Effect of TiO2–Ti and TiO2–TiN composite coatings on corrosion behavior of NiTi alloy

Two kinds of biocompatible coatings were produced in order to improve the corrosion resistance of nickel titanium (NiTi) alloy. A titanium oxide–titanium (TiO₂–Ti) composite was coated on NiTi alloy using electrophoretic method. After the coating process, the samples were heat‐treated at 1000 °C in two tube furnaces, the first one in argon atmosphere and the second one in nitrogen atmosphere at 1000 °C. The morphology and phase analysis of coatings were investigated using scanning electron microscopy and X‐ray diffraction analysis, respectively. The electrochemical behavior of the NiTi and coated samples was examined using polarization and electrochemical impedance spectroscopy tests. Electrochemical tests in simulated body fluid demonstrated a considerable increase in corrosion resistance of composite‐coated NiTi specimens compared to the non‐coated one. The heat‐treated composite coating sample in nitrogen atmosphere had a higher level of corrosion resistance compared to the heat‐treated sample in argon atmosphere, which is mainly due to having nitride phases. Copyright © 2014 John Wiley & Sons, Ltd.     

Surface‐Independent and Oriented Immobilization of Antibody via One‐Step Polydopamine/Protein G Coating: Application to Influenza Virus Immunoassay

For the construction of high‐performance biosensor, it is important to interface bioreceptors with the sensor surface densely and in the optimal orientation. Herein, a simple surface modification method that can optimally immobilize antibodies onto various kinds of surfaces is reported. For the surface modification, a mixture of polydopamine (PDA) and protein G was employed. PDA is a representative mussel‐inspired polymer, and protein G is an immunoglobulin‐binding protein that enables an antibody to have an optimal orientation. The surface characteristics of PDA/Protein G mixture‐coated substrates are analyzed and the PDA/protein G ratio is optimized to maximize the antibody binding efficiency. Moreover, the antibody‐immobilized substrates are applied to the detection of influenza viruses with the naked eye, providing a detection limit of 2.9 × 10³ pfu mL^‐1. Importantly, the several substrates (glass, SiO₂, Si, Al₂O₃, polyethylene terephthalate, polyethylene, polypropylene, and paper) can be modified by simple incubation with the mixture of PDA/protein G, and then the anti‐influenza A H1N1 antibodies can be immobilized on the substrates successfully. Regardless of the substrate, the influenza viruses are detectable after the sandwich immunoreaction and silver enhancement procedure. It is anticipated that the developed PDA/protein G coating method will extend the range of applicable materials for biosensing.     

Magnetic solid‐phase extraction based on a polydopamine‐coated Fe3O4 nanoparticles absorbent for the determination of bisphenol A,tetrabromobisphenol A, 2,4,6‐tribromophenol,and (S)‐1,1’‐bi‐2‐naphthol in environmental waters by HPLC

Polydopamine‐coated Fe₃O₄ magnetic nanoparticles synthesized through a facile solvothermal reaction and the self‐polymerization of dopamine have been employed as a magnetic solid‐phase extraction sorbent to enrich four phenolic compounds, bisphenol A, tetrabromobisphenol A, (S)‐1,1′‐bi‐2‐naphthol and 2,4,6‐tribromophenol, from environmental waters followed by high‐performance liquid chromatographic detection. Various parameters of the extraction were optimized, including the pH of the sample matrix, the amount of polydopamine‐coated Fe₃O₄ sorbent, the adsorption time, the enrichment factor of analytes, the elution solvent, and the reusability of the nanoparticles sorbent. The recoveries of these phenols in spiked water samples were 62.0–112.0% with relative standard deviations of 0.8–7.7%, indicating the good reliability of the magnetic solid‐phase extraction with high‐performance liquid chromatography method. In addition, the extraction characteristics of the magnetic polydopamine‐coated Fe₃O₄ nanoparticles were elucidated comprehensively. It is found that there are hydrophobic, π–π stacking and hydrogen bonding interactions between phenols and more dispersible polydopamine‐coated Fe₃O₄ in water, among which hydrophobic interaction dominates the magnetic solid‐phase extraction performance.     

Sol–gel titania coating on NiTi alloy with a porous titania film as interlayer

Porous titania film is prepared by alkali treatment of NiTi alloy followed by soaking treatment in HCl solution. The benefit of this porous titania film as an interlayer to improve adhesion and integrity of the sol–gel titania coating on NiTi alloy substrate is evidenced by surface morphological observations. X-ray diffraction analyses indicate the formation of Ni₄Ti₃ phase in the matrix during heat treatment of the NiTi samples. X-ray photoelectron spectroscopy results indicate that the titania coating with two dip-coating layers has completely covered the NaOH–HCl treated NiTi substrate, and potentiodynamic polarization tests show that this titania coating provides good protection for the treated NiTi substrate in 0.9% NaCl solution. Ultraviolet illumination can increase surface hydrophilicity of the NiTi samples by reducing contact angles from 60–80° to 20–10°.     

Insulin Surface Acoustic Wave Immunosensor Based on Immobilized C60/Anti‐Insulin Antibody

Immobilized fullerene C₆₀/anti‐insulin antibody was prepared and applied in shear horizontal surface acoustic wave (SH‐SAW) immunosensors to detect insulin in aqueous solutions. The immobilizations of anti‐insulin onto fullerene were studied through a C₆₀/PVC coated SH‐SAW sensor system in liquid. The partially irreversible frequency response for an anti‐insulin antibody was observed by the desorption study, which implied that fullerene could chemically react with anti‐insulin. C₆₀/anti‐insulin coating materials were successfully prepared and identified with an FTIR spectrometer. The C₆₀/anti‐insulin coated SH‐SAW immunosensors were developed and applied for detection of insulin in aqueous solutions. Within the range of normal human insulin concentration, the SH‐SAW immunosensors immobilized with C₆₀/anti‐insulin exhibited linear frequency responses to the concentration of insulin with a sensitivity of 130 Hz/pM. The SH‐SAW immunosensor immobilized with C₆₀/anti‐insulin showed a detection limit of 0.58 pM for insulin in aqueous solution. The interference of various common bio‐species in human blood, e.g. urea, ascorbic acid, tyrosine, and metal ions, to the SH‐SAW immunosensor immobilized with C₆₀/anti‐insulin for insulin was investigated. These common bio‐species interferences showed nearly no interference to the SAW immunosensors coated with C₆₀/anti‐insulin. The reproducibility of the SH‐SAW immunosensor immobilized with C₆₀/anti‐insulin for insulin was also investigated and is discussed.     

Mussel‐Inspired Polydopamine Coating for Flexible Ternary Resistive Memory

In recent years, numerous organic molecules and polymers carrying various functional groups were synthesized and used in fabrication of wearable electronic devices. Compared to previous materials that suffer from poisonousness, stiffness and complex film fabrication, we circumvent above matters by taking advantage of mussel‐inspired polydopamine as our active material to realize resistive random access memories (RRAMs). Polydopamine thin films were grown on indium tin oxide glass catalyzed by Cu₂SO₄/H₂O₂ and characterized by Fourier infrared spectroscopy (FT‐IR), UV/Vis spectroscopy and scanning electron microscopy. The Al/Polydopamine film/ITO devices possess ternary memory behavior with good ternary device yield with two threshold voltages around 1.50 V and 3.50 V, long data retention over 10⁴ s of continuous reading or 10⁴ pulse reading. The two resistance switchings are attributed to defects functioning as charge traps and the formation of conductive filaments. A flexible device based on Al/polydopamine film/ITO/polyethylene terephthalate retains its ternary memory behavior after being bent with a bending radius of 1.54 cm and bending cycles up to 5000, demonstrating good compatibility and flexibility of polydopamine.     

Zn–HA–TiO2 nanocomposite coatings electrodeposited on a NiTi shape memory alloy

In this work, zinc–hydroxyapatite (Zn–HA) and zinc–hydroxyapatite–titania (Zn–HA–TiO₂) nanocomposite coatings were electrodeposited onto a NiTi shape memory alloy, using a chloride zinc plating bath. The structure of the composite coatings was characterized by X‐ray diffraction, scanning electron microscopy and high‐resolution transmission electron microscopy. According to the results, the Zn–HA–TiO₂ coating exhibited a plate‐like surface morphology, where the addition of the nanoparticles caused to an increase in roughness. It was also found that due to applying a proper stirring procedure during co‐deposition, a homogenous dispersion of the nanoparticles in the coatings was achieved. Also, the addition of the TiO₂ nanoparticles to the Zn–HA–TiO₂ coating enhanced the microhardness and wear resistance. Copyright © 2014 John Wiley & Sons, Ltd.     

A Carbon Nanotube Layered Electrode for the Construction of the Wired Bilirubin Oxidase Oxygen Cathode

Oxidatively treated carbon nanotubes were coated on a glassy carbon surface to form a CNT‐layer. On the CNT‐layered GC surface, a redox hydrogel film of the copolymer, of polyacryamide and poly(N‐vinylimidazole) complexed with [Os(4,4′‐dichloro‐2,2′‐bipyridine)₂Cl]^+/2+ wiring bilirubin oxidase was immobilized. A good contact was achieved between the hydrogel film and the hydrophilic CNT‐layer with carboxylated CNTs. The prepared bilirubin oxidase cathode on the CNT‐layer was employed for the electrocatalytic reduction of O₂, and enhanced current and stability were observed. Electron transfers from the electrode surface O₂ molecules were analyzed. The optimal composition of the enzyme, redox polymer, and cross‐linker in the catalyst and the thickness of the CNT‐layer were determined.     

CuSO4/H2O2‐Induced Rapid Deposition of Polydopamine Coatings with High Uniformity and Enhanced Stability

Mussel‐inspired polydopamine (PDA) deposition offers a promising route to fabricate multifunctional coatings for various materials. However, PDA deposition is generally a time‐consuming process, and PDA coatings are unstable in acidic and alkaline media, as well as in polar organic solvents. We report a strategy to realize the rapid deposition of PDA by using CuSO₄/H₂O₂ as a trigger. Compared to the conventional processes, our strategy shows the fastest deposition rate reported to date, and the PDA coatings exhibit high uniformity and enhanced stability. Furthermore, the PDA‐coated porous membranes have excellent hydrophilicity, anti‐oxidant properties, and antibacterial performance. This work demonstrates a useful method for the environmentally friendly, cost‐effective, and time‐saving fabrication of PDA coatings.     

Magnetic digital microfluidics on a bioinspired surface for point‐of‐care diagnostics of infectious disease

Magnetic digital microfluidics uses magnetic force to manipulate droplets on a Teflon‐coated substrate through the added magnetic particles. To achieve a wide range of droplet manipulation, hydrophilic patterns, known as surface energy traps, are introduced onto the Teflon‐coated hydrophobic substrate. However, the Teflon‐coated substrate is difficult to modify because it is nonwettable, and existing techniques for patterning surface energy traps have many limitations. Inspired by the mussel adhesion mechanism, we use polydopamine, a bioinspired substance that adheres strongly to almost any materials, to pattern surface energy traps on the Teflon‐coated substrate with a great ease. We have optimized the polydopamine coating protocol and characterized the surface properties of the polydopamine surface energy traps. Droplet operations including particle extraction, liquid dispensing, liquid shaping, and cross‐platform transfer have been demonstrated on the polydopamine surface energy trap‐enabled magnetic digital microfluidic platform in both single‐plate and two‐plate configurations. Furthermore, the detection of hepatitis B surface antigen using ELISA has been demonstrated on the new magnetic dgitial microfluidic platform. This new bioinspired magnetic digital microfluidic platform is easy to fabricate and operate, showing a great potential for point‐of‐care applications.     

Application of NiTi alloy coated with ZrO2 as a new fiber for solid-phase microextraction for determination of halophenols in water samples

A new fiber for solid-phase microextraction (SPME) employing a metallic support coated with an inorganic material is proposed. A nitinol alloy (NiTi) was used as the support material due to its super elasticity and shape memory properties. Zirconium oxide (ZrO₂) was electrodeposited onto NiTi using chronoamperometry. The surface characteristics and morphology of the coated and uncoated support were evaluated through scanning electronic microscopy and dispersive energy microanalysis. This assembly was applied in the extraction of three halophenols from aqueous samples. A multivariate approach was used for optimization of the variables involved in the system. The Doehlert matrix was used for evaluation of the best derivatization conditions and a Box-Behnken design to obtain the best extraction conditions. In order to investigate the repeatability, one fiber was used for six extraction tests under similar conditions and the relative standard deviations (R.S.D.) were lower than 12.5%. Detection limits were lower than 0.30 ng mL⁻¹. Correlation coefficients were higher than 0.997. Extraction efficiency of the NiTi-ZrO₂ fiber was similar to a PDMS 7 μm commercial fiber, even though it had a lower coating thickness of 1.35 μm. Considering the amount extracted per unit volume, the NiTi-ZrO₂ fiber had a better extraction profile when compared to commercial fibers. The new SPME fiber has a lifetime of over 500 extractions. Thus, it is a promising alternative for low-cost analysis, as the proposed fiber is robust, and easily and inexpensively prepared.     

Photocurrent Enhancement of Dye‐Sensitized Solar Cells owing to Increased Dye‐Adsorption onto Silicon‐Nanoparticle‐Coated Titanium‐Dioxide Films

The inverse‐micellar preparation of Si nanoparticles (Nps) was improved by utilizing sodium naphthalide. The Si Nps were subsequently functionalized with 4‐vinylbenzoic acid for their attachment onto TiO₂ films of dye‐sensitized solar cells (DSSCs). The average diameter of the COOH‐functionalized Si (Si? COOH) Nps was 4.6(±1.7) nm. Depth profiling by secondary‐ion mass spectrometry revealed that the Si Nps were uniformly attached onto the TiO₂ films. The number of Ru^II dye molecules adsorbed onto a TiO₂ film that was treated with the Si? COOH Nps was 42 % higher than that on the untreated TiO₂ film. As a result, DSSCs that incorporated the Si? COOH Nps exhibited higher short‐circuit photocurrent density and an overall energy‐conversion efficiency than the untreated DSSCs by 22 % and 27 %, respectively. This enhanced performance, mostly owing to the intramolecular charge‐transfer to TiO₂ from the dye molecules that were anchored to the Si? COOH Nps, was confirmed by comparing the performance with two different Ru^II–bipyridine dyes (N719 and N749).     

Redox Properties of Ferricyanide Ion on Layer‐by‐Layer Thin Film‐Coated Gold Electrodes; Effects of Type of Polycationic Components in the Film

The surface of a gold (Au) electrode was coated with layer‐by‐layer (LbL) thin films composed of poly(vinyl sulfate) (PVS) and different type of poly(amine)s including poly(allylamine) (PAH), poly(ethyleneimine) (PEI) and poly(diallyldimethylammonium chloride) (PDDA) and redox properties of ferricyanide ion ([Fe(CN)₆]^3?) on the LbL film‐coated Au electrodes were studied. The LbL film‐coated electrodes exhibited redox response to [Fe(CN)₆]^3? ion when the outermost surface of the LbL film was covered with the cationic poly(amine)s while virtually no response was observed on the LbL film‐coated electrodes whose outermost surface was covered with PVS due to an electrostatic repulsion between [Fe(CN)₆]^3? ion and the negatively‐charged PVS layer. The redox properties of [Fe(CN)₆]^3? ion on the LbL film‐coated electrodes significantly depended on the type of polycationic materials in the LbL film. The LbL film‐coated electrodes which had been immersed in the [Fe(CN)₆]^3? solution for 15 min exhibited redox response even in a [Fe(CN)₆]^3? ion‐free buffer solution, suggesting that [Fe(CN)₆]^3? ion is confined in the films. In the buffer solution, redox peaks were observed between +0.1 and 0.4 V depending on the type of polycations in the film. Thus, [Fe(CN)₆]^3? ion can be confined in the film and the redox potential is polycation‐dependent.     

Electro-less plating nickel-phosphorus of low carbon steel using various pretreatments and an external magnetic field

Electro-less plating nickel-phosphorus (EPNP) subject to various pretreatments and an external magnetic field, are prepared onto low carbon steel (LCS). The surface hardness (SH), fatigue life (FL) and corrosion behavior (CB) of Ni-P coated are respectively obtained using a nano-indenter and a high strain low cycle fatigue life (HSLCFL) and polarization test. The experimental outcome reveals that LCS substrates that are pretreated using the proposed acid mixture (25% H₂SO₄ + 5% HCl) roughening and activation allow good Ni-P films to be deposited. In terms of EPNP, as the P content decreases, the SH and FL increase. Specimens that a coated in a Ni–P film using an external magnetic field have better mechanical performance than those that are not produced in an external magnetic field. As the external magnetic field intensity is increased, the film thickness, SH and FL increase, the concentration of P decreases and thin grains are formed on the film surface. The specimens without using an external magnetic field that are coated with a Ni–P film exhibit better resistance to corrosion than the uncoated sample of LCS. However, the Ni–P film that is coated using an external magnetic field has higher SH, so it exhibits increased resistance to corrosion.     

Evaluation of in vitro bioactivity and MG63 Oesteoblast cell response for TiO2 coated magnesium alloys

The present investigation reports TiO₂ coating on magnesium alloy AZ31 by sol–gel method via dip coating technique. TiO₂ coated surface was characterized by thin film X-ray diffraction (TF-XRD), Fourier transform infrared red (FT-IR) spectroscopy, scanning electron microscopy (SEM) with energy-dispersive X-ray (EDX) spectroscopy, atomic force microscopy (AFM) and transmission electron microscopy (TEM) techniques. From TF-XRD results, the peaks at 2θ values of 25.14, 32.12, 68.73 and 70.11 confirm the presence of TiO₂. The TiO₂ is crystalline in nature and the crystallite size is about 32.4 nm. SEM-EDX, TEM and AFM show that the coated surface is uniform and nanoporous. FT-IR analysis shows that the peak in the range of 692 cm^?1 is assigned to Ti–O–Ti stretching vibration. Contact angle measurements show that the coating is hydrophilic in nature. Bioactivity of the coating in simulated body fluid (SBF) was also examined, the hydroxyl functionalized surface greatly enhances the hydroxyapatite growth. The potentiodynamic polarization studies prove that the corrosion resistance of the TiO₂ coated surface after immersion in SBF for 7 days is improved dramatically. Cell adhesion studies confirm the increased cell attachment on TiO₂ coated surface when compared to uncoated alloy, due to less amount of Mg ion release from the substrate in the culture medium.     

Sensitive electrochemical immunoassay for chlorpyrifos by using flake-like Fe3O4 modified carbon nanotubes as the enhanced multienzyme label

A highly sensitive electrochemical immunoassay of chlorpyrifos (CPF) was developed by using a biocompatible quinone-rich polydopamine nanospheres modified glass carbon electrode as the sensor platform and multi-horseradish peroxidase-flake like Fe₃O₄ coated carbon nanotube nanocomposites as the signal label. Due to the quinone-rich polydopamine nanospheres, the platform exhibited excellent fixing capacity by simple coating of sticky polydopamine nanospheres and subsequent oxidization. By coprecipitation of Fe³⁺ and Fe²⁺ on polydopamine modified carbon nanotubes (CNTs) with the aid of ethylene glycol (EG), the flake-like Fe₃O₄ coated CNTs (CNTs@f-Fe₃O₄) were synthesized and chosen as the carrier of multi-enzyme label due to the high loading of secondary antibody (Ab₂) and horseradish peroxidase (HRP) and also the peroxidase-mimic activity of Fe₃O₄. Under the optimum conditions, the immunosensor can detect CPF over a wide range with a detection limit of 6.3 pg/mL. Besides, the high specificity, reproducibility and stability of the proposed immunosensor were also proved. The preliminary application in real sample showed good recoveries, indicating it holds promise for fast analysis of CPF in aquatic environment.     

Selective extraction of berberine from Cortex Phellodendri using polydopamine‐coated magnetic nanoparticles

A new extraction agent featuring dopamine self‐polymerized on magnetic Fe₃O₄ nanoparticles has been successfully synthesized and evaluated for the SPE of berberine from the extract of the traditional Chinese medicinal plant, Cortex Phellodendri. The nanoparticles prepared possessed a core–shell structure and showed super‐paramagnetism. It was found that these polydopamine‐coated nanoparticles exhibited strong and selective adsorption for berberine. Among the chemical components present in C. Phellodendri, only berberine was adsorbed by the nanoparticles and extracted by a following SPE procedure. Various conditions such as the amount of polydopamine‐coated nanoparticles, desorption solvent, desorption time and equilibrium time were optimized for the SPE of berberine. The purity of berberine extracted from C. Phellodendri was determined to be as high as 91.3% compared with that of 9.5% in the extract. The established SPE protocol combined advantages of highly selective enrichment with easy magnetic separation, and proved to be a facile efficient procedure for the isolation of berberine. Further, the prepared polydopamine‐coated magnetic nanoparticles could be reused for multiple times, reducing operational cost. The applicability and reliability of the developed SPE method were demonstrated by isolating berberine from three different C. Phellodendri extracts. Recoveries of 85.4–111.2% were obtained with relative standard deviations ranging from 0.27–2.05%.     

Direct Electrochemistry of Hemoglobin Immobilized on Carbon‐Coated Iron Nanoparticles for Amperometric Detection of Hydrogen Peroxide

A novel method for preparation of hydrogen peroxide biosensor was presented based on immobilization of hemoglobin (Hb) on carbon‐coated iron nanoparticles (CIN). CIN was firstly dispersed in a chitosan solution and cast onto a glassy carbon electrode to form a CIN/chitosan composite film modified electrode. Hb was then immobilized onto the composite film with the cross‐linking of glutaraldehyde. The immobilized Hb displayed a pair of stable and quasireversible redox peaks and excellent electrocatalytic reduction of hydrogen peroxide (H₂O₂), which leading to an unmediated biosensor for H₂O₂. The electrocatalytic response exhibited a linear dependence on H₂O₂ concentration in a wide range from 3.1 μM to 4.0 mM with a detection limit of 1.2 μM (S/N=3). The designed biosensor exhibited acceptable stability, long‐term life and good reproducibility.     

Effect of Poly(amine)s on the Redox Properties of Carboxymethylcellulose and Ferrocene‐modified Poly(amine) Layer‐by‐Layer Films

Multilayer films consisting of carboxymethylcellulose (CMC) and ferrocene‐modified poly(ethyleneimine) (Fc‐PEI) or poly(allylamine hydrochloride) (Fc‐PAH) were successfully prepared on a gold electrode to examine their redox properties. The redox current of (Fc‐PEI/CMC)_n film‐coated electrodes increased with the number of layers, while the (Fc‐PAH/CMC)_n film‐coated electrodes exhibited increased response only for the first eight bilayers. The (Fc‐PEI/CMC)_n and (Fc‐PAH/CMC)_n films deposited on the surface of Fc‐free multilayer film‐coated electrodes also showed a redox response. The (PEI/CMC)₅ film‐coated electrode showed redox responses in Fc‐PEI and Fc‐PAH solutions, confirming the uptake of the Fc‐polymers in the inner film. In contrast, the uptake of the Fc‐polymers in the (PAH/CMC)₅ film was severely suppressed, suggesting that different permeability of (PEI/CMC)₅ and (PAH/CMC)₅ films.     

Biomolecule‐Doped Organic/Inorganic Hybrid Nanocomposite Film for Label‐Free Electrochemical Immunoassay of α‐1‐Fetoprotein

A new electrochemical immunosensor for the detection of α‐1‐fetoprotien (AFP) was developed based on AFP antibody (anti‐AFP)‐functionalized organic/inorganic hybrid nanocomposite membrane. To fabricate such a hybrid composite membrane, 3,4,9,10‐perylenetetracarboxylic acid‐bound thionine molecules (PTCTH) were initially doped into titania colloids (TiO₂), and then gold nanoparticles and anti‐AFP were immobilized onto the composite film in turn. Comparison with the electrode fabricated only with thionine not 3,4,9,10‐perylenetetracarboxylic acid, the immunosensor with PTCTH exhibited high sensitivity and fast electron transfer. The presence of gold nanoparticles provided a good microenvironment for the immobilization of biomolecules, enhanced the surface coverage of protein, and improved the sensitivity of the immunosensor. The modified process was characterized by scanning electron microscope (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The surface topography of the membrane was investigated by scanning electron microscopy (SEM). Under optimal conditions, the proposed immunosensor exhibited a wide linear range from 2.5 to 200.0 ng/mL towards AFP with a detection limit of 0.5 ng/mL (S/N=3). The stability, reproducibility and precision of the immunosensor were acceptable. Comparison with the conventional enzyme‐linked immunosorbent assay (ELISA), the present method did not require more labeled procedures and washing steps. Significantly, the detection methodology provides a promising approach for other proteins or biosecurities.