Carbon Nanotubes

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Long Conducting and Water-Compatible Polymer/Carbon Nanotubes Nanocomposite with “Beads-on-a-String” Structure as a Highly Effective Electrochemical Sensing Material Sheng Xu, Wei Zhao, Geyu Lin, Qian Wu, Mengyi Xu, Xuewen Huang, Jing Luo, Ye Zhu, and Xiaoya Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05891 • Publication Date (Web): 02 Jan 2019 Downloaded from http://pubs.acs.org on January 3, 2019

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ACS Sustainable Chemistry & Engineering

Long Conducting and Water-Compatible Polymer/Carbon Nanotubes Nanocomposite with “Beads-on-a-String” Structure as a Highly Effective Electrochemical Sensing Material Sheng Xu †‡, Wei Zhao†, Geyu Lin†§, Qian Wu†, Mengyi Xu‡, Xuewen Huang†, Jing Luo‡, Ye Zhu*†, Xiaoya Liu*†‡ † Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, Jiangnan University, No.1800 Lihu Avenue, Wuxi 214122, Jiangsu Province, P. R. China ‡ School of Chemical and Material Engineering, Jiangnan University, No.1800 Lihu Avenue, Wuxi 214122, Jiangsu Province, P. R. China § School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Rd. Minhang District, Shanghai 200240, P. R. China

*Corresponding authors. Tel.: (+86)0510-85917763. E-mail: [email protected] (Xiaoya Liu); [email protected] (Ye Zhu).

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ABSTRACT A novel water-compatible polymer/carbon nanocomposite with “beads-on-a-string” nanostructure was developed by a facile one-step co-assembly of an amphiphilic random copolymer with multi-walled carbon nanotubes (MWCNTs), and successfully applied as effective electrode material for electrochemical sensing with high sensitivity.

An

amphiphilic

Poly(AA-co-VMc-co-EHA)

(PAVE)

photo-crosslinkable was

synthesized

random via

copolymer

one-step

radical

polymerization. Then PAVE copolymer co-assembled with MWCNTs non-covalently in selective solvent, generating water-compatible and “beads-on-a-string” structured polymer/carbon nanocomposites (PAVE-CNTs NCs) with PAVE nanoparticles as nano-sized beads and MWCNTs as micron-long conducting strings. Then PAVE-CNTs NCs were used as electrode material for electrochemical sensing of a model target namely paracetamol (PCM). The resultant sensor shows significantly wider linear detection range with lower detection limit than other sensors. The excellent sensing performance was ascribed to large surface area and high electrical conductivity of the “beads-on-a-string” structured PAVE-CNTs NCs. In addition, the sensor was utilized to measure PCM in commercial tablets and urine samples, demonstrating high practicability in medical diagnosis. The synthesis method of this hierarchically structured polymer/carbon nanocomposite is facile with mild working condition and the sensor generation process is green and energy-saving, which will 2


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encourage promising applications in chemical sensors like biosensors, and other microelectronic devices. Keywords:

Beads-on-a-string

structure;

Polymer/CNTs

Nanocomposite;

Co-assembly; Amphiphilic copolymer; Carbon nanotubes; Electrochemical sensor

3



INTRODUCTION For advanced materials design, it’s highly desirable to realize diversely excellent performance and synergetic multifunctionality from the unique hybrid structures or composite materials via self-assembly of organic/inorganic hybrid building blocks.1−6 Particularly, the hybridization of carbon nanomaterials with polymers to create polymer/carbon nanocomposites has demonstrated a myriad of powerful advantages in scientific and industrial studies.7−8 As one of the most popular carbon nanomaterials, carbon nanotubes (CNTs) with extraordinary electrical, mechanical, optical, and other physicochemical properties, have been widely used to prepare polymer/CNTs composites with the purpose of synergistically combining the merits of each individual component.9,10 Thanks to their outstanding properties, CNTs have been greatly exploited in electrochemical sensors, capacitors, actuators, solar cells, etc.11−13 It has been proved that polymer/CNTs nanocomposites as electrochemical transducers could substantially improve the performance of sensing devices, in general, and electrochemical sensors, in particular.14,15 However, due to the existence of strong intertube van der Waals interactions, CNTs are prone to aggregate into bundles which sets a great obstacle for their applications in electronic devices.16,17 To tackle this problem, either covalent or noncovalent functionalization of CNTs has been exploited as a highly efficient way to effectively improve the dispersibility of CNTs. Covalent functionalization is typically operated by introducing solvophilic 4


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groups on CNTs surface, which is apt to disrupt the intrinsic structure of CNTs.18 In contrast, noncovalent functionalization assisted by dispersants has been well regarded as a “nondestructive” way which maintains the integrity of the CNTs sidewall with preserving the desirable properties of CNTs.19 Dispersants for this strategy can be identified into several main categories, including small molecule aromatic compounds,20 surfactants,21 biomacromolecules,22 and conjugated polymers.23 In general, CNTs dispersion and stabilization are mainly achieved by electrostatic repulsion and steric repulsion.24 In recent years, amphiphilic copolymers composed of both solvophobic and solvophilic segments have been recognized as a new type of dispersants. With one segment interacting with CNTs and another dangling into solvent, these macromolecules can realize stable dispersion of CNTs by providing steric hindrance or electrostatic repulsion between CNTs.25−27 However, most of these amphiphilic copolymers are block copolymers or “regular” copolymers, for which a relatively tedious synthesis procedure is generally involved, greatly limiting their practical applications. In addition, though these copolymers are capable of deaggregating CNTs clusters, it’s still a challenge to increase the long-term dispersibility against CNTs reaggregation due to the dynamic self-assembled structure of these amphiphilic macromolecules. Compared with these “regular” copolymers, random copolymers are more easily synthesized at lower cost and thus show higher value of practical application.28 In our previous work, we have prepared a series of amphiphilic random 5



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copolymers with photo-crosslinkable segments and successfully assembled them into polymeric

nanoparticles

as

polymeric

particulate

emulsifiers,

obtaining

thermodynamically stable Pickering emulsions.29−31 Inspired by the ability of stabilizing oil phase in aqueous phase of these self-assembled amphiphilic nanoparticles, we highly anticipated whether the micron-long carbon nanotubes could also be stabilized in macromolecular assembly system. If it works, a water-compatible as well as highly conductive polymer/CNTs nanocomposite would be created in a way of great significance in practice. In this work, a novel kind of polymer/carbon nanocomposite with a “beads-on-a-string” photo-crosslinkable

morphology

was

created

amphiphilic

based

random

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

on

co-assembly

copolymer

coumarin)-co-ethylhexyl

of

a

poly(acrylic acrylate)

(Poly(AA-co-VMc-co-EHA), PAVE) with multi-walled carbon nanotubes (CNTs) and employed as effective electrode material for constructing electrochemical sensor. The prepared PAVE-CNTs nanocomposites (PAVE-CNTs NCs) were shaped with PAVE polymeric beads along one long conductive CNT string. For sensor development, glassy carbon electrode (GCE) was modified with PAVE-CNTs NCs as sensing materials. After subsequent photo-crosslinking, a robust composite sensing film based electrochemical sensor was successfully developed. Paracetamol (PCM), one of the most popular and widely used analgesic antipyretic drug, was employed as a target analyte to illustrate the performance of the PAVE-CNTs NCs based sensor. The 6


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chemical structure PAVE copolymer and photo-crosslinking reaction of PAVE inter-chains are shown in Scheme 1a. The amphiphilic random copolymer PAVE was synthesized by one-step radical copolymerization, for which the whole procedure is simple and can be easily scaled up for end applications. Thanks to the presence of photosensitive VMc units in the PAVE polymer skeleton, ultraviolet (UV) induced cross-linking reactions would occur among PAVE chains without the use of chemical cross-linking agents. The preparation process of PAVE-CNTs NCs is illustrated in Scheme 1b. The PAVE-CNTs NCs were formed by PAVE copolymers wrapping around the MWCNTs surface (noncovalent “π-π” stacking interaction) and the following co-assembly with MWCNTs (noncovalent hydrophobic effect). Notably, the bonding strength between PAVE NPs and MWCNTs would be effectively enhanced by photo-crosslinking among the polymer chains. As the polymer beads are cross-linked, physical winding effect would further prevent the PAVE beads breaking away from the constraint of MWCNTs. As a result, the water-compatible PAVE-CNTs NCs would have high structure stability and long-term dispersibility. The influence of polymer/CNTs mass ratio and pH on the structures of PAVE-CNTs NCs were investigated in detail. The surface geology and interfacial properties of PAVE-CNTs NCs based composite sensing film were also characterized. Further, the sensing performance of the sensor towards PCM was investigated in detail. Moreover, satisfactory results were also obtained for determination of PCM in real tablets and human urines, demonstrating promising potential of this strategy for biomedical 7



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diagnostics.

Scheme 1. (a) Schematic illustration of photo-crosslinking of the photo-crosslinkable and amphiphilic

poly(AA-co-VMc-co-EHA),

PAVE

copolymer.

(b)

Fabrication

of

the

“beads-on-a-string” structured PAVE-CNT NCs via one step co-assembly of PAVE copolymers and MWCNTs (the photosensitive VMc units are marked as red pendants).

EXPERIMENTAL SECTION Synthesis of amphiphilic copolymer: PAVE

The coumarin-containing 7-(4-vinylbenzyloxy)-4-methyl coumarin (VMc) was synthesized referring to reported literature29 and 1H NMR characterization is provided in Figure S1, confirming the structure of VMc monomer. The amphiphilic PAVE 8


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copolymer was prepared with a typical synthesis procedure as follows: A mixture of AA (5 mmol), VMc (5mmol) and EHA (5 mmol) were co-dissolved in dioxane, followed by introducing AIBN (1.5% with respect to the total monomer molar quantities) as initiator. The mixture was deoxygenated with N2 gas for 30 mins and sealed under N2 atmosphere, after which the reaction was activated and conducted in oil bath at 65 °C and proceeded for 24 h under vigorous stirring. The resultant PAVE copolymers were purified and collected as white powder (84.2% weight yield) after drying in vacuum at 30 oC overnight.

Preparation of the PAVE-CNTs nanocomposites

To prepare the PAVE-CNTs nanocomposites (PAVE-CNTs NCs), PAVE copolymer and pristine MWCNTs were co-dispersed in DMF, obtaining 5 mg mL−1 copolymer solutions containing different amount of MWCNTs. After sonication treatment for 2 h, 0.6 mL water was dropwise added to the mixture under magnetic stirring. For complete co-assembly of PAVE copolymers and MWCNTs, the mixture was kept stirring overnight and then 5 folds volume of water was poured into to “freeze” the structure of resultant composites. After that, the obtained mixture was dialyzed against water for eliminating DMF followed by irradiation with UV light for 15 mins. The resultant suspension was filtered through glass wool to get rid of undispersed carbon nanotubes, obtaining the PAVE-CNTs NCs dispersion.

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Fabrication of the electrochemical sensor

In brief, 10 μL PAVE-CNTs NCs dispersion was cast on the surface of clean glassy carbon electrode (GCE, Φ=3 mm) and evaporated at room temperature, followed by being exposed with UV irradiation for another 15 mins to induce more complete photo-crosslinking among the composite film, generating the electrochemical sensor. The prepared sensor was stored at room temperature before use. Characterization and measurement The detailed materials characterizations and corresponding instruments are displayed in the Supporting Information. All electrochemical experiment methods and conditions can also be found in the Supporting Information.

RESULTS AND DISCUSSION Synthesis and characterization of the PAVE copolymer

The PAVE copolymer, as shown in Scheme 1a, was synthesized using acrylic acid (AA), 7-(4-vinylbenzyloxy)-4-methyl coumarin (VMc) and ethylhexyl acrylate (EHA) as hydrophilic, photo-crosslinkable and hydrophobic monomers, respectively. As shown in Figure S2, the structure of PAVE copolymer was confirmed by 1H NMR (Details can been found in Supporting Information).

10


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Preparation and characterization of PAVE-CNTs NCs Self-assembly of the PAVE copolymers

With both hydrophilic and hydrophobic units, the PAVE copolymer can self-assemble into polymeric nanoparticles (PAVE NPs) in selective solvents. As shown in the DLS plot (Figure S3a), the mean hydrodynamic diameter of PAVE NPs is about 72.8 nm. Figure S3b shows the TEM image of PAVE NPs, from which many monodispersed spherical nanoparticles with dehydrated diameters of 30～60 nm can be observed. All above results verify the successful self-assembly of PAVE copolymer because of its amphipathicity. Therefore, it is expected that this amphiphilic copolymer would possibly serve as a macromolecular stabilizer for MWCNTs by forming integrated polymer/CNTs nanocomposite when strong interactions exist between the interfaces of copolymers and MWCNTs.

Preparation of the PAVE-CNTs NCs

To demonstrate above hypothesis, MWCNTs were incorporated in the self-assembly process of PAVE copolymers. The preparation process of PAVE-CNTs NCs was schematically shown in Scheme 1b. It should be noticed that the hydrophobic as well as the photosensitive VMc units would act as important “bridges at molecular level” to non-covalently bind up PAVE copolymers with MWCNTs. As a result, the MWCNTs can be simultaneously co-assembled with the amphiphilic PAVE copolymers, bringing forth the PAVE-CNTs NCs. 11



The co-assembly of MWCNTs with PAVE copolymers to generate PAVE-CNTs NCs was investigated by FT-IR spectra and TGA curves. Significant difference can be observed from the FT-IR spectra in Figure 1a. Pristine MWCNTs show almost no absorption peaks on account of the inert nature of carbon surfaces. After integration with PAVE copolymers, new peaks which are assigned to the characteristic absorption signals of PAVE copolymers appear in FT-IR spectrum of PAVE-CNTs NCs. The FT-IR spectra well identify the presence of PAVE copolymers on the surfaces of MWCNTs. The wrapping of PAVE copolymers on MWCNTs surfaces was also demonstrated by TGA under nitrogen atmosphere. From Figure 1b, the pristine MWCNTs were thermally stable with only a 3.6% weight loss at 800 °C probably due to the degradation of amorphous carbon and impurities.32 PAVE copolymer rapidly lost its weight with heating up from 250 °C to 500 °C and almost lost all its weight at 600 °C. PAVE-CNTs NCs showed a weight loss of 68.5 wt% starting from 200 to 600 °C, which was resulted from thermal decomposition of the PAVE copolymers adsorbed on the surfaces of MWCNTs. The mass content of MWCNTs in the PAVE-CNTs NCs is calculated to be 12.5 % based on the above TGA data. The different decomposition process between the pristine MWCNTs and PAVE-CNTs NCs proved the successful co-assembly of PAVE copolymers with MWCNTs.

12


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Figure 1. (a) FT-IR spectra and (b) TGA curves of pristine MWCNTs, PAVE copolymer and PAVE-CNTs NCs, P/C=1/0.5.

The mass ratio of polymer and CNTs (P/C), regarded as a key parameter in the co-assembly process, was investigated for obtaining an optimum preparation condition of PAVE-CNTs NCs. The dispersion state and stability of the prepared PAVE-CNTs dispersion with different P/C ratios were firstly investigated by visual observation. Figure S4 shows the photographs of pristine MWCNTs and PAVE-CNTs NCs with different P/C mass ratios, from which complete precipitation of pristine MWCNTs in water can be observed within 1 hour. In contrast, all the PAVE-CNTs dispersions with different P/C mass ratios showed no visualized precipitation even after 30 days, indicating the high long-term dispersibility of MWCNTs modified with PAVE copolymers. The morphologies of the pristine MWCNTs and PAVE-CNTs NCs with different P/C ratios were further characterized by TEM (Figure 2). As observed in Figure 2a, the pristine MWCNTs entangle with each other to form precipitates in water as a result of the strong van der Waals interaction. Figure 2b-e show TEM images the PAVE-CNTs NCs with different P/C ratios. At the mass ratio of P/C=1/1 13



with higher MWCNTs content, less mutual MWCNTs entanglements are observed with thin copolymer coating layer and some ellipsoidal nanoparticles wrapping along the surfaces of MWCNTs. Interestingly, with decreasing MWCNTs content in the co-assembly process, “beads-on-a-string” structured PAVE-CNTs NCs with PAVE copolymer as the nano-sized “beads” and MWCNTs as the micron-sized “string” were formed, as shown in Figure 2b-e. However, with increasing P/C mass ratio, more independent polymer NPs can be observed in the dispersion due to less MWCNTs as linear surpports to “string” the polymer NPs.

Figure 2. TEM images of (a) pristine MWCNTs, (b-e)PAVE-CNTs NCs with different P/C mass ratios: (b)-1/1, (c)-1/0.5, (d)-1/0.2, (e)-1/0.1, and (f) SEM image of the PAVE-CNTs NCs with P/C=1/0.5. The final PAVE concentration for all the samples is 0.25 mg mL−1.

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Formation mechanism of the PAVE-CNTs NCs

The probable formation mechanism of this “beads-on-a-string” structured PAVE-CNTs NCs in our study is shown in Scheme 2. Initially, PAVE polymer chains closely attach to MWCNTs surfaces in DMF solvent due to strong π-π interactions between MWCNTs and VMc segments with the help of ultrasonication. Upon addition of water as a poor solvent for the hydrophobic segments of PAVE, the amphiphilic copolymers wrapping around MWCNTs surfaces assemble into nano-sized colloids, resulting in “nano-sized polymeric particles decorated inorganic MWCNTs” NCs with “beads-on-a-string” appearance. This mechanism well explains the good dispersibility of the PAVE-CNTs NCs in aqueous solution as a result of steric repulsion with solvophilic hemispheres (–COOH or –COO–) along the MWCNTs. In contrast, this “beads-on-a-string” structured NCs can’t be obtained by simply mixing MWCNTs with preformed PAVE NPs (Figure S5), further convincing the co-assembly formation mechanism of this nanostructure in our strategy. Notably, the PAVE beads on MWCNTs surfaces are much larger in size (120~200 nm) than that of free PAVE NPs without MWCNTs (30~60 nm). The difference in size can be explained as follows: The self-assembly of amphiphilic copolymers in selective solvents is driven by a delicate balance between hydrophobic and hydrophilic interactions.26 Upon adding water into PAVE copolymer DMF solution, the overall solubility of hydrophobic segments decreases with the increasing of water content.

15



When the content of water reaches the point namely critical water content (CWC), phase separation takes place among these amphiphilic polymer chains. As a result, PAVE copolymers transfer from swollen random coils into aggregates, forming the spherical PAVE NPs. After the incorporation of MWCNTs in PAVE copolymers, much more PAVE polymer chains wrap surround the side-walls of MWCNTs due to the “π-π” interactions. More PAVE chains aggregate around the MWCNTs when adding water into the mixtures, leading to larger PAVE beads in size along the MWCNTs. From the view of “beads-on-a-string” morphology with more MWCNTs content, PAVE-CNTs NCs with P/C=1/0.5 was selected as a typical model for further study and use in the following work. The surface morphology of PAVE-CNTs NCs with P/C=1/0.5 was characterized by SEM shown in Figure 2f, also displaying the shape of long necklaces with PAVE NPs as beads tightly wrapping around the linear MWCNTs as strings. This hierarchically structured polymer/CNTs composite, with nano-sized polymer NPs providing high specific surface area and the micron-sized MWCNTs acting as long conducting path, is expected to show great advantage in electrochemical sensors and other electronics.

Scheme 2. Schematic illustration of the formation mechanism of the “beads-on-a-string” 16


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structured PAVE-CNTs NCs.

Influence of pH on the PAVE-CNTs NCs

Due to the existence of solvophilic AA moiety in PAVE NPs, the influence of pH on the nanostructure and morphology of the PAVE-CNTs NCs was investigated. Figure 3 shows the TEM images of PAVE-CNTs NCs with different pH values. Interestingly, “beads-on-a-string” structured morphologies with PAVE as beads decorating the MWCNTs as strings were observed for almost all the PAVE-CNTs NCs samples in a wide range of pH values. At pH 2.93, aggregations of the “beads-on-a-string” structured NCs were observed due to weak electrostatic repulsion forces as a result of high-degree protonation of carboxylic acid groups in PAVE beads (Figure 3a). When pH value was increased above 2.93, electrostatic repulsion forces among the PAVE-CNTs NCs was enhanced due to increased protonation of PAVE copolymers, bringing about better dispersion of PAVE-CNTs NCs (Figure 3b-c). However, further increases in pH resulted in disintegration of PAVE beads and fusion of the “beads-on-a-string” structured NCs (Figure 3d-e) and this phenomenon became more obvious at pH 8.94 (Figure 3f). For the PAVE-CNTs NCs, the stabilization of polymer beads on the MWCNT string is controlled by a delicate balance between two main competing interactions of hydrophilic interaction and “π-π” interaction (or hydrophobic interaction). At higher pH condition, hydrophilic interaction become stronger due to the progressive increase of deprotonated groups. As a result, PAVE 17



copolymers tend to “get rid of” the constraint of MWCNTs. However, thanks to the photo-crosslinked PAVE copolymers wrapping around the MWCNT string, it’s rather difficult for the PAVE beads to completely separate from the MWCNT string. Therefore, cloudy morphology among the PAVE-CNTs NCs was observed due to high swelling and fusion of PAVE polymer beads along the MWCNTs surfaces. As the pH values were further elevated to 9.94 and 11.08 (Figure S6) in strongly alkaline environment, the swelling behaviors of PAVE copolymers become much more obvious with some beads disintegrating from MWCNTs. This is attributed to that almost all carboxyl groups on PAVE polymer chains are deprotonated at too high pH values, resulting in awfully strong electrostatic repulsion between inter-chains at the interior of the polymer beads. The interior electrostatic repulsion could get over the restriction of the polymer chains by hydrophobic effect and even photo-crosslinking effect, giving rise to disintegrated morphologies of the PAVE-CNTs NCs. As a result, the PAVE-CNTs NCs dispersion would show poorer long-term stability in aqueous solution due to the disintegration of PAVE polymers and MWCNTs. In general, the photo-crosslinked and “beads-on-a-string” structured PAVE-CNTs NCs show great tolerance to a wide pH range, demonstrating a robust structure stability for application in harsh aqueous environment.

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Figure 3. TEM images of PAVE-CNTs NCs with different pH values: (a) pH=2.93, (b) pH=3.96, (c) pH=5.60 (original value), (d) pH=7.45, (e) pH=7.90, (f) pH=8.94. All the samples are at a mass ratio of P/C =1/0.5.

The effectiveness of photo-crosslinking

As a well-known photosensitive monomer, coumarin possesses excellent photodimerization property when being exposed to UV irradiation (λ > 310 nm) without the need of other chemical crosslinking agents.33 The presence of coumarin moiety (VMc) in the molecular structure of PAVE copolymer would endow this amphiphilic copolymer with photo-crosslinkable property. The photo-crosslinking behavior of PAVE copolymers was firstly verified by UV-vis absorbance spectroscopy. Figure 4a shows the UV-vis spectra of PAVE NPs aqueous solution with increasing 19



irradiation times by UV light (λ = 365 nm). With prolonging the irradiation time, the characteristic absorbance of coumarin group at 320 nm continuously decreased, indicating UV light-induced dimerization of neighboring coumarin moieties in PAVE NPs.34 The photodimerization degree (PD) of coumarin moieties in PAVE NPs at different irradiation time is depicted in the inset of Figure 4a, manifesting that most of coumarin moieties have been crosslinked after UV irradiation for 30 minutes with a maximum PD of about 60%. For PAVE-CNTs NCs with PAVE NPs wrapping around the CNTs surfaces, it’s expected the photo-crosslinking of PAVE polymer beads would enhance the interactions between the polymer beads and CNTs surfaces and thus improve the long-term dispersion stability of CNTs as well as the structure stability of the integrated PAVE-CNTs NCs. The effectiveness of photo-crosslinking on the long-term stability of PAVE-CNTs NCs was investigated more quantitatively by UV-vis spectroscopy. It has been reported the long-term dispersibility of polymer/CNTs NCs can be reflected by the UV-vis absorbance value at the wavelength of 500 nm. In general, a higher absorbance value indicates a better dispersibility of the polymer/CNTs NCs.35 As illustrated in Figure 4b, the absorbance of PAVE-CNTs without UV irradiation was 78% of its initial value after 30 days. In contrast, the absorbance intensity of PAVE-CNTs decayed much slower and stayed at 93% of the initial value after 30 days, suggesting the enhanced long-term dispersion stability of photo-crosslinked PAVE-CNTs NCs. Overall, photo-crosslinking could greatly enhance the dispersibility of PAVE-CNTs NCs in aqueous solution, which is 20


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attributed to tighter integration of PAVE beads with MWCNTs (the noncovalent interactions like “π-π” stacking and hydrophobic interaction as well as physical barrier from cross-linked PAVE NPs twining around the CNTs) as a result of crosslinked polymeric beads along the MWCNTs surfaces, as shown in Scheme 1b.

Figure 4. (a) UV-vis absorption spectra of PAVE NPs with different UV irradiation times, Inset: the photodimerization degree (PD) of PAVE NPs versus irradiation time (λ=365 nm, the concentration of PAVE NPs is 0.25 mg mL−1). (b) The stability of PAVE-CNTs NCs dispersion with and without UV irradiation recorded versus standing time, where At corresponds to the absorbance value at standing time of t days and A0 represents the initial value of the absorbance value recorded at the wavelength of 500 nm.

To further illustrate the effectiveness of photo-crosslinking on the structure integrity, PAVE-CNTs NCs dispersions with and without UV irradiation were treated by ultra-sonication for 30 mins, and the respective morphologies were characterized by TEM. In general, ultra-sonication will produce a transient imbalance between hydrophilic–hydrophobic interactions to strike a new balance in a complex system. As observed in Figure S7, uncross-linked PAVE-CNTs NCs show detached and irregular 21



morphology that some copolymers dissociate from MWCNTs strings to assemble into independent small aggregations, which would result in MWCNTs re-aggregation in aqueous solution. Interestingly, the photo-crosslinked PAVE-CNTs NCs still maintain well-structured “beads-on-a-string” morphology even after ultra-sonication. It seems that the PAVE-CNTs NCs structure is “frozen” by the inter-chains crosslinking among the wrapping polymer beads. The morphological difference between ultra-sonicated PAVE-CNTs NCs with and without UV irradiation well verify that photo-crosslinking would enhance the structural integrity of this “beads-on-a-string” structured NCs.

Interactions between PAVE beads and MWCNTs strings

Raman spectroscopy was conducted to confirm the interaction between the MWCNTs surfaces and the PAVE copolymers. Figure 5a illustrates the typical D-band (1311 cm−1) and G-band (1607 cm−1) attributed to disordered sp3 carbon and graphitic sp2 carbon in pristine MWCNTs, respectively. Compared to the Raman spectrum of pristine MWNTs, both the D-band and G-band show no obvious shifts, suggesting that non-covalent interaction exists between PAVE copolymers and MWCNTs. The ratio of ID/IG (defect/graphite) can quantify the defect levels in modified graphite materials. As calculated from the Raman spectra, the ratio of ID/IG of the PAVE-CNTs NCs (ID/IG =1.37) was almost identical to that of the pristine MWCNTs(ID/IG =1.42), indicating that noncovalent interactions occurred between the MWCNTs and the copolymers without structural damage to the MWCNTs.36 From the copolymer 22


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structure viewpoint, we proposed in Scheme 2 that the attachment of PAVE copolymers onto MWCNTs is driven by noncovalent “π-π” stacking interaction, which was verified by fluorescence spectroscopy. It is well-known that due to intermolecular electronic energy and charge transfer effects, MWCNTs can effectively quench the fluorescence of conjugated polymers.37 Figure 5b shows the fluorescence emission spectra of PAVE NPs and PAVE-CNTs NCs recorded with the excitation wavelength of 340 nm. PAVE NPs show strong fluorescence due to the presence of aromatic VMc units in PAVE chains. Contrastively, the fluorescence spectrum of PAVE-CNTs NCs with the same polymer content exhibits a similar emission band to that of PAVE NPs, but the intensity is significantly decreased. This significant fluorescence quenching strongly demonstrates efficient photo-induced electron transfer among the interface of VMc-containing PAVE copolymers and MWCNTs38,39 which greatly evidences the “π-π” stacking interaction between the PAVE polymeric beads and MWCNTs strings.

Figure 5. (a) Raman spectra of pristine MWCNTs and PAVE-CNTs NCs; (b) Fluorescence spectra of PAVE NPs and PAVE-CNTs NCs. 23



Fabrication and characterization of the electrochemical sensor

The “beads-on-a-string” nanostructured PAVE-CNTs NCs were used as a novel kind of electrode materials to fabricate electrochemical sensor and the fabrication process is illustrated in Scheme S1. Firstly, the PAVE-CNTs NCs dispersion with incomplete UV irradiation was drop-cast on a clean glass carbon electrode (GCE) and stand for drying naturally. Then the PAVE-CNTs NCs modified GCE was further photo-crosslinked by UV irradiation for formation of enhanced PAVE-CNTs composite film, resulting in the generation of the PAVE-CNTs NCs based electrochemical sensor as a whole. Notably, UV induced photo-crosslinking mainly took place between accessible or neighboring beads, which played a crucial role in fabricating the sensor. The structure of the PAVE-CNTs film was partly “frozen”, which remarkably strengthened the structural stability of PAVE-CNTs NCs based composite film. As a consequence, the PAVE-CNTs sensing film based electrochemical sensor would show strong resistance to solvent damage when applying in solution environment.

Morphology characterization of the sensor

The surface morphology of PAVE-CNTs NCs based sensor was investigated by SEM. As shown in Figure 6, numerous “beads-on-a-string” structured PAVE-CNTs NCs assemble on the sensor surface to give formation of an interlaced three-dimensional network film. The rough and porous surface greatly improve the 24


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specific surface area and thus provides more contacting sites for analyte, which is expected to enhance the electrochemical interaction and shorten the response time of the electrochemical sensor. What’s more, the MWCNTs in the composite film would establish numerous long conducting “electronic bridges” to facilitate electron transport among the sensing film, bringing about high sensitivity of the electrochemical sensor.

Figure 6. SEM images of the surface of PAVE-CNTs composite film on electrode surface at (a) low and (b) high magnifications.

Electrochemical characterization of the sensor

In this work, electrochemical impedance spectroscopy (EIS) was performed to study the interfacial properties of different modified surfaces by employing Fe(CN)3–/Fe(CN)4– as a redox probe.40 A typical EIS spectrum is generally presented as Nyquist plot including a semicircular part and a linear part.41 In general, the electron transfer kinetics of redox probe on electrode surfaces can be assessed by electron transfer resistance (Rct) obtained from the semicircular domain.42 As shown

25



in Figure S8, Nyquist plot of bare GCE shows an almost straight line, indicating negligible resistance of the redox probe toward electrode. After the GCE was modified with polymeric PAVE NPs, a much larger Rct (about 5000 Ω) can be observed, which is due to that the nonconductive PAVE copolymers have a large electron transfer resistance. In contrast, Rct value of PAVE-CNTs NCs/GCE is much smaller than PAVE NPs/GCE, revealing the better conductivity of PAVE-CNTs NCs/GCE provided by MWCNTs as the long conducting electrical paths. The EIS results well illustrate the vital role of MWCNTs in the PAVE-CNTs NCs for accelerating electron transfer of the sensing surface, indicating superior advantages of the “beads-on-a-string” structured PAVE-CNTs NCs as electrode materials in electrochemical sensors.

Performance of the electrochemical sensor Quantificational detection of paracetamol (PCM)

Paracetamol (PCM) is one of the most widely used analgesic and antipyretic drugs for fever, headaches, and minor pain relief.43 However, excess use of PCM would lead to severe consequences like fatal hepatoxicity and nephrotoxicity. Therefore, it’s extremely important to develop reliable electrochemical sensors for highly sensitive and selective PCM determination.44,45 PCM can undergo electrochemical oxidationreduction processes on electrode surface. However, the electrochemical reactions of PCM are irreversible with only weak oxidation reaction on bare electrode. Due to 26


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their high electron transfer capability and high conductivity, CNTs can greatly enhance

the

electro-catalytic

activity

towards

PCM,

resulting

reversible

oxidation-reduction reactions of PCM on electrode surface.46 As a proof-of-principle strategy, the prepared PAVE-CNTs NCs based sensor was used to illustrate its applicability in PCM determination. The sensing mechanism of PCM on PAVE-CNTs NCs/GCE is illustrated in Scheme S2, in which a two-electron and two-proton process are involved. During the oxidation-reduction process of PCM, the electric system was exchanged with PCM and N-acetyl-P-quinone-amine.47,48 As shown in Figure 7a, Differential pulse voltammetry (DPV) was applied to monitor the electrochemical oxidation signals of PCM at different concentrations on the PAVE-CNTs NCs based sensor surface. The oxidation peak currents recorded by DPV elevate with increasing PCM concentration as a result of electrocatalytic oxidation of PCM by the PAVE-CNTs NCs. The calibration curve of the electrochemical sensor to different PCM concentrations is shown in Figure 7b. A good linear relationship is established between ΔI (variation of peak currents before and after the presence of PCM) and the logarithm of PCM concentration in the range of 1 μM to 1000 μM. The linear regression equation was represented as ΔI(μA)= 0.07515+0.01611CPCM(μM) (R2 = 0.9993). The detection limit (LOD), defined as LOD=3σ/s (in which σ is the standard deviation of signals recorded in blank samples (n=3), and s is the slope of calibration curve), was calculated to be 0.20 μM (S/N=3).48 In addition, electrochemical performance of the PAVE-CNTs NCs based sensor was compared 27



with some previously reported carbon nanotubes based PCM sensors. As displayed in Table 1, the PCM sensor based on PAVE-CNTs NCs demonstrates much wider linear detection range and lower LOD against other polymer/CNTs based PCM sensors. It's worth noting that our sensor shows widest linear range among all PCM sensors. The outstanding sensing performance results from the synergistic effects of the nanosized PAVE beads and the micron long conductive MWCNTs strings. The interlaced and porous composite film based on these “nano-necklaces” provides much more contact sites due to large specific surface area. Furthermore, the long conductive MWCNTs electronic paths substantially accelerate electron transfer rates throughout the electrode surface, which consequently boosts the sensing performance of the electrochemical PCM sensor.

Figure 7. (a) DPV curves of the PAVE-CNTs NCs based sensor with increasing concentration of PCM in on in 0.1 M PBS (pH 7.0). PCM concentrations: 1.0, 2.0, 4.0, 6.0, 6.0, 8.0, 10, 20, 40, 60, 80, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000 μM (from bottom to top). (b) Calibration curve for PCM determination. Inset: enlarged view of the calibration curve in low PCM 28


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concentrations range from 1.0 to10 μM.

Table 1. Comparison of the analytical performance of our PAVE-CNTs based sensor with some previously reported carbon nanotubes based electrochemical sensors for PCM sensing. Electrode materials

Linear range (μM)

Detection limit (μM)

Reference

acid functionalized MWCNT

3～300

0.6

49

PDDA/PSS-MWCNTs

25～400

0.5

50

N-DHPB/MWCNT

15～270

10.0

51

AuNP-PGA-SWCNT

8.3～145.6

1.18

52

luteolin functionalized MWCNT

0.9～80

0.78

53

poly(methylene green)/MWCNT

25～200

4.3

54

poly(thionine)/MWCNT

25～250

6.0

55

poly(methacrylic acid–hemin)/MWCNT

10～90

1.1

56

PAVE-CNTs NCs

1.0～1000

0.20

This work

Anti-interference ability of PCM sensor

One important virtue of an electrochemical sensor is its anti-interference ability, which allows discrimination of interfering species in complex environment. In biological samples, PCM usually coexists with ascorbic acid (AA) and dopamine (DA).57 In this study, interferents including AA and DA were used as typical representatives to assess the anti-interference ability of the PCM sensor, and the corresponding DPV responses were recorded as depicted in Figure 8a. Figure 8a shows that PCM exhibited a well-defined anodic peak with good peak-to-peak separations from AA and DA. In addition, anodic peak of DA was completely 29



excluded in the work potential window (0 ~ 0.8 V) of the proposed sensor in this study.58 Therefore, the PAVE-CNTs NCs based sensor showed high anti-interference ability for PCM detection with the coexistence of AA and UA, which can be potentially applied in real samples.

Reproducibility and Stability of PCM sensor

Reproducibility, a key factor for a successful sensor, was evaluated by measuring the current signals of six independent PCM sensors. All these sensors were prepared and measured under identical conditions. Figure 8b shows that the relative standard deviation (RSD) was 2.70% for the six sensors at PCM concentration of 100 μM, demonstrating excellent reproducibility of the PAVE-CNTs NCs based PCM sensor. The stability was investigated with the PCM sensor stored in a sealed condition at ambient temperature over one month. By recording the current responses of the same sensor towards the same PCM concentration every 5 days，no evident fading of peak current was found and the peak current retained 93.4% of initial value after storing for 30 days. The results indicated that the PCM sensor has good storage stability, which can be attributed to the enhanced structure stability resulting from the photo-crosslinked PAVE-CNTs NCs and the composite sensing films.

30


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Figure 8. (a) DPV curve recorded on the PAVE-CNTs NCs based sensor after incubated with the mixture of PCM, dopamine (DA) and ascorbic acid (AA) in 0.1 M PBS (pH 7.0), the concentrations of all analytes are 100 μM. (b)Peaks currents of six independent PCM sensors towards 100 μM PCM in 0.1 M PBS (pH 7.0).

Preliminary analysis of real samples

The practical performance of our PCM sensor was firstly investigated by determining PCM content in commercial PCM tablets (300 mg/tablet, China, SFDA approval number H12020266) supplied by local hospital. The PCM tablet was dissolved in PBS (pH 7.0) by sonication, and diluted to a working concentration range. The content of paracetamol detected by our sensor was calculated to be 294.9 ± 2.7 mg in one tablet, which is consistent with the PCM content stated by the manufacturer. To further validate the applicability in real body fluid, a spike recovery test59,60 was conducted by adding quantified PCM to urine samples of healthy individuals. As listed in Table S1, the recoveries of PCM varies from 97.57% to 103.58% with 31



acceptable RSD values in the range of 3.32% to 5.14%. All these results well demonstrated that the PAVE-CNTs NCs based PCM sensor has great potential for practically determinating PCM contents in pharmaceutical preparations and diagnostic applications

CONCLUSIONS In conclusion, a novel kind of polymeric carbon nanocomposites with “beads-on-a-string” nanostructure was developed and successfully applied as electrode material for fabrication of electrochemical sensor. An amphiphilic photo-crosslinkable copolymer was prepared, which then co-assembled with MWCNTs driven by “π-π” interaction and hydrophobic interaction, generating the “beads-on-a-string” structured NCs with polymeric NPs as beads decorating along the linear MWCNT as a string. The photo-crosslinked PAVE-CNTs NCs exhibited high long-term dispersion stability in aqueous solution. For sensor application, the NCs were used for electrode modification followed by further photo-crosslinking without the need of any other toxic chemical cross-linking agents, leading to a robust and interlaced three-dimensional composite film on the electrode surface. As a proof of concept, the PAVE-CNTs NCs based sensor was used to determine PCM in PBS as well as in real samples. Owing to the large surface area of the “beads-on-a-string” structured” PAVE-CNTs NCs and high electrical conductivity of the MWCNTs throughout the complex sensing film, the PCM sensor showed a wide linear detection 32


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range with relative low detection limit and high selectivity in operation potential window (0 ~ 0.8 V). In addition, the PCM sensor was practically applied in PCM analysis in commercial PCM tablets and human urine samples, manifesting promising and practical application potential in medical diagnostics. The PAVE-CNTs preparation and the sensor fabrication were all implemented at mild working condition, for which the whole procedure is easy to operate, environmental-friendly, and energy-saving. In view of that more functionalities can be incorporated in the “beads-on-a-string” structured nanocomposite by virtue of the nanosized polymeric beads, this work can be generalized to apply in many other sensors and electronic devices. Ongoing work in our laboratory is aimed to further the potential of this strategy and to make it broadly available in biosensors (molecularly imprinted sensors and enzymatic biosensors) and flexible electronic devices.

ASSOCIATED CONTENT Supporting Information

Reagents and materials, 1H NMR spectra of VMc monomer and PAVE copolymer, detail characterization and measurement, DLS plot and TEM image of PAVE NPs, stability study by visual observation, TEM image of PAVE NPs/CNTs mixture, morphologies of PAVE-CNTs NCs at higher pH values, Effectiveness of photo-crosslinking on PAVE-CNTs NCs by TEM images, and comparison of the 33



analytical performance

AUTHOR INFORMATION Corresponding Author

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

ORCID

Sheng Xu: 0000-0001-9492-0585 Jing Luo: 0000-0001-9728-537X Xiaoya Liu: 0000-0003-2868-7601

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We appreciate the financial support from the National Natural Science Foundation of China (NSFC 51103064, 51803079), and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX17_1428). In addition, Sheng Xu would like to express his special thanks to Hongyang Pan and Keyu Lu in State Key Laboratory of Food Science and Technology, Jiangnan University, Haiyan Zhu and Fangping Xu in School of Chemical and Material Engineering, Jiangnan University, 34


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for their kind helps in technical characterizations.

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“Beads-on-a-string” structured and water-compatible polymer/carbon nanotubes nanocomposite as effective electrochemical sensing material with high sensitivity.

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Carbon Nanotubes

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