PEGylated Luminescent Gold Nanoclusters: Synthesis,Characterization, Bioconjugation,and Application to One‐ and Two‐Photon Cellular Imaging

Biocompatible, near‐infrared luminescent gold nanoclusters (AuNCs) are synthesized directly in water using poly(ethylene glycol)‐dithiolane ligands terminating in either a carboxyl, amine, azide, or methoxy group. The ≈1.5 nm diameter AuNCs fluoresce at ≈820 nm with quantum yields that range from 4–8%, depending on the terminal functional group present, and display average luminescence lifetimes approaching 1.5 μs. The two‐photon absorption (TPA) cross‐section and two‐photon excited fluorescence (TPEF) properties are also measured. Long‐term testing shows the poly(ethylene glycol) stabilized AuNCs maintain colloidal stability in a variety of media ranging from saline to tissue culture growth medium along with tolerating storage of up to 2 years. DNA and dye‐conjugation reactions confirm that the carboxyl, amine, and azide groups can be utilized on the AuNCs for carbodiimide, succinimidyl ester, and Cu^I‐assisted cycloaddition chemistry, respectively. High signal‐to‐noise one‐ and two‐photon cellular imaging is demonstrated. The AuNCs exhibit outstanding photophysical stability during continuous‐extended imaging. Concomitant cellular viability testing shows that the AuNCs also elicit minimal cytotoxicity. Further biological applications for these luminescent nanoclustered materials are discussed.     

Instrumentation for beam-foil spectroscopic studies in the UV-visible region

A facility for carrying out beam-foil spectroscopic studies in the UV and visible region using the 400 kV electrostatic ion accelerator at Tata Institute of Fundamental Research, Bombay is described.     

Construction of fiber-optic bundle light-collection systems and calculations of collection efficiency

This paper describes the position-sensitive light-collection system that we use in our fast-beam laser experiments. The collection system consists of fiber-optic bundles whose facets are arranged to accept light emitted from a beam of fluorescent atoms. The flexibility of the fiber bundles allows their use in scanning collection systems with precise position sensitivity. We describe calculations of geometrical collection efficiency using a numerical integration scheme and compare the results with measurements. We also compare the collection efficiencies of the different fiber bundle arrangements that we used as our apparatus evolved with the implementation of various improvements.     

Radiative lifetimes of the NaRb C(3)1Σ+ state: experiment and theory

Radiative lifetimes for 2≤v^′≤44 rovibronic C¹Σ⁺ state levels of NaRb and quenching collision cross-sections with Rb atoms have been directly measured in a thermal cell by detecting time resolved laser induced fluorescence after pulsed excitation. Many body multipartitioning theory was applied to calculate C¹Σ⁺-X¹Σ⁺ and C¹Σ⁺-A¹Σ⁺ transition dipole moments. The relevant ab initio matrix elements were converted to the C¹Σ⁺ state radiative lifetimes. The strong spin-orbit A¹Σ⁺∼ b³Π coupling effect on the total C → A transition probabilities and lifetimes of the C¹Σ⁺ state is discussed. The measured radiative lifetimes show a decrease from 61 to 34 ns as the v^′ values increase, the results being in good agreement with calculations. The averaged collisional quenching cross-section value σ=(3±1)×10^-14 cm² was determined for NaRb (C¹Σ⁺) + Rb collisions from the Stern-Volmer plots.     

Recent research using the Oxford electron beam ion trap

The presence of the σ-phase in Fe-Cr alloys (eg. Stainless steel) is important in industrial applications and from an academic point of view. The presence of the σ-phase in these alloys drastically affects their mechanical properties and their resistance to various corrosive media. In the present investigation Fe-Cr alloys containing different amounts of Mo were prepared and the transformation to the σ-phase was carried out by isothermally annealing the samples for various periods in an argon atmosphere. It will be shown that the presence of Mo has a dramatic accelerating effect on the rate of the σ-phase formation in these alloys. This revised version was published online in August 2006 with corrections to the Cover Date.     

Lifetime measurements for some levels in Be-like Cl XIV and S XIII

Lifetime measurements have been carried out for some low lying (n = 2) levels in the four-electron ions Cl XIV and S XIII using the beam-foil technique. Accurate oscillator strengths for the 2 s2 ¹ S - 2 s 2 p ¹ P o transition have been determined by the inclusion of prominent cascades in the analysis. Lifetimes of the levels of the 2 p2 ³ P j multiplet have also been measured for both ions. The results are compared with theoretical predictions and earlier measurements. Received: 29 October 1997 / Revised: 10 February 1998 / Accepted: 5 March 1998     

Theoretical study of the 5p56s – 5p6 spectra of neutral xenon

Recent high precision measurements on the lifetime of the metastable 6s[3/2]₂ state of atomic xenon display a difference with previous predictions by a factor of 2–3. In the present work, a systematic relaxation and correlation approach, which has been developed on the basis of a widely used multi-configuration Dirac-Fock method, is applied to study the electric dipole allowed E1 and forbidden M1, E2 and M2 transitions between the 5p⁵ 6s and 5p⁶ configurations. We systematically include the correlation effects which arise from all the single and double excitations from the occupied shells into the (n=6–10) active sets and the relaxation effects caused by change of the electron density between the radiative initial- and final-states. This study not only reduces greatly the existing discrepancy in the lifetime of the 6s[3/2]₂ state, but also presents rather consistent results for both the lifetime of the metastable 6s'[1/2]₀ state and the oscillator strength of the 5p⁵ 6s - 5p⁶ E1 resonant transitions.     

Temperature dependence of luminescence lifetimes in quartz under pulsed blue light stimulation

Abstract

The effect of increased thermal exposure on lifetimes during optical stimulation of luminescence was investigated in quartz over the temperature range 20—200°C. It is shown using a series of constant and dynamic temperature experimental data that lifetimes are strongly affected by temperature. Lifetimes change from about 30 μs at 20°C to about 7 μs at 200°C.     

The lowest electronic states of Ne2 +, Ar2 + and Kr2 +: comparison of theory and experiment

Non-relativistic configuration interaction (CI) ab initio calculations using large basis sets have been carried out to determine the potential curves of the first electronic states of Ne₂ ⁺, Ar₂ ⁺ and Kr₂ ⁺. The spin—orbit interaction was treated assuming that the spin—orbit coupling constant is independent of the internuclear separation (R). For Ar₂ ⁺, calculated dissociation energies and equilibrium separations are in good agreement with experimental results. The calculations for Ne₂ ⁺ suggest that the lowest vibrational level of the I(1/2u) ground state observed by threshold photoelectron spectroscopy by Hall et al. [1995, J. Phys. B: At. molec. opt. Phys., 28, 2435] and assigned to either ν = 0 or ν = 2 actually corresponds to ν = 4. The calculations also predict the I(1/2g) state of Ne₂ ⁺ and Ar₂ ⁺ to possess a double-well potential and that of Kr₂ ⁺ to be repulsive at short range and to only possess a single shallow well at large internuclear separation. The ab initio calculations provide an explanation for the observation made by Yoshii et al. [2002, J. chem. Phys., 117, 1517] that Kr₂ ⁺ and Xe₂ ⁺ dissociate after photoemission from the II(1/2u) state to the I(1/2g) state whereas Ar₂ ⁺ does not.     

Fine-structure energy levels, oscillator strengths and lifetimes in Co XV

Excitation energies from ground state for 97 fine-structure levels as well as oscillator strengths and radiative decay rates for all electric-dipole-allowed and intercombination transitions among the fine-structure levels of the terms belonging to the (1s ²2s ²2p ⁶)3s ²3p, 3s ³ p ², 3s ²3d, 3p ³, 3s3p3d, 3p ²3d, 3s3d ², 3s ²4s, 3s ²4p, 3s ²4d, 3s ²4f, and 3s3p4s configurations of Co XV are calculated, using extensive configuration-interaction (CI) wave functions, obtained with the CIV3 computer code of Hibbert. The important relativistic effects in intermediate coupling are included through the Breit-Pauli approximation via spin-orbit, spin-other-orbit, spin-spin, Darwin and mass correction terms. Small adjustments to the diagonal elements of the Hamiltonian matrices have been made. Our calculated excitation energies, including their ordering, are in excellent agreement with the experimental results and the experimentally compiled energy values of the National Institute for Standards and Technology (NIST) wherever available. The mixing among several fine-structure levels is found to be very strong, with most of the strongly mixed levels belonging to the (1s ²2s ²2p ⁶)3p ²3d and 3s3d ² configurations. The strong mixing among several fine-structure levels makes it very difficult to identify them uniquely. Perhaps, that may be the reason for the lack of both experimental and theoretical results for these levels. We believe that our extensive calculated values can guide experimentalists in identifying the fine-structure levels in their future work. From our radiative decay rates we have also calculated radiative lifetimes of some fine-structure levels. In this calculation we also predict new data for several fine-structure levels where no other theoretical and/or experimental results are available.