Michael-Addition-Mediated Photonic Crystals ... - ACS Publications

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Michael-addition-mediated photonic crystals allows pretreatmentfree and label-free sensoring of Ciprofloxacin in fish farming water Yanqiu Song, Jialei Bai, Rong Zhang, Houluo He, Chao Li, Jiang Wang, Shuang Li, Yuan Peng, Baoan Ning, Minglin Wang, and Zhixian Gao Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04655 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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Analytical Chemistry

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Michael-addition-mediated photonic crystals allows pretreatmentfree and label-free sensoring of Ciprofloxacin in fish farming water Yanqiu Songa,b,#, Jialei Baib,#, Rong Zhangb, Houluo Heb, Chao Lib, Jiang Wangb, Shuang Lib, Yuan Pengb,*, Baoan Ningb,*, Minglin Wanga,*, Zhixian Gaob [a] College of Food Science and Engineering, Shandong Agricultural University, Tai’an 271018, P. R. China [b] Tianjin Key Laboratory of Risk Assessment and Control Technology for Environment and Food Safety, Tianjin Institute of Health and Environmental Medicine, Tianjin 300050, P. R. China Corresponding Author * Y. Peng: e-mail, [email protected]; tel/fax, +86-22-84655403. * B. Ning: e-mail, [email protected]; tel/fax, +86-22-84655192. * M. Wang: e-mail, [email protected]; tel/fax, +86-531-82492410.

ABSTRACT: The abuse of antibiotics results in a large number of antibiotics residues in the environment and even causes the problem of “super bacteria”. Therefore, it is crucial to develop a powerful analytic method to monitor antibiotics quickly and simply. Photonic crystal (PC), as a sensing material, has a promising application prospects. Herein, we adventurously try to use PC to realize the pretreatment-free and label-free detection of Ciprofloxacin (CF) through Michael addition reaction. The recognition process is carried out by the Michael addition reaction between the piperazine group of CF and the o-benzoquinone group on the PC. The monodisperse microspheres with o-benzoquinone groups are prepared by polymerization and oxidation and then stacked to form PC. During the detection, the peak intensity of the PC decreases with the increasing CF concentration, and the linear range is from 2 to 512 µg/L. And the limit of detection (LOD) is 0.76 µg/L. Furthermore, it retains 97% of the initial response after stored in a petri dish at room temperature for 1 month, which shows that the PC has well stability. Moreover, the CF in fish farming water can be detected directly without any pretreatment and label, and the results are in good accordance with the LC-MS-MS results. This Michael-addition-mediated PC is accurate, easily prepared, cost-efficient and long-term stable. In addition, it’s environmentally friendly, because little organic solvent is needed during both the preparation and the detection.

INTRODUCTION Fluoroquinolone drugs, as the bactericidal drugs, play a positive role in the health of the human body, breeding industry and the improvement of economic interests. Ciprofloxacin (CF) [1-cyclopropyl-6-fluoro-1,4-dihydro-4oxo-7-(piperazinyl) quinolone-3-carboxylic acid], a fluoroquinolone drug, presents a broad-spectrum antibacterial activity, and becomes one of the most commonly used antibiotics in fish farming1,2 due to the low cost and distinct curative effect. However, its abuse can lead to the accumulation in the environment and the increase of antibiotic-resistant bacteria.3-5 Therefore, it is urgent to monitor the CF amount to prevent the accumulation in the environment and avoid the harmful influence on human. Photonic crystal (PC), as an optical sensing material, offers a convenient tool for label-free, ultrasensitive and fast detection. PC is a periodic arrangement of regularly shaped materials with different dielectric constants. It has distinct diffraction wavelength which causes its specific structural color. When the PC exposes to physical or chemical stimuli, its diffraction wavelength or intensities will be changed.6,7 Thus it has attracted much interest in the application in vitro diagnosis,8-13 food safety,14-17 and environmental monitoring.18-21 As is known to all, the recognition groups attached to the PC is the

key to achieve specific detection. The chemical reaction between specific recognition groups and targets is regarded as an alternative choice. The Michael addition is the nucleophilic addition of a carbanion or another nucleophile to an α, βunsaturated carbonyl compound.22,23 It belongs to the larger class of conjugate additions, and it’s one of the most useful methods for the mild formation of C–C bonds. Interestingly, the Ciprofloxacin (CF) contains piperazine group which can react with the o-benzoquinone through Michael addition reaction. Therefore, we adventurously try to use photonic crystals as a sensing material to detect CF through the Michael addition.24 In this work, we proposed a PC sensing material for rapid monitoring CF in fish farming water without pretreatment and label. The PC was an opal structure, which was prepared by the oxidized monodisperse microspheres polymerized by methyl methacrylate (MMA) and 3-methacrylamidopamine (DMA). This oxidized PMMA-DMA (o-PMD) microspheres were attached with the o-benzoquinone group and the piperazine group in CF could react with the o-benzoquinone group resulting in the decrease of the diffraction peak intensity of PC.25 The measuring principle of o-PMD PC was shown in Figure 1. The CF amount in fish farming water was detected directly without any pretreatment and label, and the detection process was completed within 15 min. CF-spiked fish farming


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Analytical Chemistry water was determined by the PC and LC-MS-MS method, respectively, and the results indicated that the PC presented good accuracy. This Michael-addition-mediated PC offered a

Figure

1.

Schematic

illustrations

of

the

promising application in clinical assays, food safety and environmental monitoring.

preparation

and

detection

procedure

of

the

o-PMD

PC

followed by washing with deionized water and ethanol for 5 times in glass petri dishes and dried with nitrogen.

EXPERIMENTAL SECTION Instrumentation

Synthesis and Oxidation of PMD microspheres The reflection spectra were obtained by the Ocean Optics Maya 2000 PRO fiber optic spectrometer (Ocean Optics, Dunedin, FL, USA). The SEM images were obtained from a Thermo-4800 high-resolution field emission SEM. TEM images were acquired with a JEM-2100 transmission electron microscope (JEOL). Dynamic Light Scattering (DLS) results were obtained from a Brookhaven 90 Plus laser particle size analyzer. FTIR spectra were recorded by Tensor 27 (Bruker, Germany) spectrophotometer. The constant temperature and humidity chamber was purchased from Ningbo Hai shu Sai fu Experimental Instrument Co., Ltd. 1H NMR spectra were recorded on a Varian UNITY-plus 400 NMR spectrometers. Chemical and reagents Methyl methacrylate (MMA) was purchased from Aladdin (Shanghai, China) and treated with activated carbon for 24 h prior to use. N-(3,4-dihydroxyphenethyl) Meth acrylamide (DMA) was purchased from Yuan Ye Reagent Company (Shanghai, China). Potassium persulfate (KPS) (Aladdin) was recrystallized using deionized water prior to use. Ciprofloxacin (CF), Enoxacin (EX), Norfloxacin (NF), Ofloxacin (OX), Fleroxacin (FX), Sparfloxacin (SF) and Horseradish peroxidase(HRP) were purchased from J&K. Phosphate buffer (PBS) and Tris-HCl were purchased from Sigma. The deionized water used in the experiment was laboratory homemade through ion-exchange columns. The glass slides used to the preparation of PD PC were immersed in an H2SO4/H2O2 (7:3, v/v) mixture solution for at least 24 h,

DMA was chosen as a monomer because it contained the catechol group which could be oxidized to form the obenzoquinone group. In order to obtain the monodisperse microspheres, MMA was chosen as the co-monomer which could polymerize with DMA through soap-free emulsion polymerization method. Briefly, the deionized water (25.5 mL) was mixed with the pre-polymerized solution MMADMA (an amount of DMA dissolved in 3 mL of MMA) in a three-necked round bottom flask and deoxygenated for 10 min. The mixture was heated in a water bath at 80 °C and stirred at 370 rpm (the whole process was completed under a nitrogen atmosphere). Subsequently, the KPS solution (60 mg KPS dissolved in 1.5 mL deionized water) was added to the mixture and the reaction lasted for 90 min. Finally, the PMD microspheres were obtained. Then 0.1 mM H2O2 (1 mL) was added to the PMD suspension with HRP (10.0 mg of HRP dissolved in 0.50 mL of 0.10 M phosphate buffer, pH 7.00) as a catalyst. The reaction was carried out overnight at room temperature. The oxidized microspheres were centrifuged at 4000 rpm for 10 min to remove the supernatant and precipitate, and the intermediate layer was collected to acquire microspheres with uniform particle size. The hydrodynamic diameter (Dh) of the microgel particles were measured by dynamic light scattering with a Brookhaven 90 Plus laser particle size analyzer. The transmission electron microscope (TEM) was used to observe the particle morphology of the PMD microspheres. Fourier transforms



infrared (FTIR) spectra were used to characterize the o-PMD microspheres, and PMMA and the PMD microspheres without oxidization were used as a control. 1H NMR spectra (using deuterated DMSO as solvent) were used to verify whether the o-PMD was successfully prepared, and PMMA and the PMD microspheres before oxidization were used as a control. Preparation of the o-PMD PC The o-PMD PC was prepared by the vertical convective selfassembly method. A glass bottle containing 10 mL of o-PMD microspheres (1 wt% o-PMD in deionized water) was placed in an oven at 40� °C and after 5 or 6 days, the highly ordered o-PMD PC was gained. PMMA PC (without DMA) and unoxidized PMD (uno-PMD) PC were prepared using the same method. The arrangement of the microspheres in the PC, the structure and morphology of the PC were characterized by scanning electron microscopy (SEM).

collected and observed by TEM (Figures S1a, b). The TEM results indicated that polymerization for 90 min could facilitate a better spherical shape of the PMD microsphere, and 90 min was chosen for further experiments. Different amount of DMA in a range of 20-50 mg was respectively dissolved in 3 mL MMA and polymerized for 90 min, and the products were collected and observed by TEM (Figures S1c, d, e). The TEM results showed that with the increasing amount of DMA, the spherical shape of the PMD microsphere became worse and the particle size got nonuniform. Thus, the amount of DMA was selected to be 20 mg, and the TEM image of the products was shown in Figure 2a. The Figure 2a indicated that the PMD microspheres had a uniform spherical shape and the particle size was about 250 nm.

Optical response of the o-PMD PC to the analytes The o-PMD PC was immersed in a phosphate buffer solution (pH 7.4) until it reached an equilibrium state (about 30 min). The reflection spectra of the o-PMD PC were obtained using a fiber optic spectrometer. The probe of the fiber optic spectrometer was fixed vertically over the surface of the PC. Firstly, a blank line was recorded, followed by the addition of different concentrations of CF (from 0.01 µg/L to 2896 µg/L) and the response spectra were recorded in turn. The PMMA PC and the uno-PMD PC were used as a control. The temperature in the measurement was controlled at room temperature. The analytes were dissolved in 10 mL of glacial acetic acid – water (1:19 v/v) solution. Specificity of the PC to analytes The structure analogs of Ciprofloxacin (CF), such as OX, FX and SF, were detected using this o-PMD PC to investigate the specificity of this sensing material. The o-PMD PC was immersed into PBS for 30 min and the response to the mentioned antibiotics at different concentration was recorded. Sample detection Fish farming water was collected from the Weishan Lake in Shandong. It was directly added to the o-PMD PC sensing system without any pre-treatment. Meanwhile, the samples were detected by LC-MS-MS method referencing from the National Standard Method of China (GB/T 20751-2006). Then the samples were spiked with CF (5 µg/L, 100 µg/L and 300 µg/L, respectively), and were detected both by the o-PMD PC sensing method and the LC-MS-MS method. The samples and spiked samples were tested three times. RESULTS AND DISCUSSION Optimization of the amount of DMA and polymerization time The amount of DMA and polymerization time could have an important influence on the morphology and particle size of the microsphere. The polymerization time was optimized firstly. 20 mg DMA was dissolved in 3 mL MMA and polymerized for 45 min or 90 min, respectively. Then the products were

Figure 2. a) TEM image of the o-PMD microspheres. b) DLS results of the o-PMD microspheres in aqueous solution.

Characterizations of PMD microspheres Figure 2b showed the particle size distribution of the o-PMD microspheres in aqueous solution. It indicated that the particle size distribution of the o-PMD microspheres was narrow, and the average diameter was 258.4 nm ± 5 nm, demonstrating a good uniformity and monodispersity of the o-PMD microspheres for the preparation of PC. In order to confirm whether DMA was polymerized in the microspheres and whether the oxidation was successful, FTIR and 1H NMR were used to characterize the microspheres. The FTIR spectra of PMMA and PMD microspheres were shown in Figure S2. In the blue and yellow line, the band at 1670.16 cm−1 was attributed to the C=O stretching vibration of amide, and the band at 1545.11 cm−1 was attributed to the N-H bending vibration, which was the typical absorption bands of DMA. The broad band at 3312.25 cm−1(Figure S2, blue line) was due to the O-H stretching vibration. It indicated that DMA had been successfully polymerized with MMA. However, the broad band at 3312.25 cm−1 disappeared in yellow line, and the little band at 1603.76 cm−1（Figure S2, yellow line）is due to the C=O stretching vibration of o-benzoquinone. These results confirmed that catechol groups in microspheres had been successfully oxidized to o-benzoquinone groups. The 1H NMR spectra of the uno-PMD and o-PMD microspheres were shown in Figure 3. δ8.78-8.55 (2H, s) and δ6. 61-6.43 (3H, m) (Figure 3a) indicated the successful introduction of catechol groups into the uno-PMD microspheres. The peaks at δ8.78 and δ8.55 disappeared in the 1 H NMR spectra of the o-PMD microspheres (Figure 3b), which demonstrated that the catechol groups in PMD had been oxidized to o-benzoquinone groups.


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Analytical Chemistry The response time of the o-PMD PC After the o-PMD PC reached equilibrium in the phosphate buffer solution (PBS, pH 7.4), the detection was carried out. When 2 µg/L of CF was added, the peak intensity decreased rapidly and became stable within 15 min (Figure 5). When the concentration was lower, the response time was shorter, and sometimes even within 2 min. With the increase of CF concentration, more and more binding sites were occupied, leading to longer response time, but also within 15 min. It indicated that the o-PMD PC could be used as a sensing material to realize rapid detection of CF.

Figure 5. Kinetic response of the o-PMD PC to CF (2 µg/L).

Optical response of the o-PMD PC to analytes

Figure 3. a) 1H NMR spectra of the uno-PMD microspheres. b) 1 H NMR spectra of the o-PMD microspheres.

Preparation and Characterizations of o-PMD PC The o-PMD PC was prepared by the vertical convective selfassembly method, and the morphology and the structural color of the PC were shown in Figure 4. The SEM results proved that o-PMD microspheres were closely arranged to form a multi-layer structure with highly ordered face-centered cubic lattice (Figure 4a, b). The o-PMD microspheres possessed a uniform size with diameters of approximately 258 nm. Under visible light, a green color, called “structure color” of the oPMD PC, was exhibited in Figure 4c, which was caused primarily by the Bragg diffraction.

When the o-PMD PC was stable in the detecting buffer, the CF standard solution, with the concentration from 0.01 µg/L to 2896 µg/L, was added gradually, and the diffraction spectrum was recorded using the fiber spectrophotometer. Figure 6a showed that with the increased CF concentration, the intensity of the diffraction peak decreased gradually, and the decrease amount (∆I) was linearly correlated with the logarithmic concentration of CF (Figure 6b) in the range from 2 µg/L to 512 µg/L. The R2 was 0.9993 and the limit of detection was 0.76 µg/L. This phenomenon could be explained by the combination of CF with the o-benzoquinone on the surface of the PC, which caused a change in the refractive index of the PC, leading to a decrease in the peak intensity. To further clarify the recognition properties of the o-PMD PC, PMMA PC and uno-PMD PC were used to detect CF under the same conditions and the data were shown in Figure 6c, d. According to the results, the diffraction peak intensity of PMMA PC (Figure 6c) and uno-PMD PC (Figure 6d) had a slight change with the increased CF concentration, indicating that oxidized DMA played an important role in the detection of CF, because it could provide o-benzoquinone groups which could combine with the piperazine groups of CF through the Michael addition reaction in alkaline aqueous solutions.

Figure 4. SEM (a and b) and optical (c) images of the o-PMD PC.



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Sample detection and Spiked recovery

Figure 6. Optical responses of different PC to series of CF concentrations. a) Responses of the o-PMD PC. b) The standard curve obtained from o-PMD PC. c) Responses of PMMA PC. d) Responses of uno-PMD PC.

Specificity of the o-PMD PC to analytes The responses of the o-PMD PC to CF analogs, such as Ofloxacin (OX), Fleroxacin (FX) and Sparfloxacin (SF) (structural formulas were described in Figure S4), were also investigated. OX, FX and SF were dissolved in 10 mL of glacial acetic acid – water (1:19, v/v) solution and then directly added to the detecting system, respectively. The results (Figure 7) indicated that these three antibiotics could hardly cause a decrease in the peak intensity of the PC. It was mainly due to that the hydrogen atom of piperazine groups in OX and FX were replaced by other groups, thereby failing to undergo Michael addition reaction with the o-benzoquinone, and thus it could scarcely cause a change in peak intensity. And for SF, the hydrogen atom of piperazine groups was not replaced, but the neighboring double methyl greatly increased the steric hindrance, which forbade the Michael addition between SF and the PC. Another word, the o-PMD PC had a good specificity to CF.

Fish farming water, collected from the Wei Shan Lake in Shandong, was directly added to the o-PMD PC sensing system without any pre-treatment, and the response was shown in Figure 8a. The peak intensity decreased about 35 a.u. and the amount of CF in the water was calculated to be 18.21 µg/L according to the standard curve. Meanwhile, the samples were detected by the LC-MS-MS method, and the results were 16.40 µg/L, indicating that the o-PMD PC sensing system possessed good accuracy. Then CF was spiked to the water sample and detected by the o-PMD PC (Figure 8b) and LCMS-MS method, respectively. The recoveries were listed in Table 1. According to the data, it illustrated that the PC results were in good accordance with the LC-MS-MS results, demonstrating that the o-PMD PC could be used to monitor the CF in real samples. It was important to note that the LCMS-MS could accurately detect CF in the concentration from 2 µg/L to 60 µg/L (Figure S5), thus when the concentration was over this range, it was necessary to dilute the samples gradually. While, the o-PMD PC sensing system possessed wider detection range and lower detection limit, and the samples could be monitored directly without any pretreatment. Furthermore, the o-PMD PC sensing system needed little organic solvent, avoiding the pollution to the environment.

Figure 8. a) Optical responses of the o-PMD PC to fish farming water. b) Optical responses of the o-PMD PC to fish farming water containing different concentrations of spiked CF.

Table 1. Determination results of the CF in fish farming water LC-MS-MS

o-PMD PC Amount spiked (µg/L)

Amount measured in mean (µg/L)

Recovery (%)

RSD (%)

Amount measured in mean (µg/L)

Recovery (%)

RSD (%)

0

18.21

/

0.5

16.40

/

0.7

5

23.20

103.4

2.0

21.23

96.7

3.6

100

118.40

105.3

1.5

121.35

111.0

0.9

300

318.99

105.4

2.4

326.55

107.7

1.3

Figure 7. The decrease of the diffraction peak intensity of the oPMD PC caused by different targets at different concentrations.


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Analytical Chemistry Verification of the long-term stability of the o-PMD PC

CONCLUSION

In order to verify the long-term stability of the o-PMD PC, it was stored in a petri dish at room temperature for 1 month and then used to detect different concentrations of CF (the other conditions were the same as above). The results were shown in Figure 9. The intensity of reflection peak decreased with the increase of CF concentration. Simultaneously, the corresponding peak intensity of different CF concentrations was 97% of the initial response. It showed that the o-PMD PC possessed a long-term stability.

The sensitivity and selectivity of the o-PMD PC method result from the Michael addition reaction which reaction is usually used in organic synthesis. Developing a label-free assay method based on this reaction may further expand the applications of the reaction. Furthermore, the recognition groups, o-benzoquinone group, can be easily attached to the PC through polymerization and oxidation, and little organic solvent is used during both the preparation and the detection. This approach is expected to possess promising prospect in clinical diagnosis, medicine analysis, food safety and environmental monitoring.

Figure 9. Optical responses of the o-PMD PC to series of CF concentrations after 1-month storage.

AWS15J006) and the Tianjin Research Program of Application Foundation and Advanced Technology (Grant No. 15JCYBJC51200) for funding this research project.

ASSOCIATED CONTENT Supporting Information The following Supporting Information is available online: TEM image of PMD microspheres, FTIR spectra of the PMD and PMMA microspheres, 1H NMR spectra of the PMMA microspheres and the DMA, the structural formula of CF analogues, LC-MS-MS chromatogram of the fish farming water sample.

AUTHOR INFORMATION Corresponding Author * Y. Peng: e-mail, [email protected]; tel/fax, +86-22-84655403. * B. Ning: e-mail, [email protected]; tel/fax, +86-22-84655192. * M. Wang: e-mail, [email protected]; tel/fax, +86-53182492410.

Author Contributions #These authors contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT The authors thank the National Natural Science Foundation of China (Grant No. 81472985, 81502847, 81602896,

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Michael-Addition-Mediated Photonic Crystals ... - ACS Publications

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