Lifetime Measurements Of Excited States Research Articles

Commissioning the DANTE array of BaF2 detectors at TRIUMF-ISAC using a fast-timing lifetime measurement

The Di-pentagonal Array for Nuclear Timing Experiments (DANTE) is an array of ten BaF2 detectors used in conjunction with the 8π gamma-ray spectrometer at the TRIUMF-ISAC radioactive-ion beam facility. DANTE is used to conduct direct lifetime measurements of nuclear excited states in the picosecond - nanosecond range. This, in turn, will aid in probing the collective structures of deformed nuclei. The capability of DANTE to measure nanosecond-scale lifetimes is demonstrated by using a 152Eu source. The half-life of the Iπ = 21+ state of 152Sm is measured to be 1.426 ± 0.018 ns.

Protein binding studies of luminescent rhenium(I) diimine complexes

Interaction of four luminescent rhenium(I) diimine complexes, [Re(CO)3(N–N)L]PF6 ((N–N=2,2-bipyridine, L=py-3-COOH) 1a, (N–N=2,2-bipyridine, L=py-3-CONH2) 1b, (N–N=1,10-phenanthroline, L=py-3-COOH) 2a, (N–N=1,10-phenanthroline, L=py-3-CONH2) 2b with bovine serum albumin (BSA) at physiological pH has been examined using UV–Vis absorption and luminescence spectroscopy, excited state lifetime measurement and circular dichroism (CD). In the presence of BSA, the luminescence of Re(I) complexes is quenched due to the locking-in of the probe into the protein environment. Interestingly the probe is released from the protein environment in the presence of sodium dodecyl sulfate (SDS) resulting in the restoration of the original luminescence along with a red shift in the emission maximum. These observations are explained in terms of binding constants (Ka) of probe with protein and surfactant and the nature of the binding has been investigated from Scatchard plot and Hill’s coefficient (n) value. These studies point out that the interaction between Re(I) complexes and BSA is cooperative in nature.

Photoinduced Electron Transfer and Persistent Spectral Hole-Burning in Natural Emerald

Wavelength-selective excited-state lifetime measurements and absorption, luminescence, and hole-burning spectra of a natural African emerald crystal are reported. The (2)E excited-state lifetime displays an extreme wavelength dependence, varying from 190 to 37 μs within 1.8 nm of the R(1)-line. Overall, the excited state is strongly quenched, in comparison to laboratory-created emerald (τ=1.3 ms), with an average quenching rate of ∼6 × 10(3) s(-1) at 2.5 K. This quenching is attributed to photoinduced electron transfer caused by a relatively high concentration of Fe(2+) ions. The forward electron-transfer rate, k(f), from the nearest possible Fe(2+) sites at around 5 Å is estimated to be ∼20 × 10(3) s(-1) at 2.5 K. The photoreductive quenching of the excited Cr(3+) ions by Fe(2+) is followed by rapid electron back-transfer in the ground state upon deactivation. The exchange interaction based quenching can be modeled by assuming a random quencher distribution within the possible Fe(2+) sites with the forward electron-transfer rate, k(f), given as a function of acceptor-donor separation R by exp[(R(f)-R)/a(f)]; R(f) and a(f) values of 13.5 and 2.7 Å are obtained at 2.5 K. The electron transfer/back-transfer reorganizes the local crystal lattice, occasionally leading to a minor variation of the short-range structure around the Cr(3+) ions. This provides a mechanism for spectral hole-burning for which a moderately high quantum efficiency of about ∼0.005% is observed. Spectral holes are subject to spontaneous hole-filling and spectral diffusion, and both effects can be quantified within the standard two-level systems for non-photochemical hole-burning. Importantly, the absorbance increases on both sides of broad spectral holes, and isosbestic points are observed, in accord with the expected distribution of the "photoproduct" in a non-photochemical hole-burning process.

Lanthanide-Ion-Assisted Structural Collapse of Layered GaOOH Lattice

GaOOH nanorods were prepared by hydrolysis of Ga(NO(3))(3)·xH(2)O by urea at ~100 °C in the presence of different amounts of lanthanide ions like Eu(3+), Tb(3+), and Dy(3+). On the basis of X-ray diffraction and vibrational studies, it is confirmed that layered structure of GaOOH collapses even when very small amounts of lanthanide ions (1 atom % and more) are present in the reaction medium during the synthesis of GaOOH nanorods. The incorporation of lanthanide ions at the interlayer spacing of the GaOOH lattice, followed by its reaction with OH groups that connect the layers containing edge-shared GaO(6) in GaOOH, is the reason for the collapse of the layered structure and associated amorphization. This leads to the formation of finely mixed hydroxides of lanthanide and gallium ions. These results are further confirmed by steady-state luminescence and excited-state lifetime measurements carried out on the samples. The morphology of the nanorods is maintained upon heat treatment at high temperatures like 500 and 900 °C, and during this process, the finely mixed lanthanide and gallium hydroxides facilitate diffusion of lanthanide ions into the Ga(2)O(3) lattice, as revealed by the existence of strong energy transfer with an efficiency of more than 90% between the host and lanthanide ions.

Strategies for light extraction from surface plasmons in organic light-emitting diodes

Organic light-emitting diodes (OLEDs) usually exhibit a low light-outcoupling efficiency of only 20%. Typically, more than 30% of the available power is lost to surface plasmons (SPs). Consequently, the overall efficiency could be strongly enhanced by recovering SP losses. Therefore, three suitable techniques for extracting SPs-index coupling, prism coupling, and grating coupling-are discussed from a theoretical point of view and investigated experimentally in simplified OLED-like structures. The basic physical processes are clarified by systematic variations of the involved layer thicknesses and by excited state lifetime measurements. In addition, the analysis of the results is supported by optical simulations based on a dipole model. Finally, the advantages and disadvantages of each method, their potential efficiency for recovering SP losses, as well as the applicability in OLEDs are compared.

Luminescence characteristics of YAP:Ce scintillator powders and composites

Cerium doped yttrium aluminate perovskite YAlO 3 (YAP) powders are pursued as interesting alternatives to bulk crystals for application in scintillating devices. The emissions of these materials in the near-UV and visible spectral regions originate from electric dipole transitions between 4f and 5d energy levels of Ce 3+ and largely depend on the environment occupied by the ion. In search for improved synthesis conditions that can lead to phase pure powders with optimized structural and spectroscopic characteristics, in this work we have employed the polymeric precursor (Pechini) method to prepare crystalline and amorphous YAP:Ce powders doped with 1–10 mol% Ce 3+. Interesting composite materials were also obtained by dispersing some of the YAP:Ce powders in silica xerogels. A comparative structural and spectroscopic study of all the samples was done by XRD, FT–IR, emission, excitation and excited state lifetime measurements. In agreement with previous reports, excitation at 296 nm results in intense emission in the range 315–425 nm with an average decay time of 30 ns.

Quenching of the E1 strength in 149Nd

Lifetime measurements of excited states in 149Nd have been performed using the advanced time-delayed \( \beta\)\( \gamma\)\( \gamma\)(t) method. Half-lives of 14 excited states in 149Nd have been determined for the first time or measured with higher precision. Twelve new \( \gamma\) -lines and 5 new levels have been introduced into the decay scheme of 149Pr based on results of the \( \gamma\)\( \gamma\) coincidence measurements. Reduced transition probabilities have been determined for 40 \( \gamma\) -transitions in 149Nd . Configuration assignments for 6 rotational bands in 149Nd are proposed. Enhanced E1 transitions indicate that the ground-state band and the band built on the 332.9keV level constitute a pair of the \(\ensuremath K^{\pi} = 5/2^{\pm}\) parity doublet bands. Potential energy surfaces on the \(\ensuremath (\beta_{2},\beta_{3})\) -plane have been calculated for the lowest single quasi-particle configurations in 149Nd using the Strutinski method and the axially deformed Woods-Saxon potential. The predicted occurrence of the octupole-deformed K = 5/2 configuration is in agreement with experiment. Unexpectedly low \(\ensuremath \vert D_0\vert\) values obtained for the \(\ensuremath K^{\pi} = 5/2^{\pm}\) parity doublet bands may result from cancellation between the proton and neutron shell correction contributions to \(\ensuremath \vert D_0\vert\) .

Squaraine Rotaxane as a Reversible Optical Chloride Sensor

A mechanically interlocked squaraine rotaxane is comprised of a deep-red fluorescent squaraine dye inside a tetralactam macrocycle. NMR studies show that Cl(-) binding to the rotaxane induces macrocycle translocation away from the central squaraine station, a process that is completely reversed when the Cl(-) is removed from the solution. Steady-state fluorescence and excited-state lifetime measurements show that this reversible machine-like motion modulates several technically useful optical properties, including a three-fold increase in deep-red fluorescence emission that is observable to the naked eye. The excited states were characterized quantitatively by time-correlated single photon counting, femtosecond transient absorption spectroscopy, and nanosecond laser flash photolysis. Cl(-) binding to the rotaxane increases the squaraine excited singlet state lifetime from 1.5 to 3.1 ns, and decreases the excited triplet state lifetime from >200 to 44 micros. Apparently, the surrounding macrocycle quenches the excited singlet state of the encapsulated squaraine dye and stabilizes the excited triplet state. Prototype dipsticks were prepared by adsorbing the lipophilic rotaxane onto the ends of narrow, C18-coated, reverse-phase silica gel plates. The fluorescence intensity of a dipstick increased eighteen-fold upon dipping in an aqueous solution of tetrabutylammonium chloride (300 mM) and was subsequently reversed by washing with pure water. It is possible to develop the dipsticks for colorimetric determination of Cl(-) levels by the naked eye. After dipping into aqueous tetrabutylammonium chloride, a dipstick's color slowly fades at a rate that depends on the amount of Cl(-) in the aqueous solution. The fading process is due primarily to hydrolytic bleaching of the squaraine chromophore within the rotaxane. That is, association of Cl(-) to immobilized rotaxane induces macrocycle translocation and exposure of the electrophilic C(4)O(2) core of the squaraine station, which is in turn attacked by the ambient moisture to produce a bleached product.

Interaction dynamics of hypericin with low-density lipoproteins and U87-MG cells

The natural photosensitizer hypericin exhibits potent properties for tumor diagnosis and photodynamic therapy. Fluorescent properties of hypericin along with various technical approaches have been used for dynamic studies of its interaction with low-density lipoprotein and U87 glioma cells. Evidences for hypericin release from low-density lipoprotein towards cells plasmatic membrane are addressed. Subsequent subcellular bulk flow redistribution leading to non-specific staining of intracellular membranes compartment were observed within cells. It was shown, that monomers of hypericin are the only redistributive forms. Increasing concentration of hypericin leads to the formation of non-fluorescent aggregates within low-density lipoprotein as well as within the U87 cells, and can preclude its photosensitizing activities. However, the aggregation process can only account for a part of the observed emission decrease. As shown by the excited state lifetime measurements, this fluorescence quenching actually results from a combination of aggregation process and energy transfer from monomers to aggregates. In all experiments, hydrophobic character of hypericin appears as the driving force of its redistribution process.

Photophysical and Spectroscopic Investigations on (Oligo)Thiophene-Arylene Step-ladder Copolymers. The Interplay of Conformational Relaxation and On-Chain Energy Transfer

An optical spectroscopy and photophysics study on four (oligo)thiophene-phenylene and (oligo)thiophene-naphthylene step-ladder type copolymers in solution (room and low temperature) and in the solid state (thin film) is presented. The study involves absorption, emission, and triplet-singlet difference spectra, together with quantitative measurements of quantum yields (fluorescence, intersystem crossing, internal conversion, and singlet oxygen formation), excited-state lifetimes, and singlet and triplet energies. The overall data allow for a determination of the rate constants for all decay processes and from these several conclusions could be drawn: (1) in solution the main deactivation channels are radiationless processes (S(1) approximately -->S(0) internal conversion and S(1) approximately -->T(1) intersystem crossing); (2) from time-resolved fluorescence decays in the picosecond time domain three decay components are seen: a fast decay (10-20 ps) at short wavelengths, which becomes a rising component at longer wavelengths, an intermediate decay component (120-190 ps) most probably associated to isolated conjugated segments, and a third exponential related to the emission of the fully relaxed polymer. The assignment of the fast decay component to an on-chain energy transfer/migration is based of the dependence of the decay time on the solvent viscosity in combination with the investigation of an oligomeric model compound. Here, the absence of any significant changes of the decay parameters (decay times and pre-exponential factors) upon going from a less (toluene) to a more viscous (decalin) solvent together with the monoexponential fluorescence decay of the oligomeric model compound allow us to differentiate between deactivation of the singlet excited state by conformational relaxation and on-chain energy/transfer migration.

Spectroscopy ofB13via theC13(t,He3)reaction at115AMeV

Gamow-Teller and dipole transitions to final states in $^{13}\mathrm{B}$ were studied via the $^{13}\mathrm{C}(t,^{3}\mathrm{He})$ reaction at ${E}_{t}=115A$ MeV. In addition to the strong Gamow-Teller transition to the $^{13}\mathrm{B}$ ground state, a weaker Gamow-Teller transition to a state at 3.6 MeV was found. This state was assigned a spin-parity of $3/{2}^{\ensuremath{-}}$ by comparison with shell-model calculations using the WBP and WBT interactions which were modified to allow for mixing between $n\ensuremath{\hbar}\ensuremath{\omega}$ and $(n+2)\ensuremath{\hbar}\ensuremath{\omega}$ configurations. This assignment agrees with a recent result from a lifetime measurement of excited states in $^{13}\mathrm{B}$. The shell-model calculations also explained the relatively large spectroscopic strength measured for a low-lying $1/{2}^{+}$ state at 4.83 MeV in $^{13}\mathrm{B}$. The cross sections for dipole transitions up to ${E}_{x}$$(^{13}\mathrm{B})=20$ MeV excited via the $^{13}\mathrm{C}(t,^{3}\mathrm{He})$ reaction were also compared with the shell-model calculations. The theoretical cross sections exceeded the data by a factor of about 1.8, which might indicate that the dipole excitations are ``quenched''. Uncertainties in the reaction calculations complicate that interpretation.

Luminescent and thermo-optical properties of Nd3+-doped yttrium aluminoborate laser glasses

In this work we performed a thorough spectroscopic and thermo-optical investigation of yttrium aluminoborate glasses doped with neodymium ions. A set of samples, prepared by the conventional melt-quenching technique and with Nd2O3 concentrations varying from 0.1 to 0.75 mol %, were characterized by ground state absorption, photoluminescence, excited state lifetime measurements, and thermal lens technique. For the neodymium emission at 1064 nm (F43/2→I411/2 transition), no significant luminescence concentration quenching was observed and the experimental lifetime values ranged around 70 μs. The obtained values of thermal conductivity and diffusivity of approximately 10.3×10−3 W/cm K and 4.0×10−3 cm2/s, respectively, are comparable to those of commercial laser glasses. Moreover, the fluorescence quantum efficiency of the glasses, calculated using the Judd–Ofelt formalism and luminescence decay, lies in the range from 0.28 to 0.32, larger than the typical values obtained for Nd3+ doped YAl3(BO3)4 crystals.

Sub-nanosecond lifetime measurements using the Double Orange Spectrometer at the cologne 10 MV Tandem accelerator

Conversion electron spectroscopy constitutes an important tool in nuclear structure physics. A high efficiency iron-free Orange type electron spectrometer with an energy resolution of 1–2% has been installed at a beam line of the Cologne 10MV FN Tandem Van-de-Graaff accelerator for in-beam studies of conversion electrons. In combination with a γ-ray detector array, high efficiency e-–γ-coincidences can be performed. The newly developed very fast LaBr3(Ce) scintillator detector with an energy resolution of about 4% makes it also possible to use e-–γ-coincidences for lifetime measurements of nuclear excited states. A second iron-free Orange spectrometer can be connected to perform e-–e--coincidences. Because of the higher efficiency and the better energy resolution, the use of the Double Orange Spectrometer for lifetime measurements is more powerful. Lifetimes down to 100ps and even less can be determined with an accuracy of about 10ps. The working principle of the Orange spectrometer and the setup of the Double Orange Spectrometer are described. The performances are illustrated by examples of in-beam experiments with a main focus on high precision lifetime measurements.

Spectroscopic and Crystal Structural Analyses of Zinc (II) Tetraphenylporphyrin J-aggregates

The self-assembly behavior of non-aqueous zinc (Ⅱ) tetraphenylporphyrin (ZnTPP) was studied in dry acetonitrile. A red shift in the absorption and fluorescence spectra implies that a part of ZnTPP molecules self-associate into J-aggregates in a head-to-head arrangement. The formation of ZnTPP J-aggregates is dependent on the solvents. The spectroscopic and excited state lifetime measurements indicate that the radioactive decay rate of the aggregates is two times faster than that of the monomer, which is indicative of superradiance from the aggregates. Optical microscopy of a ZnTPP microcrystal shows a rod-like molecular arrangement during crystal growth. An X-ray structural analysis and an illustration of crystal packing in ZnTPP (CH3CN) also show that one of the peripheral phenyl groups of a ZnTPP molecule interacts perpendicularly with the pyrrole ring of an adjacent porphyrin molecule. These π-π interactions are actually an edge-to-face C-H…π interaction. We, therefore, propose a structural model for the ZnTPP J-aggregates. The effects of the Zn-N coordination bond distance in a single crystal are discussed based on different packing arrangements.

Photocrystallographic analysis of [AuCl(PPh3)2] for elucidation of the crystal packing effect on the photophysical properties

A metal-ligand bond shortening of [AuCl(PPh(3))(2)] by photoexcitation was analyzed by the photocrystallographic method in the unsolvated crystal. The gradual structural change of photoexcited and ground-state molecules with cooling explains the temperature dependence of the emission spectrum and the excited-state lifetime. Actually, on cooling, the ground-state molecular structure approached the excited-state structure. As a result, the HOMO-LUMO gap of [AuCl(PPh(3))(2)] became narrower and a red shift of the absorption and emission bands were observed. Below 180 K, inhibition of the bond shortening was observed due to the intermolecular interactions, which was confirmed by the temperature dependence of the photoexcited phase cell volume, the integrated emission intensity, and the excited-state lifetime measurement.

Fluorescence Resonance Energy Transfer Studies of Prothrombin Recognition by Prothrombinase.

Prothrombinase, the membrane bound complex of factors Xa and Va, catalyzes thrombin formation by the cleavage of prothrombin at R320 followed by R271. The two cleavage sites are expected to be ~40Å apart in prothrombin. Essentially ordered cleavage arises from exosite tethering of prothrombin to the enzyme which facilitates engagement of the R320 site but precludes docking of the R271site to the active site of the catalyst. Prior cleavage at R320 appears necessary for efficient active site engagement by the R271 site. The physical bases for these observations have been investigated using fluorescence resonance energy transfer to assess the disposition of the 271 site to the active site of the catalyst. Expression and purification of a prothrombin variant containing C in place of R271 (IIC271) surprisingly yielded a product lacking free cysteines. A free thiol could be rescued following treatment with reduced glutathione and could be modified with PEG or Alexa532 maleimides yielding ~1 mol reagent incorporated /mol protein. Characterization of IIC271 modified with Alexa532 maleimide (IIC271*) established that the probe was covalently contained only in those fragments bearing residue 271. IIC271* could be cleaved normally by prothrombinase at R320 yielding a product with the expected catalytic activity. Thus, thiol rescue permits selective modification of the engineered 271 site without obvious deleterious effects. Fluorescence studies employed prothrombinase assembled using Xa containing Alexa488 covalently linked to the active site with a peptidyl chloromethylketone as donor and IIC271* as acceptor. Increasing concentrations of IIC271* yielded a large and saturable decrease in donor fluorescence. This decrease could be reversed by the addition of excess unlabelled prothrombin, was not observed in the absence of membranes and could be eliminated by the addition of EDTA. Changes in donor fluorescence exhibit the features expected for energy transfer arising from the binding of prothrombin to prothrombinase. Titrations with increasing concentrations of IIC271* yielded Kd = 40.7 ± 13.5 nM and n = 1.12 ± 0.23 mol IIC271*/mol prothrombinase. Agreement between the Kd measured using active site blocked enzyme and the previously measured Km further substantiates the primary role played by exosite binding in determining substrate affinity. Maximal energy transfer efficiency inferred from donor quenching (79.3 ± 1.7%) was confirmed by measurements of excited state lifetime. Based on the measured Forster distance (57.4 Å), this value indicates that the acceptor at the 271 site in IIC271* is ~46 Å away from the donor at the active site of Xa within prothrombinase. IIC271* was cleaved by ecarin at R320 to yield meizothrombin (mIIaC271*), stabilized by covalent inactivation and re-purified. Similar experiments performed with mIIaC271* as acceptor yielded substantial differences in the extent of donor quenching in comparison to IIC271*. Thus, major changes are evident at the 271 site following cleavage at R320. We conclude that exosite-dependent binding of prothrombin to prothrombinase constrains the substrate such that the 271 site is distant from and cannot efficiently engage the active site of the catalyst. This explains the inability of prothrombinase to efficiently cleave at R271 in prothrombin. Proteinase formation following initial cleavage at R320 reorients the 271 site with respect to the active site of prothrombinase. Our findings offer a physical explanation for how prothrombinase is able to discriminate between the cleavage sites in prothrombin resulting in the ordered cleavage of the two distant sites within the substrate.

Luminescent Dipyridophenazine-Ruthenium Probes for Liposome Membranes

Ruthenium complexes with dipyridophenazine (dppz) type ligands have several characteristics that make them good candidates for use as luminescence probes for hydrophobic environments. Most studies have concerned DNA intercalation, but also lipid membrane fluidity and liposome orientation have been assessed. We report here dipyridophenazine derivatives ([Ru(phen)2dppz]2+) substituted with one or two alkyl ether chains of different lengths aimed at finding the optimum substitution for a high quantum yield when bound to a phospholipid membrane bilayer. The orientation of membrane bound molecules is studied using flow linear dichroism (LD) with phospholipid vesicles as membrane models. LD, excitation anisotropy, steady state luminescence and excited-state lifetime measurements are used to quantitatively investigate the insertion and orientation of the complexes in the vesicles. All complexes are inserted with their long axis of the dppz moiety mainly parallel to the lipid chains, and the degree of orientation is comparable to that of the orientation probe retinoic acid. The ruthenium "head group" with its positive charge functions as a buoy at the water-membrane interface while the hydrophobic chain part embeds the complex down into the bilayer. The complex with two hexyl ether substituents (named D6) has the optimal chain length regarding membrane insertion and orientation, and together with the highest quantum yield, is the best luminescence membrane probe in the two series.

Lifetime measurements of excited states in 17C: Possible interplay between collectivity and halo effects

Lifetime measurements were performed on low-lying excited states of the neutron-rich isotope C17 using the recoil shadow method. The γ-decay mean lifetimes were determined to be 583±21(stat)±35(syst) ps for the first excited state at 212 keV and 18.9±0.6(stat)±4.7(syst) ps for the second excited state at 333 keV. Based on a comparison with the empirical upper limits for the electromagnetic transition strengths, these decays are concluded to be predominantly M1 transitions. The reduced M1 transition probabilities to the ground state were deduced to be (1.0±0.1)×10−2μN2 and (8.2−1.8+3.2)×10−2μN2, respectively, for the first and second excited states. The strongly hindered M1 strength as well as the lowered excitation energy represents unique nature of the 212-keV state.

Photoacoustic probing of fluorophore excited state lifetime with application to oxygen sensing

A new method is developed to perform local measurements of fluorophore excited state lifetimes in turbid media without collecting the fluorescence emission. The method is based on a pump-probe approach where a first laser pulse excites the dye and then a second laser pulse is used for photoacoustic probing of the transient absorption. The photoacoustic response generated by the probe pulse is recorded by an ultrasound receiver. Repeating the measurement for increasing pump-probe time delays yields a series of photoacoustic signals that are used to extract the lifetime of the excited state. The method is validated by measuring the lifetime of an oxygen sensitive dye solution at different concentrations of dissolved oxygen. The dye is pumped with a 532-nm pulsed laser and the transient absorption at 740 nm is probed using a second pulsed laser system. The photoacoustic-based results are in close agreement with those obtained from time-dependent fluorescent measurements. The method can be extended to photoacoustic lifetime imaging by using a receiver array instead of a single receiver. Potential applications of this method include tissue oxygen imaging for cancer diagnostics and mapping molecular events such as resonant energy transfer and ion collisions in a biological environment.

Spectroscopic characterization of streptavidin functionalized quantum dots

The spectroscopic properties of quantum dots can be strongly influenced by the conditions of their synthesis. In this work, we have characterized several spectroscopic properties of commercial, streptavidin functionalized quantum dots (QD525, lot 1005-0045, and QD585, lot 0905-0031, from Invitrogen). This is the first step in the development of calibration beads to be used in a generalizable quantification scheme of multiple fluorescent tags in flow cytometry or microscopy applications. We used light absorption, photoexcitation, and emission spectra, together with excited state lifetime measurements, to characterize their spectroscopic behavior, concentrating on the 400- to 500-nm wavelength ranges that are important in biological applications. Our data show an anomalous dependence of emission spectrum, lifetimes, and quantum yield (QY) on excitation wavelength that is particularly pronounced in the QD525. For QD525, QY values ranged from 0.2 at 480 nm excitation up to 0.4 at 450 nm and down again to 0.15 at 350 nm. For QD585, QY values were constant at 0.2 between 500 and 400 nm, but they dropped to 0.1 at 350 nm. We attribute the wavelength dependencies to heterogeneity in size and surface defects in the QD525, consistent with characteristics described previously in the chemistry literature. The results are discussed in the context of bridging the gap between what is currently known in the physical chemistry literature of quantum dots and the quantitative needs of assay development in biological applications.