{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,3,18]],"date-time":"2026-03-18T00:09:47Z","timestamp":1773792587307,"version":"3.50.1"},"reference-count":28,"publisher":"MDPI AG","issue":"5","license":[{"start":{"date-parts":[[2018,4,27]],"date-time":"2018-04-27T00:00:00Z","timestamp":1524787200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>We demonstrate a fast response colorimetric humidity sensor using a crosslinked poly(2-hydroxyethyl methacrylate) (PHEMA) in the form of inverse opal photonic gel (IOPG) soaked in 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM+][BF4\u2212]), a non-volatile hydrophilic room temperature ionic liquid (IL). An evaporative colloidal assembly enabled the fabrication of highly crystalline opal template, and a subsequent photopolymerization of PHEMA followed by solvent-etching and final soaking in IL produced a humidity-responsive IOPG showing highly reflective structural color by Bragg diffraction. Three IOPG sensors with different crosslinking density were fabricated on a single chip, where a lightly crosslinked IOPG exhibited the color change response over entire visible spectrum with respect to the humidity changes from 0 to 80% RH. As the water content increased in IL, thermodynamic interactions between PHEMA and [BMIM+][BF4\u2212] became more favorable, to show a red-shifted structural color owing to a longitudinal swelling of IOPG. Highly porous IO structure enabled fast humidity-sensing kinetics with the response times of ~1 min for both swelling and deswelling. Temperature-dependent swelling of PHEMA in [BMIM+][BF4\u2212] revealed that the current system follows an upper critical solution temperature (UCST) behavior with the diffraction wavelength change as small as 1% at the temperature changes from 10 \u00b0C to 30 \u00b0C.<\/jats:p>","DOI":"10.3390\/s18051357","type":"journal-article","created":{"date-parts":[[2018,4,27]],"date-time":"2018-04-27T12:04:50Z","timestamp":1524830690000},"page":"1357","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":26,"title":["Colorimetric Humidity Sensor Using Inverse Opal Photonic Gel in Hydrophilic Ionic Liquid"],"prefix":"10.3390","volume":"18","author":[{"given":"Seulki","family":"Kim","sequence":"first","affiliation":[{"name":"Department of Chemistry, Sejong University, 209 Neungdong-ro, Gwngjin-gu, Seoul 05006, Korea"}]},{"given":"Sung Gu","family":"Han","sequence":"additional","affiliation":[{"name":"Department of Chemistry, Sejong University, 209 Neungdong-ro, Gwngjin-gu, Seoul 05006, Korea"}]},{"given":"Young Gook","family":"Koh","sequence":"additional","affiliation":[{"name":"Engain Co. Ltd. Korea Bio Park BLD C-201, Seongnam 13488, Korea"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7048-7331","authenticated-orcid":false,"given":"Hyunjung","family":"Lee","sequence":"additional","affiliation":[{"name":"School of Advanced Materials Engineering, Kookmin University, 77 Jeongneung-ro, Seongbuk-gu, Seoul 02707, Korea"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6757-885X","authenticated-orcid":false,"given":"Wonmok","family":"Lee","sequence":"additional","affiliation":[{"name":"Department of Chemistry, Sejong University, 209 Neungdong-ro, Gwngjin-gu, Seoul 05006, Korea"},{"name":"Engain Co. Ltd. Korea Bio Park BLD C-201, Seongnam 13488, Korea"}]}],"member":"1968","published-online":{"date-parts":[[2018,4,27]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"829","DOI":"10.1038\/39834","article-title":"Polymerized colloidal crystal hydrogel films as intelligent chemical sensing materials","volume":"389","author":"Holtz","year":"1997","journal-title":"Nature"},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"468","DOI":"10.1038\/nphoton.2007.140","article-title":"Photonic-crystal full-colour displays","volume":"1","author":"Arsenault","year":"2007","journal-title":"Nat. Photonics"},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"466","DOI":"10.1002\/(SICI)1521-4095(199904)11:6<466::AID-ADMA466>3.0.CO;2-E","article-title":"Assembly of nanoparticles into opaline structures over large areas","volume":"11","author":"Gates","year":"1999","journal-title":"Adv. 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