Effect of solvent polarity and electrophilicity on quantum yields and solvatochromic shifts of single-walled carbon nanotube photoluminescence.

In this work, we investigate the impact of the solvation environment on single-walled carbon nanotube (SWCNT) photoluminescence quantum yield and optical transition energies (E(ii)) using a highly charged aryleneethynylene polymer. This novel surfactant produces dispersions in a variety of polar solvents having a wide range of dielectric constants (methanol, dimethyl sulfoxide, aqueous dimethylformamide, and deuterium oxide). Because a common surfactant can be used while maintaining a constant SWCNT-surfactant morphology, we are able to straightforwardly evaluate the impact of the solvation environment upon SWCNT optical properties. We find that (i) the SWCNT quantum yield is strongly dependent on both the polarity and electrophilicity of the solvent and (ii) solvatochromic shifts correlate with the extent of SWCNT solvation. These findings provide a deeper understanding of the environmental dependence of SWCNT excitonic properties and underscore that the solvent provides a tool with which to modulate SWCNT electronic and optical properties.

Structural and pH dependence of excited state PCET reactions involving reductive quenching of the MLCT excited state of [RuII(bpy)2(bpz)]2+ by hydroquinones.

The proton-coupled electron transfer (PCET) reaction between the bpz-based photoexcited (3)MLCT state of [Ru(II)(bpy)(2)(bpz)](2+) (bpy = 2,2'-bipyridine, bpz = 2,2'-bipyrazine) and a series of substituted hydroquinones (H(2)Q) has been studied by transient absorption (TA) and time-resolved electron paramagnetic resonance (TREPR) spectroscopy at X-band. When the reaction is carried out in a CH(3)CN/H(2)O mixed solvent system with unsubstituted hydroquinone, the neutral semiquinone radical (4a) and its conjugate base, the semiquinone radical anion (4b), are both observed. Variation of the acid strength in the solvent mixture allows the acid/base dependence of the PCET reaction to be investigated. In solutions with very low acid concentrations, TREPR spectra exclusively derived from radical anion 4b are observed, while at very high acid concentrations, the spectrum is assigned to the protonated structure 4a. At intermediate acid concentrations, either a superposition of spectra is observed (slow exchange between 4a and 4b) or substantial broadening in the TREPR spectrum is observed (fast exchange between 4a and 4b). Variation of substituents on the H(2)Q ring substantially alter this acid/base dependence and provide a means to investigate electronic effects on both the ET and PT components of the PCET process. The TA results suggest a change in mechanism from PCET to direct ET quenching in strongly basic solutions and with electron withdrawing groups on the H(2)Q ring system. Changing the ligand on the Ru complex also alters the acid/base dependence of the TREPR spectra through a series of complex equilibria between protonated and deprotonated hydroquinone radicals and anions. The relative intensities of the signals from radical 4a versus 4b can be rationalized quantitatively in terms of these equilibria and the relevant pK(a) values. An observed equilibrium deuterium isotope effect supports the conclusion that the post-PCET HQ(•)/Q(•-) equilibrium is the most important in determining the 4a/4b ratio at early delay times.

Driving high quantum yield NIR emission through proquinoidal linkage motifs in conjugated supermolecular arrays.

High quantum yield NIR fluorophores are rare. Factors that drive low emission quantum yields at long wavelength include the facts that radiative rate constants increase proportional to the cube of the emission energy, while nonradiative rate constants increase in an approximately exponentially with decreasing S0-S1 energy gaps (in accordance with the energy gap law). This work demonstrates how the proquinoidal BTD building blocks can be utilized to minimize the extent of excited-state structural relaxation relative to the ground-state conformation in highly conjugated porphyrin oligomers, and shows that 4-ethynylbenzo[c][1,2,5]thiadiazole (E-BTD) units that terminate meso-to-meso ethyne-bridged (porphinato)zinc (PZnn) arrays, and 4,7-diethynylbenzo[c][1,2,5]thiadiazole (E-BTD-E) spacers that are integrated into the backbone of these compositions, elucidate new classes of impressive NIR fluorophores. We report the syntheses, electronic structural properties, and emissive characteristics of neoteric PZn-(BTD-PZn)n, PZn2-(BTD-PZn2)n, and BTD-PZnn-BTD fluorophores. Absolute fluorescence quantum yield (ϕ f) measurements, acquired using a calibrated integrating-sphere-based measurement system, demonstrate that these supermolecules display extraordinary ϕ f values that range from 10-25% in THF solvent, and between 28-36% in toluene solvent over the 700-900 nm window of the NIR. These studies underscore how the regulation of proquinoidal conjugation motifs can be exploited to drive excited-state dynamical properties important for high quantum yield long-wavelength fluorescence emission.

Real-time dose-rate monitoring with gynecologic brachytherapy: Results of an initial clinical trial.

PURPOSE: A nanoscintillator-based fiber-optic dosimeter (nanoFOD) was developed to measure real-time dose rate during high-dose-rate (HDR) brachytherapy. A trial was designed to prospectively test clinical feasibility in gynecologic implants. METHODS AND MATERIALS: A clinical trial enrolled women undergoing vaginal cylinder HDR brachytherapy. The nanoFOD was fixed to the cylinder alongside two thermoluminescent dosimeters (TLDs). Treatment was delivered and real-time dose rates captured by the nanoFOD. The nanoFOD and TLD positions were identified in CT images and used to extract the treatment planning system (TPS) calculated dose. The nanoFOD and TLD cumulative doses were compared with the TPS. RESULTS: Nine women were enrolled for 30 fractions, and real-time data were available in 27 treatments. The median ratio of nanoFOD/TPS dose was 1.00 (IQR 0.94-1.02), with a TLD/TPS ratio of 1.01 (IQR 0.98-1.04). Of the nanoFOD dose measurements, 63% were within 5% of the TPS, 26% between 5 and 10% of the TPS, and the remaining 11% between 10 and 20% of the TPS dose. Of the TLD measurements, 70% were within 5% of the TPS, 22% between 5 and 10% of the TPS, and 7% between 10 and 20% of the TPS dose. CONCLUSIONS: Real-time dose-rate measurements during HDR brachytherapy were feasible using the nanoFOD and cumulative dose per fraction showed reasonable agreement to TLD and TPS doses. Additional studies to determine dose thresholds that would yield a low false alarm rate and ongoing device development efforts to improve localization of the scintillator in CT images are needed before this detector should be used to inform clinical decisions.

Fiber-optic detector for real time dosimetry of a micro-planar x-ray beam.

PURPOSE: Here, the authors describe a dosimetry measurement technique for microbeam radiation therapy using a nanoparticle-terminated fiber-optic dosimeter (nano-FOD). METHODS: The nano-FOD was placed in the center of a 2 cm diameter mouse phantom to measure the deep tissue dose and lateral beam profile of a planar x-ray microbeam. RESULTS: The continuous dose rate at the x-ray microbeam peak measured with the nano-FOD was 1.91 ± 0.06 cGy s(-1), a value 2.7% higher than that determined via radiochromic film measurements (1.86 ± 0.15 cGy s(-1)). The nano-FOD-determined lateral beam full-width half max value of 420 µm exceeded that measured using radiochromic film (320 µm). Due to the 8° angle of the collimated microbeam and resulting volumetric effects within the scintillator, the profile measurements reported here are estimated to achieve a resolution of ∼0.1 mm; however, for a beam angle of 0°, the theoretical resolution would approach the thickness of the scintillator (∼0.01 mm). CONCLUSIONS: This work provides proof-of-concept data and demonstrates that the novel nano-FOD device can be used to perform real-time dosimetry in microbeam radiation therapy to measure the continuous dose rate at the x-ray microbeam peak as well as the lateral beam shape.

Europium- and lithium-doped yttrium oxide nanocrystals that provide a linear emissive response with X-ray radiation exposure.

Eu- and Li-doped yttrium oxide nanocrystals [Y2-xO3; Eux, Liy], in which Eu and Li dopant ion concentrations were systematically varied, were developed and characterized (TEM, XRD, Raman spectroscopic, UV-excited lifetime, and ICP-AES data) in order to define the most emissive compositions under specific X-ray excitation conditions. These optimized [Y2-xO3; Eux, Liy] compositions display scintillation responses that: (i) correlate linearly with incident radiation exposure at X-ray energies spanning from 40-220 kVp, and (ii) manifest no evidence of scintillation intensity saturation at the highest evaluated radiation exposures [up to 4 Roentgen per second]. For the most emissive nanoscale scintillator composition, [Y1.9O3; Eu0.1, Li0.16], excitation energies of 40, 120, and 220 kVp were chosen to probe the dependence of the integrated emission intensity upon X-ray exposure-rate in energy regimes having different mass-attenuation coefficients and where either the photoelectric or the Compton effect governs the scintillation mechanism. These experiments demonstrate for the first time for that for comparable radiation exposures, when the scintillation mechanism is governed by the photoelectric effect and a comparably larger mass-attenuation coefficient (120 kVp excitation), greater integrated emission intensities are recorded relative to excitation energies where the Compton effect regulates scintillation (220 kVp) in nanoscale [Y2-xO3; Eux] crystals. Nanoscale [Y1.9O3; Eu0.1, Li0.16] (70 ± 20 nm) was further exploited as a detector material in a prototype fiber-optic radiation sensor. The scintillation intensity from the [Y1.9O3; Eu0.1, Li0.16]-modified, 400 µm sized optical fiber tip, recorded using a CCD-photodetector and integrated over the 605-617 nm wavelength domain, was correlated with radiation exposure using a Precision XRAD 225Cx small-animal image guided radiation therapy (IGRT) system. For both 80 and 225 kVp energies, this radiotransparent device recorded scintillation intensities that tracked linearly with total radiation exposure, highlighting its capability to provide alternately accurate dosimetry measurements for both diagnostic imaging (80 kVp) and radiation therapy treatment (225 kVp).

Dual energy converting nano-phosphors: upconversion luminescence and X-ray excited scintillation from a single composition of lanthanide-doped yttrium oxide.

We report an upconverting nanomaterial composition, [Y(2)O(3); Yb (2%), Er (1%)], that converts both X-ray and high-fluence NIR irradiation to visible light. This composition is compared to a higher Yb(3+) doped composition, [Y(2)O(3); Yb (10%), Er (1%)], that displays diminished visible X-ray scintillation, but shows enhanced red wavelength centered upconversion emission. These nanocrystals have been characterized by TEM, X-ray diffraction, power-dependent upconversion luminescence, and X-ray scintillation spectroscopy. We further demonstrate that lithium ion doping of the [Y(2)O(3); Yb (2%), Er (1%)] nanoscale composition leads to enhanced X-ray and NIR excited emission intensities through the production of nanoparticles that feature slightly enhanced sizes and increased crystallinity.