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Sensitive and selective colorimetric detection of Cu2+ in aqueous medium via aggregation of thiomalic acid functionalized Ag nanoparticles Vairaperumal Tharmaraj and Jyisy Yang* A simple and effective colorimetric method for determination of Cu2+ in real samples was developed. In this method, thiomalic acid functionalized silver nanoparticles (TMA-AgNPs) were prepared and changes in solution color, induced by the aggregation of TMA-AgNPs in the presence of Cu2+, were employed for quantitative analysis. The surface plasmon resonance (SPR) band of our synthesized TMA-AgNPs was located at 392 nm and shifted to a longer wavelength after aggregation due to the interactions between carboxylate and Cu2+. A band intensity ratio of A455/(A392
A455) was constructed and used to correlate
with the concentration of Cu2+. A linear relationship was found with a linear response up to 50 nM of Cu2+. Due to the formation of a stable carboxylate Cu2+ complex, highly sensitive detection of Cu2+ was Received 7th August 2014 Accepted 27th September 2014
achieved with the estimated detection limit approaching 1 nM. Moreover, the formation of the stable
DOI: 10.1039/c4an01449a
metal ions. In the detection of Cu2+ in real samples, results indicated that our proposed method is
www.rsc.org/analyst
simple, sensitive and selective for application in such measurements.
complex leads to high selectivity in the detection of Cu2+, which was verified by examination of 12 other
Introduction Determination of Cu2+ level is an important task because of its environmental and biological importance. Any decient or excess uptake of Cu2+ can cause various degrees of illness.1,2 To quantitatively monitor Cu2+ levels, conventional methods such as atomic spectroscopy and inductively coupled plasma (ICP) spectroscopy,3–6 infrared spectroscopy,7 and uorescence spectroscopy8,9 have been used. However, the lack of simplicity and need for skilful operation limit the spread of these techniques for practical uses. On the other hand, sensors for specic detection of Cu2+ have been constructed, which offer portability and simplicity in detection. For instance, quartz crystal microbalances,10 anodic stripping voltammetric sensors,11,12 uorescence sensors,13,14 dynamic light scattering15 and colorimetric methods16–21 have been developed by integrating the unique properties of nanostructures into sensing devices. Among them, metal nanostructure-based colorimetric sensors can not only simplify the detection process but also enable visual identication of Cu2+. To facilitate a change of color response to the concentration of analyte, the surface plasmon resonance (SPR) effect from metallic nanostructures is used and several types of nanostructure-based colorimetric methods can be categorized as anti-aggregation,16,17 aggregation,18–21 and non-aggregation methods.22,23 Changes in the physical states of the Department of Chemistry, National Chung-Hsing University, 250 Kuo-Kuang Rd, Taichung 402, Taiwan. E-mail: [email protected]; Tel: +886-422840411 ext. 514
6304 | Analyst, 2014, 139, 6304–6309
nanostructures cause a shi in SPR band position and hence, a change in solution color. For instance, a red-shi in the extinction spectrum of nanostructures aer binding with analytes has been observed and used for quantication.24–28 As reported in a previous study,29 the extinction coefficients of nanostructures can be three orders higher than those of uorescent dye molecules. This reveals that colorimetric methods based on nanostructures have the potential to be extremely sensitive means of detection. Besides sensitivity in detection, a selective mechanism is generally employed by functionalizing the surface of nanostructures to selectively interact with target analytes. For instance, metal nanoparticles have been surface functionalized with dopamine dithiocarbamate,30 bovine serum albumin31 and papain32 proteins, catechol,33 peptides34 and ssDNA35 to achieve selective recognition and signal output in the detection of Cu2+. The observed detection limits vary largely and fall in the range from 10 nM to 1000 nM. However, most of the sensing phases suffer different degrees of interference from other metal ions. To improve the selectivity and also to simplify the procedure for the preparation of surface functionalized metal nanostructures for the detection of Cu2+ by a colorimetric method, thiomalic acid (TMA) was selected to modify the surface of silver nanoparticles (AgNPs) and the detection scheme is shown in Fig. 1. As can be seen in this gure, TMA contains a thio group and two carboxylic acid groups. The thio group can interact with AgNPs for effective immobilization. The carboxylic acid group in TMA acts as a hard binding site and preferentially interacts with Cu2+ through the formation of stable complexes.36–38
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light scattering particle size analyzer (DLS, Zetasizer Nano ZS, Malvern Instruments Ltd, Worcestershire, UK) was used for determining the particle size of the TMA-AgNPs. An inductively coupled plasma mass spectrometer (ICP-MS, PE-SCIEX ELAN 6100 DRC, PerkinElmer Inc.) was used to acquire ICP-MS spectra, and an orbital shaker (TS-500, Yihder Co., Xinbei City, Taiwan) was used for shaking the samples.
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Results and discussion Basic properties of the prepared TMA-AgNPs in detection Fig. 1
Schematic diagram for the interaction of TMA-AgNPs with Cu2+.
Therefore, selectivity in the detection of Cu2+ can be greatly improved. Because the aggregation of TMA-AgNPs can be induced by Cu2+, a red shi of the SPR band is expected and an identiable color change can be obtained.
Experimental section Chemicals Thiomalic acid was purchased from Fluka. Trisodium citrate was purchased from Janssen Chimica. NaBH4 (Lancaster) and BaCl2, CaCl2, CdCl2, CoCl2, CuCl2, FeCl2, HgCl2, MgCl2, MnCl2, NiCl2, PbCl2, SnCl2, ZnCl2, and AgNO3 metal salts were purchased from various chemical companies (Aldrich, Merck, Acros and Alfa Aesar). All other chemicals were used as received without further purication. Deionized water was used throughout the experiments. Real samples of tap, lake and river water were collected using polypropylene bottles and the locations for sampling were either in or near the campus of National Chung-Hsing University (NCHU). Synthesis of thiomalic acid functionalized silver nanoparticles
AgNPs exhibit a unique optical property known as surface plasmonic effect, which usually gives an absorption band located in the visible region. By surface modication of TMA, the TMA-AgNPs can aggregate in the presence of Cu2+ to cause a red-shi of the SPR band and consequently, a change of solution color. To examine the optical properties of the prepared TMA-AgNPs, the UV-Visible spectra of AgNPs before and aer TMA modication, and the TMA-AgNPs in the presence of 100 nM Cu2+ were acquired and the obtained spectra are plotted in Fig. 2. As shown in Fig. 2, the AgNPs before surface modication with TMA exhibit a SPR band located at 392 nm. Aer modication with TMA, the SPR band did not shi but the width of the SPR band was slightly reduced. This reveals that AgNPs aer modication with TMA exhibit high surface charges that enable them to disperse uniformly in aqueous medium. Once the TMA-AgNPs interacted with Cu2+, a red-shi was observed as the maximal band moved to 423 nm with a reduction of the band intensity. This proves that Cu2+ can effectively induce the aggregation of TMA-AgNPs. Quantitative aspects of TMA-AgNPs in Cu2+ determination TMA-AgNPs interact with Cu2+ to cause an aggregation of nanoparticles and hence, a change of solution color. The required aggregation time was rst determined by mixing TMA-
Thiomalic acid functionalized AgNPs (TMA-AgNPs) were synthesized by mixing freshly prepared 10 mM AgNO3 (5 mL) and 10 mM trisodium citrate (5 mL) solutions in a reaction bottle at room temperature with vigorous stirring for approximately 15 min. Aer that, freshly prepared 0.1 mM NaBH4 solution (1 mL) was added drop wise with constant stirring at room temperature. The change in solution color from colorless to yellow indicated the formation of AgNPs. This solution was stirred for 1 h at room temperature and the resulting AgNPs were further functionalized with TMA by the addition of 10 mM TMA (5 mL) with 2 h of stirring at room temperature. Aer TMA was chemisorbed on the AgNPs, a yellow solution was acquired. Instrumentation A Genesys 10 UV-Visible spectrophotometer (Thermo Electron) was used for spectra collection. The quartz cell used had a total volume of 2 mL and a path length of 1 cm. Spectra were recorded in the absorbance mode in the range 300–600 nm. A eld emission scanning electron microscope (FE-SEM, JOEL, JSM-6700F, Tokyo, Japan) was employed to examine the morphologies of the formed TMA-AgNPs. A dynamic
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UV-Visible spectra of (a) citrate–AgNPs, (b) TMA-AgNPs (0.25 mM), (c) TMA-AgNPs after three days and (d) TMA-AgNPs in the presence of 100 nM Cu2+ in aqueous medium.
Fig. 2
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AgNPs solution with 100 nM Cu2+. By measuring the UV-Visible spectra for different shaking times, the intensity of the SPR band at 423 nm was used to estimate the minimal interaction time. The aggregation of TMA-AgNPs was fast and a shaking time of 20 min is sufficient to reach a stable SPR band intensity as can be seen in Fig. 3. To examine the response of TMA-AgNPs to Cu2+ concentration, Cu2+ solutions with concentrations ranging from 0.25 nM to 1000 nM were individually added to TMA-AgNPs solution and the corresponding UV-Visible spectra were acquired aer 20 min of shaking time. Some examples of the obtained UV-Visible spectra are plotted in Fig. 4. A photograph of solutions of TMAAgNPs with the addition of different concentrations of Cu2+ are also shown in the inset in Fig. 4. As can be seen in the UV-Visible spectra presented in Fig. 4, the SPR band of the TMA-AgNPs was shied by different degrees, which was related to the concentration of the added Cu2+. The band intensity decreased gradually with a red shi, which indicates the strong aggregation of TMA-AgNPs. Referring to the inset image in Fig. 4, the changes in solution color can also be clearly seen when the Cu2+ concentration is close to 10 nM. This reveals that the visual determination of Cu2+ is also feasible with our prepared TMAAgNPs. For quantitative analysis of Cu2+, a quantity was dened as A455/(A392 A455), where A455 and A392 are the absorption intensities at 455 nm and 392 nm, respectively. Although the absorption intensity at 423 nm is a good indication of the amount of aggregated TMA-AgNPs, its absorption intensity was partially interfered by the SPR band of non-aggregated TMAAgNPs. Therefore, the absorption intensity at 455 nm was used instead of that at 423 nm to avoid such interference. This quantity was used to correlate with the concentration of Cu2+ as plotted in Fig. 5. A linear range was observed for Cu2+ concentrations lower than 50 nM with a linear regression coefficient (R2) of 0.9977.
Variation of the UV-Visible band intensity of TMA-AgNPs in the presence of 100 nM Cu2+ with different shaking time. Fig. 3
6306 | Analyst, 2014, 139, 6304–6309
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Fig. 4 UV-Visible spectra of TMA-AgNPs (0.25 mM) upon addition of Cu2+ from 0.25 to 1000 nM in aqueous medium. The inset image is of the TMA-AgNPs (0.25 mM) in the presence of different concentrations of Cu2+.
To conrm the aggregation behavior, the as-prepared TMAAgNPs and the TMA-AgNPs in the presence of 100 nM, 500 nM and 1000 nM Cu2+ were scanned by SEM. The observed images are shown in Fig. 6. As can be seen in Fig. 6a, the as-prepared TMA-AgNPs have similar sizes and the particle size falls in the range of 10 to 30 nm. To obtain a better estimation of the particle size in solution, as-prepared TMA-AgNPs were also examined by dynamic light scattering (DLS) and the obtained average diameter of the particles was 12.0 (4.2) nm. Aer addition of 100 nM Cu2+, the TMA-AgNPs were aggregated, shown in Fig. 6b. The diameter of the aggregated particles in solution was also estimated by DLS measurement and the obtained average value was 191.8 (53.7) nm. When the concentration of Cu2+ reached 500 nM, signicantly aggregated
Fig. 5 Plot of A455/(A392 A455) vs. the concentration of Cu2+ in aqueous medium. The concentration of TMA-AgNPs was maintained at 0.25 mM with a shaking time of 20 min.
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SEM images of as-prepared TMA-AgNPs (a), TMA-AgNPs in the presence of 100 nM Cu2+ (b), TMA-AgNPs in the presence of 500 nM Cu2+ (c) and TMA-AgNPs in the presence of 1000 nM Cu2+ (d). Fig. 6
TMA-AgNPs were observed as shown in the SEM images (Fig. 6c and d). Therefore, the aggregation of TMA-AgNPs upon addition of Cu2+ causes a colorimetric response that can be proved by the SEM images and is also consistent with the phenomenon reported in the literature.39–42 Based on the SEM images, TMAAgNPs are sensitive to the presence of Cu2+ and can be used for quantitative purposes.
Selectivity of TMA-AgNPs for Cu2+ over other competitive metal ions To examine selectivity in the detection of Cu2+, 12 other metal ions with a concentration of 100 nM were individually added to 0.25 mM TMA-AgNPs colloidal solutions. These metal ions were Ba2+, Ca2+, Cd2+, Co2+, Fe2+, Hg2+, Mg2+, Mn2+, Ni2+, Pb2+, Sn2+ and Zn2+. Absence of signicant change in the UV-Visible spectrum could be observed with the exception of Hg2+ as shown in Fig. 7. The presence of Hg2+ caused a slight blue shi in the SPR band. This phenomenon is reasonable as Hg2+ tends to interact with silver to form amalgams, which causes an alteration in the morphology of the TMA-AgNPs and results in a slight blue shi. The quantity of A455/(A392 A455) was also used to calculate the responses of the examined metal ions and the results are plotted along with values obtained in the presence of Cu2+ as shown in Fig. 8. As can be observed in this gure, the aggregation of TMA-AgNPs occurs only with Cu2+ over other metal ions. The effects of coexisting cation Mg2+ on the detection of Cu2+ were also determined to verify that the TMA-AgNPs are free of metal ion interferences. With and without addition of 100 nM Mg2+ to the 100 nM Cu2+ solution, the obtained A455/(A392 A455) values were 22.6 (0.5) and 22.3 (0.5), respectively. The obtained values show no signicant difference, which indicates that commonly coexisting metal ions in water samples have a negligible effect on Cu2+ detection.
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Fig. 7 UV-Visible spectra of TMA-AgNPs (0.25 mM) in the presence of Cu2+ and various metal ions (100 nM) (Ba2+, Ca2+, Cd2+, Co2+, Fe2+, Hg2+, Mg2+, Mn2+, Ni2+, Pb2+, Sn2+ and Zn2+) in aqueous medium.
A colorimetric sensor is especially promising because the color change can be easily observed by the naked eye, thus less work and no costly equipment are required. It is also noteworthy that TMA-AgNPs can act as a visual colorimetric sensor. As observed by the image shown in the inset of Fig. 8, only Cu2+ can signicantly induce an aggregation to cause changes in solution color, whereas all other metal ion solutions remained yellow without any perceptible change even with high metal ion concentrations, i.e., 500 nM, as shown in the inset of Fig. 8.
Inuence of pH and ionic strength on the TMA-AgNPs with Cu2+ To study the inuence of solution pH in detection, 100 nM Cu2+ solution was used and its solution pH was adjusted by an aqueous solution of NaOH or HNO3. The variation of band intensity caused by solution pH was determined by ratioing the
Fig. 8 UV-Visible intensity ratio A455/(A392 A455) of TMA-AgNPs (0.25 mM) upon addition of different metal ions (Ba2+, Ca2+, Cd2+, Co2+, Fe2+, Hg2+, Mg2+, Mn2+, Ni2+, Pb2+, Sn2+ and Zn2+) (100 nM) in aqueous medium. The inset image is of the TMA-AgNPs (0.25 mM) in the presence of Cu2+ and various metal ions (500 nM).
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these samples were 27.1, 34.3 and 50.3 nM, which agreed with the measurements by our proposed method.
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Conclusions
Fig. 9 (a) UV-Visible intensity (423 nm) of TMA-AgNPs in the presence of 100 nM Cu2+, at different pH levels. (b) UV-Visible intensity (423 nm) of TMA-AgNPs in the presence of 100 nM Cu2+, with different concentrations of sodium nitrate (NaNO3).
band intensity at 423 nm to that of none pH adjusted solution. The results are plotted in Fig. 9a. As can be seen in this gure, the variations of the band intensities in the examined pH region were all less than 10%. Moreover, the variation rate was very steady in the pH range from 3 to 7. Because ionic strength in the sample solution may affect the signal in Cu2+ detection, NaNO3 was used as an additive to alter the ionic strength of the 100 nM Cu2+ solution. The variations caused by electrolytes are plotted in Fig. 9b, which are expressed as the ratio values of the band intensities to that of Cu2+ solution without added NaNO3. Based on Fig. 9b, the variation with addition of less than 300 nM NaNO3 was less than 5%. Even with a high concentration of electrolyte, the variation of the band intensity was still less than 10%. This reveals that the TMA-AgNPs prepared in this work can tolerate changes in electrolyte concentration.
Real sample analysis To demonstrate that TMA-AgNPs can be used for real sample detection, a solution of 0.25 mM TMA-AgNPs was used to detect Cu2+ in real samples. These real samples include tap, lake and river water samples. Samples aer collection were settled for 24 h to allow precipitation of possible solid materials. The upper part of the solution was taken, and these samples were used for detection without further treatment. Aer UV-Visible spectra were acquired, the values of A455/(A392 A455) were calculated and used to estimate the concentration of Cu2+ in the real sample using the standard curve constructed above. Results showed that Cu2+ concentrations of 27.6 (0.5) nM, 32.9 (0.5) nM, and 47.6 (1.1) nM were found in tap, lake and river water, respectively. The accuracy of TMA-AgNPs in determination was also checked by comparing the results obtained by ICP-MS. The ICP-MS analyses showed that the concentrations of Cu2+ in
6308 | Analyst, 2014, 139, 6304–6309
In this work, we have developed a simple colorimetric method based on thiomalic acid functionalized silver nanoparticles (TMA-AgNPs) for the selective and sensitive detection of Cu2+ in real samples. Based on UV-Visible spectra and SEM images of the TMA-AgNPs, Cu2+ can effectively and selectively induce aggregation of TMA-AgNPs and hence, a change of solution color from yellow to purple results from the signicant shi of the SPR band. Moreover, this sensing system exhibits a linear response to the concentration of Cu2+ at concentration levels lower than 50 nM in aqueous medium. Due to the obvious changes in solution color, Cu2+ could be detected by the naked eye with a visual detection limit of approximately 0.50 nM. Practical applications carried out using real water samples further indicate that this sensing system has great potential for facile real-time monitoring of Cu2+.
Acknowledgements Financial assistance from the Ministry of Science and Technology of Republic of China is gratefully acknowledged.
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Sensitive and selective colorimetric detection of Cu2+ in aqueous medium via aggregation of thiomalic acid functionalized Ag nanoparticles Vairaperumal Tharmaraj and Jyisy Yang* A simple and effective colorimetric method for determination of Cu2+ in real samples was developed. In this method, thiomalic acid functionalized silver nanoparticles (TMA-AgNPs) were prepared and changes in solution color, induced by the aggregation of TMA-AgNPs in the presence of Cu2+, were employed for quantitative analysis. The surface plasmon resonance (SPR) band of our synthesized TMA-AgNPs was located at 392 nm and shifted to a longer wavelength after aggregation due to the interactions between carboxylate and Cu2+. A band intensity ratio of A455/(A392
A455) was constructed and used to correlate
with the concentration of Cu2+. A linear relationship was found with a linear response up to 50 nM of Cu2+. Due to the formation of a stable carboxylate Cu2+ complex, highly sensitive detection of Cu2+ was Received 7th August 2014 Accepted 27th September 2014
achieved with the estimated detection limit approaching 1 nM. Moreover, the formation of the stable
DOI: 10.1039/c4an01449a
metal ions. In the detection of Cu2+ in real samples, results indicated that our proposed method is
www.rsc.org/analyst
simple, sensitive and selective for application in such measurements.
complex leads to high selectivity in the detection of Cu2+, which was verified by examination of 12 other
Introduction Determination of Cu2+ level is an important task because of its environmental and biological importance. Any decient or excess uptake of Cu2+ can cause various degrees of illness.1,2 To quantitatively monitor Cu2+ levels, conventional methods such as atomic spectroscopy and inductively coupled plasma (ICP) spectroscopy,3–6 infrared spectroscopy,7 and uorescence spectroscopy8,9 have been used. However, the lack of simplicity and need for skilful operation limit the spread of these techniques for practical uses. On the other hand, sensors for specic detection of Cu2+ have been constructed, which offer portability and simplicity in detection. For instance, quartz crystal microbalances,10 anodic stripping voltammetric sensors,11,12 uorescence sensors,13,14 dynamic light scattering15 and colorimetric methods16–21 have been developed by integrating the unique properties of nanostructures into sensing devices. Among them, metal nanostructure-based colorimetric sensors can not only simplify the detection process but also enable visual identication of Cu2+. To facilitate a change of color response to the concentration of analyte, the surface plasmon resonance (SPR) effect from metallic nanostructures is used and several types of nanostructure-based colorimetric methods can be categorized as anti-aggregation,16,17 aggregation,18–21 and non-aggregation methods.22,23 Changes in the physical states of the Department of Chemistry, National Chung-Hsing University, 250 Kuo-Kuang Rd, Taichung 402, Taiwan. E-mail: [email protected]; Tel: +886-422840411 ext. 514
6304 | Analyst, 2014, 139, 6304–6309
nanostructures cause a shi in SPR band position and hence, a change in solution color. For instance, a red-shi in the extinction spectrum of nanostructures aer binding with analytes has been observed and used for quantication.24–28 As reported in a previous study,29 the extinction coefficients of nanostructures can be three orders higher than those of uorescent dye molecules. This reveals that colorimetric methods based on nanostructures have the potential to be extremely sensitive means of detection. Besides sensitivity in detection, a selective mechanism is generally employed by functionalizing the surface of nanostructures to selectively interact with target analytes. For instance, metal nanoparticles have been surface functionalized with dopamine dithiocarbamate,30 bovine serum albumin31 and papain32 proteins, catechol,33 peptides34 and ssDNA35 to achieve selective recognition and signal output in the detection of Cu2+. The observed detection limits vary largely and fall in the range from 10 nM to 1000 nM. However, most of the sensing phases suffer different degrees of interference from other metal ions. To improve the selectivity and also to simplify the procedure for the preparation of surface functionalized metal nanostructures for the detection of Cu2+ by a colorimetric method, thiomalic acid (TMA) was selected to modify the surface of silver nanoparticles (AgNPs) and the detection scheme is shown in Fig. 1. As can be seen in this gure, TMA contains a thio group and two carboxylic acid groups. The thio group can interact with AgNPs for effective immobilization. The carboxylic acid group in TMA acts as a hard binding site and preferentially interacts with Cu2+ through the formation of stable complexes.36–38
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light scattering particle size analyzer (DLS, Zetasizer Nano ZS, Malvern Instruments Ltd, Worcestershire, UK) was used for determining the particle size of the TMA-AgNPs. An inductively coupled plasma mass spectrometer (ICP-MS, PE-SCIEX ELAN 6100 DRC, PerkinElmer Inc.) was used to acquire ICP-MS spectra, and an orbital shaker (TS-500, Yihder Co., Xinbei City, Taiwan) was used for shaking the samples.
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Results and discussion Basic properties of the prepared TMA-AgNPs in detection Fig. 1
Schematic diagram for the interaction of TMA-AgNPs with Cu2+.
Therefore, selectivity in the detection of Cu2+ can be greatly improved. Because the aggregation of TMA-AgNPs can be induced by Cu2+, a red shi of the SPR band is expected and an identiable color change can be obtained.
Experimental section Chemicals Thiomalic acid was purchased from Fluka. Trisodium citrate was purchased from Janssen Chimica. NaBH4 (Lancaster) and BaCl2, CaCl2, CdCl2, CoCl2, CuCl2, FeCl2, HgCl2, MgCl2, MnCl2, NiCl2, PbCl2, SnCl2, ZnCl2, and AgNO3 metal salts were purchased from various chemical companies (Aldrich, Merck, Acros and Alfa Aesar). All other chemicals were used as received without further purication. Deionized water was used throughout the experiments. Real samples of tap, lake and river water were collected using polypropylene bottles and the locations for sampling were either in or near the campus of National Chung-Hsing University (NCHU). Synthesis of thiomalic acid functionalized silver nanoparticles
AgNPs exhibit a unique optical property known as surface plasmonic effect, which usually gives an absorption band located in the visible region. By surface modication of TMA, the TMA-AgNPs can aggregate in the presence of Cu2+ to cause a red-shi of the SPR band and consequently, a change of solution color. To examine the optical properties of the prepared TMA-AgNPs, the UV-Visible spectra of AgNPs before and aer TMA modication, and the TMA-AgNPs in the presence of 100 nM Cu2+ were acquired and the obtained spectra are plotted in Fig. 2. As shown in Fig. 2, the AgNPs before surface modication with TMA exhibit a SPR band located at 392 nm. Aer modication with TMA, the SPR band did not shi but the width of the SPR band was slightly reduced. This reveals that AgNPs aer modication with TMA exhibit high surface charges that enable them to disperse uniformly in aqueous medium. Once the TMA-AgNPs interacted with Cu2+, a red-shi was observed as the maximal band moved to 423 nm with a reduction of the band intensity. This proves that Cu2+ can effectively induce the aggregation of TMA-AgNPs. Quantitative aspects of TMA-AgNPs in Cu2+ determination TMA-AgNPs interact with Cu2+ to cause an aggregation of nanoparticles and hence, a change of solution color. The required aggregation time was rst determined by mixing TMA-
Thiomalic acid functionalized AgNPs (TMA-AgNPs) were synthesized by mixing freshly prepared 10 mM AgNO3 (5 mL) and 10 mM trisodium citrate (5 mL) solutions in a reaction bottle at room temperature with vigorous stirring for approximately 15 min. Aer that, freshly prepared 0.1 mM NaBH4 solution (1 mL) was added drop wise with constant stirring at room temperature. The change in solution color from colorless to yellow indicated the formation of AgNPs. This solution was stirred for 1 h at room temperature and the resulting AgNPs were further functionalized with TMA by the addition of 10 mM TMA (5 mL) with 2 h of stirring at room temperature. Aer TMA was chemisorbed on the AgNPs, a yellow solution was acquired. Instrumentation A Genesys 10 UV-Visible spectrophotometer (Thermo Electron) was used for spectra collection. The quartz cell used had a total volume of 2 mL and a path length of 1 cm. Spectra were recorded in the absorbance mode in the range 300–600 nm. A eld emission scanning electron microscope (FE-SEM, JOEL, JSM-6700F, Tokyo, Japan) was employed to examine the morphologies of the formed TMA-AgNPs. A dynamic
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UV-Visible spectra of (a) citrate–AgNPs, (b) TMA-AgNPs (0.25 mM), (c) TMA-AgNPs after three days and (d) TMA-AgNPs in the presence of 100 nM Cu2+ in aqueous medium.
Fig. 2
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AgNPs solution with 100 nM Cu2+. By measuring the UV-Visible spectra for different shaking times, the intensity of the SPR band at 423 nm was used to estimate the minimal interaction time. The aggregation of TMA-AgNPs was fast and a shaking time of 20 min is sufficient to reach a stable SPR band intensity as can be seen in Fig. 3. To examine the response of TMA-AgNPs to Cu2+ concentration, Cu2+ solutions with concentrations ranging from 0.25 nM to 1000 nM were individually added to TMA-AgNPs solution and the corresponding UV-Visible spectra were acquired aer 20 min of shaking time. Some examples of the obtained UV-Visible spectra are plotted in Fig. 4. A photograph of solutions of TMAAgNPs with the addition of different concentrations of Cu2+ are also shown in the inset in Fig. 4. As can be seen in the UV-Visible spectra presented in Fig. 4, the SPR band of the TMA-AgNPs was shied by different degrees, which was related to the concentration of the added Cu2+. The band intensity decreased gradually with a red shi, which indicates the strong aggregation of TMA-AgNPs. Referring to the inset image in Fig. 4, the changes in solution color can also be clearly seen when the Cu2+ concentration is close to 10 nM. This reveals that the visual determination of Cu2+ is also feasible with our prepared TMAAgNPs. For quantitative analysis of Cu2+, a quantity was dened as A455/(A392 A455), where A455 and A392 are the absorption intensities at 455 nm and 392 nm, respectively. Although the absorption intensity at 423 nm is a good indication of the amount of aggregated TMA-AgNPs, its absorption intensity was partially interfered by the SPR band of non-aggregated TMAAgNPs. Therefore, the absorption intensity at 455 nm was used instead of that at 423 nm to avoid such interference. This quantity was used to correlate with the concentration of Cu2+ as plotted in Fig. 5. A linear range was observed for Cu2+ concentrations lower than 50 nM with a linear regression coefficient (R2) of 0.9977.
Variation of the UV-Visible band intensity of TMA-AgNPs in the presence of 100 nM Cu2+ with different shaking time. Fig. 3
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Fig. 4 UV-Visible spectra of TMA-AgNPs (0.25 mM) upon addition of Cu2+ from 0.25 to 1000 nM in aqueous medium. The inset image is of the TMA-AgNPs (0.25 mM) in the presence of different concentrations of Cu2+.
To conrm the aggregation behavior, the as-prepared TMAAgNPs and the TMA-AgNPs in the presence of 100 nM, 500 nM and 1000 nM Cu2+ were scanned by SEM. The observed images are shown in Fig. 6. As can be seen in Fig. 6a, the as-prepared TMA-AgNPs have similar sizes and the particle size falls in the range of 10 to 30 nm. To obtain a better estimation of the particle size in solution, as-prepared TMA-AgNPs were also examined by dynamic light scattering (DLS) and the obtained average diameter of the particles was 12.0 (4.2) nm. Aer addition of 100 nM Cu2+, the TMA-AgNPs were aggregated, shown in Fig. 6b. The diameter of the aggregated particles in solution was also estimated by DLS measurement and the obtained average value was 191.8 (53.7) nm. When the concentration of Cu2+ reached 500 nM, signicantly aggregated
Fig. 5 Plot of A455/(A392 A455) vs. the concentration of Cu2+ in aqueous medium. The concentration of TMA-AgNPs was maintained at 0.25 mM with a shaking time of 20 min.
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SEM images of as-prepared TMA-AgNPs (a), TMA-AgNPs in the presence of 100 nM Cu2+ (b), TMA-AgNPs in the presence of 500 nM Cu2+ (c) and TMA-AgNPs in the presence of 1000 nM Cu2+ (d). Fig. 6
TMA-AgNPs were observed as shown in the SEM images (Fig. 6c and d). Therefore, the aggregation of TMA-AgNPs upon addition of Cu2+ causes a colorimetric response that can be proved by the SEM images and is also consistent with the phenomenon reported in the literature.39–42 Based on the SEM images, TMAAgNPs are sensitive to the presence of Cu2+ and can be used for quantitative purposes.
Selectivity of TMA-AgNPs for Cu2+ over other competitive metal ions To examine selectivity in the detection of Cu2+, 12 other metal ions with a concentration of 100 nM were individually added to 0.25 mM TMA-AgNPs colloidal solutions. These metal ions were Ba2+, Ca2+, Cd2+, Co2+, Fe2+, Hg2+, Mg2+, Mn2+, Ni2+, Pb2+, Sn2+ and Zn2+. Absence of signicant change in the UV-Visible spectrum could be observed with the exception of Hg2+ as shown in Fig. 7. The presence of Hg2+ caused a slight blue shi in the SPR band. This phenomenon is reasonable as Hg2+ tends to interact with silver to form amalgams, which causes an alteration in the morphology of the TMA-AgNPs and results in a slight blue shi. The quantity of A455/(A392 A455) was also used to calculate the responses of the examined metal ions and the results are plotted along with values obtained in the presence of Cu2+ as shown in Fig. 8. As can be observed in this gure, the aggregation of TMA-AgNPs occurs only with Cu2+ over other metal ions. The effects of coexisting cation Mg2+ on the detection of Cu2+ were also determined to verify that the TMA-AgNPs are free of metal ion interferences. With and without addition of 100 nM Mg2+ to the 100 nM Cu2+ solution, the obtained A455/(A392 A455) values were 22.6 (0.5) and 22.3 (0.5), respectively. The obtained values show no signicant difference, which indicates that commonly coexisting metal ions in water samples have a negligible effect on Cu2+ detection.
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Fig. 7 UV-Visible spectra of TMA-AgNPs (0.25 mM) in the presence of Cu2+ and various metal ions (100 nM) (Ba2+, Ca2+, Cd2+, Co2+, Fe2+, Hg2+, Mg2+, Mn2+, Ni2+, Pb2+, Sn2+ and Zn2+) in aqueous medium.
A colorimetric sensor is especially promising because the color change can be easily observed by the naked eye, thus less work and no costly equipment are required. It is also noteworthy that TMA-AgNPs can act as a visual colorimetric sensor. As observed by the image shown in the inset of Fig. 8, only Cu2+ can signicantly induce an aggregation to cause changes in solution color, whereas all other metal ion solutions remained yellow without any perceptible change even with high metal ion concentrations, i.e., 500 nM, as shown in the inset of Fig. 8.
Inuence of pH and ionic strength on the TMA-AgNPs with Cu2+ To study the inuence of solution pH in detection, 100 nM Cu2+ solution was used and its solution pH was adjusted by an aqueous solution of NaOH or HNO3. The variation of band intensity caused by solution pH was determined by ratioing the
Fig. 8 UV-Visible intensity ratio A455/(A392 A455) of TMA-AgNPs (0.25 mM) upon addition of different metal ions (Ba2+, Ca2+, Cd2+, Co2+, Fe2+, Hg2+, Mg2+, Mn2+, Ni2+, Pb2+, Sn2+ and Zn2+) (100 nM) in aqueous medium. The inset image is of the TMA-AgNPs (0.25 mM) in the presence of Cu2+ and various metal ions (500 nM).
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these samples were 27.1, 34.3 and 50.3 nM, which agreed with the measurements by our proposed method.
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Fig. 9 (a) UV-Visible intensity (423 nm) of TMA-AgNPs in the presence of 100 nM Cu2+, at different pH levels. (b) UV-Visible intensity (423 nm) of TMA-AgNPs in the presence of 100 nM Cu2+, with different concentrations of sodium nitrate (NaNO3).
band intensity at 423 nm to that of none pH adjusted solution. The results are plotted in Fig. 9a. As can be seen in this gure, the variations of the band intensities in the examined pH region were all less than 10%. Moreover, the variation rate was very steady in the pH range from 3 to 7. Because ionic strength in the sample solution may affect the signal in Cu2+ detection, NaNO3 was used as an additive to alter the ionic strength of the 100 nM Cu2+ solution. The variations caused by electrolytes are plotted in Fig. 9b, which are expressed as the ratio values of the band intensities to that of Cu2+ solution without added NaNO3. Based on Fig. 9b, the variation with addition of less than 300 nM NaNO3 was less than 5%. Even with a high concentration of electrolyte, the variation of the band intensity was still less than 10%. This reveals that the TMA-AgNPs prepared in this work can tolerate changes in electrolyte concentration.
Real sample analysis To demonstrate that TMA-AgNPs can be used for real sample detection, a solution of 0.25 mM TMA-AgNPs was used to detect Cu2+ in real samples. These real samples include tap, lake and river water samples. Samples aer collection were settled for 24 h to allow precipitation of possible solid materials. The upper part of the solution was taken, and these samples were used for detection without further treatment. Aer UV-Visible spectra were acquired, the values of A455/(A392 A455) were calculated and used to estimate the concentration of Cu2+ in the real sample using the standard curve constructed above. Results showed that Cu2+ concentrations of 27.6 (0.5) nM, 32.9 (0.5) nM, and 47.6 (1.1) nM were found in tap, lake and river water, respectively. The accuracy of TMA-AgNPs in determination was also checked by comparing the results obtained by ICP-MS. The ICP-MS analyses showed that the concentrations of Cu2+ in
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In this work, we have developed a simple colorimetric method based on thiomalic acid functionalized silver nanoparticles (TMA-AgNPs) for the selective and sensitive detection of Cu2+ in real samples. Based on UV-Visible spectra and SEM images of the TMA-AgNPs, Cu2+ can effectively and selectively induce aggregation of TMA-AgNPs and hence, a change of solution color from yellow to purple results from the signicant shi of the SPR band. Moreover, this sensing system exhibits a linear response to the concentration of Cu2+ at concentration levels lower than 50 nM in aqueous medium. Due to the obvious changes in solution color, Cu2+ could be detected by the naked eye with a visual detection limit of approximately 0.50 nM. Practical applications carried out using real water samples further indicate that this sensing system has great potential for facile real-time monitoring of Cu2+.
Acknowledgements Financial assistance from the Ministry of Science and Technology of Republic of China is gratefully acknowledged.
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