Gas Sensor

We tested the detection properties of four MOX sensors toward different ozone mixtures to identify sets of sensing layers and interfering compounds concentrations most suitable for a reliable detection of ozone. The measurement campaign lasted 1 year divided in four sessions. We collected a substantial amount of measurements (more than 500) with diverse interfering gases: ammonia, ethanol, ethylene, carbon monoxide and humidity. Due to the dimension of the data set it could not be analyzed using the conventional methods generally applied for characterizing gas sensors: evaluating the sensor performance by visual inspection of the sensors responses is unfeasible. For this reason we systematically applied the exploratory data analysis methodology. We used some simple but effective statistical techniques to insight the data. This approach allows us to draw sound conclusions about the causes of variation in the data, e.g. time (sensors' long-term stability) or interfering effects of different chemical compounds. All the analysis techniques employed in this work are implemented in a software package developed at our laboratory.

The nanometer-size In 2 O 3 was synthesized via a reverse microemulsion. A new catalytic combustion-type In 2 O 3 -based H 2 gas sensor was developed based on the technology for fabricating the direct-heating-type sensor and a surface-modifying process. A dense SiO 2 layer near the surface of the sensor was formed by chemical vapor deposition (CVD) of hexamethyldisiloxane (HMDS). The SiO 2 layer, which acted as a molecular sieve, reduced the penetration of large molecular, such as C 2 H 5 OH, CH 4 , i-C 4 H 10 , into the sensing layer, resulting in the improvement of selectivity to H 2 . The sensitive properties and the working mechanism of the sensor were presented. The In 2 O 3 nanoparticles prepared by microemulsion were characterized by transmission electron microscopy and X-ray diffraction.

Abstracf-Thin films of SnOl were pdpared by a sol-gel technique using tin tetrachloride (SnC4) ahd isopropyl alcohol solution. Applying standard lithography process, high sensitivity SDOZ gas sensors were miniaturized on maeromachined Si substrates. Conductivity measurements of the films annealed at different temperatures have been carried out in dry air and in the presence of 5ppm to lOOppm ethanol. Response and recovery times OF the sensors annealed at 55O0C a t the optimum operating temperature were found to be about 5 and 40 seconds, respectively. Also sensitivity of the sensors was studied towards SQOppm CO and 1OOOppm Hz at different temperatures, indicating the satisfactory selectivity for ethanol.

Sulfur hexafluoride (SF 6) of >99.9% purity was artificially released to simulate the emission sources in the etching-thin film area of a working cleanroom in a semiconductor fab at the rate of 492 g/h. Three mobile Fourier transform infrared spectrometers (FTIRs, detection limit: 10 ppb) were used simultaneously to measure the real time SF 6 concentrations at different locations of the cleanroom. A three-dimensional numerical model was also used to predict the unsteady gas concentration distribution and the results were compared with the experimental data. Due to high dilution of the pollutant in the cleanroom, it is found that the current gas sensors may not be sensitive enough and a better monitoring system and strategy is needed to protect workers from injury and to ensure good product yield. After comparison with the validated numerical results, the well-mixed model is found to predict the peak pollutant concentrations within a reasonable range which is 0.34-1.33 times the experimental values except when the monitored distance is very close to the release point. The well-mixed model is shown to be capable of predicting a reasonable attainable maximum concentration once a pollutant leaks in the cleanroom.

TiO2 and SnO2 are the well-known sensing materials with a good thermal stability of the former and a high sensitivity of the latter. Carbon nanotubes (CNTs) have also gas sensing ability at room temperature. CNTs-included SnO2/TiO2 material was a new exploration to combine the advantages of three kinds of materials for gas-sensing property. In this work, a uniform SnO2/TiO2 solution was prepared by the sol gel process with the ratio 3:7 in mole. The CNTs with contents in the range of 0.001 0.5 wt% were dispersed in a mixed SnO2/TiO2 matrix by using an immersion-probe ultrasonic. The SnO2 TiO2 and the CNTs-included SnO2 TiO2 thin films were fabricated by the sol gel spin-coating method over Pt-interdigitated electrode for gas-sensor device fabrication and they were heat treated at 500 °C for 30 min. FE-SEM and XRD characterizations indicated that the inclusion of CNTs did not affect the particle size as well as the morphology of the thin film. The sensing properties of all as-fabricated sensors were investigated with different ethanol concentrations and operating temperatures. An interesting sensing characteristic of mixed SnO2/TiO2 sensors was that there was a two-peak shape in the sensitivity versus operating temperature curve. At the region of low operating temperature (below 280 °C), the hybrid sensors show improvement of sensing property. This result gives a prospect of the stable gas sensors with working temperatures below 250 °C.

A Pt/C heterojunction has been fabricated and tested as a conductometric hydrogen gas sensor. The carbon layer, deposited between planar Au substrate contacts using a filtered cathodic vacuum arc (with an applied substrate bias of -500 V), consisted largely of vertically oriented graphitic sheets. These sheets have good through-film electrical conductivity and contributed to a low device resistance of ~ 60 Ω. The Pt layer (of 3 nm thickness) was deposited using a Gatan Model 682 Precision Electron Coating System (PECS). A change in the device resistance of > 2% was exhibited upon exposure to 1 % hydrogen gas (in synthetic, zero humidity air) at room temperature. The time for the sensor resistance to decrease by 2 % under these conditions was 60 s and the baseline (zero hydrogen exposure) resistance remained constant to within 0.03% during and after the exposure tests.

The effects of UV light illumination on the performance of SnO and In O semiconductor gas sensors toward CO and NO are 2 2 3 2 Ž . reported. The sample were prepared with DC sputtering in Ar atmospheres by the Rheotaxial Growth and Thermal Oxidation RGTO technique and with reactive magnetron sputtering in Ar and O atmosphere, respectively. Results are quite promising for the development 2 of sensor working at room temperature. q 2000 Elsevier Science S.A. All rights reserved.

A micro-gas analysis system, μGAS, is proposed here for the continuous and highly sensitive on-site measurement of atmospheric trace gases. The μGAS presented here is comprised of a microchannel scrubber and a high-sensitivity small fluorescence detector. The key to the μGAS is the fabrication of a good gas permeable membrane on a shallow channel to allow for the efficient accumulation of the analyte into the absorbing solution. The channel is formed by micromachining, and a gas permeable membrane of any desired thickness is formed by spin-coating on a fluorosilane-treated substrate. In this work, polydimethylsiloxane was used for both the channel block and the gas permeable membrane, and were easily tightly bonded. The microchannel structure ensures a high sensitivity, and the sensitivity is inversely proportional to the membrane thickness. The device, including the air supply function, is set in only 30-mm-cube. The performance of the μGAS has been demonstrated as a H2S gas sensor. The μGAS consumes only 1ml of the reagent solution in 8h of operation with a detection limit of 1ppbv.

Complex impedance analysis and transient response studies in SnO 2 thick films show the impedance dependence on gaseous environment and temperature. Possible mechanisms responsible for the found behaviors are proposed. The influence of temperature on the oxygen diffusion into the grains annihilating oxygen vacancies was detected. In particular, we found the transition from having bulk regions unaffected by surface phenomena to the overlapping of potential barriers due to an oxidizing gas.

Abstract: - This paper describes a fully integrated acoustic sensor that combines high sensitivity, wide frequency range and low cost of batch processed miniaturized silicon components. A sputtered piezoelectric ZnO layer transforms the mechanical deflection of ...

We report about the manufacturing of sensor devices for detection of volatile organic compounds (VOCs) in the air. The sensor comprises three fully inkjet-printed layers of (i) silver interdigitated electrodes (SIDE), (ii) multi-walled carbon nanotubes (MWCNTs) and (iii) poly (styrene-co-maleic acid) partial isobutyl/methyl mixed ester (PSE). We have found that inkjet printing of MWCNT layers and a PSE layer on top yields an interfacial layer leading to the formation of a MWCNTs/PSE composite-like structure. The printed layers act as chemiresistive vapor sensors due to the change of electrical resistance in presence of selected VOCs. The sensor responses to various VOCs were studied by varying the concentration of the chemicals in the air between 50 and 500 ppm. The sensing mechanism can be understood as a fractional volume change of the inkjet-printed polymer that occurs during exposure to VOCs, resulting in a structural alteration of the MWCNT-polymer network. We have found that the sensor response can be defined as a function of the VOC's concentration. To investigate the morphologies of the inkjet-printed thin films, profilometry and scanning electron microscopy (SEM) were employed.

We investigated the effect of the reaction parameters on the structure of multi-walled carbon nanotubes containing different concentrations of nitrogen and boron. The nanotubes were produced using a 'standard' aerosol chemical vapour deposition technique in conjunction with benzylamine, triethylborane, hexane and toluene mixtures. These precursors were thermally decomposed between 800 and 1100°C under argon at atmospheric pressure. By varying the precursor concentrations, the nitrogen and boron content of the nanotubes could be altered between 0-2.2 and 0-0.5 at.% respectively. Using a typical laboratory-sized 50 cm long tube furnace, yields between 0.3 and 1.5 g of nanotubes/10 min were relatively easily achieved. Moreover, we show that doping carbon nanotubes with heteroatoms, such as B and N, can be used to control nanotube diameters, change their defect density, and manipulate their oxidation resistance within a range of ca. 170°C.

The goal of this paper is to explore the possibility to detect and identify bacteria by sensing their odor via fluctuation-enhanced sensing with commercial Taguchi sensors. The fluctuations of the electrical resistance during exposure to different bacterial odors, Escherichia coli and anthrax-surrogate Bacillus subtilis, have been measured and analyzed. In the present study, the simplest method, the measurement and analysis of power density spectra was used. The sensors were run in the normal heated and the sampling-and-hold working modes, respectively. The results indicate that Taguchi sensors used in these fluctuation-enhanced modes are effective tools of bacterium detection and identification even when they are utilizing only the power density spectrum of the stochastic sensor signal.

Recent progress in the development of a ß-alumina gas sensor for automotive applications is reported. The sensing device consists of two solid electrolytes (namely a thin film of sodium sulfate deposited on ß-alumina by appropriate treatment in a gaseous atmosphere), with two electrodes having different catalytic properties, one made of platinum, the other of gold. The ß-alumina component was prepared

This work details the fabrication and performance of a sensor for ammonia gas analysis which has been constructed via the inkjet-printed deposition of polyaniline nanoparticle films. The conducting films were assembled on interdigitated electrode arrays and characterised with respect to their layer thickness and thermal properties. The sensor was further combined with heater foils for operation at a range of temperatures. When operated in a conductimetric mode, the sensor was shown to exhibit temperaturedependent analytical performance to ammonia detection. At room temperature, the sensor responded rapidly to ammonia (t 50 = 15 s). Sensor recovery time, response linearity and sensitivity were all significantly improved by operating the sensor at temperatures up to 80 • C. The sensor was found to have a stable logarithmic response to ammonia in the range of interest (1-100 ppm). The sensor was also insensitive to moisture in the range from 35 to 98% relative humidity. The response of the sensor to a range of common potential interferents was also studied.

Surface acoustic wave (SAW) delay line (DL) oscillators offer many advantages as gas sensors. Such devices are fabricated on a piezoelectric (PE) substrate, whose propagation path is coated with a selectively sorbing film. The sorption produces a change in the SAW phase velocity, which can be detected as a frequency shift of the SAW oscillator frequency. Very often these sensors consist of a double DL oscillator circuitry which cancels the undesirable influences of the environment, e.g., temperature, pressure and so on. In practice one of the DLs Serves as a reference. In this work we present the theoretical and experimental results obtained by many authors, concerning the SAW gas sensor response. determination for three types of overlying thin films: insulating, conducting and semiconducting. The theoretical and experimental results are compared and discussed.

Ultrahigh-sensitive WO 3 nanosensors have been developed by using an interdigitated Au nano-electrode for dilute NO 2 detection. The interdigitated Au nano-electrodes with a line spacing of 200 nm and various teeth numbers were fabricated on a SiO 2 /Si substrate by means of electron beam lithography. A WO 3 thin film was deposited on the interdigitated Au nano-electrode to be a WO 3 nanosensor. The nanosensor exhibited an extremely high sensor response (Rg/Ra = 28) to 0.01 ppm NO 2 with excellent response-recovery characteristics. Interestingly, the sensor response increased with increasing the number of teeth in the interdigitated nano-electrode, i.e., the length of interface between oxide grain and Au electrode. This suggests that the nano-design of electrode structure is important for development of a highly sensitive gas sensor.

A dynamic model of a metal-oxide semiconductor gas sensor is proposed and discussed as a tool for obtaining a substantial reduction of the time needed for sensor calibration. The sensor dynamics turn out to be well represented using a simple exponential model. The prediction of the steady-state value from the initial part of the transient permits a reduction of the

Results from gas probing with various analyte vapors on high-low-loss surface transverse wave (STW) and surface acoustic wave (SAW) resonators coated with thin plasma-polymer films of hexamethyldisiloxane (HMDSO), styrene, and allyl alcohol at different polymerization conditions are presented in this paper. At the same acoustic wavelength of 7.22 m and identical film thicknesses, HMDSO-coated STW devices feature substantially higher relative sensitivities to all analytes compared to their SAW counterparts. When operated in a microwave oscillator loop, plasma-polystyrene and allyl-alcohol-coated STW devices generate strong sensor signals, even at low analyte concentrations, retaining an oscillator short-term stability in the 1 10 9 /s to 1 10 8 /s range. A 250-kHz sensor signal with 7 10 9 /s stability was obtained from a styrene coated 700-MHz STW resonator oscillator at a 1400 parts per million concentration of xylene vapor, which results in a measurement resolution of less than 40 parts per billion for xylene in the ambient air. It is shown that, with respect to sensitivity and stability over long probing periods, plasma-polymer films may become a serious competitor to the more or less unstable soft polymer coatings currently used in SAW-based gas sensors for applications in wireless systems for environmental control and protection.

A gas sensor based on a b-alumina thick ®lm was recently developed on a laboratory scale. This sensor can be successfully used for selective detection of CO and NO x , resulting from an appropriate choice of the working temperature of the sensing element. This paper deals with the approach of the industrial transfer of the sensor prototype, mainly concerning the scale-up of the screen-printing procedure from a hand-operated apparatus to an industrial production machine. Many parameters were already investigated during the laboratory development, namely the ink composition in terms of organic/inorganic component ratio, the type and amount of the liquid dispersant and rheological agent, the composition and softening temperature of the binder, the balumina/binder weight ratio. During the industrial transfer, some other parameters must be considered, such as the particle size distribution of the ceramic components of the ink, the in¯uence of the mesh number of the screen fabric on the surface texture of the b-alumina ®lm, the viscosity performances of the ink as a function of the applied shear rate and ageing. A common commercial metallic paste was also used as reference for optimizing the characteristics of the b-alumina based ink.

Detection of low concentration of air pollution, like cigarette smoke, cooking fumes, etc. is possible with the combination of an air quality sensor and data acquisition system. In present paper is presented approach for design and implementation of air quality monitoring system based on tin dioxide gas sensor, integrated temperature and humidity sensors, portable modular data acquisition system and graphical

Ahxtsrmcr -The effect of ethanol vapour and temperature was investigatcd on gas sensors fabricated from poly(viny1 acetate)\carbon black composites based around a predetermined percolation threshold. Samples with 8% carbon black loading displayed the best response to the ethanol vapour. Typical response and recovery times of 140s and 45s respectively were recorded. In addition, bridge structures were fabricated, where all four resistive elements were prepared from the same composite material and in which a novel passivation process was employed. It was observed that these bridge stmchres were significantly less affected by variations in temperature in comparison to the single sensor structures.

An integrated catalytic combustion H 2 sensor has been fabricated by using MEMS technology. Both the sensing elements and the reference elements could be integrated into the suspended micro heaters connected in a suitable circuit such as a Wheatstone configuration with low power consumption. Two sensitive elements and two reference sensors were integrated together onto a single chip. The size of chip was 5.76 mm 2 and the catalytic combustion sensor showed high response to H 2 at operating voltage of 1 V. The response and recovery times to 1000 ppm H 2 were 0.36 s and 1.29 s, respectively.

An analytical solution for the one-dimensional steady-state transport of ions in an electrolyte towards a planar electrode is obtained. This electrolyte contains more than one electroactive species and any number of non-reacting species. The mass and charge transport equations give rise to an implicit form of a set of non-linear algebraic equations which must be solved numerically. The solution is generally applicable and can deal with any kind of overpotential relation at the electrode. The analytical solution is used to determine the mass and charge transport parameters for the Fe(CN)63-/4-redox couple in a KCI solution for two different electrolyte concentrations (0.03 M Fe(CN)63-/4-+ 1.0M KCI and 0.005 M Fe(CN) 3-/4-+ 0.2M KCi). The agreement between the experimental and analytical current densities is perfect for both electrolyte solutions under investigation. It is shown that, although an excess of supporting electrolyte is added, neglecting migration results in an under/overestimation of the diffusion coefficient of ferri-/ferrocyanide of 3 to 5% for the Fe(CN) 3-/4-/KC! ratios investigated. Furthermore, a mathematical background is given for the wide range of values found in the literature for the charge transfer coefficient and the rate constants of the ferri-/ferrocyanide redox system. The same approach can also be useful for other systems, as to parameter identification procedures.

Pulsed laser deposition (PLD) technique has been successfully used to not only deposit nanostructured tin dioxide (SnO 2 ) thin ®lms, on alumina substrates, but also to concomitantly achieve their in-situ doping by Pt metal catalyst. The deposition of PLD SnO 2 ®lms was investigated at a substrate deposition temperature of 3008C under both vacuum and an oxygen background pressure of 150 mTorr. While the crystalline structure and the composition of the PLD SnO 2 ®lms were systematically determined by means of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques, respectively, their morphology and nanostructure were investigated by scanning and transmission electron microscopies (SEM and TEM, respectively). The PLD ®lms deposited under oxygen consist of polycrystalline pure SnO 2 phase, are stoechiometric and exhibit a columnar growth morphology with an apparent inter-columnar porosity. In contrast, those deposited under vacuum consist of a mixture of both a-SnO and poly-SnO 2 phases. TEM observations have revealed that the SnO 2 columns (of few 10 s nm diameter) that constitute the ®lms are, in turn, composed of almost spherical nanograins. On the other hand, the newly developed in-situ doping approach has been shown not only to provide uniform Pt-doping all through the ®lm thickness, but also to be effective in fabricating high-performance gas sensors. A Pt-doping level of $2 at.% was identi®ed as the optimal concentration that leads to the highest sensitivity at an operating temperature of the sensors as low as 2008C. The sensing performances of the developed SnO 2 sensors for detecting CO in synthetic air background, over the (10±500 ppm) CO concentration range, are presented.

In order to enable further developments in low cost wireless sensor networks (WSNs) for unobtrusive environmental monitoring, increased miniaturisation and integration of hardware is essential. This paper outlines the design concept and preliminary results for a multifunctional micromachined sensor unit, comprising CMOS-compatible temperature, humidity and gas sensors on a single silicon substrate. The sensor is being developed for integration with the highly modular and programmable Tyndall25 mote, where the ''plug and play" stackable layers incorporate communications, power supply and onboard data processing capabilities. This will enable the assembly of a miniature sensor network node complete with wireless transmission capability as well as intelligent data mining and storage.

This work present a new approach to design and fabrication of AlGaN/GaN based SAW-HEMT structures to be applied for chemical gas sensors operating in harsh environment. A direct on-chip integrated compatibility in the process technology of surface acoustic wave (SAW) structure and high electron mobility transistor (HEMT) was demonstrated. It enabled to integrate two different principles of gas sensing based on both SAW and HEMT. The functionality of two types of SAW structures and HEMT device was investigated. The measured amplitude characteristics of SAW structures revealed excitation at the centre peak frequency about 1.35 GHz and 1.7 GHz, respectively with a both good quality factor and stop-band rejection ratio. The detection electronic based on SAW oscillator was subsequently designed and simulated. It proved the expected function and properties.

Few decades, it is very difficult to Monitoring and eradicating the problem of sewage outlet system. Most of the cities and towns the clog in the sewage system will collapse to an overflow in streets. It is very dangerous for affecting respiratory and causes severe diseases. In current survey proves that hundreds of people died due to asphyxiation in India every year. The aim of present project is to surveillance and blot out the clog in the sewage system with minimum human effort. It is upper most important for this present situation. The IoT using system suggested here operates on a series of sensors and driven circuits. Gas sensor to find out level of gas formed and leakage. Ultrasonic sensor is to identify the blockage in the pipe. When the blockage is detected, immediately it will alert both server and client through IoT. Once the notification is received to the server/client, immediately the clog is rectified / cleared before the flow of water. The various kinds of detectors, sensors and driver circuits are fixed in different sections in the drainage network using Arduino Uno ATmega328P Chip. The door magnetic sensor activates the corresponding driver circuit and automatically closes the door using sliding door mechanisms. This type of sewage system is very low cost and easy maintenance. IoT based real-time when alerts the monitoring station through message, mail or any way when any manhole crosses its threshold values. This system reduces the death risk of manual scavengers who clean the underground drainage and immediately rectify the damage part. Therefore this project provides a smarter sewage management system and fills the gap between current and further research in various disciplines within the field and suggests more operational implementations in future.

Nanostructured anatase TiO 2 was produced by flame spray pyrolysis (FSP) and tested for sensing of volatile organic compounds and CO at 500 • C. The as-prepared powders were characterized by transmission/scanning electron microscopy, X-ray diffraction and nitrogen adsorption. Titania films about 30 m thick on alumina substrates interdigitated with gold electrodes were prepared by drop-coating a heptanol suspension of these powders. The films showed a high signal of n-type sensor to isoprene, acetone and ethanol at concentrations ranging from 1 to 75 ppm in dry N 2 /O 2 at 500 • C. The response (within seconds) and recovery (within minutes) times were very fast. Heat-treatment at 900 • C caused a nearly complete anatase to rutile transformation and a transition to p-type sensing behavior. That resulted in a poor sensor signal to all hydrocarbons tested and considerably longer recovery times than that of the anatase sensor. That rutile sensor could detect CO that the original, anatase sensor could not. For ethanol the sensor response changed back to n-type.

Air pollution is a serious problem in thickly populated and industrialized areas in Thailand, especially in Bangkok. The air pollution in Bangkok is abundant, especially in areas where pollution sources and the human population are concentrated. Economic growth and industrialization are proceeding at a rapid pace, accompanied by increasing emissions of air polluting sources. Furthermore, though the variety and quantities of polluting sources have increased dramatically, the development of a suitable method for monitoring the pollution causing sources has not followed at the same pace. Environmental impacts of air pollutants have impact on public health, vegetation, material deterioration etc. To prevent or minimize the damage caused by atmospheric pollution, suitable monitoring systems are urgently needed that can rapidly and reliably detect and quantify polluting sources for monitoring by regulating authorities in order to prevent further deterioration of the current pollution levels. Consequently, it is important that the current real-time air quality monitoring system, controlled by the Pollution Control Department (PCD), should be adapted or extended to aid in alleviating this problem.

The continuous evolution of nanotechnology in these years led to the production of quasi-one dimensional (Q1D) structures in a variety of morphologies such as nanowires, core-shell nanowires, nanotubes, nanobelts, hierarchical structures, nanorods, nanorings. In particular, metal oxides (MOX) are attracting an increasing interest for both fundamental and applied science. MOX Q1D are crystalline structures with well-defined chemical composition, surface terminations, free from dislocation and other extended defects. In addition, nanowires may exhibit physical properties which are significantly different from their coarse-grained polycrystalline counterpart because of their nanosized dimensions. Surface effects dominate due to the increase of their specific surface, which leads to the enhancement of the surface related properties, such as catalytic activity or surface adsorption: key properties for superior chemical sensors production.

ZnO nanocrystals (2.5-4.5 nm) were prepared by a wet chemical method based on alkaline-activated hydrolysis and condensation of zinc acetate solutions. Dropcasting of the nanocrystals onto alumina substrates allowed the fabrication of gas sensing devices, that were tested towards NO 2 , acetone and methanol and showed promising results. At low working temperature, the ZnO quantum dots based sensors are selective to nitrogen oxide, in fact a good sensitivity is shown at 200 • C at low concentration (2 ppm), while at temperature above 350 • C, high responses are obtained for acetone and methanol. The results obtained are stimulating for further developing of nano-ZnO based sensor devices.

The development of carbon nanotube-(CNTs-)based gas sensors and sensor arrays has attracted intensive research interest in the last several years because of their potential for the selective and rapid detection of various gaseous species by novel nanostructures integrated in miniature and low-power consuming electronics. Chemiresistors and chemical field effect transistors are probably the most promising types of gas nanosensors. In these sensors, the electrical properties of nanostructures are dramatically changed when exposed to the target gas analytes. In this review, recent progress on the development of different types of CNT-based nanosensors is summarized. The focus was placed on the means used by various researchers to improve the sensing performance (sensitivity, selectivity and response time) through the rational functionalization of CNTs with different methods (covalent and non-covalent) and with different materials (polymers and metals).

Zinc oxide (ZnO), tin dioxide (SnO 2 ) and compounds ZnO/SnO 2 (ZTO) nanostructures have been synthesized successfully from the vapor phase without a catalyst using three different approaches. XRD analyses showed that ZnO with a wurtzite crystal structure, SnO 2 with a rutile crystal structure and zinc stannate (ZnSnO 3 ) and/or dizinc stannate (Zn 2 SnO 4 ) were condensed from the vapor phase when Zn and/or Sn metal powders or their oxides individually or mixed were used as the starting materials. The formation of either zinc or dizinc stannate was controlled by the Zn/Sn ratio and growth technique. SEM and TEM investigations showed that ZnO grew mainly in the form of wires, rods and belts. These are believed to be originated from the common tetrapod structure of ZnO. While SnO 2 grew in the form of tetragonal rods with rectangle-like cross section and nanoparticles, ZTO grew in the form of nanobelts. The final length, width and thickness were as low as 40, 10 and 5 nm, respectively. The driving forces for growth of nanowires, nanorods, nanobelts, and nanoparticles were found to be vapor density or supersaturation, temperature, pressure and location of deposition from the source materials. The optical absorbance and photoluminescence spectra of all samples showed excitonic character at room temperature implying good crystal quality, and high photocurrent properties suggesting possible applications in nanoscaled functional devices such as optoelectronics and gas sensors.

This paper gives an overview about the principles and the technologies used in solid-state gas sensors. These devices work by measuring a physical property changed by adsorption/desorption processes and chemical reactions on the surface of a sensing element, i.e. a solid-state film of a gas-sensitive material. Some of the most used types of solid state gas sensors are here described together with novel sensor technologies in development for commercial exploitation in the future.

Two recent topics important for advancing gas sensor technology are introduced. Semiconductor gas sensors have been developed so far on empirical bases but now a fundamental theory has been made available for further developments. The theory reveals the roles of physical properties of semiconductors and chemical properties of gases in the receptor function. MEMS techniques have been applied to fabrication of micro-platforms for use in gas sensors. The micro-platforms appear to provide gas sensors with new innovative function.

A detailed overview towards the advancement of graphene based biosensors has been reviewed. The large surface area and excellent electrical conductivity of graphene allow it to act as an "electron wire" between the redox centers of an enzyme or protein and an electrode's surface. Rapid electron transfer facilitates accurate and selective detection of biomolecules. This review discusses the application of graphene for the detection of glucose, Cyt-c, NADH, Hb, cholesterol, AA, UA, DA, and H 2 O 2 . GO and RGO have been used for the fabrication of heavy metal ion sensors, gas sensors, and DNA sensors. Graphene based FETs have also been discussed in details. In all these cases, the biosensors performed well with low working potentials, high sensitivities, low detection limits, and long-term stabilities.