Table 3.

The list of most suitable covalent coupling linkers for functionalization of graphene/rGo through covalent interactions reported in the literature.

Surface functional molecule for covalent functionalization	Carbon nanomaterial functionalization feature	Interaction	Reference
Free radical addition
Aryl diazonium salt	GO reduction in presence of a surfactant (SDBS) with hydrazine followed by treatment with aryl diazonium salts	Aryl radical formation from aryl diazonium ion by elimination of nitrogen and the radical aryl moiety interact/bind to graphene sp2 surface which donates an electron	[108]
4-nitrophenyl diazonium (NPD) tetrafluoroborate	Aryl groups grafting to epitaxial graphene via reduction of 4-nitrophenyl diazonium (NPD) tetrafluoroborate	Transfer of electron to the diazonium salt from the graphene layer and its substrate	[109]
4-nitrobenzene diazonium tetrafluoroborate	Graphene nanoribbons (GNRs) and chemical functionalization with diazonium salts by oxidative unzipping of carbon nanotubes	Chemical functionalization of 4-nitrophenyl on GNR through in-situ electrolytic reduction to alter the electrical properties	[110]
4-nitrobenzene diazonium tetrafluoroborate	A layer formed on epitaxial graphene with Sic substrate by spontaneous reduction of-nitrophenyl diazobium (4-NPD) tetrafluoroborate	Band gap was observed in covalent attachment sites on modified graphene	[111]
Electrografting of aryl diazonium salts	Electrochemical reduction of diazonium salt with a supportive electrolyte medium on carbon electrode	Electrode surface modified with phenyl radical through producing the electron transfer and facilitated by the separation of dinitrogen yielding phenyl radical on carbon substrate	[112]
(4-nitrophenyl) diazonium tetrafluoroborate (NDTB)	Electrochemical controlled functionalization of active aromatic radicals CVD grown graphene on SiO2	Selectively setting of electrochemical potential bias allow the ratio control and tuning electrical properties of graphene	[113]
bis(4-trifluoromethylphenyl) iodonium tetrafluoroborate [(CF3Ph)2I+BF4−]	Anchoring of CF3C6H4-moieties by electrochemical reduction of[(CF3Ph)2I+BF4−] on epitaxial graphene-SiC substrate	covalent bonding of trifluoromethylphenylene (CF3Ph) on graphene functionalization-induced defect density through rehybridization of sp2 to sp3 state and useful for transparent electrode and sensing application	[114]
Electrochemical oxidation of α-naphthylacetate for generatingradical addition of α-naphthylmethyl groups (Naph-CH2-)	Grafting α-naphthylmethyl radicals on epitaxial graphene /SiC by electrochemical oxidation of α-naphthylacetate	Preparation of electrochemically erasable organic dielectric film and reversible bandgap engineering in graphene	[115]
In situ generation of aryl diazonium salts by treatment of 4-methoxyaniline with isopentylnitrite	Surface/chemical multi-walled carbon nanotubes (MWCNT)	Controlled molar ratio of reactants can tune the chemical modification of MWCNTs	[116]
4-nitrophenyl diazonium tetrafluoroborate	Chemical modification of monolayered graphene supported on bare SiO2, self-assembled monolayer of octadecyltrichlorosilane (OTS) on SiO2, Al2O3 (sapphire), reactive imprint lithography on hexagonal boron nitride (hBN)	Covalent functionalization of graphene on SiO2 and Al2O3 with aryl diazonium salts was more reactive compared with hBN.	[117]
	Aryl functionalization on epitaxial graphene on SiC	Aryl-functionalized graphene network reorganize the π-bonds of graphene forming C − C additional bonds at the sp3 basal plane centers thereby introducing a band gap	[118]
	Covalently bonded phenyl substituent on single and multi-layers graphene on Si wafers	Chemical reactivity of graphene edges is greater as compared with single sheet of bulk graphene due to differences in electron transfer rates	[119]
	Selective functionalization graphene with high local curvature on aryl radical-SiO2 NPs with silicon substrate	Enhanced chemical reactivity in high local curvature of graphene facilitated by NPs – because of mechanical deformation and increases the strain energy	[120]
4-nitrobenzendiazonium (4-NBD) 4-bromobenzenediazonium (4-BBD) 4-methoxybenzendiazonium (4-MBD)	Fast functionalization reactions of diazonium salts with chemical reactive CVD graphene on polydimethylsiloxane (PDMS) stretching via applying external strain	Electronic structure alteration in graphene by strain induced distortion of lattice attributing to high reactivity rate to facilitate functionalization.	[121]
Benzoyl peroxide	Functionalization of benzoyl peroxide (a precursor of phenyl radical) on mechanically exfoliated single and double layer graphene on SiO2 under laser illumination	Reaction is initiated by an electron transfer from photo-excited graphene on field-effect-transistor (GFET) to benzoyl peroxide that physisorbs and photo-excited. The photo-excited species is then decomposed into a phenyl radical that readily reacts with graphene's sp2 carbon atoms enabling introduction of sp3 defect centers	[122]
In-situ diazonium addition reaction	Covalent polystyrene chains grafting on graphene nanoplatelets	Covalent grafting on graphene with hydroxylated aryl groups through addition reaction of diazonium that polymerizes styrene through atomic transfer radical polymerization (ATRP) process	[123]
Photopolymerization: •Styrene, •Methyl methacrylate (MMA),	Styrene based UV-induced polymerization on graphene; yielding layers of homogeneous polystyrene (PS) brush on epitaxial graphene-SiC substrate and CVD grown graphene on Cu	Self-organized PS brushes covalently bound to graphene with exceptional electrical transport properties and useful for ultra-capacitors and graphene-based biosensors	[124]
Photografting and photopolymerization (SIPGP) of monomers: •N,N-Dimethylaminoethyl methacrylate (DMAEMA)and •tert-Butyl methacrylate (tBMA)	Photopolymerization of CVD grown graphene transistor on PMMA that carry pH sensitivity with DMAEMA, whereas tBMA incorporates -COOH groups for immobilization of enzyme	Polymer brush functionalization scaffold produced on graphene transistor for biosensor development	[125]
Cycloaddition reactions
•Dienophiles tetracyanoethylene (TCNE) and •Maleic anhydride (MA)	Single layer and few-layers of graphene	Diels−Alder (DA) reaction with graphene	[99]
1,3-dipolar cyclo-addition of azomethine ylide.	Graphene by graphite in pyridine followed by reaction of N-methyl-glycine and 3,4-dihydroxybenzaldehyde	OH-functionalized graphene with better dispersibility	[126]
Benzyne species generation when of carbon reacts with o-trimethylsilylphenyl triflate (TMST) and cesium fluoride (CsF)	Benzyne functionalization on epitaxial graphene and generation of graphite by fluoride-induced elimination with TMS and triflate precursors	Benzene moieties on graphene generates fluorescence signal and useful for optoelectronic applications	[127]
2-(trimethylsilyl)aryl triate	Aryl-modified graphene sheets via aryne cycloaddition through a four-membered ring formation that attached to the graphene surface	Aryn ring attachment covalently to graphene surface and useful for tuning electric and magnetic properties of graphene	[128]
Azide compounds, e.g., •Az − OH, Az-COOH, Az-NH2, Az-Br, Az-C16, Az-PEG, Az-PS	Different functional moieties (−OH, −COOH, −NH2, and − Br) and polymers (PEG, PSS)	GO is used as a graphene precursor and the reduction of GO to graphene with simultaneous. The electrically conductive graphene showedexcellent dispersibility	[129]
Azidotrimethylsilane	Chemical modification of graphene (epitaxial) with azidotrimethylsilane (ATS)	Covalent bond formation between thermally generated nitrene radicals and epitaxial graphene	[130]
Hydrogenation
Cold hydrogen plasma	Graphane ‑hydrogenated graphene after exposing with the cold hydrogen plasma	Altered electronic properties graphene via attachment of sp2 carbons hydrogen atoms that changes their hybridization state to sp3	[100]
Lithium/ammonia solution	Hydrogen coverage on single-layer CVD-grown graphene on SiO2/S by Birch reduction reaction Liquid-based method	Hydrogenation reduced the electronic conductivity and transformed graphene into an insulator and the reduction reaction is reversible.	[101]
Microwave plasma enhanced chemical vapor deposition	Introducing hydrogen in few layers CVD graphene on SiO2/Si	Turning graphene as ferromagnetic semiconductor material by hydrogen functionalization	[131]
Epitaxial graphene by heating an Ir(111) crystal	Patterned adsorption of atomic hydrogen on graphene, which is grown on Ir(111) substrate	Bandgap opening of graphene through adsorption of H atom.	[132]
Dissociation of hydrogen silsesquioxane (HSQ) induced by electrons	Chemisorbed hydrogenation of single or bilayered graphene on Si/SiO2 and photothermal activation	Localized generation of reactive species can engineer charge transport properties and electronic structure of graphene	[133]
Halogenation
SF6 and XeF2 Plasma	Plasma SF6 treatment to epitaxial graphene on SiC and plasma XeF2 fluorination on CVD graphene/SiO2	Intermediate formation between the semi-ionic and covalent bonding on fluorinated graphene (F—C bond character)	[102]
Chlorine plasma treatments	Plasma-based chlorination of graphene flakes exfoliated and CVD-grown large-area graphene	Chlorinated graphene field-effect transistors exhibits a hole doping effect and electrical conductivity increase	[134]
Gas-phase photochlorination	Covalent linking of Cl radicals to the basal C-atoms of graphene flakes on Si/SiO2 substrate	C-Cl bonds on graphene and chemical modification by photochemical chlorination and chlorinated graphene field-effect transistors demonstrated band gap opening	[135]
Photochlorination and alkylation reactions	Cholorinating epitaxial graphene on SiC using Cl reaction under UV light followed by methylation using CH3MgBr	Chemical bonding of Cl and CH3 on the graphene basal-plane	[103]
Oxidation
Plasma treatment O2/Ar	Functionalization of epitaxial graphene surface on SiC by incorporating oxygen as ether, alcohol, and carboxyl groups	Graphene's covalent modification by O2/Ar changing its electronic properties	[104]
Covalent attachment of organic molecules:
Octadecylamine (ODA)	Covalently functionalize GO nanosheets with long alkyl chains	Enhanced lipophilicity and better dispersion of GO on polypropylene through -NH2 bonds formed between carboxylic groups of GO and octadecylamine (ODA)	[136]
p-phenylene diamine (PPD)	Conductive graphene synthesis using PPD as reducing agent on ITO substrate	Graphene surfaces absorbed with OPPD via π–π stacking + protonation to form –N+ during the reaction	[137]
1-(3-aminopropyl)-3-methylimidazolium bromide	Covalent chemical attachment of 1-(3-aminopropyl)-imidazolium bromide GO nanoplatelet	Reaction between amine groups of ionic liquid and epoxy groups of GO resulting in imidazolium-modified GO that exhibited excellent dispersibility	[138]
1-(3-aminopropyl)imidazole	Coupling by covalent interaction between the acyl chloride of activated GO and 1-(3-aminopropyl)imidazole followed by treating with 1-bromobutane and positive ion exchange with either protoporphyrin IX disodium salt or NaPF6	Positively charged imidazolium functionalized graphene modulated wettability and photoactive anionic porphyrin for optoelectronic application	[139]
3-aminopropyl triethoxysilane (APTS)	GO surface functionalized with APTS through the epoxy groups	APTS –GO exhibited improved mechanical properties	[140]
phenyl isocyanate	Mixing solution-phase of polystyrene and exfoliated phenyl isocyanate-treated GO sheets followed by chemical reduction	GO covalently attached with phenyl isocyanate through an ester bond with the epoxy group that enhanced the dispensability in polystyrene polymer and electrical properties by reduction	[140]
Grafting with polymer
Butyl amine	Plasma induced fluorination of a few layer of graphene on ITO and chemically functionalized with butylamine	PEDOT:PSS-graphene based transparent conductive electrode	[141]
	Graphene sheet on ITO were treated with plasma and covalently attached fluorine. The fluorinated graphene sheets were then exposed to a polymerization initiator such as butylamine	Amino-functionalized graphene can be used to integrate with thermosetting polymers	[142]
Gamma-aminopropyltriethoxysilane (APTES)	APTES-graphene oxide was grafted with polyethylene (MA-g-PE) using maleic anhydride	Graphene as reinforcement for non-polar polymers (PE/polystyrene)	[143]