Biomedical subjects

Aleksandar Radu

Publications and source records attributed to Aleksandar Radu.

8 recordsLinked to original sources

PubMed2006

Solid contact potentiometric sensors for trace level measurements.

A simple procedure for the development of a range of polymeric ion-selective electrodes (ISEs) with low detection limits is presented. The electrodes were prepared by using a plasticizer-free methyl methacrylate-decyl methacrylate copolymer as membrane matrix and poly(3-octylthiophene) as intermediate layer deposited by solvent casting on gold sputtered copper electrodes as a solid inner contact. Five different electrodes were developed for Ag+, Pb2+, Ca2+, K+, and I-, with detection limits mostly in the nanomolar range. In this work, the lowest detection limits reported thus far with solid contact ISEs for the detection of silver (2.0 x 10(-9) M), potassium (10(-7) M), and iodide (10(-8) M) are presented. The developed electrodes exhibited a good response time and excellent reproducibility.

Calibration↗

PubMed2005

Fiber-optic microsensor array based on fluorescent bulk optode microspheres for the trace analysis of silver ions.

An optical microsensor array is described for the rapid analysis of silver ions at low parts per trillion levels. Because the ionophore o-xylylenebis(N,N-diisobutyldithiocarbamate) (Cu-I) was reevaluated and shown to exhibit excellent selectivity for silver ions, ion-selective electrode (ISE) membranes were optimized and found to exhibit the lowest reported detection limit so far (3 x 10(-10) M). A corresponding Ag+-selective fluorescent optical microsensor array for the rapid sensing of trace level Ag+ was then developed. It was fabricated using plasticized PVC-based micrometer-scale fluorescent microspheres that were produced via a sonic particle casting device. They contained 156 mmol/kg Cu-I, 10 mmol/kg 9-(diethylamino)-5-[4-(15-butyl-1,13-dioxo-2,14-dioxanodecyl) phenylimino]benzo[a]phenoxazine (chromoionophore VII, ETH 5418), 2.3 mmol/kg 1,1' '-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (internal reference dye), and 14 mmol/kg sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate and were deposited onto the etched distal end of a 3200-microm-diameter optical fiber bundle. The microarray was characterized by fluorescence spectroscopy in samples containing 10(-12)-10(-8) M AgNO3 at pH 7.4, with selectivity characteristics comparable to the corresponding ISEs. The response time of the microsensor array was found to be less than 15 min for 10(-9) M AgNO3, which is drastically shorter than earlier data on optode films (8 h) and corresponding ISEs (30 min). A detection limit of 4 x 10(-11) M for Ag+ was observed, lower than any previously reported optode or silver-selective ISE. The microsensor array was applied for measurement of free silver levels in buffered pond water samples.

Calibration↗

PubMed2005

Shifting the Measuring Range of Chloride Selective Electrodes and Optodes Based on the Anticrown Ionophore [9]Mercuracarborand-3 by the Addition of 1-Decanethiol.

Ion-selective electrodes and optodes based on the anticrown ionophore mercuracarborand-3 (MC-3) exhibit excellent selectivity to halide ions but suffer from a low upper detection limit because of very strong complexation in the sensing phase. In this work, we have successfully improved the upper detection limit and widened the working range of chloride selective electrode based on MC-3 by introducing 1-decanethiol into the membrane cocktail. With an assumed 1:2 stoichiometry between chloride and MC-3 the apparent complex formation constant was reduced from log beta = 13.4 as reported for the membrane without the addition of 1-decanethiol to log beta = 12.13, log beta = 10.84 and log beta = 5.67 with the addition of a 1:1, 1:2 and 1:4 molar ratio of 1-decanethiol to MC-3. Besides these values, obtained with the sandwich membrane method, similar shifts in the measuring range of thin optode film responses to lower chloride concentrations were observed as well. The selectivity of the modified membrane was found to be very good, with hydroxide and the other halides: bromide and iodide, as the main interferences. Based on these results, corresponding iodide selective electrodes were prepared that showed a near-Nernstian iodide slope, rather than a cationic slope as observed without thiol additive.Jonoselektywne elektrody i optody stosujace antykoronowy jonofor [9]merkuracarborand-3 (MC-3) charakteryzuja sie doskonała selektywnościa w stosunku do jonów halogenków lecz wskutek silnego kompleksowania w fazie receptora maja ograniczony zakres górnej granicy oznaczalności. W przedstawionej pracy poszerzono zakres stosowalności dzieki wprowadzeniu 1-dekanotiolu do membrany zawierajacej MC-3. Przy załozonej stechiometrii kompleksu chlorek-MC-3 równej 1:2 stała tworzenia kompleksu została obnizona, z wartości log beta = 13,4 (w przypadku membrany bez dodatku 1-dekanotiolu) do wartości log beta = 12,13; log beta = 10,84 i log beta = 5,67 (po dodaniu 1-dekanotiolu) dla stosunków molowych stezeń-1-dekanotiol:MC-3 wynoszacych odpowiednio 1:1, 1:2 i 1:4. Ponadto oprócz tych danych otrzymanych metoda warstwowej membrany, podobne przesuniecie zakresu pomiarowego obserwowano w przypadku małych stezeń chlorków dla cienkowarstwowej optody. Selektywnoś ć modyfikowanej membrany była dobra, przy czym głównymi interferentami były jony bromkowe, jodkowe i wodorotlenkowe. W oparciu o te wyniki skonstruowano elektrode czuła na jony jodkowe, charakteryzujaca sie nachyleniem anionowym charakterystyki bliskim Nernstowskiemu, a nie kationowym wykazywanym w nieobecności tiolu.

Journal Article↗

PubMed2004

Dynamic diffusion model for tracing the real-time potential response of polymeric membrane ion-selective electrodes.

A numerical solution for the prediction of the time-dependent potential response of a polymeric-based ion-selective electrode (ISE) is presented. The model addresses short- and middle-term potential drifts that are dependent on changes in concentration gradients in the aqueous sample and organic membrane phase. This work has important implications for the understanding of the real-time response behavior of potentiometric sensors with low detection limits and with nonclassical super-Nernstian response slopes. As a model system, the initial exposure of membranes containing the well-examined silver ionophore O,O' '-bis[2-(methylthio)ethyl]-tert-butylcalix[4]arene was monitored, and the large observed potential drifts were compared to theoretical predictions. The model is based on an approximate solution of the diffusion equation for both aqueous and organic diffusion layers using a numerical scheme (finite difference in time and finite elements in space). The model may be evaluated on the basis of experimentally available parameters and gives time-dependent information previously inaccessible with a simpler steady-state diffusion model. For the cases studied, the model gave a very good correlation with experimental data, albeit with lower than expected diffusion coefficients for the organic phase. This model may address numerous open questions regarding the response time and memory effects of low-detection-limit ion-selective electrodes and for other membrane electrodes where ion fluxes are relevant.

Diffusion↗

PubMed2003

Rotating disk potentiometry for inner solution optimization of low-detection-limit ion-selective electrodes.

The extent of optimization of the lower detection limit of ion-selective electrodes (ISEs) can be assessed with an elegant new method. At the detection limit (i.e., in the absence of primary ions in the sample), one can observe a reproducible change in the membrane potential upon alteration of the aqueous diffusion layer thickness. This stir effect is predicted to depend on the composition of the inner solution, which is known to influence the lower detection limit of the potentiometric sensor dramatically. For an optimized electrode, the stir effect is calculated to be exactly one-half the value of the case when substantial coextraction occurs at the inner membrane side. In contrast, there is no stir effect when substantial ion exchange occurs at the inner membrane side. Consequently, this experimental method can be used to determine how well the inner filling solution has been optimized. A rotating disk electrode was used in this study because it provides adequate control of the aqueous diffusion layer thickness. Various ion-selective membranes with a variety of inner solutions that gave different calculated concentrations of the complex at the inner membrane side were studied to evaluate this principle. They contained the well-examined silver ionophore O,O' '-bis[2-(methylthio)ethyl]-tert-butylcalix[4]arene, the potassium ionophore valinomycin, or the iodide carrier [9]mercuracarborand-3. Stir effects were determined in different background solutions and compared to theoretical expectations. Correlations were good, and the results encourage the use of such stir-effect measurements to optimize ISE compositions for real-world applications. The technique was also found to be useful in estimating the level of primary ion impurities in the sample. For an iodide-selective electrode measured in phosphoric acid, for example, apparent iodide impurity levels were calculated as 5 x 10(-10) M.

Ion-Selective Electrodes↗

PubMed2003

Improving the detection limit of anion-selective electrodes: an iodide-selective membrane with a nanomolar detection limit.

The lower detection limit and the selectivity behavior of anion-selective electrodes (ISEs) are improved by using optimized inner solutions and membrane compositions. With a membrane based on the recently described ionophore [9]mercuracarborand-3, a detection limit of 2 x 10(-9) M has been achieved for iodide. Nevertheless, the improvements are less pronounced than in the case of cation ISEs. This is mainly due to the fact that so far no anion ISE is known with the extremely high selectivities of cation ISEs. If the membrane does not contain an ionophore, leaching of the ion exchanger from the membrane into the sample is also a relevant limiting factor except for ion exchangers of very high lipophilicity.

Anions↗

PubMed2003

Plasticizer-free polymer containing a covalently immobilized Ca2+-selective ionophore for potentiometric and optical sensors.

A derivative of a known Ca2+-selective ionophore, ETH 129, was synthesized to contain a polymerizable acrylic moiety (AU-1) and covalently grafted into a methyl methacrylate-co-decyl methacrylate polymer matrix. The polymer containing AU-1 was prepared via a simple one-step homogeneous polymerization method. It exhibited mechanical properties suitable for the fabrication of plasticizer-free ion-selective membrane electrodes and bulk optode films by solvent-casting and spin-coating techniques, respectively. The segmented sandwich membrane technique was utilized to assess the binding constant of free and covalently bound ionophores to calcium and to study their diffusion coefficients in the membrane phase. Diffusion was greatly diminished for the bound ionophore. This was confirmed in ion-selective electrode membranes containing no calcium ions in the inner solution, which should normally show apparent super-Nernstian response slopes in dilute calcium solutions. The response slope was Nernstian down to submicromolar concentration levels, indicating slow mass transport of calcium in the membrane. Optical-sensing films with the new copolymer matrix, unblended and blended with PVC-DOS, also confirmed that covalently bound ionophores are fully functional for maintaining selective ion extraction and binding properties of the sensing membrane.

Biosensing Techniques↗

PubMed2002

Rational design of potentiometric trace level ion sensors. A Ag+-selective electrode with a 100 ppt detection limit.

Submicromolar to picomolar lower detection limits have recently been obtained with various polymer membrane ion-selective electrodes by minimizing biases due to ion fluxes through the membrane. For the best performance, the compositions of the membrane and inner solution should be optimized for each application. Given the number of parameters to be adjusted, it has been difficult to find the best parameters for a target sample. In this paper, a much simplified and more practical steady-state model of zero-current ion fluxes is derived, which is based on measurable parameters. The model allows one to predict achievable lower detection limits for a membrane with given selectivities. It can also be used to predict the optimal composition of the inner filling solution for the measurement of samples with a known, typical ionic background. Selectivity coefficients of monovalent and divalent analyte ions required for desired detection limits in drinking water are calculated. As an application of the proposed general recipe, a silver-selective electrode is developed on the basis of the ionophore O,O''-bis[2-(methylthio)ethyl]-tert-butylcalix[4]arene. With the predicted optimal composition of the inner electrolyte, its lower detection limit is found to be 10(-9) M or 100 ppt Ag+ with an ionic background of 10(-5) M LiNO3, which is very close to the expected value.

Cations, Divalent↗