Reactive argon-plasma activation of screen-printed carbon electrodes for highly selective dopamine determination.

Dopamine (DA) deficiency has been linked to several psychiatric disorders. Electrochemical determination of the level of DA suffers from abundant ascorbic acid (AA) and uric acid (UA) in body fluids. In this work, a facile argon (Ar) plasma treatment was utilized to enhance the electrocatalytic reactivity of screen-printed carbon electrodes (SPCEs) for selective DA detection. Surface characterization of the Ar-treated SCPEs verified that the carbon paste binders were successfully removed and single-bonded oxygenated moieties (-OH and C-O-C) were generated. Interestingly, the sharper D* and D'' Raman interbands were new key evidence of a higher exposure of carbon defect sites. Electrochemical studies further revealed that the Ar-treated SPCEs possessed faster heterogeneous electron-transfer rates, larger electroactive surface areas, and much higher conductivity when compared with untreated electrodes. As a result, the oxidation potentials of AA, DA, and UA in the mixture could be well-resolved and the current responses were significantly increased. The selective determination of DA in the presence of AA and UA by differential pulse voltammetry gave two linear responses with the limit of detection of 0.27 µM (0.15-10 µM range). Moreover, this Ar-treated SPCE had high reproducibility and good storage stability. These results suggest that Ar-plasma treatment could be a promising method to enhance the electrocatalytic properties of SPCEs for the detection of biomolecules.

Selection of cryoprotectants for freezing and freeze-drying of gold nanoparticles towards further uses in various applications.

Recently, cryopreservation of AuNPs without aggregation has been attempted to improve their long-term stability. This study investigated criteria to select cryoprotectants for AuNPs using a variety of materials, including sugar (sucrose), surfactant (Tween 20), polymers (polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP)), and biopolymer (pectin). For cryoprotective performance, UV-vis spectroscopy reveals the potential of all cryoprotectants for preventing citrate-capped AuNPs (cit-AuNPs) from irreversible aggregation under freezing. While sucrose, PVP, and pectin were more suitable than Tween 20 and PVA as cryoprotectants for lyophilization of AuNPs with the maintained redispersability. For storage and further use, Luria-Bertani agar plate, dynamic light scattering (DLS), and transmission electron microscopy (TEM) results indicate impacts of the cryoprotectant coexisted with AuNPs after resuspension and imply that washing of the restored AuNPs is encouraged. Otherwise, running the restored AuNPs through applications, such as functionalization, protein conjugation, and surface-enhanced Raman scattering (SERS), without washing the cryoprotectant could lead to inaccurate results. This study also serves as a guideline for a comprehensive practice flow of AuNP handling, encompassing the synthesis step, cryopreservation, and use after resuspension.

Luminescent nanohybrid of ZnO quantum dot and cellulose nanocrystal as anti-counterfeiting ink.

Luminescent quantum dot (QD) ink is currently a powerful tool for generating hidden information on paper substrates. Herein, we fabricated a nanohybrid ink of bacterial cellulose nanocrystal (BCNC) and UV-responsive ZnO QD via electrostatic self-assembly for improving solvent resistance and message encryption process. Under investigations on the printed areas, the nanohybrid can slightly infiltrate into the paper fibers and form a thin layer on the top of paper substrates, conferring an enhanced print permanence against wetting conditions while maintaining the daylight unobservability and its luminescent stability. The water resistance of the proposed nanohybrid ink enables developing a higher security level that the prints can be submerged in CuCl2 aqueous solutions to quench the luminescent message. The concealed message can eventually be revealed under UV light again after submerging in EDTA solution. Our ZnO QD/BCNC nanohybrid with eco-friendly nature therefore exhibits great potential as security marking ink for counterfeit protection with sustainable uses.

Silver nanoparticle/bacterial nanocellulose paper composites for paste-and-read SERS detection of pesticides on fruit surfaces.

This study aimed to develop an eco-friendly flexible surface-enhanced Raman scattering (SERS) substrate for in-situ detection of pesticides using biodegradable bacterial nanocellulose (BNC). Plasmonic silver nanoparticle- bacterial nanocellulose paper (AgNP-BNCP) composites were prepared by vacuum-assisted filtration. After loading AgNPs into BNC hydrogel, AgNPs were trapped firmly in the network of nanofibrous BNCP upon ambient drying process, resulting in 3D SERS hotspots within a few-micron depth on the substrate. The fabricated AgNP-BNCPs exhibited high SERS activity with good reproducibility and stability as demonstrated by the detection of 4-aminothiophenol and methomyl pesticide. Due to the optical transparency of BNCP, a direct and rapid detection of methomyl on fruit peels using AgNP-BNCPs can be achieved, demonstrating a simple and effective 'paste-and-read' SERS approach. These results demonstrate potential of AgNP-BNCP composites for user-friendly in-situ SERS analysis.

Zwitterionic peptide-capped gold nanoparticles for colorimetric detection of Ni2+ .

Zwitterionic nanoparticles are typically utilized as nanoprobes and delivery vehicles in nanomedicine and therapeutics due to their resistance to interferences. Their high stability also shows great potential to be applied in sensing applications. Here, we report a selective, sensitive and rapid colorimetric sensing of nickel ions (Ni2+) using zwitterionic polypeptide, EKEKEKPPPPC (EK)3, capped gold nanoparticles (AuNP-(EK)3). By taking advantage of the alternate carboxylic (-COOH)/amine (-NH2) groups, the zwitterionic peptide can function dually by being able to sense metal ions and maintain colloidal stability. Ni2+ can trigger the aggregation of the AuNP-(EK)3 nanoprobe, which results in a red-to-purple color change of the AuNP-(EK)3 solution. Our 40 nm AuNP-(EK)3 nanoprobe can detect Ni2+ as low as 34 nM within 15 min with a linear range of 60-160 nM, and is stable in soil, urine and water samples. We demonstrate that the aggregation mechanism of the nanoprobe is due to the interactions between the -NH2 group of glutamic acid at the N-terminus of the peptide and Ni2+, and the aggregation process is reversible. Furthermore, the slight modification of two amino acid sequences at the N-terminus allows the nanoprobe to retain its stability, even in a high ionic strength medium. We believe that by adjusting or extending the peptide sequences, new metal ion selective peptides could be created.

Ultrastable and Biofunctionalizable Gold Nanoparticles.

Gold nanoparticles provide an excellent platform for biological and material applications due to their unique physical and chemical properties. However, decreased colloidal stability and formation of irreversible aggregates while freeze-drying nanomaterials limit their use in real world applications. Here, we report a new generation of surface ligands based on a combination of short oligo (ethylene glycol) chains and zwitterions capable of providing nonfouling characteristics while maintaining colloidal stability and functionalization capabilities. Additionally, conjugation of these gold nanoparticles with avidin can help the development of a universal toolkit for further functionalization of nanomaterials.