Self-assembly of colloidal mesoporous (nanoporous) silica particles create a unique environment for optically active molecules. We study  nanoporous particles which have fluorescent molecules of various organic dyes entrapped inside cylindrical nanochannels. These micron size particles can be up to two others of magnitude brighter than polymeric particles of comparable size assembled with quantum dots. Comparing with the maximum fluorescence of free dye in the same volume, the particles can show fluorescence which is higher by more than three orders of magnitude. Therefore, we call these particles “ultrabright” (for the lack of a better term).

photonics_selfassembly

Ultrabright fluorescent micron size particles

The mechanism of ultrabrightness is as follows. The dye molecules are well dispersed within each cylinder. Alkane chains of surfactant molecules (used in the synthesis as the templating molecules) act as a dispersing agent along the channel.  In the perpendicular directions, silica walls play the role of the dye separator. The separation prevents the dyes from collapsing into each other, i.e., dimerization, and consequence quenching of fluorescence. It is shown in the figure below.

ultrabright

A schematic of location of nanochannels and dyes inside silica colloids.  Zoomed area of the nanochannels with the dye encapsulated inside ~ 3nm channels is shown. Alkane chains of surfactant molecules are presented as zigzag vertical lines with the headgroups adjacent to silica walls.

Recently we have synthesized nanometer-size (20-60 nm) nanoporous particles that can be called ultrabright. For example, 40nm nanoporous silica particles containing R6G dye were 34 times brighter than water dispersible (15-30nm) Cd/Se coated with Zn/S quantum dots.

Our current research is in expanding this work to multiple dyes and multiple sizes, to functionalization of the particles for biomedical labeling.

 

An example of functional labeling is shown below. Our 40nm ultrabright silica particles functionalized with folic acid were used to identify cancer cells.

cells_nnaoparticles

This research has been supported by NSF (grant CBET 0755704)

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