Near-infrared fluorescent nanocapsules with reversible response to thermal/pH modulation for optical imaging

Abstract

Polymeric near-infrared (NIR) fluorescent nanocapsules were developed, of which the fluorescence exhibited reversible response to local thermal/pH modulation. Our strategy was to use polymeric micelles made of temperature-sensitive Pluronic F-127 to encapsulate an amphiphilic NIR fluorescent dye-indocyanine green (ICG)-within the core and then cross-link the micelle corona by pH-sensitive poly(ethylenimine) (PEI). The size swelling/shrinking property of the micelles induced by temperature decrease/increase was used as a switch to control the fluorescence yield of the nanocapsules. It was found that the fluorescence yield significantly increased with the increase in temperature. The PEI cross-link made the fluorescence yield also sensitive to local pH change and enhanced intracellular delivery of the nanocapsules as well. Preliminary results suggest the NIR fluorescent probes could be potentially used as a contrast agent sensitive to local environment for translational optical imaging/sensing.

Figure 1

Absorption and emission spectra and molecular structure of ICG (A) and ICG fluorescence intensity versus solvent polarity (B), where the ICG concentration was 1 μg/mL.

Figure 2

Steps for engineering ICG-loaded Pluronic micellar nanocapsules that are cross-linked by PEI polymers. Molecular structures are also shown in this figure for the chemicals involved in engineering the polymeric ICG nanocapsules.

Figure 3

Size and zeta potential of ICG–Pluronic/PEI nanocapsules measured by DLS (A) and a zeta potential analyzer (B) over a temperature range of 22–40 °C and TEM images of the nanocapsules at different temperatures (C).

Figure 4

ICG nanocapsule fluorescence intensity versus temperature at a given pH (i.e., 7.0) (A) and fluorescence intensity versus pH at a given temperature (i.e., 22 °C) (B) in aqueous solution. cps: counts per second.

Figure 5

Reversible response of the ICG nanocapsule fluorescence to temperature (A) and pH (B) modulation in aqueous solution. For temperature modulation in (A), the pH was kept at 7.0. For pH modulation in (B), the temperature was kept at 22 °C. It took about 30 min to reach equilibrium during thermal modulation and about 10 min during pH modulation.

Figure 6

Fluorescence response of the ICG nanocapsules to the change in medium temperature at different ionic strengths.

Figure 7

ICG fluorescence images of A431 cells after incubation with PEI-cross-linked ICG nanocapsules (A) and ICG nanocapsules without PEI cross-linkers (B) for 30 min with an ICG concentration of 5 μg/mL. After washing with PBS three times, fluorescence microscopy images of the cells were acquired by a microscope (Zeiss Axiovert 200) with an ICG filter set (49030 ET, Chroma Technology Corp., excitation 775/50 nm, emission 845/55 nm) and a 20× objective (Zeiss, Ecplan-NEOFLUAR). Images were recorded using an EMCCD camera (Andor) with a gain of 200 and an exposure time of 0.2 s.