Milling-Induced "Turn-off" Luminescence in Copper Nanoclusters.

Atomically precise copper nanoclusters (NCs) attract research interest due to their intense photoluminescence, which enables their applications in photonics, optoelectronics, and sensing. Exploring these properties requires carefully designed clusters with atomic precision and a detailed understanding of their atom-specific luminescence properties. Here, we report two copper NCs, [Cu4(MNA)2(DPPE)2] and [Cu6(MNA-H)6], shortly Cu4 and Cu6, protected by 2-mercaptonicotinic acid (MNA-H2) and 1,2-bis(diphenylphosphino)ethane (DPPE), showing "turn-off" mechanoresponsive luminescence. Single-crystal X-ray diffraction reveals that in the Cu4 cluster, two Cu2 units are appended with two thiols, forming a flattened boat-shaped Cu4S2 kernel, while in the Cu6 cluster, two Cu3 units form an adamantane-like Cu6S6 kernel. High-resolution electrospray ionization mass spectrometry studies reveal the molecular nature of these clusters. Lifetime decay profiles of the two clusters show the average lifetimes of 0.84 and 1.64 µs, respectively. These thermally stable Cu NCs become nonluminescent upon mechanical milling but regain their emission upon exposure to solvent vapors. Spectroscopic data of the clusters match well with their computed electronic structures. This work expands the collection of thermally stable and mechanoresponsive luminescent coinage metal NCs, enriching the diversity and applications of such materials.

Observing Real-Time Adhesion of Microparticles on Glass Surfaces.

Fouling on glass surfaces reduces the solar panel efficiency and increases water consumption for cleaning. Superhydrophobic coatings on glass enable self-cleaning by allowing water droplets to carry away dirt particles. Observing the interaction between charged particles and surfaces provides insights into effective cleaning. Using a high-speed camera and a long-distance objective, we analyzed the in situ deposition of variously functionalized and charged silica dust microparticles on chemically treated glass. The ambient charges for the control, hydrophobic, and positively charged particles were approximately -0.5, -0.13, and +0.5 nC, respectively. We found that a positively charged particle of 2.3 ± 1.2 µm diameter adhered to hydroxylated glass in ∼0.054 s, compared to 0.40 and 0.45 s for quaternary ammonium- and fluorosilane-functionalized hydrophobic glass. Experiments suggest that quaternary ammonium-functionalized glass surfaces are about 77.8% more resistant to soiling than bare surfaces.

Nanocluster reaction-driven in situ transformation of colloidal nanoparticles to mesostructures.

Atomically precise noble metal nanoclusters (NCs) are molecular materials known for their precise composition, electronic structure, and unique optical properties, exhibiting chemical reactivity. Herein, we demonstrated a simple one-pot method for fabricating self-assembled Ag-Au bimetallic mesostructures using a reaction between 2-phenylethanethiol (PET)-protected atomically precise gold NCs and colloidal silver nanoparticles (Ag NPs) in a tunable reaction microenvironment. The reaction carried out in toluene at 45 °C with constant stirring at 250 revolutions per minute (RPM) yielded a thermally stable, micron-sized cuboidal mesocrystals of self-assembled AgAu@PET nanocrystals. However, the reaction in dichloromethane at room temperature with constant stirring at 250 RPM resulted in a self-assembled mesostructure of randomly close-packed AgAu@PET NPs. Using a host of experimental techniques, including optical and electron microscopy, optical absorption spectroscopy, and light scattering, we studied the nucleation and growth processes. Our findings highlight a strategy to utilize precision and plasmonic NP chemistry in tailored microenvironments, leading to customizable bimetallic hybrid three-dimensional nanomaterials with potential applications.

Multicolor photoluminescence of Cu14 clusters modulated using surface ligands.

Copper nanoclusters exhibit unique structural features and their molecular assembly results in diverse photoluminescence properties. In this study, we present ligand-dependent multicolor luminescence observed in a Cu14 cluster, primarily protected by ortho-carborane-9,12-dithiol (o-CBDT), featuring an octahedral Cu6 inner kernel enveloped by eight isolated copper atoms. The outer layer of the metal kernel consists of six bidentate o-CBDT ligands, in which carborane backbones are connected through µ3-sulphide linkages. The initially prepared Cu14 cluster, solely protected by six o-CBDT ligands, did not crystallize in its native form. However, in the presence of N,N-dimethylformamide (DMF), the cluster crystallized along with six DMF molecules. Single-crystal X-ray diffraction (SCXRD) revealed that the DMF molecules were directly coordinated to six of the eight capping Cu atoms, while oxygen atoms were bound to the two remaining Cu apices in antipodal positions. Efficient tailoring of the cluster surface with DMF shifted its luminescence from yellow to bright red. Luminescence decay profiles showed fluorescence emission for these clusters, originating from the singlet states. Additionally, we synthesized microcrystalline fibers with a one-dimensional assembly of DMF-appended Cu14 clusters and bidentate DPPE linkers. These fibers exhibited bright greenish-yellow phosphorescence emission, originating from the triplet state, indicating the drastic surface tailoring effect of secondary ligands. Theoretical calculations provided insights into the electronic energy levels and associated electronic transitions for these clusters. This work demonstrated dynamic tuning of the emissive excited states of copper nanoclusters through the efficient engineering of ligands.

Toward Continuous Breath Monitoring on a Mobile Phone Using a Frugal Conducting Cloth-Based Smart Mask.

A frugal humidity sensor that can detect changes in the humidity of exhaled breath of individuals has been fabricated. The sensor comprises a humidity-sensitive conducting polymer that is in situ formed on a cloth that acts as a substrate. Interdigitated silver electrodes were screen-printed on the modified cloth, and conducting threads connected the electrodes to the measurement circuit. The sensor's response to changing humidity was measured as a voltage drop across the sensor using a microcontroller. The sensor was capable of discerning between fast, normal, and slow breathing based on the response time. A response time of ∼1.3 s was observed for fast breathing. An Android-based mobile application was designed to collect sensor data via Bluetooth for analysis. A time series classification algorithm was implemented to analyze patterns in breathing. The sensor was later stitched onto a face mask, transforming it into a smart mask that can monitor changes in the breathing pattern at work, play, and sleep.