Use of deep eutectic solvents in environmentally-friendly dye-sensitized solar cells and their physicochemical properties: a brief review

A novel way to mitigate the greenhouse effect is to use dye-sensitized solar cells (DSSCs) to convert carbon dioxide from the air into useful products, such as hydrocarbons, which can also store energy from the sun, a plentiful, clean, and safe resource. The conversion of CO2 can help reduce the impacts of greenhouse gas emissions that contribute to global warming. However, there is a major obstacle in using DSSCs, since many solar devices operate with organic electrolytes, producing pollutants including toxic substances. Therefore, a key research area is to find new eco-friendly electrolytes that can effectively dissolve carbon dioxide. One option is to use deep eutectic solvents (DESs), which are potential substitutes for ionic liquids (ILs) and have similar advantages, such as being customizable, economical, and environmentally friendly. DESs are composed of low-cost materials and have very low toxicity and high biodegradability, making them suitable for use as electrolytes in DSSCs, within the framework of green chemistry. The purpose of this brief review is to explore the existing knowledge about how CO2 dissolves in DESs and how these solvents can be used as electrolytes in solar devices, especially in DSSCs. The physical and chemical properties of the DESs are described, and areas are suggested where further research should be focused.

Facilitation of Hydrate Dissociation and Structural Evolution by Major Marine Anions under Static Electric Fields

Electric fields have been proven to be capable of significantly affecting the equilibrium state of hydrates. In this study, the thermodynamic properties and structural changes of methane hydrate (MH) in various anion solutions in an electric field at 0.7 V/nm were investigated by molecular dynamics simulations. The presence of anions significantly enhances the instability of methane hydrates under electric fields, leading to a staged dissociation process. First, the anions coexist with MH to form a temporary metastable structure under the action of an electric field. Then, the migration of anions causes the dissociation of nearby hydrates and the formation of flow channels in the hydrate layer, which leads to the complete dissociation of MH after a period. The promotive effects of F-, Br-, I-, and Cl- ions were close, while SO42- was relatively weak. The anions are still in hydration shells in the MH phase, but the structure of the hydration shells differs slightly from that in solution (the coordination numbers of I- and SO42- ions increased). The migration resistances of multiple anions to cross the surface of the hydrate layer are similar. However, inside the hydrate phase, the anions with a larger radius have a higher migration resistance. It is difficult for SO42- ions to migrate inside the hydrate phase, and they tend to form a metastable structure on the hydrate surface. Combining our previous studies, SrCl2 solution has the best hydrate promotion under an electric field environment.

Theoretical Investigation of the X-ray Stark Effect in Small Molecules

We have studied the Stark effect in the soft x-ray region for various small molecules by calculating the field-dependent x-ray absorption spectra. This effect is explained in terms of the response of molecular orbitals (core and valence), the molecular dipole moment, and the molecular geometry to the applied electric field. A number of consistent trends are observed linking the computed shifts in absorption energies and intensities with specific features of the molecular electronic structure. We find that both the virtual molecular orbitals (valence and/or Rydberg) as well as the core orbitals contribute to observed trends in a complementary fashion. This initial study highlights the potential impact of x-ray Stark spectroscopy as a tool to study electronic structure and environmental perturbations at a submolecular scale.

A Computational Quantum-Based Perspective on the Molecular Origins of Life’s Building Blocks

The search for the chemical origins of life represents a long-standing and continuously debated enigma. Despite its exceptional complexity, in the last decades the field has experienced a revival, also owing to the exponential growth of the computing power allowing for efficiently simulating the behavior of matter—including its quantum nature—under disparate conditions found, e.g., on the primordial Earth and on Earth-like planetary systems (i.e., exoplanets). In this minireview, we focus on some advanced computational methods capable of efficiently solving the Schro¨dinger equation at different levels of approximation (i.e., density functional theory)—such as ab initio molecular dynamics—and which are capable to realistically simulate the behavior of matter under the action of energy sources available in prebiotic contexts. In addition, recently developed metadynamics methods coupled with first-principles simulations are here reviewed and exploited to answer to old enigmas and to propose novel scenarios in the exponentially growing research field embedding the study of the chemical origins of life.

Interfacial and bulk properties of concentrated solutions of ammonium nitrate

We conducted molecular dynamics (MD) simulations to calculate the density and surface tension of concentrated ammonium nitrate (AN) solutions up to the solubility limit of ammonium nitrate in water, by combining the SPC/E, SPCE/F and TIP4P/2005 water models with OPLS model for ammonium and nitrate ions. This is the first time that the properties of concentrated solutions of nitrates, especially AN, have been studied by molecular dynamics. We effectively account for the polarisation effects by the electronic continuum correction (ECC), practically realised via rescaling of the ionic charges. We found that, the full-charge force field MD simulations overestimate the experimental results, as the ions experience repulsion from the interface and prefer to remain in the subsurface layer and the bulk solution. In contrast, reducing the ionic charges results in the behaviour that fits well with the experimental data. The nitrate anions display a greater propensity for the interface than the ammonium cations. We accurately predict both the density and the rise in the surface tension of concentrated solutions of AN, recommending TIP4P/2005 for water and the scaled-charge OPLS model (OPLS/ECC) for the ions in the solutions. We observe that, the adsorption of anions to the interface accompanies their depletion in the subsurface layer, which is preferentially occupied by cations, resulting in the formation of the electric double layer. We demonstrate the ion deficiency for up to 3 Å below the surface and establish the requirement to include the polarisability effects in the OPLS model for AN. While these results confirmed the findings of the previous studies for dilute solutions, they are new in the solubility limit. Concentrated solutions exhibit a strong effect of the abundance of solute on the coordination numbers of ions and on the degree of ion pairing. Surprisingly, ion pairing decreases significantly at the interface compared with the bulk. The present study identifies OPLS/ECC, along with TIP4P/2005, to yield accurate predictions of physical properties of concentrated AN, with precision required for industrial applications, such as a formulation of emulsion and fuel-oil explosives that now predominate the civilian use of AN. An application of this model will allow one to predict the surface properties of supersaturated solutions of AN which fall outside the capability of the present laboratory experiments but are important industrially.

Electric-Field-Induced Effects on the Dipole Moment and Vibrational Modes of the Centrosymmetric Indigo Molecule

Intense static electric fields can strongly perturb chemical bonds and induce frequency shifts of the molecular vibrations in the so-called vibrational Stark effect. Based on a density functional theory (DFT) approach, here, we report a detailed investigation of the influence of oriented external electric fields (OEEFs) on the dipole moment and infrared (IR) spectrum of the nonpolar centrosymmetric indigo molecule. When an OEEF as intense as ∼0.1 V Å^-1 is applied, several modifications in the IR spectrum are observed. Besides the notable frequency shift of some modes, we observe the onset of new bands-forbidden by the selection rules in the zero-field case. Such a neat field-induced modification of the vibrational selection rules, and the subsequent variations of the peaks' intensities in the IR spectrum, paves the way toward the design of smart tools employing centrosymmetric molecules as proxies for mapping local electric fields. In fact, here, we show that the ratio between the IR and the Raman intensities of selected modes is proportional to the square of the local field. This indicator can be used to quantitatively measure local fields, not only in condensed matter systems under standard conditions but also in field-emitting-tip apparatus.

Interacting Ru(bpy) 3 2 + Dye Molecules and TiO2 Semiconductor in Dye-Sensitized Solar Cells

Solar energy is an alternative source of energy that can be used to replace fossil fuels. Various types of solar cells have been developed to harvest this seemingly endless supply of energy, leading to the construction of solar cell devices, such as dye-sensitized solar cells. An important factor that affects energy conversion efficiency of dye-sensitized solar cells is the distribution of dye molecules within the porous semiconductor (TiO 2 ). In this paper, we formulate a continuum model for the interaction between the dye molecule Tris(2,2 ′ -bipyridyl)ruthenium(II) (Ru(bpy) 3 2 + ) and titanium dioxide (TiO 2 ) semiconductor. We obtain the equilibrium position at the minimum energy position between the dye molecules and between the dye and TiO 2 nanoporous structure. Our main outcome is an analytical expression for the energy of the two molecules as a function of their sizes. We also show that the interaction energy obtained using the continuum model is in close agreement with molecular dynamics simulations.

Removal of As(III) from Biological Fluids: Mono- versus Dithiolic Ligands

Arsenic is one of the inorganic pollutants typically found in natural waters, and its toxic effects on the human body are currently of great concern. For this reason, the search for detoxifying agents that can be used in a so-called “chelation therapy” is of primary importance. However, to the aim of finding the thermodynamic behavior of efficient chelating agents, extensive speciation studies, capable of reproducing physiological conditions in terms of pH, temperature, and ionic strength, are in order. Here, we report on the acid–base properties of meso-2,3-dimercaptosuccinic acid (DMSA) at different temperatures (i.e., T = 288.15, 298.15, 310.15, and 318.15 K). In particular, its capability to interact with As(III) has been investigated by experimentally evaluating some crucial thermodynamic parameters (ΔH and TΔS), stability constants, and its speciation model. Additionally, in order to gather information on the microscopic coordination modalities of As(III) with the functional groups of DMSA and, at the same time, to better interpret the experimental results, a series of state-of-the-art ab initio molecular dynamics simulations have been performed. For the sake of completeness, the sequestering capabilities of DMSA—a simple dithiol ligand—toward As(III) are directly compared with those recently emerged from similar analyses reported on monothiol ligands.

Self-assembly of Organic Nanomaterials and Biomaterials: The Bottom-Up Approach for Functional Nanostructures Formation and Advanced Applications

In this paper, we survey recent advances in the self-assembly processes of novel functional platforms for nanomaterials and biomaterials applications. We provide an organized overview, by analyzing the main factors that influence the formation of organic nanostructured systems, while putting into evidence the main challenges, limitations and emerging approaches in the various fields of nanotechology and biotechnology. We outline how the building blocks properties, the mutual and cooperative interactions, as well as the initial spatial configuration (and environment conditions) play a fundamental role in the construction of efficient nanostructured materials with desired functional properties. The insertion of functional endgroups (such as polymers, peptides or DNA) within the nanostructured units has enormously increased the complexity of morphologies and functions that can be designed in the fabrication of bio-inspired materials capable of mimicking biological activity. However, unwanted or uncontrollable effects originating from unexpected thermodynamic perturbations or complex cooperative interactions interfere at the molecular level with the designed assembly process. Correction and harmonization of unwanted processes is one of the major challenges of the next decades and requires a deeper knowledge and understanding of the key factors that drive the formation of nanomaterials. Self-assembly of nanomaterials still remains a central topic of current research located at the interface between material science and engineering, biotechnology and nanomedicine, and it will continue to stimulate the renewed interest of biologist, physicists and materials engineers by combining the principles of molecular self-assembly with the concept of supramolecular chemistry.

Ab initio spectroscopy of water under electric fields

Whereas a broad range of literature exists on the spectroscopy of water in disparate conditions, infrared (IR) and Raman spectra of water subjected to electric fields have never extensively been investigated so far. Based on ab initio molecular dynamics simulations, here we present IR and Raman spectra of bulk liquid water under the effect of static electric fields. A contraction of the entire frequency range is recorded upon increasing the field intensity both in the IR and in the Raman spectra. Whilst the OH stretching band is progressively shifted toward lower frequencies - indicating a field-induced strengthening of the H-bond network - all the other bands are up-shifted by the field. Furthermore, an evident modification of the librational mode band appears in all the spectra. Finally, the order-maker action of the field emerges also from the increase of the water orientational tetrahedral order. Upon field exposure, the water structure becomes more "ice like".

Development of Nonbonded Models for Metal Cations Using the Electronic Continuum Correction

The parametrization of classical nonbonded models of metal ions has been widely addressed in the recent years. Despite the continuous development of novel and more physically inspired functional forms, the 12-6 Lennard-Jones plus Coulomb potential is still the most adopted force field in molecular dynamics (MD) codes, owing to its simple form and easy implementation. However, due to the integer formal charge, unpolarizable force fields of ions may suffer from overestimated interatomic electrostatic interactions, leading to nonphysical clustering or repulsion between such full charges. The electronic continuum correction (ECC) can fix this problem through a simple inclusion of solvent polarization effects via ionic charge rescaling. In this work, the development of novel nonbonded models for mono, divalent, and highly charged metal ions is presented. For each metal species, the ionic charge has been scaled, according to the ECC. Lennard-Jones parameters have been optimized using experimental structural and thermodynamic properties as target quantities. Performances of the proposed models are discussed and compared with the literature data, while transferability attitudes among different and well-known water models are evaluated. © 2019 Wiley Periodicals, Inc.

Optimizing a Simple Natural Dye Production Method for Dye-Sensitized Solar Cells: Examples for Betalain (Bougainvillea and Beetroot Extracts) and Anthocyanin Dyes

We present a study about the sensitizers extracted from natural resources. This paper focuses on how to select, extract and characterize natural dyes, giving some guides to establish a protocol for the whole process of fabricating and using these dyes. The influence of the extraction solvent and method, and of parameters such as pH are analyzed. Also, dye precursor and dye extract stability have been studied, as well as how the dye adsorbs onto substrates and the effect of mixing or concentrating the extracts. Results concerning betalain pigments present in bougainvillea and beetroot extracts, and anthocyanins in eggplant extracts, analyzed by using UV-Vis spectrometry, are included. As an example of application, we report procedures intended to test and enhance the dye potential as a main component of dye-sensitized solar cells (DSSCs). DSSCs mimic nature’s photosynthesis and have some advantages like an easy and low-cost fabrication procedure. Their efficiency depends on its design and fabrication process and also on the different components involved. Hence, optimizing each component is essential to achieve the best performance, and thus the dye used as a sensitizer is crucial. We fabricate cells by using a simple procedure: As the interest is focused on the sensitizer, the same consecutive steps are followed, varying only the dye extract. Among all the natural-dyes tested, beetroot extract reaches up to 0.47% cell efficiency, which is near the highest values found in literature for this pigment.

Interactions between cyclic nucleotides and common cations: an ab initio molecular dynamics study

We present the first, to the best of our knowledge, ab initio molecular dynamics (AIMD) investigation on three aqueous solutions where an abasic cyclic nucleotide model is solvated in the presence of distinct cations (i.e., Na⁺, K⁺ and Mg²⁺). We elucidate the typical modalities of interaction between those ionic species and the nucleotide moiety by first-principles numerical simulations, starting from an inner-shell binding configuration on a time scale of 100 ps (total simulation time of ∼600 ps). Whereas the strong "structure-maker" Mg²⁺ is permanently bound to one of the two oxygen atoms of the phosphate group of the nucleotide model, Na⁺ and K⁺ show binding times τ_b of 65 ps and 10-15 ps, respectively, thus reflecting their chemical nature in aqueous solutions. Furthermore, we qualitatively relate these findings to approximate free-energy barriers of the cations' unbinding obtained by means of exploratory well-tempered metadynamics. With the aim of shedding light on the features of commonly employed force-fields (FFs), classical MD simulations (almost 200 trajectories with a total simulation time of ∼18 μs) using the biomolecular AMBER FF are also reported. By choosing several combinations of the parametrization for the water environment (i.e., TIP3P, SPC/E and OPC) and cations (i.e., Joung-Cheatham, Li-Merz 12-6 and Li-Merz 12-6-4), we found significant differences in the radial distribution functions and residence times compared to the ab initio results. The Na⁺ and K⁺ ions wrongly show quasi-identical radial distribution functions and the Li & Merz 12-6-4 Lennard-Jones parameters for Mg²⁺ were found to be essential in quickly reaching the binding state consistent with AIMD.

New insights into the effect of pH on the mechanism of ofloxacin electrochemical detection in aqueous solution

We first found that identifying the protonation status and polarity of the target pollutant helped to rapidly find an optimal pH condition for its electrochemical detection, which promoted practical application.

Surface charge-dependent hydrodynamic properties of an electroosmotic slip flow

The electroosmosis effects at the interface of an aqueous NaCl solution and a charged silicon surface are studied using a molecular dynamics (MD) method. Considering a plug-like electroosmotic flow, we identified a thin interfacial layer in the immediate vicinity of the charged surface, where the flow velocity experiences almost linear spatial variations. The thickness of this interfacial layer is found to be linearly dependent on the surface charge density, with a negative slope which slightly depends on the surface hydrophobicity while being independent of the salt concentration, electric field strength, and orientation of the surface lattice. It is also found that upon increasing the surface charge density, the effective slip length first increases up to a maximum amount and then follows an almost linear reduction. We found that increasing the salt concentration drastically reduces the surface charge at which the effective slip length reaches its maximum amount. For highly concentrated solutions, therefore, the effective slip length could be assumed to change linearly in the whole range of the surface charge density, with a slope which is proportional to the square root of the electric field strength divided by the depth of the potential well assigned to the surface atoms εwall. Also, in a wide range of the surface charge density, the slip velocity is found to be a constant fraction of the electroosmotic velocity, which could be measured experimentally. Finally, by comparing the electroosmotic velocities calculated from the Stokes equation (considering both the slip and no-slip boundary conditions) with our MD results, we found that the no-slip boundary condition, which is normally used in analytical calculations, leads to a very inaccurate result for the studied system.

Alternating electric field-induced ion current rectification and electroosmotic pump in ultranarrow charged carbon nanocones

Pumping fluid in ultranarrow (sub-2 nm) synthetic channels, analogous to protein channels, has widespread applications in nanofluidic devices, molecular separation, and related fields. In this work, molecular dynamics simulations were performed to study a symmetrical sinusoidal electric field-induced electroosmotic pump in ultranarrow charged carbon nanocone (CNC) channels. The results show that the CNC channels could rectify the ion current because of the different ion flow rates in the positive and negative half circles of the sinusoidal electric field. Electroosmotic flow (EOF) rectification yielded by the ion current rectification is also revealed, and net water flow from the base to the tip of the CNC channels is observed. The simulations also show that the preferential ion current conduction direction in the ultranarrow CNC channels (from base to tip) is opposite to that in conical nanochannels with tip diameters larger than 5 nm (from tip to base). However, the preferential EOF direction is the same as that of large conical nanochannels (from base to tip). We also investigated the influences of ion concentration and the amplitudes and periods of the sinusoidal electric field on the EOF pump. The results show that high ion concentration, large amplitudes, and long periods are desired for high EOF pumping efficiency. Finally, through comparison with a constant electric field and a pressure-induced water pump, we prove that the EOF pump under an alternating electric field has a higher pump efficiency. The approach outlined in this work provides a general scheme for pumping fluid in ultranarrow charged conical nanochannels.

Stability of hydrolytic arsenic species in aqueous solutions: As3+vs. As5+

By combining ab initio molecular dynamics simulations and experiments, the stable hydrolytic species formed by As³⁺ and As⁵⁺ have been identified both in natural waters and in biologically relevant systems.

Confinement of aqueous mixtures of ionic liquids between amorphous TiO2 slit nanopores: electrostatic field induction

Electrostatic potential in the vicinity of the surface is induced when aqueous mixtures of hydrophobic and hydrophilic ionic liquids (ILs) are confined between a slit nanopore of amorphous but not crystalline TiO₂ semiconductors.