{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,1,8]],"date-time":"2026-01-08T00:32:31Z","timestamp":1767832351475,"version":"3.49.0"},"reference-count":48,"publisher":"MDPI AG","issue":"13","license":[{"start":{"date-parts":[[2023,7,5]],"date-time":"2023-07-05T00:00:00Z","timestamp":1688515200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"Tohoku University Center for Gender Equality Promotion, Tohoku University"},{"name":"Kao Foundation for Arts and Sciences"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>We demonstrate an S-shaped double-spiral microresonator (DSR) for detecting small volumes of analytes, such as liquids or gases, penetrating a microfluidic channel. Optical-ring resonators have been applied as label-free and high-sensitivity biosensors by using an evanescent field for sensing the refractive index of analytes. Enlarging the ring resonator size is a solution for amplifying the interactions between the evanescent field and biomolecules to obtain a higher refractive index sensitivity of the attached analytes. However, it requires a large platform of a hundred square millimeters, and 99% of the cavity area would not involve evanescent field sensing. In this report, we demonstrate the novel design of a Si-based S-shaped double-spiral resonator on a silicon-on-insulator substrate for which the cavity size was 41.6 \u00b5m \u00d7 88.4 \u00b5m. The proposed resonator footprint was reduced by 680 times compared to a microring resonator with the same cavity area. The fabricated resonator exposed more sensitive optical characteristics for refractive index biosensing thanks to the enhanced contact interface by a long cavity length of DSR structures. High quality factors of 1.8 \u00d7 104 were demonstrated for 1.2 mm length DSR structures, which were more than two times higher than the quality factors of microring resonators. A bulk sensitivity of 1410 nm\/RIU was calculated for detecting 1 \u00b5L IPA solutions inside a 200 \u00b5m wide microchannel by using the DSR cavity, which had more than a 10-fold higher sensitivity than the sensitivity of the microring resonators. A DSR device was also used for the detection of 100 ppm acetone gas inside a closed bottle.<\/jats:p>","DOI":"10.3390\/s23136177","type":"journal-article","created":{"date-parts":[[2023,7,6]],"date-time":"2023-07-06T00:54:41Z","timestamp":1688604881000},"page":"6177","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":4,"title":["Enhancement of Refractive Index Sensitivity Using Small Footprint S-Shaped Double-Spiral Resonators for Biosensing"],"prefix":"10.3390","volume":"23","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-9941-0995","authenticated-orcid":false,"given":"Anh","family":"Igarashi","sequence":"first","affiliation":[{"name":"Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Maho","family":"Abe","sequence":"additional","affiliation":[{"name":"Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577, Japan"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Shigeki","family":"Kuroiwa","sequence":"additional","affiliation":[{"name":"R&D Group, KOKOROMI Inc., Shinjuku-ku, Tokyo 169-0051, Japan"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Keishi","family":"Ohashi","sequence":"additional","affiliation":[{"name":"R&D Group, KOKOROMI Inc., Shinjuku-ku, Tokyo 169-0051, Japan"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Hirohito","family":"Yamada","sequence":"additional","affiliation":[{"name":"Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2023,7,5]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"63","DOI":"10.1016\/j.proeng.2011.12.016","article-title":"Label-free, multiplexed biomolecular analysis using arrays of silicon photonic microring resonators","volume":"25","author":"Washburn","year":"2011","journal-title":"Procedia Eng."},{"key":"ref_2","doi-asserted-by":"crossref","unstructured":"Park, M.K., Yiying, J.Q., Kee, J.S., Song, J., Kao, L.T.H., Fang, Q., Guo-Qiang, L., Netto, V., and Fosse, E.M.L. 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