Hiang Kwee Lee

One-step synthesis of zero-dimensional hollow nanoporous gold nanoparticles with enhanced methanol electrooxidation performance

Nanoporous gold with networks of interconnected ligaments and highly porous structure holds stimulating technological implications in fuel cell catalysis. Current syntheses of nanoporous gold mainly revolve around de-alloying approaches that are generally limited by stringent and harsh multistep protocols. Here we develop a one-step solution phase synthesis of zero-dimensional hollow nanoporous gold nanoparticles with tunable particle size (150-1,000 nm) and ligament thickness (21-54 nm). With faster mass diffusivity, excellent specific electroactive surface area and large density of highly active surface sites, our zero-dimensional nanoporous gold nanoparticles exhibit ~1.4 times enhanced catalytic activity and improved tolerance towards carbonaceous species, demonstrating their superiority over conventional nanoporous gold sheets. Detailed mechanistic study also reveals the crucial heteroepitaxial growth of gold on the surface of silver chloride templates, implying that our synthetic protocol is generic and may be extended to the synthesis of other nanoporous metals via different templates.

Surfactant-Directed Atomic to Mesoscale Alignment: Metal Nanocrystals Encased Individually in Single-Crystalline Porous Nanostructures

Composite nanomaterials are attractive for a diverse range of applications in catalysis, plasmonics, sensing, imaging, and biology. In such composite nanomaterials, it is desired, yet still challenging to create a controlled alignment between components with lattices in disparate scales. To address this challenge, we report a new concept of colloidal synthesis, in which self-assembled molecular layers control the alignment between materials during the synthesis. To illustrate this concept, self-assembled cetyltrimethylammonium bromide (CTAB) molecules are used to control interfaces in a core-shell nanocomposite with a well-defined metal nanocrystal core and a metal-organic-framework (MOF) shell, which differ in structural dimensions by orders of magnitude. We show that single metal nanocrystals are captured individually in single-crystalline MOFs, and an alignment between the {100} planes of the metal and {110} planes of the MOFs is observed. By utilizing the same concept, a layer of mesostructured silica is formed over MOF crystals. These multilayered core-shell structures demonstrate a controlled alignment across a wide range of materials, from the metal nanocrystals, extending to nanoporous MOFs and mesostructured silica.

Graphene Liquid Marbles as Photothermal Miniature Reactors for Reaction Kinetics Modulation

We demonstrate the fabrication of graphene liquid marbles as photothermal miniature reactors with precise temperature control for reaction kinetics modulation. Graphene liquid marbles show rapid and highly reproducible photothermal behavior while maintaining their excellent mechanical robustness. By tuning the applied laser power, swift regulation of graphene liquid marbles surface temperature between 21-135 °C and its encapsulated water temperature between 21-74 °C are demonstrated. The temperature regulation modulates the reaction kinetics in our graphene liquid marble, achieving a 12-fold superior reaction rate constant for methylene blue degradation than at room temperature.

Plasmonic Liquid Marbles: A Miniature Substrate‐less SERS Platform for Quantitative and Multiplex Ultratrace Molecular Detection

Inspired by aphids, liquid marbles have been studied extensively and have found application as isolated microreactors, as micropumps, and in sensing. However, current liquid-marble-based sensing methodologies are limited to qualitative colorimetry-based detection. Herein we describe the fabrication of a plasmonic liquid marble as a substrate-less analytical platform which, when coupled with ultrasensitive SERS, enables simultaneous multiplex quantification and the identification of ultratrace analytes across separate phases. Our plasmonic liquid marble demonstrates excellent mechanical stability and is suitable for the quantitative examination of ultratrace analytes, with detection limits as low as 0.3 fmol, which corresponds to an analytical enhancement factor of 5×10(8). The results of our simultaneous detection scheme based on plasmonic liquid marbles and an aqueous-solid-organic interface quantitatively tally with those found for the individual detection of methylene blue and coumarin.

Superhydrophobic-oleophobic Ag nanowire platform: an analyte-concentrating and quantitative aqueous and organic toxin surface-enhanced Raman scattering sensor.

The ultratrace detection and quantification of toxins in both water and organic liquids remains a challenge due to the random spreading and dilution of liquids on substrate-based sensors, especially for organic liquids with low surface tension. Herein, we fabricate a superhydrophobic-oleophobic (SHP-OP) 3D Ag nanowire mesh-like surface-enhanced Raman scattering (SERS) platform to overcome the random spreading issue, demonstrating ultratrace toxin sensing in both water and organic liquid. Our SHP-OP SERS platform is able to concentrate analyte solutions in water and toluene to 100-fold and 8-fold smaller areas, respectively, as compared to its omniphilic counterparts. The synergy of analyte-concentrating ability and intense SERS-enhancing properties on our SHP-OP SERS platform enables quantitative and ultratrace detection of melamine and Sudan I down to 0.1 fmol in water and toluene, respectively, using just 1 μL of analyte solution. These detection limits are 10(3)-fold lower than the regulatory limits, clearly indicating our SHP-OP SERS platform as an appealing universal ultratrace toxin sensor. The ultratrace detection of spiked melamine in liquid milk down to 100 fmol also highlights the suitability of our SHP-OP SERS platform for the sensing of food toxins in real samples.

Plasmonic Colloidosomes as Three‐Dimensional SERS Platforms with Enhanced Surface Area for Multiphase Sub‐Microliter Toxin Sensing

Colloidosomes are robust microcapsules attractive for molecular sensing because of their characteristic micron size, large specific surface area, and dual-phase stability. However, current colloidosome sensors are limited to qualitative fluorogenic receptor-based detection, which restrict their applicability to a narrow range of molecules. Here, we introduce plasmonic colloidosome constructed from Ag nanocubes as an emulsion-based 3D SERS platform. The colloidosomes exhibit excellent mechanical robustness, flexible size tunability, versatility to merge, and ultrasensitivity in SERS quantitation of food/industrial toxins down to sub-femtomole levels. Using just 0.5 μL of sample volumes, our plasmonic colloidosomes exhibit >3000-fold higher SERS sensitivity over conventional suspension platform. Notably, we demonstrate the first high-throughput multiplex molecular sensing across multiple liquid phases.

Favoring the unfavored: Selective electrochemical nitrogen fixation using a reticular chemistry approach

Originally unfavored nitrogen-to-ammonia electroconversion is now preferred over competing reaction using reticular chemistry. Electrochemical nitrogen-to-ammonia fixation is emerging as a sustainable strategy to tackle the hydrogen- and energy-intensive operations by Haber-Bosch process for ammonia production. However, current electrochemical nitrogen reduction reaction (NRR) progress is impeded by overwhelming competition from the hydrogen evolution reaction (HER) across all traditional NRR catalysts and the requirement for elevated temperature/pressure. We achieve both excellent NRR selectivity (~90%) and a significant boost to Faradic efficiency by 10 percentage points even at ambient operations by coating a superhydrophobic metal-organic framework (MOF) layer over the NRR electrocatalyst. Our reticular chemistry approach exploits MOF’s water-repelling and molecular-concentrating effects to overcome HER-imposed bottlenecks, uncovering the unprecedented electrochemical features of NRR critical for future theoretical studies. By favoring the originally unfavored NRR, we envisage our electrocatalytic design as a starting point for high-performance nitrogen-to-ammonia electroconversion directly from water vapor–abundant air to address increasing global demand of ammonia in (bio)chemical and energy industries.

Nanoporous Gold Bowls: A Kinetic Approach to Control Open Shell Structures and Size-Tunable Lattice Strain for Electrocatalytic Applications.

Controlling sub-10 nm ligament sizes and open-shell structure in nanoporous gold (NPG) to achieve strained lattice is critical in enhancing catalytic activity, but it remains a challenge due to poor control of reaction kinetics in conventional dealloying approach. Herein, a ligament size-controlled synthesis of open-shell NPG bowls (NPGB) through hetero-epitaxial growth of NPGB on AgCl is reported. The ligament size in NPGB is controlled from 6 to 46 nm by varying the hydroquinone to HAuCl4 ratio. The Williamson-Hall analysis demonstrates a higher lattice strain in smaller ligament size. In particular, NPGB with 6 nm (NPGB 6) ligament size possess the highest strain of 15.4 × 10(-3) , which is nearly twice of conventional 2D NPG sheets (≈8.8 × 10(-3) ). The presence of high surface energy facets in NPGBs is also envisaged. The best electrocatalytic activity toward methanol oxidation is observed in NPGB 6 (27.8 μA μg(-1) ), which is ≈9-fold and 3-fold higher than 8 nm solid Au nanoparticles, and conventional NPG sheets. The excellent catalytic activity in NPGB 6 is attributed to the open-shell structure, lattice strain, and higher electro-active surface area, allowing efficient exposure of catalytic active sites to facilitate the methanol oxidation. The results offer a potential strategy for designing next generation electrocatalysts.

Spinning liquid marble and its dual applications as microcentrifuge and miniature localized viscometer

Liquid marble offers an attractive droplet manipulation approach by isolating microdroplet in a nonstick encapsulating shell formed via the spontaneous coating of hydrophobic particles onto the liquid surface. While liquid marble prepared using magnetic nanoparticles enables precise spatiotemporal actuation of microdroplets, these manipulations are generally limited to simple and linear spatial maneuver of microdroplets. Herein, we demonstrate the unique and three-dimensional spinning of microliter-sized liquid marble (LM) and its subsequent dual applications as (1) the worlds smallest centrifuge and (2) a miniature and localized viscometer. Our LM is responsive to an applied rotating magnetic field, with its spinning speed programmable between 0 and 1300 rpm. This spinning generates an unprecedented centrifugal force of >2g in a LM of ∼1 mm radius. Such centrifugal force facilitates an outward and radial hydrodynamic flow in the enclosed microdroplet, enabling LM to serve as a microcentrifuge for the sedimentation of nanoparticles with >85% separation efficiency. Furthermore, we apply spinning LM as an ultrasensitive spin-to-viscosity transducer to quantify the viscosity of the external suspended liquid in the relative viscosity (η/ηwater) range of 1-70 using ≤1 mL liquid sample. Collectively, the ensemble of benefits offered by spinning LM creates enormous opportunities in the development of multifunctional micromagneto-mechanical devices as promising surface-sensitive microsensor, miniature centrifugal pump, and even microreactor with directed heat and mass transfer mechanism.

Identifying Enclosed Chemical Reaction and Dynamics at the Molecular Level Using Shell-Isolated Miniaturized Plasmonic Liquid Marble

Current microscale tracking of chemical kinetics is limited to destructive ex situ methods. Here we utilize Ag nanocube-based plasmonic liquid marble (PLM) microreactor for in situ molecular-level identification of reaction dynamics. We exploit the ultrasensitive surface-enhanced Raman scattering (SERS) capability imparted by the plasmonic shell to unravel the mechanism and kinetics of aryl-diazonium surface grafting reaction in situ, using just a 2-μL reaction droplet. This reaction is a robust approach to generate covalently functionalized metallic surfaces, yet its kinetics remain unknown to date. Experiments and simulations jointly uncover a two-step sequential grafting process. An initial Langmuir chemisorption of sulfonicbenzene diazonium (dSB) salt onto Ag surfaces forms an intermediate sulfonicbenzene monolayer (Ag-SB), followed by subsequent autocatalytic multilayer growth of Ag-SB3. Kinetic rate constants reveal 19-fold faster chemisorption than multilayer growth. Our ability to precisely decipher molecular-level reaction dynamics creates opportunities to develop more efficient processes in synthetic chemistry and nanotechnology.