Surface charge controlled nucleoli selective staining with nanoscale carbon dots

Abstract

Organelle selective imaging can reveal structural and functional characters of cells undergoing external stimuli, and is considered critical in revealing biological fundamentals, designing targeted delivery system, and screening potential drugs and therapeutics. This paper describes the nucleoli targeting ability of nanoscale carbon dots (including nanodiamond) that are hydrothermally made with controlled surface charges. The surface charges of carbon dots are controlled in the range of -17.9 to -2.84 mV by changing the molar ratio of two precursors, citric acid (CA) and ethylenediamine (EDA). All carbon dots samples show strong fluorescence under wide excitation wavelength, and samples with both negative and positve charges show strong fluorescent contrast from stained nucleoli. The nucleoli selective imaging of live cell has been confirmed with Hoechst staining and nucleoli specific staining (SYTO RNA-select green), and is explained as surface charge heterogeneity on carbon dots. Carbon dots with both negative and positive charges have better ability to penetrate cell and nucleus membranes, and the charge heterogeneity helps carbon dots to bind preferentially to nucleoli, where the electrostatic environment is favored.

Fig 1

FTIR spectrum (A), fluorescent emission spectra at the same concentration (B), and TEM (C) of carbon dots made from different molar ratio of CA: EDA. The Raman spectrum of one sample is also included as inset in C4.

Fig 2

Fluorescent images of HeLa cells incubated with 400 μg/mL of carbon dots with the molar ratio of CA: EDA at 1:0.5(A), 1:1 (B), 1:2 (C), 1:3 (D) for 24 h (under blue light excitation). Table of the Zeta potentials of carbon dots with different molar ratio of CA: EDA (E). Viability of HeLa cells incubated with various carbon dots with different concentrations for 24 h (F).

Fig 3

The fluorescent images of HeLa cells incubated with 400 μg/mL of carbon dots from CA: EDA 1:2 for 0 h (A), 1 h (B), 2 h (C), 4 h (D), 8 h (E) and 24 h (F), respectively (under blue excitation).

Fig 4

The fluorescent image of HeLa cells incubated with carbon dots from CA: EDA 1:2 and Hoechst 33342, (A) for blue bright excitation, (B) for UV bright excitation, (C) for overlap of (A) and (B). Fluorescent image of the HeLa cells stained with SYTO RNA-select green fluorescent dye (D). The fluorescent images of HeLa cell incubated with 400 μg/mL of carbon dots made from CA (E), EDA (F), and CA and PEI with CA: EDA 1:0.5 (G) and 1:2 (H) for 24 h. The fluorescent images of LN229 incubated with 400 μg/mL of carbon dots for 4 h (I).