Diamond nanoparticles, so-called nanodiamonds (NDs), are attracting growing attention, because they have the intrinsic characteristics of diamond together with the unique properties of nanometer-sized particles. In particular, biomedical applications of NDs have been investigated extensively owing to their low toxicity and amenability to various surface functionalizations. [1][2][3][4][5][6][7] One of the most promising applications of ND is biomedical imaging on the basis of nonbleaching fluorescence from the diamond core (nitrogen-vacancy, or N-V, center). [4,8,9] In such biomedical applications of ND, it should form a stable hydrosol in a physiological environment, as pointed out by Xing and Dai, [2] and Shenderova et al. [3] in their recent reviews.Although we successfully prepared a very stable hydrosol of ND functionalized with polyethylene glycol (PEG), [5] its solubility was not sufficient for biomedical applications, for example, as a platform for an imaging probe and drugdelivery system. To increase the solubility, we changed the molecular design and exchanged the linear polyethers PEG for hyperbranched polyols, because the hydroxy group is more hydrophilic than an ether group, and the hyperbranched structure can cover the nanoparticle surface much more densely than a linear chain. Polyglycerol (PG) was adopted as the hyperbranched polyol to be grafted onto the ND surface for the following three reasons: 1) PG shows high biocompatibility as well as high hydrophilicity, [10][11][12][13] 2) a PG layer can be readily constructed on the ND surface through ringopening multibranching polymerization of glycidol initiated at the functional groups on the ND surface, [14,15] and 3) PG can be functionalized further by derivatizing the periphery. [12,13] The recent use of PG for the functionalization of various nanoparticles with potential biological applications [14][15][16][17][18] prompted us to communicate our results. Herein, we report the preparation of PG-functionalized ND (ND-PG). Its extremely high solubility not only in pure water but also in buffer solutions enabled chromatographic separation of the NDs according to size. In view of cancer imaging, size control of nanoparticles is important because of enhanced permeability and retention (EPR) in solid tumors. [19] The ND used for PG functionalization, designated as ND30 herein, has a 30 nm median diameter and is prepared from bulk diamond synthesized by a static high-pressurehigh-temperature (HPHT) method. ND-PG was synthesized through ring-opening multibranching polymerization of glycidol at high temperature (Scheme 1). When the polymerization was initiated at the surface functional groups, such as hydroxy and carboxylic acid groups, the ND surface was covered with PG. However, PG without the ND core, designated as free PG, was also obtained as a side product through self-ring-opening polymerization of glycidol. To retard the side reaction initiated not from the ND surface but from glycidol, we examined the reaction conditions, including the solvent, t...

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Upon contact with biofluids, proteins are quickly adsorbed onto the nanoparticle (NP) surface to form a protein corona, which initiates the opsonization and facilitates the rapid clearance of the NP by macrophage uptake. Although polyethylene glycol (PEG) functionalization has been the standard approach to evade macrophage uptake by reducing protein adsorption, it cannot fully eliminate nonspecific uptake. Herein, polyglycerol (PG) grafting is demonstrated as a better alternative to PEG. NPs of various size and material were grafted with PG and PEG at 30, 20, and 10 wt % contents by controlling the reaction conditions, and the resulting NP-PG and NP-PEG were characterized qualitatively by IR spectroscopy and quantitatively by thermogravimetric analysis. Their resistivity to adsorption of the proteins in fetal bovine serum and human plasma were compared by polyacrylamide gel electrophoresis, bicinchoninic acid assay, and liquid chromatography-tandem mass spectrometry, giving a consistent conclusion that PG shields protein adsorption more efficiently than does PEG. The macrophage uptake was assayed by transmission electron microscopy and by extinction spectroscopy or inductively coupled plasma mass spectrometry, revealing that PG avoids macrophage uptake more efficiently than does PEG. In particular, a NP coated with PG at 30 wt % (NP-PG-h) prevents corona formation almost completely, regardless of NP size and core material, leading to the complete evasion of macrophage uptake. Our findings demonstrate that PG grafting is a promising strategy in nanomedicine to improve anti-biofouling property and stealth efficiency in nanoformulations.