Core–Shell Au@Metal–Organic Frameworks for Promoting Raman Detection Sensitivity of Methenamine

Abuse of methenamine in foodstuff is harmful to the health of people. Routine methods recommended by the national standard are indirect assays with complicated pretreatment of samples or less sensitivity. In this work, core–shell Au nanoparticles@inositol hexaphosphate@MIL-101(Fe) nanoparticles, designated as Au@MIL-101, are successfully synthesized by layer-by-layer assembly. Metal–organic framework (MOF; MIL-101)-modified AuNPs could narrow the distance between neighboring Au@MIL-101, which increases the amount of "hot spots" and contributes excellent enhancement of Raman scattering. In addition, certain target molecules could access the proximity to the "hot spots" by the strong interaction capability of MOF with −COOH groups. Taking the syngeneic effect of "hot spots" and chemical enhancement via specific binding, Au@MIL-101-based Raman protocol with huge sensitivity is developed to achieve direct detection of methenamine. It has good linearity of dynamic concentration from 3.16 × 10–6 to 1.0 × 10–8 M with correlation coefficient (R2) of 0.9908. The limit of detection reaches 5.0 × 10–10 M. As a practical application, such an Au@MIL-101-based Raman protocol could be used for the direct determination of trace methenamine in vermicelli, which meets the requirements of the national standard.