A Short Report on the Polymerization of Pyrrole and Its Copolymers by Sonochemical Synthesis of Fluorescent Carbon Dots

polymers

Brief Report A Short Report on the Polymerization of Pyrrole and Its Copolymers by Sonochemical Synthesis of Fluorescent Carbon Dots

Moorthy Maruthapandi and Aharon Gedanken *

Bar-Ilan Institute for Nanotechnology and Advanced Materials, Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel * Correspondence: [email protected]; Tel.: +972-3-5318315; Fax: +972-3-7384053

Received: 23 June 2019; Accepted: 23 July 2019; Published: 26 July 2019

Abstract: In polymer chemistry, polymerization constitutes the process of the conversion of monomers into polymers using an initiator to form polymeric chains. There are many polymerization techniques and different systems exist by which the polymers are classified. Recently, our group has reported the synthesis of polymers using both carbon dots (CDs) and UV light as initiators. In these reports, the carbon dots were used with or without UV light. The CDs produce free radicals in the presence of UV light, indicating their role as initiators. The CD surface has many unshared or unpaired electrons, making it negatively charged. The present study focuses on the use of CDs for the formation of polymers from monomers containing various functional group. The properties of the synthesized CDs and the polymers obtained from the various monomers were characterized by various analytical techniques, including Fourier-Transform Infrared (FTIR) spectroscopy, X-ray Diffraction (XRD), Thermogravimetric Analysis (TGA) and Solid-State NMR spectroscopy. This polymerization technique is of interest both from the scientific aspect and for its applicative potential. The synthesized polymers were investigated for their various applications.

Keywords: carbon dots; sonochemical method; UV-light; polypyrrole; Poly(pyrrole-co-aniline); Poly (Bis(p-aminophenyl) ether-co-pyrrole)

1. Introduction Recently, carbon dots (CDs) have been emerging as new materials demonstrating great potential for use in biomaterial and medical engineering. In the last few decades, carbon dots have begun to feature in such areas as electronic industries, solar and photovoltaic cells, aerospace, catalysis, targeting drug delivery [1–5]. CDs are a type of nanomaterial composed of a carbon core, with oxygen and hydrogen on the circumference of the 5 nm dots appearing as carboxylic, or carbonyl, or alcohol groups [6–10]. CDs have been used as new cell imaging and drug-delivery nano-platforms due to their outstanding fluorescence, biocompatibility, excellent aqueous solubility, negligible cytotoxicity, easy functionalization, biodegradation ability, and environmental friendliness. CDs have been synthesized by several methods, including microwaves and ultrasonic irradiation [11–17]. The characteristic properties of CDs are partially dependent on the particular carbon-based starting materials, which include sugars, proteins, amino acids, juice carbohydrates, milk, glucose, etc. [12,18–20]. The CDs synthesized sonochemically from polyethylene glycol (PEG-400) provide a high quantum yield (QY) of fluorescence, [21,22] and have been applied in numerous areas of science and technology due their high photoluminescence, biocompatibility, low-cost preparation, no cell cytotoxicity, and high stable fluorescence [5,21,23–29]. Moreover, the synthesis of CDs from PEG produces a good passivating layer on their surface. These sonochemically prepared CDs were used here as an initiator for

Polymers 2019, 11, 1240; doi:10.3390/polym11081240 www.mdpi.com/journal/polymers Polymers 2019, 11, 1240 2 of 15 polymerization because they form free radicals in aqueous solution, which leads to their unique functionality and their high potential in many applications [1,6–8,30–32]. The free radicals play a main role in the polymerization mechanism [18–20,33]. Sonochemistry usually yields unorganized and perturbed particles having dislocations and vacancies leading to the formation of free radicals in water. This was demonstrated, for example for metal oxide NPs comparing the formation of commercial and sonochemically made products of the same size [34]. The same happens in the current work, where the CDs prepared sonochemically has yielded OH radicals. The radicals are detected by EPR when exposed to spin traps [22]. Polypyrrole (PPY) is recognized as a conducting polymer that relies on its electrochemical responsive properties rather than on its chemical structure. PPY offers a potentially useful resource because it possesses optical, magnetic, and electronic properties comparable to those of metals or semiconductors, while also retaining its polymeric structure and properties, such as ease of processing, low toxicity, flexibility, and adjustable electrical conductivity [34–40]. However, the general applications of PPY have been mostly limited to date owing to its poor mechanical properties. In order to improve the mechanical properties, various dopants have been introduced into the polymer using certain strong initiators [39,41–43]. Our novel polymerization of PPY was developed using only CDs as initiator, without any other strong initiators [44–46]. The synthesized polypyrrole has been used as an adsorbent material for various organic dyes and for biological application [47,48]. To date, there have been only a few reports on the synthesis of polymers using CDs and UV light, all of which were reported by our group. The conventional synthesis of the conductive polymer polypyrrole usually employed various strong oxidizing agents as initiators. The best methods by which PPY is polymerized are either through chemical oxidization or electrochemically. There are many initiators that have been used for the polymerization of pyrrole, including ferric chloride, silver nitrate, ammonium persulfate, potassium persulfate, and copper (II) chloride [49–60]. These oxidizing agents, however, dangerous as they can cause eye, nose, throat, lung, and skin irritation upon contact [56]. The persulfate salts also produce asthmatic symptoms. We developed polymerization techniques for the synthesis of PPY and PPY copolymers using the CDs both with and without UV-light. Copolymers of PPY were also prepared from various monomers containing various functional group using CDs as initiators. The CD surface has many unshared or unpaired electrons, making it negatively charged [61–63]. During the course of the reaction, the positively charged pyrrole is attracted to the carbon dots, prior to the polymerization reaction. The reflux method was used to synthesize the copolymer of polypyrrole within 24 h without UV light [64]. The UV light promoted free radical generation, replacing the use of thermal heating to accomplish the polymerization. The present article reports on the synthesis of PPY and its copolymers using CDs, as well as discussing the coating of the CD onto a glass slide that served as a substrate for the polymerization of PPY.

2. Experimental Section

2.1. Preparation of CDs

30 mL of PEG-400 was placed in a 50 mL beaker in an oil bath and heated to 70 ◦C. The tip of an ultrasonic transducer was immersed in the PEG-400 and sonication was carried out for 3 h at 65% amplitude [21,22].

2.2. CDs Coated on Glass Slides In parallel, CDs were produced and coated on glass slides. The process was identical to that described above except that the sonication beaker contained a glass slide and sonication was continued for 60 min at 25 ◦C in order to deposit the preformed CDs onto the glass slide. The glass slides were washed with water and dried in an oven at 50 ◦C for 1 h under ambient atmosphere. The free CDs observed on the slides were utilized for the synthesis of the polymer. The ultrasonic waves, and particularly the micro-jets formed after the collapse of the acoustic bubbles, caused the embedding of the CDs formed on the glass [44]. After the synthesis, the polymer was washed several times with PolymersPolymers 20192019,, 1111,, x 1240 FOR PEER REVIEW 33 of of 15 15

Polymers 2019, 11, x FOR PEER REVIEW 3 of 15 times with water and ethanol to remove unreacted CDs. The lifetime of the CDs containing free electronswater and is ethanol short and to removetheir quantity unreacted is much CDs. smaller The lifetime than the of theamount CDs containingof the polymer. free electronsSonochemically is short times with water and ethanol to remove unreacted CDs. The lifetime of the CDs containing free preparedand their CDs quantity initiated is much polymerization smaller than is thedisplays amount in Scheme of the polymer. 1. Sonochemically prepared CDs electrons is short and their quantity is much smaller than the amount of the polymer. Sonochemically initiated polymerization is displays in Scheme1. prepared CDs initiated polymerization is displays in Scheme 1.

Scheme 1. Sonochemically prepared carbon dots (CDs) for polymerization. Scheme 1. Sonochemically prepared carbon dots (CDs) for polymerization. Scheme 1. Sonochemically prepared carbon dots (CDs) for polymerization. 2.3. Synthesis of Polypyrrole and Its Copolymer by CDs 2.3. Synthesis of Polypyrrole and Its Copolymer by CDs 2.3. SynthesisPyrrole (1.0of Polypyrrole g) was mixed and Itsin 30Copolymer mL of 1 byM CDsnitric acid in a 100 mL beaker at room temperature. A 3 mL Pyrroleaqueous (1.0 solution g) was containing mixed in 309 mg mL of of CDs 1 M was nitric then acid added. in a 100A similar mL beaker process at roomwas carried temperature. out to Pyrrole (1.0 g) was mixed in 30 mL of 1 M nitric acid in a 100 mL beaker at room temperature. A prepareA 3 mL aqueouspolyaniline solution using containingCDs coated 9 onto mg ofa CDsglass wasslide then rather added. than Aa CD similar dispersion process in was water. carried To 3 mL aqueous solution containing 9 mg of CDs was then added. A similar process was carried out to activateout to prepare the reaction, polyaniline the solution using CDswas coatedkept under onto UV a glass light slide (USV-18 rather EL than series a CD UV dispersion lamp and in8-Watt, water. prepare polyaniline using CDs coated onto a glass slide rather than a CD dispersion in water. To 365To activatenm wavelength) the reaction, for the3 days, solution after was which kept we under observed UV light the precipitation (USV-18 EL series of blackish-brown UV lamp and 8-Watt,solids, activate the reaction, the solution was kept under UV light (USV-18 EL series UV lamp and 8-Watt, which365 nm we wavelength) collected by for filtration. 3 days, after These which solids we we observedre washed the several precipitation times ofwith blackish-brown distilled water solids, and 365 nm wavelength) for 3 days, after which we observed the precipitation of blackish-brown solids, driedwhich weat collectedroom temperature. by ﬁltration. TheseThe polypyrrole solids were washedwas also several synthesized times with distilledusing different water and molar dried which we collected by filtration. These solids were washed several times with distilled water and concentrationsat room temperature. of nitric The acid polypyrrole and pyrrole was also[44]. synthesized The Scheme using 2 shows diﬀerent the molar chemical concentrations reaction of of dried at room temperature. The polypyrrole was also synthesized using different molar polypyrrole.nitric acid and pyrrole [44]. The Scheme2 shows the chemical reaction of polypyrrole. concentrations of nitric acid and pyrrole [44]. The Scheme 2 shows the chemical reaction of polypyrrole.

Scheme 2. The chemical reaction of polypyrrole. Scheme 2. The chemical reaction of polypyrrole. 2.4. Poly(pyrrole-co-aniline) Scheme 2. The chemical reaction of polypyrrole. 2.4. Poly(pyrrole-co-aniline) Pyrrole (1.0 g) and aniline (1.0 g) were mixed in 30 mL of 1 M nitric acid in a 100 mL beaker 2.4.at roomPoly(pyrrole-co-aniline)Pyrrole temperature. (1.0 g) and Toaniline this, (1.0 5 mL g) ofwere an aqueousmixed in solution30 mL of containing 1 M nitric acid 15 mg in ofa 100 CDs mL was beaker added. at room temperature. To this, 5 mL of an aqueous solution containing 15 mg of CDs was added. The The polymerizationPyrrole (1.0 g) and reaction aniline was (1.0 stimulated g) were bymixed illumination in 30 mL with of 1 UVM nitric light foracid three in a days,100 mL after beaker which at a polymerization reaction was stimulated by illumination with UV light for three days, after which a roomblackish-brown temperature. solid To was this, obtained 5 mL of and an collected aqueous by solution ﬁltration, containing washed several15 mg timesof CDs with was distilled added. water, The blackish-brown solid was obtained and collected by filtration, washed several times with distilled polymerization reaction was stimulated by illumination with UV light for three days, after which a blackish-brown solid was obtained and collected by filtration, washed several times with distilled

Polymers 2019, 11, 1240 4 of 15 Polymers 2019, 11, x FOR PEER REVIEW 4 of 15 Polymers 2019, 11, x FOR PEER REVIEW 4 of 15 andwater, dried and at dried room temperatureat room temperature [44]. The chemical[44]. The reaction chemical of poly(pyrrole-co-aniline)reaction of poly(pyrrole-co-aniline) is illustrates inis water,Schemeillustrates and3. in dried Scheme at 3.room temperature [44]. The chemical reaction of poly(pyrrole-co-aniline) is illustrates in Scheme 3.

Scheme 3. The chemical reaction of poly(pyrrole-co-aniline). Scheme 3. TheThe chemical chemical reaction reaction of poly(pyrrole-co-aniline). 2.5. Synthesis of Poly (Bis(p-aminophenyl)ether-co-pyrrole) 2.5. Synthesis Synthesis of of Poly Poly (Bis(p (Bis(p-aminophenyl)ether-co-pyrrole)-aminophenyl)ether-co-pyrrole) Bis(p-aminophenyl) ether (0.6 g, 3 mmol), pyrrole (0.6 g, 10 mmol), CD solution (3 mL, 9 mg), Bis(p-aminophenyl) ether (0.6 g, 3 mmol), pyrrole (0.6 g, 10 mmol), CD solution (3 mL, 9 mg), acetonitrileBis(p-aminophenyl) (15 mL), nitric ether acid (1.5(0.6 mL),g, 3 mmol),and distilled pyrrole water (0.6 (10 g, mL)10 mmol), were mixed, CD solution after which (3 mL, the 9 colormg), acetonitrile (15 mL), nitric acid (1.5 mL), and distilled water (10 mL) were mixed, after which the color acetonitrileof the monomer (15 mL), changed nitric acid from (1.5 a whitemL), and solid distilled to a purple water (10solution. mL) were The mixed, reaction after mixture which wasthe colorthen of the monomer changed from a white solid to a purple solution. The reaction mixture was then mixed ofmixed the monomerwell and refluxedchanged infrom an oila whitebath forsolid 24 toh ata purple90 °C under solution. ambient The reactionenvironment. mixture The was reaction then well and refluxed in an oil bath for 24 h at 90 C under ambient environment. The reaction mixture was mixedmixture well was and then refluxed cooled into anroom oil bathtemperature for ◦24 h andat 90 po °Cured under into ambient a beaker environment. filled with ice.The The reaction dark then cooled to room temperature and poured into a beaker filled with ice. The dark brown solids that mixturebrown solids was thenthat precipitatedcooled to room were temperature collected by andfiltration, poured washed into a several beaker times filled with with distilled ice. The water, dark precipitated were collected by filtration, washed several times with distilled water, and dried at room brownand dried solids at room that precipitated temperature were [64]. collected The chemical by filtration, reaction washed of (Bis(p-amino several times phenyl) with ether-co-pyrrole) distilled water, temperature [64]. The chemical reaction of (Bis(p-amino phenyl) ether-co-pyrrole) is demonstrates in andis demonstrates dried at room in temperatureScheme 4. [64]. The chemical reaction of (Bis(p-amino phenyl) ether-co-pyrrole) Scheme4. is demonstrates in Scheme 4.

Scheme 4. The chemical reaction of Poly (Bis(p-aminophenyl) ether-co-pyrrole). Scheme 4. The chemical reaction of Poly (Bis(p-aminophenyl) ether-co-pyrrole). 3. Physical CharacterizationScheme 4. The chemical of CDs reaction as Initiator of Poly (Bis(p-aminophenyl) ether-co-pyrrole). 3. Physical Characterization of CDs as Initiator 3. PhysicalCharacterization Characterization of the of properties CDs as Initiator of the prepared CDs, used as an initiator for the diﬀerent polymerizationCharacterization processes, of the is properties provided below.of the prepared High-resolution CDs, used transmission as an initiator electron for the microscopy different Characterization of the properties of the prepared CDs, used as an initiator for the different (HR-TEM)polymerization images processes, of the CDs is provided revealed below. them to Hi begh-resolution spherical in transmission shape, with aelectron rather narrowmicroscopy size polymerization processes, is provided below. High-resolution transmission electron microscopy distribution(HR-TEM) images (Figure of1a). the The CDs average revealed size them of the to CDs be wasspherical about in ~10 shape, nm, whichwith a is rather in contrast narrow to oursize (HR-TEM) images of the CDs revealed them to be spherical in shape, with a rather narrow size previously-prepareddistribution (Figure 1a). CDs. The CDs average embedded size of on the the CDs glass was slide about are displayed ~10 nm, which in Figure is in1b. contrast The particles to our distribution (Figure 1a). The average size of the CDs was about ~10 nm, which is in contrast to our onpreviously-prepared the glass slide present CDs. a CDs larger embedded size range on (17–50 the glas nm)s slide than are those displayed formed inin theFigure solution. 1b. The This particles larger previously-preparedon the glass slide present CDs. a CDs larger embedded size range on (17–50 the glas nm)s slide than arethose displayed formed in theFigure solution. 1b. The This particles larger onsize the appears glass slide to be present due toa larger the sonochemical size range (17–50 coating nm) bombardingthan those fo rmedthe already-coated in the solution. surface This larger and size appears to be due to the sonochemical coating bombarding the already-coated surface and

Polymers 2019, 11, 1240 5 of 15

PolymersPolymers 20192019,, 1111,, xx FORFOR PEERPEER REVIEWREVIEW 55 ofof 1515 size appears to be due to the sonochemical coating bombarding the already-coated surface and adding CDsaddingadding to the CDsCDs ﬁrst toto thethe existing firstfirst existingexisting layer and layerlayer thus andand enlarging thusthus enenlarginglarging them. them.Thethem. detailed TheThe detaileddetailed characterizations characterizationscharacterizations of XRD, ofof XRD,XRD, EPR, ZetaEPR,EPR, potential ZetaZeta potentialpotential values, values,values, and XPS andand for XPSXPS CDs forfor were CDsCDs alreadywerewere alreadyalready provided providedprovided by our byby group ourour groupgroup [44]. [44].[44].

Figure 1. HRTEM images of (a) CDs that were separated from the PEG 400 suspension and (b) deposited FigureFigure 1.1. HRTEMHRTEM imagesimages ofof ((aa)) CDsCDs thatthat werewere separatedseparated fromfrom thethe PEGPEG 400400 suspensionsuspension andand ((bb)) onto a glass side. [44]. depositeddeposited ontoonto aa glassglass side.[44].side.[44]. Recently, we reported the formation of ultrafine CDs by means of ultrasonic cavitation in PEG-400. Recently,Recently, wewe reportedreported thethe formationformation ofof ultrafineultrafine CDsCDs byby meansmeans ofof ultrasonicultrasonic cavitationcavitation inin PEG-PEG- Characterization of the products was performed employing various methods. HRTEM images 400.400. CharacterizationCharacterization ofof thethe productsproducts waswas perfperformedormed employingemploying variousvarious methods.methods. HRTEMHRTEM imagesimages (Figure2a) of the CDs reveal them to possess a spherical shape with a narrow size distribution (~6 nm). (Figure(Figure 2a)2a) ofof thethe CDsCDs revealreveal themthem toto possesspossess aa spsphericalherical shapeshape withwith aa narrownarrow sizesize distributiondistribution (~6(~6 The fluorescence emission of the CDs found in the supernatant was spread over the 420–610 nm range nm).nm). TheThe fluorescencefluorescence emissionemission ofof thethe CDsCDs foundfound inin thethe supernatantsupernatant waswas spreadspread overover thethe 420–610420–610 nmnm (Figure1b), with the excitation wavelengths being between 330 and 490 nm. The d-spacing of CDs rangerange (Figure(Figure 1b),1b), withwith thethe excitationexcitation wavelengthwavelengthss beingbeing betweenbetween 330330 andand 490490 nm.nm. TheThe d-spacingd-spacing ofof was estimated from the HRTEM and selected area electron diffraction patterns and was found to be CDsCDs waswas estimatedestimated fromfrom thethe HRTEMHRTEM andand selectedselected areaarea electronelectron diffractiondiffraction patternspatterns andand waswas foundfound 0.21 nm. toto bebe 0.210.21 nm.nm.

FigureFigure 2.2.2. ((aa)) HRTEM HRTEM imagesimages ofof thethethe CDsCDsCDs (Inset:(Inset:(Inset: lattice lattice fringes fringes ofof CDs);CDs); ((bb)) ﬂuorescencefluorescencefluorescence spectraspectra atatat didifferentdifferentﬀerent excitationexcitationexcitation wavelengths wavelengthswavelengths [ [45].45[45].].

Figure3a presents images of a CD dispersion taken under ambient light and under irradiation FigureFigure 3a3a presentspresents imagesimages ofof aa CDCD dispersiondispersion takentaken underunder ambientambient lightlight andand underunder irradiationirradiation with a hand-held, long-wave TLC lamp (365 nm excitation), highlighting the strong, blue ﬂuorescence withwith aa hand-held,hand-held, long-wavelong-wave TLCTLC lamplamp (365(365 nmnm excitation),excitation), highlightinghighlighting thethe strong,strong, blueblue of the CD dispersion resulting from such excitation. The UV-Vis absorption spectra of the CDs are fluorescencefluorescence ofof thethe CDCD dispersiondispersion resultingresulting fromfrom susuchch excitation.excitation. TheThe UV-VisUV-Vis absorptionabsorption spectraspectra ofof displayed in Figure3b. The strong absorption bands of CDs at 246 and 360 nm are due to the π–π* thethe CDsCDs areare displayeddisplayed inin FigureFigure 3b.3b. TheThe strongstrong ababsorptionsorption bandsbands ofof CDsCDs atat 246246 andand 360360 nmnm areare duedue electron transition and graphitic nature of the carbon material, respectively. toto thethe ππ––ππ** electronelectron transitiontransition andand graphiticgraphitic naturenature ofof thethe carboncarbon material,material, respectively.respectively.

Polymers 2019, 11, 1240 6 of 15 Polymers 2019, 11, x FOR PEER REVIEW 6 of 15

Figure 3. Characterization of CD products: ( a)) Optical images of the CD dispersiondispersion underunder ambientambient light and under UV-light irradiation;irradiation; (b) UV-visible absorptionabsorption spectrumspectrum of thethe CDCD dispersion,dispersion, with two mainmain maximamaxima indicated indicated by by blue blue arrows; arrows; (c) HRTEM(c) HRTEM images images of the of CD; the and CD; (d )and Fluorescence (d) Fluorescence spectra ofspectra the CD of solutionthe CD solution with excitation with excitation under a under diversity a diversity of dissimilar of dissimilar wavelengths wavelengths [64]. [64].

A magnifiedmagnified HRTEM image (Figure3 3c,c, inset)inset) ofof individualindividual CDsCDs demonstratesdemonstrates thatthat thethe latticelattice plane of CDs is 0.21 nm and ties with the graphite facet (100), in agreement with previous reports on CD microscopic images [[26].26]. The fluorescence fluorescence emis emissionsion spectra were recorded at various excitation wavelengths (330–470 nm) and maximum emission from such excitations were observed in the range of 420–600 nm. Figure 33dd displaysdisplays thethe typicaltypical fluorescencefluorescence spectraspectra ofof CDsCDs producedproduced fromfrom PEG-400PEG-400 following the sonication treatment.treatment. 3.1. Polypyrrole 3.1. Polypyrrole The formation of polypyrrole was confirmed by XRD, FT-IR and TGA (Figure4). The X-ray The formation of polypyrrole was confirmed by XRD, FT-IR and TGA (Figure 4). The X-ray diffraction of polypyrrole displayed in Figure4a, a broadband around 2 θ = 13 –45 due to being diffraction of polypyrrole displayed in Figure 4a, a broadband around 2θ = 13°–45°◦ ◦ due to being parallel to the polypyrrole structure and perpendicular to the polymer chain. The XRD pattern reveals parallel to the polypyrrole structure and perpendicular to the polymer chain. The XRD pattern the synthesized polypyrrole to be amorphous in nature. The FT-IR spectrum of polypyrrole shows in reveals the synthesized polypyrrole to be amorphous in nature. The FT-IR spectrum of polypyrrole Figure4b. The FT-IR spectrum reveals a broadband around 3261 cm 1 due to N-H stretching vibration. shows in Figure 4b. The FT-IR spectrum reveals a broadband around− 3261 cm−1 due to N-H stretching The peak at 1574 cm 1 is due to the C=C in-ring stretching vibration and the band at 1130 cm 1 is vibration. The peak at− 1574 cm−1 is due to the C=C in-ring stretching vibration and the band at 1130− due to the C–N stretching vibration. The bands at 1308 and 1078 cm 1 are due to the C–H in-plane cm−1 is due to the C–N stretching vibration. The bands at 1308 and 1078− cm−1 are due to the C–H in- vibration. The thermal stability of the synthesized polypyrrole is provided in Figure4c. The TGA plane vibration. The thermal stability of the synthesized polypyrrole is provided in Figure 4c. The analysis displays three stages of weight loss: the first stage is minor and the second and third are major TGA analysis displays three stages of weight loss: the first stage is minor and the second and third weight loss [44]. are major weight loss [44].

Polymers 2019, 11, 1240 7 of 15 PolymersPolymers 2019 2019, 11, 11, x, x FOR FOR PEER PEER REVIEWREVIEW 7 of 715 of 15

(a) (b) 100 (a) (b) 100 (c)(c) 90 90 80 80 70

60 60 Intensity(a.u.) Weight(%)loss 50 Intensity(a.u.) Transmittance (%) Weight(%)loss 50 Transmittance (%) 40 40 10 20 30 40 50 60 70 80 90 4000 3500 3000 2500 2000 1500 1000 100 200 300 400 500 600 700 800 900 -1 o 10 20 30 402θ (degree) 50 60 70 80 90 4000 3500Wavenumber 3000 2500 2000 (cm 1500) 1000 100Temperature 200 300 400 500 ( C) 600 700 800 900 2θ (degree) Wavenumber (cm-1) Temperature (oC)

Figure 4. (a) X-ray diffractogram and (b) FT-IR of polypyrrole obtained from the polymerization of Figure 4. (a) X-ray diffractogram and (b) FT-IR of polypyrrole obtained from the polymerization of Figurepyrrole, 4. and(a) X-ray (c) TGA diffractogram of polypyrrole. and (b) FT-IR of polypyrrole obtained from the polymerization of pyrrole,pyrrole, and and ( cc)) TGA TGA of of polypyrrole. polypyrrole. The particle size distribution of the polypyrrole was measured by dynamic light scattering The particle size distribution of the polypyrrole was measured by dynamic light scattering (DLS). (DLS).The Two particle different size populationsdistribution of of particle the polypy size distributionrrole was weremeasured found: by 75% dynamic of the particles light scattering were Two different populations of particle size distribution were found: 75% of the particles were micron-size, (DLS).micron-size, Two different ca. 1000–1600 populations nm; and of particle25% were size nano-size, distribution ca. 600–900were found: nm. The75% SEM of the image particles shows were µ micron-size,ca.particles 1000–1600 of 2–7ca. nm; μ1000–1600m. and The 25% difference werenm; and nano-size, between 25% were ca.the 600–900 SEMnano-size, and nm. the ca. The DLS 600–900 SEM results image nm.from shows Thethe largeSEM particles micron image of size 2–7shows m. particlesTheparticles difference of in 2–7 the between μSEMm. The being thedifference due SEM to the and between aggregation the DLS the results SEMof particles. and from the the A DLScareful large results micronlook atfrom Figure size the particles 5b large reveals micron in theeven SEMsize particlesbeingsmaller due inparticles. tothe the SEM aggregation being due of to particles. the aggregation A careful of particles. look at Figure A careful5b reveals look at even Figure smaller 5b reveals particles. even smaller particles.

FigureFigure 5. ((aa,,bb)) SEM SEM micrograms micrograms of of polypyrrole polypyrrole under under different diﬀerent magnification. magniﬁcation.

InIn order order to Figureto conﬁrm confirm 5. successful(a, bsucce) SEMssful micrograms polymerization, polymerization, of polypyrrole the 13theC solid-state 13underC solid-state different NMR NMRmagnification. was analyzed.was analyzed. The spectrum The (Figurespectrum6) is (Figure displays 6) is four displays broad four bands broad at 110,bands 122, at 110, 126, 122, 129 ppm126, 129 and ppm one and shoulder one shoulder peak at peak 142 ppm.at The142In peaksppm. order The at 122to peaks confirm ppm at and122 succe ppm 126ssful areand assigned 126polymerization, are assigned to carbon to the carbon C-1 13C and solid-stateC-1 C-2, and respectively, C-2, NMR respectively, was while analyzed. while the peakthe The at peak at 110 ppm originates from protonated C-3 and non-protonated C-7 carbon of the quinoid ring spectrum110 ppm originates(Figure 6) is from displays protonated four broad C-3and bands non-protonated at 110, 122, 126, C-7 129 carbon ppm and of the one quinoid shoulder ring peak in the at in the polypyrrole chain. The peak around 129 ppm is attributed to the protonated C-6 and C-4 and 142polypyrrole ppm. The chain. peaks Theat 122 peak ppm around and 126 129 are ppm assigned is attributed to carbon to theC-1 protonatedand C-2, respectively, C-6 and C-4 while and the the the non-protonated C-4 carbons. peaknon-protonated at 110 ppm C-4originates carbons. from protonated C-3 and non-protonated C-7 carbon of the quinoid ring in the polypyrrole chain. The peak around 129 ppm is attributed to the protonated C-6 and C-4 and the non-protonated C-4 carbons.

Polymers 2019, 11, 1240 8 of 15 Polymers 2019, 11, x FOR PEER REVIEW 8 of 15

Figure 6. Solid state 13C-NMR spectrum of polypyrrole.

13 3.2. Poly(pyrrole-co-aniline) Figure 6. Solid state C-NMR spectrum of polypyrrole.

3.2.X-ray Poly(pyrrole-co-aniline) diffractograms of poly(pyrrole-co-aniline) are given in Figure7a. A broad peak around 2θ = 18 to 40 degrees, indicating the formation of the repeated unit of both the monomers in the X-ray diffractograms of poly(pyrrole-co-aniline) are given in Figure 7a. A broad peak around 2θ polymer chain, demonstrates that the polymer structure is highly associated. The broad peak is = 18 to 40 degrees, indicating the formation of the repeated unit of both the monomers in the polymer evidence that the formation of poly(pyrrole-co-aniline) is amorphous in nature. The IR spectrum chain, demonstrates that the polymer structure is highly associated. The broad peak is evidence that1 for poly(pyrrole-co-aniline) is provided in Figure7b. The FT-IR shows a broadband at 3255 cm − , the formation of poly(pyrrole-co-aniline) is amorphous in nature. The IR spectrum for poly(pyrrole- corresponding to the N–H stretching vibration. The aromatic C–H stretching vibration band appears −1 co-aniline)1 is provided in Figure 7b. The FT-IR shows1 a broadband at 3255 cm , corresponding to the at 2984 cm− and the peaks at 1572 and 1486 cm− are attributed to the C=C stretching mode of the N–H stretching vibration. The aromatic C–H stretching vibration band appears at 2984 cm−1 and the benzenoid and quinoid rings. The peak at about 1170 cm 1 is due to the C–N stretching vibration and −1 − peaks at 1572 and 14861 cm are attributed to the C=C stretching mode of the benzenoid and quinoid the peak at 1190 cm− corresponds to the N=Quinoid-N+ stretching vibration, respectively [44]. rings. The peak at about 1170 cm−1 is due to the C–N stretching vibration and the peak at 1190 cm−1 TGA of the Poly(pyrrole-co-aniline) shows a three-step weight-loss performance in Figure7c. corresponds to the N=Quinoid-N+ stretching vibration, respectively [44]. The first major weight loss of 5% occurs within the temperature range of 70–120 C and is due to the TGA of the Poly(pyrrole-co-aniline) shows a three-step weight-loss performance◦ in Figure 7c. removal of moisture or loss water from the polymer; the second major weight loss of around 32% The first major weight loss of 5% occurs within the temperature range of 70–120 °C and is due to the occurs at around 212–490 C and is due to with removal of small oligomers; the third major weight loss removal of moisture or◦ loss water from the polymer; the second major weight loss of around 32% is 22% within a range of 455–720 C, and is due to the decomposition of the polymer. The morphology occurs at around 212–490 °C and◦ is due to with removal of small oligomers; the third major weight of the poly(pyrrole-co-aniline) was examined by SEM. Figure7e shows that poly(pyrrole-co-aniline) loss is 22% within a range of 455–720 °C, and is due to the decomposition of the polymer. The is formed as irregular shapes, ca. 400–500 nm, while the 10-micron size particles in the SEM are morphology of the poly(pyrrole-co-aniline) was examined by SEM. Figure 7e shows that due to the aggregation of the particles. Confirmation of the poly(pyrrole-co-aniline) product was poly(pyrrole-co-aniline) is formed as irregular shapes, ca. 400–500 nm, while the 10-micron size established using 13C solid-state NMR in a cross-polarization magic angle-spinning mode. The 13C particles in the SEM are due to the aggregation of the particles. Confirmation of the poly(pyrrole-co- solid-state NMR spectrum for the poly(pyrrole-co-aniline) is given in Figure7d. It comprises a broad aniline) product was established using 13C solid-state NMR in a cross-polarization magic angle- peak, centered at 127 ppm, and companied by a few peaks that appear as shoulders The spectral spinning mode. The 13C solid-state NMR spectrum for the poly(pyrrole-co-aniline) is given in Figure features all correspond to the aromatic carbons of the poly(pyrrole-co-aniline) backbone. The spectrum 7d. It comprises a broad peak, centered at 127 ppm, and companied by a few peaks that appear as demonstrates six broad resonances which can be observed at 116 ppm (shoulder), 138.0, 146.1, 154.5, shoulders The spectral features all correspond to the aromatic carbons of the poly(pyrrole-co-aniline) and 164.0 ppm. The peak at 164 ppm originates from protonated and non-protonated carbons of the backbone. The spectrum demonstrates six broad resonances which can be observed at 116 ppm quinoid part of the poly(pyrrole-co-aniline) structure. (shoulder), 138.0, 146.1, 154.5, and 164.0 ppm. The peak at 164 ppm originates from protonated and non-protonated carbons of the quinoid part of the poly(pyrrole-co-aniline) structure.

Polymers 2019, 11, 1240 9 of 15 Polymers 2019, 11, x FOR PEER REVIEW 9 of 15

FigureFigure 7. 7.( a()a) X-ray X-ray di diffractogramﬀractogram and and (b )(b FT-IR) FT-IR of polypyrroleof polypyrrole obtained obtained from from the the polymerization polymerization of (c of) TGA(c) TGA of polypyrrole, of polypyrrole, (d) solid (d) statesolid13 stateC-NMR 13C-NMR spectrum, spectrum, and (e) and SEM (e image) SEM of image poly(pyrrole-co-aniline). of poly(pyrrole-co- aniline).

Polymers 2019, 11, 1240 10 of 15

Polymers 2019, 11, x FOR PEER REVIEW 10 of 15 3.3. Poly (Bis(p-aminophenyl)ether-co-pyrrole) 3.3. Poly (Bis(p-aminophenyl)ether-co-pyrrole) The FTIR spectrum poly (Bis(p-aminophenyl)ether-co-pyrrole) is displayed in Figure8a as follows: The FTIR spectrum1 poly (Bis(p-aminophenyl)ether-co-pyrrole) is displayed in Figure 8a as broad peak at 3351 cm− is assigned to the N-H stretching vibration; an aromatic C=C–H stretching −1 follows: broad peak 1at 3351 cm is assigned to the 1N-H stretching vibration; an aromatic C=C–H vibration at 3074 cm− ; peaks at 1605 and 1501 cm− , due to the aromatic C=C stretching vibration; stretching vibration at 3074 cm−1; peaks at 1605 and 1501 cm−1, due to the aromatic C=C stretching peaks around 1340 cm 1 which are due to the C–N stretching vibration; a peak at 1246 cm 1, is due to vibration; peaks around− 1340 cm−1 which are due to the C–N stretching vibration; a peak at 1246− cm−1, the ether C–O stretching vibration; and a peak at 830 cm 1, is assigned to the out-of-plane bending is due to the ether C–O stretching vibration; and a peak at− 830 cm−1, is assigned to the out-of-plane vibration of an amine group. A summary of all signiﬁcant IR peaks and the corresponding structural bending vibration of an amine group. A summary of all significant IR peaks and the corresponding evidencestructural is givenevidence in Tableis given1[64 in]. Table 1 [64].

FigureFigure 8. 8.( a(a)) FT-IR- FT-IR- spectrum,spectrum, ((b) TGA analysis, analysis, and and (c ()c )X-ray X-ray diffractogram diﬀractogram of ofPoly Poly (Bis(p-amino (Bis(p-amino phenyl)ether-co-pyrrole).phenyl)ether-co-pyrrole).

TableTable 1. 1.Key Key peaks peaks in thein the IR absorptionIR absorption frequency frequency region region of Poly of (Bis(p-aminophenyl)ether-co-pyrrole)Poly (Bis(p-aminophenyl)ether-co- andpyrrole) the corresponding and the correspondi functionalng functional groups. groups.

PolyPoly (Bis(p-aminophenyl)ether-co-pyrrole) (Bis(p-aminophenyl)ether-co-pyrrole) MajorMajor Functional Functional Group Group Absorption Frequency Region (cm−1) Absorption Frequency Region (cm 1) 3351 − N–H (stretching vibration) 33513074 C=C–H (aromatic N–H (stretching stretching vibration) vibration) 16053074 and 1501 C=C–H C=C (stretching (aromatic stretchingvibration) vibration) 1605 and1340 1501 C–N C =(stretchingC (stretching vibration) vibration) 13401246 C–O ether C–N (stretching (stretching vibration) vibration) 1246830 out of plane C–O etherbending (stretching vibration vibration) of amine 830 out of plane bending vibration of amine The TGA weight loss of Bis(p-aminophenyl) ether exhibited three dissimilar weight loss stages (FigureThe TGA8b): (1) weight Minor loss weight of Bis(p-aminophenyl) loss (4%) at 97–135 °C, ether due exhibited to the evaporation three dissimilar of moisture weight and loss water stages (Figuremolecules;8b): (1)(2) Minor12% weight weight loss loss between (4%) at 468–664 97–135 °C,◦C, attributed due to the to the evaporation thermal removal of moisture of oligomers; and water and (3) final weight loss above 721 °C due to the thermal decomposition of the polymer. molecules; (2) 12% weight loss between 468–664 ◦C, attributed to the thermal removal of oligomers; Further demonstration of the crystalline nature of the successful polymer was achieved by XRD and (3) ﬁnal weight loss above 721 ◦C due to the thermal decomposition of the polymer. analysis,Further with demonstration the XRD spectrum of the of crystalline Poly (Bis(p-ami naturenophenyl) of the successful ether-co-pyrrole) polymer given was achievedin Figure 8c. by A XRD analysis,broad peak with around the XRD 2θ spectrum = 13.4°–26.9° of Poly can (Bis(p-aminophenyl) be ascribed to the repeated ether-co-pyrrole) units of pyrrole. given inThe c. AXRD broad spectrum shows that the copolymer is amorphous in nature, as a highly crystalline polymer would peak around 2θ = 13.4◦–26.9◦ can be ascribed to the repeated units of pyrrole. The XRD spectrum display much sharper peaks. A morphological study of the solid-state polymer was conducted using shows that the copolymer is amorphous in nature, as a highly crystalline polymer would display scanning electron microscopy (SEM), with SEM images of Poly (Bis(p-aminophenyl) ether-co- much sharper peaks. A morphological study of the solid-state polymer was conducted using scanning pyrrole) given in Figure 9a. The SEM image of Poly (Bis(p-aminophenyl)ether-co-pyrrole) reveals electron microscopy (SEM), with SEM images of Poly (Bis(p-aminophenyl) ether-co-pyrrole) given agglomerated particles with irregular spherical shapes and a broad range of diameters between 800 inand Figure 15009 a.nm. The SEM image of Poly (Bis(p-aminophenyl)ether-co-pyrrole) reveals agglomerated particles with irregular spherical shapes and a broad range of diameters between 800 and 1500 nm.

Polymers 2019, 11, 1240x FOR PEER REVIEW 11 of 15 Polymers 2019, 11, x FOR PEER REVIEW 11 of 15

Figure 9. ((aa)) SEM SEM image image and and ( (bb)) DLS DLS measurement measurement of of Poly (Bis(p-aminophenyl)ether-co-pyrrole). Figure 9. (a) SEM image and (b) DLS measurement of Poly (Bis(p-aminophenyl)ether-co-pyrrole). The particle size distribution of the Poly (Bis(p-aminophenyl)ether-co-pyrrole) was analyzed by The particle size distribution of the Poly (Bis(p-aminophenyl)ether-co-pyrrole) was analyzed by dynamicThe particle light scattering size distribution (Figure9a). of Thethe Poly particle (Bis(p-aminophenyl)ether-co-pyrrole) size diameters ranged from 600 to 2010 was nm. analyzed The DLS by dynamic light scattering (Figure 9a). The particle size diameters ranged from 600 to 2010 nm. The dynamicanalysis revealedlight scattering the copolymer (Figure of 9a). Poly The (Bis(p-aminophenyl)ether-co-pyrrole) particle size diameters ranged from to possess600 to 2010 a macro-nano nm. The DLS analysis revealed the copolymer of Poly (Bis(p-aminophenyl)ether-co-pyrrole) to possess a DLSparticle analysis size distribution. revealed the The copolymer polydispersity of Poly of (Bis(p-aminophenyl)ether-co-pyrrole) the copolymer with dispersity indices to of possess less than a macro-nano particle size distribution. The polydispersity of the copolymer with dispersity indices of macro-nano1.0, evidences particle the particulate size distribution. nature of The the polydisper polymers andsity stronglyof the copolymer signiﬁes thewith good dispersity processability indices of of less than 1.0, evidences the particulate nature of the polymers and strongly signifies the good lesscopolymer than 1.0, dispersions evidences [59 the]. particulate nature of the polymers and strongly signifies the good processability of copolymer dispersions [59]. processabilityThe successful of copo copolymerizationlymer dispersions was [59]. conﬁrmed by 13C solid-state NMR. The solid-state NMR The successful copolymerization was confirmed by 13C solid-state NMR. The solid-state NMR spectrumThe successful Poly (Bis(p-aminophenyl) copolymerization ether-co-pyrrole) was confirmed by (Figure 13C solid-state 10) displays NMR. a peak The “a”solid-state at 153.3 NMR ppm spectrum Poly (Bis(p-aminophenyl) ether-co-pyrrole) (Figure 10) displays a peak “a” at 153.3 ppm spectrumdue to the Poly carbon (Bis(p-aminophenyl) singly-bound to oxygen:ether-co-pyrrole) and a peak (Figure “b” at10) 144.1 displays ppm a attributed peak “a” toat the153.3 carbon ppm due to the carbon singly-bound to oxygen: and a peak “b” at 144.1 ppm attributed to the carbon duesingly-bound to the carbon to nitrogen. singly-bound The peak to “c”oxygen: at 136 and ppm a ispeak due to“b” the at carbon, 144.1 ppm which attributed is part of to the the C–N carbon bond singly-bound to nitrogen. The peak “c” at 136 ppm is due to the carbon, which is part of the C–N singly-boundbetween the Bis(p-aminophenyl) to nitrogen. The peak ether “c” subunit at 136 andppm the is due pyrrole to the ring. carbon, Peaks which “d” and is “g”,part atof 123.5the C–N and bond between the Bis(p-aminophenyl) ether subunit and the pyrrole ring. Peaks “d” and “g”, at 123.5 bond106.3 between ppm, are the assigned Bis(p-aminophenyl) to the aromatic ether pyrrole subunit carbons. and the Finally, pyrrole peaks ring. “e” Peaks and “d” “f” and indicate “g”, thatat 123.5 the and 106.3 ppm, are assigned to the aromatic pyrrole carbons. Finally, peaks “e” and “f” indicate that andpeaks 106.3 at 121.1 ppm, and are 118.3 assigned ppm to are the attributed aromatic to pyrrole the aromatic carbons. carbons Finally, of Bis(p-aminophenyl)peaks “e” and “f” indicate ether in that the the peaks at 121.1 and 118.3 ppm are attributed to the aromatic carbons of Bis(p-aminophenyl) ether thecopolymer peaks at unit. 121.1 and 118.3 ppm are attributed to the aromatic carbons of Bis(p-aminophenyl) ether in the copolymer unit. in the copolymer unit.

Figure 10. Solid-state 13C-NMRC-NMR spectrum spectrum of of Poly Poly (Bis(p-a (Bis(p-aminophenyl)minophenyl) ether-co-pyrrole). ether-co-pyrrole). Figure 10. Solid-state 13C-NMR spectrum of Poly (Bis(p-aminophenyl) ether-co-pyrrole). 4. Mechanism for Polymerization 4. Mechanism for Polymerization The polymerization mechanism is supported by CDs and UV light. The dots have an unshared The polymerization mechanism is supported by CDs and UV light. The dots have an unshared pair of electrons on their surface and are carry negative charges [21,31,61,63]. The amine group of the pair of electrons on their surface and are carry negative charges [21,31,61,63]. The amine group of the

Polymers 2019, 11, 1240 12 of 15

4. Mechanism for Polymerization Polymers 2019, 11, x FOR PEER REVIEW 12 of 15 The polymerization mechanism is supported by CDs and UV light. The dots have an unshared anilinepair of is electrons transformed on their into surface the protonated and are carry form negative by the nitric charges acid. [21 The,31,61 reason,63]. The for aminechoosing group the ofhigh the concentrationaniline is transformed of nitric intoacid the (1–4 protonated M) is that form it bycan the efficiently nitric acid. react The with reason base, for choosingforming thePhenyl high ammoniumconcentration ion, of nitric(C6H5 acidNH3 (1–4+) at M) room is that temperature, it can efficiently this reaction can with easily base, formingreact with Phenyl CDs. ammonium Another argumention, (C6H 5isNH that3+ )the at roomhigh temperature,concentration thisof the ion acid can easilycan accelerate react with both CDs. the Another oxidation argument and the is acidificationthat the high of concentration the amine group. of the acidBoth can influence accelerate thermodynamic both the oxidation and and kinetics, the acidification favor the ofhigh the concentrationamine group. of Both the acid. influence Electron thermodynamic negative charges and on kinetics, the CDs favor are adsorbed the high concentrationto the positively of charged the acid. ofElectron the aniline negative cations, charges through on electrostatic the CDs are interactions adsorbed to. The the positivelyrole of the UV charged light ofin thethis anilinereaction cations, is the activationthrough electrostatic of aniline, which interactions. enables The more role aniline of the UVmolecules light in to this form reaction the protonated is the activation species. of This aniline, is wellwhich known enables and more reported aniline recently molecules in tothe form literatur the protonatede [57]. Carbon species. dots This were is wellshown known to produce and reported OH radicalsrecently in in water. the literature The OH [radica57]. Carbonls remove dots one were hydrogen, shown toforming produce H2O. OH A radicalsradical NH in water.2+ group The is left OH onradicals the aromatic remove group. one hydrogen, In the next forming stage, Ha 2protonO. A radical is removed NH2+ whengroup two is left of onthese the radicals aromatic interact group. leadingIn the next to the stage, formation a proton of isthe removed polymer. when Thus two the of oxidizing these radicals agent interactis the OH leading radical. to The the formationUV light assistsof the in polymer. the formation Thus theof the oxidizing OH radicals agent [22]. is the The OH formation radical. Theof more UV lightfree radical assists is in caused the formation by the photoexcitationof the OH radicals of CDs. [22]. The The mechanism formation of for more the freeform radicalation of is polyaniline caused by theby carbon photoexcitation dot is shows of CDs. in SchemeThe mechanism 5. for the formation of polyaniline by carbon dot is shows in Scheme5.

Scheme 5. Schematic representation of the electrostatic interaction between the protonated aniline and Schemethe CDs. 5. Schematic representation of the electrostatic interaction between the protonated aniline and the CDs. 5. Conclusions 5. ConclusionsThis short report provides in-depth data on the polymerization of pyrrole and its copolymer, usingThis carbon short dots report as anprovides initiator. in-depth The disadvantages data on the andpolymerization limitations in of using pyrrole various and its other copolymer, initiators usingassociated carbon with dots the as polymerizationan initiator. The ofdisadvantage pyrrole ares noted, and limitations and how thein using use of various CDs overcomes other initiators these associatedproblems iswith highlighted the polymerization in this report. of pyrrole In addition, are noted, we note and thathow both the use polypyrrole of CDs overcomes and copolymers these problemswere easily is synthesizedhighlighted usingin this only report. CDs In as additi an initiator,on, we making note that the both polymerization polypyrrole low and cost, copolymers non-toxic, wereand requiringeasily synthesized an easy one-step using preparationonly CDs as method. an initiator, CDs aremaking recognized the polymerization as an effective andlow economicalcost, non- toxic,initiator and at requiring low concentration an easy one-step and can preparation be used for method. polymer CDs synthesis. are recognized The synthesized as an effective conductive and economicalpolypyrrole initiator produced at low using concentration CDs has also and been can used be used in applications for polymer such synt ashesis. in adsorbent The synthesized material conductivefor organic polypyrrole dyes due to produced its recyclable using CDs capacity, has also morphology, been used andin applications stability. Amongsuch as in the adsorbent available materialpolymeric for adsorbent organic dyes materials, due to the its CD-initiated recyclable capacity, polypyrrole morphology, and copolymer and macro-nanostability. Among materials the availablehave been polymeric widely explored adsorbent as ematerials,fficient adsorbent the CD-initiated materials polypyrrole for the removal and ofcopolymer different organicmacro-nano dyes materialsfrom aqueous have sources.been widely This propertyexplored has as beenefficient attributed adsorbent to the materi presenceals for of highlythe removal active of surface different sites organicin the macro-nano dyes from aqueous adsorbents. sources. In order This toproperty improve has the been performance attributed ofto the CD-initiatedpresence of highly polypyrrole, active surfaceseveral sites metal in oxidesthe macro-nano were used adsorbents. to form composites In order to with improve PPY, the especially performance for biological of the CD-initiated application. polypyrrole,The CD-initiated several polypyrrole metal oxides was were composited, used to forform example, composites with with CuO PPY, anddemonstrated especially for anbiological efficient application.antibacterial The eff CD-initiatedect against both polypyrroleE. coli and wasS. comp aureusosited,. The for present example, report with thusCuO introducesand demonstrated a novel anpolymerization efficient antibacterial method effect and demonstrates against both itsE. coli advantages and S. aureus for PPY. The and present its copolymers. report thus introduces a novel polymerization method and demonstrates its advantages for PPY and its copolymers. Funding: This research received no external funding. Funding:Conflicts This of Interest: researchThe received authors no declare external no funding. conflict of interest.

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Wang, Y.; Zhu, Y.; Yu, S.; Jiang, C. RSC Advances optical properties and analytical applications. RSC Adv. 2017, 7, 40973–40989. 2. Wang, Y.; Hu, A. Carbon quantum dots : Synthesis, properties and applications. J. Mater. Chem. C 2014, 2, 6921–6939.

Polymers 2019, 11, 1240 13 of 15

References

1. Wang, Y.; Zhu, Y.; Yu, S.; Jiang, C. RSC Advances optical properties and analytical applications. RSC Adv. 2017, 7, 40973–40989. [CrossRef] 2. Wang, Y.; Hu, A. Carbon quantum dots: Synthesis, properties and applications. J. Mater. Chem. C 2014, 2, 6921–6939. [CrossRef] 3. Li, Y.; Shu, H.; Niu, X.; Wang, J. Electronic and Optical Properties of Edge-Functionalized Graphene Quantum Dots and the Underlying Mechanism. J. Phys. Chem. C 2015, 119, 24950–24957. [CrossRef] 4. Pal, A.; Chattopadhyay, A. Conducting Carbon Dot—Polypyrrole Nanocomposite for Sensitive Detection of Picric acid. ACS Appl. Mater. Interfaces 2016, 8, 5758–5762. [CrossRef] 5. Sun, X.; Lei, Y. Trends in Analytical Chemistry. Trends Anal. Chem. 2017, 89, 163–180. [CrossRef] 6. Wang, J.; Zhang, P.; Huang, C.; Liu, G.; Leung, K.C.; Wa, J. High Performance Photoluminescent Carbon Dots for In Vitro and In Vivo Bioimaging: Effect of Nitrogen Doping Ratios. Langmuir 2015, 31, 8063–8073. [CrossRef] 7. Wang, D.; Xu, H.; Zheng, B.; Li, Y.; Liu, M.; Xiao, D. Analytical Methods selectivity for hypochlorous acid detection and its. Anal. Methods 2015, 7, 5311–5317. [CrossRef] 8. Han, C.; Wang, R.; Wang, K.; Xu, H.; Sui, M.; Li, J.; Xu, K. Biosensors and Bioelectronics Highly fl uorescent carbon dots as selective and sensitive “on-off-on” probes for iron (III) ion and apoferritin detection and imaging in living cells. Biosens. Bioelectron. 2016, 83, 229–236. [CrossRef] 9. Kumara, K.; Datta, R.; Qi, G.; Zboril, R.; Giannelis, E.P. Yellow emitting carbon dots with superior colloidal, thermal, and photochemical stabilities. J. Mater. Chem. C 2016, 4, 9798–9803. 10. Zhou, Y.; Sharma, S.K.; Peng, Z.; Leblanc, R.M. Polymers in Carbon Dots: A Review. Polymers 2017, 9, 67. [CrossRef] 11. Karasik, D. Accelerated Bone Regeneration by Nitrogen-Doped Carbon Dots Functionalized with Hydroxyapatite Nanoparticles. ACS Appl. Mater. Interfaces 2018, 10, 19373–19385. 12. Abu-ghosh, S.; Bhooshan, V.; Fixler, D.; Dubinsky, Z.; Gedanken, A.; Iluz, D. Nitrogen-doped carbon dots prepared from bovine serum albumin to enhance algal astaxanthin production. Algal Res. 2017, 23, 161–165. [CrossRef] 13. Li, H.; Kong, W.; Liu, J.; Liu, N.; Huang, H.; Liu, Y.; Kang, Z. Fluorescent N-doped carbon dots for both cellular imaging and highly-sensitive catechol detection. Carbon 2015, 91, 66–75. [CrossRef] 14. Jiang, K.; Sun, S.; Zhang, L.; Wang, Y.; Cai, C.; Lin, H. Bright-Yellow-Emissive N-Doped Carbon Dots: Preparation, Cellular Imaging, and Bifunctional Sensing. ACS Appl. Mater. Interfaces 2015, 7, 23231–23238. [CrossRef][PubMed] 15. Qian, Z.; Ma, J.; Shan, X.; Feng, H.; Shao, L. Highly Luminescent N-Doped Carbon Quantum Dots as an Effective Multifunctional Fluorescence Sensing Platform. Chem. Eur. J. 2014, 20, 2254–2263. [CrossRef] [PubMed] 16. Khajuria, D.K.; Kumar, V.B.; Karasik, D.; Gedanken, A. Fluorescent Nanoparticles with Tissue-Dependent A ffi nity for Live Zebrafish Imaging. ACS Appl. Mater. Interfaces 2017, 9, 18557–18565. [CrossRef] 17. López, C.; Zougagh, M.; Algarra, M.; Rodríguez-Castellón, E.; Campos, B.B.; Esteves Da Silva, J.C.G.; Jiménez-Jiménez, J.; Ríos, A. Microwave-assisted synthesis of carbon dots and its potential as analysis of four heterocyclic aromatic amines. Talanta 2015, 132, 845–850. [CrossRef] 18. Lim, S.Y.; Shen, W.; Gao, Z. Carbon quantum dots and their applications. Chem. Soc. Rev. 2015, 44, 362–381. [CrossRef] 19. Liu, Y.; Liu, C.Y.; Zhang, Z.Y. Graphitized carbon dots emitting strong green photoluminescence. J. Mater. Chem. A 2013, 1, 4902–4907. [CrossRef] 20. Hola, K.; Zhang, Y.; Wang, Y.; Giannelis, E.P.; Zboril, R.; Rogach, A.L. Carbon dots—Emerging light emitters for bioimaging, cancer therapy and optoelectronics. Nano Today 2014, 9, 590–603. [CrossRef] 21. Bhooshan, V.; Gedanken, A. Facile one-step sonochemical synthesis of ultrafine and stable fluorescent C-dots. Ultrason. Sonochem. 2016, 28, 367–375. 22. Kumar, V.B.; Perelshtein, I.; Lipovsky, A.; Porat, Z.; Gedanken, A. The sonochemical synthesis of Ga@C-dots particles. RSC Adv. 2015, 5, 25533–25540. [CrossRef] 23. Innocenzi, P.; Malfatti, L.; Carboni, D. Graphene and carbon nanodots in mesoporous materials: An interactive platform for functional applications. Nanoscale 2015, 7, 12759–12772. [CrossRef][PubMed] Polymers 2019, 11, 1240 14 of 15

24. Fu, M.; Ehrat, F.; Wang, Y.; Milowska, K.Z.; Reckmeier, C.; Rogach, A.L.; Stolarczyk, J.K.; Urban, A.S.; Feldmann, J. Carbon Dots: A Unique Fluorescent Cocktail of Polycyclic Aromatic Hydrocarbons. Nano Lett. 2015, 15, 6030–6035. [CrossRef][PubMed] 25. Khan, S.; Gupta, A.; Verma, N.C.; Nandi, C.K. Time-Resolved Emission Reveals Ensemble of Emissive States as the Origin of Multicolor Fluorescence in Carbon Dots. Nano Lett. 2015, 15, 8300–8305. [CrossRef] [PubMed] 26. Zheng, X.T.; Ananthanarayanan, A.; Luo, K.Q.; Chen, P. Glowing Graphene Quantum Dots and Carbon Dots: Properties, Syntheses, and Biological Applications. Small 2015, 11, 1620–1636. [CrossRef][PubMed] 27. Xu, Q.; Kuang, T.; Liu, Y.; Cai, L.; Peng, X.; Sreeprasad, T.S.; Zhao, P.; Yu, Z.; Li, N. Heteroatom-doped carbon dots: Synthesis. J. Mater. Chem. B 2016, 4, 7204–7219. [CrossRef] 28. Sciortino, A.; Cannizzo, A.; Messina, F. Carbon Nanodots: A Review—From the Current Understanding of the Fundamental Photophysics to the Full Control of the Optical Response. Carbon 2018, 4, 67. [CrossRef] 29. Liu, M.L.; Chen, B.B.; Li, C.M.; Huang, C.Z. Carbon dots: Synthesis, formation mechanism, fluorescence origin and sensing applications. Green Chem. 2019, 21, 449–471. [CrossRef] 30. Migneault, I.; Dartiguenave, C.; Bertrand, M.J.; Waldron, K.C. Glutaraldehyde: Behavior in aqueous solution, reaction with proteins, and application to enzyme crosslinking. BioTechniques 2004, 37, 790–802. [CrossRef] 31. Chong, Y.; Ge, C.; Fang, G.; Tian, X.; Wen, T.; Wamer, W.G.; Chen, C.; Chai, Z.; Yin, J. Crossover between Anti- and Pro-oxidant Activities of Graphene Quantum Dots in the Absence or Presence of Light. ACS Nano 2016, 10, 8690–8699. [CrossRef][PubMed] 32. Sun, X.; Lei, Y. Fluorescent carbon dots and their sensing applications. TrAC 2017, 89, 163–180. [CrossRef] 33. Hu, Y.; Al Awak, M.M.; Yang, F.; Yan, S.; Xiong, Q.; Wang, P.; Tang, Y.; Yang, L.; LeCroy, G.E.; Hou, X.; et al. Photoexcited state properties of carbon dots from thermally induced functionalization of carbon nanoparticles. J. Mater. Chem. C 2016, 4, 10554–10561. [CrossRef] 34. Perelshtein, I.; Lipovsky, A.; Perkas, N.; Gedanken, A.; Moschini, E.; Mantecca, P. The influence of the crystalline nature of nano-metal oxides on their antibacterial and toxicity properties. Nano Res. 2015, 8, 695–707. [CrossRef] 35. Monk, P.M.S.; Rosseinsky, D.R.; Mortimer, R.J. Electrochromic Materials and Devices Based on Viologens. In Electrochromic Materials and Devices; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2015; pp. 57–90. ISBN 9783527679850. 36. Chougule, M.A.; Pawar, S.G.; Godse, P.R.; Mulik, R.N.; Sen, S.; Patil, V.B. Synthesis and Characterization of Polypyrrole (PPy) Thin Films. Soft Nanosci. Lett. 2011, 1, 6–10. [CrossRef] 37. Sevilla, F. Chemical sensors based on conducting polymers. In Proceedings of the Asian Conference Sensors, 2003. AsiaSense 2003, Kebangsann, Malaysia, 18 July 2003; pp. 87–92. 38. Guimard, N.K.; Gomez, N.; Schmidt, C.E. Conducting polymers in biomedical engineering. Prog. Polym. Sci. 2007, 32, 876–921. [CrossRef] 39. Spitalsky, Z.; Tasis, D.; Papagelis, K.; Galiotis, C. Carbon nanotube–polymer composites: Chemistry, processing, mechanical and electrical properties. Prog. Polym. Sci. 2010, 35, 357–401. [CrossRef] 40. Park, K.-S.; Schougaard, S.B.; Goodenough, J.B. Conducting-Polymer/Iron-Redox- Couple Composite Cathodes for Lithium Secondary Batteries. Adv. Mater. 2007, 19, 848–851. [CrossRef] 41. Kang, H.; Geckeler, K. Enhanced electrical conductivity of polypyrrole prepared by chemical oxidative polymerization: Effect of the preparation technique and polymer additive. Polymer 2000, 41, 6931–6934. [CrossRef]

42. Li, J.; Zhang, Q.; Feng, J.; Yan, W. Synthesis of PPy-modiﬁed TiO2 composite in H2SO4 solution and its novel adsorption characteristics for organic dyes. Chem. Eng. J. 2013, 225, 766–775. [CrossRef] 43. Ebrahimiasl, S.; Zakaria, A.; Kassim, A.; Norleha Basri, S. Novel conductive polypyrrole/zinc oxide/chitosan bionanocomposite: Synthesis, characterization, antioxidant, and antibacterial activities. Int. J. Nanomed. 2014, 10, 217. [CrossRef][PubMed] 44. Moorthy, M.; Kumar, V.B.; Porat, Z.; Gedanken, A. Novel polymerization of aniline and pyrrole by carbon dots. New J. Chem. 2018, 42, 535–540. [CrossRef]

45. Maruthapandi, M.; Kumar, V.B.; Gedanken, A. Carbon Dot Initiated Synthesis of Poly(4,40- diaminodiphenylmethane) and Its Methylene Blue Adsorption. ACS Omega 2018, 3, 7061–7068. [CrossRef] Polymers 2019, 11, 1240 15 of 15

46. Maruthapandi, M.; Luong, H.T.; Gedanken, A. Kinetic, isotherm and mechanism studies of organic dye adsorption on poly(4,-oxybisbenzenamine) and copolymer of poly(4,-oxybisbenzenamine-pyrrole) macro-nanoparticles synthesized by multifunctional carbon dots. New J. Chem. 2019, 43, 1926–1935. [CrossRef] 47. Maruthapandi, M.; Kumar, V.B.; Luong, J.H.T.; Gedanken, A. Kinetics, Isotherm, and Thermodynamic Studies of Methylene Blue Adsorption on Polyaniline and Polypyrrole Macro-Nanoparticles Synthesized by C-Dot-Initiated Polymerization. ACS Omega 2018, 3, 7196–7203. [CrossRef] 48. Maruthapandi, M.; Nagvenkar, A.P.; Perelshtein, I.; Gedanken, A. Carbon-Dot Initiated Synthesis of Polypyrrole and Polypyrrole@CuO Micro/Nanoparticles with Enhanced Antibacterial Activity. ACS Appl. Polym. Mater. 2019, 1, 1181–1186. [CrossRef] 49. Sadki, S.; Schottland, P.; Sabouraud, G.; Brodie, N. The mechanisms of pyrrole electropolymerization. Chem. Soc. Rev 2000, 29, 283–293. 50. Liu, F.; Yuan, Y.; Li, L.; Shang, S.; Yu, X.; Zhang, Q.; Jiang, S.; Wu, Y. Synthesis of polypyrrole nanocomposites decorated with silver nanoparticles with electrocatalysis and antibacterial property. Compos. Part B Eng. 2015, 69, 232–236. [CrossRef] 51. Zhou, J.; Lü, Q.F.; Luo, J.J. Efficient removal of organic dyes from aqueous solution by rapid adsorption onto polypyrrole–based composites. J. Clean. Prod. 2018, 167, 739–748. [CrossRef] 52. Saafan, S.A. Study of Dielectric Properties of Polypyrrole Prepared using Two Different Oxidizing Agents. J. Appl. Polym. Sci. 2005, 99, 3370–3379. [CrossRef] 53. Lee, J.Y.; Kim, D.Y.; Kim, C.Y. Synthesis of soluble polypyrrole of the doped state in organic solvents. Synth. Met. 1995, 74, 103–106. [CrossRef] 54. Lin, W.; Xu, K.; Xin, M.; Peng, J.; Chen, M. Hierarchical porous polyaniline–silsesquioxane conjugated hybrids with enhanced electrochemical capacitance. RSC Adv. 2014, 4, 39508–39518. [CrossRef] 55. Patil, P.T.; Anwane, R.S.; Kondawar, S.B. Development of electrospun polyaniline/ZnO composite nanofibers for LPG sensing. Procedia Mater. Sci. 2015, 10, 195–204. [CrossRef] 56. Sapurina, I.; Blinova, N.V.; Stejskal, J.; Trchova, M. The oxidation of aniline with silver nitrate to polyaniline —Silver composites. Polymer 2009, 50, 50–56. 57. Reda, S.M.; Al-Ghannam, S.M. Synthesis and Electrical Properties of Polyaniline Composite with Silver Nanoparticles. Adv. Mater. Phys. Chem 2012, 2, 75–81. [CrossRef] 58. Ayad, M.; Zaghlol, S. Nanostructured crosslinked polyaniline with high surface area: Synthesis, characterization and adsorption for organic dye. Chem. Eng. J. 2012, 204–205, 79–86. [CrossRef] 59. Boomi, P.; Prabu, H.G.; Manisankar, P.; Ravikumar, S. Study on antibacterial activity of chemically synthesized PANI-Ag-Au nanocomposite. Appl. Surf. Sci. 2014, 300, 66–72. [CrossRef] 60. Responders, E.; Last, S.E.E. Hazardous Substance Fact Sheet; NJHealth: Ewing Township, NJ, USA, 2009. 61. Sachdev, A.; Gopinath, P. Green synthesis of multifunctional carbon dots from coriander leaves and their potential application as antioxidants, sensors and bioimaging agents. Analyst 2015, 140, 4260–4269. [CrossRef] 62. Pal, T.; Mohiyuddin, S.; Packirisamy, G. Facile and Green Synthesis of Multicolor Fluorescence Carbon Dots from Curcumin: In Vitro and in Vivo Bioimaging and Other Applications. ACS Omega 2018, 3, 831–843. [CrossRef] 63. Song, Y.; Li, H.; Lu, F.; Wang, H.; Zhang, M.; Yang, J.; Huang, J.; Huang, H.; Liu, Y.; Kang, Z. Fluorescent carbon dots with highly negative charges as a sensitive probe for real-time monitoring of bacterial viability. J. Mater. Chem. B 2017, 5, 6008–6015. [CrossRef]

64. Maruthapandi, M.; Kumar, V.B.; Levine, M.; Gedanken, A. Fabrication of poly (4,40-oxybisbenzenamine) and its conjugated copolymers initiated by easily accessible carbon dots. Eur. Polym. J. 2018, 109, 153–161. [CrossRef]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).