Luminescent Semiconductor

Sapsford, K.E., Pons, T., Medintz, I.L., and Mattoussi, H. (2006) Biosensing with luminescent semiconductor quantum dots. Sensors 6, 925-953. 

Compounds 3 and 18 - 20 have been prepared to combine luminescent semiconductors with curable units. The transformation of the monomeric compounds to siloxanes of higher molecular weight was performed by hydrolysis of the silicic esters in chloroform/ethanol solution, catalyzed by traces of hydrochloric acid for 2 days (Scheme 4). 

A dielectric oxide layer such as silica is useful as shell material because of the stability it lends to the core and its optical transparency. The thickness and porosity of the shell are readily controlled. A dense shell also permits encapsulation of toxic luminescent semiconductor nanoparticles. The classic methods of Stober and Her for solution deposition of silica are adaptable for coating of nanocrystals with silica shells [864,865]. These methods rely on the pH and the concentration of the solution to control the rate of deposition. The natural affinity of silica to oxidic layers has been exploited to obtain silica coating on a family of iron oxide nanoparticles including hematite and magnetite [866-870]. The procedures are mostly adaptations of the Stober process. Oxide particles such as boehmite can also be coated with silica [871]. Such a deposition process is not readily extendable to grow shell layers on metals. The most successful method for silica encapsulation of metal nanoparticles is that due to Mulvaney and coworkers [872—875]. In this method, the smface of the nanoparticles is functionalized with aminopropyltrimethylsilane, a bifunctional molecule with a pendant silane group which is available for condensation of silica. The next step involves the slow deposition of silica in water followed by the fast deposition of silica in ethanol. Changes in the optical properties of metal nanoparticles with silica shells of different thicknesses were studied systematically [873 75]. This procedure was also extended to coat CdS and other luminescent semiconductor nanocrystals [542,876-879]. 

Figure 3.5. Microcapsules simultaneously loaded with luminescent semiconductor and magnetic oxide nanoparticles are aligned in a magnetic field. The images were obtained with a confocal laser scanning microscope TCS Leica operating in transmission (left column) and luminescence (right column, excitation wavelength 476 nm) modes, respectively. Capsule diameter is 5.6 fim in all cases. (Reproduced from Ref. 78).

Nowadays, hybrid organic-inorganic polymer nanocomposites with luminescent semiconductor NCs are also of interest for their potential applications in electronic devices such as OLEDs. Unfortunately, polymer nanocomposite EL LEDs containing semiconductor NCs have remained a challenge. Only a few EL devices have been so far fabricated successfully [30-38,294,297-299]. Common EL devices are composed from a few layers, with one of them containing SC nanoparticles. 

FRET (fluorescence resonance energy transfer) between luminescent semiconductor nanoparticles and other fluorescent molecnles such as amino acid residues in proteins, etc. that are attached to the particles has also been observed. Examples include... 

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