Phosphinic acid functionalized carbon nanotubes for sensitive and selective sensing of chromium(VI)

Journal of Hazardous Materials 278 (2014) 559–565

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Phosphinic acid functionalized carbon nanotubes for sensitive and selective sensing of chromium(VI) Akash Deep ∗ , Amit L. Sharma, Satish K. Tuteja, A.K. Paul Central Scientific Instruments Organisation (CSIR-CSIO), Sector 30 C, Chandigarh 160030, India

h i g h l i g h t s • • • • •

SWCNTs have been conjugated with bis(2,4,4-trimethylpentyl) phosphinic acid (PA/d). SWCNT-PA/d adduct is demonstrated for electrochemical sensing of Cr(VI). Linear response is obtained for 0.01–10 ppb Cr(VI). Sensitivity and the limit of detection are 35 ± 4 nA/ppb and 0.01 ppb, respectively. Proposed sensing of Cr(VI) is selective with respect to many other metals.

a r t i c l e

i n f o

Article history: Received 8 March 2014 Received in revised form 15 June 2014 Accepted 22 June 2014 Available online 28 June 2014 Keywords: Carbon nanotubes Phosphinic acid modifier Microsensor Amperometry Cr(VI) Determination

a b s t r a c t Single-walled carbon nanotubes (SWCNTs) have been functionalized with a phosphinic acid derivative ‘bis(2,4,4-trimethylpentyl) phosphinic acid’ (PA/d). It has been achieved by treating the chlorinated SWCNTs with PA/d at 80 ◦ C. Successful functionalization and different nanomaterial properties have been investigated by UV–vis–NIR, FTIR, Raman spectroscopy, AFM and FE-SEM. PA/d conjugated SWCNTs (CNT–PA) are dispersible in some common organic solvents, e.g. CH2 Cl2 , DMF, CHCl3 , and THF. The ‘CNT–PA’ complex was spin-casted on boron doped silicon wafer. Thus fabricated sensing electrode is demonstrated for sensitive and selective electrochemical sensing of chromium(VI) ions. A linear response is obtained over a wide range of Cr(VI) concentration (0.01–10 ppb). The sensor’s sensitivity and the limit of detection are observed to be 35 ± 4 nA/ppb and 0.01 ppb, respectively. The practical utility of the proposed sensor is demonstrated by determining the Cr(VI) concentration in an industrial effluent sample and an underground water sample. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Chromium is one of the highly toxic heavy metals which is non-biodegradable and can pose severe threat to the environment and human health. Atomic absorption spectroscopy, inductively coupled plasma/mass spectrometry, inductively coupled plasma/atomic emission spectrometry and ultraviolet–visible spectroscopy have been the main techniques for analysis of chromium in various samples [1–4]. Despite of sensitivity and selectivity of the above techniques, the requirements of tedious sample preparation, pre-concentration procedures, expensive instruments, and professional expertise limit their on-site and routine applications. Development of sensor technology is important to meet the ever-growing demand of on-site high throughput

∗ Corresponding author. Tel.: +91 172 2657811x452; fax: +91 172 2657082. E-mail address: [email protected] (A. Deep). http://dx.doi.org/10.1016/j.jhazmat.2014.06.043 0304-3894/© 2014 Elsevier B.V. All rights reserved.

detections. Hanging mercury drop electrodes and mercury film electrodes were quite popular in early years of chromium sensor development. Several reviews covers their features and scope of applications [5–8]. Of the late, the focus has been more on the development of electroanalytical sensors. Carbon, graphite and gold based electrodes have been widely proposed for the electrochemical sensing of Cr(VI) [9–13]. Gold nanoparticles electrodeposited indium tin oxide (ITO) electrodes [14] and self-assemblies of gold nanoparticles [15] have been proposed for voltammetric detection of Cr(VI). Electrochemical quantification of Cr(VI) on a gold plated carbon based composite electrode has been recently reported [16]. It has been possible to detect the ppb level concentrations of Cr(VI) with the above electrochemical sensors. However, at times, interferences from other commonly associated metal ions could not be avoided. Carbon nanomaterials such as carbon nanotubes (CNTs), carbon nanofibers, and graphene have been suggested as very sensitive electrode materials for the detection of heavy metals [17–21].

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Specifically mentioning, the CNTs offer certain advantages due to their large surface area, small size, excellent electron transfer ability, and easy surface-modification. A single-walled CNT electrode has been reported for the detection of Cd2+ and Pb2+ ions [22]. The surface functionalization of the CNT helps in altering their affinity towards heavy metals. CNTs, modified with thiacalixarene (TCA), have been proposed for the selective recognition of trace Pb2+ ions [23]. Another interesting report highlights the measurement of 1 ppb of Pb2+ and 15 ppb of Cu2+ by using cysteine-modified CNTs [24]. The above cited CNTs based electrochemical sensor systems still needs to address few issues, such as sensor stability, regeneration, specificity, and sensitivity [25,26]. Proper selection of CNT surface chemistry and the electrochemical detection technique are important criteria to deliver improved resolution and selectivity. Present work first time reports a simple route of derivatizing single-walled carbon nanotubes (SWCNTs) with a metal extractant, bis(2,4,4trimethylpentyl) phosphinic acid (PA/d). The compound PA/d, or more commonly known as CYANEX 272® , is a useful reagent in the liquid–liquid extraction and separation of various metals [27–31]. However, we could find any noteworthy report on the use of PA/d in the development of selective electrochemical sensing of metals. High thermal and pH stability along with excellent loading and regeneration capabilities make the PA/d an interesting reagent to be explored for sensing applications. In this work, we report the functionalization of SWCNTs with PA/d. The derivatized SWCNTs are explored for the sensitive and selective electrochemical sensing of chromium(VI). It has been possible to achieve low detection limit, wide range of operating conditions, and much desired selectivity. Main advantages of the herein proposed technique include high sensitivity (up to sub-ppb levels), selectivity (with respect to several commonly associated metal ions), stability, re-usability, fast response and possibility of on-site detections. The functionality and selectivity of the PA/d derivatized SWCNTs are observed over a wide range of pH suggesting the capability of the proposed sensor for range of applications. 2. Experimental 2.1. Materials Single-walled carbon nanotubes (SWCNTs, >90% purity) were purchased from Nanostructured & Amorphous Materials Inc., USA. All other metal salts, solvents, acids and reagents were purchased from Sigma/Merck. Boron doped p-type (1 1 1) silicon wafers (BD-SiE, resistivity 10−3 –40  cm) were also purchased from Sigma–Aldrich. The reagent ‘2,4,4 (trimethylpentyl) phosphinic acid’ (PA/d) [Cyanex 272® , C16 H35 O2 P, molecular weight: 290] was a sample from Cytec Canada Inc. 2.2. Chlorination of SWCNT SWCNTs were chlorinated by following an earlier reported method [32]. 100 mg of single-walled carbon nanotubes were added into 30 mL mixture of concentrated HNO3 and concentrated HCl (3:1 v/v). The contents were refluxed at 80 ◦ C for 6 h. The reaction mixture was cooled, diluted with 500 mL of deionized (DI) water and then filtered through a 0.2 ␮m polytetrafluoroethylene membrane. The recovered product was washed with DI water and then re-suspended in 30 mL of concentrated HCl. The contents were again subjected to reflux (80 ◦ C) for 4 h. The reaction mixture was allowed to cool and then filtered to obtain chlorinated SWCNTs (Cl–CNT). The product was washed with DI water and stored at 25 ◦ C. The amount of Cl on the surface of SWCNTs was found to be 0.25 ± 0.02 wt.% according to the elemental analysis data obtained from inductively coupled plasma-mass spectrometry.

2.3. Adduct formation of Cl–CNT with PA/d 75 mg of the above Cl–CNT were dispersed in DMF by sonication for 30 min. The suspension was mixed with 10 mL of PA/d and the contents were refluxed at 80 ◦ C for 12 h. After the completion of the reaction, the PA/d functionalized SWCNTs (CNT–PA) were recovered by filtration. The product was finally washed with nhexane, air-dried and stored for further use. Whenever required, dispersions of the CNT–PA were obtained by sonicating 1 mg of the sample in 10 mL of dichloromethane (CH2 Cl2 ), dimethylformamide (DMF)/chloroform (CHCl3 )/tetrahydrofuran (THF) at 50 ◦ C for 1 h. 2.4. Uptake of Cr(VI) with CNT–PA A multi-element aqueous mixture of Cr(VI), Al(III), Fe(III), Mn(II), Ni(II), Cu(II), Zn(II) and Cd(II) was prepared. The acidity of the solution was kept to 0.1 mol L−1 H2 SO4 . 5 mg of the CNT–PA was added into 2 mL of the multi-element mixture [5 ppb Cr(VI) and 1 ppm each of Cr(III), Al(III), Fe(III), Mn(II), Ni(II), Cu(II), Zn(II), Cd(II)]. The contents were shaken at 25 ◦ C for 5 min and the supernatant was separated by centrifugation. Concentrations of different metals in the initial solution and in the supernatant were analyzed by inductively coupled plasma-mass spectrometry. The percentage adsorption of Cr(VI) on the CNT–PA was calculated by the formula:

%E =

[Cr]initial − [Cr]after extraction × 100 [Cr]initial

Similarly, the uptake of all the above metals with the unfunctionalized SWCNT was also investigated. 2.5. Construction of sensor Boron doped p-type (1 1 1) silicon wafers (BD-SiE) were treated with 1% (v/v) aqueous HF solution (for 20 s) and 40% (v/v) aqueous NH4 F solution (for 4 min). These were then washed with acetone, ethanol and DI water. 1 mg mL−1 of CNT–PA (in CH2 Cl2 ) was spin-casted on the above cleaned silicon surfaces (effective area 0.5 × 0.5 cm2 ). This CNT–PA modified silicon electrode “CNT–PA/BD-SiE” was used as working electrode during the electrochemical sensing of chromium(VI). For the construction of unfunctionalized SWCNT electrode, the sample of the nanotubes was first dispersed in N-methylpyrrolidone (NMP) by sonicating 10 mg of the SWCNTs in 1 mL of the solvent for 5 h at 60 ◦ C. Large aggregates from the suspension were separated by centrifugation for 30 min at room temperature. The electrode “CNT/BD-SiE” was constructed by spin casting. 2.6. Equipment Absorption characteristics of the dispersed CNT–PA were studied by UV–vis–NIR spectrophotometer (Varian Cary 5000, USA). FTIR spectrum was recorded on a spectrometer (Nicolet iS10, USA) by mixing the materials in IR grade KBr powder. Raman spectra were recorded by Raman (Invia Renishaw, UK) spectrometer using laser excitation wavelength of 785 nm. Surface morphology and elemental composition were investigated by field emission scanning electron microscope–energy dispersive X-ray spectroscope (FESEM–EDX, Hitachi S4300 SE/N, Japan). Atomic force microscopic (AFM) investigations were carried out in non-contact mode by a XE-NSOM system (Park Systems, Korea). Inductively coupled plasma-mass spectrometry (ICP-MS, ELAN DRC-e, Perkin Elmer, USA) was used for analyzing the elemental composition of some samples. Cyclic voltammetry, amperometric response and interference studies were carried out on Potentiostat system (PGSTAT 302N,

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Fig. 1. FE-SEM investigations of (A) pristine SWCNTs, (B) CNT–PA dispersion in dichloromethane, (C) CNT–PA dispersion in tetrahydrofuran.

Autolab, Metrohm, USA). Platinum wire and Ag/AgCl electrode (Cl− – 3 mol L−1 ) were used as counter and reference electrodes, respectively. Cyclic voltammograms were recorded keeping the scan rate of 200 mV/s. Amperometric response of the sensor was measured keeping the electrode potential constant to 0.2 V. 3. Results and discussion 3.1. Dispersion studies of CNT–PA Dispersibility of the PA/d functionalized SWCNTs was studied in some common organic solvents, namely CH2 Cl2 , DMF, CHCl3 , and THF. Large aggregates were removed by centrifugation (15,000 rpm, 30 min) of the suspensions. CNT–PA was readily dispersed in all the above solvents and the liquid phases thus formed were stable for several days. FE-SEM investigations (Fig. 1, representative data for CH2 Cl2 and THF) have verified the successful dispersion of CNT–PA conjugates with significant debundling of the nanotubes. EDX analysis (Fig. S1, Supplementary Information) for the selected field of view of the pristine and conjugated SWCNTs has highlighted the emergence of phosphorus as a result of successful introduction of phosphinic acid derivative on the nanotubes’ surface. UV–vis–NIR spectroscopy data for the CNT–PA dispersions (Fig. S2, Supplementary Information) in CH2 Cl2 , DMF and THF indicate good quality nanotubes dispersions with the appearance of typical resonance peaks [33–35]. Dispersion of CNT–PA in organic solvents were also investigated by AFM analysis. Imaging data of a representative sample (DMF dispersion) is shown in Fig. 2. Significant debundling of the PA/d functionalized SWCNTs is apparent from the image analysis. Region analysis has been used to measure the average heights of the individual bundles. The mean diameter of the bundles are found to be 5–10 nm clearly demonstrating significant debundling in the PA/d treated SWCNTs which is presumably responsible for the dispersion of the material.

Fig. 3. FTIR investigations of PA/d, Cl-CNT and CNT–PA.

3.2. FTIR and Raman studies FTIR spectra of the chlorinated SWCNTs, PA/d, and CNT–PA are shown in Fig. 3. Chlorinated SWCNTs are characterized with bands at 2810 and 3400 cm−1 corresponding to the C–H and O–H stretches, respectively. A band at 1625 cm−1 appears due to the graphitic structure of the SWCNTs. The band at 765 cm−1 attributes

Fig. 2. AFM investigations CNT–PA dispersion.

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Fig. 6. Adsorption of different metal ions by CNT–PA as a function of test solution acidity. Fig. 4. Raman spectra of SWCNTs and CNT–PA.

to the C–Cl vibration [36]. Pure PA/d sample shows bands at 1276 and 1095 cm−1 , corresponding to the vibrations of P O stretching and P–OH banding, respectively. Bands at 3261 and 2815 cm−1 correspond to O–H vibrations. Presence of a weak band at 980 cm−1 is due to the P–O–H group. The functionalization of SWCNTs with PA/d is characterized with disappearance of bands at 765 (C–Cl of chlorinated nanotubes) and 980, 1095 and at 3261 cm−1 (of PA/d), thus suggesting that the desired functionalization takes place via reaction between C–Cl (of nanotubes) and P–OH (of PA/d). The presence of band at 1292 cm−1 accounts for the P O stretching. A band at 1220 cm−1 can be attributed to the C–O stretching, whereas the band at 1050 cm−1 accounts for P–O–C stretching vibration. All the above observations confirm the formation of CNT–PA adduct. Another evidence of SWNTs functionalization is given by Raman spectroscopy. Raman spectra of the SWCNTs and CNT–PA are given in Fig. 4. Remarkable features can be observed in the Raman spectra due to the chemical functionalization. Tangential vibration mode (TM, due to C–C displacements) and D-band (due to sp3 C hybridization) signals give important data for the analysis. Raman spectrum of pristine SWNTs shows peaks at 1580 and 1352 cm−1 . The dominant TM peak in the chlorinated SWCNTs shifts from 1580 to 1590 cm−1 along with some increase in the peak intensity. The shift in the C–C binding energy indicates successful modification of the nanotubes. The intensity of D-band also shows notable increase after the functionalization reaction suggesting the introduction of some structural defects as a result of functionalization. An estimation of the intensity ratio of the D band to the G band (D/G ratio: [D/G]) may be used to assess the degree of functionalization. Such a study is in scope of our future communication; however at this stage, apparent increase in the D/G ratio (from 0.67 to 0.95) may be

noted as a sufficient indicator of desired functionalization. Radial breathing mode (RBM) is another Raman-active vibration which may be used to reflect the tube diameter dependent vibration frequency. The RBM of pristine and derivatized samples shows peaks at the same frequency (199 cm−1 ) thus suggesting that the functionalization has not resulted in any significant change in the tubule shape and diameter. 3.3. Uptake studies of Cr(VI) using CNT–PA The formation of CNT–PA complex has taken place according to the given schematic (Fig. 5). FTIR analysis (Fig. 3) suggests the availability of free P O groups in the CNT–PA complex. The PA/d is a well-known metal extractant and has been reported to bind with several metal ions through cation exchange route wherein the P–OH moiety is utilized. Herein proposed CNT–PA adduct formation consumes P–OH binding sites of the extractant; and therefore, the general binding efficiency of the PA/d (with respect to the metal ions) is likely to be affected. We have tested the extraction efficiency of the CNT–PA complex with respect to 5 ppb Cr(VI) and elevated concentrations (200-fold excess) of some commonly associated metal ions [e.g. Cr(III), Al(III), Fe(III), Mn(II), Ni(II), Cu(II), Zn(II), Cd(II)] as a function of solution acidity (Fig. 6). To our advantage, Cr(VI) is selectively and almost quantitatively adsorbed over a wide range of acidity (1.0 × 10−6 –0.1 mol L−1 H2 SO4 ), while the uptake of all other metal ions remains negligible. The hexavalent chromium species exists primarily as chromic acid (H2 CrO4 ) and its salts, hydrogen chromate ion (HCrO4 – ) and chromate ion (CrO4 2− ) depending on the pH. H2 CrO4 , HCrO4 – and CrO4 2− are predominant species at pH less than 1, between 1 and 6, and at about 6, respectively. In case of other metal ions, the main species are Mn+ or M(OH)x y+ (where n is the oxidation state; x/y = 1, 2, 3). At

Fig. 5. Schematic of the functionalization reaction of Cl-MWCNT with PA/d.

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Fig. 7. Schematic of Cr(VI) uptake by CNT–PA through solvation.

pH near 6, the hydrolyzed species M(OH)x y+ tend to show poor aqueous solubility. In comparison, the Cr(VI) species are better aqueous soluble over a wide range of the pH conditions. CNT–PA is a derivatized phosphinate, which show electrophilic nature. The electrophilic nature of this derivative is further responsible for its selective affinity towards Cr(VI) species ‘HCrO4 – and CrO4 2− . The Cr(VI) uptake by the CNT–PA conjugate may be explained by the solvation mechanism (Fig. 7). The uptake of metal ions through solvation mechanism has also been reported in case of some other extractants with P O linkage in their structures [37,38]. However, the kind of observed selectivity with CNT–PA towards Cr(VI) has not been reported earlier to the best of our knowledge. Absorption spectra (Fig. S3, Supplementary Information) of the Cr(VI) loaded CNT–PA dispersion has also confirmed the uptake of the metal ion with the appearance of an additional well defined Cr(VI) characteristic peak at 355 nm, which is due to the ligand to metal charge transfer [O2− → Cr(VI), metal salt used was K2 CrO4 ]. The uptake of the all the above metal ions with the unfunctionalized SWCNTs was also tested to confirm the role of functionalization. Negligible uptake was observed in case of all the investigated metals. 3.4. Electrochemical sensing of Cr(VI) with CNT–PA Selective sensing of Cr(VI) in environmental samples is a challenging task because of the co-presence of several other interfering metal ions. Most of the electrochemical techniques are based on the pre-concentration of Cr(VI) species at the electrode surface using complexing agents. These techniques suffer from undesirable interferences. In the light of the observed promising selective Cr(VI) adsorption characteristics, the CNT–PA complex has been investigated for the electrochemical sensing of Cr(VI). Fig. 8 depicts the typical cyclic voltammetric (CV) responses of CNT–PA/BD-SiE electrode in the blank sample (0.1 mol L−1 H2 SO4 ) as well as in presence of 1 ppb Cr(VI). The CV of the unfunctionalized SWCNTs (CNT/BDSiE) in 0.1 mol L−1 H2 SO4 electrolyte solution is also presented. Unfunctionalized SWCNTs are characterized with typical doublelayer behaviour. The background current (i.e. electron transfer) is likely to be due to the reduction of oxygen trapped inside the nanotubes. The feature less behaviour (absence of redox couple) may be attributed to the distribution of the nanotubes, where the electron transfer into each tube leads to the appearance of average of many closely spaced peaks. The CV studies of CNT–PA/BD-SiE in blank electrolyte are also characterized with background current. Broad redox peaks are indicative of the Faradaic process occurring on the surface of the carbon nanotubes. The Faradaic process in the case of functionalized nanotubes can be explained by the

Fig. 8. Voltammetric response of CNT–PA/BD-SiE electrode for 1 ppb Cr(VI) solution in 0.1 mol L−1 H2 SO4 , scan rate: 200 mV/s. Inset image showing the cyclic voltammograms of unfunctionalized SWCNT and PA/d functionalized SWCNT in blank electrolyte (0.1 mol L−1 H2 SO4 ) solution.

reduction/oxidation of functional groups attached on the surface of nanotubes. Compared to the above two investigations, well-defined voltammetric peak during the CV analysis of CNT–PA/BD-SiE in chromium(VI) solution can be ascribed to the three-electron reduction of Cr(VI) to Cr(III). Large capacitive current is observed here also, which indicates about the high surface area of the functionalized SWCNT. The peak current and peak position remains more or less unchanged in 20 cycles of subsequent sweeps, thereby indicating stability of the voltammetric response. The cathodic peak in this case is observed at more positive potential (0.33 V) than other substrates, e.g. indium tin oxide (0.2 V) and nano-gold (0.3 V) [14]. It has been explained in the literature that the above behaviour may be correlated with the enhanced catalytic activity provided by the high surface area of the sensing material [14]. Analytical performance of the developed electrode was evaluated by constant potential amperometry. Fig. 9 displays the amperometric response with respect to the injection of different concentrations of Cr(VI) at a constant electrode potential of 0.20 V. Linear response has been observed over a wide range of metal ion concentration (0.01–10 ppb). It took only 20 s to obtain stable

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3.5. Studies on the stability of CNT–PA/BD-SiE electrode

Fig. 9. Amperometric response of CNT–PA/BD-SiE electrode for varying (0.01–10 ppb) Cr(VI) concentrations in a continuously stirred electrolyte solution (0.1 mol L−1 H2 SO4 ), applied potential was 0.2 V, measurements were taken after current stabilization which took 20 s. Each data point is an average of three readings.

The long-term stability of the proposed electrode has been tested by using the same CNT–PA/BD-SiE electrode for 25 repetitive measurements of 0.1 ppb Cr(VI) solution in a supporting electrolyte (0.1 mol L−1 H2 SO4 ). After each test, the electrode was kept in the electrolyte for 30 s in order to strip out any conjugated metal content followed by washing with water. We observed insignificant change (coefficient of variation: 0.15%) in the peak potential and peak current for the reduction of Cr(VI). The above study confirmed the stability and regeneration capacity of the electrode. Operational stability of the proposed sensor was ascertained by carrying out voltammetric measurements (test solution of 0.1 ppb metal ion) over a period a time. The peak current was measured at regular interval (5 h) over a period of 50 h. Insignificant change in the peak current was observed during the course of the above experiment (Fig. S6, Supplementary Information). The long-term stability of the electrode was also tested by periodic study of the peak current over a period of 10 days (Fig. S6). The electrodes were stored at room temperature in a vacuum desiccator. The sensor output was almost stable for first 5 days. Thereafter, a small drop of 5% was observed during the subsequent period. It can be outlined that the proposed sensor system is reasonably stable for long-term use. The reproducibility of the results was assessed by checking the response of five different CNT-PA/BD-SiE electrodes towards the detection of 0.1 ppb Cr(VI). All the five electrodes were prepared using the same batch of the electrode material and identical experimental conditions were maintained. The response of electrodes was highly reproducible with the calculated standard deviation in the peak current to be ±0.025 ␮A. 3.6. Analysis of Cr(VI) in electroplating effluent and water samples by CNT–PA/BD-SiE electrode

Fig. 10. Amperometric response of CNT–PA/BD-SiE electrode for 0.1 ppb Cr(VI) (electrolyte: 0.1 mol L−1 H2 SO4 ) in the presence of associated metal ions. Each data point is an average of three readings.

current measurements, indicating fast response of the sensor. The estimated sensitivity and limit of detection are 35 ± 4 nA/ppb and 0.01 ppb, respectively. The practical utility of the sensor was tested by recording its amperometric response for a constant Cr(VI) concentration (0.1 ppb) in the co-presence of elevated ratios of several commonly associated metal ions {10 ppb of Cr(III), Al(III), Fe(III), Mn(II), Ni(II), Cu(II), Zn(II) and Cd(II)}. The results (Fig. 10) highlight that the CNT–PA/BD-SiE electrode tolerates all the impurities in the tested concentration range and provide selective sensing of the Cr(VI). The above sensor response studies (Figs. 9 and 10) were conducted in solutions with acidic conditions (0.1 mol L−1 H2 SO4 ) with a view of demonstrating the utility of the proposed technique to detect Cr(VI) in industrial effluents such as plating waste, etc. However, the sensor has also been tested for Cr(VI) in water samples with relatively lower acidic conditions. The CNT–PA/BD-SiE electrode offered sensitive (Fig. S4, Supplementary Information) and selective (Fig. S5, Supplementary Information) detections in samples with pH 5. The limit of detection in this case was observed to 0.1 ppb. The sensor response was also independent of other associated metals.

The practical utility of the proposed CNT–PA/BD-SiE electrode was verified by analysing the chromium(VI) contents in an electroplating effluent and a groundwater sample. The electroplating effluent sample was collected from a chrome plating industry located at Rohtak, Haryana – India; while the underground water sample was from a nearby village (Kansal, Punjab – India). The effluent sample was diluted to 1/200 before analysis (final pH ∼ 1.2). In case of water sample, it was left to react with 0.1 mol L−1 sodium hypochlorite (v/v ratio of 100/1) for 1 h to ensure the conversion of residual Cr(III) to Cr(VI). Table S1 (Supporting Information) gives the data on the composition of the above two samples. Using the standard calibration, the concentration of Cr(VI) in the effluent and water samples was found to be 16 ± 0.07 ppm and 0.18 ± 0.02 ppb, respectively by using the CNT–PA/BD-SiE sensor system. Dilution factors were incorporated wherever required. The obtained results agreed well with the data obtained by ICP-AES analysis. The experiment demonstrated the sensitive and specific performance of the proposed chromium sensor. 4. Conclusions Chlorinated SWCNTs have been functionalized with a phosphinic acid derivative, bis(2,4,4-trimethylpentyl) phosphinic acid. The resultant complex shows good dispersibility in some common organic solvents. Despite their rapid dispersibility, the PA/d conjugated nanotubes retain electronic properties as suggested by spectroscopic investigations. The availability of functional P O groups has made it possible to exploit the synthesized functional nanomaterial for selective and sensitive sensing of Cr(VI). The proposed nanosensor can be regenerated for repeated use. The detection limit is well below the WHO prescribed limits for Cr(VI) in

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drinking water. The performance of the proposed sensor in terms of the limit of detection and the range of analysis is also competitive or better with the existing electrochemical nanosensors [39–41]. Moreover, the proposed sensor can be utilized over a wide range of solution acidity. The calibration of the sensor may need to be redrawn for different pH condition. Alternatively, samples can be adjusted to the pH conditions of a given calibration curve. Acknowledgements The authors are thankful to the Director, CSIR-CSIO, Chandigarh, India for providing infrastructure facilities. The financial support from CSIR India under the project OMEGA/PSC0202/2.2.5 is gratefully acknowledged. We also thank Dr. Kamal Kumar for his help in the analysis of samples on ICP-MS. Thanks are also due to Mr. Pawan Kumar, Mr. Parveen Kumar, Mr. Girish C Mohanta, and Ms. Rajnish Kaur for helping in the experimental work. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat. 2014.06.043. References [1] N.H. Bings, A. Bogaerts, J.A.C. Broekaert, Atomic spectroscopy, Anal. Chem. 78 (2006) 3917–3946. [2] K.E. Lorber, Monitoring of heavy metals by energy dispersive X-ray fluorescence spectrometry, Waste Manag. Res. 4 (1986) 3–13. [3] R. Kunkel, S.E. Manahan, Atomic absorption analysis of strong heavy metal chelating agents in water and wastewater, Anal. Chem. 45 (1973) 1465–1468. [4] M. López-Artíguez, A. Cameán, M. Repetto, Preconcentration of heavy metals in urine and quantification by inductively coupled plasma atomic emission spectrometry, J. Anal. Toxicol. 17 (1993) 18–22. [5] M. Korolczuk, M. Grabarczyk, Determination of Cr(VI) in the presence of Cr(III) and humic acid by cathodic stripping voltammetry, Microchem. J. 72 (2002) 103–109. [6] M. Boussemart, C.M.G. van den Berg, M. Ghaddaf, The determination of the chromium speciation in sea water using catalytic cathodic stripping voltammetry, Anal. Chim. Acta 262 (1992) 103–115. [7] S. Morais, G.S. De Carvalho, J.P. Sousa, Chromium determination in osteoblastlike cell culture medium by catalytic cathodic stripping voltammetry with a mercury microelectrode, J. Trace Elem. Med. Biol. 12 (1998) 101–108. [8] M.M. Palrecha, P.K. Mathur, Adsorptive stripping voltammetric determination of chromium in gallium, Talanta 45 (1997) 433–436. [9] I. Svancara, P. Foret, K. Vytras, A study on the determination of chromium as chromate at a carbon paste electrode modified with surfactants, Talanta 64 (2004) 844–852. [10] P.M. Hallam, D.K. Kampouris, R.O. Kadara, C.E. Banks, Graphite screen printed electrodes for the electrochemical sensing of chromium(VI), Analyst 135 (2010) 1947–1952. [11] J.P. Metters, R.O. Kadara, C.E. Banks, Electroanalytical sensing of chromium(III) and (VI) utilising gold screen printed macro electrodes, Analyst 137 (2012) 896–902. [12] C.M.A. Brett, O.M.S. Filipe, C. Susanna Neves, Determination of chromium(VI) by batch injection analysis and adsorptive stripping voltammetry, Anal. Lett. 36 (2003) 955–969. [13] J. Wang, J. Wang, J. Lu, B. Tian, D. MacDonald, K. Olsen, Flow probe for in situ electrochemical monitoring of trace chromium, Analyst 124 (1999) 349–352. [14] M.-C. Tsai, P.-Y. Chen, Voltammetric study and electrochemical detection of hexavalent chromium at gold nanoparticle-electrodeposited indium tinoxide (ITO) electrodes in acidic media, Talanta 76 (2008) 533–539. [15] R. Ouyang, S.A. Bragg, J.Q. Chambers, Z.-L. Xue, Flower-like self-assembly of gold nanoparticles for highly sensitive electrochemical detection of chromium(VI), Anal. Chim. Acta 722 (2012) 1–7.

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