Structural Analysis by Enhanced Raman Scattering

Noble Metal Nanoparticle-Based Photothermal Therapy: Development and Application in Effective Cancer Therapy

Photothermal therapy (PTT) is a promising cancer therapy modality with significant advantages such as precise targeting, convenient drug delivery, better efficacy, and minimal adverse effects. Photothermal therapy effectively absorbs the photothermal transducers in the near-infrared region (NIR), which induces the photothermal effect to work. Although PTT has a better role in tumor therapy, it also suffers from low photothermal conversion efficiency, biosafety, and incomplete tumor elimination. Therefore, the use of nanomaterials themselves as photosensitizers, the targeted modification of nanomaterials to improve targeting efficiency, or the combined use of nanomaterials with other therapies can improve the therapeutic effects and reduce side effects. Notably, noble metal nanomaterials have attracted much attention in PTT because they have strong surface plasmon resonance and an effective absorbance light at specific near-infrared wavelengths. Therefore, they can be used as excellent photosensitizers to mediate photothermal conversion and improve its efficiency. This paper provides a comprehensive review of the key role played by noble metal nanomaterials in tumor photothermal therapy. It also describes the major challenges encountered during the implementation of photothermal therapy.

Tunable lipid-coated nanoporous silver sheet for characterization of protein-membrane interactions by surface-enhanced Raman scattering (SERS)

Membrane environments affect protein structures and functions through protein-membrane interactions in a wide range of important biological processes. To better study the effects from the lipid's hydrophilic and hydrophobic interaction with protein on different membrane regions, we developed the lipid-coated nanoporous silver sheets to provide tunable supported lipid monolayer/bilayer environments for in situ surface-enhanced Raman vibrational spectroscopy (SERS) characterizations. Under the controllable surface pressure, lipid monolayer/bilayer was coated along the microscopic curved surface of nanoporous silver sheets to serve as a cell membrane mimic as well as a barrier to avoid protein denaturation while empowering the high SERS enhancements from the underlying metallic bases allowing detection sensitivity at low physiological concentrations. Moreover, we fine-tuned the lipid packing density and controlled the orientation of the deposited lipid bilayers and monolayers to directly monitor the protein structures upon interactions with various membrane parts/positions. Our results indicate that lysozyme adopted the α-helical structure in both hydrophilic and hydrophobic interaction with lipid membrane. Interestingly, alpha-synuclein folded into the α-helical structure on the negatively charged lipid heads, whereas the hydrophobic lipid tails induced the β-sheet structural conversion of alpha-synuclein originated from its unstructured monomers. These direct observations on protein hydrophilic and hydrophobic interaction with lipid membrane might provide profound insights into the formation of the β-sheet-containing alpha-synuclein oligomers for further membrane disruptions and amyloid genesis associated with Parkinson's disease. Hence, with the controllability and tunability of lipid environments, our platform holds great promise for more general applications in investigating the influences from membranes and the correlative structures of proteins under both hydrophilic and hydrophobic effects.

Breaking the nanoparticle's dispersible limit via rotatable surface ligands

Achieving versatile dispersion of nanoparticles in a broad range of solvents (e.g., water, oil, and biofluids) without repeatedly recourse to chemical modifications are desirable in optoelectronic devices, self-assembly, sensing, and biomedical fields. However, such a target is limited by the strategies used to decorate nanoparticle's surface properties, leading to a narrow range of solvents for existing nanoparticles. Here we report a concept to break the nanoparticle's dispersible limit via electrochemically anchoring surface ligands capable of sensing the surrounding liquid medium and rotating to adapt to it, immediately forming stable dispersions in a wide range of solvents (polar and nonpolar, biofluids, etc.). Moreover, the smart nanoparticles can be continuously electrodeposited in the electrolyte, overcoming the electrode surface-confined low throughput limitation of conventional electrodeposition methods. The anomalous dispersive property of the smart Ag nanoparticles enables them to resist bacteria secreted species-induced aggregation and the structural similarity of the surface ligands to that of the bacterial membrane assists them to enter the bacteria, leading to high antibacterial activity. The simple but massive fabrication process and the enhanced dispersion properties offer great application opportunities to the smart nanoparticles in diverse fields.

Pinpointing Alkane Chain Length, Saturation, and Double Bond Regio- and Stereoisomers by Liquid Interfacial Plasmonic Enhanced Raman Spectroscopy

The lipids with a rich diversity of isomers face a formidable challenge in comprehensive structural analysis. The commonly used mass spectrometry-based techniques usually require a considerable number of molecules with sophisticated chemical derivatization or ion mobility separation, but the co-existing of structurally similar isomers often makes the distinction impossible. Here, we develop an alternative powerful liquid/liquid interfacial surface-enhanced Raman spectroscopy (SERS) strategy at normal temperature and pressure without any sources of ionization or radiation. This strategy generates high-resolution fingerprints in molecular chain length, C═C position, saturation, and regio- and stereoisomers of both glycerides and fatty acids and requires only trace amounts of molecules down to 1 ppb to achieve discrimination and exhibits great potentials to push the identification capability to trace levels or even the single-molecule level. According to experimental data and theoretical simulations, these targets have the amphiphilic and emulsifying properties, exhibit ordered molecular orientation and adsorption patterns, promote the co-assembly with plasmonic nanoarrays at the immiscible liquid/liquid interface, and consequently amplify the detection sensitivity. As a contrast, the typical SERS based on solid/air interfacial plasmonic nanoarrays faces the intrinsic bottleneck of extremely weak intensity and indistinguishable spectral fingerprints of lipid molecules. The vibrational fingerprints exhibit a rich range of well-resolved absorption features that are clearly diagnostic for fine structural changes and pave a new way for straightforward measurement without laborsome sample purification, enrichment, or complex derivatization. Although challenging, its unprecedented resolving power expands the potentials of SERS, serving as an ultimate analytical method to provide insights into the detailed structural features of other lipids under facile conditions in the future.

Improvements in Gold Nanorod Biocompatibility with Sodium Dodecyl Sulfate Stabilization

Due to their well-defined plasmonic properties, gold nanorods (GNRs) can be fabricated with optimal light absorption in the near-infrared region of the electromagnetic spectrum, which make them suitable for cancer-related theranostic applications. However, their controversial safety profile, as a result of surfactant stabilization during synthesis, limits their clinical translation. We report a facile method to improve GNR biocompatibility through the presence of sodium dodecyl sulfate (SDS). GNRs (120 × 40 nm) were synthesized through a seed-mediated approach, using cetyltrimethylammonium bromide (CTAB) as a cationic surfactant to direct the growth of nanorods and stabilize the particles. Post-synthesis, SDS was used as an exchange ligand to modify the net surface charge of the particles from positive to negative while maintaining rod stability in an aqueous environment. GNR cytotoxic effects, as well as the mechanisms of their cellular uptake, were examined in two different cancer cell lines, Lewis lung carcinoma (LLC) and HeLa cells. We not only found a significant dose-dependent effect of GNR treatment on cell viability but also a time-dependent effect of GNR surfactant charge on cytotoxicity over the two cell lines. Our results promote a better understanding of how we can mediate the undesired consequences of GNR synthesis byproducts when exposed to a living organism, which so far has limited GNR use in cancer theranostics.

Interfacial hydration determines orientational and functional dimorphism of sterol-derived Raman tags in lipid-coated nanoparticles

Lipid-coated noble metal nanoparticles (L-NPs) combine the biomimetic surface properties of a self-assembled lipid membrane with the plasmonic properties of a nanoparticle (NP) core. In this work, we investigate derivatives of cholesterol, which can be found in high concentrations in biological membranes, and other terpenoids, as tunable, synthetic platforms to functionalize L-NPs. Side chains of different length and polarity, with a terminal alkyne group as Raman label, are introduced into cholesterol and betulin frameworks. The synthesized tags are shown to coexist in two conformations in the lipid layer of the L-NPs, identified as "head-out" and "head-in" orientations, whose relative ratio is determined by their interactions with the lipid-water hydrogen-bonding network. The orientational dimorphism of the tags introduces orthogonal functionalities into the NP surface for selective targeting and plasmon-enhanced Raman sensing, which is utilized for the identification and Raman imaging of epidermal growth factor receptor-overexpressing cancer cells.

Seed-Mediated Synthesis of Gold Nanorods at Low Concentrations of CTAB

Although gold nanorods capped with hexadecyltrimethylammonium bromide (CTAB) have been prepared through the seed-mediated method for their use in diagnostics and therapeutics, the toxicity of AuNRs@CTAB limits their practical applications in the biomedical field. In this work, the synthesis and tuning of gold nanorods at very low concentrations of CTAB (as low as 0.008 M) was successfully achieved by using the seed-mediated method. Furthermore, we managed to optimize the growth conditions by changing the amount of seeds, AgNO₃, and/or HCl. At low CTAB concentrations, gold nanorods with tunable size and aspect ratio, high monodispersity, and high purity were obtained and studied by UV-vis spectroscopy, transmission electron microscopy, and Mie-Gans theoretical calculations. This work revealed a method of nanoparticle growth that may be extended to synthesize other nanomaterials such as Ag, Cu, Pd, and Pt at such low CTAB concentrations.

Single plasmonic nanostructures for biomedical diagnosis

The field of single nanoparticle plasmonics has grown enormously. There is no doubt that a wide diversity of the nanoplasmonic techniques and nanostructures represents a tremendous opportunity for fundamental biomedical studies as well as sensing and imaging applications. Single nanoparticle plasmonic biosensors are efficient in label-free single-molecule detection, as well as in monitoring real-time binding events of even several biomolecules. In the present review, we have discussed the prominent advantages and advances in single particle characterization and synthesis as well as new insight into and information on biomedical diagnosis uniquely obtained using single particle approaches. The approaches include the fundamental studies of nanoplasmonic behavior, two typical methods based on refractive index change and characteristic light intensity change, exciting innovations of synthetic strategies for new plasmonic nanostructures, and practical applications using single particle sensing, imaging, and tracking. The basic sphere and rod nanostructures are the focus of extensive investigations in biomedicine, while they can be programmed into algorithmic assemblies for novel plasmonic diagnosis. Design of single nanoparticles for the detection of single biomolecules will have far-reaching consequences in biomedical diagnosis.

Effects of Conformational Variation on Structural Insights from Solution-Phase Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman scattering (SERS) spectra contain information on the chemical structure on nanoparticle surfaces through the position and alignment of molecules with the electromagnetic near field. Time-dependent density functional theory (TDDFT) can provide the Raman tensors needed for a detailed interpretation of SERS spectra. Here, the impact of molecular conformations on SERS spectra is considered. TDDFT calculations of the surfactant cetyltrimethylammonium bromide with five conformers produced more accurate unenhanced Raman spectra than a simple all-trans structure. The calculations and measurements also demonstrated a loss of structural information in the CH2/CH3 scissor vibration band at 1450 cm-1 in the SERS spectra. To study lipid bilayers, TDDFT calculations on conformers of methyl phosphorylcholine and cis-5-decene served as models for the symmetric choline stretch in the lipid headgroup and the C═C stretch in the acyl chains of 1,2-oleoyl-glycero-3-phosphocholine. Conformer considerations enabled a measurement of the distribution of double-bond orientations with an order parameter of SC═C = 0.53.

The orientation of a membrane probe from structural analysis by enhanced Raman scattering

Small fluorescent molecules are widely used as probes of biomembranes. Different probes optically indicate membrane properties such as the lipid phase, thickness, viscosity, and electrical potential. The detailed molecular mechanisms behind probe signals are not well understood, in part due to the lack of tools to determine probe position and orientation in the membrane. Optical measurements on aligned biomembranes and lipid bilayers provide some degree of orientational information based on anisotropy in absorption, fluorescence, or nonlinear optical properties. These methods typically find the polar tilt angle between the membrane normal and the long axis of the molecule. Here we show that solution-phase surface enhanced Raman scattering (SERS) spectra of lipid membranes on gold nanorods can be used to determine molecular orientation of molecules within the membrane. The voltage sensitive dye 4-(2-(6-(dibutylamino)-2-naphthalenyl)ethenyl)-1-(3-sulfopropyl)-hydroxide, known as di-4-ANEPPS, is studied. Through the analysis of several peaks in the SERS spectrum, the polar angle from the membrane normal is found to be 66°, and the roll angle around the long axis of the molecule to be 305° from the original orientation. This structural analysis method could help elucidate the meaning of fluorescent membrane probe signals, and how they are affected by different lipid compositions.

Surface-Enhanced Raman Spectroscopy of Fluid-Supported Lipid Bilayers

Supported lipid bilayers are essential model systems for studying biological membranes and for membrane-based sensor development. Surface-enhanced Raman spectroscopy (SERS) stands to add considerably to our understanding of the dynamics and interactions of these systems through direct chemical information. Despite this potential, SERS of lipid bilayers is not routinely achieved. Here, we carried out the first measurements of a solid-supported lipid bilayer on a SERS-active substrate and characterized the bilayer using SERS, atomic force microscopy, surface plasmon resonance spectroscopy, ellipsometry, and fluorescence recovery after photobleaching (FRAP). The creation of a fluid, SERS-active supported lipid bilayer was accomplished through use of a novel silica-coated silver film-over-nanosphere substrate. These substrates offer a powerful new platform to couple common surface techniques that are challenging on the nanoscale, for example, ellipsometry and FRAP, with SERS for studying biological membranes and their dynamics.

Advances in surface-enhanced Raman spectroscopy (SERS) substrates for lipid and protein characterization: sensing and beyond

Surface-enhanced Raman spectroscopy (SERS) has become an essential ultrasensitive analytical tool for biomolecular analysis of small molecules, macromolecular proteins, and even cells. SERS enables label-free, direct detection of molecules through their intrinsic Raman fingerprint. In particular, protein and lipid bilayers are dynamic three-dimensional structures that necessitate label-free methods of characterization. Beyond direct detection and quantitation, the structural information contained in SERS spectra also enables deeper biophysical characterization of biomolecules near metallic surfaces. Therefore, SERS offers enormous potential for such systems, although making measurements in a nonperturbative manner that captures the full range of interactions and activity remains a challenge. Many of these challenges have been overcome through advances in SERS substrate development, which have expanded the applications and targets of SERS for direct biomolecular quantitation and biophysical characterization. In this review, we will first discuss different categories of SERS substrates including solution-phase, solid-supported, tip-enhanced Raman spectroscopy (TERS), and single-molecule substrates for biomolecular analysis. We then discuss detection of protein and biological lipid membranes. Lastly, biophysical insights into proteins, lipids and live cells gained through SERS measurements of these systems are reviewed.

US-Czech conference strengthens bilateral and multidisciplinary collaborations in nanotechnology and chemistry

There are several methods to encourage bilateral and multidisciplinary collaborations-the most oft-used and effective include technical conferences, workshops, and seminars with the optimal combination of people, agenda, and length. However, this balance is difficult to attain, especially for meetings across academic disciplines and geographic borders. For nearly two decades, the US National Academies have developed a series of bilateral meetings to bring together scientists in different countries, which appear to have addressed such challenges via concise meetings with select emerging leaders from disparate fields and sectors. The authors utilized a similar approach to promote US and Czech collaborations in nanotechnology, given their mutual investments in the area and interests in building relationships. The bilateral meeting was held in Prague and designed to focus on nanotechnology and chemistry. The goals were four-fold: (1) evaluate US and Czech capabilities and challenges in nanotechnology and chemistry, (2) identify likely research projects and funding sources for collaborations, (3) celebrate 100 years of US and Czech diplomatic relations, and (4) increase government and public support for bilateral scientific research. The two-day meeting was named the US-Czech Conference on Advanced Nanotechnology and Chemistry. A summary of the research presentations, funding opportunities, panel discussions, and potential joint projects are given herein. It is our hope that this perspective will facilitate similar bilateral meetings in other fields to promote more multidisciplinary research.