Self-assembled Au Nanoparticle-based Flexible SERS Substrates For In Situ Detection And Ultrasensitive Detection

Surface-enhanced Raman scattering(SERS)technology can achieve ultra-sensitive detection and non-destructive detection,and it has high selectivity and unaffected by the fluorescence interference.SERS has been widely used in chemical,biomedical,food detection,environmental monitoring,heritage identification and other research fields.How to prepare the excellent SERS substrate is vital for the wide application of SERS technology in many fields.Compared with traditional rigid substrates,flexible substrates have good flexibility,and can wrap the objects with complex morphology,and also they are easily integrated with other devices.Therefore,the preparation of a highly sensitive flexible substrate in a suitable manner is of great significance for the wide application of SERS technology.In this paper,the in-situ detection of trace amount of the illicit drug and the illegal color additive in liquids are realized by designing different kinds of flexible self-assembled SERS substrates.The self-assembled monolayer film of gold nanoparticles with different sizes(20 nm,40 nm,60 nm,80 nm,100 nm),together with the self-assembled multilayer films(1,2,3,and 4 layers)of gold nanoparticles with the certain size(100 nm)have been produced on flexible transparent polydimethylsiloxane(PDMS)substrate through the oil-water interface self-assembly method.To evaluate the enhancement of substrates,rhodamine 6G(R6G)was selected as a probe molecule for SERS detection.The experimental results show that in this series of self-assembled single-layer SERS substrates with different diameters of gold nanoparticles,the diameter composed of gold nanoparticles with 100 nm has the largest enhancement ability,and the calculated enhancement factor is 2.81×10~7.Besides,when 100 nm gold particles were used to construct a series of SERS substrates with different layers,the substrate with a layer number of 2 has the greatest enhancement ability,and the calculated enhancement factor is 3.92×10~7.Finite-difference time-domain(FDTD)simulation verified the experimental results.Finally,the SERS substrate with the highest enhancement was used to realize the in-situ detection of trace amount of malachite green molecules in lake water and rhodamine B molecules in mixed juice.Since SERS technology is aimed at the highly diluted molecules,how to improve the detection sensitivity is of great importance.The common SERS substrates are mostly with the hydrophilic surface.When detecting hydrophilic molecules,the droplets of the molecules to be measured will be spread on the surface of the substrate,so that the number of molecules to be detected in the laser irradiation area is reduced,which limits the further enhancement of the SERS signal.When detecting hydrophobic molecules,the inherent non-polarity of the hydrophobic molecules prevents the molecules to be measured from approaching the polar hydrophilic surface,and a slight increase in the distance between the molecules to be measured and the surface of the substrate will result in a drastic decrease in SERS signal.In order to achieve ultra-sensitive detection of hydrophilic/hydrophobic molecules,we performed a series of surface modification on gold nanoparticle self-assembled monolayer to construct interleaved hydrophilic/hydrophobic microwell arrays.Next,a hydrophilic molecule(R6G)and a hydrophobic molecule(4-mercaptopyridine)were chosen to verify the concentration-enrichment effect.Based on the hydrophilic/hydrophobic microwell surface with different wettability,ultra-sensitive detection of hydrophilic R6G molecules and hydrophobic 4-mercaptopyridine molecules is achieved,respectively.This work provides new insights for the ultra-sensitive detection and the development of multifunctional SERS substrates.