بهره برداری از یک دیسک هیدروژل حساس به pH برای تشخیص CO2

In this paper is described how hydrogel is exploited as sensor material for the detection of carbon dioxide (CO2). A pH-sensitive hydrogel disk, which swells and deswells in response to pH changes, was clamped between a pressure sensor membrane and a porous metal screen together with a bicarbonate solution. CO2 reacts with the bicarbonate solution resulting in a pH change, which is converted into a pressure by the enclosed hydrogel. This pressure is a measure for the partial pressure of CO2. The main advantage of this sensor principle is the fact that a reference electrode as required for potentiometric sensors is no longer needed.

Hydrogels are cross-linked hydrophilic polymers that can contain a large amount of water [1] and [2]. By incorporating functional groups, a hydrogel can be made stimulus-sensitive. Such stimulus-sensitive hydrogels can undergo volume changes in response to changes in stimuli [3], [4], [5], [6] and [7]. These stimuli can be pH, temperature, light, ion concentration or electric field. Applications for stimulus-sensitive hydrogels have been mostly proposed in the biomedical field [3] and [4], e.g. for controlled drug delivery, but also for sensors and actuators [5]. Hydrogel-based sensors usually consist of a particular stimulus-sensitive hydrogel, which is used as sensing element, and a transducer to convert the swelling of the hydrogel to the optical or electrical domain. Conductometric, amperometric, optical and mechanical methods have been explored to measure the hydrogel swelling [5]. In this paper, a CO2 sensor is presented which makes use of a pressure sensor as transducer and a pH-sensitive hydrogel as sensing material. Fig. 1 shows a schematic representation of the sensor. A pH-sensitive hydrogel is placed in a bicarbonate solution enclosed by a porous cover and a pressure sensor. CO2 reacts with the bicarbonate solution, resulting in a pH decrease according to the Severinghaus principle [8]. In response to the pH decrease the pH-sensitive hydrogel starts to swell but since its volume is fixed by the porous cover, a pressure will be generated (isometric conditions). This pressure is a measure for the partial pressure of carbon dioxide. The advantage of this sensor is that a reference electrode, showing typical problems like drift, leakage and fouling, is no longer needed. In future, a CO2 permeable membrane must be added to complete the sensor. Full-size image (19 K) Fig. 1. Schematic representation of the hydrogel-based PCO2 sensor. Figure options The main application of the proposed sensor is to measure the partial pressure of carbon dioxide in the stomach. High PCO2 levels can indicate that a person has gastrointestinal ischemia caused by occlusion of arteries or veins, or by general circulatory failure resulting in splanchnic hypoperfusion (insufficient blood circulation in stomach and/or intestines) [10]. Because the CO2 measurements takes place in situ, miniaturization is required to be able to insert the sensor, applied on a catheter, in the stomach through the nose. Further possible applications are in the automobile industry, in horticulture and for environmental monitoring instruments.

A hydrogel-based PCO2 sensor is demonstrated where a thin hydrogel disk is enclosed between a pressure sensor and a porous screen. Experiments showed that the sensor responded to CO2. The pressure generated by the pH-sensitive hydrogel due to the acidification by CO2 agrees reasonably well with the pressure found by interpolation of the characterization plot pressure versus pH. The redesigned sensor responded almost four times faster than the previous design. The faster response was obtained by using a thin hydrogel disk, instead of microspheres. In future the use of thin layers hydrogel will be continued because they are easy to handle, simple to make, their dimensions are well-defined by photolithography and they generate a stable pressure signal. The apparent pKa of the hydrogel should be increased to obtain a larger pressure response around pH 8 and thus, higher sensor sensitivity. This can be done by adding solvent to the hydrogel to decrease the density of titratable groups. This will also make the hydrogel faster because less protons need to diffuse into the polymer. Miniaturization will reduce the sensor response time even more. Decreasing the thickness of the hydrogel layer results in a faster swell/deswell equilibrium but also in a decrease of generated pressure and thus, accuracy. A compromise should be found where both the response time and accuracy are acceptable. Miniaturization also involves the fabrication of a silicon porous cover since enclosing thinner hydrogels is hard to achieve by applying a metal screen by hand. Furthermore the sensor has to be completed with a CO2 permeable membrane.