Advanced functional materials for environmental monitoring and applications

Resume : This is a overview of the COST Action TD1105 EuNetAir - European Network on New Sensing Technologies for Air-Pollution Control and Environmental Sustainability - funded in the COST framework. The objective of the Concerted Action (2012-16) is to develop new sensing technologies for Air Quality Control at multidisciplinary scale by coordinated research on nanomaterials, sensor-systems, air-quality modelling and standardised methods to support environmental sustainability with special focus on SMEs. This international Networking, coordinated by ENEA (Italy), includes over 80 institutions and over 170 international experts from 28 COST Countries (Austria, Belgium, Bulgaria, Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Israel, Italy, Latvia, The Former Yugoslav Republic of Macedonia, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovenia, Spain, Sweden, Switzerland, Turkey, United Kingdom) and 7 Non-COST Countries (Australia, Canada, China, Morocco, Russia, Ukraine, USA) to create a S&T critical mass in the environmental issues. This COST Action focuses on a new detection paradigm based on sensing technologies at low cost for Air Quality Control and set up an interdisciplinary top-level network to define innovative approaches in sensor nanomaterials, gas sensors, devices, wireless sensor-systems, distributed computing, methods, models, standards and protocols for environmental sustainability within the European Research Area.

Resume : Low power consumption and high specificity are two of the most sought after requirements for future gas sensors technologies. In this contribution, new device concepts capable to detect gases without the need of external power sources to activate the sensor-gas interaction or to generate a read out signal will be described [1]. It will be demonstrated that sensors featuring zero power consumption can be achieved by integrating the sensing components and the energy sources intimately, at the nanoscale. On the other hand, a rational methodology to design organic surface functionalizations that provide high selectivity towards single gas species will also be discussed [2]. Specifically, theoretical results, confirmed experimentally, indicate that precisely tuning the sterical and electronic structure of the sensor material and the organic functionalization can lead to unprecedented selectivity values, comparable to those typical of bioselective processes. Finally, the first attempts to combine both strategies in one single, solid-state, cost-effective, integrated device will also be presented. [1] M. W. G. Hoffmann, A. E. Gad, J. D. Prades, F. Hernandez-Ramirez, R. Fiz, H. Shen, S. Mathur, Nano Energy 2013, 2, 514?522. [2] M. W. G. Hoffmann, J. D. Prades, L. Mayrhofer, F. Hernandez-Ramirez, T. T. J?rvi, M. Moseler, A. Waag, H. Shen, Adv. Funct. Mater. 2013, DOI: 10.1002/adfm.201301478

Resume : The employment of metal oxide nanowires is a very powerful strategy to push the performance of gas sensing devices. Nanowires have a high surface to volume ratio and a strong interaction with the surrounding gas. Nanowire based sensors thus provide very sensitive gas detection and have the advantage of improved stability due to high crystallinity. However, efficient and reliable nanowire integration with standard CMOS technology remains a major challenge. We present gas sensor devices employing metal oxide nanowires integrated on micro hotplates devices, which are fabricated in standard 0.35µm CMOS technology combined with a MEMS etching process. Three different nanowire materials are employed for realization of the gas sensor devices: SnO2, CuO, and ZnO. The specific processes developed for the CuO and ZnO nanowires enable their direct synthesis on the CMOS micro hotplates; the SnO2 nanowires, however, require a transfer process from a specific synthesis wafer to the CMOS micro hotplates. The presented technologies can be used in a CMOS backend process and enable the fabrication of fully silicon integrated SnO2, CuO and ZnO nanowire gas sensing devices. The gas sensing performance of the nanowire based gas sensor devices to a variety of gases (CO, CO2, VOCs, H2, H2S) relevant for environmental monitoring will be presented. The approach towards a fully CMOS integrated multi-parameter gas sensing device will be discussed.

Resume : In the present study, Au-surfactant core-shell colloidal nanoparticles (Au NPs) with controlled dimension and composition were synthesized by sacrificial anode electrolysis. Transmission electron microscopy revealed that Au NPs core diameter is comprised between 8-12 nm, as a function of the electrosynthesis conditions; UV-vis spectroscopy showed that size effects affect position and width of the surface plasmon resonance peak. Moreover, surface spectroscopic characterization by XPS analysis confirmed the presence of nanosized gold phase. Controlled amounts of Au NPs were then deposited electrophoretically on CNTs networked films. The resulting hybrid materials were morphologically and chemically characterized using TEM, SEM and XPS analyses, which revealed the presence of nanoscale gold, and its successful deposition/decoration on CNTs. Au NPs/CNTs networked films were tested as active layers in a two-pole resistive NO2 sensor for sub-ppm detection. Au-NPs/CNTs exhibited a p-type response with a decrease in electrical resistance upon exposure to oxidizing NO2 gas and an increase in resistance upon exposure to reducing gases (e.g., CO, SO2). It was also demonstrated that the sensitivity of Au NP/CNTs-based sensors depends on Au-loading; therefore the impact of Au loading on gas sensing performance was investigated as a function of the working temperature, gas concentration and interfering gases.

Resume : The exponential development of nanomaterials opened the way to a new generation of devices with enhanced performances and new application fields. Among these new materials, nanocarbons are extensively investigated for the elaboration of electronic components like transistors, LEDs, electron sources and gas sensors. Since nanocarbonaceous materials offers high affinity with gases as exploited into sensing devices, we propose as complementary approach to shape nanocarbons as chemical filter for air cleaning. A large panel of pristine nanocarbons has been firstly studied: single and multi-wall nanotubes, nanowires, nanodiscs, activated carbons. Their filtering power, the interaction mechanisms involved with the target gases (O3, NO2, CO) as well as the physical and chemical properties of nanocarbons responsible for their filtering ability have been determined both from gas exposures and characterizations (EPR, Raman spectroscopy, NEXAFS). The relevance of original indigo/nanocarbons hybrid materials as O3 filter has been established too. The non-covalent functionalization of nanocarbonaceous materials by two wet chemical routes has been monitored by SEM, XRD and TGA. Performances like filtering efficiency, selectivity and sustainability have been especially assessed. As an application, we will discuss on the successful development of selective NO2 sensor-systems associating such a filter to phthalocyanine-based chemoresistor for the NO2 monitoring in ppb-range in air.

Resume : The Gate All Around GAA MOSFETs have emerged as excellent devices to provide the electrostatic integrity needed to scale down transistors to minimal channel lengths, and allowing a continuous progress in digital and analog applications. Employing this design for chemical and environment monitoring applications becomes more beneficial if the device is made in vertical cylindrical recrystallized silicon due to highly flexible process integration options. In this paper, a numerical investigation of a new pH-ISFET design, called the Junctionless Gate All Around Ion-sensitive field-effect transistor with oxide kernel (pH-JGAAISFET), is proposed, investigated and expected to improve the fabrication process and the sensitivity behavior for pH-ISFET sensor-based applications. The numerical investigation has been used to propose a new sensitivity performance and predict the device behavior. The comparison of device architectures shows that the proposed sensor exhibits a superior performance with respect to the conventional ISFET in term of electrical performances. The obtained results make the proposed sensor a promising candidate for low cost monitoring and high performance pH sensing applications.

Resume : The unique properties of SnO2 and WO3 nanocrystals which expose specific surfaces as chemical sensors have been recently reported. However, a clear assessment of the relative involvement of the crystal faces in the sensing mechanism is still under debate. To clarify this issue, we have carefully investigated the sensing behavior of SnO2 shape controlled nanoparticles in connection to the generation and the reactivity of their oxygen defects (VO•), detected by ESR, associating their abundance and reactivity to the exposed crystal faces and to the electrical response of the nanocrystals. Results indicate that the presence of the {221} and {111} surfaces enhances the generation of the VO• centers and boosts the sensing performance toward CO. A similar investigation was carried out on WO3 nanocrystals with tailored morphology and definite prominent surfaces. Their sensing performances toward NH3 were evaluated and, in order to deepen the role of the surfaces in the sensing mechanism, a comprehensive XPS investigation on the surface nitrogen species was also performed. It turned out that {020} and {002} high-energy surfaces represent privileged reactive sites for the NH3 oxidation and therefore are essential for enhancing the sensing properties. These outcomes may help to a more effective evaluation of the involvement of the crystal surfaces in the sensing mechanism and may shed some light on how crystal facet engineering can be better applied to design more efficient gas sensors.

Resume : Ammonia and amines can originate from both various human initiated sources including animal husbandry, industry and combustion, composting operation, automobiles, cooking, tobacco smoke, and natural sources as biomass burning, and biodegradation of organic matter that contain proteins or amino acids. The adverse health effects of amines and ammonia are demonstrated and it is important to detect them for health and environment protection. Due to base properties of ammonia and amines, conducting polyaniline (PANI) can be used to create inexpensive gas sensors with a very low quantification limit and relatively short response time. The principle of measurement is based on the electrical variation of the resistance of an active layer submitted to the gas. Following this principle, we developed a new low cost organic electronic sensor with PANI composites to detect ammonia and trimethylamine at room temperature. Two types of nanocomposite materials were synthesized and optimized for gas detection at ppb levels. First a PANI doped with a sulfonic acid/ polyurethane composite was studied for ammonia detection. It was demonstrated that the percolation phenomena and the rate of doping can be used to enhance the response of the sensors at ppb levels. Second PANI nanofibers synthesized by microwave treatment were used for trimethylamine detection. It was shown that the use of percolation phenomena added to this particular nanostructure can make sensors with large response at ppb levels. In both cases, it is demonstrated that nanostructured conducting polymers are suitable for new generation of nanosensors for air quality control.

Resume : After the years of intense investigations, nanomaterials have entered the phase of commercial applications in medicine as drug carriers or labels contrast agents for magnetic resonance imaging, and convenient tool for biodetection. Prominent example is the use of nanoparticles in combination with fluorescent labels for large number of biochemical tests. During last decade a new class of biological sensors emerged, which relies on the use of nanomaterials i.e. nanoparticles or nanowires, for detection of biological molecules or of products of biologically catalyzed reactions. Within this novel group of the devices, silicon nanowire based field effect transistors represent a promising route for label-free detection of biochemical species. Here we present and fully characterize the biosensor platform based on NiSi2-Si-NiSi2 nanowire heterostructures. Such devices require integration of Schottky-barriers (SB) to provide the change in contact resistance, which can be further used for sensing. We focus on the following aspects of the biosensor functioning and characterization: (i) fabrication and characterization of the SB-FET nanodevices; (ii) biochemical functionalization of the silicon nanowire sensors using aptamers, also known as “artificial antibodies”; (iii) demonstrate sensing capabilities of the developed devices.

Resume : Hybrid materials combining polypyrrole (PPy) with ionic macrocycles as counterions have been electrosynthesized at the surface of platinum interdigited electrodes. The obtained films revealed to be more homogeneous compared to PPy synthesized with small counterions, with a strongly smaller roughness. The films exhibited a higher sensitivity to ammonia (NH3), with a very good reversibility and with a stable response in a broad range of relative humidity (20-80 % RH). Overall, we present the advantages of combining ionic phthalocyanines (Pc) with PPy in enhancing the properties of the final material. Synergetic effects in the structure and properties of PPy-Pc materials open the way to their development in the field of hybrid material and their use for chemosensing. At a given RH value, the current baseline is very stable, the response to NH3 is highly reproducible, and proportional to the NH3 concentration. As a function of the RH, the relative response varies differently depending on the NH3 concentration. It increases, at 25 ppm, but increases then decreases at 45 and 90 ppm. However, at these NH3 levels, discrimination still occurs between the different concentrations. This result is very important, since it means that, whatever the RH in the range 20-80%, the device is capable to discriminate between these different NH3 concentrations.

Resume : Abstract The reduction of graphene oxide (GO) has been a subject of intense efforts in the last decade. Many efforts have been devoted to the preparation of chemically modified GO and graphene composites aiming at a better processability of graphene and modulating at wish its electronic, optical properties and electrochemical properties.1 Functionalization of graphene enables indeed this material to be processed by solvent-assisted techniques and prevents its agglomeration and thus maintaining the inherent properties of graphene crucially important for their end application. Reduced graphene oxide (rGO) is prepared through the reduction of GO using different means. The technique consists of the initial oxidation of graphite to graphite oxide, followed by the subsequent mechanical/chemical or thermal exfoliation of graphite oxide to graphene oxide (GO) sheets, and their reduction to graphene.2 Various approaches have been investigated for the reduction or reduction/functionalization of GO using chemical, electrochemical, thermal or photochemical means.2 Hydrazine monohydrate is still the most widely used reductant, mainly due to its strong reduction activity to eliminate most oxygen-containing functional groups of GO and its ability to yield stable rGO aqueous dispersions. However, with hydrazine as the reducing agent, its residual trace may strongly decrease the performance of rGO-based devices. In addition, hydrazine is a highly toxic and potentially explosive chemical. To avoid using hydrazine, many environmentally friendly and high-efficient reductants have been developed and used for the reduction of GO, including vitamin C, amino acids, reducing sugars, alcohols, hydroiodic acid, reducing metal powder, sodium citrate, tea, lysozyme, dopamine, etc.3 Herein, we report on simultaneous reduction/functionalization of graphene oxide (GO) using environmentally friendly reagent, arginine (Arg). This one-step reaction consists of a simple reaction of GO with Arg at 100°C for 4 h. Furthermore, this approach was successfully applied for the synthesis of rGO decorated with metal nanoparticles. The resulting materials have been characterized using UV-vis spectrometry, FTIR spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electrochemical measurements. Finally, the catalytic activity of rGO/metal nanoparticles has been evaluated for the reduction of nitrophenol to aminophenol.

Resume : Graphene-based gas sensors normally show ultra-high sensitivity to certain gas molecules while suffering from poor selectivity and slow response and recovery. Graphene functionalization can improve these issues but usually results in poor reproducibility. We report on surface modifications of epitaxial graphene on SiC with metal or metal-oxide nanostructures, formed by highly reproducible thin film deposition techniques, and their effect on the electronic properties of the graphene and on gas interactions at the graphene surface. Under the right decoration conditions the electronic properties of the surface remain those of graphene, while the surface chemistry can be modified to improve sensitivity, selectivity and speed of response toward e.g. nitrogen dioxide (NO2) or certain toxic volatile organic compounds (VOCs). Sensors with extremely low detection limits, selective towards these gases, can be useful for air quality control due to the high toxicity already at parts per billion (ppb) concentrations. The effect of decoration strongly depends on the choice, thickness and nanostructure of the material. Decoration with thin nano-porous Au allows reaching a NO2 detection limit below 1 ppb. Preliminary results indicate that decoration with TiO2 quantum dots can be used to selectively detect ppb concentrations of certain toxic VOCs. The presentation will include early results on other monolayer modifications of gas sensing layers like iron oxide on top of platinum.

Resume : In the last years, the ozone gas sensors have been the subject of great attention for monitoring and determination of ozone gas concentration due to its high toxicity and effects to human health. Therefore the determination and continuous monitoring of ozone concentration has taken special interest in the last years due to its high toxicity and effects to human health. The zinc oxide (ZnO) compound is one of the most promising metal oxide for gas sensing applications due to the well-known high surface-to-volume and surface conductivity . Additionally, ZnO compound has been proved to be an excellent gas-sensing material for several gases, such as CO, NOx, ozone and ethanol. In this context, specials ZnO microstructures with different morphologies and high surface/volume ratio can overcome the limitations of the current commercial sensors. Motivated by these considerations, the goal of this study consisted to synthesize ZnO nanorods directly onto a SiO2/Si substrate with interdigitated Pt electrodes via hydrothermal method and evaluated its potential use as ozone gas sensor. In this study, we investigated the ozone sensing properties of as-prepared ZnO nanorods-like structures grown directly onto a substrate containing electrodes via hydrothermal method. The sensor response was studied for ozone concentration ranges from 0.08 ppm to 1 ppm at different operating temperatures. The ZnO nanorods displayed a short response time of 25 s and a recovery time of 88 s when exposed to 1 ppm of ozone at 250°C. Our results indicate that ZnO nanorods show a good sensitivity to the presence of ozone, even when exposed to a low concentration (0.08 ppm).

Resume : Conducting polymers such as polypyrrole (PPy), polyaniline (Pani), Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) and their derivatives are very interesting materials because they combine tunable electrical transport characteristics and excellent mechanical properties. In particular, conducting polymers have been used as the active parts of gas sensors since early 1980s [1]. In comparison with most of the commercially available sensors, based usually on metal oxides and operated at high temperatures, the conducting polymer sensors have many improved characteristics. They have high sensitivities and short response time; especially, these feathers are ensured at room temperature. In this study, we fabricated a sensitive wire transducer in a chemiresistive gas sensor by a three-dimensional (3D) growth of PEDOT:PSS wires with arched shape. The wires were grown by controlled pulling of a micropipette (?Fountain-pen?) filled with a PEDOT:PSS solution (?Ink?) [2,3]. When the fountain-pen touched the substrate, a meniscus of the ink was created outside its opening and the solvent was evaporated at the same time. As the fountain-pen was pulled away, the meniscus was stretched and its cross section decreases, resulting in formation of wires. The individual positioning of grown wires was easily available by the 3D accurate control of fountain-pen. The as-prepared suspended PEDOT:PSS sensor which consisted of 10 wire array was tested for their ability to sense water, ethanol and acetone vapor gases at the room temperature. The sensor showed high sensitive, rapid, stable and reversible responses for detection of various concentrations of vapor gases, resulting from high surface area to volume ratio of the suspended wire. KEYWORDS Conducting polymer nanowire, Gas sensor, 3D suspended wire, Fountain-Pen lithography, Direct writing REFERENCES [1] Nylabder, C.; Armgrath, M.; Lundstrom, I., Proceedings of the International Meeting on Chemical Sensors, Fukuoka, Japan, 203 (1983). [2] Ji Tae Kim, Seung Kwon Seol, Jaeyeon Pyo, Ji San Lee, Jung Ho Je and G. Margaritondo, Advanced Materials 23 1968 (2011). [3] Seung Kwon Seol, Daeho Kim, Sunshin Jung and Won Suk Chang, RSC advances 2 8926 (2012).

Resume : Increasingly restrictive limitations of pollutants require the development of more accurate detection devices. Although there are already a large number of sensors from mature technologies currently on sale, the detection of some gases remains problematic (limit of detection, time response, repeatability, cross-sensitivity, ...). We present here the main results of the PEPS (PEllet Photonic Sensor) project, aiming at developing a new transducer via a technological breakthrough : the combination of photonics (insensitive to external electromagnetic disturbances and deportation of the measurement fiber optic components) and catalysis (reversibility, limited energy consumption). Indeed, catalytic reaction are often exothermic and this heat of reaction can modify the properties of optical devices. The experimental studies performed during the PEPS project highlighted the rapid and reversible response at room temperature of catalytic powders towards different concentrations of CO and H2. The thermal and optical properties of the materials used for the photonic component have also been studied. These results have been exploited for the design of the photonic transducers. The feasibility of the physical transduction principle has been demonstrated by developing different prototypes based on Bragg gratings and Multimode Interference Couplers.

Resume : Semiconductor ZnO nanowires(NWs) were prepared using the electrodeposition method in a polycarbonate template. By ultrasonication in alcohol the grown NWs were harvested and drop-casted on a SiO2/Si substrate. The optical, morphological and structural properties of the NWs were investigated. Photolithography and e-beam lithography were used for contacting single NWs. When Ti/Au metallic contacts were deposited on the NWs, ohmic electrical characteristics were obtained. Thus an appropriate framework was created for processing back gate field effect transistors, having as gate the N++ heavily doped Si substrate. The electrical measurements on the ZnO NW transistors have shown a clear increase of the source drain current with the voltage applied on the gate electrode. The parameters of the transistor were calculated and were highly improved when the surface of the NW was passivated with PMMA and AZ5214E. In this case the source drain I-V characteristics were saturating at higher applied voltages and the Ion/Ioff ratio was in the range of 10000 having a field effect mobility of about 100 cm2/(V•s). The characteristic parameters are among the highest reported in literature for such devices even when compared with NWs which were prepared by other methods, e.g. VLS or CVD. These highly sensitive, electrodeposited ZnO NW field effect transistors are excellent candidates for various sensor applications including here chemical or bio – molecules detection.

Resume : In swimming pools, chlorine (Cl2) is used as a disinfectant to minimize the risk to users from microbial contaminants. In water, Cl2 is transformed into HOCl which reacts with saliva, sweat, urine and skin, leading to the formation of several chloramines, such as monochloramine (NH2Cl), dichloramine (NHCl2) and nitrogen trichloride (NCl3). Because of its low solubility in water, NCl3 is essentially found in the air while NH2Cl preferentially remains in water. These compounds are toxic and their detection has become of great importance. However, there is currently no fast and selective method of measurement of NCl3 in the atmosphere nor of NH2Cl in water. The development of innovative chemical and colorimetric sensors for the direct detection of NCl3 and NH2Cl, is described. They are based on solid nanoporous materials synthesized via the sol-gel process. These materials display high adsorptive properties and the pores are tailored to become efficient nanoreactors. These nanoreactors, are designed to enhance a specific reaction between a probe molecule and NCl3 or NH2Cl, leading to coloured products. With matrices doped with NaI and Amylose, an NCl3 sensor is produced which can detect NCl3 at ppb level within 20 minutes in humid atmospheres (RH : 60-80 %) at ambient pool temperatures. The NCl3 concentration range covered is 5 to 250 ppb. For the detection of NH2Cl, the Berthelot reaction is exploited with the formation of a final blue-green product absorbing in the visible with a maximum centered at 640 nm. The NH2Cl sensor can measure NH2Cl in water with a detection limit of 0.1 µM (1.8 ppb). We will show the comparison of the sensors performance to the currently used, but indirect, methods during campaigns of measurements in swimming pools.

Resume : We have investigated hydrogen sensing of heavily Ga-doped ZnO (GZO) polycrystalline films. We deposited 50-nm-thick GZO films on glass substrates (@200 ºC) by ion-plating with dc arc discharge. A sintered ZnO tablet with a Ga2O3 content of 3 wt.% was used as a deposition source. Hall effect measurement results showed electrical resistivity of 2.36×10-3 Ωcm, carrier concentration of 1.89 ×1020 cm-3 and Hall mobility of 14.0 cm2/Vs. Analysis of data obtained by XRD measurements shows that the films consist of wurtzite structure with a preferred c-axis orientation perpendicular to the substrates. The gas sensing properties toward hydrogen gas (the concentration in air of 1 volume%) strongly depended on operating temperature. We found as follows: (1) a change in electric current of +0.1 to +0.2 mA at 150 ºC; (2) a change in electric current of -5.0 mA at 330 ºC. The findings imply that the gas sensing mechanism at the high operating temperature is significantly different from that at the low operating temperature. We will propose that the mechanism at the low temperature is limited by chemical reaction between hydrogen gas and oxygen atoms with a negative charge in the grain bulk, generating free carriers to increase carrier concentration, whereas a factor limiting the mechanism at the high temperature is concentration of hydrogen molecules physically adsorbed at the grain boundary, enhancing a barrier height to carriers at the grain boundaries to decrease carrier mobility.

Resume : ZnO is a fluorescence material characterized by one emission line in the near UV range of the spectrum associated with excitonic decay processes and a broad emission in the visible linked with electronic defects in the gap. It is recognized that the relative ratio between the excitonic and defect emissions depends on the crystallization and agglomeration states of ZnO. In this work we report the synthesis of ZnO nanocolumnar porous thin films by plasma enhanced chemical vapor deposition1 that, despite of their relatively porous character, only presenting an intense fluorescence emission at room temperature. It is found that the emission intensity is quite sensitive to the presence of oxygen in the medium and its evolution as a function of the oxygen concentration can be utilized for the photonic sensing of this gas in the environment. In addition, the system is also able to respond to the oxygen concentration dissolved in water and other liquid medium where the sensitivity of detection can be one or two order of magnitudes higher than that of commercial detectors based on electrochemical methods. The mechanism of detection and the development of a new generation of photonic sensors of oxygen based in pure ZnO are discussed. 1.- P. Romero-Gómez, J. Toudert, J.R. Sánchez-Valencia, A. Borrás, A. Barranco, A.R. González-Elipe. Journal of Physical Chemistry C 114, 20932–20940 (2010).

Resume : UV photodetectors are used in many applications including biological and chemical analysis, flame sensing and astronomical studies. UV can be damaging to humans and many materials, but also can be used in air, water and food purification; therefore UV monitoring can be used to assess potential harm or to control dosing. UV photodetectors typically require an external bias, but for self-sufficient sensors systems power sources such as batteries add considerably to their size and weight, and integration of sensors into multi-component nanosystems will be facilitated by self-powered functionality. We present a self-powered UV detector using a p-n heterostructure of CuSCN and ZnO nanorods produced using low temperature solution methods. At a nominal zero-applied field, the device produces a photocurrent response of 4.5 µA for a low UV (375nm) irradiance of 6.0 mW/cm2. A fast 500 ns rise and 6.7 µs decay time was recorded with a UV/visible rejection ratio of ~100. With a small applied bias of +0.1 mV a rapid detection time of 4 ns was possible. For comparison to similar heterostructures a responsivity of 9.5 A/W was measured at -5 V applied bias. The photodetector performance was attributed to the pre-existing Fermi-level alignment of the ZnO and CuSCN semiconductors, which resulted in a low turn-on voltage of ~0 V and photovoltaic behaviour. Therefore the ZnO/CuSCN UV photodetector is suitable for nanoscale applications that require rapid response times and self-sufficiency.

Resume : Nanowires (NWs) have emerged as an important topic of research due to their high aspect ratio, and their capability to be part of new device architectures. Especially monocrystalline materials are important to achieve effective and known interactions of their surface in several applications, like gas sensors. An imporante issue that prevents the use of the nanostructures in functional electronic devices is their integration into a scalable process. Here we present a method for localized site selective direct growth of inorganic NWs that avoids several technological steps, reducing uncertainty factors of the process fabrication. VLS growth of SnO2 NWs is developed on top of CMOS compatible micromembranes with top interdigitated electrodes and buried integrated heater, which provides the thermal energy required for the growth. The SnO2 NWs show high crystalline quality and once the growth is finished, no post-processing is required because of NW bundles bridge the interdigitated contacts on top of the membrane. Thus, a scalable and time efficient process has been developed. The one-step fabrication device has been proven as a low power gas sensor, showing high response levels and low response times towards CO, NH3 and NO2 in a synthetic air atmosphere. The obtained devices will be shown as suitable for environmental gas sensing monitoring. The fabrication methodology will be described and its extension to the growth of other materials will be critically discussed.

Resume : Semiconducting nanowires (NW) are very attractive materials for gas sensing applications due to their high aspect ratio, that results in strong interactions between their surface and the surrounding atmosphere, giving rise to important electrical changes in the nanomaterial. Heating the NW is required to reach the right operation temperature. When operating with one single NW, the same current that is used to measure the resistance can be used to provide the NW heating, dramatically reducing the power consumption of the device. Here we present a single metal oxide nanowire gas sensor with ultra low power consumption based on this principle. Defect-free monocrystalline SnO2 nanowires were synthesized by chemical vapor deposition of a molecular precursor [Sn(OtBu)4]. NWs were dispersed onto a substrate with prepatterned electrodes and a Focused Ion Beam machine equipped with a Pt precursor was used to contact the individual NW to the electrodes. Very low level of currents, below of 1 µA, are enough to bring the nanomaterial to temperatures in excess of 150ºC, at which the reaction between surface adsorbed oxygen and reducing or oxidizing species takes place, while allowing the resistance variation to be measured. High response and low response times have been obtained against NO2, and a mixture of NO2 with humidity in a synthetic air atmosphere. The advantages and drawbacks of the approach to develop these advanced ultra-low power consumption gas sensors will be discussed.

Resume : One of main advantages of carbon based sensors is their low temperature of operation, compared with materials such as metal oxides [1]. Among carbon materials, carbon nanofibers (CNFs) show very attractive properties for environmental gas sensing [2], at a moderate production costs. However, the knowledge about the sensing mechanisms in this material is very limited. In this work, we study of the interference of water and oxygen on the response of CNFs conductometric gas sensor, at different operation temperatures (<150ºC), and under ultraviolet light. From a fundamental perspective, our results show that, at different working conditions, different competitive mechanisms between both species on the CNFs surfaces occur. From a practical perspective, our experiments confirmed that at low temperatures (<80ºC) and low ultraviolet light intensities, CNFs are suitable for environmental relative humidity sensing. Moreover, the baseline and the responses of these sensors were highly stable and reproducible, even after several weeks of operation. The here-studied sensors were fabricated with a combination of inkjet printing and electrospray technique over flexible substrates [3]. [1] M. Penza, R. Rossi, M. Alvisi, D. Suriano, E. Serra, Pt-modified carbon nanotube networked layers for enhanced gas microsensors, Thin Solid Films. 520 (2011) 959–965. [2] O. Monereo, S. Claramunt, M.M. De Marigorta, M. Boix, R. Leghrib, J.D. Prades, et al., Flexible sensor based on carbon nanofibers with multifunctional sensing features., Talanta. 107 (2013) 239–47. [3] S. Claramunt, O. Monereo, M. Boix, R. Leghrib, J.D. Prades, a. Cornet, et al., Flexible gas sensor array with an embedded heater based on metal decorated carbon nanofibres, Sensors Actuators B Chem. 187 (2013) 401–406.

Resume : To achieve good sensing performance, in terms of response time, reversibility and baseline stability, normally imply, conductometric solid-state gas sensors require and external an external source of energy, capable of stimulating the solid-gas interactions[1]. Ultraviolet light (UV) [3] is a viable alternative to the traditional strategy of heating the sensor material [2]. Here, we present a study on the use of UV to modulate the response of flexible gas sensors based on carbon nanofibers (CNFs). The sensing device was developed with a combination of different methods including the inkjet printing for the fabrication of the electrodes and electrospray technique for the deposit of the sensing material [4]. UV illumination was chosen first, to anneal the active layer in the fabrication process and then, to fasten desorption of the gas adsorbed, achieving a stable sensor signal. UV light was also proven to be an effective method to improve reversibility and response dynamics. In comparison with temperature modulation, UV illumination seems to be much more effective to improve the sensing characteristics. Characterization on the modulation of UV light was performed, focusing in the sensing of NH3 and NO2. Sensor response, response time and recovery time were fully characterized. A sensing mechanism is proposed in order to explain and verify the behavior of the CNFs sensor. NH3 and NO2 sensing is proven to be achievable with the correct UV light modulation. [1] Z. Zanolli, R. Leghrib, A. Felten, J.-J. Pireaux, E. Llobet, J.-C. Charlier, Gas sensing with Au-decorated carbon nanotubes., ACS Nano. 5 (2011) 4592–9. [2] J.D. Fowler, M.J. Allen, V.C. Tung, Y. Yang, R.B. Kaner, B.H. Weiller, Practical chemical sensors from chemically derived graphene., ACS Nano. 3 (2009) 301–6. [3] P. Qi, O. Vermesh, M. Grecu, A. Javey, Q. Wang, H. Dai, et al., Toward Large Arrays of Multiplex Functionalized Carbon Nanotube Sensors for Highly Sensitive and Selective Molecular Detection, Nano Lett. 3 (2003) 347–351. [4] S. Claramunt, O. Monereo, M. Boix, R. Leghrib, J.D. Prades, a. Cornet, et al., Flexible gas sensor array with an embedded heater based on metal decorated carbon nanofibres, Sensors Actuators B Chem. 187 (2013) 401–406.

Resume : Capacitive-voltage measurement (C(V)), as one of the most sensitive electrochemical methods, has been investigated extensively for the detection of environmental contaminants in recent years. In this work, we reported the development of a chemical sensor based on Co(II) phthalocyanine acrylate-polymer (CPAP) as a sensitive membrane for detection perchlorate anions. For this, we have used two type of transducer, silicon nitride (Si3N4) and Hafnium oxide (HfO2) grown by Atomic Layer Deposition (ALD) on silicon [1,2]. High dielectric constants of the produced HfO2 film facilitated the processing of the chemical sensor with considerably higher input capacitances than the standard Si3N4. The surface qualities of the used transducers have been studied by contact angle measurement. We have studied the pH effect on Si/HfO2/Electrolyte capacitance values for different phosphate buffer solutions (PBS). This optimization step have allowed a sensitivity value of about 46 mV/decade. We have developed also a perchlorate sensors based on Si3N4 and HfO2 transducers functionalized with CPAP membrane and characterized by C(V) measurements for different perchlorate concentrations (from 10-7 to 10-2 M). The sensor developed with HfO2 transducer shows better performances compared to that based on Si3N4 : a larger detection range (10-6 - 10-2 M and 10-4 - 10-2 M respectively) and a lower detection limit (10-6 M and 10-4 M). The specificities of our perchlorate sensor has been tested for some interfering ions (nitrate, sulfate and carbonate).

Resume : Research on sensing mechanisms and vapor-surface interaction is the key for developing FOs to detect VOCs in environment, healthcare, food quality, and industrial process control. So far, research on FOs has focused on the configuration and the films used as sensing elements. Less attention has been paid to the effect of temperature and the interaction of VOCs with the surface of films. We report experimental results on the effect of temperature in the range 5 – 50 ºC on the response of FOs that uses films of hybrid silica xerogels as the sensing elements. The xerogel films were prepared using the sol-gel process at pH 4.5 using mixtures of methyltriethoxysilane (MTEOS) or ethyltriethoxysilane (ETEOS), and tetraethylorthosilane (TEOS) in a 30:70 molar percentage, and affixed to the end of the optical fiber by the dip-coating technique. The measuring system works under volumetric static conditions. In the range 20 – 40 ºC, the response decreased with temperature, which can be attributed to the decrease in the adsorption of VOCs, which is an exothermic process. Below 20 ºC the response didn’t change with temperature that can be mainly attributed to the condensation of the analytes in the film. For temperatures higher than 45 ºC, adsorption is scarce. At a given temperature, experimental data agree with a model based on adsorption of VOCs molecules on the films that, in turn, changes the refractive index of the external medium. The interaction energy (Ea) was obtained by applying the Arrhenius equation to the experimental data in the range 20 – 40 ºC.

Resume : Ammonia is a chemical that is encountered in many different environments over a wide range of concentrations. Sensors are required that can cover a large dynamic range of concentrations. For instance, ammonia levels in the natural atmosphere can be very low, down to sub-ppb concentration. Hence, very accurate ammonia detectors with a detection limit of 1 ppb or lower are required for measuring such concentrations. On the other hand, near intensive farming areas, ammonia concentrations are much higher, above 10 ppm. The sensors need to work under high background humidity and under a range of temperature. Polyaniline has good (thermal/environmental) stability and provides reasonable conductivities required for most application fields. The material can be reversibly doped and undoped by electrochemical or chemical oxidation or reduction. While this material has previously been used for chemical sensing of ammonia, problems of reversibility, and difficulties in making reproducible sensors have limited its use. Here we report the development of a vapour phase polymerisation technique that allows the deposition of nanolayers of polyaniline onto interdigited substrates where a heater is incorporated. The resulting structure forms a heated chemiresistor that has improved reproducibility in manufacture as well as good reversibility on binding ammonia. By adjusting the dimensions and thickness of the polymer layer, a large dynamic range of ammonia concentrations can be measured. We have eliminated the need for solubilising polyaniline, and the process is fully compatible with large scale fabrication methods including printing technologies.

Resume : Nowadays the increasing level of ground water contamination, due to highly toxic organic pollutants (HTOPs), such as the organophospate pesticides, is becoming a serious threat especially for the neurotoxic effects associated with some of them. This issue has therefore arisen the need for new catalytic materials with water remediation capabilities. The present work is aimed at developing a titania-based mesoporous film with catalytic properties toward organophosphate pesticides by combining two different approaches: the molecular imprinting and the self-assembly with a supramolecular template. The mesoporosity of the material has been obtained by using a tri-block copolymer (Pluronic F127) as a micellar template while the molecular imprinted cavities have been templated by a complex between La3 and bis-4-nitro-phenyl-phosphate. The template removal allowed opening, in one step, both the mesopores and the imprinted cavities with a simultaneous estimation of the active sites. The catalytic activity of the molecularly imprinted and not imprinted films toward the pesticide Paraoxon? has been evaluated by means of UV-Vis spectroscopy titration of the 4-nitro-phenolate released by the Paraoxon? hydrolysis. The analysis of the initial rates of molecularly imprinted and not imprinted films has shown that the presence of a very low number of molecular cavities improves the catalytic properties of the imprinted film when compared to the not imprinted films and the background hydrolysis

Resume : Palladium Platine (PdPt) has been intensively studied these last decades due to high conversion rate in hydrogen oxidation at room temperature with significant exothermic effects (up to 250 KJ/mol). These remarkable properties have been studied by measuring the temperature variations of alumina (Al2O3) supported nanosized PdPt nanoparticles exposed to different hydrogen concentrations in dry air. This catalyst is expected to be used as a sensing material for stable and reversible ultrasensitive hydrogen sensors working at room temperature (low power consumption). Structural and gas sensing characterizations and catalytic activity of PdPt/Al2O3 systems synthesized by co-impregnation will be presented. Catalytic characterizations show that the system is already active at room temperature and sharply increases with rise in temperature. Moreover, the increase of the PdPt proportion in the co-impregnation process, improves the activity and very high conversion can be reached even at room temperature. The thermal response (about 3°C) of only 1 mg of PdPt/Al2O3 is reversible and the time response is about 5 seconds. The integration of PdPt/Al2O3 powder on a flat substrate has been realized by the deposition onto the powder of a thin porous hydrophobic layer of parylene. The possibility of using PdPt in gas sensors will be discussed.

Resume : In this study, we develop tungstate materials for nanostrucutre catalytic sensors systems allowing the detection of pollutant like as methane and carbon monoxide. We report the synthesis of WO3 and SrWO4 (SWO) phases, based on the coprecipitation approach. All samples were characterized by thermogravimetric analysis, X-ray diffraction and electron microscopy to identify the structure and the morphology of each compound. The catalytic reactivity of the samples was analyzed using a reactor coupled to a combined system of Fourier Transform Infrared spectrometer (FTIR) and Mass Spectrometer (MS) apparatus. The measurements were performed with different gas carriers, from room temperature to 600ºC. The catalytic activity was determined as a function of time, temperature and gases concentration. FTIR experiments show that methane given rise to two types of reactions, partial and total oxidation, at high temperature. Unlike to carbon monoxide, the oxidation occurs at a low temperature. The reactivity should be linked to oxygen species and electron exchanged with the solids. To try to connect catalytic senssing effects with charges mobility, electrical analyses have been performed. Electrical impedance measurements have been carried out on compacted polycrystalline samples, and have been interpreted with Nyquist representations.

Resume : Research on carbon nanomaterials for applications covering electronics devices, chemical sensors etc [1] is becoming a flourishing domain. Owing to their narrow size, carbon nanotubes (CNTs) as a new class of carbon nanomaterials present noticeable properties mainly attributed to their high surface areas and their transport properties, leading especially to tailored surface reactivity. However, because of their poor solubility, CNTs have a restricted use in many applications. This problem can be overcome by functionalisation methods [2]. The functionalization is a good way for simultaneously preserving the CNTs properties and bringing additional functionalities like gas sensitivity. In this work, CNTs have been functionalized with conjugated organic molecules like phthalocyanines (Pcs) or porphyrins (Por) derivatives for potential application in the detection of BTX (Benzene-Toluene-Xylenes). With our strategy based on dispersion technique, sensing structures based on macrocycle-CNTs hybrid materials have been processed on different transducers. This presentation will be focussed on: the functionalization of the CNTs, their characterisations (UV-Vis spectroscopy, TGA, TEM, Raman analysis) and the room temperature sensitivity of these hybrids materials towards BTX and others interfering gases. References: [1] A.G. Mamalis et. al., Prec. Eng., 2004, 28, 16. [2] D.M. Guldi, et. al., Chem. Rev. 2010, 110, 6768.

Resume : Germanium (Ge) is a classical semiconducting material that constituted the substrate for the invention of the bipolar transistor, but due to its low abundance at the earth crust it is only sparsely used for certain applications. However, recently, there is a growing interest on Ge nanowires (NW) because they overcome electrical and optical properties of silicon. In this work we present for the first time, to the best of our knowledge, the development and gas test of Ge nanowire based chemoresistive gas sensors. For this, we have locally, site-specifically and energy and material efficiently grown NWs using the VLS method on the top of micromembranes equipped with surface interdigitated electrodes and a buried heater. Similarly to other chemiresistors, the GeNW surface reacts with the presence of reducing and oxidizing gases, what causes a change in its resistance. A drawback of Ge is, unlike silicon, a non uniform and non stoichiometric GeOx layer is formed, whose thickness can be limited by working in a low temperature range. Ge NWs based sensors have shown good response towards the presence hundred of ppb’s of NO2 and CO even at room temperature. The fabricated device provides a functional sensor which consumes few mW for both heating and measuring. The behavior of the Ge NW gas sensor will be presented and the possibilities for improvements of its properties through surface functionalization will be discussed.