Optical Sensing of pH and O₂ in the Evaluation of Bioactive Self-Healing Cement

Abstract

Leakage from cementitious structures with a retaining function can have devastating environmental consequences. Leaks can originate from cracks within the hardened cementitious material that is supposed to seal the structure off from the surrounding environment. Bioactive self-healing concretes containing bacteria capable of microbially inducing CaCO3 precipitation have been suggested to mitigate the healing of such cracks before leaking occurs. An important parameter determining the biocompatibility of concretes and cements is the pH environment. Therefore, a novel ratiometric pH optode imaging system based on an inexpensive single-lens reflex (SLR) camera was used to characterize the pH of porewater within cracks of submerged hydrated oil and gas well cement. This enabled the imaging of pH with a spatial distribution in high resolution (50 μm per pixel) and a gradient of 1.4 pH units per 1 mm. The effect of fly ash substitution and hydration time on the pH of the cement surface was evaluated by this approach. The results show that pH is significantly reduced from pH >11 to below 10 with increasing fly ash content as well as hydration time. The assessment of bioactivity in the cement was evaluated by introducing superabsorbent polymers with encapsulated Bacillus alkalinitrilicus endospores into the cracks. The bacterial activity was measured using oxygen optodes, which showed the highest bacterial activity with increasing amounts of fly ash substitution in the cement, correlating with the decrease in the pH. Overall, our results demonstrate that the pH of well cements can be reliably measured and modified to sustain the microbial activity.

Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications

“… 1249 Also an aza-BODIPY pH indicator was used in a ratiometric approach using a single-lens reflex camera instead of a gated CCD camera. 1250 The system was used to characterize the pH of porewater within cracks of self-healing concrete materials. It enabled imaging of pH with a high resolution (50 μm per pixel) and a gradient of 1.4 pH units per 1 mm.…”

Hydrogel-Based Optical Ion Sensors: Principles and Challenges for Point-of-Care Testing and Environmental Monitoring

“…47 Some fluoroionophores are also similar to the well-known fluorescent indicators such as Fura-2 for Ca 2+ and SBFI for Na + except that they are typically hydrophobic. 48,49 The groups of Wolfbeis, 50−54 Borisov, 55−59 Klimant, 60,61 and Koren 62,63 have reported many ion optodes by incorporating fluoroionophores into hydrogels. These thin-film optodes exhibited reversible responses and are well suited for continuous monitoring and 2D imaging of environmental or clinical samples.…”

Optode Based Chemical Imaging—Possibilities, Challenges, and New Avenues in Multidimensional Optical Sensing

“…The group of Wolfbeis demonstrated that POs can be valuable tools in medical research, in particular to find infections by imaging the chemical composition of wounds. , Others used POs in material research enabling, e.g. , pH monitoring of concrete , to find areas where the chemical composition is altered. Also in soil sciences, researchers now use POs to visualize microenvironments and to follow the distribution of important analytes , linking the chemical microenvironments to nutrient uptake and nutrient fluxes.…”