Amines are used for many applications, but their release in the environment needs to be carefully monitored and controlled. Para-nitroaniline (p-NA) is a harmful compound due to its hematoxicity, splenotoxicity, hepatoxicity, and nephrotoxicity: accordingly, its dispersion in water strongly affects human health and represents a huge threat to the environment [1
]. p-NA is resistant to chemical and biological oxidation degradation due to the presence of a nitro group linked to the aromatic ring. Therefore, the development of cost-effective membranes able to adsorb p-NA from aqueous solutions represents an alternative strategy to other treatments for wastewater purification (e.g., advanced oxidation processes, extraction, or biodegradation), and from this perspective, different types of adsorbents have been investigated and utilized to remove p-NA [4
]. Recently, electrospun fibrous mats composed of a wide variety of polymers have been successfully applied in various fields, such as nanocatalysis [10
], filtration [13
], healthcare [17
], biotechnology [20
], security, and environmental control [23
]. Polyethersulfone (PES) mats are largely used in membrane technology for water treatment due to their insolubility in water as well as their good mechanical and thermal properties [27
]. Moreover, functionalized PES fibers have proved to be effective for applications in photocatalysis [24
] and heavy metal removal [23
], and electrospun fibers functionalized with porphyrins are suitable for the detection and removal of unwanted pollutants in air and water. In fact, porphyrins and metallo-porphyrins are widely used for developing sensing systems due to their coordination sites for axial ligation and significant optical modifications which provide evidence of successful core complexation [34
]. In particular, zinc(II) meso-tetraphenyl porphyrin (ZnTPP) and meso-tetraphenyl porphyrin (H2
TPP) have been used to optically detect NH3
and gaseous amines [35
]. Therefore, the immobilization of these versatile macrocycles on surfaces is fundamental to obtain active layers devoted to molecular recognition in gas or liquid phase, and various techniques have been proposed to obtain porphyrin layers on substrates [37
In this general panorama, the aim of our work was to test the ability of porphyrin-modified PES nanofibers to remove p-NA from water. To our knowledge, little data reporting the use of electrospun mats as adsorbents to remove p-NA from water is available. In particular, herein we functionalized PES fibers with meso-tetraphenyl porphyrin (H2
TPP) or zinc(II) meso-tetraphenyl porphyrin (ZnTPP) to exploit the ability of these porphyrins to coordinate amino and/or nitro groups of p-NA (Figure 1
Porphyrin-loaded fibers were prepared using two different approaches: H2TPP or ZnTPP were immobilized on the surface of PES mats by i) dip-coating in porphyrin solutions (impregnation); or ii) by adding H2TPP or ZnTPP to the PES electrospinning solution to obtain porphyrin-doped PES mats. The presence of porphyrins on the fiber was confirmed by UV–Vis spectroscopy, fluorescence measurements, and XPS analysis, and p-NA removal from water solutions was spectrophotometrically detected and evaluated.
2. Results and Discussion
Fiber impregnation was achieved by dip-coating PES electrospun mats in 5 μM H2
TPP or ZnTPP toluene solutions. In this case, porphyrin adsorption was promoted by non-covalent interactions, which depend on several factors, including the specific surface area and mobility of dye molecules both in the liquid phase and in the interior of the polymeric scaffold. ZnTPP and H2
TPP are readily soluble in toluene whilst PES mats are only moderately soluble. SEM analysis (Figure 2
) demonstrated how the mats’ integrity was maintained after 2 h dipping in toluene-porphyrin solutions.
Moreover, impregnated mats were also extensively washed and sonicated in water after dip-coating, and UV–Vis measurements of the washing solution ruled out any porphyrin release in water, thus confirming the robustness of this non-covalent grafting.
To control the amount of adsorbed porphyrin, the fibers were dissolved in a toluene:DMF (50:50 v
) solution and the UV–Vis spectrum (Figure 3
) for H2
TPP showed an intense Soret band at 417 nm and four Q-bands at 513, 548, 590, and 647 nm, while for ZnTPP the Soret band was at 425 nm and the two expected Q-bands were at 558 and 598 nm [39
The porphyrin concentration on mats (having 2 × 2 cm2 surface area and 2.4 mg weight, dissolved in 2.5 mL of toluene:DMF 50:50 v:v) was spectrophotometrically estimated to be 0.32 μM and 0.30 μM for H2TPP and ZnTPP, respectively (~0.02% w/w for both H2TPP and ZnTPP).
The presence of p-NA in water can be spectrophotometrically estimated using the band at 381 nm (Figure S1
) and, accordingly, the ability of porphyrin-impregnated PES mats to interact with p-NA can be spectrophotometrically studied by measuring the absorbance decrease at λ = 381 nm after the dipping of untreated and porphyrin-treated PES mats (Figure S2
). Figure 4
shows the trend of normalized absorbance upon increasing the mats’ dipping time in the p-NA solution: untreated PES fibers showed negligible p-NA removal (less than 10% after 2 h), whilst a signal reduction up to 60% in the presence of H2
TPP PES mats and 70% for ZnTPP PES mats was observed (corresponding to an average p-NA residual concentration of p-NA in solution of about 2 μM).
To explain this behavior, we can invoke the ability of H2
TPP and ZnTPP to coordinate nitro and amino groups [40
]. However, it is important to note that the nitro group reduces the NH2
basicity in p-NA and, accordingly, a preferential coordination of Zn2+
with the –NO2
oxygen atoms, rather than with –NH2
nitrogen, can be hypothesized. Note that the use of polar solvent has an important effect on the Lewis acid–base interaction between Zn2+
and the donor atoms, and the presence of water can contribute to enhance π−π interactions between aromatic groups [42
]. The presence of p-NA on the fiber surface was further supported by XPS analyses, which indicated an approximate increase of 65% in the nitrogen concentration upon p-NA (data not showed).
The reported data demonstrate the importance of H2
TPP and ZnTPP in promoting p-NA adsorption on PES fibers, but the dip-coating procedure remains time-expensive and suffers from a lack of reproducibility in terms of the homogeneity of distribution of porphyrin on PES surfaces, as demonstrated by the fluorescence response (Figure 5
The second strategy we used to fabricate hybrid porphyrin/PES mats was based on the electrospinning of porphyrin and PES solutions, thus leading to porphyrin-doped mats. This approach guarantees a better control of the amount of porphyrin in the PES matrix as well as a high fabrication yield (a 20 × 40 cm2
foil can be prepared in less than 1 h). Porphyrin/PES electrospun mats and impregnated fibers could be easily distinguished by their different coloration (Figure 5
a–c), and were all characterized by strong fluorescence emissions under UV irradiation (see inset).
The amount of porphyrin in electrospun prepared mats (Figure 5
a,b) higher than that obtained for impregnated mats (Figure 5
c) was responsible for the different coloration of the fibers.
The photoluminescence spectra (λex
= 422 nm) of electrospun H2
TPP/PES or ZnTPP/PES dissolved in toluene:DMF (50:50 v
) solutions showed two emissions at 650 and 715 nm for H2
TPP/PES mats and at 600 and 650 nm for ZnTPP/PES mats (Figure S3
UV–Vis analysis of an aqueous 5 µM p-NA solution after fiber immersion (Figure 6
) indicated that p-NA preferentially absorbed on H2
Note that ZnTPP usually adopts a five-coordinate structure by the axial complexation of a solvent molecule containing electron donors such as N, O, and S [43
]. Accordingly, it is reasonable to assume changes in the electrospun solution properties (i.e., surface tension, viscosity, conductivity) arising from different interactions of H2
TPP or ZnTPP with the DMF solvent and PES sulfonic groups, and these effects might be responsible for the different geometrical arrangement and surface availability of porphyrins on the polymeric matrix. As a result, the Zn2+
in the electrospun fiber is probably less available to axially coordinate with p-NA.
The XPS analysis performed on electrospun H2
TPP/PES and ZnTPP/PES confirmed the presence of p-NA on the fiber surfaces. In particular, Figure 7
shows the XPS spectrum of the H2
TPP/PES before and after sensing of the p-NA, in the N 1s binding energy region. As already reported in a large number of studies, XPS ionizations of the four pyrrole nitrogen atoms in porphyrins, for symmetry restrictions, show two N 1s signals having a 1:3 intensity ratio [45
]. This behavior is well documented by literature data, and finds a counterpart in the present spectrum consisting of the convolution of two components at 399.3 and 400.8 eV, with a 1:3 intensity ratio [45
]. After p-NA sensing, this spectrum showed major changes, mainly due to the intensity increase of the low binding energy component now at about 399.6 eV. Moreover, there is also evidence of a very small feature at about 407 eV, in tune with the presence of the –NO2
group of the p-nitroaniline [45
]. In this context, it has already been reported that –NO2
groups reduce to –NH2
during XPS measurements because of the X-ray irradiation, and this observation is in strong agreement with both the observed intensity increase of the low binding energy component at about 399.6 eV, and with the very low intensity of the feature at 407 eV [52
]. In addition, we noted a 100% atomic concentration increase of the N 1s signal, confirming the increase of nitrogen-containing species on the PES mats upon interaction with the p-NA.
shows the XPS spectra of the ZnTPP/PES mats before and after sensing of the p-NA in the N 1s binding energy region. The increased symmetry of this porphyrin (group D4h
) is evident in the XPS of the N 1s states that now show a unique band at 401.1 eV [45
]. Upon contact with the p-NA, we noted a lower binding energy shift of the whole spectrum that now peaked at 400.6 eV [52
]. Again, we also noted a 100% atomic concentration increase of the overall N 1s signal. These results strongly confirm that the present porphyrin-doped PES mats are well-suited to sense p-NA.