Integration of Nanophotonic Structures with a Metallic Nanoporous Air Filter for Particulate Matter Monitoring

The filtration and analysis of airborne particles have received substantial attention since the COVID-19 pandemic. Previous methods of analyzing airborne particles were conducted independently using a sampler, which requires a lengthy and complicated preparation process. In this research, we introduce a new concept of filter-type surface-enhanced Raman spectroscopy (SERS) substrate that allows the simultaneous collection and analysis of ultrafine nanoparticles in the air. SERS is an analytical technique that investigates small amounts of molecules without labeling, which predominantly utilizes the localization effect of an electric field on the nanophotonic surface. Such an analysis method is advantageous over conventional chemical sensors, as a wide range of analyte molecules are detectable. Furthermore, with the recent advance in the miniaturization of spectroscopic devices, there is an increasing potential for integration with Internet of Things (IoT) applications. The proposed substrate in this study is fabricated using the scalable electrochemical deposition of copper nano-dendrites followed by appropriate post-engineering processes to create a nanoporous metal membrane. The thin porous media is suitable for the physical filtration of small particulate matter in the air, and the excellent electrical conductivity of copper allows the capture of particles via electrostatic attraction. In addition, the SERS activation capability is achieved via electron beam deposition of gold. The gold-copper nanoporous membrane demonstrates a particle capture efficiency of ~96% for 300 nm particles under a charging condition of 5 kV with a differential pressure of 121 Pa. The captured particulates on the substrate can be monitored via Raman spectroscopy. We evaluate the performance of the substrate by measuring the Raman spectra of a 1 μM Rhodamine 6G solution and a bacterial sample (<i>P. aeruginosa</i>). Characteristic peaks are observed in both cases, which indicates successful detection. Additionally, we measure the Raman spectra from aerosolized Rhodamine 6G particles as small as 150 nm. The substrate exhibits excellent mechanical properties due to the roll pressing process, which simultaneously allows it to be reused via liquid phase rinsing. Even after undergoing repeated compression tests of ~22,000 cycles at 177 kPa, the substrate maintains functionality along with its filtering and sensing properties. We further conduct numerical simulations to elucidate the localized enhancement of the Raman signal from the nanoporous metal membrane. Finite-difference time-domain (FDTD) simulations of Maxwell's equations reveal amplification of the near field between the nanorod structures. Moreover, by analyzing the electrostatic potential using the finite element method (FEM), we confirm that the particles can be spontaneously transported toward the electromagnetic hotspot.