Local Dielectric Constant Research Articles

Solvent Descriptors Guided Wide‐Temperature Electrolyte Design for High‐Voltage Lithium‐Ion Batteries

AbstractLithium‐ion batteries are increasingly required to operate under harsh conditions, particularly at high temperatures above 55 °C. However, existing electrolytes suffer from inadequate thermal stability and significant interphasial side reactions. Moreover, there is a lack of clear guidelines for developing electrolytes that can withstand high temperatures. Here a solvent screening descriptor is introduced based on dual local softness and dielectric constant. The findings indicate that solvents with moderate dielectric constants and low reactivity are ideal candidates for high‐temperature electrolytes. Among the solvents evaluated, tetraethyl orthosilicate (TEOS) is identified as a suitable option and is utilized to formulate a localized high‐concentration electrolyte (TEOS‐based LHCE). Remarkably, the 1‐Ah LiNi0.8Co0.1Mn0.1||graphite pouch cell utilizing this TEOS‐based LHCE demonstrates 95.8% capacity retention after 300 cycles at 60 °C. Interphasial analysis reveals that the TEOS‐based LHCE promotes the formation of thin, uniform LiF‐rich interphases, effectively suppressing interfacial side reactions at elevated temperatures. This screening strategy not only enhances the understanding of electrolyte performance but also paves the way for high‐throughput screening of electrolytes suitable for wide‐temperature lithium‐ion batteries.

(Invited) Exciton Dissociation in Mixed-Dimensionality SWCNT-Based Heterojunctions

Quantum-confined semiconductors provide highly tunable optical and electrical properties for a wide variety of emerging applications. Semiconducting single-walled carbon nanotubes (s-SWCNTs) have shown tremendous potential in applications ranging from digital logic, biological imaging, quantum information processing, photovoltaics, and thermoelectric energy harvesting. Energy harvesting applications rely critically upon the creation of tailored interfaces that enable the movement of energetic species (excitons, electrons, holes) in specified directions. In this talk, I will discuss the use of a variety of rationally designed “mixed-dimensionality” heterojunctions between s-SWCNTs and transition metal dichalcogenide (TMDC) monolayers to harvest visible/near-infrared photons and convert these excitations into long-lived charge-separated states. Type-II heterojunctions between s-SWCNTs and TMDCs enable rapid exciton dissociation and microsecond charge-separated state lifetimes. We have developed new TMDC/SWCNT/TMDC trilayer architectures that enable charge transfer cascades for improving charge separation yield and lifetime. These trilayer architectures allow us to probe out-of-plane carrier diffusion in the nanotube layer and the degree to which bound interfacial excitons impact the photophysics in these mixed-dimensionality heterostructures. I will also discuss the methodology by which we attempt to quantify the photoinduced exciton dissociation quantum yields in these nanoscale heterostructures. Using several heterostructure results, we are attempting to narrow in on appropriate ways to quantify the local dielectric constant and exciton size in both the SWCNT and TMDC components, each of which impact the estimated charge carrier yields. Taken together, these studies provide fundamental insights into the links between ground-state thermodynamics and excited-state dynamics for model nanoscale heterojunctions used to harvest solar energy and produce long-lived charge-separated states.

Mechanism simulation of polar and nonpolar organic solvent vapor adsorption on a multiwall carbon nanotubes paper gas sensor.

We investigate the adsorption behavior of polar and nonpolar molecules on carbon nanotube interfaces through computational simulations. Gaussian 16 was utilized to calculate the total energy of each possible molecular structure and analyze the adsorption mechanisms in stacked and inline configurations. The study reveals that nonpolar molecules favor stacked adsorption on two graphene interfaces, while polar molecules prefer inline adsorption. The findings suggest that inline adsorption of polar molecules results in minimal changes to the local dielectric constant, which may explain the absence of multi-step adsorption isotherms. The research examines the stability and energetics of molecular adsorption on graphene layers simulating CNT interfaces. Different types of molecules (polar and nonpolar) exhibit distinct adsorption behaviors, with nonpolar molecules aligning with the IUPAC type VI isotherm model and polar molecules following the Langmuir isotherm model (IUPAC type I). This study provides insight into how molecules are likely to adsorb on CNT surfaces and the impact on the local dielectric constant. This understanding has implications for the design and optimization of CNT-based sensors, particularly in detecting organic solvents and gases in various environments.

ДЖОНШЕРОВСКИЙ УНИВЕРСАЛЬНЫЙ ДИНАМИЧЕСКИЙ ОТКЛИК В ТОНКИХ ПЛЕНКАХ ПОЛИДИФЕНИЛЕНФТАЛИДА

It is known that the supramolecular structure of organic materials can have a significant effect on the conductivity of polymer films in both constant and alternating fields. In particular, the presence of associative supramolecular formations can lead to a significant increase in the local dielectric constant. At the same time, the presence of different types of supramolecular ordering can lead to the predominance of certain charge transfer mechanisms in polymer films, and, consequently, to the presence of a dependence of conductivity on the electric field strength. The degree of molecular ordering, both on the surface and in the bulk of a polymer film, can also have a significant effect on polarization. In this regard, studying the influence of the supramolecular structure on the conductivity of polymer films in an alternating electric field is an urgent task.Previously, using various methods, it was established that the supramolecular structure of films with a thickness of more than 300 nm is determined mainly by supramolecular formations of an associative structure. The structure of films with a thickness of 100 to 300 nm is determined by the mixed structure of globular and associative formations. The structure of films less than 100 nm thick is determined by supramolecular formations of a globular. It was also established by the method of thermally stimulated depolarization that three regions are observed on the dependence of the activation energy of capture centers on the thickness of the polymer film, corre- sponding to different supramolecular organizations.In this work, the dependence of the conductivity of poly (diphenylene phthalide) films in an alternating electric field in the low-frequency range from 0.1 to 106 Hz on the supramolecular structure of polymer films was studied. It was found that the type of dependence of the conductivity of polymer films on the frequency of an alternating field depends on the thickness and supramolecular structure of the polymer films, as well as the electric field strength. The applicability of the Jonsher’s model, which describes the conductivity of amorphous materials in an alternating electric field, for thin polymer films is discussed. A correlation between the power-law dependence index and the supramolecular structure of the polymer film and the influence of dipole polarization and injection of charge carriers into the bulk of the polymer film on the conductivity of the polymer film have been revealed.

Species Transport in Hydrophilic Nafion-Coated Cu Catalysts for CO2 Electroreduction

Ionomer coated catalysts have increasingly been utilized in the electrocatalytic reaction of CO2 to multi-carbon products on Cu-based GDEs1,2. Their proton conductivity coupled with the Donnan exclusion due to the charged immobilized ions have served as a means of tailoring the microenvironment of the catalyst to desired products. Whilst there are experimental efforts geared towards the tuning of the local environment to desired products, there is a lack of supporting computational models that elucidate ionic species profile within the Nafion coated catalyst in the presence of fixed negative sulfonate ion charges from Nafion and the negative charges from the electrocatalysts.On the other hand, hydrated Nafion ionomer in the presence of a supporting electrolyte forms a biphasic polymer matrix which allows the transport of CO2 in both the hydrophilic and hydrophobic regions of Nafion ionomer3. Recent literature has highlighted the transport of CO2 in hydrophobic domains but in not in hydrophilic domains4. In this work, we look a closer look at CO2 transport in hydrophilic domains. We combine mass transport generalized modified Poisson-Nernst-Planck (GMPNP) equations and molecular dynamics to elucidate the transport of reactant/product species in the local environment of Nafion coating on planar Cu catalysts.We find that the thickness of the Nafion coating, the presence of immobilized sulfonic charges, ionic concentration of electrolyte impacts the availability of CO2, pH and the concentration of cations which are important parameters for CO2 reduction reaction. We also provide a relationship between the pKa of Nafion and the local dielectric constant of the reaction’s microenvironment using molecular modelling and the corresponding impact of Nafion’s ability to effectively minimize carbonate reactions. Our results underscore the need to manage the catalyst-ionomer microenvironments for CO2 reduction on copper electrodes. This model in conjunction with further experimental endeavours can serve as a benchmark for other intricate porous catalyst models that are significant for optimizing catalyst performance for large scale CO2 reduction operations. Bibliography Bui, J. C. et al. Engineering Catalyst–Electrolyte Microenvironments to Optimize the Activity and Selectivity for the Electrochemical Reduction of CO2 on Cu and Ag. Acc. Chem. Res. 17, acs.accounts.1c00650 (2022).Kim, C. et al. Properties of the ionomer under CO 2 R-related environments Tailored catalyst microenvironments for CO 2 electroreduction to multicarbon products on copper using bilayer ionomer coatings. doi:10.1038/s41560-021-00920-8 (2022).Ren, X., Myles, T. D., Grew, K. N. & Chiu, W. K. S. Carbon Dioxide Transport in Nafion 1100 EW Membrane and in a Direct Methanol Fuel Cell. J. Electrochem. Soc. 162, F1221–F1230 (2015).García de Arquer, F. P. et al. CO2 electrolysis to multicarbon products at activities greater than 1 A cm−2. Science (80-. ). 367, 661–666 (2020). Figure 1

Multiscale modelling of species transport in hydrophilic Nafion®-coated Cu catalysts for CO2 electro-reduction

Ionomers such as Nafion are being used for engineering the microenvironment of the CO2RR towards selectivity to higher hydrocarbons. Their cationic exchange qualities mitigate homogeneous bicarbonate reactions, providing favourable conditions for production of C2-C3 products. The proximity of Nafion® layers to the negatively charged electrode surface results in complex effects that have yet to be explored computationally. We combine mass transport equations and density functional theory (DFT) calculations to model the hydrophilic moieties in Nafion® layer on a 1D planar copper electrode. We vary the Nafion® layer thickness in the thick film regime (200–1100 nm), the ionic strength of supporting aqueous electrolyte and the applied voltage to investigate the concentration profile of participating species within the Nafion® layer. We find that the bicarbonate ion concentration and local pH decrease with decreasing thickness which makes thinner films better at mitigating bicarbonates at the cost of a relatively lower pH. We observe the breakdown of Donnan exclusion effects with higher ionic strength electrolytes which allows diffusion of both co-ions and counter-ions, promoting the bicarbonate reactions. Further, we find that the pKa of Nafion® generally increases as the local dielectric constant of the reaction medium decreases. This is primarily due to the accumulation of K+ and production of low dielectric constant products such as ethanol, albeit their concentrations are low on planar Cu electrodes. The lower permittivity of the reaction medium weakens the exclusion forces and changes the morphology of Nafion® which ultimately impacts its ability to mitigate homogeneous bicarbonate reactions.

Diblock Copolymer Melt in an Electric Field: Stability of the Homogeneous State in the Random Phase Approximation

The contribution of I.Ya. Erukhimovich (1947–2022) to the creation of the theory of microphase separation in di- and triblock copolymers has been briefly analyzed. His matrix method of calculating correlation functions for multicomponent polymer systems has been applied to find the spinodal of a diblock copolymer melt in a static electric field. It has been strictly shown that for a linear dependence of local dielectric constant of the copolymer on the order parameter the spinodal condition remains the same as in the absence of the electric field. The correction to the critical value of the Flory‒Huggins parameter has been calculated for more general case of the quadratic dependence.

Preferred microenvironments of halogen bonds and hydrogen bonds revealed using statistics and QM/MM calculation studies.

Hydrogen bonds (HBs) and halogen bonds (XBs) are two essential non-covalent interactions for molecular recognition and drug design. As proteins are heterogeneous in structure, the microenvironments of protein structures should have effects on the formation of HBs and XBs with ligands. However, there are no systematic studies reported on this effect to date. For quantitatively describing protein microenvironments, we defined the local hydrophobicities (LHs) and local dielectric constants (LDCs) in this study. With the defined parameters, we conducted an elaborate database survey on the basis of 22 011 ligand-protein structures to explore the microenvironmental preference of HBs (91 966 in total) and XBs (1436 in total). The statistics show that XBs prefer hydrophobic microenvironments compared to HBs. The polar residues like ASP are more likely to form HBs with ligands, while nonpolar residues such as PHE and MET prefer XBs. Both the LHs and LDCs (10.69 ± 4.36 for HBs; 8.86 ± 4.00 for XBs) demonstrate that XBs are prone to hydrophobic microenvironments compared with HBs with significant differences (p < 0.001), indicating that evaluating their strengths in the corresponding environments should be necessary. Quantum Mechanics-Molecular Mechanics (QM/MM) calculations reveal that in comparison with vacuum environments, the interaction energies of HBs and XBs are decreased to varying degrees given different microenvironments. In addition, the strengths of HBs are impaired more than those of XBs when the local dielectric constant's difference between the XB microenvironments and the HB microenvironments is large.

Dielectric response of confined water films from a classical density functional theory perspective.

We re-examine the problem of the dielectric response of highly polar liquids such as water in confinement between two walls using simple two-variable density functional theory involving number and polarisation densities. In the longitudinal polarisation case where a perturbing field is applied perpendicularly to the walls, we show that the notion of the local dielectric constant, although ill-defined at a microscopic level, makes sense when coarse-graining over the typical size of a particle is introduced. The approach makes it possible to study the effective dielectric response of thin liquid films of various thicknesses in connection with the recent experiments of Fumagalli et al., [Science, 2018, 360, 1339-1342], and to discuss the notion of the interfacial dielectric constant. We argue that the observed properties as a function of slab dimensions, in particular the very low dielectric constants of the order of 2-3 measured for thin slabs of ∼1 nm thickness do not highlight any special properties of water but can be recovered for a generic polar solvent having similar particle size and the same high dielectric constant. Regarding the transverse polarisation case where the perturbing field is parallel to the walls, the associated effective dielectric constant as a function of slab dimensions reaches bulk-like values at much shorter widths than in the longitudinal case. In both cases, we find an oscillatory behaviour for slab thicknesses in the one nanometer range due to packing effects.

Multi-parametric sensing by multi-channel molecular fluorescent probes based on excited state intramolecular proton transfer and charge transfer processes

Considering the applications of fluorescent probes and the information they provide, their brightness of fluorescence and photostability are of paramount importance. However, in the case of steady-state fluorescence spectroscopy and fluorescence microscopy, the amount of information can be increased by the application of multi-channel probes, via a multi-band fluorophore introduced in the probe molecule. In most cases, the use of such a multi-band (or multi-channel) fluorophore can also be combined with the concomitant introduction of one or several analyte receptors. Most often, the design of ratiometric probes with multi-band fluorescence emission are based on phenomena such as photoinduced intramolecular charge transfer (ICT) or excited state intramolecular proton transfer (ESIPT). Although ICT probes were up to recently the most popular, ESIPT probes and among them 3-hydroxyflavone derivatives, were shown to be the most productive. Several general problems were resolved by this family of probes, as for example the measurement of local dielectric constant, local H-bond accepting ability, water local concentration and ATP concentration in small volumes.Incorporation of such multi-channel probes into lipid membranes allowed to measure the different membrane potentials and to detect cell apoptosis. Also, it enabled to recognize and characterize the rafts formation in different lipid bilayers and peculiar features of the charged membrane interface. Such probes are also able to provide a concentration-dependent fluorescence signals upon binding of H+, Mg2+and Ba2+ions, and thus to recognize these different cations.The multi-channel probes are effective tools in the study of interactions of macromolecules such as peptides, proteins and nucleic acids. The most useful feature is that they inform simultaneously about several physical parameters, in this way giving a better insight in the investigated system. Thus, by comparing the reviewed probes with other modern fluorescent approaches, it can be concluded they are more informative and accurate tools.

Large emergent optoelectronic enhancement in molecularly cross-linked gold nanoparticle nanosheets

A central goal in molecular electronics and optoelectronics is to translate tailorable molecular properties to larger materials and to the device level. Here, we present a method to fabricate molecularly cross-linked, self-assembled 2D nanoparticle sheets (X-NS). Our method extends a Langmuir approach of self-assembling gold nanoparticle (NP) arrays at an air-water interface by replacing the liquid sub-phase to an organic solvent to enable cross-linking with organic molecules, and then draining the sub-phase to deposit films. Remarkably, X-NS comprising conjugated oligophenylene dithiol cross-linkers (HS-(C6H4)n-SH, 1 ≤ n ≤ 3) exhibit increasing conductance with molecule length, ~6 orders of magnitude enhancement in UV-Vis extinction coefficients, and photoconductivity with molecule vs. NP contributions varying depending on the excitation wavelength. Finite difference time domain (FDTD) analyses and control measurements indicate that these effects can be modeled provided the local complex dielectric constant is strongly modified upon cross-linking. This suggests quantum hybridization at a molecule–band (q-MB) level. Given the vast number of molecules and nano-building blocks available, X-NS have potential to significantly increase the range of available 2D nanosheets and associated quantum properties.

Kinetics and thermodynamics of enzymatic decarboxylation of α,β-unsaturated acid: a theoretical study.

Enzymatic decarboxylation of α,β-unsaturated acid through ferulic acid decarboxylase (FDC1) has been of interest because this reaction has been anticipated to be a promising, environmentally friendly industrial process for producing styrene and its derivatives from natural resources. Because the local dielectric constant at the active site is not exactly known, enzymatic decarboxylation to generate β-methylstyrene (β-MeSt) was studied under two extreme conditions (ε = 1 and 78 in the gas phase and aqueous solution, respectively) using the B3LYP/DZP method and transition state theory (TST). The model molecular clusters consisted of an α-methylcinnamate (Cin) substrate, a prenylated flavin mononucleotide (PrFMN) cofactor and all relevant residues of FDC1. Analysis of the equilibrium structures showed that the FDC1 backbone does not play the most important role in the decarboxylation process. The potential energy profiles confirmed that the increase in the polarity of the solvent could lead to significant changes in the energy barriers, especially for the transition states that involve proton transfer. Analysis of the rate constants confirmed the low/no quantum mechanical tunneling effect in the studied temperature range and that inclusion of the fluctuation of the local dielectric environment in the mechanistic model was essential. Because the computed rate constants are not compatible with the time resolution of the stopped-flow spectrophotometric experiment, the direct route for generating β-MeSt after CO2 elimination (acid catalyst (2)) is unlikely to be utilized, thereby confirming that indirect cycloelimination in a low local dielectric environment is the rate determining step. The thermodynamic results showed that the elementary reactions that involve charge (proton) transfer are affected by solvent polarity, thereby leading to the conclusion that overall, the enzymatic decarboxylation of α,β-unsaturated acid is thermodynamically controlled at high ε. The entropy changes due to the generation of molecules in the active site appeared more pronounced than that due to only covalent bond breaking/formation or structural reorientation. This work examined in detail for the first time the scenarios in each elementary reaction and provided insight into the effect of the fluctuations in the local dielectric environment on the enzymatic decarboxylation of α,β-unsaturated acids. These results could be used as guidelines for further theoretical and experimental studies on the same and similar systems.

Demystifying the Stern layer at a metal-electrolyte interface: Local dielectric constant, specific ion adsorption, and partial charge transfer.

Electric double layer (EDL) represents one of the most basic concepts in electrochemistry and is pertinent to diverse engineering applications ranging from electrocatalysis to energy storage. Whereas phenomenological and coarse-grained models have been long established to describe ionic distributions in the diffuse layer, a faithful prediction of the physicochemical properties of the electrode-electrolyte interface from a molecular perspective remains a daunting challenge. In this work, we investigate the charging behavior of an Ag (111) electrode in NaF aqueous solutions leveraging experimental results and theoretical calculations based on the classical density functional theory for ion distributions in the diffuse layer and on the joint density functional theory (JDFT) for the electronic structure. When the Ag electrode is applied with a negative voltage, the surface charge density can be reasonably described by assuming a neutral Stern layer with the dielectric constant dependent on the local electric field as predicted by the Kirkwood equation. However, the specific adsorption of F- ions must be considered when the electrode is positively charged and the fluoride adsorption can be attributed to both physical and chemical interactions. Qualitatively, F- binding and partial charge transfer are supported by JDFT calculations, which predict an increased binding energy as the voltage increases. Our findings shed insight on the molecular characteristics of the Stern layer and the charge behavior of adsorbed species not specified by conventional EDL models.

Mapping the local dielectric constant of a biological nanostructured system.

The aim of this work is to determine the varying dielectric constant of a biological nanostructured system via electrostatic force microscopy (EFM) and to show how this method is useful to study natural photonic crystals. We mapped the dielectric constant of the cross section of the posterior wing of the damselfly Chalcopteryx rutilans with nanometric resolution. We obtained structural information on its constitutive nanolayers and the absolute values of their dielectric constant. By relating the measured profile of the static dielectric constant to the profile of the refractive index in the visible range, combined with optical reflectance measurements and simulation, we were able to describe the origin of the strongly iridescent wing colors of this Amazonian rainforest damselfly. The method we demonstrate here should be useful for the study of other biological nanostructured systems.

The Effect of Local Dielectric Environment on the Fluorescence Quenching Efficiency of Gold Nanoshell

By using the theory of quasi-static electrodynamics and fluorescence resonance energy transfer (FRET), local dielectric constant-dependent fluorescence quenching of gold nanoshell has been studied. Because the bonding plasmon coupling mode has stronger resonance light absorption, the bonding plasmon coupling mode-induced FRET has a greater quenching efficiency than that of anti-bonding plasmon coupling mode. Both the bonding and anti-bonding plasmon coupling mode-induced fluorescence quenching are greatly dependent on the local dielectric constant. The quenching efficiency corresponding to anti-bonding plasmon coupling mode increases as the inner core dielectric constant is increased, whereas the quenching efficiency corresponding to bonding plasmon coupling mode increases as the outer surrounding dielectric constant is increased. The corresponding physical origin could be illustrated as follows. The anti-bonding and bonding plasmon coupling modes are primarily composed of the cavity and surface plasmon, respectively. As the result of the increasing outer surrounding/inner core dielectric constant-induced shifting and widening of bonding/anti-bonding plasmonic absorption peak, the corresponding wavelength region with high quenching efficiency value also red shifts and broadens as the dielectric constant of outer surrounding/inner core is increased. Thus, the fluorescence quenching efficiency of gold nanoshell could be fine tuned by altering the local dielectric environment.

Uncovering topographically hidden features in 2D MoSe2 with correlated potential and optical nanoprobes

Developing characterization strategies to better understand nanoscale features in two-dimensional nanomaterials is of crucial importance, as the properties of these materials are many times driven by nanoscale and microscale chemical and structural modifications within the material. For the case of large area monolayer MoSe2 flakes, kelvin probe force microscopy coupled with tip-enhanced photoluminescence was utilized to evaluate such features including internal grain boundaries, edge effects, bilayer contributions, and effects of oxidation/aging, many of which are invisible to topographical mapping. A reduction in surface potential due to n-type behavior was observed at the edge of the flakes as well as near grain boundaries. Potential phase mapping, which corresponds to the local dielectric constant, depicted local biexciton and trion states in optically-active regions of interest such as grain boundaries. Finally, nanoscale surface potential and photoluminescence mapping was performed at several stages of oxidation, revealing that various oxidative states can be evaluated during the aging process. Importantly, all of the characterization performed in this study was non-destructive and rapid, crucial for quality evaluation of an exciting class of two-dimensional nanomaterials.

Mapping the conducting channels formed along extended defects in SrTiO3 by means of scanning near-field optical microscopy

Mixed ionic-electronic-conducting perovskites such as SrTiO3 are promising materials to be employed in efficient energy conversion or information processing. These materials exhibit a self-doping effect related to the formation of oxygen vacancies and electronic charge carriers upon reduction. It has been found that dislocations play a prominent role in this self-doping process, serving as easy reduction sites, which result in the formation of conducting filaments along the dislocations. While this effect has been investigated in detail with theoretical calculations and direct observations using local-conductivity atomic force microscopy, the present work highlights the optical properties of dislocations in SrTiO3 single crystals. Using the change in optical absorption upon reduction as an indicator, two well-defined arrangements of dislocations, namely a bicrystal boundary and a slip band induced by mechanical deformation, are investigated by means of scanning near-field optical microscopy. In both cases, the regions with enhanced dislocation density can be clearly identified as regions with higher optical absorption. Assisted by ab initio calculations, confirming that the agglomeration of oxygen vacancies significantly change the local dielectric constants of the material, the results provide direct evidence that reduced dislocations can be classified as alien matter embedded in the SrTiO3 matrix.

Dynamics of Binding of Lysozyme with Gold Nanoparticles: Corona Formation and its Correlation with a Naked-Eye-Based Colorimetric Approach

The interaction between colloidal gold nanoparticles (AuNPs) and lysozyme (Lyz) has been studied through spectroscopic and microscopic measurements, molecular docking simulation and colorimetric measurements to investigate corona formation and the mechanism, affinity, number of sites and stoichiometry of binding. Molecular docking simulation endorses that, at physiological pH, the interaction between basic NH2group of arginine and citrate ions is predominant than the interaction between citrate ions and the positive surface charge of the bare AuNPs that result in the desorption of citrate ions from Au surfaces and finally, electrostatic interaction between [Formula: see text] group of arginine and positive charge surface of AuNPs results in the adsorption of Lyz on Au surfaces. As observed from Benesi–Hildebrand and fluorescence analyses, the ground state complex formation between Lyz and AuNPs requires several minutes, which is approximately 11 min in the present work to attain stoichiometric ratio 1:1. CD spectra indicate insignificant or no conformational change in the secondary structure of Lyz in the presence of AuNPs. The time variation of LSPR peak position and peak height in the presence of Lyz have been studied extensively through spectroscopic and microscopic measurements. Detailed discussion on the probable time-specific roles of change in local dielectric constant, plasmon coupling and electrostatic interaction on these variations has been presented. Colorimetric change of the AuNPs-Lyz system with time has been analyzed by measuring the red, blue and green color fractions as well as its correlation with the process of corona formation and aggregation has been investigated to propose a novel naked eye colorimetric approach of studying these processes in AuNPs-Lyz system.

Probing of local polarity in poly(methyl methacrylate) with the charge transfer transition in Nile red.

The permittivity of polymers and its spatial distribution play a crucial role in the behavior of thin films, such as those used, e.g., as sensor coatings. In an attempt to develop a conclusive approach to determine these quantities, the polarity of the model polymer poly(methyl methacrylate) (PMMA) in 600 nm thin films on a glass support was probed by the energy of the charge transfer transition in the oxazine dye Nile red (NR) at 25 °C. The absorption and fluorescence spectra of NR were observed to shift to the red with increasing solvent polarity, because of the intramolecular charge transfer character of the optical transition. New types of solvatochromic plots of emission frequency against absorption frequency and vice versa afforded the Onsager radius-free estimation of the ground and excited states dipole moment ratio. With this approach the values of these dipole moments of 11.97 D and 18.30–19.16 D, respectively, were obtained for NR. An effective local dielectric constant of 5.9–8.3 for PMMA thin films was calculated from the solvatochromic plot and the fluorescence maximum of NR observed in the PMMA films. The fluorescence band of NR in the rigid PMMA films shifted to the red by 130 cm−1 with increasing excitation wavelength from 470 to 540 nm, while in a series of liquids the position of the emission maximum of NR remained constant within same range of the excitation wavelength. It is concluded that the fluorescence spectrum of NR in PMMA undergoes inhomogeneous broadening due to different surroundings of NR molecules in the ground state and slow sub-glass transition (Tg) relaxations in PMMA.

Anomalously low dielectric constant of confined water.

The dielectric constant ε of interfacial water has been predicted to be smaller than that of bulk water (ε ≈ 80) because the rotational freedom of water dipoles is expected to decrease near surfaces, yet experimental evidence is lacking. We report local capacitance measurements for water confined between two atomically flat walls separated by various distances down to 1 nanometer. Our experiments reveal the presence of an interfacial layer with vanishingly small polarization such that its out-of-plane ε is only ~2. The electrically dead layer is found to be two to three molecules thick. These results provide much-needed feedback for theories describing water-mediated surface interactions and the behavior of interfacial water, and show a way to investigate the dielectric properties of other fluids and solids under extreme confinement.