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Photochemical Modification of an Optical Fiber Tip with a Silver Nanoparticle Film: A SERS Chemical Sensor Xianliang Zheng, Dangwei Guo, Yunliang Shao, Shaojie Jia, Shuping Xu, Bing Zhao, and Weiqing Xu* State Key Laboratory of Supramolecular Structure and Materials, Jilin UniVersity, Changchun 130012, PR China
Charlie Corredor and John R. Lombardi* Department of Chemistry, The City College of New York, New York, New York 10031 ReceiVed December 20, 2007. In Final Form: January 30, 2008 We present a method of photochemical modification of an optical fiber tip with a silver nanoparticle film. The deposited silver nanoparticle film displays alternating light and dark circles, which are similar to a radial diffraction pattern. The modified optical fiber is examined as a chemical sensor for in situ detection. The modified fibers show excellent SERS activity, a low limit of detection (LOD), and good reproducibility. The maximum SERS activity of the sensor was achieved within 5.0 min of deposition. Thus, the method is also quite rapid.
1. Introduction 1970s,1
Since its discovery in the surface-enhanced Raman scattering (SERS) spectroscopy has attracted increasing interest as a typical nonlinear optical phenomenon.2 The SERS technique can provide a huge amplification of normal Raman signals when molecules approach the surface of noble metal substrates. This effect is based on the excitation of the collective oscillation of free electrons on the surface of a substrate, namely, surface plasmon resonance (SPR).3 Under SPR conditions, an intense electromagnetic field arises and is located around the surface of the substrate. In this way, both incoming light and the outgoing Raman signals can be significantly amplified.4 As we know normal Raman signals are very low in intensity because of the small cross-section (typically 10-30-10-25 cm2 per molecule), which restricts the development of Raman spectroscopy as a universal analytic tool. The employment of the SERS techniques overcomes this drawback. The intensity of a normal Raman signal can be amplified by an enhancement factor of >106 or even by as much as 1014 so that a single molecule can be detected5 when using SERS techniques. However, this huge enhancement factor is based on the employment of SERSactive substrates, such as silver nanoparticle colloids. Numerous attempts have been made to obtain efficient strategies to control the growth of silver nanoparticles and modulate their size and shape. Photochemical methods utilize the photosensitive proper* To whom all correspondence should be addressed. (W.X.) E-mail:
[email protected]. Fax: +86-431-85193421. Tel: +86-431-85168505. (J.R.L.) E-mail:
[email protected]. Fax: 212-650-6848. Tel: 212650-6032. (1) (a) Fleischmann, M.; Hendra, P. J.; MeQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163-166. (b) Jeanmaire, D. L.; Van Duyne, R. P. J. Electroanal. Chem. Interfacial Electrochem. 1977, 84, 1. (2) (a) Xia, Y. N.; Halas, N. J. MRS Bull. 2005, 30, 338-348. (b) Haes, A. J.; Haynes, C. L.; McFarland, A. D.; Schatz, G. C.; Van Duyne, R. P.; Zou, S. L. MRS Bull. 2005, 30, 368-375. (3) Haynes, C. L.; McFarland, A. D.; Van Duyne, R. P. Anal. Chem. 2005, 338A. (4) Baumberg, J. J.; Kelf, T. A.; Sugawara, Y.; Cintra, S.; Abdelsalam, M. E.; Bartlett, P. N.; Russell, A. E. Nano Lett. 2005, 5, 2262-2267. (5) (a) Nie, S.; Emory, S. R. Science 1997, 275, 1102-1106. (b) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Ltzkan, L.; Dasari, R. R.; Feld, M. S. Phys. ReV. Lett. 1997, 78, 1667-1670.
ties and abilities of the SPR response of the silver element to synthesize well-controlled silver nanoparticles in size and shape with the help of reducing agents and stabilizers. Nanoparticles in the shape of triangles,6,7 pentagons,7 disks,8 and hexagons9 have been prepared by using the photochemical reduction of silver ions in the presence of citrate and silver nanoseeds. However, these colloids have some disadvantages in applications as SERS substrate, such as their poor stabilization in suspension.10 In recent years, several photoinduced methods have been developed to deposit silver or gold nanoparticles on a glass slide or silica. Bjerneld et al.11 used a system containing silver nitrate and sodium citrate to deposit a silver nanoparticle film at the interface of a glass slide and a silver ion solution at the focus of a laser beam. Sanchez-Cortes et al.12 employed a similar method and prepared a silver nanoparticle film on several kinds of substrates for in situ detection without using citrate. However, none has attempted to apply this photoinduced method to the deposition of a silver nanoparticle film on the end of a fiber tip. This could provide a useful method for modifying optical fibers with SERS-active substrates and using them as a chemical SERS sensor. In this article, we use the photoinduced process to immobilize silver nanoparticles on the end of an optical fiber tip. The deposited silver nanoparticles give a pattern of radial diffraction, and the modified optical fibers produce a sizable enhancement of the Raman intensity of probing molecules. The modified optical fiber could be used for the in situ detection of trace species. (6) (a) Jin, R. C.; Cao, Y. W.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901-1903. (b) Jin, R. C.; Cao, C.; Hao, E.; Me´traux, G. S.; Schatz, G. C.; Mirkin, C. A. Nature 2003, 425, 487-490. (c) Xue, C.; Mirkin, C. A. Angew. Chem., Int. Ed. 2007, 46, 2036. (7) Zheng, X. L.; Xu, W. Q.; Corredor, C.; Xu, S. P.; An, J.; Zhao, B.; Lombardi, J. R. J. Phys. Chem. C 2007, 111, 14962-14967. (8) Maillard, M.; Huang, P.; Brus, L. Nano Lett. 2003, 3, 1611-1615. (9) An, J.; Tang, B.; Ning, X.; Zhou, J.; Xu, W.; Zhao, B.; Xu, W.; Corredor, C.; Lombardi, J. R. J. Phys. Chem. C 2007, 111, 18055-18059. (10) Pieczonka, N.; Aroca, R. Chem. Phys. Chem. 2005, 6, 2473-2484. (11) (a) Bjerneld, E. J.; Murty, K. V. G. K.; Prikulis, J.; Ka¨ll, M. Chem. Phys. Chem. 2002, 3, 116. (b) Bjerneld, E. J.; Svedberg, F.; Ka¨ll, M. Nano Lett. 2003, 3, 593. (12) Canamares, M. V.; Garcia-Ramos, J. V.; Gomez-Varga, J. D.; Domingo, C.; Sanchez-Cortes, S. Langmuir 2007, 23, 5210.
10.1021/la703993j CCC: $40.75 © 2008 American Chemical Society Published on Web 03/15/2008
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Figure 1. Setup for the modification process and in situ detection.
The SERS activity of the silver nanoparticle film has been quantified by a probing molecule, BPENB (1,4-bis[2-(4-pyridyl)ethenyl]-benzene), modified on the fiber tip. It has a large π-bond conjugated system and two active nitrogen atoms (as shown in Figure 4). It was originally designed and synthesized as an antiseptic by Chiang and Hartung.13 It has been employed as a compound to build up an artificial photoresponsive supramolecular system by Yam et al.14 In recent years, it has been found that this molecule possesses excellent SERS activity.15 A sample of BPENB was kindly provided by Lixin Wu from the State Key Laboratory of Supermolecular Structure and Materials, Jilin University. 2. Experimental Section 2.1. Materials. AgNO3 (99.5%) was purchased from Wako Pure Chemical Industry, Ltd. Trisodium citrate (98%) was obtained from Shanghai Chemical Reagent Co., Ltd. All chemicals used were analytic-grade reagents without further purification. BPENB was synthesized by the literature procedure.14 Silica optical fiber (100 µm in diameter) was purchased from Nanjing Chunhui Science & Technology Industrial Co., Ltd. 2.2. Sample Preparation. Growth solution was prepared by mixing 1.0 × 10-3 M AgNO3(aq) and 1.0 × 10-3M aqueous trisodium citrate in a molar ratio of 1:1, and then it was kept in the dark at room temperature for further use. The addition of citrate was found to accelerate the growth process greatly. Background spectra taken without adsorbed molecules show no interference from the citrate spectrum. Optical fiber was cut into segments of 6 cm length, and both ends were carefully polished. Plastic jackets (0.5 cm each) were removed from both ends of the fibers by dipping them in condensed sulfuric acid and then rinsing them with acetone, ethanol, and deionized water. The cleaned segments were dried in air and kept in a desiccator until use. 2.3. Laser-Induced Deposition Process. The laser-induced deposition of silver nanoparticles on the optical fiber was carried out on a cofocal micro-Raman spectrometer (Renishaw 1000, 514.5 nm laser line, 4.2 mW). The experimental setup is displayed in (13) Chiang, M.-C.; Hartung, W. H. J. Org. Chem. 1945, 10, 21. (14) Yam, V. W.; Lau, V. C.; Wu, L. X. J. Chem. Soc., Dalton Trans. 1998, 1461. (15) (a) Jia, H. Y.; Zeng, J. B.; Song, W.; An. J.; Zhao, B. Thin Solid Films 2006, 496, 281-287. (b) Li, X. L.; Xu, W. Q.; Wang, X.; Jia, H. Y.; Zhao, B.; Li, B. F.; Ozaki, Y. Thin Solid Films 2004, 457, 372-380. (c) Li, X. L.; Zhang, J. H.; Xu, W. Q.; Jia, H. Y.; Wang, X.; Yang, B.; Zhao, B.; Li, B. F.; Ozaki, Y. Langmuir 2003, 19, 4285-4290. (d) Li, X.; Xu, W.; Zhang, J.; Jia, H.; Yang, B.; Zhao, B.; Li, B.; Ozaki, Y. Langmuir 2004, 20, 1298-1304.
Figure 2. Optical images of the silver deposition film on the M end at different deposition times of (a-f) 0.0, 1.0, 2.0, 3.0, 4.0, and 6.0 min, respectively. Figure 1. A clean segment of optical fiber was fixed on a XYZ stage by a homemade fiber holder in which the position of the optical fiber can be finely adjusted. For the modification process, one end of the fiber tip (marked as the M end) was immersed in the growth solution, and then the laser beam was introduced into the fiber core. As the modification process finished, the segment was taken out and carefully rinsed with deionized water to remove the excess silver ions on the deposition film and then dried with pure N2. Optical photographs of the deposition of the silver nanoparticle film were obtained by positioning the M end at the focal point of the objective lens and were taken by the CCD camera, which was attached to the microscope. 2.4. Detection of the SERS Spectrum. SERS spectra of BPENB (1.0 × 10-5M) were extracted by the cofocal Raman spectrometer mentioned in section 2.3. Before detecting the spectrum, the M end of the segment, which was modified with the deposited film, was immersed in the BPENB solution for about 1.0 min. Then without removing the optical fiber from the BPENB solution, the SERS spectrum of BPENB was extracted by the Raman spectrometer with the detection setup being similar to that of the modification process as shown in Figure 1. 2.5. Instruments. The SERS spectrum was detected with a Renishaw 1000 microspectrometer connected to a Leica microscope with an objective lens of 10× (NA ) 0.22). The spectra were obtained under a laser power of 0.33 mW, an accumulation time of 10 s, and an excitation wavelength of 514.5 nm. The SEM micrographs were obtained on a field-emission scanning electron microscope (FESEM, JSM-6700F) with the acceleration voltage of 3 kV.
3. Results and Discussions Figure 2 shows the optical photographs of the silver nanoparticle film deposited on the M end. The photographs were taken at different deposition times, namely, 0.0, 1.0, 2.0, 3.0, 4.0, and 6.0 min, respectively. From these photographs, the deposition and growth process of the silver nanoparticle film can be legibly observed. The system investigated here is photosensitive in that no silver nanoparticles can be found to deposit on
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Figure 3. (A-C) SEM images of the silver deposited film on the fiber tip with the deposition time of about 4.0 min. (D) SEM image of deposited silver nanoparticles at the light circles. (E) SEM image of deposited silver nanoparticles at the dark circles. (F) Pattern of the laser beam as it was introduced into and transmitted through the optical fiber. (G) Size distribution of the silver nanoparticles on the M end of the optical fiber.
the M end if no laser beam is introduced into the fiber core. The M end, at this moment, was clean and smooth without any nanoparticles collected on the surface. On introducing the laser beam into the fiber core, silver nanoparticles began to form on the surface of the M end. At a deposition time of 1.0 min, a few nanoparticles were deposited on the surface of the M end, and most of them were concentrated in the center whereas only a handful were found in the surrounding area. Two minutes later, more nanoparticles formed on the surface of the M end. Interestingly, the nanoparticles deposited in the surrounding area gradually formed into a pattern of concentric rings. The silver concentric rings were spaced by some dark areas, which is similar to the case of radial diffraction. As the deposition time increased, the radial diffraction pattern on the M end was more visible as displayed in Figure 2e,f. To investigate the interesting pattern of the silver nanoparticle film deposited on the M end, we took the corresponding SEM images as shown in Figure 3. First, we scanned a large-area image of the silver nanoparticle deposition film, which is displayed in Figure 3A. As can be seen, the deposition film in the surrounding area shows a set of concentric rings alternating between light and dark. In the light circles, the deposited silver nanoparticles were large in number and relatively large in size. The silver nanoparticles in these areas appeared in the shapes of triangles, disks, square-like objects, and other polygons (Figure 3D), with their sizes narrowly dispersed in the 80-90 nm range (Figure 3G). Moreover, the nanoparticles were crowded so that they became stacked and aggregated (Figure 3D). In the dark circles, the deposited nanoparticles were greatly reduced in number, as well as in size (Figure 3E). Their sizes were measured to be about 35 nm (Figure 3G). The difference in the number of the deposited silver nanoparticles, the area coverage, and the corresponding size of the nanoparticles made the deposited film on the M end resemble alternating light and dark circles, which is very similar to the pattern of radial diffraction (Figure 3F). The possible process of silver deposition and ring pattern formation on the M end is described as follows. As noted above, the growth solution that we used in the experiment was photosensitive in that the growth process of silver nanoparticles can be controlled by tuning the light intensity as well as by
changing the excitation wavelength that we discussed previously.7 When the laser beam was introduced into the fiber core, the growth of the silver nanoparticles was triggered. The nanoparticles formed both at the M end/growth solution interface and in the solution. The nanoparticles that formed at the interface could be easily trapped on the surface of the M end and formed the deposition film. It is evident that the pattern of the laser beam (Figure 3F) is similar to that for the silver nanoparticle deposition film, especially with respect to the location of the alternating light and dark circles, which are due to the difference in light intensity caused by monochromatic interference when the laser travels through the fiber core. In other words, the growth and deposition process on the surface of the M end is not uniform. It is influenced by the dispersion of light intensity, which displays an alternating light and dark pattern on the M end. Where the light intensity is strong, the growth and deposition process is faster, and the number and size of the deposited silver nanoparticles are greatly increased. As shown in Figure 3F, the light intensity in the center area is much stronger, so the silver deposition film first formed here. In the surrounding area, the reaction preferably occurs at the location where the light is brighter. Gradually, the radial diffraction pattern formed on the surface of the M end in a manner similar to the laser intensity. It has been widely accepted that the occurrence of SPR produces an intense local electromagnetic field (LEF) around the surface of the substrate and that the incident light and the excited spectral signals would be amplified by several orders of magnitude. This phenomenon would be more remarkable in the spaces between aggregated nanoparticles, which are named hot spots, where large electromagnetic field are excited; consequently, the Raman signals would be greatly enhanced.16 The SEM images displayed in Figure 3 show that the silver nanoparticles in the deposition film are widely aggregated in that a large number of hot spots could be produced. In this case, the SERS activity of this deposition film modified on the optical fiber tip should be excellent if applied as a chemical SERS sensor. To investigate the SERS activity of the deposition film, we used BPENB (1.0 × 10-5 M) as the probing molecule. (16) Hao, E.; Schatz, G. C. Chem. Phys. 2004, 120, 357-366.
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Figure 4. (A, B) SERS spectra of BPENB (1.0 × 10-5 M) detected by the sensor prepared at different deposition times. The laser power is 0.33 mW, the accumulation time is 10 s, and the excitation wavelength is 514.5 nm. (C) Intensity of SERS signals at 1597 cm-1 as a function of deposition time for SERS sensor preparation. The inset is the structure of BPENB.
Figure 4 shows the SERS spectra of BPENB detected by the optical fiber modified with a silver nanoparticle deposition film on the M end. When detecting, the M end of the optical fiber was immersed in the BPENB solution, and the SERS signals were extracted in situ at a measurement geometry of 180° as shown in Figure 1. The set of SERS spectra shown in Figure 4 was obtained by the detection of BPENB with the substrates prepared at different deposition times, that is, 2.0, 3.0, 4.0, 4.5, 5.0, and 5.5 min. As can be seen, the Raman signal of BPENB was greatly enhanced. Several intense bands can be observed in the spectrum (shown in Figure 4A). The 1632, 1597, 1547, and 1420 cm-1 bands were assigned to the CdC and C-C stretching mode; the 1333 cm-1 band was assigned to the C-H deformation mode; the 1181 cm-1 band was assigned to the Py-C-H deformation; and the 1012 cm-1 band was the Py-C-H inplane bend.17 There was also a broad band spanning the range from 200 to 800 cm-1, which is attributed to the background of the optical fiber. We can also observe that the intensity of BPENB changed by using substrates prepared at different deposition times, which could reflect the difference in the SERS activity of these deposition films. To compare the intensity evolution of SERS spectra, we measured the peak intensity at 1597 cm-1. The measured result is shown in Figure 4C. As can be seen, the intensity at 1597 cm-1 changed as a function of deposition time: from 2.0 to 4.5 min, the intensity increased sharply and attained its maximum at 4.5 min; from 4.5 to 5.5 min, the intensity decreased gradually as the deposition time lengthened. Therefore, we can conclude that the optical fiber modified with the deposition film possesses optimum SERS activity at a deposition time of 4.5 min. To apply this modified optical fiber as a chemical sensor, other parameters should be measured (e.g., the limit of detection (LOD) and the reproducibility). First, we measured the LOD of BPENB by using the optical fiber sensor prepared at a deposition of 4.5 min. Different concentrations of BPENB were detected. (17) Ceng, J. B.; Li, X. L.; Song, W.; Xu, W. Q.; Zhao, B.; Zhang, G. Chem. Phys. Lett. 2005, 405, 344.
Figure 5. SERS spectra of BPENB at different concentrations, that is, 1.0 × 10-6, 1.0 × 10-7, 1.0 × 10-8, and 1.0 × 10-9 M, respectively, detected by the sensor prepared at a deposition time of 4.5 min. The laser power is 0.5 mW, the accumulation time is 10 s, and the excitation wavelength is 514.5 nm. The inset spectrum shows the intensity of SERS signals at 1597 cm-1 as a function of the concentration of BPENB (mol/L).
Figure 5 shows the SERS spectra of BPENB at concentrations of 1.0 × 10-6, 1.0 × 10-7, 1.0 × 10-8, and 1.0 × 10-9 M. The inset spectrum shows intensities at 1597 cm-1 as a function of the concentration of BPENB (mol/L). For the case of 1.0 × 10-9 M, the peaks in the spectrum can still be clearly identified. Therefore, the LOD of BPENB detected by this optical fiber sensor is 1.0 × 10-9 M. As can also be observed, the intensity of the SERS signal increases as the concentration of BPENB increases. Moreover, the inset spectrum shows a quasi-linear correlation between the intensity at 1597 cm-1 and the concentration of BPENB, which indicates that the concentration
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as the contrast. About 15 sensors were used, and the estimated result is shown in Figure 6. As can be seen, the intensities at 1597 cm-1 are dispersed in a relatively narrow range between 5.0 × 105 and 6.0 × 105, and the corresponding standard deviation is calculated to be 8.9%, which is as good as or even better than that for the optical fiber sensors reported previously.18 Possessing good SERS activity, low LOD, and good reproducibility, the optical fiber modified with the silver nanoparticle deposition film might be able to serve as a good chemical SERS sensor for online, in situ, and remote detection. We should also mention that when the molecule is only weakly adsorbed the fiber tip may be washed with a solvent and reused. However, if the adsorption is too strong, then the tip must be discarded.
4. Conclusions Figure 6. SERS spectra of BPENB (1.0 × 10-5 M) detected by numerous sensors prepared at a deposition time of 3 min. The laser power is 0.33 mW, and the accumulation time is 10 s. The inset spectrum represents the intensity at 1597 cm-1 collected from different sensors.
of BPENB can be determined by measuring the intensity of SERS signals. We also compared the LOD of BPENB with optical fiber sensors prepared by the reported methods.18 The optical fiber sensors prepared by two classical methods, namely, vacuum evaporation of silver islands and static assembly of silver colloids, were used as the references. The LODs of BPENB obtained by these two kinds of sensors were about 1.0 × 10-9 M,18f which is as good as for the sensor prepared by the laser-induced process. In this case, we conclude that the optical fiber sensor that we prepared here is comparable to theirs. Then, we estimated the reproducibility of the optical fiber sensor. The reproducibility is relative to the SERS activity of the sensor, which is obtained by comparing the intensities of SERS signals detected by numerous sensors prepared at a deposition time of 3 min. The intensity of BPENB at 1597 cm-1 was chosen (18) (a) Viets, C.; Hill, W. J. Mol. Struct. 2001, 565-566, 515. (b) Viets, C.; Hill, W. J. Raman Spectrosc. 2000, 31, 625. (c) Stokes, D. L.; Vo-Dinh, T. Sens. Actuators, B 2000, 69, 28. (d) Polwart, E.; Keir, C. M.; Smith, W. E.; Sadler, D. A. Appl. Spectrosc. 2000, 54, 522. (e) Xu, W. Q.; Xu, S. P.; Lu¨, Z. C.; Chen, L.; Zhao, B.; Ozaki, Y. Appl. Spectrosc. 2004, 58, 414-419. (f) Xu, W. Q.; Xu, S. P.; Hu, B.; Wang, K. X.; Zhao, B.; Xie, Y. T.; Fan, Y. G. Chem. J. Chin. U. 2004, 25, 144-147.
A photoinduced growth and deposition process was applied to modify an optical fiber tip with a silver nanoparticle film. The deposited silver nanoparticle film displayed an interesting pattern alternating with light and dark circles, which was very similar to a radial diffraction pattern. The silver nanoparticles in the light circles were relatively large in size and widely aggregated, but they were smaller and fewer in number in the dark circles. The modified optical fiber was then tested as a chemical sensor, which gave a good enhancement of BPENB, displayed a low LOD, and provided good reproducibility that is comparable to that of the sensor prepared by previous methods. The photochemical method discussed in this article has proven to be an efficient route to the preparation of good SERS-active substrates. The maximum SERS activity can be achieved with a deposition time of 4.5 min, which is very rapid. Acknowledgment. All experimental work was performed at Jilin University. This work was supported by the National Natural Science Foundation of China (nos. 20773045, 20627002, and 20573041). We are also indebted to the National Institute of Justice (Department of Justice award no. 2006-DN-BX-K034) and the City University Collaborative Incentive program (no. 80209). This work was also supported by the National Science Foundation under cooperative agreement no. RII-9353488, grants CHE-0091362, CHE-0345987, and ECS0217646, and the City University of New York PSC-BHE Faculty Research Award Program. LA703993J
