Colorimetric Sensor of Cobalt Ions in Aqueous Solution Using Gold Nanoparticles Modified with Glycyrrhizic Acid
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The present work describes simple and green method for the preparation of gold nanoparticles (AuNPs) in aqueous medium under ambient condition and their use in colorimetric detection of cobalt ion. The AuNPs were prepared by an environmentally benign method using glycyrrhizic acid (GA) which is a reducing and stabilizing agent in aqueous medium. The prepared GA-AuNPs were thoroughly characterized by using UV–visible spectroscopy, TEM, TGA, DLS, and FT-IR techniques. The analytical response was linear over the range from 50 mM to 16 μM (R 2 = 0.971) with a detection limit of 0.4 nM. The proposed diethyldithiocarbamate-AgNPs-based colorimetric method is simple and highly sensitive for the detection of cobalt ions and allows for the monitoring of cobalt ions directly with naked eye in aqueous medium.
KeywordsGold nanoparticle Surface plasmon resonance Nano sensor Nanoparticles Colorimetric sensing Co2+
The detection of metal ions in water is an important process and is more important with the increasing of industrial activities in the new world. However, the important pollutants in environment due to their toxic effect on human health are heavy metals. The heavy metal contamination exists in the aqueous waste streams, especially metal plating facilities, mining operations, and nuclear power plant. Moreover, the heavy metals in the environment are not biodegradable and tend to accumulate in living organisms and causing various adverse effects, diseases, and disorders [1, 2]. the cobalt is one of the most important heavy metal that found in certain ores of the Earth’s crust, so cobalt is used in various arrays of products and processes such as electroplating, because of its hardness and resistance to oxidation, magnet and batty manufacturing, pigments, stainless steels alloys, mining, electric cable manufacturing, automotive industries, coloring, and catalysts [3, 4]. Therefore, this element is an important metal in industries. In the other world, cobalt is beneficial for humans because it is a part of vitamin B12, and trace amount of Co(II) is essential for life but higher levels of exposure may damage human health. Therefore, the detection of cobalt ions like Co2+ in the environment is significant for health. Although a variety of methods, such as atomic absorption spectroscopy [5, 6], chemiluminescence [7, 8], and ICP-mass spectrometry [9, 10, 11], are widely used to detect cobalt ions, they have the disadvantages because of high-cost technic. Thus, the demand for methods that are simple, sensitive, and selective to detect cobalt ions is continuously increasing. Among these detection techniques, colorimetric method for detection of metal ions without any sophisticated instrumentation is very signification. With recent development in nanotechnology, the nanoparticles in different forms are developing as important colorimetric receptor for metal ions [12, 13, 14, 15, 16, 17, 18, 19]. Also, the colorimetric sensor based on gold nanoparticles is an important method of detection which allows a direct analysis of the substrate simply by the naked eye [20, 21, 22]. Recently, Maity et al. have developed useful method for the colorimetric detection of Co2+ ions by calixarene functionalized gold nanoparticles . Even though these methods successfully detected cobalt ions with high selectivity and sensitivity, unfortunately, these methods involved of hazardous organic materials for synthesis , so describing simple and green method for the preparation of gold nanoparticles (AuNPs) as colorimetric detection of metal ions in aqueous medium under ambient condition by multi-role (reducing and capping agent) has been a challenge for investigators.
Therefore, here we demonstrate the use of a simple one-pot synthetic method for water-stabilized, monodisperse AuNPs that are coated with glycyrrhizic acid on their surfaces. The glycyrrhizic acid (GA) which was extracted on licorice root not only reduces the chloroaurate but also acts as a capping agent on the surface of AuNPs for preparation of GA-AuNPs. Furthermore, there has been no report on the utilization of extracted root of glycyrrhizic acid as a reducing and capping agent for preparation of AuNPs and their uses as sensor for analysis of metal ions in water solution. Therefore, we introduced GA as a novel and green reagent for one-pot synthesis of AuNPs, and the size of the particles was systematically controlled by varying the concentration of glycyrrhizic acid and the temperature of reaction and its application as a colorimetric probe for sensing of cobalt ions in water media, resulting in appreciable changes in color and optical properties.
UV–Vis absorption spectra were acquired on a Cary 100 UV–Vis spectrometer (Varian, USA) at room temperature (23–25 °C) and using a double beam. FT-IR spectra were measured on a WQF-510 spectrophotometer pressed into KBr pellets and is reported in wave numbers (cm−1). Transmission electron microscopy was carried out on a Zeiss-EM10C-80 KV, and ultrasonic Misonix-S3000. Dynamic light scattering (DLS) measurements were performed for colloidal solutions using Malvern ZEN3600 from England. Thermogravimetric analysis (TGA) were carried out on (Netzsch STA 409PC) TG-DTA instrument from 30 to 600 °C with a scanning rate of 10 °C min−1 in the presence of nitrogen flow.
Chemical and Materials
All glassware used in the following procedure was cleaned in a bath of freshly prepared 3:1 HNO3–HCl, rinsed thoroughly in water and dried fully prior to use. All chemicals were used of analytical grade or of the highest purity available. All solutions were prepared with double-distilled, deionized water. Glycyrrhizic acid and HAuCl4 was purchased from Aldrich. All different cations, in the form of nitrate or chloride salts including Zn(NO3)2·6H2O, Co(NO3)2·6H2O, Fe(NO3)2·6H2O, Fe(NO3)3·6H2O, Pb(NO3)2, Ni(NO3)3·6H2O, Mn(NO3)2·4H2O, Cd(NO3)2, KNO3, MgCl2·6H2O, Ca(NO3)2·4H2O, Sr(NO3)2, BaCl2·2H2O, AgNO3, Hg(NO3)2·H2O, and Cu(NO3)2·3H2O) were purchased from Merck.
Synthesis of GA-AuNPs
General Procedure for the Colorimetric Determination of Co2+
For colorimetric detection of cobalt(II) ion, the metal ion detection ability of GA-AuNPs, representative alkali K+, Mg2+, Ca 2+, Sr2+, Ba2+, Ni2+, Mn2+, Cu2+, Zn2+, Hg2+, Fe3+, Fe2+, Ag+, Cd2+, Hg2+, Pb2+, and Co2+ at the same conditions and of the same concentration (500 nM) were added into 1.00 mg GA-AuNPs fresh prepared in 3 mL double-distill, respectively. The assays and the changes in the UV–Vis (A550/525) spectra were performed and monitored at room temperature. The photographs were taken with a digital camera after 10 min of mixing.
Strategy for the Colorimetric Detection of Co(II) Ion
A solution with different concentrations of Co2+ (50 nM–1.6 μM) was added to 1 mg of GA-AuNPs in 3 mL double-distilled water, and the obtained mixture was stirred at room temperature. After stirring for 10 min, UV–Vis absorption of these solutions was measured and the response curve of the ratio of absorbance of GA-AuNPs at 550 to 525 nm (A550/525) versus the concentration of Co2+ ion was plotted.
The Influence of pH on Co2+-Induced Aggregation of GA-AuNPs
GA-AuNPs were added with Co2+ (1 mL) ion (2 × 10−5 M) in 2.0 mL water solution (10 mM buffer). The buffers were pH 2–12, Britton–Robinson buffer.
Results and Discussion
Effect of Glycyrrhizic Acid Concentration
Effect of Temperature
Characterization of GA-AuNPs
Influence of pH on Co2+-Induced Aggregation of GA-AuNPs
Colorimetric Strategy to Detect Co2+ Ion
According to Mie theory, the aggregation of nanoparticles can be associated with the red shift of SPR band . The theory states that when distance between the two nanoparticles is less than the sum of their radii, the SPR band displays red shift . When AuNPs were modified with glycyrrhizic acid, the GA-AuNPs is rich of functional groups such as carboxylic acid, oxygen, and ketone indicating that these functional groups were sensitive to metal ions with complex on the surface of the gold nanoparticle [31, 32].
We assumed these groups’ strong tendency to metal ions and that the formation of this tendency shows aggregation on surface of nanoparticle. To test the selectivity of our sensor, various environmentally relevant ions such as K+, Mg2+, Ca2+, Sr2+, Ba2+, Ni2+, Mn2+, Cu2+, Zn2+, Hg2+, Fe3+, Fe2+, Ag+, Cd2+, Hg2+, Pb2+, and Co2+ with same concentration (900 nM) were added to the GA-AuNP solution separately (Fig. 6b). Figure 6c is the plot of the values of A550/A525 of GA-AuNP solution in the presence of these metal ions. The values of A550/A525 of metal ions are nearly the same as blank solution and only Co2+ has remarkable change, so this red shift of surface plasmon resonance band is clearly indicating the aggregation of GA-AuNPs. To investigate this assumption and evaluate the data obtained from UV–visible studies, the morphology of the GA-AuNPs after addition of Co2+ ions was studied by transmission electron microscopy. The TEM image of GA-AuNPs in the presence of Co2+ (900 nM) is shown in Fig. 3b. Upon adding Co2+, the TEM image of GA-AuNPs is exhibited a significant aggregation driven by Co2+ ion, and the average of diameters is more than 50 nm.
As another improvement for aggregation, we used dynamic light scattering (DLS) of the nanoparticles before (Fig. 3c) and after adding Co2+ (Fig. 3d) to GA-AuNPs. Aggregation of nanoparticles was further confirmed by DLS study. The size distribution profiles obtained from DLS data, before and after addition of metal ions, showed that the hydrodynamic radii of GA-AuNPs before addition of Co2+ are smaller than after addition of the Co2+ ion which confirmed the aggregation of the nanoparticles upon addition of Co2+.
Sensitivity and UV–Vis Titration Studies of Co2+
A comparison of various colorimetric method for detection of Co2+
Carboxyl functionalized CdS quantum dots
0.23 μg mL−1
Calixarene functionalized gold nanoparticles
Green synthesis of gold nanoparticles
Dopamine dithiocarbamate functionalized AgNPs
Cobalt Ion Detection in Real Water Samples
We have demonstrated an efficient, cost-effective, and ecofriendly approach for the synthesis of AuNPs in aqueous medium under ambient condition. The size of the GA-AgNPs was tuned by varying the concentration of the glycyrrhizic acid (GA). The prepared GA-AgNPs were characterized by UV–visible, TEM, FT-IR, TGA, and DLS. The GA-AgNPs are highly stable in aqueous medium and do not show any signs of aggregation up to several months. The synthesized GA-AgNPs were used as colorimetric sensors for selective detection Co2+ ions in aqueous medium with detection limits of 0.4 nM concentration.
We are thankful to the Department of Chemistry, Faculty of Sciences, Shahid Bahonar University of Kerman, Kerman, Iran for the support in this work.
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