Electrochemical oxidation behavior of guanosine-5��-monophosphate on a glassy carbon electrode modified with a composite film of graphene and multi-walled carbon nanotubes, and its amperometric determination

The electrochemical oxidation of guanosine-5??-monophosphate (GMP) was studied with a glassy carbon electrode modified with a composite made from graphene and multi-walled carbon nanotubes. GMP undergoes an irreversible oxidation process at an oxidation peak potential of 987?mV in phosphate buffer solution. Compared to other electrodes, the oxidation peak current of GMP with this electrode was significantly increased, and the corresponding oxidation peak potential negatively shifted, thereby indicating that the modified material exhibited electrochemical catalytic activity towards GMP. Chronocoulometry demonstrates that the material also effectively increases the surface area of the electrode and increases the amount of GMP adsorbed. Under the optimum conditions, the oxidation current is proportional to the GMP concentration in the range from 0.1 to 59.7???M with a correlation coefficient of 0.9991. The detection limit is 0.025???M (at S/N?=?3).

Carbon paste electrode modified with a binuclear manganese complex as a sensitive voltammetric sensor for tryptophan

We report on a carbon paste electrode that was modified with a binuclear manganese(II) complex by the drop-coating method. A study on the mechanism of the electro-oxidation of tryptophan (Trp) at this electrode indicated that it enables Trp to be determined with good sensitivity and selectivity. Second-order derivative linear sweep voltammetry at pH 4.1 revealed that a sensitive anodic peak appears at 812?mV (vs. SCE) whose current is proportional to the concentration of Trp in the concentration range from 0.1 to 1.0???mol?L^?1 and 1.0 to 80???mol?L^?1, with a detection limit (S/N?=?3) of 0.08???mol?L^?1 (60?s of accumulation). The method was applied to the determination of Trp in amino acid injection solutions with satisfactory results.

A lead(II) sensor based on a glassy carbon electrode modified with Fe3O4 nanospheres and carbon nanotubes

We have developed a highly sensitive and selective sensor for lead(II) ions. A glassy carbon electrode was modified with Fe₃O₄ nanospheres and multi-walled carbon nanotubes, and this material was characterized by scanning electron microscopy and X-ray diffraction. The electrode displays good electrochemical activity toward Pb(II) and gives anodic and cathodic peaks with potentials at ?496 mV and ?638 mV (vs. Ag/AgCl) in pH?6.0 solution. The sensor exhibits a sensitive and fairly selective response to Pb(II) ion, with a linear range between 20 pM and 1.6 nM, and a detection limit as low as 6.0 pM (at a signal-to noise ratio of 3). The sensor was successfully applied to monitor Pb(II) in spiked water samples.

Sensitive simultaneous determination of catechol and hydroquinone using a gold electrode modified with carbon nanofibers and gold nanoparticles

A highly sensitive electrochemical sensor for the simultaneous determination of catechol (CC) and hydroquinone (HQ) was fabricated by electrodeposition of gold nanoparticles onto carbon nanofiber film pre-cast on an Au electrode. Both CC and HQ cause a pair of quasi-reversible and well-defined redox peaks at the modified electrode in pH?7.0 solution. Simultaneously, the oxidation peak potentials of CC and HQ become separated by 112?mV. When simultaneously changing the concentrations of both CC and HQ, the response is linear between 9.0???M and 1.50?mM. In the presence of 0.15?mM of the respective isomer, the electrode gives a linear response in the range from 5.0 to 350???M, and from 9.0 to 500???M for CC and HQ, respectively, and detection limits are 0.36 and 0.86???M. The method was successfully examined for real sample analysis with high selectivity and sensitivity.

Overcoming the adverse effects of crosslinking in biosensors via addition of PEG: Improved sensing of hydrogen peroxide using immobilized peroxidase

Glutaraldehyde (GA) is widely used as a crosslinker to immobilize enzymes, for examples in biosensors, but often causes partial denaturation. We find that the proper use of poly(ethylene glycol) (PEG) during the crosslinking process can fully preserve the native state and activity of horseradish peroxidase (HRP). An amperometric biosensor was developed based on these findings for the direct determination of hydrogen peroxide. UV-Vis and FTIR spectroscopy reveal that the HRP entrapped in a polypyrrole matrix retains its native structure. The addition of PEG increases the sensitivity and stability of the biosensor and prevents many of effects caused by intra-crosslinking via GA. The biosensor was operated at a potential of ?350?mV (vs Ag/AgCl) without any mediator and gave a linear response to H₂O₂ in the 5 to 190???M concentration range. The apparent Michaelis-Menten constant is 3.37?mM, and maximal current is as high as 3.43???A. The surface of the biosensor was characterized by atomic force microscopy operated in the tapping mode.

Electrochemical oxidation of p-nitrophenol using graphene-modified electrodes, and a comparison to the performance of MWNT-based electrodes

The electrochemical oxidation of p-nitrophenol (p-NP) has been studied comparatively on a graphene modified electrode and a multiwall carbon nanotube (MWNT) electrode by using cyclic and differential pulse voltammetry. The sensors were fabricated by modifying screen-printed electrodes with graphene and MWNT nanomaterials, respectively, both dispersed in Nafion polymer. p-NP is irreversibly oxidized at +0.9?V (vs. the Ag/AgCl) in solutions of pH 7. The height and potential of the peaks depend on pH in the range from 5 to 11. In acidic media, p-NP yields a well-defined oxidation peak at +0.96?V which gradually increases in height with the concentration of the analyte. In case of differential pulse voltammetry in sulfuric acid solution, the sensitivity is practically the same for both electrodes. The modified electrodes display an unusually wide linear response (from 10???M to 0.62?mM of p-NP), with a detection limit of 0.6???M in case of the graphene electrode, and of 1.3???M in case of the MWNT electrode.

Fluorescent sensing of nitrite at nanomolar level using functionalized mesoporous silica

Aminopyrene was covalently anchored onto mesoporous silica through serial post-grafting to obtain a fluorescent solid that can be used as a sensing material for the determination of nitrite. The latter, in acidic medium, reacts with the secondary amino groups on the material to form a non-fluorescent nitroso derivative. Based on the fluorescence quenching caused by this specific reaction, a method was developed for the determination of nitrite at nanomolar levels. The range for detection of nitrite in 1.5?mol.L^?1 HCl is linear between 1.50?nM to 0.45???M and 0.45???M to 2.22???M, the detection limit being 1.10?nM and 0.307???M respectively at an S/N of 3.

A hydrogen peroxide biosensor based on direct electron transfer from hemoglobin to an electrode modified with Nafion and activated nanocarbon

A biosensor for hydrogen peroxide (HP) was developed by immobilizing hemoglobin on a glassy carbon electrode modified with activated carbon nanoparticles/Nafion. The characteristics of the sensor were studied by UV?Cvis spectroscopy and electrochemical methods. The immobilized Hb retained its native secondary structure, undergoes direct electron transfer (with a heterogeneous rate constant of 3.37?±?0.5?s^?1), and displays excellent bioelectrocatalytic activity to the reduction of HP. Under the optimal conditions, its amperometric response varies linearly with the concentration of HP in the range from 0.9???M to 17???M. The detection limit is 0.4???M (at S/N?=?3). Due to the commercial availability and low cost of activated carbon nanoparticles, it can be considered as a useful supporting material for construction of other third-generation biosensors.

Ultratrace voltammetric method for the detection of DNA sequence related to human immunodeficiency virus type 1

An ultra-trace voltammetric method was developed for the determination of single strand DNA (ss-DNA) related to the human immunodeficiency virus type 1 (HIV-1). It is based on the signal amplification of carbon nanotubes loaded with silver nanoparticles and placed on a gold microelectrode. The capture ss-DNA (a 21-mer) possessing a thiol group at the 3?? end was self-assembled onto the surface of the gold microelectrode. It was then hybridized with target HIV-1 ss-DNA (a 42-mer) and further hybridized with the electrochemical probe (a 18-mer ss-DNA) tagged with multiwall carbon nanotubes and loaded with silver nanoparticles. The resulting formation of a DNA sandwich conjugate led to a strong electrochemical oxidation signal that was linearly proportional to the concentration of HIV-1 ss-DNA in the range from 1.0 to 100?pM. The detection limit was 0.5?pM (at an S/N of 3). This was equivalent to 0.05?fmol of HIV-1 ss-DNA in a volume of 20???L. The relative standard deviation was 4.0% at 1.0?pM (n?=?11). Non-complementary ss-DNA of HIV-1 ss-DNA was effectively discriminated. This work demonstrates that the employment of the microelectrode and a sandwich hybridization model is promising in terms of sensitive and selective electrochemical detection of DNA.

Biosensor based on a glassy carbon electrode modified with tyrosinase immmobilized on multiwalled carbon nanotubes

We describe a biosensor for phenolic compounds that is based on a glassy carbon electrode modified with tyrosinase immobilized on multiwalled carbon nanotubes (MWNTs). The MWNTs possess excellent inherent electrical conductivity which enhances the electron transfer rate and results in good electrochemical catalytic activity towards the reduction of benzoquinone produced by enzymatic reaction. The biosensor was characterized by cyclic voltammetry, and the experimental conditions were optimized. The cathodíc current is linearly related to the concentration of the phenols between 0.4???M and 10???M, and the detection limit is 0.2???M. The method was applied to the determination of phenol in water samples.

Iodide-induced adsorption of lead(II) ion on a glassy carbon electrode modified with ferroferric oxide nanoparticles

Spherical Fe₃O₄ nanoparticles (NPs) were prepared by hydrothermal synthesis and characterized by scanning electron microscopy and X-ray diffraction. A glassy carbon electrode was modified with such NPs to result in a sensor for Pb(II) that is based on the strong inducing adsorption ability of iodide. The electrode gives a pair of well-defined redox peaks for Pb(II) in pH 5.0 buffer containing 10 mM concentrations of potassium iodide, with anodic and cathodic peak potentials at ?487 mV and ?622 mV (vs. Ag/AgCl), respectively. The amperometric response to Pb(II) is linear in the range from 0.10 to 44 nM, and the detection limit is 40 pM at an SNR of 3. The sensor exhibits high selectivity and reproducibility.

Electrochemical oxidation and nanomolar detection of acetaminophen at a carbon-ceramic electrode modified by carbon nanotubes: a comparison between multi walled and single walled carbon nanotubes

Carbon-ceramic electrodes (CCE) modified with carbon nanotubes were prepared, and the electrochemical behavior towards acetaminophen (ACOP) was investigated using both a bare CCE and electrodes modified with either single walled carbon nanotubes (SWCNT) or multi walled carbon nanotubes (MWCNT) in an effort to understand which of them is the better choice in terms of electrocatalyzing the oxidation of ACOP, and thus for sensing it. The SWCNT are found to be the better material in significantly enhancing the oxidation peak current and improving the reversibility of the oxidation. Under optimal conditions, linearity between the oxidation peak current and the concentration of ACOP is obtained for the concentration range from 40?nM to 85???M, with a detection limit of 25?nM. Finally, ACOP was successfully determined with the SWCNT modified electrode in pharmaceutical samples.

Biomimetic norfloxacin sensors made of molecularly-imprinted materials for potentiometric transduction

A biomimetic sensor for norfloxacin is presented that is based on host-guest interactions and potentiometric transduction. The artificial host was imprinted into polymers made from methacrylic acid and/or 2-vinyl pyridine. The resulting particles were entrapped in a plasticized poly(vinyl chloride) (PVC) matrix. The sensors exhibit near-Nernstian response in steady state evaluations, and detection limits range from 0.40 to 1.0???g?mL^?1, respectively, and are independent of pH values at between 2 and 6, and 8 and 11, respectively. Good selectivity was observed over several potential interferents. In flowing media, the sensors exhibit fast response, a sensitivity of 68.2?mV per decade, a linear range from 79???M to 2.5?mM, a detection limit of 20???g?mL^?1, and a stable baseline. The sensors were successfully applied to field monitoring of norfloxacin in fish samples, biological samples, and pharmaceutical products.

Amperometric determination of L-tyrosine by an enzymeless sensor based on a carbon ceramic electrode modified with copper oxide nanoparticles

We report on the electrochemical formation of copper oxide nanoparticles (CuO-NPs) at a carbon ceramic electrode (CCE) as a highly-porous substrate. A copper film was deposited on the surface of the CCE and derivatized in situ to give CuO-NPs by potential cycling between ?0.8 and 0.35?V in strongly alkaline solution. The electrode was characterized by scanning electron microscopy and cyclic voltammetry. The CuO-NPs exhibited excellent electrocatalytic activity toward the oxidation of L-tyrosine (L-Tyr) in responding linearly in the 2 to 1,350???M concentration range, an associated detection limit (S/N?=?3) of 160?nM, and a sensitivity of 0.61?A?M^?1?cm^?2.

Glassy carbon electrode modified with a film composed of Ni(II), quercetin and graphene for enzyme-less sensing of glucose

We present a modified glassy carbon electrode as a sensing platform for glucose. It is based on a composite film prepared from Ni(II) ion, quercetin and graphene. The sensor was characterized by cyclic voltammetry. The electron transfer coefficient, reaction rate constant and catalytic rate constant were determined and found to be 0.53, 5.4?s^?1 and 2.93?×?10³?M^?1 s^?1, respectively. The catalytic current depends linearly on the concentration of glucose in the range from 3 to 900???M, with a detection limit of 0.5???M (at an S/R of 3). The sensor exhibits good reproducibility, stability, fast response, and high sensitivity.

Direct electrochemistry of myoglobin in a layer-by-layer film on an ionic liquid modified electrode containing CeO2 nanoparticles and hyaluronic acid

We describe an ionic liquid modified electrode (CPE-IL) for sensing hydrogen peroxide (HP) that was modified by the layer-by-layer technique with myoglobin (Mb). In addition, the surface of the electrode was modified with CeO₂ nanoparticles (nano-CeO₂) and hyaluronic acid. UV-vis and FTIR spectroscopy confirmed that Mb retains its native structure in the composite film. Scanning electron microscopy showed that the nano-CeO₂ closely interact with Mb to form an inhomogeneously distributed film. Cyclic voltammetry reveals a pair of quasi-reversible redox peaks of Mb, with the cathodic peak at ?0.357?V and the anodic peak at ?0.269?V. The peak separation (??E _p) and the formal potential (E ^0??) are 88?mV and ?0.313?V (vs. Ag/AgCl), respectively. The Mb immobilized in the modified electrode displays an excellent electrocatalytic activity towards HP in the 0.6 to 78.0???M concentration range. The limit of detection is 50?nM (S/N?=?3), and then the Michaelis-Menten constant is 71.8???M. We believe that such a composite film has potential to further investigate other redox proteins and in the fabrication of third-generation biosensors.

Enhanced conductivity of a glassy carbon electrode modified with a graphene-doped film of layered double hydroxides for selectively sensing of dopamine

A strategy is presented for doping graphene into layered double hydroxide films (LDHs) as a means of improving charge transport of the LDH film in a modified glassy carbon electrode. This result in an enhanced electrocatalytic current for dopamine (DA) and a good separation of the potentials of DA, uric acid and ascorbic acid. Under selected conditions, the square wave voltammetric response of the electrode to DA is linear in the concentration range from 1.0 to 199???M even in the presence of 0.1?mM ascorbic acid, and the detection limit is 0.3???M at a signal-to-noise ratio of 3. The method was applied to the determination of DA in pharmaceutical injections with satisfactory results.

A uric acid sensor based on electrodeposition of nickel hexacyanoferrate nanoparticles on an electrode modified with multi-walled carbon nanotubes

An electrode sensitive to uric acid was prepared by electrodeposition of nickel(II) hexacyanoferrate(III) on the surface of a glassy carbon electrode modified with multi-walled carbon nanotubes. The morphology of the material was characterized by scanning electron microscopy and Fourier transform infrared spectrometry. The modified electrode were characterized via cyclic voltammetry and amperometry (i - t). It exhibited efficient electron transfer ability and a strong and fast (< 3?s) response towards uric acid which is linear in the range from 0.1???M to 18???M, with a lower detection limit of 50 nM (at an S/N ratio of 3). In addition, the electrode exhibited good reproducibility and long-term stability.

Amperometric sensing of NADH and ethanol using a hybrid film electrode modified with electrochemically fabricated zirconia nanotubes and poly (acid fuchsin)

We report on a glassy carbon electrode (GCE) modified with a film of chitosin containing acid fuchsin (AF) adsorbed onto zirconia nanotubes. The mixture was polymerized by cyclic voltammetric scannings in the potential range from - 0.8?V to +1.3?V in buffer solution to produce a hybrid film electrode (nano-ZrO₂/PAF/GCE). The morphology of the hybrid film electrode surface was characterized by scanning electron microscopy. Its electrochemical properties were studied via electrochemical impedance spectroscopy. The electrochemical response of nicotinamide adenine dinucleotide (NADH) was investigated by differential pulse voltammetry and amperometry. The results indicated that the nano-ZrO₂/PAF/GCE possesses well synergistic catalytic activity towards NADH. Compared to an unmodified GCE, the oxidation overpotential is negatively shifted by 224?mV, and the oxidation current is significantly increased. Under optimal conditions, the amperometric response is linearly proportional to the concentration of NADH in the 1.0 – 100.0?μM concentration range. Ethanol also can be determined by amperometry if alcohol dehydrogenase and NADH are added to the sample. Two linear relationships between current and alcohol concentration were obtained. They cover the range from 0.03 to 1.0?mM, and from 1.0 to 12.0?mM.

Voltammetric determination of cadmium (II) based on a composite film of a thiol-functionalized mesoporous molecular sieve and an ionic liquid

A composite film made from a thiol-functionalized mesoporous molecular sieve and an ionic liquid is introduced for use in a voltammetric sensor for Cd(II). The electrode exhibits excellent sensitivity towards Cd(II) in showing a markedly increased stripping peak current. Following the optimization of the experimental parameters, a linear response is obtained in the concentration range from 29?nM to 0.87?mM of Cd(II). The detection limit is as low as 1.0?nM (at S/N?=?3) after an accumulation at ?1.1?V for 4?min. The method was successfully applied to determine Cd(II) in water samples. Features such as large electroactive area, fast electron transfer and low background current make this electrode a promising platform for fabricating reliable electrochemical sensors for various species, such as heavy metals and environmental pollutants.