Surface-Enhanced Raman Spectroscopy And Correlation Studies Based On Solid Nanostructure Substrates

It is profound for basic and application studies to investigate single-molecule tracing,calculation and chemical structure characterization.Laser-induced fluorescence spectroscopy was the original technique to realize single-molecule detection.However,on the one hand,available molecular structure information from this spectroscopy is extremely limited.One the other hand,measurements are frequently restricted to specific environmental conditions.Compared to that,Surface-enhanced Raman Spectroscopy?SERS?is ideal for single-molecule detection,identification and abundant chemical structure information,simultaneously.As an emerging analytical tool,surface-enhanced Raman spectroscopy?SERS?not only inherits remarkable advantages of traditional Raman spectroscopy such as molecular fingerprint identification specificity,high resolution,non-destructiveness and in-situ detection,but also capable of achieving tremendous sensitivity and high selectivity.Ever since this technology was published,it has attracted attentions from various fields such as chemistry analysis,biomedical area,environmental monitoring,and food safety,etc.It is a major challenge to invent substrates with high sensitivity,reproducibility,homogeneity and cost-effectiveness simultaneously.In addition,in ultra-low concentration solution environment,due to a variety of dynamic and complex processes including molecular adsorption,desorption,surface diffusion and interface reconstruction,there are characteristic time-dependent fluctuations in signal intensity.It is important for single-molecule manipulation and real-time monitoring to study molecular dynamics on substrate surface and spectral correlation.Based on aforementioned problems,in this article,we developed multiple novel multilayer composite nanostructures fabricated by template method.SERS effect of substrates and single-molecule detection reproducibility were improved continuously.By using Monte Carlo method,molecular dynamics model in SERS environment was built and simulated.Correlation analysis of SERS for ultralow concentration detection was analyzed from both experiments and simulation aspects.The main works and innovation points are listed as follows:?1?Optimization of gold nanolayer coated porous anodic alumina oxide as SERS-active substrate.Necessary modification was performed on commonly-used simulation model of Au/AAO.Based on frequency-domain finite element method,three-dimensional localized electromagnetic field excited by incident photoelectric field and corresponding reflectivity and transmissivity were simulated,by which the SERS enhancement mechanism of Au/AAO was explained.Localized electric fields of Au/AAO were simulated over independent variables to obtain optimal SERS performance.The substrate with corresponding optimal parameters was manufactured.SERS measurement results showed that SERS properties of Au/AAO substrate were effectively improved,this simulation method is feasible and reliable for parameter optimization.The simulation optimization method has a guiding significance for future design of SERS-active substrates.?2?Investigation on preparation and SERS properties of Au/CuCl₂/AAO multilayer composite nanostructure substrate.A new nanoflower-patterned nanostructure,Cu Cl2/AAO,was innovatively developed on AAO template by bottom-up self-assembling method.This type of nanostructure can be fabricated with controllable morphology and tunable unit size throughout large area.By combining with vacuum thermal deposition technique,the nano-flower structure coated with a gold nanolayer can be employed as SERS-active substrate.After optimization of surface morphology and thickness of gold nanolayer on Au/CuCl₂/AAO substrates,it was measured that the detectable lowest concentration of Rhodamine 6G?R6G?was 10-10 M.And the enhancement factor was evaluated to be 2.30 × 107,which is improved and represented better performance than its template served as SERS-active substrate.Reproducibility and homogenous hot spots across the region of interest were also achieved.Furthermore,the promising SERS activity of the flower-patterned substrate was theoretically explained through simulation of the electromagnetic field distribution using Finite Integration Technique?FIT?.?3?Fabrication and SERS performance optimization of GNP/Au/ZnCl₂ multilayer composite nanostructure.A type of novel nano-sheet structure with large area,controllable size and stable property was innovatively developed on AAO template.It can be employed as SERS-active substrate after gold nanolayer coated and gold nanoparticles?GNP?immobilized on surface.In order to achieve optimal enhancement effect,the localized electromagnetic field was simulated by using FIT.Both of the compared simulation and experiment results showed that the modified surface achieved optimal SERS effect other than GNP/Zn Cl2/AAO,Au/Zn Cl2/AAO and GNP/quartz,which is capable of detecting probe molecule with lowest concentration at 5×10-14 M and a evaluated enhancement factor of 1.9 × 108.The stereoscopic structure of nanosheets provides longitudinal distribution for gold nanoparticles,and the enhancement is the coupling of both effects of nanoparticles and Au/Zn Cl2/AAO substrate itself.The numbers of nanoparticles and integrated electric field intensity throughout entire measurement area were increased,respectively,by which the sensitivity and reproducibility were improved for practical application.This composite substrate has improved the stability of single-molecule SERS detection that has significant potential for applications in multiple fields.?4?Molecular dynamics and SERS correlation studies on gold nanoparticle substrate surface.Based on aforementioned works,it was observed that with the decrease of detectable lowest solution concentration,SERS signal started to fluctuate obviously over time.Enhancement effect at different positions distinguish from each other,and sharply weakens with the increase of distance between molecules and hotspots.The adsorbed molecules stay on surface for a certain amount of time.Among them,only those adsorbed near hotspots contribute to tremendous SERS enhancement.Therefore,single-molecule SERS measurement results can be severely affected by adsorption situation of molecules on substrate surface.With aforementioned considerations,taking gold nanoparticles as an example substrate,molecular motion in solution with low concentration was modelled under SERS conditions.Molecules in calculation unit are in one of three possible states of motion which are free Brownian motion,adsorption and desorption.Combined with Monte Carlo method,Python was employed as programming language to simulate coordinate changes of molecules in different states until the dynamic equilibrium is achieved.Adsorption status on substrate surface was fitted with Langmuir isothermal curve and the dynamic adsorption equilibrium condition was obtained.The results indicated that there exists a certain linear relation between adsorption equilibrium constant K and ratio of surface adsorption and desorption possibilities that are defined in our simulation system.By Combining the established molecular dynamic model with localized electric field distribution calculated by FEM method,integration intensity of molecules at different positions over time and space was simulated and converted to auto-correlation spectrum.From experiment aspect,a series of SERS spectra of ultralow-concentration probe solution at same point and micro-area were scanned.SERS intensity changes over time and correspond correlation spectra were obtained,by which the real-time monitoring on single molecule adsorption configurations was realized.Simulation and experimental works both indicated that SERS spectra inevitably present distinct signal correlations from fluorescence spectra.The residence time of adsorbed molecule in our system was determined.This analogy procedure is the basis of single-molecule SERS detection correlation,which has guiding significance for selection of SERS measurement integration time and improvement of single-molecule detection reproducibility.