Investigating Electrochemical Stability and Reliability of Gold Electrode‐electrolyte Systems to Develop Bioelectronic Nose Using Insect Olfactory Receptor

This article aims to demonstrate an electrochemically stable and reliable gold electrode‐electrolyte system to develop an insect odorant receptor (Drosophila melanogaster Or35a) based bioelectronic nose. Cyclic voltammograms (CVs) and electrochemical impedance spectroscopy (EIS) of bare gold electrodes, after modification with the self‐assembled monolayer (SAM) of 6‐mercaptohexanoic acid (MHA) and after immobilization with Or35a integrated into the lipid bilayers of liposomes were conducted in the presence of four different redox probes. Potassium ferri/ferrocyanide [Fe(CN)₆]^3?/[Fe (CN)₆]^4? and hydroquinone (H₂Q) redox probes revealed variable and irreversible signals at the time scale of our measurements, with atomic force microscopy (AFM) images and x‐ray photoelectron spectroscopy (XPS) results suggesting gold surface etching due to the presence of CN^? ions in case of [Fe(CN)₆]^3?/[Fe (CN)₆]^4?. Although the hexaammineruthenium complex showed stable electrochemical behaviour at all stages of biosensor development, changes in CV and EIS readings after each surface modifications were insignificant. PBS buffer as a non‐Faradaic medium, was found to provide reliable systems for electrochemical probing of modified gold electrodes with Or35a/liposomes in aqueous media. Using this system, we have shown that this novel biosensor can detect its known odorant E2‐hexenal selectively compared to methyl salicylate down to femtomolar concentration.     

硫醇在金电极上的SA单分子层膜的电化学研究

金基底上的硫醇自组装单分子层膜(Self-asembledmonolayers,SAMs)具有良好的稳定性和有序性,因此在基础研究及应用技术等领域都受到了广泛的重视^[1].通过电化学方法测定自组装膜对溶液中电活性物质的异向电子转移的阻碍作用.     

Self‐assembly of p‐Aminothiophenol on Gold Surface: Application for Impedimetric and Potentiometric Sensing of Cobalt (II) Ions – A Comparative Study

Cobalt, despite an essential biological element, imposes threat to humans when exposed to high concentration or even to low concentration for long term which demands the development of highly sensitive and selective analytical methods for its trace analysis. In the present work, self‐assembly of p‐aminothiophenol (p‐ATP) on gold surface (Au?ATP SAM) was carried out and for the first time, applied as a platform for impedimetric and potentiometric sensing of Co²⁺. Au?ATP SAM was characterized using electrochemical techniques: cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), in the presence of two redox probes: [Fe(CN)₆]^3?/4? and [Ru(NH₃)₆]^2+/3+ to evaluate associated passivating behaviour. Au?ATP SAM completely blocked [Fe(CN)₆]^3?/4? as compared to [Ru(NH₃)₆]^2+/3+ which may be attributed to inner‐sphere and outer‐sphere ET mechanisms, respectively. Au?ATP SAM was found to exhibit excellent sensitivity towards Co²⁺ in a wider concentration range from 1.0×10^?12 M to 1.0×10^?5 M (r²=0.963) at pH 5.5 with a detection limit of 6.0×10^?13 M and superior selectivity. Further, carbon paste electrode (CPE) was prepared by incorporating p‐ATP bound gold nanoparticles and explored for potentiometric sensing of Co²⁺ which exhibited Nernstian slope of 29.2±0.2 mV/dec in linear concentration range of 1.0×10^?6 M–1.0×10^?1 M (r²=0.971) with a detection limit of 8.0×10^?7 M. The proposed sensors were successfully applied for estimation of Co²⁺ content in water samples.     

Indigo Carmine as New Label in PNA Biosensor for Detection of Short Sequence of p53 Tumor Suppressor Gene

A new electrochemical PNA hybridization biosensor for detection of a 15‐mer sequence unique to p53 using indigo carmine (IC) as an electrochemical detector is described in this work. This genosensor is based on the hybridization of target oligonucleotide with its complementary probe immobilized on the gold electrode by self‐assembled monolayer formation. Because this label is electroactive in acidic medium, the interaction between IC and short sequence of p53 is studied by differential pulse voltammety (DPV) in 0.1 M H₂SO₄. The results of electrochemical impedance spectroscopy and cyclic voltammetry in the solution of [Fe(CN)₆]^3?/4? shows no breakage in PNA‐DNA duplex. A decrease in the voltammetric peak currents of IC is observed upon hybridization of the probe with the target DNA. The influence of probe concentration on effective discrimination against non‐complementary oligonucleotides is investigated and a concentration of 10^?7 M is selected. The diagnostic performance of the PNA sensor is described and the detection limit is found to be 4.31×10^?12 M.     

Study on surface acid-base property of carboxylic acid-terminated self-assembled monolayers by cyclic voltammetry and electrochemical impedance spectroscopy

Cyclic voltammetry and electrochemical impedance spec-troscopy were used to study the surface acid-base property of carboxylic acid-terminated self-assembled monolayers(SAMs).A carboxylic acid-terminated thiol,such as thioctic acid(1,2-dithiolane-3-pentanoic acid),was self-assembled on gold electrodes.Electron transfer between the bulk solution and the SAM modified electrode was studied at different pH using Fe(CN)63-as a probe.The surface pka of thioctic acid was determined by cyclic voltammetry and electrochemical impedance spectroscopy to be 5.6 ±0.1 and 5.8±0.1,respectively.The method is compared with other methods of monolayer pKa measurement.     

Electrocatalytic Investigations of Cu(II) and Fe(III) Complexes of Salophen Derivative Schiff Bases on the Pencil Graphite Electrode

This present study describes a pencil graphite electrode surface covered with Cu(II) and Fe(III) complexes based on Salophen derivative Schiff bases in acetonitrile solution containing LiClO₄ as a supporting electrolyte. Cyclic voltammetry method was used for the surface modification procedure with 25 cycle at a sweep rate of 50 mV s^?1. Some characterization methods were used to identify of the prepared modified surfaces including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), Ultraviolet‐visible Spectroscopy (UV‐Vis), and Scanning Electron Microscopy/Energy Dispersive X‐ray Spectroscopy (SEM/SEM‐EDX). The catalytic activity of these modified surfaces on the electrochemical oxidation of catechol (CC) was investigated and they compared with each other. The results demonstrated that these modified electrodes showed perfect electrocatalytic activity on the catechol determination, however the modified electrode prepared with the Cu(II) complex has higher catalytic activity than this prepared with the Fe(III) complex thanks to its the lower detection limit.     

Nanocrystalline Tin‐Oxide Modified Electrodes and their Electrochemical Characterization

Nanocrystalline tin‐oxide particles were prepared as electrodes on the bases of ITO glass and AT‐cut quartz crystals (sputtered gold), respectively, and characterized for their electrochemical behavior. Experiments suggested that the SnO₂ particles could induce an energy barrier to the redox reactions taking place on the electrode surface. When the amount of SnO₂ exceeded ca. 10^?7 mol cm^?2, electrochemical activity demonstrated by the solution redox couples was entirely suppressed. Nevertheless, electrochemical impedance spectroscopic (EIS) measurements suggested that mutual communication between redox couples would still take place on the surface of SnO₂. For instance, although the CV curves of Fe(CN)₆^3‐/4‐ were completely blocked, the exchange current of Fe(CN)₆^3‐/4‐ could still flow through the tin‐oxide modified electrode, increasing with its concentration up to 40 mM. The propagation of electrons in the SnO₂ film was likely via a hopping mechanism. Electrochemical quartz microbalance (EQCM) measurements, in addition, suggested that a charge‐compensating cation (K⁺ or H⁺) uptake reaction may be induced as electrons were pumped to the Sn0₂ electrode, while, if electrons were removed, that could cause water desorption. Analysis based on the Frumkin adsorption isotherm showed the driving force behind the adsorption of water on SnO₂ is about ?2 kcal/mol. Nonetheless, the adsorbed water might face a competitive repulsion from acetonitrile when acetonitrile was used as the electrolyte medium.     

Molecularly Imprinted Impedimetric Sensor for Determination of Mycotoxin Zearalenone

The mycotoxin zearalenone (ZEA) prompts reproductive toxicity due to its strong estrogenic effects. In this work, an electrochemical sensor for determination of ZEA was developed by electropolymerization of a molecularly imprinted poly (o‐phenylenediamine) (PPD) film on screen‐printed gold electrode (SPGE) surface. The sensor was examined by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) using K₃[Fe(CN)₆]/K₄[Fe(CN)₆] as redox probe. The molecularly imprinted polymer (MIP) sensor showed a wide determination range from 2.50 to 200.00 ngmL^?1 for ZEA. The Limit of detection (LOD) was calculated to be 0.20 ngmL^?1, based on the signal to noise (S/N) ratio equal to 3.0. The sensor displayed good repeatability, with RSD values≤4.6 %, and maintained 93.2 % of its initial response after storage for 10 days in air at room temperature. The developed method was successfully applied for the determination of ZEA in corn flakes with mean recoveries ranged from 96.2 % to 103.8 % and RSDs within the interval of 2.1 % to 3.8 %.     

Electrocatalytic Oxidation and Sensitive Determination of Paracetamol Based on Nanosheets Self‐assembled Lindgrenite Microflowers

In this work, the nanosheet‐assembled lindgrenite microflowers (chemical formula: Cu₃Mo₂O₉) were synthesised through a simple process and low‐cost raw materials at room temperature in aqueous solution without using any surface‐active agent. The tightly interlaced nanosheets, like petals, can increase the specific surface area, which can bring about higher electrocatalytic activity and electroanalysis sensitivity. Thus, lindgrenite microflowers were prepared as an electrochemical sensor and successfully applied in the detection of paracetamol through the modified glass carbon electrode. Furthermore, this electrochemical reaction process was simulated at the ab‐initio level to reveal the catalytic mechanism, and the simulation results agreed well with electrochemical experiments. The electrochemical performance of the lindgrenite microflowers modified glassy carbon electrode (GCE) was studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The linearity of paracetamol ranged from 0.05 to 1200 μM (IT method) and 0.05 to 1000 μM (DPV method), low detection limit (0.01 μM) and high sensitivity (5.11 mA mM^?1 cm^?2) towards paracetamol. Moreover, this sensor was applied to detect paracetamol in human blood serum samples. The excellent results demonstrated that the prepared electrode not only showed a desirable linear range towards paracetamol but also exhibited practical applicability and reliability towards human serum samples detection.     

Electrochemical and Photoelectrochemical Study of Self‐assembled Monolayer of Phytic Acid on Brass

Phytic acid is an environment-friendly reagent for processing metals. The anticorrosion and inhibiting mechanism for phytic acid monolayers self-assembled on a brass (HSn70-1) electrode has been investigated by using electrochemical and photocurrent response methods. The electrochemical measurements indicate that phytic acid is liable to form surface complexes on the brass electrode, and the self-assembled monolayers (SAM) change the structure of the electric double-layer and shift the potential of zero charge positively. The photochemical measurement indicates that the brass electrode shows a p-type photoresponse owing to the formation of a Cu2O layer on its surface, and the presence of SAM weakens significantly the photoresponse, suggesting an excellent effect on anticorrosion, which is consistent with the EIS and polarization curve measurements. Adsorption of phytic acid was found to be typical of chemisorption, which can be reasonably described on the basis of the Langmuir isotherm.     

Cobalt(II) Schiff Base/Large Mesoporous Carbon Composite Film Modified Electrode as Electrochemical Biosensor for Hydrogen Peroxide and Glucose

A simple method was developed to prepare a cobalt(II) Schiff base (Co(salen))/large mesoporous carbon (LMC) composite film. The structure and electrocatalytic performance of the Co(salen)/LMC film were investigated by scanning electron microscopy (SEM) and cyclic voltammetry (CV). The Co(salen)/LMC film exhibits high electrocatalytic activity toward H₂O₂, such as low detection limit (8.5×10^?7 M) and wide linear concentration range (2.0×10^?6–8.9×10^?3 M). Furthermore, glucose oxidase (GOD) was self‐assembled on the surface of the Co(salen)/LMC film modified electrode. Determination of glucose in human blood serum with satisfying result was investigated by the resulting biosensor.     

Electrochemical Determination of L‐Lactate Using Phenylboronic Acid Monolayer‐Modified Electrodes

The surface of a gold (Au) disk electrode was modified with a self‐assembled monomolecular layer of dithiobis(4‐butylamino‐m‐phenylboronic acid) (DTBA‐PBA) to prepare L ‐lactate‐sensitive electrodes. The DTBA‐PBA‐modified electrodes exhibited an attenuated cyclic voltammogram (CV) for the Fe(CN)₆^3? ion in the presence of L ‐lactate, as a result of the formation of phenylboronate ester of L ‐lactate accompanied with the addition of OH^? ion to the boron atom. In other words, the negatively charged DTBA‐PBA monolayer blocked the electrode surface from the access of the Fe(CN)₆^3?/4? ions. Thus, the DTBA‐PBA monolayer‐modified Au electrode can be used for determining L ‐lactate on the basis of the change in redox current of Fe(CN)₆^3?/4? ions. The calibration graph useful for determining 1–30 mM L ‐lactate was obtained.     

Modification of Electrode Surfaces by Self‐Assembled Monolayers of Thiol‐Terminated Oligo(Phenyleneethynylene)s

The wire‐like properties of four S‐(4‐{2‐[4‐(2‐phenylethynyl)phenyl]ethynyl}phenyl) thioacetate derivatives, PhC≡CC₆H₄C≡CC₆H₄SAc ( 1 ), H₂NC₆H₄C≡CC₆H₄C≡CC₆H₄SAc ( 2 ), PhC≡CC₆H₂(OMe)₂C≡CC₆H₄SAc ( 3 ) and AcSC₆H₄C≡CC₆H₄C≡CC₆H₄SAc ( 4 ) (Figure 1 ), all of which possess a high degree of conjugation along the oligo(phenyleneethynylene) (OPE) backbone, were investigated as self‐assembled monolayers (SAMs) on gold and platinum electrodes by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The redox probe [Fe(CN)₆]₄^? was used in both the CV and impedance experiments. The results indicate that the thiolates derived from thioacetate‐protected precursor molecules 1 and 2 form well‐ordered monolayers on a gold electrode, whereas SAMs derived from 3 and 4 exhibit randomly distributed pinholes. The electron tunnelling resistance and fractional coverage of SAMs of all four compounds were examined using electron tunnelling theory. The analysis of the results reveal that the well‐ordered SAMs of 1 and 2 exhibit higher charge‐transfer resistance in comparison to the defect‐ridden SAMs of 3 and 4 . The additional steric bulk offered by the methoxy groups in 3 is likely to prevent efficient packing within the SAM, leading to a microelectrode behaviour, when assembled on a gold electrode surface. The protected dithiol derivative 4 probably binds to the surface through both terminal groups which prevents dense packing and leads to the formation of a monolayer with randomly distributed pinholes. Atomic force microscopy (AFM) was used to examine the morphology of the monolayers, and height images gave root‐mean‐square (RMS) roughness′s which are in agreement with the proposed SAM structures.     

Electrochemical Characterization of Gold 6‐Amino‐2‐mercaptobenzothiazole Self‐Assembled Monolayer for Dopamine Detection in Pharmaceutical Samples

A new sensor, gold‐6‐amino‐2‐mercaptobenzothiazole (6A2MBT), was fabricated via a self‐assembly procedure. Electrochemical properties of the monolayer were investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The modified electrode showed excellent antifouling property against the oxidation products of DA, allowed us to construct a dynamic calibration curve with two linear parts, 1.00×10^?6 to 3.72×10^?4 and 3.72×10^?4 to 6.42×10^?4 M DA, with correlation coefficients of 0.997 and 0.992 and a detection limit of 1.57×10^?7 M DA by using differential pulse voltammetry (DPV), respectively. Finally, the performance of the Au‐6A2MBT modified electrode was successfully tested for electrochemical detection of DA in a pharmaceutical sample.     

Nano Au/TiO2 hollow microsphere membranes for the improved sensitivity of detecting specific DNA sequences related to transgenes in transgenic plants

Gold nanoparticles (nano Au)/titanium dioxide (TiO₂) hollow microsphere membranes were prepared on the carbon paste electrode (CPE) for enhancing the sensitivity of DNA hybridization detection. The immobilization of nano Au and TiO₂ microsphere was investigated with cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The hybridization events were monitored with EIS using [Fe(CN)₆]^3?/4? as indicator. The sequence-specific DNA of the 35S promoter from cauliflower mosaic virus (CaMV35S) gene was detected with this DNA electrochemical sensor. The dynamic detection range was from 1.0×10^?12 to 1.0×10^?8 mol/L DNA and a detection limit of 2.3×10^?13 mol/L could be obtained. The polymerase chain reaction (PCR) amplification of the terminator of nopaline synthase (NOS) gene from the real sample of a kind of transgenic soybean was also satisfactorily detected.     

A Sensitive Biosensor for Determination of Cu2+ by One‐step Electrodeposition

A simple one‐step electrodeposition method is described to fabricate three dimensional ordered macroporous chitosan?prussian blue?single walled carbon nanotubes (3DOM CS?PB?SWCNTs) film onto the gold electrode surface to fabricate a copper ions (Cu²⁺)‐specific DNAzyme biosensor. The new sensing strategy for sensitive and selective detection of Cu²⁺ was based on Au nanorods (AuNRs) as signal amplification labels. The electrochemical signal of glucose increased with the concentration of Cu²⁺ increasing. The morphologies and electrochemistry of the composites were investigated by using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electrochemical techniques including cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) and so on. Linear correlations of copper ion concentration were obtained in the range from 10^?18 M to 10^?5 M, achieving with a limit of detection of 10^?19 M (S/N=3). Parameters affecting the biosensor response such as temperature, the cleavage time and the time of hybridization were optimized. This biosensor showed a wide range, low detection limit, good reproducibility and high stability. Additionally, these striking properties endow the biosensor with a great promise for analytical applications.     

Functionalization of gold cysteamine self-assembled monolayer with ethylenediaminetetraacetic acid as a novel nanosensor

Electrochemical characterization of gold cysteamine self-assembled monolayer, in situ functionalized with ethylenediaminetetraacetic acid (Au-CA-EDTA SAM), is described by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Osteryoung square wave voltammetry (OSWV). The results obtained by EIS and CV, in the presence of [Fe(CN)₆]^3−/4− redox probe, show that EDTA is successfully grafted to the surface of Au-CA electrode. Reproducible and reversible variation of the R_ct and ΔEp as a function of solution pH show that Au-CA-EDTA SAM is stable in a wide range of pH and potentials. Accumulation of the Pb²⁺ and Cu²⁺ ions on the Au-CA-EDTA SAM electrode is investigated using faradaic currents or impedimetric effects measured by OSWV and EIS, respectively. These results reveal the presence of active complexing functional groups of EDTA on the surface, and thus, the formation of Au-CA-EDTA SAM electrode. The new sensor responds to the Pb²⁺ and Cu²⁺ separately and simultaneously in a wide linear range of concentrations.     

Electrochemical and Electrocatalytic Properties of Ferrocene Incorporated in L‐Cysteine Self‐Assembled Monolayers on a Gold Electrode

Abstract

The preparation of a gold electrode modified by aminylferrocene (FcAI) covalently bound to L‐cysteine self‐assembled monolayer (L‐Cys/Au SAM) was described, and characterized by cyclic voltammogram (CV) and electrochemical impedance spectroscopy (EIS). In pH 7.4 buffers, FcAI incorporated in L‐Cys/Au SAM gave a pair of well‐defined and quasi‐reversible cyclic voltammetric peaks at 0.109 vs. saturated calomel eletrode (SCE), characteristic of Fe(II)/Fe(III) redox couples of the Fc. The apparent surface electron transfer rate constant is 6.86 s^?1 at the modified electrode. The immobilized Fc gave an excellent electrocatalytic activity for the oxidation of epinephrine (EP). The catalytic current of EP vs. its concentration has a good linear relation in the range of 1.7×10^?7–1.0×10^?4 mol/L, with the correlation coefficient of 0.9975 and detection limit of 1.8×10^?8 mol/L. The modified electrode can be used for the determination of EP in practical injection. The method is simple, quick, sensitive, and accurate.     

Electrochemical characterization of cationic surfactants’ adsorption on a disturbed n-alkanethiolate self-assembled monolayer-modified polycrystalline gold electrode

The adsorption of cetyltrimethylammonium bromide (CTAB) on disturbed n-alkanethiolate self-assembled monolayers (SAMs) was investigated by electrochemical methods with potassium ferricyanide [K₃Fe(CN)₆] as a probe. Compared with the completely restrained signal at ordinary compact n-alkanethiolate SAMs, the electrochemical response of K₃Fe(CN)₆ at the disturbed n-alkanethiolate SAMs was partly restored and became progressively reversible in the presence of increasing concentrations of CTAB, which was employed to characterize the adsorption of cationic surfactants on hydrophobic SAMs. The effect of CTAB concentration on electrochemical impedance spectroscopy (EIS) plots indicated that CTAB experienced two different types of adsorptive behavior at the disturbed n-alkanethiolate SAMs: monomer adsorption at low concentrations below 1×10^–6 M and monolayer adsorption at CTAB concentrations above 1×10^–5 M. The adsorption of a series of cationic surfactants with similar structures to CTAB on disturbed n-alkanethiolate SAMs was also explored. These surfactants had similar adsorptive behavior and showed nearly linear adsorption characteristics with the length of their hydrophobic tails.     

Molecularly Imprinted Poly-o-phenylenediamine Electrochemical Sensor for Entacapone

We have developed a molecularly imprinted polymer (MIP) electrochemical sensor for entacapone (ETC) based on an electropolymerised polyphenylenediamine (Po-PD) on a glassy carbon electrode (GCE) surface. The direct electropolymerisation of the o-phenylenediamine monomer (o-PD) was carried out with ETC as a template. The steps of electropolymerization process, template removal and binding of the analyte were tested by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) using [Fe(CN)₆]³⁻/[Fe(CN)₆]^{4 −} as a redox probe. The operation of the sensor has been investigated by differential pulse voltammetry (DPV). Under optimal experimental conditions, the response of the DPV was linearly proportional to the ETC concentration between 1.0×10⁻⁷ and 5.0×10⁻⁶ M ETC with a limit of detection (LOD) of 5.0×10⁻⁸ M. The developed sensor had excellent selectivity without detectable cross-reactivity for levodopa and carbidopa. The MIP sensor was successfully used to detect ETC in spiked human serum samples.