Electrochemical Investigation of pH‐Dependent Activity of Polyethylenimine‐Capped Silver Nanoparticles

pH-dependent Synthesis and Interactions of Fluorescent L-Histidine Capped Copper Nanoclusters with Metal Ions

In this work, L-Histidine-protected copper nanoclusters synthesized by changing the pH levels of precursor solution have been shown to display different emission wavelengths and intensities. As determined by mass spectrometry, nanoclusters Cu3L2 synthesized at acidic pH have 3 atoms in their core and emit in the greenish-yellow region, and nanoclusters Cu2L2, synthesized in the basic conditions have 2 atoms in their core and emit in the blue-green region. They are expected to have coordination through the carboxylate group and nitrogen of the imidazole ring of histidine ligand, respectively. Metal ions Mg2+, Mn2+, Zn2+, and Pb2+ selectively enhance the interaction between carboxylate - copper metal core and increase the emission intensity of Cu3L2. These metal ions weaken the interaction between imidazole nitrogen and copper metal core and quench the emission intensity of Cu2L2. As synthesized, nanoclusters exhibit good water solubility and photostability, they can act as fluorescent probes to sense the metal ions, therefore, they were utilized for the optical sensing of the mentioned metal ions. Fluorescent nanoclusters were found to sense even a very low concentration of metal ions with a limit of detection (3 σ/slope) in nanomolar range.

On-Chip Electrokinetic Micropumping for Nanoparticle Impact Electrochemistry

Single-entity electrochemistry is a powerful technique to study the interactions of nanoparticles at the liquid-solid interface. In this work, we exploit Faradaic (background) processes in electrolytes of moderate ionic strength to evoke electrokinetic transport and study its influence on nanoparticle impacts. We implemented an electrode array comprising a macroscopic electrode that surrounds a set of 62 spatially distributed microelectrodes. This configuration allowed us to alter the global electrokinetic transport characteristics by adjusting the potential at the macroscopic electrode, while we concomitantly recorded silver nanoparticle impacts at the microscopic detection electrodes. By focusing on temporal changes of the impact rates, we were able to reveal alterations in the macroscopic particle transport. Our findings indicate a potential-dependent micropumping effect. The highest impact rates were obtained for strongly negative macroelectrode potentials and alkaline solutions, albeit also positive potentials lead to an increase in particle impacts. We explain this finding by reversal of the pumping direction. Variations in the electrolyte composition were shown to play a critical role as the macroelectrode processes can lead to depletion of ions, which influences both the particle oxidation and the reactions that drive the transport. Our study highlights that controlled on-chip micropumping is possible, yet its optimization is not straightforward. Nevertheless, the utilization of electro- and diffusiokinetic transport phenomena might be an appealing strategy to enhance the performance in future impact-based sensing applications.

A fluorescent PET probe based on polyethyleneimine-Ag nanoclusters as a reversible, stable and selective broad-range pH sensor

In this work, nanoclusters (NCs) of Cu and Ag capped with hyperbranched polyethyleneimine (PEI) were prepared using chemical reduction by a one-step hydrothermal method. The PEI coated-NCs were characterized by high-resolution transmission electron microscopy, ζ potential, thermogravimetric analysis, dynamic light scattering, Fourier-transform infrared, UV-visible, and fluorescence spectroscopy. The PEI-NCs exhibited strong absorption and fluorescence, high stability, and excellent water dispersibility. The resulting PEI-NCs showed a reversible and linear response of fluorescence intensity with pH over a wide range (3-11); however, PEI-AgNCs showed a better reversibility and sensitivity than PEI-CuNCs. Unlike several types of pH sensors based on modified NCs, which are based on a nanoparticle aggregation/disaggregation mechanism, the response of our sensor is based on a photoinduced electron transfer process, which gives it a high reversibility. This method was successfully applied in pH measurements in tap water and green tea samples, with excellent results, indicating its practical utility for these applications. A visual device was obtained by immobilizing PEI-AgNCs into agarose hydrogels at different pH values. The results show that the proposed sensor can be used as a pH visual detector. Besides, the light emission of the nanosensor was corroborated by fluorescence microscopy, confirming that the nanosensor based on PEI-AgNCs has great potential to be used in cellular imaging.

Nanoparticle Characterization Through Nano-Impact Electrochemistry: Tools and Methodology Development

The field of nanomaterials has been expanding rapidly into many diverse applications within the last 20 years. With this growth, there is a significant need for new method development for the detection and characterization of nanomaterials. Understanding the physical properties of nanoscale entities and their associated reaction kinetics is crucial for monitoring their effect on environmental and human health, and in their use for practical applications. Nano-impact electrochemistry is a novel development in the field of fundamental electrochemistry that provides an ultrasensitive method for analyzing physical and redox properties of nanomaterials and their derivatives. This protocol focuses on the tools required for characterizing silver nanoparticles (AgNPs) by nano-impact electrochemistry, the preparation of microelectrodes and the methodology needed for measurement of the AgNP redox activity. The fabrication of cylindrical carbon fiber as well as gold and platinum microwire electrodes is described in detail. The analysis of nano-impact electrochemistry for the characterization of redox active entities is also outlined with examples of applications.

Single-Particle Investigation of Environmental Redox Processes of Arsenic on Cerium Oxide Nanoparticles by Collision Electrochemistry

Quantification of chemical reactions of nanoparticles (NPs) and their interaction with contaminants is a fundamental need to the understanding of chemical reactivity and surface chemistry of NPs released into the environment. Herein, we propose a novel strategy employing single-particle electrochemistry showing that it is possible to measure reactivity, speciation, and loading of As³⁺ on individual NPs, using cerium oxide (CeO₂) as a model system. We demonstrate that redox reactions and adsorption processes can be electrochemically quantified with high sensitivity via the oxidation of As³⁺ to As⁵⁺ at 0.8 V versus Ag/AgCl or the reduction of As³⁺ to As⁰ at -0.3 V (vs Ag/AgCl) generated by collisions of single particles at an ultramicroelectrode. Using collision electrochemistry, As³⁺ concentrations were determined in basic conditions showing a maximum adsorption capacity at pH 8. In acidic environments (pH < 4), a small fraction of As³⁺ was oxidized to As⁵⁺ by surface Ce⁴⁺ and further adsorbed onto the CeO₂ surface as a As⁵⁺ bidentate complex. The frequency of current spikes (oxidative or reductive) was proportional to the concentration of As³⁺ accumulated onto the NPs and was found to be representative of the As³⁺ concentration in solution. Given its sensitivity and speciation capability, the method can find many applications in the analytical, materials, and environmental chemistry fields where there is a need to quantify the reactivity and surface interactions of NPs. This is the first study demonstrating the capability of single-particle collision electrochemistry to monitor the interaction of heavy metal ions with metal oxide NPs. This knowledge is critical to the fundamental understanding of the risks associated with the release of NPs into the environment for their safe implementation and practical use.

Colloidal-electrochemical fabrication strategies for functional composites of linear polyethylenimine

Colloidal-electrochemical fabrication strategies have been developed for the deposition of linear polyethylenimine (LPEI) composite materials. Electrophoretic deposition (EPD) allowed for the fabrication of composite films containing Mn₃O₄ and ZnO nanoparticles, as well as advanced flame retardant materials, such as halloysite nanotubes and memory-type Al-Mg-Zr complex hydroxide (AMZ) in the matrix of the water-insoluble LPEI. A liquid-liquid extraction method has been designed for the agglomerate-free processing of AMZ particles. Efficient extraction was achieved using decylphosphonic acid as an extractor. A conceptually new polymer complex (PC)-EPD method has been developed, which is based on the use of LPEI-metal ion complexes. Proof-of-concept studies involved the fabrication of LPEI-Ni(OH)₂ and LPEI-MnOx nanocomposites. The composites showed valuable flame retardant and charge-storage properties. The analysis of basic EPD and PC-EPD mechanisms as well as complexing properties of LPEI has driven the development of new strategies for the fabrication of organic composites. Hemoglobin was used as a model protein for the fabrication of composite films. Another important finding was the fabrication of composites, containing cyclodextrin, which is a unique carrier of various functional organic molecules. EPD and PC-EPD are versatile methods, which allow for the deposition of novel LPEI based composites containing various functional materials.