ABSTRACT
Frequency-domain near-infrared spectroscopy (FD-NIRS) provides quantitative noninvasive measurements of tissue optical absorption and scattering, as well as a safe and accurate method for characterizing tissue composition and metabolism. However, the poor scalability and high complexity of most FD-NIRS systems assembled to date have contributed to its limited clinical impact. To address these shortcomings, we present a scalable, digital-based FD-NIRS platform capable of measuring optical properties and tissue chromophore concentrations in real-time. The system provides single-channel FD-NIRS amplitude/phase, optical property, and chromophore data at a maximum display rate of 36.6 kHz, 17.9 kHz, and 10.2 kHz, respectively, and can be scaled to multiple channels as well as integrated into a handheld format. The entire system is enabled by several innovations including an ultra-high-speed k-nearest neighbor lookup table method (maximum of 250,000 inversions/s for a large 2500x700 table of absorption and reduced scattering coefficients), embedded FPGA and CPU high-speed co-processing, and high-speed data transfer (due to on-board processing). We show that our 6-wavelength, broad modulation bandwidth (1-400 MHz) system can be used to perform 2D high-density spatial mapping of optical properties and high speed quantification of hemodynamics.
ABSTRACT
Two-dimensional molybdenum disulfide (MoS2) is emerging as a catalyst for energy and environmental applications. Recent studies have suggested the stability of MoS2 is questionable when exposed to oxidizing conditions found in water and air. In this study, the aqueous stability of 2H- and 1T-MoS2 and 2H-MoS2 protected with a carbon shell was evaluated in the presence of model oxidants (O2, NO2 -, BrO3 -). The MoS2 electrocatalytic performance and stability was characterized using linear sweep voltammetry and chronoamperometry. In the presence of dissolved oxygen (DO) only, 2H- and 1T-MoS2 were relatively stable, with SO4 2- formation of only 2.5% and 3.1%, respectively. The presence of NO2 - resulted in drastically different results, with SO4 2- formations of 11% and 14% for 2H- and 1T-MoS2, respectively. When NO2 - was present without DO, the 2H- and 1T-MoS2 remained relatively stable with SO4 2- formations of only 4.2% and 3.3%, respectively. Similar results were observed when BrO3 - was used as an oxidant. Collectively, these results indicate that the oxidation of 2H- and 1T-MoS2 can be severe in the presence of these aqueous oxidants but that DO is also required. To investigate the ability of a capping agent to protect the MoS2 from oxidation, a carbon shell was added to 2H-MoS2. In a batch suspension in the presence of DO and NO2 -, the 2H-MoS2 with the carbon shell exhibited good stability with no oxidation observed. The activity of 2H-MoS2 electrodes was then evaluated for the hydrogen evolution reaction by a Tafel analysis. The carbon shell improved the activity of 2H-MoS2 with a decrease in the Tafel slope from 451 to 371 mV dec-1. The electrode stability, characterized by chronopotentiometry, was also enhanced for the 2H-MoS2 coated with a carbon shell, with no marked degradation in current density observed over the reaction period. Because of the instability exhibited by unprotected MoS2, it will only be a useful catalyst if measures are taken to protect the surface from oxidation. Further, given the propensity of MoS2 to undergo oxidation in aqueous solutions, caution should be used when describing it as a true catalyst for reduction reactions (e.g., H2 evolution), unless proven otherwise.
