Introduction:

Development of nano-based chemical sensor is of great interest due to the increasing need of chemical sensors to monitor metal ions in day to day life. Several research studies have been reported on heavy metal ions detection in aqueous media by exploiting the novel features of functionalised nanoparticles ^[1-3]

Iron plays a very important role in biochemical and metabolic path ways ^{[4, 5]}. Especially ferric ion (Fe3+) acts as co factor in many enzymatic reactions that takes place in mitochondria ^[6-8].As both deficiency and excessive consumption trigger different disease conditions ^{[9, 10]}. Therefore, there is a need for selective and sensitive ferric ions sensor to monitor the level of ferric ions concentration. Different analytical techniques such as atomic absorption spectrometry (AAS), colorimetry and voltammetry exist to determine ferric ions concentration [11]. Although, these methods offer excellent sensitivity they are costly, time consuming, and inconvenient as they require sophisticated instrumentation. In response to these limitations fibre-optic sensors ^{[12, 13]} and fluorescence based sensors ^[14-23] were developed as sensor probes. Even though these probes provide low detection limits, the detection methods are quiet complex. Moreover, Fe3+ selective fluorescent probes are hydrophobic and incompatible with aqueous environments. Furthermore, they need sophisticated instrumentations and very complex synthesis methods are employed. Therefore, the focus is now on nanoparticles, owing to their unique optical properties and simplicity. In recent years, noble metal nanoparticles such as gold nanoparticles (AuNP) and silver nanoparticles (AgNP) have attracted researchers in the field of sensors. Capped noble metal nanoparticles were reported as colorimetric sensors for different types of analytes especially for metal ions such as Cu2+and Hg2+^[24-31]. They are capable of visual analysis where the signaling can be easily detected by naked eye. It was stated that silver nanoparticles have higher extinction coefficient value than gold nanoparticles ^[24] which enables the detection with least material consumption.

Nobel metal nanoparticles show strong absorption in the Ultra violet and visible region of the electromagnetic spectrum due to Surface Plasmon resonance (SPR) ^[32]. The SPR absorption of metal nanoparticles is being extensively studied as it is highly affected by the surroundings of the metal nanoparticles ^{[33, 34]}. This unique optical property has opened avenues in the field of sensors. The SPR of metal nanoparticles is determined by factors such as morphology of the nanoparticle of interest (size and shape), dielectric constant of the environment, and inter particle coupling of the system ^[35]. Any perturbation in the surrounding environment of the metal surface affects the SPR of metal nanoparticles. Hence, shift in the SPR band position and intensity changes occur ^{[36, 37]}. These changes can be used as a tool to evaluate the sensing performance of the sensor system for the corresponding analyte.

Regards to the facts mentioned, we present a single step synthesis of citrate capped silver nanoparticles in aqueous medium to selectively sense ferric ion under acidic conditions. Tri sodium citrate which has carboxylate groups can coordinate silver nanoparticles via –CO functionalities ^{[38, 39]}. Generally, hard acid tends to bind with hard base according to hard and soft acid base (HSAB) theory ^[40]. Ferric ion is considered as hard (Lewis) acid ^[41] and citrate is considered as hard (Lewis) base ^[40-42]. Therefore citrate can preferentially bind with ferric ion. Comparison of binding constant values of citrate –metal ion complex also ensures the selectivity of ferric ion over other metal ions. This sensor system requires only a spectrophotometer to detect ferric ion and other sophisticated instruments are not needed. Furthermore, a simple synthesis method was employed to develop the sensor system and changes in the SPR absorption were mainly focused to estimate the sensing ability. Moreover, visual detection is an additional advantage of the sensor probe.

Experimental

All chemicals were of analytical grade and used without further purification. All aqueous solutions were prepared with double distilled water. Tri sodium citrate dihydrate (Na3C6H5O7.2H₂O), AgNO3, NaBH4, Fe2 (SO4)3.2H₂O, CuNO3)2.3H₂O, FeCl2, CoCl2, Ni(NO3)2 .6H₂O, MnCl2, Zn(NO3)2 .6H₂O, KNO3, MgCl2, Na₂SO₄ and other relevant chemicals were purchased from Sigma-Aldrich. Al (NO3)3 was purchased from Merck.

Absorption spectra of samples were taken using a Shimadzu UV-3600 UV-VIS-NIR spectrophotometer. Fluorescence measurements were acquired on a Fluorolog® Horiba Jobin Yuon fluorometer. Capped silver nanoparticles were structurally characterized using Bruker Vertex – 80 Fourier transform infrared (FT-IR) spectrophotometer and Hitachi SU 6600 scanning electron microscope (SEM). Samples for SEM were prepared by placing a drop of silver nanoparticle solution on a sample stub and drying at room temperature. Average particle size was obtained using a Malvern Zetasizer (Nano –ZS) Particle size analyser. Photographs of the silver nanoparticles were taken by a Nikon CoolPix 885 digital camera

Functionalised citrate capped silver nanoparticles were synthesized through the procedure described by Zhou et al., [24] with some modifications. A mixture of 30.0 ml of sodium borohydrid (2mM) and 10.0ml of tri sodium citrate (1mM) was stirred in an ice bath for 10 minutes. After 10 minutes, a 10.0 ml portion of silver nitrate (1mM) was slowly added to the mixture and stirring was stopped soon after the addition of silver nitrate. Formation of nanoparticles was observed at this stage. Silver nanoparticles obtained from this procedure were faint yellow in colour and the colloidal solution was stored at room temperature in a dark place for further studies.

To study the metal ion sensing ability of citrate capped silver nanoparticles, metal ions such as Fe3+, Al3+, Fe2+,Co2+,Ni2+,Cu2+, Zn2+,Mn2+,Mg2+, K+ and Na+ were tested with the prepared sensor system. Equal concentrations (1x10-3 mol/dm3) of metal solutions were prepared and equal volumes (0.5 cm3) of them were added into equal volumes (5.0 cm3) of silver nanoparticle solutions. SPR absorbance measurements were performed using UV-Vis spectrophotometer, soon after the addition of metal ions to the solutions.

Results and discussion

Both structural characterisation and optical characterisation of capped silver nanoparticles were carried out using different analytical techniques such as UV-Visible absorption spectroscopy, Fluorescence spectroscopy, Fourier Transform Infrared (FT-IR) Spectroscopy, Particle size analysis and Scanning Electron Microscopy (SEM). FT-IR spectra of free sodium citrate and functionalised citrate capped silver nanoparticles are shown in Fig.1.The presence of bands at 1582 cm-1 and 1405 cm-1 corresponding to the asymmetric and symmetric C=O stretch of carboxylate ion, respectively and a wide band at 3393 cm-1 corresponding to the O-H stretch prove the surface capping of tri sodium citrate on silver nanoparticles. The band at 1281 cm-1 corresponds to the C-O stretch of pure tri sodium citrate showed a shift towards shorter wave number with decreasing intensity in the FT-IR spectrum of capped silver nanoparticles. This is due to the capping of tri sodium citrate via –CO functionality ^{[38, 39]}. The pattern of finger print region of the capped silver nanoparticle is entirely different from the pure tri sodium citrate. It proves the complete surface capping of tri sodium citrate and the absence of free tri sodium citrate in the system.

Functionalised citrate capped silver nanoparticles were optically characterised by UV-Vis absorption spectroscopy and fluorometry. SPR absorption band was observed at the wave length of 393 nm. The excitation and emission maxima of capped silver nanoparticles were observed at 463 nm and 509 nm respectively as shown in Fig.2. Raman peak of water molecules interrupted the emission spectra since silver nanoparticles were in aqueous medium. The Raman peak was easily identified by the shifting of the peak with the change of the excitation wave length.

Fig.3 (A) and (B) show the SEM images of capped silver nanoparticles which are spherical in structure. Average particle size was estimated as 14 nm by the particle size analyser. The size distribution curve by number is depicted in fig.4.

Fig.5 (B) shows the SPR absorption of citrate capped silver nanoparticles with different metal ions. Changes in absorbance values with metal ions were plotted as shown in Fig.5(C). Addition of Ferric ions(1 x 10-3) highly decreased the SPR absorption with broadening whereas other metal ions showed negligible decrease in SPR absorption. Therefore, this observation shows that this system is more sensitive to the ferric ions than the others. On the basis of this observation, we decided to establish a method to sense ferric ion. Ferric ion undergoes hydrolysis in aqueous medium under neutral condition and pH of the solution decreases with the formation of colloidal ferric hydroxide complexes ^[43]. Formation of colloidal hydroxide complexes reduces the concentration of freely available Fe3+. Therefore, the standard series of ferric ion solution were prepared under acidic conditions (pH ~1-1.5). Interfering ion studies were also carried out under same condition. The SPR absorbance values were measured after each addition of standard ferric ion series (0.5 cm3 in 5cm3 of AgNP solution). Reduction and broadening of the SPR band with the concentration of Fe3+ are clearly seen in Fig. 6(A). Change in SPR absorption (∆A) was plotted against the concentration of ferric ions. Linearity (R2 = 0.98) of the plot was observed in the concentration range of 4 x 10-4 mol dm-3 to 1 x 10-3 mol dm-3 as shown in fig.6 (B). The Limit of detection (LOD) of the capped AgNP sensor system was calculated using 3σ/S and was found to be 1.3 x 10-4 mol dm-3 where σ is the standard deviation of the blank signal, and S is the slope of the linear calibration plot. Concentrations of silver in AgNP solution were determined by AAS before and after the addition of Fe3+ ions. The concentration of silver was 0.58 ppm and remained unchanged throughout the process. It shows that there was no precipitation process took place after the addition of ferric ions. The colour change of AgNP solution after the addition of Fe3+ is shown in Fig. 5(A).

Mechanism for the analytical sensing of ferric ions silver is attributed to the electron transfer takes place on the surface of the nanoparticles. Fe3+ ions added to the system were completely converted to Fe2+ under the condition employed for this system. A qualitative study for the detection of ferrous ions was used to check the proposed mechanism. 1, 10-phenanthroline was chosen to confirm the mechanism and 1, 10-phenanthroline is a bidentate ligand which forms red colour complex ([Fe II (phen)3]2+) with ferrous ions and a blue colour complex ([Fe III (phen)3]3+) with ferric ions ^[43]. Silver ions do not produce any coloured complexes with 1, 10-phenanthroline. Therefore, the qualitative study with 1,10-phenanthroline was carried out for the confirmation of ferrous ions in AgNP after treating it with ferric ions. AgNP solutions to which ferric ion solutions were added, were treated with 1, 10-phenanthroline/ sodium citrate-citric acid buffer to check the presence of ferrous ions. As shown in Fig. 7, the formation of red colour ensures the presence of ferrous ions which were not initially present in the system. This also evidences the disappearance of ferric ions after the treatment with functionalised citrate capped silver nanoparticles. Therefore, it can be concluded that the reduction and of the SPR absorption is due to the electron transfer on the surface of the silver nanoparticles under the condition applied to the so prepared sensor system.

Table 1 :Effect of interfering metal ion on the determination of ferric ion

Interfering ion	Concentration ( mol dm-3)	Error (%)
Co2+	1×10-3	8.0
Fe2+	1 x10-3	2.5
Mg2+	1 x10-3	2.2
Mn2+	1 x10-3	3.6
Cu2+	1 x10-3	2.7
Ni2+	1 x10-3	1.8
Zn2+	1 x10-3	1.2
K2+	1 x10-3	1.3
Na2+	1 x10-3	1.0
Al2+	1 x10-3	1.42

Conclusion

This work suggests a simple, single step synthesis of citrate capped silver nanoparticles to use as optical sensor for ferric ions. Sensing performance of this sensor system was evaluated by the phenomena called Surface Plasmon Resonance (SPR) absorption. Under the optimum conditions, the calibration plot of change in SPR absorption versus concentration of ferric ion was linear within the concentration range of 4 x 10-4 mol dm-3 to 1x 10-3 mol dm-3. The limit of detection was calculated as 1.3 x 10-4 mol dm-3. Interference caused by other metal ions is almost negligible in this system. Highest interference among the metal ions tested is caused by Co2+ ion which rarely coexists with ferric ion in biological system. Therefore, the method described in this paper can be employed to selectively sense ferric ion in the presence of other metal ions in aqueous medium. The advantages of this sensor system compare to other existing sensing methods are simplicity, quick responsiveness, capable of being visually detected, hydrophilicity and cost effectiveness.

Acknowledgements

We would like to acknowledge the Sri Lanka Institute of Nanotechnology (SLINTEC) and the National Science Foundation of Sri Lanka for the great support rendered to carry out this project.

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