The tomorrow of mesoporous materials in plasmonics
A soft-enveloping synthetic strategy offers a robust approach to synthesize diverse mesoporous metal nanostructures that can be utilized in catalysis, optics, and biomedicine applications.
The paper in Nature Communications is here: http://go.nature.com/2EnWkVa
In my office, aspecimen of porous aluminium was exhibited in the book cabinet, which was prepared from my early experiments during my master stage. Just, maybe, owing to this early experience in the study of porous materials, in 2010, after I came back from Germany and worked in Xi’an Jiaotong University, an idea comes into my mind.
At that time, I started to involve in a new field of plasmonic nanooptics, and I wanted to create plasmonic hot spots for the application of surface enhanced Raman spectroscopy (SERS). In April of 2012, I went through the website of Prof. Dongyuan Zhao’s group in Fudan University in China who is a distinguished scientist in the field of mesoporous materials. Unexpectedly, I found two papers on the three-dimensional ordered mesoporous silica with ultralarge accessible mesopores1,2.
Then I contacted Prof. Zhao and explained my idea. What makes me proud is that Prof. Zhao also praised the idea and agreed to provide the materials for our further experiments.
Actually, the ordered mesoporous silica template1,2 is exactly the one that I was always seeking for the application of SERS. The characters of the mesoporous silica template1,2 are: a three-dimensional opened mesoporous structure, a large and tunable pore size from 15 nm to 37 nm, and a small wall thickness of less than ~ 5 nm.
Using this kind of ordered mesoporous silica as the template through nanocasting processes, a nanoparticle superlattice can be obtained. The particle size, around 20~30 nm, is the range of plasmonic effect; and, importantly, the uniform particle-particle gap of ~ 5 nm may result in a huge electromagnetic enhancement.
Thus, we carried out the experiments and this work was finally published in Advanced Optical Materials3. This work is the early one regarding the plasmonic nanooptics and SERS using nanocasting processes through ordered mesoporous materials as the template. A similar study using KIT-6 and SBA-15 as the template was published in Nanoscale4.
In order to investigate the influence of microstructure of particle-particle nanogaps on the electromagnetic field, we performed the in situ etching and probing using real-time SERS. A nanopore-in-nanogap hybrid plasmonic coupling mode can be created and contribute to the increase of SERS signal5.
However, until now, the controlled growth of metal nanocrystals in a confined space, i.e. mesoporous channels, has been very challenging. This is owing to, compared with oxide component, metal species have a relatively high mobility inside mesoporous silica; thus it is difficult to introduce the metal precursors into silica mesopores and suppress the migration of metal species during a reduction process6. As a result, the product may always mix with bulk particle, which can deteriorate the properties of materials. Recently, we successfully prepared pure WO3 mesostructures inside a KIT-6 mesoporous template using a heating reduction process6, while we failed in Au system by the same heating treatment.
We benefited from the expertise of our Lab members in the synthesis of noble metal nanocrystals in solution. Instead of a heating reduction process, I and Prof. You HJ developed a general soft-enveloping strategy7 to synthesize a variety of components and structures as reported in Nature Communications. The main contribution of this work is that we solved the long-term technique issue in preparing noble metal nanocrystals within a confined mesoporous channels using the soft-enveloping protocol.
These novel mesoporous materials may exhibit remarkable properties in a variety of fields such as catalyst, SERS and biological applications. In the future, we will investigate the related properties for the reported mesoscaled materials and further exploit the application of mesoporous materials in plasmonic related field.
The paper in Nature Communications is here: http://rdcu.be/GnvL
1. Wei. J., et al., Solvent evaporation induced aggregating assembly approachto three-dimensional drdered mesoporous silica with ultralargeaccessible mesopores. J. Am. Chem. Soc. 2011, 133, 20369
2. Wei. J., et al., Synthesis of dual-mesoporous silica using non-ionic diblock copolymer and cationic surfactant as co-templates, Angew. Chem. Int. Ed. 2012, 51, 6149.
3. Tian. C. F., et al., Plasmonic silver supercrystals with ultrasmall nanogaps for ultrasensitive SERS-based molecule detection. Advanced Optical Materials. 2015, 3(3), 404.
4. Tian. C. F., et al., Ordered mesoporous Ag superstructure synthesized via template strategy for Surface-Enhanced Raman scattering. Nanoscale, 2015, 7, 12318.
5. Ma. C., et al., Real-time probing nanopore-in-nanogap plasmon coupling effect on silver supercrystals with surface-enhanced Raman spectroscopy, Advanced Functional Materials, 2017, 27(2), DOI: 10.1002/adfm.201603233.
6. Zhang. L. L., et al., In situ probing of the particle-mediated mechanism of WO3-networked structures grown inside confined mesoporous channels. Small, DOI: 10.1002/smll.201702565
7. Fang. J. X., et al., A general soft-enveloping strategy in the templating synthesis of mesoporous metal nanostructures. Nature Communications, DOI:10.1038/s41467-018-02930-9.