Natércia Martins

Martins, NCT; Freire, CSR; Pinto, RJB; Fernandes, SCM; Neto, CP; Silvestre, AJD; Causio, J; Baldi, G; Sadocco, P; Trindade, T
Nanofibrillated cellulose offers new technological solutions for the development of paper products. Here, composites of nanofibrillated cellulose (NFC) and Ag nanoparticles (NP) were prepared for the first time via the electrostatic assembly of Ag NP (aqueous colloids) onto NFC. Distinct polyelectrolytes have been investigated as macromolecular linkers in order to evaluate their effects on the building-up of Ag modified NFC and also on the final properties of the NFC/Ag composite materials. The NFC/Ag nanocomposites were first investigated for their antibacterial properties towards S. aureus and K. pneumoniae microorganisms as compared to NFC modified by polyelectrolytes linkers without Ag. Subsequently, the antibacterial NFC/Ag nanocomposites were used as fillers in starch based coating formulations for Eucalyptus globulus-based paper sheets. The potential of this approach to produce antimicrobial paper products will be discussed on the basis of complementary optical, air barrier and mechanical data.

Martins, NCT; Avellan, A; Rodrigues, S; Salvador, D; Rodrigues, SM; Trindade, T
Zinc (Zn) micronutrient deficiency is a widespread problem affecting crops worldwide. Unlike conventional ionic fertilizers (Zn as salt or chelated forms), Zn-based engineered nanomaterials (ENMs) have the potential to release Zn in a controlled manner, reducing Zn losses through leaching upon application to soil. In this work, composites made of biopolymers (microcrystalline cellulose, chitosan, and alginate) and ZnO nanoparticles [NPs; 4-65% (w/w) Zn] were prepared. Their potential for Zn controlled release was tested in four agricultural soils of distinct pH and organic matter content over 30 days. While conventionally used Zn salts leached from the soil resulting in very low CaCl2-extractable Zn concentration, Zn in ZnO NPs was less labile, and ZnO biopolymers maintained a better constant supply of CaCl2-extractable Zn than all other treatments. ZnO NPs/alginate beads prepared by cross-linking with CaCl2 presented the slowest Zn release kinetics. As assessed with maize plants grown in poor Zn acidic soil (LUFA 2.1, pH 5.2), this constant Zn release from ZnO NPs/alginate beads resulted in a steadier Zn concentration in soil pore water over time. Results further indicated that ZnO NPs/alginate beads could meet the maize Zn needs while avoiding the early stage Zn toxicity induced by conventional Zn supplies, demonstrating that these ENMs are a sustainable way to supply Zn in a controlled manner in acidic soils. The impact of plant exudates on Zn bioavailability in the soil under maize-root influence (rhizosphere) is also discussed, underlying the need to study the fate of micronutrients in the rhizosphere to better predict its long-term bioavailability in bulk soils.

Martins, NCT; Angelo, J; Girao, AV; Trindade, T; Andrade, L; Mendes, A
A new composite of TiO2 (P25) and N-doped carbon quantum dots (P25/NCQD) was prepared by a hydrothermal method and was used for the first time as catalyst of the photo-oxidation of NO under UV and visible light irradiation. P25/NCQD composite exhibited a NO conversion (27.0%) more than two times higher of that observed for P25 (10%) under visible light and the selectivity of the process was increased from 37.4% to 49.3%. The composite also showed better photocatalytic performance than P25 in the UV region with increases of 36.3% on NO conversion and 16.8% on selectivity. Moreover, compared with P25, the photodegradation ratio of methylene blue was enhanced from 68% to 91% after UV irradiation for 1 h. NCQD played a crucial role on the photocatalytic activity improvement of P25/NCQD, increasing visible light absorption, slowing the recombination and improving the charge transfer.

Natércia C.T. Martins*, Sara Fateixa, Tiago Fernandes, Helena I.S. Nogueira, and Tito Trindade
Hydrophobic substrates for surface-enhanced Raman scattering (SERS) of adsorbate species are of great interest in chemical analysis because they can concentrate the analyte molecules in a small area of the surface, thereby improving the SERS sensitivity. Here, we propose a general strategy to fabricate hydrophobic paper-based substrates for SERS applications. The paper substrates have been manufactured by inkjet printing of aqueous emulsions containing Ag and polystyrene (PS) colloidal nanoparticles. In a first stage, the SERS performance of the substrates was optimized by varying the relative amounts of polymer/metal colloidal nanoparticles, the number of printing layers, and the degree of hydrophobicity of the substrates, using crystal violet as a molecular probe, which is well known for its strong SERS activity. In these conditions, the strongest surface Raman enhancement was observed for the highest water contact angle (146°) achieved. The selected Ag/PS-coated paper substrates showed wide applicability for several analytes, but in this study, a detailed analysis is provided for the pesticide thiram as a proof of concept. Several samples spiked with thiram have been analyzed by SERS, giving a detection limit of 0.024 ppm thiram in mineral water and apple juice, while in apple peel, the detection limit achieved for the same pesticide was 600 ng/cm2. We suggest that this one-step fabrication method produces a hydrophobic coating whose nanoscale features improve the SERS performance of the paper substrates.