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Lead ion detection in turbid media by pulsed photoacoustic spectrometry based on dissolution of gold nanoparticles
Sensors and Actuators B 150 (2010) 770–773
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Short communication
Lead ion detection in turbid media by pulsed photoacoustic spectrometry based on dissolution of gold nanoparticles Martín G. González, Xiangjiang Liu, Reinhard Niessner, Christoph Haisch ∗ Chair for Analytical Chemistry, Technische Universität München, Marchioninistrasse 17, 81377 Munich, Germany
a r t i c l e
i n f o
Article history: Received 1 April 2010 Received in revised form 21 July 2010 Accepted 30 July 2010 Available online 6 August 2010 Keywords: Photoacoustic Gold nanoparticles Lead ion
a b s t r a c t In this paper, we present a pulsed photoacoustic configuration for the detection of lead ion in aqueous solution based on the dissolution of gold nanoparticle. The photoacoustic signal is generated by laserinduced nanobubble. This technique allows for a sensitive measurement of size changes of the gold nanoparticles in the presence of lead ions even in turbid samples. As a first step, the best gold nanoparticle size and concentration were determined. Using 1.25 nM of gold nanoparticle with a diameter of 11 nm containing 1 mM of 2-mercaptoethanol, a linear range from 1 nM to 2.5 M and a limit of detection of 0.5 nM were obtained. Then, to study the response of our system to strong light scattering in the medium, the scattering coefficient of samples containing lead ions was incremented using spherical silica particles (diameter 500 nm). The results corroborate that the technique used in this work is almost insensitive to light scattering. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Monitoring toxic metal ions in aquatic ecosystems is an important issue because these contaminants have severe effects on human health and the environment [1]. Lead is one of the most widely used heavy metals and has a large number of industrial applications, including battery manufacturing, paint, gasoline, radiation shielding alloys, etc. Lead in paint and gasoline accounts for most of the content present in environment, and leads to serious pollution and human health problems. It is known that lead enters the human body by ingestion and inhalation, and it is associated with damage to the kidneys, the liver and the gastrointestinal tract, as well as with neurological degeneration and decreased hemoglobin production [2]. The maximum contamination level for lead in drinking water is defined by the U.S. Environmental Protection Agency (EPA) to be 75 nM [3]. Because of its toxicity, the accurate determination of Pb is critical. Several methods for Pb analysis have been developed. The commonly used methods are atomic absorption spectrometry, atomic emission spectrometry [4], and inductively coupled plasma/mass spectrometry (ICP/MS) [5]. These techniques are sensitive, accurate and allow discrimination among different metal ions. However, they are time-consuming, expensive, and/or require complex sample
Abbreviations: GNP, gold nanoparticle; PA, photoacoustic; LINB, laser-induced nanobubbles. ∗ Corresponding author. Tel.: +49 89 2180 78242; fax: +49 89 2180 99 78242. E-mail address: [email protected] (C. Haisch). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.07.058
pretreatment and sophisticated equipment. Other methods for the detection of lead ions (Pb2+ ), using chromophores [6], DNAzymes [7], oligonucleotides [8], polymers [9], antibody [10], or functionalized nanoparticles [11] have been published. However, many of these systems possess limited practical use due to different causes such as poor aqueous solubility, high costs, complicated processing or poor sensitivity and selectivity. Recently, a colorimetric, label-free, nonaggregation-based gold nanoparticle (GNP) probe has been developed for the detection of Pb2+ in aqueous solution [12]. As shown by the authors, this approach is highly sensitive as well as selective, and avoids the need for sophisticated equipment. They proved that the approach is well suitable for real-world samples. Cross-sensitivity to many other metal ions is at least a factor of 1000 lower than the system’s reaction on lead ions. However, the method is not suitable to analyze Pb2+ in scattering media; a feature which can be essential for environmental measurements. In contrast to conventional, transmission based techniques, the widely known photoacoustic (PA) technique is insensitive to light scattering within the sample [13]. In the special mode of laser-induced nanobubble formation, PA is highly sensitive to nanoparticle and especially to size changes of these particles [14], which makes it a favorable detection tool for the nanoparticle-based Pb2+ analysis. In this paper, we present a modification of the method developed by Chen et al. [12], where we replace the UV/VIS spectrometer by a pulsed photoacoustic system. This change allows the measurement of lead ions in turbid samples. As a first step, the best GNP size and concentration is studied in order to obtain a wide linear range and a low limit of detection. Next, to study the response of
M.G. González et al. / Sensors and Actuators B 150 (2010) 770–773
771
Fig. 1. Experimental setup.
our system to strong light scattering in the medium, spherical silica particles were used. The results are compared with those obtained with the colorimetric method. 2. Experimental setup The sensing strategy used in this work is the same applied by Chen et al. [12]. When the GNPs react with thiosulfate (S2 O3 2− ) in solution, Au(S2 O3 )2 3− complexes are formed instantaneously on the GNPs surfaces. Then, if Pb2+ and 2-mercaptoethanol (2-ME) are present, the GNP rapidly dissolves to form Au+ –2-ME complexes in solution. As a result, the size of the GNPs decreases significantly, allowing the quantification of the lead ions in the solution. The PA system used in this work is shown in Fig. 1. A frequencydoubled, Q-switched Nd:YAG laser (SL280 Spectron Laser System, UK; 532 nm, 6 ns pulse time, 10 Hz) was employed. The laser beam was focused by a lens (100 mm focal length) into a conventional 1 cm glass cuvette, equipped with an in-house made piezoelectric transducer on one side (side-on detection) [15]. The PA signals were amplified (HCA-100M-50k-C current amplifier, Femto, Germany) and recorded with a digital oscilloscope. All data presented here are averaged over 50 laser pulses. A fraction of the laser beam was coupled out onto a pyroelectric detector (Pyroelectric J25LPMB, Coherent, USA) for pulse energy monitoring. The oscilloscope was triggered by the Q-switch signal of the laser. The laser fluence was set above the laser fluence threshold for cavitation bubble formation. Therefore, the PA signal is generated by laserinduced nanobubbles (PA-LINB) [14,16]. The NP sizes were verified by Transmission Electron Microscopy (TEM), which were measured by a JEOL JEM 2010 instrument (JEOL GmbH; Germany).
In order to determine the optimum GNP size for our PA system, we prepared GNP of five different average sizes following the steps previously described in Refs. [17–19], with slight modifications. After the reaction between the GNP and S2 O3 2− , the lead ions and 2-ME were added, maintaining a constant ratio between the Pb2+ concentration and the GNP surface. After 3 h, the PA responses were measured (Fig. 2). As it can be appreciated, the factor = (S0 − S)/S (S0 and S are the PA response without and with Pb2+ , respectively) is inversely proportional to the GNP diameter. On the other hand, the GNP concentration depends on the background signal of our PA setup. Based on above results, we have chosen a concentration and diameter of 1.25 nM and 11 nm, respectively. As it is shown later, these values allow the lowest limit of detection and a wide linear range. For smaller particles, the relative size change is larger than for large particles, which make the detection more sensitive. As it can be seen in Fig. 3, the pulsed PA technique generally offers a good time resolution, since the response can be analyzed from pulse to pulse. However, it has to be stated that the temporal resolution of this specific lead detection scheme is limited by the kinetics of the leaching process, which is in the range of an hour.
3. Results In a first experiment, it was shown that the PA response generated by LINB is proportional to the GNP volume. Three samples, containing different amounts of lead, were measured. The ratios between the PA response with and without Pb2+ were compared with the corresponding information obtained from TEM images. In Table 1 the results are compared; similar results are found. Hence, the PA response can be considered as an indicator for the leaching process.
Fig. 2. vs. GNP diameter. The ration between the lead ion concentration and the GNP surface was kept constant.
Table 1 Comparison between the PA response and the GNP diameter change resulting from leaching. The NP sizes were verified by TEM. [Pb2+ ] (nM)
PA response (V/J)
Ratio of PA signals with and without lead S/S0
NP diameter (nm)
Ratio of particle size (TEM) with and without lead D3 /D03
0 100 250
539 ± 12 405 ± 19 238 ± 11
1.00 0.75 0.44
11.35 ± 0.97 10.46 ± 1.07 8.55 ± 1.38
1.00 0.78 0.43
772
M.G. González et al. / Sensors and Actuators B 150 (2010) 770–773
Fig. 3. vs. time. [S2 O3 2− ] = 1 mM, [Pb2+ ] = 1.5 M, [2-ME] = 1 mM.
The sensitivity of the system is of primary interest for this kind of technique. Using 1.25 nM of GNP (Dnp = 11 nm), containing 1 mM of 2-ME, a linear relationship between and the concentration of Pb2+ over the range from 1 nM to 2.5 M was obtained (Fig. 4). The limit of detection (S/N ratio = 3) was 0.5 nM. We repeated the procedure in the same way, measuring the extinction values at 520 nm with a UV/VIS spectrometer (colorimetric method). Approximately the same sensitivity was obtained. We observed saturation for concentrations larger than 3 M in both methods. In the case of the PA system, it is caused by the fact that the response generated by the NPs is lower than the background signal originating from the host liquid. In a scattering medium, the spatial distribution of the light is different from the one observed in a non-scattering liquid. The transmitted fraction of light is reduced even if no absorption takes place. In consequence, transmission based absorption measurements have a systematic error in scattering media. However, there is no such effect on the PA signal. Moreover, the piezoelectric transducer used in this work is largely insensitive to scattered light that directly impinges on the sensor surface, which is not the case for optical sensors. In order to show this advantage of the PA over the colorimetric method, the scattering coefficient of samples con-
Fig. 5. Lead ion detection in a highly scattering medium. Solid line: calibration curve as shown in Fig. 4. Squares: PA response and dots: extinction values at 520 nm. Scattering particles: 0.05 g/L of silica with 500 nm diameter. [S2 O3 2− ] = 1 mM, [GNP] = 1.25 nM, [2-ME] = 1 mM, [GNP] = 1.25 nM.
taining lead ions was incremented using spherical silica particles (diameter 500 nm). Fig. 5 shows the measurements of the PA system and the extinction values at 520 nm. As expected, the results corroborate that the PA-LINB technique is widely insensitive to light scattering, whilst the conventional method presents an error larger than 100%. Apart from the applicability for measurements in scattering media, we have no reason to assume that the performance of the PA based approach regarding selectivity and reaction to environmental conditions like temperature and pH is different from the classical UV/VIS-based approach as described in [12]. 4. Conclusions The measurement of trace contaminants in the presence of strong light scattering is an important challenge in environmental and biological analysis. In this paper we demonstrate that the pulsed photoacoustic spectrometry, based on the dissolution of GNPs, is a useful tool to monitor the presence of lead ions on highly scattering media. It is highly sensitive and suitable for the application in scattering media. Currently, we are attempting to decrease the electromagnetic background noise of the piezoelectric transducer. This way it should be possible increase sensitivity of the system by about one order of magnitude. In a next step we plan to construct a small and robust mobile system. Acknowledgements Grants from the Deutscher Akademischer Austauschdienst (DAAD), the Facultad de Ingeniería Universidad de Buenos Aires (FIUBA), and the China Scholarship Council (CSC) for two of the authors are gratefully acknowledged. References
Fig. 4. of the 2-ME/S2 O3 2− GNPs (1.25 nM, Dnp = 11 nm) in the presence of lead ions (0–10 M). Squares: PA response and dots: extinction values at 520 nm.
[1] H.L. Needleman, D. Bellinger, The health-effects of low-level exposure to lead, Annu. Rev. Public Health 12 (1991) 111–140. [2] H. Needleman, Lead poisoning, Annu. Rev. Med. 55 (2004) 209–222. [3] http://www.epa.gov/safewater/contaminants/index.html (accessed February 2010). [4] J.L. Anderson, E.F. Bowden, P.G. Pickup, Dynamic electrochemistry: methodology and application, Anal. Chem. 68 (1996) R379–R444. [5] A.L. Burlingame, R.K. Boyd, S.J. Gaskell, Mass spectrometry, Anal. Chem. 68 (1996) R599–R651.
M.G. González et al. / Sensors and Actuators B 150 (2010) 770–773 [6] K. Kavallieratos, J.M. Rosenberg, W.Z. Chen, T. Ren, Fluorescent sensing and selective Pb(II) extraction by a dansylamide ion-exchanger, J. Am. Chem. Soc. 127 (2005) 6514–6515. [7] A.K. Brown, J. Li, C.M.B. Pavot, Y. Lu, A lead-dependent DNAzyme with a twostep mechanism, Biochemistry 42 (2003) 7152–7161. [8] C.W. Liu, C.C. Huang, H.T. Chang, Highly selective DNA-based sensor for lead(II) and mercury(II) ions, Anal. Chem. 81 (2009) 2383–2387. [9] C.D. Geary, I. Zudans, A.V. Goponenko, S.A. Asher, S.G. Weber, Electrochemical investigation of Pb2+ binding and transport through a polymerized crystalline colloidal array hydrogel containing benzo-18-crown-6, Anal. Chem. 77 (2005) 185–192. [10] T.J. Lin, M.F. Chung, Using monoclonal antibody to determine lead ions with a localized surface plasmon resonance fiber-optic biosensor, Sensors 8 (2008) 582–593. [11] Z.D. Wang, J.H. Lee, Y. Lu, Label-free colorimetric detection of lead ions with a nanomolar detection limit and tunable dynamic range by using gold nanoparticles and DNAzyme, Adv. Mater. 20 (2008) 3263–3267. [12] Y.Y. Chen, H.T. Chang, Y.C. Shiang, Y.L. Hung, C.K. Chiang, C.C. Huang, Colorimetric assay for lead ions based on the leaching of Gold nanoparticles, Anal. Chem. 81 (2009) 9433–9439. [13] A. Rosencwaig, Photoacoustics and Photoacoustic Spectroscopy, Wiley, 1980. [14] G.M. González, J.X. Liu, R. Niessner, C. Haisch, Strong size-dependent photoacoustic effect on gold nanoparticles by laser-induced nanobubbles, Appl Phys. Lett. 96 (2010) 174104. [15] T. Schmid, U. Panne, R. Niessner, C. Haisch, Optical absorbance measurements of opaque liquids by pulsed laser photoacoustic spectroscopy, Anal. Chem. 81 (2009) 2403–2409. [16] S. Egerev, S. Ermilov, O. Ovchinnikov, A. Fokin, D. Guzatov, V. Kilmov, A. Kanavin, A. Oraevsky, Acoustic signals generated by laser-irradiated metal nanoparticles, Appl. Opt. 48 (2009) C38–C45. [17] K.R. Brown, D.G. Walter, M.J. Natan, Seeding of colloidal Au nanoparticle solutions. 2. Improved control of particle size and shape, Chem. Mater. 12 (2000) 306–313. [18] K.R. Brown, L.A. Lyon, A.P. Fox, B.D. Reiss, M.J. Natan, Seeding of colloidal Au nanoparticle solutions. 3. Controlled formation of conductive Au films, Chem. Mater. 12 (2000) 314–323. [19] X.J. Liu, S.Y. Huan, Y.L. Bu, G.L. Shen, R.Q. Yu, Liposome-mediated enhancement of the sensitivity in immunoassay based on surface-enhanced Raman scattering at gold nanosphere array substrate, Talanta 75 (2008) 797–803.
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Biographies Martín Germán González obtained the title of a master’s degree in Electronic Engineering at the Faculty of Engineering at the University of Buenos Aires (FIUBA) in 2003. A year later, the same institution awarded him a fellowship for the doctorate, where, after 4 years of research, he received the endorsement Summa Cum Laude. In 2009, he worked as a post-doc researcher at the Chair for Analytical Chemistry, Technische Universität München. His work focuses mainly on research and development of laser sources, control pollution instrumentation, numerical models and methods of signal processing, new spectroscopy and photothermal techniques. Xiangjiang Liu obtained a bachelor’s degree in Chemistry at the Colleague of Chemistry & Chemical Engineering, Hunan University, P.R. China in 2004. In 2008, State Key Laboratory of Bio-Chemical Sensing & Chemometrics of Hunan University awarded him a Master degree in Analytical Chemistry. Since September 2009, he works as PhD student at the Chair for Analytical Chemistry, Technische Universität München. His work mainly focuses on development of new approaches for surface-enhanced Raman scattering, photoacoustic techniques and nanoparticles preparation. Reinhard Niessner, full professor for Analytical Chemistry at Technische Universität München is director of the Institute of Hydrochemistry and holds the Chair for Analytical Chemistry. His research interests are devoted to applications of laser spectroscopy, nanoparticle characterization, and microarray technologies. The main subjects of interest are aerosols, hydrocolloids and biofilms, within hydro- and atmosphere. He received several international awards, e.g. the EmanuelMerck-Prize for Analytical Chemistry (1990), the Smoluchowski-Award for Aerosol Research of the Association for Aerosol Research (1991), the Fritz-Pregl-Medal of the Austrian Society for Analytical Chemistry (1996), and the Fresenius-Award for Analytical Chemistry of the German Chemical Society (2000). Christoph Haisch studied General Physics at the Technische Universität München, where he also achieved his doctoral degree in the field of laser-induced atomic spectrometry. After post-doc positions in Paris and Berlin, he returned in 2001 to the Chair for Analytical Chemistry at TU München, heading the research group for Applied Laser Spectroscopy. His research interests range from opto-acoustical aerosol characterization, where a soot exhaust monitor was successfully commercialized, over new imaging systems for medical diagnostics to Raman and surface-enhanced Raman spectrometry for environmental analytical applications.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Short communication
Lead ion detection in turbid media by pulsed photoacoustic spectrometry based on dissolution of gold nanoparticles Martín G. González, Xiangjiang Liu, Reinhard Niessner, Christoph Haisch ∗ Chair for Analytical Chemistry, Technische Universität München, Marchioninistrasse 17, 81377 Munich, Germany
a r t i c l e
i n f o
Article history: Received 1 April 2010 Received in revised form 21 July 2010 Accepted 30 July 2010 Available online 6 August 2010 Keywords: Photoacoustic Gold nanoparticles Lead ion
a b s t r a c t In this paper, we present a pulsed photoacoustic configuration for the detection of lead ion in aqueous solution based on the dissolution of gold nanoparticle. The photoacoustic signal is generated by laserinduced nanobubble. This technique allows for a sensitive measurement of size changes of the gold nanoparticles in the presence of lead ions even in turbid samples. As a first step, the best gold nanoparticle size and concentration were determined. Using 1.25 nM of gold nanoparticle with a diameter of 11 nm containing 1 mM of 2-mercaptoethanol, a linear range from 1 nM to 2.5 M and a limit of detection of 0.5 nM were obtained. Then, to study the response of our system to strong light scattering in the medium, the scattering coefficient of samples containing lead ions was incremented using spherical silica particles (diameter 500 nm). The results corroborate that the technique used in this work is almost insensitive to light scattering. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Monitoring toxic metal ions in aquatic ecosystems is an important issue because these contaminants have severe effects on human health and the environment [1]. Lead is one of the most widely used heavy metals and has a large number of industrial applications, including battery manufacturing, paint, gasoline, radiation shielding alloys, etc. Lead in paint and gasoline accounts for most of the content present in environment, and leads to serious pollution and human health problems. It is known that lead enters the human body by ingestion and inhalation, and it is associated with damage to the kidneys, the liver and the gastrointestinal tract, as well as with neurological degeneration and decreased hemoglobin production [2]. The maximum contamination level for lead in drinking water is defined by the U.S. Environmental Protection Agency (EPA) to be 75 nM [3]. Because of its toxicity, the accurate determination of Pb is critical. Several methods for Pb analysis have been developed. The commonly used methods are atomic absorption spectrometry, atomic emission spectrometry [4], and inductively coupled plasma/mass spectrometry (ICP/MS) [5]. These techniques are sensitive, accurate and allow discrimination among different metal ions. However, they are time-consuming, expensive, and/or require complex sample
Abbreviations: GNP, gold nanoparticle; PA, photoacoustic; LINB, laser-induced nanobubbles. ∗ Corresponding author. Tel.: +49 89 2180 78242; fax: +49 89 2180 99 78242. E-mail address: [email protected] (C. Haisch). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.07.058
pretreatment and sophisticated equipment. Other methods for the detection of lead ions (Pb2+ ), using chromophores [6], DNAzymes [7], oligonucleotides [8], polymers [9], antibody [10], or functionalized nanoparticles [11] have been published. However, many of these systems possess limited practical use due to different causes such as poor aqueous solubility, high costs, complicated processing or poor sensitivity and selectivity. Recently, a colorimetric, label-free, nonaggregation-based gold nanoparticle (GNP) probe has been developed for the detection of Pb2+ in aqueous solution [12]. As shown by the authors, this approach is highly sensitive as well as selective, and avoids the need for sophisticated equipment. They proved that the approach is well suitable for real-world samples. Cross-sensitivity to many other metal ions is at least a factor of 1000 lower than the system’s reaction on lead ions. However, the method is not suitable to analyze Pb2+ in scattering media; a feature which can be essential for environmental measurements. In contrast to conventional, transmission based techniques, the widely known photoacoustic (PA) technique is insensitive to light scattering within the sample [13]. In the special mode of laser-induced nanobubble formation, PA is highly sensitive to nanoparticle and especially to size changes of these particles [14], which makes it a favorable detection tool for the nanoparticle-based Pb2+ analysis. In this paper, we present a modification of the method developed by Chen et al. [12], where we replace the UV/VIS spectrometer by a pulsed photoacoustic system. This change allows the measurement of lead ions in turbid samples. As a first step, the best GNP size and concentration is studied in order to obtain a wide linear range and a low limit of detection. Next, to study the response of
M.G. González et al. / Sensors and Actuators B 150 (2010) 770–773
771
Fig. 1. Experimental setup.
our system to strong light scattering in the medium, spherical silica particles were used. The results are compared with those obtained with the colorimetric method. 2. Experimental setup The sensing strategy used in this work is the same applied by Chen et al. [12]. When the GNPs react with thiosulfate (S2 O3 2− ) in solution, Au(S2 O3 )2 3− complexes are formed instantaneously on the GNPs surfaces. Then, if Pb2+ and 2-mercaptoethanol (2-ME) are present, the GNP rapidly dissolves to form Au+ –2-ME complexes in solution. As a result, the size of the GNPs decreases significantly, allowing the quantification of the lead ions in the solution. The PA system used in this work is shown in Fig. 1. A frequencydoubled, Q-switched Nd:YAG laser (SL280 Spectron Laser System, UK; 532 nm, 6 ns pulse time, 10 Hz) was employed. The laser beam was focused by a lens (100 mm focal length) into a conventional 1 cm glass cuvette, equipped with an in-house made piezoelectric transducer on one side (side-on detection) [15]. The PA signals were amplified (HCA-100M-50k-C current amplifier, Femto, Germany) and recorded with a digital oscilloscope. All data presented here are averaged over 50 laser pulses. A fraction of the laser beam was coupled out onto a pyroelectric detector (Pyroelectric J25LPMB, Coherent, USA) for pulse energy monitoring. The oscilloscope was triggered by the Q-switch signal of the laser. The laser fluence was set above the laser fluence threshold for cavitation bubble formation. Therefore, the PA signal is generated by laserinduced nanobubbles (PA-LINB) [14,16]. The NP sizes were verified by Transmission Electron Microscopy (TEM), which were measured by a JEOL JEM 2010 instrument (JEOL GmbH; Germany).
In order to determine the optimum GNP size for our PA system, we prepared GNP of five different average sizes following the steps previously described in Refs. [17–19], with slight modifications. After the reaction between the GNP and S2 O3 2− , the lead ions and 2-ME were added, maintaining a constant ratio between the Pb2+ concentration and the GNP surface. After 3 h, the PA responses were measured (Fig. 2). As it can be appreciated, the factor = (S0 − S)/S (S0 and S are the PA response without and with Pb2+ , respectively) is inversely proportional to the GNP diameter. On the other hand, the GNP concentration depends on the background signal of our PA setup. Based on above results, we have chosen a concentration and diameter of 1.25 nM and 11 nm, respectively. As it is shown later, these values allow the lowest limit of detection and a wide linear range. For smaller particles, the relative size change is larger than for large particles, which make the detection more sensitive. As it can be seen in Fig. 3, the pulsed PA technique generally offers a good time resolution, since the response can be analyzed from pulse to pulse. However, it has to be stated that the temporal resolution of this specific lead detection scheme is limited by the kinetics of the leaching process, which is in the range of an hour.
3. Results In a first experiment, it was shown that the PA response generated by LINB is proportional to the GNP volume. Three samples, containing different amounts of lead, were measured. The ratios between the PA response with and without Pb2+ were compared with the corresponding information obtained from TEM images. In Table 1 the results are compared; similar results are found. Hence, the PA response can be considered as an indicator for the leaching process.
Fig. 2. vs. GNP diameter. The ration between the lead ion concentration and the GNP surface was kept constant.
Table 1 Comparison between the PA response and the GNP diameter change resulting from leaching. The NP sizes were verified by TEM. [Pb2+ ] (nM)
PA response (V/J)
Ratio of PA signals with and without lead S/S0
NP diameter (nm)
Ratio of particle size (TEM) with and without lead D3 /D03
0 100 250
539 ± 12 405 ± 19 238 ± 11
1.00 0.75 0.44
11.35 ± 0.97 10.46 ± 1.07 8.55 ± 1.38
1.00 0.78 0.43
772
M.G. González et al. / Sensors and Actuators B 150 (2010) 770–773
Fig. 3. vs. time. [S2 O3 2− ] = 1 mM, [Pb2+ ] = 1.5 M, [2-ME] = 1 mM.
The sensitivity of the system is of primary interest for this kind of technique. Using 1.25 nM of GNP (Dnp = 11 nm), containing 1 mM of 2-ME, a linear relationship between and the concentration of Pb2+ over the range from 1 nM to 2.5 M was obtained (Fig. 4). The limit of detection (S/N ratio = 3) was 0.5 nM. We repeated the procedure in the same way, measuring the extinction values at 520 nm with a UV/VIS spectrometer (colorimetric method). Approximately the same sensitivity was obtained. We observed saturation for concentrations larger than 3 M in both methods. In the case of the PA system, it is caused by the fact that the response generated by the NPs is lower than the background signal originating from the host liquid. In a scattering medium, the spatial distribution of the light is different from the one observed in a non-scattering liquid. The transmitted fraction of light is reduced even if no absorption takes place. In consequence, transmission based absorption measurements have a systematic error in scattering media. However, there is no such effect on the PA signal. Moreover, the piezoelectric transducer used in this work is largely insensitive to scattered light that directly impinges on the sensor surface, which is not the case for optical sensors. In order to show this advantage of the PA over the colorimetric method, the scattering coefficient of samples con-
Fig. 5. Lead ion detection in a highly scattering medium. Solid line: calibration curve as shown in Fig. 4. Squares: PA response and dots: extinction values at 520 nm. Scattering particles: 0.05 g/L of silica with 500 nm diameter. [S2 O3 2− ] = 1 mM, [GNP] = 1.25 nM, [2-ME] = 1 mM, [GNP] = 1.25 nM.
taining lead ions was incremented using spherical silica particles (diameter 500 nm). Fig. 5 shows the measurements of the PA system and the extinction values at 520 nm. As expected, the results corroborate that the PA-LINB technique is widely insensitive to light scattering, whilst the conventional method presents an error larger than 100%. Apart from the applicability for measurements in scattering media, we have no reason to assume that the performance of the PA based approach regarding selectivity and reaction to environmental conditions like temperature and pH is different from the classical UV/VIS-based approach as described in [12]. 4. Conclusions The measurement of trace contaminants in the presence of strong light scattering is an important challenge in environmental and biological analysis. In this paper we demonstrate that the pulsed photoacoustic spectrometry, based on the dissolution of GNPs, is a useful tool to monitor the presence of lead ions on highly scattering media. It is highly sensitive and suitable for the application in scattering media. Currently, we are attempting to decrease the electromagnetic background noise of the piezoelectric transducer. This way it should be possible increase sensitivity of the system by about one order of magnitude. In a next step we plan to construct a small and robust mobile system. Acknowledgements Grants from the Deutscher Akademischer Austauschdienst (DAAD), the Facultad de Ingeniería Universidad de Buenos Aires (FIUBA), and the China Scholarship Council (CSC) for two of the authors are gratefully acknowledged. References
Fig. 4. of the 2-ME/S2 O3 2− GNPs (1.25 nM, Dnp = 11 nm) in the presence of lead ions (0–10 M). Squares: PA response and dots: extinction values at 520 nm.
[1] H.L. Needleman, D. Bellinger, The health-effects of low-level exposure to lead, Annu. Rev. Public Health 12 (1991) 111–140. [2] H. Needleman, Lead poisoning, Annu. Rev. Med. 55 (2004) 209–222. [3] http://www.epa.gov/safewater/contaminants/index.html (accessed February 2010). [4] J.L. Anderson, E.F. Bowden, P.G. Pickup, Dynamic electrochemistry: methodology and application, Anal. Chem. 68 (1996) R379–R444. [5] A.L. Burlingame, R.K. Boyd, S.J. Gaskell, Mass spectrometry, Anal. Chem. 68 (1996) R599–R651.
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Biographies Martín Germán González obtained the title of a master’s degree in Electronic Engineering at the Faculty of Engineering at the University of Buenos Aires (FIUBA) in 2003. A year later, the same institution awarded him a fellowship for the doctorate, where, after 4 years of research, he received the endorsement Summa Cum Laude. In 2009, he worked as a post-doc researcher at the Chair for Analytical Chemistry, Technische Universität München. His work focuses mainly on research and development of laser sources, control pollution instrumentation, numerical models and methods of signal processing, new spectroscopy and photothermal techniques. Xiangjiang Liu obtained a bachelor’s degree in Chemistry at the Colleague of Chemistry & Chemical Engineering, Hunan University, P.R. China in 2004. In 2008, State Key Laboratory of Bio-Chemical Sensing & Chemometrics of Hunan University awarded him a Master degree in Analytical Chemistry. Since September 2009, he works as PhD student at the Chair for Analytical Chemistry, Technische Universität München. His work mainly focuses on development of new approaches for surface-enhanced Raman scattering, photoacoustic techniques and nanoparticles preparation. Reinhard Niessner, full professor for Analytical Chemistry at Technische Universität München is director of the Institute of Hydrochemistry and holds the Chair for Analytical Chemistry. His research interests are devoted to applications of laser spectroscopy, nanoparticle characterization, and microarray technologies. The main subjects of interest are aerosols, hydrocolloids and biofilms, within hydro- and atmosphere. He received several international awards, e.g. the EmanuelMerck-Prize for Analytical Chemistry (1990), the Smoluchowski-Award for Aerosol Research of the Association for Aerosol Research (1991), the Fritz-Pregl-Medal of the Austrian Society for Analytical Chemistry (1996), and the Fresenius-Award for Analytical Chemistry of the German Chemical Society (2000). Christoph Haisch studied General Physics at the Technische Universität München, where he also achieved his doctoral degree in the field of laser-induced atomic spectrometry. After post-doc positions in Paris and Berlin, he returned in 2001 to the Chair for Analytical Chemistry at TU München, heading the research group for Applied Laser Spectroscopy. His research interests range from opto-acoustical aerosol characterization, where a soot exhaust monitor was successfully commercialized, over new imaging systems for medical diagnostics to Raman and surface-enhanced Raman spectrometry for environmental analytical applications.