In-situ Structural Elucidation and Selective Pb2+ Ion Recognition of

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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 Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01209 • Publication Date (Web): 24 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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Illustration for the controlled electrochemical oxidation of dopamine to polydopamine and its selective Pb2+ ion recognition

ACS Paragon Plus Environment

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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 Kumara,b,c* a

Nano and Bioelectrochemistry Research Laboratory, Department of Chemistry, School of Advanced Sciences, Vellore Institute of Technology, Vellore-632 014, India

b

Carbon dioxide Research and Green Technology Centre, Vellore Institute of Technology, Vellore-632 014, India c

Institute of Biochemical and Biomedical Engineering, National Taipei University of Technology, Taipei 10608, Taiwan (R.O.C.)

KEYWORDS. Dopamine; Controlled electrochemical oxidation; Polydopamine; In-situ structural analysis; Pb2+ ion recognition.


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ABSTRACT. Owing to the versatility and biocompatibility, a self-polymerized DA (in 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 fMWCNT-Nf surface via π-π (sp2 carbon of MWCNT and aromatic e-s), 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 Eo’= 0.25 V and -0.350 vs Ag/AgCl corresponding to the surface-confined dopamine/dopamine quinone and DHI/5,6indolequinone

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-pertrillion has been demonstrated.


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1. INTRODUCTION Development of stable and functional polymers that are based on naturally occurring precursors is an important research in the interdisciplinary fields of material chemistry and biological science.1 Dopamine (DA) is an 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 a polydopamine (pDA), an inert, bio compatible and non-poisonous polymer as melanins/eumelamins, that can adhere on a variety of substrates like organic,4,5 inorganic including metals,6,7 metal oxides,8 semiconductors,9 ceramic10 and polymers.11-13 pDA has been widely used for applications like cellular

interfacing,14

energy

applications,10,12,13,15,

cancer

therapy16

nanomedicine,17

biosensing,18 drug delivery16,19 etc. Indeed, major obstacles in expanding this material are serious surface-fouling property under electrified condition20,21 and structural ambiguity.11,12,22,23 In early 2012, it has been thought that dopamine polymerize 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 resonance22 and X-ray photoelectron spectroscopy,26 following mechanisms and structural information about pDA have been postulated; oxidation of dopamine to 5,6dihydroxy (DHI) and 5,6-indolequinone (IDQ) intermediates followed by (i) self-assembling 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 saturation23; (iii) covalent linking between the benzene ring of DHI and indole ring of the IDQ.27-29 Still it is unclear that


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what kind of pDA film is formed?, what are the surface functional groups? and what is the building block of the biopolymer?. In general followings are the

problems faced while

characterizing the pDA by the conventional techniques; (i)sample collection; rather to analysis the pDA film directly, pDA aggregates collected from the dopamine solution in presence of dissolved oxygen,22,23 or chemical-oxidant such as ammonium persulphate,29 Ce(IV) ion,30 sodium hypochlorite,22 and periodate27 and solid surface (TiO2 and glass)23,28 have been used; (ii) 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 hydrogenbonding and in turn structure by organic solvent that has been 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 in-situ electrochemical quartz crystal microbalance (EQCM) technique, which contain a sensitive piezoelectric quartz crystal,

for identifying intermediates of several complex electro-organic reactions such as

phenol oxidation to hydroquinone,31 azo-bond cleavage via aniline intermediate32 and quinolinequinone 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 to trap the intermediates, that have been formed on the interface of the electrochemical reactions. Previously, there was an report relating to pDA growth studies on gold electrode using EQCM technique.34 In this work, in-situ cyclic voltammetry (CV)-EQCM technique has been 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.


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In spite of the variety of utilities,10-19,35 independent electroanalytical application of pDA film is never reported in the literature. The reason behind is the serious fouling/stripping behaviour of the pDA’s constitutes 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, pi-pi, ionic and hydrogen bonding. Meanwhile, recently, Gao et al first pointed the ion-permeability behaviour of the pDA by studying with Prussian bluemodified electrode which has been prepared by successive deposition of PB (electrochemical method in pH 3) and pDA (by ammonium persulphate assisted DA oxidation in pH 7 phosphate buffer solution) on a gold electrode surface.29 It has been concluded that pDA has zwitter-ionic property and contains interconnected intermolecular voids for alkaline ion (K+) mobility, irrespective of the film thickness. By considering the ionic property, aromatic π-electrons 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, 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 for 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, in aim 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 significant concentration in day-to-day life products, like


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toys, paints and cosmetic formulations.36 Selective and sensitive detection of Pb2+ is a demanding research interest in interdisciplinary areas of chemistry.36-39

2. EXPERIMENTAL SECTION 2.1 Chemicals and Reagents. Dopamine hydrochloride (98% purity), graphene oxide (GO; 2 mg mL-1 in water), activated charcoal (AC) and graphite nano powder (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


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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 microscope (TEM) was done using a Technai, G2 20 Twin FEI instrument. 2.3. Preparation Procedures. In first, the surface of the GCE was cleaned both mechanically (polished with 1 micron 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 V to 1 V vs. Ag/AgCl in pH 7 PBS at scan rate (v)= 50 mV s-1). GCE/f-MWCNTNf 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±2oC). Thickness of the fMWCNT-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 the 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 twenty continuous cycles in pH 7 PBS at a scan rate of 50 mV s-1 (Scheme 1AF). 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 showed qualitatively similar e-pDA products formation. Following Sauerbrey equation is referred for the conversion of frequency to mass change;31-33 Mass change


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Scheme 1. Illustration for 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). Step 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.


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(∆m) = (-1/2) fo-2 ∆f 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 S-2 ), ∆f= measured frequency change, and fo = oscillation frequency of the crystal (8 MHz). A ∆f change of 1 Hz is equivalent to a ∆m value of 1.4×10-9 g in mass. For the electrochemical sensing of Pb2+, anodic stripping voltammetric (ASV) technique in pH 4.6 acetate buffer solution was adopted. For Pb2+ pre-concentration step, the modified electrode is conditioned at -1.2 V vs Ag/AgCl for desired timings (90s). Followings are the optimal differential pulse voltammetry (DPV) parameters used in this work: potential = -1.1 V, Estep= 0.004 V, pulse width = 0.06 sec and sampling width = 0.01 sec. 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 Fig.1A,curve a. Appearance of 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 (dilution solution of DA drop-casted on GCE, GCE/DAads, ads=adsorbed) and performed CV in a blank pH 7 PBS (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 poly-dopamine (pDA) product. Hereafter the electro-oxidized 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


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A.

B.

20th cycle st 1 a.GCE /f-MWCNT-Nf@DAads

b.GCE/f-MWCNT@DAads A1

200

200

A2'

I/µA

A1 Stable

A2'

Unstable

A2

A2

a.GCE/DA

0

0 c.GCE/Nf@DAads

b.GCE/f-MWCNT-Nf

C1 C2

-200

C2' v= 50 mV s-1

-0.8

-0.4

0.0

0.4

-200 -0.8

0.8

C2'

C2

-0.4

C1

0.0

0.4

0.8

E (vs Ag/AgCl)/V

E (vs Ag/AgCl)/V C.

D. A1

e.

A1 -0.7 to -0.1V

d.

0.1 to 0.7V

300

300 c.

Estart

b. a.

0

-300 -0.8

A2

f.

A2 I/µA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

C1

C2

-0.4

0.0

a. g. f. e. d. c. b.

-300 0.4

0.8

Estart

C1

C2

-0.8

-0.4

0.0

0.4

0.8

E (vs Ag/AgCl)/V

E (vs Ag/AgCl)/V

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/f-MWCNT-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 10th first cycle) at different potential windows. Electrolyte = pH 7 PBS; Scan rate = 50 mV s-1.


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literature. In aim to prepare a stable e-pDA film, in first, a DA adsorbed f-MWCNT modified GCE (GCE/f-MWCNT@DAads; without Nf) was used and was subjected to electrochemical oxidation reaction as in Fig.1A, curve b. This time, well-defined quasi-reversible peaks at apparent electrode potentials, Eo’ = 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 (Fig.1A, curve a), the GCE/f-MWCNT 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 (Fig.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’ (Eo’= -0.15 V vs Ag/AgCl), without any marked alteration in the peak currents, relative stand deviation (RSD) between the 1st and 20th cycle is 3.7 %, were noticed (Fig.1B, curve a). This observation attributes an unusual and fouling-free polymeric product formation on the modified electrode, GCE/f-MWCNT-Nf@e-pDA. Existing 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 sulphonic acid of nafion and anionic amino functional groups of DA, pKa 8.5) and covalent (between the carboxylic functional group of f-MWCNT and e-pDA; minimal) are the likely reason for the usual stability of the e-pDA film.

These redox peaks have mixed adsorption and diffusion controlled electron-transfer

feature (slope (∂logipa/∂logv) of double logarithmic plot of ipavs v is ~0.6) and strong pH dependent property (Supporting information Fig.S1A&B). A plot of anodic Epa vs pH yielded a slope, -45±3 mV pH-1 suggesting non-Nernstian type of proton-coupled electron-transfer reaction with involvement of unequal number of proton and electron (H+/2e-) (Supporting


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information Fig.S1C&D). Based on the literature reports on chemical22,27,30,31 and electrochemical oxidation reactions of DA,20,22,25,26,34 possible mechanism for the DA-oxidation and e-pDA polymer formation reactions occurred on GCE/f-MWCNT-Nf surface is sketched as in the Scheme 1A-H. As per the mechanism, at ~0.3 V vs Ag/AgCl, DA gets reversibly electrooxidized to dopamine-quinone (A1/C1) followed by intermolecular cyclization of the amine functional

group to

reduced

form

of indole group via 1,4-Michael addition to

leucodopaminechrome (LDAC; chemical reaction) that is subsequently electro-oxidized to respective quinone derivative, dopaminechrome (DAC), reversibly (A2’/C2’). The unstable DAC compound further involved in a chemical isomerization reaction to form 5,6-dihydroxyindole (DHI) that has been involved in proton-coupled redox reaction with 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 showed marked current signals while the intermediate, A2’/C2’ peak resulted a feeble response in this work (Fig. 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 end potentials as in Fig.1C and D. It is obvious that potential started at -0.8 V and swept to 0.3 V (Fig.1C, curves c-f), where the A1/C1 redox peak is appeared (DA oxidation occurring), showed a marked A2/C2 peak current signal. Potential cycling experiments by omitting the A1 peak were resulted to nil A2/C2 response (Fig.1C, curves a & b). Similarly, potential started at +0.8 V (where DA oxidation occurs maximum) and swept to a half way, -0.4 V vs Ag/AgCl (Fig.1D, curve a), showed about twice decrement in the A1/C1 peak current response (Fig.1D). Based on the results, following important conclusions can be derived: (i) Oxidation to DA to DA-quinone at ~0.3 V vs Ag/AgCl is a key reaction; (ii) overall, e-pDA


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B.

C. d.f-MWCNT-Nf@e-pDA/Pb

0 +

(-NH; -NH4 )

3709

933 810 1743

3383

2639

+

(-NH4 )

b.f-MWCNT-Nf@e-pDA

1195

1607 1498 1280 -NH (-C-O)

2921 3049 (-NH)

D ID/IG=0.35

3934 (-HO--H2O)

2853 2934 (-CH=CH)

c.f-MWCNT-Nf@e-pDA

(Amide-II) 2113 2335 -NH (-C=C=O) -OH&-C-H

1630 -NH

(>C=O)1737

1524

G band

Intensity (AU)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1370 1218 (PO 2-) 4

a.DA 3336 1623

2931

(-NH...H) 3035

1501

ID/IG=0.43

a. f-MWCNT-Nf 1200

1400

1600 -1

Wavenumber(cm )

b.f-MCNT-Nf 4000

2352

3000

(-NH)

2131 1723

2000

1000

Wavenumber(cm-1)

Fig.2. Comparatives (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 its ID/IG ratio values and (C) FTIR/KBr responses of dopamine (a), f-MWCNT-Nf (b), f-MWCNTNf@e-pDA and f-MWCNT-Nf@e-pDA/Pb0 (d) systems.


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formation is a 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 appeared 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 microgram level and it is difficult isolate as 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 with control experiments as given in the below sections. 3.2. Physicochemical characterization of f-MWCNT-Nf@e-pDA. TEM analysis of the fMWCNT-Nf and MWCNT-Nf@e-pDA showed marked increment in the diameter of the fMWCNT after the e-pDA deposition suggesting formation of a thin layer of polymeric film on the MWCNT out surface (Fig.2A). It is expected that strong π-π interaction between the MWCNT-sp2 carbon and benzene 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 the 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 (Fig. 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 (1524 cm-1; minimal) and phosphate ion (2335 and 1218 cm-1) in addition with hydroxyl (~3900 cm-1) (Fig.2C, curve c), combined –NH, -CH and –OH functional groups ( as combined peak at 2113 cm-1), unlike to the intense bands at a finger print region, 750-1600 cm-1 corresponding to the


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Fig. 3. CV responses of various carbon wize; 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 (p-MWCNT), 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 Fig.1B, curve a. Fig.3J 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.

primary amine, C-N and –N-H stretching frequencies with the naked DA (Fig.2C, curve a). The IR observation of the optimal system is resembled with the IR of pDA agglomerates reported by Dreyer et al.22 Following are some of the conclusions derived from the above observations; (i)


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deprotonated carboxylic acid of the f-MWCNT interacted with the ammonium ion of DA/e-pDA and resulted to formation of the hybrid product. To further confirm the carboxylic group and graphitic participation, effect of carbon which doesn’t contain graphitic units (AC) and has graphitic structure with relatively lesser oxygen functional (wide info) groups (GNP, CNF, GMC, SWCNT and MWCNT) than that of the f-MWCNT were examined discreetly as in Fig.3A-H. Meanwhile, GO, which contain rich oxygen functional group was also investigated for the e-pDA formation (Fig.3D). 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 (has twice less peak current response than the f-MWCNT case) showed the best response supporting the interaction of the carboxylic acid on the e-pDA (Fig.3J) and (ii) existence of ionic structure of pDA as reported by Gao et al.29 In this work, in consideration with 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 (iii) There is an unusual amide II linkage formation between the carboxylic acid of MWCNT and amino group of pDA (IR, 1524 cm-1; minimal). Previously, it has been reported that metal catalyst like ZnO, ZnCl2, Indium, FeCl2 etc have been 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 cm-1 and 1415 cm-1)42,44 between amino group containing chitosan and carboxylic acid functionalized (a) MWCNT (impurities:- 0.6 wt%-Fe; 11×10-3wt%-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 the information, it is expected that the metal impurities in the f-MWCNT can be helped for the unusual amide linkage formation in this work.


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3.3 In-situ CV-EQCM analysis. Fig.4A is a ten continuous in-situ CV response of EQCMAu/f-MWCNT-Nf working electrode in presence of 1 mM DA in pH 7 PBS at v=50 mV s-1. Typical responses of frequency (∆f) and charge Q against the working potential were shown in Fig.4B and C. Respective plots of mass (∆m) vs Q for the overall (D) and some of the individual cycles (1st, 2nd , 3rd , 5th and 10th ; E-I) were displayed in Fig.4E-I. Note that the in-situ CV graph (Fig.14A) is resembled with the response of the GCE/f-MWCNT-Nf@DAads displayed the Fig.1B, curve a. Moreover, the individual EQCM cycle patterns were qualitatively similar to that of each other cycles (Fig.4E-I). The thickness of the polydopamine coatings is controlled by varying the number of potential cycling experiment of the e-pDA electrochemical preparation. As shown in Figure 4D, depends on the potential cycle number, variable amount of polydopamine is deposited on the subtract. For instance, three potential cycle of the pDA preparation resulted to 1.53 µg, whereas, ten potential cycle ended with 6.49 µg of polydopamine. For the molecular weight calculation following equation was used; Mw = F×∆m/∆Q, where, Q= charge passed, Mw= molar mass per electron (g mol−1 = molecular weight), and F= Faraday constant (96500).31-33 Followings 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+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 are corresponding to combination and/or individuals of the molecular fractions, mDHI, nIDQ, oDA, pNa+ and sPO42- (Scheme 1H). Meanwhile, some of the molecular weights like, 111±3 (#2 at 2nd cycle), 402.4±3 (#3, 5th cycle), 721±8 (#6, 7th cycle) (data not included) and 2475±20 g mol-1 (#5, 10th cycle) were also noticed which are corresponding to the side products like pyrrolecarboxylic acid (PCA) (111 g mol-1) and trimer


17

Page 19 of 35

A.

B.

C.

D. th

10 cycle:

th

1-10 cycle:

4000

A2

0

2000 C2

4 2 st

-20

-200

-0.4

0.0

1 cycle: 0

0

C1

0.4

0.8

-0.8

E/V vs Ag/AgCl

-0.4

0.0

0.4

0.8

-0.8

-0.4

0.0

0.4

-20

0.8

E/V vs Ag/AgCl

E/V vs Ag/AgCl

E. 1st cycle:

-0.8 V

-10

0

Q/mC

F. -0.4 V

G.

1.2

2nd cycle: 1.6

0.8

1.2

rd

3 cycle:

0V

0.4 ∆m/µg

-10

∆m/µg

0

-0.8

6

f /Hz

A1

Q/mC

I/µA

200

#7.150.7 ±3 #1'.92.8+8

#1.154.1±9

#6. 630±10 -0.5 V 0.4 V

0.0

-0.8 V #2. 78.2 ±2 g mol-1

#5.1053±8

0.4

0.8 V

0.8

#2. 111±1 -1

g mol

#3.327±4

-0.4

0.64 V #4. 66.1±3

-1

#3. 402±2 g mol

0.4

0.0

-0.8 -6

-4

-2

0

-8

2

-6

-4

-2

Q/mC

Q/mC

H.

-10

-8

-6

Q/mC

I. th

5th cycle:

10 cycle: 6.3

2.4

∆m/µg

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

5.6

2.0

#6. 1732±35

4.9

-1

#3. 402±2 g mol

1.6

#5. 2475±20

4.2

1.2 -14

-12 Q/mC

-10

-8

-26

-24

-22

-20

Q/mC

Fig.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 & D are respective charge vs E and mass (∆m) vs charge plots. E-I are typical plot of ∆m vs Q of 1st, 2nd, 3rd, 5th and 10th cycles of the EQCM-Au/f-MWCNT-Nf@epDA 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.


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Page 20 of 35

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). Part of these products have been supported by Ding et al characterization study of pDA using MALDIMS technique (110 and 402 g mol-1).26 Based on the results 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 was occurred 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 is partially resembled with the structure reported by Liebscheret al23 and Ding et al26. 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 has been claimed that maximum molecular weight formed on the film is ~402 g mol-1 based on the analysis of MALDI-MS technique, wherein, pDA sample collected from autoxidation of DA on a TiO2 surface, was analyzed.26 It is likely that upon vigorous water based sample preparation method, the high molecular products are cleaved as smaller fractions and got detected in the MALDI-MS technique. (iv) Potential regions have specific influence on the pDA film formation. In the region, -0.8 to -0.6 V (#1, Fig.4E), adsorption of DA and at -0.6 to -0.3 V (#2) stripping of molecular products like PO42- and PCA were noticed. It is expect that at these negative potentials, oxygen radical 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 region, -0.6 to 0.1 V (#3), wherein, there is a strong oxidation of DA noticed, an intense stripping of molecular

fraction

like

150.7

(DHI),

327

(DHI+DA+Na+)

and

630

g

mol-1

(DHI+2IDQ+DA+Na+) were noticed. It can be referred to the surface-fouling behaviour of the


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pDA.20,34 This stripping process was continued in the region #4, 0.6 to 0.8 V as well. This time ionic species like Na+ (66 g mole-1; 2Na+) is found to be expelled-out 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 a stable pDA units of high molecular weights fractions up to 2475 g mol-1 on the underlying surface. It is likely that in that region aza-Michael-addition, isomerization and polymerization reactions are occurred 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 the 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/f-MWCNT-Nf and GCE/f-MWCNT@e-pDA for the detection of 500ppb of Pb2+ in pH 4.6 acetate buffer solution was displayed in Fig. 5A. Prior to the analysis, the electrodes were subjected to Pb2+ pre-concentration at potential (Epre) =-1.2 V vs Ag/AgCl for 90 sec. 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 the cases. Indeed, the GCE/fMWCNT-Nf@e-pDA exhibited about five and two times higher ASV current signals than that of 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/f-MWCNT-Nf@e-pDA. Constructed 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

detection of 50 ppb of Pb2+ yielded a relative standard deviation of 4.1 (data not shown).


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Langmuir

A.

B.

C.

c. f-MWCNT-Nf@e-pDA

D.

[Pb2+]/50-600 ppb

[Pb2+]=500ppb

2+ [Cd ]

2+ [Pb ]

a.

b. c. d. e.

I / µA

ipa/µA

I/µA

40

b.f-MWCNT-Nf

a.Pb b. Pb+Ni

20

c. Pb+Ni+K

20 µA

20µA

d. Pb+Ni+K+Cd

a. Nf@e-pDA

-0.8

y=0.78+0.0734x

-0.6

-0.4

E (vs Ag/AgCl)/V E.

-1.0

-0.8

-0.6

-0.4

E(vs Ag/AgCl)/V

-0.2

e. Pb+Ni+K+Cd+Ca

0 200

400

[Pb2+]/ppb

600

-1.0

-0.8

-0.6

-0.4

E (vs Ag/AgCl)/V

Eyeliner [Pb2+] R+S3 R+S2

I/µA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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R+S1 R

Conc of Pb2+(ppb)

-0.8

10 µA -0.4

E(vs Ag/AgCl)/V

Fig. 5 (A) Comparative anodic stripping voltammetric responses of 500 ppb Pb2+on GCE/Nf@epDA (a), GCE/f-MWCNT-Nf and GCE/f-MWCNT-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 (eye-liner cosmetic sample) by standard addition approach using the GCE/f-MWCNT-Nf@e-pDA. R=Real sample; R+S1-3= Real small with standard concentrations of Pb2+ (100 ppb addition). Pre-concentration potential: -1.2 V vs Ag/AgCl; pre-concentration time: 90 sec; stirring rate: 260 rpm. Inset is the illustration for the Pb2+ pre-concentration (as Pb0) and stripping response of the GCE/fMWCNT-Nf@e-pDA surface.

Calculated detection limit value (Signal-to-noise ratio=3) is 840 ppt. Obtained value is significantly lowered than that of the previous reported (complexation route) Pb2+ detection methods based on over-oxidized polypyrrole electrode doped with 2-(2-pyridylazo) chromotropic acid anion (25 ppb)37 and DNA enzyme assembly modified electrode (62 ppb)38


21

2+

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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. Possible mechanism for the sensing of Pb2+ is sketched as in inset Fig.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 pre-conditioned 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 pre-concentration) and for the sensitive anodic stripping voltammetric analysis (Inset Fig.5). In further, to order understand the complexation process, Pb0 preconcentrated f-MWCNT-Nf@epDA (I.e., f-MWCNT-Nf@e-pDA/Pb0) was subject to FTIR analysis as in Figure 2C, curve d. These results showed following two important notifications: (i) The carbonyl signal, 1737 cm-1 is found to be completed vanished after the Pb0 immobilization on the f-MWCNT-Nf@e-pDA. This observation supports reduced form of the carbonyl (hydroquinone) active site for the binding of Pb0. It also suggests existence of marked content of carbonyl functional group with the f-MWCNT-Nf@pDA. (ii) 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 (Fig.2C, curve d). 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 Fig.5). Effect of interference such as K+, Ni+, Cd2+ and Ca2+ on the ASV detection of Pb2+ was investigated as in Fig.5D. The K+, Ni2+ and Cd2+ didn’t show any marked alteration in the current


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Page 24 of 35

signal. For the case of Cd2+, additional peak at Epa= -0.8 V was noticed, which allows multiple metal ion detection by this new test system. Indeed, when 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 e-pDA and in turn avoids Pb2+ linkage. Utilizing this technique, selective detection of toxic Pb2+ level in a cosmetic eye-liner sample was investigated by standard addition approach as in Fig.5E. About 100% recovery values were noticed in each additions of the standard Pb2+ ion indicating the applicability of the present system for electro-analytical sensor application (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 f-MWCNT-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/fMWCNT-Nf@e-pDA showed well-defined redox peaks at Eo’= 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 feature and has pH dependent property. Physicochemical characterizations of the e-pDA film by TEM, Raman and IR spectroscopic techniques reveal immobilization of polymeric DA on the f-MWCNT-Nf surface with specific functional 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


23

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sites of deprotonated carboxylic acid and Nafion sulphonic 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 to 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 upto molecular weight 2475 g mol-1, which is due to the combination of different degree of {mDHI+nIDQ+oDA+pNa++sPO42-} as a building block, wherein, m,n,o,p and s are integer (0,1,2,3----), have 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 to practical real sample applications. Indeed, Ca2+ ion showed serious interference for the detection, which is the limitation of this Pb2+ sensing work.

ASSOCIATED CONTENT Supporting Information. Effect of scan rate and solution pH on the CV responses of GCE/fMWCNT-Nf@e-pDA and its respective plot of double logarithmic of ipa or ipc vs scan rate and Epa

vs

solution

pH.

This

content

is

available

free

of

charge.

AUTHOR INFORMATION Corresponding Author Annamalai Senthil Kumar*, Emails; [email protected]; [email protected]; Phone: +91-416-2202754


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Page 26 of 35

ORCID Annamalai Senthil Kumar: 0000-0001-8800-4038 Notes The authors declare no competing financial interest. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was funded by Department of Science and Technology, Science Engineering Research Board, India. ACKNOWLEDGMENT The authors acknowledge the Department of Science and Technology – Science and Engineering Research Board (DST-SERB-EMR/2016/002818) Scheme. ASK acknowledges National Taipei University of Technology for the support of distinguished visiting professor program. ABBREVIATIONS DA, dopamine; pDA, polydopamine; e-pDA, electrochemically deposited pDA; DHI, 5,6dihydroxy indole; IDQ, 5,6-indolequinone; DAC, dopaminechrome; LDAC, leucodopaminechrome; EQCM, electrochemical quartz crystal microbalane; Nf, Nafion; AC, activated charcoal; GNP, graphite nanopowder; CNF, carbon nanofiber; GO, graphene oxide; SWCNT, single walled carbon nanotube; MWCNT, multiwalled carbon nanotube ; f-MWCNT,


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carboxylic acid functionalized MWCNT; p-MWCNT, purified MWCNT; GMC, graphitized mesoporous carbon. REFERENCES (1) Carlini, A. S.; Adamiak, L.; &Gianneschi, N. C. Biosynthetic Polymers as Functional Materials. Macromolecules, 2016, 49, 4379-4394. (2) Bodor, N.;& Simpkins, J. W. Redox Delivery System for Brain-Specific, Sustained Release of Dopamine. Science, 1983, 221, 65-67. (3) Zhang, A.; Neumeyer, J. L.; & Baldessarini, R. J. Recent Progress in Development of Dopamine Receptor Subtype-Selective Agents: Potential Therapeutics for Neurological And Psychiatric Disorders. Chem Rev, 2007, 107, 274-302. (4) Lee, H.; Dellatore, S. M.; Miller, W. M.; & Messersmith, P. B. Mussel-Inspired Surface Chemistry for Multifunctional Coatings. Science, 2007, 318, 426-430. (5) Pop-Georgievski, O.; Verreault, D.; Diesner, M. O.; Proks, V.; Heissler, S.; Rypáček, F.; Koelsch, P. Nonfouling Poly (Ethylene Oxide) Layers End-Tethered To Polydopamine. Langmuir 2012, 28, 14273-14283. (6) Liu, C. Y.; Huang, C. J. Functionalization of Polydopamine via the Aza-Michael Reaction for Antimicrobial Interfaces. Langmuir 2016, 32, 5019-5028. (7) Yang, W.; Liu, C.; Chen, Y. Stability of Polydopamine Coatings on Gold Substrates Inspected

by

Surface

Plasmon

Resonance

Imaging. Langmuir

2018,

DOI:

10.1021/acs.langmuir.7b03143


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(8) Zhang J.; Fang Q.; Duan, J.; Xu, H.; J.; Xuan, S. Magnetically Separable Nanocatalyst with Fe3O4 Core and Polydopamine-Sandwiched-Au-Nanocrystals Shell, Langmuir 2018, DOI: 10.1021/acs.langmuir.8b0030. (9)

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Scheme 1. Illustration for the concept of controlled electrochemical oxidation of dopamine and polydopamine formation


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Figure 3. Effect of carbon on the dopamine oxidation to polydopamine formation 184x169mm (150 x 150 DPI)


In-situ Structural Elucidation and Selective Pb2+ Ion Recognition of

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