Dual-wavelength Excitation Research Articles

High-sensitive thermometry and tunable dual-color luminescence based on effective energy transfer pathways of Er, Tm system

In this paper, the Er3+,Tm3+ co-doped La2Mo2O9 up-conversion luminescence phosphors are synthesized via solid state reaction. The luminescence characteristics and up-conversion processes of La2Mo2O9:Er3+,Tm3+ phosphors under the excitation of 980 and 1550 nm are investigated in detail. Tm3+ as an important energy transfer medium is the key to effectively improve the population of red-emitting energy levels (4F9/2) in the Er3+ sensitized system. Dual-color luminescence of green and red can be successfully achieved by controlling the concentration ratio of Er3+,Tm3+ co-doping under the dual-wavelength excitation. In addition, the optical temperature sensing ability of La2Mo2O9:Er3+,Tm3+ phosphors is analyzed by employing thermally coupled and non-thermally coupled energy levels. The obtained maximum absolute sensitivities are 8.33 × 10−3 K−1 under the 1550 nm excitation for non-thermally coupled energy levels. The La2Mo2O9:Er3+,Tm3+ phosphors with dual-color luminescence and high temperature sensing ability are the promising candidates in the fields of anti-counterfeiting and non-contact temperature sensing.

Opto-Chemical pH Detection of Myocardial Ischaemia Using Fluorescent Hydrogels

In this research fluorescent optochemical pH probes for the detection of ischaemia have been investigated. Myocardial ischaemia is the most prominent risk during heart surgery. During open heart surgery the heart is temporarily arrested and, since there no blood flowing, oxygen supply and removal of waste products is stopped and heart cells can be damaged. In this paper we propose a novel method to monitor the condition of the heart by placing optochemical pH sensors on several strategic places around the heart during surgery. Low cost opto-chemical pH sensors, using a HPTS (8-hydroxy-1,3,6-pyrene trisulfonic acid trisodium salt) fluorescent dye encapsulated in a thin bio-compatible hydrogel layer, were investigated for this application. Our research started with an extensive optical characterization of several types of hydrogel layers at different pH levels. Secondly a reflection probe prototype using several of these layers was designed, built and tested. Dual wavelength excitation and ratiometric detection of the fluorescent signals was used to detect the pH level. Typical output signals of 35% to 53% per pH in the range from 6.5-8.0 pH have been measured and a response time of typically 400 seconds was obtained for the prototypes. Finally based on our measurements on the HPTS layers and the reflection probe we propose an improved type of pH probe for the detection of ischaemia during open heart surgery.

Real-time whole-brain imaging of hemodynamics and oxygenation at micro-vessel resolution with ultrafast wide-field photoacoustic microscopy

High-speed high-resolution imaging of the whole-brain hemodynamics is critically important to facilitating neurovascular research. High imaging speed and image quality are crucial to visualizing real-time hemodynamics in complex brain vascular networks, and tracking fast pathophysiological activities at the microvessel level, which will enable advances in current queries in neurovascular and brain metabolism research, including stroke, dementia, and acute brain injury. Further, real-time imaging of oxygen saturation of hemoglobin (sO2) can capture fast-paced oxygen delivery dynamics, which is needed to solve pertinent questions in these fields and beyond. Here, we present a novel ultrafast functional photoacoustic microscopy (UFF-PAM) to image the whole-brain hemodynamics and oxygenation. UFF-PAM takes advantage of several key engineering innovations, including stimulated Raman scattering (SRS) based dual-wavelength laser excitation, water-immersible 12-facet-polygon scanner, high-sensitivity ultrasound transducer, and deep-learning-based image upsampling. A volumetric imaging rate of 2 Hz has been achieved over a field of view (FOV) of 11 × 7.5 × 1.5 mm3 with a high spatial resolution of ~10 μm. Using the UFF-PAM system, we have demonstrated proof-of-concept studies on the mouse brains in response to systemic hypoxia, sodium nitroprusside, and stroke. We observed the mouse brain’s fast morphological and functional changes over the entire cortex, including vasoconstriction, vasodilation, and deoxygenation. More interestingly, for the first time, with the whole-brain FOV and micro-vessel resolution, we captured the vasoconstriction and hypoxia simultaneously in the spreading depolarization (SD) wave. We expect the new imaging technology will provide a great potential for fundamental brain research under various pathological and physiological conditions.

Oxyfluoride glass-ceramics for upconversion all-optical combinational logic gate operations

All-optical logic gates operations are highly desired for optical communications and data processing because of the high computing speed, low power consumption, and parallel processing. Here, we develop a combinational upconversion (UC) all-optical logic system based on glass-ceramic (GC) material, which uses two external near-infrared excitation sources as inputs signal and UC emission as optical outputs. The prepared transparent oxyfluoride GC containing Er:NaYF4 nanocrystals could well respond to dual-wavelength excitation, realizing the enhancement of UC luminescence intensity and the adjustment of emission color based on the synergistic effect of photons. This special UC system can be used to design a variety of all-optical logic gates and combinational logic operations through observing the two luminescence emission intensities and the ratio of these emission intensities under the dual-wavelength excitation input conditions. The results provide a path for the development of novel photonic logic devices for future optical computing technologies.

Highly Sensitive Dual-Mode Optical Thermometry of Er3+/Yb3+ Codoped Lead-Free Double Perovskite Microcrystal.

In this Letter, erbium (Er3+) and ytterbium (Yb3+) codoped perovskite Cs2Ag0.6Na0.4In0.9Bi0.1Cl6 microcrystal (MC) is synthesized and demonstrated systematically to the most prospective optical temperature sensing materials. A dual-mode thermometry based on fluorescence intensity ratio and fluorescence lifetime provides a self-reference and highly sensitive temperature measurement under dual wavelength excitation at a temperature from 300 to 470 K. Combined with the white-light emission derived from self-trapped excitons (STEs), the characteristic emission peak of Er3+ ions can be observed under 405 nm laser excitation. The fluorescence intensity ratio (FIR) between perovskite and Er3+ is used as temperature-dependent probe signal, of which maximum value for relative and absolute sensitivities reaches to 1.40% K-1 and 8.20 × 10-2 K-1. Moreover, Er3+ luminescence becomes stronger with the feeding Yb3+ increasing under 980 nm laser excitation. The energy transfer of Er3+ and Yb3+ is revealed by power-dependent photoluminescence (PL) spectroscopy, and the involved upconversion mechanism pertains to the two-photon excitation process. The results reveal that the Er3+/Yb3+ codoped lead-free double perovskite MC is a good candidate for a thermometric material for the novel dual-mode design.

A mitochondrial and lysosomal targeted ratiometric probe for detecting intracellular H2S.

Based on coumarin and benzopyran derivatives, a dual-wavelength excitation ratiometric fluorescent probe, HABA, was prepared to detect H2S. The HABA probe showed good selectivity and anti-interference abilities during H2S detection. Fluorescence co-localization experiments showed that HABA had excellent localization abilities toward mitochondria and lysosomes. More importantly, HABA can not only detect exogenous H2S, but it can also detect endogenous H2S, indicating that HABA has high application potential and value in the biological field.

A water-soluble turn-on fluorescent probe for rapid discrimination and imaging of Cys/Hcy and GSH in cells and zebrafish through different fluorescent channels

Biothiols are a class of amino acids containing mercaptan (thiol, SH) groups that have similar structures but play different roles in physiological activities, making it important to effectively distinguish them. Here, a simple water-soluble probe, PMCI, was developed for the first time to rapidly discriminate Cys/Hcy and GSH by dual excitation and emission wavelengths. The introduction of an N–Me pyridinium salt thioether group into the fluorophore structure not only makes the probe highly water-soluble, but also serves as a potential reactive site for biothiols, which together with an aldehyde group allows the simultaneous differential detection of Cys/Hcy and GSH. The probe itself has no background fluorescence due to the intramolecular photoinduced electron transfer (PET) effect. After reaction with biothiols, the probe solution showed different strong fluorescent signals (green: Cys and Hcy, red: GSH). It can respond quickly to biothiols (Cys: 190 s; Hcy: 155 s; GSH: 80 s) with high selectivity and low detection limit. Moreover, the probe has been successfully used for the rapid discrimination imaging of endogenous and exogenous biothiols in cells and zebrafish through dual fluorescent channels.

High performance FIR thermometry on the basis of the redshift of CTB by dual-wavelength alternative excitation in Eu3+:YVO4.

Compared with the forbidden 4f transition of rare earth ions, the strong absorption of the charge transfer band (CTB) enabled fluorescence thermometry to have high luminescence efficiency. Based on the temperature induced redshift of CTB, a high performance fluorescence intensity ratio (FIR) thermometry performed by dual-wavelength alternative excitation was studied. By way of the rising and falling edges of CTB in Eu3+ doped YVO4, monochrome sensitivity as a function of excitation wavelength was studied in the range of 303-783 K. The excitation wavelength with the highest positive monochrome sensitivity was determined, as well as that with the negative one. The optimum FIR temperature sensing strategy is proposed, and the theoretical highest relative sensitivity (Sr) is calculated to be 1.86% K-1, with the lowest uncertainty (ΔT) of 0.1 K at 783 K.

Reduced quantum defect in a Yb-doped fiber laser by balanced dual-wavelength excitation

Two color optical pumping, both above (anti-Stokes pump or ASP) and below (Stokes pump) the lasing wavelength, was adopted to reduce the net quantum defect (QD) in a solid-state Yb-doped fiber laser. The reduction in QD was achieved by converting a substantial portion of the gain medium's phonons directly into useful photons through a dual-wavelength excitation (DWE) mechanism. Since this is achieved through the usual processes of absorption and stimulated emission associated with lasing, high efficiency can be maintained. Both time domain and power measurements are presented, demonstrating a 13.2% reduction of the system's net QD and a 13.8% reduction in the lasing threshold power. These values were limited only by the available ASP power. Laser slope efficiency, with respect to launched ASP power, was found to be as high as 38.3%. A finite difference time domain model, developed to elucidate the role of both pumps in populating the upper states, corroborated the experimental findings. The DWE concept proposed here opens the door to an “excitation-balanced” type of self-cooled fiber laser. Simulation results also suggest that the technique is scalable and conceptually applicable to other solid-state laser systems.

A Portable Ultrawideband Confocal Raman Spectroscopy System with a Handheld Probe for Skin Studies.

Confocal Raman spectroscopy (CRS) has shown potential in non-invasive skin analysis. However, current CRS systems have various limitations including a narrow detection band, large size, non-flexibility, slowness, and complexity, which hinder their clinical applications. Herein, we developed a portable ultrawideband CRS system with a fiber-based handheld probe to acquire the Raman spectra in both fingerprint and high wavenumber regions in a fast and quasi-simultaneous way. Dual-wavelength excitation with a dual-passband laser cleaning filter and high-speed fiber array multiplexer was adopted instead of a specialized grating and detector to achieve instant switching between the detection regions and improve system robustness. Preliminary in vivo results demonstrated its depth profiling capability in an ultrawide detection range for stratum corneum thickness, natural moisturizing factor, and water content quantification, indicating its great potential in a wide range of clinical and cosmeceutical applications.

Dual-foci fast-scanning photoacoustic microscopy with 3.2-MHz A-line rate

We report fiber-based dual-foci fast-scanning OR-PAM that can double the scanning rate without compromising the imaging resolution, the field of view, and the detection sensitivity. To achieve fast scanning speed, the OR-PAM system uses a single-axis water-immersible resonant scanning mirror that can confocally scan the optical and acoustic beams at 1018 Hz with a 3-mm range. Pulse energies of 45∼100-nJ are sufficient for acquiring vascular and oxygen-saturation images. The dual-foci method can double the B-scan rate to 2036 Hz. Using two lasers and stimulated Raman scattering, we achieve dual-wavelength excitation on both foci, and the total A-line rate is 3.2-MHz. In in vivo experiments, we inject epinephrine and monitor the hemodynamic and oxygen saturation response in the peripheral vessels at 1.7 Hz over 2.5 × 6.7 mm2. Dual-foci OR-PAM offers a new imaging tool for the study of fast physiological and pathological changes.

Development of Neutral Red as a pH/pCO2 Luminescent Sensor for Biological Systems

Neutral Red (NR), a eurhodin dye, is often used for staining living cells, but we demonstrated for the first time that NR can also serve as a CO2 sensor, because of NR’s unique optical properties, which change with dissolved carbon dioxide (dCO2) concentrations. In the present study, optical sensitivity of NR was quantified as a function of changes in absorption and emission spectra to dCO2 in a pH 7.3 buffer medium at eight dCO2 concentrations. NR exhibited a response time of two minutes for equilibration in pure N2 to 100% CO2 with an ~200% percent change (%∆) in emission intensity and >400%∆ in absorbance, with full reversibility. Important to its application to biological systems, NR exhibited zero sensitivity to dissolved oxygen, which has routinely caused interference for CO2 measurements. NR exhibited pH sensitive emission and excitation energies with dual excitation wavelengths at 455 nm and 540 nm, and a single emission at 640 nm. The CO2 sensing properties of NR were benchmarked by a comparison to pyranine (8-hydroxypyrene-1, 3,6-trisulfonic acid trisodium salt) (HPTS). Future studies will evaluate the feasibility of NR as an intracellular in vivo pCO2 sensor in aquatic organisms critically impacted by increasing global CO2 levels.

Dual-wavelength excitation combined Raman spectroscopy for detection of highly fluorescent samples.

As fluorescence is the major limitation in Raman scattering, near-infrared excitation wavelength (>780nm) is preferred for fluorescence suppression in Raman spectroscopy. To reduce the risk of fluorescence interference, we developed a dual-wavelength excitation combined Raman spectroscopy (DWECRS) system at 785 and 830 nm. By a common optical path, each laser beam was focused on the same region of the sample by a single objective lens, and the dual-wavelength excitation Raman spectra were detected by a single CCD detector; in addition, 785 and 830 nm excitation Raman spectra can be directly constructed as combined Raman spectrum in the DWECRS system. The results of pure peanut oil and glycerol indicate that the combined Raman spectrum cannot only reduce fluorescence interference but also keep a high signal-to-noise ratio in the high-wavenumber region. The results of dye-ethanol solutions with different concentrations show that the handheld DWECRS system can be used as a smart method to dodge strong fluorescence. Furthermore, we developed a peak intensity ratio method with the DWECRS system to distinguish different types of edible oils. The peak intensity ratio distribution chart of edible oils showed each oil normalized center was relatively independent and nonoverlapped, which can be used as the basis of edible oil classification analysis. In the future, the DWECRS system has potential to be used as a tool for more complex applications.

Dual-wavelength Rapid Excitation Raman Difference Spectroscopy System for Direct Detection of Ethanol in Illegal Beverages

A dual-wavelength rapid excitation Raman difference spectroscopy (DWRERDS) system for separation of Raman signals from background interference was developed. In the DWRERDS system, a dual-wavelength diode laser at 784.59 and 785.22 nm with volume Bragg gratings was used as the excitation light source. For each laser line, optical power could reach as high as 550 mW. Under dual-wavelength excitation mode, the DWRERDS system could switch each laser instantaneously and the excitation time of each laser was as short as 1 s. Two Raman spectra were measured with slightly shifted excitation wavelengths and subtracted from each other, resulting in a background-free difference spectrum. The DWRERDS system was applied to complex matrix background suppression, which could improve a very good signal-to-noise ratio (SNR) for Raman spectra. Although ethanol in Coca-Cola could not be detected by conventional Raman spectroscopy, the DWRERDS system could be used for the direct detection of ethanol in illegal alcoholic beverages. The experimental results showed that the limit of detection (LOD) of ethanol in Coca-Cola was volume percentage of 4.76% and the correlation coefficient for the linear regression was as high as 0.984. The DWRERDS system could be used to determine the concentration of ethanol in illegal alcoholic beverages within 1 min without any sample pretreatment.

Nitrogen-doped carbon dots for dual-wavelength excitation fluorimetric assay for ratiometric determination of phosalone.

N-doped carbon dots (N-CDs) were fabricated in a simple procedure by hydrothermal treatment of cellobiose and urea. When excited at 235 nm or 327 nm, only one emission peak at around 420 nm has been observed. With the addition of phosalone, the excitation band at 235 nm was efficiently quenched within 1 min, while the excitation band at 327 nm showed little change. Accordingly, the fluorescence of the N-CDs-phosalone mixture showed quenching under 254-nm UV light, while nearly no fluorescence quenching could be observed under 365-nm UV light. This phenomenon provides a novel anti-false-positive mechanism for phosalone identification. Therefore, the label-free ratiometric sensor for rapid, naked-eye, and anti-false-positive detection of phosalone was proposed for the first time based on the intrinsic dual-excitation N-CDs. Under the optimum experimental conditions, the linear ranges of the excitation-based ratiometric assay were 0.08~4.0 μg/mL and 4.0~14.0 μg/mL; the limit of detection was 28.5 ng/mL. The as-constructed sensor was applied to detect phosalone residue in actual samples, and results were compared with the standard gas chromatographic (GC) method. The recoveries of the established sensor were between 90.0% and 110.0% with RSD lower than 6.6%, while that for the GC method was between 92.5% and 113.0% with RSD lower than 5.8%. Results reveal that the accuracy (recovery) and precision (RSD) of the as-constructed method are comparable to the standard GC method. In this paper, dual-excitation N-doped carbon dots (N-CDs) were synthesized by a simply one-step hydrothermal method for the first time. The novel dual-excitation ratiometric sensor based on the sole intrinsic N-CDs was constructed for phosalone sensing.

Temporal and spectral unmixing of photoacoustic signals by deep learning.

Improving the imaging speed of multi-parametric photoacoustic microscopy (PAM) is essential to leveraging its impact in biomedicine. However, to avoid temporal overlap, the A-line rate is limited by the acoustic speed in biological tissues to a few megahertz. Moreover, to achieve high-speed PAM of the oxygen saturation of hemoglobin, the stimulated Raman scattering effect in optical fibers has been widely used to generate 558 nm from a commercial 532 nm laser for dual-wavelength excitation. However, the fiber length for effective wavelength conversion is typically short, corresponding to a small time delay that leads to a significant overlap of the A-lines acquired at the two wavelengths. Increasing the fiber length extends the time interval but limits the pulse energy at 558 nm. In this Letter, we report a conditional generative adversarial network-based approach that enables temporal unmixing of photoacoustic A-line signals with an interval as short as ${\sim}{38}\;{\rm ns}$, breaking the physical limit on the A-line rate. Moreover, this deep learning approach allows the use of multi-spectral laser pulses for PAM excitation, addressing the insufficient energy of monochromatic laser pulses. This technique lays the foundation for ultrahigh-speed multi-parametric PAM.

Surface-enhanced shifted excitation Raman difference spectroscopy for trace detection of fentanyl in beverages.

In recognition of the misuse risks of fentanyl, there is an urgent need to develop a useful and rapid analytical method to detect and monitor the opioid drug. The surface-enhanced shifted excitation Raman difference spectroscopy (SE-SERDS) method has been demonstrated to suppress background interference and enhance Raman signals. In this study, the SE-SERDS method was used for trace detection of fentanyl in beverages. To prepare the simulated illegal drug-beverages, fentanyls were dissolved into distilled water or Mizone as a series of test samples. Based on our previous work, the surface-enhanced Raman spectroscopy detection was performed on the beverages containing fentanyl by the prepared AgNPs and the SE-SERDS spectra of test samples were collected by the dual-wavelength rapid excitation Raman difference spectroscopy system. In addition, the quantitative relationship between fentanyl concentrations and the Raman peaks was constructed by the Langmuir equation. The experimental results show that the limits of quantitation for fentanyl in distilled water and Mizone were 10 ng/mL and 200 ng/mL, respectively; the correlation coefficients for the nonlinear regression were as high as 0.9802 and 0.9794, respectively; and the relative standard deviation was less than 15%. Hence, the SE-SERDS method will be a promising method for the trace analyses of food safety and forensics.

Analytical model of fluorescence intensity for the estimation of fluorophore localisation in biotissue with dual-wavelength fluorescence imaging

Analytical expression for the fluorescence response of a photosensitiser uniformly distributed in the superficial layer of biotissue is obtained in the diffusion approximation of radiative transfer theory, and the approach for estimating the fluorescent layer thickness based on dual-wavelength excitation of fluorescence is proposed. It is shown that the error in estimation of the fluorescent layer thickness employing the ratio of the fluorescence signals obtained at different excitation wavelengths does not exceed 30 % for the thickness range of 0.1 – 2 mm in the case of 30 %-variation of biotissue optical properties.

A coumarin-based fluorescence sensor for rapid discrimination of cysteine/homocysteine and glutathione under dual excitation wavelengths.

Biological thiols (Cys, Hcy and GSH) are crucial biomolecules in living cells and play indispensable roles in maintaining the redox homeostasis of organisms. But due to their similar molecular structure, the development of effective tools for distinguishing two or three of them remains a great difficulty. Herein, we constructed a sensitive sensor (CB) by connecting the bifunctional fluorescent reagent with coumarin derivatives for simultaneous recognition of these three thiols through different pathways. Free CB had no fluorescence; however, with gradual addition of thiols, the chlorine unit was replaced by sulfhydryl. Furthermore, the intramolecular rearrangement occurred between the amino and sulfhydryl groups of Cys/Hcy and yellow fluorescence was observed at 570 nm. However, GSH with a large structure could not undergo intramolecular rearrangement, and green fluorescence was excited at 505 nm. In this way, Cys/Hcy and GSH can be detected distinctively. Under dual excitation wavelengths, CB exhibited high selectivity and fast response to the three thiols. Furthermore, CB was successfully applied to imaging endogenous and exogenous thiols in living cells and zebrafish, providing us with a reliable tool for thiols recognition.

High-sensitivity and wide-temperature-range dual-mode optical thermometry under dual-wavelength excitation in a novel double perovskite tellurate oxide.

Novel double perovskite SrLaLiTeO6 (abbreviated as SLLT):Mn4+,Dy3+ phosphors synthesized using a solid-state reaction strategy exhibit distinct dual-emission of Mn4+ and Dy3+. High-sensitivity and wide-temperature-range dual-mode optical thermometry was exploited taking advantage of the diverse thermal quenching between Mn4+ and Dy3+ and the decay lifetime of Mn4+. The thermometric properties in the range of 298-673 K were investigated by utilizing the fluorescence intensity ratio (FIR) of Dy3+ (4F9/2→6H13/2)/Mn4+ (2Eg→4A2g) and the Mn4+ (2Eg→4A2g) lifetime under 351 nm and 453 nm excitation, respectively. The maximum relative sensitivities (SR) of the resultant SLLT:1.2%Mn4+,7%Dy3+ phosphor under 351 nm and 453 nm excitation employing the FIR technology were determined to be 1.60% K-1 at 673 K and 1.44% K-1 at 673 K, respectively. Additionally, the maximum SR values based on the lifetime-mode were 1.59% K-1 at 673 K and 2.18% K-1 at 673 K, respectively. It is noteworthy that the SR values can be manipulated by different excitation wavelengths and multi-modal optical thermometry. These results suggest that the SLLT:Mn4+,Dy3+ phosphor has prospective potential in optical thermometry and provide conducive guidance for designing high-sensitivity multi-modal optical thermometers.