Point of Care Diagnostic Ammonia Sensing by Responsive Silver Nanoparticles

Ammonia is produced and consumed in the human body as a normal part of homeostasis, with the urease activity of intestinal bacteria thought to contribute significantly to blood ammonia levels. The ammonia found in blood is largely dominated by the ionised (NH₄⁺) form although to a lesser extent the unionised NH₃ form can be found. Here ammonia levels are used to refer to the total ammonia content of both ionised and unionised ammonia.<br/>The liver plays a critical role in maintaining blood ammonia levels below the threshold considered to be hyperammonaemia (&gt;50 µM) in adult humans. Elevated blood ammonia levels can give rise to diffuse symptoms ranging from minor cognitive or behavioural changes to coma. In well-equipped hospital diagnostic labs ammonia levels are routinely analysed by enzymatic assays, however, due to the specialised equipment and cold-chain requirements their application in resource limited settings can be challenging. An alternative analytical method for determination of ammonia levels is the Berthelot reaction and its subsequent derivatives. This relies on the reaction of ammonia with the hypochlorite ion to cause conversion of phenol to the strongly blue coloured indophenol. A similar reaction scheme is exploited in this study, whereby the reaction of the hypochlorite ion with silver nanoparticles is exploited to derive ammonia sensitivity. Silver nanoparticles are produced by flame spray pyrolysis as either a powder or deposited on porous glass-fibre filter paper. The addition of hypochlorite to the silver nanoparticles causes their oxidative dissolution and therefore a strong decrease in the plasmonic absorbance. However, in the presence of ammonia the hypochlorite can be neutralised preserving the plasmonic properties of the silver nanoparticles and therefore also their colour. A thin silica coating was applied to the silver nanoparticles and optimised to provide stable ammonia determination in isotonic solutions down to physiologically relevant ammonia concentrations. Measurements were also demonstrated from simulated serum by applying a perfluorosulfonic acid polymer (Nafion™), which displays selective transport of small positively charged species. Furthermore, the silver nanoparticles directly deposited onto filter paper were used for ammonia concentration determination in a paper-based point of care naked eye format. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (ERC Grant agreement # 758705). Funding from the Karolinska Institutet, the Swedish Research Council (2021-05494), and the Swedish Foundation for Strategic Research (FFL18-0043) is kindly acknowledged.