Different electrically charged proteins result in diverse transport behaviors of plastic particles with different surface charge in quartz sand

The influence of proteins on the transport and deposition behaviors of microplastics (MPs) in quartz sand was examined at both low (5 mM) and high ionic strength (25 mM) in NaCl solutions at pH 6. Carboxylate- and amine-modified polystyrene latex microspheres with size of 200 nm were employed as negatively (CMPs) and positively surface charged MPs (AMPs), respectively, while bovine serum albumin (BSA) and bovine trypsin were utilized as representative negatively and positively charged proteins, respectively. The results showed that for two examined protein concentrations (both 1 and 10 mg/L TOC) under both ionic strength conditions, the presence of BSA increased the transport of both CMPs and AMPs, while the presence of trypsin decreased the transport of CMPs yet increased the transport of AMPs in porous media. The mechanisms driving to the changed transport of MPs induced by two types of proteins were found to be different. Particularly, steric interaction induced by BSA corona adsorbed onto CMPs surface as well as the repel effects resulted from BSA suspending in solutions were found to contribute to the enhanced CMPs transport with BSA copresent in suspensions. The increased sizes and the decreased electrostatic repulsion of CMPs due to the adsorption of trypsin onto CMPs, together with the addition of extra deposition sites due to the adsorption of trypsin onto quartz sand drove to the decreased CMPs transport with trypsin copresent in suspensions. The increased electrostatic repulsion due to the adsorption of BSA onto AMPs surfaces caused the enhanced AMPs transport with BSA in solutions. While, the decreased electrostatic attraction of AMPs due to the adsorption of trypsin onto AMPs, as well as the competition of deposition sites due to the adsorption of trypsin onto quartz sand contributed to the increased AMPs transport with trypsin copresent in suspensions. The results showed that the presence of different types of proteins would induce different transport behaviors of microplastics with different surface charge in porous media. Since proteins are widely present in aquatic systems, to more accurately predict the fate and transport of MPs in natural environments, the effects and mechanisms of proteins on the transport of MPs should be considered.

Aggregation of oxidized multi-walled carbon nanotubes: Interplay of nanomaterial surface O-functional groups and solution chemistry factors

The fast-growing production and application of carbon nanotube (CNT) materials in a variety of industrial products inevitably lead to their release to wastewater and surface water. CNT would experience oxidization in wastewater treatment plant due to the presence of large amount of disinfectants, such as H₂O₂ and O₃, which in turn affects the environmental fates and risks of CNT. In this study, oxidized CNT materials (O-CNTs) were prepared by treating CNT with H₂O₂/UV and O₃ (denoting as H₂O₂-CNT and O₃-CNT, respectively). A variety of characterizations indicated that oxygen containing groups were generated on CNT surface upon the oxidation, and the O/C ratio increased in the order of pristine CNT < H₂O₂-CNT < O₃-CNT. In the presence of Na⁺, K⁺ and Mg²⁺, the O-CNTs displayed better colloidal stability than the pristine CNT, and the stability increased with the oxidation degree (indicated by O/C ratio). This could be explained by the more negative surface charge and stronger hydrophilicity of the O-CNTs. Unexpectedly, in the presence of Ca²⁺, the most oxidized O₃-CNT exhibited the poorest colloidal stability. The abundant carboxyl groups in O₃-CNT provided effective binding sites for cation bridging effect through Ca²⁺ and led to stronger aggregation. Increasing pH was more favorable to disperse CNTs (both O-CNT and pristine CNT) in the presence of Na⁺, but much less effective in inhibiting the aggregation of O₃-CNT in presence of Ca²⁺. This could be explained by the stronger cation bridging effect due to enhanced deprotonation the -COOH groups at higher pH conditions. The calculated Hamaker constants of the CNTs decreased with the oxidation degree, implying that there was lower van der Waals force between the O-CNTs. The Derjaguin-Landau-Verwey-Overbeek (DLVO) calculation confirmed that O-CNTs had to overcome higher energy barrier and thus showed better colloidal stability than the pristine CNT in the presence of Na⁺.

Nonspecific Colloidal-Type Interaction Explains Size-Dependent Specific Binding of Membrane-Targeted Nanoparticles

Emerging biomedical applications such as molecular imaging and drug delivery often require directed binding of nanoparticles to cell-membrane receptors. The specific apparent affinity of such ligand-functionalized particles is size-dependent, an observation so far solely attributed to multivalent receptor-ligand interaction. We question the universality of this explanation by demonstrating that the binding kinetics also depends on weak, attractive colloidal-type interaction between nanoparticles and a lipid membrane. Applying label-free single-particle imaging, we correlate binding of nanoparticles targeted to a cell-mimetic lipid membrane with the distribution of nontargeted particles freely diffusing close to the membrane interface. This analysis shows that already a weak, k_BT-scale attraction present between 50 nm gold nanoparticles and the membrane renders these particles an order of magnitude higher avidity compared to 20 nm particles. A stronger emphasis on nonspecific particle-membrane interaction might thus be required to accurately predict nanoparticle targeting and other similar processes such as cellular uptake of exosomes and viruses.