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Colorimetric detection of Co2+ ion using silver nanoparticles with spherical, plate, and rod shapes Hwa Kyung Sung, Seung Yeon Oh, Chul Hwan Park, and Younghun Kim Langmuir, Just Accepted Manuscript • DOI: 10.1021/la401408f • Publication Date (Web): 25 Jun 2013 Downloaded from http://pubs.acs.org on June 30, 2013

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Colorimetric detection of Co2+ ion using silver nanoparticles with spherical, plate, and rod shapes

Hwa Kyung Sung, Seung Yeon Oh, Chulhwan Park,* and Younghun Kim∗

Department of Chemical Engineering, Kwangwoon University, Wolgye-dong, Nowon-gu, Seoul 139-701, Republic of Korea

AUTHOR

EMAIL

ADDRESS:

H.

K.

Sung

([email protected]),

S.

W.

Oh

([email protected]), C. Park ([email protected]), Y. Kim ([email protected])

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

TITLE RUNNING HEAD: Colorimetric detection of Co2+ using AgNPs

Keywords: Colorimetric detection, silver nanoparticles, heavy metal ions, surface plasmon resonance *

∗

Corresponding author. Tel.: +82-2-940-5769; fax: +82-2-941-5769.

E-mail address: [email protected] (Y. Kim), [email protected] (C. Park) 1

ACS Paragon Plus Environment

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Abstract A highly sensitive colorimetric sensing platform for the selective trace analysis for Co2+ ions is reported, based on glutathione (GSH) modified silver nanoparticles (AgNP). The shape of metallic nanoparticles used in colorimetric detection, using the unique optical properties of plasmonic nanoparticles, is almost spherical. Therefore, in this work we attempted to investigate the selective detection of heavy metal ion (Co2+), with the shape of AgNPs (nanosphere, nanoplate, and nanorod). GSH-AgNP with spherical shape shows a high sensitivity for all of the metal ions (Ni2+, Co2+, Cd2+, Pb2+, and As3+), but poor selective recognition for target metal ions. Whereas, AgNPs solution containing rod-type GSH-AgNP has a special response to Co2+, and its selective detection might be based on the cooperative effect of CTAB and GSH. Therefore, Co2+ ion could be selectively recognized using rod-type GSH-AgNPs.

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Introduction

One of the most important characteristics of metal nanoparticles (NPs) is their localized surface plasmon resonance (LSPR), which exhibits sensitivity to their size, shape, composition, and dielectric constant.1 This unique optical property in the visible spectral region originates from the excitation of the collective oscillation of conducting electrons of metal NPs.2 In particular, the plasmon properties of NPs depend strongly on the interparticle distance between pairs of (aggregated) NPs, compared to the mono-dispersed state.3 This distance-dependent LSPR absorption of metal NPs has been emerging as the basis of useful colorimetric sensors for detecting various chemicals, due to its extreme simplicity and low cost. The development of highly sensitive and selective analytical tools for heavy metal ions is of great importance, to avoid their cytotoxicity effects. Therefore, this colorimetric method, which can be observed by the naked eye, is appropriate as an on-site method for real-time detection of target heavy metal ions, due to its simple configuration and portability to on-site. To date, various metal NPs have been used as colorimetric detectors for heavy metal ions in the aqueous phase. In particular, gold (Au) and silver nanoparticles (AgNPs) offer excellent LSPR properties, exhibiting strong and well-defined color, and easy visualization of color change.2-9 Visual detection has been based on the well-known metal-ligand coordination, where metal and ligand act as electronic acceptor and donor, respectively. A simple colorimetric assay employing peptide and AuNPs with 10 nm diameter for the detection of Hg2+ and Pb2+ ions, based on the metal ions-peptide complex inducing the aggregation of AuNPs, was reported by Slocik et al.4 A highly selective detection for Co2+ in the presence of other metal ions (Hg2+, Na2+, Cu2+, and so on) was evaluated by thioglycollic acid functionalized AuNPs, which were stabilized with cetyltriammonium bromide (CTAB).5 AuNPs modified with ammonium group-terminated thiols were prepared to selectively detect Hg2+ ion, via abstraction of thiols induced to aggregate AuNPs.6 Although AgNPs have been used less extensively than AuNPs in colorimetric assays, AgNPs also have good applicability to the detection of heavy metal ions, based on color 3


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change between the dispersed and aggregated ones. Triazo-carboxyl AgNPs show a cooperative effect on the recognition of Co2+ over other metal ions tested,3 and mercaptopyridine-glutathione (GSH) modified AgNP was used as a colorimetric detector of As3+ ion.7 As described above, the metal NPs show excellent selectivity and sensitivity as colorimetric sensors, and metal NPs used in sensor assay are mostly of the spherical type.8,9 The LSPR absorption of metal NPs was changed also, with the particle’s shape. Therefore, in this work we investigated the selective detection of heavy metal ion (herein, Co2+) with the shape of AgNPs (nanospherical, nanoplate, and nanorod). It is not easy to prepare different shaped AgNPs with the same stabilizer, and thus three different AgNPs are synthesized via the seed-mediated method, using different stabilizers.10-12 In general, the bi-functionalization of metal NPs was introduced, in order to obtain selective detection of target metal ions in the presence of others.3,5,7 Therefore, GSH was here used as a common functional material, to selectively detect the target metal ion (Co2+), compared to Ni2+, Cd2+, Pb2+, and As3+ ions.

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Experimental

Preparation of AgNPs of different shapes Ag seed was prepared, by dissolving 0.1 mL of 18 mM AgNO3 and 0.1 mL of 17 mM trisodium citrate (TSC) in 20 mL of deionized (DI) water. To this solution, 0.6 mL of 10 mM NaBH4 solution was added dropwise, under vigorous stirring. This citrate-stabilized Ag seed with 3-4 nm size yield was kept in a dark place at room temperature, before usage. The growth solution was prepared by the reported method for each AgNP. Spherical AgNP (AgNP-S) was prepared using polymer stabilizer.10 85 mg of AgNO3 and 83 mg of PVP (polyvinyl pyrrolidone) were added into 10 mL of EG (ethylene glycol). The resulting solution was heated at 185oC for 20 min, and particles were separated by centrifugation, and washed with DI water several times, to remove the rest of the EG. Washed particles were mixed with 19.4 mL of DI water and 0.6 mL of 10 mM NaBH4. To synthesize silver nanoplate (AgNP-P),11 the growth solution was prepared by dissolving 0.1 g of TSC, 0.14 mL of 18 mM AgNO3, and 0.1 mL of 100 mM L-ascobic acid (AA) in 20 mL of DI water. After 0.4 mL of Ag seed was added to the growth solution, the mixture was allowed to stand without stirring, until the color changed to blue. Silver nanorod (AgNP-R) stabilized with CTAB was prepared by the reported method.12 In brief, 0.25 mL of Ag seed was added to a mixture of 10 mL of 25 mM CTAB, 0.125 mL of 10 mM AgNO3, and 0.125 mL of 100 mM AA, followed by the addition of 1 M NaOH, until the color changed to pale blue.

Preparation of GSH-AgNPs As shown in Figure 1, three different shaped AgNPs were obtained. To recognize the target metal ion, the surface of the AgNPs should be functionalized. GSH can bind to AgNPs easily through Ag-S bonds,8 and thus COO- and/or NH3+ groups of GSH-modified AgNPs could bind to positive ions, via the cooperative effect.5 Therefore, GSH was selected as the functional material to recognize the target metal ion. 0.025 g of GSH was added into as-made AgNPs solution, and the pH of the resulting mixture was adjusted to 8.0, using 1 M NaOH. Upon the addition of GSH to AgNPs, aggregation of the AgNPs was observed.13 Finally, 5


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GSH-AgNP-X was obtained, after neutralization of the AgNPs solution. Detailed analytical conditions for the preparation of AgNPs are summarized in Table S1.

Colorimetric detection and characterizations The colorimetric detection of heavy metal ions was performed at room temperature. The resulting solutions’ concentrations of GSH-AgNP-S, -P, and –R are 70, 100, and 20 ppm, respectively. A volume of 3 mL of GSH-AgNP-X solution was added to 2 mL of different concentrations of Ni2+, Co2+, Cd2+, Pb2+, and As3+ ions (5 to 700 μM). The stock solution of metal ions was adjusted to under pH 4, in order to maintain their ionic state. The concentration, ranging from 1 to 100 ppm, was prepared by using serial dilution of the stock solution. The morphology of AgNPs was analyzed using transmission electron microscopy (TEM, JEM-1010, JEOL), and UV-vis spectra (UV-18000, Shimadzu).

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Results and discussion The chelating sulfur-containing GSH ligands bind to the AgNPs surface through Ag-S bonds. Each metal ion (Mn+) can bind with GSH-AgNPs through an M-S linkage; however, there is no free SH group available for binding with M2+ ions. The remaining functional moiety, COO- groups of GSH-modified AgNPs, could bind to positive ions, and thus positive ions can bind with two or three GSH-modified AgNPs, through M-O linkage or complex.7,9,13 In the case of mercaptobenzoic acid (MBA) modified AgNPs, the capped AgNPs are linked together by carboxylate-Mn+-carboxylate coordinative couplings.14,15 Therefore, colorimetric detection herein is based on the fact that the GSH-AgNPs undergo aggregation, due to the formation of chelating complex between the metal ions and COO- groups, namely, iontemplated chelation. The addition of GSH with NaOH does not change the color of AgNPs, which indicates that there is no aggregation. GSH is linked with AgNPs through Ag-S linkage, and as a result, there are two free carboxyl groups and one amine group, which can be used for functionalization with metal ions. Although Cd2+, Cu2+, and Zn2+ are well-known to bind to the amine group, the NH2 group in the GSH-AgNPs is already protonated to NH3+,13 due to the experimental condition at pH 8. As a result, the carboxyl groups are the only binding site. GSH-AgNP-S with spherical shape shows 30 nm of diameter; it is well-dispersed in aqueous phase, and its characteristic peak in UV-vis spectroscopy is about 400 nm. When we added metal ions to GSH-AgNP-S, the AgNPs underwent aggregation, due to the formation of strong chelating complex, via carboxylate ions. As shown in Figure 2, the presence of metal ions led to red-shift of the peak at 400 nm, and emergence of a new peak, at about 550 nm in the UV-vis spectra. This red shift might be due to the change of local refractive index on the AgNPs surface, caused by the specific binding of GSH-AgNPs with metal ions, and the interparticle interaction resulting from the AgNPs assembly.13 Therefore, aggregation of AgNPs in the presence of metal ions yields both a substantial shift in the plamon band energy to longer wavelength, and a red color change. Quantitative analysis was performed by adding different concentrations of metal ions into the GSH-AgNP-S solution, and monitoring the absorption peak in the UV-vis spectra. The UV-vis absorbance ratio (A550/A400) increased linearly with the concentration (5 7


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to 400 μM) of metallic cations (Figure S1). The determination coefficient (R2) for all of the Ni2+, Co2+, Cd2+, Pb2+, and As3+ ions is high, at 0.99. A linear correlation between the absorbance ratio and concentrations of metal ions makes it suitable for the quantitative determination of target metal ions in aqueous solutions. Although the colorimetric sensitivity of GSH-AgNP-S for various metal ions is excellent, the color changes of individual metal ions are similar to each other, regardless of the ion type. That is, the colorimetric selectivity of GSH-AgNP-S is very poor, but GSHAgNP-S is applicable to a universal colorimetric sensor for various metal ions. Therefore, we examined the different shaped AgNPs, for colorimetric detection of metal ions. As shown in Figure 3a, GSH-AgNP-P showed ca. 40 nm size, and formed a distinctive blue solution. As previously reported, they have a highly stable morphology in the aqueous phase.11 As compared to the absorbance peak in GSH-AgNP-S, GSH-AgNP-P has three plasmon peaks in the UV-vis spectra (Figure 3c). Three distinctive peaks are assigned to the out-of-plane quadrupole resonance (the first peak), the in-plane quadrupole resonance (the second peak), and the in-plane dipole plasmon resonance (the third peak).16 However, after the addition of 100 ppm metal ions in GSH-AgNP-P solution, all of the plasmon peaks were eliminated, and the color of solution was transparent, as shown in Figure 3c. For 1 to 10 ppm of metal ions, the solution color of GSH-AgNP-P was very slightly changed, so it could not be recognized by the naked eye. As shown in Figure 3b, GSHAgNPs-P in the presence of metal ions found a few, and showed smaller size, as compared to GSH-AgNP-P in the absence of metal ions. In our previous report,11 a strong oxidizing agent readily takes electrons from AgNPs, and releases silver ions (Ag+) into the solution. This ion release results in a transformation of the AgNPs shape and size, and thus metal nanoparticles could finally be fully ionized. Citrate-stabilized AgNPs might be easy oxidized chemically in low pH, namely by the addition of high concentration of metal ion solutions. Consequently, it is not easy to find the GSH-AgNP-P in the TEM image (Figure 3(b)), due to the presence of few nanoparticles in solution. Therefore, GSH-AgNP-P is not suitable as a colorimetric sensor for detecting metal ions. Finally, rod-type AgNPs were prepared, and modified with GSH, to evaluate the 8


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colorimetric selectivity and sensitivity for metal ions. As shown in Figures 4a and 4c, rodtype AgNPs with above 400 nm size have a longitudinal peak at 750 nm in the UV-vis spectra, and it is noted that AgNPs have directional growth. Because of low yield for AgNPR, very small spherical AgNPs under 10 nm size co-existed with rod-type AgNPs (Figure S2a). After mixing between GSH and AgNP-R solution, spherical AgNPs with 100 nm as well as rod-type AgNPs were found which were grown from small spherical seed in AgNPR solution. GSH could act as a reducing as well as a capping agent,17 and thus small spherical particles were re-grown to larger particles. After addition of metal ions into the GSH-AgNP-R solution, the absorbance peaks in the UV-vis specta were not changed, except for Co2+. When Co2+ ion was mixed with GSH-AgNP-R, the solution color changed dramatically, from pale blue to dark green. Even though the main peak at 750 nm in the UVvis spectra was maintained, its UV absorbance band was largely changed. In the UV-vis spectra, a new shoulder peak from 300 to 550 nm emerged, and the absorption intensity of the main peak for the longitudinal band decreased. It should be noted that smaller particles in length than the rod-AgNPs were formed in the presence of Co2+ solution. As shown in Figure 4b, many spherical AgNPs with ca. 150 nm diameter, as well as rod-type AgNPs, were exhibited in the TEM image, and all of the spherical and rod-type AgNPs aggregated by formation of coordination compounds between Co2+ and functional groups (Figure 5 and S3). Although carboxyl-modified AgNPs can respond to many transition metal ions, only GSH-CTAB modified AgNPs have a special response to Co2+. By a report by Bala and co-worker,15 the metal diacetate cohesive energies and respective metal-acetate bond energy of Co2+ are higher than those of other metal ions. The metal-acetate bond energies of Co2+, Cd2+, and Pb2+ are 180, 16, and 168 kJ/mol, respectively. But that of Ni2+ is 212 kJ/mol. Specifically, the bonding energy of carboxyl-Co2+ is high, but lower than that of Ni2+. Therefore, we found the other reason for the selective recognition of Co2+. It is well known that each GSH molecule contains amine and carboxylate functionalities that provide coupling possibilities for further cross-linking to other molecules of sensing interest.17 In addition, it is reported that thioglycollic acid (TGA) functionalized CTAB modified AuNPs can selectively detect Co2+ ions.5 The surface modification system of 9


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that was similar to our case. Although their suggested mechanism is not yet clear, we can understand the selective recognition of Co2+ ion, based on the cooperative effect of CTAB and GSH. The GSH was absorbed on the surface of AgNPs through the Ag-S bond, and AgNPs were aggregated in the presence of Co2+ due to binding with chelating ligands, and CTAB separated from the AgNPs surface, because CTAB and GSH had a cooperative effect on the recognition of Co2+. It could be explained that coordination compounds were formed by Co2+, with carboxyl groups (Figure 5). Namely, coordination compounds with Co2+ formed aggregates between each AgNPs. Aggregation of nanoparticles by the abstraction of stabilizer was also found in another example. AuNPs stabilized with thiol groups successively recognized the Hg2+ ion, by the abstraction of the thiols group from the AuNPs that led to the aggregation of AuNPs.6 This mechanism is helpful in understanding the formation of aggregates of AgNPs, after the addition of Co2+ solution. Because CTAB was separated from GSH-AgNP-R, the stability of AgNPs was reduced, and Ag+ ion could be released from the AgNPs, in the presence of low pH (pH 3.61 for 1 ppm, pH 2.72 for 10 ppm, and pH 1.83 for 100 ppm of Co2+ solution). Metal silver was readily oxidized in oxygen contained solution under low pH, and Ag+ ion was released by oxidation;15 2Ag + 1/2O2 + H2O → 2Ag+ + 2OH-. Jin et al. discovered that normal room light stimulated colloidal silver nanocrystals to reform into larger nanoprisms, without addition of Ag+.18 Therefore, Ag+ ion release results in a transformation of AgNP shape, and finally, spherical AgNPs, as the most stable form, were grown by Ostwald ripening.

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Conclusions GSH modified AgNPs with spherical, plate, and rod shapes were prepared, and evaluated for the detection of metal ions by colorimetric sensing. The carboxyl group in GSH has high affinity to the transition metal ions, and thus several metallic cations were selected as target ions. Colorimetric detection is based on the fact that GSH-AgNPs undergo aggregation, due to the formation of chelating complex between metal ions and COO- groups. Spherical GSHAgNP-S was highly sensitive to all metal ions, but did not show selective detection. Meanwhile, GSH-AgNP-P with plate-type NPs in the presence of metal ions was ionized, and a few particles were found in the TEM image. In particular, GSH-AgNP-P was not suitable as a colorimetric sensor for metal ions. Finally, GSH-AgNP-R with rod-type NPs was prepared, and tested for the same metal ions. The results showed that GSH-AgNP-R solution has high sensitivity to only Co2+ ion. Because CTAB and GSH had a cooperative effect on the recognition of Co2+, CTAB separated from the GSH-AgNP-R surface, and the reducing stability of AgNPs led to them being reformed from rod-type to spherical shape. Therefore, we found that GSH-AgNP-S is applicable to a universal colorimetric sensor for various metal ions, and GSH-AgNP-R has high selectivity for the Co2+ ion.

Acknowledgements

This work was supported by the Research Grant of Kwangwoon University in 2013, and the National Research Foundation of Korea (NRF-2010-0007050).

Supporting Information Available: UV-vis absorbance ratio for GSH-AgNP-S, TEM images of AgNP-R, particle size distribution of GSH-AgNP-X, and analytical condition for metal detection. This material is available free of charge via the internet at http://pubs.acs.org.

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References (1) Vilela, D.; González M.C.; Escarpa, A. Sensing colorimetric approaches based on gold and silver nanoparticles aggregation: chemical creativity behind the assay. Anal. Chim. Acta 2012, 751, 24-43. (2) Dou, Y.; Yang, X.; Liu, Z.; Zhu, S. Homocysteine-functionalized silver nanoparticles for selective sensing of Cu2+ ions and Lidocaine hydrochloride. Colloids Surf. A 2013, 423, 20-26. (3) Yao, Y.; Tian, D.; Li, H. Cooperative binding of bifunctionalized and click-synthesized silver nanoparticles for colorimetric Co2+ sensing. ACS Appl. Mater. Interfaces 2010, 2, 684-690. (4) Slocik, J. M.; Zabinski, J.; Phillips, D. M.; Naik, R. R. Colorimetric response of peptide-functionalized gold nanoparticles to metal ions, Small 2008, 4, 548-551. (5) Zhang F.; Zeng, L.; Zhang, Y.; Wang, H.; Wu, A. A colorimetric assay method for Co2+ based on thioglycolic acid functionalized hexadecyl trimethyl ammonium bromide modified Au nanoparticles (NPs). Nanoscale 2011, 3, 2150-2154. (6) Liu, D. B.; Qu, W. S.; Chen, W. W.; Zhang, W.; Wang, Z.;Jiang, X. Highly sensitive, colorimetric detection of mercury (II) in aqueous media by quaternary ammonium group-capped gold nanoparticles at room temperature, Anal. chem. 2010, 82, 9606-9610. (7) Kalluri, J. R.; Arbneshi, T.; Khan, S. A.; Neely, A.; Candice, P.; Varisli, B.; Washington, M.; McAfee, S.; Robinson, B.; Banerjee, S.; Singh, A. K.; Senapati, D.; Ray, P. C. Use of gold nanoparticles in a simple colorimetric and ultrasensitive dynamic light scattering assay: selective detection of arsenic in groundwater. Angew. Chem. Int. Ed. 2009, 48, 9668-9671. (8) Chen, Z.; He, Y. J.; Luo, S. L.; Lin, H. L.; Chen, Y. F.; Sheng., P. T.; Li, J. X.; Chen, B. B.; Liu, C. B.; Cai, Q. Y. Label-free colorimetric assay for biological thiols based on ssDNA/silver nanoparticle system by salt amplification. Analyst 2010, 135, 1066-1069. (9) Ravindran, A.; Elavarasi, M.; Prathan, T. C.; Raichur, A. M.; Chandrasekaran, N.; Mukherjee, A. Selective colorimetric detection of nanomolar Cr (VI) in aqueous solutions using unmodified silver nanoparticles. Sensors Act. B. 2012, 166-167, 365-371. (10) Wang, H. S.; Qiao, X. L.; Chen, J. G.; Wang, X. J.; Ding, S. Y. Mechanisms of PVP in the preparation of silver nanoparticles. Mater. Chem. Phys. 2005, 94, 449-453. (11) Roh, J.; Yi, J.; Kim, Y. Rapid, reversible preparation of size-controllable silver nanoplates by chemical redox. Langmuir 2010, 26, 11621-11623. (12) Jana, N. R.; Gearheart, L.; Murphy, C. J. Wet chemical synthesis of silver nanorods and nanowires of controllable aspect ratio. Chem. Commun. 2001, 0, 617-618. (13) Beqa, L.; Singh, A. K.; Khan, S. A.; Senapati, D.; Arumugam, S. R.; Ray, P. C. Gold nanoparticlebased simple colorimetric and ultrasensitive dynamic light scattering assay for the selective detection of Pb(II) from paints, plastics, and water samples. ACS Appl. Mater. Interfaces 2011, 3, 668673. (14) Zhou, Y.; Zhao, H.; He, Y.; Ding, N.; Cao, Q. Colorimetric detection of Cu2+ using 4mercaptobenzoic acid modified silver nanoparticles. Colloids Surf. A 2011, 391, 179-183. (15) Bala, T.; Prasad, B. L. V.; Sastry, M.; Kahaly, M. U.; Waghmare, U. V. J. Interaction of different metal ions with carboxylic acid group: a quantitative study. J. Phys. Chem. A 2007, 111, 6183-6190. (16) Roh, J.; Umh H. N.; Sung H. K.; Lee, B.; Kim, Y. Repression of photomediated morphological changes of silver nanoplates. Colloids Surf. A 2012, 415, 449-453. (17) Baruwati, B.; Polshettiwar, V.; Varma, R. S. Gluathione promoted expeditious green synthesis of silver nanoparticles in water using microwaves. Green Chem. 2009, 11, 926-930. (18) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Photoinduced conversion of silver nanospheres to nanoprisims. Science 2001, 294, 1901-1903.

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List of Figures

Figure 1. Schematic diagram of the preparation of AgNPs with different shapes.

Figure 2. (a) TEM image of GSH-AgNP-S in the absence of metal ions, and (b) TEM image of GSH-AgNP-S in the presence of 10 ppm Co2+ ion (scale bar is 200 nm). (c) Absorption spectra of GSH-AgNP-S in the presence of Ni2+, Co2+, Cd2+, Pb2+, and As3+ ions. Inset pictures in (c) show the color change of GSH-AgNP-S in the presence of 10 ppm metallic cations.

Figure 3. (a) TEM image of GSH-AgNP-P in the absence of metal ions, and (b) TEM image of GSH-AgNP-P in the presence of 100 ppm Co2+ ion (scale bar is 200 nm). (c) Absorption spectra of GSH-AgNP-P in the presence of Ni2+, Co2+, Cd2+, Pb2+, and As3+ ions. Inset pictures in (c) show the color change of GSH-AgNP-P in the presence of 100 ppm metallic cations.

Figure 4. (a) TEM image of GSH-AgNP-R in the absence of metal ions, and (b) TEM image of GSH-AgNP-R in the presence of 10 ppm Co2+ ion(scale bar is 200 nm). (c) Absorption spectra of GSH-AgNP-R in the presence of Ni2+, Co2+, Cd2+, Pb2+, and As3+ ions. Inset pictures in (c) show the color change of GSH-AgNP-R in the presence of 10 ppm metallic cations.

Figure 5. A strategy for Co2+ detection using a GSH functionalized CTAB stabilized AgNPs. (The scheme was modified from Reference 5).

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Figure 1. Schematic diagram of the preparation of AgNPs with different shapes.

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Figure 2. (a) TEM image of GSH-AgNP-S in the absence of metal ions, and (b) TEM image of GSH-AgNP-S in the presence of 10 ppm Co2+ ion (scale bar is 200 nm). (c) Absorption spectra of GSH-AgNP-S in the presence of Ni2+, Co2+, Cd2+, Pb2+, and As3+ ions. Inset pictures in (c) show the color change of GSH-AgNP-S in the presence of 10 ppm metallic cations.

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Figure 3. (a) TEM image of GSH-AgNP-P in the absence of metal ions, and (b) TEM image of GSH-AgNP-P in the presence of 100 ppm Co2+ ion (scale bar is 200 nm). (c) Absorption spectra of GSH-AgNP-P in the presence of Ni2+, Co2+, Cd2+, Pb2+, and As3+ ions. Inset pictures in (c) show the color change of GSH-AgNP-P in the presence of 100 ppm metallic cations.

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Figure 4. (a) TEM image of GSH-AgNP-R in the absence of metal ions, and (b) TEM image of GSH-AgNP-R in the presence of 10 ppm Co2+ ion (scale bar is 200 nm). (c) Absorption spectra of GSH-AgNP-R in the presence of Ni2+, Co2+, Cd2+, Pb2+, and As3+ ions. Inset pictures in (c) show the color change of GSH-AgNP-R in the presence of 10 ppm metallic cations.

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Langmuir

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Figure 5. A strategy for Co2+ detection using a GSH functionalized CTAB stabilized AgNPs. (The scheme was modified from Reference 5).

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Langmuir

SYNOPSIS TOC

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Colorimetric Detection of Co2+ Ion Using Silver ... - ACS Publications

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