Chuanqing Lan,
Jingjin Zhao,
Liangliang Zhang
*,
Changchun Wen,
Yong Huang and
Shulin Zhao
*
State Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources, Key Laboratory of Ecology of Rare and Endangered Species and Environmental Protection of Ministry Education, Guangxi Normal University, Guilin, 541004, China. E-mail: zhaoshulin001@163.com; liangzhang319@163.com
First published on 28th March 2017
Surface-enhanced Raman scattering (SERS) as an unequivocal surface-sensitive technique has been considered one of the most powerful tools for identifying molecular species. However, the SERS active substrates have mainly been confined to some research on noble metals such as Pd, Au, Ag and Cu. In the present study, we describe the self-assembly of graphene quantum dots (GQDs) with Mn3O4, and the use of the resulting nanocomposite in the signal enhancement of Raman spectroscopy for non-noble metal SERS active substrates. GQDs can be used to enhance the Raman signal via an electromagnetic mechanism, but their intrinsic fluorescence overlaps with the Raman signal. Previous studies regarding tuning the energy level and energy gap to change the optical properties of GQDs mainly focused on doping heteroatoms. Assembling Mn3O4 with GQDs in nanocomposites can change the energy level of the material, and quench the fluorescence of the GQDs and adsorbed molecules to improve the Raman signal and noise. The large enhancement factor of 2.06 × 104, compared to that of graphene and graphene oxide, can be explained using a photon-induced charge transfer resonance model, which means that it not only can be used in small molecule identification but can also be used to discriminate cancer cells and normal cells.
Graphene-enhanced Raman scattering (GERS)14 occurs solely through chemical enhancement without an additional electromagnetic enhancement.15 GERS provides a cleaner SERS baseline (lower noise) through surface passivation that in turn protects the detected molecule from photocarbonization and photobleaching under the laser beam compared with noble metal substrates. Therefore, the GERS-related enhancement is greater than that offered by a bare noble-metal substrate.16 However, the EFs are relatively small. For a graphene substrate, for example, only a 2- to 17-fold EF was observed.14 Graphene substrates coated with noble-metal nanoislands (Ag and Au) also provided only moderate Raman enhancement: 7.55 × 102 for GERS-Ag and 85 for GERS-Au.16 Various works have investigated the relationship between the chemical structure of graphene and the SERS performance using graphene oxide (GO),12 graphene-quantum dot (GQDs) assembled nanotubes17 and meshed graphene.18 These studies tell us that the edge effects of graphene in the enhancement of the Raman signal may be an important focus of GERS research.
GQDs possess unique optoelectronic properties that differ from bulk graphene sheets as a result of quantum confinement19 and edge effects20 that are present in graphene structures smaller than 100 nm.21 Furthermore, theoretical calculations suggest that GQDs have surface plasmon frequencies similar to those of metal nanoparticles, in contrast to graphene sheets, which do not have these frequencies.22 In other words, GQDs can be applied to SERS using an EM. However, GQDs that possess the properties that give rise to quantum confinement effects are also prone to fluorescence. For such systems, the excitation wavelength-dependent fluorescence properties23 and upconversion24 would significantly disturb the Raman signal because the fluorescence signal would cover the Raman peaks in Raman spectroscopy. Thus, reducing the fluorescence background of the Raman spectrum can improve the detection sensitivity and make the Raman peaks more visible.25 By Lombardi's model,26 charge transfer is important because SERS would not occur with surface plasmon resonance without charge transfer, and it is therefore important to investigate the effect of introducing charge transfer from a suitable additive to the GQDs.
Photo-induced charge transfer is charge transfer induced by light. Because of the importance of this process in understanding the photophysics and photochemistry of excited states in organic molecules, it is studied extensively in many scientific fields. The product of charge transfer can be metastable because of the fast delocalization of the charges produced on the donor and acceptor (not limited to molecules).27 Based on the photo-induced charge transfer mechanism, a series of chemical and biological sensors,28 photovoltaic devices,29 photocatalysts,30 photodynamic and photothermal therapy nanomaterials,31 and Raman enhancement substrates have been developed.32 Among the SERS processes, photo-induced charge transfer is an important mechanism that occurs between the molecule and the substrate and can enhance the Raman signal. It is the key factor in making GQDs an appropriate Raman substrate. Photo-induced charge transfer applied for Raman enhancement requires a charge transfer energy between dye molecules and substrate that matches the incident light energy.33 However, the Fermi level (in a vacuum) of GQDs is about −4.2 eV to −4.4 eV,34 and the energy gap is approximately 0.4 eV.22 This also limits application of GQDs to Raman technology. Methods for tuning the energy level and gap have mainly focused on doping with heteroatoms.35,36 However, the effects are confined and cannot satisfy the demand of charge transfer energy that matches the incident light energy.
Self-assembly is an efficient strategy for making ensembles of nanostructures. The design of components that organize themselves into desired patterns and functions is key to the successful application and control of the self-assembly process.37 An assembled nanoscale composite can possess structure-controlled optical and electronic properties, and the potential for morphological diversity extends the functions of the material in electronic, electrochemical, optical and sensing applications.38–40 Reports of nanocomposites that use graphene as a building block include materials self-assembled using DNA for self-healing,41 using TiO2 for photocatalysis,42 using transition-metal oxides for electrochemical functionality,43,44 using noble metals as organic catalysts,45 and using ferrite nanoparticles for separation and purification.46 To the best of our knowledge, graphene metal oxide nanocomposites for use in SERS have not been reported. Metal or metal oxide nanoparticles were deposited on the surface of graphene, and a 3D net structure of a graphene nanocomposite was formed by self-assembly. In this case, the relationship between nano-particles and graphene is not clear. Because graphene is a material with a 2D planar structure (approximately micron scale), which is significantly larger than the nanoparticles, GQDs are more readily observed with respect to the nanoparticles. Transmission electron microscopy (TEM) images illustrate that the nanoparticles are present on the graphene surface,42,46 but it is not clear how the 3D network structure is connected. Understanding how the 3D graphene networks are connected is important so that new self-assembled graphene materials may be designed in the future.
One potentially suitable additive for the purpose of facilitating charge transfer in a SERS system is Mn3O4, which has an extensive oxide system with oxidation states ranging from +2 to +4 valences. Mn3O4 is widely used in electrodes for electrolytic supercapacitors,47 and the charge transfer process is correlated with electrode materials that can be used in Raman spectroscopy.48 Mn3O4 can exhibit absorption in the UV and visible regions from 220–730 nm owing to the combination of O2− → Mn ion charge transfer and the crystal field d → d transition,49 and as such, it has attracted the interest of researchers.50,51
Herein, we describe for the first time a one-pot synthesis of a GQD–Mn3O4 nanocomposite using multi-walled carbon nanotubes (MWCNTs) as a starting material. MWCNTs were cut into pieces (GQDs) by KMnO4. Then, the GQDs were combined with manganese ions to form the GQD–Mn3O4 nanocomposite through a process of 0D to 3D self-assembly. The purpose of using Mn3O4 was not only to connect the GQDs but also to quench the fluorescence of the GQDs as Mn3O4 can efficiently suppress the Raman spectrum fluorescence back-ground. Using Rhodamine B (RhB) as a Raman-active molecule, an EF as high as 2.06 × 104 was obtained. Compared to the EFs of previously reported GERS substrates and many semiconductor substrates, this is an approximately 2 orders of magnitude improvement in the performance of graphene SERS substrates. The prepared GQD–Mn3O4 nanocomposite possesses very good biocompatibility, which can be applied for SERS to discriminate cancer cells and normal cells (HeLa, HepG-2 cell line and 7702 cell line) for potential diagnostic applications.
Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained using a TECNAI G2 high-resolution transmission-electron microscope (Holland) with an accelerating voltage of 200 kV. The sample for TEM characterization was prepared by placing a drop of prepared solution on a carbon-coated copper grid and drying at room temperature. Scanning electronic microscopy (SEM) images were obtained using a Quanta FEG 250. The sample was dropped onto a Si wafer, allowed to evaporate and then a conductive Au coating was applied.
The fluorescence experiment was carried out using a Perkin Elmer (U.S.A.) LS-55 Molecular Spectroscopy system. FTIR spectra were recorded using a Perkin Elmer (U.S.A.) Spectrum Two™ IR spectrometer. Fluorescence lifetime data were obtained using HORIBA JobinYvon FluoroLog-3 TCSPC (France). Electrochemical data were recorded using a CHI660C workstation (Shanghai, China).
Electrochemical properties were measured using cyclic voltammetry with glassy carbon as the working electrode, platinum wire as the counter electrode and a double junction saturated calomel electrode as the reference electrode. The triple electrode system was in dry, HPLC grade, argon-purged acetonitrile and 0.1 m tetrabutylammonium per chlorate electrolyte solution. The sample was dropped onto the working electrode and dried under vacuum at 60 °C for 1 h. The initial scan voltage was 1.8 V, high voltage was 1.8 V, low voltage was −2.0 V and the scan rate was 0.05 V s−1. The data were recorded on a CHI660C workstation.
In strongly acidic conditions, manganese oxide will dissociate into constituent ions, and these building blocks will be dispersed into solution, allowing the nanocomposite to be formed by a metal ion-driven self-assembly process with the GQDs. The GQD–Mn3O4 composite was treated with 3 M HCl to destroy the Mn3O4 and was characterized by TEM (Fig. S1†). The results confirmed that the GQDs were one of the building blocks of the nanocomposite. The TEM image also demonstrates that the GQDs were well dispersed in the aqueous solution.
By measuring 158 particles, the diameter of the GQDs was determined to be in the range of 2 to 12 nm (Fig. S2†); approximately 71% of the particles were 2–4 nm in diameter. HRTEM (Fig. S3†) showed that the lattice spacing values of the GQDs were 0.21 nm and 0.24 nm, corresponding to the (100) and (112) planes of graphite, respectively.54,55 To elucidate the structure of the GQD–Mn3O4 nanocomposite, TEM characterization was performed (Fig. 1C). As shown in the figure, the GQD–Mn3O4 nanocomposite possesses a porous structure. This porous nature is reminiscent of those found in previously reported self-assembled graphene-based materials. However, the scale is different: instead of having pores on the micrometer scale, as seen in other studies (GO–metal oxides composites),46 the pores of the GQD–Mn3O4 nanocomposite are on the sub-micrometer scale (see SEM images in Fig. 2A). The SEM and TEM images all show that the nanocomposite self-assembled into a porous structure, with sub-200 nm pore sizes. This indicates that changing the scale of the building blocks, such as replacing graphene sheets with GQDs can result in significant morphological changes and potentially tunable pore sizes.
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Fig. 2 (A) SEM image of the GQD–Mn3O4 nanocomposite. (B) XRD pattern of the GQD–Mn3O4 nanocomposite. (C) XPS of the GQD–Mn3O4 nanocomposite. (D) XPS of the C 1s. (E) XPS of the Mn 2p. |
The GQD–Mn3O4 nanocomposite was also characterized using HRTEM (Fig. 1D). Each distinct crystal lattice spacing was corresponds to a particular compound and can therefore be used to determine the chemical constituents of the nano-composite. The crystal lattice spacing values of 0.21 nm and 0.24 nm belong to the GQDs, and the 0.25 nm and 0.28 nm values belong to Mn3O4, corresponding to the (211) and (103) plane of Mn3O4, respectively.56,57 After identifying the GQDs and Mn3O4, it was determined that the GQD particles connect with the Mn3O4 particles. This result is supported by the lattice fringes shown in Fig. 2B. The spacings of the lattice fringes in adjacent particles are 0.21 nm, 0.24 nm and 0.25 nm, and 0.28 nm, which appear to be fused together corresponding to the d-spacings of GQDs and Mn3O4, respectively. In addition, the self-assembly seems to be randomly oriented. Therefore, in our graphene–metal ion-driven assembly process, the function of the metal oxide nanoparticles is to connect the graphene particles to each other. According to previous research,58,59 GQDs are similar to GO with epoxy, hydroxyl, carbonyl and carboxyl groups. These functional groups of GQDs attached to the Mn ions, and in situ formation of Mn3O4 then occurred, as is described in Fig. 1B.
The X-ray diffraction (XRD) pattern (Fig. 2B) shows broad peaks centered at approximately 29°, 32°, 36°, 38°, 44° and 59°, which can be indexed to the (112), (103), (211), (004), (220) and (224) planes, respectively.60 These data match the crystal lattice spacing values of Mn3O4 in the HRTEM image (Fig. 1C). All these reflections were indexed to Mn3O4 (ICDD-JCPDS no. 24-0734). The peaks (29° and 32°) of the nanocomposite were located at the same positions as reported for Mn3O4 (ref. 49) and graphene–Mn3O4 composites.47,60
We first identified the peaks of C 1s, O 1s and Mn 2p at 284, 528 and 645 eV, respectively, by X-ray photoelectron spectroscopy (XPS) (Fig. 2C). The high-resolution XPS spectrum of C 1s (Fig. 2D) was used for chemical state analysis, and the signal was deconvoluted to peaks at 284.6 eV, 285.0 eV, 286.3 eV and 288.1 eV, which correspond to C–C/CC bonds, C–O bonds, C
O bonds and O–C
O bonds. The splitting between the peak of Mn 2p3/2 and the peak of Mn 2p1/2 is approximately 11.7 eV (Fig. 2E), which matches reports for Mn3O4.47,60
When comparing the Fourier transform infrared (FTIR) spectroscopy spectra (Fig. S4†) of the GQDs and the GQD–Mn3O4 nanocomposite, many of the same absorption bands appear: the O–H stretching vibration at 3400 cm−1, the C–H stretching vibration at 2924 cm−1, the CO stretching vibration at 1626 cm−1, the C–H deformation vibration of CH and CH2 at 1466 cm−1, the C–H deformation vibration of CH3 at 1386 cm−1 and the C–O stretching vibration at 1105 cm−1. However, differences arise in peaks belonging to the octahedral vibration of Mn3O4 in the range of 400 cm−1 to 700 cm−1. The FTIR spectra agree with those obtained for Mn3O4 in previous studies.49,61–63 The Raman spectrum (Fig. S5†) of the GQDs exhibited two peaks at 1363 cm−1 and 1596 cm−1, which are characteristic peaks of graphene. The same two peaks were observed for the GQD–Mn3O4 nanocomposite, and the sharp peak seen only in the GQD–Mn3O4 nanocomposite sample at approximately 650 cm−1 corresponds to the Mn–O vibrations in Mn3O4.58,60
UV-Vis analysis was performed on the GQDs and the GQD–Mn3O4 nanocomposite samples, and the results (Fig. S6†) support the conclusions of the FTIR analysis. The GQDs show support the conclusions of the FTIR analysis. The GQDs show a peak at 320 nm, which is attributed to the π →
π* absorption.23 Similar to reports by previous researchers,49 the GQD–Mn3O4 composite, with compositions of MnO–Mn2O3 (Mn2+ and Mn3+) and 2Mn–MnO2 (Mn2+ and Mn4+), exhibits absorption features from 210 to 730 nm. The three charge transfer transitions O2− → Mn2+ (210–250 nm), O2− → Mn3+ (350–390 nm) and O2− → Mn4+ (440–460 nm) can reasonably be attributed to the d–d crystal field transitions in octahedral Mn with oxidation states ranging from +2 to +4 valance species. Thus, we conclude that Mn2+, Mn3+ and Mn4+ composed the GQD–Mn3O4 nanocomposite. The intense and broad UV-Vis absorption of the nanocomposite is the primary reason that it can quench the fluorescence of the GQDs, as well as that of the dye molecules. Because of the self-assembled nature of the GQDs and Mn3O4 species, charge transfer occurs between the GQDs and Mn3O4 nanoparticles.
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Fig. 3 The Raman signal of RhB (0.6 mM) was recorded on the GQD–Mn3O4 nanocomposite film, GQDs and glass. |
Furthermore, even though the baseline is not subtracted from the spectrum, the peaks are clearly visible in the original data. The original spectrum exhibits two peaks at 348 cm−1 and 419 cm−1. When carefully checking the original spectrum on the GQD substrate, the two peaks at 351 cm−1 and 421 cm−1 were determined to be too obscure to be discerned easily because of masking by the fluorescence background. After subtracting the baseline (Fig. S7 and S8†), it was found that the vibrational frequencies of RhB on GQDs and on the GQD–Mn3O4 nanocomposite (Table S1,† columns 2 and 4, respectively) were consistent with the SERS results reported previously for graphene and12 metal substrates64 and from theoretical calculations.65,66
For further investigation, solutions with different concentrations of RhB (concentrations ranging from 1 μM to 600 μM) were dropped onto three different substrates (see Fig. 4A), and the signals were found to improve with increasing concentration. It is interesting that the baseline becomes flatter as the concentration increases. We chose the peaks centered at 419 cm−1 as P1, 592 cm−1 as P2, 763 cm−1 as P3, 943 cm−1 as P4, 1190 cm−1 as P5, 1361 cm−1 as P6 and 1566 cm−1 as P7. P1 is assigned to xanthene ring deformation and N–H oscillation, P2 represents –NH wagging, P3 corresponds to C–H out of the plane of the xanthene ring, P4 is xanthene ring and phenyl ring stretching, P5 is assigned to C–H bending of the xanthene ring, P6 is xanthene ring stretching, and P7 is phenyl ring stretching. By fitting these peaks and by applying the Brunauer–Emmett–Teller (BET) model14 with concentration, this model indicates that the molecules observed a model of multilayer adsorption.67 It was also found that the relative intensity of some peaks would change with the concentration, according to previous research.68,69 It can be concluded that the molecular orientation of RhB on the GQD–Mn3O4 nanocomposite would change with the concentration of RhB (Fig. 4B). To compare the signal enhancement in a more quantitative manner, we calculated the EF for each case (Fig. S7 and S8†). The largest EF value is 2.06 × 104 at 425 cm−1, and the smallest EF value is 7.95 × 102 at 1649 cm−1. These EFs are among the highest reported for graphene and other non-noble metal SERS substrates (Table S2†).
GQDs cannot quench the fluorescence of RhB Raman dye when used as a SERS substrate. Moreover, the GQDs' own fluorescence properties, including upconversion and down conversion, decrease the signal-to-noise ratio of the Raman spectrum. By self-assembling with Mn3O4, the distance between the GQDs and Mn3O4 is small; thus, charge transfer can occur, the fluorescence of the GQDs is quenched, and the nanocomposite can quench the fluorescence of the Raman dye. The lower number of peaks observed for the GQDs than for GQD–Mn3O4 can be attributed to the influence of the fluorescence background on the Raman enhancement of the GQDs.
We propose that the Raman-enhancement mechanism of the synthesized nanocomposite is an example of photon-induced charge transfer resonance. As illustrated in Fig. 4E, the energies (vacuum level) of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of RhB are −3.50 eV and −5.50 eV,70 while the energies (vacuum level) of the valence band (VB) and conduction band (CB) of the GQD–Mn3O4 nanocomposite are −5.91 eV edge and −4.39 eV, respectively, as calculated using cyclic voltammetry measurements (Fig. 4D). Compared with a previous study,71 the VB and CB of GQDs are −5.62 eV and −4.72 eV. By using the photon-induced charge transfer resonance model.26,33 The charge transfers from the VB to the LUMO at 2.41 eV and from the HOMO to the CB at 1.11 eV are allowed. Charge transfer at 2.4 eV corresponds to 514 nm; thus, the wavelength of the incident laser was selected. The other charge transfer is at 1.1 eV, corresponding to 1127 nm, which is the same as the wavelength of the surface plasmon resonance. With GQDs as the substrate, charge transfer at 2.1 eV corresponds to 590 nm, and the second is at 0.8 eV and corresponds to 1550 nm. According to our knowledge, most Raman spectrometers do not have 590 nm laser beams. Therefore, 514 nm is more convenient than 590 nm.
The Raman enhancement consists of a number of different effects: the molecular resonance (μmol) of RhB, the excitation resonance (μex) of the GQD–Mn3O4 nanocomposite and the photon-induced charge transfer resonance (μCT) from matching the energy levels between the GQDs–Mn3O4 nanocomposite and the RhB molecules. In the quantum mechanical model (calculations are given in the ESI†), via the vibronic coupling, the ground state |I〉 with the conduction-band edge |C〉 and molecular excited state |K〉 with the valence-band edge |V〉 of the semiconductor would contribute to the molecular polarizability tensor. When the energy of the incident light equals the charge–transfer transition frequency between the semiconductor valence-band edge and molecular excitation state (ω = ωVK), the polarizability tensor αρσ is composed of A, B and C, where the A term, RVK(ω), represents the plasmon resonance; the B-terms, RICK(ω) and RICV(ω), represent the charge transfer transitions from the molecule to the semiconductor; and the C-terms, RIVK(ω) and RKVC(ω), represent the charge transfer transitions from the semiconductor to the molecule.33 In this condition (ω = ωVK), with the incident laser wavelength at 514 nm, the main contributions to the Raman enhancement factors are from the A term RVK(ω) and the C terms RIVK(ω) and RKVC(ω). Therefore, application of the photon-induced charge transfer resonance model to the GQD–Mn3O4 nanocomposite experimental results indicates that large EFs could be expected. This is an effective way to design SERS substrates with high enhancement factors, tuning the energy level and energy gap to change the optical properties through self-assembly.
Footnote |
† Electronic supplementary information (ESI) available: Experimental calculations, supplementary figures and tables. See DOI: 10.1039/c7ra00997f |
This journal is © The Royal Society of Chemistry 2017 |