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Uptake of Gold Nanoparticles by Intestinal Epithelial Cells: Impact of Particle Size on their Absorption, Accumulation, and Toxicity Mingfei Yao, Lili He, David Julian McClements, and Hang Xiao J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.5b03242 • Publication Date (Web): 27 Aug 2015 Downloaded from http://pubs.acs.org on August 28, 2015

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Journal of Agricultural and Food Chemistry

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Uptake of Gold Nanoparticles by Intestinal Epithelial Cells: Impact of

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Particle Size on their Absorption, Accumulation, and Toxicity

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Mingfei Yao †, Lili He†, David Julian McClements†, §, Hang Xiao*, †, ‡

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Department of Food Science, University of Massachusetts Amherst, Amherst, MA

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College of Bioscience and Biotechnology, Hunan Agricultural University, Changsha,

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Hunan, P. R. China

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§

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Saudi Arabia

Department of Biochemistry, Faculty of Science, King Abdulaziz University, Jeddah,

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*Address correspondence to [email protected]; Tel: 413 545 2281; Fax 413 545 1262

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ABSTRACT Inorganic nanomaterials have been increasingly utilized in many consumer products,

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which has led to concerns about their potential toxicity.

At present, there is limited

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knowledge about the gastrointestinal fate and cytotoxicity of ingested inorganic

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nanoparticles. In this study, we determined the influence of particle size and

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concentration of gold nanoparticles (AuNPs) on their absorption, accumulation and

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cytotoxicity in model intestinal epithelial cells. As the mean particle diameter of the

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AuNPs decreased (from 100 to 50 to 15 nm) their rate of absorption by the intestinal

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epithelium cells increased, but their cellular accumulation in the epithelial cells decreased.

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Moreover, accumulation of AuNPs caused cytotoxicity in the intestinal epithelial cells,

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which was evidenced by depolarization of mitochondria membranes. These results

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provide important insights into the relationship between the dimensions of AuNPs and

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their gastrointestinal uptake and potential cytotoxicity.

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Keywords: Gold nanoparticles; absorption; accumulation; toxicity; intestinal epithelial

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cells; particle size: mitochondria potential.

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Journal of Agricultural and Food Chemistry

INTRODUCTION Developments in the nanotechnology field during the past decade or so have led to

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numerous applications of nanomaterial, such as in computer science, chemistry,

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cosmetics, agrochemicals, and pharmaceuticals 1, 2. Nanotechnology has also been

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utilized within the food industry to improve the microbiological safety, sensory attributes,

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nutrition value, and shelf life of foods, as well as to develop innovative sensors to

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monitor food quality and safety 3-5.

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inorganic nanoparticles have been found in a wide range of food products, including the

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nanoparticles based on silver, titanium dioxide, silica, zinc oxide, etc. 6.

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nanoparticles may be intentionally added to food products as additives because of their

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specific functional attributes (such as antimicrobial, whitening, or anticaking agents), or

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they may be unintentionally added to foods (e.g., as a contaminant in an additive that is

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supposed to contain larger particles).

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nanoparticles within food products means that humans come into contact with them more

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frequently 7. Hence, it is particularly important to establish whether there are any adverse

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effects of ingesting inorganic nanoparticles in foods on human health, especially since

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nanoparticles may have completely different absorption and toxicity patterns in

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comparison to the larger particles.

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Consequently, various types of nanomaterial such as

These

The increasing prevalence of inorganic

A number of studies suggest that ingestion of inorganic nanoparticles may have toxic 3 ACS Paragon Plus Environment

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effects. It has been reported that ingestion of nanomaterials may promote inflammatory

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bowel disease, DNA fragmentation, liver damage and pregnancy complications, as well

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as result in abnormal intestinal barrier function and increased intestinal permeability. 8-11.

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In particular, prolonged exposure to inorganic nanoparticles may have a detrimental

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impact on the function of the intestinal membrane 12. However, detailed information

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about the gastrointestinal uptake and cytotoxicity of ingested inorganic nanoparticles is

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currently unavailable, and more studies are needed in this important area.

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Gold nanoparticles (AuNPs) have been widely used in the studies of the biological

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fate and toxicity of inorganic nanoparticles because of their ease of synthesis, high

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chemical stability, and unique optical properties 13, 14. Commercially, the main

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applications of AuNPs are in the medical field, such as for drug delivery and

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photodynamic therapy. However, AuNPs have also been used in cosmetics, beverages,

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food packaging materials, and toothpastes, which leads to oral exposure of AuNPs in

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humans 14-16. However, a review on the previous studies highlighted the current poor

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understanding of the gastrointestinal uptake and potential cytotoxicity of AuNPs 17.

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Therefore, the objective of this study was to determine the effect of nanoparticle size and

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concentration on the intestinal absorption and cellular accumulation of AuNPs in the

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model intestinal epithelial cells, and to assess the potential toxicity caused by the

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exposure to AuNPs. 4 ACS Paragon Plus Environment

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Journal of Agricultural and Food Chemistry

Differentiated human intestinal cells (Caco-2 cells) can be grown on a permeable

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support to form a model intestinal epithelial monolayer. This model system can be used

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to carry out systematic studies of the impact of nanoparticle characteristics (such as size,

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surface chemistry, and concentration) on cellular responses such as absorption and

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toxicity 12, 18.

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by or transported across this type of epithelial monolayer 15. In this study, we utilized this

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model to determine the influence of AuNPs dimensions (15 nm, 50, or 100 nm) on their

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absorption, accumulation, and toxicity. The results of this study are important for

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understanding the gastrointestinal fate of inorganic nanoparticles that may be ingested.

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MATERIALS AND METHODS

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Materials

Previous studies have shown that different nanoparticles can be absorbed

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The following products were purchased from SigmaAldrich Chemicals (St. Louis,

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MO, USA): 4% osmium tetroxide (OsO4), phosphatungstic acid (PTA), nitric acid, and

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hydrochloric acid. AuNPs with different diameters (15, 50, and 100 nm) were purchased

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from NanopartzTM (Loveland, CO). A mitochondrial membrane potential probe (JC-1 dye)

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was purchased from Life Technologies (Thermo Fisher Scientific, Agawam, MA, USA).

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Hank’s balanced salt solution, glutaraldehyde , cacodylate, formvar/carbon Cu grids 200

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mesh, epoxy resin were purchased from EM Sciences (Hatfield, PA, USA).

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Cell culture

Caco-2 cells (passage 55~65) were cultured in complete Dulbecco’s modified

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essential medium (DMEM) containing high glucose, 10% fetal bovine serum (FBS), 1%

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antibiotic, and 1% amino acids as we described previously 19. Cells were seeded at 3×105

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cells/mL on transwells (Corning Inc., MA, USA) containing polyester filters (3 m pore

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size and 4.7 cm2 surface area) and grown for 21 days. The transepithelial electrical

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resistance (TEER) was monitored using a Millicell® ERS-2 epithelial voltammeter

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(World Precision Instruments, Sarasota, FL). The TEER data is presented as resistance

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per unit surface area Ω×cm2.

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Transmission electron microscopy analysis

To visualize the AuNPs, a drop of liquid sample was allowed to air-dry onto a

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carbon-coated 200 mesh copper grid, and then the sample was stained with 2% PTA.

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Samples were visualized with a transmission electron microscope (TecaniTM 12 TEM,

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FEI Company, Hillsboro, OR, USA). To visualize the uptake of AuNPs in intestinal

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epithelial cells, suspensions of AuNPs (5 µg/mL, 15, 50, or 100 nm) were applied on

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Caco-2 cell monolayers. After treatment for 0.5 h, 1 h, 2 h, 4 h, 8 h and 24 h, the media in

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both the apical and basolateral sides were removed and the monolayers were rinsed with

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Hank’s balanced salt solution (HBSS) that had been warmed to 37 ºC. The monolayers

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were immediately fixed in 2.5% glutaraldehyde and 2% paraformaldehyde (0.1 M

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sodium cacodylate buffer, pH 7.4) for 2 h at 37 ºC. They were then post-fixed in 1%

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aqueous osmium tetroxide solution and then embedded in Epoxy resin. Thin sections

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were obtained using an ultramicrotome with a diamond knife (DDK, Wilmington, DE).

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The sections were stained with uranyl acetate followed by lead citrate. The image

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samples were then viewed with the transmission electron microscope (TecaniTM 12, FEI

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Company, Hillsboro, OR, USA).

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ICP-MS analysis

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After treatment with AuNPs, the cell monolayers and medium samples were

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collected at the specific time period from the apical and basolateral sides of the transwell

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as described above. A bicinchoninic acid (BCA) kit was used to quantify the protein

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concentration of the monolayers in each well. Each sample was mixed with 0.5 mL of

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aqua regia and then the sample was diluted to 10 mL with de-ionized water. A series of

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standard solutions of Au were prepared under the same conditions. Inductively coupled

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plasma (ICP) mass spectrometry (MS) measurements were performed on a mass

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spectrometer (NexION 300X ICP, PerkinElmer, Cambridge, MA).

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Calculation of number of gold nanoparticles

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The number of gold nanoparticles in a given sample was calculated from the Au 7 ACS Paragon Plus Environment

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concentration measured by ICP-MS. The number of gold atoms per gold nanoparticle (n)

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can be estimated from the following expression 20:

=

 

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Where, D is the diameter of a gold nanoparticle and d is the diameter of a single gold

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atom, which is approximately 0.28 nm 21.

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calculated from the measured Au concentration as follows:

 =

Hence, the number of AuNPs can be



× 

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Where Cn = the number of AuNPs per unit volume, Cm = the measured mass of Au per

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unit volume, and NA is Avogadro’s constant.

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Determination of mitochondrial membrane potential

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Aqueous suspensions of AuNPs (5 µg /mL, 15, 50, or 100 nm) were applied on to

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well-differentiated Caco-2 monolayer (TEER > 200 Ω). Four hours after the treatment,

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the apical and basolateral media were removed and the monolayers were rinsed three

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times with warmed HBSS. A medium containing 10 µg/mL of JC-1 (mitochondrial

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membrane potential probe) in warmed DMEM solution was added to the Caco-2 cell

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monolayers. After the incubation for 15 minutes (37 ºC, 5% carbon dioxide, and 100%

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humidity), the monolayers were washed twice with HBSS, placed onto a microscope 8 ACS Paragon Plus Environment

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slide.

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sealed and stored overnight at 4 ºC. The microstructure of the samples was observed

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using a confocal fluorescent microscope (Olympus FV-1000, Waltham, MA). The JC-1

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treated coverslips were excited at 488 nm, and the emission was recorded simultaneously

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at 527 and 590 nm by independent detectors 22. Two-photon excitation of JC-1 labeled

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Caco-2 cell monolayers allowed for the real-time analysis of mitochondria membrane

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potential. The data were analyzed using the following expression as described

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previously23:

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After adding a fluorescent dye (DAPI) to the samples, the slides were rapidly

Ratio = 5 × Ired/Igreen

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Here Ired and Igreen are the intensities of the signals in the red and green regions of the

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spectra.

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256 levels.

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Statistical Analysis

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The factor “5” is used to transform the ratiometric results into an 8-bit scale of

All values are expressed as means ± standard deviations (SD) unless stated otherwise.

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The difference among samples were analyzed by ANOVA with significance level of p
50 nm > 15 nm (Figure

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4A).

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Quantification of the ratio of red/green florescence further

In summary, the absorption, accumulation, and transfer of AuNPs in the intestinal epithelial cells were highly dependent on their particle diameter (15, 50 or 100 nm). 17 ACS Paragon Plus Environment

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AuNPs with intermediate sizes (50 nm) were the most efficiently transported across the

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intestinal epithelium: uptaken from the apical side and excreted into the basolateral side.

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The smaller AuNPs (15 nm) were more rapidly absorbed by the intestinal epithelial cells

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and more quickly spread throughout the interior of the cells.

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AuNPs (100 nm) tended to accumulate within the intestinal epithelial cells since their rate

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of excretion into the basolateral side was slow. The increasing accumulation of the total

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mass of gold caused by AuNPs resulted in increasing mitochondria toxicity to the

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intestinal epithelial cells. Overall, this study provided detailed information on the impact

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of particle size on the absorption, accumulation and cytotoxicity of AuNPs in the

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intestinal epithelium.

Conversely, the larger

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ACKNOWLEDGMENTS This study was partially supported by grants from United States Department of

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Agriculture.

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REFERENCES

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Figure Captions

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Figure 1. Morphology of the initial gold nanoparticle suspensions (nominal diameters of

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15, 50 and 100 nm) used in this study. All images were taken under transmission electron

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microscopy as described in the Method.

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Figure 2. The absorption of AuNPs by intestinal epithelial cells. Caco-2 cell monolayers

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were incubated with AuNPs (15 nm, 50 nm, or 100 nm) for various time periods (up to 24

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hour). After thorough wash, Caco-2 cell monolayers were processed for TEM

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(transmission electron microscope) analysis to visualize AuNPs absorbed by the cells.

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AuNPs are highlighted by the circles. Arrows indicate the direction from apical to

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basolateral side across microvilli. Different magnifications were used in different images.

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Scale bar represent 500 nm. Representative images were selected from replicates of at

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least 3.

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Figure 3. Transport of AuNPs across intestinal epithelium. Change in the concentration of

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gold measured in the basolateral compartment after incubating Caco-2 cell monolayers

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with AuNPs in the apical compartment of the transwells.

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different concentrations of gold nanoparticles were used (shown in figure): 2.5, 5 and 10

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µg/mL in the apical medium.

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Figure 4.

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amount of gold measured in the Caco-2 monolayers after incubating Caco-2 cell

Suspensions containing

Accumulation of AuNPs within the intestinal epithelium. Change in the

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monolayers with 5 µg/mL of AuNPs in the apical compartment of the transwells.

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Amount of gold was expressed as either particle mass (A) or particle number (B) in the

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Caco-2 cell monolayer.

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Figure 5. The effects of AuNPs on the mitochondrial membrane potentials of intestinal

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epithelial cells.

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100 nm) for 4h separately. Mitochondria toxicity of AuNPs was visualized by confocal

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microscope using a red fluorescent dye JC-1 (A). JC-1 exhibits potential-dependent

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accumulation in mitochondria, indicated by a fluorescence emission shift from green to

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red.

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of dye aggregates that fluoresce red, whereas cells with low mitochondria membrane

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potential (damaged) remain monomeric JC-1 that fluoresce green. The red/green ratio

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was quantified using image J (B). Representative images were selected from replicates of

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3. Different letters indicate statistical difference among the three groups at the given time

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point (P