Design Of SERS Biosensors Based On Hyperbolic Metamaterials And Detection Application Research

The detection of small molecules at low concentrations is of great importance for food safety,biochemical research and medical diagnosis,and therefore biosensors with high sensitivity are of great practical importance in these fields.Currently,surface-enhanced Raman scattering（SERS）biosensors,surface plasmon resonance sensors and microfluidic electrochemical sensors have been applied to the field of small molecule detection.Among them,SERS biosensors are widely used for their ultra-high sensitivity,excellent selectivity,as well as rapid,efficient and non-destructive advantages.The enhancement of surface-enhanced Raman scattering for electromagnetic fields mainly relies on localized surface equipartition excitations（LSP）,which generate local electric fields through the collective oscillation of conduction electrons of noble metal nanoparticles,which are usually confined to the interstices of nanoparticles or sharp protrusions.Hyperbolic metamaterial is a kind of electromagnetic superstructure material,HMM is used in SERS biosensing because of their unique near-field electromagnetic wave manipulation properties and their ability to contribute to the enhancement of Raman spectra.Hyperbolic metamaterials generally consist of periodic stacks of metal/dielectric layers that meet certain conditions,which not only facilitate the propagation of bulk waves between the metal and dielectric layers,but also support the propagation of surface waves along the metal surface.The collective response is generated when surface plasmon excitations（SPP）on the surface of each metal film layer are coupled to each other,producing bulk plasmon excitations（BPP）in the hyperbolic metamaterial.The higher-order BPPs are capable of generating strong electromagnetic fields on the surface of hyperbolic metamaterials,which is important for optical applications such as waveguides and sensing.However,the excitation of BPP requires external coupling structures such as nanoantennas or gratings to satisfy the wave vector matching condition,thus decoupling the electric field,which is highly confined inside the hyperbolic metamaterial,to the surface and its near-field range.Therefore,in this study,we use silver nanoparticles as an external coupling structure to achieve excitation of BPP inside hyperbolic metamaterials and enhancement of Raman signals,and we prepare a composite SERS substrate of graphene/silver nanoparticle-coupled multilayer film and an array structure of nanoparticle-coupled hyperbolic metamaterials,respectively.（1）We first prepared a gold/alumina multilayer film structure by thermal vapor deposition,and then deposited silver nanoparticles onto the surface of monolayer graphene,and subsequently combined the two structures by inversion transfer method.The topmost layer of this composite SERS substrate is covered by graphene,which can avoid the effect on the Raman signal due to the contact between molecules and metals during the actual detection process,and can keep the silver nanoparticles from being oxidized,and also has the advantage of reducing the background fluorescence.Secondly,through the electric field simulation of the composite structure,the strong local electric field between the particles can penetrate the graphene well,and therefore,the substrate has a higher efficiency in utilizing the hot spot.The change of electric field intensity at the Au/Al₂O₃ interface of each layer also reflects the enhancement effect more intuitively,proving that the BPP in the multilayer film structure is successfully excited.This study not only demonstrates the significant enhancement effect of multilayer membrane structure on electric field,but also verifies that silver nanoparticles can be used to excite the restricted BPP.（2）For conventional two-dimensional SERS substrates with spatial constraints,the hot spot density within the laser detection range is also limited.Therefore,we introduced a three-dimensional array-based structure into the design of SERS substrates.Patterned sapphire array（PSS）structure is an arrayed substrate with large specific surface area,which has good transparency and conical sapphire pattern shape will produce more scattering of generated photons,which is very beneficial for optical applications such as optical biosensors and metamaterials.The patterned sapphire arrays were used as the basis for the preparation of hyperbolic metamaterials by thermal vapor deposition,followed by deposition of silver nanoparticles.The electric field of the structure was simulated and analyzed by comparing the electric field of different structures and the electric field variation inside the SERS substrate,and it was found that the electric field between the silver nanoparticles was significantly enhanced,indicating that the highly confined BPP in the hyperbolic metamaterial array was successfully excited and coupled to the LSP,in which the electric field in the hyperbolic metamaterial was also enhanced in an orderly manner.In addition,the SERS substrate also exhibited excellent hydrophobicity in the application detection with CV molecules as the molecules to be measured,overcoming the diffusion limit of the solution to be measured and further enhancing the Raman signal intensity to the detection limit of 10^-10 M.This study shows that silver nanoparticles as an efficient external coupling structure can excite the BPP inside the hyperbolic metamaterials well,and this array-based composite SERS biosensor is expected to be widely used in the field of small molecule detection.