###### Abstract  
This work presents the first instance of reversed-phase liquid chromatographic separation of small molecules using graphene oxide nanoparticle-modified polystyrene-divinylbenzene polymeric high internal phase emulsion (GONP PS-_co_-DVB polyHIPE) materials housed within a 200-\(\upmu\)m internal diameter (i.d.) fused silica capillary. The graphene oxide nanoparticle (GONP)-modified materials were produced as a potential strategy to increase both the surface area limitations and the reproducibility issues observed in monolithic stationary phase materials. GONP PS-_co_-DVB polyHIPEs were found to have a surface area up to 40% lower than unmodified polymeric high internal phase emulsion (polyHIPE) stationary phases. However, despite having a surface area significantly lower than that of the unmodified material, the GONP-modified polyHIPEs demonstrated superior analyte adsorption properties. Reducing the GONP material did not have any significant impact on elution order or retention factor of the analytes, which was most likely due to low GONP loading attributed to the 250-nm GONPs utilised. The lower surface area of GONP-modified polyHIPEs provided similar separation efficiency and increased repeatability from injection to injection resulting in % relative standard deviations (%RSDs) of less than 0.6%, indicating the potential offered by graphene oxide (GO)-modified polyHIPES in flow through applications such as adsorption or separation processes. polymeric high internal phase emulsion (polyHIPE); high internal phase emulsions; graphene oxide; liquid chromatography; nanoparticles; polystyrene; divinylbenzene; monolith; macroporous materials +
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as shown in particular in [10; 11; 12; 13]. As polyHIPEs are macroporous materials (pores from 10 to 100 \(\upmu\)m) they are ideal for biological separations and have been demonstrated previously in the literature [10; 11; 13]. However, fewer instances of small molecule separation using high performance liquid chromatography (HPLC) methods have been utilised [12]. Due to intrinsic large pore structures the surface area of these materials is very low, typically ranging from 3 m\({}^{2}\).g\({}^{-1}\) to 20 m\({}^{2}\).g\({}^{-1}\)[14]. Attempts to increase the surface area of these materials have consisted mainly of introducing different additional porogens, which can leave the materials with low mechanical rigidity [15; 16]. This decreased rigidity would compromise the material's ability to function as a high pressure stationary phase. An alternative potential strategy is to incorporate an additional component into the polymerisation mixture to increase the surface area of the material [17]. Previously, polymer monoliths have been chemically modified to allow for nanoparticle (NP) attachment as a means to increase surface area [18; 19]. However, the incorporation of NPs in polyHIPEs typically results in Pickering emulsions which generally form closed cell structures, making them unsuitable for use as chromatographic stationary phases [20]. Graphene oxide (GO) has been utilised in the fabrication of polymer materials such as for increasing the flame retardant ability of polymer materials for fire safety, demonstrating the importance of GO incorporation in material applications [21; 22; 23; 24; 25]. Graphene oxide nanoparticles (GONPs) are also amphiphilic, and are thus suitable for use as a Pickering agent to form an emulsion. Pickering studies using GO have resulted in the formation of polystyrene particles coated in GO and even hollow hybrid polymer GO particles [26; 27; 28; 29; 30; 31; 32]. The successful use of GONPs, particularly with emulsion polymerisation, enables the development of GONP-modified polyHIPE materials, potentially enhancing these materials in terms of stability and surface area. GONP materials have been applied as solid-phase extraction materials [33], progressing from the insoluble graphene solid-phase techniques that were being developed at the time.