Fluorescence Line-narrowing Spectroscopy Research Articles

(Invited) Design and Synthesis of Colloidal QDs for Quantum Emitters

On-demand generation of quantum photons is one of the four major targets of Quantum Information Science (QIS), identified by the NSF as one of the five “industries of the future.” Colloidal semiconductor quantum dots (QDs) are viable candidates for quantum materials, owing to their tunable, bright, and stable emission. Identifying a suitable semiconductor material and engineering long exciton coherence times is key to producing single coherent photons on demand. We show that colloidal compositionally-graded CdSe/ZnS QDs designed with a high degree of asymmetric internal strain demonstrate superior quantum emission properties compared to conventional sharp-interface CdSe/ZnS QDs. In comparison to conventional colloidal CdSe/ZnS core/shell QDs, we find that, in asymmetrically strained CdSe QDs, over six times more light is emitted directly by the bright exciton. Temperature-dependent PL dynamics and fluorescence line narrowing (FLN) spectroscopy reveals the radiative recombination rates of bright and dark excitons, the relaxation rate between the two, and the energy spectra of the quantized acoustic phonons in the QDs that can contribute to relaxation processes. We attribute the six-fold improvement to the four-fold deceleration of the bright-dark relaxation and the doubled radiative recombination rate in the strained QDs.The next step in this research is to bring these improvement from the visible to the near-IR emission, where most communication bandwidths lie. This area has been traditionally dominated by III-V materials, which are chemically challenging in the colloidal state. We design, synthesize, and characterize alternative strong near-IR materials and assess them as potential quantum emitters. Last but not least, we demonstrate applications of electrochemistry in programmable hot syntheses of colloidal QDs.

Nonequilibrium site distribution governs charge-transfer electroluminescence at disordered organic heterointerfaces

The interface between electron-donating (D) and electron-accepting (A) materials in organic photovoltaic (OPV) devices is commonly probed by charge-transfer (CT) electroluminescence (EL) measurements to estimate the CT energy, which critically relates to device open-circuit voltage. It is generally assumed that during CT-EL injected charges recombine at close-to-equilibrium energies in their respective density of states (DOS). Here, we explicitly quantify that CT-EL instead originates from higher-energy DOS site distributions significantly above DOS equilibrium energies. To demonstrate this, we have developed a quantitative and experimentally calibrated model for CT-EL at organic D/A heterointerfaces, which simultaneously accounts for the charge transport physics in an energetically disordered DOS and the Franck-Condon broadening. The 0-0 CT-EL transition lineshape is numerically calculated using measured energetic disorder values as input to 3-dimensional kinetic Monte Carlo simulations. We account for vibrational CT-EL overtones by selectively measuring the dominant vibrational phonon-mode energy governing CT luminescence at the D/A interface using fluorescence line-narrowing spectroscopy. Our model numerically reproduces the measured CT-EL spectra and their bias dependence and reveals the higher-lying manifold of DOS sites responsible for CT-EL. Lowest-energy CT states are situated ∼180 to 570 meV below the 0-0 CT-EL transition, enabling photogenerated carrier thermalization to these low-lying DOS sites when the OPV device is operated as a solar cell rather than as a light-emitting diode. Nonequilibrium site distribution rationalizes the experimentally observed weak current-density dependence of CT-EL and poses fundamental questions on reciprocity relations relating light emission to photovoltaic action and regarding minimal attainable photovoltaic energy conversion losses in OPV devices.

Wavelength-Dependent Exciton-Vibrational Coupling in the Water-Soluble Chlorophyll Binding Protein Revealed by Multilevel Theory of Difference Fluorescence Line-Narrowing.

One of the most powerful line-narrowing techniques used to unravel the homogeneous lineshapes of inhomogeneously broadened systems is difference fluorescence line-narrowing spectroscopy. When this spectroscopy was applied to multichromophoric systems so far, the spectra were analyzed by an effective two-level system approach, composed of the electronic ground state and the lowest exciton state. An effective Huang-Rhys factor was assigned for the coupling of this state to the vibrations. Here, we extend this approach by including a multilevel line shape theory, which takes into account the excitonic coupling between pigments and thereby the effect of the delocalization of the excited states explicitly. In this way, it becomes possible to extract the spectral density of the local exciton-vibrational coupling. The theory is applied to the recombinant water-soluble chlorophyll binding protein reconstituted with chlorophyll a or b and reveals a significant decrease of the Huang-Rhys factor of the local exciton-vibrational coupling with decreasing transition energy of the chlorophylls. This decrease could be due to the increase in steric interactions reducing the flexibility of the environment and red-shifting the site energy of the pigments.

Exciton Fine Structure and Lattice Dynamics in InP/ZnSe Core/Shell Quantum Dots.

Nanocrystalline InP quantum dots (QDs) hold promise for heavy-metal-free optoelectronic applications due to their bright and size-tunable emission in the visible range. Photochemical stability and high photoluminescence (PL) quantum yield are obtained by a diversity of epitaxial shells around the InP core. To understand and optimize the emission line shapes, the exciton fine structure of InP core/shell QD systems needs be investigated. Here, we study the exciton fine structure of InP/ZnSe core/shell QDs with core diameters ranging from 2.9 to 3.6 nm (PL peak from 2.3 to 1.95 eV at 4 K). PL decay measurements as a function of temperature in the 10 mK to 300 K range show that the lowest exciton fine structure state is a dark state, from which radiative recombination is assisted by coupling to confined acoustic phonons with energies ranging from 4 to 7 meV, depending on the core diameter. Circularly polarized fluorescence line-narrowing (FLN) spectroscopy at 4 K under high magnetic fields (up to 30 T) demonstrates that radiative recombination from the dark F = ±2 state involves acoustic and optical phonons, from both the InP core and the ZnSe shell. Our data indicate that the highest intensity FLN peak is an acoustic phonon replica rather than a zero-phonon line, implying that the energy separation observed between the F = ±1 state and the highest intensity peak in the FLN spectra (6 to 16 meV, depending on the InP core size) is larger than the splitting between the dark and bright fine structure exciton states.

Conformational Changes in Pigment-Protein Complexes at Low Temperatures-Spectral Memory and a Possibility of Cooperative Effects.

We employed nonphotochemical hole burning (NPHB) and fluorescence line narrowing (FLN) spectroscopies to explore protein energy landscapes and energy transfer processes in dimeric Cytochrome b6f, containing one chlorophyll molecule per protein monomer. The parameters of the energy landscape barrier distributions quantitatively agree with those reported for other pigment-protein complexes involved in photosynthesis. Qualitatively, the distributions of barriers between protein substates involved in the light-induced conformational changes (i.e., -NPHB) are close to glass-like ∼1/√V (V is the barrier height) and not to Gaussian. There is a high degree of correlation between the heights of the barriers in the ground and excited states in individual pigment-protein systems, as well as nearly perfect spectral memory. Both NPHB and hole recovery are due to phonon-assisted tunneling associated with the increase of the energy of a scattered phonon. As the latter is unlikely for simultaneously both the hole burning and the hole recovery, proteins must exhibit a NPHB mechanism involving diffusion of the free volume toward the pigment. Entities involved in the light-induced conformational changes are characterized by md(2) value of about 1.0 × 10(-46) kg·m(2). Thus, these entities are protons or, alternatively, small groups of atoms experiencing sub-Å shifts. However, explaining all spectral hole burning and recovery data simultaneously, employing just one barrier distribution, requires a drastic decrease in the attempt frequency to about 100 MHz. This decrease may occur due to cooperative effects. Evidence is presented for excitation energy transfer between the chlorophyll molecules of the adjacent monomers. The magnitude of the dipole-dipole coupling deduced from the Δ-FLN spectra is in good agreement with the structural data, indicating that the explored protein was intact.

Probing the physicochemical interactions of 3-hydroxy-benzo[a]pyrene with different monoclonal and recombinant antibodies by use of fluorescence line-narrowing spectroscopy

Characterization of interactions between antigens and antibodies is of utmost importance both for fundamental understanding of the binding and for development of advanced clinical diagnostics. Here, fluorescence line-narrowing (FLN) spectroscopy was used to study physicochemical interactions between 3-hydroxybenzo[a]pyrene (3OH-BaP, as antigen) and a variety of solvent matrices (as model systems) or anti-polycyclic aromatic hydrocarbon antibodies (anti-PAH). We focused the studies on the specific physicochemical interactions between 3OH-BaP and different, previously obtained, monoclonal and recombinant anti-PAH antibodies. Control experiments performed with non-binding monoclonal antibodies and bovine serum albumin (BSA) indicated that nonspecific interactions did not affect the FLN spectrum of 3OH-BaP. The spectral positions and relative intensities of the bands in the FLN spectra are highly dependent on the molecular environment of the 3OH-BaP. The FLN bands correlate with different vibrational modes of 3OH-BaP which are affected by interactions with the molecular environment (π-π interactions, H-bonding, or van-der-Waals forces). Although the analyte (3OH-BaP) was the same for all the antibodies investigated, different binding interactions could be identified from the FLN spectra on the basis of structural flexibility and conformational multiplicity of the antibodies' paratopes.

Pigment–Protein Interactions in Phytochromes Probed by Fluorescence Line Narrowing Spectroscopy

Fluorescence line narrowing (FLN) spectroscopy was used to study bacteriophytochromes and variants from various species in their red-absorbing Pr ground state, including phytochromes Agp1 from Agrobacterium tumefaciens , DrBphP from Deinococcus radiodurans , and RpBphP2 and RpBphP3 from Rhodopseudomonas palustris . A species-dependent narrowing of the fluorescence emission bands is observed. The results suggest varied pigment-protein interactions, possibly connected to chromophore mobility or extended water pyrrole networks inside of the differing binding pockets. Solvent water isotope exchange from H2O-based buffer to D2O-based buffer solutions was used to identify specific vibrational modes of the chromophore. In addition to the expected frequency shifts upon isotope exchange, the line narrowing efficiency is increased in deuterated compared to protonated surroundings. We conclude that proton dynamics inside of the protein binding pocket are a dominant source of spectral diffusion at low temperatures, which possibly relates to the previous observation that the electronic transition is directly coupled to proton transfer. The FLN spectra of Agp1 reconstituted with a synthesized pigment shows strong line narrowing efficiency even in protonated buffer solution. The FLN spectra of a point mutant of RpBphP3 highlight the involvement of aspartate 216 in a hydrogen bond network around the chromophore. On the basis of similar FLN characteristics in RpBphP2 and RpBphP3, we propose a similarly extended hydrogen bond network around their chromophores despite the different photoreactions leading to red- or blue-shifted absorption relative to the respective photoreceptors' ground-state absorption.

Excitonic Energy Level Structure and Pigment−Protein Interactions in the Recombinant Water-Soluble Chlorophyll Protein. I. Difference Fluorescence Line-Narrowing

Difference fluorescence line-narrowing spectroscopy at 4.5 K was employed to investigate electron-phonon and electron-vibrational coupling strengths of the lower exciton level of water-soluble chlorophyll-binding protein (WSCP) from cauliflower reconstituted with chlorophyll a or chlorophyll b, respectively. The electron-phonon coupling is found to be moderate with integral Huang-Rhys factors S in the order of 0.81-0.85. A weak dependence of S on excitation wavelength within the inhomogeneously broadened fluorescence origin band is attributed to a sizable contribution of nonresonant excitation that varies with excitation wavelength. The strongly asymmetric and highly structured one-phonon profile is characterized by a peak phonon frequency (ω(m)) of ~24 cm(-1) and further discernible peaks at 48 and 88 cm(-1), respectively. A structural assignment of this unusual one-phonon profile is proposed. As will be shown in the accompanying paper (part II) (DOI 10.1021/jp111457t), the parameters of electron-phonon coupling readily account for shape and position of the fluorescence origin bands at 666.1 and 683.8 nm for chlorophyll b- and chlorophyll a-WSCP, respectively. A rich structure of S(1)→S(0) vibrational frequencies was resolved in the wavenumber range between 180 and 1665 cm(-1) for both chlorophyll a- and chlorophyll b-WSCP. The corresponding individual Huang-Rhys factors fall in the range between 0.0011 and 0.0500. To the best of our knowledge, this is the first report of S-factors for vibrational modes of chlorophyll b. Most remarkable is the presence of two additional modes at 228 and 327 cm(-1) compared with the vibrational spectrum of chlorophyll in solution. The additional modes can most likely be attributed to H-bond formation in the vicinity of the chlorophyll molecule bound by WSCP.

Evidence of two erbium sites in standard aluminosilicate glass for EDFA

Site distributions of Er(3+)-doped aluminosilicate preforms of standard EDFA were studied by the low temperature Resonant Fluorescence Line Narrowing (RFLN) spectroscopy. Two erbium concentration samples with the same glass base were investigated. At very low erbium concentration, two classes of sites were identified, related to the number of AlO(6) octahedral linked by two oxygen edge-sharing to Er(3+) in the coordination sphere. As erbium concentration is increased, the high AlO(6) coordinated class of sites is smeared out by the optical response of the one AlO(6) coordinated class of sites.

Laser spectroscopy of Nd 3+ ions in glasses with the 0.8CaSiO 3–0.2Ca 3(PO 4) 2 eutectic composition

In this work we report the study of site-selective laser spectroscopy of Nd 3+ ions in glasses with the 0.8CaSiO 3–0.2Ca 3(PO 4) 2 eutectic composition together with a dynamic study of energy transfer among Nd 3+ ions by using time-resolved fluorescence line narrowing (TRFLN) spectroscopy. The spectral features of the time-resolved fluorescence line narrowed 4 F 3 / 2 → 4 I 9 / 2 emission spectra obtained under resonant excitation reveal the existence of spectral migration of excitation among the Nd 3+ ions. The analysis of the time evolution of the 4 F 3 / 2 → 4 I 9 / 2 narrowed emission shows that the electronic mechanism responsible for the ion–ion interaction can be identified as a dipole–dipole energy transfer process.

Fluorescence line-narrowing spectroscopy for probing purposes in bioanalytical and environmental chemistry

Fluorescence line-narrowing spectroscopy (FLNS) is a cryogenic technique that can be used to obtain high-resolution fluorescence spectra. The technique has mainly been used for fingerprint identification. However, this review focuses on the trend to use FLNS for probing purposes: obtaining information on interactions with the local environment or determining conformations.We briefly discuss the basics of FLNS, solute-matrix interactions and important instrumental aspects. We describe examples from various bioanalytical fields: DNA adducts of the aromatic carcinogen benzo[a]pyrene (BP); binding of BP metabolites to monoclonal antibodies or the estrogen receptor; FLNS studies of porphyrin-containing proteins; and, photosynthetic systems.As an illustration from the field of environmental chemistry, we discuss the use of luminescence line-narrowing spectroscopy – a special mode of FLNS – to study the complexation of lanthanides to humic substances.

Spectral Differentiation and Immunoaffinity Capillary Electrophoresis Separation of Enantiomeric Benzo(a)pyrene Diol Epoxide-Derived DNA Adducts

Antibody cross-reactivity makes separation and differentiation of enantiomeric analytes one of the most challenging problems in immunoanalytical research, particularly for the analysis of structurally related biological molecules [such as benzo( a)pyrene (BP) metabolites and BP-derived DNA adducts]. It has recently been shown that the interaction of enantiomers of BP tetrols (BPT) with a promiscuous anti-polycyclic aromatic hydrocarbon ( anti-PAH) monoclonal antibody (mAb) allowed for separation of all four enantiomeric isomers using immunoaffinity capillary electrophoresis [ Grubor, N. M. , Armstrong, D. W. , and Jankowiak, R. ( 2006) Electrophoresis 27, 1078 ] and unambiguous spectral resolution using fluorescence line narrowing spectroscopy (FLNS) [ Grubor, N. M. , Liu, Y. , Han, X. , Armstrong, D.W. , and Jankowiak, R. ( 2006) J. Am.Chem. Soc. 128, 6409 ]. Here, we expand the use of the above two methodologies to the group of biologically important molecules that are products of BP diol epoxide (BPDE)-induced DNA damage. Four diastereomeric anti-BPDE-derived deoxyguanosine (dG) adducts, that is, (+)- and (-)- anti-trans-BPDE- N (2)-dG and (+)- and (-)- anti-cis-BPDE- N (2)-dG, were electrophoretically separated and spectroscopically differentiated using 8E11 mAb raised against BP-DNA conjugates. In fluorescence line narrowing spectroscopy (FLNS) experiments, complexes of BPDE-dG adducts with mAb revealed differences in fluorescence origin band positions, bandwidths, and vibrational patterns for all four BPDE- N (2)-dG adducts. Narrow fluorescence origin bands observed for (-)- trans-BPDE-dG (70 cm (-1)) and (+)- trans-BPDE- N (2)-dG (80 cm (-1)) suggest spatial constraint within the mAb binding pocket. Broader origin bands observed for cis type adducts ( approximately 120 cm (-1)) in 8E11 mAb suggest different binding geometries and/or conformational changes, as also indicated by changes in vibrational frequencies observed for the (+)- anti-cis and (-)- anti-cis adducts complexed with mAb. FLNS revealed that binding conformations and interactions within the mAb binding pocket are different for each adduct, enabling unambiguous positive identification. The methodologies described in this manuscript could also be used for analysis of DNA adducts following enzymatic hydrolysis of BPDE-adducted DNA to free nucleosides.

High Resolution Spectral Differentiation of Enantiomers: Benzo[a]Pyrene Tetrols Complexed with a Promiscuous Antibody

The interaction of the highly cross-reactive anti-PAH monoclonal antibody with four diastereomeric benzo[a]pyrene tetrols (BPTs) is studied by means of fluorescence line-narrowing spectroscopy. It is shown that the interaction of enantiomers of cis-BPT and trans-BPT with the antibody involves different complex geometries. These spatially different ligand-protein interactions alter the relative intensities of the excited-state vibrational frequencies of immunocomplexed molecules allowing for unambiguous spectroscopic resolution of all four enantiomeric isomers. This study represents the first example of a high-resolution, fluorescence-based spectroscopic method capable of enantiospecific differentiation.

The Chemical Interaction between the Estrogen Receptor and Monohydroxybenzo[a]pyrene Derivatives Studied by Fluorescence Line-Narrowing Spectroscopy

A novel approach is presented for studying the chemical interaction between receptor binding sites and ligands. Monohydroxylated polyaromatic compounds were found to be environmentally sensitive ligands when applying a special mode of fluorescence: fluorescence line-narrowing spectroscopy (FLNS). With this technique, solvent dependencies and ligand-receptor interactions can be studied in great detail, due to the high spectral resolution and the fact that at cryogenic temperatures (4 K), no solvent reorientation effects complicate the interpretation. The FLN spectrum of a ligand bound to the receptor is compared to the spectra of the free ligand in solvent mixtures that mimic the functionalities present within the receptor's binding site. It is shown that for the well-known estrogen receptor (ER), the orientations of two xenoestrogenic ligands 3- and 9-hydroxybenzo[a]pyrene (3- and 9-OH-BaP) can be determined. The FLN results clearly indicate that an H-bond accepted by HIS524 plays a major role in the binding of these ligands to the ER. Furthermore, the spectra indicated a pi-pi stacking aromatic interaction for 9-OH-BaP with PHE404. These results are in line with molecular modeling studies published earlier.

Cross-reactivity and conformational multiplicity of an anti-polycyclic aromatic hydrocarbon mAb

Cross-reactivity and multispecific functionality of antibodies play a central role in the immune system. The Ab's promiscuity is attributed to structural flexibility and conformational multiplicity of its binding sites governed by the rearrangement of hydrogen bonding centers. However, antibodies whose recognition and binding rely on less directional hydrophobic interactions might follow different interaction pathways. We investigated interaction of anti-polycyclic aromatic hydrocarbon mAb with two biologically important cross-reactants, pyrene and benzo(a)pyrene. Complex formation was characterized by means of low-temperature laser-induced fluorescence spectroscopy in both low- and high-resolution fluorescence line-narrowing (FLN) modes. It is shown that the FLN spectroscopy can identify various haptens cross-reacted with an Ab, as well as simultaneously differentiate between free and immunocomplexed haptens. In addition, our results suggest an interesting case of an Ab binding a particular cross-reactant by adopting two distinct conformations of its binding sites. The existence of the multiple conformations for anti-polycyclic aromatic hydrocarbon mAb that are trapped at low temperature can be rationalized through the existing models for Ab binding. Finally, as revealed by FLN spectra of immunocomplexed chromophores, pi-pi interactions, rather than hydrogen bonding, play the central role in complex formation.

Identification and Quantification of the Depurinating DNA Adducts Formed in Mouse Skin Treated with Dibenzo[a,l]pyrene (DB[a,l]P) or Its Metabolites and in Rat Mammary Gland Treated with DB[a,l

Dibenzo[a,l]pyrene (DB[a,l]P) is the most potent carcinogenic polycyclic aromatic hydrocarbon and has been identified in the environment. Comparative tumorigenicity studies in mouse skin and rat mammary gland indicate that DB[a,l]P is slightly more potent than DB[a,l]P-11,12-dihydrodiol and much more potent than (+/-)-syn-DB[a,l]P-11,12-dihydrodiol-13,14-epoxide {(+/-)-syn-DB[a,l]PDE} and (+/-)-anti-DB[a,l]PDE. We report here the identification and quantification of the depurinating adducts formed in mouse skin treated with DB[a,l]P, DB[a,l]P-11,12-dihydrodiol, (+/-)-syn-DB[a,l]PDE, or (+/-)-anti-DB[a,l]PDE and rat mammary gland treated with DB[a,l]P. The biologically formed adducts were compared with standard adducts by their retention times on HPLC and their spectra obtained by fluorescence line-narrowing spectroscopy at low temperature. In mouse skin treated with DB[a,l]P, depurinating adducts comprised 99% of the total adducts. Most of the depurinating adducts were formed by one-electron oxidation, with 63% at Ade and 12% at Gua. The remainder were formed by the diol epoxide, with 18% at Ade and 6% at Gua. When mouse skin was treated with DB[a,l]P-11,12-dihydrodiol, depurinating adducts comprised 80% of the total, and the predominant one was with Ade (69%). Treatment of skin with (+/-)-syn-DB[a,l]PDE resulted in 32% depurinating adducts, primarily at Ade (25%), whereas treatment with (+/-)-anti-DB[a,l]PDE produced 97% stable adducts. The formation of depurinating adducts following treatment of rat mammary gland with DB[a,l]P resulted in approximately 98% depurinating adducts, with the major adducts formed by one-electron oxidation. Only one depurinating diol epoxide adduct was formed. Tumorigenicity, mutations, and DNA adduct data suggest that depurinating Ade adducts play a major role in the initiation of tumors by DB[a,l]P.

Energy transfer studies in Eu3+-doped lead–niobium–germanate glasses

The spectral migration of energy among Eu 3+ ions in lead-niobium-germanate glass (60GeO 2 -25PbO-15Nb205) with different Eu 3+ concentrations (0.5, 1, 3, and 5 mol%) has been investigated at low temperature by using time-resolved fluorescence line narrowing (TRFLN) spectroscopy. The spectral features of the time-resolved fluorescence line-narrowed 5 D 0 → 7 F 0 emission spectra under resonant excitation at different wavelengths along the 7 F 0 → 5 D 0 transition as a function of time, excitation wavelength, and concentration reveal the existence of energy migration between discrete regions of the inhomogeneous broadened spectral profile. The analysis of the time evolution of the 5 D 0 → 7 F 0 emission shows that the electronic mechanism responsible for the ion-ion interaction can be identified as a dipole-dipole energy transfer process.

Identification and quantitation of benzo[ a]pyrene-derived DNA adducts formed at low adduction level in mice lung tissue

The two major metabolic pathways of benzo[ a]pyrene (BP) that lead to DNA lesions are monooxygenation that results in diolepoxides (BPDE) and one-electron oxidation that yields a BP radical cation. These pathways result in formation of stable and depurinating DNA adducts, respectively. Most in vivo animal studies with BP, however, have employed dosage/DNA adduct levels several orders of magnitude higher than the DNA damage level expected from environmentally relevant exposures. Presented are results of experiments in which A/J strain mice were intraperitoneally exposed to 50-μg/g doses of BP. It is shown that non-line-narrowed fluorescence and fluorescence line-narrowing spectroscopies possess the selectivity and sensitivity to distinguish between helix-external, base-stacked, and intercalated conformations of DNA–BPDE adducts formed in lung tissue. Concentrations measured by 32P postlabeling 2 and 3 days after intraperitoneal injection were 420–430 and 600–830 amol BPDE-type adducts per μg DNA. The external and base-stacked conformations are attributed mainly to (+)- trans-anti-BPDE-N 2dG and the intercalated conformations to (+)- cis-anti adducts. A stable adduct derived from 9-OH-BP-4,5-epoxide was also detected at a concentration about a factor of 10 lower than the above concentrations. The DNA supernatants were analyzed for the presence of depurinating BP-derived adducts by capillary electrophoresis laser-induced fluorescence and high-performance liquid chromatography mass spectrometry.

Role of Fluorescence Line-Narrowing Spectroscopy and Related Luminescence-Based Techniques in the Elucidation of Mechanisms of Tumor Initiation by Polycyclic Aromatic Hydrocarbons and Estrogens

Formation of DNA adducts by various carcinogens represents the first critical event in the mechanism of tumor initiation. The carcinogenic polycyclic aromatic hydrocarbons (PAHs) are biologically activated by two major mechanisms: one-electron oxidation to produce radical cations and monooxygenation to form bay-region diol epoxides. The PAH−DNA adducts formed by these mechanisms are stable adducts that remain in DNA unless removed by repair and depurinating adducts that are lost from DNA by cleavage of the glycosyl bond. Identification of PAH−DNA adducts has relied heavily on low-temperature, laser-based fluorescence spectroscopy under non-line-narrowing (NLN) and line-narrowing (FLN) conditions. These spectroscopies can be used for chemical identification, conformational analysis, and/or probing the microenvironment of DNA (or protein) adducts. Small and co-workers have pioneered the use of FLN spectroscopy in this research. For example, the structures of the depurinating adducts formed by the PAHs benzo[a]pyrene, 7,12-dimethylbenz[a]anthracene, and dibenzo[a,l]pyrene have been elucidated. Understanding of the mechanism of tumor initiation by PAHs has relied on identifying and quantifying the DNA adducts formed. The insights gained from the study of PAH−DNA adducts enabled us to discover the estrogen metabolites that form depurinating DNA adducts and can be potential endogenous initiators of human cancer. Small and co-workers have also studied the estrogen−DNA adducts and estrogen−thioether conjugates by using FLNS and related luminescence-based techniques and have demonstrated that the level of the 4-hydroxyestrone-1-N3-adenine depurinating adduct in breast tissue from a woman with breast carcinoma was significantly higher than that in breast tissue from women without breast cancer. The fluorescence- and phosphorescence-based techniques they are developing will be applied to analyzing estrogen adducts and conjugates as biomarkers of susceptibility to breast and other types of human cancer.

Time-resolved fluorescence line narrowing spectroscopy and fluorescence quenching in Nd3+-doped fluoroarsenate glasses

Abstract The spectral migration of energy among the Nd 3+ ions in fluoroarsenate glasses of the Na 4 As 2 O 7 , BaF 2 , YF 3 system with different Nd 3+ concentrations (0.5, 2, 3, 4, and 5 mol%) has been investigated by using time-resolved fluorescence line narrowing (TRFLN) spectroscopy. The features of the time-resolved fluorescence line narrowing 4 F 3/2 → 4 I 9/2 emission spectra obtained under resonant excitation at different wavelengths along the 4 I 9/2 → 4 F 3/2 transition as a function of Nd 3+ concentration reveal the existence of energy migration between discrete regions of the inhomogeneously broadened spectral profile. The analysis of the time evolution of the 4 F 3/2 → 4 I 9/2 narrowed emission shows that the electronic mechanism responsible for the ion–ion interaction can be identified as a dipole–dipole energy transfer process. From the thermal behavior of lifetimes of the 4 F 3/2 state a T 3 dependence for the non-radiative Nd–Nd relaxation processes has been found in the 10–80 K temperature range for Nd 3+ concentrations higher than 0.5 mol% which is in agreement with a two-site non-resonant process.