Prof. Shin-Hyun Kim's group presented a molecular sensor with a microbead format for a rapid in-situ detection of harmful molecules in biological fluids or foods in a collaboration with a Korea Institute of Materials Science (KIMS) research group. As the sensor is designed to selectively concentrate charged small molecules and amplify the Raman signal, no time-consuming pretreatment of samples is required.

Raman spectra are commonly known as molecular fingerprints. However, its low intensity has restricted the use in molecular detection. Raman signal can be dramatically amplified by locating the molecules on the surface of metal nanostructures at which electromagnetic field is strongly localized. However, it is still challenging to use Raman signal for detection of small molecules dissolved in complex biological fluids. Adhesive proteins irreversibly adsorb on the metal surface, which prevents the access of small target molecules onto the metal surface. Therefore, it was prerequisite to purify the samples before analysis. However, it costs a long time and expense.

A joint team between Prof. Shin-Hyun Kim's group and Dr. Dong-Ho Kim's group in KIMS has addressed the issue by encapsulating agglomerates of gold nanoparticles using a hydrogel. The hydrogel has three-dimensional network structures so that molecules smaller than the mesh are selectively permeable. Therefore, the hydrogel can exclude relatively large protein, while allowing the infusion of small molecules. Therefore, the surface of gold nanoparticles remains intact against proteins, which accommodates small molecules. In particular, charged hydrogel concentrates oppositely-charged small molecules through Coulomb attraction. That is, the purification is autonomously done by the materials, thereby obviating the time-consuming pretreatment. As a result, Raman signal of small molecules can be selectively amplified in the absence of adhesive proteins.

Using the molecular sensors, the research team demonstrated the direct detection offipronil sulfone dissolved in the egg without sample pretreatment. Recently, insecticide-contaminated eggs spread in Europe, South Korea, and other countries, threatening the health and causing social chaos.fipronil is one of the most commonly used insecticides for veterinary medicine to combat fleas. Thefipronil is absorbed through the chicken skin, of which metabolite,fipronil sulfone, is accumulated in eggs. As thefipronil sulfone carries partial negative charges, it can be concentrated using positively-charged microgels while excluding adhesive proteins in eggs, such as ovalbumin, ovoglobulin, and ovomucoid. Therefore, Raman spectrum offipronil sulfone can be directly measured. The limit of direct detection offipronil sulfone dissolved in the egg was evaluated as 0.05 ppm.

Prof. Kim said, "The molecular sensors can be used not only for direct detection of harmful molecules in foods but also for residual drugs or biomarkers in blood or urines." This research was led by graduate student Dong Jae Kim and an article entitled "SERS-Active Charged Microgels for Size- and Charge-Selective Molecular Analysis of Complex Biological Samples" was published on October 4, 2018 in Small and featured as inside cover of the journal.