Graphene-Based Materials for Biosensors: A Review <s> Chemical Synthesis of Graphite Oxide and Graphene Oxide <s> Fluorination of graphite oxide under mild conditions gives the same X-ray diffraction pattern as that for stage 2-type graphite fluoride, (C2F)n. Well-hydrated graphite oxide shows the presence of a superlattice along the c axis. A new structure model of graphite oxide has been proposed based on these facts. The new model consists of double carbon layers linked with each other by sp3 bonds of carbon perpendicular to the carbon network. The carbonyl and hydroxyl groups are combined with the double carbon layers from above and below. This structure model is an intermediate form between two ideal structures, C8(OH)4(c0 = 8.22 × 2 A) and C8O2(c0 = 5.52 × 2 A). It has a structure with sixfold symmetry (hexagonal system 6m2) with the stacking sequence of carbon layers, A-A'/B-B'/.… The repeat distance along the c axis decreases with increasing dehydration. The composition is expressed by C8O2−x(OH)2x(0 <s> BIB001 </s> Graphene-Based Materials for Biosensors: A Review <s> Chemical Synthesis of Graphite Oxide and Graphene Oxide <s> Graphite oxide (GO) and its derivatives have been studied using 13C and 1H NMR. The 13C NMR lines at 60, 70, and 130 ppm are assigned to C−OH, C−O−C, and >CC CC< double bonds are relatively stable, while C−OH groups may condense to form C−O−C (ether) linkages. There are at least two magnetically inequivalent C−OH sites, and the structure does not necessarily possess long-range order. Water molecules interact very strongly with the structure. The results reveal a number of new structural features. <s> BIB002 </s> Graphene-Based Materials for Biosensors: A Review <s> Chemical Synthesis of Graphite Oxide and Graphene Oxide <s> Abstract Solid-state 13 C NMR spectra of graphite oxide (GO) and its derivatives prompt us to propose a new structural model. The spectra of GO treated with KI and the course of the thermal decomposition of GO reveal the presence of epoxide groups, responsible for the oxidating nature of the material. GO is built of aromatic “islands” of variable size which have not been oxidized, and are separated from each other by aliphatic 6-membered rings containing C–OH, epoxide groups and double bonds. The carbon grid is nearly flat; a small degree of warping is caused by the carbons attached to OH groups, which are in a slightly distorted tetrahedral configuration. <s> BIB003 </s> Graphene-Based Materials for Biosensors: A Review <s> Chemical Synthesis of Graphite Oxide and Graphene Oxide <s> Unilamellar colloids of graphite oxide (GO) were prepared from natural graphite and were grown as monolayer and multilayer thin films on cationic surfaces by electrostatic self-assembly. The multilayer films were grown by alternate adsorption of anionic GO sheets and cationic poly(allylamine hydrochloride) (PAH). The monolayer films consisted of 11−14 A thick GO sheets, with lateral dimensions between 150 nm and 9 μm. Silicon substrates primed with amine monolayers gave partial GO monolayers, but surfaces primed with Al13O4(OH)24(H2O)127+ ions gave densely tiled films that covered approximately 90% of the surface. When alkaline GO colloids were used, the monolayer assembly process selected the largest sheets (from 900 nm to 9 μm) from the suspension. In this case, many of the flexible sheets appeared folded in AFM images. Multilayer (GO/PAH)n films were invariably thicker than expected from the individual thicknesses of the sheets and the polymer monolayers, and this behavior is also attributed to folding... <s> BIB004 </s> Graphene-Based Materials for Biosensors: A Review <s> Chemical Synthesis of Graphite Oxide and Graphene Oxide <s> This study contributes to the sustained effort to unravel the chemical structure of graphite oxide (GO) by proposing a model based on elemental analysis, transmission electron microscopy, X-ray diffraction, 13C magic-angle spinning NMR, diffuse reflectance infrared Fourier transform spectroscopy, X-ray photoelectron spectroscopy, and electron spin resonance investigations. The model exhibits a carbon network consisting of two kinds of regions (of trans linked cyclohexane chairs and ribbons of flat hexagons with CC double bonds) and functional groups such as tertiary OH, 1,3-ether, ketone, quinone, and phenol (aromatic diol). The latter species give clear explanation for the observed planar acidity of GO, which could not be interpreted by the previous models. The above methods also confirmed the evolution of the surface functional groups upon successive oxidation steps. <s> BIB005 </s> Graphene-Based Materials for Biosensors: A Review <s> Chemical Synthesis of Graphite Oxide and Graphene Oxide <s> Graphite oxide (GO) samples were prepared by a simplified Brodie method. Hydroxyl, epoxide, carboxyl, and some alkyl functional groups are present in the GO, as identified by solid-state 13C NMR, Fourier-transform infrared spectroscopy, and X-ray photoemission spectroscopy. Starting with pyrolytic graphite (interlayer separation 3.36 A), the average interlayer distance after 1 h of reaction, as determined by X-ray diffraction, increased to 5.62 A and then increased with further oxidation to 7.37 A after 24 h. A smaller signal in 13C CPMAS NMR compared to that in 13C NMR suggests that carboxyl and alkyl groups are at the edges of the flakes of graphite oxide. Other aspects of the chemical bonding were assessed from the NMR and XPS data and are discussed. AB stacking of the layers in the GO was inferred from an electron diffraction study. The elemental composition of GO prepared using this simplified Brodie method is further discussed. <s> BIB006 </s> Graphene-Based Materials for Biosensors: A Review <s> Chemical Synthesis of Graphite Oxide and Graphene Oxide <s> The detailed chemical structure of graphite oxide (GO), a layered material prepared from graphite almost 150 years ago and a precursor to chemically modified graphenes, has not been previously resolved because of the pseudo-random chemical functionalization of each layer, as well as variations in exact composition. Carbon-13 (13C) solid-state nuclear magnetic resonance (SSNMR) spectra of GO for natural abundance 13C have poor signal-to-noise ratios. Approximately 100% 13C-labeled graphite was made and converted to 13C-labeled GO, and 13C SSNMR was used to reveal details of the chemical bonding network, including the chemical groups and their connections. Carbon-13-labeled graphite can be used to prepare chemically modified graphenes for 13C SSNMR analysis with enhanced sensitivity and for fundamental studies of 13C-labeled graphite and graphene. <s> BIB007 </s> Graphene-Based Materials for Biosensors: A Review <s> Chemical Synthesis of Graphite Oxide and Graphene Oxide <s> Graphene is a promising material for the next-generation of nanoelectronic devices, but it has been difficult to produce single-layer samples in bulk quantities. A solution-based process for the large-scale production of single-layer, chemically converted graphene has now been demonstrated and used to make field-effect devices with currents that are three orders of magnitude higher than previously reported for chemically produced graphene. <s> BIB008 </s> Graphene-Based Materials for Biosensors: A Review <s> Chemical Synthesis of Graphite Oxide and Graphene Oxide <s> Graphite oxide is one of the main precursors of graphene-based materials, which are highly promising for various technological applications because of their unusual electronic properties. Although epoxy and hydroxyl groups are widely accepted as its main functionalities, the complete structure of graphite oxide has remained elusive. By interpreting spectroscopic data in the context of the major functional groups believed to be present in graphite oxide, we now show evidence for the presence of five- and six-membered-ring lactols. On the basis of this chemical composition, we devised a complete reduction process through chemical conversion by sodium borohydride and sulfuric acid treatment, followed by thermal annealing. Only small amounts of impurities are present in the final product (less than 0.5 wt% of sulfur and nitrogen, compared with about 3 wt% with other chemical reductions). This method is particularly effective in the restoration of the π-conjugated structure, and leads to highly soluble and conductive graphene materials. <s> BIB009 </s> Graphene-Based Materials for Biosensors: A Review <s> Chemical Synthesis of Graphite Oxide and Graphene Oxide <s> An improved method for the preparation of graphene oxide (GO) is described. Currently, Hummers’ method (KMnO4, NaNO3, H2SO4) is the most common method used for preparing graphene oxide. We have found that excluding the NaNO3, increasing the amount of KMnO4, and performing the reaction in a 9:1 mixture of H2SO4/H3PO4 improves the efficiency of the oxidation process. This improved method provides a greater amount of hydrophilic oxidized graphene material as compared to Hummers’ method or Hummers’ method with additional KMnO4. Moreover, even though the GO produced by our method is more oxidized than that prepared by Hummers’ method, when both are reduced in the same chamber with hydrazine, chemically converted graphene (CCG) produced from this new method is equivalent in its electrical conductivity. In contrast to Hummers’ method, the new method does not generate toxic gas and the temperature is easily controlled. This improved synthesis of GO may be important for large-scale production of GO as well as the ... <s> BIB010 </s> Graphene-Based Materials for Biosensors: A Review <s> Chemical Synthesis of Graphite Oxide and Graphene Oxide <s> The chemistry of graphene oxide is discussed in this critical review. Particular emphasis is directed toward the synthesis of graphene oxide, as well as its structure. Graphene oxide as a substrate for a variety of chemical transformations, including its reduction to graphene-like materials, is also discussed. This review will be of value to synthetic chemists interested in this emerging field of materials science, as well as those investigating applications of graphene who would find a more thorough treatment of the chemistry of graphene oxide useful in understanding the scope and limitations of current approaches which utilize this material (91 references). <s> BIB011 </s> Graphene-Based Materials for Biosensors: A Review <s> Chemical Synthesis of Graphite Oxide and Graphene Oxide <s> The chemical reduction of graphene oxide is a promising route towards the large scale production of graphene for commercial applications. The current state-of-the-art in graphene oxide reduction, consisting of more than 50 types of reducing agent, will be reviewed from a synthetic chemistry point of view. Emphasis is placed on the techniques, reaction mechanisms and the quality of the produced graphene. The reducing agents are reviewed under two major categories: (i) those which function according to well-supported mechanisms and (ii) those which function according to proposed mechanisms based on knowledge of organic chemistry. This review will serve as a valuable platform to understand the efficiency of these reducing agents for the reduction of graphene oxide. <s> BIB012 </s> Graphene-Based Materials for Biosensors: A Review <s> Chemical Synthesis of Graphite Oxide and Graphene Oxide <s> A protein-based electrochemical biosensor was developed for detection of tau protein aimed towards electrochemically sensing misfolding proteins. The electrochemical assay monitors tau–tau binding and misfolding during the early stage of tau oligomerization. Electrochemical impedance spectroscopy was used to detect the binding event between solution tau protein and immobilized tau protein (tau–Au), acting as a recognition element. The charge transfer resistance (Rct) of tau–Au was 2.9 ± 0.6 kΩ. Subsequent tau binding to tau–Au decreased the Rct to 0.3 ± 0.1 kΩ (90 ± 3% decrease) upon formation of a tau–tau–Au interface. A linear relationship between the Rct and the solution tau concentration was observed from 0.2 to 1.0 μM. The Rct decrease was attributed to an enhanced charge permeability of the tau–tau–Au surface to a redox probe [Fe(CN)6]3−/4−. The electrochemical and surface characterization data suggested conformational and electrostatic changes induced by tau–tau binding. The protein-based electrochemical platform was highly selective for tau protein over bovine serum albumin and allowed for a rapid sample analysis. The protein-based interface was selective for a non-phosphorylated tau441 isoform over the paired-helical filaments of tau, which were composed of phosphorylated and truncated tau isoforms. The electrochemical approach may find application in screening of the early onset of neurodegeneration and aggregation inhibitors. <s> BIB013
There are three original methods used to synthesize graphene, including: (1) Brodie method (1859)  , the GpO was prepared using Ceylon as a raw material resulting in a purification to give 99.96% carbon. A boiled mixture of concentrated nitric and sulfuric acids called carbonic acid was used as an oxidizing agent. As observed from elemental analysis, the oxidized Ceylon graphite included C:O:H contents as 67.79:30.37:1.84 and the C-to-O ratio was 2.23. Therefore, the material was termed graphic acid, and was the very first sample of graphite oxide prepared experimentally. (2) Staudenmaier method (1898)  , this method is very similar to Brodie's. The graphite oxide is prepared in a mixture of concentrated sulfuric acid and fuming nitric acid. In addition, potassium chlorate oxidizing agent is also added and reacted over 4 days. By rinsing in water and dispersing in diluted hydrochloric acid, sulfonate ions were removed. Finally, the graphite oxide was dried at 60 °C for 2 days. The graphite oxide prepared by this method was found to have an elemental composition of C:O:H of 58.73:23.28:17.99. The C-to-O ratio was 2.52, which indicated the lowest degree of oxidation; (3) The Hummers and Offeman method (1958)  was developed when it was realized that the usage of nitric acid requires a lot of time for oxidizing graphite, has the potential for explosion and the release of highly corrosive vapor. The Hummers and Offeman method is a less hazardous way to oxidize graphite. The oxidizing agent is a mixture of concentrated sulfuric acid, sodium nitrate and potassium permanganate. The entire process needs 1-2 h to complete the reaction. As a result, the graphitic oxide had a C-to-O ratio between 2.1-2.9. The color of the product in aqueous solution is referred to the degree of oxidation. The product gives a bright yellow color for the most oxidized graphite while the green to black color refers to poor graphitic oxidation having too high C-to-O ratios. Currently, the Hummers and Offeman method is the most commonly used and is commonly known as the Hummers method. As a result of the oxidation of graphite there is an increase in the interlayer spacing. This means that the separation between graphene interlayers is expanded by the intercalation of oxidized contents or the polar liquids such as sodium hydroxide  as seen in solid-state 13 C-NMR BIB006 . The interlayer distance of 0.335 nm in pristine graphite increases to 0.562 nm after 1 h of oxidation and increases further to 0.737 nm after 24 h of oxidation. It is suggested that the carboxyl and alkyl groups are mostly at the edges of the GpO layers. By treatment via mechanical stirring or ultrasonication, the  There are three original methods used to synthesize graphene, including: (1) Brodie method (1859)  , the GpO was prepared using Ceylon as a raw material resulting in a purification to give 99.96% carbon. A boiled mixture of concentrated nitric and sulfuric acids called carbonic acid was used as an oxidizing agent. As observed from elemental analysis, the oxidized Ceylon graphite included C:O:H contents as 67.79:30.37:1.84 and the C-to-O ratio was 2.23. Therefore, the material was termed graphic acid, and was the very first sample of graphite oxide prepared experimentally. (2) Staudenmaier method (1898)  , this method is very similar to Brodie's. The graphite oxide is prepared in a mixture of concentrated sulfuric acid and fuming nitric acid. In addition, potassium chlorate oxidizing agent is also added and reacted over 4 days. By rinsing in water and dispersing in diluted hydrochloric acid, sulfonate ions were removed. Finally, the graphite oxide was dried at 60 • C for 2 days. The graphite oxide prepared by this method was found to have an elemental composition of C:O:H of 58.73:23.28:17.99. The C-to-O ratio was 2.52, which indicated the lowest degree of oxidation; (3) The Hummers and Offeman method (1958)  was developed when it was realized that the usage of nitric acid requires a lot of time for oxidizing graphite, has the potential for explosion and the release of highly corrosive vapor. The Hummers and Offeman method is a less hazardous way to oxidize graphite. The oxidizing agent is a mixture of concentrated sulfuric acid, sodium nitrate and potassium permanganate. The entire process needs 1-2 h to complete the reaction. As a result, the graphitic oxide had a C-to-O ratio between 2.1-2.9. The color of the product in aqueous solution is referred to the degree of oxidation. The product gives a bright yellow color for the most oxidized graphite while the green to black color refers to poor graphitic oxidation having too high C-to-O ratios. Currently, the Hummers and Offeman method is the most commonly used and is commonly known as the Hummers method. As a result of the oxidation of graphite there is an increase in the interlayer spacing. This means that the separation between graphene interlayers is expanded by the intercalation of oxidized contents or the polar liquids such as sodium hydroxide  as seen in solid-state 13 C-NMR BIB006 . The interlayer distance of 0.335 nm in pristine graphite increases to 0.562 nm after 1 h of oxidation and increases further to 0.737 nm after 24 h of oxidation. It is suggested that the carboxyl and alkyl groups are mostly at the edges of the GpO layers. By treatment via mechanical stirring or ultrasonication, the GpO can be exfoliated to GO easily with the same oxide contents. Moreover, the Hummers method can be improved by making a small modification BIB010 . The amount of potassium permanganate was doubled and the sodium nitrate was replaced by phosphoric acid in a proportion to sulfuric acid of (1:9). The improved method leads to a large amount of hydrophilic GpO being made compared to the original Hummers method. It highlights the fact that the oxidizing agents interact more with the basal planes of graphite. A high quality of GpO would promote the electrical conductivity after it was converted to graphene. Many findings make it possible to utilize GO and further composites BIB004 BIB008 . Understanding of the GpO structure is needed in order to develop modifications which would improve the mechanisms in any applications. Primarily, the GpO is composed of C, O and H atoms where the C-to-O ratio for complete oxidation is in a range of 1.5-2.5 BIB012 . To model the GpO structure, many research groups have debated and determined the precise structure using complex and advance techniques. Seven proposed structures have been presented BIB002 BIB011 BIB003 BIB009 . The first model was proposed by Hofmann and Hoist in 1939 [81] . The basal plane of graphite consists of 1,2-epoxide (C-O-C) groups across the sp 2 -hybridized carbon structure. The molecular structure is formulated as C 2 O. In 1947, Ruess  proposed a second model made up of a sp 3 -hybridized carbon structure as the basal plane whose surface is decorated with the hydroxyl (C-OH) groups. 1,3-Epoxides were placed at 1/4th of the cyclohexanes. Scholz and Boehm  suggested their model in 1969 where the epoxide and ether groups are absent in the carbon structure. They were replaced by ketone and hydroxyl groups by using regular quinoidal bases. In 1988, Nakajima and Matsuo BIB001 proposed a structural model which was similar to that of the 2-type graphite fluoride (C 2 F) n . The model showed hydroxylated graphite intercalation along the c-axis. There were carbonyl (C=O) and hydroxyl (C-OH) groups which were bonded to the basal carbon structure. The molecular formula was intermediate between C 8 (OH) BIB013 and C 8 O 2 with C 0 = 8.22 × 2 Å and C 0 = 5.52 × 2 Å, respectively. The increase in the dehydration was influential in decreasing the distance along the c-axis. This model was deduced from the elemental analysis, X-ray diffraction and chemical reactivity studies. The most well-known and widely used model is the one proposed by Lerf and Klinowski in 1996 BIB002 BIB003 . Their model was derived from the 13 C and 1 H magnetic-angle spinning nuclear magnetic resonance (NMR) data of GpO. The evidence suggested aliphatic 6-membered rings containing hydroxyl (C-OH), epoxide (C-O-C) and double-bonded carbon (>C=C<) which indicated a basal plane structure made up of aromatic islands. The basal carbon structure was not only separated by the C-OH attachments leading to wrapping, but the graphite oxide structure was reconfigured to be tetrahedral. This leads to the strong interaction with water molecules. In this model, the 1,2 epoxide and hydroxyl groups are at the basal plane while the carboxyl and hydroxyl groups are mostly at the edge of the plane. Observations  supported the model of Lerf and Klinowski, where (1) the sp 2 hybridized structure is found as a core of the graphite oxide sheet; (2) holes, a kind of defect, are present in the structure; (3) C-OH, 1,2-C-O-C groups are doped onto the structure; (4) the C=O groups are found along the edge of the holes; and (5) the oxidized regions are randomly located and no superstructures are formed. Another model was proposed by Dékány BIB005 as recently as 2006. X-ray photoemission spectroscopy (XPS), diffused reflectance infrared Fourier transform spectroscopy (FTIR), electron spin resonance (ESR), TEM, XRD and 13 C-MAS NMR were used to reveal the structural details. The Dékány model was built upon the Ruess and Scholz-Boehm models. The graphite oxide contained trans-linked cyclohexyl chairs, having 1,3-epoxides and tertiary hydroxyls, and corrugated networks of keto/quinoidal groups. In addition, phenolic groups were inserted into this model to explain the acidity of graphite oxide. Two years later in 2008, Ruoff and colleagues BIB007 synthesized and determined the structural characteristics of 13 C-labelled graphite oxide. Using 13 C-MAS NMR analysis, hydroxyl and epoxide groups were found bonded to the carbon structure. Also, the carbonyl groups were found mostly bonded to the edge of defects. The carbon structure had non-protonated functionality as detected near 100 ppm. This was narrowed down to the possible structure of graphite detected by NMR. In 2009, Gao and Ajayan BIB009 offered new insights into the graphite oxide structure. The Ajayan model suggested that 5-and 6-membered-ring lactols at a level of 100 ppm were detected in the epoxide), the 3 (lactol, O-C-O), the 63 (sp 2 hybridized carbon), the 10 (lactol:ester:acid carbonyl) and 9 (ketone carbonyl). Moreover, the GpO and GO are seen to have the acidic properties due to the ketones at the edge of carbon plane in a presence of incompletely hydrolyzed covalent sulphates, containing salts or esters of sulfuric acid. In contrast to that of usual acid properties, it originates from the presence of carboxyl groups. In fact, the acidic functional groups, i.e., vinylogous acid, on GpO and GO are originated gradually due to the interaction with water. This points out the instability of the oxide functional groups in the presence of water, leading to constantly evolving GpO and GO structures. Therefore the latest model has been proposed in terms of the dynamic-like structure.