Formation of Chiral Self-Assembled Structures of Amino Acids on Transition-Metal Surfaces: Alanine on Pd(111)

The structure and self-assembly of alanine on Pd(111) is explored using X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), reflection–absorption infrared spectroscopy (RAIRS), and scanning tunneling microscopy (STM), and supplemented by density functional theory (DFT) calculations to explore the stability of the proposed surface structures formed by adsorbing alanine on Pd(111) and to simulate the STM images. Both zwitterionic and anionic species are detected using RAIRS and XPS, while DFT calculations indicate that isolated anionic alanine is significantly more stable than the zwitterion. This observation is rationalized by observing dimeric species when alanine is dosed at ∼270 K and then cooled to trap metastable surface structures. The dimers form due to an interaction between the carboxylate group of anionic alanine with the NH₃⁺ group of the zwitterion. Adsorbing alanine at 290 K results in the formation of dimer rows and tetramers resulting in only short-range order, consistent with the lack of additional diffraction spots in LEED. The stability of various structures is explored using DFT, and the simulated STM images are compared with experiment. This enables the dimer rows to be assigned to the assembly of anionic-zwitterionic dimers and the tetramer to the assembly of two dimers in which three of the alanine molecules undergo a concerted rotation by 30°

Fast and Selective Room-Temperature Ammonia Sensors Using Silver Nanocrystal-Functionalized Carbon Nanotubes

We report a selective, room-temperature NH₃ gas-sensing platform with enhanced sensitivity, superfast response and recovery, and good stability, using Ag nanocrystal-functionalized multiwalled carbon nanotubes (Ag NC–MWCNTs). Ag NCs were synthesized by a simple mini-arc plasma method and directly assembled on MWCNTs using an electrostatic force-directed assembly process. The nanotubes were assembled onto gold electrodes with both ends in Ohmic contact. The addition of Ag NCs on MWCNTs resulted in dramatically improved sensitivity toward NH₃. Upon exposure to 1% NH₃ at room temperature, Ag NC–MWCNTs showed enhanced sensitivity (∼9%), very fast response (∼7 s), and full recovery within several minutes in air. Through density functional theory calculations, we found that the fully oxidized Ag surface plays a critical role in the sensor response. Ammonia molecules are adsorbed at Ag hollow sites on the AgO surface with H pointing toward Ag. A net charge transfer from NH₃ to the Ag NC–MWCNTs hybrid leads to the conductance change in the hybrid

Subcellular Biochemical Investigation of Purkinje Neurons Using Synchrotron Radiation Fourier Transform Infrared Spectroscopic Imaging with a Focal Plane Array Detector

Coupling Fourier transform infrared spectroscopy with focal plane array detectors at synchrotron radiation sources (SR-FTIR-FPA) has provided a rapid method to simultaneously image numerous biochemical markers in situ at diffraction limited resolution. Since cells and nuclei are well resolved at this spatial resolution, a direct comparison can be made between FTIR functional group images and the histology of the same section. To allow histological analysis of the same section analyzed with infrared imaging, unfixed air-dried tissue sections are typically fixed (after infrared spectroscopic analysis is completed) via immersion fixation. This post fixation process is essential to allow histological staining of the tissue section. Although immersion fixation is a common practice in this filed, the initial rehydration of the dehydrated unfixed tissue can result in distortion of subcellular morphology and confound correlation between infrared images and histology. In this study, vapor fixation, a common choice in other research fields where postfixation of unfixed tissue sections is required, was employed in place of immersion fixation post spectroscopic analysis. This method provided more accurate histology with reduced distortions as the dehydrated tissue section is fixed in vapor rather than during rehydration in an aqueous fixation medium. With this approach, accurate correlation between infrared images and histology of the same section revealed that Purkinje neurons in the cerebellum are rich in cytosolic proteins and not depleted as once thought. In addition, we provide the first direct evidence of intracellular lactate within Purkinje neurons. This highlights the significant potential for future applications of SR-FTIR-FPA imaging to investigate cellular lactate under conditions of altered metabolic demand such as increased brain activity and hypoxia or ischemia