Papers by Philippe Bühlmann
Analytical Chemistry, Dec 22, 2011
It is well known that the selectivity of an ion-selective electrode (ISE) depends on the stoichio... more It is well known that the selectivity of an ion-selective electrode (ISE) depends on the stoichiometry of the complexes between its ionophore and the target and interfering ions. It is all the more surprising that the possibility for the simultaneous occurrence of multiple target ion complexes with different complex stoichiometries was mostly ignored in the past. Here we report on the simultaneous formation of 1:1 and 1:2 complexes of a fluorophilic crown ether in fluorous ISE membranes, and how this results in what look like super-Nernstian responses. These increased response slopes are not caused by mass transfer limitations and can be readily explained with a phase boundary model, a finding that is supported by experimentally determined complex formation constants and excellent fits of response curves. Not only Cs + but also the smaller ions Li + , Na + , K + , and NH 4 + form 1:1 and 1:2 complexes with the fluorophilic crown ether, with cumulative formation constants of up to 10 15.0 and 10 21.0 for of the 1:1 and 1:2 complexes, respectively. Super-Nernstian responses of the type observed with these electrodes are probably not particularly rare, but lacking in the past an adequate discussion in the literature remained ignored or misinterpreted. Preliminary calculations also predict sub-Nernstian responses and potential dips of a similar origin. The proper understanding of such phenomena will facilitate the development of new ISEs based on ionophores that form complexes of higher stoichiometries. [email protected]. Supporting Information Available Figures showing potentiometric K + responses in presence of a background of Li + , Na + , or NH 4 +. Illustration of effect of K IL and K JL on response curves of membranes with 71 mol % ionic sites. Derivation of the expression for the width of super-Nernstian response ranges. Illustration of the site-to-ionophore ratio on the EMF slope in the super-Nernstian response range. Examples of complex stabilities that result in a sub-Nernstian response slope and potential dip. This material is available free of charge via the Internet at http://pubs.acs.org.
Tetrahedron, 1993
Ein erster Versuch, den Analyten mit Hilfe eines 4-Aminoacridin-5-yl-amino-substituierten 4(3H)-P... more Ein erster Versuch, den Analyten mit Hilfe eines 4-Aminoacridin-5-yl-amino-substituierten 4(3H)-Pyrimidons nicht nur durch drei, sondern durch vier Wasserstoffbriicken an den Wirt zu binden, misslang. Der Grund dazu liegt wahrscheinlich in der zu grossen Flexibility dieser Wirtverbindung. Unter Beriicksichtigung dieser Tatsache wird ein neues Wirtmolekul vorgeschlagen. Seine Fahigkeit, Kreatinin zu komplexieren, wurde mit Hilfe von Kristallstrukturdaten sowie eines Molecular Modelling-Programmes und quantenmechanischer Berechnungen evaluiert. 32 For relatively simple analytes, such as ethanol or carbon dioxide, both a synthetic host and a molecule of biochemical origin may take advantage of most analyte properties available for the recognition process. In such a case, the selectivity of a biosensor is not neces¬ sarily better than that of a sensor with a synthetic host.60 This is, in principle, also true for analyte molecules of medium and large size, but the efforts necessary for synthesizing adequate hosts become rapidly prohibitive. However, there is no reason why recognition processes in biosensors should in principle be more selective than those in sensors based on synthetic hosts. 3.3 Optodes 3.3.1 36 4.
Journal of the American Chemical Society, May 17, 2010
High-valent iron-oxo intermediates have been identified as the key oxidants in the catalytic cycl... more High-valent iron-oxo intermediates have been identified as the key oxidants in the catalytic cycles of many nonheme enzymes. Among the large number of synthetic Fe IV =O complexes characterized to date, [Fe IV (O)(N4Py)] 2+ (1) exhibits the unique combination of thermodynamic stability, allowing its structural characterization by X-ray crystallography, and oxidative reactivity sufficient to cleave C-H bonds as strong as those in cyclohexane (D C-H = 99.3 kcal mol-1). However, its redox properties are not yet well understood. In this work, the effect of protons on the redox properties of 1 has been investigated electrochemically in nonaqueous and aqueous solutions. While the cyclic voltammetry of 1 in CH 3 CN is complicated by coupling of several chemical and redox processes, the Fe IV/III couple is reversible in aqueous solution with E 1/2 = +0.41 V vs. SCE at pH 4 and involves the transfer of one electron and one proton to give the Fe III-OH species. This is in fact the first example of reversible electrochemistry to be observed for this family of nonheme oxoiron(IV) complexes. C-H bond oxidations by 1 have been studied in H 2 O and found to have reactions rates that depend on the C-H bond strength but not on the solvent. Furthermore, our electrochemical results have allowed a D O-H value of 78(2) kcal mol-1 to be calculated for the Fe III-OH unit derived from 1. Interestingly, although this D O-H value is 6-11 kcal mol-1 lower than those corresponding to oxidants such as [Fe IV (O)(TMP)] (TMP = tetramesitylporphinate), [Ru IV (O)(bpy) 2 (py)] 2+ (bpy = bipyridine, py = pyridine) and the tert-butylperoxyl radical, the oxidation of dihydroanthracene by 1 occurs at a rate comparable to those for these other oxidants. This comparison suggests that the nonheme N4Py ligand environment confers a kinetic advantage over the others that enhances the C-H bond cleavage ability of 1.
Angewandte Chemie, May 11, 2023
Journal of Fluorine Chemistry, Oct 1, 2008
Fluorous solvents are the most nonpolar, nonpolarizable phases known, whereas ions are inherently... more Fluorous solvents are the most nonpolar, nonpolarizable phases known, whereas ions are inherently polar. This makes it difficult to create salts that are soluble in a fluorous solvent. Here we present the synthesis and characterization of a new fluorophilic phosphonium salt, tris{3,5bis[(perfluorooctyl)propyl]phenyl}methylphosphonium methyl sulfate. The salt has a solubility of at least 14 mM in perfluoro(perhydrophenanthrene), perfluoro(methylcyclohexane), and perfluorohexanes. It also shows immediate potential for use as a phase-transfer catalyst in fluorous biphasic catalysis, but in this work it is used as an anion exchanger site in the first potentiometric fluorous-membrane anion-selective electrode. The membrane sensor exhibited the exceptional selectivity of 3.9 × 10 10 to 1 for perfluorooctanesulfonate over chloride, and of 2.5 × 10 7 to 1 for perfluorooctanoate over chloride. With improvements to the sensor's detection limit and lifetime, it has the potential to be an attractive alternative to the expensive, time-consuming methods currently employed for measurement of perfluorinated acids.
Meeting abstracts, 2019
A calibration-free measurement with an ionophore-doped polymeric membrane ion-selective electrode... more A calibration-free measurement with an ionophore-doped polymeric membrane ion-selective electrode requires that the phase boundary potential at the sample/sensing membrane interface is controlled by the activity of the target ion in the sample of interest, while all other phase boundary potentials in the electrochemical cell are constant and long term stable. Historically, the biggest difficulty lies in establishing a reproducible phase boundary potential at the interface of the sensing membrane and the underlying electron conductor. Efforts over several decades to use conducting polymers as an interlayer between the ion-selective membrane and an underlying electron conductor, such as a metal or carbon, have had limited success. While the performance of such devices has been much improved in terms of light sensitivity and hydrophobicity of the conducting polymer layer, devices that can be considered calibration-free are still elusive. To that end, hydrophobic redox buffers have been introduced as an alternative to conducting polymers. While hydrophilic redox buffers play central roles in all living organisms, control many geological and environmental processes, and are often utilized in the laboratory, buffering of redox potentials in hydrophobic media is a topic that has in the past been overlooked. This presentation will address principles for the use of hydrophobic redox buffers, and it will discuss recent examples of hydrophobic redox buffers suitable for the fabrication of ion-selective electrode membranes that are calibration-free. (1) Redox Buffer Capacity of Ion-Selective Electrode Solid Contacts Doped with Organometallic Complexes, Zhen, X. V.; Rousseau, C. R.; Buhlmann, P. Anal. Chem., 2018, 90, 11000-11007. (2) Paper-Based All-Solid-State Ion-Sensing Platform with a Solid Contact Comprising Colloid-Imprinted Mesoporous Carbon and a Redox Buffer, Hu, J.; Zhao, W.; Bühlmann, P.; Stein, A., ACS Appl. Nano Mat. 2018, 1, 293–301.
Meeting abstracts, 2016
A calibration-free measurement with an ion-selective membrane requires that only the phase bounda... more A calibration-free measurement with an ion-selective membrane requires that only the phase boundary potential at the sample/sensing membrane interface is variable, i.e., either dependent on the activity of the target ion in the sample, as in a potentiometric experiment, or determined by an applied potential, as in a voltammetric experiment. All the other phase boundary potentials in the electrochemical cell are constant. Unfortunately, one of the biggest challenges in the fabrication of devices comprising solid-contact membranes has been to establish a reproducible phase boundary potential at the interface of the hydrophobic ion-selective membrane and the underlying electron conductor. This talk will address our use of ion-selective membranes doped with lipophilic redox buffers, both in view of potentiometry and voltammetry. Solid contact ion-selective electrodes (ISEs) typically have an intermediate layer between the ion-selective membrane and the underlying solid electron conductor that reduces the irreproducibility and instability of the measured electromotive force (emf). Nevertheless, the electrode-to-electrode reproducibility of the emf of current solid-contact electrodes in a zero-current measurement is widely considered to be unsatisfactory. To address this problem, we reported on lipophilic redox buffers consisting of the Co(III) and Co(II) complexes with hydrophobic ligands such as 1,10-phenanthroline (e.g., [Co(phen)3]3+/2+) paired with tetrakis(pentafluorophenyl)borate as counterion [1,2]. The resulting electrodes exhibit zero-current emf values with an electrode-to-electrode standard deviation as low as 1.7 mV after conditioning of freshly prepared electrodes for 1 h. While many prior examples of solid contact ISEs also used intermediate layers that contained redox-active species, the importance of selecting a balanced ratio of the reduced and oxidized species has typically been difficult and was often ignored, contributing to emf irreproducibility. The ease of the control of the [Co(phen)3]3+ / [Co(phen)3]2+ ratio explains the high emf reproducibility, as confirmed by the emf decrease of 58 mV per tenfold increase in the ratio of the reduced and oxidized redox buffer species. The result is an electrode-to-electrode standard deviation of Eº of 0.7 mV after 1 week of exposure to KCl solution. The use of this redox buffer in combination with colloid-imprinted mesoporous carbon [3,4] will also be discussed. [1] Zou, X. U.; Cheong, J. H.; Taitt, B. J.; Bühlmann, P., Anal. Chem. 2013, 85, 9350–9355. [2] Zou, X. U.; Zhen, X. V.; Cheong, J. H.; Bühlmann, P. Anal. Chem., 2014, 86, 8687–8692. [3] Hu, J.; Ho, K. T.; Zou, X. U.; Smyrl, W. H.; Stein, A.; Bühlmann, P. Anal. Chem., 2015, 87, 2981–2987 [4] Hu, J.; Stein, A.; Bühlmann, P. Angew. Chem. Int. Ed., in press (DOI: 10.1002/anie.201603017).
Analytical Sciences, Feb 22, 2019
Polyquaternium polymers are polycationic polymers that are contained in many hair shampoos and co... more Polyquaternium polymers are polycationic polymers that are contained in many hair shampoos and conditioners and are also often added to water to remove organic and inorganic anions by floc formation. While polyquaternium analysis is not trivial, electroanalytical methods have been proposed for their detection using either irreversible emf responses or reversible potential-driven extraction into and out of polymeric sensing membranes. We present here an alternative technique for the determination of a representative polyquaternium polymer, poly(dimethylamine-co-epichlorohydrin) chloride, by equilibrium binding with a singly charged anionic surfactant, 1-dodecyl sulfate. Binding of an anionic surfactant to the polyquaternium polymer simplifies electrochemical detection as the concentration of unbound surfactant can be monitored using the equilibrium Nernstian emf response of ion-exchanger membranes. The latter can be used to determine the nature of the binding interaction and allows for the straightforward determination of polyquaternium polymer concentrations by titration.
Analytical Sciences, Sep 6, 2019
Porous glass frits are frequently used to contain the salt bridges through which reference electr... more Porous glass frits are frequently used to contain the salt bridges through which reference electrodes interface samples. Prior work with widely used glass frits with 4-10 nm diameter pores showed that, when samples have a low electrolyte strength, electrostatic screening of sample ions by charged sites on the glass surface occurs. This creates an ion-specific phase-boundary potential at the interface between the sample and frit, and it biases the potential of the reference half-cell. Use of frits with much larger pores eliminates this problem but results in the need for frequent replenishing of the bridge electrolyte. A methodical study to determine the optimum pore size has been missing. We show here that charge screening of sample ions occurs when the pore size of nanoporous glass frits is on the order of 1-50 nm and samples have a low electrolyte strength. An increase in pores size to 100 nm eliminates charge screening in samples with ionic strengths in the 1.0 M to 3.3 × 10-4 M range. However, the rates of electrolyte solution flow through frits with 1, 5, 20, 50, and 100 nm pores are still low, which makes diffusion the dominant mode of ion transport into and out of these frits. Consequently, the flow of bridge electrolyte into samples is not fast enough to prevent diffusion of ions and electrically neutral components from the sample diffusing into the salt bridge, which can result in cross contamination among samples.
Journal of Organic Chemistry, Dec 24, 2013
ACS Sensors, Apr 12, 2023
Current Opinion in Electrochemistry, 2021
Supramolecular Chemistry, 2012
ACS Sensors, 2020
Many reference electrodes with an ionic liquid-doped reference membrane contain a plasticizer tha... more Many reference electrodes with an ionic liquid-doped reference membrane contain a plasticizer that can gradually leach out into the sample. However, because many common plasticizers are known to be endocrine disruptors and may induce inflammatory reactions, they are preferably avoided for wearable or implantable sensors. Therefore, this work tested polymeric reference electrode membranes prepared by solvent casting from seven commercially available biocompatible silicones that are widely used in implantable devices. Only reference electrodes with membranes consisting of poly(3,3,3-trifluoropropylmethylsiloxane) (Fluorosilicone 1) and one of several 1-methyl-3-alkylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids provided a stable and sample-independent potential in electrolyte solutions spanning the range of electrolyte concentrations in human blood, with more hydrophobic ionic liquids performing better. Over 8 days at 37 ºC in artificial blood electrolyte solutions, the reference membranes doped with 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide exhibited a potential drift as low as 20 µV/h. In 10% animal serum, a 112 µV/h drift was observed over 5.8 days. The other six silicone materials doped with an ionic liquid either failed to form self-standing membranes or did not provide a sample-independent potential in the ionic concentration range tested. In case of the functional reference electrodes, differential scanning calorimetry confirmed good miscibility between the ionic liquid and the polymer matrix, whereas the poor miscibility of four polymer matrixes and the ionic liquids-as confirmed by differential scanning calorimetry-correlated with an undesirable sample dependence of the reference potential.
Journal of Chemical Education, 2018
This article describes an ongoing initiative of the Department of Chemistry (Chem. Dept.) at the ... more This article describes an ongoing initiative of the Department of Chemistry (Chem. Dept.) at the University of Minnesota (UMN) to support the mental health of graduate students. With the increasing pressure on students to carry out novel research, publish articles, learn a broad range of skills, and look for career opportunities, the levels of stress, anxiety, and depression among graduate students are on the rise. For tackling these issues, the UMN Chem. Dept. has adopted an approach that heavily relies on the involvement of graduate students and student empowerment. This contribution describes the results of a collaboration between a student group (Community of Chemistry Graduate Students, CCGS), the director of graduate studies of the Chem. Dept., and mental health professionals at the UMN campus health service, to provide strategies for ensuring a welcoming and productive departmental climate. It describes the events that CCGS has hosted to help to improve the mental health of students, and raise awareness and stimulate open discussions about this topic. As an early intervention strategy, the UMN Chem. Dept. revised several policies to ensure that students receive frequent feedback from their advisors. Through the collaboration of the CCGS, UMN Chem. Dept., and UMN campus health service, a survey for the evaluation of mental health and stress factors in graduate studies was developed. Findings of the survey attest to the stigma associated with mental health, as more than 40% of the graduate students responded that they did not consider consulting with a therapist, counselor, or physician even when they felt that their health was affected by the level of stress in their lives. The results also show the importance of an open and friendly environment for students who struggle with stress and mental health, as they were most likely to approach a friend rather than advisor, counselor, or physician.
Electroanalysis, 2017
This work demonstrates the remarkable stability of fluorous ion-selective electrode (ISE) membran... more This work demonstrates the remarkable stability of fluorous ion-selective electrode (ISE) membranes by exposing them to cleaning-in-place treatments (CIP) as they are used in many industrial processes. The sensing membranes consisted of Teflon AF2400 plasticized with a linear perfluoropolyether and doped with ionic sites and a H + ionophore (i. e., tris[3-(perfluorooctyl)propyl]amine, 1, or tris[3-(perfluorooctyl)pentyl]amine, 2). To mimic a typical CIP treatment, the electrodes were repeatedly exposed for 30 min to a 3.0 % NaOH solution at 90 8C (pH 12.7). ISE membranes doped with the less strongly H + binding ionophore 1 started to show reduced potentiometric response slopes and increased resistances after one exposure for 30 min to hot 3.0 % NaOH solution. No decomposition of the ionic sites and ionophore 1 at 90 8C was evident by 1 H NMR spectroscopy, suggesting that the performance of membranes doped with 1 was compromised primarily by leaching of the negatively charged ionic sites along with H + into the hot caustic solution. In contrast, even after ten exposures to hot 3.0 % NaOH for a cumulative 5 h at 90 8C, the fluorous sensing membranes doped with the more strongly H + binding ionophore 2 still showed the ability to respond with a theoretical (Nernstian) slope up to pH 12. Addition of the fluorophilic electrolyte salt methyltris[3-(perfluorooctyl)propyl] ammonium tetrakis[3, 5-bis(perfluorohexyl)phenyl]borate reduced the membrane resistance by an order of magnitude.
Analytical Chemistry, 2000
Analytical Chemistry, 2008
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Papers by Philippe Bühlmann