Synthesis And Assembly Of Anisotropic Noble Metal Nanoparticles And Their SERS Property

In recent years,surface-enhanced Raman spectroscopy(SERS)technology has been widely applied in various fields such as food safety,life sciences,and environmental monitoring due to its high sensitivity and unique fingerprint specificity.The efficient SERS substrates constructed by a simple and low-cost method are critical for SERS technology.Compared with spherical nanoparticles(NPs),anisotropic noble metal NPs possess rich morphologies,highly tunable localized surface plasmon resonance(LSPR)properties,and strong electromagnetic(EM)field,which are widely used as the building blocks for solid SERS substrates.Therefore,this thesis mainly focuses on tailoring the shape of anisotropic noble metal NPs and the distribution of SERS hotspots(intensity,density,and spatial arrangement)to construct highly sensitive and uniform SERS substrates.The LSPR properties and EM field distribution of noble metal NPs are important to the SERS performance of substrates.The optical properties of Au nanospheres(NSs)and Au nanorods(NRs)were simulated by the finite-difference time-domain(FDTD)method,and the effect of size and gap on the electric field distribution of Au NRs were also investigated.It is found that the longitudinal LSPR of Au NRs is located in the near-infrared(NIR)region,which is beneficial to the electric field enhancement at NIR excitation.Furthermore,the electric field increases with the increase of the size of Au NRs and the decrease of the gaps between the dimer.However,the longitudinal electric field of Au NRs is always stronger than that of the transverse electric field.This provides a theoretical basis for tailoring the morphology of noble metal NPs and designing ultra-sensitive SERS substrates in experiments.A method to improve the SERS properties of substrates by enhancing the intensity of single-particle SERS hotspots is proposed.Au NPs with various shapes and multi-tip were prepared by underpotential deposition(UPD)technology.Furthermore,Au NPs with different morphologies were assembled into densely arranged two-dimensional(2D)monolayer film by interfacial self-assembly technique,their SERS properties was studied,and the enhancement factor of substrates was calculated.The results show that the Au nanostars with multi-tip exhibit high single-particle SERS hotspots and the monolayer film exhibits the best SERS property.The electric field distribution of Au NPs with different morphologies was simulated by the FDTD method,which further explained the experimental results.Three-dimensional(3D)SERS substrates exhibit higher sensitivity compared to 2D monolayer film.To enhance the SERS properties of substrates,3D rough Au NR SERS substrates with high hotspots density were designed through a layer-by-layer(LBL)assembly technique.Rough Au NRs with multi-tip were prepared by UPD technology,and then SERS substrates with adjustable density from sparse to multi-layer were prepared through electrostatic interaction combined with interfacial self-assembly technology.Therefore,the density of SERS hotspots is regulated and the properties of SERS substrates is optimized.It shows that the SERS properties of substrates and the density of SERS hotspots increase with the increasing nanoparticle density,which is in line with the electric field distribution simulation results.However,the intensity of SERS signal reaches saturation when the rough Au NR reaches 4layers,owing to the limited penetration depth of laser.To further improve the SERS properties of substrates,3D Au@Ag NR monolayer film over nanosphere(MFON)composite SERS substrates was engineered by LBL assembly technique,realizing the distribution of SERS hotspots in 3D space.Au@Ag NRs were first fabricated using seed-medicated growth method,and then broadband and ultra-sensitive Au@Ag NR MFON substrates were constructed by the LBL assembly technique.It is found that assembling Au@Ag NRs onto the polystyrene(PS)spheres is beneficial to fabricate broadband SERS substrates.Compared to Au@Ag NR monolayer film,Au@Ag NR MFON substrates exhibit an improved SERS properties and enhanced electric field at different excitation wavelengths.Besides,the SERS properties of substrates is further optimized by adjusting the layer numbers of Au@Ag NR film and the types of support,respectively.It is found that the Au@Ag NR MFON substrates show high sensitivity,excellent uniformity,and a wide quantitative detection range.