Mechanism of Plasmon-Induced Catalysis of Thiolates and the Impact of Reaction Conditions

The conversion of the thiols 4-aminothiophenol (ATP) and 4-nitrothiophenol (NTP) can be considered as one of the standard reactions of plasmon-induced catalysis and thus has already been the subject of numerous studies. Currently, two reaction pathways are discussed: one describes a dimerization of the starting material yielding 4,4'-dimercaptoazobenzene (DMAB), while in the second pathway, it is proposed that NTP is reduced to ATP in HCl solution. In this combined experimental and theoretical study, we disentangled the involved plasmon-mediated reaction mechanisms by carefully controlling the reaction conditions in acidic solutions and vapor. Motivated by the different surface-enhanced Raman scattering (SERS) spectra of NTP/ATP samples and band shifts in acidic solution, which are generally attributed to water, additional experiments under pure gaseous conditions were performed. Under such acidic vapor conditions, the Raman data strongly suggest the formation of a hitherto not experimentally identified stable compound. Computational modeling of the plasmonic hybrid systems, i.e., regarding the wavelength-dependent character of the involved electronic transitions of the detected key intermediates in both reaction pathways, confirmed the experimental finding of the new compound, namely, 4-nitrosothiophenol (TP*). Tracking the reaction dynamics via time-dependent SERS measurements allowed us to establish the link between the dimer- and monomer-based pathways and to suggest possible reaction routes under different environmental conditions. Thereby, insight at the molecular level was provided with respect to the thermodynamics of the underlying reaction mechanism, complementing the spectroscopic results.

Investigating biochemical and structural changes of glycated collagen using multimodal multiphoton imaging, Raman spectroscopy, and atomic force microscopy

Advanced glycation end products (AGEs) form extracellular crosslinking with collagenous proteins, which contributes to the development of diabetic complications. In this study, AGEs-related pentosidine (PENT) crosslinks-induced structural and biochemical changes are studied using multimodal multiphoton imaging, Raman spectroscopy and atomic force microscopy (AFM). Decellularized equine pericardium (EP) was glycated with four ribose concentrations ranging between 5 and 200 mM and monitored for up to 30 days. Two-photon excited fluorescence (TPEF) and second harmonic generation (SHG) microscopic imaging probed elastin and collagen fibers, respectively. The glycated EP showed a decrease in the SHG intensities associated with loss of non-centrosymmetry of collagen and an increase of TPEF intensities associated with PENT crosslinks upon glycation. TPEF signals from elastin fibers were unaffected. A three-dimensional reconstruction with SHG + TPEF z-stack images visualized the distribution of collagen and elastin within the EP volume matrix. In addition, Raman spectroscopy (RS) detected changes in collagen-related bands and discriminated glycated from untreated EP. Furthermore, AFM scans showed that the roughness increases and the D-unit structure of fibers remained unchanged during glycation. The PENT crosslinked-induced changes are discussed in the context of previous studies of glutaraldehyde- and genipin-induced crosslinking and collagenase-induced digestion of collagen. We conclude that TPEF, SHG, RS, and AFM are effective, label-free, and non-destructive methods to investigate glycated tissues, differentiate crosslinking processes, and characterize general collagen-associated and disease-related changes, in particular by their RS fingerprints.

Inside Block Copolymer Micelles-Tracing Interfacial Influences on Crosslinking Efficiency in Nanoscale Confined Spaces

Recently, several studies have demonstrated the excellent capabilities of tip-enhanced Raman spectroscopyfor in-depth investigations of structural properties of matter with unprecedented resolution and chemical specificity. These capabilities are utilized here to study the internal structure of core-crosslinked micelles, which are formed by self-assembly of the diblock terpolymer poly(ethylene oxide)-block-poly(furfuryl glycidylether-co-tert-butylglycidyl ether). Supplementing force-volume atomic force microscopy experiments address additionally the nanomechanical properties. Particularly, TERS enables investigating the underlying principles influencing the homogeneity and efficiency of the Diels-Alder core-crosslinking process in the confined hydrophobic core. While the central core region is homogenously crosslinked, a breakdown of the crosslinking reaction is observed in the core-corona interfacial region. The results corroborate that a strong crosslinking efficiency is directly correlated to the formation of a mixed zone of the glycidyl ether and PEO corona blocks reaching ≈5 nm into the core region. Concomitantly a strong exclusion of the encapsulated bismaleimide crosslinker from the interfacial region is observed. It is conceivable that a changed structure, chemical composition and altered nanomechanical properties of this interfacial region may also influence the crosslinking efficiency across the entire core region by a modification of the solubility of the crosslinker in the interfacial core-corona region.

Targeted Suppression of Peptide Degradation in Ag-Based Surface-Enhanced Raman Spectra by Depletion of Hot Carriers

Sample degradation, in particular of biomolecules, frequently occurs in surface-enhanced Raman spectroscopy (SERS) utilizing supported silver SERS substrates. Currently, thermal and/or photocatalytic effects are considered to cause sample degradation. This paper establishes the efficient inhibition of sample degradation using iodide which is demonstrated by a systematic SERS study of a small peptide in aqueous solution. Remarkably, a distinct charge separation-induced surface potential difference is observed for SERS substrates under laser irradiation using Kelvin probe force microscopy. This directly unveils the photocatalytic effect of Ag-SERS substrates. Based on the presented results, it is proposed that plasmonic photocatalysis dominates sample degradation in SERS experiments and the suppression of typical SERS sample degradation by iodide is discussed by means of the energy levels of the substrate under mild irradiation conditions. This approach paves the way toward more reliable and reproducible SERS studies of biomolecules under physiological conditions.

Structural and Biochemical Changes in Pericardium upon Genipin Cross-Linking Investigated Using Nondestructive and Label-Free Imaging Techniques

Tissue cross-linking represents an important and often used technique to enhance the mechanical properties of biomaterials. For the first time, we investigated biochemical and structural properties of genipin (GE) cross-linked equine pericardium (EP) using optical imaging techniques in tandem with quantitative atomic force microscopy (AFM). EP was cross-linked with GE at 37 °C, and its biochemical and biomechanical properties were observed at various time points up to 24 h. GE cross-linked EP was monitored by the normalized ratio between its second-harmonic generation (SHG) and two-photon autofluorescence emissions and remained unchanged for untreated EP; however, a decreasing ratio due to depleted SHG and elevated autofluorescence and a fluorescence band at 625 nm were found for GE cross-linked EP. The mean autofluorescence lifetime of GE cross-linked EP also decreased. The biochemical signature of GE cross-linker and shift in collagen bands were detected and quantified using shifted excitation Raman difference spectroscopy as an innovative approach for tackling artifacts with high fluorescence backgrounds. AFM images indicated a higher and increasing Young's modulus correlated with cross-linking, as well as collagen structural changes in GE cross-linked EP, qualitatively explaining the observed decrease in the second-harmonic signal. In conclusion, we obtained detailed information about the biochemical, structural, and biomechanical effects of GE cross-linked EP using a unique combination of optical and force microscopy techniques in a nondestructive and label-free manner.

On the stability of microwave-fabricated SERS substrates - chemical and morphological considerations

The stability of surface-enhanced Raman spectroscopy (SERS) substrates in different organic solvents and different buffer solutions was investigated. SERS substrates were fabricated by a microwave-assisted synthesis approach and the morphological as well as chemical changes of the SERS substrates were studied. It was demonstrated that the SERS substrates treated with methanol, ethanol, or N,N-dimethylformamide (DMF) were comparable and showed overall good stability and did not show severe morphological changes or a strong decrease in their Raman activity. Toluene treatment resulted in a strong decrease in the Raman activity whereas dimethyl sulfoxide (DMSO) treatment completely preserved or even slightly improved the Raman enhancement capabilities. SERS substrates immersed into phosphate-buffered saline (PBS) solutions were observed to be rather instable in low and neutral pH buffer solutions. Other buffer systems showed less severe influences on the SERS activity of the substrates and a carbonate buffer at pH 10 was found to even improve SERS performance. This study represents a guideline on the stability of microwave-fabricated SERS substrates or other SERS substrates consisting of non-stabilized silver nanoparticles for the application of different organic solvents and buffer solutions.

Characterization of a library of vitamin A-functionalized polymethacrylate-based nanoparticles for siRNA delivery

A 60-membered library of vitamin A-functionalized P(MMA-stat-DMAEMA)-b-PPEGMA block copolymers was synthesized by RAFT polymerization. From these, nanoparticles containing genetic material were formulated and fully characterized.

Multimodal Characterization of Resin Embedded and Sliced Polymer Nanoparticles by Means of Tip-Enhanced Raman Spectroscopy and Force-Distance Curve Based Atomic Force Microscopy

Understanding the property-function relation of nanoparticles in various application fields involves determining their physicochemical properties, which is still a remaining challenge to date. While a multitude of different characterization tools can be applied, these methods by themselves can only provide an incomplete picture. Therefore, novel analytical techniques are required, which can address both chemical functionality and provide structural information at the same time with high spatial resolution. This is possible by using tip-enhanced Raman spectroscopy (TERS), but due to its limited depth information, TERS is usually restricted to investigations of the nanoparticle surface. Here, TERS experiments are established on polystyrene nanoparticles (PS NPs) after resin embedding and microtome slicing. With that, unique access to their internal morphological features is gained, and thus, enables differentiation between information obtained for core- and shell-regions. Complementary information is obtained by means of transmission electron microscopy (TEM) and from force-distance curve based atomic force microscopy (FD-AFM). This multimodal approach achieves a high degree of discrimination between the resin and the polymers used for nanoparticle formulation. The high potential of TERS combined with advanced AFM spectroscopy tools to probe the mechanical properties is applied for quality control of the resin embedding procedure.

Conformation-dependent phosphorescence emission of individual mononuclear ruthenium-(ii)-bis-terpyridine complexes

The potential of supramolecular transition metal coordination complexes to form robust, long-living, radiative charge transfer states makes this class of triplet state emitters ideal candidates for application as photosensitizes or in photonic devices. Antenna-enhanced phosphorescence experiments on single Ru2+-bis-terpyridine complexes incorporated into a thin PMMA film show that phosphorescence emission spectra can exhibit shifts depending on the local environment [J. F. Herrmann, P. S. Popp, A. Winter, U. S. Schubert and C. Höppener, ACS Photonics, 2016, 3, 1897-1906]. Here, we demonstrate that the environmentally altered spectral properties of individual dual-luminescent Ru2+-bis-terpyridine complexes in PMMA and acetonitrile can be reproduced by DFT-based vibrationally resolved Franck-Condon spectra, if the phosphorescent emission of different molecular conformations is taken into account. Furthermore, we demonstrate that the triplet emission of these complexes occurs from a metal-to-ligand charge transfer (MLCT) state.

Dumbbell gold nanoparticle dimer antennas with advanced optical properties

Plasmonic nanoantennas have found broad applications in the fields of photovoltaics, electroluminescence, non-linear optics and for plasmon enhanced spectroscopy and microscopy. Of particular interest are fundamental limitations beyond the dipolar approximation limit. We introduce asymmetric gold nanoparticle antennas (AuNPs) with improved optical near-field properties based on the formation of sub-nanometer size gaps, which are suitable for studying matter with high-resolution and single molecule sensitivity. These dumbbell antennas are characterized in regard to their far-field and near-field properties and are compared to similar dimer and trimer antennas with larger gap sizes. The tailoring of the gap size down to sub-nanometer length scales is based on the integration of rigid macrocyclic cucurbituril molecules. Stable dimer antennas are formed with an improved ratio of the electromagnetic field enhancement and confinement. This ratio, taken as a measure of the performance of an antenna, can even exceed that exhibited by trimer AuNP antennas composed of comparable building blocks with larger gap sizes. Fluctuations in the far-field and near-field properties are observed, which are likely caused by distinct deviations of the gap geometry arising from the faceted structure of the applied colloidal AuNPs.

Ordered Arrangement and Optical Properties of Silica-Stabilized Gold Nanoparticle-PNIPAM Core-Satellite Clusters for Sensitive Raman Detection

Gold-polymer hybrid nanoparticles attract wide interest as building blocks for the engineering of photonic materials and plasmonic (active) metamaterials with unique optical properties. In particular, the coupling of the localized surface plasmon resonances of individual metal nanostructures in the presence of nanometric gaps can generate highly enhanced and confined electromagnetic fields, which are frequently exploited for metal-enhanced light-matter interactions. The optical properties of plasmonic structures can be tuned over a wide range of properties by means of their geometry and the size of the inserted nanoparticles as well as by the degree of order upon assembly into 1D, 2D, or 3D structures. Here, the synthesis of silica-stabilized gold-poly(N-isopropylacrylamide) (SiO₂ -Au-PNIPAM) core-satellite superclusters with a narrow size distribution and their incorporation into ordered self-organized 3D assemblies are reported. Significant alterations of the plasmon resonance are found for different assembled structures as well as strongly enhanced Raman signatures are observed. In a series of experiments, the origin of the highly enhanced signals can be assigned to the interlock areas of adjacent SiO₂ -Au-PNIPAM core-satellite clusters and their application for highly sensitive nanoparticle-enhanced Raman spectroscopy is demonstrated.

Impact of the Nanoscale Gap Morphology on the Plasmon Coupling in Asymmetric Nanoparticle Dimer Antennas

Coupling of plasmon resonances in metallic gap antennas is of interest for a wide range of applications due to the highly localized strong electric fields supported by these structures, and their high sensitivity to alterations of their structure, geometry, and environment. Morphological alterations of asymmetric nanoparticle dimer antennas with (sub)-nanometer size gaps are assigned to changes of their optical response in correlative dark-field spectroscopy and high-resolution transmission electron microscopy (HR-TEM) investigations. This multimodal approach to investigate individual dimer structures clearly demonstrates that the coupling of the plasmon modes, in addition to well-known parameters such as the particle geometry and the gap size, is also affected by the relative alignment of both nanoparticles. The investigations corroborate that the alignment of the gap forming facets, and with that the gap area, is crucial for their scattering properties. The impact of a flat versus a rounded gap structure on the optical properties of equivalent dimers becomes stronger with decreasing gap size. These results hint at a higher confinement of the electric field in the gap and possibly a different onset of quantum transport effects for flat and rounded gap antennas in corresponding structures for very narrow gaps.

Hierarchical, guided self-assembly of preselected carbon nanotubes for the controlled fabrication of CNT structures by electrooxidative nanolithography

A fully controllable process for the fabrication of carbon nanotube assemblies is presented on the basis of a sequential electrochemical oxidation lithography process. This approach utilizes the local chemical conversion of a n-octadecyltrichlorosilane self-assembled monolayer into a template featuring polar acid groups. The capability to utilize such chemically active templates for the site-selective assembly of individual carbon nanotubes was demonstrated, and a hierarchical, sequential structuring routine to obtain crossed CNT configurations, formed by preselected carbon nanotubes, was implemented. The introduced process allows the reliable and well-controlled fabrication of tailor-made nanoscopic assemblies of nanomaterials toward their integration into complex device frameworks and could provide control over the electrical properties of the fabricated assemblies.

Self-similar gold-nanoparticle antennas for a cascaded enhancement of the optical field

We experimentally demonstrate cascaded field enhancement by means of gold nanoparticle dimer and trimer antennas. The local field enhancement is probed by single-molecule fluorescence using fluorophores with high intrinsic quantum efficiency (Q(0)>80%). Using a self-similar trimer antenna consisting of 80, 40, and 20 nm gold nanoparticles, we demonstrate a fluorescence enhancement of 40 and a spatial confinement of 15 nm. Compared with a single gold nanoparticle, the self-similar trimer antenna improves the enhancement-confinement ratio by more than an order of magnitude. Self-similar antennas hold promise for high-resolution imaging and spectroscopy, ultrasensitive detection, and efficient single-photon sources.

Background suppression in near-field optical imaging

In several recent studies, antenna-based optical microscopy has demonstrated its potential to resolve features as small as 10 nm. Most studies are concerned with well-separated features on flat surfaces, and there are only few studies that deal with samples of high feature density or even three-dimensional objects. The reason is that the external laser irradiation of the optical antenna (e.g., tip or particle) also directly irradiates the sample and therefore gives rise to a background. Here we introduce an efficient background suppression scheme that makes use of feedback modulation. The method is widely applicable and not restricted to cantilever-based scanning schemes. We apply this technique to both dense samples of dye molecules and ion channel proteins in plasma membranes and demonstrate effective background suppression and strongly improved sensitivity. The feedback modulation scheme is expected to find application for biological studies in liquid environments and for investigations of subsurface features in material science.

Imaging of membrane proteins using antenna-based optical microscopy

The localization and identification of individual proteins is of key importance for the understanding of biological processes on the molecular scale. Here, we demonstrate near-field fluorescence imaging of single proteins in their native cell membrane. Incident laser radiation is localized and enhanced with an optical antenna in the form of a spherical gold particle attached to a pointed dielectric tip. Individual proteins can be identified with a diffraction-unlimited spatial resolution of ∼50 nm. Besides determining the concentration and distribution of specific membrane proteins, this approach makes it possible to study the colocalization of different membrane proteins. Moreover, it enables a simultaneous recording of the membrane topology. Protein distributions can be correlated with the local membrane topology, thereby providing important information on the chemical and structural organization of cellular membranes.

Antenna-based optical imaging of single Ca2+ transmembrane proteins in liquids

Understanding the diversity of biological processes requires methods that can address single proteins in their natural environment and provide insights into structural and functional properties, as well as the local distribution of each individual protein. We use an optical antenna in the form of a single gold nanoparticle to localize incident laser radiation to 50 nm, significantly smaller than the diffraction limit of light. Our approach enables us to optically resolve individual plasma-membrane-bound Ca2+ pumps (PMCA4) immersed in aqueous environments and to determine the distribution of interprotein distances. We are able to correlate the protein maps with local topology. Improved antenna geometries will make it possible to resolve, identify, and probe single membrane proteins in live cells with true protein resolution of 5-10 nm.

High-resolution near-field optical imaging of single nuclear pore complexes under physiological conditions

Scanning near-field optical microscopy (SNOM) circumvents the diffraction limit of conventional light microscopy and is able to achieve optical resolutions substantially below 100 nm. However, in the field of cell biology SNOM has been rarely applied, probably because previous techniques for sample-distance control are less sensitive in liquid than in air. Recently we developed a distance control based on a tuning fork in tapping mode, which is also well-suited for imaging in solution. Here we show that this approach can be used to visualize single membrane protein complexes kept in physiological media throughout. Nuclear envelopes were isolated from Xenopus laevis oocytes at conditions shown recently to conserve the transport functions of the nuclear pore complex (NPC). Isolated nuclear envelopes were fluorescently labeled by antibodies against specific proteins of the NPC (NUP153 and p62) and imaged at a resolution of approximately 60 nm. The lateral distribution of epitopes within the supramolecular NPC could be inferred from an analysis of the intensity distribution of the fluorescence spots. The different number densities of p62- and NUP153-labeled NPCs are determined and discussed. Thus we show that SNOM opens up new possibilities for directly visualizing the transport of single particles through single NPCs and other transporters.

Scanning near-field optical microscopy of a cell membrane in liquid

The applications of scanning near-field optical microscopy to biological specimens under physiological conditions have so far been very rare since common techniques for a probe-sample distance control are not as well suited for operation in liquid as under ambient conditions. We have shown previously that our own approach for a distance control, based on a short aperture fibre probe and a tuning fork as force sensor in a tapping mode, works well even on soft material in water. By means of an electronic self-excitation circuit, which compensates for changes of the resonance frequency due to evaporation of liquid, the stability of the force feedback has now been further improved. We present further evidence for the excellent suitability of the tapping-mode-like distance control to an operation in liquid, for example, by force-imaging of double-stranded DNA. Moreover, we demonstrate that a nuclear envelope in liquid can be imaged with a high optical resolution of approximately 70 nm without affecting its structural integrity. Thereby, single nuclear pores in the nuclear envelope with a nearest neighbour distance of approximately 120 nm have been optically resolved for the first time.

Enhanced light confinement in a near-field optical probe with a triangular aperture

We present a probe concept for scanning near-field optical microscopy combining the excellent background suppression of aperture probes with the superior light confinement of apertureless probes. A triangular aperture at the tip of a tetrahedral waveguide (full taper angle approximately 90 degrees ) shows a strong field enhancement at only one rim when illuminated with light of suitable polarization. Compared to a circular aperture of equivalent size, the resolution capability is doubled without loss of brightness. For a approximately 60 nm sized triangular aperture, we measured an optical resolution <40 nm and a transmission of approximately 10(-4).