Single-particle tracking of the formation of a pseudoequilibrium state prior to charged microgel cluster formation at interfaces

The correlation between micron-sized particles and their self-assembly at fluid interfaces is important in several applications, including the stabilization of Pickering emulsions and creation of colloidosomes. In this study, through real-time visualization of the diffusion of microgel particles at the air–water interface of an aqueous pendant drop, the formation of a pseudoequilibrium state is observed prior to cluster formation. It is shown here that at the microscopic level, a pendant drop surface has nonuniform principal curvatures and exhibits positive deviatoric curvature (+∆c) gradients. The +∆c gradients confer superdiffusive motion to single ionic microgel particles and are responsible for bringing particles that are initially far apart to common sites on the interface with high curvatures. Prior to two-particle cluster formation, the balance between pairwise repulsion, capillary attraction and +∆c-induced energy that pushes the pair of particles to a high curvature creates a pseudoequilibrium state where the interparticle distance remains relatively invariant for a long period of time. This observation is also noted during higher-order cluster formation. Thereafter, a sufficiently strong long-range attraction potential is activated to facilitate cluster formation. Real-time tracking of the evolution of cluster formation provides useful insights into the interplay between various interactions experienced by ionic microgels.Ministry of Education (MOE)Published versionW. Bi acknowledges the financial support from the Heilongjiang Provincial Natural Science Foundation of China (LH2019B004). E.K.L. Yeow acknowledges the Singapore Ministry of Education MoE Tier 1 fund (RG6/18) for financial support

Unconventional multiple ring structure formation from evaporation-induced self-assembly of polymers

The formation of multiring deposits of poly(2-vinylpyridine) (P2VP) from the evaporation of a P2VP-(2,6-lutidine + water) drop on a glass substrate does not conform to the conventional pinning–depinning mechanism. Instead, ringlike deposits are formed when the droplet undergoes several cycles of spreading and receding where, for each spreading event, a P2VP ridge is formed at the contact line when the polymer flows toward the outward advancing edge. The complex interplay between an outward solutal-Marangoni flow due to a higher concentration of the polymer at the contact line and an inward solvent-Marangoni flow arising from the differences in volatilities and surface tensions of the pure solvent components plays an important role in enhancing the droplet spreading rate. The newly discovered surface patterning mechanism has important implications in the development of novel techniques for inducing self-assembly of functional materials from evaporating drops

Nontoxic colloidal particles impede antibiotic resistance of swarming bacteria by disrupting collective motion and speed

A monolayer of swarming B. subtilis on semisolid agar is shown to display enhanced resistance against antibacterial drugs due to their collective behavior and motility. The dynamics of swarming motion, visualized in real time using time-lapse microscopy, prevents the bacteria from prolonged exposure to lethal drug concentrations. The elevated drug resistance is significantly reduced when the collective motion of bacteria is judiciously disrupted using nontoxic polystyrene colloidal particles immobilized on the agar surface. The colloidal particles block and hinder the motion of the cells, and force large swarming rafts to break up into smaller packs in order to maneuver across narrow spaces between densely packed particles. In this manner, cohesive rafts rapidly lose their collectivity, speed, and group dynamics, and the cells become vulnerable to the drugs. The antibiotic resistance capability of swarming B. subtilis is experimentally observed to be negatively correlated with the number density of colloidal particles on the engineered surface. This relationship is further tested using an improved self-propelled particle model that takes into account interparticle alignment and hard-core repulsion. This work has pertinent implications on the design of optimal methods to treat drug resistant bacteria commonly found in swarming colonies.Published versio

Surface-Enhanced Raman Scattering (SERS) of Nitrothiophenol isomers chemisorbed on TiO2

Surface-enhanced Raman scattering (SERS) spectroscopy and density functional theory (DFT) calculations were used to investigate the nature of the charge-transfer (CT) process between nitrothiophenol (NTP) isomers and the n-type semiconductor, TiO2. The Raman signals of p-NTP and m-NTP that were chemisorbed onto TiO2 were significantly enhanced with respect to their corresponding neat compounds. In particular, an enhancement factor (EF) of 102–103 was observed for both p-NTP and m-NTP, with m-NTP displaying a larger EF compared to p-NTP. The Raman signal of o-NTP on TiO2 was not detectable, owing to interference from fluorescence emissions. A molecule-to-TiO2 charge-transfer mechanism was responsible for the enhanced Raman signals observed in p-NTP and m-NTP. This transfer was due to a strong coupling between the adsorbate and the metal oxide, which led to an optically driven CT transition from the HOMO of NTP into the conduction band of TiO2. Based on the mesomeric effect, the NO2 group para to the thiol had a stronger electron-withdrawing ability than the NO2 group at the meta position. A less-efficient CT transition from p-NTP to TiO2 in the surface complex resulted in a weaker Raman-signal enhancement for p-NTP compared to m-NTP. The DFT calculation determined that the HOMO and the LUMO of NTP bound to TiO2 were located entirely on the adsorbate and the semiconductor, respectively, thereby supporting the experimental findings that a molecule-to-TiO2 mechanism was the driving force behind the observed SERS effect

Versatile telluracycle synthesis via the sequential electrophilic telluration of C(sp2)–Zn and C(sp2)–H bonds

We report herein a new approach for the synthesis of tellurium-bridged aromatic compounds based on the sequential electrophilic telluration of C(sp2)–Zn and C(sp2)–H bonds with tellurium(IV) chlorides. A combination of transition metal-catalyzed (migratory) arylmetalation of alkynes and sequential telluration allows for the expedient construction of a library of functionalized benzo[b]tellurophenes. Furthermore, a variety of heteroarene-fused benzotellurophenes and other novel tellurium-embedded polycyclic aromatics can be readily synthesized from the corresponding 2-iodoheterobiaryls.MOE (Min. of Education, S’pore)Published versio

Near-infrared light-mediated photoactivation of a platinum antitumor prodrug and simultaneous cellular apoptosis imaging by upconversion-luminescent nanoparticles

Platinum-based drugs are among the most active antitumor reagents in clinical practice; their application is limited by side effects and drug resistance. A novel and personalized near-infrared (NIR) light-activated nanoplatform is obtained by combining a photoactivatable platinum(IV) prodrug and a caspase imaging peptide conjugated with silica-coated upconversion-luminescent nanoparticles (UCNPs) for the remote control of antitumor platinum prodrug activation, and simultaneously for real-time imaging of apoptosis induced by activated cytotoxicity. Upon NIR light illumination, the PtIV prodrug complex is activated at the surface of the nanoparticle and active components are selectively released which display cytotoxicity against human ovarian carcinoma A2780 cells and its cisplatin-resistant variant A2780cis cells. More importantly, the caspases enzymes triggered by cytotoxicity would effectively cleave the probe peptide, thereby allowing the direct imaging of apoptosis in living cells

Increasing antibiotic activity by rapid bioorthogonal conjugation of drug to resistant bacteria using an upconverted light-activated photocatalyst

Antibiotic vancomycin (Van) is often used as the drug of last resort to treat methicillin resistant Staphylococcus aureus. Due to the emergence of Van-resistant microbes, it is necessary to continuously design strategies to increase the efficacy of Van against resistant cells. In this study, an efficient method of bio-conjugating Van to bacteria is proposed using near-infrared (NIR)-light activation. A Nd³⁺-sensitized upconversion nanocrystal (UCNC) decorated with toluidine blue O (TB) on its surface undergoes upconverted energy transfer from the UCNC to TB when excited by 808 nm light. The photoexcited TB then catalyses the conversion of the dihydrotetrazine (dHTz) moiety in a Van-dHTz conjugate system to tetrazine which undergoes an efficient inverse electron demand Diels-Alder reaction with prior attached norbornene molecules on bacterial cell walls. The enhanced affinity of Van to bacteria by covalent bonding improves the activity of the drug against drug-resistant Enterococci, and the MIC is reduced by 6- to 7-fold as compared to neat Van. We demonstrate that the mode of action is due to increased inhibition of peptidoglycan cell wall biosynthesis. The findings in this study demonstrate that on-demand NIR-light activated bioorthogonal conjugation of Van to microbes is a viable alternative treatment in combating drug-resistant bacteria.Ministry of Education (MOE)E. K. L. Y acknowledges the Singapore Ministry of Education MoE Tier 1 fund (RG6/18) for financial support

The role of ligand acid-base reaction in the facile synthesis of Alkali Metal Neodymium Penta- And Heptafluoride Nanocrystals

We report a facile co-precipitation method of synthesizing orthorhombic phase CsNd2F7 and hexagonal phase K2NdF5 nanocrystals, which avoids the use of relatively harsh conditions (e.g., high temperatures and long reaction times in hydrothermal synthesis). By reacting MF (M = Cs and K), Nd(CH3CO2)3, oleic acid (OA), and 1-octadecene in the presence of sufficient amounts of oleylamine ligands (OAm) to ensure a non-acidic environment, CsNd2F7 and K2NdF5 nanocrystals are formed for reaction temperatures (Tr) ranging between 150 and 300 °C. When OAm is in excess of OA and Tr = 300 °C, uniformly distributed CsNd2F7 nanodiscs (∼20 nm in diameter) and sub-10 nm K2NdF5 nanodiscs with an average thickness of 2-3 unit cell layers are formed. When OAm is removed, the acidic mixture produces CsNd2F7 nanocrystals at low Tr (150-200 °C); however, a mixture of NdF3 and Cs-oleate is observed instead when Tr is increased. Similarly, in an acidic enviroment, K2NdF5 nanocrystals rapidly decompose into a mixture of K0.156Nd0.142-yF0.702-3y and y(NdF3) (y 200 °C). Therefore, the penta- and heptafluoride nanocrystals are thermodynamically stable only in a basic medium when OA is consumed, in part by its condensation reaction with OAm. We further demonstrate that the CsNd2F7 and K2NdF5 nanocrystals display good light-to-heat conversion efficiencies (16-27%) and computed tomography (CT) attenuation, making them promising theranostic photothermal imaging/therapy and CT contrast agents.Ministry of Education (MOE)We acknowledge financial support from the Singapore Ministry of Education MoE Tier 1 grants (RG89/20 and RG6/18) and Duke-NUS Block Grant

Peptide–perylene diimide functionalized magnetic nano-platforms for fluorescence turn-on detection and clearance of bacterial lipopolysaccharides

A simple and unique strategy has been successfully designed for sensitive detection and rapid clearance of bacterial lipopolysaccharides (LPS) by integration of core–shell Fe3O4@SiO2 magnetic nanoparticles with a perylene-diimide (PDI) conjugated LPS-recognition peptide.Published versio

Polysulfide anions as visible light photoredox catalysts for aryl cross-couplings

Polysulfide anions are endowed with unique redox properties, attracting considerable attentions for their applications in alkali metals-sulfur batteries. However, the employment of these anionic species in redox catalysis for small molecule synthesis remains underdeveloped due to their moderate-poor electrochemical potential in the ground state, whereas some of them are characterized by photoabsorptions in visible spectral regions. Herein, we disclose the use of polysulfide anions as visible light photoredox catalysts for aryl cross-coupling reactions. The reaction design enables single-electron reduction of aryl halides upon the photoexcitation of tetrasulfide dianions (S42-). The resulting aryl radicals are engaged in (hetero)biaryl cross-coupling, borylation, and hydrogenation in a redox catalytic regime involving S4• -/S42- and S3• -/S32- redox couples.Agency for Science, Technology and Research (A*STAR)Accepted versionFinancial support was provided by Pharma Innovation Programme Singapore (A*STAR, SERC A19B3a0014)