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Conducting Carbon Dot - Polypyrrole Nanocomposite for Sensitive Detection of Picric acid. Ayan Pal, Md Palashuddin Sk, and Arun Chattopadhyay ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11572 • Publication Date (Web): 16 Feb 2016 Downloaded from http://pubs.acs.org on February 17, 2016
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Conducting Carbon Dot - Polypyrrole Nanocomposite for Sensitive Detection of Picric acid Ayan Pal, † Md Palashuddin Sk †,‡ and Arun Chattopadhyay*†,‡ †
Department of Chemistry and ‡ Centre for Nanotechnology, Indian Institute of Technology Guwahati,
Guwahati - 781039, Assam, India.
ABSTRACT: We report the conducting nature of carbon dots (Cdots) synthesized from citric acid and ethylene diamine. Chemically synthesized conducting nanocomposite consisting of Cdots and polypyrrole (PPy) is further reported, which showed higher electrical conductiviy in comparison to the components i.e., Cdots or PPy. The conductive film of the composite material was used for highly sensitive and selective detection of picric acid in water as well as in soil. To the best of our knowledge, this is the first report on the conductivity based sensing application of Cdot nanocomposite contrary to the traditional fluorescence based sensing approaches. KEYWORDS: conducting carbon dots, polypyrrole, organic electronics, sensing, picric acid, currentvoltage (I-V) characteristics. The current surge in the research activities on carbon dots (Cdots) may be considered to have originated from the ease of their synthesis and processing, functionalization, physical especially optical and chemical properties and above all their versatile application potential. This is apparent from the rather extensive reports on their use in sensing, 1 catalysis,2,3 drug delivery,4 single- and multi-photon based bioimaging1,5 solar cells1 and light emitting devices.6
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Besides, the very recent discovery of conducting Cdots7 manifests their promise in the field of organic electronics for making Cdot based inexpensive electronic sensors and devices. On the other hand, the ubiquitous use of conducting polymers in organic electronics and their tunable electrical conductivities, based on the structure of the monomer and use of dopant provide significant opportunity for incorporating nanomaterials like Cdots in enhancing the versatility of their applications. Added to these is the vast repertoire of existing applications of organic polymers in transistors, sensors, solar cells and OLEDs.8, 9 The competitive advantages of organic materials in terms of ease of synthesis, fabrication of devices and making flexible structures,10 as compared to silicon and gallium based inorganic electronic substrates, 9 are propelling a vast array of research works in chemistry based modern electronic devices. In this regard, polypyrrole (PPy), an important and biofriendly polymer, can easily be synthesized chemically from commercially available precursors and is also amenable to functionalization before and after polymerization.11 The polymer has high chemical and environmental stability and is also electrically conducting. Added to it is the mechanical flexibility, which provides a favorable condition for device fabrication. Additionally, chemical methods allow systematic fabrication of nanoscale and higher order structures, which are useful in device fabrication.11 Also, the demonstrated ease of pursuing chemistry of Cdots under typically ‘green’ reaction conditions, makes them appropriate for composite based organic polymer devices. Herein, we report the conducting property of Cdots and further the synthesis of Cdot embedded PPy nanocomposite (Cdot-PPy), which exhibited considerably higher electrical conductivity in comparison to both Cdots and PPy. The composite also showed high sensitivity to the presence of picric acid, thus acting as a sensor for the same. Picric acid is extensively used not only for preparing explosives but also in chemical laboratories, pharmaceutical and dye industries. Because of its substantial applications, picric acid is considered as a major pollutant, which contaminates ground water and soil. The exposure 2 ACS Paragon Plus Environment
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to the acid causes irritations to skin and eye, and mutilation of the respiratory system.12 Consequently, fast and miniaturized sensitive devices for specific detection of picric acid, present both in water and in soil, are keys to provide essential information with regard to environment pollution and health hazards. For on-the-spot detection of picric acid in real samples, use of conductivity measurement based sensors are more desirable, compared to those which work based on other techniques such as change in photoluminescence property of the probe,12 gas chromatography, membrane electrode method and capillary electrophoresis.13 Although there are reports of similar composite based on carbonaceous dots such as that of graphene quantum dots with conducting polymers,14,15,16 electrical conductivity based sensors are limited in number and use of Cdot based polymer composite in making such a device is yet to be reported. The technique described here is highly selective and the device can potentially be made flexible and portable for field applications. A pictorial representation of the process for the synthesis of the composite and electrical conductivity measurement of its film form for sensing application is shown in Scheme 1.
Scheme 1. Schematic representation of the synthesis of Cdot-PPy composite and corresponding experimental set-up for I-V characteristic study of the composite film. 3 ACS Paragon Plus Environment
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The Cdot-PPy composite was synthesized using Cdots and pyrrole (Py) in HCl medium. Cdots were first prepared from citric acid and ethylene diamine based on a reported method.17 Addition of HCl solution of Py into the aqueous dispersion of Cdots, followed by H2O2, in a glass vial, led to polymerization of Py. The mixture was allowed to stir overnight at room temperature for complete polymerization. The details of synthesis procedure are included in the experimental section of the Supporting Information. UV-vis spectrum of the aqueous dispersion of the composite consisted of broad absorption bands at ~290 nm and ~468 nm (Figure 1). The characteristic absorption peak at ~290 nm is assigned to π-π* transition, which became broad and red-shifted compared to that from PPy, appearing at ~280 nm (Figure 1). Interestingly, the sharp bipolaron transition of PPy18 at 475 nm appeared as broad absorption band from 350 nm to 600 nm in the composite, which might be due to interaction (overlap) between bipolaron transition band in PPy and the n-π* transition band, centered at 360 nm in the Cdot absorption spectrum (Figure1). Further, UV-vis diffuse reflectance spectrum of the solid composite powder consisted of absorption bands at ~252, ~326 and ~490 nm (Figure S1a). The characteristic absorption at ~252 nm in the composite is assigned to π-π* transition, which became shifted compared to that of PPy appearing at ~264 nm (Figure S1a) or Cdot appearing at 224 and 258 nm in the Cdot spectrum (Figure S1b). Absorption band at 490 nm in the composite material appeared due to the bipolaron transition in the polymer. The n-π* transition band in Cdots centered at 337 nm (Figure S1b) was also shifted to 326 nm after the composite formation.
Photoluminescence (PL) spectra (Figure 1c) revealed retention of
wavelength tunable emission due to the Cdots in the composite; however, the intensity was significantly reduced as compared to Cdots. The fluorescence quantum yield (QY) of Cdot-PPy composite was found to be 0.04% at the excitation wavelength of 365 nm (details of the QY calculation are available in the Supporting Information). Electron transfer through non-radiative pathway between Cdots and PPy could have resulted in the lowering of luminescence quantum yield in the emission spectrum.19 However, 4 ACS Paragon Plus Environment
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contribution of PPy to the overall absorbance in the composite might also have led to the lowering of the measured QY. Fourier transform infrared (FTIR) spectroscopic measurement of the composite material revealed the presence of characteristics peak at 1610 cm-1 due to typical C=C stretching vibration of the PPy ring (Figure S2).20 The C=O stretching vibration in Cdot-PPy composite was shifted to 1698 cm-1 (from 1709 cm-1 ) due to hydrogen bonding interaction between C=O group present on the surface of Cdots and the N-H of Py rings in polymer as well as π-π interaction between aromatic Py rings and C=O in Cdots. Weak absorption for C=O stretching of Cdots may be due to its low concentration in the composite.
Figure 1. UV-vis absorption spectra of (a) Cdots, PPy and (b) the composite in water. (c) Emission spectra of Cdot-PPy composite. The excitation wavelengths are indicated in the legends. (d) TEM image of Cdots inside the polymer matrix. The Raman spectra of PPy and the composite material were recorded with laser wavelength set at 488 nm (Figure S3). Two typical bands appeared for PPy at 1383 cm-1 and 1574 cm-1 due to the ring 5 ACS Paragon Plus Environment
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stretching mode and the C=C backbone stretching of PPy.21 Importantly, when Cdots were introduced into the polymer, both the bands were observed at 1369 cm-1 and 1567 cm-1, respectively. The formation of nanocomposite was further confirmed by the transmission electron microscopy (TEM) study, which revealed that smaller Cdots with average size of 4.5 ± 2 nm (Figure S4) were embedded in the polymer matrix (Figure 1d), along with the presence of individual PPy coated Cdots (Figure S5a, b). Further, Cdot-PPy composite material was observed to have been semi-crystalline in nature. Crystalline nature of the material was established by selected area electron diffraction (SAED) measurement (Figure S6a).
High
resolution TEM (HRTEM) images showed the presence of fringes with lattice spacing of 0.35 and 0.28 nm (Figure 2), which correspond to the inter-planar distance between aromatic pyrrole to pyrrole rings and face-to-face stacking respectively.22,
23
However, additional fringe with
spacing of 0.20 nm could also be observed during analysis (Figure S7a, b). The X-ray diffraction (XRD) study (Figure S8) showed two broad peaks at 15º and 25º. Due to the broad diffractions from the amorphous component, it might have been difficult to observe the sharper diffractions from the crystalline component of the composite in the XRD pattern. The field-emission scanning electron microscopy (FESEM) images (Figure S9a) of the composite thin film demonstrated the presence of uneven surface of the polymeric layer, with possible incorporation of Cdots and PPy coated Cdots as represented by the presence of nanoscale particles (Figure S9b). Elemental analysis confirmed that the composite film contained 55.74% C, 5.05% H, 15.33% N and 23.88% O (as calculated).
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Figure 2. HRTEM and corresponding inverse fast Fourier transform (IFFT) images of the composite material with corresponding d- spacing of 0.35 and 0.28 nm. A possible mechanism for the formation of the composite could be proposed to be based on the interaction between Cdots and Py moiety, occurring through hydrogen bonding and π-π interactions between sp2 bonded carbon atoms of both the materials, leading to the proximity of the components.24 Subsequently, reaction with H2O2 led to the formation of polymeric layer over Cdot-surface during chemical oxidation polymerization of Py. Thermal stability studies of PPy and Cdot-PPy composite were carried out in nitrogen atmosphere at a heating rate of 10 oC/min. As shown in Figure S10, the initial weight loss for both the materials in temperature range of 70 to 130 oC was due to evaporation of residual water. Subsequent weight loss in the range of 220 to 700 oC was due to the continuous degradation of the polymer. Importantly, the
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thermogravimetric study revealed that the rate of weight loss for the composite was less as compared to that of PPy, suggesting higher thermal stability of the Cdot-PPy composite than PPy. The current vs. voltage (I-V) characteristics plot for Cdot film was recorded, revealing the electrically conducting nature of Cdots (Figure 3a). Linear I-V characteristics plot demonstrated the metallic behavior of the film. The conductivity of Cdot film could be attributed to the presence of sp2 C-C bonds, which allow conjugation of the adjacent π-bonds to form the π- and π∗-bands.25 Further introduction of Cdots into the polymeric matrix of PPy revealed the higher conductivity of the composite nanomaterial as compared to PPy. A digital photograph of the composite film and the circuit configuration for the study of I-V characteristics are presented in Figure S11 and Figure S12, respectively. The I-V characteristic plots presented in Figure 3b clearly indicated improved electrical conductivity of CdotPPy film in comparison to PPy film. The maximum conductivity for composite film was measured to be 2.60 mS m-1 and that for PPy film was 0.23 mS m-1. The details of the measurement procedures are given in the Supporting Information. PPy, being a p-type of semiconductor, mainly conducts through holes.26 Therefore, the conductivity of the composite is expected to be higher, as positive charge density increases in the polymer backbone. The charge carrier density in the composite might have been enhanced in the presence of Cdots, which are excellent electron acceptors.3, 19, 27 This resulted in the higher conductivity of the composite as compared to PPy. Concentration dependent change in conductivity was also studied by varying the Cdot concentration in the composite material. As shown in Figure S13, conductivity of the composite was observed to increase initially with the increase in Cdot content; however at higher concentrations of Cdot the conductivity decreased. The maximum conductivity of the film was obtained when it was prepared from a reaction mixture containing 4.0 mL of 7.3 mg/mL Cdot dispersion and 268 µL of pyrrole added to 4.0 mL of 1N HCl medium. Furthermore, the effects of oxidative and reductive additives were studied by adding H2O2 and ascorbic acid, respectively, to the composite film. Thus 2.0 µL of 29% H2O2 was added in this regard to the composite film. As is clear from Figure S14a, the change in the conductivity was no different from 8 ACS Paragon Plus Environment
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that obtained due to addition of water to the film. Addition of a NaBH4 as the reductive additive led to deformation of the film. Moreover, addition of ascorbic acid solution (in water) did not lead to any change in conductivity of the film (Figure S14b).
Figure 3. Plots of I-V characteristics of (a) Cdot film and (b) Cdot-PPy (CD-PPy) and PPy films. (c) The ratios of current flowing through the composite film after adding 2.0 µL 1.0 mM aqueous solution of different analytes to that of the film only. Here PA = picric acid, 2,4-DNP = 2,4-dinitrophenol, 4-NP = 4-nitrophenol, NB = nitrobenzene, PH = phenol, QN = 1,4- benzoquinone, 4-MBA= 4methoxybenzoic acid. The measurements were made at +5 V. Finally, Cdot-PPy composite film was used for the detection of picric acid, based on the change in IV characteristics (for experimental details, refer to the Supporting Information). Interestingly, we found significant increase in current due to the addition of aqueous solutions of picric acid with increasing concentrations on the composite film. Concentration dependent distinct changes in the I-V characteristics of the composite film in presence of picric acid (Figure S15) indicated higher degree of protonation to aromatic ring in the PPy backbone, thus allowing higher charge carrier density. As is illustrated in Figure 3c, the response of the composite film was much higher towards picric acid in9 ACS Paragon Plus Environment
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comparison to other nitro phenol derivatives such as 2,4-dinitrophenol (2,4-DNP), 4-nitrophenol (4-NP) and phenol (PH), thus indicating selectivity for the preferential detection of picric acid (Figure S16, S17). The I-V characteristics study of the composite film was also performed with electron donor molecule like 4-methoxy benzoic acid (MBA) and electron acceptor molecule, quinone (QN), for which the sensitivities were low (Figure S18). It is worthy to mention here that the limit of detection of picric acid was found to be 1.40 × 10-7 M (32 ppb), revealing higher sensitivity of the composite film, as compared to earlier reported results (Figure S19).28-30 Further, the results from the measurement of pH of the medium of picric acid, which was added to the composite film (Table S1), indicated minimum effect on the conductivity of the film, at least at lower concentrations of the analyte. It was observed that the Cdot-PPy composite film was also able to detect trace amount of picric acid present in soil sample (Figure S20; Experimental Procedure, refer to the Supporting Information), with appreciable change in I-V-characteristics at low concentrations of the acid. The minimum concentration of picric acid detected by the film was 5.7 ng mg-1 of the soil. In conclusion, we have reported the electrical conducting property of Cdots and also developed a facile method of synthesis of conducting Cdot-PPy nanocomposite, which showed enhanced electrical conductivity compared to both Cdot and PPy films. Further, the as-synthesized composite film exhibited higher selectivity and sensitivity towards detection of trace amount of picric acid present in aqueous phase as well as in the soil. The above results indicated the potential of Cdot - being present in a composite polymer film - for versatile sensing applications especially in flexible nano-electronics. Associated Content Supporting Information Synthesis and characterization process of Cdot-PPy nanocomposite, preparation of conductive film, I-V characteristics study, picric acid sensing and detection limit calculation, Figure S1-S23, Table S1. This 10 ACS Paragon Plus Environment
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material is available free of charge via the Internet at http://pubs.acs.org. Author Information Corresponding Author * E-mail: [email protected].
Notes The authors declare no competing financial interest.
Acknowledgements We thank the Department of Electronics and Information Technology, Government of India (Grant Number 5(9)/2012-NANO (Vol II)) for funds. Assistance from Department of Physics, Central Instruments Facility (CIF), IIT Guwahati, Dr. Nirmala Devi, Dr. Devasish Chowdhury, Kafeel Ahmad, Mitradip Bhattacharjee and Anindya Sundar Patra are acknowledged.
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19. Barman, M.; Jana, B.; Bhattacharyya, S.; Patra, A. Photophysical Properties of Doped Carbon Dots (N, P, and B) and Their Influence on Electron/Hole Transfer in Carbon Dots−Nickel (II) Phthalocyanine Conjugates. J. Phys. Chem. C 2014, 118, 20034-20041. 20. Chen, A.; Wang, H.; Li, X. One-Step Process to Fabricate Ag–Polypyrrole Coaxial Nanocables. Chem. Commun. 2005, 1863-1864. 21. Lim, S.; Pandikumar, A.; Lim, Y.; Huang, N.; Lim, H. In-situ Electrochemically Deposited Polypyrrole Nanoparticles Incorporated Reduced Graphene Oxide as an Efficient Counter Electrode for Platinum-free Dye-sensitized Solar Cells. Sci. Rep.2014, 4, 5305. 22. Carrasco, P.; Grande, H.; Cortazar, M.; Alberdi, J.; Areizaga, J.; Pomposo, J. StructureConductivity Relationships in Chemical Polypyrroles of Low, Medium and High conductivity. Synth. Met. 2006, 156, 420-425. 23. Jeeju, P.; Varma, S.; Xavier, P. A.; Sajimol, A. M.; Jayalekshmi, S. Novel Polypyrrole Films with Excellent Crystallinity and Good Thermal Stability. Mater. Chem. Phys. 2012, 134, 803808. 24. Xu, C.; Sun, J.; Gao, L. Synthesis of Novel Hierarchical Graphene/Polypyrrole Nanosheet Composites and Their Superior Electrochemical Performance. J. Mater. Chem. 2011, 21, 11253-11258. 25. Eda, G.; Lin, Y.; Mattevi, C.; Yamaguchi, H.; Chen, H.; Chen, I.; Chen, C.; Chhowalla, M. Blue Photoluminescence from Chemically Derived Graphene Oxide. Adv. Mater. 2010, 22, 505-509. 26. Chen, X.; Jenekhe, S. Bipolar Conducting Polymers: Blends of p-Type Polypyrrole and an nType Ladder Polymer. Macromolecules 1997, 30, 1728-1733. 27. Wang, X.; Cao, L.; Lu, F.; Meziani, M.; Li, H.; Qi, G.; Zhou, B.; Harruff, B.; Kermarrec, F.; Sun, Y. Photoinduced Electron Transfers with Carbon Dots. Chem. Commun. 2009, 3774-3776. 28. Huang, J.; Wang, L.; Shi, C.; Dai, Y.; Gu, C.; Liu, J. Selective Detection of Picric Acid Using Functionalized Reduced Graphene Oxide Sensor Device. Sens. Actuators B 2014, 196, 567-573. 29. Ma, J.; Lin, T.; Pan, X.; Wang, W. Graphene-Like Molecules Based on Tetraphenylethene Oligomers: Synthesis, Characterization, and Applications. Chem. Mater. 2014, 26, 4221-4229. 30. Dinda, D.; Gupta, A.; Shaw, B.; Sadhu, S.; Saha, S. Highly Selective Detection of Trinitrophenol by Luminescent Functionalized Reduced Graphene Oxide through FRET Mechanism. ACS Appl. Mater. Interfaces 2014, 6, 10722-10728.
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Letter
Conducting Carbon Dot - Polypyrrole Nanocomposite for Sensitive Detection of Picric acid. Ayan Pal, Md Palashuddin Sk, and Arun Chattopadhyay ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11572 • Publication Date (Web): 16 Feb 2016 Downloaded from http://pubs.acs.org on February 17, 2016
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Conducting Carbon Dot - Polypyrrole Nanocomposite for Sensitive Detection of Picric acid Ayan Pal, † Md Palashuddin Sk †,‡ and Arun Chattopadhyay*†,‡ †
Department of Chemistry and ‡ Centre for Nanotechnology, Indian Institute of Technology Guwahati,
Guwahati - 781039, Assam, India.
ABSTRACT: We report the conducting nature of carbon dots (Cdots) synthesized from citric acid and ethylene diamine. Chemically synthesized conducting nanocomposite consisting of Cdots and polypyrrole (PPy) is further reported, which showed higher electrical conductiviy in comparison to the components i.e., Cdots or PPy. The conductive film of the composite material was used for highly sensitive and selective detection of picric acid in water as well as in soil. To the best of our knowledge, this is the first report on the conductivity based sensing application of Cdot nanocomposite contrary to the traditional fluorescence based sensing approaches. KEYWORDS: conducting carbon dots, polypyrrole, organic electronics, sensing, picric acid, currentvoltage (I-V) characteristics. The current surge in the research activities on carbon dots (Cdots) may be considered to have originated from the ease of their synthesis and processing, functionalization, physical especially optical and chemical properties and above all their versatile application potential. This is apparent from the rather extensive reports on their use in sensing, 1 catalysis,2,3 drug delivery,4 single- and multi-photon based bioimaging1,5 solar cells1 and light emitting devices.6
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Besides, the very recent discovery of conducting Cdots7 manifests their promise in the field of organic electronics for making Cdot based inexpensive electronic sensors and devices. On the other hand, the ubiquitous use of conducting polymers in organic electronics and their tunable electrical conductivities, based on the structure of the monomer and use of dopant provide significant opportunity for incorporating nanomaterials like Cdots in enhancing the versatility of their applications. Added to these is the vast repertoire of existing applications of organic polymers in transistors, sensors, solar cells and OLEDs.8, 9 The competitive advantages of organic materials in terms of ease of synthesis, fabrication of devices and making flexible structures,10 as compared to silicon and gallium based inorganic electronic substrates, 9 are propelling a vast array of research works in chemistry based modern electronic devices. In this regard, polypyrrole (PPy), an important and biofriendly polymer, can easily be synthesized chemically from commercially available precursors and is also amenable to functionalization before and after polymerization.11 The polymer has high chemical and environmental stability and is also electrically conducting. Added to it is the mechanical flexibility, which provides a favorable condition for device fabrication. Additionally, chemical methods allow systematic fabrication of nanoscale and higher order structures, which are useful in device fabrication.11 Also, the demonstrated ease of pursuing chemistry of Cdots under typically ‘green’ reaction conditions, makes them appropriate for composite based organic polymer devices. Herein, we report the conducting property of Cdots and further the synthesis of Cdot embedded PPy nanocomposite (Cdot-PPy), which exhibited considerably higher electrical conductivity in comparison to both Cdots and PPy. The composite also showed high sensitivity to the presence of picric acid, thus acting as a sensor for the same. Picric acid is extensively used not only for preparing explosives but also in chemical laboratories, pharmaceutical and dye industries. Because of its substantial applications, picric acid is considered as a major pollutant, which contaminates ground water and soil. The exposure 2 ACS Paragon Plus Environment
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to the acid causes irritations to skin and eye, and mutilation of the respiratory system.12 Consequently, fast and miniaturized sensitive devices for specific detection of picric acid, present both in water and in soil, are keys to provide essential information with regard to environment pollution and health hazards. For on-the-spot detection of picric acid in real samples, use of conductivity measurement based sensors are more desirable, compared to those which work based on other techniques such as change in photoluminescence property of the probe,12 gas chromatography, membrane electrode method and capillary electrophoresis.13 Although there are reports of similar composite based on carbonaceous dots such as that of graphene quantum dots with conducting polymers,14,15,16 electrical conductivity based sensors are limited in number and use of Cdot based polymer composite in making such a device is yet to be reported. The technique described here is highly selective and the device can potentially be made flexible and portable for field applications. A pictorial representation of the process for the synthesis of the composite and electrical conductivity measurement of its film form for sensing application is shown in Scheme 1.
Scheme 1. Schematic representation of the synthesis of Cdot-PPy composite and corresponding experimental set-up for I-V characteristic study of the composite film. 3 ACS Paragon Plus Environment
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The Cdot-PPy composite was synthesized using Cdots and pyrrole (Py) in HCl medium. Cdots were first prepared from citric acid and ethylene diamine based on a reported method.17 Addition of HCl solution of Py into the aqueous dispersion of Cdots, followed by H2O2, in a glass vial, led to polymerization of Py. The mixture was allowed to stir overnight at room temperature for complete polymerization. The details of synthesis procedure are included in the experimental section of the Supporting Information. UV-vis spectrum of the aqueous dispersion of the composite consisted of broad absorption bands at ~290 nm and ~468 nm (Figure 1). The characteristic absorption peak at ~290 nm is assigned to π-π* transition, which became broad and red-shifted compared to that from PPy, appearing at ~280 nm (Figure 1). Interestingly, the sharp bipolaron transition of PPy18 at 475 nm appeared as broad absorption band from 350 nm to 600 nm in the composite, which might be due to interaction (overlap) between bipolaron transition band in PPy and the n-π* transition band, centered at 360 nm in the Cdot absorption spectrum (Figure1). Further, UV-vis diffuse reflectance spectrum of the solid composite powder consisted of absorption bands at ~252, ~326 and ~490 nm (Figure S1a). The characteristic absorption at ~252 nm in the composite is assigned to π-π* transition, which became shifted compared to that of PPy appearing at ~264 nm (Figure S1a) or Cdot appearing at 224 and 258 nm in the Cdot spectrum (Figure S1b). Absorption band at 490 nm in the composite material appeared due to the bipolaron transition in the polymer. The n-π* transition band in Cdots centered at 337 nm (Figure S1b) was also shifted to 326 nm after the composite formation.
Photoluminescence (PL) spectra (Figure 1c) revealed retention of
wavelength tunable emission due to the Cdots in the composite; however, the intensity was significantly reduced as compared to Cdots. The fluorescence quantum yield (QY) of Cdot-PPy composite was found to be 0.04% at the excitation wavelength of 365 nm (details of the QY calculation are available in the Supporting Information). Electron transfer through non-radiative pathway between Cdots and PPy could have resulted in the lowering of luminescence quantum yield in the emission spectrum.19 However, 4 ACS Paragon Plus Environment
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contribution of PPy to the overall absorbance in the composite might also have led to the lowering of the measured QY. Fourier transform infrared (FTIR) spectroscopic measurement of the composite material revealed the presence of characteristics peak at 1610 cm-1 due to typical C=C stretching vibration of the PPy ring (Figure S2).20 The C=O stretching vibration in Cdot-PPy composite was shifted to 1698 cm-1 (from 1709 cm-1 ) due to hydrogen bonding interaction between C=O group present on the surface of Cdots and the N-H of Py rings in polymer as well as π-π interaction between aromatic Py rings and C=O in Cdots. Weak absorption for C=O stretching of Cdots may be due to its low concentration in the composite.
Figure 1. UV-vis absorption spectra of (a) Cdots, PPy and (b) the composite in water. (c) Emission spectra of Cdot-PPy composite. The excitation wavelengths are indicated in the legends. (d) TEM image of Cdots inside the polymer matrix. The Raman spectra of PPy and the composite material were recorded with laser wavelength set at 488 nm (Figure S3). Two typical bands appeared for PPy at 1383 cm-1 and 1574 cm-1 due to the ring 5 ACS Paragon Plus Environment
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stretching mode and the C=C backbone stretching of PPy.21 Importantly, when Cdots were introduced into the polymer, both the bands were observed at 1369 cm-1 and 1567 cm-1, respectively. The formation of nanocomposite was further confirmed by the transmission electron microscopy (TEM) study, which revealed that smaller Cdots with average size of 4.5 ± 2 nm (Figure S4) were embedded in the polymer matrix (Figure 1d), along with the presence of individual PPy coated Cdots (Figure S5a, b). Further, Cdot-PPy composite material was observed to have been semi-crystalline in nature. Crystalline nature of the material was established by selected area electron diffraction (SAED) measurement (Figure S6a).
High
resolution TEM (HRTEM) images showed the presence of fringes with lattice spacing of 0.35 and 0.28 nm (Figure 2), which correspond to the inter-planar distance between aromatic pyrrole to pyrrole rings and face-to-face stacking respectively.22,
23
However, additional fringe with
spacing of 0.20 nm could also be observed during analysis (Figure S7a, b). The X-ray diffraction (XRD) study (Figure S8) showed two broad peaks at 15º and 25º. Due to the broad diffractions from the amorphous component, it might have been difficult to observe the sharper diffractions from the crystalline component of the composite in the XRD pattern. The field-emission scanning electron microscopy (FESEM) images (Figure S9a) of the composite thin film demonstrated the presence of uneven surface of the polymeric layer, with possible incorporation of Cdots and PPy coated Cdots as represented by the presence of nanoscale particles (Figure S9b). Elemental analysis confirmed that the composite film contained 55.74% C, 5.05% H, 15.33% N and 23.88% O (as calculated).
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Figure 2. HRTEM and corresponding inverse fast Fourier transform (IFFT) images of the composite material with corresponding d- spacing of 0.35 and 0.28 nm. A possible mechanism for the formation of the composite could be proposed to be based on the interaction between Cdots and Py moiety, occurring through hydrogen bonding and π-π interactions between sp2 bonded carbon atoms of both the materials, leading to the proximity of the components.24 Subsequently, reaction with H2O2 led to the formation of polymeric layer over Cdot-surface during chemical oxidation polymerization of Py. Thermal stability studies of PPy and Cdot-PPy composite were carried out in nitrogen atmosphere at a heating rate of 10 oC/min. As shown in Figure S10, the initial weight loss for both the materials in temperature range of 70 to 130 oC was due to evaporation of residual water. Subsequent weight loss in the range of 220 to 700 oC was due to the continuous degradation of the polymer. Importantly, the
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thermogravimetric study revealed that the rate of weight loss for the composite was less as compared to that of PPy, suggesting higher thermal stability of the Cdot-PPy composite than PPy. The current vs. voltage (I-V) characteristics plot for Cdot film was recorded, revealing the electrically conducting nature of Cdots (Figure 3a). Linear I-V characteristics plot demonstrated the metallic behavior of the film. The conductivity of Cdot film could be attributed to the presence of sp2 C-C bonds, which allow conjugation of the adjacent π-bonds to form the π- and π∗-bands.25 Further introduction of Cdots into the polymeric matrix of PPy revealed the higher conductivity of the composite nanomaterial as compared to PPy. A digital photograph of the composite film and the circuit configuration for the study of I-V characteristics are presented in Figure S11 and Figure S12, respectively. The I-V characteristic plots presented in Figure 3b clearly indicated improved electrical conductivity of CdotPPy film in comparison to PPy film. The maximum conductivity for composite film was measured to be 2.60 mS m-1 and that for PPy film was 0.23 mS m-1. The details of the measurement procedures are given in the Supporting Information. PPy, being a p-type of semiconductor, mainly conducts through holes.26 Therefore, the conductivity of the composite is expected to be higher, as positive charge density increases in the polymer backbone. The charge carrier density in the composite might have been enhanced in the presence of Cdots, which are excellent electron acceptors.3, 19, 27 This resulted in the higher conductivity of the composite as compared to PPy. Concentration dependent change in conductivity was also studied by varying the Cdot concentration in the composite material. As shown in Figure S13, conductivity of the composite was observed to increase initially with the increase in Cdot content; however at higher concentrations of Cdot the conductivity decreased. The maximum conductivity of the film was obtained when it was prepared from a reaction mixture containing 4.0 mL of 7.3 mg/mL Cdot dispersion and 268 µL of pyrrole added to 4.0 mL of 1N HCl medium. Furthermore, the effects of oxidative and reductive additives were studied by adding H2O2 and ascorbic acid, respectively, to the composite film. Thus 2.0 µL of 29% H2O2 was added in this regard to the composite film. As is clear from Figure S14a, the change in the conductivity was no different from 8 ACS Paragon Plus Environment
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that obtained due to addition of water to the film. Addition of a NaBH4 as the reductive additive led to deformation of the film. Moreover, addition of ascorbic acid solution (in water) did not lead to any change in conductivity of the film (Figure S14b).
Figure 3. Plots of I-V characteristics of (a) Cdot film and (b) Cdot-PPy (CD-PPy) and PPy films. (c) The ratios of current flowing through the composite film after adding 2.0 µL 1.0 mM aqueous solution of different analytes to that of the film only. Here PA = picric acid, 2,4-DNP = 2,4-dinitrophenol, 4-NP = 4-nitrophenol, NB = nitrobenzene, PH = phenol, QN = 1,4- benzoquinone, 4-MBA= 4methoxybenzoic acid. The measurements were made at +5 V. Finally, Cdot-PPy composite film was used for the detection of picric acid, based on the change in IV characteristics (for experimental details, refer to the Supporting Information). Interestingly, we found significant increase in current due to the addition of aqueous solutions of picric acid with increasing concentrations on the composite film. Concentration dependent distinct changes in the I-V characteristics of the composite film in presence of picric acid (Figure S15) indicated higher degree of protonation to aromatic ring in the PPy backbone, thus allowing higher charge carrier density. As is illustrated in Figure 3c, the response of the composite film was much higher towards picric acid in9 ACS Paragon Plus Environment
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comparison to other nitro phenol derivatives such as 2,4-dinitrophenol (2,4-DNP), 4-nitrophenol (4-NP) and phenol (PH), thus indicating selectivity for the preferential detection of picric acid (Figure S16, S17). The I-V characteristics study of the composite film was also performed with electron donor molecule like 4-methoxy benzoic acid (MBA) and electron acceptor molecule, quinone (QN), for which the sensitivities were low (Figure S18). It is worthy to mention here that the limit of detection of picric acid was found to be 1.40 × 10-7 M (32 ppb), revealing higher sensitivity of the composite film, as compared to earlier reported results (Figure S19).28-30 Further, the results from the measurement of pH of the medium of picric acid, which was added to the composite film (Table S1), indicated minimum effect on the conductivity of the film, at least at lower concentrations of the analyte. It was observed that the Cdot-PPy composite film was also able to detect trace amount of picric acid present in soil sample (Figure S20; Experimental Procedure, refer to the Supporting Information), with appreciable change in I-V-characteristics at low concentrations of the acid. The minimum concentration of picric acid detected by the film was 5.7 ng mg-1 of the soil. In conclusion, we have reported the electrical conducting property of Cdots and also developed a facile method of synthesis of conducting Cdot-PPy nanocomposite, which showed enhanced electrical conductivity compared to both Cdot and PPy films. Further, the as-synthesized composite film exhibited higher selectivity and sensitivity towards detection of trace amount of picric acid present in aqueous phase as well as in the soil. The above results indicated the potential of Cdot - being present in a composite polymer film - for versatile sensing applications especially in flexible nano-electronics. Associated Content Supporting Information Synthesis and characterization process of Cdot-PPy nanocomposite, preparation of conductive film, I-V characteristics study, picric acid sensing and detection limit calculation, Figure S1-S23, Table S1. This 10 ACS Paragon Plus Environment
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material is available free of charge via the Internet at http://pubs.acs.org. Author Information Corresponding Author * E-mail: [email protected].
Notes The authors declare no competing financial interest.
Acknowledgements We thank the Department of Electronics and Information Technology, Government of India (Grant Number 5(9)/2012-NANO (Vol II)) for funds. Assistance from Department of Physics, Central Instruments Facility (CIF), IIT Guwahati, Dr. Nirmala Devi, Dr. Devasish Chowdhury, Kafeel Ahmad, Mitradip Bhattacharjee and Anindya Sundar Patra are acknowledged.
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19. Barman, M.; Jana, B.; Bhattacharyya, S.; Patra, A. Photophysical Properties of Doped Carbon Dots (N, P, and B) and Their Influence on Electron/Hole Transfer in Carbon Dots−Nickel (II) Phthalocyanine Conjugates. J. Phys. Chem. C 2014, 118, 20034-20041. 20. Chen, A.; Wang, H.; Li, X. One-Step Process to Fabricate Ag–Polypyrrole Coaxial Nanocables. Chem. Commun. 2005, 1863-1864. 21. Lim, S.; Pandikumar, A.; Lim, Y.; Huang, N.; Lim, H. In-situ Electrochemically Deposited Polypyrrole Nanoparticles Incorporated Reduced Graphene Oxide as an Efficient Counter Electrode for Platinum-free Dye-sensitized Solar Cells. Sci. Rep.2014, 4, 5305. 22. Carrasco, P.; Grande, H.; Cortazar, M.; Alberdi, J.; Areizaga, J.; Pomposo, J. StructureConductivity Relationships in Chemical Polypyrroles of Low, Medium and High conductivity. Synth. Met. 2006, 156, 420-425. 23. Jeeju, P.; Varma, S.; Xavier, P. A.; Sajimol, A. M.; Jayalekshmi, S. Novel Polypyrrole Films with Excellent Crystallinity and Good Thermal Stability. Mater. Chem. Phys. 2012, 134, 803808. 24. Xu, C.; Sun, J.; Gao, L. Synthesis of Novel Hierarchical Graphene/Polypyrrole Nanosheet Composites and Their Superior Electrochemical Performance. J. Mater. Chem. 2011, 21, 11253-11258. 25. Eda, G.; Lin, Y.; Mattevi, C.; Yamaguchi, H.; Chen, H.; Chen, I.; Chen, C.; Chhowalla, M. Blue Photoluminescence from Chemically Derived Graphene Oxide. Adv. Mater. 2010, 22, 505-509. 26. Chen, X.; Jenekhe, S. Bipolar Conducting Polymers: Blends of p-Type Polypyrrole and an nType Ladder Polymer. Macromolecules 1997, 30, 1728-1733. 27. Wang, X.; Cao, L.; Lu, F.; Meziani, M.; Li, H.; Qi, G.; Zhou, B.; Harruff, B.; Kermarrec, F.; Sun, Y. Photoinduced Electron Transfers with Carbon Dots. Chem. Commun. 2009, 3774-3776. 28. Huang, J.; Wang, L.; Shi, C.; Dai, Y.; Gu, C.; Liu, J. Selective Detection of Picric Acid Using Functionalized Reduced Graphene Oxide Sensor Device. Sens. Actuators B 2014, 196, 567-573. 29. Ma, J.; Lin, T.; Pan, X.; Wang, W. Graphene-Like Molecules Based on Tetraphenylethene Oligomers: Synthesis, Characterization, and Applications. Chem. Mater. 2014, 26, 4221-4229. 30. Dinda, D.; Gupta, A.; Shaw, B.; Sadhu, S.; Saha, S. Highly Selective Detection of Trinitrophenol by Luminescent Functionalized Reduced Graphene Oxide through FRET Mechanism. ACS Appl. Mater. Interfaces 2014, 6, 10722-10728.
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