A Novel SERS Substrate Platform: Spatially Stacking Plasmonic Hotspots Films
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Surface-enhanced Raman scattering (SERS) technique has presented great potential in medical diagnosis, environment monitoring, and food detection due to its high sensitivity, rapid response, and fingerprint effect. Many efforts have been concentrated on all kinds of strategies to produce efficient SERS platforms. Here, we report a simple and controllable method to produce large-area efficient SERS platforms with spatially stacked plasmonic hotspots. The SERS platforms consist of double-layer metal porous films and are easily fabricated by magnetron sputtering and annealing, assisted by the evaporation of hydrofluoric acid. The stacked dual-layer metal porous films show prominent Raman enhancement and ultrasensitive SERS sensing capability for different target molecules. The detection limit is demonstrated down to 10−13 M by detecting rhodamine 6G molecules. These superior Raman properties can be mainly ascribed to the highly dense spatially stacked plasmonic hotspots formed in the dual-layer metal porous films. The simple, controllable, and scalable fabrication strategy and superior Raman performance make these platforms promising candidates for the development of inexpensive, efficient, and mass-produced SERS substrates.
KeywordsSurface-enhanced Raman scattering Localized surface plasmonic resonances Plasmonic hotspots Raman sensing Au porous film structures
Localized surface plasmonic resonance
Surface-enhanced Raman scattering
Surface-enhanced Raman scattering (SERS), as a powerful analytical approach, can provide vibration information of target molecules and has been widely investigated and applied in various fields such as bio-sensing and imaging, medical diagnosis, environmental monitoring, and food detection [1, 2, 3]. The SERS technique relies prominently on the localized surface plasmonic resonance (LSPR) of metal nanostructures [4, 5]. It is well known that the free electrons existing in metal nanostructures would oscillate coherently when they interact with the incident photon under certain conditions and thus produce the LSPR [6, 7, 8, 9, 10]. The LSPR effects will result in the tremendously enhanced local electromagnetic field near sharp corners/tips and inter-/intra-nanogaps, i.e., plasmonic hotspots [5, 8]. Strongly enhanced electromagnetic field can significantly enhance the Raman signals of target molecules in the hotspots [5, 8], leading to morphology-dependent SERS behaviors and therefore providing the basis for analysis and detection through SERS effects.
To obtain expected SERS platforms with superior Raman performance, various metal nanostructures have been broadly researched by controlling their size, shape, composition, etc. For example, metal nanoparticles, rough metal films, porous nanostructures, periodic arrays, and other hierarchy structures have been fabricated by using electron beam lithography , template-assisted technology , electroplating , and chemical reaction and etching . However, apart from the high cost, sophisticated equipment, and time-consuming synthesis, most of these platforms suffer from a limited density of plasmonic hotspots distributed in either one-dimensional or two-dimensional (2D) space. Increasing the number of plasmonic hotspots is essentially to improve the chances to trap target molecules in the hotspots. Thus, from the practical point of view, one of the key research topics is to develop facile, low-cost methods to construct large-area platforms with high-density plasmonic hotspots.
Three-dimensional (3D) morphological metal nanostructures break the traditional limitation of two-dimensional SERS platforms and supply high-density plasmonic hotspots across all spatial planes within the laser illumination volume [15, 16] and thus increase the versatility of SERS platforms . Nowadays, many strategies have been put forward to build 3D SERS platforms . For example, dielectric nanorods/pillars decorated by Ag nanoparticles have been presented by electroless depositing , physical vapor depositing , reactive ion etching together with annealing and deposition . These hybrid 3D nanostructures exhibited excellent SERS enhancement due to the formation of high-density plasmonic hotspots with quasi-3D distributed patterns in the metal nanostructures. However, the reproducibility is still an important research direction in the future. The self-assembly of Ag colloidal nanoparticles , evaporation of the droplet of citrate-Ag sols on a fluorosilylated silicon wafer , and shaking of a mixture of water, decane, and functionalized Ag nanocubes  were recently used to obtain 3D SERS platforms. These methods bridge the wet and dry state methods and might allow SERS to be more widely used in various fields. But how to stabilize the constructed 3D geometry of nanoparticles in 3D solutions is still a challenge. Multi-petal flowers assembled by metal nanoparticles as SERS platforms were also engineered by using the spark discharge, ion-induced electrostatic focusing, and e-beam lithography . The number of plasmonic hotspots super-linearly increases with the petal number, and the Raman enhancement is sufficient for the single molecule detection. However, the expensive fabrication device limits its further development. Very recently, porous nanostructures [21, 22] were considered to be high-sensitive SERS platforms. However, the fabrication of these structures needs a long time or complex operation process. Overall, the 3D plasmonic hotspot platforms overcome the long-standing limitations of SERS for the ultrasensitive detection of various target molecules and promise to transform SERS into a practical analytical technique. Therefore, it is greatly vital to design and fabricate optimal plasmonic structures to obtain successful SERS platforms with rich plasmonic hotspots.
In this work, our motif is to design and obtain optimal SERS platforms with spatially stacked plasmonic hotspots. The SERS platforms consisting of double-layer metal porous films are created by integrating simple magnetron sputtering with thermal annealing, assisted by the evaporation of hydrofluoric acid. The SERS dual-layer porous Au films possess excellent fabrication flexibility, scalability and practicability. High sensitivity and good homogeneity are realized in our experiment by detecting different molecules such as rhodamine 6G (R6G), ascorbic acid, and 4-Mercaptobutyramidine (4-MBA). The detection limit is even down to 10−13 M for the R6G molecules owing to the appearance of spatially stacked plasmonic hotspots. The experiment results suggest that the dual-layer stacked porous Au films can be used for the novel and practical SERS applications in the biomedicine, food security, and environment detection.
First, ultrathin Au films were deposited on the clean SiO2 substrates via the magnetron sputtering. The sputtering time of ultrathin Au films was controlled by the sputtering speed and time. In this work, the sputtering speed was controlled to 32 nm/min and the sputtering time was changed from 19 to 75 s. Then, the sputtered ultrathin Au films together with the clean SiO2 substrates were put into the muffle furnace to be annealed at a certain temperature (180–220 °C) for about 30 min with the annealing speed of 2 °C/min. Single-layer porous Au film structures coated on the SiO2 substrates were thus formed due to the split and melt of Au material during the annealing process. Next, the SiO2 substrate coated with the single-layer porous Au film was placed in a sealed container. A beaker filled with 0.5 M hydrofluoric acid was placed below the samples. By gentle stirring (600 r/min) at room temperature, the hydrofluoric acid vapor would be produced due to the high volatility of hydrofluoric acid. One minute later, the surface of the SiO2 substrate was etched and became rough, which greatly reduced the adsorption capacity of Au film. After that, the etched SiO2 substrate coated with a single-layer porous Au film was slowly immersed in the de-ionized water until the single-layer porous Au film was separated from the SiO2 substrate and fully suspended in the water. Last, another SiO2 substrate coated by the same single-layer porous Au film was slowly inserted in the water at a small tilted angle (approximately 45°) to avoid the damage or bending of the suspended single-layer porous Au film during the coating process [23, 24] and then dried at room temperature. Consequently, dual-layer stacked porous Au films were formed on the SiO2 substrate. In this work, the fabrication condition and parameters of each layer Au porous film in the dual-layer structures are the same. Through the above steps, the sensitive SERS platforms were prepared by a simple method with no complex procedure involved.
Structural and Optical Characterization
The morphologies of samples were observed by scanning electron microscopy (SEM, Hitachi, S3400). The samples with double-layer stacked porous Au films were infiltrated into the solution of target molecules with different concentrations for about 24 h and then dried at room temperature. The target molecules included R6G, ascorbic acid, and 4-MBA (all purchased from Aladdin). The Raman spectrometer (Horiba Scientific, LabRRm 750) was employed to measure the Raman signals of target molecules adsorbed in the samples. The spectra were collected using a × 10 objective lens with a numerical aperture of ×100 and an excitation wavelength of 633 nm. The laser powers of 0.061 mW and 0.24 mW were used in this work. The signal collection time was 10 s.
Results and Discussion
We have illustrated a facile and economical method for fabricating an excellent mechanical flexible platform based on double-layer-stacked porous Au films as the SERS-active platform. The fabrication of SERS platforms only needs to incorporate the simple sputtering, annealing, stripping, and transfer methods, assisted by the vapor of hydrofluoric acid. High sensitivity and good homogeneity are realized in our experiment by detecting R6G, ascorbic acid, and 4-MBA. The detection limit down to 10−13 M is achieved for the R6G molecules adsorbed in the dual-layer porous Au films. Moreover, the SERS dual-layer porous Au films possess excellent fabrication flexibility, scalability, and practicability. The experiment results suggest that the dual-layer stacked porous Au films can be used for the novel and practical SERS applications in the biomedicine, food security, and environment detection.
This work was support by the National Natural Science Foundation of China (51761015, 11564017, 11804134, 11464019, 11264017, and 11664015), Outstanding Youth Program (2018ACB21005), Natural Science Foundation (20181BAB201015) and Major Academic and Technical Leaders (20182BCB22002) of Jiangxi province.
Availability of data and materials
All data generated or analyzed during this study are included in this article.
LT performed the experiments, analyzed the results, and wrote the manuscript. YL, QC, YL, and LS participated in the sample fabrication and characterizations. GL, ZL, and XL contributed to the data interpretation, manuscript writing, and supervising the research. All authors read and approved the final version of the manuscript.
The authors declare that they have no competing interests.
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