Impact of crystal structure of silica nanoparticles on Rhodamine 6G adsorption : a molecular dynamics study

Understanding the mechanism of adsorption of Rhodamine 6G (R6G) to various crystal structures of silica nanoparticles (SNPs) is important to elucidate the impact of the dye size when measuring the size of the dye-SNP complex via the time-resolved fluorescence anisotropy method. In this work, molecular dynamics (MD) simulations were used to get an insight into the R6G adsorption process, which cannot be observed using experimental methods. It was found that at low pH α-Cristobalite structured SNPs have a strong affinity to R6G, however at high pH more surface silanol groups undergo ionization when compared with α-Quartz, preventing the adsorption. Therefore, α-Quartz structured SNPs are more suitable for R6G adsorption at high pH, than α-Cristobalite ones. Furthermore, it was found that stable adsorption can occur only when the R6G xanthene core is oriented flat with respect to the SNP surface, indicating that the dye size does not contribute significantly to the measured size of the dye-SNP complex. The requirement of correct dipole moment orientation indicates that only one R6G molecule can adsorb on any size SNP and the R6G layer formation on SNP is not possible. Moreover, the dimerization process of R6G and its competition with the adsorption has been explored. It has been shown that the highest stable R6G aggregate is a dimer, in this form R6G does not adsorb to the SNPs. Finally, using Steered Molecular Dynamics (SMD) with constant velocity pulling, the binding energies of R6G dimers and R6G complexes with both α-Quartz and α-Cristobalite SNPs of 40 Å diameter were estimated. These confirm that R6G adsorption is most stable on 40 Å α-Quartz at pH7, although dimerization is equally possible

Sodium silicate particle size measurements using time-resolved fluorescence anisotropy

Sodium silicates are versatile inorganic chemicals produced by combining silica sand and soda ash (sodium carbonate) under high temperature. When in aqueous solution, they are often used in coating and bonding applications. Additionally, they exhibit a range of attractive characteristics, such as being odorless and non-toxic, high strength and rigidity, resistance to high temperatures and low-cost [1].The important characteristics of silicates are the correlation between the ratio of silica to soda concentrations and the size of the species. Traditionally, the particle sizes of nanoparticles are determined using methods such as Dynamic Light Scattering (DLS) [2], Small-Angle X-Ray Scattering (SAXS) [3], Small Angle Neutron Scattering (SANS) [4] and Transmission Electron Microscopy (TEM) [5]. All these methods are far from ideal and have significant drawbacks: DLS becomes difficult for particles below 10 nm, SAXS and SANS are expensive and complex, and TEM requires complex sample preparations which can lead to alterations of particle sizes [6-8].Here, we present a new way of determining the particle sizes of sodium silicate liquors at high pH using time-resolved fluorescence anisotropy. Different from previous approach of using a single dye label, two fluorescent labels were used in this work [9,10]. Rotational times of the non-binding rhodamine B and electrostatically binding rhodamine 6G were used to determine the medium microviscosity and the silicate particle radius, respectively. This approach of using two dyes ensures that the microviscosity stays accurate in time, unlike in the case when a single dye was used. Applying this method to samples of various pH (prepared by diluting the stock solution of silicate to the concentrations of NaOH ranging from 0.2M to 2M) and different temperatures (10°C to 55°C), therecovered average particle size was found to have an upper limit of 7.0±1.2

Sodium silicate particle size measurements using time-resolved fluorescence anisotropy

We present a development of the method for determining the particle sizes in sodium silicate liquors at high pH by using the measurement of time-resolved fluorescence anisotropy. Rather than the previous approach of using a single dye label, we demonstrate the use and advantages of using two fluorescent labels. Rotational times of the non-binding rhodamine B and electrostatically binding rhodamine 6G are used to independently determine the medium microviscosity and the silicate particle radius, respectively. The anisotropy measurements were performed on the range of samples prepared by diluting the stock solution of silicate to the concentrations ranging between 0.2M and 2M of NaOH and on the stock solution at different temperatures. The recovered average particle size has an upper limit of 7.0±1.2Å

Bovine serum albumin as a platform for designing biologically active nanocarriers : experimental and computational studies

Due to the specificity of their structure, protein systems are adapted to carry various ligands. The structure of many proteins potentially allows for two types of immobilization of a therapeutic agent, either on the outer surface of the protein or within the protein structure. The existence of two active sites in BSA’s structure, the so-called Sudlow I and II, was confirmed. The conducted research involved determining the effectiveness of BSA as a potential carrier of 5-fluorouracil (5FU). 5-fluorouracil is a broad-spectrum anticancer drug targeting solid tumors. The research was carried out to estimate the physicochemical properties of the system using complementary measurement techniques. The optimization of the complex formation conditions made it possible to obtain significant correlations between the form of the drug and the effective localization of the active substance in the structure of the protein molecule. The presence of two amino groups in the 5FU structure contributes to the deprotonation of the molecule at high pH values (pH > 8) and the transition to the anionic form (AN1 and AN3). To investigate the binding affinity of the tautomeric form with BSA, UV-vis absorption, fluorescence quenching, zeta potential, QCM-D, and CD spectroscopic studies were performed. The experimental research was supported by molecular dynamics (MD) simulations and molecular docking. The simulations confirm the potential location of 5FU tautomers inside the BSA structure and on its surface

Dendrimer platforms for targeted doxorubicin delivery : physicochemical properties in context of biological responses

The unique structure of G4.0 PAMAM dendrimers allows a drug to be enclosed in internal spaces or immobilized on the surface. In the conducted research, the conditions for the formation of the active G4.0 PAMAM complex with doxorubicin hydrochloride (DOX) were optimized. The physicochemical properties of the system were monitored using dynamic light scattering (DLS), circular dichroism (CD), and fluorescence spectroscopy. The Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D) method was chosen to determine the preferential conditions for the complex formation. The highest binding efficiency of the drug to the cationic dendrimer was observed under basic conditions when the DOX molecule was deprotonated. The decrease in the zeta potential of the complex confirms that DOX immobilizes through electrostatic interaction with the carrier’s surface amine groups. The binding constants were determined from the fluorescence quenching of the DOX molecule in the presence of G4.0 PAMAM. The two-fold way of binding doxorubicin in the structure of dendrimers was visible in the Isothermal calorimetry (ITC) isotherm. Fluorescence spectra and release curves identified the reversible binding of DOX to the nanocarrier. Among the selected cancer cells, the most promising anticancer activity of the G4.0-DOX complex was observed in A375 malignant melanoma cells. Moreover, the preferred intracellular location of the complexes concerning the free drug was found, which is essential from a therapeutic point of view

Nanoparticle metrology of silicates using time-resolved multiplexed dye fluorescence anisotropy, small angle x-ray scattering and molecular dynamics simulations

We investigate the nanometrology of sub-nanometre particle sizes in industrially manufactured sodium silicate liquors at high pH using time-resolved fluorescence anisotropy. Rather than the previous approach of using a single dye label, we investigate and quantify the advantages and limitations of multiplexing two fluorescent dye labels. Rotational times of the non-binding rhodamine B and adsorbing rhodamine 6G dyes are used to independently determine the medium microviscosity and the silicate particle radius, respectively. The anisotropy measurements were performed on the range of samples prepared by diluting the stock solution of silicate to concentrations ranging between 0.2 M and 2 M of NaOH and on the stock solution at different temperatures. Additionally, it was shown that the particle size can also be measured using a single excitation wavelength when both dyes are present in the sample. The recovered average particle size has an upper limit of 7.0 ± 1.2 Å. The obtained results were further verified using small-angle X-ray scattering, with the recovered particle size equal to 6.50 ± 0.08 Å. To disclose the impact of the dye label on the measured complex size, we further investigated the adsorption state of rhodamine 6G on silica nanoparticles using molecular dynamics simulations, which showed that the size contribution is strongly impacted by the size of the nanoparticle of interest. In the case of the higher radius of curvature (less curved) of larger particles, the size contribution of the dye label is below 10%, while in the case of smaller and more curved particles, the contribution increases significantly, which also suggests that the particles of interest might not be perfectly spherical

Impact of the crystal structure of silica nanoparticles on Rhodamine 6G adsorption

Silicon is one of the most abundant elements on Earth with around 78% of Earth’s crust consisting of varioussilicon and oxygen compounds [1]. Due to this, silica nanoparticles (SNPs) are widely used nanostructures fordrug delivery, bonding and coating applications and others [2].The properties of nanoparticles strongly correlate with their size hence it is critical to have an accurateway of measuring it. Commonly used techniques such as small angle x-ray scattering (SAXS), transmissionelectron microscopy (TEM) or dynamic light scattering (DLS) have drawbacks, such as being expensive andrequiring complex sample preparation. Additionally, they might be inaccurate for particles under 10 nm size.Potential methods that can be used to measure sizes of such constructs are time-resolved fluorescenceanisotropy [3], and fluorescence recovery after photobleaching (FRAP) however, due to the size of the systemit is impossible to determine experimentally how the dye is oriented on the SNP surface. As a result, its contri-bution to the measured complex size is unknown. Fortunately, the dye and SNP interaction mechanism can bestudied using computational methods, such as molecular dynamics, which allow full insight into such processeson an atomistic scale.In this work we used molecular dynamics simulations to get an insight into the rhodamine 6G (R6G) ad-sorption process to assess the most favourable conditions for successful adsorption and determine the impactof the dye to the measured complex size. Furthermore, we found that due to the geometric constraints and therequirement of correct dipole moment orientation, only one R6G molecule can adsorb on any sized SNP, andthe R6G layer formation on the nanoparticle surface is not possible. Similar restrictions lead to the fact that thehighest stable R6G oligomer is a dimer [4]

Estimating binding energies of π-stacked aromatic dimers using force field-driven molecular dynamics

π–π stacking are omnipresent interactions, crucial in many areas of chemistry, and often studied using quantum chemical methods. Here, we report a simple and computationally efficient method of estimating the binding energies of stacked polycyclic aromatic hydrocarbons based on steered molecular dynamics. This method leverages the force field parameters for accurate calculation. The presented results show good agreement with those obtained through DFT at the ωB97X-D3/cc-pVQZ level of theory. It is demonstrated that this force field-driven SMD method can be applied to other aromatic molecules, allowing insight into the complexity of the stacking interactions and, more importantly, reporting π–π stacking energy values with reasonable precision

Impact of the Crystal Structure of Silica Nanoparticles on Rhodamine 6G Adsorption: A Molecular Dynamics Study

Understanding the mechanism of adsorption of Rhodamine 6G (R6G) to various crystal structures of silica nanoparticles (SNPs) is important to elucidate the impact of dye size when measuring the size of the dye–SNP complex via the time-resolved fluorescence anisotropy method. In this work, molecular dynamics (MD) simulations were used to get an insight into the R6G adsorption process, which cannot be observed using experimental methods. It was found that at low pH, α-Cristobalite structured SNPs have a strong affinity to R6G; however, at high pH, more surface silanol groups undergo ionization when compared with α-Quartz, preventing the adsorption. Therefore, α-Quartz structured SNPs are more suitable for R6G adsorption at high pH than the α-Cristobalite ones. Furthermore, it was found that stable adsorption can occur only when the R6G xanthene core is oriented flat with respect to the SNP surface, indicating that the dye size does not contribute significantly to the measured size of the dye–SNP complex. The requirement of correct dipole moment orientation indicates that only one R6G molecule can adsorb on any sized SNP, and the R6G layer formation on SNP is not possible. Moreover, the dimerization process of R6G and its competition with the adsorption has been explored. It has been shown that the highest stable R6G aggregate is a dimer, and in this form, R6G does not adsorb to SNPs. Finally, using steered molecular dynamics (SMD) with constant-velocity pulling, the binding energies of R6G dimers and R6G complexes with both α-Quartz and α-Cristobalite SNPs of 40 Å diameter were estimated. These confirm that R6G adsorption is most stable on 40 Å α-Quartz at pH 7, although dimerization is equally possible