Surface-enhanced Raman spectroscopy for environmental monitoring using gold clusters anchored on reduced graphene oxide

Surface-enhanced Raman spectroscopy is a strong and sensitive analysis tool that can realize single-molecule level detection and provide the fingerprint information of molecules, which has been widely applied in analysing chemistry and biomolecules and monitoring environment. However, it is still a challenge to design and prepare SERS substrates with high enhancement factor, simple synthesis, stability and reproducibility. Here, we synthesized gold clusters anchored on reduced graphene oxide (Au clusters@rGO) using co-reduction method to achieve high SERS enhancement. The substrate of gold clusters anchored on reduced graphene oxide combines the chemical enhancement of reduced graphene oxide and the electromagnetic enhancement of gold clusters, leading to an ultrahigh enhancement factor of 3.5 × 10⁷. The efficient SERS was ascribed to the high localized surface plasmon resonance (LSPR) of aggregations of gold clusters, the synergistic effect of gold clusters and reduced graphene oxide, and the charge transfer between graphene and the molecules. This research will provide an invaluable strategy to design and prepare superior-property SERS substrates.

Graphene Biodevices for Early Disease Diagnosis Based on Biomarker Detection

The early diagnosis of diseases plays a vital role in healthcare and the extension of human life. Graphene-based biosensors have boosted the early diagnosis of diseases by detecting and monitoring related biomarkers, providing a better understanding of various physiological and pathological processes. They have generated tremendous interest, made significant advances, and offered promising application prospects. In this paper, we discuss the background of graphene and biosensors, including the properties and functionalization of graphene and biosensors. Second, the significant technologies adopted by biosensors are discussed, such as field-effect transistors and electrochemical and optical methods. Subsequently, we highlight biosensors for detecting various biomarkers, including ions, small molecules, macromolecules, viruses, bacteria, and living human cells. Finally, the opportunities and challenges of graphene-based biosensors and related broad research interests are discussed.

Structural effects of silver-nanoprism-decorated Si nanowires on surface-enhanced Raman scattering

Surface enhanced Raman scattering (SERS) is an important analytical tool for the optochemical detection of molecules. The enhancement is commonly achieved by engineering (i) novel types and morphologies of plasmonic nanomaterials, and (ii) patterned or roughened supporting substrates of high surface area for increased light scattering and molecule adsorption. Si substrates can be easily and reproducibly textured for effective SERS applications. In this work, silver nanoprisms (AgNPr) coated silicon nanowire (SiNWs) of different morphologies have been prepared by metal-assisted chemical etching and tested for SERS detection of R6G dye. By varying the etching time from 5 to 30 min, the nanowires' lengths increased from 2.4 to 10.5  µm and resulted in a variable topological morphology of the substrates in terms of bundles and valleys. We found that an optimum of 10 min etching time led to the highest SERS enhancement of R6G on AgNPr/SiNWs at 612 cm^-1 Raman shift (30× compared to R6G/Si and 2× compared to R6G/AgNPr/Si), with a detection limit comparable to that of state-of-the-art performances (down to 5×10^-10  M of R6G). Such an enhancement is attributed to a middle ground between increased overall surface area of SiNWs, and the available bundle tops trapping the AgNPr and R6G molecules.

Controlled Electrodeposition of Gold on Graphene: Maximization of the Defect-Enhanced Raman Scattering Response

A reliable method to prepare a surface-enhanced Raman scattering (SERS) active substrate is developed herein, by electrodeposition of gold nanoparticles (Au NPs) on defect-engineered, large area chemical vapour deposition graphene (GR). A plasma treatment strategy is used in order to engineer the structural defects on the basal plane of large area single-layer graphene. This defect-engineered Au functionalized GR, offers reproducible SERS signals over the large area GR surface. The Raman data, along with X-ray photoelectron spectroscopy and analysis of the water contact angle are used to rationalize the functionalization of the graphene layer. It is found that Au NPs functionalization of the "defect-engineered" graphene substrates permits detection of concentrations as low as 10^-16 m for the probe molecule Rhodamine B, which offers an outstanding molecular sensing ability. Interestingly, a Raman signal enhancement of up to ≈10⁸ is achieved. Moreover, it is observed that GR effectively quenches the fluorescence background from the Au NPs and molecules due to the strong resonance energy transfer between Au NPs and GR. The results presented offer significant direction for the design and fabrication of ultra-sensitive SERS platforms, and also open up possibilities for novel applications of defect engineered graphene in biosensors, catalysis, and optoelectronic devices.

Visible light-induced Ag nanoparticle deposited urchin-like structures for enhanced SERS application

A simple and new fabrication method of Ag nanoparticle deposited "urchin-like" structures has been reported by the visible light-induced approach. The as-synthesized structures show the deposition of ∼5 nm sized Ag nanoparticles with nanogaps, which can generate highly enhanced electromagnetic fields for higher activity of surface-enhanced Raman scattering. Thus, these structures are important for highly sensitive Raman scattering activity.

Seed-mediated synthesis and SERS performance of graphene oxide-wrapped Ag nanomushroom

A facile seed-mediated method was developed to modify core-shell Ag nanosphere@PSPAA with another Ag layer for achieving an enhancement of their surface-enhanced Raman scattering (SERS) activity. Interestingly, an Ag bridge in the polymer shell connected the inner and outer Ag layers, resulting in a mushroom-like nanostructure. The outer Ag grew around the polymer shell to form the cap of the nanomushrooms (NMs) with the extension of the reaction time. The epitaxial growth mechanism of this novel nanostructure was investigated by tuning the type of seed from nanosphere to nanocube and nanorod. With the growth of the outer Ag cap, the SERS intensity of these Ag NMs increased significantly together with the red-shifting and broadening of their typical localized surface plasmon resonance band. Such a phenomenon can be attributed to the formation of SERS hotspots between the inner and outer Ag layers. The Ag NMs were then wrapped with a graphene oxide (GO) shell via static interactions. The GO-wrapped Ag NMs exhibited a further better SERS performance in terms of sensitivity, homogeneity and stability compared with non-wrapped ones, indicating that the heterostructure could be potentially useful for SERS-based immunoassay.