Surface Plasmon Resonance Imaging-MALDI-TOF Imaging Mass Spectrometry of Thin Tissue Sections

Coupling of surface plasmon resonance and mass spectrometry for molecular interaction studies in drug discovery

Various analytical technologies have been developed for the study of target-ligand interactions. The combination of these technologies gives pivotal information on the binding mechanism, kinetics, affinity, residence time, and changes in molecular structures. Mass spectrometry (MS) offers structural information, enabling the identification and quantification of target-ligand interactions. Surface plasmon resonance (SPR) provides kinetic information on target-ligand interaction in real time. The coupling of MS and SPR complements each other in the studies of target-ligand interactions. Over the last two decades, the capabilities and added values of SPR-MS have been reported. This review summarizes and highlights the benefits, applications, and potential for further research of the SPR-MS approach.

Research advances on surface plasmon resonance biosensors

The surface plasmon resonance (SPR) phenomenon is of wide interest due to its sensitivity to changes in surface refractive index for the label-free, highly sensitive and rapid detection of biomarkers. This paper reviews research progress on SPR biosensors modified with different substrate structures and surface materials, surface plasmon resonance imaging (SPRI), and SPR-enhanced electrochemiluminescent (ECL) biosensors for applications in biosensing in the last five years. This paper focuses on the research on the application of the SPR phenomenon in the field of bio-detection, reviews the sensing characteristics of SPR biosensors with substrate structures of prisms, gratings, and optical fibers, and summarizes and analyzes the sensitivity and interference resistance of SPR sensors with surface modification of different materials (high-refractive index dielectric films, metallic micro- and nanostructures, and surface antifouling materials). Considering that imaging is an important tool for biomedical detection, this paper reviews the research progress on SPRI technology in the field of biomedical detection. In addition, this paper also reviews the research progress on SPR-enhanced ECL biosensors in the field of biosensing. Finally, this paper provides an outlook on the development trends of biosensing technology in terms of portable high-precision SPR sensors, reduction of self-loss of thin film materials, optimization of image processing techniques and simplification of electrode modification for ECL sensors.

Combined Analysis Based on a Crystalline Sponge Method

The crystalline sponge (CS) method was developed as an X-ray crystallographic molecular structure analysis method that can be performed without the need for crystallization of the analyte. CS has strong molecular-recognition properties and a highly flexible framework. The amount of analyte can be reduced to a sub-milligram level. These features of the crystalline nano-space allow for determining the absolute structure of a trace analyte. In this review, we focus on the discovery of the CS method and its applications to biosynthetic products in combination with NMR spectroscopy. We also describe some examples of the CS method that are used mainly in combination with mass spectrometry (MS). Both approaches demonstrate the potential of microanalysis to determine the molecular structure of an unknown sample. Finally, we mention the use of a crystalline "nano-surface" rather than a crystalline nano-space in MS, which can detect small metabolites as well as post-translation biomolecules.

Microscopic Study on Excitation and Emission Enhancement by the Plasmon Mode on a Plasmonic Chip

Excitation and emission enhancement by using the plasmon mode formed on a plasmonic chip was studied with a microscope and micro-spectroscope. Surface plasmon resonance wavelengths were observed on one-dimensional (1D) and two-dimensional (2D) plasmonic chips by measuring reflection and transmission spectra, and they were assigned to the plasmon modes predicted by the theoretical resonance wavelengths. The excitation and emission enhancements were evaluated using the fluorescence intensity of yellow–green fluorescence particles. The 2D grating had plasmon modes of kgx45(2) (diagonal direction with m = 2) in addition to the fundamental mode of kgx(1) (direction of a square one side) in the visible range. In epifluorescence detection, the excitation enhancement factors of kgx(2) on the 1D and 2D chips were found to be 1.3–1.4, and the emission enhancement factor of kgx45(2) on the 2D chip was 1.5–1.8, although the emission enhancement was not found on the 1D chip. Moreover, enhancement factors for the other fluorophores were also studied. The emission enhancement factor of kgx(1) was shown to depend on the fluorescence quantum yield. The emission enhancement of 2D was 1.3-fold larger than that of 1D considering all azimuth components, and the 2D pattern was shown to be advantageous for bright fluorescence microscopic observation.

Rapid label-free detection of intact pathogenic bacteria in situ via surface plasmon resonance imaging enabled by crossed surface relief gratings

The unique plasmonic energy exchange occurring within metallic crossed surface relief gratings (CSRGs) has recently motivated their use as biosensors. However, CSRG-based biosensing has been limited to spectroscopic techniques, failing to harness their potential for integration with ubiquitous portable electronics. Here, we introduce biosensing via surface plasmon resonance imaging (SPRi) enabled by CSRGs. The SPRi platform is fully integrated including optics and electronics, has bulk sensitivity of 613 Pixel Intensity Unit (PIU)/Refractive Index Unit (RIU), a resolution of 10^-6 RIU and a signal-to-noise ratio of ∼33 dB. Finite-Difference Time-Domain (FDTD) simulations confirm that CSRG-enabled SPRi is supported by an electric field intensity enhancement of ∼30 times, due to plasmon resonance at the metal-dielectric interface. In the context of real-world biosensing applications, we demonstrate the rapid (<35 min) and label-free detection of uropathogenic E. coli (UPEC) in PBS and human urine samples for concentrations ranging from 10³ to 10⁹ CFU mL^-1. The detection limit of the platform is ∼100 CFU mL^-1, three orders of magnitude lower than the clinical detection limit for diagnosis of urinary tract infection. This work presents a new avenue for CSRGs as SPRi-based biosensing platforms and their great potential for integration with portable electronics for applications requiring in situ detection.

Crystalline sponge-laser desorption ionization (CS-LDI) of unsaturated cyclic organic compounds encapsulated in different electronic environments in pores

We have developed laser desorption ionization mass spectrometry (LDI-MS) of organic molecules encapsulated in a crystalline sponge (CS). Cyclic organic compounds 1,2,3,4,5-pentamethylcyclopentadiene, zerumbone, and muscone were encapsulated in CS as guest regardless of guest crystallization. Single-crystal X-ray analysis closely scrutinized the position of the guest in the pore and the interaction between the guest and the CS framework. After single-crystal X-ray analysis, the same single crystal was subjected to LDI-MS. Ionization efficiency differed markedly depending on whether π-π interaction existed or not. It is obvious that π-π interaction is the key to transferring laser energy from the CS framework to the guest for the ionization of molecule encapsulated in CS. The results suggest the importance of controlling the positions of analyte and matrix as well as noncovalent interactions, such as hydrogen bonding interaction, electrostatic interaction, and so on.

Matrix-assisted laser desorption/ionization mass spectrometric imaging for the rapid segmental analysis of methamphetamine in a single hair using umbelliferone as a matrix

Segmental hair analysis offers a longer period for retrospective drug detection than blood or urine. Hair is a keratinous fiber and is strongly hydrophobic. The embedding of drugs in hydrophobic hair at low concentrations makes it difficult for extraction and detection with matrix-assisted laser desorption/ionization (MALDI) coupled with mass spectrometric imaging (MSI). In this study, a single scalp hair was longitudinally cut with a cryostat section to a length of 4 mm and fixed onto a stainless steel MALDI plate. Umbelliferone was used as a new hydrophobic matrix to enrich and assist the ionization efficiency of methamphetamine in the hair sample. MALDI-Fourier transform ion cyclotron resonance (FTICR)-MS profiling and imaging were performed for direct detection and mapping of methamphetamine on the longitudinal sections of the single hair sample in positive ion mode. Using MALDI-MSI, the distribution of methamphetamine was observed throughout five longitudinally sectioned hair samples from a drug abuser. The changes of methamphetamine were also semi-quantified by comparing the ratios of methamphetamine/internal standard (I.S). This method improves the detection sensitivity of target drugs embedded in a hair matrix for imaging with mass spectrometry. The method could provide a detection level of methamphetamine down to a nanogram per milligram incorporated into hair. The results were also compared with the conventional high performance liquid chromatography -tandem mass spectrometry (HPLC-MS/MS) method. Changes in the imaging results over time by the MSI method showed good semi-quantitative correlation to the results from the HPLC-MS/MS method. This study provides a powerful tool for drug abuse control and forensic medicine analysis in a narrow time frame, and a reduction in the sample amount required.

Structural Analysis of Natural Products

Current mass spectrometry, nuclear magnetic resonance spectroscopy, and X-ray diffraction are presented as structure elucidation tools for analytical chemistry of natural products. Discovering new molecular entities combined with dereplication of known organic compounds represent prerequisites for biological assays and for respective applications as pharmaceuticals or molecular markers. Liquid chromatography is briefly addressed with respect to its use in mass spectrometry- and nuclear magnetic resonance-based metabolomics studies.