Liang C-K, Eller M J, Verkhoturov S V, Schweikert Emile A
Department of Chemistry, Texas A&M University, College Station, Texas, 77843, USA.
J Am Soc Mass Spectrom. 2015 Aug;26(8):1259-65. doi: 10.1007/s13361-015-1151-9. Epub 2015 May 6.
Secondary ion mass spectrometry, SIMS, is a method of choice for the characterization of nanoparticles, NPs. For NPs with large surface-to-volume ratios, heterogeneity is a concern. Assays should thus be on individual nano-objects rather than an ensemble of NPs; however, this may be difficult or impossible. This limitation can be side-stepped by probing a large number of dispersed NPs one-by-one and recording the emission from each NP separately. A large collection of NPs will likely contain subsets of like-NPs. The experimental approach is to disperse the NPs and hit an individual NP with a single massive cluster (e.g., C-60, Au-400). At impact energies of ~1 keV/atom, they generate notable secondary ion (SI) emission. Examination of small NPs (≤20 nm in diameter) shows that the SI emission is size-dependent and impacts are not all equivalent. Accurate identification of the type of impact is key for qualitative assays of core or outer shell composition. For quantitative assays, the concept of effective impacts is introduced. Selection of co-emitted ejecta combined with rejection (anticoincidence) of substrate ions allows refining chemical information within the projectile interaction volume. Last, to maximize the SI signal, small NPs (≤5 nm in diameter) can be examined in the transmission mode where the SI yields are enhanced ~10-fold over those in the (conventional) reflection direction. Future endeavors should focus on schemes acquiring SIs, electrons, and photons concurrently.
二次离子质谱法（SIMS）是表征纳米颗粒（NPs）的首选方法。对于具有大表面积与体积比的纳米颗粒，异质性是一个需要关注的问题。因此，分析应该针对单个纳米物体，而不是纳米颗粒的集合；然而，这可能很难做到或根本无法做到。通过逐一探测大量分散的纳米颗粒并分别记录每个纳米颗粒的发射，可以避开这一限制。大量的纳米颗粒集合可能包含同类纳米颗粒的子集。实验方法是将纳米颗粒分散，并用单个大质量团簇（例如，C-60、Au-400）撞击单个纳米颗粒。在约1 keV/原子的撞击能量下，它们会产生显著的二次离子（SI）发射。对小纳米颗粒（直径≤20 nm）的研究表明，二次离子发射与尺寸有关，而且撞击并非都等效。准确识别撞击类型是对核心或外壳成分进行定性分析的关键。对于定量分析，引入了有效撞击的概念。选择共同发射的喷射物并排除（反符合）基体离子，可以在射弹相互作用体积内提炼化学信息。最后，为了使二次离子信号最大化，可以在透射模式下检测小纳米颗粒（直径≤5 nm），在这种模式下，二次离子产率比（传统的）反射方向提高约10倍。未来的研究应集中在同时获取二次离子、电子和光子的方案上。
