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Fiber Optic Chemical Sensors Based on Single Walled Carbon Nanotubes Perspectives and Challenges

Optical Fibre, New DevelopmentsbasedSWCNTs and their feasibility to be successfully employed for practicaof swcnt-based nanocwill also be shown how the typical poor selectivity of SWCNTs-based sensors towards aen chemicalby usingcognition techniqueplied to fiber opticarrays, exploiting both the static and dynamical features of theChemical sensors for environmental monitoitoring is required to protect the public and thefrom toxicased intoand water Airinclude sulfur dioxide, carbon monoxide, nitrogen dioxide, andOCs, whhillsesSoil and water contaminantsmicrobiological, radioactive, inorganic, syntheapplied directly to plants and soils, and incidental releases of otheroriginate fromderground storage tanks, wasnd waste repositories Some of these contaminantsPersist formigrate through large regions of soil until they reach water rewhere thresent an ecological or human-health threat

The current monitoring methods are mainlyFalta, 2000) For this reason, a need exists for accurate, inexpensive, continuous and longoring of environmental contaminarschematically describedpart which, interactingmolecules at or within the surface undergoing physical changes, and of an opportunansducer that converts into an interpretable and quantifiable term such modification of theand external enythature, the selectivity anditynsing part should optimize specific interactions with a target analytealytes, should provide a fast and reversible diffusion of the penetrants, small rtimes and should maintain the physical state so as the geometry over91) Candidate materials for chemicalanicmetalsonductors nanostructured and poroerials(nanomaterials,molecular sieves, sol-gels, aerogels), biomolecules and combination thereoThe natural step following the selective recognition of an anfrom thehemical changes occurring at the sensing part Tincludeechanical (acoustmicromechanical), electrochemical, optical, thermal, andelectronic types Each has strengths and weaknesses relative to the particular application

ber Optic Chemical Sensors based oningle-Walled Carbon Nanotubes: Perspectives and Challengesorphological features Such characterization involveddiffraction(XRD)and Ramanon Electron Microscopy(HRTEM) andcanning Electron Microscopy (SEM)observations In particular, in Fig 4 a is reported thebe carbon materialFurther XRD analyses performed on CdA multilayers on glass substrates evidenced a CdAyer spacing of about 2for cadmium arachidate Theerformed with a SWCNT multilayer, revealed a carbon nanotubes monolayer spacing ofabout 20 nm(Penza et al, 2005_b)

Also Raman spectroscopyharacterize the fabricated LB films based on SWCNTs already depositedicroscope functioning in backscattering configuration employirHeNe laser(633 nm) and 50x and 100x objective lensesused The resultsFig 4 b, where the typical Raman spectrum of a SWCNT film is reported The characteristingentially with respect to the nanotube walls (Saito efremarkable disorder-induced "D-band peak typically in the range 1300-1400epresenting the degree of defects or dangling bonds, can be easily revealeIUU)LAmorphousSwcNT powder =1 mg20 angle (degrees)Fig 4(a)X-ray diffraction spectrum obtained from HiPco SWCNT powder(1 mg)and (b)typical Raman spectrumT film directly deposited on the optical fiber tipIn particular the observation of the two most intense G peaks (labelleand G) confirme single-walled nature of the carbon tubes while their predominant semiconductingcontrary, is broadened for metallicG to D peaksindication of an ordered structure of the deposited SWCNT overlayrth noting that, since the Raman studies have been performed on SwCNT filmIready deposited on the fiber end-face, the results shown also confirm their successfulber technologyHRTEM images of a SWCNT powdreported in Fig 5 at (a) low and (b) high magnification, confirm the nanometric dimensionthe presence between themme Fe metal particles,typical catalyst used in the HiPco production process of carbon nanotubes

Finally, in Fig 5c is reportedEM images of CdA-buffered LB SWCNT filmsposited upon a SOF tip It derf the integration of carbonanotubes with the optical fibereveals their attitude to adhere one to each otherorming bundles or ropes with a-like arrangementFig

5 HRTEM images of Swcnt powder at(a)low and (b) high magnification and (c)sEM4 Environmental monitoring applications: Experimental Resultsthe results obtained during the last years of research focused onttention on chemical trace detection in air and water, at room temperature The strongtentiality of this novel SWCNTs-based fiber optic sensing technology to be employed forn of the voc detection performance of SWCNTs-n carried out by means of an experimental setup adOptical fibeFig 6 Experimental setup used for the vapor testing

ber Optic Chemical Sensors based onSingle-Walled Carbon Nanotubes: Perspectives and ChallengeIn particular the optical fiber probes are located in a prdesigned test chambercating with a PC via standard RS-485 serial bus The controlranging from 10 to 1000 ml/min Numerous tests have been performed by usi the mass floable to drive up to eight different gas-channels, and the gas flchannel was regulated by a dedicated mass flow meter with a fulsor signals, All the experiments have been conductedcapability of SWCNT overlays of undergoing changes in theirconsequence of the adsorption of target analdemonstrated for the first time in 2004(Penza et al, 2004)

In that case LB films consisting ofwCNT bundles were transferred upon the optical fiber tip by using a buffer LB multilayerof Cda pre-deposited on the sensor surface in order to promote the CNT adhesion Theeral vOCs, such as isopropaneethanol, ethanol, toluene and xylene However, in 2005, multilayers of SwCNTs withdifferent thicknesses were successfully deposited directly upon the optical fiber surface by asensitivity Ashows the highest△R/Roexposed to 30-minutes decreasing concentration pulses of xylene vapors witlart optoelectronic sensorged in the Cda buffered configuration (2eflectance changes occurred on analyte exposure as consequence of the variation in theWCNT overlay refractive index promoted by toluene molecules adsorptio△R/ Ro occurredvapor exposure to CdA -buffered and un-bufferedSWCNTS-based opto-chemicalm temperature,

0Optical Fibre, New DevelopmentsBothrs exhibited the capability of detecting the chemical under investigaes combined with fast response, completof magnitude higher than the ones provided by the counterpart probe in the CdA-bufferedlight of this results, strongas devoted to the investigation of theing capabilities of SWCNTs-based opto-chemical sensorsed in the un-bufferedfiguration against several VocIn Fig 8 a arefour Ported the results of toluene vapor testing carried out by exposing the probeobtained confirmed the behavior of the fiber optic sensor exhibited durAR/Roas observed asf analyte molecule adsorption, A less pronounced dependence of the response time onluene concentration was noticed, revealing that different adsorption decognized, due to little thermal changes in the not perfectly thermo-stated test chamber bee vOc under investigorder to investigate the reliability of the proposed transducers, a repeatability test haseen carried out for theThe results are shown in Fig 8 b, where theAR/Ro occurred as consequence of two xylene exposures at 21vident, the opto-chemical probe demonstrated high repeatability and reliability also at veryhalve concentrations-4- sOF: 4 Mon SWCNTs-SOF:4 Mon

SWCNTsa)(b)Fig 8(a)Response of sensor SOF-4 to toluene vapors at room temperature and(b)d exposingvaporsThese results have to be consideretant temhermal variationoccurred meanwhile, Her it is evident that monitoring ofmal drifts andensation of their effects on sensor response are strictly required to not affect the systemproper fibere sensors which could be eithertegrated with the optical fiber probe( Cusano et al, 2004) Similarly, CNTs-ased fiber optic chemo-sensors demonstrated a relevant sensitivity also to humiditychanges(Consales et al, 2006_b), thus revealing the necessity of proper calibrations andmpensations of the sensor response, especially when high accuracy is requesed

ber Optic Chemical Sensors based oningle-Walled Carbon Nanotubes: Perspectives and ChallengesIn Fig 9 have been reported the calibrationSOF-4 obtained against tolualmost linear behavior in the investigatedoncentration ranges In addition, by comparing sensorSam=(△RRonico Di Natale 2001)a SOFivity to xylene(Xylene"1wo times higher than that to toluene(Stoluene=47-10-4 ppm- ')has been found Consideringachievable with the exploited interrogation unit, a resolution ofapprox 290 ppb and 120 ppb have been estimated for toluene and xylene, respectivelSOF: ILENElon(ppm)9 Calibrationor SOF-4, exposed to tolhan three orders ofmeans of a fluorosiloxane polymer-based Surface Plasmon Resonance(SPR)optical fiber(900 ppm and 190 ppm, respectivelyr toluene and xylene)(Abdelghani Jaffrezic-Renault, 2001), and more than two orders of magnitude higher than that provided bymultimodal optical fiber sensor sensitized by phenyl-modified porous silica(100 ppm and20 ppm, respectively)(Abdelmalek et al

, 1999)players number and cda buffer on sensor sensitivityHere, the influence of the number of SwCNT monolayers onconfigurations are also better discussed To this aim, four opto-chemcoated bbers of monolayers, directly depositO LB monolayers of CdA, have been simultaneously exposed to told xvleneaporsg 10 a reports the AR/Rotoluene concentration for all the tested sensors It can bethe contrary, the optical chemo-sensor coated bythicker SWCNT film, exhibited a negativeensitivity (-07-10- ppm-') This means that for this sensor the fiber-film interface reflectancefact that the film reflectance, and thus also thetrongly dependent on the thickness and refractive index ofCNTthe fact, depending on the geometric featuresof the film deposited atop the optical fiber,analyte can be obtained

UTOLUENEXYLENEng four fiber opticcoated by a different number of SwCNT monolayers to(a) toluene and (b) xylene vapFor the same reason, the Cda buffer multilayer, whose optical properties are quite similar tothose of the standard optical fibers, and thus not optimized for the particular configurationthat by choosing a buffer-linker material whose opticalgeometrical features (such as the multilayer thickness and refractive index) are welptimized for the specific configuration, one could be able to strongly enhance the SOFFrom Fig 10 b it can be seen that the four fiber optic chemo-sensorssensitivitsted, but to the optical configuration exploited

This feature could be very useful fortternanalysiselementarysors are exploited to improve the analyte discrimination(Penza et al, 2005-b)As thendoftimes(and007_b)byosure to different concentration pulses ofhe exposure time was 30 minutes) The respome the output signal needed to pass from 10% to 90%(from 90% to 10%)of the total signalon The results obtained for both chemicals are quite similar

ber Optic Chemical Sensors based oningle-Walled Carbon Nanotubes: Perspectives and Challengestime to increases withte concentration while thetime is quitenstantresponse time inwas approx 7 minutes while the maximum one, obtained for 93 ppm, was approx 11bserved in case of xylene exposure, for which the SOF response times increased fromcules inside the CNT sensitive nanocoatings It is noteworthy, however, that theesponse times of the proposed fiber optic chemo-sensors are relatively good taking intorate per exposure It is alsnoting that although these times could be reduced by depositing a lowerf carbon nanotu bes on the sof tip the chof the geometric features of the sensitiveyidering the trade-off that exists between serandd recovery)time(Consales et al, 2007 b)42 Towards fibergue:chemical trace detectid on the feasibility of exploiting the excellensing properties of CNTs for the developmeoptoelectronicapable of chemical trace detection in aqueous environments, at room temperatureis reported in Fig 11

a The SWCNTs-based opto-chemical transducers have been inserted inontainingter The presence within the test ambient of the analyte undervestigation has been promoted by its injection inside the beaker The injected volume haseen chosen, each time, in order to obtain the desired analyte concentration The pollutedwater has been continuously stirred to ensure maximum dispersion of analyte In addition,fter each exposure, the capabilities of SoF sensors to recover the initial steady state leveave been investigated by restoring the initial condition of pure water: pure water has beentinuously injected in the test chamber, while the contaminated water, previously present12 Monolayers of SWCNTSFig 11(a) Experimental setup exploited for chemical trace detection in water and (b)AR/R

ber Optic Chemical Sensors based oningle-Walled Carbon Nanotubes: Perspectives and ChallengesEach transduction principle can be implemented in a variety of configurations, andabricated by multiple approaches, resulting in many different sensing platfoA number of chemical sensors have been developed for environmentacarried operating mechanisms The most exploited transduction principles in cherhe mass change and the resistivity /conductivity change of the servironmental analytes Thefirst physical parameter is in many cases measured by the shift in the resonant frequency ofSurface Acoustic Wave(SAW)(Hartman et al, 1994; Kepley et al, 1992; Penza et a2006_a) They typicallysensitive layer(Grate, 2000), howeveSAW and QCM-based chemicaors using other sensitive coatings have been proposed(Zhang et al, 2004; Penza et al, 2005-a) Instead, resistivity/conductivity changes areainly semiconducting metal oxides and conjugated polymers)deposited between twothe name of Metal Oxide Semiconductors(MOSand Conductive Organic Polymer (COP) sensors (ames et al, 2005) During the last twoecades, however, a remarkable interest has been also focused on optical transductionment of chemical and biological quantities(Baldini et al

2006)Ansing including ellipsometiptical waveguide structures), spectroscopy of guided modes in optical ometry inRaman), interferometry(whiteveguideructures (grating coupler, resonant mirror), and surface plasmon2004; Zudans et al, 2004; Steinberg et al, 2003; Orellana, 2004; Homola et al, 1999; Mignanial 2005; Arregui et al, 2003; Brecht &z Gauglitz, 1995; Gauglitz, 1996; Boisde, 1996) Inof analyte molecules or of1o-optical transducing mediumolicahead to be remotely located from the instrumentation This feature isn temperature occur In addition the small size, light weight and highProvided the optical power density are within certain limits, fiber optic chemical sensors areluch safer in explosive environments compared w

itrs involving electrical signals,gas explgnetic interference from, for examples, power lines and electrical machineryurthermore optical fibers have the capability of carrying a hugluch greater than that carried by electrical wires Fiber optic sensing is very versatile, sincetvlength, phase and polarization of light can all be exploited aseasurement parameters,direction form independent signals This gives the possibility to monitor several chemicalswith thmultaneously monitor unwanted environme

40Optical Fibre, New Developmentsparameter variatihich could drasticallthe chemical concentrationements, such as the temperature or disturbance of the fiber Mis also relatively easying expensive source or analysisThis contribution reviews the integration of carbon nanotubes as advanced sensitivetings with optical fiber technology for the development of high performance opto-nsors exploitable for several environmental monitoring applications Inparticular, the excellent sensing capabilities of the realized photonic chemo-sensors againsOCs and other pollutants in air and water, at room temperature, will be reviewedCarbon nanotubes: advanced materials for chemicalhe search for new advanced materials is an important area of contemporary research inructured materials of different chemical composition, produced yearsus disciplGreat attention has been paid innowires or nanotubes Similarly, there has already been great interest in their prepainssyroperties and applications in the literature As matter of fact, with the development ofano-science and nano-technology, a large number of literatures on one-dimensionalanostructured materials, including tubes, rods, belts, and wires in this area, have beenpublished every year(Huang Choi, 2007) These materials have their unique structuresre-like structure whose diameter varies orexhibit unique properties and morphological lexikarticular, carbon-based nanostructuresmultifunctional and compatible with organic and inorganic systemCarbon nanotubes(CNTs), discovered by lijima in 1991 (lijima, 1991) are at the forefront ofhe novel nanoscale investigations and nanostructure effects due to thechemical, structural, optical, mechanical and thermal properties depending on their specific2001:Darbon, and can essentially be thought ofof graphite rolledto a tube to form a cylinder with diameter of few nanometers and length ranging from 1 tons

To date, CNTs are building blocks considered as the most promising funde attractive characteristicteristics for gas sensingapplications In fact, due to their unique morphology, CNTs possess the excellentilitrb molecules ofergoingheir electrical, geometrical and optical properties, such as conductivity, refractive index,thickness etc CNTs can be distinguished in SWCNTs or multi-walled carbon nanotubesMWCNTs)depending on whether only one layer or many layers of graphiteepending upon theand chirality(the way the hearranged alongS,2003) SWCNTs are aimportant variety of carbon nanotubeecause they exhibit important electrical and sensing properties that are not shared by theMWCNTs variants The purity of the CNTs affects their sensing performance, thus efficientPmetallic impurities and amorphous carbon particles havbeen developed as well(Penza et al, 2007-a

ber Optic Chemical Sensors based oningle-Walled Carbon Nanotubes: Perspectives and Challenges22 Chemical senThe special geometry of carbon nanotubes and their characteristic of being all surfaceeat potential applications as chemical sergaseous and voc molechany studies have shown thatnanotubes are robust and inert structures, their electrical properties are extremely sensitivechemical doping bymolecules, The electronicstructures ofar the semiconducting nanotubes cause measurable changesto the nanotubes electrical conductivity Nbasede highly sensitive, but they are also limited by factors such as their inabilitydsorption energies, poor diffusion kinetics and poor chargesing SWCNTs to electron withdrawing(eg NO2) or donatingdecreases the electrical resistanceWWCNTs in the transistor scheme In addition CNTs-based sensors demonstrated a fastor less in the same period itSWCNTs could be modified in presence of Oz(Collins et al, 2000) The effect of thesorption of several gasds in SWCNTs was also described (Sumanasekera et al00), as well as those of water vapor on the electrical resistance of a SwCNT(Zahab et al000) Shortly afterwards, Fujiwara et al( Fujiwara et al 2001) studied the N2 and O2sorption properties of SwcNT bundles and their structures All these studies opened thedoor to the development of chemical sensors based on CNTsSensing devices based not only on the changes in the electrical properties of CNTseasured the thermoelectric qualitative response to a variety of gases(He, N2, H,, O andH3) Sumanasekera et al

(Sumanasekeal, 2002) created a thermoelectric chemicalo measure the easily detectable and reversible thermoelectric power changes ofWCNTs when they are in contact with He, N2 and Hz Chopra et al( Chopra et al, 2003)tor coated with swCnselectively detects the qualitative presence of several gases(NH3, CO, Ar, N2 and O2)due toanges in the dielectric constant and shifts in the resonant frequenei et al(Wei et al, 2003) demonstrated a gas sensor depositing CNT bundles onto aiezoelectric quartz crystal This sensor detected CO, NOz, H2 and N2 by detecting changesIency and was more effective at highd used them to detect VOCs such as ethanol, ethylacetate and toluene by measuring theownshift in thealized with molecules enablingteraction with target chemicals thus improving thectivity of cnts-basedes In this way, different types of sensors based on molecular recognition interactionsan be developed, allowing the development of narrs that are highly selective andsensitive Chen et al (Chen et al, 2003)used a non-covalent functionalized FEt based

Optical Fibre, New DevelopmentsSWCNTs for selectivenizing target proteins in solution Azanian et al (Azamian et2002)immobilized glucose oxidase on SWCNTs and enhanced thegnitude com(Sotiropoulou et al, 2003) worked with enzymes Barone et al(Barone et al, 2002)B-D-so shed twodistinct mechanisms of signal transduction: fluorescence and charge transferecently, further interest has also been devoted to the possibility to change the opticaldyor geometrical properties of SwCNTs upon adsorptiorous environmental monitoring application, from chemical detection in air andConsoles2006a;Pet al 2004 b: p2005b:Consales et al, 2007-a)to hydrogen detection at cryogenic temperatures suitable forospace applications(Cusano et al

, 2006_b) In particular, the possibility of exploitingonitoring, characterized by ppm and sub-ppm resoluti Pable of air and water qualityandgood recovery features and fastresponses, as it will be seen in section 4 and 53 Opto-chemical sensors in reflectometric configurationThe reflectometric configuration is essentially based on a low finesse and extrinsic FabrPerot(FP) interferometer and, as sclis given by the fiber/sensitive layer interface whereas the second one is given bythe sensitive laver/external medium interfaceSingle-mode optical fiber Sensitive layer1 Schematic view of the Fabry- Perot-based configurationFirst described in 1899 by Fabry and Perot(Fabry Perot, 1899), the interferometer kitheir names makes use of multiple reflections between two closely spaced surfaces Infact the light is partially reflected each time it reaches theth each other The amof light reflected athe first interface can be calculated as the sum of the multiple reflected beams and isstrongly influenced even by very small changes of the distance between the two surfaceshe sensitive layer thickness) or its optical properties(the sensitive layer refractive indeakin cusabased sensing in the two past decades, especially for the detection and meats ofd biomedical parameters (ackson, 1994; Chan et al, 1994) All

ber Optic Chemical Sensors based oningle-Walled Carbon Nanotubes: Perspectives and Challengesthese characteristics, combined with theibilityals with the optical fibers by means of very simple, low costThe principle of operation of an optoelectronicreflectometric configuration relieson the fact that a modulation of the intensity of light reflected at the fiber-serlay interfacedue to changes in layer thickness(dsm) and complex refractiveAs matter ofe fiber-film reflectance can be expressed as(Macleod, 2001)aF4// is the overlay absorption coefficient, y and Mert are the opticalfiber and external medium refractive index, and a is the optical wavelength Thus, thereflectance changes due to the chemical interaction between sensing overlay and targetAn+△a+S·A+S→Mk+S,△dwhere sn, Sagainst thetions of the effective refractiveindex, the absorption coefficient and the overlay thickness, respectively

They strongln they have to be properly considered case by case In particular, several effects couldchangelence of the analyte molecuof all, swelling of the SWCNT nano-compositearis that leads to a consequent increase of the film thickness; also, refractive indexas expresse72; Soref Bennet, 1987: Heinrich, 1990)a change either in the real part of the refractiveindex or in the absorption coefficient could bevery low chemical concentrations are considered (as in this work), it can be asthat theanalyte molecule adsorptat constant overlay thickness(Ad(3)

Optical Fibre, New Developmentsogation systemrtantth sensors is the driety of schemes have been proposed for the intf a fiber opticbasedthe FPavelength reflectometry(Kersey Dandrige, 2001) Here the attention has been focusedquires just few widespreadrfce In addition it enables the fabrication of cost-effective, reliable, robust andents, whichfactors of crucial importance for in-situ and long-termmonitoring applications and for the desired technology transfer to the market The typicalterrogation scheme enabling the reflectance monitoring of a FP cavity realized on theistal end of an optical fiber is shown in Fig 2(Consales et al, 2007b)1310 nm and a bandwidth of approx 40 nm), a 2x2 coupler and two photodetectors

Ittput signal I that is proportional to the fiber-film interface reflectance R andat is insensitive to eventual fluctuations of the optical poweralong the wholeement chain As matter of fact, emitted light is splitted by theer and directed toput thus probe(where partial reflection occurs)and to the first(Powree) Reflected ligth is directed through the coto the) proportional to Preflectance(R) The intensity compensation is obtained by consideritignals at the two photoreceiversa·R4where a is a constant accounting for all the set-up parameters In the folloout Al/lo is considered (where lo is the output signal in thtive layer interface (AR/Ro) Synchronous detection is typicallenhance theat 500 Hz and retrieving the photodetector voltages by using a dual channel lockamplifier The minimum AR/Ro that can be detected by means of this interrogation system,culated considering the maximum scattering on theDivision Multiplexing (TDM) approach is typically exploited to perform theoon, al for a time interval of at least 10 minutes, is typically in the rasimultaneous interrogation ofeight optical probes by means of a fiber optic switch

ber Optic Chemical Sensors based onOptical switchx CoFig 2 Schematic illustration of the typical interrogation scheme adopted for the singleg of an optical cavity realized upon the fiber2 Opto-chemicalr fabricatiin films of SWCNTs with a controllable thickness is an important basisture development of their scientific understanding and technological applicationsus proposals exist for theirgle tube or thin film architectures(Bachtold et al, 2001)ere the Langmuir Blodgett(LB) technique has been chosen as way to transfer nanometerale layers of SwCNTs upon either bare optical fibersdium Arachidate (CdAbuffer-linker material, previously deposited(by the same technique) upon the fiber end inchosen as buffer material due to its peculiar amphiphilic molecular structure suitable for LBdeposition process (Takamoto et al

, 2001; Di Luccio et al, 2004)The LB-technique is one of the most promising techniques for preparing such thin films as itol of thelayer thickness, homogeneous depolayer over large areas and the possibility of making multilayer structures with varyinglayer composition(Roberts, 1990) An additional advantage of the LB technique is thatalmost any kind of solid substrate, Howeages have to be traded with the low speed of the deposition procedure as well as thelimited number of materials suitable for this technique As reted in Fig 3, themolecules of the films to depofirstly dispersed onto the surface of a sub-phastypicallynted with the hvdbic part upwards and witha reduction of the surface area occupied by each moleculesurface pressure in which the molecules are densely packed forming an highly orderedy ames Tatam, 2006) From this phase the moleculestransferred to a properlyleaned and prepared solid substrate by its dipping through the condensed Langmuir layerhase is reachedt high surface picontinuous reduction of theloving barriers is performed when the molecules are transferred from the sub-phase to thebstrate in order to keep the surface pressuret,ensuring that the solid phase ismaintained Repeated dipping of the same substrate arethosition of a thin filmlaver at a time

Optical Fibre, New DevelopmentsNOLID PHASEFor Cda buffer multilayer deposition, a solution (0953 mg/ml) of arachidic acid[CH3(CH2)s COOH] in chloroform is typically spread onto a sub-phase of deioniz(18 MO)containing 10-4 M cadmium chloride(Cdcl2) The sub-phase pH is kept constant afth the temperature fixed at 23C Thmm/min up to a surface pressure of 27 mN/m The single layers isdeposited on the SOF with a vertical dipping rate of 12 mmAfter a proper drying ofhours the coated fiberg layFor SWCNT film deposition,a(0

2 mg/ml) of SwcnT pristine material inchloroform is spread onto a sub-phase constituted by deionized w8 MQ)with 10-Mof CdClz The sub-phase pH and the temperature are kept constant at values of 60 and 23%(The monolayer is compressed with a barrier rate of 15 mm/ min up to a surfacem/min Afterdrying of 12 hours overnight, the sensing multilayers depositedmarcial SwCNiithout any purification treatment The samples areting swcnt1 hour attemperatureFs are previously accurately polished from the acrylic protection and cleaved withecision cleaver in order to obtain a smooth and plane surface Then, they are washedchloroform and dried with gaseous nitrogen to be ready for the deposition The numberdeposited CdA and or SwCNT monolayers are controlled by choosing how many times the3 Structurhological characterization of SWcNT overlaysas of the depositedB sWcnt films has been carried out in orderivestigate their structural and