as organic pollutants and oils because of:

 + surface hydrophobicity

 + high porosity

 + mechanically stable

•  NGA showed the capacity to uptake amounts of liquids up to 40 to 156 times of their own weight

 - most of the porous volume was used for oil storage

The reason for this high adsorption capacity was that oils were stored mainly in the interconnected pores formed by the oleophilic walls of the doped graphene sheets. The absorbed oil or solvents can be removed by direct combustion in air for recycled use of the NGA

II. CHEMICAL SENSORS

Ascorbic acid (AA), DA and uric acid (UA) play important roles in inducing cancer, Parkinson’s disease, Huntington’s diseases, schizophrenia, etc => Electroanalytical method has been developed for the determination of these biological substances. Electrochemical determination of these species based on anodic oxidation suffers from the oxidation peaks severely overlapping with solid electrodes

The CV (Cyclic voltammograms) behaviors of AA, DA and UA were investigated

(a) Cyclic voltammograms of 1.0 mM AA, 1.0 mM DA and 1.0 mM UA in 0.10 M PBS (pH 7.4) at NGA/GCE with a scan rate of 100 mV/s. (b) Differential pulse voltammogram (DPV) for 1.0 mM AA, 0.050 mM DA and 0.10 mM UA in 0.10 M PBS (pH 7.4) at NGA/GCE, GA/GCE and GCE. (c) DPV for different concentrations of DA from 0.50 to 160 lM containing 1.0 mM AA and 50 lM UA at NGA/GCE. Inset: plots of the anodic peak current as a function of DA concentrations. (d) Electrochemical impedance spectroscopy of NGA/GCE and GA/GCE in 2.5 mM [Fe(CN)₆]³/[Fe(CN)₆]⁴ containing 0.10 M KCl. The frequency range was selected from 0.01 to 10⁵ Hz with a perturbation amplitude of 5 mV. The initial potential was 0.10 V

•  AA was negatively charged, and the oxidation peak of AA corresponded to the oxidation of hydroxyl groups to carbonyl groups. The formation of hydrogen bonds between the NGA microlayers and AA may increase the electron transfer
•  the hydrogen bonds between the doped nitrogen atoms within the NGA layers and the hydroxyl or amine groups from DA molecule enhanced the electron transfer kinetics
• UA showed quasi-reversible electrochemical behavior with oxidation/reduction peaks on the NGA/GCE, revealing that UA was first oxidized to quinonoid, and then experienced a rapid chemical reaction, which matched to an EC mechanism

Compared to the bare GCE and GA/GCE, the oxidation potential of AA, DA and UA was distinctly negative shifted and well separated, in addition to the much higher oxidation currents using the NGA/GCE.

The difference of their oxidation potentials was large enough to distinguish each other => NGA accelerated the oxidation of AA, DA and UA, and thus decreased their overpotentials, which is the key factor to realize their simultaneous determination.

DPV (Differential pulse voltammogram) is a sensitive way to undertake electrochemical detection

 - the addition of DA into the electrochemical cell did not have significant influence on the peak

currents or peak potentials of the other two biomolecules (Fig. c)

 - the NGA/GCE showed an almost straight tail line and better ability to promote the

electron transfer than that of the GA/GCE due to the nitrogen doping and the particular 3D microstructure with multiple electron paths

=> NGA possessed preferable electroactivity in neutral media and displayed excellent electrocatalytic activity towards the oxidation of AA, DA and UA