Origin of Unusual Excitonic Absorption and Emission from Colloidal Ag2S Nanocrystals: Ultrafast Photophysics and Solar Cell

Wet chemical synthesis of TGA capped Ag2S nanoparticles and their use for fluorescence imaging and temperature sensing in living cells

In this work, we describe a simple wet chemical route for preparing silver sulfide nanoparticles (Ag2S) encapsulated with thioglycolic acid (TGA). By using Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), energy dispersive X-ray (EDS) microanalysis, transmission electron microscopy (TEM), and dynamic light scattering (DLS), we have found that these nanoparticles were enrobed by TGA molecules and they have an Ag/S ratio nearly equal to 2.2 and a nearly spherical shape with two average size populations. Photoluminescence (PL) spectroscopy has shown that these nanoparticles are highly luminescent, photostable and photobleaching resistant and they emit in the first biologic window with a band peaking in the NIR region at 915 nm. We have demonstrated through a 3-(4,5-dimethyl-thiazol-2yl)-2,5-diphenyltetrazolium bromide (MTT) assay protocol and using U-87 MG human living cells that these nanoparticles are biocompatible with a viability ratio higher than 80% for a concentration equal to 100 μg mL-1. By investigating the effect of pH, ionic strength and thermal quenching on the PL emission, we have shown that these nanoparticles provide a convenient stable tool to measure temperature in the biological range with a relative thermal sensitivity higher than 5% per °C and they may be used as suitable fluorescent probes for living cell imaging and intracellular temperature mapping.

One-Dimensional Hybrid Copper(I) Iodide Single Crystal with Renewable Scintillation Properties

Low-dimensional hybrid copper(I) halides attract considerable attention in the field of light emissions. In this work, we obtained the centimeter-sized single crystal of 1,3-propanediamine copper(I) iodide (PDACuI₃) with a solvent evaporation method. The single crystal X-ray diffraction of PDACuI₃ reveals that the [CuI₄] tetrahedra form the corner-connected chains separated by PDAs, forming a one-dimensional structure with an orthorhombic space group of Pbca. The band gap is determined to be 4.03 eV, and the room temperature photoluminescence (PL) quantum yield is determined to be 26.5%. The thermal quenching and negative thermal quenching of emission are observed via temperature-dependent PL spectra, and our study shows that the intermediate nonradiative state below the self-trapped exciton state may get involved in these temperature-dependent behaviors. The X-ray scintillation performance of PDACuI₃ single crystals is also evaluated, and the relative light output renewed to 94.3% of the fresh one after a low-temperature annealing. On the basis of our results, PDACuI₃ single crystals provide nontoxicity and renewable scintillation performance, thus showing potential application in the area of low-cost radiation detectors.

NIR-activated multi-hit therapeutic Ag2S quantum dot-based hydrogel for healing of bacteria-infected wounds

Hydrogel dressings are highly biocompatible and can maintain a moist wound environment, suggesting constructing an efficient multi-modal antibacterial hydrogel platform is a promising strategy for treating bacterial wound infections. In this work, a composite Ag₂S quantum dot/mSiO₂ NPs hydrogel (NP hydrogel) with antibacterial ability was constructed by incorporating Ag₂S quantum dots (QDs) modified by mesoporous silica (mSiO₂) into the network structure of 3-(trimethoxylmethosilyl) propyl methacrylate based on free radical polymerization. The NP hydrogel showed outstanding controllable photothermal and photodynamic characteristics under 808 nm near infrared (NIR) light irradiation, with a photothermal conversion efficiency of 57.3%. Additionally, the release of Ag⁺ could be controlled by the inherent volume change of the NP hydrogel made of N-isopropylacrylamide (NIPAAm) and acrylamide (AAm) during NIR laser exposure, with the embedded Ag₂S QDs working as a reservoir to release Ag⁺ continuously from the hydrogel matrix to achieve bactericidal activity. The synergetic effects between hyperthermia, radical oxygen species, and Ag⁺ released under NIR radiation endowed the NP hydrogel with prominent antibacterial properties against Escherichia coli (E. coli) and methicillin-resistant Staphylococcus aureus (MRSA), with an inhibition rate of 99.7% and 99.8%, respectively. In vivo wound healing experiments indicated that the NP hydrogel could enhance bacterial clearance, increase collagen coverage area and up-regulate VEGF expression, exhibiting high biocompatibility. Overall, this study proposed an efficient and highly biocompatible multi-modal therapeutic nanohydrogel, opening up a new way for developing broad-spectrum antibacterial wound dressings to treat bacterial wound infections. STATEMENT OF SIGNIFICANCE: Bacterial wound infection is still one of the most difficult medical problems. In this work, a stimulating NIR-responsive hydrogel encapsulating functional Ag₂S QDs was prepared, which showed high photothermal conversion efficiency (57.3%) and outstanding antibacterial ability under 808 nm NIR laser, killing 99.7% and 99.8% of E. coli and MRSA in 4 min, respectively. During NIR light irradiation, the release rate of Ag⁺ could be regulated by the intrinsic volume transition of the hydrogel, leading to remarkable antibacterial properties in vitro and in vivo under the combined action of hyperthermia, radical oxygen species and Ag⁺ released. This study proposed a novel multi-modal therapeutic nanohydrogel, opening up a new way for developing broad-spectrum antibacterial wound dressings to treat bacterial wound infections.

Synthesis and Bioapplications of Ag2 S Quantum Dots with Near-Infrared Fluorescence

Quantum dots (QDs) with near-infrared fluorescence (NIR) are an emerging class of QDs with unique capabilities owing to the deeper tissue penetrability of NIR light compared with visible light. NIR light also effectively overcomes organism autofluorescence, making NIR QDs particularly attractive in biological imaging applications for disease diagnosis. Considering latest developments, Ag₂ S QDs are a rising star among NIR QDs due to their excellent NIR fluorescence properties and biocompatibility. This review presents the various methods to synthesize NIR Ag₂ S QDs, and systematically discusses their applications in biosensing, bioimaging, and theranostics. Major challenges and future perspectives concerning the synthesis and bioapplications of NIR Ag₂ S QDs are discussed.

Interpreting the effects of natural organic matter on antimicrobial activity of Ag2S nanoparticles with soft particle theory

Natural organic matter (NOM) ubiquitously exists in natural waters and would adsorb onto the particle surface. Previous studies showed that NOM would alleviate the toxicity of nanomaterials, while the mechanism is seldom quantitatively interpreted. Herein, the effects of humic substances [Suwannee River fulvic acid (SRFA) and Suwannee River humic acid (SRHA)] and biomacromolecules [alginate and bovine serum albumin (BSA)] on the aggregation and antimicrobial effects of silver sulfide nanoparticles (Ag₂S-NPs) were investigated. The aggregation kinetics of Ag₂S-NPs in electrolyte solutions were in agreement with the results based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The dynamic light scattering (DLS) results showed that the SRFA, SRHA, alginate and BSA molecules coated on the Ag₂S-NPs surfaces. The NOM coating layer prevented salt-induced coagulation of Ag₂S-NPs, and the effects of BSA and SRHA on Ag₂S-NPs stabilizing were more obvious than that of SRFA and alginate. Flow cytometry analysis results suggested that BSA and SRHA were more effective on alleviating the Ag₂S-NPs induced cell (Escherichia coli) membrane damage than SRFA and alginate. After interpreting the electrophoretic mobility (EPM) data of the NOM coated Ag₂S-NPs by Ohshima's soft particle theory, it was found that the thickness of the NOM coating layers followed the orders of BSA > SRHA > alginate > SRFA. The E.coli cell membrane damage level was negatively correlated with the thickness and softness of the coating layer. NOM coating may physically alleviate the contact between NPs and E. coli cells and thus attenuate the extent of cell membrane damage caused by the NP-cell interaction. This work provides a new perspective for quantitatively interpreting the influence of NOM on the environmental behaviors and risks of nanomaterials.

Ultrafast carrier dynamics and third-order nonlinear optical properties of AgInS2/ZnS nanocrystals

Broad photoluminescence (PL) emission, a large Stokes shift and extremely long-lived radiative lifetimes are the characteristics of ternary I-III-VI semiconductor nanocrystals (NCs), such as CuInS₂ and AgInS₂. However, the lack of understanding regarding the intriguing PL mechanisms and photo-carrier dynamics limits their further applications. Here, AgInS₂ and AgInS₂/ZnS NCs were chemically synthesized and their carrier dynamics were studied by time-resolved PL spectroscopy. The results demonstrated that the surface defect state, which contributed dominantly to the non-radiative decay processes, was effectively passivated through ZnS alloying. Femtosecond transient absorption spectroscopy was also used to investigate the carrier dynamics, revealing the electron storage at the surface state and donor state. Furthermore, the two photon absorption properties of AgInS₂ and AgInS₂/ZnS NCs were measured using an open-aperture Z-scan technique. The improved third-order nonlinear susceptibility [Formula: see text] of AgInS₂ through ZnS alloying demonstrates potential application in two photon PL biological imaging.

Pressure induced photoluminescence modulation in a wide range and synthesis of monodispersed ternary AgCuS nanocrystal based on Ag2S nanocrystals

Binary Ag₂S nanocrystals (NCs) have many potential optical applications because of their low toxicity, narrow direct band gaps and near-infrared photoluminescence (PL) with high emission efficiency. However, due to its small exciton Bohr radius (2.2 nm), the PL spectra of Ag₂S NCs can only be modulated below ∼1200 nm with increasing particle size. Meanwhile, ternary silver copper chalcogenides (AgCuX, X = S, Se) have also attracted increased attention in recent years. Temperature-dependent structural phase transformation leads electrical transport to exhibit fascinating transitions between p and n type conduction, which makes AgCuS and AgCuSe ideal materials for diode or transistor devices. Nevertheless, the traditional method to synthesize these materials is mainly through melting the mixture of Ag, Cu and S/Se powder under extremely high reaction temperatures (973-1373 K) and long reaction time, forming a bulk product. Therefore, the synthesis of high quality monodispersed and size-tunable AgCuS or AgCuSe NCs is still a challenge. To address these issues, in this paper, we report using Ag₂S NCs as a template, a method to synthesize monodispersed and size-tunable β-AgCuS NCs via ion exchange and diffusion processes. Similarly, monodispersed β-AgCuSe NCs were also synthesized by this simple and reproducible strategy. This synthetic method opens new avenues for investigating the size-, morphology- and temperature-dependent phase transitions of these ternary AgCuS and AgCuSe materials. Thus, the corresponding electrical transport between p and n type conduction can be studied in the future. Furthermore, we systematically investigated the pressure-dependent PL properties and band gap modulation of monodispersed Ag₂S NCs using in situ high pressure PL and absorption spectroscopy. We found that the PL peak of 6.0 nm for Ag₂S NCs could be easily adjusted from ∼1200 to 1900 nm with increasing pressure from 0 to 5.1 GPa, which greatly extends the wavelength range of the PL peak beyond that of other approaches.

Reviving near infra-red emission of Ag2 S nanoparticles using interfacial defects in the Ag2 S@CdS core-shell structure

Ag₂ S@CdS core-shell particles were synthesized with different Cd source content as a measure of shell thickness using a pulsed microwave irradiation method. The particles were verified structurally using X-ray diffraction, energy dispersive X-ray analysis and transmission electron microscopy. Optical spectroscopy revealed that core-shells show an absorption peak at 750 nm and an emission peak located around 800 nm after 6 min of microwave irradiation. With continued microwave treatment, the NIR luminescence first vanished but it was revived after 12 min of irradiation, which was 100 nm red shifted. This new type of NIR emission in Ag₂ S with sizes greater than 5 nm is due to the proximity of a highly deficient CdS shell with strong red emission that was stable for more than 6 months in water. A mechanism has been suggested for this type of emission.

Synthesis and Characterization of Ultrathin Silver Sulfide Nanoplatelets

We report the synthesis of ultrathin silver sulfide (Ag₂S) nanoplatelets (NPLs) synthesized via a one-pot method in ethylene glycol with 3-mercaptopropionic acid serving as both the sulfur precursor and the platelet ligand. The colloidally synthesized nanoplatelets are exceptionally thin, with a thickness of only 3.5 ± 0.2 Å and a 1S exciton Bohr diameter to confinement ratio of ∼12.6. The NPL growth is shown to be quantized by layer thickness using absorption and photoluminescence (PL) spectroscopy. Transmission electron microscopy, atomic force microscopy, and X-ray diffraction analyses of the NPLs show that they correspond to the (202) plane of the β-Ag₂S structure. The PL quantum yield of these NPLs is ∼30%, suggesting their potential use in biomedical imaging. Optoelectronic properties were evaluated via sensitized photocurrent spectroscopy with the resulting spectra closely matching the distinctive absorption spectral shape of the Ag₂S NPLs.

Time resolved spectroscopy of infrared emitting Ag2S nanocrystals for subcutaneous thermometry

We report a systematic investigation on the temperature dependence of fluorescence decay dynamics of infrared emitting colloidal Ag₂S nanocrystals (NCs) with different surface coatings. The drastic lifetime reduction in the biological temperature range (20-50 °C) makes Ag₂S NCs outstanding candidates for high sensitivity subcutaneous lifetime-based thermal sensing in the second biological window (1000-1400 nm). Indeed, the lifetime thermal sensitivity of Ag₂S NCs has been found to be as large as 3-4% °C^-1 at an operating wavelength of 1250 nm. Their application for lifetime-based luminescence nanothermometry has been demonstrated through simple ex vivo experiments specially designed to elucidate the magnitude of subcutaneous thermal gradients. Experimental data were found to be in excellent agreement with numerical simulations.

Excellent green but less impressive blue luminescence from CsPbBr3 perovskite nanocubes and nanoplatelets

Green photoluminescence (PL) from CsPbBr3 nanocubes (∼11 nm edge-length) exhibits a high quantum yield (>80%), narrow spectral width (∼85 meV), and high reproducibility, along with a high molar extinction coefficient (3.5 × 10(6) M(-1) cm(-1)) for lowest energy excitonic absorption. In order to obtain these combinations of excellent properties for blue (PL peak maximum, λ max < 500 nm) emitting samples, CsPbBr3 nanocubes and nanoplatelets with various dimensions were prepared. Systematic increases in both the optical gap and transition probability for radiative excitonic recombination (PL lifetime 3-7 ns), have been achieved with the decreasing size of nanocubes. A high quantum yield (>80%) was also maintained, but the spectral width increased and became asymmetric for blue emitting CsPbBr3 nanocubes. Furthermore, PL was unstable and irreproducible for samples with λ max ∼ 460 nm, exhibiting multiple features in the PL. These problems arise because smaller (<7 nm) CsPbBr3 nanocubes have a tendency to form nanoplatelets and nanorods, eventually yielding inhomogeneity in the shape and size of blue-emitting nanocrystals. Reaction conditions were then modified achieving nanoplatelets, with strong quantum confinement along the thickness of the platelets, yielding blue emission. But inhomogeneity in the thickness of the nanoplatelets again broadens the PL compared to green-emitting CsPbBr3 nanocubes. Therefore, unlike high quality green emitting CsPbBr3 nanocubes, blue emitting CsPbBr3 nanocrystals of any shape need to be improved further.

Toward the Facile and Ecofriendly Fabrication of Quantum Dot-Sensitized Solar Cells via Thiol Coadsorbent Assistance

This paper reports a facile and environmentally friendly approach to the preparation of highly efficient quantum dot-sensitized solar cells (QDSSCs) based on a combination of aqueous CuInS2 quantum dots (QDs) and thiol coadsorbents. The photovoltaic properties of the QDSSCs were found to be dependent on the type and concentration of the thiol coadsorbent. The incorporation of thiol coadsorbents results in improved JSC and VOC because (1) they provide disulfide reductants during the QD sensitization process and (2) the coadsorbent molecules are anchored on the TiO2 surface, thus affecting the movement of the conduction band of TiO2. To the best of the our knowledge, this is the first demonstrated use of various thiol coadsorbents as reducing agents in the fabrication of high-efficiency QDSSCs. CuInS2 QDSSCs fabricated with the assistance of thioglycolic acid coadsorbents exhibited efficiencies as high as 5.90%, which is 20 times higher than that of the control device without thiol coadsorbents (0.29%). In addition, the photovoltaic properties of a device fabricated using the colloidal CuInS2 QDs coated with different bifunctional linkers were investigated for comparison. The versatility of this facile fabrication process was demonstrated in the preparation of solar cells sensitized with aqueous AgInS2 or CdSeTe QDs. The AgInS2 QDSSC showed a conversion efficiency of 2.72%, which is the highest reported for Ag-based metal sulfides QDSSCs thus far.

Impact of reaction variables and PEI/l-cysteine ratio on the optical properties and cytocompatibility of cationic Ag2S quantum dots as NIR bio-imaging probes

Near-infrared emitting semiconductor quantum dots (NIRQDs) are popular fluorescent probes due to better penetration depth and elimination of tissue autofluorescence.