Cellular imaging of endosome entrapped small gold nanoparticles.

Small gold nanoparticles (sAuNPs, <10 nm in a core diameter) have been used for drug delivery and cancer therapy due to their high payload to carrier ratio. Information about the amount and location of sAuNPs in cells and tissues is critical to many applications. However, the current detection method (i.e., transmission electron microscopy) for such sAuNPs is limited due to the extensive sample preparation and the limited field of view. Here we use confocal laser scanning microscopy to provide endosome-entrapped sAuNP distributions and to quantify particle uptake into cells. The quantitative capabilities of the system were confirmed by inductively coupled plasma-mass spectrometry, with an observed linear relation between scattering intensity and the initial cellular uptake of sAuNPs using 4 nm and 6 nm core particles. The summary of the method is: • This non-invasive imaging strategy provides a tool for label-free real-time tracking and quantification of sAuNPs using a commercially available confocal laser scanning microscope. • Scattering intensity depends on particle size. • The linear relation established between scattering intensity and uptaken gold amount enables simultaneous quantitative assessment through simple image analysis. Method details We report a simple, rapid, and non-invasive approach for the imaging of sAuNPs within cells by using a standard confocal laser scanning microscope (CLSM). No additional optical or imaging system is required for this approach. A single-wavelength laser excitation was used to excite sAuNPs within the cell, and the reflective images were recorded to explore the size-dependent visibility of the AuNPs. These studies demonstrate that sAuNPs as small as 4 nm in core size can be readily imaged. Image analysis was carried out to explore the correlation between the sAuNP scattering intensity and sAuNP quantities inside cells. Step 1: surface-functionalized gold nanoparticle synthesis Materials All the reagents/materials required for nanoparticle synthesis were purchased from Fisher Scientific, except for hydrogen tetrachloroaurate(III) hydrate, which was obtained from Strem Chemicals Inc. The organic solvents were from Pharmco-Aaper or Fisher Scientific and used as-received except for dichloromethane, which was distilled in the presence of calcium hydride. HeLa cells (human cervical-cancer cell line) were purchased from ATCC. Dulbecco’s Modified Eagle’s Medium (DMEM; Sigma, D5523) and fetal bovine serum (FBS; Fisher Scientific, SH3007103) were used in cell culture.