Anal Bioanal Chem DOI 10.1007/s00216-014-7664-4
RESEARCH PAPER
A microvolume molecularly imprinted polymer modified fiber-optic evanescent wave sensor for bisphenol A determination Yan Xiong & Zhongbin Ye & Jing Xu & Yucheng Liu & Hanyin Zhang
Received: 26 September 2013 / Revised: 23 November 2013 / Accepted: 16 December 2013 # Springer-Verlag Berlin Heidelberg 2014
Abstract A fiber-optic evanescent wave sensor for bisphenol A (BPA) determination based on a molecularly imprinted polymer (MIP)-modified fiber column was developed. MIP film immobilized with BPA was synthesized on the fiber column, and the sensor was then constructed by inserting the optical fiber prepared into a transparent capillary. A microchannel (about 2.0 μL) formed between the fiber and the capillary acted as a flow cell. BPA can be selectively adsorbed online by the MIP film and excited to produce fluorescence by the evanescent wave produced on the fiber core surface. The conditions for BPA enrichment, elution, and fluorescence detection are discussed in detail. The analytical measurements were made at 276 nm/306 nm (λex/λem), and linearity of 3×10−9–5×10−6 g mL−1 BPA, a limit of detection of 1.7×10−9 g mL−1 BPA (3σ), and a relative standard deviation of 2.4 % (n=5) were obtained. The sensor selectivity and MIP binding measurement were also evaluated. The results indicated that the selectivity and sensitivity of the proposed fiber-optic sensor could be greatly improved by using MIP as a recognition and enrichment element. Further, by modification of the sensing and detection elements on the optical fiber, the proposed sensor showed the advantages of easy fabrication and low cost. The novel sensor configuration provided a platform for monitoring other species by simply changing the Y. Xiong (*) : Z. Ye : Y. Liu : H. Zhang State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China e-mail: [email protected] Y. Xiong : Z. Ye : Y. Liu School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, China J. Xu Liaoning Entry-Exit Inspection and Quarantine Bureau, Dalian 116001, China
light source and sensing elements. The sensor presented has been successfully applied to determine BPA released from plastic products treated at different temperatures. Keywords Fiber-optic sensor . Bisphenol A . Evanescent wave . Molecularly imprinted polymer
Introduction Bisphenols are a class of chemicals with two hydroxyphenyl functionalities and include several analogues. Among the bisphenol analogues that are in commercial use, bisphenol A (BPA; Fig. 1a) has been the most widely studied. BPA is a high production volume chemical and is widely used as the chemical building block in the manufacture of polycarbonate plastics [1], the epoxy resins used in food can linings [2], and dental fillings and sealants [3]. Further, BPA has also been used as a component in the production of thermal papers (as a weakly acidic color developer) for more than 50 years [4]. Human exposure to BPA that leaches from packaging or storage containers into foods and beverages is a great public health concern, owing to the endocrine-disrupting potential of this compound. BPA can potentially interfere with the endocrine system of wildlife and humans, increase the cancer rate, reduce immune function, and impair reproduction [5,6]. Migration of BPA from polycarbonate drinking bottles and baby bottles into water and foods [7,8], migration of BPA from metallic can epoxy coatings into various food items [9,10], and migration of BPA from paper products [11] have been reported. The European Commission has established a migration limit of 600 μg kg−1 for BPA in food or food simulators from plastic materials and articles intended to come in contact with foodstuffs [12]. Owing to widespread human exposure and its toxicity, BPA has drawn considerable attention from regulatory
Y. Xiong et al.
a
CH3
HO
C
OH
CH3
b HO
CH2
OH
Fig. 1 The structures of a bisphenol A (BPA) and b bisphenol F
organizations and the general public in recent years [13]. Restrictions on the use of BPA in certain consumer products have been suggested [14–16]. For example, the US Food and Drug Administration banned the use of BPA in baby bottles and children’s drinking cups in July 2012 [15]. The manufacture, sale, or distribution of BPA has also been prohibited for some consumer products, such as reusable food or beverage containers, infant formula containers, and thermal receipt paper, by several states in the USA since 2009 [16]. The European Commission [14] and China restricted the use of BPA in plastic infant feeding bottles in January 2011 and June 2012. Thus, the determination of BPA is of vital significance for risk assessment and control of human exposure to BPA. A number of analytical methods for BPA determination have been reported, including gas chromatography coupled with mass spectrometry [17,18], liquid chromatography coupled with mass spectrometry [19,20], high-performance liquid chromatography [21], enzyme-linked immunosorbent assays [22], fluorimetry [23,24], a surface plasmon resonance biosensor [25], and electrochemical methods, including capillary electrophoresis [26] and electrochemical sensors [27–31]. However, although many detection modes can be used to quantify the amounts of BPA, strategies to achieve the specific recognition of BPA commonly adopt chromatography [17–21], which can easily discriminate impurities and provide high sensitivities. However, these chromatographic methods not only need expensive and complicated instruments, but also are laborious and time-consuming. The molecular imprinting technique is a rapidly developing technique for the preparation of polymers that possess specific recognition functions for the template molecule. In principle, molecularly imprinted polymer (MIP) is prepared by copolymerization of a template molecule (generally the analyte itself) with a functional monomer capable of interacting with the template molecule in the presence of a cross-linker to produce three-dimensional network polymers. Removal of the template molecule from the resultant polymer yields a functional polymeric matrix possessing microcavities with a threedimensional structure complementary in both shape and chemical functionality to that of the template molecule. Since its introduction by Wulff et al. [32] and Vlatakis et al. [33], the molecular imprinting technique has become a well established
and facile technique. Owing to the molecular memory, imprinted polymers are able to recognize and bind target molecules, which results in their extensive uses as solidphase-extraction matrices [34,35], antibody mimics [36], enzyme mimics [37], catalytic micromotors [38], and artificial receptors mimicking natural biological receptors, for example, in chemical sensors [39] and biochips [40]. MIPs for BPA prepared by different methods have been used as the solidphase-extraction matrices in liquid chromatography [41] and high-performance liquid chromatography [42], and as the sensing matrices for electrochemical sensors [26,43]. However, the integration of imprinted polymers into sensors and similar devices in a suitable form such as thin films, micropatterns, and nanopatterns is the main challenge of practical sensor development and requires microfabrication processes [44]. Fiber-optic sensors based on the principle of an evanescent wave [45], known as fiber-optic evanescent wave sensors (FOEWS), have attracted great attention because of their potential for in vivo measurement in biological and chemical applications [46,47]. When light travels by total internal reflection in an optical fiber core, an evanescent wave can be produced on the fiber surface and penetrates into the surrounding medium. Fluorophores around or immobilized on the core surface can be excited by the evanescent wave to produce fluorescence. This approach avoids laborious optical adjustment and is thus convenient and effective in practical applications. FOEWS for nitrite [48], oxygen [49], and pH [50] determination have been developed in our laboratory. To the best of our knowledge, there is no report of BPA determination based on FOEWS sensing. This work aimed to develop a novel MIP-based FOEWS with a microliter detection volume for BPA determination. The BPA was immobilized in MIP film and then modified on the surface of an unclad optical fiber. The fiber prepared was used as a sensing element and inserted into the optical fiber in a transparent capillary to construct the sensor. A microchannel with a small detection volume (about 2.0 μL) formed between the optical fiber and the capillary acted as a flow cell. For the proposed sensor, BPA can be selectively adsorbed online by the MIP film and then excited to produce fluorescence by the evanescent wave field produced on the fiber core surface. To decrease the light source background and improve the fluorescence signal collection efficiency, the photomultiplier (PMT) detector was designed to be parallel to the axis of the optical fiber in the proposed sensor construction for BPA determination. The spectroscopic properties were characterized by UV–vis and fluorescence measurements. The sensor performances, including sensitivity, repeatability, binding measurement, pH effect and surfactant effect, are reported in detail for the proposed BPA sensor. A detailed performance comparison of the method with other methods based on MIPs for determination of BPA was made, and the results are listed
A microvolume sensor based on molecularly imprinted polymer Table 1 Performance comparison of the proposed method with other methods based on molecularly imprinted polymers (MIPs) for determination of bisphenol A Detection method based on MIP
Limit of detection (g mL−1)
Real sample/matrix
Sensor volume and temperature effect
Reference
Capillary electrophoresis Electrochemical sensor LC HPLC Surface-enhanced Raman scattering Fiber-optic sensor
1.8×10−9 3.4×10−8 6.8×10−11 2.0×10−10 1.2×10−7 1.7×10−9
Water samples and human urine Water River water Water and milk Real samples Plastic productions
Not shown Not shown Not shown Not shown Not shown 2.0 μL 25–70 °C
[26] [31] [41] [42] [52] Present study
HPLC high-performance liquid chromatography, LC liquid chromatography
in Table 1. The results showed that all of the methods were applicable to BPA determination in real samples. However, the present MIP-based FOEWS was fabricated with a small detection volume. Further, the present sensor was applied to detection of BPA released from common plastic products at different temperatures with different solvents.
Scientific Instrument Company, China). The UV–vis absorption spectra were obtained with a UV-1800 spectrophotometer (Hitachi, Japan). A homemade flow cell was used for the sensing measurements. Solutions were delivered by two peristaltic pumps (HL-2, Shanghai Huxi, China). PTFE tubing (inner diameter 0.8 mm) was used to connect the components.
Experimental
Fabrication of the BPA-MIP microvolume sensor
Reagents and materials
Preparation of the sensor solution
B PA , β - c y c l o d e x t r i n ( β - C D ) , s o d i u m dodecylbenzenesulfonate, Triton X-100, acetonitrile, boric acid (H3BO3), phosphoric acid (H3PO4), acetic acid, ethanol, methacrylic acid (MAA), ethylene glycol dimethacrylate (EGDMA), and 2,2’-azobisisobutyronitrile (AIBN) were obtained from Kelong Chemical Reagent Company (Chengdu, China). BPA stock solution (0.001 g mL−1) was prepared in ethanol. The stock solutions of β-CD (0.01 mol L−1), sodium dodecylbenzenesulfonate (0.01 mol L−1), and Triton X-100 (0.01 mol L−1) were prepared in doubly distilled water. All stock solutions were stored in a refrigerator at 4 °C. Before use, β-CD was purified by recrystallization twice in water, followed by vacuum drying at 60 °C for 12 h, and EGDMA and MAA were distilled. All the reagents were of analytical grade except for AIBN and β-CD, which were of chemical purity grade. The plastic-clad silica optical fiber (inner diameter 300 μm, numerical aperture 0.37) was purchased from Chunhui (Nanjing, China). The silica capillary tube (inner diameter 530 μm, outer diameter 690 μm) was purchased from Yongnian Optic Fiber Plant (Hebei, China). Apparatus All fluorescence measurements were performed with a 970 CRT fluorospectrophotometer (Shanghai Precision and
A homogeneous monomer mixture of MAA (0.4 mL), EGDMA (0.02 mL), AIBN (0.001 g), and BPA (0.002 g) in ethanol (0.3 mL) was degassed under nitrogen for 10 min. This mixture was used as the sensor solution. Pretreatment of the optical fiber Part of the plastic clading (20 mm in length) of the optical fiber was declad to let the evanescent wave pass out of the optical fiber. The declad region was immersed in 0.1 mol L−1 NaOH solution for 24 h and was washed with water and ethanol and dried. Modification of optical fiber column based on BPA MIP The declad section of the fiber was immersed in the sensor solution for 5 min. Then the fiber was taken out and photopolymerization was performed under UV light at 365 nm for 2 h. These steps were repeated four times. The MIP film was less than 5 μm thick. The resulting polymer film modified on the optical fiber surface was soaked in a 0.1 mol L−1 acetic acid solution for 2 h and rinsed with deionized water in order to remove the BPA template. The MIP fabrication process is shown in Fig. 2. The photonic nonimprinted polymer was prepared using the same procedure
Y. Xiong et al.
CH2
C
C
CH3 O
CH3
CH3 O
C
OH + HO
MAA
OH + CH2
CH3
O
C
C
The BPA determination procedures with the proposed sensor OH
MAA
The sensor system used in this work is shown in part b of Fig. 3. The procedures for BPA enrichment, elution, and fluorescence detection can be summarized as the following five steps:
BPA
+ O O
Crosslinker Imprinting
CH3 HO OH O C
C
C OH
O
HO
CH3
Elution
C
O
HO OH O C
Fig. 2 The molecularly imprinted polymer (MIP) fabrication process. MAA methacrylic acid
but in the absence of BPA. All of the procedures were performed at room temperature (25 °C).
Fabrication of the BPA-MIP-based annular column sensor Part of the polyimide coating (20 mm in length) on the capillary was burned off to form a detection window for light to pass through. The annular column sensor was constructed by inserting the modified optical fiber into the capillary tube (Fig. 3, part a). To reduce friction and damage to the surface of the fiber, the capillary tube was filled with water before the optical fiber was inserted. A xenon lamp fixed in the 970 CRT fluorospectrophotometer was used as the light source and coupled with one end of the optical fiber. A PMT fixed in the 970 CRT fluorospectrophotometry was used as a detector for the fluorescence signal and was arranged parallel to the capillary tube axis. By setting the fiber optical column and capillary tube sides parallel to the PMT detector instead of setting the fiber end vertical to the PMT detector, we improved the sensor sensitivity because the light source background was decreased and the fluorescence signal collection efficiency was enhanced with this novel design.
Step 1 (separation and enrichment of BPA). In this step, pump 2 was stopped. Pump 1 caused BPA solution (or sample solution) to flow through the micro flow cell formed between the MIP-based fiber column and the capillary. BPA was then selectively absorbed on the MIP film that was modified on the fiber surface. BPA could be separated and enriched on the MIP film in this step. Step 2 (gradient elution of structural analogues and potential interfering substances). In this step, pump 2 was also stopped. Gradient eluent replacing BPA solution was caused by pump 1 to flow through the micro flow cell to eliminate potential interfering substances with appropriate elution times. Step 3 (fluorescence detection of BPA). After the potential interfering substances had been eliminated in step 2, BPA was selectively enriched on the MIP film modified on the fiber column. With use of the xenon lamp as the light source, the evanescent wave produced on the fiber core surface excited the enriched BPA to produce fluorescence. Appropriate surfactant and appropriate pH buffer solution were caused by pump 2 to flow through the flow cell to enhance the fluorescence intensity. Step 4 (pulsed elution of BPA). In this step, pulsed eluent replacing gradient eluent was caused by pump 1 to flow through the flow cell to elute the adsorbed BPA on the MIP film. Step 5 (cleaning the cavities on the MIP film). After the fluorescence intensity became stable at the blank level, water was caused by pump 1 to flow through the cell to clean the cavities on the MIP film for the next determination.
Binding measurements The binding characteristics of BPA-imprinted polymer were estimated by a dynamic method. The BPA solution was continuously flowed through the MIP column sensor at a constant flow rate. Then the absorbance of free BPA in the efflux solution was detected by the UV absorbance at 280 nm.
A microvolume sensor based on molecularly imprinted polymer
Fig. 3 The annular column sensor (a) and the MIP-based fiber-optic sensor system (b). PMT photomultiplier tube
θc =sin−1(n2/n1). BPA immobilized in the MIP film present within the penetration depth could be excited by the evanescent field to produce fluorescence.
Results and discussion Principles of evanescent wave sensing An evanescent wave is an exponentially decaying electric field that penetrates a short distance into a low refractive index medium when total internal reflection occurs at a dielectric interface [45]. The degree of penetration is often characterized by the penetration depth, dp, which is the perpendicular distance from the interface at which the electric field amplitude, E, has fallen to 1/e of its value, E0 at the interface i.e., . E ¼ E o exp −z d p : ð1Þ
The magnitude of the penetration depth is given by dp ¼
λ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 2πn1 sin2 θ−sin2 θc
λ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ; s . 2 2πn1 sin2 θ− n2 n1 ð2Þ
where λ is the vacuum wavelength, θ is the angle of incidence to the normal at the interface, and n1 and n2 are the refractive indices of the dense and rare media, respectively. Although dp is typically less than λ, it is clear from Eq. 2 that its value rises sharply as the angle of incidence approaches the critical angle
Spectral characteristics Spectra a and b in Fig. 4 show the excitation spectrum and emission spectrum of the BPA solution, respectively. As shown, the BPA solution had two excitation peaks in spectrum a at 242 and 276 nm and two emission peaks in spectrum b at 306 and 607 nm. As a result, 276 nm was chosen as the wavelength for excitation of the BPA solution as its intensity was greater than that of 242 nm. According to the basic fluorescence principle, 306 nm was chosen as the emission wavelength of the BPA solution as its peak shape and intensity match the excitation wavelength of 276 nm in spectrum a. As a result, the following measurements were made at 276 nm/ 306 nm (λex/λem). Optimization of conditions To obtain the best analytical performance of the proposed FOEWS system, several operation conditions were optimized. Optimization of sensor construction To obtain the maximum light collection efficiency, the sensing length of the optical fiber was selected to be 20 mm to match the length of the optical window of the PMT. With a distance
Y. Xiong et al.
160
1200
(a) BPA excitation spectrum (b) BPA emission spectrum 276 nm
306 nm
Fluorescence intensity
Fluorescence intensity
200
120 b
a 80
607 nm 242 nm
40
- CD effect on fluorescence intensity MSDS effect on fluorescence intensity Triton-100 effect on fluorescence intensity
1000 800 600 400 200
0 200
300
400
500
600
700
0
800
Wavelength (nm)
Fig. 4 The fluorescence excitation spectrum and emission spectrum of the BPA solution
1
2
3
4
5
6
-logC
Fig. 5 Comparison of the effects of the surfactants β-cyclodextrin (βCD), sodium dodecylbenzenesulfonate (MSDS), and Triton X-100 on BPA fluorescence enhancement
between the fiber and the capillary inner tube (dc) of about 100 μm, the whole volume of the annular microchannel was less than 10.0 μL and the volume of the detection part in front of the PMT window was about 2.0 μL. The small dc could effectively inhibit the sample dispersion caused by solution mixing, which was helpful to improve the sensor sensitivity. Further, the small volume could also speed up the sample displacement, which was beneficial to shorten the response time.
shown, because pKa of BPA is approximately 9.28 [51], the dissociation of BPA was inhibited at pH
RESEARCH PAPER
A microvolume molecularly imprinted polymer modified fiber-optic evanescent wave sensor for bisphenol A determination Yan Xiong & Zhongbin Ye & Jing Xu & Yucheng Liu & Hanyin Zhang
Received: 26 September 2013 / Revised: 23 November 2013 / Accepted: 16 December 2013 # Springer-Verlag Berlin Heidelberg 2014
Abstract A fiber-optic evanescent wave sensor for bisphenol A (BPA) determination based on a molecularly imprinted polymer (MIP)-modified fiber column was developed. MIP film immobilized with BPA was synthesized on the fiber column, and the sensor was then constructed by inserting the optical fiber prepared into a transparent capillary. A microchannel (about 2.0 μL) formed between the fiber and the capillary acted as a flow cell. BPA can be selectively adsorbed online by the MIP film and excited to produce fluorescence by the evanescent wave produced on the fiber core surface. The conditions for BPA enrichment, elution, and fluorescence detection are discussed in detail. The analytical measurements were made at 276 nm/306 nm (λex/λem), and linearity of 3×10−9–5×10−6 g mL−1 BPA, a limit of detection of 1.7×10−9 g mL−1 BPA (3σ), and a relative standard deviation of 2.4 % (n=5) were obtained. The sensor selectivity and MIP binding measurement were also evaluated. The results indicated that the selectivity and sensitivity of the proposed fiber-optic sensor could be greatly improved by using MIP as a recognition and enrichment element. Further, by modification of the sensing and detection elements on the optical fiber, the proposed sensor showed the advantages of easy fabrication and low cost. The novel sensor configuration provided a platform for monitoring other species by simply changing the Y. Xiong (*) : Z. Ye : Y. Liu : H. Zhang State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China e-mail: [email protected] Y. Xiong : Z. Ye : Y. Liu School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, China J. Xu Liaoning Entry-Exit Inspection and Quarantine Bureau, Dalian 116001, China
light source and sensing elements. The sensor presented has been successfully applied to determine BPA released from plastic products treated at different temperatures. Keywords Fiber-optic sensor . Bisphenol A . Evanescent wave . Molecularly imprinted polymer
Introduction Bisphenols are a class of chemicals with two hydroxyphenyl functionalities and include several analogues. Among the bisphenol analogues that are in commercial use, bisphenol A (BPA; Fig. 1a) has been the most widely studied. BPA is a high production volume chemical and is widely used as the chemical building block in the manufacture of polycarbonate plastics [1], the epoxy resins used in food can linings [2], and dental fillings and sealants [3]. Further, BPA has also been used as a component in the production of thermal papers (as a weakly acidic color developer) for more than 50 years [4]. Human exposure to BPA that leaches from packaging or storage containers into foods and beverages is a great public health concern, owing to the endocrine-disrupting potential of this compound. BPA can potentially interfere with the endocrine system of wildlife and humans, increase the cancer rate, reduce immune function, and impair reproduction [5,6]. Migration of BPA from polycarbonate drinking bottles and baby bottles into water and foods [7,8], migration of BPA from metallic can epoxy coatings into various food items [9,10], and migration of BPA from paper products [11] have been reported. The European Commission has established a migration limit of 600 μg kg−1 for BPA in food or food simulators from plastic materials and articles intended to come in contact with foodstuffs [12]. Owing to widespread human exposure and its toxicity, BPA has drawn considerable attention from regulatory
Y. Xiong et al.
a
CH3
HO
C
OH
CH3
b HO
CH2
OH
Fig. 1 The structures of a bisphenol A (BPA) and b bisphenol F
organizations and the general public in recent years [13]. Restrictions on the use of BPA in certain consumer products have been suggested [14–16]. For example, the US Food and Drug Administration banned the use of BPA in baby bottles and children’s drinking cups in July 2012 [15]. The manufacture, sale, or distribution of BPA has also been prohibited for some consumer products, such as reusable food or beverage containers, infant formula containers, and thermal receipt paper, by several states in the USA since 2009 [16]. The European Commission [14] and China restricted the use of BPA in plastic infant feeding bottles in January 2011 and June 2012. Thus, the determination of BPA is of vital significance for risk assessment and control of human exposure to BPA. A number of analytical methods for BPA determination have been reported, including gas chromatography coupled with mass spectrometry [17,18], liquid chromatography coupled with mass spectrometry [19,20], high-performance liquid chromatography [21], enzyme-linked immunosorbent assays [22], fluorimetry [23,24], a surface plasmon resonance biosensor [25], and electrochemical methods, including capillary electrophoresis [26] and electrochemical sensors [27–31]. However, although many detection modes can be used to quantify the amounts of BPA, strategies to achieve the specific recognition of BPA commonly adopt chromatography [17–21], which can easily discriminate impurities and provide high sensitivities. However, these chromatographic methods not only need expensive and complicated instruments, but also are laborious and time-consuming. The molecular imprinting technique is a rapidly developing technique for the preparation of polymers that possess specific recognition functions for the template molecule. In principle, molecularly imprinted polymer (MIP) is prepared by copolymerization of a template molecule (generally the analyte itself) with a functional monomer capable of interacting with the template molecule in the presence of a cross-linker to produce three-dimensional network polymers. Removal of the template molecule from the resultant polymer yields a functional polymeric matrix possessing microcavities with a threedimensional structure complementary in both shape and chemical functionality to that of the template molecule. Since its introduction by Wulff et al. [32] and Vlatakis et al. [33], the molecular imprinting technique has become a well established
and facile technique. Owing to the molecular memory, imprinted polymers are able to recognize and bind target molecules, which results in their extensive uses as solidphase-extraction matrices [34,35], antibody mimics [36], enzyme mimics [37], catalytic micromotors [38], and artificial receptors mimicking natural biological receptors, for example, in chemical sensors [39] and biochips [40]. MIPs for BPA prepared by different methods have been used as the solidphase-extraction matrices in liquid chromatography [41] and high-performance liquid chromatography [42], and as the sensing matrices for electrochemical sensors [26,43]. However, the integration of imprinted polymers into sensors and similar devices in a suitable form such as thin films, micropatterns, and nanopatterns is the main challenge of practical sensor development and requires microfabrication processes [44]. Fiber-optic sensors based on the principle of an evanescent wave [45], known as fiber-optic evanescent wave sensors (FOEWS), have attracted great attention because of their potential for in vivo measurement in biological and chemical applications [46,47]. When light travels by total internal reflection in an optical fiber core, an evanescent wave can be produced on the fiber surface and penetrates into the surrounding medium. Fluorophores around or immobilized on the core surface can be excited by the evanescent wave to produce fluorescence. This approach avoids laborious optical adjustment and is thus convenient and effective in practical applications. FOEWS for nitrite [48], oxygen [49], and pH [50] determination have been developed in our laboratory. To the best of our knowledge, there is no report of BPA determination based on FOEWS sensing. This work aimed to develop a novel MIP-based FOEWS with a microliter detection volume for BPA determination. The BPA was immobilized in MIP film and then modified on the surface of an unclad optical fiber. The fiber prepared was used as a sensing element and inserted into the optical fiber in a transparent capillary to construct the sensor. A microchannel with a small detection volume (about 2.0 μL) formed between the optical fiber and the capillary acted as a flow cell. For the proposed sensor, BPA can be selectively adsorbed online by the MIP film and then excited to produce fluorescence by the evanescent wave field produced on the fiber core surface. To decrease the light source background and improve the fluorescence signal collection efficiency, the photomultiplier (PMT) detector was designed to be parallel to the axis of the optical fiber in the proposed sensor construction for BPA determination. The spectroscopic properties were characterized by UV–vis and fluorescence measurements. The sensor performances, including sensitivity, repeatability, binding measurement, pH effect and surfactant effect, are reported in detail for the proposed BPA sensor. A detailed performance comparison of the method with other methods based on MIPs for determination of BPA was made, and the results are listed
A microvolume sensor based on molecularly imprinted polymer Table 1 Performance comparison of the proposed method with other methods based on molecularly imprinted polymers (MIPs) for determination of bisphenol A Detection method based on MIP
Limit of detection (g mL−1)
Real sample/matrix
Sensor volume and temperature effect
Reference
Capillary electrophoresis Electrochemical sensor LC HPLC Surface-enhanced Raman scattering Fiber-optic sensor
1.8×10−9 3.4×10−8 6.8×10−11 2.0×10−10 1.2×10−7 1.7×10−9
Water samples and human urine Water River water Water and milk Real samples Plastic productions
Not shown Not shown Not shown Not shown Not shown 2.0 μL 25–70 °C
[26] [31] [41] [42] [52] Present study
HPLC high-performance liquid chromatography, LC liquid chromatography
in Table 1. The results showed that all of the methods were applicable to BPA determination in real samples. However, the present MIP-based FOEWS was fabricated with a small detection volume. Further, the present sensor was applied to detection of BPA released from common plastic products at different temperatures with different solvents.
Scientific Instrument Company, China). The UV–vis absorption spectra were obtained with a UV-1800 spectrophotometer (Hitachi, Japan). A homemade flow cell was used for the sensing measurements. Solutions were delivered by two peristaltic pumps (HL-2, Shanghai Huxi, China). PTFE tubing (inner diameter 0.8 mm) was used to connect the components.
Experimental
Fabrication of the BPA-MIP microvolume sensor
Reagents and materials
Preparation of the sensor solution
B PA , β - c y c l o d e x t r i n ( β - C D ) , s o d i u m dodecylbenzenesulfonate, Triton X-100, acetonitrile, boric acid (H3BO3), phosphoric acid (H3PO4), acetic acid, ethanol, methacrylic acid (MAA), ethylene glycol dimethacrylate (EGDMA), and 2,2’-azobisisobutyronitrile (AIBN) were obtained from Kelong Chemical Reagent Company (Chengdu, China). BPA stock solution (0.001 g mL−1) was prepared in ethanol. The stock solutions of β-CD (0.01 mol L−1), sodium dodecylbenzenesulfonate (0.01 mol L−1), and Triton X-100 (0.01 mol L−1) were prepared in doubly distilled water. All stock solutions were stored in a refrigerator at 4 °C. Before use, β-CD was purified by recrystallization twice in water, followed by vacuum drying at 60 °C for 12 h, and EGDMA and MAA were distilled. All the reagents were of analytical grade except for AIBN and β-CD, which were of chemical purity grade. The plastic-clad silica optical fiber (inner diameter 300 μm, numerical aperture 0.37) was purchased from Chunhui (Nanjing, China). The silica capillary tube (inner diameter 530 μm, outer diameter 690 μm) was purchased from Yongnian Optic Fiber Plant (Hebei, China). Apparatus All fluorescence measurements were performed with a 970 CRT fluorospectrophotometer (Shanghai Precision and
A homogeneous monomer mixture of MAA (0.4 mL), EGDMA (0.02 mL), AIBN (0.001 g), and BPA (0.002 g) in ethanol (0.3 mL) was degassed under nitrogen for 10 min. This mixture was used as the sensor solution. Pretreatment of the optical fiber Part of the plastic clading (20 mm in length) of the optical fiber was declad to let the evanescent wave pass out of the optical fiber. The declad region was immersed in 0.1 mol L−1 NaOH solution for 24 h and was washed with water and ethanol and dried. Modification of optical fiber column based on BPA MIP The declad section of the fiber was immersed in the sensor solution for 5 min. Then the fiber was taken out and photopolymerization was performed under UV light at 365 nm for 2 h. These steps were repeated four times. The MIP film was less than 5 μm thick. The resulting polymer film modified on the optical fiber surface was soaked in a 0.1 mol L−1 acetic acid solution for 2 h and rinsed with deionized water in order to remove the BPA template. The MIP fabrication process is shown in Fig. 2. The photonic nonimprinted polymer was prepared using the same procedure
Y. Xiong et al.
CH2
C
C
CH3 O
CH3
CH3 O
C
OH + HO
MAA
OH + CH2
CH3
O
C
C
The BPA determination procedures with the proposed sensor OH
MAA
The sensor system used in this work is shown in part b of Fig. 3. The procedures for BPA enrichment, elution, and fluorescence detection can be summarized as the following five steps:
BPA
+ O O
Crosslinker Imprinting
CH3 HO OH O C
C
C OH
O
HO
CH3
Elution
C
O
HO OH O C
Fig. 2 The molecularly imprinted polymer (MIP) fabrication process. MAA methacrylic acid
but in the absence of BPA. All of the procedures were performed at room temperature (25 °C).
Fabrication of the BPA-MIP-based annular column sensor Part of the polyimide coating (20 mm in length) on the capillary was burned off to form a detection window for light to pass through. The annular column sensor was constructed by inserting the modified optical fiber into the capillary tube (Fig. 3, part a). To reduce friction and damage to the surface of the fiber, the capillary tube was filled with water before the optical fiber was inserted. A xenon lamp fixed in the 970 CRT fluorospectrophotometer was used as the light source and coupled with one end of the optical fiber. A PMT fixed in the 970 CRT fluorospectrophotometry was used as a detector for the fluorescence signal and was arranged parallel to the capillary tube axis. By setting the fiber optical column and capillary tube sides parallel to the PMT detector instead of setting the fiber end vertical to the PMT detector, we improved the sensor sensitivity because the light source background was decreased and the fluorescence signal collection efficiency was enhanced with this novel design.
Step 1 (separation and enrichment of BPA). In this step, pump 2 was stopped. Pump 1 caused BPA solution (or sample solution) to flow through the micro flow cell formed between the MIP-based fiber column and the capillary. BPA was then selectively absorbed on the MIP film that was modified on the fiber surface. BPA could be separated and enriched on the MIP film in this step. Step 2 (gradient elution of structural analogues and potential interfering substances). In this step, pump 2 was also stopped. Gradient eluent replacing BPA solution was caused by pump 1 to flow through the micro flow cell to eliminate potential interfering substances with appropriate elution times. Step 3 (fluorescence detection of BPA). After the potential interfering substances had been eliminated in step 2, BPA was selectively enriched on the MIP film modified on the fiber column. With use of the xenon lamp as the light source, the evanescent wave produced on the fiber core surface excited the enriched BPA to produce fluorescence. Appropriate surfactant and appropriate pH buffer solution were caused by pump 2 to flow through the flow cell to enhance the fluorescence intensity. Step 4 (pulsed elution of BPA). In this step, pulsed eluent replacing gradient eluent was caused by pump 1 to flow through the flow cell to elute the adsorbed BPA on the MIP film. Step 5 (cleaning the cavities on the MIP film). After the fluorescence intensity became stable at the blank level, water was caused by pump 1 to flow through the cell to clean the cavities on the MIP film for the next determination.
Binding measurements The binding characteristics of BPA-imprinted polymer were estimated by a dynamic method. The BPA solution was continuously flowed through the MIP column sensor at a constant flow rate. Then the absorbance of free BPA in the efflux solution was detected by the UV absorbance at 280 nm.
A microvolume sensor based on molecularly imprinted polymer
Fig. 3 The annular column sensor (a) and the MIP-based fiber-optic sensor system (b). PMT photomultiplier tube
θc =sin−1(n2/n1). BPA immobilized in the MIP film present within the penetration depth could be excited by the evanescent field to produce fluorescence.
Results and discussion Principles of evanescent wave sensing An evanescent wave is an exponentially decaying electric field that penetrates a short distance into a low refractive index medium when total internal reflection occurs at a dielectric interface [45]. The degree of penetration is often characterized by the penetration depth, dp, which is the perpendicular distance from the interface at which the electric field amplitude, E, has fallen to 1/e of its value, E0 at the interface i.e., . E ¼ E o exp −z d p : ð1Þ
The magnitude of the penetration depth is given by dp ¼
λ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 2πn1 sin2 θ−sin2 θc
λ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ; s . 2 2πn1 sin2 θ− n2 n1 ð2Þ
where λ is the vacuum wavelength, θ is the angle of incidence to the normal at the interface, and n1 and n2 are the refractive indices of the dense and rare media, respectively. Although dp is typically less than λ, it is clear from Eq. 2 that its value rises sharply as the angle of incidence approaches the critical angle
Spectral characteristics Spectra a and b in Fig. 4 show the excitation spectrum and emission spectrum of the BPA solution, respectively. As shown, the BPA solution had two excitation peaks in spectrum a at 242 and 276 nm and two emission peaks in spectrum b at 306 and 607 nm. As a result, 276 nm was chosen as the wavelength for excitation of the BPA solution as its intensity was greater than that of 242 nm. According to the basic fluorescence principle, 306 nm was chosen as the emission wavelength of the BPA solution as its peak shape and intensity match the excitation wavelength of 276 nm in spectrum a. As a result, the following measurements were made at 276 nm/ 306 nm (λex/λem). Optimization of conditions To obtain the best analytical performance of the proposed FOEWS system, several operation conditions were optimized. Optimization of sensor construction To obtain the maximum light collection efficiency, the sensing length of the optical fiber was selected to be 20 mm to match the length of the optical window of the PMT. With a distance
Y. Xiong et al.
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(a) BPA excitation spectrum (b) BPA emission spectrum 276 nm
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Fig. 4 The fluorescence excitation spectrum and emission spectrum of the BPA solution
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Fig. 5 Comparison of the effects of the surfactants β-cyclodextrin (βCD), sodium dodecylbenzenesulfonate (MSDS), and Triton X-100 on BPA fluorescence enhancement
between the fiber and the capillary inner tube (dc) of about 100 μm, the whole volume of the annular microchannel was less than 10.0 μL and the volume of the detection part in front of the PMT window was about 2.0 μL. The small dc could effectively inhibit the sample dispersion caused by solution mixing, which was helpful to improve the sensor sensitivity. Further, the small volume could also speed up the sample displacement, which was beneficial to shorten the response time.
shown, because pKa of BPA is approximately 9.28 [51], the dissociation of BPA was inhibited at pH
