Remote Raman Detection of Trace Explosives by Laser Beam Focusing and Plasmonic Spray Enhancement Methods

Remote Raman spectroscopy is a technique that can detect and identify different target molecules through Raman vibrational modes from a remote distance. However, the current remote Raman technique is restricted by poor detection sensitivity, and it is still extremely challenging for trace explosive detection. Here, in order to achieve trace explosive detection from a remote distance, we innovatively propose two enhanced Raman spectroscopy methods by using a plasmonic spray and a laser beam focusing/Raman signal collecting instrument. In brief, a facile convex lens can converge the laser beam and collect Raman scattering signals, and a plasmonic spray can be used for surface-enhanced Raman scattering. Under the combination of the above enhancement methods, we achieve remote Raman detection of a variety of trace explosives with a concentration of ∼1 μg/cm² from a distance of 30 m. These novel methods demonstrate a simple approach that significantly improves the capability of remote detection of trace chemicals for further applications.

Synthesis of monodisperse spherical AgNPs by ultrasound-intensified Lee-Meisel method, and quick evaluation via machine learning

Due to the high reactivity of Ag⁺ and uncontrolled growth process, the AgNPs produced by conventional Lee-Meisel method always exhibited larger particle size (30-200 nm) and polydisperse morphology (including spherical, triangular, and rod-like shape). An ultrasound-intensified Lee-Meisel (UILM) method is developed in this study to environmental-friendly and controllable synthesize monodisperse spherical AgNPs (~3.7 nm). Effects of Ag:citrate ratio (1:3 or 5:4), ultrasound power (300 to 1200 W) and reaction time (4 to 24 min) on the physical-chemical properties of AgNPs are investigated systematically. The transmission electron microscope (TEM) images, UV-Vis spectra, average particle size, zeta potential and pH value all demonstrate that crystallization and digestive ripening processes are facilitated in the presence of ultrasound irradiation. Therefore, both chemical reaction rate and mass transfer rate are enhanced to accelerate primary nucleation and inhibit uncontrolled particle growth, leading to the formation of monodisperse spherical AgNPs. Moreover, a machine learning approach - Decision Tree Regressor in conjunction with Shapley value analysis reveal the concentration of reactants is a more important feature affecting the particle.