Patent Publication Number: US-2006016698-A1

Title: Method and apparatus for electrochemical detection

Description:
This application claims the benefit of Taiwan Application No. 093121861, filed Jul. 22, 2004, which is herein incorporated by reference in its entirety.  
     BACKGROUND  
      I. Field of the Invention  
      The present invention relates generally to electrochemical detection, and, more particularly, to a method and apparatus for quantitatively determining the concentration of an analyte in a fluid sample.  
      II. Background of the Invention  
      In the field of biomedical techniques, biosensors have been developed to analyze human body fluids in order to diagnose potential diseases or monitor health condition. A biosensor is an analytical device that comprises at least a biological component for selective recognition of an analyte in a sample fluid and a transducer device for relaying biological signals for further analysis. For example, biosensors are typically used to monitor lactate, cholesterol, bilirubin and glucose in certain individuals. In particular, determination of the concentration of glucose in body fluids such as blood is of great importance to diabetic individuals, who must frequently check the level of glucose in their blood as a means of regulating the glucose intake in their diets and monitoring the effects of therapeutics. With proper maintenance of blood glucose through daily injections of insulin and strict control of dietary intake, the prognosis for diabetics is excellent for type-I patients. Since blood glucose levels must be closely followed in diabetic individuals, an ideal biosensor for the detection of glucose must be simple and easy to operate without compromising accuracy.  
      In electrochemistry, an interplay between electricity and chemistry concerns current, potential, and charge from an electrochemical reaction. There are generally two types of electrochemical measurements, potentiometric and amperometric. The potentiometric technique is a static technique with no current flow, which has been widely used for monitoring ionic species such as calcium, potassium, and fluoride ions. The amperometric technique is used to drive an electron-transfer reaction by applying a potential. A responsive current measured is related to the presence and/or concentration of a target analyte. Amperometric biosensors make possible a practical, fast, and routine measurement of test analyte.  
      The success in the development of the amperometric devices has led to amperometric assays for several biomolecules including glucose, cholesterol, and various drugs. In general, an amperometric biosensor includes an insulating base plate, two or three electrodes, a dielectric layer, and a region containing an enzyme as a catalyst and at least one redox mediator for introduction of electron-transfer during the enzymatic oxidation of the analyte. The reaction progresses when a sample liquid containing an analyte is added onto the reaction region. Two physical effects, mesh spread and capillary action, are commonly used to guide a uniform distribution of the applied sample on the reaction region. A controlled potential is then applied between the electrodes to trigger oxidoreduction. The test analyte is therefore oxidized and electrons are generated from the accompanying chain reaction of the enzyme and mediator. The applied electrical potential must be sufficient enough to drive a diffusion-limited electrooxidation, yet insufficient to activate irrelevant chemical reactions. After a short time of delay, the current generated by the electrochemical oxidoreduction is observed and measured and the current is correlated to the presence and/or amount of the analyte in the sample.  
      Examples of conventional techniques for amperometric detection can be found in U.S. Pat. No. 5,620,579 to Genshaw et al., entitled “Apparatus for Reduction of Bias in Amperometric Sensors” (hereinafter “the &#39;579 patent”), and U.S. Pat. No. RE. 36,268 to Szuminsky et al., entitled “Method and Apparatus for Amperometric Diagnostic Analysis” (hereinafter “the &#39;268 patent.) Each of these references proposes a different way to supply the potential to trigger the electrochemistry reaction. The &#39;579 patent discloses a method for determining the concentration of an analyte by applying a first potential, which is a burn-off voltage potential, to an amperometric sensor and then applying a second potential, which is a read voltage potential, to the amperometric sensor. A first current in response to the burn-off voltage potential and a second current in response to the read voltage potential are measured for calculating a bias correction value in order to enhance the accuracy of the analyte determination.  
      The &#39;268 patent discloses a method for quantitatively determining biologically important compounds in body fluids. The &#39;268 patent does not provide any voltage at an early stage of electrochemical reaction, avoiding unwanted power consumption at the early stage. After a span of time, a constant voltage is applied to a sample and a corresponding Cottrell current is measured.  
      The trend of new generations of biosensors focuses on the methodology of quick response time and higher resolution. It is desirable to have an apparatus or method for electrochemical detection that can achieve improved signal resolution and efficient power consumption for detection. It is also desirable to achieve detection by modifying the profile of the potential supplied to trigger the electrochemistry reaction.  
     BRIEF SUMMARY OF THE INVENTION  
      The present invention is directed to an apparatus and method that may enhance electrochemical reaction and achieve improved signal resolution. The present invention proposes a potential profile that comprises a voltage bias and an alternating part such as a sinusoidal wave to trigger the electrochemistry reaction. By supplying the potential profile, the electrochemical reaction is enhanced and results in improved signal resolution. In accordance with an embodiment of the present invention, there is provided a method for quantitatively determining an analyte that comprises adding a sample fluid containing an analyte to an electrochemical cell that includes an enzyme, applying a potential profile to the electrochemical cell, measuring a current signal for a period of measuring time through the electrochemical cell, and correlating the current signals with the concentration of the analyte.  
      Further in accordance with the present invention, there is provided an apparatus for measuring the amount of an analyte in a sample fluid that comprises a holder for holding an electrochemical cell that includes a catalyst, a waveform generator for generating a potential profile, wherein the potential profile comprises a voltage bias and an alternating part, a detector for detecting a current signal for a period of measuring time through the electrochemical cell, a memory for storing the current signal detected in the period of measuring time, and a processor for correlating the current signal with a concentration of the analyte.  
      Additional features and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The features and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.  
      It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.  
      The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one embodiment of the present invention and together with the description, serves to explain the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Reference will now be made in detail to the present embodiment of the invention, an example of which is illustrated in the accompanying drawings.  
      Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like parts.  
       FIG. 1  is a block diagram of a system for determining the concentration of an analyte contained in a sample fluid in accordance with one embodiment of the present invention;  
       FIG. 2  is a schematic diagram of an apparatus for measuring the concentration of an analyte in accordance with one embodiment of the present invention;  
       FIG. 3A  is a plot showing an experimental result of applying a constant voltage to a sample fluid containing an analyte at various concentration levels;  
       FIG. 3B  is a plot showing an experimental result of applying a potential profile to a sample fluid containing an analyte at various concentration levels in accordance with one embodiment of the present invention;  
       FIG. 3C  is a plot showing a comparison between experimental results of applying to a sample fluid a constant voltage and a potential profile;  
       FIG. 4  is a plot illustrating methods for processing a current signal in accordance with one embodiment of the present invention; and  
       FIG. 5  is a flow diagram showing a method for correlating a current signal with a concentration of an analyte in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  is a block diagram of a system  10  for determining the concentration of an analyte in a sample fluid in accordance with one embodiment of the present invention. The sample fluid includes, but not limited to, blood, lymph, saliva, vaginal and anal secretions, urine, feces, perspiration, tears, and other bodily fluids. Referring to  FIG. 1 , system  10  includes a microprocessor  12 , a waveform generator  14 , a cell  20 , a detector  21 , and a memory  26 .  
      A potential profile is set to trigger an electrochemical reaction in cell  20 . The potential profile comprises a voltage bias and an alternating part. The alternating part, having an amplitude and transmitting at a frequency, includes one of a sinusoidal wave, a triangular wave, a square wave, or a combination thereof. A volume of a test sample containing an analyte of a concentration is added to cell  20 . Microprocessor  12 , in response to the application of the test sample, enables waveform generator  14  to generate a potential in accordance with the designed profile. Various commercially available data acquisition apparatuses, such as a DAQ card manufactured by National Instruments (Austin, Tex.), can be used as waveform generator  14 . In one embodiment according to the present invention, a potential profile comprises a voltage bias of 0.4V (volts) and an alternating part, which is a sinusoidal wave having an amplitude of 0.1V and a frequency of 1 Hz (Hertz), in the case where glucose is selected as the analyte. In one aspect, the voltage bias includes a direct-current (dc) component having a constant value over a measuring period. In another aspect, the voltage bias includes a dc component which is time-varying over a measuring period. Moreover, in other embodiments according to the present invention where glucose is selected as the analyte, the voltage bias may have a value, either constant or time-varying, ranging from approximately 0.1V to 1.0V, and the sinusoidal wave may have an amplitude ranging from approximately 0.0V to 0.5V at a frequency ranging from 0.5 Hz to 100 Hz. The voltage bias, amplitude and frequency may change as cell  20  changes.  
      Although the embodiment directed towards the determination of glucose is discussed, skilled persons in the art will understand that the method and apparatus of the present invention can be used for the determination of other analytes upon selection of an appropriate catalyst such as an enzyme. Examples of the analytes include a substance metabolite such as glucose, cholesterol, triglyceride or latic acid, a hormone such as T4 or TSH, a physiological constituent such as albumin or hemoglobin, a biomarker including protein, lipid, carbohydrate, deoxyribonucleic acid or ribonucleic acid, a drug such as an antiepileptic or an antibiotic, or a non-therapeutic compound such as a heavy metal or toxin.  
      The potential profile generated by waveform generator  14  is applied to cell  20 . Cell  20 , an electrochemical cell where the electrochemical reaction takes place, contains an enzyme, which has been previously applied thereto. The electrochemical reaction occurs via at least one electron transfer agent. Given a biomolecule A, the oxidoreductive process is described by the following reaction equation:  
                 
 
      The biomolecule A is oxidized to B by an electron transfer agent C, in the presence of an appropriate enzyme. Then the electron transfer agent C is oxidized at an electrode of cell  20  
 
 C  (red)→ C ( ox )+ n e   −   (Equation 2) 
 
 where n is an integer. Electrons are collected by the electrode and a resulting current is measured. 
 
      Those skilled in the art will recognize there are many different reaction mechanisms that will achieve the same result. Equations 1 and 2 are non-limiting examples of such a reaction mechanism.  
      As an example, a glucose molecule and two ferricyanide anions in the presence of glucose oxidase produce gluconolacton, two ferrocyanide anions, and two protons by the following equation:  
                 
 
      The amount of glucose present is assayed by electrooxidizing the ferrocyanide anions to ferricyanide anions and measuring the charge passed. The process mentioned above is described by the following equation: 
 
[Fe(CN) 6 ] 4− →[Fe(CN) 6 ] 3−   +e   −   (Equation 4) 
 
      In a preferred embodiment of the invention, an appropriate enzyme for glucose is glucose oxidase, and the reagent in electrochemical cell  20  contains the following formulations: 600 u/ml of glucose oxidase, 0.4M of potassium ferricyanide, 0.1M of phosphate buffer, 0.5M of potassium chloride, and 2.0 g/dl of gelatin.  
      In another example, the amount of total cholesterol contained in a sample fluid, which may include cholesterol and cholesterol esters, is to be measured. Appropriate enzymes provided in cell  20  include cholesterol esterase and cholesterol oxidase. The cholesterol esters are hydrolyzed to cholesterol in the presence of cholesterol esterase, as given in an equation below.  
                 
 
      The cholesterol is then oxidized to cholestenone, as given in an equation below.  
                 
 
      The amount of total cholesterol is assayed by electrooxidizing the ferrocyanide anions to ferricyanide anions and measuring the charge passed. 
 
[Fe(CN) 6 ] 31  →[Fe(CN) 6 ] 3−   +e   −   (Equation 7) 
 
      Detector  21  detects an output current signal from cell  20 . Microprocessor  12  processes and analyzes the current signal, and correlates the processed current signal with the concentration of glucose. Methods for processing the current signal will be discussed in detail with reference to  FIG. 4 . Memory  26  stores the processed data and a current-concentration relationship under the same potential profile. System  10  may further include a display device (not shown) for display of the detection result.  
       FIG. 2  is a schematic diagram of an apparatus  40  for measuring the concentration of an analyte in accordance with one embodiment of the present invention. Referring to  FIG. 2 , apparatus  40  includes a holder  42 , a detector  43 , a waveform generator  44 , a microprocessor  45  and a memory  46 . Holder  42  receives and holds cell  20 . Memory  46  has been stored with, for example, a lookup table that specifies the concentration-current relationship between various concentrations of an analyte and corresponding current levels. Waveform generator  44  generates a potential profile having substantially the same profile as those used for establishing the concentration-current relationship. The potential profile is applied to cell  20 . Detector  43  detects a current signal provided from cell  20 . Microprocessor  45  processes the current signal and correlating the processed result with the concentration.  
      Cell  20  to be inserted to apparatus  40  includes conductive contacts  202 , and electrodes  204  and  206  electrically connected (not shown) to conductive contacts  202 . Electrodes  204  and  206  are disposed at a reaction region  208 , where an appropriate catalyst such as an enzyme for an analyte has been provided. When a sample liquid containing an analyte is added to cell  20  at reaction region  208 , the reaction involving the analyte and an electron transfer agent proceeds as previously described with respect to Equations 1 and 2. Later, when the potential profile from waveform generator  44  is applied to cell  20 , a current flow, generated as previously described with respect to Equations 2 and 4, is detected by apparatus  40 . The detected current level is compared with the lookup table stored in memory  46  by mapping, linear interpolation or other methods. An indicator  48  of apparatus  40  displays the glucose level for the sample liquid.  
       FIG. 3A  is a plot showing an experimental result of applying a constant voltage to a sample fluid containing an analyte at various concentrations. Referring to  FIG. 3A , a constant voltage of 0.4V is applied to sample fluids containing glucose at the concentrations of 230 mg/dl, 111 mg/dl, 80 mg/dl and 0 mg/dl, respectively. The glucose concentration of these sample fluids are determined by a colometric method based upon the reactions: 
 Glucose+O 2 +H 2 O→Gluconic acid+H 2 O 2    H 2 O 2 +Reagent H 2 O+Red dye  
      Response currents are represented by curves L 230DC , L 111DC , L 80DC  and L 0DC . At an early stage, for example, from 0 to 0.5 second, an unstable current may occur due to an unstable electrochemical reaction. Moreover, the magnitude of a response current decreases over time as the electrochemical reaction proceeds.  
       FIG. 3B  is a plot showing an experimental result of applying a potential profile to a sample fluid containing an analyte at various concentrations in accordance with one embodiment of the present invention. Referring to  FIG. 3B , a potential profile that comprises a voltage bias of 0.4V and a sinusoidal wave having an amplitude of 0.1V and a frequency of 1 Hz is applied to electrochemical cells that include glucose at the concentrations of 230 mg/dl, 111 mg/dl, 80 mg/dl and 0 mg/dl, respectively.  
      Response currents are represented by curves L 230AC , L 111AC , L 80AC  and L 0AC . According to American Diabetics Association (“ADA”), blood glucose normally falls between 50 to 100 mg/dl before meal, and rises up to a level generally less than 170 mg/dl after meal. The selected range, 0 to 230 mg/dl, which may be directed to diabetic individuals, is wider than the normal range suggested by ADA.  
       FIG. 3C  is a plot showing a comparison between experimental results of applying to a sample fluid a constant voltage and a potential profile. Referring to  FIG. 3C , curves L 111DC1  and L 111DC2  represent response current signals measured by applying constant voltages of 0.4V and 0.5V, respectively, to a sample fluid containing glucose of 111 mg/dl, and a curve L 111AC  represents a response current signal measured by applying a potential profile that comprises a voltage bias of 0.4V and a sinusoidal wave having an amplitude of 0.1V and a frequency of 1 Hz to an electrochemical cell that includes glucose of 111 mg/dl. It can be seen that the curve L 111AC  has a higher current response, and in turn a higher resolution, than the curves L 111DC1  and L 111DC2 . In particular, when the curves L 111AC  and L 111DC2  are compared to one another, the curve L 111AC  has a higher resolution than the curve L 111DC2 , which means that the method using the potential profile is advantageous.  
       FIG. 4  is a plot illustrating methods for processing a current signal in accordance with one embodiment of the present invention. Referring to  FIG. 4 , as an example of the curve L 80AC  shown in  FIG. 3B , the peaks of the curve L 80AC  are connected to form a peak curve L P80  by, for example, curve fitting. In another aspect, the valleys of the curve L 80AC  are connected to form a valley curve L V80 . To correlate the current signal with a concentration of the analyte, i.e., glucose, in a first example, the current magnitude of a peak curve of a response curve is measured at a time point during a measuring period of approximately 60 seconds. The time point should be selected from a stable current region of the response curve without the concern of any unstable reaction. In a second example, the current magnitude of a valley curve of a response curve is measured at a time point. The first and second examples as an example of response curves L 0AC , L 80AC , L 111AC  and L 230AC  are summarized in Table 1.  
      Table 1 shows experimental results of methods for correlating current signals with the amount of the analyte in the sample fluid. Specifically, the second and third columns of Table 1 refer to methods in accordance with the above-mentioned first and second examples of the present invention, respectively, where the current magnitudes are taken at the fourth second once the potential profile (the same as that shown in  FIG. 3B ) is applied. By comparison, the last column of Table 1 refers to a method for measuring the current magnitude at the fourth second once a constant voltage is applied.  
                           TABLE 1                                   Current magnitude of           Current magnitude of   Current magnitude of   a response curve at           a peak curve of a   a valley curve of a   the fourth second       Concentration of   response curve at the   response curve at the   under a constant       glucose (mg/dl)   fourth second (μA)   fourth second (μA)   voltage of 0.4 V (μA)                                                0   3.89   −1.19   1.60       80   6.88   0.46   3.72       111   9.75   2.87   7.38       230   17.62   9.24   14.91                  
 
      Moreover, in a third example, a response curve is integrated over a time period to calculate the amount of charges. In a fourth example, a peak curve of a response curve is integrated over a time period to calculate the amount of charges. In a fifth example, a valley curve of a response curve is integrated over a time period to calculate the amount of charges. The operations such as curve fitting and integration may be performed in microprocessor  12 . The third, fourth and fifth examples as an example of response curves L 0AC , L 80AC , L 111AC  and L 230AC  are summarized in Table 2.  
      Table 2 shows experimental results of other methods for correlating current signals with the amount of the analyte. Specifically, the second, third and fourth columns of Table 2 refer to methods in accordance with the above-mentioned third, fourth and fifth embodiments of the present invention, respectively, where the curves are integrated over a time period from the first to the sixth second once the potential profile is applied. By comparison, the last column of Table 2 refers to a method for integrating response curves over the same period once a constant voltage is applied.  
                               TABLE 2                                       Amount of                       charges               Amount of   Amount of   calculated by           Amount of   charges   charges   integrating a           charges   calculated by   calculated by   response curve           calculated by   integrating a peak   integrating a   from the first to           integrating a   curve of a   valley curves of a   sixth second       Concentration   response curve   response curve   response curve   under a constant       of glucose   from the first to   from the first to   from the first to   voltage of 0.4 V       (mg/dl)   sixth second (Q)   sixth second (Q)   sixth second (Q)   (Q)                                                    0   10.79   22.93   −1.10   14.57       80   24.23   40.24   8.60   28.16       111   41.41   58.89   25.98   44.07       230   81.13   103.34   60.96   88.79                  
 
       FIG. 5  is a flow diagram showing a method for correlating a current signal with a concentration of an analyte in accordance with one embodiment of the present invention. Referring to  FIG. 5 , a sample containing an analyte of a concentration is applied to a cell  20  at step  502 . Next, a potential profile including a voltage bias and an alternating part is applied to the sample at step  504 . A response current signal is then measured at step  506 . Microprocessor  12  processes the response current to derive a concentration-current relationship for the analyte at step  508 . In processing the response current, the methods in accordance with the present invention as previously described with respect to Table 1 and Table 2 may be used. The concentration-current relationship may be stored in memory  46  in the form of a lookup table.  
      The foregoing disclosure of the preferred embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents.  
      Further, in describing representative embodiments of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.