Ultrafine Fiber Raman Probe with High Spatial Resolution and

Jan 22, 2016 - Considerable interest has been shown in fiber Raman probes as powerful tools for in situ biomedical diagnosis and monitoring processes ...
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Ultrafine Fiber Raman Probe with High Spatial Resolution and Fluorescence Noise Reduction Toshiro Yamanaka,*,† Hiroe Nakagawa,†,§ Manabu Ochida,†,∥ Shigetaka Tsubouchi,†,⊥ Yasuhiro Domi,†,# Takayuki Doi,†,¶ Takeshi Abe,*,‡ and Zempachi Ogumi† †

Office of Society-Academia Collaboration for Innovation, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan

‡

S Supporting Information *

ABSTRACT: Considerable interest has been shown in fiber Raman probes as powerful tools for in situ biomedical diagnosis and monitoring processes in the materials industry. Miniaturization and high spatial resolution are required for less invasive measurements with accurate locations. In analysis of organs, widespread visible excitation light produces problematic fluorescence backgrounds. Here, we report an ultrafine fiber Raman probe that is thinner than the needle of a mosquito (labrum: 50−80 μm in diameter) with high spatial resolution (23 μm) and with a function of fluorescence background reduction. Due to the fineness and resolution, the distribution of ions in an electrolyte solution in narrow spaces could be measured. Backgrounds in spectra of liquid containing fluorescent impurity were reduced by using the probe. The probe has wide applicability for noninvasive in situ molecular diagnosis of organs and small devices.

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INTRODUCTION Remote, in situ, and nondestructive chemical analysis is important in various fields including biomedical diagnostics and monitoring processes in the materials industry.1−4 Raman spectroscopy5−13 has been used with fiber probes in which excitation light and signal light are guided by optical fibers between the portable probe and a fixed spectrometer at separate locations. Miniaturization and high spatial resolution of probes are crucial for minimally invasive (painless) analysis of subsurface tissue with accurate location when inserting the probe into a sample, especially human organs, by a needle or through a natural orifice. The diameter of the finest probes with silica-based fibers is 600 μm, and their spatial resolution is several hundreds of micrometers.8 When analyzing organisms, excitation by widespread visible laser light causes dense fluorescence backgrounds, and near-infrared light5,8,10−15 and a confocal setting have thus been used. Here, we describe a high-spatial-resolution (23 μm) fiber Raman probe thinner than the needle of a mosquito (labrum: 50−80 μm in diameter) with an unexpected function to reduce fluorescence background. The probe consists of two silica-based optical fibers of 30 μm in diameter. By utilizing the fineness and resolution, enhancement of the concentration of LiClO4 within 100 μm from the anode during electrochemical dissolution of lithium into an LiClO4-based electrolyte solution, known as a diffusion layer, was clearly observed. The concentration of LiClO4 was also measured at three different positions in deep narrow spaces of 115 μm in thickness inside a model © 2016 American Chemical Society

electrochemical device by using three probes, showing different concentrations between positions. Spectra of acetone containing fluorescent impurity were successfully obtained with reduced background at 532 nm visible excitation. The probe is fine enough to insert into the world’s thinnest 34G pain-free needle with outer and inner diameters of about 180 and 80 μm, respectively, which is widely used for selfinjection of insulin and various medicines. The probe has many possibilities of in situ noninvasive diagnostics of organs and blood contents16−19 including glucose in veins with reduced background. It is also possible to insert the probe into very narrow spaces between electrodes in various electrochemical devices such as batteries, fuel cells, and capacitors to analyze an electrolyte solution, which is necessary for the development of new devices with high performance. The present results also demonstrate a new strategy for fluorescence background reduction by enhancing spatial resolution in narrow spaces inside samples where a confocal setting, which requires the use of a large objective lens, cannot be used.

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METHODOLOGY Silica-based optical fibers are used in many probes. Recently, new types of optical fibers, (i) hollow fiber20 and (ii) photonic crystal fiber,21 have been used for fiber Raman probes. In Received: December 5, 2015 Revised: January 21, 2016 Published: January 22, 2016 2585

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fiber, and the resultant face was mirror-coated by deposition of aluminum or silver so that the excitation laser light was reflected in the direction perpendicular to the axis. The collection fiber was parallel to the excitation fiber, and the end of the collection fiber was placed in front of the trajectory of the reflected laser. The two fibers were fixed by an inorganic adhesive (see Figure S1). The excitation area (blue area) and collection area (green area) overlapped to form an analysis area as shown by hatching in Figure 1a. Figures 1c−f show trajectories of lights simulated by OptiCAD (OptiCAD Corporation, Santa Fe, New Mexico) showing how the lights are emitted from the ends of fibers into the air (refraction index n = 1) and organic solvent (n assumed to be 1.42). The refraction indexes for the clad and core were assumed to be 1.5 and 1.6, respectively, and the divergence of the input beam was assumed to be 50°. The trajectories in Figures 1e and 1f indicate the excitation area, while those in Figures 1c and 1d indicate the collection area. The simulated beams emitted from excitation and collection fibers were more parallel in the organic solvent than in air, resulting in a smaller analysis area. A similar inclined mirror was previously applied to a probe that consisted of fibers of 400−600 μm in diameter.23,24 In those works, a mirror was used to make the analysis area at a position close to the ends of the excitation and collection fibers in order to reduce absorbance of light by a liquid sample, while there was not much consideration of spatial resolution. A Raman spectrometer (NRS-3100, JASCO) and objective lens unit (RMP-102, JASCO) connected to the spectrometer were used with the present fiber probe as shown in Figure 1g. A 532 nm laser light propagated through a fiber cable and the light was focused by the objective lens on the excitation fiber. The Raman scattering light transmitted through the collection fiber was collected by the same objective lens and transmitted to the spectrometer through another fiber cable. The excitation fiber was 100 μm away from the collection fiber and outside the detection area of the objective lens (see Figures S2 and S3). In this situation, the detection efficiency of Raman scattering signals and fluorescence generated in the excitation fiber was small. Unless otherwise specified, the output power of the laser was adjusted to about 100 and 500 mW for measurements with the objective lens and with the fiber probe, respectively, resulting in an irradiation power of 10 mW on samples for both measurements.

contrast to silica-based fibers, these fibers do not generate a significant background in Raman spectra, but they are not very thin (80 to several hundreds of micrometers). Thus, silicabased fibers of 30 μm in thickness were used in the present study. Figures 1a and 1b show the structure of the fiber Raman

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Figure 1. Outline of the probe. (a) A schematic illustration of the probe. (b) Photographs of the probe. Scale bars indicate 30 μm. (c) and (d) Simulated trajectories of light emitted from the collection fiber into air (n = 1) and ethylene carbonate (n = 1.42), respectively. (e) and (f) Simulated trajectories of light emitted from the excitation fiber into air and ethylene carbonate, respectively. (g) Experimental setup for Raman spectroscopy using the probe.

RESULTS AND DISCUSSION Spatial Resolution. In order to determine the spatial resolution of the probe, a polypropylene (PP) particle of 6 μm in diameter was placed near the end of the probe, and Raman scattering spectra from the PP particle were recorded. The PP particle was attached to the end of a tungsten wire of 5 μm in diameter that was mounted on an XYZ stage (Figure 2a). The position of the PP particle in the YZ plane (Figure 2b) and also that in the XY plane (Figure 2c) were confirmed by observing a mirror image reflected by a polished face of a Ag wire of 500 μm in diameter. First X was set at zero (center of the collection fiber in the mirror image in Figure 2c), and the areas of the peak of the PP particle in Raman scattering spectra (Figure 2d) (averaged over two spectra taken with irradiation power of 10 mW for 60 s) were plotted against the position in the YZ plane (Figure 2e). Y = 0 and Z = 0 corresponded to the position of the surface of the excitation fiber and the flat surface of the collection fiber (Figures 1e and c), respectively. The resultant intensity profile had a maximum at (Y, Z) = (15, 20), and the

probe developed in the present study. This probe consisted of two fibers of 30 μm in outer diameter, 27.5 μm in diameter of the core, and about 90 mm in length. One was an excitation fiber that transmitted excitation laser light to a sample, and the other was a collection fiber that transmitted Raman scattering light from the sample to a spectrometer. The detection efficiency of Raman scattering light and fluorescence generated in the excitation fiber can be reduced when different fibers are used for excitation and collection. Dual-fiber-type probes with a single excitation fiber and a single collection fiber have long been used for various measurements.9,10,12,22 The end of the excitation fiber was cut at an angle of 45° from the axis of the 2586

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width at half-maximum (fwhm) obtained from both profiles were 23 μm. Then the PP particle was set at (Y,Z) = (15,20), where the profile had a maximum, and a peak area profile against the X axis was obtained as shown in Figure 2h. The value of fwhm was estimated to be 10 μm. This value was smaller than the value of 23 μm because laser light emitted from the excitation fiber was focused due to the curved surface of the fiber as shown in Figure 1e. High-Spatial-Resolution Measurements in a Narrow Space. The concentration of LiClO4 near a lithium anode electrode was measured as a function of distance between the probe and the surface of the electrode during electrochemical dissolution of the lithium electrode in a solution of 1 mol dm−3 LiClO4 dissolved in propylene carbonate (PC). The electrolyte solution was poured into a polypropylene bottle of 15 mm in diameter and 6 mm in height. Two lithium foils of 5 mm × 5 mm × 200 μm in size were put in the bottle and used as the anode and cathode. The foils were positioned facing each other, and the distance between them was 5 mm. The bottle with the electrodes was mounted on an XYZ stage, and scanning was performed to adjust d (Figure 3a) to the desired values while the probe placed in the bottle was fixed. Then a constant electric current of 32 mA/cm2 was applied between the two electrodes, and after 80 s, a Raman spectrum was accumulated for 30 s. The potential difference between the anode and the cathode was about 2 V. The value for d was confirmed by using a microscope (Figure 3b) before and after each spectrum was taken. The decrease in thickness of the anode Li foil due to dissolution was calculated to be 1.25 μm per 30 s at 32 mA/ cm2. Figure 3c shows a Raman spectrum taken at d = 140 μm without an electric current. The spectrum was averaged over 20 spectra taken with irradiation power of 10 mW for 15 s. Figures 3d−3g show spectra taken with an electric current at d = 115 μm, d = 55 μm, d = 35 μm, and d = 15 μm, respectively. Each spectrum was averaged over two spectra taken with irradiation power of 10 mW for 15 s. The peaks around 960 cm−1 (P1) and 930 cm−1 (P2) correspond to the −O−C(O)−O− symmetric stretching of PC and the symmetric stretching vibration of the ClO4− ion, respectively. The peak around 712 cm−1 (P3) corresponds to the ring deformation band of PC.25−27 The peak at 850 cm−1 was assigned to symmetric ring vibration.28 The spectrum at d = 115 μm shown in Figure 3d was similar to the spectrum taken without an electric current (Figure 3c), indicating that the concentration of LiClO4 did not change significantly. With decreases in d, the height of P2 increased due to increased concentration of ClO4−. In addition, the position of P2 shifted to higher wavenumbers. Three peaks were previously assigned for ClO4−, that is, (i) free solvated anions (933.8 cm−1), (ii) solvent-shared ion pairs (939.3 cm−1), and (iii) contact ion pairs (947.7 cm−1).27 The shift of the peak with decreases in d indicates transformation from free solvated anions to solvent-shared ion pairs and contact ion pairs. As d decreased, a shoulder of P3 appeared at 725 cm−1 (P3′), and it became dominant, indicating increased concentration of PC solvated with Li+. These results are consistent with the results of a previous study.25 To determine the concentration of LiClO4, reference solutions of LiClO4 dissolved in PC with various concentrations of LiClO4 were prepared, and Raman spectra of these solutions were taken. By comparing the spectra shown in Figures 3d−3g with those of the reference solutions, the concentrations of LiClO4 at various values of d were determined (see Figures S4−S7). The dependence of

Figure 2. Measurements of spatial resolution. (a) Experimental setup for determination of spatial resolution of the probe. (b) Microscopic observation to confirm the position of the PP particle relative to the probe in the YZ plane. (c) Microscopic observation to confirm the position of the PP particle relative to the probe in the XY plane. (d) Raman spectrum of the PP particle measured with the probe. The spectrum was averaged over two spectra taken with incident power of 20 mW and 60 s. (e) Area of the peak of PP plotted against the YZ plane. (f) Peak area profile plotted against the Z axis made by projection of the profile shown in (e). (g) Peak area profile plotted against the Y axis made by projection of the profile shown in (e). (h) Peak area profile plotted against the X axis at (Y,Z) = (15,20).

intensity decreased to almost zero on the lines of Y = 40 and Z = 40. Figures 2f and g show profiles projected on the Z-peak area and Y-peak area planes, respectively. The values for full 2587

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Figure 4a was constructed. Four porous polypropylene separator sheets, the thickness of two of them being 7 μm

Figure 4. Analysis at different positions inside deep narrow spaces in a model electrochemical device by multiprobes. (a) An electrochemical cell with multiprobes. (b) Spectra taken at different positions in the electrochemical cell during application of a current.

and that of the other two sheets being 25 μm, were inserted between two closely faced lithium foils, which were used for an anode and a cathode (20 mm × 17 mm × 150 μm). Three probes, A, B, and C, were placed at different positions, that is, the anode side, the middle position, and the cathode side. Each of the foils was mounted on a very flat face (±1 μm) of a stainless block, and the distance between the two lithium foils was adjusted to 115 μm (see Figure S8), which is close to the distance between electrodes in practical electrochemical devices. A solution of 2.4 mol dm−3 of LiClO4 dissolved in PC was immersed between the foils as an electrolyte solution. Before applying a current, Raman spectra taken with the three probes were almost identical, showing a concentration of 2.4 M. On the other hand, 10 min after the start of application of a current of 30 mA (8.8 mA/cm2), a clear difference was observed between spectra taken at the anode side and at the cathode side, as shown in Figure 4b. By comparing these spectra with the spectra of the reference solutions, the concentrations of LiClO4 at the anode side and the cathode side were concluded to be 2.8 ± 0.2 M and 2.0 ± 0.2 M, respectively, while the concentration at the middle position remained at 2.4 ± 0.2 M. In these measurements, high spatial resolution of the probe is necessary because signals from separators would be detected if the spatial resolution of the probe was not sufficiently high, resulting in complicated spectra. Although the spatial resolution of Raman microscopy and tip-enhanced Raman spectroscopy (TERS) is higher,30,31 these methods require large spaces around the sample for the experimental setup. The present method is the only way for high-spatial-resolution analysis with Raman spectroscopy that is applicable for interiors of electrochemical devices and other

Figure 3. Measurements of electrolyte solution during electrochemical dissolution. (a) Experimental setup for measurement of the concentration of Li+ near a lithium electrode during electrochemical dissolution. (b) Microscopic images of a lithium anode and a probe. (c) Raman spectrum of a solution of 1 mol dm−3 LiClO4 dissolved in PC at about 140 μm without an electric current. (d−g) Raman spectrum of a solution of 1 mol dm−3 LiClO4 dissolved in PC at various values of d during electrochemical dissolution of lithium. (h) Dependence of the concentration of Li+ on d.

concentration on d is shown in Figure 3h. An increase in concentration can be seen at d below 100 μm, consistent with the results of a previous study using holographic interferometry.29 The probe is useful for high-resolution analysis in deep narrower spaces, such as the interiors of various electrochemical devices. In many electrochemical devices such as batteries, fuel cells, and capacitors, the electrolyte solution is confined in a very narrow space of several tens of micrometers in thickness between the two electrodes, and it has thus been very difficult to analyze the electrolyte solution during operation of the devices. As a model of electrochemical devices, a cell shown in 2588

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and many peaks were observed when a spectrum of this sample was taken by the probe, as shown in Figure 5d (averaged over two spectra taken with irradiation power of 10 mW for 10 s). Figures 5e and 5f show details of the spectra shown in c and d after subtraction of background baselines, respectively. In the spectrum taken with the probe, three peaks indicated by arrows are clearly seen, while in the spectrum taken with the objective lens, the peaks are not clear due to strong background noise. It is known that the fluorescence background in Raman spectroscopy can be reduced by using a confocal detection setup in which only signals emitted from the small focal point of the objective lens are detected and signals emitted from outside the focal point are excluded by a slit,32 although the mechanism of fluorescence background reduction has not been completely elucidated. In measurements with the present probe, the detection area is small due to crossing of excitation and collection areas (Figure 1a), and mechanisms for background reduction similar to that in the confocal setup32 may be operative. One of the possible mechanisms may be as follows. The primary fluorescence emission, which is excited by the excitation light in the excitation area (Figure 1a), travels outside the excitation area in all directions and then induces secondary fluorescence emission. The secondary fluorescence also travels and induces third and fourth fluorescence emissions at different positions (multiple fluorescence emission). The collection fiber selectively detects the emission from the analysis area in front of the end of the collection fiber, which is the most Raman scattering-rich, containing primary fluorescence and a small portion of multiple fluorescence emission. The fiber detects less sensitively the emission from the distant part of the collection area beyond the analysis area, almost all of which is multiple fluorescence emission. The fiber does not detect emission from the other areas in which there is mostly primary and multiple fluorescence emission, resulting in the reduced background, as shown in Figures 5d and 5f. To confirm the above hypothesis, Raman spectra were measured from a small fixed detection area set on a thin film of acetone with a trace amount of rhodamine B, changing the distance L of the excitation laser from the center of the detection area, as shown in Figure 6a. A vacuum-sealed liquid sample holder (Bruker A14) was used with two quartz windows of 2 and 4 mm in thickness. The thickness of acetone was adjusted to be 23 μm by inserting a polypropylene spacer between the windows. By using a high magnification (× 100) objective lens with the objective lens head shown in Figure 1g, the radii of the detection area and the focal point of the excitation laser were adjusted to 10 and 4 μm, respectively. Figures 6b and 6c show Raman spectra taken at L = 0 and 42 μm, respectively. The relative intensity of the peak of acetone at 2920 cm−1 to the intensity of fluorescence background was lower at L = 42 μm than at L = 0 μm. The dependences of each of these intensities on L are plotted in Figure 6d. With increases in L, both intensities decreased quickly below L = 14 μm as the focal point of the excitation laser went outside the detection area. The intensity of the background remained significant even at L = 84 μm, suggesting that fluorescence was emitted from the detection area as a result of multiple excitation. If the fluorescence observed in Raman spectra above L = 14 μm consisted mainly of secondary emission, the intensity of fluorescence would decrease quickly according to approximately 1/L2 since the irradiation intensity inside the detection area by the primary fluorescence decreases according to approximately 1/L2. In the multiple emission, the memory of

deep narrow spaces. Such analysis would be very important for elucidation of degradation mechanisms of devices and for development of future devices with high performance. Fluorescence Background Reduction. The background in Raman spectra, which is mainly caused by fluorescence from samples or optical fibers, was low when the probe was used for analysis of a nonfluorescent transparent liquid since the incident laser light reflected by the transparent liquid toward the collection fiber was weak. Figure 5a shows a Raman

Figure 5. Fluorescence background reduction by the probe. (a) Raman spectrum of acetone taken with an objective lens. (b) Raman spectrum of acetone taken with the probe. (c) Raman spectrum of acetone containing a trace amount of rhodamin B taken with an objective lens. (d) Raman spectrum of acetone containing a trace amount of rhodamin B taken with the probe. (e) and (f) Details of Raman spectra shown in (c) and (d).

spectrum of acetone taken with an objective lens, which was averaged over two spectra taken with irradiation power of 10 mW for 10 s. Many peaks were observed from 300 to 3000 cm−1. Figure 5b shows a Raman spectrum of acetone taken with the probe (averaged over two spectra taken with irradiation power of 10 mW for 10 s). The red line in Figure 5b was obtained after the probe, and acetone and the bottle were eliminated. The red line was very similar to the baseline of the spectrum of acetone shown in Figure 5b, indicating that the background signals generated by the probe were small. The function of the probe to reduce fluorescence background generated in a sample was observed during analysis of acetone containing fluorescent impurity. Figure 5c shows a spectrum of acetone containing a trace amount of rhodamine B obtained with the objective lens (averaged over two spectra taken with irradiation power of 10 mW for 10 s). Due to the large background of fluorescence, only the highest peak at 2920 cm−1 was recognized. The background was significantly suppressed, 2589

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a narrow space could be measured. Two probes were inserted into a lithium ion battery, and it was found that the ion concentration in the electrolyte in the battery changed and became ununiform after charging. Backgrounds in spectra of liquid containing fluorescent impurity were reduced by using the probe at a visible excitation wavelength (532 nm). This phenomenon has been explained by a mechanism based on multiple fluorescence emission, providing a general strategy for background reduction in Raman spectroscopy. The probe has wide applicability to noninvasive in situ diagnosis inside deep narrow spaces such as the interiors of organs and electrochemical devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11894. Pictures of the probe; experimental setup; reference spectra of solutions of LiClO4 in propylene carbonate; pictures of a model electrochemical cell; power dependence and time dependence of intensities of acetone and fluorescence and analysis of a solid surface (PDF)

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Figure 6. Dependence of intensities of Raman scattering of acetone and fluorescence on the distance between the detection area and the excitation laser. (a) A sample holder and experimental setup. (b) Spectrum of acetone with a trace amount of Rhodamine B at L = 0 μm. (c) Spectrum of acetone with a trace amount of Rhodamine B at L = 42 μm. (d) Dependence of intensities of Raman scattering of acetone and fluorescence on L.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Addresses §

Hiroe Nakagawa: GS Yuasa Int Ltd., R&D Ctr, Minami Ku, Kyoto 6128520, Japan. ∥ Manabu Ochida: Technology Div., Shin-Kobe Electric Machinery Co., Ltd. St. Luke’s Tower, 8−1 Akashi-cho, Chuo-ku, Tokyo 1040044, Japan. ⊥ Shigetaka Tsubouchi: Center for Technology InnovationMaterials Hitachi, Ltd., Research & Development Group 3191292, Japan. # Yasuhiro Domi: Tottori Univ. Department of Material Science, Faculty of Engineering, Tottori 6808552, Japan. ¶ Takayuki Doi: Doshisha Univ., Department of Mol Chem & Biochem, Kyoto 6100321, Japan.

L is lost through repeated emission processes, resulting in less significant dependence of fluorescence intensity on L. The observed fluorescence in Figure 6d decreased more slowly than 1/L2. This indicates that the higher-order multiple excitation processes contribute significantly to the generation of fluorescence inside the detection area. The intensity of the peak of acetone remained above L = 42 μm, which is thought to be stray light. These results support the above hypothesis that the background reduction by the probe is due to the small analysis area. Fluorescence should be reduced also by photobleaching if the irradiation power density in the analysis area is sufficiently high. However, the irradiation power density in the present study seemed not to be sufficiently high to induce photobleaching (see Figure S9). By using the probe, it will be possible to obtain Raman spectra of fluorescent organisms in narrow spaces inside samples where the focal point in the confocal setup is difficult to place using visible excitation light, although further improvements to enhance sensitivity may be required for detection of weak signals from organisms. The probe developed in this study is also useful for analysis of a solid surface (see Figure S10), although careful alignment between the sample and the probe is needed to avoid fluorescence emission from fibers excited by scattered laser light. The probe has the possibility of application to noninvasive in situ analysis in various fields of science and technologies.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under contract from the Research & Development Initiative for Scientific Innovation of New Generation Batteries (RISING).

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CONCLUSIONS An ultrafine fiber Raman probe with spatial resolution of 23 μm and a function of fluorescence background reduction has been developed. The distribution of ions in an electrolyte solution in 2590

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DOI: 10.1021/acs.jpcc.5b11894 J. Phys. Chem. C 2016, 120, 2585−2591