I− assembly as turn-on optical probe for mercury(II) ion

Accepted Manuscript Title: Sulfur-containing, Triphenylamine-based Red-emitting Conjugated Polymer/I− Assembly as Turn-on Optical Probe for Mercury (II) Ion Author: Wei Shi Fudong Ma Zhengfeng Xie PII: DOI: Reference:

S0925-4005(15)00767-4 http://dx.doi.org/doi:10.1016/j.snb.2015.06.008 SNB 18569

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

2-4-2015 29-5-2015 1-6-2015

Please cite this article as: W. Shi, F. Ma, Z. Xie, Sulfur-containing, Triphenylaminebased Red-emitting Conjugated Polymer/Iminus Assembly as Turn-on Optical Probe for Mercury (II) Ion, Sensors and Actuators B: Chemical (2015), http://dx.doi.org/10.1016/j.snb.2015.06.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Sulfur-containing, Triphenylamine-based Red-emitting Conjugated Polymer/IAssembly as Turn-on Optical Probe for Mercury (Ⅱ) Ion

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Wei Shi a*, Fudong Mab, Zhengfeng Xie a*

Oil & Gas Field Applied Chemistry Key Laboratory of Sichuan Province, School of Chemistry

b

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and Chemical Engineering, Southwest Petroleum University, Chengdu, 610500, China

Laboratory of Chemical Biology, College of Chemical Engineering, Xinjiang Agricultural

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University, Urumqi, 830052, China

* corresponding author: Tel: +86 028 83037306; fax: +86 028 83037305

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E-mail address: [email protected], [email protected] (Wei Shi)

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[email protected] (ZF Xie)

Abstract: A type of sulfur-containing, triphenylamine-based red-emitting conjugated

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polymer, PFTD, was selected as optical probing platform for I- and Hg2+. Fluorescence of PFTD solution (in THF) was significantly quenched by the addition of I-, and the apparent color of this probing system turned from pale pink to yellow

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accordingly. Specificity for the detection of I- of PFTD was verified by competing experiments toward other common anionic ions, and the detection limit reaches ~

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2.3×10-7 M (3σ). Owing to the high affinity between I- and Hg2+, absorption and

fluorescence characteristics of PFTD/I- assembly recovered quickly with the presence

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of Hg2+. Common background cationic ions brought slight interference to the

detection of Hg2+. Detection limit of ~1.1×10-7 M (3σ) for Hg2+ can be obtained by

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PFTD/I-, suggests that PFTD can act as dual-channel, high selective, immediacy and

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sensitive optical probe for I- and Hg2+.

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optical probe

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Keywords: triphenylamine; sulfur-containing; conjugated polymer; red-emitting;

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Highlights

A type of red-emitting conjugated polymer was selected as probing platform.

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High affinity between I- and Hg2+ played key role in the detection process.

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Fluorescence of this polymer was specifically quenched by the introduction of I-. Fluorescence turn-on probing of Hg2+ was achieved by polymer/I- assembly.

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The presence of S in this polymer seems didn’t perturb the detection of Hg2+.

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1. Introduction Mercury (
) is a kind of high dangerous ion because of its inherent toxicity, and long-term exposure to high Hg2+ levels can lead to serious and permanent damage to

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the central nervous system and other organs such as heart, kidneys, lungs, etc [1-4]. Over the past decades, there are continual interests in the development of detection

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methods for Hg2+. Among the variety of available probing platforms for the detection

of Hg2+ [5-10], optical probes have drawn special concern due to the advantages they

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have, such as high selectivity, low detection limit, simplicity in sample preparation and real-time applicability, etc [11-13].

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Among these optical probes of Hg2+, conjugated polymers (CPs) based probes

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displayed the unique signal-amplification characteristic [14-17], which origins from the extended conjugation between consecutive repeating units and the inherent

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electron density along polymer backbones [14, 15]. The common designing thought

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of CPs-based Hg2+ probes were based on the strong interaction between Hg2+ and sulfur (S) atoms embedded in polymers’ structures [11, 18-22]. The working manner

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of the majority of these probes were depend on the fluorescence ‘turn-off’ mechanism, that is, the fluorescence of CPs was quenched with the addition of Hg2+ [18-22]. As compared to turn-off probes, fluorescence enhancing (turn-on) probes displayed superiority from the view-point of ease in detection (the optical signals were strengthened during the detection) and anti-interference [23-25]. On the other hand, it is well known that the introduction of S-containing moieties, such as electron-withdrawing benzo[2,1,3]thiadiazole (BT) and electron-donating thiophene groups, into CPs is an efficient and commonly adopted strategy to regulate the optoelectronic properties of CPs [26]. As a result, the construction of sulfur-containing, fluorescence ‘turn-on’ type Hg2+ probe will be a good complement

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for the development of CPs-based, multi-color Hg2+ probe. Recently, our group [27-30] and others [31-34] have reported a series of CPs-based fluorescence turn-off probes for I-. Weak and instant interaction formed

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between these CPs and I-, and the heavy atom effect of I- caused the quenching of CPs’ fluorescence [27-34]. We [27-30] and others [33, 34] further demonstrated that

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the connection between these CPs and I- could be cut off when stronger I- binding agent was introduced into CPs-I- assembly system. High association constant and

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matching ratio between Hg2+ and I- over other metal ions [35] implying that Hg2+ holds the potential to act as a suitable I- abstractor, and the efficient fluorescence

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turn-on detection of Hg2+ has been realized successfully along with this thought

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[27-30, 33, 34]. Despite of the satisfactory Hg2+ detection performance these CPs-Iprobing systems displayed, it is a pity that all of these reported CPs were

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blue-emitting materials [27-30, 33, 34], and there is no involvement of S atoms in

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their backbones. The design of CPs-based, turn-on type Hg2+ probes with longer emission bands by taking advantage of such I--Hg2+reciprocal interaction is still

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remains as a challenging task.

In this study, along with our continual interest of CPs-based optical probes for

Hg2+, a type of sulfur-containing, triphenylamine-based red-emitting CP (PFTD) was

selected as a probing platform to realize the sequential fluorescence turn-off and turn-on detection of I- and Hg2+. The corresponding detection process was systematically described here. 2. Experimental section 2.1 Measurements and Characterization Number-average (Mn) and weight-average (Mw) molecular weights were determined

by Waters GPC 2410 in tetrahydrofuran (THF) using a calibration curve of

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polystyrene standards. UV-visible absorption spectra were recorded on a SHIMADZU UV-2450 UV-vis spectrophotometer. PL spectra were recorded on HITACHIF-4500 spectrophotometer.

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2.2 Materials All reagents, unless otherwise specified, were purchased from Adamas-beta Chemical

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Co. and used without further purification. Tetrahydrofuran (THF) was distilled from sodium in the presence of benzophenone and degassed before use.

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Solutions of F-, Cl-, SO42-, SCN-, H2PO4-, CO32-, NO3-, S2- and I- were prepared from their sodium salts; Br- was prepared from its potassium salt. Solutions of Ag+,

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Al3+, Ba2+, Cu2+, Ca2+, Mg2+, Pb2+, Ni2+, Co2+, Fe3+ and Zn2+ were prepared from their nitrate salts; Hg2+ was prepared from its acetate salt. Concentration of metal solutions

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concentration stocks for next use.

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was controlled at 10-1 M in deionized water and was diluted subsequently to different

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The selected probing polymer,

Poly[(9,9-dioctyl)-2,7-fluorene-co-4-(N,N-dimethylamino)propyloxy)phenyl-4,4'-diph

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-enylamine-co-4,7-dithien- 2-yl-2,1,3-benzothiadiazole)](PFTD) (Scheme 1) was

synthesized according to the reported procedures in previous literature by our lab [36] (the simple description of the preparation and the 1H NMR spectrum of PFTD can be traced in ‘Supporting Information’ ) . PFTD possesses good solubility in common organic solvents, such as CHCl3, CH2Cl2, toluene and THF. Number-averaged molecular weight (Mn) of PFTD was evaluated by gel-permeation chromatography

(GPC) analysis, to be 8900 g/mol and with the polydispersity index (PDI) of 2.40. 3. Results and discussion 3.1 Optical properties of PFTD in dilute solution UV-vis absorption and photoluminescence properties of PFTD were investigated in

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dilute solution (in THF) and corresponding spectra are illustrated in Fig. 1. Two absorption bands, with the absorption maxima at ~ 390 and ~ 540 nm, respectively, appear in the UV-vis curve of PFTD, which can be ascribe to the π-π* transition of

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fluorene-triphenylamine (~ 390 nm) and fluorene-DBT (~ 540 nm) conjugated segments, respectively. In consistent with the finding in UV-vis spectrum, there are

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also two emission bands in the PL curve of PFTD, with the blue and red emission maxima at ~ 445 nm and ~ 625 nm, respectively. This is the reflection of the

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incomplete energy transfer between blue-emitting (fluorene-triphenylamine moiety) and red-emitting (fluorene-DBT moiety) bands in the dilute solution (with

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concentration of ~ 5.0×10-6 M) [36].

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3.2 Optical response of PFTD toward I-

As revealed in recent reports by our group and others, a series of

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arylamine/carbazole-containing conjugated oligomer and polymers displayed distinct

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optical response toward I- [27-33]. In this current case, triphenylamine group is also introduced into the backbone of PFTD. Given the relative low composition ratio of (~35%)

(as

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triphenylamine

compared

to

the

corresponding

values

of

arylamine/carbazole moieties in previously reported materials, which was higher than 50% in all cases [27-33]) and the coexistence of DBT unit in PFTD, an issue lay

before us is that whether PFTD still bears the optical response toward I-. Spectral

response of PFTD (with the concentration of ~ 5.0×10-6 M in THF) toward I- was

investigated thus to explore the above-mentioned issue. Various anionic ions were added into THF solution of PFTD, and corresponding UV-vis spectra are illustrated in Fig. 2. In the presence of fixed concentration of anionic ions, except for I-, absorption curves of PFTD almost retain their initial state in the presence of other common anionic ions. With the introduction of I-, the absorption band at ~ 540 nm is slightly

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affected, indicates that there is no obvious interrelation between fluorene-DBT segment and I-. Distinct from this, with the addition of I-, the intensity of absorption at shorter length (~ 390 nm) increased significantly. Similar phenomena were also

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reported by our group and others about arylamine/carbazole-containing compounds, which might be due to the ‘heavy-atom’ interaction exists between the excited state of

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triphenylamine-containing polymer and I- [30-33].

Detailed alterations of UV-vis spectra of PFTD with incremental I- are placed in

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Fig. S2, upon the addition of I-, absorption band at ~ 390 nm blue-shifted gradually to

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~ 365 nm, accompanied by the succeeding increase of absorption intensity at this band. The apparent color of PFTD solution turned from pale pink to desert tan (Fig.

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3b, upper), suggests that this polymer can act as colorimetric probe for I- by naked eyes.

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Fluorescence response of PFTD toward different anionic ions was investigated

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later and corresponding results are shown in Fig. 3. With the addition of I-, both emission bands at blue (~ 445 nm) and red (~ 625 nm) region were significantly

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quenched (Fig. 3). On the sharp contrast, the presence of other anions endows slight influence toward PL characteristic of PFTD (Fig. 3), suggests the specific interaction

between PFTD and I-. With the help of portable UV lamp (365 nm), such difference in fluorescence is appreciable enough to be distinguished by naked eyes (Fig. 3b,

bottom). The mechanism of fluorescence quenching in this effort might also depend on the ‘heavy-atom’ interaction between the excited state of polymer and I-, leading to

an enhancement of the spin-orbit coupling as discussed in previous reports [27-34]. -

Detailed fluorescence response of PFTD toward I is displayed in Fig. S3.

Fluorescence intensity of PFTD decreased gradually upon the addition of increasing amount of I-. Detection limit of ~2.3×10-7 M for I- (according to 3σ criteria) [37] can

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be obtained by PFTD from the corresponding Stern-Volmer curve (Fig. S3b). The interference of other background anions toward the detection of I- was also investigated and the result is displayed in Fig. S4. The quenching degree of the

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fluorescence of PFTD brought by I- retained at a same level with the presence of all of background anions, suggesting that this polymer has excellent selectivity toward I-.

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3.3 Optical response of PFTD/I- assembly toward Hg2+

According to previous reports by our group and others, the interaction between

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arylamine/carbazole and I- is weak and instant, and this type of interrelation will be lapsed with the further introduction of other stronger binding agents of I- [27-30, 33].

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A variety of cationic metal ions was thus added into PFTD/I- assembly to explore the corresponding optical response behaviors.

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As can be seen from Fig. 4a, with the addition of Hg2+, the absorbance at ~ 365

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nm decreased significantly, and the absorption characteristic of PFTD reversed to its

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pristine state on the whole. The absorption of the long wavelength region (~ 540 nm) seems slightly affected by the introduction of Hg2+. For other metal ions, the alteration

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degree in absorption is much less than that brought by Hg2+. Fig. 4b illustrates the

detailed absorption response of PFTD/I- toward the incremental addition of Hg2+. Upon the titration of Hg2+, the absorption intensity at ~ 365 nm decreased gradually, and the absorption peak red-shift back to ~ 390 nm. The appearance of probing system turned from yellow to pale pink accordingly (Fig. 5b, upper). In the case of photoluminescence, the similar recovery trend was observed when

PFTD/I- assembly was subjected to Hg2+ (Fig. 5). The introduction of other metal ions did not render massive recovery of the fluorescence of PFTD/I-, either at blue (~ 445 nm) or red (~ 625 nm) bands (Fig. 5a). On the sharp contrast, with the introduction of Hg2+, apparent recovery of fluorescence can be detected (Fig. 5a). Such difference of

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metal ions-induced fluorescence alteration is strong enough to be observed by naked eyes with portable UV lamp as exciting light source (365 nm) (Fig. 5b, bottom). Such finding in this study is in good agreement with previous reports about the

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investigation of the optical response of arylmaine/carbazole-based optical materials [27-30, 34]. The strong association between I- and Hg2+ (with association constants of

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8.3×1023 for HgI2 and 6.31×1029 for [HgI4]2-) plays a key role here for the

fluorescence recovery. That is, the addition of Hg2+ distract I- from the PFTD/I-

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complex due to the stronger association strength between them, the heavy atom effect brought by I- was thus ruined and the fluorescence recovered accordingly [27-30, 33,

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34]. It is well known that there is also high affinity between Hg2+ and sulfur, and

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several fluorescence turn-off mode, CPs-based Hg2+ probes [18-22] have been designed by taking advantage of such typical Pearson’s soft acids and bases

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interaction [38]. The result in our recent work about thymine-decorated, fluorene and

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DBT based CPs also revealed that the fluorescence of DBT and DBT-containing polymers will be quenched by the addition of Hg2+ [39]. The apparent fluorescence

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enhancement in this effort reflects that the binding between I- and Hg2+ is so strong

that the fluorescence restored even with the coexistence of S atoms in the backbone of PFTD, this is to say that here the association between I- and Hg2+ precedes that

between S and Hg2+.

As can be seen from Fig. 6a, fluorescence intensity of PFTD/I- increased

gradually with the addition of Hg2+. Detection limit of Hg2+ was evaluated as ~

1.1×10-7 M according to 3σ rule (relationship between I/I0 and Hg2+ was illustrated in Fig. 6b). Although the detection limit in this current work (~1.1×10-7 M) is higher than the maximum level (10 nM) of Hg2+ in drinking water guided by the United States Environmental Protection Agency (EPA) and that (30 nM) permitted by the

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World Health Organization (WHO), this value is comparable to those of previously reported CPs-based Hg2+ probes (the probing performance of representative neutral CPs-based Hg2+ probes has been summarized in Table S1 for facilitate to make

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comparison between our current work and previous reports). Fluorescence of probing system reaches a plateau when the concentration of

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Hg2+ above ~ 1.60×10-4 M. Incomplete fluorescence recovery of PFTD is presumably

due to the synergic effect of the formation of [HgI4]2- complex (which is a kind of

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fluorescence quenching agent [40]) and the coordination between free Hg2+ and S atoms on polymer backbone.

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To evaluate the anti-interference of PFTD/I- toward Hg2+, comparison of fluorescence of PFTD/I- with the presence of common background metal ions and

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with the further addition of Hg2+ were investigated in competition experiments. One

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can find from Fig. 7 that the presence of background metal ions didn’t bring

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significant interference to Hg2+, with the subsequent addition of Hg2+, remarkable fluorescence recovery with close magnitude was found for every metal ion.

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Response speed of PFTD/I- toward Hg2+ was also investigated and the

corresponding result tells us that I/I0 value reached a plateau within 20 seconds,

suggesting the immediacy of PFTD/I- probing system.

4 Conclusions

A type of sulfur-containing, triphenylamine-based red-emitting conjugated polymer, PFTD, was selected to act as the optical probing platform for I- and Hg2+. UV-vis and

PL properties of PFTD altered significantly with the introduction of I-. A strategy to take advantage of the high affinity between Hg2+ and I- was adopted here to realize the turn-on detection of high toxic Hg2+. The presence of sulfur atoms in PFTD didn’t

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render significant unfavorable effects for the detection. Results revealed that such complex probing system has the potential to be used as sensitive, selective and

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anti-interferential optical Hg2+ probe.

Acknowledgements

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Foundation of China (Project No. 21364013, 21262034).

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Authors greatly appreciate the financial support from the National Natural Science

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Figure Captions Fig.1. Normalized UV-vis and PL spectra (excited by 390 nm) of PFTD in THF (~ 5.0×10-6 M).

the presence of common anions (with concentration of ~ 3.5×10-4 M).

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Fig.2. UV-vis absorption spectra of PFTD (with the concentration of ~ 5.0×10-6 M) in

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Fig. 3. PL spectra (excited by 390 nm) (a) of PFTD (~ 5.0×10-6 M) in the presence of common anions (~ 3.5×10-4 M), and the corresponding visual photographs (b) under

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natural light (upper) and ultraviolet light (365 nm, provided by potable UV lamp) (bottom).

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Fig. 4. UV-vis absorption of PFTD/I- in THF (concentrations of PFTD and I- were ~ 5.0×10-6 and 3.5×10-4 M, respectively) in the presence of various metal ions (~

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1.5×10-4 M) (a), and UV-vis alterations of PFTD/I- in THF (concentrations of PFTD

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Hg2+ (b).

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and I- were ~ 5.0×10-6 and 3.5×10-4 M, respectively) in the presence of incremental

Fig. 5. PL spectra (excited by 390 nm) of PFTD/I- in THF (concentrations of PFTD

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and I- were ~ 5.0×10-6 and 3.5×10-4 M, respectively) in the presence of various metal

ions (~ 1.5×10-4 M) (a), and the corresponding visual photographs under natural light (upper) and ultraviolet light (365 nm, provided by potable UV lamp) (bottom). Fig. 6. Fluorescence alterations (excited by 390 nm) of PFTD/I- in THF

(concentrations of PFTD and I- were ~ 5.0×10-6 and 3.5×10-4 M, respectively) in the presence of incremental Hg2+ (a) (the inset is the enlarged region with [Hg2+] in the

range of 0-1.1×10-5 M), and the corresponding relationship between I/I0 and [Hg2+] (b) (inset is the corresponding fitting curve with [Hg2+] lower than 7.04×10-6 M) (values of I0 and I were obtained by integration of corresponding PL curves with the integration region from 400 to 700 nm).

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Fig. 7. Fluorescence alterations of PFTD/I- in THF (concentrations of PFTD and Iwere ~ 5.0×10-6 and 3.5×10-4 M, respectively) in the presence of various background metal ions (~ 1.5×10-4 M for each ion) and with sequential addition of Hg2+ (~

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1.5×10-4 M).

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Scheme 1 The chemical structure of PFTD.

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Biographies

Wei Shi received his PhD degree in 2007 from South China University of Technology.

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Since 2007, he works in Key Laboratory of Petroleum and Gas Fine Chemicals,

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Xinjiang University (Xinjiang, China). Since 2015, he works in Southwest Petroleum

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University (Sichuan, China) as associate professor. His current research interest is the syntheses of functional organic, polymeric and organic-inorganic complex materials

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with optoelectronic properties.

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Fudong Ma obtained his Master degree in 2014 from Xinjiang University (Xinjiang, China). He is now working in Xinjiang Agricultural University (Xinjiang, China). His

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research interests include synthesis of organic fluorescent macromolecules with

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environmentally sensing properties.

Zhengfeng Xie received his PhD degree in 2011 from Xi’an Jiaotong University.

Since 2006, he works in Key Laboratory of Petroleum and Gas Fine Chemicals, Xinjiang University (Xinjiang, China). Since 2015, he works in Southwest Petroleum University (Sichuan, China) as professor. His current research interest is the organic synthesis, industry catalysis and chemical sensors.

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Graphical Abstract (for review)

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Figure1

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Figure 2

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Figure 3a

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Figure 3b

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Figure 4a

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Figure 5a

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Figure 5b

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Figure 6b

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Figure 7

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Scheme 1

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