Łukasz Górski

Polymeric membrane electrodes with enhanced fluoride selectivity using Zr(IV)-porphyrins functioning as neutral carriers

Poly(vinyl chloride) polymeric membranes plasticized with o-NPOE (o-nitrophenyl octyl ether) or DOS (dibutyl sebacate) and containing Zr(IV)-octaethyl(OEP)- or Zr(IV)-tetraphenylporphyrins (TPP) along with lipophilic cationic additives (tridodecylmethylammonium chloride; TDMACl) are examined potentiometrically and optically with respect to their response toward fluoride. It is shown that these zirconium porphyrins can function as neutral anion carriers within the organic membranes of the electrodes. Spectrophotometric measurements of thin polymeric films indicate that the presence of lipophilic cationic sites in the form of TDMA(+) and use of lower dielectric constant plasticizer (DOS) prevents formation of metalloporphyrin dimers in the organic polymer phase, which have been observed previously in polymeric membranes formulated with the same Zr(IV) porphyrins but with lipophilic anion site additives. By preventing dimer formation, rapid and Nernstian potentiometric response of the corresponding membrane electrodes toward fluoride ion is observed. Indeed, electrodes prepared with PVC/DOS membranes containing Zr(IV)-OEP and 15mol% of TDMACl (relative to the ionophore) exhibit fast (t(95)<15s) and reversible response toward fluoride. The slope of calibration plots are near-Nernstian (-59.9mV per decade). Such electrodes display the following selectivity pattern: ClO(4)(-)>SCN(-)>F(-)>NO(3)(-)>Br(-)>Cl(-), which differs significantly from the classical Hofmeister series, with greatly enhanced potentiometric selectivity toward fluoride. The data presented herein, coupled with results from a previous study, confirm that Zr(IV) porphyrins can serve as either charged or neutral type anion carriers with respect to their enhanced interactions with fluoride when used as ionophores to prepare liquid-polymeric membrane electrodes, and that the nature of membrane additives and plasticizer dictates the response mechanism at play for given membrane formulations.

Fluoride-selective polymeric membrane electrodes based on Zr(IV)- and Al(III)-salen ionophores of various structures.

Al(III)- and Zr(IV)-salophens of novel structures were tested as anion-selective ionophores. It was shown that these compounds are highly selective to fluoride and give selectivity greatly deviating from classical Hofmeister pattern, when doped into the polymeric membrane of ion-selective electrode (ISE). The following selectivity sequence has been recorded for both ionophores: F(-) > ClO(4)(-) > SCN(-) > NO(3)(-) approximately Br(-) approximately Cl(-). The results of potentiometric and spectrophotometric measurements allow to conclude that the nature and structure of salophen ligands influence stability of ISE working parameters. An increase in salophen ligands lipophilicity results in prolongation of the ISE lifetime, most likely due to slower ionophore decomposition caused by the hydrolysis of imine bonds in salophen structure. Ion-selective electrodes (ISEs) with the most successful Al(III)-salophen exhibited a stable, fast and near-Nernstian fluoride response and a functional lifetime near 3 weeks and selectivity coefficients with log K(F(-), Y(-))(pot) as follows: -2.8 (Y(-)=Br(-)), -2.7 (Cl(-)), -2.8 (NO(3)(-)), -1.5 (SCN(-)), -1.3 (ClO(4)(-)), which is better than for other ones based on Zr(IV)- and Al(III)-salophens and salens described to date.

Electrochemical oligonucleotide-based biosensor for the determination of lead ion

The possibility of utilization of gold electrodes modified with short guanine-rich ssDNA probes for determination of Pb(2+) was examined. Interaction between guanine residues and lead ion followed by formation of G-quadruplex structures was confirmed by electrochemical impedance spectroscopy investigations. An external cationic redox label, methylene blue, was employed in voltammetric measurements for analytical signal generation. It was shown that due to the G-quadruplex formation, the oligonucleotides in the recognition layer fold, which enhances the electron transfer between methylene blue and the electrode surface. The MB current signal rises proportionally to the lead ion concentration in the range from 0.05 to 1μmol/L. The developed biosensor demonstrated high selectivity towards Pb(2+) ion, with only minor response towards interfering metal cations. The calculated limit of detection was of 34.7nmol/L. The utilization of the biosensor for Pb(2+) determination in real samples of water was also tested.

Electrochemical aptamer-based biosensors as potential tools for clinical diagnostics

Aptamers are single-stranded DNA and RNA sequences that belong to the group of “functional nucleic acids”, which exhibit both catalytic and receptor properties. Aptamers change their spatial conformation upon binding to a target analyte, and so far more than 200 sequences selective for large variety of molecules such as metal ions, organic dyes, proteins and bacteria cells have been identified. Aptamers can be applied in several fields including diagnostics, therapy and as the recognition layers for biosensors, which is one of the most promising areas. The utilization of aptamers as receptor elements in electrochemical assays requires not only the choice of an appropriate immobilization method with respect to the detected target, but also experimental conditions including the manner of analytical signal generation. The latter issue might affect the efficiency of binding between the aptamer-modified surface and analyte, and consequently the sensitivity of target molecule quantification. Herein, a review of electrochemical aptasensors dedicated to the determination of target analytes crucial in clinical diagnostics developed during the past 10 years is presented. It contains a short characterization of aptamers and their application as sensing layers in electrochemical assays, which is followed by a description of the examples of the use of aptasensors.

Electrochemical detection of DNA hybridization using metallacarborane unit.

The evaluation of novel electrochemically active label for electrochemical detection of DNA hybridization is presented. Metallacarborane units modified with iron, cobalt or chromium were investigated. The value of redox potential and relatively strong current signal facilitate usage of Fe-carborane as marker covalently attached to the ssDNA. In electrochemical genosensor the sequence complementary to UL55 gene was labeled and used as a target for biosensor device. Interactions were investigated using electrochemical and piezoelectric methods. Obtained results confirm usefulness of the designed label in electrochemical detection of DNA hybridization.

Application of mass fabricated silicon-based gold transducers for amperometric biosensors

The backside contact, silicon-based transducers with vacuum-deposited gold layer (BSC) are evaluated as the base for electrochemical biosensors construction. Their comparison with commercially available transducers with screen printed gold and traditional gold disc electrode is reported. To determine the advantages and disadvantages of each of gold surfaces mentioned above, the 6-(ferrocenyl)-hexanethiol was used as the indicator. The results revealed the usefulness of BSC chips for the formation of stable self-assembled monolayers (SAMs). After the preliminary analysis, the SAM of thiol-ssDNA was formed on the BSC transducers as recognition layer. The electrochemical analysis in methylene blue solution was carried out after ssDNA immobilization and DNA-DNA hybridization. It is shown that prepared sensors are able to recognize complementary DNA sequence, based on the change in height and the potential shifts of reduction peaks of methylene blue. Obtained results are in full agreement with literature data. The compact size of silicone-based transducers allows to significantly reduce the required volume of tested solutions.

Metalloporphyrin-based acetate-selective electrodes as detectors for enzymatic acetylcholine determination in flow-injection analysis system

In this work two Zr(IV)-porphyrins were tested as potential acetate-selective ionophores. It is shown that these compounds show increased selectivity towards acetate ion, with selectivity sequence: Cl- < NO3- < SCN- < ClO4- < acetate- < F-, when incorporated into polymeric ion-selective membranes. Among tested ionophores, Zr(IV)-tetra(tert-butylphenyl)porphyrin was found to be the best in terms of response time (20 s) and lower detection limit (2 x 10(-4) M acetate-). Designed electrodes were used as detectors in flow-injection analysis system for acetylcholine determination. Enzymatic hydrolysis was used to generate acetate ions from acetylcholine, employing acetylcholinesterase immobilized on Amberlite. The optimization of enzymatic reactions and flow-injection analysis system configuration were performed and enzyme immobilization procedures have been evaluated. It is shown that resulting flow-injection system exhibits good working parameters, such as reproducibility (standard deviation < or = 3.5%), sampling rate (60 samples per hour) and lifetime (over a week).

Electrochemical uranyl cation biosensor with DNA oligonucleotides as receptor layer.

The present study aims at the further development of the uranyl oligonucleotide-based voltammetric biosensor, which takes advantage of strong interaction between UO2(2+) and phosphate DNA backbone. Herein we report the optimization of working parameters of previously elaborated electrochemical DNA biosensor. It is shown that the sensor sensitivity is highly dependent on the oligonucleotide probe length and the incubation time of sensor in a sample solution. Consequently, the highest sensitivity was obtained for 10-nucleotide sequence and 60 min incubation time. The lower detection limit towards uranyl cation for developed biosensor was 30 nM. The influence of mixed monolayers and the possibility of developing a non-calibration device were also investigated. The selectivity of the proposed biosensor was significantly improved via elimination of adenine nucleobases from the DNA probe. Moreover, the regeneration procedure was elaborated and tested to prolong the use of the same biosensor for 4 subsequent determinations of UO2(2+).

Complexes of tetra-tert-butyl-tetraazaporphine with Al(III) and Zr(IV) cations as fluoride selective ionophores

In this work, complexes of Zr(IV) and Al(III) cations with 2,7,12,17-tetra-tert-butyl-5,10,15,20-tetraazaporphine (TAP) were tested as ionophores in plasticized PVC membranes of ion-selective electrodes. It was found that both tested ionophores show enhanced affinity towards fluoride anion. High fluoride selectivity was observed in the presence of anionic or cationic additives in the membrane, which indicates that proposed compounds work according to charged or neutral carrier mechanism, depending on membrane composition and pretreatment. tert-Butyl substituents, present in the structure of tested compounds, were supposed to prevent formation of ionophore dimers within the membrane phase. This process was found to be responsible for some unfavorable potentiometric properties of electrodes based on complexes of Zr(IV) and Al(III) cations with porphyrins (compounds closely related to tetra-tert-butyl-5,10,15,20-tetraazaporphine). As it was shown using spectrophotometrical measurements, Al(III)-TAP was not susceptible to dimerization, while dimer formation was observed for Zr(IV)-TAP. In full agreement with these observations, electrodes with membranes containing Al(III)-TAP responded in near-Nernstian and fast manner towards fluoride anion, while the employment of Zr(IV)-TAP as ionophore resulted in super-Nernstian and sluggish response. Plasticized PVC membranes doped with Al(III)-TAP and 20mol% of lipophilic anionic additives shown remarkable F(-) selectivity, with selectivity coefficients, logK(F-pot.).(Y-), as follows: -4.4 (Y(-)Br(-)), -4.3 (Cl(-)), -4.2 (NO(3)(-)), -3.6 (SCN(-)), -2.9 (ClO(4)(-)).

Potentiometric studies of complexation properties of tetrafunctionalized resorcinarene-based cavitands

Resorcinarene-based cavitands functionalized with [–NH–C(O)–CH2–(Ph)2PO] (I), [–NH–C(O)–CH2–(EtO)2PO] (II), [–NH–C(S)–NHC(O)Ph] (III) and [–NH–C(O)–NHC(O)Ph] (IV) moieties have been characterized potentiometrically. Both P-containing cavitands (I and II) form very stable complexes (log βIL > 18) for most of the examined cations as determined with the segmented sandwich membrane method. The order of the stability constants was found to be: Eu3+ > UO22+ > Pb2+, Cd2+, Sr2+ and Cu2+. Much weaker complexation occurs in the case of compounds III and IV (log βIL   Pb2+ > Cu2+ > Ag+ > UO22+ > Na+ > K+ for III and UO22+ > Cd2+ > Pb2+ > Eu3+Cu2+ > Na+ > Ag+ > K+ for IV. The response to representatives of various cation groups and the selectivity of polymeric membrane electrodes based on these compounds are presented. While cavitands functionalized with phosphine oxide or phosphonate moieties (I or II) exhibited the highest selectivity for UO22+ and Pb2+ and a pronounced discrimination of Ag+ ions, thioamide- (III) and amide-functionalized (IV) cavitands showed preferences for Ag+ and Pb2+ and reduced selectivity toward UO22+ ions. The correlation between the potentiometric selectivity and the ability of examined cavitands to form metal–ligand complexes is discussed.