Probe Excitation Research Articles

A circular eddy current probe using miniature fluxgates for multi-orientation slit evaluation in steel components

Detection of crack propagation, such as its direction and depth, is one of the important aspects in ensuring the safety and reliability of steel structures. This study presents a development of a circular eddy current excitation probe using a planar differential miniature fluxgate to detect vertical and horizontal slits. The probe utilizes a circular eddy current excitation technique that induces multi-direction eddy currents in a mild steel plate and a sensing configuration based on the tangential magnetic response. The performances of the developed probe were characterized based on line and 2-D map scans of the magnetic response due to the induced eddy currents in mild-steel samples from different orientations and depths of artificial slits. The results showed that the developed probe obtained a signal correlation with the slit depth at different slit orientations. The vertical and horizontal slits were able to be visualized from the magnetic field distribution, where the differential imaginary component had a better detection sensitivity for the vertical slits represented by the measured peak-to-peak signals. The detection of multi-orientation slits revealed that the slit orientation could be estimated from the magnetic response maps with a detection limit of 5 mm in the slit length and 0.5 mm in the slit width, respectively.

Population and decay of the HenICl(β1) clusters

The letter presents the results of studies of population and decay of the {2,0}He2ICl(β1,vβ = 0) and {1,1}He2ICl(β1,vβ = 0) clusters. Action, pump–probe and luminescence excitation spectra as well as luminescence spectra themselves have been measured. The T-shaped HeICl(E,vE = 0 andD′,vD′ = 0) complex luminescence has been observed when the {2,0}He2ICl(β1,vβ = 0) cluster is populated. It has been shown that a population of the {1,1}He2ICl(A,13,nA) cluster at the energy higher than its dissociation threshold is accompanied by the T-shaped HeICl(A,13,nA) formation.

A thickness reduction testing method for ferromagnetic materials based on variable intensity DC magnetization

The assessments of internal thickness reduction of ferromagnetic components are extremely important. Traditional nondestructive testing (NDT) methods are usually non-linear and sometimes the electromagnetic parameters of components are required. In this paper, a thickness reduction testing method based on variable intensity DC magnetization (VIDC-MAG) is proposed. Both theoretical modeling analysis and simulation modeling analysis show that a pole point of magnetic permeability μm appears with the increase of DC magnetization intensity, which corresponds to an inflection point magnetizing current im. The im decreases with the increase of the defect depth of the wall thickness reduction and shows a linear relationship. The experimental results show that the method performs well for the inner wall thickness thinning and the relative error of the measurement results is small. Additionally, the measurement results are related to the width of the thickness reduction, the probe excitation frequency, the magnetic properties of ferromagnetic material and the lift-off distance of the magnetizer. The method is applicable to the thickness variation of the cover layer and does not need to know the electromagnetic parameters of the components in advance, which has greater practical application value.

Highly hydrostable and flexible opal photonic crystal film for enhanced up-conversion fluorescence sensor of COVID-19 antibody

Efficient detection of related markers is significant for the early screening of COVID-19. Near infrared (NIR) light excited up-conversion fluorescence probes are ideal for biosensing but limited by the low luminescence efficiency. In this work, a novel highly stable opal photonic crystal (OPC) structure was designed to provide an OPC effect for up-conversion fluorescence enhancement, and sensitive Novel Coronavirus IgG up-conversion FRET-based sensor was further constructed. For the problems of water stability and mechanical stability of polymer OPC which cannot be solved for a long time, polymer spray combined with a flipped OPC film strategy is presented. Fragmented size OPC film was firmly fixed by polymer modification layer, which gave large size OPC film great water stability, mechanical stability and bending performance without affecting the fluorescence enhancement property. On this basis, the up-conversion emission intensity was enhanced significantly, and fluorescence resonant energy transfer (FRET) based Novel Coronavirus IgG antibody sensor was constructed. Monolayer up-conversion nanoparticles (UCNPs) on the surface of the polydopamine (PDA)/OPC film can make the fluorescent signal more sensitive, and effectively reduce the detection limit. The test device integrating NIR excitation and mobile phone realized the visual fast detection, showing remarkable sensing performance for COVID-19 antibodies with the limit of detection (LOD) of 0.1 ng mL−1. This detection platform will provide a more effective tool for early detection of the novel coronavirus.

Confocal Single-Pixel Imaging

Obtaining depth-selective images requires gating procedures such as spatial, nonlinear, or coherence gating to differentiate light originating from different depths of the volume of interest. Nonlinear gating requires pulsed excitation sources and excitation probes, limiting easy usage. Coherence gating also requires broadband sources and interferometry requiring specialized stable setups. Spatial gating can be used both for fluorescence and reflection geometry and various light sources and thus has the least requirements on hardware, but still requires the use of a pinhole which makes it difficult to use for photography or widefield imaging schemes. Here, we demonstrate that we can utilize a single digital micromirror device (DMD) to simultaneously function as a dynamic illumination modulator and automatically synchronized dynamic pinhole array to obtain depth-sectioned widefield images. Utilizing the multiplexed measurement advantage of single-pixel imaging, we show that the depth and ballistic light gating of the confocal single pixel imaging scheme can be utilized to obtain images through glare and multiple scattering where conventional widefield imaging fails to recover clear images due to saturation or random scattered noise.

Preparation of ultracold ions in strong magnetic field and application to gas-phase NMR spectroscopy II

This paper presents a gas-phase nuclear magnetic resonance (NMR) apparatus, which may enable a tandem combination with an ion cyclotron resonance mass spectrometer. The NMR technique is widely used for analyzing the physical and chemical properties of materials; however, it is mostly limited to materials in condensed phases. Here, we construct a gas-phase NMR apparatus to extend this technique to gas-phase molecular ions. We outline the principle of NMR detection based on a Stern-Gerlach-type experiment and describe the experimental procedures and results for the preparation and manipulation of ultracold ions, which are fundamental for the proposed NMR detection. The development of the gas-phase NMR probe and radio-frequency excitation system and the features of the apparatus are presented. We also discuss the feasibility of combining this apparatus with an ion cyclotron resonance cell for realizing an NMR apparatus for mass-selected molecular ions briefly.

A theoretical study of the ESIPT mechanism for the 2-butyl-4-hydroxyisoindoline-1, 3-dione probe

For the synthesis of two-photon excited probes, fluorophores based on excited-state intramolecular proton transfer (ESIPT) become critical. The ESIPT process of 2-butyl-4-hydroxyisoindoline-1, 3-dione is analyzed in this paper by DFT and TDDFT methods. We choose three non-protonic solvents to study their solvation effect. By examining infrared vibration spectrum data, optimized geometric structures and reduced density gradient, the hydrogen bond strengthened after photoexcitation and increased gradually with the decrease of solvent polarity. The proton transfer in the excited state is a low-barrier ultrafast process. The observed fluorescence peak in the experiment is derived from the emission of the tautomer form. Compared with the reaction barriers on the potential energy curves for different non-protonic solvents, a weaker polar solvent is more favorable to the ESIPT process.

Accurate diagnosis of hepatic fibrosis with dual detection of nitric oxide and viscosity by a ratiometric fluorescent probe

Hepatic fibrosis (HF) is a prevalent and irreversible liver disease caused by unhealthy lifestyles. However, no specific diagnosis tools, except for liver biopsy, are available for HF so far. It was reported that nitric oxide (NO) levels and viscosity in the liver are elevated with HF aggravation, thereby possibly acting as HF biomarkers. Based on this finding, a single-wavelength excited ratiometric fluorescent probe, BDP, was developed as the NO-activated and viscosity-enhanced probe for diagnosing HF first. In the presence of NO, probe BDP exhibited a distinct fluorescence change from red to orange within 12 min. Moreover, the fluorescent intensity of probe BDP could be greatly strengthened in a highly viscous environment. In biological application, probe BDP showed excellent biocompatibility and an ability to ratiometrically image exogenous and endogenous NO in HeLa cells. In addition, more obvious fluorescent signals were observed in cells with increased viscosity induced by nystatin. In particular, the probe accurately diagnosed HF in a mouse model by analyzing ratiometric fluorescence signals.

Evanescent-Field Excited Surface Plasmon-Enhanced U-Bent Fiber Probes Coated with Au and ZnO Nanoparticles for Humidity Detection

We report the design, fabrication, and testing of a humidity sensor based on an optical fiber-based evanescent wave probe. The fiber was bent into a U-shape and de-cladded at the location of the bending. The de-cladded section was coated either with Au or with ZnO nanoparticles. Humidity is detected based on the interaction in the surface plasmon resonance of the Au/ZnO nanoparticles excited by an evanescent wave of light passing through the optical fiber. The response of the U-bent fibers to humidity was investigated using a specifically designed low-voltage portable interrogation box. We found that the fibers coated with ZnO nanoparticles were able to detect a minimum 0.1% change in humidity with an average sensitivity of 143 µV/%RH and 95% linearity over the 10% to 80% humidity range. In comparison, samples coated with Au and Au + ZnO nanoparticles demonstrated a minimum change detection of 0.3% RH and 2% RH respectively. The response and recovery time of the sensor were measured to be 3 s and 4 s, respectively, for a 60% change in humidity from 20% to 80%. The entire measurement system was operated by consuming an electrical power of 1.62 W at an input voltage of 12 Vdc.

New ICT-Based Ratiometric Two-Photon near Infrared Probe for Imaging Tyrosinase in Living Cells, Tissues, and Whole Organisms

Melanoma is a type of highly malignant and metastatic skin cancer. In situ molecular imaging of endogenous levels of the melanoma biomarker tyrosinase (TYR) may decrease the likelihood of mortality. In this study, we proposed the weakly fluorescent probe 1-(4-(2-(4-(dicyanomethylene)-4H-chromen-2-yl)vinyl)phenyl)-3-(4-hydroxybenzyl)urea (DCM-HBU), which releases a strong red-shifted fluorescent signal after a TYR-mediated oxidation followed by hydrolysis of the urea linkage. The large Stokes shift of the dye is owed to the recovery of the intramolecular charge transfer (ICT) effect. The resulting probe derivate shows a highly ratiometric fluorescence output. Furthermore, the simultaneous excitation by two near-infrared (NIR) photons of the released derivative of dicyanomethylene-4H-pyran (DCM-NH2) fluorophore could avoid the usual drawbacks, such as cellular absorption, autofluorescence, and light scattering, due to an usually short wavelength of the excitation light on biological systems, resulting in images with deeper tissue penetration. In addition, the probe is useful for the quantitative sensing of TYR activity in vivo, as demonstrated in zebrafish larvae. This new ratiometric two-photon NIR fluorescent probe is expected to be useful for the accurate detection of TYR in complex biosystems at greater depths than other one-photon excited fluorescent probes.

Surface dynamics of graphite probed by soft x-ray linear dichroism in Auger-emission yield

Element-specific investigation of the dynamics of surface atoms is not a trivial task. Here, the authors show that time-resolved near-edge x-ray absorption for strongly anisotropic excitation resonances -- combined with Auger-electron detection -- yields such information, whereby the small Auger-electron escape depth provides surface sensitivity. The method is applied to highly oriented graphite in a pump-probe scheme, based on femtosecond laser excitation and x-ray probe pulses with variable polarization direction and delay time, both of which influence transient changes in the $K\phantom{\rule{0}{0ex}}V\phantom{\rule{0}{0ex}}V$ Auger yield.

Vibro-Acoustic Modulation with broadband pump excitation for efficient impact damage detection in composite materials

In the past few decades, the need for efficient and reliable Structural Health Monitoring strategies has led to the development of several approaches for damage detection and characterization purposes. Among them, the Nonlinear Vibro-Acoustic Modulation (VAM) exploits the modulation arising from the interaction of two concurrently applied driving waves, namely the probe and the pump excitations, in the presence of nonlinear scatters such as cracks and defects. Therefore, the VAM provides information on the emergence of internal damage by extracting the nonlinear modulated components of the response of a damaged system. Originally proposed for granular media, the method has shown to be effective in detecting the presence of defects also in metals and composite materials. Nonetheless, its efficacy is highly affected by the excitation frequencies, which are usually chosen among the system resonances. The need for a preliminary modal analysis and, at once, the risk of selecting pump-probe frequency combinations with low sensitivity to damage may make the procedure time-consuming and not fully reliable, preventing the VAM technique from being widely accepted as a robust monitoring tool. To overcome these limitations, a broadband excitation may be used. This study assesses the effectiveness of the VAM technique when a combination of a frequency-swept pump excitation and a mono-harmonic probe wave is applied to drive the sample. Experimental tests were conducted on a composite laminated beam mounted on an electrodynamic shaker and tested in both pristine and damaged conditions. Low-profile surface-bonded piezoceramic transducers were used for both probe excitation and sensing. Barely visible impact damage (BVID) was introduced in the composite beam to examine the potential of the approach for the detection of very small, localized damage. The results show that the use of VAM with a broadband low-frequency excitation may be an effective option for identifying nonlinearities associated with typical damage occurring in composite structures.

Ultrafast generation and detection of coherent acoustic phonons in SnS0.91Se0.09

The rapid growth of attention to tin sulfide (SnS) is due to its efficient application in the field of thermoelectricity, and its doping product SnS0.91Se0.09 can obtain a high average thermoelectric figure of merit (ZTave≈ 1.25). Although many efforts have been made to reveal the mechanisms associated with electron transport and heat conduction, the role of lattice vibration remains poorly understood. Here, we studied the coherent vibration dynamics in SnS0.91Se0.09 bulk single crystal by using the femtosecond two-color pump-probe optical reflectivity technique. The oscillation signal in the time domain of SnS0.91Se0.09 is considered to be an electron–phonon interaction. The coherent dynamics of longitudinal acoustic phonons in this coupling behavior is closely related to probe polarization and excitation power. The stability of oscillation frequency reveals the microscopic mechanism of phonon conduction, and further understands the lattice nonharmonic caused by electron–phonon interaction in light-induced.

Ultrabright two-photon excitable red-emissive fluorogenic probes for fast and wash-free bioorthogonal labelling in live cells.

Fluorogenic bioorthogonal reactions are promising tools for tracking small molecules or biomolecules in living organisms. Two-photon excitation, by shifting absorption towards the red, significantly increases the signal-to-noise ratio and decreases photodamage, while allowing imaging about 10 times deeper than with a confocal microscope. However, efficient two-photon excitable fluorogenic probes are currently lacking. We report here the design and synthesis of fluorogenic probes based on a two-photon excitable fluorophore and a tetrazine quenching moiety. These probes react with bicyclo[6.1.0]no-4-yn-9ylmethanol (BCN) with a good to impressive kinetic rate constant (up to 1.1 × 103 M-1 s-1) and emit in the red window with moderate to high turn-on ratios. TDDFT allowed the rationalization of both the kinetic and fluorogenic performance of the different probes. The best candidate displays a 13.8-fold turn-on measured by quantifying fluorescence intensities in live cells under one-photon excitation, whereas a value of 3 is sufficient for high contrast live-cell imaging. In addition, live-cell imaging under two-photon excitation confirmed that there was no need for washing to monitor the reaction between BCN and this probe since an 8.0-fold turn-on was measured under two-photon excitation. Finally, the high two-photon brightness of the clicked adduct (>300 GM) allows the use of a weak laser power compatible with in vivo imaging.

A two-photon fluorescent probe for formaldehyde detection and regeneration in living cells.

A two-photon excited fluorescent probe CMB-1 has been rationally developed for the detection and regeneration of formaldehyde based on a novel nucleophilic addition of a secondary amine to FA and subsequential alcoholysis reactivity mechanism. It enables a specific turn-on response towards formaldehyde and facilitates the monitoring of exogenous and endogenous formaldehyde in living cells via both one- and two-photon microscopy, with minimal influence on its native homeostasis and local concentration.

Two-photon excited red-green "discoloration" bioprobes for monitoring lipid droplets and lipid droplet-lysosomal autophagy.

Lipid droplets (LDs) and their autophagy by lysosomes are closely related to a variety of physiological and pathological conditions. Therefore, identifying and tracking LDs and the dynamic process of autophagy can provide useful information for the diagnostics and treatment of related diseases. However, few organic small molecule-based fluorescent probes can specifically recognize LDs and dynamically track their autophagy process. Herein, we synthesized a "discoloration" fluorescent bioprobe DPABP-BI with distinguishable features including red fluorescence emission (630 nm), large Stokes shift (145 nm), two-photon excitation and outstanding photostability and biocompatibility. In particular, LDs could be specifically identified via the red fluorescence emission of DPABP-BI (colocalization constant of 0.98), while autophagolysosomes could be visualized via the green fluorescence emission of its acid-hydrolyzed product (colocalization constant of 0.90) to track the autophagy dynamic process. In addition, DPABP-BI enabled the specific recognition of fatty substances in zebrafish larvae. In this study, a two-photon excited red light small molecule probe was constructed to identify LDs and track their autophagy dynamic process by changing the fluorescence emission wavelength.

Structural isomerism induces pH dependent AIE-coupled ESIPT: an in-depth spectroscopic exploration with application in amine vapor sensing.

A novel excited state intramolecular proton transfer probe 3-(benzo[d]thiazol-2-yl)-4-hydroxy-5-methoxybenzaldehyde (3BTHMB) was synthesized and characterized using NMR, ESIMS and single crystal diffraction studies. The steady state and time resolved studies using the time correlated single photon counting (TCSPC) technique revealed that 3BTHMB shows ESIPT reaction with a time constant of 0.10-0.15 ns and longer local emission and anion emissions of time constants 0.5-2.0 ns and 3.0-4.0 ns, respectively, that co-exist in polar solution. In a polar protic solution like methanol, the ESIPT process is damped. Interestingly, 3BTHMB shows aggregation induced emission (AIE) solely in an acidic environment, evidenced by the enhanced quantum yield of emission (∼35%) in acidic solution as well as a long lifetime component of 7.4 ns that corresponds to the lifetime in the solid state. The AIE phenomenon was explored using the DLS technique where enhancement of the hydrodynamic radius in an acidic medium was observed. The judicious positioning of the formyl group in 3BTHMB gives the mentioned probe to show AIE in an acidic medium, as opposed to its previously reported structural isomer BTHMB, which showed no AIE phenomenon as well as its parent molecule 2-(benzo[d]thiazol-2-yl)-6-methoxyphenol (TMP), which showed AIE in neutral pH. The AIE property of 3BTHMB was exploited by successful and rapid detection of amine vapor in the solid state, qualitative ratiometric detection of aqueous pH, by observing the colour change under UV light in acidic (yellow-green) and basic to neutral (blue) medium. The above results pave the way for multiutility fluorescent probes for environmental purposes along with a deep understanding of the underlying photophysical processes which bestow the probes with such outstanding utilities.

Novel V-shaped D-π-A-π-D two-photon absorption compounds with large Stokes shifts: Synthesis, optical properties, selective detection of cysteine, and imaging in living cells

The novel V-shaped donor-π-acceptor-π-donor compounds bearing an aldehyde group (1Pip, 2EtO, and 3TEG) were designed, synthesized, and evaluated as the two-photon excited fluorescence probes for the selective detection of cysteine (Cys). Their structures were fully characterized by 1H NMR, 13C NMR, FT-IR, and HRMS. The optical properties of the target compounds in various solvents were studied in details and the time-dependent density functional theory calculations were performed to shed light on their electronic structures. It is found that all of them exhibit large Stokes shifts (6331-10047 cm−1). 1Pip and 3TEG are capable of detecting Cys with high selectivity, good anti-interference ability, and favorable two-photon absorption performance. The limits of detection were determined to be equal to 0.15 μM for 1Pip and 0.73 μM for 3TEG. The possible recognition mechanism was also validated by 1H NMR and HRMS. One- and two-photon fluorescence imaging experiments indicate that 3TEG shows more satisfactory cellular membrane permeability than 1Pip, and it can successfully visualize the endogenous and exogenous Cys in living cells. These results imply that 3TEG has potential as a two-photon fluorescence probe for detecting Cys in biological samples.

A single-wavelength excited NIR fluorescence probe for distinguishing GSH/H2S and Cys/Hcy in living cells and zebrafish through separated dual-channels

Biothiols and hydrogen sulfide, as critical sulfur-containing reactive substances, serve essential functions in various human pathological processes, making it challenging to simultaneously distinguish them due to their similar reactivity and structures (−SH). Here, we rationalized the development of a single-wavelength excitation near-infrared (NIR) fluorescence probe, FC-NBD, for distinguishing GSH/H2S and Cys/Hcy by separated fluorescence dual channels. In this probe, FC-NBD, composed of coumarin-benzopyrylium derivatives linked with nitro benzoxadiazole (NBD) via ether bonds, could quantitatively and selectively distinguish GSH/H2S and Cys/Hcy with a low limit of detection (LOD) of 0.199/0.177 μM and 0.106/0.076 μM, respectively. As expected, under single-wavelength excitation (470 nm), FC-NBD demonstrated distinctly separable green and NIR fluorescence emissions towards Cys/Hcy at 550 and 660 nm, but only exhibited a noticeable NIR fluorescence emission towards GSH/H2S at 660 nm. Moreover, FC-NBD could simultaneously visualize and discriminate GSH/H2S and Cys/Hcy in living cells as well as zebrafish through green and NIR channels at a single excitation wavelength.

Germanium Monosulfide as a Natural Platform for Highly Anisotropic THz Polaritons.

Terahertz (THz) electromagnetic radiation is key to access collective excitations such as magnons (spins), plasmons (electrons), or phonons (atomic vibrations), thus bridging topics between optics and solid-state physics. Confinement of THz light to the nanometer length scale is desirable for local probing of such excitations in low-dimensional systems, thereby circumventing the large footprint and inherently low spectral power density of far-field THz radiation. For that purpose, phonon polaritons (PhPs) in anisotropic van der Waals (vdW) materials have recently emerged as a promising platform for THz nanooptics. Hence, there is a demand for the exploration of materials that feature not only THz PhPs at different spectral regimes but also host anisotropic (directional) electrical, thermoelectric, and vibronic properties. To that end, we introduce here the semiconducting vdW-material alpha-germanium(II) sulfide (GeS) as an intriguing candidate. By employing THz nanospectroscopy supported by theoretical analysis, we provide a thorough characterization of the different in-plane hyperbolic and elliptical PhP modes in GeS. We find not only PhPs with long lifetimes (τ > 2 ps) and excellent THz light confinement (λ0/λ > 45) but also an intrinsic, phonon-induced anomalous dispersion as well as signatures of naturally occurring, substrate-mediated PhP canalization within a single GeS slab.