Novel carbonate-selective potentiometric sensors based on 5,10,15-tris(4-aminophenyl)-20-phenyl porphyrinates of Cu(II) and Co(II) have been developed. Ionophore functioning mechanism and possible source of carbonate sensitivity have been evolved. Potentiometric properties of Co(II)- and Cu(II)TATPP-based sensors were compared with common carbonate-ISEs containing trifluoroacetophenone derivatives. The analytical utility of newly developed sensors has been demonstrated by measuring the bicarbonate content in human blood plasma.
An accurate detection of hydrophilic anions, carbonate in particular, in physiological fluids, seawater, industrial, and environmental samples is still a big challenge. Ion-selective potentiometric sensors represent a useful approach to this task [
In this contribution we report a development of novel carbonate-selective potentiometric sensors based on 5,10,15-tris(4-aminophenyl)-20-phenyl porphyrinates of Co(II) and Cu(II) (Co(II)TATPP and Cu(II)TATPP correspondingly). PVC solvent polymeric membranes doped with Co(II)TATPP alone and containing lipophilic cationic additive (TDACl), and films of poly-Co(II)TATPP and poly-Cu(II)TATPP electropolymerized on Pt working electrodes (WE) from various organic solvents (acetonitrile, dimethyl-formamide, pyridine) have been studied with the aim to evolve the origin of sensitivity towards carbonate ion. Potentiometric properties of Co(II)- and Cu(II)TATPP-containing sensors were compared with those based on TFAP (carbonate ionophore I, ETH-6010, and carbonate ionophore IV), Scheme
Molecular structures of studied ionophores: (a) 5,10,15-tris(4-aminophenyl)-20-phenyl porphyrinate of Cu(II) and Co(II), (b) heptyl-4-trifluoroacetylbenzoate (carbonate ionophore I), and (c) 4-butyl-
Poly(vinyl chloride) (PVC) high molecular weight; plasticizer bis(2-ethylhexyl) sebacate (DOS), heptyl-4-trifluoroacetylbenzoate (Carbonate Ionophore I), 4-butyl-
PVC membranes were prepared according to a common procedure. Membrane of 100 mg weight contained 1.5–3.5 wt% of ionophore and/or 1–6 wt% of lipophilic additive distributed in PVC/DOS (1 : 2) polymeric matrix, Table
Membrane compositions and deposition details.
Ionophore, wt% | Additive, wt% | Film/solvent | |
---|---|---|---|
I.a-d | Co(II)TATPP | — | EP/(a-d)a |
II | Cu(II)TATPP | — | EP/(d) |
III | Co(II)TATPP, 1.5% | TDACl, 1% | PVC/DOS |
IV | Co(II)TATPP, 1.5% | — | PVC/DOS |
V | Carb.Ion I, 2.7% | TDACl, 2% | PVC/DOS |
VI | Carb.Ion IV, 3.5% | TDACl, 2% | PVC/DOS |
VII | — | TDACl, 6% | PVC/DOS |
VII | PANI | — | EPb |
aSee experimental section for details.
bDeposition from 0.5 M/L aniline solution in 1 M/L H2SO4.
Freshly prepared EP and solvent polymeric membrane sensors were soaked in 0.1 M/L NaHCO3 at least for 24 hours before first measurement. The potentiometric responses of sensors have been studied in solutions of several salts in a range 10−7–10−1 M/L. The known volumes of standard salts solutions were added to 0.1 M Tris-H2SO4 buffer pH 8.6 or distilled water (with simultaneous pH control). Three replica electrodes were studied with each membrane formulation. Sensor potentials were measured versus double-junction SCE reference electrode (AMEL, Italy) and recorded using high-impedance 8-channel potentiometer LiquiLab (Ecosens, Italy). Selectivity coefficients were calculated by the separate solution method (SSM) using EMF values measured in 0.01 M salt solutions and theoretical slope values [
UV-visible spectroscopic data were acquired with a Cary-50 Scan spectrophotometer. The quartz and methacrylate cells 45 × 10 × 10 mm with a path length of 10 mm were used. 15
Arterial blood samples were taken from 5 male subjects (3 healthy persons and 2 with respiratory acidosis, samples A, C). Blood plasma was isolated by centrifugation of fresh samples and following removal of suspended blood cells. Samples were analyzed few hours after collection. At least tree replicas were performed for each plasma sample during the same day. If not analyzed immediately, samples were stored at −20°C. Standard addition method was applied to detect bicarbonate ion content in samples [
Sensor array was composed of 5 carbonate-selective electrodes and pH glass electrode. Prior to measurements in real plasma samples, array was calibrated in 25 model solutions mimicking human plasma composition. Each solution contained 4 salts; the salt concentration was similar to those in plasma and varied in the following range: 70–100 mM/L NaCl, 20–60 mM/L NaHCO3, 1–8 mM/L Na3PO4, 1 mM/L NaSal; solutions pH was fixed in a range 7.2–7.4 by addition of 0.1 M/L HCl.
Partial Least Square regression (PLS) method was applied to train multisensory array in artificial solutions mimicking human blood plasma samples and to correlate bicarbonate content determined commercial blood analyzer with multisensory array response. The autoscaling procedure was applied to the data. Since the number of measurements composing the dataset was not big enough to divide the dataset in a training and test set, a leave-one-out validation was applied. The Unscrambler v.9.1 (2004, CAMO PROSESS AS, Norway) was used for data treatment.
Porphyrin electropolymers based on polyaniline (PANI) are well studied. The electropolymerisation of mono-, bis-, tris- and tetra-2- or 4-aminophenyl substituted porphyrinates of various metals on Pt or GC WE has been previously reported by several authors [
In the present study we have focused on the development and investigation of the potentiometric behavior of sensors based on Co(II)- and Cu(II)-tris-4-aminophenyl porphyrinates due to their known sensitivity towards hydrophilic anions [
Cyclic voltammograms of PANI-Co(II)TATPP film electrodeposition on Pt WE from (a) acetonitrile; (b) DMF; (c) DMF: 0.5/L M aniline in 1 M/L H2SO4 = 1 : 1. The working and counterelectrodes were platinum, and the scan rate was 0.1 V/s. On figure X corresponds to incorporation of Co(II)TATPP in film; Y indicates the decrease of current and insulating film formation.
The details of PANI-Co(II)TATPP membrane I.d electropolymerisation from DMF/aniline solution. Peaks indicated as X show an incorporation of Co(II)TATPP in PANI film during first 5 cycles.
The incorporation of Co-TATPP in PANI backbone formed on the ITO glass electrodes was confirmed by the presence of Soret’s band (
UV-visible absorption spectra of PANI-Co(II)TATPP electropolymer (membrane I.d) deposited on ITO glass slide. The spectra of Co(II)TATPP in CH2Cl2 and bare ITO glass are given for comparison.
Potentiometric responses of poly-Co(II)TATPP electropolymerized membranes I.b–I.d deposited from DMF, pyridine, and DMF/aniline towards several anions have been studied. Membranes I.b and I.c did not show any significant response to all the tested anions probably due to the prevalence of insulating EP formation. Selectivity patterns significantly different from the Hofmeister series were detected for membrane I.d, as far for membrane II based on Cu(II)TATPP-doped PANI film, Figure
Potentiometric dynamic response of EP films towards several anions: (a) membrane I.d based on Co(II)TATPP and (b) membrane II based on Co(II)TATPP.
A high sensitivity of membranes I.d and II towards NaHCO3 concentration change could be explained either by PH influence on Co(II)TATPP- and Cu(II)TATPP-doped PANI films or by selective complexation of bicarbonate/carbonate ions by metalloporphyrins. In fact, the growth of NaHCO3 concentration increase the solution pH, and; hence, the correct determination of various forms of CO2 (i.e., CO2, H2CO3,
The pH response of membranes I.b–I.d, II, and VIII has been, hence, studied by stepwise addition of 1 M/L NaOH to the universal buffer (11.4 mM/L boric acid, 6.17 mM/L citric acid, 10 mM/L NaH2PO4, pH 2.75) and achieving the final solution pH 10. A relatively little effect of pH on electrodes with membranes I.b–I.d and II have been detected in a pH range from 6 to 10 (−9.1, −2.9, and −14.0 mV/pH correspondingly), while PANI membrane VIII has shown a significant pH response in the all examined range with a slope −41.2 mV/decade (data not shown). It can hence be assumed that Co(II)- and Cu(II)-5,10,15-tris(4-aminophenyl)-20-phenyl porphyrinates may selectively coordinate carbonate ions and are promising candidates as ionophores for carbonate-selective sensor development.
In order to evolve the source of high sensitivity towards carbonate as far as elucidating the ionophore functioning mechanism, potentiometric and optical properties of solvent polymeric PVC/DOS membranes III and IV doped with Co(II)TATPP (see Table
The comparison of the selectivity coefficients of electropolymerized (EP) and solvent polymeric membranes (PVC/DOS) containing Co(II)- and Cu(II)TATPP and TFAP derivatives (commercially available carbonate ionophore I and IV). Selectivity coefficients were evaluated by SSM method, the theoretical Nernstian slope of 29 mV/pCO3 was used in calculations.
The formation both of 1 : 1 and 2 : 1 adducts between metalloporphyrin and
UV-visible spectra of dry membrane III and soaked in 10−6–10−2 M/L solutions of NaHCO3.
On the contrary to PVC/DOS solvent polymeric membranes, no ionophore dimerization occurs in electropolymerized membrane I.d due to the rigid fixation of Co(II)TATPP inside PANI matrix. The only process that takes place in EP film is an axial coordination of target primary ion on metal center of porphyrin ionophore. This fact may explain the higher carbonate selectivity of EP membranes I.d and II over the solvent polymeric PVC membrane III.
Due to the fact of elevated
Results of developed sensors application for bicarbonate detection in plasma*.
Co(II)TATPP | Carbonate ionophore I, membrane VI | Blood analyzer | ||
Membrane I.d | Membrane III | |||
A | 51.4 | |||
B | — | 31.8 | ||
C | — | — | 42.8 | |
D | — | — | 21.2 | |
E | 28.9 |
*Data reprinted from [
The effectiveness of single sensor application for bicarbonate content analysis in human plasma has been compared to the multisensory approach. For this purpose sensor array composed of 5 carbonate-selective sensors with membranes I.d, II, III, VI, and VIII, and pH electrode has been utilized. Before application in plasma, array was calibrated in artificial solutions mimicking human plasma composition (see Section
The results of PLS model calibration and validation in multicomponent solutions mimicking plasma for bicarbonate, hydrophosphate, and salicylate ion content detection.
From PLS1 model the concentration of
Newly developed sensors prepared by formation of electropolymerized PANI film doped with 5,10,15-tris(4-aminophenyl)-20-phenyl porphyrinates of Co(II) and Cu(II) have showed a high capability to detect
The authors would like to acknowledge Dr. F. Mandoj and Dr. G. Pomarico from the Department of Chemical Science, and Technologies, University of Rome “Tor Vergata”, Rome, Italy, for porphyrin ionophores synthesis and G. Romeo and C. Andreozzi for the technical assistance.