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Increasing the production of renewable energy will be critical to achieving global sustainability goals in the coming decades. Biofuels derived from microalgae have great potential to contribute to this production. However, cultivating algae with sufficient neutral lipid content, while maintaining high growth rates, is a continual challenge in making algal-derived biofuels a reality. Previous work has shown that exposure to polymer-functionalized carbon dots can increase the lipid content of the microalgaeRaphidocelis subcapitata. This study investigates this finding, aiming to determine the mechanisms underlying this effect and if altering nanoparticle surface charge mediates the mechanism of action of the carbon dots used. Carbon dots with both negative and positive surface charges were added to microalgal cultures, and the impacts of this exposure were analyzed using high-content imaging, growth measurements, and chlorophyll content measurements. Results indicate that positively charged carbon dots induce a nano-specific increase in lipid content but also cause decreases in growth. Additionally, the mechanism of action of each nanoparticle was examined by conducting a morphological comparison to treatments with known mechanisms of action. This analysis showed that negatively charged carbon dots cause similar impacts toR. subcapitataas nitrogen deprivation. Nitrogen deprivation is known to increase lipid content in microalgae. The findings of this study suggest that carbon dots may have surface charge dependent effects on the lipid metabolism ofR. subcapitata. Future work should consider the use of carbon dots with varied surface charge densities for enhancing algae biofuel production in bioreactors. 

Global food security is a pressing issue in our society. Maintaining food security in coming years will require improving crop yield, as well as increased resiliency to abiotic and biotic stress. Nanoscale materials have increasingly been proposed as a tool which could be used to meet these challenges. However, much research is needed to optimize nanoparticle design and crop application for this to become a reality. In this study, we investigated the impact of polymer-functionalized carbon dots on tomatoes (Solanum lycopersicum L.). Tomato seeds were vacuum infiltrated with carbon dots and then grown for 3 weeks before collection of phenotypic and transcriptomic data. No changes to fresh biomass or chlorophyll content were observed, indicating that these particles can be applied without overt harm to the plant at early growth stages. In addition, changes in gene expression suggest that polymer-functionalized carbon dots can initiate the expression of biochemical pathways associated with a pathogen resistance response in tomato plants. Specifically, genes involved in ethylene signaling, ethylene production, and camalexin synthesis were upregulated. These findings suggest that seed priming with carbon dots may improve plant tolerance to biotic stress by modulating ethylene signaling pathways. Carbon dots could also be loaded with nutrients or other agrochemicals to create a multifunctional platform. Future work should focus on understanding the mechanisms by which nanoparticles can modulate ethylene signaling, enabling use of this knowledge to develop sustainable and effective nanoparticles for agricultural applications. 

Nitrogen fertilizer delivery inefficiencies limit crop productivity and contribute to environmental pollution. Herein, we developed Zn- and Fe-doped hydroxyapatite nanomaterials (ZnHAU, FeHAU) loaded with urea (∼26% N) through hydrogen bonding and metal-ligand interactions. The nanomaterials attach to the leaf epidermal cuticle and localize in the apoplast of leaf epidermal cells, triggering a slow N release at acidic conditions (pH 5.8) that promote wheat (Triticum aestivum) growth and increased N uptake compared to conventional urea fertilizers. ZnHAU and FeHAU exhibited prolonged N release compared to urea in model plant apoplast fluid pH in vitro (up to 2 days) and in leaf membranes in plants (up to 10 days) with a high N retention (32% to 53%) under simulated high rainfall events (50 mm). Foliar N delivery doses of up to 4% as ZnHAU and FeHAU did not induce toxicity in plant cells. The foliar-applied ZnHAU and FeHAU enhanced fresh and dry biomass by ∼214% and ∼161%, and N uptake by ∼108% compared to foliar-applied urea under low soil N conditions in greenhouse experiments. Controlled N release by leaf-attached nanomaterials improves N delivery and use efficiency in crop plants, creating nanofertilizers with reduced environmental impact. 

Dynamic optical modulation in response to stimuli provides exciting opportunities for designing novel sensing, actuating, and authentication devices. Here, we demonstrate that the reversible swelling and deswelling of crosslinked polymer colloidal spheres in response to pH and temperature changes can be utilized to drive the assembly and disassembly of the embedded gold nanoparticles (AuNPs), inducing their plasmonic coupling and decoupling and, correspondingly, color changes. The multi‐responsive colloids are created by depositing a monolayer of AuNPs on the surface of resorcinol‐formaldehyde (RF) nanospheres, then overcoating them with an additional RF layer, followed by a seeded growth process to enlarge the AuNPs and reduce their interparticle separation to induce significant plasmonic coupling. This configuration facilitates dynamic modulation of plasmonic coupling through the reversible swelling/deswelling of the polymer spheres in response to pH and temperature changes. The rapid and repeatable transitions between coupled and decoupled plasmonic states of AuNPs enable reversible color switching when the polymer spheres are in colloidal form or embedded in hydrogel substrates. Furthermore, leveraging the photothermal effect and stimuli‐responsive plasmonic coupling of the embedded AuNPs enables the construction of hybrid hydrogel films featuring switchable anticounterfeiting patterns, showcasing the versatility and potential of this multi‐stimuli‐responsive plasmonic system. 

Surface charge is a key characteristic of nanoparticles which has great potential to impact the interactions of nanoparticles and biological systems. Understanding the role charge plays in these interactions is key to determining the ecological risks of nanoparticle exposure and informing sustainable nanoparticle design. In this study, the model freshwater algae Raphidocelis subcapitata was exposed to carbon dots (CDs) functionalized with polymers to have positive, negative, or neutral surface charges to examine the impact of nanoparticle surface charge on nano-algae interactions. Traditional toxicological endpoints of survival and growth inhibition were measured. Additionally, morphological impacts on whole cells, individual organelles, and cellular components were quantified using high-content fluorescence microscopy, demonstrating one of the first uses of high-content imaging in microalgae. Results indicate that PEI functionalized, positively charged CDs are most toxic to green algae (EC50 42.306 μg/L), but that CDs with negative charge induce sublethal impacts on algae. PEI-CD toxicity is hypothesized to be related to electrostatic interactions between CDs and the algal cell wall, which lead to significant cell aggregation. Interestingly, morphological data suggests that exposure to both positively and negatively charged CDs leads to increased neutral lipid droplet formation, a possible indicator of nutrient stress. Further investigation of the mechanisms underlying impacts of nanoparticle surface charge on algae biology can lead to more sustainable nanoparticle design and environmental protections. 

A lack of mechanistic understanding of nanomaterial interactions with plants and algae cell walls limits the advancement of nanotechnology-based tools for sustainable agriculture. We systematically investigated the influence of nanoparticle charge on the interactions with model cell wall surfaces built with cellulose or pectin and performed a comparative analysis with native cell walls of Arabidopsis plants and green algae (Choleochaete). The high affinity of positively charged carbon dots (CDs) (46.0 ± 3.3 mV, 4.3 ± 1.5 nm) to both model and native cell walls was dominated by the strong ionic bonding between the surface amine groups of CDs and the carboxyl groups of pectin. In contrast, these CDs formed weaker hydrogen bonding with the hydroxyl groups of cellulose model surfaces. The CDs of similar size with negative (−46.2 ± 1.1 mV, 6.6 ± 3.8 nm) or neutral (−8.6 ± 1.3 mV, 4.3 ± 1.9 nm) ζ-potentials exhibited negligible interactions with cell walls. Real-time monitoring of CD interactions with model pectin cell walls indicated higher absorption efficiency (3.4 ± 1.3 10−9) and acoustic mass density (313.3 ± 63.3 ng cm–2) for the positively charged CDs than negative and neutral counterparts (p < 0.001 and p < 0.01, respectively). The surface charge density of the positively charged CDs significantly enhanced these electrostatic interactions with cell walls, pointing to approaches to control nanoparticle binding to plant biosurfaces. Ca2+-induced cross-linking of pectin affected the initial absorption efficiency of the positively charged CD on cell wall surfaces (∼3.75 times lower) but not the accumulation of the nanoparticles on cell wall surfaces. This study developed model biosurfaces for elucidating fundamental interactions of nanomaterials with cell walls, a main barrier for nanomaterial translocation in plants and algae in the environment, and for the advancement of nanoenabled agriculture with a reduced environmental impact. 

Top-side foliar application of layered double hydroxide penetrates the cuticle better, accumulates between epidermal cell walls, and improves plant performance more than bottom-side application.