DOI: 10.1002/chem.201405012
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& Nanostructures
Silver/Gold Core–Shell Nanoprism-Based Plasmonic Nanoprobes for Highly Sensitive and Selective Detection of Hydrogen Sulfide Xinjian Yang, Yuqian Ren, and Zhiqiang Gao*[a] throughput, which make them unfavorable in monitoring the continuous fluctuations of hydrogen sulfide in biological systems because of their rapid metabolism. Recently, many organic fluorescent agents have been exploited for the detection and imaging of hydrogen sulfide.[8] However, some drawbacks of most of these organic fluorescent agents are unavoidable, such as difficulty in separating unreacted agents, easy photobleaching, poor photostability, and lack of sufficient penetration depth when imaging biological tissues. Therefore, the design of selective and efficient probes for hydrogen sulfide has been one of the grand challenges. Nanotechnology provides an exciting avenue for rapid and accurate detection of analytes with high sensitivity and selectivity. Particularly, noble-metal-based nanomaterials, which display the surface plasmon resonance (SPR) absorption in the near IR (NIR) region with ultrahigh extinction coefficients, offer exciting opportunities in the construction of chemical sensing and biosensing platforms and assays with unprecedented functionalities for highly sensitive and selective detection of a wide range of analytes.[9] The distinctive spectral sensitivity of SPR to nanostructure morphology can provide the basis for versatile biological labeling, sensing, and detection platforms if changes in the nanostructure’s shape or size can be induced upon interacting with an analyte.[10] Recently, a plamonic probe was developed for sulfide mapping in living cells using gold/silver core–shell nanoparticles.[11] Though promising, this nanoprobe showed low selectivity toward hydrogen sulfide because some species can also etch the silver on the outer surface of the gold/silver core–shell nanoprobes, resulting in false signals. Therefore, it remains a challenge to develop new plasmonic probes with high sensitivity and selectivity. Herein, we present a new plamonic assay for highly sensitive and selective detection of hydrogen sulfide by using goldcoated silver nanoprisms (Ag/Au core–shell nanoprisms) as plasmonic nanoprobes. Our previous work[12] demonstrated that bare silver nanoprisms are sensitive to some species in solution, such as hydrogen peroxide, resulting in a blueshift of their SPR peak. However, these silver nanoprisms are not suitable for direct detection of analytes in complex matrixes since a separation step before target analysis is required. To address this problem, we coated the silver nanoprisms with a thin gold layer that protects the silver nanoprisms from direct reaction with many species in solution. In the presence of strong etching agents, such as hydrogen sulfide, the silver in the Ag/Au core–shell nanoprisms will be etched through the defects of the gold outlayer and converted into Ag2S. As schematically depicted in Scheme 1, the plasmonic sensing strategy is based
Abstract: A simple and highly sensitive and selective hydrogen sulfide assay utilizing plasmonic nanoprobes is presented in this report. The assay employs the etching of silver in the Ag/Au core–shell nanoprisms, accompanied by surface plasmon resonance (SPR) signal depression and shift. Briefly, thin layers of gold are first coated onto silver nanoprisms. The thin gold layer not only guarantees the high stability of the plasmonic nanoprobes but also ensures the high selectivity toward hydrogen sulfide. Once hydrogen sulfide is introduced, the silver core is converted to Ag2S mainly from its lateral walls. Moreover, the SPR peak is located in the NIR region that makes these plasmonic nanoprobes more appealing for the detection of hydrogen sulfide in real-world samples and in in vivo applications.
Hydrogen sulfide with the characteristic odor of rotten eggs, historically known as a toxic gas, has been recognized as a member of signaling molecule family along with nitric oxide (NO) and carbon monoxide (CO).[1] The production of endogenous hydrogen sulfide is related to at least three separate enzymes within different tissues and organs: cystathionine b-synthase, cystathionine g-lyase, and 3-mercaptopyruvate sulfur transferase.[2] Increasing evidence has shown that hydrogen sulfide plays vital roles in regulating intracellular redox status and other fundamental signaling processes involved in human health and disease, such as modulation of blood pressure, mediation of neurotransmission, and regulation of inflammation.[3] The abnormal levels of hydrogen sulfide have been found to be closely associated with diabetes, Alzheimer’s disease, and various types of cancer.[4] Therefore, rapid and selective detection of hydrogen sulfide is of paramount importance in understanding its physiological and pathological functions. One significant limiting factor in studying hydrogen sulfide is the lack of highly sensitive assays and sensors for its accurate detection. Classic methods using electrochemical methods,[5] colorimetry,[6] and chromatography[7] have been developed. In most cases, these methods require sophisticated instrumentation, involve cumbersome laboratory procedures, and have low [a] Dr. X. Yang, Y. Ren, Dr. Z. Gao Department of Chemistry, National University of Singapore Singapore 117543 (Singapore) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201405012. Chem. Eur. J. 2014, 20, 1 – 6
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Since the mechanism plays an important role in understanding and performing the detection of hydrogen sulfide, TEM experiments were conducted to gain insight into the mechanism of the interaction between the Ag/Au core–shell nanoprisms and hydrogen sulfide. Previous work has indicated that Ag is readily converted to Ag2S by reacting with sulfide.[14] After coating a thin gold layer on the silver nanoprisms, sulfide etching process could only occur through the defects of the Ag/Au core–shell nanostructure. As shown in the EDX mapping images (Figure S1, Supporting Information), sulfur element exists all over the nanoprisms after the incubation with sulfide. Meanwhile, because the formed Ag2S layer is very thin (3– 4 nm), no obvious difference of sulfur element distribution between edges and the plane was observed. EDX results also showed the decrease of the percentage of silver in the complex after the incubation with sulfide. It is thought that the Ag oxidation by oxygen, which is very low, first occurs at the defects of the thin Au layer. After the introduction of H2S, the formation of highly stable Ag2S, which has an extremely low dissociation constant (6.3 1050), drives further oxidation of Ag. However, as shown in Figure 1E, the TEM image indicated that sulfuration reaction mainly occurs at the lateral faces of the nanoprisms after the incubation with hydrogen sulfide. The shielding effect of the gold layer decreased its potential to rapid react with H2S. Previous work showed that each Ag nanoprism contains two main (111) facets on the triangular planes and three (110) facets on the edges.[15] Since the relative surface energies of different crystal facets of silver are in the order of g111 < g100 < g110,[16] the lateral walls would be etched easier. Furthermore, we found that some of the cores shrink from the lateral walls after the incubation with H2S (Figure S2, Supporting Information). With the ratio of Ag2S in the nanoprisms increasing, the SPR peak of the nanoprisms experienced a continuous depression and a redshift while the shortwavelength absorption derived from the Ag2S region became more prominent probably due to the high refractive index of Ag2S.[17] Electron diffraction spectroscopic (EDS) tests (Figure S3, Supporting Information) further demonstrate that the sulfuration reaction occurs on the surface of the nanoprisms. Meanwhile, the gold layer plays an important role in the transformation of Ag into Ag2S. In control experiments, we found that bare silver nanoprisms undergo different SPR peak shifts at different hydrogen sulfide concentrations (Figure S4, Supporting Information). A blue SPR peak shift appeared in the presence of hydrogen sulfide (40 mm) while a redshift occurred when hydrogen sulfide (200 mm) was used. This is because of the fact that the transformation process prefers to initiate at the corner of the nanoprisms and the shape of the nanoprisms could be retained when enough hydrogen sulfide is endow-
Figure 1. A) Absorption spectra of the Ag/Au core–shell nanoprisms collected during the incubation with 200 mm hydrogen sulfide; B) the absorption change of the SPR peak maxima and C) the SPR peak shift during the incubation with 200 mm hydrogen sulfide; TEM images D) before and E) after a period of 30 min incubation with 200 mm hydrogen sulfide.
on hydrogen sulfide etching derived shape change of the Ag/ Au core–shell nanoprisms and their corresponding surface plasmon resonance signal change. The Ag/Au core–shell nanoprisms with a high extinction coefficient and excellent stability were synthesized by following a published procedure.[13] Figure 1D shows a typical TEM image of the nanoprisms. The original triangular shapes are well retained, as expected by the uniform deposition of Au atoms on the plate surfaces. High-resolution TEM imaging demonstrates that a thin layer of Au is coated on the Ag nanoprism surface. We subsequently tested the feasibility of this plasmonic nanoprobe for hydrogen sulfide sensing. The absorption spectra, which are associated with the altering of shape and surface conditions of the nanoprisms, were studied as a function of reaction time. As shown in Figure 1A–C, it was &
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Communication ed.[18] Moreover, we incubated different interfering agents with silver nanoprisms. Our results suggested that bare silver nanoprisms are not suitable for hydrogen sulfide sensing (Figure S5, Supporting Information). The thickness of the gold layer is the key factor for the success of the assay. The selectivity will be lost when the gold coating on the silver nanoprisms is too thin, whereas the sensitivity is sacrificed when the gold layer is too thick (Figure S6, Supporting Information). We found that the thickness of gold layer can be facilely manipulated by simply adjusting the amount of chlorauric acid in the preparation of the Ag/Au core–shell nanoprisms. Next, we measured the hydrogen sulfide concentration-dependent spectra change. Figure 2A displays the results after in-
Figure 2. A) Spectral changes of the Ag/Au core–shell nanoprisms in the presence of different concentrations of hydrogen sulfide after a period of 30 min incubation; B) absorption depression at the SPR peak maxima and C) the SPR peak shift derived from (A).
Figure 3. Effect of possible interfering agents on A) the SPR absorption maxima and B) the SPR peak shift of the Ag/Au core–shell nanoprisms after a period of 30 min incubation.
cubating 0.1, 0.5, 1, 2.5, 5, 10, 25, 50, and 100 mm hydrogen sulfide with the Ag/Au core–shell nanoprisms for 30 min, respectively. From the corresponding absorption changes of the SPR maxima (Figure 2B), a linear relationship between the SPR absorption change and the hydrogen sulfide concentration from 0.1 to 10 mm was obtained. The absorption change and SPR shift leveled off when more hydrogen sulfide was introduced because of the strong protective effect of the gold shell and the formed Ag2S layer (Figure 2B, C). These results clearly demonstrated the ability of Ag to Ag2S transformation on the Ag/Au core–shell nanoprisms as an attractive strategy for hydrogen sulfide sensing with high sensitivity. To evaluate the specific nature of the Ag/Au core–shell nanoprism etching by hydrogen sulfide, we then examined possible SPR absorption peak changes of the nanoprisms when incubated with other species, most of which could work as etchants for silver in other systems.[19] As shown in Figure 3, the nanoprisms exhibited very limited change in their extinction and SPR peak shift to various halogen ions, even at a concentration as high as 0.10 m of chloride. The high selectivity toward hydrogen sulfide is due to the protection effects of gold layer and low dissociation constant comparing with other ions. Moreover, negligible changes were observed toward other common anions. Next, we investigated the reactivity of the nanoprisms toward hydrogen peroxide. Although hydroChem. Eur. J. 2014, 20, 1 – 6
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gen sulfide and hydrogen peroxide cannot coexist in one system, hydrogen peroxide produced by some organs in the living cells might generate a false signal because of its strong etching ability toward silver.[20] Again, because of the strong protective effect of the gold shell to the inner silver nanoprism cores, no obvious SPR signal changes were observed in the presence of hydrogen peroxide (200 mm). In a control experiment, we found that there was a redshift of the SPR peak when Ag-coated gold nanorods were incubated in 200 mm hydrogen peroxide solution (Figure S7, Supporting Information), thereby suggesting the superiority of the Ag/Au core–shell nanoprisms in hydrogen sulfide sensing. More importantly, it was observed that biothiols elicit only a slight change in the extinction and practically no SPR peak shifts of the nanoprisms. Because of the strong binding effect of thiols to Au, they tend to protect the nanoparticles from destruction. These results suggest that only hydrogen sulfide can produce the significant absorption decrease and SPR shift and no noticeable changes in the SPR absorption of the Ag/Au core–shell nanoprisms were observed when incubated with other competitive species. After the confirmation of the sensitivity and selectivity of this plasmonic nanoprobe for hydrogen sulfide in pure buffer 3
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Communication pH we used (Figure S10, Supporting Information). Another reason might be the significant scavenging effect toward hydrogen sulfide by proteins in serum.[22] As depicted in Figure 4, a linear relationship between dAbs and the concentration of H2S was obtained between 0.1 and 10 mm with a detection limit of 54 nm (R2 = 0.967). Meanwhile, by using the calibration curve between dl and the concentration of H2S (R2 = 0.992), the detection limit was estimated as low as 43 nm. In summary, we have presented in this work new plasmonic Ag/Au core–shell nanoprism nanoprobes for hydrogen sulfide based on its strong ability to etch the Ag/Au core–shell nanoprisms. In the presence of hydrogen sulfide, silver in the nanoprisms was converted to Ag2S mainly at the defective lateral walls, accompanied by obvious SPR absorption depression and peak shift. The thin gold layer not only guarantees the high stability of the plasmonic nanoprobes but also ensures the high selectivity toward hydrogen sulfide. Compared with most of the reported hydrogen sulfide probes, complicated synthesis steps and expensive equipment are eliminated in our assay. Moreover, the SPR peak is located in the NIR region that makes these plasmonic nanoprobes more appealing for the detection of hydrogen sulfide in real-world samples and in vivo applications.
solution, we further explored its applications in the detection of hydrogen sulfide in complex matrixes. It has been reported that the typical concentration of hydrogen sulfide in blood is 10–100 mm.[21] Accurate detection of the concentration of hydrogen sulfide in serum is the key to early diagnosis of many diseases in which hydrogen sulfide plays an important role. Many components in serum, which have significant absorptions in the UV/Vis region, limit the use of most hydrogen sulfide probes. In the proposed assay, the SPR peak locates in the NIR region, which is much more desirable for the detection of hydrogen sulfide in serum. In addition, in the absence of hydrogen sulfide no obvious SPR absorption depression or peak shifts were observed (Figure S8, Supporting Information), thereby indicating that the Ag/Au core–shell nanoprisms are stable in serum. Consequently, we studied the possibility of using these plasmonic nanoprobes to detect hydrogen sulfide in serum (Figure 4, Figure S9, Supporting Information). The
Experimental Section Synthesis of the Ag nanoprisms Silver nanoprisms were synthesized by following a procedure developed in a previous report.[12] Briefly, AgNO3 (40 mL, 0.1 m), sodium citrate (600 mL, 0.1 m), and H2O2 (112 mL, 30 %) were mixed in water at room temperature to a final volume of 40 mL. The reaction mixture was vigorously stirred for 10 min. Then, NaBH4 (400 mL, 0.1 m) was rapidly injected into this mixture, and the solution changed gradually from colorless to yellow, red, and finally blue, which indicated the formation of the silver nanoprisms.
Synthesis of the Ag/Au core–shell nanoprisms The as-prepared silver nanoprisms (20 mL) were centrifuged and washed with water twice and redispersed in water (4.5 mL). For vertical growth of the Ag nanoprisms, to this solution was added (0.50 mL, 17.5 mm in repeating vinylpyridine unit) PVP and ascorbic acid (18.7 mL, 0.50 m), and then AgNO3 (0.30 mL, 0.60 mm) was slowly added to the solution under stirring through a syringe pump at a speed of 0.1 mL min1. Afterwards, sodium citrate (150 mL, 0.10 m) and a mixture (1.5 mL) of AgNO3 (0.75 mm) and sodium citrate (1.13 mm) were added sequentially using the syringe pump at a speed of 0.1 mL min1, for lateral growth of the silver nanoprisms. The reaction was allowed to proceed for one additional hour, and the silver nanoprisms obtained were used for gold deposition without further purification. PVP (0.50 mL, 5 wt %), diethylamine (75 mL), and ascorbic acid (0.10 mL, 0.50 m) were was successively added to the above solution. A separate growth solution for Au was prepared by mixing PVP (400 mL, 5 wt. %), KI (80 mL, 0.20 m), and HAuCl4 (20 mL, 0.25 m) in water (3.0 mL). By using the syringe pump, growth solution (0.50–0.70 mL) was slowly added into the solution of Ag nanoprisms at a speed of 0.05 mL min1 to deposit the gold outlayers on the silver nanoprisms. The Ag/Au core–shell nanoprisms were ready for use after they were collected by centrifugation, washed with water, and redispersed in water.
Figure 4. A) The absorption depression at the SPR peak and B) the SPR peak shift of the Ag/Au core–shell nanoprisms after a period of 30 min incubation with different concentrations of hydrogen sulfide in serum. The insert: linear fitting curve from 0.1 to 10 mm.
plasmonic nanoprobes incubated with hydrogen sulfide displayed obviously SPR absorption depression and shifts, clearly demonstrating that the Ag/Au core–shell nanoprisms could be used for the detection of hydrogen sulfide in complicated matrixes. However, the SPR shift was smaller than that observed in pure buffer solution. It was observed that there is a smaller shift in the SPR peak at physiological pH compared with the &
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[7] J. Furne, A. Saeed, M. D. Levitt, Am. J. Physiol. Regul. Integr. Comp. Physiol. 2008, 295, R1479. [8] a) A. R. Lippert, E. J. New, C. J. Chang, J. Am. Chem. Soc. 2011, 133, 10078 – 10080;b) Y. Qian, J. Karpus, O. Kabil, S. Y. Zhang, H. L. Zhu, R. Banerjee, J. Zhao, C. He, Nat. Commun. 2011, 2, 495; c) H. Peng, Y. Cheng, C. Dai, A. L. King, B. L. Predmore, D. J. Lefer, B. Wang, Angew. Chem. Int. Ed. 2011, 50, 9672 – 9675; Angew. Chem. 2011, 123, 9846 – 9849; d) K. Sasakura, K. Hanaoka, N. Shibuya, Y. Mikami, Y. Kimura, T. Komatsu, T. Ueno, T. Terai, H. Kimura, T. Nagano, J. Am. Chem. Soc. 2011, 133, 18003 – 18005; e) S. Chen, Z. J. Chen, W. Ren, H. W. Ai, J. Am. Chem. Soc. 2012, 134, 9589 – 9592; f) J. Liu, K. K. Yee, K. K. Lo, K. Y. Zhang, W. P. To, C. M. Che, Z. J. Xu, J. Am. Chem. Soc. 2014, 136, 2818 – 2824. [9] a) O. Hess, J. B. Pendry, S. A. Maier, R. F. Oulton, J. M. Hamm, K. L. Tsakmakidis, Nat. Mater. 2012, 11, 573 – 584; b) J. N. Anker, W. P. Hall, O. Lyandres, N. C. Shah, J. Zhao, R. P. Van Duyne, Nat. Mater. 2008, 7, 442 – 453; c) R. de la Rica, M. M. Stevens, Nat. Nanotechnol. 2012, 7, 821 – 824; d) L. Rodrguez-Lorenzo, R. de La Rica, R. A. Alvarez-Puebla, L. M. LizMarzn, M. M. Stevens, Nat. Mater. 2012, 11, 604 – 607; e) H. J. Wu, J. Henzie, W. C. Lin, C. Rhodes, Z. Li, E. Sartorel, J. Thorner, P. Yang, J. T. Groves, Nat. Methods 2012, 9, 1189 – 1191. [10] P. K. Jain, X. Huang, E. I. H. l-Sayed, M. A. El-Sayed, Acc. Chem. Res. 2008, 41, 1578 – 1586. [11] B. Xiong, R. Zhou, J. Hao, Y. Jia, Y. He, E. S. Yeung, Nat. Commun. 2013, 4, 1708. [12] X. J. Yang, Y. B. Yu, Z. Q. Gao, ACS Nano 2014, 27, 4902 – 4907. [13] a) C. Gao, Z. Lu, Y. Liu, Q. Zhang, M. Chi, Q. Cheng, Y. Yin, Angew. Chem. Int. Ed. 2012, 51, 5629 – 5633; Angew. Chem. 2012, 124, 5727 – 5731; b) B. Malile, J. I. Chen, J. Am. Chem. Soc. 2013, 135, 16042 – 16045. [14] D. Seo, C. Yoo, J. Jung, H. Song, J. Am. Chem. Soc. 2008, 130, 2940 – 2941. [15] M. M. Shahjamali, M. Bosman, S. Cao, X. Huang, X. Cao, H. Zhang, S. S. Pramana, C. Xue, Small 2013, 9, 2880 – 2886. [16] Y. Xiong, Y. Xia, Adv. Mater. 2007, 19, 3385 – 3391. [17] J. M. Bennett, J. L. Stanford, E. J. Ashley, J. Opt. Soc. Am. 1970, 60, 224 – 231. [18] a) B. Liu, Z. F. Ma, Small 2011, 7, 1587 – 1592; b) J. Zeng, J. Tao, D. Su, Y. M. Zhu, D. Qin, Y. N. Xia, Nano Lett. 2011, 11, 3010 – 3015. [19] C. A. Burnyeat, R. S. Lepsenyi, I. O. Nwabuko, T. L. Kelly, Chem. Mater. 2013, 25, 4206 – 4214. [20] G. C. van de Bittner, E. A. Dubikovskaya, C. R. Bertozzi, C. J. Chang, Proc. Natl. Acad. Sci. USA 2010, 107, 21316 – 213621. [21] P. Wu, J. Y. Zhang, S. L. Wang, A. R. Zhu, X. D. Hou, Chem. Eur. J. 2013, 19, 952 – 956. [22] X. G. Shen, C. B. Pattillo, S. Pardue, S. C. Bir, R. Wang, C. G. Kevil, Free Radical Biol. Med. 2011, 50, 1021 – 1031.
Before hydrogen sulfide detection, the as-prepared Ag/Au core– shell nanoprisms (1 mL) were centrifuged once and redispersed in sodium citrate buffer (4 mL, pH 6.0, 10 mm). Then different concentrations of Na2S were added into 100 mL of the nanoprisms solution for 30 min. UV/Vis spectrophotometry was used to monitor the response of the nanoprisms toward hydrogen sulfide. After being incubated for 30 min, the absorption spectrum was recorded. As for the detection of hydrogen sulfide in serum, 1 mL the as-prepared Ag/Au core–shell nanoprisms were centrifuged and redispersed in 1 mL water. Aliquots of serum samples (75 mL) were first spiked with different concentrations of hydrogen and aliquots of the nanoprisms (25 mL) solution were added, respectively. After being incubated for 30 min, the absorption spectra were collected.
Acknowledgements This work was supported by the ASTAR-ANR program. Keywords: biosensors nanostructures · SPR
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[1] a) C. Szab, Nat. Rev. Drug Discovery 2007, 6, 917; b) O. Kabil, R. Banerjee, J. Biol. Chem. 2010, 285, 21903; c) E. Culotta, D. E. Koshland, Science 1992, 258, 1862 – 1865; d) T. Morita, M. A. Perrella, M. E. Lee, S. Kourembanas, Proc. Natl. Acad. Sci. USA 1995, 92, 1475. [2] a) S. Singh, D. Padovani, R. A. Leslie, T. Chiku, J. Biol. Chem. 2009, 284, 22457; b) T. Chiku, D. Padovani, W. Zhu, S. Singh, V. Vitvitsky, R. Banerjee, J. Biol. Chem. 2009, 284, 11601; c) N. Shibuya, M. Tanaka, M. Yoshida, Y. Ogasawara, T. Togawa, K. Ishii, H. Kimura, Antioxid. Redox Signaling 2009, 11, 703. [3] a) G. D. Yang, L. Y. Wu, B. Jiang, W. Yang, J. S. Qi, K. Cao, Q. H. Meng, A. K. Mustafa, W. T. Mu, S. M. Zhang, S. H. Snyder, R. Wang, Science 2008, 322, 587; b) R. Wang, Antioxid. Redox Signaling 2010, 12, 1061. [4] a) C. Szabo, Antioxid. Redox Signaling 2012, 17, 68 – 80; b) K. Eto, T. Asada, K. Arima, T. Makifuchi, H. Kimura, Biochem. Biophys. Res. Commun. 2002, 293, 1485; c) W. J. Cai, M. J. Wang, L. H. Ju, C. Wang, Y. C. Zhu, Cell Biol. Int. 2010, 34, 565 – 572; d) Q. H. Cao, L. Zhang, G. D. Yang, C. Q. Xu, R. Wang, Antioxid. Redox Signaling 2010, 12, 1101 – 1109. [5] N. S. Lawrence, J. Davis, L. Jiang, T. G. J. Jones, S. N. Davies, R. G. Compton, Electroanalysis 2000, 12, 1453 – 1460. [6] a) E. Fischer, Ber. Dtsch. Chem. Ges. 1883, 16, 2234 – 2236; b) M. M. F. Choi, Analyst 1998, 123, 1631 – 1634.
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COMMUNICATION & Nanostructures
Plasmonic nanoprobes: Plasmonic Ag/ Au core–shell nanoprism nanoprobes are demonstrated for the detection of hydrogen sulfide based on the strong ability of hydrogen sulfide to etch the Ag/Au core–shell nanoprisms. In the presence of hydrogen sulfide, silver in the nanoprisms is converted to Ag2S at the defective lateral walls, accompanied by an obvious surface plasmon resonance (SPR) absorption depression and peak shift (see figure).
X. Yang, Y. Ren, Z. Gao* && – && Silver/Gold Core–Shell NanoprismBased Plasmonic Nanoprobes for Highly Sensitive and Selective Detection of Hydrogen Sulfide
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Silver/Gold Core–Shell Nanoprism-Based Plasmonic Nanoprobes for Highly Sensitive and Selective Detection of Hydrogen Sulfide Xinjian Yang, Yuqian Ren, and Zhiqiang Gao*[a] throughput, which make them unfavorable in monitoring the continuous fluctuations of hydrogen sulfide in biological systems because of their rapid metabolism. Recently, many organic fluorescent agents have been exploited for the detection and imaging of hydrogen sulfide.[8] However, some drawbacks of most of these organic fluorescent agents are unavoidable, such as difficulty in separating unreacted agents, easy photobleaching, poor photostability, and lack of sufficient penetration depth when imaging biological tissues. Therefore, the design of selective and efficient probes for hydrogen sulfide has been one of the grand challenges. Nanotechnology provides an exciting avenue for rapid and accurate detection of analytes with high sensitivity and selectivity. Particularly, noble-metal-based nanomaterials, which display the surface plasmon resonance (SPR) absorption in the near IR (NIR) region with ultrahigh extinction coefficients, offer exciting opportunities in the construction of chemical sensing and biosensing platforms and assays with unprecedented functionalities for highly sensitive and selective detection of a wide range of analytes.[9] The distinctive spectral sensitivity of SPR to nanostructure morphology can provide the basis for versatile biological labeling, sensing, and detection platforms if changes in the nanostructure’s shape or size can be induced upon interacting with an analyte.[10] Recently, a plamonic probe was developed for sulfide mapping in living cells using gold/silver core–shell nanoparticles.[11] Though promising, this nanoprobe showed low selectivity toward hydrogen sulfide because some species can also etch the silver on the outer surface of the gold/silver core–shell nanoprobes, resulting in false signals. Therefore, it remains a challenge to develop new plasmonic probes with high sensitivity and selectivity. Herein, we present a new plamonic assay for highly sensitive and selective detection of hydrogen sulfide by using goldcoated silver nanoprisms (Ag/Au core–shell nanoprisms) as plasmonic nanoprobes. Our previous work[12] demonstrated that bare silver nanoprisms are sensitive to some species in solution, such as hydrogen peroxide, resulting in a blueshift of their SPR peak. However, these silver nanoprisms are not suitable for direct detection of analytes in complex matrixes since a separation step before target analysis is required. To address this problem, we coated the silver nanoprisms with a thin gold layer that protects the silver nanoprisms from direct reaction with many species in solution. In the presence of strong etching agents, such as hydrogen sulfide, the silver in the Ag/Au core–shell nanoprisms will be etched through the defects of the gold outlayer and converted into Ag2S. As schematically depicted in Scheme 1, the plasmonic sensing strategy is based
Abstract: A simple and highly sensitive and selective hydrogen sulfide assay utilizing plasmonic nanoprobes is presented in this report. The assay employs the etching of silver in the Ag/Au core–shell nanoprisms, accompanied by surface plasmon resonance (SPR) signal depression and shift. Briefly, thin layers of gold are first coated onto silver nanoprisms. The thin gold layer not only guarantees the high stability of the plasmonic nanoprobes but also ensures the high selectivity toward hydrogen sulfide. Once hydrogen sulfide is introduced, the silver core is converted to Ag2S mainly from its lateral walls. Moreover, the SPR peak is located in the NIR region that makes these plasmonic nanoprobes more appealing for the detection of hydrogen sulfide in real-world samples and in in vivo applications.
Hydrogen sulfide with the characteristic odor of rotten eggs, historically known as a toxic gas, has been recognized as a member of signaling molecule family along with nitric oxide (NO) and carbon monoxide (CO).[1] The production of endogenous hydrogen sulfide is related to at least three separate enzymes within different tissues and organs: cystathionine b-synthase, cystathionine g-lyase, and 3-mercaptopyruvate sulfur transferase.[2] Increasing evidence has shown that hydrogen sulfide plays vital roles in regulating intracellular redox status and other fundamental signaling processes involved in human health and disease, such as modulation of blood pressure, mediation of neurotransmission, and regulation of inflammation.[3] The abnormal levels of hydrogen sulfide have been found to be closely associated with diabetes, Alzheimer’s disease, and various types of cancer.[4] Therefore, rapid and selective detection of hydrogen sulfide is of paramount importance in understanding its physiological and pathological functions. One significant limiting factor in studying hydrogen sulfide is the lack of highly sensitive assays and sensors for its accurate detection. Classic methods using electrochemical methods,[5] colorimetry,[6] and chromatography[7] have been developed. In most cases, these methods require sophisticated instrumentation, involve cumbersome laboratory procedures, and have low [a] Dr. X. Yang, Y. Ren, Dr. Z. Gao Department of Chemistry, National University of Singapore Singapore 117543 (Singapore) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201405012. Chem. Eur. J. 2014, 20, 1 – 6
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Communication observed that the absorbance maxima decreases and the SPR peak shifts to long wavelength gradually with the increase of the incubation time and both of them level off within 30 min. It is well known that sulfide ions selectively and quickly react with Ag atoms to generate Ag2S in the presence of oxygen. 4 Ag þ 2 S2- þ O2 þ 2 H2 O ! 2 Ag2 S þ 4 OHScheme 1. Schematic illustration of the hydrogen sulfide sensing mechanism based on the etching of the Ag/Au core–shell nanoprisms.
Since the mechanism plays an important role in understanding and performing the detection of hydrogen sulfide, TEM experiments were conducted to gain insight into the mechanism of the interaction between the Ag/Au core–shell nanoprisms and hydrogen sulfide. Previous work has indicated that Ag is readily converted to Ag2S by reacting with sulfide.[14] After coating a thin gold layer on the silver nanoprisms, sulfide etching process could only occur through the defects of the Ag/Au core–shell nanostructure. As shown in the EDX mapping images (Figure S1, Supporting Information), sulfur element exists all over the nanoprisms after the incubation with sulfide. Meanwhile, because the formed Ag2S layer is very thin (3– 4 nm), no obvious difference of sulfur element distribution between edges and the plane was observed. EDX results also showed the decrease of the percentage of silver in the complex after the incubation with sulfide. It is thought that the Ag oxidation by oxygen, which is very low, first occurs at the defects of the thin Au layer. After the introduction of H2S, the formation of highly stable Ag2S, which has an extremely low dissociation constant (6.3 1050), drives further oxidation of Ag. However, as shown in Figure 1E, the TEM image indicated that sulfuration reaction mainly occurs at the lateral faces of the nanoprisms after the incubation with hydrogen sulfide. The shielding effect of the gold layer decreased its potential to rapid react with H2S. Previous work showed that each Ag nanoprism contains two main (111) facets on the triangular planes and three (110) facets on the edges.[15] Since the relative surface energies of different crystal facets of silver are in the order of g111 < g100 < g110,[16] the lateral walls would be etched easier. Furthermore, we found that some of the cores shrink from the lateral walls after the incubation with H2S (Figure S2, Supporting Information). With the ratio of Ag2S in the nanoprisms increasing, the SPR peak of the nanoprisms experienced a continuous depression and a redshift while the shortwavelength absorption derived from the Ag2S region became more prominent probably due to the high refractive index of Ag2S.[17] Electron diffraction spectroscopic (EDS) tests (Figure S3, Supporting Information) further demonstrate that the sulfuration reaction occurs on the surface of the nanoprisms. Meanwhile, the gold layer plays an important role in the transformation of Ag into Ag2S. In control experiments, we found that bare silver nanoprisms undergo different SPR peak shifts at different hydrogen sulfide concentrations (Figure S4, Supporting Information). A blue SPR peak shift appeared in the presence of hydrogen sulfide (40 mm) while a redshift occurred when hydrogen sulfide (200 mm) was used. This is because of the fact that the transformation process prefers to initiate at the corner of the nanoprisms and the shape of the nanoprisms could be retained when enough hydrogen sulfide is endow-
Figure 1. A) Absorption spectra of the Ag/Au core–shell nanoprisms collected during the incubation with 200 mm hydrogen sulfide; B) the absorption change of the SPR peak maxima and C) the SPR peak shift during the incubation with 200 mm hydrogen sulfide; TEM images D) before and E) after a period of 30 min incubation with 200 mm hydrogen sulfide.
on hydrogen sulfide etching derived shape change of the Ag/ Au core–shell nanoprisms and their corresponding surface plasmon resonance signal change. The Ag/Au core–shell nanoprisms with a high extinction coefficient and excellent stability were synthesized by following a published procedure.[13] Figure 1D shows a typical TEM image of the nanoprisms. The original triangular shapes are well retained, as expected by the uniform deposition of Au atoms on the plate surfaces. High-resolution TEM imaging demonstrates that a thin layer of Au is coated on the Ag nanoprism surface. We subsequently tested the feasibility of this plasmonic nanoprobe for hydrogen sulfide sensing. The absorption spectra, which are associated with the altering of shape and surface conditions of the nanoprisms, were studied as a function of reaction time. As shown in Figure 1A–C, it was &
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Communication ed.[18] Moreover, we incubated different interfering agents with silver nanoprisms. Our results suggested that bare silver nanoprisms are not suitable for hydrogen sulfide sensing (Figure S5, Supporting Information). The thickness of the gold layer is the key factor for the success of the assay. The selectivity will be lost when the gold coating on the silver nanoprisms is too thin, whereas the sensitivity is sacrificed when the gold layer is too thick (Figure S6, Supporting Information). We found that the thickness of gold layer can be facilely manipulated by simply adjusting the amount of chlorauric acid in the preparation of the Ag/Au core–shell nanoprisms. Next, we measured the hydrogen sulfide concentration-dependent spectra change. Figure 2A displays the results after in-
Figure 2. A) Spectral changes of the Ag/Au core–shell nanoprisms in the presence of different concentrations of hydrogen sulfide after a period of 30 min incubation; B) absorption depression at the SPR peak maxima and C) the SPR peak shift derived from (A).
Figure 3. Effect of possible interfering agents on A) the SPR absorption maxima and B) the SPR peak shift of the Ag/Au core–shell nanoprisms after a period of 30 min incubation.
cubating 0.1, 0.5, 1, 2.5, 5, 10, 25, 50, and 100 mm hydrogen sulfide with the Ag/Au core–shell nanoprisms for 30 min, respectively. From the corresponding absorption changes of the SPR maxima (Figure 2B), a linear relationship between the SPR absorption change and the hydrogen sulfide concentration from 0.1 to 10 mm was obtained. The absorption change and SPR shift leveled off when more hydrogen sulfide was introduced because of the strong protective effect of the gold shell and the formed Ag2S layer (Figure 2B, C). These results clearly demonstrated the ability of Ag to Ag2S transformation on the Ag/Au core–shell nanoprisms as an attractive strategy for hydrogen sulfide sensing with high sensitivity. To evaluate the specific nature of the Ag/Au core–shell nanoprism etching by hydrogen sulfide, we then examined possible SPR absorption peak changes of the nanoprisms when incubated with other species, most of which could work as etchants for silver in other systems.[19] As shown in Figure 3, the nanoprisms exhibited very limited change in their extinction and SPR peak shift to various halogen ions, even at a concentration as high as 0.10 m of chloride. The high selectivity toward hydrogen sulfide is due to the protection effects of gold layer and low dissociation constant comparing with other ions. Moreover, negligible changes were observed toward other common anions. Next, we investigated the reactivity of the nanoprisms toward hydrogen peroxide. Although hydroChem. Eur. J. 2014, 20, 1 – 6
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gen sulfide and hydrogen peroxide cannot coexist in one system, hydrogen peroxide produced by some organs in the living cells might generate a false signal because of its strong etching ability toward silver.[20] Again, because of the strong protective effect of the gold shell to the inner silver nanoprism cores, no obvious SPR signal changes were observed in the presence of hydrogen peroxide (200 mm). In a control experiment, we found that there was a redshift of the SPR peak when Ag-coated gold nanorods were incubated in 200 mm hydrogen peroxide solution (Figure S7, Supporting Information), thereby suggesting the superiority of the Ag/Au core–shell nanoprisms in hydrogen sulfide sensing. More importantly, it was observed that biothiols elicit only a slight change in the extinction and practically no SPR peak shifts of the nanoprisms. Because of the strong binding effect of thiols to Au, they tend to protect the nanoparticles from destruction. These results suggest that only hydrogen sulfide can produce the significant absorption decrease and SPR shift and no noticeable changes in the SPR absorption of the Ag/Au core–shell nanoprisms were observed when incubated with other competitive species. After the confirmation of the sensitivity and selectivity of this plasmonic nanoprobe for hydrogen sulfide in pure buffer 3
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Communication pH we used (Figure S10, Supporting Information). Another reason might be the significant scavenging effect toward hydrogen sulfide by proteins in serum.[22] As depicted in Figure 4, a linear relationship between dAbs and the concentration of H2S was obtained between 0.1 and 10 mm with a detection limit of 54 nm (R2 = 0.967). Meanwhile, by using the calibration curve between dl and the concentration of H2S (R2 = 0.992), the detection limit was estimated as low as 43 nm. In summary, we have presented in this work new plasmonic Ag/Au core–shell nanoprism nanoprobes for hydrogen sulfide based on its strong ability to etch the Ag/Au core–shell nanoprisms. In the presence of hydrogen sulfide, silver in the nanoprisms was converted to Ag2S mainly at the defective lateral walls, accompanied by obvious SPR absorption depression and peak shift. The thin gold layer not only guarantees the high stability of the plasmonic nanoprobes but also ensures the high selectivity toward hydrogen sulfide. Compared with most of the reported hydrogen sulfide probes, complicated synthesis steps and expensive equipment are eliminated in our assay. Moreover, the SPR peak is located in the NIR region that makes these plasmonic nanoprobes more appealing for the detection of hydrogen sulfide in real-world samples and in vivo applications.
solution, we further explored its applications in the detection of hydrogen sulfide in complex matrixes. It has been reported that the typical concentration of hydrogen sulfide in blood is 10–100 mm.[21] Accurate detection of the concentration of hydrogen sulfide in serum is the key to early diagnosis of many diseases in which hydrogen sulfide plays an important role. Many components in serum, which have significant absorptions in the UV/Vis region, limit the use of most hydrogen sulfide probes. In the proposed assay, the SPR peak locates in the NIR region, which is much more desirable for the detection of hydrogen sulfide in serum. In addition, in the absence of hydrogen sulfide no obvious SPR absorption depression or peak shifts were observed (Figure S8, Supporting Information), thereby indicating that the Ag/Au core–shell nanoprisms are stable in serum. Consequently, we studied the possibility of using these plasmonic nanoprobes to detect hydrogen sulfide in serum (Figure 4, Figure S9, Supporting Information). The
Experimental Section Synthesis of the Ag nanoprisms Silver nanoprisms were synthesized by following a procedure developed in a previous report.[12] Briefly, AgNO3 (40 mL, 0.1 m), sodium citrate (600 mL, 0.1 m), and H2O2 (112 mL, 30 %) were mixed in water at room temperature to a final volume of 40 mL. The reaction mixture was vigorously stirred for 10 min. Then, NaBH4 (400 mL, 0.1 m) was rapidly injected into this mixture, and the solution changed gradually from colorless to yellow, red, and finally blue, which indicated the formation of the silver nanoprisms.
Synthesis of the Ag/Au core–shell nanoprisms The as-prepared silver nanoprisms (20 mL) were centrifuged and washed with water twice and redispersed in water (4.5 mL). For vertical growth of the Ag nanoprisms, to this solution was added (0.50 mL, 17.5 mm in repeating vinylpyridine unit) PVP and ascorbic acid (18.7 mL, 0.50 m), and then AgNO3 (0.30 mL, 0.60 mm) was slowly added to the solution under stirring through a syringe pump at a speed of 0.1 mL min1. Afterwards, sodium citrate (150 mL, 0.10 m) and a mixture (1.5 mL) of AgNO3 (0.75 mm) and sodium citrate (1.13 mm) were added sequentially using the syringe pump at a speed of 0.1 mL min1, for lateral growth of the silver nanoprisms. The reaction was allowed to proceed for one additional hour, and the silver nanoprisms obtained were used for gold deposition without further purification. PVP (0.50 mL, 5 wt %), diethylamine (75 mL), and ascorbic acid (0.10 mL, 0.50 m) were was successively added to the above solution. A separate growth solution for Au was prepared by mixing PVP (400 mL, 5 wt. %), KI (80 mL, 0.20 m), and HAuCl4 (20 mL, 0.25 m) in water (3.0 mL). By using the syringe pump, growth solution (0.50–0.70 mL) was slowly added into the solution of Ag nanoprisms at a speed of 0.05 mL min1 to deposit the gold outlayers on the silver nanoprisms. The Ag/Au core–shell nanoprisms were ready for use after they were collected by centrifugation, washed with water, and redispersed in water.
Figure 4. A) The absorption depression at the SPR peak and B) the SPR peak shift of the Ag/Au core–shell nanoprisms after a period of 30 min incubation with different concentrations of hydrogen sulfide in serum. The insert: linear fitting curve from 0.1 to 10 mm.
plasmonic nanoprobes incubated with hydrogen sulfide displayed obviously SPR absorption depression and shifts, clearly demonstrating that the Ag/Au core–shell nanoprisms could be used for the detection of hydrogen sulfide in complicated matrixes. However, the SPR shift was smaller than that observed in pure buffer solution. It was observed that there is a smaller shift in the SPR peak at physiological pH compared with the &
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Before hydrogen sulfide detection, the as-prepared Ag/Au core– shell nanoprisms (1 mL) were centrifuged once and redispersed in sodium citrate buffer (4 mL, pH 6.0, 10 mm). Then different concentrations of Na2S were added into 100 mL of the nanoprisms solution for 30 min. UV/Vis spectrophotometry was used to monitor the response of the nanoprisms toward hydrogen sulfide. After being incubated for 30 min, the absorption spectrum was recorded. As for the detection of hydrogen sulfide in serum, 1 mL the as-prepared Ag/Au core–shell nanoprisms were centrifuged and redispersed in 1 mL water. Aliquots of serum samples (75 mL) were first spiked with different concentrations of hydrogen and aliquots of the nanoprisms (25 mL) solution were added, respectively. After being incubated for 30 min, the absorption spectra were collected.
Acknowledgements This work was supported by the ASTAR-ANR program. Keywords: biosensors nanostructures · SPR
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COMMUNICATION & Nanostructures
Plasmonic nanoprobes: Plasmonic Ag/ Au core–shell nanoprism nanoprobes are demonstrated for the detection of hydrogen sulfide based on the strong ability of hydrogen sulfide to etch the Ag/Au core–shell nanoprisms. In the presence of hydrogen sulfide, silver in the nanoprisms is converted to Ag2S at the defective lateral walls, accompanied by an obvious surface plasmon resonance (SPR) absorption depression and peak shift (see figure).
X. Yang, Y. Ren, Z. Gao* && – && Silver/Gold Core–Shell NanoprismBased Plasmonic Nanoprobes for Highly Sensitive and Selective Detection of Hydrogen Sulfide
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