Plasmon-Enhanced Autofluorescence Imaging of Organelles in Label-Free Cells by Deep-Ultraviolet Excitation

ATR-far-ultraviolet spectroscopy: a challenge to new σ chemistry

This review reports the recent progress on ATR-far ultraviolet (FUV) spectroscopy in the condensed phase. ATR-FUV spectroscopy for liquids and solids enables one to explore various topics in physical chemistry, analytical chemistry, nanoscience and technology, materials science, electrochemistry, and organic chemistry. In this review, we put particular emphasis on the three major topics: (1) studies on electronic transitions and structures of various molecules, which one cannot investigate via ordinary UV spectroscopy. The combined use of ATR-FUV spectroscopy and quantum chemical calculations allows for the investigation of various electronic transitions, including σ, n-Rydberg transitions. ATR-FUV spectroscopy may open a new avenue for σ-chemistry. (2) ATR-FUV spectroscopy enables one to measure the first electronic transition of water at approximately 160 nm without peak saturation. Using this band, one can study the electronic structure of water, aqueous solutions, and adsorbed water. (3) ATR-FUV spectroscopy has its own advantages of the ATR method as a surface analysis method. ATR-FUV spectroscopy is a powerful technique for exploring a variety of top surface phenomena (∼50 nm) in adsorbed water, polymers, graphene, organic materials, ionic liquids, and so on. This review briefly describes the principles, characteristics, and instrumentation of ATR-FUV spectroscopy. Next, a detailed description about quantum chemical calculation methods for FUV and UV regions is given. The recent application of ATR-FUV-UV spectroscopy studies on electronic transitions from σ orbitals in various saturated molecules is introduced first, followed by a discussion on the applications of ATR-FUV spectroscopy to studies on water, aqueous solutions, and adsorbed water. Applications of ATR-FUV spectroscopy in the analysis of other materials such as polymers, ionic liquids, inorganic semiconductors, graphene, and carbon nanocomposites are elucidated. In addition, ATR-FUV-UV-vis spectroscopy focusing on electrochemical interfaces is outlined. Finally, FUV-UV-surface plasmon resonance studies are discussed.

Autofluorescence Imaging of Living Yeast Cells with Deep-Ultraviolet Surface Plasmon Resonance

Autofluorescence in living cells on aluminum thin film was excited with deep-ultraviolet surface plasmon resonance (deep-UV SPR). Deep-UV SPR under aqueous medium was excited with Kretschmann configuration by using a sapphire prism. Deep-UV SPR is one of the promising techniques for high-sensitive autofluorescence imaging of living cells without staining. Label-free observation is significant for the structural analysis of living cells. We demonstrated the high-sensitive autofluorescence imaging of living yeast cells with deep-UV SPR. We applied a high refractive index prism, such as sapphire, which is suitable for the observation of specimens in aqueous medium, to excite deep-UV SPR. Although typical autofluorescence from living cells is buried in background noise, deep-UV SPR enhances the autofluorescence signal. The deep-UV SPR excitation of an aluminum thin film through a sapphire prism was investigated theoretically and experimentally. It showed that the fluorescence intensities are increased 2.8-fold. Deep-UV SPR enhanced the autofluorescence of cell structures, and yeast cells were found to be very sensitive. As a result, for water-immersed specimens, the sapphire-prism-based Kretschmann configuration excited SPR in deep-UV. Findings from this study suggest that deep-UV SPR can be considered an effective technique for attaining high-sensitivity observation of biological samples.

Optical/electrochemical methods for detecting mitochondrial energy metabolism

This review highlights the biological importance of mitochondrial energy metabolism and the applications of multiple optical/electrochemical approaches to determine energy metabolites. Mitochondria, the main sites of oxidative phosphorylation and adenosine triphosphate (ATP) biosynthesis, provide the majority of energy required by aerobic cells for maintaining their physiological activity. They also participate in cell growth, differentiation, information transmission, and apoptosis. Multiple mitochondrial diseases, caused by internal or external factors, including oxidative stress, intense fluctuations of the ionic concentration, abnormal oxidative phosphorylation, changes in electron transport chain complex enzymes and mutations in mitochondrial DNA, can occur during mitochondrial energy metabolism. Therefore, developing accurate, sensitive, and specific methods for the in vivo and in vitro detection of mitochondrial energy metabolites is of great importance. In this review, we summarise the mitochondrial structure, functions, and crucial energy metabolic signalling pathways. The mechanism and applications of different optical/electrochemical methods are thoroughly reviewed. Finally, future research directions and challenges are proposed.

ATR-far-ultraviolet spectroscopy in the condensed phase-The present status and future perspectives

Far-ultraviolet (FUV) spectroscopy in the region of 140-200 nm of condensed-phase has received keen interest as a new electronic spectroscopy. The introduction of the attenuated total reflection (ATR) technique to the FUV region has opened a new avenue for FUV spectroscopy of liquids and solids. ATR-FUV spectroscopy enables the study of electronic structures and transitions of most types of molecules. It also holds great promise for a variety of applications, i.e., from the application to basic sciences to practical applications. In this review, the characteristics and advantages of ATR-FUV spectroscopy in the condensed phase are described first; then, a brief historical overview is provided. Next, the ATR-FUV spectroscopy instrumentation is outlined. After these introductory parts, a variety of AFT-FUV spectroscopy applications are introduced, starting from applications to investigations of electronic structure and transitions of alkanes, graphenes, and polymers. Then, time-resolved ATR-FUV spectroscopy is discussed. The applications to materials research, such as the research on inorganic semiconductors and ionic liquids, follow. In the last part, the FUV spectroscopy perspective is emphasized.

Enhanced Surface Plasmon Resonance Wavelength Shifts by Molecular Electronic Absorption in Far- and Deep-Ultraviolet Regions

In this study, surface plasmon resonance (SPR) wavelength shifts due to molecular electronic absorptions in the far-ultraviolet (FUV, < 200 nm) and deep-ultraviolet (DUV, < 300 nm) regions were investigated by attenuated total reflectance (ATR) spectroscopy. Due to the strong absorption in the DUV region, N,N-dimethylformamide (DMF) significantly increased the SPR wavelength shift of Al film. On the other hand, no such shift enhancement was observed in the visible region for Au film because DMF does not have absorbance compared to non-absorbing materials such as water and alcohols. The enhanced SPR wavelength shift, caused by the overlap between SPR and molecular resonance wavelengths in FUV-DUV region, is expected to result in high sensitivity for resonant materials.

Dual-channel extraordinary ultraviolet transmission through an aluminum nanohole array

Ultraviolet (UV) surface plasmon (SP) has distinct applications in UV filters, high-density optical storage, spectral enhancement, optical detectors, and nanolithography, which are closely related to plasmon-induced extraordinary optical transmission (EOT). However, such EOT in the UV region has not been the subject of detailed research. We report UV transmission based on theoretical research using the finite-difference time-domain method, by modulating the Al thickness, hole size, array periodicity, and SiO₂ overlayer thickness. It is notable that we can obtain dual-channel UV transmission peaks with excellent qualities such as high transmissivity, zero cross-talk, narrow bandwidth, and perfect symmetry, by optimizing the parameters. The UV transmission peaks have been discovered to non-monotonously shift with increasing hole size. Although array periodicity has great influence on the transmission peak position, the peak energy in the UV region is much less than the value predicted by the well-known periodicity-related surface plasmon polariton (SPP) wavelength equation; the energy discrepancy in the UV region can reach above 20%, which is much larger than the value (typically 4%) in the visible-infrared region. Furthermore, the SiO₂ overlayer may significantly modify the transmission properties. The Al nanohole arrays have also been found to exhibit distinct multi-band UV electric field enhancement properties with special interface effect and size effect. Such extraordinary dual-channel UV transmission with zero cross-talk, based on a very simple Al nanohole array, has promising application in dual-channel UV filters, high-density optical storage, and plasmon-enhanced fluorescence/Raman spectroscopy, which generally involves two wavebands (writing/reading storage or exciting/emission wavelengths). This study is expected to broaden our fundamental understanding of the UV EOT phenomenon, and provide references for experimental research and application of deep-UV and near-UV-related dual-band plasmonic devices.