SERS-active MIL-100(Fe) sensor for sensitive detection of organic dyes and its enhancement mechanism study

Abstract

Metal organic frameworks (MOFs) has been considered as an ideal molecular capture material and has been widely used in many fields, including storage, catalytic, drug delivery and molecular separation due to its unique porous structure. MOFs contain organic linkers and metallic nodes, in which the aromatic ligands of MOFs have a strong π-π interaction with the aromatic structure of many organics, and the metal nodes are conducive to the coordination adsorption of the polarized parts of the molecule. Therefore, MOFs has a broad application prospect in the molecular detection field. Especially in recent years, researchers have continuously reported a series of SERS-active MOFs, which enriches the scope of metal-free semiconductor SERS platforms. In this study, MIL-100(Fe) was synthesized via hydrothermal method following the reported procedure with slight modifications, and then was characterized by scanning electron microscope (SEM), powder X-ray diffraction (PXRD) and porosity analyzer. Upon the adsorption of methyl blue (MB) and methylene orange (MO) onto MIL-100(Fe), different adsorption capability of MIL-100(Fe) to dyes were observed from their adsorption behavior curves. We found for the first time that MIL-100(Fe) can serve as an ideal SERS-active substrate for the capture and recognition of MB and MO but with different detection limits, 10−7 mol/L for MB and 10−5 mol/L for MO, respectively, indicating MIL-100(Fe) presents selective enhancement properties for these two organic dye molecules. In order to further increase the sensitivity of MIL-100(Fe) substrate for MO detection, the detection has been accomplished by depositing concentrated gold nanoparticle colloid onto the MIL-100(Fe) substrate, thus forming SERS “hot spots” which enable amplify the Raman signal of MO due to the electromagnetic enhancement (EM) effect. Density functional theory (DFT) was carried out to evaluate the binding position, molecular distance, binding energy and energy gap between dyes molecules and MIL-100(Fe). The energy gap between the MB and MIL-100(Fe) levels is 1.59 eV, close to the laser excitation energy ( E L=1.58 eV). This resonance suggests that the chemical enhancement (CE) mechanism may be operating, accounting for the high SERS activity of MIL-100(Fe) in MB detection. In addition, the adsorption energy between MB and MIL-100(Fe) is stronger than that of MO, this together with the CE accounts for the selective enhancing effect of MIL-100(Fe) on the two dye molecules.