Near-Infrared-Excitable SERS Measurement Using Magneto-Responsive Metafluids for in Situ Molecular Analysis

Metal nanoparticle clusters are regarded as metamaterials, and dispersions of nanoparticle clusters are regarded as metafluids. Surface-enhanced Raman scattering (SERS) from molecules adsorbed on the nanoparticle clusters is one of the notable properties of metafluids. SERS is expected to permit the realization of single-molecule detection in chemical and biological samples, especially cells and tissues. However, most SERS measurements have been done on substrates; local information on cells and tissues have been hard to obtain. SERS active particles can be used to measure the local information on cells. To analyze biological samples using SERS, the SERS substrate should be excitable in the near-infrared (NIR) region to ensure high transparency in biological tissues. Furthermore, transporting the SERS particles to the desired position is crucial for obtaining high resolution. Sizes of SERS-active particles also affect to the resolutions. In this report, gold nanoparticle clusters based on polymer core-shell particles incorporating magnetic Fe₃O₄ nanoparticles were prepared via a self-assembly method. Structures, LSPR absorption, SERS signals, magnetic responsibility of prepared particles were analyzed by electron microscope, UV-vis spectrum, Raman measurement, and optical microscope observation under magnetic flux, respectively. The enhancement factor of the SERS signal was determined by the size of composited gold nanoparticles. Furthermore, the migration direction of the gold nanoparticle cluster composite particles in aqueous media was successfully controlled by the application of an external magnetic field.