Sensitive Detection By Surface Enhanced Raman Spectroscopy Based On Enrichment Of Molecules And Modulation Of Optical Path

Surface enhanced Raman spectroscopy（SERS）technology has been widely used in many fields,such as biochemical analysis,because it can obtain ultra-high sensitivity through the enhancement of huge electromagnetic field induced by surface plasmon polaritons,and even has the detection level of single molecular in the laboratory.However,in the practical application of SERS,especially in complex liquids,there are still many problems,such as the difficulty of enrichment of trace molecules in the enhanced area,which leads to its sensitive detection ability far from being demonstrated.Based on SERS enhancement principle and its practical application environment,SERS sensitivity can be improved from two aspects:detection samples（enrichment of molecules,selective capture,etc.）and enhancement of optical path（enhancement of substrate and modulation of optical path）.In this paper,the sensitive detection of SERS is studied from two aspects:enrichment of molecules and modulation of optical path.At present,SERS application scenario has been expanded from solid to liquid,and then online real-time detection is carried out.However,in the liquid medium,on the one hand,because of the Brownian motion of molecules,it is not easy to adsorb in hot spots;On the other hand,because the hot spot area is very narrow,it is a space of sub-wavelength range,and it is difficult to realize the sensitive SERS detection of trace molecules in low concentration solution.Therefore,researchers have made relevant explorations in molecules enrichment,including electric preconcentration method,liquid-liquid extraction method,etc.,to improve SERS sensitivity by enriching trace molecules in hot spots.However,there are still some problems such as long enrichment time and low SERS sensitivity.Based on this,we have carried out the research of enriching trace molecules in hot spots to improve the sensitivity of SERS detection,namely SERS detection assisted by tribo-electrophoresis and SERS microfluidic chip assisted by droplet extraction.In addition,the sensitivity of SERS detection can be improved by modulating optical path,such as the wavelength and polarization.Here we focus on the new SERS technology-remote SERS.The realization of remote SERS is based on light-excited surface plasmon waveguide,which can avoid the problem of high-power laser damage to samples in traditional SERS in-situ detection,especially to biological cells.However,due to the low coupling efficiency of optical waveguide,the detection sensitivity of remote SERS in application is not high.Therefore,in order to improve its sensitivity,we propose a remote SERS detection system with two-beam coherence enhancement.Therefore,in this paper,sensitive detection of surface enhanced Raman spectroscopy is mainly studied from two aspects:enrichment of molecules（triboelectric preconcentration,microdroplet extraction）and modulation of optical path（polarization,phase）:（1）The SERS detection sensitivity of aqueous solution is generally lower than that of solid phase because of the difficulty of enriching trace molecules in hot spots.Therefore,in this paper,a new type of ultra-sensitive SERS detection system based on electric preconcentration technology is proposed.The system uses triboelectric nanogenerator to provide electrostatic driving force to attract the target molecules in the solution to the SERS active substrate,thus realizing ultra-sensitive SERS detection of low concentration target molecules.Due to the unique output characteristics of high voltage and low current of friction nano-generator,compared with the enrichment efficiency of traditional DC power supply（10 min）,the triboelectric preconcentration technology can effectively enrich NB molecules(10^-8 M)in 30 s.Moreover,the experimental results show that this new triboelectric preconcentration SERS system can identify molecules at 10^-15 M level,and further selectively detect molecules with opposite charges under alternating electric field.Finally,we also designed a self-powered triboelectric preconcentration SERS detection device,which uses environmental mechanical energy to provide energy for the triboelectric nanogenerator,thus realizing outdoor real-time high-sensitivity SERS detection.This new SERS system has the advantages of high efficiency,super-sensitivity,ultra-portability,self-power,etc.,so it can provide a wider application prospect for SERS detection field.（2）The SERS detection sensitivity of high viscosity oil phase fluid is low because of the slow diffusion of trace molecules and the complex composition of oil solvent.Therefore,in this paper,a SERS microfluidic chip assisted by droplet extraction is designed,which is used for highly sensitive and effective detection and real-time monitoring of trace molecules in complex oil phase.Firstly,based on the different solubility of molecules in water/oil,the target molecule（fluorescein）in complex mineral oil was enriched into water droplets by droplet extraction technology.Using the real-time fluorescence intensity of fluorescein molecules in water droplets,the extraction performance was studied and optimized.Then,the water droplets with the best extraction effect were applied to Raman detection of furfural molecules in complex mineral oil（furfural molecules are an important feature to evaluate the aging state of oil-paper insulation equipment）,and the limit of detection（LOD）reached 26 ppb.In order to further improve the detection sensitivity,we injected the SERS-enhanced substrate into the micro-droplets to realize the detection of furfural molecules at a lower concentration.Compared to traditional detection technology of trace substances in complex oil（HPLC）,our method greatly simplifies the process of measurement,reduce the volume of sample,fasten the time of measurement and exhibit application prospect of real-time monitoring with high sensitivity,which not only promotes the development of oil quality but also enlarges the knowledge of Raman spectrum in chem/bio sensor.（3）Due to the low coupling efficiency of surface plasmon waveguide,the sensitivity of remote SERS is not high.In this work,we demonstrated that the remote SERS signals could be enhanced by 100%through the subwavelength interference in dual path excited Ag branched nanowire dimer and nanowire-nanoparticle systems.Our experiment has revealed that the remote SERS intensities could be modulated by the polarization and phase differences of two incident light illuminating at two nanowire terminals.The simulated electromagnetic field distributions through the FDTD method indicate the subwavelength interference occurs on Ag nanowire,which causes the Raman intensities collected at a remote site is greatly influenced by the coherent superposition of propagating surface plasmon polaritons.Our work on this coherent enhancement could not only promote the application of remote SERS,but also enlarge the research on light manipulating in subwavelength.