Accepted Manuscript

Non-covalent interactions for synthesis of new graphene based composites

Xuqiang Ji, Liang Cui, Yuanhong Xu, Jingquan Liu

PII: S0266-3538(14)00380-7

DOI: http://dx.doi.org/10.1016/j.compscitech.2014.10.018

Reference: CSTE 5967

To appear in: Composites Science and Technology

Received Date: 31 July 2014

Revised Date: 17 October 2014

Accepted Date: 21 October 2014

Please cite this article as: Ji, X., Cui, L., Xu, Y., Liu, J., Non-covalent interactions for synthesis of new graphene

based composites, Composites Science and Technology (2014), doi: http://dx.doi.org/10.1016/j.compscitech.

2014.10.018

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers

we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and

review of the resulting proof before it is published in its final form. Please note that during the production process

errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

1

Non-covalent Interactions for Synthesis of New Graphene Based

Composites

Xuqiang Ji, Liang Cui, Yuanhong Xu, Jingquan Liu*

College of Chemical Science and Engineering, Laboratory of Fiber Materials and

Modern Textile, The Growing Base for State Key Laboratory, Qingdao University,

Qingdao 266071, P. R. China.

*Corresponding author: Tel: +86 053283780128; fax: +86 053283780128

Email address: jliu@qdu.edu.cn

ABSTRACT

Compared with covalent modification methods, non-covalent modifications will not

destroy graphene’s intrinsic structure, thus its excellent properties can be preserved,

therefore it has been extensively utilized to prepare composite materials. In this feature

article, we like to present our recent work on the preparation of graphene based

composite materials with polymers, small molecules and biomolecules via non-covalent

interactions and the characterization of their structures and properties. The applications

in the preparation of mechanical property-enhanced and electrical conductivity-tunable

composites, solar cells and other fields are explored. We also discussed the challenges

and future trends.

Keywords: A. Polymer-matrix composites; A. Nano composites; B. Electrical properties;

B. Mechanical properties; Non-covalent interactions.

1. Introduction

2

Graphene has proved to have remarkable properties, such as high thermal

conductivity, superior mechanical properties and excellent electronic transport

properties due to its highly conjugated 2D honeycomb structure [1]. These intrinsic

properties of graphene have attracted enormous interest to explore its potential

applications in a myriad of devices [2], including future generations of high speed and

radio frequency logic devices, solar cells, ultra-thin carbon films, electronic circuits,

sensors, transparent and flexible electrodes for displays and thermally and electrically

conducting reinforced nanocomposites [3-5].

As a nanofiller, graphene is preferred over other conventional nanofillers, like

graphite, carbon black and carbon nanotube owing to its high surface area, aspect ratio,

tensile strength, thermal and electrical conductivity, flexibility and transparency [6, 7].

Generally speaking, graphene based composites can be realized through either covalent

bondings or non-covalent interactions. As shown in Table 1 the non-covalent methods

like π–π stacking interactions are mostly frequently used to prepare graphene based

composites since graphene’s natural structure is unaffected. Furthermore, non-covalent

modifications are easy to achieve over the entire graphene surface and reversible in

some cases [8], therefore, they are especially preferred when graphene’s superb intrinsic

properties are required.

Inspired by the advantages of non-covalent modification methods, we have made

great effort on the preparation of graphene based composite materials and exploration of

their applications. In this feature article, the preparation, structure and properties of

3

varied graphene based composite materials, including pH and thermal responsive

graphene/polymer composites; conductivity-tunable graphene composites with

micron-level thickness, graphene/TiO2 composites and graphene/polymer composite

directly prepared from graphite via non-covalent interactions have been presented. Their

applications in the preparation of mechanical property-enhanced and electrical

conductivity-tunable composites, solar cells and other fields have also been discussed.

Some researches closely related to this topic are also briefly discussed.

Preparation, structure and properties of graphene based composite materials via

non-covalent interactions

2.1. Thermal and pH responsive graphene/polymer composites

Thermal-responsive polymer materials are very promising for varied applications in

the field of nanoscience, nanotechnology, nanomedicine and biomedical science [15,

16], due to their unique feature to respond reversibly or irreversibly to small changes in

the environmental temperature [17]. To improve thermal-responsive polymer’s

responsiveness for more applications including actuators, switches, robots, sensors,

drug/gene deliveries, etc [18-20], graphene has been adopted into various

thermal-responsive polymers to prepare graphene/polymer composites via non-covalent

methods [21]. Our early work was focused on the preparation of thermosensitive

graphene/polymer composites by attaching thermo-responsive PNIPAAm onto the basal

planes of graphene sheets via non-covalent π–π stacking interaction. Pyrene-terminated

PNIPAAm was synthesized using reversible addition fragmentation chain transfer

4

(RAFT) polymerization via a pyrene-functional RAFT agent (Fig. 1a). Aqueous

solutions of the graphene/polymer composites were stable and thermosensitive. The

lower critical solution temperature (LCST) of pyrene-terminated PNIPAAm was

measured to be 33 °C. Very interestingly, when the pyrene-functional polymer was

attached to graphene, the resultant composites were also thermosensitive in aqueous

solutions exhibiting a reversible suspension behavior at 24 °C (Fig. 1b) [13]. As an

extension, thermo-responsive polymers of oligoethylene glycol acrylate (OEG-A) and

diethylene glycol ethyl ether acrylate (DEG-A) with tunable LCSTs depending on the

feed ratio of the monomers were prepared using the same RAFT mechanism (Fig. 1a).

The LCSTs of the copolymer of OEG-A and DEG-A were tuned from 31 to 82 °C.

When these thermo-sensitive copolymers were attached onto graphene basal planes, the

as-prepared graphene/polymer composites exhibited reduced LCST from 22 to 72 °C

(Fig. 1c). These results revealed that the polymeric properties could be imparted to

graphene composites via surface modification [22].

Similar to the thermal-responsive graphene/polymer composite, RAFT

polymerization and π–π stacking interaction are also used to prepare pH-sensitive

graphene/polymer composites. In our recent work, pH sensitive graphene-polymer

composites were prepared by the modification of graphene basal planes with

pyrene-terminated poly(2-N,N′-(dimethyl amino ethyl acrylate) (PDMAEA) and

poly(acrylic acid) (PAA) via π-π stacking interaction (Fig. 1a). The pyrene-terminal

PDMAEA and PAA were synthesized using RAFT polymerization with a

5

pyrene-functional RAFT agent. The as-prepared graphene/polymer composites were

found to have phase transfer behavior between aqueous and organic media at different

pH values (Fig. 1d). More importantly, self-assembly of the two oppositely charged

graphene-polymer composites afforded layer-by-layer structures as evidenced by SEM

and quartz crystal microbalance (QCM) measurements [14].

2.2. Conductivity-tunable graphene composites with micron-level thickness

Combining small molecules or macromolecules with graphene has been used to

prepare graphene based nanocomposites, allowing materials conductivity to meet the

requirements of various application [23]. In our recent work, three different mono- or

bi-functional small molecules were synthesized ad used to modify graphene nanosheets.

The monoaryl diazonium salts (MDS) (Fig. 2a) and bifunctional aryl diazonium salts

(BDS) (Fig. 2b) were employed to destruct the in-sheet conjugation of graphene via

covalent bonding to tune the electrical conductivity (EC) of graphene papers (Fig. 2d,

2e). A bipyrene molecular wire (BPMW) was the third molecule that was used to

modify graphene via non-covalent interactions (Fig. 2c). As expected a significant boost

in EC was observed for graphene paper modified with BPMW in comparison to BDS

and even more so in comparison with MDS (Fig. 2e) [24]. In comparison, the electrical

conductivity of our graphene papers can be tuned from 0.1 to 10000 S m−1 , which is

much broader than those obtained by Su et al. (from 190 to1390 S m−1) [25].

Macromolecules are also used to prepare conductivity tunable grahene/polymer

composites. Recently we adopted a facile “graft from” method to prepare

6

molecular-level dispersed graphene-polymer composites with tunable electrical

conductivity (Fig. 3a). The positive-charged poly(dimethyl aminoethyl acrylate),

negative-charged PAA and neutral polystyrene were prepared by ‘‘graft from’’

methodology using RAFT polymerization. Graphene composites of different polymers

with the same polymerization degree exhibited similar conductivity. However, when the

polymer chain was designed as random copolymer, the conductivity was significantly

decreased. It was also observed that the longer the grafted polymer chains the lower the

conductivity (Fig. 3b) [26]. Conductivity of graphene–polystyrene (PS) composite

papers is increasing with the higher graphene contents as expected (Fig. 3c).

2.3. Nitrogen-doped graphene/TiO2 composites with higher electrocatalytic activity

TiO2 semiconductor has been considered as one of the best photocatalytic materials

because of its long-term thermodynamic stability, strong oxidizing power, and relative

non-toxicity. However, the photo produced electrons and holes in TiO2 may experience

a rapid recombination, which significantly diminishes the efficiency of the

photocatalytic reaction [27]. Graphene/TiO2 composites demonstrated lower

recombination rates, increased efficiency in electron transport, and enhanced light

scattering [20]. This has spurred increasing interest to synthesize the graphene/TiO2

nanocomposites as photocatalysts for degradation of pollutants (dyes, bacteria, and

volatile organic pollutant) as well as water splitting to H2 and some other aspects [28,

29].

In our recent work, the preparation of nitrogen-doped TiO2/graphene nanohybrids and

7

their application as counter electrode for dye-sensitized solar cell (DSSC) are presented.

These nanohybrids are prepared by self-assembly of pyrene modified H2Ti3O7

nanosheets and graphene in aqueous medium via π−π stacking interactions, followed by

thermal calcination at different temperatures in ammonia atmosphere to afford

nitrogen-doped TiO2/graphene nanohybrids (Fig. 4a). H2Ti3O7 nanosheets were

synthesized from TiOSO4·xH2O by a hydrothermal reaction at 150 °C for 48 h. (Fig. 4b).

Moreover, the performances of the as-prepared nanohybrids as counter electrode

materials for DSSC indicated that the nanohybrids prepared at higher nitridation

temperature would lead to higher short-circuit current density than those prepared at

lower nitridation temperature, indicating that it can be utilized as a low-cost alternative

to Pt for DSSCs and other applications [30].

2.4. Amphiphilic graphene/polymer composites directly prepared from graphite

exfoliation

Graphene/polymer composites are usually prepared from graphene and polymer via

multi-step processes. Graphene precursor used for composites is usually prepared from

the reduction of graphene oxide (GO), its size is usually small and a lot of defects

retained, therefore, graphene’s intrinsic properties are compromised. Furthermore, high

homogeneity is difficult to achieve since the graphene sheets readily aggregate during

the processing due to their hydrophobicity and strong π-π stacking interactions in

between. Therefore, it is imposing to explore more efficient method to prepare high

quality graphene/polymer composites.

8

Recently, we designed amphiphilic block copolymers with multi-pyrene side groups

to replace surfactant or amphiphiles to exfoliate graphene directly from graphite in both

aqueous and organic media with the aid of sonication. The pyrene-functionalised

amphiphilic block copolymer, poly(pyrenemethyl acrylate)-b-poly[(polyethylene glycol)

acrylate] (polyPA-b-polyPEG-A), was prepared using RAFT polymerization of a

pyrene-functionalised monomer to afford a homopolymer (polyPA), followed by

copolymerization with PEG-A using polyPA as the macroRAFT agent (Fig. 5a).

Obviously, it was a simple and efficient means that applied amphiphilic polymer as a

matrix to directly exfoliate graphene from graphite to prepare graphene/polymer

composites in just one step. The as-prepared graphene/polymer composite films

exhibited increased tensile strength and tunable conductivity. These composites can be

used to produce graphene sheets with large size and enhanced conductivity via just the

simple thermal annealing under nitrogen atmosphere (Fig. 5b and 5c) [31]. The

influence of the relative amount of copolymer to graphite on graphene exfoliation

efficiency was also investigated. The yield was found to be 78% at the copolymer to

graphite ratio of 40, which is higher than 50% as reported by Geng et al. [32].

2.5. The related applications

Besides the above work on graphene based composites, we have also utilized

non-covalently modified graphene for varied applications in adsorption, chemo-sensing,

bio-sensing, drug delivery and so on. It is well-known that graphene exhibits superb

electron mobility. Our study on the influence of graphene on the electrical

9

communication through organic layers fabricated on graphite and gold electrodes

revealed that the immobilization of graphene could significantly enhance the electron

transfer between the redox probe and the underlying electrodes (Fig. 6I) [33]. Graphene

itself has huge surface area; therefore, it can be an excellent adsorbent for the adsorption

of varied materials. After π−π stacking modification with pyrene butyric acid, graphene

exhibited the higher adsorption capacity of up to 92.3 mg/g for Cu2+, 85.8 mg/g for Cd2+

and 124.2 mg/g for Pb2+ at pH of 5 (Fig. 6II) [34].

Graphene modified electrodes can also be used as chemo-sensors, for example,

graphene modified gold electrode via π-π stacking interaction exhibited enhanced

analytical sensitivity toward Cu2+ and Pb2+. The linear detection range for Cu2+ analysis

is 1.5–20 nM and for Pb2+ is 0.4–20 nM. (Fig. 6III) [35]. Graphene’s good conductivity

made it a good electron transfer relay for the fabrication of biosensors. We have

successfully fabricated glucose oxidase (GOx) enzyme electrodes with controlled

alternate enzyme and graphene layers via non-covalent π−π stacking interaction. Such

multi-layered enzyme electrodes with controlled nanostructure exhibited reliable

application in human serum samples analysis with high detection sensitivity, good

stability and repeatability. A broad linear detection limit of 0.2 to 40 mM was obtained

(Fig. 6IV) [36]. Graphene composites with positively charged polymers have been

successfully employed to load negatively charged DNA fragments for controlled release

(Fig. 6V) [37]. Since a lot of drug molecules contain large aromatic groups and

chargeable groups (like amino or carboxylic acid), these drugs can be loaded via

10

non-covalent π−π stacking interactions and released by static repulsion process, which

has been successfully evidenced by us (Fig. 6VI) [8].

3. Conclusion and future trends

Strictly to say, graphene is highly conjugated 2D nanocarbon material. Reduction of

GO is currently the most frequently adopted method to produce graphene in large scale.

Since very few functional groups will be remained on graphene after fully reduction,

covalent modifications of graphene have to be realized via the destruction of graphene’s

in-plane double bonds. As a result, graphene’s intrinsic properties will be significantly

compromised. In comparison, non-covalent modifications exhibited incomparable

advantages: graphene’s properties being maximally protected; simple preparation

process etc. Driven by these advantages graphene has been successfully modified by

polymers, small molecules and biomolecules etc. via non-covalent interactions to afford

varied functional composite materials with enhanced mechanical properties, tunable

electrical conductivity and potential applications for solar cells, drug delivery and

sensing.

Graphene-based polymer nanocomposites represent one of the most technologically

promising developments. However, there are still many challenges that must be

addressed for these nanocomposites to reach their full potential. The poor interfacial

adhesion with graphene/polymer composites in the absence of covalent bonding or

non-covalent interactions underscore the importance of the platelet surface chemistry in

reinforcement and the need for continued progress in this direction. Further property

11

improvements in graphene-based composites could be achieved by improved

morphological control. Defects and wrinkles in platelets are likely to influence their

reinforcing capabilities, and so exfoliation and/or dispersion techniques that promote a

more elongated morphology could conceivably further improve the mechanical

properties of these composites. Despite these challenges, polymer nanocomposites have

already found huge applications in various fields and their scientific impact is expected

to rise significantly in the future.

Acknowledgements

JL thanks the NSF of China (51173087), NSF of Shandong (ZR2011EMM001), NSF of

Qingdao (12-1-4-2-2-jch) and Taishan Scholars Program of Shandong Province for

financial support.

References

[1] Blake P, Brimicombe PD, Nair RR, Booth TJ, Jiang D, Schedin F, et al.

Graphene-based liquid crystal device. Nano Lett 2008; 8(6): 1704-8.

[2] Gilje S, Han S, Wang M, Wang KL, Kaner RB. A chemical route to graphene for

device applications. Nano Lett 2007; 7(11): 3394-8.

[3] Kim KS, Zhao Y, Jang H, Lee SY, Kim JM, Kim KS, et al. Large-scale pattern

growth of graphene films for stretchable transparent electrodes. Nature 2009;

457(7230): 706-10.

[4] Wu J, Agrawal M, Becerril HA, Bao Z, Liu Z, Chen Y, et al. Organic light-emitting

diodes on solution-processed graphene transparent electrodes. ACS Nano 2009; 4(1):

12

43-8.

[5] Wang X, Zhi L, Müllen K. Transparent, conductive graphene electrodes for

dye-sensitized solar cells. Nano Lett 2008; 8(1): 323-7.

[6] Miranda R, de Parga ALV. Graphene: surfing ripples towards new devices. Nat

Nanotechnol 2009; 4(9): 549-50.

[7] Sinha Ray S, Okamoto M. Polymer/layered silicate nanocomposites: a review from

preparation to processing. Prog Polym Sci 2003; 28(11): 1539-641.

[8] Cao M, Fu A, Wang Z, Liu J, Kong N, Zong X, et al. Electrochemical and

theoretical study of π–π stacking interactions between graphitic surfaces and pyrene

derivatives. J Phys Chem C 2014; 118(5): 2650-9.

[9] Kim J, Cote LJ, Kim F, Huang J. Visualizing graphene based sheets by fluorescence

quenching microscopy. J Am Chem Soc 2010; 132(1): 260-7.

[10] Han TH, Lee WJ, Lee DH, Kim JE, Choi E-Y, Kim SO. Peptide/graphene hybrid

assembly into core/shell nanowires. Adv Mater (Weinheim, Ger) 2010; 22(18):

2060-4.

[11] Rafiee MA, Rafiee J, Wang Z, Song H, Yu Z, Koratkar N. Enhanced mechanical

properties of nanocomposites at low graphene content. ACS Nano 2009; 3(12):

3884-90.

[12] Sutter P, Sadowski JT, Sutter EA. Chemistry under cover: tuning metal-graphene

interaction by reactive intercalation. J Am Chem Soc 2010; 132(23): 8175-9.

[13] Liu J, Yang W, Tao L, Li D, Boyer C, Davis TP. Thermosensitive graphene

13

nanocomposites formed using pyrene-terminal polymers made by RAFT

polymerization. J Polym Sci, Part A: Polym Chem 2010; 48(2): 425-33.

[14] Liu J, Tao L, Yang W, Li D, Boyer C, Wuhrer R, et al. Synthesis, characterization,

and multilayer assembly of pH sensitive graphene-polymer nanocomposites.

Langmuir 2010; 26(12): 10068-75.

[15] Kelley EG, Albert JN, Sullivan MO, Epps III TH. Stimuli-responsive copolymer

solution and surface assemblies for biomedical applications. Chem Soc Rev 2013;

42(17): 7057-71.

[16] Gil ES,Hudson SM. Stimuli-reponsive polymers and their bioconjugates. Prog

Polym Sci 2004; 29(12): 1173-222.

[17] Jochum FD,Theato P. Temperature-and light-responsive smart polymer materials.

Chem Soc Rev 2013; 42(17): 7468-83.

[18] Al-Mashat L, Shin K, Kalantar-Zadeh K, Plessis JD, Han SH, Kojima RW, et al.

Graphene/polyaniline nanocomposite for hydrogen sensing. J Phys Chem C 2010;

114(39): 16168-73.

[19] Ma X, Li Y, Wang W, Ji Q, Xia Y. Temperature-sensitive poly

(N-isopropylacrylamide)/graphene oxide nanocomposite hydrogels by in situ

polymerization with improved swelling capability and mechanical behavior. Eur

Polym J 2013; 49(2): 389-96.

[20] Yang N, Zhai J, Wang D, Chen Y,Jiang L. Two-dimensional graphene bridges

enhanced photoinduced charge transport in dye-sensitized solar cells. ACS Nano

14

2010; 4(2): 887-94.

[21] Loh KP, Bao Q, Ang PK,Yang J. The chemistry of graphene. J Mater Chem 2010;

20(12): 2277-89.

[22] Liu J. Imparting polymeric properties to graphene nanosheets by surface

modification via π-π stacking. Aust J Chem 2011; 64(10): 1414.

[23] Eda G, Chhowalla M. Graphene-based composite thin films for electronics. Nano

Lett 2009; 9(2): 814-8.

[24] Liu J, Wang R, Cui L, Tang J, Liu Z, Kong Q, et al. Using molecular level

modification to tune the conductivity of graphene papers. J Phys Chem C 2012;

116(33): 17939-46.

[25] Su Q, Pang S, Alijani V, Li C, Feng X,Müllen K. Composites of graphene with

large aromatic molecules. Adv Mater (Weinheim, Ger) 2009; 21(31): 3191-5.

[26] Cui L, Liu J, Wang R, Liu Z,Yang W. A facile "graft from" method to prepare

molecular-level dispersed graphene-polymer composites. J Polym Sci, Part A:

Polym Chem 2012; 50(21): 4423-32.

[27] Robert D. Photosensitization of TiO2 by MxOy and MxSy nanoparticles for

heterogeneous photocatalysis applications. Catal Today 2007; 122(1): 20-6.

[28] Zhang X, Li H, Cui X, Lin Y. Graphene/TiO2 nanocomposites: synthesis,

characterization and application in hydrogen evolution from water photocatalytic

splitting. J Mater Chem 2010; 20(14): 2801-6.

[29] Zhou K, Zhu Y, Yang X, Jiang X, Li C. Preparation of graphene-TiO2 composites

15

with enhanced photocatalytic activity. New J Chem 2011; 35(2): 353-9.

[30] Wang R, Wu Q, Lu Y, Liu H, Xia Y, Liu J, et al. Preparation of nitrogen doped

TiO2/graphene nanohybrids and application as counter electrode for dye-sensitized

solar cells. ACS appl mater inter 2014; 6(3): 2118-24.

[31] Liu Z, Liu J, Cui L, Wang R, Luo X, Barrow CJ, et al. Preparation of

graphene/polymer composites by direct exfoliation of graphite in functionalised

block copolymer matrix. Carbon 2013; 51: 148-55.

[32] Geng J, Kong B-S, Yang SB,Jung H-T. Preparation of graphene relying on

porphyrin exfoliation of graphite. Chem Commun (Cambridge, U K) 2010; 46(28):

5091-3.

[33] Zong X, Kong N, Liu J, Yang W, Cao M,Gooding JJ. The Influence of graphene on

the electrical communication through organic layers on graphite and gold

electrodes. Electroanalysis 2014; 26(1): 84-92.

[34] Kong N, Huang X, Cui L,Liu J. Surface modified graphene for heavy metal Ions

adsorption. Sci Adv Mater 2013; 5(8): 1083-9.

[35] Kong N, Liu J, Kong Q, Wang R, Barrow CJ,Yang W. Graphene modified gold

electrode via π–π stacking interaction for analysis of Cu2+ and Pb2+. Sens Actuators,

B 2013; 178: 426-33.

[36] Liu J, Kong N, Li A, Luo X, Cui L, Wang R, et al. Graphene bridged enzyme

electrodes for glucose biosensing application. Analyst 2013; 138(9): 2567-75.

[37] Liu J, Liu Z, Luo X, Zong X,Liu J. RAFT controlled synthesis of biodegradable

16

polymer brushes on graphene for DNA binding and release. Macromol Chem Phys

2013; 214(20): 2266-75.

Fig. 1. (a) A schematic depiction of the synthesis of graphene-polymer composites. a1:

pyrene–PNIPAAm; a2: pyrene-PDMAEA or PAA; a3: pyrene-OEG-A or DEG-A.

Adapted from Ref.s [13, 14, 22] with permission of John Wiley＆Sons, Inc. American

Chemical Society and CSIRO. (b) Thermal turbidity testing of the graphene–PNIPAAm

composite in H2O. Reproduced from Ref. [13] with permission of John wiley＆Sons,Inc.

(c) LCSTs of graphene-OEG-A or DEG-A composites. Reproduced from Ref. [22] with

permission of CSIRO. (d) pH sensitivity testing of graphene-polymer composites of

PDMAEA (d11-d14) and PAA (d21-d24) at different pH values. Reproduced from Ref.

[14] with permission of American Chemical Society.

Fig. 2. Schematic illustrating the modification of graphene sheets with (a) MDS, (b)

BDS, (c) BPMW. (d) EC of graphene papers fabricated with graphene sheets modified

with MDS at different ratios of MDS to GDB before (filled square) and after (filled

circle) annealing and (e) with BDS at different molar ratios of BDS to GDB (filled

square) and BPMW at different molar ratios of BPMW to a quarter of the phenyl rings

(filled circle). Adapted from Ref. [24] with permission of American Chemical Society.

Fig. 3. (a) The modification of graphene with pyrene functional RAFT agent via π-π

stacking interactions and the subsequent in situ polymerization to directly grafting

polymer on graphene. (b) Conductivity of papers of graphene and graphene composites

with different polymers and (c) Conductivity of graphene–PS composite papers with

17

different graphene contents. Adapted from Ref. [26] with permission of Wiley

Periodicals, Inc.

Fig. 4. (a) Schematic illustration for the surface modification of H2Ti3O7 with

(3-aminopropyl) triethoxysilane (APTES) and 1-pyrenebutyric acid, followed by the

attachment of graphene and nitridation at different temperatures. (b) H2Ti3O7/graphene

nanohybrids calcined at 700 °C in NH3 atmosphere for 1 h: A, TEM image; B, Selected

area electron diffraction rings pattern; C, TEM bright field image with higher

magnification and energy dispersion spectrum profile; D, High-resolution TEM; E, Fast

Fourier transformation image. Adapted from Ref. [30] with permission of American

Chemical Society.

Fig. 5. (a) A schematic showing the direct exfoliation of graphene from graphite using

amphiphilic block copolymers with multi-pyrene pendent groups, which were

synthesized using the RAFT mechanism and the high temperature treatment to prepare

graphene. (b) TEM image of thermally annealed graphene directly exfoliated from

graphite. (c) AFM image and the height profile of thermally annealed graphene.

Adapted from Ref. [31] with permission of Elsevier Ltd.

Fig. 6. I Graphene enhanced electrical communication; II pyrene butyric acid modified

graphene for heavy metal adsorption; III Graphene modified gold electrode for analysis

of Cu2+ and Pb2+; IV Graphene bridged enzyme electrodes for glucose biosensing

application; V Surface modified graphene for DNA binding and release; VI Controlled

release of charged drugs by static repulsion process. Adapted from Refs [8, 33-37] with

18

permission of American Chemical Society, Wiley-VCH Verlag GmbH＆Co. KGaA,

Weinheim, Amer Scientific Publishers, Elsevier B.V. and Royal Society of Chemistry.

Table 1

Contents of non-covalent interactions and their key applications

Interaction type Key applications Refs.

Van der Waals force Improved imaging techniques [9]

Electrostatic interaction Self-assembly composite materials [10]

Hydrogen bonding Enhanced materials’ strength [11]

Coordination bonds Graphene/ transition metals composite [12]

π–π stacking interaction Smart materials [13], [14]

19

20

21

22

23

24