Optoelectronic Properties of Self-Assembled Nanostructures of

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Invited Feature Article

Optoelectronic Properties of Self-assembled Nanostructures of Polymer Functionalized Polythiophene and Graphene Nabasmita Maity, Radhakanta Ghosh, and Arun K. Nandi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04387 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 4, 2018

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Optoelectronic Properties of Self-assembled Nanostructures of Polymer Functionalized Polythiophene and Graphene Nabasmita Maitya, Radhakanta Ghosha and Arun K. Nandi* Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700 032, INDIA

Abstract: In this highlight, we have discussed the variation of optoelectronic properties with the aggregation style of polythiophene (PT) graft copolymers and polymer modified graphene systems. Grafting of flexible polymers on PT chain exhibit several self-organized patterns under various conditions, causing different optical and electronic properties, arising from the different conformational states of the conjugated chain. Graphene, a zero band gap material, is functionalized with polymer by both covalent and noncovalent way to bring finite band gap importing

new

optoelectronic

properties.

The

polymer

triggered

self-assembled

nanostructures of PT and graphene based materials bring unique optical/electronic properties suitable for sensing toxic ions, nitroaromatics, surfactants; drug delivery and also for fabricating molecular logic gates, electronic rectifier, photo current devices etc.

*

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

a

Both the authors contributed equally

ACS Paragon Plus Environment

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Introduction: Over the past decades, a large number of scientists has dedicated their enormous efforts to develop receptive macromolecules that can be crafted into new smart materials with the properties required for numerous practical applications. Two of such demanding applications are in the field of optics and electronics. The rapid development of nanotechnology begets the demand of better performance of optoelectronic materials, and it becomes essential to design new materials that will meet these demands. The field of optoelectronics merges two disciplines; therefore, its emergence is not surprising because lots of applications require a collaboration of both optical and electronic properties. Thus, fabrication of a polymeric system bequeathed with optoelectronic properties become an attractive area of interest. Amongst the different types of fluorescent and electroactive materials, the unique and tuneable optoelectronic properties of polythiophene (PT) and graphene make them promising candidates for this purpose. Although, PT and graphene belong to entirely different families, but, their properties unite them to achieve unprecedented optoelectronic applications. The discovery of polythiophene (PT) attracts a special attention in the field of material science for its intrinsic tuneable optoelectronic properties, originating from the nanoscale self-assembled structure of its derivatives.1 In the meantime, discovery of graphene (2004)2 has stimulated immense interest for both fundamental science and practical applications due to the unique optoelectronic properties of graphene based self-assembled nanomaterials.3,4 Both these systems at their pristine state suffer from insolubility in common solvents, particularly in water. Now, solution processable materials are always cost-effective and handier for various technological applications. Therefore, to overcome the solubility problem, one way is the polymer functionalization which also have the capability of forming nanostructured self-assembly that would originate new and tuneable optoelectronic properties


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depending on the nature of the assembly. Thus, both PT and graphene based derivatives are continuing to be extensively studied for various technological applications e.g. optoelectronic, imaging, drug delivery, sensing devices etc.5-21 Generally, PT based systems form either planer or non-planer conformation, dictated by its extended-ordered or coiled-random backbone conformation.22 It often self-assembles through side-chain aggregation to form π-stacked structure, leading to superior optical and electrical properties.23 Thus self-assembly is strategically used to manipulate the molecular arrangement for developing better optoelectronic devices.24 It is normally difficult to control the aggregation nature and optical properties of PT in the most effective way for more extensive delocalization of the π-electrons without using any hazardous solvent or annealing process. However, it is sometimes possible to drive the molecular structure towards the desired type of aggregation specially in water using simple external stimuli such as temperature, pH, and ions.22,

25-28

To alleviate the solubility problem of pristine PT and in

order to tune its properties, various flexible polymers are grafted via “grafting from” or “grafting to” approach.29-31 The interactions among pendant groups and /or with PT backbone and / or with solvent molecules are responsible for assembly or disassembly, inducing conformational change in PT backbone varying its optoelectronic response. On the other hand, graphene, the two-dimensional monolayer network of sp2hybridised carbon atoms, is difficult to process in common solvents and being a zero bandgap material, it is incapable of showing any spectral transition. Thus, functionalization of graphene is the primary requirement to overcome the poor dispersibility as well as to open up finite band gap resulting in fluorescence property for wide applications.17-19 So, graphene derivatives i.e. graphene oxide (GO), reduced graphene oxide (RGO) and graphene quantum dots (GQDs) are extensively investigated.32-34 In last few years, a number of fantastic review articles dealing with the properties and applications of graphene based nanomaterials are


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published, summarizing that the functionalized substituents and functionalization methods can tune the band-gap and hence the properties.20,21,35-37 Recently, functionalization by polymers has gained growing popularity as it facilitate the self-assembly imparting unique new properties. Polymers have multifarious properties e.g. easy solution processability, flexibility, phase behaviour, self-aggregation property, stimuli responsiveness etc.38,39 Thus, surface modification of graphene by polymers is an elite way to modulate the selfaggregation, offering new avenues for applications in multiple areas. In this feature article, a concise account of optical and electronic properties modulation of the self-assembled polythiophene and graphene-based nanomaterial systems is presented, where the self-assembly is governed by attaching flexible polymers and the formation of effective type of aggregates decides the final outcome. Polymer Functionalized Polythiophene based Nanomaterials: The correlation between molecular arrangements of the PT conjugated chain and their resulting optoelectronic properties are unravelled by many research groups extensively in last few years.1,14,29,30,40 The delocalized π-electrons on the PT main chain are responsible for all its unique optical and electrical properties which depend on its backbone conformations, controlled by its aggregation nature. Here, we would consider how the (i) optical and (ii) electrical properties are tuned through the assembly-disassembly of PT graft copolymers. Optical properties: Generally, the photophysical properties of PT derivatives are dominated by the side-chain or inter-chain interaction induced chemical and electronic structures of π-conjugated backbones. PT main chain tends to form aggregates due to π-π stacking in aqueous media because of their amphiphilic structures (hydrophobic backbones and hydrophilic side


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groups).23 However, at optimized condition the hydrophilic side chains break the aggregates leading to well-defined organisation to get improved photo physical properties. In 1987, Heeger and co-workers first reported an water soluble PT derivative,41 and a considerable amount of research in this field has been made by Inganas, Leclerc, McCullough, Shinkai and their coworkers.8-11 Such water-soluble PT derivatives have mainly ionic groups as a small pendent hydrophilic moieties from PT backbone. However, the molecular mechanism that drive the observed optical and electrochemical effects of highly densed PT graft copolymers are still poorly understood. Costanzo and Stokes in 2002, first forwarded an idea to incorporate highly densed poly(methyl acrylate) chains from PT macroinitiator showing improved photo luminescent properties.23 The synthesized polymer was not water soluble due to hydrophobicity of poly(methyl acrylate) chains, but the side chains help to prevent any aggregation of the PT backbone. This steric effect of poly(methyl acrylate) chains causes twisting of PT backbone resulting a decrease in conjugation length. Hence, the nonradiative decay paths of PT excitons are diminished leading to 20 times hike of fluorescence intensity relative to that of PT macroinitiator with a quantum yield of 40% in the solid state.23 In 2005, Balamurugan et al. successfully synthesized a highly water-soluble, thermally responsive poly(thiophene-g-N-isopropylacrylamide) [molar mass ≈ 2×106] molecular brush with an unique thermochromic behavior.26 They demonstrated the temperature-induced structural transition of poly(N-isopropyl acrylamide) (PNIPAM) in water leading to a change in the effective conjugation length of PT backbone with a sharp and reversible changes of colour and absorption intensity at 30-35 °C. The PT backbone adopts a roughly disordered, randomcoiled structure at the temperatures below lower critical solution temperature (LCST) of PNIPAM, and a more ordered globular structure above LCST due to the combined steric interactions between the PNIPAM and PT backbone and the hydrophilic/hydrophobic environment afforded by the PNIPAM segments.26 In 2008, Winnik and co-workers provided


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direct evidence for the molecular mechanism that drives the pH-induced conformational changes of a polythiophene graft poly(N,N-dimethylaminoethyl methacrylate) (PT-g-PDMA) [number average molecular weight (Mn)=190000, polydispersity index (PDI) = 1.9, degree of polymerization (DP) of grafted polymer= 690].22 At low pH due to the repulsive interactions among the protonated poly(N,N-dimethyl aminoethyl methacrylate) (PDMAEMA) side chains, the PT backbone adopts less folded and twisted conformation (Figure 1a) consisting primarily more circular individual molecules with diameters in the range from 20 to 45 nm

(a)

water (pH 8)

(b)

(c)

water (pH 2)

(d)

Figure 1. (a) Schematic representation for the pH dependent molecular conformational transition of PT-g-PDMA with colour change of the polymer solution in water ; AFM images (phase) from dilute aqueous solution at (b) pH 2, and (c) pH 8, and (d) FL spectra of PT-g-PDMA in water at pH 8 and at pH 2. Reproduced from Wang et. al.22 Copyright 2008 American Chemical Society. (Figure 1b) than at high pH where PT exists as elongated clusters of molecules containing multiple cores with diameters of 35-70 nm (Figure 1c). This causes a colour change of the solution from yellow to dark orange (Figure 1a) with a significant hike in emission intensity (Figure 1d).22 Using the pH dependent protonation and deprotonation of PDMAEMA segments, we have developed a fully polymeric fluorescent molecular logic gate where fluorescence property of PT-g-PDMA (PD) doped methyl cellulose (MC) hydrogel42 is taken as an output and temperature and pH are used as inputs. The number of thiophene monomer


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units present in the macroinitiator (i.e. average graft density) is about 141 and the Mn of PD is 173000 with PDI value of 2.1, hence the average graft length of PDMAEMA chain is 6. The hydrophobic nature increases comparatively at pH 9.2 leading to reduction in the microenvironmental polarity near the PT backbone and enhances its fluorescence intensity than that at pH 4 or pH 7. Similarly at 45 °C, a significant enhancement in emission intensity of the PD doped aqueous MC solution is observed due to an increase of hydrophobicity of the medium with thermal gelation of MC embedding PD micelles and producing fibre-like structure via self-assembly.42 In the same year, we have reported a series of water-soluble thermosensitive poly(ethylene glycol)-based polythiophene graft copolymers and successfully tuned the phase transition and optical properties with systematically varying the copolymer composition.27 The

π-π*

absorption

band

of

the

backbone

shows

a

red

shift

in

PT-g-

poly(diethyleneglycolmethylethermethacrylate)-copoly(oligoethyleneglycolmethylethermethacrylate) (PT-g-P(MeO2MA-co-OEGMA, PTDO) copolymers from that of PT-g-PMeO2MA, (PTD) due to the extension of PT conjugation length in the former. A sharp increase in emission intensity of PTD solution is noticed at the LCST due to collapse of the radiating PMeO2MA fibrils forming a shell on the PT nanosphere. PTD shows LCST at 21 °C which gradually increases from 31 to 43 °C with increase in OEGMA concentration. Hence, the exact transition temperature at the physiological range in the PTDO copolymer can be achieved by fixing the composition of MeO2MA and OEGMA, to construct precession biosensors for working under physiological condition. Next we have grafted temperature responsive PMeO2MA and pH responsive PDMAEMA chains from PT backbone for logic operation and nitroaromatic sensing.28 PT-gP(MeO2MA-co-DMAEMA) (PTDM, Mn = 442600 and PDI=1.5) exhibits a sharp increase in fluorescence intensity and particle size above its LCST only at pH 9.2. In this situation both PMeO2MA and non-protonated PDMAEMA segments collapse on the PT core and therefore,


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a more compact arrangement of the core forms. Using the pH and temperature as inputs and the fluorescence intensity as an output, this is the first report using polythiophene as the fluorescence probe system to act as a fully polymeric AND logic gate. This graft copolymer can also be used for detection of nitroaromatics such as picric acid, dinitro phenol, etc. in the solid/solution state originating from the quenching of PT excitons by the electron deficient nitro substituted aromatic rings. Thus the water soluble PT can act as a multifunctional polymer, which may extend its applications in sensing, computing, etc.43 A fluorometric sensor for the detection of toxic cyanide (CN-) ion in water using the self-assembly of a cationic polythiophene (PT-g-DMA, Mn=173000, PDI=2.1) with iodide counter ion (CPT-I) is recently investigated in our laboratory.44 The absorption spectra of PTg-DMA and CPT-I systems in aqueous solution exhibit absorption maxima at 424 and 446 nm, respectively. The repulsion between positively charged –NMe3+ groups causes, the PT backbone into more uncoiled and planar conformation in CPT-I from that of PT-g-DMA. The CPT-I has lower emission intensity than PT-g-DMA due to the excitonic energy transfer from PT excitons to iodide ions which is a well-known fluorescence quencher for its “heavy-atom effect” increasing the rate of inter system crossing causing nonradiative decay.45 When the iodide anions are replaced by gradual addition of cyanide ions, the fluorescence intensity shows a step by step hike. Thus CPT-I exhibits very high sensitivity, selectivity, and quick responsiveness towards CN- ion over large number of other anions with limit of detection 3.2 ppb. Ionic amphiphiles can interact with oppositely charged surfactant molecules through electrostatic and hydrophobic interactions13,14,24 leading to the formation of compound miceller aggregation. This concept has been successfully deployed in the aggregation of a polythiophene-graft-polyampholyte (PTP) to tune its optical responseby properly choosing the ionic nature of the surfactants.46 The PTP is synthesised by hydrolysis of poly(tert-butyl


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methacrylate) segments (PTBMA) of PT-g-poly(N,N- dimethylaminoethyl methacrylate)-copoly(tert-butyl methacrylate) [Mn= 102000 and graft length of ~7]. The PTP exhibits water solubility via formation of small sized miceller aggregates (Figure 2a). In acidic media, the polycationic PTP (formed due to the protanation of PDMAEMA) co-assembles producing compound micelles (Figure 2b) with the anionic surfactant, sodium dodecyl benzene sulfonate (SDBS), via electrostatic interaction resulting in a 20 nm blue-shift in the absorption maximum with an increase in the fluorescence intensity. The aggregates can be disassembled through the addition of a cationic surfactant cetyltrimethylammonium bromide (CTAB) which triggers PT chains to return back to its relatively straightened conformation

(a)

(b)

(d)

(c)

At pH 2.7,

= SDBS,

At pH 9.2,

= CTAB,

= CTAB. = SDBS.

Single micelle

Aggregation of PTP-

Disaggregation of PTP-

of PTP.

Surfactant forming

Surfactant Compound

compound micelle.

micelle.

Figure 2. SEM images of (a) PTP producing individual micelles in water, (b) PTP + SDBS aggregated complex forming compound micelles, (c) Reversible fluorescence turn “on” and “off” behaviour of PTP with increasing amounts of SDBS then CTAB sequentially in acidic water solution, and (d) Schematic aggregation-disaggregation processes of PTP with ionic surfactants at acidic and basic pH. Reproduced from Ghosh et. al.46 Copyright 2016 American Chemical Society.


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showing 10 nm red-shifts in absorption maximum, regenerating the optical properties of the free PTP. Conversely, in basic media, the carboxylic acid group is deprotonated and the PTP co-assembles with CTAB, causing a 27 nm blue shift in absorption maximum and an increase of fluorescence intensity, which can also be subsequently reversed through the addition of SDBS. The polymer shows repeatedly reversible fluorescence on and off with the sequential addition of differently charged ionic surfactants response in both acidic (Figure 2c) and basic medium. Thus, PTP has potential for surfactant induced reversible fluorescence turn on and off using ionic surfactant (SDBS and CTAB) through assembly and de-assembly of ionic aggregates (Figure 2d). Recently we have tuned the optoelectronic properties of PT chains utilizing the position of Hofmeister iodide ion by varying pH or by adding polyanionic RNA in [polythiophene-g-poly{(N,N,N-trimethylamino iodide)ethylmethacrylate-co-methacrylic acid}, APT].45 Here, APT is synthesized by hydrolysis of PTBMA segments followed by quaternization

of

PDMAEMA

segments

of

PT-g-poly(N,N-dimethylaminoethyl

methacrylate)-co-poly(tert-butyl methacrylate) (Mn = 595520 g/mol with average graft length of 28). The π-π*absorption band (λabs = 364 nm) shows appreciable blue shift (Figure 3b) and so also in its emission signal (λem = 532 nm) (Figure 3c) in acidic medium due to the threading of grafted chains upon PT backbone which creates twisting of thiophene units deviating from planarity (Figure 3a). The co-operative effect of undissociated -COOH and quaternary ammonium group immobilizes chaotropic I- ion near to the apolar PT chain causing threading of grafted chains. As medium pH is increased, the ionization of –COOH group promotes dethreading of PT backbone, releasing quencher iodide ions from the vicinity of PT backbone resulting red shift in absorption (λabs = 445 nm) (Figure 3b) and emission signal (λabs = 558 nm) with a sharp hike (390 times) in fluorescence intensity (Figure 3c). With increase of pH, morphology changes from multi vesicular aggregate with vacuoles


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(b)

(a)

(c)

(d)

(e)

(f)

Figure 3. (a) Schematic appearance of threading-dethreading processes of grafted chains of APT with change of pH, where yellow ball indicates sulfur atom, blue ball indicates -N(Me)3+ group, red ball indicates I− ion, grey ball indicates -COOH group, and brown ball indicates -COO−group, (b) UV-vis absorption spectra (c) fluorescence spectra of APT at acidic and basic pH; FESEM micrographs of APT forming at pH 2 (d), at pH 4.5 (e), and at pH 9 (f). Reproduced from Ghosh et. al.45 Copyright 2017 American Chemical Society. (Figure 3d) to smaller size vesicles (Figure 3e) and finally to nanofibrillar network (Figure 3f). The aggregation induced changes in morphology and optical properties of APT is also tuned by conjugating with RNA via duplex formation with significant hike of fluorescence for displacing the quencher iodide ions.


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Electronic properties: The electronic properties of the PT based conjugated polymer are encouraging for device fabrication due to its semi-conducting nature.5 The non-conducting grafted polymer diminishes the conductivity of PT but it can be somewhat regained and tuned applying appropriate conditions like grafting of nitroxide radical polymer, conjugating with another conducting polymer, embedding nanoparticles and free ions, etc.45,47-49 Nitroxide radical polymers have huge potential for battery applications as while charging it transformed into oxoammonium cations on oxidation which again back to nitroxides by reduction during discharging.49 In 2012, Lin et al. have successfully grafted a nitroxide radical polymer, poly(2,2,6,6-tetramethylpiperidin-1-oxyl-4-yl methacrylate) from PT backbone (PEBBT-gPTMA).49 The electrochemical properties in organic radical batteries show that the polymer has good electrochemical stability and cyclybility as it inhibits the dissolution into the electrolyte solvents. Thus PEBBT-g-PTMA has potential as a cathode-active material in a rechargeable organic radical battery. In the same year, using the reducing ability of PT-gDMA, we have successfully synthesised and stabilized Au nanoparticles (NPs) in water and tuned its morphological as well as electronic properties with RNA depending upon its assembled structure.47 The Au NPs embedded PT based polymer shows 6 times higher dcconductivity than that of pure polymer due to the hopping process for charge carriers via Au NPs. This system also shows symmetric negative differential resistance (NDR) with a maximum NDR ratio of 64. This is due to the fact that during the preparation of Au NPs a large amount of charge is adsorbed on Au NPs causing hindrance to charge flow until attaining a higher voltage when it releases the trapped charges completely, giving almost zero current.47 In another work, we have demonstrated that PT-g-poly(methacrylic acid), PTMA (synthesised from hydrolysis of PT-g-PTBMA, Mn=212000, PDI=1.5) can be used both as a template and a dopant for the synthesis of polyaniline (PANI) nanostructures motivating its


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(a)

(b)

(d)

(c)

(e)

Figure 4. SEM images of (a) PTMA with spheroidal shape, (b) PTPA14 hybrid with rod shape morphology (c) PTPA120 hybrid with helical nanorod shape, (d) photocurrent cycles of PTPA12 hybrid show reversible turn “on”and turn “off” by switching the white light illumination on and off, respectively, (e) Nyquist plot of PTPA12 hybrid with its equivalent circuit. Reproduced with permission from ref. 48. Copyright 2014 Royal Society of Chemistry. morphological and electronic properties.48 PTMA-doped PANI (PTPA) hybrids can selfassemble into either non-helical or single-handed helical nanorod with some small size spheroidal morphology depending upon the amount of PANI ratio. PTMA itself has spheroidal morphology (Figure 4a) whereas PTPA hybrids have nanorod morphology (Figure 4b). On increasing aniline concentration with respect to PTMA the nanorod becomes helical in nature, along with some spheroids (Figure 4c). PTPA hybrid exhibits dcconductivity to a maximum of 4.1×10-2 Scm-1, whereas PTMA has conductivity value of only 5.6×10-7 Scm-1. The PTPA hybrid exhibits reproducible photo-conductivity by alternate “On” and “Off” switching of white light illumination with semiconducting nature (Figure 4d) and this photocurrent generation is the property of only PTPA hybrids as no appreciable


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photocurrent is observed for either PT macroinitiator or poly(methacrylic acid) doped PANI. The core–shell type structure of PTMA with a coating of conducting PANI is responsible for the good electrical conduction, because the doped PANI produces an extra electrical conduction path. The molecular arrangement of PANI and PT in the PTPA hybrids may store charges through resonance stabilization yielding a moderate capacitance value; hence, the single semicircle in the Cole-Cole plot of real and imaginary parts of complex impedance is obtained (Figure 4e). The threading and dethreading of grafted chains in APT45 influences the electrical properties of the system as apparent from the impedance spectra (Nyquist plot) (Figure 5). At pH 2 and pH 4.5, APT shows semicircle in the impedance spectra due to the presence of an

Figure 5. Nyquist plots of APT from (a) pH 2 and pH 4.5 and from (b) pH 9 with their corresponding equivalent circuit. Reproduced from Ghosh et. al.45 Copyright 2017 American Chemical Society. equivalent circuit containing capacitance (C) and resistance (R2) in parallel to each other (Figure 5a), whereas at pH 9 a semicircle and a linear hike corresponds to the presence of Warburg impedance (W) at lower frequency region (Figure 5b). At lower pH, the loss of planarity of the PT chain and the shield resisting interchain hopping of charge carriers for the threaded non-conducting grafted chain causes hindrance to charge flow. APT has maximum capacitance value at pH 2, as the vesicle surface (cf. Figure 3d) can easily store the charges


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but at pH 4.5, due to the creation of partial negative charges, there occurs some charge annihilation resulting the lowest capacitance value. At pH 9, APT shows some increase in capacitance with Warburg impedance which arises due to mass transfer as complete dethreading makes I- ions free to move on the nanofibrillar network (cf. Figure 3f).45 Polymer functionalized graphene-based nanomaterials: Unlike graphene, its oxidative derivatives like GO, RGO, GQDs etc. possess sp3 domains in sp2 carbon network, importing electronic band gap, which are intrinsically related with the shapes, sizes, topology and symmetry of sp2 carbon domains.20,21 Owing to the radiative recombination of electron-hole pairs at the confined sp2 carbon domains fluorescence arises in graphene derivatives. In polymer functionalized graphene, self-assembly of grafted polymer chains play a crucial role on the aggregation and disaggregation of it, regulating the π-π stacking, modulating the band gap and changing the optical as well as electronic properties. Optical properties: Kundu et. al. have reported functionalization of GO with methyl cellulose (MC) and poly(vinyl alcohol) (PVA) in a noncovalent approach, where the fluorescence (PL) of GO (Figure 6b) has enhanced in a large extent with increasing the polymer concentration at pH 4.50,51 The pendent -OH groups of MC and PVA interact with the oxygenated functional groups (epoxy, -COOH, -OH) of GO via H-bonding (Figure 6a), which prevent restacking of GO, causing passivation of electron-hole recombination and thus, resulting in the enhancement of PL intensity. Highly green emitting ribbons and fibrils of self-assembled GO-MC and GO-PVA hybrids, respectively, are observed under fluorescence microscope (Figure 6c). Covalent attachment of a stimuli responsive hydrophilic polymer PNIPAM from GO surface52 has increased the fluorescence intensity of GO significantly with a gradual blue


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1. GO 2. GO-PVA 0.5 3. GO-PVA 1.0 4. GO-PVA 2.0

4 438 nm

3

c

5 4 3

2

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1

1 0

105µm 400

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Figure 6. (a) Schematic representation of supramolecular interactions between GO and PVA, (b) PL spectra of GO and GO-PVA hybrids of different composition at pH 4, (c) Fluorescence micrographs of GO-PVA1. Reproduced from Kundu et. al.51 Copyright 2012 American Chemical Society. shift on increasing PNIPAM content. At 25 oC, the N-H and -C=O groups of PNIPAM form intermolecular H-bonding with the water molecules, preventing GO from restacking, causing passivation of GO. Moreover, as evidenced from the Raman spectra lowering of sp2 domain size, reduction of GO planarity, defect level transition occur during polymerization and these are responsible for the restriction of nonradiative hopping of excitons, contributing to both the gradual increase in PL intensity and blue shift of the emission peak. The self-aggregation of PNIPAM chains with increasing temperature in the GO grafted PNIPAM hybrid (1:50 w/w) provides change in PL intensity with temperature at pH 7, indicating a phase transition temperature (LCST) at 36 oC and thus, it is used as a thermo-responsive fluorescent nanocarrier of both hydrophobic and hydrophilic drugs.52 Grafting of another stimuli responsive polymer, PDMAEMA (Mn~1500 with a PDI of 1.15) to RGO (RGP) has been reported to modulate the fluorescence property of RGO with change in pH of the medium.53 RGP exhibits two emission peaks at 407 and 461 nm for pH 4, but it shows a broad emission peak at 447 nm for pH 7 and at 449 nm for pH 9.2 (Figure 7a). It has been suggested that at pH 4, pendent -NMe2 groups of PDMAEMA remain fully protonated and because of the flexible coil nature of PDMAEMA chains, these -NHMe2+ groups dope graphene ring (p-type doping), producing secondary holes in its sp2 domains, which is supported by the Raman


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V= 1 V= 0 V= 2 V= 3 V= 4 V= 5 Figure 7. (a) PL spectra of RGP in different pH (inset: PL spectrum of RGP at pH 4), (b) Current-voltage (I-V) characteristic curves of RGP at pH 7, (c) (i-vi) Schematic representation of NDR mechanism. The green and blue rectangles represent the DOS of the two electrodes (right and left). Two red lines and black dotted line between them represent the positions of the HOMO, LUMO and polaronic band, respectively, of RGP (scattering region) with different bias voltages (Fermi energy is considered as zero). Reproduced with permission from ref. 53. Copyright 2014 Royal Society of Chemistry. spectral analysis. The radiative decay of excitons to the two types of holes i.e. the graphitic holes and the holes produced by p-type doping are responsible for the generation of two emission peaks at acidic pH. At pH 9.2, the -NMe2 groups dope the graphitic rings using their non-bonding electrons and this n-type doping of RGO shifts the Fermi level causing band gap reduction which causes both an increase in the PL intensity as well as in the red shift of the peak as compared to the main emission peak (407 nm) at pH 4. Removal of protons from the


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-NMe2 groups at basic pH prohibit additional excitonic decay path, producing a single emission peak. Similarly, at pH 7, most of the -NMe2 groups remain non protonated with a small extent of protonated -NMe2 groups producing single emission peak with a small hump. Thus polymer assisted doping of graphene tunes the band gap as well as the fluorescence property.53 Further, the effect of self-assembly of a diblock copolymer on the fluorescence property of GO is recently studied by grafting poly(ε-caprolactone)-block poly(N,N’dimethylaminoethyl methacrylate) (PCL13-b-PDMAEMA117) from GO surface, designated as GPCLD. Here, the subscripts 13 and 117 represent the DP of grafted PCL and PDMAEMA chains, respectively.54 Owing to the stimuli responsive nature of PDMAEMA and hydrophobic-hydrophilic balance of the block copolymer, GPCLD exhibits a phase transition temperature of 32 oC at pH 9.2. On the basis of increased hydrophobicity of PDMAEMA chains with rise in temperature, GPCLD shows a schizophrenic self-assembly behaviour at this pH, indicating a change in morphology from vesicles (Figure 8a) at 26 oC to annular ring (Figure 8b) (30 oC) to gigantic aggregates formation at 38 oC.54 With change in the pH of the medium from 9.2 to 4, morphological transformation is also observed for GPCLD. Depending on the degree of protonation of pendent –NMe2 groups of PDMAEMA, GPCLD transforms from blowhole vesicles (Figure 8c) (pH 7) to core-shell (Figure 8d) at pH 4. Moreover, both the phase transition and self-assembly behaviour of GPCLD can be reversibly tuned by CO2 and N2 gas flow (Figure 8e), which are studied in a delicate way by monitoring the change in PL intensity (Figure 8f, inset). A model illustrating the formation of different aggregates is presented in Figure 8g. Thus, GPCLD behaves as a good CO2 gas sensor at pH 9.2, at 38 oC and it has been used for CO2 triggered release of hydrophobic guest molecules.54 Again, fluorescence spectra of GPCLD at pH 4 and 9.2 (Figure 9b) provides the concept of localized p- and n-type doping of graphene on the basis of protonation and deprotonation of pendent -NMe2 groups of PDMAEMA in presence of the spacer PCL block.


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Figure 8. HRTEM images of GPCLD at (a) 26 oC, (b) 30 oC in pH 9.2 and at 26 oC in (c) pH 7 and (d) pH 4, (e) Digital images of GPCLD solution in pH 9.2 buffer at 26 oC, 38 oC and after CO2 purging at 38 oC, 46 oC, (f) PL Intensities (λmax: 467 nm) vs. temperature plots of GPCLD at pH 9.2 during heating, after CO2 and N2 purging (inset: Reversible change in PL Intensity upon alternate bubbling of CO2 and N2 at 38 oC), (g) Schematic representation of shape modulation of the GPCLD (green tinge at the outer region indicates the fluorescence of polymer grafted GO; Reproduced with permission from ref. 54. Copyright 2016 John Wiley and Sons. Moreover, the chain length alteration of each block is observed to affect the localized doping of graphene significantly, changing the fluorescence behaviour of GPCLD. Comparing the PL spectra of GPCLD1 [GO-g-(PCL13-b-PDMAEMA158)] and GPCLD2 [GO-g-(PCL3-bPDMAEMA60)] with that of GPCLD [GO-g-(PCL13-b-PDMAEMA117)], it is noticed that as the hydrophilic to hydrophobic ratio increases, the extent of localized doping decreases and delocalized doping starts appearing.55 Recently, we have reported a PCL grafted GQDs


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Figure 9. (a) Schematic representation of doping mechanism, (b) PL spectra of GPCLD at pH 4, 7, and 9.2 at 25 °C, (c) Log σ vs. 1000/T plot of GPCLD at pH 4, 7 and 9.2.; Current-voltage (IV) characteristic curves of GPCLD at (d) 27°C and (e) 37 °C at pH 9.2. Reproduced from Maity et. al.55. Copyright 2017 American Chemical Society. system having PCL of different DP (3, 7, 15, and 21), where the length of tethered chains plays a key role on the modulation of self-assembly and hence the optical behaviour in chloroform.56 With increase in the length of PCL chains, the optical and morphological analysis reveals the alteration in the aggregation pattern from J-aggregates to H-aggregates, causing morphological transformation from toroid to spheroid to rodlike structures. Here, as the chain length decreases the van der Waals force and H-bonding interactions between PCL chains become weaker, inducing GQDs to slide into loose packing arrangement producing Jaggregates. Similarly, with increase in the chain length interchain interactions increases, favouring face to face stacking, causing the H-aggregates formation. Functionalized GQDs with lower chain lengths of PCL (DP=3 and 7), also assembles into columnar hexagonal (Colh) liquid crystalline mesophases in chloroform at higher concentration (15 wt%). But, no


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such behavior is observed at higher chain lengths of PCL under identical conditions indicating that variation of PCL chain length plays a crucial role in the self-assembly. To understand the effect of conducting polymers on the fluorescence, Lau and co-workers have studied the PL emission peak of GQDs having diameter of 3.2 nm, which is red shifted from 454 to 489 nm in polyaniline (PANI)-GQDs hybrid and exhibits further red shifting from 489 to 515 nm with increasing mole number of aniline from 1 to 5 mmol. It is proposed that electron donating amine groups of aniline form a p-π conjugated system with electron withdrawing aromatic GQDs, resulting in elevation of primary HOMO to higher energy. Increased amount of aniline is expected to contribute to the enhancement of electron delocalization densities, causing reduction of optical band gap, thus justifying red shift of PL emission peak.57 Similar type of phenomenon is also reported decreasing the intensity of PL emission peak of PT-g-P(MeO2MA-co-DMAEMA) with addition of GQDs due to the energy transfer from donor PT to the acceptor GQDs.58 Electronic properties: Layek et. al. reported influence of PVA matrix on the aggregation of sulfonated graphene (SG) enhancing the dc conductivity of the system.59 With increase in SG content (0.3, 1, 3 and 5 wt%), PVA-SG hybrids self-assemble in different way showing different morphology and exhibit semiconducting nature for fibrils, rectification behaviour for rod like morphology and electronic memory for dendrites, where an easier hopping of charge carriers in dendrites contribute to the highest conductivity (9 x 10-5 S/cm) compared to the others. Functionalization of graphene with poly(vinyl chloride) (PVC) also improves electrical conductivity as covalent attachment of PVC helps to strengthen graphene dispersion, which causes a change in alignment of graphene from random to parallel oriented to the direction of stretching during deformation.60 Thus, PVC/graphene nanocomposites with very low


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graphene content exhibits a sharp transition from insulating to conducting material. Tan el. al. developed poly(styrene-co-methyl methacrylate) (P(St-co-MMA)) functionalized graphene to prepare conductive composites where the controllable interfacial distribution of graphene improves electrical conductivity of immiscible P(St-co-MMA).61 Effect of polymer modification of graphene on its electronic properties by grafting stimuli responsive polymers is investigated by our group. As discussed above, polymeric selfassembly under different pH and temperatures tune the band gap of graphene system,53,55 which has affected the electronic properties of graphene. The dc-conductivity of RGP at pH 4 is 2 orders higher than that at pH 9.2, indicating the contribution from additional hole conductivity due to p-type doping at acidic pH. The current(I)-voltage(V) behaviour of RGP reveals semiconducting nature both at pH 4 and 9.2 but at pH 7, interestingly, it exhibits bimodal NDR with rectification property (Figure 7b). This is because of the fact that RGP is partially doped by a portion of protonated -NMe2 groups along with another portion of deprotonated -NMe2 groups of PDMAEMA, forming intimate mixture of p-n junctions, which result in NDR and rectification property. The bimodal NDR is explained using a density of state model (Figure 7c), where apart from the HOMO and LUMO a polaronic state is formed due to partial p- and n-type doping of GO at pH 7.53 Moreover, in GPCLD, where PCL13-block-PDMAEMA117 is grafted from GO, the polymer assisted p- and n-type doping of graphene (Figure 9a) is also observed to alter the electronic properties of graphene with change in temperature and pH.55 As evident from impedance spectra, the Nyquist plot of GPCLD exhibits a semicircle at higher frequencies along with a spike at lower frequencies at pH 4; at pH 7 spike is transformed to an arc and at pH 9.2 only a spike is noticed instead of semicircle at higher frequencies, which further confirms different types of doping of graphene at different pH.55 As GPCLD exhibits schizophrenic behaviour with varying temperature and pH,54 the dc-conductivity of GPCLD is also found to vary with temperature


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and pH (Figure 9c) depending on different types of doping aroused from coiling and decoiling of polymer chains. Thus, GPCLD shows rectification at pH 7 and interesting NDR property at both pH 9.2 and 4 in its I-V behaviour. In addition, temperature variable I-V plots at pH 9.2 (Figure 9d, 9e) and pH 7 reveal existence of doping, dedoping and redoping of graphene, indicating a strong influence of coiling and decoiling of grafted polymers on the electronic properties of graphene. Here, it would be worth mentioning that in RGP doping by PDMAEMA occurs throughout the GO causing delocalized doping of graphene yielding semiconducting nature.53 But, the spacer PCL segment in diblock architecture helps in selective doping by PDMAEMA to a particular region of graphene i.e. localized doping, giving NDR at pH 9.2 in GPCLD. The variation of chain length of hydrophobic spacer also influences the doping of graphene, which is reflected in the I-V behaviour of the system. Unlike GPCLD, GPCLD1 with relatively lower hydrophobic to hydrophilic ratio of grafted polymers shows semiconducting behaviour at pH 9.2 arising from delocalized doping. However, with increase in the hydrophobic to hydrophilic ratio GPCLD2 exhibits small NDR at this pH due to localized doping.55 Li and co-workers have reported CO2-switchable graphene aqueous dispersions, where they have used series of CO2-switchable star copolymers (Mn=242-433 kDa) having hyperbranched polyethylene (HBPE) core with multiple

poly[(2-(dimethylamino)ethyl

methacrylate)-co-(2-(diethylamino)ethyl

methacrylate)] (P(DMAEMA-co-DEAEMA)) arms. In these dispersions, interactions of the star copolymer with graphene are affected by CO2/N2-bubbling.62 At 45 oC under N2 gas bubbling graphene flakes remain aggregated, which after CO2-bubbling redisperse to form stable dispersion (Figure 10a). The reversible aggregation and redispersion cycles occur repeatedly with successive CO2/N2-bubbling (Figure 10b), causing cyclic change in the conductivity (Figure 10c). Figure 10d represents the TEM image of redispersed graphene after multiple CO2/N2 reversible cycle.62


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Figure 10. (a) Schematic representation of preparation of graphene dispersions in HBPEP(DMAEMA-co-DEAEMA)s and their reversible aggregation and redispersion with N2/CO2 treatment, (b) digital images, (c) conductivity and (d) TEM image of redispersed graphene flakes after multiple CO2/N2 reversible cycle. Reproduced with permission from ref. 62. Copyright 2016 Royal Society of Chemistry. Conducting polymers also guide the morphology, providing better conductive paths for easier electron transportation, which leads to the modulation of the electronic properties of self-assembled nanostructures for the fabrication of opto-electronic devices. Morphology of polyaniline (PANI) changes from nanotube to a flat rectangular nanopipe (FRNP) when PANI is produced from amino-functionalized reduced graphene oxide (a-RGO) (Figure 11a, 11b), causing a large increase in the photocurrent than that of PANI. Based on the respective donor and acceptor property of PANI and a-RGO, bulk heterojunction (BHJ) is produced in the PANI-a-RGO hybrid (Figure 11c), which reduces electron-hole recombination leading to the observed photocurrent increment.63 On in situ polymerization of aniline with GQDs,


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a

b

c

Figure 11. (a) PANI-a-RGO, (b) Schematic representation of PANI nanotube and PANI-a-RGO

FRNP formation, (c) BHJ model of PANI-a-RGO FRNP hybrid by donor (PANI) and acceptor (a-RGO). Reproduced with permission from ref. 63. Copyright 2013 Elsevier. morphology of PANI has changed from flakes to rods with increasing concentration of GQDs and highest photoresponse is observed for perfectly organized rods. Compared to the flat on lamella of flakes, the closely packed cylindrical lamella of rod structures help in easier charge separation and movement under illuminated condition due to the optimum matching of the energy levels of donor PANI and acceptor GQDs.64 Song and co-workers have developed a hierarchical nanocomposite of PANI nanowire arrays on RGO sheets showing enhanced electrochemical performance in supercapacitor applications because of low internal resistance, easier charge transfer and ion conductivity.65 Zhai and co-workers have manipulated electrical properties of RGO via supramolecular interactions with poly(3-hexyl thiophene) (P3HT) nanowires. P3HT nanowires (Mn=14,800 with a PDI of 1.2) on RGO surface connect individual RGO monolayers, which provide variation in electrical properties of RGO, leading to its application in nanometer-scale electronics.66 Poly [3-(2-hydroxyethyl)2,5-thienylene] grafted RGO (PHET-g-RGO) self-assembles into fibrillar network


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morphology, where PHET fibres (Mn=19,470 with a PDI of 1.20) remain appended on the RGO surface producing a good electron transport promoter. This facilitates an effective charge separation on the dye adsorbed on it with a negative charge on the semiconducting PHET-g-RGO. It acts as an efficient photo electrode in dye sensitized solar cell.67 Thus, different morphology provides different results indicating great impact of self-aggregation pattern on final outcome.68,69 Summary and Outlook: In summary, the influence of polymer triggered self-assembly on the optical and electronic properties of PT graft copolymers and graphene derivatives are discussed. The optoelectronic property of PT is tuned by applying various conditions to get the desired result for making the fully polymeric AND logic gate, photo-conductive device, sensors like pH, temperature, nitoaromatic, toxic ions, surfactants, biomolecules etc. Also, self-assembly of GO/GQDs with polymers attached both by covalent and noncovalent way has been tuned producing interesting optoelectronic properties. These properties are used to sense toxic ions, nitro explosives, gas flows, to carry drug molecules as well as to fabricate electronic rectifier, semiconductors, solar cell etc. Hence different new optoelectronic materials are imported by polymer modification of polythiophene and graphene, suitable for wide range of applications. Thus, polymer driven self-assembly of electroactive polymers and carbon nanomaterials is a new exciting route to explore and tune the optical/electronic properties of the polymeric material appropriate for different applications, including dielectric, supercapacitor, photovoltaics, sensor, cell imaging, drug delivery etc. In this area apart from PT, other conducting polymers, piezoelectric/ferroelectic polymers may be self-assembled by grafting stimuli-responsive polymers which can generate new properties to make the polymers smart enough for application in different transducing devices. Further, conjugating with different metal or semiconducting nanoparticles of these stimuli responsive electroactive


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polymers and carbon nanomaterials would enhance the optoelectronic property significantly because of interaction with the plasmon band of nanoparticles. Functionalization of graphene with stimuli responsive polymers in a judicious manner would enable to generate myriad of new smart materials. The chain length of the grafted polymer/ block copolymer can enable the different types of aggregate formation (J/H) hence dictating the morphology of the assembly as well as its properties. Judicious choice of the polymeric chain can evolve different liquid crystalline morphology and hence would be appropriate for display applications. The polymer assisted doping can be further extended with metal nanoparticles which would generate novel properties. So this polymer triggered self-assembly of different materials is an exciting new field to generate myriads of smart soft materials suitable for use in optoelectronic appliances, energy generation and storage, sensor, biotechnology etc. Acknowledgements: We acknowledge SERB, New Delhi (grant number EMR/2016/005302) for financial support. N.M and R.G acknowledge CSIR for providing the fellowship. References: 1.

Mei, J.; Bao, Z. Side Chain Engineering in Solution-Processable Conjugated Polymers. Chem. Mater. 2014, 26, 604-615.

2.

Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666-669.

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Huang, X.; Yin, Z.; Wu, S.; Qi, X.; He, Q.; Zhang, Q.; Yan, Q.; Boey, F.; Zhang, H. Graphene-Based Materials: Synthesis, Characterization, Properties, and Applications. Small 2011, 7, 1876-1902.

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Chang, H.; Wu, H. Graphene-Based Nanomaterials: Synthesis, Properties, and Optical and Optoelectronic Applications. Adv. Funct. Mater. 2013, 23, 1984-1997.


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Bao, Z.; Lovinger, A. J. Soluble Regioregular Polythiophene Derivatives as Semiconducting Materials for Field-Effect Transistors.Chem. Mater. 1999, 11, 26072612.

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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. J. Mater. Chem. 2011, 21, 13780-13784.

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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.

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Nilsson, K. P. R.; Inganäs, O. Chip and Solution Detection of DNA Hybridization Using a Luminescent Zwitterionic Polythiophene Derivative. Nat. Mater. 2003, 2, 419-424.

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Ho, H.-A.; Leclerc, M. Optical Sensors Based on Hybrid Aptamer/Conjugated Polymer Complexes. J. Am. Chem. Soc. 2004, 126, 1384-1387.

10. Ewbank, P. C.; Loewe, R. S.; Zhai, L.; Reddinger, J.; Sauve, G.; McCulloug, R. D. Regioregular Poly(thiophene-3-alkanoic acid)s: Water Soluble Conducting Polymers Suitable for Chromatic Chemosensing in Solution and Solid State. Tetrahedron 2004, 60, 11269-11275. 11. Li, C.; Numata, M.; Takeuchi, M.; Shinkai, S. A. Sensitive Colorimetric and Fluorescent Probe Based on a Polythiophene Derivative for the Detection of ATP. Angew. Chem. Int. Ed. 2005, 44, 6371-6374. 12. Yao, Z.; Li, C.; Shi, G. Optically Active Supramolecular Complexes of Water-Soluble Achiral Polythiophenes and Folic Acid: Spectroscopic Studies and Sensing Applications. Langmuir 2008, 24, 12829-12835. 13. Yao, Z.; Li, Y.; Li, C.; Shi, G. Disassembly of Conjugated Polyelectrolyte Aggregates and Their Application for Colorimetric Detection of Surfactants in Water. Chem. Commun. 2010, 46, 8639-8641.


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14. Li, C.; Shi, G. Polythiophene-Based Optical Sensors for Small Molecules. ACS Appl. Mater. Interfaces 2013, 5, 4503-4510. 15. 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. 16. Evans, R. C. Harnessing Self-Assembly Strategies for the Rational Design of Conjugated Polymer Based Materials. J. Mater. Chem. C 2013, 1, 4190-4200. 17. Eda, G.; Chhowalla, M. Chemically Derived Graphene Oxide: Towards Large-Area Thin-Film Electronics and Optoelectronics. Adv. Mater. 2010, 22, 2392-2415. 18. Liu, Z.; Robinson, J. T.; Sun, X.; Dai, H. PEGylated Nanographene Oxide for Delivery of Water-Insoluble Cancer Drugs. J. Am. Chem. Soc.2008, 130, 10876-10877. 19. Lu, C. H.; Yang, H. H.; Zhu, C. L.; Chen, X.; Chen, G. N. A Graphene Platform for Sensing Biomolecules. Angew. Chem. Int. Ed. 2009, 48, 4785-4787. 20. Allen, M. J.; Tung, V. C.; Kaner, R. B. Honeycomb Carbon: A Review of Graphene. Chem. Rev. 2010, 110, 132-145. 21. Wan, X.; Huang, Y.; Chen, Y. Focusing on Energy and Optoelectronic Applications: A Journey for Graphene and Graphene Oxide at Large Scale. Acc. Chem. Res 2012, 45, 598-607. 22. Wang, M.; Zou, S.; Guerin, G.; Shen, L.; Deng, K.; Jones, M.; Walker, G. C.; Scholes G. D.; Winnik, M. A. A Water-Soluble pH-Responsive Molecular Brush of

Poly(N,N-

dimethylaminoethyl methacrylate) Grafted Polythiophene. Macromolecules 2008, 41, 6993-7002. 23. Costanzo, P. J.; Stokes, K. K. Synthesis and Characterization of Poly(methyl acrylate) Grafted

from

Poly(thiophene)

to

Form

Solid-State

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Materials.

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Optoelectronic Properties of Self-assembled Nanostructures of Polymer Functionalized Polythiophene and Graphene Nabasmita Maitya, Radhakanta Ghosha and Arun K. Nandi* Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700 032, INDIA

__________________________________________________________ *

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

a

Both the authors contributed equally


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Short Biodata of Authors Ms Nabasmita Maity received her B.Sc. and M.Sc. degree in Chemistry from Vidyasagar University, West Midnapore and joined at Indian Association for the Cultivation of Science as a ‘CSIR’ research fellow under the guidance of Prof. Arun. K. Nandi. Her research interest includes synthesis of stimuli-responsive graft copolymers from graphene, graphene quantum dots and poly(vinylidene fluoride) backbone for different applications Mr. Radhakanta Ghosh received his B.Sc. and M.Sc. degree in Chemistry from Burdwan University, Burdwan and joined at Indian Association for the Cultivation of Science as a ‘CSIR’ research fellow under the supervision of Prof. Arun. K. Nandi. His research interest is synthesis of polythiophene based graft copolymers by using ATRP technique and preparation of polythiophene based sensor. Prof. Arun K. Nandi obtained Ph.D degree on “Studies on Polymer-Polymer and Polymer-Solvent mixing” and joined Chemistry Department, North Bengal University, Darjeeling. He did post-doctoral work at Florida State University with Prof. L. Mandelkern in crystallization of polymers. In 1992 he was appointed at Polymer Science Unit of Indian Association for the Cultivation of Science and he is presently holding senior professor position. His research interests focus on polymer blends, polymer crystallization, polymer and supramolecular gels, polymer nanocomposites, polymer grafting, biomolecular hybrids and polymer photovoltaics. He is author of more than 210 papers and supervised 27 Ph.D students.


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Optoelectronic Properties of Self-Assembled Nanostructures of

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