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Carbon nanotubes-coated multi-transducing sensors for VOCs detection
Sensors and Actuators B 111–112 (2005) 171–180
Carbon nanotubes-coated multi-transducing sensors for VOCs detection M. Penza a,∗ , G. Cassano a , P. Aversa a , F. Antolini a , A. Cusano b , M. Consales b , M. Giordano c , L. Nicolais c a
ENEA, C.R. Brindisi, Materials and New Technologies Unit, SS. 7, Appia, km 714, 72100 Brindisi, Italy b Department of Engineering, University of Sannio, Corso Garibaldi 104, 82100 Benevento, Italy c Institute for Composite and Biomedical Materials, CNR, P.le Tecchio 80, 80124 Napoli, Italy Available online 10 August 2005
Abstract Langmuir–Blodgett (LB) films consisting of tangled bundles of single-walled carbon nanotubes (SWCNTs) have been used as sensing nanomaterials onto three different types of sensory systems using complementary transducing principles as surface acoustic waves (SAWs), quartz crystal microbalance (QCM) and standard silica optical fiber (SOF) for volatile organic compounds (VOCs) detection, at room temperature. The multi-transducing probes have been configured as 433 MHz SAW oscillator, 10 MHz QCM oscillator, and SOF reflectometrybased system at a wavelength of 1310 nm. A nanocomposite film of SWCNTs embedded in a cadmium arachidate (CdA) matrix was deposited by LB methodics onto SAW sensors. A LB multilayer of SWCNTs-onto-CdA buffer material was deposited onto QCM and SOF sensors. Highly sensitive, repeatable and reversible responses of the SWCNTs sensors indicate feasible VOCs detection in a wide mmHg vapor pressures range. The VOCs sensing performance of the SWCNTs-based multi-transducing sensors will be discussed. © 2005 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; SAW and QCM acoustic gas sensors; Optical fiber gas sensors
1. Introduction Since their discovery [1], the carbon nanotubes (CNTs) have been extensively studied as nanostructured materials in the both forms of single-walled [2] and multi-walled [1] aggregates for many nanoscience applications due to their unique, outstanding and useful physical characteristics. The CNTs have attracted attention in the fabrication of field effect transistors [3], memories [4], logic gates [5], field emission displays [6], supercapacitors [7], and nanoscale devices [8–10]. A single-walled carbon nanotube (SWCNT) can be visualized as a graphene sheet rolled up into a cylinder consisting of an one-dimensional nanotubular wire with only surface-arranged carbon atoms [11]. Due to their peculiar hollow structure, nanosized morphology and high surface area (>1500 m2 /g), the carbon nanotubes are ideal ∗ Corresponding author. Tel.: +39 0831 201422; fax: +39 0831 201581/423. E-mail address: [email protected] (M. Penza).
0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.06.055
candidates for highly sensitive gas adsorption, hence they are strongly attractive as gas sensor materials [12–16]. Recently, this mix of properties has prompted researchers to develop carbon nanotubes based gas sensors [17–22] making single-walled carbon nanotubes (SWCNTs) an promising sensing nanomaterial for a new class of chemical molecular sensors. The carbon nanotubes grown by the current synthesis techniques [23] (laser ablation, CVD, arc-discharge) are arranged in tangled nets of nanotubules or ropes of aggregated nanochains. The Langmuir–Blodgett (LB) process is a suitable method for depositing defect-free, molecularly ordered ultra-thin films with controlled thickness and orientation. This technique allows fine surface modifications in a film of carbon nanotubes with a highly controlled manipulation to implement molecularly self-organizing nanomaterials. Composite [24] of SWCNTs incorporated in a foreign matrix and multilayer [25] of SWCNTs onto buffer materials attract considerable interest as highly sensitive materials for chemical sensing applications. The LB method is very suitable for covering electrodes array and optical fiber surface
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with a few CNTs or bundles of CNTs to study the sensing properties of monolayers of such nanomaterials. Here, sensing properties of this class of materials have been investigated by using three different types of transducers with complementary principles of operation: a surface acoustic wave (SAW) two-port resonator 433.92 MHz oscillator based on ST,X-quartz substrate, a quartz crystal microbalance (QCM) vibrating at 10 MHz based on AT-cut quartz substrate, a standard silica optical fiber (SOF) using near infrared optical reflectometry at a wavelength of 1310 nm. The main aim of the integration of multi-transducing sensors is that the simultaneous use of chemical detectors with complementary transducing principles can efficiently increase the amount of information extracted from a multi-transducer and multi-sensor array by pattern recognition methods for chemical analysis of VOCs. In this work, SAW and QCM acoustic and SOF optical gas sensors using LB films of nanostructured SWCNTs as composite and multilayer for VOCs detection, at room temperature, have been reported.
2. Experimental 2.1. Langmuir–Blodgett films of carbon nanotubes The pristine material of SWCNTs was purchased from Carbon Nanotechnologies Inc. (Houston, USA). The SWCNTs were produced according to the HiPco process [26] with high pressure CO molecules dissociation at about 1000 ◦ C. The Langmuir–Blodgett films were prepared using HiPco SWCNTs onto acoustic and optical transducers according to a vertical dipping method, as reported in the scheme of Fig. 1. The film deposition system used was a KSV 5000 LB trough system. The geometries of LB films deposition were a multilayer of SWCNTs onto a buffer material of LB cadmium arachidate (CdA) and a nanocomposite of SWCNTs embedded in a CdA matrix. Multilayer: The cadmium arachidate has been used as buffer material to promote the adhesion of CNTs onto transducers. The CdA films have been prepared by LB technique using arachidic acid [CH3 (CH2 )18 COOH] in chloroform spread onto a subphase of deionized water
(18 M) containing 10−4 M cadmium chloride (CdCl2 ). The LB process parameters for preparing CdA films are reported elsewhere [27]. A multilayer with a different number of CdA monolayers has been implemented according to the various transducers used. The LB films of SWCNTs-onto-CdA buffer multilayer have been deposited onto acoustic and optical transducers. For LB SWCNTs film deposition, a solution (0.2 mg/ml) of single-walled carbon nanotubes in chloroform was spread onto a subphase constituted by deionized water (18 M) with 10−4 M CdCl2 . The subphase pH was 6.0 and the temperature was 23 ◦ C. The monolayer was compressed with a barrier rate of 15 mm/min up to a surface pressure of 45 mN/m. The single layer was deposited onto the different sensors with a dipping rate of 3 mm/min. The transfer ratio of the monolayer from subphase to hydrophobic surface of a sensor already coated by CdA film was in the range 0.5–0.7. After a proper drying of 12 h overnight, the sensing SWCNTs multilayer deposited onto CdA was ready for vapor testing. The X-ray diffraction confirmed the monolayer spacing of about 2.0 nm for the SWCNTs deposited. The carbon nanotubes have been implemented to fabricate a multilayer of two monolayers onto the CdA buffer material. The QCM and SOF sensors were used as transducers to deposit the SWCNTs multilayer. Nanocomposite: LB composite of SWCNTs incorporated in a CdA matrix was prepared. A solution (0.734 mg/ml) of arachidic acid [CH3 (CH2 )18 COOH] in chloroform (CHCl3 ) and a separate solution (0.202 mg/ml) of single-walled carbon nanotubes in chloroform were prepared. A volume of 400 l was taken from each solution to form a mixed solution with a total volume of 800 l. This mixed solution was accurately dispersed and stirred in an ultrasonic bath for 1 h. Then, only a volume of 200 l was used for LB composite film deposition. This solution of 200 l was spread onto a subphase constituted by deionized water (18 M) with 10−4 M CdCl2 . The subphase pH was 6.0 and the temperature was 19 ◦ C. The monolayer of nanocomposite was compressed with a barrier rate of 15 mm/min up to a surface pressure of 27 mN/m. The single composite layer was deposited onto the different sensors with a dipping rate of 14 mm/min. The transfer ratio of the monolayer from subphase to hydrophobic surface of a sensor was in the range 0.6–0.7. The weight
Fig. 1. Scheme of vertical dipping deposition of LB films.
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Fig. 2. SEM images of multilayer of CNTs-onto-CdA on (a) QCM and (b) SOF sensors.
ratio of nanocomposite obtained SWCNTs:CdA was as 1:3.625. After a proper drying of 12 h overnight, the sensing nanocomposite deposited were ready for vapor testing. A composite of two monolayers of SWCNTs embedded in CdA matrix was prepared. The SAW sensors were used as transducers to deposit the SWCNTs nanocomposite. Fig. 2 shows typical SEM images of SWCNTs deposited by Langmuir–Blodgett technique onto QCM quartz substrate and silica optical fiber. Both transducers were coated by cadmium arachidate as buffer layer to promote adhesion of carbon nanotubes onto surface of sensors. The surface texture caused by the presence of nanotubes appears clearly with the SWCNTs randomly distributed in tangled networks of nanotubular chains and densely aggregated mats. This surface distribution of the nanotubes enhances the gas adsorption in the nanomaterial. The mean diameter of carbon nanotubes is 1–5 nm and the mean length of the nanotubes is 1–10 m.
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period was of 7 m for 433.92 MHz device. The high Q (>2500) and low insertion loss (<15 dB) make these devices extremely stable when incorporated in an oscillator circuit with frequency output. Each SAW two-port resonator was inserted into an oscillating loop with a low-noise IC-based RF amplifier configured as a modified Colpitts scheme. The oscillator was designed and implemented on a printed circuit board by using SMD electronic components and low-voltage circuitry (5 V, 20 mA). The average baseline noise, intended as maximum frequency change over a time interval as exposure time, for SAW 433.92 MHz devices has been measured as low as 10 Hz, over a 6-min time interval. A conventional 10 MHz QCM has been used consisting of a circularly shaped AT-cut quartz crystal with a diameter of 10 mm and a thickness of 0.1 mm. The Al electrodes on both sides of the quartz were 0.10 m thick and 4 mm in diameter. The QCM resonator was inserted as frequencycontrol element in an oscillating loop close by a low-noise IC-based RF amplifier. The noise of the uncoated 10 MHz QCM sensor was 0.5 Hz in 10 min. The resonant frequency of the QCM-based oscillator was the sensor output. The SAW and QCM resonators have been inserted in a PCB board with oscillator circuitry based on SMD components, as shown in Fig. 3. This electronic board is able to allocate up to 12 sensing acoustic elements (6 SAW + 6 QCM) with multiplexed read-out. The output frequency of SAW and QCM acoustic sensors was measured by a frequency counter (Agilent 53132A) with a multiplexed read-out by a switch unit (Agilent 34970A) driving two 50 4 × 1 rf multiplexers (Agilent 34905A). A standard silica optical fiber was used as optical sensor. The diameter of the fiber and the core was of 125 and 9 m. The sensing probe was prepared by stripping the fiber protective coating (1–2 cm) from the fiber end, which was properly cleaved to obtain a planar cross-section, where the nanostructured layer was deposited. The distance from fiber end for deposition of carbon nanotubes was 5 mm. Reflectance measurements have been performed by lighting the optical fiber sensing interface with a superluminescent diode (40 nm bandwidth) operating at a wavelength of 1310 nm. A 2 × 2 in-
2.2. Acoustic and optical sensors systems design Commercially available SAW two-port resonators (433.92 MHz: Siemens, R2632) operating at 433.92 MHz are used as acoustic elements. The resonators (4.0 mm × 1.0 mm × 0.5 mm) based on ST-cut,X-propagation quartz substrate are mounted on a 3-pin TO39 case. The IDTs
Fig. 3. PCB board integrating 6 SAW and 6 QCM oscillating sensors.
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2.3. Chemical sensing setup
Fig. 4. Reflectometric system based on standard silica optical fiber.
fiber coupler connects the light source, sensing interface and two receiving channels: one for the reflected signal detection and the other one for incident signal monitoring to compensate optical intensity. Synchronous detection has been implemented with the light source externally amplitude modulated at 500 Hz and the sensor outputs retrieved by using a dual channel lock-in amplifier. Here, the normalized optoelectronic sensor output consists of the reflected signal from fiber-coated end divided by the source signal. In this configuration, any effect able to modify the refractive index of the sensing layer changes the normalized output of the optical sensor [28]. The reflectometric experimental system used for optical sensing is shown in Fig. 4.
The experimental setup used for chemical testing of acoustic and optical sensors is shown in Fig. 5. The acoustic and optical sensors have been located in the same test cell (1000 ml volume) for VOCs exposure measurements in simultaneous detection. The VOCs vapors were generated by the bubbling method. Nitrogen was used as reference gas and carrier gas to transport the individual VOCs in the test cell. The test ambient was controlled and switched with some three-way valves operated manually. The total flow rate per exposure was kept constant at 1000 ml/min. The gas flow rate was controlled by a mass flowmeter driven by a controllerunit communicating with a PC via standard RS-485 serial bus. The vapor pressure of six individual VOCs tested of ethanol, methanol, isopropanol, acetone, ethylacetate, toluene was in the range 15–150, 50–200, 20–150, 100–650, 30–270, 10–110 mmHg, respectively. The sensing experiments were conducted at room temperature by simultaneous exposure of acoustic and optical sensors to VOCs. The temperature, monitored by a J-type thermocouple properly located in the test chamber, was measured by a 20-channel multiplexer 2-wire card (Agilent 34901A) incorporated in a data-logger (Agilent 34970A). The output frequency of the SAW and QCM acoustic sensors was measured by a frequency counter (Agilent 53132A) with a multiplexed read-out by a switch unit (Agilent 34970A) driving the two 50 4 × 1 rf multiplexers (Agilent 34905A). Data were collected and stored for fur-
Fig. 5. Scheme of experimental setup for chemical testing of acoustic and optical sensors.
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ther analysis in a PC machine interfaced by a GPIB card in Agilent-VEE software ambient. The output signal of the optical fiber sensor was the normalized optical reflectance measured by the previously described stand-alone system. The optical sensor data were stored in the notebook, which controlled the data acquisition process in LabView software ambient by a NI-DAQ card.
3. Results and discussion 3.1. SAW sensors using SWCNTs filler composite film Fig. 6 shows a typical transient response of two ST-cut quartz-based SAW oscillating 433 MHz sensors, one coated by two monolayers of LB CdA film and other coated by two monolayers of LB nanocomposite of SWCNTs embedded in a CdA matrix. They are exposed, at room temperature and relative humidity of about 10% at 28 ◦ C, to various 5min pulses of toluene decreasing concentrations ranging from 139 to 7 ppm. The SAW sensor response decreases upon a given concentration of analyte for both detectors due to mass loading of vapor molecules adsorbed into sensing material. However, after switching to recovery ambient of nitrogen, the sensors signal overcrosses the baseline and then returns to a stable reference level. This behaviour is repeated for each exposure and for both sensors. The reasons have to be investigated. Probably, this effect should be attributed to CdA film instead of presence of SWCNTs in the nanocomposite. A comparison of SAW sensitivity to toluene between nanocomposite LB film of SWCNTs embedded in CdA and LB CdA film has been performed. The results obtained,
Fig. 7. Comparison of the SAW sensitivity to toluene between nanocomposite 27.5 wt.% LB film of SWCNTs embedded in CdA and LB CdA film, at room temperature. The thickness of sensing material for both SAW sensors is two LB monolayers.
shown in Fig. 7, indicate that the frequency shift as a function of toluene concentration is fitted by a linear regression for both SAW sensors, coated by two monolayers of LB film. The toluene sensitivity of 0.04371 kHz/ppm of SWCNTs nanocomposite is about twice that of unembedded LB CdA film. The vapor sensitivity of the nanocomposite of SWCNTs embedded in a CdA matrix can be enhanced by optimizing the weight ratio SWCNTs/CdA. This better sensitivity of the SWCNTs nanocomposite towards other VOCs tested has been also measured. Moreover, the increased sensitivity of chemical sensors based on composite of CNTs has been reported in literature [24] for acid vapors. 3.2. QCM sensors using SWCNTs-on-CdA multilayer film
Fig. 6. Transient response of SAW 433 MHz sensor coated by (a) CdA film and (b) nanocomposite 27.5 wt.% of SWCNTs embedded in CdA, at room temperature, to different concentrations of toluene.
Fig. 8 shows typical transient responses of the two QCM 10 MHz sensors, one coated by 20 monolayers of LB CdA and other coated by two monolayers of SWCNTs-onto-20 monolayers LB CdA, exposed, at room temperature, to repeated 6-min pulses of various vapor pressures of ethylacetate. Upon individual VOC ambient, the QCM resonant frequency decreases due to mass loading with a response time in the range of 1–2 min and, then the signals are recovered upon N2 reference ambient. In the comparison for each tested VOC vapor pressure, the highest frequency change has been measured for the SWCNTs-functionalized QCM sensor. The high surface area of carbon nanomaterial is responsible for the high vapor sensitivity. The repeatability of the QCM response is quite good, although drift in the baseline of SWCNTs-based sensor is measured. The calibration curves of the QCM 10 MHz sensors coated by a multilayer of 2 monolayers of SWCNTs-onto-20 monolayers LB CdA buffer, and exposed, at room temperature, to five VOCs of ethanol, methanol, isopropanol, ethylacetate and toluene in a wide range of vapor pressures have been
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Fig. 8. Transient response of CdA-coated QCM 10 MHz sensor, with and without SWCNTs, at room temperature, to ethylacetate.
Fig. 10. Transient response of CdA-coated SOF sensor, with and without SWCNTs, at room temperature, to toluene.
reported in Fig. 9. The QCM frequency shift for a given oscillator increases with the mass of the molecules adsorbed into layer. Effects of saturation are measured in the range of the highest vapor pressures tested. The VOCs sensitivity of SWCNTs-based QCM sensor is extremely improved up to 1–2 orders of magnitude higher with respect to the better VOCs QCM detectors based on phthalocyanines and their phthalocyaninato metal complexes [29].
and without SWCNTs sensing overlayer. Fig. 10 shows the typical transient responses to toluene at different vapor pressures for both optical fiber sensors. As in the case of mass sensitive QCM sensors, the presence of SWCNTs overlayer leads to a significant increase in the vapor sensitivity of an average factor of four to six times, even with a extremely thin thickness (4.0 nm) of the SWCNTs overlayer. The measured reflectance decreases upon exposure of the sorbed molecules of VOC investigated by monitoring the changes in the refraction index of the sensitive layer. Fig. 11 shows the calibration curves of the SOF sensors coated by a multilayer of 2 monolayers of SWCNTs-onto-20
3.3. SOF sensors using SWCNTs-on-CdA multilayer film Experimental demonstration of silica optical fiber sensors for VOCs detection has been carried out by two probes with
Fig. 9. Calibration curves of QCM 10 MHz sensors coated by SWCNTsonto-CdA multilayer, at room temperature, for five VOCs. The thickness of SWCNTs film is 2 LB monolayers. The thickness of CdA buffer material is 20 LB monolayers.
Fig. 11. Calibration curves of SOF sensors coated by SWCNTs-onto-CdA multilayer, at room temperature, for six VOCs. The thickness of SWCNTs film is 2 LB monolayers. The thickness of CdA buffer material is 20 LB monolayers.
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monolayers LB CdA buffer, and exposed, at room temperature, to six VOCs of ethanol, methanol, isopropanol, acetone, ethylacetate and toluene in a wide range of vapor pressures. From these results, and considering the minimum detectable normalized output, a vapor pressure resolution of less than 1 mmHg is obtainable. It is worth to note that sensor sensitivity and thus resolution can be significantly enhanced by tailoring the SWCNTs overlayer thickness or by properly designing in terms of buffer layer thickness and refractive index. 3.4. Cross-relationships between QCM and SOF sensors using SWCNTs-on-CdA multilayer film
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Table 1 Cross-sensitivity of the SWCNTs-onto-CdA (2 LB monolayers-onto-20 LB monolayers) multilayer-coated QCM and SOF sensors, at room temperature VOC
Ethanol Methanol Isopropanol Acetone Ethylacetate Toluene
SWCNTs-onto-CdA LB multilayer Average sensitivity of 10 MHz QCM (Hz mmHg−1 )
Average sensitivity of SOF 10−4 (mmHg−1 )
3.460 1.969 4.149 0.043 1.558 7.089
1.861 1.011 −0.803 −0.035 0.397 3.834
The average sensitivity of the both sensors has been used as defined in the text.
In order to investigate the vapor sensing properties of carbon nanotubes to VOCs, repeated vapor exposures have been realized by measuring the sensor response, at room temperature, for two transducers QCM and SOF coated by a multilayer consituted of 2 LB monolayers SWCNTs-onto20 monolayers LB CdA buffer material. Cross-sensitivity of the two multi-transducing sensors to individual VOCs tested of ethanol, methanol, isopropanol, acetone, ethylacetate, and toluene has been measured. In this study, the average sensitivity, Sm , of a QCM device coated with a sensing material may be defined as the mean value of the n responses to different vapor pressures of the same organic solvent: n
Sm =
1 fi (Hz mmHg−1 ) n pi
(1)
i=1
where fi is the steady-state frequency change of the sensor exposed to vapor pressure pi of a given i-solvent tested. Also, the average sensitivity, SV , of a SOF sensor coated by a sensing layer was obtained according to the following equation: n
SV =
1 Vi (mmHg−1 ) n pi
(2)
i=1
where n takes into account the various responses to different vapor pressures for each VOC investigated; pi represents the actual change in the vapor pressure responsible for a consequent sensor output variation, Vi . The results obtained have been reported in Table 1 showing the average sensitivity of QCM and SOF sensors both coated by a multilayer of 2 LB SWCNTs monolayers-onto-20 LB CdA buffer monolayers to VOCs under test. High sensitivity of the SWCNTs-coated sensors has been detected. Both sensors measured highest sensitivity to toluene, then to alcohols considered. SOF sensor detected isopropanol and acetone by reversing the sign of the response. The chemical patterns for six VOCs examined have been reported in Fig. 12. These are important features for VOCs pattern recognition purposes. In order to study the correlation between QCM and SOF sensors coated by SWCNTs-onto-CdA, the responses towards some tested VOCs of isopropanol, ethylacetate and acetone have been cross-referenced, at room temperature.
Fig. 12. Chemical patterns of VOCs using (a) QCM and (b) SOF sensors coated by a SWCNTs-onto-CdA (2 monolayers-onto-20 monolayers) LB multiplayer, at room temperature.
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addition, these specific slopes in the correlation plot can be used to recognize the analytes under test. Fig. 14 reports the typical on-line responses, at room temperature, of two multi-transducing sensors – a QCM sensor coated by two monolayers of SWCNTs-onto-CdA and a SOF sensor coated by two monolayers of SWCNTs-onto-CdA – upon decreasing 10-min steps of ethanol vapor pressure in the range from 28 to 14 mmHg. The results indicate that continuous real-time detection of the ethanol by SWCNTs-coated acoustic and optical sensors is demonstrated with better resolution for the QCM sensor. 4. Conclusions Fig. 13. Correlation between QCM and SOF sensors based on SWCNTsonto-CdA (2 monolayers-onto-20 monolayers) multilayer for isopropanol, acetone, and ethylacetate, at room temperature.
The results of the inter-relationships are depicted in Fig. 13. The plot fits a linear regression of the data for each vapor examined, with a negative slope for isopropanol and acetone, and a positive slope for ethylacetate. These results indicate an evident correlation of the sensing mechanisms based on the change in the mass and refractive index of the chemosensitive material for QCM and SOF sensors, respectively. In
In summary, three different multi-transducing sensors as SAW, QCM and SOF coated by single-walled carbon nanotubes buffered with cadmium arachidate multilayer have been fabricated and characterized for VOCs detection, at room temperature. Langmuir–Blodgett multilayered films with ordered molecular structure of carbon nanotubes have been successfully transferred onto surface of the QCM acoustic and optical sensors. A nanocomposite of LB SWCNTs embedded in a LB CdA matrix has been fabricated as chemically sensitive interface onto SAW devices for VOCs detection. High sensitivity of carbon nanotubes to VOCs tested has been measured as multilayer and as nanocomposite, at room temperature. Good correlation of the sensing mechanisms of the change in the mass and refractive index of the SWCNTs-onto-CdA for the QCM and SOF sensors has been also measured. On-line detection and calibration curves of SAW, QCM and SOF sensors based on SWCNTs have been reported. Finally, the development of carbon nanotubes sensors based on complementary transducing principles is very promising for sensing applications of multi-transducer and multi-sensor array by using pattern recognition techniques for efficient VOCs chemical analysis, at room temperature. Acknowledgements This work was partially supported by ENEA under project Pro.Te.Ma. The authors are indebted to L. Capodieci for SEM observations.
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Fig. 14. Transient responses of (a) QCM and (b) SOF multi-transducing sensors coated by carbon nanotubes films onto CdA buffer to ethanol decreasing pulses, at room temperature.
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Biographies Michele Penza born in 1964, graduated in physics from the University of Bari (Italy) in 1990. He was a fellow of the Istituto Nazionale Fisica Materia (INFM) in 1991. Since 1992, he has worked at PASTISCNRSM, Brindisi (Italy) first as fellow and then as researcher of the technical staff. His main scientific activities are PVD preparation and characterization of thin films for resistive and acoustic sensing devices, SAW gas and vapor sensors, acoustic sensors, sensor arrays for chemical detection, sensor nanostructured materials, pattern recognition. He has coauthored peer-review 60 scientific papers and has attended international conferences, workshops and schools. He was abroad for scientific training at the Russian Academy of Sciences in 1993, and at other Italian scientific organizations. He is IEEE member, UFFCS member, EDS member, member of Italian Physical Society, member of Italian Association on Sensors and Microsystems (AISEM). Since 2001, he is a researcher of the technical staff of ENEA, Italian National Agency for New Technologies, Energy and the Environment, in the Materials and New Technologies Unit. He currently serves as referee for various journals and magazines on chemical sensors, sensor materials, materials science. Gennaro Cassano born in 1963, obtained electrotechnical diploma in 1984. Since 1990, he was fellow at CNRSM and then starting from 1992, he has worked as technician in the staff of PASTIS-CNRSM. His main activity is devoted to electrical characterization of thin-film gas sensors. He spent a 1-year-period for technical and scientific training at CNRMASPEC, Parma (Italy) and attended to numerous technical workshops. Since 2002, he is in the technical staff of ENEA, Italian National Agency for New Technologies, Energy and the Environment. Patrizia Aversa born in 1963, obtained chemical diploma in 1982. Since 1990, she was fellow at CNRSM and then starting from 1992, she has worked as technician in the staff of PASTIS-CNRSM. Her main activity is the Langmuir–Blodgett deposition of thin films for gas sensors applications. She spent a 1-year-period for technical and scientific training at Chemistry Department, University of Pisa (Italy) and attended to numerous technical workshops and schools. Since 2002, she is in the technical staff of ENEA, Italian National Agency for New Technologies, Energy and the Environment. Francesco Antolini born in 1964, graduated in chemistry at the University of Perugia (Italy) in 1990. From 1990 up to 1994, he worked on protein thin film technology and surfaces modification via Langmuir–Blodgett technique at the University of Genova in which he obtained the PhD in Biophysicis in 1993. From 1995 up to 1999, he spent its post-doctoral activity at the University of Perugia working on biophysical characterization of proteins and on clinical and food biochemistry. From 2000 to 2001, he worked at the Department of Engineering section of Material Science of the University of Perugia in the field of surface chemical modification. Since 2001, he get a permanent position as a researcher at the Material Science unit of ENEA working in the field of chemical surface modification and nanostructured material synthesis. He has co-authored 20 scientific papers and patents and has attended international workshops and schools. He is a member of Italian Chemical Society and F.I.S.V.
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Andrea Cusano was born in Caserta, Italy in 1971. He received the master degree in electronic engineering in 1998 from University of Naples Fedrico II. He finished his PhD course in 2002 focused on the development of optoelectronic sensors based on fiber optic technology for smart materials and structures. At present time he is permanent researcher at University of Sannio, Benevento, Italy. His field of interest is in the area of fiber optic sensors, integrated optics, optoelectronic device for telecommunication applications. His interest in fiber optic sensors is focused on structural applications involving fiber Bragg grating sensors and on chemical sensing based on reflectometric refractive index measurements. Marco Consales was born in 1979 in Benevento, Italy. He received the master degree in telecommunications engineering in January 2004 from University of Naples Federico II, Italy. At present time he is a PhD student at University of Sannio, Benevento, Italy. His field of interest is in the area of fiber optic chemical sensors based on reflectometric refractive index measurements. Michele Giordano, research scientist, received the degree in chemical engineering from University of Naples “Federico II” and the engineer’s diploma qualification in 1992. He completed his PhD in materials engineering in 1996. He joined the CNR in 1996. The main research field is in the area of polymer-based composites. His principal research interests in thermoset based composites manufacturing technologies include: modeling (Resin Transfer Molding, pultrusion), development (autoclave) and monitoring systems (Fiber Optic). Expertise has been developed in the area of damage dynamics of composite structures via acoustic emission techniques.
Luigi Nicolais is a full professor of Polymer Technology at University of Naples “Federico II” and adjunct professor at Universities of Connecticut in Storrs and Washington in Seattle. He is Prepost of the Schools of Science, Engineering and Architecture of University of Napes “Federico II”. He is the author of more than 300 papers on scientific journals, 15 patents and he is also the editor of 6 books. He is member of the editorial boards of the following scientific journals: “Polymer Engineering”, SPE Pub.; “Composite Science and Technology”, Elsevier Pub.; “Adhesion Science and Technology”, UNV Science Press BV; “New Polymeric Materials”, VNU Science Press BV; “Polymer News”, Gordon and Breach Science Pub.; “Journal of Applied Polymer Science”, John Wiley & Sons; “Science and Engineering of Composite Materials”, Freund Publishing House Ltd.; “Advanced Composites Letters”, Woodhead Pubishing Limited; “Composite Interfaces”, VSP Denmark; “Applied Composite Materials”, Rapid Communications and Reviews Kluwer Academic Publishers. He was awarded of the SAMPE (Society for the Advancement of Materials Technology) honor certificate and of the “G. Dorsi” and “Scanno” prizes, and of the Gold Medal of the Academy of the Fourty. Professor Nicolais significantly contributed to development of knowledge in the field of composite materials, rheology, energy and mass diffusion through polymers, materials for biomedical application. The main areas of interest include: biodegradable and natural polymeric systems, damage dynamics, liquid crystalline polymers, packaging design, thermoplastic foams, polymer-based nanostructured composites, conductive composites, polymer blends & reactive processing.
Carbon nanotubes-coated multi-transducing sensors for VOCs detection M. Penza a,∗ , G. Cassano a , P. Aversa a , F. Antolini a , A. Cusano b , M. Consales b , M. Giordano c , L. Nicolais c a
ENEA, C.R. Brindisi, Materials and New Technologies Unit, SS. 7, Appia, km 714, 72100 Brindisi, Italy b Department of Engineering, University of Sannio, Corso Garibaldi 104, 82100 Benevento, Italy c Institute for Composite and Biomedical Materials, CNR, P.le Tecchio 80, 80124 Napoli, Italy Available online 10 August 2005
Abstract Langmuir–Blodgett (LB) films consisting of tangled bundles of single-walled carbon nanotubes (SWCNTs) have been used as sensing nanomaterials onto three different types of sensory systems using complementary transducing principles as surface acoustic waves (SAWs), quartz crystal microbalance (QCM) and standard silica optical fiber (SOF) for volatile organic compounds (VOCs) detection, at room temperature. The multi-transducing probes have been configured as 433 MHz SAW oscillator, 10 MHz QCM oscillator, and SOF reflectometrybased system at a wavelength of 1310 nm. A nanocomposite film of SWCNTs embedded in a cadmium arachidate (CdA) matrix was deposited by LB methodics onto SAW sensors. A LB multilayer of SWCNTs-onto-CdA buffer material was deposited onto QCM and SOF sensors. Highly sensitive, repeatable and reversible responses of the SWCNTs sensors indicate feasible VOCs detection in a wide mmHg vapor pressures range. The VOCs sensing performance of the SWCNTs-based multi-transducing sensors will be discussed. © 2005 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; SAW and QCM acoustic gas sensors; Optical fiber gas sensors
1. Introduction Since their discovery [1], the carbon nanotubes (CNTs) have been extensively studied as nanostructured materials in the both forms of single-walled [2] and multi-walled [1] aggregates for many nanoscience applications due to their unique, outstanding and useful physical characteristics. The CNTs have attracted attention in the fabrication of field effect transistors [3], memories [4], logic gates [5], field emission displays [6], supercapacitors [7], and nanoscale devices [8–10]. A single-walled carbon nanotube (SWCNT) can be visualized as a graphene sheet rolled up into a cylinder consisting of an one-dimensional nanotubular wire with only surface-arranged carbon atoms [11]. Due to their peculiar hollow structure, nanosized morphology and high surface area (>1500 m2 /g), the carbon nanotubes are ideal ∗ Corresponding author. Tel.: +39 0831 201422; fax: +39 0831 201581/423. E-mail address: [email protected] (M. Penza).
0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.06.055
candidates for highly sensitive gas adsorption, hence they are strongly attractive as gas sensor materials [12–16]. Recently, this mix of properties has prompted researchers to develop carbon nanotubes based gas sensors [17–22] making single-walled carbon nanotubes (SWCNTs) an promising sensing nanomaterial for a new class of chemical molecular sensors. The carbon nanotubes grown by the current synthesis techniques [23] (laser ablation, CVD, arc-discharge) are arranged in tangled nets of nanotubules or ropes of aggregated nanochains. The Langmuir–Blodgett (LB) process is a suitable method for depositing defect-free, molecularly ordered ultra-thin films with controlled thickness and orientation. This technique allows fine surface modifications in a film of carbon nanotubes with a highly controlled manipulation to implement molecularly self-organizing nanomaterials. Composite [24] of SWCNTs incorporated in a foreign matrix and multilayer [25] of SWCNTs onto buffer materials attract considerable interest as highly sensitive materials for chemical sensing applications. The LB method is very suitable for covering electrodes array and optical fiber surface
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with a few CNTs or bundles of CNTs to study the sensing properties of monolayers of such nanomaterials. Here, sensing properties of this class of materials have been investigated by using three different types of transducers with complementary principles of operation: a surface acoustic wave (SAW) two-port resonator 433.92 MHz oscillator based on ST,X-quartz substrate, a quartz crystal microbalance (QCM) vibrating at 10 MHz based on AT-cut quartz substrate, a standard silica optical fiber (SOF) using near infrared optical reflectometry at a wavelength of 1310 nm. The main aim of the integration of multi-transducing sensors is that the simultaneous use of chemical detectors with complementary transducing principles can efficiently increase the amount of information extracted from a multi-transducer and multi-sensor array by pattern recognition methods for chemical analysis of VOCs. In this work, SAW and QCM acoustic and SOF optical gas sensors using LB films of nanostructured SWCNTs as composite and multilayer for VOCs detection, at room temperature, have been reported.
2. Experimental 2.1. Langmuir–Blodgett films of carbon nanotubes The pristine material of SWCNTs was purchased from Carbon Nanotechnologies Inc. (Houston, USA). The SWCNTs were produced according to the HiPco process [26] with high pressure CO molecules dissociation at about 1000 ◦ C. The Langmuir–Blodgett films were prepared using HiPco SWCNTs onto acoustic and optical transducers according to a vertical dipping method, as reported in the scheme of Fig. 1. The film deposition system used was a KSV 5000 LB trough system. The geometries of LB films deposition were a multilayer of SWCNTs onto a buffer material of LB cadmium arachidate (CdA) and a nanocomposite of SWCNTs embedded in a CdA matrix. Multilayer: The cadmium arachidate has been used as buffer material to promote the adhesion of CNTs onto transducers. The CdA films have been prepared by LB technique using arachidic acid [CH3 (CH2 )18 COOH] in chloroform spread onto a subphase of deionized water
(18 M) containing 10−4 M cadmium chloride (CdCl2 ). The LB process parameters for preparing CdA films are reported elsewhere [27]. A multilayer with a different number of CdA monolayers has been implemented according to the various transducers used. The LB films of SWCNTs-onto-CdA buffer multilayer have been deposited onto acoustic and optical transducers. For LB SWCNTs film deposition, a solution (0.2 mg/ml) of single-walled carbon nanotubes in chloroform was spread onto a subphase constituted by deionized water (18 M) with 10−4 M CdCl2 . The subphase pH was 6.0 and the temperature was 23 ◦ C. The monolayer was compressed with a barrier rate of 15 mm/min up to a surface pressure of 45 mN/m. The single layer was deposited onto the different sensors with a dipping rate of 3 mm/min. The transfer ratio of the monolayer from subphase to hydrophobic surface of a sensor already coated by CdA film was in the range 0.5–0.7. After a proper drying of 12 h overnight, the sensing SWCNTs multilayer deposited onto CdA was ready for vapor testing. The X-ray diffraction confirmed the monolayer spacing of about 2.0 nm for the SWCNTs deposited. The carbon nanotubes have been implemented to fabricate a multilayer of two monolayers onto the CdA buffer material. The QCM and SOF sensors were used as transducers to deposit the SWCNTs multilayer. Nanocomposite: LB composite of SWCNTs incorporated in a CdA matrix was prepared. A solution (0.734 mg/ml) of arachidic acid [CH3 (CH2 )18 COOH] in chloroform (CHCl3 ) and a separate solution (0.202 mg/ml) of single-walled carbon nanotubes in chloroform were prepared. A volume of 400 l was taken from each solution to form a mixed solution with a total volume of 800 l. This mixed solution was accurately dispersed and stirred in an ultrasonic bath for 1 h. Then, only a volume of 200 l was used for LB composite film deposition. This solution of 200 l was spread onto a subphase constituted by deionized water (18 M) with 10−4 M CdCl2 . The subphase pH was 6.0 and the temperature was 19 ◦ C. The monolayer of nanocomposite was compressed with a barrier rate of 15 mm/min up to a surface pressure of 27 mN/m. The single composite layer was deposited onto the different sensors with a dipping rate of 14 mm/min. The transfer ratio of the monolayer from subphase to hydrophobic surface of a sensor was in the range 0.6–0.7. The weight
Fig. 1. Scheme of vertical dipping deposition of LB films.
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Fig. 2. SEM images of multilayer of CNTs-onto-CdA on (a) QCM and (b) SOF sensors.
ratio of nanocomposite obtained SWCNTs:CdA was as 1:3.625. After a proper drying of 12 h overnight, the sensing nanocomposite deposited were ready for vapor testing. A composite of two monolayers of SWCNTs embedded in CdA matrix was prepared. The SAW sensors were used as transducers to deposit the SWCNTs nanocomposite. Fig. 2 shows typical SEM images of SWCNTs deposited by Langmuir–Blodgett technique onto QCM quartz substrate and silica optical fiber. Both transducers were coated by cadmium arachidate as buffer layer to promote adhesion of carbon nanotubes onto surface of sensors. The surface texture caused by the presence of nanotubes appears clearly with the SWCNTs randomly distributed in tangled networks of nanotubular chains and densely aggregated mats. This surface distribution of the nanotubes enhances the gas adsorption in the nanomaterial. The mean diameter of carbon nanotubes is 1–5 nm and the mean length of the nanotubes is 1–10 m.
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period was of 7 m for 433.92 MHz device. The high Q (>2500) and low insertion loss (<15 dB) make these devices extremely stable when incorporated in an oscillator circuit with frequency output. Each SAW two-port resonator was inserted into an oscillating loop with a low-noise IC-based RF amplifier configured as a modified Colpitts scheme. The oscillator was designed and implemented on a printed circuit board by using SMD electronic components and low-voltage circuitry (5 V, 20 mA). The average baseline noise, intended as maximum frequency change over a time interval as exposure time, for SAW 433.92 MHz devices has been measured as low as 10 Hz, over a 6-min time interval. A conventional 10 MHz QCM has been used consisting of a circularly shaped AT-cut quartz crystal with a diameter of 10 mm and a thickness of 0.1 mm. The Al electrodes on both sides of the quartz were 0.10 m thick and 4 mm in diameter. The QCM resonator was inserted as frequencycontrol element in an oscillating loop close by a low-noise IC-based RF amplifier. The noise of the uncoated 10 MHz QCM sensor was 0.5 Hz in 10 min. The resonant frequency of the QCM-based oscillator was the sensor output. The SAW and QCM resonators have been inserted in a PCB board with oscillator circuitry based on SMD components, as shown in Fig. 3. This electronic board is able to allocate up to 12 sensing acoustic elements (6 SAW + 6 QCM) with multiplexed read-out. The output frequency of SAW and QCM acoustic sensors was measured by a frequency counter (Agilent 53132A) with a multiplexed read-out by a switch unit (Agilent 34970A) driving two 50 4 × 1 rf multiplexers (Agilent 34905A). A standard silica optical fiber was used as optical sensor. The diameter of the fiber and the core was of 125 and 9 m. The sensing probe was prepared by stripping the fiber protective coating (1–2 cm) from the fiber end, which was properly cleaved to obtain a planar cross-section, where the nanostructured layer was deposited. The distance from fiber end for deposition of carbon nanotubes was 5 mm. Reflectance measurements have been performed by lighting the optical fiber sensing interface with a superluminescent diode (40 nm bandwidth) operating at a wavelength of 1310 nm. A 2 × 2 in-
2.2. Acoustic and optical sensors systems design Commercially available SAW two-port resonators (433.92 MHz: Siemens, R2632) operating at 433.92 MHz are used as acoustic elements. The resonators (4.0 mm × 1.0 mm × 0.5 mm) based on ST-cut,X-propagation quartz substrate are mounted on a 3-pin TO39 case. The IDTs
Fig. 3. PCB board integrating 6 SAW and 6 QCM oscillating sensors.
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2.3. Chemical sensing setup
Fig. 4. Reflectometric system based on standard silica optical fiber.
fiber coupler connects the light source, sensing interface and two receiving channels: one for the reflected signal detection and the other one for incident signal monitoring to compensate optical intensity. Synchronous detection has been implemented with the light source externally amplitude modulated at 500 Hz and the sensor outputs retrieved by using a dual channel lock-in amplifier. Here, the normalized optoelectronic sensor output consists of the reflected signal from fiber-coated end divided by the source signal. In this configuration, any effect able to modify the refractive index of the sensing layer changes the normalized output of the optical sensor [28]. The reflectometric experimental system used for optical sensing is shown in Fig. 4.
The experimental setup used for chemical testing of acoustic and optical sensors is shown in Fig. 5. The acoustic and optical sensors have been located in the same test cell (1000 ml volume) for VOCs exposure measurements in simultaneous detection. The VOCs vapors were generated by the bubbling method. Nitrogen was used as reference gas and carrier gas to transport the individual VOCs in the test cell. The test ambient was controlled and switched with some three-way valves operated manually. The total flow rate per exposure was kept constant at 1000 ml/min. The gas flow rate was controlled by a mass flowmeter driven by a controllerunit communicating with a PC via standard RS-485 serial bus. The vapor pressure of six individual VOCs tested of ethanol, methanol, isopropanol, acetone, ethylacetate, toluene was in the range 15–150, 50–200, 20–150, 100–650, 30–270, 10–110 mmHg, respectively. The sensing experiments were conducted at room temperature by simultaneous exposure of acoustic and optical sensors to VOCs. The temperature, monitored by a J-type thermocouple properly located in the test chamber, was measured by a 20-channel multiplexer 2-wire card (Agilent 34901A) incorporated in a data-logger (Agilent 34970A). The output frequency of the SAW and QCM acoustic sensors was measured by a frequency counter (Agilent 53132A) with a multiplexed read-out by a switch unit (Agilent 34970A) driving the two 50 4 × 1 rf multiplexers (Agilent 34905A). Data were collected and stored for fur-
Fig. 5. Scheme of experimental setup for chemical testing of acoustic and optical sensors.
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ther analysis in a PC machine interfaced by a GPIB card in Agilent-VEE software ambient. The output signal of the optical fiber sensor was the normalized optical reflectance measured by the previously described stand-alone system. The optical sensor data were stored in the notebook, which controlled the data acquisition process in LabView software ambient by a NI-DAQ card.
3. Results and discussion 3.1. SAW sensors using SWCNTs filler composite film Fig. 6 shows a typical transient response of two ST-cut quartz-based SAW oscillating 433 MHz sensors, one coated by two monolayers of LB CdA film and other coated by two monolayers of LB nanocomposite of SWCNTs embedded in a CdA matrix. They are exposed, at room temperature and relative humidity of about 10% at 28 ◦ C, to various 5min pulses of toluene decreasing concentrations ranging from 139 to 7 ppm. The SAW sensor response decreases upon a given concentration of analyte for both detectors due to mass loading of vapor molecules adsorbed into sensing material. However, after switching to recovery ambient of nitrogen, the sensors signal overcrosses the baseline and then returns to a stable reference level. This behaviour is repeated for each exposure and for both sensors. The reasons have to be investigated. Probably, this effect should be attributed to CdA film instead of presence of SWCNTs in the nanocomposite. A comparison of SAW sensitivity to toluene between nanocomposite LB film of SWCNTs embedded in CdA and LB CdA film has been performed. The results obtained,
Fig. 7. Comparison of the SAW sensitivity to toluene between nanocomposite 27.5 wt.% LB film of SWCNTs embedded in CdA and LB CdA film, at room temperature. The thickness of sensing material for both SAW sensors is two LB monolayers.
shown in Fig. 7, indicate that the frequency shift as a function of toluene concentration is fitted by a linear regression for both SAW sensors, coated by two monolayers of LB film. The toluene sensitivity of 0.04371 kHz/ppm of SWCNTs nanocomposite is about twice that of unembedded LB CdA film. The vapor sensitivity of the nanocomposite of SWCNTs embedded in a CdA matrix can be enhanced by optimizing the weight ratio SWCNTs/CdA. This better sensitivity of the SWCNTs nanocomposite towards other VOCs tested has been also measured. Moreover, the increased sensitivity of chemical sensors based on composite of CNTs has been reported in literature [24] for acid vapors. 3.2. QCM sensors using SWCNTs-on-CdA multilayer film
Fig. 6. Transient response of SAW 433 MHz sensor coated by (a) CdA film and (b) nanocomposite 27.5 wt.% of SWCNTs embedded in CdA, at room temperature, to different concentrations of toluene.
Fig. 8 shows typical transient responses of the two QCM 10 MHz sensors, one coated by 20 monolayers of LB CdA and other coated by two monolayers of SWCNTs-onto-20 monolayers LB CdA, exposed, at room temperature, to repeated 6-min pulses of various vapor pressures of ethylacetate. Upon individual VOC ambient, the QCM resonant frequency decreases due to mass loading with a response time in the range of 1–2 min and, then the signals are recovered upon N2 reference ambient. In the comparison for each tested VOC vapor pressure, the highest frequency change has been measured for the SWCNTs-functionalized QCM sensor. The high surface area of carbon nanomaterial is responsible for the high vapor sensitivity. The repeatability of the QCM response is quite good, although drift in the baseline of SWCNTs-based sensor is measured. The calibration curves of the QCM 10 MHz sensors coated by a multilayer of 2 monolayers of SWCNTs-onto-20 monolayers LB CdA buffer, and exposed, at room temperature, to five VOCs of ethanol, methanol, isopropanol, ethylacetate and toluene in a wide range of vapor pressures have been
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Fig. 8. Transient response of CdA-coated QCM 10 MHz sensor, with and without SWCNTs, at room temperature, to ethylacetate.
Fig. 10. Transient response of CdA-coated SOF sensor, with and without SWCNTs, at room temperature, to toluene.
reported in Fig. 9. The QCM frequency shift for a given oscillator increases with the mass of the molecules adsorbed into layer. Effects of saturation are measured in the range of the highest vapor pressures tested. The VOCs sensitivity of SWCNTs-based QCM sensor is extremely improved up to 1–2 orders of magnitude higher with respect to the better VOCs QCM detectors based on phthalocyanines and their phthalocyaninato metal complexes [29].
and without SWCNTs sensing overlayer. Fig. 10 shows the typical transient responses to toluene at different vapor pressures for both optical fiber sensors. As in the case of mass sensitive QCM sensors, the presence of SWCNTs overlayer leads to a significant increase in the vapor sensitivity of an average factor of four to six times, even with a extremely thin thickness (4.0 nm) of the SWCNTs overlayer. The measured reflectance decreases upon exposure of the sorbed molecules of VOC investigated by monitoring the changes in the refraction index of the sensitive layer. Fig. 11 shows the calibration curves of the SOF sensors coated by a multilayer of 2 monolayers of SWCNTs-onto-20
3.3. SOF sensors using SWCNTs-on-CdA multilayer film Experimental demonstration of silica optical fiber sensors for VOCs detection has been carried out by two probes with
Fig. 9. Calibration curves of QCM 10 MHz sensors coated by SWCNTsonto-CdA multilayer, at room temperature, for five VOCs. The thickness of SWCNTs film is 2 LB monolayers. The thickness of CdA buffer material is 20 LB monolayers.
Fig. 11. Calibration curves of SOF sensors coated by SWCNTs-onto-CdA multilayer, at room temperature, for six VOCs. The thickness of SWCNTs film is 2 LB monolayers. The thickness of CdA buffer material is 20 LB monolayers.
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monolayers LB CdA buffer, and exposed, at room temperature, to six VOCs of ethanol, methanol, isopropanol, acetone, ethylacetate and toluene in a wide range of vapor pressures. From these results, and considering the minimum detectable normalized output, a vapor pressure resolution of less than 1 mmHg is obtainable. It is worth to note that sensor sensitivity and thus resolution can be significantly enhanced by tailoring the SWCNTs overlayer thickness or by properly designing in terms of buffer layer thickness and refractive index. 3.4. Cross-relationships between QCM and SOF sensors using SWCNTs-on-CdA multilayer film
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Table 1 Cross-sensitivity of the SWCNTs-onto-CdA (2 LB monolayers-onto-20 LB monolayers) multilayer-coated QCM and SOF sensors, at room temperature VOC
Ethanol Methanol Isopropanol Acetone Ethylacetate Toluene
SWCNTs-onto-CdA LB multilayer Average sensitivity of 10 MHz QCM (Hz mmHg−1 )
Average sensitivity of SOF 10−4 (mmHg−1 )
3.460 1.969 4.149 0.043 1.558 7.089
1.861 1.011 −0.803 −0.035 0.397 3.834
The average sensitivity of the both sensors has been used as defined in the text.
In order to investigate the vapor sensing properties of carbon nanotubes to VOCs, repeated vapor exposures have been realized by measuring the sensor response, at room temperature, for two transducers QCM and SOF coated by a multilayer consituted of 2 LB monolayers SWCNTs-onto20 monolayers LB CdA buffer material. Cross-sensitivity of the two multi-transducing sensors to individual VOCs tested of ethanol, methanol, isopropanol, acetone, ethylacetate, and toluene has been measured. In this study, the average sensitivity, Sm , of a QCM device coated with a sensing material may be defined as the mean value of the n responses to different vapor pressures of the same organic solvent: n
Sm =
1 fi (Hz mmHg−1 ) n pi
(1)
i=1
where fi is the steady-state frequency change of the sensor exposed to vapor pressure pi of a given i-solvent tested. Also, the average sensitivity, SV , of a SOF sensor coated by a sensing layer was obtained according to the following equation: n
SV =
1 Vi (mmHg−1 ) n pi
(2)
i=1
where n takes into account the various responses to different vapor pressures for each VOC investigated; pi represents the actual change in the vapor pressure responsible for a consequent sensor output variation, Vi . The results obtained have been reported in Table 1 showing the average sensitivity of QCM and SOF sensors both coated by a multilayer of 2 LB SWCNTs monolayers-onto-20 LB CdA buffer monolayers to VOCs under test. High sensitivity of the SWCNTs-coated sensors has been detected. Both sensors measured highest sensitivity to toluene, then to alcohols considered. SOF sensor detected isopropanol and acetone by reversing the sign of the response. The chemical patterns for six VOCs examined have been reported in Fig. 12. These are important features for VOCs pattern recognition purposes. In order to study the correlation between QCM and SOF sensors coated by SWCNTs-onto-CdA, the responses towards some tested VOCs of isopropanol, ethylacetate and acetone have been cross-referenced, at room temperature.
Fig. 12. Chemical patterns of VOCs using (a) QCM and (b) SOF sensors coated by a SWCNTs-onto-CdA (2 monolayers-onto-20 monolayers) LB multiplayer, at room temperature.
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addition, these specific slopes in the correlation plot can be used to recognize the analytes under test. Fig. 14 reports the typical on-line responses, at room temperature, of two multi-transducing sensors – a QCM sensor coated by two monolayers of SWCNTs-onto-CdA and a SOF sensor coated by two monolayers of SWCNTs-onto-CdA – upon decreasing 10-min steps of ethanol vapor pressure in the range from 28 to 14 mmHg. The results indicate that continuous real-time detection of the ethanol by SWCNTs-coated acoustic and optical sensors is demonstrated with better resolution for the QCM sensor. 4. Conclusions Fig. 13. Correlation between QCM and SOF sensors based on SWCNTsonto-CdA (2 monolayers-onto-20 monolayers) multilayer for isopropanol, acetone, and ethylacetate, at room temperature.
The results of the inter-relationships are depicted in Fig. 13. The plot fits a linear regression of the data for each vapor examined, with a negative slope for isopropanol and acetone, and a positive slope for ethylacetate. These results indicate an evident correlation of the sensing mechanisms based on the change in the mass and refractive index of the chemosensitive material for QCM and SOF sensors, respectively. In
In summary, three different multi-transducing sensors as SAW, QCM and SOF coated by single-walled carbon nanotubes buffered with cadmium arachidate multilayer have been fabricated and characterized for VOCs detection, at room temperature. Langmuir–Blodgett multilayered films with ordered molecular structure of carbon nanotubes have been successfully transferred onto surface of the QCM acoustic and optical sensors. A nanocomposite of LB SWCNTs embedded in a LB CdA matrix has been fabricated as chemically sensitive interface onto SAW devices for VOCs detection. High sensitivity of carbon nanotubes to VOCs tested has been measured as multilayer and as nanocomposite, at room temperature. Good correlation of the sensing mechanisms of the change in the mass and refractive index of the SWCNTs-onto-CdA for the QCM and SOF sensors has been also measured. On-line detection and calibration curves of SAW, QCM and SOF sensors based on SWCNTs have been reported. Finally, the development of carbon nanotubes sensors based on complementary transducing principles is very promising for sensing applications of multi-transducer and multi-sensor array by using pattern recognition techniques for efficient VOCs chemical analysis, at room temperature. Acknowledgements This work was partially supported by ENEA under project Pro.Te.Ma. The authors are indebted to L. Capodieci for SEM observations.
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Fig. 14. Transient responses of (a) QCM and (b) SOF multi-transducing sensors coated by carbon nanotubes films onto CdA buffer to ethanol decreasing pulses, at room temperature.
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Biographies Michele Penza born in 1964, graduated in physics from the University of Bari (Italy) in 1990. He was a fellow of the Istituto Nazionale Fisica Materia (INFM) in 1991. Since 1992, he has worked at PASTISCNRSM, Brindisi (Italy) first as fellow and then as researcher of the technical staff. His main scientific activities are PVD preparation and characterization of thin films for resistive and acoustic sensing devices, SAW gas and vapor sensors, acoustic sensors, sensor arrays for chemical detection, sensor nanostructured materials, pattern recognition. He has coauthored peer-review 60 scientific papers and has attended international conferences, workshops and schools. He was abroad for scientific training at the Russian Academy of Sciences in 1993, and at other Italian scientific organizations. He is IEEE member, UFFCS member, EDS member, member of Italian Physical Society, member of Italian Association on Sensors and Microsystems (AISEM). Since 2001, he is a researcher of the technical staff of ENEA, Italian National Agency for New Technologies, Energy and the Environment, in the Materials and New Technologies Unit. He currently serves as referee for various journals and magazines on chemical sensors, sensor materials, materials science. Gennaro Cassano born in 1963, obtained electrotechnical diploma in 1984. Since 1990, he was fellow at CNRSM and then starting from 1992, he has worked as technician in the staff of PASTIS-CNRSM. His main activity is devoted to electrical characterization of thin-film gas sensors. He spent a 1-year-period for technical and scientific training at CNRMASPEC, Parma (Italy) and attended to numerous technical workshops. Since 2002, he is in the technical staff of ENEA, Italian National Agency for New Technologies, Energy and the Environment. Patrizia Aversa born in 1963, obtained chemical diploma in 1982. Since 1990, she was fellow at CNRSM and then starting from 1992, she has worked as technician in the staff of PASTIS-CNRSM. Her main activity is the Langmuir–Blodgett deposition of thin films for gas sensors applications. She spent a 1-year-period for technical and scientific training at Chemistry Department, University of Pisa (Italy) and attended to numerous technical workshops and schools. Since 2002, she is in the technical staff of ENEA, Italian National Agency for New Technologies, Energy and the Environment. Francesco Antolini born in 1964, graduated in chemistry at the University of Perugia (Italy) in 1990. From 1990 up to 1994, he worked on protein thin film technology and surfaces modification via Langmuir–Blodgett technique at the University of Genova in which he obtained the PhD in Biophysicis in 1993. From 1995 up to 1999, he spent its post-doctoral activity at the University of Perugia working on biophysical characterization of proteins and on clinical and food biochemistry. From 2000 to 2001, he worked at the Department of Engineering section of Material Science of the University of Perugia in the field of surface chemical modification. Since 2001, he get a permanent position as a researcher at the Material Science unit of ENEA working in the field of chemical surface modification and nanostructured material synthesis. He has co-authored 20 scientific papers and patents and has attended international workshops and schools. He is a member of Italian Chemical Society and F.I.S.V.
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Andrea Cusano was born in Caserta, Italy in 1971. He received the master degree in electronic engineering in 1998 from University of Naples Fedrico II. He finished his PhD course in 2002 focused on the development of optoelectronic sensors based on fiber optic technology for smart materials and structures. At present time he is permanent researcher at University of Sannio, Benevento, Italy. His field of interest is in the area of fiber optic sensors, integrated optics, optoelectronic device for telecommunication applications. His interest in fiber optic sensors is focused on structural applications involving fiber Bragg grating sensors and on chemical sensing based on reflectometric refractive index measurements. Marco Consales was born in 1979 in Benevento, Italy. He received the master degree in telecommunications engineering in January 2004 from University of Naples Federico II, Italy. At present time he is a PhD student at University of Sannio, Benevento, Italy. His field of interest is in the area of fiber optic chemical sensors based on reflectometric refractive index measurements. Michele Giordano, research scientist, received the degree in chemical engineering from University of Naples “Federico II” and the engineer’s diploma qualification in 1992. He completed his PhD in materials engineering in 1996. He joined the CNR in 1996. The main research field is in the area of polymer-based composites. His principal research interests in thermoset based composites manufacturing technologies include: modeling (Resin Transfer Molding, pultrusion), development (autoclave) and monitoring systems (Fiber Optic). Expertise has been developed in the area of damage dynamics of composite structures via acoustic emission techniques.
Luigi Nicolais is a full professor of Polymer Technology at University of Naples “Federico II” and adjunct professor at Universities of Connecticut in Storrs and Washington in Seattle. He is Prepost of the Schools of Science, Engineering and Architecture of University of Napes “Federico II”. He is the author of more than 300 papers on scientific journals, 15 patents and he is also the editor of 6 books. He is member of the editorial boards of the following scientific journals: “Polymer Engineering”, SPE Pub.; “Composite Science and Technology”, Elsevier Pub.; “Adhesion Science and Technology”, UNV Science Press BV; “New Polymeric Materials”, VNU Science Press BV; “Polymer News”, Gordon and Breach Science Pub.; “Journal of Applied Polymer Science”, John Wiley & Sons; “Science and Engineering of Composite Materials”, Freund Publishing House Ltd.; “Advanced Composites Letters”, Woodhead Pubishing Limited; “Composite Interfaces”, VSP Denmark; “Applied Composite Materials”, Rapid Communications and Reviews Kluwer Academic Publishers. He was awarded of the SAMPE (Society for the Advancement of Materials Technology) honor certificate and of the “G. Dorsi” and “Scanno” prizes, and of the Gold Medal of the Academy of the Fourty. Professor Nicolais significantly contributed to development of knowledge in the field of composite materials, rheology, energy and mass diffusion through polymers, materials for biomedical application. The main areas of interest include: biodegradable and natural polymeric systems, damage dynamics, liquid crystalline polymers, packaging design, thermoplastic foams, polymer-based nanostructured composites, conductive composites, polymer blends & reactive processing.