Characterization of nano-particles and organic dye molecules

Several research projects on the characterization of nano-particles and organic dyes have been carried out. Examples of this work are:  

Single Quantum Dot imaging

The room-temperature luminescence of single CdSe/ZnS core-shell quantum dots was investigated by spectrally and temporally resolved confocal microscopy. A large (30 nm) blue shift was observed in the emission wavelength during excitation in air. In nitrogen athmosphere, no blue shift was observed. The blue shift in air is ascribed to a 1 nm shrinkage of the CdSe core by photooxidation. Photobleaching occurs about 4 times faster in air than in nitrogen, indicating the formation of nonradiative recombination centers during photooxidation. The initial light output is higher in air than in nitrogen, which may be due to a reduction of the defect state lifetime by oxygen using confocal microscopy in combination with spectral imaging.

silica shell nano particles
Electron micrograph of gold core - silica shell nano particles

Surface plasmon enhanced fluorescence in Metal-core silica shell colloids

Modifications of the emission rate and quantum yield of fluorophores by metal surfaces have been widely studied in the past decades. Most of this work concerns fluorophores located at a fixed distance with respect to the metal. We studied enhancement of fluorescence in a system consisting of a metal-core, a silica spacer shell of variable thickness and a dye labeled shell.

Our experiments are in agreement with the theoretical calculations and showed that:

  1. The effective quantum yield of the fluorophores can be substantially increased in the presence of a metal-core.
  2. In the presence of the metal-core the lifetime of the fluorophores was decreased depending on the type of metal, the thickness of the spacer and the type of fluorophore.
  3. Comparison of the results for the different fluorophores suggests that the fluorescence enhancement is due to an increased radiative rate that is caused by interaction with surface plasmons.  The results strongly depend on the relative locations of the plasmon band and the absorption and emission bands of the fluorophore.

Recent results on colloids with a gold core and a FAM (carboxy-fluorescein) labeled shell yielded a fluorescence enhancement factor of 6.8 and results on silver core Cascade yellow labeled shell particles yielded fluorescence enhancement factors as high as 12.5. We note that the distance between the dye and the metal is a critical parameter. Only at distances larger than about 15 nm fluorescence enhancement can be observed.

Tovmachenko, O.G., C. Graf, D.J.v.d. Heuvel, A.v. Blaaderen, and H.C. Gerritsen. 2006. Fluorescence Enhancement by Metal-Core/Silica-Shell Nanoparticles. Advanced Materials 18:91-95 

Characterization of julolidine molecular rotor dyes

We carried out a Fluorescent Lifetime Imaging (FLIM) study on so-called “molecular rotor” dyes. The dyes effectively provides a contrast between different lipids and mixtures in model membranes.  These new dyes may be able to be used to provide information on free volume and dynamic processes in membranes that have heretofore been unavailable.  By attaching the julolidine core to phospholipids, a drastic improvement is seen in the localization of the dye inside of the membrane.  A comparison of the contrasting ability of these rotor dyes versus LAURDAN is presented. 

Non linear spectroscopy of ZnO nanowires

ZnO nanowires were studied using TPE in combination with spectral imaging. The results were published in Physical Review Letters. Exciton Polaritons Confined in a ZnO Nanowire Cavity," published in Physical Review Letters 97, 147401 (2006)