The Use of Conductive Polymers Embedded Macro Porous Pei and Ionic Liquid Form of Pei Cryogels for Potential Conductometric Sensor Application to CO2

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Article The Use of Conductive Polymers Embedded Macro Porous Pei and Ionic Liquid Form of Pei Cryogels for Potential Conductometric Sensor Application to CO2

Sahin Demirci 1 and Nurettin Sahiner 1,2,3,* 1 Canakkale Onsekiz Mart University, Faculty of Science and Arts, Department of Chemistry & Nanoscience and Technology Research and Application Center (NANORAC), Terzioglu Campus, 17100 Canakkale, Turkey; [email protected] 2 Department of Chemical and Biomolecular Engineering, University of South Florida, Tampa, FL 33620, USA 3 Department of Ophthalmology, Morsani College of Medicine, University of South Florida, 12901 Bruce B. Downs Blvd, MDC21, Tampa, FL 33612, USA * Correspondence: [email protected]

Received: 19 February 2020; Accepted: 11 March 2020; Published: 13 March 2020

Abstract: Polyethyleneimine (PEI) cryogels with interconnected superporous morphology were synthesized via the cryopolymerization technique. Then, conductive polymers, poly(Aniline) (PANi), poly(Pyrrole) (PPy), and poly(Thiophene) (PTh) were prepared within these PEI cryogels. Then, the conductive polymer embedding PEI composites’ characterization was carried morphologically using scanning electron microscope (SEM) by means of Fourier Transform Infrared Radiation (FT-IR) spectrometer, and by means of electrical conductivity measurements using an electrometer. Among all the prepared cryogel conductive polymer composites, the highest value in terms of conductivity was determined for PEI/PANi cryogel composites with 4.80 10 3 S.cm 1. Afterward, to prepare polymeric ionic liquid (PIL) forms of PEI × − − and PEI/PANi composites. To assess the effect of anions on the conductivities of the prepared composites, PEI-based cryogels were anion ex-changed after protonation with HCl by treatment + of aqueous solutions of sodium dicyanamide (Na [N(CN)2]−), ammonium hexafluorophosphate + + + (NH4 [PF6]−), sodium tetrafluoroborate (Na [BF4]−), and potassium thiocyanate (K [SCN]−), separately. Furthermore, PEI-based cryogel composites and their PIL forms were tested as a sensor for CO2 gas. The higher conductivity changes were observed on bare PEI cryogel and + PEI [BF4]− PIL cryogels with 1000-fold decrease on conductivity upon 240 min CO2 exposure. + The sensitivity and recovery percent of bare PEI and PEI [BF4]− PIL cryogels were shown almost the same with a two-fold decrease in the presence of 0.009 mole of CO2 gas, and approximately 30% recovery after the fifth consecutive reuse.

Keywords: PEI cryogel; polymeric ionic liquid cryogel; cryogel/conductive polymer composite; polymeric CO2 sensor; conductometric sensor

1. Introduction

Carbon dioxide (CO2) exist in the mixture of atmospheric gas and fossil fuels (coal, natural gas, petroleum, and derivatives) as a result of industrial discharge and forest ﬁres and natural disasters. Among the thermal power plants, oil reﬁneries, and petrochemical plants, hydrogen and cement plants are the major producers of CO2 [1]. The contribution of CO2 is 80%–85% in the atmosphere, derived from fossil fuels, and about 15%–20% from the respiration of living things and the decomposition of organic matter from microscopic organisms [2]. In addition to the rapid increase of fossil fuel use, the destruction of forests and vegetation plankton, which consume tons of CO2 during photosynthesis, have reached

J. Compos. Sci. 2020, 4, 27; doi:10.3390/jcs4010027 www.mdpi.com/journal/jcs J. Compos. Sci. 2020, 4, 27 2 of 14

the highest level of CO2 in the atmosphere over the past 160,000 years and analyses project that this increase will continue [3]. The emission of produced CO2 gas into the world atmosphere is an international issue due to the influence of greenhouse gases with this uncontrolled emission that the Kyoto protocol tried to limit. Therefore, the selective separation of CO2 from gas mixtures, detection of CO2, and harmless storage "CO2 sequestration" is considered of paramount significance [4]. Current approaches in CO2 sensing, including spectrophotometry [5,6], solid-electrolyte electrochemical sensors [7], and semiconductor metal oxide-based sensors [8], have disadvantages such as cost, volume/size, or high energy consumption and so on. Researchers have been seeking to discover new methods to overcome these disadvantages in CO2 detection methods. Following Severinghaus’s approach, some types of alkyl amine were used as CO2 determination layers, producing carbonic acid and carbamates contributing to the pH change in the amine phase [9]. The reaction or interaction between amine groups and CO2 occurs in three possible ways such as the acid-based reaction, decomposition, and base-catalyzed CO2 hydration, depending on the type of amine groups (primary, secondary, or tertiary) and relative humidity [10]. Therefore, it is a practical approach to consider PEI as it contains all different types of amines (primary, secondary, and tertiary amines) to use CO2 interacting material as composite polymeric sensing applications. Additionally, the branched structure of PEI can provide an intermedia to the acid-base reaction, structurally helping to realize the reaction that acidifies amines for high CO2 loading capacity in base catalyzed hydration and results in good amine recovery [10]. In this study, PEI cryogels with superporous morphology were used as a template for in situ conductive PANi, PPy, and PTh polymer syntheses. In addition, PEI cryogels were turned into the corresponding IL forms by employing anion-exchange reactions or the protonated PEI at room + + + + temperature. The aqueous solutions of Na [N(CN)2]−, NH4 [PF6]−, Na [BF4]−, and K [SCN]− were used as an anion source in the preparation of PIL PEI cryogels. Next, this PIL PEI cryogels were also used as a template in conductive PANi synthesis. The potential sensor application of prepared PEI, PEI/PANi, PEI/PPy, and PEI/PTh cryogel composites were investigated based on electrical conductivity values after CO2 gas exposure at various times at 200 mL/min flow rate. The potential sensor application of PEI-based PIL cryogels were also tested, and the sensitivity and percent recovery (to attain the % initial sensitivity) were investigated.

2. Materials and Methods

2.1. Materials Polyethyleneimine (PEI, 50 wt% soln. in water, Mn: 1200, Mn:1800, and Mn:60,000, d:1.08, Sigma Aldrich, St. Louis, MO, USA), glycerol diglycidyl ether (GDE 100%, Sigma Aldrich, ≤ St. Louis, MO, USA) were used in the synthesis of PEI cryogels. Hydrochloric acid (HCl, 36.5–38%, Sigma Aldrich, St. Louis, MO, USA) were used for the protonation of PEI cryogels. Sodium dicyanamide + + (Na [N(CN)2]−, 96%, Aldrich, St. Louis, MO, USA), ammonium hexaﬂuorophosphate (NH4 [PF6]−, + 99%, Aldrich, St. Louis, MO, USA), sodium tetraﬂuoroborate (Na [BF4]−, 97%, Merck, Darmstadt, + Germany), and potassium thiocyanate (K [SCN]−, 99%, Merck, Darmstadt, Germany) were used for preparation of polymeric ionic liquid PEI cryogels as an anion source. Aniline (ANi, 99%, Sigma-Aldrich, St. Louis, MO, USA), pyrrole (Py, 98%, Aldrich, St. Louis, MO, USA), and thiophene (Th, 99%, Aldrich, St. Louis, MO, USA) were used as monomers in the synthesis of the corresponding conductive polymers. Ammonium persulfate (APS, 98%, Sigma-Aldrich, St. Louis, MO, USA), and iron (III) chloride hexahydrate (FeCl3.6H2O, 98%, Acros, Carlsbad, CA, USA) were used as an initiator for in situ synthesis of conductive polymers. Chloroform (CH3Cl, 99%, Riedel de Haen, Seelze, Germany) was used for in situ synthesis of PTh. The CO2 gas tank was purchased from local vender, Turkey. Pure ethanol (99.9%, Sigma-Aldrich, St. Louis, MO, USA) was used in washing of cryogels and throughout the experiments. J. Compos. Sci. 2020, 4, 27 3 of 14

2.2. Synthesis of Macro Porous PEI and PIL PEI Cryogels The epoxy-amine reaction at cryogenic conditions was exploited in the synthesis of PEI cryogels as reported earlier by our group and the preparation of PIL forms of PEI cryogels was carried out via anion exchange reactions [11]. Additionally, the details about synthesis of PEI cryogels, and preparation of PIL forms of PEI cryogels are given in Supporting Information.

2.3. In situ Preparation of Conductive Polymers within PEI Cryogels In the in-situ synthesis of PANi, PPy, and PTh, conductive polymers within super macroporous PEI cryogels was carried out according to earlier reports [12,13]. In brief, PEI-based cryogels were placed into ANi, Py, and Th monomers in liquid form to load these monomers into PEI cryogels. Then, the PEI cryogels containg ANi, Py, and Th monomers were placed into initiator systems for the polymerization within PEI cryogel pores. The details about in situ synthesis of PAN, PPy, and PTh conductive polymers within PEI cryogels are also provided in Supporting Information.

2.4. Instruments The details about used instruments for the characterization and sensor application are given in the Supporting Information.

2.5. Conductometric Sensor Application of PEI-Based Composites against CO2 Gas PEI-based cryogels and their PANi, PPy, and PTh conductive polymer composites, and also PIL PEI cryogels were tested as a sensor against CO2 gas by means electrical conductivity changes upon CO2 exposure. The PEI-based cryogels were exposed to 200 mL/min flow rate of CO2 gas for 30, 60, 120, and 240 min, and the difference on conductivities before and after CO2 treatments were measured. In the sensitivity studies, the exposure of CO2 gas with a 200 mL/min flow rate for 1, 5, 15, 30, 45, 60, 75, 90, 105, 120, and 240 min were also carried out. Furthermore, the recovery percentage studies were done by thermal methods in which 240 min of CO2 gas treated PEI-based cryogels, which were placed into an oven at 50 ◦C for 60 min, and re-exposed to CO2 gas again at the same conditions (200 mL/min flow rate). These cycles were repeated for five times by measuring the conductivity in between every cycle.

3. Results

3.1. Synthesis and Characterization of PEI Cryogel/Conductive Polymer Composites As mentioned, PEI cryogels were prepared by using epoxy-amine reaction at cryogenic conditions (T = 18 C). The linking of PEI chains with GDE molecules generated supermacroporous PEI cryogels − ◦ with interconnected pores [11]. The preparation of the IL forms of PEI cryogels and their use in adsorption/separation of some organic dyes were also reported in detail [11]. Among the various molecular weights of branched PEI, Mn: 1800 was found to possess better mechanical strength than the others [11]. Therefore, PEI cryogels that were prepared by using Mn: 1800 g/mole was used as a template for in situ conductive polymers, e.g., PANi, PPy, and PTh synthesis. The schematic presentation of PEI cryogels and then the in situ conductive polymers syntheses within PEI cryogel as composites is illustrated in Figure S1 [12,13]. Before polymerization of ANi, Py, and Th, known weight PEI cryogel pieces was placed into 10 mL of corresponding monomers for about 30 min to load monomers into the pores of PEI cryogels. Then, the ANi loaded PEI cryogels were weighed again to determine the loaded amount of ANi into PEI cryogels and then placed into a 1:1.25 (APS:ANi) mole ratio APS containing 1 M HCl solution to initiate the in situ polymerization of ANi molecules within PEI cryogels under a constant stirring rate at 250 rpm for about 15 min. The Py and Th loaded PEI cryogels were also similarly placed into 0.5 M 250 mL FeCl3 solution in DI water at room temperature for 2 h, and in 0.3 M 100 mL FeCl3 solution in chloroform at 65 ◦C for 16 h to initiate the corresponding J. Compos. Sci. 2020, 4, 27 4 of 14 in situ polymerization of Py and Th within PEI cryogels, respectively. After that, the PEI conductive polymer containing cryogels were washed with DI water exhaustively and dried in oven at 50 ◦C for 12 h. J. Compos. Sci. 2020, 4, x FOR PEER REVIEW 4 of 14 The FT-IR spectra of PEI/PANi, PEI/PPy, and PEI/PTh cryogel conductive polymer composites are givenThe in FT-IR Figure spectra S2 to of conﬁrm PEI/PANi, the synthesisPEI/PPy, and of conductivePEI/PTh cryogel PANi, conductive PPy, and polymer PTh polymers composites within PEIare cryogels. given in From Figure the S2 FT-IR to confirm spectrum, the synthesis the most of notable conductive peaks PANi, of PEI PPy, cryogel and PTh were polymers observed within as N-H 1 1 1 stretchingPEI cryogels. at 3276 From cm− the, C-H FT-IR stretching spectrum, at the 2951 most and notable 2842 cm peaks− , and of PEI C-N-C cryogel stretching were observed peak at 1300 as N-H cm − . Additionally,stretching at upon 3276 crosslinkingcm-1, C-H stretching PEI with at GDE,2951 and the 2842 peaks cm for-1, and O-H C-N-C bending stretching and C-O peak stretching at 1300 cm from- secondary1. Additionally, alcohols upon obtained crosslinking from PEI the with reactions GDE, ofthe amines peaks for with O-H epoxies bending were and observedC-O stretching at 1112 from and 1 1 1043secondary cm− , respectively. alcohols obtained Moreover, from thethe observedreactions of peaks amines at 1570with cmepoxies− match were the observed benzenoic-quinonic at 1112 and -1 1 -1 nitrogen1043 cm vibration,, respectively. at 1486 Moreover, cm− aromatic the observed C-C peaks, peaks at 1305at 1570 cm-1 cm aromatic match the amine benzenoic-quinonic peak, and C-N-C 1 -1 peaksnitrogen at 1145 vibration, cm− belonging at 1486 cm to aromatic PANi conductive C-C peaks, polymers at 1305 cm-1 [14] conformingaromatic amine the peak, PEI/PANi and C-N-C cryogel composites.peaks at 1145 Similarly, cm-1 belonging FT-IR spectrum to PANi of conductive PEI/PPy cryogel polymers composites [14] conforming were also the shown PEI/PANi characteristic cryogel 1 peakscomposites. of PPy such Similarly, as =C–H FT-IR band spectrum in-plane of PEI/PPy vibrations cryogel at 1360 composites cm− , and were N–C also stretching shown characteristic vibrations at peaks of1 PPy such as =C–H band in-plane vibrations at 1360 cm-1, and N–C stretching1 vibrations at 1228 cm− , and the peaks, to the presence of PPy observed 789 and 1021 cm− , and =C–H assigned to 1228 cm-1, and the peaks, to the presence of PPy1 observed 789 and 1021 cm-1, and =C–H assigned to the out of plane stretching vibration at 923 cm− [15]. Furthermore, the characteristic peaks of PTh the out of plane stretching vibration at 923 cm-1 [15]. Furthermore, the characteristic1 peaks of PTh were also seen in FT-IR spectrum of PEI/PTh cryogel composites at 1324 and 1424 cm− associated with were also seen in FT-IR spectrum of PEI/PTh cryogel composites at 13241 and 1424 cm-1 associated C-H in-plane bending vibrations of Th molecules, and peak at 789 cm− for α-α connection between with C-H in-plane bending vibrations of Th molecules, and peak at 789 cm-1 for α-α connection Th molecules, which was generated due to the PTh structure. This conﬁrms the in situ synthesis of between Th molecules, which was generated due to the PTh structure. This confirms the in situ PTh within the PEI cryogel [16]. synthesis of PTh within the PEI cryogel [16]. The SEM images of PEI/conductive polymer cryogel composites are demonstrated in Figure1. The SEM images of PEI/conductive polymer cryogel composites are demonstrated in Figure 1. In FigureIn Figure1a, 1a, the the clear clear and and open open pores pores of of bare bare PEI PEI cryogels cryogels werewere clearly seen seen with with varied varied pore pore sizes sizes µ betweenbetween 1–50 1–50m. µm.

FigureFigure 1. SEM1. SEM images images ofprepared of prepared (a) bare (a) Polyethyleneiminebare Polyethyleneimine (PEI) (PEI) cryogel, cryogel, (b) PEI (b/PANi,) PEI/PANi, (c) PEI (/PPy,c) andPEI/PPy, (d) PEI /andPTh (d cryogel) PEI/PTh composites. cryogel composites.

OnOn the the other other hand, hand, it it is is apparently apparently seenseen from SEM images images of of PEI/PANi, PEI/PANi, PEI/PPy, PEI/PPy, and and PEI/PTh PEI/PTh cryogelcryogel composites composites in in Figure Figures1b–d 1b–1d the the existence existence of of conductive conductive polymerspolymers of PANi, PANi, PPy, PPy, and and PTh, PTh, respectively, within PEI cryogels by filing and/or plugging the pores and pore walls of PEI cryogels respectively, within PEI cryogels by ﬁling and/or plugging the pores and pore walls of PEI cryogels with these guest conductive polymers. The extent of in situ synthesized conductive PANi, PPy, and with these guest conductive polymers. The extent of in situ synthesized conductive PANi, PPy, and PTh PTh polymers within PEI cryogels were calculated gravimetrically and results were summarized in polymers within PEI cryogels were calculated gravimetrically and results were summarized in Table S1, Table S1, as 5.9 ± 0.4 g of PANi/g of PEI cryogel, 5.1 ± 0.1 g of PPy/g of PEI cryogel, and 3.2 ± 0.3 g of as 5.9 0.4 g of PANi/g of PEI cryogel, 5.1 0.1 g of PPy/g of PEI cryogel, and 3.2 0.3 g of PTh/g of PTh/g± of PEI cryogel. ± ± PEI cryogel. 3.1.1 Conductivity Measurements Conductivity Measurements The electrical conductivity of PEI, PEI/PANi, PEI/PPy, and PEI/PTh cryogel composites were The electrical conductivity of PEI, PEI/PANi, PEI/PPy, and PEI/PTh cryogel composites were determined from ohmic region of I-V curves obtained using an electrometer. The schematic determinedpresentation from of ohmic electrometer region of setup I-V curves used obtained in the conductivity using an electrometer. measurements The schematic of PEI/conductive presentation polymer cryogel composite are given in Figure S3. It was clearly seen from digital camera images that the white color of PEI cryogels turned to black upon situ synthesis conductive PANi and PPy polymers, and brown with the in-situ synthesis of conductive PTh polymers, respectively. The I-V curves from the conductivity measurements of PEI-based composite cryogels done by connecting the

J. Compos. Sci. 2020, 4, 27 5 of 14 of electrometer setup used in the conductivity measurements of PEI/conductive polymer cryogel composite are given in Figure S3. It was clearly seen from digital camera images that the white color of PEIJ. Compos. cryogels Sci. 2020 turned, 4, x FOR to black PEERupon REVIEW situ synthesis conductive PANi and PPy polymers, and brown 5with of 14 the in-situ synthesis of conductive PTh polymers, respectively. The I-V curves from the conductivity electrodes on carbon tape that glued on both sides of PEI-based cryogel composite cylinders. Then, measurements of PEI-based composite cryogels done by connecting the electrodes on carbon tape that the conductivity of a PEI-based cryogel composite were calculated from an ohmic region of I-V curves glued on both sides of PEI-based cryogel composite cylinders. Then, the conductivity of a PEI-based by using Equations (1) and (2). cryogel composite were calculated from an ohmic region of I-V curves by using Equations (1) and (2). V = I × R (1) V = I R (1) × σ = (1/R) × (l/A) (2) σ = (1/R) (l/A) (2) where ‘V’ is applied voltage, ‘I’ is current, ‘R’ is× the bulk resistance, ‘σ’ is conductivity, ‘1/R’ is whereresistivity, ‘V’ is ‘l’ applied is the voltage,thickness, ‘I’ and is current, ‘A’ is the ‘R’ iscross-sectional the bulk resistance, area of ‘σ the’ is conductivity,sample. ‘1/R’ is resistivity, ‘l’ is theThe thickness, comparison and ‘A’of isconductivity the cross-sectional values areaof PEI, of the PEI/PANi, sample. PEI/PPY, and PEI/PTh cryogel compositesThe comparison are presented of conductivity in Figure values 2a. The of PEI,measur PEIed/PANi, conductivity PEI/PPY, andvalues PEI /arePTh also cryogel summarized composites in areTable presented S1. The inconductivity Figure2a. of The bare measured PEI cryogels conductivity were increased values areto 7500 also-, summarized 220-, 3- folds in after Table in S1.situ Thesynthesis conductivity of PANi, of barePPy, PEIand cryogels PTh polymers, were increased respectively. to 7500-, As 220-,the maximum 3- folds after extent in situ of increase synthesis was of PANi,observed PPy, for and PEI/PANi PTh polymers, cryogel, respectively. the conductive As the PANi maximum polymer extent was of chosen increase for was IL observedforms of forPEI PEIcryogels./PANi cryogel, the conductive PANi polymer was chosen for IL forms of PEI cryogels.

1,E+00

1,E-01 Conductivity (a) ) -1 1,E-02

1,E-03

1,E-04

1,E-05 Conductivity (S/cm

1,E-06

1,E-07 PEI PEI/PANi PEI/PPy PEI/PTh

+ - 1- PEI cryogels 4-PEI [PF6] PIL cryogel + - + - 2- PEI Cl PIL cryogels 5-PEI [BF4] PIL cryogel + - + - 3- PEI [N(CN)2] PIL cryogels 6-PEI [SCN] PIL cryogel PEI based cryogels Insitu PANi synthesized PEI based cryogels

1,E-02 (b) )

-1 1,E-03 ~30 fold

1,E-04 fold ~300 ~20 fold ~8 M fold

1,E-05 ~7500 fold

1,E-06 Conductivity (S/cm ~300 K ~300 K fold 1,E-07

1,E-08

1,E-09 123456

FigureFigure 2.2. ((aa)) Comparison Comparison of of the the measured measured conductivities conductivities of bare of bare PEI PEI cryogel, cryogel, PEI/PANi, PEI/PANi, PEI/PPy, PEI/PPy, and and PEI/PTh cryogel composites, and (b) PANi containing PEI PIL cryogels composites. PEI/PTh cryogel composites, and (b) PANi containing PEI PIL cryogels composites.

J. Compos. Sci. 2020, 4, 27 6 of 14

PIL PEI cryogels were prepared in accordance with the literature and provided in Supplementary Information [11]. The protonated PEI cryogels by using 1 M HCl solution were treated with various + + + + IL agents such as Na [N(CN)2]−, NH4 [PF6]−, Na [BF4]−, and K [SCN]− to exchange the anions to produce IL PEI cryogels with different anions. IL PEI cryogels were acquired by replacing Cl- anions on protonated PEI cryogels with dicyanamide ([N(CN)2]−), hexafluorophosphate ([PF6]−), tetrafluoroborate ([BF4]−), and thiocyanate ([SCN]−) anions. In addition, the anion exchange reactions were confirmed with FT-IR spectrum [11]. As mentioned previously, these IL PEI cryogels were used as a templateJ. Compos. Sci. for 2020 conductive, 4, x FOR PEER PANi REVIEW polymer and the results of the conductivity measurements 6 of 14 of the prepared IL PEI-based cryogel composites were compared and illustrated in Figure2b. Conductivity 4 valuesreactions were also were summarized confirmed with in FT-IR Table spectrum S2. The conductivity[11]. As mentioned of PEI previously, cryogels these increased IL PEI to cryogels 1.64 10− were used5 as a1 template for conductive7 PANi 8 polymer1 and the results of the conductivity× 2.16 10− S.cm− from 6.40 10− 3.45 10− S.cm− with 250-fold increase after protonation ± measurements× of the prepared× IL PEI-based± × cryogel composites were compared and illustrated+ in with 1 M HCl solutions (IL forms of PEI). On the other hand, the conductivity of PEI Cl− cryogels Figure 2b. Conductivity values were also summarized in Table S2. The conductivity of PEI cryogels also changed after treatment with IL forming agents and the conductivity values of 1.68 10 9 increased to 1.64 × 10-4 ± 2.16 × 10-5 S.cm-1 from 6.40 × 10-7 ± 3.45 × 10-8 S.cm-1 with 250-fold increase× − ± 3.21 10 5, 1.76 10 8 1.26 10 9, 1.55 10 5 2.85 10 6, and 1.09 10 5 1.11 10 6 S cm 1 ×after− protonation× with− ± 1 M HCl× solutions− × (IL forms− ± of PEI).× On− the other hand,× − the± conductivity× − of − that arePEI+ 100Cl- cryogels 000-, 10 also 000-, changed 10-, and after 15-fold treatment decrease with forIL forming [N(CN) agents2]−, [PF and6]− the, [BF conductivity4]−, and [SCN] values− anions,of respectively,1.68 × 10-9 were ± 3.21 measured. × 10-5, 1.76 × Then, 10-8 ± after1.26 × the 10-9 in-situ, 1.55 × 10 synthesis-5 ± 2.85 × of 10 the-6, and conductive 1.09 × 10-5 PANi± 1.11 × polymer 10-6 S cm within-1 thesethat IL PEIare 100 cryogel 000-, also10 000-, show 10-, some and 15-fold increase decrease on the for measured [N(CN)2]- conductivities , [PF6]- , [BF4]- , ofand all [SCN] PIL PEI- anions, cryogels. The conductivityrespectively, were of IL measured. PEI cryogels Then, were after found the in-situ to increase synthesis as of 30-, the 8 conductive 000 000-, 300 PANi 000-, polymer 300-, andwithin 20-fold uponthese in situ IL PEI synthesis cryogel ofalso conductive show some PANiincrease polymers. on the measured The conductivity conductivities values of all PIL ofall PEI PEI-based cryogels. PIL and compositeThe conductivity cryogels of IL provide PEI cryogels some were appealing found to properties increase as as 30-, they 8 000 can 000-, be used300 000-, to determine 300-, and 20- and/or sensefold various upon moleculesin situ synthesis because of conductive of not just PANi functional polymers. groups The ofconductivity PEI but also values the existenceof all PEI-based of diverse PIL and composite cryogels provide some appealing properties as they can be used to determine IL forming anions as well as the existence of conductive PANi within the pores of cryogels. There are and/or sense various molecules because of not just functional groups of PEI but also the existence of few reportsdiverse onIL forming the use anions of cryogels as well and as the cryogel existence/conductive of conductive polymer PANi composites within the aspores a sensor of cryogels. to various gasesThere or molecules are few reports [12,17 on,18 the]. use of cryogels and cryogel/conductive polymer composites as a sensor to various gases or molecules [12,17,18]. 3.2. Sensor Application 3.2 Sensor Application 3.2.1. PEI/Conductive Polymer Cryogel Composites as Sensing Material 3.2.1 PEI/Conductive Polymer Cryogel Composites as Sensing Material The potential usage of PEI for CO2 capture studies encouraged us to employ a PEI-based super porous structureThe potential for ausage potential of PEI sensorfor CO2 application.capture studies It encouraged was further us knownto employ that a PEI-based IL materials super along withporous PEI, e.g., structure composite for a potential cryogels sensor for COapplication.2 gas possess It was further great potentialknown that [19 IL– materials21]. Therefore, along with a basic experimentalPEI, e.g., set-up,composite as showncryogels in for Figure CO32 ,gas is arranged possess great to measure potential the [19–21]. change Therefore, in the conductivity a basic of experimental set-up, as shown in Figure 3, is arranged to measure the change in the conductivity of PEI cryogel before and after CO2 exposure. PEI cryogel before and after CO2 exposure.

Exhaust Cryogel sample Glass-fibre

Manometer

CO2

Flow rate 200 mL/min

CO2 Gas

Figure 3. The schema of the presentation of CO2 gas exposure on PEI-based cryogels and Figure 3. The schema of the presentation of CO2 gas exposure on PEI-based cryogels and composites. composites.

As shown in the figure, the tube of CO2 gas was attached to a flow-meter to adjust flow rate of CO2 to 200 mL/min and then contact the-PEI based cryogels at this constant flow rate at various time intervals, e.g., 30, 60, 120, and 240 min. The change in the electrical conductivity of PEI and PEI/PANi,

J. Compos. Sci. 2020, 4, 27 7 of 14

As shown in the figure, the tube of CO2 gas was attached to a flow-meter to adjust flow rate of CO2 to 200 mL/min and then contact the-PEI based cryogels at this constant flow rate at various time J. Compos. Sci. 2020, 4, x FOR PEER REVIEW 7 of 14 intervals, e.g., 30, 60, 120, and 240 min. The change in the electrical conductivity of PEI and PEI/PANi, PEI/PPy, and PEI/PTh cryogel composites after CO2 treatment at various time intervals were measured PEI/PPy, and PEI/PTh cryogel composites after CO2 treatment at various time intervals were and the corresponding graphs are given in Figure4a–d, respectively. measured and the corresponding graphs are given in Figures 4a–4d, respectively.

1,E-06 1,E+00

) Before After (a) ) (b)

-1 -1 Before After 1,E-07 1,E-01

1,E-08 1,E-02

1,E-09 1,E-03

1,E-10 1,E-04 Conductivity (S.cm Conductivity (S.cm 1,E-11 1,E-05 30 60 120 240 30 60 120 240 Time (min) Time (min)

1,E-02 1,E-04

) (c) ) (d)

-1 Before After -1 Before After 1,E-03 1,E-05

1,E-04 1,E-06

1,E-05 1,E-07

1,E-06 1,E-08 Conductivity (S.cm Conductivity (S.cm 1,E-07 1,E-09 30 60 120 240 30 60 120 240 Time (min) Time (min)

FigureFigure 4.4.The The change change in in conductivities conductivities of( aof) bare(a) bare PEI cryogel,PEI cryogel, (b) PEI (b/PANi,) PEI/PANi, (c) PEI/ PPy,(c) PEI/PPy, and (d) PEI and/PTh (d) cryogel composites under CO gas exposure. [200 mL/min ﬂow rate]. PEI/PTh cryogel composites under2 CO2 gas exposure. [200 mL/min flow rate].

The measured electrical conductivity values of PEI-based cryogel composites before and after The measured electrical conductivity values of PEI-based cryogel composites before and after exposure of CO2 gas at various exposure times are summarized in Table1. The conductivity of PEI exposure of CO2 gas at various exposure times are summarized in Table 1. The conductivity of PEI cryogels was found to decrease with the increase of CO2 exposure time of at least a 200 mL/min cryogels was found to decrease with the increase of CO2 exposure time of at least a 200 mL/min flow ﬂow rate. The decrease in the extent of conductivity of PEI cryogels were increased with the rise rate. The decrease in the extent of conductivity of PEI cryogels were increased with the rise of of exposure time of CO2 as approximately 10-, 90-, 800-, and 1000-fold decrease for 30, 60, 120, and exposure time of CO2 as approximately 10-, 90-, 800-, and 1000-fold decrease for 30, 60, 120, and 240 240 min exposure of CO2 gas. min exposure of CO2 gas.

Table 1. The conductivities of bare PEI and PEI conductive polymer composites before and after CO2 Table 1. The conductivities of bare PEI and PEI conductive polymer composites before and after CO2 gas exposure at various times. gas exposure at various times. 1 Conductivity (S.cm− ) -1 Cryogel Composite Conductivity (S.cm ) Cryogel Composite 30 min 60 min 120 min 240 min 30 min 60 min 120 min 240 min 8 8 8 8 Before 7.0 10− -8 7.1 10− -8 8.3 10− 9.2-8 10− -8 PEI Before 7.0× × 109 ×7.1 ×10 10 × 8.310 × 10 × 11 9.2 × 10 PEI After 6.9 10− 7.6 10− 1.0 10− 9.7 10− × 3-9 × 3 -10 × 3 -10× 3 -11 AfterBefore 4.56.9 ×10 10− 4.3 7.610 ×− 10 4.6 101.0− × 104.9 10− 9.7 × 10 PEI/PANi × 2 × 2 × 2 × 3 After 1.1 10− -3 1.1 10− -3 2.3 10− 8.5-3 10− -3 Before 4.5× × 104 ×4.3 ×4 10 × 4.64 × 10 × 4 4.9 × 10 PEI/PANi Before 1.3 10− 1.3 10− 1.2 10− 1.4 10− PEI/PPy × 5-2 × 5 -2 × 5 -2 × 4 -3 AfterAfter 7.51.1 ×10 10− 6.5 1.110 −× 10 9.2 10−2.3 × 101.4 10− 8.5 × 10 × 6 × 6 × 6 × 6 Before 2.1 10− -4 2.0 10− -4 1.8 10− 2.2-4 10− -4 PEI/PTh Before 1.3× × 107 ×1.3 ×9 10 × 1.29 × 10 × 9 1.4 × 10 PEI/PPy After 1.7 10− 5.7 10− 5.9 10− 6.5 10− × -5 × -5 × -5 × -4 After 7.5 × 10 6.5 × 10 9.2 × 10 1.4 × 10 -6 -6 -6 -6 Before 2.1 × 10 2.0 × 10 1.8 × 10 2.2 × 10 PEI/PTh -7 -9 -9 -9 After 1.7 × 10 5.7 × 10 5.9 × 10 6.5 × 10

It is known that, with the exposure of CO2 to PEI cryogels results in carbamate formation, which reduce the overall electron donating effect and, therefore, upon carbamate formation, geometric deformations in the polymers’ layer may lead to reduction on the electrical conductivity [22]. The

J. Compos. Sci. 2020, 4, 27 8 of 14

It is known that, with the exposure of CO2 to PEI cryogels results in carbamate formation, which reduce the overall electron donating eﬀect and, therefore, upon carbamate formation, geometric deformations in the polymers’ layer may lead to reduction on the electrical conductivity [22]. The increase in the conductivity of PEI/PANi cryogel composites after exposure of CO2 gas as 2.5-, 2.5-, 5-, and 2-fold after 30, 60, 120, and 20 min of CO2 exposure. This increase on the conductivity of PEI/PANi cryogel composites can be explained with the formation of carbamates and carbonic acids that potentially dopes PANi leading to increased conductivity of PANi [23]. On the other hand, there was no observed signiﬁcant change in the conductivity of PEI/PPy cryogel composites after 30, 60, 120, and 240 min of CO2 exposure. There is a slight change in conductivity as single digit results are reported for PEI/PPy composites upon 240 min of CO2 exposure. This can be perceived as unchanged. The conductivity values for PEI/PPy cryogel composites were measured as 1.44 10 4 3.25 10 5, × − ± × − and 1.43 10 4 1.87 10 5 before and after 240 min exposure of CO gas. It was reported that the × − ± × − 2 saturation of PPy to CO2 gas was realized in 2 min exposure, and the conductivity of PPy was increased with more CO2 exposure [24]. On the other hand, there is no change observed in the conductivity of composite PPy with polyamide (PA) with more CO2 exposure. This was explained with very rapid, and reversible processing of doping and undoping of composites of PEI/PPy cryogel composites under CO2 gas exposure [24]. The conductivity of PEI/PTh cryogel composites also decreased as 12-, 350-, 310-, and 330-fold after the CO2 exposure times of 30, 60, 120, and 240 min, respectively. The higher conductivity change was observed for bare PEI cryogels with 10-, 90-, 800-, and 1000-fold decrease upon 30, 60, 120, and 240 min of CO2 exposure, respectively.

3.2.2. The Use of IL Forms of Pei Cryogels

The use of PIL materials on CO2 capture studies in the literature [25–29] urged us to study the change in the conductivity of conductive polymer containing IL PEI cryogels compounds after CO2 gas exposure. Therefore, IL forms of the PEI cryogel were also treated with CO2 for 30, 60, 120, and 240 min, and the change in their electrical properties was determined. The corresponding + + + + + graphs of PEI Cl−, PEI [N(CN)2]−, PEI [PF6]−, PEI [BF4]−, and PEI [SCN]− IL cryogels are given in Figure5a–e, respectively, and summarized in Table2. Form Figure5a, it was observed that the + 5 6 6 conductivity value of PEI Cl− cryogels decreased from 1.7 10− 1.8 10− to 3.8 10− 1.2 7 5 6 7 8 ×5 ± ×7 ×8 ± ×9 10− , 1.8 10− 2.7 10− to 8.5 10− 6.2 10− , 1.9 10− 8.9 10− to 7.7 10− 3.6 10− , × 5 ± × 6 × 8 ± × 9 ×1 ± × × ± × and 2.4 10− 1.5 10− to 8.0 10− 2.4 10− S.cm− by 30, 60, 120, and 240 min CO2 exposure, × ± × × ± × + respectively. There are no observed signiﬁcant changes in the conductivity of PEI [N(CN)2]−, as shown + in Figure5b. The changes of the conductivity of PEI [PF6]− PIL cryogels are given in Figure5c, and it is clear that the conductivity of PEI+[PF ] PIL cryogels decreased from 4.8 10 10 4.6x10 11 to 8.5 6 − × − ± − 10 11 3.6 10 12 S.cm 1 upon 30 min, and from 7.8 10 10 3.9 10 11 to 7.6 10 11 4.9 × − ± × − − × − ± × − × − ± × 10 12 S.cm 1 upon 60 min, and from 4.3 10 10 4.2 10 12 to 4.1 10 11 3.9 10 12 S.cm 1 upon − − × − ± × − × − ± × − − 120 min, and from 3.7 10 10 4.2 10 11 to 3.8 10 11 4.7 10 12 S.cm 1 upon 240 min CO times. × − ± × − × − ± × − − 2 Table 2. The change in conductivities of bare polyethyleneimine (PEI) and IL forms of PANi containing

composites after CO2 gas exposure at various times.

The Decrease in Conductivity (Fold) PEI+ PEI+ PEI+ PEI+ PEI+ PEI Cl− [N(CN)2]− [PF6]− [BF4]− [SCN]− 0 0 0 0 0 0 0 30 10 4.4 1 5 10 13.3 60 100 20 1 10 66.7 30 120 1000 250 1 10 1000 160 240 1000 300 1 10 1000 300 J. Compos. Sci. 2020, 4, 27 9 of 14 J. Compos. Sci. 2020, 4, x FOR PEER REVIEW 9 of 14

1.E-03 )

-1 Before (a) + - 1.E-04 After PEI Cl 1.E-05 1.E-06 1.E-07 Conductivity (S.cm 1.E-08 30 60 120 240 Time (min)

1.E-07 1.E-07 + - ) ) Before (b) + - (c) PEI [PF6] PEI [N(CN)2] -1

-1 Before 1.E-08 1.E-08 After After 1.E-09 1.E-09 1.E-10 1.E-10 1.E-11 1.E-11 Conductivity (S.cm Conductivity (S.cm 1.E-12 1.E-12 30 60 120 240 30 60 120 240 Time (min) Time (min)

1.E-05 Before 1.E-05 ) (d) + - ) Before (e) PEI+ [SCN]- -1

PEI [BF ] -1 1.E-06 After 4 1.E-06 After 1.E-07 1.E-07 1.E-08 1.E-08 1.E-09 1.E-09 Conductivity (S.cm 1.E-10 Conductivity (S.cm 1.E-10 30 60 120 240 30 60 120 240 Time (min) Time (min)

+ + + + FigureFigure 5.5. TheThe changingchanging on conductivities of (a) PEI+ClCl-,− (,(b)b PEI) PEI+[N(CN)[N(CN)2]- 2(]c−) PEI(c) PEI+ [PF6[PF]-, (d6])− PEI,(d+) [BF PEI4]- + [BF, and4]− (,e and) PEI (+e )[SCN] PEI -[SCN] PIL cryogels− PIL cryogels under CO under2 gas CO exposure.2 gas exposure. [200 mL/min [200 mL flow/min rate]. ﬂow rate].

+ + Moreover,Moreover, thethe conductivityconductivity changes on PEI+[BF4][BF4]- −, ,and and PEI PEI+[SCN][SCN]- PIL− PIL cryogels cryogels are are given given in + inFigures Figures 5d5 dand and 5e5e respectively, respectively, and and it it was was observed observed that that the the conductivities conductivities of of PEI PEI+[BF4][BF4]-, and−, + 7 8 andPEI+[SCN] PEI [SCN]- PIL −cryogelPIL cryogel composites composites were decreased were decreased from 8.2 from × 10-7 8.2 ± 6.5 10× 10− -8 and6.5 3.2 10× −10-7and ± 5.2×10 3.2 -8 7 8 8 9 8 9 × 1 ± × × 10to− 8.3 ×5.2 10-810 ±− 3.2to × 8.3 10-9 and10− 2.43.2 × 10-810 ±− 1.6and × 10 2.4-9 S.cm10−-1 for1.6 30 min,10− andS.cm from− for 8.5 30 × min,10-7 ± and 3.6from × 10-8 8.5 and 7 ± × 8 × ± 7 × 8 × ±10 × 11 9 10× 102.9− × 103.6-7 ± 4.210 −× 10and-8 to 2.9 8.6 ×10 10−-10 ± 4.28.5 × 10−-11 andto 8.6 1.8 ×10 10− -9 ± 1.48.5 × 1010-10− S.cmand-1 for 1.8 24010 min− exposure1.4 10− of ±1 × × ± × × ± × × ± × S.cm− for 240 min exposure of CO2, respectively. As shown in Table2, the order of higher change in CO2, respectively. As shown in Table 2, the order of higher change in the conductivities of IL forms + thePEI-based conductivities cryogels of are IL bare forms PEI PEI-based with 1000- cryogels fold, PEI are+Cl bare- PIL PEI cryogel with with 1000- 300-fold, fold, PEI PEICl+[BF4]− PIL- cryogelPEI PIL + + with 300-fold, PEI [BF4]− PEI PIL+ cryogels- with 1000-fold, and PEI [SCN]− PIL cryogels with 165-fold cryogels with 1000-fold, and PEI [SCN] PIL cryogels with 165-fold were observed after 240 min CO2 were observed after 240 min CO gas exposure. As reported, the interaction between ILs and CO is gas exposure. As reported, the2 interaction between ILs and CO2 is strongly dependent on 2the strongly dependent on the solubility of CO2 in ILs, and the interaction- between [N(CN) ]− anions and solubility of CO2 in ILs, and the interaction between [N(CN)2] anions and CO2 is very2 low due to its CO is very low due to its basicity [30]. Therefore, this can explain the almost constant conductivity basicity2 [30]. Therefore, this can explain the almost constant conductivity values of PEI+[N(CN)2]- PIL values of PEI+[N(CN) ] PIL cryogels against diﬀerent CO exposure times or its independence from cryogels against different2 − CO2 exposure times or its independence2 from CO2 exposure time. CO2 exposure time. TheTable reason 2. The for change the constant in conductivities conductivity of bare value polyethyleneimine after 120 min exposure (PEI) and to COIL forms2 can beof explainedPANi withcontaining the saturation composites of PEI-based after CO cryogels2 gas exposure after at 120 various min exposuretimes. time for CO2 gas at a 200 mL/min ﬂow rate. The saturation of cryogels with CO2 is preventing the change of conductivity as steady state is reached. The Decrease in Conductivity (Fold) + + + + + + The FT-IR` spectrum of barePEI PEI cryogels,PEI PEI Cl−, PEI [BFPEI4] −, and PEI [SCN] − PEIPIL cryogels were PEI - - - recorded before and after 240 min of- CO2 exposure. The corresponding FT-IR spectra that- were forced Cl [N(CN)2] [PF6] [BF4] [SCN] in the 1650–1850 cm 1 wavenumbers’ range shown in Figure S4a–d for bare PEI cryogels, PEI+Cl , 0 − 0 0 0 0 0 0 − PEI+[BF ] , and PEI+[SCN] PIL cryogels, respectively. It was clearly seen from FT-IR spectrum of 4 − 30 10 − 4.4 1 5 10 13.3 PEI-based cryogels after 240 min of CO exposure, the peak of C=O coming from the adsorbed CO at 60 100 20 2 1 10 66.7 30 2 120 1000 250 1 10 1000 160 240 1000 300 1 10 1000 300

J. Compos. Sci. 2020, 4, 27 10 of 14

1 1 + 1 + 1754 cm− for the PEI cryogel, at 1722 cm− for the PEI Cl− PIL cryogel, at 1743 cm− for the PEI [BF4]− 1 + PIL cryogel, and at 1752 cm− for the PEI [SCN]− PIL cryogels were observed. Therefore, it is evident that CO2 gas molecules adsorbed by PEI-based cryogels caused a signiﬁcant change in conductivity of IL forms of PEI-based composites.

3.2.3.J. Compos. Sensitivity Sci. 2020, 4 of, x PEI-BasedFOR PEER REVIEW Cryogels as a CO2 Gas Sensor 10 of 14 + As the highest change in the conductivity of bare PEI and PEI [BF4]− PIL cryogels upon The reason for the constant conductivity value after 120 min exposure to CO2 can be explained CO2 exposure were observed, their sensitivity against CO2 gas exposure at 200 mL/min flow rate. with the saturation of PEI-based cryogels after 120 min exposure time for CO2 gas at a 200 mL/min The sensitivity studies were done at 200 mL/min CO2 exposure for 1, 5, 15, 45, 60, 75, 90, 105, 120, flow rate. The saturation of cryogels with CO2+ is preventing the change of conductivity as steady state and 240 min, respectively, to bare PEI and PEI [BF4]− PIL cryogels. is reached. + The amount of CO2 gas to bare PEI and PEI [BF4]− PIL cryogels was calculated as 0,009, 0.045, + - + - + - 0.135,The 0.27, FT-IR 0.40, 0.54,spectrum 0.68, 0.81,of bare 0.94, PEI 1.08, cryogels, and 2.16 PEI molesCl , PEI upon[BF 1,4] 5,, and 15, 30,PEI 45,[SCN] 60, 75, PIL 90, cryogels 105, 120, were and 240recorded min exposure before and times, after respectively. 240 min of CO It is2 exposure. clearly seen The from corresponding Figure6 that FT-IR the changes spectra on that conductivity were forced in the 1650–1850 cm-1 wavenumbers’+ range shown in Figure S4a–d for bare PEI cryogels, PEI+Cl-, of both bare PEI and PEI [BF4]− PIL cryogels start from the first min CO2 exposure with 0.009 mole of PEI+[BF4]-, and PEI+[SCN]- PIL cryogels, respectively. It was clearly seen from FT-IR spectrum of PEI- CO2 with approximately a two-fold conductivity decrease. On the other hand, it was also observed thatbased the cryogels changes after on conductivity 240 min of CO values2 exposure, of bare the PEI peak cryogels of C=O become coming constant from the with adsorbed a 741-fold CO decrease2 at 1754 cm-1 for the PEI cryogel, at 1722 cm-1 for the PEI+Cl- PIL cryogel, at 1743 cm-1 +for the PEI+[BF4]- PIL at 0.81 mole of CO2 exposure in 90 min, whereas the conductivity values of PEI [BF4]− PIL cryogels cryogel, and at 1752 cm-1 for the PEI+[SCN]- PIL cryogels were observed. Therefore, it is evident that become constant by 15 min slower than the PEI cryogel with a 762-fold decrease at 0.94 mole of CO2 CO2 gas molecules adsorbed by PEI-based cryogels caused a significant+ change in conductivity of IL exposure in 105 min. This means that the sensitivity of bare PEI and PEI [BF4]− PIL cryogels are almost forms of PEI-based composites. the same with a change in the conductivity values for two-fold in the presence of 0.009 mole of CO2 gas. The reason for no change in conductivity or constant conductivity after 1.08 mole of CO2 exposure 3.2.3 Sensitivity of PEI-Based Cryogels as a CO2 Gas Sensor in 120 min was confirmed from the results in Table2. This can also be explained by the saturation of + - PEI-basedAs the cryogels highest with change CO2 inleading the conductivity to no more changeof bare inPEI conductivity and PEI [BF with4] PIL further cryogels CO2 uponexposure CO2 anymore.exposure Additionally,were observed, it cantheir be sensitivity seen from against Figure 6CO that2 gas the exposure limit of detection at 200 mL/min of the preparedflow rate. PEI, The + andsensitivity PEI [BF studies4]− PIL were cryogels done for at CO2002 mL/mingas were CO obtained2 exposure as 0.009for 1, moles5, 15, 45, of CO60, 275,with 90, approximately105, 120, and 240 a two-foldmin, respectively, decrease in to the bare conductivity PEI and PEI of+[BF both4]- PIL cryogels. cryogels.

10000

1000

100

PEI cryogel 10

Change Change conductivityin (Fold) + - PEI+[BF4]-PEI [BF4] cryogelcryogel

1 0 0.5 1 1.5 2 2.5

The amount of CO2 (mole)

+ FigureFigure 6. 6.The The comparison comparison of of sensitivities sensitivities of of bare bare PEI PEI cryogel cryogel and and PEI PEI[BF+[BF4]4−]- cryogelcryogel underunder didifferentﬀerent

amountsamounts of of CO CO22exposure exposure[200 [200mL mL/min/min ﬂow flow rate]. rate].

According to the American Association of Heating, Refrigerating and Air Conditioning Engineers The amount of CO2 gas to bare PEI and PEI+[BF4]- PIL cryogels was calculated as 0,009, 0.045, (ASHRAE)0.135, 0.27, Standard0.40, 0.54, 62-1989, 0.68, 0.81, the 0.94, CO2 1.08,concentration and 2.16 moles in the upon environment 1, 5, 15, 30, should 45, 60, not 75, exceed 90, 105, 1000 120, ppm and for240 healthy min exposure breathing times, [31 respectively.]. The CO2 concentration It is clearly seen emitted from Figure in the 6 industrial that the changes sector ison much conductivity higher than this value and causes a serious increase in the greenhouse eﬀect, which leads to a disruptive of both bare PEI and PEI+[BF4]- PIL cryogels start from the first min CO2 exposure with 0.009 mole of natural environment for living creatures to breathe and live healthy. The concentration of CO in the CO2 with approximately a two-fold conductivity decrease. On the other hand, it was also observed2 that the changes on conductivity values of bare PEI cryogels become constant with a 741-fold decrease at 0.81 mole of CO2 exposure in 90 min, whereas the conductivity values of PEI+[BF4]- PIL cryogels become constant by 15 min slower than the PEI cryogel with a 762-fold decrease at 0.94 mole of CO2 exposure in 105 min. This means that the sensitivity of bare PEI and PEI+[BF4]- PIL cryogels are almost the same with a change in the conductivity values for two-fold in the presence of 0.009 mole of CO2 gas. The reason for no change in conductivity or constant conductivity after 1.08 mole of CO2 exposure in 120 min was confirmed from the results in Table 2. This can also be explained by the saturation of PEI-based cryogels with CO2 leading to no more change in conductivity with further CO2 exposure

J. Compos. Sci. 2020, 4, x FOR PEER REVIEW 11 of 14

anymore. Additionally, it can be seen from Figure 6 that the limit of detection of the prepared PEI, and PEI+[BF4]- PIL cryogels for CO2 gas were obtained as 0.009 moles of CO2 with approximately a two-fold decrease in the conductivity of both cryogels. According to the American Association of Heating, Refrigerating and Air Conditioning J. Compos.Engineers Sci. 2020 (ASHRAE), 4, 27 Standard 62-1989, the CO2 concentration in the environment should not exceed11 of 14 1000 ppm for healthy breathing [31]. The CO2 concentration emitted in the industrial sector is much higher than this value and causes a serious increase in the greenhouse effect, which leads to a atmosphere was reported as 411.77 ppm in July 2019 and increased day-by-day [32]. In this study, disruptive natural environment for living creatures to breathe and live healthy. The concentration of the concentration of CO2 is 396 25 ppm, and the PEI-based cryogels in the form of PIL that is used as CO2 in the atmosphere was reported± as 411.77 ppm in July 2019 and increased day-by-day [32]. In a prospective sensor revealed a two-fold reduction in conductivity showing the potential sensor use this study, the concentration of CO2 is 396 ± 25 ppm, and the PEI-based cryogels in the form of PIL of thethatprepared is used as cryogel a prospective composite sensor materials. revealed Since a two-fold this concentration reduction isin lessconductivity than half showing of the value the of thepotential hazardous sensor limit, use these of the cryogels prepared in cryogel the form composit of PILe show materials. great Since potential this concentration as sensors. The is less potential than applicationshalf of the ofvalue these of kindsthe hazardous of CO2 sensorslimit, these and cryo detectorsgels in the are form possible of PIL in show the chimney great potential of different as industrialsensors. factories,The potential households, applications mining of these shafts, kinds etc. of thatCO2 mightsensors allow and detectors us to monitor are possible the amount in the of COchimney2 emitted of/generated different orindustrial released, factories, and also households, be used to determine mining shafts, the accumulation etc. that might that allow may occurus to in indoormonitor living the environments amount of CO such2 emitted/generated as home, hotel, hostel, or released, and so on.and Therefore,also be used the preparedto determine PEI-based the cryogelsaccumulation can be employedthat may occur for these in indoor purposes. living environments such as home, hotel, hostel, and so on. Therefore, the prepared PEI-based cryogels can be employed for these purposes. 3.2.4. Reusability of PEI-Based Cryogel as a CO2 Gas Sensor 3.2.4 Reusability of PEI-Based Cryogel as a CO+ 2 Gas Sensor The recovery studies of bare PEI and PEI [BF4]− PIL cryogels were done by treating 240 min CO2 + + - gas exposedThe recovery bare PEI studies and PEI of bare[BF4 ]PEI− PIL and cryogels PEI [BF at4] 50PIL◦C cryogels for 60 min. were In done brief, by the treating conductivity 240 min ofCO bare2 + + - PEIgas and exposed PEI [BF bare4]− PEIPIL and cryogels PEI [BF treated4] PIL withcryogels CO2 atgas 50 under°C for 60 a200 min. mL In/ minbrief, flow the conductivity rate for 240 minof bare were + - recorded.PEI and AfterPEI [BF that,4] PIL these cryogels PIL cryogels treated werewith CO placed2 gas intounder an a oven 200 mL/min at 50 ◦C flow for 60rate min, for and240 min the changewere in conductivitiesrecorded. After were that, alsothese recorded. PIL cryogels The were corresponding placed into recovery an oven graphat 50 °C of for bare 60 PEImin, cryogel and the is change given in in conductivities were also recorded. The corresponding recovery graph of bare PEI 10cryogel is given 8 Figure7a. It is clear that the conductivity of bare PEI cryogels decreased to 1.1 10− from 7.9 10− in Figure1 7a. It is clear that the conductivity of bare PEI cryogels8 decreased1 to 1.1× × 10-10 from 7.9 × 10- S.cm− with a 740-fold decrease, and then increased to 6.5 10− S.cm− after placing in a 50 ◦C oven 8 -1 × -8 -1 for 60S.cm min. with These a 740-fold cycles weredecrease, carried and outthen for increased five times to 6.5 and × the10 percentS.cm after recovery placing was in a calculated 50 °C oven for for 60 min. These cycles were carried out for five times and the percent recovery was calculated for bare PEI cryogels as 82%, 64%, 40%, and 32%, respectively, for 2nd, 3rd, 4th, and 5th cycle, respectively. bare PEI cryogels as 82%, 64%, 40%, and 32%, respectively, for 2nd, 3rd, 4th, and 5th cycle, respectively. The decrease in the conductivity of bare PEI cryogels upon five cycles were decreased to approximately The decrease in the conductivity of bare PEI cryogels upon five cycles were decreased to 630-, 410-, 180-, and 50- fold from 740-fold for 2nd, 3rd, 4th, andnd 5thrd cycles,th respectively.th On the other approximately 630-, 410-, 180-, and+ 50- fold from 740-fold for 2 , 3 , 4 , and 5 cycles, respectively. hand, the recovery studies of PEI [BF4]− PIL cryogels were also done and a corresponding graph is On the other hand, the recovery studies of PEI+[BF4]- PIL cryogels were also done and a corresponding given in Figure7b. The conductivity of PEI +[BF ] PIL cryogels after 240 min of CO exposure was graph is given in Figure 7b. The conductivity of PEI4 −+[BF4]- PIL cryogels after 240 min of 2CO2 exposure decreasedwas decreased to 316, to 151, 316, 84, 151, and 84, 57-fold and 57-fold from from 760-fold 760-fold for 2nd, for 2 3rd,nd, 3rd 4th,, 4th and, and 5th 5th cycles, respectively.

1.E-05 1.E-04 (a) PEI (b) + - BeforeBefore BeforeBefore PEI [BF4] 1.E-06 1.E-05 AfterAfter AfterAfter ) o ) o -1 After 60 min treatment at 50 C 1.E-07 After 120min tretament at 50 oC -1 1.E-06 AfterAfter 120 60 min treatment at 50 oCC

1.E-08 1.E-07

1.E-09 1.E-08

1.E-10 1.E-09 Conductivity Conductivity (S.cm Conductivity Conductivity (S.cm 1.E-11 1.E-10

1.E-12 1.E-11 12345 12345 Number of cycle Number of cycle

Figure 7. The percent recovery studies of (a) bare PEI cryogel and (b) PEI++ [BF4]- cryogel under 240 Figure 7. The percent recovery studies of (a) bare PEI cryogel and (b) PEI [BF4]− cryogel under 240 min CO2 gas exposure [200 mL/min flow rate]. min CO2 gas exposure [200 mL/min ﬂow rate].

++ - TheThe calculated calculated percent percent recovery recovery for for bare bare PEI PEI[BF[BF44]− PILPIL cryogels cryogels after after 240 240 min min of of CO CO2 exposure2 exposure werewere 47%, 47%, 43%, 43%, 38%, 38%, and and 31% 31% for for 2nd, 2nd 3rd,, 3rd, 4th,4th, and and 5 5thth cycles, cycles, respectively. respectively. The The comparison comparison of ofpercent percent recovery values of bare PEI and PEI+ +[BF4]- PIL cryogels revealed that percent recovery of PEI and recovery values of bare PEI and PEI [BF4]− PIL cryogels revealed that percent recovery of PEI and PEI+ +[BF4]- PIL cryogels are almost the same after up to five consecutive uses, but, during the 2nd use, PEI [BF4]− PIL cryogels are almost the same after up to ﬁve consecutive uses, but, during the 2nd + use, bare PEI cryogels showed better recovery than PEI [BF4]− PIL cryogels. These kinds of behavior can be attributed to the existence of bigger anions in PIL for PEI that may interact with adsorbed CO2 diﬀerently than bare PEI chains. J. Compos. Sci. 2020, 4, 27 12 of 14

4. Conclusions It was demonstrated that PEI cryogels can be utilized as a template for the in-situ synthesis PANi, PPy, and PTh conductive polymers as polymer-polymer composites. It was found that the PANi containing PEI cryogels were shown higher conductivity values with 4.80 10 3 S.cm 1 than the PPy × − − and PTh containing PEI composites cryogels. The conductivities of the IL form of PEI cryogels with various anions revealed that the conductivity values strongly dependent on the types and sizes of the anions. The conductivity of bare PEI cryogels was increased and decreased with the treatment of different IL agents. The higher conductivity was observed for Cl− anions that contain PEI cryogels 4 1 + with 1.60 10− S.cm− and the lowest conductivity was observed at PEI [N(CN)2]− PIL cryogels × 9 1 with 1.68 10− S.cm− . The existence of PANi within the IL form of PEI cryogels also increased the × + + + conductivity values with 30-, 8M-, 300K-, 300-, and 20-fold for PEI Cl−, PEI [N(CN)2]−, PEI [PF6]−, + + PEI [BF4]−, and PEI [SCN]− PIL cryogels, respectively. The potential sensory use of PEI-based cryogels for CO2 at a 200 mL/min flow rate disclosed that the higher change in the conductivities were for + PEI and PEI [BF4]− cryogels with a 1000-fold conductivity decrease after 240 min of CO2 exposure. The sensitivity and percent recovery studies were also shown almost the same results with a two-fold + decrease on the conductivity of PEI and PEI [BF4]− PIL cryogels in the presence of 0.009 mole CO2 and + approximately 30% recovery after the fifth usage for both bare PEI and PEI [BF4]− PIL cryogels were + accomplished. Therefore, both bare PEI and PEI [BF4]− PIL cryogels are encouraging materials that can be used as a sensor or an adsorbent for CO2. Moreover, since all components of these composites can be sensitive to different analytes, the parameters such as the size and the types of the porous polymeric networks, the types and size of the anions, the existence of other conductive polymers, and even metal ions, organic compounds, and gas molecules, as well as the environmental factors, e.g., moisture and temperature, can be taken into consideration for the design of PEI-based viable CO2 adsorbents or sensors.

Supplementary Materials: The following are available online at http://www.mdpi.com/2504-477X/4/1/27/s1. Figure S1: The schematic presentation of in situ synthesis of conductive PANi, PPy, and PTh polymers within PEI cryogels. S: FT-IR spectra of bare PEI cryogel, PEI/PANi, PEI/PPy, and PEI/PTh cryogel composites. Figure S3: Digital camera images of PEI/PANi, PEI/PPy, and PEI/PTh cryogel composites and the schematic presentation of experimental setup used in electrical conductivity measurements. Figure S4: The FT-IR spectra between 1 + + + 1650-1850 cm− wavenumber region of (a) bare PEI cryogel (b) PEI Cl−, (c) PEI [BF4]−, and (e) PEI [SCN]− PIL cryogels before and after 240 min of CO2 exposure. [200 mL/mim flow rate]. Table S1: The conductivities of bare and conductive polymer composites embedded a PEI cryogel composite and the amounts of polymers within PEI cryogels. Table S2: The effect of the anion on the electrical conductivity of bare PEI cryogels and their PANi containing IL forms of composites. Supporting Information: Synthesis of macro porous PEI and PIL PEI cryogels, In situ preparation of conductive polymers within PEI cryogels, Instruments. Author Contributions: Conceptualization, N.S. Formal analysis, S.D. Funding acquisition, N.S. Investigation, N.S. and S.D. Methodology, N.S. and S.D. Resources, N.S. Supervision, N.S. Writing—original draft, S.D. Writing—review & editing, N.S. All authors have read and agreed to the published version of the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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