Synthesis and functionalization of magnetite nanoparticles (Fe
3
O
4
) was achieved with the view to covalently bind both cholesterol oxidase and cholesterol esterase biorecognition agents for the ...development of free and total cholesterol biosensors. Prior to enzyme attachment, Fe
3
O
4
was functionalized with 3-aminopropyltriethoxysilane (APTES) and polyamidoamine (PAMAM) dendrimer. Characterization of the material was performed by FT-IR and UV spectroscopy, SEM/EDX surface analysis and electrochemical investigations. The response to cholesterol and its palmitate ester was examined using cyclic voltammetry. Optimum analytical performance for the free cholesterol biosensor was obtained using APTES-functionalized magnetite with a sensitivity of 101.9 μA mM
−1
cm
−2
, linear range 0.1–1 mM and LOD of 80 μM when operated at 37 °C. In the case of the total cholesterol biosensor, the best analytical performance was obtained using PAMAM dendrimer-modified magnetite with sensitivity of 73.88 μA mM
−1
cm
−2
and linear range 0.1–1.5 mM, with LOD of 90 μM. A stability study indicated that the free cholesterol biosensors retained average activity of 98% after 25 days while the total cholesterol biosensors retained 85% activity upon storage over the same period.
Graphical abstract
Schematic representation of cholesterol esterase and oxidase loaded magnetic nanoparticles (Fe
3
O
4
@APTES or Fe
3
O
4
@APTES-PAMAM) generating hydrogen peroxide from cholesterol palmitate.
Keggin‐type polyoxometalates (POMs), (n‐Bu4N)4AsVMo11O40 (AsVMo11) and (n‐Bu4N)4HAsV2Mo10O40 (AsV2Mo10), have been immobilised with the conducting polymer poly (3,4‐ethylenedioxythiophene) (PEDOT) on ...the surface of the glassy carbon electrode via chronocoulometry technique. The voltammetric behaviour of these modified electrodes was investigated with cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM). The resulting films showed the surface adsorbed behaviour and dependence on the pH of the electrolyte solution. By using AC impedance, it was found that the resulting POM‐doped PEDOT films were highly conductive. The redox switching stability of the polymer films was investigated, and it was found that the AsVMo11/PEDOT hybrid film is significantly more stable for continuous potential sweep than the AsV2Mo10/PEDOT hybrid film. Additionally, the EQCM measurements confirm that during the redox potential cycling, the anions in the supporting electrolyte replace the POMs anion doped onto the film.
This article demonstrates the immobilization of Keggin‐type polyoxometalates (POMs), such as (n‐Bu4N)4AsVMo11O40 and (n‐Bu4N)4HAsV2Mo10O40, within a poly(3,4 ethylenedioxythiophene) (PEDOT) matrix on glassy carbon and gold electrodes. Electrochemical techniques, including cyclic voltammetry electrochemical impedance spectroscopy, and electrochemical quartz crystal microbalance are employed to study the electrochemical behaviour of the immobilized POMs. This study sheds light on the fundamental electrochemical behavior of immobilized Keggin POMs, offering insights into their potential applications.
Nanocomposites of Pt:Au (ratios 1:1, 1:2, 1:3 and 1:4) on activated carbon were synthesized using a novel synthetic route, leading to materials with surface enrichment of Pt(0) with high catalytic ...performance for the direct electro-oxidation of glucose. The overall metallic content (Pt
+
Au) was 20% (w/w) and the bimetallic nanoparticles were found to be uniformly distributed in the activated carbon (80%) matrix. The surface morphology and composition of the synthesized materials were characterised using scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDX), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry was employed in order to confirm the typical metallic electrochemical response of Pt, Au and Pt–Au nanomaterials. The optimum nanocomposite material (Pt
1Au
3/C) for glucose electro-oxidation exhibited a sensitive (4.7
μA
mM
−1
cm
−2, limit of detection
=
2.0
μM) response to glucose over the range 0–10
mM (
r
2
=
0.99) at
E
app
=
0.35
V vs Ag/AgCl under neutral conditions (phosphate buffer solution, pH 7.4). The electrocatalysts showed an excellent response to glucose in comparison to other structurally related sugars (no/negligible response by xylose, maltose, galactose and fructose) and normally co-existing electroactive species (minimal response for ascorbic acid, acetaminophen, uric acid and dopamine). Overall, the materials examined demonstrated excellent glucose sensing capability, while eliminating the pH, temperature sensitivity and lifetime issues associated with enzyme based glucose systems and demonstrate great promise in the quantification of glucose in real clinical samples.
In this work, Preyssler‐type POM (NH4)14NaP5W30O110.44H2O (NH4P5W30), has been synthesised and its electrochemical behaviour in solution was examined at the surface of glassy carbon (GC) and gold ...electrodes. Furthermore, multilayer assemblies of NH4P5W30 POM were constructed onto the surfaces of GCE, gold electrode, and gold quartz electrode via the electrostatic Layer‐by‐Layer (LBL) technique employing polyethyleneimine as the cationic layer and POM as an anionic layer. Cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and electrochemical quartz crystal microbalance measurements (EQCM) were used to monitor the LBL assembly as the NH4P5W30 POM layer was being built. These techniques revealed significant differences in film growth. The multilayer film exhibited well‐defined redox couples associated with POM's tungsten‐oxo framework and showed surface‐confined behaviour up to 100 mVs−1 on both the GC and gold electrodes. The pH dependency and stability of the film were investigated. EIS demonstrated that when the POM layer was the outer layer, the layers were less conductive, and resistance increased as the number of layers increased. In addition, the charge transfer resistance values (Rct) for the layers were calculated. The solvation of ions into the film associated with POM redox activity was studied employing an in‐situ EQCM.
Employment of layer‐by‐layer technique for the immobilisation of Preyssler type polyoxometalate using polyethyleneimine polymer on a 4‐mercaptobenzoic acid self‐assembled gold electrode. Cyclic voltammetry and electrochemical impedance spectroscopy were used to characterise the electrochemical behaviour of the modified films. An in‐situ electrochemical quartz crystal microbalance was employed for studying the solvation of electrolyte ions into film during potential switching.
A tetra Ru-substituted polyoxometalate Na10{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2 (Ru4POM) has been successfully immobilised onto glassy carbon electrodes and indium tin oxide (ITO) coated glass slides ...through the employment of a conducting polypyrrole matrix and the layer-by-layer (LBL) technique. The resulting Ru4POM doped polypyrrole films showed stable redox behavior associated with the Ru centres within the Ru4POM, whereas, the POM’s tungsten-oxo redox centres were not accessible. The films showed pH dependent redox behavior within the pH range 2–5 whilst exhibiting excellent stability towards redox cycling. The layer-by-layer assembly was constructed onto poly(diallyldimethylammonium chloride) (PDDA) modified carbon electrodes by alternate depositions of Ru4POM and a Ru(II) metallodendrimer. The resulting Ru4POM assemblies showed stable redox behavior for the redox processes associated with Ru4POM in the pH range 2–5. The charge transfer resistance of the LBL films was calculated through AC-Impedance. Surface characterization of both the polymer and LBL Ru4POM films was carried out using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Initial investigations into the ability of the Ru4POM LBL films to electrocatalytically oxidise water at pH 7 have also been conducted.
An electrochemical method to determine alanine aminotransferase (ALT) activity over its normal and elevated physiological range was developed based upon detection of
l-glutamate at a glutamate ...oxidase-modified platinum electrode. Measurements were carried out in the presence of ALT co-substrates
l-alanine and α-ketoglutarate and current response from either the oxidation of hydrogen peroxide or the re-oxidation of the mediator ferrocene carboxylic acid was employed. The enzyme electrode was tested over a 6-month period and found to retain 79% of its original activity towards ALT detection with >200 measurements performed over this time. Signals associated with interfering electroactive species (ascorbic acid and uric acid) were eliminated using background subtraction at a denatured glutamate oxidase enzyme electrode. The sensitivity of the device was found to be 0.845
nA
U
−1
L ALT with
t
90
=
180
s, linear range 10–1000
U
L
−1 and LOD of 3.29
U
L
−1 using amperometry at
E
app
=
0.4
V vs. Ag/AgCl at 308
K (35
°C).
The electrochemistry of the water oxidation catalyst, Rb8K2{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2 (Rb8K2-1(0)) has been studied in the presence and absence of potassium cations in both hydrochloric and ...sulfuric acid solutions by transient direct current (dc) cyclic voltammetry, a steady state dc method in the rotating disk configuration and the kinetically sensitive technique of Fourier transformed large-amplitude alternating current (ac) voltammetry. In acidic media, the presence of potassium ions affects the kinetics (apparent rate of electron transfer) and thermodynamics (reversible potentials) of the eight processes (A′/A to H/H′) that are readily detected under dc voltammetric conditions. The six most positive processes (A′/A to F/F′), each involve a one electron ruthenium based charge transfer step (A′/A, B′/B are RuIV/V oxidation and C/C′ to F/F′ are RuIV/III reduction). The apparent rate of electron transfer of the ruthenium centers in sulfuric acid is higher than in hydrochloric acid. The addition of potassium cations increases the apparent rates and gives rise to a small shift of reversible potential. Simulations of the Fourier transformed ac voltammetry method show that the B′/B, E/E′, and F/F′ processes are quasi-reversible, while the others are close to reversible. A third RuIV/V oxidation process is observed just prior to the positive potential limit via dc methods. Importantly, the ability of the higher harmonic components of the ac method to discriminate against the irreversible background solvent process allows this (process I) as well as an additional fourth reversible ruthenium based process (J) to be readily identified. The steady-state rotating disk electrode (RDE) method confirmed that all four Ru-centers in Rb8K2-1(0) are in oxidation state IV. The dc and ac data indicate that reversible potentials of the four ruthenium centers are evenly spaced, which may be relevant to understanding of the water oxidation electrocatalysis. A profound effect of the potassium cation is observed for the one-electron transfer process (G/G′) assigned to RuIII/II reduction and the multiple electron transfer reduction process (H/H′) that arise from the tungstate polyoxometalate framework. A significant shift of E°′ to a more positive potential value for process H/H′ was observed on removal of K+ (∼100 mV in H2SO4 and ∼50 mV in HCl).
Herein we report the use of scanning electrochemical microscopy (SECM) together with electrochemical and spectroscopic techniques to develop and characterise a stable and uniformly reactive ...chemically modified platinum electrode for NADH electrocatalysis. In order to achieve this, a range of different approaches for thionine entrapment within an electropolymerised poly (3,4-ethylendioxythiophene) (PEDOT) film were evaluated using SECM imaging in the presence of NADH, demonstrating the uniformity of the reactive layer towards NADH oxidation. The effect of electrolyte type and time scale employed during PEDOT electropolymerisation was examined with respect to thionine loading and the resulting charge transport diffusion coefficient (DCT) estimated via chronoamperometry. These studies indicated a decrease in DCT as thionine loading increased within the PEDOT film, suggesting that charge transport was diffusion limited within the film. Additionally, thionine functionalised nanotubes were formed, providing a stable support for lactate dehydrogenase entrapment while lowering the rate of thionine leaching, determined via SECM imaging. This enabled lactate determination at Eapp = 0.0 V vs Ag/AgCl over the range 0.25–5 mM in the presence of 1 mM NAD+.
•Scanning electrochemical microscopy examination of thionine within an poly(3,4-ethylendioxythiophene) (PEDOT) film.•NADH electrocatalysis and lactate biosensing enabled coupled with electrochemical/spectroscopic and surface investigations.
In this contribution, a nanoarchitectural approach was employed to produce a nanolayer of polyoxometalate (POM) on the surface of a glassy carbon electrode (GCE) to achieve a higher surface area with ...higher electrocatalytic activity toward the electrochemical hydrogen evolution reaction (HER). To accomplish this, the well-known layer-by-layer (LbL) technique was employed, which involved the alternate adsorption of the POM, Na0.3N(C4H9)47.7 (Mo3O8)4(O3PC(O)(C3H6NH2CH2C4H3S)PO3)4, abbreviated as (TBA)Mo12(AleThio)4, and polyethyleneimine (PEI) polymer. This nanolayered electrode exhibited catalytic properties toward the HER in 0.5 M H2SO4 with the resulting polarization curves indicating an increase in the HER activity with the increasing number of POM layers, and the overpotential required for this reaction was lowered by 0.83 V when compared with a bare GCE. The eighth PEI/(TBA)Mo12(AleThio)4 bilayer exhibited a significantly lower HER overpotential of −0.077 V at a current density of 10 mA cm–2. Surface characterization of the LbL-assembled nanolayers was carried out using X-ray photoelectron spectroscopy, atomic force microscopy, and scanning electron microscopy. We believe that the synergetic effect of the positively charged PEI polymer and the catalytically active molybdate POM is the cause for the successful response to the electrochemical HER.
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