Plasmonic Spots Research Articles

Ultrasensitive inkjet-printed based SERS sensor combining a high-performance gold nanosphere ink and hydrophobic paper

We report the fabrication of a low-cost paper-based SERS platform with enhanced sensing capabilities. The sensor consists of a highly concentrated glycerol/ethanol dispersion of ca. 77 nm Au nanospheres (AuSph) inkjet-printed on hydrophobic chromatographic paper forming circular sensing spots of 1 mm in diameter. The AuSph ink exhibits high stability for 1 year. Interestingly, we demonstrate that the modification of the paper surface with hydrophobic ligands improve the sensing capabilities by confining the analyte solution in the AuSph sensing platform. It contributes to both concentrate the analyte, as well as, to decrease the sample volume to just a few μl. We also analyze the effect of the amount of the nanoparticles, tunable by the number of printing cycles, in the SERS performance of the plasmonic spots. The results show that 5 printing cycles give rise to sensing platforms with great SERS response in terms of intensity and uniformity. The spot-to-spot reproducibility is also analyzed observing good results even for AuSph spots from different printed papers. The paper-based SERS platform was tested towards the detection of crystal violet (CV) and the fungicide thiram. Additionally, applying a digital protocol for SERS analysis, a good linear correlation between the digital counts (or positive events) and the analyte concentration was obtained at the single-molecule SERS regime. In both cases, the quantification region threshold was 10−11 M. The great sensitivity performance of the inkjet-printed SERS substrate is reflected by the low sample volume needed (only 2 μL). The inkjet-printed SERS substrate and the fabrication method demonstrated to be efficient, reliable, reproducible and robust, with a cost less than $ 0.01 per spot.

Forming Sub-32-nm High-Aspect Plasmonic Spot via Bowtie Aperture Combined with Metal-Insulator-Metal Scheme

We theoretically utilize bowtie aperture combined with metal-insulator-metal (MIM) scheme to obtain sub-32-nm (lambda/12) high-aspect plasmonic spots. The improvement of the depth profile is attributed to the asymmetry electromagnetic mode excitation in MIM structure and the decaying compensation of the reflective Ag layer. A theoretical near-field exposure model has been used to evaluate the exposure depth in the photoresist. It is demonstrated that the exposure depth of the sub-32-nm plasmonic spot is more than 20 nm, which is about four times of the bowtie aperture without MIM scheme. The influences of the air gap tolerance and the ridge gap size of bowtie aperture are also discussed.

High-resolution 2D plasmonic fan-out realized by subwavelength slit arrays

The authors demonstrate a two dimensional (2D) plasmonic fan-out spot array by using subwavelength sized slit arrays. Near field scanning optical microscope (NSOM) is employed to examine intensity distributions of the generated fan-out plasmonic spots, showing good agreement with finite-difference time-domain (FDTD) simulation results. The plasmonic fan-out spots with full width half-maximum (FWHM) of 0.34 lambda(0) are optimized by various design parameters associated with the subwavelength slit as well as polarization states.