WO2019135251A1 - Organic field effect transistor (ofet) for the detection of gram negative/positive bacteria, method of detection and fabrication of the transistor - Google Patents

Organic field effect transistor (ofet) for the detection of gram negative/positive bacteria, method of detection and fabrication of the transistor Download PDF

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Publication number
WO2019135251A1
WO2019135251A1 PCT/IN2018/050849 IN2018050849W WO2019135251A1 WO 2019135251 A1 WO2019135251 A1 WO 2019135251A1 IN 2018050849 W IN2018050849 W IN 2018050849W WO 2019135251 A1 WO2019135251 A1 WO 2019135251A1
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layer
ofet
bacteria
sensing system
dielectric layer
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PCT/IN2018/050849
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French (fr)
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Anamika DEY
Ashish Singh
Deepanjalee DUTTA
Siddhartha Sankar GHOSH
Parameswar Krishnan Iyer
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Indian Institute Of Technology, Guwahati
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
    • G01N27/403—Cells and electrode assemblies
    • G01N27/414—Ion-sensitive or chemical field-effect transistors, i.e. ISFETS or CHEMFETS
    • G01N27/4145—Ion-sensitive or chemical field-effect transistors, i.e. ISFETS or CHEMFETS specially adapted for biomolecules, e.g. gate electrode with immobilised receptors
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K10/00—Organic devices specially adapted for rectifying, amplifying, oscillating or switching; Organic capacitors or resistors having potential barriers
    • H10K10/40—Organic transistors
    • H10K10/46—Field-effect transistors, e.g. organic thin-film transistors [OTFT]
    • H10K10/462—Insulated gate field-effect transistors [IGFETs]
    • H10K10/468—Insulated gate field-effect transistors [IGFETs] characterised by the gate dielectrics
    • H10K10/474—Insulated gate field-effect transistors [IGFETs] characterised by the gate dielectrics the gate dielectric comprising a multilayered structure
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K10/00—Organic devices specially adapted for rectifying, amplifying, oscillating or switching; Organic capacitors or resistors having potential barriers
    • H10K10/40—Organic transistors
    • H10K10/46—Field-effect transistors, e.g. organic thin-film transistors [OTFT]
    • H10K10/462—Insulated gate field-effect transistors [IGFETs]
    • H10K10/484—Insulated gate field-effect transistors [IGFETs] characterised by the channel regions
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K85/00—Organic materials used in the body or electrodes of devices covered by this subclass
    • H10K85/761—Biomolecules or bio-macromolecules, e.g. proteins, chlorophyl, lipids or enzymes

Definitions

  • the present invention relates to transistor based biosensor. More specifically, the present invention is directed to develop an ultra-low voltage operated, highly stable, Organic Field Effect Transistor (OFET) based bio-sensing system for detection of Gram positive and Gram negative bacteria and a method for fabricating such OFET based bio-sensing system.
  • OFET Organic Field Effect Transistor
  • the present OFET based bio- sensing system is particularly useful for detecting and quantifying different biological species/Gram positive bacteria/Gram negative bacteria.
  • OFETs Organic Field Effect Transistors
  • a transistor-based biosensor normally has been in high demand because of its high sensitivity since the single device can be able to sense the analyte and as well as can amplify its response [Ref: R. M. Owens, G. G. Malliaras, MRS Bull. 2010, 35, 449; Q. T. Zhang, V. Subramanian, Biosens. Bioelectron. 2007, 22, 3182; C. Bartic, A. Campitelli, S. Borghs, Appl. Phys. Lett. 2003,
  • bacteria sensor is one of the promising since bacterial contamination is a major health hazard especially in the context of food safety, environmental monitoring, and the pharmaceutical industry. It assumes even greater significance in case of pathogens as the presence of even a single cell may lead to serious health risk. Thus, rapid quantification of bacteria is imperative for clinical diagnosis, food safety, therapeutic strategies, and for reducing potential infections [Ref: P. Lin, F. Yan, J. J. Yu,
  • OECTs Organic Electro-Chemical Transistors
  • the basic object of the present invention is to develop an ultra-low voltage operated OFET based bio-sensing system which would be adapted to detect and quantify Gram positive bacteria/Gram negative bacteria .
  • Another object of the present invention is to develop an ultra-low voltage operated OFET based bacteria sensor which can be used as flexible, low cost and disposable bio-sensors.
  • Yet another object of the present invention is to develop an ultra-low voltage operated n-type organic semiconductor based transistor based bacteria sensor which would be operable below 2V and exhibit high sensitivity in detection of the Gram positive bacteria/Gram negative bacteria .
  • a still further object of the present invention is to develop a simple, low cost method for fabricating the ultra-low voltage operated n-type organic semiconductor based transistor based bacteria sensor for detecting and quantifying Gram positive bacteria/Gram negative bacteria.
  • an ultra-low voltage operated Organic Field Effect Transistor (OFET) device for bio-sensing system for the detection of Gram positive bacteria and/or Gram negative bacteria comprising a base substrate; a gate electrode deposited on said base substrate; layered hybrid dielectrics deposited on said gate electrode having a combination of high-k and low-k triple layer dielectric system enabling reduced operating voltage; n-type organic semiconducting layer on top of said layered hybrid dielectrics; a source electrode and a drain electrode.
  • OFET Organic Field Effect Transistor
  • the source electrode and the drain electrode are deposited on the top side of the n-type organic semiconducting layer providing space there between for adsorbing the Gram positive and/or Gram negative bacteria to be detected directly over said n-type organic semiconducting layer.
  • an ultra-low voltage operated Organic Field Effect Transistor (OFET) based bio- sensing system for the detection of Gram positive bacteria and/or Gram negative bacteria comprising an OFET device having a base substrate; a gate electrode deposited on said base substrate; layered hybrid dielectrics deposited on said gate electrode having top and bottom dielectric layer of low dielectric constant (k) based dielectric materials and intermediate dielectric layer of high dielectric constant (k) based dielectric material; n-type organic semiconducting layer deposited on top of said layered hybrid dielectrics; a source electrode and a drain electrode deposited on the top side of said n- type organic semiconducting layer; and
  • OFET Organic Field Effect Transistor
  • Gram positive and/or Gram negative bacterial bio-analyte deposited on the n- type semiconducting layer between the source electrode and the drain electrode for the adsorption of Gram positive and/or Gram negative bacteria to be detected over the active channel and ensuring the direct contact of the bacterial bio-analyte with n-type organic semiconducting active channel.
  • the layered hybrid dielectrics of the OFET device comprises said bottom dielectric layer of low-k dielectric material on the gate electrode constituting a thin barrier layer in between the intermediate dielectric layer and the gate electrode to prevent gate leakage current of the OFET device and isolate the intermediate dielectric layer from said gate electrode; said intermediate dielectric layer of metal oxide nano-particle based high-k dielectric material on said bottom dielectric layer to reduce operating voltage and threshold voltage of the OFET device by induction of charges in the semiconducting active layer through the intermediate dielectric layer resulting improvement of charge carrier mobility in the active channel; and said top dielectric layer of low-k dielectric material on said metal oxide nano- particle based high-k dielectric material acting as buffer layer in between the high-k dielectric material and the active n-type organic semiconducting layer to reduce micro cracks on the surface of the high-k dielectric layer and prevent degradation of the n-type organic semiconductor of the active n-type organic semiconducting layer by protecting direct it
  • the layered hybrid dielectrics of the OFET device includes the low-k dielectric material for the bottom dielectric layer having k value preferably within 10-12, the low-k dielectric material for the top dielectric layer having k value preferably within 3-9 and high-k metal oxide nano-particle based dielectric material for the intermediate dielectric layer having k value preferably within 20-30 are selected to enable the intermediate dielectric layer dominant dielectric layer in the layered hybrid dielectrics.
  • the gate electrode of the OFET device comprises thermally deposited metal film preferably having thickness more than 200 nm with gate contact for application of gate voltage to the OFET device.
  • the bottom dielectric layer of the OFET device comprises a portion of the metal film oxidized through anodic oxidation preferably having thickness of with k-value ⁇ 10-12 to prevent the gate leakage up to 5 mA, whereby rest of the thick metal film operates as the gate electrode.
  • the intermediate dielectric layer of the OFET device comprises solution processable spin coated metal oxide NPs based high-k dielectric material deposition on the bottom dielectric layer having thickness of ⁇ 100-110 nm.
  • the top dielectric layer of the OFET device comprises spin coated dielectric material deposition on the intermediate dielectric layer having thickness of 80 ⁇ 200 nm and k value 3-9.
  • the n-type organic semiconducting layer based active channel of the OFET device includes thermal deposition of n-type monomer thin film having thickness of 60-100 nm.
  • the source electrode and the drain electrode of the OFET device includes thermally deposited metallic source-drain metal contact deposited on the top side of said n-type organic semiconducting layer to provide n-type organic semiconducting layer based active channel having channel length (L) and width (W) of 10-80 pm and 500-1000 pm respectively.
  • the adsorbing bacterial layer having the Gram positive bacteria and/or Gram negative bacteria above the semiconducting active channel of the OFET device and direct contact with said semiconducting active channel alters mobility of the active channel depending on the total surface charges of the cell wall and the outer wall structural arrangement of the adsorbed bacteria thus indicating nature of the bacteria.
  • the adsorbing bacterial layer with the Gram positive bacteria having negatively charged teichoic acid in the bacterial cell wall and much well-arranged bacterial cell wall compared to Gram Negative bacteria constitutes an additional channel along with the original active channel of the OFET device and thereby increase flow rate of charge carrier in the channel resulting increase in drain current of the OFET device, whereby presence of the negatively charged teichoic acid in the bacteria cell wall increases overall charge density at the active channel of the OFET device resulting shifting of the threshold voltage towards more negative.
  • the adsorbing bacterial layer with the Gram negative bacteria having uneven outer wall of the cell constitutes a resistive path along the original active channel and thereby decrease flow rate of charge carrier in the channel resulting decrease in the drain current of the OFET device, whereby presence of the Gram negative bacteria in adsorbing layer decrease the charge density at the channel resulting shifting of the threshold voltage towards more positive.
  • fabricating the OFET device by involving the base substrate; depositing the gate electrode on said base substrate; fabricating layered hybrid dielectric structure on said gate electrode by involving low-k dielectric materials in top and bottom dielectric layers and high-k dielectric material in intermediate dielectric layer; fabricating n-type organic semiconducting layer based active channel on top of said layered hybrid dielectric structure; depositing the source electrode and the drain electrode deposited deposited on the top side of said n-type organic semiconducting layer; and depositing the adsorbing bacterial layer on the n-type semiconducting layer between the source electrode and the drain electrode to the Gram positive and/or Gram negative bacteria to be detected over the active channel ensuring the adsorbed bacteria in said adsorbing layer stays above, and in direct contact with n-type organic semiconducting active channel.
  • the base substrate of the OFET device preferably includes glass substrate cleaned by acidic piranha solution (3: 1 ratio of H 2 S0 4 : H 2 0 2 ) or PET substrate cleaned by detergent.
  • the deposition of the gate electrode includes thermally depositing metal through a shadow mask on the base substrate forming the metal film preferably having thickness about 200 nm with gate contact.
  • the fabrication of the layered hybrid dielectric structure of the OFET device comprises oxidizing upper surface of the metal film by anodic oxidation to grow the bottom dielectric layer with k-value ⁇ 10-12 and thickness 13-60 nm; preparing sol-gel solution of the metal-oxide NPs based dielectric material having k-value ⁇ 20-30 and spin coating the sol-gel solution on the bottom dielectric layer to grow the intermediate dielectric layer of thickness ⁇ 100- 110 nm and k-value ⁇ 20-30; preparing solution of dialectic material having k-value 3-9 and spin coating the dielectric solution on the intermediate dielectric layer to grow the top dielectric layer of thickness ⁇ 80 - 200 nm and k-value ⁇ 3-9.
  • the fabrication of the n-type organic semiconducting layer based active channel comprises preparing n-type monomer; applying a thin film of the n-type monomer on the layered hybrid dielectrics having thickness of 60-100 nm by involving thermal deposition technique in organic thermal deposition chamber, wherein the monomer is sublimed during deposition to ensure the substrate temperature can be kept constant, ranging from room temperature to 150°C under 10 7 mbar pressure.
  • the deposition of the source electrode and the drain electrode includes thermally depositing metallic source-drain contact of thickness 80-130 on the n-type organic semiconducting layer through a shadow mask to define the active channel on the n-type organic semiconducting layer having length (L) and width (W) of 10-80 pm and 500-1000 pm respectively.
  • the fabrication of the adsorbing layer on the n-type semiconducting layer includes involving bacteria based analyte in de-ionized water media; drop-casting the bacteria based analyte solution on the active channel between the source and the drain electrode; vacuum drying the drop-casted solution under dark to evaporate the water and forming the adsorbing layer with the bacteria.
  • a method for detection of the Gram positive bacteria and/or Gram negative bacteria by involving the OFET based bio-sensing system comprising the steps of driving the OFET device by applying a gate voltage and a drain voltage to the device; measuring drain current and gate current before forming the adsorbing bacterial layer on the active channel of the OFET device; measuring drain current and gate current after forming the adsorbing bacterial layer on the active channel of the OFET device;
  • the present method for detection of the Gram positive bacteria and/or Gram negative bacteria by involving the OFET based bio-sensing system includes applying gate voltage and drain voltage between about 0 V and 2 V; comparing the measured current values of the OFET device corresponding to the pre and post formation of the adsorbing layer with the adsorbed bacteria on the active channel of the OFET device including noting increment in mobility value of the OFET device from 0.30 cm 2 /Vs in pre-formation of the adsorbing layer to the mobility value about 0.50 cm 2 /Vs in post-formation of the adsorbing layer with the adsorbed Gram positive bacteria with a negative threshold voltage shift from 0.2 V to about -0.5 V in the transfer curves of the OFET device; noting decrement in mobility value of the OFET device from 0.30 cm 2 /Vs in pre-formation of the adsorbing layer to the mobility value about 0.27 cm 2 /Vs in post-formation of the adsorbing layer with the adsorbed Gram negative bacteria with a positive
  • Figure 1 shows schematic representation of a preferred embodiment of the OFET based bio-sensing system with a top contact bottom-gate configuration for the detection of biological species such as Gram positive bacteria, Gram negative bacteria in accordance with the present invention.
  • Figure 2 shows Drain Characteristics of the fabricated n-type OFET based bio- sensing system in accordance with an embodiment of the present invention.
  • Figure 3 shows Transfer characteristics of the fabricated n-type OFET based bio-sensing system in accordance with an embodiment of the present invention.
  • Figure 4 shows FESEM images of the bacteria layers (with 10 3 dilution) on 40 pm channel of n-type OFETs in accordance with the present invention.
  • Figure 5 shows Transfer Characteristics of the fabricated n-type OFET based bio-sensing system with and without bacteria cell wall in accordance with an embodiment of the present invention.
  • the present invention discloses an ultra-low voltage operated, highly stable, Organic Field Effect Transistor (OFET) based bio-sensing system for the detection of biological species such as Gram positive bacteria, Gram negative bacteria.
  • the OFET based bio-sensing system of the present invention advantageously includes a combination of high-k and low-k triple-layer dielectric structure which significantly lower the operating voltage of the OFET and prevent its gate leakage current.
  • the present invention also discloses fabrication of ultra-low operating voltage organic thin film transistor sensor comprising of triple-layer dielectric system which are compatible with any desired substrate on simple glass slide or PET plastic substrate and can be used for the detection of the Gram positive and Gram negative bacteria with n-type organic semiconductor as the active layer under room temperature conditions.
  • FIG. 1 shows a schematic illustration of a preferred embodiment of the present OFET based bio-sensing system (1) which includes a base substrate (2), a gate electrode (3), hybrid multilayered dielectrics (4-5-6), a n-type organic semiconducting layer (7), a source electrode (8) and a drain electrode (9).
  • the OFET based bio-sensing system (1) further includes adsorbing bacterial layer (10) between the source electrode (8) and the drain electrode (9) for the detection Gram positive and Gram negative bacteria.
  • the base substrate (2) constitutes the lowest level of the OFET device of the present OFET based bio-sensing system (1).
  • the base substrate (2) is preferably made of glass or PET and provides structural stability to the entire system (1).
  • the gate electrode (3) which is basically a metal film with gate contact is thermally deposited on the base substrate (2).
  • the layered hybrid dielectrics (4-5-6) of the OFET based bio-sensing system (1) is fabricated on the gate electrode (3).
  • the multilayered hybrid dielectric structure of the present system as shown in the figure 1 includes three dielectric layers of dielectric materials having different dielectric constant (k) viz. top dielectric layer (4), bottom dielectric layer (6) and intermediate dielectric layer (5).
  • the entire layered hybrid dielectrics has very important role in lowering operating voltage of the OFET device of the present OFET based bio-sensing system (1).
  • the bottom dielectric layer (6) of the layered hybrid dielectrics comprises dielectric material with low dielectric constant (k) (preferably k-value ⁇ 10-12) and it is deposited on the gate electrode (3) by using anodic oxidation method to constitute a thin barrier layer in between the intermediate dielectric layer (5) and the gate electrode (3).
  • the bottom low-k dielectric layer (6) is basically provided to prevent gate current leakage and avoid any direct contact between the intermediate dielectric layer (5) and the gate electrode (3).
  • the bottom dielectric layer (6) is constituted by oxidizing upper surface portion of the metallic film of the gate electrode (3) through anodic oxidation.
  • the intermediate dielectric layer (5) comprises dielectric material having high dielectric constant (k-value ⁇ 20-30) and it is fabricated over the bottom low-k dielectric layer (6) by spin coating sol-gel solution of the high-k dielectric material onto the bottom dielectric layer (6).
  • the intermediate dielectric layer (5) reduces the operating voltage of the OFET and simultaneously helps to improve the sensitivity of the OFET based bio-sensing system (1) by induction of charges in the semiconducting layer based active channel through the intermediate dielectric layer resulting improvement of charge carrier mobility in the active channel.
  • the top dielectric layer (4) of biocompatible dielectric material with low k value (having k- value ⁇ 3-9) is deposited over the intermediate dielectric layer (5) to act as a buffer layer in between the intermediate layer (5) and the semiconducting layer (7) deposited over the layered hybrid dielectrics.
  • the top dielectric layer (4) is basically provided to reduce micro cracks on the surface of the intermediate dielectric layer (5) and prevent the degradation of n-type semiconductor by avoiding the direct contact between the intermediate dielectric layer and the n-type semiconducting layer (7).
  • the dielectric constant of the dielectric materials corresponding to the layered hybrid dielectric structure are selectively chosen to ensure that the intermediate dielectric layer becomes more dominant layer compared to the other two.
  • the n-type semiconducting layer (7) which is deposited over the hybrid layered dielectrics comprise n-type organic semiconductor and constitutes active channel of the OFET device for the detection of Gram positive and Gram negative bacteria.
  • the source electrode (8) and the drain electrode (9) are deposited on the top side of the n-type organic semiconducting layer (7) defining the effective length and width of the active channel.
  • the adsorbing bacterial layer (10) is grown on the n-type semiconducting layer (7) based active channel between the source electrode (8) and the drain electrode (9) of the OFET device of the present OFET based bio-sensing system (1) for immobilization of the Gram positive and / or the Gram negative bacteria over the active channel such that the bacteria adsorbed in this layer (10) stays above, and in direct contact with, the semiconductor of the active channel .
  • IDS C 0X p e (W/2L) (V GS -V Th ) 2 ,
  • I D s is the drain current
  • C ox is the capacitance per unit area of the gate dielectric layer
  • p e is the field effect electron mobility
  • W and L are the active channel width and length
  • V GS and V Th are the gate voltage and threshold voltage of the OFET device respectively.
  • the bacteria based analyte is taken in de-ionized water media and 1 pl_ of the solution is drop casted on the effective active channel of the OFET device and vacuum dried to evaporate the water and form the adsorbing bacterial layer.
  • FESEM image may also be taken after the immobilization.
  • the drain current of the OFET of the system (1), post immobilizations of the bacteria is again calculated under dark and vacuum condition and presence of the Gram positive or Gram negative bacteria in the adsorbing layer is determined based on the variation of the drain current.
  • the accompanying figure 2 describes the drain characteristics of the present n-type OFET based bio-sensing system.
  • the black line signifies the characteristics of the system without bacteria, whereas the colored line signifies the system properties in presence of bacteria.
  • drain current of the OFETs increased in presence of Gram positive bacteria due to the presence of negatively charged teichoic acid in the bacterial cell wall, which further create an additional channel along with the original active channel of the OFETs and help to increase the flow rate of charge carrier in the channel.
  • drain current increases.
  • the Gram negative bacteria are also negatively charge, it is observed completely opposite behavior from Figure 2(c) and 2(d). This is due to uneven outer wall structure of the cell wall of Gram negative bacteria compared to Gram positive, which creates more resistive path along the channel as a result of which drain current decreases.
  • the accompanying figure 3 shows the transfer characteristics of the present n-type OFET based bio-sensing system.
  • the black line signifies the characteristics of the device without the bacteria whereas the colored line signifies the device properties in presence of bacteria as like Figure 2.
  • the figure 3 actually supported the explanation of the result shown in the figure 2. Due to the symmetrical structure of the Gram positive bacteria and the presence of negatively charged teichoic acid in the cell wall, the overall charge density increases at the channel of the OFETs. As a results The V Th values of Figure 3(a) and 3(b) shifted towards more negative, whereas due to the decrement of charge density at the channel in presence of Gram negative bacteria, the V Th values of Figure 3(c) and 3(d) shifted towards more positive.
  • the accompanying figure 4 shows the FESEM images of bacterial layers on the active channel of the present n-type OFET based bio-sensing system.
  • the present invention also discloses a novel method for fabricating the bio- sensing system for the detection of Gram positive and Gram negative bacteria by involving ultra-low operating voltage n-type OFET device.
  • the method for fabricating the bio-sensing system by involving, ultra-low voltage operated, stable n-type OFET device with a top contact bottom-gate configuration are summarized in below-
  • Step 1 Substrate cleaning method :
  • the glass slides or the PET are cut into (1 cm x 2.5 cm) square substrates.
  • the glass substrate is then cleaned by dipping the substrate in acidic piranha solution (3: 1 ratio of H 2 S04: H20 2 ) for 1 hour. After that the glass substrate is vigorously washed 8-10 times by using de-ionized water to remove the acidic layer on the substrate surface and then dried at 100 °C on hot plate.
  • the same is first cleaned with detergent 2-3 times and then vigorously washed 8-10 times by de-ionized water.
  • Step 2 Gate electrode and first low-k dielectric layer deposition by anodic oxidation method :
  • the metal thin film having thickness of 200 nm is thermally deposited on its top through a shadow mask to constitute the gate electrode.
  • Upper surface of the metal film of the gate electrode preferably having thickness of ⁇ 13-60 nm is oxidized by anodic oxidation method to grow the bottom dielectric layer having k- value ⁇ 10-12. The rest of the metal film is used for the gate contact. The function of this bottom dielectric layer is to prevent the gate leakage up to 5 mA and to prevent the direct contact of the intermediate dielectric layer with the gate electrode.
  • the intermediate dielectric layer is prepared by sol-gel method .
  • This intermediate dielectric layer can be used for low voltage operations and can be the type as listed below in Table 2.
  • the sol-gel solution of any of the high-k dielectric material listed in the Table is prepared and then spin coated on top of the top dielectric layer in such a way that the thickness of the layer is ⁇ 100- 110 nm.
  • This synthesized dielectric layer has k-value range from 20-30.
  • Step 4 Deposition of top low-k dielectric layer:
  • the top dielectric layer is fabricated over the intermediate dielectric layer by spin coating technique.
  • solution of the possible dielectric material having k value ⁇ 3-9 is spin coated on the intermediate dielectric layer in such a way that it can be used as a buffer layer in between dielectric layers and the n-type active semiconducting layer. It is also used to prevent the degradation of n-type semiconductor by protecting direct contact with the intermediate dielectric layer.
  • the possible top dielectric layer materials which can be used for this purpose are listed below in Table 3. TABLE-3
  • Step 5 Deposition of n-type active layer and source-drain electrodes:
  • a n-type semiconducting thin film of monomer is deposited on the layered hybrid dielectrics by thermal deposition technique in an organic thermal deposition chamber to constitute the n-type active organic semiconducting layer.
  • the monomer is sublimed in such a way that during deposition, the substrate temperature can be kept constant, ranging from room temperature to 150°C under 10 7 mbar pressure.
  • the thickness of the film is varied between 60-100 nm.
  • 80-130 nm thick metallic source-drain contact (preferably Ag) is thermally deposited on two opposite side of the n-type active organic semiconducting layer through a shadow mask to constitute the active channel of length (L) and width (W) of 10-80 pm and 500-1000 pm respectively.
  • the bacteria based analyte is taken in de-ionized water media and 1 pL of the solution is drop casted on the effective channel and vacuum dried overnight under dark to evaporate the water.
  • FESEM image is taken after the immobilization.
  • the all the electrical properties are again characterized by Keithley 4200-SCS semiconductor parameter analyzer under dark and vacuum condition in presence of the bacteria.
  • the present invention provides a method for detection of the Gram positive bacteria and/or Gram negative bacteria by involving the present OFET based bio-sensing system comprising the following steps:
  • Step i driving the OFET device by applying a gate voltage and a drain voltage to the device;
  • Step ii measuring drain current and gate current before forming the adsorbing bacterial layer on the active channel of the OFET device;
  • Step iii measuring drain current and gate current after forming the adsorbing layer with the adsorbed bacteria on the active channel of the OFET device;
  • Step iv comparing the measured current values of the OFET device corresponding to the pre and post formation of the adsorbing bacterial layer on the active channel of the OFET device and determining the nature of the bacteria in the adsorbing layer based on the variation of the measured currents.
  • the gate voltage and drain voltage applied may be between about 0 V and 2 V.
  • the n-type organic semiconductor based low-cost, disposable and OFET device of the present OFET based bio-sensing system containing triple-layer dielectric system exhibits excellent sensitivity in presence of Gram positive and Gram negative bacteria.
  • this triple-layer dielectric layer containing OFET device is operated under very low voltage ( ⁇ 2V) and are compatible with different low cost substrate like pristine PET substrate and this combination of dielectric layer can be used for the detection of several biological species by changing only different suitable active layer material.
  • ⁇ 2V very low voltage

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Abstract

The present invention discloses an ultra-low voltage operated OFET based bio-sensing system for the detection of biological species5 including Gram positive bacteria and/or Gram negative bacteria comprising an OFET device having a base substrate, a gate electrode deposited on said base substrate, layered hybrid dielectrics deposited on said gate electrode having top and bottom dielectric layer of low dielectric constant (k) based dielectric materials and intermediate dielectric layer of high dielectric constant (k)10 based dielectric material, n-type organic semiconducting layer based active channel deposited on top of said layered hybrid dielectrics, a source electrode and a drain electrode deposited on the top side said n-type organic semiconducting layer; and adsorbing layer grown on the n-type semiconducting layer between the source electrode and the drain electrode to15 adsorbed the Gram positive and/or Gram negative bacteria to be detected over the active channel ensuring the adsorbed bacteria in said adsorbing layer stays above, and in direct contact with n-type organic semiconducting active channel.

Description

ORGANIC FIELD EFFECT TRANSISTOR (OFET) FOR THE DETECTION OF GRAM NEGATIVE/POSITIVE BACTERIA, METHOD OF DETECTION AND
FABRICATION OF THE TRANSISTOR
FIELD OF THE INVENTION:
The present invention relates to transistor based biosensor. More specifically, the present invention is directed to develop an ultra-low voltage operated, highly stable, Organic Field Effect Transistor (OFET) based bio-sensing system for detection of Gram positive and Gram negative bacteria and a method for fabricating such OFET based bio-sensing system. The present OFET based bio- sensing system is particularly useful for detecting and quantifying different biological species/Gram positive bacteria/Gram negative bacteria.
BACKGROUND OF THE INVENTION:
Organic Field Effect Transistors (OFETs) have been extensively studied since the past few decades because of their promising applications in various chemical and biological sensing applications [Ref: J. Janata ; M. Josowicz, Nat. Mater. 2003 , 2, 19; Z. H. Sun , J. H. Li, C. M. Liu, S. H. Yang, F. Yan, Adv. Mater. 2011, 23, 3648; F. Yan, H. Tang, Expert Rev. Mol. Diagn. 2010, 10, 547; M. Berggren, A. Richter-Dahlfors, Adv. Mater. 2007, 19, 3201 ]
A transistor-based biosensor normally has been in high demand because of its high sensitivity since the single device can be able to sense the analyte and as well as can amplify its response [Ref: R. M. Owens, G. G. Malliaras, MRS Bull. 2010, 35, 449; Q. T. Zhang, V. Subramanian, Biosens. Bioelectron. 2007, 22, 3182; C. Bartic, A. Campitelli, S. Borghs, Appl. Phys. Lett. 2003,
82, 475].
In addition, organic materials are more compatible with biological elements, which is very essential for an effective sensor device. Compared to other inorganic biosensors, the low cost, flexible and ease to fabricate OFET seems to be one of the ideal disposable sensing devices which can deliver accurate results in the new generation of flexible electronics [Ref: L. Jagannathan, V. Subramanian, Biosens. Bioelectron. 2009, 25, 288; Q. T. Zhang, L. Jagannathan, V. Subramanian, Biosens. Bioelectron. 2010, 25, 972; G. Scarpa ; A. L. Idzko, A. Yadav, E. Martin , S. Thalhammer ; IEEE Trans. Nanotechnol. 2010, 9, 527 ; H. U. Khan, J. Jang, J.-J. Kim, W. Knoll, Biosens. Bioelectron. 2011, 26, 4217].
Among the biosensors, bacteria sensor is one of the promising since bacterial contamination is a major health hazard especially in the context of food safety, environmental monitoring, and the pharmaceutical industry. It assumes even greater significance in case of pathogens as the presence of even a single cell may lead to serious health risk. Thus, rapid quantification of bacteria is imperative for clinical diagnosis, food safety, therapeutic strategies, and for reducing potential infections [Ref: P. Lin, F. Yan, J. J. Yu,
H. L. W. Chan, M. Yang, Adv. Mater. 2010, 22, 3655; P. Lin , X. Luo, I. M.
Hsing, F. Yan, Adv. Mater. 2011, 23, 4035; K. Krishnamoorthy, R. S. Gokhale, A. Q. Contractor, A. Kumar, Chem. Commun. 2004, 820]
But the sensing of bacteria or other living cells are generally detected by Organic Electro-Chemical Transistors (OECTs) in literature since it can be easily fabricated and mainly operated under low operating voltage. On the other hand the main disadvantage of OECT is slower response time compared to OFETs and it cannot be easily handled since it contains electrolytic solution [Ref: M. H. Bolin, K. Svennersten, D. Nilsson, A. Sawatdee, E. W. H. Jager, A. Richter-Dahlfors, M. Berggren, Adv. Mater. 2009, 21, 4379].
Thus there has been a need for developing new ultra-low voltage operated OFET based bacteria sensor which successfully overcomes all the disadvantages of OFET over OECT and adapted to be used as flexible, cheap and disposable bio-sensors.
OBJECT OF THE INVENTION:
It is thus the basic object of the present invention is to develop an ultra-low voltage operated OFET based bio-sensing system which would be adapted to detect and quantify Gram positive bacteria/Gram negative bacteria . Another object of the present invention is to develop an ultra-low voltage operated OFET based bacteria sensor which can be used as flexible, low cost and disposable bio-sensors. Yet another object of the present invention is to develop an ultra-low voltage operated n-type organic semiconductor based transistor based bacteria sensor which would be operable below 2V and exhibit high sensitivity in detection of the Gram positive bacteria/Gram negative bacteria .
A still further object of the present invention is to develop a simple, low cost method for fabricating the ultra-low voltage operated n-type organic semiconductor based transistor based bacteria sensor for detecting and quantifying Gram positive bacteria/Gram negative bacteria.
SUMMARY OF THE INVENTION:
Thus according to the basic aspect of the present invention there is provided an ultra-low voltage operated Organic Field Effect Transistor (OFET) device for bio-sensing system for the detection of Gram positive bacteria and/or Gram negative bacteria comprising a base substrate; a gate electrode deposited on said base substrate; layered hybrid dielectrics deposited on said gate electrode having a combination of high-k and low-k triple layer dielectric system enabling reduced operating voltage; n-type organic semiconducting layer on top of said layered hybrid dielectrics; a source electrode and a drain electrode.
In the above ultra-low voltage operated Organic Field Effect Transistor (OFET) device for bio-sensing system, the source electrode and the drain electrode are deposited on the top side of the n-type organic semiconducting layer providing space there between for adsorbing the Gram positive and/or Gram negative bacteria to be detected directly over said n-type organic semiconducting layer.
According to another aspect in the present invention there is provided an ultra-low voltage operated Organic Field Effect Transistor (OFET) based bio- sensing system for the detection of Gram positive bacteria and/or Gram negative bacteria comprising an OFET device having a base substrate; a gate electrode deposited on said base substrate; layered hybrid dielectrics deposited on said gate electrode having top and bottom dielectric layer of low dielectric constant (k) based dielectric materials and intermediate dielectric layer of high dielectric constant (k) based dielectric material; n-type organic semiconducting layer deposited on top of said layered hybrid dielectrics; a source electrode and a drain electrode deposited on the top side of said n- type organic semiconducting layer; and
Gram positive and/or Gram negative bacterial bio-analyte deposited on the n- type semiconducting layer between the source electrode and the drain electrode for the adsorption of Gram positive and/or Gram negative bacteria to be detected over the active channel and ensuring the direct contact of the bacterial bio-analyte with n-type organic semiconducting active channel.
In an embodiment of the present OFET based bio-sensing system, the layered hybrid dielectrics of the OFET device comprises said bottom dielectric layer of low-k dielectric material on the gate electrode constituting a thin barrier layer in between the intermediate dielectric layer and the gate electrode to prevent gate leakage current of the OFET device and isolate the intermediate dielectric layer from said gate electrode; said intermediate dielectric layer of metal oxide nano-particle based high-k dielectric material on said bottom dielectric layer to reduce operating voltage and threshold voltage of the OFET device by induction of charges in the semiconducting active layer through the intermediate dielectric layer resulting improvement of charge carrier mobility in the active channel; and said top dielectric layer of low-k dielectric material on said metal oxide nano- particle based high-k dielectric material acting as buffer layer in between the high-k dielectric material and the active n-type organic semiconducting layer to reduce micro cracks on the surface of the high-k dielectric layer and prevent degradation of the n-type organic semiconductor of the active n-type organic semiconducting layer by protecting direct it's contact with the high-k metal oxide nano-particle based dielectric layer.
In an embodiment of the present OFET based bio-sensing system, the layered hybrid dielectrics of the OFET device includes the low-k dielectric material for the bottom dielectric layer having k value preferably within 10-12, the low-k dielectric material for the top dielectric layer having k value preferably within 3-9 and high-k metal oxide nano-particle based dielectric material for the intermediate dielectric layer having k value preferably within 20-30 are selected to enable the intermediate dielectric layer dominant dielectric layer in the layered hybrid dielectrics.
In an embodiment of the present OFET based bio-sensing system, the gate electrode of the OFET device comprises thermally deposited metal film preferably having thickness more than 200 nm with gate contact for application of gate voltage to the OFET device.
In an embodiment of the present OFET based bio-sensing system, the bottom dielectric layer of the OFET device comprises a portion of the metal film oxidized through anodic oxidation preferably having thickness of with k-value ~10-12 to prevent the gate leakage up to 5 mA, whereby rest of the thick metal film operates as the gate electrode.
In an embodiment of the present OFET based bio-sensing system, the intermediate dielectric layer of the OFET device comprises solution processable spin coated metal oxide NPs based high-k dielectric material deposition on the bottom dielectric layer having thickness of ~100-110 nm.
In an embodiment of the present OFET based bio-sensing system, the top dielectric layer of the OFET device comprises spin coated dielectric material deposition on the intermediate dielectric layer having thickness of 80 ~ 200 nm and k value 3-9.
In an embodiment of the present OFET based bio-sensing system, the n-type organic semiconducting layer based active channel of the OFET device includes thermal deposition of n-type monomer thin film having thickness of 60-100 nm.
In an embodiment of the present OFET based bio-sensing system, the source electrode and the drain electrode of the OFET device includes thermally deposited metallic source-drain metal contact deposited on the top side of said n-type organic semiconducting layer to provide n-type organic semiconducting layer based active channel having channel length (L) and width (W) of 10-80 pm and 500-1000 pm respectively.
In an embodiment of the present OFET based bio-sensing system, the adsorbing bacterial layer having the Gram positive bacteria and/or Gram negative bacteria above the semiconducting active channel of the OFET device and direct contact with said semiconducting active channel alters mobility of the active channel depending on the total surface charges of the cell wall and the outer wall structural arrangement of the adsorbed bacteria thus indicating nature of the bacteria.
In an embodiment of the present OFET based bio-sensing system, the adsorbing bacterial layer with the Gram positive bacteria having negatively charged teichoic acid in the bacterial cell wall and much well-arranged bacterial cell wall compared to Gram Negative bacteria constitutes an additional channel along with the original active channel of the OFET device and thereby increase flow rate of charge carrier in the channel resulting increase in drain current of the OFET device, whereby presence of the negatively charged teichoic acid in the bacteria cell wall increases overall charge density at the active channel of the OFET device resulting shifting of the threshold voltage towards more negative.
In an embodiment of the present OFET based bio-sensing system, the adsorbing bacterial layer with the Gram negative bacteria having uneven outer wall of the cell constitutes a resistive path along the original active channel and thereby decrease flow rate of charge carrier in the channel resulting decrease in the drain current of the OFET device, whereby presence of the Gram negative bacteria in adsorbing layer decrease the charge density at the channel resulting shifting of the threshold voltage towards more positive.
According to another aspect in the present invention there is provided a method for fabricating the OFET based bio-sensing system comprising the steps of
fabricating the OFET device by involving the base substrate; depositing the gate electrode on said base substrate; fabricating layered hybrid dielectric structure on said gate electrode by involving low-k dielectric materials in top and bottom dielectric layers and high-k dielectric material in intermediate dielectric layer; fabricating n-type organic semiconducting layer based active channel on top of said layered hybrid dielectric structure; depositing the source electrode and the drain electrode deposited deposited on the top side of said n-type organic semiconducting layer; and depositing the adsorbing bacterial layer on the n-type semiconducting layer between the source electrode and the drain electrode to the Gram positive and/or Gram negative bacteria to be detected over the active channel ensuring the adsorbed bacteria in said adsorbing layer stays above, and in direct contact with n-type organic semiconducting active channel.
In the present method for fabricating the OFET based bio-sensing system, the base substrate of the OFET device preferably includes glass substrate cleaned by acidic piranha solution (3: 1 ratio of H2S04: H202) or PET substrate cleaned by detergent.
In the present method for fabricating the OFET based bio-sensing system, the deposition of the gate electrode includes thermally depositing metal through a shadow mask on the base substrate forming the metal film preferably having thickness about 200 nm with gate contact.
In the present method for fabricating the OFET based bio-sensing system, the fabrication of the layered hybrid dielectric structure of the OFET device comprises oxidizing upper surface of the metal film by anodic oxidation to grow the bottom dielectric layer with k-value ~10-12 and thickness 13-60 nm; preparing sol-gel solution of the metal-oxide NPs based dielectric material having k-value ~20-30 and spin coating the sol-gel solution on the bottom dielectric layer to grow the intermediate dielectric layer of thickness ~100- 110 nm and k-value ~20-30; preparing solution of dialectic material having k-value 3-9 and spin coating the dielectric solution on the intermediate dielectric layer to grow the top dielectric layer of thickness ~80 - 200 nm and k-value ~3-9. In the present method for fabricating the OFET based bio-sensing system, the fabrication of the n-type organic semiconducting layer based active channel comprises preparing n-type monomer; applying a thin film of the n-type monomer on the layered hybrid dielectrics having thickness of 60-100 nm by involving thermal deposition technique in organic thermal deposition chamber, wherein the monomer is sublimed during deposition to ensure the substrate temperature can be kept constant, ranging from room temperature to 150°C under 10 7 mbar pressure.
In the present method for fabricating the OFET based bio-sensing system, the deposition of the source electrode and the drain electrode includes thermally depositing metallic source-drain contact of thickness 80-130 on the n-type organic semiconducting layer through a shadow mask to define the active channel on the n-type organic semiconducting layer having length (L) and width (W) of 10-80 pm and 500-1000 pm respectively.
In the present method for fabricating the OFET based bio-sensing system, the fabrication of the adsorbing layer on the n-type semiconducting layer includes involving bacteria based analyte in de-ionized water media; drop-casting the bacteria based analyte solution on the active channel between the source and the drain electrode; vacuum drying the drop-casted solution under dark to evaporate the water and forming the adsorbing layer with the bacteria.
According to another aspect in the present invention there is provided a method for detection of the Gram positive bacteria and/or Gram negative bacteria by involving the OFET based bio-sensing system comprising the steps of driving the OFET device by applying a gate voltage and a drain voltage to the device; measuring drain current and gate current before forming the adsorbing bacterial layer on the active channel of the OFET device; measuring drain current and gate current after forming the adsorbing bacterial layer on the active channel of the OFET device;
comparing the measured current values of the OFET device corresponding to the pre and post formation of the adsorbing layer with the adsorbed bacteria on the active channel of the OFET device and determining the nature of the bacteria in the layer based on the variation of the measured currents.
The present method for detection of the Gram positive bacteria and/or Gram negative bacteria by involving the OFET based bio-sensing system, includes applying gate voltage and drain voltage between about 0 V and 2 V; comparing the measured current values of the OFET device corresponding to the pre and post formation of the adsorbing layer with the adsorbed bacteria on the active channel of the OFET device including noting increment in mobility value of the OFET device from 0.30 cm2/Vs in pre-formation of the adsorbing layer to the mobility value about 0.50 cm2/Vs in post-formation of the adsorbing layer with the adsorbed Gram positive bacteria with a negative threshold voltage shift from 0.2 V to about -0.5 V in the transfer curves of the OFET device; noting decrement in mobility value of the OFET device from 0.30 cm2/Vs in pre-formation of the adsorbing layer to the mobility value about 0.27 cm2/Vs in post-formation of the adsorbing layer with the adsorbed Gram negative bacteria with a positive threshold voltage shift from 0.2 V to about 0.5 V in the transfer curves of the OFET device; and noting level of variation in the mobility value and the threshold voltage shift depending variation of the concentration of both the bacteria.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1 shows schematic representation of a preferred embodiment of the OFET based bio-sensing system with a top contact bottom-gate configuration for the detection of biological species such as Gram positive bacteria, Gram negative bacteria in accordance with the present invention. Figure 2 shows Drain Characteristics of the fabricated n-type OFET based bio- sensing system in accordance with an embodiment of the present invention.
Figure 3 shows Transfer characteristics of the fabricated n-type OFET based bio-sensing system in accordance with an embodiment of the present invention.
Figure 4 shows FESEM images of the bacteria layers (with 10 3 dilution) on 40 pm channel of n-type OFETs in accordance with the present invention. Figure 5 shows Transfer Characteristics of the fabricated n-type OFET based bio-sensing system with and without bacteria cell wall in accordance with an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS:
The present invention discloses an ultra-low voltage operated, highly stable, Organic Field Effect Transistor (OFET) based bio-sensing system for the detection of biological species such as Gram positive bacteria, Gram negative bacteria. The OFET based bio-sensing system of the present invention advantageously includes a combination of high-k and low-k triple-layer dielectric structure which significantly lower the operating voltage of the OFET and prevent its gate leakage current. The present invention also discloses fabrication of ultra-low operating voltage organic thin film transistor sensor comprising of triple-layer dielectric system which are compatible with any desired substrate on simple glass slide or PET plastic substrate and can be used for the detection of the Gram positive and Gram negative bacteria with n-type organic semiconductor as the active layer under room temperature conditions.
Reference is first invited from the accompanying figure 1 which shows a schematic illustration of a preferred embodiment of the present OFET based bio-sensing system (1) which includes a base substrate (2), a gate electrode (3), hybrid multilayered dielectrics (4-5-6), a n-type organic semiconducting layer (7), a source electrode (8) and a drain electrode (9). The OFET based bio-sensing system (1) further includes adsorbing bacterial layer (10) between the source electrode (8) and the drain electrode (9) for the detection Gram positive and Gram negative bacteria.
As shown in the referred figure 1, the base substrate (2) constitutes the lowest level of the OFET device of the present OFET based bio-sensing system (1). The base substrate (2) is preferably made of glass or PET and provides structural stability to the entire system (1). The gate electrode (3) which is basically a metal film with gate contact is thermally deposited on the base substrate (2).
The layered hybrid dielectrics (4-5-6) of the OFET based bio-sensing system (1) is fabricated on the gate electrode (3). The multilayered hybrid dielectric structure of the present system as shown in the figure 1 includes three dielectric layers of dielectric materials having different dielectric constant (k) viz. top dielectric layer (4), bottom dielectric layer (6) and intermediate dielectric layer (5). The entire layered hybrid dielectrics has very important role in lowering operating voltage of the OFET device of the present OFET based bio-sensing system (1).
The bottom dielectric layer (6) of the layered hybrid dielectrics comprises dielectric material with low dielectric constant (k) (preferably k-value~10-12) and it is deposited on the gate electrode (3) by using anodic oxidation method to constitute a thin barrier layer in between the intermediate dielectric layer (5) and the gate electrode (3). The bottom low-k dielectric layer (6) is basically provided to prevent gate current leakage and avoid any direct contact between the intermediate dielectric layer (5) and the gate electrode (3).
In a preferred embodiment, the bottom dielectric layer (6) is constituted by oxidizing upper surface portion of the metallic film of the gate electrode (3) through anodic oxidation.
The intermediate dielectric layer (5) comprises dielectric material having high dielectric constant (k-value ~20-30) and it is fabricated over the bottom low-k dielectric layer (6) by spin coating sol-gel solution of the high-k dielectric material onto the bottom dielectric layer (6). The intermediate dielectric layer (5) reduces the operating voltage of the OFET and simultaneously helps to improve the sensitivity of the OFET based bio-sensing system (1) by induction of charges in the semiconducting layer based active channel through the intermediate dielectric layer resulting improvement of charge carrier mobility in the active channel.
Since mobility depends upon the n-type organic semiconducting layer (7) surface morphology, smoothness of the intermediate dielectric surface is vital, together with a biocompatible, low-k dielectric that is used as a top dielectric layer, before the active n-type organic semiconducting layer deposition. The top dielectric layer (4) of biocompatible dielectric material with low k value (having k- value ~3-9) is deposited over the intermediate dielectric layer (5) to act as a buffer layer in between the intermediate layer (5) and the semiconducting layer (7) deposited over the layered hybrid dielectrics. The top dielectric layer (4) is basically provided to reduce micro cracks on the surface of the intermediate dielectric layer (5) and prevent the degradation of n-type semiconductor by avoiding the direct contact between the intermediate dielectric layer and the n-type semiconducting layer (7).
In the present system the dielectric constant of the dielectric materials corresponding to the layered hybrid dielectric structure are selectively chosen to ensure that the intermediate dielectric layer becomes more dominant layer compared to the other two.
The n-type semiconducting layer (7) which is deposited over the hybrid layered dielectrics comprise n-type organic semiconductor and constitutes active channel of the OFET device for the detection of Gram positive and Gram negative bacteria. The source electrode (8) and the drain electrode (9) are deposited on the top side of the n-type organic semiconducting layer (7) defining the effective length and width of the active channel.
The adsorbing bacterial layer (10) is grown on the n-type semiconducting layer (7) based active channel between the source electrode (8) and the drain electrode (9) of the OFET device of the present OFET based bio-sensing system (1) for immobilization of the Gram positive and / or the Gram negative bacteria over the active channel such that the bacteria adsorbed in this layer (10) stays above, and in direct contact with, the semiconductor of the active channel .
In the present OFET based bio-sensing system (1), before the immobilizations of the bacteria, all the electrical properties are measured under dark and vacuum condition. The drain current of the OFET of the system (1) is calculated by using the equation :
IDS=C0X pe (W/2L) (VGS-VTh)2, Where, IDs is the drain current, Cox is the capacitance per unit area of the gate dielectric layer, pe is the field effect electron mobility, W and L are the active channel width and length, VGS and VTh are the gate voltage and threshold voltage of the OFET device respectively. For the immobilization of the Gram positive and/or Gram negative bacteria, the bacteria based analyte is taken in de-ionized water media and 1 pl_ of the solution is drop casted on the effective active channel of the OFET device and vacuum dried to evaporate the water and form the adsorbing bacterial layer. For confirmation of the presence of bacteria on the channel, FESEM image may also be taken after the immobilization.
The drain current of the OFET of the system (1), post immobilizations of the bacteria is again calculated under dark and vacuum condition and presence of the Gram positive or Gram negative bacteria in the adsorbing layer is determined based on the variation of the drain current.
Reference in this regards are invited from the accompanying figures 2 and 3, which shows Pre and Post bacteria immobilization Drain and Transfer Characteristics of the present n-type OFET based bio-sensing system.
The accompanying figure 2 describes the drain characteristics of the present n-type OFET based bio-sensing system. The black line signifies the characteristics of the system without bacteria, whereas the colored line signifies the system properties in presence of bacteria. From Figure 2(a) and 2(b) it would be apparent that drain current of the OFETs increased in presence of Gram positive bacteria due to the presence of negatively charged teichoic acid in the bacterial cell wall, which further create an additional channel along with the original active channel of the OFETs and help to increase the flow rate of charge carrier in the channel. As a result, drain current increases. Further, though the Gram negative bacteria are also negatively charge, it is observed completely opposite behavior from Figure 2(c) and 2(d). This is due to uneven outer wall structure of the cell wall of Gram negative bacteria compared to Gram positive, which creates more resistive path along the channel as a result of which drain current decreases.
The accompanying figure 3 shows the transfer characteristics of the present n-type OFET based bio-sensing system. The black line signifies the characteristics of the device without the bacteria whereas the colored line signifies the device properties in presence of bacteria as like Figure 2. The figure 3 actually supported the explanation of the result shown in the figure 2. Due to the symmetrical structure of the Gram positive bacteria and the presence of negatively charged teichoic acid in the cell wall, the overall charge density increases at the channel of the OFETs. As a results The VTh values of Figure 3(a) and 3(b) shifted towards more negative, whereas due to the decrement of charge density at the channel in presence of Gram negative bacteria, the VTh values of Figure 3(c) and 3(d) shifted towards more positive. The accompanying figure 4 shows the FESEM images of bacterial layers on the active channel of the present n-type OFET based bio-sensing system.
Reference is now invited from the accompanying figure 5 which shows the transfer characteristic of one Gram positive and Gram negative bacteria with and without the cell wall. For removing the bacteria cell wall, 1 mg/mL solution of chicken egg white lysozyme is prepared and out of this solution 50 pg/mL working solution is made. A 50 pL of this is added to 1 mL of each of both Gram positive and Gram negative bacteria culture and incubated for 30 min at 37 °C. Then the suspension is centrifuged at 10000 rpm for 1 min and supernatant is removed. The pellet is then re-dispersed for further measurements. This experiment strongly revels that the deflection in the OFETs properties in presence of bacteria are obtained only due to the interaction of the cell wall of the bacteria. The Table 1 given below explain the result obtained in the whole process which also implies that the limit of detection through this method is quite compatible compared to some other available costly techniques.
TABLE- 1
Figure imgf000018_0001
The present invention also discloses a novel method for fabricating the bio- sensing system for the detection of Gram positive and Gram negative bacteria by involving ultra-low operating voltage n-type OFET device. The method for fabricating the bio-sensing system by involving, ultra-low voltage operated, stable n-type OFET device with a top contact bottom-gate configuration are summarized in below-
Step 1 : Substrate cleaning method :
Two different types of substrates viz. (a) Microscope glass slides and (b) PET substrate having compatibility with the dielectric layers can be used as the base substrate.
To form the base substrate, the glass slides or the PET are cut into (1 cm x 2.5 cm) square substrates.
The glass substrate is then cleaned by dipping the substrate in acidic piranha solution (3: 1 ratio of H2S04: H202) for 1 hour. After that the glass substrate is vigorously washed 8-10 times by using de-ionized water to remove the acidic layer on the substrate surface and then dried at 100 °C on hot plate.
For the plastic sheet substrate, the same is first cleaned with detergent 2-3 times and then vigorously washed 8-10 times by de-ionized water.
Following this the substrate is dried separately by N2 gas flash under room temperature.
Step 2: Gate electrode and first low-k dielectric layer deposition by anodic oxidation method :
After cleaning the substrate, the metal thin film having thickness of 200 nm is thermally deposited on its top through a shadow mask to constitute the gate electrode. Upper surface of the metal film of the gate electrode preferably having thickness of ~13-60 nm is oxidized by anodic oxidation method to grow the bottom dielectric layer having k- value ~10-12. The rest of the metal film is used for the gate contact. The function of this bottom dielectric layer is to prevent the gate leakage up to 5 mA and to prevent the direct contact of the intermediate dielectric layer with the gate electrode. Step 3: Synthetic and deposition of metal oxide NPs based second high-k dielectric layer:
The intermediate dielectric layer is prepared by sol-gel method . This intermediate dielectric layer can be used for low voltage operations and can be the type as listed below in Table 2. The sol-gel solution of any of the high-k dielectric material listed in the Table is prepared and then spin coated on top of the top dielectric layer in such a way that the thickness of the layer is ~100- 110 nm. This synthesized dielectric layer has k-value range from 20-30. TABLE-2
Figure imgf000020_0001
Step 4: Deposition of top low-k dielectric layer:
After fabrication of the intermediate dielectric layer, to reduce micro cracks on the surface of the intermediate dielectric layer, the top dielectric layer is fabricated over the intermediate dielectric layer by spin coating technique. For this, solution of the possible dielectric material having k value ~ 3-9 is spin coated on the intermediate dielectric layer in such a way that it can be used as a buffer layer in between dielectric layers and the n-type active semiconducting layer. It is also used to prevent the degradation of n-type semiconductor by protecting direct contact with the intermediate dielectric layer. The possible top dielectric layer materials which can be used for this purpose are listed below in Table 3. TABLE-3
Figure imgf000021_0001
Step 5: Deposition of n-type active layer and source-drain electrodes: A n-type semiconducting thin film of monomer is deposited on the layered hybrid dielectrics by thermal deposition technique in an organic thermal deposition chamber to constitute the n-type active organic semiconducting layer. The monomer is sublimed in such a way that during deposition, the substrate temperature can be kept constant, ranging from room temperature to 150°C under 10 7 mbar pressure. The thickness of the film is varied between 60-100 nm. After that 80-130 nm thick metallic source-drain contact (preferably Ag) is thermally deposited on two opposite side of the n-type active organic semiconducting layer through a shadow mask to constitute the active channel of length (L) and width (W) of 10-80 pm and 500-1000 pm respectively.
Step 6: Immobilizations of Gram positive and Gram negative bacteria and characterizations of the fabricated devices: Before the immobilizations of the bacteria, all the electrical properties are measured by Keithley 4200-SCS semiconductor parameter analyzer under dark and vacuum condition. The mobility of the device is calculated from the saturated region by using the equation : IDS=C0Xpe(W/2L) (VGS-VTh)2, where IDs is the drain current, Cox is the capacitance per unit area of the gate dielectric layer, pe is the field effect electron mobility, W and L are the channel width and length, VGS and VTh are the gate voltage and threshold voltage respectively. For the immobilization of the Gram positive and Gram negative bacteria, the bacteria based analyte is taken in de-ionized water media and 1 pL of the solution is drop casted on the effective channel and vacuum dried overnight under dark to evaporate the water. For confirmation of the presence of bacteria on the channel, FESEM image is taken after the immobilization. The all the electrical properties are again characterized by Keithley 4200-SCS semiconductor parameter analyzer under dark and vacuum condition in presence of the bacteria.
In a further aspect, the present invention provides a method for detection of the Gram positive bacteria and/or Gram negative bacteria by involving the present OFET based bio-sensing system comprising the following steps:
Step i : driving the OFET device by applying a gate voltage and a drain voltage to the device;
Step ii : measuring drain current and gate current before forming the adsorbing bacterial layer on the active channel of the OFET device;
Step iii : measuring drain current and gate current after forming the adsorbing layer with the adsorbed bacteria on the active channel of the OFET device;
Step iv: comparing the measured current values of the OFET device corresponding to the pre and post formation of the adsorbing bacterial layer on the active channel of the OFET device and determining the nature of the bacteria in the adsorbing layer based on the variation of the measured currents.
The gate voltage and drain voltage applied may be between about 0 V and 2 V. The n-type organic semiconductor based low-cost, disposable and OFET device of the present OFET based bio-sensing system containing triple-layer dielectric system exhibits excellent sensitivity in presence of Gram positive and Gram negative bacteria. In presence of Gram positive bacteria there is a significant increment in the mobility value [from 0.30 cm2/Vs (bare device) to 0.50 cm2/Vs] of the OFET device of the present system with a negative threshold voltage shift [from 0.2 V (bare device) to -0.5 V] observed in the transfer curves whereas in case of Gram negative bacteria, the mobility value decreases [from 0.30 cm2/Vs (bare device) to 0.27cm2/Vs] with a positive threshold voltage shift [from 0.2 V (bare device) to 0.5 V] as observed. The variation of the concentration of both the bacteria showed similar property. By using this easily fabricated, low-cost, disposable device one can easily distinguish and quantify the Gram positive and Gram negative bacteria. Moreover, this triple-layer dielectric layer containing OFET device is operated under very low voltage (~2V) and are compatible with different low cost substrate like pristine PET substrate and this combination of dielectric layer can be used for the detection of several biological species by changing only different suitable active layer material.

Claims

WE CLAIM:
1. An ultra-low voltage operated Organic Field Effect Transistor (OFET) device for bio-sensing system for the detection Gram positive bacteria and/or Gram negative bacteria comprising a base substrate; a gate electrode deposited on said base substrate; layered hybrid dielectrics deposited on said gate electrode having a combination of high-k and low-k triple layer dielectric system enabling reduced operating voltage; n-type organic semiconducting layer based active on top of said layered hybrid dielectrics; a source electrode and a drain electrode.
2. The ultra-low voltage operated Organic Field Effect Transistor (OFET) device for bio-sensing system as claimed in claim 1, wherein the source electrode and the drain electrode deposited on the top side of the n-type organic semiconducting layer providing space there between for adsorbing the Gram positive and/or Gram negative bacteria to be detected directly over said n-type organic semiconducting layer.
3. An ultra-low voltage operated Organic Field Effect Transistor (OFET) based bio-sensing system for the detection of Gram positive bacteria and/or Gram negative bacteria comprising an OFET device having a base substrate; a gate electrode deposited on said base substrate; layered hybrid dielectrics deposited on said gate electrode having top and bottom dielectric layer of low dielectric constant (k) based dielectric materials and intermediate dielectric layer of high dielectric constant (k) based dielectric material; n-type organic semiconducting layer based active channel deposited on top of said layered hybrid dielectrics; a source electrode and a drain electrode deposited on the top side of said n- type organic semiconducting layer; and bacterial layer grown on the n-type semiconducting layer between the source electrode and the drain electrode to adsorbed the Gram positive and/or Gram negative bacteria to be detected over the active channel ensuring the adsorbed bacteria in said adsorbing layer stays above, and in direct contact with n-type organic semiconducting active channel.
4. The OFET based bio-sensing system as claimed in anyone of claims 1 to 3, wherein the layered hybrid dielectrics of the OFET device comprises said bottom dielectric layer of low-k dielectric material on the gate electrode constituting a thin barrier layer in between the intermediate dielectric layer and the gate electrode to prevent gate leakage current of the OFET device and isolate the intermediate dielectric layer from said gate electrode; said intermediate dielectric layer of metal oxide nano-particle based high-k dielectric material on said bottom dielectric layer to reduce operating voltage and threshold voltage of the OFET device by induction of charges in the semiconducting layer based active channel through the intermediate dielectric layer resulting improvement of charge carrier mobility in the active channel; and said top dielectric layer of low-k dielectric material on said intermediate dielectric layer acting as buffer layer in between the intermediate layer and the active n-type organic semiconducting layer to reduce micro cracks on the surface of the intermediate dielectric layer and prevent degradation of the n- type organic semiconductor of the active n-type organic semiconducting layer by protecting direct contact of it's with the intermediate dielectric layer.
5. The OFET based bio-sensing system as claimed in anyone of the claims 1 to 4, wherein the layered hybrid dielectrics of the OFET device includes the low-k dielectric material for the bottom dielectric layer having k value preferably within 10-12, the low-k dielectric material for the top dielectric layer having k value preferably within 3-9 and high-k dielectric material for the intermediate dielectric layer having k value preferably within 20-30 are selected to enable the intermediate dielectric layer dominant dielectric layer in the layered hybrid dielectrics.
6. The OFET based bio-sensing system as claimed in anyone of claims 1 to 5, wherein the gate electrode of the OFET device comprises thermally deposited metal film preferably having thickness more than 200 nm with gate contact for application of gate voltage to the OFET device.
7. The OFET based bio-sensing system as claimed in anyone of claims 1 to 6, wherein the bottom dielectric layer of the OFET device comprises a portion of the metal film oxidized through anodic oxidation preferably having thickness of with k-value ~10-12 to prevent the gate leakage up to 5 mA, whereby rest of the thick metal film operates as the gate electrode.
8. The OFET based bio-sensing system as claimed in anyone of claims 1 to 7, wherein the intermediate dielectric layer of the OFET device comprises solution processable spin coated metal oxide NPs based high-k dielectric material deposition on the bottom dielectric layer having thickness of ~100- 110 nm.
9. The OFET based bio-sensing system as claimed in anyone of claims 1 to 8, wherein the top dielectric layer of the OFET device comprises spin coated dielectric material deposition on the intermediate dielectric layer having thickness of 80 ~ 200 nm and k value 3-9.
10. The OFET based bio-sensing system as claimed in anyone of claims 1 to 9, wherein the n-type organic semiconducting layer based active channel of the OFET device includes thermal deposition of n-type monomer thin film having thickness of 60-100 nm.
11. The OFET based bio-sensing system as claimed in anyone of claims 1 to 10, wherein the source electrode and the drain electrode of the OFET device includes thermally deposited metallic source-drain metal contact deposited on the top side of said n-type organic semiconducting layer to provide n-type organic semiconducting layer based active channel having channel length (L) and width (W) of 10-80 pm and 500-1000 pm respectively.
12. The OFET based bio-sensing system as claimed in anyone of claims 1 to 11, wherein the adsorbing bacterial layer having the adsorbed bacteria above the semiconducting active channel of the OFET device and direct contact with said semiconducting active channel alters mobility of the active channel depending on cell wall structure of the adsorbed bacteria thus indicating nature of the adsorbed bacteria.
13. The OFET based bio-sensing system as claimed in anyone of claims 1 to 12, the adsorbing bacterial layer with the adsorbed Gram positive bacteria having negatively charged teichoic acid in the bacterial cell wall and much uniform bacterial cell wall compared to Gram Negative bacteria constitutes an additional channel along with the original active channel of the OFET device and thereby increase flow rate of charge carrier in the channel resulting increase in drain current of the OFET device, whereby presence of the negatively charged teichoic acid in the bacteria cell wall increases overall charge density at the active channel of the OFET device resulting shifting of the threshold voltage towards more negative.
14. The OFET based bio-sensing system as claimed in anyone of claims 1 to
13, In an embodiment of the present OFET based bio-sensing system, the adsorbing bacterial layer with the adsorbed Gram negative bacteria having uneven outer wall of the cell constitutes a resistive path along the original active channel and thereby decrease flow rate of charge carrier in the channel resulting decrease in the drain current of the OFET device, whereby presence of the Gram negative bacteria in adsorbing layer decrease the charge density at the channel resulting shifting of the threshold voltage towards more positive.
15. A method for fabricating the OFET based bio-sensing system as claimed in anyone of the claims 1 to 14 comprising
fabricating the OFET device by involving the base substrate; depositing the gate electrode on said base substrate; fabricating layered hybrid dielectric structure on said gate electrode by involving low-k dielectric materials in top and bottom dielectric layers and high-k dielectric material in intermediate dielectric layer; fabricating n-type organic semiconducting layer based active channel on top of said layered hybrid dielectric structure; depositing the source electrode and the drain electrode deposited on the top side of said n-type organic semiconducting layer; and fabricating the adsorbing bacterial layer on the n-type semiconducting layer between the source electrode and the drain electrode to adsorbed the Gram positive and/or Gram negative bacteria to be detected over the active channel ensuring the adsorbed bacteria in said adsorbing layer stays above, and in direct contact with n-type organic semiconducting active channel.
16. A method for fabricating the OFET based bio-sensing system as claimed in claim 15, wherein the base substrate of the OFET device preferably includes glass substrate cleaned by acidic piranha solution (3: 1 ratio of H2S04: H202) or PET substrate cleaned by detergent.
17. A method for fabricating the OFET based bio-sensing system as claimed in claim 15 or 16, wherein the deposition of the gate electrode includes thermally depositing metal through a shadow mask on the base substrate forming the metal film preferably having thickness about 200 nm with gate contact.
18. A method for fabricating the OFET based bio-sensing system as claimed in anyone of claims 15 to 17, wherein the fabrication of the layered hybrid dielectric structure of the OFET device comprises oxidizing upper surface of the metal film by anodic oxidation to grow the bottom dielectric layer with k-value ~10-12 and thickness 13-60 nm; preparing sol-gel solution of the metal-oxide NPs based dielectric material having k-value ~20-30 and spin coating the sol-gel solution on the bottom dielectric layer to grow the intermediate dielectric layer of thickness ~100- 110 nm and k-value ~20-30; preparing solution of dialectic material having k-value 3-9 and spin coating the dielectric solution on the intermediate dielectric layer to grow the top dielectric layer of thickness ~80 - 200 nm and k-value ~3-9.
19. A method for fabricating the OFET based bio-sensing system as claimed in anyone of claim 15 to 18, wherein fabrication of the n-type organic semiconducting layer based active channel comprises preparing n-type monomer;
applying a thin film of the n-type monomer on the layered hybrid dielectrics having thickness of 60-100 nm by involving thermal deposition technique in organic thermal deposition chamber, wherein the monomer is sublimed during deposition to ensure the substrate temperature can be kept constant, ranging from room temperature to 150°C under 10 7 mbar pressure.
20. A method for fabricating the OFET based bio-sensing system as claimed in anyone of claim 15 to 19, wherein deposition of the source electrode and the drain electrode includes thermally depositing metallic source-drain contact of thickness 80-130 on the n-type organic semiconducting layer through a shadow mask to define the active channel on the n-type organic semiconducting layer having length (L) and width (W) of 10-80 pm and 500- 1000 pm respectively.
21. A method for fabricating the OFET based bio-sensing system as claimed in anyone of claim 15 to 20, wherein fabrication of the adsorbing layer on the n- type semiconducting layer includes involving bacteria based analyte in de-ionized water media; drop-casting the bacteria based analyte solution on the active channel between the source and the drain electrode; vacuum drying the drop-casted solution under dark to evaporate the water and forming the adsorbing layer with the adsorbed bacteria.
22. A method for detection of the Gram positive bacteria and/or Gram negative bacteria by involving the OFET based bio-sensing system as claimed in anyone of the claims 1 to 14, comprising driving the OFET device by applying a gate voltage and a drain voltage to the device; measuring drain current and gate current before forming the adsorbing bacterial layer with the adsorbed bacteria on the active channel of the OFET device; measuring drain current and gate current after forming the adsorbing layer with the adsorbed bacteria on the active channel of the OFET device; comparing the measured current values of the OFET device corresponding to the pre and post formation of the adsorbing layer with the adsorbed bacteria on the active channel of the OFET device and determining the nature of the bacteria in the adsorbing layer based on the variation of the measured currents.
23. A method for detection of the Gram positive bacteria and/or Gram negative bacteria by involving the OFET based bio-sensing system as claimed claim 22, includes applying gate voltage and drain voltage between about 0 V and 2 V; comparing the measured current values of the OFET device corresponding to the pre and post formation of the adsorbing layer with the adsorbed bacteria on the active channel of the OFET device including noting increment in mobility value of the OFET device from 0.30 cm2/Vs in pre-formation of the adsorbing layer to the mobility value about 0.50 cm2/Vs in post-formation of the adsorbing layer with the adsorbed Gram positive bacteria with a negative threshold voltage shift from 0.2 V to about -0.5 V in the transfer curves of the OFET device; noting decrement in mobility value of the OFET device from 0.30 cm2/Vs in pre-formation of the adsorbing layer to the mobility value about 0.27 cm2/Vs in post-formation of the adsorbing layer with the adsorbed Gram negative bacteria with a positive threshold voltage shift from 0.2 V to about 0.5 V in the transfer curves of the OFET device; and noting level of variation in the mobility value and the threshold voltage shift depending variation of the concentration of both the bacteria.
PCT/IN2018/050849 2018-01-04 2018-12-18 Organic field effect transistor (ofet) for the detection of gram negative/positive bacteria, method of detection and fabrication of the transistor WO2019135251A1 (en)

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