Deferoxamine-Based Materials and Sensors for Fe(III) Detection

Abstract

Deferoxamine (DFO) is a siderophore widely studied for its ability to bind iron(III) strongly. Thanks to its versatility, it is suitable for several clinical and analytical applications, from the recognized iron(III) chelation therapy to the most recent applications in sensing. The presence of three hydroxamic functional groups enables Deferoxamine to form stable complexes with iron(III) and other divalent and trivalent metal ions. Moreover, the terminal amino group in the DFO molecule, not involved in metal ion complexation, allows modification or functionalization of solid phases, nanoobjects, biopolymers, electrodes and optical devices. This review summarizes and discusses deferoxamine-based applications for the chelation and recognition of Fe(III).

1. Introduction

Deferoxamine or desferrioxamine (DFO) is a trihydroxamic acid siderophore discovered in 1950 as a metabolite of the Streptomyces pilosus, a soil bacterium. Subsequently, it has been found that other species of marine and terrestrial actinomycetes can produce deferoxamine and other siderophores [1].

A siderophore (siderophore means iron-carrier in Greek) is an iron(III) chelator produced by microorganisms for scavenging the metal ion from inorganic and biological sources and delivering it to cells where it is recognized and transported across the cell membranes [2,3]. More than 500 siderophores are known, and the structure has been characterized for 270 of them [3,4]. Based on the moieties structure, siderophores can be classified into hydroxamate, carboxylate, catecholate and phenolate [5]. DFO belongs to the hydroxamate class since it presents a linear acyclic structure with trihydrosamic functional groups (see Figure 1).

The acid–base properties of deferoxamine have been deeply studied and reviewed [6,7,8,9,10,11,12,13,14], highlighting the protonable terminal amino group (log K₁ = 10.84) and the three weakly acidic hydroxamic groups (log K₂ = 9.46, log K₃ = 9.00 and log K₄ = 8.30) [14]. Figure 2 shows the distribution diagram of the differently protonated species of DFO.

The very high affinity of DFO for iron(III) derives from its characteristic structure, with the three bidentate hydroxamic groups able to envelop Fe(III), forming a very stable octahedral complex (see Figure 3).

Several data concerning solution equilibria studies of the system DFO/Fe(III) are reported in the literature, confirming the formation of a 1:1 complex, in equimolar metal/ligand conditions, and also in very acidic media, i.e., when the hydroxamic groups are completely protonated. At pH ranging from 1 to 10, the dominant specie is the complex [FeHL]⁺ with a log β of about 41.5; at higher and lower pHs, the main species are respectively [FeL] with a log β of about 31, and [FeH₂L]²⁺ with a log β of about 42.5 [6,8,9,11,12,13,14,15].

The distribution diagram of Fe(III)/DFO complexes as a function of the solution pH is shown in Figure 4.

Iron overload is the most common human metal toxicity condition worldwide. Under physiological conditions, the body’s iron levels depend on homeostatic controls of iron uptake, distribution and storage; these factors are mainly regulated by gastrointestinal absorption of the metal ion by diet and bone marrow erythropoietic activity. Iron absorption is regulated by proteins such as hephaestin and ferroportin [16]. Several factors may affect iron absorption, including the amount of diet iron, its chemical form (heme iron, ferrous and ferric), the iron ligands in the gut (phosphates, phenols and sugars) and the presence of other potential chelators (i.e., drugs) [17]. Pathological conditions may arise from gastrointestinal absorption (primary hemochromatosis) or red blood cell transfusions (secondary hemochromatosis). Genetic diseases with consequent hemoglobin mutation, such as β-thalassemia, can result in anemia, for which the typical therapy is blood transfusions. An indirect consequence of this treatment is a high concentration in the body of Non-Transferrin-Bound Iron (NTBI), which can accumulate in organs such as the liver [18,19]. Accumulation of NTBI can cause oxidative damage to the organs leading to the patient’s death after an average of 20 years [20]. Therefore, chelation is the only effective way to remove iron overload in transfusion-dependent patients.

Deferoxamine (marketed by Novartis^© under the brand name Desferal^®) is the first-choice treatment, despite its severe side effects and disadvantages, such as the high cost and poor compliance [13,21]. These aspects lead to developing and researching new drugs for chelating therapy or new strategies for DFO’s drug delivery.

However, the capability of DFO to strongly bind Fe(III), its physical–chemical proprieties such as the good water solubility and the low molecular weight associated with its relatively low-cost have spread the use of DFO in various fields [21], especially for the development of Fe(III) sensors.

The present paper is a comprehensive review of DFO-based materials and sensors reported in the literature of the last 20 years, providing a critical discussion of their applications for Fe(III) chelation and recognition in biological and environmental samples. In particular, DFO-immobilized materials such as biopolymers used for drug delivery, polymers, nanoparticles, sorbents and the functionalization of the active surfaces of electrodes and optical sensors are described.

2. DFO-Based Biopolymers for Chelation and Detection of Fe(III)

Deferoxamine, commercialized by Novartis with the brand name Desferal^® is the first US FDA-approved clinical iron chelator. It was included in the therapy of iron-overload diseases due to its pretty good therapeutic outcomes thanks to its strong iron-binding efficacy. Since its clumsy pharmacokinetic behavior (plasma half-life of about 20 min) and several side effects, the clinical use of DFO is progressively decreasing. Several approaches, such as loading nanocarriers or functionalizing biopolymers with DFO, have been proposed to enhance the therapeutic effects of deferoxamine [22]. These strategies have focused on improving DFO’s half-life reducing the administration frequency, and minimizing the side effects [23].

Initial attempts began in the 1970s with liposomal formulations [24,25,26], but since the US FDA did not approve the liposomal drugs until the 1990s, it is not surprising that these first attempts were not effectively transferred to the clinic [23].

Many papers reported the development of nanochelators with demonstrated efficiency by in vitro trials, but most of them were not yet applied as iron-overload treatments in vivo [23].

In this scenario, high DFO-loaded (of about 80%) nanoparticles assisted by polyphenols were developed with both efficient iron chelation and reactive oxygen species-scavenging properties [22]. In particular, a series of self-assembled DFO-polyphenol conjugates nanoparticles were prepared and tested for Fe(III) and ROS removal. In vitro and in vivo experiments were carried out. The best results were obtained with DFO-gallic acid nanoparticles thanks to the formation of iron complexes with both DFO and polyphenol. This study seems to be a promising strategy for improving the therapeutic effect on iron(III)-overload patients.

With the aim of developing an oral drug delivery system for deferoxamine, polymeric micelles were prepared and characterized [27]. The micelles formulation was optimized by an experimental design. All polymeric micelles increased the iron(III) complexing ability compared to the free DFO drug. Optimized polymeric micelle consisted of Tween 80 and Span 20 surfactants and Poloxamer^® as polymer, demonstrating more than 97% iron(III) chelation. Moreover, they showed perfect rat intestine permeation and good stability, making them up-and-coming as DFO’s drug delivery system.

Another kind of self-assembled polymeric micelle was developed for chelating and selectively detecting Fe(III) both in vitro and in iron-overloaded cells [28]. The micelles comprised Pluronics F127 polymer and Pluronics F127–DFO polymer conjugates, and they could encapsulate tetraphenylethylene (TPE). The fluorescence of TPE, determined by the aggregation-induced state in the micelle, can be quenched in the presence of iron(III), as shown in Figure 5. The DFO–based polymeric micelles demonstrated the ability to retain the iron-chelation properties of the free deferoxamine, exhibiting slight cytotoxicity compared to the free drug. Moreover, the fluorescence “turn-off” mechanism can detect the presence of iron(III) and monitor the chelation process in iron-overload cell models. This study could be a proof of concept for a future design of an effective in vivo therapy system.

As bio-polymeric materials, polysaccharides, such as starch, dextrose and chitosans, were conjugated with DFO mainly to extend the drug’s half-life. Clinical trials with starch-conjugated DFO as a long-lasting formulation started in 2007 [29]. This work evaluated the pharmacokinetics, iron excretion and safety of starch-conjugated DFO in patients with β-thalassemia. However, the clinical development of the formulation stopped after this study.

Recently, two kinds of chitosan nanoparticles loaded with DFO for the slow release of the drug were suggested [30,31]. In both cases, nanoparticles were prepared by the ionotropic gelation method [32], involving a solution of DFO and chitosan (i.e., a polycation polymer) in which tripolyphosphate (i.e., a polyanionic counter ion) was added dropwise. The DFO release and the iron-chelation efficiency were tested in vitro in cell culture models, and other studies are required to evaluate the pharmacological performances of this formulation in vivo.

Chitosan/alginate hydrogels were also proposed as lengthy delivery systems of deferoxamine. In particular, DFO-based chitosan/alginate hydrogel alone or DFO-encapsulated into a composite of hydrogel and poly(d,l-lactide-co-glycolide, PLGA) biodegradable microspheres were studied in vitro [33]. The composite resulted in the most effective delivery system since the DFO is strongly entrapped in the hydrogel network and is gradually released by diffusion. The formulation, wholly biodegradable and biocompatible, coupled with excellent results, seems promising for clinical applications.

Cyclodextrin-based polyrotaxanes were also reported as efficient carriers for several drug delivery systems [34]. Polyrotaxanes are supramolecular materials constituted by a linear polymer chain, and many cyclic molecules screwed on its linear axis stopped with two large end groups at the extremes of the chain. The main advantage of preparing polyrotaxanes-drug conjugates is that despite their small dimension, they can extend the blood circulation of several drugs [35]. In 2016, Liu et al. [36] synthesized polyrotaxanes by threading multiple α-cyclodextrin rings onto poly(ethylene glycol) bis(amine) chains capped with enzymatically cleavable bulky Z-L phenylalanine. Then the hydroxy groups of the cyclodextrin moieties were oxidized into aldehydes and conjugated with the terminal amine of DFO to obtain the polyrotaxane-DFO (see Figure 6).

The Fe(III) chelating properties of the developed materials were checked by UV–vis spectrophotometry. In vitro studies considering iron-overloaded macrophages demonstrated the ability of polyrotaxane-DFO to decrease the drug’s cytotoxicity while keeping its chelating properties unaltered. The same research group had recently proposed an improved version of polyrotaxane-DFO, incorporating (ROS)-sensitive thioketal groups into the polyrotaxane platform [37]. In vivo experiments demonstrated how ROS-induced dissociation of the chelator into small parts of 2 nm size drastically increased fecal and urine elimination of excess iron(III). Moreover, this nano-drug showed excellent biocompatibility since no adverse effects were detected in the organs analyzed, proving to be a promising alternative to free DFO.

In the last 20 years, many attempts have been devoted to conjugating DFO to a water-soluble polymer, such as polyethylene glycol (PEG), to enhance its pharmacokinetics. For example, DFO was conjugated to PEG-containing copolymers to develop high blood compatible and long-circulating macromolecular chelator [38,39]. PEG has been applied in several formulations since the FDA approved it as a pharmaceutical polymer material [40,41,42]. However, the main drawbacks of PEG-containing copolymers are the wide molecular weight and low loading efficacy of the active pharmaceutical ingredients; thus, multi-armed PEGs were recently exploited thanks to their higher efficiency in drug loading compared to the classical linear polymers [43,44]. In this field, Yu et al. [45] proposed a star-like 8-arm-polyethylene glycol conjugate with DFO (see Figure 7), demonstrating that it can be a promising candidate as a long-circulating, less toxic iron chelator to be used in the treatment of Fe(III)-overload patients.

Table 1. DFO-based biopolymers for chelation and detection of Fe(III).

Material	DFO Concentration	Trials	Ref.
Polyphenol-DFO nanoparticles	80% w/w	in vitro and in vivo	[22]
Unilamellar and multilamellar liposomes encapsulated DFO	10–12% w/w	in vitro and in vivo	[24]
Unilamellar and multilamellar liposomes encapsulated DFO	5–28% w/w	in vitro and in vivo	[25]
Liposomes encapsulated DFO	28.5% w/w	in vivo	[26]
DFO-loaded polymeric micelles	4.5–6.1% w/w	in vitro and in vivo	[27]
DFO- Tetraphenylethene encapsulated polymeric micelles	12.5% w/w	in vitro	[28]
Starch-conjugated DFO	>50% w/w	in vivo	[29]
DFO-loaded chitosan-nanoparticles	20, 45 and 75 w/w	in vitro	[30]
DFO-nanogel	8.5% w/w	in vitro	[31]
Chitosan/alginate hydrogel encapsulated DFO	10–25% w/w	in vitro	[33]
Polyrotaxane–DFO conjugates	35.7% w/w	in vitro and in vivo	[36,37]
DFO-based copolymers polyethylene glycol methacrylate	5.5–15% w/w	in vitro	[39]
Star Like Eight-Arm Polyethylene Glycol-DFO Conjugate	~100% w/w	in vitro and in vivo	[45]

summarizes the DFO-based biopolymers discussed in this paragraph.

3. Solid Phase Chelating Materials for Fe(III) Sensing

The ability of deferoxamine to strongly complex Fe(III), its physical–chemical proprieties and the presence in its structure of a terminal amino group allowed the development of DFO-based solid sorbents for iron sensing. Several strategies have been applied to anchor deferoxamine to different solid substrates, which will be summarized and commented on in this paragraph.

Among various materials, mesoporous silica was the most widely employed since it was particularly attractive for its large specific surface area, cost-effective production, biocompatibility and ease of functionalization [46].

DFO was generally grafted on the silica particles’ surface by covalent bonding. For example, Biesuz et al. presented the synthesis of DFO self-assembled monolayers (DFO-SAM) on the mesoporous silica MCM-41 by a one-pot strategy [47]. This synthetic approach was optimized by an experimental design, and the procedure consisted of DFO’s dissolution in dimethyl sulfoxide followed by a reaction with (3-glycidyloxypropyl)trimethoxysilane (GPTMS) at 90 °C under overnight stirring. MCM-41 is then added to the mixture obtaining the final product (see Figure 8).

The characterization of the material revealed that the iron uptake kinetic at pH 2.5 was relatively fast, less than 2 h. From the sorption isotherms, the maximum sorption capacity of iron(III), obtained in optimized conditions, was about 0.3 mmol/g. The promising results, coupled with those obtained by applying the material to urine samples spiked with the Fe(III) [48], were confident that this functionalized mesoporous silica could be used for iron detection in environmental and biological samples.

Recently, stellate mesoporous silica nanoparticles grafted with deferoxamine were developed [49]. Nanoparticles were synthesized by a sol-gel approach and then functionalized with deferoxamine B (DfoB) by a three-step procedure: firstly, the nanoparticles’ surface was modified with aminopropyltriethoxysilane through a condensation reaction in ethanol; then, the amine groups previously inserted were converted into carboxylic groups by a reaction with succinic anhydride (SA) and finally, the carboxylic functionalities, after activation with 1-ethyl-3,3-dimethylaminopropylcarbodiimide hydrochloride reacted with the terminal amino group of DFO. Figure 9 shows the synthetic pathway.

The material obtained showed performances similar to those of the DFO-SAM previously described [47], confirming the efficient and selective removal of Fe(III) from biological samples.

Pawlaczyk and Schroeder reported a similar approach [50]. In that case, two commercial amorphous silica with microparticles’ surfaces modified by isocyanate and maleimide groups were functionalized with DFO by a reaction between the terminal free amino group of deferoxamine with the isocyanate or maleimide group of the silica. The obtained materials presented a good sorption capacity for Fe(III) of about 1.5–2 mmol/g at pH 2.45, indicating the high adsorptive potential of the DFO-functionalized materials. The process was spontaneous and endothermic, but the iron(III) uptake required at least 5–10 h. In the same paper, the authors proposed other kinds of DFO-based hybrid material; in particular, the most promising was that of magnetite encapsulated in silica nanoparticles. Since the good performance, probably due to the nanostructured texture, the material was applied to a competitive test in vitro to evaluate its iron(III) scavenging properties from the biological complex protoporphyrin IX−Fe(III) (hemin). The excellent results can lead to a further study of the material for its application in clinical or biological fields.

Polysaccharides such as agarose and alginates were also functionalized with deferoxamine for developing biocompatible sorbents with high porosity, good chemical stability and high loading capacity. For example, Yehuda et al. [51] immobilized DFO on pre-activated Sepharose gels aiming to develop a slow-release Fe(III) fertilizer. The most promising product was achieved by functionalizing (p-nitrophenyl)chloroformate activated Sepharose in the presence of (Dimethylamino)pyridine, obtaining a good affinity for iron(III) and a maximum sorption capacity of 0.14 mmol/g at pH 4.5.

More recently, deferoxamine-grafted alginate hydrogel was synthesized by an amidation reaction between sodium alginate and DFO; its application as a wound cover material was considered [52].

In the following

Table 2. DFO-based solid phase chelating materials for Fe(III) detection.

Material	Equilibration Time (min)	Sorption Capacity, qmax (mmol Fe(III)/g)	Ref.
DFO-SAM on mesoporous silica MCM41	200	0.3	[47]
DFO-grafted stellate silica nanoparticles	30	0.48	[49]
DFO-grafted amorphous silica	300–600	1.5–2	[50]
DFO-immobilized Sepharose gel	1440	0.14	[51]
DFO-grafted alginate hydrogel	n.d. 1	n.d. 1	[52]

¹ n.d. = not determined.

, the over-described DFO-based solid phase chelating materials for Fe(III) detection are reported.

4. DFO-Based Sensors for Fe(III) Detection

Due to the property of DFO to strongly coordinate Fe(III), it is applied for detecting and sensing iron in biological and environmental samples. In particular, optical and electrochemical signals were transduced to obtain the quantitative data.

In this section, deferoxamine-based sensors and methods for Fe(III) detection are summarized and discussed.

4.1. Optical Sensors

The intense red-orange color of the 1:1 Fe(III)/DFO complex, with a peak of maximum absorption of about 430 nm at neutral pH, has been exploited for developing colorimetric methods (see Figure 10) [53].

Starting from the simple strategy of deferoxamine application as a reagent for UV–vis spectrophotometric analysis of Fe(III) in aqueous solutions [53], some DFO-based colorimetric sensors were realized by functionalizing different substrates, such as mesoporous silica [48,54,55] or filter paper [56,57]. The original idea was to develop low-cost, eco-friendly devices with a simple signal read-out, easily convertible into analytical data.

DFO-based mesoporous silica was grafted as self-assembled monolayers on the silica surfaces according to the above-described one-pot synthesis [47] and applied for the colorimetric sensing of free Fe(III) in samples with unknown iron content [48,54]. The procedure consists of sorbing the metal cation on the modified silica and directly measuring the absorbance of the product. Afterward, due to a deep characterization of the complexation equilibria of Fe(III) on the solid phase, the free iron(III) concentration (pFe) can be assessed from the measured absorbance. Figure 11 shows the images of the DFO-based mesoporous silica before and after contact with Fe(III) [55].

The LOD referred to the total iron(III) content is about 10 µM, but the free iron quantification can be lower than 10⁻²⁵ M, depending on the pH and sample’s composition [54].

Deferoxamine-functionalized filter paper-based devices have been proposed (DFO-papers) [56]. They were prepared by a two steps procedure, i.e., a halogenation reaction, followed by the introduction of the DFO molecules, as reported in Figure 12.

The colorimetric response of the DFO–papers to the presence of Fe(III) ions in aqueous solutions allowed the detection with the naked eye and the metal ion quantification. From the scan of the DFO–paper, the RGB indexes were acquired, and their correlation with Fe(III) concentration was obtained by Principal Components Analysis (PCA); in particular, the first component, PC1, that explained the maximum variance was correlated with the metal ion content so providing the dose-response curve. The LOD of 3 µM was obtained, and the approach successfully permitted the detection of Fe(III) in water samples by a cheap and ready-to-use sensor [56].

However, the main drawbacks of using silica or paper-based sensors are their fragility since they can decompose in alkaline media; besides, silica is unwieldy when it has to be inserted into the spectrophotometric cuvettes.

Ethylene-vinyl alcohol copolymer, EVOH, was also exploited [58]; in this case, a random copolymer was prepared by covalently bonding the free amino group of DFO to the alcoholic functionalities of the EVOH and using carbonyldiimidazole as a coupling agent, according to the synthetic pathway reported in the following Figure 13. Despite the low loading capacity of about 0.01 mmol/g, the polymer that was prepared was malleable, and it was pressed to obtain thin films sensors without compromising the sorbent properties; moreover, it also provided an excellent response to the sorption of Fe(III) from aqueous solutions.

Fluorescent device technology offers relatively cheap and easy-to-use equipment that can be miniaturized and applied for in situ analysis [59]. In this scenario, DFO-based fluorescent probes and sensors were reported. For example, Delattre et al. developed a carbazole–desferrioxamine fluorescent chemosensor for Fe(III) determination in the airborne particulate matter [60]; DFO acted as the ionophore and carbazole as fluorophore unit. The sensor presented excellent selectivity and a very low detection limit of about 20 nM, confirming its efficacy in sensing Fe(III) at a trace level in atmospheric particulate.

Su et al. proposed other fluorescent nanosensors by immobilization of a fluoresceine–DFO bifunctional probe into mesoporous silica, employing three different synthetic strategies [61]. In particular, encapsulation of the probe in surfactant micelles during the synthesis of the silica, direct impregnation into the mesochannels of the mesoporous material and covalent binding with previously inserted propylamine groups in the porous silica surface were considered. Significant results were obtained only with the material prepared with the last synthetic pathway, i.e., by covalent binding of the probe; the minimum quantifiable Fe(III) concentration was about 0.1 µM, and the selectivity was good since only Cu(II) caused quenching of the sensor fluorescent emission.

Nanosensors functionalized with deferoxamine have been proposed aiming to determine trace levels of iron(III). For example, π-plasmon absorption in the UV region of deferoxamine-modified carbon nanotubes was exploited for sensitive and selective detection of Fe(III) [62]. The surface of single walls carbon nanotubes (SWCNs) was firstly modified with PL-PEG-COOH (carboxyl terminated phospholipid–polyethylene glycol); then, DFO was covalently anchored to the carboxylic group of the SWCNs-PL-PEG-COOH via N-(3-dimethylaminopropyl)-N0-ethylcarbodiimide hydrochloride (EDC)/N-hydroxysuccinimide (NHS) coupling reaction. The UV–vis–NIR spectra, after additions of increasing concentrations of Fe(III) to the modified nanotubes, showed a quenching of the nanotubes’ absorption peak at 270 nm, with a response extremely sensitive since a significant lowering of the absorbance also occurred with picomolar Fe(III) content. The possible electron transfer between the DFO-Fe(III) complex and the carbon nanotubes was a tentative interpretation of the mechanism. Although, the high sensitivity and the limited linearity range (from 10 to 90 pM) makes the method unsuitable for environmental or biological sample analysis.

An array of poly(3,4-ethylenedioxythiophene) (PEDOT) nanowires doped with DFO for Fe(III) detection at trace concentrations was developed [63]. The nanowires were deposited from a solution containing EDOT and DFO by lithographically patterned electrodeposition on a glass substrate. The through-nanowire electrical resistance was measured vs. iron(III) concentration, and a linear relationship was found in the range 10⁻⁸–10⁻⁴ M with a LOD of 10 nM Fe(III). Despite the very low detection limit, the selectivity was not wholly proven since only Zn(II) was tested as interferent. Moreover, the possible application of the sensor to complex matrix samples was not verified.

Surface-Enhanced Raman Scattering (SERS) based on DFO-functionalized silver nanoparticles for iron(III) sensing has been proposed [64]. Firstly, DFO–iminothiolane was synthesized to provide the deferoxamine molecules with a moiety containing sulfur atoms for realizing stable and localized S–Ag bonds with the nanoparticles. Thus, the so-obtained product was used to coat the silver nanoparticles’ surface. The ability of the nanocomposites obtained to bind Fe(III) was proven by SERS measurements, but no data were reported to demonstrate the applicability of these nanocomposites as an analytical sensing device.

A small-size, low-cost sensor obtained by deposition of a DFO self-assembled monolayer (DFO-SAM) on the gold surface of a surface plasmon resonance (SPR) platform was developed for the selective detection of Fe(III) [65]. The sensor was based on SPR transduction in connection with a plastic optical fiber (POF). The optical response was achieved due to the Fe(III)-DFO interaction on the sensor’s surface. A scheme of the sensor is reported in Figure 14.

A linear relationship between the resonance wavelength variation, Δλ and Fe(III) concentration was obtained from 2 × 10⁻⁶ M to 3 × 10⁻⁵ M. Although a fairly innovative approach, the relatively high LOD, compared with that claimed by other sensors and with the concentration levels in real life samples, makes the method ineffective for trace iron(III) analysis.

Iron is a bioactive element in seawater able to regulate photosynthetic carbon dioxide drawing and its transfer from surface waters by phytoplankton in more than 40% of the world’s oceans [66]. Iron quantification in seawater is a real challenge since its extremely low concentration (less than 1 nM) due to the scarce solubility of Fe(III) in oxygen-rich seawater and the high iron demand by phytoplankton. Consequently, sample contamination during the whole analytical process can occur. Moreover, marine Fe(III) determinations are additionally problematic due to the seawater complex matrix, which could interfere during the trace analyses [67].

An interesting study [68] presented a nanostructured silica-based method for detecting dissolved iron(III) in natural seawater. In particular, DFO was covalently linked to a mesoporous silica film surface through a 3-(triethoxysilyl)propylsuccinic anhydride linker. The changes in the infrared spectrum of the DFO-modified silica film after Fe(III) complexation (monitoring the band at 560 cm⁻¹) provide the accurate dosage of iron(III) in seawater samples. A very low LOD of about 50 pM was obtained, allowing the Fe(III) detection in subarctic Pacific waters without interferences.

4.2. Electrochemical Sensors

To date, the development of modified electrodes with DFO for voltammetric or potentiometric analysis has been reported in a few papers.

One pioneering study was that proposed in 1993 by Arrigan et al., who described a voltammetric determination of Fe(III) using a Nafion-coated glassy carbon electrode incorporating DFO or other hydroxamic siderophores [69]. DFO was immobilized in the Nafion-coated electrode by an ion-exchange reaction between the sulfonic groups of the Nafion film and the protonated amino group of the deferoxamine in mesylate form. Differential Pulse Voltammetry (DPV) was applied for iron(III) detection and applying 10 min of preconcentration, the LOD of the method was about 0.5 µM.

An electrochemical DFO-based sensor for Fe(III) determination in wine was developed by Norocel and Gutt [70]. It must be underlined that in food analysis, for trace elements, speciation, i.e., the quantification of any single species, is often required to understand their biological activity [71]. Fe(III) speciation in wine is extremely important since the iron(III) compounds present are insoluble and undesirable, for instance, Fe(III) speciation is stated to be associated with oxidative spoilage and stalling [72]. Consequently, Fe(III) content is a crucial parameter for wineries to guarantee the quality of the wine after bottling [73]. In this scenario, different methods were proposed for iron(III) speciation in wines [74,75,76,77,78,79]; most of all is based on spectrophotometric or chromatographic techniques requiring bulky and expensive instruments, not suitable for rapid and in situ analysis. Therefore, the demand for Fe(III) sensors in food analysis are necessary. Electrochemical sensors meet the requirements because they do not alter the speciation, are economical, fast and do not necessitate bulky instrumentation [80].

The purpose of the method proposed by Norocel and Gutt [70] was the development of an electrochemical sensor for Fe(III) detection in wines using DFO as the recognition element. Deferoxamine was immobilized on the graphite working electrode of a screen-printed cell using benzophenone as an immobilizer agent for improving ligand stability. Fe(III) quantification was performed by cyclic voltammetry, obtaining a LOD of 16 µM.

In the Shervedani and Akrami [81] paper, deferoxamine is immobilized as a self-assembled monolayer (SAM) on the gold surface of a voltammetric disk electrode. The electrode functionalization required the deposition of 3-mercapto propionic acid (MPA) on the clean gold surface, subsequently, its activation by N-(3-dimethyl aminopropyl)-N0-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) and finally the reaction with DFO via amide formation with the free terminal amino group of the molecule [82]. Figure 15 schematizes the functionalization process.

Electrochemical impedance spectroscopy (EIS), in combination with cyclic voltammetry (CV) and square wave voltammetry (SWV), was applied for the sensor’s characterization and application for Fe(III) sensing in several samples. Very high sensitivity and a low LOD of about 0.02 nM are the interesting figures of merit of the method.

The self-assembled monolayers (SAMs) approach in sensor development presents various pros, such as the easy preparation procedure, great stability, and adaptability due to the simple incorporation of several functionalities [83]. SAMs-modified gold disk electrodes were traditionally suggested since their high sensitivity and selectivity toward different ions and molecules. However, the increased request for low-cost, easy-to-prepare, and disposable sensors for in situ analysis encouraged the employment of screen-printing electrodes [84]. In this context, screen-printed cells with the gold surface of the working electrode functionalized with deferoxamine were recently proposed [85]. A similar approach to that previously described [82] was applied. The gold-ink electrode surface was characterized, before and after modification, by determining the area and the double-layer capacitance. DPV analyses were performed after accumulating Fe(III) at the open circuit potential in solution at pH 1 for 2 min. The method’s pros are the in situ application, simple electrode surface modification, low reactives’ consumption of reagents, good precision and a low detection limit of 0.5 nM.

DFO-based sensors for Fe(III) analysis, commented above, are summarized in the following

Table 3. Optical and electrochemical DFO-based sensors for iron(III) detection.

Receptor	Analytical Method	Sensitivity	RDS %	LOD (M)	Ref.
Deferoxamine (aqueous solution)	UV–vis spectroscopy	2764.8 cm−1 M−1	2	1.4 × 10−4	[53]
DFO-SAM on mesoporous silica	UV–vis spectroscopy	23,337 M−1	10–15	1 × 10−5	[48,54]
DFO immobilized on filter paper	Colorimetry	n.d. 1	n.d. 1	3 × 10−6	[56,57]
DFO@Ethylene-vinyl alcohol copolymer	Colorimetry	n.d. 1	n.d. 1	n.d. 1	[58]
carbazole–desferrioxamine	Fluorescence	n.d. 1	n.d. 1	n.d. 1	[60]
fluoresceine–DFO@mesoporous silica	Fluorescence	n.d. 1	n.d. 1	1 × 10−7	[61]
DFO-modified carbon nanotubes	UV–vis–NIR spectroscopy	0.0128 pM−1	n.d. 1	1 × 10−11	[62]
DFO-PEDOT	Impedance spectroscopy	n.d. 1	n.d. 1	1 × 10−8	[63]
DFO-functionalized Ag nanoparticles	surface-enhanced raman scattering (SERS)	n.d. 1	n.d. 1	n.d. 1	[64]
DFO-SAM on Au surface	Plastic optical fiber–Surface plasmon resonance (POF-SPR)	1.5 × 10−5 nm M−1	n.d. 1	1 × 10−6	[65]
DFO on nanostructured silica	IR spectroscopy	0.0372 pM−1	15	5 × 10−11	[68]
DFO-immobilized Nafion-coated glassy carbon electrode	Differential pulse voltammetry (DPV)	n.d. 1	n.d. 1	5 × 10−7	[69]
DFO-modified graphite screen-printed electrode	Cyclic voltammetry (CV)	2.93 × 10−5 µA cm2 mmol−1	6–13	1.6 × 10−5	[70]
DFO-SAM on gold disk electrode	Square wave voltammetry (SWV)	3.7 µA (p[Fe(III)/µM])−1	2.2	2 × 10−11	[81]
DFO-SAM on gold screen-printed electrode	Differential pulse voltammetry (DPV)	0.421 µA nM−1	n.d. 1	5 × 10−10	[85]

¹ n.d. = not determined.

.

5. Conclusions

The siderophore deferoxamine (DFO), as a strong iron chelator, has been widely studied. Despite the unquestionable efficacy of DFO in iron chelation, its several side effects have limited its application in clinical therapy. Other drawbacks are the poor compliance of thalassemic patients subjected to long-term subcutaneous DFO treatment and the high cost of the drug.

Several strategies, including the use of deferoxamine-loaded nanocarriers, protection by surface-active agents, or the conjugation of DFO with polymers, have been proposed to improve deferoxamine’s bioavailability, half-life (reducing the administration frequency) and minimize its side effects. Nevertheless, these new drug-delivery systems have much room for improvement, and most still require in vivo trials and approval for commercialization.

On the other end, the DFO’s ability to coordinate Fe(III) with high complexation constants have made it efficiently employed for developing selective and sensitive methods and sensors.

Various DFO-based solid sorbents have been proposed; most of them have used nanostructured silica or biopolymers as a substrate. Despite the quite good selectivity of these materials, the high cost of production and the low sorption capacity make them applicable only for in-lab-scale and unsuitable for scaling up.

DFO has been exploited in sensing technology to develop selective and sensitive Fe(III) detection devices.

Due to its terminal amino group, deferoxamine can be covalently immobilized to different surfaces or nanomaterials that, coupled with different transduction systems, can sense iron(III).

The intense red-orange color of Fe(III)/DFO complexes, low-cost and disposable colorimetric devices have mainly been proposed, but with limits of detection not enough for trace analysis. More sensitive sensors were obtained by functionalizing the electrodes’ surfaces and applying voltammetric techniques to enhance the LOD.

The current trend is the development of cheap, disposable, miniaturized devices, expanding the fields of DFO applications.

The example of deferoxamine can be considered a proof of concept with a view to reusing banned drugs or molecules, providing them with a new use destination.