Dualism of Sensitivity and Selectivity of Porphyrin Dimers in

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Dualism of sensitivity and selectivity of porphyrin dimers in electroanalysis Grzegorz Lisak, Takashi Tamaki, and Takuji Ogawa Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b04179 • Publication Date (Web): 07 Mar 2017 Downloaded from http://pubs.acs.org on March 7, 2017

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Analytical Chemistry

Dualism of sensitivity and selectivity of porphyrin dimers in electroanalysis Grzegorz Lisak1,2,3,*, Takashi Tamaki4, Takuji Ogawa4 1

Johan Gadolin Process Chemistry Centre, Laboratory of Analytical Chemistry, Åbo Akademi University, Biskopsgatan 8, Åbo-Turku 20500, Finland 2

School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

3

Nanyang Environment and Water Research Institute, 1 Cleantech Loop, CleanTech, Singapore 637141, Singapore

4

Department of Chemistry, Graduate School of Science, Osaka University, 1-1 Machikaneyamacho, Toyonaka, Osaka 560-0043, Japan Corresponding author: Email: [email protected] Telephone number: +6567904737

Abstract This work uncovers the application of porphyrin dimers for the use in electroanalysis, such as potentiometric determination of ions. It also puts in question a current perception of an occurrence of the super-Nernstian response, as a result of the possible dimerization of single porphyrins within an ion-selective membrane. To study that, four various porphyrin dimers were used as ionophores, namely freebase-freebase, Zn−Zn, Zn-freebase and freebase-Zn. Since the Zn-freebase and freebase-Zn porphyrin dimers carried both anion- and cation-sensitive porphyrin units, their application in ISEs was utilized in both anion- and cation-sensitive sensors. In respect to the lipophilic salt added, both porphyrins dimers were found anion- and cation-sensitive. This allowed using a single molecule as novel type of versatile ionophore (anion- and cationselective), simply by varying the membrane composition. All anion-sensitive sensors were perchlorate-sensitive, while the cation-selective sensors were silver-sensitive. The selectivity of the sensors depended primarily on the porphyrin dimers in the ion-selective membrane. Furthermore, the selectivity of cation-sensitive dimer based sensors was found significantly superior to the ones measured for the single porphyrin unit based sensors (precursors of the porphyrin dimers). Thus, the dimerization of single porphyrins may actually be a factor to increase or modulate porphyrin selectivity. Moreover, in the case of cation-sensitive sensors, the 1 ACS Paragon Plus Environment


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selectivity vastly depended on the order of porphyrin units in the dimer. This opens a new approach of regulating and adjusting sensitivity and selectivity of the sensor through the application of complex porphyrin systems with more than one porphyrin units with mix sensitive porphyrins. INTRODUCTION Porphyrins are truly remarkable compounds. Their application is widely spread among various fields including photonics, catalysis, molecular electronics, biomedicine and sensors. In the chemical sensor field, porphyrins are continuously investigated in chromatography, preconcentration techniques, spectroscopy and electroanalysis.1-4 There are two main types of porphyrins, freebase- and metalloporphyrins. The versatility of porphyrins makes them utilized in the wide range of anion and cation sensing applications. They pose a range of functionalities allowing them to be applied in analyses of gas and liquid phases via the possibility of electropolymerisation of porphyrin films on electrode substrates, electrocatalytic activity of metalloporphyrins, cross sensitivity towards inorganic gasses (electronic nose), volatile compounds and receptors in ion-selective electrodes and optodes for the determination of ions in liquid samples, and application in multi sensor electronic tongues.5-12 In electroanalysis, porphyrins are usually incorporated in ion-selective membrane (ISM). They act as ionophores for a wide range of anionic and cationic analytes. The freebase porphyrins are cation-selective. They act as neutral carriers in ISM, forming with cations a “sitting-top” complex.13,14 On the other hand, metalloporphyrins are anion-selective. They act as Lewis acids in ISM, forming axial coordination between anion and the central metal ion. Positively, they often show non-Hofmeister selectivity patterns. The selectivity of such anion- or cation-selective sensors may be regulated by the modification of the porphyrin macrocycle structure and/or the addition of peripheral substituents, as well as in case of metalloporphyrins, the change of the central atom.15-18 In the ion sensor design, porphyrin is trapped in the liquid membrane consisting of plasticized polymeric matrix. Commonly an ion additive in form of lipophilic salt is added to the ion-selective membrane. The additive in ISM works as ion-exchanger, helping the electrode to equilibrate with the analyte in the sample solution and maintaining Nernstian response of the sensor.19 Some of the porphyrin-based sensors, however, exhibit super Nernstian response. A 2 ACS Paragon Plus Environment

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dimerization of porphyrins in the ISM, via hydroxide ion bridges, was proposed as possible explanation of this phenomenon. Furthermore, such dimerization was found to have a negative influence on the lifetime of the sensor owning to the possible crystallization of the porphyrin when in contact with aqueous media.20,21 This finding triggered the new direction in the porphyrin based sensor research towards solving the problem of dimerization of porphyrins in the ISM.22-24 In this work we purposely apply four synthetized porphyrin dimers as ionophores for the use in chemical sensing. The dimers used in this study were: freebase-freebase (8-HH); Zn−Zn (8-ZnZn), Zn-freebase (8-ZnH) and freebase-Zn (8-HZn). They were investigated for the use in chemical sensing of anions and cations. Furthermore, the 8-ZnH and 8-HZn porphyrins carried both anion- and cation-sensitive porphyrin units thus the new type of ionophore was investigated and proposed, which can be applied either as anion- or cation-selective, driven by the composition of the ion-selective memebrane. EXPERIMENTAL SECTION Reagents and materials. Cadmium nitrate hexahydrate, Cd(NO3)2 × 4H2O; copper nitrate trihydrate, Cu(NO3)2 × 3H2O; strontium nitrate Sr(NO3)2; lead(II) nitrate Pb(NO3)2, lithium acetate (LiAc) and sodium fluoride, NaF were purchased from Sigma-Aldrich (Steinheim, Germany), potassium chloride, KCl; zinc nitrate hexahydrate, Zn(NO3)2 × 6H2O; sodium thiocyanate, NaSCN;

tridodecylmethylammonium chloride, TDMACl; potassium tetrakis(4-

chlorophenyl)borate (KTClPB); 2-nitrophenyl octyl ether, o-NPOE; poly(vinyl chloride) of high molecular weight, PVC and tetrahydrofuran, THF were purchased from Fluka (Buchs, Switzerland), nitric acid, HNO3; silver(I) nitrate, AgNO3 and sodium bromide, NaBr were purchased from J.T. Baker (Central Valley, USA) while magnesium nitrate hexahydrate, Mg(NO3)2 × 6H2O; calcium nitrate tetrahydrate, Ca(NO3)2 × 4H2O; cobalt nitrate hexahydrate, Co(NO3)2 × 6H2O; nickel nitrate hexahydrate, Ni(NO3)2 × 6H2O; sodium nitrate, NaNO3; potassium nitrate, KNO3; sodium perchlorate hydrate, NaClO4 × H2O; sodium acetate, NaAc; sodium iodide, NaI; sodium salicylate, NaSal and sodium hydroxide, NaOH were purchased from Merck (Darmstadt, Germany). Porphyrins investigated as ionophores in ion-selective electrodes are shown in Figure 1, namely: Freebase-Freebase Porphyrin Dimer, 8-HH; Zn−Zn Porphyrin Dimer, 8-ZnZn; Zn-Freebase Porphyrin Dimer, 8-ZnH and Freebase-Zn Porphyrin Dimer, 83 ACS Paragon Plus Environment


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HZn. Porphyrin dimers used as ionophores as well as their precursors, single unit porphyrins (1H, 1-Zn, 2-H and 2-Zn) were synthesized according to the procedures described elsewhere and their molecular structures were confirmed by means of NMR, UV-vis, FT-IR and HRMS.25,26 Aqueous solutions were prepared with freshly deionized water of 18.2 MΩ cm resistivity obtained with the ELGA purelab ultra water system (High Wycombe, United Kingdom).

Figure 1. Porphyrin dimers used as ionophores in ISM. Electrodes and potentiometric measurements with ISEs. The glassy carbon electrodes with a teflon body were firstly cleaned by polishing, using mesh paper with 0.3 µm Al2O3, washed with deionized water and 99.5% ethanol, left in an ultrasonic deionized water bath for 300 s and finally washed with water and 99.5% ethanol, and dried in open air. Then, electropolymerisation of poly(3,4-ethylenedioxythiophene) (PEDOT) doped either with polystyrene sulfonate (PSS–) (8-HH, (+)8-ZnH, (+)8-HZn, (+)8-HZn:(+)ZnH, 1:1, 1-H and 2-H ISEs) or with chloride (Cl–) (8-ZnZn, (–)8-ZnH, (–)8-HZn, (–)8-HZn:(–)ZnH, 1:1, 1-Zn and 2-Zn ISEs) was performed in a three-electrode cell containing aqueous solution of 0.01 mol dm–3 of monomer (3,4-ethylenedioxythiophene, EDOT) and 0.1 mol dm–3 PSS– or 0.1 mol dm–3 of KCl. Thus the prefix (+) indicates that the electrodes were prepared as cation sensitive while prefix (–) indicates that electrodes were prepared as anion sensitive by applying different solid contact and different ionic additives while using the same mixed sensitive porphyrin dimers. During the electrodeposition the glassy carbon electrode served as the working electrode, a single junction 4 ACS Paragon Plus Environment

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Ag/AgCl/3 mol dm–3 KCl (6.0733.100, Herisau, Switzerland) served as the reference electrode, and a glassy carbon rod served as the counter electrode. The electropolymerisation was done galvanostatically by applying a current corresponding to the current density of 0.2 mA cm–2 to the working electrode for 714 s .27 Then the electrodes were washed with deionized water and left for water evaporation for three hours. Then the ion-selective membrane was drop casted from appropriate membrane cocktail on top of PEDOT:PSS or PEDOT:Cl. Membrane cocktails composition is presented in the Table 1. All the porphyrin dimers in this work were used as not charged (neutral) molecules, thus they were further on treated as neutral ionophores in ISEs (within the sensors pH stability region). In this situation, the molar ratios of porphyrin dimers and single porphyrins to the ionic additives were approximately 1.5 and 3, respectively. Table 1. The composition of the ion-selective membranes cocktails (in weight %). ISE

Porphyrin

TDMACl

KTClPB

o-NPOE

PVC

mol(P)/mol(S)*

TDMACl

-

1.0

-

66.0

33.0

N.D.

KTClPB

-

-

1.0

66.0

33.0

N.D.

8-ZnZn

1.0

0.2

-

66.0

32.8

1.5

8-HH

1.0

-

0.2

66.0

32.8

1.4

(–)8-ZnH

1.0

0.2

-

66.0

32.8

1.6

(–)8-HZn

1.0

0.2

-

66.0

32.8

1.6

(+)8-ZnH

1.0

-

0.2

66.0

32.8

1.4

(+)8-HZn

1.0

-

0.2

66.0

32.8

1.4

(–)8-ZnH:(–)8-HZn, 1:1

0.5:0.5

0.2

-

66.0

32.8

1.5

(+)8-ZnH:(+)8-HZn, 1:1

0.5:0.5

-

0.2

66.0

32.8

1.5

1-Zn

1.0

0.2

-

66.0

32.8

3.3

2-Zn

1.0

0.2

-

66.0

32.8

3.0

1-H

1.0

-

0.2

66.0

32.8

3.1

2-H

1.0

-

0.2

66.0

32.8

2.8

*mol(P)/mol(S): mol ratio between the ionophore (P, porphyrin) and the salt additive (S, either TDMACl or KTClPB)

The membrane cocktails were prepared by dissolving in 2 ml of THF a total weight of 200 mg of each membrane components. Then, portions of 20 µl of the membrane cocktail were applied onto the electrode every 10 minutes (longer if necessary, until evaporation of the THF) until a final volume of 60 µl of the cocktail for each single electrode was attained. Electrodes were then left in open air for overnight evaporation of the residual solvent. Three electrodes were prepared according to the same procedure for each membrane composition. The name of the electrode indicated the ionophores/porphyrins used. In addition to single ionophore based membranes, two mix ionophore based membranes were investigated, namely: (+)8-HZn:(+)ZnH (1:1) and (–)8HZn:(–)ZnH (1:1). Such unconventional compositions of ISMs were done in order to investigate 5 ACS Paragon Plus Environment


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how selectivity changes when two competing ionophores with different selectivities to main over interfering ions are applied within the same ISM. The potentiometric measurements were performed using an EMF16 Interface (Lawson Labs Inc., USA). The potentiometric response of all ISEs was measured against the double junction (Ag/AgCl/3 mol dm–3 KCl/10–1 LiAc) reference electrode. The potentiometric measurements were always performed with decreasing activities of primary ion in the standard solution, from ~10–1 to 10–8 mol dm–3. The automatic dilutions of stock solution were performed using two Metrohm Dosino 700 instruments equipped with burets of 50 mL capacity (Herisau, Switzerland). The pumps were programmed to dilute the sample solution with freshly deionized water (18.2 MΩ cm) every 5 min. Measurements were carried out in a 100 ml glass beaker. The activity coefficients were calculated according to the Debye-Hückel approximation. The measurements were performed at room temperature, 22-23oC. The pH sensitivity of porphyrin dimer-based cation-selective ISEs. Prior to the selectivity measurement, the pH stability was investigated only for cation-selective dimer-based sensors (8-HH, (+)8-ZnH, (+)8-HZn and (+)8-HZn:(+)ZnH, 1:1) in the pH range from 2 to 10 by measuring the EMF of all ISEs in 0.1 mol dm–3 KNO3 at various pH. The pH was adjusted by small additions of strongly concentrated HNO3 and NaOH. The additions were planned to change tenfold the pH from pH 2 to 10 without significantly altering KNO3 concentration in the solution. After each addition the EMF of the electrodes was recorded only after stabilization of the potential readout (approx. 3 min). Selectivity measurements. For all electrodes, the separate solution method (SSM)28 was used to determine unbiased selectivity coefficients for primary over the relevant interfering ions.29-31 Except of porphyrin dimer-based ISEs, single unit porphyrins were also used to determine if the dimerization of single porphyrins is beneficial on the selectivity of such sensors. Figure 2 presents four variations of single unit porphyrins that were used as the precursors in the synthesis of the investigated porphyrin dimers as ionophores, namely: 10, 20-bis(3, 5diisopentoxyphenyl) porphyrin (1-H), (10, 20-bis(3, 5-diisopentoxyphenyl) porphyrinato)zinc(II) (1-Zn), 10-(4-hydroxyphenyl)-5, 15-Bis(3, 5-diisopentoxyphenyl)porphyrin (2-H) and (10-(4Hydroxyphenyl)-5, 15-Bis(3, 5-diisopentoxyphenyl)porphyrinato)zinc(II) (2-Zn).25,26

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Figure 2. Single unit porphyrins used as the precursors for the synthesis of porphyrin dimers. 25,26 In all cases, unconditioned electrodes were used for the measurement. The potentiometric data was collected according to the procedure described in previous section. To avoid problems with possible changes of ISEs standard potential, all measurements were performed in a single run, within 4 hours. The swift three point EMF measurements were performed (5 min in each of interfering and main ion solutions, 0.1, 0.01 and 0.001 mol dm–3) from which the data was used to calculate selectivity coefficients. For the anion-selective sensors (8-ZnZn, (–)8-ZnH, (–)8HZn, (–)8-HZn:(–)ZnH, 1:1, 1-Zn and 2-Zn) the EMF was recorded in the following sequence of anions: F–, Ac–, Cl–, NO3–, Br–, I–, Sal–, SCN–, ClO4–. All investigated salts had sodium as counter cation. For the cation-selective sensors (8-HH, (+)8-ZnH, (+)8-HZn, (+)8-HZn:(+)ZnH, 1:1, 1-H and 2-H) the EMF was recorded in the following sequence of cations (in brackets is given pH value of 0.1 mol dm–3 of each salt): Mg2+ (pH= 5.8), Ca2+ (pH= 5.7), Co2+ (pH= 5.1), Zn2+ (pH= 4.9), Cu2+ (pH= 4.0), Ni2+ (pH= 5.3), Pb2+ (pH= 3.6), Cd2+ (pH= 4.9), Sr2+ (pH= 5.5), Na+ (pH= 6.9), K+ (pH= 6.8), Ag+ (pH= 4.6). All investigated salts had nitrate as counter anion. The pH was measured using pH meter (Metrohm, Herisau, Switzerland).

The slope of

perchlorate-selective (for anion-selective) and silver-selective (for cation-selective) electrodes and the potential of ClO4–-ISEs in perchlorate and in the interfering ion solutions and Ag+-ISEs in silver and in the interfering ion solutions (close to Nernstian slope and assumption of a constant standard potential of ClO4–-ISEs and Ag+-ISEs regardless the measuring solution) were used to calculate selectivity coefficient of ClO4–-ISEs for perchlorate over interfering ions and for Ag+ISEs for silver over interfering ions, respectively. The uncertainties of selectivity coefficients were obtained from the measurement performed with three identical electrodes.



RESULTS AND DISCUSSION Sensitivity of the porphyrin dimers. The electrochemical response of ISEs with porphyrin dimers based ion-selective membranes is shown in Figure 3. In previous works, the dimerization of the porphyrins in ISM was proposed as the cause of super-Nernstian response toward some ions.20,21 In our case, despite deliberately using chemically bonded porphyrins (in form of dimers) none of the sensors exhibited super-Nernstian response. This finding brings up an issue if the dimerization of single porphyrins, that may undoubtedly occur to some extent in the ion-selective memebrane20,

21

, is the cause of the occurrence of super-Nernstian response

using the porphyrin based sensors. 450

(-)8-HZn

400 350 300 250 EMF / mV

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200

8-ZnZn

(-)8-ZnH

(-)8-HZn:(-)8-ZnH, 1:1 (+)8-HZn

150 100 50

(+)8-HZn:(+)8-ZnH, 1:1 8-HH

0 -50

(+)8-ZnH

-100 -8

-7

-6

-5

-4

-3

-2

-1

log ai

Figure 3. Electrochemical response of porphyrin dimer-based anion-selective sensors (8-ZnZn, (– )HZn, (–)ZnH and (–)HZn:(–)ZnH, 1:1) to perchlorate and cation-selective sensors (8-HH, (+)HZn, (+)ZnH and (+)HZn:(+)ZnH, 1:1) to silver. As expected 8-ZnZn and 8-HH porphyrin dimers-based sensors exhibited anionic and cationic behavior, respectively. The 8-ZnZn based sensors were characterized with Nernstian response (between 10–1.12 and 10–4.01 mol dm–3 ClO4–) of –57.3 ± 0.5 mV dec–1 and the low detection limit of 10–5.5 ± 0.1 mol dm–3 ClO4–, while 8-HH based sensors were characterized with Nernstian response (between 10–1.13 and 10–4.01 mol dm–3 Ag+) of 57.9 ± 1.5 mV dec–1 and the low detection limit of 10–4.7 ± 0.1 mol dm–3 Ag+. The mixed porphyrin dimers 8-ZnH and 8-HZn were tested with lipophilic additives typical for anion- or cation-selective sensors. Thus (–)8-ZnH and (–)8-HZn based sensor contained tridodecylmethylammonium chloride (TDMACl), while (+)88 ACS Paragon Plus Environment

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ZnH and (+)8-HZn based sensors contained potassium tetrakis(4-chlorophenyl)borate (KTClPB) in their ISMs. Additionally, a more complexed, double mixed porphyrin dimers system were also used as ionophores, namely: (–)8-HZn:(–)8-ZnH, 1:1 and (+)8-HZn:(+)8-ZnH, 1:1. In all cases of mixed unit porphyrin dimers used as ionophores, the addition of the lipophilic salt to ISM was found to be the sole factor, in respect to their sensitivity toward various analytes. It was found that the anionic and cationic sensitivity of mixed sensitivity porphyrin dimers depended on the lipophilic salt used as the additive in ISM. Thus when the TDMACl was used all (–)8-ZnH, (–)8HZn and (–)8-HZn:( –)8-ZnH, 1:1 based sensors exhibited anionic sensitivity, while when KTClPB was used all (+)8-ZnH, (+)8-HZn and (+)8-HZn:(+)8-ZnH, 1:1 based sensors exhibited cationic sensitivity. The (–)8-ZnH, (–)8-HZn and (–)8-HZn:( –)8-ZnH, 1:1 based sensors were characterized with Nernstian response (between 10–1.12 and 10–4.01 mol dm–3 ClO4–) of –57.5 ± 0.9, –57.9 ± 0.6 and 57.7 ± 1.1 mV dec–1 and the low detection limit of 10–5.5 ± 0.1, 10–6.0 ± 0.1 and 10–5.8 ± 0.1

mol dm–3 ClO4–, respectively. On the other hand, the (+)8-ZnH, (+)8-HZn and (+)8-

HZn:(+)8-ZnH, 1:1 based sensors were characterized with Nernstian response (between 10–1.13 and 10–4.01 mol dm–3 Ag+) of 56.0 ± 2.0, 55.1 ± 0.7 and 56.1 ± 1.7 mV dec–1 and the low detection limit of 10–4.8 ± 0.1, 10–5.0 ± 0.1, 10–4.9 ± 0.1 mol dm–3 Ag+, respectively. The switchability of the sensor responses to anions or cations, using the same ionophores (8-ZnH and 8-HZn), but with different ionic additives (TDMACl and KTClPB), may be explained by the generalized mechanism of the ISEs response. Potentiometric sensors’ response mechanism is based on the charge separation at the ion-selective membrane │ sample solution interface. Firstly, the charge separation occurs due to the analyte (anions or cations) attraction by the ionophore. Secondly, to constrain the electroneutrality of the ISM, the ion-exchange process occurs between analyte and the mobile ion from the lipophilic additive. In this respect, there is accumulation of single charged ions at membrane sample solution interface while the counter ions are left out in the solution at the close proximity to the ISM.32-34 The TDMACl carry mobile chloride (Cl–) ion while KTClPB carry mobile potassium (K+) ion. Naturally, during the response formation the ISM containing TDMACl will exchange chloride ions while ISM containing KTClPB will exchange potassium ions in favor of primary ion entering the membrane. In 8-ZnH and 8-HZn based sensors, the ionic sensitivity of the sensor was then dictated by the lipophilic salt used for preparation of the membrane. As the ionophore in ISM attracted both ions from the solution to the ion-selective membrane │ sample solution interface, it is the ionic additive’s mobile ion that could only 9 ACS Paragon Plus Environment


exchange with one type of the ion from the sample solution (condition of electroneutrality of ISM) and in turn determined the sensitivity of the ISM. Such IESs response was possible as all the porphyrin dimers were neutral ionophores, thus the ionic additives in the ISM were the charge baring moieties and the sole factor dictating the sensitivity, converting the sensor to work either as anion- or cation-selective sensors.

Moreover, the response of such ISEs was obtained

regardless the fact that both porphyrins dimers carried anion- and cation-sensitive porphyrin units within the porphyrin dimer. This fact shows that mixed porphyrin unit dimers may be considered as the novel type of ionophores that have both cation- and anion-sensitive properties (switchable ion-sensitivity by only changing the composition of the membrane). Furthermore, all cationsensitive porphyrin dimers-based sensors were investigated for their potential stability window at different pH. To perform that, the potential response of ISEs was measured in 0.1 mol dm–3 KNO3 after small additions of concentrated HNO3 and NaOH in order to change the pH of the solution. The pH sensitivity of the cation-selective sensors was investigated and the results are shown in Figure 4. For all cation-selective ISEs (8-HH, (+)HZn and (+)ZnH), the pH independent response was found to be between pH=4 and 8. At acidic pH all ISEs exhibited proton sensitivity, thus the useful pH range of the sensors was found to be in slightly acidic and neutral pH. The potentiometric measurements were done in a pH independent response (between 4 and 8) range assuring that any of the porphyrin dimers would get protonated, thus assuring they stay as neutral ionophores.

350 pH independent response 300

EMF / mV

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250 (+)8-ZnH 200

(+)8-HZn

150

8-HH

100 2

4

6

8

10

pH


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Figure 4. Response of porphyrin dimer-based cation-selective sensors (8-HH, (+)HZn and (+)ZnH) to pH in the presence of 0.1 mol dm–3 KNO3 . The selectivity of porphyrin dimers. The selectivity of all porphyrin dimers based sensors was investigated and compared to ISEs without porphyrin dimers as ionophores in ISM, containing only TDMACl or KTClPB. The results from selectivity measurements applying separate solution method (SSM) are presented in Table 2 and Table 3 for TDMACl and KTClPB and porphyrin dimers based sensors, respectively. TDMACl based sensors were more selective to salicylate than to perchlorate. Selectivities of all anion-sensitive porphyrin dimers based sensors to ClO4– were better than the ones obtained with TDMACl based sensors, except of selectivity of 8-ZnZn based sensors towards perchlorate over fluoride, acetate, chloride and iodide which were found comparable to TDMACl based sensors. Similarly, selectivities of all cation-sensitive porphyrin dimers based sensors to Ag+ were better than the ones obtained only for KTClPB based sensors, except of selectivity of 8-HH based sensors towards perchlorate over sodium that was found comparable, and selectivity of 8-HH and (+)8-HZn based sensors towards perchlorate over potassium that was found worse than the ones obtained for KTClPB based sensors. Table 2. Observed electrode slopes (mV dec–1, calculated between aion≈ 10–1 and 10–3 mol dm–3) and selectivity coefficients obtained according to separate solution method for porphyrin-based anion-sensitive sensors (8-ZnZn, (–)HZn, (–)ZnH and (–)HZn:(–)ZnH, 1:1) towards perchlorate over interfering ions. The uncertainty was calculated from measurements performed using three identical electrodes. Interfering

TDMACl

ion / j

slope

݈‫ܭ݃݋‬஼௟ைర– ,௝

slope

8-ZnZn

F– Ac– Cl– NO3– Br– I– Sal– SCN– ClO4–

–52.6 ± 0.5 –53.1 ± 1.5 –57.1 ± 1.2 –60.8 ± 0.5 –54.2 ± 0.6 –52.7 ± 2.0 –57.9 ± 1.8 –64.6 ± 1.9 –61.1 ± 0.6

–4.19 ± 0.23 –4.69 ± 0.31 –3.42 ± 0.27 –1.58 ± 0.18 –2.69 ± 0.23 –1.73 ± 0.07 0.54 ± 0.29 –0.94 ± 0.03 0

–60.1 ± 4.0 –53.6 ± 0.7 –58.5 ± 0.9 –54.9 ± 0.3 –54.9 ± 0.3 –58.6 ± 0.2 –59.7 ± 0.9 –58.0 ± 0.9 –59.6 ± 0.3

௣௢௧

(–)8-ZnH ௣௢௧

݈‫ܭ݃݋‬஼௟ைర– ,௝ –4.23 ± 0.14 –4.67 ± 0.04 –4.24 ± 0.03 –2.73 ± 0.04 –3.49 ± 0.05 –1.78 ± 0.03 –1.90 ± 0.16 –1.12 ± 0.14 0

slope –52.3 ± 1.2 –50.5 ± 0.8 –61.0 ± 0.3 –60.1 ± 1.0 –56.9 ± 0.9 –58.5 ± 0.9 –60.2 ± 0.9 –62.7 ± 0.9 –57.6 ± 0.7

(–)8-HZn ௣௢௧

݈‫ܭ݃݋‬஼௟ைర– ,௝ –5.32 ± 0.18 –5.57 ± 0.12 –4.36 ± 0.07 –2.63 ± 0.06 –3.68 ± 0.03 –1.86 ± 0.02 –1.96 ± 0.05 –1.32 ± 0.03 0

slope –50.1 ± 0.9 –51.0 ± 1.6 –57.5 ± 1.6 –59.0 ± 0.6 –54.5 ± 0.1 –53.7 ± 0.2 –59.5 ± 0.1 –63.0 ± 2.0 –60.0 ± 0.1

(–)8-ZnH:(–)8-HZn, 1:1 ௣௢௧

݈‫ܭ݃݋‬஼௟ைర– ,௝ –5.05 ± 0.07 –5.37 ± 0.02 –4.36 ± 0.04 –2.46 ± 0.12 –3.38 ± 0.06 –1.91 ± 0.02 –2.22 ± 0.14 –1.22 ± 0.07 0

௣௢௧

slope

݈‫ܭ݃݋‬஼௟ைర– ,௝

–57.0 ± 1.6 –50.8 ± 0.7 –58.4 ± 1.0 –59.5 ± 1.1 –55.2 ± 0.2 –55.7 ± 0.7 –60.7 ± 1.1 –59.6 ± 1.4 –57.7 ± 1.1

–5.15 ± 0.04 –5.53 ± 0.08 –4.27 ± 0.11 –2.42 ± 0.08 –3.57 ± 0.02 –1.91 ± 0.03 –2.05 ± 0.07 –1.25 ± 0.12 0

Table 3. Observed electrode slopes (mV dec–1, calculated between aion≈ 10–1 and 10–3 mol dm–3) and selectivity coefficients obtained according to separate solution method for porphyrin-based cation-sensitive sensors (8-HH, (+)HZn, (+)ZnH and (+)HZn:(+)ZnH, 1:1) towards silver over 11 ACS Paragon Plus Environment


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interfering ions. The uncertainty was calculated from measurements performed using three identical electrodes. Interfering

KTClPB

ion / j

slope

Mg2+ Ca2+ Co2+ Zn2+ Cu2+ Ni2+ Pb2+ Cd2+ Sr2+ Na+ K+ Ag+

17.5 ± 2.5 26.7 ± 1.0 22.8 ± 0.7 26.6 ± 0.9 34.5 ± 1.3 38.8 ± 2.3 24.2 ± 0.1 31.8 ± 1.9 21.9 ± 2.2 52.0 ± 1.7 64.9 ± 0.1 56.0 ± 2.3

8-HH ௣௢௧ ݈‫ܭ݃݋‬஺௚శ ,௝

–4.63 ± 0.50 –3.83 ± 0.36 –3.59 ± 0.43 –4.39 ± 0.43 –2.12 ± 0.53 –5.00 ± 0.51 –3.38 ± 0.51 –4.56 ± 0.20 –4.12 ± 0.49 –4.87 ± 0.44 –4.37 ± 0.51 0

(+)8-ZnH ௣௢௧ ݈‫ܭ݃݋‬஺௚శ ,௝

slope 21.7 ± 0.5 19.1 ± 0.8 18.0 ± 1.8 27.0 ± 1.6 37.7 ± 0.2 37.7 ± 0.8 29.6 ± 1.6 26.4 ± 2.0 20.6 ± 0.5 40.0 ± 3.1 56.0 ± 0.9 61.6 ± 0.2

–4.92 ± 0.23 –5.98 ± 0.22 –5.79 ± 0.12 –5.58 ± 0.20 –4.28 ± 0.14 –5.36 ± 0.04 –4.04 ± 0.06 –4.72 ± 0.07 –5.11 ± 0.16 –4.92 ± 0.17 –2.74 ± 0.05 0

slope 21.4 ± 0.1 26.8 ± 1.9 28.3 ± 1.6 24.4 ± 2.8 32.9 ± 1.6 26.8 ± 1.1 25.6 ± 1.9 35.4 ± 0.9 15.0 ± 2.7 42.5 ± 0.2 59.9 ± 1.4 59.4 ± 1.9

(+)8-HZn ௣௢௧ ݈‫ܭ݃݋‬஺௚శ ,௝

–7.15 ± 0.44 –6.34 ± 0.11 –5.81 ± 0.59 –6.15 ± 0.45 –4.67 ± 0.58 –5.82 ± 0.29 –3.72 ± 0.20 –4.93 ± 0.11 –4.95 ± 0.50 –6.25 ± 0.50 –4.14 ± 0.40 0

slope 18.8 ± 1.5 16.1 ± 2.8 23.8 ± 2.3 21.9 ± 2.3 32.3 ± 0.8 26.7 ± 1.2 30.4 ± 0.4 35.0 ± 0.9 10.9 ± 0.5 42.6 ± 0.1 59.2 ± 0.1 57.4 ± 2.2

(+)8-ZnH:(+)8-HZn, 1:1 ௣௢௧ ݈‫ܭ݃݋‬஺௚శ ,௝ .

–6.34 ± 0.38 –5.13 ± 0.59 –4.71 ± 0.47 –5.97 ± 0.52 –4.06 ± 0.49 –5.93 ± 0.22 –4.07 ± 0.60 –4.93 ± 0.52 –5.90 ± 0.04 –4.45 ± 0.59 –3.09 ± 0.55 0

slope 22.1 ± 1.8 24.1 ± 0.5 28.8 ± 0.4 22.5 ± 1.4 33.8 ± 2.2 25.9 ± 1.5 29.5 ± 0.6 28.8 ± 1.8 12.7 ± 0.7 46.6 ± 1.8 59.3 ± 1.6 56.1 ± 1.7

௣௢௧

݈‫ܭ݃݋‬஺௚శ,௝ –6.84 ± 0.39 –5. 07 ± 0.42 –5.49± 0.28 –6.08 ± 0.18 –4.14 ± 0.20 –6.00 ± 0.13 –3.90 ± 0.10 –5.03 ± 0.11 –5.66 ± 0.06 –5.23 ± 0.30 –3.89 ± 0.22 0

The pH of all solutions (except of 0.1 mol dm–3 Pb(NO3)2) was within the pH independent response of cation-sensitive porphyrin dimers based sensor (pH between 4 and 8). Thus the improved selectivity coefficients toward primary ions over interfering ions proved that the ISEs were not influenced by the pH and the selectivity of all sensors depended on the presence of porphyrin dimers as ionophores and not to the presence of lipophilic additives in ISMs. In this way the presence of the lipophilic salt in ISM is responsible for the initial conversion of the mix sensitivity porphyrin based sensor (exchanging only cations or anions from the sample solution), but then the ionophore in ISM is responsible for the preference of which cation or which anion actually enters the membrane. The comparison of the selectivity toward primary ion over interfering ions between 8ZnZn, (–)8-ZnH, (–)8-HZn, (–)8-HZn:(–)ZnH, 1:1 and 8-HH, (+)8-ZnH, (+)8-HZn, (+)8HZn:(+)ZnH, 1:1 based sensors is presented in Table 2 and 3, respectively. The change in the selectivity coefficients of each sensor is presented visually in Figures 5 a and b.


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1

a

0

-1

Sal-

ClO-4

log Kpot ClO

4 ,j

SCNNO-3

-2

I-

-3

BrCl-

-4

F-

-5

Ac-

-6

TDMACl

8-ZnZn

(-)8-ZnH

(-)8-HZn

(-)8-HZn: (-)8-ZnH, 1:1

0 -1

log Kpot Ag+,j

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b Ag+

-2

Cu2+ Pb2+

-3

Co2+

-4

Ca2+ Sr2+ + K Zn2+

-5

Cd2+ Mg2+

-6

+

Na

Ni2+

-7 KTClPB

8-HH

(+)8-HZn

(+)8-ZnH

(+)8-HZn: (+)8-ZnH,1:1

Figure 5. Comparison of selectivity of various porphyrin dimer systems to a) perchlorate (anionselective sensors; TDMACl, 8-ZnZn, (–)HZn, (–)ZnH and (–)HZn:(–)ZnH, 1:1) and to b) silver (cation-selective sensors; KTClPB, 8-HH, (+)HZn, (+)ZnH and (+)HZn:(+)ZnH, 1:1). For 8-ZnZn based sensor, the response toward perchlorate ion over other interfering ions was found similar or worse than for (–)8-ZnH and (–)8-HZn based sensors. The selectivity towards fluoride and acetate (weakly interfering ions) was improved applying mix sensitivity porphyrin dimers. The selectivity did not significantly vary between (–)8-ZnH and (–)8-HZn based sensors, thus in general the position of the anion- and cation-sensitive porphyrin unit in the porphyrin dimers did not influence the abilities of the sensor in response to perchlorate over interfering 13 ACS Paragon Plus Environment


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ions. Similarly, for 8-HH based sensor, the response toward silver ion over other interfering ions was found similar or worse than for (+)8-ZnH and (+)8-HZn based sensors. The selectivity towards magnesium and calcium (weakly interfering ions) was improved applying mix sensitivity porphyrin dimers. The selectivity between (+)8-ZnH and (+)8-HZn based sensors varied and (+)8-ZnH based sensors was found superior to the (+)8-HZn based sensors especially because of the fact that (+)8-ZnH based sensors were described to have better selectivity toward silver ion over sodium and potassium (strongly interfering ions). In this way, the position of the anion- and cation-sensitive porphyrin unit in the porphyrin dimers had an influence on the abilities of the sensor in response to silver ions over other interfering ions. To summarize, in both cases the application of mix sensitive porphyrin dimers resulted in better selectivity than the selectivity of the sensors based on dimmers consisting of only freebase or metalloporphyrin units. The 8-ZnZn and 8-HH porphyrin dimers may interact with two ions (an ion per a porphyrin center) of the same kind (anions or cations) at the same time. Thus they act in ISM similarly as single porphyrins, except of having a higher concentration of ion binding sites. Contrary, the 8-ZnH and 8-HZn porphyrin dimers can get into the complex formation with only one ion (only one porphyrin center is sensitive to the ion), while other porphyrin center is unoccupied and can act as side group regulating the selectivity of the sensor. The selectivities of anion-sensitive porphyrin dimers were found to nearly follow the Hoffmeister series35, and more importantly, did not differ much from each other, thus the approach of modulating the selectivity of the sensors by changing the position of the single units of porphyrins in the dimers was unsuccessful. On the other hand, the selectivity of cation-sensitive porphyrin dimers was dependent on the position of the single units of porphyrins in the dimers. The (+)8-ZnH based sensors were found more selective to silver than other interfering ions than (+)8-HZn based sensors. The second porphyrin unit had hydroxyl group (–OH) inbuilt onto the porphyrin ring. The hydroxyl group has also affinity towards cations.36 Thus the proximity of cation binding hydroxyl group to another cation binding porphyrin unit seems to be crucial in the improvement of the selectivity of the whole ionophore. If the distance between cation binding porphyrin unit and the hydroxyl was smaller ((+)8-ZnH) synergistic effect selectivity towards cations was observed. Although the hydroxyl is expected to have a weaker affinity towards cations than the porphyrin, the hydroxyl can take part in complexation by uptaking ions and either fixing them, intramolecular passing them further to more selective porphyrin dimer center and/or supporting 14 ACS Paragon Plus Environment

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the formation of the “sitting-top” complex. Owing to this, in the most cases in which the complexation constants between the hydroxyl and the cations were significant, the improvement in the selectivity of the (+)8-ZnH based sensor was observed. While in case of complexation constant between hydroxyl ion and the cation, e.g. Sr2+ being marginal, such improvement was not registered.36 Thus by changing the arrangement of the porphyrin units in the porphyrin dimers and functionalization of porphyrin units, the selectivity of the sensors can be regulated/adjusted to the specific analytical applications. In order to check the influence of different ionophores mixed together in ISM on the selectivity of sensors, the (–)8-ZnH and the (–)8-HZn based sensors were compared to the mixed (–)8-HZn:(–)ZnH, 1:1 based sensors, while the (+)8-ZnH and the (+)8-HZn based sensors were compared to the mixed (+)8-HZn:(+)ZnH, 1:1 based sensors (Table 1 and 2). In case of (–)8HZn:(–)ZnH based sensors, the sensitivity coefficients to ClO4– over F–, Br– and Sal– were approximately the mean values of selectivities obtained with (–)8-ZnH and (–)8-HZn when used separately. Similarly, in case of (+)8-HZn:(+)ZnH, 1:1 based sensors, the sensitivity coefficients to Ag+ over Mg2+, Co2+, Zn2+ and Pb2+ were approximately the mean values of selectivities obtained with (+)8-ZnH and (+)8-HZn when used separately. Despite that, in both sensor types, the (–)8-HZn:(–)ZnH, 1:1 and the (+)8-HZn:(+)ZnH, 1:1, the selectivity coefficients were similar or closer to the ones obtained with (–)8-HZn and (+)8-HZn, respectively. The (–)8-HZn and (+)8HZn based sensors were described with worse selectivity coefficients toward primary ion over interfering ions than (–)8-ZnH and (+)8-ZnH based sensors. Thus it can be concluded that the selectivity coefficient, in case of equal contribution of two different ionophores in ISM, is influenced primarily by the presence of the least selective ionophore. Without any optimization of the ion-selective membrane composition (as it is in this particular approach) the use of a single ionophore with inbuilt selective units, such as porphyrin dimers, is a better approach to adjust the selectivity of the sensor than the use of mixture of two separate ionophores in one ISM. On the other hand, as it was shown by Mikhelson et al., the application of two ionophores in ISM can be optimized for their concentration by using multispecies approximation. In this approach, the variability/improvement of the selectivity of IESs may be obtained by changing the measurement conditions and composition of ISM.37-39 Such optimization, however, was not performed in this study.



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Finally, single unit porphyrins, the precursors of the investigated porphyrin dimers, were also used as ionophores. This was performed to investigate if the dimerization of porphyrins brings additional positive effect on the selectivity of ISEs based on such ionophores. Tables 4 a and b present selectivity coefficients of four variations of single unit porphyrins used as ionophores: a) anion-sensitive for perchlorate (1-Zn and 2-Zn) and b) cation-selective for silver (1-H and 2-H). Tables 4 a and b. Observed electrode slopes (mV dec–1, calculated between aion≈ 10–1 and 10–3 mol dm–3) and selectivity coefficients obtained according to separate solution method for single unit porphyrin-based a) anion-sensitive (1-Zn and 2-Zn) and b) cation- sensitive (1-H and 2-H) sensors towards perchlorate and silver over interfering ions, respectively. The uncertainty was calculated from measurements performed using three identical electrodes. a

b

Interfering ion / j

1-Zn

2-Zn

slope

݈‫ܭ݃݋‬

F– Ac– Cl– NO3– Br– I– Sal– SCN– ClO4–

–58.9 ± 1.0 –56.8 ± 0.8 –55.9 ± 0.3 –56.1 ± 0.2 –56.2 ± 0.8 –58.1 ± 0.4 –61.4 ± 0.9 –58.2 ± 0.4 –60.2 ± 0.2

–5.73 ± 0. 03 –5.50 ± 0.01 –4.61 ± 0.08 –3.08 ± 0.01 –3.66 ± 0.01 –1.82 ± 0.02 –1.95 ± 0.02 –0.74 ± 0.01 0

௣௢௧

– ஼௟ைర ,௝

.

௣௢௧

slope

݈‫ܭ݃݋‬஼௟ைర– ,௝

–110.2 ± 5.5 –54.0 ± 0.6 –59.9 ± 0.2 –54.6 ± 0.1 –53.7 ± 0.2 –58.5 ± 0.8 –59.8 ± 0.2 –57.4 ± 0.2 –60.5 ± 0.2

–4.41 ± 0.19 –5.15 ± 0.09 –4.43 ± 0.17 –2.78 ± 0.09 –3.55 ± 0.09 –1.91 ± 0.01 –1.68 ± 0.11 –0.90 ± 0.13 0

Interfering ion / j Mg2+ Ca2+ Co2+ Zn2+ Cu2+ Ni2+ Pb2+ Cd2+ Sr2+ Na+ K+ Ag+

1-H slope 21.8 ± 0.5 20.6 ± 0.1 30.0 ± 0.6 25.9 ± 2.3 33.2 ± 0.3 25.4 ± 0.3 29.4 ± 0.6 24.6 ± 0.5 22.1 ± 0.2 43.0 ± 0.6 52.1 ± 0.5 56.5 ± 0.9

௣௢௧

݈‫ܭ݃݋‬஺௚శ,௝ –5.65 ± 0.08 –5.41 ± 0.08 –4.72 ± 0.02 –4.64 ± 0.12 –3.33 ± 0.04 –4.35 ± 0.01 –3.18 ± 0.02 –3.83 ± 0.06 –4.06 ± 0.10 –4.56 ± 0.22 –1.99 ± 0.15 0

2-H slope 23.9 ± 0.8 21.0 ± 0.9 29.5 ± 1.0 25.2 ± 1.9 35.2 ± 0.8 28.0 ± 2.0 30.7 ± 0.3 23.3 ± 0.6 18.0 ± 1.3 43.1 ± 0.9 53.5 ± 0.8 53.2 ± 0.3

௣௢௧

݈‫ܭ݃݋‬஺௚శ,௝ –5.04 ± 0.10 –5.19 ± 0.08 –3.83 ± 0.04 –4.22 ± 0.17 –3.05 ± 0.17 –4.23 ± 0.04 –3.13 ± 0.03 –3.70 ± 0.02 –4.10 ± 0.36 –5.07 ± 0.10 –1.81 ± 0.10 0

The selectivity of porphyrin dimer-based ISEs (8-ZnZn, (–)8-HZn and (–)8-HZn) to perchlorate over other interfering ions, in comparison to the selectivity of ISEs based on single anionsensitive porphyrins (1-Zn and 2-Zn), did only improve in case of ISEs selectivity to perchlorate over the most interfering ion, thiocyanate. On the other hand, the selectivity of porphyrin dimerbased ISEs (8-HH, (+)8-HZn and (+)8-HZn) to silver over other interfering ions did improve significantly (nearly in all cases) in comparison to the selectivity of ISEs based on single cationsensitive porphyrins (1-H and 2-H). The improvement of the selectivity of the porphyrin dimerbased ISEs indicates that porphyrin dimers as ionophores, having two porphyrin rings, may coordinate much stronger complexes with primary ion (here silver) than their single unit porphyrins precursors. This indicates that purposely fixing single porphyrins into dimers may actually be a factor to increase/modify porphyrin systems selectivity to primary ion. 16 ACS Paragon Plus Environment

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CONCLUSIONS The four different porphyrin dimers were investigated as ionophores in chemical sensing of anions and cations. All anion-sensitive sensors exhibited the most selective behavior toward perchlorate ion, while the cation-selective sensors exhibited the most selective behavior towards silver ion. Sensors with 8-ZnZn and 8-HH porphyrin dimers as ionophores were found to be anion- and cation-sensitive, respectively. Since the porphyrin dimers were neutral ionophores, the porphyrin dimers with mix sensitivity porphyrin units (anion- and cation-sensitive) were described with lipophilic ion additive dependent response toward primary ion. If the sensor contained TDMACl as lipophilic salt both porphyrin dimers, (–)8-ZnH and (–)8-HZn, were characterized with anion-selective response. Consequently, if the sensor contained KTClPB as lipophilic salt, both porphyrin dimers, (+)8-ZnH and (+)8-HZn, were characterized with cationselective response. In this respect, the primary response depended on the lipophilic salt added to the ISM, while the selectivity of the sensors toward primary ion over interfering ions depended on the porphyrin dimers used in preparation of the ISM. In this respect, a novel universal ionophore type (anion- and cation-selective) convertible to anion- or cation-sensitivity, by varying the membrane composition, was developed. Moreover, those mix sensitivity porphyrin dimers were characterized with better selectivity towards primary over interfering ions than same porphyrin unit dimers. In anion-selective sensors the selectivity pattern was similar despite the order of the porphyrin units in the dimer. Contrary, the selectivity of cation-sensitive sensors hugely depended on the order of porphyrin units in the dimer. The (+)8-ZnH based sensors were found the most selective, among investigated dimers, toward silver over other interfering ions. Moreover, in case of cation-selective ISEs, the selectivity of the porphyrin dimers based sensors was improved to the sensors based on their single unit porphyrin precursors. That fact is opening a new possibility in synthesis and application of more complexed porphyrin structures for the improvement of the selectivity of the porphyrins as ion sensors. The application of mix sensitive porphyrin dimers may open a new and an exciting chapter in electroanalysis, by the possibility of not only regulating the selectivity of the sensor via modification of the porphyrin macrocycle structure and/or addition of peripheral substituents, but 17 ACS Paragon Plus Environment


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also by regulating/adjusting the sensitivity and selectivity by the development and application of complexed porphyrin structures (containing freebase and metalloporphyrins) and the order of porphyrin units in such porphyrin structures. AUTHOR INFORMATION Corresponding Author *[email protected] telephone number: +6567904737 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS GL acknowledge Åbo Akademi Research Foundation, Magnus Ehrnrooth Foundation and Academy of Finland for financial support. REFERENCES (1) Lvova L., Di Natale C., Paolesse R., Sens. and Actuat. B 2013, 179, 21– 31. (2) Mathew S., Yella A., Gao P., Humphry-Baker R., Curchod B.F.E., Ashari-Astani N, Tavernelli I., Rothlisberger U., Nazeeruddin K., Grätzel M., Nat. Chem. 2014, 6, 242–247. (3) Auwärter W., Écija D., Klappenberger F., Barth J.V., Nat. Chem. 2015, 7, 105–120. (4) Zhang C., Chen P., Dong H., Zhen Y., Liu m., Hu W., Adv. Mat. 2015, 27, 5379-5387. (5) Buhlmann P., Pretsch E., Bakker E., Chem. Rev. 1998, 98, 1593-1687. (6) Albert K.J., Lewis N.S., Schauer C.L., Sotzing G.A., Stitzel S.E., Vaid T.P., Walt D.R., Chem. Rev. 2000, 100, 2595-2626. (7) Paolesse R., Lvova L., Nardis S., Di Natale C., D’Amico A., Lo Castro F., Microchim. Acta 2008, 163, 103-112. (8) Rock F., Barsan N., Weimar U., Chem. Rev. 2008, 108, 705-725. (9) Ma Q., Ai S., Yin H., Chen Q., T. Tang T., Electrochim. Acta 2010, 55, 6687-6694. (10) Liu X., Feng H., Liu X., Wong D.K.Y., Anal. Chim. Acta 2011, 689, 212-218. (11) Sivalingam Y., Martinelli E., Catini A., Magna G., Pomarico G., Basoli F., Paolesse R., Di Natale C., J. Phys. Chem. C 2012, 116, 9151-9157. 18 ACS Paragon Plus Environment

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