Impedimetric immunoglobulin G immunosensor based on chemically modified graphenes

Immunosensors which display high sensitivity and selectivity are of utmost importance to the biomedical field. Graphene is a material which has immense potential for the fabrication of immunosensors. For the first time, we evaluate the immunosensing capabilities of various graphene surfaces in this work. We propose a simple and label-free electrochemical impedimetric immunosensor for immunoglobulin G (IgG) based on chemically modified graphene (CMG) surfaces such as graphite oxide, graphene oxide, thermally reduced graphene oxide and electrochemically reduced graphene oxide. Disposable electrochemical printed electrodes were first modified with CMG materials before anti-immunoglobulin G (anti-IgG), which is specific to IgG, was immobilized. The principle of detection lies in the changes in impedance spectra of the redox probe after the attachment of IgG to the immobilized anti-IgG. It was found that thermally reduced graphene oxide has the best performance when compared to the other CMG materials. In addition, the optimal concentration of anti-IgG to be deposited onto the modified electrode surface is 10 μg ml(-1) and the linear range of detection of the immunosensor is from 0.3 μg ml(-1) to 7 μg ml(-1). Finally, the fabricated immunosensor also displays selectivity for IgG.     

Determining the thickness of chemically modified graphenes by scanning probe microscopy

The thickness of unreduced and chemically reduced graphene oxide sheets deposited on different substrates was measured by different scanning probe microscopy (SPM) variants. Inaccurate and inconsistent results are obtained when thickness is derived as a sheet-to-substrate height, which is the typically employed approach to determine such a parameter. Measuring overlapped regions between different sheets leads to more realistic thickness values, which clearly reflect, for example, the removal of oxygen functionalities from graphene oxide following chemical reduction. The results underline the precautions that are required to draw valid conclusions from SPM-derived thickness data of chemically modified graphenes.     

The production of chemically converted graphenes from graphite fluoride

The liquid-phase extraction of graphene fluoride by exfoliation of graphite fluoride in dimethylformamide and subsequent reduction with triethylsilane or zinc particles yields the corresponding chemically converted graphene, namely reduced graphene fluoride. The formation of either graphene fluoride (fluorographene) or graphene monolayers in the course of the reaction was demonstrated by several microscopy techniques. Fluorine elimination after reduction was verified by elemental analysis and infrared/Raman spectroscopy.     

Impedimetric microbial biosensor based on single wall carbon nanotube modified microelectrodes for trichloroethylene detection

Contamination of soils and groundwaters with persistent organic pollutants is a matter of increasing concern. The most common organic pollutants are chlorinated hydrocarbons such as perchloroethylene and trichloroethylene (TCE). In this study, we developed a bacterial impedimetric biosensor for TCE detection, based on the immobilization of Pseudomonas putida F1 strain on gold microelectrodes functionalized with single wall carbon nanotubes covalently linked to anti-Pseudomonas antibodies. The different steps of microelectrodes functionalization were characterized by electrochemical impedance and atomic force spectroscopies, and analytical performances of the developed microbial biosensor were determined. The impedimetric biosensor response was linear with TCE concentration up to 150 μg L⁻¹ and a low limit of detection (20 μg L⁻¹) was achieved. No significant loss of signal was observed after 4 weeks of storage at 4 °C in phosphate buffer saline pH 7 (three to four measurements a week). After 5 weeks, 90% of the initial value still remained. cis-1,2-Dichloroethylene and vinylchloride, the main TCE degradation products, did not significantly interfere with TCE. The microbial sensor was finally applied to the determination of TCE in natural water samples spiked at the 30, 50 and 75 μg L⁻¹ levels. Recoveries were very good, ranging from 100 to 103%.     

A sensitive electrochemical aptasensor based on water soluble CdSe quantum dots (QDs) for thrombin determination

A novel aptamer biosensor with easy operation and good sensitivity, specificity, stability and reproducibility was developed by immobilizing the aptamer on water soluble CdSe quantum dots (QDs) modified on the top of the glassy carbon electrode (GCE). Methylene blue (MB) was intercalated into the aptamer sequence and used as an electrochemical marker. CdSe QDs improved the electrochemical signal because of their larger surface area and ion centers of CdSe QDs may also had a major role on amplifying the signal. The higher ion concentration caused more combination of aptamer which caused larger signal. The thrombin was detected by differential pulse voltammetry (DPV) quantitatively. Under optimal conditions, the two linear ranges were obtained from 3 to 13 μg mL⁻¹ and from 14 to 31 μg mL⁻¹, respectively. The detection limit was 0.08 μg mL⁻¹ at 3σ. The constructed biosensor had better responses compared with that in the absence of the CdSe QDs immobilizing. The control experiment was also carried out by using BSA, casein and IgG in the absence of thrombin. The results showed that the aptasensor had good specificity, stability and reproducibility to the thrombin. Moreover, the aptasensor could be used for detection of real sample with consistent results in comparison with those obtained by fluorescence method which could provide a promising platform for fabrication of aptamer based biosensors.     

Thin films of carbon nanotubes and chemically reduced graphenes for electrochemical micro-capacitors

We report the making of chemically reduced graphene (CRG) sheets separated by layer-by-layer-assembled multi-walled carbon nanotubes (MWCNTs) for electrochemical micro-capacitor applications. Submicron thin films of amine-functionalized MWCNTs (MWCNT-NH₂) and CRG derived from graphene oxides, were shown to be cross-linked with amide bonds having high packing densities of ∼70%. These carbon-only electrodes were found to have large volumetric capacitance of in an acidic electrolyte (0.5 M H₂SO₄). The electrode capacitance in a neutral electrolyte (1 M KCl) was found much lower, which supported the hypothesis that the observed high capacitances in the acidic electrolyte can be attributed primarily to redox reactions between protons and surface oxygen-containing groups on carbon.     

Synthesis and characterization of pH-responsive hydrogels based on chemically modified Arabic gum polysaccharide

This work describes the preparation of a pH-responsive hydrogel from Arabic gum (AG) chemically modified with glycidyl methacrylate (GMA). This report first describes the chemical modification of AG and next the synthesis and characterization of the hydrogel obtained. An appropriate mixture of water and DMSO was used to dissolve AG and GMA. The presence of GMA groups in the modified structure of Arabic gum (AG-MA) was detected by ¹³C NMR, ¹H NMR, and FT-IR techniques. The cross-linking reaction of AG-MA leads to formation of an AG-MA hydrogel, which was characterized by solid-state ¹³C-CP/MAS NMR and FT-IR spectroscopy. Morphology was visualized by scanning electron microscopy. It was observed in water uptake tests that AG-MA hydrogels showed significant pH dependence, which affected the water absorption transport mechanism. In the studied pH range, it was found that the transport mechanism of water into AG-MA hydrogel was controlled by Fickian diffusion and polymer relaxation (anomalous transport). At high pH values, the water transport profile became more dependent on polymer relaxation. This effect was attributed to the increase in the ionized groups of glucuronic acid segments, which contributed to electrostatic repulsion among the groups and led the gel polymer network to expand. AG-MA hydrogels exhibited pH-responsive, demonstrating them to be appropriate materials for further tests as drug carriers.     

Studies in chemically modified celluloses. IV. Lactones in chemically modified celluloses

Cellulose has been modified by hydrolysis followed by oxidation with chlorous acid to produce gluconic carboxyl groups, which can form only δ-lactones. A method has been described for reducing these gluconic acid groups by sodium borohydride into alcoholic groups. Iodometric and alkalimetric methods were used for estimating the carboxyl value of various chemically modified samples.     

Electrochemically assisted metal uptake by cation exchange based chemically modified electrodes

Carbon paste electrodes containing powdered cation exchange resin (Amberlite IR120) have been fabricated and applied to the uptake of copper ion in aqueous solution. The copper uptake reaction was carried out at open circuit and at different cathodic potentials. Linear potential sweep voltammetry was used to quantify the accumulated ions. With the application of an optimum cathodic potential, the amount of copper absorbed increased four to nine-fold in comparison with similar uptake at open circuit conditions. The effect of different resin size employed in the chemically modified electrodes, on the sensitivity of the electrode response was also studied as a function of peak current.     

A novel fluorescent aptasensor based on single-walled carbon nanohorns

Single-walled carbon nanohorns have been used to construct an aptasensor for the first time. This novel aptasensor was successfully used for the detection of thrombin with high sensitivity and excellent selectivity. This thrombin aptasensor has a detection limit of as low as 100 pM.     

Rheology of chemically modified triglycerides

The rheology of chemically modified plant oils was determined to aid in their processing for polymers and other applications. Epoxidized and acrylated triglycerides were derived from various plant oils and model triglycerides. The viscosity of the unmodified oils decreased as the level of oil unsaturation increased. However, the viscosity of the epoxidized oils increased slightly as the level of epoxidation increased. Furthermore, the viscosity of the acrylated oils increased exponentially as the level of acrylation increased because of increased polarity. In addition, the viscosity of the acrylated oils increased as the average distance of the acrylate groups from the fatty acid chain ends decreased. Chemically modified and unmodified oils did not exhibit any shear‐thinning behavior or any memory of shear history. The temperature dependence of the viscosity followed the Arrhenius law, and the activation energy decreased linearly with the level of acrylation. The addition of comonomers, such as styrene, reduced the viscosity of the acrylated oils exponentially as the comonomer concentration increased. However, the glass‐transition temperature and modulus of the triglyceride‐based polymers decreased as the styrene content decreased. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 774–783, 2005     

Studies in chemically modified celluloses. I. Oxidative susceptibility of chemically,modified cellulose

The oxidative susceptibility to different oxidizing agents such as dichromate–sulfuric acid, dichromate–oxalic acid and sodium hypochlorite in presence of leuco Cibanone Orange R of cellulose oxidized with sodium metaperiodate and its corresponding borohydride-reduced product was studied and compared with that of cellulose. From the changes in the chemical properties of the oxidixed products, an attempt was made to study the mechanism of oxidation of periodate oxycellulose and sodium borohydride-reduced periodate oxycellulose with the above mentioned oxidizing agents.     

Studies in chemically modified celluloses. X. Affinities of direct dyes for chemically modified celluloses

A series of chemically modified celluloses with different functional groups present in various extents were prepared by separately oxidizing cotton cellulose by potassium metaperiodate, potassium dichromate–sulfuric acid, potassium dichromate–oxalic acid and further modifying them by treatment with chlorous acid and sodium borohydride. These were dyed with two purified direct dyes to equilibrium at 95°C, and their thermodynamic affinities for these substrates were determined, assuming various extents of ionization of carboxyl groups present in them. It has been shown that the affinity decreases with increase in carboxyl value of the oxycelluloses if the carboxyl groups are assumed to be un-ionized and increases with increasing carboxyl value if they are considered to be fully ionized.     

Studies in chemically modified celluloses. V. Affinities of vat dyes for chemically modified celluloses

Cotton cellulose was separately oxidized by potassium metaperiodate, potassium dichromate–sulfuric acid, and potassium dichromate–oxalic acid at two oxidation levels, and the resulting oxycelluloses were further modified by treatment with chlorous acid and sodium borohydride in order to prepare a series of chemically modified celluloses with different functional groups. These fiber substances were dyed with leuco-flavanthrone at 40°C for different periods till equilibrium was reached. Adsorption isotherms were obtained with three purified vat dyes, and thermodynamic affinity of the dyes for the modified celluloses was determined, taking into consideration the adsorption of hydroxyl ions by cellulosic material and of sodium ions by carboxylate groups of oxycellulose. The presence of aldehyde and carboxyl groups is shown to decrease the rate of dyeing as well as the affinity of the dyes.     

Swelling properties of chemically modified oil palm empty fruit bunch based polyurethane composites

In this study, the diffusion of various types of solvent in oil palm empty fruit bunch/polyurethane composites, produced from chemically modified empty fruit bunches, was investigated. The solubility parameters and polymer–solvent interaction parameters of the produced composites were determined. The void contents of the composites were also determined before swelling tests to eliminate the free solvent present in the system. From the results obtained, we found that the diffusion of the solvents was dependent on the compatible group available and the voids present in the system. The solubility parameters of the empty fruit bunch/polyurethane composites with different degrees of chemical modification were 11.6 and 11.7 (cal/cm^?3)^1/2. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Studies in chemically modified celluloses. II. Some aspects of physical properties of chemically modified celluloses

The changes in physical properties such as moisture regain, tensile strength, elongation at break, bursting strength, and resistance to abrasion of cellulose modified with different oxidizing agents such as dichromate–sulfuric acid, sodium hypochlorite in presence of leuco vat dyes, and potassium metaperiodate and of the corresponding products obtained as a result of reduction with sodium borohydride and those by oxidation with chlorous acid have been studied. Results are critically discussed in the light of existing knowledge on the changes that occur in the fine structure as well as in the chemical structure of cellulose during the mentioned modifications. It is pointed out that complex changes in physical properties of cotton cellulose brought about by chemical modification could hardly be evaluated and interpreted in terms of one or two physical or chemical parameters.     

Impedimetric detection of dopamine on poly(3-aminophenylboronic acid) modified skeleton nickel electrodes

Detection of biologic compounds in particular dopamine is usually based on the complexation between boronic acid groups and diols. For this reason the development of new sensors based on direct monitoring of boronic acid–diol complexation is attractive. A measurable electric response due to a change in the dopamine concentration can be achieved on electrodes modified with boronic groups. In this work a modified electrode has been obtained by electropolymerization of 3-aminophenylboronic acid in aqueous solutions on a preformed polyaniline layer electrochemically deposited on smooth and skeleton nickel electrodes. The modified electrodes have been tested as impedimetric sensors for the detection of dopamine in aqueous phosphate buffer at pH = 7.4. Both sensors gave a linear response for dopamine concentrations between 10^?5 and 10^?10 mol L^?1. Poly(3-aminophenylboronic acid) modified skeleton nickel electrode has the advantage of an increased specific surface area, that lead to a high density of boronic acid groups and hence to a better sensitivity.     

Thermogravimetric analysis of some chemically modified celluloses

Thermal degradation of carboxymethylcellulose, cellulose acetate, grafted wood pulp, and viscose rayon have been studied using thermogravimetric analysis and differential thermal analysis. The values of rate constant K and the activation energy E were calculated from the kinetic of weight loss using the theory of the first-order reaction. The activation energy of grafted cellulose and viscose rayon was > cellulose acetate > carboxymethylcellulose.     

Polyelectrolyte behavior of chemically modified cotton cellulose

Ion exchange and sorption of charged molecules such as dyes, crosslinking agents, etc., by cotton cellulose very much depend on the charge on cellulose, which in turn is known to depend on the external pH of the medium. In order to understand the role of various fixed groups in cellulose in its polyelectrolyte behavior, the standard cellulose was deliberately oxidized with nitrogen dioxide and sodium periodate, and the sorption of formaldehyde and dimethylol dihydroxyethylene urea by these oxycelluloses as well as by alkali-treated nitrogen dioxide–oxycellulose was investigated. The results obtained are interpreted in terms of polyampholyte behavior of oxycellulose. The nitrogen dioxide–oxycellulose is positively charged and is a polycation in acidic medium due to protonation of aldehyde groups while it is negatively charged and acts as a polyanion in slightly acidic to alkaline medium due to dissociation of carboxyl groups.     

Studies on chemically modified hen egg white and gelatin composites

Hen egg white (A) and gelatin (G) composites at different proportions were prepared and optimized by their stress–strain behavior. The optimized composite (AG) was graft copolymerised with methyl methacrylate (MMA). The AG and its graft copolymer AGMMA were characterized for their mechanical strength, water absorption capacity, Fourier transform infrared spectra, and circular dichroism (CD) spectroscopy. It was evident from the results that AGMMA possesses higher mechanical strength and lower water absorption capacity than does AG. The FTIR spectra of AG and AGMMA confirm the grafting phenomena of MMA onto AG, and the CD spectrum of AG shows two negative bands at around 207 and 222 nm and a positive band at around 191.76 nm, which confirms the α‐helix in the composite. The α‐helical structure in AG was broken and converted into unordered random coil in AGMMA because of the reaction conditions while grafting MMA onto AG, which was confirmed by the increase in mechanical strength of AGMMA. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 318–322, 2006