- Email: [email protected]
CdS quantum dots
Sensors and Actuators B 143 (2010) 535–538
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Detecting hydrogen sulfide by using transparent polymer with embedded CdSe/CdS quantum dots Huan Xu a , Ji’en Wu b , Chih-Hsin Chen a , Lianhui Zhang b , Kun-Lin Yang a,∗ a b
Department of Chemical and Bimolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore Institute of Molecular Cell and Biology, 61 Biopolis Drive, Proteos, Singapore 138673, Singapore
a r t i c l e
i n f o
Article history: Received 29 May 2009 Received in revised form 19 August 2009 Accepted 28 September 2009 Available online 6 October 2009 Keywords: Fluorescent sensor CdSe/CdS quantum dots Polymer Hydrogen sulfide Poly(dimethylsiloxane)
a b s t r a c t Hydrogen sulfide (H2 S) is a hazardous vapor commonly found in oil fields and industrial processes. In this paper, we first reported the fluorescence quenching of CdSe/CdS quantum dots (QDs) by hydrogen sulfide in dichloromethane. This response is highly specific because no response to other volatile organic vapors can be observed. Second, we embedded QDs in films of poly(dimethylsiloxane) (PDMS), a transparent and gas-permeable polymer, for detecting low level of hydrogen sulfide. After exposing the film to 10 ppm of hydrogen sulfide, the film loses its fluorescence within 1 min. However, the fluorescence recovers completely after the film is left in the open air for 2 h. Compared with other types of hydrogen sulfide sensors, the film sensor with embedded QDs is more portable, easy-to-operate and has low-cost. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen sulfide (H2 S) is a well-known poisonous gas, which is often generated as a by-product in many industrial processes. Inhalation of 500–1000 ppm of H2 S for 30 min can lead to loss of consciousness or even death because of respiratory paralysis or asphyxiation [1]. According to National Institute for Occupational Safety and Health (NIOSH), the H2 S concentration immediately dangerous to life or health (IDLH) is 100 ppm, and the recommended exposure limit (REL) is 10 ppm for a maximum duration of 10 min. Although human noses are extremely sensitive to the presence of H2 S in the environments, it cannot serve as a long-term H2 S detectors because of olfactory fatigue [2]. Furthermore, if the concentration of H2 S is over 100 ppm, one’s ability to smell H2 S is severely compromised because of temporary paralysis of olfactory nerves. Therefore, many methods and sensors have been developed to detect the presence of H2 S [3–8]. However, they often require trained personnels and complex instrumentation, which limit their applications in the fields. Highly fluorescent core/shell CdSe/CdS quantum dots (QDs) are ideal candidates for sensing applications because of their high quantum yield, photostability, extremely large surface-to-atom ratio and sensitivity to surface ligands [9–11]. However, fluorescent QDs only have been applied to detect the presence of
∗ Corresponding author. Tel.: +65 6516 6614; fax: +65 6779 1936. E-mail address: [email protected] (K.-L. Yang). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.09.049
Escherichia coli, nucleic acid, DNA, protein and copper ions in aqueous solution in previous studies [12–16]. Although some studies have shown that the fluorescence of CdSe QDs can be quenched by thiols in the aqueous solution [4,17–19], core/shell CdSe/CdS QDs have never been used for detecting H2 S before. Furthermore, most past studies about QDs were focused on the fluorescence quenching phenomenon in aqueous solutions or organic solvents which are not practical for gas sensing applications. To address this issue, we developed a polymeric film with embedded CdSe/CdS QDs to detect the presence of H2 S in the atmosphere. Because QDs embedded in the polymer have higher stability compared to QDs dispersed in solutions, this type of film sensor may also offer a solution to the stability issue encountered in many QD-based sensors. 2. Experimental 2.1. Materials Cadmium oxide (≥99%), stearic acid (95%), octadecylamine (ODA, 90%), trioctylphosphine oxide (TOPO, 99%), trioctylphosphine (TOP, 90%), selenium powder (99.999%) and sulfur powder (99.998%) were purchased from Sigma–Aldrich (Singapore). H2 S (100 ppm) was purchased from National Oxygen (Singapore). Slygard 184 silicone elastomer kit for making poly(dimethylsiloxane) (PDMS) polymer was purchased from Dow Corning (U.S.A.). All solvents used in this study were HPLC grades. Water was purified by using a Milli-Q system (Millipore).
536
H. Xu et al. / Sensors and Actuators B 143 (2010) 535–538
Fig. 1. Fluorescent images of CdSe/CdS QDs in dichloromethane under the excitation of 365 nm UV. To study the fluorescence quenching phenomenon. In (B–F), the dichloromethane is also mixed with (B) 100 ppm H2 S, (C) 10 L of water, (D) 2 L of hexane, (E) 2 L of 1-butylamine and (F) 2 L of 1-heptanol.
2.2. Synthesis of CdSe/CdS QDs Highly fluorescent CdSe/CdS QDs were synthesized following reported procedures with slight modifications [9,10,20–22]. In a Schlenk flask, mixture of 0.2 mmol of CdO and 0.8 mmol of stearic acid were heated to 150 ◦ C until they become a colorless solution. After the solution was cooled to room temperature, ODA (1.5 g) and TOPO (0.5 g) were added to the solution. This solution was then reheated to 200 ◦ C under nitrogen protection. Subsequently, a selenium solution (made by dissolving 0.2 mmol of Se in 2.0 g of TOP) was injected quickly. The reaction is continued until yellow green fluorescence can be observed under an UV lamp. To grow the shell structure, we first mixed 0.42 mol of sulfur, 1.0 g
Fig. 3. Detection limit for H2 S. CdSe/CdS QDs in dichloromethane was mixed with (A) 0 mL, (B) 1 mL, (C) 2 mL, (D) 3 mL and (E) 4 mL of 100 ppm H2 S vapor. The detection limit is approximately 2 mL of 100 ppm H2 S.
TOP and 1.0 g TOPO together and heated the mixture to 150 ◦ C to get a clear solution (solution 1). Next, 0.2 mmol of Se and 2.0 g of TOP were mixed together and heated to 150 ◦ C to get another clear solution (solution 2). Solutions 1 and 2 were injected into the reaction mixture mentioned above, and the growth of CdSe/CdS QDs was monitored continuously by checking the fluorescence of the solution. The heater was removed when the solution appears orange color. The solution was then cooled to room temperature and chloroform was added to the solution. The solution was kept in a fridge at 4 ◦ C. Before use, the stock solution was diluted by using dichloromethane to a desired concentration. The unit of concentration reported herein is optical density of a solution at 570 nm (OD570 ), which was determined by using a UV–vis spectrometer. 2.3. Preparation of PDMS films Films of PDMS were prepared according to a standard protocol. PDMS prepolymer and curing agent were mixed together in a 10:1 ratio and cured at 50 ◦ C for 12 h. To incoporate QDs in PDMS, films of PDMS were immersed in dichloromethane containing CdSe/CdS QDs (OD570 = 0.3) for 2 h at 4 ◦ C. Then, the films were withdrawn from the solution, rinsed with acetone and blown dried. 2.4. Characterization of QDs solution All UV–vis spectra were obtained by using a UV/vis spectrometer (Cary 50, Varian, Australia). In all experiments, standard quartz cuvettes were used. Fluorescence spectra were obtained by using a luminescence spectrometer (LS 50B, PerkinElmer, U.S.A.). 3. Result and discussion The CdSe/CdS QDs used in this study were synthesized following a revised literature procedure [9,10,20–22]. Fig. 1A shows that
Fig. 2. Time-series (A) UV–vis spectra and (B) fluorescent spectra of CdSe/CdS QDs in dichloromethane after the injection of 100 ppm H2 S. Both spectra show that the fluorescence of CdSe/CdS QDs can be quenched within 15 s by H2 S.
Fig. 4. Fluorescent images of PDMS films with embedded CdSe/CdS QDs and their responses to (A) air, (B) 10 ppm H2 S, (C) water, (D) hexane, (E) 1-butylamine, and (F) 1-heptanol.
H. Xu et al. / Sensors and Actuators B 143 (2010) 535–538
537
Fig. 5. Reversibility of fluorescent response to H2 S. The sample was exposed to 10 ppm of H2 S for 1 min and air for 2 h alternatively.
our CdSe/CdS QDs in dichloromethane (OD570 = ∼0.03) give strong fluorescence at 590 nm when they are excited at 365 nm, similar to the results reported earlier [10]. To test whether H2 S can quench the fluorescence, 100 ppm H2 S vapor was injected into the solution. After 10 s, the solution turns to white (Fig. 1B), which suggests that H2 S can quench the orange fluorescence of the CdSe/CdS QDs. For comparison, we also injected 10 L of water, 2 L of hexane, 2 L of 1-butylamine and 2 L of 1-heptanol into the QD solution, but they do not quench the fluorescence of QDs (Fig. 1C–F). This result indicates that the fluorescence quenching phenomenon is highly specific. We attributed the quenching effect to the displacements of surface ligands by H2 S. Previous studies have shown that alkylthiols can rapidly displace weakly bound amines from the surfaces of CdSe/CdS QDs, leading to fluorescence quenching [4,17–19]. Although H2 S is not an alkylthiol, it also has a sulfur atom and can act as a Lewis acid. Therefore, it is able to substitute the weakly bound ligands such as TOPO or ODA and quenches the fluorescence. To better understand the kinetics of fluorescence quenching after the injection of 100 ppm H2 S vapor, time-series UV–vis spectra (Fig. 2A) and fluorescent spectra (Fig. 2B) of QDs in dichloromethane were recorded. Fig. 2A shows that the broad absorption peak between 240 and 540 nm increases with time after the injection of H2 S for 15 s. However, the spectra do not change further after 15 s. Meanwhile, the emission peak at 590 nm decreases with time for the first 15 s while a new emission peak emerges between 400 and 550 nm as shown in Fig. 2B. The change in the fluorescence spectra is consistent with the different optical appearances of the CdSe/CdS QDs before and after the injection of H2 S as shown in Fig. 1. Similar to the UV–vis spectra, the fluorescence spectra do not change further after 15 s. The fast fluorescence quenching phenomenon associated with the CdSe/CdS QDs makes them suitable for the detection of H2 S in the ppm range. Next, to determine the detection limit, different volumes of H2 S were injected into 1 mL of dichloromethane containing CdSe/CdS QDs (OD570 = ∼0.03). Fig. 3C shows that 2 mL of 100 ppm H2 S can partially quench the fluorescence and 4 mL of 100 ppm H2 S can quench the orange fluorescence completely (Fig. 3E). Thus, our result shows that the detection limit is approximately 2 mL of 100 ppm H2 S. Results shown above demonstrate that the fluorescence of CdSe/CdS QDs in dichloromethane can be quenched by H2 S. To avoid the use of organic solvent and prepare a solvent-free H2 S sensor, CdSe/CdS QDs were embedded in PDMS, because this polymer is transparent and is permeable to gases [23–25]. To test the response of PDMS films with embedded QDs to H2 S vapor, these films were exposed to 10 ppm of H2 S, saturated water, hexane, 1-butylamine and 1-heptanol vapors, respectively. Fig. 4B shows that the film of PDMS with embedded QDs loses its fluorescence after the exposure to 10 ppm H2 S for 1 min. In contrast, other films exposed to water, hexane and 1-heptanol vapors for 2 h do not show any decrease in fluorescence (Fig. 4C, D and F). The only exception is the one exposed to saturated 1-butylamine vapor. After 2 h of exposure, the film gives greenish fluorescence as shown in Fig. 4E. Thus, we can conclude that vaporous amines are potential inter-
ference for the H2 S sensor reported here. This is expected because 1-butylamine is a good ligand; it may replace ODA or TOPO from the surfaces of QDs and shift the emission peak of the fluorescence [26]. To determine the reversibility of the response of the PDMS films with embedded QDs, we exposed a film to 10 ppm of H2 S and air alternatively. Fig. 5 shows that after the exposure to H2 S, the fluorescence is quenched after 1 min in the presence of H2 S, but the fluorescence completely recovers to its original intensity after 2 h in the air. The result demonstrated that the adsorption of H2 S on the surface of QDs is fully reversible. In fact, the experiment can be repeated several times without losing any fluorescence intensity as shown in Fig. 5. The observation suggests that the QDs embedded in the PDMS film can absorb H2 S vapor reversibly. We note that this is different from QDs in dichloromethane, which cannot regain their fluorescence even after purging with nitrogen for 1 min. The difference between these two configurations is that the H2 S has high solubility in the dichloromethane (0.53 M) such that the purging process cannot remove H2 S from dichloromethane easily [27]. However, H2 S has low solubility in the PDMS film and it can absorb by PDMS reversibly depending on the H2 S concentration in the surrounding environments [28,29]. Therefore, when the PDMS film is exposed to air again, H2 S escapes from the PDMS film, and original ligands recombine with the QDs and fluorescence is regenerated. 4. Conclusion In conclusion, CdSe/CdS QDs have been successfully exploited for the detection of H2 S vapor through a fluorescence quenching mechanism. Two different configurations have been demonstrated. The first one is dichloromethane with dispersed QDs. In this system, 100 ppm H2 S vapor is able to quench the fluorescence of QDs within 15 s, and the detection limit is approximately 2 mL of 100 ppm H2 S. The second system is a thin PDMS film with embedded QDs. Our results show that 10 ppm of H2 S vapor can quench the fluorescence of QDs embedded in films of PDMS within 1 min. The film does not show any response to water, hexane, 1-heptanol, but shows some response to 1-butylamine. However, only QDs embedded in films of PDMS show fully reversible response to H2 S vapor. Films of PDMS with embedded QDs offer many advantages such as high portability and low cost over traditional gas sensors. These polymers can also be coated on safety equipments such as goggles or used as a standalone sensor to detect the presence of H2 S vapor in the atmosphere. Acknowledgement This work was supported by research funding from the National University of Singapore, the Agency for Science, Technology and Research (A*STAR) in Singapore. References [1] R.O. Beauchamp, J.S. Bus, J.A. Popp, C.J. Boreiko, D.A. Andjelkovich, P. Leber, A critical review of the literature on hydrogen sulfide toxicity, CRC Crit. Rev. Toxicol. 13 (1984) 25–97. [2] W. Puaca, W. Szahun, K. Linke, Catalytic determination of sulfide in blood, Analyst 120 (1995) 939–941.
538
H. Xu et al. / Sensors and Actuators B 143 (2010) 535–538
[3] X. Xue, L. Xing, Y. Chen, S. Shi, Y. Wang, T. Wang, Synthesis, H2 S sensing properties of CuO-SnO2 core/shell PN-junction nanorods, J. Phys. Chem. C 112 (2008) 12157–12160. [4] K.J. Wallace, S.R. Cordero, C.P. Tan, V.M. Lynch, E.V. Anslyn, A colorimetric response to hydrogen sulfide, Sens. Actuators B 120 (2007) 362–367. [5] N.S. Lawrence, J. Davis, R.G. Compton, Analytical strategies for the detection of sulfide: a review, Talanta 52 (2000) 771–784. [6] Z. Pawlak, A.S. Pawlak, Modification of iodometric determination of total and reactive sulfide in environmental samples, Talanta 48 (1999) 347–353. [7] L.S. Bark, A. Rixon, The spectrofluorimetric determination of sulphides, Analyst 95 (1970) 786–790. [8] R.L. Benner, D.H. Stedman, Universal sulfur detection by chemiluminescence, Anal. Chem. 61 (1989) 1268–1271. [9] X. Peng, M.C. Schlamp, A.V. Kadavanich, A.P. Alivisatos, Epitaxial growth of highly luminescent CdSe/CdS core/shell nanocrystals with photostability and electronic accessibility, J. Am. Chem. Soc. 119 (1997) 7019–7029. [10] J. Li, Y.A. Wang, W.Z. Guo, J.C. Keay, T.D. Mishima, M.B. Johnson, X. Peng, Largescale synthesis of nearly monodisperse CdSe/CdS core/shell nanocrystals using air-stable reagents via successive ion layer adsorption and reaction, J. Am. Chem. Soc. 125 (2003) 12567–12575. [11] A.M. Munro, I.J. Plante, M.S. Ng, D.S. Ginger, Quantitative study of the effects of surface ligand concentration on CdSe nanocrystal photoluminescence, J. Phys. Chem. C 111 (2007) 6220–6227. [12] M. Bruchez, M. Moroonne, P. Gin, S. Weiss, A.P. Alivisatos, Semiconductor nanocrystals as fluorescent biological labels, Science 281 (2003) 2013–2016. [13] P. Zhong, Y. Yu, J. Wu, Z. Long, C. Liang, Synthesis of mercaptoehtylaminecoated CdSe/CdS nanocrystals and their use for DNA probe, Anal. Sci. 23 (2007) 1085–1089. [14] F. Huang, G. Chen, Preparation and application of l-cysteine-modified CdSe/CdS core/shell nanocrystals as a novel fluorescence probe for detection of nucleic acid, Spectrochim. Acta. A 70 (2008) 318–323. [15] Y. Lai, Y. Yu, P. Zhong, J. Wu, Z. Long, C. Liang, Development of novel quantum dots as fluorescent sensors for application in highly sensitive spectrofluorimetric determination of Cu2+ , Anal. Lett. 39 (2006) 1201–1209. [16] M.D. Hirschey, Y.J. Han, G.D. Stucky, A. Butler, Imaging Escherichia coli using functionalized core/shell CdSe/CdS quantum dots, J. Biol. Inorg. Chem. 11 (2006) 663–669. [17] W. Guo, J.J. Li, Y.A. Wang, X. Peng, Conjugation chemistry and bioapplications of semiconductor box nanocrystals prepared vis dendrimer bridging, Chem. Mater. 15 (2003) 3125–3133. [18] W. Guo, J.J. Li, Y.A. Wang, X. Peng, Luminescent CdSe/CdS core/shell nanocrystals in dendron boxes: superior chemical, photochemical and thermal stability, J. Am. Chem. Soc. 125 (2003) 3901–3909. [19] Y. Liu, M. Kim, Y. Wang, Y.A. Wang, X. Peng, Highly luminescent, stable, and water-soluble CdSe/CdS core-shell dendron nanocrystals with carboxylate anchoring groups, Langmuir 22 (2006) 6341–6345. [20] L. Qu, X. Peng, Control of photoluminescence properties of CdSe nanocrystals in growth, J. Am. Chem. Soc. 124 (2002) 2049–2055. [21] M.A. Malik, P. O’Brien, N. Revaprasadu, A Simple route to the synthesis of core/shell nanoparticles of chalcogenides, Chem. Mater. 14 (2002) 2004–2010. [22] W.W. Yu, X. Peng, Formation of high-quality CdS and other II–VI semiconductor nanocrystals in noncoordinating solvents: tunable reactivity of monomers, Angew. Chem., Int. Ed. 41 (2002) 2368–2371.
[23] S.A. Stern, B.D. Bhide, Permeability of silicone polymers to ammonia and hydrogen sulfide, J. Appl. Polym. Sci. 38 (1989) 2131–2147. [24] B. Wilks, M.E. Rezac, Properties of rubbery polymers for the recovery of hydrogen sulfide from gasification gases, J. Appl. Polym. Sci. 85 (2002) 2436– 2444. [25] Y. Xia, G.M. Whitesides, Soft lithography, Angew. Chem. Int. Ed. 37 (1998) 550–575. [26] C. Bullen, P. Mulvaney, The effects of chemisorption on the luminescence of CdSe quantum dots, Langmuir 22 (2006) 3007–3013. [27] A.F. Barnabas, D. Sallin, B.R. James, Mechanistic aspects of the reaction of hydrogen sulfide with dinuclear palladium(I) complexes containing bis(dipheny1phosphino)methane and related ligands, Can. J. Chem. 67 (1989) 2009–2015. [28] A.Y. Nazzal, L.H. Qu, X.G. Peng, M. Xiao, Photoactivated CdSe nanocrystals as nanosensors for gases, Nano Lett. 3 (2003) 819–822. [29] M. Kuno, J.K. Lee, B.O. Dabbousi, F.V. Mikulec, M.G. Bawendi, The band edge luminescence of surface modified CdSe nanocrystallites: probing the luminescing state, J. Chem. Phys. 106 (1997) 9869–9882.
Biographies Huan Xu received her PhD degree (2006) in the Department of Chemistry from the National University of Singapore. She is currently a research fellow at the Department of Chemical and Biomolecular Engineering in the National University of Singapore. Her present work is focused on developing chemical sensors and biosensors. Ji’en Wu received his PhD degree (2002) in the Department of Chemistry from the National University of Singapore. Then he worked as Research Associate in the School of Chemistry, Bristol, UK for 3 years. Currently, he is a research fellow in the Institute of Molecular Cell and Biology, Singapore. Chih-Hsin Chen is a research fellow at the Department of Chemical and Biomolecular Engineering, National University of Singapore. He received his PhD degree (2006) in Department of Chemistry, National Taiwan Normal University before becoming a post-doctoral researcher in the Institute of Chemistry, Academia Sinica. His present work is focused on DNA microarray and DNA microcontact printing. Lianhui Zhang received his graduate training in microbiology at the University of Adelaide and obtained his PhD in 1993. He continued his research on microbial diseases as an Australian Research Council Post-doctoral Research Fellow, and then a team leader. He was appointed as a principal investigator at the former Institute of Molecular Agrobiology in 1998 and became a member of Institute of Molecular and Cell Biology in 2002. Kun-Lin Yang is an assistant professor at the Department of Chemical and Biomolecular Engineering, National University of Singapore. He was awarded his PhD degree (2002) in Environmental Engineering from Georgia Institute of Technology before becoming a post-doctoral researcher in the Department of Chemical and Biological Engineering at University of Wisconsin in Madison. His present research interests include advanced materials, molecular engineering, surface chemistry, and chemical sensing.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Detecting hydrogen sulfide by using transparent polymer with embedded CdSe/CdS quantum dots Huan Xu a , Ji’en Wu b , Chih-Hsin Chen a , Lianhui Zhang b , Kun-Lin Yang a,∗ a b
Department of Chemical and Bimolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore Institute of Molecular Cell and Biology, 61 Biopolis Drive, Proteos, Singapore 138673, Singapore
a r t i c l e
i n f o
Article history: Received 29 May 2009 Received in revised form 19 August 2009 Accepted 28 September 2009 Available online 6 October 2009 Keywords: Fluorescent sensor CdSe/CdS quantum dots Polymer Hydrogen sulfide Poly(dimethylsiloxane)
a b s t r a c t Hydrogen sulfide (H2 S) is a hazardous vapor commonly found in oil fields and industrial processes. In this paper, we first reported the fluorescence quenching of CdSe/CdS quantum dots (QDs) by hydrogen sulfide in dichloromethane. This response is highly specific because no response to other volatile organic vapors can be observed. Second, we embedded QDs in films of poly(dimethylsiloxane) (PDMS), a transparent and gas-permeable polymer, for detecting low level of hydrogen sulfide. After exposing the film to 10 ppm of hydrogen sulfide, the film loses its fluorescence within 1 min. However, the fluorescence recovers completely after the film is left in the open air for 2 h. Compared with other types of hydrogen sulfide sensors, the film sensor with embedded QDs is more portable, easy-to-operate and has low-cost. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen sulfide (H2 S) is a well-known poisonous gas, which is often generated as a by-product in many industrial processes. Inhalation of 500–1000 ppm of H2 S for 30 min can lead to loss of consciousness or even death because of respiratory paralysis or asphyxiation [1]. According to National Institute for Occupational Safety and Health (NIOSH), the H2 S concentration immediately dangerous to life or health (IDLH) is 100 ppm, and the recommended exposure limit (REL) is 10 ppm for a maximum duration of 10 min. Although human noses are extremely sensitive to the presence of H2 S in the environments, it cannot serve as a long-term H2 S detectors because of olfactory fatigue [2]. Furthermore, if the concentration of H2 S is over 100 ppm, one’s ability to smell H2 S is severely compromised because of temporary paralysis of olfactory nerves. Therefore, many methods and sensors have been developed to detect the presence of H2 S [3–8]. However, they often require trained personnels and complex instrumentation, which limit their applications in the fields. Highly fluorescent core/shell CdSe/CdS quantum dots (QDs) are ideal candidates for sensing applications because of their high quantum yield, photostability, extremely large surface-to-atom ratio and sensitivity to surface ligands [9–11]. However, fluorescent QDs only have been applied to detect the presence of
∗ Corresponding author. Tel.: +65 6516 6614; fax: +65 6779 1936. E-mail address: [email protected] (K.-L. Yang). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.09.049
Escherichia coli, nucleic acid, DNA, protein and copper ions in aqueous solution in previous studies [12–16]. Although some studies have shown that the fluorescence of CdSe QDs can be quenched by thiols in the aqueous solution [4,17–19], core/shell CdSe/CdS QDs have never been used for detecting H2 S before. Furthermore, most past studies about QDs were focused on the fluorescence quenching phenomenon in aqueous solutions or organic solvents which are not practical for gas sensing applications. To address this issue, we developed a polymeric film with embedded CdSe/CdS QDs to detect the presence of H2 S in the atmosphere. Because QDs embedded in the polymer have higher stability compared to QDs dispersed in solutions, this type of film sensor may also offer a solution to the stability issue encountered in many QD-based sensors. 2. Experimental 2.1. Materials Cadmium oxide (≥99%), stearic acid (95%), octadecylamine (ODA, 90%), trioctylphosphine oxide (TOPO, 99%), trioctylphosphine (TOP, 90%), selenium powder (99.999%) and sulfur powder (99.998%) were purchased from Sigma–Aldrich (Singapore). H2 S (100 ppm) was purchased from National Oxygen (Singapore). Slygard 184 silicone elastomer kit for making poly(dimethylsiloxane) (PDMS) polymer was purchased from Dow Corning (U.S.A.). All solvents used in this study were HPLC grades. Water was purified by using a Milli-Q system (Millipore).
536
H. Xu et al. / Sensors and Actuators B 143 (2010) 535–538
Fig. 1. Fluorescent images of CdSe/CdS QDs in dichloromethane under the excitation of 365 nm UV. To study the fluorescence quenching phenomenon. In (B–F), the dichloromethane is also mixed with (B) 100 ppm H2 S, (C) 10 L of water, (D) 2 L of hexane, (E) 2 L of 1-butylamine and (F) 2 L of 1-heptanol.
2.2. Synthesis of CdSe/CdS QDs Highly fluorescent CdSe/CdS QDs were synthesized following reported procedures with slight modifications [9,10,20–22]. In a Schlenk flask, mixture of 0.2 mmol of CdO and 0.8 mmol of stearic acid were heated to 150 ◦ C until they become a colorless solution. After the solution was cooled to room temperature, ODA (1.5 g) and TOPO (0.5 g) were added to the solution. This solution was then reheated to 200 ◦ C under nitrogen protection. Subsequently, a selenium solution (made by dissolving 0.2 mmol of Se in 2.0 g of TOP) was injected quickly. The reaction is continued until yellow green fluorescence can be observed under an UV lamp. To grow the shell structure, we first mixed 0.42 mol of sulfur, 1.0 g
Fig. 3. Detection limit for H2 S. CdSe/CdS QDs in dichloromethane was mixed with (A) 0 mL, (B) 1 mL, (C) 2 mL, (D) 3 mL and (E) 4 mL of 100 ppm H2 S vapor. The detection limit is approximately 2 mL of 100 ppm H2 S.
TOP and 1.0 g TOPO together and heated the mixture to 150 ◦ C to get a clear solution (solution 1). Next, 0.2 mmol of Se and 2.0 g of TOP were mixed together and heated to 150 ◦ C to get another clear solution (solution 2). Solutions 1 and 2 were injected into the reaction mixture mentioned above, and the growth of CdSe/CdS QDs was monitored continuously by checking the fluorescence of the solution. The heater was removed when the solution appears orange color. The solution was then cooled to room temperature and chloroform was added to the solution. The solution was kept in a fridge at 4 ◦ C. Before use, the stock solution was diluted by using dichloromethane to a desired concentration. The unit of concentration reported herein is optical density of a solution at 570 nm (OD570 ), which was determined by using a UV–vis spectrometer. 2.3. Preparation of PDMS films Films of PDMS were prepared according to a standard protocol. PDMS prepolymer and curing agent were mixed together in a 10:1 ratio and cured at 50 ◦ C for 12 h. To incoporate QDs in PDMS, films of PDMS were immersed in dichloromethane containing CdSe/CdS QDs (OD570 = 0.3) for 2 h at 4 ◦ C. Then, the films were withdrawn from the solution, rinsed with acetone and blown dried. 2.4. Characterization of QDs solution All UV–vis spectra were obtained by using a UV/vis spectrometer (Cary 50, Varian, Australia). In all experiments, standard quartz cuvettes were used. Fluorescence spectra were obtained by using a luminescence spectrometer (LS 50B, PerkinElmer, U.S.A.). 3. Result and discussion The CdSe/CdS QDs used in this study were synthesized following a revised literature procedure [9,10,20–22]. Fig. 1A shows that
Fig. 2. Time-series (A) UV–vis spectra and (B) fluorescent spectra of CdSe/CdS QDs in dichloromethane after the injection of 100 ppm H2 S. Both spectra show that the fluorescence of CdSe/CdS QDs can be quenched within 15 s by H2 S.
Fig. 4. Fluorescent images of PDMS films with embedded CdSe/CdS QDs and their responses to (A) air, (B) 10 ppm H2 S, (C) water, (D) hexane, (E) 1-butylamine, and (F) 1-heptanol.
H. Xu et al. / Sensors and Actuators B 143 (2010) 535–538
537
Fig. 5. Reversibility of fluorescent response to H2 S. The sample was exposed to 10 ppm of H2 S for 1 min and air for 2 h alternatively.
our CdSe/CdS QDs in dichloromethane (OD570 = ∼0.03) give strong fluorescence at 590 nm when they are excited at 365 nm, similar to the results reported earlier [10]. To test whether H2 S can quench the fluorescence, 100 ppm H2 S vapor was injected into the solution. After 10 s, the solution turns to white (Fig. 1B), which suggests that H2 S can quench the orange fluorescence of the CdSe/CdS QDs. For comparison, we also injected 10 L of water, 2 L of hexane, 2 L of 1-butylamine and 2 L of 1-heptanol into the QD solution, but they do not quench the fluorescence of QDs (Fig. 1C–F). This result indicates that the fluorescence quenching phenomenon is highly specific. We attributed the quenching effect to the displacements of surface ligands by H2 S. Previous studies have shown that alkylthiols can rapidly displace weakly bound amines from the surfaces of CdSe/CdS QDs, leading to fluorescence quenching [4,17–19]. Although H2 S is not an alkylthiol, it also has a sulfur atom and can act as a Lewis acid. Therefore, it is able to substitute the weakly bound ligands such as TOPO or ODA and quenches the fluorescence. To better understand the kinetics of fluorescence quenching after the injection of 100 ppm H2 S vapor, time-series UV–vis spectra (Fig. 2A) and fluorescent spectra (Fig. 2B) of QDs in dichloromethane were recorded. Fig. 2A shows that the broad absorption peak between 240 and 540 nm increases with time after the injection of H2 S for 15 s. However, the spectra do not change further after 15 s. Meanwhile, the emission peak at 590 nm decreases with time for the first 15 s while a new emission peak emerges between 400 and 550 nm as shown in Fig. 2B. The change in the fluorescence spectra is consistent with the different optical appearances of the CdSe/CdS QDs before and after the injection of H2 S as shown in Fig. 1. Similar to the UV–vis spectra, the fluorescence spectra do not change further after 15 s. The fast fluorescence quenching phenomenon associated with the CdSe/CdS QDs makes them suitable for the detection of H2 S in the ppm range. Next, to determine the detection limit, different volumes of H2 S were injected into 1 mL of dichloromethane containing CdSe/CdS QDs (OD570 = ∼0.03). Fig. 3C shows that 2 mL of 100 ppm H2 S can partially quench the fluorescence and 4 mL of 100 ppm H2 S can quench the orange fluorescence completely (Fig. 3E). Thus, our result shows that the detection limit is approximately 2 mL of 100 ppm H2 S. Results shown above demonstrate that the fluorescence of CdSe/CdS QDs in dichloromethane can be quenched by H2 S. To avoid the use of organic solvent and prepare a solvent-free H2 S sensor, CdSe/CdS QDs were embedded in PDMS, because this polymer is transparent and is permeable to gases [23–25]. To test the response of PDMS films with embedded QDs to H2 S vapor, these films were exposed to 10 ppm of H2 S, saturated water, hexane, 1-butylamine and 1-heptanol vapors, respectively. Fig. 4B shows that the film of PDMS with embedded QDs loses its fluorescence after the exposure to 10 ppm H2 S for 1 min. In contrast, other films exposed to water, hexane and 1-heptanol vapors for 2 h do not show any decrease in fluorescence (Fig. 4C, D and F). The only exception is the one exposed to saturated 1-butylamine vapor. After 2 h of exposure, the film gives greenish fluorescence as shown in Fig. 4E. Thus, we can conclude that vaporous amines are potential inter-
ference for the H2 S sensor reported here. This is expected because 1-butylamine is a good ligand; it may replace ODA or TOPO from the surfaces of QDs and shift the emission peak of the fluorescence [26]. To determine the reversibility of the response of the PDMS films with embedded QDs, we exposed a film to 10 ppm of H2 S and air alternatively. Fig. 5 shows that after the exposure to H2 S, the fluorescence is quenched after 1 min in the presence of H2 S, but the fluorescence completely recovers to its original intensity after 2 h in the air. The result demonstrated that the adsorption of H2 S on the surface of QDs is fully reversible. In fact, the experiment can be repeated several times without losing any fluorescence intensity as shown in Fig. 5. The observation suggests that the QDs embedded in the PDMS film can absorb H2 S vapor reversibly. We note that this is different from QDs in dichloromethane, which cannot regain their fluorescence even after purging with nitrogen for 1 min. The difference between these two configurations is that the H2 S has high solubility in the dichloromethane (0.53 M) such that the purging process cannot remove H2 S from dichloromethane easily [27]. However, H2 S has low solubility in the PDMS film and it can absorb by PDMS reversibly depending on the H2 S concentration in the surrounding environments [28,29]. Therefore, when the PDMS film is exposed to air again, H2 S escapes from the PDMS film, and original ligands recombine with the QDs and fluorescence is regenerated. 4. Conclusion In conclusion, CdSe/CdS QDs have been successfully exploited for the detection of H2 S vapor through a fluorescence quenching mechanism. Two different configurations have been demonstrated. The first one is dichloromethane with dispersed QDs. In this system, 100 ppm H2 S vapor is able to quench the fluorescence of QDs within 15 s, and the detection limit is approximately 2 mL of 100 ppm H2 S. The second system is a thin PDMS film with embedded QDs. Our results show that 10 ppm of H2 S vapor can quench the fluorescence of QDs embedded in films of PDMS within 1 min. The film does not show any response to water, hexane, 1-heptanol, but shows some response to 1-butylamine. However, only QDs embedded in films of PDMS show fully reversible response to H2 S vapor. Films of PDMS with embedded QDs offer many advantages such as high portability and low cost over traditional gas sensors. These polymers can also be coated on safety equipments such as goggles or used as a standalone sensor to detect the presence of H2 S vapor in the atmosphere. Acknowledgement This work was supported by research funding from the National University of Singapore, the Agency for Science, Technology and Research (A*STAR) in Singapore. References [1] R.O. Beauchamp, J.S. Bus, J.A. Popp, C.J. Boreiko, D.A. Andjelkovich, P. Leber, A critical review of the literature on hydrogen sulfide toxicity, CRC Crit. Rev. Toxicol. 13 (1984) 25–97. [2] W. Puaca, W. Szahun, K. Linke, Catalytic determination of sulfide in blood, Analyst 120 (1995) 939–941.
538
H. Xu et al. / Sensors and Actuators B 143 (2010) 535–538
[3] X. Xue, L. Xing, Y. Chen, S. Shi, Y. Wang, T. Wang, Synthesis, H2 S sensing properties of CuO-SnO2 core/shell PN-junction nanorods, J. Phys. Chem. C 112 (2008) 12157–12160. [4] K.J. Wallace, S.R. Cordero, C.P. Tan, V.M. Lynch, E.V. Anslyn, A colorimetric response to hydrogen sulfide, Sens. Actuators B 120 (2007) 362–367. [5] N.S. Lawrence, J. Davis, R.G. Compton, Analytical strategies for the detection of sulfide: a review, Talanta 52 (2000) 771–784. [6] Z. Pawlak, A.S. Pawlak, Modification of iodometric determination of total and reactive sulfide in environmental samples, Talanta 48 (1999) 347–353. [7] L.S. Bark, A. Rixon, The spectrofluorimetric determination of sulphides, Analyst 95 (1970) 786–790. [8] R.L. Benner, D.H. Stedman, Universal sulfur detection by chemiluminescence, Anal. Chem. 61 (1989) 1268–1271. [9] X. Peng, M.C. Schlamp, A.V. Kadavanich, A.P. Alivisatos, Epitaxial growth of highly luminescent CdSe/CdS core/shell nanocrystals with photostability and electronic accessibility, J. Am. Chem. Soc. 119 (1997) 7019–7029. [10] J. Li, Y.A. Wang, W.Z. Guo, J.C. Keay, T.D. Mishima, M.B. Johnson, X. Peng, Largescale synthesis of nearly monodisperse CdSe/CdS core/shell nanocrystals using air-stable reagents via successive ion layer adsorption and reaction, J. Am. Chem. Soc. 125 (2003) 12567–12575. [11] A.M. Munro, I.J. Plante, M.S. Ng, D.S. Ginger, Quantitative study of the effects of surface ligand concentration on CdSe nanocrystal photoluminescence, J. Phys. Chem. C 111 (2007) 6220–6227. [12] M. Bruchez, M. Moroonne, P. Gin, S. Weiss, A.P. Alivisatos, Semiconductor nanocrystals as fluorescent biological labels, Science 281 (2003) 2013–2016. [13] P. Zhong, Y. Yu, J. Wu, Z. Long, C. Liang, Synthesis of mercaptoehtylaminecoated CdSe/CdS nanocrystals and their use for DNA probe, Anal. Sci. 23 (2007) 1085–1089. [14] F. Huang, G. Chen, Preparation and application of l-cysteine-modified CdSe/CdS core/shell nanocrystals as a novel fluorescence probe for detection of nucleic acid, Spectrochim. Acta. A 70 (2008) 318–323. [15] Y. Lai, Y. Yu, P. Zhong, J. Wu, Z. Long, C. Liang, Development of novel quantum dots as fluorescent sensors for application in highly sensitive spectrofluorimetric determination of Cu2+ , Anal. Lett. 39 (2006) 1201–1209. [16] M.D. Hirschey, Y.J. Han, G.D. Stucky, A. Butler, Imaging Escherichia coli using functionalized core/shell CdSe/CdS quantum dots, J. Biol. Inorg. Chem. 11 (2006) 663–669. [17] W. Guo, J.J. Li, Y.A. Wang, X. Peng, Conjugation chemistry and bioapplications of semiconductor box nanocrystals prepared vis dendrimer bridging, Chem. Mater. 15 (2003) 3125–3133. [18] W. Guo, J.J. Li, Y.A. Wang, X. Peng, Luminescent CdSe/CdS core/shell nanocrystals in dendron boxes: superior chemical, photochemical and thermal stability, J. Am. Chem. Soc. 125 (2003) 3901–3909. [19] Y. Liu, M. Kim, Y. Wang, Y.A. Wang, X. Peng, Highly luminescent, stable, and water-soluble CdSe/CdS core-shell dendron nanocrystals with carboxylate anchoring groups, Langmuir 22 (2006) 6341–6345. [20] L. Qu, X. Peng, Control of photoluminescence properties of CdSe nanocrystals in growth, J. Am. Chem. Soc. 124 (2002) 2049–2055. [21] M.A. Malik, P. O’Brien, N. Revaprasadu, A Simple route to the synthesis of core/shell nanoparticles of chalcogenides, Chem. Mater. 14 (2002) 2004–2010. [22] W.W. Yu, X. Peng, Formation of high-quality CdS and other II–VI semiconductor nanocrystals in noncoordinating solvents: tunable reactivity of monomers, Angew. Chem., Int. Ed. 41 (2002) 2368–2371.
[23] S.A. Stern, B.D. Bhide, Permeability of silicone polymers to ammonia and hydrogen sulfide, J. Appl. Polym. Sci. 38 (1989) 2131–2147. [24] B. Wilks, M.E. Rezac, Properties of rubbery polymers for the recovery of hydrogen sulfide from gasification gases, J. Appl. Polym. Sci. 85 (2002) 2436– 2444. [25] Y. Xia, G.M. Whitesides, Soft lithography, Angew. Chem. Int. Ed. 37 (1998) 550–575. [26] C. Bullen, P. Mulvaney, The effects of chemisorption on the luminescence of CdSe quantum dots, Langmuir 22 (2006) 3007–3013. [27] A.F. Barnabas, D. Sallin, B.R. James, Mechanistic aspects of the reaction of hydrogen sulfide with dinuclear palladium(I) complexes containing bis(dipheny1phosphino)methane and related ligands, Can. J. Chem. 67 (1989) 2009–2015. [28] A.Y. Nazzal, L.H. Qu, X.G. Peng, M. Xiao, Photoactivated CdSe nanocrystals as nanosensors for gases, Nano Lett. 3 (2003) 819–822. [29] M. Kuno, J.K. Lee, B.O. Dabbousi, F.V. Mikulec, M.G. Bawendi, The band edge luminescence of surface modified CdSe nanocrystallites: probing the luminescing state, J. Chem. Phys. 106 (1997) 9869–9882.
Biographies Huan Xu received her PhD degree (2006) in the Department of Chemistry from the National University of Singapore. She is currently a research fellow at the Department of Chemical and Biomolecular Engineering in the National University of Singapore. Her present work is focused on developing chemical sensors and biosensors. Ji’en Wu received his PhD degree (2002) in the Department of Chemistry from the National University of Singapore. Then he worked as Research Associate in the School of Chemistry, Bristol, UK for 3 years. Currently, he is a research fellow in the Institute of Molecular Cell and Biology, Singapore. Chih-Hsin Chen is a research fellow at the Department of Chemical and Biomolecular Engineering, National University of Singapore. He received his PhD degree (2006) in Department of Chemistry, National Taiwan Normal University before becoming a post-doctoral researcher in the Institute of Chemistry, Academia Sinica. His present work is focused on DNA microarray and DNA microcontact printing. Lianhui Zhang received his graduate training in microbiology at the University of Adelaide and obtained his PhD in 1993. He continued his research on microbial diseases as an Australian Research Council Post-doctoral Research Fellow, and then a team leader. He was appointed as a principal investigator at the former Institute of Molecular Agrobiology in 1998 and became a member of Institute of Molecular and Cell Biology in 2002. Kun-Lin Yang is an assistant professor at the Department of Chemical and Biomolecular Engineering, National University of Singapore. He was awarded his PhD degree (2002) in Environmental Engineering from Georgia Institute of Technology before becoming a post-doctoral researcher in the Department of Chemical and Biological Engineering at University of Wisconsin in Madison. His present research interests include advanced materials, molecular engineering, surface chemistry, and chemical sensing.