Membrane Surface-Enhanced Raman Spectroscopy for Cholesterol-Modified Lipid Systems: Effect of Gold Nanoparticle Size

Plasmonic nano-bowls for monitoring intra-membrane changes in liposomes, and DNA-based nanocarriers in suspension

Programmable nanoscale carriers, such as liposomes and DNA, are readily being explored for personalized medicine or disease prediction and diagnostics. The characterization of these nanocarriers is limited and challenging due to their complex chemical composition. Here, we demonstrate the utilization of surface-enhanced Raman spectroscopy (SERS), which provides a unique molecular fingerprint of the analytes while reducing the detection limit. In this paper, we utilize a silver coated nano-bowl shaped polydimethylsiloxane (PDMS) SERS substrate. The utilization of nano-bowl surface topology enabled the passive trapping of particles by reducing mobility, which results in reproducible SERS signal enhancement. The biological nanoparticles' dwell time in the nano-trap was in the order of minutes, thus allowing SERS spectra to remain in their natural aqueous medium without the need for drying. First, the geometry of the nano-traps was designed considering nanosized bioparticles of 50-150 nm diameter. Further, the systematic investigation of maximum SERS activity was performed using rhodamine 6 G as a probe molecule. The potential of the optimized SERS nano-bowl is shown through distinct spectral features following surface- (polyethylene glycol) and bilayer- (cholesterol) modification of empty liposomes of around 140 nm diameter. Apart from liposomes, the characterization of the highly crosslinked DNA specimens of only 60 nm in diameter was performed. The modification of DNA gel by liposome coating exhibited unique signatures for nitrogenous bases, sugar, and phosphate groups. Further, the unique sensitivity of the proposed SERS substrate displayed distinct spectral signatures for DNA micelles and drug-loaded DNA micelles, carrying valuable information to monitor drug release. In conclusion, the findings of the spectral signatures of a wide range of molecular complexes and chemical morphology of intra-membranes in their natural state highlight the possibilities of using SERS as a sensitive and instantaneous characterization alternative.

A Quantitative Approach to Characterize the Surface Modification on Nanoparticles Based on Localized Dielectric Environments

Nanoparticles (NPs) are utilized for the functionalization of composite materials and nanofluids. Although oxide NPs (e.g., silica (SiO2)) exhibit less dispersibility in organic solvents or polymers due to their hydrophilic surface, the surface modification using silane coupling agents can improve their dispersibility in media with low dielectric constants. Herein, SiO2 NPs were functionalized using octyltriethoxysilane (OTES, C8) and dodecyltriethoxysilane (DTES, C12), wherein the degrees of surface modification of SiO2@C8 and SiO2@C12 were quantitatively evaluated based on the ratio of modifier to surface silanol group (θ) and the volume fraction of organic modifier to total particle volume (ϕR). The variations of surface properties were revealed by analyzing the Hansen solubility parameters (HSP). Particularly, the surface modification using OTES or DTES significantly affected the polarity (δP) of NPs. The local dielectric environments of surface-modified SiO2 NPs were characterized using a solvatochromic dye, Laurdan. By analyzing the peak position of the steady-state emission spectrum of Laurdan in a NP suspension, the apparent dielectric environments surrounding NPs (εapp) were obtained. A good correlation between ϕR and εapp was observed, indicating that ϕR is a reliable quantity for understanding the properties of surface-modified NPs. Furthermore, the generalized polarization (GP) of NPs was investigated. The surface-modified SiO2 NPs with higher ϕR (≥0.15) exhibited GP > 0, suggesting that the modifiers are well-organized on the surface of NPs. The localized dielectric environment surrounding NPs could be predicted by analyzing the volume fraction of nonpolar moieties derived from modifiers. Alternatively, εapp and GP can be utilized for understanding the properties of inorganic-organic hybrid NPs.

Leaflet by Leaflet Synergistic Effects of Antimicrobial Peptides on Bacterial and Mammalian Membrane Models

Antimicrobial peptides (AMPs) offer significant hope in the fight against antibiotic resistance. Operating via a mechanism different from that of antibiotics, they target the microbial membrane and ideally should damage it without impacting mammalian cells. Here, the interactions of two AMPs, magainin 2 and PGLa, and their synergistic effects on bacterial and mammalian membrane models were studied using electrochemical impedance spectroscopy, atomic force microscopy (AFM), and fluorescence correlation spectroscopy. Toroidal pore formation was observed by AFM when the two AMPs were combined, while individually AMP effects were confined to the exterior leaflet of the bacterial membrane analogue. Using microcavity-supported lipid bilayers, the diffusivity of each bilayer leaflet could be studied independently, and we observed that combined, the AMPs penetrate both leaflets of the bacterial model but individually each peptide had a limited impact on the proximal leaflet of the bacterial model. The impact of AMPs on a ternary, mammalian mimetic membrane was much weaker.

Structure and Interaction of Ceramide-Containing Liposomes with Gold Nanoparticles as Characterized by SERS and Cryo-EM

Due to the great potential of surface-enhanced Raman scattering (SERS) as local vibrational probe of lipid-nanostructure interaction in lipid bilayers, it is important to characterize these interactions in detail. The interpretation of SERS data of lipids in living cells requires an understanding of how the molecules interact with gold nanostructures and how intermolecular interactions influence the proximity and contact between lipids and nanoparticles. Ceramide, a sphingolipid that acts as important structural component and regulator of biological function, therefore of interest to probing, lacks a phosphocholine head group that is common to many lipids used in liposome models. SERS spectra of liposomes of a mixture of ceramide, phosphatidic acid, and phosphatidylcholine, as well as of pure ceramide and of the phospholipid mixture are reported. Distinct groups of SERS spectra represent varied contributions of the choline, sphingosine, and phosphate head groups and the structures of the acyl chains. Spectral bands related to the state of order of the membrane and moreover to the amide function of the sphingosine head groups indicate that the gold nanoparticles interact with molecules involved in different intermolecular relations. While cryogenic electron microscopy shows the formation of bilayer liposomes in all preparations, pure ceramide was found to also form supramolecular, concentric stacked and densely packed lamellar, nonliposomal structures. That the formation of such supramolecular assemblies supports the intermolecular interactions of ceramide is indicated by the SERS data. The unique spectral features that are assigned to the ceramide-containing lipid model systems here enable an identification of these molecules in biological systems and allow us to obtain information on their structure and interaction by SERS.

Characterization of entrapment behavior of polyphenols in nanostructured lipid carriers and its effect on their antioxidative activity

Polyphenols are widely used as antioxidant agents to protect human health. Resveratrol, kaempferol, and quercetin have been reported to have potent antioxidant activity; however, these compounds have many problems related to their practical application, such as instability and insolubility. Thus, a nanostructured lipid carrier (NLC) was utilized as a drug delivery system (DDS) to overcome these limitations. This study investigated the particle stability, drug loading performance, and antioxidant activity of polyphenols-incorporated NLCs. The particle size and distribution were suitable for DDS applications, and all the samples demonstrated good stability after 2 months of storage. Based on Raman spectroscopy analysis, polyphenols were successfully encapsulated in NLCs. Quantitative high-performance liquid chromatography analysis indicated that NLCs could load resveratrol more than kaempferol and quercetin. In addition, NLCs have successfully improved all the antioxidant activity per unit concentration of polyphenol (specific antioxidant activity) compared to the free polyphenols. Quercetin-incorporated NLCs showed the highest specific antioxidant activity. This result is the opposite of entrapment efficiency and actual antioxidant activity, most likely influenced by the location of entrapped polyphenol molecules. As it was performed, NLCs are highly recommended to be applied as an antioxidant delivery system.

Multimodal Investigation into the Interaction of Quinacrine with Microcavity-Supported Lipid Bilayers

Quinacrine is a versatile drug that is widely recognized for its antimalarial action through its inhibition of the phospholipase enzyme. It also has antianthelmintic and antiprotozoan activities and is a strong DNA binder that may be used to combat multidrug resistance in cancer. Despite extensive cell-based studies, a detailed understanding of quinacrine's influence on the cell membrane, including permeability, binding, and rearrangement at the molecular level, is lacking. Herein, we apply microcavity-suspended lipid bilayers (MSLBs) as in vitro models of the cell membrane comprising DOPC, DOPC:Chol(3:1), and DOPC:SM:Chol(2:2:1) to investigate the influence of cholesterol and intrinsic phase heterogeneity induced by mixed-lipid composition on the membrane interactions of quinacrine. Using electrochemical impedance spectroscopy (EIS) and surface-enhanced Raman spectroscopy (SERS) as label-free surface-sensitive techniques, we have studied quinacrine interaction and permeability across the different MSLBs. Our EIS data reveal that the drug is permeable through ternary DOPC:SM:Chol and DOPC-only bilayer compositions. In contrast, the binary cholesterol/DOPC membrane arrested permeation, yet the drug binds or intercalates at this membrane as reflected by an increase in membrane impedance. SERS supported the EIS data, which was utilized to gain structural insights into the drug-membrane interaction. Our SERS data also provides a simple but powerful label-free assessment of drug permeation because a significant SERS enhancement of the drug's Raman signature was observed only if the drug accessed the plasmonic interior of the pore cavity passing through the membrane. Fluorescent lifetime correlation spectroscopy (FLCS) provides further biophysical insight, revealing that quinacrine binding increases the lipid diffusivity of DOPC and the ternary membrane while remarkably decreasing the lipid diffusivity of the DOPC:Chol membrane. Overall, because of its adaptability to multimodal approaches, the MSLB platform provides rich and detailed insights into drug-membrane interactions, making it a powerful tool for in vitro drug screening.

Surface enhanced Raman scattering for probing cellular biochemistry

Surface enhanced Raman scattering (SERS) from biomolecules in living cells enables the sensitive, but also very selective, probing of their biochemical composition. This minireview discusses the developments of SERS probing in cells over the past years from the proof-of-principle to observe a biochemical status to the characterization of molecule-nanostructure and molecule-molecule interactions and cellular processes that involve a wide variety of biomolecules and cellular compartments. Progress in applying SERS as a bioanalytical tool in living cells, to gain a better understanding of cellular physiology and to harness the selectivity of SERS, has been achieved by a combination of live cell SERS with several different approaches. They range from organelle targeting, spectroscopy of relevant molecular models, and the optimization of plasmonic nanostructures to the application of machine learning and help us to unify the information from defined biomolecules and from the cell as an extremely complex system.

Detection of Hypertension-Induced Changes in Erythrocytes by SERS Nanosensors

Surface-enhanced Raman spectroscopy (SERS) is a promising tool that can be used in the detection of molecular changes triggered by disease development. Cardiovascular diseases (CVDs) are caused by multiple pathologies originating at the cellular level. The identification of these deteriorations can provide a better understanding of CVD mechanisms, and the monitoring of the identified molecular changes can be employed in the development of novel biosensor tools for early diagnostics. We applied plasmonic SERS nanosensors to assess changes in the properties of erythrocytes under normotensive and hypertensive conditions in the animal model. We found that spontaneous hypertension in rats leads (i) to a decrease in the erythrocyte plasma membrane fluidity and (ii) to a decrease in the mobility of the heme of the membrane-bound hemoglobin. We identified SERS parameters that can be used to detect pathological changes in the plasma membrane and submembrane region of erythrocytes.

Design and Application of Near-Infrared Nanomaterial-Liposome Hybrid Nanocarriers for Cancer Photothermal Therapy

Liposomes are attractive carriers for targeted and controlled drug delivery receiving increasing attention in cancer photothermal therapy. However, the field of creating near-infrared nanomaterial-liposome hybrid nanocarriers (NIRN-Lips) is relatively little understood. The hybrid nanocarriers combine the dual superiority of nanomaterials and liposomes, with more stable particles, enhanced photoluminescence, higher tumor permeability, better tumor-targeted drug delivery, stimulus-responsive drug release, and thus exhibiting better anti-tumor efficacy. Herein, this review covers the liposomes supported various types of near-infrared nanomaterials, including gold-based nanomaterials, carbon-based nanomaterials, and semiconductor quantum dots. Specifically, the NIRN-Lips are described in terms of their feature, synthesis, and drug-release mechanism. The design considerations of NIRN-Lips are highlighted. Further, we briefly introduced the photothermal conversion mechanism of NIRNs and the cell death mechanism induced by photothermal therapy. Subsequently, we provided a brief conclusion of NIRNs-Lips applied in cancer photothermal therapy. Finally, we discussed a synopsis of associated challenges and future perspectives for the applications of NIRN-Lips in cancer photothermal therapy.

Spectroscopy of model-membrane liposome-protein systems: complementarity of linear dichroism, circular dichroism, fluorescence and SERS

A range of membrane models have been developed to study components of cellular systems. Lipid vesicles or liposomes are one such artificial membrane model which mimics many properties of the biological system: they are lipid bilayers composed of one or more lipids to which other molecules can associate. Liposomes are thus ideal to study the roles of cellular lipids and their interactions with other membrane components to understand a wide range of cellular processes including membrane disruption, membrane transport and catalytic activity. Although liposomes are much simpler than cellular membranes, they are still challenging to study and a variety of complementary techniques are needed. In this review article, we consider several currently used analytical methods for spectroscopic measurements of unilamellar liposomes and their interaction with proteins and peptides. Among the variety of spectroscopic techniques seeing increasing application, we have chosen to discuss: fluorescence based techniques such as FRET (fluorescence resonance energy transfer) and FRAP (fluorescence recovery after photobleaching), that are used to identify localisation and dynamics of molecules in the membrane; circular dichroism (CD) and linear dichroism (LD) for conformational and orientation changes of proteins on membrane binding; and SERS (Surface Enhanced Raman Spectroscopy) as a rapidly developing ultrasensitive technique for site-selective molecular characterisation. The review contains brief theoretical basics of the listed techniques and recent examples of their successful applications for membrane studies.

Plasmonic nanobowtiefluidic device for sensitive detection of glioma extracellular vesicles by Raman spectrometry

Cancer cells shed into biofluids extracellular vesicles (EVs) - nanoscale membrane particles carrying diagnostic information. EVs shed by heterogeneous populations of tumor cells offer a unique opportunity to access biologically important aspects of disease complexity. Glioblastoma (GBM) exemplifies cancers that are incurable, because their temporal dynamics and molecular complexity evade standard diagnostic methods and confound therapeutic efforts. Liquid biopsy based on EVs offers unprecedented real-time access to complex tumour signatures, but it is not used clinically due to inefficient testing methods. We report on a nanostructured microfluidic-device that employs SERS for unambiguous identification of EVs from different GBM cell populations. The device features fabless plasmonic nanobowties for label-free and non-immunological SERS detection of EVs. This nanobowtiefluidic device combines the advanced characteristics of plasmonic nanobowties with a high throughput sample-delivery system for concentration of the analytes in the vicinity of the detection site. We showed theoretically and experimentally that the fluidic device assists the monolayer distribution of the EVs, which dramatically increase the probability of EV's existence in the laser illumination area. In addition, the optimized fabless nanobowtie structures with an average electric field enhancement factor of 9 × 10⁵ achieve distinguishable and high intensity SERS signals. Using the nanobowtiefluidic and micro-Raman equipment, we were able to distinguish a library of peaks expressed in GBM EV subpopulations from two distinct glioblastoma cell lines (U373, U87) and compare them to those of non-cancerous glial EVs (NHA) and artificial homogenous vesicles (e.g. DOPC/Chol). This cost-effective and easy-to-fabricate SERS platform and a portable sample-delivery system for discerning the sub-population of GBM EVs and non-cancerous glial EVs may have broader applications to different types of cancer cells and their molecular/oncogenic signature.

Surface Plasmon Localization-Based Super-resolved Raman Microscopy

We investigate label-free measurement of molecular distribution by super-resolved Raman microscopy using surface plasmon (SP) localization. Localized SP was formed with plasmonic nanopost arrays (PNAs) for measurement of the molecular distribution in HeLa cells. Compared with conventional Raman microscopy on gold thin films, PNAs induce a localized near-field, which allows for the enhancement of the peak signal-to-noise ratio by as much as 4.5 dB in the Raman shifts. Super-resolved distributions of aromatic amino acids and lipids (C-C stretching and C-H₂ twist mode) as targets in HeLa cells were obtained after image reconstruction. Results show almost 4-fold improvement on average in the lateral precision over conventional diffraction-limited Raman microscopy images. Combined with axial imaging in an evanescent field, the results suggest an improvement in optical resolution due to superlocalized light volume by more than an order of magnitude over that of conventional diffraction-limited Raman microscopy.

Targeting self-assembly peptide for inhibiting breast tumor progression and metastasis

The ubiquitous interactions between tumor cells and the surrounding microenvironment contribute to tumor metastasis, interrupting these communications has, therefore, a great potential for antimetastasis therapy. Here, we describe an in situ self-assembly strategy that limits direct contact between tumor cells and the tumor microenvironment (TME). In this strategy, the Lys-Leu-Val-Phe-Phe (KLVFF) peptide motifs are targeted to the tumor by hyaluronic acid (HA) functionalized liposomes and spontaneously undergo self-assembly to form nanofibers with a net-like structure wrapping around tumor cells. The fibrous nanostructures bury the membrane protrusions and thus hinder the migration and invasion of tumor cells, especially the transmigration through the fenestrated endothelium. The nanofibril coatings on tumor cells significantly block tumor cells induced platelet aggregation in vitro and prevent the adhesion of platelet around circulating tumor cells (CTCs) in vivo, thus limit the pro-metastasis effect of platelets and prevent the early metastasis. Furthermore, the nano-nets stably retain in the primary tumor site for over 72 h and effectively prevent the activation of intratumoral platelet, which suppress tumor progression and the spontaneous lung metastasis in 4T1 breast cancer mice model. Our study paves a promising avenue to combat tumor metastasis by regulating the interactions between tumor cells and the TME.