Nano-Gap Magnetophoresis with Raman Spectroscopic Detection

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ANALYTICAL SCIENCES DECEMBER 2010, VOL. 26 1211 Magnetophoresis 13 is especially a unique migration phenomenon, which is caused by the magnetic interaction between a particle and the magnetic field gradient. Theoretically, the magnetophoretic velocity, vm, of a spherical particle in a liquid with a radius (r) is represented by v r B B x m p f d d = ( ) , 2 9 2 0 χ χ ημ - (1) where (χp χf) is the difference in the magnetic susceptibility between the particle, χp, and the medium, χf; B the magnetic flux density, dB/dx the gradient of the magnetic flux density, η the viscosity of the medium, and μ0 the magnetic susceptibility of the vacuum. The sign of vm is positive, when the particle migrates to the positive direction of the x axis. This equation was derived based on the assumption that the magnetic force on a spherical particle balances with the drag force on the particle all the way. The observed magnetophoretic velocity can afford the magnetic susceptibility of the particle, provided that the value of r is known, or experimentally determined. 4 As for micrometer-sized particles, an optical microscope can be used to determine the radius of a particle in a liquid. However, in the case of sub-micron or nanometer-sized particles, an alternative method has to be used. Recently, an innovative approach, named nano-gap method, has been reported. 57 The original nano-gap device was constructed by a flat glass plate and a convex or a cylindrical lens for trapping particles in the gap. The gap distance was precisely measured by observing interference fringes generated by the flat glass plate and the lens. In the present study, we invented a flat plate nano-gap device to determine the size of nano/microparticles in a liquid, and combined it with the magnetophoretic velocity analysis method to determine the magnetic susceptibility of a single particle in a liquid. Furthermore, Raman microscope spectroscopy was combined to detect or characterize the trapped particle. The performance of the newly constructed method was demonstrated by an analysis of suspended particulate matter (SPM) in traffic air. Experimental The polystyrene latex particles used in the experiments were 3 μm (3.015 ± 0.158 μm) and 500 nm (516 ± 11 nm) (Polyscience Inc., Pennsylvania, USA) in diameter. A 1.0 M MnCl2 (GR, Katayama Kagaku) aqueous solution ( χ p = 1.58 × 10 –4 ) was used as a paramagnetic medium of the sample particles. Traffic suspended particle matter (SPM) less than 4 μm was collected in the Osaka area on a fluorocarbon resin-treated glass filter. A filter with collected black SPM was immersed in a 1.0 M MnCl2 solution and the SPM was dispersed in the solution. The nano-gap magnetophoresis device was constructed by stacking 3 cover glass plates (18 × 18 mm, Matunami Glass, Tokyo, Japan) with glue, as illustrated in Fig. 1. Both sides of the gap were sealed with glue (Araldite, Hantsman, USA) to make a flow channel with 6 mm width. Interference fringes were observed between plate A and plate B under a microscope (U-DP1XC, Olympus, Japan) with 520 nm irradiation light. A small strip of paper (3 × 40 mm, Kimwiper ® ) was inserted at the end of the nano-gap for absorbing and evaporating water to generate a passive continuous flow in the nano-gap channel. The linear flow velocity, vf, was steady for several tens of minutes. Nano-gap magnetophoresis was performed by inserting the nano-gap device into the gap of iron pole pieces attached to a pair of permanent magnets (Fig. 2). The maximum magnetic field gradient at the edge of pole pieces, which was measured by a Gauss meter (HGM-3000P, ADS Inc., Japan), was B(dB/dx) = 1100 T 2 m –1 . A 5-μL portion of the sample solution was put on the sample inlet of the end of the glass plate B. The sample particles were introduced by passive flow, and the velocity of the migrating particle, vp, in the magnetophoresis region was observed. The magnetophoretic velocity, vm, was obtained by the relationship vm = vp vf. A calibration line between the gap distance h and the trapped position of a particle l in the nano-gap device was obtained from 2010 © The Japan Society for Analytical Chemistry To whom correspondence should be addressed. E-mail: [email protected] J. L. Y. present address: Department of Medicine, Trinity School of Medicine, Ratho Mill, Saint Vincent and the Grenedines. Nano-Gap Magnetophoresis with Raman Spectroscopic Detection Makoto KAWANO,* Jennifer L. YOUNG,* and Hitoshi WATARAI* , ** *Department of Chemistry, Graduate School of Science, Osaka University,Toyonaka, Osaka 5600043, Japan **Institute for NanoScience Design, Osaka University, Toyonaka, Osaka 5608531, Japan As a tool to determine the magnetic susceptibility of a nano/microparticle in a liquid, a nano-gap magnetophoresis device was fabricated using tilted flat glass plates and a permanent magnet. The nano-gap device was not only used to determine the size of particles, but also directly provided measurements of Raman spectra of fractionally trapped particles in the nano-gap device. The performance of the new hyphenated method was demonstrated by an analysis of suspended particle matter in traffic air. (Received November 17, 2010; Accepted November 25, 2010; Published December 10, 2010) Rapid Communications

Transcript of Nano-Gap Magnetophoresis with Raman Spectroscopic Detection

Page 1: Nano-Gap Magnetophoresis with Raman Spectroscopic Detection

ANALYTICAL SCIENCES DECEMBER 2010, VOL. 26 1211

Magnetophoresis1–3 is especially a unique migration phenomenon, which is caused by the magnetic interaction between a particle and the magnetic field gradient. Theoretically, the magnetophoretic velocity, vm, of a spherical particle in a liquid with a radius (r) is represented by

vr

B Bx

mp f d

d=

( ),2

9

2

0

χ χηµ−

(1)

where (χp – χf) is the difference in the magnetic susceptibility between the particle, χp, and the medium, χf; B the magnetic flux density, dB/dx the gradient of the magnetic flux density, η the viscosity of the medium, and μ0 the magnetic susceptibility of the vacuum. The sign of vm is positive, when the particle migrates to the positive direction of the x axis. This equation was derived based on the assumption that the magnetic force on a spherical particle balances with the drag force on the particle all the way. The observed magnetophoretic velocity can afford the magnetic susceptibility of the particle, provided that the value of r is known, or experimentally determined.4 As for micrometer-sized particles, an optical microscope can be used to determine the radius of a particle in a liquid. However, in the case of sub-micron or nanometer-sized particles, an alternative method has to be used. Recently, an innovative approach, named nano-gap method, has been reported.5–7 The original nano-gap device was constructed by a flat glass plate and a convex or a cylindrical lens for trapping particles in the gap. The gap distance was precisely measured by observing interference fringes generated by the flat glass plate and the lens. In the present study, we invented a flat plate nano-gap device to determine the size of nano/microparticles in a liquid, and combined it with the magnetophoretic velocity analysis method to determine the magnetic susceptibility of a single particle in a liquid. Furthermore, Raman microscope spectroscopy was combined to detect or characterize the trapped particle. The performance of the newly constructed method was demonstrated

by an analysis of suspended particulate matter (SPM) in traffic air.

Experimental

The polystyrene latex particles used in the experiments were 3 μm (3.015 ± 0.158 μm) and 500 nm (516 ± 11 nm) (Polyscience Inc., Pennsylvania, USA) in diameter. A 1.0 M MnCl2 (GR, Katayama Kagaku) aqueous solution (χp = 1.58 × 10–4) was used as a paramagnetic medium of the sample particles. Traffic suspended particle matter (SPM) less than 4 μm was collected in the Osaka area on a fluorocarbon resin-treated glass filter. A filter with collected black SPM was immersed in a 1.0 M MnCl2 solution and the SPM was dispersed in the solution.

The nano-gap magnetophoresis device was constructed by stacking 3 cover glass plates (18 × 18 mm, Matunami Glass, Tokyo, Japan) with glue, as illustrated in Fig. 1. Both sides of the gap were sealed with glue (Araldite, Hantsman, USA) to make a flow channel with 6 mm width. Interference fringes were observed between plate A and plate B under a microscope (U-DP1XC, Olympus, Japan) with 520 nm irradiation light. A small strip of paper (3 × 40 mm, Kimwiper®) was inserted at the end of the nano-gap for absorbing and evaporating water to generate a passive continuous flow in the nano-gap channel. The linear flow velocity, vf, was steady for several tens of minutes.

Nano-gap magnetophoresis was performed by inserting the nano-gap device into the gap of iron pole pieces attached to a pair of permanent magnets (Fig. 2). The maximum magnetic field gradient at the edge of pole pieces, which was measured by a Gauss meter (HGM-3000P, ADS Inc., Japan), was B(dB/dx) = 1100 T2 m–1. A 5-μL portion of the sample solution was put on the sample inlet of the end of the glass plate B. The sample particles were introduced by passive flow, and the velocity of the migrating particle, vp, in the magnetophoresis region was observed. The magnetophoretic velocity, vm, was obtained by the relationship vm = vp – vf.

A calibration line between the gap distance h and the trapped position of a particle l in the nano-gap device was obtained from

2010 © The Japan Society for Analytical Chemistry

† To whom correspondence should be addressed.E-mail: [email protected]. L. Y. present address: Department of Medicine, Trinity School of Medicine, Ratho Mill, Saint Vincent and the Grenedines.

Nano-Gap Magnetophoresis with Raman Spectroscopic Detection

Makoto KAWANO,* Jennifer L. YOUNG,* and Hitoshi WATARAI*,**†

*DepartmentofChemistry,GraduateSchoolofScience,OsakaUniversity,Toyonaka,Osaka560–0043,Japan**InstituteforNanoScienceDesign,OsakaUniversity,Toyonaka,Osaka560–8531,Japan

As a tool to determine the magnetic susceptibility of a nano/microparticle in a liquid, a nano-gap magnetophoresis device was fabricated using tilted flat glass plates and a permanent magnet. The nano-gap device was not only used to determine the size of particles, but also directly provided measurements of Raman spectra of fractionally trapped particles in the nano-gap device. The performance of the new hyphenated method was demonstrated by an analysis of suspended particle matter in traffic air.

(Received November 17, 2010; Accepted November 25, 2010; Published December 10, 2010)

Rapid Communications

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1212 ANALYTICAL SCIENCES DECEMBER 2010, VOL. 26

the observed interference fringes of the irradiated light (520 nm), as shown in Fig. 3. The details used to obtain the corrected diameter d is described in Supporting Information. The trapping behavior of the particles was observed by the reflection light or

fluorescence images (in the case of 500 nm particles) using a microscope. CCD images captured with a PC were analyzed. Using the observed radius of the particle by the nano-gap method, the magnetic susceptibilities of the particles were finally determined by Eq. (1).

After the nano-gap megnetophoresis measurement, the SPM particles which were trapped in the nano-gap device were characterized by a semi-confocal Raman microscope spectroscopy (Nanofinder® 30, Tokyo Instruments, Tokyo, Japan) with 100× objective.

Results and Discussion

SizemeasurementofpolystyreneparticlesThe average particle diameter of 50 particles of 3 μm

polystyrene particles, measured by the nano-gap device, was 2.983 ± 0.031 μm, which was slightly smaller than the suggested value by the company, 3.015 ± 0.158 μm. This may be due to the fact that the nano-gap method measures the shorter axis of the particles, if the particles are not strictly spherical. The average diameter of 500 nm polystyrene particles was 511 ± 12 nm, which was close to the suggested value of 516 ± 11 nm.

The distance between the observed dark lines was 40.4 μm (Fig. 3). Since we can observe the position of the particle with an accuracy of 1 μm, the limit of the accuracy of the particle diameter measurement is estimated to be 5 nm. The realistic minimum size of a particle to be discriminated by the nano-gap method will be about 50 nm, corresponding to 10 μm in the lateral distance.

Nano-gapmagnetophoresisofSPMThe velocity of SPM particles moving with the passive flow

was decreased when approaching the edge of the pole piece, indicating that the SPM particle was diamagnetic. The magnetophoretic velocity was determined from the migration

Fig. 2 Schematic drawing of the magnetophoretic velocity measurement by the nano-gap magnetophoresis device combined with a Raman microscopic system. The magnetophoretic strength, B(dB/dx), was shown as a function of the distance from the edge of the pole piece.

Fig. 3 Upper: an example of the interference fridges observed with 520 nm light between the flat plates of the nano-gap cell, where the dark lines were indicated by the number m. Lower: a linear calibration equation of h = 0.00484l + 0.194, calculated from the dark lines, where h0 is 0.194 μm and the gap angle is 0.276°.

Fig. 1 Schematic drawing of (a) the side-view of the nano-gap magnetophoresis device inserted into the magnet and (b) the topview of the nano-gap cell.

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ANALYTICAL SCIENCES DECEMBER 2010, VOL. 26 1213

velocity in the region of 500 μm outside from the edge of the pole piece, where the value of B(dB/dx) is positive (Fig. 2). The average magnetic susceptibility of SPM particles with a diameter of 2.0 ± 0.5 μm was determined to be χp = –7.2 ± 0.3 × 10–6, suggesting soot carbon as the main component. This is clearly different from the magnetic susceptibility of a polystyrene particle (χp = –8.21 × 10–6). To obtain more information concerning the chemical composition of SPM particles, the Raman microscopic method was applied.

RamanmicroscopicanalysisofSPMA traffic SPM particle trapped in the nano-gap cell was

directly observed by Raman microscopy (Fig. 4(a)). Figure 4 shows the Raman spectra of a traffic SPM particle as well as those of a SPM from motorbike exhaust (Fig. 4(b)) and a polystyrene particle (Fig. 4(c)). The Raman spectra of SPM showed two main peaks, which were assigned to the D band (1330 cm–1) and the G band (1600 cm–1) of graphite.8,9 Although the spectrum of SPM was slightly different depending on the particle, the G band of Fig. 4(a) shifted to a lower wave number, and was more broadened, and the D band of Fig. 4(a) was larger than that of Fig. 4(b), which suggested a higher content of the disordered sp2 carbon in the traffic SPM.10 It seems that the traffic SPM particles included amorphous structured carbon generated from vehicles other than motorbikes. The Raman spectra of the SPM particles were clearly different from those of

polystyrene particles, which showed a strong aromatic peak at 1600 cm–1 (Fig. 4(c)). Raman spectroscopy will provide useful information to discriminate the origin of trapped particles.

In conclusions, we have invented a novel particle-analysis method combining magnetophoresis, a nano-gap method and Raman microscopy. Using the radius determined by the nano-gap method, the magnetic susceptibility of the particle in a liquid was determined. Nano-gap magnetophoresis with the Raman detection method is the most promising method for nano/microparticle magnetic analysis, which must be required in fields including the magnetic nano-material industry and magnetic bio-nano-particle technology.

Acknowledgements

This research was financially supported by Grant-in-Aid for Scientific Research (A) (No. 21245022) of the Ministry of Education, Culture, Sports, Science and Technology of Japan.

Supporting Information

Supporting information is available free of charge via the Internet at http://www.jsac.or.jp/analsci/.

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Fig. 4 Raman microscope spectra of (a) a SPM particle (3 μm) trapped in the nano-gap device, (b) a SPM particle (3 μm) from motorbike exhaust and (c) a polystyrene particle (6 μm).