Fabrication And Application Of Highly-Sensitive Surface-Enhanced Raman Scattering Sensors

Surface-enhanced Raman scattering(SERS)is a powerful spectroscopic technique.Since its first observation in 1970s,SERS has been developed into a more mature technique under the efforts of researchers.SERS possesses wide applications in various fields,including chemical and biological sensing,medical imaging,and environment and food safety field because of its unique advantages of high sensitivity(detection of trace molecules and even single molecule),fingerprint specificity,and nondestructive detection.However,one of the main challenges that limit its extensive applications is the lacking of effective SERS substrate.Therefore,designing and fabricating the ideal SERS active sensors with high sensitivity,uniformity,and application compatibility are extremely significant for practical applications of this technique.We know hot spots plays a key role in the SERS substrate,so a desirable SERS substrate should contain a large number of high-density hotspots.Based on above research background,this paper aims at designing and synthesizing highly-sensitive SERS sensors,the effective SERS substrates are mainly prepared by bottom-up methods.We explore the Raman enhancement mechanism by theoretical simulation.This paper also shows the practical application of the SERS sensors.The main research contents and results of this paper are list as follows:(1)The noble metal(such as gold,silver,copper)nanoparticles can produce local surface plasmon resonance effect under the irradiation of incident light,thereby generating great electromagnetic field enhancement.Therefore,noble metal nanoparticles are widely used in SERS field.The wet chemical synthesis method is the most commonly used method for synthesizing metal nanoparticles,but it is difficult to obtain highly monodisperse nanoparticles on the substrate using this method,and the surface of the synthesized nanoparticles often contains surfactants that are difficult to remove.This will affect the application of nanoparticles in SERS and catalysis field.Based on above challenge,We developed a chemical vapor deposition method to prepare high-quality gold nanoparticles.By further adjusting the experimental parameters,we successfully synthesized single crystal gold nanosheets.In the synthesis process,we only use chloroauric acid as the precursor without adding any reducing agent.Chloroauric acid decomposes under heating conditions,and the decomposition products are further deposited on the substrate,after a series of processes,we obtain the gold nanoparticles/nanosheets.The whole process is carried out in a tube furnace with nitrogen as the carrier gas.The main advantage of the vapor deposition method is that nanoparticles/sheets without any surface agent and contamination on the surface can be obtained.The development of this method is very important for the subsequent preparation of highly-sensitive SERS substrates.(2)Constructing extremely small nanogap is the most effective method to obtain highly sensitive SERS substrate.The disadvantage of the top-down fabrication method is that it is costly,time-consuming,and it is difficult to define sub-5 nm gap.So we design and synthesize a three-dimensional-stacked gold nanoparticles on Ti O₂ nanosheets for SERS sensor by a simple and low cost method,and we achieve highly-sensitive SERS detection using this substrate.The key to this structure is the ultra-thin sub-5 nm gap layer between two stacked gold nanoparticles.The Raman measurement results show that this SERS substrate can achieve extremely low detection limit(10 f M).Further control experiments and electromagnetic simulations indicate the Raman enhancement mechanism of this SERS substrate.Finally,in order to prove the application of this SERS substrate in actual detection,we demonstrate that the fabricated SERS substrate can be used for the trace analysis of melamine in milk.The SERS substrate has potential in practical applications.(3)We design and fabricate a SERS sensor which can be used for on-site detection by a simple method.This substrate is a sandwich configuration,the lower layer is a gold film substrate with optical thickness,the middle layer is probe molecular modified on the gold film,and the upper layer is a flexible polyvinyl alcohol(PVA)film embedded with noble metal nanoparticles.During the Raman detection process,the probe molecules are adsorbed on the gold film substrate,and then the flexible PVA film with noble metal nanoparticles is attached closely to the gold film by the post-assembly method.In such a configuration,the analyte molecules are confined at the interfaces between plasmonic nanoparticles and the metallic film to achieve the great enhancement of Raman signal.This research work provides a method for enhancing Raman spectroscopy with simple operation,low cost,high sensitivity and accurate results.Through further improvement,this substrate can be made into a fully flexible SERS substrate,which can detect pesticide residues on the fruits surface and the water pollution.Taking the detection of pesticide residues as an example,the advantage of this method is that it has better compatibility in practical applications.In addition,we use gold film instead of gold nanostructures to extract the pesticide,so the metal nanostructures can be well protected.Therefore,this SERS sensor has great application potential in actual detection.(4)A large number of studies have shown that porous metal nanostructure is a very effective SERS sensor.In this work,we used a simple method to prepare a nanoporous gold bowl structure for the SERS substrate.Importantly,in this structure,we can adjust ligament size by adjusting the experimental parameters to optimize the detection performance of the SERS substrate.The advantage of the nanoporous gold bowl structure used for SERS sensor is that it has a rough surface.On the other hand,we found that there is a strong local electromagnetic field in the nanopore area of the nanoporous gold bowl structure by FDTD simulation.So this structure contains a large number of hot spots,which can be used to enhance the Raman signal of the analyte molecule.The Raman measurement results show that the SERS substrate achieves a very low detection limit of 10^-15 M for 4-MBT molecules,indicating that the SERS substrate we prepared can be used for ultra-sensitive detection of molecules.Finally,we used this SERS substrate to detect caffeine.