Schiff Base Ligand Coated Gold Nanoparticles for the Chemical Sensing of Fe ( III ) Ions

New Schiff base-coated gold nanoparticles (AuNPs) of type AuNP@L (where L: thiolated Schiff base ligand) have been synthesized and characterized using various spectroscopic techniques. The AuNPs and AuNP@L were imaged by transmission electron microscopy (TEM) and were confirmed to be well-dispersed, uniformly distributed, spherical nanoparticles with an average diameter of 8–10 nm. Their potential applications for chemosensing were investigated in UV-Vis and fluorescence spectroscopic studies.TheAuNP@Lexhibited selectivity for Fe in an ethanol/watermixture (ratio 9 : 1 v/v).The absorption and emission spectral studies revealed a 1 : 1 binding mode for Fe, with binding constants of 8.5 × 105 and 2.9 × 10M, respectively.


Introduction
In recent years, gold nanoparticles (AuNPs) have attracted substantial attention for their extensive application in drug delivery [1,2], magnetic resonance imaging (MRI) [3,4], X-ray computed tomography (X-ray CT) [5], catalysis [6], biosensing [7,8], and so forth because their size, shape, and surface functionalization are easily controlled through the ligands and corresponding metal complexes.One of the typical applications of AuNPs in current research is the colorimetric detection of metal ions in the environment as well as in physiological systems because they possess excellent optical properties, such as high extinction coefficients and distance-dependent plasmonic absorption [9,10].However, the challenge moving forward is to prevent aggregation of the nanoparticles in high-ionic-strength solutions because aggregation restricts the broad and practical application of AuNPs in the detection of ionic species [11,12].
Furthermore, the detection of Fe(III) at trace levels is relevant because iron, with its chemical versatility, is essential for the proper functioning of numerous organisms in the entire spectrum of the biological system [13].In the human body, iron is one of the most essential trace elements; deficiency of ferric ion (Fe(III)) in the body causes anemia, hemochromatosis, liver damage, diabetes, Parkinson's disease, and cancer [14][15][16].Ferric ions also play critical roles in the growth and development of living cells and catalyze numerous biochemical processes [17].However, the physiological abundance of Fe(III) causes imbalance, triggering the failure of multiple organs, such as the heart, pancreas, and liver [18,19].In this regard, the judicious selection and proper design of an adequate receptor are vital.Numerous studies on the development of Schiff base chemosensors for the detection of Hg(II), Zn(II), Al(III), and other ions have recently been reported in the literature [20][21][22].However, the availability of chemosensors for Fe(III) that have a high detection threshold is rather limited and the amount of material required to detect a signal is high.
Conventional detection of Fe 3+ relies on several standard analytical techniques such as inductively coupled plasma atomic emission spectrometry (ICP-AES) [23], inductively coupled plasma mass spectrometry (ICPMS) [24,25], atomic absorption spectrometry (AAS) [26], and voltammetry [27].However, these methods are expensive, bulky, and time consuming because they require tedious pretreatment procedures for sample preparation.However, fluorescence microscopy, which is based on optical fluorescence, is a simple, easy, inexpensive, and highly selective tool for studying the localization, trafficking, and expression levels of biomolecules and metal ions within living cells [28].Most Fe 3+ sensing methods are based on an organic chemosensor that either undergoes fluorescence quenching because of the paramagnetic nature of ferric ion [29] or undergoes a "turn on" mechanism [30].
In the search of a new chemosensor with high sensitivity and a very low detection limit for Fe(III), the combination of nanotechnology and a metal binding unit became an obvious choice.AuNPs, which exhibit good optical properties as signaling units as well as the ability to carry higher payloads on their surface and ligands with strong coordinating elements, have enabled the development of a suite of highly efficient chemosensors.However, the literature contains few reports of their application.For example, Zhang et al. reported excellent dispersion AuNPs for detecting sugars by the hypsochromic surface plasmon resonance (SPR) shift [31].Bai et al. also reported 4-piperazinyl-1,8naphthalimide functionalized AuNPs for Fe(III) recognition, and their results were highly encouraging [32].
Here, we report the synthesis of a thiolated Schiff base ligand by the reaction of salicylaldehyde and 4aminothiophenol, followed by its subsequent anchoring onto the surface of AuNPs through replacing citrate as a stabilizing agent.The results of the characterization of the ligand and the resulting surface-functionalized AuNP@L are described.The efficiency of the AuNP@L as a chemosensor is also reported here on the basis of the results of fluorescence and UV-Vis studies.

Experimental
2.1.General Remarks.All of the chemicals and solvents were purchased from Sigma-Aldrich.The 1 H and 13 C NMR spectra and chemical shifts were recorded in deuterated chloroform (CDCl 3 ) on a JEOL 500 MHz spectrometer.FT-IR spectra were collected on Nicolet (Thermo Scientific) spectrometer using iTR as a sample holder in the wavenumber range from 600 to 4000 cm −1 .Absorption spectra were collected at room temperature in the 4000-400 cm −1 range using a JASCO-670 spectrophotometer, and emission spectra were acquired on a Fluorolog (Horiba) system.Diffraction data were collected on a Rigaku model MiniFlex II diffractometer equipped with a Cu-K radiations source.The data were acquired over the 2 range between 25 and 110 ∘ .The surface morphology of the NPs was discerned by field-emission scanning electron microscopy (FESEM) on a microscope (LYRA 3 Dual Beam, Tescan) operated at 30 kV.FESEM samples were prepared from either a suspension or a dry powder.The energydispersive X-ray spectra for the chemical and elemental analyses of NPs were also collected using an X-Max detector by Oxford, Inc. TEM was performed on a Philips CM200 operated at 200 kV; for the sample preparation, one drop of the aqueous AuNP@L solution was spread onto a 200-mesh copper carbon grid and allowed to dry at room temperature.

Synthesis of Schiff Base Ligand. The thiolated bidentate
Schiff base ligand was prepared (Scheme 1) according to a procedure reported in the literature [33,34].To an ethanolic solution of salicylaldehyde, an equimolar amount of 4aminothiophenol was added, and the mixture was refluxed at 90 ∘ C for 5 h.The yellow precipitate was filtered, purified by recrystallization from methanol, and finally dried under vacuum to obtain a 91% yield.

Synthesis of AuNP@L.
AuNPs coated with citrate (AuNP@Cit) were prepared using the citrate (Cit) reduction method in deionized water (Scheme 2).HAuCl 4 ⋅3H 2 O (0.33 g, 1 mmol) in 500 mL of water was refluxed in a 1 L round-bottom flask equipped with a condenser.The mixture was stirred vigorously under argon for 30 min.Trisodium citrate (10 mL, 1.14 g, 3.88 mmol) solution was rapidly added, which resulted in a color change from yellow to purple.After the mixture was boiled for another 10 min, the heating mantle was removed and the mixture was allowed to cool at room temperature.AuNP@L was prepared as follows.To the freshly prepared AuNP@Cit (50 mL), Schiff base ligand (5 mg in 0.5 mL methanol) was added in one portion and stirred for 5 h at room temperature.The AuNP@L precipitated upon the addition of an equal amount of acetone.The nanoparticles were collected by centrifugation and washed successively with water and acetone to remove the unreacted ligands.

Results and Discussion
3.1.Synthesis and Characterization.The 2-[(4-mercaptophenyl)imino methyl] phenol Schiff base (L) was prepared by the reaction between salicylaldehyde and 4-aminothiophenol in ethanol (1 : 1 mole/mole) under reflux conditions for 5 h (Scheme 1).The resulting yellow solid was recrystallized from methanol in 94% yield.The formation of the imino ligand was confirmed by 1 H and 13 C NMR, which showed a characteristic olefinic proton shift at 8.9 ppm, supported by the olefinic carbon shift at 160.2 ppm (see supporting information in Supplementary Material available online at http://dx.doi.org/10.1155/2015/101694).The FT-IR spectra of the imino ligand showed peaks at 1614 cm −1 and 3448 cm −1 , corresponding to the vibration modes of the C=N and -OH groups, respectively.As a result of coordination of the bare ligand with the Fe 3+ , the C=N band shifted to a lower wavenumber (1609 cm −1 ), indicating the formation of a metal complex.A similar trend was observed for the phenolic group upon participation in coordination with the metal center.The surface of the AuNP@Cit nanoparticles was functionalized via the one-step addition of the ligand in a minimum amount of methanol solution (Scheme 2).Optimization of the Au-toligand molar ratio was critical for the preparation of AuNP@L because excess ligand resulted in aggregation and precipitation.The formation of the AuNP@Cit and AuNP@L was confirmed by spectroscopic techniques.For instance, in the case of the citrate-coated nanoparticles, visible absorption spectra showed a shift of the absorption band ( max ) from 525 nm to 530 nm for the ligand-modified moieties.This observed shift was attributed to the surface plasmon vibration in the ligandmodified particles.The binding of the thiolated-imino ligand to the Au surface was further confirmed by the disappearance of -SH stretches in the FT-IR spectrum, indicating Au-S bond formation [35].The TEM image (Figure 1) shows uniformly distributed spherical particles with an average diameter of 8-10 nm.The peaks at 2 = 38.2,44.4,64.5, 77.5, and 81.7 ∘ in the XRD pattern correspond to the (111), ( 200), ( 220), (311), and (222) planes in the AuNPs and are identical to those reported in the literature (JCPDS card number: 00-004-0784) [36].The uniform anchoring of ligands onto the surface of nanoparticles is demonstrated by the energy-dispersive Xray spectroscopy (EDX) element mapping images in Figures 2(a) and 2(b).Thiols were uniformly anchored onto the Au surface.The structural composition was demonstrated by EDX (Figure 2(c)); carbon, nitrogen, and sulfur were observed to be present on the AuNP surface.A high loading of Schiff base ligand was confirmed from thermogravimetric analysis (TGA), which showed 21.5% weight loss in the temperature range from 0 to 800 ∘ C (ramp rate: 10 ∘ C/min), corresponding to the decomposition of the organic ligand.

UV-Vis Absorption Studies.
Preliminary results of the UV-Vis absorption and fluorescent emission studies revealed that the AuNP@L exhibited selectivity toward ferric ions (10 M) in a 9 : 1 ethanol/water system.As evident in Figure 3, in the absence of ligand, the peak at 525 nm corresponds to the SPR of AuNPs.Upon attachment of the ligand, this peak red-shifted to 530 nm.Moreover, an additional absorption band appeared at 350 nm; this band was attributed to the - * transition, which is likely favored by the planar orientation enforced by the intramolecular hydrogen bonding in AuNP@L [37].The addition of Fe 3+ causes the plasmonic absorption peak to shift again from 530 to 559 nm.
Interestingly, the presence of other metal ions did not influence the UV-Vis signature, indicating that no aggregation occurred, similar to the observed behavior of ferric ions.However, upon further ingress of ferric ions in the solution containing AuNP@L, the absorption band at 350 nm was gradually but systematically quenched, whereas that at 530 nm was synchronously shifted to 559 nm, as shown in Figure 4.The 530 to 559 nm shift in the plasmonic absorption band with a gradual increase in the Fe 3+ concentration indicates cation-induced aggregation of AuNPs.The shift in the peak is linear up to 1 equivalent of Fe 3+ (Figure 4 inset), indicating the formation of a 1 : 1 complex with a strong affinity (binding constant: 8.5 × 10 5 M −1 ; estimated error ≤ 10%) [38].

Photoluminescence Studies.
The results of the photoluminescence studies of AuNP@L with iron (concentration: 10 M) in 10% (v/v) water/ethanol are shown in Figure 5.The emission peak at 491 nm upon excitation with 390 nm radiation resulted from the intramolecular charge transfer (ICT) between the imine groups and the phenolic groups of the ligands in AuNP@L.The addition of ferric ions quenched the fluorescent emission due to chelation-enhanced quenching (CEQ) because Fe 3+ is paramagnetic.The quenching in the presence of iron provides a very fast and efficient nonradiative  decay of the excited states due to the electron or energy transfer between the cations and the ligands.

Competition with Other Metal
Ions.The selectivity and tolerance of AuNP@L for Fe 3+ over other cations were investigated by adding 10 equivalents of various metal ions to 10 M of AuNP@L (Figure 6).Partial quenching occurred with Al 3+ , Cu 2+ , Hg 2+ , and Zn 2+ , as shown in Figure 5, whereas the molecular fluorescence was quenched to a maximum level with Fe 3+ , indicating that AuNP@L exhibited the highest sensitivity for ferric ion detection.
This observation was attributed to the difference in the coordinative interaction energy for various cations that otherwise do not substantially differ in ionic size.Thus, this energy difference can be exploited for discriminative purposes, especially for fluorescent sensing [39].Fe 3+ exhibits high thermodynamic affinity for phenolic-C=N and -OH groups, which is a hybrid of the imino nitrogen of the amine and the oxygen of the phenol ring; this hybrid is formed as a result of the strong tendency of phenol to undergo deprotonation during complex formation, with fast metal-toligand binding kinetics that are otherwise not possible with other transition-metal ions.The estimated detection limit of AuNP@L is 1.2 M for Fe 3+ .Although the fluorescence turn on approach is more effective than switch off approach, the probe, AuNP@L, was selective to Fe 3+ compared to the other biologically relevant metal ions (Cu 2+ , Zn 2+ , etc.).It also has a comparable detection limit of 1.2 M for Fe 3+ .Moreover the absorption and emission spectral studies showed a 1 : 1 binding mode for Fe 3+ , with strong binding constants of 8.5 × 10 5 and 2.9 × 10 5 M −1 , respectively.The obtained detection limit is comparable with the literature data for the detection of the Fe 3+ in different system (Table 1).

Conclusion
In summary, we have prepared a simple and sensitive nanogold-based Schiff base chemosensor that exhibits high selectivity toward ferric ions compared to other cations in a water/ethanol mixture.The AuNP@L was characterized by UV-visible absorption spectroscopy, photoluminescence, TGA, and TEM.The detection limit for Fe 3+ ions was estimated to be 1.2 M, without interference from other metal ions.The binding mode was 1 : 1 and the binding constants were 8.5 × 10 5 M −1 and 2.9 × 10 5 M −1 , as calculated from the results of absorption and emission titrations, respectively.Thus from sensing point of view this probe can be used in the physiological system with good selectivity and sensitivity for the detection of Fe 3+ .

Table 1 :
Comparison of the detection limit AuNPs@L with similar system.