Griess reaction-based paper strip for colorimetric/fluorescent/SERS triple sensing of nitrite

This study demonstrates a novel strategy for colorimetric/fluorescent/surface enhanced Raman scattering (SERS) triple-mode sensing of nitrite based on Griess reaction modulated gold nanorods (GNRs)-Azo-gold nanoparticles (GNPs) (GNRs-Azo-GNPs) assembly. The p-aminothiophenol (PATP)-capped GNRs (GNRs@PATP) and 1,8-diaminonaphthalene (DAP)-modified GNPs (GNPs@DAP) are first synthesized through Au-S and Au-N bonds, respectively. Upon being excited at 365nm, the dispersion of GNRs-GNPs (GNRs@PATP and GNPs@DAP) emits cyan fluorescence. Next, the addition of nitrite into the GNRs-GNPs induces the formation of GNRs-Azo-GNPs assembly, resulting in the enhancement of color and decrease of fluorescence. Therefore, the GNRs-Azo-GNPs assembly can not only be used as a naked-eye indicator of nitrite changed from orange-yellow to purple, but also as a highly selective ''fluorescence quenching'' probe due to the fluorescence resonance energy transfer (FRET) between azo-moiety and DAP. The limit of detection (LOD) for nitrite is 0.05μM by colorimetry and 0.01μM by fluorescence. Meanwhile, the GNRs-Azo-GNPs assembly possesses controllable core-satellites nanostructures and enables on-field SERS detection of nitrite with the LOD of 0.8nM. More importantly, the GNRs-GNPs sensing system can not only be used as a paper-based test strip for on-site fast screening of nitrite with a high sensitivity and selectivity, but also as a SERS substrate for reliable quantitative analysis of nitrite. This study offers a new method for on-site visual detection of nitrite in human urine and meat products, as well as provides a strategy for designing multi-mode sensing platform for various applications.