G. Bharath

Solvent-free mechanochemical synthesis of graphene oxide and Fe3O4–reduced graphene oxide nanocomposites for sensitive detection of nitrite

We report a versatile and eco-friendly approach to prepare large-scale defect-free high quality graphene nanosheets from graphite by simple mechanochemical ball milling in the presence of KMnO4 and aspartic acid. Two foremost concerns such as surface chemistry and physical properties must be considered for potential application of the functionalized ball milled graphene. The surface chemistry was studied for the functionalized graphene anchored with 10, 20 and 30 wt% of Fe3O4 nanoparticles prepared by a simple hydrothermal process. The obtained samples were systematically studied by a variety of analytical and spectroscopic techniques to understand the structural, morphological, functional, compositional, electrical and magnetic properties. An electrochemical sensor was developed based on the prepared nanocomposite loaded on a glassy carbon electrode (GCE). The sensor based on the modified GCE exhibits good electrocatalytic activity, high sensitivity and stability for the detection of nitrite. The current response was linear over two different ranges between 0.5 and 58 μM with a wide range of 0.5 μM–9.5 mM, and low detection limit and sensitivity of 0.03 μM and 202.5 μA mM−1 cm−2 respectively. In addition, validation of the applicability of the prepared biosensor was carried out by detecting nitrite in tap, river and rain water samples.

Enzymatic electrochemical glucose biosensors by mesoporous 1D hydroxyapatite-on-2D reduced graphene oxide

A novel hydrothermal process was used for the preparation of hydroxyapatite (HAp) nanorods on two-dimensional reduced graphene oxides (RGO). The hydrothermal reaction temperature improves the crystallinity of HAp and partially reduces graphene oxide (GO) to RGO. The crystalline structure, chemical composition and morphology of the prepared nanocomposites were characterized by using various analytical techniques. Nanorods of HAp with a diameter and length of ∼32 and 60-85 nm were grown on basal planes and edges of the layered RGO sheets. The estimated specific surface area and pore-size distribution are 120 m2 g-1 and 5.6 nm, respectively. We also report the direct electrochemistry of glucose oxidase (GOx) on 1D HAp-on-2D RGO nanocomposite-modified glassy carbon electrode (GCE) for glucose sensing. The electrocatalytic and electroanalytical applications of the proposed RGO/HAp/GOx-modified GCE were studied by cyclic voltammetry (CV) and amperometry. The increased electron rate constant of 3.50 s-1 was obtained for the modified GCE. The reported biosensor exhibits a superior detection limit and higher sensitivity ca. 0.03 mM and 16.9 μA mM-1 cm-2, respectively, with a wide linear range of 0.1-11.5 mM. The tremendous analytical parameters of the reported sensor surpass those of related modified electrodes and are promising for practical industrial applications.

Shape evolution and size controlled synthesis of mesoporous hydroxyapatite nanostructures and their morphology dependent Pb(II) removal from waste water

Nanostructured hydroxyapatite (n-HAp) with tuneable morphology was successfully synthesized by varying the process parameters using a hydrothermal process with CTAB and PEG as surfactants. Systematic experiments were carried out to investigate the influences of process parameters on morphology. The morphology of n-HAp can be modified from nanorods to spheres by replacing the surfactant CTAB with PEG. The prepared materials were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and field emission scanning electron microscopy (FESEM). The specific surface area (SSA) and pore size were determined by N2 adsorption–desorption isotherms. The obtained specific surface area of the nanorods is greater compared to the nanospheres of HAp. These nanostructures of HAp have been used for removal of Pb(II) ions from waste water. The kinetic mechanism was best described by a pseudo-second order model and the isotherm data were fitted well by the Langmuir isotherm and Freundlich model. The adsorption of Pb(II) was found to be 714.14 and 526.31 mg g−1 for the HAp nanorods and nanospheres respectively. The effect of pH, contact time and initial concentration of Pb(II) were also studied through batch experiments.

Enhanced electrocatalytic activity of gold nanoparticles on hydroxyapatite nanorods for sensitive hydrazine sensors

Well-designed noble metals and ceramic nanoarchitectures are significantly important for the development of high performance, selective, sensitive and cost effective electrochemical sensors. Here, we report gold (Au) nanoparticles (NPs) uniformly dispersed on hydroxyapatite (HAp) nanorods forming particles on rod nanoarchitectures for sensitive hydrazine sensors. The Au/HAp nanocomposites were prepared by a versatile hydrothermal precipitation method. The precipitated citrate-stabilized Au NPs were 6–8 nm in size and strongly anchored onto rod-shaped HAp with a diameter of 10 nm and length of 65 nm. The structural, chemical, and electrochemical properties and growth mechanism of the Au nanoparticles on the HAp nanorods (NRs) are presented. Progress toward the application of hybrid nanocomposites in electrochemical oxidation of hydrazine is reviewed. Compared to Au NPs, the incorporation of Au NPs into HAp NRs favored the adsorption of hydrazine, thus bringing hydrazine much closer to the catalytic sites of Au NPs and then increasing the efficiency of hydrazine oxidation in neutral solution. The amperometric (i–t) hydrazine sensor, using the as-prepared Au/HAp as the electrochemical catalyst, shows a wide linear response range of 0.5–1429 μM, a lower detection limit (0.017 μM) and very high sensitivity of 0.5 μA μM−1 cm−2. Furthermore, the Au/HAp nanocomposites showed an excellent anti-interference property towards the various organic and inorganic electroactive compounds, and good inter-electrode and intra-electrode reproducibility. Our present technique shows both qualitative and quantitative measurement of hydrazine in various water samples with high sensitivity, cost effectiveness and rapid analysis time.

Edge-carboxylated graphene anchoring magnetite-hydroxyapatite nanocomposite for an efficient 4-nitrophenol sensor

The surface chemistry and physical properties of edge-carboxylated graphene (ECG) have to date been over looked in terms of understanding the real world practical applications. The accurate identification of each possible oxygenated group on the surface of the basal plane as well as the edges of ECG is necessary to understand the properties for their potential multifunctional applications. Herein, we report the use of a simple high energy ball mill to prepare a large scale production of ECG from natural graphite flakes through interaction with aspartic acid under solid conditions. These 2 dimensional ECG sheets were anchored with magnetite-hydroxyapatite (m-HAp) using a simple hydrothermal process. The prepared materials were systematically investigated by various analytical techniques to realize the structural, morphological, compositional and functional properties. These m-HAp dispersed ECG sheets can be further used to modify the glassy carbon electrode (GCE) for the sensitive and selective detection of 4-nitrophenol (4-NP) by cyclic voltammogram (CV) and differential pulsed voltammetry (DPV). The high specific surface area of 130 m2 g−1 for the m-HAp on ECG displays an excellent catalytic activity with reversible redox behavior of 4-NP. The modified electrode possesses a good detection limit and high sensitivity of 0.27 μM and 0.587 μA μM−1 cm−2, respectively, towards 4-NP, rendering practical industrial applications.

Hydroxyapatite nanoparticles on dendritic α-Fe2O3 hierarchical architectures for a heterogeneous photocatalyst and adsorption of Pb(II) ions from industrial wastewater

A facile surfactant free hydrothermal process was used to prepare dendritic α-Fe2O3 and hydroxyapatite (HAp) nanoparticles dispersed on dendritic α-Fe2O3 nanostructures. The dendrite consists of a 1 μm long central trunk with secondary branches of 80 nm. The prepared nanocomposite with a mesoporous structure exhibits a high specific surface area. A possible formation mechanism for the dendrite is proposed. These HAP/α-Fe2O3 nanocomposites were further used to degrade methyl violet (MV) dye using photodegradation and adsorb Pb(II) ions from industrial waste water through an adsorption process. These investigations clearly confirm the extremely fast degradation and adsorption of the dye and Pb(II) ions from the aqueous solution. The experimental adsorption data also very well fit with the pseudo-second-order model. The results confirm the new method has advantages of both a photodegradation and adsorption process for the removal of various wastewater pollutants. The present method is more energetic, cost effective, sustainable and also easily recycled after the adsorption process with a high recovery ratio due to the magnetic response of the nanocomposite.

Facile in situ growth of Fe3O4 nanoparticles on hydroxyapatite nanorods for pH dependent adsorption and controlled release of proteins

A general one-pot hydrothermal process was used to prepare different sizes of Fe3O4 nanoparticles dispersed on hydroxyapatite nanorods with CTAB as a surfactant. We also explore the role of hydrothermal reaction temperature and the surfactant on the crystallinity and formation of the rod like morphology of HAp. The obtained nanoparticles are systematically studied by X-ray powder diffraction (XRD), Fourier-transform infrared spectroscopy, Raman spectroscopy, field emission scanning electron microscopy (FESEM) with EDS for elemental mapping, transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) nitrogen sorptometry and vibrating sample magnetometry (VSM). The as-synthesized Fe3O4/HAp nanocomposites are further explored to study the pH dependent protein adsorption and controlled release using hemoglobin (Hb) as a model protein. A maximum protein adsorption (Qo) of 166.67 mg g−1 is observed for the Fe3O4/HAp nanocomposite and it increases to 200.07 mg g−1 upon increasing the concentration of Fe3O4 nanoparticles. The pH controlled sustained release process is observed for Hb at various pH values of 4.0, 7.4 and 9.0 in phosphate buffer saline (PBS) solution at room temperature. The maximum protein release was obtained for the lower pH values. The dosage dependent in vitro cytotoxicity assays are also performed to confirm biocompatibility of the prepared samples.

Solution combustion synthesis and physico-chemical properties of ultrafine CeO2 nanoparticles and their photocatalytic activity

Ultrafine cerium oxide (CeO2) nanoparticles have been confirmed to be capable photocatalysts for environmental remediation because of their strong redox ability, long-term stability, nontoxicity, and cost effectiveness. CeO2 nanoparticles were successfully synthesized by a solution combustion method using cerium nitrate and urea. The physical, chemical, thermal and optical properties of the as-prepared samples were characterized using various analytical techniques. X-ray diffraction data confirm that the synthesized nanocrystalline CeO2 samples have cubic structure with an average grain size of 10, 8 and 19 nm corresponding to as-prepared, 300 and 600 °C annealed samples, respectively. The HRTEM results confirmed that the synthesized nanoparticles exhibit good polycrystalline nature with spherical ultrafine nanoparticles having size in the range of ∼9 nm. The UV/vis spectrum shows a maximum absorption at 294 nm and the band gap of CeO2 was tuned by adjusting the annealing temperature. In the PL spectra, a strong and broad emission band was observed at 425 nm due to the presence of a blue shift in the visible region. The photocatalytic activities of annealed CeO2 nanoparticles were investigated by photodegradation of methylene blue (MB) under UV light irradiation. It was found that the ultra-fine CeO2 nanoparticles exhibited better photocatalytic activity than that of already available commercial photocatalysts. Based on the investigation, these ultra-fine CeO2 nanoparticles possess adaptable potential applications for wastewater purification.

Enhanced hydroxyapatite nanorods formation on graphene oxide nanocomposite as a potential candidate for protein adsorption, pH controlled release and an effective drug delivery platform for cancer therapy

Herein, we report an eco-friendly hydrothermal synthesis of one dimensional (1D) hydroxyapatite (HAp) nanorods which were grown on two dimensional (2D) graphene oxide (GO) sheets (1D-on-2D) using creatine phosphate as a biomolecular source for phosphorus. Comprehensive analyses were performed to study the structural, morphological and bio-physical properties of the as-prepared nanocomposites using several analytical techniques. Enthrallingly, the as-prepared nanocomposite was explored to study the adsorption–desorption property of bovine serum albumin (BSA) on the HAp/GO nanocomposite. Notably, the observed data of adsorption–desorption process confirm the faster adsorption of BSA on HAp/GO nanocomposite. A maximum protein adsorption (Qo) of 350 mg g−1 in a neutral pH was attained for this nanocomposite and maximum release was obtained at a lower pH of 4. In addition, andrographolide, a model anticancer drug, was effectively loaded onto the HAp/GO nanocomposite through electrostatic and hydrophobic interactions. Experimental results showed that the HAp/GO nanocomposite not only exhibited good biocompatibility but also acted as good carrier for anticancer drugs, thereby greatly reducing the side effects of free anticancer drugs administered at high doses. Also, we include the results of the MTT assay of andrographolide loaded nanocomposite in a normal cell line (HaCaT) for selective anti-cancer activity. Therefore, the as-synthesized HAp/GO nanocomposite may act as a promising drug delivery platform due to its good biocompatibility, pH sensitivity, high drug loading efficiency, selectivity, and excellent biodegradability.