UV-Visible Spectroscopy Detection of Iron(III) Ion on Modified Gold Nanoparticles With a Hydroxamic Acid

The present work describes the preparation of gold nanoparticles (AuNPs) functionalized with hydroxamic acid and the use of them in UV-visible spectroscopy detection of iron(III) ions. The prepared AuNPs were thoroughly characterized by using UV-visible spectroscopy, TEM, and 1H NMR techniques. The newly synthesized hydroxamic acid-AuNPs are brown in color due to the intense surface plasmon absorption band centered at 527 nm. In the presence of Fe(III), the surface plasmon absorption band is centered at 540 nm. However, the sensitivity of hydroxamic acid-AuNPs towards other metal ions such as Mg(II), Ca(II), Ag(I), Cu(II), Mn(II), Cr(II), Ni(II), Co(II),Fe(II), Hg(II), and Pb(II) can be negligible. This highly selective sensor allows a direct quantitative assay of Fe(III) with a UVvisible spectroscopy detection limited to 45.8 nM.

Experiment. All compounds were obtained from Sigma-Aldrich and Merck. Milli-Q water was used in the experiments, and the solvents and reagents were used as received without further purifi cation. All glassware used in the procedure was cleaned in a bath of freshly prepared 3:1 HCl-HNO 3 rinsed thoroughly in water and dried in the open air. UVvis absorption spectra were acquired on a Cary 100 UV-vis spectrometer (Varian, USA) at room temperature (23-25 o C). 1 H NMR spectra were recorded on a Bruker spectrometer operating at 200 MHz in CDCl 3 and DMSO-d. Transmission electron microscopy (TEM) was carried out on a Philips CM 200 FEG scanning transmission electron microscope at 200 keV. A pH meter (Metrohm AG, 827) was used for the pH adjustment.
The synthesis of C4-AuNPs (1) was carried out with a modified Brust method as described previously [29] using octanethiol (C4) (2.7 mmol) as ligands for nanoparticles preparation in the course of gold reduction with sodium borohydride. Nanoparticles were purified by precipitation and centrifugation using ethanol or a mixture of ethanol/acetone, and finally C4-AuNPs were dispersed in THF and stored in the dark.
Hydroxamic acid-AuNPs (3) was prepared by dissolving 40 mg (0.05 mmol) of 11-mercapto-AuNPs in 10 mL of dichlorometane, and then 70 mg (0.25 mmol) of trifl ic anhydride (Tf 2 O) was added and the mixture was stirred at room temperature for 20 min. Then 0.3 mmol of acid hydroxylammonium chloride was added to the mixture and the mixture was stirred at room temperature for 90 min. The solvent was removed via rotary evaporation and washed copiously with water and acetone (3:7, v/v) to remove the unreacted starting materials.
A solution with a different concentration of Fe(III) ion in 2 mL acetonitrile was added to 1 mg of hydroxamic acid-AuNPs, and the obtained mixture was stirred at room temperature to offer enough time for reaction. The fi nal concentrations of Fe(III) in each solution are 0-200 nM. After being stirred for another 30 min, the UV-vis absorption of these solutions was measured and the response curve of the ratio of absorbance of hydroxamic acid-AuNPs at A650/527 versus the concentration of Fe(III) ion was plotted.
Resul ts and Ddiscussion. The synthesis of buthanethiol gold nanoparticle (C4-AuNPs) with hydroxamic acid is planned as shown in Scheme 1. The C4-AuNPs (1) were synthesized following the Brust procedure [31], and we prepared 11-mercapto-AuNPs (2) following the Murray model for the place exchange reaction using an excess amount of 11-mercaptoundecanoic acid as an incoming ligand. Therefore, for preparation of hydroxamic acid-AuNPs, 11-mercapto-AuNPs was put into reaction with hydroxylammonium chloride in the presence of Tf 2 O [17]. The success of the thiol exchange process can be proved by utilizing 1 H NMR spectroscopy. Figure 1A shows that the 1 H NMR spectrum of C4-AuNPs has only broad Scheme 1. Scheme of synthesis and assembly of hydroxamic acid-AuNPs. resonances at 0.88 and 1.25-1.75 ppm attributed to protons on the terminal methyl group and methylene protons of the alkanethiolate chain. Figure 1B represents the 1 H NMR spectra of 11-mercapto-AuNPs and shows that the new signals appeared at 2.13 ppm in addition to the signals observed for C4-AuNPs. These new peaks are assigned to the protons of α methylene unit adjacent to the carboxylic acid groups. Figure 1C represents the 1 H NMR spectra of hydroxamic acid-AuNPs (3) and shows that the new signals appeared at 5.2-5.4 ppm and 8.2-8.3 ppm in addition to the signals observed for 11-mercapto-AuNPs. These new peaks are assigned to the protons of the hydroxyl and the protons of the amine group. In addition, the UV-Vis absorption spectra of C4-AuNPs, 11-mercapto-AuNPs, and hydroxamic acid-AuNPs are shown in Fig. 2a.
Princ iple of the proposed method for Fe(III) ion detection. The aggregation of nanoparticles can be associated with the red shift of the SPR band according to the Mie theory [32]. This theory states that when the distance between the two nanoparticles is less than the sum of their radii, the SPR band displays a red shift, broadening and decreasing in intensity [33].The transition of nanoparticles from dispersion to aggregation exhibits a distinct change in color, and this is due to the coupling of plasmon absorbance as a result of their proximity to each other [34]. However, the latter originates from surface plasmon bands related to collective oscillations of the electrons at the surface of NPs interacting with the electromagnetic field of the incoming light. For the gold surface, plasmon bands are located in the visible region of the electromagnetic spectrum and can be effectively controlled by the shape of NPs [35,36]. On the other hand, the gold nanoparticle with >5 nm diameter is brown or brown red, so when the aggregation occurs, the surface plasmon resonance shifts to lower energies; thus, this low energy band results from the coupling of the plasmon resonance of equidistant neighboring hydroxamic acid-AuNPs [37,38], causing the absorption and scattering peaks to occur at longer wavelengths. The aggregation of AuNPs was induced by the binding of hydroxamic acid-AuNPs N and O group to Fe 3+ ion as a result of bonding between the two or three molecules, leading to a SPR change, as shown in Scheme 2, prior to the addition of Fe(III) ion, and the color change is easily detected with the naked eye. It is obvious that the change in plasmon absorbance spectroscopy of the nanoparticles solution is associated with the increase in the size of aggregated nanoparticles [27]. The ability of hydroxamic acid to form a complex with Fe(III) ion was previously reported [39]. This motivated us to fabricate a simple nanoparticles based UV-visible spectroscopy sensor for Fe(III) ion based on the SPR changes. Accordingly, the ability of the prepared nanoparticles to detect Fe(III) ions was investigated in detail using UV-Vis spectroscopy and TEM analyses.
It is necessary to examine the optimum conditions for a sensor. We investigated responsive conditions in our sensor including the pH of solutions. The value of A650/525, which could characterize the ratio of the dispersed to the aggregated forms of AuNPs, was used as an indicator for performance evaluation. The response of A650/530 was achieved over the pH range 4.24-11.51 in Fig. 2b. In addition, Fig. 2b presents the UV/Vis spectrum of hydroxamic acid-AuNPs; it shows its original SPR band at 537 nm. After adding a drop of an acid solution (pH 4.24), a signifi cant redshift of the band from 537 to 546 nm was observed. Upon the addition of one drop of an alkaline solution (pH 9.32) to the resulting solution, the SPR band shifted from 546 nm back to 536 nm. To obtain better performance, the pH at 9.32 was chosen to avoid aggregation of particles. This is attributed to interparticle interactions: at a low pH the hydroxamic acid groups facilitate interparticle interactions through hydrogen bonding and an increase in hydrophobicity, resulting in the formation of particle aggregates; however, particle aggregation is disfavored due to the repulsive interactions between negatively charged carboxylate particles [40]. Scheme 2. The possible mechanism of the phenomenon of binding the free hydroxamic acid-AuNPs with Fe(III) ion.   To investigate the metal recognition ability of hydroxamic acid-AuNPs, we tested the selectivity of this method in the presence of various metal ions. The same concentration (5 × 10 -7 M) of these metals was added into the solution of hydroxamic acid-AuNPs. The color of the solution containing Fe(III) was determined by the presence of surface plasmon resonance at 525 nm, while other metals had no effect on the surface plasmon resonance. With increasing concentrations of Fe(III) ion, the surface plasmon resonance shifted to red. The red shift clearly indicated the aggregation of hydroxamic acid-AuNPs (Fig. 3a). In other words, the morphology of the functionalized hydroxamic acid-AuNPs before and after addition of Fe(III) ions was studied by transmission electron microscopy (TEM) as shown in Fig. 4. The average particle diameter for hydroxamic acid-AuNPs, deduced from the TEM image (Fig. 4a), is 3 ± 0.5 nm, and the image shows the shape of these modifi ed hydroxamic acid-AuNPs is regular and close to spherical with a reasonable degree of monodispersity. The TEM image of this nanoparticle acid in the presence of Fe(III) is shown in Fig. 4b. Upon adding Fe(III), the SPR of hydroxamic acid-AuNPs acid solution changes, and the TEM image of hydroxamic acid-AuNPs exhibited a signifi cant aggregation driven by Fe(III) ions, and the average diameters are 15 ± 3 nm. These images clearly show the transition from dispersion to aggregation in response to Fe(III) recognition. The UV-visible spectroscopy response is thus attributed to aggregation induced by the complex between hydroxamic acid and Fe(III) ions (Scheme 2). The UV-Vis absorbance values (A 630 /A 527 ) increase with increasing concentration of Fe(III) ions in the system and show a linear correlation for target analytics in the range 0-200 nM (R 2 = 0.9776) in Fig. 3b, and the detection limit is 45.8 nM on the basis of S/N = 3.
Conclusions. We synthesized a highly sensitive UV-visible spectroscopy sensor for the detection of Fe 3+ using gold nanoparticles modifi ed with hydroxamic acid and characterized by UV-visible spectroscopy, TEM and 1 H NMR techniques. This new method offered several advantages over other Fe 3+ detection techniques. First, the UV-vis spectrometer method did not require complicated and expensive instruments. Second, Fe 3+ was the only metal ion that induced aggregation of hydroxamic acid-AuNPs, resulting in a corresponding plasmonic absorption shift from 527 to 650 nm. The method allowed us to detect concentrations up to 45.8 nM. These advantages make this method quite promising for rapid detection of Fe 3+ in aqueous solutions.