Abstract:
Design of a disposable screen printed electrode (SPE) for sensing percentage glycated hemoglobin using electrochemistry is disclosed. SPE has four electrodes, one working electrode for the detection of glycated hemoglobin, one working electrode for the detection of hemoglobin and the other two electrodes are counter and reference electrodes that are common for both detection schemes. It also has a cellulose acetate membrane with lysis agents and surfactant embedded in it. Lysis agents lyse erythrocytes and release hemoglobin. Surfactants modify hemoglobin structure and enhance the rate the electron transfer and thereby the output signal during the electrochemical analysis. The SPE is low cost and user friendly. The only input from the user is a drop of blood.

Description:
RELATED APPLICATIONS 
       [0001]    The present application is related to and claims priority from co-pending India provisional patent application entitled, “Disposable Screen Printed Electrode For Percentage Glycated Hemoglobin Sensor”, application serial number: 1318/CHE/2011, filed on 18 Apr. 2011, attorney docket number: IISC-303-INPR, naming as inventors Vanjari et al. and is incorporated in its entirety herewith. 
       TECHNICAL FIELD 
       [0002]    Embodiments of the present disclosure relate generally to disposable sensors, and more specifically to a disposable sensor for measuring of percentage glycated hemoglobin. 
       BACKGROUND 
       [0003]    As is well known in the relevant arts, glycated hemoglobin (GHb) is formed in a non-enzymatic glycation pathway by hemoglobin&#39;s exposure to plasma glucose, over 120-day lifespan of the red blood cell (RBC). A buildup of glycated hemoglobin within the red cell, therefore, reflects the average level of glucose to which the cell has been exposed during its life. It is always expressed as percentage of total hemoglobin. 
         [0004]    Measurement of glycated hemoglobin levels may provide more accurate information on blood glucose levels over a long period of time (typically 120 days). In comparison, fasting blood glucose measurements merely indicate blood glucose levels over a much shorter period of time (typically on a daily basis). Estimation of percentage GHb may be valuable, for example, in devising optimum treatment strategies for long term management of diabetes. Various methods for the measurement of glycated hemoglobin are known in the art. Although techniques such as immunoassay, ion-exchange chromatography, electrophoresis, boronate affinity chromatography and high pressure liquid chromatography along with electrospray ionization mass spectrometry are commonly used in the laboratories, these techniques may be time consuming and may require bulky equipments. Other techniques to reduce the size of such equipments exploit the adsorption of GHb onto various materials like Zirconia nanoparticles, Aminophenyl boronic acid agarose beads, Thiophene-3-boronic acid on gold particles. However such materials may be associated with high costs, and designing these instruments may be complex. US patent no. 20100089774A1 discloses a screen printed electrode (SPE) system for detection of glycated hemoglobin and hemoglobin. However this method of detection involves elaborate use of buffers and reagents and uses potentiometric techniques to detect GHb. Addition of precise amount of liquid reagents, needs a lab technician with relevant experience and hence cannot be used by a common unskilled user. The present invention is directed towards the above problem and relates to easy to use disposable sensors for measuring percentage glycated hemoglobin in a blood sample. The inventors propose using a membrane modified with lysing agents, so that it can be used in miniaturized point of care devices for hemoglobin or glycated haemoglobin measurement. This technique eliminates the need for mixing in a separate lysis buffer which is a huge advantage in terms of making a point of care device both more user friendly and more robust. The inventors also propose the methodology for the specific detection of GHb and Hb. 
       SUMMARY 
       [0005]    A disposable sensor for measuring glycated hemoglobin (GHb) and hemoglobin (Hb) has a GO-APBA(graphene oxide-3-Aminophenylboronic acid) coated working electrode and a GO modified working electrode. The sensor further has a reference electrode for maintaining potential and a counter electrode for maintaining charge balance and a salt-surfactant modified porous cellulose acetate membrane for lysing erythrocytes of blood cell samples and releasing hemoglobin molecules. The electrodes of the sensor are screen printed electrodes (SPE) and the lysis membrane is stitched on top of these electrodes. Detection of GHb is done using electrochemical impedance spectroscopy. A potential is applied between working electrode and reference electrode and the impedance between working electrode and counter electrode is determined. In another embodiment a two electrode system is used. The potential is applied between working and reference electrodes and the resultant current is measured between working and reference electrodes. A counter electrode is not used. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1A  depicts the design of a SPE strip to be used for the detection of GHb. 
           [0007]      FIG. 1B  shows the cross-sectional arrangement of the SPE strip. The layer  110  represents a membrane on to which a sample of blood is applied. The membrane is capable of lysing the erythrocytes in the blood and filtering the hemoglobin molecules through the pores. The filtrate comprising the hemoglobin molecules comes in contact with the SPE strip,  120  underneath, that can now detect and measure the hemoglobin and GHb present in the filtrate. 
           [0008]      FIG. 2  shows a photograph of casted cellulose acetate membrane with a cross-sectional view of the membrane used for lysis and isolation of cellular components of a biological sample, in an embodiment of the present invention. 
           [0009]      FIG. 3  depicts an alternative embodiment of the SPE strip. 
           [0010]      FIG. 4  is a schematic representation of the reaction between GO and APBA to form the product GO-APBA. The amide linkage formed in the presence of coupling reagent EDC is shown. 
           [0011]      FIG. 5  represents the interaction between GO-APBA and GHb. 
           [0012]      FIGS. 6A and 6B  are respective Nyquist plots for GO-APBA modified electrode and for GO-modified electrode. 
           [0013]      FIG. 7  is a plot of charge transfer resistance with respect to the concentration of Hb. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Various embodiments are described below with several examples for illustration. Example embodiments will be described with reference to the accompanying drawings briefly described below. 
       1. Sensor 
       [0015]      FIG. 1A  and  FIG. 1B  show plan and elevation views respectively of a sensor  100  for the detection and measurement of hemoglobin (Hb) and GHb in a sample of blood, in an embodiment. Sensor  100  shown in elevation view in  FIG. 1B  is shown containing a membrane  110  and a screen printed electrode (SPE)  120 . Membrane  110  is attached or stitched on the SPE  120 . 
         [0016]    Area formed by length  130  and width  140  in plan view of  FIG. 1A  represents the portion of sensor  100  containing membrane  110  and the portion of SPE  120  containing the electrodes for measurement of hemoglobin and glycated hemoglobin (GHb). This SPE system has four electrodes and is a combination of two individual three electrode systems, The electrodes shown numbered  131 ,  132 ,  133  and  134  respectively represent a first working electrode (W 1 ) for measurement of Hb1Ac, a second working electrode (W 2 ) for measurement of Hb, a reference electrode (R) and a counter electrode (C). Electrodes (R) and (C) are common for both the GHb and Hb measurements. Portion  150  contains electrical paths/tracks  151 ,  152 ,  153  and  154  connected respectively to  131 ,  132 ,  133  and  134 , and facilitates connection of the electrodes to a device used for performing the GHb and Hb measurements. In an alternate embodiment the electrode  134  and the conductive track  154  can be eliminated. The SPE system then has three electrodes and is a combination of two individual two electrode systems. 
         [0017]    Referring now to  FIG. 1B , layer  125  is a substrate of sensor  100 , and is made of poly-vinyl chloride (PVC). Any flexible substrate suitable for screen printing can be used as for example polymethyl methacrylate, epoxy-fiber composites, epoxy carbon composites, polyimide composites, phenolic strips and the like. Layer  121  of SPE  120  contains the electrodes  131 ,  132 ,  133  and  134  described above. Layer  121  is made up of either electroactive carbon and/or other electrode materials. Layer  122  is constructed using electro-active carbon. The contacts for the electrodes are made up of two layers viz., a conductive silver layer ( 124 ) and an electroinactive graphite layer ( 123 ) for protecting silver layer ( 124 ) from oxidation. In an alternate embodiment the silver track can be replaced by any highly conductive track such as copper, indium, brass, tin, alloys of metals like gold and so on. Layers  121 - 125  are typically printed using screen printing techniques. 
         [0018]    Layer  110  ( FIG. 2 ) represents a porous membrane comprising at least one embedded lysing agent used for lysing of cells in a sample of blood that is applied onto the membrane. A cross-sectional view and the mechanism of lysis of cells, when a drop of blood is applied onto porous membrane  110 , are conceptually depicted in  FIG. 2 . Layer  210  represents a blood sample to be analyzed. Layer  220  represents porous membrane embedded with lysing agents. Layer  230  represents the filtrate (Hb and GHb components of erythrocytes) obtained by the porous action of the porous membrane  110 . 
         [0019]    The porous membrane of the invention may comprise embedded salts including, but not limited to ammonium chloride, potassium bicarbonate, saponin b and lithium salts. One skilled in the art will be able to optimize the lysis conditions for isolation of Hb and GHb from a sample of blood. The membrane may further comprise a hydrophilic surfactant like sodium dodecyl sulphate (SDS), cetyl trimethylammonium bromide (CTAB), octyl phenol ethoxylate (Triton X-100), polyethylene glycol tert-octylpheny ether (Triton X-114), the zwitterionic detergent (CHAPS), nonyl phenoxypolyethoxy ethanol (NP-40), polysorbate 20 (Tween 20). Didodecyldimethylammonium bromide(DDAB), Hexyltrimethylammonium bromide (HTAB), ethoxylated alcohols or phenols, betaines, Lauryl mono ethanol, sulphosuccinates, and so on. In some embodiments a double membrane system may be used. The membranes may be stitched together. One membrane may have lysis salts to isolate hemoglobin and detect GHb. The other membrane may contain salts and surfactant to enhance the electron transfer rate of hemoglobin. 
         [0020]    The densely porous membrane layer may face electrode side while a porous layer may face air side. The pore size of the membrane can be decided based on the cell or protein that needs to be isolated. The diameter of hemoglobin is 5.5 nm and the size of human body cells ranges from 2 to 120 μm. In order to isolate other cells and allow only hemoglobin to pass through, it is therefore sufficient to have an average pore diameter of 1.5 μm or less. The dense skin layer has pores with an average diameter of 1.5 μm. The presence of macrovoids of diameters greater than 10 μm in the porous sub layer helps in allowing blood to pass through easily. Hence it is preferable to place the blood on top of the porous sub layer. 
       2. Working 
       [0021]    In one embodiment of the invention, the working electrode  131  has been modified by the methods described below, for the adsorption of glycated hemoglobin. As the working electrodes  131  and  132  come in contact with the filtrate comprising Hb and GHb, the electrode  131  will adsorb GHb while the working electrode  132  detects the total Hb, facilitating the transfer of charge to and from the GHb or total Hb when a potential is applied between  131  or  132  and the reference electrode  133 . The electrode  131  may be modified by appropriate compounds and processes, as described below, to enable the adsorption of GHb. Similarly, the electrode  132  may be designed for the detection of total hemoglobin. The electrode,  133  has a known potential to gauge the potential of the working electrode, while also balancing the charge added or removed by the working electrode. Its only role is to act as reference in measuring and controlling the potential of the working electrodes  131  or  132  and at no point does it pass any current. The electrode  133 , may be made of electroactive carbon or conductive Ag/AgCl paste. The counter electrode,  134  passes all the current needed to balance the current observed at the working electrode  131  or  132 . The working electrodes  131  and  132  may be of at least 1 mm diameter and the reference and counter electrodes,  133  and  134  may be rings with a width of at least 0.5 mm. 
         [0022]    The distance between the working electrode and reference electrode or the reference electrode and counter electrode may be at least 0.5 mm. The presence of Hb and GHb can be detected by amperometric techniques such as cyclic voltammetry, differential pulse voltammetry and chronoamperometry. Electrochemical impedance spectroscopy may also be used. 
         [0023]    The sensor of the present invention employs a three electrode system wherein the role of maintaining constant potential and balancing the charge are achieved by two electrodes, the reference electrode  133  and the counter electrode  134 . In an alternative embodiment a two electrode system wherein the same electrode maintains a constant potential while passing current to counter redox events at the working electrode can be used. 
         [0024]    Although the embodiments of the present invention describe a three electrode system, the invention cannot be limited to this. Other systems which have more electrodes, but their design principles are generally the same as the three electrode system may also fall within the scope of the invention. The electrodes may be formed by the technique of screen printing, other printing methods known in the art to form electrodes may be used. The sensor of the present invention made by the process of screen printing may be referred to as “Screen Printed Electrode or SPE” interchangeably. Although the electrodes of the present invention have been formed by screen printing, the invention cannot be construed to be limiting to screen printing. 
         [0025]    In an alternative embodiment of the invention, two or more membranes with different parameters e.g., pore size, nature of embedded salts and surfactants, may be used together. One example provided here as depicted in  FIG. 3  may comprise one membrane with lysing salts for isolation of hemoglobin and GHb. The other membrane may have lysing salts as well as the surfactant such as CTAB that is capable of denaturing hemoglobin. The detection of hemoglobin may be performed using the methods known in the art. 
       3. Synthesis of Composite for Specific Detection of GHb and its Use Thereof to Detect GHb. 
       [0026]    Glassy carbon is widely used electrode in electroanalytical techniques. The term “Glassy Carbon” refers to non graphitizing carbon which combines glassy and ceramic properties with those of non graphitizing carbon allowing the electrode to have high resistance to temperature and chemical attack. 
         [0027]    It is well known in the art that Graphene Oxide (GO) can act as an electrode material and has vast applications in developing biosensors. Graphene Oxide can be synthesized from exfoliated graphite by established methods known in the art. In the present invention the Hummers method has been used for the preparation of GO. Accordingly one working electrode of the current invention has been coated with GO. 
         [0028]    Another aspect of the present invention is the reaction between GO with 3-aminophenyl boronic acid (APBA) to form a composite (GO-APBA) that can be coated on the working electrode. Since the ability of APBA to bind covalently to GHb is well documented in the art, the working electrode can be used in the SPE of the current invention for the detection of GHb. GO has carboxylic groups. In an embodiment, the carboxylic groups of GO are made to react with amine groups of 3-Aminophenylboronic acid to form the composite GO-APBA. Further, any boronic acid compound containing amine group can be made to react with GO and a new composite containing boronic acid can be formed by following the protocol described below. 
         [0029]    Synthesis of GO-APBA: An aspect of the invention is the chemical modification of GO with APBA. Accordingly GO at a concentration of 1 mg/ml and 10 mM of ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) were added to deionized water and stirred continuously for 36 hrs. A 5 mM concentration of APBA was added to the mixture and stirred at room temperature for another 24 hrs. EDC being a coupling reagent aids in the formation of amide bond as shown in  FIG. 4 . The suspension was filtered and washed several times with water and ethanol, to remove any physically adsorbed APBA. The material was then dried in vacuum using silicagel. 
         [0030]    Another aspect of the invention is the design of a modified electrode. The term “modified electrode” as used herein refers to an electrode whose surface has been coated with compounds or materials that are suitable for a particular application. In the present invention the GO-APBA prepared as described above was used to coat the working electrode such that the modified working electrode is capable of binding the GHb. The coating of electrodes can be done by any of the methods known in the art. In the invention described, a dispersion of GO-APBA prepared as described above, was drop cast on the GCE and allowed to dry for 2 hours and is used to detect GHb using Electrochemical Impedance spectroscopy. The same can be carried out with the working electrode of SPE ( 131 ) to detect GHb on SPE. 
         [0031]    Modification of Cellulose acetate membranes: A further aspect of the invention is the preparation of modified cellulose acetate membranes for isolating total Hb. Thus, 10% (w/w) Cellulose acetate is dissolved in 80% (w/w) acetone and is mixed with 10% (w/w) water in which the lysing salts NH4Cl and KHCO3 are dissolved. A small amount of surfactant tween 80 is also added to ensure that the surface of the membrane is hydrophylic in nature. The mixture is sonicated to obtain a homogenous transparent solution. Membranes are cast using film casting knife on a nicely polished, cleaned BK-7 glass slides. Initial film casting thickness of 300 μm is enough to get porous membranes of 2 μm pore size which is sufficient to isolate hemoglobin. Within a few seconds after the casting, the slides were transferred into an inert atmosphere chamber and the solvents were allowed to evaporate in controlled environment. 
         [0032]    The lysing salts get embedded into the membrane. It is known that surfactants enhance electron transfer rate of hemoglobin by denaturing it structure and releasing heme. Membranes can be modified with surfactants CTAB and the filtrate from these membranes contains denatured hemoglobin which can be analyzed electrochemically. Other salts and surfactants and their combinations may be used in the modification of the membranes. 
         [0033]      FIG. 2  shows a cross sectional view of the membrane. These salts act as lysing agents and the cellulose acetate membrane in which the salts are embedded acts as a filter. The salts are embedded inside the membrane during the membrane casting process, which is a controlled process as compared to the precipitation process. Unwanted cells get filtered out by the pores in the cellulose acetate membrane. 
         [0034]    In another embodiment, these cellulose acetate membranes can be replaced with lysing agent modified nylon meshes. Though Hb is isolated in this case, the unwanted cells are not filtered out. Other polymeric meshes modified to suit the lysing requirements may also be used. 
       4. Detection of Glycated Hemoglobin 
       [0035]    The GHb present in the filtrate has an affinity for APBA and gets immobilized on to the surface of the modified electrode,  131  through cis-diol bonds of glucose to boronic acid moiety. The chemical interaction of GHb with GO-APBA compound is shown in  FIG. 5 . A potential of 0.2V is applied between the working electrode and the counter electrode, and the impedance offered by the electrode for the electron transfer is measured between the working and counter electrodes. 
         [0036]    The chemical adsorption of GHb inhibits the electron transfer rate of the redox couple [Fe(CN) 6 ] 3− /[Fe(CN) 6 ] 4−  thereby increasing the charge transfer resistance (R a ). The diameter of the semicircle in the Nyquist plot in  FIG. 6A , which is R ct , increases with increase in the concentration of GHb. In order to eliminate the possible reason that physisorption may play a role in increase of R ct , an experiment was performed using GO modified GCE. The corresponding Nyquist plots are shown in the  FIG. 6B . The variation in R ct  is small and is not systematic in this case. This proves that the increase in the charge transfer resistance is only due to the chemisorption of GHb onto the electrode surface. The increase in normalized charge transfer resistance is linear with respect to the concentration of GHb and is shown in  FIG. 7 . 
         [0037]    Detection of Hemoglobin: It is well known in the art that hemoglobin consists of four protein chains with four heme portions (Fe 2+ ) buried deep inside the bulky hemoglobin molecule. The present invention exploits the redox potential and electron transfer ability of the heme portions. In one embodiment the method involves measuring the total hemoglobin in a sample by electrochemically measuring current due to Fe 2+ /Fe 3+  redox reactions. The electrode potential is thus fixed at a level that allows for the heme molecule on the electrode surface to undergo electron transfer reaction. Therefore the current output observed will be proportional to the heme present which in turn is proportional to the concentration of hemoglobin in a test sample. To make the heme centers buried inside the molecule to be released before applying voltage, a current enhancing surfactant may be used. The surfactants used may be selected from Sodium Dodecyl Sulfate (SDS), Cetyltrimethylammonium Bromide (CTAB), DDAB, Tween 80 and TritonX-100 (TX-100). Standard amperometric techniques such as cyclic voltammetry, differential pulse voltammetry and chronoamperometry can be used to detect hemoglobin 
         [0038]    SPE action: SPE ( FIG. 1 ) makes contact with the filtrate of the blood sample, obtained by the porous action of the membrane  110 . The filtrate may contain the analyte, for example, Hb and GHb. As used herein an “analyte” may refer to any substance, chemical or biological constituent that is determined in an analytical procedure, for example the electroanalytical technique described below. In the present invention a drop of blood is applied onto layer  110  shown in  FIG. 1B . Following the lysis of the blood and filtration action of the porous membrane, Hemoglobin molecules are collected under the membrane such that the filtrate comes in contact with the electrodes of the SPE layer  121  of  FIG. 1B . GHb can be detected on the electrode  131  which is modified with GO-APBA compound as described above using electrochemical impedance spectroscopy or other amperometric techniques such as cyclic voltammetry, differential pulse voltammetry and chronoamperometry. For the detection of hemoglobin, electrode  132  is modified with Graphene oxide or surfactants like DDAB, SDS to detect Hb using standard amperometric techniques like cyclic voltammetry, differential pulse voltammetry and chronoamperometry. In another embodiment, the electrode  132  need not be modified, Instead the membrane can be embedded with surfactants like SDS, DDAB. Then filtrate is a denatured hemoglobin which can be detected directly on the electrode  132 .