Sensors and Actuators B 138 (2009) 625–630
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Poly(vinyl alcohol) capped silver nanoparticles as localized surface plasmon resonance-based hydrogen peroxide sensor E. Filippo ∗ , A. Serra, D. Manno Dipartimento di Scienza dei Materiali, Università del Salento, I-73100 Lecce, Italy
a r t i c l e
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Article history: Received 30 January 2009 Received in revised form 23 February 2009 Accepted 23 February 2009 Available online 9 March 2009 Keywords: Biosensor Hydrogen peroxide Localized surface plasmon resonance Nanoparticles
a b s t r a c t A colorimetric hydrogen peroxide sensor based on localized surface plasmon resonance of poly(vinyl alcohol) capped silver nanoparticles is introduced. The silver nanoparticles are directly synthesized in the PVA matrix by a one-step method based on the reduction of the inorganic precursor AgNO3 through thermal treatment in aqueous medium. No other reagent is used. These nanoparticles are characterized using UV–vis spectroscopy, transmission electron microscopy and X-ray diffraction. Then they are used for the preparation, characterization and calibration of a highly sensitive, cost-effective and renewable localized surface plasmon resonance-based hydrogen peroxide sensor. The silver nanoparticles have the catalytic ability for the decomposition of hydrogen peroxide; then the decomposition of hydrogen peroxide induces the degradation of silver nanoparticles. Hence, a remarkable change in the localized surface plasmon resonance absorbance strength could be observed. As a result, the yellow colour of the silver nanoparticle–polymer solution was gradually changed to transparent colour. Furthermore, when this transparent solution was subjected to thermal treatment, it became again yellow and the UV–vis spectroscopy confirmed that nanoparticles were again formed, suggesting the renewability of this sensor. The determination of reactive oxygen species such as hydrogen peroxide has possibilities for applying to medical and environmental applications. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The analytical determination of hydrogen peroxide represents an important topic that has relevance pertaining to environmental, pharmaceutical, clinical and industrial research, as reported in many recent works. These studies include the determination of hydrogen peroxide in rainwater [1] and in disinfectant preparations [2], as well as its usage in textile, paper, and food industries [3]. Hydrogen peroxide determination can be carried out using several analytical techniques, such as titrimetry [4], spectrophotometry [5], and chemiluminescence [6]. Previously, we also proposed the fabrication of a new amperometric, nanostructured sensor for the analytical determination of hydrogen peroxide by immobilizing silver nanoparticles in a poly(vinyl alcohol) (PVA) film on a platinum electrode, which was performed by direct drop-casting silver nanoparticles reduced by ethanol and capped in a PVA colloidal suspension [7]. Cyclic voltammetry experiments yielded evidence that silver nanoparticles facilitate hydrogen peroxide reduction, showing excellent catalytic activity. Technique choice is determined by several factors, which can constitute an important impediment to effective application of the technique. In this context, localized sur-
∗ Corresponding author. Tel.: +39 832 297101; fax: +39 832 297100. E-mail address: emanuela.fi
[email protected] (E. Filippo). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.02.056
face plasmon resonance (LSPR)-based optical chemical sensors and biosensors are an appropriate low-cost and rapid alternative, or at least a complementary choice, with respect to the aforementioned techniques. LSPR is well known to be excited on nanostructured noble metals such as gold and silver. Noble metals have intrigued people for centuries because of their optical properties, which include the display of a bright glow in various attractive colours. As described by the Mie theory, these properties are strongly dependent on the size, shape, interparticle distance, and the local environment of the noble metal nanostructures [8]. Briefly, as the size of the metal structures decreases from the bulk-scale to the nano-scale, the movement of electrons through the internal metal framework is restricted. As a result, metal nanoparticles display specific extinction bands in the UV–vis spectra when the incident light resonates with the conduction band electrons on their surfaces. These charge density oscillations are simply defined as LSPR [9]. The LSPR resonant wavelength is extremely sensitive to the local environment around the nanoparticles, allowing for the development of the nanoparticle-based LSPR sensing devices and detection methods. The LSPR-based optical chemical sensor and biosensor, which are excited using noble metal nanomaterials such as gold and silver, have been expected to realize the highly sensitive detection of target molecules in medical applications [10–18]. In particular,
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recently Endo et al. reported the development of a localized surface plasmon resonance-based optical biosensor using polyvinylpyrrolidone coated silver nanoparticles for the detection of hydrogen peroxide [19]. The aim of our work is to propose the fabrication of a hydrogen peroxide sensor which was simplified, low-cost and first of all renewable. This work was based on two separate steps. The first step was the synthesis of silver nanoparticles directly in the PVA matrix by a one-step method based on a thermal treatment of the host polymeric matrix containing silver nitrate salt. No other reducing agent was used. Vinyl polymers having high density of polar groups in side chain can easy stabilise metal nanoparticles grown in the matrix at nanometric dimension because they wrap the particles avoiding the agglomeration [20]. These nanoparticles were characterized using UV–vis spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). The second step was the preparation and characterization of a renewable, good sensitive, cost-effective and simplified LSPR-based hydrogen peroxide sensor. The selectivity and calibration of this LSPRbased hydrogen peroxide sensor were evaluated by using UV–vis spectrophotometer. We are interested in the use of PVA because its wide range of potential applications in optical, pharmaceutical, medical, and membrane fields. In fact it is a water soluble polymer and allows the development of environment-friendly material processes [21]. Furthermore, silver has great potentials for applying to various applications as catalyst and sterilization [22]. 2. Experimental 2.1. Materials Silver nitrate (AgNO3 , 99%) and poly(vinyl alcohol) were from Sigma–Aldrich and used as received for the synthesis of silver nanoparticle–poly(vinyl alcohol) composites. Thirty percent (v/v) hydrogen peroxide solution as target molecule, sodium dihydrogenphosphate and disodium hydrogenphosphate for buffer preparation were purchased from Wako Pre Chemical Industries, Ltd. (Osaka, Japan). Deionized ultra-filtered water prepared with a Milli-Q water purification system has been used throughout the experiments. All glassware was washed by ultrasonication in a mixture of Millipore water and nonionic detergent, followed by thorough rinsing with Millipore water and ethanol for many times to get rid of any remnants of nonionic detergent and dried prior to use [23]. 2.2. Instrumentation A Varian Cary-100 spectrophotometer was used for the evaluation of the optical characteristics of the LSPR-based optical hydrogen peroxide sensor. Solution spectra were obtained by measuring the absorption of the prepared nanoparticle dispersions in a quartz cuvette with a 1-cm optical path and were recorded in the range between 300 and 800 nm. X-ray diffraction measurements were carried out in the reflection mode on a Mini Flex Rigaku model diffractometer with Cu K␣ radiation ( = 0.154056 nm). The X-ray diffraction data were collected at a scanning rate of 0.02 degrees per second in 2 ranging from 15◦ to 85◦ . Samples for XRD patterns were prepared by putting few drops of the solutions on glass substrate and drying slowly in air. TEM images and electron diffraction patterns were taken using a JEOL 2010 transmission electron microscope operated at 160 kV, representing the suitable acceleration voltage to obtain a sufficient resolution and minimal radiation damage of the material.
Specimens for TEM observations were prepared by drop-casting two drops of freshly prepared solutions containing Ag nanoparticles onto standard carbon supported 600-mesh copper grid and drying slowly in air naturally. 2.3. Synthesis of silver nanoparticles–PVA composites In a typical synthesis, PVA (0.5 g) was dissolved in hot deionized water (25 ml) under magnetic stirring in order to obtain a colourless transparent PVA solution. An aqueous solution of silver nitrate (8 mM, 250 ml) was prepared and added to the PVA solution to induce the Ag+ → Ag reaction. The mixture was kept on boiling for about 2 h, under the heating conditions (90 ◦ C) and the vigorous stirring. Under these reaction conditions, the apparent colour in the resulting sample Ag–PVA (consists of derived Ag-metal nanoparticles, which are dispersed and embedded in the PVA polymer matrix) deepens gradually from an achromatic colour in the beginning to a faint yellowish equilibrium colour. In order to retain a stable Ag–PVA colloid structure, which consists of Ag nanoparticles capped in PVA molecules, the sample was cooled at 20–25 ◦ C temperature just after the reaction [24]. 3. Results and discussions 3.1. Optical and structural characterization of Ag–PVA solution The absorption spectrum of the Ag–PVA solution is shown in Fig. 1a. The appearance of a sharp plasmon peak at ∼420 nm, due to surface plasmon resonance phenomena of the electrons in the conduction bands of silver, indicated the formation of silver colloids with nanometer-sized dimensions [25]. The formation of Ag nanoparticles after thermal treatment in the PVA matrix is also confirmed by X-ray diffraction analysis (Fig. 1b). All the prominent peaks at 2 values of about 38.1◦ , 44.3◦ , 64.4◦ , 77.4◦ and 81.6◦ representing the 111, 200, 220, 311 and 222 Bragg’s reflections of fcc structure of silver [26]. The peak centred at about 19◦ and the one close to 23 ◦ C are relative to the PVA crystalline phase and can be observed after the thermal treatment of PVA amorphous phase [21]. Typical TEM image of the silver nanoparticles dispersed in the polymer matrix is presented in Fig. 2, with the corresponding size distribution; it shows non agglomerated, well dispersed Ag nanoparticles, with a size ranging from 6 to 24 nm and a bimodal size distribution. According to the previous observations, the PVA molecules serve the two basic purposes of a reducing and a capping agent. As a result, when adding AgNO3 to a hot PVA solution, the continuous magnetic stirring refines PVA molecules of refreshed reactive surfaces in smaller PVA molecules, which serve as reactive reaction centres to operate a controlled Ag+ → Ag reaction in divided reaction groups. The Ag-metal particles obtained in this way keep to be embedded in a dispersed structure polymer matrix of refined PVA molecules at out-set of the reaction. No separate Ag-metal synthesis is involved. The silver nitrate reduction with the formation of Ag(0) in the polymeric film was promoted by the thermal induced degradation of the PVA host matrix occurred through the mechanism reported as follows: R–OH + Ag+ →R–O–Ag + H+ R–O + Ag → –R O + Ag0 R–OH + Ag+ →R O–Ag0 + H+ Here, R OH represents a PVA monomer, R O represents a monomer in a partially oxidized PVA at the reaction surfaces while H+ is an acid byproduct HNO3 [24].
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Fig. 1. (a) Absorption spectrum and (b) XRD pattern of the prepared Ag–PVA solution.
Fig. 2. (a) Typical TEM image of the silver nanoparticles embedded in PVA matrix with (b) the corresponding size distribution histogram.
3.2. Evaluation of Ag–PVA solution as LSPR-based optical hydrogen peroxide sensor For the evaluation of the optical characteristics of Ag–PVA solution as LSPR-based hydrogen peroxide sensor, 1000 l of different concentrations of hydrogen peroxide solutions diluted with 20 mM phosphate buffer (pH 7.4) were introduced into the Ag–PVA solution in a quartz cuvette; the change in its optical characteristics with time (0, 2, 4, 6, 10, 14, 20, 30, 60 and 80 min) was carried out by monitoring the peak at 420 nm in the UV–vis spectra. Fig. 3 shows the change of the optical characteristics of LSPRbased optical hydrogen peroxide sensor with time in the visible region (300–800) due to the introduction of 1 mM hydrogen peroxide solution. It was evident that after 80 min a drastic change in its optical characteristics could be observed and the yellow colour of the solution was changed to transparent colour. The change of the colour of the Ag–PVA solution and hence the LSPR absorbance change are due to the catalytic ability of silver for the decomposition of hydrogen peroxide [27]; the catalytic decomposition of hydrogen peroxide induces the degradation of silver nanoparticles. Hence, a remarkable change in the LSPR absorbance strength could be observed. In order to confirm that the LSPR absorbance strength change was affected only by hydrogen peroxide, phosphate buffer and distilled water were introduced into the Ag–PVA composite solution. No LSPR absorbance strength change could be observed. Based on the above mentioned mechanism of the LSPR-based optical sensor and experimental conditions, the evaluation of the
calibration characteristics was carried out. For this purpose, different concentrations of hydrogen peroxide solutions were introduced into the Ag–PVA composite solution and the change upon time in the optical characteristics were investigated. The relationship between the absorbance strength change after 80 min and hydrogen peroxide concentrations and between the absorbance strength
Fig. 3. LSPR optical characteristics change with time due to the introduction of 1 mM hydrogen peroxide.
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Fig. 4. (a) Relationship between the absorbance strength change after 80 min and hydrogen peroxide concentrations; (b) relationship between the absorbance strength change and time for the different hydrogen peroxide concentration.
change and time for the different hydrogen peroxide concentration could be observed, as illustrated in Fig. 4a and b, respectively. Furthermore, the detection limit of this optical sensor was 10−6 M. In order to further confirm the above mentioned mechanism, 10−5 M and 10−2 M hydrogen peroxide solutions were introduced in the Ag–PVA solution for 60 min; then some drops of these solutions were put on a glass substrate and dried at room temperature in air for some minutes. Subsequently, X-ray diffraction analysis was performed and the results are shown in Fig. 5. For comparison, it was also shown the diffraction pattern of the starting Ag–PVA solution. The well defined peak centred at about 19◦ and the small diffraction peak close to 23◦ are relative to the PVA crystalline phase, as already stated. It is evident that the intensity of the peaks relative to fcc crystalline silver decreased in the case of the sample treated with 10−5 M hydrogen peroxide solution and disappeared in the case of the sample treated with 10−2 M hydrogen peroxide solution. Nevertheless, the low amount of silver and the small size of metallic particles can explain the broadened reflection observed at 39◦ and the absence of other well defined diffraction peaks characteristic of silver [21], suggesting that the nanoparticles were slightly decomposed by 10−5 M hydrogen peroxide solution. The absence of the all diffraction peaks characteristic of silver indicated that a more concentrated hydrogen peroxide solution induced the com-
Fig. 5. XRD pattern of the Ag–PVA solution after the introduction of 10−5 M (curve b) and 10−2 M (curve c) hydrogen peroxide for 60 min and dried on a glass substrate for some minutes. It was also shown the diffraction pattern of the starting Ag–PVA solution (curve a).
plete decomposition of nanoparticles. These results are in good agreement with UV–vis spectroscopic observations. Moreover, compared with the XRD pattern of the Ag–PVA composite, two additional peaks are observed at 2 equal to about 29.5◦ and 32.6◦ ; they can be attributed to AgNO3 [21]. After X-ray diffraction analysis, the Ag–PVA solution treated with 10−2 M hydrogen peroxide solution was again annealed at a temperature of 90 ◦ C for about 1 h. It was observed that the colourless solution became yellow. The UV–vis spectrum acquired on this annealed solution is shown in Fig. 6 and it revealed the presence of a broad peak at around 420 nm. The spectrum of Ag sol regenerated in PVA matrix is less symmetric: shoulders were observed in the range of higher wavelengths, indicating the presence of some aggregate particles [28]. This observation confirms that the diffraction peaks at 2 equal to about 29.5◦ and 32.6◦ observed in Fig. 5 can be attributed to AgNO3 . In the renewed Ag–PVA solution, 10−2 M hydrogen peroxide solution was again injected in order to investigate the change in the optical characteristics with time by monitoring the peak at 420 nm in the UV–vis spectrum. It was evident that the renewed solution kept the same optical characteristics of the starting Ag–PVA solution, as shown in Fig. 7, confirming that Ag–PVA is a good candidate as renewable hydrogen peroxide LSPR sensor. We believed that the renewability of the sensor is linked to the fact that the thermal treatment of the host matrix gave rise only to a partial oxidation of the polymer that promotes the reduction of the colourless Ag+ ions to characteristic yellow metallic silver
Fig. 6. Absorption spectrum of the renewed Ag–PVA solution.
E. Filippo et al. / Sensors and Actuators B 138 (2009) 625–630
Fig. 7. Relationship between the absorbance strength change and time after the introduction of 10−2 M hydrogen peroxide for the starting Ag–PVA solution (1st Test) and the renewed Ag–PVA solution (2nd Test).
nanoparticles that result efficiently stabilised by the electron donor OH groups of the PVA [20]. Moreover, recent works concerning the evolution of the polymer structure as a function of the annealing temperature [21], demonstrated that the thermal treatment for longer time or at temperatures higher than 90 ◦ C gives rise to cross-linked PVA, which is insoluble in water. In particular, it was observed that the hybrid films prepared from an aqueous solution of PVA and silver nitrate thermally treated for 1 h at 110 ◦ C evidenced some insoluble fractions and the films treated 1 h at 160 ◦ C became even totally insoluble [21]. So we can suppose that the reduction of Ag+ ions into metallic silver nanoparticles can continue until the thermal treatment of PVA gave rise to a total oxidation or cross-linking of PVA host matrix. 4. Conclusions In the present investigation, Ag–PVA solutions were prepared through a simple chemical route, characterized by transmission electron microscopy, UV–vis spectroscopy and X-ray diffraction and used to realize a renewable, very fast and simple hydrogen peroxide sensor. In fact, these samples were tested as LSPR-based optical sensor for the detection and determination of hydrogen peroxide. A drastic LSPR absorbance change, which depends on the hydrogen peroxide concentration and time, could be observed and related to the degradation of silver nanoparticles. The measure of LSPR absorbance strength decrease makes possible to determine the concentration of hydrogen peroxide molecules. The thermal treatment of the transparent solution obtained after the degradation of the silver nanoparticles in Ag–PVA composite solution due to the introduction of hydrogen peroxide, caused the formation of new silver nanoparticles, as confirmed by UV–vis spectroscopy. The renewed Ag–PVA solution could be again used as hydrogen peroxide sensor. This detection method has a potential application in medical and environmental monitoring as a renewable, simplified and lowcost sensor and provides a pathway to sensing experiments with extremely simple, small, light, robust and low-cost instrumentation. References [1] R.C. Matos, J.J. Pedrotti, L. Angnes, Flow-injection system with enzyme reactor for differential amperometric determination of hydrogen peroxide, Anal. Chim. Acta 441 (1) (2001) 73–79.
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Biographies Emanuela Filippo received her physics degree and PhD degree both from University of Lecce, Italy in 2000 and 2005, respectively. Since 2000, she works at the Department of Material Science as fellow. Her research interest includes metal nanostructures synthesis and characterization and the study of thin films for applications in technology.
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Antonio Serra received his physics degree from University of Lecce, Italy in 1994. Actually, he is an associate professor of applied physics at the University of Salento. His main research activity includes structural, electrical and optical properties of semiconductor materials for applications in technology.
Daniela Manno received her physics degree from University of Lecce, Italy in 1985. Actually, she is an associate professor at experimental physics at the University of Salento. Her mean research activity concerns the characterization of nanostructured materials by means of transmission and scanning electron microscopy.
