Chlorine species sensing using pseudo-graphite

Methods, electrodes, and sensors for chlorine species sensing using pseudo-graphite are disclosed. In one illustrative embodiment, a method may include coating a pseudo-graphite material onto a surface of an electrode substrate to produce a pseudo-graphite surface. The method may also include exposing the pseudo-graphite surface to a sample to detect chlorine species in the sample.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No. 16/292,317 (titled “Chemical Oxygen Demand Sensing Using Pseudo-Graphite”), Ser. No. 16/292,320 (titled “pH Sensing Using Pseudo-Graphite”), Ser. No. 16/292,322 (titled “Technologies Using Pseudo-Graphite Composites”), Ser. No. 16/292,323 (titled “Technologies Using Nitrogen-Functionalized Pseudo-Graphite”), and Ser. No. 16/292,325 (titled “Technologies Using Surface-Modified Pseudo-Graphite”), all of which were filed on Mar. 5, 2019, by the co-applicants of the present application. The disclosures of the foregoing patent applications are all incorporated herein by reference in their entirety.

BACKGROUND

The present disclosure relates to electrodes for chlorine species detection, and more particularly to the utilization of pseudo-graphite electrodes for the detection of chlorine species.

Generally, some electrodes in a sensor are capable of detecting chlorine species, for example, chlorine species such as hypochlorite in a liquid. The electrodes may be utilized to detect a concentration of chlorine species in the liquid by applying an electrical potential through the electrode and measuring a resultant signal. However, most of these electrodes have high cost and low performance. Additionally, the electrode performance may be reduced due to fouling of the electrode or environmental interferences.

SUMMARY

According to an aspect of the disclosed embodiments, a method may include coating a pseudo-graphite onto a surface of an electrode substrate to produce a pseudo-graphite surface. The method may also include exposing the pseudo-graphite surface to a sample to detect chlorine species in the sample.

In some embodiments, the method includes modifying the pseudo-graphite surface with an electrochemically sensitive material to alter a sensing property of the electrode to enhance the electrode for chlorine species detection. The method may include modifying the pseudo-graphite surface comprises adding amine groups to the pseudo-graphite surface to enhance the electrode for chlorine species detection. The amine groups may be added to the pseudo-graphite surface by a Kolbe electro-oxidation of a carbamate group. The method may include grafting the amine radicals to the pseudo-graphite surface. The method may include modifying the pseudo-graphite surface with gold nanoparticles.

In some embodiments, providing an electrode may include providing a working electrode. The working electrode may be modified to enhance the electrode for chlorine species detection. The method may also include providing a counter electrode. The method may include detecting chlorine species by measuring a current flow between the working electrode and the counter electrode. The method may include applying at least one of a steady state potential or a known potential to the working electrode.

In some embodiments, the pseudo-graphite may have fast heterogeneous electron transfer at a basal plane. The pseudo-graphite may have a corrosion resistance greater than graphitic materials. A carbon content of the pseudo-graphite may include 80-90% sp2 carbon and 10-20% sp3 carbon.

According to another aspect of the disclosed embodiments, an electrode may include an electrode substrate with a surface. A pseudo-graphite may be coated onto the surface of the electrode substrate to produce a pseudo-graphite surface for detection of chlorine species in a sample.

In some embodiments, the pseudo-graphite may be modified with an electrochemically sensitive material to alter a sensing property of the electrode to enhance the electrode for chlorine species detection. The pseudo-graphite surface may include amine groups to enhance the electrode for chlorine species detection. The pseudo-graphite surface may be modified by Kolbe electro-oxidation of carbamate groups to produce amine radicals. The amine radicals may be grafted to the pseudo-graphite surface. The pseudo-graphite may be modified with gold nanoparticles.

In some embodiments, the pseudo-graphite may have fast heterogeneous electron transfer at a basal plane. The pseudo-graphite may have a corrosion resistance greater than graphitic materials. A carbon content of the pseudo-graphite may include 80-90% sp2 carbon and 10-20% sp3 carbon.

According to yet another aspect of the disclosed embodiments, a sensor may include a working electrode and a counter electrode. An electrical source may supply at least one of a current or voltage to the working electrode. A measurement circuit may measure a resultant signal from the working electrode. The working electrode may include an electrode substrate with a surface. A pseudo-graphite may be coated onto the surface of the electrode substrate to produce a pseudo-graphite surface that enhances the electrode for chlorine species detection.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring toFIG.1, in one illustrative embodiment, a sensor device10includes an electrochemical cell12configured to be positioned within a liquid having a chemical species. The electrochemical cell12houses a working electrode14, a counter electrode16, and a reference electrode18. In some embodiments, the electrochemical cell12only houses the working electrode14and the counter electrode16, and does not include a reference electrode18. In other embodiments, the reference electrode18and working electrode14may be combined into a single electrode. The working electrode14is electrically coupled to a source30. The source30may be a current source or a voltage source. Each electrode14,16,18is coupled to a measuring circuit32that is configured to measure current or voltage, depending on the type of source30.

The sensor device10may be used to detect chlorine species, such as chlorine or hypochlorite. In an embodiment where the source30is a current source, the source30applies a known current to the working electrode14. The measuring circuit32detects a resultant current between the working electrode14and the counter electrode16. By comparing the resultant current to a current at the reference electrode18, a concentration of chemical species in the liquid may be detected.

In an embodiment where the source30is a voltage source, the source30applies a known voltage to the working electrode14that is held at a controlled potential relative to a reference. The measuring circuit32detects a resultant voltage or current between the working electrode14and the counter electrode16. By comparing the resultant voltage to a voltage at the reference electrode18, a concentration of chemical species in the liquid may be detected.

Referring now toFIG.2, in one illustrative embodiment, the working electrode14includes a substrate50having at least one surface52with a coatable surface54. In some embodiments, the electrode14is a composite electrode. The coatable surface54may be selected from at least one of nanosprings, nanotubes, diatomites, a metal, glass, mica, germanium, and silicon (including porous high surface area electrochemically etched silicon). The metal may be selected from copper or iron. The coatable surface54may possess suitable thermal stability, chemical stability at fabrications temperatures and surface chemistry to have a pseudo-graphite applied thereon. The coatable surface54may also have relatively low thermal expansion between the deposition conditions and room temperature. For synthesis/fabrication purposes this includes stable ceramics such as SiO2(which includes micro- and nano-sized structures such as nanosprings and diatomites), as well as other ceramics like Al2O3(including halloysite and anodized aluminum oxide membranes), MgO, iron oxides, silicon, cenospheres, and the like. It also includes suitable carbons such as graphite fibers and carbon black and some high temperature tolerant metals such as tungsten and molybdenum. A pseudo-graphite54is coated onto the surface52of the substrate50. The illustrative embodiment shows the pseudo-graphite54coated on two surfaces52of the substrate50. In some embodiments, the pseudo-graphite is only coated on a single surface52of the substrate50. In some embodiments, the pseudo-graphite54is coated around the substrate50. The pseudo-graphite54may be modified with an electrochemically sensitive material62to alter a sensing property of the electrode14to enhance the electrode14for free chlorine species detection.

As used in the present disclosure, “pseudo-graphite” refers to an allotrope of carbon that is graphite-like, but that has one or more improved properties as compared to graphite and to graphene. These improved properties may include fast heterogeneous electron transfer (HET) at a basal plane of the pseudo-graphite and/or corrosion resistance greater than graphite and graphene. In some embodiments, the pseudo-graphite may be a nanocrystalline-graphite that is in Stage-2 of Ferrari's amorphization trajectory between amorphous carbon and graphite. In some embodiments, the pseudo-graphite has a nanocrystallite size of 1.5 nm, as measured by X-Ray Diffraction (XRD). The pseudo-graphite may have a layered morphology but, in contrast to graphites and graphenes, has a resistance to monoloyer exfoliation. Instead, pseudo-graphite typically exfoliates in thick films of several hundred monolayers at a time.

In some embodiments, the pseudo-graphite may have a sp2/sp3 carbon ratio of about 85/15. In other embodiments, the carbon content of the pseudo-graphite may include between 80-90% sp2 carbon and 10-20% sp3 carbon. In still other embodiments, the carbon content of the pseudo-graphite may include between 75-95% sp2 carbon and 5-25% sp3 carbon. By contrast, typical graphites and graphenes both are near 100% sp2 carbon. For clarity, the pseudo-graphite can contain additional elements besides carbon. For instance, some pseudo-graphites include about 11 atomic % hydrogen.

The appearance of pseudo-graphite may be similar to a crystalline graphite but differs in that both the basal and edge planes (EP) have facile heterogeneous electron transfer (HET) kinetics. The basal plane (BP) of graphites have a barrier to HET as these materials are zero-band gap semiconductors. On the other hand, structural defects within the molecular planes of BP pseudo-graphite may increase density of electronic states (DOS) near the Fermi-level with corresponding HET rates. With the Fe(CN)63-/4-redox probe, BP and EP pseudo-graphite have achieved a standard HET rate (k0) of 10-2 cm/s. Other distinguishing features can include slow hydrogen evolution kinetics and/or molecular planes that are impervious to sub-surface electrolyte intercalation, making the pseudo-graphite more resistant to corrosion than graphites and graphenes. These features can provide a wide electrochemical potential window of 3 V at 200 μA/cm2 in 1 M H2SO4, which surpasses other sp2 carbon electrodes by 1 V and provides pseudo-graphite similar properties to boron-doped diamond. Sensing of strongly oxidizing species, e.g. free chlorine, requires resistance to corrosion along with fast HET rates.

Illustrative examples of “pseudo-graphite,” and methods of producing such materials, are disclosed in each of U.S. Pat. No. 9,691,556, U.S. Patent Application Publication No. 2012/0228555, and Humayun Kabir et al., “The sp2-sp3 carbon hybridization content of nanocrystalline graphite from pyrolyzed vegetable oil, comparison of electrochemistry and physical properties with other carbon forms and allotropes,” published in Carbon, volume 144, pages 831-840. The entire disclosures of each of the foregoing references are incorporated herein by reference.

While the pseudo-graphite54itself possesses many advantageous electrochemical properties, modifying the pseudo-graphite54with other chemical groups may improve the range of functionality and efficacy of the pseudo-graphite54for various applications. Such functionalization can provide improved electrode characteristics for a variety of applications. One such type of application is the sensing of free chlorine species.

In one embodiment, gold nanoparticles are incorporated onto the pseudo-graphite54. The gold nanoparticles may be deposited pre-formed onto the pseudo-graphite54or may be formed on the pseudo-graphite54. Illustratively, the presence of gold nanoparticles may enhance the sensitivity of the pseudo-graphite54for detecting chlorine species. Such nanoparticles may be formed onto the pseudo-graphite54surface by direct electroreduction of gold salts at the pseudo-graphite, or by ALD/CVD or PVD of a thin layer of gold on to the graphenic surface optionally followed by a thermal annealing step to form the gold nanoparticles. Such nanoparticles can be deposited from pre-made gold nanoparticles by methods such as are known in the art including spin-coating, dip-coating, doctor-blading, and electrophoretic deposition.

In another embodiment, the pseudo-graphite54comprises amines. Illustratively, the amine groups may be coupled to the pseudo-graphite54to enhance sensitivity of the pseudo-graphite54for chlorine species measurement. Such amine groups may be created through Kolbe electro-oxidation of carbamic acid or salts thereof such as ammonium carbamate to produce amine radicals which are covalently grafted directly to the pseudo-graphite54.

Referring now toFIG.5, a method400of detecting chlorine species may include voltammetric or amperometric techniques. At block402, chlorine species are detected by continuous amperometric techniques such as applying a static potential between the working electrode14and the reference electrode18and passing a current between the working electrode14and counter electrode16to enable the applied potential to be maintained, at block404. At block406, the current which flows between the working electrode14and the counter electrode16is measured. In one embodiment, applied potential forms such as a steady state applied potential are applied, at block408. Chronoamperometry is then utilized to measure the flow of the potential between the working electrode14and the counter electrode16, at block410. In another embodiment, a known potential is applied for a known amount of time, at block412. The current after the known amount of time is measured, at block414. Such techniques may be applied using a variety of pseudo-graphite material-based electrodes. For instance native pseudo-graphite material may be used for performing the sensing operation. In some embodiments, the measurement may be made utilizing amperometric analysis, potentiometric analysis, cyclic voltammetric analysis, square-wave voltammetric analysis, etc.

In some embodiments, a multi-electrode system may be utilized to improve the reliability of chlorine species measurement in various environmental conditions. For example, the working electrode14may be incorporated with additional electrodes such as a pH measuring electrode system so that the chlorine species measurement (which is pH dependent) may be conducted without requiring the addition of reagents to buffer the pH for the system. Also for example, additional electrodes may be incorporated for measuring the conductivity of the solution so that the chlorine species measurement signal may be calibrated with input from the conductivity measurement. For example, by impressing a sufficient current density through a set of physically proximate electrodes (e.g. native pseudo-graphite material, diamond-like coated pseudo-graphite material, boron-doped diamond, etc.) the pH of the system may be altered.

In some embodiments, a multi-electrode system may be composed with electrodes containing a plurality of electrode surfaces (such as containing a native pseudo-graphite material electrode, an amine-functionalized pseudo-graphite material electrode, and a gold-nanoparticle-functionalized pseudo-graphite material electrode and potentially other electrodes). In many cases the signal of each electrode to the parameter of interest (chlorine species) may be sensitive to other parameters (e.g. pH, etc.) but will commonly have differential sensitivity for such other parameters. Therefore, by measuring the apparent signal from each and correlating the differences in the apparent signals the desired signal can be more reliably extracted. And, if desired the other interfering parameter can often be inferred as well.

Upon exposure to hypochlorite, a pseudo-graphite may experience fouling over time which decreases the sensitivity of the electrode for amperometric detection. However, the pseudo-graphite may be regenerated by applying a cathodic potential (e.g. −1.6 V in 0.1 M phosphate buffer solution at pH 7 for 5 minutes). This procedure can substantially recover the sensitivity of the pseudo-graphite material electrode.

Free chlorine is widely used in water disinfection in order to inactivate pathogenic microorganisms such asEscherichia coliand eryptosporidium. Free chlorine is also used in a variety of other applications spanning from household to the agriculture and food industries. In water treatment, concentration of free chlorine must fall within the range of 20-100 μM according to WHO (World Health Organization) standard. In case of industrial processes the concentration tends to fall within 10-10,000 μM. Analytical techniques for free chlorine sensing include spectrophotometry, iodimetry, chemiluminiscence, catalyst-assisted flow injection and electrochemistry. The most widely used technique for municipal water samples is colorimetry based on N,N′-diethyl-p-phenylenediamine (DPD) which has narrow concentration linear range and cannot be applied in continuous on-line monitoring systems. In contrast, electrochemical methods offer the promise of a cost effective, portable and rapid detector with continuous monitoring and little or no sample preparation.

In an experiment, cyclohexanol (99.94%), elemental sulfur (99.5%, sublimed) and sodium hypochlorite (5% m/m), potassium iodide (101.1%), potassium iodate (99.6%), sodium thiosulfate (99.5-101.0%), glacial acetic acid (99.9%) and starch (1% w/v), potassium monophosphate (99.6%), potassium diphosphate (99.8%) and potassium chloride (99.7%), paraffin were obtained. All aqueous solutions were prepared with deionized water purified by passage through an activated carbon purification cartridge. Pseudo-graphite deposition targets were constructed from quartz tubes cut into 2 cm×0.5 cm wafers. Hypochlorite solutions were standardized by iodometric titration and used within three days.

Pseudo-graphite flakes were synthesized using cyclohexanol and sulfur. All electrochemical studies were conducted in a three-electrode undivided cell with graphite rod counter electrode and Ag/AgCl/3M NaCl (aq) (0.209 V vs SHE) reference electrode and using a CV-50W potentiostat. Chronoamperometric studies at −0.15 V vs. Ag/AgCl were conducted under mass transport aided conditions by stirring at 800 rpm with a controlled growth mercury electrode cell stand.

The reduction of free chlorine proceeds as in the following equation:
ClO−+2e−+2H+=Cl−+H2O E0′=1.49 (vs SHE) at pH 7

This reaction was examined by both chronoamperometric (CA) (at −0.15 V) and cyclic voltammetric (CV) (at 50 mV/s) methods on pseudo-graphite electrodes (as shown inFIG.4) at pH 7.0 in various concentrations of free chlorine. The CV peak potential (Ep) appears at −0.15 V (vs Ag/AgCl) in 1 mM free chlorine (FIG.4B). The corresponding calibration curves are shown inFIG.5. For that figure the CA (FIG.5A) current densities were collected fromFIG.4Aat 120 seconds. For the CV calibration curve (FIG.5B) Epcurrent densities are considered. The CA and CV linear ranges are 200-2,200 and 0-5,000 μM respectively and the sensitivities are 55.24 and 215.83 μA/mM-cm2respectively (n=5). The CA and CV limit of detections (S/N=3) of free chlorine are 0.5 and 1.0 μM respectively.

A comparison of LOD, linear range and sensitivity for free chlorine determination on different materials is shown inFIG.6. Overall, pseudo-graphite has a good combination of LOD, linear range and sensitivity relative to other electrode materials. The LOD and the linear range of the pseudo-graphite-based sensor gives it more flexibility in use relative to other sensors. It is also noteworthy that pseudo-graphite is expected to be a much lower cost material than the others mentioned inFIG.6.

Dissolved oxygen is a possible interference for free chlorine detection as indicated by the formal potential for its reduction.
O2+4H++4e−→2H2O E0′=0.816 V (vs SHE) at pH 7

FIG.7Ashows that that the CV reduction peak for dissolved oxygen (air saturation) is −0.45 V which is separated by 300 mV from free chlorine (Ep=−0.15 V).FIG.8, Rows I and II highlight the results of the CA calibration curve for free chlorine determination in presence of dissolved O2(FIG.7B) and under N2purge (FIG.5A). Both have the same slopes and intercept demonstrating that O2(aq) is not an interference with this sensor.

FIG.8shows the effects of other potential interferences. In that study the effects of 100 μL spikes from 10 mM of free chlorine (NaOCl), NaNO3, Na2SO4, NaCl and CaCO3are measured by chronoamperometry. The first two spikes of free chlorine solutions (at 120 and 240 seconds) give proportional responses via Reaction1. The pseudo-graphite electrode did not respond to the introduction of the other salts. The spike at 840 seconds indicates that the electrode still gives a proportional response to free chlorine.

The response of the pseudo-graphite electrode is found stable after 4 days of continuous exposure to 1 mM free chlorine solution in 0.1 M phosphate buffer solution at pH 7.0 (Figure S1, see supporting information).FIG.9, Row III shows the sensitivity (slope), intercept and correlation values for each day obtained by chronoamperometry at −0.15 V. After 7 days of continuous exposure in free chlorine solution pseudo-graphite electrodes experience a 37% loss in sensitivity (from 56.8 to 35.7 μA/mM-cm2) as summarized inFIG.9, Row III. An in-situ regeneration protocol was developed to extend the life time of this sensor. This process applied −1.6 V (vs Ag/AgCl) for 10 min in 0.1 M phosphate buffer solution, pH=7.0. The regenerated pseudo-graphite electrode recovered 94% of the initial sensitivity (FIG.9, Row IV). Literature reports free chlorine sensor electrodes with signal stabilities from hours to several months (seeFIG.6). Again when considering LOD, linear range, sensitivity along with sensor lifetime, pseudo-graphite electrodes are very competitive with literature.

The focus on electrochemical free chlorine sensors have been with materials of relatively high costs. Pseudo-graphite will be inexpensive relative to these materials (seeFIG.6) with competitive detection limit, linear range, and sensitivity. Furthermore, pseudo-graphite is not effected from possible common aqueous species and interferences O2, Ca2+, Na+, NO3−, SO42−, Cl−and CO32−. A significant feature of pseudo-graphite is long term signal stability and the ability to recover sensitivity with the eventual fouling of the electrode surface. Relative to recent advances in fluorescence method and classical colorimetric method (N,N-diethyl-p-phenylenediamine), the presented technique has much wider linear range (0.05-15 vs. 0-5000 μM). Furthermore, the pseudo-graphite-based method is more rapid and offers continuous monitoring capabilities. Another feature is that pseudo-graphite electrodes can be fabricated into a variety of geometries including micron and smaller dimensions. These qualities indicate possible application ranging from home use to embedded sensors where durability and continuous monitoring is required.

While certain illustrative embodiments have been described in detail in the figures and the foregoing description, such an illustration and description is to be considered as exemplary and not restrictive in character, it being understood that only illustrative embodiments have been shown and described and that all changes and modifications that come within the spirit of the disclosure are desired to be protected. There are a plurality of advantages of the present disclosure arising from the various features of the methods, systems, and articles described herein. It will be noted that alternative embodiments of the methods, systems, and articles of the present disclosure may not include all of the features described yet still benefit from at least some of the advantages of such features. Those of ordinary skill in the art may readily devise their own implementations of the methods, systems, and articles that incorporate one or more of the features of the present disclosure.