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Supporting Information for
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A calibrated Graphene-based chemi-sensor for sub-ppm NO2 detection operating at room
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temperature
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Filiberto Ricciardella,* Ettore Massera, Tiziana Polichetti, Maria Lucia Miglietta and
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Girolamo Di Francia
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ENEA Research Center, Piazzale E. Fermi, 1, Portici (Napoli), I-80055, Italy
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Corresponding author: filiberto.ricciardella@enea.it
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Optimization of device conductance value
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The chemiresistor conductance value has been optimized in order to have a conductance
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around 10-5 S by varying the volume of the dispensed suspension until the achievement of the
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proper film conductivity: This particular conductivity range allowed to exploit at the best the
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electronics of our experimental setup.
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After preparing different graphene dispersions by chemical exfoliation, few microliters of
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suspension were deposited by drop-casting onto quartz substrate and the sheet resistance was
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measured through four-probes instrument (NNPSON RESISTAGE RG8).
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The flake average thickness in the deposited film was estimated equal to 200 nm through
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profilometric analysis (KLA TENCOR P-10) so that a conductivity value ranging between
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104÷105 S/m corresponds to a sheet resistance of drop-casted films in the range of kΩ/sq.
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Seebeck effect measurement
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Exploiting the thermoelectric effect, this qualitative method allows to identify the type of the
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carrier in semiconductors. In our case, this analysis appears to be particularly useful because the
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interaction between graphene and analyte is strongly influenced by the majority carriers.1
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On alumina strip (3 x 1 cm2), two pads at a distance of about 5 mm were created by spreading
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drops of silver paste. Few microliters of graphene dispersion were deposited between the
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metallic contacts. Once one pad was heated at 450°C, a positive voltage equal to 3mV was
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measured between the electrodes. This behavior, typical of the p-type doped material, has been
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inspected for each graphene dispersion, confirming the type of the material doping.
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Alumina transducer details
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The sketch of the transducer used as sensor substrate is depicted in Figure S1.
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Figure S1. Sketch of transducer used as substrate for sensor
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All the finger dimensions have been designed by ourselves and customized by 3M taking into
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account our need and the structure of the test chamber. The graphene film covers the gold
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interdigitated electrodes (IDE) for an area of about 49 mm2. Volt-amperometric measurements
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were performed by applying the two probes on the larger pads of the transducer. The probes have
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a diameter of about 500 m.
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Gas Sensor Characterization System and test protocols towards NO2
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Figure S2. Picture of Gas Sensor Characterization System (left) and corresponding circuital
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scheme (right).
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The device is located in a stainless steel test chamber (see fig. S2) placed in a thermostatic box.
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The chamber is provided of an electrical grounded connector for bias and conductance measure
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as shownon the right in fig. S2 .
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A constant flow (500sccm) of the gas carrier, i.e., synthetic air crosses the test chamber. The
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carrier can be properly humidified through a water bubbler placed in a thermostatic bath. In this
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environment, characterized by controlled temperature and humidity, the conductance value of the
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device in its equilibrium state is firstly measured (baseline); after that, an intentional disruption
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of the equilibrium state is produced by introducing Nitrogen dioxide in a controlled amount and
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by mixing it with the gas carrier via pneumatic valves and through programmable Mass Flow
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Controllers. In the field of environmental monitoring, the NO2 concentrations needed to be
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detected are lower than few parts-per-million (ppm), this poses a fundamental issue in
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reproducing as much as possible these conditions in lab. In addition, the reproducibility of these
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levels of concentration in the lab equipment appears no immediate since the pipes and the core of
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test chamber is made of electro-polished stainless steel. This means that the NO2 molecules can
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spread over and adhere also to the chamber walls, reducing the effective analyte concentration
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interacting with the sensor. In order to minimize the effects of these limitations, within 30
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minutes before the measurements, the maximum available concentration of NO2 is fluxed into
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the chamber for 15 minutes through a protocol controlled via software. In order to check the
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degree of agreement between the gas concentrations set in the protocols and those present into
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the chamber, tests have been performed through an FTIR (THERMO ANTARIS IGSS) equipped
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with a cryogenic detector and a cell having an optical path equal to 10 m. All these operations
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guarantee the reproducibility of the condition of the test chamber. Hardware and software
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implemented on a work station allow to control and record environmental parameters, device
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bias and output signal, making possible to perform customizable automated tests on devices
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(Protocols).
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In Figure S2 a picture of the stainless steel testing chamber (on the left) and the circuital
scheme for the electrical characterizations (on the right) are reported.
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In the field of environmental monitoring, the NO2 concentrations needed to be detected are
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lower than few parts-per-million (ppm), this poses a fundamental issue in reproducing as much
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as possible these conditions in lab. In addition, the reproducibility of these levels of
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concentration in the lab equipment appears no immediate since the pipes and the core of test
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chamber is made of electro-polished stainless steel. This means that the NO2 molecules can
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spread over and adhere also to the chamber walls, reducing the effective analyte concentration
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interacting with the sensor. In order to minimize the effects of these limitations, within 30
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minutes before the measurements, the maximum available concentration of NO2 is fluxed into
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the chamber for 15 minutes through a protocol controlled via software. In order to check the
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degree of agreement between the gas concentrations set in the protocols and those present into
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the chamber, tests have been performed through an FTIR (THERMO ANTARIS IGSS) equipped
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with a cryogenic detector and a cell having an optical path equal to 10 m. All these operations
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guarantee the reproducibility of the condition of the test chamber.
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An example of standard protocol for sensing measurement at fixed NO2 concentration is set as
follows:
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1200 seconds in carrier gas at constant relative humidity (RH) and temperature
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15 seconds exposure to 4300 parts-per-billion (ppb) of NO2
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225 seconds exposure to 350 parts-per-billion (ppb) of NO2
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180 seconds at zero gas flow
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1200 seconds of flushing with carrier gas.
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The first step, also called baseline, is required to stabilize the conductance value and to
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determine the starting point of the response at the gas inlet. Since the test chamber volume is
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about 40 cl, a “double step” for the gas inlet is needed to more rapidly reach the steady state
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conditions in terms of gas concentrations: in the first 15 sec the gas flow is set to very high
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concentration value; in the subsequent 225 sec the gas flow is set to the target concentration. The
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flushing step is used to recover the conductance to the initial condition.
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The typical conductance dynamic response of the sensor upon the just described run is showed
in Fig. 4 in the paper; in that case, the carrier gas is N2 and the RH is set at 50%.
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The standard protocol for the sensing measurements referred to Fig. 5 in the main text consists
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of different pulses of NO2 followed by short recovery steps, operating in conditions of humidity
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and carrier gas similar to those reported in the last runs. For all the concentrations, except for the
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exposure to 50 ppb, the protocol is set as follows:
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
2 minutes exposure to NO2
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4 minutes flushing with carrier gas
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In the case of 50 ppb, the only change regards the exposure time window that requires 4
minutes to achieve an appreciable conductance variation.
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Likewise, for the sensor calibration, the test protocol is constituted by sequential steps at
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different analyte concentrations: after the initial acquisition of the baseline, the device was
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exposed to NO2 for 2 minutes and left to recover in inert gas flow for 28 minutes assisting the
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partial desorption of the gas and avoiding the poisoning of the sensing film.
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Derivative Method and calibration protocol
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The employed method for the sensor analysis and calibration is focused on the derivative of
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the signal output. In particular, the derivative is calculated as the average of the incremental ratio
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according to the following relation implemented by OriginPro® software:
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𝒇′ (𝒙𝒊 ) =
𝟏 𝒚𝒊+𝟏 − 𝒚𝒊 𝒚𝒊 − 𝒚𝒊−𝟏
[
+
]
𝟐 𝒙𝒊+𝟏 − 𝒙𝒊 𝒙𝒊 − 𝒙𝒊−𝟏
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where xi and yi are generic time instant and value conductance, respectively.
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When the sensor is exposed to several sequential steps at different analyte concentrations (see
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Figure 5 in the text) for the same exposure time, the effects of the algorithm on the signal is well-
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rendered. The linear trend of the maxima with the concentration leaps out, clearly highlighting an
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increase of one order of magnitude upon NO2 exposure passing from 100 ppb to 1000 ppb. This
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biunivocal corrispondance has been exploited to the aim to calibrate the sensor according to a
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above mentioned calibration protocol.
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The error bars in the calibration plot (Figure 6 in the paper) have been calculated by
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considering the difference between two subsequent points in the neighborhood of the maximum
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of each derivative peak.
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The employed analysis method allows to compare devices fabricated starting from different
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batches and/or having a large different parameters such as initial conductance and response to the
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analyte. At the same time, the method reveals so versatile that it can be extended to all devices
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affected by slow and incomplete recovery.
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State of the art for the sensors operating at Room Temperature
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A brief comparison between our sensor results gas sensor devices operating at Room
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Temperature and based on sensing materials other than graphene, is reported herein.
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Sensitive film based on nanostructured metal oxides
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Metal oxides are considered the best option for this kind of application and SnO2 is one of the
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most investigated material. This oxide is a very good choice for sensors working at high
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temperatures (>350°C), but when RT scenarios have to be considered, their performances
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strongly get worse and different approaches (i.e. nanostructured morphologies) have to be
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considered. For instance, Khuspe et al. show that a fairly high response (about 20%) can be
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obtained although at much higher concentration (100 ppm).2 However, the device shows a very
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short time of response (7s) but the operating temperature is still fairly high, around 200°C . In
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comparison, our device is certainly slower but it is much more sensitive (10 times at least) and it
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truly operates at RT.
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ZnO is another widely investigated metal oxide for NO2 detection. Shishiyanu et al. report on
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a doping process also finalized to decrease the device operating temperature. They find a
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sensitivity around a few %/ppm at RT (therefore again our device seems more promising) with a
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response time in the order of minutes (comparable to our device).3
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Sensitive film based on an organic material.
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Some conjugated polymers have been investigated as possible sensitive materials exploiting
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the “doping” effects that some chemicals can exhibit during the molecule-polymer interaction. In
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the recent paper, Ji et al., it is for instance shown that a high change of the relative response at
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RT can be observed (about 20%) but at 5 ppm of NO2, with a time of response that strictly
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follows the gas pulse (tens of minutes).4 Also in this case our device seems to be more
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promising, at least in terms of response time.
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Sensitive film based on Carbon Nanotubes (CNT).
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CNTs based sensors show a very interesting response to NO2. The most widely cited papers, in
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this respect, is the paper by L. Valentini et al. Their device shows a 6% response to 100 ppb and
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are sensitive to the gas up to 10 ppb. However the device operates at a fairly high temperature
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(165 °C) and, more importantly, the characterizations have been performed in dry air, that is, far
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from normal operating conditions.5 Our device operates, on the contrary, at relative humidity
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concentrations similar to those normally observed in urban environments.
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REFERENCES
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2. G. D. Khuspe, R. D. Sakhare, S. T. Navale, M. A. Chougule, Y. D. Kolekar, R. N. Mulik, R.
C. Pawar, C. S. Lee, V. B. Patil, Ceram. Int. 39, 8673 (2013).
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3. S. T. Shishiyanu, T. S. Shishiyanu, O. I. Lupan, Sens. Actuators B 107, 379 (2005).
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4. S. Ji, H. Wang, T. Wang, D. Yan, Adv. Mat 25, 1755 (2013).
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5. L. Valentini, I. Armentano, J. M. Kenny, C. Cantalini, L. Lozzi, S. Santucci, Appl. Phys.
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Lett. 82, 961 (2003).
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