Construction of Chitosan-Modified Naphthalimide Fluorescence Probe for Selective Detection of Cu2

A chitosan-based Cu2+ fluorescent probe was designed and synthesized independently using the C-2-amino group of chitosan with 1, 8-naphthalimide derivatives. A series of experiments were conducted to characterize the optical properties of the grafted probe. The fluorescence quenching effect was investigated based on the interactions between the probe and common metals. It was found that the proposed probe displayed selective interaction with Cu2+ over other metal ions and anions, reaching equilibrium within 5 min.

Naphthalimide-Piperazine Derivatives as Multifunctional "On" and "Off" Fluorescent Switches for pH, Hg2+ and Cu2+ Ions

Novel 1,8-naphthalimide-based fluorescent probes NI-1 and NI-2 were designed and screened for use as chemosensors for detection of heavy metal ions. Two moieties, methylpyridine (NI-1) and hydroxyphenyl (NI-2), were attached via piperazine at the C-4 position of the napthalimide core resulting in a notable effect on their spectroscopic properties. NI-1 and NI-2 are pH sensitive and show an increase in fluorescence intensity at around 525 nm (switch "on") in the acidic environment, with pK_a values at 4.98 and 2.91, respectively. Amongst heavy metal ions only Cu²⁺ and Hg²⁺ had a significant effect on the spectroscopic properties. The fluorescence of NI-1 is quenched in the presence of either Cu²⁺ or Hg²⁺ which is attributed to the formation of 1:1 metal-ligand complexes with binding constants of 3.6 × 10⁵ and 3.9 × 10⁴, respectively. The NI-1 chemosensor can be used for the quantification of Cu²⁺ ions in sub-micromolar quantities, with a linear range from 250 nM to 4.0 μM and a detection limit of 1.5 × 10^-8 M. The linear range for the determination of Hg²⁺ is from 2 μM to 10 μM, with a detection limit of 8.8 × 10^-8 M. Conversely, NI-2 behaves like a typical photoinduced electron transfer (PET) sensor for Hg²⁺ ions. Here, the formation of a complex with Hg²⁺ (binding constant 8.3 × 10³) turns the green fluorescence of NI-2 into the "on" state. NI-2 showed remarkable selectivity towards Hg²⁺ ions, allowing for determination of Hg²⁺ concentration over a linear range of 1.3 μM to 25 μM and a limit of detection of 4.1 × 10^-7 M.

A highly selective and sensitive fluorescent sensor based on a 1,8-naphthalimide with a Schiff base function for Hg2+ in aqueous media

A new fluorescent sensor, N-allyl-4-[(2-(3-methoxysalicylaldimino)ethylamino]-1,8-naphthalimide (HL), for Hg2+ has been developed where the Schiff base substituent acts as a recognition group. This sensor shows a large Stokes shift of 3535–4042 cm−1 and a general fluorescence quantum yield of 0.05, 249–0.11, 866 in organic solvents of different polarity as expected. It also exhibits highly selective and a sensitive response to Hg2+ (Ф Hg+HL/Ф HL = 2.28) over other metal ions (Na+, K+, Ca2+, Mg2+, Al3+, Pb2+, Fe3+, Ni2+, Zn2+, Cu2+, Ag+, Co2+, Cr3+, Mn2+ and Cd2+) in solution (DMF/Tris-HCl buffer, 1:1, v/v, pH = 7.2). The Hg2+-induced fluorescence enhancement at 526 nm is proportional to the concentration of Hg2+ in the range of 0.5–4.0 µm with a detection limit of 0.18 µm. Based on the fluorescence titration and a Job’s plot analysis, the metal-to-ligand ratio of the complex is 2:1 with a binding constant of 1.56 × 1012 m −1.

A ratiometric fluorescence platform based on carbon dots for visual and rapid detection of copper(II) and fluoroquinolones

A simple smartphone-integrated ratiometric fluorescent sensing system for visual determination of copper ions (Cu²⁺) and fluoroquinolones (FQs) was developed based on carbon dots (CDs) which were synthesized through the high-temperature pyrolysis of citric acid. In this system, with the fluorescence resonance energy transfer effect between CDs and 2,3-diaminophenazine (oxOPD), the detection of Cu²⁺ and ciprofloxacin (CIP, an example for FQs) was realized. Cu²⁺ catalyzes the oxidation of OPD to form oxOPD with yellow fluorescence, resulting in the quenching of CDs. In addition, CIP can inhibit the catalytic activity of Cu²⁺ and induce the recovery of CDs fluorescence. Under the excitation of 400 nm, the changes of CDs fluorescence at 472 nm and oxOPD fluorescence at 556 nm were monitored. The detection results showed that the sensing system exhibited good selectivity and sensitivity to Cu²⁺ and CIP with the limit of detection of 2.32 × 10^-8 mol L^-1 and 0.2 ng mL^-1, respectively. In addition, a smartphone was developed as a portable analyzer to capture the change of fluorescence color and quickly analyze the concentration of Cu²⁺ and CIP. The proposed smartphone-based sensing platform has satisfactory sensitivity, and it has application prospects for detecting Cu²⁺ and FQs in food safety monitoring.