Article pubs.acs.org/JPCC
Enhanced Stability of Reduced Graphene Oxide Colloid Using CrossLinking Polymers Akshaya Kumar Swain† and Dhirendra Bahadur*,‡ †
IITB Monash Research Academy, Department of Metallurgical Engineering and Materials Science, IIT Bombay, Mumbai 400076, India ‡ Department of Metallurgical Engineering and Materials Science, IIT Bombay, Mumbai 400076, India S Supporting Information *
ABSTRACT: Reduced graphene oxide (RGO) synthesized by subsequent oxidation and reduction suffers from agglomeration within a few hours/days of its preparation. The stability of RGO is enhanced (no agglomeration for more than 23 months) by allowing two cross-linking polymers that prevent restacking; this increases the solubility and biocompatibility of RGO. The RGO dispersion follows an electrosteric stabilization mechanism. Various theoretical models were adopted to understand this behavior by estimating the potential barrier for agglomeration, surface free energy, and Hansen solubility parameters of the dispersion. As-prepared RGO is found to be biocompatible and has luminescent properties.
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INTRODUCTION Chemistry-rich graphite oxide (GO) and its derivatives are amphiphilic in nature due to the presence of a hydrophobic basal plane and hydrophilic edges.1 So, the flocculation of reduced graphene oxide (RGO) that leads to agglomeration is spontaneous by virtue of its dominant hydrophobic interaction over an RGO−solvent interaction. Thus, using a single surfactant or a polymer with one repeating monomer is insufficient to deal with the wide range of forces that are present in RGO dispersions.2−9 This may also be due to the presence of two different length scales (thickness in Å and lateral size in μm) in GO derivatives. This property makes it behave both as a molecule and a colloid.1 RGO, prepared chemically by the classical oxidation− reduction technique, is cheap, easy, and scalable to realize commercial applications on a large scale. The various applications of RGO in major industries such as paints, coatings, pharmaceuticals, and the like demand that its stability be maintained for prolonged periods. Thus, there have been many attempts to make a stable graphene/RGO suspension.2−10 In fact, Hernandez et al. tested 40 solvents in order to facilitate the understanding of the solubility of graphene and to increase its stability in a dispersion.11 Park et al. reported colloidal dispersions of RGO in a variety of organic solvent mixtures.12 Lotya et al. obtained a maximum stability of ∼6 weeks, using a surfactant.2 Jo et al. obtained a maximum stability of six months for RGO by noncovalent functionalization using conducting polymers. 10 RGO, with excess hydrazine/reducing agents, gets agglomerated readily.13 A summary of the stability of graphene/RGO, which has already been reported, is presented in Table S1 (Supporting Information). Even though the solubility of GO/RGO in a wide range of solvents has been studied, the prolonged stability © 2014 American Chemical Society
of these dispersions remains a challenge. This limits the use of RGO in major industrial applications. In this study, we describe a simple technique that uses two cross-linking polymers (CLPs), polyvinylpyrrolidone (PVP) and poly(vinyl alcohol) (PVA) during reduction, in order to enhance the stability of RGO dispersion significantly. The cross-linking between PVA and PVP is a well established fact in the literature. The cross-link generally occurs at the site of radical formation on the OH group of PVA and/or at the main chain of the polymers. The OH groups in PVA and the CO groups in PVP cross-link through hydrogen bonding.14 Also, the dispersion is biocompatible due to the presence of the two polymers (PVP and PVA). This renders it a potential material for various industrial applications (including biomedical applications) that require a colloid to stand for a few years without sedimentation or agglomeration. To the best of our knowledge, there is no report so far that claims stabilization of a RGO dispersion for more than six months.
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SYNTHESIS AND CHARACTERIZATION OF MATERIALS Natural graphite powder (purity >99.99%, average particle size few μg/mL), which might result from oxidative stress and reactive oxygen species that are abundantly present in both GO and RGO.52,53 In addition, aggregation of RGO in the dispersion blocks the cells to obtain sufficient nutrients for the growth of the cells.54 Thus, a welldispersed stable dispersion would facilitate cell growth. In general, the biocompatibility of a material depends mainly on
how well proteins mediate the interactions between the cell and the corresponding material. The cellular interaction can be tuned by functionalizing the RGO with suitable polymers. Since RGO is linked to the polymers (PVA and PVP) through defect sites, its biocompatibility will be governed mainly by the cell− polymer interaction. The cell membrane finds itself in a biofriendly environment that comprises these two polymers and the media mostly, forming an extra cellular matrix for the cells.55 This matrix provides the essential nutrients for cell growth resulting in a biocompatible system. Our experiments suggest an excellent method for the stabilization of RGO and making it biocompatible. The biocompatibility of SRG comes from the presence of the two excellent biocompatible polymers (PVP/PVA) that are bonded to RGO. Thus, we expect this material to be a potential material for various bioapplications.
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CONCLUSION To conclude, we report a simple approach to obtain high solubility and enhanced stability of RGO by using cross-linking polymers as an add-on to classical GO reduction. The experimental data supports the theoretical arguments obtained through various models. The DLVO theory predicts a potential barrier for the agglomeration. SFE of the polymer−matrix demands steric stabilization, and HSPs of the constituents of SRG confirms the solubility. SRG was found to be biocompatible with luminescent properties. Thus, we believe that SRG would be a potential candidate for several promising industrial applications including bioapplications that demand prolonged stability of a dispersion.
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ASSOCIATED CONTENT
* Supporting Information S
Experimental methods for preparation of GO, SRG, PVPA; sample preparation techniques for various characterizations; Table S1 represents the summary of the stabilization studies of graphene/RGO; Table S2 represents the LW and AB components of PVA, PVP, water, and PTFE; and Figure S1 represents the morphology of dried SRG on a silicon substrate. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +91 22 2576 7632; fax: +91 22 2572 6975; e-mail:
[email protected]. 9455
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank DST-Nanomission and IITBMonash Research Academy for the financial support. REFERENCES
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