Article pubs.acs.org/Langmuir
Identification of Metalloporphyrins with High Sensitivity Using Graphene-Enhanced Resonance Raman Scattering Bo-Hyun Kim,† Daechul Kim,† Sungho Song,† DongHyuk Park,‡ Il-Suk Kang,§ Dae Hong Jeong,∥ and Seokwoo Jeon*,†,⊥ †
Department of Material Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea ‡ Department of Applied Organic Materials Engineering, Inha University, Incheon 402-751, Korea § National Nanofab Center, KAIST, Daejeon 305-701, Korea ∥ Department of Chemistry Education, Seoul National University, Seoul 151-742, Korea ⊥ KAIST Institute for the Nanocentury, Graphene Research Center, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea S Supporting Information *
ABSTRACT: Graphene-enhanced resonance Raman scattering (GERRS) was performed for the detection of three different metallo-octaethylporphyrins (M-OEPs; M = 2H, FeCl, and Pt) homogeneously thermal vapor deposited on a graphene surface. GERRS of M-OEPs were measured using three different excitation wavelengths, λex = 405, 532, and 633 nm, and characterized detail vibrational bands for the identification of M-OEPs. The GERRS spectra of Pt-OEP at λex = 532 nm showed ∼29 and ∼162 times signal enhancement ratio on graphene and on graphene with Ag nanoclusters, respectively, compared to the spectra from bare SiO2 substrate. This enhancement ratio, however, was varied with M-OEPs and excitation wavelengths. The characteristic peaks and band shapes of GERRS for each M-OEP were measured with high sensitivity (100 pmol of thermal vapor deposited Pt-OEP), and these facilitate the selectively recognition of molecules. Also, the peaks shift and broadening provide the evidence of the interaction between graphene and M-OEPs through the charge transfer and π-orbital interaction. The increase of graphene layer induced the decrease of signal intensity and GERRS effect was almost not observed on the thick graphite flakes. Further experiments with various substrates demonstrated that the interaction of single layer of graphene with molecule is the origin of the Raman signal enhancement of M-OEPs. In this experiment, we proved the graphene is a good alternative substrate of Raman spectroscopy for the selective detection of various metalloporphyrins with high sensitivity.
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INTRODUCTION
that is simple and easily accessible even after deposition on substrates. Among various spectroscopic methods of molecular detection, Raman spectroscopy is ideal for nondestructive studies of materials including biological compounds.5,6 Resonance Raman spectroscopy (RRS) using excitation wavelengths close to the optical band gap of molecules has been used to identify chemical compounds.6,7 Recent advances in surface-enhanced Raman scattering (SERS) techniques have enabled the detection and characterization of materials with a single-molecular detection level.8−10 The surface-enhanced resonance Raman spectroscopy (SERRS) combining these two methods not only magnifies Raman signal enhancement factors but also is accompanied by useful quenching of
Metalloporphyrins are compounds formed by a combination of porphyrin with metal ions, playing a major role in many enzymes and photosynthesis.1 In recent decades, various synthetic porphyrin derivatives have been developed for biosensors, solid catalysts, organic semiconductors, lightemitting devices, etc.2,3 However, despite extensive work on the synthesis and utilization of metalloporphyrin systems, the metalloporphyrin detection has not been actively studied. The preferred diagnostic tests for porphyrin are two types: biochemical (chromatography, plasma fluorescence scanning, and enzyme assay) and DNA test.4 These tests are not only cumbersome and slow but also are less specific and may not be indicative for the metalloporphyrin. Moreover, their detection limit of porphyrin is so low that reliable data cannot be obtained. Thus, there is a need for a detection method for metalloporphyrin with high sensitivity (100 nm) graphite flake can be found at the bottom right corner in Figure 4c. The Raman mapping, drawn with the peak intensity of ν7 ∼ 679 cm−1, which is a significant band at λex = 532 and 405 nm regardless of center ions, obviously shows that intensity decreases as a function of graphene layer thickness. It is obvious that in the single layer the signal intensity of GERRS was the strongest, and the intensity decreased as number of layers increased. The GERRS spectra measured from single (1), double (2), thin (∼5 nm) flake (3), and thick (>100 nm) flake (4) are compared in Figure 4d. As the thickness varied from single layer to double layer, the intensity of the 670 cm−1 band decreased to half that of the single layer and then almost disappeared for thick flake. This relation was generally observed for other M-OEP molecules and confirmed in general (Figure
S5), demonstrating that single-layered graphene is the best material for an effective GERRS substrate. The cause of decreased signal intensity on multilayer graphene is still obscure, but it might be ascribed to the interaction between graphene layers which results in the electronic states splitting.42 This interlayer interaction between graphene layers induces weak interaction of absorbed molecules with graphene, inducing low polarizability of molecules. GERRS on oxidized graphene showed little enhanced Raman signal (Figure S6), demonstrating that the defect or the epoxide group on the graphene surface, determining the size of the sp2 domain within graphene, has an important role in signal enhancement of Raman scattering.23 Doping State of Graphene and Substrate Effect on GERRS. We also investigated the effect of graphene doping 2965
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Ministry of Science, ICT & Future Planning, Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2012M3A7B4049807), and the GRC project of KAIST Institute for the NanoCentury.
state on the GERRS signal intensity using a surface modification of substrate. GERRS spectra were measured from graphene transferred on the substrates modified by silane molecules with functional groups of CH3, CF3, and NH2, respectively (Figure 5a). Among spectra, the largest intensity was observed from the neutralized one, which might be originated from the relatively easy induced dipole of absorbed molecules on the neutral graphene. Figure 5b compares the RRS intensity of Pt-OEP measured from various substrates. The intensity increased as the substrate changed from SiO2 to graphene. It should be mentioned that the degree of enhancement from graphene/Au was higher than the only-Au substrate, and the highest level was observed from graphene/ SiO2. The same measurements were taken for 2H- and FeOEPs as shown in Figure S7. These substrate-dependent Raman spectra suggest that graphene has not only an interaction of charge transfer but also an effect on molecular polarizability through π-orbital interaction: however, further study is still needed to fully understand this phenomena.
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ABBREVIATIONS GERRS, graphene enhanced resonance Raman spectroscopy; M-OEP, M(Pt, FeCl, 2H)-octaethylporphyrin; CM, chemical mechanism; CVDG, chemical vapor deposited graphene; TVD, thermal vapor deposition.
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CONCLUSION The resonance Raman spectra for metalloporphyrins showed the enhanced signal intensity based on the graphene substrate, where the excitation wavelength was an important factor to get an efficient enhancement effect. The enhancement ratio was a maximum of 29 in Pt-OEP, varying with M-OEPs and vibrational modes. Also, the detailed analysis of Raman bands facilitated the identification of M-OEPs. The band broadening and peaks shift implied the interaction between M-OEPs and graphene via charge transferring and π-orbital interaction. The GERRS signal was increased 5.6 times on Ag nanoclusters deposited graphene compared to that on grapheme without Ag; however, due to the simultaneously enhanced photoluminescence, the latter was a better substrate for Raman signal detection of M-OEPs. The substrate-dependent Raman spectra showed that the single-layer graphene is the most effective Raman substrate. Our result demonstrates that single-layered graphene is an efficient Raman substrate for practical detection of metalloporphyrin molecules with high sensitivity and selectivity.
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ASSOCIATED CONTENT
S Supporting Information *
Detailed procedures for the preparation of graphene and molecule deposition, UV−vis absorption, detailed assignment of characteristic GERRS bands of M-OEPs at different excitation wavelengths, Ag nanoclusters image, and RRS comparison measured from various substrate (Figures S1−S7 and Tables S1−S3). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel +82-42-350-3342; e-mail
[email protected] (S.J.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the KUSTAR-KAIST Institute, Korea, under the R&D program supervised by KAIST, a grant (Code No. 2011-0031630) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the 2966
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