Wey Yang Teoh

Progress in Heterogeneous Photocatalysis: From Classical Radical Chemistry to Engineering Nanomaterials and Solar Reactors

The field of heterogeneous photocatalysis has expanded rapidly in the last four decades, having undergone various evolutionary phases related to energy and the environment. The two most significant applications of photocatalysis are geared toward solar water splitting and the purification of air and water. Notably, the interdisciplinary nature of the field has increased significantly, incorporating semiconductor physics, surface sciences, photo and physical chemistry, materials science, and chemical engineering. Whereas this forms the basis on which the field continues to grow, adequate bridging of multidisciplinary knowledge remains essential. By recalling some of the classical fundamentals of photocatalysis, this Perspective provides contemporary views on heterogeneous photochemical conversion, encompassing charge transport characteristics, radical chemistry and organic degradation mechanisms, photocatalyst design, and photoreactor engineering.

Cytotoxic origin of copper(II) oxide nanoparticles: Comparative studies with micron-sized particles, leachate, and metal salts

The work investigates the source of toxicity of copper oxide nanoparticles (CuO NPs) with respect to its leaching characteristic and speciation. Complexation-mediated leaching of CuO NPs by amino acids was identified as the source of toxicity toward Escherichia coli, the model microorganism used in the current study. The leached copper-peptide complex induces a multiple-fold increase in intracellular reactive oxygen species generation and reduces the fractions of viable cells, resulting in the overall inhibition of biomass growth. The cytotoxicity of the complex leachate is however different from that of equivalent soluble copper salts (nitrates and sulfates). A pH-dependent copper speciation during the addition of copper salts gives rise to uncoordinated copper ions, which in turn result in greater toxicity and cell lysis, the latter of which was not observed for CuO NPs even at comparable pH. Since leaching did not occur with micrometer-sized CuO, no cytotoxicty effect was observed, thus highlighting the prominence of materials toxicity at the nanoscale.

Reversible Antimicrobial Photoswitching in Nanosilver

Nanosilver has emerged as one of the most commercialized nanomaterials, particularly as antimicrobial agents with interesting applications such as wound dressings, textiles, water and air purification, self-sterilizing polymer films, and bone implants. In comparison to bulk counterparts, nanosilver exhibits superior antimicrobial activity toward various microorganisms. Its reactivity is in part driven by a high specific surface area and enhanced surface electronic effects. Incorporation of silver nanoparticles into bacteria was recognized by the surface and intracellular uptake of Escherichia coli, which potentially results in the binding of silver to sulfur-containing membranes and cytoplasmic proteins. Exposure of E. coli and Staphylococcus aureus to dissolved silver ions affects the DNA replication ability as well as inactivates the expression of ribosomal subunit proteins and enzymes vital for adenosine 50-triphosphate (ATP) production. Microscopic studies revealed the condensation and concentration of DNA in the center of the cell and detachment of the cytoplasm membrane from the cell wall upon treatment with silver ions. It was further reported recently that the antimicrobial mechanism of silver nanoparticles was related to protein/membrane damage of E. coli but not to DNA damage. While the antimicrobial effect of nanosilver is widely recognized and has triggered extensive research interest, the ‘‘tuning’’ of its antimicrobial activity, to the best of our knowledge, has never been reported. This is important especially for applications involving direct antimicrobial interactions with human cells such as those in self-sterilizing

The stabilization and bio-functionalization of iron oxide nanoparticles using heterotelechelic polymers

Iron oxide nanoparticles (IONPs) are important tools for nanobiotechnology applications. However, aqueous instability and non-specific biodistribution problems limit the applications of IONPs. Considering this, α-phosphonic acid, ω-dithiopyridine functionalized polymers were synthesized via the reversible addition–fragmentation chain transfer (RAFT) polymerization and used for stabilizing and biofunctionalizing IONPs. A new trithiocarbonate RAFT agent bearing dimethyl phosphonate group was utilized in the synthesis of well-defined telechelic polymers of styrene, oligoethylene glycol acrylate (OEG-A) and N-isopropylacrylamide (NIPAAm). IONPs were grafted with α-phosphonic acid, ω-dithiopyridine functionalized poly(OEG-A) through the α-chain end of the polymer, as evidenced by FTIR-ATR, XPS and zeta potential measurements. Using TGA results, the grafting density of the polymer chains was calculated between 0.12 and 0.23 chains/nm2 particle depending on the molecular weight of the polymer. DLS measurements indicated that the particles grafted with poly(OEG-A) larger than 10 000 g/mol were stable in water for several days and the mean diameter of the particles was between 40 and 130 nm depending on the molecular weight of the polymer. Moreover, particles stabilized with poly(OEG-A) with a Mn = 62 000 g/mol were stable in phosphate buffer (pH 6.5, 0.1 M) containing varying concentrations of BSA. Polymer-stabilized IONPs were successfully functionalized with two different peptides, i.e. reduced glutathione as a model peptide and NGR motif as a tumor-targeting peptide through the ω-dithiopyridine functionality of the polymer, as measured by XPS and zeta potential analysis. Poly(OEG-A)-stabilized IONPs were also found to be resistant to protein adsorption.

Anti-fouling magnetic nanoparticles for siRNA delivery

Iron oxide nanoparticles (IONPs), with a diameter of 8 nm, have been coated with two different polymers, i.e. poly(oligoethylene glycol) methyl ether acrylate (P(OEG-A)) and poly(dimethylaminoethyl acrylate) (P(DMAEA)). The polymers were attached to the nanoparticle surface using two different strategies, with the aim of creating an internal layer of P(DMAEA) and an outer shell of P(OEG-A). The subsequent polymer-stabilized IONPs were characterized using ATR, XPS and TGA, proving the presence of polymers on the IONP surfaces with a grafting density ranging from 0.05 to 0.22 chain per nm2. High grafting densities were demonstrated when the two homopolymers were assembled on the surfaces of the IONPs simultaneously. The polymer composition at the surfaces of the IONPs could be controlled by manipulating the feed ratio P(OEG-A)–P(DMAEA) present in solution. These hybrid organic–inorganic particles (70–150 nm) proved to be stable in both water and 50 vol% fetal bovine serum (FBS). In addition, zeta-potential measurements confirmed that P(OEG-A) chains effectively mask the positive charge originating from P(DMAEA) thereby limiting protein adsorption on these particles. Hybrid nanoparticles were exploited for the complexation of siRNA, thereby generating IONP siRNA nano-carriers with anti-fouling P(OEG-A) shells. The transfection efficiency was measured using human neuroblastoma SHEP cells both in the presence and in the absence of a magnetic field in FBS. The transfection efficiency was determined by both fluorescence microscopy and flow cytometry. Cytotoxicity studies revealed that the IONP carriers were non-toxic to SHEP cells. In addition, studies on the proton transverse relaxation enhancement properties of these stabilized IONPs indicated high relaxivities (∼160 s−1 per mM of Fe).

Flame preparation of visible-light-responsive BiVO4 oxygen evolution photocatalysts with subsequent activation via aqueous route.

Visible-light-active BiVO(4) photocatalyst prepared by a one-step flame spray pyrolysis demonstrates the structural evolution from amorphous to crystalline scheelite-tetragonal and further to scheelite-monoclinic (the photocatalytic active phase). Up to 95% scheelite-monoclinic content, the rest being scheelite-tetragonal, can be achieved in situ by exposing the collection filter to higher flame temperature. Reasonable activity in terms of photocatalytic O(2) evolution was obtained with the increase in crystallinity and scheelite-monoclinic content. Although analogous postcalcination of BiVO(4) improves crystallization and phase transformation, it inevitably induces vacancy defects that are detrimental to the photocatalytic activity. Hence a facile aqueous acid treatment on the flame-made BiVO(4) is introduced, which in the presence of small addition of Bi and V promotes full transformation to scheelite-monoclinic and reduces charge trapping defects. As a result, the photocatalytic O(2) evolution activity was increased by a remarkable 5 folds compared to the best performing untreated flame-made BiVO(4).

Cellular uptake and reactive oxygen species modulation of cerium oxide nanoparticles in human monocyte cell line U937

Cerium oxide nanoparticles (nanoceria) are promising materials for intracellular oxygen free radical scavenging providing a potential therapy for reactive oxygen species (ROS)-mediated inflammatory processes. In this study rhombohedral-shaped nanoceria were synthesized by flame spray pyrolysis with tuneable particle diameters between 3 and 94 nm by changing the liquid precursor flow rate. Monocytes and macrophages are major players in inflammatory processes as their production of ROS species has important downstream effects on cell signalling. Therefore, this study examined the ability of the nanoceria to be internalised by the human monocytic cell line, U937, and scavenge intracellular ROS. U937 cells activated in the presence of phorbol 12-myristate 13-acetate (PMA) were found to be more responsive to the nanoceria than U937 cells, which may not be surprising given the role of monocyte/macrophages in phagocytosing foreign material. The smaller particles were found to contain more crystal lattice defects with which to scavenge ROS, however a greater proportion of both the U937 and activated U937 cell populations responded to the larger particles. Hence all nanoceria particle sizes examined in this study were equally effective in scavenging intracellular ROS.

Experimental validation of proton transverse relaxivity models for superparamagnetic nanoparticle MRI contrast agents

Analytical models of proton transverse relaxation rate enhancement by magnetic nanoparticles were tested by making measurements on model experimental systems in a field of 1.4 T. Proton relaxivities were measured for five aqueous suspensions of iron oxide (maghemite) nanoparticles with nominal mean particle sizes of 6, 8, 10, 11, and 13 nm. Proton relaxivity increased with mean particle size ranging from 13 s(-1) mM Fe(-1) for the 6 nm sample, up to 254 s(-1) mM Fe(-1) for the 13 nm sample. A strong correlation between the measured and predicted values of the relaxivity was observed, with the predicted values being consistently higher than the measured values. The results indicate that the models give a reasonable agreement with experimental results and hence can be used as the basis for the design of new magnetic resonance imaging contrast and labelling agents.

Hierarchical growth of SnO2 nanostructured films on FTO substrates: structural defects induced by Sn(II) self-doping and their effects on optical and photoelectrochemical properties

Direct hydrothermal growth of Sn(II)-doped SnO2 films on fluorine-doped tin oxide (FTO) substrates results in the formation of upstanding SnO2 nanosheet arrays covered by hierarchical SnO2 nanoflowers. The n-type semiconductor films show extended photoresponse in the visible spectrum arising from the coexistence of Sn(II) dopant ions and oxygen vacancies in these hierarchical SnO2 nanostructures, which leads to a narrowed bandgap. Photoluminescence spectroscopy revealed that the emission in the UV, blue and red spectral ranges is related to the evolution of Sn(II) dopants and oxygen vacancies with annealing temperature, whereas oxygen vacancies are mostly responsible for visible emission. The Sn(II)-doped SnO2 films show higher photocurrent when sensitized with narrow bandgap CdS nanoparticles, serving as efficient electron acceptors.

Induced Adaptation of Bacillus sp. to Antimicrobial Nanosilver

The natural ability of Bacillus sp. to adapt to nanosilver cytotoxicity upon prolonged exposure is reported for the first time. The combined adaptive effects of nanosilver resistance and enhanced growth are induced under various intensities of nanosilver-stimulated cellular oxidative stress, ranging from only minimal cellular redox imbalance to the lethal levels of cellular ROS stimulation. An important implication of the present work is that such adaptive effects lead to the ultimate domination of nanosilver-resistant Bacillus sp. in the microbiota, to which nanosilver cytotoxicity is continuously applied.