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Digital pH test strips for in-field pH monitoring using iridium oxide-reduced graphene oxide hybrid thin films Jiang Yang, Tae Joon Kwak, Xiao-Dong Zhang, Robert L McClain, Woo-Jin Chang, and Sundaram Gunasekaran ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.6b00385 • Publication Date (Web): 28 Sep 2016 Downloaded from http://pubs.acs.org on October 5, 2016
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Title: Digital pH test strips for in-field pH monitoring using iridium oxide-reduced graphene oxide hybrid thin films Authors: Jiang Yang1,2, Tae Joon Kwak3, Xiaodong Zhang4, Robert McClain5, Woo-Jin Chang3,6*, Sundaram Gunasekaran1* 1
Department of Biological Systems Engineering, University of Wisconsin-Madison, 460 Henry Mall, Madison, WI 53706, USA 2Environment, Energy and Natural Resources Center, Department of Environmental Science and Engineering, Fudan University, No. 220, Handan Road, Shanghai, 200433, China 3Department of Mechanical Engineering, University of Wisconsin-Milwaukee, 3200 North Cramer Street, Milwaukee, WI 53211, USA 4Department of Physics, School of Science, Tianjin University, 92 Weijin Road, Tianjin, 300354, China 5 Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, WI, 53706, USA 6School of Freshwater Sciences, University of Wisconsin-Milwaukee, 600 East Greenfield, Milwaukee, WI 53204, USA *Corresponding authors at:
[email protected] (W.-J. Chang) and
[email protected] (S. Gunasekaran)
Abstract While pH test strips are not decimally accurate and only semi-quantitative, the more accurate and automated laboratory-based pH meters lack disposability and portability. Benefitting from advantages of both sides and filling the gap of unmet needs in between, we integrated a miniaturized paper-fluidic electrochemical pH platform coupled with a portable pH device powered by consumer batteries to make digital pH test strips for truly in-field measurements. To overcome the inability to form the smooth and uniform pH-sensitive functionality on screenprinted carbon, i.e. anodically electrodeposited iridium oxide thin films (AEIROFs), reduced graphene oxide was used as substrate for synthesis of homogeneous metal oxide-nanocarbon hybrid thin films, IrO2-reduced graphene oxide (IrO2-rGO), as the sensing moiety with strong substrate adhesion. A hydrophobic barrier-patterned paper micropad (µPAD) with programmed flow rates can locally accelerate linear wicking speed to boost delivery and response time. IrO2rGO shows slightly super-Nernstian linear responses from pH 2-12 with small hysteresis, fast response time, reproducible performances and low sensitivities to interfering ionic species and dissolved oxygen. A miniaturized portable pH device made of high-impedance buffer amplifier and off-the-shelf digital multimeter provides results consistent with those of a commercial pH meter equipped with a glass electrode. Our system combines the accuracy of pH meters with the low-cost and convenience of pH strips, making it ideal for resource-limited settings. Key words: Iridium oxide, graphene, paper-fluidic, potentiometric sensor, pH sensor, electrochemistry, programmed flow rate, hydrophobic patterning The pH value is an important parameter in ensuring quality and sustainability of environments. Maintaining pH within a well-defined range is critical for aquatic ecosystems, as most living organisms are adapted to a narrow optimal pH window typically from 6.5 to 8.2. Even minor deviations can cause fatal consequences. Low pH levels, for instance, not only destroy
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physiological systems of fish, decrease biological availability, and increase their mortality1, but also increase the solubility and toxicity of detrimental chemicals and heavy metals and make water corrosive2. A minor increase in pH can change a lake from oligotrophic to eutrophic and lead to excessive algal bloom3. Two common methods for pH measurements are pH strips and pH meters. The pH test strips are paper pads impregnated with pH-responsive color-changing molecules which are inexpensive and easy to use. However, the values measured with pH strips need to be subjectively contrasted to a standard color chart and sometimes inherent human eyes are unable to accurately, sensitively and consistently read the displayed color. Other factors such as improper storage, exposure to moisture and light, contamination during storage and handling and intrinsic interfering colors of samples can all adversely affect results. While paper strips sufficiently meet regular needs on daily basis, environmental analysis where pH changes are small requires quantitative and precise measurements. Researchers typically use a laboratory pH meter. These meters can be rather expensive and not disposable including those hand-held versions, while the glass electrodes on pH meters require special care in maintenance and periodic calibration. They are also known to suffer from acid/alkaline errors, temperature instability, and mechanical fragility4. The pH meters also require relatively large sample volumes and clean containers. It is advantageous to have a robust and quantitative pH measurement system that combines the accuracy and robustness of pH meters with the simplicity and inexpensiveness of pH strips. Recent progresses in ionophore-based pH sensors are typical examples meeting these needs5. Microfluidic systems have long been proven as powerful candidates for inexpensive and portable analytical devices. The most widely used microfluidic fabrication technology is the combination of polydimethylsiloxane (PDMS) and soft-lithography. The equipment required to make masks can be highly specialized and expensive, even requiring the use of a clean room facility. Recently, paper microfluidic devices have overcome these limitations and are ideally suited for resourcepoor settings where expensive equipment are unavailable or precious samples are limited6. Paperfluidics are conveniently combined with electrochemical detection to make measurement systems miniaturized for portable and inexpensive devices such as at-home blood glucose monitoring systems. The porous paper matrix allows for controlled sample wicking, size-dependent filtration, and elimination of undesired impurities and interferences. Paper as a measurement substrate only minimally affects analyte diffusion and electrochemical detection6a, 7. The small sample volumes in such devices can also minimize interferences from vibration and convection during measurements. Patterned microfluidic pads have been used to wick and deliver liquid samples to the sensing electrodes for detection of heavy metal ions and glucose6. Noncomplex portable electrochemical devices can be constructed with pencil drawing8, enzyme paper7b, paper potentiometric cell9, textile-printed electrode-array band10, bandage11 and origami paper12. More recently solid-contact ion-selective electrode for in-field ammonium measurements were demonstrated13. However, most of these devices rely on laboratory signal readout. It is reasonable to conceive that combining the cost and convenience from pH strips and the accuracy and precision from pH meters can be a powerful tool and realized by a paper-fluidic electrochemical system as digital pH strip to truly conduct out-of-lab measurements. Additionally, a simple, inexpensive but broadly applicable and scalable approach to manufacture functional paper devices with programmed flow-rates and hydrophobic barriers is called for as an unmet need.
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AEIROFs are non-toxic low-impedance thin film electrode materials widely used as highperformance pH electrodes. These electrodes show extraordinary sensitivity and minimal electrochemically irreversible processes such as electrode dissolution and water electrolysis14. They have outstanding mechanical strength and are tolerant to ultrasonication, corrosion, repeated dehydration/hydration processes, certain organic solvents and autoclaving15. However, their poor adhesion to the underlying substrate and susceptibility to delamination and degradation has been a major challenge and largely limited the practical applications of AEIROFs15. These limitations may be overcome using carbon scaffolds to support AEIROFs deposition, and IrOx-carbon nanotube hybrid materials have been shown to increase charge storage capacity and stability14a. Such functional thin films as scaffolds to further anchor nanoparticles and enhance electrodeelectrolyte electron transfers are appealing for different analytical applications16. Herein, we employed rGO for deposition of AEIROFs thin film as functional pH-sensing inorganic-carbon hybrid material. In general, chemically reduced graphene oxide has a low surface area and electronic conductivity, and thermal reduction at high temperature damages the graphene platelets as local pressure builds up with gaseous CO2 causing imperfections and vacancies. However, electrochemical reduction of graphene oxide is green, rapid and scalable for the production of high-quality rGO with high carbon-to-oxygen ratio, resulting in structures close to that of pristine graphene. IrO2-rGO thin film displayed little cracks and defects with dispersed surface-anchored IrO2 nanoparticles. This rGO-assisted strategy can be generalized to make other hybrid thin film materials. The as-constructed IrO2-rGO nano-hybrid thin film electrode with flow rate-programmed µPADs presented a slightly super-Nernstian response and outperformed the sensitivity of -51.7 mV/pH using sol-gel synthesized IrOx for pH sensing17. When integrated within a paper-fluidic device and combined with a hand-held digital voltmeter, we constructed a completely portable pH measurement system.
Results and discussion Fluidic behavior in the flow-rate programmed µPAD The µPADs was visually tested using a methylene blue dye solution only to wick inner hydrophilic regions in a uniform phase (Fig. 1B). The flow of the dye solution is confined by the hydrophobic barriers without observing any leakage or delamination. It is vital to quantitatively monitor fluidic flow pattern in real-time to precisely control sample deliveries. The experimental and simulated flow profile data are displayed in Fig. 2A and 2B, respectively, and the calculated travel distances of the wetted region, i.e., the length from inlet to the center of front end of absorbed water, is compared with experimental results (Fig. 2C). The water flow rate spreading through the wicking region of µPADs is one-dimensional horizontal motion, and it gradually decreases over time as determined by the tangent slope. This relationship can also be described by Lucas-Washburn equation18 by ignoring shape effects and assuming a fully-saturated flow at a clearly-defined fluid font in a capillary tube (Eqn 1): ∆ = − =
(1)
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Where, Rc is the radius of tube, ∆p is the net driving pressure between wetting pw and non-wetting fluids pnw, is the liquid surface tension and is the equilibrium liquid contact angle on the capillary. Considering the Hagen-Poiseuille equation that relates volumetric flow-rate to the pressure difference along the capillary (Eqn 2)19: ∆ =
(2)
Where, is the liquid viscosity, L is the wicked distance and Q is the volumetric flow-rate
equivalent to the rate of wetted length changes, i.e., . Combining Eqn 1 and Eqn 2, we obtain an expression for wetting velocity V which is inversely related to the wicking distance: =
(3)
When water encounters the circular region at the end of the rectangular area, the fluidic velocity increased due to the gradual increase of channel width (Rc in Eqn 3) in the circular area19-20. In contrast, the liquid traveling speed decreased with narrower channels as it passes the first halfcircle. Overall, the flow rate rapidly decreased in the constant-width rectangular area and then increased when the shape of the channel changes from rectangle into circle (width increasing), and then decreased again when the width of the channel in the circular area changes from diverging to converging (width decreasing) (Fig. 2C). Results from both experiment and simulation clearly indicate these velocity inflection points as marked by arrows in Fig. 2C. This design provides a local boost of linear wicking rate more than 100%. Such controlled flow traveling scheme by programmable shape change is desirable for fast delivery while still keeping sample volumes minimal using narrower delivery channels. The diffusion-based fluidic behaviors in µPADs between experiment and simulation were statistically consistent as verified by t-test with 95% confidence level (p = 0.925). Surface characterization of IrO2-rGO Fig. 3A shows the surface morphology of the rGO film with a typical wrinkled texture composed of flexible and ultrathin graphene sheets spreading through the uneven screen-printed carbon substrate. Electrochemical GO reduction is rapid, irreversible and controllable without reducing reagents21. rGO film can also be grown on other different substrates such as glassy carbon, indium tin oxide (ITO) glass and gold, with a similar folded-sheet morphology22. IrO2-rGO nanohybrid thin film (Fig. 3B) was observed with homogeneously dispersed small crystallites in nanoscaled grain (106.9 ± 17.2 nm, Fig. S1) on the entire surface. These anchored nanoparticles greatly increase the surface area of the hybrid film and provide additional electrochemically active sites. The underlying nano-hybrid thin film appears uniform and smooth, with hardly any observable cracks or mud structures, providing good substrate adhesion. When AEIROFs are smooth and uncracked, their analytical performances are insensitive to minor changes (± 20%) in deposition conditions such as time and current density23. Additionally, we found out that AEIROFs cannot form an integral thin film on graphitic carbon substrates without rGO under the same deposition condition. This is primarily caused by the different surface structures, electrochemical properties and hydrophobicities of screen-printed carbon and rGO (Fig. 3C),
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leading to substantially different efficiencies and behaviors in depositions at the interface. In fact, large aggregates were formed without rGO (Fig. S2) and we thus conceive that the good filmforming ability24 and indispensable electrochemical properties of rGO at solid-liquid interface play a key role during the deposition processes of thin films. Oxalates in the deposition solution can trigger complex formation and prevent precipitation of iridium in alkaline media. The deposition of IrO2 onto rGO is achieved by anodic oxidation of oxalated Ir(IV) compounds with CO2 release and concomitant Ir(IV) oxide formation at the anode surface by hydrophobic interactions25, as in Eqn 4: [Ir(COO) (OH) )]*
IrO + 2CO + 2H O + 2e*
(4)
Large contact angles measured for bare (110.6°) and IrO2-modified (122.5°) electrode surfaces are indicative of their hydrophobic nature (Fig. 3C). Although it is generally known that rGO is far more hydrophobic than GO22a, it is still evident that rGO is more hydrophilic than bare graphitic carbon surface, with an enormously decreased contact angle of 56.7°. As such, IrO2rGO nano-hybrid thin film displayed an improved hydrophilicity (62.8°) compared to modification by only IrO2. This increase in hydrophilicity is highly favorable in paper-fluidic devices to achieve more efficient and stable sensing and better electrical contact between paper and electrodes, similar to what was observed before12,7b. Failure of rapid liquid delivery and slow mass transport to wet electrode surfaces could lead to failure of sensing26. Additionally, such improved hydrophilicity may play a key role during the synthesis of smooth AEIROFs. Cyclic voltammograms (CV) of rGO-SPEs exhibited a pair of weak and broad characteristic redox peaks between -0.1 V and -0.2 V in de-aerated PBS (Fig. 3D), which are attributed to the remaining unreduced oxygen-containing groups in rGO21. It has been widely reported that during extensive electrochemical reduction by cathodic cycling, majority of unstable oxygen species can be removed in an effective way without reducing reagents and the rest small portions are electrochemically-stabilized and do not harm their electrochemical properties in electrocatalysis and energy storage22b, 27. After anodic deposition of IrO2, two redox peak pairs appeared with anodic ones at +0.2 V and +0.65 V (A1 and A2) and cathodic ones at -0.25 V and +0.45 V (C1 and C2), similar to previous reports on the redox reactions of Ir3+/Ir4+ couples in a two-step manner28. Ir3+ is first partially (A1) and then completely (A2) oxidized into Ir4+ species. The cathodic charge storage capacity (CSCc) is representative of both Faradaic one-electron-transfer reaction between Ir4+ and Ir3+ (pseudo-capacitance) and double-layer capacitance14a. Using time integral of the cathodic current flow at a relatively slow potential sweep rate, CSCc of IrO2-rGO is calculated to be 17.6 mC·cm-2 (Fig. 3D) which is higher than those of anodically oxidized IrO2 film (AIROF)29, activated amorphous or crystalline IrO2 thin film30 and sputtered IrO2 thin film31. These results demonstrate AEIROFs was successfully and efficiently deposited on rGO with large active surface area. Analytical performances A general mechanism of oxides for pH sensing was described by Fog and Buck32 to be attributed to the ion exchange processes within the OH group-bearing surfaces. Different IrO2-based electrodes exist in nature due to different approaches of synthesis. A number of possible sensing
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mechanisms are proposed involving pH-dependent redox intercalation equilibrium between the two oxidation states (Ir(IV) and Ir(III)) of Ir oxides33. As in the case of electrochemicallysynthesized Ir oxides, the predominant state is the hydrated form25a which delivers superNernstian responses with a slope superior than that of anhydrous forms at -59 mV/pH derived from -2.303RT/F. The mechanism of completely hydrated form can be explained using Eqn 525 and Eqn 6: 2[IrO2 (OH)2 ∙ 2H O)]* + 3H 0 + 2e* 2[IrO 20[IrO
2− 3− 2 (OH)2 ∙2H2 O)] /[Ir2 O3 (OH)2 ∙3H2 O)] 2−
2 (OH)2 ∙2H2 O)]
5 1 ln 67 [:
