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

Abstract

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.

Keywords: 1, 8-naphthalimide; Cu2+; chitosan; fluorescence.

Figure 1

Synthetic scheme for compound P, “*” represents the repeated units.

Figure 2

(a) FTIR pattern of LCS, LCS-a, LCS-b and P; (b) ¹H NMR of LCS-b and P, “a–d” stand for H form -NH, -NH₂, -N=CH- and -OH groups, respectively; “*” represents the repeated units.

Figure 3

(a) Selectivity of P (20 ppm) in the presence of common metal ions (100 µM) including Na⁺, K⁺, Ca²⁺, Mg²⁺, Cd²⁺, Hg²⁺, Ag⁺, Cr³⁺, Fe³⁺ and Al³⁺; (b) absorption spectra of P (20 ppm) for metal ions (100 µM); (c) fluorescence response of P (20 ppm) to Cu²⁺ (100 µM) in the presence of other metal ions (100 µM); (d) fluorescence response of P (20 ppm) to Cu²⁺ (100 µM) in the presence of anion ions including HCO₃⁻, NO₃⁻, CO₃²⁻, F⁻, SO₄²⁻, C₂O₄²⁻ and HPO₄²⁻ (100 µM) in the aqueous-ethanol media (pH 7.0, 20 mM HEPES, v:v = 1:9).

Figure 4

(a) Fluorescence spectra of P (20 ppm) in the presence of different amounts of Cu²⁺ (0.5–9 μM) in the aqueous-ethanol media (pH 7.0, 20 mM HEPES, v:v = 1:9). Inset: Linear fluorescence intensity at 561 nm of P (20 ppm) upon addition of Cu²⁺ (0.5–9 μM); (b) absorption spectra of P (20 ppm) in the presence of different amounts of Cu²⁺ (0–0.6 μM) in the aqueous-ethanol media (pH 7.0, 20 mM HEPES, v:v = 1:9). Inset: Absorbance ratio at 499 nm and 462 nm of P (20 ppm) upon addition of Cu²⁺ (0–0.6 μM); (c) the reversibility experiment: I. P (20 ppm), II. P (20 ppm) + Cu²⁺ (10 μM), III. P (20 ppm) + Cu²⁺ (10 μM) + EDTA (10 μM), IV. P (20 ppm) + Cu²⁺ (10 μM) + EDTA (100 μM), V. P (20 ppm) + Cu²⁺ (10 μM) + EDTA (100 μM) + Cu²⁺ (10 μM); VI. P (20 ppm) + Cu²⁺ (10 μM) + EDTA (100 μM) + Cu²⁺ (100 μM).

Figure 5

Comparison of selectivity of LCS-a, LCS-b and P (20 ppm) for Cu²⁺ (100 µM) in the aqueous-ethanol media (pH 7.0, 20 mM HEPES, v:v = 1:9).

Figure 6

(a) Effect of pH on fluorescence spectra of P (20 ppm) and P (20 ppm) for Cu²⁺ (100 µM); (b) Effect of time on the recognition between Cu²⁺ (100 µM) and P (20 ppm); (c) effect of water content on fluorescence spectra of P (20 ppm) and P (20 ppm) for Cu²⁺ (100 µM).