Optical sensor technology offers significant opportunities in the field of medical research and clinical diagnostics, particularly for the detection of small numbers of molecules in highly diluted solutions. Several methods have been developed for this purpose, including label-free plasmonic biosensors based on metamaterials. However, the detection of lower-molecular-weight (<500 Da) biomolecules in highly diluted solutions is still a challenging issue owing to their lower polarizability. In this context, we have developed a miniaturized plasmonic biosensor platform based on a hyperbolic metamaterial that can support highly confined bulk plasmon guided modes over a broad wavelength range from visible to near infrared. By exciting these modes using a grating-coupling technique, we achieved different extreme sensitivity modes with a maximum of 30,000 nm per refractive index unit (RIU) and a record figure of merit (FOM) of 590. We report the ability of the metamaterial platform to detect ultralow-molecular-weight (244 Da) biomolecules at picomolar concentrations using a standard affinity model streptavidin-biotin.

Hydrogen production via photoelectrochemical water-splitting is a key source of clean and sustainable energy. The use of one-dimensional nanostructures as photoelectrodes is desirable for photoelectrochemical water-splitting applications due to the ultralarge surface areas, lateral carrier extraction schemes, and superior light-harvesting capabilities. However, the unavoidable surface states of nanostructured materials create additional charge carrier trapping centers and energy barriers at the semiconductor-electrolyte interface, which severely reduce the solar-to-hydrogen conversion efficiency. In this work, we address the issue of surface states in GaN nanowire photoelectrodes by employing a simple and low-cost surface treatment method, which utilizes an organic thiol compound (i.e., 1,2-ethanedithiol). The surface-treated photocathode showed an enhanced photocurrent density of -31 mA/cm at -0.2 V versus RHE with an incident photon-to-current conversion efficiency of 18.3%, whereas untreated nanowires yielded only 8.1% efficiency. Furthermore, the surface passivation provides enhanced photoelectrochemical stability as surface-treated nanowires retained ∼80% of their initial photocurrent value and produced 8000 μmol of gas molecules over 55 h at acidic conditions (pH ∼ 0), whereas the untreated nanowires demonstrated only <4 h of photoelectrochemical stability. These findings shed new light on the importance of surface passivation of nanostructured photoelectrodes for photoelectrochemical applications.

Solar-based water sanitation is an environmentally friendly process for obtaining clean water that requires efficient light-to-heat-to-vapour generation. Solar-driven interfacial evaporation has potential, but the inability to control interfacial evaporators for solar tracking limits efficiency at large solar zenith angles and when using optical concentration. Furthermore, clogging affects the efficiency of the device. Here, we create a super-wicking and super-light-absorbing (SWSA) aluminium surface for efficient solar-based water sanitation. The measured evaporation rate exceeds that of an ideal device operating at 100% efficiency, which we hypothesize resulted from a reduced enthalpy of vaporization within the microcapillaries. Limited solar absorber-water contact for water transport minimizes heat losses to bulk water and maximizes heat localization at the SWSA surface. The device can be mounted at any angle on a floating platform to optimize incident solar irradiance and can readily be integrated with commercial solar-thermal systems. With a design that is analogous to bifacial photovoltaic solar panels, we show a 150% increase in efficiency compared with a single-sided SWSA. Given the open capillary channels, the device surface can be easily cleaned and reused. Using the SWSA surface to purify contaminated water, we show a decrease in the level of contaminants to well below the WHO and EPA standards for drinkable water.

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The extreme anisotropic permittivity of hyperbolic metamaterials (HMMs) represent a unique opportunity to realize effective bulk metastructures with extraordinary optical properties over a broad frequency range from visible to terahertz. [1] HMMs are artificial uniaxial materials that exhibit hyperbolic dispersion because the out of plane dielectric constant (ε zz = ε ⊥ ) has an opposite sign to the in-plane dielectric constants (ε xx = ε yy = ε ll ). HMMs can be classified into two types based on the sign of their dielectric components, i.e., type I (−ε ⊥ and ε ll ) and type II (ε ⊥ and −ε ll ). In comparison to isotropic materials showing elliptical dispersion, HMMs support propagation of optical modes across the structure with infinitely large momentum (high-k modes) in the effective medium limit, [2,3] irrespective of Hyperbolic metamaterials (HMMs) have emerged as a burgeoning field of research over the past few years as their dispersion can be easily engineered in different spectral regions using various material combinations. Even though HMMs have comparatively low optical loss due to a single resonance, the noble-metal-based HMMs are limited by their strong energy dissipation in metallic layers at visible frequencies. Here, the fabrication of noble-metal-free reconfigurable HMMs for visible photonic applications is experimentally demonstrated. The low-loss and active HMMs are realized by combining titanium nitride (TiN) and stibnite (Sb 2 S 3 ) as the phase change material. A reconfigurable plasmonic biosensor platform based on active Sb 2 S 3 -TiN HMMs is proposed, and it is shown that significant improvement in sensitivity is possible for small molecule detection at low concentrations. In addition, a plasmonic apta-biosensor based on a hybrid platform of graphene and Sb 2 S 3 -TiN HMM is developed and the detection and real-time binding of thrombin concentration as low as 1 × 10 −15 m are demonstrated. A biosensor operating in the visible range has several advantages including the availability of sources and detectors in this region, and ease of operation particularly for point-of-care applications.

Electrochemical reduction of CO 2 provides an opportunity to produce fuels and chemicals in a carbon-neutral manner, assuming that CO 2 can be captured from the atmosphere. To do so, requires efficient, selective, and stable catalysts. In this study, we report a highly mesoporous metallic Cu catalyst prepared by electrochemical reduction of thermally nitrided Cu foil.Under aqueous saturated CO 2 reduction conditions, the Cu 3 N-derived Cu electrocatalyst produces virtually no CH 4 , very little CO, and exhibits a Faradaic efficiency of 68% to C 2+ products (C 2 H 4 , C 2 H 5 OH, and C 3 H 7 OH) at a current density of ~18.5 mA cm -2 and a cathode potential of -1.0 V vs. the reversible hydrogen electrode (RHE). Under these conditions, the catalyst produces more oxygenated products than hydrocarbons. We show that surface roughness is a good descriptor of catalytic performance. The roughest surface reached 98% CO utilization efficiency for C 2+ product formation from CO 2 reduction and the ratio of oxygenated to hydrocarbon products correlates with the degree of surface roughness. These effects of surface roughness are attributed to the high population of under-coordinated sites as well as a high pH environment within the mesopores and adjacent to the surface of the catalyst.

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