(469c) Stability of Carbon Dots in Natural Waters: Implications for Sensing and the Environment

Carbon dots are a class of carbon-based spherical nanomaterial with emission properties in the visible range. Their lower cost, comparable stability and biocompatibility make them an excellent material for environmental sensors. Photoluminiscent nanomaterials are of interest for environmental sensing due to their high chemical and photo stability. Quantum dots have been extensively explored for nanosensor fabrication and present several desirable properties: high quantum yield, ease of fabrication and stability. However, their toxicity and potential impact to environment and biological systems have raised concerns and limited its application.

As their field of use grows and more C-dots are being manufactured, the probability of them reaching the environment as a contaminant also increases. The objective of this work is to assess the behavior of C-dots in natural waters, in order to estimate their implications on the environment and potential limitation to their use in sensing applications. C-dots are known as hydrophilic, very stable nanoparticles. However, when suspended in complex water matrices, i.e. natural waters, nanoparticle stability may be compromised by changes in ionic strength, dissolved organic matter, or natural colloidal particles such as clay or mineral particles.

The size and surface charge of the C-dots were measured as a function of pH and ionic strength given by CaCl₂ and NaCl, as models of divalent and monovalent cations in water. Another important water component is natural organic matter (NOM), and humic acid (Aldrich) was used to mimic its presence. In order to test the stability of the C-dots, both size distribution and zeta potential measurements were conducted, in a variety of water chemistries. The results were modeled by the Derjaguin and Landau, Verwey and Overbeek (DLVO) theory of colloidal stability. Aggregation was also observed to influence their photoluminescence properties, and therefore may limit their use in sensing applications. In order to understand the relationship between stability and fluorescence, C-dots fluoresce was investigated in different water matrixes as well as in the presence of dinitro toluene (DNT), that is known to produce emission quenching.  In the presence of DNT, the fluorescence was observed to decrease over time thus resulting in the hypothesized quenching effect, which is concentration dependent.

C-dots showed improved stability compared to other nanomaterials with similar sizes but different chemical composition. However, increasing ionic strengths (up to 100 mM) caused significant aggregation which could negatively impact its use as signaling particle for sensors and medical imaging as well as modify the environmental implications of their use, by inducing changes in their fate and transport in natural systems.