Functionalization of PET Track-Etched Membranes by UV-Induced Graft (Co)Polymerization for Detection of Heavy Metal Ions in Water

Article Functionalization of PET Track-Etched Membranes by UV-Induced Graft (co)Polymerization for Detection of Heavy Metal Ions in Water

Maxim V. Zdorovets 1,2,3,* , Ilya V. Korolkov 1,2,*, Arman B. Yeszhanov 1,2 and Yevgeniy G. Gorin 1,2

1 L.N.Gumilyov Eurasian National University, Satpaev str., 5, Nur-Sultan 010008, Kazakhstan; [email protected] (A.B.Y.); [email protected] (Y.G.G.) 2 The Institute of Nuclear Physics, Ibragimov str., 1, Almaty 050032, Kazakhstan 3 Ural Federal University, Mira str. 19, Ekaterinburg 620002, Russia * Correspondence: [email protected] (M.V.Z.); [email protected] (I.V.K.)

Received: 14 October 2019; Accepted: 11 November 2019; Published: 13 November 2019

Abstract: Nowadays, water quality monitoring is an essential task since environmental contamination and human exposure to heavy metals increased. Sensors that are able to detect ever lower concentrations of heavy metal ions with greater accuracy and speed are needed to effectively monitor water quality and prevent poisoning. This article shows studies of the modification of flexible track-etched membranes as the basis for the sensor with various polymers and their influence on the accuracy of detection of copper, cadmium, and lead ions in water. We report the UV-induced graft (co)polymerization of acrylic acid (AA) and 4-vinylpyridine (4-VPy) on poly(ethylene terephthalate) track-etched membrane (PET TeMs) and use them after platinum layer sputtering in square wave anodic stripping voltammetry (SW-ASV) for detection of Cu2+, Cd2+, and Pb2+. Optimal conditions leading to functionalization of the surface and retention of the pore structure were found. Modified membranes were characterized by SEM, FTIR, X-ray photoelectron spectroscopy (XPS) and colorimetric analysis. The dependence of the modification method on the sensitivity of the sensor was shown. Membrane modified with polyacrylic acid (PET TeMs-g-PAA), poly(4-vinylpyridine) (PET TeMs-g-P4VPy), and their copolymer (PET TeMs-g-P4VPy/PAA) with average grafting yield of 3% have been found to be sensitive to µg/L concentration of copper, lead, and cadmium ions. Limits of detection (LOD) for sensors based on PET TeMs-g-PAA are 2.22, 1.05, and 2.53 µg/L for Cu2+, Pb2+, and Cd2+, respectively. LODs for sensors based on PET TeMs-g-P4VPy are 5.23 µg/L (Cu2+), 1.78 µg/L (Pb2+), and 3.64 µg/L (Cd2+) µg/L. PET TeMs-g-P4VPy/PAA electrodes are found to be sensitive with LODs of 0.74 µg/L(Cu2+), 1.13 µg/L (Pb2+), and 2.07 µg/L(Cd2+). Thus, it was shown that the modification of membranes by copolymers with carboxylic and amino groups leads to more accurate detection of heavy metal ions, associated with the formation of more stable complexes.

Keywords: track-etched membranes; graft polymerization; stripping voltammetry; 4-vinylpyridine; acrylic acid

1. Introduction Currently, development of analytical methods for detection of heavy metal ions is urgent task since toxic ions such as As, Pb, Cd, Hg, Ni, and others are widely used in industry and released into the environment aﬀecting human health [1,2]. Accumulation of heavy metal ions in the body can cause diﬀerent diseases such as cancer, schizophrenia; kidney, lung, liver, skin diseases, etc. The World Health Organization has established the maximum permissible concentration of toxic

Polymers 2019, 11, 1876; doi:10.3390/polym11111876 www.mdpi.com/journal/polymers Polymers 2019, 11, 1876 2 of 16 metals in water, which, for instance, for Pb2+ is 0.01 mg/L[3]. Methods of absorption spectroscopy, mass spectrometry, and X-ray fluorescence spectroscopy can be used to detect metal ions in trace level [4]. However, these methods have drawbacks including bulkiness, high cost, time-consuming, etc. In this regard, interest is growing in the search for portable, inexpensive, and sensitive methods of heavy metal ion analysis. One such method is electrochemical method based on stripping voltammetry [5]. This method is currently widely used both in the field of chemical analysis of substances present in trace-level concentrations [6,7], and to study the mechanism of electrode reactions, the properties of solid electrodes, the adsorption of substances, etc.; it is inexpensive, fast, and portable. Moreover, advantages of the method are: the ability to determine a significant number (more than 9 10 40) of chemical elements and many organic compounds; low detection limits (10− –10− M); high selectivity and good metrological characteristics; ease of computerization and automation. The stripping voltammetry is usually performed with a three-electrode system containing a reference electrode, auxiliary electrode, and working electrode (WE). Electrodes based on mercury and bismuth are the most commonly used for standard tests. Mercury and bismuth are toxic and difficult to use, and new materials are currently under considerable attention [8]. For such purpose, materials based on carbon nanotubes [9], carbon thin film [10], graphene and graphene oxide [11], metal nanoparticles [11–14], and polymers and membranes [15–17] are used as WE. Moreover, the number of publications has been growing on the use of track-etched membranes (TeMs) based on polyvinylidene fluoride (PVDF) as a dual-electrode [18–23]. TeMs are thin (5–24 µm), light, and flexible polymeric films with pores with narrow pore-size distribution. It is possible to control the pore size from 30–50 nm to 3–6 microns, as well as the shape of the channel, which can be cylindrical, tapered, and others [24,25], subjected to relatively easy modification by functional polymers, which makes them an attractive object for research in voltammetric measurements as WE. To increase the selectivity and sensitivity of the analysis, modification of the TeMs surface with functional monomers is carried out. Barsbay et al. [20] grafted poly(acrylic acid) (PAA) into the nanochannels of ß-PVDF TeMs by controlled radical polymerization using reversible addition-fragmentation chain transfer (RAFT) agents. Such grafted membranes were transformed into membrane electrode by deposition of gold layer (50 nm) and found to be sensitive to sub-ppb Pb2+ concentration. Poly(4-vinyl pyridine) (P4VPy) grafted into PVDF was found to be effective in detection of mercury [19,26] with concentrations ng/L after 24 h of absorption and µg/L after 2 h of absorption. Penaeva et al. [18] reported the electron-beam-induced graft polymerization of bis[2-(methacryloyloxy)ethyl] phosphate onto PVDF membrane for the detection of uranium (VI) in ppb concentrations from 20 to 100 ppb using square wave cathodic stripping voltammetry. At the same time, there are no works in the literature on the modifications of TeMs for stripping voltammetry by copolymers with different nature. However, it is known that such copolymers form more stable complexes with heavy metal ions [27,28], which ultimately will improve the properties of the sensor [29]. Such polymers can be PAA and P4VPy. PAA [20,30] and P4VPy [19,31] were found to be effective modifier of electrodes for the detection of lead, cadmium, cobalt, and others. Their copolymers can form more stable complexes with heavy metal ions [32,33]. Thus, using PAA-P4VPy copolymers as modifying agents can improve the versatility, sensitivity as well as enable its use in various electrochemical applications at a much larger scale. The current study is focused on investigating the effect of copolymer with carboxylic and amino groups or individual polymers on sensitivity of heavy metal ion electrochemical detection. Modification of TeMs based on PET was performed by simple method of UV-induced graft polymerization of acrylic acid (AA) and 4-vinylpyridine (4VPy) and their copolymers. Based on such modified membranes, sensors were prepared by platinum sputtering. These sensors were tested for detection of copper, lead, and cadmium ions in the concentration range from 0.25 to 12.5 µg/L by the method of stripping voltammetry. Polymers 2019, 11, 1876 3 of 16

2. Materials and Methods

2.1. Materials and Instruments Acrylic acid, 4-vinylpyrridine, benzophenone, N,N-dimethylformamide, and ethanol (reagent grade) were purchased from Sigma-Aldrich. Acrylic acid was purified by distillation, 4-vinylpyrridine was purified by Al2O3 column chromatography, and benzophenone was cleaned by recrystallization. Sodium acetate, lead, cadmium, and copper standard solutions (Sigma-Aldrich, Hong Kong, China) were analytical grade and used without further purification. Deionized water purchased from Akvilon-D301 (18.2 MΩ) was used for preparing all the solutions. A PalmSens EmStat 3+ potentiostat (Houten, The Netherlands) was used for voltammetric measurements.

2.2. Preparation of the Membranes and Their Modification TeMs were prepared by irradiation of 12 µm thick PET films with krypton ions with an energy of 1.75 MeV/nucleon and ion fluence of 4.3 107 ion/cm2 using the DC-60 accelerator in Institute of Nuclear · Physics (Nur-Sultan, Kazakhstan). Membranes with average pore diameters of 400 nm were obtained by photosensitization and chemical etching in 2.2 M NaOH at 85 ◦C. Modification of poly(ethylene terephthalate) track-etched membranes (PET TeMs) was performed by photoinduced graft (co)polymerization of AA and 4-VPy. The samples were ultrasonicated for 10 min (in water) to remove any pollutions and then soaked in 5% benzophenone (BP) solution in N,N-dimethylformamide for 24 h, dried, quickly washed in ethanol and placed in a monomer mixture solution. UV-induced graft polymerization was performed using UV lamp OSRAM Ultra Vitalux E27 (UVA: 315–400 nm, 13.6 W; UVB: 280–315 nm, 3.0 W). It should be noted that UV-lamp heat the sample, without any cooling, the temperature increased to 85 ◦C. Air cooling allows to decrease the temperature to 35 ◦C. Finally, the samples were washed in water, dried at 50 ◦C, and weighed to determine the degree of grafting.

2.3. Membrane Characterization FTIR analysis was performed using Agilent Cary 600 Series FTIR Spectrometer with attenuated 1 1 total reflectance (ATR) accessory at scan range from 400 to 4000 cm− , resolution 4.0 cm− . X-ray photoelectron spectra were recorded on a Thermo Scientific K-Alpha spectrometer in the Ural Center for Shared Use “Modern Nanotechnology” Yekaterinburg, Russia. The pressure in the analysis chamber was maintained at 2 10 6 Pa or lower. Scanning electron microscope JEOL × − JSM-7500F was used for pore diameters measurements and morphology characterization. To estimate effective membrane pore sizes, the gas flow rate was used at a pressure drop of 20 kPa [34]. Colorimetric assay was done according to recommendations described in References [35,36].

2.4. Anodic Stripping Voltammetry Measurements Sensors based on modiﬁed PET TeMs were obtained according to References [19,21] by magnetron sputtering of platinum with average thickness of 40–50 nm using mask on both sides of the membrane. These surfaces were connected to potentiostat EmStat 3+ (PalmSens) through 0.4 mm diameter copper cables, which were glued to metalized membrane using silver paste (Sigma-Aldrich, Seoul, South Korea). Connections were isolated by ﬁngernail varnish and wax. Diameter of platinum surfaces is 5 mm. One side of membrane was used as working electrode, another side as counter electrode, Ag/AgCl electrode in 1 M KCl solution was used as reference electrode. Electrochemical analysis was performed by square wave anodic stripping voltammetry (SW-ASV) using standard solution of Cu2+, Pb2+, and Cd2+ in 0.1 M sodium acetate. Before the measurements, sensors were kept in analyzed solution for a certain time. Then applying 1.2 V for deposition time of 120 s and scanning from 1 V − − to 1 V at a frequency of 50 Hz and an amplitude of 20 mV, SW-ASV was performed. Polymers 2019, 11, 1876 4 of 16

Polymers3. Results 2019, 11 and, x FOR Discussion PEER REVIEW 4 of 16

3.1.Optimization UV-Induced Graft of irradiation Polymerization time, of distance 4-vinylpyridine from the on PETUV-lamp TeMs and monomer concentration for the effectiveOptimization graft polymerization of irradiation of time, 4-VPy distance on PET from TeMs the are UV-lamp presented and in monomer Figure 1. concentration Water–ethanol for mixturethe eﬀ ective(30% by graft volume) polymerization was used as of solvent. 4-VPy onThe PET choice TeMs of 30% are presented water–ethanol in Figure mixture1. Water–ethanol is due to the peculiaritiesmixture (30% of bymonomer volume) dissolution. was used as The solvent. solvent The should choice dissolve of 30% water–ethanol the monomer mixture well, at is the due same to the timepeculiarities it should of monomernot dissolve dissolution. the previously The solvent adsorbed should dissolve onto thethe monomermembrane well, photosensitizer at the same time (benzophenone)it should not dissolve in order the to previously reduce its adsorbed transition onto into the the membrane solution photosensitizerand reduce the (benzophenone)side-reaction of in homopolymerization.order to reduce its transition into the solution and reduce the side-reaction of homopolymerization.

[VP]=3% 3.0 [VP]=3% a Т - 370С b 40 Т - 370С 2.5 Solvent - H2O-C2H5OH Solvent - H2O-C2H5OH L=10 см 2.0 30 Time - 60 min

1.5 20 1.0 Degree of grafting, % grafting, Degree of Degree of grafting, % grafting, Degree of 0.5 10

0.0 0 10 15 20 25 30 35 40 45 50 55 60 65 6 8 10 12 14 16 Time, min Distance from UV-source, сm

50 180 Time - 60 min Time-1 h 0 с Т - 37 С 160 d Solvent - H2O-C2H5OH L=7 сm 40 Solvent - H2O-C2H5OH 140 L=7 сm 85 0С 120 30 100 80 20 60 Degree of grafting, % grafting, Degree of 37 0С Degree of grafting, % grafting, Degree of 40 10 20 0 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Monomer concentrtion, % Monomer concentration, % Figure 1. Grafting degree depends on irradiation time (a), distance to UV-source (b), monomer Figureconcentration 1. Grafting (c), anddegree temperature depends (ond). irradiation time (a), distance to UV-source (b), monomer concentration (c), and temperature (d). As can be seen from Figure1, there is an increase in the degree of grafting, measured gravimetrically withAs an can increase be seen in irradiation from Figure time, 1, concentration,there is an increase temperature, in the and degree a decrease of grafting, in the distance measured from gravimetricallythe UV source. with an increase in irradiation time, concentration, temperature, and a decrease in the distanceA sharp from increase the UV in sour thece. degree of grafting is observed (from 3 to 45%) with a decrease in the distanceA sharp to theincrease UV-source in the from degree 10 toof 7grafting cm. An is increase observed in temperature(from 3 to 45%) from with 37 ◦ aC decrease to 85 ◦C leadsin the to distancean increase to thein UV the-source degree from of grafting 10 to 7 fromcm. An 15% increase to 167% in (at temperature a concentration from of37 monomer°C to 85 °C 2%, leads a distance to an increaseof 7 cm in). Suchthe degree a sharp of increasegrafting can from lead 15% to to a signiﬁcant167% (at a changeconcentration in the pore of monomer structure 2%, of the a distance membranes. of 7 cm). AtSuch the a same sharp time, increase a prerequisite can lead forto a the significant further use change of such in membranes the pore structure as sensors of the is the membranes. preservation of theAt porethe same structure time, of a theprerequisite membranes; for therefore,the further the use samples of such were membranes studied by as scanning sensors electronis the preservmicroscopyation (SEM).of the Examplespore structure of SEM of images the membranes with the preservation; therefore, ofthe the samples pore structure were arestudied presented by scanningin Figure electron2. Data frommicroscopy SEM and (SEM). gas permeability Examples of are SEM summarized images with in Table the preservation1. of the pore structure are presented in Figure 2. Data from SEM and gas permeability are summarized in Table 1.

Polymers 2019, 11, 1876 5 of 16 Polymers 2019, 11, x FOR PEER REVIEW 5 of 16

(a) (b)

Figure 2. SEMSEM images images of of poly(ethylene poly(ethylene terephthalate) terephthalate) track track-etched-etched membrane membraness (PET (PET TeMs TeMs)) before before (a) (anda) and after after UV- UV-graftinggrafting of 4- ofvinylpyridine 4-vinylpyridine (4VPy (4VPy)) for 15 for min 15 ( minb), 60 (b min), 60 ( minc) (at ( cconstant) (at constant T = 37°, T L= =37 10◦, cm,L = 10monomer cm, monomer concertation concertation 3%), and 3%), after and grafting after grafting with grafting with grafting degree degree167% ( 167%d). (d). Table 1. Pore sizes for grafted PET TeMs at various conditions. As can be seen, with an increase in the degree of grafting, a uniform decrease in pore diameter occurs. According to gas permeability test, the pore diameter decreased from 400 nmPore to 355, 350, and Distance Size 230 nm, respectivelyTime of , atConcentration irradiation for 15, 30, and 60 min. Grafting Eﬀective № from Temperature, (from Porosity, Grafting, of Degree, Pore Sample UV-Lamp, C SEM % min Monomer,% ◦ % Size, nm Table 1. Pore sizes cmfor grafted PET TeMs at various conditions. Analysis), nm Distance Pore size 1Time of 0 - - - -Effective 400 5 405 25 22 № Concentration from Temperature, Grafting ± (from± SEM Porosity, 2grafting, 15 3 10 37 0.5pore 355 size,4 365 17 17 sample of monomer,% UV-lamp, °C degree,% ± analysis),± % 3min 30 3 10 37 1 350nm 6 361 15 16.5 ± ± 4 60 3cm 10 37 3 230 6 253 nm22 7 ± ± 1 50 60- 3- 7 37- 45- 400 0 ± 5 405 0 ± 25 - 22 2 615 603 310 15 37 37 0.5 0.1 357355 ± 64 376365 22± 17 17 17 ± ± 3 730 603 110 737 37 101 145350 ± 46 167361 21± 15 4 16.5 ± ± 4 860 603 1.510 737 37 113 154230 ± 66 176253 15± 22 4 7 ± ± 5 960 603 27 737 37 45 16 110 0 5 103 012 2 - ± ± 6 1060 603 315 737 37 0.1 45 357 0 ± 6 376 0 ± 22 - 17 7 1160 601 17 737 85 10 18 145 0 ± 4 167 0 ± 21 - 4 8 1260 601.5 1.57 737 85 11 32 154 0 ± 6 176 0 ± 15 - 4 9 1360 602 27 737 85 16716 110 0 ± 5 103 0 ± 12 - 2 10 60 3 7 37 45 0 0 - 11 60 1 7 85 18 0 0 - As can be seen, with an increase in the degree of grafting, a uniform decrease in pore diameter 12 60 1.5 7 85 32 0 0 - occurs.13 According60 to gas2 permeability7 test, the85 pore diameter167 decreased0 from 4000 nm to- 355, 350, and 230 nm, respectively, at irradiation for 15, 30, and 60 min. Figure 3 shows FTIR-ATR spectroscopy that was used to elucidate changes on PET TeMs surface after 4-VPy grafting. The main absorption bands were at 3435 cm−1 (О–Н), 2975 cm−1

Polymers 2019, 11, 1876 6 of 16

Figure3 shows FTIR-ATR spectroscopy that was used to elucidate changes on PET TeMs surface Polymers 2019, 11, x FOR PEER REVIEW 6 of 16 1 1 after 4-VPy grafting. The main absorption bands were at 3435 cm− (O–H), 2975 cm− (aromatic C–H), 1 1 1 (aromatic2915 cm− С(aliphatic–Н), 2915 CH), cm−1 1715(aliphatic cm− CH),(C=O), 1715 1615, cm 1470,−1 (С=О), 1430, 1615, 1409 1470, cm− 1430,(aromatic 1409 vibrationscm−1 (aromatic of the 1 1 vibrationscarbonPolymers skeleton), of 2019 the, 11carbon, 1238x FOR PEER cmskeleton),− REVIEW(stretching 1238 cm vibrations−1 (stretching of C vibrations (O)–O bonds), of C (O) and–O 980 bonds), cm− and(O–CH 6980 of 162 )[cm 37−1 ]. The main diﬀerence between the spectra of the initial and modiﬁed PET TeMs is related to P4VPy, (О–CH2) [37]. The main difference−1 between the spectra−1 of the initial and modified− 1PET TeMs is (aromatic С–Н), 2915 cm 1(aliphatic CH), 1715 cm (С=О), 1615,1 1470, 1430, 1409 cm (aromatic absorption bands at 1595 cm (C=C aryl.), 1450,−1 and 1410 cm (C–N). Moreover,−1 an increase in the related to P4VPy, absorption −bands at 1595 cm−1 (C=C aryl.), 1450− , and 1410 cm (С–N). Moreover,−1 vibrations of the carbon skeleton), 1238 cm (stretching vibrations1 of C (O)–O bonds), and 980 cm degree of grafting leads to increasing peak height at 1595 cm− . −1 an increase(О–CH in2) the[37] degree. The main of grafting difference lead betweens to increasing the spectra peak of heightthe initial at 1595and modifiedcm . PET TeMs is related to P4VPy, absorption bands at 1595 cm−1 (C=C aryl.), 1450, and 1410 cm−1 (С–N). Moreover, −1 an increase in the degree0.20 of grafting leads to increasing peak height PET at TeMs-g-P4VPy 1595 cm .(16%) PET TeMs-g-P4VPy (10%) PET TeMs-g-P4VPy (3%) 0.20 PETPristine TeMs-g-P4VPy PET TeMs (16%) PET TeMs-g-P4VPy (10%) 0.15 PET TeMs-g-P4VPy (3%)

Pyridine ring band Pristine PET TeMs C-N 0.15 Pyridine ring band C-N 0.10

0.10 Absorbance Absorbance 0.05

0.05

0.00 0.00 3000 2500 2000 1500 1000 500 3000 2500 2000 1500 1000 500 Wavenumber, cm-1 Wavenumber, cm-1 Figure 3. ATR-FTIR spectra of PET TeMs modified with 4-VPy at different grafting degrees. FigureFigure 3. ATR 3. -ATRFTIR-FTIR spectra spectra of PET of PET TeMs TeMs modified modified with with 44--VPy atat different different grafting grafting degrees. degrees. X-ray photoelectron spectroscopy (XPS) analysis was carried out for further investigation of the surfacesX-ray of Xphotoelectron- PETray photoelectron TeMs after spectroscopy graftingspectroscopy of (XPS 4-VPy (XPS) analysis) analysis with grafting was was carriedcarried degrees outout for offor 3%further further and investigation 10%.investigation Survey of the spectraof the surfacesof initialsurfaces of PET PET of TeMs TeMsPET TeMs consistafter after grafting of grafting carbon of 4of- (71.9%)VPy 4-VPy with with and grafting grafting oxygen degreesdegrees (28.1%). ofof 3 3 Graft%% and and polymerization10%. 10%. Survey Survey spectra spectra of of 4-VPy of initialat grafting initialPET TeMsPET degree TeMs consist of consist 3% of lead carbon of carbon to detection (71.9%) (71.9%) andof and7% oxygen oxygen nitrogen, (28.1(28.1%). 7.8% GraftGraft of oxygen,polymerization polymerization and 85.2% of 4of-VPy of4-VPy carbon.at at grafting degree of 3% lead to detection of 7% nitrogen, 7.8% of oxygen, and 85.2% of carbon. graftingHigh-resolution degree of N1s 3% spectra lead to of detection PET TeMs-g-P4VPy of 7% nitrogen, at different 7.8% grafting of oxygen, degrees and represents 85.2% of onecarbon. peak High-resolution N1s spectra of PET TeMs-g-P4VPy at different grafting degrees represents one peak Highat 399-resolution eV related N1s to nitrogenspectra of of PET P4VPy TeMs [38-g].-P4VPy High-resolution at different C1s grafting spectra degrees of PET represents TeMs are one consist peak of at 399 eV related to nitrogen of P4VPy [38]. High-resolution C1s spectra of PET TeMs are consist of at 399 eV related to nitrogen of P4VPy [38]. High-resolution C1s spectra of PET TeMs are consist of threethree main main peaks peaks at 285, at 285, 286.6, 286.6 and, and 289 289 eV eV related related toto C–CC–C/C/C–H,–H, C C–O–C,–O–C, and and C=O C=, respectively,O, respectively, also also threeπ–π* mainπ shake–π* peaks shake up was upat detected285, was 286.6 detected at, and 292 at eV.289 292 GrafteV eV. related Graft polymerization polymerization to C–C/C– ofH, P4VPy ofC – P4VPyO–C lead, and lead to C=O disappearanceto disappearance, respectively, of of peaksalso πat–π* 286.6 shakepeaks and at up 289 286.6 was eV, and probablydetected 289 eV, atassociatedprobably 292 eV. associated withGraft full polymerization with covering full covering of very of of P4VPy top very surface top lead surface with to disappearance graftedwith grafted polymer. of peaksThis ispolymer.at also 286.6consistent Thisand is289 also witheV, consistent probably a decrease with associated in a intensitiesdecrease with in of intensities C–Ofull covering and of C C=–O Oof peaksand very C=Oin top O1speaks surface spectra in O1s with presented spectra grafted in polymer.Figurepresented4d. This is in also Figure consistent 4d. with a decrease in intensities of C–O and C=O peaks in O1s spectra presented in Figure 4d.

PET TeMs -g-P4VPy (3%) PET TeMs-g-P4VPy (10%) N1s 50000 1.00 PET TeMs-g-P4VPy (10%) PET TeMs-g-P4VPy (45%) PET TeMs PET TeMs -g-P4VPy (3%) PET TeMs-g-P4VPy (10%) N1s 50000 1.00 40000 O 1s PET TeMs-g-P4VPy (10%) PET TeMs-g-P4VPy (45%) PETC 1s TeMs 0.75

40000 30000 O 1s

C 1s 0.75 0.50 20000

30000 Intensity [a.u.] Intensity [a.u.] O 2s 0.50 N1s 0.25 10000 20000 Intensity [a.u.] Intensity [a.u.] O 2s

0 N1s 0.250.00 10000 600 500 400 300 200 100 0 404 402 400 398 396 Binding Energy, eV Binding Energy,eV 0 0.00 600 500 400 300 200 100 0 404 402 400 398 396 Binding Energy,eV Binding Energy,(a) eV (b)

Figure 4. Cont. (a) (b)

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1.25 PET TeMs-g-P4VPy (3%) C1s 1.25 PET TeMs-g-P4VPy (3%) O1s PET TeMs-g-P4VPy (10%) PET TeMs-g-P4VPy (10%) 1.25 C-C/C-H 1.00 PET PET TeMs TeMs-g-P4VPy (3%) C1s PETPET TeMsTeMs-g-P4VPy (3%) O1s 1.25 C-O C=O PET TeMs-g-P4VPy (10%) 1.00 PET TeMs-g-P4VPy (10%) C-C/C-H 1.00 PET TeMs PET TeMs C-O C=O 0.75 1.00 0.75 0.75 0.75 0.50 C-O-C/COH 0.50 Intensity [a.u.] Intensity [a.u.] 0.50 C-O-C/COH 0.50 Intensity [a.u.] C=O Intensity [a.u.] 0.25 0.25 ∗ C=O π−π shake up 0.25 0.25 π−π∗ 0.00 shake up 0.00 295 290 285 280 538 536 534 532 530 528 526 0.00 0.00 295 Binding290 Energy, eV 285 280 538 536 534Binding532 Energy,eV530 528 526 Binding Energy, eV Binding Energy,eV

(c) (d) (c) (d) Figure 4. XPS survey (a), high resolution N1s (b), C1s (c), and O1s (d) spectra of PET TeMs before FigureFigure 4. 4.XPS XPS survey survey (a(a),), high high resolutionresolution N1sN1s ((bb),), C1sC1s ((cc),), andand O1sO1s ((dd)) spectraspectra ofof PETPET TeMsTeMs beforebefore and and after 4-VPy grafting at degrees of grafting of 3% and 10%. afterand 4-VPyafter 4- graftingVPy grafting at degrees at degrees of grafting of grafting of 3% of 3 and% and 10%. 10%. 3.2. UV-Induced Graft Copolymerization of 4-Vinylpyridine and Acrylic Acid on PET TeMs 3.2.3.2. UV-Induced UV-Induced GraftGraft CopolymerizationCopolymerization of of 4 4-Vinylpyridine-Vinylpyridine and and Acrylic Acrylic Acid Acid on onPET PET TeMs TeMs In order to study the effect of polymers of various nature on the detection ability, graft InIn orderorder to to study study the the effect eﬀect of ofpolymers polymers of various of various nature natureon the ondetection the detection ability, graft ability, copolymerizationgraftcopolymerization copolymerization of of4-VPy 4-VPyof with 4-VPy with AA AA with was was AAstudied. studied. was studied.AA AA cont contains AAains negatively containsnegatively negatively charged carboxylcarboxyl charged groups groups carboxyl (ingroups contrast(in contrast (in with contrast with positively positively with charged positively charged P4VPy), P4VPy), charged which which P4VPy), can can also also which formform can complexescomplexes also form withwith complexes heavyheavy metal metal with ions. ions. heavy GraftingmetalGrafting ions.polymerization polymerization Grafting polymerizationwas was carried carried out out wasin in a awater carried water––ethanolethanol out in medium amedium water–ethanol by varying medium thethe concentration concentration by varying of of the monomersconcentrationmonomers and and ofthe monomersthe rat ratio ioof ofmonomers, and monomers, the ratio while ofwhile monomers, thethe irradiationirradiation while the timetime irradiation and temperaturetemperature time and remained temperatureremained constantremainedconstant - 1 h- constant—1 1and h and 37 37° C, ° h C,respectively. and respectively. 37 ◦C, respectively. The The results results are Theare presented presented results are inin presented FigureFigure 5. in Figure5.

4040 [Monomer]=3%[Monomer]=3% Т -Т 37- 370С0С Solvent - H O-C H OH 3030Solvent - H2O-C2 2H2 5OH5 L=10L=10 сm сm 2020

1010 Degree of grafting, % grafting, Degree of Degree of grafting, % grafting, Degree of 0 0 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Monomer ratio of 4-VPy, % Monomer ratio of 4-VPy, %

Figure 5. Figure 5. The dependence of of the the ratio ratio of of the the monomers monomers on on the the degree degree of grafting. of grafting. Figure 5. The dependence of the ratio of the monomers on the degree of grafting. It can be seen from the graph that the largest mass gain is observed at the ratio of the monomers of It can be seen from the graph that the largest mass gain is observed at the ratio of the monomers 4-VPy:AAofIt 4 can-VPy:AA be= seen90:10, = 90:10, from probably probablythe graph due due that to to the the the greater largest greater tendency mass tendency gain of ofis 4-VPy observed4-VPy to to polymerization polymerizationat the ratio of the comparedcompared monomers toto AA (r (4-VPy) = 5, r (AA) = 0.5 [39]. The minimum value of grafting degree is achieved when the ratio of of 4AA-1VPy:AA (r1 (4- VPy)= 90:10, 2= 5, probably r2 (AA) = due0.5 [39]to the. The greater minimum tendency value of of 4 grafting-VPy to degreepolymerization is achieved compared when the to AAmonomers ratio(r1 (4 of-VPy) monomers is 50:50.= 5, r2 According(AA)is 50:50. = 0.5 According to[39] Reference. The to minimum Reference [39], this value[39] is due, this of to graftingis the due formation to degree the formation ofis associates,achieved of associates, when which the lead ratiotowhich aof decrease monomers lead to in a thedecrease is reactivity50:50. in According the of reactivity monomers to Referenceof monomers (Figure 6[39]). (Figure, this is 6) due. to the formation of associates, which lead to a decrease in the reactivity of monomers (Figure 6).

Polymers 2019, 11, 1876 8 of 16 Polymers 2019,, 11,, x FOR PEER REVIEW 8 of 16

O H C C C 2 H N OH

Figure 6. Interaction of acrylic acid and 4-vinylpyridine monomers. Figure 6. InteractionInteraction ofof acrylicacrylic acidacid andand 44--vinylpyridine monomers. The composition of the grafted copolymer was studied by colorimetry using dyes toluidine blue The composition of the grafted copolymer was studied by colorimetry using dyes toluidine blue (TB) and orange acid (AO) [35,36,40]. TB has speciﬁcity to carboxyl groups of PAA grafted chains and (TB)(TB) andand orangeorange acidacid (AO)(AO) [35,36,40][35,36,40].. TBTB hashas specificityspecificity toto carboxylcarboxyl groupsgroups ofof PAAPAA graftgrafted chains AO has speciﬁcity to P4VPy. Schematically, the process is presented in Figure7. The results of the and AO has specificity to P4VPy. Schematically, the process is presented in Figure 7.. TheThe resultsresults ofof analysis are presented in Table2. thethe analysisanalysis areare presentedpresented inin TableTable 2.2.

Figure 7. Schematic representation of the colorimetric analysis of the grafted copolymer on PET TeMs. Figure 7. Schematic representation of the colorimetric analysis of the grafted copolymer on PET TeMs.Table 2. The composition of the grafted copolymer PET-TeMs-g-PAA/poly(4-vinylpyridine) (P4VPy) depending on the AA/4VPy ratio. Table 2. The composition of the grafted copolymer PET--TeMs--g--PAA/poly(4--vinylpyridine) ((P4VPy)) Composition of Grafted Monomerdepending Mixture on the Composition AA/4VPy ratio.. Dye Concentration, µM/g Copolymer Monomer mixture composition Dye concentration, µМ/g Composition of grafted copolymer [AA] [4-VPy] TB AO m1 m2 [AA][AA] [4[4--VPy] ТB AO m11 m22 90 10 11.9 11.1 51.7 48.3 90 7010 30 11.9 21.4 11.1 89.5 51.7 19.3 48.3 80.7 70 5030 50 21.4 26.6 89.5 72.1 19.3 27.0 80.7 73.0 50 3050 70 26.6 25.7 72.1 81.9 27.0 23.9 73.0 76.1 30 1070 90 25.7 34.0 81.9 44.4 23.9 43.4 76.1 56.6 10 90 34.0 44.4 43.4 56.6 As can be seen from the Table2, the ratio of the polymer units in the chain 50:50 is achieved with the compositionAs can be of theseen monomer from the mixture Table 2, AA:4-VPy the ratio= of90:10, the thispolymer is due units to the in high the polymerizationchain 50:50 is achieved tendency withof 4-VPy. the However,composition with of anthe increase monomer in the mixture concentration AA:4--VPy of 4-VPy, = 90:10, a decrease this is in due its concentrationto the high polymerizationin the grafted polymer tendency is observed,of 4--VPy. probablyHowever, due with to an the increase formation in ofthe a concentration homopolymer, of which 4--VPy, was a decreaseclearly recorded in its concentration during the experiment. in the grafted The polymer observed is eobserved,ﬀects are probably in good agreement due to the with formation previously of a homopolymer,published works which [41]. was clearly recorded during the experiment. The observed effects are in good agreementSEM images with previously of the surface published of modiﬁed works PET [41][41] TeMs.. at various monomer ratios are shown in Figure8. SEM images of the surface of modified PET TeMs at various monomer ratios are shown in Figure 8..

Polymers 2019, 11, 1876 9 of 16 Polymers 2019, 11, x FOR PEER REVIEW 9 of 16

АA/4-VPy=90:10

АA/4-VPy =70:30 АA/4-VPy =50:50

АA/4-VPy=30:70 АA/4-VPy=10:90

Figure 8.8. SEMSEM images images of of PET PET TeMs TeMs modiﬁed modified with with graft graft copolymerization copolymerization of AA andof AA 4VPy and at 4VPy diﬀerent at differentmonomer monomer ratios. ratios.

SEM images clearlyclearly showshow aa changechange inin surface surface morphology. morphology With. With increasing increasing of of grafting grafting degree degree at atmonomer monomer ratio ratio AA AA/4/4-VPy-VPy= 30:70 = 30:70 and and 10:90, 10:90, surface surface topography topography inhomogeneity inhomogeneity with with inclusions inclusions can canbe observed. be observed. Moreover, Moreover, changes changes in porein pore diameters diameters occurred, occurred, and and thickness thickness of of grafted grafted copolymer copolymer is isdi ﬀdifferenterent between between pore pore diameter diameter of pristine of pristin PETe TeMsPET andTeMs modiﬁed and modified one. However, one. However, the pore diameterthe pore diametervaries slightly. varies For slightly. a more accurateFor a more study accurate of the change study in of pore the diameter, change thein gaspore permeability diameter, the method gas waspermeability used. The method results was are presentedused. The inresults Figure are9. presented in Figure 9.

Polymers 2019, 11, x FOR PEER REVIEW 10 of 16 PolymersPolymers 20192019,, 1111,, x 1876 FOR PEER REVIEW 1010 of of 16 16

[Monomer]=3% [Monomer]=3%0 Т - 37 0С 400 SolventТ - 37 С - H O-C H OH 400 Solvent - H2 O-C2 H5 OH L=10 сm 2 2 5 380 L=10 сm 380 360 360 340 340 Diameter of pores, nm

Diameter of pores, nm 320 320 VP 0 10 20 30 40 50 60 70 80 90 100 VP 0 10 20 30 40 50 60 70 80 90 100 AA 100 90 80 70 60 50 40 30 20 10 0 AA 100 90 80 70 60 50 40 30 20 10 0 Monomer ratio, % Monomer ratio, % FigureFigure 9. 9.PorePore diameters diameters of of PET PET TeMs TeMs at at different diﬀerent monomer monomer ratios ratios.. Figure 9. Pore diameters of PET TeMs at different monomer ratios. TheThe gas gas permeability permeability data data obtained obtained are are in in good good agreement agreement with with SEM SEM images. images. There There is is greater greater The gas permeability data obtained are in good agreement with SEM images. There is greater porepore overgrowth overgrowth with with an an increase increase in in the the proportion proportion of of 4 4-VPy-VPy in in the the monomer monomer mixture. mixture. pore overgrowth with an increase in the proportion of 4-VPy in the monomer mixture. ToTo investigate investigate chemical chemical compos compositionition of of the the surface surface after after graft graft polymerization, polymerization, FTIR FTIR and and XPS XPS To investigate chemical composition of the surface after graft polymerization, FTIR and XPS analysisanalysis were were performed. performed. analysis were performed. FTIRFTIR-ATR-ATR spectra spectra are are presented presented in in Figure Figure 10 10. .In In addition addition to to the the main main peaks peaks related related to to the the PET PET FTIR-ATR spectra are presented in Figure 10. In addition to the1 main peaks related to the PET membrane,membrane, shifting shifting of CC=C=C arylaryl ofof P4VPy P4VPy ring ring from from 1595 1595 to 1575to 1575 cm −cmtogether−1 togethe withr with recording recording of peaks of membrane, shifting1 of C=C aryl of P4VPy ring1 from 1595 to 1575 cm−1 together with recording of peaksat 1640 at cm1640− (COO cm−1 −(COO) and at−) 3500–3100and at 3500 cm–−3100(OH) cm was−1 (OH) observed was indicatingobserved formationindicating complexingformation peaks at 1640 cm−1 (COO−) and at 3500–3100 cm−1 (OH) was observed indicating formation complexingbetween PAA between and P4VPy. PAA and P4VPy. complexing between PAA and P4VPy.

1.2 PET TeMs-g-P4VPy/AA -10:90 1.2 PET PET TeMs-g-P4VPy/AA TeMs-g-P4VPy/AA -30:70 -10:90 Pristine PET TeMs-g-P4VPy/AA PET TeMs -30:70 1.0 Pristine PET TeMs 1.0 1575

0.8 1595 1575

0.8 1595 1640

0.6 1640 0.6 OH OH 0.4 Absorbance 0.4 Absorbance

0.2 0.2

0.0 0.0

3500 3000 2500 2000 1500 1000 500 3500 3000 2500 2000 1500 1000 500 Binding Energy, eV Binding Energy, eV Figure 10. ATR-FTIR spectra of PET TeMs modified with 4-VPy/AA at different monomer ratios. Figure 10. ATR-FTIR spectra of PET TeMs modified with 4-VPy/AA at different monomer ratios. SurveyFigure 10. XPS ATR spectra-FTIR spectra (Figure of PET 11a) TeMs of modified PET TeMs-g-P4VPy with 4-VPy/AA/AA at =different30:70 consistmonomer of rati Cos. (84.2%), O (9.3%),Survey and XPS N spectr (6.5%)a; (Figure PET TeMs-g-P4VPy 11a) of PET TeMs/AA-g=-P4VPy/AA=30:7010:90 consist of consist C (79.3%), of C (84.2%), O (16.4%), O (9.3%) and N, Survey XPS spectra (Figure 11a) of PET TeMs-g-P4VPy/AA=30:70 consist of C (84.2%), O (9.3%), and(4.3%). N (6.5%); High-resolution PET TeMs C1s-g-P4VPy/AA spectra show = 10 us:90 that consist with increasing of C (79.3%), of AA O content, (16.4%) an, and increase N (4.3%). in the and N (6.5%); PET TeMs-g-P4VPy/AA = 10:90 consist of C (79.3%), O (16.4%), and N (4.3%). Highpeak-resolution intensity atC1s289 spectra eV corresponding show us that to with C=O increasing bond has occurred.of AA content, At a monomer an increase mixture in the ratio peak of High-resolution C1s spectra show us that with increasing of AA content, an increase in the peak intensity4-VPy:AA at =28910:90, eV whichcorresponding leads to theto C=O formation bond ofhas copolymers occurred. onAt PETa monomer TeMs with mixture a ratio ratio of 50:50, of intensity at 289 eV corresponding to C=O bond has occurred. At a monomer mixture ratio of 4-twoVPy:AA peaks =appear 10:90, which at high-resolution leads to the N1s formation XPS spectra of copolymers at 399 eV and on 401.8PET TeMs eV. The wi presenceth a ratio of of a second50:50, 4-VPy:AA = 10:90, which leads to the formation of copolymers on PET TeMs with a ratio of 50:50, twopeak peaks at 401.8 appear eV also at confirmshigh-resolution the assumption N1s XPS that spectra a complex at 399 is eV formed and 401.8 between eV.PAA The andpresence P4VPy of [38 a ]. two peaks appear at high-resolution N1s XPS spectra at 399 eV and 401.8 eV. The presence of a

Polymers 2019, 11, x FOR PEER REVIEW 11 of 16

second peak at 401.8 eV also confirms the assumption that a complex is formed between PAA and Polymers 2019, 11, 1876 11 of 16 P4VPy [38].

25000 PET TeMs-g-P4VPy/PAA-10/90 PET TeMs-g-P4VPy/PAA -30/70 399.0 eV N1s 22500 PET TeMs-g-P4VPy/PAA-30/70 1.00 PET TeMs-g-P4VPy/PAA -10/90

20000 С 1s

17500 0.75 15000 O 1s 401.8 eV 12500 0.50 10000 Intensity [a.u] Intensity [a.u.] 7500 N 1s

O 2s 0.25 5000

2500

0 0.00 600 500 400 300 200 100 0 404 402 400 398 396 Bindning Energy,eV Binding Energy,eV

(a) (b)

PET TeMs-g-P4VPy/PAA -30/70 PET TeMs-g-P4VPy/PAA -10/90 C-C/C-H C1s 1.25 O1s 1.00 PET TeMs-g-P4VPy/PAA -10/90 PET TeMs-g-P4VPy/PAA -30/70 PET TeMs PET TeMs C-O C=O 1.00 0.75

0.75 C-O-C/COH 0.50 0.50 Intensity [a.u.] Intensity [a.u.] C=O 0.25 0.25 π−π∗ shake up

0.00 0.00 295 290 285 280 538 536 534 532 530 528 526 Binding Energy [eV] Binding Energy,eV

Figure 11.11. XPSXPS surveysurvey (a(a),), high-resolution high-resolution N1s N1s (b (),b), C1s C1s (c ),(c and), and O1s O1s (d) ( spectrad) spectra of PET of PET TeMs TeMs before before and afterand after graft graft copolymerization copolymerization of 4-VPy of 4-VPy and AAand onAA PET on TeMsPET TeMs at di ﬀaterent different monomer monomer ratios. ratios.

3.3.3.3 Electrochemical Electrochemical D Detectionetection For the electrochemical detection, PET TeMs modifiedmodified with AA according to our previously published paper [[36]36] PET TeMs modifiedmodified withwith 4-VPy4-VPy andand 4-VPy4-VPy/AA/AA inin monomermonomer ratio 10:90 (this monomer ration leadlead to formation 50:50 grafted PAA/P4VPy PAA/P4VPy onon thethe membrane,membrane, which in turn lead to the formationformation of a moremore stablestable complexcomplex withwith heavyheavy metalmetal ionsions [[32]32]).). In all casescases thethe graftinggrafting degreedegree was aroundaround 3%, 3%, in in which which the the chemical chemical modification modification of the of surface the surface was achieved was achieved by complexing by complexing groups togroups heavy to metal heavy ions metal (HMI) ions together (HMI) withtogether the preservationwith the preservation of the pore of structure the pore of structure the membranes. of the Thesemembranes. modified Thesemembranes modified were membranes converted were to sensorsconverted by magnetronto sensors sputteringby magnetron of platinum sputtering from of bothplatinum sides from of the both membranes sides of the with membranes a thickness with of 40–50a thickness nm. Oneof 40 side–50 nm. of the One membrane side of the was membrane used as workingwas used electrode, as working another electrode, side another as counter side electrode, as counter thus electrode, distance thus between distance these between two electrodes these two is µ equalelectrodes to membrane is equal thicknessto membrane (12 m) thickness since graft (12 polymerization µm) since graft at 3%polymerization of grafting degree at 3% does of grafting not lead todegree a significant does not change lead to in a membranesignificant change thickness. in membrane thickness. 2+ 2+ 2+ SW-ASVSW-ASV waswas performedperformed usingusing standardstandard solution solution of of Cu Cu2+,, PbPb2+, and Cd2+ inin electrolyte electrolyte of of 0.1 M µ sodium acetate. HMIs were detected simultaneously in the same concentrations from 0.025 µg/Lg/L to µ 12 µg/L.g/L. ForFor the the optimization optimization of adsorptionof adsorption time, time, sensors sensors based onbased modified on modified PET TeMs PET were TeMs immersed were in 12.5 µg/L solution for certain time from 10 min to 120 min. Then, in present of reference electrode (Ag /AgCl, 1M KCl) at applied potential of 1.2 V during 120 s, deposition of HMI and their reduction − Polymers 2019, 11, x FOR PEER REVIEW 12 of 16 immersed in 12.5 µg/L solution for certain time from 10 min to 120 min. Then, in present of reference electrode (Ag/AgCl, 1M KCl) at applied potential of −1.2 V during 120 s, deposition of HMI and their Polymersreduction2019 was, 11, 1876 performed. After electrodeposition, a potential scan from −1 to 1 V was perform12ed of 16in order to oxidize of HMI at redox potential of Cd2+ around −0.84 V, Pb2+ at −0.52 V, and Cu2+ at 0.13 V, respectively. Results are presented in Figure 12. Optimal adsorption time for all HMI is 30 min. It was performed. After electrodeposition, a potential scan from 1 to 1 V was performed in order should be noted that nonmodified membrane does not show any− signal for HMI in this detection to oxidize of HMI at redox potential of Cd2+ around 0.84 V, Pb2+ at 0.52 V, and Cu2+ at 0.13 V, range. Then sensors were tested in different concentration,− calibration curves− are presented in Figure respectively. Results are presented in Figure 12. Optimal adsorption time for all HMI is 30 min. 13. The sensors modified with PAA and P4VPy/PAA were found to be more sensitive for copper and It should be noted that nonmodified membrane does not show any signal for HMI in this detection lead ions over cadmium ions, while sensors modified with P4VPy show less selectivity. Limits of range. Then sensors were tested in different concentration, calibration curves are presented in Figure 13. detection (LOD) for sensors based on PET TeMs-g-PAA are 2.22, 1.05, and 2.53 µg/L for Cu2+, Pb2+, The sensors modified with PAA and P4VPy/PAA were found to be more sensitive for copper and lead and Cd2+, respectively. LODs for sensors based on PET TeMs-g-P4VPy are 5.23 µg/L (Cu2+), 1.78 µg/L ions over cadmium ions, while sensors modified with P4VPy show less selectivity. Limits of detection (Pb2+), and 3.64 µg/L (Cd2+) µg/L. PET TeMs-g-P4VPy/PAA electrodes are found to be sensitive with (LOD) for sensors based on PET TeMs-g-PAA are 2.22, 1.05, and 2.53 µg/L for Cu2+, Pb2+, and Cd2+, a LODs of 0.74 µg/L(Cu2+), 1.13 µg/L (Pb2+), and 2.07 µg/L(Cd2+). In most cases, good correlation respectively. LODs for sensors based on PET TeMs-g-P4VPy are 5.23 µg/L (Cu2+), 1.78 µg/L (Pb2+), between concentration and current was found and R2 is achieved to 0.992. Thus, these sensors can be and 3.64 µg/L (Cd2+) µg/L. PET TeMs-g-P4VPy/PAA electrodes are found to be sensitive with a LODs used for SW-ASV detection of copper, lead and cadmium in the µg/L range. of 0.74 µg/L(Cu2+), 1.13 µg/L (Pb2+), and 2.07 µg/L(Cd2+). In most cases, good correlation between Furthermore, it is seen that LODs of cadmium ion is higher than LODs of copper and lead ions, concentration and current was found and R2 is achieved to 0.992. Thus, these sensors can be used for this fact is in a good correlation with complexing ability of this ion with polymers: cadmium reacts SW-ASV detection of copper, lead and cadmium in the µg/L range. weaker with polymers than lead and copper [42,43].

220 PET TeMs-g-PAA 200 Cu2+ PET TeMs-g-PAA 200 Pb2+ Cd2+ 180 Pb2+ Cu2+ 150

160 A µ µΑ 140 100 120 Current, 100 Peak hight, Peak hight,

80 Cd2+ 50

40 0 -1.0 -0.5 0.0 0.5 1.0 0 20 40 60 80 100 120 Potential, V Immersion time, min

(a) (b)

100 Cu2+ PET TeMs-g-P4VPy Cu2+ PET TeMs-g-P4VPy/PAA Pb2+ Pb2+ Cd2+ 150 Cd2+ A A µ µ 100 50 Peak hight, Peak hight, Peak height, Peak height, 50

0 0 0 20 40 60 80 100 120 0 20 40 60 80 100 120 Immersion time, min Immersion time, min

FigureFigure 12.12. SW-ASVSW-ASV analysisanalysis curvecurve afterafter absorptionabsorption timetime ofof 3030 minmin ((aa)) andand kineticskinetics ofof adsorptionadsorption asas functionfunction ofof HMIHMI peakpeak heightheight (12.5(12.5µ µg/Lg/Lconcentration), concentration),deposition deposition at at −1.21.2 VV forfor 120120 ss ((bb––dd).). −

Polymers 2019, 11, 1876 13 of 16 Polymers 2019, 11, x FOR PEER REVIEW 13 of 16

PET TeMs-g-PAA PET TeMs-g-PAA PET TeMs-g-P4VPy PET TeMs-g-P4VPy 160 80 PET TeMs-g-P4VPy/AA PET TeMs-g-P4VPy/AA

140 y=32.018+3.356x; R2=0.974 60 120 y=38.377+9.482x; R2=0.979 µΑ µΑ y=15.471+2.307x; R2=0.992 100 40

y=28.195+9.151x; R2=0.989 Current, Current, 80 Current, 20 60 y=1.088+2.575x; R2=0.980 40 y=35.279+1.871x; R2=0.972 0 20 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 Concentration of Cu2+, µg/L Concentration of Cd2+, µg/L

(a) (b) 200 PET TeMs-g-PAA 180 PET TeMs-g-P4VPy PET TeMs-g-P4VPy/AA 160

140 y=29.023+10.686x; R2=0.991 120 µΑ 100

y=19.445+9.517x; R2=0.992

Current, Current, 80

40 y=22.591+3.669x; R2=0.989 20

0 0 2 4 6 8 10 12 14 Concentration of Pb2+, µg/L

(c)

FigureFigure 13.13. Calibration curves curves of of peak peak currents currents for for Cu Cu2+2 +(a(),a Cd), Cd2+ (2b+),( band), and Pb2+ Pb (c2)+ concentration(c) concentration for the for the SW-ASVSW-ASV afterafter 30 min of of adsorption adsorption in in appropriate appropriate HMI HMI solution solution in in0.1 0.1 M Msodium sodium acetate acetate electrolyte. electrolyte.

4. ConclusionsFurthermore, it is seen that LODs of cadmium ion is higher than LODs of copper and lead ions, this fact is in a good correlation with complexing ability of this ion with polymers: cadmium reacts This work demonstrates the methods of modification of track-etched membranes based on weaker with polymers than lead and copper [42,43]. poly(ethylene terephthalate) by methods of UV-induced graft polymerization of 4-vinylpyridine, acrylic acid and copolymerization of 4-vinylpyridine and acrylic acid for application as detectors for 4. Conclusions electrochemical detection of heavy metal ions using square-wave anodic stripping voltammetry. OptimalThis conditions work demonstrates of modification the methodswere found of leading modification to creation of track-etched of anchors for membranes heavy metal based ions on poly(ethylenecomplexation terephthalate)and prevention by of methods pore structure of UV-induced of the membranes. graft polymerization Based on ofsuch 4-vinylpyridine, modified acrylicmembranes, acid anddetectors copolymerization were prepared of by 4-vinylpyridine platinum sputtering and through acrylic acidthe mask for application on both sides as of detectors the formembranes. electrochemical These detectionsensors can of be heavy used metal for detection ions using of square-wavecopper, lead, anodicand cadmium stripping ions voltammetry. in the Optimalconcentration conditions range offrom modification 0.25 to 12.5 were µg/L. found Membranes leading tomodified creation with of anchors acrylic foracid heavy and acrylic metal ions complexationacid/4-vinylpyridine and prevention showed more of pore selectivity structure to of Pb the2+ membranes.and Cu2+ rather Based than on to such Cd2+ modified. Modification membranes, by detectorscopolymerization were prepared with different by platinum polymer sputtering nature (polyacrylic throughthe acid mask and poly on both-4-vinylpyridine) sides of the membranes.leads to Thesemore accurate sensors cansimultaneous be used for heavy detection metal of ions copper, detection. lead, and cadmium ions in the concentration range fromAuthor 0.25 Co tontributions: 12.5 µg/L. C Membranesonceptualization, modified M.V.Z. with and acrylic I.V.K.; acidmethodology, and acrylic I.V. acidK.; validation,/4-vinylpyridine Y.G.G. showedand moreA.B.Y. selectivity; investigation, to Pb A.B2+ .andY. and Cu 2Y+.G.ratherG.; data than curation, to Cd2 +Y..G Modification.G.; writing—original by copolymerization draft preparation, with I.V. diK.fferent; polymerwriting— naturereview and (polyacrylic editing, M.V. acidZ.; and visualization, poly-4-vinylpyridine) I.V.K.; supervision, leads M.V. to moreZ.; project accurate administration, simultaneous M.V.Z. heavy; funding acquisition, M.V.Z. metal ions detection.

Polymers 2019, 11, 1876 14 of 16

Author Contributions: Conceptualization, M.V.Z. and I.V.K.; methodology, I.V.K.; validation, Y.G.G. and A.B.Y.; investigation, A.B.Y. and Y.G.G.; data curation, Y.G.G.; writing—original draft preparation, I.V.K.; writing—review and editing, M.V.Z.; visualization, I.V.K.; supervision, M.V.Z.; project administration, M.V.Z.; funding acquisition, M.V.Z. Funding: The research was funded by the Ministry of Energy of the Republic of Kazakhstan (technological program, #74 on 02.04.2018). Acknowledgments: The research was funded by the Ministry of Energy of the Republic of Kazakhstan (technological program, #74 on 02.04.2018). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Kumar, P.; Kim, K.H.; Bansal, V.; Lazarides, T.; Kumar, N. Progress in the sensing techniques for heavy metal ions using nanomaterials. J. Ind. Eng. Chem. 2017, 54, 30–43. [CrossRef] 2. Cui, L.; Wu, J.; Ju, H. Electrochemical sensing of heavy metal ions with inorganic, organic and bio-materials. Biosens. Bioelectron. 2015, 63, 276–286. [CrossRef] 3. Guidelines for Drinking-Water Quality, 4th ed.; World Health Organization: Geneva, Switzerland, 2011; pp. 383–385. ISBN 9789241548151. 4. Pierce, D.T.; Zhao, J.X. Trace Analysis with Nanomaterials; Wiley-VCH: Weinheim, Germany, 2015; p. 418. ISBN 978352763. 5. Dai, X.; Wu, S.; Li, S. Progress on electrochemical sensors for the determination of heavy metal ions from contaminated water. J. Chin. Adv. Mater. Soc. 2018, 6, 91–111. [CrossRef] 6. Lu, Y.; Liang, X.; Niyungeko, C.; Zhou, J.; Xu, J.; Tian, G. A review of the identification and detection of heavy metal ions in the environment by voltammetry. Talanta 2018, 178, 324–338. [CrossRef][PubMed] 7. Abdulla, M.; Ali, A.; Jamal, R.; Bakri, T.; Wu, W.; Abdiryim, T. Electrochemical sensor of double-thiol linked PProDOT@Si composite for simultaneous detection of Cd(II), Pb(II), and Hg(II). Polymers 2019, 11, 815. [CrossRef] 8. Brett, C.M.A.; Brett, A.M.O. Electrochemistry: Principles, Methods, and Applications; Oxford University Press: Oxford, UK, 1993; p. 464. 9. Huang, H.; Chen, T.; Liu, X.; Ma, H. Ultrasensitive and simultaneous detection of heavy metal ions based on three-dimensional graphene-carbon nanotubes hybrid electrode materials. Anal. Chim. Acta 2014, 852, 45–54. [CrossRef] 10. Gao, C.; Yu, X.-Y.; Xu, R.-X.; Liu, J.-H.; Huang, X.-J. AlOOH-Reduced Graphene Oxide Nanocomposites: One-Pot Hydrothermal Synthesis and Their Enhanced Electrochemical Activity for Heavy Metal Ions. ACS Appl. Mater. Interfaces 2012, 4, 4672–4682. [CrossRef] 11. Liu, X.; Yao, Y.; Ying, Y.; Ping, J. Recent advances in nanomaterial-enabled screen-printed electrochemical sensors for heavy metal detection. TrAC Trends Anal. Chem. 2019, 115, 187–202. [CrossRef] 12. Zhao, W.; Ge, P.-Y.; Xu, J.-J.; Chen, H.-Y. Catalytic Deposition of Pb on Regenerated Gold Nanofilm Surface and Its Application in Selective Determination of Pb 2+. Langmuir 2007, 23, 8597–8601. [CrossRef] 13. Ouyang, R.; Bragg, S.A.; Chambers, J.Q.; Xue, Z.-L. Flower-like self-assembly of gold nanoparticles for highly sensitive electrochemical detection of chromium(VI). Anal. Chim. Acta 2012, 722, 1–7. [CrossRef] 14. Wang, N.; Kanhere, E.; Miao, J.; Triantafyllou, M.S. Nanoparticles-modified chemical sensor fabricated on a flexible polymer substrate for cadmium(II) detection. Polymers 2018, 10, 694. [CrossRef] 15. Gouveia-Caridade, C.; Brett, C.M.A. Strategies, Development and Applications of Polymer-Modified Electrodes for Stripping Analysis. Curr. Anal. Chem. 2008, 4, 206–214. [CrossRef] 16. Kim, Y.; Amemiya, S. Stripping Analysis of Nanomolar Perchlorate in Drinking Water with a Voltammetric Ion-Selective Electrode Based on Thin-Layer Liquid Membrane. Anal. Chem. 2008, 80, 6056–6065. [CrossRef] [PubMed] 17. Abdalla, N.S.; Amr, A.E.-G.E.; El-Tantawy, A.S.M.; Al-Omar, M.A.; Kamel, A.H.; Khalifa, N.M. Tailor-Made Specific Recognition of Cyromazine Pesticide Integrated in a Potentiometric Strip Cell for Environmental and Food Analysis. Polymers 2019, 11, 1526. [CrossRef][PubMed] Polymers 2019, 11, 1876 15 of 16

18. Pinaeva, U.; Dietz, T.C.; Al Sheikhly, M.; Balanzat, E.; Castellino, M.; Wade, T.L.; Clochard, M.C. Bis[2-(methacryloyloxy)ethyl] phosphate radiografted into track-etched PVDF for uranium (VI) determination by means of cathodic stripping voltammetry. React. Funct. Polym. 2019, 142, 77–86. [CrossRef] 19. Bessbousse, H.; Zran, N.; Fauléau, J.; Godin, B.; Lemée, V.; Wade, T.; Clochard, M.-C. Poly(4-vinyl pyridine) radiografted PVDF track etched membranes as sensors for monitoring trace mercury in water. Radiat. Phys. Chem. 2016, 118, 48–54. [CrossRef] 20. Barsbay, M.; Güven, O.; Bessbousse, H.; Wade, T.L.; Beuneu, F.; Clochard, M.-C. Nanopore size tuning of polymeric membranes using the RAFT-mediated radical polymerization. J. Membr. Sci. 2013, 445, 135–145. [CrossRef] 21. Bessbousse, H.; Nandhakumar, I.; Decker, M.; Barsbay, M.; Cuscito, O.; Lairez, D.; Clochard, M.-C.; Wade, T.L. Functionalized nanoporous track-etched β-PVDF membrane electrodes for lead(ii) determination by square wave anodic stripping voltammetry. Anal. Methods 2011, 3, 1351. [CrossRef] 22. Mizuguchi, H.; Shibuya, K.; Fuse, A.; Hamada, T.; Iiyama, M.; Tachibana, K.; Nishina, T.; Shida, J. A dual-electrode flow sensor fabricated using track-etched microporous membranes. Talanta 2012, 96, 168–173. [CrossRef] 23. Mizuguchi, H.; Sakurai, J.; Kinoshita, Y.; Iiyama, M.; Kijima, T.; Tachibana, K.; Nishina, T.; Shida, J. Flow-based Biosensing System for Glucose Fabricated by Using Track-etched Microporous Membrane Electrodes. Chem. Lett. 2013, 42, 1317–1319. [CrossRef] 24. Apel, P.Y.; Blonskaya, I.V.; Dmitriev, S.N.; Orelovich, O.L.; Sartowska, B.A. Ion track symmetric and asymmetric nanopores in polyethylene terephthalate foils for versatile applications. Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 2015, 365, 409–413. [CrossRef] 25. Apel, P.Y. Fabrication of functional micro- and nanoporous materials from polymers modified by swift heavy ions. Radiat. Phys. Chem. 2019, 159, 25–34. [CrossRef] 26. Pinaeva, U.; Lairez, D.; Oral, O.; Faber, A.; Clochard, M.-C.; Wade, T.L.; Moreau, P.; Ghestem, J.-P.; Vivier, M.; Ammor, S.; et al. Early Warning Sensors for Monitoring Mercury in Water. J. Hazard. Mater. 2019, 376, 37–47. [CrossRef][PubMed] 27. Ashraf, S.; Cluley, A.; Mercado, C.; Mueller, A. Imprinted polymers for the removal of heavy metal ions from water. Water Sci. Technol. 2011, 64, 1325–1332. [CrossRef][PubMed] 28. Zhao, G.; Huang, X.; Tang, Z.; Huang, Q.; Niu, F.; Wang, X. Polymer-based nanocomposites for heavy metal ions removal from aqueous solution: A review. Polym. Chem. 2018, 9, 3562–3582. [CrossRef] 29. Zhang, Y.; Vallin, J.R.; Sahoo, J.K.; Gao, F.; Boudouris, B.W.; Webber, M.J.; Phillip, W.A. High-Affinity Detection and Capture of Heavy Metal Contaminants using Block Polymer Composite Membranes. ACS Cent. Sci. 2018, 4, 1697–1707. [CrossRef] 30. Oyagi, M.O.; Onyatta, J.O.; Kamau, G.N.; Guto, P.M. Validation of the polyacrylic acid/glassy carbon differential pulse anodic stripping voltammetric sensor for simultaneous analysis of lead(II), cadmium(II) and cobalt(II) ions. Int. J. Electrochem. Sci. 2016, 11, 3852–3861. [CrossRef] 31. Ling, J.L.W.; Ab Ghani, S. Poly(4-vinylpyridine-co-aniline)-modified electrode—Synthesis, characterization, and application as cadmium(II) ion sensor. J. Solid State Electrochem. 2013, 17, 681–690. [CrossRef] 32. Kabanov, N.M.; Kozhevnikova, N.A.; Kokorin, A.I.; Rogacheva, V.B.; Zezin, A.B.; Kabanov, V.A. Study of the structure of a polyacrylic acid-Cu (II)-poly-4-vinylpyridine ternary polymeric metal complex. Polym. Sci. U.S.S.R. 1979, 21, 2090–2097. [CrossRef] 33. Turmanova, S.; Vassilev, K.; Boneva, S. Preparation, structure and properties of metal-copolymer complexes of poly-4-vinylpyridine radiation-grafted onto polymer films. React. Funct. Polym. 2008, 68, 759–767. [CrossRef] 34. Mulder, M. Transport in Membranes. In Basic Principles of Membrane Technology; Springer Netherlands: Dordrecht, The Netherlands, 1996; pp. 210–279. 35. Korolkov, I.V.; Mashentseva, A.A.; Güven, O.; Gorin, Y.G.; Zdorovets, M.V. Protein fouling of modified microporous PET track-etched membranes. Radiat. Phys. Chem. 2018, 151, 141–148. [CrossRef] 36. Korolkov, I.V.; Mashentseva, A.A.; Güven, O.; Zdorovets, M.V. Modification of Track-Etched PET Membranes by Graft Copolymerization of Acrylic Acid and N-Vinylimidazole. Pet. Chem. 2017, 57, 1233–1241. [CrossRef] 37. Holland, B.; Hay, J. The thermal degradation of PET and analogous polyesters measured by thermal analysis–Fourier transform infrared spectroscopy. Polymer 2002, 43, 1835–1847. [CrossRef] Polymers 2019, 11, 1876 16 of 16

38. Zhou, X.; Goh, S.H.; Lee, S.Y.; Tan, K.L. XPS and FTi.r. studies of interactions in poly(carboxylic acid)/poly(vinylpyridine) complexes. Polymer 1998, 39, 3631–3640. [CrossRef] 39. Fujimori, K.; Trainor, G.T.; Costigan, M.J. Complexation and rate of polymerization of acrylic acid and methacrylic acid in the presence of poly(4-vinylpyridine) in dilute methanol solution. J. Polym. Sci. Polym. Chem. Ed. 1984, 22, 2479–2487. [CrossRef] 40. Hennig, A.; Borcherding, H.; Jaeger, C.; Hatami, S.; Würth, C.; Hoffmann, A.; Hoffmann, K.; Thiele, T.; Schedler, U.; Resch-Genger, U. Scope and Limitations of Surface Functional Group Quantification Methods: Exploratory Study with Poly(acrylic acid)-Grafted Micro- and Nanoparticles. J. Am. Chem. Soc. 2012, 134, 8268–8276. [CrossRef] 41. Masuda, S.; Minagawa, K.; Tsuda, M.; Tanaka, M. Spontaneous copolymerization of acrylic acid with 4-vinylpyridine and microscopic acid dissociation of the alternating copolymer. Eur. Polym. J. 2001, 37, 705–710. [CrossRef] 42. Moulay, S.; Bensacia, N. Removal of heavy metals by homolytically functionalized poly(acrylic acid) with hydroquinone. Int. J. Ind. Chem. 2016, 7, 369–389. [CrossRef] 43. Biswas, T.K.; Yusoff, M.M.; Sarjadi, M.S.; Arshad, S.E.; Musta, B.; Rahman, M.L. Ion-imprinted polymer for selective separation of cobalt, cadmium and lead ions from aqueous media. Sep. Sci. Technol. 2019. [CrossRef]

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