We report on the fabrication of self-organized titanium oxide nanotube arrays of enhanced surface area prepared by anodic oxidation of a pure titanium sheet in electrolyte solutions containing potassium fluoride (KF) or sodium fluoride (NaF). The effects of electrolyte composition and concentration, solution pH, and the anodic potential on the formation of nanotubes and dimensions of the resulting nanotubes are detailed. Although nanotube arrays of length greater than 500 nm are not possible with hydrofluoric acid containing electrolytes [G.K. Mor, O.K. Varghese, M. Paulose,N. Mukherjee, C.A. Grimes, J. Mater. Res. 18, 2588 (2003)], by adjusting the pH of a KF containing electrolyte to 4.5 using additives such as sulfuric acid, sodium hydroxide, sodium hydrogen sulfate, and/or citric acid, we could increase the length of the nanotube-array to approximately 4.4 μm, an order of magnitude increase in length. The as-prepared nanotubes are composed of amorphous titanium oxide. Independent of the electrolyte composition, crystallization of the nanotubes to anatase phase occurred at temperatures ⩾280 °C. Rutile formation occurred at the nanotube-Ti substrate interface at temperatures near 480 °C. It appears geometry constraints imposed by the nanotube walls inhibit anatase to rutile transformation. No disintegration of the nanotube array structure is observed at temperatures as high as 580 °C. The excellent structural and crystal phase stability of these nanotubes make them promising for both low- and high-temperature applications.

The synthesis and testing of two gold-thiolate monolayer-protected (nano)clusters as interfacial layers on a dual-chemiresistor vapor sensor array are described. Responses (changes in dc resistance) to each of 11 organic solvent vapors are rapid, reversible, and linear with concentration at low vapor concentrations, becoming sublinear at higher concentrations. Limits of detection (LODs) range from 0.1 to 24 parts per million and vary inversely with solvent vapor pressure. When configured as a GC detector and used to analyze 0.5-L preconcentrated samples of the 11-vapor mixture, the array provides LODs of < or = 700 parts per trillion for most vapors, comparing favorably with those from an integrated array of polymer-coated surface acoustic wave sensors configured and tested similarly. This first report on the use of such an array as a GC detector shows that the combination of response patterns and GC retention times improves capabilities for vapor recognition compared to the sensor array alone or to single-detector GC systems. Spray-coated nanocluster thin films can be deposited reproducibly and exhibit response stability in air that ranges from fair to excellent for up to several months. Scaling the active device area down by a factor of 16 has no significant effect on sensitivity. Implications of these results for portable vapor sensing systems are discussed.

Driven by the urgent demand of detecting trace amounts of pentachlorophenol (PCP) in contaminative water, a label-free immunosensor with ultra sensitivity and high selectivity was constructed based on a hybrid CdSe(x)Te(1-x) (0 ≤ x ≤ 1) nanocrystal (NCs)-modified TiO(2) nanotube (NT) arrays for the first time. The CdSe(x)Te(1-x) NCs were photoelectrodeposited on inner and outer space of the TiO(2) NTs, leading to high photoelectrical conversion efficiency in the visible region. PCP antibodies are covalently conjugated on the TiO(2) NTs due to the large surface area and good biocompatibility. Since the photocurrent is highly dependent on the TiO(2) surface properties, the specific interaction between PCP and the antibodies results in a sensitive change in the photocurrent, with a limit of detection (LOD) of 1 pM. High sensor-to-sensor reproducibility is achieved. The sensor was applied for the direct analysis of river water samples.

TiO(2) nanotube arrays (NTAs) are modified with PbS nanoparticles by successive ionic layer adsorption and reaction (SILAR) or electrodeposition, with an aim towards tuning the photoelectrochemical cell to the visible region. The PbS modification of the TiO(2) NTAs results in an increase in the visible light adsorption, however the increase in photocurrent is dependent on the modification method. PbS/TiO(2) NTAs prepared by SILAR and electrodeposition show, respectively, photocurrents of 11.02 and 5.72 mA/cm(2). The increase in photocurrent is attributed to enhanced charge separation efficiency and improved electron transport.

Thick-film magnetoelastic sensors vibrate mechanically in response to a time varying magnetic excitation field. The mechanical vibrations of the magnetostrictive magnetoelastic material launch, in turn, a magnetic field by which the sensor can be monitored. Magnetic field telemetry enables contact-less, remote-query operation that has enabled many practical uses of the sensor platform. This paper builds upon a review paper we published in Sensors in 2002 (Grimes, C.A.; et al. Sensors 2002, 2, 294–313), presenting a comprehensive review on the theory, operating principles, instrumentation and key applications of magnetoelastic sensing technology.

No abstract

This paper describes a wireless, remote query glucose biosensor using a ribbonlike, mass-sensitive magnetoelastic sensor as the transducer. The glucose biosensor is fabricated by first coating the magnetoelastic sensor with a pH-sensitive polymer and upon it a layer of glucose oxidase (GOx). The pH-responsive polymer swells or shrinks, thereby changing mass, respectively, in response to increasing or decreasing pH values. The GOx-catalyzed oxidation of glucose produces gluconic acid, inducing the pH-responsive polymer to shrink, which in turn decreases the polymer mass. In response to a time-varying magnetic field, a magnetoelastic sensor mechanically vibrates at a characteristic resonance frequency, the value of which inversely depends on sensor mass loading. As the magnetoelastic films are magnetostrictive, the vibrations launch magnetic flux that can be remotely detected using a pickup coil. Hence, changes in the resonance frequency of a passive magnetoelastic transducer are detected on a remote query basis, without the use of physical connections to the sensors.The sensitivity of the glucose biosensors decreases with increasing ionic strength; at physiological salt concentrations, 0.6 mmol/L of glucose can be measured. At glucose concentrations of 1-10 mmol/L, the biosensor response is reversible and linear, with the detection limit of 0.6 mmol/L corresponding to an error in resonance frequency determination of 20 Hz. Since no physical connections between the sensor and the monitoring instrument are required, this sensor can potentially be applied to in vivo and in situ measurement of glucose concentrations.

No abstract