Selective fluoride ion sensing in aqueous medium using ultrathin film of functionalized single-walled carbon nanotubes

The presence of fluoride ion (F - ) in potable water above its permissible limit (1–4 ppm) poses serious health hazards. Hence, detection of fluoride in potable water is essential. The π-electron rich single-walled carbon nanotubes can interact with F- to form semi-covalent C-F bond which can act as a basis for F - sensing in aqueous medium. Here, a single layer of octadecylamine functionalized single-walled carbon nanotubes (ODA-SWCNTs) was transferred onto solid substrates by the Langmuir–Schaefer (LS) method and employed for sensing of F- in aqueous medium by recording piezo and electrochemical responses, simultaneously using an electrochemical quartz crystal microbalance. The lowest detectable concentration and range of detectable concentration of fluoride ion were found to be 0.5 ppm and 0.5–145 ppm, respectively. The analysis of the LS film of ODA-SWCNTs before and after interaction with fluoride ion by Raman spectroscopy and grazing angle x-ray diffraction measurement reveals perturbation of π-electrons of the SWCNTs due to semi-covalent binding of the fluoride with the carbon atom of the nanotubes. The sensor showed a good selectivity towards the F- in the presence of some heavy metal ions. Testing of the sensor towards F- in tap water obtained from some local region showed a good accuracy.


Introduction
Fluoride ion (F -) is one of the most abundant contaminants in water which can pose serious threats to human health.The intake of higher concentrations (>1-4 ppm) [1] of fluoride ion through drinking water may lead to serious health problems such as dental fluorosis and even cancer.The surface and ground water in some region can gets contaminated by the dissolution of fluoride ions from the various minerals.Water gets contaminated with fluoride ion due to the release of effluents by industries such as aluminum, steel and fertilizers.Hence, precise detection of F -in drinking water is very important.Reports in the literature indicate that the F -can be sensed by using an ion-selective electrode, fluoride hydrogen bonding, boron fluoride complexation, and desilylation [2][3][4].The reported sensing protocols lack high resolution, sensitivity, and cost-effectiveness.Apart from the sensing techniques, the functional material also plays an important role in a sensing device.A variety of materials used for the F -sensing is available in the literature viz.porphyrin, fluorescein dye (fluorescein isothiocyanate (FITC-OSi)), boron dipyrromethene compound and carbon nanotubes (CNTs) [5].The use of CNTs for sensing contaminants in water medium has gained a lot of interest because of the unique properties which include viz.rich in π-electrons, tunable surface chemistry, adjustable pore size and groove in CNT bundles [6].The surface chemistry and grooves of CNTs can be used to facilitate the adsorption of both organic as well as inorganic contamination in an aqueous medium [7].Choi et al have reported sensing of anions (AcO-, Cl-, Br-, NO3-) using the functionalized SWCNTs.The chemiresistive based sensing was performed in acetonitrile solvent [8]. on It has been reported that the surface of the glassy carbon electrode was modified with multiwalled carbon nanotubes (MWCNTs) and a composite of MWCNTs with isothiocyanate and utilized them for F -ion sensing using electrochemical platform [9].Although the MWCNTs are cheaper, the sensing merits of SWCNTs are far better than that of MWCNTs [10][11][12].Such enhanced performance is due to structural difference between them leading to improved physicochemical properties for sensing application.Due to the single walled structure, SWCNTs have a larger specific area and has the higher density of active sites for the analyte interactions.The surface of a device can be tailored for better performance by the deposition of singlewalled carbon nanotubes(SWCNTs) in form of ultrathin film.The ultrathin film offers not only an enormous gain in surface-to-volume (SV) ratio but also facilitates charge transfer mechanism during electrochemical processes [13,14] .The simple and economical way to deposit an ultrathin film on a solid substrate is to transfer the Langmuir film (LF) from the air-water (a/w) interface by either a vertical or horizontal dipping mechanism popularly known as Langmuir-Blodgett (LB) or Langmuir-Schaefer (LS) method, respectively [15,16].Therefore, it is essential to obtain a stable LF at the a/w interface for LB/LS film deposition during device fabrication.Though SWCNTs are purely hydrophobic in nature, it can be chemically functionalized by organic ligands e.g.amine, carboxylic, or hydroxyl to enhance the physicochemical properties and processability during the device fabrication.The present study employs octadecylamine functionalized SWCNTs (ODA-SWCNTs) for the formation of ultrathin film on the surface of a device for F -sensing in an aqueous medium.The F -are known to interact strongly with the π-electrons of the CNTs and form a semi-covalent C-F bond [17][18][19].In addition, the amine group of the ODA-SWCNTs can form amine-fluoride complex through hydrogen bonding [20][21][22].Therefore, the functional layer of an ultrathin film of ODA-SWCNTs can act as a suitable candidate for the sensing of F -in the aqueous medium.
Here, we utilized the ultrathin LS film of aligned ODA-SWCNTs as the functional layer in electrochemical quartz crystal microbalance (eQCM) for F -detection in aqueous medium.The eQCM technique is considered as one of the most suitable, inexpensive, and easy-to-handle techniques for sensing applications [23].The SWCNTs can be aligned in the ultrathin film by deposition through LB/LS method.An eQCM was set up in the laboratory for simultaneous multi-parameter viz.electrochemical and piezoresponse measurement.The sensing performance by assessing multi-parameters is reliable and can enhance the accuracy of decision making [24].Large morphological changes were observed in the LS film of ODA-SWCNTs before and after interaction with the F -.The Raman spectroscopy and grazing angle x-ray diffraction (GIXD) analysis indicated the signature of perturbation of π-electron of the SWCNTs to form a semi-covalent (C-F) bond.The sensor showed good selectivity towards F -in the presence of some heavy metal ions viz.Cd +2 , Pb +2 and Hg +2 .The sensor showed good accuracy when tested using real samples from tap water.

Experimental
ODA-SWCNTs (P5-SWNT) were purchased from Carbon Solution Inc., USA.The carbonaceous purity of the material was > 90%.The bundle length and bundle diameter of ODA-SWCNTs is in the range of 500 nm − 5 μm and 20-50 nm, respectively.The diameter of individual SWCNT is 1.55 ± 0.1 nm.Sodium fluoride (NaF) was purchased from Merck Life Science.We have developed an electrochemical quartz crystal microbalance (eQCM) sensor by integrating a QCM (Stanford Research System, USA) with a handheld potentiostat (CHI 1200 C) which allows the simultaneous measurement of piezo and electrochemical responses [24].The AT-cut 5 MHz quartz wafer having a diameter of 24.5 mm possesses a co-centrally circular gold patch of diameter 10 mm.The circular gold patch facilitates the immobilization of ligands through thiol-gold interaction.The quartz and Si wafers were made hydrophobic by depositing self-assembled monolayer (SAM) of octadecanethiol (ODT) and hexamethyldisilazane, respectively.
The details of surface manometry and GIXD are presented in Supplementary Material.A single layer of ODA-SWCNTs was transferred onto ODT functionalized hydrophobic quartz wafer and the hydrophobic silicon substrates through the LS technique at a target surface pressure of 20 mN m −1 .The transfer ratio was found to be 0.8 ± 2. The ODA-SWCNTs functionalized quartz wafer was connected as a working electrode of the potentiostat.The platinum wire and Ag/AgCl 2 electrodes were utilized as counter and reference electrodes, respectively.The piezo and electrochemical responses were recorded simultaneously, in the presence of supporting electrolyte (HCl, pH4).A fixed range of the potential from 0.5 V to −2.5 V was applied on the working electrode at a rate of 0.1 V s −1 and the corresponding current was recorded.The piezo and electrochemical responses were recorded for a wider concentration range of F -in the aqueous medium.In piezoelectric response, the change in resonance frequency (Δf) of the functionalized quartz wafer due to its interaction with F -can be related to the change in mass per unit area (Δm) of quartz wafer using the Sauerbrey relation [25] Δf = −CΔm, where C = 56.6Hz cm 2 /μg, a constant dependent on the experimental parameters.The resolution of the QCM and potentiostat were 0.1 Hz and 0.3 pA, respectively.The morphology and composition of the films of ODA-SWCNTs before and after interaction with F -were studied using field emission scanning electron microscope (FESEM) (FEI, APREO with Aztec standard).The Raman spectra and GIXD curves of LS film of ODA-SWCNTs before and after interaction with F -were obtained to investigate the structural changes of ODA-SWCNTs after the interaction with F -in aqueous medium.The Raman spectra were obtained using LabRAM HR Evolution (Horiba).The excitation wavelength for Raman spectroscopy was 633 nm (He-Ne laser) focusing through X50 objective lens on the sample for 60 s.The spectra collected were average of 50 scans.The films were scanned at several places to ensure the repeatability of the data.All experiments were carried out at 22 °C.

Results & discussion
The surface manometry indicated that the LF of ODA-SWCNTs at the a/w interface is very stable and exhibited gas and liquid-like phases (Supplementary Material).The ultrathin film of ODA-SWCNTs was deposited onto hydrophobically treated Si substrate by the LS method and the morphology was obtained using FESEM.The morphology was compared with that of the spin coated film deposited onto the similar Si substrate by spreading 20 μl of the chloroform solution of ODA-SWCNTs while spinning the substrate at a rate of 900 rotations per minute for about 120 s.
The bundle of ODA-SWCNTs in the LS film are found to be aligned in the FESEM image (figure 1(b)) whereas a dense random network of the bundle of the nanotubes can be observed in the spin-coated film (figure 1(a)).The distribution of the nanotubes was non-uniform in the spin coated film of ODA-SWCNTs.There are few reports in literature wherein the aligned SWCNTs in the ultrathin film deposited by LB or LS techniques [26][27][28] can offer superior sensing performance as compared to the random network of the nanotubes [13,27,29,30].The ultrathin LS film of ODA-SWCNTs can act as a potential functional layer for sensing F -in aqueous medium using the developed eQCM.
On potential scan from positive to the negative direction (0.5 V to −2.5 V), two cathodic current signals at the potential of −0.6 V and −1.2 V were observed as shown in figure 2(a).A strong electronic interaction occurs between lone-pair electrons of F -and carbon of the nanotubes which can form semi-covalent (C-F) bond [19].This bonding may prohibit the π-electron activity in ODA-SWCNTs.The formation of C-F bond can induced effects in ODA-SWCNTs structure [18].Additionally, the amine group of ODA-SWCNTs can form complex with F -to yield amine-fluoride [20][21][22].The anodic current was observed during the reverse scan of applied potential.As applied potential scan towards a positive direction, the fluorine bonded with the ODA-SWCNTs was oxidized into F -.This is the oxidation process corresponding to which adsorbed fluorine may dissociate from the surface of the quartz wafer.As no peak appeared during this scan (figure 2(a)) which indicates the quasi-reversible nature of the electrochemical response of F -sensing.The bindings of F -with ODA-SWCNTs can change the mass of the functional layer over quartz wafer which can be detected by monitoring change in resonance frequency (Δf) with respect to a reference as measured using eQCM.Figure 2(b) shows the variation in Δf as a function of time for two different concentrations of F -in aqueous medium.These were measured simultaneously during the potential scan of the cyclic voltammetry.It can be observed that the decrease in Δf with time corresponding to the cathodic trace (0.5 V to −2.5 V) during the electrochemical measurement can represent adsorption of mass.This may indicate the F -bind with the functional layer of ODA-SWCNTs during the cathodic trace.Similarly, the increase in Δf during the anodic trace (−2.5 V to 0.5 V) represents loss of mass due to desorption of fluoride from the functional layer of LS film of ODA-SWCNTs.The cycle of adsorption and desorption repeats on sweeping of voltage during electrochemical process.The minimum value of Δf for a given concentration of F -suggests the maximum degree of adsorption of mass on the functional layer of ODA-SWCNTs at that concentration.The sensing performance of F -in aqueous medium was evaluated using the functional layer of ODA-SWCNTs deposited by LS and spin coating techniques.The piezoresponse data in terms of Δm for different concentration of F -in aqueous medium is shown in figure 3. The piezo and electrochemical response data of spin coated film is shown in SI.The magnitude of the both the piezoelectric and electrochemical responses were low as compared to that of LS film of ODA-SWCNTs (Fig. S4).The response curves measured at difference cycles was less stable.This is due to random nature and non-uniform distribution of the bundles of nanotubes in the spin coated film.Although, a linear trend (figure 3) was observed in case of spin coated film, the range of detectable concentration and lowest detectable concentration (LDC) of the F -was found to be merely 14 to 84 ppm and 14 ppm, respectively.Since, the sensitivity can be related to the slope of such linear curve [24], the sensitivity of the spin-coated film is found to be 10.1 ± 0.4 ng/cm 2 /ppm.The LS film  of ODA-SWCNTs can detect F -in a wider concentration range of 0.5 to 145 ppm with a very low LDC of 0.5 ppm.The curve shows different linear trend in two different concentration ranges.The estimated sensitivity in these concentration ranges are 24.4 ± 1.3 ng/cm 2 /ppm from 0.5 to 55 ppm and 12.0 ± 0.6 ng cm −2 /ppm from 55 to 145 ppm.The spin coated nanotubes yield saturated response beyond 90 ppm whereas such saturation was found to be beyond 150 ppm in case of LS film of ODA-SWCNTs.The sensing performance of the LS film of ODA-SWCNTs was found to be superior than the spin-coated film in terms of concentration range, LDC and sensitivity of the sensor.The enhanced sensing performance of the LS film of ODA-SWCNTs is due to enormous gain in surface-to-volume ratio and ordered assembly of the nanotubes in the ultrathin film regime.The aligned nanotubes in ultrathin film can enhance the charge transfer mechanism and density of surface active hot-spots.These feature not only enhances the adsorption of the analytes but also improves the electrochemical responses.However, such enhancement was not observed in the drop-casted film due to the random network of the nanotubes and higher thickness of the film [27,31].Oliveira et al [32] have compared gas sensing performance of LS and drop casting film of neutral random polyalkylthiophenes (P3AT relying on the electrical response of the film towards the ammonia.The electrical conductivity of LS film was found to be higher with the good reproducibility at each cycle of the sensing than the drop casting film, due to stable and organised structure of the LS film.The sensitivity of the sensor depends upon the surface interaction between the analyte and sensing layer.The surface interaction is highly influenced by the nature of receptor, its stability and well-organization and S-V ratio.In addition to this the physicochemical properties of deposited molecules, and their nature of aggregation governs the sensing performance [32].The fluoride ion has been detected using ion-selective electrode, colorimetric method, chemosensing and fluorosensing methods.These methods are complex and less reliable towards F -.The colorimetric method relies on the dye indicator, which may not be sensitive like nanomaterialbased sensor.It yields qualitative results, less selectivity, and less reliable as presence of other analytes can also lead to change in color.Mostly such detections were carried out in the solvents rather than aqueous medium using UV-vis and fluorosensing techniques.The sensing of F -using the LS films of ODA-SWCNTs in the present case was performed in aqueous medium.It exhibited a very low LDC (0.5 ppm) and a very wide detectable concentration range (0.5-145 ppm).
The absolute value of cathodic, anodic current as well as fluoride peak current is plotted as a function of the concentration of F -in aqueous medium (figure 4).The cathodic current increases linearly with increasing the concentrations of F -.During an anodic trace of the applied potential, no significant peak appeared therefore the oxidizing process of F -was weaker than the reducing process.This represents the quasi-reversible electrochemical process.Therefore, the change in anodic current (figure 4(a)) for the concentration of F -was negligible.The absence of a significant peak during the anodic trace of the applied potential in an electrochemical experiment indicates that the reduction process is more favourable or dominant than the oxidation process.The irreversible electrochemical processes may be due to kinetics of the ions, thermodynamics of the electrochemical processes, concentration of the ionic species, and the electrode materials.In the present case, the weaker oxidation process may reflect the complex formation of F -with -NH 2 of the ODA-SWCNTs [33].The rate of change of cathodic and anodic current was found to be 3.91 μA ppm −1 and −0.144 μA ppm −1 , respectively.The rate of change of fluoride peak current was found to be 0.348 μA ppm −1 .The FESEM image shows a remarkable change in the morphology of ODA-SWCNTs functional layer after interaction with F -.The regular circular domain can be seen (figure 5(a)) on the functional layer of ODA-SWCNTs due to adsorption and nucleation of F -. Elemental mapping images for three elements viz.silicon, carbon and fluorine were obtained from the same area of the topographic image (figure 5 Due to perturbation of π-electron by forming the semi-covalent C-F bonds during sensing of F -by ODA-SWCNTs, the structure of the functional layer may change.The change in the structure of ODA-SWCNTs after interaction with F -was investigated though grazing angle x-ray diffraction (GIXD) and Raman spectroscopy.The characteristic Raman features from SWCNTs like radial breathing mode (RBM at 173 cm −1 ), disorderinduced mode (D-band at 1322 cm −1 ), and tangential mode (G-band at 1588 cm −1 ) [34] are observed in the spectra (figure 6).The D band is associated with the presence of defects in the structure of SWCNTs [34,35] whereas the G band indicates the sp 2 -bonded carbon atoms in SWCNTs.The Raman spectrum of ODA-SWCNTs (figure 6(a)) shows the RBM of SWCNTs at 173 cm −1 [36,37].The intensity of Raman peak for RBM decreases on the interaction of ODA-SWCNTs with F -(figure 6(b)).This indicates that the number of radial breathing mode is reduced due to the formation of a semi-covalent C-F bond with the carbon nanotubes.The diameter of the carbon nanotube can be estimated from the RBM frequency of Raman spectrum of ODA-SWCNTs LS film, using the equation [38], 248 RBM w = where d is the tube diameter in nm and ω RBM is RBM frequency in cm −1 .Thus, the estimated diameter of ODA-SWCNTs is 1.43 nm.As observed from the spectra (figure 6) there are no shift in frequency for the RBM mode.However, the intensity value decreases significantly on interaction with the F -.The diminishing trend in the RBM band may indicate the interaction of the ions intratubular.The ratio of for the D and G band (I D /I G ) indicates the degree of disorder in the structure of carbon nanotubes [34].The I D /I G ratio of ODA-SWCNTs was found to be 0.25.After the interaction of ODA-SWCNTs with F -, the ratio of I D /I G was found 0.52.An increase of the I D /I G ratio suggests a loss of aromaticity in the rings of SWCNTs due to a strong interaction with F - [39].The estimated intensity of I RBM /I G before and after the interaction with fluoride ion were found to be 0.471 and 0.240, respectively.After interaction with F -ion, the intensity of both G band and RBM band reduced which can indicate the increase in defects in the nanotubes.The nature of interaction of F - with ODA-SWCNTs is studied using GIXD (Supplementary Material).The Raman and GIXD measurements clearly indicate strong interaction of the F -ion with the ODA-SWCNTs.Such interaction can support the complexation and encapsulation of the ions inside the tubes.
In continuation to this, we further studied the interference sensing of F -with heavy metal ions (Pb +2 , Cd +2 , Hg +2 ) for the selective detection of F -.The composite sensing of F --Hg +2 , F --Pb +2 and F --Cd +2 were carried out using the LS film of ODA-SWCNTs as the functional layer in eQCM.The piezo and electrochemical responses were collected by as a function of concentration of F -(2-100 ppm) in the aqueous medium for a given concentration of the heavy metal ion.Similarly, the peak strength corresponding to each of the ions were recorded as a function of the analytes in the aqueous medium.The linear part of the curves was used for the calculation of sensitivity.Figure 7 shows the sensitivity dependence on the concentration of different species of heavy metal ions in the aqueous medium during the F − sensing.
The bar diagram in figure 7(a) indicates that the sensitivity during the sensing of F -in the aqueous medium does not change due to the presence of Hg +2 and Cd +2 ions at difference concentration.The average piezoresponse sensitivity for F -sensing in aqueous medium in the presence of Cd +2 ion is ∼2.25 times greater than that of Hg +2 ion.The sensitivity appears increasing monotonically due to the increase in concentration of the Pb +2 ion in the aqueous medium.Interestingly, the bar diagram corresponding to the electrochemical sensitivity (figure 7(b)) shows a monotonic linear increase in the values at different rates due to the increase in  concentration of any of the heavy metal ion species in the aqueous medium.The CNTs can act as both electron donor and electron acceptor.The adsorption sites and binding energy of these metal ions are reported to be different [40].In the present experimental condition, we found that the piezo sensitivity towards Cd +2 is highest due to the abundance of favourable adsorption sites on the LS film of ODA-SWCNTs.As per the data, the algorithm formulated for the selective detection of F -in the presence of other heavy metal ions viz Cd +2 , Hg +2 and Pb +2 as follows: (a) if the sensitivity of the piezoresponse and electrochemical response changes then the Pb +2 ion is present in the aqueous medium and the calibration curve for composite sensing of F --Pb +2 can be used for the detection of concentration of the ions.(b) if the sensitivity of the piezoresponse remains invariant and electrochemical response changes then the Hg +2 or Cd +2 ion may present.These ionic species can be identified by observing the fact that the piezoresponse sensitivity of Cd +2 is more than twice as that of Hg +2 .Once, the signature of the presence of these heavy ions are identified in the aqueous medium during the sensing of F -, the respective calibration curves for the composite sensing data can be used to measure the respective concentration of the ionic species.
We have applied the principal component analysis (PCA) on the eQCM sensor data and found that the first three principal components (PC1, PC2, and PC3) retain most of the information (∼82%).Therefore, the clustering was done among PC1, PC2, and PC3 as shown in figure 8.It can be seen that all the sample points of the anionic F -as well as cationic metal ions are well separated from each other.The good separability of the data indicates less interference and good selectivity towards the sensing of these ions measured through the proposed functionalized eQCM.

Detection of F − in real samples from tap water
Tap water samples were collected from three different regions of Rajasthan viz.Jhunjhunu, Pilani and Bikaner.A standard fluoride meter (FL700, USA) was used to measure the precise concentration of the F − ion in the sample of tap water.The concentration of F − ion in the tap water of Jhunjhunu, Pilani and Bikaner were measured to be 1.3 ppm, 1.8 ppm and 0.5 ppm, respectively.The collected water sample was inserted in the eQCM setup to record the electrochemical and piezoelectric responses, simultaneously.The data analysis reveals concentration of F -in tap water to be very close to that of obtained from the standard fluoride meter.The results are presented in table 1.

Conclusion
Reports in the literature indicated that the fluoride ion can interact with CNTs to form a semi-covalent C-F bond.Also, the amine group of the ODA-SWCNTs can interact with fluoride ion in the aqueous medium to form amine-fluoride complex.The ultrathin film of aligned nanotubes can offer superior physicochemical properties as compared to that of a random network of the nanotube.Therefore, strategically, we fabricated ultrathin film of ODA-SWCNTs through LS method and employed it as a functional layer in eQCM sensing transducer and performed a systematic simultaneous measurement of piezo and electrochemical responses as a function of concentration of F -in the aqueous medium.The detectable concentration range of F -in the aqueous medium was found to be 0.5 to 145 ppm.The Raman spectroscopy and GIXD measurement suggested that the F -interact with SWCNTs by forming a semi-covalent C-F bond.The F atoms can bond inwardly towards the inner core of the nanotubes.The sensor showed a good selectivity towards the F -in the presence of some heavy metal ions in the aqueous medium.The sensing of F -in tap water from the local areas showed a good accuracy when compared with the measurement from a commercial standard.This study indicates the potential of an ultrathin film of ODA-SWCNTs for sensing F -in aqueous medium using the eQCM.

Figure 1 .
Figure 1.FESEM images of the (a) spin coated and (b) LS film of ODA-SWCNTs.

Figure 2 .
Figure 2. Response curves using eQCM during F -sensing using LS film of ODA-SWCNTs.(a) Cyclic voltammetry curve (b) change in resonance frequency (Δf) versus time.Both the responses were obtained simultaneously.The dashed line in (b) is shown to indicate initial/final voltage during voltage sweep.The scan rate was at 0.1 V s −1 and the pH of the aqueous medium was 4.

Figure 3 .
Figure 3. Change in mass per unit area (Δm) as a function of concentration of F -in aqueous medium obtained from functional layers: LS film (red) and spin coated film (black) of ODA-SWCNTs onto quartz wafer.Mass on the surface of quartz wafer increases and decreases during the oxidation and reduction process, respectively.The change in mass was calculated from the change in resonance frequency using Sauerbrey relation.The measurement was carried out on two different electrodes each functionalized with LS and spin coated film independently.At one particular concentration 3 sets of experiments were carried out.

Figure 4 .
Figure 4. (a) Change in cathodic and anodic current versus concentration (b) change in peak current of F -versus concentration of F -in aqueous medium.The data were collected from the CV curve for the concentration of the F -ion.
(a)) and are shown in figure 5(b).The presence of fluorine can be observed as circular domains and bright specks all over the region.The circular domains as observed in topographic image showed the abundance of fluorine atoms.This clearly indicates the signature of nucleation and growth of fluoride domains due to its strong interaction with ODA-SWCNTs.

Figure 5 .
Figure 5. (a) FESEM images of the film of ODA-SWCNTs after the interaction with F -(b) element mapping image show the distribution of elements in a sensing layer after interaction with F -.The scanning was done on LS film of ODA-SWCNTs deposited onto silicon wafer and kept immersed in aqueous medium with F-ion.The sample was dried in desiccator and then the scanned using FESEM.

Figure 7 .
Figure 7. Bar diagram showing the sensitivity of (a) piezoelectric response and (b) electrochemical response for the composite sensing of F -and heavy metal ions in the aqueous medium.The chlorine salts of the heavy metal ions were used.

Figure 8 .
Figure 8. Principle component analysis (PCA) plot of the sensing of heavy metal ions as well as F -using the functional layer as LS film of ODA-SWCNTs.

Table 1 .
Concentration of F − ion in tap water obtained from nearby areas in Rajasthan India.The measurement was done using a standard fluoride meter and the proposed sensing protocol in this article.Tap water sample collected from nearby areas of Rajasthan, India Concentration of F − (ppm) obtained from standard fluoride meter (FL 700) Concentration of F − (ppm)