Biocompatible luminescent silicon quantum dots for imaging of cancer cells

Abstract

Luminescent silicon quantum dots (Si QDs) have great potential for use in biological imaging and diagnostic applications. To exploit this potential, they must remain luminescent and stably dispersed in water and biological fluids over a wide range of pH and salt concentration. There have been many challenges in creating such stable water-dispersible Si QDs, including instability of photoluminescence due their fast oxidation in aqueous environments and the difficulty of attaching hydrophilic molecules to Si QD surfaces. In this paper, we report the preparation of highly stable aqueous suspensions of Si QDs using phospholipid micelles, in which the optical properties of Si nanocrystals are retained. These luminescent micelle-encapsulated Si QDs were used as luminescent labels for pancreatic cancer cells. This paves the way for silicon quantum dots to be a valuable optical probe in biomedical diagnostics.

Figure 1

Schematic of surface functionalization of silicon quantum dots followed by micellar encapsulation and a table indicating the compounds (R1) grafted onto the silicon nanoparticles and the functional groups (R2) commercially available on the phospholipids.

Figure 2

Characterization of micelle-encapsulated Si QDs: A) PL intensity vs. Si:phospholipid mass ratio. B) Optical spectra of Si-QDs in chloroform and micelle encapsulated QDs in water: PL (orange) absorbance (pink), PLE (blue). Solid lines are ethyl undecylenate grafted particles; dotted lines are micelle encapsulated Si QDs. The inset photos are vials of styrene grafted (left) and micelle encapsulated (right) Si QDs under ambient lighting (top) and UV illumination (bottom).

Figure 3

TEM images of A and B) ethyl undecylenate grafted Si QDs cast from a chloroform dispersion; C) and D) micelle encapsulated Si QDs, fixed with 4% formaldehyde and cast from water; E) all of the silicon QDs contained in a single micelle; and F) silicon QDs visible within the micelle

Figure 4

Photoluminescence intensity from micelle encapsulated Si-QDs vs A) temperature and B) pH.

Figure 5

Confocal microscopic visualization of live pancreatic cancer cells treated with A) amine terminated micelle encapsulated Si QDs. B) Tf-Conjugated micelle encapsulated Si.QDs. From left to right, the panels show the transmission image, luminescence image, and an overlay of the two. The scale bars are 17.29 μm and 24.87 μm in A and B, respectively.