In Situ pH Measurement of Water Droplets Using Flash-Freeze Surface-Enhanced Raman Spectroscopy
In Situ pH Measurement of Water Droplets Using Flash-Freeze Surface-Enhanced Raman Spectroscopy
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DOI：
10.1021/acs.estlett.2c00181
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发表时间：
2022-05-10
影响因子：
10.9
通讯作者：
Vikesland, Peter J.
中科院分区：
文献类型：
--
作者：
Huang, Qishen;Vikesland, Peter J.
为了更好地了解水滴和散装水化学成分的差异以及单个大气水滴的可变性，需要对单个水滴的酸碱度进行表征。在这项研究中，我们使用闪速冷冻表面增强拉曼光谱(SERS)作为一种手段来表征由低溶质浓度环境水样和pH纳米探针混合产生的微米级液滴的pH。这些样本包括弗吉尼亚州布莱克斯堡周围地区的溪水、雨水和积雪融化。与室温SERS相比，闪速冷冻SERS保留了初始液滴的性质，并最大限度地减少了由于SERS热点附近的局部热产生而造成的水分损失。此外，对于低溶质浓度的液滴，闪速冷冻SERS方法相对于pH纸提供了更高的灵敏度。我们观察到，闪速冷冻SERS测得的pH值与室温下测得的pH值大致相当，但不会导致加热诱导液滴失稳。使用闪速冷冻SERS，我们观察到大多数环境样品的液滴和总体pH是相当的。一个雨水样本显示，与其大样本相比，水滴的pH值更高。根据电感耦合等离子体质谱结果，该雨水的离子组成与其他收集的样品有显著差异，从而表明液滴组成对液滴pH的影响。
Characterization of single droplet pH is required to better understand differences in droplet and bulk water chemistries and the variability of individual atmospheric droplets. In this study, we demonstrate the use of flash-freeze surface-enhanced Raman spectroscopy (SERS) as a means to characterize the pH of micrometer-sized droplets generated from mixtures of low solute concentration environmental water samples and a pH nanoprobe. These samples included stream waters, rain, and snowmelt from areas surrounding Blacksburg, Virginia. Compared to room-temperature SERS, flash-freeze SERS preserves the initial droplet properties and minimizes water loss due to localized heat generation in the vicinity of SERS hot spots. Moreover, the flash-freeze SERS approach provides greater sensitivity relative to pH paper for low solute concentration droplets. We observed that pH values measured by flash-freeze SERS were generally comparable with those obtained at room temperature, but did not result in heating-induced droplet destabilization. Using flash-freeze SERS, we observed that droplet and bulk pH were comparable for most environmental samples. One rainwater sample showed a higher droplet pH compared to its bulk sample. Upon the basis of ICP-MS results, the ionic composition of this rainwater was significantly different from the other collected samples, thus suggesting the effect of droplet composition on droplet pH.