3D Proton Bragg Peak Visualization and Spot Shape Measurement with Polymer Gel Dosimeters

Proton pencil beam scanning is a dynamic beam delivery technique with excellent conformability to the tumor volume. The accuracy of spot size and scanning positions will have a significant effect on the delivered dose distribution. We employed polymer gel dosimeters to measure the spot size and the scanning positions for the Shanghai Advanced Proton Therapy facility (SAPT). Polymer gel dosimeters (MAGAT-f and PAGAT) were utilized to measure the full width at half maximum (FWHM) of the beam spot at various depths on the basis of their MRI readouts. The correlation between the spot FWHM and standard deviation (σ) was analyzed at different depths. The measured Bragg peak range was compared with the Monte Carlo (MC) simulation. Three-dimensional volume rendering of the Bragg peak was reconstructed for the 3D visualization to measure the spot size three-dimensionally. The R2 dose–response curve was investigated with polymer gel dosimeters. The deviations of the Bragg peak ranging between measurement and simulation were 0.13% and −0.53% for MAGAT-f and PAGAT, respectively. Our results ascertain the feasibility of a polymer gel dosimeter to measure the spot size and positions of a proton pencil beam

Dose-Area Product Determination and Beam Monitor Calibration for the Fixed Beam of the Shanghai Advanced Proton Therapy Facility

Research conducted to-date, makes use of the IBA-Lynx scintillating screen and radiochromic film to analyze the proton field uniformity for dose-area product (DAP) determination. In this paper, the machine log file based reconstruction is proposed to calculate the field uniformity to simplify the measurement. In order to calculate the field uniformity, the dose distribution is reconstructed based on the machine log file with matRad (an open source software for analytical dose calculation in MATLAB). After acquisition of the dose distribution, the field flatness and symmetry are calculated automatically for different proton energies. A comprehensive comparison of DAP determined with Bragg peak chamber (BPC) and Markus chamber (MC) is presented. The actual delivered dose is reconstructed with the log file to analyze the lateral dose distribution of the scanned field. DAP of different energies are calculated ranging from 70.6 MeV to 235 MeV. The percentage difference is calculated, illustrating the DAP discrepancy between the MC and BPC to the mean value. The percentage difference ranges from −0.19% to 1.26%. The variation between DAP measured with the BPC and MC peaks at −2.5%. The log file based reconstruction to calculate field uniformity can be an alternative for DAP determination. The direct method using a large-area Bragg peak chamber is investigated. The two methods to determine DAP and calibrate beam monitor illustrate consistent results

Thiolate-Protected Au₂₄(SC₂H₄Ph)₂₀ Nanoclusters: Superatoms or Not?

We report a new gold thiolate cluster with molecular purity. Electrospray ionization (ESI) mass spectrometry in conjunction with thermogravimetric analysis (TGA), elemental analysis (EA), and ¹H NMR, unambiguously determined the composition of the as-prepared Au nanocluster to be Au₂₄(SC₂H₄Ph)₂₀. The optical absorption spectrum of this cluster shows a highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO−LUMO) transition at 765 nm, indicating quantum confinement of electrons in the particle. The HOMO−LUMO gap (∼1.5 eV) of Au₂₄(SR)₂₀ is much smaller than that of Au₂₀(SR)₁₆ (∼2.1 eV) but slightly larger than that of Au₂₅(SR)₁₈ (∼1.3 eV). The number of valence electrons in Au₂₄(SC₂H₄Ph)₂₀ is 4e, which is not predicted by the superatom model

Intramolecular Metal Exchange Reaction Promoted by Thiol Ligands

The synthesis of an alloy nanocluster that is atomically precise is the key to understanding the metal synergy effect at the atomic level. Using the Ag2Au25(SR)18 nanocluster as a model, we reported a third approach for the metal exchange reaction, that is, intramolecular metal exchange. The surface adsorbed metal ions (i.e., Ag) can be exchanged with the kernel metal atoms (i.e., Au) that are promoted by thiol ligands. The exchanged gold atoms can be further stripped by the thiol ligands, and produce the AgxAu25−x(SR)18− nanocluster

New Insights into the Bonding Properties of [Ag25(SR)18]- Nanoclusters from X-ray Absorption Spectroscopy

Atomically precise metal nanoclusters have attracted significant interest due to their molecule-like properties. [Ag25(SR)18]- is one of the Ag nanoclusters having a unique structure similar to its Au counterpart but different from most other Ag nanoclusters. In this study, a new five-shell fitting method was developed to analyze the extended X-ray absorption fine structure (EXAFS) spectra of [Ag25(SR)18]- to provide more insights into its bonding properties. This new method was successfully applied to compare the bond lengths as the temperature changed from 300 K to 90 K. Interestingly, the metal core of [Ag25(SR)18]- shows negative thermal expansion behaviour that is not observed for Au25(SR)18. These unique bonding properties of [Ag25(SR)18]- could be related to the Ag4 tetrahedral units found in the metal core, which are absent in Au25(SR)18. These new findings about its bonding properties can provide a better understanding of the structure-property relationship of [Ag25(SR)18]-. This new EXAFS analysis method could be applied to gain insights into the bonding properties of other metal nanoclusters

Site-Specific Electronic Properties of [Ag25(SR)18]- Nanoclusters by X-Ray Spectroscopy

We present the site-specific electronic properties of Ag25(SR)18 and Au25(SR)18 using X-ray spectroscopy experiments and quantum simulations. To overcome the final state effect observed in X-ray photoelectron spectroscopy (XPS), a unique method was developed to reliably analyze the charge transfer behavior of the NCs. Density functional theory calculations were combined with XPS to provide more insight into the electronic properties of the NCs. The differences in the XPS valence bands of these two NCs were further compared and interpreted using the relativistic effect. The first derivative of the X-ray absorption near-edge structure (XANES) spectrum was further used as a tool to sensitively probe the bonding properties of Ag25(SR)18. By combining the experimental XANES data and their site-specific quantum simulations, the large impact of the staple motif on the bonding properties of the NC was demonstrated. These findings highlight the unique electronic properties of each atomic site in Ag25(SR)18; the effective X-ray analysis techniques developed here can offer new opportunities for the site-specific study of other NCs

Size focusing: a methodology for synthesizing atomically precise gold nanoclusters

Controlling nanoparticles with atomic precision, somewhat like the way organic chemists control small molecules by organic chemistry principles, is highly desirable for nanoparticle chemists. Recent advances in the synthesis of gold nanoparticles have opened the possibility to precisely control the number of gold atoms in a particle. In this Perspective, we will discuss a size-focusing methodology that has been developed in the synthesis of a number of atomically monodisperse ultrasmall gold nanoparticles (also called nanoclusters). We focus our discussion on thiolate-stabilized Au nanoclusters (referred to as Aun(SR)m, where n and m are the respective number of metal atoms and ligands). The underlying principle of this size-focusing process is primarily related to the peculiar stability of certain sized Aun(SR)m nanoparticle, that is, “survival of the robustest”, much like the natural law “survival of the fittest”. We expect that this universal size-focusing method will ultimately allow for preparing a full series of size-discrete, atomically monodisperse nanoparticles that span the size regimes of both nonplasmonic nanoclusters and plasmonic nanocrystals. These well-defined nanoparticles will be of major importance for both fundamental science research and technological applications