âMulti-Component Template Effectsâ- Preparation of Highly Porous

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“Multi-Component Template Effects”- Preparation of Highly Porous Polyaniline Nano Rods using Crude Lemon Juice and its Application for Selective Detection of Catechol Vineeta Gautam, Karan Pratap Singh, and Vijay Laxmi Yadav ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03705 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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ACS Sustainable Chemistry & Engineering

Full mailing address of authors

1. Dr. Karan Pratap Singh President Mahamana Innovative Technologies Welfare Society 121- Station road, Bijauriya, Nawabganj, Bareilly, Uttar Pradesh, India, 262406 E-mail: [email protected] 2. Dr. Vijay Laxmi Yadav Associate Professor Department of Chemical Engineering and Technology Indian Institute of Technology (Banaras Hindu University) Varanasi, Uttar Pradesh, India 221005 E-mail: [email protected] 3. Vineeta Gautam Research Scholar Department of Chemical Engineering and Technology Indian Institute of Technology (Banaras Hindu University) Varanasi, Uttar Pradesh, India 221005 E-mail: [email protected]

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“Multi-Component Template Effects”- Preparation of Highly Porous Polyaniline Nanorods using Crude Lemon Juice and its Application for Selective Detection of Catechol Vineeta Gautam1, 2$, Karan P. Singh2**, Vijay L. Yadav1* 1. Department of Chemical Engineering & Technology, IIT-BHU, Varanasi-221005, U.P., India 2. Mahamana Innovative Technologies Welfare Society, Nawabganj, Bareilly-262406, U.P., India Abstract: This paper deals with the preparation and characterization of three different polyaniline samples, namely, PANI-HCl (synthesis in inorganic acid), PANI-Citric acid (synthesis in organic acid) and PANILemon (synthesis in crude lemon juice – biological derived acidic solution). PANI-Lemon is a lowdensity polymer with well-defined nanorod shape morphology (~ 100 nm in diameter and ~ 300-600 nm in length). Lemon juice manipulates the morphology of PANI because it has lower pKa and the matrix molecules produce template like effects. The distinct structure of PANI-Lemon confirmed from the microscopic and spectroscopic analysis. In the same protonating condition, the alignment of chains is the dominating factor which governs the conductivity of polyaniline. PANI-Lemon was successfully applied for the electrocatalytic detection of catechol. The selective interaction of PANI-Lemon with catechol leads to the generation of a new redox center. PANI-Lemon modified carbon paste electrode exhibited


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high sensitivity (49.68 µAµM-1cm-2), specificity, wide linear range (5 µM -100 mM), low detection limit (2.1 µM), and less response time (2 s). Based on our results, we propose an eco-friendly concept “Multicomponent Template Effects” to bring nano-scale morphological manipulation using suitable natural extract. This concept equally implemented to synthetic polymers and other nanoparticles.

Keywords: Multi-component Template Effects, Porous Polyaniline Nanorods, Cyclic Voltammetry, Catechol-Sensor, Electron Microscopy.

INTRODUCTION Novel nanomaterials have been prepared by applying specific synthesis protocols. PANI is an excellent and attractive conducting polymer for a wide range of electrochemical and optical applications. Traditionally, PANI is prepared by free radical chemical polymerization in the solution of organic or inorganic acids. Nature of acid and associated anionic dopants bring significant variation in ionic strength, doping level, molecular forces, spatial arrangements, and inter/intra chain charge transport. Different polyaniline nanostructures have been developed, such as – nano-rods, globular, nano-sphere, nanotubes, nano-sheet, core-shell, thin films and fibers.1-4 The degree of oxidation and protonation of PANI chains are very important parameters, which controls the electrical and optical properties. During doping condition, the imine nitrogen of PANI protonated without changing the number of pi electrons in the chain, and the generated polarons/bipolarons participate in the charge transfer mechanism. In a medium of the protonic acid of small molecular size (HCl and H2SO4), a globular morphology observed.5 Hung et al. reported that the morphology of polyaniline depends upon the functionality and substitution of associated anions of organic acids. The morphology of polyaniline depends upon the functionality and substitution of associated anions of organic acids. 6 The pKa value of dopant acids strongly correlated with the spectral and redox properties of polyaniline.7 Macdiarmid et al. described the concept of doping, structural information and conductivity mechanism of the 3 ACS Paragon Plus Environment


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polyaniline doped with nonoxidizing protonic acid (HCl).8 Structural and morphological characteristics of polyaniline, prepared in HCl and H2SO4, have been studied by our research group. We have concluded from the SEM and TEM images that PANI-H2SO4 has compact and aligned morphology than PANI-HCl, the significant differences observed due to longer induction period in sulphuric acid and bivalent sulphate anion. Thus, the nature of acids and the type of associated anionic dopants governs the kinetics of polymerization.9 Liu et al reported the PANI nanorods formation and its uniform deposition in the vertically pores of caron aerosol. 3 The shape and size of nanomaterials could be easily controlled by directed synthesis, which is an efficient, time-saving, cost-effective and bottom-up approach. Temporary template skeletons direct the spontaneous immobilization/force deposition of the building blocks of a material. Many solids have been used as hard template structures, such as - membrane, silica, metal oxides and nanoparticles. Materials or their precursors can be filled in the template by using suitable deposition methods, such as – sol-gel process, electrochemical polymerization, vapor deposition, casting, supercritical fluid, chemical and photochemical. After deposition, the templates were scaffolded with a suitable remover. Recent advancements reveal that self-assembled structure involves donor/acceptor forces, metal-ligand coordination, hydrogen bonding, π–π stacking, solvophobic repulsion, and/or electrostatic forces. Besides these, soft templates were also used for the same purpose. Soft template synthesis (also called as endotemplate synthesis) does not have any rigid solid structures.

10-14

Hard and soft template synthesis have

distinct merits and demerits. The hard template provides better control over the shape and size of the products; however, it is a multi-step, complicated, costly and time-consuming procedure. The removal of template structure present in the developed material is a tedious process and requires hazardous chemicals. The soft template has been proven to be more efficient, requires less synthetic steps and easy removal of template molecules by the simple process. Chiou and coworkers reported alignment of PANI fibers under controlled deposition on different substrates.15-16 Pruna and coworkers have reported


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aluminum oxide template assisted electro-polymerization of PANI nanotubes.17 Montilla et.al. reported template assisted the growth of PANI by using porous sol-gel films.18 Porosity of polyaniline is an important factor, it depends on the synthesis process parameter, additive used and the refining steps.2,19-22 Doping-dedoping-redoping is a convenient way to achieve specific property and desired level of doping.23 Scientists and environmental activists are advocating green manufacturing protocols, environment-friendly chemicals and clean technologies for the global sustainable development. Conventional doping of PANI with HCl is liable to dedoping, the evolution of HCl on evaporation may damage the device. Industrial production of PANI uses strong inorganic acids, discharge corrosive acidic waste in the water bodies which adversely impact on the ecosystem. Lemon juice is environment-friendly and biocompatible acidic solution, it is a low-cost material for the green synthesis of PANI. Lemon juice consists many weak organic acids, which facilitate the slow polymerization reaction and directional alignment of polyaniline chains. Helali et al. studied the important characteristics of lemon juice having composition as ; moisture (89.7%), total solid (10.3%), total suspended solids (8.7%), total sugar (4.2%), pH (2.54), citric acid (0.53%), vitamin-C (35.08 mg/100g), a sugar: acid ratio (7.97%) and ash (0.39%).24 Naseem and coworkers reported environment-friendly oxidative chemical polymerization of PANI using lemon juice.25 The main objective of this research work is to study the spectroscopic, morphological and electrochemical characteristics of the three PANI samples (PANI-Lemon, PANI-HCl, and PANI-Citric acid) and used for electrochemical sensor. Many phenolic compounds have been widely used in different chemical industries. Catechol or pyrocatechol or 1, 2-dihydroxybenzene (molecular formula C6H6O2), is commonly used in medicines, cosmetics, dye developers, pesticides/fungicides, and antioxidants. It is released into the environment during its manufacturing and uses. Hence, its detection is very important for environmental protection and public health.

Its exposure may cause many metabolic disorders, e.g. Atherosclerosis, Parkinson,

Alzheimer, Diarrhea, depression of central nervous system, gastrointestinal, respiratory, and cardiovascular problems. Enzymatic and nonenzymatic detection of catechol has been reported by many researchers.26-41 5 ACS Paragon Plus Environment


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Experimental section Materials. Aniline (Merk India Limited) was purified by vacuum distillation and stored in the refrigerator prior to use. Ammonium peroxidisulphate (APS) was used as received from Qualigens Fine Chemicals (India) and citric acid was purchased from SD fine chemicals. Graphite powder and Nujol oil were purchased from Sigma Aldrich. Lemon juice was freshly extracted from fruits and diluted with double distilled water. All other solutions were prepared in double distilled water. Phosphate buffer solutions (0.1 M PBS) were prepared by using disodium hydrogen phosphate, potassium chloride, and potassium dihydrogen phosphate, and pH were adjusted by using hydrochloric acid or sodium hydroxide. Instrumental analysis. Scanning electron microscope (SEM), High resolution-scanning electron microscope (HR-SEM), Transmission electron microscope (TEM) and Atomic force microscope (AFM) techniques were used to explore morphological information. SEM and HR-SEM observations were performed, using Quanta 200, FEI of USA (SEA) PTE Ltd., Singapore and Novananosem respectively, at suitable voltages and magnifications. TEM examination was made using FEI, TECHNAI 20G2 electron microscope at an accelerating voltage of 200KV. AFM characterization was performed with the DI Nanoscope IIIa microscope of the LNLS, in non-contact mode, NSC-10-50, » 20N/m at » 260KHz. The crystalline properties of materials were investigated using Ultima IV X-ray diffractometer with CuKα radiation (λ = 1.5404 Å). The textural properties (surface area, pore size distribution and pore volume) were measured, using a volumetric gas adsorption apparatus at 77 K with a Micromeritics ASAP 2020 physisorption instrument. Before each measurement, the samples were kept at 50 oC in vacuum for 12 hours to remove adsorbed foreign molecules. Fourier transform infrared (FTIR) spectra were recorded in the range of 500 - 4000 cm-1, with potassium bromide (KBr) pellets at room temperature, using Perkin Elemer Spectrum version 10.03.05. Electronic structures were analyzed using Varian Carry 100 Bio dual beam UV-Visible Spectrophotometer, in the wavelength region 200 to 900 nm, at a scanning rate 400 nm/min, using the paste of 0.05 g sample in Nujol oil. NMR spectra were obtained by Bruker Ac (250 MHz) spectrometer using DMSO-d6 solution. Electrochemical studies were carried out, using CH600D electrochemical workstation, CH Instruments U.S.A., using a three electrode configuration. Modified 6 ACS Paragon Plus Environment

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carbon paste, Ag/AgCl (saturated with KCl) and Pt wire were used as working, reference and counter electrodes, respectively. Polymerization of aniline. Vacuum distilled aniline was mixed with filtered and diluted lemon juice at the ice-cold temperature. To investigate the effect of lemon juice concentration, two PANI-Lemon samples were prepared in the solutions of different concentrations; 0.5 % [PANI-Lemon (L)] and 2 % [PANI-Lemon (H)]. Dilute solution of ammonium peroxydisulfate (0.1 M) was used to initiate polymerization reactions. APS was added dropwise in the reaction mixture under stirred condition. After 2 hours of continuous stirring, the reaction mixture was kept undisturbed for 24 hours in order to complete polymerization.9 The polymerization reactions in lemon juice solution have longer induction period and slow kinetics than others. Initially, the dark brown precipitate was obtained which turn into green color at the later stage (indicates “Emeraldine salt” form of PANI). The polymeric material was filtered and washed several times with 0.1 M HCl solution and final washing was done with acetone. Washing of precipitate removes the residual monomers, the oxidizing agent, oligomers, and other soluble contents of lemon juice. PANI-HCl, PANI-Citric acid were prepared as control systems, by following the same procedure in 0.1 M HCl and 0.1 M citric acid solutions, respectively. The obtained materials were weighed and calculate the yield of the reactions- PANI-HCl: 85%; PANI-Citric acid: 90%; PANI-Lemon 78%. Fabrication of carbon paste electrode. 10 mg material was mixed with 100 mg fine quality graphite powder to form a homogeneous paste with 10 µl Nujol oil. Out of this, 2 mg paste was filled in a glass capillary (2 mm diameter) and a clean copper wire was used for the connection. Results and discussion The conceptual hypothesis “Multi-component Template Effects”. Fascinating nanostructures present in nature motivate scientists to develop novel nanomaterials by adopting the mimic approach. During the biosynthesis of the nanostructure within a natural system (i.e. living cell), the medium consists of many chemical and biochemical molecules. Synergism plays an important role in any biosynthesis, matrix molecules impacts on the morphology of natural nanomaterials. Many research groups have prepared 7 ACS Paragon Plus Environment


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specific shape and size nanoparticles by using bio-extracts as an environment-friendly option.42-45 In the present research work, dilute lemon juice was used for the synthesis of PANI. On comparative analysis of microscopic images, PANI-Lemon has directional synthesis and specific shape nanostructure, whereas PANI-Citric acid and PANI-HCl have globular and irregular morphology. Based on our results, we are proposing a hypothetical concept “Multi-component Template Effects”, important assumptions and aspects given as supplementary information (c.f. S1). The core ideas of this study is to illustrate the template like effects produced by natural molecules on a synthetic system. This concept (in the present format) concerns with the morphological manipulations of synthetic materials, excluding the identity and role of individual molecules and changes associated with them. Further improvements are open for discussion. Mechanism of polyaniline nanorods formation. PANI has been prepared by oxidative chemical polymerization of aniline in acidic solution, and the mechanism has well reported the literature.

2-4,42

PANI synthesis has three major stages; nucleation, initial growth and secondary growth. Phenazine, a constituent unit of oligomers, act as a template and it supports self-assembly during secondary growth of PANI. The acidity and nature of associated anions greatly influence the oxidation of aniline. The initial phase (induction period) of polymerization varies in different acids. Anilinium ions combined with monomers and followed chain propagation reaction. Doping is charge-transfer reactions between the organic polymers and ionic dopants, which effectively change the morphology, electrical, electrochemical properties, geometric parameters (bond length and bond angles). The conductivity of PANI depends on the degree of protonation and oxidation, spatial arrangement, and chain length. 2, 3, 46-47 In the strong acidic solutions, speedy and preferred growth in nucleation leads to the formation of numerous short chains. During the secondary growth phase, these short chains were elongated and entangled as agglomeration. The protonated chains agglomerate in compact coil-conformation due to twist defects between aromatic rings, in weakly acidic solution, low oxidizing agent and rapid movement of monomers could suppress secondary growth and nanofibrous structure produced.9


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It is very interesting to consider two contrasting effects on the conductivity of PANI. On one hand, strong acid produces highly protonated PANI chains (leads to higher conductivity) but the higher nucleation of PANI could result in irregular agglomerated globular morphology (leads to lower conductivity). Randomly aligned chains have impeded charge transfer due to intra-chain hopping and inter-chain tunneling. On the other hand, weak acid has low protonation level (leads to low conductivity), however, polymer chains align in a regular fashion which results in specific shape nanostructures. The morphological barrier for charge transport could be overcome by spatially arrangements of polymer chains in some regular fashion (leads to better conductivity). We here in proposed that protonation level and spatial arrangement of chains are key factors responsible for the electrical and electrochemical properties of PANI, however, under the similar protonating condition, the spatial arrangement is the dominating factor. The three PANI samples prepared in this work, significantly differing in doping level and type of dopants, i.e. PANI-HCl has chloride ions, PANI-Citric acid has citrate anion (citric acid is a tribasic acid) and PANI-Lemon has multiple dopants (crude lemon juice contains a mixture of organic acids, such as; citric acid, malic acid, ascorbic acid, trace amounts of tartaric acid, fumaric acid and others). In addition, lemon juice contains minor amounts of glucose and other sugars. The three solutions significantly differ in the acidity (pH of 0.1 M HCl, 0.1 M citric acid and crude lemon juice are 0.8, 3.0 and 2.3, respectively). The low acidity of dilute lemon juice, and slow kinetics facilitated the directional synthesis of the specific nanostructure of PANI. Post-synthesis dedoping/redoping process brings the desired level of doping without perturbing the spatial arrangements of material. Lemon juice assisted polymerization reaction showed a comparatively longer induction period. Biological molecules, present in the lemon juice, significantly manipulate the polymer-polymer interface. A higher fraction of linear assembles chains is attributed to the electrostatic repulsion between the same charge of chains, and strong interactions between amine and imine groups in adjacent chains via hydrogen bonds. It was observed that induction period was shortened with the increase in the concentration of lemon juice. 9 ACS Paragon Plus Environment


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The proposed green approach of PANI synthesis used less amount of oxidizing agent and natural acidic medium. As most of the components of lemon juice are water soluble so purity issues could be easily resolved by adopting appropriate washing procedures. During the washing process, most of the soluble contents, residual molecules, and side products were separated out from the prepared polymer. Morphological studies. SEM analysis. SEM images of PANI-HCl, PANI-Citric acid and PANI-Lemon at low different resolutions have been investigated. PANI-Lemon is interconnected polymeric mass having pores/ hollow space up to 0.5 to 10 µm (Figure 1 C, D). The 3D porous network of nanorods indicates that PANILemon

is

a

low-density

material,

and may

be a

suitable host

matrix

to

immobilize

nanoparticles/catalyst/bio-molecules for sensor/biosensor application, separation, and solid-state doping catalysis. Molecules of lemon juice put combined effects on the morphology of PANI. Thus, this spatial alignment of chains supports the concept of “Multi-component Template Effects”. Slow nucleation process, linearly aligned chains and template like effects of the matrix have facilitated the formation of polyaniline nanorods. On the other hand, irregular globular morphology of PANI-Citric acid and PANIHCl was observed, as reported by some earlier researchers (Figure 1 A B). 22,45-49 HR-SEM images of the PANI samples provide an insight into the morphology of the materials at the nano level. The three PANI samples exhibited very distinct morphology (c.f. Figure S2 A, B, C). PANI-Lemon has rod shape nanostructures with dimension; length ~ 600 nm in diameter and ~ 100 nm (magnification 400016 X). We observed long strip like structures at magnification 2000 X, which attributed to the combination of nanorods. We inferred that the systematic orientation of chains facilitated charge transport, rapid proton exchange, and better electroactivity. HR-SEM image of PANI-Lemon (L) and PANI-Lemon (H) reveals that the length of PANI nanorods varies with the concentration of lemon juice. At higher concentration, shorter nanorods were observed due to more concentrated nucleation (c.f. Figure S2D). PANI-HCl has an irregular lump like morphology, attributed to random agglomeration and coiling of chains (c.f. Figure S2A, the image at 10 ACS Paragon Plus Environment

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100000 X and 580164 X). Citric acid is a tribasic acid and crosslinked two or more polyaniline chains through ionic bonds. In addition, hydroxyl, carboxylic groups, and bonded water molecule contributed to form a network of hydrogen bonds. These interconnected chains lead to the formation of specific globular porous structure (c.f. Figure S 2 B, the image at 100000X and 575023 X).

Figure 1. SEM images (A) PANI-HCl (B) PANI-Citric acid (C and D) PANI-Lemon. TEM analysis. TEM images provide very fine structural information on the nanoscale. PANI-Lemon has interconnected nanorods, sub-nano porosity, regular surface topology; with an average diameter of a single nanorod 100 - 125 nm and length ~ 300 - 500 nm (Figure 2 D, E, and F). On the other hand, PANIHCl and PANI-citric acid have irregular shape and size architecture (Figure 2 A, B and C). Tiwari et. al. reported the morphological modification by using different acid and additives on PANI-HCl and PANIH2SO4 by TEM images.9 PANI chains could be oriented by blending with other polymers, applying high pressure, and applying an electric field. PANI nanofibers have been prepared by electrochemical 11 ACS Paragon Plus Environment


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polymerization.50 Additional TEM images (at different magnification) and electron diffraction patterns for three PANI samples provide an insight into the morphology and further support the concept of “Multicomponent Template Effects” (c.f. Figure S3; A, B, C). PANI-Lemon has a 3D porous network of porous nanorods (c.f. Figure S3 C). Zoomed image of the surface, indicated that the nanorod has numerous small cavities ~ 2 – 4 nm (c.f. Figure S3 D). PANI-H2SO4 exhibited irregular compact morphology ( c.f. Figure S3D). The bright electron diffraction rings confirm that PANI-Lemon has an alignment of chains in an orderly fashion due to various intermolecular forces, multi-component template effects, a network of hydrogen bonds, and pi-pi stacking of aromatic rings, X-ray powder diffraction analysis also supported the result (c.f. Figure S4). It is to be noted, very few research papers have been reported in the literature regarding the formation of poly-crystalline PANI (show sharp concentric rings in electron diffraction).54 To investigate the surface properties, AFM studies have been carried out for PANI-HCL and PANILemon at the nanoscale. AFM image strongly supports the fact that the polymerization in the presence of dilute lemon juice effectively manipulated the morphology, so rod shape nanostructure observed, whereas PANI-HCl has aggregated irregular morphology (c.f. Figure S5).


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Figure 2. TEM images at different magnifications; PANI-HCl (A, B), PANI-Citric acid, and (C) and PANI-Lemon (D, E and F). Inset - Electron diffraction pattern.



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Surface area and porosity analysis. The measurement of surface area, pore size, and pore volume are very crucial parameters to optimize the material for many technological applications. The surface area was measured by the theory of Brunauer–Emmet–Teller (BET). Pore size distribution, total pore volume measured by Brunauer–Joyner–Hallenda (BJH) method. Surface parameters are significantly dependent on the mode of synthesis, the additive used and sample processing for the analysis. Standard procedures have been followed to measure surface parameters for different samples as reported earlier. 46, 51-53 The adsorption/desorption isotherms, surface area, pore analysis, data point are given as supplementary information (c.f. Figure S6 A, B, C). A sharp increase in the adsorbed volume of N 2 and a hysteresis loop at a high relative pressure (≈ P/Pº = 0.95) are the typical characteristics of mesoporous materials. The specific surface areas calculated by the BET equation for PANI-HCl, PANI-Citric acid, PANI-Lemon (L) and PANI-Lemon (H) were 11.6 m2/g, 10.9 m2/g, 3.7 m2/g and 13.5 m2/g, respectively. The surface area of PANI-Lemon (L) is significantly lower than PANI-HCl and PANI-citric acid. It can be attributed to the formation of the long nanorod and spatial arrangement of small particles. PANILemon (H) (13.5 m2/g) has a larger surface area than PANI-Lemon (L) (3.7 m2/g), attributed to the formation of short nanorods (confirmed from HR-SEM images). PANI-Lemon has lower pore volume and larger pore sizes than PANI-HCl and PANI-Citric acid due to the compact arrangement of PANI particles and strong inter-particle interactions. Lemon juice contains a mixture of organic acid and sugar molecules, and their hydroxyl and carboxylic groups participated in the formation of hydrogen bonds. On comparing the four samples, PANI-Lemon (L) has a smaller pore volume, whereas PANI-HCl has the largest pore volume. Pores with the diameter greater than 10 nm contributed the maximum to the pore volume. The result suggests that PANI-Lemon exhibited type-IV isotherms, which specified the formation of mesoporous structure (2 - 50 nm). With an increase in concentration of lemon juice, pore diameter and pore volume do not change to a greater extent;


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PANI-Lemon (L)

PANI-Lemon(H)

Pore volume

0.02 cm³/g

0.08 cm³/g

Pore Diameter

17.2 nm

17.6 nm

The presence of hollow space in the polymer matrix also confirmed from microscopic images. Thus, we infer that PANI-Lemon has mesoporous and rod-shaped structure. Spectroscopic analysis FTIR. FTIR analysis of polyaniline and its composite has been reported by our group. N–H stretching band (~ 3400 cm−1), OH stretching of physically adsorbed water molecules (3200 cm−1) CH stretching (2900–3100 cm–1), exhibit presence of benzenoid and quinoid ring vibration (1487 and 1573 cm–1), to C– N/C=N (1305 cm–1), N=Q=N produce sharp and strong bands (1130 cm–1) are CCC ring in-plane deformation due to 1240 cm–1.9,46,47 FTIR spectra provide structural information, i.e functional groups, aggregation and alignment of chains, and the level of doping. The spectra obtained have all the characteristic peaks for polyaniline, as reported in the literature. Important stretching frequencies have been summarized and compared, given as supplementary information (c.f. Table S7). The IR spectrum of the PANI-HCl shows seven principal absorptions respective functional groups; ~ 3500 cm−1, 1577 cm−1, 1499 cm−1, 1297 cm−1, 1231 cm−1, 1130 cm−1, 806 cm−1. Similar peaks observed by other samples, however, they exhibited a slight shift in the peaks positions and intensity change, attributed to the effects of dopants and the structural modifications. In the spectra of PANI-HCl and PANI-Citric acid, the peak at 805 cm-1 is attributed to the out-of-plane bending of C-H, and C-N ring stretching, whereas PANI-Lemon exhibited at 791 cm-1, The peak shift toward lower energy indicate that lemon juice certainly configured the chain arrangements. The strong peak at 1130 cm-1 attributed to N=Q=N structure for PANI-HCl and PANI-Citric acid, however, this peak shift to 1132 cm-1 having intensity for PANI-Lemon (Figure 3 A, C, and E), attributed to feasible delocalization of pi electrons and good conductivity. Peak at 1231 cm-1 in PANI-HCl and PANI-Citric acid spectra attributed to CCC ring in-plane deformation; CN stretching, this peak is shifted to higher energy 1238 cm-1 with higher intensity. Sharp and intense peak confirmed minimum distortion 15 ACS Paragon Plus Environment


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from its mean position. Benzenoid units of PANI chains bonded with each other through a network of weak hydrogen bonds, such structure supported linear arrangement of PANI chains. PANI-Lemon shows a broad peak in the aromatic region due to dynamic proton exchange. Symmetrically arranged polyaniline chains exhibited minimum steric hindrance to the rapid proton exchange process, the result also supported by NMR spectra (c.f. Figure S8). Stretching frequency 1297 cm-1, attributed to C-N/ C=N, does not show any change in all the three samples. Relative higher peak intensities for N-quinoid than N-benzenoid rings (at 1568 cm-1 and at 1478 cm-1) confirmed more quinoid units and suggested emeraldine form of PANI. PANI-Lemon exhibited a sharp peak at ~ 1650 cm-1 (CH ring in-plane bending), attributed to chains linearly arranged in a plane. No additional peaks or bands have appeared in the spectra of PANILemon, which confirmed that the most of the lemon juice contents could easily wash out from PANI matrix, however, effects on the morphology persisted and the final material has no significant impurities. Comparative FTIR spectra for PANI samples before and after catechol treatment, indicate the specific interactions with catechol (Figure 3 B, D, F). Significant shift observed for the peaks correspond to aromatic units (1570 cm-1 to 1588 cm-1, 1495 cm-1 to 1466 cm-1, 1238 cm-1 to 1242 cm-1). Peaks at frequencies 1588 cm-1 and 1141 cm-1 exhibited significant shift and decrease in intensity, which indicate quinoid rings in PANI-Lemon mainly participated in the interaction with catechol. Electronic spectra. UV-Visible spectroscopy is an important technique to identify forms of PANI, variation in electronic structures, conformational changes, and chain arrangement. The important electron absorption band was observed due to transition between HOMO to LUMO ; ~ 320 nm - attributed to ππ* transition in the benzenoid ring, ~ 418 nm attributed to the polaron and bipolarons, ~ 610-650 attributed to exciton formation and π-π* transition of quinoid rings, and ~ 800 due to protonation of imine site. The dispersion of polarons bands between adjacent tetrameric units depends on the geometric arrangement of the polymer chains.47,48 Electronic spectra reveal the structural changes and interaction of PANI with catechol; PANILemon showed red-shift with higher intensity for the band at 353 nm (assigned to the π-π* transition) indicates better charge delocalization and linear arrangements of chains (c.f. Figure S9). Linearly 16 ACS Paragon Plus Environment

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arranged PANI chains exhibit a better inter-chain charge transfer, whereas twist defects of aromatic rings isolated the polarons of individual tetrameric units. PANI-Lemon exhibited changes in absorption band at 353 nm (assigned for pi-pi transition) indicate that PANI-Lemon effectively interacts with catechol molecule (in accordance with FTIR analysis). Other than that, additional peaks appear at 277 nm and 271 nm. Peak intensity decrease at 352 nm and 353 nm and higher absorption in the tail region 400 nm to 800 nm.



Page 18 of 50

Figure 3. FTIR spectra for (A) PANI-HCl, (C) PANI-Citric acid and (E) PANI-Lemon; After Catechol treatment. (B) PANI-HCl, (D) PANI-Citric acid and (F) PANI-Lemon


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Electrochemical Studies. Electrochemical characterization of conducting polymers have immense importance for following applications; batteries, corrosion protection, electrochemical sensors, electrooptic and electrochromic devices. Cyclic voltammetry. CV analysis of PANI-HCl, PANI-Citric acid, and PANI-Lemon were carried out at the rate 20 mV/s, potential range -400 mV to 800 mV using three electrode system, Ag/AgCl as a reference electrode, PANI modified carbon paste as working and a platinum wire as counter electrode. CV of PANI and its composites depends on the nature of anionic dopant, the degree of protonation and oxidation state of polyaniline.46,47 In the acidic solution, cyclic voltammograms showed three important redox couples corresponding to following transitions; poly-leucoemeraldine/emeraldine salt, degradation products or over-oxidized products, and poly-emeraldine salt / poly-pernigraniline. Anodic and cathodic scans are associated with doping/dedoping process of protons and anions, respectively. Well shaped redox couples of PANI in 0.1 M HCl indicate its electroactivity and conductivity (Figure 4A). PANI-Lemon showed higher peak currents than PANI-HCl and PANI-Citric acid, attributed to better charge transfer. In ordinary conditions, irregularly arranged PANI has some isolated redox sites, which does not participate in the charge-transfer process. The significant rise in peak current values justifies the impact of the lemon juice extract on the electrochemical properties of PANI. Ionic bonds between NH+ and anionic dopants stabilized the positive charge of PANI chains. The strength of these bonds is pH dependent and can easily break at higher and neutral pH. Small size dopant molecules/ions (such as chloride ion) could easily leach out from the polymer matrix (leads to deprotonation).To overcome, a variety of bulky anionic dopants have been used.1,48,49,51 Figure 4 B shows the CV of the three PANI samples in phosphate buffer (pH 7.0), which indicate that PANI-Lemon is more electroactive than PANI-HCl and PANI-Citric acid. Symmetrically arranged PANI chains have suppressed deprotonation process, rapid proton exchange between primary and secondary amines, and a network of hydrogen bonds (broad singlet for aromatic protons ~ 7 ppm in the NMR spectra of PANILemon, confirms the symmetric arrangement of chain and rapid proton exchange reactions). Evaluation of proton doping/dedoping on the electrochemistry of the prepared materials, cyclic voltammograms were 19 ACS Paragon Plus Environment


Page 20 of 50

recorded in a series of working solutions, pH ranging from 1-7 (c.f. Figure S10; A, B and C). Anodic and cathodic peak current decreases as the pH of the solution increase due to deprotonation.

Figure 4. Cyclic voltammogram of carbon paste electrode modified with PANI-HCl, PANI-Citric acid and PANI-Lemon, (A) in 0.1 M HCl (B) in PBS-7.


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Figure 5. Cyclic voltammogram of modified carbon paste electrode; blank, PANI-HCl and PANILemon, PANI-Citric acid in PBS-7, (A) With 0.1 M potassium ferricyanide. (B) With 0.1 M catechol. For comparing electroactivity, cyclic voltammograms were measured in the PBS solution (pH-7) at the scan rate of 20 mVs-1, using Potassium ferricyanide as a redox marker (Figure 5A). No significant signal was observed for plain carbon paste electrode. PANI-Lemon showed better redox behavior in terms of higher current and sharp peaks. So, we inferred that modified electrodes are more sensitive to detect an electroactive species. Similarly, the cyclic voltammogram in presence of catechol solution indicate that PANI-Lemon gives a better response than PANI-HCl and PANI-Citric acid (Figure 5B). Although, PANI-HCl has a larger surface area than PANI-Lemon, it exhibited lower current response. This interesting observation



Page 22 of 50

could be explained by considering the fact that the irregular arrangement of PANI chains has larger spatial charge impedance (leads to low conductivity). We herein propose that under similar protonation condition, linear alignment of chains is a governing factor for higher peak current rather than the surface area. This fact also supported by the cyclic voltammograms of PANI-Lemon (L) and PANI-HCL (H) in the absence/presence of catechol (c.f. Figure S11). In spite of a significant difference in the surface area of two PANI samples, same cyclic voltammetric response were observed. The HR-SEM images have suggested that PANI-Lemon (L) has the longer nanorods than PANI-Lemon (H), but similar chain arrangement. Thus, in the same protonating condition, the alignment of PANI chains significantly decide the conductivity of materials. CV analysis suggested that PANI-Lemon is a better electrode material than PANI-HCl and PANI-Citric acid.

AC Electrochemical Impedance Spectroscopy (EIS). EIS is an effective method to investigate the electronic features of surface-modified electrodes. The Nyquist plot was used to analyze the interfacial properties of PANI-Lemon, PANI-HCl and PANI-Citric acid in PBS solution at pH-1 (Figure 6A).The Nyquist diameter (real axis value at lower frequency intercept) reveals that PANI-Lemon modified electrode has lower charge transfer resistance (RCT) due to better electron-transfer, efficient anionic doping, porosity and linear alignment of chains. Alignment of chains and protonation are major factors which decide the amount of charge transfer and conductivity of the system. 51 Although PANI-HCl has the smallest particle size and a larger surface area and greater protonation, it showed greater impedance due to a large number of inter-chain charge transfer. PANI-Citric acid has intermediate impedance due to more regular morphology (confirmed from HR-SEM images). We inferred that PANI nanorods have guided spatial arrangement, extended charge delocalization, easy inter-chain charge transfer, minimum charge

drainage

and

short

diffusion path

lengths.

Primarily,

conductivity

depends

upon

the protonation level, but under similar protonation conditions, the spatial arrangement of chains could be a dominating factor.


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Protonation is a very important factor which impacts on the bulk conductivity of PANI. So, we measured the impedance of PANI-Lemon in the solutions of different pH (Figure 6B). Stable positive charge promotes the charge delocalization along the conjugated structures of polyaniiline. Interface resistance was increased significantly in the solutions above pH 2, and sudden increment in resistance was observed at pH 6 due to deprotonation.

Figure 6. Nyquist plots (A) PANI-HCl, PANI-Citric acid, PANI-Lemon modified carbon paste electrodes in dilute HCl (B) PANI-Lemon modified carbon paste electrodes in the PBS solution at different pH. Detection of catechol. Polyaniline exists in three common oxidation states - leucoemeraldine base (fully reduced), emeraldine base

(half-oxidized)

and pernigraniline base

(fully

oxidized).

Partially 23

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oxidized emeraldine salt behaves as a conductor, whereas fully reduced and oxidized forms act as an insulator (figure 7C). Conducting form of PANI has alternate dynamic benzenoid-quinoid structures. The relative ratio of benzenoid and quinoid units indicate from the corresponding intensity ratio in FTIR and NMR spectra. Three important factors which control the conductivity of PANI, i.e. (a) degree of protonation (b) degree of oxidation (c) spatial arrangement. The stability of positive charge on the chain depends on the degree of protonation, in the neutral solution dedoping leads the loss of conductivity and electroactivity. The delocalization of charge depends upon the length of the chain and proton exchange reactions. Conductivity and structural changes associated with the proton exchange reactions have been examined by spectral analysis.55,56 Catechol follows the reversible oxidation reaction involving two electrons and two protons exchanges. The protons released during oxidation process was accepted by the PANI, which led to an increase in conductivity and electroactivity. Linearly aligned polyaniline structure shows dominated proton exchange reaction and it accepted the proton released during catechol oxidation at the electrode surface. All the three PANI samples were treated with 0.1 mM catechol solution and kept undisturbed for 6-8 hours and then separated out. UV-Visible has shown intense absorption peaks between 400 - 600 nm for PANI-Lemon, which indicate that PANI-Lemon could effectively interact with catechol and it catalyzes the formation of colored aromatic pigment “benzoquinone” (confirmed from red color for catechol solution obtained after treatment) (figure 7 A, B).


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Figure 7. (A) UV-Visible spectra of catechol (B) Photograph of catechol solution after treating with PANI (a) PANI-HCl (b) PANI-Lemon (c) PANI-Citric acid) (C) Different forms of polyaniline

The electrocatalytic activity of the PANI-Lemon modified electrode was investigated by cyclic voltammetry in the presence of different catechol solutions (5 µM - 100 mM, PBS-7 as supporting electrolyte, scan rate 20 mVs-1) (Figure 8A). Under mild conditions, catechol produce benzoquinone by quasi-reversible two-electron, two proton redox reaction. PANI-Lemon facilitate electrocatalytic oxidation of catechol at the electrode surface; C6H6O2 [Catechol] + ½ O2  C6H4O2 [Ortho-quinone] + 2e- + 2H+ PANI (Oxidized) + H+  PANI (Reduced) PANI (Reduced)  PANI (Oxidized) + e- (to electrode) 25 ACS Paragon Plus Environment


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The anodic current increased continuously with each subsequent addition of catechol, attributed to catechol oxidation and higher protonation level of PANI (improve electroactivity and conductivity). The calibration plot indicates (Figure 8B) a linear range from 5 µM to 100 mM (R2 = 0.9773), limit of detection 2.1 µM (S/N = 3) and sensitivity of 49.68 μA mM-1 cm-2. The limit of detection (LOD) was determined using the 3 SD /slope ratio, where SD is the standard deviation of the anodic current values of 10 responses in cyclic voltammograms of the blank. Thus, the present sensor has extended linear range, high sensitivity, and low detection limit. Catechol sensors have been fabricated by using many other materials such as, goldnano particle/cobalt hexacyanoferrate/SBA-15, poly(aniline-co-o-aminophenol), poly(N-vinyl pyrrolidone), PANI/polyacrylonitrile,

silicon/tyrosinase,

Fe3O4/PANI/chitosan,

Fe3O4–SiO2,

MWCNTs/MnO2,

agarose–guar gum, graphene oxide/multi-walled carbon nanotubes/terthiophene, poly(malachite green)/MWCNTs, molecularly imprinted polymers, PANI/Ionic liquid, and DNA/graphene oxide.26-41 The performance of the proposed sensor was compared with some other reported polyaniline based catechol sensors, summarized in table-1.

Table 1. Comparasion of performance of the proposed sensor with other polyaniline based catechol sensors

Electrode Material

Detection Limit and Sensitivity

Linear Range and Response Time

Reference

poly(aniline-co-p-aminophenol) poly(aniline-co-o-aminophenol) -

0.8µM -

Chena et. al. (28)

Polyaniline/ laccase

2.0 µM

Fe3O4/ Polyaniline /Laccase/Chitosan Biocomposite Polyaniline/ polyphenol oxidase

0.4 μM & 126.76 µAmM-1 -

5 to 500 µM 5-80 µM & 10 Sec 3.2 to 19.6 µM 0.5 to 80 µM, & 8 Sec 0.05 –165.5 µM -

Mu et. al. (32) Nazari et. al. (33) Sadeghi et. al. (34) Sethuraman et.al. (36) 26


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Polyaniline/ polyphenol oxidase

1.25–150µM -

0.3 to 51 µM -

Tan et.al. (37)

Polyaniline/Polyacrylonitrile/ Polyphenol oxidase Polyaniline nano rods

2.03 AM−1 cm−2 2.1 µM & 49.68 µAµM-1cm-2

0.05 –7.5 µM 5 µM to 100 mM 2 second

Xue et. al. (39) (Current work)

Figure 8. (A) Cyclic Voltammograms of PANI-Lemon modified carbon paste electrode in the presence of different concentration of catechol in PBS-7, (B) Calibration plot.



Page 28 of 50

Differential pulse voltammetry and chronoamperometry. DPV and chronoamperometry of PANILemon were carried out in PBS-7.0 using different concentrations of catechol (c.f. Figure S12). It is interesting to note that the blank experiment with PANI-Lemon showed two peaks at potential ~30 mV and ~320 mV, but after the addition of catechol, a third peak was observed at ~600 mV. With further addition of catechol, there was an increment of current at all three peak potential, however, the third peak shows a relatively higher current response. It is inferred that the newly generated redox center on PANILemon is more sensitive for catechol detection. Chronoamperometric experiments were performed at single potential step, + 0.5 V vs. Ag/AgCl, the resultant current was measured with respect to time. PANI-Lemon exhibited catalytic oxidation of catechol and anodic current increased with subsequent addition. The faradaic chronoamperometric responses were in good agreement with the CV measurements. Response of other analytes. The cyclic voltammetric responses of different analytes using PANI-Lemon modified electrode in PBS-7 (c.f. Figure S13). The response of other interfering substances was quantitatively plotted as the percentage increase in the anodic current with respect to blank peak current (figure 9). It was found that catechol exhibited higher anodic peak current response. The material showed good response for ascorbic acid and phenol too.


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Figure-9. Increment of anodic peak current in the CV response of different analytes on PANILemon modified carbon paste electrode.

Conclusions

On the basis of the above results and discussions, we have drawn following important conclusions. Natural extract assisted synthesis could manipulate the nanostructures having novel properties. The important aspects and presumptions of “Multi-component Template Effects” have been discussed. This concept potentially implemented to synthesize other polymers and nanoparticles. It is possible that this template like effect may be produce due to the combined



Page 30 of 50

impact of several molecules. The synergistic effect of different molecules is a matter of further research. Lemon juice assisted polymerization is a cost-effective and environment-friendly method the synthesis of PANI. Some molecules of lemon juice produce a template like effects on the morphology of PANI. The polymerization reaction in lemon juice followed sluggish kinetics, and have longer induction period due to lower pKa value of lemon juice. This condition provides an good opportunity for self-assembling of PANI chains. SEM, HR-SEM, TEM and AFM images confirm the formation of rod shape nanostructures (dimension: diameter ~ 100 nm, length ~ 300 - 600 nm). FTIR and UV-Visible spectra provided valuable structural information. The distinct molecular structure of PANI-Lemon is confirmed from the intensity ratio of peaks for benzenoids and quinoids units and blue shift for N=Q=N stretching vibrations. PANI-Lemon is found to be superior to PANI-HCl and PANI-Citric acid, in terms of more quinoid units, linear alignment of the chains, extended dispersion of polaron bands, higher conductivity, and better electroactivity. XRD and electron diffraction patterns of PANI-Lemon support polycrystalline structure, this confirms that there is a regular arrangement of chains in the rod-shaped nanostructures. We have proposed that in the similar protonation conditions, (a) spatial arrangement of chains is the dominating factor which decides conductivity and electroactivity of PANI (due to better inter-chain charge transfer), (b) The alignment of chains is more significant structural parameter than size and surface area. PANI-Lemon modified carbon paste capillary electrode was successfully used for catechol sensing. On interacting with catechol, generate a new redox-active site, which is more sensitive to catechol. The developed catechol sensor exhibited high sensitivity (49.68 µAµM-


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1

cm-2), wide linear concentration range (5 µM - 100 mM), low detection limit (2.1 µM) and

lower response time (2 s). Thus, PANI-Lemon is a better electrode material than PANI-HCl and PANI-Citric acid due to large surface area, porous architecture, low interfacial resistance and good electro-catalytic property.

AUTHORS INFORMATION Corresponding Author *

Dr.

Vijay

Laxmi

Yadav,

Associate

Professor,

IIT-BHU,

Varanasi,

E-mail:

[email protected] ** Dr. Karan Pratap Singh, President, MITWS, Bareilly, E-mail: [email protected], Phone no.: +919457566259 $ Miss. Vineeta Gautam, Research Scholar, IIT-BHU, Varanasi E-mail: [email protected], Phone no +918005449669 Acknowledgements The authors are thankful to Head, Department of Chemical Engineering and Technology, IIT-BHU for providing necessary laboratory resources including BET surface analysis; Prof. O. N. Srivastava and Mr. Dinesh Jaiswal, Department of Physics, BHU for providing SEM and TEM facilities; Mr. Amit Kumar, Department of Chemistry, BHU for providing FTIR UV-Visible spectroscopy facilities; Mr. Brij Bihari, Incharge University Science Instrumentation, Centre-Level II for fabrication of electrochemical cell; Head, Central Instrumental Facilities for AFM, NMR, HR-SEM, XRD analysis, Ms. Vineeta Gautam is very much thankful to MHRD for providing Senior Research Fellowship and contingency grant to carry out this research work. The authors also would like to give its gratitude to the members of MITWS, and Prof. U.S. Rai, Department of Chemistry, BHU for giving their valuable suggestions and continuous intellectual support.



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Associated content Details of supplementary information S1: Important presumptions of the concept “Multi-component Template Effects” S2: HR-SEM images S3: TEM images S4: XRD S5: AFM images S6: BET surface area and BJH pore size distribution S7: Comparative table of FT-IR frequency S8: NMR analysis S9: UV-Visible spectra S10: Effect of pH on cyclic voltammograms S11: Cyclic voltammograms of PANI (L) and PANI (H) S12: Differential Pulse Voltammetry and Chronoampereometry S13: Cyclic Voltammetric response of different analytes References

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For Table of Contents Use Only

Synopsis Nanorod shaped polyaniline nanostructure were prepared in lemon juice extract and apply for the selective electrochemical detection of catechol.



Figure 1. SEM images (A) PANI-HCl (B) PANI-Citric acid (C and D) PANI-Lemon. 161x150mm (300 x 300 DPI)


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Figure 2. TEM images at different magnifications; PANI-HCl (A, B), PANI-Citric acid, and (C) and PANILemon (D, E and F). Inset - Electron diffraction pattern. 219x271mm (300 x 300 DPI)



Figure 3. FTIR spectra for (A) PANI-HCl, (C) PANI-Citric acid and (E) PANI-Lemon; After Catechol treatment. (B) PANI-HCl, (D) PANI-Citric acid and (F) PANI-Lemon 214x258mm (300 x 300 DPI)


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Figure 4. Cyclic voltammogram of carbon paste electrode modified with PANI-HCl, PANI-Citric acid and PANILemon, (A) in 0.1 M HCl (B) in PBS-7. 72x29mm (300 x 300 DPI)



Figure 5. Cyclic voltammogram of modified carbon paste electrode; blank, PANI-HCl and PANI-Lemon, PANICitric acid in PBS-7, (A) With 0.1 M potassium ferricyanide. (B) With 0.1 M catechol. 126x190mm (300 x 300 DPI)


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Figure 6. Nyquist plots (A) PANI-HCl, PANI-Citric acid, PANI-Lemon modified Graphite paste electrodes in dilute HCl (B) PANI-Lemon modified Graphite paste electrodes in the PBS solution at different pH. 123x181mm (300 x 300 DPI)



Figure 7. (A) UV-Visible spectra of catechol (B) Photograph of catechol solution after treating with PANI (a) PANI-HCl (b) PANI-Lemon (c) PANI-Citric acid) (C) Different forms of polyaniline 198x216mm (300 x 300 DPI)


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Figure 8. (A) Cyclic Voltammograms of PANI-Lemon modified carbon paste electrode in the presence of different concentration of catechol in PBS-7, (B) Calibration plot. 131x207mm (300 x 300 DPI)



Figure-9. Increment of anodic peak current in the CV response of different analytes on PANI-Lemon modified carbon paste electrode. 64x49mm (300 x 300 DPI)


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