Department of Analytical Chemistry, University of Vienna , Währingerstrasse 38, 1090 Vienna, Austria.
ACS Nano. 2013 Feb 26;7(2):1129-36. doi: 10.1021/nn306024a. Epub 2013 Jan 29.
Gold nanoparticles (GNPs) are often used as colloidal carriers in numerous applications owing to their low-cost and size-controlled preparation as well as their straightforward surface functionalization with thiol containing molecules forming self-assembling monolayers (SAM). The quantification of the ligand density of such modified GNPs is technically challenging, yet of utmost importance for quality control in many applications. In this contribution, a new method for the determination of the surface coverage of GNPs with thiol containing ligands is proposed. It makes use of the measurement of the gold-to-sulfur (Au/S) ratio by inductively coupled plasma mass spectrometry (ICP-MS) and its dependence on the nanoparticle diameter. The simultaneous ICP-MS measurement of gold and sulfur was carefully validated and found to be a robust method with a relative standard uncertainty of lower than 10%. A major advantage of this method is the independence from sample preparation; for example, sample loss during the washing steps is not affecting the results. To demonstrate the utility of the straightforward method, GNPs of different diameters were synthesized and derivatized on the surface with bifunctional (lipophilic) ω-mercapto-alkanoic acids and (hydrophilic) mercapto-poly(ethylene glycol) (PEG)(n)-carboxylic acids, respectively, by self-assembling monolayer (SAM) formation. Thereby, a size-independent but ligand-chain length-dependent ligand density was found. The surface coverage increases from 4.3 to 6.3 molecules nm⁻² with a decrease of ligand chain length from 3.52 to 0.68 nm. Furthermore, no significant difference between the surface coverage of hydrophilic and lipophilic ligands with approximately the same ligand length was found, indicating that sterical hindrance is of more importance than, for example, intermolecular strand interactions of Van der Waals forces as claimed in other studies.
金纳米粒子（GNPs）由于其低成本、尺寸可控的制备以及与含硫分子形成自组装单层（SAM）的简单表面功能化，常被用作胶体载体，应用于众多领域。此类修饰后的 GNPs 的配体密度定量分析具有一定的技术难度，但对于许多应用中的质量控制至关重要。本研究提出了一种测定含硫配体修饰的 GNPs 表面覆盖率的新方法。该方法利用电感耦合等离子体质谱法（ICP-MS）测量金与硫的比值（Au/S），并与纳米粒子的直径相关联。我们对金和硫的同时 ICP-MS 测量进行了仔细的验证，发现这是一种可靠的方法，其相对标准不确定度低于 10%。该方法的一个主要优点是其不依赖于样品制备；例如，在洗涤步骤中样品损失不会影响结果。为了证明该简便方法的实用性，我们合成了不同直径的 GNPs，并通过自组装单层（SAM）形成，分别在表面上用双官能（亲脂性）ω-巯基烷酸和（亲水性）巯基聚（乙二醇）（PEG）（n）-羧酸修饰。由此，我们发现了一种与粒径无关但与配体链长相关的配体密度。当配体链长从 3.52nm 减少至 0.68nm 时，表面覆盖率从 4.3 个分子/nm²增加至 6.3 个分子/nm²。此外，我们还发现亲水性和疏水性配体的表面覆盖率没有显著差异，其配体长度相近，这表明空间位阻比其他研究中提出的范德华力分子间链相互作用更为重要。
