Proton-Resistant Quantum Dots: Stability in Gastrointestinal Fluids and Implications for Oral Delivery of Nanoparticle Agents

Abstract

Semiconductor quantum dots (QDs) have shown great promise as fluorescent probes for molecular, cellular and in-vivo imaging. However, the fluorescence of traditional polymer-encapsulated QDs is often quenched by proton-induced etching in acidic environments. This is a major problem for QD applications in the gastrointestinal tract because the gastric (stomach) environment is strongly acidic (pH 1-2). Here we report the use of proton-resistant surface coatings to stabilize QD fluorescence under acidic conditions. Using both hyperbranched polyethylenimine (PEI) and its polyethylene glycol derivative (PEG grafted PEI), we show that the fluorescence of core-shell CdSe/CdS/ZnS QDs is effectively protected from quenching in simulated gastric fluids. In comparison, amphiphilic lipid or polymer coatings provide no protection under similarly acidic conditions. The proton-resistant QDs are found to cause moderate membrane damage to cultured epithelial cells, but PEGylation (PEG grafting) can be used to reduce cellular toxicity and to improved nanoparticle stability.

Figure 1. Schematic diagrams of (a) acid-etchable and (b) proton-resistant quantum dots

Acid etching leads to surface defects and fluorescence quenching, as observed for QDs coated with amphiphilic polymers or lipids. The use of cationic “proton-resistant” surface coatings prevents free protons from reaching the nanocrystal surface, thus protecting QDs from acid-induced etching.

Figure 2

Optical absorbance and fluorescence spectral changes showing acid-induced etching of lipid-encapsulated QDs in simulated gastric fluids (pH 2). Note the blue shift in the absorbance peak (short dashes) and quenching of the fluorescence signal (long dashes).

Figure 3

(a) Comparison of fluorescence signals between traditional amphiphilic PEG-coated QDs and proton-resistant QDs in simulated gastric fluids. Under similarly acidic conditions, QDs coated with traditional amphiphilic polymers (octylamine-modified polyacrylic acid) are both quenched and precipitated. (b) Absorbance and fluorescence spectra of proton-resistant QD at different etching times. Notice that there is no blue-shift to either absorbance or fluorescence and that > 50% of the fluorescence intensity remains at 60 min.

Figure 4

Visualization of (a) traditional and (b) proton-resistant quantum dots before and after acid etching in simulated gastric fluids for 60 minutes. The initial fluorescence images were obtained by neutralizing SGF with sodium bicarbonate and then adding QD solution, while the etched QD fluorescence images were obtained by incubating QDs in SGF for 60 min at 37° C.

Figure 5

Cellular cytotoxicity of proton-resistant QDs measured by lactate dehydrogenase (LDH) release in Caco-2 cells as a result of membrane damage. Triton X-100, a cell permeabilizing agent, was used as the positive control and represented the maximum possible LDH release (100% LDH). The cell culture media was used as the negative control (0% LDH release).