Active CMOS Sensor Array For Electrochemical Biomolecular Detection

Abstract

Electrochemical sensing of biomolecules eliminates the need for bulky optical instruments required in traditional fluorescence-based sensing assays. Integration of the sensor interface electrodes and active electrochemical detection circuitry on CMOS substrates miniaturizes the sensing platform, enhancing portability for point-of-care applications, while enabling high-throughput, highly-parallel analysis. One embodiment includes a four-by-four active sensor array for multiplexed electrochemical biomolecular detection in a standard 0.25-μm CMOS process. Integrated potentiostats, including control amplifiers and dual-slope ADCs, stimulate the electrochemical cell and detect the current flowing through on-chip gold electrodes at each sensor site resulting from biomolecular reactions occurring on the chip surface. Post-processing techniques for fabricating biologically-compatible surface-electrode arrays in CMOS that can withstand operation in harsh electrochemical environments are described. Demonstrations showing example operation of the active CMOS array for biomolecular detection include cyclic voltammetry of a reversible redox species, DNA probe density characterization, and quantitative and specific DNA hybridization detection in real time.

1 . A system for electrochemical sensing of biomolecules, comprising:
 an integrated circuit;   one or more working electrodes on the integrated circuit, the one or more working electrodes configured to receive one or more biomolecular probes, a desired potential maintained through one or more reference electrodes, the one or more working electrodes configured to form a portion of one or more corresponding potentiostats; and   a digitizing circuit on the integrated circuit configured to measure a signal indicative of a biomolecule sensing operation in real time.   
     
     
         2 . The system of  claim 1 , wherein the integrated circuit is a complementary metal-oxide-semiconductor (CMOS) chip comprising a top metal layer operably connected to one or more vias for routing electrical signals. 
     
     
         3 . The system of  claim 2 , wherein the chip is fabricated in a 2.5-V, 5-metal, 0.25-μm CMOS process. 
     
     
         4 . The system of  claim 1 , wherein the one or more working electrodes comprise square, gold electrodes. 
     
     
         5 . The system of  claim 2 , wherein the one or more working electrodes are adhered to the top metal layer with an adhesion layer. 
     
     
         6 . The system of  claim 5 , wherein the adhesion layer comprises titanium. 
     
     
         7 . The system of  claim 1 , wherein the digitizing circuit comprises a dual-slope analog-to-digital converter circuit. 
     
     
         8 . The system of  claim 1 , wherein the one or more working electrodes are in contact with an electrolyte solution, and the electrolyte solution includes one or more target molecules. 
     
     
         9 . A method for electrochemical sensing of biomolecules, comprising:
 providing one or more working electrodes on an integrated circuit, a desired potential maintained through one or more reference electrodes, the one or more working electrodes configured to bind one or more biomolecular probes, the one or more working electrodes configured to form a portion of one or more corresponding potentiostats; and   providing a digitizing circuit on the integrated circuit configured to receive a signal resulting from an electrochemical measurement operation to measure one or more aspects of a biomolecular reaction in real time.   
     
     
         10 . The method of  claim 9 , further comprising:
 binding one or more biomolecular probes at the one or more working electrodes.   
     
     
         11 . The method of  claim 9 , wherein the integrated circuit is a complementary metal-oxide-semiconductor (CMOS) chip comprising a top metal layer operably connected to one or more vias for routing electrical signals. 
     
     
         12 . The method of  claim 11 , wherein the chip is fabricated in a 2.5-V, 5-metal, 0.25-μm CMOS process. 
     
     
         13 . The method of  claim 9 , wherein the one or more working electrodes comprise square, gold electrodes. 
     
     
         14 . The method of  claim 11 , wherein the one or more working electrodes are adhered to the top metal layer with an adhesion layer. 
     
     
         15 . The method of  claim 14 , wherein the adhesion layer comprises titanium. 
     
     
         16 . The method of  claim 9 , wherein the digitizing circuit comprises a dual-slope analog-to-digital converter circuit. 
     
     
         17 . The method of  claim 9 , wherein the electrochemical measurement operation includes cyclic voltammetry, linear-sweep voltammetry, square-wave voltammetry, ac voltammetry, ac impedance, or electrochemical impedance spectroscopy techniques. 
     
     
         18 . The method of  claim 9 , further comprising:
 analyzing the biomolecular reaction to quantify surface target coverages.   
     
     
         19 . The method of  claim 9 , wherein the biomolecular reaction is indicative of quantitative and specific detection of biomolecules. 
     
     
         20 . The method of  claim 9 , wherein the biomolecular reaction is indicative of DNA sensing. 
     
     
         21 . A method of manufacture of a CMOS-based array for electrochemically measuring a biomolecular reaction, comprising:
 defining one or more openings in a passivation layer of a CMOS chip to expose a top metal layer;   depositing an adhesion layer at one of the openings, the adhesion layer in electrical communication with the top metal layer;   depositing a metal layer on the adhesion layer to form a working electrode at least one of the openings, the working electrode in electrical communication with the adhesion layer, the working electrode configured to bind one or more biomolecular probes, the working electrode configured to form a portion of a potentiostat; and   electrically connecting an on-chip digitizing circuit to the working electrode, the digitizing circuit configured to measure an electrical signal resulting from the biomolecular reaction.   
     
     
         22 . The method of  claim 21 , wherein defining the one or more openings includes using a wet etch process to selectively remove the top metal layer. 
     
     
         23 . The method of  claim 21 , wherein the deposition of the adhesion layer includes an electron-beam deposition technique. 
     
     
         24 . The method of  claim 21 , further comprising:
 encapsulating bond wires in a chemically resistant epoxy to shield the wires from exposure to an electrolyte.   
     
     
         25 . The method of  claim 21 , wherein a layer of polydimethylsiloxane is included between the chip and a top plate, the polydimethylsiloxane layer preventing leakage of the electrolyte. 
     
     
         26 . The method of  claim 21 , further comprising:
 forming a reservoir above the chip to hold the electrolyte.   
     
     
         27 . A system for electrochemical sensing of biomolecules, comprising:
 means for binding one or more biomolecular probes at one or more working electrodes of an integrated circuit, the integrated circuit including one or more working electrodes, a desired potential maintained through one or more reference electrodes, and a digitizing circuit on the integrated circuit configured to measure a signal indicative of a biomolecule sensing operation in real time, the one or more working electrodes configured to form a portion of one or more corresponding potentiostats; and   means for performing an electrochemical measurement operation to measure one or more aspects of a biomolecular reaction in real time.   
     
     
         28 . The system of  claim 27 , wherein the integrated circuit is a complementary metal-oxide-semiconductor (CMOS) chip comprising a top metal layer operably connected to one or more vias for routing electrical signals. 
     
     
         29 . The system of  claim 27 , wherein the electrochemical measurement operation includes cyclic voltammetry, linear-sweep voltammetry, square-wave voltammetry, ac voltammetry, ac impedance, or electrochemical impedance spectroscopy techniques. 
     
     
         30 . The system of  claim 27 , further comprising:
 analyzing the biomolecular reaction to quantify surface target coverages.