Article pubs.acs.org/ac
Triphosphate Ion-Selective Electrode Based on Zr-Porphyrin Complex Yuma Yagi, Shuhei Masaki, Tetsuki Iwata, Daisuke Nakane, Takashi Yasui, and Akio Yuchi* Nagoya Institute of Technology, Graduate School of Engineering, Gokiso, Showa, Nagoya, Japan, 466-8555 S Supporting Information *
ABSTRACT: Ion-selective electrode using zirconium(IV) complex with octaethylporphin (H2oep) as a carrier showed high selectivity to triphosphate (TP, H5tp) against other hydrophilic anions including diphosphate and phosphate. The electroactive species was identified to be [(Zr4(oep)4(Htp)2] (TP/Zr ratio of 0.5) of the unique structure; triphosphates are recognized by one Zr atom through three O atoms on three different P atoms and by another Zr atom through two O atoms on two terminal P atoms and are also involved in complementary intermolecular hydrogen bonding to be surrounded by four porphyrin complexes. In contrast, Zr(IV) in the other complex with tetraphenylporphin has the higher Lewis acidity, due to the electron-withdrawing property of phenyl rings and, at the higher TP concentration, forms a species having a TP/Zr ratio of unity, which precipitates to lose the electroactivity. The electrode was successfully applied to monitor hydrolysis of TP that provides diphosphate and phosphate.
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selectivity. A mononuclear Zn complex with benzenesulfonamide of dipicolylamine showed a selectivity pattern of ATP > DP attributed to the stronger hydration of DP.7 Another mononuclear Zn complex with terpyridine derivative showed comparable optical responses to both ATP and DP at the concentration comparable to that of the probe Zn complex (50 μmol L−1) but could differentiate DP from other phosphate species at much lower concentrations (20−180 nmol L−1) due to formation of DP/Zn = 1:3 species.8 A dinuclear Zn complex with a reagent having two picolylamines at two ortho- positions of phenol showed the selectivity of DP > ATP is attributed to the difference in electric charge.9 Another dinuclear Zn complex with a reagent having two picolylamines at Z-positions on tetraphenylethene showed the selectivity to TP by aggregation-induced emission.10 The change in central metal as well as modification of the chelating reagent structure may be promising in order to achieve a novel selectivity. Moreover, adoption of the phase transfer to an organic phase instead of reactions in aqueous solution may enhance the selectivity due to the necessity of charge neutralization, although more extended dehydration of POs and more lipophilicity of the resulting species is required. Potentiometry by ion-selective electrode utilizing the phase transfer of phosphate has been reported with use of the carriers such as bis(guanidinium) compounds,11 cyclic polyamines,12 thiourea derivatives of calixarene,13 organotin compounds,14,15 and uranyl-salophen complexes.16,17 In contrast, no ISE has been reported for TP and DP.18−20
ecognition and detection of anions have been attracting attention, because of their ubiquitousness and important roles in biological and environmental systems.1,2 Compared with cations, anions are not so reactive in general but, except halides, have inherent structural features of occasionally carrying groups or atoms at the peripheral positions to be anchored. Thus, simultaneous operation of weak but plural interactions such as hydrogen-bonding and van der Waals interaction increases the strength of interaction with and give selectivity to some specific anion. Such anion recognition grows up to one of the major fields of supramolecular chemistry, and optical sensing is especially studied for application to modern imaging.3 Among anions, phosphorus oxoacids (POs) including condensed ones [phosphoric acid (MP, H3mp), diphosphoric acid (DP, H4dp), triphosphoric acid (TP, H5tp)) and their adenosine esters (AMP, ADP, ATP)] are one of the most challenging items, because of their biological functions and difficulty in recognition due to strongly hydrating tendencies. For example, DP interacts with bis(urea) derivatives by hydrogen-bonding and provides a characteristic fluorescence change in dehydrated DMSO, but the change is extremely diminished in water even in the presence of a surfactant.4 In order to overcome the disadvantage of the dehydration process, the stronger interaction of Lewis acid−base reaction has been widely used. Zinc complexes with neutral chelating reagents are almost exclusively adopted to utilize the electrostatic attraction as well.5 For example, an organic field-effect transistor modified by a most simple Zn complex with a dipicolylamine derivative responds to ATP, ADP, AMP, DP, and MP at the 10−5 mol L−1 level.6 This method is selective for the functional group of phosphate but cannot be used, e.g., to monitor hydrolysis of ATP that provides AMP and DP. A variety of chelating reagents have been reported to increase the © 2017 American Chemical Society
Received: September 22, 2016 Accepted: March 7, 2017 Published: March 7, 2017 3937
DOI: 10.1021/acs.analchem.6b03754 Anal. Chem. 2017, 89, 3937−3942
Article
Analytical Chemistry
Figure 1. Change in absorption spectrum and molar ratio plot on the reaction of Zr-OEP (a) and Zr-TPP (b) in CB with TP in aqueous phase. CZr/ 10−5 mol L−1: 5.2 (a), 4.0 (b). CAcOH/10−4 mol L−1: 5.2 (a), 4.0 (b). Wavelength/nm: (a) □, 401.5;, ○, 396.5; (b) □, 419.0; and ○, 413.0.
Optode. An aliquot (200 μL) of the same tetrahydrofuran solution as that for ISE membrane was developed on a silicate plate rotated at 1000 rpm by a spin coater (MS-A100, Mikasa) for 10 s. The thickness of the membrane was 3 μm. The plate was immersed in the solution in the optical cell of 1 cm length. The UV−vis spectrum was directly recorded. ESI-MS Analysis. A series of chloroform solutions of [Zr(tpp) (OH)2] were shaken with aqueous solutions containing the stoichiometric amounts of respective anions; pH was adjusted at 2.0 with HCl. The extracts were diluted twice with acetonitrile and were subjected to ESI-MS (Synapt G2 HDMS, Waters) in positive ion mode. X-ray Crystallography. A single crystal of Zr-TPP-TP complex was mounted on a glass fiber. X-ray measurements were made on a Rigaku Mercury CCD area detector with graphite monochromated Mo−Kα radiation. The data were collected at a temperature of −100 ± 1 °C to a maximum 2θ value of 20.0°. A total of 1200 oscillation images were collected. The structure was solved by direct methods28 and expanded using Fourier techniques.29 The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were refined isotropically using the riding model.
Inspired by the strong but reversible affinity of POs to zirconium(IV),21−23 the phase transfer of these acids into organic solvents such as chlorobenzene (CB) and o-nitrophenyloctylether (o-NPOE) using the Zr(IV) complexes of porphyrins [POR, H2por: tetraphenylporphin (TPP, H2tpp); octaethylporphin (OEP, H2oep)], which had been proposed as a carrier for citrate, carboxylates, and fluoride,24−26 was examined in this study. On the basis of the results, potentiometric determination of TP in the presence of DP and MP has been achieved by using Zr-OEP as a carrier. The lack of carrier function of Zr-TPP is also discussed.
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EXPERIMENTAL SECTION Reagents. The two Zr-PORs were prepared according to the literature24,27 and were isolated as OH-type complexes, [Zr(por) (OH)2]. Na5P3O10, Na4P2O7·10H2O, NaH2PO4· 2H2O (reagent grade, Wako), disodium salt of ATP, ADP, and AMP (Oriental Ind.) were used as the sources of POs. Tridodecylmethylammonium chloride (TDMACl) and sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB) (Aldrich) were used as lipophilic additives. Potentiometry. The membrane was prepared by slow evaporation of 1.5 mL of a tetrahydrofuran solution containing [Zr(por) (OH)2] (2 mg), 2-nitrophenyl octyl ether (o-NPOE, 132 mg), and poly(vinyl chloride) (66 mg). The membranes containing the Zr complex in addition to 10 and 20 mol % TDMACl or NaTFPB and the membrane based on only TDMACl were also prepared for comparison. The thickness of the membrane was around 100 μm. As the internal solution of ISE, a 10−2 mol L−1 NaCl and 10−2 mol L−1 anion solution was used. The ISE was, unless otherwise stated, conditioned with a relevant anion solution at 10−2 mol L−1 overnight. The electromotive force of the following cell was measured with a potentiometer at 25.0 ± 0.1 °C: Ag/AgCl|KCl(3.33 mol L−1) | test solution|membrane|internal solution|Ag/AgCl. Two-Phase Reaction between CB and Water. A volume of 1 mL of a CB solution of Zr-POR (5.2 × 10−5 mol L−1 for Zr-OEP and 4.0 × 10−5 for Zr-TPP) was shaken with 1 mL of an aqueous solution containing a relevant anion in a Teflon tube at 25 °C for 3 h; acetic acid (5.2 × 10−4 mol L−1 for ZrOEP and 4.0 × 10−4 for Zr-TPP) was added to the aqueous phase, unless otherwise stated. After phase separation, the UV− vis spectrum of the CB phase was recorded using the optical cell of 1 mm length.
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RESULTS AND DISCUSSION Preliminary Potentiometric Studies. The potential responses of ISEs based on Zr-PORs to POs were preliminary examined after conditioning with respective salt solutions at 10−2 mol L−1. The response of Zr-OEP was instantaneous to any anion at 10−2 mol L−1, while the response of Zr-TPP was fast to TP and DP but took 1 h to MP (Figure S-1). The equilibrium potential was in the order of MP > DP > TP for ZrOEP and MP ≈ TP > DP for Zr-TPP. The effect of pH on the potential response to each anion was examined (Figure S-2); pH was adjusted by addition of HCl. The Zr-OEP showed a potential response to TP at above 10−6 mol L−1 (linear response of −15 mV/decade between 10−5.0 and 10−1.5 mol L−1) at pH 1.5. The response to DP was observed at above 10−5 mol L−1 (−15 mV/decade between 10−5.0 and 10−1.0 mol L−1 at pH 2.0), while the response to MP was weak (−30 mV/decade between 10−3.0 and 10−1.0 mol L−1 at pH 2.5). The Zr-TPP, in contrast, showed no response to TP, a weak response to DP, and a linear potential response to MP (−30 mV/decade between 10−4.0 and 10−1.0 mol L−1) at pH 3.0. Other related anions, such as ATP and ADP, showed 3938
DOI: 10.1021/acs.analchem.6b03754 Anal. Chem. 2017, 89, 3937−3942
Article
Analytical Chemistry weak responses at concentration >10−4.0 mol L−1, while AMP exhibited no responses. In summary, Zr-OEP is promising for potentiometry of TP. In order to confirm the striking difference between the behaviors of two Zr-PORs as a carrier for TP, a change in the calibration curve by conditioning of ISEs with TP solution was studied (Figure S-3). Under the present condition acidified with HCl in the absence of acetic acid, transformation to [Zr2(por)2(OH)3]+,Cl− concomitantly proceeded, depending on the concentrations of TP and HCl. The ISE based on ZrOEP showed a calibration curve characteristic of uptake of TP into the membrane just after starting.30 After overnight, a calibration curve with a linear portion (−15 mV decade−1) indicated completion of the conditioning. The ISE based on ZrTPP, on the other hand, definitely showed a response (−30 mV decade−1) just after starting or after 30 min, while the response was completely lost after overnight. Characterization of Electroactive Species by TwoPhase Reactions between CB and Water. In order to confirm the scientific basis of the TP-ISE based on Zr-OEP and to elucidate the striking difference in performance between ZrOEP and Zr-TPP, the electroactive species formed by conditioning of the membrane with POs were characterized using the two-phase reactions between CB and water. The twophase reaction of Zr-TPP with common acids (HA: HClO4. HNO3, HCl, CH3COOH) had been elucidated.24 The four species, [Zr(tpp) (OH)2], [Zr2(tpp)2(OH)3]+,A−, [Zr(tpp) (OH)A], and [Zr(tpp)A2], were formed in different extents depending on the coordinating ability and lipophilicity of A−. On the basis of the results of this research, acetic acid of 10−4 mol L−1 level in addition to HCl of 10−2 mol L−1 was added to the aqueous phase, so as to form [Zr(por) (AcO)2] with the absorption maximum at the longer wavelengths and to give large shifts by the reactions with POs. Triphosphate. The spectral changes of the Soret band accompanied by the two-phase reactions of Zr-PORs in CB with an aqueous TP solution are shown in Figure 1. The absorption maximum was shifted to a shorter wavelength (from 401.5 to 396.5 nm with an isosbestic point at 399.0 nm for ZrOEP; from 419.0 to 413.0 nm with an isosbestic point at 416.0 nm for Zr-TPP). The molar ratio plots in insertion clearly indicated the reaction stoichiometry of Zr-POR/TP = 2:1 for both complexes. Although the stability constants were too large to be accurately evaluated, the reactivity of Zr-OEP was lower than Zr-TPP according to the curvature at CTP/CZr = 0.5 in the molar ratio plots. Moreover, when the CB solutions of the products after phase separation were diluted by 50- or 200times, the absorption maximum of each complex was slightly shifted to a longer wavelength. This suggested the presence of more than two products. The ESI-MS of Zr-TPP-TP showed peaks at m/z values corresponding to [Zr2(tpp)2(Htp)] and [Zr4(tpp)4(Htp)2] (M and 2M, [M + H+]+, [M + Na+]+, [2M + H+]+, [2M + Na+]+) with characteristic fine structures assignable to the abundance of Zr isotopes (Figure S-4). The ESI-MS of Zr-OEP-TP showed substantially the same results. Diphosphate. The two-phase reactions with DP also showed shifts of the Soret band to a shorter wavelength as shown in Figure S-5a,b (to 396.0 nm for Zr-OEP; to 412.0 nm for ZrTPP). The molar ratio plot for Zr-TPP indicated the reaction stoichiometry of Zr-TPP/DP = 2:1 (Figure S-5d). In the reaction of Zr-OEP, in contrast, an excess of DP was essential to complete the reaction. The apparent stability constant was successfully obtained for Zr-OEP, based on the same
stoichiometry as that of Zr-TPP (Figure S-5c); the constant of Zr-TPP was too large to be evaluated. In contrast to the reaction with TP, the resulting species showed no spectral change upon dilution. The ESI-MS of Zr-TPP-DP showed peaks at m/z values corresponding to [Zr2(tpp)2(dp)] (M, [M + H+]+, [M + Na+]+) (Figure S-6). Dimeric structures with μ4-P2O7-bridging have been reported for several metal ions with auxiliary chelating reagents.31 Phosphate. The two-phase reaction of Zr-OEP with MP showed a shift of the Soret band to a shorter wavelength (to 397.0 nm) with a systematic shift of the cross point of spectra (Figure S-7a). A large excess of MP was essential to complete the reaction. The reaction of Zr-TPP showed a shift to shorter wavelength of 414.0 nm and to a slightly longer wavelength of 415.0 nm in a stepwise manner (Figure S-7b,c). Since two reactions overlapped in Zr-OEP, the change in absorption spectrum could not be analyzed. In contrast, the equilibrium analysis for the spectral changes of Zr-TPP at the lower MP concentration (
