Improved biocompatibility of surface functionalized dendrimer- entrapped gold nanoparticles{

Gold nanoparticles (NPs) have recently received immense scientific and technological interest because of their extensive applications in biology, catalysis, and nanotechnology. In most of these applications, the gold NP surfaces are modified with different functionalities. For instance, alkanethiol and alkylamine molecules are used for phase transfer of gold NPs, and oligonucleotide molecules are linked onto gold NPs for the subsequent spatial organization of nanocrystals and for the detection of particular DNA sequences. One unique approach to the preparation of gold NPs is through the use of poly(amidoamine) (PAMAM) dendrimers as templates. Although functionalized dendrimers have been used to prepare dendrimer-entrapped or dendrimer-stabilized gold or other noble metal NPs with different functionalities, in most circumstances dendrimer-entrapped gold NPs (Au DENPs) are prepared using amine-terminated PAMAM dendrimers. This is due to the commercial availability of this material, but this yields particles with high cytotoxicity and non-specific membrane binding due to the amine surface on the dendrimers, limiting the biological application of these particles. Preparation of non-toxic, biocompatible {(Au)n-PAMAM} DENPs is of great importance for applications in various biological systems. It is well documented that decreasing the surface charge of amine-terminated PAMAM dendrimers toward neutral reduces their toxicity. In this present study, we have developed a new, facile approach to surface modification of Au DENPs by replacing the terminal amine groups of the dendrimers after the entrapment of Au NPs. Au DENPs formed using ethylenediamine core amineterminated generation 5 PAMAM dendrimers (G5.NH2) as templates were reacted with acetic anhydride or glycidol molecules to form acetamide or hydroxyl-functionalized Au DENPs (see Scheme 1). The Au DENPs formed after surface functionalization are stable, water-soluble, and display similar sizes, size distributions, and optical properties as the original DENPs, however the surface charge changes and the biocompatibility is significantly improved. Using this approach, one can directly tailor the surface functionalities of preformed Au DENPs. To our best knowledge, this is the first example of modifying preformed Au DENP surfaces through conventional organic reactions with dendrimer surface groups. Surface modifications of the {(Au)51.2-G5?NH2} DENPs to yield acetamides and hydroxyl groups were confirmed by NMR measurements (H NMR and C NMR spectra of Au DENPs with amine, acetamide, and hydroxyl groups and the corresponding dendrimers G5?NH2, G5?NHAc, and G5?NGlyOH are shown in the Electronic Supplementary Information (ESI), Figs. S1–S3).{ The NMR spectra of {(Au)51.2-G5?NH2}, {(Au )51.2-G5?NHAc}, and {(Au)51.2-G5?NGlyOH} DENPs are very similar to those of G5?NH2, G5?NHAc, and G5?NGlyOH dendrimers. 27–29 The slight down-field shift of –CH2– proton signals in {(Au )51.2G5?NH2} (from 3.20, 3.14, 2.73, 2.62, 2.53, 2.33 ppm of G5?NH2 to 3.37, 3.21, 2.99, 2.75, 2.55, 2.38 ppm, respectively) are potentially due to the strong interaction between some of the dendrimer terminal amine groups and the Au NPs. The slight variation of the NMR spectrum of {(Au)51.2-G5?NGlyOH} from that of G5?NGlyOH appears to be the result of an incomplete hydroxylation reaction of the dendrimer surface amines due to the strong interaction with Au NPs. The incomplete hydroxylation reaction is also verified by polyacrylamide gel electrophoresis (PAGE), where remaining positive charge is detected (see below). In contrast, the NMR spectra of both {(Au)51.2-G5?NHAc} and G5?NHAc are similar because the acetylation reaction is very fast, and the interaction of dendrimer terminal amines with Au NPs does not affect the reaction. The optical properties of the modified Au DENPs were investigated using UV-Vis spectrometry. {(Au)51.2-G5?NH2}, {(Au)51.2-G5?NHAc}, and {(Au )51.2-G5?NGlyOH} DENPs (Fig. 1) exhibit a similar absorption behavior, with surface plasmon bands around 510 nm, indicating their similar size and size distribution. We also found that the UV-Vis spectra of {(Au)51.2-G5?NH2}, {(Au )51.2-G5?NHAc}, and {(Au )51.2-G5? NGlyOH} DENPs did not change at pH ranging from 5.3 to 8.5 (ESI, Fig. S4),{ indicating that the Au DENPs are stable and the acid–base chemistry of dendrimers does not affect the stability of Michigan Nanotechnology Institute for Medicine and Biological Sciences, University of Michigan, Ann Arbor, MI, 48109, USA { Electronic supplementary information (ESI) available: Experimental section, additional characterization and cytotoxicity analysis of Au DENPs and the corresponding dendrimer derivatives. See DOI: 10.1039/ b612972b Scheme 1 Reactions for modifying Au DENPs prepared using amineterminated G5.NH2 dendrimers as templates. COMMUNICATION www.rsc.org/softmatter | Soft Matter