Bundesanstalt für Materialforschung und -prüfung (BAM), Richard-Willstätter-Straße 11, 12489 Berlin, Germany.
Anal Methods. 2020 Nov 14;12(42):5046-5054. doi: 10.1039/d0ay01504k. Epub 2020 Oct 9.
Raman spectroscopy is becoming a commonly used, powerful tool for structural elucidation and species identification of small liquid samples, e.g. in droplet-based digital microfluidic devices. Due to the low scattering cross sections and the temporal restrictions dictated by the droplet flow, however, it depends on amplification strategies which often come at a cost. In the case of surface-enhanced Raman scattering (SERS), this can be an enhanced susceptibility towards memory effects and cross talk, whereas resonant and/or stimulated Raman techniques require higher instrumental sophistication, such as tunable lasers or the high electromagnetic field strengths which are typically provided by femtosecond lasers. Here, an alternative instrumental approach is discussed, in which stimulated Raman scattering (SRS) is achieved using the single fixed wavelength output of an inexpensive diode-pumped solid-state (DPSS) nanosecond laser. The required field strengths are realized by an effective light trapping in a resonator mode inside the interrogated droplets, while the resonant light required for the stimulation is provided by the fluorescence signal of an admixed laser dye. To elucidate the underlying optical processes, proof-of-concept experiments are conducted on acoustically levitated droplets, mimicking a highly reproducible and stable digital fluidic system. By using isotope-labeled compounds, the assignment of the emitted radiation as Raman scattering is firmly corroborated. A direct comparison reveals an amplification of the usually weak spontaneous Stokes emission by up to five orders of magnitude. Further investigation of the optical power dependence reveals the resulting gain to depend on the intensity of both, the input laser fluence and the concentration of the admixed fluorophore, leaving SRS as the only feasible amplification mechanism. While in this study stable large droplets have been studied, the underlying principles also hold true for smaller droplets, in which case significantly lower laser pulse energy is required. Since DPSS lasers are readily available with high repetition rates, the presented detection strategy bears a huge potential for fast online identification and characterization routines in digital microfluidic devices.
拉曼光谱技术正成为一种常用于对小体积液体样本进行结构解析和物种鉴定的强大工具，例如在基于液滴的数字微流控设备中。然而，由于散射截面较低，以及液滴流动所带来的时间限制，该技术依赖于放大策略，而这些策略往往需要付出代价。在表面增强拉曼散射（SERS）的情况下，这可能会导致对记忆效应和串扰的敏感性增强，而共振和/或受激拉曼技术则需要更高的仪器复杂性，例如可调谐激光器或通常由飞秒激光器提供的高强度电磁场。本文讨论了一种替代的仪器方法，其中使用廉价的二极管泵浦固态（DPSS）纳秒激光器的单一固定波长输出来实现受激拉曼散射（SRS）。所需的场强是通过在被检测液滴内的谐振器模式中有效捕获光来实现的，而用于刺激的共振光则由混合激光染料的荧光信号提供。为了阐明潜在的光学过程，在模拟高度重现性和稳定性的数字流体系统的声悬浮液滴上进行了概念验证实验。通过使用同位素标记的化合物，明确地证实了所发射辐射是拉曼散射。直接比较表明，通常较弱的自发斯托克斯发射的放大倍数高达五个数量级。进一步研究光功率依赖性表明，所得增益取决于输入激光通量和混合荧光染料浓度的强度，从而使 SRS 成为唯一可行的放大机制。虽然在这项研究中研究了稳定的大液滴，但基本原理也适用于较小的液滴，在这种情况下，所需的激光脉冲能量显著降低。由于 DPSS 激光器具有高重复率，因此所提出的检测策略在数字微流控设备中的快速在线识别和特征分析方面具有巨大潜力。
