Dyes and Pigments 184 (2021) 108832 Available online 10 September 2020 0143-7208/© 2020 Elsevier Ltd. All rights reserved. Colorimetric recognition of multiple frst-row transition metals: A single water-soluble chemosensor in acidic and basic conditions Rosita Diana a , Ugo Caruso b , Luigi Di Costanzo a , Francesco Silvio Gentile b , Barbara Panunzi a, * a Department of Agriculture, University of Napoli Federico II, Portici, NA, Italy b Department of Chemical Sciences, University of Napoli Federico II, Napoli, Italy A R T I C L E INFO Keywords: Colorimetric enhancer Chemosensor Multimetal First-row transition ions ABSTRACT A new tripodal chemosensor named PoPAP for colorimetric-enhancing detection of four metal ions from frst-row transition series by combining pyridyl and phenolic binding sites with a highly conjugated pyridinyl-1,3,4- oxadiazolyl-phenyl moiety was designed, synthetized, and structurally analyzed. The chemosensor PoPAP is highly water soluble, it gives a selective fast colorimetric response to cations of iron, vanadium, copper and cobalt, depending on pH conditions, and it can discriminate among different oxidation states. PoPAP sensing ability was explored by absorbance spectroscopic studies, “naked-eye” perception, and UV–visible titration. The chemosensor-metal ions binding constants (Ka) and stoichiometry of host-guest complex in aqueous media were determined by classical analytical methods based on UV–visible data. The coordination geometry of PoPAP copper (II) complex was elucidated by crystal structure analysis and used as starting point for ab-initio calcu- lations to analyze the interaction between ligand and metal. 1. Introduction Selective recognition of metal ions attracts multiple interests due to its role for chemical and biological processes and owing to environ- mental pollutants analysis [1–3]. Fast-responsive and selective low-cost approach for the determination of metal cations is a challenging task due to costly analytical instruments [4–7]. Chemical sensors are based on molecular devices able to transform chemical information into elec- trical, magnetic or optical signals [8–10]. This attractive research area derives from possible applications for real time selective response to analytes, easy handling and smart utilization [11–13]. Colorimetric sensors have the advantage to produce a desirable on-site “naked-eye” response. One of the most ambitious goal is the development of a single probe for “naked-eye” multimetal sensing since most of available che- mosensors were targeted to highly one- or two-metals selective deter- mination [14–26]. Herein, we present the design, synthesis and characterization of a unique sensor able to recognize several transition metal ions sharing important biological functions. The metals in living organisms fall into two classes: the alkali and alkaline earth metals and the transition metals. For many biological processes most of the frst-row transition metals are essential nutrients and are present in the human body as “trace elements” functioning with specifc oxidation states. Among them, vanadium has a regulatory function in phosphate metabolic processes. Cobalt is involved with vitamin B12, repair myelin, and form haemoglobin and some enzymes. Copper plays a role in making red blood cells, preserving nerve cells and the immune system. Iron is critical in electron transfer, oxygen trans- port, and RNA and DNA synthesis. Environmentally, the same metals in higher concentrations and/or in different oxidation states have poten- tially toxic effects and could be present as pollutants. The goal of a unique chemosensor for vanadium, iron, cobalt, and copper ions is important for biochemical and analytical purposes. Among the optical chemosensors, tripodal sensors are multidentate chelating ligands containing conjugated chemical groups and fexible linkers acting in concert toward the analyte [27–31]. By chelating a given metal ion, the sensor undergoes conformational and electronic changes, leading to a specifc absorbance spectrum and color. For this study, we designed a tripodal sensor for transition metals with many advantageous properties. The new receptor named PoPAP is a real-time multivalent “naked-eye” tool, highly water-soluble, that cannot be saturated with most common metals and is able of sensing in acidic and basic conditions. In this study, we also correlated sensor analytical properties with the understanding of how it functions. The chemosensor PoPAP contains a PAP group (2-(((pyridin-2-yl) * Corresponding author. E-mail address: barbara.panunzi@unina.it (B. Panunzi). Contents lists available at ScienceDirect Dyes and Pigments journal homepage: http://www.elsevier.com/locate/dyepig https://doi.org/10.1016/j.dyepig.2020.108832 Received 18 July 2020; Received in revised form 31 August 2020; Accepted 2 September 2020