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Related Concept Videos

meta-Directing Deactivators: –NO2, –CN, –CHO, –⁠CO2R, –COR, –CO2H01:13

meta-Directing Deactivators: –NO2, –CN, –CHO, –⁠CO2R, –COR, –CO2H

6.6K
All meta-directing substituents are deactivating groups. These substituents withdraw electrons from the aromatic ring, making the ring less reactive toward electrophilic substitution. For example, the nitration of nitrobenzene is 100,000 times slower than that of benzene because of the deactivating effect of the nitro group. The first step in an electrophilic aromatic substitution is the addition of an electrophile to form a resonance-stabilized carbocation. The energy diagrams for...
6.6K
2° Amines to N-Nitrosamines: Reaction with NaNO201:20

2° Amines to N-Nitrosamines: Reaction with NaNO2

5.4K
Secondary amines react with nitrous acid to form N-nitrosamines, as depicted in Figure 1. Nitrous acid, a weak and unstable acid, is formed in situ from an aqueous solution of sodium nitrite and strong acids, such as hydrochloric acid or sulfuric acid, in cold conditions. In the presence of an acid, the nitrous acid gets protonated. The subsequent loss of water results in the formation of the electrophile known as nitrosonium ion.
5.4K
SN2 Reaction: Kinetics02:14

SN2 Reaction: Kinetics

10.2K
Kinetic Studies and Significance
In a chemical reaction, a relationship exists between the concentration of reactants and the rate at which the reaction proceeds. The study to measure this relationship is known as the kinetics of a chemical reaction. Kinetic studies are used to deduce the rate law of a chemical reaction, which provides information about the species involved during the transition state of the rate-determining step. Thus, kinetic studies help to derive the mechanism of a...
10.2K
SN2 Reaction: Mechanism02:27

SN2 Reaction: Mechanism

17.2K
The kinetic studies of SN2 reactions suggest an essential feature of its mechanism: it is a single-step process without intermediates. Here, both the nucleophile and the substrate participate in the rate-determining step.
The presence of the more electronegative halogen in the substrate creates a polarized carbon-halide bond. The halide pulls the electron cloud generating an electrophilic center at the carbon atom. Thus, the carbon atom carries a partial positive charge while the halide has a...
17.2K
SN2 Reaction: Transition State02:26

SN2 Reaction: Transition State

11.7K
An SN2 reaction of an alkyl halide is a single-step process in which bond formation between the nucleophile and the substrate and bond breaking between the substrate and the halide occurs simultaneously through a transition state without forming an intermediate.
When the nucleophile approaches the electrophilic carbon with its lone pairs, the halide acts as a leaving group and moves away with the electron-pair bonded to the carbon. Dotted partial bonds represent the bonds being formed or broken...
11.7K
SN2 Reaction: Stereochemistry02:23

SN2 Reaction: Stereochemistry

11.6K
In an SN2 reaction, the nucleophilic attack on the substrate and departure of the leaving group occurs simultaneously through a transition state. As the nucleophile approaches the substrate from the back-side, the configuration of the substrate carbon changes from tetrahedral to trigonal bipyramidal and then back to tetrahedral, leading to an inversion in the configuration of the product.
If the substrate is an achiral molecule at the α-carbon, the inversion of configuration is not...
11.6K

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An Integrated System to Remotely Trigger Intracellular Signal Transduction by Upconversion Nanoparticle-mediated Kinase Photoactivation
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Single upconversion nanoparticle imaging at sub-10 W cm-2 irradiance.

Qian Liu1,2, Yunxiang Zhang1,2, Chunte Sam Peng1,2

  • 1Department of Physics, Stanford University, Stanford, California 94305, United States.

Nature Photonics
|July 2, 2019
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Summary

Lanthanide-doped upconversion nanoparticles (UCNPs) show enhanced brightness for single-molecule imaging. New core-shell-shell UCNPs enable sensitive live cell tracking at low excitation power.

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Synthesis of Core-shell Lanthanide-doped Upconversion Nanocrystals for Cellular Applications
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Synthesis of Core-shell Lanthanide-doped Upconversion Nanocrystals for Cellular Applications

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Area of Science:

  • Nanotechnology
  • Biophotonics
  • Materials Science

Background:

  • Lanthanide-doped upconversion nanoparticles (UCNPs) offer photostable, near-infrared-excited luminescence for bioimaging.
  • Sub-50 nm UCNPs exhibit weak luminescence, requiring high excitation power (>10 kWcm⁻²) unsuitable for live cell applications.

Purpose of the Study:

  • To systematically characterize single-particle luminescence of various UCNP formulations under low excitation power.
  • To develop brighter UCNPs for practical single-molecule and live cell imaging.

Main Methods:

  • Investigated single-particle luminescence of UCNPs across a 10⁶ incident power range (down to 8 Wcm⁻²).
  • Synthesized and characterized core-shell-shell (CSS) UCNPs (NaYF₄@NaYb₁₋ₓF₄:Erₓ@NaYF₄) with varying Er³⁺ doping and Yb³⁺ sensitization.
  • Analyzed luminescence enhancement mechanisms, including the role of inert shells and power-dependent effects.

Main Results:

  • A CSS UCNP structure demonstrated significantly higher brightness compared to conventional UCNPs.
  • At 8 Wcm⁻², 8% Er³⁺ CSS particles showed a 150-fold luminescence enhancement.
  • Identified high Yb³⁺ content and inert shells as key factors for enhanced brightness and reduced energy loss.
  • Revealed power-dependent luminescence enhancement from the inert shell, reconciling previous measurement discrepancies.

Conclusions:

  • Developed significantly brighter UCNPs suitable for low-power bioimaging.
  • The CSS structure and optimized composition overcome limitations of previous UCNPs for single-particle detection.
  • These advanced UCNPs facilitate low-irradiance cellular and single-molecule tracking.