Hanne McPeak

The development of new microelectrode gas sensors: an odyssey. Part 1. O2 and CO2 reduction at unshielded gold microdisc electrodes

Abstract In this paper we describe the reduction of O 2 and CO 2 , when both molecules are always present and the balance gas is N 2 , at unshielded gold microdisc electrodes in dimethyl sulphoxide (DMSO) in a standard reaction cell. We show that both gases can be reduced in the presence of each other, in the range 3–15 vol.% CO 2 and 10–30 vol.% O 2 , with minimal cross-interference from “feedforward” and “feedback” reaction products from the individual O 2 and CO 2 reduction processes. Well-separated O 2 and CO 2 reduction processes were obtained with gold microdisc electrodes when the polarizing voltage was swept typically at 0.1–0.5 V s −1 , and steady state limiting currents were measured which were proportional to the O 2 and CO 2 concentrations.

The development of new microelectrode gas sensors: an odyssey. Part 2. O2 and CO2 reduction at membrane-covered gold microdisc electrodes

Abstract In a previous paper we described the reduction of O 2 and CO 2 , in the presence of each other, at unshielded microdisc electrodes in an aprotic solvent with minimal cross-interference between the competing O 2 and CO 2 reduction reactions. In this new work, we describe a practical Clark-type membrane-covered sensor for the simultaneous measurement of O 2 and CO 2 . The sensor comprises a gold disc microelectrode, housed in a PTFE holder and covered with a PTFE membrane, with dimethylsulphoxide (DMSO) as the solvent and with a silver quasi-reference electrode. The polarizing voltage is swept from 0 to −2.4 V vs. Ag at a typical sweep rate of 0.25 or 0.5 V s −1 and the O 2 and CO 2 concentrations are determined from the limiting currents of the well separated O 2 and CO 2 reduction waves. We demonstrate that the addition of up to 10% v/v H 2 O to the solvent does not appear to compromise the ability of the sensor to analyse O 2 and CO 2 simultaneously.

A fibre optic oxygen sensor that detects rapid PO2 changes under simulated conditions of cyclical atelectasis in vitro

Highlights • Real time detection of cyclical atelectasis is fundamental for individualised mechanical-ventilation therapy in ARDS.• Intra-arterial oxygen sensors could be used to detect the breath-by-breath oscillations in PO2 during cyclical atelectasis.• The fidelity with which oxygen sensors can detect these arterial PO2 oscillations depends on the sensors’ speed of response.• We present a system for testing fast-response fibre optic oxygen sensors under simulated conditions of cyclical atelectasis.• We show that a prototype fibre optic oxygen sensor, compatible with clinical use, can detect rapid PO2 changes in vitro.

The development of new microelectrode gas sensors : an odyssey. Part III. O2 and N2O reduction at unshielded and membrane-covered gold microdisc electrodes

In two previous papers we described: (a) the reduction of O2 and CO2, in the presence of each other, in an aprotic solvent with minimal cross-interference between the competing O2 and CO2 reduction reactions; (b) a practical Clark-type membrane-covered sensor for the simultaneous measurement of O2 and CO2. The sensor comprised a gold disc microelectrode, housed in a PTFE holder and covered with a PTFE membrane, with dimethyl sulphoxide as the solvent and with an Ag quasi-reference electrode. In this new work, we describe the reduction of O2 and N2O, in the presence of each other, with this same sensor when the microelectrode was first unshielded and then membrane-covered. No cross-interference from the two reactions was evident, and the problem N2O bubble build-up on the microelectrode surface, from the reduction of N2O, was minimised by sweeping the polarising voltage from 0 to −2.4 V (Ag) at a typical sweep rate of 0.1 to 0.5 V s−1. The O2 and N2O concentrations were determined from the limiting currents of the well-separated O2 and reduction N2O waves. We also demonstrated that the addition of up to 100% v/v H2O to the solvent did not appear to comprimise the ability of the sensor to analyse O2 and N2O simultaneously.

The development of new microelectrode gas sensors: an odyssey: Part IV1. O2, CO2 and N2O reduction at unshielded gold microdisc electrodes

Abstract In this work, we describe the reduction of O 2 , CO 2 and N 2 O, in the presence of each other, at an unshielded gold microdisc electrode, in dimethylsulfoxide (DMSO) as the solvent and with a Ag quasi-reference electrode. Carbon dioxide and nitrous oxide, when present on their own in N 2 , have been shown previously to be reduced at very similar potentials. So, it was not necessarily anticipated that gas mixtures containing both these two gases could be determined analytically. However, remarkably, a well resolved new CO 2 wave, which was ideal for analytical purposes, was observed under our experimental conditions in the presence of O 2 and N 2 O. Furthermore, and most importantly, no cross-interference from the O 2 , N 2 O, and the new CO 2 reactions, was evident when the N 2 O concentration was in excess of that of CO 2 (the normal anaesthetic expired gas and blood gas analysis situation). The O 2 , CO 2 and N 2 O concentrations were determined from the limiting currents of the well-separated waves.

Electrochemical reduction of the anaesthetic gas enflurane (2-chloro-1,1,2-trifluoroethyl difluoromethyl ether)

The electrochemical reduction of the inhalation anaesthetic agent enflurane is reported at a variety of microelectrode substrates (Au, Ag, Cu, Pt and glassy carbon) with electrode dimensions varying from 10 to 60 μm. The solvents water, dimethyl sulfoxide and acetonitrile were investigated along with the supporting electrolytes potassium chloride, tetrabutylammonium hexafluorophosphate (TBAHFP) and various tetraalkylammonium perchlorates. The use of a gold microelectrode with dimethyl sulfoxide solvent and tetraethylammonium perchlorate (TEAP) as the supporting electrolyte was found to give well-defined voltammetry. Linear calibration curves were obtained between 0 and 2% v/v (gaseous additions) or up to 135 mM (gravimetric additions), offering scope for the development of a rapid, inexpensive electrochemical gas sensor.

Experimental investigation of the effect of polymer matrices on polymer fibre optic oxygen sensors and their time response characteristics using a vacuum testing chamber and a liquid flow apparatus

Very fast sensors that are able to track rapid changes in oxygen partial pressure (PO2) in the gas and liquid phases are increasingly required in scientific research – particularly in the life sciences. Recent interest in monitoring very fast changes in the PO2 of arterial blood in some respiratory failure conditions is one such example. Previous attempts to design fast intravascular electrochemical oxygen sensors for use in physiology and medicine have failed to meet the criteria that are now required in modern investigations. However, miniature photonic devices are capable of meeting this need. In this article, we present an inexpensive polymer type fibre-optic, oxygen sensor that is two orders of magnitude faster than conventional electrochemical oxygen sensors. It is constructed with biologically inert polymer materials and is both sufficiently small and robust for direct insertion in to a human artery. The sensors were tested and evaluated in both a gas testing chamber and in a flowing liquid test system. The results showed a very fast T90 response time, typically circa 20 ms when tested in the gas phase, and circa 100 ms in flowing liquid.

Optimizing design for polymer fiber optic oxygen sensors

The development of a clinically useful fiber-optic oxygen sensor based on oxygen fluorescence quenching is described in this paper. The fiber optic oxygen sensor was formed by coating a thin polymer matrix, which contains an oxygen sensitive fluorophore, on the tapered end of a polymer optical fiber. Three acrylate polymers have been used for the matrix, and the sensitivity and time-response of the oxygen sensors were tested. The results showed that the sensitivity and time response of the sensors can be modified using different polymer matrices. Using these modifications, a very fast time response of the polymer fiber-based oxygen sensor could be readily achieved and the fastest T10-90 response time were <;100 ms.

Intra-breath arterial oxygen oscillations detected by a fast oxygen sensor in an animal model of acute respiratory distress syndrome.

Background There is considerable interest in oxygen partial pressure (Po2) monitoring in physiology, and in tracking Po2 changes dynamically when it varies rapidly. For example, arterial Po2 (PaO2) can vary within the respiratory cycle in cyclical atelectasis (CA), where PaO2 is thought to increase and decrease during inspiration and expiration, respectively. A sensor that detects these PaO2 oscillations could become a useful diagnostic tool of CA during acute respiratory distress syndrome (ARDS). Methods We developed a fibreoptic Po2 sensor (<200 µm diameter), suitable for human use, that has a fast response time, and can measure Po2 continuously in blood. By altering the inspired fraction of oxygen (FIO2) from 21 to 100% in four healthy animal models, we determined the linearity of the sensors signal over a wide range of PaO2 values in vivo. We also hypothesized that the sensor could measure rapid intra-breath PaO2 oscillations in a large animal model of ARDS. Results In the healthy animal models, PaO2 responses to changes in FIO2 were in agreement with conventional intermittent blood-gas analysis (n=39) for a wide range of PaO2 values, from 10 to 73 kPa. In the animal lavage model of CA, the sensor detected PaO2 oscillations, also at clinically relevant PaO2 levels close to 9 kPa. Conclusions We conclude that these fibreoptic PaO2 sensors have the potential to become a diagnostic tool for CA in ARDS.

Respiratory oscillations in alveolar oxygen tension measured in arterial blood

Arterial oxygen partial pressure can increase during inspiration and decrease during expiration in the presence of a variable shunt fraction, such as with cyclical atelectasis, but it is generally presumed to remain constant within a respiratory cycle in the healthy lung. We measured arterial oxygen partial pressure continuously with a fast intra-vascular sensor in the carotid artery of anaesthetized, mechanically ventilated pigs, without lung injury. Here we demonstrate that arterial oxygen partial pressure shows respiratory oscillations in the uninjured pig lung, in the absence of cyclical atelectasis (as determined with dynamic computed tomography), with oscillation amplitudes that exceeded 50 mmHg, depending on the conditions of mechanical ventilation. These arterial oxygen partial pressure respiratory oscillations can be modelled from a single alveolar compartment and a constant oxygen uptake, without the requirement for an increased shunt fraction during expiration. Our results are likely to contribute to the interpretation of arterial oxygen respiratory oscillations observed during mechanical ventilation in the acute respiratory distress syndrome.