Development and application of a ratiometric nanosensor for measuring pH inside the gastrointestinal tract of zooplankton

Abstract

The pH within the gastrointestinal (GI) tract of zooplankton regulates the bioavailability of nutrients and inorganic toxins (metals, nanoparticles) and the breakdown of ingested food. However, measuring the spatial distribution of pH in the GI tract of zooplankton is challenging because the size of the GI tract is extremely small and its micro-environment is complex. Here, we developed a silica-doped ratiometric nanosensor capable of mapping pH in the whole GI tracts of zooplankton. The nanosensors, 79.0 ± 25.0 nm in diameter, were prepared by conjugating pH sensitive dye – fluorescein 5-isothiocyanate and pH insensitive dye (as a reference) – Rhodamine B, to an inert silica nanoparticle. The new nanosensors have a pH sensing range from 5 to 8 which is suitable for determining the pH of the gastrointestinal tract of microorganisms. To demonstrate the application of the nanosensors, we fed them to a model organism Daphnia magna neonates to map the pH inside their GI tracts. The results show that the pH was 5.5–6.0 at the anterior section of the GI tract and up to 7.2 in the posterior section. Overall, the pH within the D. magna GI tract was significantly lower than the surrounding aqueous medium (pH = 7.8). This indicates that D. magna neonates are able to regulate their internal pH. Such a low pH in the anterior part of the GI tract of D. magna can increase the solubility of nutrients (e.g., iron oxides) or toxins (e.g., silver nanoparticles) by orders of magnitude, providing a potentially important source of soluble nutrients and/or toxins in surface waters.