Abstract:
A method for real-time detection of polymerase chain reaction (PCR) is provided. The method includes: manufacturing a reactor having a plurality of electrodes; immobilizing a PCR primer to surfaces of electrodes; injecting a mixture for a PCR into the reactor to perform the PCR on the surfaces of electrodes and measuring an impedance of the PCR product.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This application claims the priority of Korean Patent Application No. 10-2004-0009841, filed on Feb. 14, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
         [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a method for real-time detection of polymerase chain reaction (PCR), and more particularly, to a method for detecting whether or not PCR is proceeding well by measuring in real-time the impedance change of PCR products as PCR is proceeding.  
         [0004]     2. Description of the Related Art  
         [0005]     Polymerase chain reaction (PCR) is mixing DNA samples, polymerase and deoxyribose Nucleoside TriPhosphate (dNTP), adjusting the temperature of the mixture appropriately and amplifying DNA.  
         [0006]     The conventional PCR detection employs gel electrophoresis to show only the qualitative result of the amplified DNA after PCR is finished, and has many problems including an accuracy problem in quantitative detection of DNA. To solve this, real-time PCR devices have been developed, which enables the quantitative analysis of DNA by detecting the magnitude of fluorescence that is in proportion to concentration of amplified DNA, through an optical detection system.  
         [0007]     So far, real-time PCR chips need a variety of optical apparatuses such as laser sources, micro mirrors, microscopes, and filters, and at the same, expensive fluorescent dye is used. In addition, for the purpose of using less samples and quickly performing analysis as well as decrease reduction in size of devices, PCR chips using a micro-electro-mechanical system (MEMS) have been developed. However, the real-time PCR chips have a problem in reducing the size in case of fluorescent detection, and an economic problem in case of using MEMS.  
         [0008]     As another example of the conventional real-time PCR chip detection, there is a method for measuring impedance while PCR is proceeding in a solution. Theoretically, if PCR is performed in a solution, the impedance should show a sequential tendency as the PCR product increases. However, actually, a variety of materials are mixed such that it is difficult to find the tendency in relation to the PCR product. In addition, when the impedance is measured, as time passes by, the measured signal of the PCR product decreases due to non-singular adhesion of components of PCR samples other than DNA, for example, enzymes or PCR reaction buffers, in which the components are adhered to electrodes. Accordingly, the result according to the PCR cycle lacks repeatability. Also, due to the non-singular adhesion, change occurs on the surfaces of electrodes such that it is difficult to measure the PCR product itself generated in the solution.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention provides a method by which a reactor having various types of electrodes is manufactured; a polymerase chain reaction (PCR) primer is immobilized to electrodes and then a PCR mixture is injected inside the reactor; and while solid PCR, in which PCR proceeds on the surfaces of electrodes, is performed, impedance is measured in real time to determine whether PCR is proceeding well.  
         [0010]     According to an aspect of the present invention, there is provided a polymerase chain reaction detection method including: manufacturing a reactor having a plurality of electrodes; immobilizing a polymerase chain reaction primer to surfaces of electrodes; and injecting a mixture for a polymerase chain reaction into the reactor to perform the PCR on the surfaces of electrodes and measuring an impedance of the polymerase chain reaction product. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0012]      FIG. 1  is a flowchart of a method for real-time detection of PCR according to the present invention;  
         [0013]      FIG. 2  illustrates a micro PCR chip having 17 pairs of different electrode structures;  
         [0014]      FIG. 3A  illustrates that the impedance of a micro PCR chip is measured using an impedance measuring apparatus;  
         [0015]      FIG. 3B  illustrates that an oligonucleotide is bound to an electrode;  
         [0016]      FIG. 4A  illustrates an impedance equivalent circuit of a PCR mixture;  
         [0017]      FIG. 4B  is a Nyquist plot expressing an imaginary component with respect to a real component of an impedance calculated according to the equivalent circuit of  FIG. 4A ;  
         [0018]      FIGS. 5A and 5B  illustrates change in an imaginary component with respect to a real component of impedance measured with respect to hybridization in PCR chips  25  and  26  of  FIG. 2 ;  
         [0019]      FIGS. 6A and 6B  are showing the impedance of  FIGS. 5A and 5B  as the magnitude of impedance with respect to frequency;  
         [0020]      FIGS. 7A and 7B  are showing the impedance of  FIGS. 5A and 5B  as the phase of impedance with respect to frequency;  
         [0021]      FIGS. 8A and 8B  illustrates change in an imaginary component with respect to a real component of impedance measured with respect to annealing cycle in PCR chips  15  and  16  of  FIG. 2 ;  
         [0022]      FIGS. 9A and 9B  are showing the impedance of  FIGS. 8A and 8B  as the magnitude of impedance with respect to frequency; and  
         [0023]      FIGS. 10A and 10B  are showing the impedance of  FIGS. 8A and 8B  as the phase of impedance with respect to frequency. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown.  
         [0025]     Referring to the flowchart of  FIG. 1  showing a method for real-time detection of PCR according to the present invention, first, a reactor having an electrode structure to be used in the present invention is designed and manufactured in step  11 . The reactor is a micro PCR chip. For this, electrodes to be used are patterned on a glass substrate. Materials to be used as the electrodes are sputtered and then lifted off. On another glass substrate, reaction chambers are etched and then, inlets and outlets are formed by using sandblast. The manufactured two glass substrates are fusion-bonded such that a predetermined amount of a PCR mixture solution can flow into the inside of the chip. The electrode structure can be designed in a variety of ways by modifying an interval between electrodes, a number of electrodes, a shape of each electrode, an area of each electrode, etc.  
         [0026]     In the present embodiment, as shown in  FIG. 2 , a micro PCR chip having 17 pairs of different electrode structures is manufactured. Reference number  20  indicates a reaction chamber in a PCR chip into which a PCR mixture is injected, and reference number  21  indicates an electrode located in the reaction chamber  20 . Among this variety of electrode structures, according to conditions such as composition of the PCR mixture, a kind of a buffer, and salt content, an appropriate electrode structure is selected. A reference number written inside a chip represent the chip number.  
         [0027]     Next, a PCR primer is immobilized in step  12 . In the present embodiment, solid PCR is performed in order to enable to read impedance change with a high sensitivity. The solid PCR means that the PCR is proceeding on surface of an electrode. For this, the PCR primer should be immobilized to the surface of the electrode. First, a PCR primer is synthesized, which is introduced a chemical group bound well to the electrode according to a kind of electrode material. Materials with high conductivity such as gold or platinum are desirable for the electrode materials. In the present embodiment, gold is used. The synthesized PCR primer is solved in a buffer to be injected into a reaction chamber, and then is immobilized through using a reaction such as a self assembled monolayer (SAM) reaction. At this time, in the primer to be immobilized, any one type of an upstream and a downstream may be immobilized, or both can be immobilized. In the present embodiment, immobilization is performed by using an upstream primer in which the chemical group is replaced with a sulfhydryl group that has a good reactivity to the electrode material.  
         [0028]     Next, impedance of the solid PCR is measured in real time in step  13 . Into the reactor in which the primer is immobilized, a PCR mixture including a PCR buffer, dNTP, MgCL 2  and Taq. polymerase enzyme, is injected. While proceeding temperature cycles appropriate to the PCR, an impedance signal is measured at an extension temperature in each cycle. Here, the impedance signal is not only measured in each cycle but can also be measured after all of the cycles are completed.  
         [0029]      FIG. 3A  illustrates that the impedance of a micro PCR chip is measured by using an impedance measuring apparatus  31 . Reference number  30  indicates a micro PCR chip.  FIG. 3B  illustrates that an oligonucleotide is bound to an electrode  21 .  
         [0030]      FIG. 4A  illustrates an impedance equivalent circuit of a PCR mixture. Referring to  FIG. 4A , R s  denotes a resistance of the mixture, R 1  denotes a resistance between the mixture and the electrode, C 1  denotes a capacitance between the mixture and the electrode, R 2  denotes a resistance on the surface of the electrode, and C 2  denotes a capacitance on a surface of the electrode. Impedance of the equivalent circuit of  FIG. 4A  can be calculated as the following equations 1: 
   Z=R   s   +[R   1   −1   +jωC   1 ] −1   +[R   2   −1   +jωC   2 ] −1      Z   1   =R   s   +R   1 /[1+ω 2   R   1   2   C   1   2   ]+R   2 /[1+ω 2   R   2   2   C   2   2 ]   Z″=−ωR   1   2   C   1 /[1+ω 2   R   1   2   C   1   2   ]−ωR   2   2   C   2 /[1+ω 2   R   2   2   C   2   2 ]  (1)  
 Here, Z′ denotes a real component of Z, and Z″ denotes an imaginary component of Z. If the impedance is measured, the analysis of the measured impedance through the equivalent circuit can be performed in step  14 . The analysis, for example, can be performed to find which PCR proceeds more strongly, by using a Nyquist plot of the imaginary component with respect to the real component of the impedance calculated in the equation 1, as shown in  FIG. 4B . That is, it can be learned which reaction is the strongest among in the mixture, between the mixture and the electrode, and on the surface of the electrode. In  FIG. 4B , reference number  40  indicates a location of R s , reference number  41  indicates a location of R s +R 1 , and reference number  42  indicates a location of R s +R 1 +R 2 . A radius of a semicircle of reference number  43  is R 1 /2, and that of reference number  44  is R 2 /2. Referring to  FIG. 4B , shapes can vary with respect to values of R 1 and R   2 . Accordingly, in order to enable easier impedance measuring, gene amplification can be confirmed by making a surface effect stronger through the solid PCR. 
 
         [0031]      FIGS. 5 through 7  show impedance changes with respect to hybridization in PCR chips  25  and  26  of  FIG. 2 .  FIGS. 5A, 6A , and  7 A show the impedance changes when a linker is immobilized to an electrode, while  FIGS. 5B, 6B , and  7 B show the impedance changes when the primer is immobilized to an electrode.  
         [0032]     More specifically,  FIGS. 5A and 5B  show change in Z″ in relation to Z′ in the equation 1. Hybridization times are 1 minute, 5 minutes, 30 minutes, and 1 hour, respectively. In the two graphs, reference numbers  50  and  53  indicate impedances measured before the linker and the primer, respectively, are immobilized to an electrode. Reference numbers  51  and  54  are impedances measured after the linker and the primer, respectively, are immobilized. Referring to  FIGS. 5A and 5B , there are differences in impedances before and after immobilization of the linker and the primer. Accordingly, it can be seen that the linker and the primer are well immobilized to the electrode. Meanwhile, with regard to the results of measuring impedances with respect to the hybridization, reference number  52  indicates the result when the linker is immobilized, and shows that there is no impedance change due to the hybridization even after an hour. When the primer is immobilized to the electrode and the hybridization proceeds for 30 minutes, there is no change as indicated by reference number  55 . However, when the hybridization proceeds for an hour, an impedance change is measured as indicated by reference number  56  such that it can be seen that gene is amplified.  
         [0033]      FIGS. 6A and 6B  show magnitudes of impedances, respectively, with respect to frequency. Reference numbers  60  and  63  indicate measured values before the linker and the primer, respectively, are immobilized to an electrode, and reference numbers  61  and  64  indicate measured values after the linker and the primer, respectively, are immobilized to an electrode. Reference number  62  indicates measured values in case that the hybridization proceeds for a minute, 5 minutes, 30 minutes, and an hour, when the linker is immobilized. According to the graph, there is no change in measured values. In case that the primer is immobilized, reference number  65  indicates measured values on the hybridization for 30 minutes, and reference number  66  on the hybridization for an hour. According to the graph, it can be seen that change in the magnitude of the impedance is measured and accordingly, that gene is amplified.  
         [0034]      FIGS. 7A and 7B  show phases of impedances, respectively, with respect to frequency. Reference numbers  70  and  73  indicate measured values before the linker and the primer, respectively, are immobilized to an electrode, and reference numbers  71  and  74  indicate measured values after the linker and the primer, respectively, are immobilized to an electrode. Reference number  72  indicates measured values in case that the hybridization proceeds for a minute, 5 minutes, 30 minutes, and an hour, when the linker is immobilized. In case that the primer is immobilized, reference number  75  indicates measured values on the hybridization for 30 minutes, and reference number  76  indicates measured values on the hybridization for an hour. It can be seen that change in the phases of the impedance is measured and accordingly, that gene is amplified.  
         [0035]     According to  FIGS. 5 through 7 , any one among the magnitude of the imaginary component with respect to the real component of the impedance, the magnitude with respect to frequency, and the phase with respect to frequency, can show whether the gene is amplified.  
         [0036]      FIGS. 8 through 10  show values obtained by measuring solid PCR in chips  15  and  16  among the chips shown in  FIG. 2 . An object gene of PCR is that of a Hepatitis B virus (HBV).  FIGS. 8A, 9A , and  10 A show the measured values when a linker is immobilized to an electrode, while  FIGS. 8B, 9B , and  1 B show the measured values when a primer is immobilized to an electrode.  
         [0037]      FIGS. 8A and 8B  show change in Z″ with respect to Z′ in the equation 1 while annealing processes of 1, 20, and 40 cycles, respectively, are performed. When the linker is immobilized, there is little change in the measured impedances in  FIG. 8A . When the primer is immobilized, however, the changes in the measured values are great enough to show that the gene is amplified.  
         [0038]      FIGS. 9A and 9B  show magnitudes of impedances, respectively, with respect to frequency. Reference numbers  90  and  93  indicate measured values, respectively, when 1 cycle annealing is performed, reference numbers  91  and  94  indicate measured values, respectively, when 20 cycle annealing is performed, and reference numbers  92  and  95  indicate measured values, respectively, when 40 cycle annealing is performed.  
         [0039]      FIGS. 10A and 10B  show phases of impedances, respectively, with respect to frequency. Reference numbers  100  and  103  indicate measured values, respectively, when 1 cycle annealing is performed, reference numbers  101  and  104  indicate measured values, respectively, when 20 cycle annealing is performed, and reference numbers  102  indicates measured values when 40 cycle annealing is performed.  
         [0040]     According to the graphs, as in  FIGS. 8A and 8B , when the linker is immobilized, there is little change in the measured values. When the primer is immobilized, however, the changes in the measured values are great with increasing frequency of the annealing cycles, and accordingly, it can be seen that the gene is amplified.  
         [0041]     According to the present invention, while solid PCR is performed, the impedance is measured such that the sensitivity can be improved better than the measuring in the solution. In addition, the non-singular adhesion to the surface of an electrode can be removed such that measuring the PCR product in real time with high repeatability is enabled.