Label-Free Ultrasensitive Detection of Abnormal Chiral Metabolites in Diabetes

Enantiomer-Specific Colorimetric Tandem Assays for Salivary d-Alanine Associated with Gastric Cancer

Salivary d-alanine (d-Ala) and d-proline (d-Pro) are of concern for their potential in the noninvasive diagnosis of gastric cancer (GC). Most reports have succeeded in determining the total concentration of d-Ala and d-Pro. However, for personalized diagnosis and better elucidation of the underlying specific correlation of d-Ala (or d-Pro) with GC, it is desirable to determine the specific concentration of d-Ala or d-Pro. Herein, we propose an enantiomer-specific tandem assay of d-Ala based on the colorimetric reaction between 2,4-dinitrophenylhydrazine and pyruvic acid generated from the deamination of d-Ala catalyzed by d-amino acid oxidase, which is easily distinguished from l-form amino acids, d-Pro, and many other species. A linear concentration range is established from 20 to 400 μmol/L with a limit of detection of 1.01 μmol/L. Real saliva sample tests reveal that the levels of d-Ala in GC cases are remarkably higher than those in healthy individuals, which offers a simple and low-cost strategy for GC diagnosis. Simultaneously, the total concentrations of d-Ala and d-Pro in saliva are determined. Hence, the concentration of d-Pro and the proportion of d-Ala could be calculated, which further provides more molecule- and individual-specific information. This research may offer a convenient method for noninvasive diagnosis of GC and pave a new route to explore the potentials of rare d-form amino acids in disease diagnosis and treatment.

Synergetic Multichiral Covalent Organic Framework for Enantioselective Recognition and Separation

In enantiomer recognition and separation, a highly enantioselective approach with universal applicability is urgently desired but hard to realize, especially in the case of chiral molecules. To resolve the trade-off between enantioselectivity and universality, a glutathione (GSH) and methylated cyclodextrins (MCD)-functionalized covalent organic framework (GSH-MCD COF) with porosity and abundant chiral surfaces is presented that was designed and synthesized for recognition and separation of various enantiomers. As expected, the GSH-MCD COF can be used as chiral stationary phases for the separation of various enantiomers, including aromatic alcohols, aromatic acids, amides, amino acids, and organic acids, with performance and versatility even superior to some widely used commercial chiral chromatographic columns. Furthermore, the synthesized GSH-MCD COF shows high enantioselectivity for the rapid recognition and identification of enantiomers and chiral metabolites when coupling to Raman spectroscopy. Molecular simulations suggest that the COF provides a confined microenvironment for cyclodextrins and peptides that dictates the separation and recognition capability. This work provides a strategy to synthesize synergetic multichiral COF and achieve separations and recognitions of enantiomers in complex samples.

Nanophotonic Enhanced Chiral Sensing and Its Biomedical Applications

Chiral sensing is crucial in the fields of biology and the pharmaceutical industry. Many naturally occurring biomolecules, i.e., amino acids, sugars, and nucleotides, are inherently chiral. Their enantiomers are strongly associated with the pharmacological effects of chiral drugs. Owing to the extremely weak chiral light-matter interactions, chiral sensing at an optical frequency is challenging, especially when trace amounts of molecules are involved. The nanophotonic platform allows for a stronger interaction between the chiral molecules and light to enhance chiral sensing. Here, we review the recent progress in nanophotonic-enhanced chiral sensing, with a focus on the superchiral near-field and enhanced circular dichroism (CD) spectroscopy generated in both the dielectric and in plasmonic structures. In addition, the recent applications of chiral sensing in biomedical fields are discussed, including the detection and treatment of difficult diseases, i.e., Alzheimer's disease, diabetes, and cancer.

Probing Interactions between Chiral Plasmonic Nanoparticles and Biomolecules

Chiral plasmonic nanoparticles (and their assemblies) interact with biomolecules in a variety of different ways, resulting in distinct optical signatures when probed by circular dichroism spectroscopy. These systems show promise for biosensing applications and offer several advantages over achiral plasmonic systems. Arguably the most notable advantage is that chiral nanoparticles can differentiate between molecular enantiomers and can, therefore, act as sensors for enantiomeric purity. Furthermore, chiral nanoparticles can couple more effectively to chiral biomolecules in biological systems if they have a matching handedness, improving their effectiveness as biomedical agents. In this article, we review the different types of interactions that occur between chiral plasmonic nanoparticle systems and biomolecules, and discuss how circular dichroism spectroscopy can probe these interactions and inform how to optimize systems for biosensing and biomedical applications.

Advancing chirality analysis through enhanced enantiomer characterization and quantification via fast Fourier transform capacitance voltammetry

The exploration of the chiral configurations of enantiomers represents a highly intriguing realm of scientific inquiry due to the distinct roles played by each enantiomer (D and L) in chemical reactions and their practical utilities. This study introduces a pioneering analytical methodology, termed fast Fourier transform capacitance voltammetry (FFT-CPV), in conjunction with principal component analysis (PCA), for the identification and quantification of the chiral forms of tartaric acid (TA), serving as a representative model system for materials exhibiting pronounced chiral characteristics. The proposed methodology relies on the principle of chirality, wherein the capacitance signal generated by the adsorption of D-TA and L-TA onto the surface of a platinum electrode (Pt-electrode) in an acidic solution is harnessed. The capacitance voltammograms were meticulously recorded under optimized experimental conditions. To compile the final dataset for the analyte, the average of the FFT capacitance voltammograms of the acidic solution (without the presence of the analyte) was subtracted from those containing the analyte. A distinct arrangement was obtained by employing PCA as a linear data transformation method, representing D-TA and L-TA in a two/three-dimensional space. The outcomes of the study reveal the successful detection of the two chiral forms of TA with a considerable degree of precision and reproducibility. Moreover, the proposed method facilitated the establishment of two linear response ranges for the concentration values of each enantiomer, spanning from 1 to 20 µM, and 50 to 500 µM. The respective detection limits were also determined to be 0.4 µM for L-TA and 1.3 µM for D-TA. These findings underscore the satisfactory sensitivity and efficiency of the proposed method in both qualitative and quantitative assessments of the chiral forms of TA.

Hot-Electron Dynamics Mediated Medical Diagnosis and Therapy

Surface plasmon resonance excitation significantly enhances the absorption of light and increases the generation of "hot" electrons, i.e., conducting electrons that are raised from their steady states to excited states. These excited electrons rapidly decay and equilibrate via radiative and nonradiative damping over several hundred femtoseconds. During the hot-electron dynamics, from their generation to the ultimate nonradiative decay, the electromagnetic field enhancement, hot electron density increase, and local heating effect are sequentially induced. Over the past decade, these physical phenomena have attracted considerable attention in the biomedical field, e.g., the rapid and accurate identification of biomolecules, precise synthesis and release of drugs, and elimination of tumors. This review highlights the recent developments in the application of hot-electron dynamics in medical diagnosis and therapy, particularly fully integrated device techniques with good application prospects. In addition, we discuss the latest experimental and theoretical studies of underlying mechanisms. From a practical standpoint, the pioneering modeling analyses and quantitative measurements in the extreme near field are summarized to illustrate the quantification of hot-electron dynamics. Finally, the prospects and remaining challenges associated with biomedical engineering based on hot-electron dynamics are presented.

Vibrational circular dichroism unveils hidden clues

Infrared chiral plasmonic metamaterials based on perpendicularly positioned nanorods enable surface-enhanced vibrational circular dichroism for more selective and sensitive identification of protein fingerprints and enantioselective sensing, which creates a new pathway for chemical or biomedical applications.

Chirality in Light-Matter Interaction

The scientific effort to control the interaction between light and matter has grown exponentially in the last 2 decades. This growth has been aided by the development of scientific and technological tools enabling the manipulation of light at deeply sub-wavelength scales, unlocking a large variety of novel phenomena spanning traditionally distant research areas. Here, the role of chirality in light-matter interactions is reviewed by providing a broad overview of its properties, materials, and applications. A perspective on future developments is highlighted, including the growing role of machine learning in designing advanced chiroptical materials to enhance and control light-matter interactions across several scales.

Expanding chiral metamaterials for retrieving fingerprints via vibrational circular dichroism

Circular dichroism (CD) spectroscopy has been widely demonstrated for detecting chiral molecules. However, the determination of chiral mixtures with various concentrations and enantiomeric ratios can be a challenging task. To solve this problem, we report an enhanced vibrational circular dichroism (VCD) sensing platform based on plasmonic chiral metamaterials, which presents a 6-magnitude signal enhancement with a selectivity of chiral molecules. Guided by coupled-mode theory, we leverage both in-plane and out-of-plane symmetry-breaking structures for chiral metamaterial design enabled by a two-step lithography process, which increases the near-field coupling strengths and varies the ratio between absorption and radiation loss, resulting in improved chiral light-matter interaction and enhanced molecular VCD signals. Besides, we demonstrate the thin-film sensing process of BSA and β-lactoglobulin proteins, which contain secondary structures α-helix and β-sheet and achieve a limit of detection down to zeptomole level. Furthermore, we also, for the first time, explore the potential of enhanced VCD spectroscopy by demonstrating a selective sensing process of chiral mixtures, where the mixing ratio can be successfully differentiated with our proposed chiral metamaterials. Our findings improve the sensing signal of molecules and expand the extractable information, paving the way toward label-free, compact, small-volume chiral molecule detection for stereochemical and clinical diagnosis applications.

Detection and analysis of chiral molecules as disease biomarkers

The chirality of small metabolic molecules is important in controlling physiological processes and indicating the health status of humans. Abnormal enantiomeric ratios of chiral molecules in biofluids and tissues occur in many diseases, including cancers and kidney and brain diseases. Thus, chiral small molecules are promising biomarkers for disease diagnosis, prognosis, adverse drug-effect monitoring, pharmacodynamic studies and personalized medicine. However, it remains difficult to achieve cost-effective and reliable analysis of small chiral molecules in clinical procedures, in part owing to their large variety and low concentration. In this Review, we describe current and emerging techniques that detect and quantify small-molecule enantiomers and their biological importance.

The interaction of tryptophan enantiomers with model membranes is modulated by polar head type and physical state of phospholipids

The mutual influence of chiral bioactive molecules and supramolecular assemblies is currently being studied in many research fields, including medical-pharmaceutical applications. Model membranes of phospholipids, such as the zwitterionic dipalmitoylphosphatidylcholine (DPPC) and the anionic dipalmitoylphosphatidylglycerol (DPPG), interact with a variety of chiral compounds that include amino acids. In this work, the interaction of tryptophan enantiomers, L-Trp and D-Trp, on DPPC and DPPG bilayers was investigated by using differential scanning calorimetry, attenuated total reflectance-Fourier transform infrared and spin-label electron spin resonance spectroscopies as well as molecular docking simulations. The results show that Trp enantiomers slightly perturb the bilayer thermotropic phase transitions. For both membranes, O atoms in the carbonyl groups have a propensity to act as acceptors of a (weak) hydrogen bond. The Trp chiral forms also promote formation of hydrogen bonds and/or hydration in the PO₂^- moiety of the phosphate group, especially for the DPPC bilayer. In contrast, they interact more closely with the glycerol group of DPPG polar head. Only for DPPC bilayers, both enantiomers increase the packing of the first hydrocarbon chain segments for temperatures through the gel state, whereas they do not affect the lipid chain order and mobility in the fluid state. The results are consistent with a Trp association in the upper region of the bilayers without permeation in the innermost hydrophobic region. The findings suggest that neutral and anionic lipid bilayers are differently sensitive to amino acid chirality.

Optothermal rotation of micro-/nano-objects

Due to its contactless and fuel-free operation, optical rotation of micro-/nano-objects provides tremendous opportunities for cellular biology, three-dimensional (3D) imaging, and micro/nanorobotics. However, complex optics, extremely high operational power, and the applicability to limited objects restrict the broader use of optical rotation techniques. This Feature Article focuses on a rapidly emerging class of optical rotation techniques, termed optothermal rotation. Based on light-mediated thermal phenomena, optothermal rotation techniques overcome the bottlenecks of conventional optical rotation by enabling versatile rotary control of arbitrary objects with simpler optics using lower powers. We start with the fundamental thermal phenomena and concepts: thermophoresis, thermoelectricity, thermo-electrokinetics, thermo-osmosis, thermal convection, thermo-capillarity, and photophoresis. Then, we highlight various optothermal rotation techniques, categorizing them based on their rotation modes (i.e., in-plane and out-of-plane rotation) and the thermal phenomena involved. Next, we explore the potential applications of these optothermal manipulation techniques in areas such as single-cell mechanics, 3D bio-imaging, and micro/nanomotors. We conclude the Feature Article with our insights on the operating guidelines, existing challenges, and future directions of optothermal rotation.

Optothermal rotation of micro-/nano-objects in liquids

Controllable rotation of micro-/nano-objects provides tremendous opportunities for cellular biology, three-dimensional (3D) imaging, and micro/nanorobotics. Among different rotation techniques, optical rotation is particularly attractive due to its contactless and fuel-free operation. However, optical rotation precision is typically impaired by the intrinsic optical heating of the target objects. Optothermal rotation, which harnesses light-modulated thermal effects, features simpler optics, lower operational power, and higher applicability to various objects. In this Feature Article, we discuss the recent progress of optothermal rotation with a focus on work from our research group. We categorize the various rotation techniques based on distinct physical mechanisms, including thermophoresis, thermoelectricity, thermo-electrokinetics, thermo-osmosis, thermal convection, and thermo-capillarity. Benefiting from the different rotation modes (i.e., in-plane and out-of-plane rotation), diverse applications in single-cell mechanics, 3D bio-imaging, and micro/nanomotors are demonstrated. We conclude the article with our perspectives on the operating guidelines, existing challenges, and future directions of optothermal rotation.

Recent Advances in Ultrathin Chiral Metasurfaces by Twisted Stacking

Artificial chiral nanostructures have been subjected to extensive research for their unique chiroptical activities. Planarized chiral films of ultrathin thicknesses are in particular demand for easy on-chip integration and improved energy efficiency as polarization-sensitive metadevices. Recently, controlled twisted stacking of two or more layers of nanomaterials, such as 2D van der Waals materials, ultrathin films, or traditional metasurfaces, at an angle has emerged as a general strategy to introduce optical chirality into achiral solid-state systems. This method endows new degrees of freedom, e.g., the interlayer twist angle, to flexibly engineer and tune the chiroptical responses without having to change the material or the design, thus greatly facilitating the development of multifunctional metamaterials. In this review, recent exciting progress in planar chiral metasurfaces are summarized and discussed from the viewpoints of building blocks, fabrication methods, as well as circular dichroism and modulation thereof in twisted stacked nanostructures. The review further highlights the ever-growing portfolio of applications of these chiral metasurfaces, including polarization conversion, information encryption, chiral sensing, and as an engineering platform for hybrid metadevices. Finally, forward-looking prospects are provided.

Plasmonic Chiral Metasurface-Induced Upconverted Circularly Polarized Luminescence from Achiral Upconversion Nanoparticles

Chirality induction, transfer, and manipulation have aroused great interest in achiral nanomaterials. Here, we demonstrate strong upconverted circularly polarized luminescence from achiral core-shell upconversion nanoparticles (UCNPs) via a plasmonic chiral metasurface-induced optical chirality transfer. The Yb³⁺-sensitized core-shell UCNPs with good dispersity exhibit intense upconversion luminescence of Tm³⁺ and Nd³⁺ through the energy transfer process. By spin-coating the core-shell UCNPs on this chiral metasurface, strong enhancement and circular polarization modulation of upconversion luminescence can be achieved due to resonant coupling between surface plasmons and upconversion nanoparticles. In the UCNPs-on-metasurface composite, a significant upconversion luminescence enhancement can be achieved with a maximum enhancement factor of 32.63 at 878 nm and an overall enhancement factor of 11.61. The luminescence dissymmetry factor of the induced upconverted circularly polarized luminescence can reach 0.95 at the emission wavelength of 895 nm. The UCNPs-on-metasurface composite yields efficient modulation for the emission intensity and polarization of UCNPs, paving new pathways to many potential applications in imaging, sensing, and anticounterfeiting fields.

Chiral Plasmonic Shells: High-Performance Metamaterials for Sensitive Chiral Biomolecule Detection

Low-cost and large-area chiral metamaterials (CMs) are highly desirable for practical applications in chiral biosensors, nanophotonic chiral emitters, and beyond. A promising fabrication method takes advantage of self-assembled colloidal particles, onto which metal patches with defined orientation are created using glancing angle deposition (GLAD). However, using this method to make uniform and well-defined CMs over macroscopic areas is challenging. Here, we fabricate a uniform large-area colloidal particle array by interface-mediated self-assembly and precisely control the structural handedness of chiral plasmonic shells (CPSs) using GLAD. Strong chiroptical signals arise from twisted currents at the main, corner, and edge of CPSs, allowing a balance between strong chiroptical and high transmittance properties. Our shell-like chiral geometry shows excellent sensor performance in detecting chiral molecules due to the formation of uniform superchiral fields. Systematic investigations optimize the interplay between peak and null point resonances in different CPSs and result in a record consistency chiral sensor parameter U, i.e., 3.77 for null points and 0.0867 for peaks, which are about 54 and 1.257 times larger than the highest value (0.068) of previously reported CMs. The geometrical chirality, surface plasmonic resonance, chiral surface lattice resonance, and chiral sensor performance evidence the chiroptical effect and the excellent chiral sensor performance.

Ultrathin Suspended Chiral Metasurfaces for Enantiodiscrimination

Chiral metasurfaces can exhibit a strong circular dichroism, but it is limited by the complicated fabrication procedure and alignment errors. Here, a new type of self-aligned suspended chiral bilayer metasurface with only one-step electron beam lithography exposure is demonstrated. A significant optical chirality of 221° µm^-1 can be realized using suspended metasurfaces with a thickness of 100 nm. Furthermore, this study experimentally demonstrates that such a structure is capable of label-free discrimination of the chiral molecules at zeptomole level, exhibiting a much higher sensitivity (orders of magnitude) compared to the conventional circular dichroism spectroscopy. The fundamental principles for chiral sensing using molecules-metasurfaces interactions are explored. Benefiting from the giant chiroptical response, the proposed metadevice may offer promising applications for ultrathin circular polarizers, chiral molecular detectors, and asymmetry information processing.

Bubble-pen lithography: Fundamentals and applications: Nanoscience: Special Issue Dedicated to Professor Paul S. Weiss

Developing on-chip functional devices requires reliable fabrication methods with high resolution for miniaturization, desired components for enhanced performance, and high throughput for fast prototyping and mass production. Recently, laser-based bubble-pen lithography (BPL) has been developed to enable sub-micron linewidths, in situ synthesis of custom materials, and on-demand patterning for various functional components and devices. BPL exploits Marangoni convection induced by a laser-controlled microbubble to attract, accumulate, and immobilize particles, ions, and molecules onto different substrates. Recent years have witnessed tremendous progress in theory, engineering, and application of BPL, which motivated us to write this review. First, an overview of experimental demonstrations and theoretical understandings of BPL is presented. Next, we discuss the advantages of BPL and its diverse applications in quantum dot displays, biological and chemical sensing, clinical diagnosis, nanoalloy synthesis, and microrobotics. We conclude this review with our perspective on the challenges and future directions of BPL.

Chiral biosensing at both interband transition and plasmonic extinction regions using twisted-stacked nanowire arrays

Chiral metal nanostructures that exhibit strong chiroptical properties and enhanced light-matter interactions have recently attracted great interest due to their potential applications including chiral sensing and asymmetric synthesis. Most studies in this field focused on chiral sensing using circular dichroism (CD) responses at the plasmonic extinction region. In comparison, little is known about their CD responses at interband transition regions and their utility in chiral biosensing. Herein, we constructed a series of twisted-stacked silver nanowire arrays (TNAs) featuring CD signals at both the interband transition and plasmonic extinction regions and that are independently controllable. These TNAs are highly sensitive towards protein secondary structures. Proteins containing more β-sheets are more sensitive toward strong chiral plasmonic fields, whereas proteins rich in α-helices tend to generate larger CD shifts at the interband transition region of TNAs. The mutually independent optical activities at the interband transition and plasmonic extinction regions complement each other, providing more sensitivity and reliability in chiral biosensing.

Heat-Mediated Optical Manipulation

Progress in optical manipulation has stimulated remarkable advances in a wide range of fields, including materials science, robotics, medical engineering, and nanotechnology. This Review focuses on an emerging class of optical manipulation techniques, termed heat-mediated optical manipulation. In comparison to conventional optical tweezers that rely on a tightly focused laser beam to trap objects, heat-mediated optical manipulation techniques exploit tailorable optothermo-matter interactions and rich mass transport dynamics to enable versatile control of matter of various compositions, shapes, and sizes. In addition to conventional tweezing, more distinct manipulation modes, including optothermal pulling, nudging, rotating, swimming, oscillating, and walking, have been demonstrated to enhance the functionalities using simple and low-power optics. We start with an introduction to basic physics involved in heat-mediated optical manipulation, highlighting major working mechanisms underpinning a variety of manipulation techniques. Next, we categorize the heat-mediated optical manipulation techniques based on different working mechanisms and discuss working modes, capabilities, and applications for each technique. We conclude this Review with our outlook on current challenges and future opportunities in this rapidly evolving field of heat-mediated optical manipulation.