Fluorescent and Colorimetric Electrospun Nanofibers for Heavy-Metal Sensing

Abstract

The accumulation of heavy metals in the human body and/or in the environment can be highly deleterious for mankind, and currently, considerable efforts have been made to develop reliable and sensitive techniques for their detection. Among the detection methods, chemical sensors appear as a promising technology, with emphasis on systems employing optically active nanofibers. Such nanofibers can be obtained by the electrospinning technique, and further functionalized with optically active chromophores such as dyes, conjugated polymers, carbon-based nanomaterials and nanoparticles, in order to produce fluorescent and colorimetric nanofibers. In this review we survey recent investigations reporting the use of optically active electrospun nanofibers in sensors aiming at the specific detection of heavy metals using colorimetry and fluorescence methods. The examples given in this review article provide sufficient evidence of the potential of optically electrospun nanofibers as a valid approach to fabricate highly selective and sensitive optical sensors for fast and low-cost detection of heavy metals.

Keywords: electrospinning; heavy metals; optical sensors.

Figure 1

Scheme of electrospun nanofibers (ESNFs) modified by distinct nanomaterials for applications in optical sensors for heavy-metal detection. The lower part of the figure provides a schematic showing the principles of optical detection by the fluorescence quenching (left) or by the color change (right) of the ESNFs in the presence of the heavy metal ion.

Figure 2

(a) (i) Schematic representation for the fabrication of fluorescent nanofibrous membrane (NFM), (ii) absorbance of FNFM after being immersed into the aqueous solution of metal ions and (iii) UV–vis absorption spectra in the presence of Hg(II) with various concentrations. Adapted and reprinted with permission from [27]. Copyright 2017 Elsevier. (b) Schematic illustration of (i) trypsin and (ii) Cu(II) sensing based on the CPBQD/PMMA (CsPbBr₃ perovskite quantum dots/polymethylmethacrylate), (iii) Photoluminescence (PL) spectra of the CPBQD/PMMA in an aqueous medium of different Cu(II) concentrations and (iv) relationship between the PL intensity and Cu(II) concentration. Reprinted with permission from [32]. Copyright 2017 Royal Society of Chemistry. (c) (i and ii) Schematic illustration of multifunctional sensory electrospun nanofibers (ESNFs) synthesized from poly(NIPAAm-co-NMA-co-AA), 1-benzoyl-3-[2-(2-allyl-1,3-dioxo-2,3-dihydro-1Hbenzo[de]isoquinolin-6-amino)-ethyl]-thiourea (BNPTU) and Fe₃O₄ blends with magnetic fluorescence emission, (iii) absorption spectra, (iv) variation of absorption spectra for different metal ions, (v) variation in the normalized PL spectra for different metal ions and (vi) fluorometric response ESNFs. Reprinted with permission from [28]. Copyright 2017 MDPI. (d) (i) Schematic illustration of the selectivity of carbon quantum dot (CD), (ii) CD/mesoSiO₂/PAN nanofibers for Fe(III) detection, (iii) PL response and (iv) linear fit of the PL intensity towards Fe(III) of the CD/mesoSiO₂/PAN nanofibers. Reprinted with permission from [29]. Copyright 2016 Springer. (e) (i) Fluorescence spectra of PNNR2 solution in methanol/Tris–HCl for different concentrations of Cu(II) ions and (ii) Stern–Volmer plots of PNNR2 in different states for Cu(II) detection. Adapted and reprinted with permission from [33]. Copyright 2016 Springer.

Figure 2

(a) (i) Schematic representation for the fabrication of fluorescent nanofibrous membrane (NFM), (ii) absorbance of FNFM after being immersed into the aqueous solution of metal ions and (iii) UV–vis absorption spectra in the presence of Hg(II) with various concentrations. Adapted and reprinted with permission from [27]. Copyright 2017 Elsevier. (b) Schematic illustration of (i) trypsin and (ii) Cu(II) sensing based on the CPBQD/PMMA (CsPbBr₃ perovskite quantum dots/polymethylmethacrylate), (iii) Photoluminescence (PL) spectra of the CPBQD/PMMA in an aqueous medium of different Cu(II) concentrations and (iv) relationship between the PL intensity and Cu(II) concentration. Reprinted with permission from [32]. Copyright 2017 Royal Society of Chemistry. (c) (i and ii) Schematic illustration of multifunctional sensory electrospun nanofibers (ESNFs) synthesized from poly(NIPAAm-co-NMA-co-AA), 1-benzoyl-3-[2-(2-allyl-1,3-dioxo-2,3-dihydro-1Hbenzo[de]isoquinolin-6-amino)-ethyl]-thiourea (BNPTU) and Fe₃O₄ blends with magnetic fluorescence emission, (iii) absorption spectra, (iv) variation of absorption spectra for different metal ions, (v) variation in the normalized PL spectra for different metal ions and (vi) fluorometric response ESNFs. Reprinted with permission from [28]. Copyright 2017 MDPI. (d) (i) Schematic illustration of the selectivity of carbon quantum dot (CD), (ii) CD/mesoSiO₂/PAN nanofibers for Fe(III) detection, (iii) PL response and (iv) linear fit of the PL intensity towards Fe(III) of the CD/mesoSiO₂/PAN nanofibers. Reprinted with permission from [29]. Copyright 2016 Springer. (e) (i) Fluorescence spectra of PNNR2 solution in methanol/Tris–HCl for different concentrations of Cu(II) ions and (ii) Stern–Volmer plots of PNNR2 in different states for Cu(II) detection. Adapted and reprinted with permission from [33]. Copyright 2016 Springer.

Figure 3

(a) (i) UV–vis for different concentrations of Pb(II) and (ii) nanofiber images showing a visual color change. Adapted and reprinted with permission from [45]. Copyright 2016 Elsevier. (b) (i) Schematic illustration of the preparation of PANI-LBNF sensing membranes by the combination of electrospinning and hydrazine treatment, (ii) reflectance spectra and (iii) optical colorimetric responses of the PANI-LBNF sensor strips after incubation in different Hg(II) concentrations. Adapted and reprinted with permission from [47]. Copyright 2014 Royal Society of Chemistry. (c) (i) Digital photo images of the loading optically active species amount and the corresponding samples after incubation with Pb(II), (ii) kinetic absorption response of the strips as a function of time and (iii) time-dependent visualization of CIE Lab color changes versus Pb(II) concentration. Adapted and reprinted with permission from [44]. Copyright 2014 Elsevier.