Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2001-06-08
2004-06-08
Noguerala, Alex (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S415000, C204S435000, C204S431000
Reexamination Certificate
active
06746583
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a miniature, differential-type sensor being able to quickly measure levels of carbon dioxide dissolved in liquid solutions, which functions with a working electrode composed of unbuffered hydrogel and a pH-sensitive gas-permeable membrane, and a reference electrode composed of buffered hydrogel and a pH-sensitive gas-permeable membrane. More particularly, the present invention relates to the introduction of carbonic anhydrase into the unbuffered hydrogel to reduce the hydration time of carbon dioxide, thereby quickly measuring carbon dioxide levels, and the introduction of a controlled content of bicarbonate ions into the unbuffered hydrogel to improve the sensitivity of the sensor.
BACKGROUND OF THE INVENTION
Quantification of carbon dioxide levels can be applied to various fields. For example, it can be applied for analyzing clinical samples in the medical diagnostic fields, for regulating combustion processes in chemical analysis fields, for diagnosing the severity of the green house effect, and for measuring the indexes related to aquatic ecosystem in an environmental field. The exact measurement of dissolved carbon dioxide becomes increasingly significant.
In a human body, carbon dioxide is present in a small quantity as a metabolic end product, resulting mainly from the metabolism of foods. When the partial pressure of carbon dioxide in blood is 40 mmHg or higher, carbon dioxide is released from erythrocytes. The free carbon dioxide is dissolved in blood plasma and hydrated to form carbonic acid (H
2
CO
3
), which is in turn dissociated into hydrogen ion (H
+
) and bicarbonate ion (HCO
3
+
). Since the total content of carbon dioxide (CO
2
gas, H
2
CO
3
, HCO
3
−
, CO
3
2−
) in blood plasma affects the acid-base balance and pH of blood, as well as being an index for pulmonary ventilation, alveolar gas exchange capacity, and the quantity of the gas transferred to somatic tissues from blood, it is very important to accurately measure the levels of carbon dioxide dissolved in blood.
Carbon dioxide also plays an important role in the field of the ecosystem. For example, carbon dioxide is in an equilibrium state between air and water. In natural water, bicarbonate ions (HCO
3
−
) act as a buffering reagent and keep the pH constant. As the concentration of carbon dioxide in the water is closely related to the health of aquatic ecosystems, it is important to accurately measure levels of carbon oxide dissolved in water in order to detect changes of the aquatic ecosystem.
For measuring concentrations of carbon dioxide, two types of carbon dioxide gas sensors are known: a Severinghaus-type carbon dioxide gas sensor, wherein an external reference electrode, a pH-sensitive electrode and a gas-permeable membrane are simultaneously housed in one sensor body; and a differential-type carbon dioxide gas sensor, wherein a working electrode and a reference electrode are separated in different sensor body.
As shown in
FIG. 1
, the Severinghaus-type carbon dioxide gas sensor comprises an external reference electrode
14
, a working electrode
16
with pH-sensitive membrane
11
, a gas-permeable membrane
12
, and an unbuffered solution
13
in one sensor body
17
. When the Severinghaus-type carbon dioxide gas sensor is immersed in a sample solution
18
of interest, a potential is generated by the sensor and displayed on a voltmeter
15
.
Such a Severinghaus-type carbon dioxide gas sensor suffers from difficulty in the fabrication and miniaturization of the sensor, because the reference electrode is incorporated inside the sensor. In addition, another disadvantage of Severinghaus-type carbon dioxide gas sensor is that it cannot be used when the level of carbon dioxide is low. That is, at a low level of carbon dioxide, the Severinghaus-type carbon dioxide sensor is so slow in sensing rate and recovery rate that it cannot be used in automatic gas sensing systems. Furthermore, Severinghaus-type carbon dioxide gas sensor suffers from the disadvantage of being poor in detection limit.
FIG. 2
illustrats a differential-type carbon dioxide gas sensor, characterized in that a working electrode is separated from a reference electrode. The differential-type carbon dioxide gas sensor comprises a working electrode
20
composed of an unbuffered inner reference solution
13
and a pH-sensitive gas-permeable membrane
19
; and a reference electrode
21
composed of a buffered inner reference solution
22
and the same pH-sensitive gas-permeable membrane
19
as that in a working electrode.
In the differential-type carbon dioxide gas sensor, charge separation and the accompanying potential difference occur at 4 different sites: E
outer1
between the pH-sensitive gas-permeable membrane
19
of the working electrode
20
and the sample solution
18
; E
outer2
between the pH-sensitive gas-permeable membrane
19
of the reference electrode
21
and the sample solution
18
; E
inner1
between the pH-sensitive gas-permeable membrane
19
of the working electrode
20
and the unbuffered inner reference solution
13
; and E
inner2
between the pH-sensitive gas-permeable membrane
19
of the reference electrode
21
and the buffered inner reference solution
22
. When such charge separations occur, E
outer1
and E
outer2
have the same value and thus can be counterbalanced, as the same pH-sensitive gas-permeable membranes are used. On the other hand, the charge separation E
inner2
between the pH-sensitive gas-permeable membrane
19
of the reference electrode
21
and the buffered inner reference solution
22
is maintained at a constant value as the reference solution is buffered. Therefore, a change in carbon dioxide levels in a sample solution
18
causes only the charge separation E
inner1
between the pH-sensitive gas-permeable membrane
19
and the unbuffered inner reference solution
13
of the working electrode
20
, so that the resulting potential change enables the carbon dioxide levels of the sample solution to be quantitatively detected.
However, the conventional differential-type carbon dioxide gas sensor also suffers from long period of response time for sensing and recovery at a low level of carbon dioxide. Also, its detection limits are still not satisfactory. However, the differential-type carbon dioxide gas sensors are easier to miniaturize than the Severinghaus-type one.
Microchip-based carbon dioxide gas sensor may be fabricated in small sizes since all parts thereof including electrolyte layers can be fabricated as a layered structure. It can be also used as a multi-sensor capable of detecting various ions and gas species simultaneously with one chip. Additionally, the mass production of the microchip-based sensor can be achieved, resulting in reduced production cost. Furthermore, the small size of its sensing site makes it possible to detect even a trace amount of samples.
SUMMARY OF THE INVENTION
In this invention, we combined two advanced technologies in an attempt to obtain a planar microchip-based carbon dioxide sensing device with faster-preconditioning and response characteristics for being dissolved carbon dioxide measurement in physiological samples: one is a differential sensing arrangement to facilitate the micro-fabrication of potentiometric carbon dioxide electrodes, and the other is the use of carbonic anhydrase to shorten total measurement time. The pH-sensitive polymeric membranes adapted for use in constructing a differential carbon dioxide sensor system in this work function as both a gas-permeable membrane and an internal pH-sensing element. In the differential configuration, the carbon dioxide electrode is made with an unbuffered recipient layer including carbonic anhydrase, hence the pH changes are promoted and detected. The reference electrode, on the other hand, employs a strongly buffered hydrogel layer; therefore diffused carbon dioxide cannot change the pH in the recipient layer. In addition, the pH and ion response signals of the outer membrane surfaces (on the sample side) at bo
Cha Geun Sig
Lee Jae Seon
Lee Min Hyung
Nam Hakhyun
Shin Jae Ho
Bachman & LaPointe P.C.
i-Sens Inc.
Noguerala Alex
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