Surface-Imprinted Magnetic Carboxylated Cellulose Nanocrystals for


Dec 22, 2016 - carboxylated cellulose nanocrystals ([email protected]@MIPs) for the separation and purification of six fluoroquinolones (FQs) from egg sample...
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Surface-imprinted magnetic carboxylated cellulose nanocrystals for the highly selective extraction of six fluoroquinolones from egg samples Yan Fei Wang, Yang Guang Wang, Xiao Kun Ouyang, and Li Ye Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12206 • Publication Date (Web): 22 Dec 2016 Downloaded from http://pubs.acs.org on December 24, 2016

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Surface-imprinted magnetic carboxylated cellulose nanocrystals for the highly selective extraction of six fluoroquinolones from egg samples Yan–Fei Wang, Yang–Guang Wang, Xiao–kun Ouyang*, Li–Ye Yang

School of Food and Pharmacy, Zhejiang Ocean University, Zhoushan 316022, P.R. China

*Corresponding author. Tel.: +86–580–2554781. Fax: +86–580–2554781. E–mail address: [email protected]

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Abstract:

We herein describe a novel adsorbent based on molecularly imprinted polymers (MIPs)

on

the

surface

of

magnetic

carboxylated

cellulose

nanocrystals

([email protected]@MIPs) for the separation and purification of six fluoroquinolones (FQs) from egg samples. The obtained [email protected]@MIPs not only exhibited a large surface area and specific recognition towards FQs, but were also easily gathered and separated from the egg samples using an external magnetic field. The morphologies and surface groups of the [email protected]@MIPs were assessed by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM), X-ray diffraction (XRD), thermogravimetric analysis

(TGA),

Fourier

transform

infrared

spectroscopy

(FTIR),

and

Brunauer–Emmett–Teller (BET) surface area analysis. The [email protected]@MIPs exhibited high selectivity towards six structurally similar FQs. An enrichment approach was established for the measurement of six FQs from egg samples using [email protected]@MIPs coupled to high-performance liquid chromatography (HPLC). The recovery of spiked FQs ranged from 75.2 to 104.9% and limit of detection (LOD) was in the range of 3.6–18.4 ng g−1 for the six FQs. Therefore, the proposed method is a promising technique for the enrichment, separation, and determination of FQs from biomatrices.

Keywords: carboxylated cellulose nanocrystals, fluoroquinolones, molecularly imprinted polymers, high-performance liquid chromatography, biomatrices.

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1. Introduction Molecularly imprinted polymers (MIPs) have received impressive attention due to their excellent affinity and selectivity towards molecular targets.1-2 To date, MIPs have been widely applied in catalysis,3 chemical/biosensor devices,4 solid-phase extraction,5 controlled drug release,6 and chromatographic separation.7 However, issues such as poor site accessibility, high diffusion barriers, and low affinity binding have limited the application of MIPs.8 To address these issues, many studies have been carried out using surface imprinting techniques to investigate viable nano-sized support materials.9 Surface molecular imprinting refers to a polymerization reaction on the surface of a solid substrate, such that the imprinted sites are scattered on the surface of the imprinted polymers or the outer surface of solid substrates.10 Support materials are therefore critical for such techniques. To date, the majority of studies have employed various materials as carriers, including carbon nanotubes (CNTs),11 graphene,12 and chitosan,13 among others,14 due to their strong mechanical strength, chemical stabilities , and high specific surface areas. However, some materials suffer from limitations, such as high preparation costs and difficult modification processes, which limit the applications of MIPs.15 Therefore, new support materials must be developed to increase the utility of MIPs. We selected carboxylated cellulose nanocrystals (CCNs) as novel support materials for MIPs to broaden the range of available substrate materials. As cellulose is the most generous renewable green material in nature, cellulose nanocrystals (CNCs) have received increasing attention as important materials in biomaterial science

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because they are lightweight and exhibit a high mechanical strength, biocompatibility, high aspect ratio, reactive surface –OH groups, broad chemical modifying capacity, large specific surface area, high availability, and biodegradability.16 Thus, CNCs have great potential for application as adsorbents,17 templates for photocatalysts,18 electrode materials for capacitive deionization,19 and stabilizing agents.20 Notably, their high specific surface area makes them promising candidates for such applications, as they exhibit strong affinities and high adsorption capacities towards various chemicals. The addition of carboxyl groups to the surface of CNCs through carboxylation yields CCNs, which have a more extensive range of applications than CNCs. CCNs are inexpensive, nontoxic, and pose no serious environmental concerns; all of which provide an impetus for their use as support materials for MIPs. The separation of MIPs from solutions or samples is also an issue that prevents the wide-scale application of MIPs. Typical separation methods include centrifugation and filtration; however, these methods are time-consuming and inefficient. Magnetic materials, e.g. Fe3O4, have therefore been employed in MIPs to aid separation.21 We herein propose that the combination of these three promising concepts (i.e., MIPs, CCNs, and Fe3O4) in a single system will lead to the generation of novel [email protected]@MIP materials exhibiting multifunctional performances, high selectivities, high adsorption capacities for molecular targets, and facile separation by an external magnetic field. Thus, we

herein

present the preparation of [email protected]@MIPs

via

copolymerization using ciprofloxacin (CIP) as the template, carboxylated cellulose

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nanocrystals (CCNs) as carriers, 2-methylacrylic acid (MAA) as the functional monomer, divinylbenzene (DVB) as the cross-linker, and Fe3O4 as the magnetic material. The prepared [email protected]@MIPs was applied in the separation of fluoroquinolones (FQs) (Figure 1) from egg samples. FQs are one of the most important categories of antibiotics, and as such, they are widely used in veterinary and human medicine. However, the leftover of FQs in foods of animal origin are potentially harmful to human health. Therefore, it is essential to develop an efficient method for their removal from foods. OH

O

OH

F

O

O

OH O

F

N

N

N

NH

F

O

O

N

N O

N F

N

NH

ciprofloxacin

ofloxacin

lomefloxacin

(CIP)

(OFX)

(LOM)

OH

OH O F

O N

F N

N O

OH O

O

O

N O

NH

NH2

NH

O N

NH2

F

O N

N F

NH

NH S O O

gatifloxacin

moxifloxacin

sparfloxacin

sulfamethoxazole

(GAT)

(MFX)

(SPX)

(SMZ)

Figure 1 The molecular structures of ciprofloxacin (CIP), ofloxacin (OFX), lomefloxacin (LOM), gatifloxacin (GAT), moxifloxacin (MFX), sparfloxacin (SPX), and sulfamethoxazole (SMZ).

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2. Experimental 2.1 Materials Analytical standards (purity >99%, used as received) of ciprofloxacin (CIP), ofloxacin (OFX), lomefloxacin (LOM), gatifloxacin (GAT), moxifloxacin (MFX), sparfloxacin (SPX), and sulfamethoxazole (SMZ), as well as analytical grade microcrystalline cellulose (MCC), ammonium persulfate (APS), citric acid, ferrous chloride tetrahydrate (FeCl2•4H2O), ferric chloride hexahydrate (FeCl3•6H2O), methacrylic acid (MAA), benzoyl peroxide (BPO), divinylbenzene (DVB), and polyethylene glycol (PEG) were purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China). Analytical grade ethanol, acetonitrile (ACN), acetic acid, ammonia solution (NH3•H2O), methanol, and dimethyl sulfoxide (DMSO) were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). HPLC grade acetonitrile was purchased from Oceanpak Alexative Chemical, Ltd. (Gothenburg, Sweden). The egg samples were delivered from a local market in Zhoushan (Zhejiang, China). Deionized water was obtained by a Milli-Q® water purification system (Millipore, Molsheim, France).

2.2 Preparation of CCNs CCNs were prepared according to a literature method22 with minor modifications. MCC (6 g) was introduced into an aqueous APS solution (200 mL, 2 M), and subjected to ultrasonication for 4 h at 60 °C with stirring. The suspension was transferred into centrifuge tubes (50 mL) and centrifuged for 10 min at 9000 rpm,

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then washed three times with deionized water. The obtained colloid was dissolved in an aqueous citric acid solution (200 mL, 1 M), and the resulting mixture was further stirred and subjected to ultrasonication for 2 h at 60 °C. The final product was acquired by centrifugation of the suspension for 10 min at 9000 rpm and was washed three times with deionized water (30 mL). Finally, to obtain an aqueous solution of the desired CCNs, deionized water (50 mL) was added, and the solution was subjected to dialysis to give a solution pH of 7.

2.3 Preparation of the [email protected] The [email protected] were prepared according to a literature procedure23 with minor modifications. An aqueous solution of CCNs (100 mL, 0.2 wt.%) was added to a three-necked flask and purged for 30 min with N2, followed by the addition of FeCl3•6H2O (0.8 g), FeCl2•4H2O (0.3 g), and NH3•H2O (~5 mL) to adjust the solution pH to 9–10. Thereafter, the mixture was stirred for 1 h while maintaining the temperature at 70 °C and then cooled to room temperature (25 °C). The resulting [email protected] were washed three times using deionized water and ethanol and separated using an external magnetic field.

2.4 Preparation of the [email protected]@MIPs The template molecules (CIP, 1.0 mmol) and functional monomers (MAA, 4.0 mmol) were mixed in DMSO (20 mL) under magnetic stirring for 2 h to allow them to self-assemble via hydrogen bonding interactions. [email protected] (0.5 g) and PEG

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(0.4 g) were then added to a solution of DMSO/H2O (9:1 v/v, 100 mL), and mixed with DVB cross-linker (1 mL), BPO initiator (0.5 g) and the pre-assembled solution under ultrasonication for 10 min. The mixture was continuously stirred under an atmosphere of N2 for 12 h while maintaining its temperature at 60 °C. The obtained nanoparticles were washed three times with ethanol and deionized water and were separated using a magnetic field. Subsequently, the CIP template was removed using a mixture of 10% acetic acid/methanol (v/v) solution under ultrasonication for 10 min until CIP cannot be detected by HPLC. Finally, the [email protected]@NIPs were rewashed with deionized water and freeze-dried. The magnetic carboxylated cellulose nanocrystal molecularly non-imprinted polymers ([email protected]@NIPs) were also synthesized using the above procedures, but without the addition of template molecules.

2.5 Characterization The morphologies of the synthesized nanoparticles were determined by TEM (Lorentz, JEM-2100, JOEL, Japan), and the surface groups were investigated by using FTIR (Tensor II, Bruker, Germany). Phase identification was detected by XRD (D8 ADVANCE with DAVINCI, Bruker, Germany). Magnetic properties were characterized using physical property measurement system (PPMS, Model-9, Quantum Design, USA), while surface elemental analysis was carried out via multifunctional XPS (AXIS ULTRA DLD, Shimadzu, Japan). In addition, thermogravimetric analysis was measured using TGA (Pyris Diamond TG/DTA,

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Perkin-Elmer, USA). Nitrogen adsorption isotherms were obtained using a Micromeritics ASAP 2020M apparatus (ASAP 2020M, Micromeritics, USA). Furthermore, the determination of the surface area was carried out using the Brunauer-Emmett-Teller (BET) method.24 Finally, the Barrett-Joyner-Halenda (BJH) model with desorption branches of the isotherms was employed to calculate the pore volumes and pore size distributions.

2.6 Chromatographic conditions For HPLC, an Agilent 1200 system (Agilent, Santa Clara, CA, USA) equipped with an autosampler (G1329A), degasser unit (G1322A), quaternary pump (G1311A), column thermostat (G1316A), and diode-array detector (G1315D) was used. A ZORBAX SB-C18 (5 µm particle size, 150 mm × 4.6 µm) analytical column was used for separation. The mobile phase (flow rate = 1 mL min−1) was composed of acetonitrile (A) and 0.05% triethylamine phosphate aqueous solution (pH 2.50) (B). The linear gradient elution program was: 0–2 min, 86.0% (B); 2–5 min, 86.0–74.0% (B); 5–8 min, 74.0% (B). 292 nm and 20 µL were the detection wavelength and the injection volume, respectively, and the column temperature was kept at 25 °C.

2.7 Binding characteristics The adsorption isotherm was investigated by mixing either [email protected]@MIPs or [email protected]@NIPs (10 mg) in CIP solutions (2 mL) of various concentrations (50–850 mg L−1) and incubating the samples at 25 °C for 2 h at 150 rpm. The

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[email protected]@MIPs were then collected under an external magnetic field, and the quantity of FQ (i.e., CIP) in the supernatant was determined by HPLC. The equilibrium adsorption capacity (Q) was described by the following equation: Q =

(C 0 − C e )V m

,

(1)

where C0 and Ce (mg L−1) are concentrations of the FQs in the initial and adsorption equilibrium solutions, respectively, V (mL) is the volume of the used solution, and m (mg) is the weight of [email protected]@MIPs or [email protected]@NIPs. The Freundlich and Langmuir isotherms were used to estimate the adsorption mechanism of the [email protected]@MIPs. The Freundlich equation25 is expressed as follows: lg Q = lg K F +

lg C e , n

(2)

where Q (mg g−1) is the quantity of adsorbed CIP at equilibrium, Ce (mg L−1) is the CIP concentration that remains in solution at equilibrium, and KF and n are Freundlich constants associated with the adsorption interaction, and adsorption intensity, respectively. In addition, the Langmuir equation26 can be expressed as follows: Ce C 1 = e + Q Qm Qm K L

,

(3)

where Qm (mg g−1) is the saturated adsorption capacity, and KL (L mg−1) is the Langmuir binding constant. Sulfamethoxazole (SMZ) was chosen as the reference compound to assess the selectivity of [email protected]@MIPs towards the target compounds. SMZ is a widely

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used sulfonamide, and its residues exist in the environment alongside FQs. A working solution of FQs and SMZ was all prepared at an initial concentration of 20 µg mL−1. [email protected]@MIPs (10 mg) was incubated with the working solution (10 mL) for 2 h at 25 °C and 150 rpm. Then the mixture was separated under an external magnetic field, and the compounds present in the supernatant were determined by HPLC. The selectivity coefficient (SC) and imprinting factor (IF)27 were used to estimate the selective adsorption of the [email protected]@MIPs, which are defined in equations 4 and 5, respectively: IF =

Q MIP , Q NIP

(4)

where QMIP and QNIP (mg g−1) are the adsorption quantity of the target molecule or competitor molecule by the [email protected]@MIPs and [email protected]@NIPs, respectively. SC =

QFQs QSMZ

,

(5)

where QFQs and QSMZ (mg g−1) are the quantities of FQs and SMZ, respectively, adsorbed by the [email protected]@MIPs. The stability and recyclability of the [email protected]@MIPs were investigated at an initial

FQ

concentration

of

1 µg mL−1.

Following

adsorption,

the

[email protected]@MIPs were eluted by 10% acetic acid/methanol (v/v) solution, and the recovered [email protected]@MIPs were re-used as the adsorbent for recycling experiments. All extraction experiments were performed in triplicate.

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2.8 Optimization of the extraction procedure An egg sample was transferred to a beaker and homogenized by stirring. Then homogenized egg sample (1.0 g) was then soaked in an Erlenmeyer flask (50 mL capacity) containing the working standard solution (200 µL, 5 µg mL−1) and deionized water (9 mL) were introduced. The resulting mixture was shaken by hand for 1 min. Subsequently, the [email protected]@MIPs (5–25 mg) were added, and were machine-shaken

at

25 °C

for

5–100 min,

prior

to

separation

of

the

[email protected]@MIPs from the egg samples using a magnetic field. The recovered adsorbent was sequentially washed with deionized water (2 mL) and aqueous acetonitrile (2 mL, 10%) three times, followed by addition of 2 mL of acetic acid/methanol with different ratios (1-30%; v/v), solution was then added, and the mixture was subjected to ultrasonication to elute the template for 1 min. The elution process was carried out for three times. The eluted solutions were then combined and concentrated to dryness under a stream of N2. Finally, the dried sample was re-dissolved in the mobile phase (1 mL), vortexed for 30 s, and prior to HPLC analysis filtered through a 0.22-µm polytetrafluoroethylene membrane.

2.9 Application of [email protected]@MIPs for the extraction of six FQs from egg samples A homogenized egg sample (1.0 g) was spiked with the six FQs at three different concentration levels (i.e., 0.5 µg g−1, 1 µg g−1, and 2 µg g−1). Deionized water (9 mL)

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and the [email protected]@MIPs (10 mg) were then added, and adsorption-desorption experiments

were

conducted

under

optimized

conditions

(10 mg

[email protected]@MIPs, 80 min adsorption time, 10% acetic acid/methanol (v/v) for elution). Finally, the FQ contents were determined by HPLC.

3. Results and Discussion We herein report the preparation of CNCs containing abundant hydroxyl and a few carboxyl groups by the oxidation of MCC by APS. Following further modification with citric acid, the as-prepared CCN surfaces were rich in carboxyl groups, which could interact with hydroxyl groups distributed on the Fe3O4 surface to produce [email protected] This association renders the materials magnetic, and thereby allows their facile collection from solution via a magnetic separation process. For the imprinted polymerization of the functional monomer MAA, CIP was used as the template, with PEG as the dispersant, DVB as the crosslinking agent, BPO as the initiator, and [email protected] as the carrier. Elution of the CIP template afforded the desired [email protected]@MIPs. The FQs have a 4-oxo-1,4-dihydroquinoline skeleton, which contains a pyridine ring with a carboxyl group, a fluorine atom and a piperazinyl group, located at positions 3, 6, and 7, respectively. Therefore, the template CIP has proton-accepting and hydrogen-bonding functional groups that can interact with the polar carboxyl group in the MAA monomers that are bonded on the [email protected]@MIPs. Adsorbents based on CNCs (or its carboxylated derivatives; i.e. CCNs) can remove a wide variety of pollutants from wastewater.22, 28 Their effectiveness as adsorbents arises from properties such as their high specific surface area, good mechanical strength, biodegradability, large number of active sites, and high functionality. In

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addition, CCNs have a large number of carboxylic groups (more than CNCs), which are effective in the binding and adsorption processes. However, the post-adsorption separation of CCNs requires high speed centrifugation, which limits their applications in many fields. Thus, CCNs were combined with magnetic Fe3O4 to facilitate their post-adsorption separation. Furthermore, the combination of [email protected] with MIPs also enhances the selective behavior of the materials. The large surface area of CCNs provides enough sites for the molecular imprinting process, and the active carboxyl and hydroxyl groups of CCNs surface enable the molecular imprinting of the CCNs surface. Thus, the functional monomer can bind to the CCNs surface form hydrogen bonds with the target FQ molecules to enhance their uptake.

3.1 TEM analysis The morphologies of the CCNs, [email protected], and [email protected]@MIPs were studied using TEM (Figure 2). As shown in Figure 2a, the large spherical particles formed from the aggregation of CCNs.29 Indeed, these aggregations were formed by a self-assembly process from short cellulose rods and their fragments via interfacial hydrogen bonds

30

In addition, as shown in Figure 2b, during the synthesis of Fe3O4

@CCNs, the presence of Fe3O4 improves the dispersion of the CCNs; correspondingly, less aggregation of the CCNs particles occurs. The Fe3O4 nanoparticles are immobilized on the surface of CCNs by interactions between the carboxyl groups of CCNs and the hydroxyl groups of Fe3O4. The Fe3O4 permeates the spherical CCNs such that [email protected] are well dispersed and do not aggregate into larger spherical particles.23 On the other hand, Figures 2c and 2d indicate that there are some

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aggregations in the formed [email protected]@MIPs. However, the presence of MIPs affected the final formed structure, because the MIP layers were present around [email protected] as a surface-imprinted layer with thickness ranging from 20 to 30 nm.

(a)

(b)

(c)

(d)

Figure 2 TEM images of (a) the CCNs, (b) the [email protected], (c) and (d) the [email protected]@MIPs in different magnification.

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3.2 XRD analysis Figure

3a

shows the

XRD patterns of the CCNs,

[email protected], and

[email protected]@MIPs, which were examined to elucidate the crystal phase of the functionalized nanoparticles further. The characteristic peaks of the CCNs shown at -

2θ = 14.92°, 16.45°, and 22.73° in Figure 3a(1) correspond to the (101), (101), and (002) planes, respectively, and represent the typical diffraction pattern of cellulose I crystals.31 For the [email protected] (Figure 3a(2)) and [email protected]@MIPs (Figure 3a(3)), the six peaks at 2θ = 30.28°, 35.41°, 43.40°, 53.80°, 57.22°, and 62.79 ° correspond to the (220), (311), (400), (422), (511), and (440) reflections, respectively, which were consistent with the JCPDS card (19-629) for Fe3O4.32 The XRD patterns, therefore, confirmed that the modification did not change the Fe3O4 phase. Furthermore, the characteristic peaks of CCNs were not observed in plots (2) and (3) of Figure 3a because the CCNs sheets cannot pile up each other to form crystalline structures after being embedded with the magnetic nanoparticles.33

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0

422 440 511 220 311 400 -400 (a) 20 40 60 2 Theta (° )

Weight (%)

Transmittance (%)

(2) [email protected]

400

90

80

(1) Fe3O4

-10.03%

(2) [email protected]

- 24.92%

60

100

- 21.42% (c)

(3) [email protected]@MIPs

30 0 60

200

1.2

(1) CCNs 1160 1059 (2) [email protected] 1620 2900 0.6 3440 1730 1430 1110 (b) (3) [email protected]@MIPs580 4000 3000 2000 1000 Wavenumbers (cm-1) 200 adsorption desorption

Pore Volume (cm3 g-1)

(1) CCNs (3) [email protected]@MIPs

800

Magnetization (emu g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Intensity (a.u.)

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400 600 800 1000 Temperature (°C) (1)[email protected]

(d) 0 0.0

0.3 0.6 0.9 Relative Pressure (P/P0)

30 0 -30

(2)[email protected]@MIPs (e)

-60 -30000 -15000

0

after 10s (f)

15000 30000

Magnetic field (Oe)

Figure 3 (a) XRD patterns of (1) the CCNs, (2) the [email protected], and (3), the [email protected]@MIPs. (b) FTIR spectra of (1) the CCNs, (2) the [email protected], and (3) the [email protected]@MIPs. (c) TGA curves of (1) Fe3O4, (2) the [email protected], and (3) the [email protected]@MIPs. (d) N2 adsorption-desorption isotherms for the [email protected]@MIPs. (e) VSM magnetization curves for (1) the [email protected], and (2) the [email protected]@MIPs. (f) Photographic images showing the magnetic separation of [email protected]@MIPs.

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3.3 FTIR analysis The FTIR spectra of the CCNs, [email protected], and [email protected]@MIPs are shown in Figure 3b. The values of 1430 and 2900 cm−1 for the observed peaks were attributed to the bending and stretching vibrations of saturated C–H and –CH2– bonds, respectively, while those at 1160, 1059, and 1110 cm−1 belong to C–C bond vibrations, the vibration of the C–O bond of the cellulose alcohol, and an ether C–O bond of the nanocrystalline cellulose, respectively. In addition, the broad peak at 3440 cm−1 corresponds to an O–H stretching vibration, while that at 1620 cm−1 corresponds to the stretching vibration of the carboxyl groups.34 Thus, the FTIR spectra suggest that the CCNs were successfully synthesized and modified with carboxyl groups. As shown in the [email protected] spectrum (Figure 3b (2)), the new peak present at 580 cm−1 was assigned to the Fe–O stretching vibrations, which indicated that the magnetic nanoparticles and CCNs had been successfully synthesized. Moreover, the band at 3440 cm−1 weakened because of the strong hydrogen bonding between Fe3O4 and the CCNs. Following MIP modification to yield the [email protected]@MIPs, a new band appeared at 1730 cm−1 (Figure 3b (3)), which was attributed to the C=O stretching vibrations of MAA. These results confirm that the imprinted layer was successfully grafted on the [email protected]

3.4 TGA analysis Figure

3c

shows

the

TGA

curves

of

the

Fe3O4,

[email protected],

and

[email protected]@MIPs samples. The first degradation step was caused by evaporation

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of residual water and moisture from the samples, with weight losses 5.8, 4.0, and 2.5% at

ambient

temperature

(0-150°C)

for

the

Fe3O4,

[email protected],

and

[email protected]@MIPs samples respectively. The second degradation step was due to the thermal decomposition of the polymer and was observed at higher temperatures in the case of the [email protected] (Figure 3c (2)) and [email protected]@MIPs (Figure 3c (3)). Previous reports indicate that CCNs exhibit a characteristic degradation peak between 280 and 320 °C that correlates with the degradation of the pyranose rings along the CCNs backbone. During this process, the CCNs lose approximately 78–80% of their weight,30 and as such, the content of CCNs in the [email protected] composite could be estimated at 9.7–10%. In contrast, the degradation mechanism for MIPs consists of two stages. Firstly, conversion into poly(methacrylic anhydride) (PMAN) takes place at 220 °C, and secondly, decomposition occurs at elevated temperatures (i.e., ≤400 °C) as a consequence of PMAN fragmentation.35-36 In the case of the [email protected]@MIPs, the added MIPs layer causes an increase in weight loss from 10.03 to 24.92%, in addition to generating a third degradation step at 400–560 ºC. In this third step, a further weight loss of 21.42% was recorded, corresponding to the decomposition of the MIPs layer,37 which clearly demonstrates the success of the imprinting process.

3.5 BET analysis The N2 adsorption-desorption isotherms of the [email protected]@MIPs are shown in Figure 3d. Similar IV-type curves with an H3-hysteresis loop were observed,

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indicating mesoporous structures containing a number of silt holes.38 Three regions can be observed in the adsorption isotherms as follows: (1) When P/P0 lies between 0 and 0.6, a monolayer-multilayer adsorption behavior exists; (2) when P/P0 increases from 0.6 to 0.9, capillary condensation is observed; and (3) when P/P0 lies between 0.9 and 1.0, surface multilayer adsorption occurs. In addition, the surface area of the [email protected]@MIPs was calculated to be 76.5 m2 g−1 using the multipoint BET (Brunauer-Emmett-Teller) method39 (P/P0 = 0.06–0.19), which was a larger value than that of the [email protected] (i.e., 67.3 m2 g–1). This finding suggests that the imprinting process

was

beneficial

for

the

adsorption

of

target

molecules

on

the

[email protected]@MIPs. Finally, average pore diameter and the total pore volume of the [email protected]@MIPs, calculated by the BJH method, were determined to be 13.4 nm and 0.242 cm3 g−1, respectively.40

3.6 VSM analysis Figure 3e shows the magnetic hysteresis loops for the [email protected] and [email protected]@MIPs at 25 °C. Both materials exhibited superparamagnetic behavior, with

the

saturated

magnetization

values

for

the

[email protected]

and

[email protected]@MIPs being 57.09 and 33.46 emu g−1, respectively. The imprinted layer on the [email protected] surface influenced the magnetic response of Fe3O4, causing a decrease in the magnetization value of the [email protected]@MIPs. However, the [email protected]@MIPs still exhibited magnetic separation under an external magnetic field, as shown in Figure 3f.

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3.7 XPS analysis To further explore the surface chemical composition of the [email protected]@MIPs, XPS survey spectra, and high-resolution scans were obtained for the main elements. As shown in Figure 4a, the survey spectrum revealed three dominant peaks at 281.4, 530.3, and 710.6 eV, which assigned to C 1s, O 1s, and Fe 2p, respectively. The XPS spectra for Fe 2p, Fe 3p, C 1s, and O 1s of the [email protected]@MIPs are shown in Figures 4b–e, respectively, while Figure 4f shows the XPS spectra of the O 1s peak of the [email protected] In the Fe 2p spectra (Figure 4b), peaks ascribing to Fe 2p3/2 and Fe 2p1/2 were observed with binding energies of 708.1 and 721.8 eV. The absence of a satellite peak at ~714 eV, which is typical of the Fe2O3 phase, suggested the formation of Fe3O4 in the [email protected]@MIPs.38 Furthermore, the proportion of Fe2+ to Fe3+ in the magnetic nanoparticles was approximately 1:2, which confirms the presence of Fe3O4 (see Figure 4c).41 The XPS spectra of C1s (Figure 4d) showed four component peaks with binding energies of 281.8, 283.7, 285.4, and 287.2 eV, which corresponded to the C–C, C–O, O–C–O, and O–C=O bonds, respectively.42 In addition, following imprinting, the major O 1s peaks at 530.1, 531.1, and 533.2 eV, corresponding to the C=O, C–O, and COO– bonds, shifted to lower binding energies of 527.0, 527.5, and 530.1 eV, respectively (see Figures 4e and 4f).43 Moreover, the intensity of the COO– bond increased, clearly suggesting the successful grafting of MAA on the [email protected]

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4000

Fe2p O1s

40000

Fe2p 1/2

CPS

CPS

60000

C1s

Fe2p 3/2

3000

20000 0 (a) Survey 1000 800

600

400

200

2000 (b) Fe 2p 740 730

0

720 B.E.(eV)

B.E.(eV)

400

1800

CPS

200

1200

3000

60

55 50 B.E.(eV)

45

0

40

C-O 527.5 C=O 527.0

2000 1000

3000 1500

(e) O1s 538 536 534 532 530 528 526 524 522

0

C-C 281.8

(d) C1s 292

4500

COO530.1

CPS

65

700

O-C-O O-C=O 285.4 287.2

600

100 (c) Fe 3p

710

C-O 283.7

CPS

3+ Fe 2+ Fe

300

CPS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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288 284 B.E.(eV) C-O 531.1

280

276

C=O 530.1

COO533.2 (f) O1s 538 536 534 532 530 528 526 524 522

B.E.(eV)

B.E.(eV)

Figure 4 XPS survey spectra of (a) the [email protected]@MIPs. XPS spectra of (b) the Fe 2p, (c) the Fe 3p, (d) the C 1s, and (e) the O 1s of [email protected]@MIPs. (f) High-resolution scan of the O 1s spectra of the [email protected]

3.8 Adsorption isotherm curves Further studies were performed to test the adsorption isotherms of CIP on the [email protected]@MIPs. Figure 5a presents the adsorption isotherm curves of the [email protected]@MIPs and [email protected]@NIPs with different initial concentrations of CIP. As expected, the equilibrium adsorption capacity of CIP on the [email protected]@MIPs

increased

with

increasing

initial

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CIP

concentration.

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Interestingly, the quantity of CIP bonded to the [email protected]@MIPs was 50 mg g−1, which is greater than that found for the [email protected]@NIPs (i.e., 29 mg g−1), and is expected to be due to the imprinting effect. The equilibrium adsorption data were explored based on Langmuir and Freundlich isothermal adsorption models. The linear regression equation for the Langmuir isotherm was Ce/Q = 0.0134Ce + 3.6909 (R2 = 0.8417), and as such, the results of Qm and KL were 68.97 mg g−1 and 0.0036 L mg−1, respectively. Similarly, the linear equation for the Freundlich isotherm was log Q = 0.2217 log Ce − 0.1599 (R2 = 0.9851), and the values of n and KF were 4.51 and 1.45, respectively. These results confirm that the adsorption of CIP on the [email protected]@MIPs was better fitted to the Freundlich isotherm model.

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-1

Adsorption Capacity (mg g

(a) 40

20 [email protected]@MIPs [email protected]@NIPs

0 0

200

120 OFX

FQs recovery (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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)

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400 600-1 Concentration (mg L ) CIP

LOM

GAT

800 SPX

MFX

(b) 90 60 30 0 1

2

3 4 5 6 Adsorption-desorption cycle

7

8

Figure 5 (a) Adsorption isotherm curves for CIP on [email protected]@MIPs and [email protected]@NIPs. (b) Reusability of the [email protected]@MIPs, using the following conditions: Initial FQ concentration = 1 µg mL−1 standard working solution; adsorption dose = 10 mg; adsorption time = 80 min; eluent solution = 10% acetic acid/methanol (v/v).

3.9 Adsorption selectivity The adsorption capacities of the [email protected]@MIPs and [email protected]@NIPs towards the various FQs were investigated using SMZ as the reference compound to

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illustrate the molecular recognition ability of the [email protected]@MIPs. The results are summarized in Table 1. Table 1 Selectivity of the [email protected]@MIPs and [email protected]@NIPs towards different FQs Analyte

QMIPa (mg g-1)

QNIPa (mg g-1)

IFb

SCc

OFX

7.24

2.59

2.80

3.80

CIP

11.31

4.75

2.38

6.64

LOM

6.77

3.31

2.05

3.75

GAT

7.88

4.59

1.72

4.08

SPX

5.59

1.98

2.83

2.77

MFX

8.08

4.94

1.63

3.91

SMZ

1.30

1.36

0.96



a

QMIP and QNIP (mg g-1) represent the binding quantity of FQs or reference compound on the [email protected]@MIPs and [email protected]@NIPs; b

IF = QMIP/QNIP, cSC = QFQs/QSMZ.

The IF (QMIP/QNIP) and SC (QFQs/QSMZ) values were used to indicate the selectivities of the [email protected]@MIPs and [email protected]@NIPs towards the various FQs and to the competitor molecule (SMZ). The IF values for OFX, CIP, LOM, GAT, SPX, and MFX were calculated as 2.80, 2.38, 2.05, 1.72, 2.83, and 1.63, respectively. The [email protected]@MIPs revealed a much greater adsorption capacity towards FQs than the [email protected]@NIPs, suggesting that the [email protected]@MIPs have a relatively high affinity towards FQs. Moreover, the SC values of OFX, CIP, LOM, GAT, SPX, and MFX were 3.80, 6.64, 3.75, 4.08, 2.77, and 3.91, respectively, which indicated

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that the [email protected]@MIPs exhibited excellent selectivity towards the FQs compared to the reference compound, SMZ. Notably, the adsorption capacity of SMZ on the [email protected]@MIPs was similar to that on the [email protected]@NIPs, which illustrated that there was no specificity between SMZ and the [email protected]@MIPs. Table 1 also shows that the SC values were high, confirming the high specificity and adsorption selectivity of the [email protected]@MIPs to FQs owing to specific imprinting sites. Thus, the [email protected]@MIPs could be applied in the separation of different FQs.

3.10 Regeneration As adsorbent reusability is a key factor for sustainability, we conducted adsorption-desorption experiments with eight different FQs using the same [email protected]@MIPs over eight cycles to evaluate the reusability of the material. As shown in Figure 5b, a gradual decline to 53.5–65.1% in the recovery of FQs by [email protected]@MIPs was observed after eight cycles. This decrease was perhaps due to the irreversible and partial loss of the active sites following FQ binding, or through active site destruction upon washing, thus leading to their unavailability for further adsorption cycles.44 Overall, the [email protected]@MIPs exhibited good repeatability and reproducibility, which appeared unaffected by either ultrasonic or acidic treatments.

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3.11 Adsorption Studies 3.11.1 Effect of adsorbent quantity Dose response studies were carried out using different quantities of adsorbent (5–25 mg) for the recovery of FQs from egg substrates (1.0 g). The results shown in Figure 6a indicate that 10 mg of [email protected]@MIPs was optimal for the extraction and recovery of FQs from this medium. In addition, an increase in the quantity of adsorbent beyond 10 mg decreased the recovery of the various FQs, likely because of incomplete desorption.45

120

60

30

OFX CIP LOM GAT

SPX (a) MFX 0 5 10 15 20 25 Amount of [email protected]@MIPs (mg)

FQ recovery (%)

90 FQ recovery (%)

90 60 30 0

(b) 0

20 40 60 80 Adsorption Time (min)

OFX CIP LOM GAT SPX MFX 100

90 FQ recovery (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

OFX CIP LOM 30 GAT SPX (c) MFX 0 1% 5% 10% 15% 20% 25% 30% Acetic acid/Methanol (v/v)

Figure 6 Optimization of extraction conditions (n = 3). (a) Effect of [email protected]@MIPs quantity on FQ recovery. (b) Effect of adsorption time on FQ recovery. (c) Effect of elution solvent on FQ recovery.

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3.11.2 Effect of adsorption equilibrium time The shaking times were varied between 5 and 100 min to investigate the effect of adsorption time. Figure 6b indicated that the recovery of the various FQs increased with the increasing adsorption time over the initial 80 min, after which time a plateau was reached. Therefore, we selected 80 min as the optimal adsorption time. Furthermore, the adsorption time of our materials was equivalent to that of the surface-imprinted polymer [email protected],46 and was more rapid than that of the imprinted polymer nanoparticle [email protected] employing mesoporous carbon nanoparticles (MCNs) as the carrier.

3.11.3 Effect of elution solvent To determine the optimal elution solvent, a variety of acetic acid/methanol mixtures (i.e., 1–30%, v/v) were evaluated, and the results are shown in Figure 6c. Poor recoveries were observed when low (1%) or high ratios (30%) of acetic acid/methanol (v/v) were employed, while satisfactory recoveries were obtained using 10% acetic acid/methanol (v/v). In addition, a range of ultrasonication times was studied for the elution process. However, the results have shown that the desorption/ultrasonication time had no significant influence on FQ recovery; therefore, an optimal ultrasonication time of 1 min was established.

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3.11.4 Application of [email protected]@MIPs for the separation of six FQs from egg samples As the sample matrix can greatly affect the signal of the target and enhance the background noise, two calibration curves were built for each analyte to qualitatively assess the matrix effects, i.e., one in a solvent, and the other in a blank egg extract. Calibration curves were obtained using peak area against analyte concentration values. Thus, by comparing the slopes of the two calibration curves (Table 2), a matrix effect (ME)48 in the range of 0.85 to 1.04 was observed, indicating that the application of [email protected]@MIPs for the extraction of FQs from egg samples yielded satisfactory results. Table 2 Linear equations and matrix effects

Analyte

a

Linear equation

R2

b

Linear equation of R2

c

ME

matrix effect OFX

y = 36.356x - 3.9494

0.9994

y = 31.070x - 1.5981

0.9968

0.85

CIP

y = 31.731x - 8.9654

0.9929

y = 29.449x - 7.9994

0.9943

0.93

LOM

y = 40.396x - 3.9079

0.9998

y = 34.974x - 3.2646

0.9982

0.87

GAT

y = 43.636x - 2.8880

0.9998

y = 38.398x + 3.9301

0.9903

0.88

SPX

y = 43.539x - 2.6496

0.9999

y = 45.281x - 3.9371

0.9969

1.04

MFX

y = 54.950x - 3.1681

0.9998

y = 47.790x + 2.616

0.9951

0.87

a,b

linear range = 0.25-10 µg mL-1.

c

ME = matrix effect, i.e., the ratio of the calibration curve slopes from the blank egg extract and the solvent.

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To

assess

the

applicability

of

the

Page 30 of 46

[email protected]@MIPs

for

the

adsorption/desorption of FQs, limit of quantitation (LOQ), the linearity, limit of detection (LOD), accuracy, and precision of the system were examined. The accuracy was assessed by the analysis of egg samples spiked with the six FQs at levels of 0.5, 1, and 2 µg g−1. As Table 3 shows the recovery values ranged from 75.2 to 104.9%, indicating that the proposed method exhibited acceptable recoveries and accuracies. The intraday precision was investigated based on RSD% values using six replicates on the same day, while the interday precision was determined by taking measurements during five different days over a two-week period. The RSD% values of the intra- and interday experiments were acceptable, ranging from 1.5 to 10.9% (Table 3). In addition, the LOD (S/N = 3) and LOQ (S/N = 10) defined the ratio of signal-to-noise (S/N) and ranged from 3.6 to 18.4 ng g−1 and from 12.0 to 61.4 ng g−1, respectively (Table 3). Table 3 Recovery of the six FQs in spiked egg samples Average recoverya,% (RSD,%)

LOD

LOQ

2 µg g-1

(ng g-1)

(ng g-1)

Fluoroquinolones 0.5 µg g-1b

1.0 µg g-1

OFX

89.8(6.5c,8.9d) 81.5(4.8,7.7)

75.3(3.6,8.7)

17.8

59.2

CIP

81.1(3.2,7.6)

77.0(5.9,8.4)

81.2(6.6,7.9)

18.4

61.4

LOM

79.8(2.6,10.0)

76.8(7.8,8.3)

75.9(3.5,7.3)

8.9

29.7

GAT

79.8(4.5,10.1)

80.5(1.5,8.6)

75.3(5.9,8.0)

5.3

17.7

SPX

87.0(8.4,10.1)

75.2(4.2,9.9)

76.6(5.7,10.3)

4.6

15.3

MFX

99.6(7.9,7.2)

104.9(2.7,4.3) 95.4(7.4,10.9)

3.6

12.0

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a

Measured in 1 day (n = 6 replicates).

b

Spiked level.

c

Intraday, n = 6 replicates in 1 d.

d

Interday, n = 6 replicates in 5 d over 2 w.

Through comparison of the FQs adsorption capacities of previously reported MIPs, we could conclude that the [email protected]@MIPs exhibited the higher value of adsorption capacity (i.e., 50 mg g−1) compared to only MIPs (i.e., 43 mg g−1) that employ the same monomer and template.49 In addition, the [email protected]@MIPs also exhibited an improved adsorption compared to [email protected] (i.e., 48 mg g−1)50 based on carbon nanotubes (CNTs) as magnetic (M) support materials, MIPs prepared by surface-initiated chain transfer polymerization (32 mg g−1),46 and MIPs prepared with biological fluids for solid phase extraction (30 mg g−1).51 These comparisons indicate that CCNs are promising carriers and that they provide a large surface area for imprinted sites.

Table 4 Comparison of the [email protected]@MIPs absorbent with other MIPs materials. Adsorbent

Adsorption capacity(mg g-1)

Adsorption time (min)

Ref.

MIPs

32

60

46

[email protected]

41

120

47

[email protected]

48

40

50

MIPs

30



51

[email protected]@MIPs

50

80

This work

MCNTs = magnetic carbon nanotubes.

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MCNs = mesoporous carbon nanoparticles.

The adsorption time of our materials was equivalent to that of the surface-imprinted polymer [email protected],46 and was faster than that of imprinted polymer nanoparticle [email protected] that employed mesoporous carbon nanoparticles (MCNs) as a carrier.

Table 5 shows a comparison between this method and previously reported FQ extraction methods. The results indicated that this method has a comparable or better recovery than previously reported methods,52-55 and has lower or comparable values of LOD and LOQ compared with protein precipitation–HPLC,28 pressurized liquid extraction–HPLC–FL22 and MIPs–SPE–HPLC–DAD. 56

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Table 5 Extraction and determination of FQs with different methods. Extraction method

Analysis Method

Analytes

Pressurized liquid extraction

HPLC–FL

CIP, SARA, ENROa

Protein precipitatio n

HPLC

PAZ, CIP, LEVOa

[email protected] MIPc

b

Linear range

HPLC–DAD

17–24

30–41

µg g-1

ng g-1

ng g-1

0.10–20 µg mL-1

0.1



µg mL-1

6

15

-1

16 fluoroquin olones

Online SPEe

LC–MS/MS

SPE

LC–MS/MS

MFX

HPLC–FL

CIP, DANO, ENRO, LEVO, NOR, SARAa

HPLC–DAD

CIP, ENRO, NOR, MARa

HPLC–DAD

CIP, OFX, GAT, LOM, SPX, MFX



MIPs–SPE

MIPs–SPE

[email protected] [email protected]

a

0.095–3 µg mL-1

Ref .

66–89

22

85.2–99. 3

28

52

ng mL

ng mL

0.01–4.23

0.04–12.93 ng mL-1

65.0–12 3.0

53

1.0

92.0–10 5.9

54

62–102

55

41.8–-96 .4

56

75.2–10 4.9

this wor k

-1

ng mL-1

0.001–2.5 µg mL-1

Recover y(%)

79.1–85. 3

GAT µg mL

f

LOQ

0.05–1

0.80–450 d

LOD



-1

ng mL-1

0.001–0.012 ng mL-1

0.012–0.14 ng mL-1

0.039–1.26 µg mL-1

18

39

ng mL-1

ng mL-1

0.25–10

3.6–18.4

12.0–61.4

µg mL-1

ng g-1

ng g-1

PAZ = pazufloxacin, LEVO = levofloxacin, SARA = sarafloxacin, ENRO =

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enrofloxacin, DANO = danofloxacin, NOR = norfloxacin, MARBO = marbofloxacin. b

FL = fluorescence detector.

c

MCNTs = magnetic carbon nanotubes.

d

DAD = diode array detector.

e

SPE = solid phase extraction.

f

MS = mass spectrometry.

The corresponding HPLC chromatograms are shown in Figure 7, with Figure 7a indicating that the resolution between each peak was greater than 1.5. Furthermore, the peak response was low for the spiked egg samples that lacked pre-enrichment by [email protected]@MIPs (Figure 7b). However, following pre-treatment of the blank egg samples with [email protected]@MIPs, the number of impurities was reduced, and no signals corresponding to the FQs were observed (Figure 7c). Finally, the signals corresponding to the various FQs in the spiked egg samples treated with [email protected]@MIPs could be clearly observed (Figure 7d). These results indicate that the [email protected]@MIPs are applicable in the enrichment and separation of FQs from complex biomatrices.

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GAT SPX

MFX

LOM OFX

(a)

mAU

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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CIP

(b) (c) (d)

1

2

3

4

5

6

7

t (min)

Figure 7 HPLC chromatograms of (a) A standard mixed solution of the six FQs (i.e., OFX, CIP, LOM, GAT, SPX, MFX); (b) Egg samples spiked with the six FQs without [email protected]@MIP pre-enrichment; (c) Blank egg samples following [email protected]@MIP

extraction;

and

(d)

Spiked

egg

samples

following

[email protected]@MIP extraction. All FQs were spiked at a concentration of 2 µg g−1.

4. Conclusions In summary, a novel molecularly surface-imprinted polymer based on [email protected] (CCNs = carboxylated cellulose nanocrystals) was successfully prepared. Owing to their high specific surface area, high availability, and low-cost modification, CCNs are among the most promising materials for molecularly imprinted polymer (MIP) supports. Thus, the prepared [email protected]@MIPs showed excellent selectivity, a large adsorption capacity (50 mg g−1), and suitable saturation

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magnetization. Our novel material was then employed for selective extraction, purification, and HPLC-based determination of six different fluoroquinolones (FQs), namely ciprofloxacin, ofloxacin, lomefloxacin, gatifloxacin, moxifloxacin, and sparfloxacin,

from

egg

samples.

We

propose

that

magnetic

molecularly

surface-imprinted polymers are a suitable alternative for the extraction, purification, and separation of various compounds from complex matrices.

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

Funding Sources The authors are grateful for the support of National Natural Science Foundation of China (21476212) and the Foundation of Science and Technology Department of Zhejiang Province (2017C33126).

Notes The authors declare that there are no conflicts of interest.

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254x190mm (96 x 96 DPI)

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