Owing to its nondestructive nature, Surface-enhanced Raman Spectroscopy (SERS) has been widely recognized as an effective monitoring tool for in situ reaction process study. In comparison to other sensing techniques, SERS technique possesses a unique ability to near real-time monitor a reaction process in situ without destructing the innate reaction process. As such, SERS techniques can realize the nature of transient reaction intermediates and mechanistic pathways involved in heterogeneously catalyzed chemical reactions. It is said that SERS technique is capable of providing useful information for mechanistic studies and reaction system optimization. Thus, characterization of the binding and/or reaction of adsorbates at the surfaces of metals through the SERS techniques is of great importance for understanding and improving catalytic reactions. Nevertheless, insufficient sensitivity and reproducibility resulting from the poor SERS substrate quality cause great limitation of SERS as a in situ monitoring technique for a wide variety of reaction systems.

To tackle the issues mentioned above, a research group in Centre of Environmental and Energy Nanomaterials (CEEN), Institute of Solid State Physics, Hefei Institutes of Physical Science, made progress in fabrication of novel three-dimensional (3D) Fe₃O₄@Au@Ag nanoflowers assembled magnetoplasmonic chains via a magnetic field induced assembly and an in situ reduction method, followed by in situ SERS monitoring for kinetic studies of a catalytic reaction process as a dual-function SERS substrate. The related work was published in Journal of Materials Chemistry A entitled 3D Fe₃O₄@Au@Ag nanoflowers assembled magnetoplasmonic chains forin situSERS monitoring of plasmon-assisted catalytic reactions 

The research group fabricated one-dimensional (1D) assembled magnetoplasmonic nanochains (MPNCs) using Fe₃O₄@Au core–shell nanoparticles (NPs) via a magnetic field induced assembly.

With the help of silver growth solution, the 3D Fe₃O₄@Au@Ag nanoflowers assembled magnetoplasmonic chains (Fe₃O₄@Au@Ag NAMPCs) were prepared by in situ reduction method. The heterogeneous epitaxial growth mechanism was proposed to explain the growth process of Fe₃O₄@Au@Ag NAMPCs.

The Fe₃O₄@Au@Ag NAMPCs possessed a large number of hot spots within highly ordered structure and used as the SERS substrate to enhance the sensitivity (2.2×10⁹ of SERS enhancement factor), uniformity and reproducibility of Raman signal (the relative standard deviation (RSD) of the Raman intensity all bellowed 15%).

Moreover, the Fe₃O₄@Au@Ag NAMPCs have double roles both as an optical enhancer for SERS detection and as a catalytic substrate for the catalytic reduction of 4-nitrothiophenol (4-NTP) to p,p’-dimercaptoazobenzene (DMAB).

Through their study, researchers found that the reaction can be dramatically influenced by varying the duration of laser exposure and laser power. In this sense, the combined catalytic reactor with built-in non-destructive detection would be widely applicable in future chemical research.

This study was sponsored by the National Natural Science Foundation of China, Anhui Provincial Natural Science Foundation of China and CAS/SAFEA International Partnership Program for Creative Research Teams of Chinese Academy of Sciences, China.

SERS monitoring studies of the catalytic reaction on the Fe3O4@Au@Ag NAMPCs. (A) Schematic of SERS monitoring in the catalytic reaction of 4-NTP dimerization to form DMAB on the Fe₃O₄@Au@Ag NAMPCs; (B) The SERS spectra of the Fe₃O₄@Au@Ag NAMPCs substrate functionalized with 4-NTP at different time intervals; (C) The color coded intensity 2D SERS mapping of 4-NTP dimerization into DMAB at an excitation power of 1 mW; (D) Determination of the rate constants for the reaction of 4-NTP into DMAB at an excitation power of 1 mW; The SEM image of (E) the Fe₃O₄@Au@Ag NAMPCs and (F) the magnified image of the Fe₃O₄@Au@Ag NAMPCs (Image by ZHOU Hongjian)