In situ synthesis of silver nanoparticle decorated vertical nanowalls in a microfluidic device for ultrasensitive in-channel SERS sensing

A microfluidic device with integrated novel silver nanoparticle (Ag NPs) decorated nanowall structures was fabricated via in situ electrodeposition of Cu-core/C-sheath nanowalls, followed by a facile in-channel silver galvanic replacement reaction method at room temperature. The integrated microfluidic devices with Ag NPs decorated nanowalls, serving as a highly active Raman substrate, were then applied for in-channel surface-enhanced Raman scattering (SERS) sensing using crystal violet as the model compound. The results show that the presence of a PDMS microfluidic channel does not significantly affect the Raman signal and crystal violet as low as 50 pM is prominently detected. The calculated apparent enhancement factor is 1.1 6 109, which allows for the ultra-sensitive detection of analytes in microfluidic devices. The superior enhancement of the in-channel Raman signal and excellent analyte sensitivity can be attributed to the synergistic effect of several distinct traits of the Ag NPs decorated nanowalls - a large available surface for analyte adsorption, a large number of nanocavities surrounded by nanowalls laterally confining the surface plasmons, and numerous hot spots resulting from close-contacted Ag NPs and/or the proximate edges of Ag decorated nanowalls. Because preparation of the active SERS substrate and subsequent finger-print SERS detection is completed within a microfluidic device, the developed method opens a new venue to integrate active SERS substrate within microfluidic channels and provides an excellent microfluidic SERS sensor platform for ultrasensitive and selective chemical and biological sensing.