Naphthalimide-based optical turn-on sensor for monosaccharide recognition using boronic acid receptor

Abstract

A highly selective and sensitive fluorescent sensor for the determination of fructose is developed. The fluorescent sensor was prepared by incorporating a new naphthalimide dye with a planar structure as a selectophore and graphene oxide (GO) nanoplatelets as a quencher for rapid optical detection of fructose. The designed probe, made with the high fusion loop-containing dye, along with the GO nanoplatelets, detected fructose over the other monosaccharides very well. The proposed sensor displays a linear response range of 7 × 10^-5 to 3 × 10^-2 M with a low limit of detection of 23 × 10^-6 M in solution at pH 7.4. This sensor shows a good selectivity towards fructose with respect to other saccharides. The proposed sensor was then applied to the determination of fructose in human plasma with satisfactory results.

Scheme 1. Synthesis procedure for NOPB probe: (a) DMF, NBS, room temperature, 75%; (b) glacial acetic acid, Na₂Cr₂O₇, reflux 3 h, 80%; (c) o-phenylenediamine, glacial acetic acid, reflux 7 h, 73%; (d) N-piperazine, 2-methoxyethanol, reflux 7 h, 20%; (e) (2-bromomethylphenyl) boronic acid pinacol ester, dry THF/MeOH, reflux 7 h, 25%.

Fig. 1. (a) Absorption spectra of NOPB in various solvents, (b) emission spectra of NOPB in various solvents.

Fig. 2. (a) UV-vis absorption spectrum of 15 μg mL⁻¹ GO aqueous dispersion (b) SEM of used GO.

Fig. 3. (a) Absorption changes spectra of the NOPB probe in the presence of various amounts of graphene oxide. [NOPB] = 5 μM, [GO] = 0–55 μg ml⁻¹, 50 mM PBS, pH = 7.4, λ_ex = 391 nm, (b) NOPB–GO with subtracted NOPB peak. [NOPB] = 5 μM, [GO] = 5 μg ml⁻¹, 50 mM PBS, pH = 7.4, λ_ex = 391 nm.

Fig. 4. (a) Fluorescence quenching spectra of NOPB in the presence of various concentration of GO. [NOPB] = 5 μM, [GO] = 0–55 μg ml⁻¹, 50 mM PBS, pH = 7.4, λ_em = 519 nm, λ_ex = 391 nm; inset; photograph of NOPB solution before and after GO addition under UV light, (b) linear range of fluorescence quenching of NOPB at 519 nm by addition of GO (0–23 μg mL⁻¹), λ_ex = 391 nm.

Fig. 5. AFM images of (a) GO (b) NOPB/GO.

Fig. 6. (a) Raman spectra of GO, (b) Raman spectra of NOPB@GO.

Fig. 7. (a) Fluorescence emission of NOPB probe and UV-vis absorption of graphene oxide, (b) electron transfer from energy levels between NOPB dye and graphene oxide.

Scheme 2. The planar structure of NOPB probe.

Fig. 8. Fluorescence emission changes relative to the pH in the presence and absence of sugars in 50 mM PBS, [fructose] = 50 mM, λ_ex = 391 nm. (a) NOPB (5 μM); (b) NOPB–GO (5 μM).

Fig. 9. (a) Fluorescence change of NOPB in the presence of various amounts of fructose (0–2 M) in PBS (50 mM, pH 7.4). (b) The linear range of fluorescence change of NOPB at 519 nm in the presence of fructose (λ_ex = 391 nm).

Fig. 10. (a) Fluorescence change of NOPB–GO (5 μM) in the presence of fructose (0–2 M) in PBS (50 mM, pH 7.4). (b) The linear range of fluorescence change of NOPB–GO at 515 nm in the presence of fructose (λ_ex = 418 nm).

Fig. 11. (a) Fluorescence change of NOPB probe (5 μM) in the presence of 50 mM of different sugars in PBS (50 mM, pH 7.4, λ_ex = 418 nm), (b) fluorescence change of NOPB–GO (5 μM with 40 μg mL⁻¹ GO) in the presence of 50 mM of different sugars in PBS (50 mM, pH 7.4, λ_ex = 418 nm), (c) plots of (I_max − I₀)/(I_c − I₀) against C⁻¹ for NOPB and NOPB–GO with fructose [NOPB] = 5 μM, [NOPB–GO] = 5 μM, 50 mM PBS, pH 7.4, (d) intensity changes of [NOPB] = 5 μM and [NOPB–GO] = 5 μM in the presence of fructose and upon addition of different saccharides (50 mM) in PBS (50 mM, pH 7.4).

Fig. 12. Fluorescence change of different NOPB probes and NOPB–GO sensors (5 μM) in the presence of 10 mM of fructose in PBS (50 mM, pH 7.4, λ_ex = 418 nm).

Scheme 3. Schematic representation of signal generation for designed fluorogenic probe (NOPB) by fructose binding and its signal amplification in the presence of GO.