A Study On Mechanism And Experiments Of Carbon Nanotubes-Ag Nanoparticles Composites For Surface-enhanced Raman Scattering

Since the surface enhanced Raman scattering(SERS)has been discovered,it has become a powerful tool for molecular detection,which due to its high-sensitivity,high-resolution,non-pretreatment,non-invasive and non-destructive.It has a wide range of applications in the environmental testing,food safety,life science and other fields.Combining SERS with microfluidic technology is expected to achieve high performance qualitative detection of trace substances.The SERS effect is the result of the interaction of substrates,the excitation of light source and the probe molecule.In view of the shortcomings about the current SERS substrates,such as low sensitivity and stability,and poor reusability,compared with other materials,carbon nanotubes(CNTs)have many advantages,such as large surface area,high surface reactivity and strong adsorption capacity,and combined with Ag nanoparticles which have strong electromagnetic enhancement effect,therefore,we propose carbon nanotubes films/arrays-Ag nanoparticles composite structures as highly active SERS substrates,to improve the enhancement factor,stability(effective life)as well as to reduce the cost of preparation and complexity of fabrication process.The carbon nanotubes films/arrays-Ag nanoparticles composites have been systematically studied from the preparation process,structure characterization,SERS properties and electromagnetic enhancement mechanism.The specific research contents are as follows:(1)The preparation process of carbon nanotubes films-Ag nanoparticles(CNTF-Ag NPs)composites was studied.The CNTF-Ag NPs SERS substrate was prepared by two methods: one is the chemical reduction method,which was completed in one step,the preparation process was simple,the cost was low,and the enhancement effect was obvious;the other was the chemical self-assembly method,the prepared carbon nanotubes suspension was mixed with silver sol,then ultrasound,the prepared Ag nanoparticles on the CNTF-Ag NPs substrate were controllable.The enhancement effect of CNTF-Ag NPs substrate prepared by chemical reduction reaction method was better than that of chemical self-assembly method,and the enhancement factor was about 1~2 orders of magnitude higher.The preparation method could also be extended to other materials or structures that could react chemically,such as reduced graphene oxide,gold nanoparticles,etc.and provided a reference for our preference in the experimental program.(2)The optical properties and SERS activities of CNTF-Ag NPs composites which prepared by chemical reduction reaction were systematically studied.The surface morphology,element composition and optical properties of carbon nanotubes,Ag nanoparticles and their composites were analyzed by scanning electron microscopy(SEM),X-ray diffraction(XRD),ultraviolet-visible spectrophotometer and Raman spectroscopy.A small area uniformity test was carried out on CNTF-Ag NPs composite by Raman position mapping,the relative standard deviation(RSD)value was less than 20%.The detection limit of CNTF-Ag NPs substrate was down to 10-14 mol/L with rhodamine 6G(R6G)as probe molecules,and the experimental enhancement factor(EF)was ~109.In addition,the stability and uniformity of CNTF-Ag NPs composite substrate were studied experimentally.The results showed that the R6 G Raman signal obtained after the sample was placed in the air for 65 days was reduced to about 15% ~ 16% compared to the results of one day,and the large area uniformity RSD value was less than 20%.(3)Carbon nanotubes arrays-Ag nanoparticles(CNTA-Ag NPs)composite SERS substrate was proposed,and its preparation method and optical properties were studied.The CNTA-Ag NPs substrate was prepared by magnetron sputtering and high temperature annealing.Compared with CNTF-Ag NPs substrate,the uniformity of the CNTFA-Ag NPs was improved,and the maximum RSD value was about 18%.The effects of different annealing temperatures and times of Ag nanoparticles on carbon nanotubes were studied by Ostwald ripening theory.The average particle size of the Ag nanoparticles increased with annealing temperature and time increase,the size distribution was mainly followed the Wagner distribution,and its growth mechanism was mainly determined by the chemical reaction growth mechanism.(4)The SERS properties such as Raman enhancement properties,stability,uniformity and reusability of CNTA-Ag NPs composites substrates were systematically studied.4-mercaptobenzoic acid(4-MBA)was used as probe molecule,the detection limit was 10-10 mol/L,the EF > 107,the values of RSD < 20%.The effective life of CNTA-Ag NPs could be reach more than 90 days,which was longer than that of CNTF-Ag NPs substrate prepared by chemical methods.Reusability experiments showed that it was better to use ethanol to clean the substrate than using deionized water to clean the substrate,ethanol cleaned more cleanly,however,after washing with ethanol,the Raman intensity of the probe on the substrate decreased by 5% ~ 10%.(5)The SERS effects of the evaporation process of the probe on the carbon nanotubes films/arrays-Ag nanoparticles composites were analyzed in detail.The time-course SERS mapping method was used for evaporation,the R6 G and 4-MBA were used as probe molecules.It was found that the Raman signals of the probe molecules increased first and then decreased with the increased of evaporation time.The main reason was that the capillary force caused the change of the "hot spots" of the substrate,which led to the change of the electromagnetic field enhancement.(6)The electromagnetic enhancement simulation of CNTF-Ag NPs and CNTA-Ag NPs composites were carried out.Combined with the experimental parameters,the electromagnetic field simulation model was established by finite difference time domain(FDTD)method,the simulation results were used to prove the correctness of the experimental results,and analyzed in detail.The results show that there were two kinds of "hot spots" in the carbon nanotubes-Ag nanoparticles composites: one existing between the carbon nanotubes and Ag nanoparticles,and the other existing between the adjacent Ag nanoparticles on the surface of carbon nanotubes.For CNTF-Ag NPs and CNTA-Ag NPs composite structures,the electromagnetic enhancement factor were ~109 and 2.7×107,respectively.(7)The SERS experiment of microfluidic system based on CNTF-Ag NPs enhanced substrate was preliminarily explored.The feasibility of Raman testing of enhanced substrate in microchannel was verified,but compared to the open SERS substrate,the SERS signal in micro-channel decreased by 87.6%.