Selective potentiometric determination of uric acid with uricase immobilized on ZnO nanowires

In this study, a potentiometric uric acid biosensor was fabricated by immobilization of uricase onto zinc oxide (ZnO) nanowires. Zinc oxide nanowires with 80-150 nm in diameter and 900 nm to 1.5 μm in lengths were grown on the surface of a gold coated flexible plastic substrate. Uricase was electrostatically immobilized on the surface of well aligned ZnO nanowires resulting in a sensitive, selective, stable and reproducible uric acid biosensor. The potentiometric response of the ZnO sensor vs Ag/AgCl reference electrode was found to be linear over a relatively wide logarithmic concentration range (1-650 μM) suitable for human blood serum. By applying a Nafion^® membrane on the sensor the linear range could be extended to 1-1000 μM at the expense of an increased response time from 6.25 s to less than 9 s. On the other hand the membrane increased the sensor durability considerably. The sensor response was unaffected by normal concentrations of common interferents such as ascorbic acid, glucose, and urea.     

A glucose oxidase immobilization platform for glucose biosensor using ZnO hollow nanospheres

A good route (template-directed synthetic route) for the fabrication of ZnO hollow nanospheres (ZnO-HNSPs) was proposed. ZnO hollow nanosphere is a wonderful platform to immobilize glucose oxidase for glucose biosensor owing to the high specific surface area and high isoelectric point (IEP). Along with nafion and glucose oxidase (GOD), a glucose sensor was designed. Nafion/ZnO-HNSPs/GOD/GCE displays higher catalytic activity toward the glucose oxidation than Nafion/ZnO nano-Flowers/GOD/GCE. Linear response was obtained over a concentration range from 5.0 × 10⁻³ mM to 13.15 mM with a detection limit of 1.0 μM (S/N = 3), and the sensitivity was 65.82 μA/(mM cm²). Satisfyingly, the Nafion/ZnO-HNSPs/GOD/GCE could effectively avoid the interferences from the common interfering species such as uric acid (UA), ascorbic acid (AA), dopamine (DA) and fructose. The Nafion/ZnO-HNSPs/GOD modified electrode allows high sensitivity, excellently selective, stable, and fast amperometric sensing of glucose and thus is promising for the future development of glucose sensors.     

Potentiometric urea biosensor based on urease immobilized by an electrosynthesized poly(o-phenylenediamine) film with buffering capability

A simple and novel potentiometric biosensor for urea detection was prepared by employing an electrosynthesized polymer with buffering capability. It was obtained by deposition of a weighed amount of urease (Ur) at a glassy carbon (GC) electrode followed by immobilization by an electrosynthesized poly-o-phenylenediamine (PPD) film. An unconventional “upside-down” (UD) geometry was employed for the electrochemical cell. The response of GC/Ur/PPD sensor is linear with urea concentration in the range 10 μM to 1 mM (15 mV/mM, R² = 0.9999) due to buffering capability of PPD film, which represents a novel role of electrosynthesized polymers in their application to biosensors. At higher concentrations, the more common Nernstian response (28 mV/decade, R² = 0.9987) is observed. The sensor exhibits a sufficient sensitivity for practical determinations, rapid response and long term stability.     

A single ZnO tetrapod-based sensor

Transferable ZnO tetrapods were grown by an aqueous solution method. An individual ZnO tetrapod-based sensor was fabricated by in situ lift-out technique and its ultraviolet (UV) and gas sensing properties were investigated. This single tetrapod-based device responds to the UV light rapidly and showed a recovery time of about 23 s. The sensitivity of a single ZnO tetrapod sensor to oxygen concentration was also investigated. We found that when UV illumination is switched off, the oxygen chemisorption process will dominate and assists photoconductivity relaxation. Thus relaxation dynamics is strongly affected by the ambient O₂ partial pressure as described.We also studied the response of ZnO tetrapod-based sensor in various gas environments, such as 100 ppm H₂, CO, i-butane, CH₄, CO₂, and SO₂ at room temperature. It is noted that ZnO tetrapod sensor is much more sensitive to H₂, i-butane and CO. It is demonstrated that a ZnO tetrapod exposed to both UV light and hydrogen can provide a unique integrated multiterminal architecture for novel electronic device configurations.     

A novel ammonia sensor based on high density, small diameter polypyrrole nanowire arrays

A novel sensor based on the PPy nanowire arrays with high density, small diameter (about 50 nm) presented a significantly high surface-to-volume ratio. Conducting polypyrrole (PPy) nanowire arrays were synthesized by electropolymerization in the anodic aluminum oxide (AAO) template which was fabricated by two-step anodizing process. Then, two sides of the PPys/AAO were pasted by Si substrates and the AAO template was etched by HF acid. At last, the gold wires were attached on Si substrates, and the sensor came into being. The sensor was sensitive to ammonia at room temperature and showed relatively high response in low concentration and comparatively short response and recovery time.     

High sensitive and selective formaldehyde sensors based on nanoparticle-assembled ZnO micro-octahedrons synthesized by homogeneous precipitation method

Nanoparticle-assembled ZnO micro-octahedrons were synthesized by a facile homogeneous precipitation method. The ZnO micro-octahedrons are hexagonal wurtzite with high crystallinity. Abundant structure defects were confirmed on ZnO surface by photoluminescence. Gas sensors based on the ZnO micro-octahedrons exhibited high response, selectivity and stability to 1–1000 ppm formaldehyde at 400 °C. Especially, even 1 ppm formaldehyde could be detected with high response (S = 22.7). It is of interest to point out that formaldehyde could be easily distinguished from ethanol or acetaldehyde with a selectivity of about 3. The high formaldehyde response is mainly attributed to the synergistic effect of high contents of electron donor defects (Zn_i and V_O) and highly active oxygen species (O²⁻) on the ZnO surface.     

Study of an amperometric glucose sensor based on Pd-Ni/SiNW electrode

An amperometric glucose sensor based on Pd-Ni/SiNW electrode has been investigated. The silicon nanowire (SiNW) electrodes were first fabricated by chemical etching, and then nickel and palladium particles were deposited onto the surfaces of SiNWs via electroless co-plating technique followed by annealing in nitrogen atmosphere at 350 °C for 300 s. The morphology of Pd-Ni/SiNW electrode was characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD). The sensor performance was characterized by cyclic voltammetry (CV) and fixed potential amperometry techniques. In 0.1 M KOH alkaline medium with different glucose concentrations, the sensor shows an excellent sensitivity of 190.72 μA mM⁻¹ cm⁻² with the detection limit (S/N ratio = 3) of 2.88 μM. And it also exhibits superior anti-interference properties to the species including ascorbic acid (AA), uric acid (UA) and 4-acetamidophenol (AP). All results demonstrate that this Pd-Ni/SiNW electrode is a candidate with great potential for glucose detection.     

Gas sensing properties of ZnO thin films prepared by microcontact printing

In situ patterned zinc oxide (ZnO) thin films were prepared by precipitation of Zn(NO₃)₂/urea aqueous solution and by microcontact printing of self-assembled monolayers (SAMs) on Al/SiO₂/Si substrates. The visible precipitation of Zn(OH)₂ from the urea containing Zn(NO₃)₂ solution was enhanced by increasing the reaction temperature and the amount of urea. The optimized condition for the ZnO thin films was found to be the Zn(NO₃)₂/urea ratio of 1/8, the precipitation temperature of 80 °C, the precipitation time of 1 h and the annealing temperature of 600 °C, respectively. SAMs are formed by exposing Al/SiO₂/Si to solutions comprising of hydrophobic octadecylphosphonic acid (OPA) in tetrahydrofuran and hydrophilic 2-carboxylethylphosphonic acid (CPA) in ethanol. The ZnO thin film was then patterned with the heat treatment of Zn(OH)₂ precipitated on the surface of hydrophilic CPA. The ZnO gas sensor was exposed to different concentrations of C₃H₈ (5000 ppm), CO (250 ppm) and NO (1000 ppm) at elevated temperatures to evaluate the gas sensitivity of ZnO sensors. The optimum operating temperatures of C₃H₈, CO and NO gases showing the highest gas sensitivity were determined to be 350, 400 and 200 °C, respectively.     

High-sensitivity NO2 gas sensors based on flower-like and tube-like ZnO nanomaterials

Hierarchical flower-like and 1D tube-like ZnO architectures were synthesized by a microemulsion-based solvothermal method. Technologies of XRD, SEM and TEM were used to characterize the morphological and structural properties of the products. The influence of the flower-like and tube-like morphologies on their NO₂ sensing properties was investigated. The experimental results showed that high-sensitivity NO₂ gas sensors were fabricated. The sensitivity of the tube-like ZnO gas sensor was much higher than that of the flower-like ZnO gas sensor and the tube-like ZnO gas sensor exhibited shorter response time. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technique was employed to investigate the NO₂ sensing mechanisms. Free nitrate ions, nitrate and nitrite were the main adsorbed species during the adsorption, and NO also existed in the initial period of surface reoxidation. Furthermore, N₂O was formed via NO⁻ and N₂O₂⁻ stemmed from NO and increased upon rising temperature. Moreover, the PL spectra and the XPS spectra further proved that the intensity of donors (oxygen vacancy (V_O) and zinc interstitial (Zn_i)) and surface oxygen species (O₂⁻ and O₂) involved in the gas sensing mechanism leaded to the different sensitivities.     

Gas sensing behavior of polyvinylpyrrolidone-modified ZnO nanoparticles for trimethylamine

Gas sensors based on polyvinylpyrrolidone (PVP)-modified ZnO nanoparticles with different molar ratios of Zn²⁺: PVP were prepared by a sol–gel method. Morphology of the sensors was characterized by field emission-scanning electron microscopy (FE-SEM), which indicated that the sensor with a molar ratio of Zn²⁺: PVP = 5:5 showed uniform morphology. Moreover, the sensor exhibited fairly excellent sensitivity and selectivity to trimethylamine (TMA). The response and recovery time of the sensor were 10 and 150 s, respectively. Finally, the mechanism for the improvement in the gas sensing properties was discussed.     

Synthesis of triazolo-thiadiazole fluorescent organic nanoparticles as primary sensor toward Ag and the complex of Ag as secondary sensor toward cysteine

Fluorescent organic nanoparticles (FONs) have received considerable attention in the past few years, since the material holds great flexibility in materials synthesis and optical properties. In this study, we report a novel Ag⁺-selective turn-on fluorescent chemosensor based on the triazolo-thiadiazole (TTD) FONs, which show a significant fluorescence enhancement to silver ions among fourteen metal ions due to the formation of Ag–FONs cation complex, and also exhibit a lowest detectable concentration of 2.87 × 10⁻⁹ M. Upon the addition of Cysteine (Cys), a thiol-containing amino acid, the fluorescence intensity of the colloidal solution decreases significantly with a limit detection concentration of 2.58 × 10⁻⁷ M, indicating that Cys can form the Ag–Cys complex. Thus FONs are a potential primary sensor toward Ag⁺ and a secondary sensor toward Cys. The method is a basis for further two-component recognition study of TTD FONs. The possible mechanism is also discussed.     

Influence of morphology and structure geometry on NO2 gas-sensing characteristics of SnO2 nanostructures synthesized via a thermal evaporation method

SnO₂ microwires, nanowires and rice-shaped nanoparticles were synthesized by a thermal evaporation method. The diameters of microwire and nanowire were 2 μm and 50-100 nm, respectively, with approximately the same length (∼20 μm). The size of nanoparticles was about 100 nm. It was confirmed that the as-synthesized products have SnO₂ crystalline rutile structure. The sensing ability of SnO₂ particle and wire-like structure configured as gas sensors was measured. A comparison between the particle and wire-like structure sensors revealed that the latter have numerous advantages in terms of reliability and high sensitivity. Although its high surface-to-volume ratio, the nanoparticle sensor exhibited the lowest sensitivity. The high surface-to-volume ratio and low density of grain boundaries is the best way to improve the sensitivity of SnO₂ gas sensors, as in case of nanowire sensor which exhibited a dramatic improvement in sensitivity to NO₂ gas.     

Preparation of ZnO nanorods by microemulsion synthesis and their application as a CO gas sensor

Zinc oxide nanorods with different surface area were synthesized by surfactant assisted microemulsion method. The alkyl chain length of surfactant would affect aspect ratio of ZnO nanorods. ZnO nanorods synthesized by ethyl benzene acid sodium salt (EBS), which is surfactant with short alkyl chain length, show higher aspect ratio than ones by dodecyl benzene sulfonic acid sodium salt (DBS). These nanorods had diameters in the range of 80–300 nm and length of up to several microns. The Brunauer–Emmett–Teller (BET) surface area of the ZnO nanorods was strongly affected by the morphology of the nanorods. The BET surface area of the nanorods synthesized with EBS was higher than the surface area of the nanorods synthesized with DBS (20.2 and 14.1 m²/g for EBS and DBS, respectively). The response of ZnO nanorods to CO in air was strongly affected by surface area, defects and oxygen vacancies. The results demonstrate that the microemulsion synthesis is an easy and useful method to synthesize ZnO nanorods with large aspect ratio, which may enhance their gas sensing properties.     

Ultra-sensitive polysilicon wire glucose sensor using a 3-aminopropyltriethoxysilane and polydimethylsiloxane-treated hydrophobic fumed silica nanoparticle mixture as the sensing membrane

In this paper a highly sensitive glucose biosensor is proposed based on a polysilicon (poly-Si) wire structure coated with 3-aminopropyltriethoxysilane (γ-APTES) mixed with polydimethylsiloxane-treated hydrophobic fumed silica nanoparticles (NPs) as the sensing membrane. The γ-APTES and fumed silica NPs mixture was directly transferred to and coated onto the poly-Si wire region with the help of a focus-ion-beam (FIB) processed capillary atomic-force-microscope (C-AFM) tip. After the necessary curing and UV illumination processes, the resultant sensor showed an extremely wide linear detection range from 0.1 μM to 10 mM with a channel current sensitivity as high as 5.33 A mM⁻¹ cm⁻² (or a channel conductance sensitivity of 70 μS mM⁻¹), and a detection limit as low as 10 nM can be achieved. Our experimental results showed that the poly-Si wire sensor has good selective analysis and operational stability on glucose detection under a 10:1 concentration ratio of glucose and uric acid. Its linear range and lowest detection limit remain virtually unimpaired in the presence of uric acid.     

Ascorbic acid imprinted polymer-modified graphite electrode: A diagnostic sensor for hypovitaminosis C at ultra trace ascorbic acid level

A new kind of molecularly imprinted polymer-modified graphite electrode was fabricated by “grafting-to” approach, incorporating sol–gel technique, for the detection of acute deficiency in serum ascorbic acid level (SAAL), manifesting hypovitaminosis C. The modified electrode exhibited ascorbic acid (AA) oxidation at less positive potential (0.0 V) than the earlier reported methods, resulting in a limit of detection as low as 6.13 ng mL⁻¹ (RSD = 1.2%, S/N = 3). The diffusion coefficient (1.096 × 10⁻⁵ cm² s⁻¹), rate constant (7.308 s⁻¹), and Gibb's free energy change (−12.59 kJ mol⁻¹) due to analyte adsorption, were also calculated to explore the kinetics of AA oxidation. The proposed sensor was found to enhance sensitivity substantially so as to detect ultra trace level of AA in the presence of other biologically important compounds (dopamine, uric acid, etc.), without any cross interference and matrix complications from biological fluids and pharmaceutical samples.     

Gold nanowire array electrode for non-enzymatic voltammetric and amperometric glucose detection

The non-enzymatic voltammetric and amperometric detection of glucose using a gold nanowire array electrode is described. The voltammetric detection of glucose was performed by cyclic and differential-pulse voltammetry. The detection of glucose by partial and direct oxidation of glucose during the anodic and cathodic potential sweeps was shown in cyclic voltammetry. An unusual decrease in overpotential for partial oxidation of glucose on a Au NW array electrode was observed. A linear differential-pulse voltammetric response for partial oxidation of glucose was observed up to a glucose concentration of at least 20 mM with a sensitivity of 41.9 μA mM⁻¹ cm⁻² and detection limit below 30 μM (signal-to-noise ratio of 3) for glucose oxidation at low potentials, where the influence of possible intermediates can be avoided. The amperometric response was also linear up to a glucose concentration of 10 mM with a sensitivity of 309.0 μA mM⁻¹ cm⁻². The wide dynamic range and high sensitivity, selectivity and stability, as well as good biocompatibility of the Au NW electrode make it promising for the fabrication of non-enzymatic glucose sensors.     

基于微接触印刷的ZnO紫外传感器特性研究

采用微接触印刷技术和水热生长方法在硅基底上实现了ZnO种子层的图案化转移与纳米线阵列的可控制备。利用X射线衍射(XRD)、能量色散谱(EDS)和扫描电子显微镜(SEM)等测试手段对制备的ZnO纳米线晶体结构、化学组分以及表面形貌进行了表征,并对制备的ZnO纳米线传感器进行了紫外特性测试。测试结果表明:随着紫外光强度的增加,传感器的光暗电流比和光响应度也随之增加。当紫外传感器偏压在4.5 V时,其光暗电流比为80.8,响应度可达4.05 A/W。     

Architecture of electrospun carbon nanofibers–hydroxyapatite composite and its application act as a platform in biosensing

A biofunctional hybrid composite was constructed by assembling hydroxyapatite (HA) onto carboxylic group-functionalized carbon nanofibers (FCNFs). The FCNFs was obtained from acid treatment of carbon nanofibers (CNFs) which were synthesized by the combination of electrospinning and thermal treatment processes. The obtained carbon nanofibers–hydroxyapatite composite (FCNFs–HA) was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and FTIR spectroscopy. Cytochrome c (Cyt c) was successfully immobilized in this three-dimensional FCNFs–HA composite and the electron transfer rate constant (k_s) was evaluated to be 3.66 s⁻¹ according to Laviron's equation. And the surface coverage (Γ*) was estimated to be 8.1 × 10⁻¹⁰ mol cm⁻². Cyt c immobilized in FCNFs–HA composite exhibited a good electrocatalytic activity towards the reduction of hydrogen peroxide (H₂O₂). The catalytic current is linear to the H₂O₂ concentration in the range of 2.0 μM to 8.7 mM (r = 0.9996; n = 28), and the detection limit was 0.3 μM based on the criterion of a signal-to-noise ratio of 3.     

A photoluminescence-based quantum semiconductor biosensor for rapid in situ detection of Escherichia coli

This work describes a novel method of detecting Escherichia coli using photoluminescence (PL) emission from III–V quantum semiconductor (QS) devices functionalized with two different antibody-based architectures. The first approach employed self-assembled monolayers of biotinylated polyethylene glycol thiols to immobilize biotinylated antibody via neutravidin. In the second approach, we used QS microstructures coated with a thin layer of Si₃N₄ allowing direct functionalization with E. coli antibodies through hydrofluoric acid etching and glutaraldehyde-based reticulation. Atomic force, optical and fluorescence microscopy measurements were used to assess the immobilization process. Depending on the biosensing architecture, density of the immobilized bacteria was observed in the range of 0.5–0.7 bacteria/100 μm². The detection of E. coli at 10⁴ CFU/ml was achieved within less than 120 min of the bacteria exposure. It is expected that an even better sensitivity threshold could be achieved following further optimization of the method.     

Fabrication and gas-sensing properties of hierarchically porous ZnO architectures

Hierarchically three-dimensional (3D) porous ZnO architectures are synthesized by a template-free, economical aqueous solution method combined with subsequent calcination. First, the precursors of interlaced and monodisperse basic zinc nitrate (BZN) nanosheets are prepared. Then calcination of the precursors produces hierarchically 3D porous ZnO architectures composed of interlaced ZnO nanosheets with high porosity resulting from the thermal decomposition of the precursors. The products are characterized by X-ray diffraction, thermogravimetric-differential thermalgravimetric analysis, scanning electron microscopy, transmission electron microscopy, and Brunauer-Emmett-Teller N₂ adsorption-desorption analyses. The BET surface area of the hierarchically porous ZnO nanostructures was calculated to be 12.8 m² g⁻¹. Compared with ZnO rods, the as-prepared porous ZnO nanosheets exhibit a good response and reversibility to some organic gases, such as ethanol and acetone. The responses to 100 ppm ethanol and acetone are 24.3 and 31.6, respectively, at a working temperature of 320 °C. These results show that the porous ZnO architectures are highly promising for gas sensor applications, as the gas diffusion and mass transportation in sensing materials are significantly enhanced by their unique structures. Moreover, it is believed that this solution-based approach can be extended to fabricate other porous metal oxide materials with a unique morphology or shape.