Enzyme-free detection of hydrogen peroxide from cerium oxide nanoparticles immobilized on poly(4-vinylpyridine) self-assembled monolayers.

A simple enzyme-free spectrophotometric detection of hydrogen peroxide is demonstrated based on its colorimetric reaction with oxygen deficient cerium oxide nanoparticles (CNPs). This colorimetric sensitivity of CNPs towards H2O2 increases significantly with decreasing crystallite size due to an increase in the surface area as well as the concentration of Ce3+ on the surface. The origin of this colorimetric reaction was studied using DFT that suggests the adsorption of peroxide and oxygen molecules on ceria nanoparticles creates new states in the electronic structure leading to transitions absorbing in the visible region of the electromagnetic spectrum. For detection, a single layer of nanoparticles was immobilized on transparent microscopic glass slides using self-assembled monolayers (SAMs) of poly(4-vinylpyridine) (PVP). Cluster-free and uniform immobilization of nanoparticles was confirmed from atomic force microscopy (AFM) and helium ion microscopy (HIM). UV-Visible absorption measurements showed a concentration dependent increase in absorbance from immobilized CNPs that were exposed to increasing concentrations (10-400 µM) of hydrogen peroxide. The immobilized CNPs can be baked at 80 °C after initial use to regenerate the sensor for reuse. The development of a direct, reusable, enzyme-free and dye-free peroxide sensing technology is possible and can be immediately applied in various areas, including biomedicine and national security.

Effect of self-assembled monolayers on charge injection and transport in poly(3-hexylthiophene)-based field-effect transistors at different channel length scales.

Charge injection and transport in bottom-contact regioregular-poly(3-hexylthiophene) (rr-P3HT) based field-effect transistors (FETs), wherein the Au source and drain contacts are modified by self-assembled monolayers (SAMs), is reported at different channel length scales. Ultraviolet photoelectron spectroscopy is used to measure the change in metal work function upon treatment with four SAMs consisting of thiol-adsorbates of different chemical composition. Treatment of FETs with electron-poor (electron-rich) SAMs resulted in an increase (decrease) in contact metal work function because of the electron-withdrawing (-donating) tendency of the polar molecules. The change in metal work function affects charge injection and is reflected in the form of the modulation of the contact resistance, R(C). For example, R(C) decreased to 0.18 MΩ in the case of the (electron-poor) 3,5-bis-trifluoromethylbenzenethiol treated contacts from the value of 0.61 MΩ measured in the case of clean Au-contacts, whereas it increased to 0.97 MΩ in the case of the (electron-rich) 3-thiomethylthiophene treated contacts. Field-effect mobility values are observed to be affected in short-channel devices (<20 µm) but not in long-channel devices. This channel-length-dependent behavior of mobility is attributed to grain-boundary limited charge transport at longer channel lengths in these devices.

Application of surface chemical analysis tools for characterization of nanoparticles.

The important role that surface chemical analysis methods can and should play in the characterization of nanoparticles is described. The types of information that can be obtained from analysis of nanoparticles using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary-ion mass spectrometry (TOF-SIMS), low-energy ion scattering (LEIS), and scanning-probe microscopy (SPM), including scanning tunneling microscopy (STM) and atomic force microscopy (AFM), are briefly summarized. Examples describing the characterization of engineered nanoparticles are provided. Specific analysis considerations and issues associated with using surface-analysis methods for the characterization of nanoparticles are discussed and summarized, with the impact that shape instability, environmentally induced changes, deliberate and accidental coating, etc., have on nanoparticle properties.

Role of outer-membrane cytochromes MtrC and OmcA in the biomineralization of ferrihydrite by Shewanella oneidensis MR-1.

In an effort to improve the understanding of electron transfer mechanisms at the microbe-mineral interface, Shewanella oneidensis MR-1 mutants with in-frame deletions of outer-membrane cytochromes (OMCs), MtrC and OmcA, were characterized for the ability to reduce ferrihydrite (FH) using a suite of microscopic, spectroscopic, and biochemical techniques. Analysis of purified recombinant proteins demonstrated that both cytochromes undergo rapid electron exchange with FH in vitro with MtrC displaying faster transfer rates than OmcA. Immunomicroscopy with cytochrome-specific antibodies revealed that MtrC co-localizes with iron solids on the cell surface while OmcA exhibits a more diffuse distribution over the cell surface. After 3-day incubation of MR-1 with FH, pronounced reductive transformation mineral products were visible by electron microscopy. Upon further incubation, the predominant phases identified were ferrous phosphates including vivianite [Fe(3)(PO(4))(2)x8H(2)O] and a switzerite-like phase [Mn(3),Fe(3)(PO(4))(2)x7H(2)O] that were heavily colonized by MR-1 cells with surface-exposed outer-membrane cytochromes. In the absence of both MtrC and OmcA, the cells ability to reduce FH was significantly hindered and no mineral transformation products were detected. Collectively, these results highlight the importance of the outer-membrane cytochromes in the reductive transformation of FH and support a role for direct electron transfer from the OMCs at the cell surface to the mineral.