Fluorescence turn-on detection of cyanide anion based on viologen-quenched water-soluble hyperbranched polymer

Polymer 54 (2013) 1323e1328

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Fluorescence turn-on detection of cyanide anion based on viologenquenched water-soluble hyperbranched polymer Seongwon Seo a, b, Daigeun Kim a, Geunseok Jang a, Jongho Kim a, Taek Seung Lee a, * a

Organic and Optoelectronic Materials Laboratory, Department of Advanced Organic Materials and Textile System Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea b Kolon Plastics Inc., 1018 Ungmyung-dong, Gimcheon, Gyeongbuk 740-180, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 November 2012 Received in revised form 2 January 2013 Accepted 6 January 2013 Available online 11 January 2013

The synthesis of an anionically-functionalized, hyperbranched conjugated polymer with sulfonic acid end groups is presented. Due to the presence of sulfonate groups in the hyperbranched polymer, the electrostatic interaction with viologen-based cation has a major impact on the quenching of the polymer. The electrostatic complex dissociates into individual molecules upon addition of cyanide anions, due to the more favorable interaction between the viologen-based cations and cyanide anions, resulting in the restoration of fluorescence. This change in fluorescence can be employed as a highly selective and sensitive sensing signal for the presence of cyanide anions. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Conjugated polymers Fluorescence Sensors

1. Introduction Fluorescent conjugated polymers with intriguing optical and electrical properties have gained a great deal of attention in the field of chemical and biosensors [1]. The unique p
p* conjugated electronic structure of conjugated polymers enables a rapid transport of excitation energy along the backbone to energy receptors, resulting in considerable amplification of an optical signal through a collective response [2]. These advantages have led to the exploration of conjugated polymers as promising chemo- and biosensor candidates. Thus, great efforts have been made to develop new conjugated polymer-based sensors [3aei]. Conjugated polymers with triphenylamine units have been widely used as donor moieties for probing device as well as semiconducting devices, taking advantage of their unique properties such as high hole mobility [3jem]. Once the correct water-soluble functionalities are introduced into the hydrophobic, rigid conjugated backbone, a great variety of sensing systems for analytes such as saccharides [4], anions [5], amino acids [6], proteins [7], enzymes [8], DNAs [9], and RNAs [10] can be revealed. Water-soluble conjugated polymers, also known as

* Corresponding author. E-mail address: [email protected] (T.S. Lee). URL: http://www.onom.re.kr 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.01.008

conjugated polyelectrolytes, can be prepared with pendant groups of sulfonate, phosphonate, carboxylate, or tertiary ammonium ions [11]. Though the sufficient water-soluble functionalities can be incorporated into the hydrophobic backbones of conjugated polymers to allow their use as biomolecular sensing platforms, it is still challenging to prepare completely water-soluble conjugated polymers [12]. Hyperbranched polymers have attracted considerable attention over the past decade because of their unusual chemical properties [13aec]. Compared with very regular dendritic polymers, hyperbranched polymers are easier to synthesize as well as exhibit comparable properties [13d]. The hyperbranched structure has some application advantages over its linear counterpart, due to its high solubility and good processability. Moreover, it is possible to incorporate desirable functions, such as optoelectronic characteristics, to these hyperbranched architectures by controlling the nature of the pending groups [14]. It is also reported that hyperbranched polymers can be exploited to decrease the strong aggregation tendency of conjugated polymers in aqueous media and, thus, develop highly fluorescent conjugated polymers [15]. The specific detection of anions is an ever-active field due to their crucial roles in industrial, biological, and environmental processes [16]. Among all anions, cyanide is of particular interest as it is highly toxic when absorbed through the lungs, skin, and gastrointestinal tract, leading to vomiting, loss of consciousness, and eventual death [17].

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Recently, fluorescent sensing techniques enabling the determination of cyanide are of tremendous significance, due to their high sensitivity, simplicity, and real-time detection. The cyanide anion is nucleophilic enough to form stable complexes with various molecules and materials, and thus many detection methods using this complex formation have been reported [18]. Herein, we report the design of a new water-soluble hyperbranched conjugated polymer with anionic sulfonate groups, representing an indirect sensing technique for cyanide anions. The hyperbranched polymer has numerous anionic functionalities (sulfonate groups), which determine water-solubility as well as help the formation of electrostatic complex with cationic quencher (based on viologen). The quenched, non-emissive polymer (by complex with cationic quencher) becomes fluorescent upon contact with cyanide anions, which proportionally restore its original fluorescence. This fluorescence “turn-on” principle has shown significant advantages in chemo/biosensing and has found various important applications because of its ease of measuring low-concentrations, due to the “dark” background [19]. Thus it is possible to obtain a highly selective and sensitive cyanide-sensor using cationically charged quencher.

2. Experimental 2.1. Materials 1,4-Butane sultone, benzene-1,4-diboronic acid, 4bromobenzaldehyde, tris(4-bromophenyl)amine, and tetrakis(triphenylphosphine)palladium(0) were purchased from Aldrich. Bromine was purchased from Junsei and hydroquinone was purchased from Samchun. All reagents were used without further purification. 2.2. Instrumentation The 1H NMR and 13C NMR spectra were obtained using a Bruker DRX-300 spectrometer (Korea Basic Science Institute). The UVevis spectra were recorded on a PerkinElmer Lambda 35 spectrometer. The FT-IR spectra were recorded on a Bruker Tensor 27 spectrometer. Elemental analysis was performed on an Elemental Analyzer EA 1108 (Fisons Instruments). The photoluminescence spectra were taken from a Varian Cary Eclipse fluorescence spectrophotometer.

Br O

Br

SO3H

Br

N

O B

+

+

O

Br O

O

Pd(0), K2CO3

O

DMF

B

SO3H Br 1

2 OH HO B KO3S O

KO3S

O

Br O

SO3K

SO3K

O

Br

N

SO3K

O O

SO3K

N

KO3S

N

Br

N O KO3S

O

O

HO B OH KO3S

O Br

SO3K

P1

R

N

N

R R = -(BOH)2 3 -H 4 Scheme 1. Synthesis of hyperbranched polymer P1 and chemical structures of 3 and 4.

S. Seo et al. / Polymer 54 (2013) 1323e1328

Absorbance

200 0.2 100

0.0 300

0 600

400 500 Wavelength (nm)

Fluorecence Intensity (arb. unit)

300

0.4

Fig. 1. Absorption (-) and fluorescence (C) spectra of P1 in aqueous solution. For the absorption spectrum, the concentration of P1 is 1.0  10
5 M. Excitation wavelength corresponds to the absorption maximum.

Fluorescence Intensity (arb. unit)

300

200

100

0

400

500

600

700

Wavelength (nm) Fig. 2. Changes in the fluorescence spectra of P1 upon addition of N,N0 -4,40 -bis(benzyl-4-boronic acid)bipyridinium dibromide (3) in aqueous solution: [P1] ¼ 1.0  10
5 M; [3] ¼ 0 (-); 1.0  10
8 (B); 1.0  10
7 (:); 1.0  10
6 (C); 1.0  10
5 M (6). The fluorescence spectra were obtained using an excitation wavelength lex ¼ 356 nm.

2.3. Hyperbranched polymer synthesis (P1) To a 100 mL round-bottomed flask were added 1 (1 g, 1.85 mmol), 4,4,5,5-tetramethyl-2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) phenyl)-1,3,2-dioxaborolane (0.38 g, 2.31 mmol), tris(4-bromophenyl)

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amine (0.148 g, 0.308 mmol), 2 M aqueous potassium carbonate solution (18 mL) and dry DMF (30 mL) under nitrogen. After addition of tetrakis(triphenylphosphine)palladium(0) (5 mol%) as a catalyst, the mixture was heated to 120  C. The mixture was then stirred for 40 h, after which it was poured into methanol. The precipitate was redissolved in deionized water and the aqueous solution was dialyzed using a membrane of 12.4 kD MWCO (molecular weight cut-off) for 3 days. The polymer was obtained after freeze-drying to yield 0.41 g (40.2%). 1H NMR (300 MHz, D2O): d ¼ 7.5e6.9 (m), 4.1e3.9 (m), 3.0e2.8 (m),1.9e1.8 (m) ppm. 13C NMR (75 MHz, D2O): d ¼ 155.14,150.35,138.15, 131.02, 129.65, 115.57, 70.26, 69.42, 51.12, 28.10, 21.42, 21.31 ppm. FT-IR (KBr, cm
1): 727 (SeO), 1155 (aryl CeN), 1379 (S]O), 1611 (C]C), 2944 (CeH). Anal. Calcd for C179H184N1O59S14: C, 49.58; H, 4.29; N, 0.32; S, 10.87. Found: C, 47.58; H, 4.12; N, 0.32; S, 10.26. 3. Results and discussion The hyperbranched polymer P1 containing triphenylamine and pheneylene units was synthesized by the simple “A2 þ B2 þ C3” type based on the Suzuki cross-coupling polymerization in the presence of palladium as a catalyst. The synthetic procedure for the phenylene monomer 1 with sulfonic acid groups and routes to the polymerization are illustrated in Scheme 1. Monomer 1 was prepared with good yield according to the published methods [3h,6a]. Due to the presence of the trifunctional monomer, the reaction time of polymerization was experimentally determined to avoid gelation or precipitation. Thus, the precise control of the reaction time and the stoichiometry of the difunctional monomers and the tribromo monomer are essential to obtain a water-soluble hyperbranched polymer. The 1H NMR and 13C NMR spectra in D2O, FT-IR spectra, and elemental analysis of P1 confirmed the expected structure. Both NMR and IR spectra showed the presence of phenylene groups as well as alkyl sulfonate groups in the polymer. Gel permeation chromatography (GPC) was carried out to determine the molecular weight for P1 using an aqueous mobile phase, but adsorption of the polymer to the GPC columns was observed and the polymer did not elute, even after a long time [7g,20a]. Matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) was also attempted, but feasible data could not be obtainable. It should be emphasized that the molecular weight of P1 should be more than 12,400 because the polymer was dialyzed for 3 days using a membrane of 12,400 cut-off. As expected, P1 showed good solubility in aqueous solution due to the presence of water-soluble sulfonate groups at the side chain of the hyperbranched polymer. The hyperbranched polymer P1 shows a strong absorption at 349 nm in aqueous solution at a concentration of 1  10
5 M, which corresponds to a p
p* transitions (Fig. 1). The emission maximum

Scheme 2. Working principle of P1-3 complex as a sensor system in response to cyanide anion.

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Fluorescence Intensity (arb. unit)

b

a

200

P1

P1-3

100

0

Fluorescence Intensity (arb. unit)

300

400

500

600

40 -

P1-3 + CN

30

Wavelength (nm)

-

P1-3 + mixed anions + CN

P1-3

20

P1-3 + ClP1-3 + mixed anions P1-3 + each anion

10

0 400

700

500

600

700

Wavelength (nm)

b

300 1

200 P1

P1-4

100

0

c

(I-Io)/Io

Fluorescence Intensity (arb. unit)

a

S. Seo et al. / Polymer 54 (2013) 1323e1328

400

500

600

700

0

CN- Br-

F-

AcO- Cl-

I-

Mix Mix+CN

anions

Wavelength (nm)

Fig. 4. (a) Changes in the fluorescence intensities and (b) relative fluorescence intensities of P1-3 complex upon addition of each anion (Br
, F
, AcO
, Cl
, and I
) and anion mixtures in the presence and absence of cyanide anion. [P1] ¼ 1.0  10
5 M, [3] ¼ 1.0  10
5 M, [anion] ¼ 2.0  10
4 M. I and I0 correspond to the fluorescence intensity of P1-3 complex in the presence and absence of anions at 416 nm, respectively. Excitation wavelength lex ¼ 365 nm.

2.5 2.0

ln (I/I0)

1.5 1.0 0.5 0.0 0

1

2 3 log [CN ] / µM

4

Fig. 3. Changes in the fluorescence spectra of (a) P1-3 complex and (b) P1-4 complex upon addition of cyanide anions in aqueous solution. [P1] ¼ 1.0  10
5 M, for (a) [3] ¼ 1.0  10
5 M, [CN
] ¼ 0 (,); 1.0  10
5 (C); 1.0  10
4 (B); 1  10
3 (:); 1.0  10
2 M (6), for (b) [4] ¼ 1.0  10
5 M, [CN
] ¼ 0 (,); 1.0  10
5 (C); 1.0  10
4 (B); 1.0  10
3 M (6). Excitation wavelength lex ¼ 356 nm. (c) Relative intensity changes where I and I0 correspond to the fluorescence intensity of P1-3 (-), P1-4 (:), or P2-3 (C) complexes in the presence and absence of cyanide anions at 416 nm for P1 and 418 nm for P2, respectively.

of P1 was observed at 415 nm with strong blue emission in aqueous solution. It seems that marked differences in spectral maxima are not observed between P1 and its linear counterpart polymers with similar structures compared to the previous results [20].

It is known that the viologen analogs act as efficient quenchers for conjugated polymers [4a,21]. Two kinds of cationic viologen compounds were synthesized as demonstrated in Scheme 1. The structure of compound 3 features the presence of two boronic acids at both ends of benzyl group, while 4 does not contain such boronic acid groups. Anionically charged P1 (functionalized with sulfonate groups) should exhibit electrostatic interaction with cationically charged molecules. Two viologen-based cations were used in this study, including 3 and 4. The fluorescence quenching studies with different quenchers were carried out in aqueous solution with a P1 concentration of 1  10
5 M. The concentrations of the cationic quenchers varied from 10
8e10
5 M levels. The quenching effect of 3 is illustrated in Fig. 2. It is presumed that electrostatic quenching through the formation of a complex is due to the strong electrostatic interaction between anionic P1 and cationic 3 or 4 quenchers [22]. The quenching constant (KSV) was calculated by the Sterne Volmer relationship:

ðF0 =FÞ ¼ 1 þ KSV ½Q  This provides a quantitative correlation between the loss of fluorescence intensity (F0/F) and the concentration of added quencher [Q]. KSV, the slope of the plot, is the SterneVolmer constant. In the case of P1-3, KSV was found to be 3.94  106 M
1

S. Seo et al. / Polymer 54 (2013) 1323e1328

1327

Fig. 5. Photographs of (a) P1, (b) P1-3 complex, P1-3 complex with (c) F
, (d) Br
-, (e) Cl
, (f) I
, (g) AcO
, (h) CN
, anion mixtures (F
þ Br
þ Cl
þ I
þ AcO
) in the (i) absence of and (j) presence of CN
in aqueous solution under UV irradiation of 365 nm. [P1] ¼ 1.0  10
5 M, [3] ¼ 1.0  10
5 M, [anion] ¼ 2.0  10
4 M.

(Fig. S2). Quencher 4 also induced a quenching of P1 similar to the case of 3 (KSV of 3.58  106 M
1). Because the fluorescence of P1 could be effectively quenched by the addition of a cationically charged species (3 or 4) forming an electrostatic complex, the water-soluble, conjugated polymers with the anionic sulfonate side groups are good candidates for the cation-sensing platforms via turn-off mode [2d,21,23]. Moreover, if the interaction between the cationically charged 3 and anionic P1 was interrupted or 3 did not bind to P1 anymore, the fluorescence of P1 would be restored as shown in Scheme 2. That is, negative P1 binds to positive 3 via electrostatic force and becomes nonfluorescent. By addition of cyanide into P1-3 complex, the P1-3 complex dissociates into individual molecules forming new complex of 3-CN
, resulting in isolated, re-emissive P1. As expected, qualitative analysis showed that the addition of cyanide anions to the P1-3 complex increased the fluorescence of P1 proportionally, indicating dissociation of P1-3 complex by formation of more favorable 3-CN
complex (Fig. 3a). Further increase in the concentration of cyanide anions led to a gradual increase in fluorescence. When the added concentration of cyanide was 1  10
2 M, the fluorescence intensity could be restored to about 50% that of the original intensity of P1. It was not possible to attain 100% of fluorescence recovery to the original intensity of P1, presumably due to the fact that all the viologen cations 3 were not removed from the P1-3 complex. To elucidate the fluorescence “turn-on” mechanism is involved with the more favorable complex formation of boronic acid groups in 3 and cyanide anion, 4, which does not have boronic acid groups, was used as a cation quencher instead of 3. After formation of the non-emissive P1-4 complex, successive additions of cyanide anions into the P1-4 complex were carried out and changes in the fluorescence spectra were recorded. As shown in Fig. 3b, no spectral changes were observed before and after addition of cyanide anions, confirming that the presence of boronic acid groups is essential for this sensing mechanism, which involves the reaction of nucleophilic cyanide and electron-deficient boron atom as shown in the below reaction [24].

R

OH B OH

CN

CN-

B CN R

observed upon exposure to other anions at concentrations of 2  10
4 M, and most anions brought about a further decrease in fluorescence. It is known that the most anions quench the emission of fluorophores [18d] and, thus, most anions investigated here exhibited decrease in the fluorescence of P1-3 complex. The critical strong point of the cyanide-selective P1-3 complex is the negligible interference from other competing anions. Fig. 4 shows the fluorescence changes of the P1-3 complex in the presence of cyanide anions and other anion mixtures such as Br
, F
, AcO
, Cl
, and I
. The restoration of initial fluorescence of P1 was not affected by the presence of other mixed anions, but only by the presence of the cyanide anions. This negligible interference from other anions indicates that the P1-3 complex has remarkable selectivity toward cyanide anions. The limit of detection (LOD) for cyanide anion was determined to be 1.0  10
6 M according to a calculation method of the previously reported method [6deg]. It is reported that the physiological safeguard level of cyanide is 20e 100 mM in water, this system can be utilized in this application [18e]. The recovery of fluorescence of 3-quenched P1 upon exposure to cyanide anions can be observed with the naked-eye as shown in Fig. 5. The remarkable selectivity toward cyanide anions is based on the preferred binding of cyanide anions to boronic acid. This sensing mechanism involving intermolecular interaction would be another challenge of using the hyperbranched polymer P1, which has functional groups (sulfonate groups) to interact with cationically charged 3.

4. Conclusions A water-soluble, hyperbranched polymer was synthesized via Suzuki cross-coupling polymerization method. The ability of viologen-based cation (3) to quench the emission of P1 in aqueous solution and the ability of cyanide anions to bind preferably to 3 and remove 3 from P1-3 complex were exploited to create a simple and effective cyanide-sensor. As expected, the restoration of blueemission color was observed upon addition of cyanide anion into P1-3 complex. The detection limit of cyanide was found to be 1  10
6 M.

CN

This result can be clearly seen in Fig. 3c, indicative of efficient restoration of initial fluorescence of P1 upon exposure to cyanide anion. Thus the results confirmed the proposed mechanism illustrated in Scheme 2. Spectral changes were not observed by simple addition of cyanide anions into the aqueous solution of P1, not P1-3 complex (data not shown here). To evaluate the cyanide anion-selective nature of P1-3 complex, the influence of other anions (Br
, F
, AcO
, Cl
, and I
) on the spectral changes of P1-3 complex was investigated as shown in Fig. 4. Virtual enhancement of fluorescence intensity could not be

Acknowledgments This research was financially supported by research fund of Chungnam National University in 2011.

Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2013.01.008.

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