Intrinsic Decay Rate Research Articles

Homogeneous Silver-Coated Nanoparticle Substrates for Enhanced Fluorescence Detection

A simple method has been developed for the deposition of uniform silver-coated nanoparticles on glass substrates, with a homogeneous distribution shown by scanning electron microscopy (SEM). UV-visible spectroscopy and energy-dispersive X-ray analysis (EDX) have been used to characterize both the optical density and elemental content of the deposited nanoparticles. The fluorescence enhancement was investigated using a monolayer of FITC-conjugated human serum albumin (FITC-HSA) and tested using laser scanning microscopy at 488 nm excitation wavelength. The enhancement factor was calculated from individual spectra recorded with a Fluorolog-Tau-3 spectrofluorometer. We identified the nanoparticle growth regime which led to fluorescence enhancement. Such enhancement is detectable when Au core-Ag shell nanoparticles increased their size to 47 nm, in agreement with theoretical estimates. The origin of this enhancement for appropriate size nanoparticles is attributed to the effect of an increased excitation rate from the local field enhanced by the interaction of incident light with the nanoparticles and/or higher quantum yield from an increase of the intrinsic decay rate of the fluorophore. We thus demonstrated that the Au core-Ag shell nanostructures on glass surfaces are promising substrates for fluorescence enhancement with outstanding macroscopic homogeneity. This important feature will pave the way for the application of our substrates in biotechnology and life sciences such as imaging and sensing of biomolecules in proteomics.

Interaction between amorphous silicon nanoclusters and neodymium ions

The luminescent infrared transitions in Nd3+ can be activated via a transfer mechanism from amorphous silicon nanoclusters. The Nd photoluminescence (PL) has some unusual characteristics, including a weak temperature dependence of the PL intensity. The data are explained using a simple rate equation model which enables an effective nanocluster-to-neodymium transfer time of ∼0.15μs to be extracted. This is short enough to dominate the intrinsic nanocluster decay rates at low temperatures but long enough to imply that the coupling between the nanoclusters and the Nd ions is, in fact, weaker than for Nd-doped bulk silicon or other semiconductors.

Photoluminescence quantum efficiency of dense silicon nanocrystal ensembles inSiO2

The photoluminescence decay characteristics of silicon nanocrystals in dense ensembles fabricated by ion implantation into silicon dioxide are observed to vary in proportion to the calculated local density of optical states. A comparison of the experimental $1∕e$ photoluminescence decay rates to the expected spontaneous emission rate modification yields values for the internal quantum efficiency and the intrinsic radiative decay rate of silicon nanocrystals. A photoluminescence quantum efficiency as high as $59%\ifmmode\pm\else\textpm\fi{}9%$ is found for nanocrystals emitting at $750\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ at low excitation power. A power dependent nonradiative decay mechanism reduces the quantum efficiency at high pump intensity.

The effects of supramolecular assembly on exciton decay rates in organic semiconductors

We present time-resolved photoluminescence measurements on two series of oligo-p-phenylenevinylene (OPV) materials that are functionalized with quadruple hydrogen-bonding groups. These form supramolecular assemblies with thermotropic reversibility. The morphology of the assemblies depends on the way that the oligomers are functionalized; monofunctionalized OPVs (MOPVs) form chiral, helical stacks while bifunctionalized OPVs (BOPVs) form less organized structures. These are therefore model systems to investigate the effects of supramolecular assembly, the effects of morphology, and the dependence of oligomer length on the radiative and nonradiative rates of pi-conjugated materials. The purpose of this work is to use MOPV and BOPV derivatives as model systems to study the effect of intermolecular interactions on the molecular photophysics by comparing optical properties in the dissolved phase and the supramolecular assemblies. A simple photophysical analysis allows us to extract the intrinsic radiative and nonradiative decay rates and to unravel the consequences of interchromophore coupling with unprecedented detail. We find that interchromophore coupling strongly reduces both radiative and intrinsic nonradiative rates and that the effect is more pronounced in short oligomers.

Enhanced lanthanide luminescence using silver nanostructures: opportunities for a new class of probes with exceptional spectral characteristics.

The photophysical effects of europium tetracycline immobilized in thin polyvinyl alcohol films coated onto silver nanostructures have been investigated. Complimentary to recent reports from our laboratories that the close proximity of luorophores to silver nanostructures can enhance their intrinsic radiative decay rate, we show that up to a 16-fold enhancement in lanthanide luminescence is possible, accompanied by a notable reduction in luminescence lifetime. These results suggest the potential future development of a new class of significantly brighter lanthanide based probes with exceptional spectral properties, which can probably undergo significantly more excitation-emission event cycles due to the reduced lifetime, substantially increasing detectability.

Stereodifferentiation in the formation and decay of the encounter complex in bimolecular electron transfer with photoactivated acceptors

Experimental evidence has been obtained for the involvement of encounter complexes between both enantiomers of a pi,pi* triplet excited ketone and a chiral phenol or indole. Determination of the pre-equilibrium constants (K(EC)) and the intrinsic decay rate constants (kd) indicates a significant stereodifferentiation in both steps of the quenching process.

SOLUTION OF THE ORTHOPOSITRONIUM LIFETIME PUZZLE

The intrinsic decay rate of orthopositronium formed in SiO 2 powder is measured using the direct 2γ correction method such that the time dependence of the pick-off annihilation rate is precisely determined. The decay rate of orthopositronium is found to be 7.0396±0.0012(stat.)±0.0011(sys.)μs-1, which is consistent with our previous measurements with about twice the accuracy. Results agree well with the O(α2) QED prediction, and also with a result reported very recently using nanoporous film.

Concentration quenching of fluorescence from the ( 3P 0 + 3P 1) manifold in heavily-doped Pr 3+: ZBAN glasses

Fluorescence waveforms from the ( 3P 0 + 3P 1) manifold in Pr 3+ doped ZBAN glass at wavelengths of 520, 635 and 695 nm were measured for Pr 3+ concentrations from 4 to 12 mol%. The waveforms were found to be non-exponential with decay rates rapidly increasing with Pr 3+ concentration and independent of whether the 3P 0 or the 3P 1 level was excited. The multipolar energy transfer model was used to analyse the waveforms and this showed that concentration quenching was due to cross-relaxation by dipole–dipole interaction. The critical concentration, at which the cross-relaxation rate equals the intrinsic decay rate, was found to be of 2.06 × 10 26 m −3 (1.20 mol%). There was no evidence of excitation diffusion for Pr 3+ concentrations of up to 12 mol%.

Luminescent Blinking from Silver Nanostructures.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Precision measurement of orthopositronium decay rate using SiO2 powder

The intrinsic decay rate of orthopositronium formed in SiO2 powder is measured using the direct 2γ correction method such that the time dependence of the pick-off annihilation rate is precisely determined using high energy-resolution germanium detectors. As a systematic test, two different types of SiO2 powder are used with consistent findings. The intrinsic decay rate of orthopositronium is found to be 7.0396±0.0012(stat)±0.0011(sys) μs−1, which is consistent with previous measurements using SiO2 powder with about twice the accuracy. Results agree well with a recent O(α2) QED prediction, varying 3.8–5.6 experimental standard deviations from other measurements.

Luminescent Blinking from Silver Nanostructures.

Silver nanostructures deposited on glass showed luminescent blinking when excited at a high 442 nm irradiance. The irradiance required to photoactivate the silver, was dependent on the nature of the silver nanostructures. Silver fractal-like structures were found to be highly emissive, requiring only ≈30 W/cm(2) for photoactivation as compared to silver island films and spin-coated silver colloids, which required a significantly higher irradiance, > 100 W/cm(2), to observe similar luminescent emission. In contrast to our recent findings for gold colloids, foci with different color blinking were also observed, with an increase in luminescence intensity as a function of time. We place these findings in context with recent work from our laboratory which employs these silver nanostructures for applications in metal-enhanced fluorescence, a relatively new phenomenon in fluorescence, whereby metallic particles, colloids, and fractal-like structures can modify the intrinsic radiative decay rate of close proximity fluorescent species. These effects are a consequence of localized changes in photonic mode density around the fluorophores, and we can now report are typically observed at significantly lower illumination intensities as compared to those required to photoactivate silver. Subsequently, our findings strongly suggest that the enhanced fluorescence emission of fluorophores positioned in close proximity to metallic silver structures is not due to either intrinsic silver blinking, or indeed the silver luminescence pumping the fluorophore. Further, the intrinsic luminescence properties of silver reported here, suggest a new class of luminescence probes and labels.

Adiabatic change in the Smoluchowski equation: Orientational diffusion of polar particles

A system described by a Smoluchowski equation in configuration space is subject to slow change of external fields. The distribution function is studied by expanding it in instantaneous eigenfunctions of the adjoint Smoluchowski operator leading to an adiabatic approximation for the distribution function which is linear in the rates of change of external fields. The general result is applied to a magnetic dipole subject to slowly changing external magnetic field. Both eigenvalues and eigenfunctions are obtained analytically by a series expansion method. For slow change of field magnitude, the eigenfunction solution is compared numerically with the adiabatic approximation and with an independent numerical solution of the Smoluchowski equation. The case of slow rotation of a field of constant magnitude and an example of cyclic change involving both change of magnitude and direction of the field are studied. Significant nonequilibrium components of magnetization are generated and persist throughout the duration of slow change of external field. For the system to be regarded as in thermal equilibrium in the instantaneous external field, the rate of change of the external field must be very much slower than the intrinsic decay rates of the system.

Microscopic theory of ultrafast electron dynamics on metal surfaces

Ultrafast electron dynamics in image states of metals are investigated by Green function method involving off-diagonal elements of self-energy functions, which represent virtual electron transitions between different image states. The diagonal and off-diagonal elements of Green functions, which represent the electron densities of the image states with quantum numbers from 3 to 6 and the probability amplitude of electron propagation between different image states, respectively, are calculated by treating the self-energy functions within the GW approximation. The virtual electron transitions between different image states account for occurrence of quantum beats and acceleration and deceleration of the decay of electron densities, as compared with the intrinsic decay rates determined by the imaginary parts of the diagonal elements of the self-energy functions. It is quantitatively demonstrated that the effects of the virtual transitions are marked especially for image states with large quantum numbers.

Effect of point defects on the decay of the longitudinal optical mode

The longitudinal optical mode at the zone center often has the highest frequency in the phonon spectrum, so that its phonons cannot be scattered elastically into other modes. Their decay rate Γ is enhanced by defects above the intrinsic anharmonic rate Γ ah by the combined action of anharmonic processes and defects such as point defects. This can occur by a second-order process. The phonon is scattered first into a neighboring mode. This intermediate state is followed by anharmonic decay. The intrinsic anharmonic decay rate Γ ah is almost independent of the intermediate state mode, so that Γ ah can be factored out of the second-order decay rate. An expression of the overall decay rate is obtained. A different result can be obtained by convoluting the point defect scattering over the anharmonic line width. This procedure was used numerically for isotopic mixtures of Ge and Si. An algebraic expression for the convolution is given here. These different approaches are compared to each other and compared to the observed line broadening in the isotopic mixtures. The second-order theory is also applied to line broadening due to Si solutes and their associated Al vacancies in aluminum nitride.

Influence of Large Metal Cations on the Photophysical Properties of Texaphyrin, a Rigid Aromatic Chromophore

The excited-state properties, including singlet oxygen quantum yields, of a series of metallotexaphyrins (M-Tex), containing coordinated paramagnetic and also diamagnetic lanthanide(III) and other cations, are reported as are the solution-phase magnetic susceptibilities of the paramagnetic species. It is found that the singlet (1.593−1.638 eV) and triplet excited-state (1.478−1.498 eV) energies are only marginally affected by the choice of coordinated metal species. By contrast, photophysical parameters that are directly associated with the intrinsic decay rates of, for example, the singlet excited state, such as fluorescence lifetimes, reveal a strong dependence on the nature of the coordinated metal species. In particular, for the series of diamagnetic metals including Y−Tex, In−Tex, Lu-Tex, and Cd−Tex, an increase in atomic number leads to notably shorter lifetimes (τfluorescence(Y-Tex) = 1298 ps, τfluorescence(Lu-Tex) = 414 ps), a result that is interpreted as a heavy atom effect. The paramagnetic species, as a general rule, give rise to much shorter fluorescence lifetimes (τfluorescence = 99−515 ps) as compared to their diamagnetic analogues and are seen to fluoresce weakly, with fluorescence quantum yields (ΦF = 0.0002−0.0028) that are at least 1 order of magnitude smaller than those found for the corresponding diamagnetic species (ΦF = 0.015−0.04). Similar trends were also noted for the intersystem crossing rates and the triplet lifetimes, a finding that is interpreted in terms of an enhanced coupling between the singlet excited and triplet states or triplet excited and singlet ground states, respectively. The magnetic moments of the paramagnetic lanthanide(III) texaphyrin complexes were found to correlate well with the fluorescence lifetimes and the intersystem crossing rates, an observation that, along with other findings, including analyses of diamagnetic texaphyrin complexes, is considered consistent with the presence of covalent interactions between the texaphyrin ligand and the various coordinated metal centers.

Avalanche up-conversion processes in Pr, Yb-doped materials

The general characteristics of the ‘photon-avalanche’ up-conversion process are presented. The Pr,Yb system was spectroscopically investigated in Pr;Yb:YLiF 4 and Pr,Yb:YAlO 3 with respect to the realization and understanding of this process. Whereas Pr;Yb:YLiF 4 exhibits an avalanche-type excitation mechanism under infrared pumping, no such behavior is observed for Pr,Yb:YAlO 3. A time dependent rate equation model was developed to describe these different behaviors using experimentally determined parameters. The absence of an avalanche-type excitation mechanism in Pr,Yb:YAlO 3 is mainly caused by (i) a better spectral overlap between the excited state absorption in the Pr and the ground state absorption of Yb, (ii) higher transfer rates between Yb and Pr and within the Pr, and (iii) lower intrinsic decay rates from the participating excited energy levels ( 3P 0,1,2 and 1G 4).

Time‐Resolved Studies of the Excited‐State Dynamics of meso‐Tetra(hydroxylphenyl)chlorin in Solution

Meso-tetra(hydroxyphenyl)chlorin (m-THPC) is a new photosensitizer developed for potential use in photodynamic therapy (PDT) for cancer treatment. In PDT, the accepted mechanism of tumor destruction involves the formation of excited singlet oxygen via intermolecular energy transfer from the excited triplet-state dye to the ground triplet-state oxygen. Femtosecond transient absorption measurements are reported here for the excited singlet state dynamics of m-THPC in solution. The observed early time kinetics were best fit using a triple exponential function with time constants of 350 fs, 80 ps and > or = 3.3 ns. The fastest decay (350 fs) was attributed to either internal conversion from S2 to S1 or vibrational relaxation in S2. Multichannel time-resolved absorption and emission spectroscopies were also used to characterize the excited singlet and triplet states of the dye on nanosecond to microsecond time scales at varying concentrations of oxygen. The nanosecond time-resolved absorption data were fit with a double exponential with time constants of 14 ns and 250 ns in ambient air, corresponding to lifetimes of the S1 and T1 states, respectively. The decay of the T1 state varied linearly with oxygen concentration, from which the intrinsic decay rate constant, ki, of 1.5 x 10(6) s-1 and the biomolecular collisional quenching constant, kc, of 1.7 x 10(9) M-1 s-1 were determined. The lifetime of the S1 state of 10 ns was confirmed by fluorescence measurements. It was found to be independent of oxygen concentration and longer than lifetimes of other photosensitizers.

Time-Resolved Studies of the Excited-State Dynamics of meso-Tetra(hydroxylphenyl)chlorin in Solution

Abstract— Meso-tetra(hydroxyphenyl)chlorin (m-THPC) is a new photosensitizer developed for potential use in photodynamic therapy (PDT) for cancer treatment. In PDT, the accepted mechanism of tumor destruction involves the formation of excited singlet oxygen via intermolecular energy transfer from the excited triplet-state dye to the ground triplet-state oxygen. Femtosecond transient absorption measurements are reported here for the excited singlet state dynamics of m-THPC in solution. The observed early time kinetics were best fit using a triple exponential function with time constants of 350 fs, 80 ps and 3.3 ns. The fastest decay (350 fs) was attributed to either internal conversion from S2 to S1 or vibrational relaxation in S2. Multichannel time-resolved absorption and emission spectroscopies were also used to characterize the excited singlet and triplet states of the dye on nanosecond to microsecond time scales at varying concentrations of oxygen. The nanosecond time-resolved absorption data were fit with a double exponential with time constants of 14 ns and 250 ns in ambient air, corresponding to lifetimes of the S1 and T1 states, respectively. The decay of the T1 state varied linearly with oxygen concentration, from which the intrinsic decay rate constant, ki, of 1.5 × 106 s−1 and the bimolecular collisional quenching constant, kc, of 1.7 × 109M−1 s−1 were determined. The lifetime of the S1 state of 10 ns was confirmed by fluorescence measurements. It was found to be independent of oxygen concentration and longer than lifetimes of other photosensitizers.

Magnetic Field Effects on the Emission from the B State of Gaseous Halogen and Interhalogen Molecules

Magnetic field effects on the emission from the B3Π0+u state of Br2 and Cl2 and the B3Π0+ state of IBr and ICl have been investigated with the aid of excitation spectroscopy under various external magnetic fields up to 7.5 kG. The magnetic quenching was observed for all the investigated vibrational levels of Br2 and the v‘ = 2 level of IBr, whereas no detectable magnetic quenching was observed for the other levels of IBr and for all the investigated levels of Cl2 and ICl. The magnetic shortening of emission lifetimes was also observed for Br2. A linear relationship was found between the magnetic quenching efficiency and the magnetic shortening efficiency. From this it is derived that the former can be represented by the ratio of magnetically induced increment in the nonradiative decay rate to the intrinsic decay rate. This relation can explain the observed dependence of magnetic quenching of Br2 and IBr upon vibrational and rotational levels. The magnetic quenching efficiency observed for Br2 and IBr was found to be linearly dependent upon the square of the magnetic field strength. This indicates that the observed magnetic quenching is caused by the direct mechanism.

Modification of the mitochondrial F1-ATPase epsilon subunit, enhancement of the ATPase activity of the IF1-F1 complex and IF1-binding dependence of the conformation of the epsilon subunit.

Treatment of bovine heart submitochondrial particles with a low concentration of 2-hydroxy-5-nitrobenzyl bromide (HNB), a selective reagent for the Trp residue of the epsilon subunit [Baracca, Barogi, Lenaz and Solaini (1993) Int. J. Biochem. 25, 1269-1275], enhances the ATP hydrolytic activity of the particles exclusively when the natural inhibitor protein IF1 is present. Similarly, isolated F1 [the catalytic sector of the mitochondrial H+-ATPase complex (ATP synthase)] treated with the reagent has the ATPase activity enhanced exclusively if IF1 is bound to it. These experiments suggest that the modification of the epsilon subunit decreases the inhibitory activity of IF1, eliciting the search for a relationship between the epsilon subunit and the inhibitory protein. Certainly, a reverse relationship exists because HNB binds covalently to the isolated F1 exclusively when the inhibitory protein is present. This finding is consistent with the existence of the epsilon subunit in different conformational states depending on whether IF1 is bound to F1 or not. Support for this assertion is obtained by measurements of the intrinsic phosphorescence decay rate of F1, a probe of the Trp epsilon subunit conformation in situ [Solaini, Baracca, Parenti-Castelli and Strambini (1993) Eur. J. Biochem. 214, 729-734]. A significant difference in phosphorescence decay rate is detected when IF1 is added to preparations of F1 previously devoid of the inhibitory protein. These studies indicate that IF1 and the epsilon subunit of the mitochondrial F1-ATPase complex are related, suggesting a possible role of the epsilon subunit in the mechanism of regulation of the mitochondrial ATP synthase.