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Core–Shell Metal–Organic Frameworks/Molecularly Imprinted Nanoparticles as Absorbents for the Detection of Pyrraline in Milk and Milk Powder Huilin Liu, Lin Mu, Xiaomo Chen, Jing Wang, Shuo Wang, and Baoguo Sun J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.6b05429 • Publication Date (Web): 12 Jan 2017 Downloaded from http://pubs.acs.org on January 14, 2017

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Journal of Agricultural and Food Chemistry

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Core–Shell

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Nanoparticles as Absorbents for the Detection of Pyrraline in Milk and

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Milk Powder

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Huilin Liu1, Lin Mu1, Xiaomo Chen1, Jing Wang1,* Shuo Wang1,2 and Baoguo

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Sun1

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1

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and Business University (BTBU), 11 Fucheng Road, Beijing, 100048, China.

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2

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University of Science and Technology, Tianjin, 300457, China.

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Metal–Organic

Frameworks/Molecularly

Beijing Engineering and Technology Research Center of Food Additives, Beijing Technology

Key Laboratory of Food Nutrition and Safety, Ministry of Education of China, Tianjin

*Corresponding author: Jing Wang

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Tel: (86 10) 68984545;

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Fax: (86 10) 68985456;

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Imprinted

Email: [email protected]

ACS Paragon Plus Environment


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Abstract

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A novel core–shell metal–organic framework coated with a dummy template molecularly

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imprinted polymer (MOF@DMIP) was synthesized by one-pot bulk polymerization for the detection

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of pyrraline in food samples. The pyrraline analog pyrrolidine-3-carboxylic acid was used as the

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template because of its lower cost, and MIL-101 was used as the MOF core owing to its numerous

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inherent advantages, including high chemical and hydrothermal stability. MIL-101@DMIP was used

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to detect trace pyrraline in foods by solid-phase extraction combined with high performance liquid

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chromatography. It exhibited the advantages of faster mass transport, excellent sensitivity and

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selectivity. Under optimum conditions, the detection limit of this system was 40.7 µg L-1, and a

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linear range was from 5×10−7 to 2×10−3 mol L−1, within relative standard deviations of 4.46%–6.87%.

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The recoveries ranged from 92.23%-103.87%, indicating the excellent ability of the prepared MIL-

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101@DMIP to recognize pyrraline in complex food matrices, and its potential for application in

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pyrraline detection.

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Keywords: Advanced glycation end products; pyrraline; mental–organic frameworks; molecular

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imprinted polymer; solid-phase extraction; HPLC


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Introduction

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Advanced glycation end-products (AGEs) have been studied extensively since they were first

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proposed by Brownlee in 1984.1 AGEs are a set of stable end products formed through advanced

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Maillard reactions, and include pyrraline, pentosidine, crossline, carboxymethyl-lysine and others.2

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According to histopathological studies, AGEs accumulate in different tissues and organs of the

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human body and play a significant role in aging and the development of other chronic diseases, such

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as diabetic nephropathy, Alzheimer’s disease, and atherosclerosis.3-5 Pyrraline (2-amino-6-(2-formyl-

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5-hydroxymethyl-1-pyrrolyl)-hexanoic acid), one of the AGEs, was first identified and described by

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Nakayama in 1980.6 Investigators have found that the concentration of pyrraline in the serum of

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diabetics patients is higher than that in healthy people.7 In addition, immunological studies have

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shown that pyrraline accumulates in the glomerular basement membrane of patients with

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atherosclerosis and ultimately leads to kidney failure. As a result, pyrraline has received great

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attention in recent years both as a potential risk to human health, as well as an important index for

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quantitative AGEs detection.8 Chromatographic methods have been employed in the detection and

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measurement of trace pyrraline. Kazuhiro applied the OasisTM HLB solid-phase extraction (SPE)

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combined with high performance liquid chromatography (HPLC) to monitor urinary pyrraline.9

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Thomas employed ion-exchange chromatography with a photodiode array for the quantification of

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pyrraline in model systems.10



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Molecular imprinting technology (MIT) is a discovered method for the detection of target

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analytes on the molecular level.11,12 The resulting molecularly imprinted polymers (MIPs) are

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considered to be promising recognition elements with high affinity, selectivity, and stability, and can

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be used to monitor trace substances in the fields of SPE, chromatographic separation, biological

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sensors and clinical medicinal analysis.13-15 MIP-based column separation technology have often

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been applied to separate and enrich the alkaloids, flavones, and polyhydric phenols from traditional

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Chinese herbs.16 Zhang et al. developed a single-hole hollow molecularly imprinted polymer

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combined with SPE for the preconcentration of Sudan I dyes in chilli sauce samples.17 Tang et al.

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employed MIPs and used SPE coupled with HPLC to effectively separate and determine the

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concentrations of clenbuterol in pork and potable water samples.18 Tan et al. applied molecularly

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imprinted TiO2 in the discrimination of fruit punch using a fluorescent sensor array for carboxylic

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acids.19 Zhu et al. prepared MIPs using theanine as a dummy template for the enrichment, and

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applied them as SPE sorbents in the determination of eighteen amino acids in tobacco.20 MIPs were

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deposited as thin films on an Au electrode to fabricate an electrochemical chemosensor for the

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determination of human serum albumin.21 All of these MIPs demonstrated good sensitivity and

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selectivity towards templates. However, to enhance the efficiency metal-organia frameworks (MOFs)

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were introduced in the field of MIPs.

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MOFs have aroused great attention owing to their potential applications in numerous fields.

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MOFs are porous organic–inorganic materials with a cubic three-dimensional structure, formed by


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linking metal ions to organic bridging ligands.22 Studies have demonstrated that MOFs can be used

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in gas storage, sensing, drug delivery, separation and catalysis applications because of their good

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chemical and thermal stabilities and porous nature.23-25 Especially the MOFs was used in the SPE

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application. Dai et al. employed MIL-101(Cr) as a SPE sorbent for the detection of sulphonamides in

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water samples combined with UPLC-MS/MS.26 Lv et al. applied MOF-5 as SPE adsorbent coupled

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with HPLC for the determination of thiol compounds from wastewater.27 Huang et al. introduced

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MOFs as sorbent for the pre-concentration and enrichment of five organochlorine pesticides from

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water samples combined with gas chromatography–mass spectrometry.28 Wang et al. applied

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magnetic graphene@polydopamine@Zr-MOF as the SPE sorbents for the pre-concentration of

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bisphenols in water samples.29 Ma et al. prepared magnetic MOFs as magnetic SPE adsorbents for

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the separation of four kinds of pyrazole/pyrrole pesticides in water samples with HPLC.30 Recently,

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MOFs was also used as the MIP core. Qian et al. firstly prepared a core-shell MIP by coating the

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MIP shell onto the surface of the MOF, which shows a homogeneous polymer film, cubic shape,

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thermal stability, and exhibits a higher specific surface area and a faster transfer-mass speed

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compared with that of the bulk MIP.31 Madiha et al. developed an electrochemical sensor for the

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detection of tetracycline, based on a gold electrode surface modiﬁed with MIP microporous-MOFs.32

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Zofia et al. prepared a surface MIP film with MOFs. It enlargement increased the analyte

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accessibility to imprinted molecular cavities.33



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In this study, a strongly binding dummy template MIP (DMIP) was synthesized on the MOF

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surface using pyrrolidine-3-carboxylic acid (PCC) as the dummy template. The MOF@DMIP was

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used for the adsorption and enrichment of pyrraline in food samples. Here, MIL-101 was used as the

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MOF core, owing to its numerous inherent advantages, which includes its high chemical and

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hydrothermal stability. MIL-101@DMIP exhibited such advantages as faster mass transport

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excellent sensitivity and selectivity towards targets when compared with traditional MIPs. Finally,

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we developed the technique of molecularly imprinted solid-phase extraction (MISPE) combined with

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HPLC for the detection of pyrraline in foods.

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Materials and Methods

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Materials and Reagents. All reagents were at least of analytical grade. A pure standard sample

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of pyrraline was purchased from Toronto Research Chemicals Incorporated (Toronto, Canada).

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Pseudo-template PCC (95%) was obtained from Tokyo Chemical Industry (Tokyo, Japan). Nε-

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carboxymethyllysine (CML), 3-pyrroline and pyrrole was purchased from J&K Scientific, Ltd.

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(Beijing, China). Other chemicals used for the polymer synthesis were (3-aminopropyl)

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triethoxysilane, (APTES, 98%,; Sinopharm Chemical Reagent Co., Ltd., Beijing, China), tetraethyl

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orthosilicate (TEOS, 98%,; J&K Scientific, Ltd.), and acetic acid (99.5%, Sinopharm Chemical

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Reagent Co., Ltd.). Doubly deionized water (18.2 MΩ cm−1) was obtained from a Water Pro water

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purification system (Labconco, Kansas, MO, USA). Other chemicals used for the MIL-101 synthesis

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were chromium (III) nitrate nonahydrate (99%, Strem Chemicals, Inc., Massachusetts, USA),


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terephthalic acid (99%, Sinopharm Chemical Reagent Co., Ltd.), hydrofluoric acid (40%, Xilong

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Chemical Co., Ltd., Guangdong, China). Acetonitrile was of HPLC grade (99.9%, J & K Scientific,

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Ltd.). Milk and milk powder were provided by the local market.

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Characterization. Scanning electron microscopy (SEM; S-4800, Hitachi, Japan) was used to

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observe the surface morphology of MIL-101@DMIP. Fourier transform infrared (FT-IR) spectra

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(4000–400 cm−1) were recorded for KBr pellets in an FT-IR spectrophotometer (Vertex 70vxrd,

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Bruker, Germany). X-ray diffraction (XRD) experiments were performed on a D/max-2500

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diffractometer (Rigaku, Japan) to characterize MIL-101 and MIL-101@DMIP. CuKa radiation

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(λ=1.5418 Å), a scan speed of 8° min−1 and a step size of 0.02° in 2θ were used. Thermogravimetric

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analysis (TGA) was performed on a PTC-10A thermal gravimetric analyzer (Rigaku, Japan) under

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air from room temperature to 700 °C at a ramp rate of 26 °C min−1.

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Synthesis of MIL-101. MIL-101 was prepared according to the method of Hwang et al. with

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some modifications.35 Typically, 0.8 g of terephthalic acid, 2 g of Cr(NO3)3·9H2O, and 63 µL of 40%

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hydrofluoric acid were mixed thoroughly with 35 mL of ultra pure water and transferred into a 100

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mL Teflon-lined reaction kettle. The reaction kettle was then heated at 220 °C in a muffle furnace for

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8 h to give a suspension of green crystals of MIL-101. The precipitate was collected by filtration

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after ultrasonic washing with water and centrifuging at 4000 rpm for 15 min. Finally, the obtained

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MIL-101 was heated in water bath for 5 h at 70 °C, and then heated in ethanol for 3 h at 60°C. Green



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crystals appeared after adding 200 mL of 30 mM NH4F and heating at 60 °C for 3 h. The resulting

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MIL-101 was filtered off and activated under reduced pressure at 30 °C for 12 h.

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Synthesis of MIL-101@DMIP. A pyrraline-imprinted polymer based on MIL-101 was

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synthesized using a one-pot modified bulk polymerization technique. MIL-101@DMIP was

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synthesized as follows: to a 25 mL round bottom flasks, PCC (0.2 g), acetonitrile (3 mL), the

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prepared MIL-101 (0.02 g), and APTES (0.9 mL) were added, which was stirred for 60 min.

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Subsequently, TEOS (2.2 mL) and acetic acid (0.2 mol L−1, 1 mL) were added. After stirring for 30

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min, the pre-polymerized product was sealed and incubated in a glycerol bath at 60 °C for 20 h. The

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obtained MIL-101@DMIP was extracted by Soxhlet extraction (200 mL of acetonitrile/acetic acid;

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9:1, v/v) to remove the template until no PCC was detected by HPLC. To evaluate the performance

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of MIL-101@DMIP, non-molecularly imprinted polymers (NIP) coated MIL-101 (MIL-101@NIP)

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was prepared in the same way as described above, only in the absence of the template. The DMIP

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without MOFs was prepared by the similar process as MIL-101@DMIP, but without MIL-101.

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Static Adsorption and Selectivity Tests. To investigate the adsorption behavior of MIL-

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101@DMIP, static adsorption and selectivity tests were performed. In the static adsorption test, 20

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mg of MIL-101@DMIP and MIL-101@NIP were each exposed to 2 mL of PCC solutions of various

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concentrations (4.3–430 µg mL−1) at room temperature for 12 h. After centrifugation, the

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concentration of the free PCC in the supernatant solution was determined by HPLC and the amount

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of the adsorbed PCC was estimated.


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To evaluate the selectivity of MIL-101@DMIP towards pyrraline, a selectivity test was carried

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out. CML, 3-pyrroline and pyrrole were used as the competitive substrate owing to analogous

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structure. Selective binding was performed by fixing the concentration of CML, 3-pyrroline, pyrrole

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and pyrraline at 5×10−6 mol L−1, and loading 400 µL of both solutions on to the SPE cartridge.

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Elution was detected by HPLC.

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HPLC Procedures. The HPLC conditions were taken from Manuel et al. with some

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modifications.36 The standard solution was prepared by dissolving 5 mg of pyrraline standards in 10

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mL of water and diluting the solution gradually from 2×10−3 to 5×10−8 mol L−1. Then, 5 µL aliquots

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of these solutions were injected into a Shimadzu HPLC instrument equipped with an InertsilTM ODS-

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SP analytical column (4.6×250 mm). The column was eluted using an acetonitrile/water gradient.

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Solvent A was trifluoroacetic acid (TFA, 1 mL/L), solvent B was water/acetonitrile (1:1, v/v). The

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proportion of solvent B was changed from 0% to15%, 20%, 100%, and 0% at 1, 10, 30, 35, 40, and

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45 min, respectively. The flow rate was 1.0 mL min−1 and the ultraviolet detection wavelength was

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297 nm.

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Preparation of Food Samples. Pre-treatment of real samples was carried out using the

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methods of Khan et al. and Yoshihara et al. with slight modifications.37,38 Three kinds of milk

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samples, each kind with two brands, and six kinds of milk powder samples were used in the study.

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To a centrifuge tube 0.5 mL of milk samples was added and then dissolved in 2.5 mL of methanol.

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MIL-101@DMIP cartridges were prepared by packing 60 mg of MIL-101@DMIP into a 3 mL



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empty SPE cartridge and fixed using frits on both sides. The polymer bed was preconditioned with 3

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mL of methanol followed by 3 mL of water. Then, 1 mL of the supernatant sample was loaded onto

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the cartridge, and the cartridge was washed with 2 mL of water. Finally, pyrraline was eluted with 3

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mL of methanol (5% ammonia solution). The SPE flow rates of sampling and eluation were 0.5

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mL/min, respectively. The eluent was evaporated to dryness by ventilating with nitrogen. The dried

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residue was then dissolved in 1 mL of water for analytical HPLC.

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For milk powder samples, 0.5 mg of samples was dissolved in 2.5 mL of methanol and the remaining steps were the same as those for the milk samples. Statistical Analysis. The adsorption capacity (Q, µg g−1) was calculated from the concentrations of pyrraline before and after adsorption onto MIL-101@DMIP using the following equation, 5

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Q = 1000000(C0 − Ci)×V×M/m

(1)

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where C0 (mol L−1) is the initial concentration of pyrraline solution, Ci (mol L−1) is the pyrraline

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concentration of the supernatant solution after the adsorption process, V (L) is the volume of the

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initial pyrraline solution, M (g mol−1) is the molar mass of pyrraline, and m (g) is the mass of MIL-

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101@DMIP or MIL-101@NIP.

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The selectivity of MIL-101@DMIP towards pyrraline was determined by the following equations,

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Kd = {(Co-Ci)/Ci}×V/m

(2)

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K = Kd(pyrraline)/Kd(competitors)

(3)


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Where Co (mol L−1) and Ci (mol L−1) represent the initial and final concentrations of the substrates, respectively, V (mL) is the volume of the solution, and m (g) is the mass of the substrates. Data are expressed as mean ± standard deviation, and each value is the mean of triplicate

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measurements. IBM SPSS Statistics 20.0 was applied to analyze the data.

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Results and Discussion

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Synthesis of MIL-101@DMIP. A one-step bulk polymerization strategy was used to prepare

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MIL-101@DMIP, the general scheme for the preparation process is illustrated in Figure 1. PCC was

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used as the dummy template owing to its analogous structure and lower cost compared with pyrraline.

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MIL-101 was used as the core of the core–shell composite structure, embedded into an MIP shell.

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This aimed to improve the adsorption efficiency of traditional MIP. The MIP formed on the surface

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of MIL-101 through hydrogen-bond interaction of PCC and APTES (functional monomer), and

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cross-linking by TEOS.

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Morphological Characterization of the Polymers. As shown in Figure 2a, the average diameter

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of the octahedral MIL-101 particles were ranged from 300-500 nm, and their surfaces were smooth.

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Figure 2b shows that the MIP fully encapsulated the MIL-101 after polymerization. However, after

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encapsulation, MIL-101@DMIP featured a spherical structure with an average size were about 400-

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600 nm. The surface of MIL-101@DMIP was not smooth, which dotted with specific cavities left by

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the template.



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FT-IR Spectra. FT-IR spectroscopy was employed to characterize the formation of synthesized

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MIL-101, MIL-101@DMIP, and MIL-101@NIP, as illustrated in Figure 3a, 3b, and 3c, respectively.

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As shown in Figure 3a, the band observed at 746 cm−1 in the spectrum of MIL-101 was attributed to

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Cr–O. The bands observed at 1168 cm−1 and 1540-1695 cm−1 were attributed to O–C–O and C=C,

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and indicate that benzene rings and carboxylic groups were incorporated into MIL-101. Finally, the

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band observed at 3362 cm−1 was attributed to adsorbed water. Figure 3b shows that the spectrum of

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MIL-101@DMIP displays a band at around 1540-1695 cm−1, which demonstrates the existence of

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MIL-101. The bands observed at 780 cm−1 and 1038 cm−1 were attributed to N–H and O–Si–O,

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respectively. These bands indicate the presence of the functional monomer and the cross linker, and

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confirm the successful encapsulation process between MIL-101 and the DMIP. The spectrum of

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MIL-101@NIP in Figure 3c contains similar bands to those in the spectrum in Figure 3b, showing

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that MIL-101@DMIP and MIL-101@NIP possess similar chemical structures.

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XRD Analysis. MIL-101 and MIL-101@DMIP were further analyzed by XRD. The main

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characteristic peaks in the diffractograms of MIL-101, shown in Figure 4a, were observed at 2θ 7.98°

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and 9.06°. While in Figure 4b，it is noteworthy that the intensity of the MIL-101 XRD peaks

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decreased significantly after the surface-imprinting process as a consequence of the successful

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coating procedure.

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TGA Analysis. TGA was conducted to determine the thermal stability of MIL-101 and MIL-

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101@DMIP. As shown in Figure 5a, MIL-101 displayed an initial weight loss of 11.12% below


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200 °C owing to water loss from the framework. A second weight loss of 5.18% observed at 200–

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360 °C was ascribed to the disintegration of the OH/F groups of MIL-101. The thermo gram of MIL-

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101@MIP is shown in Figure 5b. Compared with the slope of Figure 5a, that of Figure 5b seemed

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gentle. Only 1.08% weight loss within the temperature range of 25-100 °C was observed. Most of the

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real sample detections are carried out in the temperature range. When the temperature was from 100

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to 200 °C, the weight of MIL-101@MIP was reduced 10.31%. However, only one significant weight

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loss of was observed within the range of 100–600 °C for MIL-101@DMIP. This phenomenon shows

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that the synthesized MIL-101@MIP feature good thermal stability, which would be a desirable

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attribute in relevant applications.

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Adsorption Isotherm. As shown in Figure 6, the adsorption capacities of MIL-101@DMIP

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increase with an increase in the initial concentration of PCC. The adsorption capacity of the MIL-

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101@DMIP (up to 21.7 mg/g) was about 14.5 times of MIL-101@NIP (up to 1.5 mg/g). It indicates

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that MIL-101@DMIP exhibits a higher binding affinity for PCC than does MIL-101@NIP, and that

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the specific recognition sites were successfully established during the molecular imprinting process.

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Specificity of MIL-101@DMIP. CML, 3-pyrroline and pyrrole have a similar chemical

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structure to pyrraline, and were thus employed to investigate the binding selectivity of MIL-

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101@DMIP. The structures of 3-pyrroline, pyrrole and pyrraline have similar ring structure, but with

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different substituent groups. CML has the similar recognition sites of lysine but without ring

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structure. As illustrated in Table 1, the adsorption capacity of MIL-101@DMIP for pyrraline is



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significant difference with 3-pyrroline, pyrrole and CML. The results summarize the data (Kd and K)

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which were obtained in competitive binding experiments. The large Kd value of the MIL-

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101@DMIP was an indication of its high selectivity for pyrraline over the competitors due to the

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tailor-made cavities in the MIL-101@DMIP. The distribution coefficients for the analytes on the

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MIL-101@DMIP were 58.82, 1.37, 46.59, and 4.38, respectively. The selectivity coefficients for the

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three competitors were 713.00, 41.94, and 475.33 for the MIL-101@DMIP. It indicates that MIL-

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101@DMIP shows good selectivity towards pyrraline molecules.

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Validated Method for the MIL-101@DMIP. The practicality of the developed method was

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validated by comparison with OASISTM HLB 3cc extraction cartridges (HLB)9, MIP-without MOF,

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and MIL-101@DMIP. The results of the three methods were shown in Table S1. It indicated that the

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prepared MIL-101@DMIP column was significant advantage with the HLB, and MIP-without MOF

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column. When the HLB cartridge was applied to extraction, the recoveries of pyrraline were lower

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than 90%. But the recovery of MIL-101@DMIP is higher than 90%, which indicated that the

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prepared MIL-101@DMIP column accurately detected pyrraline. The recovery of MIL-101@DMIP

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is higher than MIP-without MOF, especially at a low concentration. It indicated that the MIL-101

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material enhance the efficiency for target detection.

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Reproducibility and Precision of MIL-101@DMIP. The reproducibility of pyrraline detection

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by MIL-101@DMIP was investigated by measuring the peak area at a pyrraline concentration of

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2.5×10-5 M. After each measurement, 400 µL of pyrraline solution was reloaded onto the SPE


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cartridge，the absorbed pyrraline was eluted by methanol (5% ammonia) and quantified by HPLC.

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A relative standard deviation of 4.53% was obtained for four repeated detections. This suggests that

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the repeatability of the new method is suitable for practical application.

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Detection of Pyrraline in Foods by MIL-101@DMIP. To assess the practicality of the

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prepared MIL-101@DMIP and MIL-101@NIP, these compounds were employed as sorbents for the

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extraction of pyrraline from food samples. The extracted pyrraline content was measured by HPLC,

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and a detection limit of 40.7 µg/L (S/N ≥ 3), and a linear range of 5×10−7–2×10−3 mol L−1 were

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found. To demonstrate the feasibility and reliability of the developed and optimized method, it was

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applied to the analysis of milk and milk powder samples for the determination of pyrraline. As

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shown in Table 2, the pyrraline was not detected in the nutritional milk powder for women, high

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calcium milk powder for children, high zinc milk powder for students, and whole milk powder. It

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was the highest in nutritional powder milk for quinquagenarian (162.78 µg/g). The pyrraline was

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detected in all milk samples, ranged from 11.63-17.75 µg/mL. Pyrraline is formed by the non-

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enzymatic reaction of reducing sugars with side-chain amino groups of milk and milk powder,

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during the process of heating, sterilization, pasteurization or storage.

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The recovery was calculated by comparing the analytical signals of pyrraline obtained from

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spiked high calcium milk samples with 0.36, 4.39 and 28.07 µg/mL pyrraline concentrations. As

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shown in Table S2, the recoveries ranged from 92.23% to 103.87%, with relative standard deviations

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of 4.46%–6.87%, which indicates the excellent ability of the prepared MIL-101@DMIP for



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recognizing pyrraline in complex food matrices, as well as its potential for application in the

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detection of pyrraline.

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Conclusion. In this work, MIL-101@DMIP was synthesized by one-pot bulk polymerization.

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The synthesized MIL-101@DMIP exhibited a high adsorption capacity and excellent binding affinity,

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and was successfully employed as a sample pre-treatment in the HPLC detection of pyrraline

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residues in food samples. Our innovative synthetic route provides a new method for the production

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of MIL-101@DMIP with faster mass transport speed, tunable pore structure, high specific

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recognition ability, and high selectivity. This holds promise for future applications in the food,

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agricultural, and aquacultural industries.

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AUTHOR INFORMATION

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Corresponding Author

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*(J.W.) Phone: +86 10-68984545. Fax: +86 10-68985456. Email: [email protected].

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Funding

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This work was supported by the National Natural Science Foundation of China (No. 31571940, and

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No. 31501559), Beijing Excellent Talents Funding for Youth Scientist Innovation Team, and the

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Construction of Scientific Research Base and Innovation Platform (No. 19008001226).

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Notes

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The authors declare no competing financial interest.


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389

FIGURE CAPTIONS

390

Figure 1. Schematic of the molecular imprinting process for the synthesis of MIL-101@DMIP.

391

Figure 2. Scanning electron micrographs of (a) MIL-101, and (b) MIL-101@DMIP.

392

Figure 3. Fourier transform infrared spectra of (a) MIL-101, (b) MIL-101@DMIP, and (c) MIL-

393

101@NIP.

394

Figure 4. X-ray diffraction patterns of (a) MIL-101 and (b) MIL-101@DMIP.

395

Figure 5. Thermogravimetric analysis of (a) MIL-101 and (b) MIL-101@DMIP.

396

Figure 6. Binding isotherms of MIL-101@DMIP and MIL-101@NIP.



397

Table 1 The specificity of MIL-101@DMIP. Sorbents

MIP

pyrraline

58.82

CML

1.37

3-Pyrroline

46.59

Pyrrole

4.38

pyrraline

14.26

CML

0.02

3-Pyrroline

0.34

Pyrrole

0.03

CML

713.00

3-Pyrroline

41.94

Pyrrole

475.33

Loading capacity (µg/g)

Kd

K

398 399


Page 24 of 32

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400


Table 2 The determination of pyrraline in milk and milk powder samples (n=3).

401

Samples

Content (µg/g or µg/mL)

Nutritional powder milk for women

0

High calcium milk powder for children

0

Nutritional powder milk for quinquagenarian

160.67± 3.15

High zinc milk powder for students

0

Whole milk powder

0

Modified milk powder

162.78 ± 0.04

High calcium milk 1

15.21 ± 0.31

High calcium milk 2

17.75 ± 0.41

Pure milk 1

15.43 ± 0.28

Pure milk 2

14.25 ± 0.04

Breakfast milk 1

11.63 ± 0.21

Breakfast milk 2

12.73 ± 0.19

402



40x32mm (600 x 600 DPI)


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19x7mm (600 x 600 DPI)



40x28mm (600 x 600 DPI)


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18x6mm (600 x 600 DPI)



18x7mm (600 x 600 DPI)


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40x28mm (600 x 600 DPI)



19x3mm (600 x 600 DPI)


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Molecularly Imprinted

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