Surface enhanced optical spectroscopies for bioanalysis

Surface enhancement can provide improved detection sensitivity in a range of optical spectroscopies. When applied to bioanalysis these enhanced techniques allow for the detection of disease biomarkers at lower levels, which has a clear patient benefit. However, to achieve widespread clinical use of surface enhanced techniques there remain several "grand challenges". In this review we consider the substrates employed to achieve enhancement before reviewing each enhanced optical technique in detail; surface plasmon resonance, localised surface plasmon resonance, surface enhanced fluorescence, surface enhanced infrared absorption spectroscopy and surface enhanced (resonance) Raman spectroscopy. Finally we set out the "grand challenges" currently facing the field.     

Application of Nonlinear Optical Microscopy for Imaging Skin†

Recent advances in the use of nonlinear optical microscopy (NLOM) in skin microscopy are presented. Nonresonant spectroscopies including second harmonic generation, coherent anti‐Stokes Raman and two‐photon absorption are described and applications to problems in skin biology are detailed. These nonlinear techniques have several advantages over traditional microscopy methods that rely on one‐photon excitation: intrinsic 3D imaging with <1 μm spatial resolution, decreased photodamage to tissue samples and penetration depths up to 1000 μm with the use of near‐infrared lasers. Thanks to these advantages, nonlinear optical spectroscopy has become a powerful tool to study the physical and biochemical properties of the skin. Structural information can be obtained using the response of endogenous chemical species in the skin, such as collagen or lipids, indicating that optical biopsy may replace current invasive, time‐consuming traditional histology methods. Insertion of specific probe molecules into the skin provides the opportunity to monitor specific biochemical processes such as skin transport, molecular penetration, barrier homeostasis and ultraviolet radiation‐induced reactive oxygen species generation. While the field is quite new, it seems likely that the use of NLOM to probe structure and biochemistry of live skin samples will only continue to grow.     

Spectroscopic techniques to characterize the spin state: Vibrational,optical, Mössbauer,NMR, and X-ray spectroscopy

Basic principles and recent work using probes including Mössbauer, optical, X-ray, and vibrational spectroscopies to follow the transitions in spin crossover complexes are reviewed. X-ray spectroscopy, being a relatively lesser-known probe, is discussed in more detail. X-ray spectroscopic methods have been used in the spin crossover field mostly to elucidate surface phenomena, ultrafast dynamics, and high pressure effects, but recent technical developments provide perspectives for a more widespread use of these powerful techniques in home laboratories.     

Determination of molecular surface structure, composition, and dynamics under reaction conditions at high pressures and at the solid-liquid interface

In the last two decades, surface-science experiments and techniques have been developed to focus on obtaining molecular information under reaction conditions at high pressures (near or above 1 bar) and liquid interfaces. This Minireview describes the results of these studies obtained by surface-sensitive laser spectroscopies, scanning tunneling microscopy, and X-ray spectroscopies usually practiced at a synchrotron light source. The use of model surfaces, single crystals, and monodisperse nanoparticles with variable size (1-10 nm) and shape facilitates meaningful interpretation of the experimental data. These methods allow evaluation of the molecular structures of intermediates, oxidation states of metals, and mobility of adsorbants. New techniques that are likely to make major contributions to the investigation of surfaces under reaction conditions are also discussed.     

Chiroptical spectroscopy and the validation of crystal structure stereochemical assignments

The absolute stereochemistry of chiral molecules is ideally established to atomic resolution by X-ray crystallographic analysis. However, chiroptical spectroscopies, namely electronic circular dichroism (ECD), optical rotatory dispersion (ORD), vibrational circular dichroism (VCD) and Raman optical activity (ROA), play important complementary roles in establishing relative and absolute sterochemistries as well as allowing determinations of optical purity. A brief summary of chiroptical spectroscopies is presented, along with guidance to their advantages and disadvantages. The application of ECD to verifying that single crystals selected for crystallographic analysis are indeed representative of bulk material is described.     

Initial photoinduced dynamics of the photoactive yellow protein.

The photoactive yellow protein (PYP) is the photoreceptor protein responsible for initiating the blue-light repellent response of the Halorhodospira halophila bacterium. Optical excitation of the intrinsic chromophore in PYP, p-coumaric acid, leads to the initiation of a photocycle that comprises several distinct intermediates. The dynamical processes responsible for the initiation of the PYP photocycle have been explored with several time-resolved techniques, which include ultrafast electronic and vibrational spectroscopies. Ultrafast electronic spectroscopies, such as pump-visible probe, pump-dump-visible probe, and fluorescence upconversion, are useful in identifying the timescales and connectivity of the transient intermediates, while ultrafast vibrational spectroscopies link these intermediates to dynamic structures. Herein, we present the use of these techniques for exploring the initial dynamics of PYP, and show how these techniques provide the basis for understanding the complex relationship between protein and chromophore, which ultimately results in biological function.     

Study of the chemical copolymerization of 2-aminoterephthalic acid and aniline.: Synthesis and copolymer properties

Aniline (ANI) and 2-aminoterephthalic acid (2ATA) copolymers, with different compositions, were prepared by chemical oxidative polymerization varying the feed ratio. The new materials have been characterized by techniques such as XPS, FTIR, Raman spectroscopies and thermal analysis. It has been checked that 2ATA units are included in the polymer backbone. Different properties have been studied as solubility, conductivity, optical absorption, fluorescence and electroactivity. The copolymers are soluble in aqueous alkaline medium and show clear electroactivity in aqueous acid medium.     

Recent advancements in optical DNA biosensors: exploiting the plasmonic effects of metal nanoparticles

The emerging field of plasmonics, the study of electromagnetic responses of metal nanostructures, has revealed many novel signal enhancing phenomena. As applied to the development of label-free optical DNA biosensors, it is now well established that plasmon-based surface enhanced spectroscopies on nanostructured metal surfaces or metal nanoparticles can markedly improve the sensitivity of optical biosensors, with some showing great promise for single molecule detection. In this review, we first summarize the basic concepts of plasmonics in metal nanostructures, as well as the characteristic optical phenomena to which plasmons give rise. We will then describe recent advances in optical DNA biosensing systems enabled by metal nanoparticle-derived plasmonic effects, including the use of surface enhanced Raman scattering (SERS), colorimetric methods, "scanometric" processes, and metal-enhanced fluorescence (MEF).     

Ultrafast UV spectroscopy: from a local to a global view of dynamical processes in macromolecules

The aim of this Perspective article is to cover recent developments in the application of femtosecond UV spectroscopy to understand molecular dynamics, and outlining potential future directions in this area. With several examples from recent literature the added-value of these techniques and their capability to study in real time changes in structure, dynamics and electrostatic fields of macromolecules in a site-specific fashion, as well as to uncover concerted dynamics in biomolecules, will be shown and discussed. The emerging fields of UV pulse-shaping techniques and UV optical nonlinear spectroscopies will be discussed to outline their potential to generate a novel family of coherent nonlinear spectroscopies for spectroscopic and microscopic applications.     

Optical and spectroscopic characteristics of oleate adsorption as revealed by FTIR analysis

Fourier transform infrared transmission (FTIR/TS), external reflection (FTIR/ERS), and internal reflection (FTIR/IRS) spectroscopies are three important sampling techniques for the study of adsorbed surfactants. The optical and spectral characteristics of a three-phase system were calculated using theoretical simulation and discussed based on experimental results for oleate adsorption at the air/water interface and at the water/fluorite interface. It is shown that a thorough understanding of the optical properties and spectral characteristics from FTIR analysis helps to improve the experimental design and explanation of experimental results and is important to properly quantify surfactant interfacial adsorption phenomena.     

Preparation of Ion Exchange Films for Solid-Phase Spectrophotometry and Solid-Phase Fluorometry

ABSTRACT

Atomic spectroscopy has dominated the field of trace inorganic analysis because of its high sensitivity and selectivity. The advantages gained by the atomic spectroscopies come with the disadvantage of expensive and often complicated instrumentation. Solid-phase spectroscopy, in which the analyte is preconcentrated on a solid medium followed by conventional spectrophotometry or fluorometry, requires less expensive instrumentation and has considerable sensitivity and selectivity. The sensitivity gains come from preconcentration and the use of chromophore (or fluorophore) developers and the selectivity is. achieved by use of ion exchange conditions that favor the analyte in combination with speciative chromophores. Little work has been done to optimize the ion exchange medium (IEM) associated with these techniques. In this paper we present a method for making ion exchange polymer films which considerably simplify the solid-phase spectroscopic techniques. They are mechanically sturdy and optically transparent in the ultraviolet and visible portion of the spectrum, which makes them suitable for spectrophotometry and fluorometry.     

Exciton coupling of tetra(p-hydroxyphenyl)porphyrin controlled by substituents of counterions in Triton X-100 micellar solution

The interactions of nonionic meso-tetra(p-hydroxyphenyl)porphyrin (THPP) with CX3COOH (X = F, Cl, Br) in Triton X-100 (TX) micellar solution have been investigated by optical absorption, resonance light-scattering, and fluorescence spectroscopies. The double red-shifted absorption bands and strong resonance light scattering (RLS) signal imply that the assemblies induced by trihalo acetic acids belong to J-aggregates. The fluorescence of porphyrin is quenched due to the aggregate formation. The kinetics of assemblies trigged by CBr3COOH is studied via stopped-flow techniques. No characteristics of autocatalyzed reactions are observed, and there is only a log phase. The nature of the exciton coupling of transition dipole moment can be systematically changed by the haloid substituents of the organic counteranion.     

Trends in single-cell analysis by use of ICP-MS

The analysis of single cells is a growing research field in many disciplines such as toxicology, medical diagnosis, drug and cancer research or metallomics, and different methods based on microscopic, mass spectrometric, and spectroscopic techniques are under investigation. This review focuses on the most recent trends in which inductively coupled plasma mass spectrometry (ICP-MS) and ICP optical emission spectrometry (ICP-OES) are applied for single-cell analysis using metal atoms being intrinsically present in cells, taken up by cells (e.g., nanoparticles), or which are artificially bound to a cell. For the latter, especially element tagged antibodies are of high interest and are discussed in the review. The application of different sample introduction systems for liquid analysis (pneumatic nebulization, droplet generation) and elemental imaging by laser ablation ICP-MS (LA-ICP-MS) of single cells are highlighted. Because of the high complexity of biological systems and for a better understanding of processes and dynamics of biologically or medically relevant cells, the authors discuss the idea of “multimodal spectroscopies.”     

New heteropolynuclear metallomesogens: copper(II), palladium(II), nickel(II) and oxovanadium(IV) chelates with [3]ferrocenophane‐containing Schiff's base and β‐aminovinylketone

New mesogenic heteropolynuclear complexes of Cu(II), Pd(II), Ni(II) and VO(IV) with the [3]ferrocenophane‐containing Schiff's base, and Cu(II) and Pd(II) complexes with the [3]ferrocenophane‐containing β‐aminovinylketone have been synthesised. The obtained heterometallic mesogenes are identified by elemental analysis, proton nuclear magnetic resonance, infrared and ultraviolet–visible spectroscopies. Liquid crystalline properties are studied by thermal polarising optical microscopy and differential scanning calorimetry techniques. Both ligands and heteropolynuclear complexes exhibit thermotropic nematic and smectic C mesophases in various temperature ranges except for the Ni(II) complex. Mesomorphism of the prepared complexes is correlated with the geometry of their central chelate core. Considerably broader mesophases and lower transition temperatures are achieved in the synthesised metallomesogens by using the alkylidene‐bridged ferrocene as a building unit.     

Combining in situ characterization methods in one set-up: looking with more eyes into the intricate chemistry of the synthesis and working of heterogeneous catalysts

Several in situ techniques are known which allow investigations of catalysts and catalytic reactions under real reaction conditions using different spectroscopic and X-ray methods. In recent years, specific set-ups have been established which combine two or more in situ methods in order to get a more detailed understanding of catalytic systems. This tutorial review will give a summary of currently available set-ups equipped with multiple techniques for in situ catalyst characterization, catalyst preparation, and reaction monitoring. Besides experimental and technical aspects of method coupling including X-ray techniques, spectroscopic methods (Raman, UV-vis, FTIR), and magnetic resonance spectroscopies (NMR, EPR), essential results will be presented to demonstrate the added value of multitechnique in situ approaches. A special section is focussed on selected examples of use which show new developments and application fields.     

Biomedical tissue phantoms with controlled geometric and optical properties for Raman spectroscopy and tomography

To support the translation of Raman spectroscopy into clinical applications, synthetic models are needed to accurately test, optimize and validate prototype fiber optic instrumentation. Synthetic models (also called tissue phantoms) are widely used for developing and testing optical instrumentation for diffuse reflectance, fluorescence, and Raman spectroscopies. While existing tissue phantoms accurately model tissue optical scattering and absorption, they do not typically model the anatomic shapes and chemical composition of tissue. Because Raman spectroscopy is sensitive to molecular composition, Raman tissue phantoms should also approximate the bulk tissue composition. We describe the fabrication and characterization of tissue phantoms for Raman tomography and spectroscopy. These phantoms have controlled chemical and optical properties, and also multilayer morphologies which approximate the appropriate anatomic shapes. Tissue phantoms were fabricated to support on-going Raman studies by simulating the human wrist and rat leg. Surface meshes (triangle patch models) were generated from computed tomography (CT) images of a human arm and rat leg. Rapid prototyping was used to print mold templates with complex geometric patterns. Plastic casting techniques used for movie special effects were adapted to fabricate molds from the rapid prototypes, and finally to cast multilayer gelatin tissue phantoms. The gelatin base was enriched with additives to model the approximate chemistry and optical properties of individual tissue layers. Additional studies were performed to determine optimal casting conditions, phantom stability, layer delamination and chemical diffusion between layers. Recovery of diffuse reflectance and Raman spectra in tissue phantoms varied with probe placement. These phantoms enable optimization of probe placement for human or rat studies. These multilayer tissue phantoms with complex geometries are shown to be stable, with minimal layer delamination and chemical diffusion.     

Deduction of structural information of interfacial proteins by combined vibrational spectroscopic methods

We demonstrate both theoretically and experimentally that the combination of vibrational spectroscopic techniques on samples can be used to deduce more detailed structural information of interfacial proteins and peptides. Such an approach can be used to elucidate structures of proteins or peptides at interfaces, such as at the solid/liquid interface or in cell membranes. We also discuss that the controlled perturbations may provide more measured parameters for structural studies on such proteins and peptides. In this paper, we will demonstrate that optical spectroscopic techniques such as polarized Fourier transform infrared spectroscopy (FTIR), sum frequency generation (SFG) vibrational spectroscopy, and higher order nonlinear vibrational spectroscopies can be used to deduce different and complementary structural information of molecules at interfaces (e.g., orientation information of certain functional groups and secondary structures of interfacial proteins). Also, we believe that controlled perturbations on samples, such as variation of sample temperature, application of electrical fields, and alternation of substrate roughness, can provide more detailed information regarding the interfacial structures of proteins and peptides. The development of nonlinear vibrational spectroscopies, such as SFG and four-wave mixing vibrational spectroscopy, to examine interfacial protein and peptide structures, and introduction of external perturbations on samples should be able to substantially advance our knowledge in understanding structures and thus functions of proteins and peptides at interfaces.     

Palladium Catalyzed Synthesis of Fluorine-containing 3-Biaryl-1-ferrocenyl-2-propene-1-one

In the last two decades, molecular-based second-order nonlinear optical (NLO) chromo-phores1, have attracted much interest because of their potential applications in emerging 2opto-electronic technologies. These efforts have mainly focused on organic systems.More recently, organometallic molecules have been investigated as well. In comparisonto common organic molecules, they offer a large variety of novel structures3, . The 4possibility of high environmental stability, and divers…     

Unusual electric-field-induced transformations in the dark conglomerate phase of a bent-core liquid crystal

Unusual behaviour of the dark conglomerate (DC) phase seen in an oxadiazole-based achiral bent-core liquid crystal, which has not previously been reported for the DC phase of other liquid crystals, is described. Under polarising optical microscopy, we see no domains of opposite handedness in the ground state of the DC phase. However, it shows unusual transformations when an electric field is applied to the system. On increasing the electric field, at first the domains of opposite handedness become visible and then they grow in size and slowly the sample transforms to a monochiral or single-handed form which is followed by a nonchiral state at very high fields. The threshold electric fields required to achieve these changes are temperature dependent and the transformations are seen irrespective of the frequency of the applied electric field (100 Hz to 5 kHz), type of the waveform (sine, square and triangular) and the thickness (1.5 μm to 15 μm) or the geometry (planar and twisted) of the device used. Further, there is no field-induced high birefringence texture observed even though sufficiently large electric field (~22 V/μm) has been applied across the devices. The nature of the behaviour is investigated by various techniques such as optical microscopy, conoscopy, circular dichroic and Raman spectroscopies, electro-optics and dielectric spectroscopy. The possible physical phenomena behind these changes are discussed in detail.