Selective Microsensor Research Articles

All-Solid-State PVC-Membrane Cu(II)-Selective Potentiometric Microsensor Based on a Novel Calix[4]arene Derivative

In this study, 25,27-dihydroxy-26,28-bis(ethoxycarbonylbutyoxy)-5,11,17,23-p-tert-butylcalix[4]arene was used as an ionophore for the preparation of a novel all-solid-state type microsensor to make a selective potentiometric response towards Cu(II) ions. The optimum membrane composition of the microsensor was determined as 2% (w/w) ionophore, 67% (w/w) bis (2-ethylhexyl) sebacate (DOS), 30% (w/w) polyvinylchloride (PVC), and 1%(w/w) potassium tetrakis 4-chlorophenyl borate (KTpClPB). The Cu(II)-selective microsensor exhibited a highly selective Nernstian response of 31.3±2.1 mV slope (R2: 0.997) with a short response time (15 s) over the concentration range of 10−5 to 10−1 mol L−1 of Cu(II) ions for 8 weeks with no significant difference in potentials. The microsensor was effectively performed in the pH range of 5.0–8.0, with low detection limit as 7.63×10−7 mol L−1, and a temperature range of 15−40 °C. The prepared microsensor was successfully used both as an indicator electrode in the potentiometric titration of Cu(II) ions with EDTA and also in the determination of the Cu(II) content of environmental water samples. When the potentiometric results were compared with ICP-OES, they were found to be in good agreement with ICP-OES results at a 95% confidence level.

Dinuclear Pb(II) monomer complex: Synthesis, characterization, and application of Pb(II) ion-imprinted polymer as a selective potentiometric microsensor

A Pb(II) centered [Pb2(maa)4(vim)2] complex was synthesized using methacrylic acid (maaH) and 1-vinylimidazole (vim) containing vinylic groups suitable for polymerization. The structure of the Pb(II) monomer complex was elucidated by single-crystal X-ray diffraction method, fourier transform infrared spectroscopy (FT-IR), and thermogravimetric analysis (TGA). A new Pb(II) ion-imprinted polymer (Pb(II)-IIP) was prepared with [Pb2(maa)4(vim)2] complex as functional monomer, ethylene glycol dimethacrylate (EGDMA) as crosslinker and 2,2′-azobisisobutyronitrile (AIBN) as initiator, to use in a Pb2+ selective sensor design. The prepared Pb(II)-IIP was characterized by FT-IR, TGA, energy-dispersive X-ray spectroscopy (EDX), and scanning electron microscopy (SEM). Pb(II)-IIP was used as an ionophore in the structure of polyvinylchloride (PVC) membrane of the all-solid-state ion-selective microsensor to obtain a selective potentiometric response for Pb2+ ions. The Pb2+-selective microsensor exhibited a highly selective Nernstian response over a wide linear concentration range of 10−6 to 10−2 mol.L−1 with a slope of 29.4 mV per decade (R2 = 0.9987) in the pH range of 3.0 to 7.0. The response time of the microsensor was very short (less than 15 s) and the detection limit was determined as 8.30 × 10−8 mol.L−1. The lifetime of the microsensor was calculated as 45 days.

Highly Selective Microsensor for Monitoring Trace Phosphine in the Environment

Monitoring P flux at the Earth's surface-atmosphere interface has many challenges. Therefore, the development of a technology with high selectivity and high sensitivity to in situ trace PH3 in aquatic or sedimentary environments has become a priority. Herein, an amperometric PH3 microsensor meeting the above conditions is developed. The sensor is equipped with a Au-coated Pt working electrode (WE) and a Pt guard electrode (GE) positioned in an outer glass casing. The WE and GE are polarized at a fixed value of +150 mV with respect to a pseudo-reference electrode. The outer casing is filled with an acid electrolyte solution, and the tip is sealed using a thin silicone membrane. Mixed gases from the environment diffuse through the first layer of the silicone membrane, and the major H2S disruptor is eliminated by a ZnCl2-propylene carbonate trap positioned in the front of the microsensor. Later, the gases diffuse into an electrolytic solution through the second layer of the silicone membrane, and PH3 is selectively oxidized into H3PO4 on the Au-coated Pt WE. This electrochemical oxidation thereby creates a current that is proportional to the concentration of PH3 (>2 nmol·L-1). With the aid of the H2S trap casing and selective catalysis, the effects of other gases on the microsensor can be ignored in terms of environmental monitoring. An example from the sedimentary profile shows that high PH3 accumulations are found 13 mm below the sediment surface.

An electrochemical investigation into the effects of local and systemic administrations of sodium nitroprusside in brain extracellular fluid of mice

Sodium nitroprusside (SNP) is a nitric oxide (NO)-donor drug used clinically to treat severe hypertension, however, there are limitations associated with its mechanism of action that prevent widespread adoption. In particular, its impact on cerebral hemodynamics is controversial and direct evidence on its effects are lacking. Electrochemical methods provide an attractive option to undertake real time neurochemical measurements in situ using selective microsensors. Herein, we report the novel application of an existing platinum (Pt)-Nafion® sensor to measure the release of NO from SNP under in vitro and in vivo conditions. Initially, the temporal release of NO was measured and the effect of the reducing agent, ascorbic acid (AA), was elucidated in vitro. A combined microdialysis/NO sensor construct was implanted into the striatum of anaesthetised mice and the local perfusion of 10 mM SNP with/without AA resulted in increased NO concentration detected using the Pt-Nafion® sensor. Subsequently, the NO sensor, coupled with carbon paste electrodes (CPEs) for the electrochemical measurement of O2, were applied to investigate SNP effects in freely moving mice. A complex mechanism of action was identified that infers NO inhibition and biphasic O2 dynamics. The preliminary findings within support a strong cerebrovascular effect of systemic SNP administration that warrants careful consideration for clinical use.

A novel copper selective sensor based on ion imprinted 2-vinylpyridine polymer

A novel potentiometric Cu2+-selective microsensor has been developed that is based upon ion imprinted 2-vinylpyridine polymer. The polymer was synthesized using Cu(II) ions, 2-vinylpyridine, methacrylic acid, and ethylene glycol dimethacrylate as a template, functional monomer, and cross-linker, respectively. The resultant polymer was used as ionophore to obtain a selective potentiometric response towards Cu(II) ions in the structure of the PVC membrane. The detection limit of the microsensor was determined to be 8.4 × 10−7mol/L, and its response time was considerably short (less than 15 s). The prepared microsensor exhibited a near-Nernstian response for Cu(II) ions over the concentration range of 10−1to 10−6mol/L, with a slope of 28.5 mV per decade over 2 months, and without any considerable divergence in potentials. The microsensor was effectively performed in a pH range between 4.0 and 7.0 and used as an indicator electrode in the potentiometric titration of Cu(II) ions with EDTA. The proposed microsensor has been successfully demonstrated for the determination of copper in a number of environmental water samples. The obtained potentiometric results were in good harmony with the results obtained by the AAS method.

Highly sensitive detection of hydrogen peroxide at a carbon nanotube fiber microelectrode coated with palladium nanoparticles

We report on a carbon nanotube (CNT) fiber microelectrode coated with palladium nanoparticles (PdNPs) and enabling electrochemical sensing of hydrogen peroxide (H2O2). The synergistic effects of the CNT fibers (good mechanical strength and large surface area) and of the PdNPs (high electrocatalytic activity) result in a microelectrode for H2O2 that exhibits a 2-s response time, a detection limit as low as 2 μM, a sensitivity of 2.75 A cm−2 M−1, and a linear response range from 2 μM to 1.3 mM (R = 0.9994). The sensor is also selective and not interfered by potentially competing species in biological fluids, thus representing an inexpensive but highly sensitive and selective microsensor for H2O2.

Molecularly imprinted poly[tetra(o-aminophenyl)porphyrin] as a stable and selective coating for the development of voltammetric sensors

Molecularly Imprinted poly[tetra(o-aminophenyl)porphyrin] is presented as an electrosynthesized coating for the development of selective microsensors. A carbon fiber microelectrode has been modified with this covering by cyclic voltammetry in the potential range from −0.15 to 1.0 V (scan rate 50 mV s−1) obtaining a microsensor able to recognize the template molecule. To carry out this study dopamine was chosen as model to work with, since this molecule presents electroactivity as it can be oxidized on carbon electrodes. The MIP sensor presented a linear response to the dopamine concentration in the range from 10−6 to 10−4 M with good repeatability (RSD 6.22%). The selectivity shown by the microsensor was found to be excellent as it was able to differentiate between different catecholamines. A method has been developed for the determination of dopamine in brain tissue samples using the MIP microsensor.

A parylene-protected nitrate selective microsensor on a carbon fiber cross section

This work describes a novel design for a potentiometric microsensor for nitrate (NO 3 −) ion based on doped polypyrrole (PPy(NO 3 −)) films. First, 6–7 μm diameter single carbon fiber filaments are coated with a thin (1–10 μm) insulating layer of parylene C (poly( o-chloro- p-xylylene)). This preparatory step is followed by electrodeposition of a PPy(NO 3 −) sensing layer on the open carbon fiber cross section. Parylene C provides an excellent protective coating for the fiber while maintaining its flexibility at micro-dimensions. Examination by scanning electron microscopy confirms that the sensing material (PPy(NO 3 −)) is deposited in the form of micro-beads with diameter of 30–40 μm at the fiber tip. The resulting nitrate sensor exhibits an improvement of approximately half an order of magnitude of NO 3 − concentration in sensitivity and linear response range relative to a previously reported PPy(NO 3 −)-based sensor for which the sensing layer was deposited on the carbon fiber walls. The overall sensor geometry, a micro-dimensioned sensing portion accompanied by a flexible, parylene-insulated body, forms a sensor for nitrate with a unique form factor that can be widely used for in situ measurements of variety of environmental samples.

Development and environmental application of a nitrate selective microsensor based on doped polypyrrole films

This work describes the development and testing of a sensitive and selective potentiometric nitrate microsensor based on doped polypyrrole films. Utilizing 6–7 μm carbon fibers as a substrate for pyrrole electropolymerization allowed fabrication of flexible, miniature and inexpensive sensors for in situ monitoring of nitrate. The sensors have a rapid response (several seconds) and in their characteristics are competitive with expensive commercial nitrate ion selective electrodes (ISE), exhibiting Nernstian behavior (slopes 54 ± 1 mV per log cycle of nitrate concentration ( n = 8), at T = 22 °C), a linear response to nitrate concentrations spanning three orders of magnitude (0.1–10 −4 M or 6200–6.2 ppm of NO 3 −), and a detection limit of (3 ± 1) × 10 −5 M (1.25–2.5 ppm). After a 2-month-period, the response was unchanged, and after 4.5 months, one version of the electrode continued to exhibit significant sensitivity to nitrate. Several polypyrrole nitrate microsensors were embedded sequentially downstream from a point source of nitrate solution in an intermediate scale physical groundwater model as a test of their performance under simulated environmental conditions. The microsensors responded appropriately to the approaching nitrate solution front, demonstrating dispersion and attenuation of the nitrate concentrations that increased with distance from the source.

Development of Metal-Based Microelectrode Sensor Platforms by Chemical Vapor Deposition

A novel approach to the fabrication of metal mircoelectrodes is described that employs chemical vapor deposition (CVD) techniques and electrode modification strategies. Two specific examples for the fabrication of copper and silver microelectrodes are described, which utilize a combination of CVD coating, electroetching and electroplating. Initially, a 25 μm diameter tungsten wire is concentrically coated with an insulating layer of silica with excellent adhesion between the two surfaces. Tungsten metal is then readily removed by electrochemical etching in hydroxide solution to afford a well-defined microcavity at the electrode tip. For the purpose of demonstrating the concept of introducing specific selectivity into the device as well as fabricating well-defined metal microelectrodes, copper and silver were electrodeposited into the microcavity from aqueous solutions of the respective cations. Both modified electrodes were characterized by electrochemistry and microscopy, and subsequently used as electrochemical sensors for the detection of glucose and chloride ions. This methodology offers a facile approach to the development of highly selective microsensors with well-defined electrode sensing elements.

Continuous Observation of Nitric Oxide Production in the Fetal Rat Brain during Uteroplacental Ischemia

Objectives: To demonstrate real-time changes in nitric oxide (NO) production within fetal rat brain during uteroplacental ischemia and subsequent reperfusion. Methods: Using a selective microsensor for NO, changes in NO electrocurrent in the brains of 10 fetal rats at gestational day 20 were observed during and after 30 min occlusion of uterine vessels in anesthetized pregnant rats. Results: The NO electrocurrent reached 397 ± 71.0% of the control level 30 min after occlusion and increased throughout the observation (p < 0.05) until placental administration of 1 M of L-NAME. In contrast, no significant changes in NO electrocurrent were found in 7 sham operated rats. Conclusion: An NO-specific microelectric sensor detected excessive NO production by fetal rat brains in response to uteroplacental ischemia.

Chemically modified microelectrodes designed for the electrochemical determination of nitric oxide in biological systems

AbstractSince the identification of nitric oxide (NO) as an endothelial‐derived relaxing factor, several research groups are now working to develop an understanding of the mechanism by which this free‐radical gas is synthesized, diffused and reacted from and within various cells and different biological tissues. Thus, it appears that measuring NO in biological models is very difficult because of its low concentration and fleeting existence. Indeed, NO directly reacts extremely fast with superoxide and other very active free radicals of biological importance. Most of the techniques for assaying NO release use indirect methods for estimating endogeneous NO, relying on measurements of secondary species. In fact, NO in biological systems may be directly assayed by three nonelectrochemical strategies (EPR, spectrophotometry and chemiluminescence) but these strategies suffer from being ex situ detection techniques with poor selectivity or sensitivity. The desire to measure a small amount of NO release in situ has led to an active area of research involving the design of microsensors and new amperometric microelectrode probes are now developed to detect NO. The use of electrochemistry as a potential way to do so is very promising and the reality is that surface electrode modification is needed to make the electrode material selective for NO. Therefore, the design of modified electrode surfaces using organized layers combined with microelectrodes is attractive as possible selective microsensors for the electrochemical detection of NO. A number of new amperometric approaches have reached this goal. This article consists of a critical review of electrochemical methods for the detection and determination of nitric oxide in biological materials to clarify some aspects of the reported microsensors.

Electrochemical microsensor for the detection of Na+ ions

Abstract Thin vitreous films were elaborated by microelectronic techniques (vacuum evaporation, r.f. sputtering) and structural and electrical properties were characterized respectively by X-ray diffractometry and complex impedance spectroscopy measurements. Morphology and composition of thin films were examined by scanning electron microscopy with energy dispersive spectroscopic microprobe. Based on these thin films, Na + -selective microsensors were built up with a sodium aluminosilicate glass as the sensitive membrane and thin film of borophosphate as the ionic bridge acting as an interfacial layer providing an ionic and electronic exchange between the collector and the sensitive membrane. Electrochemical behaviour and sensitivity results regarding the microsensor were investigated. Microsensors exhibit near-Nerstian response in the Na + -concentration range of 10 −1 –10 −4 mol/l with a detection limit close to 5 × 10 −4 M and good reproducibility.