Fabrication And Properties Investigation Of SERS Substrates Based On Controllable Silver Nanostructures

Surface-enhanced Raman Scattering（SERS）is an abnormal optical phenomenon in which the Raman signals of molecules adsorbed on rough metal surfaces or in sol system are dramatically enhanced.It has been developed as one of the most efficient tool in surface and interface detection,as well as a powerful analytical way in biological medicine,analytical chemistry,food and environmental safety inspection due to its ultra-high sensitivity and the accurate and abundant information it offered.To make the most use of SERS technology,the key point is to fabricate active substrates with controllable hot spots.The basic principles of synthesizing SERS substrates mainly include improving the sensitivity,increasing controllability and reproducibility,as well as reducing the preparation cost.Previous reports suggest that noble metal nanomaterials such as gold and silver are the most stable ones that can be used in fabricating SERS substrates with high activity.Compared with Au nanomaterials,the SERS activity of Ag is higher,but its fabrication controllability is weaker.Therefore,the controllable fabrication of Ag nanostructures has been an important project these years.In this paper,we have skillfully fabricated several kinds of Ag nanostructures with different morphologies mainly using an efficient and low-cost method.Based on the idea of controllable fabrication,we also analyzed the important meaning of regulating hot spots in studying the activity of SERS substrates.The main results are as follows:（1）By using the low-cost,simple and environment-friendly method--the displacement reaction between Aluminum foils and AgNO₃solution,a series of typical silver dendrite micro-nanostructures with different coverages and morphologies were finally obtained by regulating the concentration of reactants and reaction time.They offered lots of nanoscale gaps and tips,thereby enhancing the SERS activity.We find that the SERS sensitivity increased when the density and coverage ratio of dendrites were improved.Crystal Violet（CV）was used as probe molecules,and the detection limit was about 10^-7 M.Moreover,the possible mechanism was inferred roughly through the observation of morphologies;（2）It is interesting that after the pretreatment on Al surfaces,a series of novel hierarchical silver nanostructures were achieved by employing surfactant（CTAB）to control the velocity of displacement reaction,with a higher density of hot spots and good uniformity in a large scale.A high SERS analytical enhancement factor of⁶.7×10⁸ was achieved mainly due to the abundant hot spots in these special hierarchical nanostructures,which is sufficient for single-molecule detection.Importantly,the three dimensional hierarchical structure can be firm for a long time without any evidence of collapse or deformation,which enables the long-term stability when serving as SERS substrates;（3）Usually,it is difficult to fabricate silver nanoparticles with uniform morphologies.In our work,large-scale silver NPs（nanoparticles）with uniform morphologies were successfully synthesized on zinc foils with controllable size by regulating the temperature of the displacement reaction.The result illustrates that the relationship between reaction temperature and the final SERS intensity is nonlinear;there exists a Favorable Temperature during this procedure.Here in our system,when the temperature was 70°C,the average size of silver NPs was approximately 88 nm in diameter（belongs to the range of hot particles）,and the high surface-average enhancement factor of³.86×10⁷ was achieved;furthermore,the SERS signals showed that the values of relative standard deviation（RSD）in the intensity of the main vibration modes were less than 10%,demonstrating excellent reproducibility of the silver NPs.