Fluorescent copper nanoparticles: recent advances in synthesis and applications for sensing metal ions

Tandem copper and gold nanoclusters for two-color ratiometric explosives detection

We report a sensory platform for the determination of common explosive species (e.g., TNT, PETN, RDX) based on the differential response from two different luminescent metal nanoclusters. In particular, whereas the red emission from bovine serum albumin-protected gold nanoclusters was strongly quenched by nitro-, nitrate-, and nitroamine-containing explosive organic molecules, blue-emitting glutathione-capped copper nanoclusters proved inert to quenching by these same analytes, instead showing evidence for aggregation-induced emission enhancement (AIEE). As a result, this discrete gold/copper nanocluster pairing provides a dual-probe, ratiometric (red-to-blue) system signaling the presence of TNT and other common explosives. This strategy opens up new potential for nanocluster-based analyte signaling, with implications to fluorescence resonance energy transfer (FRET) strategies as well.

Label-free and sensitive microRNA detection based on a target recycling amplification-integrated superlong poly(thymine)-hosted copper nanoparticle strategy

Poly(thymine)-hosted copper nanoparticles (poly T-CuNPs) have emerged as a promising label-free fluorophore for bioanalysis, but its application in RNA-related studies is still rarely explored. Herein, by utilizing duplex-specific nuclease (DSN) as a convertor to integrate target recycling mechanism into terminal deoxynucleotidyl transferase (TdT)-mediated superlong poly T-CuNPs platform, a specific and sensitive method for microRNA detection has been developed. In this strategy, a 3'-phosphorylated DNA probe can hybridize with target RNA and then be cut by DSN to produce 3'-hydroxylated fragments, which can be further tailed by TdT with superlong poly T for fluorescent CuNPs synthesis. As proof of concept, an analysis of let-7d was achieved with a good linear correlation between 20 and 1000 pM (R² = 0.9965) and a detection limit of 20 pM. Moreover, both homologous and heterologous microRNAs were also effectively discriminated. This strategy might pave a brand-new way for designing label-free and sensitive microRNA assays.

A Novel Detection Method of Human Serum Albumin Based on the Poly(Thymine)-Templated Copper Nanoparticles

In this work, we developed a facile fluorescence method for quantitative detection of human serum albumin (HSA) based on the inhibition of poly(thymine) (poly T)-templated copper nanoparticles (CuNPs) in the presence of HSA. Under normal circumstances, poly T-templated CuNPs can display strong fluorescence with excitation/emission peaks at 340/610 nm. However, in the presence of HSA, it will absorb cupric ion, which will prevent the formation of CuNPs. As a result, the fluorescence intensity will become obviously lower in the presence of HSA. The analyte HSA concentration had a proportional linear relationship with the fluorescence intensity of CuNPs. The detection limit for HSA was 8.2 × 10⁻⁸ mol·L⁻¹. Furthermore, it was also successfully employed to determine HSA in biological samples. Thus, this method has potential applications in point-of-care medical diagnosis and biomedical research.

Fabrication of Stable and Luminescent Copper Nanocluster-Based AIE Particles and Their Application in β-Galactosidase Activity Assay

Thiolated copper nanoclusters (CuNCs) with aggregation-induced emission characteristic are becoming a novel luminescent material, but it is still a challenging task to retain its bright luminescence in a neutral solution. In this work, we report a new copper nanocluster with aggregation-induced emission (AIE) enhancement property using a hydrophobic molecule as the protecting ligand, and brightly luminescent AIE particles of copper nanocluster were prepared via hydrophobic interaction. These CuNCs AIE particles possess uniform rod-like shapes, with sizes in hundreds of nanometer, and an intense luminescence; more importantly, its luminescence remains stable in neutral and alkaline solutions. It is found that 4-nitrophenol is able to effectively quench the luminescence of CuNC AIE particles through strong hydrophobic interaction and electron transfer between them. This strong quenching effect was adopted to develop a luminescent assay for β-galactosidase at physiological condition. This work presents a demonstration of preparing CuNC AIE particles with bright luminescence at neutral condition and gives an example of the use of AIE particles in monitoring the enzyme activity.

Engineering a red emission of copper nanocluster self-assembly architectures by employing aromatic thiols as capping ligands

Luminescent Cu nanoclusters (NCs) are potential phosphors for illumination and display, but the difficulty in achieving full-color emission greatly limits practical applications. On the basis of our previous success in preparing Cu NC self-assembly architectures with blue-green and yellow emission, in this work, Cu NC self-assembly architectures with strong red emission are prepared by replacing alkylthiol ligands with aromatic thiols. The introduction of aromatic ligands is able to influence the ligand-to-metal charge transfer and/or ligand-to-metal-metal charge transfer, thus permitting the tuning of the emission color and enhancing of the emission intensity. The emission color can be tuned from yellow to dark red by choosing the aromatic ligands with different conjugation capabilities, and the photoluminescence quantum yield is up to 15.6%. Achieving full-color emission Cu NC self-assembly architectures allows the fabrication of Cu NC-based white light-emitting diodes.

Multi-stimuli responsive copper nanoclusters with bright red luminescence for quantifying acid phosphatase activity via redox-controlled luminescence switch

Thiolate-protected copper nanoclusers (CuNCs) are emerging as a promising class of luminescent materials since its unique optical properties such as aggregation-induced emission (AIE) and intriguing molecular-like behavior have been explored for sensing application. In this work, multi-stimuli responsive property of CuNCs was first investigated in depth and further adopted to develop a reliable and sensitive ACP assay. Penicilamine-capped CuNCs from a facile one-pot synthesis possess bright red luminescence and distinctive multi-stimuli responsive behaviors. Its sensitive and reversible response in luminescence to pH and temperature is originated from its inherent AIE property, and can be constructed as luminescent nanoswitches controlled by these external stimuli for precisely monitoring the change of environmental pH or temperature. The specific redox-responsive behavior of CuNC aggregates is found from severe luminescence quenching in the presence of a small amount of ferric or silver ions, and this sensitive response in luminescence to the preceding species is proved to be due to the conversion of Cu(II) from copper atoms with lower valence inside CuNCs. The luminescence switch of CuNC aggregates controlled by specific external potentials is further utilized to design a novel detection strategy for ACP activity. The great difference in luminescence quenching of CuNCs induced by iron(III) pyrophosphate (FePPi₂) complex and free ferric ions enables us to quantitatively monitor ACP level by the luminescence change as variation of ACP activity in the assay solution. This assay is able to detect ACP level as lower as 0.8 U/L, and covers a broad linear scope of 100.0 U/L. This work reports redox-responsive property of CuNCs and its underlying nature due to the oxidation of its interior copper atoms, and provides a sensitive assay method for ACP activity which is sufficiently sensitive for practical measurement in real samples.

Synthesis, optical properties and applications of light-emitting copper nanoclusters

Metal nanoclusters (NCs) containing a few to a few hundreds of atoms bridge the gap between nanoparticles and molecular compounds. The last decade evidenced impressive developments of noble metal NCs such as Au and Ag. Copper is an earth abundant, inexpensive metal from the same group of the periodic table, which is increasingly coming into focus for NC research. This review specifically addresses wet chemical synthesis methods, optical properties and some emerging applications of Cu NCs. As surface protecting templates/ligands play an important role in the stability and properties of Cu NCs, we classified the synthetic methods by the nature of the capping agents. The optical properties of Cu NCs are discussed from the point of view of the effects of the metal core, surface ligands and environment (solvents and aggregation) on the emission of the clusters. Applications of luminescent Cu NCs in biological imaging and light emitting devices are considered.

Renal Clearable Ag Nanodots for in Vivo Computer Tomography Imaging and Photothermal Therapy

Albumin-stabilized Ag nanodots (ANDs) are prepared by a one-step biomineralization method. The highly crystallized nanodots have ultrasmall sizes (approximately 5.8 nm) and robust X-ray attenuation (5.7313 HU per mM Ag). The unlabeled ANDs are directly excreted from the body via the urine after in vivo X-ray computer tomography (CT) imaging application. ANDs could be used as CT imaging agents and effective photothermal therapy agents. Tumor growth inhibition reaches 90.2% after photothermal treatment with ANDs. ANDs are promising tools for in vivo CT imaging and clearable near-infrared-triggered theranostic agents.

Cation-driven luminescent self-assembled dots of copper nanoclusters with aggregation-induced emission for β-galactosidase activity monitoring

The self-assembly of CuNCs was driven by aluminum cations and they had a sensing application in the monitoring of β-galactosidase activity.

Synthesis of ultra – stable copper nanoclusters and their potential application as a reversible thermometer

We report a strategy for the synthesis of luminescent copper nanoclusters that demonstrate potential application as a thermometer.

Hydrothermal synthesis of nitrogen and boron doped carbon quantum dots with yellow-green emission for sensing Cr(vi), anti-counterfeiting and cell imaging

Nitrogen and boron co-doped carbon quantum dots with great stability in high-salt conditions and good photostability are prepared through hydrothermal method and utilized for fluorometric detection of Cr(vi), anti-counterfeiting and cell imaging.

Stretchable and Thermally Stable Dual Emission Composite Films of On-Purpose Aggregated Copper Nanoclusters in Carboxylated Polyurethane for Remote White Light-Emitting Devices

Stretchable, mechanically stable films with thermally stable dual emission peaked in the blue and orange spectral range are fabricated by condensation and aging of carboxylated polyurethane in the presence of on-purpose aggregated copper nanoclusters. The aggregation of copper clusters leads to the enhancement of their emission in the orange, while polyurethane matrix contributes with the blue emission band, with an overall photoluminescence quantum yield of the films as high as 18%. Composite Cu nanoclusters/polyurethane films are sufficiently transparent over the visible spectral range and are absorbing in the UV range; more than 90% of their emission intensity is preserved after 10 times of cycle of stretch and recovery, as well as aging of up to 10 h at 90 °C, making them useful for optoelectronic devices. Remote white light-emitting devices (LEDs) have been fabricated by placing a down-conversion layer of composite Cu nanoclusters/polyurethane film separated through a silicone resin spacer from the UV LED chip, with Commission Internationale de l'Eclairage color coordinates of (0.34, 0.29), and a high color rendering index of 87.

A tunable nanopore sensor for the detection of metal ions using translocation velocity and biphasic pulses

A tunable resistive pulse sensor, utilising a polyurethane nanopore, has been used to characterise nanoparticles as they traverse the pore opening. Herein we demonstrate that the translocation speed, conductive and resistive pulse magnitude, can be used to infer the surface charge of a nanoparticle, and act as a specific transduction signal for the binding of metal ions to ligands on the particle surface. Surfaces of silica nanoparticles were modified with a ligand to demonstrate the concept, and used to extract copper(ii) ions (Cu²⁺) from solution. By tuning the pH and ionic strength of the solution, a biphasic pulse, a conductive followed by a resistive pulse is recorded. Biphasic pulses are becoming a powerful means to characterise materials, and provide insight into the translocation mechanism, and herein we present their first use to detect the presence of metal ions in solution. We demonstrate how combinations of translocation speed and/or biphasic pulse behaviour are used to detect Cu²⁺ with quantitative responses across a range of pH and ionic strengths. Using a generic ligand this assay allows a clear signal for Cu²⁺ as low as 1 ppm with a short 5-minute incubation time, and is capable of measuring 10 ppm Cu²⁺ in the presence of 5 other ions. The method has potential for monitoring heavy metals in biological and environmental samples.

A nuclease-assisted label-free aptasensor for fluorescence turn-on detection of ATP based on the in situ formation of copper nanoparticles

Owing to their promising advantages in biochemical analysis, aptamer-based sensing systems for the fluorescence detection of important biomolecules are being extensively investigated. Herein, we propose a turn-on fluorescent aptasensor for label-free detection of adenosine triphosphate (ATP) by utilizing the in situ formation of copper nanoparticles (CuNPs) and the specific digestion capability of exonuclease I (Exo I). In this assay, the addition of ATP can effectively hinder the digestion of aptamer-derived oligonucleotides due to the G-quadruplex structure. Accordingly, the remaining poly thymine at 5'-terminus of substrate DNA can serve as an efficient template for red-emitting fluorescent CuNPs with a Mega-Stokes shifting in buffered solution, which can be used to evaluate the concentration of ATP. This method is cost-effective and facile, because it avoids the use of traditional dye-labeled DNA strands and complex operation steps. Under optimized conditions, this method achieves a selective response for ATP with a detection limit of 93nM, and exhibits a good detection performance in biological samples.

A gadolinium MOF acting as a multi-responsive and highly selective luminescent sensor for detecting o-, m-, and p-nitrophenol and Fe3+ ions in the aqueous phase

A new Gd-MOF constructed with a π-conjugated ligand shows highly luminescent selective sensing of o-, m-, and p-nitrophenol and Fe³⁺ ions in an aqueous system.

Unique quenching of fluorescent copper nanoclusters based on target-induced oxidation effect: a simple, label-free, highly sensitive and specific bleomycin assay

A novel label-free fluorescence biosensor for bleomycin (BLM) detection was developed by combining the excellent fluorescence behavior of copper nanoclusters (CuNCs) and the unique oxidation capability of BLM–Fe²⁺ complex toward CuNCs.

Selective molecular recognition on calixarene-functionalized 3D surfaces

Calixarene based various 3D surface materials with unique signal amplification in molecular recognition are presented, including quantum dots (QDs), metal nanoparticles (NPs), nanotubes, and mesoporous silica.

Facile preparation of pH-responsive AIE-active POSS dendrimers for the detection of trivalent metal cations and acid gases

AIE-active POSS dendrimers, exhibiting AIE effects and pH-responsive properties, were employed as sensitive fluorescent probes for trivalent metal cations and acid gases.