  Citation: Galhano, J.; Marcelo, G.A.; Santos, H.M.; Capelo-Martínez, J.L.; Lodeiro, C.; Oliveira, E. Development of Cyanine 813@Imidazole-Based Doped Supported Devices for Divalent Metal Ions Detection. Chemosensors 2022, 10, 80. https://doi.org/10.3390/ chemosensors10020080 Academic Editor: Francesco Dell’Olio Received: 6 January 2022 Accepted: 9 February 2022 Published: 14 February 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). chemosensors Article Development of Cyanine 813@Imidazole-Based Doped Supported Devices for Divalent Metal Ions Detection Joana Galhano 1 , Gonçalo A. Marcelo 1 , Hugo M. Santos 1,2 , José Luis Capelo-Martínez 1,2 , Carlos Lodeiro 1,2 and Elisabete Oliveira 1,2, * 1 BIOSCOPE Research Group, LAQV-REQUIMTE, Chemistry Department, NOVA School of Science and Technology, FCT-NOVA, Universidade NOVA de Lisboa, 1099-085Lisbon, Portugal; j.galhano@campus.fct.unl.pt (J.G.); goncalo.marcelo9@gmail.com (G.A.M.); hmsantos@fct.unl.pt (H.M.S.); jlcm@fct.unl.pt (J.L.C.-M.); cle@fct.unl.pt (C.L.) 2 PROTEOMASS Scientific Society, Rua dos Inventores, Madam Parque, Caparica Campus, 2825-182 Caparica, Portugal * Correspondence: ej.oliveira@fct.unl.pt Abstract: A NIR cyanine@imidazole derivative Cy1 was synthesized and evaluated as a metal ion sensor in solution. Cy1 was shown to be very sensitive to all metal ions tested, presenting a blue shift in the absorption from 668 nm to 633 nm, followed by a change in colour from pale green to blue with Zn 2+ , Cd 2+ , Co 2+ , Ni 2+ and Hg 2+ ions. Despite the blue shift in the absorption, a decrease at 633 nm (with a colour change from pale green to colourless), as well as a quenching in the emission intensity at 785 nm were observed for Cu 2+ ions. The results show the formation of sandwich complexes of two ligands per metal ion with the highest association constant observed for Cu 2+ (Log K ass.abs = 14.76 ± 0.09; Log K ass.emis. = 14.79 ± 0.06). The minimal detectable amounts were found to be 31 nM and 37 nM, with a naked eye detection of 2.9 ppm and 2.1 ppm for Hg 2+ and Cu 2+ ions, respectively. These results prompted us to explore the applicability of Cy1 by its combination with nanomaterials. Thus, Cy1@ doped MNs and Cy1@ doped PMMA nanoparticles were synthesized. Both nanosystems were shown to be very sensitive to Cu 2+ ions in water, allowing a naked-eye detection of at least 1 ppm for Cy1@ doped MNs and 7 ppm for Cy1@ doped PMMA. This colourimetric response is an easy and inexpensive way to assess the presence of metals in aqueous media with no need for further instrumentation. Keywords: cyanine@imidazole derivative; colourimetric; metal ions; PMMA nanoparticles; mesoporous silica nanoparticles 1. Introduction The development of new methodologies for the detection of analytes in samples is an increasingly studied topic in the scientific community [1]. These methods are particularly important when detecting the presence of analytes in aqueous environments [2,3]. For example, levels of metal ions such as copper and cadmium, amongst others, in drinking water are very tightly regulated by governmental and non-governmental entities, since the presence of these metals above a particular concentration can cause severe health problems, once ingested [4,5]. The maximum allowable concentration (MAC) is a regulated unit that defines the maximum concentration of a particular analyte, present in drinking water [6]. These values can be described by governments or by other organizations, such as the World Health Organization (WHO) [7], or the European Union [8], and can be different, depending on the regulating unit. For example, the MAC for copper defined by the WHO is 2 ppm, a similar value to that described by the European Union; however, in the United States, this value is described as 1.3 ppm [9]. Other examples include mercury ions by the same entities. The MACs for this analyte are 6, 2 and 1 ppb, defined by the WHO, United States Government and European Union respectively. These concentrations are usually in Chemosensors 2022, 10, 80. https://doi.org/10.3390/chemosensors10020080 https://www.mdpi.com/journal/chemosensors