WO2023069453A1 - Methods of quantitating carbon dioxide hydration in aqueous solutions - Google Patents

Abstract

Provided are methods and devices for quantitating CO₂ hydration in aqueous solutions. In some embodiments, the methods comprise disposing a sample of the aqueous solution in an optical cell, wherein the sample is disposed as a layer having a thickness of from 5 to 200 µm. Such methods further comprise performing transmission Fourier transform infrared (FTIR) spectroscopy on the sample layer to determine CO₂(aq) vibrational features in the sample, and quantitating CO₂ hydration in the aqueous solution based on the CO₂(aq) vibrational features. Also provided are methods and devices for quantitating HCO₃ ^- and CO₃ ^2- ions in aqueous solutions by enhanced Raman spectroscopy, as well as methods and devices for determining the pH of aqueous solutions by electrochemical cyclic voltammetry.

Description

METHODS OF QUANTITATING CARBON DIOXIDE HYDRATION IN AQUEOUS SOLUTIONS

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 63/256,877, filed October 18, 2021 , which application is incorporated herein by reference in its entirety.

INTRODUCTION

Understanding CO₂ dissolution in aqueous solutions is of fundamental importance and has implications relating to energy, the environment and consumer industries. As a greenhouse gas, excessive CO₂ emissions from burning of fossil fuels has led to negative environmental and social consequences. Absorption of CO₂(g) by aqueous solvents is a promising approach to CO₂ capture and storage. CO₂ is also a popular flavoring molecule in many types of consumer drinks. Dissolved carbon species in aqueous solutions are typically probed by titration and total carbon (TC) measurements, and further analyzed according to chemical equilibria. Tracking of gas phase pressure, composition, and electrical or thermal conductivity allowed one to glean the kinetics of CO₂ hydration. Direct speciation and kinetics analysis of dissolved CO₂(aq) based on in-situ techniques, especially for high pressures conditions are challenging.

To combat climate change and close the carbon cycle, conversion of CO₂ into fuels and value-added products via photo-/electro-catalytic CO₂ reduction reactions (CO₂RR) is attractive. In CO₂RR, hydrated CO₂(aq) serves as the active carbon species being reduced to CO, hydrocarbons and oxygenates, but CO₂(aq) concentration is rarely measured directly and commonly estimated by chemical equilibria and diffusion behavior of the species involved. Approaches are needed for dynamic carbon species monitoring (HCO₃-, CO₃ ^2-, and especially CO₂(aq)) under representative CO₂RR conditions including 002(g) bubbling and high pressures, which should facilitate understanding the mechanism and devising rational reaction schemes.

SUMMARY

Provided are methods of quantitating CO₂ hydration in aqueous solutions. In some embodiments, the methods comprise disposing a sample of the aqueous solution in an optical cell, wherein the sample is disposed as a layer having a thickness of from 5 to 200 pm. Such methods further comprise performing transmission Fourier transform infrared (FTIR) spectroscopy on the sample layer to determine CO₂(aq) vibrational features in the sample, and quantitating CO₂ hydration in the aqueous solution based on the CO₂(aq) vibrational features. Also provided are methods of quantitating HCO₃- and CO3²' ions in aqueous solutions by enhanced Raman spectroscopy, as well as methods of determining the pH of aqueous solutions by electrochemical cyclic voltammetry.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1a-1e: CO₂ hydration monitored in micro-IR optical cell, a, Transmittance FTIR spectrum (resolution = 0.96 cm^-1) of H2O after bubbling with 30 seem Ar, and of 0.5 M NaHCO₃ solution after bubbling with 30 seem 002(g), both for 30 min. Inset: Zoom-in of 2300 - 2400 cm^-1 range, showing CO₂(aq) V3 (C=O asymmetric stretching) IR absorption band; A schematic of the micro-IR cell used, with thin (< ~ 100 pm) aqueous sample films sandwiched between borosilicate glass windows for FTIR measurements, b, Background subtracted CO₂(aq) V3 peak at f = 2343.1 cm ¹ (resolution = 0.06 cm ¹) measured in water equilibrated with 1 atm 002(g), showing a 4.4 cm ¹ redshift from fundamental V3 vibrational frequency fo = 2347.5 cm'¹ due to interactions with H2O. c, COSMO-RS simulation of Gibbs free energy change AG = -0.149 kcal/mol during CO₂ hydration in atmosphere, suggesting the high reversibility between 002(g) and CO₂(aq). d, FTIR spectrum of ¹²C02(g) (resolution = 0.06 erm¹), revealing the coupling of quantized molecular rotational transitions with 0=0 vibration. Sharp peaks correspond to transitions of increasing (AJ > 0, R-branch) and decreasing (AJ < 0, P-branch) rotational energy levels located at each side of fundamental V3 frequency fo following Boltzmann distribution. Small ¹³C02(g) signals corresponding to ~ 1.1% natural abundance of ¹³C are observed near 2270 cm'¹, e, Dynamic monitoring of CO₂ hydration in 0.5 M NaHCO₃ solution under 30 seem 002(g) bubbling (resolution = 0.06 cm ¹). Sharp 002(g) absorption peaks diminished from ~ 2 min and disappeared after ~ 20 min, suggesting quenching of CO₂ molecular rotations by solvating H2O molecules and during 002(g) to CO₂(aq) elevation.

FIG. 2a-2f: CC faq) quantification in solutions important to CO₂ capture, storage, photo-Zelectro-reduction, and to consumer products, a, Background subtracted CO₂(aq) V3 IR absorbance spectra (resolution = 0.96 erm¹) measured in a 0.5 M NaHCO₃ solution equilibrated with 1 atm 002(g) for 30 min in micro-IR cells with thickness d = 12.2, 24.7, 51 .5, and 103.0 pm respectively (gas-liquid interphase area of ~ 1 .5 cm²), b, Integrated CO₂(aq) IR absorbance A plotted against cell thickness d, showing a high linearity (R²=0.998). Error bars were obtained from 3 parallel measurements, c, Calibration curves for CO₂(aq) integrated absorbance vs. theoretically calculated concentrations in micro-IR cells with different cell thicknesses. The pure H2O and 0.5, 1 .0 M NaHCO₃ solutions were equilibrated with air or 1 atm 002(g) for 30 min before micro-IR measurements. Integrated CC>2(a<7) absorbance A in micro-IR cells of d= 12.2, 24.7, 51 .5, and 103.0 pm was correlated with calculated CO₂(aq) concentration under corresponding equilibrating condition for linear regression. Error bars were obtained from 3 parallel experiments. CO₂(aq) molar extinction coefficient E = 1.74 * 10⁴ M’¹-cnr² according to Beer’s law. d, Dynamic monitoring of CO₂ hydration in H2O under 30 seem 002(g) bubbling or 1 atm 002(g) equilibrating. With CO₂(aq) vs. time fit by an exponential function of y = yo + a * exp (-x / th), a faster CO₂ hydration kinetics and a ~ 1.2 times higher CO₂(aq) concentration after 40 min were observed under bubbling than equilibrating in 1 atm 002(g). Error bars were obtained from 3 parallel experiments, e, Dynamic monitoring of CO₂ hydration in 0.5 M KHCO3 solution under 30 seem CO₂(g) bubbling and 1 atm 002(g) equilibrating conditions respectively, showing slower initial hydration kinetics and lower CO₂(aq) equilibrium concentrations than those observed in pure water in D. Error bars were obtained from 3 parallel experiments, f, Quantitation of CO₂(a<7) in commercial drinks by micro-IR technique. CO₂(aq) concentration in brut champagne may be underestimated due to its high packaging pressure.

FIG. 3a-3d: In-situ kinetic monitoring of CO₂ hydration under high pressures, a, Dynamic evolution of CO₂ V3 IR peak in H2O when 58 atm CO₂(g) was applied in a micro-IR cell (resolution = 0.96 cm^-1 , cell thickness d = 6 pm, and gas-liquid interphase area ~ 1.5 cm²). ¹²CO₂(g) branches diminished from ~ 1 min and tiny ¹³CO₂(aq) signal was observed. Inset: A schematic of micro-IR cell equipped with a stainless steel frame to sustain high pressure conditions, b, CO₂(aq) V3 IR spectra of H2O equilibrated with 15 - 58 atm 002(g) for 30 min (resolution = 0.96 cm ¹, d = 6.0 pm), showing significant absorbance increase at higher pressures, c, Calibration curve for CO₂(aq) quantification under 1 - 58 atm 002(g) partial pressures. Measured CO₂(aq) concentration (derived from integrated absorbance and molar extinction coefficient) was correlated with 002(g) partial pressure, affording a reciprocal fitting of [CChtaq)] ' = 30.18 * ptCC ) ' + 0.1550 with R² = 0.9997, in close agreement with the ECO₂ N model. Error bars were obtained from 3 parallel experiments, d, CO₂ hydration kinetics in water under 30 and 58 atm 002(g) respectively. With CO₂(aq) evolutions fit by exponential functions, a faster hydration kinetics was observed under high pressures (M ~ 5.2 and 6.3 min respectively) than the 1 atm condition in Fig. 2d red curve (fo ~ 14 min). Error bars were obtained from 3 parallel experiments.

FIG. 4a-4d: Kinetic monitoring of CO₂ dehydration in aqueous solutions, a, Dynamic monitoring of CO₂ V3 IR band in H2O (resolution = 0.96 cm^-1 , cell thickness d= 51 .5 pm, and gas-liquid interphase area ~ 1.5 cm²) after 002(g) bubbling stopped and solution exposed to air. CO₂(aq) absorbance decreased and 002(g) P7R-branches appeared over time, suggesting the dehydration of dissolved CO₂(aq) to 002(g). b, CO₂ dehydration kinetics in supersaturated water after bubbling stopped. With solution pH increasing from 3.71 to a near equilibrium value of 5.67 over 3 h standing in air, CO₂(aq) concentration gradually decreased from ~ 40 to 17 mM, far from reversing supersaturation. CO₂(aq) evolution vs. time fit by y = yo + a * exp (-x / tdh) suggested a much slower dehydration kinetics than hydration, leading to the hysteresis of dissolved CO₂(aq) in liquid phase. Error bars were obtained from 3 parallel experiments, c, Dehydration time constant t plotted vs. initial CO₂(aq) concentration in CO₂(g) equilibrated H2O under conditions indicated, seawater and 0.5 M KHCO3 under 1 atm CO₂(g), and commercial under/near packaged pressures. The purple line connects data points for pure H2O under various CC>2(g) equilibrating conditions. Note all data points for salt solutions and consumer drinks are located below the purple line for pure H2O, suggesting quicker CO₂(aq) dehydration kinetics than in water, d, The measured H2O bending-libration combination IR absorption band of H2O in air and 15 - 58 atm CC>2(g), 0.5 M KHCO3 solution and seawater in air, and commercial drinks in/near packing pressures (resolution = 0.96 cm^-1, absorbance intensity normalized for clear comparison). Systematic redshifts from pure H2O bending-libration peak at 2144 cm ¹ revealed the weakening of hydrogen bonding network in all solute-rich solutions, in which strong solute-water interactions outcompeted CO₂ hydration and accelerated dehydration.

FIG. 5a-5e: a, Schematics of the optical cell for pGOLD Raman spectroscopy, using a quartz disc window and a pGOLD deposited glass disc substrate (shown in the photos) for enhanced Raman scattering in solution phase. SEM image of pGOLD disc revealed isolated Au islands of 50 - 200 nm randomly dispersing on the disc surface, with gaps of 10 - 50 nm in between, b, Raman spectra of acidified 2 M K2CO3 solution (pH = 10.0) measured by a 532 nm laser on pGOLD disc and polycrystalline Au foil substrate respectively. On the pGOLD substrate, Raman peak area for both ions increased by ~ 8 times compared to on polycrystalline Au foil, c, The pGOLD Raman spectra of 2 M K2CO3 solution acidified by diluted HCI, taken under 532 nm laser wavelength. With the solution pH decreased from 13.3 to 8.0 during HCI titration, v(C-O) intensity continued decreasing and v(C-OH) continued increasing, suggesting the evolution of dominant ionic carbon species in the solution changing from CO₃ ^2- to HCO₃-. d, Calculated concentrations of bicarbonate and carbonate ions in the acidified 2 M K2CO3 solution at different pH. The calculation was based on the dissociation equilibrium of HCO₃- ion. e, Calibration curves for the quantification of HCO₃- and CO3²- ions by the pGOLD Raman technique using 532 nm laser. 2 M K2CO3 solution was acidified by stepwise HCI titration, and Raman spectra were recorded at multiple pH points. Integrated HCO₃- and CO₃ ^2- Raman peak areas were normalized by the corresponding peak areas recorded with an acidified 2 M K2CO3 (pH = 10.0) as the standard solution. The standard solution was measured every time together with unknown samples, which helped to normalize the Raman measurement conditions such as the laser power (see Experimental Section for details). Normalized HCO₃- and CO₃ ^2- peak areas were then correlated with calculated ion concentrations in the solution at corresponding pH (see Computational Methods for details) to derive the calibration curves.

FIG. 6a-6d: a, Dynamic micro-IR spectra of 002(g) bubbling into 2 M KOH solution at 30 seem (cell thickness d = 51.5 pm, IR resolution = 0.96 cm^-1 , 1.1 mm thick borosilicate glass windows, and background H2O absorption subtracted by polynomial fitting), b, CO₂(aq) evolution and solution pH (measured by pH meter) during 30 seem CO₂(g) bubbling into 2 M KOH. Error bars were obtained from 3 parallel experiments. CO₂(aq) showed a rapid increase after solution pH < ~ 9 when most OH- was consumed. Equilibrium was reached after ~ 230 min with a CO₂(aq) concentration of ~ 26.0 mM. c, Dynamic pGOLD Raman spectra of the 2 M KOH solution recorded every 20 min upon 30 seem CO₂(g) bubbling start, showing that CO₃ ^2- concentration decreased after an initial increase, while HCO₃- kept increasing until an equilibrium was reached after ~ 3 h. The dominant carbon species evolved from CO3²’ to HCO₃- during continuous CO₂ bubbling, d, Dynamic monitoring of HCO₃- and CO3²’ concentrations by pGOLD Raman technique during 1 atm CO₂ bubbling into 2 M KOH solution at 30 seem. CO3²’ concentration increased in the first 40 min then decreased to a very low level, and HCO₃- concentration kept increasing to ~ 2 M after 3 hours bubbling.

FIG. 7a-7c: Derivation of the onset interconversion potential between Zn(0) and Zn(ll) species from Zn-CV curves. In a, 2 M KOI titrated by dilute KOH to pH = 7.0, b, 2 M K2CO3 titrated by dilute HCI to pH = 9.0, c, 2 M K2CO3 titrated by dilute HCI to pH = 12.5, Zn foil was used as WE for CV scans with Ag/AgCI as RE. The onset oxidation/reduction potential Φ_Ox and was derived by extending the corresponding peaks on the curve, and the onset interconversion potential 0 used for the following pH calculation was set to be the average of Φ_Ox and Φ_red.

FIG. 8a-8b: a, Dynamic monitoring of HCO₃- and CO3²’ concentrations by pGOLD Raman during and after 58 atm CO₂ capture in 2 M K2CO3 solution. CO3²’ concentration in the solution kept decreasing while HCO₃- kept increasing during the 6 h CO₂ absorption. CO₂ pressure was released to atmosphere after 6 h, and no significant ion concentration change was observed in the solution after 1 h standing in air. b, Dynamic monitoring of pH and CO₂(aq) concentration in 2 M K2CO3 solution during and after 58 atm CO₂ capture. The pH values were derived by pGOLD Raman/Zn-CV method, as well as calculated based on Raman quantification of [HCO3 ] I [CO3² ] in the slightly alkaline to neutral range (pH = 8 ~ 11 ). CO₂(aq) concentration experienced significant increase after the solution pH decreased to < 9. No significant pH change was observed after CO₂ pressure was released from 58 atm to atmosphere over 1 h, while CO₂(aq) concentration showed a large decrease. DETAILED DESCRIPTION

Before the methods of the present disclosure are described in greater detail, it is to be understood that the methods are not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the methods will be limited only by the appended claims.

Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the methods. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed within the methods, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the methods.

Certain ranges are presented herein with numerical values being preceded by the term “about.” The term “about” is used herein to provide literal support for the exact number that it precedes, as well as a number that is near to or approximately the number that the term precedes. In determining whether a number is near to or approximately a specifically recited number, the near or approximating unrecited number may be a number which, in the context in which it is presented, provides the substantial equivalent of the specifically recited number.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the methods belong. Although any methods similar or equivalent to those described herein can also be used in the practice or testing of the methods, representative illustrative methods are now described.

All publications and patents cited in this specification are herein incorporated by reference as if each individual publication or patent were specifically and individually indicated to be incorporated by reference and are incorporated herein by reference to disclose and describe the materials and/or methods in connection with which the publications are cited. The citation of any publication is for its disclosure prior to the filing date and should not be construed as an admission that the present methods are not entitled to antedate such publication, as the date of publication provided may be different from the actual publication date which may need to be independently confirmed. It is noted that, as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as “solely,” “only” and the like in connection with the recitation of claim elements, or use of a “negative” limitation.

It is appreciated that certain features of the methods, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the methods, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. All combinations of the embodiments are specifically embraced by the present disclosure and are disclosed herein just as if each and every combination was individually and explicitly disclosed, to the extent that such combinations embrace operable processes and/or compositions. In addition, all sub-combinations listed in the embodiments describing such variables are also specifically embraced by the present methods and are disclosed herein just as if each and every such sub-combination was individually and explicitly disclosed herein.

As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present methods. Any recited method can be carried out in the order of events recited or in any other order that is logically possible.

METHODS FOR QUANTITATING DISSOLVED CO₂ IN LIQUIDS BY INFRARED SPECTROSCOPY

Aspects of the present disclosure include infrared spectroscopy (IR)-based methods for quantitating dissolved CO₂ in liquids. Measurement of dissolved CO₂ has important applications in healthcare monitoring and consumer goods quality control, yet is difficult to measure directly. Common methods include titration, measuring off-gas pressure, electrical conductivity or calculating chemical equilibria, all of which require a secondary calculating to determine the concentration of dissolved CO₂. Described herein are infrared (IR) spectroscopy techniques that overcome the typical challenge of CO₂ peaks being overshadowed by water. The IR cell creates a thin film of solution where the water does not absorb all the light, allowing the CO₂ signal to be resolved and quantified. This configuration tolerates high pressure systems (e.g., up to 60 atm) and still allows for accurate CO₂ quantification. From CO₂ capture to consumer drinks and healthcare monitoring via dissolved CO₂ in blood, the methods of the present disclosure provide reliable, accurate and direct approaches for in situ measurement of dissolved CO₂.

In certain aspects, provided are methods of quantitating CO₂ hydration in an aqueous solution, the methods comprising disposing a sample of the aqueous solution in an optical cell, wherein the sample is disposed as a layer having a thickness of from 5 to 200 pm. Such methods further comprise performing transmission Fourier transform infrared (FTIR) spectroscopy on the sample layer to determine CO₂(aq) vibrational features in the sample, and quantitating CO₂ hydration in the aqueous solution based on the CO₂(aq) vibrational features.

According to some embodiments, the thickness of the sample layer is from 5 to 100 pm. For example, the thickness of the sample layer may be, e.g., from 5 to 90 pm, 5 to 80 pm, 5 to 70 pm, 5 to 60 pm, or 5 to 50 pm.

In certain embodiments, the sample layer is at a CO₂(g) partial pressure of from 10- 60 atm when performing the FTIR spectroscopy. For example, the sample layer may be at a CC>2(g) partial pressure of from 40-60 atm when performing the FTIR spectroscopy.

According to some embodiments, the methods further comprising quantitating HCO₃- and CO3²- ions in the aqueous solution. In certain embodiments, the quantitating is by Raman spectroscopy, and the quantitating is carried out as described in the section below entitled Methods for Quantitating HCO₃- and CO₃ ^2- Anions by Raman Spectroscopy and/or Example 2 in the Experimental section herein. For example, according to some embodiments, the methods of quantitating HCO₃- and CO3²’ anions comprise disposing a sample of the aqueous solution in an optical cell, wherein the sample is disposed as a layer on a plasmonic gold thin film, performing enhanced Raman spectroscopy on the sample layer disposed on the plasmonic gold thin film, and quantitating HCO₃- and CO3²’ ions in the aqueous solution based on the enhanced Raman spectroscopy. Detailed guidance for quantitating HCO₃- and CO3²- anions by Raman spectroscopy according to embodiments of the present disclosure is provided in Example 2 below.

In certain embodiments, the present methods of quantitating CO₂ hydration further comprising determining the pH of the aqueous solution. According to some embodiments, determining the pH of the aqueous solution is carried out as described in the section below entitled Electrochemical Methods for Solution pH Measurements and/or Example 3 in the Experimental section herein. For example, according to some embodiments, determining the pH of the aqueous solution comprises performing electrochemical cyclic voltammetry on a sample of the aqueous solution. In certain embodiments, performing electrochemical cyclic voltammetry comprises disposing a sample of the aqueous solution in an electrolyzer comprising a working electrode, a reference electrode, and a counter electrode. Such performing may further comprise applying a potential across the electrodes, measuring a current associated with the sample, and determining the pH of the aqueous solution based on the current associated with the sample. Detailed guidance for determining the pH of an aqueous solution by electrochemical cyclic voltammetry according to embodiments of the present disclosure is provided in Example 3 below.

The methods of quantitating CO₂ hydration in an aqueous solution may be performed on any aqueous solution of interest. In certain embodiments, the aqueous solution is from a natural body of water. Non-limiting examples of natural bodies of water include an ocean, a bay, a river, or a lake. According to some embodiments, the aqueous solution is a beverage. As used herein, a “beverage” is any potable liquid, including those other than water. Non- limiting examples of beverages include soda, sparkling water, seltzer, sparkling wine, and champagne. In certain embodiments, the aqueous solution is whole blood or a fraction thereof, e.g., serum or plasma. Serum is the liquid fraction of whole blood that is collected after the blood is allowed to clot. The clot may be removed by centrifugation and the resulting supernatant, designated serum, may be removed, e.g., using a Pasteur pipette. Plasma is produced when whole blood is collected in tubes that are treated with an anticoagulant. The blood does not clot in the plasma tube. The cells may be removed by centrifugation. The supernatant, designated plasma may be carefully removed from the cell pellet using, e.g., a Pasteur pipette.

Devices that find use in quantitating CO₂ hydration in an aqueous solution are also provided. In certain embodiments, such devices comprise an optical cell adapted to contain a sample disposed as a layer having a thickness of from 5 to 200 pm, e.g., a thickness of from 5 to 100 pm, or a thickness of from 5 to 50 pm. In certain embodiments, the optical cell is operably coupled to a FT-IR spectrometer. As used herein, “operably coupled” means that the two elements are in a functional relationship with each other.

According to some embodiments, such devices include one or more non-transitory computer-readable media comprising instructions stored thereon, which when executed by one or more processors, cause the one or more processors to perform transmission Fourier transform infrared (FTIR) spectroscopy on the sample layer to determine CO₂(aq) vibrational features in the sample, and quantitate CO₂ hydration in the aqueous solution based on the CO₂(aq) vibrational features.

Non-limiting examples of devices that find use in quantitating CO₂ hydration in an aqueous solution according to embodiments of the present disclosure are provided in the Experimental section below. METHODS FOR QUANTITATING HCOS' AND COS²' IONS BY RAMAN SPECTROSCOPY

Additional aspects of the present disclosure include methods for quantitating HCO₃- and CO₃ ^2- ions in aqueous solutions. The methods are based at least in part on the inventors’ discovery that Raman spectroscopic probing of liquids over nanostructured plasmonic gold substrates allows for dynamic quantification of HCO₃- and CO₃ ^2- anions during CO₂ dissolution and escaping.

In certain embodiments, provided are methods of quantitating HCO₃- and CO₃ ^2- ions in an aqueous solution, the methods comprising disposing a sample of the aqueous solution in an optical cell, wherein the sample is disposed as a layer on a plasmonic gold thin film. Such methods further comprise performing enhanced Raman spectroscopy on the sample layer disposed on the plasmonic gold thin film, and quantitating HCO₃- and CO₃ ^2- ions in the aqueous solution based on the enhanced Raman spectroscopy.

According to some embodiments, the plasmonic gold thin film comprises isolated gold islands have an average greatest dimension of 25 to 300 nm separated by gaps having an average greatest dimension of 5 to 100 nm. For example, in certain embodiments, the isolated gold islands have an average greatest dimension of 50 to 200 nm separated by gaps having an average greatest dimension of 10 to 50 nm.

In some instances, the enhanced Raman spectroscopy comprises irradiating the sample layer disposed on the plasmonic gold thin film with a laser at from 500 to 800 nm. According to some embodiments, the enhanced Raman spectroscopy comprises irradiating the sample layer disposed on the plasmonic gold thin film with a laser at from 600 to 650 nm, optionally at about 633 nm.

Detailed guidance for quantitating HCO₃- and CO₃ ^2- ions by Raman spectroscopy according to embodiments of the present disclosure is provided in Example 2 below.

The methods of the present disclosure for quantitating HCO₃- and CO₃ ^2- ions may be performed on any aqueous solution of interest. In certain embodiments, the aqueous solution is from a natural body of water. Non-limiting examples of natural bodies of water include an ocean, a bay, a river, or a lake. According to some embodiments, the aqueous solution is a beverage. As used herein, a “beverage” is any potable liquid, including those other than water. Non-limiting examples of beverages include soda, sparkling water, seltzer, sparkling wine, and champagne. In certain embodiments, the aqueous solution is whole blood or a fraction thereof, e.g., serum or plasma.

Devices that find use for quantitating HCO₃- and CO₃ ^2- ions by Raman spectroscopy are also provided. In certain embodiments, such devices comprise an optical cell comprising a plasmonic gold thin film and adapted to contain a sample of an aqueous solution disposed as a layer on the plasmonic gold thin film. According to some embodiments, the plasmonic gold thin film comprises isolated gold islands have an average greatest dimension of 25 to 300 nm separated by gaps having an average greatest dimension of 5 to 100 nm. For example, in certain embodiments, the isolated gold islands have an average greatest dimension of 50 to 200 nm separated by gaps having an average greatest dimension of 10 to 50 nm.

In some instances, such devices are operably coupled to a laser, e.g., a laser adapted for irradiating the sample at from 500 to 800 nm, e.g., at from 600 to 650 nm, optionally at about 633 nm.

According to some embodiments, such devices include one or more non-transitory computer-readable media comprising instructions stored thereon, which when executed by one or more processors, cause the one or more processors to perform enhanced Raman spectroscopy on the sample layer disposed on the plasmonic gold thin film, and quantitate HCO₃- and CO₃ ^2- ions in the aqueous solution based on the enhanced Raman spectroscopy.

Non-limiting examples of devices that find use in quantitating HCO₃- and CO₃ ^2- ions in an aqueous solution by Raman spectroscopy according to embodiments of the present disclosure are provided in the Experimental section below.

ELECTROCHEMICAL METHODS FOR SOLUTION pH MEASUREMENTS

Aspects of the present disclosure also include electrochemical methods for solution pH measurements. The methods find use in a variety of contexts, including for pH probing of aqueous solutions in closed systems under high pressures. In certain embodiments, provided are electrochemical cyclic voltammetry methods of determining the pH of an aqueous solution, the methods comprising disposing a sample of the aqueous solution in an electrolyzer comprising a working electrode, a reference electrode, and a counter electrode. The methods further comprise applying a potential across the electrodes, measuring a current associated with the sample, and determining the pH of the aqueous solution based on the current associated with the sample.

According to some embodiments, the working electrode is a zinc (Zn) working electrode. In other embodiments, the working electrode is a nickel (Ni) working electrode.

In certain embodiments, the reference electrode is an Ag/AgCI reference electrode. In some instances, the counter electrode is a Pt counter electrode. According to some embodiments, the reference electrode is an Ag/AgCI reference electrode and the counter electrode is a Pt counter electrode. Detailed guidance for determining the pH of an aqueous solution by electrochemical cyclic voltammetry according to embodiments of the present disclosure is provided in Example 3 below.

The methods of the present disclosure for determining the pH of an aqueous solution by electrochemical cyclic voltammetry may be performed on any aqueous solution of interest. In certain embodiments, the aqueous solution is from a natural body of water. Non-limiting examples of natural bodies of water include an ocean, a bay, a river, or a lake. According to some embodiments, the aqueous solution is a beverage. As used herein, a “beverage” is any potable liquid, including those other than water. Non-limiting examples of beverages include soda, sparkling water, seltzer, sparkling wine, and champagne. In certain embodiments, the aqueous solution is whole blood or a fraction thereof, e.g., serum or plasma.

Devices that find use in determining the pH of an aqueous solution by electrochemical cyclic voltammetry are also provided. In certain embodiments, such devices comprise an electrolyzer comprising a working electrode, a reference electrode, and a counter electrode. According to some embodiments, the working electrode is a zinc (Zn) working electrode. In other embodiments, the working electrode is a nickel (Ni) working electrode. In certain embodiments, the reference electrode is an Ag/AgCI reference electrode. In some instances, the counter electrode is a Pt counter electrode. According to some embodiments, the reference electrode is an Ag/AgCI reference electrode and the counter electrode is a Pt counter electrode.

According to some embodiments, such devices include one or more non-transitory computer-readable media comprising instructions stored thereon, which when executed by one or more processors, cause the one or more processors to apply a potential across the electrodes, measure a current associated with the sample, and determine the pH of the aqueous solution based on the current associated with the sample.

Non-limiting examples of devices for determining the pH of an aqueous solution by electrochemical cyclic voltammetry according to embodiments of the present disclosure are provided in the Experimental section below.

Devices, Systems and Non-Transitory Computer-Readable Media

A variety of processor-based devices and systems may be employed to implement any of the embodiments of the present disclosure relating to the infrared spectroscopy (IR)- based methods for quantitating dissolved CO₂, methods for quantitating HCO₃- and CO₃ ^2- ions in aqueous solutions, and electrochemical methods for solution pH measurements. Such devices and systems may include system architecture wherein the components of the system are in electrical communication with each other using a bus. System architecture can include a processing unit (CPU or processor), as well as a cache, that are variously coupled to the system bus. The bus couples various system components including system memory, (e.g., read only memory (ROM) and random access memory (RAM), to the processor.

System architecture can include a cache of high-speed memory connected directly with, in close proximity to, or integrated as part of the processor. System architecture can copy data from the memory and/or the storage device to the cache for quick access by the processor. In this way, the cache can provide a performance boost that avoids processor delays while waiting for data. These and other modules can control or be configured to control the processor to perform various actions. Other system memory may be available for use as well. Memory can include multiple different types of memory with different performance characteristics. Processor can include any general purpose processor and a hardware module or software module, such as first, second and third modules stored in the storage device, configured to control the processor as well as a special-purpose processor where software instructions are incorporated into the actual processor design. The processor may essentially be a completely self-contained computing system, containing multiple cores or processors, a bus, memory controller, cache, etc. A multi-core processor may be symmetric or asymmetric.

To enable user interaction with the computing system architecture, an input device can represent any number of input mechanisms, such as a microphone for speech, a touch- sensitive screen for gesture or graphical input, keyboard, mouse, motion input, speech and so forth. An output device can also be one or more of a number of output mechanisms. In some instances, multimodal devices and systems can enable a user to provide multiple types of input to communicate with the computing system architecture. A communications interface can generally govern and manage the user input and system output. There is no restriction on operating on any particular hardware arrangement and therefore the basic features here may easily be substituted for improved hardware or firmware arrangements as they are developed.

The storage device is typically a non-volatile memory and can be a hard disk or other types of computer-readable media which can store data that are accessible by a computer, such as magnetic cassettes, flash memory cards, solid state memory devices, digital versatile disks, cartridges, random access memories (RAMs), read only memory (ROM), and hybrids thereof.

The storage device can include software modules for controlling the processor. Other hardware or software modules are contemplated. The storage device can be connected to the system bus. In one aspect, a hardware module that performs a particular function can include the software component stored in a computer-readable medium in connection with the necessary hardware components, such as the processor, bus, output device, and so forth, to carry out various functions of the disclosed technology.

Embodiments within the scope of the present disclosure may also include tangible and/or non-transitory computer-readable storage media or devices for carrying or having computer-executable instructions or data structures stored thereon. Such tangible computer- readable storage devices can be any available device that can be accessed by a general purpose or special purpose computer, including the functional design of any special purpose processor as described above. By way of example, and not limitation, such tangible computer-readable devices can include RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other device which can be used to carry or store desired program code in the form of computer-executable instructions, data structures, or processor chip design. When information or instructions are provided via a network or another communications connection (either hardwired, wireless, or combination thereof) to a computer, the computer properly views the connection as a computer-readable medium. Thus, any such connection is properly termed a computer- readable medium. Combinations of the above should also be included within the scope of the computer-readable storage devices.

Computer-executable instructions include, for example, instructions and data which cause a general purpose computer, special purpose computer, or special purpose processing device to perform a certain function or group of functions. Computer-executable instructions also include program modules that are executed by computers in stand-alone or network environments. Generally, program modules include routines, programs, components, data structures, objects, and the functions inherent in the design of specialpurpose processors, etc. that perform tasks or implement abstract data types. Computerexecutable instructions, associated data structures, and program modules represent examples of the program code means for executing steps of the methods disclosed herein. The particular sequence of such executable instructions or associated data structures represents examples of corresponding acts for implementing the functions described in such steps.

Other embodiments of the disclosure may be practiced in network computing environments with many types of computer system configurations, including personal computers, hand-held devices, multi-processor devices and systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, and the like. Embodiments may also be practiced in distributed computing environments where tasks are performed by local and remote processing devices that are linked (either by hardwired links, wireless links, or by a combination thereof) through a communications network. In a distributed computing environment, program modules may be located in both local and remote memory storage devices.

Notwithstanding the appended claims, the present disclosure is also defined by the following embodiments:

1 . A method of quantitating CO₂ hydration in an aqueous solution, the method comprising: disposing a sample of the aqueous solution in an optical cell, wherein the sample is disposed as a layer having a thickness of from 5 to 200 pm; performing transmission Fourier transform infrared (FTIR) spectroscopy on the sample layer to determine CO₂(aq) vibrational features in the sample; and quantitating CO₂ hydration in the aqueous solution based on the CO₂(a<7) vibrational features.

2. The method according to embodiment 1 , wherein the thickness of the sample layer is from 5 to 100 pm.

3. The method according to embodiment 2, wherein the thickness of the sample layer is from 5 to 50 pm.

4. The method according to any one of embodiments 1 to 3, wherein the sample layer is at a CO₂(g) partial pressure of from 10-60 atm when performing the FTIR spectroscopy.

5. The method according to embodiment 4, wherein the sample layer is at a CO₂(g) partial pressure of from 40-60 atm when performing the FTIR spectroscopy.

6. The method according to any one of embodiments 1 to 5, further comprising quantitating HCO₃' and CO₃ ^2- ions in the aqueous solution.

7. The method according to embodiment 6, wherein quantitating HCO₃- and CO₃ ^2- ions in the aqueous solution comprises: disposing a sample of the aqueous solution in an optical cell, wherein the sample is disposed as a layer on a plasmonic gold thin film; performing enhanced Raman spectroscopy on the sample layer disposed on the plasmonic gold thin film; and quantitating HCO₃ ^_ and CO₃ ^2- ions in the aqueous solution based on the enhanced Raman spectroscopy.

8. The method according to embodiment 7, wherein the plasmonic gold thin film comprises isolated gold islands have an average greatest dimension of 25 to 300 nm separated by gaps having an average greatest dimension of 5 to 100 nm. 9. The method according to embodiment 8, wherein the isolated gold islands have an average greatest dimension of 50 to 200 nm separated by gaps having an average greatest dimension of 10 to 50 nm.

10. The method according to any one of embodiments 7 to 9, wherein the enhanced Raman spectroscopy comprises irradiating the sample layer disposed on the plasmonic gold thin film with a laser at from 500 to 800 nm.

11 . The method according to embodiment 10, wherein the enhanced Raman spectroscopy comprises irradiating the sample layer disposed on the plasmonic gold thin film with a laser at from 600 to 650 nm, optionally at about 633 nm.

12. The method according to any one of embodiments 1 to 9, further comprising determining the pH of the aqueous solution.

13. The method according to embodiment 12, wherein determining the pH of the aqueous solution comprises performing electrochemical cyclic voltammetry on a sample of the aqueous solution.

14. The method according to embodiment 13, wherein performing electrochemical cyclic voltammetry comprises: disposing a sample of the aqueous solution in an electrolyzer comprising a working electrode, a reference electrode, and a counter electrode; applying a potential across the electrodes; measuring a current associated with the sample; and determining the pH of the aqueous solution based on the current associated with the sample.

15. The method according to embodiment 14, wherein the working electrode is a zinc (Zn) working electrode.

16. The method according to embodiment 14, wherein the working electrode is a nickel (Ni) working electrode.

17. The method according to any one of embodiments 14 to 16, wherein the reference electrode is an Ag/AgCI reference electrode and the counter electrode is a Pt counter electrode.

18. A method of quantitating HCO₃- and CO₃ ^2- ions in an aqueous solution, the method comprising: disposing a sample of the aqueous solution in an optical cell, wherein the sample is disposed as a layer on a plasmonic gold thin film; performing enhanced Raman spectroscopy on the sample layer disposed on the plasmonic gold thin film; and quantitating HCO₃ ^_ and CO₃ ^2- ions in the aqueous solution based on the enhanced Raman spectroscopy.

19. The method according to embodiment 18, wherein the plasmonic gold thin film comprises isolated gold islands have an average greatest dimension of 25 to 300 nm separated by gaps having an average greatest dimension of 5 to 100 nm.

20. The method according to embodiment 19, wherein the isolated gold islands have an average greatest dimension of 50 to 200 nm separated by gaps having an average greatest dimension of 10 to 50 nm.

21 . The method according to any one of embodiments 18 to 20, wherein the enhanced Raman spectroscopy comprises irradiating the sample layer disposed on the plasmonic gold thin film with a laser at from 500 to 800 nm.

22. The method according to embodiment 21 , wherein the enhanced Raman spectroscopy comprises irradiating the sample layer disposed on the plasmonic gold thin film with a laser at from 600 to 650 nm, optionally at about 633 nm.

23. An electrochemical cyclic voltammetry method of determining the pH of an aqueous solution, the method comprising: disposing a sample of the aqueous solution in an electrolyzer comprising a working electrode, a reference electrode, and a counter electrode; applying a potential across the electrodes; measuring a current associated with the sample; and determining the pH of the aqueous solution based on the current associated with the sample.

24. The method according to embodiment 23, wherein the working electrode is a zinc (Zn) working electrode.

25. The method according to embodiment 23, wherein the working electrode is a nickel (Ni) working electrode.

26. The method according to any one of embodiments 23 to 25, wherein the reference electrode is an Ag/AgCI as reference electrode and the counter electrode is a Pt counter electrode.

27. The method according to any one of embodiments 1 to 26, wherein the aqueous solution is from a natural body of water.

28. The method according to embodiment 27, wherein the aqueous solution is from an ocean, a bay, a river, or a lake.

29. The method according to any one of embodiments 1 to 26, wherein the aqueous solution is a beverage. 30. The method according to embodiment 29, wherein the beverage is soda, sparkling water, seltzer, sparkling wine, or champagne.

31 . The method according to any one of embodiments 1 to 26, wherein the aqueous solution is whole blood or a fraction thereof.

The following examples are offered by way of illustration and not by way of limitation.

EXPERIMENTAL

Example 1 - Infrared Spectroscopy of Carbon Dioxide Hydration

Described in this example are infrared spectroscopy-based methods and devices which circumvent the overwhelming infrared absorption of water by devising a thin (e.g., < ~ 100 pm) liquid micro-IR cell for transmission Fourier transform infrared (FTIR) spectroscopy of aqueous solutions, revealing CO₂(aq) vibrational features over the water background and enabling in-situ dynamic probing and quantitating CO₂(aq) in solutions at up to ~ 60 atmospheres CO₂(g) pressures. During CO₂ hydration, the gaseous CO₂(g) v₃ (C=O asymmetric stretching vibration) IR absorption band with P and R branches comprised of sharp spectra lines of molecular rotational transitions evolved to a single absorption peak of CO₂(aq), suggesting quenching of CO₂ molecular rotations for hydrated CO₂(aq). Quantitative CO₂(aq) in aqueous solutions were measured, following Beer’s law with a ~ 1 mM (~ 44 mg/L) detection limit. CO₂(aq) concentration was elucidated in pure water, seawater, KHCO₃, KCI, K₂SO₄, and KOH solutions under typical CO₂ capture and reduction conditions, and in household drinks (coke, sparkling water, champagne, and wine), affording an effective way of CO₂ analysis for both fundamental studies and consumer industries. Dynamic micro-IR probing revealed more rapid CO₂ hydration than dehydration kinetics in aqueous solutions, leading to the supersaturation and hysteresis of dissolved CO₂(aq). Slowdowns of CO₂ hydration and accelerations of dehydration in solute-rich solutions and drinks suggested competition of CO₂ solvation with strong solute - H₂O interactions in the presence of salts/ions, alcohols and other additive molecules in liquid phase, leading to weakened CO₂ - H₂O molecular interactions.

A micro-IR cell was designed with two borosilicate glass (or CaF₂) windows sandwiching a thin (thickness d ~ 10-100 pm, set by thin PTFE spacers) aqueous layer (Fig. 1 a). The microscale path lengths were critical to revealing CO₂ v₃ band (C=O asymmetric stretching at ~ 2350 cm'¹) without diminishing due to IR adsorption by the H₂O bending-libration combination mode in the same ~ 1900 - 2400 cm^-1 region. With 0.5 M NaHCO₃ solution in 51 .5 pm thick micro-IR cell under CO₂(g) bubbling or equilibrated in 1 atm CO₂(g) (see Methods for details), a dip in the spectrum at ~ 2350 cm^-1 was observed (Fig. 1 a, IR resolution of 0.96 cm^-1 with 1.1 mm borosilicate glass cell windows). Upon background subtraction of high resolution (0.06 cm^-1) spectrum a symmetric peak at 2343.1 cm'¹ was obtained (Fig. 1 b), attributed to the main hydrated carbon species of CO₂(aq). Calculation based on Conductor-like Screening Model for Real Solvents (COSMO-RS) revealed a slight Gibbs free energy decrease of 0.149 kcal/mol for CO₂ hydration in H₂O (Fig. 1 c, see Methods for details) suggesting the high reversibility of CO₂(g) and CO₂(aq) interconversion, and close to the experimental value of 0.24 kcal/mol. CO₂(aq) peak exhibited a 4.4 cm^-1 redshift (marked by dashed line in Fig. 1d and 1 e) from fundamental CO₂ v₃ vibrational frequency f₀ = 2347.5 cm^-1, consistent with density functional theory (DFT) result of the decreasing trend in v₃ frequency due to CO₂ interactions with H₂O molecules).

In contrast to CO₂(aq), gaseous CO₂(g) showed two broad asymmetric branches of v₃ band at 2336 and 2362 cm'¹ in FTIR spectrum. The branches exhibited sharp narrowly spaced peaks on the two sides of the pure v₃ fundamental frequency f₀ = 2347.5 cm'¹ (Fig. 1d) at 0.06 cm'¹ resolution, corresponding to the R-branch and P-branch of CO₂(g) rotational-vibrational spectrum with molecular rotational transitions AJ > 0 and AJ < 0 respectively. A much weaker ¹³CO₂(g) P-branch (corresponding to ~ 1.1% natural abundance of ¹³C) were observed in the 2500 - 2800 cm'¹ range with its R-branch merging/overlapping with ¹²CO₂(g) P-branch. Comparison of high resolution CO₂(g) spectrum with the single narrow CO₂(aq) peak observed at 2343.1 cm'¹ without P/R-branch suggested quenching of rotational transitions and inhibition of CO₂(aq) molecular rotations by the surrounding solvating H₂O molecules.

The CO₂ hydration dynamics were monitored in the micro-IR cell for 0.5 M NaHCO₃ solution under 30 seem CO₂(g) bubbling (see Methods for details). When bubbling started, the initial CO₂(aq) peak (~ 5 mM formed via HCO₃ ^_ equilibria, see Methods for details) increased accompanied by rotational P- and R-branches due to coexistence of CO₂(g) (Fig. 1 e). The CO₂(g) P/R-branch decreased in ~ 2 min and disappeared after 20 min of bubbling, leaving a symmetric single peak at 2343.1 cm^-1 corresponding to CO₂(aq) in equilibrium with the NaHCO₃ solution under bubbling.

A calibration curve for CO₂(aq) quantification was derived by correlating the integrated absorbance A (integrated over wavenumber in unit of cm'¹) of CO₂(aq) v₃ peak (Fig. 2a) with equilibrium CO₂(aq) concentration [CO₂(aq)] calculated from CO₂ dissolution equilibria and Henry’s Law in water and in 0.5, 1 .0 M NaHCO₃ solutions equilibrated with air or 1 atm CO₂(g) (see Methods for details). High linearities in integrated CO₂(aq) absorbance A vs. solution thickness d (12.2 - 103 pm, Fig. 2b) and vs. calculated [CO₂(aq)] (Fig. 2c) were observed. The molar extinction coefficient E of CO₂(aq) was derived from Beer’s law A = E - [CO₂(aq)] ■ d o be 1.74 * 10⁴ M’¹-crrr², giving CO₂(aq) concentrations of 34.2 mM and 23.1 mM in 1 atm CO₂(g) equilibrated H₂O and 0.5 M NaHCO₃ solution respectively based on the measured micro-IR absorbances, within 5% of literature results. The integrated absorbance A was found superior to peak absorbance (peak height) in yielding a constant E independent of the instrument FTIR resolution used to acquire spectra. CO₂(aq) detection limit by micro-IR was determined to be ~ 1 mM (~ 44 mg/L, see Methods for details) at 0.96 cm'¹ IR resolution. Applying the calibration curve, CO₂(aq) was first quantified in various solutions under CO₂(g) bubbling, a condition widely employed for CO₂ photo-Zelectro-reduction to fuels. Under 30, 50 and 100 seem CO₂(g) bubbling rate, CO₂(aq) in water reached 39.4, 50.1 , and 59.9 mM respectively in 30 min, - 1.2, 1 .5, and 1 .8 times higher than the concentration in water equilibrated with 1 atm CO₂(g) without bubbling. [CO₂(aq)] vs. time were fit to exponential functions, showing faster CO₂ hydration kinetics under CO₂(g) bubbling than 1 atm equilibrating (Fig. 2d). Similar dynamic monitoring of CO₂(g) bubbling or 1 atm equilibrating in 0.5 M KHCO3 solution and seawater was performed, and a co-existence of CO₂(g) and CO₂(aq) v₃ absorption bands was observed at time points when only CO₂(aq) was observed in pure water. Deconvolutions of CO₂(aq) peaks (see Methods for details) showed an obviously slower initial CO₂(aq) formation processes in these salty solutions (Fig. 2e) than in pure water. Also, the equilibrium CO₂(aq) concentrations in KHCO3, KCI, K₂SO₄, KOH solutions and seawater were all lower than in H₂O under the same bubbling or equilibrating conditions. These results corroborated the salting-out effect and revealed reduced CO₂ hydration in the presence of strong ion - H₂O interactions in solutions.

Due to the importance of CO₂(aq) as a determinant of taste, micro-IR measurements were performed of dissolved CO₂(aq) in commercial drinks, a ubiquitous task routinely done in industry by tracking electrical or thermal signals of released gaseous CO₂(g) instead of direct CO₂(aq) quantifications. With micro-IR, dissolved CO₂(aq) was measured in drinks including sparkling water, Coca-Cola®, grape wine, and brut champagne (see Methods for details). These liquids contained various solutes such as ethanol, sugar, minerals and flavoring agents, without interfering with acquiring clear and quantifiable CO₂(aq) peaks in micro-IR spectra. The lowest CO₂(aq) concentration (~ 0.4 g/L) was observed in grape wine, the highest (~ 10 g/L) was observed in champagne, and similar concentrations of - 6 g/L were observed in sparkling water and Coca-Cola® (Fig. 2f), matching with reported levels in these liquids.

For CO₂ capture, storage and photo-/electro-reduction in aqueous solutions, high CO₂(g) partial pressure conditions are widely investigated, but in-situ probing of CO₂(aq) has been rare. The micro-IR cell was equipped with a stainless-steel frame and 6 mm thick CaF₂ windows for CO₂(aq) monitoring in fully sealed aqueous samples under high CO₂(g) pressures up to 58 atm (Fig. 3a, see Methods for details). For pure H₂O equilibrated under 15 - 58 atm CO₂(g) partial pressures for 30 min, CO₂(aq) concentration was derived at each pressure from the measured integrated CO₂(aq) absorbance (Fig. 3b) and molar extinction coefficient and correlated it with CO₂(g) partial pressure. A reciprocal fitting of CO₂(aq) concentration and CO₂(g) pressure afforded + 0.1550 with R² = 0.9997 (Fig. 3c), corroborating previous

experimental data and the ECO₂N module analysis (an extended Henry’s law) developed for CO₂ aquifers.

Dynamic monitoring of CO₂ hydration in water under high CO₂(g) pressures by micro-IR revealed much faster kinetics than under 1 atm CO₂(g) (Fig. 3d). COSMO-RS simulation resulted in AG = -0.106 kcal/mol of CO₂ hydration under 58 atm, smaller than that of -0.149 kcal/mol under atmosphere, suggesting deviation from the linear Henry’s law in the high pressure range and reduced thermodynamic driving forces for CO₂ hydration as CO₂(aq) concentration increased. Redshifts of the H₂O bending-libration combination IR absorption band were observed under elevating CO₂(g) pressures, signaling increased disruption of the hydrogen bonding network of water when more CO₂(g) dissolved, agreeing with reduced hydration AG at higher pressures.

CO₂ supersaturation and dehydration kinetics in aqueous solutions are of fundamental and practical interests. Micro-IR was employed herein to investigate these phenomena quantitively. Upon exposing a CO₂(g) bubbled H₂O (under 30 seem bubbling for 30 min) to air, continuous probing of CO₂ v₃ absorption band (Fig. 4a, see Methods for details) found that during 3 h standing in air, CO₂(aq) concentration in the supersaturated solution decreased from 39.9 to 17.2 mM (Fig. 4b), with visible tiny gas bubbles released and pH increased from 3.71 to a near equilibrium level of 5.67. By fitting to exponential functions (R² = 0.985), much slower CO₂ dehydration kinetics

= 46.1 min) were observed than hydration (th = 9.8 min, Fig. 2d) in water, suggesting a large hydration-dehydration hysteresis and significant retention of dissolved CO₂(aq) over long times. Under ambient conditions, reducing the CO₂(aq) supersaturated water to half concentration of ~ 20 mM would take 84.7 min, much longer than the 23 min needed to increase from 20 to ~ 40 mM under bubbling. An ultralong time was expected to reach the level of ~ 0.01 mM for water equilibrated in air. Slow dehydration of supersaturated CO₂(aq) was also observed in 0.5 M KHCO3 and seawater equilibrated with 1 atm CO₂(g), and H₂O equilibrated with high pressure CO₂(g). CO₂ supersaturation in oceans and lakes have been observed and the mechanisms are still under active investigation. The observed CO₂(aq) hysteresis and slow supersaturation reversal could play important roles.

To glean the lasting times of the tastes of drinks upon opening seals, CO₂(aq) dehydration kinetics were measured for sparkling water, grape wine, Coca-Cola®, and brut champagne upon exposure to air and derived the dehydration time constant of tdh = 22.5, 15.5, 9.6, and 7.4 min respectively. Generally, accelerated CO₂(aq) dehydration kinetics were observed for all of the tested solutions over 1 atm CO₂(g) equilibrated pure water (Fig. 4c), suggesting reduced favorability of CO₂ hydration in aqueous solutions in the presence of salts/ions, sugar, alcohol, and other solutes (including high concentrations of dissolved CO₂(aq)). That is, the ions and other solutes outcompete the hydration of CO₂(g). Interactions of these solutes with H₂O were reflected by obvious redshifts of H₂O bending-libration combination band from f= 2144 cm'¹ (for pure water in air) (Fig. 4d), suggesting weakened hydrogen bonding network in the presence of solutes that led to disfavored CO₂ - H₂O molecular interactions slowing down CO₂ hydration and accelerating dehydration.

By performing widely accessible transmission FTIR with < ~ 100 microns thick liquid cells, the diminishing of CO₂ v₃ vibrational signatures by the H₂O bending-libration mode was prevented, revealing quenching of molecular rotation of water solvated CO₂ and affording a spectroscopic approach to CO₂(aq) quantitation and hydration/dehydration kinetic monitoring for a wide range of solutions important to CO₂ capture/storage/reduction and consumer products. The micro-IR approach is convenient and applicable to in-situ probing of CO₂ solvation in aqueous solutions and extendable to non-aqueous systems for fundamental understanding and real-world applications.

In summary, CO₂ hydration in aqueous solutions is of wide ranging importance from CO₂ capture, storage and photo-/electro-reduction in the fight against global warming, to CO₂ analysis in various liquids including natural waterbodies and consumer drinking products. Described herein is a micro-scale infrared (IR) spectroscopy technique for dynamical monitoring and quantitating CO₂ hydration in aqueous solutions. The quantized CO₂(g) rotational states transitions were observed to quench when hydrated to CO₂(aq), with dissolved CO₂(aq) equilibrium concentration following Beer’s law and an extended Henry’s law up to high pressures. CO₂ hydration and dehydration kinetics important to energy/environmental fields and the drink industry were probed, revealing large hydration-dehydration hysteresis and ultralong times needed to reverse supersaturated CO₂(aq) in liquid phase, which could have implications to CO₂ levels in lakes and oceans.

METHODS FOR EXAMPLE 1

Transmission FTIR spectroscopy and micro-IR optical cell. Transmission FTIR spectra of all liquid samples were obtained on a Nicolet iS50 FT-IR Spectrometer using the micro- IR cell connecting with a sample reservoir. Borosilicate glass disc windows for the cell were purchased from McMaster-Carr Supply Company. The stainless steel cell frame, CaF₂ disc windows, and PTFE spacers for the cell were purchased from Harrick Scientific Products Inc. The stainless steel frame was screwed tightly with O-rings onto the cell windows to ensure good sealing under high pressures. During FTIR measurements, the optical cell was fixed in the FTIR chamber with continuous N₂ purging at 30 scfh. Background subtraction was applied before sample testing to exclude the IR absorption of instrument, gas phase in the FTIR chamber, and cell windows.

FTIR measurements of CO₂(g). The FTIR chamber was purged by 30 scfh N₂ for > 2 h to remove air from the chamber. Then with continuous N₂ purging, CO₂(g) was flowed into the chamber at 10 seem for 5 min and recorded IR spectra at resolutions of 0.06 and 0.96 cm^-1.

Micro-IR measurements of NaHCO₃ solutions equilibrating with air. 5 mL 0.5 and 1 .0 M NaHCO₃ solutions were prepared and transferred into 15 mL vials with gas-liquid interphase area of ~ 1 .5 cm², and the headspace in the vials was filled with air. After standing still for 1 h, 0.5 mL top surface liquid samples in the vials were transferred by pipette into the micro-IR optical cell with 1 .1 mm thick borosilicate glass windows and 12.2, 24.7, 51 .5, and 103.0 pm thick PTFE spacers. This sampling process was finished in - 15 s to avoid potential changes in the liquid samples. FTIR spectra were recorded at a resolution of 0.96 cm^-1.

Micro-IR measurements of H₂O, seawater, NaHCO₃ and KHCO3 solutions equilibrating with 1 atm CO₂(g). 5 mL deionized water, seawater (collected in November 2020 from San Gregorio State Beach, California, US), 0.5 and 1.0 M NaHCO₃ solutions, and 0.5 M KHCO3 solution were prepared and transferred into 15 mL vials with gas-liquid interphase area of - 1 .5 cm², and the headspace in the vials was filled with 1 atm CO₂(g). After every 10 min, 0.5 mL top surface liquid samples in the vials were transferred by pipette into the micro-IR optical cell with 1 .1 mm thick borosilicate glass windows and 12.2, 24.7, 51 .5, 103.0, and 204.2 pm thick PTFE spacers. This sampling process was finished in - 15 s to avoid potential changes in the liquid samples, and CO₂(g) was refilled into the headspace of the vials after sampling. FTIR spectra were recorded at resolutions of 0.06, 0.24, and 0.96 cm^-1.

Micro-IR measurements of H₂O, NaHCO₃, KHCO3, KCI, K2SO4, and KOH solutions under CO₂(g) or Ar bubbling. 5 mL deionized water, 0.5 M NaHCO₃ solution, 0.1 , 0.5, 1 .0, 2.0 M KHCO3 solutions, 0.1 , 0.5, 1.0, 2.0 M KCI solutions, 0.05, 0.25, 0.5 M K₂SO4 solutions, and 0.1 , 0.5, 1.0, 2.0 M KOH solutions were prepared and transferred into the sample reservoir connected with micro-IR optical cell. With a CO₂(g) or Ar tubing reaching deep inside the reservoir, valve V₂ was kept closed and the solutions were bubbled with 30, 50, and 100 seem CO₂(g) or Ar. After every 10 min, valve V₂ and V₃ were opened for the CO₂(g) bubbled liquid entering and filling the 51 .5 pm thick micro-IR cell equipped with 1 .1 mm thick borosilicate glass windows. The valves were quickly closed after > 300 pL liquid flowing out of V3 (for flushing the channels with fresh samples) and FTIR spectra were then recorded at resolutions of 0.06 and 0.96 cm'¹.

Micro-IR measurements of sparkling water, coke, grape wine, and brut champagne. Since sparkling water (S. Pellegrino Sparkling Natural Mineral Water, 500 mL, PRD 02.12.21 02), Coca-Cola® (Coca-Cola Original Taste, 500 mL, AUG1621 UVD1249), and brut champagne (Kirkland Signature Champagne Brut, 750 mL, MA 3761 -01 -00666) were packed under pressures higher than 1 atm, these drinks were stored in 4 °C for 24 h before transferring 0.5 mL of them into 12.2 pm thick micro-IR cell equipped with 6 mm thick CaF₂ windows, and quickly sealed the cell within - 30 s of opening the cooled drinks to minimize escaping of dissolved CO₂. After - 5 - 10 min of the liquids warming to room temperature, FTIR spectra were recorded at a resolution of 0.96 cm^-1. CO₂ solubility increases by - 2 times in water with temperature decreasing from 25 to 4 °C⁵⁹, however, the measured CO₂(aq) concentration in carbonated drinks especially champagne could still be underestimated due to the very high concentration of CO₂(aq) and some inevitable escaping during sample transfer. For the measurement of grape wine (SparrowHawk 2018 Reserve Chardonnay Napa Valley, 750 mL, 2107 B3 1 20190227 17 41 ), 0.5 mL sample was transferred into the micro-IR cell without cooling and FTIR spectrum was recorded at a resolution of 0.96 cm^-1.

Micro-IR measurements of H₂O equilibrating with high pressure CO₂(g). 2 mL deionized water was transferred into the sample reservoir connected with micro-IR optical cell (with 6 mm CaF₂ windows, cell thickness d= 6.0 pm). The stainless steel cell frame was screwed tightly with O-rings onto CaF₂ windows to ensure good sealing under high pressures. With valve V₂ opened, V₃ closed, and Vi connected to CO₂ cylinder, 15 - 58 atm CO₂(g) was applied to the reservoir with a gas-liquid interphase area ~ 1.5 cm². IR spectra were recorded from 0.5 to 30 min at a resolution of 0.96 cm^-1.

Micro-IR measurements of CO₂ dehydration from H₂O, seawater, and 0.5 M KHCO3 solution after bubbled by CO₂(g) or equilibrated with 1 atm CO₂(g), and from commercial drinks. 5 mL of the liquids (saturated with CO₂(aq) under respective conditions) were stored in 15 mL vials with the headspace filled with air (gas-liquid interphase area of ~ 1 .5 cm²). At various times after standing for 20 to 180 min, 0.5 mL top surface liquid samples in the vials were transferred by pipette into the micro-IR optical cell with 1.1 mm thick borosilicate glass windows and 51 .5 pm thick PTFE spacers for measuring CO₂(aq) concentration. This sampling process was finished in ~ 15 s to avoid potential changes in the liquid samples. FTIR spectra were recorded at a resolution of 0.96 cm^-1.

Micro-IR measurements of CO₂ dehydration from H₂O after equilibrated with 30 and 60 atm CO₂(g). After 2 mL of H₂O was equilibrated with 30 or 60 atm CO₂(g) for 30 min in 6.0 pm thick micro-IR cell equipped with 6 mm thick CaF₂ windows, the gas pressure was released and the sample reservoir was opened to expose the liquids to air. FTIR spectra were recorded at a resolution of 0.96 cm'¹ after 10 and 20 min of standing. The micro-IR cell was then switched to 51 .5 pm thick PTFE spacers and 1 .1 mm thick borosilicate glass windows for CO₂ dehydration monitoring after 30 - 180 min exposure to air. This was done since CO₂(aq) decayed quickly to much lower levels in the initial ~ 30 min, after which switching to a thicker cell/longer optical path gave higher signals and provided more accurate quantification of CO₂(aq) at lower concentrations. FTIR spectra were recorded at a resolution of 0.96 cm'¹.

Micro-IR measurements of water, seawater, and 0.5 M KHCO3 solution for probing H₂O bending-libration combination IR absorption band. 0.5 mL deionized water, seawater, or 0.5 M KHCO3 solution was transferred by pipette into the micro-IR optical cell equipped with 6 mm thick CaF₂ windows and 6.0 or 12.2 pm thick PTFE spacers. FTIR spectra were recorded at a resolution of 0.96 cm'¹.

Data processing of FTIR spectra. Raw FTIR spectra obtained from the instrument with a scale of Transmittance T (%) on Y axis were converted to Absorbance A by: A = - log(T) for further quantitative analysis. To obtain background subtracted CO₂ v₃ (C=O asymmetric stretching) IR absorption bands in ~ 2300 - 2400 cm'¹ range for liquid samples, baselines (H₂O bend + libration combination peaks) in absorbance spectra were fitted by polynomial functions and subtracted using OriginPro (version: 2017b 9.4).

For CO₂ v₃ bands with single CO₂(aq) peaks at 2343.1 cm-1 , the absorbance peaks were fitted by Voigt functions and integrated over wavenumber to obtain peak areas (in unit of cm^-1) for quantification. And for CO₂ v₃ bands with convoluted CO₂(aq) and CO₂(g) signals at 0.96 cm- ¹ IR resolution, each convoluted v₃ band was fitted with one Voigt function and two Asymmetric Double Sigmoidal (Asym2Sig) functions in representing the single peak of CO₂(aq) and two asymmetric branches of CO₂(g), with fixed peak width and position parameters. CO₂(aq) peak width and position parameters were obtained from previous Voigt fitting of pure CO₂(aq) v₃ peak in water at the same IR resolution, and CO₂(g) parameters were from the fitting of CO₂(g) FTIR spectra by Asym2Sig functions. CO₂(aq) peaks obtained from deconvolution were integrated over wavenumber for quantitative kinetics analysis.

Determination of detection limit in micro-IR measurements by standard deviation analysis of parallel experiments. During micro-IR quantification of CO₂(aq), liquid samples were measured parallelly for 3 times, and the three integrated absorbance peak areas were averaged. Standard deviation (SD) of the 3 parallel measurements was calculated by:

where x, refers to the integrated peak area in each parallel experiment, and x refers to the average value. The detection limit <p of CO₂(aq) was then derived by:

where E refers to molar extinction coefficient of CO₂(aq) derived from working curve. Parallel measurements of 0.5 M NaHCO₃ solution equilibrated with air were used for determining CO₂(aq) detection limit at 0.96 cm^-1 IR resolution using a 51 .5 pm thick micro-IR cell.

Calculation of equilibrium concentrations of CO₂(aq), H2CO3, HCO3 , CO3² , H⁺, and OH in H2O equilibrated with air or 1 atm CO₂(g), and in 0.5 M NaHCO₃ solution equilibrated with 1 atm CO₂(g). CO₂ hydration in aqueous systems leads to the existing of multiple carbon species including CO₂(aq), H₂CO₃, HCO₃', and CO₃ ^2- in liquid phase at equilibrium states. The relationship between equilibrium concentrations of these species can be derived from the dissociation equations of carbonic acid: (1 ) (2)

And due to the high reversibility of conversions between H₂CO₃ and CO₂(aq), the first dissociation process of H₂CO₃ can be described with an apparent dissociation constant involving CO₂(aq)¹⁸: (3)

Since aqueous solutions are electrically neutral, there is the charge conservation equation of ion species in H₂O:

[H⁺] = [HCO₃ ] + 2 * [CO₃ ² ] + [OH ], (4a) and in sodium bicarbonate solution:

[Na⁺] + [H⁺] = [HCO₃ ] + 2 * [CO₃ ² ] + [OH ], (4b)

And according to the dissociation equilibrium of water, the relationship of [H⁺] and [OH ] in liquid phase can be described as:

H₂O [H⁺] + [OH ], k_w = [H⁺] * [OH ] = 10'¹⁴. (5)

According to Henry’s law, the concentration of dissolved CO₂(aq) in pure water or dilute solutions (< ~ 2 M) is linearly dependent on the partial pressure of CO₂(g) in the gas phase:

[CO₂(aq)] = k_p * p(CO₂), (6) where p(CO₂) refers to CO₂(g) partial pressure, and k_p refers to Henry’s constant of CO₂ in specific liquids at 25 °C. For pure water, k_p = 0.034 M/atm at room temperature⁴⁷; and for 0.5 M NaHCO₃ solution, k_p = 0.024 M/atm⁴⁶. Note that these constants are applicable under near atmospheric conditions and deviate under very high pressures.

Based on the above, calculated were the equilibrium concentrations of species involved in CO₂ hydration in H₂O and NaHCO₃ solution under air or 1 atm CO₂(g) conditions using the dissociation equilibrium equations (1) - (3) and (5), ionic charge conservation equation (4), and Henry’s law equation (6). MATLAB (version: R2016a) codes were employed for the calculations above.

Calculation of equilibrium concentrations of CO₂(aq), H₂CO₃, HCO₃ , CO3² , H⁺, and OH in 0.5 and 1.0 M NaHCO₃ solutions equilibrated with air. For bicarbonate or carbonate solutions equilibrating with a gas phase that has very low CO₂(g) partial pressures (such as air), species equilibria suggests CO₂(aq) mainly coming from the conversion of bicarbonate or carbonate ions instead of CO₂(g) dissolution. Thus, instead of applying Henry’s law, a carbon conservation equation for those solutions is introduced:

[H₂CO₃] + [CO₂(aq)] + [HCO₃] + [CO₃ ² ] = c(carbon), (7) where c(carbon) refers to the initial concentration of the bicarbonate or carbonate salt added for preparing the solution.

Calculated were the equilibrium concentrations of species involved in CO₂ hydration in NaHCO₃ solutions under air based on dissociation equilibrium equations (1 ) - (3) and (5), ionic charge conservation equation (4), and carbon conservation equation (7). MATLAB codes were employed for the calculations above.

Example 2 - Measurement of CO₃ ^2- and HCC>3~ Anions by Enhanced Raman Spectroscopy

Described in this example is an enhanced Raman spectroscopy approach (sometimes referred to herein as ‘pGOLD Raman’) on nanostructured plasmonic gold substrates to measure CO₃ ^2- and HCO₃ ^_ anions applicable to in-situ dynamic monitoring under a wide pressure range up to 58 atm. Raman spectroscopy has previously been employed to characterize HCO₃ ^_ and CO₃ ^2- in solutions with limited sensitivities especially for the HCO₃ ion. Here, a nanostructured plasmonic gold thin film was deposited on borosilicate glass discs to serve as the substrate of the liquid optical cell paring with a quartz disc window (Fig. 5a), and enhanced Raman measurements were performed by focusing lasers on the liquid layer on the nanostructured pGOLD surface. The pGOLD substrate showed isolated Au islands of 50 - 200 nm in size patched together on the glass surface, with gaps of 10 - 50 nm between the islands (Fig. 5a). Ultraviolet- visible (UV-vis) spectroscopy showed broad plasmon resonances of the pGOLD film in a wavelength range covering all three of the Raman lasers (wavelengths of 532, 633, and 785 nm) with the strongest resonance at ~ 620 nm. The pGOLD Raman spectrum of a HCI adjusted 2 M K₂CO₃ solution (pH = 10.0) recorded using a 532 nm laser showed two prominent peaks at 1015 and 1065 cm^-1, corresponding to stretching of the C-OH bond in HCO₃‘ and C-0 bond in CO₃ ^2- respectively (Fig. 5b). Compared to the Raman measurement on a polycrystalline gold foil under the same condition, pGOLD Raman afforded greater peak area by ~ 8 times for the two ions (Fig. 5b). Switching to longer wavelength lasers afforded Raman enhancement on pGOLD over gold foil by ~ 90 times and ~ 40 times for 633 nm and 785 nm lasers respectively. The Raman enhancement increased by switching 532 nm laser to 785 nm laser and peaked with 633 nm laser followed the same trend of optical absorption/extinction intensity at these wavelengths of the pGOLD substrate, indicating the plasmonic enhanced Raman scattering. The pGOLD Raman detected down to ~ 10 mM for HCO₃ ^_ and ~ 1 mM for CO₃ ^2- based on standard deviation analysis (see Computational Methods for details). Note that the enhancement effect of < 100 times was orders of magnitude lower than that for molecules absorbed on plasmonic substrates. A rationale for this result was that solution phase Raman on the pGOLD substrate detected the entire volume of solution under the laser focus, unlike typical surface enhanced Raman scattering (SERS) detecting densely packed molecules on the hot spots on a plasmonic substrate. In the laser focal volume of the solution, enhancement of Raman scattering decays in power law of

where d is the distance from Raman active species to the plasmonic surface.

To obtain Raman calibration curves for quantifying HCO₃' and CO₃ ²' ion concentrations, we prepared 2 M K₂CO₃ solution and stepwise titrated using diluted HCI. Using the 532 nm laser, pGOLD Raman measurements were recorded during solution pH decreasing from the original 13.3 to the final 8.0. Integrated Raman peak areas of HCO₃ ^_ and CO₃ ^2- were normalized by the corresponding peak areas recorded with an acidified 2 M K₂CO₃ solution (pH = 10.0) as the standard solution (see Experimental Section for details), and correlated with the calculated ion concentration at different pH values (see Computational Methods for details) by linear regression, resulting in two calibration curves with high linearity (Fig. 5c).

CO₂ bubbling into KOH solutions is a chemical absorption process due to CO₂ reacting with OH- to form CO₃ ^2- and HCO₃ ^_ ions. To monitor the kinetics of the absorption, we bubbled 1 atm CO₂ at 30 seem into 0.1 , 0.5, 1 , and 2 M KOH solutions and investigated speciation by micro- IR and pGOLD Raman spectroscopy dynamically. Over CO₂ bubbling CO₃ ²' concentration decreased after an initial increase while HCO₃ ^_ kept increasing, leading to the dominant carbon species evolving from CO₃ ^2- to HCO₃' (Fig. 6d). The pH value of the solutions during continuous CO₂ bubbling was calculated based on pGOLD Raman measurement of [HCO₃ ] I [CO₃ ² ] (in the slight alkaline to neutral pH range of ~ 8 - 11 , both CO₃ ^2- and HCO₃' were accurately quantified by pGOLD Raman) and also by direct pH-meter measurement. The results were in excellent agreement (Fig. 6e), confirming pGOLD Raman as a reliable spectroscopic approach to quantitative ionic speciation in aqueous solutions. The CO₂(aq) concentration quantified by micro- IR stayed low due to dominant chemical reaction between CO₂ and KOH, until the solution pH decreased to < ~ 9 when most KOH was consumed (Fig. 6e). Monitoring the pH revealed that equilibrium was reached in 0.1 , 0.5, 1.0, and 2.0 M KOH after CO₂ bubbling for 20, 60, 80, and 230 min respectively (Fig. 6e). Longer stabilizing times in KOH solutions of higher concentration were due to stronger HCO₃7CO₃ ^2- buffering effects. The KOH solutions afforded equilibrium CO₂ loading of ~ 1 - 1.7 mol CO₂ per mol solute. Despite the slow reaction kinetics requiring long equilibrating times, KOH is a CO₂ absorber of high absorption capacity.

Concentrated K₂CO₃ solutions are commonly introduced as CO₂ absorber for large scale CO₂ capture due to its low cost and ease of regeneration. To explore high pressure CO₂ capture in K₂CO₃ system, we maintained 58 atm CO₂(g) over 2 M K₂CO₃ solution and monitored HCO₃' and CO₃ ^2- concentration hourly by pGOLD Raman spectroscopy. We observed continuous decreasing of CO₃ ²' concentration and increasing of HCO₃' concentration, and the dominant carbon species in the solution changed from CO₃ ²' to HCO₃' in 4 h (Fig. 7a). When CO₂ pressure was released to atmosphere after 6 h of absorption, we observed no signification changes in the concentrations of both ions over more than 1 h exposure to air (Fig. 7a), suggesting high stability of the captured CO₂ in the form of dissolved ionic species at room temperature.

To probe the pH of 2 M K₂CO₃ solution during CO₂ capture under 58 atm in a closed system, we employed a homemade high pressure electrolyzer and performed an electrochemical zinc cyclic voltammetry method (sometimes referred to herein as “Zn-CV”) involving cyclic voltammetry (CV) scans with Zn foil as working electrode (WE), Ag/AgCI as reference electrode (RE), and Pt foil as counter electrode (CE). We observed that the onset potential of interconversions between Zn(0) and Zn(ll) species shifts with solution pH, which when combined with HCO₃ ^_ or CO₃ ^2- concentration determined by pGOLD Raman allowed pH calculation based on the Nernst equation (see Computational Methods for details). In HCI acidified 2 M K₂CO₃ solution, we confirmed that the pGOLD-Raman/Zn-CV derived pH values were in good agreement with direct pH meter measurements, suggesting the feasibility of this technique for pH monitoring in the alkaline range in closed high pressure systems.

CO₂(aq) monitoring by micro-IR and pH by Raman/Zn-CV during 58 atm CO₂ capture in 2 M K₂CO₃ solution showed that considerable CO₂(aq) concentration was observed only when solution pH decreased to < 9 (Fig. 7b), which was expected since CO₂ reacted rapidly with CO₃ ^2- to form HCO₃‘ at higher pH values and afforded no/little CO₂(aq). Upon releasing the CO₂ pressure to atmosphere, the solution pH and anionic carbon species showed no significant change within 1 hour (Fig. 7b), with CO₂(aq) decreasing obviously but still in a supersaturated level (Fig. 7b). High concentrations of dissolved/hydrated CO₂ escaped from liquid to gas phase but with a slow kinetics to fully reverse supersaturation.

We observed formation of white crystallized precipitates in the 2 M K₂CO₃ solution after 6 h of CO₂ capture under 58 atm. The precipitates were collected by centrifugation and redissolved in water, and the anionic carbon species were quantified by pGOLD Raman spectroscopy yielding ~ 1 .8 mol KHCO₃(s) per liter K₂CO₃ solution. These combined with 0.65 M CO₂(aq) and 2.2 M HOGS- in the solution suggested a total CO₂ absorption capacity of 58.3 g per mol of solute in 2 M K₂CO₃ under 58 atm CO₂. We also conducted CO₂ capture in 2 M K₂CO₃ under 30 atm and 5 M K₂CO₃ under 58 atm with carbon species and pH evolution monitored dynamically. CO₂ absorption showed a slower kinetics under a lower CO₂ pressure or in a solution of higher ionic strength.

K₂CO₃ solutions showed higher CO₂ absorption capacity than high loading primary and secondary amine solvents (typically < 35 g CO₂ per mol of solute). Due to the ease of regeneration for CO₂ saturated K₂CO₃ solutions by mild heating, the high pressure K₂CO₃ system is a highly effective and economical approach to large scale CO₂ enrichment and storage.

Enhanced Raman spectroscopy of liquid samples in micro-cell. We obtained the Raman spectra by Horiba Jobin Yvon Raman Spectroscopy equipped with PilotPC 500 Laser Diode Controller and Spectra- Physics Laser Exciter as 532, 633, and 785 nm laser light sources. Mitutoyo 50x M Plan APO NIR Objective with a working distance of 2.0 cm was used to focus the laser onto the substrate. Quartz disc of 1 .0 mm was used (the same as used for the microcell optical window) for 1 atm experiment and 3.0 mm was used for up to 58 atm pressure conditions. The pGOLD deposited borosilicate glass disc of 6.0 mm was used as substrate for both 1 atm and high pressure conditions. For dynamic monitoring of liquid samples, Raman spectra were recorded with 10 - 30 scan cycles over an acquisition time of 30 s. For measurement of stable samples, Raman spectra were recorded with 30 - 60 scan cycles with an acquisition time of 30 s to improve the sensitivity. HCI acidified 2 M K₂CO₃ solution (pH = 10.0) was used as a standard solution for normalization and sample quantification (see Computational Methods for details). pGOLD Raman measurement of acidified 2 M K2CO3 solution. 50 mL 2.0 M K₂CO₃ solution was prepared and stepwise titrated by dilute HCI, with the solution pH monitored by pH meter. At desired pH points, 50 pL aqueous samples were transferred into the liquid optical cell with spacer thickness d = 103.0 pm. 532 nm laser beam was adjusted under microscope mode to focus on the pGOLD glass substrate. Raman spectrum at each pH point was recorded with 30 - 60 scan cycles at a laser exposure time of 30 s. HCI acidified 2 M K₂CO₃ solution (pH = 10.0) was used as a standard solution for normalization. pGOLD Raman measurement of KOH solutions during 1 atm CO₂(g) bubbling. 5 mL 0.1 , 0.5, 1.0, 2.0 M KOH solutions were prepared and transferred into the sample reservoir connected with the liquid optical cell (Fig. S1 ). With the cover of the reservoir kept open and valve V₂ close, a tube with 30 seem CO₂(g) flow was reached into the liquids for CO₂ bubbling. After every 20 min, we opened valve V₂ and V₃ to sample the optical cell with a spacer of d = 103.0 pm. 532 nm laser beam was adjusted under microscope mode to focus on the pGOLD glass substrate. Raman spectrum for each sample was recorded with 10 - 30 scan cycles over 30 s. To switch to new liquid samples, we kept valve V₂ and V₃ open for ~ 30 s to have the cell and channels flushed by the new liquid sample in reservoir, then closed them and recorded a new spectrum. HCI acidified 2 M K₂CO₃ solution (pH = 10.0) was used as a standard solution for normalization.

High pressure pGOLD Raman measurement of K2CO3 during and after equilibrating with high pressure CO₂(g). 5 mL 2.0 and 5.0 M K₂CO₃ solutions were prepared and transferred into the sample reservoir connected with the liquid optical cell (Fig. S1 ). With the cover of the reservoir closed, valve Vi connected to CO₂ cylinder and V₂ closed, 30 - 58 atm CO₂ was applied to the head space in the reservoir. After equilibrating for every 1 h, we opened V₂ to sample the optical cell with a spacer of d = 103.0 pm. 532 nm laser beam was adjusted under microscope mode to focus on the pGOLD glass substrate. Raman spectrum for each sample was recorded was recorded over 10 - 30 scan cycles in 30 s. To switch to new liquid samples, we closed V₂ and opened V₃ to have the cell and channels flushed by the new liquid sample in reservoir, then closed V₃ and opened V₂ to inject new liquid samples and recorded new spectra. When CO₂ pressure was released, we opened the cover of sample reservoir to make the liquids inside directly contacting with air. After standing for 1 h, we sampled the cell and recoded the Raman spectrum. HCI acidified 2 M K₂CO₃ solution (pH = 10.0) was used as a standard solution for normalization.

Calculation of HCOs and CO3² concentrations in the acidified K2CO3 solution at different pH values. During the step-wise titration by diluted HCI, CO₃ ^2- in K₂CO₃ solution is continuously converting to HCO₃‘, with a constant concentration of total inorganic carbon ions. According to the dissociation of HCO₃‘:

And according to the carbon ions conservation: c(carbon) = [CO₃ ² ] + [HCO₃], where c(carbon) refers to the initial concentration of total inorganic carbon ions in the K₂CO₃ solution before acidification, i.e., the initial concentration of CO₃ ^2-. Thus, with a known pH value, the concentration of HCO₃ ^_ and CO₃ ^2- can be derived by the above two equations. The calculated ion species distribution change in 2 M K₂CO₃ solution during titration is shown in Fig. S14b.

Normalization of pGOLD Raman peak areas for CO3² and HCO₃-. To avoid the influence of laser power fluctuation on the pGOLD Raman detection accuracy, the tested CO₃ ^2- and HCO₃- peak areas in liquid samples were divided by the corresponding peak area for each ion in diluted HCI acidified 2 M K₂CO₃ solution (pH = 10.0). This acidified K₂CO₃ solution (pH = 10.0) was regarded as a standard solution for Raman normalization, and was tested every time before unknown sample quantifications to calibrate the laser power.

Determination of detection limit in micro-IR and pGOLD Raman measurements by parallel experiments standard deviation analysis. During micro-IR and pGOLD Raman speciation, liquid samples were tested in parallel experiments for 3 times, and average normalized peak areas were obtained for further quantifications. The standard deviation (SD) of 3 parallel measurements can be calculated by:

where x, refers to the normalized peak area in each parallel experiment, and x refers to the average value. The concentration detection limit of a specific species was then derived by 2 * SD divided by the slope of the corresponding calibration curve. Parallel testing results of CO₂(aq) IR peak areas in 0.3 and 0.5 M NaHCO₃ solutions equilibrating with air were used for micro-IR detection limit calculation. Parallel testing results of HCO₃ ^_ and CO₃ ^2- Raman peak areas in acidified 2 M K₂CO₃ solutions (pH = 8.5 and 9.0) were used for pGOLD Raman detection limit calculation. Example 3 - Measurement of Solution pH Under High Pressure Conditions by

Electrochemical Zinc Cyclic Voltammetry

Derivation of solution pH based on Nernst equation in the neutral to alkaline region. In aqueous solutions containing inorganic carbon species, Zn(ll) exists as Zn²⁺, ZnCO₃(s), and ZnO^2- in different pH ranges⁶⁷. At a slightly acidic condition, Zn(0) can be oxidized to Zn²⁺ with proper potential applied:

Zn²⁺ + 2 e = Zn, Ei° = -0.7618 V vs. SHE.

The conversion between Zn(0) and Zn²⁺ does not influenced by the solution pH value since no H⁺ or OH- ion is involved in the reaction. However, since we are using a KCI saturated Ag/AgCI reference electrode,

where

is the tested onset potential of the interconversion reactions between Zn²⁺ and Zn(0), derived from the CV curves using Zn foil as the working electrode (Fig. S25). So we have

In a neutral or slightly alkaline solution, the dominated Zn(ll) species becomes ZnCO₃(s): ZnCO₃ + 2 e' = Zn + CO₃ ²',

E₂° = -1.049 V vs. SHE.

The potential under a non-standard condition can be expressed by Nernst equation:

where R is the universal gas constant, 7 is the temperature in kelvins, z is the number of electrons transferred in the reaction, and Fis the Faraday constant. According to the dissolution equilibrium of bicarbonate: k₂ = ([H⁺] * [CO₃ ² ]) / [HCO₃] = 4.84 * 10 ” M, we substitute [CO₃ ² ] in Nernst equation:

Since we are using a KCI saturated Ag/AgCI reference electrode,

, where is the onset potential of ZnCO₃(s) and Zn(0) interconversion derived

from the CV curves. In summary, we have: 0.197

for slightly alkaline conditions, where [CO₃ ² ] can be quantified by pGOLD Raman spectroscopy, or:

0.197

for near neutral conditions, where [HCO₃ ] can be quantified by pGOLD Raman spectroscopy.

At a highly alkaline condition, Zn(ll) exists as and the conversion between Zn(0)

and Zn(ll) becomes:

And the potential under a non-standard condition is:

Based on the water dissociation k_w = [H⁺] * [OH ] = 10^-14 we can substitute [OH ]:

And since we are using a KCI saturated Ag/AgCI reference electrode,

where

is the onset potential of ZnO2²' and Zn(0) interconversion derived from the CV curves. Thus, we have:

where s unknown, but derivable if we perform CV scans in a solution of a known pH

value (such as 0.1 M KOH solution, pH = 13.0) with the same electrochemical parameters as Zn- CV experiments for sample testing.

The relationship between solution pH and tested onset potential <t> is divided into 3 regions based on the dominant Zn(ll) species involved in the redox reactions, and the boundaries of these regions are influenced by the concentration of ionic carbon species and Zn-CV scan parameters. Assuming

the boundary pH values of these regions at specific conditions become derivable.

Accordingly, the preceding merely illustrates the principles of the present disclosure. It will be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and conditional language recited herein are principally intended to aid the reader in understanding the principles of the invention and the concepts contributed by the inventors to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents and equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. The scope of the present invention, therefore, is not intended to be limited to the exemplary embodiments shown and described herein.

Claims

WHAT IS CLAIMED IS:

1 . A method of quantitating CO₂ hydration in an aqueous solution, the method comprising: disposing a sample of the aqueous solution in an optical cell, wherein the sample is disposed as a layer having a thickness of from 5 to 200 pm; performing transmission Fourier transform infrared (FTIR) spectroscopy on the sample layer to determine CO₂(aq) vibrational features in the sample; and quantitating CO₂ hydration in the aqueous solution based on the CO₂(aq) vibrational features.

2. The method according to claim 1 , wherein the thickness of the sample layer is from 5 to 100 pm.

3. The method according to claim 1 , wherein the sample layer is at a CO₂(g) partial pressure of from 10-60 atm when performing the FTIR spectroscopy.

4. The method according to claim 1 , further comprising quantitating HCO₃‘ and CO₃ ^2- ions in the aqueous solution.

5. The method according to claim 4, wherein quantitating HCO₃‘ and CO₃ ^2- ions in the aqueous solution comprises: disposing a sample of the aqueous solution in an optical cell, wherein the sample is disposed as a layer on a plasmonic gold thin film; performing enhanced Raman spectroscopy on the sample layer disposed on the plasmonic gold thin film; and quantitating HCO₃‘ and CO₃ ^2- ions in the aqueous solution based on the enhanced Raman spectroscopy.

6. The method according to claim 5, wherein the plasmonic gold thin film comprises isolated gold islands have an average greatest dimension of 25 to 300 nm separated by gaps having an average greatest dimension of 5 to 100 nm.

7. The method according to claim 5, wherein the enhanced Raman spectroscopy comprises irradiating the sample layer disposed on the plasmonic gold thin film with a laser at from 500 to 800 nm.

8. The method according to claim 1 , further comprising determining the pH of the aqueous solution.

9. The method according to claim 8, wherein determining the pH of the aqueous solution comprises performing electrochemical cyclic voltammetry on a sample of the aqueous solution.

10. The method according to claim 9, wherein performing electrochemical cyclic voltammetry comprises: disposing a sample of the aqueous solution in an electrolyzer comprising a working electrode, a reference electrode, and a counter electrode; applying a potential across the electrodes; measuring a current associated with the sample; and determining the pH of the aqueous solution based on the current associated with the sample.

11 . The method according to claim 10, wherein: the working electrode is a zinc (Zn) working electrode or a nickel (Ni) working electrode; and/or

12. A method of quantitating HCO₃‘ and CO₃ ^2- ions in an aqueous solution, the method comprising: disposing a sample of the aqueous solution in an optical cell, wherein the sample is disposed as a layer on a plasmonic gold thin film; performing enhanced Raman spectroscopy on the sample layer disposed on the plasmonic gold thin film; and quantitating HCO₃‘ and CO₃ ^2- ions in the aqueous solution based on the enhanced Raman spectroscopy.

13. The method according to claim 12, wherein the plasmonic gold thin film comprises isolated gold islands have an average greatest dimension of 25 to 300 nm separated by gaps having an average greatest dimension of 5 to 100 nm.

14. The method according to claim 12, wherein the enhanced Raman spectroscopy comprises irradiating the sample layer disposed on the plasmonic gold thin film with a laser at from 500 to 800 nm.

15. An electrochemical cyclic voltammetry method of determining the pH of an aqueous solution, the method comprising: disposing a sample of the aqueous solution in an electrolyzer comprising a working electrode, a reference electrode, and a counter electrode; applying a potential across the electrodes; measuring a current associated with the sample; and determining the pH of the aqueous solution based on the current associated with the sample.

16. The method according to claims 1 to 15, wherein the aqueous solution is from a natural body of water.

17. The method according to claim 16, wherein the aqueous solution is from an ocean, a bay, a river, or a lake.

18. The method according to any one of claims 1 to 15, wherein the aqueous solution is a beverage.

19. The method according to claim 18, wherein the beverage is soda, sparkling water, seltzer, sparkling wine, or champagne.

20. The method according to any one of claims 1 to 15, wherein the aqueous solution is whole blood or a fraction thereof.