Stoichiometry, Length, and Wall Thickness Optimization of TiO2 Nanotube Array for Efficient Alcohol Sensing

Unveiling the potential of polymer cholesteric liquid crystal interpenetrating networks as a label-free alcohol biochemical sensor

In this study, an optical sensor is developed, incorporating hydrogen-bonded photonic array dots containing poly(acrylic acid) (PAA) within a polymer cholesteric liquid crystal interpenetrating polymer network (PCLCIPN) framework, thereby effectively controlling porosity. This methodology involves the fabrication of a porous photonic film, subsequent infusion with a hydrogel (PAA), and precise UV-curing to generate patterned array dots. The sensor exhibits exceptional discriminatory capability between methanol and ethanol, accurately discerning their varying concentrations within alcohol solutions. The optical sensing performance of the film is rigorously evaluated through continuous monitoring of wavelength shifts in the transmission spectrum across various alcohol concentrations. Notably, the observed wavelength shifts demonstrate a linear correlation with the concentration of alcohol, thereby enabling precise quantitative analysis of the alcohol solutions. The sensor exhibits a sensitivity of 0.44 nm/% for ethanol concentrations ranging from 5% to 60%, increasing to 2.1 nm/% for concentrations between 60% and 80%. Similarly, for methanol, sensitivities of 0.68 nm/% (5-60%) and 2.2 nm/% (60-80%) are recorded. Remarkably, this sensitivity trend extends seamlessly to 1 : 1 ethanol/methanol ratios, with values of 0.49 nm/% (5-60%) and 2.25 nm/% (60-80%). Furthermore, these sensors demonstrate colorimetric response to different alcohols, rendering them accessible and cost-effective biosensors for visual detection, thus obviating the necessity for complex analytical instruments.

Metal Oxide Semiconductor Nanostructure Gas Sensors with Different Morphologies

There is an increasing need for the development of low-cost and highly sensitive gas sensors for environmental, commercial, and industrial applications in various areas, such as hazardous gas monitoring, safety, and emission control in combustion processes. Considering this, resistive-based gas sensors using metal oxide semiconductors (MOSs) have gained special attention owing to their high sensing performance, high stability, and low cost of synthesis and fabrication. The relatively low final costs of these gas sensors allow their commercialization; consequently, they are widely used and available at low prices. This review focuses on the important MOSs with different morphologies, including quantum dots, nanowires, nanofibers, nanotubes, hierarchical nanostructures, and other structures for the fabrication of resistive gas sensors.

Alcohol Selective Optical Sensor Based on Porous Cholesteric Liquid Crystal Polymer Networks

A responsive hydrogen-bonded cholesteric liquid crystal polymer (CLCP) film with controlled porosity was fabricated as an optical sensor to distinguish between methanol and ethanol in alcohol solutions. To facilitate responding the alcohols, porosity was generated by removing the nonreactive liquid crystal agent, and the hydrogen bridges of CLCP were broken. The sensitivities of CLCPs to ethanol and methanol were obtained by monitoring the wavelength shifts of the transmission spectrum at different alcohol concentrations and ratios of methanol/ethanol. Changes in the central wavelength of the CLCP network transmission spectrum allowed the methanol–ethanol ratio to be discriminated. A linear relationship between wavelength shift of CLCP networks and alcohol concentration was obtained experimentally, and the sensor characteristics were explored. The sensitivities of the CLCPs were 1.35 and 0.18 nm/% to ethanol and methanol, respectively. The sensing sensitivity of cholesteric networks to alcohol molecules increased as the methanol–ethanol ratio declined. Therefore, CLCP could act as a stimuli-responsive material to distinguish the concentrations of acetone and ethanol in mixed solutions. Furthermore, the impact of UV intensity for curing a CLC mixture on the sensing sensitivity to the different alcohol concentrations was also studied. The higher UV intensity could enhance the sensitivity to alcohol molecules and distinguishing ability between methanol and ethanol.

C60-encapsulated TiO2nanoparticles for selective and ultrahigh sensitive detection of formaldehyde

The current study concerns development of fullerene-C₆₀-encapsulated TiO₂nanoparticles hybrid for an efficient detection of volatile organic compounds (VOCs). The nanocomposite was synthesized via chemical route by using hydrated fullerene-C₆₀and sol-gel derived undopedp-type TiO₂nanoparticles. The nanocomposite was characterized morphologically and structurally comparing with pure C₆₀clusters and pure TiO₂nanoparticles as the reference materials. The average diameter of the C₆₀-encapsulated TiO₂nanoparticles was 150 nm whereas the average diameters of C₆₀clusters and pure TiO₂nanoparticles were 161 nm and 18 nm respectively. Therefore, all the materials were implemented in interdigitated electrode based planner structured sensors and tested towards multiple VOCs. However, C₆₀-TiO₂composite exhibited its natural selectivity towards formaldehyde with a very high sensitivity for the concentration range of 1-1000 ppm. C₆₀-encapsulated TiO₂nanoparticles depicted more than double response magnitude (117%) than the pure TiO₂nanoparticle (48%) and pure C₆₀particles (40%) and appreciably fast response/recovery (12 s/331 s) towards 100 ppm of formaldehyde at 150 °C. However, the efficient VOC sensing was achieved in C₆₀-encapsulated TiO₂sensors possibly due to the extreme reactive surface provided by the oxygen functionalized C₆₀and easy electronic exchange between ambient and the TiO₂nanoparticles through C₆₀layers. The combined properties of both C₆₀and TiO₂lead to the formation of a promising nanocomposite which provided better sensing characteristics than that of the pure materials.

Electroless deposition of Pd/Pt nanoparticles on electrochemically grown TiO2 nanotubes for ppb level sensing of ethanol at room temperature

This work presents a comparative sensing study of three sensors based on pristine TiO₂ nanotubes, Pd loaded TiO₂ nanotubes, and Pt loaded TiO₂ nanotubes. Pristine TiO₂ nanotubes were synthesized using an electrochemical anodization method and an electroless plating method was used for the uniform deposition of noble metal nanoparticles of either Pd or Pt over the surface of TiO₂ nanotubes. The samples were thoroughly characterized by XRD, FESEM, EDS, TEM, and XPS techniques. The sensitivity of all three sensors was investigated at room temperature (300 K) for different volatile organic compounds like ethanol, methanol, 2-propanol, acetone, and benzene. The results revealed that loading of Pd and Pt nanoparticles improved the response magnitude of the sensor remarkably as these noble metals possess better oxygen dissociation capability than pristine TiO₂. The Pd-TiO₂ nanotube sensor exhibited a maximum response magnitude of 20-98% towards 100-1000 ppb of ethanol at room temperature. Notably, the formation of Pd/Pt-TiO₂ discrete heterojunctions on the surface of TiO₂ nanotubes was found to be responsible for enhanced sensitivity of the sensors.

Multiple nano-filaments based efficient resistive switching in TiO2 nanotubes array influenced by thermally induced self-doping and anatase to rutile phase transformation

In this paper, the impact of thermally induced self-doping and phase transformation in TiO₂ based resistive random-access memory (ReRAM) is discussed. Instead of a thin film, a vertically aligned one-dimensional TiO₂ nanotube array (TNTA) was used as a switching element. Anodic oxidation method was employed to synthesize TNTA, which was thermally treated in the air at 350 °C followed by further annealing from 350 °C to 650 °C in argon. Au/TiO₂ nanotube/Ti resistive switching devices were fabricated with porous gold (Au) top electrode. The x-ray diffraction results along with Raman spectra evidently demonstrate a change in phase of crystallinity from anatase to rutile, whereas photoluminescence spectra revealed the self-doping level in terms of oxygen vacancies (OV) and Ti interstitials (Ti_i) as the temperature of thermal treatment gets increased. The electrical characterizations establish the bipolar and electroforming free resistive switching in all the samples. Among those, the ReRAM sample S₃ thermally treated at 550 °C displayed the most effective resistive switching properties with R _OFF/R _ON of 10² at a read voltage of -0.6 V and a SET voltage of -2.0 V. Moreover, the S₃ sample showed excellent retention performance for over 10⁶ s, where stable R _OFF/R _ON ≈ 107 was maintained throughout the experiment.

1D Titanium Dioxide: Achievements in Chemical Sensing

For the last two decades, titanium dioxide (TiO₂) has received wide attention in several areas such as in medicine, sensor technology and solar cell industries. TiO₂-based gas sensors have attracted significant attention in past decades due to their excellent physical/chemical properties, low cost and high abundance on Earth. In recent years, more and more efforts have been invested for the further improvement in sensing properties of TiO₂ by implementing new strategies such as growth of TiO₂ in different morphologies. Indeed, in the last five to seven years, 1D nanostructures and heterostructures of TiO₂ have been synthesized using different growth techniques and integrated in chemical/gas sensing. Thus, in this review article, we briefly summarize the most important contributions by different researchers within the last five to seven years in fabrication of 1D nanostructures of TiO₂-based chemical/gas sensors and the different strategies applied for the improvements of their performances. Moreover, the crystal structure of TiO₂, different fabrication techniques used for the growth of TiO₂-based 1D nanostructures, their chemical sensing mechanism and sensing performances towards reducing and oxidizing gases have been discussed in detail.

Oxygen vacancy modulation of titania nanotubes by cathodic polarization and chemical reduction routes for efficient detection of volatile organic compounds

In this work, we have synthesized a highly ordered TiO₂ nanotube array by an electrochemical anodization method. Then the oxygen vacancy level of the TiO₂ nanotubes was tuned by two different methods: i.e. (i) cathodic polarization by the application of a reverse potential and (ii) chemical reduction using a reducing agent (e.g. hydrazine hydrate) treatment at elevated temperature. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) confirmed there was no morphological deformation of TiO₂ nanotubes after the modulation of oxygen vacancies. X-ray diffraction spectroscopy (XRD) and TEM both confirmed the formation of highly crystalline anatase (101). The oxygen vacancy level of all the TiO₂ nanotubes was tested progressively with photoluminescence (PL) spectra, Raman spectra, energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectra (XPS). All the spectroscopy results confirmed the successful reduction of TiO₂ nanotubes with different levels of defects. All the nanotube samples with various oxygen vacancy levels were transformed to Au/TiO₂ nanotube/Ti type sandwich structured sensor devices and tested under exposure to 100 ppm of ethanol in air. Highly doped cathodic reduced nanotubes offered very high response magnitudes of 99.64% and 60% under exposure to 100 ppm of ethanol at 200 °C and 50 °C, respectively. Chemically reduced TiO₂ nanotubes offered moderate response magnitudes of 75.4% and 80% at 150 °C and 200 °C under exposure to 100 ppm of ethanol, which was found to be the best among all the samples due to the appreciably fast response (155 s) and recovery time (779 s). The developed sensors showed adequate stability and selectivity towards ethanol with a moderate dynamic range (20 to 200 ppm of ethanol) of detection. A general relation was drawn based on the experimental findings of this work to estimate the response magnitude of nanoscale metal oxide gas sensors with various levels of oxygen vacancies.

Detection of Four Distinct Volatile Indicators of Colorectal Cancer using Functionalized Titania Nanotubular Arrays

Screening of colorectal cancer is crucial for early stage diagnosis and treatment. Detection of volatile organic compounds (VOCs) of the metabolome present in exhaled breath is a promising approach to screen colorectal cancer (CRC). Various forms of volatile organic compounds (VOCs) that show the definitive signature for the different diseases including cancers are present in exhale breathe. Among all the reported CRC VOCs, cyclohexane, methylcyclohexane, 1,3-dimethyl- benzene and decanal are identified as the prominent ones that can be used as the signature for CRC screening. In the present investigation, detection of the four prominent VOCs related to CRC is explored using functionalized titania nanotubular arrays (TNAs)-based sensor. These signature biomarkers are shown to be detected using nickel-functionalized TNA as an electrochemical sensor. The sensing mechanism is based on the electrochemical interaction of nickel-functionalized nanotubes with signature biomarkers. A detailed mechanism of the sensor response is also presented.

Synthesis of In2O3 nanoparticle/TiO2 nanobelt heterostructures for near room temperature ethanol sensing

The obtained In₂O₃ nanoparticle/TiO₂ nanobelt heterostructures exhibit a high sensitive toward ethanol at near room temperature of 45 °C and low detection limit of 1 ppm.

Fast Growth of Highly Ordered TiO2 Nanotube Arrays on Si Substrate under High-Field Anodization

ABSTRACT

Highly ordered TiO₂ nanotube arrays (NTAs) on Si substrate possess broad applications due to its high surface-to-volume ratio and novel functionalities, however, there are still some challenges on facile synthesis. Here, we report a simple and cost-effective high-field (90-180 V) anodization method to grow highly ordered TiO₂ NTAs on Si substrate, and investigate the effect of anodization time, voltage, and fluoride content on the formation of TiO₂ NTAs. The current density-time curves, recorded during anodization processes, can be used to determine the optimum anodization time. It is found that the growth rate of TiO₂ NTAs is improved significantly under high field, which is nearly 8 times faster than that under low fields (40-60 V). The length and growth rate of the nanotubes are further increased with the increase of fluoride content in the electrolyte.

GRAPHICAL ABSTRACT

Highly ordered TiO₂ nanotube arrays (NTAs) on Si substrate have been fabricated by high-field anodization method. A high voltage (90-180 V) leads to a high growth rate of TiO₂ NTAs (35-47 nm s^-1), which is nearly 8 times faster than the growth rate under low fields (40-60 V). Furthermore, the current density-time curves recorded during the anodization provide a facial method to determine the optimal anodization parameters, leading to an easy obtaining of the desired nanotubes.

Metal Chelation Assisted In Situ Migration and Functionalization of Catalysts on Peapod-Like Hollow SnO2 toward a Superior Chemical Sensor

Rational design of nanostructures and efficient catalyst functionalization methods are critical to the realization of highly sensitive gas sensors. In order to solve these issues, two types of strategies are reported, i.e., (i) synthesis of peapod-like hollow SnO₂ nanostructures (hollow 0D-1D SnO₂ ) by using fluid dynamics of liquid Sn metal and (ii) metal-protein chelate driven uniform catalyst functionalization. The hollow 0D-1D SnO₂ nanostructures have advantages in enhanced gas accessibility and higher surface areas. In addition to structural benefits, protein encapsulated catalytic nanoparticles result in the uniform catalyst functionalization on both hollow SnO₂ spheres and SnO₂ nanotubes due to their dynamic migration properties. The migration of catalysts with liquid Sn metal is induced by selective location of catalysts around Sn. On the basis of these structural and uniform functionalization of catalyst benefits, biomarker chemical sensors are developed, which deliver highly selective detection capability toward acetone and toluene, respectively. Pt or Pd loaded multidimensional SnO₂ nanostructures exhibit outstanding acetone (R _air /R _gas = 93.55 @ 350 °C, 5 ppm) and toluene (R _air /R _gas = 9.25 @ 350 °C, 5 ppm) sensing properties, respectively. These results demonstrate that unique nanostructuring and novel catalyst loading method enable sensors to selectively detect biomarkers for exhaled breath sensors.

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

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-10,000 ppm) is demonstrated for methanol detection.