Tris-(2-aminoethyl)amine-Intercalated Graphene Oxide as an Efficient 2D Material for Cerium-Ion Fluorescent Sensor Applications.

In this work, we report the covalent functionalization of tris(2-aminoethyl)amine (tren) onto the graphene oxide surface by the ring opening of epoxide with primary amine moieties. The effect of intercalation creates the covalent coordinating moieties in between the basal planes of graphene oxide by increasing the interlayer spacing of 1.08 nm from 0.75 nm and thereby decreasing 2θ of 8.17 deg from 10.46 deg. Because of the intercalation of tren, the aminoalcohol moieties are formed in the GO planes and the intercalated material is characterized by the spectroscopic (IR, XPS, UV, and Fluorescence) and microscopic techniques. The DFT calculation shows the newly formed C-N bond length of 1.484 Å and enhanced energy gap due to functionalization. The intercalated material shows a good selective fluorescent chemosensor for the cerium ion in aqueous solution. The reversibility and its interference with other competing metal ions have been studied. The enhanced fluorescence upon the addition of cerium ions is due to the intramolecular charge transfer and photoinduced electron transfer processes in the sp2- and sp3-hybridized carbon networks.

Introduction

Recently, carbon based two-dimensional material research has been focused much on the oxidative derivative of graphene called graphene oxide (GO) for further chemical functionalization through the oxygen-containing functional groups such as epoxides, hydroxyls, and terminal carboxylic acids.[1−5] These class of intercalated materials find enormous application toward optoelectronics,[6] solar cells,[7] supercapacitors,[8] field emission transistors,[9] smart sensors,[10] nanocomposites, batteries,[11] and biomedical device applications.[12] Additionally, surface modification of GO has fascinated great attention due to the presence of both sp2- and sp3-carbon networks and extended possibility for covalent functionalization of heteroatoms (−NH, −OH, and −SH), ensuing assured applications in the advanced technologies.[13] GO exhibits the photoluminescence property due to the disruption of the carbon matrix during oxidation or covalent functionalization process,[14] which contain the paired electron holes that are localized with the sp2- and sp3- hybridized carbon domain.[15] Chemical functionalization of GO was classified into two categories: (a) covalent functionalization due to the ring opening of epoxide by the nucleophilic attack[16,17] or amide formation through the activation of terminal carboxylic acids[18] and (b) noncovalent functionalization through Van der Waals interaction on the surface of GO.[19,20] It was reported that the structure disorder leads to the fluorescence due to the optical transition from the localized states in the π–π* transitions gap of sp2-carbon through the intercalation process and also by the quantum confinement effect.[21,22]

Shang et al.,[23] have reported that the presence of functionalized groups from the sp3-hybridized carbons causes the fluorescence. The noncovalent interaction of anthracene–imidazolium salt with the reduced GO hybrid material exhibits strong fluorescence with a quantum yield of 0.29 due to the electron transfer between GO and the anthracene moieties.[24] Eda et al.,[25] have reviewed the tunable optoelectronic properties of chemically derived GO materials with its heterogeneous electronic structure.[26] The origin of fluorescence has been derived from the recombination of the electron–hole in the nearby localized states of conduction band and the wide range of the valence band.[27] It is worth noting that the functionalization of enriched heteroatoms in the GO planes lead to the formation of sp3-sites which eventually contributes to the fluorescence.[28]

In this present work, we report the covalent functionalization of GO with tris(2-aminoethyl)amine (GO–tren) for the creation of covalent coordinating moieties in between the GO planes. The choice of the tren molecule for the intercalation is to create covalent coordination moiety on the surface of GO through the epoxide ring opening which eventually forms aminoalcohol moieties with penta coordination for metal complexation. Because of the intercalation, a significant enhancement of interlayer d-spacing is observed, and thereby, it opens an avenue for metal coordination. It is our interest to study the fluorescent behavior of the GO–tren material with the formation of disordered sp2- and sp3-carbon networks upon intercalation and perform with various metal ions in the tetrahydrofuran–water mixture. Upon the addition of the cerium metal ion, the enhanced fluorescence was observed and suggested the intramolecular charge transfer (ICT).

Results and Discussion

Material Preparation

The starting material, GO, was synthesized by the previously reported method.[29] The covalent functionalization of GO was performed in the mixture of solvents, tetrahydrofuran–water in a well-dispersed condition. The resulting solution appeared black after 3 days, and the crude intercalated material was centrifuged. The black solid was washed with cold tetrahydrofuran to remove the unreacted tren, which was confirmed by thin-layer chromatography. The amino alcohol moieties were formed in the GO surface due to the epoxide ring opening by the nucleophilic attack of the primary amine moiety in tren. The remaining intercalated GO material was characterized by spectroscopic and microscopic techniques. The schematic representation of covalent intercalation of the tren molecule on the surface of GO is shown in Figure .

Figure 1

Schematic representation of covalent intercalation of tren in the GO basal planes.

DFT Calculations

Density field theory (DFT) calculations are performed to understand the surface modification of GO due to the covalent intercalation of the tren molecule. The optimized structure of GO, tren, and GO–tren material is shown in Figure S1, and their geometrical parameters are listed in Table S1 (refer the Supporting Information). The optimized energy of GO and tren at the HF/6-31g level of theory is found to be −69 209.46 and −12 370.46 eV, respectively. The single-point calculation at the B3LYP/6-311++g** level predicts the energy of the GO and tren to be −69 674.83 and −12 460.56 eV, respectively. Upon intercalation, the optimized energy of GO–tren is −81 594.23 eV and single-point energy is −82 153.16 eV.

The nucleophilic attack on the epoxy rings in GO sheets leads to the formation of hydroxyl group referred to the amino alcohol moiety. A significant interaction between C(24) of GO and N(1) of the tren with a C–N bond length of 1.484 Å is identified. The notable variation in the geometrical parameters between the intercalated material and the monomers is observed (Table ). The interaction energies of the complex calculated at ab initio and DFT levels are found to be −14.31 eV (−0.526 Hartrees) and −17.77 eV (−0.653 Hartrees), which confirm strong intercalation of GO and tren. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for GO[30] and its complex are calculated at B3LYP/6-31G* level of theory, and it is depicted in Figure .

Table 1

Calculated Chemical Reactivity Descriptors at B3LYP/6-31G* Level of Theorya

materials	I	A	ΔE	χ	η	ζ	μ	ω
GO	4.498	2.395	2.103	3.447	1.052	0.951	–3.447	5.647
GO–tren	4.428	2.217	2.211	3.323	1.106	0.904	–3.323	4.992

I is the ionization potential, A is the electron affinity, ΔE is the LUMO–HOMO gap, χ is the electronegativity, η is global hardness, ζ is global softness, μ is the chemical potential, and ω is the lectrophilicity index. All of the values are given in terms of eV and are calculated by using HOMO and LUMO energies.

Figure 2

Spatial distributions of HOMO and LUMO for (a) GO and (b) GO–tren.

The highest electrophilicity (5.647 eV) and electronegativity (3.447 eV) are observed for GO which denotes that it accepts more electrons. The high global hardness (1.106 eV) and lower softness (0.904 eV) obtained for GO–tren confirms the molecular stability of the complex.[31] The HOMO and LUMO energies of GO are −4.498 and −2.395 eV and henceforth their energy gap (ΔE) value is found to be 2.103 eV. These values are in good agreement with the GO-based material functionalized with aminoazobenzene reported by Gupta et al.[32] The Gibbs free energy corrected to zero point energy for GO–tren is calculated to be −1.354 kJ/mol and the negative value clearly indicates the intercalation between the planes. The number of benzene rings in the GO sheets and the intercalating molecule does have an effect on the orbital energy values as well explained by Mondal et al.,[33] The hybridization of GO orbitals due to tren increases the orbital band energy value to 2.211 eV. Demir and Akman[34] have reported that the lowest energy gap has high polarizability, high chemical reactivity, and low kinetic stability. In our case, the GO monomer has a comparatively smaller energy gap which indicates that it readily reacts with amine for the formation of intercalated materials.

Characterization of GO–Tren

The powder X-ray diffraction (XRD) of GO–tren is performed to examine the effect of the intercalated structure of GO and its crystallinity upon the functionalization of the tren molecule. The XRD patterns of GO and GO–tren are presented in Figure . The intercalated GO–tren material shows an enhanced interlayer spacing of 1.08 nm from 0.75 nm with the decrease of 2θ(002) = 8.17° from 10.46° of GO, which indicates that the tren is intercalated covalently in between the basal planes of GO. It confirms that the nucleophilicity of the primary amine attacks the epoxide moieties on the GO surface leading to the formation of amino alcohol moieties, and thereby, the enhanced interlayer d-spacing between the GO layers is observed. The mean crystallite thickness (t) of 23 Å (72 Å for GO) was calculated for the GO–tren material using the Debye–Scherrer equation (refer the Supporting Information), and the number of layers for the functionalized GO–tren was theoretically calculated as 2.3.

Figure 3

Powder XRD pattern of GO and intercalated GO–tren.

SEM and TEM Images

The surface morphology, layer arrangement, effect of intercalation, and the nanostructures of as-prepared functionalized GO–tren and GO are characterized by scanning electron microscopy (SEM) images (Figure a,b). The SEM images show the sheet- and flake-like structures with relatively large surface area can be seen. The images of the sheets reveal the penetration of intercalated tren to the stack arrangement through the edge to center process which eventually increased the space between the GO sheets. Upon covalent functionalization, the changes in the morphology of the intercalated GO material are seen using transition electron microscopy (TEM). TEM images of GO–tren and GO are shown in Figure c,d. It clearly shows the formation of few layers, and its selected area electron diffraction (SAED) pattern indicates the existence of plane crystallinity upon intercalation in the few sheets of GO.[35] The intensity of diffractions is decreased, as compared to the GO, which corresponds to the presence of tren moieties on the surface of GO layers.

Figure 4

(a) SEM image of GO, (b) SEM Image of GO–tren, (c) TEM Images of GO, (d) fridges of the TEM Image of GO (insight SAED Pattern), (e) TEM images of GO–tren, and (f) fridges of the TEM image of GO–tren (insight SAED Pattern).

XPS Analysis

The surface chemical composition and chemical state of the GO–tren material was studied by X-ray photoelectron spectroscopy (XPS) analysis. The survey spectrum and deconvoluted spectra are shown in Figure a. The survey spectrum of GO–tren shows the significant variations from the GO spectrum, in terms of C 1s binding energies and the presence of the new N 1s band at 399–405 eV which confirms the successful functionalization on the GO planes. The deconvoluted bands of C 1s spectra appears at 284.9, 285.6, 286.9, 289.9, 292.3, and 293.5 eV corresponding to the C 1s C–C/C=C, C–N, C–OH of hydroxyl, C=O of carboxylic acid, O–C=O bonds, respectively. The N 1s spectrum shows the deconvoluted band peaks at 399.2, 400.8, and 404.9 eV attributed to the N–H, NH2, and N protonated bonds, respectively (Figure c). In the case of the O 1s spectrum, the strong peaks at 531.9, 534.7, and 536.8 corresponds to the C=O, C–O, and O–singly bounded to aromatic carbons, respectively.

Figure 5

(a) XPS survey spectrum of GO–tren, (b) XPS O 1s spectrum of GO–tren, (c) XPS N 1s spectrum of GO–tren, and (d) XPS O 1s spectrum of GO–tren.

The Fourier transform infrared (FT-IR) spectrum of GO–tren confirms the intercalation of tren into the basal plane of GO by shifting the epoxide C–O stretching in GO to 1021 cm–1, indicating the formation of primary alcohol moieties. It shows in Figure a that the peaks at 3417 and 2923 cm–1 correspond to NH and OH stretching frequencies in −NH, −NH2, and −OH bonds, respectively. The peaks appearing at 1632, 1436, and 875 cm-1 are attributed to NH bending, NH deformative vibrations from the NH bonds, and C–N stretching vibrations in GO–tren, respectively.

Figure 6

(a) IR spectra of GO–tren and GO and (b) UV spectra of GO–tren and GO.

The UV spectrum of GO–tren is compared with GO to confirm the impact of intercalation in the GO sheets. The strong absorption peak at 230 nm corresponds to π–π* transitions of aromatic C=C bonds and a shoulder at 301 nm is attributed to n–π* transitions of C=O bonds. The GO–tren material that shows the redshift to 242 nm (Figure b) is due to the functionalization on the basal planes, and n–π* transitions appear at 324 nm which also confirms no reduction of GO during the functionalization.

The thermal stability of the GO and GO–tren is compared by thermogravimetric analysis (TGA), which are shown in Figure . GO–tren shows a slight weight loss of 15–20% due to the removal of residual water molecules and few labile oxygen-containing moieties on the surface of GO until 210 °C. Thereafter, the slow decomposition with around 30% of weight loss of functionalized groups is observed between 230 and 900 °C.[36] The improved thermal stability of GO–tren is observed mainly due to the covalent intercalation of the tren molecule.[37]

Figure 7

TGA curves of GO and GO–tren.

Selectivity of Metal Ion Studies

The fluorescence behavior of GO–tren nanosheets in the presence of various metal ions is studied, and their corresponding fluorescence spectra are shown in Figure a. The covalent-functionalized GO–tren material shows a remarkable enhancement of fluorescent intensity at the emission maximum at 332 nm upon the addition of cerium metal ions (100 equiv) to the receptor-dispersed solution of GO–tren. It is observed that the appearance of a peak with the redshift of the fluorescent intensity at 405 nm at an excitation of 275 nm wavelength shows a significant enhancement of fluorescence when compared to the other metal ions. It is evident that the disordered carbon network is formed by the intercalation of the tren moiety on the basal planes of GO, and it is highly selective toward cerium ions compared to other metal cations. The intercalated GO–tren material detects cerium ions as an “off-on” fluorescent probe in the mixture of solvents (tetrahydrofuran–water) at neutral pH.

Figure 8

(a) GO–tren with metal ions for selectivity, (b) interference with other competent metal ions, as shown in Figure a, the fluorescence titrations of GO–tren was performed by the addition of various concentrations of cerium metal ions (10–100 equiv). It is observed that the fluorescent intensity is gradually increased and reached the saturation level around 100 equiv. It is worth noting to report the shifting of the peak to 405 nm, indicating the possibility of ICT between the GO and the intercalated amine within the GO sheets.[36] Because of the photoelectron transfer, the quenching of fluorescence intensity is blocked and thereby it creates a path for the ICT process.

Figure 9

(a) Fluorescence spectra of the GO–tren material with various concentrations of cerium and (b) reversibility with the external addition of ethylenediaminetetraacetic acid (EDTA).

Interaction with Other Competent Metal Ions for Selectivity

The interference of GO–tren with other competent metal ions for the selectivity of cerium ions is performed through the competitive complex formation (Figure b). The change in fluorescent intensity is recorded upon the addition of equimolar concentration of other metal ions (100 equiv) to the dispersed test solution containing cerium ions. It is observed that no transmetallation takes place with the detection of cerium ions which confirms that the GO–tren material detects cerium ions selectively.

Reversibility of GO–tren and Ce3+ with EDTA

The binding behavior of Ce3+ ions with GO–tren in the presence of the hexadentate ligand; EDTA was performed in the solvent mixture tetrahydrofuran–water (1:1 v/v, Tris buffer-30 mM at pH 7.2). The fluorescence intensity was measured by the addition of 100 equiv of EDTA to the dispersed solution with the variation of time at an excitation of 275 nm. After 48 h, the slow disappearance of the fluorescence signal (Figure b) indicates the cerium metal ions reversible binding with EDTA which normally forms Ce3+–EDTA stable complex. It is also evident that the complexation of cerium ions with GO–tren is relatively weaker than EDTA due to the availability of higher coordination sites.

Because of the intercalation of tren with dispersed GO, the strained epoxide ring is cleaved by the nucleophilic attack of the primary amine in the tren molecule and an additional coordination site in the GO planes is formed. It is also suggested that another primary amine reacts with the GO plane and forms interlayer spacing with a distance of 1.08 nm. Upon the addition of cerium ions, the available lone pair is involved in bonding with the electron-deficient Ce3+ ion that inhibits the photoinduced electron transfer process. However, the higher vibrational energy in the excited states of the graphene sheet is shared through an ICT in the form of nonradiative decays with a significant fluorescence shift.

Conclusions

The intercalation of graphene oxide with tren is performed by the nucleophilic ring opening of epoxide for the formation of covalent aminoalcohols on the surface of the GO. The functionalized GO–tren material shows the increased interlayer d-spacing compared with the normal GO due to the covalent functionalization. DFT calculation also supports the intercalation of GO–tren with the slight variation in the band gap and disordered sp2- and sp3-hybridized carbon networks exist in the intercalated material. The fluorescent behavior of the intercalated material is studied with various metal ions for the selectivity and its reversibility of binding for the cerium ion detection. The enhancement fluorescence in aqueous medium is plausibly due to the photoinduced electron transfer and ICT process. Eventually, the creation of covalent coordination moieties in between the GO basal plane pays huge impact in the sensing of selective metal ions or removal of heavy metal ions in the aqueous medium toward the environment applications.

Experimental Sections

Synthesis of GO–tren

To the dispersed solution of GO (2 gm) in 200 mL of tetrahydrofuran and water (20:80 v/v), tris(2-ethylamino)ethylamine (tren, 2 gm) in 20 mL of tetrahydrofuran was added over a period of 3 h under vigorous stirring at RT. The solution was stirred for 3 days at RT to get black color solution. The dispersed mixture was centrifuged, and the black solid was washed thrice with tetrahydrofuran (80 mL) to remove unreacted tren. The resulting black solid was washed and centrifuged with water (3 × 100 mL). The wet solid was washed with ethanol (3 × 80 mL) and diethylether thrice to obtain the GO–tren material as a free-flow black solid.

Characterization of GO–tren

XRD patterns for as-prepared functionalized materials were recorded on a Shimadzu XRD-6000 Powder X-ray diffractometer at 40 kV voltages and 30 mA current. FT-IR spectra were recorded on IR-Prestise-21, Shimadzu. Scanning electron microscopy was carried out on JEOL model JSM-6390 for all of the samples. Absorption spectra were recorded on a shimadzu UV-240 spectrophotometer. Raman spectra were recorded on a Horiba–Jobin Raman spectrometer with a 514 nm laser at a power. High-resolution transmission electron microscopy and SAED measurements were conducted on a JEOL JEM-2100 for the surface morphology of GO and GO–tren. Fluorescence measurements were recorded in a Jasco FP-8200 spectrofluorometer with a quartz cuvette of 1 cm path length.