Synthesis,characterizations of silica-coated gold nanorods and its applications in electroanalysis of hemoglobin

Gold nanorods (GNRs) were synthesized by a seed–mediated growth approach followed by TEOS polymerization leading to the formation of silica layer surrounding the gold nanorod core. TEM images showed that the silica-coated gold nanorods (GNRs@SiO₂) were dispersed with an average aspect ratio of 3.1 for the GNRs cores and a uniform thickness of the silica shell. The core/shell nanocomposites were further used as efficient supports for the immobilization of hemoglobin (Hb) to fabricate a novel biosensor. The immobilized Hb showed an enhanced electron transfer for its heme Fe(III) to Fe(II) redox couple. This biosensor showed an excellent bioelectrocatalytic activity towards H₂O₂ with a linear range from 8.0 × 10⁻⁷ to 6.1 × 10⁻⁵ M, and the detection limit was 6.0 × 10⁻⁸ M at 3σ. The apparent Michaelis–Menten constant of the immobilized hemoglobin was calculated to be 0.13 mM.     

Gelatin-functionalized carbon nanotubes for the bioelectrochemistry of hemoglobin

In this paper, we compared the use of gelatin-functionalized carbon nanotubes (CNTs) as substrates for Hemoglobin (Hb) immobilization and as electrodes for electrochemical reduction of the absorbed Hb. The non-covalently gelatin-functionalized CNTs possessed an improved solubility in aqueous solution and may have an enhanced biocompatibility with Hb. The characteristics of Hb/gelatin-CNTs composite films were studied by using UV–vis spectroscopy, FTIR spectroscopy and electrochemical methods. The immobilized Hb showed a couple of quasi-reversible redox peaks with a formal potential of −0.35 V (vs. SCE) in 0.10 M pH 7.0 phosphate buffer solution (PBS). The surface concentration of electroactive Hb immobilized on gelatin-CNT/GC electrode was about 4.34 × 10⁻¹⁰ mol cm⁻².     

Direct electrochemistry and electrocatalysis of hemoglobin entrapped in composite matrix based on chitosan and CaCO3 nanoparticles

The direct electron transfer between hemoglobin (Hb) and the underlying glassy carbon electrode (GCE) can be readily achieved via a high biocompatible composite system based on biopolymer chitosan (CHT) and inorganic CaCO₃ nanoparticles (nano-CaCO₃). Cyclic voltammetry of Hb-CHT/nano-CaCO₃/GCE showed a pair of stable and quasi-reversible peaks for HbFe(III)/Fe(II) redox couple in pH 7.0 buffer. The electrochemical reaction of Hb immobilized in CHT/nano-CaCO₃ composite matrix exhibited a surface-controlled process accompanied by electron and proton transfer. The electron transfer rate constant was estimated to be 1.8 s⁻¹. This modified electrode showed a high thermal stability up to 60 °C. The apparent Michaelis–Menten constant was calculated to be 7.5 × 10⁻⁴ M, indicating a high catalytic activity of the immobilized Hb toward H₂O₂. The interaction between Hb and this nano-hybrid material was also investigated using FT-IR and UV–vis spectroscopy, indicating that Hb retained its native structure in this hybrid matrix.     

Using flowerlike polymer–copper nanostructure composite and novel organic–inorganic hybrid material to construct an amperometric biosensor for hydrogen peroxide

A new type of amperometric hydrogen peroxide biosensor was fabricated by entrapping horseradish peroxidase (HRP) in the organic–inorganic hybrid material composed of zirconia–chitosan sol–gel and Au nanoparticles (ZrO₂–CS–AuNPs). The sensitivity of the biosensor was enhanced by a flowerlike polymer–copper nanostructure composite (pPA–FCu) which was prepared from co-electrodeposition of CuSO₄ solution and 2,6-pyridinediamine solution. Several techniques, including UV–vis absorption spectroscopy, scanning electron microscopy, cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy were employed to characterize the assembly process and performance of the biosensor. The results showed that this pPA–FCu nanostructure not only had excellent redox electrochemical activity, but also had good catalytic efficiency for hydrogen peroxide. Also the ZrO₂–CS–AuNPs had good film forming ability, high stability and good retention of bioactivity of the immobilized enzyme. The resulting biosensors showed a linear range from 7.80 × 10^?7 to 3.7 × 10^?3 mol L^?1, with a detection limit of 3.2 × 10^?7 mol L^?1 (S/N = 3) under optimized experimental conditions. The apparent Michaelis–Menten constant was determined to be 0.32 mM, showing good affinity. In addition, the biosensor which exhibits good analytical performance, acceptable stability and good selectivity, has potential for practical applications.     

Ferrocene-modified Fe3O4@SiO2 magnetic nanoparticles as building blocks for construction of reagentless enzyme-based biosensors

A novel amperometric glucose biosensor was developed by entrapping glucose oxidase (GOD) in chitosan (CS) composite doped with ferrocene monocarboxylic acid-modified magnetic core-shell Fe₃O₄@SiO₂ nanoparticles (FMC-AFSNPs). It is shown that the obtained magnetic bio-nanoparticles attached to the surface of a carbon paste electrode (CPE) with the employment of a permanent magnet showed excellent electrochemical characteristics and at the same time acted as mediator to transfer electrons between the enzyme and the electrode. Under optimal conditions, this biosensor was able to detect glucose in the linear range from 1.0 × 10⁻⁵ to 4.0 × 10⁻³ M with a detection limit of 3.2 μM (S/N = 3). This immobilization approach effectively improved the stability of the electron transfer mediator and is promising for construction of biosensor and bioelectronic devices.     

Direct electrochemistry of hemoglobin and glucose oxidase in electrodeposited sol–gel silica thin films on glassy carbon

This work points out that electrogeneration of silica gel (SG) films on glassy carbon electrodes (GCEs) can be applied to immobilize biomolecules – hemoglobin (Hb) or glucose oxidase (GOD) or both of them in mixture – without preventing their activity. These proteins were physically entrapped in the sol–gel material in the course of the electro-assisted deposition process applied to form the thin films onto the electrode surface. SG films were prepared from a precursor solution by applying a suitable cathodic potential likely to induce a local pH increase at the electrode/solution interface, accelerating thereby polycondensation of the silica precursors with concomitant film formation. Successful immobilization of proteins was checked by various physico-chemical techniques. Both Hb and GOD were found to undergo direct electron transfer, as demonstrated by cyclic voltammetry. GCE–SG–Hb gave rise to well-defined peaks at potentials E_c = −0.29 V and E_a = −0.17 V in acetate buffer, corresponding to the Fe^III/Fe^II redox system of heme group of the protein, while GCE–SG–GOD was characterized by the typical signals of FAD group at E_c = −0.41 V and E_a = −0.33 V in phosphate buffer. These two redox processes were also evidenced on a single voltammogram when both Hb and GOD were present together in the same SG film. Hb entrapped in the silica thin film displayed an electrocatalytic behavior towards O₂ and H₂O₂ in solution, respectively in the mM and μM concentration ranges. Immobilized GOD kept its biocatalytic properties towards glucose. Combined use of these two proteins in mixture has proven to be promising for detection of glucose in solution via the electrochemical monitoring of oxygen consumption (decrease of the oxygen electrocatalytic signal).     

Direct electrochemistry and electrocatalysis of hemoglobin in sodium alginate film on a BMIMPF6 modified carbon paste electrode

A room temperature ionic liquid (RTIL) modified carbon paste electrode was constructed based on the substitute of paraffin with 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIMPF₆) as binder for carbon paste. Direct electrochemistry and electrocatalytic behaviors of hemoglobin (Hb) entrapped in the sodium alginate (SA) hydrogel film on the surface of this carbon ionic liquid electrode (CILE) were investigated. The presence of IL in the CILE increased the electron transfer rate and provided a biocompatible interface. Hb remained its bioactivity on the surface of CILE and the SA/Hb modified electrode showed a pair of well-defined, quasi-reversible cyclic voltammetric peaks with the apparent standard potential (E^0′) at about −0.344 V (vs. SCE) in pH 7.0 Britton–Robinson (B–R) buffer solution, which was attributed to the Hb Fe(III)/Fe(II) redox couple. UV–Vis absorption spectra indicated that heme microenvironment of Hb in SA film was similar to its native status. Hb showed a thin-layer electrochemical behavior in the SA film with the direct electron transfer achieved on CILE without the help of electron mediator. Electrochemical investigation indicated that Hb took place one proton with one electron electrode process and the average surface coverage of Hb in the SA film was 3.2 × 10⁻¹⁰ mol/cm². The immobilized Hb showed excellent electrocatalytic responses to the reduction of H₂O₂ and nitrite.     

Highly efficient enzyme immobilization by nanocomposites of metal organic coordination polymers and carbon nanotubes for electrochemical biosensing

New nanocomposites with multi-walled carbon nanotubes (MWCNTs) embedded in metal-organic coordination polymers (MOCPs) were successfully prepared as highly efficient matrices of enzyme immobilization for sensitive electrochemical biosensing. NaAuCl₄ was pre-adsorbed on the MWCNTs to act as anchor sites to further coordinate with ligand benzenedithiol and form MOCPs. The formation of MWCNTs-MOCPs one-pot entrapped glucose oxidase (GOx) with a ratio close to 100% and exhibited enhanced mass-transfer over MOCPs. Thus MWCNTs-MOCPs-modified electrodes present superior enzymatic catalysis performance of greatly enhanced sensitivity (136 μA cm^− 2 mM^− 1) and magnitudes-lower detection limit (48 nM), being superior to most analogues.     

Layer-by-layer assembled DNA functionalized single-walled carbon nanotube hybrids for arsenic(III) detection

Based on layer-by-layer assembled DNA functionalized single-walled carbon nanotube hybrids, a DNA biosensor for the detection of arsenic(III) in a nearly physiological pH environment was developed. The redox process between arsenic(III) and arsenic(0) on the biosensor was proved. The growth of those hybrids on glassy carbon electrode was monitored by detecting arsenic(III). The arsenic(III) current on the biosensor was similar over a broad pH range (3.0–8.0) and the limit of detection (S/N = 3) was 0.05 μg L⁻¹ at pH 7.0. The biosensor can be reused up to 16 times.     

Electrochemical biosensor based on hairpin DNA probe using 2-nitroacridone as electrochemical indicator for detection of DNA species related to Chronic Myelogenous Leukemia

In this article, a new kind of hairpin DNA Electrochemical biosensor using nitroacridone as electrochemical indicator was first designed, and used to detect BCR/ABL fusion gene in Chronic Myelogenous Leukemia (CML). The results indicated that in pH 7.0 Tris–HCl buffer solution, the oxidation peak current was linear with the concentration of complementary strand in the range of 6.2 × 10⁻⁸–3.1 × 10⁻⁷ mol/l with a detection limit of 5.3 × 10⁻⁹ mol/l. This new hairpin DNA electrochemical biosensor demonstrates its excellent specificity for single-base mismatch and complementary (dsDNA) after hybridization, and this probe has been used for assay of PCR product of a real sample with satisfactory result.     

Electrodeposited PbO2 thin film on Ti electrode for application in hybrid supercapacitor

PbO₂ thin films were prepared by pulse current technique on Ti substrate from Pb(NO₃)₂ plating solution. The hybrid supercapacitor was designed with PbO₂ thin film as positive electrode and activated carbon (AC) as negative electrode in the 5.3 M H₂SO₄ solution. Its electrochemical properties were determined by cyclic voltammetry (CV), charge–discharge test and electrochemical impedance spectroscopy (EIS). The results revealed that the PbO₂/AC hybrid supercapacitor exhibited large specific capacitance, high-power and stable cycle performance. In the potential range of 0.8–1.8 V, the hybrid supercapacitor can deliver a specific capacitance of 71.5 F g^?1 at a discharge current density of 200 mA g^?1(4 mA cm^?2) when the mass ratio of AC to PbO₂ was three, and after 4500 deep cycles, the specific capacitance remains at 64.4 F g^?1, or 32.2 Wh Kg^?1 in specific energy, and the capacity only fades 10% from its initial value.     

Direct electrochemistry and electrocatalysis of hemoglobin immobilized in bimodal mesoporous silica and chitosan inorganic–organic hybrid film

Hemoglobin modified electrode was successfully fabricated to realize direct electrochemistry by immobilizing of Hemoglobin (Hb) in bimodal mesoporous silica (BMS) and chitosan (CS) inorganic–organic hybrid film. Here, BMS acted as a support to immobilize Hb due to its large pores and CS acted as a binder to increase film adherence and stabilizer to prevent the leakage of Hb. The resulting electrode (Hb/BMS/CS) gave a well-defined, reversible redox couple for HbFe(III)/Fe(II) with a formal potential of about −0.32 V (vs. Ag/AgCl) in pH 7.0 phosphate buffer solution. Hb/BMS/CS electrode showed a better electrocatalytial performance to H₂O₂ with wider linear detection range, lower detection limit, and higher sensitivity than that at electrode without BMS. The improved electrocatalytic performance for Hb/BMS/CS electrode was possibly contributed to BMS bimodal structure, whose large pores with 10–40 nm provide favorable conditions for protein immobilization and small pores with 2–3 nm avoid the mass-transfer limitations. In addition, UV–Vis and FTIR spectra indicated that Hb well maintained its native structure in the hybrid film.     

A microtubular all CNT gas diffusion electrode

We report a microtubular gas diffusion electrodes made of multi-walled carbon nanotubes (MWCNT). The electrodes were prepared by inside-out cake filtration of an aqueous MWCNT suspension onto a microfiltration hollow fiber (HF) membrane, followed by washing out the surfactant, drying and removal of the all CNT microtube from the HF membrane. Length, outer diameter, and wall thickness of the tubular electrodes are: up to 44 cm, ~ 1.7 mm and 275 μm, respectively. The BET surface area is 200 m²/g with a porosity of 48–67% and an electrical conductivity of ~ 20 S/cm. Application of this microtubular Gas Diffusion Electrodes (GDE) was studied for the oxygen reduction reaction (ORR) in divided and undivided electrochemical cells. Oxygen supply into the lumen of the tubular electrodes resulted in much higher current densities for ORR than in experiments where the electrolyte was saturated by bubbling with pure oxygen. Within the 0.25–1.0 bar pressure (gauge) region, higher ORR rates were achieved at lower pressure. We also show that H₂O₂ production is possible using the new GDE. We propose to use such novel electrodes for the fabrication of tubular electrochemical reactors, e.g. fuel cells, H₂O₂ generators, CO₂ reduction and other processes that involve GDE application.     

A high-performance DNA biosensor using polyhydroxylated fullerenol as 3D matrix for probe immobilization

A novel electrochemical DNA biosensor has been developed using water-soluble polyhydroxylated fullerenols as 3D matrix platform for probe DNA immobilization. Owing to the multiple merits including the unique spherical 3D nanostructure, rich OH on the outside surface, and good water-solubility of fullerenol platform, the developed biosensor revealed high probe loading density (2.24 × 10¹³ strands cm^− 2) and fast hybridization kinetics. Also, a wide linear range from 1.0 fM to 1.0 nM with a detection limit down to 0.17 fM in target DNA detection was obtained.     

Direct electrochemistry of hemoglobin and its electrocatalytic effect based on its direct immobilization on carbon ionic liquid electrode

Direct electrochemistry of hemoglobin (Hb) has been achieved by its direct immobilization on carbon ionic liquid electrode (CILE). CILE was immersed in a solution containing Hb and ionic liquid, octylpyridinium chloride ([OcPy][Cl]), to directly immobilize Hb on CILE. Cyclic voltammetry of modified electrode exhibited quasi-reversible peaks corresponding to Hb. The oxidation and reduction peak potentials of immobilized Hb in acetate buffer solution, pH 5.0 and at a scan rate of 0.1 V s⁻¹ were obtained at about –150 mV and –290 mV, respectively. The average surface coverage of the electroactive Hb adsorbed on the electrode surface was calculated as 8.4 × 10⁻¹¹ mol cm⁻². Hb retained its bioactivity on modified electrode and showed excellent electrocatalytic activity towards oxygen, hydrogen peroxide and nitrite. Hydrogen peroxide can be determined in the range of 1.0 × 10⁻⁴–5.0 × 10⁻³ M.     

Novel poly (neutral red) nanowires as a sensitive electrochemical biosensing platform for hydrogen peroxide determination

Poly (neutral red) nanowires (PNRNWs) have been synthesized for the first time by the method of cyclic voltammetric electrodeposition using porous anodic aluminum oxide (AAO) template and were examined by scanning electron microscopy (SEM) and transmission electron microscope (TEM). Moreover, horseradish peroxidase (HRP) was encapsulated in situ in PNRNWs (denoted as PNRNWs–HRP) by electrochemical copolymerization for potential biosensor applications. The PNRNWs showed excellent efficiency of electron transfer between the HRP and the glassy carbon (GC) electrode for the reduction of H₂O₂ and the PNRNWs–HRP modified GC electrode showed to be excellent amperometric sensors for H₂O₂ at −0.1 V with a linear response range of 1 μM to 8 mM with a correlation coefficient of 0.996. The detection limit (S/N = 3) and the response time were determined to be 1 μM and <5 s and the high sensitivity is up to 318 μA mM⁻¹ cm⁻².     

Ultra-sensitive cholesterol biosensor based on low-temperature grown ZnO nanoparticles

A high-sensitive cholesterol amperometric biosensor based on the immobilization of cholesterol oxidase (ChOx) onto the ZnO nanoparticles has been fabricated which shows a very high and reproducible sensitivity of 23.7 μA mM^?1 cm^?2, detection limit (based on S/N ratio) 0.37 ± 0.02 nM, response time less than 5 s, linear range from 1.0 to 500.0 nM and correlation coefficient of R = 0.9975. A relatively low value of enzyme’s kinetic parameter (Michaelis–Menten constant) ～4.7 mM has been obtained which indicates the enhanced enzymatic affinity of ChOx to Cholesterol. To the best of our knowledge, this is the first report in which such a very high-sensitivity and low detection limit has been achieved for the cholesterol biosensor by using ZnO nanostructures modified electrodes.     

Direct-growth of polyaniline nanowires for enzyme-immobilization and glucose detection

Amperometric enzyme biosensor based on the glucose oxidase (GOx) incorporated polyaniline nanowires (PANI-NWs) on carbon cloth (CC) electrode was demonstrated. The simple, direct-growth of PANI-NWs on CC, via electrochemical polymerization, provides free-standing, template-independent, hence almost (interfacial) defects-free nanostructures. The defect-free interfaces, along with the excellently sensitive organic nanostructured-surface, as evident from its significantly large effective surface area (24 times the geometric area) for redox-sensing, allows efficient entrapment/immobilization and sensing of biomolecules, via rapid electron-transfer at NWs-CC. The GOx-immobilized PANI-NWs/CC, even in initial unoptimized stage, exhibited an excellent sensitivity, ～2.5 mA mM^?1 cm^?2, to glucose, over detection range 0–8 mM, adequate for clinical monitoring of human glucose levels. The report clearly reveals a cost-effective simple system possessing enormous potentiality for biosensors, bioenergy and bioelectronics applications.     

Quantum dot-based electrochemical DNA biosensor using a screen-printed graphite surface with embedded bismuth precursor

This work reports the development of screen-printed quantum dots (QDs)-based DNA biosensors utilizing graphite electrodes with embedded bismuth citrate as a bismuth precursor. The sensor surface serves both as a support for the immobilization of the oligonucleotide and as an ultrasensitive voltammetric QDs transducer relying on bismuth nanoparticles. The utility of this biosensor is demonstrated for the detection of the C634R mutation through hybridization of the biotin-tagged target oligonucleotide with a surface-confined capture complementary probe and subsequent reaction with streptavidin-conjugated PbS QDs. The electrochemical transduction step involved anodic stripping voltammetric determination of the Pb(II) released after acidic dissolution of the QDs. Simultaneously with the electrolytic accumulation of Pb on the sensor surface, the embedded bismuth citrate was converted in situ to bismuth nanoparticles enabling ultra-trace Pb determination. The biosensor showed a linear relationship of the Pb(II) peak current with respect to the logarithm of the target DNA concentrations from 0.1 pmol L^− 1 to 10 nmol L^− 1, and the limit of detection was 0.03 pmol L^− 1. The biosensor exhibited effective discrimination between a single-base mismatched sequence and the fully complementary target DNA. These “green” biosensors are inexpensive, lend themselves to easy mass production, and hold promise for ultrasensitive bioassay formats.     

Amperometric enzyme biosensor for hydrogen peroxide via Ugi multicomponent reaction

A novel strategy based on the Ugi multicomponent reaction was employed for immobilizing horseradish peroxidase on sodium alginate-coated gold electrode. The electrode was employed for constructing an amperometric biosensor device using 1 mM hydroquinone as electrochemical mediator. The electrode showed linear response (poised at −300 mV vs Ag/AgCl) toward H₂O₂ concentration between 70 μM and 8.8 mM at pH 7.0. The biosensor reached 95% of steady-state current in about 12 s and its sensitivity was 33.8 mA/M cm². The electrode retained full initial activity after 30 days of storage at 4 °C in 50 mM sodium phosphate buffer, pH 7.0.