Radhakanta Ghosh

Water soluble polythiophenes: preparation and applications

This review describes the synthesis of different water soluble polythiophenes and their versatile applications. Solubility in water is essential for developing sensors for different bio-molecules and polythiophene derivatives are excellent candidates due to their important optoelectronic properties. A pristine polythiophene chain is hydrophobic and it exhibits aqueous solubility after attachment/grafting of ionic pendent groups or hydrophilic polymer chains on its backbone. A concise account of the different synthetic procedures of preparing water soluble polythiophene is described and all the specific techniques relevant to the synthesis of water soluble polythiophene using cationic, anionic pendent groups and grafting of hydrophilic polymers are discussed. Different grafting processes e.g. “grafting from” and “grafting to” techniques using click chemistry and atom transfer radical polymerization (ATRP) are described in detail. Detections of different bio-molecules such as DNA, RNA, polypeptides, polysaccharides, ATP, UDP and ADP from the excellent opto-electronic properties of aqueous polythiophenes are discussed. The reports on fluorescence based specific sensing of metal ions, and nitro-aromatics using water soluble polythiophenes are also embodied with an up-to-date description of the optoelectronic device applications such as logic gates, molecular thermometers, photovoltaic cells etc. Finally, a summary and outlook is presented discussing the future scope of research on this important subject.

Water soluble stimuli-responsive star copolymers with multiple encapsulation and release properties

A series of three arm star shaped (random/block) and linear water soluble copolymers are synthesised by atom transfer radical polymerization (ATRP) using di(ethylene glycol) methyl ether methacrylate (DEGMA) and 2-(dimethylamino) ethyl methacrylate (DMAEMA). The structure and composition of the block and random copolymers are characterized by 1H NMR spectra and gel permeation chromatography (GPC). The self-assembly of these copolymers, investigated by dynamic light scattering (DLS), exhibits that below the lower critical solution temperature (LCST) of pDEGMA all the copolymers are soluble in water and possess lower particle size but above its LCST particle size increases particularly in a basic medium. On addition of 8-anilino-1-naphthalenesulfonic acid (ANS) the particle size increases by ∼10 times below the LCST. Both the DLS and fluorescence studies using a hydrophobic fluorescent dye, exhibit temperature triggered encapsulation and pH triggered release. All the copolymers exhibit highly reversible multiple aggregation at different temperature and pH conditions and require increased temperature for the aggregation with decrease in pH of the medium. The random star copolymer [3-arm-p(DEGMA40-co-DMAEMA18)] exhibits aggregate formation under physiological conditions (37 °C and pH 7.5) and with decreasing the pH to 6.5 the aggregates dissociate. The MTT assay and cell morphology indicate that the three arm star and linear random copolymers have lower cytotoxicity against normal CHO-K1 cells having lower positive zeta potential values.

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

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 center 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 (0.025 mg/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 benzenesulfonate (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 with CTAB, thus deassembling the PTP-SDBS aggregates. At pH 9.2, these turn on and turn off responses 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 deassembling 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 transform infrared spectroscopy. The morphology of the above systems studied by transmission and scanning electron microscopy also supports the above aggregation and disaggregation process.

Cationic polythiophene for specific detection of cyanide ions in water using fluorometric technique

The toxic cyanide ion has been detected in water at a minimum level of 4.4 ppb (0.17 μM) which is far below the lethal dose. A new water soluble cationic polythiophene graft poly(N,N-dimethylaminoethyl methacrylate) (PTDMA) with iodide counter ion (CPT-I) is synthesised for this purpose. The CPT-I has lower fluorescence intensity than that of PTDMA suggesting excitonic energy transfer from PT backbone to iodide ions. When the iodide anions are substituted by cyanide ions the fluorescence intensity shows an enormous increase depending on its concentration. The increase of the fluorescence intensity remains intact even in the presence of individual or a mixture of other anions e.g. Br−, Cl−, F−, I−, NO3−, ClO4−, OH−, HCO3−, AcO−, CO32−, SO42−, Fe(CN)64−, Fe(CN)63− and SCN−. A histogram of the fluorescence intensity of CPT-I with the addition of 19.6 μM of each of the above anions suggests a nine fold increase in the fluorescence intensity with the cyanide ion whereas other ions exhibit a negligible increase. A calibration curve has been drawn from the addition of different amounts of cyanide ions to the CPT-I solution. CPT-I exhibits very high sensitivity, quick response and high selectivity towards CN− ion over a large number of other anions with a low detection limit in water.

Light-Induced Conformational Change of Uracil-Anchored Polythiophene-Regulating Thermo-Responsiveness

Tuning the electronic structure of a π-conjugated polymer from the responsive side chains is generally done to get desired optoelectronic properties, and it would be very fruitful when light is used as an exciting tool that can also affect the backbone chain conformation. For this purpose, polythiophene- g-poly-[ N-(6-methyluracilyl)- N, N-dimethylamino chloride]ethyl methacrylate (PTDU) is synthesized. On exposure to diffuse sunlight, the uracil moieties of the grafted chains cause the absorption maximum of PTDU solution to show gradual blue shift of 87 nm and a gradual blue shift of 46 nm in the emission maximum, quenching its fluorescence with time. These effects occur specifically at the absorption range of polythiophene (PT) chromophore on direct exposure of light of different wavelengths, and the optimum wavelength is found to be 420 nm. Impedance study suggests a decrease in charge transfer resistance upon exposure because of conformational change of PTDU. Theoretical study indicates that on exposure to visible light, uracil moieties move toward the backbone to facilitate photoinduced electron transfer between the PT and the uracil, attributing to the variation in optoelectronic properties. Morphological and light-scattering studies exhibit a decrease in particle size because of coiling of the PT backbone and squeezing of the grafted chain on light exposure. The transparent orange-colored PTDU solution becomes hazy with a hike in emission intensity on addition of sodium halides and becomes reversibly transparent or hazy on heating or cooling. The screening of cationic centers of PTDU by varying halide anion concentration tunes the phase transition temperature. Thus, the light-induced variation in the backbone conformation is responsible for tuning the optoelectronic properties and regulates the thermos-responsiveness of the PTDU solution in the presence of halide ions.

Optoelectronic Properties of Self-Assembled Nanostructures of Polymer Functionalized Polythiophene and Graphene

In this Feature Article, we discuss the variation of optoelectronic properties with the aggregation style of polythiophene (PT) graft copolymers and polymer-modified graphene systems. Grafting of flexible polymers on a PT chain exhibits 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 polymers both covalently and noncovalently to create a 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, and surfactants, for drug delivery, and also for fabricating molecular logic gates, electronic rectifiers, photocurrent devices, etc.

Candle soot derived carbon nanodot/polyaniline hybrid materials through controlled grafting of polyaniline chains for supercapacitors

A general approach for controlled grafting of polyaniline (PANI) chains from the surface of nano-structured carbon materials following a ‘grafting from’ strategy through oxidative polymerization of aniline is reported. Fluorescent spherical carbon nano-dots (CNDs) are obtained through HNO3 oxidation followed by size separation from an easily available and cheap source like candle soot. Oxidative polymerization of aniline under dilute acid conditions is conducted in the presence of CNDs tethered to the mono-amine group of N,N′-bis(4′-aminophenyl)-1,4-quinonediimine (APQD, in the emeraldine state) (CNDT), as the initiator. The appreciably lower oxidation potential of APQD compared to aniline directs the growth of PANI chains preferentially from these moieties acting as ‘seeds’. Furthermore, the conditions are optimized to complete the entire polymerization within the duration of the induction period (IP) for the oxidative polymerization of aniline under similar conditions to ensure the grafting of PANI chains from the CND surface only. The attachment of APQD moieties to the CND surface and hence the formation of PANI chains are confirmed by using FTIR and XPS techniques. The covalently attached carbon nano-dot/polyaniline shows a remarkably higher specific capacitance of 972 F g−1 with 90% retention after 2000 cycles, in comparison to only 482 F g−1 for the non-covalently attached composite. The excellent electrochemical performance is attributed to the formation of the nano-structured composite material.

Influence of Hofmeister I– on Tuning Optoelectronic Properties of Ampholytic Polythiophene by Varying pH and Conjugating with RNA

A significant tuning of optoelectronic properties of polythiophene (PT) chains due to Hofmeister iodide (I-) ion is demonstrated in ampholytic polythiophene [polythiophene-g-poly{(N,N,N-trimethylamino iodide)ethyl methacrylate-co-methacrylic acid}, APT] at different pHs. In acidic medium, the absorption and emission signals of PT chromophore exhibit appreciable blue shift in the presence of I- as counteranion only. The cooperative effect of undissociated -COOH and quaternary ammonium groups immobilize I- near the apolar PT chain causing threading of grafted chains and hence twisting of the backbone attributing to the blue shift. As medium pH is increased, dethreading of the PT backbone occurs due to ionization of -COOH group, releasing quencher iodide ions from the vicinity of the PT chains resulting in a red shift in absorption and a sharp hike in fluorescence intensity (390 times) for an increase of excitons lifetime. With an increase of pH, morphology changes from a multivesicular aggregate with vacuoles to smaller size vesicles and finally to nanofibrillar network structure. Dethreading is also found when APT interacts with RNA showing a significant hike of fluorescence (22 times) for displacing iodide ions forming a nanofibrillar network morphology. Threading and dethreading also affect the resistance, capacitance, and Warburg impedance values of APT. Molecular dynamics simulation of a model APT chain in a water box supports the threading at lower pH where the iodide ions pose nearer to the PT chain than that at higher pH causing dethreading. So the influence of Hofmeister I- ion is established for tuning the optoelectronic properties of a novel PT based polyampholyte by changing pH or by conjugating with RNA.