Talanta 131 (2015) 678–683

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Conjugated polyelectrolyte-stabilized silver nanoparticles coupled with pyrene derivative for ultrasensitive fluorescent detection of iodide Yi Xiao a, Ye Zhang a, Hongmei Huang a,n, Youyu Zhang a, Beilei Du a, Fang Chen a, Qiao Zheng a, Xiaoxiao He b, Kemin Wang b,nn a Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education), College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, PR China b State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, College of Biology, Hunan University, Changsha 410082, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 10 May 2014 Received in revised form 4 August 2014 Accepted 6 August 2014 Available online 19 August 2014

A new sensing system is developed for selective and ultrasensitive detection of iodide based on the inner filter effect (IFE) between conjugated polyelectrolyte-stabilized silver nanoparticles (P1–AgNPs) and 4-oxo-4-(pyren-1-ylmethoxy) butanoic acid (probe 1). P1–AgNPs are designed to be capable of functioning as a composite light-absorber. Meanwhile, probe 1 is selected as an ideal fluorophore because its emission spectrum can perfectly overlap with the absorption band of P1–AgNPs. The intense fluorescence of probe 1 quenched by P1–AgNPs can be efficiently recovered after adding H2O2 and I
via the oxidation-etching and regeneration of P1–AgNPs. Under the optimum conditions, this facile IFEbased approach allows the sensitive and selective determination of I
in tap water, iodized salt and urine with the detection limit as low as 0.3 nM (S/N ¼3). Interestingly, the regenerated AgNPs is in smaller size and well-dispersed perhaps due to the particular role of P1 as a polymer localizer containing pyridinyl and sulfonate groups. & 2014 Elsevier B.V. All rights reserved.

Keywords: Silver nanoparticles Conjugated polyelectrolyte Pyrene derivative Iodide Fluorescence

1. Introduction In recent years, substantial efforts have been devoted to the design and synthesis of sensors capable of binding and sensing anions because of their pivotal roles in biological, chemical, and environmental processes [1–9]. Among these anions, iodide (I
) is extremely crucial for controlling many metabolic pathways and activities since thyroid hormones, triiodothyronine (T3) and thyroxine (T4) released by the thyroid gland are closely related to the concentrations of I
. Iodide deficiency or excess can result in human diseases and disorders [10]. To date, various methods, e.g., kinetic determination of iodide using the Sandell–Kolthoff reaction [11], optical emission spectrometry (OES) [12], ion chromatography [13], size-exclusion chromatography (SEC) coupled with ICP MS [14], capillary electrophoresis [15], and indirect atomic absorption spectrometry (IAAS) [16], have been used for the determination of iodide in foods, pharmaceutical products and biosamples. Nevertheless, the development of new, ultrasensitive n

Corresponding author. Tel.: þ 86 731 88872576; fax: þ 86 731 88872531. Corresponding author. E-mail addresses: [email protected] (H. Huang), [email protected] (K. Wang). nn

http://dx.doi.org/10.1016/j.talanta.2014.08.025 0039-9140/& 2014 Elsevier B.V. All rights reserved.

analytical approaches for I
(e.g., detection limit o1 nM) without complex pretreatment is highly desirable. Noble metal nanoparticles (e.g., Ag and Au NPs) have emerged as attractive platforms for developing new sensors mainly due to their intrinsic properties such as high extinction coefficient, size-dependent optical properties, and convenient surface modification [17–19]. Compared to conventional analytical methods, the NPs-based sensing techniques can provide enhanced probing sensitivity [20,21]. In addition, fluorescence technique is a powerful tool for the detection of analytes owing to its simplicity, high sensitivity and selectivity, and facile imaging in vivo [22–26]. More recently, Tseng et al. reported fluorescein-5-isothiocyanate-modified gold nanoparticles (FITC–AuNPs) for selective turn-on fluorescence detection of I
[27]. It was also demonstrated by Huang and co-workers that FITC-labeled bovine serum albumin self-adsorbed on fluorescent gold nanoparticles (FITC
BSA
AuNPs) could be employed for the selective sensing of I
[28]. Accordingly, improved sensitivity (limits of detection 10 and 50 nM, respectively) was obtained by the aforementioned NPs-based I
sensors compared with other I
fluorescence sensors reported in literature [25]. Conjugated polyelectrolytes (CPEs) featuring a π-delocalized backbone and ionic pendant groups are among the most important tools for fluorescence detection of small ions, biomolecules

Y. Xiao et al. / Talanta 131 (2015) 678–683

Turn-off

679

Turn-on

(1) H2O2 (2) I -

: P1 : AgNPs

: AgI, I - or I2

SO 3Na

O

NaO 3S

N

: probe 1

SO 3 Na

O

:

O N

n

O

O

n:m=1

m

O

OH O

NaO 3 S

probe 1

P1

Scheme 1. Schematic illustration of the fluorescent iodide detection based on the inner filter effect between P1–AgNPs and probe 1.

and cells [29,30]. However, despite the pioneer work on the determination of DNA and cancerous cells, CPEs-based metal NPs used for the highly sensitive analysis of iodide anion remain relatively unexplored [31,32]. On the other hand, pyrene has been used for the development of fluorescent probe with ultrasensitivity (e.g., detection limit for DNA in the fM range) due to its spatial sensitivity and long fluorescence lifetime [33]. Herein, we firstly design a fluorescence sensor for turn-on iodide detection based on the inner filter effect (IFE) between pyrene derivative and conjugated polyelectrolyte-stabilized silver nanoparticles (P1–AgNPs). The IFE-based approach is considerably more flexible, simple and sensitive when the absorption band of the absorbent dye possesses a perfect spectral overlap with the excitation and/or emission bands of the fluorophore [34,35]. The principle of our method is illustrated in Scheme 1. The fluorescence of probe 1 can be quenched in the presence of P1–AgNPs. When H2O2 and I
are added to the mixed solution of probe 1 and P1–AgNPs, the fluorescence of probe 1 can be restored. This facile IFE-based approach has the following advantages during the determination of I
in real samples: (1) ultrasensitivity with the detection limit as low as 0.3 nM compared to conventional fluorescence-based techniques; (2) requiring no complex pretreatment of sample; (3) good recovery; (4) utilization of relatively inexpensive AgNPs [36]. Furthermore, the regenerated AgNPs is obviously in smaller size, which could be potentially generalized to design welldispersed and highly stable nanoparticles.

2. Experimental 2.1. Materials P1 was synthesized according to our previous report [37]. Probe 1 was obtained according to literature procedures [38]. The chemical structures of P1 and probe 1 were shown in Scheme 1. AgNO3 and other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) and used as received without further purification. Citric acid–disodium hydrogen phosphate buffer solutions (H3Cit–Na2HPO4 pH 3.0–8.0) were prepared with 0.2 M Na2HPO4 and 0.1 M citric acid. The aqueous buffer solutions (pH 9.0 and 10.0) were prepared with 0.1 M sodium carbonate and 0.1 M sodium bicarbonate. The concentrations of the stock solutions of P1 (in deionized water)

and probe 1 (in methanol) are 2.4  10
4 M and 2.0  10
4 M, respectively. Tap water and iodized salt were collected from Hunan Normal University and Hunan Xiangli Salt Chemical Co., Ltd. (Jinshi, China), respectively. Urine samples obtained from three healthy volunteers were filtered through a 0.22 μm membrane and stored at 4 1C. P1-stabilized AgNPs were prepared by a modified method reported in the literature [39]. Briefly, 0.6 mL of 0.02 M AgNO3 and 0.2 mL of 0.24 mM P1 were added into 50 mL of water under stirring, then 0.48 mL of freshly prepared 0.25 M NaBH4 was added rapidly under vigorous stirring. The resulting yellow colloidal silver solution was stirred for 30 min and stored at 4 1C overnight. The size and morphology of the silver nanoparticles were investigated by transmission electron microscopy (TEM). The concentration of as-prepared P1–AgNPs was calculated to be 2.4  10
4 M assuming that all silver in the AgNO3 was reduced. 2.2. Apparatus Fluorescence spectra were obtained on an F-4500 fluorometer (Hitachi Co., Japan). UV–visible absorption spectra were taken on a UV-2450 spectrophotometer (Shimadzu Co., Japan). All pH values were measured using a PHS-3C (Shanghai Pengshun Scientific Instrument Co., China) pH meter with a combined glass–calomel electrode. TEM was performed on a JEM-2010 (JEOL, Japan) operated at 200 kV. Each sample for TEM characterization was prepared by delivering a drop of colloidal solution on carboncoated copper grid and drying at room temperature. The zeta potential of the AgNPs was measured using a Zetasizer 3000HS analyzer (Malvern, UK). 2.3. Detection of iodide A freshly prepared solution containing P1–AgNPs, buffer solution, probe 1 and H2O2 was placed in a quartz cell (10.0 mm width). This mixture was incubated for 1 h before recording the fluorescence spectra at room temperature. After the introduction of KI, this solution was incubated for another 5 min before recording the fluorescence spectra. The concentrations of stock solutions containing I
or other anions are 0.01 M unless otherwise stated.

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Y. Xiao et al. / Talanta 131 (2015) 678–683 1200

3.1. The inner filter effect of P1–AgNPs on probe 1

1000

P1, a novel conjugated polyelectrolyte with specific coordinating pyridyl segments, was chosen as the protective agent for AgNPs. As good electron donors, pyridine moieties in P1 spontaneously coordinate with AgNPs to form a stable complex P1–AgNPs, similar to the previous case of 4,4-bipyridine-stabilized silver nanoparticles [40]. As a new composite nanomaterial, P1–AgNPs presented remarkable stability in aqueous solution for months under ambient conditions. Pyrene derivative can act as a sensitive fluorescent probe [33,41]. Fig. 1 shows the fluorescence emission and excitation spectra of probe 1 and the absorption spectrum of P1–AgNPs. Probe 1 exhibits a maximum emission at 398 nm (Fig. 1b) and aqueous P1–AgNPs (no fluorescence is observed) display an intense absorption at 395 nm (Fig. 1c). It is obvious that the emission spectrum of probe 1 perfectly overlaps with the absorption band of P1–AgNPs, which is critical for the IFE-based fluorescent assay [34,35,42]. Thus the emission light of probe 1 should be easily absorbed by P1–AgNPs, leading to the fluorescence quenching of probe 1 when the two components coexist. It can be seen from Fig. 2 that a gradual decrease in the fluorescence of probe 1 appeared when increasing the concentration of P1–AgNPs. The fluorescence of probe 1 was almost completely quenched as 2.4  10
4 M P1–AgNPs was added. Since both probe 1 and P1–AgNPs (the zeta potential of P1–AgNPs is
22.1 mV) possess negative charges, it may indicate that there is neither complex formation nor energy transfer between them. All these results suggest that the decrease in probe 1 fluorescence intensity could be mainly attributed to the IFE between P1–AgNPs and probe 1. 3.2. Influence of the detection conditions The detection conditions such as pH, concentrations of P1–AgNPs and H2O2 were then optimized for the IFE-based fluorescent assay (see Figs. S1–S3). Waite et al. reported that the interaction between citrate-coated AgNPs and H2O2 is pH-dependent [43]. Wang and coworkers also demonstrated that pH is an important parameter influencing the detection of I
when using colorimetric sensing system [18]. In our sensing system, H2O2 may decompose under alkaline environments and pyridyl moieties of P1 tend to protonate in a strongly acidic environment. As shown in Fig. S1, slightly acidic environments (pH¼4.0–6.0) is favorable for I
sensing. The fluorescence restoration reached a maximum at pH 6.0 after the addition

1.0

0.8

0.8

a

0.6

0.6

0.4

0.4

0.2

b

0.2

c

0.0

Normalized Absorbance

Normalized FL Intensity

1.0

0.0 200

250

300

350

400

450

500

550

600

Fluorescence Intensity / a.u.

3. Results and discussion

a 800

600

K

400

200

0 360

380

400

420

440

460

480

500

Wavelength / nm Fig. 2. Fluorescence emission spectra (λex ¼ 345 nm) of 5.0  10
7 M probe 1 in the presence of various concentrations of P1–AgNPs. Curves a to k: 0, 2.4  10
6, 2.4  10
5, 4.8  10
5, 7.2  10
5, 9.6  10
5, 1.2  10
4, 1.44  10
4, 1.68  10
4, 1.92  10
4 and 2.4  10
4 M in H3Cit–Na2HPO4 buffer solution (pH 6.0).

of I
. The pH used in the following experiments was therefore set at 6.0. The relative amount of P1–AgNPs in the detection system also affected the fluorescence response toward I
. As shown in Fig. S2, 2.28  10
4 M P1–AgNPs gave the best response when the sensitivity and linear range were taken into account. As shown in Fig. S3, the higher concentration of H2O2 would make the oxidation-etching of P1–AgNPs more efficient, which could be important for the fluorescence restoration of probe 1. Thus, the concentration of H2O2 used in the following experiments was 5.0  10
5 M. 3.3. Fluorescence restoration of probe 1 by I
Under the above optimum conditions of pH ¼ 6.0, [P1–AgNPs] ¼ 2.28  10
4 M and [H2O2] ¼5.0  10
5 M, the fluorescence detection of I
was carried out. As shown in Fig. 3, the fluorescence intensity of probe 1 increases obviously at low concentration of I
, and it increases steadily at high I
concentration. Two linear curves were obtained when measured in the range of 5.0  10
10– 1.0  10
5 M (Ir/Iq ¼1.863 þ0.082 log [I
], correlation coefficient R¼0.991) and 2.0  10
5–9.0  10
5 M (Ir/Iq ¼6.265 þ1.029 log [I
], correlation coefficient R¼ 0.979). Ir and Iq represent the fluorescence intensity of P1–AgNPs/probe 1 system in the presence and absence of I
at 398 nm, respectively. The full curve in Fig. 3B displays a sigmoidal progression, thereby indicating that multiple equilibria are involved in the process [44]. Since Ag þ can be produced from the oxidation of AgNPs in the presence of H2O2, the former equilibrium presumably involves the formation of AgI due to its low Ksp (8.5  10
17) [45] and the latter equilibrium may involve the AgI-promoted formation of I2 [18]. Notably, the limit of detection (LOD) of the above IFE-based approach is obtained as low as 0.3 nM at S/N ¼3. These results show that this method has excellent sensitivity, high correlation and a broad range for the quantitative analysis of I
. Representative iodide measurement methods, including widely practiced Sandell–Kolthoff reaction and novel Ag or AuNPs-based fluorescence sensing, are summarized in Table 1. 3.4. Selectivity of the sensing system

Wavelength / nm Fig. 1. Fluorescence excitation (a) and emission (b) spectra of probe 1 and absorption spectrum (c) of P1–AgNPs. [probe 1] ¼5.0  10
7 M, [P1–AgNPs] ¼ 2.4  10
4 M, [P1] ¼9.6  10
7 M.

To evaluate the selectivity of the P1–AgNPs/probe 1-based IFE system for I
sensing, different aqueous anions such as Br
, Cl
, F
, IO3
, HPO4 2
, H2 PO4
, PO4 3
, SO3 2
, CO3 2
, EDTA2
,

Y. Xiao et al. / Talanta 131 (2015) 678–683

C2 O4 2
and NO3
were also investigated. As exhibited in Fig. 4, the addition of 2.5  10
5 M I
to the P1–AgNPs/probe 1 sensing system resulted in a dramatic increase in fluorescence intensity. In comparison, when 2.5  10
5 M of each anion mentioned above was added,

681

only a slight change in the fluorescence intensity of probe 1 could be observed, thereby indicating that the IFE system provides good selectivity for I
over various other anions. 3.5. Analysis of I
in real sample

130

The good selectivity and ultrasensitivity of the P1–AgNPs/ probe 1-based IFE system suggest that the novel method might be directly applied to determine I
in real samples. Iodide levels in tap water, iodized salt and urine samples were measured according to the experimental procedure. Particularly, the determination of urinary I
concentration is significant for the diagnosis of iodine deficiency since 90% of total iodine in urine is present as iodide [46]. The results are shown in Table 2, which exhibits excellent recovery of  98% (average value). The recovery values were between 91% and 108% in the presence of interfering species (e.g., 50- and 100-fold Cl
), indicating that the proposed method can be simple and practical for the sensing of iodide.

120

Fluorescence Intensity / a.u.

110 100 90

+I

80 70 60 50 40 30 20

3.6. Working mechanism for I
sensing

10 0 360

380

400

420

440

460

480

500

520

Wavelength / nm

2.2 2.0

P1–AgNPs functioning as a novel composite absorber can be readily tuned after the addition of H2O2 and I
due to the fact that H2O2 and I
may lead to the P1–AgNPs oxidation-etching and regeneration, respectively. Thus the optical property of P1–AgNPs will be significantly changed and the IFE between P1–AgNPs and probe 1 will be disrupted. UV–vis spectroscopic measurements and TEM analysis were performed to obtain insights into the 0.7

Ir/Iq

1.8

0.6

1.6

0.5

( Ir-Iq) / Iq

1.4 1.2 1.0 -10

-9

-8

-7

-6

-5

-4

0.4 0.3 0.2

-3 0.1

log[I-] Fig. 3. (A) Fluorescence emission spectra of the P1–AgNPs/probe 1 system with different concentrations of I
. Down to up: 0, 5.0  10
10, 1.0  10
9, 6.0  10
9, 1.0  10
8, 6.0  10
8, 1.0  10
7, 6.0  10
7, 1.0  10
6, 6.0  10
6, 1.0  10
5, 2.0  10
5, 3.0  10
5, 4.0  10
5, 5.0  10
5, 6.0  10
5, 7.0  10
5, 8.0  10
5 and 9.0  10
5 M. (B) Fluorescence recovery of P1–AgNPs/probe 1 system with various concentrations of I
. Ir and Iq are the fluorescence intensity with and without I
. [probe 1] ¼ 5.0  10
7 M, [P1–AgNPs] ¼2.28  10
4 M, [H2O2] ¼ 5.0  10
5 M, pH¼ 6.0. Excitation and emission wavelengths of probe 1 were 345 nm and 398 nm, respectively.

0.0 -0.1

1

2

3

4

5

6

7

8

9

10

11

12

13

Fig. 4. The Fluorescence response of the P1–AgNPs/probe 1 system containing 5.0  10
7 M probe 1, 5.0  10
5 M H2O2 and 2.28  10
4 M P1–AgNPs in the presence of 2.5  10
5 M I
or other anions at pH 6.0: (1) I
, (2) Br
, (3) Cl
, (4) F
, (5) IO3
, (6) HPO4 2
, (7) H2 PO4
, (8) PO4 3
, (9) SO3 2
, (10) CO3 2
, (11) EDTA2
, (12) C2 O4 2
, and (13) NO3
.

Table 1 Comparison of different iodide measurement methods. Ref.

Detection method

System

Linear range (nM)

LOD (nM)

[11] [12] [13] [14] [15] [16] [20] [25] [28] [27] This work

Sandell–Kolthoff reaction Optical emission spectrometry (OES) Ion chromatography ICP MS Capillary electrophoresis (CE) IAAS Raman spectroscopy Fluorescence sensor Fluorescence nanosensor Fluorescence detection Fluorescence detection

CHIP-MSFIA DBD–OES Concentrator column SEC-ICP MS tITP-CE AgI–CN
Rh6G-adsorbed AuNPs Oligopyrrole derivative FITC–BSA–AuNPs FITC–AuNPs P1–AgNPs

37–591 788–7.88  104 1.6–788 – 0–315 0–78.8 0.2–47.3 100–6.0  103 1–1.0  103 10.0–600.0 0.5–1.0  104

37 236 0.8 7.9 1.6 9.5 0.2 90 50 10.0 0.3

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Y. Xiao et al. / Talanta 131 (2015) 678–683

Table 2 Detection of iodide in real sample. All measurements were performed in 2.0 mL PBS, [probe 1] ¼5.0  10
7 M, [P1–AgNPs] ¼2.28  10
4 M, pH¼ 6.0. 20.0 μL of urine or water sample was added into PBS, [Cl
] of iodized salt sample in 2.0 mL PBS was 1.0  10
6 M. Sample

Spiked (nM)

Detected (nM)

Recovery (%)

Tap water

0 10 20

not detected 9.8 20.2

– 98 101

Iodized salt

0 10 20

not detected 10.0 19.2

– 100 96

Urine (child)

0 10 20

6.7 17.5 26.1

– 108 97

Urine (man)

0 10 20

9.1 18.3 29.5

– 92 102

Urine (woman)

0 10 20

11.8 21.3 30.0

– 95 91

3.0

a

2.5

Absorbance / a.u.

b 2.0

c 1.5

d

1.0 0.5 0.0 200

300

400

500

600

Wavelength / nm Fig. 5. Absorption spectra of P1–AgNPs in the presence of probe 1 (a); P1–AgNPs in the presence of H2O2 (5.0  10
5 M) and probe 1 (b); P1–AgNP in the presence of H2O2 (5.0  10
5 M), probe 1 and I
(5.0  10
5 M) (c); P1–AgNP in the presence of H2O2 (5.0  10
5 M), probe 1 and I
(5.0  10
4 M) (d). [probe 1]¼ 5.0  10
7 M, [P1–AgNPs]¼2.28  10
4 M, pH¼6.0.

mechanism of the P1–AgNPs-based I
sensor. As presented in Fig. 5, the absorbance of P1–AgNPs is found to significantly decrease with increasing concentration of I
and an obvious red shift (  12 nm, b-d) in the spectra can be observed, which could be ascribed to the AgI, I
or I2 absorbed on the surface of P1–AgNPs in the presence of iodide [18,47]. As a result, the IFE between P1–AgNPs and probe 1 will be damaged. In addition, Fig. 6 reveals the typical process involving P1–AgNPs oxidationetching and regeneration. As judged from about 150 individual particles for each sample, the mean diameters of AgNPs were 17 nm (Fig. 6A), 7 nm (Fig. 6B) and 4 nm (Fig. 6C), respectively. Importantly, the AgNPs can be regenerated with a smaller diameter (Fig. 6A and C) after the addition of H2O2 and I
via the P1–AgNPs oxidation-etching (Fig. 6B), which could be attribute to the particular role of P1 as a polymer localizer for AgNPs, similar to the previous case of poly(o-methoxyaniline) in stabilizing silver nanoparticles by coordination through the nitrogen atoms to AgNPs [48]. In the case of citrate-stabilized AgNPs, however, AgNPs aggregated after interacted with iodide in the presence of

Fig. 6. TEM images of P1–AgNPs (A) and P1–AgNPs before (B) and after (C) adding I
(5.0  10
5 M) in the presence of probe 1 and H2O2 (5.0  10
5 M) at pH¼6.0.

H2O2 since the absorption of AgI could neutralize the negative charges of citrate on the surface of AgNPs (Fig. S4) [18]. Thus, as a new conjugated polyelectrolyte containing pyridinyl and sulfonate groups, P1 demonstrated the novel role of CPE in the design of well-dispersed and highly stable nanoparticles.

4. Conclusions In summary, we have demonstrated a new and facile method for sensitive and selective detection of iodide anion based on the inner filter effect between P1–AgNPs and probe 1 due to their

Y. Xiao et al. / Talanta 131 (2015) 678–683

perfectly spectral overlap. The fluorescence of probe 1 quenched by P1–AgNPs can be efficiently recovered after the addition of H2O2 and I
. The oxidation-etching and regeneration of P1–AgNPs can be verified by UV–vis and TEM analysis, which providing not only a promising method for the design of well-dispersed and highly stable nanoparticles but also an insight into the new role of conjugated polyelectrolyte. In addition, new CPE could endow novel sensory system with superior performance (i.e., regeneration of nanoparticles), which implies repeated sensing of iodide might be achieved. Under the proper concentration of H2O2 and P1–AgNPs at pH 6.0, the detection limit for I
is as low as 0.3 nM. Acknowledgments Financial support for this research from the National Natural Science Foundation of China (20805014), Natural Science Foundation of Hunan Province (09JJ3018), the Scientific Research Fund of Hunan Provincial Education Department (11C0809 and 12B077) and Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, is gratefully acknowledged. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.talanta.2014.08. 025. References [1] V. Amendola, L. Fabbrizzi, M. Licchelli, A. Taglietti, in: K. Bowman-James, A. Bianchi, E. García-Españ a (Eds.), Anion Coordination Chemistry, Wiley-VCH, Weinheim, 2012, pp. 521–552. [2] V.K. Gupta, A.K. Singh, P. Singh, A. Upadhyay, Sens. Actuators B Chem. 199 (2014) 201–209. [3] V.K. Gupta, A.K. Singh, S. Bhardwaj, K.R. Bandi, Sens. Actuators B Chem. 197 (2014) 264–273. [4] M.R. Ganjali, V.K. Gupta, M. Hosseini, Z. Rafiei-Sarmazdeh, F. Faridbod, H. Goldooz, A.R. Badiei, P. Norouzi, Talanta 88 (2012) 684–688. [5] V.K. Gupta, A.K. Jain, M.K. Pal, A.K. Bharti, Electrochim. Acta 80 (2012) 316–325. [6] M. Hosseini, V.K. Gupta, M.R. Ganjali, Z. Rafiei-Sarmazdeh, F. Faridbod, H. Goldooz, A.R. Badiei, P. Norouzi, Anal. Chem. Acta 715 (2012) 80–85. [7] V.K. Gupta, L.P. Singh, S. Chandra, S. Kumar, R. Singh, B. Sethi, Talanta 85 (2011) 970–974. [8] A.K. Jain, V.K. Gupta, L.P. Singh, P. Srivastava, J.R. Raisoni, Talanta 65 (2005) 716–721. [9] R. Prasad, V.K. Gupta, A. Kumar, Anal. Chem. Acta 508 (2004) 61–70. [10] P. Pienpinijtham, X.X. Han, S. Ekgasit, Y. Ozaki, Anal. Chem. 83 (2011) 3655–3662.

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[11] F.Z. Abouhiat, C. Henríquez, B. Horstkotte, F.E. Yousfi, V. Cerdà, Talanta 108 (2013) 92–102. [12] Y.L. Yu, S. Dou, M.L. Chen, J.H. Wang, Analyst 138 (2013) 1719–1725. [13] Y. Bichsel, U. von Gunten, Anal. Chem. 71 (1999) 34–38. [14] L.F. Sanchez, J. Szpunar, J. Anal. At. Spectrom. 14 (1999) 1697–1702. [15] K. Ito, T. Ichihara, H. Zhuo, K. Kumamoto, A.R. Timerbaev, T. Hirokawa, Anal. Chim. Acta 497 (2003) 67–74. [16] P. Bermejo-Barrera, L.M. Fernández-Sánchez, M. Aboal-Somoza, R.M. AnlloSendín, A. Bermejo-Barrera, Microchem. J. 69 (2001) 205–211. [17] J. Ai, Y. Xu, B. Lou, D. Li, E. Wang, Talanta 118 (2014) 54–60. [18] G.L. Wang, X.Y. Zhu, Y.M. Dong, H.J. Jiao, X.M. Wu, Z.J. Li, Talanta 107 (2013) 146–153. [19] B. Yang, X.B. Zhang, W.N. Liu, R. Hu, W. Tan, G.L. Shen, R.Q. Yu, Biosens. Bioelectron. 48 (2013) 1–5. [20] S.S.R. Dasary, P.C. Ray, A.K. Singh, T. Arbneshi, H. Yu, D. Senapati, Analyst 138 (2013) 1195–1203. [21] P.K. Jain, X.H. Huang, I.H. EL-Sayed, M.A. EL-Sayed, Acc. Chem. Res. 41 (2008) 1578–1586. [22] K. Wang, X. He, X. Yang, H. Shi, Acc. Chem. Res. 46 (2013) 1367–1376. [23] P. Zhu, S. Huang, M. Li, N. Ding, B. Peng, L. Kong, Y. Bo, J. Fluoresc. 21 (2011) 179–186. [24] S. Lin, M.T. Morris, P.C. Ackroyd, J.C. Morris, K.A. Christensen, Biochemistry 52 (2013) 3629–3637. [25] S. Nabavi, N. Alizadeh, Sens. Actuators B Chem. 200 (2014) 76–82. [26] J.N. Zhao, J.H. Deng, Y.H. Yi, H.T. Li, Y.Y. Zhang, S.Z. Yao, Talanta 125 (2014) 372–377. [27] Y.M. Chen, T.L. Cheng, W.L. Tseng, Analyst 134 (2009) 2106–2112. [28] S.C. Wei, P.H. Hsu, Y.F. Lee, Y.W. Lin, C.C. Huang, ACS Appl. Mater. Interfaces 4 (2012) 2652–2658. [29] S.W. Thomas III, G.D. Joly, T.M. Swager, Chem. Rev. 107 (2007) 1339–1386. [30] B. Liu, G.C. Bazan (Eds.), Conjugated Polyelectrolytes: Fundamentals and Applications, Wiley-VCH, Weinheim, 2013. [31] Y. Wang, B. Liu, A. Mikhailovsky, G.C. Bazan, Adv. Mater. 22 (2010) 656–659. [32] A. Bajaj, O.R. Miranda, I.B. Kim, R.L. Phillips, D.J. Jerry, U.H.F. Bunz, V.M. Rotello, Proc. Natl. Acad. Sci. U. S. A. 106 (2009) 10912–10916. [33] J. Huang, Y. Wu, Y. Chen, Z. Zhu, X. Yang, C.J. Yang, K. Wang, W. Tan, Angew. Chem. Int. Ed. 50 (2011) 401–404. [34] L. Shang, S.J. Dong, Anal. Chem. 81 (2009) 1465–1470. [35] L. Xu, B.X. Li, Y. Jin, Talanta 84 (2011) 558–564. [36] A. Desireddy, B.E. Conn, J.S. Guo, B. Yoon, R.N. Barnett, B.M. Monahan, K. Kirschbaum, W.P. Griffith, R.L. Whetten, U. Landman, T.P. Bigioni, Nature 501 (2013) 399–402. [37] Y.Q. Hu, Y. Xiao, H.M. Huang, D.L. Yin, X.M. Xiao, W.H. Tan, Chem. Asian J. 6 (2011) 1500–1504. [38] Y.J. Li, H. Li, Y.L. Li, H.B. Liu, S. Wang, X.R. He, N. Wang, D.B. Zhu, Org. Lett. 7 (2005) 4835–4838. [39] L. Shang, C.J. Qin, T. Wang, M. Wang, L.X. Wang, S.J. Dong, J. Phys. Chem. C 111 (2007) 13414–13417. [40] H.B. Li, F.Y. Li, C.P. Han, Z.M. Cui, G.Y. Xie, A.Q. Zhang, Sens. Actuators B Chem. 145 (2010) 194–199. [41] S. Nagatoishi, T. Nojima, B. Juskowiak, S. Takenaka, Angew. Chem. Int. Ed. 44 (2005) 5067–5070. [42] Y.L. Tang, Y. Liu, A. Cao, Anal. Chem. 85 (2013) 825–830. [43] D. He, S. Garg, T.D. Waite, Langmuir 28 (2012) 10266–10275. [44] D. Aldakov, M.A. Palacios, P. Anzenbacher Jr., Chem. Mater. 17 (2005) 5238–5241. [45] M. Luo, X. Hou, C. He, Q. Liu, Y. Fan, Anal. Chem. 85 (2013) 3715–3722. [46] Y.J. Li, Y.T. Tseng, B. Unnikrishnan, C.C. Huang, ACS Appl. Mater. Interfaces 5 (2013) 9161–9166. [47] P. Mulvaney, Langmuir 12 (1996) 788–800. [48] A. Dawn, P. Mukherjee, A.K. Nandi, Langmuir 23 (2007) 5231–5237.