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Within cell plasma membranes, unsaturated lipids are asymmetrically distributed over the inner and outer leaflets, offering an attractive local structural feature. However, the mechanism to keep lipid transmembrane asymmetry and the closely related transmembrane movement (flip-flop) for unsaturated lipids remain poorly understood. Here, we applied sum frequency generation vibrational spectroscopy to investigate this lipid transmembrane asymmetry upon mimicking the cell membrane homeostatic processes. On the one hand, unsaturated lipids were found to hinder the flip-flop process and preserve lipid transmembrane asymmetry in model cell membranes, owing to the steric hindrance caused by their bent tails. On the other hand, local unsaturated lipids in the mixed unsaturated/saturated lipid bilayer were conducive to the formation of the local asymmetry. Therefore, lipid unsaturation can be recognized as an intrinsic key factor to form and maintain lipid transmembrane asymmetry in cell membranes. [ABSTRACT FROM AUTHOR]

Development of new strategies with high antimicrobial capability against complex bacterial infections is still challenging. Herein, a three‐in‐one synergistic antimicrobial platform is presented based on gallium–carbenicillin framework coated defect‐rich hollow TiO2 nanoshells (H‐TiO2−x@MOF), for simultaneous eradicating methicillin‐resistant Staphylococcus aureus (MRSA) and Pseudomonas aeruginosa (PA) in complex infections, which are the two most common bacteria in wounds. The metal gallium can disrupt bacterial antioxidation system using a "Trojan horse" strategy by substituting iron in the antioxidant enzymes, and finally increase bacterial susceptibility to oxidants. Meanwhile, oxygen‐deficient hollow TiO2−x nanoshells (H‐TiO2−x NSs) can efficiently bind to the bacteria and promote local generation of abundant reactive oxygen species (ROS) under visible‐light irradiation. Thus, the combination of gallium (antioxidant enzyme inhibitor) and black H‐TiO2−x NSs (ROS generator) constitutes a photocatalyzed oxidative stress amplifier that can boost ROS accumulation to destroy pathogens thoroughly. In addition, carbenicillin (Car), as the organic ligand coordinated to gallium ion, also acts as a broad‐spectrum antibacterial agent against PA and shares the responsibility for combating complex infections simultaneously. In view of the superior antibacterial ability, accelerated healing of infected wounds, and good biosafety, the H‐TiO2−x@MOF potentially provides an alternative antibacterial agent to combat complex bacterial infections. [ABSTRACT FROM AUTHOR]

Distinct annexin V binding behaviours in Ca2+-dependent and Ca2+-independent cases were comparatively investigated using sum frequency generation vibrational spectroscopy. It was discovered that binding affected the molecular arrangement of both membrane leaflets, and the initial Ca2+-independent binding went through a transition with annexin V reorientation to a more stable state upon adding Ca2+. [ABSTRACT FROM AUTHOR]

Silica or calcium fluoride (CaF 2 ) substrate-supported poly(methyl methacrylate) (PMMA) thin films as insulating layers are commonly used in photoelectric/photovoltaic devices to improve the efficiency or stability of these devices. However, a comparative investigation of molecular structures at buried PMMA/silica and PMMA/CaF 2 interfaces under thermal stimuli remains unexplored. In this study, we qualitatively and quantitatively revealed different molecular orderings and orientations of PMMA at two interfaces before and after annealing using sum frequency generation (SFG) vibrational spectroscopy. SFG vibrations were carefully assigned by using various deuterated PMMAs. SFG results indicated that, at the buried PMMA/silica interface, the side OCH 3 groups were prone to lie down before annealing and tended to stand up after annealing. In contrast, the case was the opposite at the buried PMMA/CaF 2 interface. The relative hydrophobicity/hydrophilicity of the two substrates and the developed hydrogen bonds upon annealing at the buried PMMA/silica interface, which is absent at the CaF 2 surface, are believed to be the driving forces for different interfacial molecular structures. This study benefits the molecular-level understanding of the interfacial local structural relaxation of polymers at buried interfaces and the rational design of photoelectric/photovoltaic devices from the molecular level.

In this work, we prepared quaternized carbon dots (CDs) with simultaneous antibacterial and bacterial differentiation capabilities using a simple carboxyl-amine reaction between lauryl betaine and amine-functionalized CDs. The obtained quaternized CDs have several fascinating properties/abilities: (1) A long fluorescence emission wavelength ensures the exceptional bacterial imaging capability, including the super-resolution imaging ability; (2) the polarity-sensitive fluorescence emission property leads to significantly enhanced fluorescence when the quaternized CDs interact with bacteria; (3) the presence of both hydrophobic hydrocarbon chains and positively charged quaternary ammonium groups makes the CDs selectively attach to Gram-positive bacteria, realizing the bacterial differentiation; (4) excellent antimicrobial activity is seen against Gram-positive bacteria with a minimum inhibitory concentration of 8 μg/mL for Staphylococcus aureus. Besides, the quaternized CDs are highly stable in various aqueous solutions and exhibit negligible cytotoxicity, suggesting that they hold great promise for clinical applications. Compared to the traditional Gram staining method, the selective Gram-positive bacterial imaging achieved by the quaternized CDs provides a much simpler and faster method for bacterial differentiation. In summary, by combining selective Gram-positive bacterial recognition, super-resolution imaging, and exceptional antibacterial activity into a single system, the quaternized CDs represent a novel kind of metal-free nanoparticle-based antibiotics for antibacterial application and a new type of reagent for efficient bacterial differentiation.

Sum frequency generation (SFG) vibrational spectroscopy has been widely employed to investigate molecular structures of biological surfaces and interfaces including model cell membranes. A variety of lipid monolayers or bilayers serving as model cell membranes and their interactions with many different molecules have been extensively studied using SFG. Here, we conducted an in-depth investigation on polarization-dependent SFG signals collected from interfacial lipid monolayers using different experimental geometries, i.e., the prism geometry (total internal reflection) and the window geometry (external reflection). The different SFG spectral features of interfacial lipid monolayers detected using different experimental geometries are due to the interplay between the varied Fresnel coefficients and second-order nonlinear susceptibility tensor terms of different vibrational modes (i.e., ss and as modes of methyl groups), which were analyzed in detail in this study. Therefore, understanding the interplay between the interfacial Fresnel coefficients and χ((2)) tensors is a prerequisite for correctly understanding the SFG spectral features with respect to different experimental geometries. More importantly, the derived information in this paper should not be limited to the methyl groups with a C3v symmetry; valid extension to interfacial functional groups with different molecular symmetries and even chiral interfaces could be expected.