Solid‐state nanopores are fascinating objects that enable the development of specific and efficient chemical and biological sensors, as well as the investigation of the physicochemical principles ...ruling the behavior of biological channels. The great variety of biological nanopores that nature provides regulates not only the most critical processes in the human body, including neuronal communication and sensory perception, but also the most important bioenergetic process on earth: photosynthesis. This makes them an exhaustless source of inspiration toward the development of more efficient, selective, and sophisticated nanopore‐based nanofluidic devices. The key point responsible for the vibrant and exciting advance of solid nanopore research in the last decade has been the simultaneous combination of advanced fabrication nanotechnologies to tailor the size, geometry, and application of novel and creative approaches to confer the nanopore surface specific functionalities and responsiveness. Here, the state of the art is described in the following critical areas: i) theory, ii) nanofabrication techniques, iii) (bio)chemical functionalization, iv) construction of nanofluidic actuators, v) nanopore (bio)sensors, and vi) commercial aspects. The plethora of potential applications once envisioned for solid‐state nanochannels is progressively and quickly materializing into new technologies that hold promise to revolutionize the everyday life.
The virtues of working with solid‐state nanopores are being increasingly recognized by the scientific community. Although in the past decades the focus has been mostly on their fundamental aspects, for several years now nanopore research has started to shift toward specific practical applications. The latest developments in this fascinating research field are brought together and summarized.
Periodic structures with a sub‐wavelength pitch have been known since Hertz conducted his first experiments on the polarization of electromagnetic waves. While the use of these structures in ...waveguide optics was proposed in the 1990s, it has been with the more recent developments of silicon photonics and high‐precision lithography techniques that sub‐wavelength structures have found widespread application in the field of photonics. This review first provides an introduction to the physics of sub‐wavelength structures. An overview of the applications of sub‐wavelength structures is then given including: anti‐reflective coatings, polarization rotators, high‐efficiency fiber–chip couplers, spectrometers, high‐reflectivity mirrors, athermal waveguides, multimode interference couplers, and dispersion engineered, ultra‐broadband waveguide couplers among others. Particular attention is paid to providing insight into the design strategies for these devices. The concluding remarks provide an outlook on the future development of sub‐wavelength structures and their impact in photonics.
With the recent development of silicon photonics and high resolution lithography, periodic structures with a sub‐wavelength pitch have found widespread application. This review provides an introduction to the physics of sub‐wavelength structures and an extensive overview of their applications in waveguide devices, including: high efficiency fiber‐chip couplers, wavelength multiplexers, athermal waveguide and ultra‐broadband waveguide couplers.
Phonon polaritons-light coupled to lattice vibrations-in polar van der Waals crystals are promising candidates for controlling the flow of energy on the nanoscale due to their strong field ...confinement, anisotropic propagation and ultra-long lifetime in the picosecond range
. However, the lack of tunability of their narrow and material-specific spectral range-the Reststrahlen band-severely limits their technological implementation. Here, we demonstrate that intercalation of Na atoms in the van der Waals semiconductor α-V
O
enables a broad spectral shift of Reststrahlen bands, and that the phonon polaritons excited show ultra-low losses (lifetime of 4 ± 1 ps), similar to phonon polaritons in a non-intercalated crystal (lifetime of 6 ± 1 ps). We expect our intercalation method to be applicable to other van der Waals crystals, opening the door for the use of phonon polaritons in broad spectral bands in the mid-infrared domain.
Segmenting silicon waveguides at the subwavelength scale produce an equivalent homogenous material. The geometry of the waveguide segments provides precise control over modal confinement, effective ...index, dispersion and birefringence, thereby opening up new approaches to design devices with unprecedented performance. Indeed, with ever-improving lithographic technologies offering sub-100-nm patterning resolution in the silicon photonics platform, many practical devices based on subwavelength structures have been demonstrated in recent years. Subwavelength engineering has thus become an integral design tool in silicon photonics, and both fundamental understanding and novel applications are advancing rapidly. Here, we provide a comprehensive review of the state of the art in this field. We first cover the basics of subwavelength structures, and discuss substrate leakage, fabrication jitter, reduced backscatter, and engineering of material anisotropy. We then review recent applications including broadband waveguide couplers, high-sensitivity evanescent field sensors, low-loss devices for mid-infrared photonics, polarization management structures, spectral filters, and highly efficient fiber-to-chip couplers. We finally discuss the future prospects for subwavelength silicon structures and their impact on advanced device design.
Summary
The kinetics of wood formation in angiosperms are largely unknown because their complex xylem anatomy precludes using the radial position of vessels and fibers to infer their time of ...differentiation.
We analyzed xylogenesis in ring‐porous ash (Fraxinus angustifolia) and diffuse‐porous beech (Fagus sylvatica) over 1 yr and proposed a novel procedure to assess the period of vessel and fiber enlargement using a referential radial file (RRF).
Our approach captured the dynamics of wood formation and provided a robust estimation of the kinetics of vessel and fiber enlargement. In beech, fibers and vessels had a similar duration of enlargement, decreasing from 14 to 5 d between April and July. In ash, wide vessels formed in April enlarged at a rate of 27 × 103 μm2 d−1, requiring half the time of contemporary fibers (6 vs 12 d), and less time than the narrower vessels (14 d) formed in May.
These findings reveal distinct cell‐type‐dependent mechanisms for differentiation in diffuse‐porous and ring‐porous trees, enhancing our understanding of angiosperm wood cell kinetics. Our approach presents an effective method for investigating angiosperm wood formation and provides a more accurate representation of vessel and fiber morphogenesis in wood formation models.
The design of an all‐plastic field‐effect nanofluidic diode is proposed, which allows precise nanofluidic operations to be performed. The fabrication process involves the chemical synthesis of a ...conductive poly(3,4‐ethylenedioxythiophene) (PEDOT) layer over a previously fabricated solid‐state nanopore. The conducting layer acts as gate electrode by changing its electrochemical state upon the application of different voltages, ultimately changing the surface charge of the nanopore. A PEDOT‐based nanopore is able to discriminate the ionic species passing through it in a quantitative and qualitative manner, as PEDOT nanopores display three well‐defined voltage‐controlled transport regimes: cation‐rectifying, non‐rectifying, and anion rectifying regimes. This work illustrates the potential and versatility of PEDOT as a key enabler to achieve electrochemically addressable solid‐state nanopores. The synergism arising from the combination of highly functional conducting polymers and the remarkable physical characteristics of asymmetric nanopores is believed to offer a promising framework to explore new design concepts in nanofluidic devices.
The integration of conductive poly(3,4‐ethylenedioxythiophene) (PEDOT) into asymmetric solid‐state nanopores leads the way to the construction of all‐plastic field‐effect nanofluidic diodes. The conducting layer acts as a gate electrode by changing its electrochemical state upon the application of different voltages. PEDOT‐based electrochemically addressable nanofluidic devices display three well‐defined voltage‐controlled transport regimes: cation‐rectifying, nonrectifying, and anion rectifying regimes.
The ability to modulate the surface chemical characteristics of solid-state nanopores is of great interest as it provides the means to control the macroscopic response of nanofluidic devices. For ...instance, controlling surface charge and polarity of the pore walls is one of the most important applications of surface modification that is very relevant to attain accurate control over the transport of ions through the nanofluidic architecture. In this work, we describe a new integrative chemical approach to fabricate nanofluidic diodes based on the self-polymerization of dopamine (PDOPA) on asymmetric track-etched nanopores. Our results demonstrate that PDOPA coating is not only a simple and effective method to modify the inner surface of polymer nanopores fully compatible with the fabrication of nanofluidic devices but also a versatile platform for further integration of more complex molecules through different covalent chemistries and self-assembly processes. We adjusted the chemical modification strategy to obtain various configurations of the pore surface: (i) PDOPA layer was used as primer, precursor, or even responsive functional coating; (ii) PDOPA layer was used as a platform for anchoring chemical functions via the Michael addition reaction; and (iii) PDOPA was used as a reactive layer inducing the metallization of the pore walls through the in situ reduction of metallic precursors present in solution. We believe that the transversal concept of integrative surface chemistry offered by polydopamine in combination with the remarkable physical characteristics of asymmetric nanopores constitutes a new framework to design multifunctional nanofluidic devices employing soft chemistry-based nanofunctionalization techniques.
Properties of reflection and transmission spectral filters based on Bragg gratings in subwavelength grating (SWG) metamaterial waveguides on silicon-on-insulator platform have been analyzed using ...proprietary 2D and 3D simulation tools based on Fourier modal method and the coupled-mode theory. We also demonstrate that the coupled Bloch mode theory can be advantageously applied to design of Bragg gratings in SWG waveguides. By combining different techniques, including judiciously positioning silicon loading segments within the evanescent field of the SWG waveguide and making use of its dispersion properties, it is possible to attain sub-nanometer spectral bandwidths for both reflection and transmission filters in the wavelength range of 1550 nm while keeping minimum structural features of the filters as large as 100 nm. Numerical simulations have also shown that a few nanometer jitter in the size and position of Si segments is well tolerated in our filter designs.
There is currently high interest in developing nanofluidic devices whose iontronic output is defined by biological interactions. The fabrication of a phosphate responsive nanofluidic diode by using ...the biological relevant amine–phosphate interactions is shown. The fabrication procedure includes the modification of a track‐etched asymmetric (conical) nanochannel with polyallylamine (PAH) by electrostatic self‐assembly. PAH is the arcaetypical model of polyamine and it is further used to address the nanochannels with phosphate responsivity. In order to explore the influence that phosphate in solution has in the conductance of the modified nanochannels, current–voltage measurements using different concentrations of phosphates are performed. Furthermore, to have a complete physicochemical understanding of the system, experimental data is analyzed using a continuous model based on Poison–Nernst–Planck equations and compared with results obtained from stochastic Monte Carlo simulations.
The ionic transport properties of nanochannels decorated with polyamines can be fine‐tuned by the presence of inorganic phosphates. Monte Carlo simulations reveal that these functional properties arise from a complex interplay between the polyamine and phosphate counterions, which, in turn, determine the concentration of mobile carriers inside the nanochannels.
Molecular design of biosensors based on enzymatic processes taking place in nanofluidic elements is receiving increasing attention by the scientific community. In this work, we describe the ...construction of novel ultrasensitive enzymatic nanopore biosensors employing “reactive signal amplifiers” as key elements coupled to the transduction mechanism. The proposed framework offers innovative design concepts not only to amplify the detected ionic signal and develop ultrasensitive nanopore-based sensors but also to construct nanofluidic diodes displaying specific chemo-reversible rectification properties. The integrated approach is demonstrated by electrostatically assembling poly(allylamine) on the anionic pore walls followed by the assembly of urease. We show that the cationic weak polyelectrolyte acts as a “reactive signal amplifier” in the presence of local pH changes induced by the enzymatic reaction. These bioinduced variations in proton concentration ultimately alter the protonation degree of the polyamine resulting in amplifiable, controlled, and reproducible changes in the surface charge of the pore walls, and consequently on the generated ionic signals. The “iontronic” response of the as-obtained devices is fully reversible, and nanopores are reused and assayed with different urea concentrations, thus ensuring reliable design. The limit of detection (LOD) was 1 nM. To the best of our knowledge, this value is the lowest LOD reported to date for enzymatic urea detection. In this context, we envision that this approach based on the use of “reactive signal amplifiers” into solid-state nanochannels will provide new alternatives for the molecular design of highly sensitive nanopore biosensors as well as (bio)chemically addressable nanofluidic elements.
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