利用AFM研究单个气溶胶颗粒

海洋会向大气释放巨量的气溶胶。它们选择性地吸收太阳辐射，参与云团的成核，进而影响全球的气候和环境。为了更好地理解气溶胶的作用，科学家需要有合适的工具，对单个气溶胶颗粒进行亚微米级别的表征。最基本的一点是，科学家需要知道这些气溶胶是以什么状态存在的，气态、液态、抑或是固态？

通过牛津仪器MFP3D原子力显微镜，爱荷华大学的科学家研究了不同种类的海洋气溶胶，其中包含无机盐颗粒、有机酸颗粒以及糖类颗粒。

他们将不同种类气溶胶沉积在硅片上，使用轻敲模式进行扫描。MFP3D原子力显微镜的高分辨率能够精确地测定气溶胶的形貌参数。测试在MFP3D配套的湿度样品腔中进行，可以在测试过程中保持已知、稳定的湿度环境。形貌测试之后，科学家通过AFM力曲线测量了颗粒的粘弹性性质，力曲线所提供的纳米力学信息可以用确定这些颗粒的物态。

AFM结果表明，颗粒的形貌和物态之间存在相关性。因此，仅仅通过形貌测试，就可以半定量地辨别颗粒的状态，这提供了一种快速鉴别气溶胶颗粒的新方法。

Measurements of individual sea spray aerosol particles showed correlations between viscoelastic properties, morphology, and phase state that could lead to quicker and simpler assessment of particle phase state.

Sea spray is one of many atmospheric aerosols that impact climate and the environment by absorbing solar radiation, nucleating clouds, and other mechanisms. To model these effects more accurately, tools capable of characterizing individual, sub-micrometer-sized aerosol particles are needed. In particular, techniques to assess a particle’s phase state—whether it is liquid, semisolid, or solid—would be valuable but remain rare.

Towards this goal, University of Iowa researchers examined several different components of sea spray aerosol including inorganic salts, organic acids, and saccharides with an AFM methodology. Nanomechanical measurements revealed correlations between a component’s viscoelastic response distance (VRD), relative indentation depth (RID), and phase state. Morphology imaging further showed that particle aspect ratio could be directly linked to VRD and RID, and thus to phase state.

The results therefore indicate that phase state can be semi-quantitatively evaluated by morphology imaging alone, without prior knowledge of chemical composition. This ability provides a new strategy for rapidly assessing aerosol components that may prove useful for a range of atmospheric studies.

Instrument used

MFP-3D

Techniques used

AFM measurements were made on aerosol particles deposited on silicon wafers. Specifically, components comprised inorganic salts (NaCl and MgSO4), organic acids (malonic, glutaric, azelaic, and palmitic acids), saccharides (glucose, sucrose, and raffinose), lipopolysaccharide from Escherichia coli, and inorganic–organic binary chemical mixtures (NaCl–malonic acid, NaCl–glucose, and MgSO4–glucose). Force curve measurements and tapping mode imaging of topography and phase were performed on the MFP-3D AFM. MFP-3Ds provide the highest performance and lowest-noise force measurements at a low price point. With the Humidity Sensing Cell, a constant relative humidity of ~20% was maintained throughout the experiments. 

Citation: K. Ray, H. Lee, M. Gutierrez et al., Correlating 3D morphology, phase state and viscoelastic properties of individual substrate-deposited particles. Anal. Chem. 91, 7621 (2019).

https://doi.org/10.1021/acs.analchem.9b00333

Note: The data shown here are reused under fair use from the original article, which can be accessed through the article link above.