In Situ Structural Elucidation and Selective Pb2+ Ion Recognition of

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Article Cite This: Langmuir 2018, 34, 7048−7058

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In Situ Structural Elucidation and Selective Pb2+ Ion Recognition of Polydopamine Film Formed by Controlled Electrochemical Oxidation of Dopamine K. S. Shalini Devi,† Sharu Jacob,† and Annamalai Senthil Kumar*,†,‡,§

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†

Nano and Bioelectrochemistry Research Laboratory, Department of Chemistry, School of Advanced Sciences, Vellore Institute of Technology, Vellore-632 014, India ‡ Carbon dioxide Research and Green Technology Centre, Vellore Institute of Technology, Vellore-632 014, India § Institute of Biochemical and Biomedical Engineering, National Taipei University of Technology, Taipei 10608, Taiwan (R.O.C.) S Supporting Information *

ABSTRACT: Owing to the versatility and biocompatibility, a self-polymerized DA (in the presence of air at pH 8.5 tris buffer solution) as a polydopamine (pDA) film has been used for a variety of applications. Indeed, instability under electrified condition (serious surface-fouling) and structural ambiguity of the pDA have been found to be unresolved problems. Previously, pDA films (has hygroscopic and insoluble property) prepared by various controlled chemical oxidation methods have been examined for the structural analysis using ex situ solid-state NMR and mass spectroscopic techniques. In this work, a new in situ approach has been introduced using an electrochemical quartz crystal microbalance (EQCM) technique for the improved structural elucidation of pDA that has been formed by a controlled electrochemical oxidation of DA on a carboxylic acid functionalized multiwalled carbon nanotube-Nafion (cationic perfluoro polymer) modified electrode (f-MWCNT-Nf) system in pH 7 phosphate buffer solution. Key intermediates like 5,6-dihydroxy indole (DHI; 150.7 g mol−1), dopamine (154.1 g mol−1), Na+, PO42−, and polymeric product of high molecular weight, 2475 g mol−1, have been trapped on f-MWCNT-Nf surface via π−π (sp2 carbon of MWCNT and aromatic es), covalent (amide-II bonding, minimal), hydrogen, and ionic bonding and identified its molecular weights successfully. The new pDA film system showed well-defined peaks at E°′ = 0.25 V and −0.350 vs Ag/AgCl corresponding to the surface-confined dopamine/dopamine quinone and DHI/5,6-indolequinone redox transitions without any surface-fouling complication. As an electroanalytical application of pDA, selective recognition of Pb2+ ion via {(pDA)-hydroquinone-Pb0} complexation with detection limit (signal-to-noise ratio = 3) 840 part-per-trillion has been demonstrated.

1. INTRODUCTION Development of stable and functional polymers that are based on naturally occurring precursors is an important research area in the interdisciplinary fields of material chemistry and biological science.1 Dopamine (DA) is a redox-active neurotransmitter that controls the key brain functions of humans and animals.2,3 Owing to the self-oxidation property in aerated alkaline solution (pH 8.5 tris buffers is an optimal),4 DA polymerizes easily as polydopamine (pDA), an inert, biocompatible, and nonpoisonous polymer as melanins/ eumelamins, that can adhere on a variety of organic,4,5 and inorganic substrates including metals,6,7 metal oxides,8 semiconductors,9 ceramics,10 and polymers.11−13 pDA has been widely used for applications like cellular interfacing,14 energy applications,10,12,13,15 cancer therapy,16 nanomedicine,17 biosensing,18 and drug delivery.16,19 Indeed, major obstacles in expanding this material are the serious surface-fouling property under electrified condition 20,21 and structural ambiguity.11,12,22,23 In early 2012, it was thought that dopamine © 2018 American Chemical Society

polymerized in a manner similar to that of hydroquinone and catechol.4,11,12,24,25 Indeed, based on several ex situ characterizations using instruments like time-of-flight secondary ion mass spectrometry (TOF-SIMS),26 high-performance liquid chromatography,27 positive ion mode electrospray (ES+) ionization coupled high-resolution mass spectrometry (HRMS), 23 matrix-assisted laser desorption/ionization (MALDI)-MS,22 solid state nuclear magnetic resonance,22 and X-ray photoelectron spectroscopy, 26 the following mechanisms and structural information about pDA have been postulated: oxidation of dopamine to 5,6-dihydroxy (DHI) and 5,6-indolequinone (IDQ) intermediates followed by (i) selfassembling with dopamine via π−π and hydrogen bondings;22 (ii) covalent linking between the benzene rings of several DHI/ IDQ and dopamine units with different degrees of saturation;23 Received: April 12, 2018 Revised: May 21, 2018 Published: May 24, 2018 7048

DOI: 10.1021/acs.langmuir.8b01209 Langmuir 2018, 34, 7048−7058

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Scheme 1. Illustration of the Adsorption of Dopamine (DAads) on GCE/f-MWCNT-Nf Surface (A), Its Controlled Electrochemical Oxidation to Dopamine Quinone Formation (B), aza-Michael Addition Reaction to Form Leucodopaminechrome (LDAC) (C) and Its Electrochemical Oxidation to Respective Dopaminechrome (DAC) formation (D), Isomerization to 5,6-Dihydroxyindole (DHI) Formation (E), and Its Electrochemical Oxidation to 5,6-Indolequinone (IDQ) Formation Followed by Polymerization Reaction (F−H)a

Steps D(I) and D(II) are side products of the DA oxidation reaction. H(I−III) are structures of e-pDA and its fractions. * = Molecular weights identified by in-situ EQCM technique. a

hygroscopic and insolubility nature of the pDA (it is highly insoluble in water, acid, moderate alkaline conditions, and all common organic solvents);11,12,22 (iii) disruption of the hydrogen bonding and in turn structure by organic solvent used to isolate the partial structure of the pDA; (iv) ambiguity in the structure of the off-line isolated products/aggregates with the original solid state pDA.28 Meanwhile, our group has been working with the in situ electrochemical quartz crystal microbalance (EQCM) technique, which contains a sensitive piezoelectric quartz crystal, for identifying intermediates of several complex electro-organic reactions such as phenol

(iii) covalent linking between the benzene ring of DHI and indole ring of the IDQ.27−29 It is still unclear what kind of pDA film is formed, what the surface functional groups are, and what the building blocks of the biopolymer are. In general, the problems faced while characterizing the pDA by conventional techniques are as follows: (i) sample collection methodrather than analysis of the pDA film directly, pDA aggregates collected from the dopamine solution in the presence of dissolved oxygen22,23 or chemical-oxidant such as ammonium persulfate,29 Ce(IV) ion,30 sodium hypochlorite,22 and periodate27 and solid surface (TiO2 and glass)23,28 have been used; (ii) 7049

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Figure 1. Continuous CV responses of (A) GCE with 1 mM DA (a), GCE/f-MWCNT@DAads (b), and GCE/Nf@DAads (c), and (B) GCE/fMWCNT-Nf@DAads (after the experiment it became GCE/f-MWCNT-Nf@e-pDA) (a) and GCE/f-MWCNT-Nf (b). (C,D) Responses of freshly prepared GCE/f-MWCNT-Nf@DAads (after tenth first cycles) at different potential windows. Electrolyte = pH 7 PBS; Scan rate = 50 mV s−1.

oxidation to hydroquinone,31 azo-bond cleavage via aniline intermediate,32 and quinoline-quinone product formation from quinoline33 by performing controlled electrochemical oxidation/reduction reactions of the precursors on a graphitic carbon electrode, like MWCNT chemically modified electrodes. Due to the strong π−π interaction, MWCNT helps trap the intermediates that have been formed on the interface of the electrochemical reaction. Previously, there was an report relating to pDA growth studies on gold electrode using EQCM technique.34 In this work, the in situ cyclic voltammetry (CV)-EQCM technique was used to precisely identify the mechanism and structural detail of the pDA film by studying with a carboxylic acid functionalized multiwalled carbon nanotube (f-MWCNT)/anionic polymer Nafion (Nf)-chemically modified electrode (f-MWCNT-Nf) in pH 7 phosphate buffer solution. In spite of the variety of utilities,10−19,35 independent electroanalytical application of pDA film has never been reported in the literature. The reason behind is the serious fouling/stripping behavior of the pDA’s constituents like DHQ, IDQ, and dopamine upon the electrochemical studies.20,21 One way to make a stable pDA film is to trap the intermediates in between several chemical and physical interactions like covalent, π−π, ionic, and hydrogen bonding. Meanwhile, recently, Gao et al. first pointed the ion-permeability behavior of the pDA by studying with Prussian blue-modified electrode was prepared by successive deposition of PB (electrochemical method in pH 3) and pDA (by ammonium persulfate assisted

DA oxidation in pH 7 phosphate buffer solution) on a gold electrode surface.29 It was concluded that pDA has a zwitterionic property and contains interconnected intermolecular voids for alkaline ion (K+) mobility, irrespective of the film thickness. By considering the ionic property, aromatic πelectrons, and hydrogen bonding ability of the pDA, we introduce a new underlying matrix, f-MWCNT-Nf, for studying a controlled electrochemical dopamine oxidation, trapping and identification of the intermediates and polymeric product (designated as f-MWCNT-Nf@e-pDA, e-pDA = electrochemically deposited pDA). The idea behind this is that the carboxylic functionalization can provide chemical interactions with the free amino-functional (amide II bonding) and hydrogen groups (hydrogen bonding); MWCNT core can help for the π−π interaction; anionic polymer of Nafion can support the stabilization of the cationic species and migration of alkali metal ion (Na+ in this work) for the surface-fouling free stabilization of the pDA and its intermediates. Finally, to demonstrate electroanalytical application, selective recognition of Pb2+ ion using the f-MWCNT-Nf@e-pDA has been demonstrated as a model system. Note that Pb2+ is a toxic heavy metal that has been found in significant concentration in products used in daily life, like toys, paints, and cosmetic formulations.36 Selective and sensitive detection of Pb2+ is a demanding research interest in interdisciplinary areas of chemistry.36−39 7050

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(DPV) parameters used in this work: potential = −1.2 V, Estep = 0.004 V, pulse width = 0.06 s, and sampling width = 0.01 s.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Reagents. Dopamine hydrochloride (98% purity), graphene oxide (GO; 5 mg mL−1 in water), activated charcoal (AC), and graphite nanopowder (GNP; ∼98% purity, 400 nm, metal oxide content = 3.7) were purchased from SRL Chemicals, India. 5% Nafion dissolved in lower aliphatic alcohol, graphitized mesoporous carbon (GMC, purity >99.95%, 90% carbon basis, outer diameter: 10−15 nm; inner diameter: 2−6 nm; length 0.1−10 μm, metal oxide content = 5.2), single-walled carbon nanotube (SWCNT, 50−70 wt % of carbon basis, outer diameter 1−1.5 nm, metal impurity content = 24.4%), carbon nanofiber (CNF; graphitized, iron free, >99.9 wt % carbon basis, 100 nm × 20−200 μm), and carboxylic acid functionalized multiwalled carbon nanotube (f-MWCNT; ∼80% purity on carbon basis, >8% carboxylic acid functionalized, size 9.5 nm × 1.5 μm) were purchased from Sigma-Aldrich, United States. All other chemicals were used without any purification and were of analytical grade quality. Double distilled water (DD water) was prepared using alkaline potassium permanganate (KMnO4). pH 7 phosphate buffer solution (PBS) was used as the supporting electrolyte with ionic strength I = 0.1 mol L−1. A pH 4.6 sodium acetate−acetic acid buffer solution was used as a secondary electrolyte for Pb2+ related studies. 2.2. Instrumentation. Voltammetric measurements were accomplished using a CHI Model 440B workstation (a bipotentiostat, USA) with a sample volume of 10 mL. The three-electrode system consisted of a glassy carbon electrode (GCE) of surface area 0.0707 cm2 as a working electrode, Ag/AgCl as a reference electrode, and platinum wire as the auxiliary electrode. In situ CV-EQCM experiments were carried out using a gold single crystal electrode (EQCM-Au) of geometric surface area, 0.196 cm2. For FTIR analysis, a JASCO 4100 spectrophotometer (Japan) instrument was used with the KBr method. Raman spectroscopy analysis was carried out using AZILTRON, PRO 532 (USA) with a λ = 532 nm laser excitation source. Transmission electron microscopy (TEM) was done using a Technai, G2 20 Twin FEI instrument. 2.3. Preparation Procedures. First, the surface of the GCE was cleaned both mechanically (polished with 1 μm alumina powder, cleaned with acetone and followed by DD water) and electrochemically (by performing CV for ten continuous cycles in the potential window −0.2 to 1 V vs Ag/AgCl in pH 7 PBS at scan rate (v) = 50 mV s−1). GCE/f-MWCNT-Nf was prepared by drop casting a 5 μL suspension of 3 mg f-MWCNT powder dispersed 0.5% Nafion solution (50 μL from 5% Nf solution in 500 μL ethanol) on a cleaned GCE electrode followed by drying in air for 5 ± 1 min at room temperature (T = 25 ± 2 °C). Thickness of the f-MWCNT-Nf film can be controlled by varying the casting volume. DA modification was done by coating 5 μL of 3 mg of DA dissolved 500 μL ethanol on the GCE/f-MWCNT-Nf surface (GCE/f-MWCNT-Nf@DAads; ads = adsorbed). In a similar way, other DAads-carbon based chemically modified electrodes were prepared. For the preparation of e-pDA modified electrodes, respective DAads systems were potential cycled by CV technique in a window of −0.8 V to +0.8 V for 20 continuous cycles in pH 7 PBS at a scan rate of 50 mV s−1 (Scheme 1A−F). For the in situ EQCM analysis, 1 mM DA dissolved pH 7 PBS was used as a test system with EQCM-Au/f-MWCNT-Nf as a working electrode. Note that both DA-adsorption and -solution based techniques have shown qualitatively similar e-pDA product formation. The following Sauerbrey equation is referred for the conversion of frequency to mass change:31−33 mass change (Δm) = (−1/2)fo−2 Δf A k ρ1/2, where A is the area of the EQCM-Au (0.196 cm2), ρ = density of the crystal (2.648 g cm−3), k = shear modulus of the crystal (2.947 × 1011 g cm S2−), Δf = measured frequency change, and f 0 = oscillation frequency of the crystal (8 MHz). A Δf change of 1 Hz is equivalent to a Δm value of 1.4 × 10−9 g in mass. For the electrochemical sensing of Pb2+, an anodic stripping voltammetric (ASV) technique in pH 4.6 acetate buffer solution was adopted. For the Pb2+ preconcentration step, the modified electrode is conditioned at −1.2 V vs Ag/AgCl for desired time (90 s). Following are the optimal differential pulse voltammetry

3. RESULTS AND DISCUSSION 3.1. Electrochemical Oxidation of DA on Various Electrodes. Initial experiment was carried out with an unmodified GCE in 1 mM DA dissolved pH 7 PBS as in Figure 1A, curve a. The appearance of a marked irreversible peak at anodic potential Epa = 0.5 ± 0.01 V vs Ag/AgCl in the first CV cycle followed by continuous decrement of the peak current response in the subsequent cycles was noticed. This observation is true when the DA is adsorbed on GCE (a dilution solution of DA drop-cast on GCE, GCE/DAads, ads = adsorbed) and CV is performed in a pH 7 PBS blank (data not enclosed). Based on the literature reports20,22,25,26,34 and from the obtained results, it is proposed that the electrochemical oxidation of DA proceeds through DA-quinone-like intermediate formation followed by several chemical reactions to form a polydopamine (pDA) product. Hereafter the electrooxidized DA is represented as e-pDA. Owing to the insulating nature,20 pDA has showed continuous decrement in the current response (Figure 1A, curve a). This is the reason why there is no electro-analytical application of pDA reported in the literature. In order to prepare a stable e-pDA film, first, a DA adsorbed f-MWCNT modified GCE (GCE/f-MWCNT@ DAads; without Nf) was used and was subjected to an electrochemical oxidation reaction as in Figure 1A, curve b. This time, well-defined quasi-reversible peaks at apparent electrode potentials E°′ = 0.25 ± 0.005 (A1/C1) and −0.35 ± 0.005 (A2/C2) and with an unstable voltammetric peak current response were noticed (GCE/f-MWCNT@e-pDA). Indeed, when compared to GCE (Figure 1A, curve a), the GCE/fMWCNT underlying system yielded better performance for the e-pDA formation. Meanwhile, as a control, GCE/Nf@DAads system was examined for the respective e-pDA film formation, but failed to show any redox peak (Figure 1A, curve c). Interestingly, when the DA adsorbed f-MWCNT-Nf mixture modified GCE was used, i.e., GCE/f-MWCNT-Nf@DAads, a stable CV response with marked redox peak at A1/C1, A2/C2, and A2′/C2′ (E°′ = −0.15 V vs Ag/AgCl), without any marked alteration in the peak currents (relative standard deviation (RSD) between the first and 20th cycle is 3.7%), were noticed (Figure 1B, curve a). This observation attributes an unusual and fouling-free polymeric product formation on the modified electrode, GCE/f-MWCNT-Nf@e-pDA. The existence of multiple interactions such as π−π (between sp2 carbon and e-pDA’s benzene ring), hydrogen bonding (oxygen and hydrogen functional groups), ionic (cationic sulfonic acid of nafion and anionic amino functional groups of DA, pKa 8.5), and covalent (between the carboxylic functional group of fMWCNT and e-pDA; minimal) is the likely reason for the usual stability of the e-pDA film. These redox peaks have mixed adsorption and diffusion controlled electron-transfer features (slope (∂logipa/∂logv) of double logarithmic plot of ipa vs v is ∼0.6) and strong pH dependent property (Supporting Information Figure S1A,B). A plot of anodic Epa vs pH yielded a slope,−45 ± 3 mV pH−1, suggesting a non-Nernstian type of proton-coupled electron-transfer reaction with the involvement of an unequal number of protons and electrons (H+/2e−) (Supporting Information Figure S1C,D). Based on the literature reports on chemical22,27,30,31 and electrochemical oxidation reactions of DA,20,22,25,26,34 a possible mechanism for the DA-oxidation and e-pDA polymer formation reactions 7051

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Figure 2. Comparative (A) TEM images of f-MWCNT-Nf and f-MWCNT-Nf@e-pDA, (B) Raman spectroscopic responses of f-MWCNT-Nf (a) and f-MWCNT-Nf@e-pDA (b) with ID/IG ratio values, and (C) FTIR/KBr responses of dopamine (a), f-MWCNT-Nf (b), f-MWCNT-Nf@e-pDA and f-MWCNT-Nf@e-pDA/Pb0 (d) systems.

occurring on the GCE/f-MWCNT-Nf surface is sketched in the Scheme 1A−H. As per the mechanism, at ∼0.3 V vs Ag/AgCl, DA gets reversibly electro-oxidized to dopamine-quinone (A1/ C1) followed by intermolecular cyclization of the amine functional group to a reduced form of the indole group via 1,4Michael addition to leucodopaminechrome (LDAC; chemical reaction) that is subsequently electro-oxidized to the respective quinone derivative, dopaminechrome (DAC), reversibly (A2′/ C2′). The unstable DAC compound was further involved in a chemical isomerization reaction to form 5,6-dihydroxyindole (DHI) that has been involved in a proton-coupled redox reaction with the respective quinone form, 5,6-indole-quinone (IDQ) (A2/C2) (Scheme 1E and F). It is obvious that two proton-coupled electron-transfer reactions, A1/C1 and A2/C2, have shown marked current signals while the intermediate,

A2′/C2′ peak resulted a feeble response in this work (Figure 1B, curve a). In order to understand the mechanistic pathway of the DA oxidation, potential segment analyses were carried out with freshly prepared GCE/f-MWCNT-Nf@DAads by varying the starting and ending potentials as in Figure 1C and D. It is clear that potential started at −0.8 V and swept to 0.3 V (Figure 1C, curves c−f), where the A1/C1 redox peak appeared (DA oxidation occurring), showing a marked A2/C2 peak current signal. Potential cycling experiments by omitting the A1 peak resulted in no A2/C2 response (Figure 1C, curves a and b). Similarly, potential started at +0.8 V (where DA oxidation occurs maximum) and swept to halfway, −0.4 V vs Ag/AgCl (Figure 1D, curve a), showing about twice the decrement in the A1/C1 peak current response (Figure 1D). Based on these results, the following important conclusions can be derived: (i) 7052

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Figure 3. CV responses of various carbon size; activated charcoal (AC), graphite nanopowder (GNP), carbon nanofiber (CNF), graphene oxide (GO), single walled carbon nanotube (SWCNT), multiwalled carbon nanotube (MWCNT, with metal impurities), purified-MWCNT (pMWCNT), graphite mesoporous carbon (GMC), and f-MWCNT with Nafion modified systems for e-pDA film formation in pH 7 PBS at v = 50 mV s−1. Other conditions as in Figure 1B, curve a. Part j is a plot of A1 peak current responses (GCE/Carbon-Nf@e-pDA) versus respective carbon electrodes. Insets are cartoon/pictures of various carbons used in this work.

rings of pDA assisted the film formation on the surface. This result is supported by Raman spectroscopic characterization of f-MWCNT-Nf@e-pDA with appreciable decrement in D (disordered graphitic structure, sp3 carbon) and G (ordered graphitic structure, sp2 carbon) band intensity ratio, I/IG (0.35) when compared with the f-MWCNT-Nf system (Figure 2B). FT-IR response of the f-MWCNT-Nf@e-pDA showed specific bands corresponding to carbonyl (1737 cm−1), amine/ ammonium (1370 cm−1), amide II (1524 cm−1; minimal), and phosphate ion (2335 and 1218 cm−1) in addition to hydroxyl (∼3900 cm−1) (Figure 2C, curve c), combined −NH, −CH, and −OH functional groups (as combined peak at 2113 cm−1), unlike the intense bands in the fingerprint region, 750− 1600 cm−1 corresponding to the primary amine, C−N and −N−H stretching frequencies with the naked DA (Figure 2C, curve a). The IR observation of the optimal system resembles the IR of pDA agglomerates reported by Dreyer et al.22 Following are some of the conclusions derived from the above observations: Deprotonated carboxylic acid of the f-MWCNT interacted with the ammonium ion of DA/e-pDA and resulted in formation of the hybrid product. To further confirm the carboxylic group and graphitic participation, the effect of carbon which does not contain graphitic units (AC) and has

Oxidation to DA to DA-quinone at ∼0.3 V vs Ag/AgCl is a key reaction. (ii) Overall, e-pDA formation is coupled chemical− electrochemical steps with involvement of proton-coupled electron transfer reaction along with aza-Michael addition, isomerization, and polymerization steps as sketched in Scheme 1A−H. (iii) Free and surface-confined DAs are expected to be responsible for the redox peak appearing at A1/C1 peak potential. (iv) Redox response of DAC (A2′/C2′) is relatively weaker than that of the A1/C1 and A2/C2 (DHI/IDQ) processes. At this stage it is difficult to propose an exact structural detail of the e-pDA. Note that e-pDA produced on the electrode surface is at the microgram level and it is difficult to isolate in its natural form from the complex matrix. In order to solve the problem several ex situ and in situ characterization studies were performed in addition to control experiments as given in the following sections. 3.2. Physicochemical Characterization of f-MWCNTNf@e-pDA. TEM analysis of the f-MWCNT-Nf and MWCNT-Nf@e-pDA showed marked increment in the diameter of the f-MWCNT after the e-pDA deposition suggesting formation of a thin layer of polymeric film on the MWCNT outer surface (Figure 2A). It is expected that strong π−π interaction between the MWCNT-sp2 carbon and benzene 7053

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Figure 4. In situ CV-EQCM responses of EQCM-Au/f-MWCNT-Nf with 1 mM DA in pH 7 PBS at v = 50 mV s−1 (A,B). C and D are respective charge vs E and mass (Δm) vs charge plots. E−I are typical plots of Δm vs Q of first, second, third, fifth, and tenth cycles of the EQCM-Au/fMWCNT-Nf@e-pDA film formation. Insets are the calculated molecular mass (Mw) values at different regions. Inset figure is the illustration of the EQCM-Au working electrode and its DA electrochemical oxidation reaction products.

graphitic structure with relatively fewer oxygen functional (wide info) groups (GNP, CNF, GMC, SWCNT, and MWCNT) than that of the f-MWCNT was examined discretely in Figure 3A−H. Meanwhile, GO, which contains rich oxygen functional groups was also investigated for the e-pDA formation (Figure 3D). With a qualitatively similar voltammetric response with graphitic carbon materials but with respect to stability and defined redox peak current, the carboxylic acid funtionalized MWCNT and GO (with half the peak current response compared to the f-MWCNT case) showed the best response supporting the interaction of the carboxylic acid on the e-pDA (Figure 3J) and existence of ionic structure of pDA as reported by Gao et al.29 In this work, with consideration of the stability of the pDA molecule, the cationic charge is placed on the free

amino terminal rather than the indole nitrogen position of pDA as proposed by Gao et al. (Scheme 1).29 There is an unusual amide II linkage formation between the carboxylic acid of MWCNT and the amino group of pDA (IR, 1524 cm−1; minimal). Previously, it was reported that metal catalysts like ZnO, ZnCl2, indium, FeCl2, etc., assisted the coupling of amine and carboxylic acid groups.40,41 In fact, our group observed such amide II linkage formation (as specific IR signals at 1535 and 1415 cm−1)42,44 between amino group containing chitosan and carboxylic acid functionalized (a) MWCNT (impurities: 0.6 wt % Fe; 11 × 10−3 wt % Ni; 1.6 × 10−3 wt % Co)42 and (b) carbon nanoblack (impurities: 0.42 wt % Fe; 0.18 wt % Ni43)44 that have been modified on glassy carbon electrode surface. Based on this information, it is expected that the metal 7054

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Figure 5. (A) Comparative anodic stripping voltammetric responses of 500 ppb Pb2+on GCE/Nf@e-pDA (a), GCE/f-MWCNT-Nf, and GCE/fMWCNT-Nf@e-pDA in pH 4.6 acetate buffer solution. (B) ASV of Pb2+ at various concentrations and (C) its calibration plot. (D) Effect of interference of various metal ions and (E) typical real sample analysis (eyeliner cosmetic sample) by standard addition approach using the GCE/fMWCNT-Nf@e-pDA. R = Real sample; R+S1−3= Real small with standard concentrations of Pb2+ (100 ppb addition). Preconcentration potential: −1.2 V vs Ag/AgCl; preconcentration time: 90 s; stirring rate: 260 rpm. Inset is the illustration of the Pb2+ preconcentration (as Pb0) and stripping response of the GCE/f-MWCNT-Nf@e-pDA surface.

+IDQ}), 66.1 ± 3 (#4, 2Na+), 1053 ± 8 (#5;{Na++3DHI +3IDQ+DA}), 630 ± 10 (#6;{ Na++DHI+2IDQ+DA}), and 150.7 ± 3 g mol−1 (#7) (Scheme 1). In general, the values correspond to combinations and/or individuals of the molecular fractions, mDHI, nIDQ, oDA, pNa+, and sPO42− (Scheme 1H). Meanwhile, some of molecular weights like 111 ± 3 (#2 at second cycle), 402.4 ± 3 (#3, fifth cycle), 721 ± 8 (#6, seventh cycle) (data not included), and 2475 ± 20 g mol−1 (#5, 10th cycle) were also noted that correspond to the side products like pyrrolecarboxylic acid (PCA) (111 g mol−1) and trimer complex of {2DHI+PCA} (402 g mol−1) and pDA fractions of the DA oxidation products like {Na++DHI+2IDQ +DA+PO42−} and {Na++8DHI+7IDQ+DA}, respectively (Scheme 1H). Some of these products have been supported by Ding et al.’s characterization study of pDA using MALDIMS technique (110 and 402 g mol−1).26 Based on the results, the following conclusions can be derived: (i) pDA is an ionic structure. Na+ and PO42−ions were trapped in the pDA for the charge neutrality. (ii) Layer-by-layer formation of pDA units occurs on the underlying surface. (iii) A complex mixture composed of {mDHI+nIDQ+oDA+pNa++sPO42−}n is a building block, wherein m, n, o, p, and s are integers, of the pDA network. The proposed structure partially resembles the structure reported by Liebscher et al.23 and Ding et al.26 Indeed, positioning of DA in the end, trapping of the electrolyte ions used (Na+ and PO42−), and high molecular weights of pDA up to 2475 g mol−1 are new observations in this work. In the previous report by Ding et al., it was claimed that

impurities in the f-MWCNT can be a factor in the unusual amide linkage formation in this work. 3.3. In Situ CV-EQCM Analysis. Figure 4A is a ten continuous in situ CV response of EQCM-Au/f-MWCNT-Nf working electrode in the presence of 1 mM DA in pH 7 PBS at v = 50 mV s−1. Typical responses of frequency (Δf) and charge Q against the working potential were shown in Figure 4B and C. Respective plots of mass (Δm) vs Q for the overall (D) and some of the individual cycles (first, second, third, fifth, and tenth; E−I) were displayed in Figure 4E−I. Note that the in situ CV graph (Figure 4A) is similar to the response of the GCE/f-MWCNT-Nf@DAads displayed in Figure 1B, curve a. Moreover, the individual EQCM cycle patterns were qualitatively similar to that of the other cycles (Figure 4E−I). The thickness of the polydopamine coatings is controlled by varying the number of potential cycling experiments of the epDA electrochemical preparation. As shown in Figure 4D, depending on the potential cycle number, a variable amount of polydopamine is deposited on the substrate. For instance, three potential cycles of the pDA preparation resulted in 1.53 μg, whereas ten potential cycles ended with 6.49 μg of polydopamine. For the molecular weight calculation the following equation was used: Mw = F × Δm/ΔQ, where Q = charge passed, Mw = molar mass per electron (g mol−1 = molecular weight), and F = Faraday constant (96 500).31−33 Following are the representative Mw values calculated at different potential regions of the first cycle: 154.1 ± 9 (#1; DA), 92.8 ± 2 (#1′; PO42−), 78.2 ± 2 (#2; unknown), 327 ± 5 (#3; {Na++DHI 7055

DOI: 10.1021/acs.langmuir.8b01209 Langmuir 2018, 34, 7048−7058

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Langmuir maximum molecular weight formed on the film is ∼402 g mol−1 based on the MALDI-MS analysis, wherein the pDA sample collected from autoxidation of DA on a TiO2 surface was analyzed.26 It is likely that using the vigorous water based sample preparation method, the high molecular weight products are cleaved as smaller fractions and were detected by the MALDI-MS technique. (iv) Potential regions have specific influence on the pDA film formation. In the −0.8 to −0.6 V region (#1, Figure 4E), adsorption of DA was noted, and at −0.6 to −0.3 V (#2), stripping of molecular products like PO42− and PCA. It is expect that, at these negative potentials, oxygen radicals like O2- and O• are formed on the interface that helped to cleave the DA as PCA and other smaller fractions (side products, Scheme 1). In the −0.6 to 0.1 V region (#3), wherein there is a strong oxidation of DA noticed, an intense stripping of the molecular fraction like 150.7 (DHI), 327 (DHI+DA+Na+), and 630 g mol−1 (DHI+2IDQ+DA +Na+) was noticed. It can be referred to the surface-fouling behavior of the pDA.20,34 This stripping process was continued in region #4, 0.6 to 0.8 V, as well. This time, ionic species like Na+ (66 g mol−1; 2Na+) are found to be expelled from the electrode surface. On the other hand, in the cathodic direction, + 0.8 to −0.8 V (#5−#7), a steep weight gain in the response was noticed which may be due to the formation of stable pDA units of high molecular weight fractions up to 2475 g mol−1 on the underlying surface. It is likely that in the region of azaMichael-addition, isomerization and polymerization reactions occurr at a maximum rate (Scheme 1B−F). Finally, potential crossing at 0 V has led to favorable adsorption of DA and/or DHI fractions. The entire process is repeated in the next cycle on the underlying surface. Overall, identification of ionic structure and high molecular weight of pDA are new observations in this work. 3.4. GCE/f-MWCNT-Nf@pDA for Pb2+ Recognition. Comparative ASV responses of GCE/Nf@e-pDA, GCE/fMWCNT-Nf, and GCE/f-MWCNT@e-pDA for the detection of 500 ppb of Pb2+ in pH 4.6 acetate buffer solution was displayed in Figure 5A. Prior to the analysis, the electrodes were subjected to Pb2+ preconcentration at potential Epre = −1.2 V vs Ag/AgCl for 90 s. A specific voltammetric response at Epa = −0.6 ± 0.05 V vs Ag/AgCl, which is due to the redox potential of Pb2+/0,45 was uniformly noticed in all cases. Indeed, the GCE/f-MWCNT-Nf@e-pDA exhibited about five and two times higher ASV current signals than the signals of the GCE/ Nf@e-pDA and GCE/f-MWCNTa-Nf systems, evidencing the best electrochemical recognition of the Pb2+ ion by the GCE/fMWCNT-Nf@e-pDA. The onstructed calibration plot was linear for the Pb2+ concentration in a window, 50−600 ppb, with a current sensitivity and regression values of 73.4 nA ppb−1 and 0.9994, respectively. Six repeated detections of 50 ppb of Pb2+ yielded a relative standard deviation of 4.1% (data not shown). Calculated detection limit value (signal-to-noise ratio = 3) is 840 ppt. The obtained value is significantly lower than that of the previous reported (complexation route) Pb2+ detection methods based on overoxidized polypyrrole electrode doped with 2-(2-pyridylazo) chromotropic acid anion (25 ppb)37 and DNA enzyme assembly modified electrode (62 ppb)38 and comparable with that of the DNA functionalized porphyrinic metal−organic framework (680 ppt)39 and 2.6 V vs Ag/AgCl pre-anodized glassy carbon electrode (700 ppt).45 A possible mechanism for the sensing of Pb2+ is sketched as in inset Figure 5. Note that, in the literature, it has been pointed that carbonyl/

hydroxyl functional groups are effective for Pb2+ immobilization (as Pb0).45 To achieve a thick layer of the oxygen functional layer, the carbon electrode (glassy carbon electrode) was preconditioned at +2.6 V vs Ag/AgCl in 0.01 M H2SO4.45 In this work, the carbonyl/hydroquinone group attached pDA served as an active site for the Pb2+ preconcentration via formation of a weak complex between the hydroquinone and Pb0 (apart from the Pb0-carboxylic acid and amino-group interactions; Nafion-SO3− avoids Pb0-neutral species preconcentration) and for the sensitive anodic stripping voltammetric analysis (inset in Figure 5). Furthermore, in order to understand the complexation process, Pb0 preconcentrated fMWCNT-Nf@e-pDA (i.e., f-MWCNT-Nf@e-pDA/Pb0) was subjected to FTIR analysis as in Figure 2C, curve d. These results showed the following two important notes: (i) The carbonyl signal, 1737 cm−1 is found to have completely vanished after the Pb0 immobilization on the f-MWCNT-Nf@ e-pDA. This observation supports the reduced form of the carbonyl (hydroquinone) active site for the binding of Pb0. It also suggests the existence of a significant content of carbonyl functional group with the f-MWCNT-Nf@pDA. (ii) There is a predominant appearance of the NH (primary amine) stretching frequencies at 3049, 2921 (asymmetric and symmetric stretching), 1607, 1468 (−NH bending), 933, and 810 cm−1 (NH wagging) after the Pb0 immobilization (Figure 2C, curve d). The exact structural detail is unclear. Indeed, it is likely that part of the Pb0 ion immobilized on the free amino terminals of the pDA that are turned after the metal ion deposition (inset Figure 5). The effect of interference such as K+, Ni+, Cd2+, and Ca2+ on the ASV detection of Pb2+ was investigated as in Figure 5D. The K+, Ni2+, and Cd2+ did not show any marked alteration in the current signal. For the case of Cd2+, an additional peak at Epa = −0.8 V was noticed, which allows multiple metal ion detection by this new test system. Indeed, Ca2+ ion exposure will collapse the ASV of the Pb2+ (limitation of this work). It is likely that Ca2+ion can form strong ionic bonding with the epDA and in turn avoid Pb2+ linkage. By utilizing this technique, selective detection of toxic Pb2+ level in a cosmetic eyeliner sample was investigated by the standard addition approach as in Figure 5E. About 100% recovery values were noticed in each addition of the standard Pb2+ ion indicating the applicability of the present system for electro-analytical sensor applications (Supporting Information Table S1). This information provides a new platform for selective and stable electroanalytical applications of the polydopamine films.

4. CONCLUSION Controlled electrochemical oxidation of dopamine on fMWCNT-Nf modified glassy carbon electrode to form a stable and surface confined polymeric product of DA, e-pDA (electrochemical deposited polydopamine), in pH 7 phosphate buffer solution has been successfully demonstrated. Control experiments with GCE/f-MWCNT, GCE/Nf, and GCE-based underlying surfaces have failed to show any such stable pDA product formation. The GCE/f-MWCNT-Nf@e-pDA showed well-defined redox peaks at E°′= 0.25 (A1/C1) and −0.35 V vs Ag/AgCl (A2/C2) corresponding to the redox transition of DA/DA-quinone and IDA/IDQ functionalized e-pDA film. These redox peaks are mixed adsorption and diffusion controlled in electron-transfer and have pH-dependent properties. Physicochemical characterizations of the e-pDA film by TEM, Raman, and IR spectroscopic techniques reveal 7056

DOI: 10.1021/acs.langmuir.8b01209 Langmuir 2018, 34, 7048−7058

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Langmuir

SWCNT, single walled carbon nanotube; MWCNT, multiwalled carbon nanotube; f-MWCNT, carboxylic acid functionalized MWCNT; p-MWCNT, purified MWCNT; GMC, graphitized mesoporous carbon

immobilization of polymeric DA on the f-MWCNT-Nf surface with specific functionalities of carbonyl, amine-II, Na+, phosphate ion, hydroxyl, and N−H units. It has been proposed that multiple interactions such as hydrogen bonding, amide-II linking (covalent bonding; minimal), ionic interaction (with anionic sites of deprotonated carboxylic acid and Nafion sulfonic acid with cationic site of pDA (ammonium group)), and π−π interaction (sp2 carbon of MWCNT and aromatic π e−s) have helped the e-pDA form as a stable film on the surface. Most importantly, unclear structural information about the pDA has been successfully elucidated using in situ CV-EQCM studies. Formation of pDA layer up to molecular weight of 2475 g mol−1, which is due to the combination of different degrees of {mDHI+nIDQ+oDA+pNa++sPO42−} as a building block, wherein m, n, o, p, and s are integers (0, 1, 2, 3, ···), has been identified. For the first time in the literature, pDA film modified electrode has been successfully applied to selective electro-analytical sensing of Pb2+ via anodic stripping voltammetric analysis suitable for practical real sample applications. Indeed, Ca2+ ion showed serious interference for the detection, which is the limitation of this Pb2+ sensing work.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b01209. Effect of scan rate and solution pH on the CV responses of GCE/f-MWCNT-Nf@e-pDA and its respective plot of double logarithmic of ipa or ipc vs scan rate and Epa vs solution pH (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Phone: +91-416-2202754. ORCID

Annamalai Senthil Kumar: 0000-0001-8800-4038 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by Department of Science and Technology, Science Engineering Research Board, India. The authors acknowledge the Department of Science and Technology − Science and Engineering Research Board (DST-SERB-EMR/2016/002818) Scheme. A.S.K. acknowledges National Taipei University of Technology for the support of his distinguished visiting professor program.

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ABBREVIATIONS DA, dopamine; pDA, polydopamine; e-pDA, electrochemically deposited pDA; DHI, 5,6-dihydroxy indole; IDQ, 5,6indolequinone; DAC, dopaminechrome; LDAC, leucodopaminechrome; EQCM, electrochemical quartz crystal microbalane; Nf, Nafion; AC, activated charcoal; GNP, graphite nanopowder; CNF, carbon nanofiber; GO, graphene oxide; 7057

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