Room Temperature, Multiphasic Detection of Explosives, and Volatile Organic Compounds Using Thermodiffusion Driven Soret Colloids

Achieving a label-free spectroscopic platform for multiphasic analytical detection in real-time, ambient conditions is extremely challenging due to the fundamental dichotomy between ultrahigh sensitivity and reliability of detection. Addressing these challenges, we demonstrate a versatile surface-enhanced Raman scattering (SERS) platform capable of multiphasic, reliable detection with ultrahigh sensitivity. The SERS platform is extremely sensitive and relies on vapor molecules present at the solid-vapor or liquid-vapor interface for reliable detection in ambient conditions (298 K, 1 atm). We observe that metal nanoparticles, subjected to a temperature gradient, migrate and self-assemble into precise nanoparticle assemblies, in a nanoscale analogue of Soret effect. The formation and monodispersity of these nanoparticles assemblies, termed Soret colloids (SCs), is kinetically controllable using the thermal gradient. Soret colloids exhibit excellent size uniformity (monodispersity index, MDI similar to 0.8) and strong interplasmon coupling to generate uniform and intense electromagnetic hot-spots enabling multiphasic, reliable (relative standard deviation, RSD < 5%), instantaneous (<60 s), ambient (298 K, 1 atm) and spectroscopic detection through SERS. Thereby, we demonstrate SERS detection of analytes with a wide range of vapor pressures (10(-9)-10(-1) atm) such as 2,4,6 trinitrotoluene (TNT) and volatile organic compounds (VOCs). Besides, extremely reliable (RSD< 5%), liquid-state SERS detection is also enabled with SCs over a broad concentration range (10(-16) - 10(-6) M) extending to single-molecular sensitivity. Besides fundamentally overcoming the trade-off between high sensitivity and reliability, the vapor-phase detection protocol and platform demonstrated here presents transformative opportunities for real-time detection of explosives, medical diagnostics by breath-analysis and pollution monitoring.