Graphene based electrochemical sensors have also been employed to detect environmental contaminants (paraoxon, nitromethane, heavy metal ions, hydroquinone and catechol, methyl jasmonate, hydrazine), pharmaceutical compounds (paracetamol, aminophenol, aloe-emodin, Rutin,etc), industrial compounds (ethanol) and explosives (TNT).
Dopamine (DA) is an important neurotransmitter, deficiency of which underlies Parkinson’s diseases. DA detection is challenged by its low physiological concentration (0.01 mM–1 mM) and interference from much more abundant ascorbic acid (AA) and uric acid (UA).
A chitosan–RGO composite electrode for DA detection was demonstrated in literature. A linear detection range (5–200 mM) was achieved in the presence of a large excess of AA or UA (500 mM). In addition, it showed that the chitosan–RGO electrode outperformed the electrode made of chitosan and multi-walled carbon nanotubes.
An electrochemical sensor was also demonstrated to selectively detect dopamine with a LOD of 0.01 mM based on a composite electrode made of Nafion and N-(trimethoxysilylpropyl) ethylenediamine triacetic acid (EDTA) modified RGO.
The high performance arises from several reasons:
(1) dopamine can interact with RGO via p–p interaction;
(2) EDTA groups, combined with ionic sulfuric groups of Nafion, can concentrate DA from the solution;
(3) EDTA groups linked to the RGO surface promote electron transfer as evidenced by the narrower potential separation between the anodic and cathodic peaks (DEp);
(4) the oxygen containing functional groups on RGO block the diffusion of AA and thus eliminate its interference.
In another work, detection of DA at 5 nM was realized in the presence of excess AA using a b-cyclodextrin/RGO nanocomposite electrode. b-Cyclodextrin functionalization assists dispersion of RGO sheets, and greatly improves the electrochemical performance.
As compared with the bare RGO electrodes, the b-cyclodextrin/RGO electrodes exhibited a two orders- of-magnitude-lower LOD, attributable, at least in part, to the faster electron transfer rate (DEp was reduced from 115 mV to 73 mV).
AA and UA sensors have also been developed using graphene materials. An AA sensor using graphene nano-sheets exfoliated in liquid by dimethylformamide (DMF). A UA sensor was constructed by self-assembling gold nanoparticles (AuNPs) onto pyrenebutyrate functionalized RGO (PFG) sheets. A LOD of 0.2 mM was obtained.
Novel microwave plasma enhanced CVD method was utilized to obtain multilayer graphene nanoflake films (MGNFs) vertically grown on a silicon substrate. DA, AA, and UA can be unambiguously distinguished by three well-defined peaks that appeared in the cyclic voltammogram (CV). Furthermore, near-ideal electron transfer kinetics was evidenced by the narrow DEp (61.5 mV at the scan rate of 10 mV s-1) which is close to the ideal value of 59 mV. Such a fast electron transfer process is due to the abundant edge planes and defects on the nanoflakes, unique electronic structure of graphene, and the good electrical contact between MGNFs and silicon substrate.
Cholesterol is an essential constituent of cell membranes. However, undesired accumulation of cholesterol and its esters causes critical health problems, such as heart diseases, cerebral thrombosis, and atherosclerosis.
A sensitive amperometric sensor based on functionalized RGO sheets has been developed for detection of cholesterol and its esters with a LOD of 0.2 mM.161 Cholesterol esterases and cholesterol oxidases were loaded onto the electrode to catalyze the hydrolysis of cholesterol and its esters, and consequently, generate H2O2. Platinum nanoparticles decorated on RGO sheets, in turn, catalyze the electrochemical oxidization of H2O2. Nafion coating was used at the same time to block other irrelevant analytes (e.g., ascorbate and urate).
Extracted and edited from “Biological and chemical sensors based on graphene materials by Yuxin Liu, Xiaochen Dong and Peng Chen in Chemical Society Reviews, 2012″