烷氧磺酸盐功能化的聚乙撑二氧噻吩水凝胶

杜 然 张学同

(1北京理工大学材料学院,北京100081;2北京大学化学与分子工程学院,北京100871)

1 Introduction

Two dimensional(2D)polymers are expected to have great impact on fundamental science and applied engineering,1and they may be used as building blocks for fabrication of three dimensional(3D)bulk materials due to their amazing mechanical or electrical properties.Utilization of covalent bonding or self-assembly of small fragments can allow the formation of 2D polymers.For example,Müllen et al.2have synthesized polycyclic aromatic hydrocarbon by combination of Diels-Alder,cyclotrimerization,and oxidative cyclodehydrogenation reactions.Jiang et al.3,4have prepared photoconductive covalent organic frameworks by self-condensation of pyrenediboronic acid,and further synthesized porphyrin-based crystalline covalent porous framework by condensation reactions under solvothermal conditions.Recently,2D covalent-organic-framework films on single-layer graphene with vertical alignment and long-range order have been revealed by Dichtel et al.,5and Lehnʹs group6has synthesized the grid-type inorganic architectures such as[4×4]Pbgrid obtained by self-assembly from lead(II)ions and ligands containing tridentate binding sites. Multi-strand compounds,or so called ladder-type polymers can also be regarded as candidates for 2D polymers,but most of them are less than quadruple-strand.7,8It is worth to mention that Shinkai et al.9,10have reported sheet-like aligned conjugated polymers via the action of supramolecular bundling molecules serving as“clips”,providing a general mean to fabricate complex 2D conjugated polymers.Template method is the other important way in fabricating of the 2D polymers.For example,Asakusa et al.11have employed specific surfactant micelles as templates to confine monomers,and then produced 2D films after crosslinking.Nowadays,Gölzhäuser et al.12have synthesized Janus nanomembranes functionalized with different groups on each side by exposure of self-assembled amphiphilic molecules on gold surface to electrons,and post-modification by different fluorescent dyes on each side of nanosheets has enabled the effective fluorescence energy transfer.Utilization of interface polymerization by complexation between terpyridine units of adjacent monomers and appropriate metal salts supplied from the water subphase,the first mono-layered,metal-complexed sheet(large than 500 μm×500 μm)has been prepared.13However,all above mentioned syntheses suffered from insolubility of products,or relatively small fragments,or difficulty in isolation of 2D structure from the resulting aggregations.Whatʹs more,self-assemblies of these 2D polymers to 3D hydrogel networks have been hardly exploited.

On the other hand,the hydrogel is a kind of soft matter with 3D network containing a large amount of water.It usually shows excellent biocompatibility,mass transfer ability,and stimuli-responsive properties,exhibiting huge potential in biosensors,controlled drug release,chemical separations,actuators,and so on.14-19Several groups have reported conducting polymer hydrogels(CPHs)with some unusual properties in comparison with traditional hydrogels.20-24For example,Chen et al.20synthesized poly(3-thiopheneacetic acid)gels by crosslinking polymers with dicyclohexylcarbodiimide reaction.By taking advantage of the crosslinkable group of poly(styrenesulfonate)(PSS),Dai et al.21polymerized PSS-stabilized 3,4-ethylenedioxythiophene(EDOT)with excess Fe(NO3)3·9H2O and in situ crosslinked the poly-EDOT(PEDOT)/PSS,resulting in ionic crosslinked PEDOT/PSS hydrogel just in one step.Recently,our group prepared conducting polymer hydrogels free of non-conductive component by imparting water-soluble pendant group in EDOT and subsequently polymerizing and crosslinking simultaneously.22Nowadays polyaniline(PANI)hydrogels has also been successfully prepared via chemical or ionic crosslinking.23,24

However,in most cases,it is linear polymeric chain,or 0D quantum dot,or 1D nanostructure that serves as the building block to self-assemble into various hydrogels.25,26Only a few inorganic hydrogels such as clay27or graphene and its derivatives28,29are reported to possess the 2D building blocks.Hence, it is fascinating if 2D building blocks contained hydrogels could be made by 1D polymer chains,and the resulting 2D polymer hydrogels may demonstrate some unusual properties compared with traditional hydrogels.

In our previous study,22we have synthesized conducting polymer hydrogels with unusual 2D building blocks in one step via a combination of oxidative coupling polymerization and non-covalent crosslinking of an alkoxysulfonate-functionalized poly(3,4-ethylenedioxythiophene).Herein we present a systematic investigation on the morphological,electrical,electrochemical,and adsorptive properties of the resulting conducting polymer hydrogels,and introduce a synthetic strategy for the synthesis of the conducting polymer hydrogels based on simultaneous use of one chemical as an oxidant and one multivalent metal ion as an ionic crosslinker.The building blocks of the resulting conducting polymer hydrogels could be controlled to exhibit an interesting dimension evolution from 0D monomer micelles to 2D nanosheets by simply altering reaction conditions.In addition,The as-synthesized conducting polymer hydrogels exhibit the specific capacitance in the range of 30-60 F·g-1with better rate performance than that of PEDOT-PSS counterpart.30And the dried conducting polymer gels possess the selective adsorption and controlled release towards some dye molecules.Finally,the over-oxidation mechanism of gels has been proposed based on both chemical and electrochemical investigations.

2 Experimental

2.1 Materials

(2,3-dihydrothien[3,4-b][1,4]dioxin-2-yl)methanol(EDTM) and butane sulfone were purchased from Nanjing Clasien Pharmaceutical&Chemical Co.,Ltd.and Wuhan Fengfan Chemical Co.,Ltd.,respectively.Other regents such as ammonium persulfate(APS),ferric chloride,lanthanum chloride hexahydrate,antimonic chloride,sodium sulfate,basic fuchsine,etc., were purchased from Beijing Chemical Reagents Company. All regents are of analytical grade and used as received.Sodium 4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)methoxybutane-1-sulfonate(EDOT-S)was synthesized according to the procedure described in our previous study.22

2.2 Synthesis of PEDOT-S hydrogels

In a typical procedure,EDOT-S monomer(48 mg,0.145 mmol)was dissolved in 100 μL deionized water.After stirring for a few minutes,a mixture of APS(39.8 mg,0.174 mmol) and CrCl3·6H2O(387.1 mg,1.45 mmol)dissolved in 543 μL deionized water were poured into above solution containing monomers.The mixture was stirred for ca 30 s before standing. The PEDOT-S hydrogel was obtained 1-2 days later.The PEDOT-S hydrogels were also synthesized by using APS,Fe-Cl3,or the mixture of both as the oxidant,which has been described elsewhere.22

2.3 Over-oxidation

PEDOT-S solution with a concentration of 40 mg·mL-1was synthesized by mixing the EDOT-S with 1.2 equiv.(the equiv. here means the molar ratio of substances)of APS in a certain amount of deionized water for 5 days in 30°C water bath to ensure the complete reaction.Then,the resulting PEDOT-S aqueous solution was mixed with 1,10,80 equiv.of H2O2aqueous (30%)respectively.After reaction for 3 days in 30°C water bath,the resulting aqueous solution was further diluted to 0.1 mg·mL-1for UV-Vis spectroscopy measurement.Electrochemical over-oxidation has been performed on three-electrode system with PEDOT-S hydrogel modified graphite rod as a working electrode.

2.4 Dyestuff adsorption and desorption

The PEDOT-S hydrogels were freeze-dried before conducting dyestuff adsorption.Dried gel with ca 1.0 mg was dropped into 9 mL basic fuchsine aqueous solution with certain concentration,followed by UV-Vis measurement after 30°C water bath for 3 days.The adsorptive capacity was calculated by absorbance difference of basic fuchsine aqueous solution before and after adsorption.Desorption of basic fuchsine was carried out by addition of different amount of cetyl trimethyl ammonium bromide(CTAB)into the aqueous solution containing adsorbed-dyestuff gel and remained at 30°C for 1 day before UV-Vis measurement.The concentration of dyestuff is determined from the standard curve of absorbance versus concentration obtained in the same conditions.

2.5 Techniques

Scanning electron microscopy(SEM)images were taken by a Hitachi S-4800 field-emission-gun scanning electron microscope at 5-10 kV.The sample for SEM was prepared by directly putting freezing-dried samples on the conductive tape.X-ray photoelectron spectroscopy(XPS)was conducted by using an AXIS Ultra spectrometer with a high-performance Al monochromatic source operated at 15 kV.The XPS spectra were taken after all binding energies were referenced to the C 1s neutral carbon peak at 284.8 eV,and the elemental compositions were determined from peak area ratios after correction for the sensitivity factor for each element.Electrochemistry properties were conducted with CHI610D electrochemical workstation (from CHI Instruments,Inc.,Shanghai,China).The three-electrode system composed of Pt wire as a counter electrode,Ag/ AgCl(3 mol·L-1KCl)as a reference electrode,and PEDOT-S hydrogel modified graphite rod made by addition of 50 μL uniform suspension of hydrogels onto the surface of the graphite rod,followed by standing in the air for 24 h,as a work electrode was used.Conductivity measurement was conducted on a Keithley 4200 Semiconductor Characterization System via a four-probe method at room temperature.UV-Vis spectroscopy was performed on UV-6100 double beam spectrophotometer (Shanghai Mapada).Nitrogen adsorption measurement has been conducted at ASAP 2010(Micromeritics,USA)with degasification temperature of 150°C.

3 Results and discussion

The polymerization mechanism is shown in Fig.1.The EDOT-S monomers were oxidatively polymerized by APS,FeCl3, or the mixture of APS and MClx,where M represents Fe,La, Ce,Cr or Sb.With the polymerization going on,PEDOT-S hydrogels formed by either ionic crosslinking between metal ions and sulfonate group,or the π-π stacking of the polymer chains.

3.1 Dimension evolution of the PEDOT-S hydrogels

When the APS or the mixture of APS and FeCl3was employed as oxidant,the hydrogels with 2D nanosheets could be formed.With the amphiphilic nature of EDOT-S,the monomers self-assembled into 0D micelles in aqueous solution and gradually formed 2D sheets upon polymerization.This results from the enhanced π-π interaction of the conjugated length of polymer,31which could overtake the hydrophobic interaction of the monomer in original micelles.Hence,the polymer chains stacked in a face-to-face fashion and were perpendicular to the polymerization direction.The mechanism of this kind of di-mension evolution could be verified by cryo-TEM and dynamic light scattering(DLS).22

Fig.1 (a)Polymerization of EDOT-S into conducting polymer, and(b)schematic model for the formation of conducting polymer hydrogels via either ionic crosslinking or π-π stackingThe oxidant used herein could beAPS,MCl3,or the mixture of them, where M represents Fe,La,Ce,Cr,or Sb.

On the other hand,the dimension evolution could also be interpreted as the dimension change of the building blocks of the resulting hydrogels,and both reaction temperature and initial monomer concentration have played significant roles during this dimension evolution process.Take PEDOT-S hydrogel prepared with FeCl3as oxidant as an example as shown in Fig.2. When the initial monomer concentration was 25 mg·mL-1and the reaction took place at 25°C,the resulting PEDOT-S precipitated in the form of dimensionless nanoparticles.However, when the initial monomer concentration was fixed but the reaction temperature was increased to 80°C,the resulting PEDOTS hydrogel with 2D nanosheets as the building blocks was observed.The relatively high temperature could not only facilitate the formation of the 3D network of the hydrogels,but also induce the dimension evolution of the building blocks from 0D nanoparticles to 2D nanosheets.This temperature induced dimensionality variety is quite similar to that reported for Cd(II)/ ip/bmb polymers.32In our case,at relatively low reaction temperature,polymerization rate would be much slow,leading to much lower conversion and much more oligomers formed in comparison with that conducted at relatively high temperature. Since the formation of nanosheets depends on the strong π-π interaction between polymeric chains with enough conjugated length,it is reasonable that oligomers with short conjugated length synthesized at relatively low temperature maintain dimensionless nanoparticles similar to spheric micelles in aqueous solution.The size enlargement of the nanoparticles compared with the micelles could be attributed to crosslinking effect of the Fe3+ions.33While the temperature was kept constant at 30°C,when monomer concentration was less than 25 mg· mL-1,only precipitate rather than hydrogel was obtained,and the 0D nanoparticles were observed from SEM image as shown in Fig.3.When the monomer concentration reached 35 mg·mL-1,the hydrogel was formed,and the building blocks turned into bulk stuff,which could be regarded as the transition state between 0D nanoparticles and 2D nanosheets.To further increase monomer concentration to 65 mg·mL-1,the hydrogel formed by 2D nanosheets with little defects was observed.According to the formation mechanism of the 2D nanosheets mentioned in Fig.1b,π-π stacking is the main driving force for fabrication of 2D nanosheets.Theoretically,since each Fe3+ion combines threeions by electrostatic interaction to balance its positive charge,it is inevitable that in order to meet the energy and geometry requirements,metal ions will hamper the π-π stacking.In other words,the introduction of excessive multivalent Fe3+ions will unavoidable interfere with the formation of 2D nanosheets,at least at low monomer concentration.However at high monomer concentration,the reduction of distance among formed PEDOT-S macromolecules enhances the π-π interaction.When monomer concentration reaches a threshold,π-π stacking overwhelms the electrostatic interaction between Fe3+andand thus the dimension of hydrogel building blocks evolves from 0D to 2D.

Fig.2 SEM images of PEDOT-S hydrogels synthesized at 25°C(a)and 80°C(b)The initial monomer concentrations(Cmonomer)were both 25 mg·mL-1. The oxidant used here was FeCl3.

Therefore in a broad sense,dimension evolution of PEDOT-S hydrogels can be interpreted by two perspectives: formation of hydrogels from 0D monomer micelles to macroscopical hydrogels with 2D nanosheets as the building blocks, and the dimension change of the building blocks of the resulting hydrogels from 0D nanoparticles to 2D nanosheets induced by altering the reaction temperature or monomer concentration.

3.2 Metal-ion effect

Fig.3 SEM images of PEDOT-S hydrogels synthesized at 30°C in water bathCmonomerwas 5,25,35,50,65 mg·mL-1,respectively.The oxidant used here was FeCl3.

Previously we have discussed the synergistic effect of hydrogels prepared by using the mixture of APS and FeCl3as the oxidant.22In that case,ultra-fast synthesis of the conducting polymer hydrogel was clarified due to the synergistic effect of both oxidized agent of APS and catalyzed effect of FeCl3.34The role of Fe3+ions in crosslinking of the PEDOT-S macromolecules can be evidenced from XPS measurement.22The existence of the characteristic iron peaks suggested remained iron in the gel matrix,which is attributed to the electrostatic interaction between Fe3+ions andgroups bonded to PEDOT-S.35

As the expansion of synthetic method,we demonstrate here that the simultaneous use of one chemical as an oxidant and one multivalent metal ion as an ionic crosslinker is a feasible method for synthesis of the conducting polymer hydrogel by taking advantage of pendant group on polymer chains.Several formulae have been used to prepare PEDOT-S hydrogels.APS or FeCl3was chosen as an oxidant while one of the CaCl2, BaCl2,AlCl3,CrCl3,SbCl3,LaCl3,and CeCl3was chosen as a multivalent ionic crosslinker.The results showed that only one of the CrCl3,SbCl3,LaCl3,and CeCl3serving as the crosslinker could induce the formation of hydrogels as shown in Fig.4, while one of the rest was employed as the crosslinker,only conducting polymer precipitate or homogeneous solution was observed.Taken the valance state of the metal ions and the solubility product constant(KSP)of metal ions withinto consideration,it is obvious that the higher the valance state of the metal is,or the lower the KSPis,the easier the hydrogels are formed.The observed experiment result,namely only CrCl3, SbCl3,LaCl3,and CeCl3could induce the formation of hydrogels when one of them was used together with APS,was consistent with above tendency.

Fig.4 Digital photos of PEDOT-S hydrogels synthesized by using APS(a),the mixture ofAPS and FeCl3(b),FeCl3(c),and the mixture ofAPS and one of CrCl3·6H2O,SbCl3,LaCl3·6H2O, and CeCl3·7H2O(from left to right)(d)as the oxidant

To deeply investigate the effect of the different metal ions on the morphology of the resulting hydrogels fabricated by this synthetic strategy,SEM was used.We can see from Fig.5 that when CrCl3or SbCl3was used,instead of quasi-2D sheets observed in hydrogels formed by using APS,FeCl3or the mixture of APS and FeCl3as an oxidant,the massive structures were obtained.This can be probably attributed to the difference of the coordination ability and ionic radius of the different metal ions.To our surprise,it is clear that when the hydrogels were formed by using the mixture of APS and rare earth metal salts such as LaCl3or CeCl3,there were lots of regular cubic stuffs within the gel matrix.It may indicate the formation of inorganic crystals.However,when APS was replaced with FeCl3,the morphology of the hydrogels formed by using rare earth metal salts as crosslinkers was totally different.In this case,dimensionless nanoparticles appeared.This variety in morphology and dimensionality of the hydrogels may attribute to the introduction of Fe3+ions destroying the crystal structure formed in the presence of only multivalent rare earth metal ions.The further mechanism and effect of these metal ions on the conductivity and rheology of the resulting PEDOT-S hydrogels are still under investigation.

3.3 Conductivity

Fig.5 SEM images of PEDOT-S hydrogels synthesized by various combination of oxidants and metal salts(a)APS and CrCl3·6H2O,(b)APS and SbCl3,(c)APS and LaCl3·6H2O,(d)APS and CeCl3·7H2O,(e)FeCl3and LaCl3·6H2O,(f)FeCl3and CeCl3·7H2O

Table 1 Conductivity of PEDOT-S wet and dried hydrogels synthesized by using different oxidants

Introducing water-soluble and crosslinkable pendant group to monomer enables us to prepare conducting polymer hydrogels free of non-conducting components,leading to the relatively high conductivity in our hydrogels,which could be seen from the Table 1.On the other hand,from the prospect of the state of materials,we can see from Table 1 that the conductivity of the wet hydrogel is about one order of magnitude less than the dried gel,indicating the existence of electronic conductance which is contributed from the conjugated skeleton of the conducting polymer.It was reported that for the materials with electronic conduction mechanism,the conductivities of the bulk materials,namely the compressed dry pellets,are more than 4 order of magnitude higher than their corresponding hydrogel counterparts.29,35Hence,it is obvious that some other factors have played significant roles in determining the conductivity of as-prepared products in our case.Since the PEDOT-S macromolecules are bonded with plenty of ionized sulfonic-acid groups,the presence of a large amount of water can induce the ionization of―SO3H,and thus the conductivity of the PEDOT-S hydrogels is the sum of the ionic conductance and electronic conductance.After drying,the ionic conductance is negligible since the―SO3H can not ionize without the proper solvent,while the electronic conductance is greatly enhanced because of both the closer intermolecular distance and the disappearance of water.The net result is only one order of magnitude difference in conductivity between two states of the conducting polymer.Therefore,both electronic conductance and ionic conductance have a significant effect on the conductivity of PEDOT-S hydrogels,30resulting in high conductivity of our hydrogels among the electrically conductive hydrogels.36

Furthermore,the oxidants used during polymerization have a great influence on the conductivity of the resulting PEDOT-S hydrogels.It seems that FeCl3has an adverse impact while the mixture of FeCl3and APS have a substantially positive effect on the conductivity of the resulting gel materials.We suppose that when hydrogels were synthesized with FeCl3,the excess amount of Fe3+ions served as ionic crosslinkers limited the spatial orientation of polymers by electrostatic interaction betweenand Fe3+,and thus reduced the degree of overlap between aromatic rings,resulting in relatively low conductivity for both wet hydrogels and compressed dry pellets.While hydrogels were synthesized with oxidants with less or no Fe3+ions,the conducting polymeric chains had inclined to approach each other by a“face-to-face”manner via π-π stacking and easily formed quasi 2D nanosheets,and thus the conductivity of the resulting PEDOT-S hydrogels was largely enhanced.

In addition,it can be seen from Table 1 that the higher the initial monomer concentration,the higher the conductivity of the resulting wet gels,and thus the hydrogel synthesized with APS as an oxidant possesses the highest conductivity due to its highest initial monomer concentration.This is due to denser polymeric conducting network formed during synthesis when higher initial monomer concentration is used.Obviously this concentration effect can be eliminated after the hydrogels is dried,and thus the dried gels prepared with APS or the mixture of APS and FeCl3as an oxidant have shared comparable conductivity,which is one order of magnitude higher than that of those prepared with FeCl3as an oxidant.

3.4 Electrochemical properties

CV curves of the PEDOT-S hydrogels were recorded in 1.0 mol·L-1Na2SO4aqueous solution.It can be seen from Fig.6(a) that in comparison with the hydrogels formed by using APS as an oxidant,it is evident that the hydrogels formed by using FeCl3or the mixture of FeCl3and APS as an oxidant show the peaks of redox process of iron ions around 0.4 V versus Ag/ AgCl reference electrode,proving the existence of Fe3+ions as the crosslinker remained in the hydrogel matrix,which is in consistent with the XPS analysis.35The anodic peak locating at 1 V corresponds to the over-oxidation of PEDOT-S,which will be discussed in detail in the following section.The anodic and catholic peaks centered at ca-0.07 and-0.15 V,respectively, is attributed to the redox process of PEDOT-S.37Whatʹs more, the CV curves show a rectangular shape,characteristic of a electrochemical capacitor.30,38The specific capacitance of the PEDOT-S hydrogels is estimated to be ca 30-60 F·g-1,which is ca one order of magnitude higher than PEDOT:PSS hydrogels reported elsewhere.30When scan rate was increased to as high as 800 mV·s-1,the envelop-like shape of CV curve was still maintained as shown in Fig.6(b).This indicates that the PEDOT-S hydrogel electrode is with good rate performance. By plotting curve of average current density versus scan rate, perfect linear relationship is obtained,which means that the electrochemistry process is non-diffusion-limited when scan rate is even up to 800 mV·s-1.37

3.5 Chemical and electrochemical over-oxidation

Chemical over-oxidation reaction was performed by addition of a large excess of hydrogen peroxide(H2O2)to PEDOT-S and the change was monitored by UV spectra, which was illustrated in Fig.7(a).We could see that when the different equiv.of H2O2was added,the UV spectra of PEDOT-S underwent a series of changes.The peak located around 800 nm,characteristic of p-doped PEDOT-S,39was immediately decreased upon addition of H2O2.At the same time, the peak at 430 nm representing π-π*transition37,39was also fell off gradually,suggesting the decrease of the conjugated length of the PEDOT-S.When oxidant was further put in,the colour of the mixture turned from dark black to nearly colourless,and all peaks disappeared when a large amount of H2O2were added,which indicates complete destroy of the conjugated structure of the polymer.If the resulting colourless mixture was dialyzed against the deionized water,there were hardly any products collected,indicating the degradation of the PEDOT-S macromolecules.

On the other hand,electrochemical over-oxidation was also observed as shown in Fig.7(b).When the PEDOT-S hydrogel electrode was used to record cyclic voltammograms in the sweep voltage range from-0.4 to 1.0 V,with increase of the scanning times,the anodic peak current which was responsible for over-oxidation of PEDOT-S40,41was dramatically decreased, so did the average current density,indicating structure destroy of the PEDOT-S hydrogels.

Fig.6 Cyclic voltammogram(CV)curves of PEDOT-S hydrogel electrode recorded in 1.0 mol·L-1Na2SO4aqueous solution(a)and scan rate dependence of the CV curves of the PEDOT-S hydrogel electrodes at the scan rate(v)range of 50-800 mV·s-1(b)Inset in(b)is the curves of average current density versus scan rate.

According to the chemical and electrochemical over-oxidation phenomena and in comparison with the literature reported elsewehere,40,41we can come to the conclusion that PEDOT-S hydrogel underwent gradually oxidation process,and eventually the destroy of conjugated structure led to the collapse of hydrogels,i.e.,gel-sol transition of conducting polymer hydrogels.

3.6 Adsorption and desorption of dyestuff

It is well known that various carbon materials,such as starch-derived carbon aerogels,activated carbon aerogels,and tubular structured ordered mesoporous carbon,showed quite high adsorptive capacity to a wide range of dyes,such as crystal violet,reactive blue,acid red,fuchsine basic,and so on.42-44Recently,self-assembled graphene oxide/DNA composite hydrogels have showed very high dye-loading capacity for safranine O(ca 960 mg·g-1GO)due to the combination of strong electrostatic interactions between dye molecules and hydrogel matrix,large specific surface area,and abundant conjugated domains of GO sheets.22However these porous materials always obey the physical adsorption mechanism,or at least quite a few adsorptive capacities are attributed to their large specific surface area,and thus they can not show selective adsorption properties.For the other absorbents,such as β-cyclodextrin polymers45or fly ash46generated in thermal power plants,show much smaller adsorptive capacity(＜60 mg·g-1)towards fuchsine,though the later is a kind of low cost absorbent.Chitosan and its derivatives are another kind of absorbents received considerable attention in recent years due to its excellent performance in wastewater treatment,which is reviewed by Crini et al.47Chitosan-based absorbents exhibit satisfactory adsorptive capacity though their specific surface areas are not so large.

Fig.7 UV spectra of PEDOT-S hydrogels treated with different amount of H2O2(a)and cyclic voltammograms of PEDOT-S hydrogel electrode recorded in 1.0 mol·L-1H2SO4at the 2nd,4th, 7th,10th,and 15th scanning cycles,respectively(b)The concentrations of corresponding H2O2concentration used in (a)were 0,0.01,0.10,and 0.82 mg·mL-1,respectively.

On the other hand,desorption of the absorbed dyes,or in other words,regeneration of absorbents is preferred from an economics and environment point of view.Solvent extraction and thermal annealing are two major methods for regeneration of absorbents,but the later is expected to decompose dyes.46De et al.48have carefully investigated adsorption and desorption of eosin dye and showed that by using appropriate surfactant,about 70%adsorbed dyes could be desorbed.It is a new way to achieve desorption without destroying the dyestuff,but the high surfactant concentration is necessary and the desorption degree is not totally satisfactory.

In our case,the dried PEDOT-S gels have exhibited amazing properties in selective dyestuff adsorption which is demonstrat-ed in Fig.8a.It is clear that dried gels prefer to adsorb cationic dyestuff,such as basic fuchsine(BF).The adsorptive capacity can reach around 164 mg·g-1.When the anionic dyestuffs, such as acid fuchsine(AF)and rhodamine B(RB)were used, they were hardly adsorbed by the dried PEDOT-S gels and only had an adsorptive capacity below 39 mg·g-1.Taking a glance at the structure of these dyestuffs,we can see that they all share π-conjugated system bonded with amino group.However,besides basic group,AF contains three sulfonic acid groups while RB contains carboxyl group.Then let us have a look at molecular structure of PEDOT-S.The conducting polymers possess linear π-conjugated system like that of above dyestuffs,and there are lots of sulfonic acid groups attached to the polymer skeleton.Combining with the selectivity in dye adsorption,we suppose that it is an electrostatic interaction having played the most important role in our case,while π-conjugated structure may have some auxiliary effect.Since PEDOT-S macromolecules contain large conjugated system, they show good affinity to all three dyestuffs via π-π stacking. However,plenty of negative-charged―SO3-groups on polymer skeletons have facilitated adsorption of BF which is bonded with positive-charged amino groups.On the other hand,the existence of acid groups(―SO3H or―COOH)makes that adsorptive capacity of RB and AF are extremely lower than that of BF,attributing to the electrostatic repulsion of acid groups between dyestuffs and PEDOT-S macromolecules.Nitrogen adsorption measurements show that the dried gels only have a BET surface area of 2.9 m2·g-1.The extremely low surface area of the dried gels further proves that adsorption of dyes obeys electrostatic interaction rather than utilization of the large specific area in the case of carbon aerogels.This also guarantees the effectiveness of the selectivity in adsorption.

Fig.8 Adsorptive capacity of dried PEDOT-S hydrogels toward different dye molecules(a)and controlled release of adsorbed dye molecules upon addition of the different amount of CTAB(b)

In addition to selective adsorption,the adsorbed dye such as BF can be desorbed in a controlled way by addition of an appropriate surfactant.As shown in Fig.8b,when cetyl trimethyl ammonium bromide(CTAB)was gradually added with sequence of 0.3,0.5,0.8,1.8,10 equiv.(from now on,define equiv.as the mass ratio of the CTAB to dried gels here)into dried gel solution containing adsorbed dyestuff,the adsorption intensity of the BF characteristic peaks in UV spectra gradually increased,which means that adsorbed BF in the gel matrix was gradually desorbed.Over 90%of adsorbed BF dye molecules were desorbed when total addition of CTAB reached 10 equiv.,which is more favourable than those reported elsewhere.42Another ten percent of BF remained in the gel matrix, which may be due to π-π interaction between BF and PEDOT-S macromolecules.The mechanism on controlled desorption is quite different from the report by De et al.48De et al.argued that it was the solubilization of surfactant micelles accounting for desorption of dyes loaded in absorbents.However,in our case,when 1.8 equiv.of CTAB(～0.2 mg·mL-1in aqueous solution)was added,ca 80%of BF dyes were desorbed.It is known that the concentration of CTAB is less than its cmc(the cmc of CTAB is about 9×10-4mol·L-1,equal to 0.33 mg·mL-149),hence the solubilization of micelles can not be the mechanism on desorption.We suggest that desorption process may also have something with electrostatic interaction. Since CTAB is a kind of cationic surfactant bearing a positivecharged quaternary ammonium group,it may compete with BF to combine negative-charged sulfonic acid groups bonded to PEDOT-S and extrude BF out of gels.The more the CTAB is added,the stronger the competition they are.Thus,the desorbed amount of BF in the gel matrix can be easily controlled by simply controlling the addition amount of CTAB.

4 Conclusions

The alkoxysulfonate-functionalized PEDOT hydrogels evolved from 0D monomer micelles to 2D nanosheets as building blocks have been synthesized by oxidative coupling polymerization.Oxidant(APS or FeCl3)and multivalent metal ions as the ionic crosslinker used during synthesis have played a significant role in determining the electrical properties of the resulting PEDOT-S hydrogels.The multivalent metal ions have a significant effect on the morphology and structure of the resulting PEDOT-S hydrogels.Dimension evolution of the building blocks of the PEDOT-S hydrogel from 0D nanoparticles to 2D nanosheets was also observed by adjusting the reaction temperature or initial monomer concentration.The resulting PEDOT-S hydrogels possess excellent conductivity due to existence of both electronic and ionic conductance.The dried PEDOT-S gels exhibit significant selective adsorption capability towards some dye molecules through electrostatic interaction and the adsorbed dyestuffs can be released in a controlled way by addition of cationic surfactant with controlled amount.Cyclic voltammogram investigation shows that the resulting PEDOTS hydrogels hold an electrochemical capacitance of 30-60 F· g-1with good rate performance.The resulting PEDOT-S hydrogels can be further over-oxidized by using strong oxidants or applying the high electrochemical potential and thus they could exhibit gel-sol transition behaviour under proper external stimuli.

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