Surface-enhanced Raman scattering has developed into a mature analytical technique useful in various applications; however, the reproducible fabrication of a portable SERS substrate with high sensitivity and good uniformity is still an ongoing pursuit. Reported herein is a rapid fabrication method of an inexpensive SERS substrate that enables sub-nanomolar detection of molecular analytes. The SERS substrate is obtained by application of silver nanoparticles (Ag NPs)-based ink in precisely design patterns with the aid of an in-house assembled printer equipped with a user-fillable pen. Finite-difference time-domain (FDTD) simulations show a 155-times Ag NP electric field enhancement for Ag nanoparticle pairs with particle spacing of 2 nm. By comparing the SERS performance of SERS substrate made with different support matrices and fabrication methods, the PET-printed substrate shows optimal performance, with an estimated sensitivity enhancement factor of 107. The quantitative analysis of rhodamine 6G absorbed on optimized SERS substrate exhibits a good linear relationship, with a correlation coefficient (R2) of 0.9998, between the SERS intensity at 610 cm−1 and the concentration in the range of 0.1 nM—1μM. The practical low limit detection of R6G is 10 pM. The optimized SERS substrates show good stability (at least one month) and have been effectively tested in the detection of cancer drugs, including doxorubicin and metvan.

Materials that undergo a phase transition from metallic to insulating, or metal−insulator transition (MIT), materials have become widely popular for their potential in emerging technologies due to their drastic conductivity change upon transitioning. Notable among the MIT materials is vanadium dioxide (VO 2 ), and ongoing efforts are focused on tuning its MIT phase transition temperature (TMIT). In this report, VO 2 germanium-doped nanoparticles with various germanium dopant levels were synthesized via a hydrothermal route and used in a simple single-switch antenna. Powder X-ray diffraction (XRD) analysis shows a monoclinic phase (M1) for both the pure and Gedoped VO 2 nanomaterials at room temperature, with no change in the diffraction pattern in the Ge-doped samples at low doping percentages; the M1 phase for both pure and Ge-doped VO 2 was further confirmed by Raman spectroscopy. Energy-dispersive X-ray spectroscopy (EDS) showed Ge uniformly distributed in the nanomaterials. The nanoparticles' morphology, imaged by fieldemission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM), reveals a morphology change from nanoparticles to nanosheets with increased dopant concentration. Ge-doped VO 2 nanoparticle dispersions were used to print a single switch in an antenna solely obtained through a facile printing process. A vector network analyzer used to characterize the antenna performance showed that the germanium doping successfully changed the transition temperature of the material, demonstrating the capability of controlling the antenna operation frequencies as a function of material doping. Density functional theory (DFT) shows that substituting Ge into a V site of the crystal structure distorts the lattice and reduces the band gap at high doping percentages. These results provide insight into the potential of smart switches fabricated from Ge-doped VO 2 .