Graphene oxide is prepared by exfoliation of graphite  oxide. The later is obtained by oxidation of graphite powder usually by Hammer's method. During this process hydrophilic groups like -COOH, -OH are formed. By modifying oxidation conditions, you can generate more hydrophilic groups. AFM images are expected to be different for a  highly hydrophilic surface and a relatively less hydrophilic surface.

Hi there,

You can use copolymer Methacrylic acid to enhance the hydrophilicity of graphene oxide 

Dear Soheila,

Atttached is a recent review on Graphene Oxide (GO) entitled " The Preparation of Graphene Oxide and Its Derivatives and

Their Application in Bio-Tribological Systems " published in Lubricants 2014, 2, 137-161; doi:10.3390/lubricants2030137.

Abstract: Graphene oxide (GO) can be readily modified for particular applications due to the existence of abundant oxygen-containing functional groups. Graphene oxide-based materials (GOBMs), which are biocompatible and hydrophilic, have wide potential applications in biomedical engineering and biotechnology. In this review, the preparation and characterization of GO and its derivatives are discussed at first. Subsequently, the biocompatibility and tribological behavior of GOBMs are reviewed. Finally, the applications of GOBMs as lubricants in bio-tribological systems are discussed in detail.

2. Preparation of GO

Although the strong σC–C bond and π bond give graphene exceptional physical properties, it also contributes to the weak reactivity of graphene, which, in turn, limits its use in engineering applications.

GO has an abundance of oxygenated functional groups, providing possible use in various applications, after chemical modification.

Graphite, the raw material for preparing GO, consists of polycrystalline particles or granules and can be selected from natural and synthetic sources. Natural graphite is the most common source and is used in a wide range of applications that use chemical modifications [8]. It is because natural graphite contains numerous localized defects in its π-structure that these may serve as seeding points for

chemical reaction processes [3].

The route to prepare GO involves two main steps; see Figure 1. Firstly, graphite powder is oxidized to produce graphite oxide, which can be readily dispersed in water or another polar solvent due to the presence of hydroxyl and epoxide groups across the basal planes of graphite oxide and carbonyl and carboxyl groups located at the edges [9–11]. Secondly, the bulk graphite oxide can be exfoliated by

sonication to form colloidal suspensions of monolayer, bilayer or few-layer GO sheets in different solvents [3]. The critical point of preparing GO is the selection of suitable oxidizing agents to oxidize graphite.

In addition, I am attaching a paper entitled "Graphene oxide as surfactant sheets " published in Pure Appl. Chem., Vol. 83, No. 1, pp. 95–110, 2011.

doi:10.1351/PAC-CON-10-10-25; © 2010 IUPAC, Publication date (Web): 1 December 2010

Abstract: Graphite oxide sheet, now referred to as graphene oxide (GO), is the product of chemical oxidation and exfoliation of graphite powders that was first synthesized over a century ago. Interest in this old material has resurged in recent years, especially after the discovery of graphene, as GO is considered a promising precursor for the bulk production of graphene-based materials. GO sheets are single atomic layers that can readily extend up to tens of microns in lateral dimension. Therefore, their structure bridges the typical length scales of both chemistry and materials science. GO can be viewed as an unconventional type

of soft material as it carries the characteristics of polymers, colloids, membranes, and as highlighted in this review, amphiphiles. GO has long been considered hydrophilic due to its excellent water dispersity, however, our recent work revealed that GO sheets are actually amphiphilic with an edge-to-center distribution of hydrophilic and hydrophobic domains.

Thus, GO can adhere to interfaces and lower interfacial energy, acting as surfactant. This new property insight helps to better understand GO’s solution properties which can inspire novel material assembly and processing methods such as for fabricating thin films with controllable microstructures and separating GO sheets of different sizes. In addition, GO can be used as a surfactant sheet to emulsify organic solvents with water and disperse insoluble materials

such as graphite and carbon nanotubes (CNTs) in water, which opens up opportunities for creating functional hybrid materials of graphene and other π-conjugated systems.

CONCLUSIONS

Despite being considered hydrophilic for the past century, GO is a unique two-dimensional amphiphile that can adhere to gas–water, liquid–water, and solid–water interfaces and lower interfacial energy [29].

Its amphiphilicity is tunable by solution pH values, sheet size, and degree of reduction. This new insight allows better understanding of the solution property of GO, which is helpful for designing better assembly

and processing techniques for graphene thin films [18,28]. Well-defined microstructure–property relationship of graphene monolayers can be established by tuning the assembly of these surfactant sheets [34]. Size-dependent amphiphilicity inspires novel size separation methods such as water surface

filtration and emulsion extraction. GO sheets can be used as dispersing agents to create colloidal dispersions of insoluble materials, especially π-conjugated aromatic systems such as conjugated polymers, CNTs, and graphite, thus enabling their solution processability. As a novel surfactant, GO offers a few

advantages. For example, as a colloidal surfactant with large dimension, they can be easily recovered (e.g., by filtration). They can be readily converted to chemically modified graphene, rendering the final complex of surfactant-dispersed phase electrically conductive. When GO is used to disperse carbonbased materials, it allows for the creation of clean carbon–carbon junctions that would otherwise be

contaminated by common molecular surfactants or degraded by covalent functionalization, which should facilitate charge transport across the interfaces. Since surfactants are routinely used in chemical and material industries as well as in our daily life, perhaps what this new surfactant system can do is only limited by our imagination.

Hoping this will be helpful

surfactants to be coated on to the surface of hydrophilic or hydrophobic graphene oxide to get modified properties or structure of Graphene oxide. electrical properties and chemical properties will change.