Direct Electrochemistry of Myoglobin in Titanate Nanotubes Film

Adsorption, aggregation and sedimentation of titanium dioxide nanoparticles and nanotubes in the presence of different sources of humic acids

Environmental behavior, bioavailability and risks posed by TiO₂, nanomaterials (TiO₂ NMs) in surface waters are affected by morphologies of the particles and geochemistry, including pH, inorganic and organic matter. Here, the adsorption, aggregation and sedimentation of TiO₂ nanoparticles (TiO₂ NPs) and nanotubes (TiO₂ NTs) were investigated in the presence of Elliott Soil humic acid (HA_E) and Suwannee River humic acids (HA_S). The adsorption amount of HA on TiO₂ NMs was inversely proportional to pH of solution. Maximum adsorption amount of HA on the surface of TiO₂ NMs follows the order TiO₂ NPs + HA_E (236.05 mg/g) > TiO₂ NTs + HA_E (146.05 mg/g) > TiO₂ NTs + HA_S (70.66 mg/g) > TiO₂ NPs + HA_S (37.48 mg/g). Stability of TiO₂ NPs and TiO₂ NTs largely depended on their isoelectric point, morphology and solution pH in the absence of HA. Dispersion of TiO₂ NMs was enhanced with solution pH deviated from the isoelectric point of nanomaterials due to electrostatic repulsion. Moreover, tubular structures of TiO₂ NTs with higher length-diameter ratio seem to aggregate more easily than dose sphere-like TiO₂ NPs. This might be due to their spherical structure enhancing steric repulsion. Notably, the adsorption of HA led to disagglomeration and significant stability of TiO₂ NPs and TiO₂ NTs due to steric hindrance under varying solution pH. In addition, adsorption time, concentration and sources of HA also influenced suspension/sedimentation behavior of TiO₂ NPs and TiO₂ NTs, and aromatic-rich HA_E stabilized TiO₂ NMs suspension more aggressively than aliphatic-rich HA_S.

Dispersion of titanate nanotubes for nanomedicine: comparison of PEI and PEG nanohybrids

In the present study, we report the dispersion of titanate nanotubes (TiONts) via polymer grafting (PolyEthylene Glycol, PEG) or polymer adsorption (polyethylene imine, PEI) where different TiONts/polymer ratios have been investigated. The TiONts/PEI and TiONts/PEG nanohybrids were characterized by scanning and transmission electron microscopy as well as by zeta potential measurements in order to determine both their dispersion state and stability in water (at different pH for zetametry). The nature of the chemical bonds at the surface of these nanohybrids was investigated by Fourier-transformed infrared (FTIR) spectroscopy while the grafting densities of PEG on the nanotubes were quantified by thermogravimetric analyses (TGA). The nanohybrids reported here are promising tools for biotechnology applications due to their tubular morphology, their very good dispersion in water and the reactivity of their surface.

Leaf-templated synthesis of 3D hierarchical porous cobalt oxide nanostructure as direct electrochemical biosensing interface with enhanced electrocatalysis

A novel three-dimensional (3D) hierarchical porous cobalt oxide (Co3O4) architecture was first synthesized through a simple, cost-effective and environmentally friendly leaf-templated strategy. The Co3O4 nanoparticles (30-100 nm) with irregular shapes were interconnected with each other to form a 3D multilayer porous network structure, which provided high specific surface area and numerous electrocatalytic active sites. Subsequently, Co3O4 was successfully utilized as direct electrochemical sensing interface for non-enzymatic detection of H2O2 and glucose. By using chronoamperometry, the current response of the sensor at +0.31 V was linear with H2O2 concentration within 0.4-200 μM with a low limit of detection (LOD) of 0.24 μM (S/N=3) and a high sensitivity of 389.7 μA mM(-1) cm(-2). Two linear ranges of 1-300 μM (with LOD of 0.1 μM and sensitivity of 471.5 μA mM(-1) cm(-2)) and 4-12.5 mM were found at +0.59 V for glucose. In addition, the as-prepared sensor showed excellent stability and anti-interference performance for possible interferents such as ascorbic acid, uric acid, dopamine, acetaminophen and especially 0.15 M chloride ions. Similarly, other various metal oxide nanostructures may be also prepared using this similar strategy for possible applications in catalysis, electrochemical sensors, and fuel cells.

Low-temperature conversion of titanate nanotubes into nitrogen-doped TiO2 nanoparticles

We report on the lowest synthesis temperature recorded for nitrogen-doped titanate nanostructures and their subsequent conversion into N-doped TiO₂.

Employing denaturation for rapid electrochemical detection of myoglobin using TiO2 nanotubes

An alternative antibody-free strategy for the rapid electrochemical detection of cardiac myoglobin has been demonstrated here using hydrothermally synthesized TiO₂ nanotubes (Ti-NT). The denaturant induced unfolding of myoglobin led to easy access of the deeply buried electroactive heme center and thus the efficient reversible electron transfer from protein to electrode surface. The sensing performance of the Ti-NT modified electrodes were compared vis a vis commercially available titania and GCEs. The tubular morphology of the Ti-NT led to facile transfer of electrons to the electrode surface, which eventually provided a linear current response (obtained from cyclic voltammetry) over a wide range of Mb concentration. The sensitivity of the Ti-NT based sensor was remarkable and was equal to 18 μA mg^-1 ml (detection limit = 50 nM). This coupled with the rapid analysis time of a few tens of minutes (compared to a few days for ELISA) demonstrates its potential usefulness for the early detection of acute myocardial infarction (AMI).

Halloysite clay nanotubes as a ceramic "skeleton" for functional biopolymer composites with sustained drug release

Halloysite, naturally occurring clay nanotubes, is described as an additive for functional polymer composites. Due to the empty tubular lumen capable of being loaded with chemically active agents, halloysite provides additional functions (drug delivery, antibiotic, flame-retardant, anticorrosion, and crack self-healing) to polymeric composites synergistically combined with enhanced tensile, impact and adhesive strength. Doping loaded clay nanotubes into a polymeric matrix provides a kind of ceramic "skeleton", and these "skeleton bones" are loaded with functional chemicals like real bones loaded with marrow. Tunable controlled release of active agents through synthesis of artificial nano-caps at the tube endings and halloysite lumen enlargement by selective etching allowed for tubular nanocontainers with chemicals release time from 10 to 200 h and a loading capacity of ca. 30 wt%. Halloysite is well mixable with polymers of high and medium polarities without any surface modification.

Conjugation of cytochrome c with hydrogen titanate nanotubes: novel conformational state with implications for apoptosis

We show that hydrogen titanate (H(2)Ti(3)O(7)) nanotubes form strongly associated reversible nano-bio-conjugates with the vital respiratory protein, cytochrome c. Resonance Raman spectroscopy along with direct electrochemical studies indicate that in this nano-bio-conjugate, cytochrome c exists in an equilibrium of two conformational states with distinctly different formal redox potentials and coordination geometries of the heme center. The nanotube-conjugated cytochrome c also showed enhanced peroxidase activity similar to the membrane-bound protein that is believed to be an apoptosis initiator. This suggests that such a nanotube-cytochrome c conjugate may be a good candidate for cancer therapy applications.

Hierarchically structured one-dimensional TiO2 for protein immobilization, direct electrochemistry, and mediator-free glucose sensing

A novel one-dimensional hierarchically structured TiO(2) (1DHS TiO(2)) was synthesized by a solvothermal method using multiwalled carbon nanotubes (MWCNTs) as a template and evaluated for the immobilization of protein and biosensing applications. Characterization studies showed that the 1DHS TiO(2) possessed an anatase crystalline structure and a large surface area with narrow pore size distribution. Fast direct electron transfer was observed for glucose oxidase (GOx) immobilized on the 1DHS TiO(2), and excellent electrocatalytic performance for glucose detection can be obtained without a mediator. The glucose sensor based on the GOx/1DHS TiO(2)-modified electrode had a high sensitivity of 9.90 μA mM(-1) cm(-2) and a low detection limit of 1.29 μM. The fabricated biosensor displayed good selectivity and long-term stability, indicating that the novel structured TiO(2) is a promising material for the immobilization of biomolecules and the fabrication of third-generation biosensors.

Extreme superomniphobicity of multiwalled 8 nm TiO2 nanotubes

We report unprecedented superomniphobic characteristics of nanotube-structured TiO(2) surface fabricated by electrochemical etching and hydrothermal synthesis process, with the wettability contact angles for water and oil both being ∼174° or higher. A tangled forest of ∼8-nm-diameter, multiwalled nanotubes of TiO(2) was produced on the microtextured Ti surface, with the overall nanotube length controlled to 150 nm by adjusting the processing time. Wettability measurements indicate that the nanotube surface is extremely nonwettable to both water and oil. The contact angle of the 8 nm TiO(2) nanotube surface after perfluorosilane coating is extremely high (178°) for water droplets indicating superhydrophobic properties. The contact angle for oil, measured using a glycerol droplet, is also very high, about 174°, indicating superoleophobic characteristics. These dual nonwetting properties, superomniphobic characteristics, are in sharp contrast to the as-made TiO(2) nanotubes which exhibit superhydrophilic properties with a contact angle of essentially ∼0°. Such an extreme superomniphobic material made by a simple and versatile method can be useful for a variety of technical applications. It is interesting to note that all three properties can be obtained with identical nanotube structures. A nanometer-scaled structure introduced by hydrothermally grown TiO(2) nanotubes is an effective air trapping nanostructure in enhancing the amphiphobic (superomniphobic) wettability.

Development of highly fluorescent silica nanoparticles chemically doped with organic dye for sensitive DNA microarray detection

Increasing the sensitivity in DNA microarray hybridization can significantly enhance the capability of microarray technology for a wide range of research and clinical diagnostic applications, especially for those with limited sample biomass. To address this issue, using reverse microemulsion method and surface chemistry, a novel class of homogenous, photostable, highly fluorescent streptavidin-functionalized silica nanoparticles was developed, in which Alexa Fluor 647 (AF647) molecules were covalently embedded. The coating of bovine serum albumin on the resultant fluorescent particles can greatly eliminate nonspecific background signal interference. The thus-synthesized fluorescent nanoparticles can specifically recognize biotin-labeled target DNA hybridized to the microarray via streptavidin-biotin interaction. The response of this DNA microarray technology exhibited a linear range within 0.2 to 10 pM complementary DNA and limit of detection of 0.1 pM, enhancing microarray hybridization sensitivity over tenfold. This promising technology may be potentially applied to other binding events such as specific interactions between proteins.

A self assembled monolayer based microfluidic sensor for urea detection

Urease (Urs) and glutamate dehydrogenase (GLDH) have been covalently co-immobilized onto a self-assembled monolayer (SAM) comprising of 10-carboxy-1-decanthiol (CDT) via EDC-NHS chemistry deposited onto one of the two patterned gold (Au) electrodes for estimation of urea using poly(dimethylsiloxane) based microfluidic channels (2 cm × 200 μm × 200 μm). The CDT/Au and Urs-GLDH/CDT/Au electrodes have been characterized using Fourier transform infrared (FTIR) spectroscopy, contact angle (CA), atomic force microscopy (AFM) and electrochemical cyclic voltammetry (CV) techniques. The electrochemical response measurement of a Urs-GLDH/CDT/Au bioelectrode obtained as a function of urea concentration using CV yield linearity as 10 to 100 mg dl(-1), detection limit as 9 mg dl(-1) and high sensitivity as 7.5 μA mM(-1) cm(-2).

Significant reduction of harmful compounds in tobacco smoke by the use of titanate nanosheets and nanotubes

Titanate nanosheets and nanotubes have first been introduced into cigarette filter, a great range of harmful compounds including tar, nicotine, ammonia, hydrogen cyanide, selected carbonyls and phenolic compounds can be reduced efficiently.

Preparation of a porous composite film for the fabrication of a hydrogen peroxide sensor

A series of dopant-type polyaniline-polyacrylic acid composite (PAn-PAA) films with porous structures were prepared and developed for an enzyme-free hydrogen peroxide (H₂O₂) sensor. The composite films were highly electroactive in a neutral environment as compared to polyaniline (PAn). In addition, the carboxyl group of the PAA was found to react with H₂O₂ to form peroxy acid groups, and the peroxy acid could further oxidize the imine structure of PAn to form N-oxides. The N-oxides reverted to their original form via electrochemical reduction and increased the reduction current. Based on this result, PAn-PAA was used to modify a gold electrode (PAn-PAA/Au) as a working electrode for the non-enzymatic detection of H₂O₂. The characteristics of the proposed sensors could be tuned by the PAA/PAn molar ratio. Blending PAA with PAn enhanced the surface area, electrocatalytic activity, and conductivity of these sensors. Under optimal conditions, the linear concentration range of the H₂O₂ sensor was 0.04 to 12 mM with a sensitivity of 417.5 μA/mM-cm². This enzyme-free H₂O₂ sensor also exhibited a rapid response time, excellent stability, and high selectivity.

A novel biosensor based on sol-gel poly () (PVA)/(titanium dioxide)TiO2 hybrid material

A novel hydrogen peroxide biosensor has been fabricated based on Hb entrapped poly(vinyl alcohol) (PVA)/Titanium dioxide (TiO2) hybrid material. Multi-walled carbon nanotubes (MWCNTs) were then dispersed into the composite matrix. It was found that such hybrid material could retain the native biocatalytic activity of the entrapped Hb by electrochemical experiments. In addition, MWCNTs enhanced catalytic performance of hydrogen peroxide and promoted electronic transfer. Effects of some experimental variables such as the amount of MWCNTs, concentration of enzyme, amounts of modifier on the current response of the biosensor were investigated. A linear calibration graph was obtained in the concentration range of H2O2 from 0.5 to 2.7 μM (linear regression coefficient = 0.997) with a detection limit of 0.01 μM (S/N = 3). The apparent Michaelis-Menten constant Km was 0.997 μM. The biosensor displayed excellent repeatability, high sensitivity, long-term stability, and good selectivity. The recovery of H2O2 in samples was testified with satisfactory results.

TiO2 nanotubes as solid-phase extraction adsorbent for the determination of polycyclic aromatic hydrocarbons in environmental water samples

An analytical method based on TiO2 nanotubes solid-phase extraction (SPE) combined with gas chromatography (GC) was established for the analysis of seven polycyclic aromatic hydrocarbons (PAHs): acenaphtylene, acenaphthene, anthracene, fluorene, phenanthrene, fluoranthene and pyrene. Factors affecting the extraction efficiency including the eluent type and its volume, adsorbent amount, sample volume, sample pH and sample flow rate were optimized. The characteristic data of analytical performance were determined to investigate the sensitivity and precision of the method. Under the optimized extraction conditions, the method showed good linearity in the range of 0.01-0.8 microg/mL, repeatability of the extraction (RSD were between 6.7% and 13.5%, n = 5) and satisfactory detection limits (0.017-0.059 ng/mL). The developed method was successfully applied to the analysis of surface water (tap, river and dam) samples. The recoveries of PAHs spiked in environmental water samples ranged from 90% to 100%. All the results indicated the potential application of titanate nanotubes as solid-phase extraction adsorbents to pre-treat water samples.

Iron phosphate nanostructures synthesized by microwave method and their applications in biosensing

A fast, simple microwave heating method has been developed for synthesizing iron phosphate (FePO(4)) nanostructures. The nanostructures were characterized and confirmed by transmission electronic microscopy (TEM), scanning electronic microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), x-ray photoelectron spectroscopy (XPS), x-ray powder diffraction (XRD), Fourier transform infrared (FT-IR), and UV-vis spectroscopy. The morphology and the size of the nanomaterials are significantly influenced by the concentration of the precursors and the kinds of surfactants. The nanostructures have been employed as an electrode substrate to immobilize myoglobin (Mb) and to facilitate the direct electron transfer (DET) reaction of the protein. After being immobilized on the nanomaterials, Mb can keep its natural structure and undergo effective DET reaction with a pair of well-defined redox peaks at - (330 ± 3.0) mV (pH 6.8) and an apparent electron transfer rate constant of 5.54 s(-1). The Mb-FePO(4)/GC electrode displays good features in the electrocatalytic reduction of H(2)O(2), and thus can be used as a biosensor for detecting substrates with a low detection limit (5 ± 1 µM), a wide linear range (0.01-2.5 mM), a high sensitivity (ca. 85 ± 3 µA mM(-1) cm(-2)), as well as good stability and reproducibility. Therefore, FePO(4) nanomaterials can become a simple and effective biosensing platform for the integration of proteins/enzymes and electrodes, which can provide analytical access to a large group of enzymes for a wide range of bioelectrochemical applications.