Visible Excitation Wavelengths Research Articles

(Invited) Advanced Carbon Nanotube Fluorescence Spectrometry for Novel Applications

Instrumental advances in near-IR fluorescence spectroscopy are enabling new types of measurements involving single-wall carbon nanotubes (SWCNTs). Two unique systems will be described. The first is a two-dimensional fluorescence-detected circular dichroism (FDCD) spectrometer. In this, SWCNT samples are excited by a spectrally selected supercontinuum laser beam that is switched between left- and right-circular polarization in an electro-optic modulator. Near-infrared sample fluorescence emitted in the backward direction is captured and directed to a scanning monochromator with a cooled InGaAs single-channel detector. After amplification and high precision digitization, the modulated signal component is extracted by computer-based phase sensitive detection. The system can measure a sample’s E22 circular dichroism in four spectral modes: 1) conventional FDCD, with scanned visible excitation wavelength and spectrally integrated (zero-order grating) emission detection; 2) Emission-specific FDCD, with scanned visible excitation wavelengths and selected emission wavelength; 3) Emission-scanned FDCD, with selected visible excitation wavelength and scanned emission wavelengths; 4) Excitation-Emission FDCD, with excitation and emission wavelengths both scanned to give two-dimensional data sets. This instrument can spectroscopically resolve enantiomer signals from a single (n,m) species in a racemic SWCNT sample.In a parallel project, developments in SWCNT fluorescence spectrometry are advancing nanotube-based strain measurement technology toward commercialization. Because SWCNT emission wavelengths vary systematically with axial strain, nanotubes in a thin coating on a specimen can serve as optically interrogated strain gauges. We apply this effect to measure strain maps through hyperspectral imaging of SWCNT fluorescence. A rotated band pass filter is used to capture a set of images in multiple spectral slices, from which a custom computer program deduces strain at each of ~105 image pixels and compiles strain maps. We will describe how this apparatus has evolved from a lab prototype into a compact portable system that can make measurements in industrial settings.

Excited state electronic structure and dynamics in diblock π-conjugated oligomers

The excited state electronic structure and ultrafast energy transfer in two diblock oligomers F3-TBT and F3-mP-TBT was explored (F3 = tri-(9,9-dioctylfluorene), TBT = 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole, mP = meta-phenylene). These diblock oligomers feature strongly coupled, π-conjugated F3 and TBT segments that have different bandgaps. F3-TBT is a fully π-conjugated chromophore in which the two conjugated segments are directly linked. By contrast, in F3-mP-TBT the chromophores are separated by a meta-linked phenylene unit. The objective of this study was to discern whether the Franck-Condon excited state produced by selective excitation of the F3 moiety in fully conjugated F3-TBT gives rise to a localized or fully delocalized excited state. The oligomers’ excited state dynamics were probed by steady-state UV–visible absorption, fluorescence and excitation wavelength dependent ultrafast time resolved transient absorption (TA) spectroscopy. Density functional theory (DFT) calculations were applied to provide insight concerning the electronic structure of the oligomers. The diblock oligomers feature distinct absorption bands due to transitions mainly localized on the F3 (high energy, 3.5 eV) and TBT (low energy, 2.75 eV) segments. Ultrafast TA spectroscopy with excitation corresponding to the low energy TBT unit reveals that the initial excitation is localized on TBT and does not vary with time. By contrast, in both oligomers excitation corresponding to high energy F3 produces an excited state that evolves rapidly (200–300 fs) into the lower energy state that is localized on the TBT chromophore. The ultrafast process is attributed to relaxation of an excited state that is predominantly localized on F3 into a lower energy state that is localized on TBT. The observation of an F3-localized excited state is unexpected for F3-TBT, given that the molecule is fully π-conjugated.

Development and Application of Ruthenium(II) and Iridium(III) Based Complexes for Anion Sensing.

Improvements in the design of receptors for the detection and quantification of anions are desirable and ongoing in the field of anion chemistry, and remarkable progress has been made in this direction. In this regard, the development of luminescent chemosensors for sensing anions is an imperative and demanding sub-area in supramolecular chemistry. This decade, in particular, witnessed advancements in chemosensors based on ruthenium and iridium complexes for anion sensing by virtue of their modular synthesis and rich chemical and photophysical properties, such as visible excitation wavelength, high quantum efficiency, high luminescence intensity, long lifetimes of phosphorescence, and large Stokes shifts, etc. Thus, this review aims to summarize the recent advances in the development of ruthenium(II) and iridium(III)-based complexes for their application as luminescent chemosensors for anion sensing. In addition, the focus was devoted to designing aspects of polypyridyl complexes of these two transition metals with different recognition motifs, which upon interacting with different inorganic anions, produces desirable quantifiable outputs.

Parameterization of a Refined Model Aimed at Simulating Laser-Induced Incandescence of Soot Using a Visible Excitation Wavelength of 532 nm

Laser-induced incandescence (LII) is one of the most powerful techniques for soot detection in combustion media. It is therefore commonly used to perform experiments in lab-scale flames and industrial combustors with a view to elucidating the formation mechanisms leading to combustion-generated fine carbonaceous particles while assessing their intrinsic properties. Quantitatively interpreting LII measurements, however, requires a firm knowledge of the optical properties of soot, including their wavelength-dependent absorption function (). Among the approaches used to evaluate such a crucial parameter, one can implement a LII model to derive the value which has to be set to reproduce a series of LII signals measured in a well-characterized environment. In this context, the present work aims at parameterizing a refined LII model built upon a comprehensive version of soot heat- and mass-balance equations for assessment when using a visible excitation wavelength of 532 nm. The proposed model integrates terms representing the saturation of linear, single- and multi-photon absorption processes, cooling by sublimation, conduction, radiation and thermionic emission, in addition to mechanisms depicting soot oxidation and annealing, non-thermal photodesorption of carbon clusters, as well as corrective factors accounting for the shielding effect and multiple scattering (MS) within aggregates. To parameterize this advanced simulation tool, an optimization procedure coupling design of experiments with a genetic algorithm-based solver was implemented. Doing so allowed to estimate the values of different factors involved in absorption and sublimation terms, including the multi-photon absorption cross-section for C2 photodesorption, the saturation coefficients for linear- and multi-photon absorption, as well as the value. Obtained parameters turned out to be well-suited to reproduce a set of LII signals acquired in a Diesel flame. While leading to predictions merging on a single curve with measured data, the so-parameterized model notably led to infer values of 0.3 and 0.38 when considering or neglecting MS within aggregates, respectively. Finally, the / ratio estimated based on data collected herein and in a former modeling work was found to be consistent with results issued from two-excitation wavelength LII measurements previously reported in the literature.

Relationships between the Physicochemical Properties of Dissolved Organic Matter and Its Reaction with Sodium Borohydride.

The reaction of dissolved organic matter (DOM) with sodium borohydride has been used to understand the geographic origin of DOM and investigate the photophysical model underlying DOM's optical properties. However, the physicochemical properties of DOM (e.g., molecular size and charge) that influence the kinetics and ultimate reducibility of DOM by borohydride remain poorly characterized. Herein, we studied the kinetics of DOM-borohydride reactions by recording absorbance and fluorescence spectra at a high temporal frequency (every ∼10 min for 24 h) for a diverse set of DOM isolates of aquatic and soil origin. The reducibility of DOM by sodium borohydride (as judged by relative removal of initial absorbance) varied appreciably among the DOM samples studied, with soil humic substances being less reducible than aquatic humic substances and natural organic matter. While statistically significant correlations were found between the reducibility of DOM and descriptors of molecular size, these descriptors were not able to differentiate the reducibility of soil versus aquatic DOM isolates that had similar bulk properties. Thus, it appears that the extent of absorbance removal by borohydride is largely driven by the origin of the humic substance isolate (aquatic vs soil) instead of molecular size or charge. Borohydride reduction resulted in increased fluorescence emission across UV and visible excitation wavelengths. However, the enhanced emission at visible excitation decreased over a time period of hours to days, suggesting that reduction of an important subset of DOM chromophores is reversible. This reversibility in fluorescence emission is consistent with the small role of quinones in the absorbance of DOM but suggests a more important role for quinone-containing charge-transfer contacts in the fluorescence of DOM, particularly at visible excitation wavelengths.

Determination of ammonium in natural water using a quinoline-based o-dialdehyde fluorescent reagent with visible excitation wavelength.

In this paper, a novel and stable fluorescent reagent, quinoline-2,3-dicarbaldehyde (QDA), is synthesized as a probe to detect ammonium in natural water. Ammonium reacts with QDA in the presence of SO32- and Ca2+ to form a fluorescence product, which has maximum excitation and emission wavelengths at 429 nm and 518 nm. The concentration of reagents, the reaction temperature, the reaction time, and the pH in the final solution are investigated and optimized. The interferences of typical organic nitrogen and inorganic compounds are evaluated, and results prove that most volatile amines have little or negligible effect. Under the optimized conditions, this method provides a limit of detection of 0.065 μmol L-1, a calibration range of 0.216-9 μmol L-1, and reproducibility (with a relative standard deviation) of 1.9% for 1.5 μmol L-1 ammonium. For water sample analysis, the proposed method provides comparable results to those of the conventional o-phthalaldehyde method but has longer reagent stability (42 days).

Fabrication and Electrochemical Properties of Three-Dimensional (3D) Porous Graphitic and Graphenelike Electrodes Obtained by Low-Cost Direct Laser Writing Methods

The development of three-dimensional (3D) porous graphitic structures is of great interest for electrochemical sensing applications as they can support fast charge transfer and mass transport through their extended, large surface area networks. In this work, we present the facile fabrication of conductive and porous graphitic electrodes by direct laser writing techniques. Irradiation of commercial polyimide sheets (Kapton tape) was performed using a low-cost laser engraving machine with visible excitation wavelength (405 nm) at low power (500 mW), leading to formation of 3D laser-induced graphene (LIG) structures. Systematic correlation between applied laser dwell time per pixel (“dwell time”) and morphological/structural properties of fabricated electrodes showed that conductive and highly 3D porous structures with spectral signatures of nanocrystalline graphitic carbon materials were obtained at laser dwell times between 20 and 110 ms/pix, with graphenelike carbon produced at 50 ms/pix dwell time, with comparable properties to LIG obtained with high cost CO2 lasers. Electrochemical characterization with inner and outer sphere mediators showed fast electron transfer rates, comparable to previously reported 2D/3D graphene-based materials and other graphitic carbon electrodes. This work opens the way to the facile fabrication of low-cost, disposable electrochemical sensor platforms for decentralized assays.

Fast and sensitive delineation of brain tumor with clinically compatible moxifloxacin labeling and confocal microscopy.

Delineation of brain tumor margins during surgery is critical to maximize tumor removal while preserving normal brain tissue to obtain optimal clinical outcomes. Although various imaging methods have been developed, they have limitations to be used in clinical practice. We developed a high-speed cellular imaging method by using clinically compatible moxifloxacin and confocal microscopy for sensitive brain tumor detection and delineation. Moxifloxacin is a Food and Drug Administration (FDA) approved antibiotic and was used as a cell labeling agent through topical administration. Its strong fluorescence at short visible excitation wavelengths allowed video-rate cellular imaging. Moxifloxacin-based confocal microscopy (MBCM) was characterized in normal mouse brain specimens and visualized their cytoarchitecture clearly. Then, MBCM was applied to both brain tumor murine models and two malignant human brain tumors of glioblastoma and metastatic cancer. MBCM detected tumors in all the specimens by visualizing dense and irregular cell distributions, and tumor margins were easily delineated based on the cytoarchitecture. An image analysis method was developed for automated detection and delineation. MBCM demonstrated sensitive delineation of brain tumors through cytoarchitecture visualization and would have potentials for human applications, such as a surgery-guiding method for tumor removal.

Generation of Microsecond Charge-Separated Excited States in Rhenium(I) Diimine Complexes: Driving Force Is the Dominant Factor in Controlling Lifetime.

A transition-metal-based donor-(linker)-acceptor system can produce long-lived charge transfer excited states using visible excitation wavelengths. The ground- and excited-state photophysical properties of a series of [ReCl(CO)3(dppz-(linker)-TPA)] complexes, with varying donor and acceptor energies, have been systematically studied using spectroscopic techniques (both vibrational and electronic) supported by computational chemistry. The long-lived excited state is 3ILCT in nature for all complexes studied, characterized through transient absorption and emission, transient resonance Raman (TR2), and time-resolved infrared (TRIR) spectroscopy and TDDFT calculations. Modulation of the donor and acceptor energies results in changes of the 3ILCT lifetime by 1 order of magnitude, ranging from 6.1(±1) μs when a diphenylamine donor is used to 0.6(±0.2) μs when a triazole linker and triphenylamine donor is used. The excited-state lifetime may be rationalized by consideration of the driving force within the framework of Marcus theory and appears insensitive to the nature of the linker.

Molecular Structure, DFT, Vibrational Spectra with Fluorescence Effect, Hirshfeld Surface, Docking Simulation and Antioxidant Activity of Thiazole Derivative

Abstract2‐(4‐methylphenoxy)‐N‐(4‐(4‐bromophenyl) thiazol‐2‐yl) acetamide compound was obtained via a multistep synthesis sequence processes, which is characterized by 1H NMR, 13C NMR and its three‐dimensional structure has been confirmed by single crystal X‐ray diffraction method. The supramolecular structure of the molecule revealed the stability of crystal packing with diverse intermolecular interactions. Density functional theory calculations (DFT) at B3LYP 6–311++G(d,p) level has been used to predict the molecular geometry and were in good agreement with the experimental data. Moreover, the theoretical calculations were carried out to appraise the electronic structure, vibrational spectra, natural bond orbital analysis, molecular electrostatic potential, frontier molecular orbitals and global reactivity descriptors. Raman spectra with fluorescence effect was examined with visible and near IR excitation wavelengths. Hirshfeld surface and energy framework analysis revealed the important intermolecular contacts and interaction energies. The antioxidant activity of this synthesized compound was predicted by in silico docking study on human peroxiredoxin 5 protein. The potential antioxidant activity was investigated by 1,1‐diphenyl‐1‐picrylhydrazyl (DPPH) free radical screening and compared with standard drug ascorbic acid.

Hemicyanine-linked pyrimidine mimics as solvatochromic fluorophores with visible excitation wavelengths

The design of solvatochromic fluorescent nucleosides with visible excitation wavelengths is a goal towards the generation of modified oligonucleotides for fluorimetry-based molecular imaging. Herein, two hemicyanine-linked C5-2′-deoxyuridine nucleosides (PyI-dU and APPy-dU) have been synthesized by first generating hemicyanine-alkyne precursors that were attached via the alkyne moiety to 5-iodo-2′-deoxyuridine (5-I-dU) by Sonogashira coupling. The photophysical properties of the hemicyanine-linked dU probes have been characterized and compared to the corresponding properties of the hemicyanine-alkyne precursors. The nucleoside probe PyI-dU exhibits optical features that mimic the properties of the free hemicyanine-alkyne precursor, while the APPy-dU probe displays more favorable optical properties (longer excitation wavelength, brighter emission in water) than its precursor that is ascribed to π-stacking interactions between the hemicyanine dye with the dU nucleobase. Overall, probe APPy-dU is a superior solvatochromic fluorophore than PyI-dU suggesting its greater utility for fluorescent imaging applications.

Deep non-contact photoacoustic initial pressure imaging

Photoacoustic imaging techniques have been extensively developed for biomedical applications, including functional and molecular imaging, due in part to their high optical contrast, high spatial resolution, and non-ionizing imaging properties. However, there are currently depth limitations in cellular-resolution, optically focused photoacoustic microscopy systems. In addition, most common photoacoustic systems need to be in contact with the sample through an ultrasound medium. In this work, by taking advantage of large photoacoustic initial pressures, all-optical non-contact optical resolution photoacoustic imaging is reported at depths beyond the optical transport mean-free path of the excitation wavelength. The proposed technique is called deep photoacoustic remote sensing (dPARS) microscopy. Visible pulsed excitation wavelengths are used to produce large initial-pressure-induced refractive index modulations in absorbing targets. These localized pressure rises create transient variations to the local scattering properties, which are detected as back-reflected intensity modulations from a deep-penetrating interrogation beam and do not require an interferometric detection pathway. Experiments demonstrate that dPARS is capable of providing optical resolution images to depths of 2.5 mm in tissue-mimicking scattering media. Signal-to-noise ratio ∼50 dB is reported for in vivo imaging of microvascular networks. Also, imaging of single red blood cells, oxygen saturation mapping, and deep-vascular imaging applications are demonstrated. dPARS’s capabilities such as remote sensing, deep optical resolution imaging, and high signal-to-noise ratio, may yield new opportunities for several pre-clinical and clinical applications.

Solvent effect on multiple emission and ultrafast dynamics of higher excited states

We present ultrafast-depopulation-dynamics of higher-lying-excited-states using femtosecond fluorescence up-conversion techniques for two near-infrared (NIR) tricarbocyanine dyes (IR144 and IR140) in primary alcohols. With visible excitation wavelengths, such dyes show two distinct emission-bands with large peak wavelength difference: one at the visible region: S2 → S0, and the other at NIR region: S1 → S0. We show that exact band-positions, intensities, and fluorescence-decay-timescales (τ) depend strongly on viscosity and polarity of solvents. Interestingly, though the faster component of τ increased for IR144 with increasing viscosity and chain-length of alcohols, the reverse was seen for IR140, indicating the possible formation of ion-pair of IR140 with alcohols.

Multispectral fluorescence imaging of human ovarian and fallopian tube tissue for early-stage cancer detection.

With early detection, 5-year survival rates for ovarian cancer exceed 90%, yet no effective early screening method exists. Emerging consensus suggests over 50% of the most lethal form of the disease originates in the fallopian tube. Twenty-eight women undergoing oophorectomy or debulking surgery provided informed consent for the use of surgical discard tissue samples for multispectral fluorescence imaging. Using multiple ultraviolet and visible excitation wavelengths and emissions bands, 12 fluorescence and 6 reflectance images of 47 ovarian and 31 fallopian tube tissue samples were recorded. After imaging, each sample was fixed, sectioned, and stained for pathological evaluation. Univariate logistic regression showed cancerous tissue samples had significantly lower intensity than noncancerous tissue for 17 image types. The predictive power of multiple image types was evaluated using multivariate logistic regression (MLR) and quadratic discriminant analysis (QDA). Two MLR models each using two image types had receiver operating characteristic curves with area under the curve exceeding 0.9. QDA determined 56 image type combinations with perfect resubstituting using as few as five image types. Adaption of the system for future in vivo fallopian tube and ovary endoscopic imaging is possible, which may enable sensitive detection of ovarian cancer with no exogenous contrast agents.

Multi-NIR-Emissive Materials based on Heterolanthanide Molecular Assemblies

ABSTRACTErxYb(3-x)Q9 (1, x = 1, 2), NdYb2Q9 (2), NdEr2Q9 (3) and NdErYbQ9 (4), have been obtained through a simple molecular strategy, by controlling reactant stoichiometry. In 2-4, the templating effect of the molecular framework allows to control the metal distribution across the coordination sites where the central position is occupied by the larger Nd ion and the terminal ones by the almost vicariants Yb3+ and Er3+, drastically reducing molecular speciation. Remarkably, 4 represents the first example of a discrete molecular entity containing three different Ln ions simultaneously emitting in three different spectral regions in the NIR, upon single visible wavelength excitation. Highly transparent and homogeneous doped silica glasses have been prepared, which show the same optical properties of the incorporated complex in solution.

Strain-induced band alignment in wurtzite/zinc-blende InAs heterostructured nanowires

We study band alignment in wurtzite/zinc-blende polytype InAs heterostructured nanowires using temperature-dependent resonance Raman measurements. Nanowires having two different wurtzite fractions are investigated. Using visible excitation wavelengths in resonance Raman measurements, we probe the electronic band alignment of these semiconductor nanowires near a high-symmetry point of the Brillouin zone (${E}_{1}$ gap). The strain in the crystal structure, as revealed from the shift of the phonon mode, explains the observed band alignment at the wurtzite/zinc-blende interface. Our experimental results are further supported by electronic-structure calculations for such periodic heterostructured interface.

White light emitting Ho3+-doped CdS nanocrystal ingrained glass nanocomposites

We report the generation of white light from Ho3+ ion doped CdS nanocrystal ingrained borosilicate glass nanocomposites prepared by the conventional melt-quench method. Near visible 405 nm diode laser excited white light emission is produced by tuning the blue emission from the Ho3+ ions, green band edge, and orange-red surface-state emissions of the nanocrystalline CdS, which are further controlled by the size of the nanocrystals. The absorption and emission spectra evidenced the excitation of Ho3+ ions by absorption of photons emitted by the CdS nanocrystals. The high color rendering index (CRI = 84–89) and befitting chromaticity coordinates (x = 0.308–0.309, y = 0.326–0.338) of white light emission, near visible harmless excitation wavelength (405 nm), and high absorbance values at excitation wavelength point out that these glass nanocomposites may serve as a prominent candidate for resin free high power white light emitting diodes.

Surface Structure of Aerobically Oxidized Diamond Nanocrystals.

We investigate the aerobic oxidation of high-pressure, high-temperature nanodiamonds (5–50 nm dimensions) using a combination of carbon and oxygen K-edge X-ray absorption, wavelength-dependent X-ray photoelectron, and vibrational spectroscopies. Oxidation at 575 °C for 2 h eliminates graphitic carbon contamination (>98%) and produces nanocrystals with hydroxyl functionalized surfaces as well as a minor component (<5%) of carboxylic anhydrides. The low graphitic carbon content and the high crystallinity of HPHT are evident from Raman spectra acquired using visible wavelength excitation (λexcit = 633 nm) as well as carbon K-edge X-ray absorption spectra where the signature of a core–hole exciton is observed. Both spectroscopic features are similar to those of chemical vapor deposited (CVD) diamond but differ significantly from the spectra of detonation nanodiamond. The importance of these findings to the functionalization of nanodiamond surfaces for biological labeling applications is discussed.

Multicenters in Ce3+ visible emission of YAG ceramics

New aspects on the Ce3+ in YAG transparent ceramics, obtained from absorption and emission spectra in the 10–300K range, under different visible excitation wavelengths, along with emission decays are presented. The spectra and lifetime of regular CeY3+ center are similar to those reported for single crystals. In the attempt to reveal perturbed Ce3+ centers by defects, the 5d→4f emission spectra under pulsed 532nm (5ns) excitation (below the Ce3+ absorption band) were recorded. The spectra and decays (at different wavelengths) are function on temperature, revealing at least three types of Ce3+ new emitting centers. The low temperature spectra, different from those of CeY3+ center, are dominated by a set of strongly perturbed centers with very short lifetimes. At least two other types of perturbed centers were separated by using wavelength and temperature dependences of decays. The spectral characteristics of these perturbed Ce3+ centers by lattice defects are analyzed in terms of the effects of the structural changes induced by Ce3+ doping on the interaction with defects, by using recent reported results.

Quantification of reactive oxygen species generation by photoexcitation of PEGylated quantum dots.

Photocatalytic generation of reactive oxygen species (ROS) from quantum dots (QDs) has been widely reported yet quantitative studies of ROS formation and their quantum yields are lacking. This study investigates the generation of ROS by water soluble PEGylated CdSe/ZnS QDs with red emission. PEGylation of QDs is commonly used to confer water solubility and minimise uptake by organs of the reticuloendothelial system; therefore studies of ROS formation are of biomedical relevance. Using non-photolytic visible wavelength excitation, the superoxide anion radical is shown to be the primary ROS species generated with a quantum efficiency of 0.35%. The yield can be significantly enhanced in the presence of the electron donor, nicotinamide adenine dinucleotide (NADH), as demonstrated by oxygen consumption measurements and electron paramagnetic resonance spectroscopy with in situ illumination. Direct production of singlet oxygen is not detectable from the QDs alone. A comparison is made with ROS generation by the same QDs complexed with a sulfonated phthalocyanine which can generate singlet oxygen via Förster resonance energy transfer between the QDs and the phthalocyanine.