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May 31, 2017 - ABSTRACT: A functional ionic microgel sensor array was developed by using 1-(2- pyridinylazo)-2-naphthaleno (PAN)- and bromothymol blue...
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Functionalized Ionic Microgel Sensor Array for Colorimetric Detection and Discrimination of Metal Ions Xianjing Zhou,†,§ Jingjing Nie,‡ and Binyang Du*,† †

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China § Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, China ‡ Department of Chemistry, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: A functional ionic microgel sensor array was developed by using 1-(2pyridinylazo)-2-naphthaleno (PAN)- and bromothymol blue (BTB)-functionalized ionic microgels, which were designed and synthesized by quaternization reaction and anionexchange reaction, respectively. The PAN microgels (PAN-MG) and BTB microgels (BTB-MG) were spherical in shape with a narrow size distribution and exhibited characteristic colors in aqueous solution in the presence of various trace-metal ions, which could be visually distinguished by the naked eye. Such microgels could be used for the colorimetric detection of various metal ions in aqueous solution at submicromolar levels, which were lower than the U.S. Environmental Protection Agency standard for the safety limit of metal ions in drinking water. A total of 10 species of metal ions in aqueous solution, Ba2+, Cr3+, Mn2+, Pb2+, Fe3+, Co2+, Zn2+, Ni2+, Cu2+, and Al3+, were successfully discriminated by the as-constructed microgel sensor array combined with discriminant analysis, agglomerative hierarchical clustering, and leave-one-out cross-validation analysis. KEYWORDS: metal ions, microgel, multivariate analysis, sensor array, trace analysis

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INTRODUCTION With the rapid development of mining, machinery manufacturing, chemicals, electronics, and instrumentation industries, wastewaters containing heavy metal ions are increasingly discharged into the environment, especially in developing countries.1 These excessive levels of metal will be accumulated in organisms. Because of their nonbiodegradable nature and high solubility in water, heavy metal ions bring serious threats to human health, such as acting as carcinogen, mutagen, and teratogen, as well as cause damage to the central nervous system, endocrine system, kidneys, bones, and so on.2−4 Therefore, the development of new techniques for a simple, low-cost, rapid, and sensitive detection of metal ions, especially heavy metal ions, in aqueous solution is of utmost importance. Several analytical methods, including atomic absorption spectrometry, fluorescence,5,6 spectrophotometry,7,8 and inductively coupled plasma spectrometry,9 have been so far developed for the accurate determination of metal ions, offering high sensitivity and selectivity. However, these methods are time-consuming and expensive as well as require intense technical training because of the complex instrumentation. Colorimetric sensor array system10−13 has emerged as an effective and simple approach for the detection and recognition of a wide variety of analytes, such as toxic gases,14−16 explosives,17−19 drinks and food,20 biomolecules,21−23 pesticides,24 and heavy metal ions.25 Rather than highly specific receptors, colorimetric sensor arrays use cross-responsive © 2017 American Chemical Society

sensing elements to imitate mammalian olfactory and gustatory systems, which are vividly called “chemical nose” or “chemical tongue”.12 The color changes induced by reactions between analyte and sensing elements of an array can be quantified by digital imaging and multivariate statistical techniques.26 In the present study, a functional ionic microgel sensor array was designed and constructed for colorimetric detection and discrimination of metal ions in aqueous solution. 1-(2Pyridinylazo)-2-naphthaleno (PAN)- and bromothymol blue (BTB)-functionalized ionic microgels were used to construct the sensor array. PAN is a complex metric indicator, which has been used in analytical procedures with about 45 different metal ions.27 However, PAN is insoluble in water. Herein, PAN is introduced into the microgel networks, which allows it to directly interact with the metal ions in the aqueous solution. BTB is a sensitive pH indicator so that the minor changes in pH caused by adding various metal ions might result in the color change of BTB-functionalized microgels. As shown in Scheme 1, PAN-functionalized ionic microgels (PAN-MGs) were synthesized via quaternization reaction during the surfactant-free emulsion copolymerization of N-isopropylacrylamide (NIPAm) and 1-vinylimidazole (VIM) in the presence of 1,6-dibromohexane and PAN.28 BTB-functionalized ionic Received: May 5, 2017 Accepted: May 31, 2017 Published: May 31, 2017 20913

DOI: 10.1021/acsami.7b06337 ACS Appl. Mater. Interfaces 2017, 9, 20913−20921

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Synthesis of PAN- and BTB-Functionalized Ionic Microgels via Quaternization Reaction and Anion-Exchange Reaction

Figure 1. Representative TEM images and the corresponding size distributions of (A) PAN-MGs and (B) BTB-MGs. Synthesis of PAN-MGs. PAN-functionalized ionic microgels (PAN-MGs) were prepared as described previously.28 Briefly, NIPAm (0.2264 g, 2 mmol), VIM (27 μL, 0.3 mmol), 1,6dibromhexane (30 μL, 0.2 mmol), and deionized water (45 mL) were added to a three-necked flask. Then, the mixture was heated to 70 °C with vigorous stirring under nitrogen. After 30 min, 5 mL of AIBA aqueous solution (5 mg/mL) was added into the solution to initiate the polymerization. After 1 h, 1 mL of DMF solution of PAN (42 mg, 0.17 mmol) was added dropwise into the reaction flask. The reaction was then kept at 70 °C for 24 h. After polymerization, the obtained suspensions were dialyzed in DMF for 2 days and in deionized water for 1 week. DMF was changed every 6 h and deionized water was changed every day during dialysis. The concentration of PAN-MGs (CPAN‑MG = 4.4 mg/mL) was determined by freeze-drying part of the sample. Synthesis of BTB-MG. BTB-functionalized ionic microgels (BTBMG) were prepared by anion-exchange reaction of normal thermosensitive ionic microgels [i.e., poly(N-isopropylacrylamide-co-

microgels (BTB-MGs) were obtained by exchanging the counteranions (i.e., Br−) of poly(NIPAm-co-VIM)/1,6-dibromohexane ionic microgels (N-MGs) with BTB anions.29 The colloidal properties and swelling porous structures of the functional microgels are responsible for their quick response to metal ions in aqueous solutions, resulting in color change.

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EXPERIMENTAL SECTION

Materials. NIPAm, 1,6-dibromhexane, and BTB were obtained from Tokyo Chemical Industry Co. Ltd. VIM was purchased from Aladdin. 2,2′-Azobis(2-methylpropionamidine)dihydrochloride (AIBA) and PAN were purchased from Sigma-Aldrich. N,NDimethylformamide (DMF), sodium hydroxide, and hydrogen chloride were purchased from Sinopharm Chemical Reagent Co. Ltd. All commercial chemicals were used as received without further purification. 20914

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Figure 2. Hydrodynamic diameters of (A) PAN-MGs and (B) N-MGs and BTB-MGs measured by DLS as a function of measuring temperature. Insets show the corresponding size distributions of PAN-MGs and BTB-MGs at 25 °C by DLS.

Figure 3. UV−vis absorption spectra of (A) PAN-MGs and (B) BTB-MGs at various pH values. (C) Digital images of PAN-MGs and BTB-MGs at different pH values. (D) Hydrodynamic diameters of PAN-MGs and BTB-MGs as a function of pH values. Concentrations of PAN-MGs and BTBMGs were 0.11 and 0.22 mg/mL, respectively. The pH value of the microgel suspensions was adjusted by NaOH or HCl. For colorimetric detection of metal ions, the given amount of metal-ion solution ( 12).27 Although immobilization of PAN into microgels will improve the dispersion stability of PAN in aqueous solutions, it still exhibits relative hydrophobic properties in neutral, weakly acidic, and alkaline solutions but not in strong acids and alkalis. That is why the absorption peak intensities of PAN-MG in weakly acidic and alkaline solutions are obviously lower than those in strong acids and alkalis. For BTB-MG, the absorption peak at 431 nm did not change in acidic solution but gradually weakened and eventually disappeared, and three new absorption peaks at 206, 392, and 618 nm clearly appeared with further increasing pH value to 13, as shown in Figure 3B. BTB acts as a weak acid in solution. It can thus be in protonated or deprotonated form, appearing yellow or blue, respectively, as shown in Figure 3C. The deprotonation of the neutral form results in a highly conjugated structure, accounting for the difference in UV−vis absorption spectra and colors. An intermediate produced in the deprotonation mechanism is responsible for the greenish color in the neutral solution.30 Figure 3D shows that the hydrodynamic diameters of PAN-MGs and BTB-MGs remained unchanged in the pH range of 2−13, as measured by DLS. These results show that in the aforementioned wide range of pH, the colorimetric behaviors and particle sizes of PAN-MGs and BTB-MGs were almost unaffected. On the basis of the results of Figure 3, PAN-MG suspensions with pH values of 2, 7, and 12 and BTB-MG suspensions with a pH value of 7 were chosen to construct the microgel sensor array for the detection of metal ions in aqueous solution. The UV−vis adsorption spectra of PAN-MG and BTB-MG suspensions in the presence of different metal ions were different and dependent of pH, as shown in Figure S2. Previous reports state that the heterocyclic azo dyestuff PAN could complex with metals through the heterocyclic nitrogen atom, the ortho-hydroxyl group, and the azo nitrogen nearest to the phenolic ring, giving two stable, five-membered chelate rings.31 For our system, the heterocyclic nitrogen atom of PAN had quaternized with brominated alkanes so that the PAN-MGs could only chelate with metals through the ortho-hydroxyl group and the azo nitrogen nearest to the phenolic ring of PAN, forming a stable, five-membered ring. Figure S3 shows the possible complex structures of PAN-MGs with metal ions. The UV−vis adsorption spectra showed that the absorption peak positions of PAN-MG suspensions complexed with different metals are obviously different at pHs 2, 7, and 12. It also exhibited different colors for different metal ions at different pH values, as shown in the insets of Figure S2. The type of metal ions could be visually distinguished from the colors of microgel suspensions by the naked eye. Metal ions with different valence electron structures, ionic radii, or unoccupied orbitals could coordinate with azo nitrogen and the ortho-hydroxyl group of PAN, which causes different ligand field transition or charge transfer.32,33 In addition, hydrogen ions or hydroxide ions in acidic or alkaline solutions would have competition or promotion for complexing with metals and ligands. As a result, the positions of the absorption peak and colors of PAN-MG complexed with different metals at different pHs are different. For BTB-MGs at pH 7, the UV−vis adsorption spectra and colors changed after adding Cr3+, Al3+, Fe3+, and Cu2+. The intensity of the characteristic absorption peak of BTB at 431 nm increased, whereas that at 618 nm decreased. As a consequence, the color of BTB-MGs changed from green to

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RESULTS AND DISCUSSION Figure 1 shows the representative TEM images of obtained PAN-MGs and BTB-MGs, with the inset showing the size distribution counted from TEM results. Both microgels were spherical in shape with a narrow size distribution. The average diameters of PAN-MGs and BTB-MGs were about 187 ± 12 and 234 ± 14 nm, respectively. The PAN-MGs and BTB-MGs with PNIPAm segments exhibited thermosensitive character, as shown in Figure 2. The hydrodynamic diameters of PAN-MGs and N-MGs decreased with increasing the measuring temperature from 25 to 60 °C. With increasing solution temperature, PNIPAm became hydrophobic and insoluble in aqueous solution, leading to the shrink and collapse of microgels. However, after anion exchange with BTB, the hydrodynamic diameters of BTB-MGs increased with increasing the measuring temperature above the lower critical solution temperature of PNIPAm segments. The large volume of BTB ions would lead to the instability of microgels and hence the formation of large aggregates at higher temperature. The hydrodynamic diameters of PAN-MGs and BTB-MGs measured by DLS at 25 °C were 408 ± 10 and 326 ± 10 nm, respectively, which were larger than those obtained from TEM images. It was reasonable because the microgels swelled in aqueous solution at low temperatures but collapsed on copper grids after drying. For BTB-MGs, the larger volume of counterions caused the microgels to have a lower swelling ratio, thus leading to a smaller hydrodynamic diameter and a larger dried diameter than PAN-MGs. Insets in Figure 2 show the size distributions of PAN-MGs and BTB-MGs at 25 °C, as measured by DLS, which further confirmed that the obtained microgels had narrow size distributions. The thermosensitive behaviors of both PAN-MGs and BTB-MGs were reversible. Both PANMGs and BTB-MGs exhibited long-term stability in aqueous solutions. After storing for more than 6 months, aggregation or precipitation was not observed in these microgels. PAN-MGs and BTB-MGs exhibited a characteristic adsorption peak at 471 and 431 nm, respectively, in neutral aqueous solutions by UV−vis spectroscopy (Figure S1), indicating the presence of PAN and BTB moieties in the microgels. The PAN and BTB contents of PAN-MGs and BTBMGs were calculated to be approximately 3.5 and 19.6 wt %, respectively. It is important to note that the references were the N-MGs with the same concentration, which did not show any adsorption peak. We first studied the effect of pH on the characteristic adsorption and hydrodynamic diameters of PANMGs and BTB-MGs. The concentrations of PAN-MGs and BTB-MGs were 0.11 and 0.22 mg/mL, respectively. For PANMGs, the absorption peak showed hypsochromic and bathochromic shifts on protonation and ionization in acid and alkali solutions, respectively, as shown in Figure 3A. PAN is insoluble 20916

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Figure 4. Color maps of the colorimetric sensor array after exposure to metal ions (A) and the corresponding difference image (B). The concentrations of PAN-MGs and BTB-MGs were 0.88 and 0.10 mg/mL, respectively. Note that PAN-X and BTB-X are the abbreviations of PANMGs at pH X and BTB-MGs at pH X.

active indication range of BTB is in the pH range of 6.0−7.6, which is the reason why BTB was suitable for detecting the pH change caused by adding metal ions in the present study. However, because of the buffer action of PB solution, the pH of the BTB-MG PB solution changed much less than that of the BTB-MG aqueous solution. Figure S5B shows the UV−vis adsorption spectra of BTB-MG PB solutions (pH 7) in the presence of different metal ions. The characteristic absorption peaks of BTB-MGs changed after adding Cr3+, Al3+, Fe3+, and Cu2+. However, because the PB solution had a buffering effect on pH, resulting in a reduced detection sensitivity of BTBMGs, we chose BTB-MG aqueous solution for the subsequent metal-ion detection. For PAN-MGs, one coordination site of PAN was used for the linkage to the microgel producing a positive charge. This positive charge might reduce the binding affinity of PAN to the metal ions. Figure S6 shows the UV−vis adsorption spectra of PAN DMF solution and PAN-MG aqueous solution by sequential addition of Cu2+ at pH 7. It is important to note that the concentrations of PAN in both solutions were the same. For PAN DMF solution, the absorption peak of PAN at 471 nm gradually weakened and new absorption peaks at 554 nm clearly appeared with increasing concentration of Cu2+. On the contrary, for PAN-MG aqueous solution, the absorption peak of PAN-MG at 471 nm disappeared and two new absorption peaks at 548 and 424 nm gradually increased with increasing concentration of Cu2+. The normalized intensities of the adsorption peak at 554 nm for the PAN DMF solution and the adsorption peak at 548 nm for the PAN-MG aqueous solution are shown in Figure S7A. With the increase of Cu2+ concentration, the PAN DMF solution responded faster and quickly reached saturation with a narrow linear concentration range of 0−7 μM.40 However, a linear dependence of normalized intensity for the adsorption peak at 548 nm on the concentration of Cu2+ was observed up to 19 μM although the sensitivity of the PAN-MG aqueous solution toward Cu2+ was lower than that of the PAN DMF solution. No saturation was observed for the PAN-MG aqueous solution after adding 19 μM Cu2+. Probably, the positive charge of the quaternization site of PAN slows the binding of PAN and metal ions, which might hence increase the saturation concentration of the metal ions. In other words, the PAN-MG aqueous solution has a larger linear detection range of concentration for metal ions than the PAN DMF solution. The effects of concentrations of PAN-MGs on the change of adsorption peaks were also studied in the presence of Cu2+ at pH 7. Figure S7B shows A548nm/A468nm as a function of [Cu2+]/ [PAN] for PAN-MG aqueous solutions with different concentrations. It is important to note that [Cu2+]/[PAN]

yellow, as shown in Figure S2D. BTB could present in two forms: acidic form (yellow), with an absorption peak at 431 nm, and basic form (blue), with an absorption peak at 618 nm (Figure S4), owing to p−p* and n−p* transitions, respectively.34,35 BTB-MGs in a neutral aqueous solution exhibit a color between yellow and blue, appearing green. The addition of Cr3+, Al3+, Fe3+, or Cu2+ resulted in the loss of acid− base balance and acidity of the solution, thereby causing BTB to indicate yellowing. Figure S4A shows the A431nm/A618nm ratios of the corresponding UV−vis adsorption spectra for BTB-MGs in the presence of 9 μM metal ions in Figure S2D. The spectral changes of BTB-MGs with Al3+ and Fe3+ were most obvious, and the absorption peaks of Cr3+ and Cu2+ changed as well. Because BTB is a pH indicator, the pH value of the solution has a significant impact on the color of BTB-functionalized microgels. We then compared the detection sensitivities of metal ions with BTB-MGs in aqueous solution and phosphate buffer solution (PB, 0.01 M) at the same pH value of 7, as shown in Figure S5. Interestingly, the colors of BTB-MGs in aqueous and PB solutions were different (Figure S5A). It was understandable because the chemical environments were different in the two solutions, leading to the different colors of BTB-MGs. The colors of BTB-MG PB solutions were almost unchanged when adding different metal ions at concentrations of 30 μM, except for Cu2+. A slight color change was observed on adding 30 μM Cu2+. In contrast, the BTB-MG aqueous solution had a much higher sensitivity toward adding metal ions such as Cr3+, Al3+, Fe3+, and Cu2+. The pH values of BTB-MGs with 30 μM Cu2+ in aqueous and PB solutions were measured to be 6.05 and 6.84, respectively. Metal ions are Lewis acids and form metal aqua ions in aqueous solution.36 The metal aqua ions undergo hydrolysis, and the aqua cations behave as acids in terms of Brønsted−Lowry acid−base theory.37 The dissociation constant, pKa, for hydrolysis reaction is more or less related to the charge-to-size ratio of the metal ion.38 Ions with large divalent cations, such as Ba2+, Mn2+, and Pb2+, have a pKa of 7.8 or more and would not normally be classed as acids, but small divalent ions and trivalent ions, such as Cu2+, Cr3+, Al3+, and Fe3+, are weak acids, with pKa comparable to that of acetic acid. The pKa values of metal ions recorded in Lange’s Handbook of Chemistry39 and the corresponding pH values measured by a pH meter are listed in Table S1. It is important to note that the pH values were measured by adding 30 μM metal nitrates to deionized water at an initial pH of 7. The smaller the value of pKa, the stronger the acid. Thus, Cu2+, Cr3+, Al3+, and Fe3+ behave as weak acids and cause the acidity of BTB-MG aqueous and PB solutions, leading to the color change of the corresponding solutions. It is also important to note that the 20917

DOI: 10.1021/acsami.7b06337 ACS Appl. Mater. Interfaces 2017, 9, 20913−20921

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Figure 5. (A) DA plots and (B) dendrogram from AHC of the colorimetric sensor array responses to metal ions. The concentrations of PAN-MGs and BTB-MGs were 0.44 and 0.05 mg/mL, respectively.

represented the ratio of molar concentration for Cu2+ to PAN in PAN-MG aqueous solutions. The intensity ratio, A548nm/ A468nm, first increased after adding Cu2+ and reached a plateau value when the value of [Cu2+]/[PAN] reached approximately 3 for all concentrations of PAN-MGs. It meant that three Cu2+ ions could be approximately complexed with one PAN moiety in PAN-MGs. However, for PAN in DMF, saturation occurred at a molar concentration ratio of 1. The microgels contained large amounts of amide and imidazole groups that could coordinate with metal ions so that it required more Cu2+ for PAN moieties to reach the plateau value of A548nm/A468nm. The apparent binding ratio of [Cu2+]/[PAN] is 3 for PAN-MGs. Thus, the quaternization of PAN to microgels will decrease the binding affinity and binding rate of PAN toward metal ions but increase the linear sensing concentration range of metal ions. Furthermore, PAN is insoluble in aqueous solution and will precipitate out of DMF when more than 10% of water was added. The PAN-MGs with quaternized fixation of PAN in the cross-linked network allow the direct detection of metal ions in aqueous solution. The colorimetric microgel sensor array was then constructed and applied to detect and discriminate metal ions. Figure 4 presents the images of the colorimetric microgel sensor array response against various metal ions (each with concentration of 35 μM). The concentrations of PAN-MGs and BTB-MGs are 0.88 and 0.10 mg/mL, respectively. It is worth noting that the sample codes are abbreviated in Figure 4. The number in the sample code represents the corresponding pH value (e.g., PAN2 is the abbreviation of PAN-MGs at pH 2). As seen in Figure 4A, the colorific alteration maps of PAN-MGs and BTB-MGs in the presence of different metal ions differ from each other. It is important to note that the color of metal salt solutions with the concentration studied has no impact on the sensing results. There is no UV−vis adsorption for pure water after adding 9 μM various metal ions. The salt aqueous solutions with a concentration of 35 μM remained transparent. When subtracting from the image of blank microgels, the resulting image offered a clearly noticeable color variation for various metal ions, as shown in Figure 4B. It is worth noting that the difference image was obtained from the RGB images by digitally deducting the color of “blank” microgels from the color “after exposure to metal ions” in red, green, and blue color channels, generating ΔRGB data as follows:26 ΔR = |R m − R b|

ΔG = |Gm − G b|

(2)

ΔB = |Bm − B b|

(3)

where m corresponds to “after exposure to metal ions”; b corresponds to “blank”; and ΔR, ΔG, and ΔB are the color differences. Multivariate analysis was used to quantitatively evaluate the responses of these metal ions.41−43 The output data from DA of the 10 metal ions as well as the blank control sample were plotted with respect to their first two principal factors, as shown in Figure 5A. The concentrations of PAN-MGs, BTB-MGs, and metal ions were 0.44 mg/mL, 0.05 mg/mL, and 10 μM, respectively. The colorimetric responses of the microgel sensor arrays, which were constructed with 0.44 mg/mL PAN-MGs and 0.05 mg/mL BTB-MGs, against various metal ions are shown in Figure S8. The contribution of the first two PCs was 87.6% of the total variance (PC1 = 67.5%; PC2 = 20.1%). The DA results revealed that each metal ion formed tight clusters with a substantial separation between each other. The colorimetric responses to Cu2+ were slightly more distributed compared to those toward other metal ions. Besides, overlapping was found between Ba2+ and the blank control sample. LOOCV analysis was also employed to estimate how accurately the predictive model will perform in practice. LOOCV involves using one observation as the validation set and the remaining observations as the training set. This is repeated on all ways to cut the original sample on a validation set of one observations and a training set. The LOOCV result reveals 99% identification accuracy for the detection of 10 metal ions, only Ba2+ and the blank control sample are easily confused. The AHC shows similar results (Figure 5B); in all of the experimental trials, all metal and blank control samples were accurately identified against one another with no error. These results demonstrated that the colorimetric microgel sensor array successfully discriminated the 10 species of metal ions in aqueous solution. We further studied the concentration dependence of the colorimetric microgel sensor array responding to different metal ions, as shown in Figure 6. The colorimetric microgel sensor array exhibited strong responses with concentrations of 0.1−50, 1−50, and 3−50 μM for detecting Co2+, Zn2+, and Cu2+, respectively. The concentration curves plotted from the DA for other metal ions are listed in Figure S9. The results indicated that the detection limits of the colorimetric microgel sensor array for detecting metal ions were at low micromolar or

(1) 20918

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Figure 6. Difference color maps of the colorimetric microgel sensor array after exposure to Co2+ (A), Zn2+ (C), and Cu2+ (E) with various concentrations. DA plots of the colorimetric microgel sensor array for detecting Co2+ (B), Zn2+ (D), and Cu2+ (F) with various concentrations. Note that PAN-X and BTB-X are the abbreviations of PAN-MGs at pH X and BTB-MGs at pH X.

where k is a factor with the value of 3, Sb is the standard

nanomolar level. This microgel sensor array might have potential application under real conditions. The pH value of targeted solution containing metal ions could be first adjusted to the corresponding pH values of microgel solutions before being subjected to the detection with the microgel sensor array. The spectral detection limits of PAN-MGs and BTB-MGs for metal ions were also investigated and calculated from the intensity ratios in the linear range (as shown in Figures 7 and S10), according to the 3α IUPAC criteria40,44 DL =

kS b m

deviation of the blank, and m is the slope of the calibration graph in the linear range. Sb was found to be 0.056% from six successive blank measurements. The calculated DL values of PAN-MGs and BTB-MGs for metal ions as well as the drinking water standards by the U.S. Environmental Protection Agency (EPA)45,46 are listed in Table 1. The detection limits of most metal ions are far less than the maximum level of metal ions in

(4)

the U.S. EPA standard. 20919

DOI: 10.1021/acsami.7b06337 ACS Appl. Mater. Interfaces 2017, 9, 20913−20921

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Figure 7. (A) UV−vis adsorption spectra of PAN-MG suspensions at pH 2 by sequenced addition of Cu2+. (B) The corresponding intensity ratios (A548nm/A459nm) as a function of Cu2+ concentration.

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Table 1. Detection Limits of PAN-MGs and BTB-MGs for Various Metal Ions in Aqueous Solutions and the Corresponding Drinking Water Regulations from U.S. EPA metal ions Cu

2+

Co2+ Fe3+ Ni2+ Pb2+

Zn2+ Mn2+ Al3+ Cr3+

DL (nM)

detectors PAN-MG PAN-MG PAN-MG PAN-MG PAN-MG PAN-MG BTB-MG PAN-MG PAN-MG PAN-MG PAN-MG BTB-MG PAN-MG PAN-MG PAN-MG BTB-MG BTB-MG

pH pH pH pH pH pH

2 7 12 7 12 7

pH pH pH pH

7 12 7 12

pH 7 pH 12 pH 12

34 16 29 15 80 93 84 112 31 28 210 112 18 21 11 40 24

*E-mail: [email protected]. ORCID

U.S. EPA

Binyang Du: 0000-0002-5693-0325

20 μM

Notes

The authors declare no competing financial interest.

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no limit listed

ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (Nos. 21674097 and 21322406), the second level of 2016 Zhejiang Province 151 Talent Project, Science Foundation of Zhejiang Sci-Tech University (No. 16062194-Y), Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry (201601), Changchun Institute of Applied Chemistry, and Chinese Academy of Sciences for financial support.

6 μM 680 nM 72 nM

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76 nM 909 nM 2−8 μM 2 μM (total)

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(1) Sharma, R. K.; Agrawal, M.; Marshall, F. Heavy Metal Contamination of Soil and Vegetables in Suburban Areas of Varanasi, India. Ecotoxicol. Environ. Saf. 2007, 66, 258−266. (2) Tan, L.; Chen, Z.; Yan, Z.; Wei, X.; Li, Y.; Chi, Z.; Wei, X.; Hu, X. Dual Channel Sensor for Detection and Discrimination of Heavy Metal Ions Based on Colorimetric and Fluorescence Response of the AuNPs-DNA Conjugates. Biosens. Bioelectron. 2016, 85, 414−421. (3) Zhou, X.; Nie, J.; Xu, J.; Du, B. Thermo-Sensitive Ionic Microgels via Post Quaternization Cross-Linking: Fabrication, Property, and Potential Application. Colloid Polym. Sci. 2015, 293, 2101. (4) Liu, L.; Lin, H. Paper-Based Colorimetric Array Test Strip for Selective and Semiquantitative Multi-Ion Analysis: Simultaneous Detection of Hg2+, Ag+, and Cu2+. Anal. Chem. 2014, 86, 8829− 8834. (5) Hou, J.; Dong, J.; Zhu, H.; Teng, X.; Ai, S.; Mang, M. A Simple and Sensitive Fluorescent Sensor for Methyl Parathion Based on LTyrosine Methyl Ester Functionalized Carbon Dots. Biosens. Bioelectron. 2015, 68, 20−26. (6) Yan, X.; Li, H.; Wang, X.; Su, X. A Novel Fluorescence Probing Strategy for the Determination of Parathion-Methyl. Talanta 2015, 131, 88−94. (7) Wang, X.; Yang, Y.; Dong, J.; Bei, F.; Ai, S. LanthanumFunctionalized Gold Nanoparticles for Coordination−Bonding Recognition and Colorimetric Detection of Methyl Parathion with High Sensitivity. Sens. Actuators, B 2014, 204, 119−124. (8) Yan, X.; Li, H.; Yan, Y.; Su, X. Selective Detection of ParathionMethyl Based on Near-Infrared CuInS2 Quantum Dots. Food Chem. 2015, 173, 179−184.

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CONCLUSIONS In summary, a functional ionic microgel sensor array was designed and constructed by using PAN-functionalized ionic microgel suspensions with pH values of 2, 7, and 12 and BTBfunctionalized ionic microgel suspensions with a pH of 7. The microgel sensor array could be applied for colorimetric detection and discrimination of various metal ions in aqueous solution at low micromolar or nanomolar level. Finally, nine species of heavy-metal ion, Ba2+, Cr3+, Mn2+, Pb2+, Fe3+, Co2+, Zn2+, Ni2+, and Cu2+, and one species of light-metal ion, Al3+, were successfully discriminated.

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AUTHOR INFORMATION

Corresponding Author

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06337. Additional UV−vis absorption spectrum, complex structures, DA plots and fitting curves of PAN-MGs and BTB-MGs in the presence of various metal ions at various pH values (PDF) 20920

DOI: 10.1021/acsami.7b06337 ACS Appl. Mater. Interfaces 2017, 9, 20913−20921

Research Article

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DOI: 10.1021/acsami.7b06337 ACS Appl. Mater. Interfaces 2017, 9, 20913−20921