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Plasmonics

, Volume 13, Issue 4, pp 1315–1323 | Cite as

Colorimetric Sensor of Cobalt Ions in Aqueous Solution Using Gold Nanoparticles Modified with Glycyrrhizic Acid

  • Changiz Karami
  • Mohammad Ali Taher
Article

Abstract

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.

Keywords

Gold nanoparticle Surface plasmon resonance Nano sensor Nanoparticles Colorimetric sensing Co2+ 

Introduction

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 calix[4]arene functionalized gold nanoparticles [23]. Even though these methods successfully detected cobalt ions with high selectivity and sensitivity, unfortunately, these methods involved of hazardous organic materials for synthesis [24], 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.

Experimental

Instrumental

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

Changiz.karami@ gmail.comThe synthesis of GA-AuNPs is depicted in Scheme 1. Briefly, 5 mL of freshly prepared (0.01 g/25 mL) of glycyrrhizic acid as a stabilizer and reducing agent was added to (5 mL, 10−3 M) HAuCl4 solution and 10 mM of NaOH solution was added at 25 °C. After 1.5 h, the light-yellow color of solution was changed to red color, which confirms that the reduction of Au3+ to AuNPs. The AuNPs dispersed solution was stored at 4 °C prior to our analysis. In order to control the size and surface plasmon resonance (SPR) band of AuNPs, we studied the various influencing parameters such as effect of glycyrrhizic acid concentration, temperature, and time for formation of AuNPs with SPR peak 525 nm and red color and these were measured by UV visible absorption spectroscopy and digital camera. The mixture obtained was purified by repeated centrifugation at 4000 rpm for 20 min with adding water/ethanol 15/5 to obtain the fresh GLY-AuNPs solution.
Scheme 1

Schematic illustration of 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

The effect of the concentration of glycyrrhizic acid on the synthesis and functionalization of GA-AuNPs was investigated and that the GA was used as agent for reduction of Au3+ and as a capping agent, and this characteristic of GA that it exhibits both reducing and stabilizing groups allows functionalizing GA-AuNPs in one-pot synthetic as shown in Scheme 1. The different concentrations of GA (0.5–2.5 mg mL−1) for preparation of GA-AuNPs were measured by using UV–visible spectrometry. As shown in Fig. 3, as you knew, optical absorption spectrum of gold nanoparticles is a good indicator of their size and shape [25]. UV–Vis absorption spectrum of the gold nanoparticles show a sharp peak centered at 525 nm, corresponding to particles smaller ranging in diameter [26], so with the SPR shifts from about 525 nm up to 550 nm, the particle sizes increase in diameter ranging. The absorbance and the position of the SPR band of GA-AuNPs are strongly dependent on the concentration of glycyrrhizic acid. Due to this fact, it can be noticed that the SPR band at 525 nm become more intense and sharp by using GA concentration at 1.00 mg mL−1, but the SPR band of other concentrations of GA (0.5, 1.5, 2, and 2.5 mg mL−1) became broad and color change from red to blue. As a result of broadening absorption band in the UV–visible spectra, the size of nanoparticle increases to larger diameter [27, 28]. Therefore, we selected 1.00 mg mL−1 of GA as the best concentration for formation of GA-AuNPs with good SPR peak intensity and red monodispersion (Fig. 1).
Fig. 1

UV–visible absorption spectra of GA-AuNPs by using Gg as reducing and as capping agent at different concentrations (0.5–2.5 mg/L)

Effect of Temperature

To investigate the synthesis of GA-AuNPs, different reaction temperatures from 25 to 100 °C was recorded with UV–visible spectra. As shown in Fig. 2, at room temperature, the SPR band became sharper at 525 nm. It was observed that the SPR peak was broad at reaction temperatures 50, 75, and 100 °C, respectively. We also observed that the color of GA-AuNPs solution was also changed from red to blue color at 50, 75, and 100 °C, which confirms that their size of nanoparticle increases to larger diameter. Based on the above results, we carried out GA-AuNP synthesis at room temperature [29].
Fig. 2

UV–visible absorption spectra of GA-AuNPs by using Gg (1.00 mg/L) as reducing and as capping agent at different reaction temperatures from 25 to100 °C

Characterization of GA-AuNPs

The gold nanoparticles synthesized were characterized by TEM, FT-IR, and UV–Vis spectroscopy. The TEM image in Fig. 3a shows a typical TEM image of GA-AuNPs. We can see that GA-AuNPs are dispersed in aqueous solution uniformly with an average size of 12 ± 3 nm. The FT-IR spectra of glycyrrhizic acid and GA-AuNPs are shown in Fig. 4. Comparing their spectra, the significant features that can be seen for the characteristic of these spectra absorption peaks at 3406, 2828, 1612, 1420, and 1028 cm−1 were found in glycyrrhizic acid (Fig. 4a), so this peak was added to the FT-IR spectra of GA-AuNPs (Fig. 4b), which suggest that the glycyrrhizic acid was capped on the surface of the gold nanoparticles.
Fig. 3

a TEM image of GA-AuNPs with an average size of 12 ± 3 nm. b The TEM image of GA-AuNPs in the presence of (900 nM) Co2+ ion with the average of diameters is 50 ± 5 nm. The dynamic light scattering of the nanoparticles c before adding of Co2+ ions and d after adding of Co2+ ions

Fig. 4

a The FT-IR spectra of glycyrrhiza acid. b The FT-IR spectra GA-AuNPs

To investigate the thermal stability of glycyrrhizic acid AuNPs, TGA was recorded from carefully weighed powders of pure glycyrrhizic acid and glycyrrhizic acid stabilized AuNPs (Fig. 5a, b) samples. The mass loss at the started around 120–320 °C which could be due to the decomposition of physically adsorbed water molecules and pyrolysis of other functional groups. However, another decomposition was observed around 310–450 °C that could be due to the decomposition of the remaining functional groups. Whereas in the case of GA-AuNPs, the initial weight loss observed at 120 °C attributed to the physically adsorbed water molecules in the GA-AuNPs and a small weight loss was observed from 300 to 400 °C, due to decomposition of capping materials on the surface of the AuNPs. The improved thermal stability of GA-AuNPs could be due to strong interaction of capping materials or glycyrrhizic acid on the surface of gold nanoparticles (Fig. 5b).
Fig. 5

a TGA curves of glycyrrhiza acid. b TGA curves of GA-AuNPs

Influence of pH on Co2+-Induced Aggregation of GA-AuNPs

To investigate the pH range in which GA-AuNPs can effectively detect Co2+, pH titration of GA-AuNPs was carried out. In (Fig. 6a), the absorbance ratio (A550/A525) of GA-AuNPs was low and constant in the pH range 5–9. This indicates that GA-AuNPs were stable in the pH range 5–9. To obtain a better performance, we chose the pH range 5–9.
Fig. 6

a Influence of pH on the UV–Vis spectra of GA-AuNPs. b UV–Vis absorption of GA-AuNPs in presence of different metal ions with same concentration (900 nM), pH 7.0. c Values of A550/A525 the GA-AuNPs in the presence of different metal ions with same concentration (900 nM), pH 7.0

Colorimetric Strategy to Detect Co2+ Ion

According to Mie theory, the aggregation of nanoparticles can be associated with the red shift of SPR band [29]. The theory states that when distance between the two nanoparticles is less than the sum of their radii, the SPR band displays red shift [30]. 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+

To evaluate the sensitivity of this method and investigate the minimum detectable concentration of Co2+ in aqueous solution by color change and monitoring the UV–Vis absorbance values, different concentrations of aqueous solution of cobalt ion were added to a solution of the GA-gold nanoparticles (GA-AuNPs) (1 mg) at room temperature in pH 7.0. For limit detection of Co2+, the characteristic SPR peak was determined by using UV–Vis spectra. Furthermore, in addition of more Co2+ ion to the GA-AuNP solution, the absorbance peak of GA-AuNPs increased and red shift from 525 to 550 nm by increasing the concentration of cobalt(II) ion and there is a linear relationship (y = 0.0001x + 0.8772, R 2 = 0.971) between the absorbance intensity changes and the concentration of Co2+ ion over the range from 50 to 1600 nM at A550/525 in (Fig. 7a). Thus, we suggest that GA-AgNPs can be used for the colorimetric detection of Co2+ with the limit of detection (LOD) 0.4 nM on the basis of signal-to noise ratio (S/N) of 3 (Fig. 7b).
Fig. 7

a Change in surface plasmon resonance absorption of GA-AuNPs in the presence of different concentrations from 50 to 1600 nM of Co2+ ion , pH 7.0, b Increase in the relative sensitivity of the detection system with respect to the concentration of the Co2+ ions

Interference Studies

In order to study the influence of other metal ions on Co2+ aggregated to GA-AgNPs, competitive experiments were carried out with Co2+ (900 nM) in the presence of other metal ions, such as K+, Mg2+, Ca2+, Sr2+, Ba2+, Ni2+, Mn2+, Cu2+, Zn2+, Hg2+, Fe3+, Fe2+, Ag+, Cd2+, Hg2+, and Pb2+ (Fig. 8). The plasmonic absorption shift caused by the mixture of Co2+ with another metal ion was similar to that caused solely by Co2+. This indicates that the other metal ions did not interfere in the binding of GA-AgNPs with Co2+. This finding is consistent in that the Co2+ is the only metal ion that can induce the aggregation of GA-AgNPs.
Fig. 8

Absorbance ratio (A550/A525) of GA-AuNPs the presence of metal ions. Blue bars represent the addition of single metal ion (900 nM); red bars represent the mixture of Co2+ (900 nM) with another metal ion (1500 nM)

In addition, to show the advantages of our proposed sensor, a comparison table of the performances with different Co2+ sensors is shown in Table 1. From Table 1, it can be seen that the detection limit of our proposed sensor 0.4 for Co2 is acceptable and the linear range of our work is relatively wider [23, 33, 34, 35].
Table 1

A comparison of various colorimetric method for detection of Co2+

Probe

Target

LOD

R 2

Time (min)

Ref.

Carboxyl functionalized CdS quantum dots

Co2+

0.23 μg mL−1

0.9996

5

[33]

Calix[4]arene functionalized gold nanoparticles

Co2+

10−9 M

0.9921

2

[23]

Green synthesis of gold nanoparticles

Co2+

10 nM

0.9806

5

[34]

Dopamine dithiocarbamate functionalized AgNPs

Co2+

14 μM

0.993

10

[35]

GA-AuNPs

Co2+

0.4 nM

0.971

10

This work

Cobalt Ion Detection in Real Water Samples

For investigation sensitivity and selectivity of GA-AgNPs, drinking water as a real sample is tested. According to previous methods [34], these samples were spiked with known concentration of cobalt ions to GA-AgNPs. The responses for drinking water were monitored by UV–visible spectroscopy. By measuring the SPR bound, it was determined that the ability of the nanosensor to detect the amounts Co2+ ions in tap water increased and red shift in the absorption intensity. As a result, annoying agents such as organic and inorganic materials were less effective on the determined of cobalt ions. The lowest detectable concentration of Co2+ was estimated to be 3.50 nM in drinking water sample (Fig. 9). The metal ion detection ability of the GA-AgNPs was significantly different when tested with real water samples in comparison to that of distilled water. As in the real water samples, more possibilities are there for dissolved organic and inorganic materials.
Fig. 9

Plot of absorbance intensity difference versus concentrations of Co2+ in drinking water samples

Conclusion

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.

Notes

Acknowledgments

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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Copyright information

© Springer Science+Business Media, LLC 2017

Authors and Affiliations

  1. 1.Department of Chemistry, Faculty of SciencesShahid Bahonar University of KermanKermanIran

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