Chemically-Modulated Photoluminescence of Graphene Oxide for Selective Detection of Neurotransmitter by “Turn-On” Response

Designing artificial nanomaterials capable of selectively detecting targets without the use of expensive and fragile antibodies is of great interest in the applications of nanomedicine. Here, we show that the photoluminescence (PL) of graphene oxide (GO) was chemically modulated for the selective detection of a neurotransmitter without the use of antibodies. GO was functionalized with nitrotriacetic acid (NTA) on which four different metal ions were chelated (M-NTA-GO), which led to its different PL responses to neurotransmitters. In particular, the Cu-NTA-GO hybrid was able to selectively detect norepinephrine at nanomolar concentrations in a simple manner via its “turn-on” PL. Moreover, it was successfully applied to the selective detection of norepinephrine secreted from living PC-12 cells

Structuring Pd Nanoparticles on 2H-WS₂ Nanosheets Induces Excellent Photocatalytic Activity for Cross-Coupling Reactions under Visible Light

Effective photocatalysts and their surface engineering are essential for the efficient conversion of solar energy into chemical energy in photocatalyzed organic transformations. Herein, we report an effective approach for structuring Pd nanoparticles (NPs) on exfoliated 2H-WS₂ nanosheets (WS₂/PdNPs), resulting in hybrids with extraordinary photocatalytic activity in Suzuki reactions under visible light. Pd NPs of different sizes and densities, which can modulate the photocatalytic activity of the as-prepared WS₂/PdNPs, were effectively structured on the basal plane of 2H-WS₂ nanosheets via a sonic wave-assisted nucleation method without any reductants at room temperature. As the size of Pd NPs on WS₂/PdNPs increased, their photocatalytic activity in Suzuki reactions at room temperature increased substantially. In addition, it was found that protic organic solvents play a crucial role in activating WS₂/PdNPs catalysts in photocatalyzed Suzuki reactions, although these solvents are generally considered much less effective than polar aprotic ones in the conventional Suzuki reactions promoted by heterogeneous Pd catalysts. A mechanistic investigation suggested that photogenerated holes are transferred to protic organic solvents, whereas photogenerated electrons are transferred to Pd NPs. This transfer makes the Pd NPs electron-rich and accelerates the rate-determining step, i.e., the oxidative addition of aryl halides under visible light. WS₂/PdNPs showed the highest turnover frequency (1244 h^–1) for photocatalyzed Suzuki reactions among previously reported photocatalysts

Optical Detection of Enzymatic Activity and Inhibitors on Non-Covalently Functionalized Fluorescent Graphene Oxide

It has been of great interest to measure the activity of acetylcholinesterase (AChE) and its inhibitor, as AChE is known to accelerate the aggregation of the amyloid beta peptides that underlie Alzheimer’s disease. Herein, we report the development of graphene oxide (GO) fluorescence-based biosensors for the detection of AChE activity and AChE inhibitors. To this end, GO was non-covalently functionalized with phenoxy-modified dextran (PhO-dex-GO) through hydrophobic interaction; the resulting GO showed excellent colloidal stability and intense fluorescence in various aqueous solutions as compared to pristine GO and the GO covalently functionalized with dextran. The fluorescence of PhO-dex-GO remarkably increased as AChE catalyzed the hydrolysis of acetylthiocholine (ATCh) to give thiocholine and acetic acid. It was found that the turn-on fluorescence response of PhO-dex-GO to AChE activity was induced by protonation of carboxyl groups on it from the product of the enzymatic hydrolysis reaction, acetic acid. On the basis of its turn-on fluorescence response, PhO-dex-GO was able to report kinetic and thermodynamic parameters involving a maximum velocity, a Michaelis constant, and an inhibition dissociation constant for AChE activity and inhibition. These parameters enable us to determine the activity of AChE and the efficiency of the inhibitor

Single-Step and Rapid Growth of Silver Nanoshells as SERS-Active Nanostructures for Label-Free Detection of Pesticides

We explored a single-step approach for the rapid growth of Ag nanoshells (Ag NSs) under mild conditions. Without predeposition of seed metals, a uniform and complete layer of Ag shells was rapidly formed on silica core particles within 2 min at 25 °C via single electron transfer from octylamine to Ag⁺ ions. The size and thickness of the Ag NSs were effectively tuned by adjusting the concentration of silica nanoparticles (silica NPs) with optimal concentrations of AgNO₃ and octylamine. This unusually rapid growth of Ag NSs was attributed to a significant increase in the reduction potential of the Ag⁺ ions in ethylene glycol (EG) through the formation of an Ag/EG complex, which in turn led to their facile reduction by octylamine, even at room temperature. A substantial enhancement in the surface-enhanced Raman scattering (SERS) of the prepared Ag NSs was demonstrated. The Ag NSs were also utilized as SERS-active nanostructures for label-free detection of the pesticide thiram. The Ag NS-based SERS approach successfully detected thiram on apple peel down to the level of 38 ng/cm² in a label-free manner, which is very promising with respect to its potential use for the on-site detection of residual pesticides

Polymer-Mediated Formation and Assembly of Silver Nanoparticles on Silica Nanospheres for Sensitive Surface-Enhanced Raman Scattering Detection

To impart a desired optical property to metal nanoparticles (NPs) suitable for surface-enhanced Raman scattering (SERS) applications, it is crucial to assemble them in two or three dimensions in addition to controlling their size and shape. Herein, we report a new strategy for the synthesis and direct assembly of Ag NPs on silica nanospheres (AgNPs-SiNS) in the presence of poly­(ethylene glycol) (PEG) derivatives such as PEG-OH, bis­(amino)-PEGs (DA-PEGs), and <i>O</i>,<i>O</i>′-bis­(2-aminopropyl)­PEG (DAP-PEG). They exhibited different effects on the formation of Ag NPs with variable sizes (10–40 nm) and density on the silica surface. As the molecular weight (MW) of DA-PEGs increased, the number of Ag NPs on the silica surface increased. In addition, DAP-PEG (MW of 2000), which has a 2-aminopropyl moiety at both ends, promoted the most effective formation and assembly of uniform-sized Ag NPs on a silica surface, as compared to the other PEG derivatives with the same molecular weight. Finally, we demonstrated that AgNPs-SiNS bearing 4-fluorobenzenethiol on its surface induced the strong SERS signal at the single-particle level, indicating that each hybrid particle has internal hot spots. This shows the potential of AgNPs-SiNS for SERS-based sensitive detection of target molecules