DNA/RNA, Uracil And Choline Grafted Carbon Electrodes For Bioelectrochemical Sensing And In-vivo Monitoring Of Dopamine

Based on detailed reviews on nucleic acid (DNA and RNA) sensors and neurotransmitter detections in cerebrum, systematic research was carried out for electrochemical sensor fabrications by surface modification of bioactive species involving DNA, RNA, uracil and choline etc. SEM, XPS and electrochemical techniques involving cyclic voltammetry (CV) and differential pulse voltametry (DPV) were used for characterization of these electrodes. These sensors were used for electrochemical sensing of bioactive molecules including dopamine (DA), epinephrine (EP), noepinephrine (NE), uric acid (UA), ascorbic acid (AA), tryptophan (Trp) and tyrosine (Tyr) etc. A Nafion/Ch dual layer modified carbon fiber electrode was fabricated and used for in-vivo determination and monitoring the DA release in mouse cerebrum. The details are as follows:A poly(p-aminobenzensulfonic acid) dimer modified glassy carbon electrode (Pp-ABSA/GCE) was prepared by CV electro-polymerization under optimized condition. Then, a thin film of ct-DNA was deposited on the Pp-ABSA/GCE, obtaining a bi-layer composite modified electrode, DNA/Pp-ABSA/GCE. The Pp-ABSA deposited forms a grass-like 2-D fractal patterned nano lattice on the GCE surface. The 2-D nano clusters of Pp-ABSA have good electric conductivity and catalytic activity toward electrochemical oxidations of DA and UA. The composite electrode DNA/Pp-ABSA/GCE presented superior electrocatalytic activity towards the oxidation of DA, UA and AA, whose DPV oxidation peak appeared at 0.308,0.172 and -0.004 V (vs. SCE) respectively, allowing simultaneous determination of these species in a mixture. This electrode showed obvious ability for increasing the current response for DA and UA through surface enrichment, and reducing interference of AA by repelling. With 1000 fold coexistence of AA, this electrode can be used to determine DA and UA selectively with a detecting limit of 88 nmol/L and 0.19μmol/L, respectively. The current sensitivity was about 4 and 5 fold of at DNA/GCE and Pp-ABSA/GCE for DA and UA. The super sensing ability of this hi-layer electrode was from the synergic effect of DNA and nano-Pp-ABSA clusters. It was proposed that theπ-electron stacking of the DNA base pairs and the conjugated aromatic rings of the conductive dimer may have interactions, which might provide a perfect pathway for charge transport (CT) from the conducting DNA chains to the GCE and facilitate the electron transfer processes.For comparison with the DNA grafted electrode, a RNA grafted GCE (RNA/GCE) was prepared by electrochemical deposition of RNA at the optimized potential of 1.8 V (vs. SCE). It was found that this electrode presents strong electro-catalytic ability for UA, DA and AA, whose anodic DPV peaks appeared at 0.312, 0.168 and -0.016 V, respectively, similar to the DNA grafted electrode. The RNA/GCE has strong enrichment effect for DA and superior exclusive ability for AA. The co-existence of 5000 fold AA did not interfere with the detection of DA or UA. In the presence of 2.0 mmol/L AA, a linear range of 0.37 to 36μmol/L with a detection limit of 0.2μmol/L for DA, and in the range of 0.74 to 73μmol/L with a detection limit of 0.36μmol/L for UA were obtained. The modified electrode shows excellent selectivity, good sensitivity and good stability.For investigation of key factors for the catalytic activity of DNA/RNA modified electrodes, a series of surface grafted GCE sensors were prepared by electrochemical grafting of ct-DNA, RNA, poly(dA), poly(dT), poly(dG), poly(dC), adenine (Ade), uracil (Ura), guanine (Gua) and cytidine (Cyt), respectively. These grafted GCEs were examined for electro-oxidation of Trp. It was found that those molecules rich of G and C segments will give better catalytic performance. Both purines and pyrimidine modified electrodes exhibited catalytic ability. Thus, we found that the bases, as the building block of nucleic acid, are the key elements for generating electro-catalytic abilities. We focused the study on the uracil grafted GCE (Ura/GCE) for Trp, Tyr, DA, EP, NE, UA and AA oxidation reactions, and the mechanism for the catalytic activity was proposed based on molecules interactions. The molecular recognition ability was related to the molecular structure. It indicates that the molecules which have the N 6-member heterocyclic ring could more easily interact with uracil grafted on the GCE through H bond andπelectrons action. An "arm effect" of side chains in the molecules was discussed as a significant interaction factor.Finally, we fabricated a choline (Ch) covalently grafted carbon fiber electrode (Ch/CFME), which was then covered by a thin layer of Nation, forming bi-layer modified electrode, Nation/Ch/CFME. Due to the electrostatic interaction between positively charged Ch molecules and negatively charged Nation film, the Nafion/Ch/CFME was stable. In combination of the electrocatalytic activity of Ch monolayer and protecting ability of Nation layer, Nafion/Ch/CFME has good property for in-vivo determination of DA. This electrode was successfully used to detect DA release in mouse cerebrum, and obvious DA release pulses were monitored with needle stimulations on a certain site "Fengfu"of the skull.