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Cycle performance of sol–gel optical sensor based on localized surface plasmon resonance of silver particles
Sensors and Actuators B 113 (2006) 382–388
Cycle performance of sol–gel optical sensor based on localized surface plasmon resonance of silver particles Noritsugu Hashimotoa,b,∗ , Tadanori Hashimotob , Taichi Teranishib , Hiroyuki Nasub , Kanichi Kamiyab a
Ceramics Laboratory, Industrial Research Division, Mie Science and Technology Promotion Center, 788 Higashiakuragawa, Yokkaichi-shi, Mie 510-0805, Japan b Department of Chemistry for Materials, Faculty of Engineering, Mie University, 1577 Kurimamachiya-cho, Tsu-shi, Mie 514-8507, Japan Received 15 September 2004; received in revised form 1 March 2005; accepted 24 March 2005 Available online 19 April 2005
Abstract Silver particles generated by the evaporation–condensation method were deposited on the sol–gel derived silica film or the silica glass substrate to form SPR sensors, and their SPR sensing property was examined. When the sensing measurement was repeated, deposited silver particles gradually came off, and optical absorbance was decreased. To avoid these phenomena, silver particles were overcoated with silica thin film prepared by sonogel method. As a result, by adjusting the silica concentration of starting sol or the thickness of sono-silica overcoating, cycle performance of the sensor was improved. © 2005 Elsevier B.V. All rights reserved. Keywords: Optical sensor; Silver particle; Surface plasmon resonance; Cycle performance; Thin film; Evaporation–condensation method; Sol–gel method
1. Introduction Noble metal particles, such as gold, silver and copper, are very attractive because of their unique optical absorption in the visible light range caused by the localized surface plasmon resonance (LSPR). Optical properties based on the LSPR have been widely studied for the thin films dispersed with such metal particles prepared by the sol–gel [1,2], the evaporation–condensation [3] and sputtering [4] methods, and in addition, for metal particles-containing bulk glasses prepared by the melt-quenching [5], ion implantation [6] and gamma irradiation [7]. It is well known that the LSPR absorption wavelength of fine metal particles is a function of the dielectric constant of the medium surrounding the metal particles. The absorption coefficient, α, as a function of wavelength is given by the following equation [8]: ∗
Corresponding author. Tel.: +81 593312381; fax: +81 593317223. E-mail address: [email protected] (N. Hashimoto).
0167-0987/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.03.033
α=
2 18πpd 3/2 λ (1 + 2d )2 + 2 2
(1)
where p is the volume fraction of metal, λ is the light wavelength, d is the dielectric constant of surrounding medium, 1 and 2 are the real and imaginary dielectric constants of metal, respectively. The α has a maximum value at the LSPR wavelength under the following condition: 1 + 2d = 0
(2)
Generally, the d is expressed as the following equation using the refractive index, n: d = n2
(3)
Accordingly, from Eqs. (2) and (3), the LSPR absorption wavelength is also determined by n of the surrounding medium of metal particles. Recently, bio- and chemical sensors based on surface plasmon resonance (SPR) coupled with evanescent waves have been studied [9]. Furthermore, the application to the optical
N. Hashimoto et al. / Sensors and Actuators B 113 (2006) 382–388
sensor based on LSPR to identify the liquids has been proposed [10–12], which is the transmission type of SPR sensor monitoring the absorption peak position or intensity of transmitted probe-light at a fixed wavelength. This type of sensor may have some advantages over conventional ones monitoring the reflected light. Namely, it requires no prism coupler to attenuate the total reflection, so that the optical system is simpler and more compact. Furthermore, in principal, this sensor can be composed of only one particle, and the very small optical sensor which is applicable in a new field may be realized. In our previous work [13], the LSPR-based sensor film consisting silver particles was prepared by three simple methods, i.e., the sol–gel method, the evaporation–condensation method and combination of them. It was found that the deposition of silver particles by the evaporation–condensation method on the sol–gel derived silica film was effective to develop a highly sensitive optical sensor. However, the decrease of optical absorbance of the sensor film was encountered when it was repeatedly used, in other words, cycle performance of the sensor was not satisfying. In the present study, in order to improve cycle performance in the repetitive SPR sensing measurements, thin silica film was put on silver particles as the overcoating. The immersion of such a sensor film in ethanol was repeated 20 times, and cycle performance defined as the decrease of absorbance with cycle number was discussed. In addition, the sensitivity was evaluated using various immersion liquids with different refractive indices.
2. Experimental 2.1. Preparation of the sensor film The silver particles deposited-silicas (Ag/silica glass substrate and Ag/sol–gel silica film/silica glass substrate hereafter abbreviated Ag/substrate and Ag/SiO2 /substrate, respectively) were prepared by the evaporation–condensation method and the sol–gel method [3]. In the case of Ag/substrate, silver particles were deposited on the silica glass substrate directly. On the contrary, in the case of Ag/SiO2 /substarte, silver particles were deposited not on the glass substrate but on sol–gel derived silica film formed on the silica glass substrate. Fig. 1 shows the flowchart for preparing the Ag/SiO2 /substrate type sensor film. The synthetic silica (9 mm × 19 mm) with the thickness of 1 mm, which was purchased from Furuuchi chemical, was used as a substrate. Thin silica film was made on the substrate by the sol–gel dip–coating method, and silver particles were deposited on it using the evaporation–condensation method. To make the coating solution, tetraethoxysilane (TEOS; Wako Pure Chemical Industries) was diluted with 2methoxyethanol (CH3 OCH2 CH2 OH; Nacalai Tesque) and stirred at room temperature for 10 min, to which water containing hydrochloric acid (HCl; Kanto Kagaku)
383
Fig. 1. Flowchart for preparing Ag/SiO2 /substrate type sensor film.
was added under stirring. Additional stirring was carried out for 2 h. The molar ratio of chemicals in the resultant solution was TEOS:CH3 OCH2 CH2 OH:H2 O:HCl = 1:10:5:0.05. The dip–coating of the silica glass substrate was carried out with a withdrawal velocity of 3 cm min−1 , followed by drying the coating gel film at 100 ◦ C for 5 min. The film was made only on one side of the substrate by masking the other side. Silver granules (Soekawa chemical, purity 99.99%) were heated at 950 ◦ C in the ceramic pipe of 11 mm (i.d.) in the flow of N2 (5 L min−1 ), then generated silver gas was cooled to form silver nanoparticles, which were transported into the deposition chamber to be deposited on the silica glass substrate (Ag/substrate) or the sol–gel derived silica film made on the silica glass substrate (Ag/SiO2 /substrate) for 2 h at room temperature. Both of the composite films thus obtained were annealed at 200 ◦ C for 5 min to settle silver particles on silicas. In order to prevent the silver particles to come off the sol–gel derived silica film, silver particles were overcoated with another thin silica film prepared by sonogel method in which the sol–gel reaction of TEOS was progressed in the ultra-sonically agitated solution without adding HCl. The flowchart for preparing the thin silica overcoating on silver particles by sonogel method is shown in Fig. 2. The molar ratio of chemicals in the coating solution was TEOS:CH3 OCH2 CH2 OH:H2 O = 1:10:5. By means of sonication for 4 h after adding distilled water, the hydrolysis of TEOS was accomplished sufficiently. The resultant solution for coating was diluted with 2-methoxyethanol 200, 600, 800 and 1000 times in the volume to achieve various concentrations of silica. The coatings using these solutions were made by dip–coating method on the silver particles-deposited sol– gel derived silica film (SiO2 /Ag/SiO2 /substrate). Another sensor which was made by depositing silver particles directly on the silica glass substrate was
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Fig. 2. Flowchart for preparing thin silica coating on silver particles by sonogel method.
also overcoated with thin silica film prepared by sonogel method (SiO2 /Ag/substrate). The sonicated coating solution was diluted with 2-methoxyethanol 1000 times in the volume. Furthermore, more concentrated coating solution, TEOS:CH3 OCH2 CH2 OH:H2 O = 1:7:5, was also prepared under the sonication to fabricate SiO2 /Ag/substrate sensor. 2.2. Characterization, and measurements of the sensitivity and cycle performance X-ray diffraction measurements (XRD; Rigaku, RINT 2500, Cu K␣ source) were carried out to confirm the existence of silver particles in the composites. The absorption spectrum was measured using UV–vis spectrophotometer (Shimadzu, UV-3100) in the wavelength range between 200 and 800 nm. The surface morphology or microstructure, and the crosssection of the films were examined by the field-emission scanning electron microscope (FE-SEM; Hitachi, S-4100). SPR sensing properties were evaluated as follows. The absorption spectra of the films immersed in liquids of different refractive indices, n’s were measured in a quartz cell with 10 mm path length by UV–vis spectrophotometer and the LSPR absorption peak wavelength, λLSPR , was determined. Liquids used were distilled water (n = 1.333), methanol (CH3 OH; Nacalai Tesque, n = 1.337), ethanol (C2 H5 OH; Kyowa Sangyo, n = 1.370), 1-pentanol (C5 H11 OH; Wako Pure Chemical Industries, n = 1.421) and carbon disulfide (CS2 ; Nacalai Tesque, n = 1.674) (n’s were values at 434 nm). The sensitivity was defined as dλ/dn, i.e., the LSPR peak shift caused by a unit change in refractive index of the immersion liquid. For the evaluation of cycle performance of the film sensors, the absorption spectrum measurement was repeated 20 times for the film which immersed in ethanol. Absorbance, A, at λLSPR in the absorption spectrum was determined for each run. Then, cycle performance was defined as dA/dN, where N is the cycle number, i.e., the change of absorbance with the cycle number. The sensitivity was also evaluated for the film after the cycle performance measurement, namely, after the 20 times immersion in ethanol.
Fig. 3. XRD pattern of Ag/substrate type sensor film.
3. Results and discussion 3.1. Characterization of the sensor film Fig. 3 shows the XRD patterns of the composite film sensor of Ag/substrate structure. Two diffraction peaks are observed around 2θ = 38◦ and 44◦ , which are assigned as (1 1 1) and (2 0 0) diffraction lines of silver (JCPDS 4-783), respectively, the presence of silver particles in the film being confirmed. Fig. 4 shows the FE-SEM images of silver particles deposited on sol–gel derived silica film (a) without and (b) with the sono-silica overcoating, before and after 20 times immersion in ethanol. When the films without overcoatings on silver particles were immersed in different liquids, it was observed that small silver particles came off, leaving relatively large particles on the substrate. It is considered that the area of larger particles adhering to the substrate are large compared with that of small particles, and so the bonding between particles and sol–gel derived silica film is stronger. In Fig. 4(b), it is noticed that the diameter of silver particles is about 20 nm, and remains almost unchanged even after 20 times measurements. This fact indicates that the thin silica overcoating prepared by sonogel method is effective to suppress the silica particles to come off the underlying sol–gel derived silica film. FE-SEM image of the cross-section of SiO2 /Ag/ SiO2 /substrate composite film sensor is shown in Fig. 5. It is seen from FE-SEM observation that the film thickness is about 200 nm, and silver particles are successfully overcoated with the sono-silica film. 3.2. Cycle performance Fig. 6 shows the absorption spectrum change with the cycle number, N, for the SiO2 /Ag/SiO2 /substrate film in which silver particles have been overcoated with sono-silica using undiluted coating solution. Absorption spectra exhibited a peak due to LSPR around 400 nm, and no discernible change
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385
Fig. 4. FE-SEM images of surface for sensor film (a) Ag/SiO2 /substrate and (b) SiO2 /Ag/SiO2 /substrate.
of λLSPR with N was observed. On the other hand, absorbance at λLSPR was decreased as N was increased. These facts indicate that the repetitive measurement may not affect the sensitivity but cause the decrease of absorbance. Next to be discussed is cycle performance of the film sensor. Relationships between absorbance at λLSPR and the cycle number for the composite films of SiO2 /Ag/SiO2 /substrate and SiO2 /Ag/substrate are shown in Fig. 7(a) and (b), respectively. The “control” in Fig. 7(a) represents the data for the film without the sono-silica overcoating of silver
particles. It is seen that absorbance decreases in the N range of 1–10 and approached almost constant value above N = 11 because silver particles fixed loosely to sol–gel derived silica underlying film have come off during earlier measurements, and those fixed tightly have been left on it. Then, cycle performance, dA/dN, was calculated using the data with N = 1–10. The slope of linear lines in Fig. 7 corresponds to dA/dN. In Fig. 7(a), the rise and fall of absorbance for the film of SiO2 /Ag/SiO2 /substrate was seen, so that the dA/dN was not calculated. This suggests that the
Fig. 5. FE-SEM image of cross-section SiO2 /Ag/SiO2 /substrate structure.
Fig. 6. Absorption spectrum SiO2 /Ag/SiO2 /substrate.
for
sensor
film
with
changes
with
cycle
number
for
386
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Fig. 7. Relationship between absorbance at peak position and cycle number for (a) SiO2 /Ag/SiO2 /substrate and (b) SiO2 /Ag/substrate. The solutions with various concentrations were used for a coating on silver particles. Control corresponds data for the film without a coating on silver particles.
sono-silica is not overcoated on the underlying gel film successfully. As a result, the dielectric constant of the medium surrounding silver particles for SiO2 /Ag/SiO2 /substrate may be complicated compared with SiO2 /Ag/substrate, which causes the rise and fall of the absorbance. In addition, it is seen in Fig. 7(b) that the decrease of absorbance for the Ag/SiO2 /substrate without the sono-silica overcoating (“control”) is larger than other composite film with the overcoating (SiO2 /Ag/substrate). It should be noticed that sono-silica overcoating of silver particles is very useful to fix the silver particles to the underlying sol–gel silica film, as has been mentioned on the basis of FE-SEM observation. 3.3. SPR sensing property Fig. 8(a) and (b) shows the relationship between LSPR absorption peak wavelength and the refractive index of the liquid for the SiO2 /Ag/substrate and SiO2 /Ag/SiO2 /substrate composite film sensors, respectively. Solid lines in Fig. 8 are drawn by means of the least square fitting of data. From this figure, it is considered that all the composite films are
Fig. 8. Relationship between absorption peak wavelength due to LSPR, λLSPR , and refractive index of immersion liquids, n.
applicable for sensor to identify the liquids because LSPR absorption peak wavelength shifts linearly more or less with the refractive index change. Then, we estimated the sensitivity, dλ/dn, from the slope of fitted linear line, and summarized the results of the sensitivity together with cycle performance in Table 1. From Table 1, it was found that the sensitivity of the composite films was increased with increasing the degree of dilution of sono-silica coating solution. Eventually, it can be stated that when thinner sono-silica overcoating was made on silver particles, silver particles were exposed to more extent to the immersion liquids, and accordingly higher sensitivity was achieved in the composite film sensor. Furthermore, as seen in Table 1, dA/dN is decreased with increasing the degree of dilution of the solution for the sonosilica overcoating. It has been reported that the increase of the degree of dilution or the decrease of the concentration of silica in the coating solution leads to the decrease the thickness of the resultant silica coating film. Eventually, it can be stated that when thicker sono-silica overcoating was made on silver particles, silver particles were fixed to the substrate tightly, and accordingly higher cycle performance was achieved in the composite film sensor. In the present study, we obtained the lowest absolute value of dA/dN as −2.3 × 10−3 with 59.4 nm of dλ/dn for
N. Hashimoto et al. / Sensors and Actuators B 113 (2006) 382–388
387
Table 1 Cycle performance and the sensitivity of sensor films with coating on silver particles Sample Control SiO2 /Ag/SiO2 /substrate SiO2 /Ag/SiO2 /substrate SiO2 /Ag/SiO2 /substrate SiO2 /Ag/SiO2 /substrate SiO2 /Ag/SiO2 /substrate SiO2 /Ag/substrate SiO2 /Ag/substrate SiO2 /Ag/substrate a
Dilution of coating solution
Cycle performance, dA/dN
Sensitivity, dλ/dn (nm)
– Undiluted 200 600 800 1000 Concentrateda Undiluted 1000
−9.7 × 10−3
67.8 37.4 63.4 56.2 80.3 88.9 59.4 63.9 74.6
– – – – – −2.3 × 10−3 −6.8 × 10−3 −8.6 × 10−3
The molar ratio of coating solution was TEOS:CH3 OCH2 CH2 OH:H2 O = 1:7:5.
the composite film of SiO2 /Ag/substrate using concentrated overcoating solution. Although this sensitivity was slightly small compared to that for the film of Ag/substrate reported in our previous work [13], cycle performance was successfully improved by means of sono-silica overcoating on silver particles. It was concluded that the thin silica overcoating on silver particles was effective on improvement of cycle performance. 4. Conclusions Optical sensor based on LSPR was prepared by the evaporation–condensation method combined with the sol– gel method. Thin silica film prepared by sonogel method was overcoated on silver particles to improve cycle performance. To estimate the cycle performance, dA/dN, absorption spectra for sensor film immersed in ethanol were measured 20 times, and the sensitivity was also determined using various immersion liquid with different refractive indices. As a result, sono-silica overcoating on silver particles was effective to avoid the decrease of absorbance through the suppression of silver particles to come off. Especially, cycle performance of the composite film of SiO2 /Ag/substrate was successfully improved from dA/dN of −9.7 × 10−3 to −2.3 × 10−3 by overcoating, while the sensitivity, dλ/dn, was still high as 59.4 nm. References [1] J. Matsuoka, R. Mizutani, S. Kaneko, H. Nasu, K. Kamiya, K. Kadono, T. Sakaguchi, M. Miya, Sol–gel processing and optical nonlinearity of gold colloid-doped silica glass, J. Ceram. Soc. Jpn. 101 (1993) 53–58. [2] M. Epifani, C. Giannini, L. Tapfer, L. Vasanelli, Sol–gel synthesis and characterization of Ag and Au nanoparticles in SiO2 , TiO2 , and ZrO2 thin films, J. Am. Ceram. Soc. 83 (2000) 2385– 2393. [3] N. Hashimoto, T. Hashimoto, H. Nasu, K. Kamiya, Preparation of silver thin films consisting of nano-sized particles by the evaporation– condensation method and its linear and nonlinear optical properties, J. Ceram. Soc. Jpn. 112 (2004) 204–209 (in Japanese). [4] L. Yang, G.H. Li, L.D. Zhang, Effects of surface resonance state on the plasmon resonance absorption of Ag nanoparticles embedded in partially oxidized amorphous Si matrix, Appl. Phys. Lett. 76 (2000) 1537–1539.
[5] Y. Hamanaka, N. Hayashi, A. Nakamura, S. Omi, Dispersion of thirdorder nonlinear optical susceptibility of silver nanocrystal-glass composites, J. Lumin. 87–89 (2000) 859–861. [6] G. Battaglin, P. Calvelli, E. Cattaruzza, R. Polloni, E. Borsella, T. Cesca, F. Gonella, P. Mazzoldi, Laser-irradiation effects during Z-scan measurement on metal nanocluster composite glasses, J. Opt. Soc. Am. B 17 (2000) 213–218. [7] A. Pan, Z. Yang, H. Zheng, F. Liu, Y. Zhu, X. Su, Z. Ding, Changeable position of SPR peak of Ag nanoparticles embedded in mesoporous SiO2 glass by annealing treatment, Appl. Surf. Sci. 205 (2003) 323– 328. [8] G. Mie, Beitrage zer optik truber meiden speziell kolloidaler matallosungen, Ann. Phys. (Leipzig) 25 (1908) 377–445. [9] J. Homola, S.S. Yee, G. Gauglitz, Surface plasmon resonance sensors: review, Sens. Actuators B: Chem. 54 (1999) 3–15. [10] T. Okamoto, I. Yamaguchi, T. Kobayashi, Local plasmon sensor with gold colloid monolayers deposited upon glass substrates, Opt. Lett. 25 (2000) 372–374. [11] H. Xu, M. K¨all, Modeling the optical response of nanoparticle-based surface plasmon resonance sensors, Sens. Actuators B: Chem. 87 (2002) 244–249. [12] A.J. Haes, R.P. Van Duyne, A nanoscale optical biosensor: sensitivity and selectivity of an approach based on the localized surface plasmon resonance spectroscopy of triangular silver nanoparticles, J. Am. Chem. Soc. 124 (2002) 10596–10604. [13] N. Hashimoto, T. Hashimoto, K. Mori, H. Nasu, K. Kamiya, Effective deposition of nano-sized silver particles on silica to develop a sensitive local plasmon-based SPR sensor, J. Ceram. Soc. Jpn. (2004) S576– S578 (Supplement 112-1, PacRim5 Special Issue).
Biographies Noritsugu Hashimoto received his BS and MS degrees in chemical engineering from Osaka Prefecture University in 1996 and 1998, respectively, and received PhD degree in material science from Mie University in 2004. He is currently a researcher at Ceramics Laboratory, Industrial Research Division, Mie Science and Technology Promotion Center, Yokkaichi, Mie, Japan since 1998. Recently, his research interests focus on the optical properties of nano-sized metal particles. Tadanori Hashimoto received his BS and MS degrees in Chemistry for Materials from Mie University in 1989 and 1991, respectively, and received PhD degree in molecular engineering from Kyoto University in 1995. He joined Kyoto University as a research associate since 1994 till 1995. He is currently a research associate of Chemistry for Materials, Faculty of Engineering, Mie University, Tsu, Mie, Japan since 1995. Recently, his research interests focus on the ecologically sustainable optical glasses. Taichi Teranishi received his BS degree in Chemistry for Materials from Mie University in 2004, and has been a student in Chemistry for Materials, Graduate School of Engineering, Mie University, Tsu, Mie, Japan.
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Hiroyuki Nasu received his BS, MS and PhD degrees in industrial chemistry from Kyoto University in 1978, 1980 and 1983, respectively. He joined University of California at Los Angeles as a post-doctoral researcher since September 1983–February 1986. In 1986, he joined Hiroshima University as a research associate. He has been an associate professor of Chemistry for Materials, Faculty of Engineering, Mie University, Tsu, Mie, Japan since 1989.
Kanichi Kamiya received his BS, MS and PhD degrees in applied chemistry from Nagoya University in 1964, 1966 and 1969, respectively. He joined Toyota Physical and Chemical Research Institute as a researcher since 1969– 1971. During the period, he joined State University of New York at Buffalo as a post-doctoral researcher since October 1969–August 1970. In 1971, he joined Mie University as an associate professor, and has been a professor of Chemistry for Materials, Faculty of Engineering, Mie University, Tsu, Mie, Japan since 1983.
Cycle performance of sol–gel optical sensor based on localized surface plasmon resonance of silver particles Noritsugu Hashimotoa,b,∗ , Tadanori Hashimotob , Taichi Teranishib , Hiroyuki Nasub , Kanichi Kamiyab a
Ceramics Laboratory, Industrial Research Division, Mie Science and Technology Promotion Center, 788 Higashiakuragawa, Yokkaichi-shi, Mie 510-0805, Japan b Department of Chemistry for Materials, Faculty of Engineering, Mie University, 1577 Kurimamachiya-cho, Tsu-shi, Mie 514-8507, Japan Received 15 September 2004; received in revised form 1 March 2005; accepted 24 March 2005 Available online 19 April 2005
Abstract Silver particles generated by the evaporation–condensation method were deposited on the sol–gel derived silica film or the silica glass substrate to form SPR sensors, and their SPR sensing property was examined. When the sensing measurement was repeated, deposited silver particles gradually came off, and optical absorbance was decreased. To avoid these phenomena, silver particles were overcoated with silica thin film prepared by sonogel method. As a result, by adjusting the silica concentration of starting sol or the thickness of sono-silica overcoating, cycle performance of the sensor was improved. © 2005 Elsevier B.V. All rights reserved. Keywords: Optical sensor; Silver particle; Surface plasmon resonance; Cycle performance; Thin film; Evaporation–condensation method; Sol–gel method
1. Introduction Noble metal particles, such as gold, silver and copper, are very attractive because of their unique optical absorption in the visible light range caused by the localized surface plasmon resonance (LSPR). Optical properties based on the LSPR have been widely studied for the thin films dispersed with such metal particles prepared by the sol–gel [1,2], the evaporation–condensation [3] and sputtering [4] methods, and in addition, for metal particles-containing bulk glasses prepared by the melt-quenching [5], ion implantation [6] and gamma irradiation [7]. It is well known that the LSPR absorption wavelength of fine metal particles is a function of the dielectric constant of the medium surrounding the metal particles. The absorption coefficient, α, as a function of wavelength is given by the following equation [8]: ∗
Corresponding author. Tel.: +81 593312381; fax: +81 593317223. E-mail address: [email protected] (N. Hashimoto).
0167-0987/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.03.033
α=
2 18πpd 3/2 λ (1 + 2d )2 + 2 2
(1)
where p is the volume fraction of metal, λ is the light wavelength, d is the dielectric constant of surrounding medium, 1 and 2 are the real and imaginary dielectric constants of metal, respectively. The α has a maximum value at the LSPR wavelength under the following condition: 1 + 2d = 0
(2)
Generally, the d is expressed as the following equation using the refractive index, n: d = n2
(3)
Accordingly, from Eqs. (2) and (3), the LSPR absorption wavelength is also determined by n of the surrounding medium of metal particles. Recently, bio- and chemical sensors based on surface plasmon resonance (SPR) coupled with evanescent waves have been studied [9]. Furthermore, the application to the optical
N. Hashimoto et al. / Sensors and Actuators B 113 (2006) 382–388
sensor based on LSPR to identify the liquids has been proposed [10–12], which is the transmission type of SPR sensor monitoring the absorption peak position or intensity of transmitted probe-light at a fixed wavelength. This type of sensor may have some advantages over conventional ones monitoring the reflected light. Namely, it requires no prism coupler to attenuate the total reflection, so that the optical system is simpler and more compact. Furthermore, in principal, this sensor can be composed of only one particle, and the very small optical sensor which is applicable in a new field may be realized. In our previous work [13], the LSPR-based sensor film consisting silver particles was prepared by three simple methods, i.e., the sol–gel method, the evaporation–condensation method and combination of them. It was found that the deposition of silver particles by the evaporation–condensation method on the sol–gel derived silica film was effective to develop a highly sensitive optical sensor. However, the decrease of optical absorbance of the sensor film was encountered when it was repeatedly used, in other words, cycle performance of the sensor was not satisfying. In the present study, in order to improve cycle performance in the repetitive SPR sensing measurements, thin silica film was put on silver particles as the overcoating. The immersion of such a sensor film in ethanol was repeated 20 times, and cycle performance defined as the decrease of absorbance with cycle number was discussed. In addition, the sensitivity was evaluated using various immersion liquids with different refractive indices.
2. Experimental 2.1. Preparation of the sensor film The silver particles deposited-silicas (Ag/silica glass substrate and Ag/sol–gel silica film/silica glass substrate hereafter abbreviated Ag/substrate and Ag/SiO2 /substrate, respectively) were prepared by the evaporation–condensation method and the sol–gel method [3]. In the case of Ag/substrate, silver particles were deposited on the silica glass substrate directly. On the contrary, in the case of Ag/SiO2 /substarte, silver particles were deposited not on the glass substrate but on sol–gel derived silica film formed on the silica glass substrate. Fig. 1 shows the flowchart for preparing the Ag/SiO2 /substrate type sensor film. The synthetic silica (9 mm × 19 mm) with the thickness of 1 mm, which was purchased from Furuuchi chemical, was used as a substrate. Thin silica film was made on the substrate by the sol–gel dip–coating method, and silver particles were deposited on it using the evaporation–condensation method. To make the coating solution, tetraethoxysilane (TEOS; Wako Pure Chemical Industries) was diluted with 2methoxyethanol (CH3 OCH2 CH2 OH; Nacalai Tesque) and stirred at room temperature for 10 min, to which water containing hydrochloric acid (HCl; Kanto Kagaku)
383
Fig. 1. Flowchart for preparing Ag/SiO2 /substrate type sensor film.
was added under stirring. Additional stirring was carried out for 2 h. The molar ratio of chemicals in the resultant solution was TEOS:CH3 OCH2 CH2 OH:H2 O:HCl = 1:10:5:0.05. The dip–coating of the silica glass substrate was carried out with a withdrawal velocity of 3 cm min−1 , followed by drying the coating gel film at 100 ◦ C for 5 min. The film was made only on one side of the substrate by masking the other side. Silver granules (Soekawa chemical, purity 99.99%) were heated at 950 ◦ C in the ceramic pipe of 11 mm (i.d.) in the flow of N2 (5 L min−1 ), then generated silver gas was cooled to form silver nanoparticles, which were transported into the deposition chamber to be deposited on the silica glass substrate (Ag/substrate) or the sol–gel derived silica film made on the silica glass substrate (Ag/SiO2 /substrate) for 2 h at room temperature. Both of the composite films thus obtained were annealed at 200 ◦ C for 5 min to settle silver particles on silicas. In order to prevent the silver particles to come off the sol–gel derived silica film, silver particles were overcoated with another thin silica film prepared by sonogel method in which the sol–gel reaction of TEOS was progressed in the ultra-sonically agitated solution without adding HCl. The flowchart for preparing the thin silica overcoating on silver particles by sonogel method is shown in Fig. 2. The molar ratio of chemicals in the coating solution was TEOS:CH3 OCH2 CH2 OH:H2 O = 1:10:5. By means of sonication for 4 h after adding distilled water, the hydrolysis of TEOS was accomplished sufficiently. The resultant solution for coating was diluted with 2-methoxyethanol 200, 600, 800 and 1000 times in the volume to achieve various concentrations of silica. The coatings using these solutions were made by dip–coating method on the silver particles-deposited sol– gel derived silica film (SiO2 /Ag/SiO2 /substrate). Another sensor which was made by depositing silver particles directly on the silica glass substrate was
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Fig. 2. Flowchart for preparing thin silica coating on silver particles by sonogel method.
also overcoated with thin silica film prepared by sonogel method (SiO2 /Ag/substrate). The sonicated coating solution was diluted with 2-methoxyethanol 1000 times in the volume. Furthermore, more concentrated coating solution, TEOS:CH3 OCH2 CH2 OH:H2 O = 1:7:5, was also prepared under the sonication to fabricate SiO2 /Ag/substrate sensor. 2.2. Characterization, and measurements of the sensitivity and cycle performance X-ray diffraction measurements (XRD; Rigaku, RINT 2500, Cu K␣ source) were carried out to confirm the existence of silver particles in the composites. The absorption spectrum was measured using UV–vis spectrophotometer (Shimadzu, UV-3100) in the wavelength range between 200 and 800 nm. The surface morphology or microstructure, and the crosssection of the films were examined by the field-emission scanning electron microscope (FE-SEM; Hitachi, S-4100). SPR sensing properties were evaluated as follows. The absorption spectra of the films immersed in liquids of different refractive indices, n’s were measured in a quartz cell with 10 mm path length by UV–vis spectrophotometer and the LSPR absorption peak wavelength, λLSPR , was determined. Liquids used were distilled water (n = 1.333), methanol (CH3 OH; Nacalai Tesque, n = 1.337), ethanol (C2 H5 OH; Kyowa Sangyo, n = 1.370), 1-pentanol (C5 H11 OH; Wako Pure Chemical Industries, n = 1.421) and carbon disulfide (CS2 ; Nacalai Tesque, n = 1.674) (n’s were values at 434 nm). The sensitivity was defined as dλ/dn, i.e., the LSPR peak shift caused by a unit change in refractive index of the immersion liquid. For the evaluation of cycle performance of the film sensors, the absorption spectrum measurement was repeated 20 times for the film which immersed in ethanol. Absorbance, A, at λLSPR in the absorption spectrum was determined for each run. Then, cycle performance was defined as dA/dN, where N is the cycle number, i.e., the change of absorbance with the cycle number. The sensitivity was also evaluated for the film after the cycle performance measurement, namely, after the 20 times immersion in ethanol.
Fig. 3. XRD pattern of Ag/substrate type sensor film.
3. Results and discussion 3.1. Characterization of the sensor film Fig. 3 shows the XRD patterns of the composite film sensor of Ag/substrate structure. Two diffraction peaks are observed around 2θ = 38◦ and 44◦ , which are assigned as (1 1 1) and (2 0 0) diffraction lines of silver (JCPDS 4-783), respectively, the presence of silver particles in the film being confirmed. Fig. 4 shows the FE-SEM images of silver particles deposited on sol–gel derived silica film (a) without and (b) with the sono-silica overcoating, before and after 20 times immersion in ethanol. When the films without overcoatings on silver particles were immersed in different liquids, it was observed that small silver particles came off, leaving relatively large particles on the substrate. It is considered that the area of larger particles adhering to the substrate are large compared with that of small particles, and so the bonding between particles and sol–gel derived silica film is stronger. In Fig. 4(b), it is noticed that the diameter of silver particles is about 20 nm, and remains almost unchanged even after 20 times measurements. This fact indicates that the thin silica overcoating prepared by sonogel method is effective to suppress the silica particles to come off the underlying sol–gel derived silica film. FE-SEM image of the cross-section of SiO2 /Ag/ SiO2 /substrate composite film sensor is shown in Fig. 5. It is seen from FE-SEM observation that the film thickness is about 200 nm, and silver particles are successfully overcoated with the sono-silica film. 3.2. Cycle performance Fig. 6 shows the absorption spectrum change with the cycle number, N, for the SiO2 /Ag/SiO2 /substrate film in which silver particles have been overcoated with sono-silica using undiluted coating solution. Absorption spectra exhibited a peak due to LSPR around 400 nm, and no discernible change
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Fig. 4. FE-SEM images of surface for sensor film (a) Ag/SiO2 /substrate and (b) SiO2 /Ag/SiO2 /substrate.
of λLSPR with N was observed. On the other hand, absorbance at λLSPR was decreased as N was increased. These facts indicate that the repetitive measurement may not affect the sensitivity but cause the decrease of absorbance. Next to be discussed is cycle performance of the film sensor. Relationships between absorbance at λLSPR and the cycle number for the composite films of SiO2 /Ag/SiO2 /substrate and SiO2 /Ag/substrate are shown in Fig. 7(a) and (b), respectively. The “control” in Fig. 7(a) represents the data for the film without the sono-silica overcoating of silver
particles. It is seen that absorbance decreases in the N range of 1–10 and approached almost constant value above N = 11 because silver particles fixed loosely to sol–gel derived silica underlying film have come off during earlier measurements, and those fixed tightly have been left on it. Then, cycle performance, dA/dN, was calculated using the data with N = 1–10. The slope of linear lines in Fig. 7 corresponds to dA/dN. In Fig. 7(a), the rise and fall of absorbance for the film of SiO2 /Ag/SiO2 /substrate was seen, so that the dA/dN was not calculated. This suggests that the
Fig. 5. FE-SEM image of cross-section SiO2 /Ag/SiO2 /substrate structure.
Fig. 6. Absorption spectrum SiO2 /Ag/SiO2 /substrate.
for
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Fig. 7. Relationship between absorbance at peak position and cycle number for (a) SiO2 /Ag/SiO2 /substrate and (b) SiO2 /Ag/substrate. The solutions with various concentrations were used for a coating on silver particles. Control corresponds data for the film without a coating on silver particles.
sono-silica is not overcoated on the underlying gel film successfully. As a result, the dielectric constant of the medium surrounding silver particles for SiO2 /Ag/SiO2 /substrate may be complicated compared with SiO2 /Ag/substrate, which causes the rise and fall of the absorbance. In addition, it is seen in Fig. 7(b) that the decrease of absorbance for the Ag/SiO2 /substrate without the sono-silica overcoating (“control”) is larger than other composite film with the overcoating (SiO2 /Ag/substrate). It should be noticed that sono-silica overcoating of silver particles is very useful to fix the silver particles to the underlying sol–gel silica film, as has been mentioned on the basis of FE-SEM observation. 3.3. SPR sensing property Fig. 8(a) and (b) shows the relationship between LSPR absorption peak wavelength and the refractive index of the liquid for the SiO2 /Ag/substrate and SiO2 /Ag/SiO2 /substrate composite film sensors, respectively. Solid lines in Fig. 8 are drawn by means of the least square fitting of data. From this figure, it is considered that all the composite films are
Fig. 8. Relationship between absorption peak wavelength due to LSPR, λLSPR , and refractive index of immersion liquids, n.
applicable for sensor to identify the liquids because LSPR absorption peak wavelength shifts linearly more or less with the refractive index change. Then, we estimated the sensitivity, dλ/dn, from the slope of fitted linear line, and summarized the results of the sensitivity together with cycle performance in Table 1. From Table 1, it was found that the sensitivity of the composite films was increased with increasing the degree of dilution of sono-silica coating solution. Eventually, it can be stated that when thinner sono-silica overcoating was made on silver particles, silver particles were exposed to more extent to the immersion liquids, and accordingly higher sensitivity was achieved in the composite film sensor. Furthermore, as seen in Table 1, dA/dN is decreased with increasing the degree of dilution of the solution for the sonosilica overcoating. It has been reported that the increase of the degree of dilution or the decrease of the concentration of silica in the coating solution leads to the decrease the thickness of the resultant silica coating film. Eventually, it can be stated that when thicker sono-silica overcoating was made on silver particles, silver particles were fixed to the substrate tightly, and accordingly higher cycle performance was achieved in the composite film sensor. In the present study, we obtained the lowest absolute value of dA/dN as −2.3 × 10−3 with 59.4 nm of dλ/dn for
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Table 1 Cycle performance and the sensitivity of sensor films with coating on silver particles Sample Control SiO2 /Ag/SiO2 /substrate SiO2 /Ag/SiO2 /substrate SiO2 /Ag/SiO2 /substrate SiO2 /Ag/SiO2 /substrate SiO2 /Ag/SiO2 /substrate SiO2 /Ag/substrate SiO2 /Ag/substrate SiO2 /Ag/substrate a
Dilution of coating solution
Cycle performance, dA/dN
Sensitivity, dλ/dn (nm)
– Undiluted 200 600 800 1000 Concentrateda Undiluted 1000
−9.7 × 10−3
67.8 37.4 63.4 56.2 80.3 88.9 59.4 63.9 74.6
– – – – – −2.3 × 10−3 −6.8 × 10−3 −8.6 × 10−3
The molar ratio of coating solution was TEOS:CH3 OCH2 CH2 OH:H2 O = 1:7:5.
the composite film of SiO2 /Ag/substrate using concentrated overcoating solution. Although this sensitivity was slightly small compared to that for the film of Ag/substrate reported in our previous work [13], cycle performance was successfully improved by means of sono-silica overcoating on silver particles. It was concluded that the thin silica overcoating on silver particles was effective on improvement of cycle performance. 4. Conclusions Optical sensor based on LSPR was prepared by the evaporation–condensation method combined with the sol– gel method. Thin silica film prepared by sonogel method was overcoated on silver particles to improve cycle performance. To estimate the cycle performance, dA/dN, absorption spectra for sensor film immersed in ethanol were measured 20 times, and the sensitivity was also determined using various immersion liquid with different refractive indices. As a result, sono-silica overcoating on silver particles was effective to avoid the decrease of absorbance through the suppression of silver particles to come off. Especially, cycle performance of the composite film of SiO2 /Ag/substrate was successfully improved from dA/dN of −9.7 × 10−3 to −2.3 × 10−3 by overcoating, while the sensitivity, dλ/dn, was still high as 59.4 nm. References [1] J. Matsuoka, R. Mizutani, S. Kaneko, H. Nasu, K. Kamiya, K. Kadono, T. Sakaguchi, M. Miya, Sol–gel processing and optical nonlinearity of gold colloid-doped silica glass, J. Ceram. Soc. Jpn. 101 (1993) 53–58. [2] M. Epifani, C. Giannini, L. Tapfer, L. Vasanelli, Sol–gel synthesis and characterization of Ag and Au nanoparticles in SiO2 , TiO2 , and ZrO2 thin films, J. Am. Ceram. Soc. 83 (2000) 2385– 2393. [3] N. Hashimoto, T. Hashimoto, H. Nasu, K. Kamiya, Preparation of silver thin films consisting of nano-sized particles by the evaporation– condensation method and its linear and nonlinear optical properties, J. Ceram. Soc. Jpn. 112 (2004) 204–209 (in Japanese). [4] L. Yang, G.H. Li, L.D. Zhang, Effects of surface resonance state on the plasmon resonance absorption of Ag nanoparticles embedded in partially oxidized amorphous Si matrix, Appl. Phys. Lett. 76 (2000) 1537–1539.
[5] Y. Hamanaka, N. Hayashi, A. Nakamura, S. Omi, Dispersion of thirdorder nonlinear optical susceptibility of silver nanocrystal-glass composites, J. Lumin. 87–89 (2000) 859–861. [6] G. Battaglin, P. Calvelli, E. Cattaruzza, R. Polloni, E. Borsella, T. Cesca, F. Gonella, P. Mazzoldi, Laser-irradiation effects during Z-scan measurement on metal nanocluster composite glasses, J. Opt. Soc. Am. B 17 (2000) 213–218. [7] A. Pan, Z. Yang, H. Zheng, F. Liu, Y. Zhu, X. Su, Z. Ding, Changeable position of SPR peak of Ag nanoparticles embedded in mesoporous SiO2 glass by annealing treatment, Appl. Surf. Sci. 205 (2003) 323– 328. [8] G. Mie, Beitrage zer optik truber meiden speziell kolloidaler matallosungen, Ann. Phys. (Leipzig) 25 (1908) 377–445. [9] J. Homola, S.S. Yee, G. Gauglitz, Surface plasmon resonance sensors: review, Sens. Actuators B: Chem. 54 (1999) 3–15. [10] T. Okamoto, I. Yamaguchi, T. Kobayashi, Local plasmon sensor with gold colloid monolayers deposited upon glass substrates, Opt. Lett. 25 (2000) 372–374. [11] H. Xu, M. K¨all, Modeling the optical response of nanoparticle-based surface plasmon resonance sensors, Sens. Actuators B: Chem. 87 (2002) 244–249. [12] A.J. Haes, R.P. Van Duyne, A nanoscale optical biosensor: sensitivity and selectivity of an approach based on the localized surface plasmon resonance spectroscopy of triangular silver nanoparticles, J. Am. Chem. Soc. 124 (2002) 10596–10604. [13] N. Hashimoto, T. Hashimoto, K. Mori, H. Nasu, K. Kamiya, Effective deposition of nano-sized silver particles on silica to develop a sensitive local plasmon-based SPR sensor, J. Ceram. Soc. Jpn. (2004) S576– S578 (Supplement 112-1, PacRim5 Special Issue).
Biographies Noritsugu Hashimoto received his BS and MS degrees in chemical engineering from Osaka Prefecture University in 1996 and 1998, respectively, and received PhD degree in material science from Mie University in 2004. He is currently a researcher at Ceramics Laboratory, Industrial Research Division, Mie Science and Technology Promotion Center, Yokkaichi, Mie, Japan since 1998. Recently, his research interests focus on the optical properties of nano-sized metal particles. Tadanori Hashimoto received his BS and MS degrees in Chemistry for Materials from Mie University in 1989 and 1991, respectively, and received PhD degree in molecular engineering from Kyoto University in 1995. He joined Kyoto University as a research associate since 1994 till 1995. He is currently a research associate of Chemistry for Materials, Faculty of Engineering, Mie University, Tsu, Mie, Japan since 1995. Recently, his research interests focus on the ecologically sustainable optical glasses. Taichi Teranishi received his BS degree in Chemistry for Materials from Mie University in 2004, and has been a student in Chemistry for Materials, Graduate School of Engineering, Mie University, Tsu, Mie, Japan.
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Hiroyuki Nasu received his BS, MS and PhD degrees in industrial chemistry from Kyoto University in 1978, 1980 and 1983, respectively. He joined University of California at Los Angeles as a post-doctoral researcher since September 1983–February 1986. In 1986, he joined Hiroshima University as a research associate. He has been an associate professor of Chemistry for Materials, Faculty of Engineering, Mie University, Tsu, Mie, Japan since 1989.
Kanichi Kamiya received his BS, MS and PhD degrees in applied chemistry from Nagoya University in 1964, 1966 and 1969, respectively. He joined Toyota Physical and Chemical Research Institute as a researcher since 1969– 1971. During the period, he joined State University of New York at Buffalo as a post-doctoral researcher since October 1969–August 1970. In 1971, he joined Mie University as an associate professor, and has been a professor of Chemistry for Materials, Faculty of Engineering, Mie University, Tsu, Mie, Japan since 1983.