Time-resolved Diffuse Reflectance Spectroscopy Research Articles

Femtosecond time-resolved diffuse reflectance study on facet engineered charge‐carrier dynamics in Ag3PO4 for antibiotics photodegradation

The contamination of antibiotics micro-pollutants in water bodies causes increasing concerns currently. Semiconductor-based photocatalysis has been considered as an efficient method for the removal of contaminations from aquatic environment, while its catalytic performance always depends on facets in semiconductor. To deeply understand the relation between facet and catalytic process, herein we take the different morphologies of Ag3PO4 as the cases to investigate the charge‐carrier dynamics in different facets of Ag3PO4 for the photodegradation of antibiotic ciprofloxacin (CIP), sulfamethoxazole (SMX), and tetracycline (TC). The degradation results show that Ag3PO4 tetrahedrons with dominated {111} facets exhibit 1.83 times higher CIP degradation rate than Ag3PO4 cubes with dominated {100} facets. Femtosecond time-resolved diffuse reflectance (fs-TDR) spectroscopy is used to give the detail charge-carrier dynamics in different facets of Ag3PO4 in accordance with the antibiotics pollutants degradation rate, which provides a more direct and visual proof.

Near Bandgap Excitation Inhibits the Interfacial Electron Transfer of Semiconductor/Cocatalyst.

Understanding the ultrafast interfacial electron transfer (IET) process is essential for establishing the structure-property relationship of the semiconductor/cocatalyst system for photocatalytic H2 evolution. However, the IET kinetics for the near bandgap excitation has not been reported. Herein, we investigate the IET kinetics of g-C3N4/Pt as a semiconductor/cocatalyst prototype by femtosecond time-resolved diffuse reflectance spectroscopy. We find that the near bandgap excitation of g-C3N4 inhibits the IET of g-C3N4/Pt due to electron deep trapping, resulting in a markedly decreased apparent quantum efficiency for photocatalytic H2 evolution. This work complements the kinetic understanding for the photocatalytic mechanism of the semiconductor/cocatalyst system in its whole light absorption range.

Shallow trap state-enhanced photocatalytic hydrogen evolution over thermal-decomposed polymeric carbon nitride.

We investigate the photogenerated electron kinetics of a thermal-decomposed polymeric carbon nitride (TCN) synthesized in air using femtosecond time-resolved diffuse reflectance spectroscopy. We find that the oxygen functional groups in TCN contribute to the formation of reactive shallow trap states for photogenerated electrons, leading to an enhanced activity for photocatalytic hydrogen evolution.

Shallow Trap State-Induced Efficient Electron Transfer at the Interface of Heterojunction Photocatalysts: The Crucial Role of Vacancy Defects.

Constructing vacancies has been demonstrated to be an effective way to modulate charge flow in semiconductor photocatalysts. However, the role of vacancies in the interfacial electron transfer (IET) of heterojunction photocatalysts remains poorly understood, which hinders the general design of heterojunction photocatalysts. Herein, by taking g-C3N4/MoS2 as a heterojunction photocatalyst prototype, we unravel that vacancies play a critical role in the IET of heterojunction photocatalysts. Theoretical simulations, combined with femtosecond time-resolved diffuse reflectance spectroscopy, give a clear physical picture that N vacancy states act as shallow trap states (STSs) for photogenerated electrons and thereby facilitate the IET process due to a large energy difference between STSs and charge separation states. Moreover, the excess electrons left by the loss of N atoms (producing N vacancies) could partially transfer to MoS2 to generate STSs in the forbidden band of MoS2, where the transferred photogenerated electrons could be further trapped to efficiently drive H2 evolution. This work suggests a promising strategy to tune IET of heterojunction photocatalysts for achieving highly efficient photocatalytic reactions.

In vivo label-free optical monitoring of structural and metabolic remodeling of myocardium following infarction.

Cardiac remodeling following myocardial infarction (MI) involves structural and functional alterations in the infarcted and remote viable myocardium that can ultimately lead to heart failure. The underlying mechanisms are not fully understood and, following our previous study of the autofluorescence lifetime and diffuse reflectance signatures of the myocardium in vivo at 16 weeks post MI in rats [Biomed. Opt. Express 6(2), 324 (2015)25780727], we here present data obtained at 1, 2 and 4 weeks post myocardial infarction that help follow the temporal progression of these changes. Our results demonstrate that both structural and metabolic changes in the heart can be monitored from the earliest time points following MI using label-free optical readouts, not only in the region of infarction but also in the remote non-infarcted myocardium. Changes in the autofluorescence intensity and lifetime parameters associated with collagen type I autofluorescence were indicative of progressive collagen deposition in tissue that was most pronounced at earlier time points and in the region of infarction. In addition to significant collagen deposition in infarcted and non-infarcted myocardium, we also report changes in the autofluorescence parameters associated with reduced nicotinamide adenine (phosphate) dinucleotide (NAD(P)H) and flavin adenine dinucleotide (FAD), which we associate with metabolic alterations throughout the heart. Parallel measurements of the diffuse reflectance spectra indicated an increased contribution of reduced cytochrome c. Our findings suggest that combining time-resolved spectrofluorometry and diffuse reflectance spectroscopy could provide a useful means to monitor cardiac function in vivo at the time of surgery.

Redox Dynamics of Pd Supported on CeO2–ZrO2 during Oxygen Storage/Release Cycles Analyzed by Time-Resolved in Situ Reflectance Spectroscopy

In situ time-resolved diffuse reflectance spectroscopy provided the redox dynamics of Pd nanoparticles supported on an oxygen storage material CeO2–ZrO2 (CZ) under lean/rich perturbation conditions. Because the reflectance at 450 nm is sensitive to the Pd oxidation state but is not affected by the redox of Ce3+/Ce4+ species of CZ, the real-time Pd redox can be monitored every second during oxygen storage/release in simulated engine combustion exhaust gas (CO–C3H6–NO–O2) corresponding to gasoline air-to-fuel ratios of 14.1 (rich) and 15.0 (lean). Although a large amount of O2 was stored by CZ upon the rich-to-lean switch, the rate of Pd oxidation during this event was found to be much more moderate compared to that with a reference catalyst, Pd/Al2O3. Because rapid oxygen uptake by CZ reduces the local O2 partial pressure near the surface, the oxidation of Pd should be retarded. This can preserve active metallic Pd and thus contribute to longer retention of high NO reduction efficiency even under the lean ...

Facet Effects of Ag3 PO4 on Charge-Carrier Dynamics: Trade-Off Between Photocatalytic Activity and Charge-Carrier Lifetime.

Silver phosphate (Ag3 PO4 ) is a promising visible-light-driven photocatalyst with a strong oxidation power and exceptionally high apparent quantum yield of O2 evolution. Although engineering Ag3 PO4 facets is widely known to enhance its photocatalytic activity, most studies have explained its facet effect by calculating surface energies. Herein, the charge carrier dynamics in three kinds of Ag3 PO4 crystals (mixed facets, cubic, and tetrahedral structures) were first investigated using single-particle photoluminescence microscopy and femtosecond time-resolved diffuse reflectance spectroscopy. As a result, we clarified that the disagreement between the photocatalytic activities (dye degradation and O2 evolution) of different Ag3 PO4 facets are the consequence of trade-off between catalytic activity and lifetime of photogenerated charge carriers in addition to surface energy.

Charge Carrier Dynamics in TiO2 Mesocrystals with Oxygen Vacancies for Photocatalytic Hydrogen Generation under Solar Light Irradiation

There is a great concern about black TiO2 prepared by H2 treatment because of its ability to enhance light harvesting of TiO2. Black TiO2 shows different photocatalytic activities compared with white TiO2. However, the mechanism of photocatalytic reaction has not been clearly understood. Here, femtosecond time-resolved diffuse reflectance (fs-TRDR) spectroscopy and single-particle photoluminescence measurements were applied to gain better understanding about the relation between oxygen vacancy, charge transfer, lifetime of photogenerated charge, and photocatalytic activity. We prepared reduced TiO2 mesocrystals (R-TMC) through simple solid-state chemical reduction at moderate temperature 350 °C. R-TMC has nearly two times higher photocatalytic activity for H2 production under solar light irradiation. The presence of oxygen vacancies and Ti3+ was studied by electro paramagnetic resonance and X-ray photoelectron spectroscopy. This work confirms the findings of previous studies that enhancement of light abso...

Multiwavelength time-resolved near-infrared spectroscopy of the adult head: assessment of intracerebral and extracerebral absorption changes.

An optical technique based on diffuse reflectance measurement combined with indocyanine green (ICG) bolus tracking is extensively tested as a method for the clinical assessment of brain perfusion at the bedside. We report on multiwavelength time-resolved diffuse reflectance spectroscopy measurements carried out on the head of a healthy adult during the intravenous administration of a bolus of ICG. Intracerebral and extracerebral changes in absorption were estimated from an analysis of changes in statistical moments (total number of photons, mean time of flight and variance) of the distributions of times of flight (DTOF) of photons recorded simultaneously at 16 wavelengths from the range of 650-850 nm using sensitivity factors estimated by diffusion approximation based on a layered model of the studied medium. We validated the proposed method in a series of phantom experiments and in-vivo measurements. The results obtained show that changes in the concentration of the ICG can be assessed as a function of time of the experiment and depth in the tissue. Thus, the separation of changes in ICG concentration appearing in intra- and extracerebral tissues can be estimated from optical data acquired at a single source-detector pair of fibers/fiber bundles positioned on the surface of the head.

2D/2D Heterostructured CdS/WS2 with Efficient Charge Separation Improving H2 Evolution under Visible Light Irradiation.

Efficient water splitting for H2 evolution under visible light irradiation has attracted more attention for solving the global environmental and energy issues, but it is still a major challenge to develop an earth-abundant and efficient photocatalyst. Herein, we report two-dimensional (2D)/2D heterostructured CdS/WS2 (CdS/WS2), composed of nanosheet CdS (CdS) and nanosheet WS2 (WS2), as an efficient photocatalyst for H2 evolution. As a noble metal-free visible light-driven catalyst for H2 evolution, CdS/WS2 with 10 wt % WS2 exhibited the largest H2 evolution rate of 14.1 mmol g-1 h-1 under visible light irradiation to be 8 times larger than that of pure CdS. The lifetime and dynamics of photogenerated electrons were evaluated by femtosecond time-resolved diffuse reflectance spectroscopy, indicating that WS2 works as an electron-trapping site and a cocatalyst to cause H2 evolution under visible light irradiation. This work suggests that CdS/WS2 has great potential as a low-cost and highly efficient photocatalyst for water splitting.

Electron transfer dynamics of quaternary sulfur semiconductor/mos2 layer-on-layer for efficient visible-light h2 evolution

Photocatalytic water reduction into H2 provides huge charm for simultaneously resolving fossil energy shortage and environmental issues. Seeking photocatalysts with characteristics of low-cost, environmental friendliness and visible light response, and accelerating the charge separation remain significant challenges to develop high efficient solar-to-H2 conversion systems. Here, quaternary sulfur semiconductor Zn0.4Ca0.6In2S4 (ZCIS) microspheres composed of cross-linked nanosheets and ZCIS microspheres modified with MoS2 (ZM-X) were designed and prepared via hydrothermal methods. Detailed characterizations indicated that the layer structured MoS2 nanosheets were mainly deposited on the surface of ZCIS nanosheet to form a 2D-2D structure that greatly increased the contact surface area for charge transfer, which was critical to the high photocatalytic performance for H2 evolution. Femtosecond time-resolved diffuse reflectance spectroscopy was used to evaluate the transfer dynamics of photogenerated electrons in ZCIS and ZM-X and revealed that an additional decay route within 5 ps for the photogenerated electrons in ZCIS after combining with MoS2 appeared which could be ascribed to the electron injection from ZCIS to MoS2. In addition, it was also demonstrated that ZM-3.0 (ZCIS with 3 wt% MoS2 loading amount) exhibited the fastest electron injection within only 1.12 ps and the highest efficient injection efficiency of 69.9%. As a result, ZM-3.0 exhibited the evolution rate of 3.5 μmol h-−1 under visible light irradiation (λ ≥ 420 nm), which was 27 times higher than that of ZCIS (0.13 μmol h−1). The results indicate that the quaternary sulfur semiconductor has great potential as a new kind of photocatalysts for solar energy conversion.

Time-resolved fluorescence (TRF) and diffuse reflectance spectroscopy (DRS) for margin analysis in breast cancer.

One of the major problems in breast cancer surgery is defining surgical margins and establishing complete tumor excision within a single surgical procedure. The goal of this work is to establish instrumentation that can differentiate between tumor and normal breast tissue with the potential to be implemented in vivo during a surgical procedure. A time-resolved fluorescence and reflectance spectroscopy (tr-FRS) system is used to measure fluorescence intensity and lifetime as well as collect diffuse reflectance (DR) of breast tissue, which can subsequently be used to extract optical properties (absorption and reduced scatter coefficient) of the tissue. The tr-FRS data obtained from patients with Invasive Ductal Carcinoma (IDC) whom have undergone lumpectomy and mastectomy surgeries is presented. A preliminary study was conducted to determine the validity of using banked pre-frozen breast tissue samples to study the fluorescence response and optical properties. Once the validity was established, the tr-FRS system was used on a data-set of 40 pre-frozen matched pair cases to differentiate between tumor and normal breast tissue. All measurements have been conducted on excised normal and tumor breast samples post surgery. Our results showed the process of freezing and thawing did not cause any significant differences between fresh and pre-frozen normal or tumor breast tissue. The tr-FRS optical data obtained from 40 banked matched pairs showed significant differences between normal and tumor breast tissue. The work detailed in the main study showed the tr-FRS system has the potential to differentiate malignant from normal breast tissue in women undergoing surgery for known invasive ductal carcinoma. With further work, this successful outcome may result in the development of an accurate intraoperative real-time margin assessment system. Lasers Surg. Med. 50:236-245, 2018. © 2018 Wiley Periodicals, Inc.

Redox Dynamics of Rh Supported on ZrP2O7 and ZrO2 Analyzed by Time-Resolved In Situ Optical Spectroscopy

In situ time-resolved diffuse reflectance spectroscopy was first applied to supported Rh catalysts (0.4 wt % Rh/ZrO2 and Rh/ZrP2O7) under dynamic three-way catalysis conditions fluctuating between fuel-lean and fuel-rich gas atmospheres. The optical absorption at 650 nm was found to decrease upon lean-to-rich switching of the gas feed, which led to the reduction of Rh oxide (Rh3+) to metallic Rh (Rh0), followed by a reversible increase upon back switching rich-to-lean. The kinetic analysis suggested that the reduction of Rh3+ to Rh0 was faster than the reoxidation over Rh/ZrP2O7, whereas the reduction was comparable with or slower than the reoxidation over Rh/ZrO2. The activation energy of Rh/ZrP2O7 for the reduction, 13.6 kJ mol–1, was smaller than that for the oxidation, 48.7 kJ mol–1, which contrasted with those of Rh/ZrO2 (21.4 and 34.1 kJ mol–1, respectively). These results were closely associated with the higher NO reduction activity of Rh/ZrP2O7 than Rh/ZrO2 under a lean-gas atmosphere because Rh w...

Au/La2 Ti2 O7 Nanostructures Sensitized with Black Phosphorus for Plasmon-Enhanced Photocatalytic Hydrogen Production in Visible and Near-Infrared Light.

Efficient utilization of solar energy is a high-priority target and the search for suitable materials as photocatalysts that not only can harvest the broad wavelength of solar light, from UV to near-infrared (NIR) region, but also can achieve high and efficient solar-to-hydrogen conversion is one of the most challenging missions. Herein, using Au/La2 Ti2 O7 (BP-Au/LTO) sensitized with black phosphorus (BP), a broadband solar response photocatalyst was designed and used as efficient photocatalyst for H2 production. The optimum H2 production rates of BP-Au/LTO were about 0.74 and 0.30 mmol g-1 h-1 at wavelengths longer than 420 nm and 780 nm, respectively. The broad absorption of BP and plasmonic Au contribute to the enhanced photocatalytic activity in the visible and NIR light regions. Time-resolved diffuse reflectance spectroscopy revealed efficient interfacial electron transfer from excited BP and Au to LTO which is in accordance with the observed high photoactivities.

Integrated Time-Resolved Fluorescence and Diffuse Reflectance Spectroscopy Instrument for Intraoperative Detection of Brain Tumor Margin

Time-resolved fluorescence (TRF) and diffuse reflectance (DR) spectroscopy are two optical biopsy modalities that have been studied in tumor diagnosis. Combination of TRF and DR spectroscopy allows us to obtain more features such as fluorescence intensity, lifetime, and optical properties; thus, potentially improving the tissue diagnostic accuracy. In this paper, an integrated TRF-DR spectroscopy instrument was developed to acquire TRF spectra as well as spatially resolved diffuse reflectance spectra in sequence for intraoperative detection of brain tumor margin. The performance of TRF-DR spectroscopy instrumentation was calibrated and evaluated using endogenous biomolecules, tissue phantoms, and ex vivo brain tumor specimens. The results demonstrated that the TRF-DR system is capable to retrieve the fluorescence and optical properties accurately.

Ultrafast charge separation dynamics in opaque, operational dye-sensitized solar cells revealed by femtosecond diffuse reflectance spectroscopy.

Efficient dye-sensitized solar cells are based on highly diffusive mesoscopic layers that render these devices opaque and unsuitable for ultrafast transient absorption spectroscopy measurements in transmission mode. We developed a novel sub-200 femtosecond time-resolved diffuse reflectance spectroscopy scheme combined with potentiostatic control to study various solar cells in fully operational condition. We studied performance optimized devices based on liquid redox electrolytes and opaque TiO2 films, as well as other morphologies, such as TiO2 fibers and nanotubes. Charge injection from the Z907 dye in all TiO2 morphologies was observed to take place in the sub-200 fs time scale. The kinetics of electron-hole back recombination has features in the picosecond to nanosecond time scale. This observation is significantly different from what was reported in the literature where the electron-hole back recombination for transparent films of small particles is generally accepted to occur on a longer time scale of microseconds. The kinetics of the ultrafast electron injection remained unchanged for voltages between +500 mV and –690 mV, where the injection yield eventually drops steeply. The primary charge separation in Y123 organic dye based devices was clearly slower occurring in two picoseconds and no kinetic component on the shorter femtosecond time scale was recorded.

Facile preparation of nitrogen and fluorine codoped TiO2 mesocrystal with visible light photocatalytic activity

N-doped TiO2 mesocrystal was prepared through simple hydrothermal treatment with triethanolamine, while F-doped TiO2 mesocrystal was easily prepared by simple stirring of TiO2 mesocrystal with NaF at room temperature. The crystal structure and surface area of TiO2 mesocrystal were not affected by the N- and F-doping. The prepared samples were characterized by X-ray diffraction (XRD), N2 adsorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV–vis diffuse reflectance spectroscopy. N,F-codoped TiO2 mesocrystal showed the highest photocatalytic activity for the degradation of Rhodamine B (RHB) and 4-nitrophenol (4-NP) under the visible light irradiation in 90% and 62% yield, respectively, suggesting the importance of the midgap level due to N 2p above the valence band, and the synergetic effect of N and F doping. Femtosecond time-resolved diffuse reflectance (TDR) spectroscopy was used to clarify the charge-separation, trapping and recombination mechanism, indicating an important role of F for increasing the photocatalytic activity.

In Situ Fluorine Doping of TiO2 Superstructures for Efficient Visible-Light Driven Hydrogen Generation.

With the aid of breakthroughs in nanoscience and nanotechnology, it is imperative to develop metal oxide semiconductors through visible light-driven hydrogen generation. In this study, TiOF2 was incorporated as an n-type F-dopant source to TiO2 mesocrystals (TMCs) with visible-light absorption during the topotactic transformation. The crystal growth, structural change, and dynamic morphological evolution, from the initial intermediate NH4 TiOF3 to HTiOF3, TiOF2, and F-doped TMCs, were verified through in situ temperature-dependent techniques to elucidate the doping mechanism from intermediate TiOF2. The visible-light efficiencies of photocatalytic hydrogen were dependent on the contents of the dopant as compared with the pure TMC and a controled reference. Using femtosecond time-resolved diffuse reflectance spectroscopy, the charge-transfer dynamics were monitored to confirm the improvement of charge separation after doping.

165 Label-free autofluorescence lifetime to assess changes in myocardial fibrosis and metabolismin vivoin a doxorubicin cardiomyopathy heart failure model

We are developing an autofluorescence (AFL) lifetime-based technique to characterise the signatures associated with histological, metabolic and functional changes in myocardial disease. AFL spectroscopy exploits the properties of a number of endogenous molecules and offers the potential to avoid exogenous label use that could potentially alter the physiological environment. AFL measurements of molecules such as NADH and flavins may report energetic state changes, whilst signatures from matrix such as collagen inform of structural alterations. We investigated the application of a fibre-optic based single-point instrument combining time-resolved spectrofluorometry and diffuse reflectance spectroscopy to a doxorubicin cardiomyopathy heart failure model (DOX-HF) in vivo . Sprague-Dawley male rats received 1.25 mg/kg doxorubicin or vehicle (0.9% NaCl) via tail vein injection weekly for 8 weeks. AFL signals were measured in vivo in week 11, once heart failure phenotype was well established. DOX-HF(n = 8) LVEF 49.6% vs. 80.4% in controls (n = 4)(**p = 0.006). At 11 weeks, significant differences in AFL signals between DOX-HF(n = 6) and control (n = 3) in LV, RV and LV posterior wall were observed (*p Our instrument has sensitivity to characterise in vivo changes in DOX-HF model without use of exogenous compounds. Incorporation into coronary catheters or pacing leads could offer additional diagnostic information and monitoring. Identification of earlier changes could permit development of a screening strategy to identify cancer patients developing cardiomyopathy to permit judicious adjustment of chemotherapy.

Kinetics of Distance-Dependent Recombination between Geminate Charge Carriers by Diffusion under Coulomb Interaction

The survival probability of electron–hole pairs decays following a power law given by t–1/2 at long times when geminate recombination proceeds under diffusion and Coulomb interaction. Although the power law decay was known, the dependence of the decay amplitude on the strength of the Coulomb interaction, distance dependence of the intrinsic recombination rate, and the diffusion coefficient has not yet been fully understood. In this manuscript, we show analytical expressions on the amplitude of the asymptotic decay and compare the results with the overall kinetics obtained by numerical calculations. The results were applied to the measured data on the kinetics of carriers in LaTiO2N (LTON) solid photocatalyst measured using time-resolved diffuse reflectance spectroscopy (TRDR). The kinetic parameters of carriers were estimated and the presence of trap states was suggested. We also discuss the generalization of the results to the case that the diffusion is dispersive. The dispersive transport of carriers is considered to originate from the carrier transport by successive transitions among trap states with various release times.