Uptake kinetics of silver nanoparticles by plant: relative importance of particles and dissolved ions.

Quantifying the relative bioavailability of particles versus ions is a key step toward understanding the mechanisms of bioaccumulation and toxicity of silver nanoparticles (AgNPs). Here we investigated AgNP uptake kinetics by wheat Triticum aestivum L. at different ages (i.e. 15- and 30-day after germination) in hydroponics. The concentration-dependent accumulation of AgNPs under the experimental conditions, in which AgNP dissolution in bulk suspension and at the biological interface was ruled out, confirmed the direct uptake of particles. This was further validated by the detection of Ag-containing particles within plants by single particle inductively coupled plasma mass spectrometry. Plants differentiated particles and dissolved ions, with uptake rate constants of particles and dissolved ions 4.3 ± 0.6 to 5.2 ± 0.6 and 288.6 ± 13.4 to 450.6 ± 63.4 L kg-1 h-1, respectively. Plant age appeared to have a negligible influence on uptake rate constant ratio of particles to ions. As a result, the relative contribution of particulate uptake to overall AgNP accumulation varied as a function of AgNP dissolution in nature. Particulate uptake was dominant when the dissolution was less than 1.2%. These findings help clearing up the inconsistency of uptake mechanisms of AgNPs in literature and could be used to predict their environmental impact and significance.