A facile one-step, eco-friendly, and cost-effective approach for the formation of copper oxide (CuO) nanowires by a green method using saponin-rich Sapindus mukorossi fruit extract (SMFE). The physio-chemical characteristics of the synthesized CuO nanowires have been characterized by X-ray Diffractometry (XRD), X-ray Photoelectron Spectroscopy (XPS), FT-IR (Fourier Transform Infrared Spectroscopy, FE-SEM (Scanning Electron Microscopy), and High-Resolution Transmission Electron Microscopy (HR-TEM). Further, the electrocatalytic activity of the CuO nanowires synthesized with SMFE has been investigated, and they have been used as dopamine (DA) sensors. Because of their unique properties, the CuO nanowires/GCE exhibited remarkable electrochemical response for the detection of DA with enhanced current response. The anodic current demonstrated that the CuO nanowires/GCE linearly detects the concentration of DA over the range of 0.1 µM to 0.105 mM of DA with a regression co-efficient of 0.9960. The obtained results illustrated that the synthesized CuO nanowires can easily stimulate the electron transfer reaction between DA and the nanowires modified electrode with the improvement of the conductivity and stability of the electrode. This remarkable electrocatalytic property of CuO nanowires makes it a unique electrochemical sensor for the detection of DA. Furthermore, the sensor is free from the interference of ascorbic acid, uric acid, and other interfering species. Moreover, the anti-interference performance also showed that the CuO nanowires/GCE could be employed for the determination of DA in real samples with good selectivity and sensitivity.

A simple and facile method for the simultaneous preparation of water soluble, fluorescent graphene quantum dots (GQDs) and reduced graphene oxide (RGO) is proposed and demonstrated in the present work. This particular method involves constant current electrolysis of a graphite rod either in an aqueous solution or in absolute ethanol medium consisting of sodium methoxide as an electrolyte. The process of electrolysis is carried out at a fixed current density for varied durations. Further chemical reduction of the resultant electrochemically exfoliated graphitic solution produces a clear supernatant solution and a black precipitate. Depending upon the solvent medium, duration of electrolysis and nature of the reducing agents, the size and in turn the emission colour of GQDs could be altered. In this work specifically GQDs that emit blue, green and yellow colours have been prepared. Structural, morphological and chemical characteristics of these GQDs are analyzed by using microscopic [scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM)], spectroscopic (UV-Vis spectroscopy, Fourier transform infrared (FTIR), X-ray diffraction (XRD), Raman) and electrochemical techniques. Microscopic images obtained using HRTEM analysis clearly reveal the formation of lattice fringe patterns for these multi-coloured GQDs along with uniform particle size distribution. Further these fluorescent GQDs are utilized for bio-imaging applications and cytotoxicity studies using Staphylococcus aureus and Escherichia coli bacteria. Similarly, the resultant black precipitate is identified as a few layered reduced graphene oxide and the structural and morphological characterization is carried out using SEM, FTIR, XRD and Raman spectroscopic studies. Further electrochemical investigation indicates the possible application of such layered RGO as a supercapacitor electrode material possessing a fast response time in the order of seconds for delivering the stored power.

Modern day hospital treatments aim at developing electrochemical biosensors for early diagnosis of diseases using unconventional human bio-fluids like sweat and saliva by monitoring the electron transfer reactions of target analytes. Such kinds of health care diagnostics primarily avoid the usage of human blood and urine samples. In this context, here we have investigated the electron transfer reaction of a well-known and commonly used redox probe namely, potassium ferro/ferri cyanide by employing artificially simulated bio-mimics of human sweat and saliva as unconventional electrolytes. Typically, electron transfer characteristics of the redox couple, [Fe(CN)₆]^3−/4− are investigated using electrochemical techniques like cyclic voltammetry and electrochemical impedance spectroscopy. Many different kinetic parameters are determined and compared with the conventional system. In addition, such electron transfer reactions have also been studied using a lyotropic liquid crystalline phase comprising of Triton X-100 and water in which the aqueous phase is replaced with either human sweat or saliva bio-mimics. From these studies, we find out the electron transfer reaction of [Fe(CN)₆]^3−/4− redox couple is completely diffusion controlled on both Au and Pt disc shaped electrodes in presence of sweat and saliva bio-mimic solutions. Moreover, the reaction is partially blocked by the presence of lyotropic liquid crystalline phase consisting of sweat and saliva bio-mimics indicating the predominant charge transfer controlled process for the redox probe. However, the rate constant values associated with the electron transfer reaction are drastically reduced in presence of liquid crystalline phase. These studies are essentially carried out to assess the effect of sweat and saliva on the electrochemistry of Fe^2+/3+ redox couple.

Selective and sensitive non-enzymatic electrochemical detection of cysteine (CySH) is achieved in the present work using a polydopamine capped silver nanoparticles (Ag–PDA) modified indium tin oxide (ITO) electrode. Efficient redox properties, synergistic effects and the specific steric hindrance associated with the Ag–PDA core–shell nanoparticles provide higher selectivity and larger sensitivity for CySH detection over other competitive bio-thiols namely, homo-cysteine and glutathione. A simple one-step method is used for the preparation of Ag–PDA core–shell nanoparticles. The structure, morphology and composition of Ag–PDA nanoparticles are characterized by using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-visible (UV-vis) and Fourier transform infra red (FTIR) spectroscopic techniques. Electrochemical characteristics are investigated by using cyclic voltammetry (CV) and linear sweep voltammetry (LSV). These studies clearly reveal the formation of Ag–PDA nanocomposite on the ITO electrodes and their corresponding redox properties. Non-enzymatic electrochemical detection of CySH is carried out using Ag–PDA modified ITO electrodes in 0.1 M PBS (pH = 5.0) aqueous solution. Under the optimized conditions, this particular electrochemical biosensor exhibits a perfect linear calibration plot in the concentration range between 0.05 μM and 300 μM. Further, a sensitivity value of 0.023 μA μM⁻¹ and a lower detection limit of 0.02 μM are determined for CySH detection. Moreover this Ag–PDA modified ITO electrode is applied further for the determination of CySH in human blood serum samples and the results are promising and satisfactory, suggesting the possible analytical application of this biosensor for the determination of CySH in biological samples.