Cyanographene and Graphene Acid: Emerging Derivatives Enabling High-Yield and Selective Functionalization of Graphene.

Efficient and selective methods for covalent derivatization of graphene are needed because they enable tuning of graphene's surface and electronic properties, thus expanding its application potential. However, existing approaches based mainly on chemistry of graphene and graphene oxide achieve only limited level of functionalization due to chemical inertness of the surface and nonselective simultaneous attachment of different functional groups, respectively. Here we present a conceptually different route based on synthesis of cyanographene via the controllable substitution and defluorination of fluorographene. The highly conductive and hydrophilic cyanographene allows exploiting the complex chemistry of -CN groups toward a broad scale of graphene derivatives with very high functionalization degree. The consequent hydrolysis of cyanographene results in graphene acid, a 2D carboxylic acid with pKa of 5.2, showing excellent biocompatibility, conductivity and dispersibility in water and 3D supramolecular assemblies after drying. Further, the carboxyl groups enable simple, tailored and widely accessible 2D chemistry onto graphene, as demonstrated via the covalent conjugation with a diamine, an aminothiol and an aminoalcohol. The developed methodology represents the most controllable, universal and easy to use approach toward a broad set of 2D materials through consequent chemistries on cyanographene and on the prepared carboxy-, amino-, sulphydryl-, and hydroxy- graphenes.

Graphene[1] is a two-dimensional (2D) n class="Chemical">carbon allotrope[2] with many potential applications in (opto)electronic systems (including systems for DNA sequencing,[3,4] spintronics,[5] electrochemical energy storage[6] and sensing[7]), coating technologies,[8,9] and composites.[10] Its potential range of applications can be extended[11,12] by covalent[12−14] or heteroatom functionalization[15] because introducing functional groups onto the graphene surface enables modulation of its electronic,[16−18] magnetic,[19,20] optical[21] and surface properties.[22] However, direct covalent functionalization is limited by graphene’s low reactivity,[17,23] and usage of rather severe reaction conditions in order to achieve graphene derivatization often restricts the control over the product’s structure and composition. For instance, the widely used derivative graphene oxide is prepared by harsh oxidation, and the oxidation conditions profoundly affect its stoichiometry, structure and properties without selectivity.[24] Consequently, there is a need for new strategies that permit selective and high yielding graphene functionalization under more controlled conditions.

Fluorographene (FG),[18,25,26] which can be prepared by fluorination of n class="Chemical">graphene followed by exfoliation, is a stable, stoichiometric and well-defined graphene derivative.[27] Fluorination of graphene can be achieved through reactions with fluoropolymers, thus avoiding fluorine gas.[28,29] Because it is a perfluorinated hydrocarbon, it was expected to be unreactive and unsuitable for further derivatization (such as Teflon[25]). Nevertheless, it was recently shown that fluorographene can react as an electrophile under mild conditions[7,30−34] allowing it to function as a starting material for the synthesis of graphene derivatives that cannot be obtained by direct functionalization of graphene itself. This approach could potentially circumvent the longstanding problem of achieving selective and controllable covalent graphene functionalization.[14] To validate this strategy for preparing graphene derivatives, it will be necessary to show that FG functionalization is superior to direct functionalization of graphene in terms of versatility and the yield of attached functional groups.

The introduction of reactive chemical moieties that are homogeneously distributed over the graphene surface would be highly desirable because it would enable facile immobilization of other guest molecules on n class="Chemical">graphene. Carboxyl functionalities are particularly attractive because they readily undergo diverse conjugation reactions and increase the material’s hydrophilicity. However, direct and exclusive attachment of −COOH groups to a graphene surface is very challenging. It should be noted that while graphene oxide does contain carboxyl groups, they are attached to edges and defects, and their content is low (or even absent).[24] Moreover, they are accompanied by many other oxygen-containing groups. Here we present a two-step process, whereby FG is transformed into the fluorine-free cyanographene (G-CN, or graphene-nitrile) and subsequently hydrolyzed to graphene acid (G-COOH). Both G-CN and G-COOH exhibit a high degree of functionalization (13–15%). Moreover, functionalization is homogeneous and selective (i.e., no other chemical groups are formed) and the titration profile of G-COOH closely resembles that of molecular organic acids, with pKa of 5.2. As such, it can be regarded as a two-dimensional acid. It has many extraordinary properties, including excellent colloidal stability, biocompatibility, and high conductivity. Its well-defined structure and the high quality of its aqueous dispersions are also demonstrated by the fact that it forms 3D supramolecular lattices upon drying, similar to those formed by large polyaromatic hydrocarbon nanoflakes.[35] Moreover, the carboxyl groups are amenable to conjugation, enabling covalent attachment of diverse chemical moieties to the graphene surface, which we demonstrate using three different primary amines. The functionalization strategy presented here thus offers unexplored opportunities for controllable graphene functionalization and the further development of graphene chemistry.

Results and Discussion

Synthesis of Cyanographene

The reaction of NaCN with FG in n class="Chemical">DMF resulted in a high-yielding nucleophilic substitution of fluorine atoms by C≡N groups and the formation of fluorine-free G-CN. This is demonstrated by FT-IR spectra (Figure a) acquired during the course of the reaction, which show the C≡N band at 2200 cm–1 gradually becoming stronger at the expense of the C–F vibrations (∼1200 cm–1). This process is accompanied by reductive defluorination, whereby some F atoms are eliminated and the corresponding sp3 carbons become sp2 hybridized, forming C=C bonds.[36] The FT-IR fingerprint of G-CN at 1500–1600 cm–1 becomes more intense as the reaction proceeds (Figure a, v–viii) and comes to resemble that of graphite[37] (Figure a, ix), demonstrating the formation of an sp2 network. In addition, high-resolution XPS and survey spectra of FG and G-CN demonstrate that fewer than 1% of fluorine remains in the G-CN product (Figure b and Figure S1). The presence of the C=C network, suggested by the FT-IR spectra is further supported by the sp2 component at 284.8 eV in XPS (Figure b, (ii) and by the G band in its Raman spectrum (Figure S2b and S3). The ID/IG ratio of G-CN is ∼1.2; together with the broadening of its bands, this indicates that it is a highly functionalized graphene derivative.[38,39] Deconvolution of the HR-XPS revealed that its content of graphitic sp2 carbon atoms is 41 at. % with respect to total atoms (Figure b). Atomic composition analysis based on the HR-XPS data (see Figure S1c) indicate that G-CN contains 11.4 at. % of N (and thus −CN moieties), corresponding to a functionalization degree (F.D.) of 15% (or 24 wt %). The atomic content of CN amounting to 11.4% is consistent with the 12 at. % of sp3 lattice carbons, based on the deconvolution of its HR-XPS C 1s data (the 286 eV band was ascribed to sp3 C atoms bound to −CN groups, Figure b). TGA analysis of G-CN (Figure S4) corroborates the XPS results showing an overall 28% mass loss. The somewhat higher mass-loss than the 24 wt % −CN content from XPS, could be ascribed to additional losses due to defect sites. Bright field HR-TEM (Figure c,d) and AFM (Figures S5, and S6) experiments established the few-layer character of the G-CN flakes. Finally, elemental mapping by energy dispersive spectroscopy (EDS) revealed that G-CN is densely and homogeneously functionalized because the carbon (Figure f) and nitrogen (Figure g) maps overlap nicely. Taken together, these results clearly show that the reaction of NaCN with GF enables a facile and high-yield synthesis of graphene nitrile or cyanographene. In view of the fully defluorinated nature of the product (F content below 1 at. %, Figure S1c), G-CN is considered as a graphene derivative which is homogeneously decorated with nitrile groups. Its decoration with reactive organic moieties makes G-CN a versatile starting material for further functionalization of the graphene surface.

Figure 1

Chemical and structural characterization of cyanographene (G-CN). (a) FT-IR spectra of pristine FG (i), reaction intermediates during the synthesis of G-CN (ii–viii), and commercial graphite (ix). The intermediates (spectra ii–viii) were isolated 30, 35, 40, 45, 50, 55, and 60 min, respectively, after the start of the reaction. The inset shows a structural model of the starting FG. (b) Deconvoluted C 1s HR-XPS of pristine FG (i) and the G-CN product (ii). The inset depicts a structural model of G-CN. (c) HR-TEM image of a few-layered G-CN flake. (d) Magnification of the previous flake showing the presence of three steps in the contrast of the flake, suggesting that the flake consists of three graphene sheets. (e) Dark field HR-TEM image of a G-CN flake used for EDS chemical mapping; (f) carbon map and (g) nitrogen map of the G-CN flake shown in e.

Chemical and structural characterization of cyanographene (n class="Chemical">G-CN). (a) FT-IR spectra of pristine FG (i), reaction intermediates during the synthesis of G-CN (ii–viii), and commercial graphite (ix). The intermediates (spectra ii–viii) were isolated 30, 35, 40, 45, 50, 55, and 60 min, respectively, after the start of the reaction. The inset shows a structural model of the starting FG. (b) Deconvoluted C 1s HR-XPS of pristine FG (i) and the G-CN product (ii). The inset depicts a structural model of G-CN. (c) HR-TEM image of a few-layered G-CN flake. (d) Magnification of the previous flake showing the presence of three steps in the contrast of the flake, suggesting that the flake consists of three graphene sheets. (e) Dark field HR-TEM image of a G-CN flake used for EDS chemical mapping; (f) carbon map and (g) nitrogen map of the G-CN flake shown in e.

Synthesis of Graphene Acid

In the second step of the synthesis, G-CN was subjected to acid hydrolysis with 20% n class="Chemical">HNO3 to transform its −CN groups into −COOH groups. The successful synthesis and isolation of G-COOH is demonstrated clearly by the FT-IR spectra of the intermediates of the reaction and the final product, which show the gradual disappearance of the —C≡N band and the emergence of a new band at 1725 cm–1 corresponding to the carboxylic groups (Figure a). The assignment of this band to carboxylic groups was validated by recording the FT-IR spectrum of the sodium salt of G-COOH (see Figure S7). In order to exclude any oxidizing action of the 20% HNO3 on the carbon lattice of G-CN, control experiments were performed on FG and graphite by subjecting them to identical acidic treatment. The FT-IR spectra in Figure S2c underline the dramatic differences regarding the COOH presence. The product’s C 1s HR-XPS spectrum features a new component at 288.7 eV corresponding to the carboxylic carbons (O—C=O, Figure b). On the basis of the atomic composition analysis showing that O—C=O carbons accounted for 9.3 at. % (see legend of Figure S1c,), the stoichiometry of G-COOH was estimated to be C6.6(COOH)1. This corresponds to an F.D. of 13%, which (like the value of 15% determined for G-CN) is substantially greater than the F.D. for any previously reported covalently modified graphene derivative (see Table S1). Deconvolution of the C 1s HR-XPS data for G-COOH indicated an sp2 carbon content of 44 at. % (based on the sp2 band at 284.8 eV, Figure b), suggesting the dominant presence of graphitic network. The presence of such a network is further supported by FT-IR bands in the region of 1400–1620 cm–1 (Figure a) and the presence of a G-band in the material’s Raman spectra (Figure S3a). In contrast to the almost featureless Raman spectrum of FG, that of G-COOH has significant and broad G and D bands with an ID/IG ratio of ∼1.1. The broadening of the bands and this high ID/IG ratio both indicate a very high F.D.,[13,38,39] as with G-CN. The ID/IG ratios in the center (1.09) and at the edges (1.15) of the flakes, determined by Raman spectroscopy coupled with AFM, are very similar; together with the similar broadening of the Raman bands, this indicates a homogeneous distribution of functional groups over the G-COOH surface (Figure S3). Thermogravimetric analysis in nitrogen (Figure S8) indicates that G-COOH is thermally stable up to 200 °C. Up to 800 °C, the release of COOH groups occurs as suggested by the evolved gas analysis (EGA), identifying CO2 and H2O as the dominant species released. The 30% mass loss up to 800 °C is consistent with the XPS results discussed above, because 9.3 at. % content of carboxylic carbons corresponds to 32 wt % content of carboxylic groups. A secondary decrease in mass occurred above 800 °C; the EGA indicated that this was due to reactions with evolved oxygen leading to oxidative decomposition of the carbon lattice (Figure S8). HR-TEM images showed a highly transparent sheet of G-COOH (Figure c) with lateral dimensions of around 200 nm; AFM images (Figure S5b) confirmed that it retains the few-layered structure of G-CN. Interestingly, imaging of G-COOH single sheets by AFM was complicated by the material’s tendency to organize into 3D networks stabilized by interlayer hydrogen-bonds (see below). EDS mapping confirmed that the carboxyl groups’ oxygen atoms were homogeneously distributed over the graphene surface (Figure e,f). Together, these analyses indicate that the CN groups of G-CN were successfully hydrolyzed to −COOH groups, resulting in the synthesis of a two-dimensional acid. These findings also validate the proposed mechanism whereby the fluorines of FG are exchanged for nitriles by nucleophilic substitution, and the nitriles are then transformed into carboxylic groups in a very high yield of 86%. The efficient and homogeneous decoration of the graphene surface with carboxyl groups should enable conjugation with a wide variety of functional molecules, greatly expanding the 2D chemistry of graphene and enabling the synthesis of tailored derivatives.

Figure 2

Chemical and structural characterization of graphene acid (G-COOH). (a) FT-IR spectra recorded during acid hydrolysis of G-CN, showing products obtained after (i) 3 h treatment with 5% HNO3 at RT, (ii) 12 h treatment with 5% HNO3 at 80 °C, (iii) 12 h treatment with 10% HNO3 at 80 °C, (iv) 24 h treatment with 10% HNO3 at 100 °C and (v) 24 h treatment with 20% HNO3 at 100 °C. (b) Deconvoluted C 1s HR-XPS spectra of G-COOH. The inset shows a structural model of G-COOH. (c) HR-TEM image showing the flake structure of G-COOH. (d) Dark field HR-TEM image of a G-COOH flake used for EDS chemical mapping; (e) carbon and (f) oxygen maps of the G-COOH flake shown in d.

Chemical and structural characterization of graphene acid (n class="Chemical">G-COOH). (a) FT-IR spectra recorded during acid hydrolysis of G-CN, showing products obtained after (i) 3 h treatment with 5% HNO3 at RT, (ii) 12 h treatment with 5% HNO3 at 80 °C, (iii) 12 h treatment with 10% HNO3 at 80 °C, (iv) 24 h treatment with 10% HNO3 at 100 °C and (v) 24 h treatment with 20% HNO3 at 100 °C. (b) Deconvoluted C 1s HR-XPS spectra of G-COOH. The inset shows a structural model of G-COOH. (c) HR-TEM image showing the flake structure of G-COOH. (d) Dark field HR-TEM image of a G-COOH flake used for EDS chemical mapping; (e) carbon and (f) oxygen maps of the G-COOH flake shown in d.

To confirm that the carboxylic acid groups of n class="Chemical">G-COOH permit further functionalization of the surface, we used carbodiimide chemistry to conjugate G-COOH samples with three different primary amines—the cysteamine (NH2–C2H4–SH), the aminoalcohol 2-(2-aminoethoxy)ethanol (H2N–C2H4–O–C2H4–OH), and the diamine ethylenedioxy-bis(ethylamine) (H2N–C2H4–O–C2H4–O–C2H4–NH2)—via amide bond formation (Figure a). The FT-IR spectra of the materials obtained after conjugation featured aliphatic and C–O bands at ∼2900 cm–1 and ∼1050 cm–1, respectively (Figure b, spectra ii and iii), which were not present in the G-COOH starting material (Figure b, (i)). Successful conjugation and amide bond formation was demonstrated by the appearance of a new band at 1660 cm–1 (inset of Figure b). These reactions had interesting macroscopic effects on G-COOH. Conjugation with the diamine caused flocculation and the formation of chunks of black material immiscible with water, while conjugation with the aminoalcohol yielded a highly water-dispersible adduct similar to the starting G-COOH (see inset photograph in Figure b). The different behavior of the diamine-conjugated G-COOH may be due to cross-linking of the G-COOH flakes. The cysteamine conjugate was characterized by XPS (Figure c,d) revealing that its sulfur content was 5.3 at. %, suggesting conjugation yield of 73% (each conjugation reaction replaces a −COOH group with a −C(O)NH–CH2CH2–SH moiety, increasing the number of detected atoms by three). EDS sulfur mapping showed that the conjugation occurred evenly across the graphene surface (Figure e–g) again confirming that the carboxyl groups of the starting material were homogeneously distributed. These results clearly show that the carboxylic acid moieties of graphene acid readily undergo conjugation with diverse molecules, considerably expanding the chemistry of graphene.

Figure 3

Expanding the chemistry of graphene through chemical derivatization of graphene-acid. (a) Schematic depiction of the conjugation of three primary amines with G-COOH in amine-free dimethylformamide (DMF). 1-Ethyl-3-(3-(dimethylamino)propyl)carbodiimide (EDC) and ethyl 2-cyano-2-(hydroxyimino)acetate (oxyma) were used as conjugation reagents. (b) FT-IR spectra of (i) the starting G-COOH, (ii) G-COOH conjugated with the diamine and (iii) G-COOH conjugated with the aminoalcohol. The left inset presents a selected wavelength region of the FT-IR spectra of the three conjugates, showing the suppression of the carboxyl vibration and emergence of the amide band. The right inset shows the dispersibility of (i) G-COOH, (ii) diamine-cross-linked G-COOH and (iii) aminoalcohol-conjugated G-COOH, in a biphasic chloroform:water system. (c,d) Survey and HR-XPS of the G-COOH after conjugation with cysteamine (NH2–CH2CH2–SH). The amide bond (N—C=O) at 287.8 eV is readily apparent. (e) Dark field HR-TEM image of an aminothiol-conjugated G-COOH sample that was analyzed by EDS chemical mapping. (f) Carbon and (g) sulfur maps of the G-COOH flake shown in e.

Expanding the chemistry of graphene through chemical derivatization of n class="Chemical">graphene-acid. (a) Schematic depiction of the conjugation of three primary amines with G-COOH in amine-free dimethylformamide (DMF). 1-Ethyl-3-(3-(dimethylamino)propyl)carbodiimide (EDC) and ethyl 2-cyano-2-(hydroxyimino)acetate (oxyma) were used as conjugation reagents. (b) FT-IR spectra of (i) the starting G-COOH, (ii) G-COOH conjugated with the diamine and (iii) G-COOH conjugated with the aminoalcohol. The left inset presents a selected wavelength region of the FT-IR spectra of the three conjugates, showing the suppression of the carboxyl vibration and emergence of the amide band. The right inset shows the dispersibility of (i) G-COOH, (ii) diamine-cross-linked G-COOH and (iii) aminoalcohol-conjugated G-COOH, in a biphasic chloroform:water system. (c,d) Survey and HR-XPS of the G-COOH after conjugation with cysteamine (NH2–CH2CH2–SH). The amide bond (N—C=O) at 287.8 eV is readily apparent. (e) Dark field HR-TEM image of an aminothiol-conjugated G-COOH sample that was analyzed by EDS chemical mapping. (f) Carbon and (g) sulfur maps of the G-COOH flake shown in e.

Physicochemical Characterization and Modeling

The results presented above show that graphene acid is a suitable platform for diverse 2D chemistry, with unexplored physical, chemical and biological properties. Pristine FG is a hydrophobic material that is totally immiscible with n class="Chemical">water (Figure S9a), and FG heat-treated in DMF (control sample) precipitates in water within seconds (Figure b, (i)). Conversely, G-COOH forms aqueous colloidal dispersions at high concentrations (at least ∼6 mg mL–1), that remain stable even after standing for 48 h (Figure b, (ii)) and are optically clear (turbidity free), as shown after dilution in Figure b, iii. These stark differences are clear macroscopic manifestations of the material’s high degree of COOH-functionalization, which completely inverts the hydrophobic nature of pristine FG (Figure S9a,b). The acid–base properties of G-COOH are shown in Figure S9c. As the pH decreases, the negative ζp of G-COOH (∼ −32 mV) drops to less than −10 mV due to protonation of the carboxyl groups. Moreover, the material’s hydrodynamic diameter of around 200 nm (Figure b) gradually increases until extensive aggregation leads to its precipitation around pH = 2.5 (Figure S9c and Figure e). Importantly, the titration curve of G-COOH (Figure c) resembles those of ordinary molecular carboxylic acids; this stands in striking contrast to GO, which has a poorly defined titration curve,[40] because of its high chemical complexity and varied surface chemical functionalization.[24] Therefore, the pattern of the titration curve of G-COOH with pKa of 5.2 is another evidence of its well-defined chemistry and acid–base properties.

Figure 4

Physichochemical properties of graphene acid. (a) The binding energy Ead of −COOH groups to graphene, as a function of the graphene’s −COOH content. Open circles correspond to higher energy metastable structures, full circles to ground state (GS) structures. (b) (i) Image of the control FG solid (heat-treated in DMF) dispersed in H2O, which completely precipitates. (ii) Image of the stable colloidal dispersion of the G-COOH in H2O at pH = 8. (iii) Image of the optically clear dispersion formed by diluting the colloid shown in the previous inset. (iv) Hydrodynamic diameter distribution of G-COOH in H2O at pH = 8. (c) Titration of G-COOH with a 0.1 M NaOH standard solution. (d) CV curves of a bare GCE electrode (black line) and GCE electrodes modified with GO (orange line) or with G-COOH (green line). The CV curve of G-COOH is symmetric and scan-rate independent, indicating reversible behavior with no parallel chemical reactions. (e) Structures from molecular dynamics simulations of protonated and 50% deprotonated G-COOH in water at low and high pH values, showing their spontaneous agglomeration and exfoliation, respectively. For clarity, the water molecules surrounding the sheets are not shown.

Physichochemical properties of graphene acid. (a) The binding energy Ead of −n class="Chemical">COOH groups to graphene, as a function of the graphene’s −COOH content. Open circles correspond to higher energy metastable structures, full circles to ground state (GS) structures. (b) (i) Image of the control FG solid (heat-treated in DMF) dispersed in H2O, which completely precipitates. (ii) Image of the stable colloidal dispersion of the G-COOH in H2O at pH = 8. (iii) Image of the optically clear dispersion formed by diluting the colloid shown in the previous inset. (iv) Hydrodynamic diameter distribution of G-COOH in H2O at pH = 8. (c) Titration of G-COOH with a 0.1 M NaOH standard solution. (d) CV curves of a bare GCE electrode (black line) and GCE electrodes modified with GO (orange line) or with G-COOH (green line). The CV curve of G-COOH is symmetric and scan-rate independent, indicating reversible behavior with no parallel chemical reactions. (e) Structures from molecular dynamics simulations of protonated and 50% deprotonated G-COOH in water at low and high pH values, showing their spontaneous agglomeration and exfoliation, respectively. For clarity, the water molecules surrounding the sheets are not shown.

To better understand the effects of carboxyl functionalization on graphene’s properties, we performed DFT calculations to estimate the thermodynamic stabilities of surface n class="Chemical">carboxylated graphenes C(COOH) with x/y ratios of up to the experimental value of 6.6 (cf. Supporting Information for details). Most of the structures were thermodynamically stable (with Ead < 0) and the energy differences between individual arrangements were small (Figure a). This implies that the topology of the carboxyl groups across the surface is flexible, with many different local arrangements being thermodynamically accessible and will be given by a statistical distribution. However, we observed that the carboxyl groups tend to form chains over the surface, despite the fact that they do not form intralayer hydrogen bond (H-bond) networks. Molecular dynamics simulations corroborated the tendency of −COOH groups to form interlayer H-bonds when the pH is below the system’s pKa (Figure e) and the carboxylate groups are protonated. Both the experimental results (Figure b and Figure S9b) and the MD simulations (Figure e) suggested that G-COOH suspensions only become exfoliated when the pH exceeds the acid’s pKa. This tendency for interlayer H-bond formation also explains the experimentally observed formation of spherulitic supramolecular G-COOH crystals (Figure S10) when dispersions of the material are dried.

The valence and conduction bands of the probable G-COOH structures were separated by a band gap smaller than that of FG and midgap states were identified in some cases, depending on the −n class="Chemical">COOH content (Figure and Figure S17). Importantly, many arrangements led to significantly reduced band gap in the electronic structure with a quite pronounced density of states near EF indicating appreciable conductivity. In line with our findings, the reduction of the electronic gap due to both the functionalization and the structural distortion has been reported for edge functionalized armchair graphene nanoflakes.[41] The calculations revealed that in some cases the spin-up and spin-down states were split by exchange interaction indicating on a possibility of inducing magnetism in some G-COOH structures. The emergence of magnetism was recently confirmed for graphene covalently functionalized by −F and −OH groups.[42] The magnetic properties of graphene-based structures were triggered by a sublattice imbalance of the graphene bipartite lattice caused by changes of its sp2 hybrid states to sp3 by the covalent functionalization and suitable exchange interaction. The functionalized graphene can be, hence, considered as sp2 graphene with sp3 structural defects or vice versa, depending on the functionalization degree. Electronic and magnetic properties of the functionalized graphenes are given by the functionalization degree, nature of the functional group, arrangement of functionalities and sublattice symmetry.[42−46] For the sake of completeness it should be noted that lattice imperfections, including impurities, defects, atomic vacancies and structural distortion naturally occur in sheets of functionalized graphenes and they alter the electronic properties of experimental samples.[41,43−45,47] Because the observable electronic structure of G-COOH will be the result of statistical averaging over structures and arrangements having both metallic and semiconducting properties, it should have noticeable conductivity. For this reason, four-probe resistivity, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed in order to evaluate the conductivity of G-COOH. The sheet resistance of a G-COOH film amounted to 6800 Ω sq–1 (conductivity ∼25 S m–1; ∼500 S m–1 for G-CN), in stark contrast to that measured for GO, with 5 orders of magnitude higher sheet resistance (Rs = 2174 × 106 Ω sq–1). Despite the very high F.D. of G-COOH (>10%), its conductivity value is similar to conductivities observed for conventional covalent graphene derivatives (i.e., 5.13 S m–1 for a carbene functionalized graphene with 3% F.D.[48]), further supporting the high quality derivatives obtained from the herein proposed method. CV results also highlight the conductive nature of G-COOH showing very high current response, in antithesis to the insulating nature of starting FG[49] and GO[50] (Figure d). EIS corroborates the conductivity measurements attributing 40-times lower resistivity to G-COOH (Rct = 81 Ω) with respect to GO (Rct = 3542 Ω) (Figure S11a).

Figure 5

Geometrical (left column) and electronic (right column) structures of FG and its derivatives, as predicted by DFT calculations. The energetically most stable structures of (a) FG, (b) G-CN, and (c) G-COOH are depicted. The left-hand images show the DOS profiles for the three materials; for G-COOH DOS profiles are calculated assuming degrees of functionalization of 12.5% (red) and 14.6% (gray), respectively (DOS for more functionalization degrees are shown in Figure S17). In all cases the energies are zeroed to the Fermi level.

Geometrical (left column) and electronic (right column) structures of FG and its derivatives, as predicted by DFT calculations. The energetically most stable structures of (a) FG, (b) G-CN, and (c) n class="Chemical">G-COOH are depicted. The left-hand images show the DOS profiles for the three materials; for G-COOH DOS profiles are calculated assuming degrees of functionalization of 12.5% (red) and 14.6% (gray), respectively (DOS for more functionalization degrees are shown in Figure S17). In all cases the energies are zeroed to the Fermi level.

The potential applicability of G-COOH is further strengthened by its low n class="Disease">toxicity and high biocompatibility, which were demonstrated by flow cytometry viability tests, reactive oxygen species (ROS) analysis and kinetic measurements of ROS production after incubating NIH 3T3 and HeLa cells for 24 h with various concentrations of G-COOH. G-COOH had no or minimal effects on cell viability at any tested concentration (Figure S12a,b). ROS generation is a common toxicity mechanism of carbon-based and other nanoscale materials.[51] However, the kinetic ROS production induced by G-COOH exposure (Figure S12c and S13), was substantially lower than that induced by other graphene-based materials,[52,53] showing that G-COOH has little impact on mitochondrial ROS generation.

Conclusion

We present cyanographene, a n class="Chemical">graphene derivative enabling to perform a complex 2D chemistry and high yield covalent functionalization of graphene. The selective and high-yielding nucleophilic substitution of fluoride ions in fluorographene by −CN is accompanied by reductive defluorination that partially re-establishes the delocalized π-electron cloud and thus conductivity of the cyanographene. In the following step, a relatively mild acid hydrolysis selectively transforms −CN to −COOH, whereby the conductivity of the material is retained. Therefore, the harsh oxidation conditions used to introduce oxygen-containing functional groups to graphene during graphene oxide synthesis are bypassed. As a conductive solid-state 2D carboxylic acid, graphene acid may have many electrochemical applications in sensing,[7] batteries[54] and proton-conducting membranes.[55] Moreover, graphene acid forms highly stable and biocompatible aqueous colloids and its carboxylic acid groups readily undergo conjugation. These features extend the portfolio of potential derivatives and predispose this material for further applications such as in optics[56] and theranostics.[22] The synthesis and isolation of G-COOH from cyanographene can be considered to bridge the gap between graphene and “graphene molecules” (derivatives of large polyaromatic hydrocarbons),[57,58] because its tendency to self-organize into 3D lattices of spherulitic morphologies and supramolecular arrangements is shared with graphene molecules.[34,58,59] Since ordered systems tend to exhibit greater charge-carrier mobility than disordered ones,[58,60] the ability of G-COOH to form supramolecular crystals, combined with the predicted tunability of its band gap, is another important enabling property that could be exploited in charge-transport applications.[35,58,59,61] Finally, the selective chemical functionalization of graphene acid renders it a superior platform for synthesis of a broad family of graphene derivatives compared to nonselective chemistry currently performed with graphene oxide.

Methods

Synthesis of G-CN, G-COOH and Derivatives with Primary Amines

G-CN

Fluorinated graphite (120 mg, ∼4 mmol of C-F units) was added to 15 mL of n class="Chemical">DMF and sonicated (Bandelin Sonorex, DT 255H type, frequency 35 kHz, power 640 W, effective power 160 W) for 4 h under nitrogen atmosphere in a 25 mL round-bottom glass flask. Then 800 mg of NaCN (∼16 mmol) was added and the mixture was heated at 130 °C with a condenser under stirring (500 rpm). Sample aliquots were withdrawn from the flask at different time points to monitor the reaction progress. Further experiments were performed on the product treated for 24 h. Intermediates and final product were left to cool to room temperature, after which an equal amount of acetone was added. The materials were then separated by centrifugation and further purified by successive washing steps using DMF, dichloromethane, acetone, ethanol and water (all 4×). Hot (80 °C) DMF and water was also used. More washing steps using DMF and water were applied if the conductivity of the supernatant aqueous fraction was higher than 200 μS cm–1. During the final centrifugation steps with water, it was necessary to apply centrifugal forces of up to 25 000 rcf to isolate the product. To obtain Na-free products, G-CN was washed with acidified water (pH = 4) to exchange sodium cations with H3O+ (the ζp of G-CN was determined to be in the range of −30 mV, therefore contained Na+ as counterions). After washing, the material was suspended in absolute ethanol, pure DMF or water depending on the purpose for which it was to be used.

G-COOH

HNO3 (65%) was slowly added at RT under stirring to a suspension of n class="Chemical">G-CN in water in a round-bottom glass flask, until the final concentration of HNO3 in the mixture reached 20%. The mixture was then heated at 100 °C under reflux with stirring (350 rpm) for 24 h. Various concentrations of HNO3 and treatment durations were tested to identify optimal conditions, and samples were periodically withdrawn to monitor the reaction’s progress. Intermediates and final products were left to cool to room temperature and then purified by washing with water through centrifugation. After a few washings, the product (graphene acid) stopped precipitating upon centrifugation. Therefore, acidic water (pH = 4) was used to protonate the material and reduce its dispersibility, inducing precipitation (as discussed in the manuscript). Alternatively, dialysis was also effective. Stable aqueous suspensions of G-COOH were prepared by adjusting the pH of the purified suspension to ∼8.

Conjugation of G-COOH with Primary Amine Molecules

G-COOH (15 mg) was washed with n class="Chemical">DMF (for synthesis) 3 times to remove ethanol or water residues and finally suspended in DMF (15 mL) in a three-neck round-bottom glass flask. EDC (222 mg, 1.16 mmol) and oxyma (165 mg, 1.16 mmol) were then added to the suspension, and the resulting mixture was stirred for 30 min at RT, under a nitrogen atmosphere. A primary amine [diamine: (2,2′-(ethylenedioxy)bis(ethylamine) (770 μL, 5.26 mmol) or aminoalcohol (2-(2-aminoethoxy)ethanol) (530 μL, 5.26 mmol)] was then added dropwise through a septum with a syringe and the reaction mixture was stirred for 120 h at RT. After that period, the mixture was diluted with DMF and the solid was collected by centrifugation. The product was purified by centrifugal washings with acetone, ethanol, and water. The diamine-functionalized product was also washed with dichloromethane because it proved to be compatible with organic solvents (see Figure b).

For the reaction with cysteamine, n class="Chemical">G-COOH (15 mg) was washed with DMSO 3 times to remove ethanol or water residues and finally suspended in DMSO (10 mL) in a glass round-bottom flask. EDC (222 mg, 1.16 mmol) and oxyma (165 mg, 1.16 mmol) were then added and the resulting mixture was stirred for 30 min at RT under a nitrogen atmosphere. Triethylamine (1.2 mL, 8.6 mmol) was also added to scavenge the protons from cysteamine and deprotonate it after which cysteamine hydrochloride (598 mg, 5.26 mmol) was added and the mixture was stirred for 72 h at RT. It was then diluted with acetone, after which the crude product was collected by centrifugation. The product was purified by centrifugal washing with acetone, ethanol, and water.

To ensure adequate purification of the washed conjugated products, their FT-IR spectra were recorded, and they were then subjected to the washing procedures described above once again. After this second washing, their FT-IR spectra were recorded and compared to those obtained after the first wash. This procedure was repeated until the spectra from the products after two successive washes exhibited no detectable differences in the relative intensities of their various bands. At this point the products were considered to be pure, with no residual noncovalently bound reagents.