Sensing of molecular analytes by probing the effects of their interaction with microwaves is emerging as a cheap, compact, label-free and highly sensitive detection and quantification technique. Microstrip ring-type resonators are particularly favored for this purpose due to their planar sensing geometry, electromagnetic field enhancements in the coupling gap and compatibility with established printed circuit board manufacturing. However, the lack of selectivity in what is essentially a permittivity-sensing method is an impediment to wider adoption and implementation of this sensing platform. By placing a polycrystalline anatase-phase TiO2 nanotube membrane in the coupling gap of a microwave resonator, we engineer selectivity for the detection and differentiation of methanol, ethanol and 2-propanol. The scavenging of reactive trapped holes by aliphatic alcohols adsorbed on TiO2 is responsible for the alcohol-specific detection while the different short chain alcohols are distinguished on the basis of differences in their microwave response. Electrodeless microwave sensors which allow spectral and time-dependent monitoring of the resonance frequency and quality factor provide a wealth of information in comparison with electrode-based resistive sensors for the detection of volatile organic compounds. A high dynamic range (400 ppm–10000 ppm) is demonstrated for methanol detection.

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Selective microwave sensors exploiting the interaction of analytes with trap states in TiO2 nanotube arrays

1D Metal Oxide Semiconductor Materials for Chemiresistive Gas Sensors: A Review

Steady-state (SRMC) and time-resolved microwave photoconductivity (TRMC) are key techniques used to perform the contact-less determination of carrier density, transport, trapping, and recombination parameters in charge transport materials such as organic semiconductors and dyes, inorganic semiconductors, and metal–insulator composites, which find use in conductive inks, thin film transistors, light-emitting diodes, photocatalysts, and photovoltaics. We present the theory, design, simulation, and fabrication of a planar microwave ring resonator operating at 5.25 GHz with a quality factor of 224, to perform SRMC and TRMC measurements. Our method consists of measuring the resonance frequency (f0) and Q-factor of the microwave resonator with the sample to be probed placed in a defined sensitive region of the resonator where the microwave field is highly concentrated. We also provide proof of concept measurements of the time-resolved microwave photoresponse of anatase-phase TiO2 nanotube array membranes (TNTAM...

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Time-Resolved Microwave Photoconductivity (TRMC) Using Planar Microwave Resonators: Application to the Study of Long-Lived Charge Pairs in Photoexcited Titania Nanotube Arrays

Nanoparticle-mediated bio-sensing promoted the development of novel sensors in the front of medical diagnosis. In the present study, we have generated and examined the potential of titanium dioxide (TiO2) crystalline nanoparticles with aluminium interdigitated electrode biosensor to specifically detect single-stranded E.coli O157:H7 DNA. The performance of this novel DNA biosensor was measured the electrical current response using a picoammeter. The sensor surface was chemically functionalized with (3-aminopropyl) triethoxysilane (APTES) to provide contact between the organic and inorganic surfaces of a single-stranded DNA probe and TiO2 nanoparticles while maintaining the sensing system’s physical characteristics. The complement of the target DNA of E. coli O157:H7 to the carboxylate-probe DNA could be translated into electrical signals and confirmed by the increased conductivity in the current-to-voltage curves. The specificity experiments indicate that the biosensor can discriminate between the complementary sequences from the base-mismatched and the non-complementary sequences. After duplex formation, the complementary target sequence can be quantified over a wide range with a detection limit of 1.0 x 10-13M. With target DNA from the lysed E. coli O157:H7, we could attain similar sensitivity. Stability of DNA immobilized surface was calculated with the relative standard deviation (4.6%), displayed the retaining with 99% of its original response current until 6 months. This high-performance interdigitated DNA biosensor with high sensitivity, stability and non-fouling on a novel sensing platform is suitable for a wide range of biomolecular interactive analyses.

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Titanium Dioxide Nanoparticle-Based Interdigitated Electrodes: A Novel Current to Voltage DNA Biosensor Recognizes E. coli O157:H7.

We report on a very significant enhancement of the thermal, chemical, and mechanical stability of self-organized TiO2 nanotubes layers, provided by thin Al2O3 coatings of different thicknesses prepared by atomic layer deposition (ALD). TiO2 nanotube layers coated with Al2O3 coatings exhibit significantly improved thermal stability as illustrated by the preservation of the nanotubular structure upon annealing treatment at high temperatures (870 °C). In addition, a high anatase content is preserved in the nanotube layers against expectation of the total rutile conversion at such a high temperature. Hardness of the resulting nanotube layers is investigated by nanoindentation measurements and shows strongly improved values compared to uncoated counterparts. Finally, it is demonstrated that Al2O3 coatings guarantee unprecedented chemical stability of TiO2 nanotube layers in harsh environments of concentrated H3PO4 solutions.

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Atomic Layer Deposition Al2O3 Coatings Significantly Improve Thermal, Chemical, and Mechanical Stability of Anodic TiO2 Nanotube Layers

We report a simple water assisted crystallization process for electrochemically synthesized TiO2 nanotube (TNT) arrays at room temperature (30 °C) and near-room temperature (50 °C). The TNT array samples kept immersed in de-ionized water at room temperature for durations from 1 day to 6 days show a distinct anatase phase confirmed by both X-ray diffraction and transmission electron microscopy. When the process is repeated at an elevated temperature of 50 °C, the crystallization is accelerated with the appearance of an anatase phase within 8 h of treatment. Interestingly, addition of 0.05 M HCl or HNO3 to the DI water during treatment helped in forming the rutile phase of TiO2, along with the dominant anatase phase. By carefully choosing the nanotube parameters (length and diameter) it has been possible to retain the nano-tubular morphology, which is advantageous for applications such as solar cells and gas sensing. Finally, we show the gas sensing and photo-electrochemical characteristics of the DI water crystallized TNT arrays. The samples show good sensitivity and appreciable values of photo-current under the given experimental conditions.

Water assisted crystallization, gas sensing and photo-electrochemical properties of electrochemically synthesized TiO2nanotube arrays

The anatase phase of titanium dioxide (TiO2) plays an important role in applications such as photo-catalysis and photo-voltaics. The stability of this crystalline phase is however limited to about 600 °C because of the onset of anatase to rutile phase transformation. In the case of aligned and oriented nanostructures such as electrochemically anodized TiO2 nanotube arrays (TNTs), the retention of the aligned and oriented morphology, in addition to a stable anatase phase, poses further challenge for high temperature processing of such materials. Here we report a simple solvothermal processing step for the as-prepared, amorphous TNTs that helps in retaining the nanotube architecture along with a stable anatase phase up to a temperature of about 800 °C. It is observed that the smoother walls of amorphous TNTs are transformed into particulate/granular morphology as a result of the solvothermal treatment that confines anatase crystallites preventing further growth during high temperature annealing. A phonon-scattering mechanism at the particle interfaces is proposed to explain the very limited crystallite growth and preservation of the anatase phase. Furthermore, the robust and compact morphology of the nanotube arrays induced by solvothermal treatment also contributes to their ability to retain tubular architecture at higher temperatures. We also report gas sensitivity and photo-current density of the solvothermally treated anatase TNTs annealed at 800 °C that show significant enhancement of these properties over the conventional TNT samples annealed at 550 °C. The reported work thus presents an important advancement in the synthesis and structural modification of TiO2 nanotube arrays, which not only leads to enhancement of properties but also opens up possibilities for high temperature applications.

Anatase TiO2 nanotube arrays with high temperature stability

Highly aligned, vertically oriented TiO2 nanotube arrays formed by electrochemical anodization have been extensively investigated in the recent times because of various potential applications such as in solar cells, hydrogen generation, CO2 reduction, gas sensors and as biocompatible material. The as prepared nanotubes, however, are amorphous and require a thermal annealing process to achieve the desired crystallographic phase. Here we report a simple solvothermal technique by which the crystallinity of the nanotube arrays can be tailored at temperatures around 200 °C to achieve the anatase phase without destroying the tubular morphology. In this alcohol-based solvothermal process, the crystallinity can be enhanced by changing the solvent from methanol to isobutanol and the sample treated with isobutanol for 2 h shows crystallinity and strain comparable to that of a sample annealed in a conventional furnace at 550 °C. A mechanism for the solvothermal crystallization and the enhancement of crystallinity ha...

B. Manmadha Rao

Papers

Water assisted crystallization, gas sensing and photo-electrochemical properties of electrochemically synthesized TiO2nanotube arrays

Anatase TiO2 nanotube arrays with high temperature stability

Solvothermal Processing of Amorphous TiO2 Nanotube Arrays: Achieving Crystallinity at a Lower Thermal Budget

Chemi-Capacitive Gas Sensors Based on TiO2 and BaTiO3 Nanotube Arrays