Adsorption Of Other Molecules Research Articles

Highly sensitive and selective room-temperature gas sensors based on B6N6H6 monolayer for sensing SO2 and NH3: A first-principles study

We applied first-principles calculations and studied the structures and electronic properties of SO2, NH3, H2, N2, CH4, H2O, N2O, CO, CO2, HCN, and H2S molecules adsorbed on the B6N6H6 monolayer to understand its performance as a sensor. The crystalline structure of the B6N6H6 monolayer remains thermodynamic stable under T = 1000 K, and the rippled structure is more stable than the planar one. The NH3 and SO2 molecules are chemically and physically, respectively, adsorbed on the B6N6H6 monolayer with moderate adsorption energy and charge transfer. By adsorbing NH3 and SO2, B6N6H6 monolayer electronic properties (especially conductivity) could be adjusted. Adsorption of other molecules did not have any similar effects. The recovery times of NH3 and SO2 desorption from the monolayer were only 0.42 ms and 1.1 s, respectively, at room temperature. The existence of humidity would have, to a certain extent, influence on the sensing of SO2 or NH3 gas using B6N6H6 monolayer. It is thus predicted that the B6N6H6 monolayer could be a room-temperature NH3 and SO2 sensor in a dry environment with high selectivity and sensitivity and fast response and recovery time. We also believe that the B6N6H6 monolayer will be a good candidate for NH3 work-function-type gas sensors.

Poly‐L‐lysine‐coated α‐lactalbumin nanoparticles: preparation, effect of pH, and stability under in vitro simulated gastrointestinal conditions

AbstractBACKGROUNDThe development of bioactive compound delivery systems using protein‐based nanoparticles as carriers is a rapidly growing field of study. α‐Lactalbumin (α‐La) nanoparticles are adequate for this task because they are nonantigenic, biodegradable, and easily modifiable via the surface physical adsorption of other molecules. Here, the effect of poly‐L‐lysine (PLL) coating of α‐La nanoparticles on the stability against enzymatic degradation in the gastrointestinal tract was investigated.RESULTSα‐La nanoparticles were prepared by desolvation with acetone. It was observed that the pH of the initial protein suspension did not have a significant effect on the size, polydispersity index (PDI), or ζ‐potential of the nanoparticles, so the subsequent experiments were performed at pH 9, where 151.2 nm nanoparticles with a PDI of 0.072 and a ζ‐potential of −29.7 mV were obtained. The nanoparticles coating was performed with two MW‐range PLL at three concentrations. Coating efficiency increased as the polymer concentration increased and this was independent of the PLL MW. The best coating efficiency (90–95%) was obtained in the case of the 1 mg mL−1 PLL coating. Most PLL‐coated nanoparticles showed a good stability to the conditions of in vitro simulated gastrointestinal digestion compared with the uncoated nanoparticles.CONCLUSIONIt can be concluded that the acetone desolvation method at pH 9 is adequate to obtain α‐La nanoparticles with good characteristics. The process of PLL coating of nanoparticles showed an increase in efficiency with the increase in concentration of the polymer. The resulting coated nanoparticles had enhanced resistance to simulated gastrointestinal conditions. © 2021 Society of Chemical Industry (SCI).

The atomic structure and dynamics at the CaCO3 vaterite-water interface: A classical molecular dynamics study.

Classical molecular and lattice dynamics were applied to explore the structure and dynamics of water on different surfaces of vaterite, the least abundant calcium carbonate polymorph. Surfaces were generated starting from the three possible structural models for vaterite (monoclinic, hexagonal/trigonal, and triclinic) and pre-screened using their surface energies in an implicit solvent. Surfaces with energies lower than 0.55 J/m2 were then run in explicit water. The majority of these surfaces dissolve in less than 100 ns, highlighting the low stability of this phase in abiotic environments. Three stable surfaces were identified; they exhibited only minor structural changes when in contact with explicit water and did not show any tendency to dissolve during 1 µs molecular dynamics simulations. The computed water density profiles show that all these surfaces have two distinct hydration layers. The water residence time at the various calcium sites was computed to be within 0.7 and 20.5 ns, which suggests that specific Ca ions will be more readily available to bind with organic molecules present in solution. This analysis is a step forward in understanding the structure of this complex mineral and its role in biomineralization, as it provides a solid theoretical background to explore its surface chemistry. In particular, this study provides realistic surface models and predicts the effect of water exchange at the surface active sites on the adsorption of other molecules.

Adsorption, Gas-Sensing, and Optical Properties of Molecules on a Diazine Monolayer: A First-Principles Study.

Using first-principles calculations, the structural, electronic, and optical properties of CO2, CO, N2O, CH4, H2, N2, O2, NH3, acetone, and ethanol molecules adsorbed on a diazine monolayer were studied to develop the application potential of the diazine monolayer as a room-temperature gas sensor for detecting acetone, ethanol, and NH3. We found that these molecules are all physically adsorbed on the diazine monolayer with weak adsorption strength and charge transfer between the molecules and the monolayer, but the physisorption of only NH3, acetone, and ethanol remarkably modified the electronic properties of the diazine monolayer, especially for the obvious change in electric conductivity, showing that the diazine monolayer is highly sensitive to acetone, NH3, and ethanol. Further, the adsorption of NH3, acetone, and ethanol molecules remarkably modifies, in varying degrees, the optical properties of the diazine monolayer, such as work function, absorption coefficient, and the reflectivity, whereas adsorption of other molecules has infinitesimal influence. The different adsorption behaviors and influences of the electronic and optical properties of molecules on the monolayer show that the diazine monolayer has high selectivity to NH3, acetone, and ethanol. The recovery time of NH3, acetone, and ethanol molecules is, respectively, 1.2 μs, 7.7 μs, and 0.11 ms at 300 K. Thus, the diazine monolayer has a high application potential as a room-temperature acetone, ethanol, and NH3 sensor with high performance (high selectivity and sensitivity, and rapid recovery time).

Exploring quantification in a mixture using graphene-based surface-enhanced Raman spectroscopy

Quantitative analysis is a demanding challenge for surface-enhanced Raman scattering (SERS). A number of methods have been developed using different SERS-active substrates for quantitative detection of certain species. However, for a target analyte that is mixed with other molecules, the competitive adsorption of other molecules on the SERS substrate becomes the major disturbance to quantitative analysis or even qualitative detection. Whether it is possible to quantitatively detect a single analyte in a mixture of molecules using SERS has remained unclear. In this work, we explored the possibility of quantitative detection of multiple analytes by using graphene-based SERS (G-SERS). Graphene provides an atomically flat surface for homogeneous adsorption of molecules, and its own Raman scattering intensity can serve as internal standard in SERS for quantitative analysis. Our results show that the adsorption constants, the concentration and the Raman scattering cross sections of competing species are the most important factors to be considered. The findings could be helpful for quantitative analysis of target analyte in practical complex systems.

Tuning electronic and optical properties of MoS2monolayer via molecular charge transfer

Density functional theory computations were performed to investigate the adsorption of four organic molecules, including tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE), tetrathiafulvalene (TTF) and benzyl viologen (BV) on the basal plane of MoS2 monolayer (MoS2ML). There mainly exist non-covalent weak interactions between these organic molecules and MoS2ML with considerable charge transfer. Due to the adsorption of organic molecules, the band gap of MoS2ML can be efficiently reduced as the flat molecular levels lie in the band gap region of MoS2ML. Moreover, the adsorption of TCNQ can significantly enhance the optical absorption of MoS2ML in the infrared region of solar spectrum, whereas the adsorption of other molecules has negligible effect on the optical properties of MoS2ML. Our computations provide a flexible approach towards tuning the electronic and optical properties of MoS2ML.

Synthetic oligonucleotides: AFM characterisation and electroanalytical studies

One of the most important steps in designing more sensitive and stable DNA based biosensors is the immobilisation procedure of the nucleic acid probes on the transducer surface, while maintaining their conformational flexibility. MAC Mode AFM images in air demonstrated that the oligonucleotide sequences adsorb spontaneously on the electrode surface, showing the existence of pores in the adsorbed layer that reveal big parts of the electrode surface, which enables non-specific adsorption of other molecules on the uncovered areas. The electrostatic immobilisation onto a glassy carbon electrode followed by hybridisation with a complementary sequence and control with a non-complementary sequence was studied using differential pulse voltammetry and electrochemical impedance spectroscopy. Changes in the oxidation currents of guanosine and adenosine were observed after hybridisation events as well as after control experiments. Modification of the double layer capacitance that took place after hybridisation or control experiments showed that non-specific adsorption of complementary or non-complementary sequences occur allowing the formation of a mixed multilayer.

Unusual Forms of Molecular Hydrogen Adsorption by Cu+1 Ions in the Copper-Modified ZSM-5 Zeolite

DRIFT and IR transmittance spectra of H2 adsorbed at 77 K or at room temperature by the copper-modified ZSM-5 zeolite pre-evacuated or pre-reduced in CO at 873 K indicated several unusual forms of adsorbed hydrogen. H–H stretching frequencies of adsorbed species at 3075–3300 cm−1 are by about 1000 cm −1 lower than in the free hydrogen molecules. This indicates unusually strong perturbation of adsorbed hydrogen by reduced Cu +1 ions that has been never before reported neither for hydrogen nor for adsorption of other molecules by any cationic form of zeolites or oxides.

Molecular structure of cysteamine monolayers on silver and gold substrates: Comparative studies by surface-enhanced Raman scattering

Monolayers of cysteamine (2-aminoethanethiol) frequently work as linkage layers for adsorption of other molecules on metal surfaces. We compared the structure of cysteamine monolayers formed on gold and silver and the influence of various electrolytes on the structure of monolayers formed on both substrates. The monolayers formed on silver contain significantly higher portion of a trans conformer than monolayers on gold. Probably monolayers on silver are self-assembled in such a way that higher portion of the amino groups is unbonded to the surface, thus being available for attaching other molecules. The structure of cysteamine monolayers formed on gold is considerably more stable and resistive to the influence of electrolytes as compared to the silver substrate. The greater stability of the monolayers on Au surface was ascribed to the strong affinity of the amine groups toward this metal.

Influence of electrolytes on the structure of cysteamine monolayer on silver studied by surface‐enhanced Raman scattering

AbstractMonolayers of cysteamine (2‐aminoethanethiol) frequently work in aqueous solutions of electrolytes as linkage layers for adsorption of other molecules on metal surfaces. We determined the influence of various ions on the structure of the cysteamine monolayer with surface‐enhanced Raman scattering (SERS). The surrounding (pH, electrolytes) considerably influences the conformation of adsorbed cysteamine molecules. Direct spectral evidence of the interaction of cysteamine with an Ag surface through AgN bonds in the gauche conformation is given. It was found that chlorides and iodides significantly increase the proportion of the gauche conformer. Probably these anions additionally stabilize the gauche conformer by the formation of hydrogen bonds with NH2 groups in addition to the direct NAg bond. Raman measurements also revealed that cations, which can form strong coordination bonds with amino groups (Cu2+, Ni2+ or Co2+), change the structure of the cysteamine layer by transforming the trans into a gauche conformer. In contrast, an increase in the proportion of trans conformers is observed with increase in the concentration of H+ in the ambient electrolyte. Possible explanations for these experimental findings are discussed. Copyright © 2001 John Wiley & Sons, Ltd.

Characterisation by drop tensiometry and by ellipsometry of the adsorption layer formed at the air/champagne wine interface

A foam ring composed of small bubbles on the surface of a champagne glass is one of its hallmarks. The equilibrium state of that ring is linked with the rate of formation and of disappearance of bubbles. The stability of bubbles is usually ascribed to the occurrence and to the properties of an adsorption layer formed at the gas/liquid interface. Our goal is to characterise such an adsorption layer at the gas/wine interface in order to understand its role in bubble stability. Alcohol in wine lowers the surface tension to 49 mN/m. The adsorption of other molecules may cause a further decrease of 2 mN/m. Such a situation makes the study of adsorption by surface tension measurement inaccurate. To overcome this problem, we have diluted the wine four times with water before its surface tension measurement by pendant drop shape analysis. In these conditions, ethanol lowers the surface tension to 64 mN/m and the adsorption of other molecules of the wine can be monitored over 6–8 mN/m. The usual behaviour of such a diluted wine is a lowering of the surface tension during at least 20 min after drop formation. Since the role of macromolecules on the foaming properties of wine had been previously observed, we have chosen to evaluate the effect of this fraction of the wine molecules on its surface properties. Thus, wines were ultrafiltrated on a membrane with a 10 000 molecular mass cut-off. The ultrafiltrate (UF) does not show any decrease of its surface tension over a 20-min period while the ultraconcentrate (UC) has a kinetics similar to that of unfiltered wine. Mixtures of UF and UC have behaviours intermediate between those of these products. A technological treatment of the wine with bentonite, believed to lower the content of macromolecules, yields a wine similar to UF. The effect of ultrafiltration was also analysed by spectroscopic ellipsometry. UF has a spectrum similar to that of a water/alcohol mixture with the same ethanol content and its ellipticity is stable during at least 20 min. On the contrary, wine or UC show spectra with the features of an adsorption layer and those characteristics increase during more than 20 min. Two varieties of vine were compared: ‘Chardonnay’ and ‘Pinot noir’. The former is known to have better foaming properties than the latter. Its surface properties measured in this study are also more pronounced than those of Pinot noir. However, the representation of the dilational modulus against the surface pressure (which, in some instances, may be a mathematical transformation of the state equation) puts all the samples (wines, UF and UC of each) on the same master curve, a fact in favour of a common nature for all the adsorption layers. It can be concluded that surface properties of champagne wines are mostly determined by ethanol and by macromolecules with a molecular mass larger than 10 000. Moreover, the adsorption layers seem to have the same nature, irrespective of the vine variety and of the concentration ratio of the wine.

Study of the adsorption of a hydrophobic peptide onto carbon surfaces by capacitance measurements

The adsorption of a hydrophobic peptide (β-CN (193–209)) onto glassy carbon and a carbon support of membrane was studied by double-layer capacitance measurements using electrochemical impedance spectroscopy (SIE). The kinetics of adsorption was investigated by recording the changes that occurred in the double-layer capacitance after adding the peptide. The change in capacitance was interpreted in terms of the number of close-contact areas between the peptide and the surface. A two-consecutive reaction model was used to describe peptide adsorption. The first step was attributed to the adsorption reaction itself and the second to a change in the conformation of the adsorbed peptide molecules. The corresponding theoretical equation of the variation of capacitance with time fits our results. Moreover, it allowed determination of the time constants for both reactions and three double-layer capacitances: the bare surface and the surfaces covered with monolayers of the peptide in states 1 and 2. For glassy carbon, the activation energies are 4.7 and 3.5 kJ mol−1 for the first and second step of the reaction and the reaction orders are 0.64 and 0.35, respectively. These values are different for each step. This confirms the model of two separate reactions to describe adsorption. The first step comprises two elementary mechanisms. The first may correspond to the adsorption of the peptide onto the surface. The second may be consistent with a modification of the adsorbed molecules induced by the adsorption of other molecules over them. The second step of the process requires the contribution of peptides of the solution for it to occur. The changes in the double-layer capacitance depend on the peptide bulk concentration. This suggests that the peptide adsorbs in a different conformation depending on its bulk concentration. Peptide polymerisation could be responsible for this phenomenon.

Monolayer physical adsorption in narrow pores. Apparent surface dimension

Monolayer physical adsorption has been considered, taking into consideration the intrinsic volume of the adsorbate molecules. Since an adsorbed molecule occupies not only the site on the surface but also some of the neighboring volume, it creates steric difficulties for the adsorption of other molecules and leads to underestimation of the measured surface area. As a result, this value depends on the size of the adsorbate molecules and the apparent surface dimension can be introduced, even if the surface of narrow pores has no irregularities of atomic scale size. This effect was shown by simulation of adsorption on the surface of Menger sponge. Experimental data for measuring D-values on silica gels with different pore size distributions are in line with this effect.

Thinning and opening of carbon nanotubes by oxidation using carbon dioxide

THE discovery1 and bulk synthesis2 of carbon nanotubes has stimulated great interest. It has been suggested that these structures may have useful electronic3–5 and mechanical6 properties, and these might be modified by introducing foreign materials into the nanotubes. But the tubes are invariably capped at the ends. Ajayan and lijima7 have succeeded in drawing molten material (lead or one of its compounds) into the tubes by heating them in the presence of lead and oxygen; less than 1% of the tubes in the sample studied could be filled in this way. Here we report that heating in carbon dioxide gas can result in the partial or complete destruction of the tube caps and stripping of the outer layers to produce thinner tubes. In some cases, we have thinned the extremity of tubes to a single layer. The opened tubes can be regarded as nanoscale test-tubes for adsorption of other molecules, and this controlled method of thinning may allow studies of the properties of single tubes.

Microwave Spectroscopic Investigation of the Kinetics of the Heterogeneous Ammonia-Deuterium Exchange

A method is described for analyzing a mixture of gases using microwave spectroscopy. Relatively simple microwave measurements are employed on single resolved collision broadened spectral lines. For binary mixtures, only the peak absorption coefficients need to be measured if the microwave collision diameters are known. Mixtures of isotopically substituted molecules can also be analyzed because the microwave absorption lines of the isotopic species are easily resolved. The technique is applied to determining the mole fraction of NH3 in mixtures of deuterated ammonias, and to a study of the kinetics of the NH3—D2 isotopic exchange reaction on a singly promoted iron catalyst. It is found that rates of desorption of molecules from surfaces are markedly increased on adsorption of other molecules, a fact that can be explained only in terms of strong interactions between adsorbed molecules. The rate of the exchange reaction is proportional to the square root of the deuterium pressure. With increasing ammonia pressure the rate passes through a maximum, showing that the mechanism involves reaction between adsorbed ammonia and adsorbed deuterium atoms. The activation energy diminishes significantly with increasing ammonia pressure, providing additional evidence for the existence of strong interactions between adsorbed molecules.