Electric-field modulated energy transfer in phosphorescent material- and fluorescent material-codoped polymer light-emitting diodes

The excited-state energy transfer widely exists in mixed-material systems and devices. The modulation of an electric field on the energy transfer in photoluminescence has been demonstrated. However, to date, no studies on the electric-field modulation of the excited-state energy transfer in organic optoelectronic devices have been reported. Herein, we investigate the effect of an electric field on the energy transfer in the poly(N-vinylcarbazole) (PVK) thin films doped with iridium(iii)[bis(4,6-difluorophenyl)pyridinato-N,C2']-tetrakis(1-pyrazolyl)borate (Fir6) and 5,6,11,12-tetraphenylnaphthacene (rubrene) (PVK:Fir6:rubrene) and the corresponding light-emitting diodes. Combined with the Onsager model describing electric-field enhanced exciton dissociation, we find that the electric field increases the rate of Dexter energy transfer from Fir6 to rubrene in the films and the diodes. The voltage-dependent color shift in the PVK:Fir6:rubrene light-emitting diodes can be explained by the electric-field enhanced Dexter energy transfer from Fir6 to rubrene. Our findings are important for the control of energy transfer process in organic optoelectronic devices by an electric field for desirable applications.

Synthesis and hybridization of CuInS2 nanocrystals for emerging applications

Copper indium sulfide (CuInS2) is a ternary A(I)B(III)X(VI)2-type semiconductor featuring a direct bandgap with a high absorption coefficient. In attempts to explore their practical applications, nanoscale CuInS2 has been synthesized with crystal sizes down to the quantum confinement regime. The merits of CuInS2 nanocrystals (NCs) include wide emission tunability, a large Stokes shift, long decay time, and eco-friendliness, making them promising candidates in photoelectronics and photovoltaics. Over the past two decades, advances in wet-chemistry synthesis have achieved rational control over cation-anion reactivity during the preparation of colloidal CuInS2 NCs and post-synthesis cation exchange. The precise nano-synthesis coupled with a series of hybridization strategies has given birth to a library of CuInS2 NCs with highly customizable photophysical properties. This review article focuses on the recent development of CuInS2 NCs enabled by advanced synthetic and hybridization techniques. We show that the state-of-the-art CuInS2 NCs play significant roles in optoelectronic and biomedical applications.

Recent Development in Sensitizers for Lanthanide-Doped Upconversion Luminescence

The attractive features of lanthanide-doped upconversion luminescence (UCL), such as high photostability, nonphotobleaching or photoblinking, and large anti-Stokes shift, have shown great potentials in life science, information technology, and energy materials. Therefore, UCL modulation is highly demanded toward expected emission wavelength, lifetime, and relative intensity in order to satisfy stringent requirements raised from a wide variety of areas. Unfortunately, the majority of efforts have been devoted to either simple codoping of multiple activators or variation of hosts, while very little attention has been paid to the critical role that sensitizers have been playing. In fact, different sensitizers possess different excitation wavelengths and different energy transfer pathways (to different activators), which will lead to different UCL features. Thus, rational design of sensitizers shall provide extra opportunities for UCL tuning, particularly from the excitation side. In this review, we specifically focus on advances in sensitizers, including the current status, working mechanisms, design principles, as well as future challenges and endeavor directions.

Ti3C2-MXene decorated with nanostructured silver as a dual-energy acceptor for the fluorometric neuron specific enolase detection

Nanohybrids of two-dimensional (2D) layered materials have shown fascinating prospects towards the fabrication of highly efficient fluorescent immunosensor. In this context, a nanohybrid of ultrathin Ti₃C₂-MXene nanosheets and silver nanoparticles (Ag@Ti₃C₂-MXene) has been reported as a dual-energy acceptor for ultrahigh fluorescence quenching of protein-functionalized graphene quantum dots (anti-NSE/amino-GQDs). The Ti₃C₂-MXene nanosheets are decorated with silver nanoparticles (AgNPs) to obsolete the agglomeration and restacking through a one-pot direct reduction method wherein the 2D Ti₃C₂-MXene nanosheets acted both as a reducing agent and support matrix for AgNPs. The as-prepared nanohybrid is characterized by various techniques to analyze the optical, structural, compositional, and morphological parameters. The quenching efficiency and energy transfer capability between the anti-NSE/amino-GQDs (donor) and Ag@Ti₃C₂-MXene (acceptor) have been explored through steady state and time-resolved spectroscopic studies. Interestingly, the Ag@Ti₃C₂-MXene nanohybrid exhibits better quenching and energy transfer efficiencies in contrast to bare Ti₃C₂-MXene, AgNPs and previously reported AuNPs. Based on optimized donor-acceptor pair, a fluorescent turn-on biosensing system is constructed that revealed improved biosensing characteristics compared to Ti₃C₂-MXene, graphene and AuNPs for the detection of neuron-specific enolase (NSE), including higher sensitivity (∼771 mL ng^-1), broader linear detection range (0.0001-1500 ng mL^-1), better LOD (0.05 pg mL^-1), and faster response time (12 min). Besides, remarkable biosensing capability has been observed in serum samples, with fluorescence recovery of ∼98%.

NIR light‐triggered peroxynitrite anion production via direct lanthanide‐triplet photosensitization for enhanced photodynamic therapy

Peroxynitrite anion (ONOO−), a product derived from reaction between reactive oxygen species (ROS) and nitric oxide (NO), is considered to be a more toxic reactive specie than most ROS for...

Degradation of upconverting nanoparticles in simulated fluids evaluated by ratiometric luminescence

The effect of artificially simulated fluids on the optical properties of upconversion nanoparticles and the degradation mechanism was systematically studied.

Current advances in bio-fabricated quantum dots emphasising the study of mechanisms to diversify their catalytic and biomedical applications

Quantum dots (QDs), owing to their single atom-like electronic structure due to quantum confinement, are often referred to as artificial atoms. This unique physical property results in the diverse functions exhibited by QDs. A wide array of applications have been achieved by the surface functionalization of QDs, resulting in exceptional optical, antimicrobial, catalytic, cytotoxic and enzyme inhibition properties. Ordinarily, traditionally prepared QDs are subjected to post synthesis functionalization via a variety of methods, such as ligand exchange or covalent and non-covalent conjugation. Nevertheless, solvent toxicity, combined with the high temperature and pressure conditions during the preparation of QDs and the low product yield due to multiple steps in the functionalization, limit their overall use. This has driven scientists to investigate the development of greener, environmental friendly and cost-effective methods that can circumvent the complexity and strenuousness associated with traditional processes of bio-functionalization. In this review, a detailed analysis of the methods to bio-prepare pre-functionalized QDs, with elucidated mechanisms, and their application in the areas of catalysis and biomedical applications has been conducted. The environmental and health and safety aspects of the bio-derived QDs have been briefly discussed to unveil the future of nano-commercialization.

Aggregation-Enhanced Energy Transfer for Mitochondria-Targeted ATP Ratiometric Imaging in Living Cells

Förster resonance energy transfer (FRET) from fluorescent nanoparticles to fluorescent dyes is an attractive approach for bioanalysis in living cells. However, the luminescence of the nanoparticle donor/acceptor has not been effectively used to produce highly efficient FRET because the distance between the energy donor and energy acceptor is often larger than the effective FRET radius (about 10 nm) and the uncontrolled rotational and translational diffusion of luminophores. Here, we develop an aggregation-enhanced energy transfer strategy that can overcome the impedance for effective energy transfer. The functional nanoprobes, named TPP-CDs-FITC, are carbon dots (CDs) functionalized with triphenylphosphine (TPP) and ∼117 fluorescein 5-isothiocyanate (FITC) on the surface. In dispersed solution, the 3.8 nm TPP-CDs-FITC show weak FRET efficiency (15.4%). After TPP-instructed mitochondrial targeting, enhanced FRET efficiency (53.2%) is induced due to the aggregation of TPP-CDs-FITC selectively triggered by adenosine triphosphate (ATP) in the mitochondria. The enhanced FRET efficiency can be attributed to the joint effect of the augment of numbers of FITC acceptors within 10 nm from dispersed 117 to aggregated 5499 and the restricted rotational and translational motions of TPP-CDs donors and FITC acceptors. Ultimately, we successfully observe the fluctuations of ATP levels in the mitochondria using the aggregation-enhanced energy transfer strategy of the TPP-CDs-FITC nanodevice.

Overcoming thermal quenching in upconversion nanoparticles

Thermal quenching that is characterized by loss of light emission with increasing temperature is widely observed in luminescent materials including upconversion nanoparticles, causing problems in technological applications such as lighting, displays, and imaging. Because upconversion processes involve extensive intra-particle energy transfer that is temperature dependent, methods have been established to fight against thermal quenching in upconversion nanoparticles by engineering the energy transfer routes. In this minireview, we discuss the origin of thermal quenching and the role of energy transfer in thermal quenching. Accordingly, recent efforts in overcoming thermal quenching of upconversion are summarized.

Including fluorescent nanoparticle probes within injectable gels for remote strain measurements and discrimination between compression and tension

The ability to remotely and non-invasively monitor and measure the strain within injectable gels used to augment soft tissue is highly desirable. Such information could enable real-time monitoring of gel performance and bespoke gel design. We report progress towards this goal using two fluorescent particle probe systems included within two different injectable gels. The two injectable gels have been previously studied in the contexts of intervertebral disc repair and stretchable gels for cartilage repair. The two fluorophore particle probes are blue or near-infrared (NIR) emitting and are present at very low concentrations. The normalised photoluminescence (PL) intensity from the blue emitting probe is shown to equal the compressive deformation ratio of the gels. Furthermore, the normalised ratio of the PL intensities for the blue and NIR probes varies linearly with deformation ratio over a wide range (from 0.2 to 3.0) with a seamless transition from compression to tension. Hence, PL can discriminate between compression and tension. The new approach established here should apply to other gels and enable remote detection of whether a gel is being compressed or stretched as well as the extent. This study may provide an important step towards remotely and minimally invasively measuring the strain experienced by load-supporting gels in vivo.

Effect of different silica coatings on the toxicity of upconversion nanoparticles on RAW 264.7 macrophage cells

Upconversion nanoparticles (UCNPs), consisting of NaYF₄ doped with 18% Yb and 2% Er, were coated with microporous silica shells with thickness values of 7 ± 2 and 21 ± 3 nm. Subsequently, the negatively charged particles were functionalized with N-(6-aminohexyl)-3-aminopropyltrimethoxysilane (AHAPS), which provide a positive charge to the nanoparticle surface. Inductively coupled plasma optical emission spectrometry (ICP-OES) measurements revealed that, over the course of 24h, particles with thicker shells release fewer lanthanide ions than particles with thinner shells. However, even a 21 ± 3 nm thick silica layer does not entirely block the disintegration process of the UCNPs. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays and cell cytometry measurements performed on macrophages (RAW 264.7 cells) indicate that cells treated with amino-functionalized particles with a thicker silica shell have a higher viability than those incubated with UCNPs with a thinner silica shell, even if more particles with a thicker shell are taken up. This effect is less significant for negatively charged particles. Cell cycle analyses with amino-functionalized particles also confirm that thicker silica shells reduce cytotoxicity. Thus, growing silica shells to a sufficient thickness is a simple approach to minimize the cytotoxicity of UCNPs.

Microscale Self-Assembly of Upconversion Nanoparticles Driven by Block Copolymer

Lanthanide-based upconversion nanoparticles can convert low-energy excitation to high-energy emission. The self-assembled upconversion nanoparticles with unique structures have considerable promise in sensors and optical devices due to intriguing properties. However, the assembly of isotropic nanocrystals into anisotropic structures is a fundamental challenge caused by the difficulty in controlling interparticle interactions. Herein, we report a novel approach for the preparation of the chain-like assemblies of upconversion nanoparticles at different scales from nano-scale to micro-scale. The dimension of chain-like assembly can be fine-tuned using various incubation times. Our study observed Y-junction aggregate morphology due to the flexible nature of amphiphilic block copolymer. Furthermore, the prepared nanoparticle assemblies of upconversion nanoparticles with lengths up to several micrometers can serve as novel luminescent nanostructure and offer great opportunities in the fields of optical applications.

Fluorescent Labelling of Glycans with FRET-Based Probes in a Gold(III)-Mediated Three-Component Coupling Reaction

Single-site multifunctionalization of glycans is of importance in biological studies considering its crucial role in mediating biological events and human diseases. In this paper, a novel approach for multifunctional labelling of glycans has been developed featuring the use of fluorescence resonance energy transfer-based (FRET-based) probes for fluorescent labelling of glycans through a gold(III)-mediated three-component coupling reaction. Oxidation of glycans into aldehydes followed by the A³ -coupling reaction with FRET-based probes resulted in the single-site formation of fluorescent propargylamine products. The conversion of labelled glycans can be revealed by ratiometric analysis of the FRET signals. This labelling approach results in multifunctionalization of glycans with high selectivity and conversion between 66 and 69 %.

Recent advances in the synthesis and application of Yb-based fluoride upconversion nanoparticles

This review focuses on recent progress in the development of Yb-based upconversion nanoparticles and their emerging technological applications.

Er3+-to-dye energy transfer in DNA-coated core and core/shell/shell upconverting nanoparticles with 980 nm and 808 nm excitation of Yb3+ and Nd3+

FRET from upconversion nanoparticles to dyes using 980 nm and 808 nm excitation.

Thermal Enhancement of Upconversion by Negative Lattice Expansion in Orthorhombic Yb2 W3 O12

Thermal quenching of photoluminescence represents a significant obstacle to practical applications such as lighting, display, and photovoltaics. Herein, a novel strategy is established to enhance upconversion luminescence at elevated temperatures based on the use of negative thermal expansion host materials. Lanthanide-doped orthorhombic Yb₂ W₃ O₁₂ crystals are synthesized and characterized by in situ X-ray diffraction and photoluminescence spectroscopy. The thermally induced contraction and distortion of the host lattice is demonstrated to enhance the collection of excitation energy by activator ions. When the temperature is increased from 303 to 573 K, a 29-fold enhancement of green upconversion luminescence in Er³⁺ activators is achieved. Moreover, the temperature dependence of the upconversion luminescence is reversible. The thermally enhanced upconversion is developed as a sensitive ratiometric thermometer by referring to a thermally quenched upconversion.

The Bioavailability, Biodistribution, and Toxic Effects of Silica-Coated Upconversion Nanoparticles in vivo

Lanthanide-doped upconversion nanoparticles can convert long wavelength excitation radiation to short wavelength emission. They have great potential in biomedical applications, such as bioimaging, biodetection, drug delivery, and theranostics. However, there is little information available on their bioavailability and biological effects after oral administration. In this study, we systematically investigated the bioavailability, biodistribution, and toxicity of silica-coated upconversion nanoparticles administrated by gavage. Our results demonstrate that these nanoparticles can permeate intestinal barrier and enter blood circulation by microstructure observation of Peyer's patch in the intestine. Comparing the bioavailability and the biodistribution of silica-coated upconversion nanoparticles with oral and intravenous administration routes, we found that the bioavailability and biodistribution are particularly dependent on the administration routes. After consecutive gavage for 14 days, the body weight, pathology, Zn and Cu level, serum biochemical analysis, oxidative stress, and inflammatory cytokines were studied to further evaluate the potential toxicity of the silica-coated upconversion nanoparticles. The results suggest that these nanoparticles do not show overt toxicity in mice even at a high dose of 100 mg/kg body weight.

Development of Structure-Switching Aptamers for Kanamycin Detection Based on Fluorescence Resonance Energy Transfer

The structure-switching aptamers are designed for the simple and rapid detection of kanamycin based on the signal transduction principle of fluorescence resonance energy transfer (FRET). The structure switch is composed of kanamycin-binding aptamers and the complementary strands, respectively labeled with fluorophore and quencher, denoted as FDNA and QDNA. In the absence of kanamycin, FDNA and QDNA form the double helix structure through the complementary pairing of bases. The fluorophore and the quencher are brought into close proximity, which results in the fluorescence quenching because of the FRET mechanism. In the presence of kanamycin, the FDNA specifically bind to the target due to the high affinity of aptamers, and the QDNA are dissociated. The specific recognition between aptamers and kanamycin will obstruct the formation of structure switch and reduce the efficiency of FRET between FDNA and QDNA, thus leading to the fluorescence enhancement. Therefore, based on the structure-switching aptamers, a simple fluorescent assay for rapid detection of kanamycin was developed. Under optimal conditions, there was a good linear relationship between kanamycin concentration and the fluorescence signal recovery. The linear range of this method in milk samples was 100-600 nM with the detection limit of 13.52 nM (3σ), which is well below the maximum residue limit (MRL) of kanamycin in milk. This method shows excellent selectivity for kanamycin over the other common antibiotics. The structure-switching aptamers have been successfully applied to the detection of kanamycin spiked in milk samples with the satisfying recoveries between 101.3 and 109.1%, which is well-consistent with the results from LC-MS/MS. Due to the outstanding advantages of facile operation, rapid detection, high sensitivity, excellent specificity, and low cost, the application and extension of this strategy for rapid determination of antibiotics in food samples may greatly improve the efficiency in food safety and quality supervision.

Graphene Oxide as an Optical Biosensing Platform: A Progress Report

A few years ago, crucial graphene oxide (GO) features such as the carbon/oxygen ratio, number of layers, and lateral size were scarcely investigated and, thus, their impact on the overall optical biosensing performance was almost unknown. Nowadays valuable insights about these features are well documented in the literature, whereas others remain controversial. Moreover, most of the biosensing systems based on GO were amenable to operating as colloidal suspensions. Currently, the literature reports conceptually new approaches obviating the need of GO colloidal suspensions, enabling the integration of GO onto a solid phase and leading to their application in new biosensing devices. Furthermore, most GO-based biosensing devices exploit photoluminescent signals. However, further progress is also achieved in powerful label-free optical techniques exploiting GO in biosensing, particularly using optical fibers, surface plasmon resonance, and surface enhanced Raman scattering. Herein, a critical overview on these topics is offered, highlighting the key role of the physicochemical properties of GO. New challenges and opportunities in this exciting field are also highlighted.