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In this study, we report a new type of flexible and free standing paper electrochemical sensor fabricated by integrating reduced graphene oxide (rGO), poly(glycine) and polyphenol oxidase (PPO) enzyme. For the sensor development, firstly, electrochemical polymerization of glycine at the rGO paper electrode was carried out by the cyclic voltammetric method, and then the poly(glycine)/rGO paper electrode was further utilized as a matrix to immobilize the PPO enzyme purified from potato. The developed PPO-poly(glycine)/rGO immobilized paper sensor was characterized by scanning electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. Enzyme aggregation and distribution within the poly(glycine)/rGO paper electrode were studied using a fluorescence microscope. The electrochemical behavior of catechole (CC) on PPO-poly(glycine)/rGO immobilized paper electrode was investigated by cyclic voltammetry and amperometry. The sensor performed well in the concentration range of 0.1–800 µM, with a detection limit of 0.07 µM and a sensitivity of 0.0512 mA µM cm−2. more...

We proposed a new type of freestanding Catalase (EC 1.11.1.6) (Cat) modified paper electrode based on Fe3O4 crystals decorated freestanding reduced graphene oxide (rGO) paper (Cat-Fe3O4/rGO) using a facile electrodeposition method. This enzyme modified paper electrode was used for the specific determination of hydrogen peroxide (H2O2). Structural, chemical, and morphological characterization of the Fe3O4 crystals formed on rGO paper was investigated by (i) using scanning electron microscopy (SEM), (ii) energy-dispersive X-ray spectroscopy (EDS), (iii) X-ray photoelectron spectroscopy (XPS), (iv) X-ray diffraction spectroscopy (XRD), (v) Raman spectroscopy, (vi) electrochemical impedance spectroscopy (EIS) and (vii) cyclic voltammetry (CV) techniques. The Cat enzyme was successfully adsorbed on the Fe3O4 nanocrystals surface which was demonstrated by fluorescence microscopy. The Cat-Fe3O4/rGO paper electrode showed excellent electrocatalytic activity toward H2O2. It is found that Cat-Fe3O4/rGO paper electrode shows a wide linear range, high sensitivity, and a low detection limit for H2O2 detection. more...

We studied the production and electrochemical performance of NiCo2S4 electrodeposited reduced graphene oxide (rGO) paper for use as a freestanding flexible electrode for the simultaneous determination of folic acid (FA) and ascorbic acid (AA). Production of rGO paper was achieved through vacuum filtration of the aqueous dispersion of graphene oxide, followed by chemical reduction in hydrogen iodide solution. NiCo2S4/rGO paper material was fabricated with a simple approach using electrodeposition of hydrothermal synthesized NiCo2S4 nanoparticles directly on rGO paper. The physicochemical properties of as-prepared flexible NiCo2S4/rGO paper were characterized by means of field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray powder diffraction patterns. This freestanding paper electrode demonstrated high sensitivity, a wide linear range, and a low detection limit towards the simultaneous detection of FA and AA. NiCo2S4/rGO paper sensor, possessing easy preparation, low cost, and high flexibility, was tested in real samples with promising recoveries. This study exhibited that the approach of the electrodeposition of bimetallic sulfides on freestanding graphene paper substrates has great potential to develop high-performance flexible electrochemical sensors. more...

The flexible, free-standing, durable SnS2-gC3N4/rGO paper material was prepared by vacuum filtration of the composite dispersion containing GO and the SnS2-gC3N4 composite structure which is synthesized by a simple hydrothermal method. The as-prepared SnS2-gC3N4/GO composite paper was converted to SnS2-gC3N4/rGO paper material by applying a chemical reduction process. SnS2-gC3N4/rGO paper material was characterized by using FESEM, XRD, Raman, XPS, and EIS spectroscopy techniques. The SnS2-gC3N4 composite, a porous flower-like structure, uniformly intercalated between the graphene nanosheets exhibited excellent mechanical stability, greatly improved active surface areas, and enhanced surface porosity, in comparison with the pristine rGO paper. SnS2-gC3N4/rGO composite paper material show a highest specific capacitance of 532 F g-1 at 1 A g-1 and good cycling stability with capacitance retention of 60 % at 4 A g-1 after 5000 cycles. Moreover, SnS2-gC3N4/rGO composite paper demonstrate an excellent supercapacitor performance compared to the gC3N4/rGO paper and pristine rGO paper. The flexible symmetric supercapacitor based on SnS2-gC3N4/rGO composite electrode exhibits remarkable mechanical flexibility, high capacitive performance (403 F g-1 at 1 A g-1) and high cycle stability (63.8% at 4 A g-1 after 1500 cycles). As a result, SnS2-gC3N4/rGO composite paper material is expected to be a promising material for high-performance energy storage applications. more...

The present work demonstrates the fabrication and sensing performance of a free-standing paper electrode composed of boron oxide (B2O3) and graphene for the application of the simultaneous detection of melatonin (MEL) and ascorbic acid (AA). The flexible paper electrode (B-RGO) was fabricated through vacuum filtration of homogeneous composite dispersion (B-GO) consisting of B2O3 in GO, followed by chemical reduction with HI. The structural, chemical and morphological characterization of B-RGO composite paper was performed by using various techniques. This flexible paper electrode exhibited high sensitivity, wide linear range and low detection limit for the simultaneous determination of MEL and AA. The outstanding performance of B-RGO paper electrode could be attributed to the round-edged rod-like B2O3 structures between graphene layers, which provide an increment of the surface area and the electrochemical activity of the composite paper. Furthermore, this paper electrode showed high sensitivity and selectivity for the determination of MEL and AA in real sample analysis. more...

In this study, malate dehydrogenase (MDH) enzyme was immobilized on a cation octadecylamine (ODA+) monolayer at the air–water interface. Pressure–area (π–A) isotherm studies confirmed that the ODA–MDH system formed a stable monolayer at the air–water interface. The as-prepared MDH–ODA monolayer was transferred onto an indium tin oxide coated glass substrate (ITO) by the Langmuir–Blodgett (LB) method as an MDH–ODA/ITO LB film and characterized using FT-IR, SEM and UV-Vis absorption spectroscopy. The catalytic activity of the enzyme immobilized electrode (MDH–ODA/ITO) was assayed by sensing malic acid (MA) in the range of 10.0–50.0 mM. The high and preserved enzymatic activity of MDH in in vitro media was explored by following the absorbance (A340 nm) of nicotinamide adenine dinucleotide (NADH). Moreover, the highly sensitive electrochemical biosensor behavior of the MDA–ODA/ITO electrode for MA detection was displayed by cyclic voltammetry studies. The electrochemical studies revealed that a voltammetric current from the MDA–ODA/ITO electrode was obtained, while the ODA/ITO electrode did not show this current response. The MDA–ODA/ITO ​demonstrated sensitive electrochemical sensor ability for quantification of MA in both standard solutions and real samples. more...