Dept. of Electrical Engineering, University of California at Santa Cruz, Santa Cruz, CA 95064, USA
Lei Tian, Rebecca Newhouse, Shaowei Chen and Jin Z. Zhang* Dept. of Chemistry and Biochemistry, University of California at Santa Cruz, Santa Cruz, CA
95064, USA.
ABSTRACT
A hollow core waveguide (HCW) with silver nanoparticles (SNPs) coated on the inner wall has been demonstrated for molecular detection based on surface enhanced Raman scattering (SERS). With rhodamine 6G (R6G) as an analyte molecule and two types of silver nanoparticles (SNPs) as double SERS substrates, the inner wall coated HCW (IWCHCW) exhibits significantly higher sensitivity than previous fiber SERS probes with only one SERS substrate. Two kinds of HCW are used in the experiment, liquid core photonic crystal fiber (LCPCF) and hollow silica waveguide (HSW). SERS signal obtained with either an LCPCF or a HSW IWCHCW is over ten times that obtained in direct detection using a single SERS substrate. The improvement of the SERS sensitivity is attributed to the additional enhancement of the electromagnetic field by the double SERS substrate “sandwich” structure with one substrate coated on the inner wall of the HCW and the other mixed in the sample solution. Furthermore, With an LCPCF IWCHCW, the SERS signal is around 100 times as strong as that in direct detection when measured from the processed fiber tip. This is attributed to the additional R6G/SNPs solution in the fiber pit, increased coupling efficiency due to surface plasmon resonance in the SNPs in the same region, and further increased electromagnetic field in the same region due to nano-structures introduced during the collapse of the cladding holes. The simple architecture and high sensitivity of the inner wall coated HCW make it promising for molecular detection in various analytical and sensing applications.
Molecular sensors based on surface enhanced Raman scattering1-3 (SERS) and optical fibers4-12 have been widely used in medical, environment and other medical or biological detections recently due to their unique advantages, such as molecular specificity, high sensitivity and flexibility.
The first single-fiber multimode SERS fiber probe was demonstrated in 1991 by Mullen et al.13 In the following years, various configurations were tested, such as flat, angled and tapered14-16 fibers; but the small number of SERS substrate nanoparticles involved in the active region due to relatively small areas of contact limited the sensitivity for these fiber probes. In order to involve more particles in the SERS activity, hollow core photonic crystal fiber17 (HCPCF) was tested in our group. By dipping a HCPCF probe into the analyte/SERS substrate solution, the solution will be absorbed into the hollow regions of the PCF by capillary effect. After it was dried, the analyte SERS signal was obtained along with the fiber Raman background. However, with the solution filling all the hollow regions of the PCF, both the excitation light and the Raman signals from the analyte would lose their confinement due to the decreased refractive index contrast, therefore, the disappearance of the photonic bandgap. This would in turn limit in vivo and in vitro applications of such a HCPCF SERS sensor.
In our recent study, a liquid core photonic crystal fiber18 (LCPCF) has been demonstrated to involve more SERS particles in the active region, leading to increased sensitivity, which is higher than that obtained by directly focusing the laser beam at the SNP/analyte mixture without using any fibers. The measured enhancement is attributed to the confinement of both the excitation light and the liquid sample inside the micro cavity.
In addition, a double SERS substrate “sandwich” structure19 for fiber probes has been proposed and shown by us recently to enhance SERS sensitivity. The key idea of the “sandwich” structure is to use two types of silver nanoparticles (SNPs) as SERS substrates simultaneously. In our earlier work, one SERS substrate (e.g. SNPs) was coated on the tip of the normal multimode fiber and the other was mixed with analyte molecules in the sample solution. When the tip was dipped into the sample solution, the SNPs with testing molecules would randomly bind with the SNPs on the tip to form a “sandwich” structure. The highly enhanced electromagnetic field between the two SNPs introduced significantly stronger SERS signals than those detected otherwise, including signals directly detected by focusing the laser beam at the SNP/analyte mixture without using any fibers.
Based on the two enhancement effects due to the micro cavity18 and the “sandwich” structure19, it is of interest to test the possibility of combining the two ideas together to achieve further enhancement. In this paper, we demonstrate a SERS probe, namely the inner wall coated hollow core waveguide (IWCHCW), with sensitivity which is two orders of magnitude higher than that of direct detection. The hollow core is for the introduction of the micro cavity effect and the inner wall coating is one part of the “sandwich” structure (the other one is in the sample solution). Two types of hollow core waveguides (HCWs) are used in our experiments: hollow core photonic crystal fiber (HCPCF) and hollow silica waveguide (HSW). The advantage of the HCPCF is its low transmission loss resulting in better light confinement and hence higher sensitivity. However, the small core size (5 μm in diameter) makes light coupling challenging and any mechanical disturbance can lead to significant noise due to the unstable coupling. As for the HSW, its relatively large core (300 μm in diameter) makes the coupling much easier and more stable. However, the radiation loss is relatively high for the HSWs in this experiment due to poor light confinement at the operating wavelength. This
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problem could be solved by using different laser wave lengths. Therefore, in our experiments carried out to date, there is a tradeoff between HCPCF and HSW as discussed in more detail in the following. We believe with the reduction of the transmission loss, HSWs, are the most promising for high sensitivity and low cost SERS detection applications.
The schematic of the IWCHCW sensor is illustrated in Fig. 1. The inner surface of the hollow core is coated with one type of SNP. The second type of SNP is mixed with the analyte, e.g., rhodamine 6G (R6G), in the solution. The solution enters the hollow core from the bottom end via the capillary effect, forming random “sandwich” structures with the analyte molecules inbetween the two types of SNPs. The excitation light is coupled into the core of hollow core waveguide from the top end and propagates in it. The confined light interacts with the sample solution, which contained SNPs with the analyte molecules absorbed on the nanoparticle surface. The SERS signal from the sample propagates back through the hollow core waveguide and is collected by the Raman spectrometer. The light source is a 633 nm diode laser inside the Renishaw micro-Raman spectrometer with a leica microscope and a 50× objective lens.
Fig. 1. Schematic of the inner wall coated hollow core waveguide (IWCHCW) sensor
The HCPCF used in our experiments was purchased from Thorlabs, Inc. (Model HC-633-01) with its cross section shown in Figure 2b. A fusion splicer (Model FITEL S175) was used to seal the cladding holes at one end of the fiber20 in order to create a LCPCF. The HSW used in our experiments was a sample from Polymicro Technologies, Arizona. As shown in Fig. 2c, a HSW is a capillary tube containing four layers, AgI film, silver film, silica tube, and acrylate coating from the inside out. This particular HSW was designed to transmit high power laser beams for IR applications. Since the wavelength of the laser source used in this paper is 633 nm, the radiation loss is relatively high. In our experiment, a 5 cm HSW was prepared by cutting carefully at both ends.
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The laser output power was 4.00 mW and at the far end of the HCW the power dropped to 0.200 mW. The light confinement was slightly improved after coating SNPs on the inner wall of HSW. The far end output power increased to 0.255 mW after the coating.
Fig.2. (a) Hollow core waveguide (HCW); (b) Cross section of hollow core photonic crystal fiber (HCPCF); (c) Cross section of
hollow silica waveguide (HSW) and four different layers of the HSW.
2.2 Synthesis of the silver nanoparticles (SNPs)
The silver nanoparticles (SNPs) used in the sample solution were synthesized using Lee and Meisel protocol21. Silver nitrate was used as the metal precursor and sodium citrate as the reducing agent. Formation of the silver nanoparticles was monitored by UV-vis spectroscopy using a HP 8452A spectrometer with 2 nm resolution. SNPs synthesized have an average diameter of about 25 nm as determined using Transmission Electron Microscope (TEM, Model JEOL JEM 1200EX). The nanoparticles made by this method in aqueous solution have a typical UV-vis spectrum with the characteristic broad surface plamon band peaked around 405 nm, as shown in Fig. 3.
Fig. 3. UV-visible absorption spectrum of silver nanoparticles (SNPs). Inset: The TEM picture of SNPs.
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Core Size Histogram of Ag-Thiol NP's
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The SNPs coated on the inner wall were prepared by following a literature procedure22. Briefly, 0.25 mmol of AgNO3 was dissolved in 5 mL of nanopure H2O, into which was added 40 mL of toluene with 0.75 mmol of tetraoctylammonium bromide (TOABr) under vigorous stirring. Then ca. 400 μL of dodecanethiol was added into the solution. In a separate beaker, 2.5 mmol of NaBH4 was dissolved in 10 mL of H2O. While the Ag-containing mixture was rapidly stirred, the reducing agent was added, and the solution turned dark brown quickly, signifying the formation of silver nanoparticles. The reaction was allowed to proceed for several hours. Upon completion of the reaction, the mixture was washed with nanopure water for 3 times. The toluene phase was then collected, and dried under reduced pressure with a rotary evaporator. Excessive thiol and TOABr ligands were then removed by rinsing the collected black solids with copious methanol. Then the Ag nanoparticles (Ag NP) were collected by centrifugation and redispersed in dichloromethane (DCM) for further use. The average core diameter is 5 ± 2 nm, as shown in Fig.4.
Fig. 4. (a)TEM micrograph of AgC6 nanoparticles. (b) Size histogram with an average core size of 5±2 nm.
2.3 Inner wall coating procedure
An injector was used to help coat the SNPs onto the inner wall of HCW. The HCW was mounted inside a syringe needle and a sticky tape was used to close the gap between the syringe needle and the HCW. After dipping the other end of HCW into the SNPs solution and pulling the injector to pump the SNPs solution into the core of the HCW, the hollow core was quickly filled with the SNP solution. Then the HCW was dried for 20 min to remove the organic component from the silver particles. The dipping procedure was repeated to form a multilayer of SNPs on the inner wall surface of the HCW.
2.4 Sample preparation
The sample solution in this study was prepared with R6G molecules at a concentration of 10-6 M and sodium chloride (NaCl, 10mM) was added to induce SNP aggregate formation. Starting with aqueous R6G solution (10-4M), SNPs was added to dilute the R6G solutions. 30 μL of the R6G solution and 270 μL of the SNPs colloid were mixed and therefore we obtained 300 μL sample with a concentration of 10-5 M of R6G molecules. Then 30 μL of the resulting solution was added to 270 μL of the SNPs colloid again to obtain a sample solution with an R6G concentration of 10-6 M. The solutions were incubated for about 10 minutes at room temperature and then activated with 15 μL
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NaCl solution. Raman measurements were performed about 20 minutes after the introduction of salt.
3. RESULTS AND DISCUSSION The inner wall coated LCPCF probe was used in two different modes to detect R6G molecules. When the cladding-sealed end of the coated LCPCF was dipped into the sample solution, the sample solution would enter the central core due to the capillary effect until it reached the unsealed end of the coated LCPCF. One way to detect the sample solution is to focus light and collect the SERS signal from the unsealed end of the coated LCPCF, and the other is from the sealed end of the coated LCPCF. The experimental results are shown in Figure 5a. The detectable SERS signals from both the unsealed end and the sealed end are much better than that obtained from the direct sampling method. Furthermore, the signal obtained from the unsealed end is nearly 10 times better than that from the direct sampling and the signal from the sealed end is around 100 times better than that from the direct sampling. Compared with the results of only LCPCF configuration18, which is slightly higher than that obtained from the direct sampling, it is evident that the inner wall coating indeed further enhances the signal presumably due to the “sandwich” effect.
An extra enhancement due to the structure of the sealed end was also observed in this experiment. As shown above, the SERS signal from the sealed end of the coated LCPCF was 10 times better than that from the unsealed end. The only difference between the two ends was the geometrical shape. The sealed tip was concave as a result of heating in the fusion splicer, i.e., the sealed end appeared as a “bowl” shape, as shown in Fig. 13 of Ref. 20. There was more sample solution in the “bowl” shaped structure and another possible reason for the enhancement could be the surface plasmon resonance (SPR) of the SNPs in the ”bowl” that helped couple more light into the core of the fiber. In addition, inside the fiber pit there are nano-structures introduced during the collapse of cladding holes when heated by a fusion splicer. When coated with SNPs, these nano-structures can further enhance the electromagnetic field, therefore, further increase the SERS sensitivity.
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Fig. 5. SERS spectra of R6G molecules at the concentration of 10-6 M by using different detection methods. (a) Comparison of direct sampling and using coated liquid core photonic crystal fiber (LCPCF); Inset: the enlarged SERS spectrum of direct sampling; (b)
Comparison of direct sampling and using coated hollow silica waveguide (HSW).
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The results obtained using inner wall coated HSW to detect R6G molecules are shown in Figure 5b. The SERS signal from the HSW is around 5 times better than that from the direct sampling. For comparison, we also calculated the effective number of molecules involved in the SERS activity from the direct sampling and the coated HSW, respectively. The laser spot was a circle with a diameter of 6 μm. The sample solution was put in a small cap to do the direct sampling. The height of the solution was 0.5 cm, so the effective volume of the sample solution involved in SERS is around 1.41×10-4 μL. On the other hand, the core diameter of the HSW is 300 μm and the effective length in the core is around 4 cm. Thus the effective volume of sample solution in the core is 1.13×10-3 μL. In addition, the laser power is 4.00 mW and after passing through the HSW is 0.255 mW. Besides, the SERS intensity from the coated HSW is around five times better than the direct sampling. Taking into account the above factors, the enhancement per molecule per photon using coated HSW is about 10 times better than direct sampling.
4. CONCLUSION In summary, a simple inner wall coated hollow core waveguide (IWCHCW) has been proposed and demonstrated as a highly sensitive SERS probe for molecular detection. With the R6G as a model analyte molecule, the SERS signal obtained with either an LCPCF or a HSW IWCHCW is over ten times that obtained in direct sampling using a single substrate. With an LCPCF IWCHCW, the SERS signal is around 100 times as strong as that in direct sampling when measured from the processed fiber tip. This is attributed to the additional R6G/SNPs solution in the fiber pit, increased coupling efficiency due to SPR in the SNPs in the same region, and further increased electromagnetic field in the same region due to nano-structures introduced during the collapse of the cladding holes. Further investigation is underway for the mechanism for this additional enhancement. The IWCHCW is not only highly sensitive due to the “sandwich” structure but also low in cost, making it promising for a variety of analytical and sensing applications.
5. ACKNOWLEDGEMENTS We acknowledge the financial support from the National Science Foundation, ECCS-0401206, ECCS-0823921, and the UC MICRO grant. We thank Prof. Karamjeet Arya at San Jose State University for helpful discussions on the initial "sandwich" structure idea.
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