Upconversion fluorescent and X-ray-sensitive bifunctional nanoprobes for assessing the penetrability of inorganic nanoparticles in the digestive system

Abstract

Nanotechnology is receiving increasing attention due to its fantastic advantages and potential applications in nanofood and nanomedicine. However, the safety of touching manufactured nanoparticles is still uncertain for human beings. Here, we track inorganic nanoparticles in the digestive system of the mouse through upconversion fluorescence and X-ray imaging, and try to demonstrate whether or not the inorganic nanoparticles will penetrate the digestive system to enter the blood system. Lanthanide-doped upconversion nanoparticles, which can convert infrared light to visible light and are simultaneously sensitive to X-rays, were selected as model inorganic nanoparticles. The investigation clarifies that even the ultrathin nanoparticles (∼5 nm) could not penetrate the digestive tract to enter the bloodstream or surrounding tissues, but were gradually excreted out. Our results help assess the safety of inorganic nanoparticles potentially used in nanofood and nanomedicine.

Scheme 1. Schematic illustration of the distribution of size-controlled upconversion nanoparticles (UCNPs) in the digestive system and blood circulation system (e.g., stomach, small intestine, large intestine, heart, liver, lung, kidney) after a mouse was fed with a mixture of fodder and UCNPs.

Fig. 1. TEM images of upconversion nanoparticles (a) NaGdF₄:2% Yb³⁺/0.5% Er³⁺; (b) NaGdF₄:18% Yb³⁺/2% Er³⁺; (c) NaGdF₄:28% Yb³⁺/2% Er³⁺; (d) NaLuF₄:10% Gd³⁺/18% Yb³⁺/2% Er³⁺. The images (e–h) show the corresponding high-resolution TEM images of (a–d), respectively. (i) Histogram of the size distribution of the OA-UCNPs from TEM images shown in (a–d). EDX spectra of (b) and (d) are shown in (j) and (k), respectively. The four kinds of nanocrystals are referred to as UCNPs-1, UCNPs-2, UCNPs-3 and UCNPs-4 in the following discussion.

Fig. 2. (a) Upconversion fluorescence spectra of the oleic acid and polyethylene glycol (PEG) coated NaGdF₄:Yb³⁺,Er³⁺ nanoparticles, under the excitation of a 980 nm near infrared laser. Insets: Left picture demonstrates the good solubility of upconversion nanoparticles in cyclohexane, while the middle and right pictures show the emission of NaGdF₄:Yb³⁺,Er³⁺ nanoparticles with OA and PEG coatings, respectively. (b) FT-IR spectra of OA-UCNPs and PEG-UCNPs.

Fig. 3. Time-dependent upconversion luminescence (UCL) imaging of the isolated organs from mice fed with various sizes of nanoparticles (e.g., UCNPs-1, UCNPs-2, UCNPs-3 and UCNPs-4, dose: 16.5 mg/20 g) with a coating of oleic acid for different times, using a green band pass filter (nanoparticles dispersed in the fodder). The major imaged organs: stomach, small intestine, large intestine, heart, liver, lung, and kidney. The bright field and UCL images of organs are shown in column 1 and column 2–6, respectively.

Fig. 4. Left column: Histogram of the integral luminescence intensity in isolated organs of the mouse fed with various sizes of nanoparticles (e.g., UCNPs-1, UCNPs-2, UCNPs-3 and UCNPs-4, dose: 16.5 mg/20 g) for different time. Error bars were based on triplet integral calculations. Right column: The corresponding integral luminescence intensity ratios of UCL_{small intestine}/UCL_stomach and UCL_{large intestine}/UCL_stomach.

Fig. 5. Middle column: Time-dependent upconversion luminescence imaging of the isolated organs from mice fed with UCNPs-2 coated with polyethylene glycol (PEG) dispersed in fodder for different times (from 3 hours to 5 days). Dose of UCNPs-2 is 10.0 mg/20 g. Left column: Conventional optical bio-imaging of organs using bright field. Right column: The overlay of the left and middle columns with green band pass filter (GBPF). 1: stomach; 2: small intestine; 3: large intestine; 4: heart; 5: liver; 6: lung; 7: kidney.

Fig. 6. In vivo X-ray CT imaging of the mouse before and after oral administration of NaGdF₄:Yb/Er (UCNPs-2 with OA coating) (25 mg) for different time periods, which can directly and roughly track the nanoparticles in the digestive system. Left panel: maximum intensity projection (MIP); middle panel: the corresponding 3D volume-rendered (VR), in vivo CT images of the mouse; right panel: lateral view of 3D VR CT images.