Spectroscopic sensing of eight metal ions in aqueous solutions using silver nanoparticles

Anthropogenic releases from different outlets of industry, municipal sewage and the road trac can give rise to higher concentrations of the heavy metals in the food commodities which imposes a threat to human health and environment. A simple silver nanoparticle (Ag NPs) used for the sensing of heavy metal ions, Cd 2+ , Cu 2+ , Fe 2+ , Hg 2+ , Mn 2+ , Ni 2+ , Pb 2+ and Zn 2+ in aqueous solution is described by qualitative and quantitatively using spectroscopic tool. FE-SEM and TEM images conrmed that the particles are spherical in shape with an average diameter of 23.4 nm. In presence of heavy metal ions with Ag NPs, a new peak at around 925, 898, 643, 665, 688, and 838 nm of Cd 2+ , Cu 2+ , Fe 2+ , Hg 2+ , Mn 2+ and Zn 2+ appeared in addition to the peak found at 410 nm of Ag NPs. Further, the addition of Ni 2+ and Pb 2+ metal ion solution with Ag NPs increased the SPR band from 410 nm to 436 and 462 nm respectively. Citrate functionalized Ag NPs are aggregated in solution in the presence of divalent metal ions by an ions-template chelating process and easily measurable change in the UV-vis absorption spectrum of the particles. Further, studies also conrmed the interaction of Ag NPs with metal ions using FT-IR spectroscopy. The proposed method was found to be useful for simple UV-vis spectroscopic sensing of metal ions in aqueous solutions and real contaminated samples.


Introduction
Heavy metals are commonly de ned as elements that have a density at least 5 times higher than of water. Their presence in the soil can be of natural and anthropogenic origin. Due to natural processes in the earth's crust, the soil usually contains low concentrations of heavy metals. However, different anthropogenic activities lead to an increase of heavy metals concentration above the natural level in aquatic ecosystems. As heavy metals are not biodegradable, they accumulate in the environment and enter the food chain as bioaccumulation. Further, excessive intake of heavy metals into living organisms causes many harmful consequences, including death (Zwolak et al. 2019). Various heavy metals such as Cd 2+ , Cu 2+ , Fe 2+ , Hg 2+ , Mn 2+ , Ni 2+ , Pb 2+  inductively coupled plasma mass spectrometry (ICP-MS) and electrochemical sensing platforms (Kumeria et al. 2013) offer excellent sensitivity, multi-element analysis but, they are high expensive, time consuming, skill dependent and use non-portable accessories.
In recent years, nanomaterials-based sensing/detection of metal ions due to their optical properties with high extinction co-e cient at the visible region for improving the performance of sensors in terms of sensitivity, limit of detection, selectivity and reproducibility (Zheng et Kim et al (2001) reported the sensing of spectroscopically silent heavy metal ions (Pb 2+ ) using 11-mercaptoundecanoic acid stabilized gold nanoparticles. Wang et al (2010) reported the detection of Hg 2+ ions using unmodi ed silver nanoparticles and mercury speci c oligonucleotides used as sensors (Zheng et al. 2004). Green synthesized silver nanoparticles using aqueous extract of Hedysarum alpinum plant used for calorimetrically detection of Hg 2+ . Zhou et al (2012) reported silver nanoparticles co-functionalized with 4-mercapto benzoic acid and melamine as a probe for colorimetric detection of Mn 2+ . Green synthesis of L-tyrosine-stabilized silver nanoparticles under ambient sunlight irradiation for colorimetric detection of heavy metal ions (Hg 2+ and Pb 2+ ) as reported. Recently, Wang et al (2020) reported that the carrageenan stabilized silver nanoparticles for effective detection of Cu 2+ and S 2− ions in aqueous solution. In the present investigation, sodium citrate stabilized silver nanoparticles for sensing/detection of various metal ions (Cd 2+ , Cu 2+ , Fe 2+ , Hg 2+ , Mn 2+ , Ni 2+ , Pb 2+ and Zn 2+ ) in aqueous solution using UV-vis spectroscopic technique. Further, metal ion interactions with silver nanoparticles were studied using FT-IR spectroscopy.

Synthesis of Ag NPs
Synthesis of silver nanoparticles using sodium citrate as reducing agents was done according to the literature procedure (Kamat et al. 1998) with slight modi cation. Brie y, 100 mL of AgNO 3 (10 mg) aqueous solution and heated until it begins to boil. 2 mL of sodium citrate (30 mg) solution was added, and heating continued till the color was yellowish brown color which indicates formation of Ag NPs nanoparticles.

Characterization of Ag NPs
The formation of Ag NPs was monitored using a UV-visible spectrophotometer (Shimadzu UV-1800) in the range of 200-1000 nm. Particle's size and shape of the citrate reduced Ag NPs were determined using FE-SEM (Supra 55-Carl Zeiss, Germany) and TEM (FEI Technai, instruments) operating at an accelerating voltage of 120kVA.

Sensing/ detection of Metal ions
The spectroscopic detection of aqueous heavy metal ions was studied using Ag NPs solution at room temperature. To demonstrate the effect of heavy metal ions on Ag NPs, 1 mL concentrations of heavy metal ions were added one at a time to 500 µL of Ag NPs and the resulting mixture was then allowed to stand for 10 min at room temperature, during which the colour change depending upon the metal ions. The intensity of this colour gradually increased with the increase of heavy metal ion concentration. UV-Vis absorption spectra from all samples were analyzed carefully to correlate the changes of SPR spectra with respect to concentrations of Cd 2+ , Cu 2+ , Fe 2+ , Hg 2+ , Mn 2+ , Ni 2+ , Pb 2+ and Zn 2+ using UV-visible Spectrophotometer (UV 1800) Shimadzu, Japan. Spectra of the adsorbents before and after Cd 2+ , Cu 2+ , Fe 2+ , Hg 2+ , Mn 2+ , Ni 2+ , Pb 2+ and Zn 2+ binding were recorded with a FT-IR analysis using a PerkinElmer 1600 infra-red spectrometer with a pellet of powered potassium bromide.

Results And Discussion
Addition of sodium citrate into the beakers containing aqueous solution of AgNO 3 led to the change in the colour of the solution from colorless to brownish yellow within reaction duration due to excitation of surface plasmon resonance (SPR) vibrations in Ag NPs. The colour of the solution is brownish yellow indicating formation of Ag NPs (Figure 1 (Figure 12). Based on the present study and previous literature report, the conceivable predicted mechanisms of Ag NPs interaction with metal ions which is shown in Figure 13.

Conclusions
Herein, we report the Ag NPs as a cost-effective sensor for the detection of toxic metal ions in water. The spectroscopic tool such as UV-vis spectroscopy was used for detection and of various heavy metal ions (Cd 2+ , Cu 2+ , Fe 2+ , Hg 2+ , Mn 2+ , Ni 2+ , Pb 2+ and Zn 2+ ) in aqueous medium with the detection limits of nM concentrations and easily visualized with the naked eye by the rapid color change from a brownish yellow to light blue color observed. From the noteworthy mention that spectral changes and fast colour changes of ions with the addition of nanoparticles, we conclude that ions can potentially become a selective detection for the qualitative detection of the used nanoparticles and could be used as a visual marker.   FT-IR spectra of (a) AgNPs; (b) Pb(NO) 3 metal salt and (c) AgNPs interaction with Pb 2+ metal ion Figure 12 FT-IR spectra of (a) AgNPs; (b) ZnSO 4 metal salt and (c) AgNPs interaction with Zn 2+ metal ion Figure 13 Schematic illustration of Ag nanoparticles interaction with metal ions