Graphene Oxide (GO) and Reduced Graphene Oxide: Applications

Graphene oxide (GO, product number 763705) is an exceptional material observed as a solitary monomolecular layer of graphite with numerous oxygen encompassing functionalities like epoxide, carbonyl, carboxyl and hydroxyl groups.^1,2,3

Awareness about GO grew hugely following graphene, a single layer of graphite, first being isolated and studied.⁴ It was originally hoped that GO might be a synthetic precursor to graphene.⁵ When the electrically insulating GO is lessened, the concentrated graphene oxide (GO, product number 777684) formed bears a resemblance to graphene but holds residual oxygen and additional heteroatoms in addition to structural imperfections.^6,7

A production process for rGO that is more resemblant of a pristine graphene is an incredible test in the field. However, because rGO can be created as a thin film from an aqueous dispersion of GO in water and has reasonable conductivity, it is good for application in electronic devices.^8,9,10,11,12 Further to being components in electronic devices, GO and rGO have been employed in nanocomposite materials,^13,14 polymer composite materials,¹³ energy storage,¹⁴ biomedical applications,^15,16,17 catalysis^18,19,20 and as a surfactant²¹ with some intersections across these areas.

Synthesis and Reduction of GO

A lot of current techniques for the synthesis of GO are founded on the technique originally described by Hummers, where graphite is oxidized by a solution of potassium permanganate in sulfuric acid.²² Decreasing GO by employing hydrazine is described.⁵ Despite this, hydrazine is extremely toxic and can possibly functionalize the GO with nitrogen heteroatoms.²³ Due to these problems, substitutes to hydrazine, including NaBH₄,¹⁴ ascorbic acid,²⁴ and HI^25,26 and others have been employed for the lessening of GO. GO can be lessened as a thin film or in an aqueous solution. Reduction techniques were recently appraised.⁶

Electronics

Many electronic devices were created employing GO as a primary material for one of the components at a minimum. One of these devices is a graphene founded field effect transistor (GFET).^27,28 Field effect transistors (FETs) that use rGO have been employed as chemical sensors^29,30,31 and biosensors. An illustration of GFET sensors can be seen in Figure 1.³¹ GFETs employing functionalized rGO as the semiconductor are sometimes utilized as biosensors to spot hormonal catecholamine molecules,³² avidin³³ and DNA.³⁴ In a further investigation, GO that was functionalized with glucose oxidase and placed on an electrode was employed as an electrochemical glucose sensor.³⁵

Visible light transparent electrodes are critical for both light emitting diodes (LEDs) and solar cell devices. Because GO may be handled in solution, employing rGO as a transparent electrode is a suitable substitute for other transparent electrodes like ITO for such devices.^36,37 As well as being a transparent electrode, rGO has been employed as a hole transport layer in polymer solar cells and LEDs.^38,39

Figure 1. (a) Typical back-gate GFET on Si/SiO₂ substrate used as gas sensor. (b) Typical solution-gate GFET on flexible polyethylene terephthalate (PET) substrate used as chemical and biological sensor in aqueous solution. Reproduced from reference 31 with permission from the Centre National de la Recherche Scientifique (CNRS) and The Royal Society of Chemistry. 

Energy Storage

Nanocomposites of rGO have been employed for high-capacity energy storage in lithium-ion batteries. In these investigations, electrically insulating metal oxide nanoparticles were adsorbed onto rGO to grow the performance of the materials in batteries.^{40,41,42,43,44} For instance, the energy storage capability and cycle constancy was proven to rise for Fe₃O₄ on rGO versus unadulterated Fe₃O₄ or Fe₂O₃ (Figure 2).⁴³ Large surface area rGO has been created, employing microwaves for exfoliation and lessening of GO. The large surface area rGO created is beneficial as an energy storing material in supercapacitors.^45,46

Figure 2. (a) Discharge/charge profiles of the GNS/Fe₃O₄ composite. (b) Cycling performance of the commercial Fe₃O₄ particles, GNS/Fe₃O₄ composite and bare Fe₂O₃ particles at a current density of 35 mA/g Solid symbols, discharge; hollow symbols, charge. (c) Cycling performance of the GNS/Fe₃O₄ composite at a current density of 700 mA/g for 100 cycles. (d) Rate performance of the commercial Fe₃O₄ particles, GNS/Fe₃O₄ composite, and bare Fe₂O₃ particles at different current densities. GNS = rGO. Adapted with permission from reference 43. Copyright 2010 American Chemical Society.

Biomedical Applications

One application of GO in the biomedical field is as an element in drug delivery structures. Functionalized nanographene oxide (nGO, product number 795534) is employed in numerous investigations about targeted distribution of anti-cancer drugs. Polyethylene glycol (PEG) functionalized nGO with SN38, a camptothecin (product number H0165) derivative, adsorbed onto the surface (nGO‒PEG‒SN38) was employed as a water and serum soluble foundation of the drug.⁴⁷ This study, nGO‒PEG‒SN38 was revealed to be three orders of magnitude more impactful than irinotecan (CPT-11), an FDA accepted SN38 prodrug, at decreasing the cell capability of human colon cancer cell lines HTC-116 (Figure 3).⁴⁷

The success of nGO‒PEG‒SN38 was comparable to SN38 in DMSO as displayed in Figure 3.⁴⁷ Melanoma skin cancer in mice was treated applying photothermal ablation therapy with a near-infrared laser and nGO that was functionalized with PEG and hyaluronic acid and distributed transdermally.⁴⁸ Magnetite was adsorbed onto GO loaded with the anti-cancer drug doxorubicin hydrochloride (DXR, product numbers D1515 and 44583) in a different study for targeted distribution of the drug to certain sites employing magnets.⁴⁹

Figure 3. In vitro cell toxicity assay. (a) Relative cell viability (versus untreated control) data of HCT-116 cells incubated with CPT-11, SN38, and NGO‒PEG‒SN38 at different concentrations for 72 h. Free SN38 was dissolved in DMSO and diluted in PBS. Water soluble NGO‒PEG‒SN38 showed similar toxicity as SN38 in DMSO and far higher potency than CPT-11. (b) Relative cell viability data of HCT-116 cells after incubation with nGO-PEG with (red) and without (black) SN38 loading. Plain NGO‒PEG exhibited no obvious toxicity even at very high concentrations. Error bars were based on triplet samples. NGO = nGO. Adapted with permission from reference 47. Copyright 2008 American Chemical Society. 

Biosensors

GO and rGO have been employed as parts of numerous systems created to find biologically appropriate molecules. GO has been employed as a fluorescence quenching material in biosensors that apply the fluorescence resonance energy transfer (FRET) effect. In a study by Lu et al., single-stranded DNA (ssDNA) with a fluorescence tag was discovered to tie noncovalently to GO following quenching of the fluorescence of the tag.⁵⁰ Adding a complementary ssDNA detached the tagged DNA from the GO surface and reinstated the fluorescence. The FRET impact was additionally employed with fluorescein categorized ATP aptamer to sense ATP as low as 10 μM.⁵¹ Folic acid-functionalized GO was employed as an element in a system employed to sense human cervical cancer and human breast cancer cells.⁵²

MiliporeSigma provides good quality graphene oxide products for state-of-the-art and progressive materials study requirements.

Acknowledgments

Produced from materials originally authored by Avery Luedtke, Ph.D., from MiliporeSigma

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