A turn-on fluorescent probe for Hg2+ detection by using gold nanoparticle-based hybrid microgels

Sensors and Actuators B 228 (2016) 767–773

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

A turn-on fluorescent probe for Hg2+ detection by using gold nanoparticle-based hybrid microgels Yecang Tang ∗ , Yi Ding, Ting Wu, Liying Lv, Zhicheng Yan College of Chemistry and Materials Science, Anhui Normal University, The Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecule-Based Materials, Wuhu 241000, China

a r t i c l e

a b s t r a c t

i n f o

Article history: Received 15 October 2015 Received in revised form 20 January 2016 Accepted 23 January 2016 Available online 27 January 2016 Keywords: Fluorescence turn-on Mercury detection Hybrid microgels Gold nanoparticle

Hybrid microgels with Au nanoparticles (AuNPs) immobilized in poly(N-isopropylacrylamideco-2-(dimethylamino)ethylmethacrylate) (P(NIPAM-co-DMA)) microgels have been prepared. The as-synthesized hybrid microgels showed excellent thermo- and pH-responsive properties and high stabilities. 1-pyrenebutyric acid (PBA) molecules were then adsorbed on the AuNP surfaces via electrostatic interactions to form PBA-AuNPs/P(NIPAM-co-DMA) composites that were used as probes for the detection of Hg2+ . The fluorescence of PBA was effectively quenched through fluorescence resonance energy transfer between AuNPs and PBA molecules. However, the addition of Hg2+ ions caused the fluorescence turn-on dramatically and 16 times fluorescence enhancement was obtained, which is attributed the fact that Hg2+ ions replace the adsorbed PBA molecules. Under the optimal conditions, the fluorescence intensity was proportional to the concentration of Hg2+ ions in the range of 0.16–1.60 ␮M with a detection limit of 31 nM. Meanwhile, some other metal ions did not interfere with the detection of Hg2+ , suggesting a good selectivity for Hg2+ ions sensing. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Mercury, a well-known highly toxic heavy metal, possesses serious threats to human health and the environment. Long-term exposure to mercury, even at very low level concentrations, can cause serious and permanent damage to the central nervous and endocrine system [1–3]. One of the most common and stable forms of mercury pollution is solvated Hg2+ ions. Therefore, the development of highly sensitive and selective methods for detecting Hg2+ ions is of great significance. To achieve that, various methods are currently available for the measurement of Hg2+ , such as atomic absorption/emission spectroscopy [4,5], inductively coupled plasma mass spectrometry [6], electrochemistry [7–9], colorimetry [2,10,11], fluorescence spectroscopy [12–22] and so on. Among them, the fluorescence spectroscopy offers significant advantages over other detection methods because of its high sensitivity, simplicity, inexpensiveness, and in situ detection capability [23]. To date, a number of fluorescent probes for Hg2+ ions have been reported and can be categorized into two types by fluorescence enhancement or quenching. The first type is the turn-on probe,

∗ Corresponding author. Fax: +86 5533869303. E-mail address: [email protected] (Y. Tang). http://dx.doi.org/10.1016/j.snb.2016.01.112 0925-4005/© 2016 Elsevier B.V. All rights reserved.

which exhibits fluorescence enhancement in the presence of Hg2+ ions [23–27]. The second type is the turn-off probe, based on the fluorescence quenching of the fluorophores upon Hg2+ complexation [16,19,20,28]. The turn-on probe is more preferable than the turn-off probe because the latter can produce false positive signals caused by other quenchers in practical samples [25,26]. Thus, it would be advantageous to design turn-on fluorescent probes to detect Hg2+ ions. Gold nanoparticles (AuNPs) have an extremely high extinction coefficient and a broad absorption spectrum, which allow them to be employed as exquisite quenchers of fluorescent dyes through energy and/or electron transfer process. AuNP-based fluorescence assays are emerging as alternative approaches for metal ions, small molecules and DNA detection, providing high sensitivity and specificity [28,29]. For example, Chang et al. [27] have reported a rhodamine B (RB)-AuNPs system for the sensing of Hg2+ ions. The fluorescence of RB adsorbed onto AuNP surfaces is strongly quenched. The presence of Hg2+ ions causes the RB molecules to be detached from the AuNPs, leading to a large turn-on fluorescence enhancement. Its selectivity can be improved by modifying the AuNP surfaces with thiol ligands and adding a chelating ligand to the sample solutions. However, AuNP-based probes are generally unstable and easily aggregated in real samples owing to their high surface energy, which limits their practical application. To overcome this limitation, bovine serum albumin has been used to

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Table 1 Comparation of different probes for the determination of Hg2+ ion. Methods

Probe

Linear range

Detection limit

Ref.

Colorimetric Colorimetric Colorimetric SERS SERS Fluorescence Fluorescence Fluorescence Fluorescence Fluorescence Fluorescence Fluorescence Fluorescence

l-tyrosine–AuNPs Citrate–AuNPs CTAB–stabilized anisotropic nanogolds Magnetic silica sphere @Au core/shell BismuthiolII–AuNPs PEG–AuNPs H2 O2 –amplex ultraRed–AuPs RB–PEG–AuNPs Conjugated polymer–based film sensor LBL on polymer particles Composite turn–on Au nanodots–PNIPAM PBA-AuNPs/P(NIPAM–co–DMA)

33–300 nM 1–1000 nM 0–10,000 nM 0.1–1000 nM 0–150 nM 5–100 nM 5–1000 nM 10–50 nM 200–30,000 nM 2000–80,000 ␮M 40–4000 nM 2–20 nM 160–1600 nM

53 nM 2.9 nM 122 nM 0.1 nM 30 nM 2.24 nM 4.0 nM 2.3 nM 100 nM 200 nM 32 nM 1.9 nM 31 nM

[46] [2] [10] [47] [48] [49] [50] [32] [51] [17] [24] [36] This work

Abbreviations: surface-enhanced Raman scattering (SERS), hexadecyl trimethyl ammonium bromide (CTAB), polyethylene glycol (PEG), Rhodamine B (RB).

stabilize rhodamine 6G/AuNPs probe that could sense mercury ions under high salt solutions [30,31]. By employing similar principle, Hg2+ probe based on rhodamine B isothiocyanate-poly(ethylene glycol)-comodified AuNPs that can be well-dispersed in various complex solutions has been designed [32]. Microgels have been used as templates for the in situ synthesis of inorganic nanoparticles. A major advantage arises from its polymer networks, which can effectively hinder nanoparticles aggregation and thus enhance their chemical stability. In particular, the porosity affords an opportunity for a rapid diffusion of analytes into and out of the networks [33]. Thus, hybrid microgels combining inorganic nanoparticles and organic polymer networks have attracted significant interest in the last decade. Their unique properties have been utilized in organic catalysis [34] and metal-enhanced fluorescence [35]. Recently, poly(N-isopropylacrylamide) (PNIPAM) microgels incorporated with gold nanodots have emerged as a selective and sensitive probe for the detection of mercury ions through Hg2+ -induced photoluminescence quenching of gold nanodots. More importantly, these hybrid microgels can be easily purified by a simple centrifugation and display great stability against salt [36]. Nevertheless, few studies have concentrated on using hybrid microgel-based turn-on fluorescent probes for Hg2+ ions detection. In this work, we synthesized hybrid microgels with AuNPs immobilized in the thermo-and pH-responsive poly(Nisopropylacrylamide-co-2-(dimethylamino) ethylmethacrylate) (P(NIPAM-co-DMA)) microgels. This hybrid microgel was developed as a selective turn-on fluorescent probe for the detection of Hg2+ ions. As shown in Scheme 1, negatively charged 1pyrenebutyric acid (PBA) dyes were first adsorbed into positively charged AuNPs/P(NIPAM-co-DMA) hybrid microgels, which resulted in the fluorescence quenching of PBA. However, in the presence of Hg2+ , the fluorescence of PBA switched to turn-on. 2. Experimental 2.1. Materials N-isopropylacrylamide (NIPAM, 98%), 2(dimethylamino)ethylmethacrylate (DMA, 99%) and 1-pyrenebutyric acid (PBA) were purchased from Aladdin Chemical Co., Ltd. NIPAM was purified by recrystallization from a benzene/n-hexane mixture. DMA was distilled under reduced pressure. N,N -methylenebisacrylamide (MBA, 98%), ammonium persulfate (APS, AR), sodium dodecylsulfate (SDS, AR), chloroauric acid hydrated (HAuCl4 ·4H2 O, AR), and sodium borohydride (NaBH4 , 96%) were obtained from Sinopharm Chemical Co., Ltd. MBA was purified by recrystallization from methanol and APS was recrystallized from the mixed solvent of ethanol and deionized

water. Other chemical reagents were of analytical grade and used as received. The buffer solution was freshly prepared with KH2 PO4 , Na2 HPO4 ·12H2 O, and Na3 PO4 ·12H2 O. Deionized water was used throughout the experiments. P(NIPAM-co-DMA) microgels with the molar ratio of 4:1 for NIPAM to DMA were prepared by a dispersion polymerization method reported previously [37]. 2.2. Characterization The morphologies of the hybrid microgels were observed by using a Tecnai G20 transmission electron microscope (TEM) with an accelerating voltage of 200 kV. Few drops of dilute aqueous dispersion were dropped onto a carbon-coated copper grid. The average hydrodynamic radius (≺Rh ) was measured by an ALV/DLS/SLS-5022F spectrometer with a multi- digital time correlation (ALV5000). A cylindrical 22 mW UNIPHASE He-Ne laser (0 = 632 nm) was used as the light source. Each sample was filtered through a 0.8 ␮m nylon filter to remove dust. All dynamic laser light scattering (DLS) measurements were performed at a scattering angle of 90◦ . Thermogravimetric analysis (TGA) was conducted on an SDT Q600 under argon atmosphere. The samples were heated from 20 to 700 ◦ C at a rate of 10 ◦ C min−1 . UV–vis absorption spectra were measured on a Hitachi U-4100 spectrophotometer. Fluorescence spectra were obtained by using a Hitachi F-4500 spectrofluorometer equipped with an R3896 red-sensitive multiplier and 1 cm quartz cuvette. 2.3. Synthesis of AuNPs/P(NIPAM-co-DMA) hybrid microgels 25 mL of P(NIPAM-co-DMA) microgels (2.8 mg mL−1 ) and 10 mL of aqueous HAuCl4 solution (1 mg mL−1 ) were mixed and stirred overnight at room temperature under nitrogen atmosphere. The mixture was dialyzed against deionized water for 2 h to remove unbound AuCl4 − ions. Subsequently, the dispersion was cooled in an ice bath and 1 mL of freshly prepared NaBH4 solution (0.5 mg mL−1 ) was slowly added. The reduction reaction was allowed to proceed for 12 h. The resultant hybrid microgels were further purified by dialyzing against deionized water and stored in a refrigerator at 4 ◦ C with a concentration of 0.5 mg mL−1 . 2.4. Fluorescence quenching of PBA by the hybrid microgels A stock solution of PBA (10 ␮M) was prepared with deionized water. 250 ␮L of PBA and different aliquots of hybrid microgels (0.5 mg mL−1 ) were mixed in a series of colorimetric tubes and then diluted to 1.25 mL. After equilibration at room temperature for 30 min, the fluorescence spectra of the mixtures were recorded with an excitation wavelength of 346 nm. The slit widths of excitation and emission were 10 and 5 nm, respectively.

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Scheme 1. Schematic illustration of the fluorescent sensing of Hg2+ ions with PBA-AuNPs/P(NIPAM-co-DMA) composites.

2.5. Procedures for Hg2+ determination 250 ␮L of PBA (10 ␮M) and 500 ␮L of hybrid microgels (0.5 mg mL−1 ) were mixed and then various amounts of Hg2+ stock solution (20 ␮M) were added. The final volume was adjusted to 1.25 mL with PBS buffer (10 mM, pH 6.0) and the mixture was equilibrated for 30 min before the spectral measurement. The fluorescence spectra were measured under the same experimental conditions as described above. In order to examine the selectivity of this probe toward Hg2+ ions, various relevant metallic ions including Co2+ , Ba2+ , Pb2+ , Cu2+ , Fe2+ , Fe3+ , Al3+ , Mg2+ , Ca2+ , Zn2+ , Na+ , Ag+ and K+ (12.8 ␮M) were analyzed in the absence and presence of 1.28 ␮M Hg2+ . 3. Results and discussion 3.1. Preparation of AuNPs/P(NIPAM-co-DMA) hybrid microgels

3.2. Thermo- and pH-responsive properties of hybrid microgels It is well known that P(NIPAM-co-DMA) microgels have excellent thermo- and pH-responsive characters and display a responsive swelling-shrinking behavior[37,38]. After embedded

Fig. 1. A typical TEM image of AuNPs/P(NIPAM-co-DMA) hybrid microgels.

2+

Absorbance

Similar to the preparation of AgNPs/P(NIPAM-co-DMA) [37]. the hybrid microgels were prepared via a two-step process. First, P(NIPAM-co-DMA) microgels were synthesized by a dispersion polymerization in aqueous solution. Second, AuNPs/P(NIPAM-coDMA) hybrid microgels were obtained by an in situ reduction of AuCl4 − ions incorporated into the microgels using NaBH4 as a reducing agent. A typical TEM image for the as-prepared AuNPentrapped microgels is shown in Fig. 1. Clearly, the spheric AuNPs with an average diameter of 4.0 ± 0.5 nm were homogeneously distributed within the P(NIPAM-co-DMA) microgels, and their weight percentage content was calculated to be 19.8% based on TGA weight loss, indicating the successful synthesis of AuNPembedded hybrid microgels. This should be due to the electrostatic interaction between AuCl4 − ions and protonated tertiary amine groups as well as the coordination of Au atoms with the nitrogen atoms in the polymer chains. The UV–vis absorption spectrum of AuNPs/P(NIPAM-co-DMA) hybrid microgels exhibits a distinctive surface plasmon resonance (SPR) peak at 524 nm (Fig. 2), which is assigned to the characteristic absorption band of AuNPs. In addition, the SPR band position as well as the absorption intensity had little change, even after the hybrid microgels was stored for 50 days at room temperature (data not shown), indicating the hybrid microgels was very stable.

0.6

PBA-AuNPs/P(NIPAM-co-DMA)+ Hg ions PBA-AuNPs/P(NIPAM-co-DMA) AuNPs/P(NIPAM-co-DMA) PBA

0.4 0.2 0.0 300

400

500

600

700

800

Wavelength / nm Fig. 2. UV–vis absorption spectra of PBA, AuNPs/P(NIPAM-co-DMA), PBAAuNPs/P(NIPAM-co-DMA), and PBA-AuNPs/P(NIPAM-co-DMA) with Hg2+ ions.

with AuNPs, the hybrid microgels may also possess similar properties. DLS was used to measure the ≺Rh  of the hybrid microgels at various temperatures and the results are displayed in Fig. 3. As expected, the ≺Rh  gradually decreased with increasing temperature at different pHs, indicating that AuNPs/P(NIPAM-co-DMA) hybrid microgels maintained the thermo-responsive property. From Fig. 4, we can find that as the pH was adjusted from 4.0 to

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FL intensity / a.u.

150

12.0

pH 4.0 pH 7.4 pH 10.0

/ nm

140

F0/F

2400 1800

130

0

4.0 0.0 0.00

1200

120

8.0

0.05

0.10

0.15

0.20 -1

[Hybrid microgels] / mgmL -1

0.2 mg mL

600

110

0 20

25

30

o

35

40

45

T/ C Fig. 3. Temperature dependence of the average hydrodynamic radius (≺Rh ) for AuNPs/P(NIPAM-co-DMA) hybrid microgels with pH 4.0, 7.4 and 10.0. The concentration of AuNPs/P(NIPAM-co-DMA) is 0.15 mg mL−1 .

375

400

425

450

475

500

Wavelength / nm Fig. 5. Fluorescence emission spectra of PBA with increasing concentrations of AuNPs/P(NIPAM-co-DMA) with ex = 346 nm. From top to bottom: 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, and 0.20 mg mL−1 . Inset: dependence of F0 /F on the hybrid microgel concentration, where F0 and F represent the fluorescence intensity of PBA at 398 nm in the absence and presence of the hybrid microgels.

145

/ nm

140 135 130 125

4

6

8

10

pH Fig. 4. pH dependence of the average hydrodynamic radius (≺Rh ) for AuNPs/P(NIPAM-co-DMA) hybrid microgels at 20 ◦ C. The concentration of AuNPs/P(NIPAM-co-DMA) is 0.15 mg mL−1 .

10.0, the ≺Rh  decreased from 142.5 to 130.1 nm, indicating the pH-responsive behavior of the hybrid microgels. When the pH of external environment is increased, the amino groups of PDMA segments are progressively deprotonated, which is responsible for the collapse of the hybrid microgels and the decrease in ≺Rh  [39]. 3.3. The fluorescence quenching of PBA by AuNPs/P(NIPAM-co-DMA) PBA molecule is an important fluorescent probe because its outstanding optical properties, such as high quantum yield and long fluorescence lifetime. It can interact with various molecules via ␲–␲ stacking, strong hydrogen bonding or electrostatic attraction, which make it extensively used as probes for the distinct fields of study [40–42]. Fig. 5 shows the fluorescence emission spectra of PBA with increasing concentrations of AuNPs/P(NIPAMco-DMA). Upon addition of the hybrid microgels to PBA solution, the fluorescence intensity of PBA significantly decreased, whereas its maximum emission peak did not show obvious shift. For comparison, we also recorded the fluorescence spectra of PBA in the presence of 0.12 mg mL−1 P(NIPAM-co-DMA) microgels. As shown in Fig. S1 in Supporting information, the fluorescence intensity shows a slight increase. These results suggest that the PBA fluorescence was quenched more likely through the interactions between AuNPs and PBA molecules [43]. In the present work, the electrostatic interaction and hydrogen bond between carboxyl units of the PBA and amine groups of the DMA made PBA molecules diffuse through the P(NIPAM-co-DMA) microgel network and reach to AuNP surfaces. The efficient energy transfer from confined PBA

to AuNPs resulted in the fluorescence quenching. The more the concentration of hybrid microgels, the more PBA molecules interacted with AuNPs, exhibiting a higher-efficiency fluorescence quenching. A linear relationship between the F0 /F ratio and the hybrid microgel concentration was found in the low concentration range (inset of Fig. 5). However, when further increasing the hybrid microgel concentration, the relationship displayed an upward deviation from the linearity, which was associated with the superquenching efficiency of AuNPs [32]. As the hybrid microgel concentration was 0.20 mg mL−1 , the quenching efficiency of PBA was over 92% relative to the fluorescence of unbound pure PBA. This superquenching efficiency is also required to maximize the fluorescent change and achieve an efficient turn-on detection of small amounts of analyte. 3.4. Detection of Hg2+ based on PBA-AuNPs/P(NIPAM-co-DMA) Next, we measured the fluorescence changes of PBAAuNPs/P(NIPAM-co-DMA) upon addition of varied amounts of Hg2+ ions. As shown in Fig. 6A, the fluorescence intensity increased successively in the Hg2+ concentration range from 0 to 3.04 ␮M, whereas the spectral widths and maximum emission wavelengths did not show obvious change. When the concentration of Hg2+ was 3.04 ␮M, the fluorescence recovered to 83.7% of the initial intensity of pure PBA, and the fluorescence intensity increased 16-fold relative to the quenched PBA. A control experiment was performed in the absence of AuNPs/P(NIPAM-co-DMA). There was no significant change in the emission spectrum of PBA. To understand the mechanism of the fluorescence recovery, we investigated the changes of the SPR band of PBA-AuNPs/P(NIPAM-co-DMA) dispersion in the absence and presence of Hg2+ ions. As shown in Fig. 2, the SPR peak shifted from 524 to 518 nm upon the addition of 1.6 ␮M Hg2+ , and the color of the solution had little change. Similar results were previously reported by Radhakumary et al. [44] and Morris et al. [45] for the interaction of AuNPs with Hg2+ ions, where the SPR band of AuNPs in the prescence of Hg2+ shrifted to a short wavelength results from the formation of a core/shell structure. In addition, Hg2+ ions most likely form stable complexes with the amine groups in the PDMA segments. These collective results suggest that Hg2+ ions displace the PBA molecules attached on the AuNP surfaces, leading to a significant enhancement in the fluorescence. On the basis of the restored fluorescence, a fluorescent turn-on method for the determination of Hg2+ was proposed. To optimize the fluorescence restoration of PBA for sensing Hg2+ , the effect of pH in a range from 6.0 to 10.0, incubation time and temperature were investigated. The fluorescence quenching efficiency of PBA in the presence of AuNPs/P(NIPAM-co-DMA) was higher in

c

771

6.0

A

2000

2+

Hg

PBA-AuNPs/P(NIPAM-co-DMA) + other ions 2+ PBA-AuNPs/P(NIPAM-co-DMA) + other ions + Hg ions

5.0

1500

F/F0

FL intensity / a.u.

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1000 500

3.0 2.0

0

375

400

425

450

475

500

1.0

Wavelength / nm

0.0

20.0 15.0

OF-F0P/F0

6.0

OF-F0P/F0

4.0

4.5

2+

2+

2+

blank Hg Co Ba

B

2+

Pb

2+

Cu

Fe

2+

3+

Fe

Al

3+

2+

2+

Mg Ca

2+

Zn

+

Ag

+

K

Na

+

Fig. 7. Fluorescence intensity ratio (F/F0 ) of PBA-AuNPs/P(NIPAM-co-DMA) with or without Hg2+ (1.28 ␮M) with the coexistence of competing metal ions (12.8 ␮M).

3.0 1.5 0.0

10.0

0.0

0.4

0.8

1.2

1.6

‚Hg2+„/ µM

Table 2 Analytical results for the detection of Hg2+ in drinking water.

5.0 0.0 0.0

0.8

1.6

2.4

3.2

‚Hg2+„/ µM Fig. 6. (A) Fluorescence spectra of PBA-AuNPs/P(NIPAM-co-DMA) system in the presence of increasing concentrations of Hg2+ ions with ex = 346 nm. The concentration of Hg2+ ions in samples (solid curves) is 0, 0.16, 0.32, 0.48, 0.64, 0.80, 0.96, 1.12, 1.28, 1.44, 1.60, 1.76, 1.92, 2.08, 2.24, 2.40, 2.56, 2.72, 2.88, 3.04 ␮M, respectively. (B) A relationship between the fluorescence intensity ratios [(F − F0 )/F0 ] and the concentrations of Hg2+ ions, where F and F0 represent the fluorescence intensity at 398 nm in the presence of different concentrations of Hg2+ and the absence of Hg2+ , respectively.

the weak acid solutions than in alkaline media, maybe resulting from the strong electrostatic interaction in weakly acid solutions. However, the pH had little effect on the fluorescence restoration of PBA by Hg2+ ions. The higher quenching efficiency is favorable for the fluorescence turn-on analysis of Hg2+ ions [43]. In addition, the swollen hybrid microgels facilitated the analyte to be quickly transported within the polymer networks and close to or detach from the AuNPs. For these reasons, the pH of 6.0 was adopted in the further experiment. From Fig. S2 (Supporting information), we can find that the relative fluorescence intensity decreased with temperature, implying that it is not conducive to recover the fluorescence of PBA at high temperature. So, the subsequent experiments were carried out at room temperature. At room temperature, time-dependent measurements on the fluorescence of PBA were conducted. Upon the addition of AuNPs/P(NIPAM-co-DMA), the fluorescent intensity of PBA was dramatically decreased, and then reached a steady value within 15 min (Fig. S3 in Supporting information). This fluorescence was recovered within 3 min upon the addition of Hg2+ ions and then kept stable (Fig. S4 in Supporting information). Based on the above two points, an incubation time of 30 min was selected in the subsequent experiments. Under the optimized experiment conditions, we investigated the sensitivity of the proposed probe for Hg2+ in aqueous solution. The fluorescence intensity ratios ((F − F0 )/F0 ) was plotted against the Hg2+ concentration as shown in Fig. 6B, in which F and F0 represent the fluorescence intensity at 398 nm in the presence of different concentrations of Hg2+ and the absence of Hg2+ , respectively. A linear relationship was observed between the values of (F − F0 )/F0 and the concentration of Hg2+ ion over the range of 0.16–1.6 ␮M (inset in Fig. 6B). The calibration curve could be expressed as (F − F0 )/F0 = −0.252 + 3.827cHg2+ (c: ␮M) with a cor-

*

Sample

Hg2+ added/nM

Hg2+ found/nM

Sample-1 Sample-2 Sample-3 Sample-4 Sample-5

160 320 480 640 800

165.8 321.6 486.1 612.3 800.8

± ± ± ± ±

4.50 12.7 20.8 16.1 22.3

Recovery (%) mean ± RSD, n = 3 103.6 100.5 101.3 95.70 100.1

± ± ± ± ±

2.8 4.0 4.3 2.5 2.8

The standard deviation of each sample was obtained by three measurements.

relation coefficient of R2 = 0.988. The detection limit was 31 nM, estimated according to the 3/k, where  represents the standard deviation of eleven blank measurements and k is the slope of the calibration curve. The present probe for Hg2+ is compared with other advance optical methods (Table 1). The results show that the linear range and the detection limit of our present probe were comparable and even better than those of other probes. More importantly, the PBA-AuNPs/P(NIPAM-co-DMA) exhibits high stable. To assess the selectivity of the as-formed composites for Hg2+ , various environmentally relevant metal ions, including Co2+ , Ba2+ , Pb2+ , Cu2+ , Fe2+ , Fe3+ , Al3+ , Mg2+ , Ca2+ , Zn2+ , Ag+ , Na+ and K+ ions, were examined under identical conditions (Fig. 7). Clearly, these ions (12.8 ␮M) did not cause any significant changes in the fluorescence intensity. On the other hand, the addition of Hg2+ (1.28 ␮M) resulted in a noticeable enhancement of F/F0 ratio, indicating the present system exhibited high selectivity for the detection of Hg2+ ions. The effects of various coexisting ions on the fluorescence intensities of dispersion were further investigated by mixing the interfering ions with Hg2+ ions. It can be seen that the dispersion had similar behavior to that containing pure Hg2+ , suggesting that the coexistence of selected metal ions did not interfere with the selectivity of the developed Hg2+ probe. To test the practicality of the proposed method, this turn-on optical Hg2+ ions probe was used to determine the concentration of Hg2+ in drinking water by the standard addition method. From Table 2, it can be seen that the detected concentrations of Hg2+ were close to those of the added Hg2+ ions. In addition, the recoveries were in the range of 95.7–103.6% and the relative standard deviations were all less than 5.0%. The results indicated that the developed probe has satisfactory accuracy and reproducibility. 4. Conclusions In summary, we designed a sensitive and turn-on fluorescent method for the determination Hg2+ ions based on PBA-AuNPs/P(NIPAM-co-DMA) composites. The AuNPs/P(NIPAM-

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co-DMA) hybrid microgels exhibited thermo- and pH-responsive properties and great stability. The fluorescence of PBA was quenched significantly by AuNPs/P(NIPAM-co-DMA) through energy transfer from PBA to AuNPs. Hg2+ ions displace the PBA molecules attached on the AuNP surfaces that led to the fluorescence recovery of PBA. The fluorescence turn-on response is linearly proportional to the concentration of Hg2+ in the range of 0.16–1.6 ␮M with detection limit 31 nM. Moreover, the probe can recognize Hg2+ in presence of other abundant metal ions with high selectivity. The present results offer valuable information and create a good platform for detecting of Hg2+ . Thus, this strategy could be used as a general approach to construct other microgels based fluorescence probes. Acknowledgements The financial support from the Natural Science Foundation of Anhui Province, China (11040606M61), the Natural Science Foundation of the Anhui Higher Education Institutions of China (KJ2011A137) and the Innovation Funds of Anhui Normal University is gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2016.01.112. References [1] Q. Wei, R. Nagi, K. Sadeghi, S. Feng, E. Yan, S.J. Ki, R. Caire, D. Tseng, A. Ozcan, Detection and spatial mapping of mercury contamination in water samples using a smart-phone, ACS Nano 8 (2014) 1121–1129. [2] G. Sener, L. Uzun, A. Denizli, Lysine-promoted colorimetric response of gold nanoparticles: a simple assay for ultrasensitive mercury(II) detection, Anal. Chem. 86 (2014) 514–520. [3] Y.H. Lin, W.L. Tseng, Ultrasensitive sensing of Hg2+ and CH3 Hg+ based on the fluorescence quenching of lysozyme type VI-stabilized gold nanoclusters, Anal. Chem. 82 (2010) 9194–9200. [4] Z. Zhu, G.C. Chan, S.J. Ray, X. Zhang, G.M. Hieftje, Microplasma source based on a dielectric barrier discharge for the determination of mercury by atomic emission spectrometry, Anal. Chem. 80 (2008) 8622–8627. [5] H. Erxleben, J. Ruzicka, Atomic absorption spectroscopy for mercury, automated by sequential injection and miniaturized in lab-on-valve system, Anal. Chem. 77 (2005) 5124–5128. [6] X.Y. Jia, D.R. Gong, Y. Han, C. Wei, T.C. Duan, H.T. Chen, Fast speciation of mercury in seawater by short-column high-performance liquid chromatography hyphenated to inductively coupled plasma spectrometry after on-line cation exchange column preconcentration, Talanta 88 (2012) 724–729. [7] E. Xiong, L. Wu, J. Zhou, P. Yu, X. Zhang, J. Chen, A ratiometric electrochemical biosensor for sensitive detection of Hg2+ based on thymine-Hg2+ -thymine structure, Anal. Chim. Acta 853 (2015) 242–248. [8] S.J. Liu, H.G. Nie, J.H. Jiang, G.L. Shen, R.Q. Yu, Electrochemical sensor for mercury(II) based on conformational switch mediated by interstrand cooperative coordination, Anal. Chem. 81 (2009) 5724–5730. [9] L. Zhou, W. Xiong, S. Liu, Preparation of a gold electrode modified with Au–TiO2 nanoparticles as an electrochemical sensor for the detection of mercury(II) ions, J. Mater. Sci. 50 (2014) 769–776. [10] L.-H. Jin, C.-S. Han, Eco-friendly colorimetric detection of mercury(II) ions using label-free anisotropic nanogolds in ascorbic acid solution, Sens. Actuators A 195 (2014) 239–245. [11] C.J. Yu, W.L. Tseng, Colorimetric detection of mercury(II) in a high-salinity solution using gold nanoparticles capped with 3-mercaptopropionate acid and adenosine monophosphate, Langmuir 24 (2008) 12717–12722. [12] X. Wan, T. Liu, S. Liu, Thermoresponsive core cross-linked micelles for selective ratiometric fluorescent detection of Hg2+ ions, Langmuir 27 (2011) 4082–4090. [13] C. Guo, J. Irudayaraj, Fluorescent Ag clusters via a protein-directed approach as a Hg(II) ion sensor, Anal. Chem. 83 (2011) 2883–2889. [14] H. Wei, Z. Wang, L. Yang, S. Tian, C. Hou, Y. Lu, Lysozyme-stabilized gold fluorescent cluster: synthesis and application as Hg2+ sensor, Analyst 135 (2010) 1406–1410. [15] B. Adhikari, A. Banerjee, Facile synthesis of water-soluble fluorescent silver nanoclusters and Hg(II) sensing, Chem. Mater. 22 (2010) 4364–4371. [16] J. Xie, Y. Zheng, J.Y. Ying, Highly selective and ultrasensitive detection of Hg2+ based on fluorescence quenching of Au nanoclusters by Hg2+ –Au+ interactions, Chem. Commun. 46 (2010) 961–963.

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Biographies Yecang Tang is an associated professor of College of Chemistry and Materials Science, Anhui Normal University, China. He obtained his Ph.D. from University of Science and Technology of China in 2008 and M.S. from Anhui Normal University in 2002. Her research interests include the development of nanocomposite materials for sensing applications and polymer science. Yi Ding obtained his B.Sc. degree from Anhui University of Technology in 2011. He is currently studying for his M.Sc. degree in College of Chemistry and Materials Science, Anhui Normal University, China. He is engaged in the synthesis of nanocomposites for sensing applications. Ting Wu is a teacher of Urban Construction College of Anhui Jianzhu University, China. She obtained her B.Sc. and M.Sc. degree from Anhui Normal University in 2011 and 2014, respectively. Her research interests focus on the design and fabrication of nanomaterials. Liying Lv received her B.Sc. degree from Guangdong University of Petrochemical Technology in 2013. She is currently pursuing his M.Sc. degree in College of Chemistry and Materials Science, Anhui Normal University. Her scientific interest is developing fluorescent probes based on nanocomposites. Zhicheng Yan is an undergraduate student in the College of Chemistry and Materials Science, Anhui Normal University.