Ionophore

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Eric Bakker - One of the best experts on this subject based on the ideXlab platform.

  • Ionophore-Based Voltammetric Ion Activity Sensing with Thin Layer Membranes
    2016
    Co-Authors: Maria Cuartero, Gaston A Crespo, Eric Bakker
    Abstract:

    As shown in recent work, thin layer ion-selective multi-Ionophore membranes can be interrogated by cyclic voltammetry to detect the ion activity of multiple species simultaneously and selectively. Additional fundamental evidence is put forward on ion discrimination with thin multi-Ionophore-based membranes with thicknesses of 200 ± 25 nm and backside contacted with poly-3-octylthiophene (POT). An anodic potential scan partially oxidizes the POT film (to POT+), thereby initiating the release of hydrophilic cations from the membrane phase to the sample solution at a characteristic potential. Varying concentration of added cation-exhanger demonstrates that it limits the ion transfer charge and not the deposited POT film. Voltammograms with multiple peaks are observed with each associated with the transfer of one type of ion (lithium, potassium, and sodium). Experimental conditions (thickness and composition of the membrane and concentration of the sample) are chosen that allow one to describe the system by a thermodynamic rather than kinetic model. As a consequence, apparent stability constants for sodium, potassium, and lithium (assuming 1:1 stoichiometry) with their respective Ionophores are calculated and agree well with the values obtained by the potentiometric sandwich membrane technique. As an analytical application, a membrane containing three Ionophores was used to determine lithium, sodium, and potassium in artificial samples at the same location and within a single voltammetric scan. Lithium and potassium were also determined in undiluted human plasma in the therapeutic concentration range

  • thin layer Ionophore based membrane for multianalyte ion activity detection
    Analytical Chemistry, 2015
    Co-Authors: Gaston A Crespo, Maria Cuartero, Eric Bakker
    Abstract:

    A concept is introduced that allows one to detect the activity of multiple ions simultaneously and selectively with a single ion-selective membrane. This is demonstrated with ∼300 nm thin plasticized PVC membranes containing up to two Ionophores in addition to a lipophilic cation-exchanger, overlaid on an electropolymerized poly-3-octylthiophene (POT) film as the electron to ion transducer. The ion-selective membranes are formulated under Ionophore depleted conditions (avoiding excess of Ionophore over ion-exchanger), which is purposely different from common practice with ion-selective electrodes. Cyclic voltammetry is used to interrogate the films. An anodic scan partially oxidizes the POT underlayer, which results in the expulsion of cations from the membrane at an appropriate potential. During the scan of a membrane containing multiple Ionophores, the least bound ion is expelled first, giving distinct Gaussian peak shaped ion transfer voltammetric waves that are analyzed in terms of their peak potential. These potentials are found to change with the logarithm of the ion activity, in complete analogy to ion-selective electrodes, and multiple such waves are observed with multiple Ionophores that exhibit no obvious interference from the other Ionophores present in the membrane. The concept is established with lithium and calcium Ionophores and accompanied by a response model that assumes complete equilibration of the membrane at every applied potential. On the basis of the model, diffusion coefficients in the membrane or aqueous phase bear no influence on the peak potentials as long as thin layer behavior is observed, further confirming the analogy to a potentiometric experiment. Idealized ion transfer waves are narrower than experimental findings, which is explained by a broader than expected anodic peak for the oxidation of conducting polymer. The correspondence between experiment and theory is otherwise excellent in terms of thin layer behavior and Nernstian shift of the peaks with analyte concentration.

  • nitrite selective electrode based on cobalt ii tert butyl salophen Ionophore
    Electroanalysis, 2014
    Co-Authors: Bastien Neel, Gaston A Crespo, Majid Ghahraman Asfhar, Marcin Pawlak, Denis Dorokhin, Eric Bakker
    Abstract:

    A series of polymeric nitrite-selective electrodes containing a new lipophilic Ionophore Co(II) tert-butyl-salophen is reported. The stability of Co(II) Ionophores within a PVC-based membrane was investigated by leaching experiments. Different membrane compositions were explored in order to reach the lowest possible limit of detection for a PVC-based nitrite selective polymeric membrane electrode containing this Ionophore. The optimal electrode showed a limit of detection of 2×10−6 M and exhibited four orders of magnitude of discrimination over nitrate, chloride and bromide. The electrodes were evaluated in undiluted human urine and attest to the robustness of the Ionophore.

  • multiplexed flow cytometric sensing of blood electrolytes in physiological samples using fluorescent bulk optode microspheres
    Analytical Chemistry, 2007
    Co-Authors: Katarzyna Wygladacz, Robert Retter, Michael Bell, Eric Bakker
    Abstract:

    Polymeric bulk optode microsphere ion sensors in combination with suspension array technologies such as analytical flow cytometry may become a power tool for measuring electrolytes in physiological samples. In this work, the methodology for the direct measurement of common blood electrolytes in physiological samples using bulk optode microsphere sensors was explored. The simultaneous determination of Na+, K+, and Ca2+ in diluted sheep blood plasma was demonstrated for the first time, using a random suspension array containing three types of mixed microsphere bulk optodes of similar size, fabricated from the same chromoIonophore without additional labeling. Sodium Ionophore X, potassium Ionophore III, and grafted AU-1 in poly(butyl acrylate) were the Ionophores used in the bulk optode microsphere ion sensors for Na+, K+, and Ca2+, respectively, in combination with the cation-exchanger NaTFPB (sodium tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate) and the same concentration of the chromoIonophore ETH 5294 ...

  • effect of lipophilic ion exchanger leaching on the detection limit of carrier based ion selective electrodes
    Analytical Chemistry, 2001
    Co-Authors: Eric Bakker
    Abstract:

    The equilibrium partitioning of lipophilic ion-exchanger salts from ion-selective polymeric membrane electrodes (ISEs) and its possible effect on the lower detection limit of these sensors is described. Predictions are made on the basis of various parameters, including the knowledge of tetraphenylborate potassium salt partitioning constants, the selectivity of Ionophore-free ion-exchanger membranes, and Ionophore stability constants in the membrane. Ion-exchanger lipophilicities are significantly increased if the membrane contains an Ionophore that strongly binds the primary ion. Predicted detection limits are on the order of 10-5−10-8 M for Ionophore-free membranes, and may reach levels as low as 10-18 M with adequate Ionophores in the membrane. Experiments are performed for well-described lead-selective membranes containing different tetraphenylborate derivatives, and detection limits appear to be independent of the ion-exchanger used. However, they are much higher if a more hydrophilic carborane cation...

Philippe Buhlmann - One of the best experts on this subject based on the ideXlab platform.

  • potentiometric selectivities of Ionophore doped ion selective membranes concurrent presence of primary ion or interfering ion complexes of multiple stoichiometries
    Analytical Chemistry, 2019
    Co-Authors: Ibrahim Yilmaz, Li D Chen, Xin V Chen, Evan L Anderson, Rosenildo Correa Da Costa, John A. Gladysz, Philippe Buhlmann
    Abstract:

    The selectivities of Ionophore-doped ion-selective electrode (ISE) membranes are controlled by the stability and stoichiometry of the complexes between the Ionophore, L, and the target and interfering ions (Izi and Jzj, respectively). Well-accepted models predict how these selectivities can be optimized by selection of ideal Ionophore-to-ionic site ratios, considering complex stoichiometries and ion charges. These models were developed for systems in which the target and interfering ions each form complexes of only one stoichiometry. However, for a few ISEs, the concurrent presence of two primary ion complexes of different stoichiometries, such as ILzi and IL2zi, was reported. Indeed, similar systems were probably often overlooked and are, in fact, more common than the exclusive formation of complexes of higher stoichiometry unless the Ionophore is used in excess. Importantly, misinterpreted stoichiometries misguide the design of new Ionophores and are likely to result in the formulation of ISE membranes ...

  • calibration free Ionophore based ion selective electrodes with a co ii co iii redox couple based solid contact
    Analytical Chemistry, 2014
    Co-Authors: Xu U Zou, Xue Zhen, Jia H Cheong, Philippe Buhlmann
    Abstract:

    A high electrode-to-electrode reproducibility of the emf response of solid contact ion-selective electrodes (SC-ISEs) requires a precise control of the phase boundary potential between the ion-selective membrane (ISM) and the underlying electron conductor. To achieve this, we introduced previously Ionophore-free ion exchanger membranes doped with a well controlled ratio of oxidized and reduced species of a redox couple as redox buffer and used them to make SC-ISEs that exhibited highly reproducible electrode-to-electrode potentials. Unfortunately, Ionophores were found to promote the loss of insufficiently lipophilic species from the Ionophore-doped ISMs into aqueous samples. Here we report on an improved redox buffer platform based on equimolar amounts of the much less hydrophilic Co(III) and Co(II) complexes of 4,4′-dinonyl-2,2′-bipyridyl, which makes it possible to extend the redox buffer approach to Ionophore-based ISEs. For example, K+-selective electrodes based on the Ionophore valinomycin exhibit e...

  • cyanide selective electrode based on zn ii tetraphenylporphyrin as Ionophore
    Analytical Chemistry, 2012
    Co-Authors: Li D Chen, Xu U Zou, Philippe Buhlmann
    Abstract:

    Receptors that exhibit high selectivity are essential for potentiometric cyanide sensors. Therefore, CN– binding to metallotetraphenylporphyrins with different metal centers (i.e., Co(II), Co(III), Zn(II), Ni(II), Cu(II), and Fe(III)) was investigated. All these metalloporphyrins were found to function as neutral Ionophores. Co(III) and Fe(III) tetraphenylporphyrins with their positive charges seemed likely to bind up to two axial CN– ligands, but only the Co(III) porphyrin was found to strongly bind a second CN– ligand. The electrode membranes doped with Zn(II) tetraphenylporphyrin provided the highest selectivity over chloride (logKCN–,Cl–pot = −3.71, as opposed to −0.36 for an Ionophore-free ISE) and were optimized by adjusting the site-to-Ionophore ratio to achieve the highest CN– selectivity, with special consideration of interfering ions present in gold mining applications. The Zn(II) tetraphenylporphyrin-based CN–-selective electrodes exhibited the best discrimination of OH–; no pH effect was obser...

  • ion selective electrodes with Ionophore doped sensing membranes
    Supramolecular Chemistry: From Molecules to Nanomaterials, 2012
    Co-Authors: Philippe Buhlmann, Li D Chen
    Abstract:

    Recent work in the field of Ionophore-based ion-selective electrodes (ISEs) has greatly improved our understanding of the thermodynamics and kinetics that describe the response and selectivity of these sensors. These efforts resulted in the lowering of detection limits from micro- to subpicomolar concentrations, improvements of selectivities by many orders of magnitude, and major advancements in biocompatibility and long-term stabilities. This chapter summarizes the current state of the art for an audience that is new to the field, introducing the basic concepts of ISE theory that replaced in recent years the empirical approach of the early ISE history. It illustrates, with specific examples, the design principles of host-guest chemistry that have been used to develop Ionophores for ISEs, and describes with a minimum of equations the recently developed sophisticated concepts for the efficient use of these Ionophores in ISEs. Importantly, this chapter shows not only how Ionophores are used in modern potentiometry to develop new ISEs but also illustrates how Ionophore-based potentiometry can support host-guest chemistry by providing tools to determine thermodynamic properties of Ionophores, such as stoichiometries and stabilities of their complexes, using only a minimum amount of Ionophore. Keywords: Ionophore; ion-selective electrode; potentiometry; complex stability; detection limit; sensor; chemical sensor; carrier; receptor; ISE

  • highly selective detection of silver in the low ppt range with ion selective electrodes based on Ionophore doped fluorous membranes
    Analytical Chemistry, 2010
    Co-Authors: Melissa A Fierke, John A. Gladysz, Rosenildo Correa Da Costa, Andreas Stein, Philippe Buhlmann
    Abstract:

    Ionophore-doped sensing membranes exhibit greater selectivities and wider measuring ranges if their membrane matrixes are noncoordinating and solvate interfering ions poorly. This is particularly true for fluorous phases, which are the least polar and polarizable condensed phases known. In this work, fluorous membrane matrixes were used to prepare silver ion-selective electrodes (ISEs). Sensing membranes composed of perfluoroperhydrophenanthrene, sodium tetrakis[3,5-bis(perfluorohexyl)phenyl]borate, and one of four fluorophilic Ag+-selective Ionophores with one or two thioether groups were investigated. All electrodes exhibited Nernstian responses to Ag+ in a wide range of concentrations. Their selectivities for Ag+ over interfering ions were found to depend on host preorganization and the length of the −(CH2)n− spacers separating the coordinating thioether group from the strongly electron withdrawing perfluoroalkyl groups. ISEs based on the most selective of the four Ionophores, that is, 1,3-bis(perfluor...

M G Elbardicy - One of the best experts on this subject based on the ideXlab platform.

  • comparative study of 2 hydroxy propyl beta cyclodextrin and calixarene as Ionophores in potentiometric ion selective electrodes for neostigmine bromide
    Talanta, 2011
    Co-Authors: Amira M Elkosasy, Marianne Nebsen, Mohamed Abd K Elrahman, Maissa Y Salem, M G Elbardicy
    Abstract:

    Three novel neostigmine bromide (NEO) selective electrodes were investigated with 2-nitrophenyl octyl ether as a plasticiser in a polymeric matrix of polyvinyl chloride (PVC). Sensor 1 was fabricated using tetrakis(4-chlorophenyl)borate (TpClPB) as an anionic exchanger without incorporation of an Ionophore. Sensor 2 used 2-hydroxy propyl β-cyclodextrin as an Ionophore while sensor 3 was constructed using 4-sulfocalix-8-arene as an Ionophore. Linear responses of NEO within the concentration ranges of 10(-5) to 10(-2), 10(-6) to 10(-2) and 10(-7) to 10(-2) mol L(-1) were obtained using sensors 1, 2 and 3, respectively. Nernstian slopes of 51.6 ± 0.8, 52.9 ± 0.6 and 58.6 ± 0.4 mV/decade over the pH range of 4-9 were observed. The selectivity coefficients of the developed sensors indicated excellent selectivity for NEO. The utility of 2-hydroxy propyl β-cyclodextrin and 4-sulfocalix[8]arene as Ionophores had a significant influence on increasing the membrane sensitivity and selectivity of sensors 2 and 3 compared to sensor 1. The proposed sensors displayed useful analytical characteristics for the determination of NEO in bulk powder, different pharmaceutical formulations, and biological fluids (plasma and cerebrospinal fluid (CSF)) and in the presence of its degradation product (3-hydroxyphenyltrimethyl ammonium bromide) and thus could be used for stability-indicating methods.

  • comparative study of 2 hydroxy propyl beta cyclodextrin and calixarene as Ionophores in potentiometric ion selective electrodes for neostigmine bromide
    Talanta, 2011
    Co-Authors: Amira M Elkosasy, Marianne Nebsen, Mohamed Abd K Elrahman, Maissa Y Salem, M G Elbardicy
    Abstract:

    Abstract Three novel neostigmine bromide (NEO) selective electrodes were investigated with 2-nitrophenyl octyl ether as a plasticiser in a polymeric matrix of polyvinyl chloride (PVC). Sensor 1 was fabricated using tetrakis(4-chlorophenyl)borate (TpClPB) as an anionic exchanger without incorporation of an Ionophore. Sensor 2 used 2-hydroxy propyl β-cyclodextrin as an Ionophore while sensor 3 was constructed using 4-sulfocalix-8-arene as an Ionophore. Linear responses of NEO within the concentration ranges of 10 −5 to 10 −2 , 10 −6 to 10 −2 and 10 −7 to 10 −2  mol L −1 were obtained using sensors 1, 2 and 3, respectively. Nernstian slopes of 51.6 ± 0.8, 52.9 ± 0.6 and 58.6 ± 0.4 mV/decade over the pH range of 4–9 were observed. The selectivity coefficients of the developed sensors indicated excellent selectivity for NEO. The utility of 2-hydroxy propyl β-cyclodextrin and 4-sulfocalix[8]arene as Ionophores had a significant influence on increasing the membrane sensitivity and selectivity of sensors 2 and 3 compared to sensor 1. The proposed sensors displayed useful analytical characteristics for the determination of NEO in bulk powder, different pharmaceutical formulations, and biological fluids (plasma and cerebrospinal fluid (CSF)) and in the presence of its degradation product (3-hydroxyphenyltrimethyl ammonium bromide) and thus could be used for stability-indicating methods.

Elzbieta Malinowska - One of the best experts on this subject based on the ideXlab platform.

  • complexes of tetra tert butyl tetraazaporphine with al iii and zr iv cations as fluoride selective Ionophores
    Analytica Chimica Acta, 2009
    Co-Authors: łukasz Gorski, Mariusz Pietrzak, Monika Mroczkiewicz, Elzbieta Malinowska
    Abstract:

    Abstract 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 20 mol% of lipophilic anionic additives shown remarkable F− selectivity, with selectivity coefficients, log K F − , Y − pot. , as follows: −4.4 (Y− Br−), −4.3 (Cl−), −4.2 (NO3−), −3.6 (SCN−), −2.9 (ClO4−).

  • polymeric membrane electrodes with improved fluoride selectivity and lifetime based on zr iv and al iii tetraphenylporphyrin derivatives
    Analytica Chimica Acta, 2007
    Co-Authors: Mariusz Pietrzak, Mark E. Meyerhoff, Elzbieta Malinowska
    Abstract:

    Abstract Novel aluminum(III)- and zirconium(IV)-tetraphenylporhyrin (TPP) derivatives are examined as fluoride-selective Ionophores for preparing polymer membrane-based ion-selective electrodes (ISEs). The influence of t -butyl- or dichloro-phenyl ring substituents as well as the nature of the metal ion center (Al(III) versus Zr(IV)) on the anion complexation constants of TPP derivative Ionophores are reported. The anion binding stability constants of the Ionophores are characterized by the so-called “sandwich membrane” method. All of the metalloporphyrins examined form their strongest anion complexes with fluoride. The influence of plasticizer as well as the type of lipophilic ionic site additive and their amounts in the sensing membrane are discussed. It is shown that membrane electrodes formulated with the metalloporphyrin derivatives and appropriate anionic or cationic additives exhibit enhanced potentiometric response toward fluoride over all other anions tested. Since selectivity toward fluoride is enhanced in the presence of both anionic and cationic additives, the metalloporphyrins can function as either charged or neutral carriers within the organic membrane phase. In contrast to previously reported fluoride-selective polymeric membrane electrodes based on metalloporphyrins, nernstian or near-nernstian (−51.2 to −60.1 mV decade −1 ) as well as rapid ( t t -butyltetraphenylporphyrin as the Ionophore provides fluoride sensors with prolonged (7 months) functional lifetime.

  • lead selective electrodes based on thioamide functionalized calix 4 arenes as Ionophores
    Analytica Chimica Acta, 1994
    Co-Authors: Elzbieta Malinowska, Zbigniew Brzozka, Krzysztof Kasiura, Richard J M Egberink, David N. Reinhoudt
    Abstract:

    Lead selective electrodes based on a di- and tetrathioamide functionalized calix [4] arene as Ionophores were investigated. The Pb(II)-response functions exhibited almost theoretical Nernstian slopes in the activity range 10?6?10?2M of lead ions. For both Ionophores a preference for lead over other cations was observed. The best results were obtained for membranes containing the tetrathioamide derivative (Ionophore 1), potassium tetrakis(4-chlorophenyl) borate and o-nitrophenyl octyl ether as a plasticizer. This electrode showed greatly improved selectivity over copper, zinc and cadmium ions as compared with commercial solid-state lead selective electrodes.

  • anion selective membrane electrodes based on metalloporphyrins the influence of lipophilic anionic and cationic sites on potentiometric selectivity
    Talanta, 1994
    Co-Authors: Eric Bakker, Elzbieta Malinowska, Robert D Schiller, Mark E. Meyerhoff
    Abstract:

    The role of lipophilic anionic and cationic additives on the potentiometric anion selectivities of polymer membrane electrodes prepared with various metalloporphyrins as anion selective Ionophores is examined. The presence of lipophilic anionic sites (e.g. tetraphenylborate derivatives) is shown to enhance the non-Hofmeister anion selectivities of membranes doped with In(III) and Sn(IV) porphyrins. In contrast, membranes containing Co(III) porphyrins require the addition of lipophilic cationic sites (e.g. tridodecylmethylammonium ions) in order to achieve optimal anion selectivity (for nitrite and thiocyanate) as well as rapid and reversible Nernstian response toward these anionic species. These experimental results coupled with appropriate theoretical models that predict the effect of lipophilic anion and cation sites on the selectivities of membranes doped with either neutral or charged carrier type Ionophores may be used to determine the operative Ionophore mechanism of each metalloporphyrin complex within the organic membrane phase.

David J Nisbet - One of the best experts on this subject based on the ideXlab platform.

  • effects of the antibiotic Ionophores monensin lasalocid laidlomycin propionate and bambermycin on salmonella and e coli o157 h7 in vitro
    Journal of Applied Microbiology, 2003
    Co-Authors: Thomas S. Edrington, T R Callaway, P D Varey, R O Elder, Elizabeth Kutter, Andrew D Brabban, Kenneth M. Bischoff, Y. S. Jung, Robin C Anderson, David J Nisbet
    Abstract:

    T.S. E DRINGTON, T .R. C ALLAWAY, P .D. V AREY, Y .S. J UNG, K .M. B ISCHOFF, R .O. E LDER, R . C . A N D E R S O N , E . K U T T E R , A . D . B R A B B A N A N D D . J . N I S B E T . 2003. Aims: To examine the effects of Ionophores on Salmonella and Escherichia coli O157:H7 in pure and mixed ruminal fluid cultures. Methods and Results: Four Salmonella serotypes (Dublin, Derby, Typhimurium, and Enteriditis) and two strains of E. coli O157:H7 (ATCC 43895 and FDIU 6058) were cultured in the presence of varying concentrations of Ionophores (monensin, lasalocid, laidlomycin propionate, and bambermycin) in pure and mixed ruminal fluid cultures. Bacterial growth rates in pure culture were not affected (P >0 AE10) by Ionophores at concentrations up to 10 times the approximate rumen Ionophore concentration under normal feeding regimens. Likewise, Ionophores had no effect (P >0 AE10) on Salmonella or E. coli CFU plated from 24-h ruminal fluid incubations. Ionophore treatment decreased (P <0 AE01) the acetate : propionate ratio in ruminal fluid cultures as expected. Conclusions: Ionophores had no effect on the foodborne pathogens Salmonella and E. coli O157:H7 in vitro. Significance and Impact of the Study: The results suggest that Ionophore feeding would have little or no effect on Salmonella or E. coli populations in the ruminant.

  • effect of feeding the Ionophores monensin and laidlomycin propionate and the antimicrobial bambermycin to sheep experimentally infected with e coli o157 h7 and salmonella typhimurium
    Journal of Animal Science, 2003
    Co-Authors: Thomas S. Edrington, T R Callaway, R O Elder, Kenneth M. Bischoff, Robin C Anderson, Kenneth J Genovese, David J Nisbet
    Abstract:

    Escherichia coli O157:H7 and Salmo- nella are widely recognized as important agents of food- borne disease with worldwide distribution. The use of Ionophores in feeding growing ruminants is widespread in the United States and has attracted recent interest due to the apparent temporal relationship between ini- tial Ionophore use and the increase in human E. coli O157:H7 cases. Two experiments were conducted to evaluate the effects of short-term feeding of Ionophores on fecal shedding, intestinal concentrations, and anti- microbial susceptibility of E. coli O157:H7 and S. typhi- murium in growing lambs. Sixteen lambs were used in each experiment, four lambs per treatment group: monensin, laidlomycin propionate, bambermycin, and a control treatment. Lambs were fed a grain and hay (50:50) diet with their respective Ionophore for 12 d before experimental inoculation with E. coli O157:H7

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