Gold Nanoparticles-Modified Tapered Fiber Nanoprobe ... - IEEE Xplore

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 8, APRIL 15, 2014

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Gold Nanoparticles-Modified Tapered Fiber Nanoprobe for Remote SERS Detection Zhenyi Chen, Zhangmin Dai, Na Chen, Shupeng Liu, Fufei Pang, Bo Lu, and Tingyun Wang, Fellow, IEEE

Abstract— This letter presents a surface-enhanced Raman scattering (SERS) nanoprobe based on gold nanoparticlesmodified tapered optical fiber and demonstrates its ability to perform remote Raman detection. The nanoscale tapered fiber with the tip size of 40.7 nm was made by heated pulling and chemical etching methods. The gold nanoparticles, prepared beforehand by the Frens method with a microwave heating process, were deposited on the tapered surface of the nanoprobe with the electrostatic self-assembly technology. Raman spectra of Rhodamine 6G (R6G) molecules were measured, using this SERS nanoprobe in an optrode remote detection mode. Considerably high signal-to-noise ratio and high sensitivity were achieved. The detection limit for R6G aqueous solution reaches 10−8 mol/L. Index Terms— Optical fiber sensors, nanostructures, Raman scattering, spectroscopy.

I. I NTRODUCTION

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AMAN scattering is known as the fingerprint of a molecule, though it is exceedingly weak due to its limited scattering cross section between 10−30 and 10−25. Many enhanced methods were developed to address this weakness [1]. Surface-enhanced Raman scattering (SERS) has been extensively studied for its extraordinary ability to enhance the weak Raman scattering signal since it was first proposed in 1974 [2]. Twofold enhancement mechanisms were generally accepted as its principle, which include electromagnetic mechanism and chemical mechanism [1]. The SERS substrates with different morphologies (size, shape et al.) have a great influence on the plasmon resonance. Hence, various structural substrates were proposed and investigated [3]–[5]. In recent years, the combined technology of SERS and optical fiber sensing emerges and attracts public attention.

Manuscript received December 5, 2013; revised February 9, 2014; accepted February 10, 2014. Date of publication February 13, 2014; date of current version March 20, 2014. This work was supported in part by the Natural Science Foundation of China under Grants 61027015, 61177088, and 61107076, in part by the National Program on Key Basic Research Project (973 Program) under Grant 2012CB723405, and in part by the Key Laboratory of Specialty Fiber Optics and Optical Access Networks under Grants SKLSFO2012-01 and SKLSFO2011-02. Z. Chen, Z. Dai, S. Liu, F. Pang, and T. Wang are with the Key Laboratory of Specialty Fiber Optics and Optical Access Networks, Shanghai University, Shanghai 200072, China (e-mail: [email protected]; daizhangmin4262@ 163.com; [email protected]; [email protected]; [email protected]). N. Chen is with the Key Laboratory of Specialty Fiber Optics and Optical Access Networks, Shanghai University, Shanghai 200072, China, and also with the Physics of Lasers, Atoms and Molecules Laboratory, Lille 1 University, IRCICA Research Institute for Advanced Communication, Villeneuve d’Ascq Cedex 59658, France (e-mail: [email protected]). B. Lu is with the Laboratory for Microstructures, Shanghai University, Shanghai 200444, Shanghai, China (e-mail: [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2014.2306134

This technology show advantages such as the capability to identify molecular structure, the high sensitivity of the SERS, and remote sensing with a flexible optical fiber. This class of devices with remote capabilities is often called “optrode” [6], in which the optical fiber is used as the waveguide to transmit both the forward excitation light and the backward Raman scattering light collected from the sample. SERS detection is often provided with noble metal substrates coated on the fiber endface. Many enhancing substrates with surface morphologies such as metal nanoparticle, nanorod and nanopillar, were used to detect Raman spectra of chemicals (R6G, trans-bis(4-pyrigyl)ethane) [7]–[10] and biological analytes (proteins, bacteria, et al.) [11]. Liquid core photonic crystal fiber probe (LCPCF) and hollow core photonic crystal fiber probe (HCPCF) were also developed for a low detectable concentration of 10−10 mol/L R6G mixed with silver nanoparticles [12], [13]. Applying SERS to single cell detection is currently a hot research area. Nanoscale optical fibers and nanopipettes play an important role in biological cell study for the challenges of small cell size and the in vivo demand of minimal invasion [14]–[16], which offer a good playground of intracellular SERS Raman spectra measurement and analysis. However, an appropriate tip size of optical fiber is crucial for in vivo Raman measurement of biological cells. Tapered fiber based optrode has the potential to accomplish this task, which becomes the motivation of this letter. The objectives of this letter are to develop an SERS nanoprobe based upon tapered optical fiber and to verify its feasibility in the application of remote optrode configuration under the condition of minimal invasion to the living cells. Besides, according to the theory of evanescent wave, tapered fiber not only transmits the evanescent field to the surroundings, but also increases the evanescent field magnitude [17], [18]. Therefore, it has great potential to enhance the reflections of laser and increase the ability of collecting light, which contributes to a summation of weakly Raman intensities [19]. Prior works of silver-nanoparticles modified tapered fiber-optic with optimized geometry by double static etching demonstrated the minimal detection on a low concentration of 10−8 mol/L of R6G in remote detection mode [20], [21]. Although silver is generally regarded with better enhancement effect than gold substrate in many cases, considering silver substrate has certain toxicity for biological sample measurement, further efforts should also be dedicated to new small-size, high sensitive SERS optrode with good biocompatibilities. In this letter, we prepared nanoscale tapered multimode optical fiber with the processes of heated pulling combined

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invasive. The dimension of tapered fiber was a trade-off between above two factors for the future intracellular Raman spectra detection and analysis. B. Preparation of Gold Nanoparticles

Fig. 1. (a) SEM photograph of the tapered optical fiber. (b) TEM photograph of gold nanoparticles prepared by Frens method and microwave heating method. (c) SEM photographs of nanoprobe modified with gold nanoparticles. (d) Distribution of gold nanoparticles on the nanoprobe surface.

with chemical etching. Gold nanoparticles were coated on the tip surface of this nanoscale fiber by using electrostatic selfassembly technology. Raman spectra of Rhodamine 6G (R6G) aqueous solutions with different concentrations were detected in the optrode remote detection mode by gold nanoparticlesmodified nanoprobes. II. E XPERIMENTAL M ETHODS A. Nanoscale Tapered Optical Fiber Fabrication The optical fiber used in our experiments is a multimode fiber with a core/cladding diameter of 50/125µm. We fabricated the tapered fiber through two steps: heated pulling and chemical etching [22]–[24]. In the former step, electrode discharge method was adopted. The fiber was pulled to form a tapered structure by a commercial optical fiber splicer. The discharge intensity and the discharge time are programmable in that step. The discharge intensity and time were set as 100 and 400 ms respectively. In the latter step, to perform chemical etching procedure, the tapered fiber was perpendicularly immersed into 40% (w) hydrofluoric (HF) acid solution, which was covered with an organic liquid (isooctane) to reduce the evaporation of the acid. The etching time was set about 10 min. After etching, the fiber was rinsed with deionized water. The final result of this process is shown in Fig. 1. Fig. 1(a) shows the scanning electron microscopy (SEM) photograph of the finished tapered fiber. The tip dimension is 40.7 nm and the taper angle is 30°. The cone length was measured at around 320µm. Compared with those made by conventional laser heated pulling or static chemical etching, this tapered fiber has the advantages of smooth surface and small tip dimension. In theory, to a certain degree, larger taper angle and shorter cone length has better ability to collect Raman scattering light. However, considering the possible application in intracellular Raman detection, a shallow taper angle is preferred in order to make the intervention less

According to the Frens method [25], gold colloids were prepared by means of microwave heating method. The preparation process is described as the following steps. First, 1 mL 1% (w) chloroauric acid (HAuCl4 ) solution was diluted to 0.01% (w). Then, the solution was heated to boiling. Second, 0.5 mL 1% (w) trisodium citrate (Na3 C6 H5 O7 ·2H2 O) solution was added to the boiling solution and the mixture solution was kept in boiling state. Finally, the gold colloids turned garnet red. Nanoparticles with different sizes show different colors in sol solution due to different doses of Na3 C6 H5 O7 ·2H2 O solution [26]. Fig. 1(b) shows the transmission electron microscopy (TEM) photograph of gold nanoparticles. It can be seen that these gold nanoparticles have a regular shape and homogeneous distribution. The diameters of most nanoparticles range from 50 nm to 60 nm. Only a small number of particles have the size either as large as 70 nm or in the magnitude around 40 nm. The average dimension was calculated to be 55 nm. Based on our previous studies of SERS dependence on nanoparticle size (diameter from 13 nm to 70 nm) [26], gold nanoparticles with diameters between 50 nm and 60 nm have the optimal enhancement effect for the R6G’s Raman spectra detection. C. Gold Nanoparticles-Modified Nanoprobe To form the SERS active substrate on its surface, the tapered optical fiber nanoprobe was deposited with gold nanoparticles through electrostatic self-assembly technology [27], [28]. First, the fiber was cleaned in piranha solution (3:1 mixture of 96% concentrated sulfuric acid and 30% hydrogen peroxide) for 30 min. Then, it was soaked in the mixture solution with 5%v/v deionized water, 5%v/v (3-Aminopropyl) trimethoxysilane (APTMS, 97%) and 90%v/v ethanol for 30min. In the next step, it was kept in the incubator at 90° for 30 min. Right after each step mentioned above, the fiber was washed thoroughly with deionized water and ethanol to remove the residual chemicals or impurities. Finally, the fiber was immersed in the gold sol for 48h. The length of distal fiber modified with gold nanoparticles was approximately 2.4 mm. The SEM photograph of this modified nanoprobe is presented in Fig. 1(c) and (d). The surface of probe becomes rough after the process of gold nanoparticles depositing, which is clearly shown in Fig. 1(c). From Fig. 1(d) we can see that the gold nanoparticles were uniformly distributed on the probe surface. D. SERS Measurement To verity the performances of the SERS nanoprobe made with above method, R6G molecules were used as the standard sample. Their Raman spectra were detected by our SERS

CHEN et al.: GOLD NANOPARTICLES-MODIFIED TAPERED FIBER NANOPROBE

Fig. 2.

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Experimental setup of remote Raman spectra detection.

nanoprobe combined with a commercial confocal microRaman spectrometer (Renishaw inVia plus). The experimental setup of the SERS nanoprobe remote detection is shown in Fig. 2. In the experiments, the SERS nanoprobe was cut into a segment of 15–20 cm with a flat end surface for laser injection. A CW laser with the exciting line at 633 nm was injected into this optical fiber end with a 10× microscope objective. The Raman scattering light, which was collected from the sample by the SERS detection end, was transmitted through the same optical fiber to the same objective, and then entered the Raman spectrometer. The laser power, which was measured from the nanoprobe detection end, was about 2.6 mW. The Raman spectrometer exposure time was set to 10 seconds and three spectra were accumulated for each measurement. Considering its tiny size, the SERS nanoprobe was dipped into the R6G aqueous solution for about 10 hours before the Raman measurement to provide enough time for the R6G molecule to be adsorbed. In order to keep consistency, above experimental conditions were fixed for all the Raman spectra measurements below. III. R ESULTS AND D ISCUSSION R6G solutions with four different concentrations (10−5 mol/L, 10−6 mol/L, 10−7 mol/L, and 10−8 mol/L) were used as the detection samples. Prior to adsorbing R6G molecules on the probe’s tip surface, Raman spectrum of this modified nanoprobe was measured as shown in Fig. 3(a) (curve a). This curve presents the background Raman spectrum of optical fiber by itself. The other four curves (b, c, d, and e) in Fig. 3(a) indicated the Raman spectra of R6G solutions with four different concentrations of 10−5 mol/L, 10−6 mol/L, 10−7 mol/L and 10−8 mol/L respectively. The Raman peak positions at 614 cm−1 , 773 cm−1 , 1187 cm−1 , 1365 cm−1 , 1509 cm−1 , 1575 cm−1 , and 1652 cm−1 of R6G molecule were detected and marked in the figure. Fig. 3(b) shows the Raman spectra from 1000 cm−1 to 1800 cm−1 with the baseline subtracted for clarity. For each spectrum, the baseline is the broken line composed of a series of the segments, each of whom is acquired by connecting the two lowest points of each Raman peak. It can be seen from the figure that the intensity in every peak position gets weaker and weaker with the decrease of R6G concentration. However, three Raman characteristic peak positions (1365 cm−1 , 1575 cm−1 , and 1652 cm−1 ) can still be identified at the minimum concentration of 10−8 mol/L. In the optrode configuration, the detection limit for R6G aqueous solution reaches

Fig. 3. (a) SERS spectra of R6G with different concentrations (curve b. 10−5 mol/L, curve c. 10−6 mol/L, curve d. 10−7 mol/L, and curve e. 10−8 mol/L) and background (curve a) of nanoprobe. (b) SERS spectra of R6G with different concentrates with background subtracted (b. 10−5 mol/L, c. 10−6 mol/L, d. 10−7 mol/L, and e. 10−8 mol/L) and a comparison spectrum of the tapered fiber nanoprobe without gold nanoparticles coating measured with 10−1 mol/L R6G aqueous solution.

10−8 mol/L. In Fig. 3(b), we also give a comparison spectrum of the tapered fiber without gold nanoparticles coating in green line to show the SERS enhancement effect of gold nanoparticles modified on the nanoprobe. The concentration of the R6G aqueous solution is 10−1 mol/L for this comparison spectrum measurement. We can see from the figure that this measurement (10−1 mol/L) has the comparable Raman intensity with the detection limit (10−8 mol/L) of the coated SERS nanoprobe. These results obviously prove that this kind of gold nanoparticle-modified tapered fiber nanoprobe possesses good enhancement effect. Furthermore, in order to study their reproducibility as SERS sensors, six different SERS nanoprobes were prepared under the same condition, and then they were all utilized to detect R6G solution (10−6 mol/L) in the remote mode. The measurement was repeated three times for each SERS nanoprobe and the average processing was performed with these three results for each sensor. The results are shown in Fig. 4. We can see from the figure that the Raman peak positions have good consistency for different probes. However, the Raman intensities have a little difference, which can be accounted as the differences among the various probes. Those differences may include different tapered angle, non-identical surface

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Fig. 4. Raman spectra of R6G (10−6 mol/L) was detected using six different nanoprobes.

distribution of the nanoparticles on the probe surface and etc. Overall, these results still indicate good reproducibility of SERS nanoprobe fabrication. IV. C ONCLUSION To sum up, we proposed the structure and fabricating method of the tapered optical fiber based SERS nanoprobe. Its ability to perform remote Raman detection was demonstrated in experiments. By heated pulling and chemical etching methods, we realized the tapered fiber with the nanoscale tip size of 40.7 nm. The gold nanoparticles were prepared and were deposited successfully on the nanoprobes surface by the electrostatic self-assembly technology. The optrode remote detection mode was adopted in the experiments. The result indicates that detection limit of R6G aqueous solution has reached as low as 10−8 mol/L. Considerable high signalto-noise ratio and high sensitivity were achieved. This kind of small dimension, high sensitive remote SERS nanoprobe opens new window on in vivo intracellular Raman spectra detection in the near future. More efforts should be spent on some aspects such as improving the reusability, suppressing the backward Raman scattering of the fiber probe itself to get higher signal-to-noise ratio and long-distance remote measurement. R EFERENCES [1] A. Campion and P. Kambhampati, “Surface-enhanced Raman scattering,” Chem. Soc. Rev., vol. 27, no. 4, pp. 241–250, 1998. [2] M. Fleischmann, P. J. Hendra, and A. J. McQuillan, “Raman spectra of pyridine adsorbed at a silver electrode,” Chem. Phys. Lett., vol. 26, no. 2, pp. 163–166, 1974. [3] X. M. Lin, Y. Cui, Y. H. Xu, B. Ren, and Z. Q. Tian, “Surface-enhanced Raman spectroscopy: Substrate-related issues,” Anal. Bioanal. Chem., vol. 394, no. 7, pp. 1729–1745, 2009. [4] K. Faulds, R. E. Littleford, D. Graham, G. Dent, and W. E. Smith, “Comparison of surface-enhanced resonance Raman scattering from unaggregated and aggregated nanoparticles,” Anal. Chem., vol. 76, no. 3, pp. 592–598, Feb. 2004. [5] C. J. Orendorff, L. Gearheart, N. R. Jana, and C. J. Murphy, “Aspect ratio dependence on surface enhanced Raman scattering using silver and gold nanorod substrates,” Phys. Chem. Chem. Phys., vol. 8, no. 1, pp. 165–170, 2006.

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