Nucleotide Adsorption on Iron(III) Oxide Nanoparticle Surfaces: Insights into Nano-Geo-Bio Interactions Through Vibrational Spectroscopy

Small molecules dominate organic phosphorus in NaOH-EDTA extracts of soils as determined by 31P NMR

Understanding the composition of organic phosphorus (P) in soils is relevant to various disciplines, from agricultural sciences to ecology. Despite past efforts, the precise nature of soil organic P remains an enigma, especially that of the orthophosphate monoesters, which dominate 31P NMR spectra of NaOH-EDTA extracts of soils worldwide. The monoester region often exhibits an unidentified, broad background believed to represent high molecular weight (MW) P. We investigated this monoester background using 1D 31P NMR and 2D 1H31P NMR, as well as 31P transverse relaxation (T2) measurements to calculate its intrinsic linewidth and relate it to MW. Analyzing seven soils from different ecosystems, we observed linewidths of 0.5 to 3 Hz for resolved monoester signals and the background, indicating that it consists of many, possibly >100, sharp signals associated with small (<1.5 kDa) organic P molecules. This result was further supported by 2D 1H31P NMR spectra revealing signals not resolved in the 1D spectra. Our findings align with 31P NMR studies detecting background signals in soil-free samples and modern evidence that alkali-soluble soil organic matter consists of self-assemblies of small organic compounds mimicking large molecules.

NaCl, MgCl2, and AlCl3 Surface Coverages on Fused Silica and Adsorption Free Energies at pH 4 from Nonlinear Optics

We employ amplitude- and phase-resolved second harmonic generation experiments to probe interactions of fused silica:aqueous interfaces with Al3+, Mg2+, and Na+ cations at pH 4 and as a function of metal cation concentration. We quantify the second-order nonlinear susceptibility and the total interfacial potential in the presence and absence of a 10 mM screening electrolyte to understand the influence of charge screening on cation adsorption. Strong cation:surface interactions are observed in the absence of the screening electrolyte. The total potential is then employed to estimate the total number of absorbed cations cm-2. The contributions to the total potential from the bound and mobile charges were separated using Gouy-Chapman-Stern model estimates. All three cations bind fully reversibly, indicating physisorption as the mode of interaction. Of the isotherm models tested, the Kd adsorption model fits the data with binding constants of 3-30 and ∼300 mol-1 for the low (<0.1 mM) and high (0.1-3 mM) concentration regimes, corresponding to adsorption free energies of -13 to -18 and -24 kJ mol-1 at room temperature, respectively. The maximum surface coverages are around 1013 cations cm-2, matching the number of deprotonated silanol groups on silica at pH 4. Clear signs of decoupled Stern and diffuse layer nonlinear optical responses are observed and found to be cation-specific.

Formic and acetic acid pKa values increase under nanoconfinement

Organic acids are prevalent in the environment and their acidity and the corresponding dissociation constants can change under varying environmental conditions. The impact of nanoconfinement (when acids are confined within nanometer-scale domains) on physicochemical properties of chemical species is poorly understood and is an emerging field of study. By combining infrared and Raman spectroscopies with molecular dynamics (MD) simulations, we quantified the effect of nanoconfinement in silica nanopores on one of the fundamental chemical reactions-the dissociation of organic acids. The pKa of formic and acetic acids confined within cylindrical silica nanopores with 4 nm diameters were measured. MD models were constructed to calculate the shifts in the pKa values of acetic acid nanoconfined within 1, 2, 3, and 4 nm silica slit pores. Both experiments and MD models indicate a decrease in the apparent acid dissociation constants (i.e., increase in the pKa values) when organic acids are nanoconfined. Therefore, nanoconfinement stabilizes the protonated species. We attribute this observation to (1) a decrease in the average dielectric response of nanoconfined aqueous solutions where charge screening may be decreased; or (2) an increase in proton concentration inside nanopores, which would shift the equilibrium towards the protonated form. Overall, the results of this study provide the first quantification of the pKa values for nanoconfined formic and acetic acids and pave the way for a unifying theory predicting the impact of nanoconfinement on acid-base chemistry.

Distinguishing different surface interactions for nucleotides adsorbed onto hematite and goethite particle surfaces through ATR-FTIR spectroscopy and DFT calculations

Geochemical interfaces can impact the fate and transport of aqueous species in the environment including biomolecules. In this study, we investigate the surface chemistry of adsorbed nucleotides on two different minerals, hematite and goethite, using infrared spectroscopy and density functional theory (DFT) calculations. Attenuated total reflectance-Fourier transform infrared spectroscopy is used to probe the adsorption of deoxyadenosine monophosphate (dAMP), deoxyguanosine monophosphate (dGMP), deoxycytidine monophosphate (dCMP), and deoxythymidine monophosphate (dTMP) onto either hematite or goethite particle surfaces. The results show preferential adsorption of the phosphate group to either surface. Remarkably, surface adsorption of the four nucleotides onto either hematite or goethite have nearly identical experimental spectra in the phosphate region (900 to 1200 cm^-1) for each mineral surface yet are distinctly different between the two minerals, suggesting differences in binding of these nucleotides to the two mineral surfaces. The experimental absorption frequencies in the phosphate region were compared to DFT calculations for nucleotides adsorbed through the phosphate group to binuclear clusters in either a monodentate or bidentate bridging coordination. Although the quality of the fits suggests that both binding modes may be present, the relative amounts differ on the two surfaces with preferential bonding suggested to be monodentate coordination on hematite and bidentate bridging on goethite. Possible reasons for these differences are discussed.

Adsorption of glycine at the anatase TiO2/water interface: Effects of Ca2+ ions

Adsorption reactions of amino acids (AAs) on TiO₂ nanoparticles (NPs) play an important role in the available nutrients in soils and sediments. The pH effects on glycine adsorption have been studied, but little is known about its coadsorption with Ca²⁺ at the molecular level. Combined attenuated total reflectance Fourier transform infrared (ATR-FTIR) flow-cell measurements and density functional theory (DFT) calculations were used to determine the surface complex and corresponding dynamic adsorption/desorption processes. The structures of glycine adsorbed onto TiO₂ were closely associated with its dissolved species in the solution phase. The presence of Ca²⁺ exerted different influences on glycine adsorption within pH 4-11, thus affecting its migration rate in soils and sediments. The mononuclear bidentate complex at pH 4-7, involving the COO^- moiety of zwitterionic glycine, remained unchanged in the absence and presence of Ca²⁺. At pH 11, the mononuclear bidentate complex with deprotonated NH₂ can be removed from the TiO₂ surface upon coadsorption with Ca²⁺. The bonding strength of glycine on TiO₂ was much weaker than that of the Ca-bridged ternary surface complexation. Glycine adsorption was inhibited at pH 4 but was enhanced at pH 7 and 11.

Oxide- and Silicate-Water Interfaces and Their Roles in Technology and the Environment

Interfacial reactions drive all elemental cycling on Earth and play pivotal roles in human activities such as agriculture, water purification, energy production and storage, environmental contaminant remediation, and nuclear waste repository management. The onset of the 21st century marked the beginning of a more detailed understanding of mineral aqueous interfaces enabled by advances in techniques that use tunable high-flux focused ultrafast laser and X-ray sources to provide near-atomic measurement resolution, as well as by nanofabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nanometer-scale measurements has uncovered scale-dependent phenomena whose reaction thermodynamics, kinetics, and pathways deviate from previous observations made on larger systems. A second key advance is new experimental evidence for what scientists hypothesized but could not test previously, namely, interfacial chemical reactions are frequently driven by "anomalies" or "non-idealities" such as defects, nanoconfinement, and other nontypical chemical structures. Third, progress in computational chemistry has yielded new insights that allow a move beyond simple schematics, leading to a molecular model of these complex interfaces. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying solid surface and the immediately adjacent water and aqueous ions, enabling a better definition of what constitutes the oxide- and silicate-water interfaces. This critical review discusses how science progresses from understanding ideal solid-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years and identifying challenges and future opportunities for the community to address. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines will continue to be critical to achieving this great aspiration.

Continuing progress in the field of two-dimensional correlation spectroscopy (2D-COS): Part III. Versatile applications

In this review, the comprehensive summary of two-dimensional correlation spectroscopy (2D-COS) for the last two years is covered. The remarkable applications of 2D-COS in diverse fields using many types of probes and perturbations for the last two years are highlighted. IR spectroscopy is still the most popular probe in 2D-COS during the last two years. Applications in fluorescence and Raman spectroscopy are also very popularly used. In the external perturbations applied in 2D-COS, variations in concentration, pH, and relative compositions are dramatically increased during the last two years. Temperature is still the most used effect, but it is slightly decreased compared to two years ago. 2D-COS has been applied to diverse systems, such as environments, natural products, polymers, food, proteins and peptides, solutions, mixtures, nano materials, pharmaceuticals, and others. Especially, biological and environmental applications have significantly emerged. This survey review paper shows that 2D-COS is an actively evolving and expanding field.

Differential Surface Interactions and Surface Templating of Nucleotides (dGMP, dCMP, dAMP, and dTMP) on Oxide Particle Surfaces

The fate of biomolecules in the environment depends in part on understanding the surface chemistry occurring at the biological-geochemical (bio-geo) interface. Little is known about how environmental DNA (eDNA) or smaller components, like nucleotides and oligonucleotides, persist in aquatic environments and the role of surface interactions. This study aims to probe surface interactions and adsorption behavior of nucleotides on oxide surfaces. We have investigated the interactions of individual nucleotides (dGMP, dCMP, dAMP, and dTMP) on TiO₂ particle surfaces as a function of pH and in the presence of complementary and noncomplementary base pairs. Using attenuated total reflectance-Fourier transform infrared spectroscopy, there is an increased number of adsorbed nucleotides at lower pH with a preferential interaction of the phosphate group with the oxide surface. Additionally, differential adsorption behavior is seen where purine nucleotides are preferentially adsorbed, with higher surface saturation coverage, over their pyrimidine derivatives. These differences may be a result of intermolecular interactions between coadsorbed nucleotides. When the TiO₂ surface was exposed to two-component solutions of nucleotides, there was preferential adsorption of dGMP compared to dCMP and dTMP, and dAMP compared to dTMP and dCMP. Complementary nucleotide base pairs showed hydrogen-bond interactions between a strongly adsorbed purine nucleotide layer and a weaker interacting hydrogen-bonded pyrimidine second layer. Noncomplementary base pairs did not form a second layer. These results highlight several important findings: (i) there is differential adsorption of nucleotides; (ii) complementary coadsorbed nucleotides show base pairing with a second layer, and the stability depends on the strength of the hydrogen bonding interactions and; (iii) the first layer coverage strongly depends on pH. Overall, the importance of surface interactions in the adsorption of nucleotides and the templating of specific interactions between nucleotides are discussed.

In-situ quantitative monitoring the organic contaminants uptake onto suspended microplastics in aquatic environments

Microplastics act as a source of organic contaminants in aquatic environments and thus affect their environmental fate and toxicity. Because of the weak and reversible interactions between microplastics and organic species, the organic coronas vary with their surrounding environments. Thus, in order to evaluate the possible environmental risks of microplastics, methods for evaluating the dynamic uptake of organic contaminants onto suspended microplastics in aquatic environments are greatly desired. In this work, a UV-vis spectroscopy-based approach was developed for in-situ monitoring organic contaminants uptake onto suspended microplastics after correcting the light scattering interference from microplastics suspensions and establishing the nonlinear relationship between concentration and light absorbance of organic species. The inverse adding-doubling method based on radiative transfer theory was adopted to correct the light scattering effect of suspensions. Then, the resulting mixed absorption spectra were decomposed to calculate the concentrations of the aqueous and adsorbed organic species simultaneously with a nonlinear calibration method. The uptake processes of bisphenol A and p-nitrophenol onto nylon 66 microparticles were monitored with this approach and confirmed by high-performance liquid chromatography analysis. The approach was validated by applying it to natural water samples, and the equilibrium adsorption capacity was found to be interfered mainly by the protein-like substances. This approach has high accuracy, good reproducibility, remarkable universality, and ease of handling, and also provides a potential tool for characterizing the corona formation process on suspended particles both in natural and artificial environments, such as eco-corona formation and engineering surface modification on nano/micro-particles.

Photoacoustic Enhancement of Ferricyanide-Treated Silver Chalcogenide-Coated Gold Nanorods

Plasmonic gold nanorods (AuNRs) are often employed as photoacoustic (PA) contrast agents due to their ease of synthesis, functionalization, and biocompatibility. These materials can produce activatable signals in response to a change in optical absorbance intensity or absorbance wavelength. Here, we report a surprising finding: Ag₂S/Se-coated AuNRs have a ~40-fold PA enhancement upon addition of an oxidant but with no change in absorption spectra. We then study the mechanism underlying this enhancement. Electron micrographs and absorption spectra show good colloidal stability and retention of the core-shell structure after potassium hexacyanoferrate(III) (HCF) addition, ruling out aggregation and morphology-induced PA enhancement. X-ray diffraction data showed no changes, ruling out crystallographic phase changes upon HCF addition, thus leading to induced PA enhancement. Attenuated total reflectance-Fourier transform infrared spectroscopy and zeta potential analysis suggest that PA enhancement is driven by the irreversible displacement of hexadecyltrimethylammonium bromide with HCF. This is further confirmed using elemental mapping with energy-dispersive X-ray analysis. PA characterization after HCF addition showed a four-fold increase in the Grüneisen parameter (Γ), thus resulting in PA enhancement. The PA enhancement is not seen in uncoated AuNRs or spherical particles. Two possible mechanisms for PA enhancement are proposed: first, the photo-induced redox heating at the Ag₂S/Se shell-HCF interface, resulting in an increase in temperature-dependent Γ, and second, an enhanced electrostriction response due to HCF adsorption on a layered plasmonic nanoparticle surface, resulting in a high thermal expansion coefficient (β) that is directly proportional to Γ.

Adsorption of serine at the anatase TiO2/water interface: A combined ATR-FTIR and DFT study

Knowledge of the adsorption reactions between serine and minerals is critical to understanding the geochemical processes of amino acids (i.e., mobility, bioavailability, and degradation) in the environment. Attenuated total reflectance Fourier-transform infrared (ATR-FTIR) flow-cell measurements were used to distinguish the inner- and outer-sphere complexation and reveal the dynamic adsorption and desorption processes of each surface complex at the molecular level. Density functional theory (DFT) calculations were applied to determine the structures of the surface complexes and to justify the peak assignments of the serine dissolved in solution and adsorbed on TiO₂. The structures of interfacial serine were governed by pH conditions but were not affected by the changes in adsorption time and serine concentration. The ATR-FTIR spectra and the results of DFT calculations resolved two different bidentate inner-sphere coordination, involving the COO^- group of the serine zwitterion at pH 4-8 and the serine anion at pH 10. The dynamic adsorption processes of these two surface complexes conformed to the pseudo-second-order kinetic model. The stable inner-sphere complexation could not be entirely removed from the TiO₂ surface upon serine desorption. In addition to reducing the migration rate in the environment, the bidentate inner-sphere coordination contributes to the potential degradation of the serine NH₃⁺ and NH₂ groups. Our research provides new insights into serine adsorption and desorption, facilitating further understanding of the fate and transport of amino acids in the environment.

Competitive Carboxylate-Silicate Binding at Iron Oxyhydroxide Surfaces

Dissolved silicate ions in wet and dry soils can determine the fate of organic contaminants via competitive binding. While fundamental surface science studies have advanced knowledge of binding in competitive systems, little is still known about the ranges of solution conditions, the time dependence, and the molecular processes controlling competitive silicate-organic binding on minerals. Here we address these issues by describing the competitive adsorption of dissolved silicate and of phthalic acid (PA), a model carboxylate-bearing organic contaminant, onto goethite, a representative natural iron oxyhydroxide nanomineral. Using surface complexation thermodynamic modeling of batch adsorption data and chemometric analyses of vibrational spectra, we find that silicate concentrations representative of natural waters (50-1000 μM) can displace PA bound at goethite surfaces. Below pH ∼8, where PA binds, every bound Si atom removes ∼0.3 PA molecule by competing with reactive singly coordinated hydroxo groups (-OH) on goethite. Long-term (30 days) reaction time and a high silicate concentration (1000 μM) favored silicate polymer formation, and increased silicate while decreasing PA loadings. The multisite complexation model predicted PA and silicate binding in terms of the competition for -OH groups without involving PA/silicate interactions, and in terms of a lowering of outer-Helmholtz potentials of the goethite surface by these anions. The model predicted that silicate binding lowered loadings of PA species, and whose two carboxylate groups are hydrogen- (HB) and metal-bonded (MB) with goethite. Vibrational spectra of dried samples revealed that the loss of water favored greater proportions of MB over HB species, and these coexisted with predominantly monomeric silicate species. These findings underscored the need to develop models for a wider range of organic contaminants in soils exposed to silicate species and undergoing wet-dry cycles.

Environmental Aspects of Oxide Nanoparticles: Probing Oxide Nanoparticle Surface Processes Under Different Environmental Conditions

Surface chemistry affects the physiochemical properties of nanoparticles in a variety of ways. Therefore, there is great interest in understanding how nanoparticle surfaces evolve under different environmental conditions of pH and temperature. Here, we discuss the use of vibrational spectroscopy as a tool that allows for in situ observations of oxide nanoparticle surfaces and their evolution due to different surface processes. We highlight oxide nanoparticle surface chemistry, either engineered anthropogenic or naturally occurring geochemical nanoparticles, in complex media, with a focus on the impact of (a) pH on adsorption, intermolecular interactions, and conformational changes; (b) surface coatings and coadsorbates on protein adsorption kinetics and protein conformation; (c) surface adsorption on the temperature dependence of protein structure phase changes; and (d) the use of two-dimensional correlation spectroscopy to analyze spectroscopic results for complex systems. An outlook of the field and remaining challenges is also presented.