AS1411 aptamer modified carbon dots via polyethylenimine-assisted strategy for efficient targeted cancer cell imaging

Abstract

Objectives: Carbon dots (CDs), as a fascinating class of fluorescent carbon nanomaterials, have been proven to be powerful tools in the field of bioimaging and biosensing due to their small size, suitable photostability and favourable biocompatibility. However, the cellular uptake of free CDs lacks selectivity and the same negative charges as cell membranes may cause inefficient cell internalization. In this study, an efficient detecting and targeting nanosystem was developed based on the DNA aptamer AS1411 modified CDs with polyethyleneimine (PEI) as connecting bridge.

Materials and methods: Hydrothermally prepared CDs were assembled with positive-charged PEI, followed by conjugation with AS1411 through electrostatic interaction to form CDs-PEI-AS1411 nanocomplexes. The CDs, CDs-PEI and CDs-PEI-AS1411 were characterized by transmission electron microscopy (TEM), fourier transform infrared (FTIR) spectra, UV-vis spectra, zeta potential measurements and capillary electrophoresis characterizations. The cytotoxicity investigation of the CDs-PEI-AS1411 and CDs-PEI in both MCF-7 and L929 cells was carried out by the CCK-8 assay. The cellular uptake of the CDs-PEI-AS1411 was studied with confocal microscopy and flow cytometry.

Results: The as-prepared nanosystem possessed good photostability and no obvious cytotoxicity. On the basis of the confocal laser scanning microscope observation and the flow cytometry studies, the cellular uptake of CDs-PEI-AS1411 nanosystem in MCF-7 cells was significantly higher than that of L929 cells, which revealed the highly selective detection ability of nucleolin-positive cells.

Conclusions: The results of this study indicated that the CDs-PEI-AS1411 nanosystem had a potential value in cancer cell targeted imaging.

Keywords: AS1411 aptamer; carbon dots; polyethylenimine; targeted imaging.

Scheme 1

Predicted mechanism of the different cellular uptake behaviour of CDs‐PEI‐AS1411 with the nucleolin‐positive MCF‐7 cancer cells and nucleolin‐negative L929 fibroblast cells

Figure 1

Transmission electron microscopy images of CDs (A), CDs‐PEI (B) and CDs‐PEI‐AS1411 (C). (D) Particle size distribution of CDs, CDs‐PEI and CDs‐PEI‐AS1411. Scale bar = 50 nm

Figure 2

(A) Zeta potential values of CDs, CDs‐PEI and CDs‐PEI‐AS1411. (B) FTIR spectra of CDs, PEI, CDs‐PEI and CDs‐PEI‐AS1411. (C) UV‐vis spectra of AS1411, CDs‐PEI and CDs‐PEI‐AS1411. (D) Electropherograms of size marker and CDs‐PEI‐AS1411

Figure 3

(A) Fluorescence spectra of CDs, CDs‐PEI and CDs‐PEI‐AS1411. Inset: optical images of CDs (left), CDs‐PEI (middle) and CDs‐PEI‐AS1411 (right) in aqueous solution under UV light. (B) The fluorescence decay curves of CDs, CDs‐PEI and CDs‐PEI‐AS1411. (C) Fluorescent intensity of CDs‐PEIAS1411 under 360 nm irradiation at different intervals of time. (D) Fluorescence profiles of CDs‐PEI‐AS1411 in water showing aqueous stability up to 15 d preservation time

Figure 4

Cell viability of L929 cells (A) and MCF‐7 cells (B) exposed to various concentrations of CDs‐PEI‐AS1411 (at the AS1411 concentrations of 100, 200, 400, 600 and 800 nmol/L) and the relative amounts of CDs‐PEI for 24 h treatment. *P < .05, **P < .01 vs control; #P < .05 vs CDs‐PEI‐treated group. Data are shown as mean ± SD. (n = 3)

Figure 5

Intracellular uptake analyses. (A) Confocal microscopy images of MCF‐7 cells and L929 cells incubated with the CDs‐PEI‐AS1411 for 6 h and 12 h, respectively (blue, CDs; green, cytoskeleton stained with FITC‐labelled phalloidin). (B) Flow cytometry profiles of MCF‐7 cells and L929 cells treated with the CDs‐PEI‐AS1411 for 6 h and 12 h, respectively