Construction Of Novel Ag-TiO₂ Composite SERS Substrates And Its Applications For Analysis And Detection

In recent years,surface-enhanced Raman scattering(SERS)research based on noble metal-semiconductor composite nanomaterials and its analytical applications have become a current research hotspot.In addition to the electromagnetic enhancement(EM)caused by noble metal surface plasmon resonance(LSPR),the SERS enhancement effect also contributes to the charge transfer(CT)-induced enhancement between the adsorbed molecules and the SERS substrate.For TiO₂,its wide band gap(3.2 e V)leads to a high electron-hole recombination rate and a weak CT effect,which is not conducive to SERS detection applications.In this paper,a novel TiO₂-Ag composite SERS substrate was constructed by functional modification on the basis of traditional TiO₂-Ag composite nanomaterials.That is:using the biomimetic polymer-polydopamine(PDA)-derived nitrogen-doped carbon layer(NC)and PDA itself to improve the absorption of visible light by TiO₂,inhibit the recombination of photogenerated electrons and holes,and obtain better CT enhancement effect,The synthesized material was applied to the SERS rapid detection of the hypoglycemic drug phenformin(PHE)in human urine and two food colorants in pepper food,and satisfactory detection results were obtained,which is a useful tool for drug safety and food safety monitoring.The specific research contents are as follows:Part 1:Synthesis of TiO₂@NC/Ag nanomaterials and its SERS and antibacterial applicationsThe bifunctional applications of noble metal-TiO₂heterojunctions are an interesting topic,such as antibacterial agents and chemical sensors via surface-enhanced Raman spectroscopy(SERS).In this study,we reported the design and the fabrication of TiO₂spheres coated with an ultrathin nitrogen-doped carbon(NC)shell and subsequent loading Ag nanoparticles.This strategy endowed TiO₂spheres with a narrow bandgap(2.34 e V)and high fluorescence quenching ability.The TiO₂@NC/Ag composites exhibited excellent charge transfer-induced SERS performance with high enhancement factor of 8.0×10~6and good uniformity.The composites were further developed as a SERS sensor for rapid detection of phenformin hydrochloride in human urine with a low detection limit of 5 n M.In the antibacterial application,TiO₂@NC/Ag showed a promising bactericidal feature against Escherichia coli under low-power LED(6 W)irradiation,which immensely emerged the synergistic antibacterial role of TiO₂@NC and Ag.The inhibition rate under the LED irradiation for 8 h increased from 59.8%to 87.6%as compared to the rate in the dark.Part 2:Synthesis of TNRs@PDA/Ag nanoarrays and their application in SERS detection of illegal food dyesBifunctions of strong surface-enhanced Raman scattering(SERS)and enrichment abilities enabled by low-cost plasmonic metal-semiconductor nanoarrays are important for ultrasensitive SERS detection of illegal food dyes.To achieve the bifunctions,we reported the preparation and application of a SERS substrate of.TiO₂nanorod arrays@PDA/Ag(TNRs@PDA/Ag).Utilizing the structural features of PDA with the conjugated?structure and abundant hydrophilic groups,the modified PDA acted as an electron transfer mediator to promote visible-light absorption and electron transfer efficiency in the synergetic charge transfer system.Also,its structural features endowed the enrichment mediator of PDA for the efficient enrichment of rhodamine B(Rh B)and crystal violet(CV)from hydrogen bonding,?–?stacking and electrostatic interactions.A p H response of the modified PDA on the SERS activity was accordingly obtained.To estimate the feasibility of the SERS detection based on TNRs@PDA/Ag,separate and simultaneous detection of the two illegal food dyes was conducted.The corresponding linear equations were obtained,along with limits of detection of 1 n M for Rh B and 5 n M for CV.Even in real chili food,separate and multiplex detection was achieved with satisfactory results.The highly sensitive and multiplex SERS detection of dyes based on the bifunctional TNRs@PDA/Ag will offer great potential for practical applications in food safety.