DNAzyme-Based Plasmonic Nanomachine for Ultrasensitive Selective Surface-Enhanced Raman Scattering Detection of Lead Ions via a Particle-on-a-Film Hot Spot Construction

We propose a highly sensitive and selective surface-enhanced Raman scattering (SERS) method for determining lead ions based on a DNAzyme-linked plasmonic nanomachine. A metallic nanoparticle-on-a-film structure was built through a rigid double-stranded bridge linker composed of a DNAzyme and its substrate. This DNAzyme could be activated by lead ions and catalyze a fracture action of the substrate. Thus, the double chain structure of DNA would turn into a flexible single strand, making the metal nanoparticles that connected to the terminal of DNAzyme fall to the surface of the metal film. Hereby, a narrow gap close to 2 nm generated between metal nanoparticles and the metal film, exhibiting a similar effect of a “hot spot” and remarkably enhancing the signal of randomly dispersed Raman-active molecules on the surface of metal film. By measuring the improvement of SERS intensity of the Raman-active molecules, we realized the lowest detection concentration of Pb²⁺ ions to 1.0 nM. This SERS analytical method is highly selective and can be extended universally to other targets via the accurate programming of corresponding DNA sequences

Waveguide-Enhanced Surface Plasmons for Ultrasensitive SERS Detection

We design an ultrasensitive surface-enhanced Raman scattering (SERS) substrate based on waveguide-enhanced surface plasmons (SPs). An optical waveguide was exploited to concentrate and restrict the electromagnetic (EM) energy of the incident light, and Ag nanoparticles that were assembled on the waveguide surface were used to enhance the EM field further by means of SP resonance. The enhancement factor (EF) of the incident EM field can reach 10³ on the two sides of nanoparticles, and a 10⁸–10¹² EF of SERS is expected. This waveguide-assisted isolated nanoparticle substrate can reach a comparable SERS enhancement capability to that of gap-type SERS hot spots. In addition, this SERS substrate is applicable to the SERS detection of large molecules (biomacromolecules etc.), which cannot be placed in traditional gap-type hot spots