Necklace-like molecularly imprinted nanohybrids based on polymeric

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Applications of Polymer, Composite, and Coating Materials

Necklace-like molecularly imprinted nanohybrids based on polymeric nanoparticles decorated multi-walled carbon nanotubes for highly sensitive and selective melamine detection Sheng Xu, Geyu Lin, Wei Zhao, Qian Wu, Jing Luo, Wei Wei, Xiaoya Liu, and Ye Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08558 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 6, 2018

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ACS Applied Materials & Interfaces

Necklace-like molecularly imprinted nanohybrids based on polymeric

nanoparticles

decorated

multi-walled

carbon

nanotubes for highly sensitive and selective melamine detection

Sheng Xu, Geyu Lin, Wei Zhao, Qian Wu, Jing Luo, Wei Wei, Xiaoya Liu*, Ye Zhu* Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, Jiangsu Province, P.R. China

* Corresponding authors. Tel.: (+86)510-85917763. E-mail addresses: [email protected] (X. Y. Liu)；[email protected] (Y. Zhu)

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ABSTRACT In

this

study,

molecularly

imprinted

nanohybrids

with

“necklace-like”

nanostructures were developed based on self-assembled polymeric nanoparticles decorating multi-walled carbon nanotubes (MWCNTs) by employing melamine as template

molecules.

An

amphiphilic

acid-co-(7-(4-vinylbenzyloxy)-4-methyl

copolymer

coumarin)-co-ethylhexyl

poly(acrylic acrylate)

(poly(AA-co-VMc-co-EHA), PAVE) containing photosensitive coumarin units was synthesized firstly. Then, the PAVE copolymers were co-assembled with MWCNTs in the presence of template molecules, generating photosensitive molecularly imprinted nanohybrids (MIP-MWCNTs) with “necklace-like” structures. Subsequently, the MIP-MWCNTs nanohybrids were used to modify electrode surface followed by photo-polymerization of the coumarin units in the nanohybrids, leading to a network architectured complex film. After extracting melamine molecules by electrolysis, a melamine MIP sensor was successfully developed. The as prepared sensor exhibited a significantly wide linear range (1.0×10–12 mol L–1 ~ 1.0×10–6 mol L−1) and a low detection limit (5.6×10−13 mol L−1) for melamine detection. High selectivity of the sensor toward melamine was well demonstrated with respect to other melamine analogues and interferents. Furthermore, the MIP sensor showed high stability and reproducibility. The excellent performance of the MIP sensor can be attributed to the unique nanostructure of the complex film provided by these “necklace-like” nanohybrids. On one hand, the nanosized polymeric MIP nanoparticles (NPs) along


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the MWCNTs increase the effective electrode surface area and thus offer high melamine binding capacity. On the other hand, the MWCNTs in MIP-MWCNTs nanohybrids serve as “electronic bridges” to accelerate the electron transfer among the complex MIP film. More importantly, the MIP sensor was practically used to monitor melamine in milk samples, demonstrating a promising feature for applications in food analysis like milk and other food products including milk powder, infant formula and animal feed. Considering the ease of polymeric nanoparticles functionalization, the “necklace-like” nanohybrids would be extended to wider applications in many other sensors and devices.

Keywords: Molecular imprinted nanohybrids; Self-assembly; MIP sensor; MWCNTs; Melamine



1. Introduction Molecular imprinting has proven to be a crucial approach for creating tailor-made binding sites which are complementary to template molecules in size, shape and functional groups.1 Thanks to their advantages of strong structure designability, high recognition selectivity and high application practicality, the resulting molecularly imprinted polymers (MIPs) with affinity and specificity towards template molecules, have been widely applied in chromatographic separation, sample pretreatment, pharmaceuticals, environmental pollutants and many other fields.2,3 However, traditional MIPs are generally prepared as monoliths, thin films and microspheres. Due to their bulky forms, most recognition sites locate inside the depths of these MIP materials, which greatly limits the extraction and rebinding efficiency of template molecules. In addition, the amount of recognition sites is also limited due to the relatively low surface area.4 In order to address these issues, MIPs in the form of nanoparticles have been exploited as a new member in the family of MIP materials. Due to the larger total surface areas provided by nanosized MIPs, it’s expected that most of the imprinted sites would situate at the materials surface and much more imprinted sites would be generated. As a result, template molecules can remove from the MIP nanoparticles faster and more completely, leading to faster rebinding kinetics and higher binding capacity.5–7 A variety of attractive methods has been used to obtain nanosized MIPs.8 However, for most of these methods, a large amount of organic and toxic solvents are inevitably needed. In addition, since some factors like


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polymerization temperature, the polarity of solvents and stirring speed would have great effects on the particle size, the reaction conditions for preparing nanosized MIPs need to be strictly controlled,9,10 which limits the wider and more practical applications of the MIPs in real aqueous systems. To tackle these problems, our group have developed lots of MIP nanoparticles (MIP NPs) by combining molecular imprinting with macromolecular self-assembly in the past decade.11-13 Driven by some noncovalent binding interactions, a great many template molecules were successfully entrapped into the polymeric NPs in the self-assembly or co-assembly process. However, the sensitivity of these sensors was generally far from satisfactory because that most of amphiphilic copolymers used in the imprinted nanoparticles are normally non-conductive. To combat the issue even further, conductive components are expected to be incorporated into the polymeric MIPs for improving the sensor sensitivity. Recently, carbon nanomaterials are gaining ever-increasing advance in sensor systems as sensing elements or building blocks due to their diversely intrinsic and excellent physicochemical properties from the structural diversity.14,15 By combining carbon nanomaterials (carbon nanotubes, graphene, carbon dots, etc.) with other nanomaterials, a myriad of novel nanocomposites with synergistic functionality have been created and applied in sensors with powerful performance.16-18 Comparing with other conventional counterparts, carbon nanomaterials based sensors generally shows higher sensitivities.19,20 In particular, carbon nanotubes, a cylindrical carbon materials



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with well-ordered arrangement of carbon atoms linked via sp2 bonds, have been greatly exploited in biosensing platforms as attractive candidate materials thanks to the distinct properties like high conductivity, a high ratio of length to diameter, biocompatibility, robust mechanical strength and chemical stability.21-23 Herein, we prepared a novel form of MIP nanohybrids based on self-assembled polymeric nanoparticles decorating multi-walled carbon nanotubes (MWCNTs). The nanohybirds were shaped in “necklace-like” appearance by incorporating MWCNTs in the co-assembly process of a photo-crosslinkable copolymer poly(acrylic acid-co-(7-(4-vinylbenzyloxy)-4-methyl

coumarin)-co-ethylhexyl

acrylate)

(poly(AA-co-VMc-co-EHA), PAVE) and template molecules (Scheme 1). As a proof of concept, melamine, an industrial chemical which has been illegally applied to generate the illusion of high protein content by mixing in our daily foods and milks,24,25 was selected as a representative template molecule according to functional moieties in PAVE copolymer. A melamine MIP sensor was successfully developed based on the “necklace-like” MIP-MWCNTs nanohybrids as molecularly imprinted materials. The polymeric MIP nanoparticles may offer high melamine binding capacity while the MWCNTs in the nanohybrids serve as “electronic bridges” to accelerate the electron transfer among the complex MIP film. As a result, the proposed MIP sensor exhibited high selectivity toward melamine with respect to its analogues and other interfering species. In addition, the sensor also showed high sensitivity and stability. To illustrate its practicability, the as-prepared sensor was also


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successfully applied in detecting melamine in liquid milk samples.

Scheme 1. Schematic representation of fabricating the MIP sensor: (a) Preparation of “necklace-like” MIP-MWCNTs nanohybrids. (b) Fabrication process of the MIP sensor. (c) The interaction mechanism of template molecules and PAVE copolymers.

2. Experimental 2.1 Reagents and materials

Acrylic acid (AA), ethylhexyl acrylate (EHA), N, N-dimethyl formamide (DMF), K3[Fe(CN)6], CaCl2, glucose (Glu), KCl, ascorbic acid (AA), NaH2PO4, K4[Fe(CN)6] and Na2HPO4 were purchased from Sinopharm Chemical Reagent Co., Ltd. The



7-(4-vinylbenzyloxy)-4-methyl coumarin (VMc) was synthesized according to our previous work.26 Azobisisobutyronitrile (AIBN), melamine, cyanuric acid (CA), p-phenylenediamine (PPDA), m-phenylenediamine (MPDA), aniline and lysine (Lys) were supplied by Aladdin Chemical Reagent Co., Ltd (China). MWCNTs were donated by Chengdu Organic Chemicals Institute (China). The deionized water (18.2 MΩ·cm) was purified by a Millipore Milli-Q system. All the other chemicals were of analytical grade and used as received.

2.2 Synthesis of amphiphilic PAVE copolymer

PAVE copolymer was synthesized by a typical free-radical copolymerization (shown in Scheme 2). In brief, 0.36 g of AA (5 mmol), 1.46 g of VMc (5 mmol), and 0.92 g of EHA (5 mmol) were dissolved in dioxane (40 mL) at a molar ratio of 1:1:1 in a round-bottom flask, followed by adding 0.04 g of azobisisobutyronitrile (AIBN) as initiator. The mixture was then degassed three times and sealed to keep a vacuum in the flask. After stirring for 30 min, the flask was incubated in an oil bath (65 °C) to proceed the reaction and kept stirring continuously for 24 h. The obtained copolymers were purified by re-precipitation into petroleum ether for three times, dialysis against ultrapure water and then were lyophilized to obtain white PAVE copolymer powder (84.2% yield).


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Scheme 2. Synthetic route of poly(AA-co-VMc-co-EHA) (PAVE) copolymer.

2.3 Preparation of molecularly imprinted nanohybrids MIP-MWCNTs

To prepare the imprinted nanohybrids, 50 mg of PAVE copolymer and 1.0 mg of MWCNTs

were

co-dispersed

in

DMF

(10

mL)

to

obtain

a

PAVE

copolymer/MWCNTs dispersion. Sonication treatment of the mixtures for 1 h was conducted to ensure the complexation of PAVE copolymers with MWCNTs. 10 mL of 2.5 mg/mL melamine aqueous solution was then dropwise added into the PAVE/MWCNTs mixture. The photosensitive MIP-MWCNTs nanohybrids with good water dispersity were obtained after dialysis against pure water. Nonimprinted photosensitive nanohybrids (NIP-MWCNTs) were prepared with the same method but without melamine molecules.

2.4 Fabrication of MIP sensor

The fabrication process of MIP sensor was illustrated in Scheme 1. Firstly, 10 μL of MIP-MWCNTs solution prepared in advance was cast on clean glassy carbon electrode (GCE) surface. After water evaporation, the MIP-MWCNTs nanohybrids



modified electrode was placed under ultraviolet light (60 W) to lock the recognition sites and enhance the bonding strength among the MIP-MWCNTs complex film. After irradiation for 30 min, the electrode was electrolyzed under 1.4 V for one hour in 0.1 M PBS (pH=7.0) to elute template molecules, generating the melamine MIP sensor. As a comparison, a nonimprinted sensor (NIP sensor) was prepared synchronously using nonimprinted photosensitive nanohybrids without melamine molecules (NIP-MWCNTs). 2.5 Characterization and measurement The detail information of characterization and measurement is shown in supporting information.

3. Results and discussion 3.1 Structural characterization of PAVE copolymer

The structure of this photosensitive and amphiphilic PAVE copolymer was characterized by 1H NMR. As shown in Figure S1, the peaks at 6.3-7.8 ppm (H4-7, H9-11) and the peak at 6.1-6.4 ppm (H12) are ascribed to aromatic protons of the coumarin groups. The multipeaks between 0.6-2.1 ppm (H1, H14, H16-25) are assigned to methyl and methylene protons in both PAVE main chain and EHA pendant. The signals at 3.8 ppm (H15) correspond to the methylene protons in EHA. All these signals confirmed the successful synthesis of the photosensitive PAVE copolymer.


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3.2 Preparation and characterization of MIP-MWCNTs nanohybrids

The amphiphilic PAVE copolymer, which contains the hydrophilic units (AA) and the hydrophobic units (EHA and VMc), can self-assemble into water-dispersive polymeric nano-aggregation. To confirm the possibility of PAVE as an effective molecularly imprinted polymer, the self-assembly behaviors of PAVE copolymers without and with the addition of melamine were investigated (denoted as NIP NPs and MIP NPs, respectively). From the DLS plots (Figure S2), the mean hydrodynamic radius of the NIP NPs is about 84.5 nm. In contrast, MIP NPs with melamine templates showed larger size with mean hydrodynamic radius of about 111.5 nm. TEM characterization was also used to visualize the morphologies of NIP NPs (Figure S3) and MIP NPs (Figure 1(a1)). As shown, both NIP and MIP NPs were all in spherical shape but with dehydrated diameters of 40～50 nm and 70～80 nm, respectively. All the TEM and DLS results demonstrate the successful entrapment of melamine molecules by co-assembly with PAVE copolymers, which is in accordance with previous report.4



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Figure 1. TEM and SEM images of the MIP nanoparticles (a1, a2) and the MIP-MWCNTs nanohybrids (b1, b2) with MWCNTs as the sting cores (black arrows) and MIP NPs as the beads (red arrows).

It should be noticed that the hydrophobic and photosensitive VMc units, may serve as “non-covalent binders” between the PAVE copolymers and the conductive component MWCNTs.27 By employing MWCNTs and template molecules in the self-assembly process of PAVE copolymer, it’s expected to obtain a novel kind of molecularly

imprinted

nanohybrids

MIP-MWCNTs.

The

preparation

of

MIP-MWCNTs nanhybrids was schematically shown in Scheme 1(a). Driven by the self-assembly of PAVE copolymers, the MWCNTs and melamine templates were co-assembled into nanohybrids simultaneously.


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To verify that melamine molecules were incorporated in MIP NPs, FT-IR spectra of melamine molecules, PAVE copolymers, MIP NPs and MIP-MWCNTs nanohybrids were recorded as shown in Figure 2. Compared to PAVE copolymer (curve b), MIP NPs (curve c) and MIP-MWCNTs (curve d) exhibit distinct new peaks from about 3467 cm−1 and 3415 cm−1. Coincidentally, these two peaks are in accordance with the typical absorption peaks (asymmetric and symmetric stretching vibrations of free N-H bonds) of melamine (curve a). The FT-IR results well implied the successful incorporation of melamine molecules into the assembled MIP NPs as well as the MIP-MWCNTs nanohybrids.

Figure 2. FT-IR spectra of (a) melamine molecules, (b) PAVE copolymer, (c) MIP NPs and (d) MIP-MWCNTs nanohybrids.

The morphologies of MIP-MWCNTs nanohybrids were observed by TEM and SEM. For comparison, TEM and SEM images of both the MIP NPs (Figure 1(a) and the pristine MWCNTs (Figure S4) are also provided. As shown in Figure 1(b), the



nanohybrids exhibit “necklace-like” structured morphology which looks like PAVE copolymer nanoparticles were stringed together with the long MWCNTs string. For more clear observation of the structure, an enlarged view of MIP-MWCNTs was provided in Figure S5, demonstrating the “necklace-like” MIP-MWCNTs was formed with the MWCNTs as the string core (black arrows) and the melamine-containing MIP NPs (red arrows) as the beaded sheath. Based on the morphological observation and previous reported literatures,28-31 the formation of “necklace-like” nanohybrids may be explained as follows. In DMF solvent, PAVE polymer chains with VMc moieties firstly attach to MWCNTs surfaces due to the existence of strong π-π interactions. With the addition of melamine aqueous solution, the solubility of DMF toward hydrophobic segments in the amphiphilic PAVE copolymers decreases due to the incorporation of water as poor solvent. When the water content reaches a critical value, more PAVE chains meet the phase separation condition and thus the PAVE copolymers self-assemble into spherical colloids along the surfaces of the MWCNTs while incorporating melamine simultaneously, resulting in the “necklace-like” nanohybrids with MIP nanoparticles as the “beads” and MWCNTs as the “axis”. The nanometer size of the MIP NPs can provide a larger number of binding sites, more complete removal of imprinted molecules and better site accessibility owing to their high specific surface area, while long conducting MWCNTs strings can greatly improve the electrical conductivity, demonstrating the great potential of this novel “necklace-like” MIP-MWCNTs nanohybrids for applications in molecular imprinting


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and electrochemical sensors. Notably, thanks to the solvophilic hemisphere of the PAVE copolymer along the MWCNTs, this “necklace-like” nanohybrids showed good long-term dispersion stability through steric repulsion. The dispersion stability of MIP-MWCNTs in water was first studied by visual observation. As shown in Figure S6, pristine MWCNTs completely settled down in water due to strong interactions between inter-tubes. However, no visualized precipitation can be observed of the MIP-MWCNTs dispersion even after 60 days storage. In addition, the stability of MIP-MWCNTs was quantitatively evaluated by UV-vis spectroscopy (Figure S7), demonstrating the excellent long-term dispersion stability. The noncovalent “π-π” interactions strongly integrate PAVE polymers and MWCNTs, while steric and electrostatic repulsion with solvophilic hemispheres along the surface of MWCNT prevent nanotubes re-aggregation, leading to the long-term stability of aqueous dispersion of MIP-MWCNTs. To investigate the interactions between PAVE copolymers and MWCNTs at molecular-level, Raman spectra of pristine MWCNTs and MIP-MWCNTs were recorded and shown in Figure 3A. Pristine MWCNTs exhibit typical D-band (1310 cm−1) from disordered sp3 carbon atoms and G-band (1611 cm−1) from sp2 carbon atoms.32 For the MIP nanoparticles decorated MWCNTs, no significant changes were observed of both the D- and G-bands, suggesting that non-covalent interaction drive the integration of MIP nanoparticles and the MWCNTs without damaging the pristine MWCNTs structures.



Fluorescence spectroscopy was further applied to probe the existing noncovalent interactions. Due to intermolecular charge and electronic energy transfer between aromatic polymers and MWCNTs, MWCNTs can quench the fluorescence of conjugated polymer.26,33 As shown in Figure 3B, MIP NPs show strong fluorescence from aromatic VMc units in PAVE polymer chain. In contrast, the MIP-MWCNTs nanohybrids with the same polymer content showed significantly decreased fluorescence intensity with respect to that of free MIP NPs. This significant fluorescence quenching of MIP-MWCNTs nanohybrids strongly demonstrates an efficient energy transfer between VMc aromatic moieties and MWCNTs, confirming the existence of noncovalent π–π stacking interactions in MIP-MWCNTs.34

Figure 3. (A) Raman spectra of pristine MWCNTs (a) and MIP-MWCNTs nanohybrids (b); (B) Fluorescence spectra of MIP NPs (a) and MIP-MWCNTs nanohybrids (b).

3.3 Characterization of the MIP sensor

For application, the “necklace-like” MIP-MWCNTs nanohybrids were used as molecularly imprinted materials in fabricating MIP sensor. As illustrated in Scheme


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1(b), the MIP sensor was prepared by directly casting MIP-MWCNTs dispersion on GCE. After evaporating water, the MIP-MWCNTs modified GCE was then exposed to ultraviolet irradiation for photo-crosslinking. Subsequently, the melamine molecules were removed by electrolysis at 1.4 V in PBS solutions, obtaining the melamine MIP sensor.

3.3.1 Surface characterization of the MIP sensor

Figure 4. SEM images of MIP-MWCNTs film on sensor surface with (a) low and (b) high magnifications.

The surface morphologies of the MIP sensor were investigated by SEM (Figure 4). The “necklace-like” structured MIP-MWCNTs nanohybrids agglomerate on the electrode surface, forming cross-linked network complex film with rough topology. It should be noted that the large amount of coumarin moieties in PAVE copolymers play a key role in forming the cross-linked network. The photodimerization of coumarin moieties under UV irradiation (λ > 310 nm) can lead to cross-linking between polymer chains.35,36 On the one hand, photo-crosslinking may produce a rigid nano-architectured structures around melamine molecules and well maintain the



characteristic of imprinted cavities after melamine extraction, which is favorable for melamine rebinding effectively and selectively. Photo-crosslinking can also enhance the structure stability of the MIP complex film, leading to expected high stability of sensor performance in testing environment. On the other hand, MWCNTs in the MIP film may act as “electronic bridges”, accelerating electron transfer from the recognition sites to the electrode. The thickness and roughness of MIP-MWCNTs film were also measured with a stylus surface profiler.37,38 As shown in Table S1, the thicknesses of the NIP-MWCNTs and MIP-MWCNTs are similar with high root mean square of surface roughness (Rq), indicating the rough and porous surface provided by the interpenetrating network with long “necklaces”. For the MIP-MWCNTs film, extraction of melamine molecules did not affect the thickness of MIP-MWCNTs, which can be attributed to the robustness of the cross-linked complex film. However, after the extraction process, Rq increased slightly which may due to the removal of melamine molecules from the nanohybrids. To further verify the extraction of melamine from the sensor film, FT-IR spectra of the MIP-MWCNTs film before and after melamine extraction process were measured and provided in Figure S8. As shown, the characteristic absorption peaks of melamine are nearly invisible compared to the original MIP-MWCNTs film, which evidences the successful extraction of melamine templates and fabrication of the MIP sensor with recognizable melamine cavities.


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3.3.2 Electrochemical characterization of the MIP sensor

The interfacial properties of different modified electrodes were probed by EIS analysis. The general Nyquist plot of an EIS spectrum is composed of a semicircular part and a linear part, which correspond to electron transfers and the diffusion phenomena accordingly. The electron transfer resistance (Rct), which is derived from the diameter of semicircular part, can be used to evaluate the electron transfer behaviors of the redox probe at electrode interface.39,40 EIS spectra of different modified electrodes were shown in Figure 5 (A). As demonstrated, bare GCE shows a very small Rct value (about 50 Ω) as the Nyquist plot present at an almost straight line. For the electrode modified with MIP NPs, a much higher Rct value (about 6.3 kΩ) can be observed due to that the non-conductive MIP NPs hinder electron transfer to a great extent. However, Rct value of the MIP-MWCNTs/GCE decrease sharply compared to the MIP NPs/GCE, demonstrating the excellent conductive interface of MIP-MWCNTs/GCE due to the accelerated transport of electrons provided by the long-conducting MWCNTs. Though slightly smaller Rct values for non-imprinting samples due to the absence of template molecules, similar change in Rct values can be observed between the NIP NPs/GCE and the MIP-MWCNTs/GCE. The EIS results reveal that MWCNTs in the MIP-MWCNTs nanohybrids greatly improve the electrical conductivity of the sensor, implying superior advantages of the MIP-MWCNTs nanohybrids with “necklace-like” structure as molecularly imprinted materials in MIP electrochemical sensors.



Figure 5. (A) Nyquist plots of different modiﬁed electrodes including (a) bare GCE, (b) NIP NPs/GCE, (c) MIP NPs/GCE, (d) NIP-MWCNTs/GCE and (e) MIP-MWCNTs/GCE. (B) CV curves of bare GCE (a), MIP-MWCNTs/GCE before (b) and after (c) UV irradiation.

Due to the photosensitive property of PAVE copolymer, the MIP-MWCNTs nanohybrids based MIP sensor were crosslinked by UV irradiation. To probe the photocrosslinking behavior of MIP-MWCNTs film, CV was conducted as an effective tool by employing K3[Fe(CN)6] as probe (Figure 5(B)). CV curve of bare GCE shows a couple of quasireversible redox peaks, which is typically from potassium ferricyanide as redox probe (curve a). After modification of MIP-MWCNTs (curve b), the currents of redox peaks became much weaker as a result of the shielding efficiency of the probe toward electrode surface. After UV irradiation (curve c), the current slightly decreased due to the hindered diffusion of redox probe across electrode surface, indicating that photo-crosslinking make the MIP-MWCNTs film more compact and thus would greatly enhance the structure stability of the complex MIP-MWCNTs film.


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3.4 Performance of the MIP sensor 3.4.1 Optimization of experimental conditions

For MIP sensors, extraction and rebinding of template molecules directly influence the electrochemical performance in detecting target analytes. Therefore, variables including the electrolysis time for removing the melamine and the melamine rebinding time (incubation time) were optimized. DPSV was applied to for optimizing conditions by employing [Fe(CN)6]3−/4− solution as probe. The oxidation peak current of redox probe was used to evaluate the optimum condition. In general, larger melamine binding capacity would lead to decreased peak current of the redox probe. As details shown in Figure S9, 60 min and 5 min were used as the optimal electrolysis time for melamine extraction and the optimal incubation time for melamine rebinding in this work, respectively.

3.4.2 Quantificational detection of melamine

Under optimal conditions, the prepared MIP sensor was applied for detecting in an indirect way by taking [Fe(CN)6]3−/4− as electrochemical probe. As displayed in Figure 6(a), the increasing melamine concentration leads to decreased peak current, implying more serious obstruction effects of the rebinding melamine molecules on the electron-transfer across electrode surface. Figure 6(b) shows a significant linear relationship between the ΔI (variation of oxidation peak current) and logarithm of melamine concentration ranging from 1.0×10−12 ～ 1.0×10−6 mol L−1. The linear



regression equation was ΔI(μA)=42.63+1.70lgCmelamine (mol L−1) with a correlation coefficient of 0.9958. The detection limit is calculated to be of 5.6×10−13 mol L−1 (S/N=3). Comparatively, the peak currents of NIP sensor in Figure 6(b) showed no significant change with increasing melamine concentration and were all at very low values. Accordingly, by calculating the ratio of the slopes of linear relationship of MIP sensor and NIP sensor, a high imprinting factor (IF)41 value of 13.1 with respect to melamine was obtained. In addition, the electrochemical performance of our MIP-MWCNTs based sensor was compared with some previously prepared melamine sensors. As listed in Table S2, the linear range of MIP-MWCNTs based sensor is much wider than other melamine sensors. In addition, the MIP-MWCNTs based sensor has a much lower detection limit towards melamine determination. Notably, the detection linear range of our MIP sensor covers 1.0×10−12～1.0×10-6 mol L−1, which is the widest among all melamine sensors. The excellent performance was due to synergistic effects brought by nanosized MIP beads and long conducting MWCNTs strings. The MIP NPs along the MWCNTs show large specific surface area, providing extremely abundant imprinting sites. Furthermore, the photo-crosslinking of the polymeric MIP NPs creates robust interpenetrating network, which not only well maintain the structure of imprinting cavities for but also greatly enhance the structural robustness of MIP-MWCNTs based sensor film. In addition, the MWCNTs throughout the “necklace-like” nanohybrids based MIP film act as “electronic bridges”, which greatly accelerate electron transfer rates. As a result, the


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“necklace-like” MIP-MWCNTs nanohybrids based MIP sensor showed outstanding performance toward melamine detection.

Figure 6. (a) DPSV curves of the MIP sensor incubated with different melamine concentrations tested in 5 mM [Fe(CN)6]3−/4− aqueous solution. melamine concentrations: 0, 1.0×10−15 , 1.0×10−14, 1.0× 10−13, 1.0×10−12, 1.0×10−11, 1.0×10−10, 1.0×10−9, 1.0×10−8, 1.0×10−7, 1.0×10−6 and 1.0×10−5 mol L−1 (from top to bottom). Inset: enlarged view of DPSV plots in the melamine concentrations from 1.0×10−12 to 1.0×10−6 mol L−1. (b) Calibration curve of the MIP sensor and NIP sensor versus melamine concentrations.

3.4.2 Selectivity of the MIP sensor

Selective recognition of target analyte with discriminating other interfering species is one of the most important analytical factors for a MIP sensor.42 In order to assess the selectivity, interferents including cyanuric acid (CA), p-phenylenediamine (PPDA), m-phenylenediamine (MPDA), aniline, glucose (Glu), ascorbic acid (AA), lysine (Lys) and calcium ion (Ca2+) were used as typical representatives. CA, PPDA, MPDA and aniline were chosen because of their structural similarities to melamine. Glu, AA, Lys and Ca2+ were investigated because of the large coexistence with



melamine in real samples (for example, liquid milk, milk powder). To assess the selectivity, DPSV curves of melamine and its possible interferents in a wide range of concentrations on the MIP sensor were measured and shown in Figure 7(a). Notably, the MIP sensor showed much higher current responses toward melamine in all concentrations than its structural analogues and possible interferents. Even for the analogues, no perceivable difference of current variations can be observed. The selectivity was evaluated with the slope ratios (ratio of the slopes of calibration plots for interfering species and melamine, separately)43 as melamine (1), CA (0.14), PPDA (0.08), MPDA (0.05), Aniline (0.04), Glu (0.07), AA (0.09), Lys (0.04) and Ca2+ (0.02), respectively. These results indicate that the proposed MIP sensor has the highest selectivity toward melamine. Figure 7(b) shows the current responses of melamine, its structural analogues and interferents at the same concentration on the MIP sensor and MIP senor. The ratios of peak currents of CA, PPDA, MPDA, Aniline, Glu, AA, Lys and Ca2+ to melamine at the MIP sensor are 0.17, 0.15, 0.14, 0.15, 0.13, 0.10, 0.06 and 0.03, respectively, which clearly verify the high selectivity of our MIP-MWCNTs based sensor for melamine molecules. The origin of highly selective recognition should be attributed to melamine selective binding sites in the “necklace-like” nanohybrids created by the imprinting process (Scheme 1(c)). These tailored sites are just fitted for the shape, size as well as functional groups of melamine that even other analogues cannot perfectly bind to melamine cavities though structurally similar. Therefore, the


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significantly smaller peak current values of these structurally similar molecules (CA, PPDA, MPDA and aniline) as well as other interferents (Glu, AA, Lys and Ca2+) demonstrated extremely high selectivity of our MIP-MWCNTs sensor for target analyte. The high selectivity can be attributed to that photo-crosslinking of PAVE copolymers after incorporating template molecules can form cross-linked rigid framework around melamine templates, which locks both the shape and size of recognition cavities in MIP beads after melamine extraction and thus greatly improve the selectivity of the MIP sensor.

Figure 7. (a) Current responses of the MIP-MWCNTs based on MIP sensor for different concentrations

of

melamine,

cyanuric

acid

(CA),

p-phenylenediamine

(PPDA),

m-phenylenediamine (MPDA), aniline, glucose (Glu), ascorbic acid (AA), lysine (Lys) and calcium ion (Ca2+), respectively. (b) Current responses of the MIP sensor and NIP sensor for pure melamine and some possible interferents at the same concentration of 1.0×10−8 mol L−1 (Error bars: n=3). Inset: molecular structures of melamine and its several structural analogues.



3.4.3 Reproducibility and stability of the MIP sensor

A successful sensor needs to be reproductive with high stability when used in practical applications. The reproducibility of MIP-MWCNTs based sensor was measured by recording the responses of five independent melamine sensors under the same preparation conditions. An acceptable accuracy with relative standard deviation of 4.64 % at 1.8×10−8 mol L−1 melamine concentration was demonstrated, indicating excellent reproducibility. The stability of the MIP sensor was evaluated over 30 days at ambient temperature. By testing the peak currents of the same sensor in melamine solution every 3～5 days, no obvious change in peak current can be observed after 25 days. In addition, current response kept 92.6% of initial value after 30 days, which indicates that our MIP sensor has excellent stability. The high stability of electrochemical response is resulted from high structure stability of the crosslinked MIP-MWCNTs complex film. Photo-crosslinking among the complex film could greatly enhance the bonding strength between the MIP beads and MWCNT string as well as form a physical barrier to prevent MIPs from getting rid of MWCNTs as the cross-linked polymer NPs twining around the MWCNTs. As shown in Figure S10, the “necklace-like” MIP-MWCNTs nanohybrids based MIP film still showed cross-linked network structure even after 30 testing times, demonstrating the structure robustness of the MIP sensor resulting from photo-crosslinking of the MIP-MWCNTs nanohybrids.


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3.4.4 Detection of melamine in real samples

The practicability of our MIP sensor in real samples was conducted through a spike recovery test44 by adding melamine into the pretreated liquid milk bought from the supermarket. As listed in Table 1, the recovery of spiked samples varied from 96.4% to 104.6% with highly acceptable RSD values in the range of 2.29%～4.46%. These recovery data demonstrated that our MIP sensor has satisfactory practical analytical utility for monitoring melamine contents in real food samples. Table 1. Recoveries of melamine in liquid milk samples. Standard value

Found

Recovery

RSD(n=3)

(μmol L−1)

(μmol L−1)

(%)

(%)

1

0.0005

0.000523

104.6

2.29

2

0.005

0.00482

96.4

4.46

3

0.5

0.486

97.2

4.01

Samples

4. Conclusions In conclusion, a novel MIP nanohybrids (MIP-MWCNTs) with “necklace-like” morphology was prepared based on molecularly imprinted polymeric nanoparticles decorating MWCNTs nanohybrids and the “necklace-like” MIP-MWCNTs were successfully applied to develop an intercrossed MIP network film on electrode surface, fabricating a melamine MIP sensor with excellent performance. The nanosized polymeric MIP nanoparticles in the MIP film increase the effective electrode surface area and thus offer high melamine binding capacity. Photo-crosslinking creates robust



polymer network among MIP-MWCNTs nanohybrids, which well maintains the spatial structure of imprinted cavities, providing much more effective recognition sites. In addition, the structural stability of MIP-MWCNTs complex films was greatly enhanced. Moreover, the MWCNTs in nanohybrids serve as “electronic bridges” to accelerate the electron transfer in the complex film. As a result, the MIP-MWCNTs based MIP sensor showed high selectivity and sensitivity for melamine determination. In addition, the MIP sensor also demonstrated high stability and reproducibility. Satisfactory results were obtained by applying the sensor in detecting melamine in liquid milk samples, demonstrating a promising feature of the proposed MIP sensor for applications in food analysis like liquid milk and other food products including milk powder, infant formula and animal feed. In light of functionalization versatility of polymeric nanoparticles, this strategy can be generalized to many other MIP systems, thus has great application prospects in biosensors and other electronic devices.

Associated content Supporting Information

1

H NMR spectrum of PAVE copolymer; DLS plots of NIP NPs and MIP NPs; TEM

image of NIP NPs; Morphology of pristine MWCNTs observed by TEM and SEM; Enlarged view of part of the MIP-MWCNTs; Stability studies by visual observation


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and UV-Vis spectroscopy; Surface roughness and thickness of the sensor surfaces; FT-IR characterization of MIP-MWCNTs film before and after extraction of melamine molecules; Optimization of experimental conditions including electrolysis time and incubation time; Comparison of different MIP sensors for detection of melamine; Structural stability of the sensor film by SEM.

Author information Corresponding Author

*E-mail: [email protected] (X. Y. Liu). *E-mail: [email protected] (Y. Zhu).

ORCID

Sheng Xu: 0000-0001-9492-0585 Xiaoya Liu: 0000-0003-2868-7601

Acknowledgments We acknowledge financial support from the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX17_1428), the Natural Science Foundation of China (NSFC 51573072) and the MOE & SAFEA for the 111 Project (B13025).



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Necklace-like molecularly imprinted nanohybrids based on polymeric

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