Enhancing analytical accuracy of intravascular electrochemical oxygen sensors via nitric oxide release using S-nitroso-N-acetyl-penicillamine (SNAP) impregnated catheter tubing

Implantable medical devices are an integral part of primary/critical care. However, these devices carry a high risk for blood clots, caused by platelet aggregation on a foreign body surface. This study focuses on the development of a simplified approach to create nitric oxide (NO) releasing intravascular electrochemical oxygen (O₂) sensors with increased biocompatibility and analytical accuracy. The implantable sensors are prepared by embedding S-nitroso-N-acetylpenacillamine (SNAP) as the NO donor molecule in the walls of the catheter type sensors. The SNAP-impregnated catheters were prepared by swelling silicone rubber tubing in a tetrahydrofuran solution containing SNAP. Control and SNAP-impregnated catheters were used to fabricate the Clark-style amperometric PO₂ sensors. The SNAP-impregnated sensors release NO under physiological conditions for 18 d as measured by chemiluminescence. The analytical response of the SNAP-impregnated sensors was evaluated in vitro and in vivo. Rabbit and swine models (with sensors placed in both veins and arteries) were used to evaluate the effects on thrombus formation and analytical in vivo PO₂ sensing performance. The SNAP-impregnated PO₂ sensors were found to more accurately measure PO₂ levels in blood continuously (over 7 and 20 h animal experiments) with significantly reduced thrombus formation (as compared to controls) on their surfaces.

Ion detection with photonic crystal microcavities

We have experimentally demonstrated a cation and anion sensor by using short linear photonic crystal microcavities with an embedded quantum dot active region. The photonic crystal microcavity covered with an ion-selective polymer forms a submicrometer optical detection system sensitive to small changes of perchlorate anion (ClO4(-)) and calcium cation (Ca2+) concentrations.

Retention behavior of large polycyclic aromatic hydrocarbons on metalloprotoporphyrin-silica stationary phases

The retention behavior of large polycyclic aromatic hydrocarbons (LPAHs) (> or = 7 rings) on newly developed metalloplotoporphyrin (MProP)-silica stationary phases is examined and the results are compared to previously reported data for retention of the same solutes on commercially available phases. HPLC columns packed with FeProP-silica are shown to exhibit unique shape selectivity for LPAH retention, with the planar LPAHs always retained much longer than corresponding non-planar solutes. Solute planarity, length to breadth ratio (L/B value), and number of carbon atoms within the LPAHs are all demonstrated to contribute to the retention sequence observed. Further, the retention of LPAH solutes on FeProP-silica phases is shown to be more predictable than on other reversed-phase columns, with the elution sequence constant regardless of the mobile phase composition. Due to the extremely high planar selectivity of FeProP-silicas with respect to LPAH retention, it is envisioned that columns packed with these phases could be used in conjunction with existing commercial columns to devise Inethods for more efficient separation of complex mixtures of LPAHs in environmental and other samples.

Rotating electrode potentiometry: lowering the detection limits of nonequilibrium polyion-sensitive membrane electrodes

A rotating electrode configuration is evaluated as a means to lower the detection limits of newly devised polyion-sensitive membrane electrodes (PSEs). Planar potentiometric polycation and polyanion PSEs are prepared by incorporating tridodecylmethylammonium chloride and calcium dinonylnaphthalenesulfonate, respectively, into plasticized PVC or polyurethane membranes and mounting disks of such films on an electrode body housed in a conventional rotating disk electrode apparatus. Rotation of the PSEs at 5000 rpm results in an enhancement in the detection limits toward heparin (polyanion) and protamine (polycation) of at least 1 order of magnitude (to 0.01 unit/mL for heparin; 0.02 microg/mL for protamine) over that observed when the EMF responses of the same electrodes are assessed using a stir-bar to achieve convective mass transport. A linear relationship between omega(-1/2), where omega is the rotating angular frequency, and C1/2, the polyion concentration corresponding to half the total maximum deltaEMF response toward the polyion species, is observed. It is further shown that the rotating polycation sensor can be used as an end-point detector to greatly enhance (relative to nonrotated indicator electrode) the analytical resolution and precision for measurement of low concentrations of heparin when such samples are titrated with protamine. The theoretical basis for lowering the detection limits by rotating PSEs is discussed based on the unique nonequilibrium response mechanism of such sensors.

Origin of non-Nernstian anion response slopes of metalloporphyrin-based liquid/polymer membrane electrodes

The origin of the non-Nernstian potentiometric anion response behavior exhibited by several metalloporphyrin-based liquid/polymeric membrane electrodes is examined. UV-visible spectrophotometry of organic-phase solutions and thin plasticized PVC films containing In(III) and Ga(III) octaethylporphyrins suggests that, in the absence of preferred axial coordination anions, the metalloporphyrins form hydroxide ion bridged dimers within the organic phases, as indicated by a significant blue shift of the Soret band in the visible spectrum. As increasing levels of the preferred anions are added, the degree of dimerization decreases and the intensity of the Soret band corresponding to the monomer species increases. Observation of Nernstian responses with membranes doped with picket fence-type In(III) and Ga(III) porphyrins not capable of forming hydroxide bridged structures (as determined by UV-visible spectroscopy) confirms that dimerization is likely responsible for the super-Nernstian slopes of membrane electrodes formulated with the non-picket fence species. A phase boundary model based on simultaneous binding equilibria of hydroxide ions with two metalloporphyrins to form the dimeric species, while the target anions bind with metalloporphyrins to form neutral 1:1 complexes, is shown to fully predict the observed non-Nernstian behavior. The prospect of utilizing this anion-dependent dimer-monomer metalloporphyrin equilibrium to fabricate anion-selective optical sensors using thin films of metalloporphyrin-doped polymers is also discussed.

Optical detection of polycations via polymer film-modified microtiter plates: response mechanism and bioanalytical applications

Microtiter plate wells modified with thin (approximately 20 microm) polymeric films capable of optically sensing macromolecular protamine and other polycationic species are described. The plates are prepared by coating the bottom of each well of a conventional 96-well polypropylene plate with an adherent polymer film (a mixture of poly(vinyl chloride) and polyurethane) containing a lipophilic 2',7'-dichlorofluorescein derivative. Surprisingly, optical response toward polycations is shown to result from the extraction of the fluorescein derivative from the polymer film into a lyophobic colloidal phase at the sample/film interface. This new phase is likely composed of a micellular-type ion pair complex between the analyte polycation from aqueous sample phase and the deprotonated form of the fluorescein derivative. Accumulation of the deprotonated fluorescein species in this interfacial region induces an absorbance change measured at 540 nm. Optimized plates can be used to sense protamine concentrations in the range of 0-100 microg/mL in 10 min with little or no response to physiological levels of common cationic species (Na+, K+, Ca2+, etc.). The modified plates are shown to be useful as simple optical detectors for measuring heparin levels in plasma via titrations with protamine and for monitoring protease activities (trypsin and plasmin) that cleave polycationic peptides/proteins such as protamine into smaller peptide fragments that are not detected by the sensing films. Assays for "clot busting" plasminogen activators (streptokinase, urokinase, and tissue plasminogen activator) are also demonstrated using this relatively simple microtiter plate-based polycation detection system.

Nature of immobilized antibody layers linked to thioctic acid treated gold surfaces

Utilization of 125I-labeled IgG enables an investigation of protein immobilized to gold electrodes sputter deposited on microporous nylon membranes, including the precise nature of the surface-protein bond (i.e. covalent or non-specific adsorption), physical location of the immobilized protein (i.e. on the surface of the gold electrode or within the pores of the membrane), and the amount of protein immobilized. This is accomplished by comparing the mass of protein immobilized to gold surfaces that have been treated in several different fashions, as well as, deposition of the gold on nylon membranes that have been treated differently. It is shown that these microporous gold electrodes, proposed previously for conducting novel non-separation electrochemical enzyme immunoassays, consist of multiple protein layers non-specifically adsorbed. Approximately, half of the total adsorbed protein is immobilized to the gold surface with the remaining protein bound within the pores on the nylon membrane.

Conversion of a polysaccharide to nitric oxide-releasing form. Dual-mechanism anticoagulant activity of diazeniumdiolated heparin

We describe heparin/diazeniumdiolate conjugates that generate nitric oxide (NO) at physiological pH. Like the heparin from which they were prepared, they inhibit thrombin-induced blood coagulation. Unlike heparin, they can also inhibit and reverse ADP-induced platelet aggregation (as expected for an NO-releasing agent), suggesting potential utility as dual-action antithrombotics.

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.

Improving the thromboresistivity of chemical sensors via nitric oxide release: fabrication and in vivo evaluation of NO-releasing oxygen-sensing catheters

The development and in vivo analytical performance of a nitric oxide (NO)-releasing amperometric oxygen sensor with greatly enhanced thromboresistivity are reported. Gas permeable coatings formulated with cross-linked silicone rubber (SR) containing NO-generating compounds (diazeniumdiolates) are shown to release NO for extended periods of time (> 20 h) while reducing platelet adhesion and activation. Oxygen-sensing catheters prepared by dip-coating the NO-releasing films over the outer SR tubes of the implantable devices display similar analytical response properties in vitro (sensitivity, selectivity, response times) when compared to analogous sensors prepared without the NO release coatings. Superior analytical accuracy (relative to blood PO2 values measured in vitro) and greatly reduced thrombus formation on the outer surface of the sensors are observed in vivo (in canine model) with the NO release PO2 sensors compared to control sensors (without NO release) implanted simultaneously within the same animals. Based on these preliminary studies, the use of NO release polymers to fabricate catheter-style chemical sensors may be a potential solution to lingering biocompatibility and concomitant performance problems encountered when attempting to employ such devices for continuous intravascular measurements of blood gases and electrolytes.

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.

Electrochemical assay of plasminogen activators in plasma using polyion-sensitive membrane electrode detection

Two relatively simple electrochemical assay methods suitable for the measurement of plasminogen activators (including urokinase (u-PA), streptokinase (SK), and tissue plasminogen activator (t-PA)) in plasma samples are described. In one approach, the initial rate of decrease in the potentiometric response of a polycation-sensitive membrane electrode toward protamine is monitored after addition of a preincubated reaction mixture containing the sample and exogenous plasminogen. The plasmin formed from plasminogen by the activators catalyzes the decomposition of the arginine-rich protamine substrate, yielding smaller polycationic fragments that are not sensed by the electrode. Alternately, the sample, plasminogen, and protamine can be incubated together, and the remaining protamine in this reaction mixture can be measured at a fixed point in time by placing the electrode into the mixture and recording the electromotive force response. Working curves found with both methods for plasma samples spiked with varying levels of the activators cover the expected therapeutic activity ranges found in the plasma of patients treated with these "clot-busting" drugs.

Shape-selective separation of polycyclic aromatic hydrocarbons on protoporphyrin-silica phases. Effect of surface porphyrin distribution on column efficiency

The chromatographic performance of various metalloprotoporphyrin-silica (MProP-silica) packing materials prepared using different porphyrin immobilization schemes is examined. Column efficiency and solute resolution for the shape-selective separation of polycyclic aromatic hydrocarbons (PAHs) can be improved significantly by preparing phases with lower porphyrin coverages and with a more homogeneous distribution of the porphyrin species on the surface. The latter is accomplished by spreading/diluting the number of aminopropyl reactive sites on the silica surface via mixing an inert methyltrimethoxysilane with 3-aminopropyltriethoxysilane during this preliminary reaction step. Subsequent covalent attachment of the ProP via amide bonds to the pendant amine sites results in a more even distribution of the porphyrins on the surface. Band shapes and retention times as a function of injected solute concentration as well as HPLC separation of various test mixtures of PAHs (including standard reference material SRM 869) are used to confirm the enhanced performance of these so-called "spread" phases. Changes in the nature of the immobilized porphyrin distribution on the silica surface are further probed by a coupled redox/UV-Vis absorbance method, and results suggest a decrease in the number of ProP species immobilized as aggregates on the surface.

Electrochemical sensors for polyionic macromolecules: development and applications in pharmaceutical research

The first electrochemical sensors responsive towards polyionic heparin and protamine - two clinically important polymeric drugs - have been fully developed. The response mechanism of these sensors has been completely elucidated. As well as their significance in measuring blood heparin levels in clinically relevant concentration ranges, these polyion sensors could also find broad applications in pharmaceutical research, such as in the study of the binding events between heparin (or protamine) and other polycationic (or polyanionic) macromolecules. In addition, the sensors could be employed in the design of blood assays for a range of clinically important proteases and their inhibitors by utilizing either protamine or specially designed synthetic polypeptides as the substrates

Bioanalytical applications of polyion-sensitive electrodes

Recent developments in the design and bioanalytical applications of polyion-sensitive electrodes (PSEs) are reviewed. The general electrochemical principles governing the potentiometric response of such polymer membrane-based devices are summarized and new directions for the use of these novel sensors are detailed. These new directions include basic fundamental studies aimed at determining the thermodynamics of polyion extraction into ion exchanger-doped polymeric membranes, new methods to quantitate the anticoagulant drug heparin in whole blood via titrations with polycationic protamine, selective assays of protease activities (and inhibitors of such activities) using natural and synthetic polyionic peptides as substrates, and novel homogeneous immunoassay schemes based on potentiometric polyion detection.

Determination of low-molecular-weight heparins and their binding to protamine and a protamine analog using polyion-sensitive membrane electrodes

A polycation-sensitive membrane electrode based on the ion-exchanger dinonylnaphthalene sulfonate has previously been developed and used as an end-point detector for the determination of unfractionated heparin in whole blood samples via simple potentiometric titration with protamine. Herein, we report the application of the same methodology for the quantitation of a commercial low-molecular-weight heparin (LMWH) preparation (Fragmin) in whole blood samples at concentrations up to 2 U/ml. Further, an analogous polyanion (heparin)-sensitive electrode is used to estimate the binding constants between protamine and various LMWH preparations. The equilibrium constants (Keq) and the number of binding sites per mole of heparin (n) are determined by recasting the data in the form of a Scatchard plot. Results show that the average molecular weight and molecular weight distribution of the LMWH preparation are important parameters affecting their binding with protamine. Comparable binding constants are obtained for the same LMWH preparations titrated with a synthetic protamine analog, [+18RGD] [acetyl-EA(R2A2R2A)4R2GRGDSPA-NH2].

Influence of nonionic surfactants on the potentiometric response of ion-selective polymeric membrane electrodes designed for blood electrolyte measurements

The effect of the nonionic surfactants Brij 35 and Triton X-100 on the selectivity of neutral carrier-based ion-selective electrodes (ISEs) commonly used for measurements of electrolytes in whole blood is investigated. Studies are conducted with plasticized PVC membranes doped with several neutral ionophores commonly employed to prepare clinically useful potassium, calcium, and sodium ISEs. An observed increase in the electrodes' EMF values upon the addition of surfactant to the test solution suggests a change in the ion selectivity of the polymeric membranes in the presence of Brij 35 or Triton X-100. For membranes doped with K(+)-selective valinomycin, the effect of nonionic surfactants is relatively small. However, in the case of calcium-selective membranes prepared with ETH 1001 and ETH 129, nonionic surfactants, especially Triton X-100, decrease the selectivity for calcium over potassium cations by nearly 1 order of magnitude. Such behavior is even more dramatic for sodium-selective membranes, with the degree of surfactant-induced loss of ion selectivity dependent on the specific sodium ionophore employed, the lipophilic tetraphenylborate derivative content of the membrane, and the surfactant type. A detailed theoretical model is presented to explain the effect of nonionic surfactants on the EMF response function of cation-selective polymeric membrane electrodes. Experimental results are in good agreement with theoretical predictions based on known binding constants for ionophores and surfactants with given cations.

Improved protamine-sensitive membrane electrode for monitoring heparin concentrations in whole blood via protamine titration

An improved protamine-sensitive electrode based on a polymeric membrane doped with the charged ion exchanger dinonylnaphthalenesulfonate (DNNS) is used for monitoring heparin concentrations in whole blood. The electrode exhibits significant nonequilibrium potentiometric response to polycationic protamine over the concentration range of 0.5-20 mg/L in undiluted whole-blood samples. The sensor can serve as a simple end point detector for the determination of heparin via potentiometric titrations with protamine. Whole-blood heparin concentrations determined by the electrode method (n > or = 157) correlate well with other protamine titration-based methods, including the commercial Hepcon HMS assay (r = 0.934) and a previously reported potentiometric heparin sensor-based method (r = 0.973). Reasonable correlation was also found with a commercial chromogenic anti-Xa heparin assay (r = 0.891) with corresponding plasma samples and appropriate correction for whole-blood hematocrit levels. Whereas a significant positive bias (0.62 kU/L; P < 0.001) is observed between the anti-Xa assay and the protamine sensor methods, insignificant bias is observed between the protamine sensor and the Hepcon HMS tests (0.08 kU/L; P = 0.02). The possibility of fully automating these titrations offers a potentially simple, inexpensive, and accurate method for monitoring heparin concentrations in whole blood.

Catheter-type sensor for potentiometric monitoring of oxygen, pH and carbon dioxide

The fabrication and analytical performance of a catheter-type electrode suitable for potentiometric monitoring of PO2, pH and PCO2 in flowing blood is described. The catheter electrode is based on impregnating a single segment of dual-lumen gas permeable silicone rubber tubing with the proton ionophore tridodecylamine to impart H+ permselectivity to both inner and outer walls of the tubing. One lumen is filled with an unbuffered bicarbonate solution and the other lumen is filled with a strong buffer. By inserting Ag/AgCl reference electrode wires in each lumen and a cobalt electrode in the buffered lumen, simultaneous potentiometric detection of PO2, pH and PCO2 is achieved. The response of cobalt electrode to PO2 arises from a steady-state mixed potential owing to slow oxidation of cobalt and simultaneous reduction of oxygen on the surface of the cobalt electrode. The response towards PCO2 is completely analogous to the response mechanism of a conventional Severinghaus PCO2 sensor (i.e., change in pH of the bicarbonate solution). Continuous measurements of PO2, pH and PCO2 during 4-5 h blood pump studies using the catheter electrodes correlate well with conventional bench-top blood gas analyzer (PO2: r2 = 0.992; pH: r2 = 0.940; PCO2: r2 = 0.993.

Photo-cross-linked decyl methacrylate films for electrochemical and optical polyion probes

Potentiometric and optical polyion probes based on photo-cross-linked thin films of decyl methacrylate (DMA) are described, and the effects of film composition on the response toward heparin are examined in detail. In accordance with existing theory governing potentiometric polyion response, lowering the amounts of plasticizer and tridodecylmethylammonium chloride ion exchanger within the film enhances its sensitivity toward heparin. Varying the cross-linker content of a DMA-based film, however, provides an additional mechanism to regulate its physical structure and, hence, the observed potentiometric polyion response. Films with low hexanedioldimethacrylate cross-linker content yield optimal potentiometric heparin detection limits (0.04 microM), suggesting a lower diffusion coefficient within such films, apparently due to interactions between adjacent pendant decyl groups. Increasing crosslinker content interrupts these interactions and facilitates diffusion. This knowledge is applied to optimize optical heparin sensing via DMA films covalently attached to glass substrates. When used in a limited volume/fixed exposure time measurement mode, such optically sensitive films can detect clinically relevant levels of heparin (0.5-5 units/mL) in undiluted human plasma.

Synthesis of N-acetylneuraminyl-alpha 2,3(6)lactose-malate dehydrogenase conjugate for detecting sialic acid terminal groups on glycoproteins via homogeneous lectin-based enzyme-linked binding assay

An N-acetylneuraminyl-alpha 2,3(6)lactose-malate dehydrogenase (MDH-Lac-Neu5Ac) conjugate is prepared via an isothiocyanate conjugation method using a p-aminophenethylamino derivative of sialyllactose. The newly synthesized conjugate can be utilized as a reagent in a novel homogeneous lectin-based, enzyme-linked, competitive binding assay (1-3) for probing the specific carbohydrate structure and content of intact glycoproteins. The enzymatic activity of the MDH-Lac-Neu5Ac conjugate is shown to be significantly inhibited (35%) by sialic acid-binding lectin, Limax flavus agglutinin (LFA), and this inhibition is reversed by mucin, a glycoprotein possessing sialic acid terminals. The asialo form of mucin, however, binds weakly to LFA, yielding no substantial increase in the MDH-Lac-Neu5Ac activity at comparable glycoprotein concentrations. Use of the newly synthesized conjugate in conjunction with LFA or other lectins capable of binding sialic acid may provide a rapid and convenient way to detect the presence and relative amount of sialic acid terminal groups within intact glycoprotein structures.

Electrochemical assay of proteinase inhibitors using polycation-sensitive membrane electrode detection

A simple and sensitive electrochemical method suitable for real time detection of trypsin-like proteinase inhibitors is described. The method is based on utilizing a protamine (a polycationic substrate for trypsin-like proteinases)-sensitive membrane electrode to monitor, potentiometrically, the initial rate of protamine decomposition upon the addition of a proteinase-antiproteinase test solution. In the presence of proteinase inhibitors, the initial rate of change in electromotive force is dependent on the concentration of proteinase inhibitor in the sample solution. The feasibility of this new assay method is demonstrated by detecting four inhibitors of trypsin-like proteinases: alpha1-antiproteinase inhibitor, alpha2-macroglobulin, aprotinin, and soybean inhibitor, using trypsin as the indicator proteinase. The efficacy of inhibition by each species, as expressed by I50 values (concentration of the inhibitor that induces 50% of the maximum proteinase inhibition), is shown to correlate well with literature values for the association constant of the proteinase-antiproteinase complex (k[assoc]). The proposed electrochemical assay for aprotinin is examined further using trypsin, plasmin, and kallikrein as the proteinase indicator reagents. It is found that the trypsin-aprotinin system offers the highest sensitivity and lowest detection limit for aprotinin detection. Application of the proposed method for measuring aprotinin in pretreated plasma samples is also reported.

Selective monitoring of peptidase activities with synthetic polypeptide substrates and polyion-sensitive membrane electrode detection

A novel method to monitor specific peptidase activities in biological samples as complex as undiluted plasma/blood is described. The approach is based on the design of synthetic polypeptide substrates in which di- or triarginine sequences are linked to each other via one or more other amino acids recognized specifically by the peptidase to be determined. Detection of chymotrypsin and renin activities using synthetic substrates P4 (F-R-R-R-F-V-R-R-F-NH2) and P5 (R-R-R-L-L-R-R-L-L-R-R-R), respectively, serves to demonstrate the principles of this new assay system. A polyion-sensitive membrane electrode, prepared by doping polymer films with dinonylnaphthalene-sulfonate (DNNS), is shown to exhibit significant nonequilibrium electromotive force (EMF) responses toward these and other polycationic substrates at microgram/milliliter levels under physiological conditions. The same electrode, however, exhibits much smaller total EMF response toward the shorter fragments of the synthetic peptides generated by peptidase activity; hence, the addition of peptidase to a solution containing the synthetic substrate yields a change in electrode EMF response, the rate of which is proportional to the activity of peptidase present. Other synthetic polycationic peptides as well as natural polycationic peptides (e.g., protamine) that lack specific cleavage sites for chymotrypsin and renin, yet are detected by the DNNS-based membrane electrode, do not elicit any significant change in EMF response in the presence of the peptidases, confirming the feasibility and utility of the proposed bioanalytical method.

Retention behavior of amino acids and peptides on protoporphyrin-silica stationary phases with varying metal ion centers

Various metalloprotoporphyrins (MProP) covalently linked to silica supports are examined as novel immobilized metal ion affinity chromatography (IMAC) stationary phases for separations of amino acids/peptides. Under reversed-phase HPLC conditions, the MProP-silicas exhibit high affinity toward L-histidine via metal-nitrogen axial ligation interactions, with an increasing degree of histidine retention highly dependent on the specific metal ion (M) in the center of the protoporphyrin (ProP) structure: Fe(III) > Ni(II) > Cu(II) > Zn (II) approximately Cd(II). Aromatic amino acids (i.e., L-trytophan and L-phenylalanine) are also retained on MProP columns through pi-pi interactions with the immobilized porphyrins, with the greatest affinity for L-trytophan observed on CuProP-silica columns. Peptides rich in L-histidine and L-tryptophan residues are selectively retained on most of the MProP-silica phases examined; however, the addition of an organic modifier and/or lowering the pH of the mobile phase can be used independently to attenuate the pi-pi and metal ion-nitrogen ligation interactions, respectively. Reproducible separations of His-Phe and trytophan releasing hormone are achieved on a FeProP-silica column even after extensive washing with 50 mM EDTA, demonstrating a fundamental advantage of the new MProP-silica over existing IMAC stationary phases, in which the metal ion is anchored weakly to the support via immobilized iminodiacetate and related ligands.

Mixed potential response mechanism of cobalt electrodes toward inorganic phosphate

The potentiometric response mechanism of a previously reported phosphate ion-sensitive electrode based on a surface-oxidized cobalt metal is examined. Beyond response to phosphate, the cobalt electrode is found to respond to changes in the partial pressure of oxygen in the sample solution. the potentiometric response toward phosphate ions and molecular oxygen is shown to depend on the sample stirring rate as well as the pH, ionic strength, and nature of the buffer salts present within the test solution. X-ray photoelectron spectroscopy studies of the cobalt electrodes, in conjunction with cyclic voltammetric measurements, suggest that the potentiometric response originates from a mixed potential resulting from the slow oxidation of cobalt and simultaneous reduction of both oxygen and Co2+ at the surface of the electrode. In contrast to an originally proposed host-guest mechanism, the present mixed potential mechanism more accurately explains behavior of oxidized cobalt electrodes in the presence of phosphate and oxygen species.

A new method of measuring heparin levels in whole blood by protamine titration using a heparin-responsive electrochemical sensor

OBJECTIVE

To determine the ability of a new electrochemical sensor to determine heparin levels in whole blood and to correlate the heparin levels as determined by this heparin-responsive sensor (HRS) with heparin levels as determined by the Hepcon assay system (Medtronic Hemotec, Parker, CO).

DESIGN

Methods comparison study.

SETTING

A large academic medical center.

INTERVENTIONS

The heparin levels of 162 samples from 24 patients undergoing cardiopulmonary bypass were determined by the HRS system and by the Hepcon system. In 21 samples, heparin levels as measured by anti-Xa activity were determined as well.

MEASUREMENTS AND MAIN RESULTS

HRS-determined values correlated highly with Hepcon-determined values (r = 0.942) and with anti-Xa determined values (r = 0.905). Bias +/- precision comparing the HRS and Hepcon methods was 0.211 +/- 0.478 U/mL.

CONCLUSIONS

HRS determined that whole blood heparin levels correlate well with Hepcon-determined levels. These limited results indicate that further development and testing of this new technology are warranted.

Influence of nonionic surfactants on the potentiometric response of hydrogen ion-selective polymeric membrane electrodes

The influence of poly(ethylene oxide)-based nonionic surfactants (i.e., Triton X-100 and Brij 35) in the sample phase on the response properties of hydrogen ion-selective polymeric membrane electrodes containing mobile (lipophilic amines) or covalently bound (aminated-poly-(vinyl chloride)) hydrogen ion carriers is reported. In the presence of these nonionic surfactants, membrane electrode response toward interfering cation activity (e.g., Na+) in the sample phase is increased substantially and the pH measuring range shortened. The degree of cation interference for pH measurements is shown to correlate with the basicity of the hydrogen ion carrier doped within the membrane phase. The observed deterioration in selectivity arises from the partitioning of the surfactant into the membrane and concomitant extraction of metal cations by the surfactants in the organic phase. The effect of nonionic surfactants on pH electrodes prepared with aminated-PVC membranes is shown to be more complex, with additional large shifts in EMF values apparently arising from multidentate interactions between the surfactant molecules and the polymeric amine in the membrane, leading to a change in the apparent pKa values for the amine sites. The effects induced by nonionic surfactants on the EMF response function of hydrogen ion-selective polymeric membrane electrodes are modeled, and experimental results are shown to correlate well with theoretical predictions.

Homogeneous enzyme-based binding assay for studying glycosaminoglycan interactions with macromolecules and peptides

A simple and rapid homogeneous enzyme-based binding assay is described to study the degree of interaction between glycosaminoglycans and various macromolecules/peptides. The method is based on the homogeneous inhibition of a highly positively charged enzyme, acid deoxyribonuclease II (EC 3.1.22.1), by glycosaminoglycan polyanions, such as heparin, chondroitin 4-sulfate, and dermatan sulfate. Catalytic activity of DNase II is inhibited to nearly 100% by relatively small amounts of these glycosaminoglycan molecules. In the presence of species that bind these polyanions, the activity of the enzyme is regained in an amount proportional to the concentration of the species present. Thus, the relative binding affinities of various species with a given GAG can be assessed rapidly by comparing the concentration of the compound required to reverse the enzyme inhibition to 50% of the maximum value (ED(50) values). The feasibility of this binding assay principle is demonstrated by measuring the ED(50) values of five macromolecules: polylysine, polyarginine, protamine, low-density lipoprotein (LDL), and high-density lipoprotein (HDL), using heparins of different size, as well as chondroitin 4-sulfate and dermatan sulfate as the GAG polyanions. The applicability of the assay method is further extended to study GAG-peptide interactions. A variety of small synthetic peptides (8-13 amino acid residues) derived from the heparin-binding domains of protamine and type IV collagen are used as model peptide species. Relative GAG-binding affinities of these macromolecules/peptides are compared to previous literature values and data obtained via a new electrode-based titration method.

Thrombogenic properties of untreated and poly(ethylene oxide)-modified polymeric matrices useful for preparing intraarterial ion-selective electrodes

In vitro platelet adhesion studies are used to compare the thrombogenic properties of various polymer matrices useful for preparing implantable ion-selective membrane electrodes. Conventional plasticized poly(vinyl chloride) and alternate polyurethane materials (Tecoflex, Pellethane) doped with proton- (tridodecylamine) and potassium-selective (valinomycin) ionophores are shown to be potentially thrombogenic. Incorporation of high molecular weight block copolymers of poly(ethylene oxide) and poly(propylene oxide) (e.g., Pluronic F108 and Tetronic 1508) within ion-selective membranes reduces platelet adhesion. A more marked decrease in platelet adhesion is, however, observed when the Tecoflex-based membranes are coated with a thin photo-cross-linked layer of poly(ethylene oxide). Such surface-modified membranes are shown to retain potentiometric ion response properties (i.e., selectivity, response times, response slopes, etc.) essentially equivalent to untreated membranes.

Novel nonseparation sandwich-type electrochemical enzyme immunoassay system for detecting marker proteins in undiluted blood

A novel nonseparation electrochemical enzyme immunoassay (NEEIA) for detecting marker proteins in undiluted blood is described. The approach is based on preferential electrochemical measurement of surface-bound enzyme-labeled reporter antibody (E-Ab), relative to an excess of this reagent in the sample solution. NEEIAs are carried out on microporous membranes coated with a thin, circular area of gold. The gold serves simultaneously as a working electrode and solid phase for immobilized capture anti-protein antibodies. In the assay, analyte protein is incubated concurrently with the Ab-coated gold surface and excess E-Ab conjugate. Detection of bound E-Ab is achieved by introducing the substrate for the enzyme through the back side of the membrane. The product of bound E-Ab is detected immediately by oxidation or reduction at the gold electrode, and the resulting current is proportional to the concentration of protein in the sample. The feasibility of the NEEIA approach is demonstrated via the detection of prostate-specific antigen in undiluted plasma samples (n = 64), with alkaline phosphatase as the label. Use of multiple gold films deposited on the same porous membrane to perform simultaneous NEEIAs is also described.

Novel nonseparation sandwich-type electrochemical enzyme immunoassay system for detecting marker proteins in undiluted blood

A novel nonseparation electrochemical enzyme immunoassay (NEEIA) for detecting marker proteins in undiluted blood is described. The approach is based on preferential electrochemical measurement of surface-bound enzyme-labeled reporter antibody (E-Ab), relative to an excess of this reagent in the sample solution. NEEIAs are carried out on microporous membranes coated with a thin, circular area of gold. The gold serves simultaneously as a working electrode and solid phase for immobilized capture anti-protein antibodies. In the assay, analyte protein is incubated concurrently with the Ab-coated gold surface and excess E-Ab conjugate. Detection of bound E-Ab is achieved by introducing the substrate for the enzyme through the back side of the membrane. The product of bound E-Ab is detected immediately by oxidation or reduction at the gold electrode, and the resulting current is proportional to the concentration of protein in the sample. The feasibility of the NEEIA approach is demonstrated via the detection of prostate-specific antigen in undiluted plasma samples (n = 64), with alkaline phosphatase as the label. Use of multiple gold films deposited on the same porous membrane to perform simultaneous NEEIAs is also described.

Potentiometric anion selectivity of polymer membranes doped with palladium organophosphine complex

The potentiometric anion selectivity of polymer membrane-based electrodes formulated with a palladium organophosphine complex (benzylbis(triphenylphosphine)palladium(II) chloride) as the membrane active component is examined. The electrode is shown to exhibit a non-Hofmeister selectivity pattern with a significantly enhanced response toward nitrite over the concentration range of 10 microM-10 mM (log-linear range) and a detection limit 5.0 microM. The effect of lipophilic anionic (tetraphenylborate derivatives) and cationic (tetraalkylammonium) site additives within the membrane on the anion selectivity is examined in detail. Addition of both cationic and anionic sites is shown to improve potentiometric anion selectivity, suggesting that the palladium complex may operate simultaneously as a neutral and charged carrier-type ionophore within the polymer membrane phase. Using optimal membrane formulations (with added 20-30 mol % cationic sites), the sensors prepared with the palladium complex do not exhibit proton/hydroxide response in the range of pH 3.5-12, a potential advantage over previously reported nitrite electrodes prepared with Co(III) corrins and porphyrin complexes.

Clinical application of disposable heparin sensors. Blood heparin measurements during open heart surgery

The authors previously reported the development of an ion selective electrode type heparin sensor consisting of a specially formulated polymer membrane doped with tridodecylmethylammonium chloride as the heparin complexing agent. They also demonstrated the feasibility of measuring blood heparin levels by protamine titration, using a disposable copper wire sensor coated with the heparin sensing membrane to probe the titration end point. In this article, the results of further titration studies conducted on 44 clinical whole blood specimens obtained from 8 patients undergoing open heart surgery were reviewed. Samples were taken from patients at four different stages during the bypass surgery: 1) before heparin administration; 2) immediately after heparin administration; 3) within 30 min to 3 hr after heparin administration; and 4) within 30 min after protamine administration. Heparin anticoagulant activity in these samples was monitored by the activated clotting time assay, whereas heparin concentrations were measured by protamine titration using either the Hepcon HMS Titrator (Medtronic HemoTec Inc., Englewood, CO) or the coated wire heparin sensor to determine titration end points. Results indicate that heparin levels determined by the sensor method were in good agreement with those determined by the Hepcon HMS Titrator. When the heparin concentrations estimated by the two methods show significant discrepancy (> 1.0 unit/ml), the sensor method seems to provide more precise values, as verified by an additional chromogenic heparin assay. The overall time required to complete the titration process and heparin measurement with a pre made heparin sensor was less than 3 min. Clinically, the heparin sensor could be used as a safeguard to precisely monitor heparin levels during surgical procedures. Alternatively, the sensor could be used to assess the accurate protamine dose required for full heparin reversal.

Optical detection of macromolecular heparin via selective coextraction into thin polymeric films

Thin plasticized polymer films, poly(vinyl chloride) doped with a specific ion pairing quaternary ammonium compound, tridodecylmethylammonium chloride, and a lipophilic pH indicator, 3-hydroxy-4-(4-nitrophenylazo)phenyl octadeconate, are shown to exhibit significant and analytically useful optical response toward macromolecular heparin. The response mechanism is based on favorable extraction of heparin into the bulk organic film, owing to the specific ion-pairing complexation reaction between the quaternary ammonium species and the polyanion. A simultaneous coextraction of hydrogen ions results in protonation of the pH chromophore and hence a change in the optical absorbance of the polymeric film. When used in a limited volume/fixed exposure (10 min) detection mode, film absorbances change as a function of the initial heparin concentration in the range of 0.2-3.0 units/mL (1.2-18 micrograms/mL). The practical measurement response time is controlled by heparin diffusion through the stagnant diffusion layer adjacent to the surface of the film as well as within the bulk of the polymer film and is shown to increase with the molecular weight of the heparin species tested. No optical response to heparin is observed when a strong heparin complexing agent (e.g., protamine) is present in the test solution, suggesting that the polymer film can be used to conveniently monitor heparin-protamine (or other antagonist) titrations. The theory relating to the operation of the sensing film in either the equilibrium or the kinetic mode and the selectivity of the optimized film to heparin relative to small anions are presented.

Protamine-sensitive polymer membrane electrode: characterization and bioanalytical applications

A polymeric membrane electrode that exhibits significant and analytically useful potentiometric response to submicromolar levels of the heparin antagonist, protamine, is reported. The sensor is prepared by incorporating a lipophilic cation exchanger, potassium tetrakis(4-chlorophenyl)borate (KTpClPB) (at 1 wt%), within a specially formulated polymer membrane composed of 33 wt% 2-nitrophenyl octyl ether (2-NPOE), and 66 wt% poly(vinyl chloride) (PVC). When the polymer film is mounted in an appropriate electrode body, the resulting membrane electrode responds reproducibly to protamine via a nonequilibrium quasi-steady-state change in the phase boundary potential at the membrane/sample interface. Such response can be used to directly monitor, via classical potentiometric titrations, the binding between protamine and a variety of native (porcine and beef) as well as low-molecular-weight heparins. Scatchard analysis of the EMF titration data provides binding constants and stoichiometries for protamine-heparin interactions. The electrode can be further used to follow the enzymatic digestion of protamine by trypsin. In the presence of a given level of protamine, initial rates of potential decrease (-dE/dt) are shown to be linearly related to trypsin activity in solution over the range of 0-130 units/ml. The speed and simplicity of the protamine sensor make it an attractive alternative to classical methods for studying the interaction of protamine with other biologically important macromolecules as well as the proteolytic activity and reaction kinetics of trypsin.

Selectivity of polymer membrane-based ion-selective electrodes: self-consistent model describing the potentiometric response in mixed ion solutions of different charge

Despite its well-documented limitations, the semiempirical Nicolsky-Eisenman equation is used throughout the existing analytical literature to describe the selectivity of modern polymer membrane-based ion-selective electrodes (ISEs). In this paper, a new quantitative description for the response/selectivity function based on ion-extraction equilibria at the sample/membrane interface is presented. The proposed selectivity formalism clearly illustrates the range of validity for the conventional Nicolsky-Eisenman formalism. Extended equations are derived describing the electrode response in an exact manner, particularly with respect to analyte and interfering ions of different charge. The expression obtained corresponds to the matched potential method proposed previously by Christian and co-workers on the basis of solely empirical observations. Selectivity coefficients required for a given analytical problem with a predefined maximum error can now be predicted more accurately. Such predictions with respect to analyte and interfering ions of varying charges differ by 1-2 orders of magnitude in comparison to the selectivity values required on the basis of the extended Nicolsky-Eisenman formalism.

Response mechanism of polymer membrane-based potentiometric polyion sensors

The potentiometric response mechanism of a previously reported polymer membrane-based electrode sensitive to the polyanion heparin is established. Based on transport and extraction studies, the heparin response is attributed to a nonequilibrium change in the phase boundary potential at the sample/membrane interface. While true equilibrium polyion response, obtained for low heparin concentrations only after very long equilibration times (> 20 h), yields the expected Nernstian response slope of < 1 mV/decade, the observed large and reproducible EMF response to clinically relevant heparin concentrations (approximately 10(-7) M) during typical measurement periods (2-5 min) is ascribed to a steady-state kinetic process defined by the flux of the polyion both to the surface and into the bulk of the polymer membrane. A model describing this nonequilibrium response is presented. With this model, the uniqueness of the polymer membrane composition (e.g., very low plasticizer content, strictly controlled cationic site concentration, etc.) required to achieve analytically useful heparin response becomes clear. Practical working conditions and limitations of the sensor are discussed. To support the generality of the steady-state model proposed, corresponding EMF response data for a newly developed membrane electrode sensitive to a polycationic protein (protamine) are also presented. It is shown that the protamine-responsive membrane electrode appears to operate via the exact same kinetic mechanism as the heparin sensing system.

A disposable, coated wire heparin sensor

The development of an ion-selective electrode heparin sensor consisting of a specially formulated polymer membrane doped with tridodecylmethylammonium chloride as the heparin complexing agent was recently reported. Because of the simple nature of the membrane technology used, the authors envisioned that the sensor could be configured as a disposable single-use device for rapid clinical or bedside measurement of heparin in a small, discrete sample. To explore this possibility, an inexpensive, disposable heparin sensor was created by dip-coating a copper wire with the specially formulated heparin-sensing polymeric membrane. Coated wire heparin sensors with a broad range of membrane thicknesses, prepared by repeatedly dipping the wire in the membrane solution for various times, were examined. Data show that increasing the membrane thickness of the sensor to a certain degree (more than 10 microns) enhanced the sensor's potentiometric response to heparin, although the time required to achieve 90% of the steady-state potential change was also prolonged. In addition, increasing membrane thickness also magnified the stirring effect on the sensor's response. In undiluted plasma samples, the coated-wire sensor with an optimized membrane thickness yielded a significant (5 to 30 mV) and reproducible response to heparin in a clinically relevant concentration range (0.5 to 12 units/ml, respectively). The clinical utility of the coated wire heparin sensor was shown using the sensor during protamine titration of heparinized plasma to assess the titration end-point. Preliminary results showed that the titration end-points determined by the heparin sensor strongly correlated with those determined by the activated partial thromboplastin time clotting assay. The overall time requirement to complete the titration process using a set of prefabricated coated wire heparin sensors, however, was less than 3 minutes. Further titration studies using undiluted clinical whole blood samples are in progress.

Separation-free sandwich enzyme immunoassays using microporous gold electrodes and self-assembled monolayer/immobilized capture antibodies

A novel separation-free sandwich-type enzyme immunoassay for proteins is performed by designing an electrochemical detection system that enables preferential measurement of surface-bound enzyme-labeled antibody relative to the excess enzyme-labeled reagent in the bulk sample solution. In this initial model system, the assay is carried out using gold-coated microporous nylon membranes (pore size 0.2 micron) which are mounted between two chambers of a diffusion cell. The membrane serves as both a solid phase for the sandwich assay and the working electrode in the three-electrode amperometric detection system. The capture monoclonal antibody is immobilized covalently on the gold side of the membrane via a self-assembled monolayer of thioctic acid. In the separation-free sandwich assay, both model analyte protein (human chorionic gonadotropin; hCG) and alkaline phosphatase labeled anti-hCG (ALP-Ab) are incubated simultaneously with the immobilized capture anti-hCG antibody. Surface-bound ALP-Ab is spatially resolved from the excess conjugate in the bulk sample solution by introducing the enzyme substrate (4-aminophenyl phosphate) through the back side of the porous membrane. The substrate diffuses rapidly through the porous membrane where it first encounters bound ALP-Ab at the gold surface. The enzymatically generated product, aminophenol, is detected immediately by oxidation at the gold electrode (at +0.19 V vs Ag/AgCl), and the magnitude of current is directly proportional to the concentration of hCG in the sample. The response time after substrate addition is less than 1 min, although maximum response toward the analyte protein requires a sample/conjugate preincubation time of 30 min with the porous electrode.(ABSTRACT TRUNCATED AT 250 WORDS)