Synthesis And Functionalization Of Dendrimer-Based Metal Nanoparticles For Biomedical Applications

With the development of nanotechnology, more and more nanoparticles (NPs) show their great potential in biomedical applications, in which desirable stability and biocompatibility are essential. The aim of this research is to develop dendrimer/metal NPs-based computed tomography (CT) contrast agents with desirable properities for CT imaging of cancer. We employed poly(amidoamine)(PAMAM) dendrimers as the platform to develop several kinds of gold, silver, and their alloy NPs-based CT contrast agents. Taking advantage of the modifiable dendrimer surface, multifunctional hybrid NPs can be formed, meeting certain requirements of applications.Based on the previous work of dendrimer/gold and dendrimer/silver NPs, here firstly dendrimer/gold-silver alloy NPs were controllably synthesized and their abilities as contrast agents for CT imaging were investigated. After finding nanowires (NWs) formed in certain gold/silver molar ratios, the experimental parameters were optimized and their feasibility of functionalization was investigated. Considering the advantage of gold, multifunctional dendrimer/gold NPs were synthesized for in vitro and in vivo CT imaging of cancer cells. In order to reduce the cost, low generation dendrimers were employed to form gold NPs for CT imaging applications. The methods and results are as follows:1) Using amine-terminated generation5PAMAM dendrimer (G5.NH2) as templates or stabilizers and NaBH4as reducing agent, Au-Ag alloy NPs with different components can be formed by simply regulating the molar ratio of dendrimer/gold atom/silver atom. Followed by an acetylation reaction to transform the dendrimer terminal amines to acetyl groups, the surface charge of the particles was changed toward neutral. UV-vis spectra indicate that the formed NPs are alloy with tunable optical properties. TEM studies reveal that the size of the alloy NPs is variable depending on both dendrimer/Au atom/Ag atom molar ratio and the acetylation modification. Under constant total dendrimer/metal atom molar ratio, the size of the alloy NPs decreases with gold composition. For particles with similar metal composition, the size of the particles becomes a little larger after acetylation. The formed Au-Ag alloy NPs are stable and cytocompatible. X-ray absorption coefficient measurements show that the attenuation of the alloy NPs is dependent on both gold content and the particle surface characteristics. With the Au content increasing and after acetylation, the corresponding CT value of the particles increases.2) Using G5.NH2as stabilizers and reducing agents, Au-Ag alloy NPs with different components can be formed at room temperature. By regulating the Au/Ag feed molar ratio, the formed NPs experience a shape evolution from spherical particles and polyhedrons to curved NWs. The alloy NPs display desirable stability. X-ray absorption coefficient measurements show that the attenuation of the binary NPs is dependent on both gold content and the particle surface characteristics. With the Au content increasing and after acetylation, the corresponding CT value of the particles increases. After that, a wide range of synthetic parameters such as reaction time, temperature, solvent, and additive were systematically investigated to explore their effects on the morphology of the Au NWs. The optimized synthesis protocol is as follows:G5.NH2as stabilizers, dendrimer/Au/Ag molar ratio at1/15/5, reaction performed at40℃for48h in aqueous solution. Our data show that the selection of these parameters is essential to obtain relatively uniform Au NWs, which are associated with the transverse growth control of Au NWs and the proper reduction rate of Ag(I). A new growth mechanism involving a synergic facet-dependent deposition/reduction of Ag(I) and anisotropic migration of Au atoms is proposed, which is based on density functional theory calculations. This may provide guidance for synthesis of other kinds of NWs in aqueous solution and also make the formation of other functionalized NWs possible.3) Based on the previous chapter, following a similar synthetic procedure, we were aiming to prepare NWs with functionalized surface. Using G5.NH2as stabilizers, which were pre-modified with folic acid (FA) or fluorescein isothiocyanate (FI), Au-Ag alloy NPs can be formed in the water bath following an acetylation reaction. To our surprise, spherical Au-Ag alloy NPs were formed. They display good water solubility and colloidal stability, and excellent cytocompatibility in a given concentration range. It can be concluded that modification of dendrimer surface may influence their cluster state in aqueous solution, which may further affect the shape of the alloy NPs. With the much better X-ray attenuation property than that of conventional iodine-based CT contrast agent Omnipaque and the ability to be specifically uptaken by cancer cells overexpressing high affinity FA receptor, the developed FA-targeted Au-Ag alloy NPs are able to be used as a nanoprobe for targeted CT imaging of cancer cells in vitro.4) To generate tumor-targeting CT imaging nanoprobes, G5.NH2were firstly modified with FI and polyethylene glycol (PEG)-linked lactobionic acid (LA), which can be employed as templates to form dendrimer entrapped Au NPs (LA-Au DENPs). The formed LA-Au DENPs display good stability, desirable cytocompatibility in the given concentration range, and specifically targeting ability towards human hepatocellular carcinoma cells in vitro. X-ray attenuation measurements show that the LA-Au DENPs display much stronger attenuation intensity than Omnipaque at the same concentration of radiodense element (Au or iodine). Importantly, the LA-modified Au DENPs are able to be used as an efficient nanoprobe for targeted CT imaging of human hepatocellular carcinoma in vitro and xenografted tumor model in vivo via LA-mediated active targeting.5) In consideration of the high cost of high generation dendrimer, development of low generation dendrimer based CT contrast agents may be more potential for practical applications. In this chapter, amine-terminated generation2PAMAM dendrimers (G2.NH2) were employed as stabilizers to form dendrimer-stabilized Au NPs (Au DSNPs) via a simple hydrothermal approach without addition of reducing agents, followed by surface modification of FA and acetyl groups. Au DSNPs with desirable stability and cytocompatibility can be prepared via optimizing the synthesis parameters. X-ray attenuation coefficient measurements show that nontargeted acetylated Au DSNPs display approximately similar X-ray attenuation property to that of Omnipaque. In contrast, FA-targeted Au DSNPs displayed better X-ray attenuation intensity than Omnipaque. The acetylated Au DSNPs show much better performance in CT imaging of the major organs of rats in vivo than Omnipaque. Importantly, the FA-modified Au DSNPs enable targeted CT imaging of cancer cells in vitro and xenograft tumor model in vivo via FA receptor-mediated active targeting pathway.6) Lastly, we show that G2.NH2firstly modified with PEG chains can be employed as templates to form Au NPs via a chemical reduction protocol. The formed PEGylated Au DENPs are quite stable and show desirable cytocompatibility in the given concentration range. In addition, the PEGylated Au DENPs display much stronger X-ray attenuation intensity than Omnipaque at the same concentration of radiodense element (Au or iodine). The formed particles can be used for CT imaging of heart and bladder of mice and xenografted tumor model in vivo.