Fiber Refractometers for in situ detection of Aquatic Methane – FRAME

As a response to the critical need for a better comprehension of the CH4 cycle, the 2nd greenhouse gas (GHG) after CO2, in aquatic environments, FRAME aims to design an innovative fiber refractometric sensor system multiplexing two different technologies based on (1) multimode (MMFs), and (2) photonic crystal fibers (PCFs). A cutting-edge performance targeting a resolution of 5 nmol/L dissolved CH4 over a range from 10 nmol/L to 100 µmol/L with a relatively fast response time of less than 2 minutes in the natural environment is envisaged by harnessing the combined performance of MMF and PCF-based refractometers, designed with the aid of AI and innovative accelerated numerical methods.

To cover the four decades in dynamic range, the proposed sensor will operate over three specific refractive index (RI) zones due to three intervening optical mechanisms: (1) Zone 1: evanescent waves (EWs), to detect the extreme lowest limit of CH4 concentration, (2) Zone 2: combined EWs and critical mode loss, for trace CH4 levels from sub-nmol/L to 2000 nmol/L, and (3) Zone 3: external total internal or Fresnel reflections (external TIR), for CH4 concentration beyond 2000 nmol/L. To selectively detect CH4, the sensing fibers from the MMF and PCF refractometers will incorporate cryptophane-sensitized sensing films whose bulk RI varies proportionally as a function of CH4 concentration that can then be detected as an optical power variation. A novelty PCF will specifically be designed using rapid AI-aided numerical tools to enhance external TIR to significantly extend the PCF’s Zone 3 range (to 100 µmol/L) while simultaneously achieving 5 nmol/L resolution. A differential configuration, coupled to synchronous detection, will be implemented to reduce common-mode noises originating from (1) optical power variations, (2) temperature and pressure variations, and (3) mechanical vibrations (target: <5 nmol/L noise floor), thereby enhancing the signal to noise ratio and sensitivity, as well as significantly improving their response, repeatability, and stability. In the PCF sensor, an innovative dual core microstructure will be designed to provide intrinsic differential sensing capability.

Another major novelty is the interchangeability of the sensor probe encapsulating the sensing fibers which will be designed to be easily replaceable to mitigate wear and tear, and damage to the sensing elements and for sensor reset (e.g. during maintenance). It will also incorporate a UV-controlled anti-biofouling system to periodically irradiate and decompose bio-organic substance built-up on the sensitized film’s surface.

Upon successful implementation, FRAME envisions to contribute to current and future detection and monitoring efforts of CH4 as a major GHG as part of national environmental strategies and of the UN’s 13th Sustainable Development Goal. The high-level outputs (TRL4-5) envisaged in FRAME are expected to be transferable to industry while simultaneously benefitting the project Partners in cutting-edge transdisciplinary research in environmental sensor development. The results exploitation and dissemination strategy proposed is further expected to lead to future collaborations with external collaborators.

Finally, the versatile sensing nature of the proposed sensor can also open future pathways to detecting other gas species and, more importantly, complex molecules present at low concentrations in water known as aquatic pollutants (wastewater, endocrine-disrupting compounds, pesticides, insecticides, etc) by using molecularly imprinted polymer technologies. Identifying, monitoring and locating these toxicological compounds can thus have a strong impact to empower future water security within the broader sustainable development context.