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Poly(aspartic acid) Electrospun Nanofiber Hydrogel MembraneBased Reusable Colorimetric Sensor for Cu(II) and Fe(III) Detection Caidan Zhang,*,† Haidong Li,† Qiaozhen Yu,† Lin Jia,‡ and Lynn Yuqin Wan§ †
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Key Laboratory of Yarn Materials Forming and Composite Processing Technology of Zhejiang Province, College of Materials and Textile Engineering, Jiaxing University, Jiaxing, Zhejiang 314001, China ‡ College of Textile, Henan University of Engineering, Zhengzhou, Henan 450007, China § Advanced Fibrous Material Laboratory, Advanced Materials and Process Engineering Laboratory, University of British Columbia, Vancouver V6T 1Z4, Canada S Supporting Information *
ABSTRACT: Electrospun nanofiber membrane (ENM) with huge specific surface area is an ideal solid substrate for sensors. However, only a few ENMs are developed into colorimetric sensors and it is even more challenging to fabricate multiple-ion-responsive ENM-based colorimetric sensor. In this study, benefiting from the excellent metal ion adsorption ability of poly(aspartic acid) (PASP) and high specific surface area of nanofibers, a reusable colorimetric sensor utilizing PASP electrospun nanofiber hydrogel membrane (ENHM) was designed to detect Cu2+ and Fe3+ in aqueous solution with simple filtration. The sensor based on PASP−ENHM exhibited high sensitivity and selectivity, and colorimetric responses for Cu2+ and Fe3+ detection could be observed by the naked eye. Upon exposure to Cu2+ aqueous solution, the color of the sensor changed from white to blue with a naked eye detection limit of 0.3 mg/L, while it turned from white to yellow with a detection limit of 0.1 mg/L for Fe3+ detection. Furthermore, this sensor was reusable after metal ion extraction by the desorption process. chemosensor.9−11 Chemosensor can be physically adsorbed or chemically linked onto electrospun nanofibers.12−14 In a ENMimmobilized sensor, the chemosensor is responsible to bind with the target substance for sensing, and the ENM provides a large quantity of active sites for chemosensor contacting target substance to improve sensor’s sensibility. In another approach, ENM can be utilized to adsorb the target substance from the sample solution first and then chemosensor is applied to detect the concentrated target substance.15 However, only a few ENMs have been employed for fabrication of colorimetric sensors as most of chemosensors can hardly be electrospun into ENMs directly. To construct a ENM-based colorimetric sensor for metal ion detection, it is a challenging to develop a electrospinning material polymer with metal ions adsorbability. Poly(aspartic acid) (PASP) is a synthetic poly(amino acid) with several unique characteristics including metal ions binding capacity, biocompatibility, biodegradability, water solubility, and low toxicity. The development and application of PASPbased polymers have gained remarkable attention in a variety of fields. The low-cost and easy synthesis creates growing possibilities of PASP application in numerous industries. It is
1. INTRODUCTION Nowadays, widespread existence of free metal ions in water are potentially threatening to the ecosystem and public health. The detection of metal ions is becoming essential.1,2 In the past decades, various equipment have been used to detect and analyze metal ions, such as atomic absorption spectrometry, atomic emission spectrometry, inductively coupled plasma mass spectrometry, and so on.3,4 Although these strategies are widely accepted and can achieve accurate results, they have disadvantages of complicated manipulation, high cost, and time consuming. To overcome these disadvantages, various rapid and sensitive detection techniques for metal ions have been developed.5−7 In particular, colorimetric sensing approach has attracted considerable attention because of its simplicity of operation, rapidity, low cost, and direct visual perception. To increase sensitivity, most colorimetric sensors are dispersed in sample solution for full contact to target substance, but the uneven and unsteady dispersion of the sensors often induces unstable detection results. In order to avoid this problem, chemosensor can be assembled on a solid substrate.8 In this case, the selection of a solid substrate is extremely important as it can significantly affect the sensitivity of the sensor. Electrospun nanofiber membrane (ENM) has the merits of huge specific surface area, high porosity, controllable structure, and easy functionalization. It is an ideal solid substrate for © XXXX American Chemical Society
Received: July 9, 2019 Accepted: August 15, 2019
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Scheme 1. Fabrication Procedure of PASP−ENHM and Ion Detection Process of PASP−ENHM-Based Sensor
already accessible on a commercial scale as Baypure DS 100 produced by Lanxess (Germany). Moreover, a current need for eco-friendly materials as chelating agent, dispersant, and antiscalant provides an opportunity for PASP to be employed as a new generation of products in water treatment field. Although many studies have been demonstrated the metal ion binding capacity of PASP, the dissolution property limits its application as a electrospinning material and ENM-based colorimetric sensor. In this study, we devoted efforts to design a PASP-based colorimetric sensor with a nanofibrous structure for Cu2+ and Fe3+ detection. PASP was fabricated into the electrospun nanofiber hydrogel membrane (ENHM) by electrospinning of its precursor and further treatment. Then PASP−ENHM was designed to detect Cu2+ and Fe3+ in aqueous solution by the filtering method. Because of the strong metal ion adsorptivity of PASP and the large specific surface area of ENHM, numerous Cu2+ or Fe3+ ions were accumulated on PASP− ENHM after filtration. The PASP−ENHM exhibited naked eye perceptible color changes as the concentration of Cu2+ or Fe3+ in the aqueous solution changes. Additionally, the ions adsorbed by PASP−ENHM can be removed by the desorption process, and therefore the PASP−ENHM-based colorimetric sensor was recoverable and reusable.
Figure 1. FTIR spectrum of PASP−ENHM.
of −OH and −NH2 groups.23 The bands at 1638 and 1548 cm−1 were assigned to the amide-I/II bands.24 The band at 1548 cm−1 was also attributed to the asymmetric stretching of −COO−. The band at 1390 cm−1 was associated with the symmetric stretching of −COO−.25 It indicated that PASP− ENHM had a high amount of amino groups, carboxylic groups, and amide groups. These functional groups endowed PASP− ENHM excellent metal-ion binding property. 2.2. Basic Mechanism for Visual Detection of Cu2+ and Fe3+. Cu2+ dissolved in water transforms into Cu(H2O)62+ and appears blue.26 The color is invisible to the naked eye and even difficult to detect by the UV−vis spectrophotometer if the concentration of Cu2+ is too low. As shown in Figure 2A, a stronger UV−vis absorption signal was detected when the concentration of Cu2+ increased from 0.1 to 100 mg/L, which suggests the higher the Cu2+ concentration the better the detectability. The color of the sensor depends on the amount of Cu2+ captured by PASP− ENHM. Because of the excellent Cu2+ adsorptivity of PASP and the large surface area of ENHM, Cu2+ ions will be captured by PASP−ENHM as the Cu2+ aqueous solution filtering through. The more Cu2+ ions accumulate on the PASP−ENHM-based sensor, the deeper color the sensor will be. The color becomes visible to the naked eye when the accumulated Cu2+ reaches a certain level. Fe 3+ can be detected by the PASP−ENHM-based colorimetric sensor too, although PASP−ENHM adsorbs less Fe3+ ions from Fe3+ aqueous solution as shown in our previous study.27 PASP−ENHM deswells in Fe3+ solution28,29 and Fe3+ tends to form Fe(OH)3 micelles with a large diameter, which prevent Fe3+ ions from diffusing into PASP−ENHM. When
2. RESULTS AND DISCUSSION 2.1. Preparation and Properties of PASP−ENHM. The cross-linked hydrogel network can barely be directly prepared by the electrospinning process. Instead, ENHMs are commonly fabricated by electrospinning of the precursor of the hydrogel and further postprocess.16,17 PASP hydrogel can be prepared by hydrolysis of cross-linked polysuccinimide (PSI).18,19 Therefore, in this study, PSI with the linear molecular structure was selected as the electrospinning precursor for PASP−ENHM preparation. High-molecular weight PSI (Mw 31 500) was synthesized from L-aspartic acid (L-ASP) with the catalyst of phosphoric acid20−22 and electrospun into PSI−ENM. The obtained PSI−ENM was then cross-linked and hydrolyzed into PASP−ENHM, as shown in Scheme 1. The resultant PASP nanofibers had a larger diameter than PSI nanofibers, while the nanofibrous geometric characteristic was still retained. The Fourier transform infrared spectroscopy (FTIR) spectrum of PASP−ENHM was presented in Figure 1. The broad band appearing at 3279 cm−1 was assigned to stretching B
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PASP−ENHM with a fixed thickness of 0.1 mm was used in subsequent experiments. 2.4. Colorimetric Determination of Cu2+. The colorimetric response of the sensor to Cu2+ aqueous solution with different concentrations was observed by the naked eye and then recorded by spectro-colorimeter. As shown in Figure 4A,
Figure 2. UV−vis absorption spectra of aqueous solutions with different concentration of Cu2+ (A) and Fe3+ (B).
the filtrating method is adopted as demonstrated in this study, Fe3+ solution is forced to pass through the PASP−ENHM and thus Fe 3+ ions are captured by the PASP−ENHM. Consequently, the PASP−ENHM sensor shows the change of color as the amount of Fe3+ ions captured by the PASP− ENHM changed. As shown in Figure 2B, when the concentration of Fe3+ aqueous solution is above 10 mg/L, the solution appeared yellow, which could be detected by naked eye and UV−vis spectrophotometer. 2.3. Effect of PASP−ENHM Thickness. To investigate the effect of PASP−ENHM thickness on the color response, sensors prepared by different thickness (0.01−0.3 mm) of PASP−ENHM were utilized for the detection of 1 mg/L Cu2+ and 1 mg/L Fe3+ aqueous solution, respectively. As shown in Figure 3, sensors appeared blue for Cu2+ detection and yellow
Figure 4. (A) Reflectance spectra and (B) optical colorimetric response of sensors for Cu2+ detection, (C) color-differentiation map based on L*a*b values.
the reflection spectra of the sensors varied upon the concentration change of Cu2+. With 0.1 mg/L Cu2+ solution, the color change of the sensor was hardly detectable and the reflectance curve of the sensor was almost the same as that of the blank sample. As Cu2+ concentration increased, the reflectance intensity of sensors at 660 nm decreased progressively. The reflectance spectrum of the sensor that was used to test 100 mg/L Cu2+ solution started to overlap with the one used for 30 mg/L Cu2+ solution, which suggests that the PASP−ENHM sensor is almost saturated with Cu2+ ions when the concentration of the solution reached 30 mg/L and can hardly capture more Cu2+ ions, therefore, the color of the sensors stopped changing. It is hence concluded that the effective detection limit of the sensor for Cu2+ is around 0.3− 30 mg/L. Figure 4B shows the color change of sensors corresponding to the Cu2+ concentration variation. The color of the sensors demonstrated distinguishable changes from white to blue upon the increase of Cu2+ concentration, which could be clearly identified by the naked eye or a charge-coupled device (CCD) camera. The naked eye detection limits were 0.3−30 mg/L which agreed well with the reflectance spectra. When Cu2+ concentration reached 1 mg/L, the color sensor became clearly distinguishable, and China’s Ministry of Health limits the maximum levels of copper to 1 mg/L, suggesting that the sensor is viable naked eye chemosensors for Cu2+ detection in drinking water. To avoid the individual differences in color identification, a converting method was introduced to standardize the color with L*a*b values. The L*a*b parameters of the free sensor were 90.63, −0.88, and 0.07, respectively, and the sensor showed a white color. With the increase of Cu2+ concentration from 0.3 to 30 mg/L, the L*a*b parameters all decreased (Table S1). Therefore, the color of the sensors change to blue gradually. With the L*a*b
Figure 3. Effect of PASP−ENHM thickness on the color response for Cu2+ and Fe3+ detection.
for Fe3+ detection. The color of the sensors was indistinguishable by the naked eye at a thickness of 0.01 mm. When the thickness increased to 0.05 mm, the color of the sensor can be easily detected by the naked eye. The sensor showed deeper color when its thickness increased 0.1 mm. Thicker PASP− ENHM had a larger surface area and provided more active sites, which thus captured more Cu2+ or Fe3+ ions and hence appeared deeper in color. However, the color of the sensor stopped changing as the thickness continuously increased because of the fixed amount of Cu2+ and Fe3+ in sample solutions limited the maximum amount of Cu2+ and Fe3+ ions that could be captured by the sensor. Based on this test result, C
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values, the color-differentiation map can be easily generated for Cu2+ concentration identification, as shown in Figure 4C. To evaluate the antidisturbance capability of the PASP− ENHM-based sensor for Cu2+ detection, competitive experiment was conducted by adding Cu2+ (1 mg/L) into test solution in the presence of other metal ions (1 mg/L), including Ca2+, Mg2+, Sn2+, Zn2+, Ni2+, Pb2+, Co2+. As shown in Figure 5, these metal ions did not exhibit observable
Figure 6. (A) Reflectance spectra and (B) optical colorimetric responses of sensors for Fe3+ detection, (C) color-differentiation map based on L*a*b values.
seen in Figure 7, the color of the sensors barely changed with the presence of other metal ions, suggesting the interference of
Figure 5. EDS spectra of sensors after detection of Cu2+ (A) and Cu2+ in the presence of different metal ions (B).
interference for Cu2+ detection, although Pb and Sn element on PASP−ENHM were detected by energy-dispersive spectrometer (EDS). Pb2+ and Sn2+ tended to form white precipitation in water, which could be partially captured by PASP−ENHM as the test solution filtering through and detected by EDS. Because the white background was used by the sensor, the captured Pb2+ and Sn2+ have no effect on Cu2+ detection. In fact, it is reported that PASP has the complexing ability toward Hg2+, Cd2+, Ca2+, Zn2+, and so forth.30,31 Compared with PASP, PASP hydrogel cross-linked by ethylenediamine (ED) has abundant −CO−NH− and −NH2 functional groups on the side chain, which have a stronger complexing ability toward Cu2+. Moreover, the strength of complexation decreases in the following order: Cu2+ > Pb2+ ≈ Co2+ > Zn2+ > Mn2+.32 Consequently, Cu2+ has a higher tendency to bind to PASP−ENHM and forms more stable complexes compared with other metal ions.33 2.5. Colorimetric Determination of Fe3+. Besides the capability of Cu2+ detection, the PASP−ENHM is also suitable for colorimetric detection of Fe3+. As shown in Figure 6, the reflectance intensity of the sensors at 430 nm decreased as the Fe3+ concentration increased. When the concentration of Fe3+ increased from 0.1 to 10 mg/L, the color of the sensors changed from white to light yellow, then deep yellow, which was distinguishable by the naked eye according to CCD camera picture and color-differentiation map. It is worth pointing out that the permissible contamination level of Fe3+ in drinking water regulated by China’s Ministry of Health (GB5749-2006) is 0.3 mg/L, which is well covered by the detection limits of this sensor. To examine the effect of other metal ions on the selectivity of this sensor for Fe3+ detection, competition experiment was also conducted by detecting Fe3+ in the presence of various 1 mg/L metal ions (Ca2+, Mg2+, Sn2+, Zn2+, Ni2+, Pb2+, Co2+). As
Figure 7. EDS spectra of sensors after detection of Fe3+ (A) and Fe3+ in the presence of different metal ions (B).
surveyed metal ions was negligible though the EDS spectrum proved the adsorption of Pb and Sn on PASP−ENHM, similar to the competitive experiment for Cu2+ detection. In test solution, Fe3+ ions in the form of Fe(OH)3 micelles were easy to be captured on PASP−ENHM and appeared yellow. Other metal ions had less effect on the formation of Fe(OH)3 micelles and the Fe3+ detection, which gave the selectivity performance to the PASP−ENHM-based sensor. 2.6. Coexistence Study for Cu2+ and Fe3+. To investigate the color response of the PASP−ENHM sensor when Cu2+ and Fe3+ coexist, solution containing different concentrations of Cu2+ and Fe3+ was tested. As shown in Figure 8, the sensors displayed different colors according to variation of the concentration of Cu2+ and Fe3+. When 0.1 mg/ L Fe3+ was added into Cu2+ solution, the color of the sensors changed from bright blue to yellow-green, indicating that Cu2+ ions and Fe3+ ions were both adsorbed on the PASP−EHNM and sensor appeared blend colors of blue and yellow. In the D
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3. CONCLUSIONS A new reusable colorimetric sensor based on PASP−ENHM was developed for naked eye detection of Cu2+ and Fe3+, by taking advantage of the large specific surface area of the nanofibers and the excellent metal ion binding property of PASP. The PASP−ENHM-based sensors exhibited an distinguishable color change from white to blue with Cu2+ concentration varied from 0.3 to 30 mg/L, and from white to yellow with Fe3+ concentration increased from 0.1 to 10 mg/L. The naked eye detection limits of Cu2+ and Fe3+ were as low as 0.3 and 0.1 mg/L, respectively. The existence of other metal ions showed negligible interference for Cu2+ and Fe 3+ detection. The discrimination between Cu2+ and Fe3+ under their coexistence could be distinguished by the naked eye too. The high sensitivity, wide naked eye detection limit and the reusability of PASP−ENHM-based sensor for Cu2+ and Fe3+ suggest that it is a viable, economic, and straightforward Cu2+ and Fe3+ sensor, and can potentially be widely used for drinking water Cu2+ and Fe3+ pollution detection.
Figure 8. (A) Optical colorimetric responses of the colorimetric and (B) color-differentiation map based on L*a*b values for Cu2+ and Fe3+ detection.
case of Fe3+ detection, the color of the sensors generated by Cu2+ was masked, which was attributed to the competitive adsorption of Fe3+ over Cu2+. 2.7. Reusability of PASP−ENHM-Based Sensor. The reusability of the PASP−ENHM-based sensor was investigated by repeated desorption and detection tests. The sensors after 1 mg/L Cu2+ detection were immersed in hydrochloric acid (HCl) and ethylene diamine tetraacetic acid (EDTA) solution in turn to dissolve Cu2+ captured on PASP−ENHM and reverted to the sensor’s original unpolluted white color. As shown in Figure 9, PASP−ENHM-based sensors demonstrated
4. EXPERIMENTAL SECTION 4.1. Reagents and Apparatus. L-ASP and phosphoric acid (85 wt % in H2O) were purchased from Aladdin. N,NDimethylformamide (DMF), ED, sodium hydroxide (NaOH), and HCl were purchased from Fisher Scientific. Metal salts (CaCl2, FeCl3, ZnCl2, SnCl2, NiCl2, CuCl2, Pb(NO3)2, AgNO3) and EDTA were supplied by Sigma-Aldrich. Distilled water was used throughout the study. All chemicals were of analytical grade purity and used without further purification. The morphology and chemical composition of the nanofiber membranes were examined by the scanning electron microscope (S3000N, Hitachi Ltd., Japan) and EDS. The FTIR spectrum was recorded with an FTIR spectroscope with a diamond attenuated total reflection sampling accessory (Frontier, PerkinElmer, USA). Visible absorption spectra were obtained on a UV−vis spectrophotometer (Cary 5000, Varian, USA) with a 1.0 cm quartz cell in the wavelength range from 400 to 700 nm. A spectro-colorimeter (Labscan XE, HunterLab, USA) was used for determining reflectance spectra and L*a*b values of the PASP−ENHM-based colorimetric sensor. 4.2. Preparation of PASP−ENHM. PSI, the precursor of PASP, was synthesized and electrospun into the nanofiber membrane according to our previously reported method.34 PSI was polymerized by L-ASP in the presence of phosphoric acid. The PSI nanofiber membrane was prepared under a typical electrospinning procedure. Thirty weight percentage PSI solution was obtained by dissolving PSI in DMF with stirring for 12 h at 50 °C. Then, the solution was fed into a syringe equipped with a steel needle (18 G). Electrospinning was carried out using NEU (nanofiber electrospinning unit, Kato Tech., Japan) with an applied voltage of 16 kV, the feed rate was 0.6 mm/min and the tip-to-collector distance was 18 cm. To acquire thickness uniformity of PSI−ENM, the target and traverse speed were set at 1 m/min and 3 cm/min, respectively. PSI−ENM with different thicknesses were fabricated by varying electrospinning time. Then PSI−ENM was cut into a 40 mm × 40 mm square for test. PASP−ENHM was produced from PSI−ENM in two steps. First, PSI−ENM was immersed in 0.3 mol/L ED solution for 2 h under room temperature for cross-linking reaction. Then, 0.2 mol/L NaOH solution was added into the solution to turn PSI
Figure 9. Sensor color change upon alternate detection and desorption of 1 mg/L Cu2+ and 1 mg/L Fe3+.
stable sensing property for repeated detection tests. Moreover, similar results were obtained with repeated detection tests of 1 mg/L Fe3+. The reusable principle of PASP−ENHM sensors is: metal ions are bound to PASP−ENHM by amide groups and carboxylic groups which are protonated at low pH value and lose the capability of capturing metal ions, as shown in Scheme 2. Consequently, the metal ions on PASP−ENHM are Scheme 2. Schematic of the Protonation of Amine Groups and Carboxylic Groups
removed. In the Fe3+ detection test, Fe(OH)3 intercepted by PASP−ENHM are converted back to Fe3+ and dissolved back to the solution, and EDTA with excellent strong complexation ability to metal ions formed complexes with Cu2+ and Fe3+, which contributed to the metal ion removal from PASP− ENHM. Then, the PASP−ENHM without complexes and precipitate recovered its sensing ability and was reusable. E
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(4) Wang, X.; He, M.; Xie, J.; Xi, J.; Lu, X. Heavy metal pollution of the world largest antimony mine-affected agricultural soils in Hunan province (China). J. Soils Sediments 2010, 10, 827−837. (5) Khairy, G. M.; Duerkop, A. Dipsticks and sensor microtiterplate for determination of copper (II) in drinking water using reflectometric RGB readout of digital images, fluorescence or eyevision. Sens. Actuators, B 2019, 281, 878−884. (6) Wang, X.; Lou, Y.; Ye, X.; Chen, X.; Fang, L.; Zhai, Y.; Zheng, Y.; Xiong, C. Green chemical method for the synthesis of chromogenic fiber and its application for the detection and extraction of Hg2+ and Cu2+ in environmental medium. J. Hazard. Mater. 2019, 364, 339−348. (7) Li, C.-Y.; Zou, C.-X.; Li, Y.-F.; Tang, J.-L.; Weng, C. A new rhodamine-based fluorescent chemosensor for Fe3+ and its application in living cell imaging. Dyes Pigm. 2014, 104, 110−115. (8) Ju, H.; Lee, M. H.; Kim, J.; Kim, J. S.; Kim, J. Rhodamine-based chemosensing monolayers on glass as a facile fluorescent “turn-on” sensing film for selective detection of Pb2+. Talanta 2011, 83, 1359− 1363. (9) Zhang, N.; Qiao, R.; Su, J.; Yan, J.; Xie, Z.; Qiao, Y.; Wang, X.; Zhong, J. Recent Advances of Electrospun Nanofibrous Membranes in the Development of Chemosensors for Heavy Metal Detection. Small 2017, 13, 1604293. (10) Ding, B.; Wang, M.; Wang, X.; Yu, J.; Sun, G. Electrospun nanomaterials for ultrasensitive sensors. Mater. Today 2010, 13, 16− 27. (11) Zhang, Y.; Li, S.; Pan, G.; Yang, H.; Qile, M.; Chen, J.; Song, Q.; Yan, D. Stretchable nanofibrous membranes for colorimetric/ fluorometric HCl sensing: Highly sensitive charge-transfer excited state. Sens. Actuators, B 2018, 254, 785−794. (12) Hu, Y.; Chen, L.; Jung, H.; Zeng, Y.; Lee, S.; Swamy, K. M. K.; Zhou, X.; Kim, M. H.; Yoon, J. Effective Strategy for Colorimetric and Fluorescence Sensing of Phosgene Based on Small Organic Dyes and Nanofiber Platforms. ACS Appl. Mater. Interfaces 2016, 8, 22246− 22252. (13) Hu, L.; Yan, X.-W.; Li, Q.; Zhang, X.-J.; Shan, D. Br-PADAP embedded in cellulose acetate electrospun nanofibers: Colorimetric sensor strips for visual uranyl recognition. J. Hazard. Mater. 2017, 329, 205−210. (14) Li, Y.; Si, Y.; Wang, X.; Ding, B.; Sun, G.; Zheng, G.; Luo, W.; Yu, J. Colorimetric sensor strips for lead (II) assay utilizing nanogold probes immobilized polyamide-6/nitrocellulose nano-fibers/nets. Biosens. Bioelectron. 2013, 48, 244−250. (15) Li, Y.; Wen, Y.; Wang, L.; He, J.; Al-Deyab, S. S.; El-Newehy, M.; Yu, J.; Ding, B. Simultaneous visual detection and removal of lead(II) ions with pyromellitic dianhydride-grafted cellulose nanofibrous membranes. J. Mater. Chem. A 2015, 3, 18180−18189. (16) Wang, J.; Sutti, A.; Wang, X.; Lin, T. Fast responsive and morphologically robust thermo-responsive hydrogel nanofibres from poly(N-isopropylacrylamide) and POSS crosslinker. Soft Matter 2011, 7, 4364−4369. (17) Wu, S.; Duan, B.; Qin, X.; Butcher, J. T. Nanofiber-structured hydrogel yarns with pH-response capacity and cardiomyocytedrivability for bio-microactuator application. Acta Biomater. 2017, 60, 144−153. (18) Sharma, S.; Dua, A.; Malik, A. Polyaspartic acid based superabsorbent polymers. Eur. Polym. J. 2014, 59, 363−376. (19) Gyarmati, B.; Mészár, E. Z.; Kiss, L.; Deli, M. A.; László, K.; Szilágyi, A. Supermacroporous chemically cross-linked poly (aspartic acid) hydrogels. Acta Biomater. 2015, 22, 32. (20) Koski, A.; Yim, K.; Shivkumar, S. Effect of molecular weight on fibrous PVA produced by electrospinning. Mater. Lett. 2004, 58, 493− 497. (21) Tao, J.; Shivkumar, S. Molecular weight dependent structural regimes during the electrospinning of PVA. Mater. Lett. 2007, 61, 2325−2328. (22) Nakato, T.; Tomida, M.; Kusuno, A.; Shibata, M.; Kakuchi, T. Synthesis of poly(succinimide-amide) by acid-catalyzed polyconden-
into PASP by hydrolysis and hence PASP−ENHM was obtained. 4.3. Colorimetric Sensing of Cu2+ and Fe3+ Aqueous Solutions. PASP−ENHM supported by a nonwoven fabric was served as the colorimetric sensor. The sensor was fixed onto a filtering unit, as shown in Scheme 1. The bottom diameter of the filter cup was 35 mm. Two hundred milliliter aqueous solution with predetermined amount of metal ions was used as sample solution for each filter test. The sample solution was poured into the filter cup and forced to pass through the sensor by the force of gravity. Then colorimetric response of the sensor was examined qualitatively by the naked eye and recorded quantitatively by a spectro-colorimeter, respectively. Photoshop was employed to generate colordifferentiation map according to the L*a*b values acquired by the spectro-colorimeter. In order to evaluate the reusability of the PASP−ENHMbased colorimetric sensors, desorption experiments were conducted to remove metal ions from the used sensors. First, the sensors were used for Cu2+ (1 mg/L) or Fe3+ (1 mg/L) detection. After the detection tests, metal ions were removed from the used sensors by immersing the sensors in 1 mol/L HCl solution for 5 min and followed by washing with distilled water several times, then 0.5 mol/L EDTA solution and distilled water several times. The detection and desorption processes were repeated in several cycles.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b02109. L*a*b values of sensors after Cu2+ detection with different concentrations and L*a*b values of sensors after Fe3+ detection with different concentrations (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Caidan Zhang: 0000-0003-0307-3371 Qiaozhen Yu: 0000-0002-0401-8801 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Open Project Program of Key Laboratory of Yarn Materials Forming and Composite Processing Technology of Zhejiang Province (no. MTC201905) and Jiaxing Project of Science and Technology (SQGY201900428).
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REFERENCES
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DOI: 10.1021/acsomega.9b02109 ACS Omega XXXX, XXX, XXX−XXX
