A recent study published in Chemosensors presents a unique donor-π-acceptor luminous dye as a sensor for Cu2+ ions. The 3,5-diphenyl-dicyanomethylene-4-H-pyran (DCM) that makes up the fluorophoric core has extended styryl chains at positions 2 and 6 for metal coordination and is used for copper (II) detection.
Fluorescent Chemosensors Coupled to Metal Binding Moieties
Due to their great sensitivity compared to other sensor systems, fluorescent chemosensors have received much attention in recent decades. Fluorescent chemosensors provide continuous monitoring and high spatiotemporal resolution of tested samples. Sensors related to metal-binding moieties have been developed in the past.
A vast array of compounds define where the chelating moiety is crucial to the selectivity of a particular metal cation. The di-(2-picolyl)-amine (DPA) group, which numerous research groups have widely examined, is one successful example in particular.
Need For Development of Sensor Systems for Copper Detection
The importance of various cations, given their functions in biological processes and their intrinsic toxicity to the environment, has aided in developing innovative sensor systems for particular metal ions. Copper is an essential catalytic cofactor for several metalloenzymes in the human body, along with iron and zinc.
Copper is a crucial trace element for both plants and wildlife. However, due to high levels of integration and bioaccumulation into the food chain, it is poisonous to fish and aquatic life in specific quantities. Long-term exposure can also result in severe food poisoning, which can eventually cause lifelong liver and kidney damage.
Due to its role in creating reactive oxygen species, it has been connected to several neurodegenerative illnesses such as Alzheimer's and Wilson's diseases. There is a need to develop sensor systems for copper detection to avoid these harmful effects.
Chemosensors with Optical Properties in the Near-Infrared Region
Developing chemosensors with emission and absorption above 700 nm has attracted significant research attention. The region above 700 nm is called a near-infrared region. Since light in this spectral area can penetrate deeper into cellular tissues, enabling in vivo monitoring with little radiation harm, this particular type of sensor is very pertinent for biological samples.
The least amount of fluorescence background and light scattering with visible light are significant benefits of optical sensors working in the near-infrared band, especially in situations where less invasive integration of artificial illumination is more appropriate.
Development of 3,5-Diphenyl-Dicyanomethylene-4H-Pyran (DCM) Core Near-Infrared Fluorescent Chemosensor
Dicyanomethylene-4H-pyran (DCM) fluorophores have been reported to absorb and emit near-infrared light owing to strong intramolecular charge transfer characteristics. High charge transfer shifts the optical properties towards lower energies in polar media due to the strong stabilization of the excited charge transfer state.
In this study, Karkosik et al. developed a 3,5-diphenyl-dicyanomethylene-4H-pyran (DCM) core near-infrared fluorescent chemosensor with an extended push-pull -system and two DPA units for metal ion detection. The researchers then thoroughly characterized this selectivity and sensitivity of the sensor towards metal ions, both in solution and on paper support.
In this study, researchers created a unique donor-π-acceptor DPA-chemosensor system based on a DCM-4H-pyran fluorophore that can produce light in the near-infrared region of the electromagnetic spectrum and responds only to copper (II).
With a limit of detection in the nanomolar range, the chemosensor exhibited a strong binding constant with Cu2+ in solution. This device can quickly screen copper (II) in drinking water supplies or waste waters in mining plants, according to paper test strips embedded with the chemosensor.
The chemosensor dye exhibited a considerable bathochromic shift in the emission, reaching a maximum at about 750 nm in polar liquids, according to optical characterization. This solvatochromic activity, which resulted in very large Stokes shifts (up to 6700 cm−1), was indicative of the chromophoric system's strong intramolecular charge transfer nature.
A strong quenching effect was seen upon Cu2+ coordination, with a 1:1 binding stoichiometry, indicating that only one DPA unit can effectively chelate Cu2+, rendering the second DPA inactive. The chemosensor showed no changes in its optical properties over a wide pH range (2–12). The binding constant indicated extremely high sensitivity and a LOD of 90.1 nM, which was found to be 7.5 107 M−1.
Comparison tests showed that despite excesses of other mono- and divalent cations, the chemosensor was highly selective for Cu2+. The strongest interferents were Co2+ and Ni2+, particularly in the luminous response.
This chemosensor has the potential to be used in the detection of Cu2+ content in aqueous solutions for in-field applications. Paper test strips made with the embedded sensor demonstrated a fluorometric response in the presence of varied copper (II) concentrations.
Karkosik, A., & Moro, A. J. (2022). An NIR Emissive Donor-π-Acceptor Dicyanomethylene-4H-Pyran Derivative as a Fluorescent Chemosensor System towards Copper (II) Detection. Chemosensors, 10(8), 343. https://www.mdpi.com/2227-9040/10/8/343/htm