Reduced Graphene Oxide-based Covalent Hybrid Film Electrode Self-assembled with Gold Nanoparticles for the Enzyme-Free Amperometric Sensing of Serum Uric Acid.

Purine metabolism in the human body leads to the production of uric acid (UA) at the end. But an abnormal level of UA in the human body creates health problems. The sensing and quantification of UA is essentially required to prevent and diagnose hypertension, arthritis, gout, hyperuricemia or Lesch-Nyhan syndrome, etc. Herein, the development of a sensing platform for the measurement of UA using Au nanoparticle-based hybrid self-assembly is described. The self-assembling of a thiol-terminated silicate network functionalized graphene oxide hybrid on a polycrystalline Au surface yields a three-dimensional assembly. The oxygen functionalities of the self-assembly were partially reduced by an NaBH4 treatment. The free -SH groups of the self-assembly were successfully used for the immobilization of Au nanoparticles by chemisorption. The nanoparticle-based hybrid self-assembly is highly sensitive toward UA, and shows a wide linear response with a detection limit of 40 nM UA (S/N = 7) without interference from co-exiting ascorbic acid. Its practical application was demonstrated using human serum samples.

Rational functionalization of reduced graphene oxide with an imidazole group for the electrochemical sensing of bisphenol A - an endocrine disruptor.

Reduced graphene oxide has been rationally functionalized with histamine for the highly sensitive and selective electrochemical determination of bisphenol A (BPA). Histamine is covalently attached to graphene oxide by amide coupling and the oxygen functionalities of graphene oxide are partially reduced electrochemically. The facilitated electrochemical oxidation of BPA was achieved in neutral pH with functionalized reduced graphene oxide. The pH of the reaction controls the oxidation of BPA. Electrochemical oxidation is not favourable at pH less than the pKa of histamine due to the protonation of the imidazole nitrogen. The electrode is highly sensitive (1727.29 ± 12.48 nA µM-1 cm-2) towards BPA and shows a linear response up to 30 µM of BPA. It could detect as low as 0.03 nM of BPA (S/N = 5). The other coexisting analytes do not interfere with the voltammetric measurement of BPA.

Covalent functionalization and electrochemical tuning of reduced graphene oxide for the bioelectrocatalytic sensing of serum lactate.

Lactate is a byproduct of glycolysis and serum lactate can be used as a non-invasive biomarker in risk-stratifying patients with several life-threatening diseases. Herein, we describe the bioelectrocatalytic sensing of lactate using a covalently functionalized reduced graphene oxide (rGO)-based material. The development of a lactate biosensor involves the covalent functionalization of rGO with the p-nitrophenyl moiety, electrochemical generation of a surface-confined redox mediator and immobilization of l-lactate dehydrogenase (LDH). The covalently functionalized rGO was characterized by XRD, XPS, FTIR, Raman, resistivity and electrochemical measurements. The covalent attachment of the nitrophenyl moiety on the basal plane of the carbon network significantly influences the capacitive properties of rGO. The chemically functionalized rGO was electrochemically tuned to generate a redox mediator (rGO-PhNHOH). An electrochemically generated redox couple (PhNHOH/PhNO) exhibits reversible voltammetric response at ∼-0.06 V with a surface coverage of (7.19 ± 0.26) × 10-9 mol cm-2. The redox couple efficiently mediates the oxidation of NADH at 0.04 V, which is ∼600 mV less positive potential than the unmodified electrode. The electrode is highly sensitive towards NADH and it could detect as low as 0.4 µM NADH at the potential of 40 mV at neutral pH without any interference from co-existing bioanalytes. A lactate biosensor was developed using the rGO-PhNHOH functional material and lactate dehydrogenase. The surface-confined redox mediator could successfully detect the enzymatically generated NADH at 40 mV. The biosensor is highly sensitive (10.57 ± 0.38 nA µM-1 cm-2) and shows linear response up to 90 µM of lactate. It could detect lactate as low as 2.5 µM without any interference from other analytes. This biosensor has been successfully used to quantify human serum lactate and the results are in excellent agreement with those obtained by the clinical method.

A Rationally Designed Thymidine-Based Self-Assembled Monolayer on a Gold Electrode for Electroanalytical Applications.

A self-assembled monolayer (SAM) of 1-(3,5-epidithio-2,3,5-trideoxy-ß-D-threo-pentofuranosyl)thymine (EFT) on a gold electrode was prepared and characterized by Raman spectral and electrochemical measurements. Voltammetric and electrochemical impedance measurements show that the SAM of EFT on a Au electrode impedes the electron-transfer reaction. The SAM of EFT was successfully used for the voltammetric sensing of urate in neutral solution. The coexisting ascorbate anion does not interfere and therefore the EFT-based electrode was able to quantify urate at the micromolar level in the presence of a large excess amount of ascorbate. To demonstrate the practical applications, the amount of urate in two different human serum samples was quantified by using the EFT-based electrode; the results are in good agreement with those determined by the clinical method. DFT calculations show that both ascorbate and urate have noncovalent interactions including hydrogen-bonding interactions with EFT.