Interaction of Zwitterionic and Ionic Monomers with Graphene Surfaces

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Interaction of Zwitterionic and Ionic Monomers with Graphene Surfaces Suguna Perumal, Atchudan Raji, and In Woo Cheong Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00975 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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Langmuir

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Interaction of Zwitterionic and Ionic Monomers with

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Graphene Surfaces

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Suguna Perumala, Atchudan Rajic, and In Woo Cheonga,b*

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a

Department of Applied Chemistry, School of Engineering, Kyungpook National University,

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Buk-gu, Daehak-ro 80, Daegu 41566, South Korea b

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Department of Nano-Science and Technology, Graduate School, Kyungpook National University, Buk-gu, Daehak-ro 80, Daegu 41566, South Korea

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School of Chemical Engineering, Yeungnam University, Gyeongsan 38541, Republic of

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Korea

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Submitted to

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Langmuir

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*Corresponding Author:

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In Woo Cheong, Ph.D.

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Professor, Department of Applied Chemistry, School of Engineering, Department of Nano-

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Science and Technology, Graduate School, Kyungpook National University E-mail:

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[email protected]; Tel.: +82 53 950 7590; Fax: +82 53 950 6594

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ACS Paragon Plus Environment

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Abstract

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Measurement of the interaction force between two materials provides important

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information on various properties, such as adsorption, binding or compatibility for coatings,

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adhesion, and composites. The interaction forces of zwitterionic and ionic monomers with

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graphite platelets (G) and reduced graphene oxide (rGO) surfaces were systematically

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investigated using atomic force microscopy (AFM) in air and water. The monomers

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examined were 2-(methacryloyloxy)ethyl 2-(trimethylammonio)ethyl phosphate (MPC), [2-

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(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium

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(acryloyloxy)ethyl]trimethylammonium chloride (ATC), and 2-methyl-2-propene-1-sulfonic

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acid sodium (MSS). The AFM studies revealed that MSS and SBE monomers with sulfonate

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units have stronger interaction forces with G surface in air, and MPC and ATC monomers

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with quaternary ammonium units have higher interaction forces in water. In the case of rGO

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surface, the monomers with quaternary ammonium units showed stronger interactions

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regardless of the medium. These interactions could be rationalized by the interaction

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mechanism between the monomers with graphene surfaces, such as cation–π for MPC and

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ATC, anion–π for MSS and SBE, respectively. Overall, cation-π interactions were effective in

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water, while anion-π interactions are effective in the air with G surface. The adhesion values

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of MPC, SBE, ATC, and MSS on rGO were lower than the values measured on G surface.

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Among the monomers, MPC showed the highest dispersibility for aqueous graphene

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dispersions. Further, the adsorption of MPC on G and rGO surfaces was verified by high-

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resolution transmission electron microscopy and X-ray diffraction patterns.

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Keywords: Zwitterions; Atomic force microscopy; Cation–π interactions; Anion–π

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interactions; Graphene Dispersions

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hydroxide

(SBE),

[2-

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1. Introduction

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Graphene is a 2D allotrope of carbon with a hexagonal lattice structure.1-3 It has

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incredible properties such as high conductivity, high mechanical strength,4,5 and a high

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surface area.6 These extraordinary properties make graphene and graphene-related materials

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an appropriate addition to a wide range of applications such as solar cells, supercapacitors,

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conductive thin films, inkjet printing, and polymer composites.7-13 Almost all of these

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applications require single- to few-layered graphene. Many researchers have proposed that

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single- to few-layered graphene can be obtained using a liquid-phase exfoliation method.14

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This method usually leads to a non-covalent functionalized graphene surface in the presence

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of surfactants or dispersants.15 This approach has resolved two major problems: scalable

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graphene synthesis and graphene dispersibility in common solvents.16–19 However,

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considerable challenges remain, such as high graphene concentration, stable dispersions for

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extended periods, and dispersion in an appropriate solvent for the specific application. Our

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recent article reports the direct measurement of the adhesion force between vinyl monomers

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and graphene surfaces.20 The adhesion forces were measured for different types of vinyl

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monomers on three different graphene surfaces, graphite, chemical vapor deposition graphene,

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and reduced graphene oxide. Among the monomers, 4-vinyl pyridine (VP) showed the

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highest adhesion force value with all graphene surfaces and the block copolymer prepared

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from VP presented stable graphene dispersion in ethanol13,21 and methanol22 as compared

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with styrene and pyrene methacrylate as a graphene-philic monomer. Thus, it is very

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important to select a monomer having high affinity for graphene in block copolymer

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synthesis when the block copolymer is considered for a graphene dispersant. Indeed,

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knowledge of the specific interactions would make the design or selection of suitable

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dispersants or surfactants much easier. One of the bottlenecks in the modification of graphene 3


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by non-covalent interactions is an understanding the interaction forces between physically

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anchoring molecules of dispersants or surfactants onto the graphene surface, which contrasts

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those involving lyophilic moieties that which are well known. Attractive interaction forces

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can be determined from an analysis of the force-distance (F-d) curve obtained through atomic

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force microscopy (AFM).23-26 There are only a few reports about adhesion force studies for

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graphene.23–25 To the best of our knowledge, only one article reports the direct measurement

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of the adhesion force between vinyl monomers and graphene surfaces.20 There are many

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reports available regarding the functionalization of graphene surface with non-ionic polymers

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or surfactants by non-covalent functionalization method.3,13-22 However, the interactions

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involved between ionic or zwitterion surfactants and graphene surface are not yet well

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established, remain unclear, indeed further clarification are required. In this aspect, we report

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an adhesion force study between graphene surfaces and zwitterionic, quaternary ammonium,

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and sulfonate monomers. Our idea is to study the interactions between quaternary ammonium,

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sulfonate, and phosphate units with graphene surfaces. It is very difficult to determine the

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specific interaction such as electrostatic interaction or hydrogen bonding etc., between

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quaternary ammonium, sulfonate, and phosphate units with graphene surfaces by AFM study.

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However, overall interaction strength of the abovementioned functional units with graphene

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surfaces can be obtained which can suggest the best candidates to prepare stable graphene

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dispersion in aqueous phase.

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In recent years, functionalization of graphene surfaces using ionic surfactants, also

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biological and biomedical applications of graphene such as drug delivery, bio-imaging, and

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biosensing, have attracted considerable attention.27,28 For example, graphene oxide complexes

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with 2-(methacryloyloxy)ethyl phosphorylcholine (GeneO-MPC), and modified polyethylene

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(PE/GeneO-MPC) showed good antibacterial and anticoagulation properties.29,30 Since 4


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graphene itself does not disperse in water due to its strong hydrophobicity, non-covalent

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functionalized graphene has been considered to improve its dispersibility. For instance,

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reduced graphene oxide (rGO) sheets dispersed in water using anionic, non-ionic, and

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zwitterionic surfactants.29-32 Although zwitterionic surfactants or polymers have been used to

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prepare rGO composites, graphene composites using graphite platelets (G) with zwitterionic

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or ionic monomers have yet to be reported.

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To this end, this article reports the results from an adhesion force study between

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MPC, [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (SBE), [2-

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(acryloyloxy)ethyl]trimethylammonium chloride (ATC), and 2-methyl-2-propene-1-sulfonic

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acid sodium (MSS) monomers and graphene surfaces, including G and rGO. The interactions

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were scrutinized using AFM and were further correlated with online turbidity measurements

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of graphene dispersion.

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2. Materials and Methods

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2.1 Materials

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MPC was purchased from TCI (96%, Tokyo, Japan). SBE (97%), ATC solution 80

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wt% in H2O, and MSS (98%) were purchased from Sigma-Aldrich (MA, USA) and used

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without further purification. Hydrofluoric acid (HF, 50%) was purchased from Duksan (Seoul,

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Korea). G (xGnP® Graphene Nanoplatelets, XG Sciences, USA) was kindly donated from

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RIST (Pohang, Korea). rGO (rGO-V20-100, 4–8% oxygen) was also donated from Standard

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Graphene (Korea). NSC36 and PPP-NCHR were used as AFM contact mode non-contact

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mode cantilevers respectively, from Park Systems (Suwon, Korea). Double distilled water

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was used for all experiments.

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2.2 Characterization

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XPS analyses for G and rGO flakes were performed on a K-Alpha XPS System

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(Thermo Fisher Scientific, U.K.) with a monochromated Al Kα source (1486.6 eV). Narrow

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scans of C1s and O1s energy levels were carried out for G and rGO samples. CasaXPS

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instrument software was used for the deconvolution of the high-resolution XPS spectra.

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Pelletized G and rGO flakes were used to measure the F-d curves by an AFM (XE7, Park

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Systems, Korea). The surface morphology of the graphene samples was characterized by the

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AFM equipped with an XEI image processor in non-contact mode at ambient atmosphere

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using a PPP-NCHR cantilever. The F-d curves were obtained using contact mode NSC36

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cantilever. All images were obtained at a scan rate of 1 Hz with a scan size of 9.09 µm × 9.09

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µm. The spring constant of the modified cantilevers was 0.9 N/m, measured by Park Systems.

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All measurements were done at room temperature (~26 °C and humidity (23-27%). The

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adhesion force between the modified cantilever and graphene surfaces was calculated from

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the F-d curves using software XEI, an image processing program for scanning probe

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microscope data developed by Park Systems. F-d curve measurements were done as in our

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earlier report.20 Morphology of graphene dispersion was also investigated using HR-TEM

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(200 kV, Titan G2 ChemiSTEM Cs probe, FEI Company, USA). The samples were prepared

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by spraying the dispersion solutions in water onto 200-mesh carbon-coated copper grids

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(CF200-Cu, Electron Microscopy Sciences, USA), followed by convective drying under air.

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FE-SEM (15 kV, Hitachi SU8220, Tokyo, Japan) measurement was performed to observe the

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surface of cantilevers before and after the modifications. XRD analysis was performed using

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a Rigaku D/MAX-2500 diffractometer (Tokyo, Japan) with CuKα as the radiation source (λ =

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1.5404 Å). The XRD patterns for graphene and the dispersions were recorded at a scattering

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angle 2θ in the range of 5–50°. 6


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2.3 Preparation of dispersions for the online turbidity measurements

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The weight ratios of graphene to the monomers were optimized to observe the

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destabilization kinetics clearly by online-turbidity analyses: 10 mg of monomer and 5 mg of

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G in 20 mL of water were sonicated for 6 h in a bath sonicator (40 kHz, SH-120, 190 W,

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Saehan, Korea). Here, the temperature was maintained at 23 °C using a water circulator to

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avoid temperature rise during ultrasonication. Time-evolution backscattering and

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transmission were measured for 18 h using a Turbiscan (LAB, Formulaction L’Union,

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France). The Turbiscan stability index (TSI) of the vial versus time was calculated and

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compared to investigate the destabilization kinetics of the dispersions.20,21 In the case of

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dispersions using rGO, 1 mg of rGO shows better destabilization kinetics than G; thus, 1 mg

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of rGO and 2 mg of monomers were sonicated and the online turbidity measurements were

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performed as described above for G. The dispersions at < 1 mg of rGO and < 5 mg of G could

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not be tested because the beam could not transmit through the sample because of high

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concentration of the solution.

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2.4 TSI value calculations

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From the transmission data obtained using Turbiscan LAB, Formulaction Co.,

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L’Union, France, Turbiscan Stability Index (TSI) value in the predefined zone (middle or top)

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of the vial bottle versus aging time was calculated using the following equation:20,21

TSI =

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∑

∑ scan (h) − scan (h) i −1

i

h

H

i

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where h is the height of the measured point, i is the backscattering intensity at a measured

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point, i – 1 is a comparison of the ith intensity with the previous intensity, and H is the total

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selected height. 7


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3. Results and Discussion

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The molecular structures of all monomers used for the investigation are shown in

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Figure 1. Four different (MPC, SBE, ATC, and MSS) monomers were used and the

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interactions of these monomers with G and rGO surfaces in air and water were investigated

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thoroughly.

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Figure 1. Chemical structures of the zwitterionic (MPC, SBE), quaternary ammonium (ATC),

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and sulfonate (MSS) monomers used in this work.

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3.1 Mechanisms involved in the modification of AFM cantilevers

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In order to study the interactions between two surfaces, AFM tips can be modified by

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various methods, such as AFM tip coating, where the AFM surface a thin chromium adhesive

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layer is deposited, followed by vapor deposition of the gold layer will be done.33 Several

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methods are available to functionalize the AFM tip chemically.33,34 AFM tip surface can be

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modified chemically with macromolecules, by electrografting process, silanization reaction 8


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process (via Si-O-Si) grafting of aliphatic chains onto the silicon surface, silanization with 3-

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aminopropyl triethoxysilane, and by hydrosilylation process via Si-C bonds.33 Here, we

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adopted a wet chemical approach to Si-C bond formation by hydrosilylation.35-37 This method

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is easy, comfortable to handle AFM tips, and monomers can be directly inserted on AFM tip

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surface, does not need any stepwise coating for adhesion, e.g., Au/Ti or Au/Cr. Densely

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packed alkyl monolayers are prepared on silicon surface from alkenes and hydrogen-

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terminated silicon using peroxide as radical initiator.37 Monolayers of hydrocarbons are

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prepared on the porous silicon surface by refluxing hydrogen-terminated silicon surface and

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alkene in high boiling-point solvents, such as toluene, benzene, xylene, and so on.36

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Formation of hydrocarbon monolayers on silicon surface was performed by UV illumination

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of hydrogen-terminated silicon surface with an alkene.35 In every case, hydrogen-terminated

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silicon surface was obtained by treating either with aqueous HF or NH4F solutions.35-37

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In the present work, native oxide on the NSC36 cantilever tip was removed by

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immersing the cantilever in 2% aq. solution of hydrofluoric acid (HF) for 1 min, and which

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could form a hydrogen-terminated silicon surface (Step 1 in Figure 2). The hydrogen-

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terminated surfaces undergo hydrosilylation with 1 M aq. solution of unsaturated monomers,

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MPC, SBE, ATC, and MSS on the surface of the cantilever (Step 2 in Figure 2). The

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hydrosilylation reaction was initiated with heat ( ATC > SBE ≈ MSS. However, the adhesion

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force values for SBE, ATC, and MSS are almost within tolerance. The XPS results confirm

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the presence of many functional groups on the rGO surface. Thus, interaction units of MPC,

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ATC, MSS, and SBE might interfere with the oxygen groups on the rGO surface leading to

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low adhesion values. In addition, as explained in Section 3.2, MSS monomers can dissociate

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in water and show low adhesion force values. However, the results in water reveal that MPC

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and ATC with quaternary ammonium units show higher adhesion forces with rGO surface

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than SBE and MSS with sulfonate units.

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Figure 7. Histogram of the adhesion forces between MPC, SBE, ATC, and MSS-modified

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AFM tips and rGO surface in (a) air and (b) water. 22


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3.7 Interactions of MPC, SBE, ATC, and MSS with rGO surface

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The interactions of rGO with MPC, SBE, ATC, and MSS in air and water are

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illustrated in Figure 8 and can be explained as follows: The overall adhesion values of MPC,

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SBE, ATC, and MSS on rGO are lower than the values measured on G surface. As described

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in Section 3.6, the functional groups on the rGO surface affect the interaction of phosphate

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and sulfonate units of MPC, SBE, and MSS. Accordingly, MPC have strong anion and cation-

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π interactions, and MPC can have hydrogen bonds with functional groups such as carboxylic

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acid and hydroxyl.58,59

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Figure 8. Illustration of the interactions of MPC, ATC, SBE, and MSS with rGO surface in

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air and water.

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However, the adhesion values of MPC on rGO and G surfaces are similar. This

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means that anion-π and cation-π interactions are effective when compared to hydrogen

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bonding. Lin. et. al.,60 suggest that all possible binding between surfactants adsorbed on the

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graphene surface should be considered. Thus in the present work, hydrogen bonding between 23


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functional groups of MPC, SBE, MSS, and ATC with water molecules and with rGO

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functional groups are taken in account to discuss the results. ATC showed the unexpected and

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unpredicted result of high adhesion value than compared to MPC, SBE, and MSS. In case of

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SBE, strong anion-π and cation-π interactions exist and in addition, sulfonate group might

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involve in hydrogen bonding with rGO functional groups and thus results in low adhesion

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value than MPC and ATC. Along with anion-π interaction, MSS would have hydrogen bonds

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with rGO functional groups lead to a small decrease in adhesion value than SBE. Reported

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articles suggest the possibility of an interaction of a head group of surfactants with functional

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groups on rGO surface through electrostatic interactions.15,49 Reported article and the

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previous work20 confirm that the functional groups on rGO are involved in the interactions

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between MPC, ATC, SBE, and MSS and rGO surface. In addition, in the present work, the

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water molecules also might involve in the interaction with anion-π, cation-π interactions, and

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hydrogen bonds. Many of these aspects affect the interactions between MPC, ATC, SBE, and

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MSS and rGO, thus the adhesion values on rGO in water are very low and are in error range.

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3.8 rGO dispersions stabilities with MPC, SBE, ATC, and MSS in water

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As with the studies using G, the adhesion forces between the monomers and rGO

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surface were compared in terms of dispersion stability. Figure 9(a) shows photographs of the

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rGO dispersions stabilized with and without monomers immediately after sonication and after

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18 h. The dispersion stability was further confirmed by online turbidity measurements. Figure

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9(b) presents the TSI values of the dispersions as a function of time and Figure S20-S24

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describe the destabilization phenomena of the dispersions by examining the extent of light

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transmission versus sample height for 18 h after sonication.

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As seen in Figure 9(a), all dispersions are opaque immediately after sonication, 24


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indicating that the dispersions are stable. rGO has settled down rapidly in the dispersions rGO,

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rGO-SBE, rGO-ATC, and rGO-MSS monomers with time and after 18 h the TSI values are

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57.2, 23.1, 16.5, and 45.3, respectively. rGO shows a steep increase in the TSI value,

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indicating that rapid sedimentation occurs over time. rGO-SBE, rGO-ATC, and rGO-MSS

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dispersions show slow and steady aggregation and sedimentation of rGO over the time with

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high TSI values. This is because of the weak interactions of MSS, SBE, and ATC with rGO in

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water. However, the rGO-MPC dispersion is darker and show low TSI value indicating stable

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dispersion. Figure 9(a) and (b) show the order of dispersions from darker to colorless or less

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dark and the TSI value in decreasing tendency as rGO-MPC > rGO-ATC > rGO-SBE > rGO-

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MSS > rGO. This trend shows good agreement with the adhesion force study, MPC > ATC >

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SBE > MSS. Table 1 shows the comparison of adhesion and TSI values from AFM and online

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turbidity measurements, respectively. Adhesion values in water does not show distinct

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changes with MPC, SBE, ATC, and MSS; however, only rGO-MPC dispersion shows low

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TSI value of 0.21 indicating a stable dispersion from online turbidity measurement. All other

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dispersions, rGO, rGO-SBE, rGO-MSS, and rGO-ATC show high TSI value indicate unstable

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dispersions.

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and XRD.

Thus, only the rGO-MPC dispersion was further characterized using HR-TEM

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Figure 9. (a) Photographs and (b) Time-evolution TSI curves of rGO dispersions with and

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without monomers for 18 h.

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Table 1. Comparison of adhesion and TSI values of MPC, SBE, ATC, and MSS on G and

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rGO surfaces in air and water Graphene Surface Monomer MPC SBE ATC MSS

G Adhesion Value (nN) Air Water 10.00 7.38 35.30 3.47 3.00 3.60 30.22 1.52

TSI value (-) 5.94 26.16 38.87 52.70

rGO Adhesion Value (nN) Air Water 7.00 0.68 5.38 0.46 8.53 0.55 3.15 0.45

TSI Value (-) 0.21 19.85 13.4 42.24

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3.9 HR-TEM and XRD analyses for stable rGO-MPC dispersion

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The HR-TEM image at low-magnification in Figure 10(a) shows graphene with a

11

curled morphology, consisting of a thin wrinkled sheet structure similar to that observed in

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the AFM topographic image (Figure S4). The higher magnification image in Figure 10(b)

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also shows a wrinkled morphology. As seen in Figure 10(c–f), the corresponding elemental 26


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mappings of C, N, O, and P from the selected area indicate a uniform distribution of MPC on

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the rGO surface. The obtained rGO-MPC and rGO were characterized by powder XRD as

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shown in Figure 10(g).

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The XRD patterns of rGO and rGO-MPC exhibit broad diffraction peaks at 2θ =

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25.48° and 22.34°, respectively. These broad peaks suggest that rGO and rGO-MPC are

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poorly ordered stacking direction of the graphene sheets,61 reveals rGO and rGO-MPC are

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largely comprised of few-layered graphene sheets. The calculated interlayer distance using

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Bragg’s equation presents d-spacings of 0.360 and 0.405 nm with FWHM values as 23.16

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and 23.15 for rGO and rGO-MPC, respectively. An increase in the interlayer distance of rGO-

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MPC and the shift in the XRD pattern prove that MPC facilitates the complete or partial

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exfoliation of rGO. It has been reported that MPC was used along with PE/perylene to

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prepare graphene nanocomposites.29,30 In the present work, stable graphene dispersion was

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obtained with the MPC monomer in the absence of additional surfactants.

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Figure 10. HR-TEM images of graphene dispersions (rGO-MPC), (a) low resolution and (b)

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high-resolution and corresponding elemental mapping (c) C, (d) O, (e) N, and (f) P. (g) XRD

17

patterns of rGO and rGO-MPC.

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4. Conclusions

2

In summary, the adhesion force between quaternary ammonium unit, phosphate, and

3

sulfonate units of SBE, MSS, MPC, and ATC with G and rGO surfaces were measured using

4

F-d analyses. The interactions were postulated as cation-π and anion-π interactions are

5

involved between quaternary ammonium units and sulfonate units, respectively, with G and

6

rGO surfaces. The results of AFM studies were directly compared with stabilities of graphene

7

dispersions that prepared using monomers, MPC, SBE, ATC, and MSS with G and rGO by

8

online turbidity measurements. In AFM study, MPC and ATC showed high adhesion values

9

than SBE and MSS on G surface reveals cation-π interaction is effective in water. The

10

adhesion force changes of MPC, ATC, SBE, and MSS observed are not significant on rGO

11

water due to the involvement of functional groups on rGO surface and the interaction of

12

phosphate or sulfonate units with water molecules. However, MPC shows relatively high

13

adhesion values rGO surfaces in water. In online turbidity measurements, MPC with G and

14

rGO showed stable dispersion than other dispersions. Among the monomers, MPC only

15

showed the highest adhesion values on G or rGO in water. Thus, only G-MPC and rGO-MPC

16

were further analyzed using HR-TEM and XRD analyses. These measurements confirmed the

17

distribution of MPC on G and rGO surfaces. The interlayer distance increase confirmed from

18

XRD suggests that the MPC monomer facilitates the exfoliation of G and rGO. AFM study in

19

the air showed anion-π interaction was effective, thus SBE and MSS showed higher adhesion

20

values on G surface than MPC and ATC.

21

Thus, this work will help in choosing monomers for the preparation of polymers and

22

to prepared graphene composites not only in water such as liquid-phase exfoliation but also in

23

air through a solvent-free method such as ball-milling. Further, this work suggests that

24

quaternary ammonium, sulfonate, and phosphate units will be good candidates for 28


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1

functionalizing graphene surfaces in air and water. In addition, a graphene-stabilized MPC

2

polymer would be a promising candidate for bio-application because of its phosphocholine

3

unit.

4 5

Acknowledgments

6

This work was supported by the Ministry of Trade, Industry and Energy, Korea (Grants No.

7

10044338, 10067082, and 10070241). We thank Mr. Jong Hwan Baek (Leanontech Co.,

8

Korea) for his help and valuable suggestion regarding the Turbiscan data analysis. We are

9

also grateful to Mr. Soo Hyung Eun and Mr. Chang Young Chang from Park Systems for the

10

measurements of spring constants for the modified cantilevers.

11 12

Conflicts of Interest

13

The authors declare no conflict of interest.

14 15

Appendix A. Supplementary material

16

Supplementary data related to this article is available free of charge on the ACS Publications

17

website at DOI:

18 19

References

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Interaction of Zwitterionic and Ionic Monomers with Graphene Surfaces

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