Polymer optical fiber SERS sensor with gold nanorods

Optics Communications 282 (2009) 439–442

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Polymer optical fiber SERS sensor with gold nanorods Zhiguo Xie, Jun Tao, Yonghua Lu *, Kaiquan Lin, Jie Yan, Pei Wang, Hai Ming * Department of Physics, University of Science and Technology of China, Anhui Key Laboratory Optoelectronic Science and Technology, Hefei, Anhui 230026, China

a r t i c l e

i n f o

Article history: Received 7 July 2008 Received in revised form 26 September 2008 Accepted 10 October 2008

PACS: 33.20.Fb 07.07.Df 42.81.Pa

a b s t r a c t A surface-enhanced Raman scattering sensor is developed by etching polymer optical fiber and coating with gold nanorods. The SERS sensing experiments are demonstrated with the analyte molecules of rhodamine 6G (R6G) at 514.5 nm laser excitation. The results show that a strong fiber Raman background scattering overwhelm the R6G molecule Raman signal in common optrod configuration, but a distinct R6G SERS spectrum with 9 order magnitude enhancement can be observed while directly focusing light on the probe. Further modeling indicates the enhancement is attributed to both nanorods local field and their coupling. Ó 2008 Elsevier B.V. All rights reserved.

Keywords: Surface-enhanced Raman scattering Polymer optical fiber Nanorods

1. Introduction Surface-enhanced Raman scattering (SERS) has been extensively used for molecules identification in chemical, biological and environmental industries due to amplifying molecular Raman signal with many orders of magnitude [1]. The development of rugged, relatively inexpensive reproducible substrates is critical to the application of SERS as an analysis technology. Recently, an exciting technique is to combine metal nanostructure with the optical fiber, which makes the substrate portable, robust and stable. Many optical fiber SERS sensors are reported and the related excellent results indicate great potential in molecules sensing [2–4]. The inherent flexibility and durability of the sensors contribute to safe handling and using in vivo environment. However, to fabricate a conventional silica fiber into a SERS probe, it requires of removing partially protective coating, which brings a problem of fragility and vulnerability from silica material. To overcome this problem, the use of polymer optical fiber (POF) is an optional means. The POF has shown a more flexible, effective, and robust performance in the sensing of displacement [5], pressure [6] and humidity [7]. Moreover, the POF exhibits low cost and perfect biocompatibility making them suitable for biomedical applications [8,9]. In this paper, a plastic optical fiber SERS sensor is developed with gold nanorods. We utilize a convenient drying process to de* Corresponding authors. Tel.: +86 551 3607223; fax: +86 551 3601745. E-mail addresses: [email protected] (Y. Lu), [email protected] (H. Ming). 0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.10.018

posit gravitationally a gold particles layer onto a specially treated POF tip with chemical etching technology, and use the metalcoated POF probe to demonstrate the SERS sensing of R6G molecules. These results show the potential application of the POF in SERS sensor. 2. Experiment The polymer optical fiber in the experiment is purchased from Toray Industries, called SI-POF (SH4001), a 1.0 mm fiber diameter and 2.2 mm jacket diameter. We use etching method [10] to prepare the POF tip in order to enable the monitoring of small area in potential application in chemical and biological field. The fiber is cut into segment of 10 cm and the jacket is removed, and then is immersed in a cyclopentanone solution. The profile of the tip can be controlled by solvent evaporation speed. Fig. 1a shows the POF tip used in our experiment, about 2–3 lm in diameter. Gold nanorods are stable and sensitive in biochemistry, and they exhibit higher surface-enhanced Raman cross sections than nanosphere [11]. The shape of nanorods can be tuned to modify the electromagnetic distribution, and it has been shown that large SERS enhancement factors can be achieved by the specially aligned nanorods [12]. Gold nanorods are synthesized using seed-mediated growth method [13] in aqueous surfactants. Gold seed of 4 nm average diameters are prepared by chemical reduction of HAuCl4 by NaBH4 in the presence of trisodium citrate. To synthe-

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Absorption

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Fig. 1. (a) Photograph of POF taper with 400 microscope, (b) TEM (transmission electron of microscopy) image of the gold nanorods, and (c) the UV–Vis absorption of the gold nanorods.

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Raman shift (cm-1) Fig. 2. (a) Experimental schematic diagram in optrode configuration and (b) Raman spectrum, Curve A: obtained from the metal-coated taper tip with R6G molecules. Curve B: obtained from the same length plastic optical fiber, both with excitation power 0.3 mW, scan time 10 s.

size gold nanorods, HAuCl4 is reduced by ascorbic acid in the presence of seed, CTAB and NaOH. Gold nanorods are separated form spherical nanoparticles by centrifugation. The TEM image of gold nanorods is shown in Fig. 1b, the width and length are about 15 nm and 45 nm, respectively. The UV–Vis absorption of gold nanorods is given in Fig. 1c. The transverse plasmon resonant wavelength of the nanorods is 520 nm, the longitudinal plasmon resonant wavelength is 650 nm. Several drops of gold nanorods colloids are placed directly on the POF tip and dried naturally. With water evaporation, the more and more particles are trapped on the POF tip, and a gold particle layer is formed on POF tip. The cluster of nanorods can induce more ‘‘hot spot” for enormous SERS enhancement. We use a 514.5 nm argon ion laser as the excitation source and a Raman spectrometer RAMANLOG 6 as the detector. The POF SERS probe is dipped into R6G solution (10 9 M) and then taken out for dryness. SERS experiments are carried out with common ‘‘optrod”

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Raman shift (cm-1) Fig. 3. (a) Experimental schematic diagram for focusing light on the tip and (b) Curve A: R6G molecules SERS signal obtained from the metal-coated taper tip; Curve B: signal from the same concentration R6G solution absorbed on glass slide without gold nanorods, both with excitation power 0.3 mW, scan time 3 s.

configuration [2,4,14], in which the fiber is used to both transport the exciting laser radiation and collect the Raman scattering from analyte, as shown in Fig. 2a. The Raman spectrum is obtained with power 0.3 mW and scan time 10 s as shown in curve A in Fig. 2b. The same length plastic optical fiber is tested as seen in curve B in Fig. 2b. No R6G SERS signal is observable in Fig. 2b curve A, except for molecules fluorescence background. The Raman background of POF overwhelms the spectrum of R6G dye, whose major bands appear between 1000 cm 1 and 1800 cm 1. The Raman spectrum of POF extends to 3000 cm 1 [15,16], so the optrod configuration is restricted for POF SERS in organic molecules sensing. Another experiment is carried out by focusing directly the light on the tip with an inverted microscope objective, which is also used for collecting the signal. The spectrum is achieved in Fig. 3b curve A. all peaks are consistent with previous report on R6G molecules [17,18]. For a comparison, the same concentration analyte is measured on glass slide without gold nanorods, the R6G strong fluorescent spectrum is shown in Fig. 3b curve B.

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Fig. 4. Cross sectional view of the E-field enhancement distribution, the incident light is perpendicular the view face; the light polarization is parallel to the longitudinal axis of gold nanorods in (a,b,c) and is perpendicular to the axis in (d,e,f).

3. Results and discussion In Fig. 3b, the two spectra of the same concentration R6G molecules are collected on the metal-coated POF tip and on glass slide. The distinct SERS signal is obtained with the gold nanorods-coated POF tip as seen in Fig. 3b curve A. No regular Raman signal is detectable on glass slide as shown in Fig. 3b curve B. R6G molecules are highly fluorescent excited with 514.5 nm laser, but when the molecules adsorbed on metal substrates, the fluorescence is quenches and the Raman intensity is dramatically enhanced [19]. Generally, the ratio between the SERS intensity of per molecule and the regular intensity of per molecule is used to account for SERS enhancement factor EF. But the regular Raman signal can be hardly detected for R6G molecules on 514.5 nm excitation. So we estimate the enhancement in the comparison of Raman scattering cross section and fluorescence cross section as described in Ref. [20]. The laser is focused on the 1 lm2 scale by the object lens, we can approximately consider the number of molecules on POF tip and on glass slide as the same order magnitude. The Raman peak height at 1365 cm 1 is about 50 counts/ mW/s in Fig. 3b Curve A, while the fluorescence signal of the corresponding position is roughly 280 counts/mW/s in Fig. 3b Curve B. So Raman signal is about two orders of magnitude less than the fluorescence signal. Since the fluorescence cross section for R6G is on the order of 10 16 cm2, the conventional Raman scattering cross section is on the order of 10 30 cm2 at 514.5 nm [21]. Subtracting the enhancement of 4 order magnitude from resonance Raman scattering [20], SERS enhancement can arrive at 9 order magnitude, which can be attributed to the enhancement of the electric field between nanorods in the cluster [12]. To clarify the effect of the nanorods assembly on the enhancement of local field, we simulate electromagnetic field distribution with DDSCAT code [22]. The structural parameters of the nanorods is from TEM images in Fig. 1b, length 45 nm and width 15 nm, exaction wavelength is 514.5 nm from the above experiment. Due to the shape anisotropy, nanorods can assembly via two orientation modes: end-to-end and side-by-side [23,24]. The space between nanorods is about 3 nm, obtained from Ref. [23]. Fig. 4 illustrates a cross sectional view of the E-field enhancement distribution. The E field enhancement of signal nanorod occurs at the tips of the cylinder, which is consistent with the so-called lightening-rod effect. A stronger enhancement can be observed from the junctions be-

tween nanoparticles with appropriate polarization light owing to field coupling. 4. Conclusion In conclusion, a plastic optical fiber SERS probe is presented. The sensing experiment is demonstrated using 514.5 nm laser. Common optrod configuration brings strong plastic optical fiber Raman background, and covers over molecules Raman information. By focusing the light directly on the tip, a clear SERS signal with the enhancement of 9 order magnitude can be achieved. The enhancement mechanism is simulated, and the results show the enhancement can be attributed to the local field of gold nanorods and their coupling, which can induce larger enhancement. Acknowledgements This work is supported by the National Key Basic Research Program of China No. 2006CB302905, the Key Program of National Natural Science Foundation of China No. 60736037, the National Natural Science Foundation of China No. 10704070 and the High Technology Research and Development Program of China under Grant No. 2007AA06Z420 the Science and Technological Fund of Anhui Province for Outstanding Youth (08040106805). References [1] C.L. Haynes, A.D. McFarland, R.P. Van Duyne, Anal. Chem. 77 (2005) 338. [2] D.L. Stokes, T. Vo-Dinh, Sens. Actuators, B – Chem. 69 (2000) 28. [3] Y. Zhang, C. Gu, A.M. Schwartzberg, J.Z. Zhang, Appl. Phys. Lett. 87 (2005) 23105. [4] A. Lucotti, G. Zerbi, Sens. Actuators, B – Chem. 121 (2007) 356. [5] F. Jimenez, J. Arrue, G. Aldabaldetreku, G. Durana, J. Zubia, O. Ziemann, C.A. Bunge, Appl. Opt. 46 (2007) 6256. [6] M. Firak, D. Radosevic, Exp. Therm. Fluid Sci. 25 (2001) 311. [7] C.M. Tay, K.M. Tan, S.C. Tjin, C.C. Chan, H. Rahardjo, Microwave Opt. Technol. Lett. 43 (2004) 387. [8] F. Baldini, P. Bechi, S. Bracci, F. Cosi, F. Pucciani, Sens. Actuators, B – Chem. 29 (1995) 164. [9] N.B. Kosa, Proc. SPIE 1592 (1991) 114. [10] D.F. Merchant, P.J. Scully, N.F. Schmitt, Sens. Actuators, A – Phys. 76 (1999) 365. [11] B. Nikoobakht, M.A. El-Sayed, J. Phys. Chem. A 107 (2003) 3372. [12] S.B. Chaney, S. Shanmukh, R.A. Dluhy, Y.P. Zhao, Appl. Phys. Lett. 87 (2005) 031908. [13] B. Nikoobakht, M.A. EL-Sayed, Chem. Mater. 15 (2003) 1957.

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