Photonically active bowtie nanoassemblies with chirality continuum.

Chirality is a geometrical property described by continuous mathematical functions1-5. However, in chemical disciplines, chirality is often treated as a binary left or right characteristic of molecules rather than a continuity of chiral shapes. Although they are theoretically possible, a family of stable chemical structures with similar shapes and progressively tuneable chirality is yet unknown. Here we show that nanostructured microparticles with an anisotropic bowtie shape display chirality continuum and can be made with widely tuneable twist angle, pitch, width, thickness and length. The self-limited assembly of the bowties enables high synthetic reproducibility, size monodispersity and computational predictability of their geometries for different assembly conditions6. The bowtie nanoassemblies show several strong circular dichroism peaks originating from absorptive and scattering phenomena. Unlike classical chiral molecules, these particles show a continuum of chirality measures2 that correlate exponentially with the spectral positions of the circular dichroism peaks. Bowtie particles with variable polarization rotation were used to print photonically active metasurfaces with spectrally tuneable positive or negative polarization signatures for light detection and ranging (LIDAR) devices.

Glycosylated quantum dots as fluorometric nanoprobes for trehalase.

Trehalase is an important enzyme in the metabolic cascades of many organisms, catalysing the hydrolysis of the disaccharide trehalose. Herein we describe the first examples of fluorometric nanoprobes for detection of trehalase, based on trehalose-functionalised quantum dots (QDs). QDs cross-linked with trehalose form aggregates, which are released upon enzymatic cleavage of the trehalose glycosidic bond proportionally to the enzyme concentration, offering a unique and efficient approach for specific sensing of this biologically important enzyme.

Hydrothermal synthesis of chiral carbon dots.

Nanocolloids that are cumulatively referred to as nanocarbons, attracted significant attention during the last decade because of facile synthesis methods, water solubility, tunable photoluminescence, easy surface modification, and high biocompatibility. Among the latest development in this reserach area are chiral nanocarbons exemplified by chiral carbon dots (CDots). They are expected to have applications in sensing, catalysis, imaging, and nanomedicine. However, the current methods of CDots synthesis show often contradictory chemical/optical properties and structural information that required a systematic study with careful structural evaluation. Here, we investigate and optimize chiroptical activity and photoluminescence of L- and D-CDots obtained by hydrothermal carbonization of L- and D-cysteine, respectively. Nuclear magnetic resonance spectroscopy demonstrates that they are formed via gradual dehydrogenation and condensation reactions of the starting amino acid leading to particles with a wide spectrum of functional groups including aromatic cycles. We found that the chiroptical activity of CDots has an inverse correlation with the synthesis duration and temperature, whereas the photoluminescence intensity has a direct one, which is associated with degree of carbonization. Also, our studies show that the hydrothermal synthesis of cysteine in the presence of boric acid leads to the formation of CDots rather than boron nitride nanoparticles as was previously proposed in several reports. These results can be used to design chiral carbon-based nanoparticles with optimal chemical, chiroptical, and photoluminescent properties.

Circular Dichroism Spectroscopy as a Powerful Tool for Unraveling Assembly of Chiral Nonluminescent Aggregates of Photosensitizer Molecules on Nanoparticle Surfaces.

Recent developments in nanoscience and nanotechnology significantly help improve the properties of traditional materials. A striking example of this is the formation of hybrid nanostructures based on nanoparticles and photosensitizer molecules, the potential range of applications of which extends from photovoltaics to biomedicine. However, the creation of new and effective hybrid nanomaterials of this form inevitably entails new challenges, one of which is a common and critical problem of aggregation of both nanoparticles and photosensitizer molecules. Therefore, a fundamental challenge is to determine the presence of these aggregates, which will produce a significant step toward creating a new generation of materials and devices of broad-spectrum applicability. Here we report on the key role of circular dichroism spectroscopy as a tool to detect the formation of nonluminescent aggregates of chlorin e6, a second-generation photosensitizer, in a hybrid nanostructure with ZnS:Mn quantum dots. These aggregates are active acceptors of photoexcitation energy from quantum dots and limit the photophysical properties of the whole nanostructure. It has been established that circular dichroism spectroscopy reveals the presence of nonluminescent molecule aggregates at chlorin e6 concentrations of ∼10-6 mol/L, which compares very favorably to absorption spectroscopy which does not show any direct indications of aggregation up to ∼10-5 mol/L. This result demonstrates the promise and importance of using circular dichroism spectroscopy in the study of organic/inorganic hybrid nanostructures.

Chirality Analysis of Complex Microparticles using Deep Learning on Realistic Sets of Microscopy Images.

Nanoscale chirality is an actively growing research field spurred by the giant chiroptical activity, enantioselective biological activity, and asymmetric catalytic activity of chiral nanostructures. Compared to chiral molecules, the handedness of chiral nano- and microstructures can be directly established via electron microscopy, which can be utilized for the automatic analysis of chiral nanostructures and prediction of their properties. However, chirality in complex materials may have multiple geometric forms and scales. Computational identification of chirality from electron microscopy images rather than optical measurements is convenient but is fundamentally challenging, too, because (1) image features differentiating left- and right-handed particles can be ambiguous and (2) three-dimensional structure essential for chirality is 'flattened' into two-dimensional projections. Here, we show that deep learning algorithms can identify twisted bowtie-shaped microparticles with nearly 100% accuracy and classify them as left- and right-handed with as high as 99% accuracy. Importantly, such accuracy was achieved with as few as 30 original electron microscopy images of bowties. Furthermore, after training on bowtie particles with complex nanostructured features, the model can recognize other chiral shapes with different geometries without retraining for their specific chiral geometry with 93% accuracy, indicating the true learning abilities of the employed neural networks. These findings indicate that our algorithm trained on a practically feasible set of experimental data enables automated analysis of microscopy data for the accelerated discovery of chiral particles and their complex systems for multiple applications.

Effect of Chiral Ligand Concentration and Binding Mode on Chiroptical Activity of CdSe/CdS Quantum Dots.

Chiroptically active fluorescent semiconductor nanocrystals, quantum dots (QDs), are of high interest from a theoretical and technological point of view, because they are promising candidates for a range of potential applications. Optical activity can be induced in QDs by capping them with chiral molecules, resulting in circular dichroism (CD) signals in the range of the QD ultraviolet-visible (UV-vis) absorption. However, the effects of the chiral ligand concentration and binding modes on the chiroptical properties of QDs are still poorly understood. In the present study, we report the strong influence of the concentration of a chiral amino acid (cysteine) on its binding modes upon the surface of CdSe/CdS QDs, resulting in varying QD chiroptical activity and corresponding CD signals. Importantly, we demonstrate that the increase of cysteine concentration is accompanied by the growth of the QD CD intensity, reaching a certain critical point, after which it starts to decrease. The intensity of the CD signal varies by almost an order of magnitude across this range. Nuclear magnetic resonance and Fourier transform infrared data, supported by density functional theory calculations, reveal a change in the binding mode of cysteine molecules from tridentate to bidentate when going from low to high concentrations, which results in a change in the CD intensity. Hence, we conclude that the chiroptical properties of QDs are dependent on the concentration and binding modes of the capping chiral ligands. These findings are very important for understanding chiroptical phenomena at the nanoscale and for the design of advanced optically active nanomaterials.

Impact of Shell Thickness on Photoluminescence and Optical Activity in Chiral CdSe/CdS Core/Shell Quantum Dots.

Core/shell quantum dots (QDs) are of high scientific and technological importance as these nanomaterials have found a number of valuable applications. In this paper, we have investigated the dependence of optical activity and photoluminescence upon CdS shell thickness in a range of core-shell structured CdSe/CdS QDs capped with chiral ligands. For our study, five samples of CdSe/CdS were synthesized utilizing successive ion layer adsorption and reaction to vary the thickness of the CdS shell from 0.5 to 2 nm, upon a 2.8 nm diameter CdSe core. Following this, a ligand exchange of the original aliphatic ligands with l- and d-cysteine was carried out, inducing a chiroptical response in these nanostructures. The samples were then characterized using circular dichroism, photoluminescent spectroscopy, and fluorescence lifetime spectroscopy. It has been found that the induced chiroptical response was inversely proportional to the CdS shell thickness and showed a distinct evolution in signal, whereas the photoluminescence of our samples showed a direct relationship to shell thickness. In addition, a detailed study of the influence of annealing time on the optical activity and photoluminescence quantum yield was performed. From our work, we have been able to clearly illustrate the approach and strategies that must be used when designing optimal photoluminescent optically active CdSe/CdS core-shell QDs.