(397bh) Robust SERS Substrates With Hotspots On a Large Scale Derived From Massive Nanogaps Between Nano-Structured and Massed Silver Surfaces

Robust SERS substrates with hotspots on a large scale from massive nanogaps can be fabricated by assembling Ag nanocubes on the massed Ag surface via 1, 2-ethanedithiol monolayer as an ultrathin spacer. Noble metal nanoparticles such as gold nanoparticles and silver nanoparticles are used as SERS-active substrates and referred to as hotspots because the localized surface plasmon resonance (LSPR) occurs on the surface of metal nanoparticles. The hotspot effect is commonly observed from a magnified perspective during SERS measurement when nanoparticles are gathered together. Ag surfaces were linked in a nano-scale gap for incurring the substantial increase with several orders of magnitude in SERS signals due to the plasmonic interaction. Plasmonic resonance creates highly enhanced local electric fields in confined nanoscale locations. In order to take the advantage of strong LSPR around nanogaps, we envisage that it might generate hotspots on a large scale by assembling Ag nanocubes on the massed Ag surface due to its massive surface. Ag nanocubes are synthesized by polyol method and be assembled homogeneously on the massed Ag surface via 1, 2-ethanedithiol.

X-ray spectroscopyis used to confirm the existence of nanogaps. With the binding energy shift of Ag3d core level and feature peaks of the sulfur atoms on the substrate treated by 1, 2-ethanedithiol, we can know that Ag nanocubes self-assembled on the massed Ag surface by Ag-S chemical bonds and the nanogaps were formed between Ag nanocubes and the massed Ag surface.

 The plasmonic interaction between Ag nanocubes and the massed Ag surface was corroborated by UV-Vis spectra because the frequency change of plasmon resonance indicates the coupling of Ag nanocubes and the massed Ag surface. The strength and frequency change of plasmon resonance indicates the strong electrical field coupling between Ag nanocubes and the massed Ag surface. Since the intensity of Raman is proportional to the fourth power of the electric field strength, the huge changes in the electromagnetic field of Ag nanocubes self-assembled on Ag surface result in the substantial increase with several orders in magnitude in SERS signals. The rhodamine 6G (R6G) is used as a model compound to confirm SERS results.

Three subatrates were used in Raman measurments: (a)10^|1 M R6G solution on a microscope slide, (b) 10^|7 M R6G solution on SA, and (c) for 10^|9 M R6G solution on SA. With the homogeneous distribution of Ag nanocubes, and strong LSPR, the SERS results prove the supreme performance of the robust substrate by detecting 10^|9 Mrhodamine 6G solution with high sensitivity (enhancement factor 2.8×10⁸), high reliability (6.6% standard deviation from 20-sites measurements), and high precision (calibration line with 99.9% correlation coefficient). on the basis of 1 cm × 2 cm area. Furthermore, the straight calibration line with 99.9% correlation coefficient is obtained by using the concentration of R6G solution from 10^|6 M to 10^|9 M, confirming its practical application in quantitative analysis.

It is very difficult to arrange nanoparticles regularly on a substrate in the general case. The substrate with high degree of regularity used for Raman quantitative analysis is usually manufactured by FIB or E-beam etching which is heavy in cost and time. Our robust substrate, prepared by assembling Ag nanocubes on the massed Ag surface represents the homogeneous distribution of nanocubes, possessing the high EF and the low standard deviation for the potential application in quantitative analysis.