Pt(dithiolene)-Based Colorimetric Chemosensors for Multiple Metal-Ion Sensing

: Colorimetric chemosensors are widely employed for in-ﬁeld analysis to detect transition metal ions in real-time with the naked eye. Colorimetric chemosensors have attracted considerable attention because they can conveniently provide quantitative and qualitative information at a low cost. However, the development of colorimetric chemosensors for multiple-ion sensing where metal cations coexist has been limited. For this reason, we developed a new type of transition metal ion sensing material by selectively replacing functional groups on (diphosphine)Pt(dmit) molecules. The terminal groups of the diphosphine ligand were successfully substituted by the cyclohexyl groups, increasing the electron density of the thione moiety. Due to the electron donation ability of the cyclohexyl terminal groups, the proposed chemosensing material was able to selectively detect the mixture of Hg 2+ , Cu 2+ , and Ag + in the presence of many types of interfering cations. To gain insight into the binding mechanisms between the metal ions and the developed (dchpe)Pt(dmit) molecule, density functional theory calculations were also performed.


Introduction
Repeated exposure to heavy metal ions, such as mercury, lead, copper, and silver from the environment can cause harm to human beings and the ecosystem [1,2]. This issue is complicated by the fact that these elements also have a wide range of important uses in industrial applications. For example, mercury is used in thermometers and sphygmomanometers, while copper and silver are used in the electrical and pharmaceutical industries, as well as in renewable energy. To protect ecosystems from potential toxicity from heavy metal ions and to utilize them for industrial use, it is essential to develop an accurate and cost-effective sensing material.
Colorimetric chemosensors are widely employed for the real-time detection of transition metal ions with the naked eye in field samples. Many studies have reported the use of colorimetric and fluorometric sensors to monitor the trace of transition metal ions from environmental samples [3][4][5][6]. Because colorimetric chemosensors produce qualitative (i.e., a visible change in color) and quantitative (i.e., via UV-vis spectroscopy) information at a lower cost and with less effort, they have received greater focus than fluorometric sensors. There was a tremendous effort to enhance the selectivity and sensitivity of the sensors. There was a tremendous effort to enhance the selectivity and sensitivity of the colorimetric chemosensors toward specific single ions and multiple ions with single sensors [7][8][9][10][11][12][13]. It is, however, barely investigated to identify the mixture of heavy metal ions, where a wide range of other metal ions coexist in the environment. In our previous work, Pt-dithiolene-based materials with a diphosphine ligand, denoted as (dxpe)Pt(dmit), were examined by substituting the terminal groups with electron-donating groups to selectively detect mercury cations. Of the tested (dxpe)Pt(dmit) molecules, where dxpe was either 1,2-bis(diphenylphosphino)ethane (dppe) or 1,2-bis[bis(pentafluorophenyl)phosphino]ethane (dfppe) and dmit was 1,3-dithiole-2-thione-4,5-dithiolate, the (dfppe)Pt(dmit) molecule was able to selectively detect Hg 2+ , while the (dppe)Pt(dmit) molecule detected Hg 2+ , Cu 2+ , and Ag + [14,15]. Inspired by the (dppe)Pt(dmit) sensor that can detect three types of single ions, the development of colorimetric chemosensors capable of detecting multiple ions was studied. The developed (dchpe)Pt(dmit) material can detect every single Hg 2+ , Cu 2+ , and Ag + ions, while it can also distinguish the combination of each ion simultaneously in the situation of various interfering cations coexisting (dchpe: (1,2-bis(dicyclohexylphosphino)ethane).
The structural difference between (dppe)Pt(dmit) and (dchpe)Pt(dmit) is the type of functional group. The (dchpe)Pt(dmit) molecule is synthesized by replacing the phenyl groups of the diphosphine ligand by cyclohexyl groups. With the effect of the electron-donating ability of cyclohexyl groups, the electron density of binding sites in (dchpe)Pt(dmit) was increased. Figure 1 presents a schematic diagram of the interaction between (dchpe)Pt(dmit) and metal ions. The (dchpe)Pt(dmit) is a chelate complex composed of a bidentate dmit ligand and a diphosphine ligand substituted with cyclohexyl groups and both bidentate ligands are connected to a central Pt 2+ ion. The thione moiety (>C=S) in the dmit ligand is an active site that binds to the target metal ions.

General Characterization
Melting points were determined using a Stuart SMP3 (Barloworld Scientific Ltd., Burlington, NJ, USA). Elemental analysis (C, H, and S) of (dxpe)Pt(dmit) was conducted Sustainability 2021, 13, 8160 3 of 15 using a PerkinElmer 2400 Series II installed at the National Center for Inter-University Research Facilities (NCIRF) at Seoul National University, Seoul, Korea. Measurement of the matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectra was performed with a Voyager-DETM STR Biospectrometry Workstation (Applied Biosystems Inc., Waltham, MA, USA) installed at NCIRF (Seoul, Korea).
Electrochemical properties were assessed at room temperature with a computercontrolled potentiostat (ZIVE SP2 Electrochemical Workstation) using a standard threeelectrode system with a Pt-disk working electrode, Pt wire as the counter electrode, and saturated Ag/AgCl as the reference electrode. Cyclic voltammograms were obtained in a mixed CH 3 CN/CH 2 Cl 2 (1:1 v/v%) solution with 1.0 mM samples and a 0.1 M n-Bu 4 N·ClO 4 supporting electrolyte at a scan rate of 100 mVs −1 . Ferrocene was used as a calibrant after each set of measurements, and all potentials reported were quoted with reference to the Fc/Fc + redox couple (E 1/2 = 0.439 V in CH 3 CN/CH 2 Cl 2 [1:1 v/v%]).

X-ray Diffraction (XRD) for Molecular Structure Analysis
X-ray structure analysis was performed by a single-crystal diffraction method at the Korea Basic Science Institute (KBSI, Western Seoul Center, Seoul, Korea). A yellow needle-like crystal (0.10 × 0.19 × 0.19 mm 3 ) of (dchpe)Pt(dmit) was taken up with Paratone oil and mounted on a Bruker SMART CCD diffractometer equipped with a graphitemonochromated Mo Kα (λ = 0.71073 Å) radiation source and a nitrogen cold stream (223 K). Data collection and integration were performed on a SMART (Bruker, Madison, WI, USA, 2000) and SAINT-Plus (Bruker, 2001). Absorption correction was performed by a multi-scan method implemented in SADABS. The structure was solved by direct methods and refined by full-matrix least-squares on F 2 using SHELXTL. All non-hydrogen atoms wererefined anisotropically, and hydrogen atoms were added to their geometrically ideal positions.

Density Functional Theory (DFT) Calculations
All calculations related to the electronic structure were carried out using the projectoraugmented wave (PAW) method for describing ionic cores as implemented in the Vienna Ab initio Simulation Package [18]. First-principles calculations for the geometrical optimization of the chemosensor materials were carried out based on the density functional theory (DFT) using generalized gradient approximation (GGA) within the Perdew-Burke-Ernzerhof (PBE) functional [19,20]. A plane-wave basis set with a kinetic-energy cut-off of 400 eV and 1 × 1 × 1 Monkhorst-Pack k-point mesh was used. The electronic optimization steps were converged self-consistently over 10 −5 eV per formula unit. The cations adsorbed on each (dchpe)Pt(dmit) molecule in the most stable state were directly relaxed using energy optimization from the initial position. The increase and decrease of electron density around the molecules are marked in gray and purple. In this research, the unit for the charge density was e a 0 −3 , where e is the elementary charge and a 0 is the Bohr radius.
Yield (dchpe)Pt(dmit) A dichloromethane solution (3 mL) of (dchpe)PtCl 2 (0.05 mmol, 34.5 mg) was added to an acetone solution (3 mL) of n-Bu 2 Sn(dmit) (0.05 mmol, 21.4 mg) under stirring overnight at room temperature. The solvent of the mixture solution was reduced via evaporation, followed by the addition of dichloromethane to precipitate the red solid in a vacuum. The product was filtered and recrystallized from dichloromethane/hexane using dry ice.
Yield (dppe)Pt(dmit) [7] A methanol solution (60 mL) of (dppe)PtCl 2 (1.0 mmol, 664 mg) was added to a methanol suspension of Na 2 (dmit), and refluxed under stirring for 24 h. The rose-red product was isolated by filtration, washed with methanol, and recrystallized from dichloromethane/hexane. (dfppe)PtCl 2 [8] A mixture of dichloromethane solution (11 mL) of (COD)PtCl 2 (0.5 mmol, 187 mg) and acetone solution (38 mL) of the dfppe ligand (0.5 mmol, 379 mg) was stirred overnight at room temperature. The mixture solution was evaporated, and the solid was treated with MeOH. The white product was isolated by filtration and recrystallized from dichloromethane/methanol. (dfppe)Pt(dmit) [8] A dichloromethane suspension (30 mL) of (dfppe)PtCl 2 (0.5 mmol, 513 mg) was added to an acetone solution (30 mL) of n-Bu 2 Sn(dmit) (0.5 mmol, 214 mg) under stirring overnight at room temperature in a nitrogen atmosphere. After the mixture solution was evaporated to reduce the volume, a minimum amount of dichloromethane was added to precipitate the red solid in a vacuum. The product was filtered and recrystallized from dichloromethane/methanol.

Metal-ion Sensing with a Pt 2+ -Chemosensor
UV-vis measurements were taken to monitor the colorimetric sensing of single metal ions using (dxpe)Pt(dmit) chemosensors. CH 3 CN/CH 2 Cl 2 solutions (1:1 v/v%) of metal perchlorate (0.4 mM, 1.0 mL) and (dxpe)Pt(dmit) (0.4 mM, 1.0 mL) were mixed to produce a solution of metal ions and the chemosensor with a concentration of 0.2 mM (1:1 v/v%). The absorbance of the mixed solution was measured immediately after mixing the 2 solutions and compared with that of a blank solution containing the chemosensor only.
The multi-metal ion sensing ability of the chemosensors was also examined. CH 3 CN/ CH 2 Cl 2 solutions of metal perchlorates (10 mM each) were mixed in equal amounts and then diluted to a concentration of 0.4 mM. The concentration of each metal ion was maintained at 0.4 mM in all mixed solutions, except for Hg 2+ , Cu 2+ , and/or Ag + , which were mixed in different combinations. Mix I was the mixture solution containing all metal ions without Hg 2+ , Cu 2+ and Ag + , and Mix II was the mixture solution containing all metal ions without Cu 2+ and Hg 2+ . A 1.0 mL solution of (dxpe)Pt(dmit) (0.4 mM) was added to 1.0 mL of the mixed-metal solution (0.4 mM), and the UV-vis absorbance was measured.

Chemistry
The (dchpe)Pt(dmit) was synthesized and the structure was identified by analyzing XRD, MALDI-TOF mass spectroscopy, UV-vis, 31 P NMR, and cyclic voltammetry plots. The (dchpe)Pt(dmit) was synthesized as a new chemosensor by reacting (dchpe)PtCl 2 with the dmit-transferring reagent n-Bu 2 Sn(dmit), as shown in Scheme 1. The product was recrystallized using a two-solvent system. A mixed hot solution of dichloromethane and hexane (1:1 v/v%) with (dchpe)Pt(dmit) was rapidly cooled using dry ice to obtain single crystals. (dfppe)Pt(dmit) was also synthesized in the same way except for the crystallization at room temperature [8]. This preparation strategy is very convenient for isolating the final product because the side-product (n-Bu 2 SnCl 2 ) is easily separated using simple filtration. In contrast, (dppe)Pt(dmit) was prepared by mixing (dppe)PtCl 2 with the very reactive Na 2 (dmit) [7].
UV-vis absorption data for the (dxpe)Pt(dmit) complexes represent the π→π* transition of the >C=S moiety in the dmit ligand. Those absorption bands exhibited a hypsochromic shift from (dchpe)Pt(dmit) (λmax = 470 nm) to (dfppe)Pt(dmit) (λmax = 448 nm) via (dppe)Pt(dmit) (λmax = 463 nm). That is, the absorption energy for the π→π* transition in the dmit ligand was positively affected by the diphosphine ligand with the electronegativity of the substituted functional groups [23].This trend was also observed in the 31 P NMR spectra of the complexes (Figure 3), with the triplets centered at 69.5 ppm for (dchpe)Pt(dmit), 46.6 ppm for (dppe)Pt(dmit), and 12.7 ppm for (dfppe)Pt(dmit) and with Pt satellites due to Pt-P coupling. The difference in the Pt-P coupling of these compounds is also related to the diphosphine ligand [24]. A diphosphine ligand with a high electron-donating ability undergoes a chemical shift in the low field. Therefore, (dchpe)Pt(dmit), which had the highest electron-donating ability, was de-shielded the most, and (dfppe)Pt(dmit), which had the lowest electron-donating ability, was shielded the most. This is due to the relationship between the electron density of the central metal and that of the phosphorous lone pair. When the electron-donating ability of the phosphine ligand increases, the electron density of the phosphorous lone pair is removed by donating electrons to the metal orbital, resulting in a downfield shift. UV-vis absorption data for the (dxpe)Pt(dmit) complexes represent the π→π* transition of the >C=S moiety in the dmit ligand. Those absorption bands exhibited a hypsochromic shift from (dchpe)Pt(dmit) (λmax = 470 nm) to (dfppe)Pt(dmit) (λmax = 448 nm) via (dppe)Pt(dmit) (λmax = 463 nm). That is, the absorption energy for the π→π* transition in the dmit ligand was positively affected by the diphosphine ligand with the electronegativity of the substituted functional groups [23].This trend was also observed in the 31 P NMR spectra of the complexes (Figure 3), with the triplets centered at 69.5 ppm for (dchpe)Pt(dmit), 46.6 ppm for (dppe)Pt(dmit), and 12.7 ppm for (dfppe)Pt(dmit) and with Pt satellites due to Pt-P coupling. The difference in the Pt-P coupling of these compounds is also related to the diphosphine ligand [24]. A diphosphine ligand with a high electrondonating ability undergoes a chemical shift in the low field. Therefore, (dchpe)Pt(dmit), which had the highest electron-donating ability, was de-shielded the most, and (dfppe)Pt(dmit), which had the lowest electron-donating ability, was shielded the most. This is due to the relationship between the electron density of the central metal and that of the phosphorous lone pair. When the electron-donating ability of the phosphine ligand increases, the electron density of the phosphorous lone pair is removed by donating electrons to the metal orbital, resulting in a downfield shift.  Consequently, with the UV-vis and 31 P NMR spectroscopic data, it is suggested that the electronegativity of substituted functional groups on the diphosphine ligand affects the electron density of dmit.

Colorimetric and UV-vis Studies
The color change is strongly associated with the electronegativity of the substituted functional groups on the diphosphine ligand. The wider range of metal ions can be detectable by activating the binding site of the (dxpe)Pt(dmit) molecule with substituted EDGs. The binding site of (dxpe)Pt(dmit) molecules is S atom in the thione (>C=S) moiety in the dmit ligand. The dmit ligand is an electron-rich region categorized as a soft base because it bears many π-electrons not only in the >C=S double bond, but also the lone pairs on the S atom. Based on Pearson's acid-base principle, a soft base is likely to interact with soft acids, such as Hg 2+ rather than hard acids, such as Na + and K + [25]. Considering the standard reduction potential (Eº/V at 298.15 K in water) of the 18 metal ions used in this study, Hg 2+ (Eº = +0.85 V) was expected to interact strongly with the thione moiety on the dmit ligand [24]. It was also expected that Cu 2+ (Eº = +0.34 V) and Ag + (Eº = +0.80 V) would bind as the number of electrons in the thione moiety became richer (Table 3).

Colorimetric and UV-Vis Studies
The color change is strongly associated with the electronegativity of the substituted functional groups on the diphosphine ligand. The wider range of metal ions can be detectable by activating the binding site of the (dxpe)Pt(dmit) molecule with substituted EDGs. The binding site of (dxpe)Pt(dmit) molecules is S atom in the thione (>C=S) moiety in the dmit ligand. The dmit ligand is an electron-rich region categorized as a soft base because it bears many π-electrons not only in the >C=S double bond, but also the lone pairs on the S atom. Based on Pearson's acid-base principle, a soft base is likely to interact with soft acids, such as Hg 2+ rather than hard acids, such as Na + and K + [25]. Considering the standard reduction potential (E • /V at 298.15 K in water) of the 18 metal ions used in this study, Hg 2+ (E • = +0.85 V) was expected to interact strongly with the thione moiety on the dmit ligand [24]. It was also expected that Cu 2+ (E • = +0.34 V) and Ag + (E • = +0.80 V) would bind as the number of electrons in the thione moiety became richer (Table 3).
With this basis, preliminary experiments were carried out to analyze the selective reactivity of three types of (dxpe)Pt(dmit) chemical sensors. The perchlorate salts of 18 metal ions were first added to the chemosensor solution under the same conditions, and Figure 5 shows the color changes (M n+ = Li + , Na + , K + , Cs + , Mg 2+ , Ca 2+ , Ba 2+ , Al 3+ , Pb 2+ , Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Ag + , Cd 2+ and Hg 2+ ). When the metal ions interacted with the chemosensors, the color of the mixed solution changed immediately. The (dchpe)Pt(dmit) and (dppe)Pt(dmit) chemosensors detected Cu 2+ , Ag + , and Hg 2+ because the electron density of the >C=S moiety is enriched by the EDGs (i.e., cyclohexyl groups in (dchpe)Pt(dmit) and phenyl groups in (dppe)Pt(dmit)). The solution changed Sustainability 2021, 13, 8160 9 of 15 from yellow to deep navy when (dchpe)Pt(dmit) and (dppe)Pt(dmit) interact with Hg 2+ . Furthermore, a slight color change could also be observed with the naked eye when they interacted with Cu 2+ and Ag + . On the other hand, the (dfppe)Pt(dmit) ligand showed selective reactivity only toward the Hg 2+ ion with a color change from yellow to purple. The sensing strength of (dfppe)Pt(dmit) is decreased because of the pentaflurophenyl groups, which are strong electron withdrawing groups (EWGs). Because of the lack of electrons on the >C=S moiety, the interaction between (dfppe)Pt(dmit) and the metal ions became relatively weak. All species are aqueous unless otherwise indicated, and the standard electrode potential is defined as the potential (volts, V) of the half reaction to the reference electrode at 298.15 K in water.
UV-vis measurements were taken to monitor the colorimetric sensing of single metal ions using (dchpe)Pt(dmit) chemosensors. The interaction characteristics of (dchpe)Pt(dmit) and (dppe)Pt(dmit) were analogous, but their absolute values vary significantly. (The UV-vis absorbance spectra of (dppe)Pt(dmit) and (dfppe)Pt(dmit) were examined in our previous work [15]). Figure 6a shows the changes of the absorbance peak of the π→π* transition when (dchpe)Pt(dmit) interacts with single metal ions and the corresponding absorbance ratio (Figure 6b). The UV-vis absorbance spectra show the interaction between the (dchpe)Pt(dmit) and the metal ions. A bathochromic shift is shown from 470 nm to 578 nm (Hg 2+ ), 506 nm with a broad band at 700-800 nm (Cu 2+ ), and 472 nm (Ag + ) while the other peaks of metal ions are unchanged. when (dchpe)Pt(dmit) and (dppe)Pt(dmit) interact with Hg . Furthermore, a slight color change could also be observed with the naked eye when they interacted with Cu 2+ and Ag + . On the other hand, the (dfppe)Pt(dmit) ligand showed selective reactivity only toward the Hg 2+ ion with a color change from yellow to purple. The sensing strength of (dfppe)Pt(dmit) is decreased because of the pentaflurophenyl groups, which are strong electron withdrawing groups (EWGs). Because of the lack of electrons on the >C=S moiety, the interaction between (dfppe)Pt(dmit) and the metal ions became relatively weak. UV-vis measurements were taken to monitor the colorimetric sensing of single metal ions using (dchpe)Pt(dmit) chemosensors. The interaction characteristics of (dchpe)Pt(dmit) and (dppe)Pt(dmit) were analogous, but their absolute values vary significantly. (The UV-vis absorbance spectra of (dppe)Pt(dmit) and (dfppe)Pt(dmit) were examined in our previous work [15].) Figure 6a shows the changes of the absorbance peak of the π→π* transition when (dchpe)Pt(dmit) interacts with single metal ions and the corresponding absorbance ratio (Figure 6b). The UV-vis absorbance spectra show the interaction between the (dchpe)Pt(dmit) and the metal ions. A bathochromic shift is shown from 470 nm to 578 nm (Hg 2+ ), 506 nm with a broad band at 700-800 nm (Cu 2+ ), and 472 nm (Ag + ) while the other peaks of metal ions are unchanged.
To gain insight into the binding mechanisms between the metal ions and the (dchpe)Pt (dmit) molecule, DFT calculations were performed. The sulfur atom in the dmit moiety was identified as a binding site, which is capable of forming a chelate complex. The (dchpe)Pt(dmit) molecule acts as a monodentate ligand by donating non-bonding electrons from the sulfur atom to the metal ions. Based on the experimental results, (dchpe)Pt(dmit) exhibited the strongest interaction with Hg 2+ , followed by Cu 2+ and then Ag + . To verify the differential selectivity of (dchpe)Pt(dmit) for Hg 2+ , Cu 2+ , Ag + and Na + ions on an atomic scale, we calculated the adhesion energy between the active site of (dchpe)Pt(dmit) and the metal ions. In Figure 8, hybridized electrons are shaded in purple (charge accumulation) and grey (charge depletion) and the sulfur atom has a high probability of reacting with heavy metal cations via electrostatic attraction.

Mechanism Study
To determine the ratio of complexation between (dxpe)Pt(dmit) and Hg 2+ , Job's plots for Hg(ClO4)2 were drawn. The mole fraction (χ) of (dchpe)Pt(dmit), (dppe)Pt(dmit), and (dfppe)Pt(dmit) was found to be 0.48, 0.50, and 0.51, respectively (Figure 7). This means that (dxpe)Pt(dmit) formed a 1:1 complex with Hg 2+ in solution, irrespective of the electron-withdrawing ability of the diphosphine ligand. To gain insight into the binding mechanisms between the metal ions and the (dchpe)Pt(dmit) molecule, DFT calculations were performed. The sulfur atom in the dmit moiety was identified as a binding site, which is capable of forming a chelate complex. The (dchpe)Pt(dmit) molecule acts as a monodentate ligand by donating non-bonding electrons from the sulfur atom to the metal ions. Based on the experimental results, (dchpe)Pt(dmit) exhibited the strongest interaction with Hg 2+ , followed by Cu 2+ and then Ag + . To verify the differential selectivity of (dchpe)Pt(dmit) for Hg 2+ , Cu 2+ , Ag + and Na + ions on an atomic scale, we calculated the adhesion energy between the active site of (dchpe)Pt(dmit) and the metal ions. In Figure 8, hybridized electrons are shaded in purple (charge accumulation) and grey (charge depletion) and the sulfur atom has a high probability of reacting with heavy metal cations via electrostatic attraction.   The adhesion energy was examined based on the differential charge density of the sulfur atom using Equation (1). ρtotal indicates the charge density of the sulfur atom when the detecting material and metal cations interact, and ρ(dchpe)Pt(dmit) is the charge density of the sulfur atom when single molecules are present without interacting. The results are summarized in Table 4

Multiple-ion Sensing Ability
The cyclohexyl group is a stronger electron donor than the phenyl group. Both are regarded as EDGs, however, the phenyl group has also electron-withdrawing ability, resulting from the presence of more electronegative sp2 hybridized carbon. For this reason, of the (dxpe)Pt(dmit)-based detection materials that have been proposed, (dchpe)Pt(dmit) showed stronger sensitivity and selectively to distinguish the combination of Hg 2+ , Cu 2+ , and Ag + ions than (dppe)Pt(dmit) or (dfppe)Pt(dmit) molecules. To investigate the possibility of multiple-ion sensing, various metal ions were mixed and examined using UV-vis spectroscopy. Here, three types of (dxpe)Pt(dmit) sensing molecule were examined. In this analysis, Mix I and Mix II solutions are prepared, Mix I contains all metal ions except Hg 2+ , Cu 2+ , and Ag + , while Mix II contains all metal ions except Hg 2+ and Cu 2+ . Mix I was used to examine the multiple-ion sensing behavior of (dchpe)Pt(dmit) and Here, the sulfur atom is shown in yellow, Pt in grey, carbon in dark brown, H in pink, and P in purple.
The adhesion energy was examined based on the differential charge density of the sulfur atom using Equation (1). ρ total indicates the charge density of the sulfur atom when the detecting material and metal cations interact, and ρ (dchpe)Pt(dmit) is the charge density of the sulfur atom when single molecules are present without interacting. The results are summarized in Table 4 and the calculated values show a good agreement with the experimental results.

Multiple-ion Sensing Ability
The cyclohexyl group is a stronger electron donor than the phenyl group. Both are regarded as EDGs, however, the phenyl group has also electron-withdrawing ability, resulting from the presence of more electronegative sp2 hybridized carbon. For this reason, of the (dxpe)Pt(dmit)-based detection materials that have been proposed, (dchpe)Pt(dmit) showed stronger sensitivity and selectively to distinguish the combination of Hg 2+ , Cu 2+ , and Ag + ions than (dppe)Pt(dmit) or (dfppe)Pt(dmit) molecules. To investigate the possibility of multiple-ion sensing, various metal ions were mixed and examined using UV-vis spectroscopy. Here, three types of (dxpe)Pt(dmit) sensing molecule were examined. In this analysis, Mix I and Mix II solutions are prepared, Mix I contains all metal ions except Hg 2+ , Cu 2+ , and Ag + , while Mix II contains all metal ions except Hg 2+ and Cu 2+ . Mix I was used to examine the multiple-ion sensing behavior of (dchpe)Pt(dmit) and (dppe)Pt(dmit), while Mix II was used to test (dfppe)Pt(dmit). As a first step, a base solution was prepared by adding 1.0 eq of Mix I or Mix II to (dxpe)Pt(dmit) solutions. Hg 2+ , Cu 2+ , and Ag + ions were then added to the base solution and mixed with various combinations of ions. The color change and absorbance peaks are presented in Figure 9. The absorbance peak of the base solution for (dchpe)Pt(dmit), (dppe)Pt(dmit), and (dfppe)Pt(dmit) was observed at 470 nm, 463 nm, and 449 nm, respectively, with no change in color. The color changes compared to Figures 5 and 9a, illustrated the noticeable effect of interfering metal cations. When (dchpe)Pt(dmit) interacted only with single Hg 2+ , Cu 2+ , and Ag + ions, the color changed to navy, deep orange, and deep yellow, respectively ( Figure 5); however, in the presence of coexisting cations, the color became ultramarine blue, rose pink, and bright yellow, respectively. The same trend was also observed for (dppe)Pt(dmit) and (dfppe)Pt(dmit). In particular, the (dfppe)Pt(dmit) molecule showed excellent property to distinguish not only single Cu 2+ , and Hg 2+ ions, but also multiple ions (Cu 2+ and Hg 2+ ) (in Figure 9a,d).
Sustainability 2021, 13, x FOR PEER REVIEW 13 of 16 excellent property to distinguish not only single Cu 2+ , and Hg 2+ ions, but also multiple ions (Cu 2+ and Hg 2+ ) (in Figure 9a,d).  This molecule, however, distinguishes only two types of molecule, Cu 2+ and Hg 2+ . In contrast, (dchpe)Pt(dmit) and (dppe)Pt(dmit) exhibited potential to detect three types of metal ions, even in the situation where interfering cations coexist. Of the three types of sensing molecule, (dchpe)Pt(dmit) is the most suitable sensing material for practical use as an analysis tool because it produces more well-distributed color change than (dppe)Pt(dmit), and it can detect three types of metal ions. The color distribution is presented as a CIE 1931 color space chromaticity diagram in Figure 10.  This molecule, however, distinguishes only two types of molecule, Cu 2+ and Hg 2+ . In contrast, (dchpe)Pt(dmit) and (dppe)Pt(dmit) exhibited potential to detect three types of metal ions, even in the situation where interfering cations coexist. Of the three types of sensing molecule, (dchpe)Pt(dmit) is the most suitable sensing material for practical use as an analysis tool because it produces more well-distributed color change than (dppe)Pt(dmit), and it can detect three types of metal ions. The color distribution is presented as a CIE 1931 color space chromaticity diagram in Figure 10. Based on the RGB values in Figure 10a, the colors were expressed as a CIE 1931 color space chromaticity diagram. The chromaticity diagram can be used to determine how the human eye will perceive light with a given spectrum [26]. Color can be divided into brightness and chromaticity. For instance, white is a bright color, while grey is considered to be a less-bright version of that same white. In other words, the chromaticity of white and grey are the same while their brightness differs. In the CIE color space, x and y are projective coordinates and the colors of the chromaticity diagram occupy a region of the real projective plane, and an outer curved boundary is a spectral locus, with the wavelengths shown in nanometers. The chromaticity is then specified by the two derived parameters x and y, with two of the three normalized values being a function of all three tristimulus values X, Y, and Z. The parameters were calculated based using Equations (2) and (3).
The red circles in Figure 10a show that the colors are well distributed, which indicates that (dchpe)Pt(dmit) is an excellent candidate as a detection material by identifying the type of metal ions in a mixed solution. In contrast, the red circles in Figure 10b for (dppe)Pt(dmit) are more closely grouped together. The proposed (dchpe)Pt(dmit) chemosensor thus appears to have high potential for practical use with real-life environmental samples because it can detect three types of single ions and multiple ions simultaneously.

Conclusions
In this study, (dchpe)Pt(dmit) material was synthesized with a two-step reaction pathway and subsequent structural confirmation was conducted using XRD, MALDI-TOF, and CV. The Job's plot and DFT have been used to explain the stoichiometric ratio and binding energies. For field use of colorimetric chemosensors, highly selective and sensitive reactivity for heavy metal ions is required. Considering the concentration limit of Hg 2+ for drinking water from EPA (Environmental Protection Agency, USA) regulation, Figure 10. CIE 1931 color space chromaticity diagrams after the interaction between (a) the (dchpe)Pt(dmit) ligand and single/multiple ions, (b) the (dppe)Pt(dmit) ligand and single/multiple ions, and (c) the (dfppe)Pt(dmit) ligand and single/multiple ions. The colors of the multiple ions (Cu 2+ + Hg 2+ , Cu 2+ + Ag + , Hg 2+ + Ag + and Cu 2+ + Hg 2+ + Ag + ) are marked by the red circles.
Based on the RGB values in Figure 10a, the colors were expressed as a CIE 1931 color space chromaticity diagram. The chromaticity diagram can be used to determine how the human eye will perceive light with a given spectrum [26]. Color can be divided into brightness and chromaticity. For instance, white is a bright color, while grey is considered to be a less-bright version of that same white. In other words, the chromaticity of white and grey are the same while their brightness differs. In the CIE color space, x and y are projective coordinates and the colors of the chromaticity diagram occupy a region of the real projective plane, and an outer curved boundary is a spectral locus, with the wavelengths shown in nanometers. The chromaticity is then specified by the two derived parameters x and y, with two of the three normalized values being a function of all three tristimulus values X, Y, and Z. The parameters were calculated based using Equations (2) and (3). (2) The red circles in Figure 10a show that the colors are well distributed, which indicates that (dchpe)Pt(dmit) is an excellent candidate as a detection material by identifying the type of metal ions in a mixed solution. In contrast, the red circles in Figure 10b for (dppe)Pt(dmit) are more closely grouped together. The proposed (dchpe)Pt(dmit) chemosensor thus appears to have high potential for practical use with real-life environmental samples because it can detect three types of single ions and multiple ions simultaneously.

Conclusions
In this study, (dchpe)Pt(dmit) material was synthesized with a two-step reaction pathway and subsequent structural confirmation was conducted using XRD, MALDI-TOF, and CV. The Job's plot and DFT have been used to explain the stoichiometric ratio and binding energies. For field use of colorimetric chemosensors, highly selective and sensitive reactivity for heavy metal ions is required. Considering the concentration limit of Hg 2+ for drinking water from EPA (Environmental Protection Agency, USA) regulation, the sensitivity of (dchpe)Pt(dmit) is not sufficient to be used in field analysis [27]. Although the sensitivity still remains as a limitation, the point of this paper is to suggest the possibility of ion mixture sensing ability and methodologies to control the selectivity. Here proposed (dchpe)Pt(dmit) showed good selectivity toward three types of single metal ions (i.e., Hg 2+ , Cu 2+ , Ag + ) and could analyze the combinations of multiple metal ions over interfering ions. By finely tuning the functional groups the selectivity was successfully enhanced. With the effect of the substituted cyclohexyl functional groups, the electron density on the sulfur atom of thione moiety is increased, facilitating the binding with heavy metal ions.