Real-time prostate-specific antigen detection with prostate-specific antigen imprinted capacitive biosensors

Prostate specific antigen (PSA) is a valuable biomarker for early detection of prostate cancer, the third most common cancer in men. Ultrasensitive detection of PSA is crucial to screen the prostate cancer in an early stage and to detect the recurrence of the disease after treatment. In this report, microcontact-PSA imprinted (PSA-MIP) capacitive biosensor chip was developed for real-time, highly sensitive and selective detection of PSA. PSA-MIP electrodes were prepared in the presence of methacrylic acid (MAA) as the functional monomer and ethylene glycol dimethacrylate (EGDMA) as the cross-linker via UV polymerization. Immobilized Anti-PSA antibodies on electrodes (Anti-PSA) for capacitance measurements were also prepared to compare the detection performances of both methods. The electrodes were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM) and cyclic voltammetry (CV) and real-time PSA detection was performed with standard PSA solutions in the concentration range of 10 fg mL⁻¹–100 ng mL⁻¹. The detection limits were found as 8.0 × 10⁻⁵ ng mL⁻¹ (16 × 10⁻¹⁷ M) and 6.0 × 10⁻⁴ ng mL⁻¹ (12 × 10⁻¹⁶ M) for PSA-MIP and Anti-PSA electrodes, respectively. Selectivity studies were performed against HSA and IgG and selectivity coefficients were calculated. PSA detection was also carried out from diluted human serum samples and finally, reproducibility of the electrodes was tested. The results are promising and show that when the sensitivity of the capacitive system is combined with the selectivity and reproducibility of the microcontact-imprinting procedure, the resulting system might be used successfully for real-time detection of various analytes even in very low concentrations.     

Anodic stripping voltammetry at a new type of disposable bismuth-plated carbon paste mini-electrodes

A new type of disposable carbon paste mini-electrodes (CPmEs), with dimensions in the 50-300 μm range, have been fabricated by heat-shrinking the end-tip of plastic micropipette tips and filling them with carbon paste. The CPmEs have been characterized by microscopic and electrochemical means and tested as substrates for in situ plated Bi film electrodes (BiF-CPmEs), used in the determination of heavy metals by square wave anodic stripping voltammetry (SWASV). It was found that this new class of CPmEs combines the advantages of carbon paste electrodes (readily renewable surface and high surface area) with those of near-microelectrode behaviour (no stirring or electrolyte excess needed). During SWASV experiments in unstirred Pb(II) and Cd(II) solutions well-shaped stripping peaks were obtained whose height varied linearly with analyte concentration in the wide 1 × 10⁻⁸ to 10⁻⁶ M range, both in acetate buffer and unbuffered solutions. Under optimal conditions detection limits of 8 × 10⁻¹⁰ and 1.3 × 10⁻⁹ M were achieved for Pb(II) and Cd(II), respectively and in a trial application, these metal ions have been determined in a spiked tap water sample using a BiF-CPmE.     

Applications of nanostructured Pt-Au hybrid film for the simultaneous determination of catecholamines in the presence of ascorbic acid

Nanostructured platinum-gold (Pt–Au) hybrid film modified glassy carbon electrode (GCE) was fabricated by electro-deposition method in the presence of 2 × 10⁻⁴ mol l⁻¹ l-cysteine. To examine the surface morphological analysis, the (Pt–Au) hybrid film were electrochemically deposited on transparent semiconductor indium tin oxide (ITO) electrodes for scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) studies. From the SEM analysis, it was observed that the deposited nanoplatinum (250–400 nm) was formed as a cauliflower-shaped structure with the gold nanoparticles (30–90 nm). The concentration variation of additive l-cysteine results in the formation of cauliflower-shaped platinum nanoparticles. Further, the Pt–Au hybrid film modified GCE could be used for the detection of catecholamine neurotransmitters epinephrine (EP), norepinephrine (NEP) individually and in the presence of ascorbic acid (AA) in pH 7 phosphate-buffered solutions (PBS). Furthermore, the proposed Pt–Au hybrid film could be applied for the detection of epinephrine in injection solution and ascorbic acid from commercially available vitamin C tablets.     

Fabrication of highly sensitive and selective nanocomposite film based on CuNPs/fullerene-C60/MWCNTs: An electrochemical nanosensor for trace recognition of paracetamol

Designing an electrochemical sensor for versatile clinical applications is a sophisticated task and how dedicatedly functionalized composite materials can perform on this stage is a challenge for today and tomorrow's Nanoscience and Nanotechnology. In the present work, we demonstrate a new strategy for the development of novel electrochemical sensor based on catalytic nanocomposite film. Fullerene-C₆₀ and multi-walled carbon nanotubes (MWCNTs) were dropped on the pre-treated carbon paste electrode (CPE) and copper nanoparticles (CuNPs) electrochemically deposited on the modified CPE to form nanocomposite film of CuNPs/C₆₀/MWCNTs/CPE. In this work, an electrochemical method based on square wave voltammetry (SWV) employing CuNPs/C₆₀/MWCNTs/CPE has been presented for the recognition and determination of paracetamol (PT). Developed electrochemical sensor was characterized using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronocoulometry. The composite film made the fabricated sensor to display high sensitivity and good selectivity for PT detection. The influence of the optimization parameters such as pH, accumulation time, deposition potential, scan rate and effect of loading of composite mixture of C₆₀-MWCNTs and CuNPs on the electrochemical performance of the sensor were evaluated. A linear range from 4.0 × 10⁻⁹ to 4.0 × 10⁻⁷ M for PT detection was obtained with a detection limit of 7.3 × 10⁻¹¹ M. The fabricated sensor was successfully applied to the detection of PT in biological samples with good recovery ranging from 99.21 to 103%.     

Electrochemistry and scanning electron microscopy of polyaniline/peroxidase-based biosensor

An amperometric biosensor was prepared by in situ deposition of horseradish peroxidase (HRP) enzyme on a polyaniline (PANI)-doped platinum disk electrode. The PANI film was electrochemically deposited on the electrode at 100 mV s⁻¹/Ag-AgCl. Cyclic voltammetric characterization of the PANI film in 1 M HCl showed two distinct redox peaks, which prove that the PANI film was electroactive and exhibited fast reversible electrochemistry. The surface concentration and film thickness of the adsorbed electroactive species was estimated to be 1.85×10⁻⁷ mol cm⁻² and approximately 16 nm, respectively. HRP was electrostatically immobilized onto the surface of the PANI film, and voltammetry was used to monitor the electrocatalytic reduction of hydrogen peroxide under diffusion-controlled conditions. Linear responses over the concentration range 2.5×10⁻⁴ to 5×10⁻³ M were observed. Spectroelectrochemistry was used to monitor the changes in UV-vis properties of HRP, before and after the catalysis of H₂O₂. The biosensor surface morphology was characterized by scanning electron microscopy (SEM) using PANI-doped screen-printed carbon electrodes (SPCEs) in the presence and absence of (i) peroxidase and (ii) peroxide. The SEM images showed clear modifications of the conducting film surface structure when doped with HRP, as well as the effect of hydrogen peroxide on the morphology of biosensor.     

Multilayered construction of glucose oxidase on gold electrodes based on layer-by-layer covalent attachment

A feasible approach to construct multilayered enzyme film on the gold electrode surface for use as biosensing interface is described. The film was fabricated by alternate layer-by-layer deposition of periodate-oxidized glucose oxidase (GOx) and poly(allylamine) (PAA). The covalent attachment process was followed and confirmed by electrochemical impedance spectroscopy (EIS). X-ray diffraction (XRD) experiments revealed that the film was homogeneous and formed in an ordered manner with a thickness of 2.6 ± 0.1 nm per bilayer. The gold electrodes modified with the GOx/PAA multilayers showed excellent electrocatalytical response to the oxidation of glucose when ferrocenemethanol was used as an artificial redox mediator, which was studied by cyclic voltammetry (CV). From the analysis of voltammetric signals, the coverage of active enzyme on the electrode surface was estimated, which had a linear relationship with the number of GOx/PAA bilayers. This suggests that the analytical performance such as sensitivity, detection limit, and so on, is tunable by controlling the number of attached bilayers. The six GOx/PAA bilayer electrode exhibited a sensitivity of 15.1 μA mM⁻¹ cm⁻² with a detection limit of 3.8 × 10⁻⁶ M. In addition, the sensor exhibited good reproducibility and stability.     

A disposable electrochemical sensor for the determination of indole-3-acetic acid based on poly(safranine T)-reduced graphene oxide nanocomposite

A disposable electrochemical sensor for the determination of indole-3-acetic acid (IAA) based on nanocomposites of reduced graphene oxide (rGO) and poly(safranine T) (PST) was reported. The sensor was prepared by coating a rGO film on a pre-anodized graphite electrode (AGE) through dipping-drying and electrodepositing a uniform PST layer on the rGO film. Scanning electron microscopic (SEM) and infrared spectroscopic (IR) characterizations indicated that PST-rGO formed a rough and crumpled composite film on AGE, which exhibited high sensitive response for the oxidation of IAA with 147-fold enhancement of the current signal compared with bare AGE. The voltammetric current has a good linear relationship with IAA concentration in the range 1.0 × 10⁻⁷-7.0 × 10⁻⁶ M, with a low detection limit of 5.0 × 10⁻⁸ M. This sensor has been applied to the determination of IAA in the extract samples of several plant leaves and the recoveries varied in the range of 97.71-103.43%.     

Optimisation of mercury film deposition on glassy carbon electrodes: evaluation of the combined effects of pH, thiocyanate ion and deposition potential

The combined effects of pH, thiocyanate ion and deposition potential in the characteristics of thin mercury film electrodes plated on glassy carbon surfaces are evaluated. Charges of deposited mercury are used as an experimental parameter for the estimation of the effectiveness of the mercury deposition procedure. The sensitivity of the anodic stripping voltammetry (ASV) method for the determination of lead at in situ and at ex situ formed thin mercury films are also examined. It was concluded that, in acidic solutions (pH 2.5-5.7) and fairly negative deposition potentials, e.g. −1.3 to −1.5 V, thiocyanate ion promotes the formation of the mercury film, in respect both to the amount of deposited mercury and to the mercury deposition rate. Also, the mercury coatings produced in thiocyanate solutions are more homogeneous, as depicted by microscopic examinations. In the presence of thiocyanate there is no obvious advantage of using high concentrations of mercury and/or high deposition times for the in situ and ex situ preparation of the mercury film electrodes. The optimised thin mercury film electrode ex situ prepared in a 5.0 mM thiocyanate solution of pH 3.4 was successfully applied to the ASV determination of lead and copper in acidified seawater (pH 2). The limit of detection (3σ) was 6×10⁻¹¹ M for lead and 2×10⁻¹⁰ M for copper for a deposition time of 5 min. Relative standard deviations (R.S.D.s) of <1.2% were obtained for determinations at the nanomolar of concentration level.     

Evaluation of thin mercury film rotating disk electrode to perform absence of gradients and Nernstian equilibrium stripping (AGNES) measurements

In the present work, the applicability of thin mercury film on a rotating disk electrode (TMF-RDE), to assess the free metal ion concentration by the absence of gradients and Nernstian equilibrium stripping (AGNES), is evaluated. The thickness of the mercury film and several AGNES parameters has been optimized. A nominal 16 nm film is chosen due to the higher signal (faradaic current) relative to the value of the noise (capacitive current). Due to the smaller volume to area ratio, the deposition time needed to reach a certain preconcentration factor (Y) is much shorter than in larger electrodes, like the HMDE. The limit of detection (3σ) for lead(II) is 7.4 × 10⁻⁹ M and 7.2 × 10⁻⁸ M for a Y of 5000 (deposition time of 150 s) and 1000 (deposition time of 100 s), respectively. A specific mathematical treatment is developed in order to subtract a corrected blank taking into account the degradation of the thin film (presumably, falling down of drops). The couple TMF-RDE/AGNES is successfully applied for speciation purposes in the systems Pb(II)-latex nanospheres and Pb(II)-IDA (iminodiacetic acid), where the stability constants calculated for both systems agree with values reported in the literature.     

Electrode Modified with Langmuir–Blodgett (LB) Film of Calixarenes for Preconcentration and Stripping Analysis of Hg(II)

A new type of current sensor, Langmuir–Blodgett (LB) film of calixarene on the surface of glassy carbon electrode (GCE) was prepared for determination of mercury by anodic stripping voltammetry (ASV). An anodic stripping peak was obtained at 0.15 V (vs. SCE) by scanning the potential from ?0.6 to +0.6 V. Compared with a bare GCE, the LB film coated electrode greatly improves the sensitivity of measuring mercury ion. The fabricated electrode in a 0.1 M H₂SO₄+0.01 M HCl solution shows a linear voltammetric response in the range of 0.07–40 μg L^?1 and detection limit of 0.04 μg L^?1 (ca. 2×10^?10 M). The high sensitivity, selectivity, and stability of this LB film modified electrode demonstrates its practical application for a simple, rapid and economical determination of Hg²⁺ in a water sample.     

Poly(3,4-ethylenedioxithiophene)/MnO2 composite electrodes for electrochemical capacitors

Composite electrodes of poly(3,4-ethylenedioxithiophene) and manganese oxide (PEDOT/MnO₂) have been prepared by electrodeposition of manganese oxide over PEDOT-modified titanium substrate. The PEDOT layers are deposited on titanium by potentiostatic deposition at 1.4 V and at two different temperatures: 5 and 25 °C (named PEDOT₍₅₎ and PEDOT₍₂₅₎, respectively). The electrodes are characterized by field emission gun scanning electron microscopy (FEG-SEM) and their electrochemical performances are evaluated by using cyclic voltammetry (CV) in 1 molL⁻¹ Na₂SO₄. The results show an improvement in the specific capacitance (C_s) of the oxide due to the presence of the polymer layer. Considering only the MnO₂ mass, the C_s values of the electrodes Ti/MnO₂, Ti/PEDOT₍₅₎/MnO₂ and Ti/PEDOT₍₂₅₎/MnO₂, estimated by the CV technique, are 151, 159 and 199 Fg⁻¹ at 10 mVs⁻¹ respectively. The micrographies of electrodes show that the polymer layer leads to very significant changes in the morphology of the oxide layers, which in turn generates the improvement observed in the capacitive property.     

Titania nanotubes decorated with gold nanoparticles for electrochemiluminescent biosensing of glycosylated hemoglobin

A glycated hemoglobin (HbA1c) biosensor with high performance has been constructed in this work. Here the fructosyl amino acid oxidase was immobilized onto a pre-functionalized indium tin oxide glass with titania nanotubes decorated with gold nanoparticles. The property of nanocomposite was characterized by transmission electromicroscopy, scanning electron microscopy, electrochemistry and spectroscopy. Under the optimum conditions, fructosyl valine was detected by this biosensor. It exhibited a linear detection range from 4.0 × 10⁻⁹ M to 7.2 × 10⁻⁷ M, and a limit of detection for 3.8 × 10⁻⁹ M at the signal-to-noise ratio of 3. Thus the HbA1c level in whole blood samples of healthy individuals or diabetic patients were evaluated with designed biosensor after pre-treatment of hydrolysis. The results of our detection were closely consistent with that of the standard method. At the same time, our biosensor has some advantages including high sensitivity, disposable usage and low cost, which implies its great promising application in point-of-care testing of HbA1c.     

Electrochemical study and flow injection analysis of paracetamol in pharmaceutical formulations based on screen-printed electrodes and carbon nanotubes

Acetaminophenol or paracetamol is one of the most commonly used analgesics in pharmaceutical formulations. Acetaminophen is electroactive and voltammetric mechanistic studies for the electrode processes of the acetaminophenol/N-acetyl-p-quinoneimine redox system are presented. Carbon nanotubes modified screen-printed electrodes with enhanced electron transfer properties are used for the study of the electrochemical-chemical oxidation mechanism of paracetamol at pH 2.0.Quantitative analysis of paracetamol by using its oxidation process (in a Britton-Robinson buffer solution pH 10.0) at +0.20 V (vs. an Ag pseudoreference electrode) on an untreated screen-printed carbon electrode (SPCE) was carried out. Thus, a cyclic voltammetric based reproducible determination of acetaminophen (R.S.D., 2.2%) in the range 2.5 × 10⁻⁶ M to 1 × 10⁻³ M, was obtained. However, when SPCEs are used as amperometric detectors coupled to a flow injection analysis (FIA) system, the detection limit achieved for paracetamol was 1 × 10⁻⁷ M, one order of magnitude lower than that obtained by voltammetric analysis. The repeatability of the amperometric detection with the same SPCE is 2% for 15 successive injections of 10⁻⁵ M acetaminophen and do not present any memory effect.Finally, the applicability of using screen-printed carbon electrodes for the electrochemical detection of paracetamol (i.e. for quality control analysis) was demonstrated by using two commercial pharmaceutical products.     

Electrochemical sensors based on binuclear cobalt phthalocyanine/surfactant/ordered mesoporous carbon composite electrode

A new ordered mesoporous carbon (OMC) composite modified electrode was fabricated for the first time. Binuclear cobalt phthalocyaninehexasulfonate sodium salt (bi-CoPc) can be adsorbed onto didodecyldimethylammonium bromide (DDAB)/OMC film by ion exchange. UV-vis spectroscopy, scanning electron microscopy (SEM) and electrochemical methods were used to characterize the composite film. The cyclic voltammograms demonstrate that the charge transfer of bi-CoPc is promoted by the presence of OMC. Further study indicated that bi-CoPc/DDAB/OMC film is the excellent electrocatalyst for the electrochemical reduction of oxygen in a neutral aqueous solution and hemoglobin (Hb) at lower concentrations. Additionally, as an amperometric 2-mercaptoethanol (2-ME) sensor, this modified electrode shows a wider linear range (2.5 × 10⁻⁶ to 1.4 × 10⁻⁴ M), high sensitivity (16.5 μA mM⁻¹) and low detection limit of 0.6 μM (S/N = 3). All these confirm the fact that the new composite film may have wide potential applications in biofuel cells, biological and environmental sensors.     

A hydrogen peroxide biosensor based on Langmuir-Blodgett technique: Direct electron transfer of hemoglobin in octadecylamine layer

The present paper describes the modification of hemoglobin (Hb)-octadecylamine (ODA) Langmuir-Blodgett (LB) film on a gold electrode surface to develop a novel electrochemical biosensor for the detection of hydrogen peroxide. Atomic force microscopy (AFM) image of Hb-ODA LB film indicated Hb molecules existed in ODA layer in a well-ordered and compact form. The immobilized Hb displayed a couple of stable and well-defined redox peaks with an electron transfer rate constant of 4.58 ± 0.95 s⁻¹ and a formal potential of −185 mV (versus Ag/AgCl) in phosphate buffer (1.0 mM, pH 5.0) contain 0.1 M KCl at a scan rate of 200 mV s⁻¹, characteristic of Hb heme Fe(III)/Fe(II) redox couple. The formal potential of Hb heme Fe(III)/Fe(II) redox couple in ODA film shifted linearly between pH 5 and 8 with a slope of −23.8 mV pH⁻¹, suggesting that proton took part in electrochemical reaction. The ODA could accelerate the electron transfer between Hb and the electrode. This modified electrode showed an electrochemical activity to the reduction of hydrogen peroxide (H₂O₂) without the aid of any electron mediator.     

Mercury(II) selective sensors based on AlGaN/GaN transistors

This work presents the first polymer approach to detect metal ions using AlGaN/GaN transistor-based sensor. The sensor utilised an AlGaN/GaN high electron mobility transistor-type structure by functionalising the gate area with a polyvinyl chloride (PVC) based ion selective membrane. Sensors based on this technology are portable, robust and typically highly sensitive to the target analyte; in this case Hg²⁺. This sensor showed a rapid and stable response when it was introduced to solutions of varying Hg²⁺ concentrations. At pH 2.8 in a 10⁻² M KNO₃ ion buffer, a detection limit below 10⁻⁸ M and a linear response range between 10⁻⁸ M-10⁻⁴ M were achieved. This detection limit is an order of magnitude lower than the reported detection limit of 10⁻⁷ M for thioglycolic acid monolayer functionalised AlGaN/GaN HEMT devices. Detection limits of approximately 10⁻⁷ M and 10⁻⁶ M in 10⁻² M Cd(NO₃)₂ and 10⁻² M Pb(NO₃)₂ ion buffers were also achieved, respectively. Furthermore, we show that the apparent gate response was near-Nernstian under various conditions. X-ray photoelectron spectroscopy (XPS) experiments confirmed that the sensing membrane is reversible after being exposed to Hg²⁺ solution and rinsed with deionised water. The success of this study precedes the development of this technology in selectively sensing multiple ions in water with use of the appropriate polymer based membranes on arrays of devices.     

Functionalized graphene oxide for the fabrication of paraoxon biosensors

There is an increasing need to develop biosensors for the detection of harmful pesticide residues in food and water. Here, we report on a versatile strategy to synthesize functionalized graphene oxide nanomaterials with abundant affinity groups that can capture histidine (His)-tagged acetylcholinesterase (AChE) for the fabrication of paraoxon biosensors. Initially, exfoliated graphene oxide (GO) was functionalized by a diazonium reaction to introduce abundant carboxyl groups. Then, N_α,N_α-bis(carboxymethyl)-l-lysine hydrate (NTA-NH₂) and Ni²⁺ were anchored onto the GO based materials step by step. AChE was immobilized on the functionalized graphene oxide (FGO) through the specific binding between Ni-NTA and His-tag. A low anodic oxidation potential was observed due to an enhanced electrocatalytic activity and a large surface area brought about by the use of FGO. Furthermore, a sensitivity of 2.23 μA mM⁻¹ to the acetylthiocholine chloride (ATChCl) substrate was found for our composite covered electrodes. The electrodes also showed a wide linear response range from 10 μM to 1 mM (R² = 0.996), with an estimated detection limit of 3 μM based on an S/N = 3. The stable chelation between Ni-NTA and His-tagged AChE endowed our electrodes with great short-term and long-term stability. In addition, a linear correlation was found between paraoxon concentration and the inhibition response of the electrodes to paraoxon, with a detection limit of 6.5 × 10⁻¹⁰ M. This versatile strategy provides a platform to fabricate graphene oxide based nanomaterials for biosensor applications.     

High sensitivity Schottky junction diode based on monolithically grown aligned polypyrrole nanofibers: Broad range detection of m-dihydroxybenzene

Aligned p-type polypyrrole (PPy) nanofibers (NFs) thin film was grown on n-type silicon (100) substrate by an electrochemical technique to fabricate Schottky junction diode for the efficient detection of m-dihydroxybenzene chemical. The highly dense and well aligned PPy NFs with the average diameter (∼150–200 nm) were grown on n-type Si substrate. The formation of aligned PPy NFs was confirmed by elucidating the structural, compositional and the optical properties. The electrochemical behavior of the fabricated Pt/p-aligned PPy NFs/n-silicon Schottky junction diode was evaluated by cyclovoltametry (CV) and current (I)-voltage (V) measurements with the variation of m-dihydroxybenzene concentration in the phosphate buffer solution (PBS). The fabricated Pt/p-aligned PPy NFs/n-silicon Schottky junction diode exhibited the rectifying behavior of I–V curve with the addition of m-dihydroxybenzene chemical, while a weak rectifying I–V behavior was observed without m-dihydroxybenzene chemical. This non-linear I–V behavior suggested the formation of Schottky barrier at the interface of Pt layer and p-aligned PPy NFs/n-silicon thin film layer. By analyzing the I–V characteristics, the fabricated Pt/p-aligned PPy NFs/n-silicon Schottky junction diode displayed reasonably high sensitivity ∼23.67 μAmM⁻¹cm⁻², good detection limit of ∼1.51 mM with correlation coefficient (R) of ∼0.9966 and short response time (10 s).     

Fabrication and characterisation of high performance polypyrrole modified microarray sensor for ascorbic acid determination

In this study, gold microelectrode array (Au/MEA) with electrode of 12 μm diameter was fabricated by photolithography technique. Subsequently, polypyrrole (Ppy) modified gold microarrays sensor (Ppy/Au/MEA) was prepared by cyclic voltammetry technique. The deposition potential range and number of cycles were optimised in order to get optimum thickness of Ppy film. Scanning Electron Microscope and Atomic Force Microscope investigations reveal that Ppy coating formed at 3 cycles is porous with thickness of 1.5 μm which exhibiting high catalytic current for ascorbic acid (AA) in square wave technique (SWV). In contrast to earlier sensors designs, these Ppy/Au/MEA sensors exhibits lower detection limit (LOD) of 10 nm towards AA at physiological conditions. It also exhibits enhanced sensitivity (2.5 mA cm⁻² mM⁻¹) and long range of linear detection limit from 10 nm to 2.8 mM. In the same way, polypyrrole modified macro Au (Ppy/Au/MA) biosensor was also fabricated and its electro catalytic property towards AA was compared with that of Ppy/Au/MEA. The Ppy/Au/MA exhibits sensitivity of only 0.27 mA cm⁻² mM⁻¹, LOD of 5 μM and linear range of 10 μM to 2.2 mM. Hence, our investigations indicate that the Ppy/Au/MEA could serve as highly sensitive sensor for AA than any of the earlier designs. So, the Ppy/Au/MEA electrode was utilised for determination AA in a wide variety of real samples.     

An optical sensor based on H-acid/layered double hydroxide composite film for the selective detection of mercury ion

A novel optical chemosensor was fabricated based on 1-amino-8-naphthol-3,6-disulfonic acid sodium (H-acid) intercalated layered double hydroxide (LDH) film via the electrophoretic deposition (EPD) method. The film of H-acid/LDH with the thickness of 1 μm possesses a well c-orientation of the LDH microcrystals confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The fluorescence detection for Hg(II) in aqueous solution was performed by using the H-acid/LDH film sensor at pH 7.0, with a linear response range in 1.0 × 10⁻⁷ to 1.0 × 10⁻⁵ mol L⁻¹ and a detection limit of 6.3 × 10⁻⁸ mol L⁻¹. Furthermore, it exhibits excellent selectivity for Hg(II) over a large number of competitive cations including alkali, alkaline earth, heavy metal and transitional metals. The specific fluorescence response of the optical sensor is attributed to the coordination between Hg(II) and sulfonic group in the H-acid immobilized in the LDH matrix, which was verified by NMR spectroscopy and UV–vis spectra. In addition, density functional theory (DFT) calculation further confirms that the coordination occurs between one Hg²⁺ and two O atoms in the sulfonic group, which is responsible for the significant fluorescence quenching of the H-acid/LDH film. The results indicate that the H-acid/LDH composite film can be potentially used as a chemosensor for the detection of Hg²⁺ in the environmental and biomedical field.