Designing Plasmonic Janus Microgels for Direct Raman Detection of Small Molecules in Pristine Samples

Raman spectroscopy enables molecular identification since each molecular species has different molecular vibrational modes. However, the probability of Raman scattering is low, limiting its pragmatic use as a detection method. When metal nanostructures or metal nanoparticles (NPs) are illuminated, a strong electric field is generated nearby the metal surface through surface plasmon resonance at the specific wavelength of the incident light. This leads to the enhanced Raman signal of target molecules in the vicinity of the metal surface, referred to as Surface Enhanced Raman Scattering (SERS). For target molecules to approach the metal surface, metal needs to be protected from contamination by adhesives such as protein. Therefore, pretreatment is required to remove adhesives from samples before SERS detection.<br/> The way to obviate the pretreatment is a metal nanoparticle-embedded hydrogel microparticle. The hydrogel network serves as a protection layer for the infusion of target molecules and the exclusion of adhesives. However, preparing gold nanoparticles (AuNPs) and encapsulating AuNPs with a hydrogel network using microfluidics is time-consuming. In addition, signal uniformity is not achievable due to the aggregation of AuNPs. In that sense, micromolding is an alternative to microfluidics, which can remarkably shorten the time taken to produce plasmonic microgel. Moreover, a gold precursor is reduced to AuNPs in the presence of PEGDA providing the nucleation sites. Combining these two, microgels where AuNPs are uniformly embedded are quickly produced. This not only obviates the preparation of AuNPs but also provides the signal uniformity of the microgel. <br/> Here, we design charge-selective plasmonic Janus microcylinders produced by the micromolding method. Microcylinders make up the charge-selective plasmonic microgel for SERS detection and the photonic crystal. By imparting the charge to the SERS microgel, SERS sensitivity is enhanced through the concentration of oppositely charged molecules. Since the charge of the microgel is not distinguishable from the naked eye, photonic color plays a role as an indicator of the charge of the plasmonic microgel. To fabricate Janus microcylinders, a solution containing gold precursor, water, PEGDA monomer, charged monomer, and photoinitiator is infiltrated into the cylindrical hole arrays of the PDMS mold. The residual solution is removed, followed by the evaporation of water and the curing of the solution by UV for 10 min. As a result, the SERS microgel whose height is half the cylindrical hole is produced. Subsequently, silica-ETPTA suspension is infiltrated into the remaining holes of the PDMS molds on a slide glass. Suspension is cured and plasmonic Janus microcylinders are collected through blading. To develop vivid structural color, Janus microcylinders are immersed in HF solution to etch out the silica NPs. As the SERS sensitivity of microcylinders varies with the concentration of the photoinitiator and gold precursor, concentrations are optimized by measuring the SERS spectra of R6G molecules. A higher concentration of charged monomer concentrates more oppositely charged molecules, but it may interrupt the formation of AuNPs due to the competition between charged monomer and gold precursor toward PEGDA. Therefore, the concentration of charged monomer is also optimized and the internal structure is investigated to observe the AuNP density. Optimized microgels exhibit signal uniformity in any spot, indicating the uniform dispersion of AuNPs inside microgels. As proof of concept, we detected charged molecules using our plasmonic Janus microcylinder. Since specific photonic color indicates the charge of the plasmonic microgels, molecules with a specific charge can be detected using the oppositely charged microcylinder. In the mixture of positively and negatively charged molecules, each molecule is identified with high sensitivity from the microcylinder based on photonic colors.