EPSRC-SFI: An ocean microlab for autonomous dissolved inorganic carbon depth profile measurement

自前工业化时代以来，大气中的二氧化碳浓度显著增加，导致全球变暖。现在一个主要的担忧是，海洋对二氧化碳的吸收可能会达到饱和，导致全球变暖的速度更快，后果比预期的要严重得多。了解海洋二氧化碳系统对于气候变化模型至关重要，气候变化模型为我们的预测提供了信息，但由于技术挑战，海洋测量二氧化碳，特别是溶解无机碳（DIC）形式的二氧化碳，严重缺乏。我们需要在全球数千个地点进行定期测量，深度可达2公里。到目前为止，精确的DIC现场测量涉及大型和昂贵的地面仪器，例如红外吸收或质谱，并且它们的小型化在所需的精度下是不可行的。该项目的目的是开发一种精确测量DIC的新方法，但也可以小型化，从而使全球浮动部署成为可能。目前，Argo网络由约3000个无系绳电池驱动的浮标组成，分布在世界各地的海洋上。它们可以自主操作，在公园1.5公里的深度漂流，每10天上升到水面，在途中测量温度和盐度深度剖面。这些数据随后被传送到卫星上，如此循环往复。这两个参数可以在每个深度瞬间测量，而DIC量化需要耗时的化学分析。在实验室中，标准校准技术将DIC从海水中分离为二氧化碳气体，然后通过膜转移到试剂（NaOH）中，导致电导率降低。通过适当的设计和校准，可以将测量到的电导率变化转换为DIC浓度。然而，气体交换所需的时间阻止了即时测量，但在Argo浮子循环中，有一个10天的停留窗口，可以允许进行这种交换，并且有大量的样品。因此，我们的目标是将实验室设置的每个功能单元小型化，并将它们集成到一个芯片上的微流体实验室中，以满足Argo浮子集成所施加的严格的尺寸，功率，成本和可靠性限制。这是一个巨大的挑战；到目前为止，微流体研究主要集中在生物医学应用上，这有一套完全不同的标准，对想法和改进进行必要的海洋测试非常困难和昂贵，而技术挑战似乎无法克服。电导率测量在概念上相对简单，易于小型化。然而，与光学技术相比，精度要低得多，并且由于需要使用极小的样品体积（~100 nL）而加剧了这一点。深度分辨率取决于在浮子上升和停留时间内收集、存储和随后分析的样品数量。最终优选为~100个样本，深度分辨率为20m。这需要100个流体回路和至少100个阀门被制造在一个10 x 10 x 2厘米的装置。这种高分辨率的通道模式在通道之间的粘接和样品泄漏方面造成了很大的困难，极端恶劣的环境、高压和长期部署加剧了这种困难。由于需要在多层结构中密封膜，这种情况进一步受到挑战。最好的膜材料（透气性和离子阻隔性）是非常疏水的，并且抵抗与其他材料结合。最后，目前还没有能够在压力变化高达200个大气压的环境中运行的纳米/微升阀门技术。迄今为止，大多数有限的研究都集中在气动阀上。在这个项目中，我们需要发现和开发新的刺激响应阀材料，并找到一种方法将这些材料结合到多个微流体通道中。

CO2 concentration in the atmosphere has increased significantly since pre-industrial times leading to global warming. There is now a major concern that ocean absorption of CO2 may be saturating, leading to more rapid global warming and much more serious consequences than predicted. Understanding the ocean CO2 system is of fundamental importance for climate change models that inform our predictions but ocean measurement of CO2, particularly in the form of dissolved inorganic carbon (DIC), is severely lacking due to technical challenges. We need regular measurements, down to a depth of 2 km, from thousands of locations world-wide. Accurate field measurements of DIC up to now have involved large and expensive surface instruments, e.g. infra-red absorption or mass spectrometry, and their miniaturisation is not feasible at the required accuracy. The aim of this project is to develop a new method of measuring DIC that is accurate, but which can also be miniaturised so that worldwide float deployment becomes a possibility. At present, the Argo network consists of ~3000 untethered battery-operated floats located across the world's oceans. They operate autonomously, drifting at a park depth of 1.5 km and every 10 days they rise to the surface, measuring the temperature and salinity depth profiles on the way. This data is then transmitted to satellite and the cycle repeats. These two parameters can be measured instantaneously at each depth whereas DIC quantification requires time-consuming chemical analysis. In the laboratory, the standard calibration technique separates DIC from seawater as CO2 gas which then transfers across a membrane into a reagent (NaOH), resulting in a decrease in conductivity. With appropriate design and calibration, the measured change in conductivity can be converted to DIC concentration. The time required for gas exchange however prevents instantaneous measurement but with the Argo float cycle, there is a 10-day park window where this exchange could be allowed to occur, and with a large number of samples. Our objectives therefore are to miniaturise each of the functional units of the laboratory setup and integrate them into a single microfluidic lab on chip which can meet the severe size, power, cost and reliability limits imposed by the Argo float integration. This presents an immense challenge; microfluidics research up to now has focussed mainly on biomedical applications which have an entirely different set of criteria, essential ocean testing of ideas and refinements is very difficult and expensive, while technical challenges can appear insurmountable. Conductivity measurement is relatively simple in concept and is readily miniaturised. However, the accuracy is much lower compared to optical techniques and this is exacerbated by the need to use extremely small sample volumes, (~100 nL). The depth resolution depends on the number of samples collected, stored, and subsequently analysed within float rise and park times respectively. The ultimate preference is ~100 samples, giving a depth resolution of 20m. This requires 100 fluid circuits and at least 100 valves to be fabricated in a 10 x 10 x 2 cm device. Such high-resolution channel patterning creates major difficulties with regards to bonding and sample leakage between channels, exacerbated by the extremely harsh environment, high pressure and the long-term deployment. This situation is further challenged by the need to seal a membrane within a multilayer structure. The best membrane materials (gas permeable and ion blocking) are very hydrophobic and resist bonding to other materials. Finally, there is no nano/micolitre valve technology that could operate in an environment where pressures vary up to 200 atmospheres. Most of the limited research to date has focussed on pneumatic valves. In this project we need to discover and develop new stimuli responsive valve materials and find a way to incorporate these into multiple microfluidic channels.