Mark E. Meyerhoff

ANION-SELECTIVE MEMBRANE ELECTRODES BASED ON METALLOPORPHYRINS: THE INFLUENCE OF LIPOPHILIC ANIONIC AND CATIONIC SITES ON POTENTIOMETRIC SELECTIVITY

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.

Preparation and characterization of hydrophobic polymeric films that are thromboresistant via nitric oxide release

The preparation of hydrophobic polymer films (polyurethane and poly(vinyl chloride)) containing nitric oxide (NO)-releasing diazeniumdiolate functions is reported as a basis for improving the thromboresistivity of such polymeric materials for biomedical applications. Several different approaches for preparing NO-releasing polymer films are presented, including: (1) dispersion of diazeniumdiolate molecules within the polymer matrix; (2) covalent attachment of the diazeniumdiolate to the polymer backbone; and (3) ion-pairing of a diazeniumdiolated heparin species to form an organic soluble complex that can be blended into the polymer. Each approach is characterized in terms of NO release rates and in vitro biocompatibility. Results presented indicate that the polymer films prepared by each approach release NO for variable periods of time (10-72 h), although they differ in the mechanism, location and amount of NO released. In vitro platelet adhesion studies demonstrate that the localized NO release may prove to be an effective strategy for improving blood compatibility of polymer materials for a wide range of medical devices.

Implantable chemical sensors for real-time clinical monitoring: progress and challenges

Recently, progress has been made in the development of implantable chemical sensors capable of real-time monitoring of clinically important species such as PO(2), PCO(2), pH, glucose and lactate. The need for developing truly biocompatible materials for sensor fabrication remains the most significant challenge for achieving robust and reliable sensors capable of monitoring the real-time physiological status of patients.

Nitric oxide releasing silicone rubbers with improved blood compatibility: preparation, characterization, and in vivo evaluation.

Nitric oxide (NO) releasing silicone rubbers (SR) are prepared via a three-step reaction scheme. A diamino triaminoalkyltrimethoxysilane crosslinker is used to vulcanize hydroxyl terminated polydimethylsiloxane (PDMS) in the presence of ambient moisture and a dibutyltin dilaurate catalyst so that the respective diamine triamine groups are covalently linked to the cured SR structure. These amine sites are then diazeniumdiolated, in situ, when the cured SR is reacted with NO at elevated pressure (80 psi). Although nitrite species are also formed during the NO addition reaction, in most cases the diazeniumdiolated polymer is the major product within the final SR matrix. Temperature appears to be the major driving force for the dissociation of the attached diazeniumdiolate moieties, whereas the presence of bulk water bathing the SR materials has only minimal effect on the observed NO release rate owing to the low water uptake of the SR matrices. The resulting SR films/coatings release NO at ambient or physiological temperature for up to 20 d with average fluxes of at least 4 x 10(10) mol x cm(-2) x min(-1) (coating thickness > or = 600 microm) over first 4 h, comparable to the NO fluxes observed from stimulated human endothelial cells. The NO loading and concomitant NO release flux of the SR material are readily adjustable by altering the diamine triamine loading and film/coating thickness. The new NO releasing SR materials are shown to exhibit improved thromboresistance in vivo, as demonstrated via reduced platelet activation on the surface of these polymers when used to coat the inner walls of SR tubings employed for extracorporeal circulation in a rabbit model.

Reduced platelet activation and thrombosis in extracorporeal circuits coated with nitric oxide release polymers

Objective: To determine whether the use of nitric oxide (NO)‐releasing polymers coated onto the inner surface of extracorporeal circuits can reduce platelet consumption and activation in the absence of systemic heparinization using a rabbit model of venovenous extracorporeal circulation. Design: Prospective, controlled trial Setting: Research laboratory at an academic medical institution. Subjects: New Zealand White Rabbits Interventions: Anesthetized, tracheotomized, and ventilated New Zealand White rabbits were injected with freshly prepared, 111In(oxine)3 labeled single donor platelets through the external jugular vein. After baseline measurements, these animals were placed on venovenous extracorporeal circulation through a 1‐m control circuit or NO test circuit for 4 hrs at a blood flow rate of 109‐118 mL/min via roller pump. Four groups were studied: systemically heparinized control circuits, systemically heparinized NO test circuits, nonheparinized control circuits, and nonheparinized NO test circuits. Platelet counts, fibrinogen levels, and plasma free indium levels were measured hourly. Circuits were rinsed and retained for gamma counting after the 4‐hr run or when the circuit clotted. Four animals, one from each group, did not receive radiolabeled platelets so that the circuits could be preserved for scanning electron microscopic examination after the 4‐hr study. Measurements and Main Results: Platelet consumption was significantly reduced in both the heparinized and nonheparinized NO test groups when compared with the controls (p < .0001 and p < .0004, respectively). Platelet adhesion to the extracorporeal circuits was significantly reduced in the nonheparinized test circuits when compared with the controls (p < .05). Scanning electron microscopic examination of the circuits revealed that in the absence of heparin and in the presence of a NO‐releasing surface, platelets retained their spherical nonactivated shape. Conclusions: The incorporation of NO into the surface of extracorporeal circuits reduces platelet consumption and eliminates the need for systemic heparinization in a rabbit model of extracorporeal circulation.

Role of axial ligation on potentiometric response of Co(III) tetraphenylporphyrin-doped polymeric membranes to nitrite ions

Abstract Two types of Co(III) tetraphenylporphyrins, Co(III)TPPX (I) and Co(III)(N)TPPX (II), where X = C1− or NO−2 and N = C5H5N or C6H5CH2C5H4N, are used as ionophores to prepare nitrite responsive polymeric membrane electrodes. The influence of the initial axial ligand (X− and N) on the operative ionophore mechanism of these metalloporphyrins within the solvent polymeric membranes is examined. Results from potentiometric and electrodialysis experiments suggest that in the presence of nitrite in the test sample and internal solution, both types of Co (III) porphyrins studied (I and II) act as neutral carriers and that the addition of lipophilic cationic sites (e.g., tridodecylmethylammonium ions (TDMA+)) to the organic membrane is essential to improve the selectivity and long term stability of sensors prepared with these species. Membranes formulated with (I) or (II) in the nitrite form along with TDMACl in plasticized PVC films exhibit the following selectivity sequence: SCN− > NO−2 ~ C1O−4 > Sal− > NO−3 > Br− > C1−. Membrane electrodes with added lipophilic cationic sites are shown to exhibit rapid, fully reversible and Nernstian response towards nitrite ions in the concentration range of 10−1–10−5 M, with good long term stability.

Recent Advances in the Development and Analytical Applications of Biosensing Probes

Abstract The term “biosensor” has become a fashionable buzzword in recent analytical literature. While scientists have used the word to describe any number of innovative devices and instrumental systems, we believe that the two most widely accepted definitions are as follows:

Screen printing: a technology for the batch fabrication of integrated chemical-sensor arrays

Abstract The commercialization of integrated chemical sensor has been slowed by the difficulty of device encapsulation and membrane application. For reasons of both cost and reproducibility, the sensor-specific structures should be mass fabricated, as are the microelectronics. The challenge lies in merging the standard semiconductor process sequence with the non-standard steps used to form the transducers. We demonstrate that screen printing can be used to partition the fabrication into two distinct sequences, semiconductor processing and sensor-specific steps. This simplifies process development and evolution, and makes semiconductor foundry services available for manufacturing sensors. After conventional semiconductor processing, our wafers have silver epoxy contacts screen printed on the aluminum sensor pads; the silver forms a stable chemical interface to the membranes, and the epoxy makes a strong physical bond to them. Next, the polymeric membranes are applied and patterned with screen printing. Membranes of different compositions can be deposited on the various sites of a multisensor chip by simply repeating the screen print/cure cycle. We show that the electrochemical performance of mass-fabricated passive and active sensor arrays is comparable to that of conventional liquid-junction ion-selective electrodes.

Measurement of Tear Glucose Levels with Amperometric Glucose Biosensor/Capillary Tube Configuration

An amperometric needle-type electrochemical glucose sensor intended for tear glucose measurements is described and employed in conjunction with a 0.84 mm i.d. capillary tube to collect microliter volumes of tear fluid. The sensor is based on immobilizing glucose oxidase on a 0.25 mm o.d. platinum/iridium (Pt/Ir) wire and anodically detecting the liberated hydrogen peroxide from the enzymatic reaction. Inner layers of Nafion and an electropolymerized film of 1,3-diaminobenzene/resorcinol greatly enhance the selectivity for glucose over potential interferences in tear fluid, including ascorbic acid and uric acid. Further, the new sensor is optimized to achieve very low detection limits of 1.5 ± 0.4 μM of glucose (S/N = 3) that is required to monitor glucose levels in tear fluid with a glucose sensitivity of 0.032 ± 0.02 nA/μM (n = 6). Only 4-5 μL of tear fluid in the capillary tube is required when the needle sensor is inserted into the capillary. The glucose sensor was employed to measure tear glucose levels in anesthetized rabbits over an 8 h period while also measuring the blood glucose values. A strong correlation between tear and blood glucose levels was found, suggesting that measurement of tear glucose is a potential noninvasive substitute for blood glucose measurements, and the new sensor configuration could aid in conducting further research in this direction.

Polymer membrane-based ion-, gas- and bio-selective potentiometric sensors

Recent progress in the design of new polymer membrane-based potentiometric ion-, gas- and bio-selective electrodes in chemistry laboratories at the University of Michigan (Ann Arbor) is reviewed. Emphasis is placed on describing the performance of devices for measuring anions (e.g., salicylate, thiocyanate, chloride and heparin) and gases (e.g., ammonia, carbon dioxide and oxygen) in biological samples, both in vitro and in vivo. Beyond direct measurement of key ions and gases in complex matrices, some of the new membrane electrode systems reported can serve as base transducers for the development of biosensors containing integrated biological reagents, including enzymes and antibodies. New approaches for mass fabricating solid-state ion and biosensor devices as well as future directions for research in the entire field of polymer membrane sensors are also described.