(232d) pH Responsive Nanoparticle Films for Biofilm Microenvironment Evaluation

Bacterial infections from biofilms formed on medical devices (e.g. catheters or implants), if not detected sufficiently early, may result in fatal sepsis (Shirtliff and Leid, 2009). However, due to the local nature of such infections, detection using non-invasive sampling techniques can be difficult and the problem is often only recognized when the medical device malfunctions or a more systemic infection develops.

Therefore, an attractive solution is to functionalise the implant surface in such a way that an antimicrobial is self-trigger released in the presence of biofilm on the surface. Due to the dense nature of biofilms provided by their characteristic extracellular matrix, they can often have heterogeneous microenvironments, which differ drastically in their composition from their surroundings. One such difference can be the pH of the biofilm; regions of low pH may form even when biofilm is grown in buffered pH 7.4 medium (Schlafer et al. 2015). Drug delivery platforms using calcium carbonate and calcium phosphate nanoparticles, targeted to both tumours and biofilms, have been developed by exploiting their pH dependent dissolution (Dong et al. 2016). Such nanoparticles are stable at pHs around 7 found in healthy tissue but quickly dissolve at lower pHs.

An attractive class of nanoparticles for spectral monitoring are phosphorescent nanoparticles (nanophosphors). These are multicomponent materials consisting of a host matrix and rare-earth dopant which is responsible for the emitted radiation. By choosing the dopant appropriately the emission wavelength may be tuned as show by Sotiriou et al. (2011). Their optical and physical stability offer some attractive advantages over fluorescent dyes and semiconducting nanoparticles which often suffer from either photobleaching, blinking effects and/or toxicity.

Such nanophosphors can be made using flame spray pyrolysis (FSP) in a single-step and directly deposited in-situ onto substrates. This allows for the facile deposition of thin nanoparticle films which can subsequently be stabilised by in-situ flame annealing. Here, phosphorescent nanoparticles of calcium carbonate and calcium phosphate doped with europium were synthesised using FSP. Upon monitoring their luminescence in solution a clear decrease in luminescence intensity was observed at pH 4, which was attributed to the dissolution of the nanoparticles. The rate at which this luminescent signal decreases was observed to depend on the pH of the solution.

Moreover, in order to yield a ratiometric readout, yttrium oxide (Y₂O₃) which is stable in the pH ranges of interest (pH 4-7) was chosen as the host matrix for a reference nanophosphor system. The most intense peak of interest in the europium doped calcium phosphate and calcium carbonate systems occurs at approximately 620 nm, thus by doping Y₂O₃ with terbium which has an intense emission at 545 nm a reference emission independent of pH can be provided. Y₂O₃:Tb was first deposited onto a glass substrate followed bydeposition of calcium phosphate nanophosphors. The substrate was then placed in a custom flow cell and the effect of various buffer solutions assessed. Thereafter, biofilms of S. aureus and E. coli were grown directly on the substrates and the luminescence monitored to demonstrate the use of this system in detecting the formation of acidic microenvironments at the substrate-biofilm interface.

REFERENCES

Shirtliff, M.; Leid, J. (2009) The Role of Biofilms in Device-Related Infections, Springer.

Schlafer, S., et al. (2015) Appl. Environ. Microbiol. 81, 1267–1273.

Dong, Z. et al. (2016) Biomaterials, 110, 60–70.

Sotiriou, G. A. et al. (2011) J. Phys. Chem. C 115, 1084–1089.