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

自前工业时代以来，大气中的二氧化碳浓度显著增加，导致全球变暖。现在人们主要担心海洋对二氧化碳的吸收可能会饱和，导致全球变暖速度更快，后果比预期严重得多。了解海洋CO2系统对于气候变化模型至关重要，这些模型为我们的预测提供了信息，但由于技术挑战，海洋CO2测量，特别是溶解无机碳（DIC）的形式，严重缺乏。我们需要从世界各地的数千个地点进行定期测量，深度可达2公里。到目前为止，DIC的精确现场测量涉及大型和昂贵的表面仪器，例如红外吸收或质谱，并且它们的重复测量在所需的精度上是不可行的。该项目的目的是开发一种新的测量DIC的方法，该方法不仅准确，而且还可以被简化，以便在全球范围内部署浮子成为可能。目前，阿尔戈网络由分布在世界各大洋的约3 000个无系留电池驱动的浮标组成。它们自主运行，在公园1.5公里的深度漂流，每隔10天上升到水面，测量途中的温度和盐度深度分布。然后，这些数据被传输到卫星，循环重复。这两个参数可以在每个深度即时测量，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CO2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，测量的电导率变化可以转换为DIC浓度。然而，气体交换所需的时间妨碍了即时测量，但在Argo浮标循环中，有一个10天的停泊窗口，可以允许进行这种交换，并且有大量的样品。因此，我们的目标是简化实验室设置的每个功能单元，并将它们集成到单个微流控芯片实验室中，以满足Argo浮标集成所带来的严重尺寸，功率，成本和可靠性限制。这提出了一个巨大的挑战;到目前为止，微流体研究主要集中在具有完全不同标准的生物医学应用上，对想法和改进的基本海洋测试非常困难和昂贵，而技术挑战似乎是无法克服的。电导率测量在概念上相对简单，并且易于实现。然而，与光学技术相比，准确度要低得多，并且由于需要使用极小的样品体积（~100 nL）而加剧了这一点。深度分辨率取决于分别在浮子上升和停止时间内采集、存储和随后分析的样本数量。最终优选约100个样本，深度分辨率为20米。这需要在10 x 10 x 2 cm的设备中制造100个流体回路和至少100个阀。这种高分辨率通道图案化在通道之间的结合和样品泄漏方面产生了重大困难，这由于极端恶劣的环境、高压和长期部署而加剧。这种情况进一步受到需要将膜密封在多层结构内的挑战。最好的膜材料（气体可渗透和离子阻挡）是非常疏水的，并且抵抗与其他材料的粘合。最后，没有纳/微升阀门技术可以在压力变化高达200个大气压的环境中运行。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纳入多个微流体通道。

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.