Patent Publication Number: US-11378539-B2

Title: Fabrication of iridium oxide pH sensors and sensory arrays

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to the field of sensors, and more particularly to the fabrication of amorphous iridium oxide film pH sensors. 
     BACKGROUND 
     pH sensors are used in clinics, laboratories and industrial factories since many biological and chemical reaction mechanisms are pH dependent. Conventional glass-type electrodes have been widely used; however, they have certain disadvantages in specific applications. The glass rod sensor configuration is difficult to use for in vivo biomedical, clinical, or food monitoring applications due to the brittleness of glass, size limitations, and the lack of deformability. To achieve small sizes and robust design, ion-sensitive field-effect transistor (iSFET) pH sensors [1-5], optical fiber pH sensors [1, 6-11], hydrogel film pH sensors [12-14], and solid state pH sensors [1, 15-18] have been proposed. iSFET sensors have power consumption concerns due to the field-effect transistor (FET) operational requirements [19]. Hydrogel film pH sensors utilize the physical properties of the pH-responsive swelling and shrinking polymer to measure resistance changes [12]. The sensor structure design and polymer layer fabrication process can be complicated and expensive [13]. Optical pH sensors also have power consumption issues due to the use of light sources. A system including optical devices could be expensive and unsuitable for implantation [1, 7-8, 10-11]. 
     Various solid-state metal oxides have been investigated for pH sensing electrodes [1, 15] including PtO 2 , IrOx, RuO 2 , OsO 2 , Ta 2 O 5 , RhO 2 , TiO 2 , and SnO 2  as the pH sensing films. The pH sensitivity, selectivity, working range, and hysteresis determine sensing performance. IrOx, RuO 2 , and SnO 2  have demonstrated more advantages in sensor performance for various applications [22]. RuO 2  [18, 20] and SnO 2  [21] show near Nernstian responses in wide pH ranges. However, SnO 2  and RuO 2  present hysteresis and drift problems leading to potential calibration issues and unstable responses [20, 21]. Iridium oxide film (IROF) has exhibited outstanding stability over wide pH ranges, rapid response, less hysteresis, and high durability; these characteristics have been demonstrated at high temperature up to 250° C. [23]. 
     There are different fabrication methods for iridium oxide films including sputtering deposition [23, 24], electrochemical deposition [25-29], thermal oxidation [23], and sol-gel [30-32] processes. The sputtering iridium oxide film (SIROF) deposition process is costly due to the target cost. The oxygen and argon pressure ratios, position of the target, deposition rate, and RF powers during the fabrication processes all affect the pH sensing parameters such as potential drifts and redox interference [22]. Anodic electrochemical deposition presents an economical way for iridium oxide thin film fabrication. The anodic iridium oxide thin film (AIROF) process is based on electrolysis of a solution containing iridium complexes. The iridium tetrachloride compound has been widely used as a deposition agent [26-29] such as the commonly used Yamanaka solution [26]. The pH value of the deposition solution, solution temperature and current density control affect the deposition efficiency [26-29]. A precise power supply system as potentiostate is required in the electro-deposition process for thickness and film quality control. Thermal oxidation processes require a high temperature ranging from 500 to 800° C. [17, 22]. The film made by thermal oxidation can be thicker than the AIROF with more stable potentials [22, 23]. However, the film surface has a tendency to crack after the high temperature treatment. The adhesion property of the cracked film then becomes an issue. The high temperature treatment also becomes a limitation during sensor fabrication, especially for the use of polymer and photoresist, which often can not survive at a temperature above 200° C. The sol-gel IROF deposition process has been demonstrated [32] with dip coating [32, 33] and heat treatment [31, 32] procedures. Sol-gel deposition provides a simpler and economical fabrication approach. 
     Integrating pH sensors in microfluidic systems can provide in-situ measurement functionality for micro-scale fluidic processing and analyzing applications, especially for medicine synthesis, bioanalysis of drugs and environmental specimens, and biological studies of cells. Conventional methods to monitor pH levels inside microfluidic channels using pH responsive dyes or tag-based indicators possess several disadvantages, including potential contamination, photo-bleaching of the indicators, and narrow dynamic responses. Thus miniaturized electrochemical pH sensors for microfluidic systems have been proposed to overcome these issues while providing real-time and time-lapsed measurement results. 
     There is a need for miniaturized microelectrode pH sensors for microfluidic systems and for a reproducible method to mass produce micro-scale pH sensors to be used in microfluidic systems. 
     SUMMARY OF THE INVENTION 
     The present invention provides a fabrication method for sol-gel based iridium oxide (IrOx) thin film pH sensors and microelectrodes. The invention further provides microelectrode arrays produced by the fabrication methods and microfluidic devices including the microelectrodes and microelectrode arrays. 
     In one aspect, the invention is a method for fabricating a microelectrode pH sensor. As an initial step at least one metal layer is deposited on a substrate. Desirably, a first layer and a second layer are deposited on a substrate. Desirably the first layer is chromium and the second layer is gold. The chromium and gold layers are coated with a photoresist layer and an electrode is defined photolithographically. The gold and chromium layers are etched to define the electrode. 
     A microfluidic mold is supplied which includes a reservoir region and a channel. In one embodiment the microfluidic mold is made by pouring and curing polydimethylsiloxane (PDMS) on a silicon wafer comprising a patterned SU-8 mold. The PDMS layer is peeled off of the silicon wafer to form the microfluidic mold. The mold is made to correspond to the configuration of the electrode, as explained below. 
     The microfluidic mold is aligned onto the electrode-substrate assembly created as above so that a pathway is defined from the reservoir region through the channel to the electrode. Next, an iridium oxide sol-gel solution is poured into the reservoir so that it flows through the channel and coats the electrode sensor. 
     To finish the pH sensor, the microfluidic mold is detached from the glass substrate, the pH sensor is cured and coated with a photoresist, and the sensing sites are opened. 
     As an additional step, one or more metal layers are deposited on the substrate by forming a pattern on the encapsulation layer, depositing at least one metal layer on the pH sensing array, and then opening the sensing electrode sites. 
     In another aspect, the invention is a method for fabricating a microelectrode sensor array following a similar method as above for the electrode sensor. At least one metal layer is deposited on a substrate. Desirably, first and second layers are deposited on a substrate. Desirably the first layer is chromium and the second layer is gold. The chromium and gold layers are coated with a photoresist layer and pattern of microelectrodes is defined photolithographically. The gold and chromium layers are etched to define the pattern of microelectrode array. 
     A microfluidic mold is supplied which includes one or more reservoir regions and an array of channels leading from the reservoir region(s). In one embodiment the microfluidic mold is made by pouring and curing polydimethylsiloxane (PDMS) on a silicon wafer comprising a patterned SU-8 mold. The PDMS layer is peeled off of the silicon wafer to form the microfluidic mold. The mold is made to correspond to the configuration of the electrode sensor array, as explained below. 
     The microfluidic mold is aligned onto the electrode array-substrate assembly created as above so that a pathway is defined from the reservoir region(s) through the channels to the electrodes. Next, an iridium oxide sol-gel solution is poured into the reservoir(s) so that it flows through the channels and coats the electrode sensors. 
     To finish the pH sensor array, the PDMS layer is detached from the glass substrate, the pH sensor array is cured and coated with a photoresist and the sensing sites are opened. 
     As an additional step, one or more metal layers are deposited on the substrate by forming a pattern on the encapsulation layer, depositing at least one metal layer on the pH sensing array, and then opening the sensing electrode sites. 
     Desirably the substrate is a glass substrate. The chromium and gold may be deposited onto the glass substrate by any known method. Preferably, the chromium and gold layers are deposited onto the glass substrate via e-beam evaporation. The photoresist may comprise any known photoresist composition and is preferably SU-8. In certain aspects, the coating steps are accomplished via a spin-coating process. Typically, after the sol-gel solution is dispensed onto the array, excess sol-gel solution is retreated from the reservoir and the device is dried. After the PDMS is detached from the glass substrate, the sensor array is thermally treated. 
     The invention further provides an apparatus comprising the microelectrode array produced according to the fabrication method described above integrated onto a microfluidic device. 
     The advantages of the fabrication methods include that a small amount of sol-gel material can be deposited onto the metal electrode site. In addition, multiple sensors can be made by one deposition step of injecting the solution into the reservoir and the sensor electrodes performance will be uniform since the fabrication micro environment is identical for all sensors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further benefits and advantages of the present invention will become more apparent from the following description of various embodiments that are given by way of example with reference to the accompanying drawings: 
         FIG. 1  illustrates a pH sensor in accordance with one embodiment of the present invention. 
         FIG. 2  illustrates an array of pH sensors in accordance with another embodiment of the present invention. 
         FIGS. 3A-3H  illustrate a method for fabricating a pH sensor in accordance with the methods of the present invention. Gold and Chromium are deposited on a glass slide ( FIG. 3A ); a NR-0 photoresist layer and pattern microelectrodes are spin-coated ( FIG. 3B ); the Au and Cr layers are etched ( FIG. 3C ); PDMS is poured on a silicon wafer with patterned SU-8 mold ( FIG. 3D ); the PDMS layer is peeled off the silicon wafer ( FIG. 3E ); the PDMS piece is aligned onto the microelectrodes ( FIG. 3F ); an IrOx sol-gel solution is dispensed and the IrOx cured ( FIG. 3G ); photoresist is spin-coated and pattern sensing sites for the micro pH sensors are opened ( FIG. 3H ). 
         FIGS. 4A-4D  illustrate a PDMS reservoir placed on top of a microelectrode array ( FIG. 4A ); a model of micro-channels to dispense sol-gel solution on the microelectrodes ( FIG. 4B ); a microscope image of IrOx on an electrode after coating ( FIG. 4C ); and an SEM photo of the surface of the IrOx microelectrode. 
         FIGS. 5A-5C  illustrate an experimental setup of a microfluidic device using a micro pH sensor array ( FIG. 5A ); the design of the microfluidic device ( FIG. 5B ); and pH spatial and temporal responses of the sensor array after adding 0.1M HCl solution to the reservoir. 
         FIG. 6  shows a micrograph of the electrode array and illustrates the voltage response of two pH electrodes vs. an Ag/AgCl reference electrode. 
         FIG. 7  illustrates the method of fabrication including additional steps 9 and 10 of depositing additional silver and chromium layers onto the glass substrate. 
     
    
    
     DETAILED DESCRIPTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention. 
     To facilitate the understanding of this invention, a number of terms are defined below. Terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present invention. Terms such as “a”, “an” and “the” are not intended to refer to only a singular entity, but include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments of the invention, but their usage does not delimit the invention, except as outlined in the claims. 
     Examples of configurations of pH sensors for use in the present invention are provided in  FIG. 1  and  FIG. 2 . Now referring to  FIG. 1 , a pH sensor  100  in accordance with one embodiment of the present invention is shown. The pH sensor  100  includes a substrate  102 , an amorphous iridium oxide film sensor electrode  104  disposed on the substrate  102  and a reference electrode  106  disposed on the substrate  102  in close proximity to the amorphous iridium oxide film sensor electrode  104 . In addition, a first electrical contact pad  108  is disposed on the substrate  102  and is electrically connected to the amorphous iridium oxide sensor  104  via a conductive trace, connector, or pathway  112 . Similarly, a second electrical contact pad  110  is disposed on the substrate  102  and electrically connected to the reference electrode via a conductive trace, connector, or pathway  114 . As will be described in more detail below, the amorphous iridium oxide film sensor electrode  104  provides a potential in reference to the reference electrode  106  that varies according to a pH of a substance contacting the amorphous iridium oxide film sensor electrode  104  and the reference electrode  106 . Note that the present invention is not limited to the specific orientation shown in  FIG. 1 . Moreover,  FIG. 1  is not to scale. As a result, any suitable orientation of the components can be used, and the components can be of any suitable geometric shape. One example of a process for fabricating the pH sensor  100  will be described in more detail below. 
     Referring now to  FIG. 2 , an array  200  of pH sensors in accordance with another embodiment of the present invention is shown. As shown, the pH sensor array  200  includes four iridium oxide film pH sensors  100   a ,  100   b ,  100   c  and  100   d  disposed on a substrate  102 . The array can contain any number of pH sensors  100 . A reference electrode  106  is disposed on the substrate  102  in close proximity to each corresponding amorphous iridium oxide film sensor electrode  104 . A first electrical contact pad  108  corresponding to each amorphous iridium oxide film sensor electrode  104  is disposed on the substrate  102  and electrically connected to the corresponding amorphous iridium oxide sensor  104  via a conductive trace, connector, or pathway  112 . A second electrical contact pad  110  corresponding to each reference electrode  106  is disposed on the substrate  102  and electrically connected to the corresponding reference electrode  106  via a conductive trace, connector, or pathway  114 . The amorphous iridium oxide film sensor electrodes  104  provide a potential in reference to the reference electrodes  106  that varies according to a pH of a substance contacting the amorphous iridium oxide film sensor electrodes  104  and the reference electrodes  106 . Note that the present invention is not limited to the specific orientation shown in  FIG. 2 . Moreover,  FIG. 2  is not to scale. As a result, any suitable orientation of the components can be used, and the components can be of any suitable geometric shape. 
     The fabrication processes are depicted in  FIGS. 3A-3H  and  FIG. 7 . As an initial step, at least one metal layer, here shown as layers of chromium  204  and gold  206  are deposited on a substrate  202 , preferably by e-beam evaporation ( FIG. 3A ). Desirably, for some applications, the substrate  202  is glass and optically transparent providing a compatible means for studies of cells. However, the substrate can be any appropriate material, such as a flexible polymer. In preferred aspects, the thickness of the chromium layer  204  is between 15 nm and 40 nm, preferably about 20 nm, and the thickness of the gold layer  206  is between 80 nm and 150 nm, preferably about 100 nm. 
     Photolithography and etching processes are carried out to define the array configuration. As shown in  FIG. 3B  a photoresist layer  208  and pattern microelectrodes are spin-coated onto the Cr/Au layers  204 / 206 . The Au and Cr layers  204 / 206  are then etched as illustrated in  FIG. 3C  and the photoresist layer  208  is removed as shown in  FIG. 3F . 
     The microfluidic mold  220  with micro-dispensing channels  222  was fabricated as shown in  FIG. 3D  and  FIG. 3E . PDMS was poured onto a SU-8 mold  214  patterned with the micro-dispensing Channels  222  on a silicon wafer  216 . After curing, a reservoir  223  ( FIG. 4A ) was created by punching out a hole of 3-mm diameter and then the PDMS layer  212  is peeled off the silicon wafer  216  ( FIG. 3E ) to form the PDMS device  220 . 
     The microfluidic device  220  can be made out of a variety of materials, the important aspects are that it defines one or more reservoir regions  223  and one or more fluidic channels  222  appropriately arranged for the sensor  100  or sensor array  200 . 
     The PDMS device  220  was aligned on top of the glass substrate  202  with each micro-dispensing channel  222  in contact with each metal electrode  224 , as shown in  FIG. 3F  and  FIG. 4A . 
     The IrOx sol-gel solution was prepared generally according to the method published in [1]. One gram of anhydrous iridium chloride (IrCl4) was dissolved in 42 ml of ethanol. 10 ml of acetic acid was added in the solution. The coating solution was stirred by a magnetic rod for one hour. A small volume of this sol-gel solution  225  was dispensed into the PDMS reservoir  223  and allowed to automatically dispense across the coating sites  227  of each of the microelectrodes  224  as depicted in  FIG. 3G . The set-up is illustrated in  FIG. 4B . 
     When the sol-gel solution with high concentration of ethanol was dripped into the reservoir  223 , it quickly flowed through the channels  222  toward the patterned electrodes  224 . Excessive sol-gel solution was removed from the reservoir  223  and the device was dried at 50° C. for 10 minutes. The PDMS device  220  was then detached from the glass substrate  202  before the sensor array  200  was thermally treated at 300° C. for 5 hours to form the anhydrous IrOx thin-film  225 .  FIG. 4C  shows iridium oxide film covered electrodes  224  after electrodeposition. EDAX analysis and SEM pictures, shown in  FIG. 4D , confirmed the elements present on the electrode surface. Finally, a layer of SU-8 photoresist  226  was spin-coated and patterned to open only the sensing sites  228  as shown in  FIG. 3H . 
     Optional additional steps are illustrated in  FIG. 7 . After step  7 , dispensing and curing the IrOx  225 ; and removal of the microfluidic mold (step not shown), a coating of photoresist  226  is applied and then etched to form the desired pattern on the substrate  202 . One or more additional metal layers  230 ,  232  are deposited to the device, to the substrate  202 , and then the photoresist layer  226  is removed. 
     Microfluidic structures designed for specific studies were placed onto the glass substrate to incorporate the pH sensor array. 
     The thickness of the IrOx layer deposited onto the electrodes was measured by a profilometer. A layer of IrOx at least 0.7 μm thick was deposited on the gold electrodes. The thickness can be controlled with the viscosity of solution and microfluidic channel designs. 
     Three possible mechanisms have been proposed for pH dependent redox equilibrium between two oxidation states of the iridium oxide [46] as 
                         Ir   2     ⁢     O   3       +     6   ⁢     H   +       +     6   ⁢     e   -         ↔       2   ⁢   Ir     +     3   ⁢     H   2     ⁢   O               (   1   )                   IrO   2     +     4   ⁢     H   +       +     4   ⁢     e   -         ↔     Ir   +     2   ⁢     H   2     ⁢   O               (   2   )                   2   ⁢     IrO   2       +     2   ⁢     H   +       +     2   ⁢     e   -         ↔         Ir   2     ⁢     O   3       +       H   2     ⁢   O               (   3   )               E   =         E   0     -     2.303   ⁢     RT   F     ⁢           ⁢   pH       =       E   o     -     59.16   ⁢           ⁢   pH                 (   4   )               
where: E 0  is the standard electrode potential with a value of 926 mV;
 
     F is the Faraday&#39;s constant with a value of 96,487 coul/equiv; and 
     R is the gas constant with a value of 8.314 joules/deg. 
     RT/F is equal to 25.688 at 25° C. The pH potential sensitivity is 59 mV/pH if space charges are formed [22, 46] which is called the Nernstian response. The pH sensitivity of our sensor is based on the super-Nernstian potential response [44] as the sensitivity will be higher than 59 mV/pH. 
     The sensitivity of the IrOx pH sensor was validated by pH titration tests at room temperature. DI water, HCl, and KOH were used during the titrations. Three sensor electrodes in an array were used to demonstrate the linear super-Nernstian response. The results showed sensitivity ranging from −71.5 mV/pH to −81.7 mV/pH. Potential responses with seven different pH levels from 11.8 to 1.92 resulted in sensitivity between −69.6 mV/pH and −71.8 mV/pH as HCl was dripped in diluted KOH solution for titration. The different sensitivities may be caused by the state of oxide [49]. The sensitivity of IrOx increases when the oxidation state reduces. A higher potential coincided with a high valence oxide and vice versa [49]. During the tests, our pH electrode array on flexible substrate showed higher sensitivity, compared to the electrode formed by other methods which typically have sensitivities in the range of 55-70 mV/pH [22, 47-48]. 
     The response time of the pH sensor was measured in three different tests. The first test was from the acid to alkaline condition by quickly dripping 0.1M KOH into an acidic solution contacting the sensor. The second test was from alkaline to acid by quickly dripping 0.1M HCl into an alkaline solution contacting the sensor. The third test involved dripping diluted HCl droplets directly on the dry sensing electrode surface of the sensor. The response time of pH electrodes is defined as the time needed for the potential change to reach 90% within the equilibrium value of potential [22]. 
     A response time of 0.9 second with a measured potential step change from pH 4 to 11 was obtained. From pH 12 to 3.5, the response time was about 2 seconds. With the dry electrode, adding pH 4 solution induced a potential step change with a response time of 0.8 s. The response times did vary due to the solution mixture, thus repeated experiments were conducted. Generally speaking the pH sensor responds to added solution within a time period of 2 s. 
     Compared with the response times of 5-15 seconds reported in literatures [57-58], this response time of about 2 seconds for our IrOx flexible pH sensor was shorter and consistent with different pH level changes. This may be due to the better quality of the IrOx film with appropriate coating and annealing processes in our sol-gel fabrication. The bulk pH solution needs to equilibrate the liquid in the pores of iridium oxide film in which the process increases the response time. Thus, although our sol-gel IrOx film has lower porosity, which provides less sensitivity than porous AIROF and SIROF, the sensor responds more quickly. 
     Responses of the IrOx electrodes in a series of different pH levels were tested. The diluted KOH solution was titrated by 1M of HCl dripping from pH 12.1 to 1.5. The pH values were recorded by the Hanna pH sensor. During the test, the IrOx pH sensor potentials followed the pH values coordinately and kept stable in each pH step. Three tests showed the electrode behaved similarly with little difference, having a linear response from pH 12.1 to 1.5. 
     The micro pH sensor array was integrated with a PDMS microfluidic device.  FIG. 5A  shows the experiment setup while  FIG. 5B  displays the dimension of the microfluidic device that had two reservoirs connected through a microfluidic flow tunnel with a height of 10 μm and a width of 100 μm. All five pH sensors shared the same standard Ag/AgCl reference as shown in  FIG. 5A . The two reservoirs and the microfluidic flow tunnel were filled with phosphate buffered saline solution (pH 7.0). A chemical mixing incident was created by adding 0.1M HCl solution into the right reservoir. The micro pH sensors placed along the microfluidic flow tunnel monitored the change in pH of the solution corresponding to the diffusion of H +  ions.  FIG. 5C  displays the pH values indicated by different pH sensors located at different distances from the reservoir containing HCl. The pH values of the sensors decreased with time. After 1200 seconds, all sensors indicated pH values of around 1.5, which means the concentration of H +  ions inside the microfluidic flow tunnel became homogeneous due to diffusion. The experiment thus demonstrated the use of the microelectrodes to spatially and temporally monitor pH change in microfluidic devices. 
     The fabrication process described herein produces sol-gel-based micro pH sensors with a wide sensing range, robustness, and high durability for long-term experiments. Sol-gel coating solution was automatically distributed through micro-dispensing channels made of PDMS to the sensors with sensing sizes of 100×100 μm 2 . The fabrication approach described herein effectively enhances the reproducibility and mass production ability of the micro-scale pH sensors. The sensitivities of the sol-gel-based pH sensors produced using the methods of the present invention remained in the range of 51.0-56.1 mV/pH after three days of fabrication. The micro-electrode array was integrated to a microfluidic device to spatially and temporally monitor pH changes in a chemical mixing process. 
       FIG. 6  shows a micrograph of a pH electrode array inside the transparent PDMS microfluidic structure. IrOx was coated on each electrode within an area of 100×100 μm 2 . The temporal response of the sensor array in a microfluidic flow tunnel with a 100-μm width connecting two chambers was tested. The chambers and tunnel were initially filled with DI water. Voltage responses of the 1st and the 5th electrodes were recorded versus a standard Ag/AgCl reference electrode when a droplet of buffer solution with pH 4 was added into one chamber which was closer to the 1st sensor. The buffer solution diffused through the microfluidic tunnel toward the other chamber. The 1st sensor reached a higher potential indicating a lower pH level than that of the 5th sensor due to the effect of dilution. The 1st sensor also responded more quickly than the 5th sensor indicating that the sensor detected the pH change first as the pH 4 buffer diffused through the tunnel. The sensitivities of sensors were tested within the range of 51.0-56.1 mV/pH. 
     The present invention can be used to provide in-situ measurement functionality for micro-scale fluidic processing and analyzing applications, especially for medicine synthesis, bioanalysis of drugs and environmental specimens and biological studies of cells. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification, but only by the claims. 
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