Accepted Manuscript Title: A highly selective molecularly imprinted electrochemiluminescence sensor for ultra-trace beryllium detection Author: Jianping Li Fei Ma Xiaoping Wei Cong Fu Hongcheng Pan PII: DOI: Reference:

S0003-2670(15)00216-0 http://dx.doi.org/doi:10.1016/j.aca.2015.02.038 ACA 233752

To appear in:

Analytica Chimica Acta

Received date: Revised date: Accepted date:

17-11-2014 10-2-2015 11-2-2015

Please cite this article as: Jianping Li, Fei Ma, Xiaoping Wei, Cong Fu, Hongcheng Pan, A highly selective molecularly imprinted electrochemiluminescence sensor for ultra-trace beryllium detection, Analytica Chimica Acta http://dx.doi.org/10.1016/j.aca.2015.02.038 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.

A highly selective molecularly imprinted electrochemiluminescence sensor for ultra-trace beryllium detection Jianping Li∗, Fei Ma, Xiaoping Wei, Cong Fu, Hongcheng Pan

Guangxi Key Laboratory of Electrochemical and Magnetochemical Function Materials, College of Chemistry and Bioengineering, Guilin University of Technology, Guilin 541004, PR China



Corresponding author. Tel.:+86 773 5898551; fax: +86 773 8990404. E-mail address: [email protected] (Porf. J. Li)

Graphical abstract

A novel molecular imprinted electrochemiluminescence sensor was fabricated for ultra-trace Be2+ detection with an extremely lower detection limit based on the luminol–H2O2 ECL system.

Highlights

● A novel molecular imprinted electrochemiluminescence sensor was fabricated for ultra-trace Be2+ detection. ● Imprint cavities in the MIPs from elution the Be-PAR complex could provide more recognition sites for analytes. ● ECL emission produced by the luminol–H2O2 ECL system, which was applied to detect Be2+. ● It gave an extremely lower detection limit (2.35×10−11 mol/L) than the reported methods.

Abstract A new molecularly imprinted electrochemiluminescence (ECL) sensor was proposed for highly sensitive and selective determination of ultratrace Be2+ determination. The complex of Be2+ with 4-(2-Pyridylazo)-resorcinol (PAR) was chosen as the template molecule for the molecularly imprinted polymer (MIP). In this assay, the complex molecule could be eluted from the MIP, and the cavities formed could then selectively recognize the complex molecules. The cavities formed could also work as the tunnel for the transfer for probe molecules to produce sound responsive signal. The determination was based on the intensity of the signal, which was proportional to the concentrations of the complex molecule in the sample solution, and the Be2+ concentration could then be determined indirectly. The results showed that in the range of 7×10−11 mol/L to 8.0×10−9 mol/L, the ECL intensity had a linear relationship with the Be2+ concentrations, with the limit of detection of 2.35×10−11 mol/L. This method was successfully used to detect Be2+ in real water samples.

Keyword: Be2+; molecularly imprinted; high selectivity; electrochemiluminescence

1. Introduction Beryllium is an insidious carcinogenic poison due to its difficulty in biodegradation [1]. The general public is mainly exposed to trace amounts of beryllium via dietary intake. Water-soluble Be2+ may pose a serious threat to human health since it is accumulated in the skin and soft tissue through food and water in one’s daily diet. [3-5]. Be2+ is at trace levels in natural and waste waters [2], it can be detected through atomic absorption spectrometry [6], atomic emission spectrometry [7], and fluorospectrophotometry [8]. However, these methods either have insufficient sensitivity and selectivity, or need expensive apparatus and complicated sample pretreatment. Moreover, the detection might also be hindered by the interference compounds in samples. Thus, it is very important to develop highly sensitive and selective detection method for ultra-trace beryllium in water and food samples. MIP are nanoporous polymeric material that can selectively recognize target molecules [9-11], which is called the template molecule. They have been used a lot in chromatography [12], sensors [13], and solid-phase extraction [14] for its good selectivity. MIP-based sensors are widely

employed to detect organic compounds and biomacromolecules [11, 15-19]. However, it could also be used for the determination of metal ions, such as Cu2+ [20], Pb2+ [21], Eu3+ [22], Hg2+ [23], Zn2+ [24], and other metal ions [25]. But Be2+ detection using molecularly imprinted polymer sensor has not been reported yet. ECL sensors have good sensitivity [26]. MIP sensors via ECL measurement could facilitate the highly sensitive and selective detection with a relatively simple sample pretreatment procedure [27, 28]. This study is the first to fabricate a MIP-ECL sensor for ultra-trace Be2+ determination. The complex molecule was used as the template molecule to copolymerize with the functional monomer and form the MIP. After the elution of the complex from MIP, the imprinted cavities formed could act as the tunnel for probe to reach the surface of the electrode and produce ECL. When the MIP-electrode was immersed in samples with PAR, the complex was specifically recognized by the cavities through the space size and the binding sites. With the cavities retaken by the complex molecules, the ECL intensity decreased. The mechanism of the sensor is shown in Scheme 1. Scheme 1

2. Experimental Section 2.1 Apparatus and Reagents ECL measurement was carried out using an ECL analysis system (MPI-E, Xi’an Remax Analysis Instrument Co. Ltd., Xi’an, China). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on an electrochemical workstation (CHI 660D, Shanghai Chenhua Instrument Co. Ltd., Shanghai, China). The classic three-electrode system consisted of a MIP-modified Au electrode (2 mm diameter) as the working electrode, a potassium chloride-saturated Ag/AgCl electrode as the reference electrode, and a Pt wire electrode as the auxiliary electrode. pH measurements were conducted using a PHS-3D digital pH meter (Shanghai Precision & Scientific Instrument Co., Ltd., Shanghai, China). The infrared (IR) assay was performed on a Nicolet iS10 (Thermo Fisher Scientific, USA). PAR, o-phenylenediamine (o-PD), trishydroxymethyl aminomethane (Tris), BeSO4·4H2O,

acetic acid, and sodium acetate were purchased from Sinopharm Chemical Reagent Co., Ltd. Shanghai, China. Luminol (>98%) was obtained from Yingrun Biotechnologies Inc. Changsha, China. The luminol stock solution (1×10−3 mol/L) was prepared by dissolving 0.177 g luminol in 0.1 mol/L NaOH solution and stored in the dark for further use. The H2O2 stock solution (3×10−3 mol/L) was stored in a refrigerator for further use. A Tris–HCl buffer solution (0.05 mol/L, pH 7.8) was prepared by adjusting the pH of a 0.1 mol/L Tris solution by adding a 0.1 mol/L HCl solution. All other reagents were analytical grade and were used without purification. Ultrapure water obtained from a water purification system (>18 MΩ; Youpu Super Water Co., Ltd., Chengdu, China) was used to prepare the solution.

2.2 Preparation of the Be–PAR complex According to related references [29,30], PAR (0.42 g) was dissolved in ethanol (120 mL) at 50 °C and then added with 0.17 g of BeSO4·4H2O. NaOH (0.1 mol/L) was dropped into the solution under stirring until flocs formed. The solution was stirred for 4 h at a constant temperature and then allowed to cool to room temperature overnight before filtration. The precipitate was washed thrice with ethanol and then vacuum-dried at 40 °C for 12 h to yield the Be–PAR complex. This complex was characterized by IR spectrometry. The curves compared with PAR are shown in Fig. 1. The N=N stretching vibration of the Be–PAR complex red-shifted from 1479 cm−1 to 1469 cm−1 probably because of the electron-drawing effect from the metal ion. A strong wide adsorption peak belonging to the hydroxyl group appeared around 3600 cm−1–3200 cm−1, and an association peak appeared around 2700 cm−1–3100 cm−1 in PAR. The hydroxyl group chelated with Be2+ and disrupted the association. The results verify the formation of the Be–PAR complex. Fig. 1

2.3 Construction of MIP sensor The surface of the Au electrode was polished with fine-grade aqueous alumina slurries (1.0, 0.3, and 0.05 µm grain size) on chamois leather; then, the electrode was sequentially washed with HNO3 (50% in volume), ethyl alcohol, and deionized water. Both MIP and non-molecularly

imprinted polymer (NIP) sensors were constructed by electropolymerizing o-PD on the surfaces of the electrode. The MIP were polymerized by CV at 0 V–0.8 V for 30 cycles with a scanning rate of 0.05 V/s in a NaAc–HAc solution (pH 5.2) containing 1×10−3 mol/L Be–PAR and 3×10−3 mol/L o-PD. The modified electrode was washed with deionized water and then dried at room temperature before further use (25 °C). The NIP was prepared under the same conditions only without the addition of the Be–PAR complex. After electropolymerization, the MIP and NIP sensors were washed with methanol and acetic acid mixed solution (V:V = 8:1) for 3 min to remove Be–PAR and other possible adsorbates on the surface of the imprinted membrane.

2.4 CV and ECL measurement methods Electrochemical characterization was carried out in 0.05 mol/L K3[Fe(CN)6]/K4[Fe(CN)6] containing 0.5 mol/L KCl. The CV measurement was carried out at −0.2 V–0.6 V and 100 mV/s. EIS was carried out in the frequency range of 0.1 Hz–100000 Hz and at an alternating voltage of 5 mV. ECL measurement was carried out in 10 mL of 0.05 mol/L Tris–HCl buffer solution (pH 7.8) with 600 µL of luminol stock solution and 30 µL of H2O2 stock solution at −0.2 V–0.7 V (vs. SCE) and 100 mV/s. The voltage of the photomultiplier tube was set to 800 V at a sampling rate of 10 T/s and a magnification time of 3. After each use, the MIP sensor was washed by methanol and acetic acid mixed solution (8:1 in volume) for 3 min for renewal of the MIP. Then, it was stored at room temperature in the dark.

3. Results and Discussion 3.1 Electropolymerization of the MIP film MIP electropolymerization is irreversible. As shown in figure S1, o-PD showed an oxidation peak at 0.35 V. With the increase in cycle number, the peak current decreased and then almost disappeared after around 30 cycles. This demonstrated the formation of MIP film which has lower conductivity on the surface of the electrode.

3.2 Characterization of MIP and NIP sensors After the elution of the Be–PAR complex, the cavities left can serve as the tunnel for the transfer of the probe. The number of cavities can influence the intensity of the current. As shown in the CV curves (Fig. 2), the peak value dramatically decreased (from curve a to curve b) after the polymerization step. With the elution of the complex, the current increased from curve b to c. The current dropped again from curve c to d after the readsorption of the complex molecule. This result could verify that the cavities formed could be occupied again by the complex. The readsorption of PAR (curve e) and Be2+ ion itself (curve f) only had minimal changes compared with curve c. This finding could verify that the cavities could only recognize the complex and not the metal ion and the ligand. The CV characterization of the NIP sensor is shown in the insert of Fig. 2. The current also dramatically decreased (curve g) after the polymerization. However, the current remained unchanged after the elution (curve h). This result could demonstrate that no cavities were formed during the NIP formation. Fig. 2

EIS was also used to identify the changes in the surface status during the steps mentioned. EIS measurement was performed to characterize the resistance of the sensor after the treatment steps. As shown in Fig. 3, the resistance of the bare electrode was very small (curve a). After the polymerization, the resistance dramatically increased (curve b). This result could verify that the formed MIP film had low conductivity. When the Be–PAR complex was removed, the resistance considerably decreased to curve c. When the sensor was immersed in 5×10−10 mol/L (d) and 5×10−9 (e) complexes, the cavities were occupied by the complex. The resistance of the sensor increased as the concentration of the complex increased. This proved that the sensor could recognize the complex and be used for the quantitative analysis of Be2+. Fig. 3

3.3 Optimization of the determination solution According to a related reference [31], a 0.05 mol/L Tris–HCl buffer solution was used as the substrate solution for the ECL test. The pH of the Tris–HCl solution was in the range of 7.4–8.6 by the MIP reabsorbed in the 5×10−10 mol/L Be–PAR solution. The results showed that the ECL intensity was high at pH 7.8. When the pH continued to increase, the base line increased, with worse peak shape. Thus, pH 7.8 was chosen for further experiments. To achieve the best ECL results, the ratio between the luminol and H2O2 was also optimized. In 0.05 mol/L Tris–HCl (pH 7.8), the ECL intensities peaked with 600 µL of 1 mmol/L luminol (curve a) solution and 30 µL of 3 mmol/L H2O2 (curve b) solution (figure S2). Thus, 600 µL of luminol and 30 µL of H2O2 were used for the subsequent experiments.

3.4 Optimization of the readsorption time After the elution step, the MIP sensor was immersed in 5×10−10 mol/L Be–PAR solution, and ECL and CV measurements were carried out every 3 or 5 min. As shown in figure S3 (curve b), the ECL intensity remained almost the same after 9 min, indicating that the readsorption reached the balance at approximately 9 min. As shown in figure S3 (curve a), the CV results could also verify the results that the readsorption could reach balance at approximately 9 min. The readsorption for the complex with smaller concentrations also reached balance within 9 min. Thus, 9 min was selected.

3.5 Calibration curve Under the optimized experimental conditions, the sensor was immersed in solutions with different Be–PAR concentrations before the ECL measurement. As shown in Fig. 4, the ECL intensity decreased with increasing complex concentrations. In the range of 7×10−11–800×10−11 mol/L, the decreased ECL intensities (∆I) showed a linear relationship to Be2+ concentration (c). This relationship can be expressed as ∆I = 6.14c (10−11 mol/L) + 297.50 (R2 = 0.9938). The detection limit (DL) could reach 2.35×10−11 mol/L (DL = 3δb/K). The results showed that the method showed higher sensitivity compared with other common methods (Table 1).

Fig. 4

Table 1

3.6 Selectivity, stability, and reproducibility of the MIP sensor To investigate the selectivity of the MIP sensor for Be2+ determination, the interference of some potential ions in real samples was tested. The result demonstrated that Ca2+, Mg2+, and Al3+ had no influence (ECL signal change within ±5%) on the detection of 5×10−10 mol/L Be2+ when their concentrations were less than 5000 times that of Be2+. The maximum amounts of other foreign ions that can be tolerated in Be2+ detection are listed in Table 2. This result could further verify the high sensitivity of the sensor to Be2+, indicating that the specific recognition capability of the sensor was greatly enhanced by the usage of the complex as a template. Table 2

The reproducibility of the sensor was evaluated by testing the ECL intensity of 5×10−10 mol/L Be–PAR for five times using one MIP sensor and five different sensors prepared in the same solution. The relative standard limits (RSD) were 3.14% and 4.83%, respectively. These data prove the good reproducibility of the sensor. The stability of one sensor was also tested by the ECL intensity of the sensor after the adsorption of 5×10−10 mol/L Be–PAR. When not in use, the sensor was stored in a refrigerator at 4 °C. The ECL intensity did not significantly change (within ±5%) after 5, 10, and 15 d.

4 Determination of beryllium in samples In order to verify the performance of the MIP sensor, it was apply to determination Be2+ in well water and rain water about 1 km away from a steel plant in Xichang County, Sichuan Province of China, Bottled water and real rice sample purchased from local market was also tested. 5 g of the rice sample were incinerated in porcelain crucible at 600 ºC for 2 h and the ash was dissolved in 2 mL 0.5 mol/L HNO3. The solution was transferred into a 100 mL volumetric flask, diluted with deionized water. The sample solution was diluted for a 10 time. 50 µL of 1.0 mmol/L PAR was

added to 10 mL of sample solution and stirred for 30 min and the experimental conditions ware

exactly the same as previously mentioned. Standard addition recovery test was performed, and the results are summarized in Table 3. The recovery rates ranged from 90.0% to 101.0%. Table 3

5 Conclusions The method of preparing MIP sensor for Be2+ determination using Be-PAR complex as template molecule provides an effective way to improve the recognization ability of the MIP. The cavities formed after template elution work as the tunnels of the probe transfer, which produce sound responsive signal with sensitive ECL emission. The method is highly selective and sensitive, with low cost and good stability for the ultra-trace Be2+ detection. It is predicted to provide a new strategy to fabricate MIP sensors for the detection of other metal ions.

Acknowledgement We gratefully acknowledge the financial support from Natural Science Foundation of China (No. 21165007, 21375031).

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Scheme 1 Mechanism of recognization and measurement of the MIP sensor.

Fig. 1 IR spectra of the PAR and Be–PAR complex

Fig. 2 CVs of the modified electrode in different conditions a. Bare gold electrode; b. MIP-electrode; c. MIP-electrode after template removal; d. MIP-electrode after readsorption of complex; e. MIP-electrode after readsorption of PAR; f. MIP-electrode after readsorption of metal; g. NIP-electrode; h. NIP-electrode after elution.

Fig. 3 AC impedances of MIP films in different conditions a. Bare gold electrode; b. MIP-electrode; c. MIP-electrode after template removal; d. MIP-electrode after rebinding with 5×10−10 mol/L complex; e. MIP-electrode after rebinding with 5×10−9 mol/L complex.

Fig. 4 ECL of MIP sensor after rebinding in different concentrations of complex. a–k: 0, 7, 20, 40, 50, 80, 100, 200, 400, 600, 800×10−11 mol/L of complex respectively.

Tables Table 1. Comparison of the parameters of the methods on Be2+ determination Table 2. Maximum allowable times of foreign ions for Be2+ determination Table 3. Sample test result and recovery (n=5) Table 1 Reference Method

Linear ranges (mol/L)

DL (mol/L) No.

a

ICP–AESa

7.5×10−10–1.0×10−8

1.3×10−10

[7]

Fluorescence spectrometry

1.0×10−7–1.9×10−6

4.2×10−9

[8]

GF–AASb

1.0×10−7–4.0×10−4

2.95×10−8

[32]

Electrochemical sensor

1.0×10−8–1.0×10−2

4.0×10−9

[33]

Mercury film electrode

1.1×10−6–6.6×10−6

2.8×10−8

[34]

This experiment

7×10−11–8×10−9

2.35×10−11

--

b

Inductively coupled plasma atomic emission spectrometry; Graphite furnace atomic absorption spectrometry

Table 2 Maximum Ion

Maximum Ion

multiple

Maximum Ion

multiple

multiple

Bi3+

1000

Co2+

500

Zn2+

1000

Ba2+

600

Fe3+

1000

Br−

1000

Cr3+

1000

In3+

1000

PO43−

1000

Cd2+

1000

Mn2+

800

SO42−

500

Cu2+

800

Pb2+

800

SiO33−

800

Table 3

Samples

Found

RSD

10−10

%

Added

Total found

RSD

Recovery

10−10 mol/L

10−10 mol/L

%

%

mol/L Rain water

-

-

5.00

4.96

2.46

99.2

Bottle water

-

-

5.00

4.95

1.34

99.0

Well water

5.25

1.80

5.00

10.30

2.28

101.0

GBW10043 c

2.55 d

1.60

5.00

7.30

1.38

95.0

Rice Sample

-

-

5.00

4.80

1.50

96.0

c

GBW10043 is the rice reference sample (i.e., Chinese National Standard Reference Materials), in which the content of Be2+ is 2.1×10−9 g/g. d The content of Be2+ is 2.04×10−9 g/g.

Scheme 1

PAR

Transmittance (%)

2700-3100 1479

PAR-Be

2+

3200-3600 1469

4000

3500

3000

2500

2000

1500

-1

Wavenumbers (cm )

Fig. 1

0.6

a

0.4

e c d h

0.0 6

-0.2 -0.4

I / µA

I /m A

0.2

b

g h

3

f

g

0 -3 -0.2 0.0 0.2 0.4 0.6 E/V

-0.6 -0.2

0.0

0.2

E/V

Fig. 2

0.4

0.6

1000

2500 2000

e

Z" / Ω

a c

1500

d

1000

b

500 e

0

ac d

0

1000

2000

3000

4000

5000

6000

Z' / Ω

Fig. 3 5600 5000 4000

I / Counts

∆I

4200

3000 2000

a

1000

2800

0 0

k

200

400 600 800 -11 c / 10 mol ⋅ L-1

1400

0 5

10

15

t/s

Fig. 4

20

25

A highly selective molecularly imprinted electrochemiluminescence sensor for ultra-trace beryllium detection.

A new molecularly imprinted electrochemiluminescence (ECL) sensor was proposed for highly sensitive and selective determination of ultratrace Be(2+) d...
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