“Point‐and‐Shoot” Synthesis of Metallic Ring Arrays and Surface‐Enhanced Optical Spectroscopy

Abstract

The realization of optically active structures with direct‐write printing has been challenging, particularly in spatially constrained microfluidic devices which are essential for point‐of‐care (POC) applications. The existing techniques are limited by resolution, accessibility, and multistep fabrication constraints. “Point‐and‐shoot” strategies to achieve site‐specific fabrication of optically active Ag rings and on‐demand targeted surface‐enhanced optical spectroscopy are reported. Stable microbubbles over an Au nanoisland (AuNI) substrate are generated using a continuous‐wave laser at low power (≈0.5 mW µm−2). Analytical modeling of bubble generation process substantiates the evolution of ring morphology and its power dependence. The tunable Ag rings exhibit surface plasmon resonances in the mid‐IR regime from 3.8 to 4.6 µm, while the AuNI shows visible region response. The Ag ring over the AuNI imparts intensified surface‐enhanced Raman spectroscopy (SERS) activity owing to amplified hot spots at Ag ring/AuNI interface. As an example, SERS and surface‐enhanced infrared spectroscopy of rhodamine 6G, crystal violet, and 2,4,6‐trinitrotoluene molecules, respectively, are demonstrated. The applicability of this technique to perform in situ fabrication and SERS sensing in microfluidic channels is shown. Using a simple in situ approach toward optically active structures, our technique can synergize multiple surface‐enhanced optical spectroscopies to facilitate POC applications.

Digital Assembly of Colloidal Particles for Nanoscale Manufacturing

“…f) Dark‐field optical image of a Ag ring array fabricated using BP with Ag precursor ink. Adapted with permission . Copyright 2018, WILEY‐VCH.…”

“…Active BP allows synthesis and manufacturing of optically active structures from metallic precursor inks. One example is the formation of an Ag ring structure as shown in Figure f . By using a Ag precursor ink and controlled formation of the bubble, direct synthesis and immobilization of Ag rings at the microbubbles could be achieved.…”

“…The Ag rings were used for obtaining the R6G SERS spectra (bottom). Adapted with permission . Copyright 2018, WILEY‐VCH.…”

“…A host of applications has been proposed for the more recently developed BP technique . Among them, the in situ fabrication of Ag rings within the microfluidic channel for SERS is quite appealing.…”

“…Among them, the in situ fabrication of Ag rings within the microfluidic channel for SERS is quite appealing. The noninvasive, remote, and light‐controlled BP allows fabrication of plasmonic structures like the Ag rings inside spatially confined microfluidic channels without any constraints to the channel design, making it compatible for various lab‐on‐a‐chip applications . Figure d (top) shows the microfluidic chip where the Ag precursor ink is introduced to fabricate Ag rings using BP.…”