Functionalized Ionic Microgel Sensor Array for Colorimetric Detection

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Functionalized Ionic Microgel Sensor Array for Colorimetric Detection and Discrimination of Metal Ions Xianjing Zhou, Jingjing Nie, and Bin-Yang Du ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on June 6, 2017

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ACS Applied Materials & Interfaces

Functionalized Ionic Microgel Sensor Array for Colorimetric Detection and Discrimination of Metal Ions Xianjing Zhou, †ξ Jingjing Nie,‡ and Binyang Du †* †

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China ξ

Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, China ‡

Department of Chemistry, Zhejiang University, Hangzhou 310027, China

ABSTRACT A

functional

ionic

microgel

sensor

array

was

developed

by

using

1-(2-pyridinylazo)-2-naphthaleno (PAN) and bromothymol blue (BTB) functionalized ionic microgels, which were designed and synthesized by quaternization reaction and anion exchange reaction, respectively. The PAN-MG and BTB-MG microgels were spherical in shape with narrow size distribution and exhibited characteristic color in aqueous solution with the presence of various trace metal ions, which could be visually distinguished by naked eyes. Such microgels could be used for colorimetric detection of various metal ions in aqueous solution at sub-micromolar level, which were lower than the U. S. EPA standard for the safety limit of metal ions in drinking water. Ten species of metal ions in aqueous solution, i.e. 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

*

Corresponding author. E-mail: [email protected]. 1

ACS Paragon Plus Environment


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INTRODUCTION With the rapid development of mining, machinery manufacturing, chemicals, electronics, and instrumentation industries, waste waters containing heavy metal ions are increasingly discharged into the environment, especially in developing countries.1 The excessive levels of metal will be accumulated in organisms. Owing to the non-biodegradable and high solubility in water, heavy metal ions bring a fatal injury to human health, such as acting as carcinogen, mutagen and teratogen, as well as causing damage to the central nervous system, endocrine system, kidneys, bones, etc.2-4 Therefore, the development of new techniques for simple, low cost, rapid, and sensitive detection of metal ions, especially heavy metal ions, in aqueous solution is a target of great importance. Several analytical methods, including atomic absorption spectrometry, fluorescence,5-6 spectrophotometry,7-8 and inductively coupled plasma spectrometry9 have been so far developed for 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 complicated instrumentation. Colorimetric sensor array system10-13 has emerged as an effective and simple approach toward 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 sensing elements to imitate mammalian olfactory and gustatory systems, 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 work, a functional ionic microgel sensor array was designed and constructed for colorimetric

detection

and

discrimination

of

metal

ions

in

aqueous

solution.

1-(2-Pyridinylazo)-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 2


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water-insoluble.

Herein, PAN is introduced into the microgel networks, which allows it to directly

interact with metal ions in 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 functional ionic microgels (PAN-MG) were synthesized via quaternization reaction during the surfactant free emulsion copolymerization (SFEP) of N-isopropylacrylamide (NIPAm) and 1-vinylimidazole (VIM) with the presence of 1,6-dibromohexane and PAN.28 Whereas, the BTB functional ionic microgels (BTB-MG) were obtained by exchanging the counter anions (i.e. Br-) of poly(NIPAm-co-VIM)/1,6-dibromohexane ionic microgels (N-MG) with BTB anions.29 The colloidal properties and swelling porous structures of the functional microgels allow the quick response to the metal ions in aqueous solutions, resulting in the colorimetric and color change.

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

NN

OS OO

C O NH

BTB BTB-MG

N-MG

NIPAm

N

+

Br

Br

PAN Quaternization

PAN-MG

Br-C6H12-Br

VIM

N

N Br

O

O

Br

N

N x

O

O C NH

Br O S OO

N

N

Br

N

Br O

N N

Anion Exchange

OO S O

+

Br

N

OH

Br

y

N N

OH Br

N

N

N N

N

EXPERIMENTAL SECTION 3



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Materials. N-isopropylacrylamide (NIPAm), 1, 6-dibromhexane and bromothymol blue (BTB) were obtained from Tokyo Chemical Industry Co. LTD. 1-Vinylimidazole (VIM) was purchased from Aladdin.

2,2'-Azobis

(2-methylpropionamidine)

1-(2-pyridinylazo)-2-naphthaleno

(PAN)

were

dihydrochloride

purchased

from

(AIBA) Aldrich-Sigma.

and N,

N-dimethylformamide (DMF), sodium hydroxide, and hydrogen chloride were from Sinopharm Chemical Reagent Co. LTD. All commercial chemicals were used as received without further purification.

Synthesis of PAN Functionalized Ionic Microgels (PAN-MG). The PAN functionalized thermo-sensitive ionic microgels (PAN-MG) were prepared as described previously.28 Briefly, NIPAm (0.2264 g, 2 mmol), VIM (27 µL, 0.3 mmol), 1, 6-dibromhexane (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 mins, 5mL AIBA aqueous solution (5 mg/mL) was added into the solution to initiate the polymerization. One hour later, 1 mL DMF solution of PAN (42 mg, 0.17mmol) was added dropwise into the reaction flask. The reaction was then kept at 70 °C for 24 hours. 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-MG microgels (CPAN-MG = 4.4 mg/mL) was determined by freeze-dry a part of sample.

Synthesis of BTB Functionalized Ionic Microgels (BTB-MG). The BTB functionalized thermo-sensitive ionic microgels (BTB-MG) were prepared by anion exchange

reaction

of

normal

thermo-sensitive

ionic

microgels

[i.e.

poly(N-isopropylacrylamide-co-1-vinylimidazole)/1,6-dibromohexane] with BTB. The normal thermo-sensitive ionic microgels were prepared as described previously.29 Then, BTB (250 mg, 0.4 4


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mmol) and NaOH (4 mg, 0.1 mmol) were added into the suspension of microgels (5 mg/mL, 40 mL) with vigorously stirring for anion exchange reaction. The reaction lasted for 3 hours at room temperature and was purified by several centrifugations and redispersion processes with the supernatant replaced by deionized water. The concentration of BTB-MG microgels (CBTB-MG = 2.3 mg/mL) was determined by freeze-dry a part of sample.

Colorimetric Detection of Metal Ions in Aqueous Solutions. Various concentrations of nitric acid salts, i.e. Ba2+, Cr3+, Al3+, Mn2+, Pb2+, Fe3+, Co2+, Zn2+, Ni2+, and Cu2+ were prepared in deionized water. 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 (less than 20 µL) was added into 2 mL suspensions of PAN-MG or BTB-MG microgels. The color changes of PAN-MG and BTB-MG microgels after adding metal ions were monitored qualitatively by naked-eye and quantitatively by using UV-visible spectrometry. A colorimetric microgel sensor array for detection metal ions was then constructed by spotting suspensions of PAN-MG and BTB-MG microgels into 48-well plate. Given amounts of metal ion solutions were then added into the colorimetric microgel sensor array and imaged using a Canon SLR camera. The images before or after adding metal ions were digitally deducted using Adobe Photoshop with subtract the RGB values of “before” image from “after” image. These RGB values were processed using statistical analysis program Statistical Product and Service Solutions (SPSS) and analyzed by discriminant analysis (AD), leave one-out cross-validation analysis (LOOCV), and agglomerative hierarchical clustering (AHC).

Characterization. Dynamic light scattering (DLS) of PAN-MG and BTB-MG microgels were measured by a 90 Plus Particle Size Analyzer (Brookhaven Instruments Corp.) at scattering angle θ of 90°. The wavelength of laser light λ was 635 nm. Transmission electron microscopy (TEM) measurements 5



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were carried out by a JEOL JEM-1200 electron microscope operated at an acceleration voltage of 60 kV. UV-visible spectra were recorded on a Cary 100 instrument (Varian Australia Pty Ltd.). pH values were measured by pH meter (FE20, Mettler Toledo, Inc.).

RESULTS AND DISCUSSION

Figure 1. Representative TEM images and the corresponding size distributions of (A) PAN-MG and (B) BTB-MG microgels.

Figure 1 shows the representative TEM images of obtained PAN-MG and BTB-MG microgels. Inset shows the size distribution counted from TEM results. Both microgels were spherical in shape with narrow size distribution. The average diameters of PAN-MG and BTB-MG microgels were about 187 ± 12 nm and 234 ± 14 nm, respectively. The PAN-MG and BTB-MG microgels with PNIPAm segments exhibited thermo-sensitive character, as shown in Figure 2. The hydrodynamic diameters of PAN-MG and N-MG microgels (N-MG) decreased with increasing the measuring temperature from 25 oC to 60 oC. With increasing the 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-MG microgels increased with raising the measuring temperature above the lower critical solution temperature 6


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(LCST) 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-MG and BTB-MG microgels measured by DLS at 25 °C were 408 ± 10 nm and 326 ± 10 nm, respectively, which were larger than those results from TEM images. It was reasonable because the microgels swelled in aqueous solution at low temperatures while collapsed on copper grids after drying. For BTB-MG microgels, the larger volume of counter ions caused the microgels to have a lower swelling ratio, thus leading to a smaller hydrodynamic diameter and a larger dried diameter for BTB-MG microgels than those of PAN-MG microgels. The inset of Figure 2 shows the size distribution of PAN-MG and BTB-MG microgels at 25 °C as measured by DLS, which further confirmed that the obtained microgels were with narrow size distributions. The thermo-sensitive behaviors of both PAN-MG and BTB-MG microgels were reversible. Both PAN-MG and BTB-MG microgels exhibited long term stability in aqueous solutions. After storing for more than six months, aggregation or precipitation was not observed for these microgels.

350

Hydrodynamic Diameters (nm)

400

B

100 75 50 25 0

380 400 420 440 Hydrodynamic Diameters (nm)

300 250 200 20

heating cooling 30

40

50

60

800 700 600

BTB-MG heating BTB-MG cooling N-MG

500

Intensity Weighted

450 Intensity Weighted

A Hydrodynamic Diameters (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400 300 200

100 75 50 25 0 285 300 315 330 345 Hydrodynamic Diameters (nm)

100 20

30

o

40

50

60

o

Temperature ( C)

Temperature ( C)

Figure 2. The hydrodynamic diameters of PAN-MG (A), N-MG and BTB-MG (B) microgels measured by DLS as a function of measuring temperature. The inset showed the size distribution of corresponding microgels, i.e. PAN-MG microgels in (A) and BTB-MG microgels in (B), at 25 °C by DLS.

7



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The PAN-MG and BTB-MG microgels exhibited characteristic adsorption peak at 471 nm and 431 nm in neutral aqueous solutions by UV-visible spectroscopy (Figure S1), respectively, indicating the existence of PAN and BTB moieties in microgels. The PAN and BTB contents of PAN-MG and BTB-MG microgels were calculated to be ca. 3.5 w.t.% and 19.6 w.t.%, respectively. Note that the references were the N-MG microgels with the same concentration, which didn’t show any adsorption peak. We first studied the effect of pH on the characteristic adsorption and hydrodynamic diameters of PAN-MG and BTB-MG microgels. The concentrations of PAN-MG and BTB-MG microgels were 0.11 mg/mL and 0.22 mg/mL, respectively. For PAN-MG microgels, 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 in water, dilute acids and alkalis, but can be dissolved in strong acid (pH < 2) and strong alkali (pH > 12).27 Although immobilization of PAN into microgels will improve the dispersion stability of PAN in aqueous solutions, it still exhibits relatively hydrophobic properties in neutral, weakly acidic and alkaline solutions, rather than those in strong acid and alkali. That’s why the absorption peak intensity of PAN-MG in weakly acidic or alkaline solutions is obviously lower than that in strong acid and alkali. For BTB-MG, the absorption peak at 431 nm didn’t change in acidic solution, but gradually weakened and eventually disappeared, and three new absorption peaks at 206 nm, 392 nm, 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-visible absorption spectra and colors. An intermediate of the deprotonation mechanism is responsible for the greenish color in neutral solution.30 Figure 3D shows that the hydrodynamic diameters of PAN-MG and BTB-MG microgels remained unchanged in the range of pH values from 2 to 13 as measured by DLS. These results show that in a wide range of pH values from 2 to 13, the colorimetric behaviors and particle sizes of PAN-MG and BTB-MG microgels were 8


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almost unaffected. Based on the results of Figure 3, PAN-MG microgel suspensions with pH values of 2, 7 and 12 as well as BTB-MG microgel suspensions with pH value of 7 were chosen to construct the microgel sensor array for the detection of metal ions in aqueous solution.

A 1.4

B

4

BTB-MG

PAN-MG pH 2 pH 4 pH 6 pH 8 pH 10 pH 12

1.0 0.8

pH 3 pH 5 pH 7 pH 9 pH 11 pH 13

pH 2 pH 4 pH 6 pH 8 pH 10 pH 12

3

Absorbance (a.u.)

1.2

Absorbance (a.u.)

0.6 0.4 0.2

2

pH 3 pH 5 pH 7 pH 9 pH 11 pH 13

1

0

0.0 300

400

500

600

700

800

300

400

Wavelength (nm)

500

600

700

800

Wavelength (nm)

D Hydrodynamic Diameters (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


800 PAN-MG BTB-MG

600

400

200

0

2

4

6

8

10

12

14

pH

Figure 3. UV-visible absorption spectra of (A) PAN-MG and (B) BTB-MG microgels at various pH values. (C) The photo pictures of PAN-MG and BTB-MG microgels at different pH values. (D) Hydrodynamic diameters of PAN-MG and BTB-MG microgels as a function of pH values. The concentrations of PAN-MG and BTB-MG microgels were 0.11 mg/mL and 0.22 mg/mL, respectively.

The UV-vis adsorption spectra of PAN-MG and BTB-MG microgel suspensions with the presence of different metal ions were different and dependent on the pH values, as shown in Figure S2. Documents recorded 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 9



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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-MG microgels 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-MG microgels with metal ions. The UV-vis adsorption spectra showed that the absorption peak positions of PAN-MG microgel suspensions complexed with different metals are obviously different at pH = 2, 7 and 12, respectively. It also exhibited different colors for different metal ions at different pH values as shown in the inset of Figure S2. The type of metal ions could be visually distinguished from the colors of microgel suspensions by naked eyes. Metal ions with different valence electron structures, ionic radii, or unoccupied orbitals could coordinate with azo nitrogen and ortho-hydroxyl group of PAN, which cause 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 of metals and ligands. As a result, the positions of absorption peak and colors of PAN-MG complexed with different metals at different pH values are different. For BTB-MG microgels at pH 7, the UV-vis adsorption spectra and colors changed after adding Cr3+, Al3+, Fe3+, and Cu2+. The intensity of characteristic absorption peak of BTB at 431 nm increased while that at 618 nm decreased. As a consequence, the color of BTB-MG microgels changed from green to yellow, as shown in Figure S2D. BTB could present in two forms: the acidic form (yellow color) with an absorption peak at 431 nm and the basic form (blue color) with an absorption peak at 618 nm (Figure S4), owing to p - p* and n - p* transitions, respectively.34-35 The BTB-MG microgels in a neutral aqueous solution exhibit the color between yellow and blue, showing green. The addition of Cr3+, Al3+, Fe3+, or Cu2+ resulted in the broken of acid-base balance and the acidity of the solution, thereby causing BTB to indicate yellowing. Figure S4A shows the A431nm/A618nm ratios of corresponding UV-vis adsorption spectra for BTB-MG microgels with the 10


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presence of 9 µM metal ions in Figure S2D. The spectral changes of BTB-MG microgels with Al3+ and Fe3+ were most obvious, besides, the absorption peaks of Cr3+ and Cu2+ also changed. Because BTB is a pH indicator, the pH value of solution has a great impact on the color of BTB functionalized microgels. We then compared the detection sensitivities of metal ions with BTB-MG microgels in aqueous solution and phosphate buffer solution (PB, 0.01M) at the same pH value of 7, as shown in Figure S5. Interestingly, the colors of BTB-MG microgels 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 color of BTB-MG microgels. The colors of BTB-MG microgel PB solutions were almost unchanged when adding 30 µM different metal ions, respectively, except for Cu2+.

Slight color change was caused when adding 30 µM Cu2+. In contrast,

BTB-MG microgel aqueous solution had a much higher sensitivity toward adding metal ions like Cr3+, Al3+, Fe3+, and Cu2+. The pH values of BTB-MG microgels 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 ions 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 whose pKa is comparable to that of acetic acid. Metal ions’ pKa values recorded in Lange's Handbook of Chemistry

39

and the corresponding pH values measured by pH meter were listed in

Table S1. Note that the pH values were measured by adding 30 µM metal nitrates to the deionized water at an initial pH of 7. The smaller the value of pKa is, the stronger the acid. Thus, Cu2+, Cr3+, Al3+ and Fe3+ behave as weak acid and cause the acidity of BTB-MG microgel aqueous and PB solutions, leading to the color change of the corresponding solutions. Note that the active indication range of BTB is in the pH range of 6.0 to 7.6, which is the reason why BTB was suitable for 11



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Page 12 of 25

detecting the pH change caused by adding metal ions in the present work. However, due to the buffer action of PB solution, the pH of BTB-MG microgel PB solution changed much less than that of BTB-MG microgel aqueous solution. Figure S5B shows the UV-vis adsorption spectra of BTB-MG microgel PB solutions (pH 7) with the presence of different metal ions.

The characteristic

absorption peaks of BTB-MG microgels changed after adding Cr3+, Al3+, Fe3+, and Cu2+.

However,

because PB solution had a buffering effect on pH, resulting in a reduced detection sensitivity of BTB-MG microgels, we chose BTB-MG microgel aqueous solution for the subsequent metal ion detection. For PAN-MG microgels, 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 microgel aqueous solution by sequenced addition of Cu2+ at pH 7, respectively. 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 the concentration of Cu2+. Whereas, for PAN-MG microgel aqueous solution, the absorption peak of PAN-MG at 471 nm disappeared and two new absorption peaks at 548 nm and 424 nm gradually increased with increasing the concentration of Cu2+.

The normalized intensities

of the adsorption peak at 554 nm for PAN DMF solution and the adsorption peak at 548 nm for PAN-MG microgel aqueous solution were shown in Figure S7A. With the increase of Cu2+ concentration, PAN DMF solution responded faster and quickly reached saturation with a narrow linear concentration range of 0 - 7µM.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 PAN-MG microgel aqueous solution toward Cu2+ was lower than that of PAN DMF solution.

No saturation was observed for PAN-MG microgel aqueous solution after adding 19 µM

Cu2+. Probably, the positive charge of the quaternization site of PAN slows down the binding of PAN 12


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and metal ions, which might hence increase the saturation concentration of the metal ions. In another word, the PAN-MG microgel aqueous solution has a larger linear detection range of concentration for metal ions than that of PAN DMF solution. The effects of concentrations of PAN-MG microgels on the change of adsorption peaks were also studied with the presence of Cu2+ at pH = 7. Figure S7B shows the A548nm/A468nm ratio as a function of [Cu2+]/[PAN] for PAN-MG microgel aqueous solutions with different concentrations. Note that [Cu2+]/[PAN] represented the ratio of molar concentration for Cu2+ to PAN in PAN-MG microgel aqueous solutions. The intensity ratio of A548nm/A468nm first increased after adding Cu2+ and reached a plateau value when the value of [Cu2+]/[PAN] reached ca. 3 for all concentrations of PAN-MG microgels. It meant that three Cu2+ ions could be approximately complexed with one PAN moiety in PAN-MG microgels. While 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-MG microgels. 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 from DMF when more than 10% of water was added.

The PAN-MG microgels with quaternized fixation of PAN in the crosslinked

network allow the direct detection of metal ions in aqueous solution.

Figure 4. The color maps of the colorimetric sensor array after exposure to metal ions (A) and the corresponding difference image (B). The concentrations of PAN-MG and BTB-MG microgels were 13



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0.88 mg/mL and 0.10 mg/mL, respectively. Note that “PAN-X” and “BTB-X” are the abbreviation of “PAN-MG microgels at pH X” and “BTB-MG microgels at pH X”.

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-MG and BTB-MG microgels are 0.88 mg/mL and 0.10 mg/mL, respectively. Note that the sample codes are abbreviated in Figure 4. The number in the sample code presents the corresponding pH value. (E.g. “PAN-2” is the abbreviation of “PAN-MG microgels at pH 2”.) As seen in Figure 4A, the colorific alteration maps of PAN-MG and BTB-MG microgels in the presence of different metal ions differ from each other. Note that the color of metal salt solutions with the concentration studied has not impact on the sensing results. There is not UV-vis adsorption for pure water after adding 9 µM various metal ions. The salt aqueous solutions with concentration of 35 µM remained transparent. When subtracting to the image of blank microgels, the corresponding difference image offered clearly noticeable color variation for various metal ions, as shown in Figure 4B. Note that the difference image was obtained from the RGB images by digitally deducting the color of “blank” from the color “after exposure to metal ions” in red, green, and blue color channels, generating ∆RGB data as follows:26

∆R = | − | (1) ∆G = | − | (2) ∆B = | − | (3) where m corresponded to “after exposure to metal ions”; b corresponded to “blank”. ∆R, ∆G, ∆B are the color differences.

14


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A

40

Zn

2+

20

PC2 (20.1%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Co Ni

2+

Mn

2+

2+ 2+

0

Cu

Ba Blank

2+

Pb

2+

Fe -20

Cr Al

3+

3+

3+

-40 -100

-80

0

20

40

PC1 (67.5%)

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

Multivariate analysis was employed to quantitatively evaluate responses of these metal ions.41-43 The output data from discriminant analysis (DA) of the ten metal ions as well as the blank control sample were plotted with respect to their first two principal factors, as shown in Figure 5A. Concentrations of PAN-MG microgels, BTB-MG microgels, and metal ions were 0.44 mg/mL, 0.05 mg/mL, and 10 µM, respectively. The colorimetric responses of microgel sensor arrays, which were constructed with 0.44 mg/mL PAN-MG and 0.05 mg/mL BTB-MG microgels, against various metal ions were 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 as compared to other metal ions. Besides, overlapping was found between Ba2+ and blank control sample. Leave one-out cross-validation (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 detection of 10 metal ions, only Ba2+ and 15



blank control sample are easily confused. The agglomerative hierarchical clustering (AHC) shows similar results (Figure 5B), in all of the experimental trials, each 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 ten species of metal ions in aqueous solution. We further studied the concentration dependence of the colorimetric microgel sensor array in responding to different metal ions, as shown in Figure 6. The colorimetric microgel sensor array exhibited strong responses with the concentrations of 0.1 - 50 µM, 1 - 50 µM, and 3 - 50 µM for detecting Co2+, Zn2+, and Cu2+, respectively. The concentration curves plotted from the discriminant analysis for other metal ions were 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 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 it was subjected to the detection with the microgel sensor array. 30

B

2+

Co 15

PC2 (11.3%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 25

0

50

0.1 0

20

1

10

3 5

-15

(µM) -30

-25

0

PC1 (87.2%)

16


25

30

D

2+

Zn 15

PC2 (0.4%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3 5

1

20

0

50

0 0.1 10

-15

(µM) -30

-100

-50

0

50

100

PC1 (99.1%) B

F

2+

Cu

15

5

PC2 (6.1%)

Page 17 of 25

3 0

10 20

0.1 0

50

1

-15

(µM) -25

0

25

PC1 (92.2%)

Figure 6. The difference color maps of the colorimetric microgel sensor array after exposure to Co2+ (A), Zn2+ (C), and Cu2+ (E) with various concentrations. Discriminant analysis 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 abbreviation of “PAN-MG microgels at pH X” and “BTB-MG microgels at pH X”.

The spectral detection limits of PAN-MG and BTB-MG microgels for metal ions were also investigated and calculated from the intensity ratios in the linear range (as shown in Figure 7 and Figure S10), according to the 3α IUPAC criteria:40, 44 =

(4)

where k is a factor with the value of 3, Sb is the standard deviation of the blank and m is the slope of the calibration graph in the linear range. The Sb was 0.056% from six successive blank 17



measurements. The calculated DL of PAN-MG and BTB-MG microgels for metal ions as well as the drinking water standards by the U. S. Environmental Protection Agency (EPA)

45-46

were listed in

Table 1. The detection limits of most metal ions are far less than the maximum level of metal ions in U. S. EPA standard. 0.6

B

A

1.2 2+

0.5

19 µM

0.4 0.3 0.2 0.1

2+

PAN-MG Cu pH 2

0.0 200

300

400

0.8 0.6 0.4

A548nm/A459nm = 0.049 * C + 0.260 2

0 µM 500

PAN-MG Cu pH 2

1.0

A548nm/A459nm

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 25

R = 0.995

0.2 600

700

800

0

4

8

12

16

20

2+

Wavelength (nm)

Concentration of Cu (µM)

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

Table 1. The detection limits of PAN-MG and BTB-MG microgels for various metal ions in aqueous solutions and the corresponding drinking water regulations from U. S. EPA. Metal ions 2+

Cu

Co2+ Fe3+ Ni2+ Pb2+

Detectors

DL (nM)

PAN-MG pH2

34

PAN-MG pH7

16

PAN-MG pH12

29

PAN-MG pH7

15

PAN-MG pH12

80

PAN-MG pH7

93

BTB-MG

84

PAN-MG pH7

112

PAN-MG pH12

31

PAN-MG pH7

28

PAN-MG pH12

210 18


U. S. EPA 20 µM

no limit listed 6 µM 680 nM 72 nM

Page 19 of 25

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Zn2+ Mn2+

BTB-MG

112

PAN-MG pH7

18

PAN-MG pH12

21

76 nM

PAN-MG pH12

11

909 nM

3+

BTB-MG

40

2 - 8 µM

3+

BTB-MG

24

2 µM (total)

Al

Cr

CONCLUSION 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 as well as BTB functionalized ionic microgel suspensions with 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. Nine species of heavy metal ions, i.e. Ba2+, Cr3+, Mn2+, Pb2+, Fe3+, Co2+, Zn2+, Ni2+, and Cu2+, and one species of light metal ion, i.e. Al3+, were successfully discriminated.

ASSOCIATED CONTENT

Supporting Information The additional UV-visible absorption spectrum, complex structures, discriminant analysis plots and fitting curves of PAN-MG and BTB-MG microgels with the presence of various metal ions at various pH values. This information is available free of charge via the Internet at http://pubs.acs.org/.

ACKNOWLEDGEMENTS 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), and Open Research Fund of State Key Laboratory

19



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of Polymer Physics and Chemistry (201601), Changchun Institute of Applied Chemistry, Chinese Academy of Sciences for financial supports.

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TOC graph

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Functionalized Ionic Microgel Sensor Array for Colorimetric Detection

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