Salt-Induced Adsorption and Rupture of Liposomes on Microplastics.

Microplastics have attracted considerable attention because of concerns regarding their environmental risks to living systems. The interaction between the lipid bilayer and microplastics is important for examining the potential harm to biological membranes in the presence of microplastics. In addition, membrane coatings may change the surface and colloidal properties of microplastics. Herein, phosphatidylcholine (PC) lipids, whose headgroup is most common in cell membranes, were used as model lipids. The adsorption and rupture of PC liposomes on microplastics were systematically studied. We found that divalent metal ions, such as Mg2+ and Ca2+, facilitate liposome adsorption onto microplastics and induce 40-55% liposome leakage at 2.5 mM. In contrast, to achieve a similar effect, 300 mM Na+ was required. Adsorption and rupture followed the same metal concentration requirements, suggesting that liposome adsorption was the rate-limiting step. After adsorption with liposomes, microplastics became more hydrophilic and were better dispersed in water. A similar behavior was observed for all five types of tested microplastics, including PP, PE, PVC, PET, and PS. Leakage also occurred in ocean water. This study provides fundamental insights into the interactions between liposomes and microplastics and has implications for the colloidal and transport properties of microplastics.

Thin Layer Chromatography-Freeze Surface-Enhanced Raman Spectroscopy: A Powerful Tool for Monitoring Synthetic Reactions.

Thin layer chromatography (TLC) is widely used to confirm the formation of the target compound in chemical synthesis. The key issue in TLC is spot identification as it primarily relies on retention factors. Coupling of TLC with surface-enhanced Raman spectroscopy (SERS), which gives direct molecular information, is an appropriate choice, to overcome this challenge. However, interference from the stationary phase and impurities on the nanoparticles added for SERS measurements significantly degrades TLC-SERS efficiency. It was found that freezing effectively eliminates such interferences and dramatically improves the performance of TLC-SERS. In this study, TLC-freeze SERS is applied to the monitoring of four chemically important reactions. The proposed method can identify the product and side-products of similar structures, detect compounds with high sensitivity, and provide information of quantities that allows the reliable determination of the reaction time based on kinetic analysis.

Freeze Surface-Enhanced Raman Scattering Coupled with Thin-Layer Chromatography: Pesticide Detection and Quantification Case.

Thin-layer chromatography (TLC) is widely used in various branches of chemical science to separate components in complex mixtures because of its simplicity. In most cases, analyte spots are visually detected by fluorescence, and the retention factor (Rf) is determined from the distance traveled by the analyte. Further characterizations are often necessary to identify separated chemicals because molecular information other than Rf is not available. Surface-enhanced Raman scattering (SERS) has been coupled with TLC to complement molecular information. In previously reported TLC-SERS, metal nanoparticle suspension was dropped onto analyte spots to obtain SERS spectra. This approach is simple and efficient for SERS measurements on the TLC plate but has limited sensitivity for several reasons, such as the low solubility of analytes in the dropped solution, difficult control of nanoparticle aggregation, and interference from the stationary phase. We recently showed that freezing enhances SERS sensitivity by a factor of ∼103. Freezing simultaneously concentrates analytes and silver nanoparticles (AgNPs) in a freeze concentrated solution, where aggregation of AgNPs is facilitated, allowing sensitive freeze SERS (FSERS) measurements. Here, we discuss FSERS measurements on TLC plates to demonstrate the superiority of this combination, i.e. TLC-FSERS. Freezing enhances SERS sensitivity by freeze concentration and facilitated aggregation of AgNPs and, in addition, eliminates interference from the stationary phase. Under the optimized condition, TLC-FSERS enables the on-site detection of pesticides at the nM level. The use of the SERS signal from adenine added as the internal standard allows us to quantify pesticides. Applications to a commercial green tea beverage are also demonstrated.

Surface-enhanced Raman scattering of DNA bases using frozen silver nanoparticle dispersion as a platform.

Raman spectroscopy is a powerful method to characterize molecules in various media. Although surface-enhanced Raman scattering (SERS) is often employed to compensate for the intrinsically poor sensitivity of Raman spectroscopy, there remain serious tasks, such as simple preparations of SERS substrates, sensitivity control, and reproducible measurements. Here, we propose freezing as an efficient way to overcome these problems in SERS measurements using DNA bases as model targets. Solutes are expelled from ice crystals and concentrated in the liquid phase upon freezing. Silver nanoparticles (AgNPs) are also concentrated in the liquid phase to aggregate with Raman target analytes. The SERS signal intensity is maximized when the AgNP concentration exceeds the critical aggregation value. Freezing allows up to 5000 times enhancements of the SERS signal. Thus, an efficient SERS platform is prepared by simple freezing. The simultaneous detection of four DNA bases effectively eliminates variations of signal intensities and allows the reliable determination of concentration ratios.

Quantification using statistical parameters derived from signal intensity distributions in surface enhanced Raman scattering (SERS).

Raman spectroscopy is a powerful method, which provides information on molecular structures, conformations, interactions etc. However, its applications are severely restricted because of low sensitivity. Although surface enhanced Raman scattering (SERS) significantly enhances sensitivity and enables single-molecular detection, quantification by this method is still challenging because of large signal fluctuations. In the present study, the signal intensity distributions (SIDs) in SERS of adenine and thymine on the silver nanoparticle (AgNP) platform are analyzed based on more than 10000 spectra to pursue the possibility of SERS quantification. The signals always involve large fluctuations but show statistically relevant patterns. SIDs are well represented by the exponentially modified Gaussian function, which is characterized by reproducible parameters. Thus, robust quantification is feasible using the parameters derived from the SIDs. At least 200 spectra for a given concentration are necessary to derive reproducible parameter values from the SID. The mean signal intensity determined from the SIDs is proportional to the adenine concentration in the range of 10-75 µM. However, this parameter becomes independent of the adenine concentration in the lower concentration range. In such concentrations, minor events, which give distinct SERS spectra, occasionally occur but have only marginal impacts on the mean signal intensity. The corrected standard deviation of the SID, which is estimated from the complementary error function, well represents the minor events and provides a clear correlation with the concentration in the range of 0.5-7.5 µM. Furthermore, the quantification in the nanomolar range is made possible by the incorporation of sample freezing, which enables to enrich target analytes and AgNPs in a liquid phase confined by ice.

Hydration of bromide at reverse micelle interfaces studied by X-ray absorption fine structure.

Nanoconfined water exhibits various interesting properties, which are not only of fundamental importance but also of practical use. Because reverse micelles (RMs) provide versatile ways to prepare nanoconfined water, the understanding of their physicochemical properties is essential for developing efficient applications. Although the water properties in the RMs could be affected by its interaction with the RM interface, the details have not been well understood. This study focuses on the local structures of Br- in hexadecyltrimethylammonium bromide (HTAB) RMs formed in chloroform and 10% hexanol/heptane. The dependence in Br- hydration on the molar ratio of water to HTAB (w) is investigated using X-ray absorption fine structure (XAFS). These systems cover a wide range of w values (0-30) and allow us to study the impact of this parameter on the local structure of Br- at the RM interface, which comprises water, surfactant headgroups, and organic solvent components. The presence of multiple scattering paths complicates the XAFS spectra and makes it difficult to analyze them using standard fitting methods. The linear combination of the spectra corresponding to the individual scattering paths captures the molecular processes that occur at the RM interface upon increasing w. The maximum hydration number of Br- is found to be 4.5 at w > 15, suggesting that although most of the ions remain at the interface as partly hydrated ions, some of them dissociate as completely hydrated ones.