Enhancing multiphoton upconversion through energy clustering at sublattice level.

The applications of lanthanide-doped upconversion nanocrystals in biological imaging, photonics, photovoltaics and therapeutics have fuelled a growing demand for rational control over the emission profiles of the nanocrystals. A common strategy for tuning upconversion luminescence is to control the doping concentration of lanthanide ions. However, the phenomenon of concentration quenching of the excited state at high doping levels poses a significant constraint. Thus, the lanthanide ions have to be stringently kept at relatively low concentrations to minimize luminescence quenching. Here we describe a new class of upconversion nanocrystals adopting an orthorhombic crystallographic structure in which the lanthanide ions are distributed in arrays of tetrad clusters. Importantly, this unique arrangement enables the preservation of excitation energy within the sublattice domain and effectively minimizes the migration of excitation energy to defects, even in stoichiometric compounds with a high Yb(3+) content (calculated as 98 mol%). This allows us to generate an unusual four-photon-promoted violet upconversion emission from Er(3+) with an intensity that is more than eight times higher than previously reported. Our results highlight that the approach to enhancing upconversion through energy clustering at the sublattice level may provide new opportunities for light-triggered biological reactions and photodynamic therapy.

Kinetic control and thermodynamic selection in the synthesis of atomically precise gold nanoclusters.

This work presents a combined approach of kinetic control and thermodynamic selection for the synthesis of monodisperse 19 gold atom nanoclusters protected by thiolate groups. The step of kinetic control allows the formation of a proper size distribution of initial size-mixed Au(n)(SR)(m) nanoclusters following the reduction of a gold precursor. Unlike the synthesis of Au(25)(SR)(18) nanoclusters, which involves rapid reduction of the gold precursor by NaBH(4) followed by size focusing, the synthesis of 19-atom nanoclusters requires slow reduction effected by a weaker reducing agent, borane-tert-butylamine complex. The initially formed mixture of nanoclusters then undergoes size convergence into a monodisperse product by means of a prolonged aging process. The nanocluster formula was determined to be Au(19)(SC(2)H(4)Ph)(13). This work demonstrates the importance of both kinetic control of the initial size distribution of nanoclusters prior to size focusing and subsequent thermodynamic selection of stable nanoclusters as the final product.

Effects of local and core body temperature on grip force modulation during movement-induced load force fluctuations.

Impaired manual functioning often occurs when the hands are exposed to cold temperatures, but the underlying mechanism is not clearly understood. Tactile feedback is thought to provide important information during object manipulations in order to scale and regulate grip forces; however, topical anaesthetic-induced tactile sensation impairments may not realistically simulate the systemic neuromuscular impairment of the whole hand that could occur during cold temperature exposure. In two experiments, we studied the impact of (1) local hand cooling [thermoneutral finger skin temperature, cold (<8 degrees C)] and (2) core body temperature (thermoneutral core body temperature, pre-heated by 0.5 degrees C, pre-cooled by 0.5 degrees C) with cold hands on manual dexterity and the ability to control and co-ordinate grip forces during a cyclical load-lifting task. In Experiment 1 (n = 10), hand cooling significantly decreased Purdue Pegboard performance (P = 0.002), while increasing grip force by approximately 5 N during the cyclical load-lifting task compared to thermoneutral (P = 0.037). The temporal co-ordination of grip and load forces was unaffected by hand cooling. In Experiment 2 (n = 11), pegboard performance was impaired following hand cooling (P < 0.001), and to a greater extent when the body was pre-cooled (p < 0.001). However, neither grip force (P = 0.99) nor the temporal co-ordination of grasping and lifting forces (P = 0.85) were affected by core body temperature. These data support the existence of a robust centrally controlled feedforward system able to anticipate the dynamics of manual manipulations and accordingly regulate the temporal co-ordination of fingertip forces during object manipulation. This centrally controlled mechanism appears to differ from the mechanisms governing other aspects of manual dexterity.