On the atomic structure of thiol-protected gold nanoparticles: a combined experimental and theoretical study.

In the present work new findings on the structure of the S-Au interface are presented. Theoretical calculations using a new semiempirical potential, based on density functional theory and a bond-order Morse potential, are employed to simulate the adsorption process in a more realistic way. The simulation results reveal the formation of gold adatoms on the nanoparticle surface and high surface disorder due to the strong S-Au bond. Experimental data were acquired by aberration (Cs) corrected scanning transmission electron microscopy (STEM) using a high angle annular dark field detector (HAADF) that showed a great similarity with the theory predicted.

Extended exposure to a discriminated, limited-hold temporal schedule does not produce stimulus control.

Four rats were exposed to two different stimuli (either lights or tones), each stimulus being correlated with independent probabilities of water delivery in a temporally defined schedule. The schedule consisted of a 60 s T cycle with 30 s t(D) and t(-) successive subcycles; t(D) was correlated with a probability of water delivery of 1.0 and t(-) was correlated with a probability of water delivery of 0.0. The schedule was maintained during 180 sessions and extended for 25 extra sessions omitting the stimulus in t(-). The four rats showed low frequencies of responding, response frequency being slightly higher in t(-) than in t(D). The percentage of lost reinforcers was independent of response frequency. The rats which lost less reinforcers were those which obtained more water deliveries during the first 15 cycles of each session. These results show that stimulus control does not develop in limited-hold temporal schedules, and that response-reinforcer effectiveness may depend on the initial contact with reinforcers in the first cycles of the session.

Effects of auditory stimuli correlated with different probabilities of water delivery in a limited-hold temporal schedule.

Four rats were exposed to two different tone frequencies, each tone being correlated with independent probabilities of water delivery in a temporally defined schedule. The schedule consisted of a 60-s T cycle, with 30-s t(D) and t(Delta) succesive subcycles. t(D) and t(Delta) were assigned complementary probabilities of water delivery (1.0-0, 0.75-0.25, 0.5-0.5 and 1.0-0) in succesive phases. Each of the first three experimental phases was followed by five consecutive probe sessions using the same probability values, and in which water was delivered independently of responding at the end of each subcycle. Three additional rats were exposed to the same conditions, except that only one tone was used in t(D) and t(Delta). The highest frequencies of responding were observed in two of the rats exposed to differential tone frequencies, an effect that was correlated with lower percentages of lost water deliveries, shorter response latencies, and less variability in the intervals between successive water presentations. In the last phase in which probabilities of water delivery in t(D) and t(Delta) were 1.0 and 0, respectively, response frequency was always higher in t(Delta). These results suggest that the number of water deliveries obtained in the early cycles of every session were responsible for the total number of responses and its correlated effects.