Surface-Enhanced Raman Scattering Performance Research Of Silver/Titanium Dioxide Three-Dimensional Arrays

Surface-enhanced Raman spectroscopy (SERS), which can identify the structure of the material due to its high sensitivity, is called the fingerprint spectrum. The basic principle is that the interaction of the incident light and nearly free electrons on the surface of the metal, surface plasma resonance (SPR) makes the local electric field enhanced leading to enhancing SERS signals. SERS techniques not only include the advantage of Raman spectra but overcome the defect of low Raman spectrum signal strength, so it is widely used in many fields and gets more attention.Based on widely application of SERS technology for micro/trace organic pollutants in environmental detection, harmful material detection and broad application in the biological molecules biological detection, etc, how to prepare high SERS-active substrate with recyclability to reduce the waste had become hot topics in the study of Raman.In this paper we designed the silver nanoparticles (Ag Nanopaticles, Ag NPs) composite titanium dioxide nanorods array (TiO2 nanorod array, TRA) of the composite structure substrate (Ag nanopaticles/TiO2 nanorod array, Ag/TRA) for the dye molecules of water pollution detection and analysis of organic molecule ATP and ADP in biology. TiO2 nanostructures loaded noble Ag nanoparticles (NPs) should have many advantages over other photoreaction methods because Ag could not only enhance TiO2 photocatalytic efficiency under visible light irradiation due to the strong interaction between the incident light and electrons in noble metal and also Ag deposits have higher SPR to provide high SERS activity. Actually, the deposited Ag NPs would act as traps to capture the photoinduced electrons and holes, leading to the reduction of electron-hole recombination in photocatalytic processes. Additionally, recent attempts to improve the SERS effect have focused on the construction of three-dimensional (3D) nanoarchitectures with nanogap rich metal NPs, which could generate more effective and adequate’hot spots’for the creation of a strong and tunable electromagnetic field within the detection volume. Therefore, based on the peculiar properties of Ag and TiO2,3D Ag NP-decorated TiO2 nanostructures would be excellent candidates for molecular sensing with high SERS sensitivity and good recyclability. In the paper, we had described the morphology structure of the Ag/TRA, SERS features, Raman enhancement mechanism and recyclable studies in detail, at the same time the groundbreaking research of adenosine triphosphate (ATP)、adenosine diphosphate (ADP) SERS tests were innovative and important guiding significance in the field of biology about ATP. The main content of the paper is as follows:First, the preparation of Ag/TRA SERS-active substrate. Multifunctional Ag nanoparticle-decorated TiO2 nanorod arrays were prepared by two simple processes:TiO2 nanorod arrays were firstly fabricated by the hydrothermal route and then Ag NPs were decorated on the nanorods by the chemical reduction impregnation method. Basal sweep electron microscope (SEM) and transmission electron microscopy (TEM) images shows that the diameters of prepared titanium dioxide nanorods and the silver nanoparticles were about 100 nm and 30 nm respectively. And the spacings of rutile titanium dioxide and silver grown in the direction of the (111), were 0.320 nm and 0.238 nm respectively. This three dimensional structure has two kinds of hot spots conducive to Raman enhancement. X-ray diffraction (XRD) furtherly illustrates the titanium dioxide and silver successful composited together well. Uv-vis absorption spectrum results show that the compound Ag NPs greatly enhance the visible light absorption ability, which is advantageous to the titanium dioxide to visible light absorption in order to enhance its photocatalysis ability. Based on the features of Ag and TiO2,3D structure of silver nanoparticles TiO2 nanoarray with high sensitivity and good recyclability will makes it an appropriate substrate of molecular detection of candidate targets.Second, the Ag/TiO2 substrate was employed as an active and recyclable substrate for SERS tests of dyes. The results show that the detection limit for rhodamine 6G (R6G) was as low as 10-7 M and the Raman enhancement factor was as large as 105 through saturated adsorption method. The relative standard deviation (RSD) calculated from 100 points of the substrate is lower than 22% indicating good reproducibility. The enhancement factor (EF) of the electromagnetic enhancement from the 3D finite difference time domain (3D-FDTD) simulation is lower than 105, this is ascribed to that some other enhancement are from the scattering of the complex structure and the charge transfer between TRA and molecules. After calibrating the Raman peak intensities of R6G, it could be quantitatively detected. More importantly, the photocatalytic activity of TiO2 provides self-clean capability of the formed SERS substrate which is now recyclable and can be used to degrade many Ag surface adsorbates such as R6G, MO, CR and MB after exposure the substrate under visible light. The absorbed small molecules can all be rapidly and completely removed from the SERS substrate which is successfully reused for four times without the decrease of accuracy and sensitivity. Our results reveal that the unique recyclable property not only paves a new way to solve the single-use problem of traditional SERS substrates but also provides more SERS platforms for multiple detections of other organic molecular species.Third, the SERS application of Ag/TRA in the field of biology. The ATPrADP molar ratio is an important physiological factor. However, in previous literatures, ATP and ADP could not be distinguished by Raman spectrum due to the high similarity of molecular structure. To challenge this problem, we considered that the y phosphate group may interact with adenine group to cause a different variation of the Raman spectrum than that of ADP; thus, the Ag/TiO2 arrays substrate was used to detect the ATP:ADP molar ratios. The detection limits as low as 10-7 M for ATP and ADP are obtained. The SERS study on various ATP:ADP molar ratios demonstrate that ATP:ADP could be distinguished and the quantitative detection of ATP percent was well achieved. Moreover, a reasonable principle is proposed to explain the achieved linear relationship between the area ratio of the Raman peaks centered at-685 and-731 cm-1 and the molar ratio. The results reveal that a new method had been developed for based quantitative detection of ATP:ADP molar ratio based on Ag/TiO2 arrays substrate by the SERS method. In addition to that, two kinds of reaction equations were used to achieve the quantitative detection of ATP by difference method successfully, which is a new method of quantitative ATP indirectly. This part of ATP research has important guiding significance in the field of biology about ATP.