From Rough Diamonds to High Precision Low-Drift pH Sensors

In the pharmaceuticals industry, pH is a crucial factor for mediating chemical reactions, as it affects reaction yield and product quality. In the production of biological medicines, pH control is critical for ensuring the highest safety standards during manufacturing; thus, it requires accurate measurement of pH to reduce waste and ensure that end-products are of the correct composition and safe for consumption. Nanodiamonds can be functionalised to emit light due to fluorescence from the implanted nitrogen vacancies and other defects in their structures. As the intensity of the fluorescence has a strong dependence on the pH, nanodiamonds are ideal for pH monitoring. However, the detonation-based manufacturing methods produce non-uniform nanodiamonds, with varying size and agglomeration, thus they must be characterised prior to usage, using transmission electron microscopy (TEM), scanning electron microscopy (SEM) or atomic force microscopy for nanostructure, high-resolution TEM for size distribution and crystallinity, infrared spectroscopy for functional groups, and dynamic light scattering or centrifugal fractionation for size dispersion. Despite the useful photoluminescence properties of nanodiamonds, they have not been previously integrated into commercial devices, thus this study discusses the design and production of a nanodiamond-based pH sensor. The sensor is constructed using a functionalised glass slide with amine-hydroxyl bonds. Protocols have been designed to ensure consistent suspensions of nanodiamonds, and currently I have been developing nanodiamond immobilisation methods to allow photometric transduction. This work aims to ultimately develop a pH sensor with the accuracy of ±0.1 pH units, with negligible drift for one month of continuous use before replacement or recalibration.