Mildly reduced graphene oxide-Ag nanoparticle hybrid films for surface-enhanced Raman scattering
© Li et al; licensee Springer. 2012
Received: 16 February 2012
Accepted: 3 April 2012
Published: 3 April 2012
Large-area mildly reduced graphene oxide (MR-GO) monolayer films were self-assembled on SiO2/Si surfaces via an amidation reaction strategy. With the MR-GO as templates, MR-GO-Ag nanoparticle (MR-GO-Ag NP) hybrid films were synthesized by immersing the MR-GO monolayer into a silver salt solution with sodium citrate as a reducing agent under UV illumination. SEM image indicated that Ag NPs with small interparticle gap are uniformly distributed on the MR-GO monolayer. Raman spectra demonstrated that the MR-GO monolayer beneath the Ag NPs can effectively quench the fluorescence signal emitted from the Ag films and dye molecules under laser excitation, resulting in a chemical enhancement (CM). The Ag NPs with narrow gap provided numerous hot spots, which are closely related with electromagnetic mechanism (EM), and were believed to remarkably enhance the Raman signal of the molecules. Due to the co-contribution of the CM and EM effects as well as the coordination mechanism between the MR-GO and Ag NPs, the MR-GO-Ag NP hybrid films showed more excellent Raman signal enhancement performance than that of either Ag films or MR-GO monolayer alone. This will further enrich the application of surface-enhanced Raman scattering in molecule detection.
Surface-enhanced Raman scattering (SERS) is an alternative to fluorescence (FL) detection. It greatly amplifies the Raman signal of analytes adsorbed on metal surface . Due to their good chemical stability and bio-compatibility, Au and Ag films are typically regarded as the good SERS substrates [2, 3]. To enhance the Raman signal from adsorbed molecules, a rough metal surface with different morphologies is normally needed for highly sensitive SERS substrates . Many methods have been employed to fabricate various SERS-active morphologies, such as electrochemically roughening metal foils, depositing rough films on substrates, spraying metal colloids on substrates, plasma treating metal films, and fabricating nanostructures by lithographic techniques or in porous anodic alumina templates [2, 5–8]. However, the fabrication processes either have the rigorous requirement for equipment or give a limited Raman sensitivity. Furthermore, both the Au and Ag films have a strong photoluminescence (PL) background under laser excitation, which leads to a huge difficulty in obtaining the detailed molecular vibrational information, especially the fingerprints of FL molecules .
Graphene, a single-layered planar sheet of carbon atoms arranged in a hexagonal pattern, has been receiving increased attention due to its exceptional electronic, thermal, optical, and mechanical properties [10–13]. Recently, graphene-enhanced Raman scattering (GERS) has attracted much interest because it does not support the electromagnetic mechanism (EM) but only supports the chemical enhancement (CM), which always coexists with and is overlapped by the former mechanism [12–16]. It was found that n-layer mechanically exfoliated graphene (MEG) and mildly reduced graphene oxide (MR-GO) can effectively quench the FL signal from probe molecules and result in an excellent Raman signal enhancement effect with an enhancement factor of 2 to 17, depending on the vibrational modes of molecules [13, 14, 17, 18]. Simulations and experiments also indicated that MEG monolayer could effectively quench the PL signals from Au or Ag films under laser excitation due to the charge transfer or resonance energy transfer from metal films to the MEG substrates [19–21]. This implies that the combination of graphene with Ag or Au films will be a more excellent SERS-active substrate than Au or Ag films alone due to the Raman signal cumulative effect resulting from metal-induced electromagnetic field, GERS effect, and the resonance energy transfer between metal particles and graphene films. Unfortunately, until now, only a few reports focus on the SERS effect of graphene-metal hybrid material based on micro-size randomly distributed MEG substrates [20, 22, 23]. Furthermore, the distribution of metal particles on graphene surface via e-beam evaporation is nonuniform, greatly weakening the EM-induced SERS effect. Therefore, it is urgently needed to find a simple method for fabricating a large-area, uniformly distributed graphene-metal nanoparticle (NP) hybrid material.
In previous studies, it was found that MR-GO has the optimized GERS effect among all RGO samples with different reduction durations. Based on the result and considering the matrix role of graphene oxide/reduced graphene oxide, in this study, we fabricated the MR-GO-Ag NP hybrid films using the self-assembled MR-GO monolayer as the template, with sodium citrate as a reducing agent under UV illumination. With rhodamine 123 (Rh123) as a probe, the Raman signal enhancement of the as-prepared MR-GO-Ag NP hybrid films was investigated, and the contributions of graphene and Ag NPs were discussed and analyzed.
Natural graphite (grade 500 mesh) was purchased from Qingdao Tianhe Graphite Co., Ltd. (Qingdao, China). Other reagents, concentrated sulfuric acid (98%), potassium permanganate, hydrogen peroxide (30%), hydrazine hydrate (24% to 26%), ammonium hydroxide (28%), Rh123, and 3-aminopropyltriethoxysilane (APTES), were obtained from Sigma-Aldrich Corporation (Singapore). Silver nitrate was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
Preparation of GO and MR-GO
Graphite oxide was synthesized by using a modified Hummers' method and described as elsewhere . Exfoliation of graphite oxide to graphene oxide was achieved by ultrasonication of dispersion in an Elma S30 (Elma Hans Schmidbauer GmbH & Co. KG, Singen, Germany) ultrasonic bath for 2 h. The obtained brown dispersion was then subjected to 30 min of centrifugation at 6,000 rpm to remove unexfoliated graphite oxide. For the chemical conversion of GO to MR-GO, 5 ml of the resulting supernate was uniformly mixed with 5.0 μl of hydrazine hydrate and 35 μl of ammonium hydroxide in an 8-ml glass vial and heated at 90°C for 10 min. The concentration of black MR-GO colloid solution was estimated to be approximately 0.1 mg/ml and employed in the succeeding self-assembly process.
Self-assembly of single-layer MR-GO on Si substrate
Prior to self-assembly, all substrates were treated in Piranha solution (H2SO4/H2O2 = 3:1, v:v) to clean the organic compounds and provide a hydroxylated surface. After being thoroughly rinsed with deionized (DI) water and dried by N2 flow gas, the substrates were immersed into the freshly prepared APTES solution (5 mM, acetone/water = 5:1, v:v) for 10 min. Then, the substrates were washed with acetone and immersed into the as-synthesized MR-GO solution for 20 min. Finally, the substrates were rinsed with copious DI water to remove the excess MR-GO nanosheet and obtain the MR-GO monolayer on SiO2/Si substrates.
Fabrication of MR-GO-Ag NP hybrid films
For the synthesis of MR-GO-Ag NP hybrid films, MR-GO monolayer-covered SiO2/Si substrates were immersed into a beaker containing 20 ml of 1 mM AgNO3 aqueous solution and 1 ml of 1% sodium citrate aqueous solution. Then the beaker was placed under the UV light and illuminated for 0.5 h. Finally, the substrates were taken out, thoroughly washed with DI water to remove the residual sodium citrate and AgNO3 solution, and dried by N2 flow gas.
Adsorption of dye molecules on sample surface
Rh123 molecules were adsorbed on the MR-GO-Ag NP surface by simply soaking the SiO2/Si substrates into the ethanol solution of Rh123 with a concentration of 1 μM for 60 min. The excess Rh123 molecules were removed from the sample surface by carefully rinsing the substrates into pure ethanol solution.
Raman spectra were recorded using a Renishaw system (Renishaw Pte Ltd., Singapore) with an excitation energy of 514 nm. Surface morphology of MR-GO monolayer was obtained by LEO 1550 (Angstrom Scientific Inc., Ramsey, NJ, USA) field emission scanning electron microscopy (FESEM) and Nanoscope IIIa atomic force microscopy (AFM) (Digital Instruments/Veeco Metrology, Singapore).
Results and discussion
The excellent SERS effect of MR-GO-Ag NP hybrid films is closely correlated with their typical structures and compositions. It can be accounted for by considering the following reasons: (a) the strong EM enhancement effect from Ag NPs, (b) the obvious GERS effect resulting from MR-GO monolayer, and (c) the interaction between the Ag NPs and MR-GO monolayer. It is well known that Ag is a SERS-active metal. Under laser excitation, the strong electromagnetic coupling produced between NPs will greatly increase the SERS intensity of adsorbed molecules. The enhancement effect is roughly proportional to |E|4 (where |E| is the intensity of the electromagnetic field produced on metal surface) and can get to 108 or more. Wang et al.  reported that when the interparticle gap of Ag NPs is reduced below 15 nm, the Raman signal intensity becomes significantly strong. In our experiment, the interparticle gap of Ag NPs on MR-GO monolayer ranges from 3 to 15 nm, in some cases, even smaller. We believe that the uniformly distributed Ag NPs with small interparticle gap provide a high concentration of so-called hot spots, which greatly boost the Raman signal intensity of Rh123 molecules adsorbed on their surface. For the contribution of GERS effect, as mentioned above, the residual oxygen-contained groups on MR-GO surface could provide an extra electric field under laser excitation, which will further increase the intensity of electric field produced on the MR-GO-Ag NP surface and lead to a stronger Raman signal intensity of Rh123 molecules on MR-GO-Ag NP hybrid films than on Ag films alone. Additionally, Rh123 is conjugated and contains the benzene ring in their molecular structure, similar to that of graphene. When it is deposited on MR-GO-Ag NP hybrid films, the vibrational signal of Rh123 is notably enhanced due to the π-π stacking. The excellent SERS performance of MR-GO-Ag NP hybrid films is also closely correlated to the interaction between Ag NPs and underlying MR-GO monolayer. On one hand, due to the presence of Ag NP hot spots, the local electric field produced on MR-GO-Ag hybrid film under laser excitation is greatly enhanced. On the other hand, the charge transfer between graphene and Ag NPs is also important and should be considered. Recent reports indicate that, when graphene is in contact with Ag, it gains electron density as its Fermi level is shifted down to nearly -3.5 eV, implying that the charge-transfer complexes can be formed between grapheme and Ag NPs [32, 33]. This will make graphene receive the excited electronic and vibrational excitations resulting from Ag NPs and Rh123 molecules under laser excitation [14, 34]. As a result, the FL signals resulting from Rh123 and Ag NPs are remarkably suppressed, and correspondingly, Raman peaks of Rh123 are greatly protruded, especially within the wavelength region of 200 to 1,000 cm-1. We believe that this coordination mechanism between the Ag NPs and MR-GO plays a significant role in the excellent SERS performance of MR-GO-Ag NPs.
We have successfully synthesized the MR-GO-Ag NP hybrid films with MR-GO as the template and sodium citrate as the reducing agent under UV illumination. By using the Rh123 molecule as a probe, we also explored the application of as-synthesized MR-GO-Ag NPs in SERS. It was found that the MR-GO-Ag NP hybrid material is a better SERS substrate than sputtered Ag films or MR-GO monolayer alone. In the MR-GO-Ag system, the presence of MR-GO monolayer not only effectively quenches the strong FL background from Rh123 and Ag films under laser excitation but also greatly increases Raman signal of Rh123 molecule. The coordination mechanism between Ag NPs and MR-GO nanosheets plays a significant role in the excellent SERS performance of MR-GO-Ag NP hybrid films. We believe that our results could further expand the applications of surface-enhanced Raman scattering in molecule detection.
This work was supported by Temasek Laboratories@NTU (TL@NTU) Research Seed Funding.
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