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Surfactant Triggered Fluorescence Turn “on/off” Behavior of a Polythiophene – graft - Polyampholyte Radhakanta Ghosh, Sandip Das, Dhruba P Chatterjee, and Arun K. Nandi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01928 • Publication Date (Web): 27 Jul 2016 Downloaded from http://pubs.acs.org on August 1, 2016

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Surfactant Triggered Fluorescence Turn “on/off” Behavior of a Polythiophene – graft - Polyampholyte

Radhakanta Ghosha, Sandip Dasa, Dhruba P. Chatterjeeb and Arun K. Nandia* a

Polymer Science Unit, Indian Association for the cultivation of Science, Jadavpur, Kolkata700 032, INDIA

ABSTRACT: Polythiophene graft polyampholyte (PTP) is synthesized using N,N-dimethylaminoethyl methacrylate and tert-butyl methacrylate monomers by grafting from polythiophene backbone, followed by hydrolysis. The resulting polymer exhibits aqueous solubility via formation of small sized miceller aggregates with hydrophobic polythiophene at the centre and radiating polyionic side chains (cationic or anionic depending on the pH of the medium) at the outer periphery. The critical micelle concentration of PTP in acidic solution (0.025mg/mL, pH=2.7) is determined from fluorescence spectroscopy. PTP exhibits reversible fluorescence on and off response in both acidic and basic medium with the sequential addition of differently charged ionic surfactants, repeatedly. The fluorescence intensity of PTP at pH 2.7 increases with the addition of an anionic surfactant, sodium dodecyl benzene sulfonate (SDBS), due to the self-aggregation forming compound micelles. The fluorescence intensity of these solutions again decreases on addition of a cationic surfactant, cetyltrimethylammonium bromide (CTAB), because of assembling of SDBS *a

b

For correspondence: Arun K. Nandi, Email: [email protected]

Dept. of Chemistry, Presidency University, Kolkata-700 073

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ACS Paragon Plus Environment

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with CTAB, thus de-assembling the PTP-SDBS aggregates. At pH 9.2, these turn on and turn off response are also shown by PTP with the sequential addition of cationic surfactant (CTAB) and anionic surfactant (SDBS), respectively. This result shows that PTP has potential for surfactant induced reversible fluorescence turn on and off using ionic surfactant (SDBS and CTAB) through self-assembling and de-assembling of the ionic aggregates. The reversible aggregation and disaggregation process of PTP with the surfactants at both acidic and basic pH is supported from dynamic light scattering and Fourier transformed infrared spectroscopy. The morphology of the above systems studied by transmission and scanning electron microscopy also supports the above aggregation and disaggregation process. INTRODUCTION: From its discovery, conjugated polymers are continuing major research curiosity both from academic and technological view point for its profound applications in material science.1-7 Amongst the conjugated polymers, polythiophene is the most widely used conjugated polymer because of its unique opto-electronic property. It has widespread applications in polymer solar cells,8-10 field effect transistors (FETs),

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polymer light emitting diodes

(PLEDs),12 sensors of toxic anions, cations, nucleic acid, protein, polysaccharides, polypeptides, human thrombin etc13-15 and other opto-electronic devices (e.g. molecular logic gate,16 molecular thermometer17 etc). Polythiophene based molecular tools are more advantageous from the small molecular systems because of its co-operative signal amplification capability of each segment through the conjugated backbone even in presence of very small perturbation.13 On exposure to different analytes, the conjugated backbone of substituted polythiophene undergoes conformational and/or aggregational changes, altering the effective conjugation length.18,

19

A major disadvantage of these polymers is its

insolubility in aqueous medium, an essential requirement for biological and environmental 2


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monitoring. Grafting of different hydrophilic polymers or attachment of ionic pendant groups upon polythiophene backbone makes it water soluble.20,

21

The pendant chains can act as

triggers for the sensor response by providing selective receptor sites for the analyte detection.13, 14, 16 Polyampholytes are polymers bearing both cationic and anionic repeat units.22 There are only few reports of polyampholytes prepared by controlled copolymerization of cationic and anionic monomers and they can show special properties by tuning of the ratio and/or distance of the two charges.22-24 Very recently Zhang and Hoogenboom have reported polyampholytes with controlled equimolar ratio of charges exhibiting upper critical solution temperature (UCST) in alcohol-water mixtures due to neutralization of charges.25 However, to our knowledge there are no reports where polyampholytes are produced by grafting cations and anions from the conducting polymer chains. Certainly, if such a polyampholyte can be synthesized from the important fluorescent polymer polythiophene, it would be very much useful, particularly for analytical purposes exploiting its fluorescence property. Surfactants have wide applications in domestic cleaning, house products such as shampoos, cosmetics and industries such as paints, textiles, pesticides, pharmaceuticals, mining, oil recovery etc. making it one of the foremost elements of environmental pollutants.26,

27

In presence of oppositely charged surfactant molecules, the ionic

polythiophene forms stable aggregate through hydrophobic, dipolar and strong electrostatic interactions.19,

28

The attractive electrostatic forces between the components are extremely

important for aggregational and conformational changes of polymer backbone significantly altering the fluorescence property. The property of the polymer-surfactant aggregate highly depends on the nature of the polyelectrolyte and certainly the aggregation above critical micelle concentration (CMC) of the surfactant is very much important to cause a dramatic 3


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change in optical properties.29 In the literature there are several reports upon the interaction between water soluble polythiophene and surfactant molecules.19, 29-31 Recently, Wang et al. have reported water soluble cationic polythiophene derivative, poly[N,N,N-trimethyl-4(thiophen-3-ylmethylene)-cyclohexanaminium chloride] (PTCA-Cl), based colorimetric and fluorometric detection of anionic surfactants (SDBS, SLS etc.).32 In the same year Evans et al. have also reported self assembly behaviour of the cationic polymer-surfactant complex formed by electrostatic interaction and phase transition of it using optical property of the polythiophene.33 In both cases the synthesized polymer specifically interacts with the anionic surfactants. However, to the best of our knowledge, there are no reports where the polyampholytes are used to bind surfactants. It would be exciting to study the aggregation behavior at different pH of the medium as the nature and amount of charge of the polyampholyte chain would vary with pH. So, surfactants of different charges may bind with the polyampholyte, at different pH and the aggregation may be delineated through fluorescence measurement and morphological investigations. Here, we report the synthesis and characterization of polythiophene graft polyampholyte (PTP) using N,N-dimethylaminoethyl methacrylate (DMAEMA) and tertbutyl methacrylate (TBMA) monomers from the polythiophene (PT) backbone. The atom transfer radical polymerization (ATRP) is used followed by hydrolysis for this purpose. The interaction between water soluble polythiophene graft polyampholyte and different surfactants (cationic and anionic) is studied from Fourier transformed infrared spectroscopy (FTIR) and the aggregation / disaggregation process is monitored from the on and off response in fluorescence study at both acidic and basic pH. The polymer shows reversible fluorescence change in both acidic and basic medium with the sequential addition of oppositely charged surfactants in a repeated fashion and it may be termed as 4


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surfactochromism. Attempt is made to understand the self-assembly / de-assembly of PTP with the surfactants from the dynamic light scattering (DLS), TEM and SEM microscopy. EXPERIMENTAL: Materials and purification: The monomer 3-thiophene ethanol and the ligand 1,1,4,7,10,10 - hexamethyl triethylene tetramine (HMTETA, Sigma Aldrich, USA) were used as received. The monomers N, N dimethylaminoethyl methacrylate (DMAEMA) and tert-butyl methacrylate (TBMA) (Sigma Aldrich, USA) were purified by passing through basic alumina and neutral alumina column, respectively. The catalyst CuCl was (Loba Chemicals, Mumbai) purified by washing with 10% HCl in water followed by methanol and diethyl ether in inert atmosphere. The solvents such as anisole, dichloromethane (DCM), chloroform, methanol etc (Loba Chemicals, Mumbai) were purified by distillation and HPLC water was used throughout the work. Preparation of thiophene initiator 3-[1-ethyl-2-(2-bromoisobutyrate)] thiophene (TI): 3-thiophene ethanol (20 mmol) was dissolved in dry DCM (30 ml) in a 100 ml round-bottom flask and triethylamine (22 mmol) was added with continuous stirring at 0 °C under nitrogen atmosphere. 2-bromoisobutyryl bromide (BIB, 22 mmol), diluted with 10 ml dry DCM, was added into the reaction mixture drop wise using a pressure-equalizer and was stirred for 24 hrs. The reaction mixture was first filtered and the filtrate was washed repeatedly with 1% HCl, saturated NaHCO3, brine solution and distilled water, respectively. The organic layer was separated by using a separatory funnel and was passed through anhydrous Na2SO4 to remove any water. For further purification, silica column chromatography was performed in a solvent mixture of hexane / ethyl acetate (95/5, in volume ratio). After the solvent evaporation, a brown color liquid was obtained as a final product (yield-75%). 1H NMR (CDCl3): δ= 1.9 (6H, s), 3.0 (2H, t), 4.3 (2H, t), 6.9-7.2 ppm (aromatic ring protons).34, 35 and 5


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C NMR (CDCl3) δ= 29.4, 30.9, 55.9, 65.9, 121.9, 125.8, 128.4, 137.8, 171.8 ppm. (Figure

S1a & b).34, 35 Molecular weight of TI is 277.9 which is obtained from high resolution mass spectrum (HRMS) (Figure S2). Preparation

of

polythiophene

macro-initiator

2,

5-poly(3-[1-ethyl-2-(2-

bromoisobutyrate )]thiophene (PTI): In a 250 ml round bottom flux, purged with N2, anhydrous FeCl3 (15 mmol) was dispersed in 30 ml dry chloroform. Then TI (3.5 mmol in 30 ml of dry chloroform) was added drop wise into the mixture and was stirred overnight at 30 °C. It was then added into the excess amount of methanol with continuous stirring. The solid precipitate was separated, repeatedly washed with methanol and finally it was soxhlet extracted with methanol for a whole day. The extract was dried under vacuum at 40 °C for three days and the solid product was then dissolved in 150 ml CHCl3. Then it was refluxed with additional 100 ml of concentrated ammonia solution to remove the trace amount of FeCl3. The organic layer was separated, concentrated and was precipitated out into excess amount of methanol. The precipitate was separated and was dried under vacuum at 40 °C for three days. A red solid was obtained as a final product (yield60%). The number average molecular weight ( M n ) of PTI is about 40,000 (PDI = 2.84). The number of thiophene monomer units present in the PTI is about 144 (M.W of TI unit = 277.9).1H NMR (CDCl3): δ= 1.9 (6H), 3.2 (2H), 4.4 (2H), 6.9-7.2 ppm (aromatic protons) (Figure S3a).35 13C NMR (CDCl3) δ= 29.4, 30.9, 55.9, 65.9, 121.9, 125.8, 128.4, 137.8, 171.8 ppm (Figure S3b).35 Synthesis of polythiophene-g-poly(N, N dimethylaminoethyl methacrylate)-co-poly(tertbutyl methacrylate) (PT-g-pDMAEMA-co-pTBMA): PTI (25 mg), anisole (2 ml), CuCl (10 mg) were taken in a N2 purged reaction vessel (8 × 2.5 cm). The monomers DMAEMA and TBMA (0.5 ml of each) were next injected into the reaction tube under nitrogen purged condition. The ligand HMTETA (40 µL) was lastly 6


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injected into the reaction mixture and the reaction was allowed to stir for 6 hrs at 60 0C. The reaction mixture was then precipitated into the excess petroleum ether. The precipitated out polymer was isolated, re-dissolved in THF and was re-precipitated into excess petroleum ether. This process was repeated for three times to remove any trace amount of monomer entrapped within the polymer. Then the polymer was dissolved in THF and was passed though silica column to remove the copper catalyst. Finally by evaporating the solvent, the pure polymer ( M n =102000, PDI=1.14) was obtained. 13C NMR (CDCl3) δ= 23, 28-36, 45.8, 57-67, 81.13, 120-140, 178 ppm (Figure S4). Hydrolysis of PT-g-pDMAEMA-co-pTBMA: PT-g-pDMAEMA-co-pTBMA (200 mg) was dissolved in 20 ml dry DCM in a nitrogen purged reaction vessel. 1 ml of trifluoroacetic acid (TFA) was injected into it drop wise with continuous stirring at 30 °C. The reaction mixture was stirred for 24 hrs and a precipitate was obtained. The solid precipitate was repeatedly washed with DCM for several times to remove trace amount of TFA. Then the product was dried under vacuum for 2 days and a yellow solid hydrolyzed PT-g-pDMAEMA-co-pTBMA (PTP) was collected (Scheme 1). 1

H &

13

C NMR spectra: The 1H NMR spectra of the TI, PTI and PT-g-pDMAEMA-co-

pTBMA were recorded in CDCl3 and the 1H NMR spectrum of PTP was recorded in D2O on a 500 MHz Bruker instrument. The

13

C NMR spectra of TI, PTI and PT-g-pDMAEMA-co-

pTBMA were also measured in CDCl3 using the same instrument. High resolution mass spectra (HRMS): The high resolution mass spectrum of TI was made in CH2Cl2 by micromass Q-Tof micro™ instrument. Gel Permeation Chromatography: The molar mass of PTI and PT-g-pDMAEMA-copTBMA were calculated using the gel permeation chromatography (GPC) experiment. The GPC experiments were performed on a Waters instrument equipped with Waters 1515 pump,

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Waters 2414 differential refractive index detector and three µ-Styragel columns. Here polystyrene was used as the standard and tetrahydrofuran was used as the solvent. UV-vis and photoluminescence (PL) Spectroscopy: The UV-vis spectra of the samples were taken in aqueous solutions (0.05% w/v) from 190 to 1100 nm using a UV-vis spectrophotometer (Hewlett-Packard, model 8453) at 25 °C. The photoluminescence (PL) spectra of the samples in aqueous solutions (0.15% w/v) were measured in a Fluoromax-3 instrument (Horiva Jovin Yvon) at 25 °C. The sample was taken in a quartz cell of 1 cm path length. It was excited at 417 nm and the emission scans were recorded from 440 to 800 nm using a slit width of 1.5 nm with an increment of 1 nm wavelength having an integration time of 0.1 s. Fourier Transform Infrared Spectroscopy (FTIR): The FTIR spectra of all the dried samples were performed in a Shimadzu FTIR instrument (model 8400S) from KBr pellets. Dynamic Light Scattering (DLS): The DLS experiments of the PTP solutions (0.15% w/v) at different pHs were performed using a Malvern instrument with a He-Ne laser at an angle 173° at 25°C. The zeta potential values of the aqueous solution of PTP (0.15% w/v) were measured using the same DLS instrument at 25 °C. Transmission Electron Microscopy (TEM): The morphology of PTP with the addition of surfactants was monitored by TEM instrument (JEOL, 2010EX) operated at an acceleration voltage of 200 kV and fitted with a CCD camera. A small drop of aqueous solution of each sample was drop casted on a carbon-coated copper grid, dried in air at 30 °C and was finally preserved at vacuum for 2 days before the TEM images were taken. Scanning Electron Microscope (SEM): In case of SEM study, a small amount of the aqueous solutions were dropped on cover slip, dried in air at 30 °C and were kept under vacuum at 60 0C for two days. Then the dried samples were platinum coated and morphology was observed using a field emission scanning electron microscope (Jeol GSM-5800). 8


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Result and Discussion: Synthesis and Characterization: The synthetic scheme of forming polythiophene-g-poly(N, N dimethylaminoethyl methacrylate)-co-poly(tert-butyl methacrylic acid) (PTP) is presented in Scheme 1. At first thiophene initiator (TI) is synthesized by a simple coupling reaction between 3-thiophene ethanol and 2-bromoisobutyryl bromide using Et3N as a base. The synthesis of polythiophene

Scheme 1. Synthetic procedure for the synthesis of PTP. macro-initiator (PTI) is made by oxidative polymerization of TI using FeCl3 as initiator. The polythiophene-g-poly(N,

N

dimethylaminoethyl

methacrylate)-co-poly(tert-butyl

methacrylate) is then synthesized by copolymerizing the DMAEMA and TBMA monomers in presence of CuCl/HMTETA as catalyst/ligand system from the polythiophene (PT) backbone of PTI by ATRP technique. Then the tert-butyl groups of the pTBMA segments of the polythiophene based graft copolymer are hydrolyzed using TFA to produce the polythiophene graft polyampholyte (PTP). The 1H NMR and

13

C NMR spectra of TI with

their peak assignments are shown in Figure S1a & b. The HRMS data in Figure S2 confirmed the formation of TI with molecular weight 277.9. The 1H NMR spectrum of PTI (Figure S3) clearly shows that both the peaks ‘b’ and ‘c’ become broader and splitted than those of TI due to polymerization causing head-tail regioirregularity. The H-T regioregularity is calculated 9


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from the ratio of signal intensity of the two component signals of ‘b’ and ‘c’, showing a value of 64 %. In Figure 1, the 1H NMR spectra of the PT-g-pDMAEMA-co-pTBMA and PTP confirm the structure of the graft copolymers. The signals resonate at δ value 2.27, 2.56 and 4.05 ppm for the corresponding proton ‘j’, ‘i’ and ‘h’ of grafted DMAEMA chains, respectively and the signal appearing at δ 1.49 ppm corresponds to proton ‘e’ of grafted TBMA chains of PT-g-pDMAEMA-co-pTBMA copolymer. The signals appearing at δ 0.95 and 1.1 ppm are of ‘f’ protons of both DMAEMA and TBMA chains of the polymer. However, on hydrolysis those ‘j’, ‘i’ and ‘h’ protons of DMAEMA chains appears at δ =2.9, 3.5 and 4.3 ppm, respectively and the tert-butyl groups of grafted TBMA chains (‘e’ protons) at δ 1.49 ppm disappears due to hydrolysis.25

Figure 1. 1H NMR spectra of (a) PT-g-pDMAEMA-co-pTBMA in CDCl3 and (b) PTP in 10 D2O along with their peak assignments. ACS Paragon Plus Environment

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So, the aforementioned 1H NMR data clearly exhibits that, after hydrolysis all the distinct peaks of the pDMAEMA segments remains but those of the tert-butyl groups of grafted TBMA chains disappear confirming the formation of PTP. The downfield shift of the ‘j’, ‘i’ and ‘h’ protons in the hydrolyzed sample is attributed to the protonation of the (-NMe2) groups and hence deshielding of the protons. The molar ratio of grafted TBMA/DMAEMA is 1.09 which is calculated of from the ratio of peak area of ‘e’ protons of grafted TBMA chains and ‘h’ protons of grafted DMAEMA chains. Hence 1H-NMR study confirms the presence of about equimolar ratios of DMAEMA and hydrolyzed TBMA repeating units in the grafted chains of PTP. The molecular weight of the polymer, calculated from 1H NMR spectroscopy is 198000. Peaks for ‘e’, ‘h’ and ‘a’ are considered for calculation of M n of PT-gpDMAEMA-co-pTBMA, namely PT144-g-pDMAEMA527-co-pTBMA574 (SI). Figure S5 shows the GPC traces of PTI and PT-g-pDMAEMA-co-pTBMA. The GPC analysis shows the M n of PTI ~ 40,000 with dispersity 2.8 and that of PT-g-pDMAEMA-copTBMA are found to be 102,000 with dispersity 1.14. The molecular weights measured from GPC for the grafted copolymer may not be accurate but the GPC traces show a clean sweep towards lower elution volume than that of PTI clearly indicating the formation of graft copolymer possesing larger hydrodynamic volume than that of PTI. The large discrepancy between the NMR and GPC molecular weight of PT-g-pDMAEMA-co-pTBMA may be attributed to the use of linear polystyrene as the calibration standard due to the nonavailability of any grafted/branched copolymer as the GPC standard.34-36 Additionally, in order to understand polymerization to take place from the initiator site of PTI we have compared the 13C NMR spectra of PTI with that of PT-g-pDMAEMA-co-pTBMA in Figure S3b and S4. The result indicates that the intensity of 13C NMR peak of ‘b’ carbon of PTI (65.9 ppm) has reduced sharply due to formation of long chain of high molar mass of PT11


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g-pDMAEMA-co-pTBMA.

The

repeated

attachment

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of

monomeric

units

during

polymerizations makes the chemical shift of the pristine ‘b’ carbon to upfield region (near 3036 ppm). However exact assignment of the same is very difficult due to the complex nature of the spectrum there. UV-Vis Absorption Spectra: Figure 2 shows the absorbance spectra of aqueous solution of PTP with anionic or cationic surfactant at acidic and basic pH. Under acidic conditions, when pH of the medium is 2.7, presumably, the basic -NMe2 groups (having pKa ~ 7.5)

16

are all protonated making the

entire polymeric system polycationic. In Figure 2a, the acidic solution of PTP shows maximum absorbance at 435 nm attributed to the π-π* transition of polythiophene backbone. Upon addition of SDBS, the polymer shows a blue shift of absorbance peak to 415 nm. The 20 nm blue shift indicates that the conjugation length of polymer is decreased due to aggregation of PTP with SDBS. The electrostatic interaction between the negatively charged sulfonate groups of SDBS and the positively charged -NHMe2+ group of pDMAEMA segments is the driving force of aggregation at this low pH. Hence, charge neutralization occurs decreasing the repulsion between the positive charges of the grafted pDMAEMA segments. As a result, the polymer adopts randomly coiled structure with twisted backbone, attributing to the blue shift of absorption peak due to the decreased conjugation length. With the addition of CTAB at the same solution, the absorption peak of PTP shows a red shift to 425 nm indicating that the polymer chains almost reverts to the pristine conjugated form. Probably, the self assembled PTP-SDBS ionic complex becomes de-assembled with the addition of CTAB. The stronger ionic interaction and also the synergistic effect due to the hydrophobic interaction between two alkyl chains of SDBS and CTAB plays the important role in the dissociation of PTP-SDBS aggregate with addition of CTAB.28,

32, 37

The

interaction consists of two parts: (i) enthalpic and (ii) entropic and the later contribution is 12


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lower in the PTP-SDBS system than SDBS–CTAB system because of longer length of PTP chains. However, the enthalpic contribution is almost same in both the cases as same ionic interaction predominates here. This de-assembly of PTP-SDBS complex triggers the coiling of PT chains and therefore it returns back to the relatively straightened PT chains. At pH 2.7 PTP PTP+SDBS (3µmol) PTP+SDBS+CTAB (1.4µmol) 415nm 435nm 425nm

400

500 Wavelength (nm)

At pH 9.2

(b)

Absorbance (a.u.)

(a)

Absorbance (a.u.)

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600

PTP PTP+CTAB (2.3µmol) PTP+CTAB+SDBS (5µmol) 408nm 435nm 425nm

400

500 Wavelength (nm)

600

Figure 2. UV visible spectra of PTP (0.05% w/v, 2ml solution) in aqueous solution with added surfactants (as indicated) in (a) acidic (pH 2.7) and (b) basic (pH 9.2) condition. Similarly at pH 9.2, (Figure 2b), when the polymer system act as a polyanionic, the PTP shows maximum absorbance at 435 nm, which is blue shifted to 408 nm with the addition of CTAB. In this basic pH, the driving force for forming PTP-CTAB aggregates is the ionic interaction between negatively charged hydrolyzed pTBMA segments (i.e. -COO-) with the positively charged quaternary ammonium groups of CTAB. The absorbance is further red shifted to 425 nm upon the addition of SDBS into that PTP-CTAB solution. This disaggregation of PTP-CTAB aggregates in presence of SDBS at basic pH is also for the strong ionic interaction and synergistic effect among oppositely charged surfactant. A similar coiling-decoiling of PT chains is also proposed here to explain this phenomenon. In this case coiling of PT chain is occurring due to the interaction of negatively charged side chains of PTP system with the cationic sites of CTAB and is again getting decoiled when SDBS is added to the system. This type of blue and red shifts of absorbance maxima confirms that the 13


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polymer at acidic or basic pH forms aggregates with the oppositely charged surfactants SDBS or CTAB by self assembled ionic complex formation and disassemble with the addition of CTAB or SDBS, respectively. These results therefore clearly indicate the surfactochromism behaviour of PTP at both low and high pH is due to reversible coiling-decoiling phenomenon of backbone PT chain of PTP. It is to be noted from the figure 2(a, b) that there is an increase of absorbance values of base line for PTP+SDBS/PTP+CTAB systems than that of PTP and it may be attributed to micro-aggregation of the above systems, causing increased scattering intensity by the larger size particles.38 Fluorescence Spectra: PTP shows good fluorescence property and in Figure S6 the fluorescence intensity is plotted with concentration at pH 2.7. From the plot the critical micelle concentration (CMC) of PTP at acidic medium is found to be 0.025 mg/mL. Figure 3(a,b) and Figure 4(a,b) exhibit the increase and decrease of fluorescence intensity of PTP in presence of ionic surfactant at acidic and basic solutions, respectively. The fluorescence intensity of PTP increases with gradual addition of anionic surfactant SDBS at acidic solution (pH 2.7, figure 3a) and with the addition of cationic surfactant CTAB at basic solution (pH 9.2, figure 4a) indicating the quenching of polythiophene excitons by solvent molecules becomes restricted during the PTP-SDBS or PTP-CTAB aggregate formation, respectively.35, 39 In the acidic pH, the PTP becomes aggregated with the addition of SDBS to its micelle by ionic complex formation between the anionic sites of the surfactant and the cationic sites of the PTP i.e. -NHMe2+. Thus quenching of fluorescence, occurring from the transfer of polythiophene excitons to the solvent molecules, is restricted causing an increase of fluorescence intensity. In case of addition of CTAB in that solution, strong interaction between SDBS and CTAB occurs dissociating the aggregated PTP-SDBS complex resulting quenching of polythiophene

14


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excitons by water molecules. Thus the fluorescence intensity again decreases to almost its initial intensity (Figure 3b).

(a)

500

600

PL Intensity (a.u.)

PL Intensity (a.u.)

With addition of SDBS

PTP 0.71 µ mol 1.4 µ mol 4.2 µ mol 5.7 µ mol 7.1 µ mol 7.8 µ mol 8.5 µ mol 9.2 µ mol

At pH 2.7

(b)

At pH 2.7

700

PTP+SDBS(9.2 µ mol) now CTAB is added 0.71 µ mol 1.42 µ mol 2.13 µ mol 2.84 µ mol 3.55 µ mol 4.26 µ mol

With addition of CTAB

500

Wavelength (nm)

600

700

Wavelength (nm)

Figure 3. PL intensity vs wavelength plot of PTP (0.15% w/v, 2ml solution) in aqueous solution with increasing amounts of (a) SDBS (as indicated) and (b) CTAB (as indicated) at pH 2.7. Excitation wavelength: 417 nm.

500

600

At pH 9.2 PTP+ 7 µ mol CTAB now SDBS is added 1.42 µ mol 4.20 µ mol 7.00 µ mol 9.80 µ mol 12.6 µ mol 15.4 µ mol

(b) PL Intensity (a.u.)

At pH 9.2 PTP 1.41 µ mol With 2.8 µ mol addition 4.2 µ mol of CTAB 5.6 µ mol 7 µ mol

(a) PL Intensity (a.u.)

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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700

With addition of SDBS

500

Wavelength (nm)

600

700

Wavelength (nm)

Figure 4. PL intensity vs wavelength plot of PTP (0.15% w/v, 2ml solution) in aqueous solution with increasing amounts of (a) CTAB (as indicated) and (b) SDBS (as indicated) at pH 9.2. Excitation wavelength: 417 nm. Similarly, in case of basic solution (Figure 4a), the ionic complex between PTP-CTAB occurs due to the interaction between the cationic sites of CTAB and COO- sites of PTP, restricting the quenching of PTP excitons with the solvent molecules causing an increase of fluorescence intensity. The fluorescence intensity of that PTP-CTAB solution also decreases with the addition of SDBS (Figure 4b). It is to be noted that upon addition of CTAB to PTP 15


Langmuir

solution (Figure 4a), there is a marked increase in the emission intensity on going from 4.2 to 5.6 micromoles. No definite reason can be put forward and a probable reason may be the more flexible nature of CTAB facilitating an easier approach to PTP. The rigidity of benzene ring of SDBS causes relatively more hindrance to the approach of SDBS to PTP. Hence there is a sharp hike in PL intensity on addition of CTAB than that of SDBS (cf. Figure 3a and 4a). For breaking of the compound micelle similar observation is also noticed, probably due to the same reason (cf. Figure 3b and 4b). Figure 5 and S7, shows that these processes are highly reversible in both acidic and

Intensity (a.u.)

basic pH and the fluorescence intensity repeatedly increases and decreases with the addition

µm

7.1 0

BS SD of

10 .65

14 .20

3. 55

1

At pH 2.7

) ol m

(µ

Ad dit ion of CT AB (

.20 14 5 0.6

7 .1 0

1

3.5 5

ol)

0.0 0

.30 21 5 7.7

n tio di Ad

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

Page 16 of 31

Figure 5. Reversible turn “on” and “off” plot of fluorescence intensity of PTP (0.15% w/v, 2ml solution) in aqueous solution with increasing amounts of SDBS then CTAB in pH 2.7. of SDBS and CTAB or reverse, respectively. The reversibility in terms of fluorescence intensity generating fluorescence 'on' and 'off' mechanism is also observed here when the surfactant molecule with reverse polarity is added in individual cases. The increase in fluorescence emission intensity may be attributed to the removal of solvent water molecules 16


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with coiling of the PT backbone chains which minimize the non-radiative deactivation of the excimers with solvent molecules. This is certainly a clear case of surfactochromism of the polythiophene graft polyampophylate. This dehydration of the polymer backbone due to coiling of backbone PT chains is also observed visually by the development of turbidity upon addition of positive or negative surfactant on respective PTP solution which becomes transparent again upon addition of the opposite type of surfactant. The CMC of SDBS (pH 2.0) is reported to be 0.45 mM40 and the critical aggregation concentration (CAC) of the present system (PTP+SDBS) is found to be 2.10 mM in 0.15% (w/v) solution of PTP at pH 2.7 (Figure S8). The increase of CAC values than that of CMC values indicate some amount of SDBS first forms complex with PTP and the rest SDBS produce micelles producing the compound micelles with that of PTP. At pH 9.2 the CMC of CTAB is found to be 0.83 mM (Figure S9)

41, 42

and the CAC of PTP+CTAB system in

0.15% (w/v) PTP solution is measured to be 2.03 mM (Figure S10). Here also the increase of CAC value than that of CMC value indicate some amount of the CTAB first forms complex with PTP and the rest CTAB produce micelles producing the compound micelles with that of PTP. The fluorescence intensity increases significantly at the CAC in both the cases. We have calculated the mole ratios of the active part of PTP system (protonated pDMAEMA at pH 2.7 and deprotonated polymethacrylic acid (pMAA) units at pH 9.2) interacting with oppositely charged surfactants SDBS and CTAB, from 1H NMR analysis (SI).

The degree of

polymerization (DP) of pDMAEMA is 527 and that of pMAA is 574. This shows 7.88 x10-6 mole DMAEMA units are present in 2 ml 0.15% (w/v) solution of PTP and onset of PTPSDBS aggregation occurs when 4.4x10-6 mole SDBS (CAC) is added. Similarly, at pH 9.2, in the same PTP solution containing 8.5x10-6 mole of methacrylic acid (MAA) units, the CAC of PTP-CTAB is 4.2x10-6 mole CTAB. It is interesting to note that at the CAC the mole ratio

17


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of ionic pendant of PTP and surfactants is ~2:1 at both the pH, indicating similar nature of aggregation in the PTP-surfactant conjugates. Dynamic Light Scattering: This surfactant triggered coiling of backbone PT chains and hence developed turbidity must be associated with increase in sizes of the particles of the PTP solution. At pH 2.7 Figure 6a shows a bimodal nature of the intensity-size distribution for PTP. 43 The Z-average value of particle size of pristine PTP solution is ~106 nm indicating a molecularly dispersed solution with an almost extended chain configuration because of repulsion between the cations (NHMe2+) of the grafted chains. The polymer solution is transparent (right side of Figure 6a) but after the addition of 9.2 micromoles SDBS (total concentration = 4.32 mM, which is above its CMC (0.45 mM at pH 2)40 and also the CAC of PTP-SDBS system) it becomes opaque (right side of Figure 6b). This is due to the aggregated complex formation between PTP and SDBS, and the Z-average particle size measured from DLS increases to ~2846 nm (Figure 6b). But after subsequent addition of 4.26 micromoles CTAB, the Z-average particle size again decreases to ~117 nm and the solution becomes transparent (right side of Figure 6c). This is really very interesting observation indicating reversible changes of particle size with consecutive addition of surfactants of opposite charges. It is to be noted that on addition of SDBS the bimodal distribution transforms to monomodal with a large increase of size producing compound micelle. On further addition of CTAB the bimodal distribution reverts with a decrease of size due to breaking of compound micelle. In a similar fashion at pH 9.2 (Figure S11a), the intensity-size distribution is also bimodal, showing the Z-average particle size of PTP to be 76 nm. It increases sharply to 980 nm after addition of 7 micromoles CTAB (overall concentration = 3.24 mM) (Figure S11b) which is above of its CMC 0.83 mM41, 42 and also CAC of PTP-CTAB system. The reason is the same

18


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PDI PdI=0.475

Intensity (Percent)

10 8 6 4 2 0

(a) PTP

1

10

100

1000

Intensity (Percent)

10000

Size (d.nm)

PDI PdI=0.256

30

20

10

0 1

10

(b) PTP+SDBS

100

1000

10000

Size (d.nm)

PDI PdI=0.242

12 Intensity (Percent)

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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10 8 6 4 2 0 1

10

(c) PTP+SDBS+CTAB

100

1000

10000

Size (d.nm)

Figure 6. Dynamic light scattering data of (a) PTP (0.15% w/v, 2ml solution) in aqueous solution, (b) PTP+SDBS aggregated complex (after addition of 9.2 micromoles SDBS) and (c) PTP+SDBS+CTAB disaggregated state (after addition of 4.26 micromoles CTAB) in pH 2.7. Snapshots and PDI values of the solutions are also shown at the side. as discussed above; however, the degree of increase of size is not as large as in the former case. No definite reason may be given here and this may be due to difference between the sizes of the respective micelles. As above, after addition of 15.4 micromoles SDBS (overall concentration 6.63 mM) the Z-average size decreases to 203 nm (Figure S11c). Here also the effect is qualitatively the same but it cannot come back exactly to the pristine size of the PTP. It might be possible that some CTAB still remains adhered to the PTP chain increasing the Zaverage size of PTP. As above, on addition of CTAB the bimodal nature transforms to monomodal with a large increase of size producing compound micelle. On further addition of 19


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SDBS the bimodal distribution again come back with a decrease of size due to breaking of compound micelle. We have made a control DLS experiment of SDBS-CTAB system (9.2 micromoles SDBS + 4.26 micromoles CTAB in 2ml solution) at pH 2.7 (Figure S12a) and also at pH 9.2 with CTAB-SDBS system without PTP (Figure S12b). The intensity-size distribution curves clearly indicate bimodality, probably due to the mixed micelle formation of CTAB and SDBS. The polymer PTP contains both cationic and anionic charges and its proportion is dependent on pH of the medium. Consequently, it is expected to have an isoelectric point as it is commonly found in proteins due to their zwitter ionic character. We have made a potentiometric titration from zeta potential measurement of PTP (0.15%, w/v) solution using the DLS instrument at different pH (Figure S13) and the isoelectric point is found to be pH 6.18. At this pH we have added SDBS or CTAB to the PTP solution, but we do not observe any significant change of fluorescence intensity like those at pH 2.7 and at pH 9.2 (Figure S14). This has been attributed to the overall nonionic nature of PTP at this isoelectric point. Correlation data of all DLS results and Cumulants Fit of the samples are presented in Figure S15, S16, S17, S18, S19, S20, S21, S22. FTIR Spectra: To understand the nature of interaction between the PTP with the surfactants FTIR spectra of PT-g-pDMAEMA-co-pTBMA, PTP and PTP with the surfactants at pH 2.7 are presented in Figure S23a where it is apparent that the >C=O peak of PT-g-pDMAEMA-co-pTBMA appearing at 1735 cm-1 on hydrolysis shifts to 1680 cm-1 indicating the formation of carboxylic acid. The hump at 1735 cm-1 may be due to the >C=O vibration of the initiator attached to the PT backbone. The lowering of force constant of carbonyl group of carboxylic acid might have occurred due to the H-bonding interactions of it with -NHMe2+ of PTP. Interestingly, on addition of SDBS to PTP system this 'C=O' stretching vibration shifts to 20


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1702 cm-1. This is probably due to the ionic interactions of cationic quaternary nitrogen containing groups of PTP with the anionic head groups of SDBS significantly decreasing the above H-bonding interactions. On addition of CTAB to the PTP-SDBS system the ionic interaction between PTP and SDBS decreases due to the inter - interaction between SDBS and CTAB, thus shifting back the >C=O peak to 1686 cm-1 characterizing the formation of carboxylic acid group and re-forming H-bonds with the free -NHMe2+ group of PTP. At pH 9.2, in Figure S23b, the carboxylate ion of PTP shows a vibration band at 1690 cm-1 which shifts to 1683 cm-1 indicating ionic interaction between carboxylate ion of PTP with the cations of CTAB. On further addition of SDBS the carboxylate ion band shifts back to 1690 cm-1. These results therefore indicate a reversibility of the interactions of PTP with the surfactants at different pH conditions. Morphology: It is necessary to observe the change of morphology of PTP after addition of surfactants at both the acidic and basic conditions. The TEM micrographs of Figure 7 indicate that at pH 2.7, PTP has spheoroidal morphology (Figure 7a) (average dimension = 12 ±3nm) with a lesser dense corona surrounding it. This indicates a core-shell type micellar structure where the core is composed of hydrophobic polythiophene units and surrounding it the lesser dense grafted chains particularly the protonated DMAEMA chains exist forming the shell (inset of Figure 7a). On addition of SDBS above its CMC an aggregation of PTP micelles through SDBS micelles occurs causing a giant size aggregated structure (average dimension = 1200 ±100 nm) (Figure 7b) forming compound micelle. Upon addition of CTAB the aggregation of PTP through SDBS is lost and the PTP particles are found to be in lesser aggregated form (average dimension = 21 ±2nm) (Figure 7c). These particles tend to assemble in a quasiaggregated structure on the CTAB-SDBS matrix.

21


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(a)

(b)

200 nm

(c)

200 nm

Figure 7. TEM images of (a) PTP, (b) PTP+SDBS aggregated complex and (c) PTP+SDBS+CTAB disaggregated state at pH 2.7. The FESEM images (Figure 8) also clearly indicate the aggregation of PTP with SDBS at pH 2.7 followed by disaggregation on addition of CTAB. In Figure 8a, the spheroidal morphology of PTP is observed, but on addition of SDBS the spheres are found

(a)

(b)

(c) Figure 8. SEM images of (a) PTP, (b) PTP+SDBS aggregated complex and (c) PTP+SDBS+CTAB disaggregated state at pH 2.7. 22


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to be much bigger due to aggregation of PTP with the SDBS micelles forming compound micelles (Figure 8b). In Figure 8c the disaggregation of the giant spheres on addition of CTAB is clearly noticed producing back the smaller size PTP spheroids. However, here some swelled domains are observed at the micrograph and these may be produced due to the self organization of the two surfactants. At pH 9.2, the morphology of PTP is quite different and it exhibits a typical structure which might be produced by the agglomeration of PTP spheroids. At this pH the carboxylic acid groups of hydrolyzed pTBMA remain ionized, but the pDMAEMA chains remain unionized. As a result the PT chain along with the grafted pDMAEMA chains tends to aggregate in spheroidal structure and the ionized carboxylic acid group of hydrolyzed pTBMA remains dispersed surrounding it forming the agglomeration of PTP spheroids (Figure 9a). On addition of the cationic surfactant CTAB above its CMC, the anionic carboxylic group of hydrolyzed pTBMA form complexes causing a giant aggregated

(a)

(b)

(c)

Figure 9. TEM images of PTP (a), PTP+CTAB aggregated complex (b) and (c) PTP +CTAB+SDBS disaggregated state at pH 9.2. 23


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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

compound micelle structure of PTP-CTAB (average dimension = 800 ±50nm) (Figure 9b). On further addition of SDBS the giant aggregated structure breaks into smaller size quasi aggregates (Figure 9c) over the assembled SDBS-CTAB matrix. A comparison of DLS sizes with respective TEM/SEM sizes indicate that DLS sizes are bigger than TEM/SEM images because DLS results afford the hydrodynamic sizes where some solvent remains trapped. The aggregation and disaggregation behavior of PTP can be well understood from the generalized Scheme-2. Because of the multiple aggregation of PTP with the micelles of SDBS/CTAB at acidic/basic pH by electrostatic interaction the size of the conjugate increases abruptly. Such a large particle size immediately collapses upon addition of oppositely charged surfactants because of inter-surfactant interaction between CTAB and SDBS which is stronger than polymer-surfactant interaction for bulkier size of the polymer chain. Consequently, the polymer chain back to its original configuration showing its initial size.

Scheme 2. Schematic aggregation-disaggregation processes with enhancement and reduction of fluorescence of PTP with ionic surfactants at acidic and basic pH. Conclusion: PTP exhibits aqueous solubility via formation of small sized miceller aggregates (CMC=0.025mg/mL at pH2.7) with hydrophobic polythiophene at the core and radiating polyionic side chains (cationic or anionic depending on the pH of the medium) at the shell. On sequential addition of differently charged ionic surfactants the polymer shows reversible blue and red shift of its absorbance, also reversible fluorescence on and off response in both 24


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acidic and basic medium. This result shows that PTP has potential for surfactant induced reversible fluorescence turn on and off using ionic surfactant (SDBS and CTAB) through ionic aggregates formation and de-assembly of that aggregates. Acknowledgements: We gratefully acknowledge DST New Delhi (grant no SB/SI/OC11/2013) for financial support. RG and SD acknowledges CSIR and DST “INSPIRE” program respectively for providing the fellowship. Supporting Information: Calculation of molecular weight of PT-g-pDMAEMA-co-pTBMA, 1H,

13

C NMR and Mass

spectra of TI, 1H, 13C NMR spectrum, GPC traces of PTI and PT-g-pDMAEMA-co-pTBMA, Fluorescence spectra of PTP, DLS and FTIR spectra with surfactants are presented in Supporting Information. It is available free of charge via the internet at http://pubs.acs.org. References: 1. Cobo, I.; Li, M.; Sumerlin, B. S.; Perrier, S., Smart hybrid materials by conjugation of responsive polymers to biomacromolecules. Nat Mater 2014, 14, 143-159. 2. Liu, B.; Bazan, G. C., Homogeneous Fluorescence-Based DNA Detection with WaterSoluble Conjugated Polymers. Chemistry of Materials 2004, 16, 4467-4476. 3. Jagur-Grodzinski, J., Electronically conductive polymers. Polymers for Advanced Technologies 2002, 13, 615-625. 4. Heredia, K. L.; Bontempo, D.; Ly, T.; Byers, J. T.; Halstenberg, S.; Maynard, H. D., In Situ Preparation of Protei-"Smart" Polymer Conjugates with Retention of Bioactivity. Journal of the American Chemical Society 2005, 127, 16955-16960.

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5. Nilsson, K. P. R.; Inganas, O., Chip and solution detection of DNA hybridization using a luminescent zwitterionic polythiophene derivative. Nat Mater 2003, 2, 419-424. 6. Ho, H.-A.; Leclerc, M., Optical Sensors Based on Hybrid Aptamer/Conjugated Polymer Complexes. Journal of the American Chemical Society 2004, 126, 1384-1387. 7. Massoumi, B.; Jaymand, M., Conducting poly(vinyl chloride)-graft-polythiophene: synthesis, characterization, and materials properties. Journal of Materials Science: Materials in Electronics 2016, 27, (3), 2267-2275. 8. Yao, K.; Chen, L.; Chen, Y.; Li, F.; Wang, P., Influence of water-soluble polythiophene as an interfacial layer on the P3HT/PCBM bulk heterojunction organic photovoltaics. Journal of Materials Chemistry 2011, 21, 13780-13784. 9. Tao, Q.; Xia, Y.; Xu, X.; Hedstrom, S.; Backe, O.; James, D. I.; Persson, P.; Olsson, E.; Inganas, O.; Hou, L.; Zhu, W.; Wang, E., D-A1-D-A2 Copolymers with Extended Donor Segments for Efficient Polymer Solar Cells. Macromolecules 2015, 48, 1009-1016. 10. Shit, A.; Nandi, A. K., Interface engineering of hybrid perovskite solar cells with poly(3thiophene acetic acid) under ambient conditions. Physical Chemistry Chemical Physics 2016, 18, 10182-10190. 11. Sun, Y.; Lu, X.; Lin, S.; Kettle, J.; Yeates, S. G.; Song, A., Polythiophene-based fieldeffect transistors with enhanced air stability. Organic Electronics 2010, 11, 351-355. 12. Ahn, T.; Lee, H.; Han, S.-H., Effect of annealing of polythiophene derivative for polymer light-emitting diodes. Applied Physics Letters 2002, 80, 392-394. 13. McQuade, D. T.; Pullen, A. E.; Swager, T. M., Conjugated Polymer-Based Chemical Sensors. Chemical Reviews 2000, 100, 2537-2574. 26


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14. Ghosh, R.; Das, S.; Chatterjee, D. P.; Nandi, A. K., Cationic polythiophene for specific detection of cyanide ions in water using fluorometric technique. RSC Advances 2015, 5, 92564-92572. 15. Yao, Z.; Yang, Y.; Chen, X.; Hu, X.; Zhang, L.; Liu, L.; Zhao, Y.; Wu, H.-C., Visual Detection of Copper(II) Ions Based on an Anionic Polythiophene Derivative Using Click Chemistry. Analytical Chemistry 2013, 85, 5650-5653. 16. Das, S.; Chatterjee, D. P.; Samanta, S.; Nandi, A. K., Thermo and pH responsive water soluble polythiophene graft copolymer showing logic operation and nitroaromatic sensing. RSC Advances 2013, 3, 17540-17550. 17. Shiraki, T.; Dawn, A.; Tsuchiya, Y.; Shinkai, S., Thermo- and Solvent-Responsive Polymer Complex Created from Supramolecular Complexation between a Helix-Forming Polysaccharide and a Cationic Polythiophene. Journal of the American Chemical Society 2010, 132, 13928-13935. 18. Rajwar, D.; Ammanath, G.; Cheema, J. A.; Palaniappan, A.; Yildiz, U. H.; Liedberg, B., Tailoring Conformation-Induced Chromism of Polythiophene Copolymers for Nucleic Acid Assay at Resource Limited Settings. ACS Applied Materials & Interfaces 2016, 8, 83498357. 19. Kim, B.-S.; Osada, Y., Tuning the thermochromism of water-soluble poly(3thiopheneacetic acid) conjugated polymer by complexation with zwitterionic polymer. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2006, 280, 237-240. 20. Das, S.; Chatterjee, D. P.; Ghosh, R.; Nandi, A. K., Water soluble polythiophenes: preparation and applications. RSC Advances 2015, 5, 20160-20177.

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21. Jaymand, M.; Hatamzadeh, M.; Omidi, Y., Modification of polythiophene by the incorporation of processable polymeric chains: Recent progress in synthesis and applications. Progress in Polymer Science 2015, 47, 26-69. 22. Kudaibergenov, S. E.; Ciferri, A., Natural and Synthetic Polyampholytes, 2a Functions and Applications. Macromolecular Rapid Communications 2007, 28, 1969-1986. 23. Kudaibergenov, S.; Jaeger, W.; Laschewsky, A., Polymeric Betaines: Synthesis, Characterization, and Application. In Supramolecular Polymers Polymeric Betains Oligomers, Springer Berlin Heidelberg: 2006; pp 157-224. 24. Pafiti, K. S.; Elladiou, M.; Patrickios, C. S., "Inverse Polyampholyte" Hydrogels from Double-Cationic Hydrogels: Synthesis by RAFT Polymerization and Characterization. Macromolecules 2014, 47, 1819-1827. 25. Zhang, Q.; Hoogenboom, R., UCST behavior of polyampholytes based on stoichiometric RAFT copolymerization of cationic and anionic monomers. Chemical Communications 2015, 51, 70-73. 26. Ying, G.-G., Fate, behavior and effects of surfactants and their degradation products in the environment. Environment International 2006, 32, 417-431. 27. Gizzi, C.; Papoff, P.; Barbara, C. S.; Cangiano, G.; Midulla, F.; Moretti, C., Old and new uses of surfactant. The Journal of Maternal-Fetal & Neonatal Medicine 2010, 23, 41-44. 28. Yao, Z.; Li, Y.; Li, C.; Shi, G., Disassembly of conjugated polyelectrolyte aggregates and their application for colorimetric detection of surfactants in water. Chemical Communications 2010, 46, 8639-8641.

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29. Evans, R. C., Harnessing self-assembly strategies for the rational design of conjugated polymer based materials. Journal of Materials Chemistry C 2013, 1, 4190-4200. 30. Li, Y.; Bai, H.; Li, C.; Shi, G., Colorimetric Assays for Acetylcholinesterase Activity and Inhibitor Screening Based on the Disassembly-Assembly of a Water-Soluble Polythiophene Derivative. ACS Applied Materials & Interfaces 2011, 3, 1306-1310. 31. Li, C.; Shi, G., Polythiophene-Based Optical Sensors for Small Molecules. ACS Applied Materials & Interfaces 2013, 5, 4503-4510. 32. Wang, L.; Feng, Q.; Wang, X.; Pei, M.; Zhang, G., A novel polythiophene derivative as a sensitive colorimetric and fluorescent sensor for anionic surfactants in water. New Journal of Chemistry 2012, 36, 1897-1901. 33. Evans, R. C.; Knaapila, M.; Willis-Fox, N.; Kraft, M.; Terry, A.; Burrows, H. D.; Scherf, U., Cationic Polythiophene-Surfactant Self-Assembly Complexes: Phase Transitions, Optical Response, and Sensing. Langmuir 2012, 28, 12348-12356. 34. Lin, C. H.; Chau, C.M.; Lee, J. T., Synthesis and characterization of polythiophene grafted with a nitroxide radical polymer via atom transfer radical polymerization. Polymer Chemistry 2012, 3, 1467-1474. 35. Das, S.; Samanta, S.; Chatterjee, D. P.; Nandi, A. K., Thermosensitive water-soluble poly(ethylene glycol)-based polythiophene graft copolymers. Journal of Polymer Science Part A: Polymer Chemistry 2013, 51, 1417-1427. 36. Kuila, A.; Maity, N.; Chatterjee, D. P.; Nandi, A. K., Temperature triggered antifouling properties of poly(vinylidene fluoride) graft copolymers with tunable hydrophilicity. J. Mater. Chem. A 2015, 3, 13546-13555. 29


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37. An, Y.; Bai, H.; Li, C.; Shi, G., Disassembly-driven colorimetric and fluorescent sensor for anionic surfactants in water based on a conjugated polyelectrolyte/dye complex. Soft Matter 2011, 7, 6873-6877. 38. Manna, S.; Nandi, A. K., Piezoelectric β Polymorph in Poly(vinylidene fluoride)Functionalized Multiwalled Carbon Nanotube Nanocomposite Films. J. Phys. Chem. C 2007, 111, 14670-14680. 39. Lakowicz, J. R., Principles of Fluorescence Spectroscopy, Springer, 3rd edn, 2006. 40. Yin, J.; Shi, S.; Hu, J.; Liu, S., Construction of Polyelectrolyte-Responsive Microgels, and Polyelectrolyte Concentration and Chain Length-Dependent Adsorption Kinetics. Langmuir 2014, 30, 9551-9559. 41. Ray, G. B.; Chakraborty, I.; Ghosh, S.; Moulik, S. P.; Palepu, R., Self-Aggregation of Alkyltrimethylammonium Bromides (C10-, C12-, C14-, and C16TAB) and Their Binary Mixtures in Aqueous Medium: A Critical and Comprehensive Assessment of Interfacial Behavior and Bulk Properties with Reference to Two Types of Micelle Formation. Langmuir 2005, 21, 10958-10967. 42. Chakraborty, T.; Chakraborty, I.; Ghosh, S., Sodium Carboxymethylcellulose-CTAB Interaction: A Detailed Thermodynamic Study of Polymer-Surfactant Interaction with Opposite Charges. Langmuir 2006, 22, 9905-9913. 43. Bahadori, F.; Dag, A.; Durmaz, H.; Cakir, N.; Onyuksel, H.; Tunca, U.; Topcu, G.; Hizal, G., Synthesis and Characterization of Biodegradable Amphiphilic Star and Y-Shaped Block copolymers as Potential Carriers for Vinorelbine. Polymers 2014, 6, 214-242.

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Surfactant Triggered Fluorescence Turn “on/off” Behavior of a Polythiophene – graft - Polyampholyte

Radhakanta Ghosha, Sandip Dasa, Dhruba P. Chatterjeeb and Arun K. Nandia* a

Polymer Science Unit, Indian Association for the cultivation of Science, Jadavpur, Kolkata700 032, INDIA, Email: [email protected]


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