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Virtual Journal for Biomedical Optics

| EXPLORING THE INTERFACE OF LIGHT AND BIOMEDICINE

  • Editor: Gregory W. Faris
  • Vol. 3, Iss. 11 — Oct. 22, 2008
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Water-assisted femtosecond laser machining of electrospray nozzles on glass microfluidic devices

Ran An, Michelle D. Hoffman, Margaret A. Donoghue, Alan J. Hunt, and Stephen C. Jacobson  »View Author Affiliations


Optics Express, Vol. 16, Issue 19, pp. 15206-15211 (2008)
http://dx.doi.org/10.1364/OE.16.015206


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Abstract

Using water-assisted femtosecond laser machining, we fabricated electrospray nozzles on glass coverslips and on assembled microfluidic devices. Machining the nozzles after device assembly facilitated alignment of the nozzles over the microchannels. The basic nozzle design is a through-hole in the coverslip to pass liquids and a trough machined around the through-hole to confine the electrospray and prevent liquid from wicking across the glass surface. Electrospray from the nozzles was stable with and without pressure-driven flow applied and was evaluated using mass spectra of the peptide bradykinin.

© 2008 Optical Society of America

1. Introduction

Miniaturization and integration of electrospray devices has been the subject of intense research and development, due to the ability to generate charged, fine droplets for deposition, sorting, and dispersion of materials. Miniaturized electrospray devices coupled with mass spectrometers offer benefits including minimal sample consumption, fast analysis, and high sensitivity for chemical analysis. Well-fabricated nozzles are crucial to generate disperse ionized electrospray required to obtain reliable mass spectra of analytes. Typically, fused silica capillaries pulled to a narrow tip are used as electrospray sources, which offer simplicity, but are not easily integrated onto microfluidic platforms. Since initial demonstrations of electrospray from microfluidic devices [1

1. R. S. Ramsey and J. M. Ramsey, “Generating electrospray from microchip devices using electroosmotic pumping,” Anal. Chem. 69, 1174–1178 (1997). [CrossRef]

, 2

2. Q. Xue, F. Foret, Y. M. Dunayevskiy, P. M. Zavracky, N. E. McGruer, and B. L. Karger, “Multichannel microchip electrospray mass spectrometry,” Anal. Chem. 69, 426–430 (1997). [CrossRef] [PubMed]

], several approaches have been used and are reviewed [3

3. I. M. Lazar, J. Grym, and F. Foret, “Microfabricated devices: A new sample introduction approach to mass spectrometry,” Mass Spectrom. Rev. 25, 573–594 (2006). [CrossRef] [PubMed]

, 4

4. S. Koster and E. Verpoorte, “A decade of microfluidic analysis coupled with electrospray mass spectrometry: An overview,” Lab Chip 7, 1394–1412 (2007). [CrossRef] [PubMed]

]. In particular, microfabrication techniques have been adopted to form nozzles directly on microfluidic devices [5

5. L. Licklider, X. Q. Wang, A. Desai, Y. C. Tai, and T. D. Lee, “A micromachined chip-based electrospray source for mass spectrometry,” Anal. Chem. 72, 367–375 (2000). [CrossRef] [PubMed]

9

9. L. Wang, R. Stevens, A. Malik, P. Rockett, M. Paine, P. Adkin, S. Martyn, K. Smith, J. Stark, and P. Dobson, “High-aspect-ratio silica nozzle fabrication for nano-emitter electrospray applications,” Microelectron. Eng. 84, 1190–1193 (2007). [CrossRef]

]. Specific examples include high aspect ratio (height to width) nozzles fabricated in silicon [6

6. G. A. Schultz, T. N. Corso, S. J. Prosser, and S. Zhang, “A fully integrated monolithic microchip electrospray device for mass spectrometry,” Anal. Chem. 72, 4058–4063 (2000). [CrossRef] [PubMed]

]; however, silicon substrates are difficult to adapt to microfluidic separation devices that operate using electrokinetic transport. Also, laser ablation techniques have been used to form nozzles in poly(methylmethacrylate) [7

7. M. Schilling, W. Nigge, A. Rudzinski, A. Neyer, and R. Hergenroder, “A new on-chip ESI nozzle for coupling of MS with microfluidic devices,” Lab Chip 4, 220–224 (2004). [CrossRef] [PubMed]

] and polyimide [8

8. N. F. Yin, K. Killeen, R. Brennen, D. Sobek, M. Werlich, and T. V. van de Goor, “Microfluidic chip for peptide analysis with an integrated HPLC column, sample enrichment column, and nanoelectrospray tip,” Anal. Chem. 77, 527–533 (2005). [CrossRef] [PubMed]

] substrates. Compared to polymer substrates, glass and silica substrates combine chemical inertness with excellent mechanical, optical, and electrical properties, but obtaining high aspect ratio features on glass using standard microfabrication techniques remains difficult.

Damage by femtosecond lasers is remarkably reproducible and has been widely studied for material processing due to its unique capabilities of high precision and three-dimensional micro- and nanomachining in transparent dielectrics [10

10. P. P. Pronko, S. K. Dutta, D. Du, and R. K. Singh, “Thermophysical effects in laser processing of materials with picosecond and femtosecond pulses,” J. Appl. Phys. 78, 6233–6240 (1995). [CrossRef]

14

14. C. Momma, S. Nolte, B. N. Chichkov, F. von Alvensleben, and A. Tunnermann, “Precise laser ablation with ultrashort pulses,” Appl. Surf. Sci. 110, 15–19 (1997). [CrossRef]

]. This technique can be enhanced with the presence of water during machining, which efficiently removes ablation debris so that obstruction of features can be avoided [15

15. Y. Li, K. Itoh, W. Watanabe, K. Yamada, D. Kuroda, J. Nishii, and Y. Y. Jiang, “Three-dimensional hole drilling of silica glass from the rear surface with femtosecond laser pulses,” Opt. Lett. 26, 1912–1914 (2001). [CrossRef]

26

26. R. An, J. D. Uram, E. C. Yusko, K. Ke, M. Mayer, and A. J. Hunt, “Ultrafast laser fabrication of submicrometer pores in borosilicate glass,” Opt. Lett. 33, 1153–1155 (2008). [CrossRef] [PubMed]

]. Moreover, the machined surfaces are smooth [20

20. K. Ke, E. F. Hasselbrink, and A. J. Hunt, “Rapidly prototyped three-dimensional nanofluidic channel networks in glass substrates,” Anal. Chem. 77, 5083–5088 (2005). [CrossRef] [PubMed]

, 22

22. R. An, Y. Li, Y. Dou, D. Liu, H. Yang, and Q. Gong, “Water-assisted drilling of microfluidic chambers inside silica glass with femtosecond laser pulses,” Appl. Phys. A-Mater. Sci. Process. 83, 27–29 (2006). [CrossRef]

], and a number of microfluidic components such as high aspect ratio microholes, microchambers, and nanopores have been successfully fabricated [15

15. Y. Li, K. Itoh, W. Watanabe, K. Yamada, D. Kuroda, J. Nishii, and Y. Y. Jiang, “Three-dimensional hole drilling of silica glass from the rear surface with femtosecond laser pulses,” Opt. Lett. 26, 1912–1914 (2001). [CrossRef]

26

26. R. An, J. D. Uram, E. C. Yusko, K. Ke, M. Mayer, and A. J. Hunt, “Ultrafast laser fabrication of submicrometer pores in borosilicate glass,” Opt. Lett. 33, 1153–1155 (2008). [CrossRef] [PubMed]

]. Here, we use water-assisted laser machining to fabricate electrospray nozzles on glass coverslips and assembled microfluidic devices. As seen in Fig. 1, the basic nozzle design consists of a hole through the coverslip and a trough machined around the through-hole to confine the electrospray and prevent liquids from wicking across the glass surface. Of particular interest is the ability to machine nozzles after microfluidic device fabrication, allowing us to place nozzles over any microchannel at any location on the microfluidic device. This circumvents having to align nozzles fabricated in a coverslip over microchannels fabricated in a glass substrate during the bonding step.

Fig. 1. Schematic of the laser machining procedure. Using a 1.3 NA objective, the laser is focused through a coverslip onto the glass/water interface on the side distal to the objective. First, a through-hole is drilled through the coverslip, followed by machining of a trough around the through-hole.

2. Experimental procedure and results

Fig. 2. (a). Schematic of the electrospray nozzle with typical dimensions. Scanning electron microscope images of the (a) top and (b) 30° side views of a nozzle machined in a glass coverslip.

To characterize the electrospray, a coverslip containing a machined nozzle was mounted on a glass microscope slide using a poly(dimethylsiloxane) (PDMS) gasket. We sandblasted an access hole in the glass slide (AEC Air Eraser, Paasche Airbrush Co.) and epoxied a threaded fitting (N-333, Upchurch Scientific, Inc.) to the backside to connect a fused-silica capillary (25 µm i.d., 360 µm o.d., Polymicro Technologies). The capillary was used to couple the electrical potential and pressure-driven flow to the nozzle, which was positioned 3 mm from the inlet on the mass spectrometer. The electrospray experiments were conducted on a Q-TOF Ultima Global mass spectrometer (Waters Corp.) using a scan time of 1 s for all experiments. The capillary was connected to a syringe pump (780100C, Cole-Parmer), which was used to prime the nozzle with a 50/50 water/acetonitrile solution with 0.1% (v/v) formic acid. To induce electrospray, a potential of 3.5 kV relative to the inlet on the mass spectrometer was applied through a tee union connected to the capillary. Electrospray was evaluated for four 30-min periods with pressure-driven flow applied (15 µL/hr) and two 30-min periods without pressure-driven flow. Over these time periods, the electrospray was stable as indicated by monitoring the major ion peaks resulting from unpolymerized residues extracted from the PDMS gasket into the water/acetonitrile solution. These major ions had mass-to-charge ratios (m/z) of 503, 519, 725, and 741. Similar results were obtained using a 50/50 water/methanol solution with 0.1% (v/v) formic acid. During these experiments, the electrospray was not visible using the optics on the mass spectrometer, suggesting the electrospray was very fine and fluid exiting the aperture was not wicking across the coverslip surface.

After demonstrating electrospray with the nozzles fabricated on coverslips, we machined nozzles on fully assembled microfluidic devices. The water-assisted laser machining allows direct fabrication of the nozzles at arbitrary locations over the microchannels of assembled microfluidic devices, circumventing issues with aligning the nozzles over the microchannels prior to device assembly. We first fabricated a microfluidic device with a single microchannel (Fig. 3) using standard fabrication procedures [27

27. Z. Zhuang, J. A. Starkey, Y. Mechref, M. V. Novotny, and S. C. Jacobson, “Electrophoretic analysis of N-glycans on microfluidic devices,” Anal. Chem. 79, 7170–7175 (2007). [CrossRef] [PubMed]

]. Microchannels, 20 µm deep and 40 µm wide, were etched into white crown glass substrates (B270, Telic Co.). Holes were sandblasted at the ends of the channel to provide fluid access. No. 1 coverslips (24 mm × 50 mm, VWR, Inc.) were bonded to the B270 substrates by hydrolyzing both pieces in NH4OH:H2O2:H2O (2:1:2), rinsing thoroughly with water, bringing the substrate and coverslip into contact with each other, and annealing at 350°C for 20 h. Threaded fittings (N-124S, Upchurch Scientific, Inc.) were epoxied over the sandblasted holes to allow pressure and electrical connections to be made.

Because the microfluidic device thickness was > 1 mm, the 200-µm working distance of the 1.3 NA objective used to fabricate the nozzles in Fig. 2 was too short to focus through the microchip and machine nozzles on the surface opposite the objective. Consequently, the setup was reconfigured to machine the nozzle on the surface near the objective, as shown in Fig. 3(b). A temporary coverslip was placed on the near side of the microchip where the nozzle is machined, and an air objective with a 700-µm working distance (0.75 NA, Plan-Neofluar, Carl Zeiss, Inc.) was used instead of the high NA objective. During machining, the microchannel, as well as the gap between the microchip and temporary coverslip (~70 µm), was filled with water. This allowed water-assisted machining on the near side of the microchip, while preventing water and machining debris from contacting the objective.

Fig. 3. (a). Schematics of the top and side views of a microfluidic device with an integrated electrospray nozzle. (b). Schematic of the nozzle machining process on a microfluidic device with a temporary coverslip and water layer underneath the microfluidic device.

Similar to the process used to form the nozzles in the coverslips, the through-hole was machined from the microchannel in the direction of the objective. The procedure was the same as that on the coverslip with step sizes of 80, 300, and 300 nm in the radial, azimuthal, and vertical directions, respectively. The laser energy was 60 nJ/pulse. After the through-hole was completed, we carved the trough in step sizes of 100, 300, and 300 nm in the radial, azimuthal, and vertical directions, respectively. Although the less tightly focused laser beam from the 700-µm working distance objective produced larger machining voxels, we did not change the step sizes significantly for the machining because machining beginning from the front surface is not as efficient as from the rear surface [16

16. R. An, Y. Li, Y. P. Dou, Y. Fang, H. Yang, and Q. H. Gong, “Laser micro-hole drilling of soda-lime glass with femtosecond pulses,” Chin. Phys. Lett. 21, 2465–2468 (2004). [CrossRef]

]. Single and multiple nozzles were formed over the microchannels, and Fig. 4(a) shows a transmitted light image of a nozzle machined over a microchannel. In addition, these devices were used to electrospray the peptide bradykinin (20 µg/mL) in a 50/50 water/acetonitrile solution with 0.1% (v/v) formic acid. A typical mass spectrum obtained in positive ion mode is shown in Fig. 4(b). The singly charged bradykinin has an m/z of 1060, and the doubly charged species has an m/z of 530. Mass spectra identical to Fig. 4(b) were obtained using a standard capillary electrospray tip (PicoTip Emitter, 15 µm i.d. tip, New Objective, Inc.).

Fig. 4. (a). Transmitted light image of an electrospray nozzle fabricated directly over a microchannel in an assembled microfluidic device. The image in (a) is a composite of two images taken at different focal planes. (b) Mass spectrum of bradykinin electrosprayed from the microfluidic device through the nozzle. The most intense peak at m/z = 530 is doubly charged bradykinin.

3. Summary

We have demonstrated fabrication of high quality electrospray nozzles on glass coverslips and on assembled microfluidic devices using water-assisted femtosecond laser machining. This approach permits integration of electrospray nozzles onto glass microfluidic devices, thus simplifying construction of integrated microfluidic systems which employ mass spectrometric detection.

Acknowledgments

This work was supported in part by NIH R21 EB006098 for AJH and by NIH P41 RR018942 and Pfizer, Inc., St. Louis, MO for SCJ. The SEM images were taken using the FEI Nova Nanolab at the Electron Microbeam Analysis Laboratory (EMAL) at North Campus, University of Michigan. We thank Drs. Yehia Mechref and Milan Madera at Indiana University for assistance with the mass spectrometry experiments.

References and links

1.

R. S. Ramsey and J. M. Ramsey, “Generating electrospray from microchip devices using electroosmotic pumping,” Anal. Chem. 69, 1174–1178 (1997). [CrossRef]

2.

Q. Xue, F. Foret, Y. M. Dunayevskiy, P. M. Zavracky, N. E. McGruer, and B. L. Karger, “Multichannel microchip electrospray mass spectrometry,” Anal. Chem. 69, 426–430 (1997). [CrossRef] [PubMed]

3.

I. M. Lazar, J. Grym, and F. Foret, “Microfabricated devices: A new sample introduction approach to mass spectrometry,” Mass Spectrom. Rev. 25, 573–594 (2006). [CrossRef] [PubMed]

4.

S. Koster and E. Verpoorte, “A decade of microfluidic analysis coupled with electrospray mass spectrometry: An overview,” Lab Chip 7, 1394–1412 (2007). [CrossRef] [PubMed]

5.

L. Licklider, X. Q. Wang, A. Desai, Y. C. Tai, and T. D. Lee, “A micromachined chip-based electrospray source for mass spectrometry,” Anal. Chem. 72, 367–375 (2000). [CrossRef] [PubMed]

6.

G. A. Schultz, T. N. Corso, S. J. Prosser, and S. Zhang, “A fully integrated monolithic microchip electrospray device for mass spectrometry,” Anal. Chem. 72, 4058–4063 (2000). [CrossRef] [PubMed]

7.

M. Schilling, W. Nigge, A. Rudzinski, A. Neyer, and R. Hergenroder, “A new on-chip ESI nozzle for coupling of MS with microfluidic devices,” Lab Chip 4, 220–224 (2004). [CrossRef] [PubMed]

8.

N. F. Yin, K. Killeen, R. Brennen, D. Sobek, M. Werlich, and T. V. van de Goor, “Microfluidic chip for peptide analysis with an integrated HPLC column, sample enrichment column, and nanoelectrospray tip,” Anal. Chem. 77, 527–533 (2005). [CrossRef] [PubMed]

9.

L. Wang, R. Stevens, A. Malik, P. Rockett, M. Paine, P. Adkin, S. Martyn, K. Smith, J. Stark, and P. Dobson, “High-aspect-ratio silica nozzle fabrication for nano-emitter electrospray applications,” Microelectron. Eng. 84, 1190–1193 (2007). [CrossRef]

10.

P. P. Pronko, S. K. Dutta, D. Du, and R. K. Singh, “Thermophysical effects in laser processing of materials with picosecond and femtosecond pulses,” J. Appl. Phys. 78, 6233–6240 (1995). [CrossRef]

11.

E. N. Glezer, M. Milosavljevic, L. Huang, R. J. Finlay, T. H. Her, J. P. Callan, and E. Mazur, “Three-dimensional optical storage inside transparent materials,” Opt. Lett. 21, 2023–2025 (1996). [CrossRef] [PubMed]

12.

C. B. Schaffer, A. Brodeur, J. F. Garcia, and E. Mazur, “Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy,” Opt. Lett. 26, 93–95 (2001). [CrossRef]

13.

M. D. Perry, B. C. Stuart, P. S. Banks, M. D. Feit, V. Yanovsky, and A. M. Rubenchik, “Ultrashort-pulse laser machining of dielectric materials,” J. Appl. Phys. 85, 6803–6810 (1999). [CrossRef]

14.

C. Momma, S. Nolte, B. N. Chichkov, F. von Alvensleben, and A. Tunnermann, “Precise laser ablation with ultrashort pulses,” Appl. Surf. Sci. 110, 15–19 (1997). [CrossRef]

15.

Y. Li, K. Itoh, W. Watanabe, K. Yamada, D. Kuroda, J. Nishii, and Y. Y. Jiang, “Three-dimensional hole drilling of silica glass from the rear surface with femtosecond laser pulses,” Opt. Lett. 26, 1912–1914 (2001). [CrossRef]

16.

R. An, Y. Li, Y. P. Dou, Y. Fang, H. Yang, and Q. H. Gong, “Laser micro-hole drilling of soda-lime glass with femtosecond pulses,” Chin. Phys. Lett. 21, 2465–2468 (2004). [CrossRef]

17.

Y. Iga, T. Ishizuka, W. Watanabe, K. Itoh, Y. Li, and J. Nishii, “Characterization of micro-channels fabricated by in-water ablation of femtosecond laser pulses,” Jpn. J. Appl. Phys. Part1 43, 4207–4211 (2004). [CrossRef]

18.

D. J. Hwang, T. Y. Choi, and C. P. Grigoropoulos, “Liquid-assisted femtosecond laser drilling of straight and three-dimensional microchannels in glass,” Appl. Phys. A-Mater. Sci. Process. 79, 605–612 (2004). [CrossRef]

19.

R. An, Y. Li, Y. P. Dou, H. Yang, and Q. H. Gong, “Simultaneous multi-microhole drilling of soda-lime glass by water-assisted ablation with femtosecond laser pulses,” Opt. Express 13, 1855–1859 (2005). [CrossRef] [PubMed]

20.

K. Ke, E. F. Hasselbrink, and A. J. Hunt, “Rapidly prototyped three-dimensional nanofluidic channel networks in glass substrates,” Anal. Chem. 77, 5083–5088 (2005). [CrossRef] [PubMed]

21.

T. N. Kim, K. Campbell, A. Groisman, D. Kleinfeld, and C. B. Schaffer, “Femtosecond laser-drilled capillary integrated into a microfluidic device,” Appl. Phys. Lett. 86, 201106 (2005). [CrossRef]

22.

R. An, Y. Li, Y. Dou, D. Liu, H. Yang, and Q. Gong, “Water-assisted drilling of microfluidic chambers inside silica glass with femtosecond laser pulses,” Appl. Phys. A-Mater. Sci. Process. 83, 27–29 (2006). [CrossRef]

23.

J. D. Uram, K. Ke, A. J. Hunt, and M. Mayer, “Submicrometer pore-based characterization and quantification of antibody-virus interactions,” Small 2, 967–972 (2006). [CrossRef] [PubMed]

24.

J. D. Uram, K. Ke, A. J. Hunt, and M. Mayer, “Label-free affinity assays by rapid detection of immune complexes in submicrometer pores,” Angew. Chem.-Int. Edit. 45, 2281–2285 (2006). [CrossRef]

25.

S. Lee, J. L. Bull, and A. J. Hunt, “Acoustic limitations on the efficiency of machining by femtosecond laser-induced optical breakdown,” Appl. Phys. Lett. 91, 023111 (2007). [CrossRef]

26.

R. An, J. D. Uram, E. C. Yusko, K. Ke, M. Mayer, and A. J. Hunt, “Ultrafast laser fabrication of submicrometer pores in borosilicate glass,” Opt. Lett. 33, 1153–1155 (2008). [CrossRef] [PubMed]

27.

Z. Zhuang, J. A. Starkey, Y. Mechref, M. V. Novotny, and S. C. Jacobson, “Electrophoretic analysis of N-glycans on microfluidic devices,” Anal. Chem. 79, 7170–7175 (2007). [CrossRef] [PubMed]

OCIS Codes
(190.7110) Nonlinear optics : Ultrafast nonlinear optics
(230.4000) Optical devices : Microstructure fabrication
(350.3390) Other areas of optics : Laser materials processing

ToC Category:
Laser Micromachining

History
Original Manuscript: August 19, 2008
Revised Manuscript: September 5, 2008
Manuscript Accepted: September 5, 2008
Published: September 11, 2008

Virtual Issues
Vol. 3, Iss. 11 Virtual Journal for Biomedical Optics

Citation
Ran An, Michelle D. Hoffman, Margaret A. Donoghue, Alan J. Hunt, and Stephen C. Jacobson, "Water-assisted femtosecond laser machining of electrospray nozzles on glass microfluidic devices," Opt. Express 16, 15206-15211 (2008)
http://www.opticsinfobase.org/vjbo/abstract.cfm?URI=oe-16-19-15206


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References

  1. R. S. Ramsey and J. M. Ramsey, "Generating electrospray from microchip devices using electroosmotic pumping," Anal. Chem. 69, 1174-1178 (1997). [CrossRef]
  2. Q. Xue, F. Foret, Y. M. Dunayevskiy, P. M. Zavracky, N. E. McGruer, and B. L. Karger, "Multichannel microchip electrospray mass spectrometry," Anal. Chem. 69, 426-430 (1997). [CrossRef] [PubMed]
  3. I. M. Lazar, J. Grym, and F. Foret, "Microfabricated devices: A new sample introduction approach to mass spectrometry," Mass Spectrom. Rev. 25, 573-594 (2006). [CrossRef] [PubMed]
  4. S. Koster and E. Verpoorte, "A decade of microfluidic analysis coupled with electrospray mass spectrometry: An overview," Lab Chip 7, 1394-1412 (2007). [CrossRef] [PubMed]
  5. L. Licklider, X. Q. Wang, A. Desai, Y. C. Tai, and T. D. Lee, "A micromachined chip-based electrospray source for mass spectrometry," Anal. Chem. 72, 367-375 (2000). [CrossRef] [PubMed]
  6. G. A. Schultz, T. N. Corso, S. J. Prosser, and S. Zhang, "A fully integrated monolithic microchip electrospray device for mass spectrometry," Anal. Chem. 72, 4058-4063 (2000). [CrossRef] [PubMed]
  7. M. Schilling, W. Nigge, A. Rudzinski, A. Neyer, and R. Hergenroder, "A new on-chip ESI nozzle for coupling of MS with microfluidic devices," Lab Chip 4, 220-224 (2004). [CrossRef] [PubMed]
  8. N. F. Yin, K. Killeen, R. Brennen, D. Sobek, M. Werlich, and T. V. van de Goor, "Microfluidic chip for peptide analysis with an integrated HPLC column, sample enrichment column, and nanoelectrospray tip," Anal. Chem. 77, 527-533 (2005). [CrossRef] [PubMed]
  9. L. Wang, R. Stevens, A. Malik, P. Rockett, M. Paine, P. Adkin, S. Martyn, K. Smith, J. Stark, and P. Dobson, "High-aspect-ratio silica nozzle fabrication for nano-emitter electrospray applications," Microelectron. Eng. 84, 1190-1193 (2007). [CrossRef]
  10. P. P. Pronko, S. K. Dutta, D. Du, and R. K. Singh, "Thermophysical effects in laser processing of materials with picosecond and femtosecond pulses," J. Appl. Phys. 78, 6233-6240 (1995). [CrossRef]
  11. E. N. Glezer, M. Milosavljevic, L. Huang, R. J. Finlay, T. H. Her, J. P. Callan, and E. Mazur, "Three-dimensional optical storage inside transparent materials," Opt. Lett. 21, 2023-2025 (1996). [CrossRef] [PubMed]
  12. C. B. Schaffer, A. Brodeur, J. F. Garcia, and E. Mazur, "Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy," Opt. Lett. 26, 93-95 (2001). [CrossRef]
  13. M. D. Perry, B. C. Stuart, P. S. Banks, M. D. Feit, V. Yanovsky, and A. M. Rubenchik, "Ultrashort-pulse laser machining of dielectric materials," J. Appl. Phys. 85, 6803-6810 (1999). [CrossRef]
  14. C. Momma, S. Nolte, B. N. Chichkov, F. von Alvensleben, and A. Tunnermann, "Precise laser ablation with ultrashort pulses," Appl. Surf. Sci. 110, 15-19 (1997). [CrossRef]
  15. Y. Li, K. Itoh, W. Watanabe, K. Yamada, D. Kuroda, J. Nishii, and Y. Y. Jiang, "Three-dimensional hole drilling of silica glass from the rear surface with femtosecond laser pulses," Opt. Lett. 26, 1912-1914 (2001). [CrossRef]
  16. R. An, Y. Li, Y. P. Dou, Y. Fang, H. Yang, and Q. H. Gong, "Laser micro-hole drilling of soda-lime glass with femtosecond pulses," Chin. Phys. Lett. 21, 2465-2468 (2004). [CrossRef]
  17. Y. Iga, T. Ishizuka, W. Watanabe, K. Itoh, Y. Li, and J. Nishii, "Characterization of micro-channels fabricated by in-water ablation of femtosecond laser pulses," Jpn. J. Appl. Phys. Part 1 43, 4207-4211 (2004). [CrossRef]
  18. D. J. Hwang, T. Y. Choi, and C. P. Grigoropoulos, "Liquid-assisted femtosecond laser drilling of straight and three-dimensional microchannels in glass," Appl. Phys. A-Mater. Sci. Process. 79, 605-612 (2004). [CrossRef]
  19. R. An, Y. Li, Y. P. Dou, H. Yang, and Q. H. Gong, "Simultaneous multi-microhole drilling of soda-lime glass by water-assisted ablation with femtosecond laser pulses," Opt. Express 13, 1855-1859 (2005). [CrossRef] [PubMed]
  20. K. Ke, E. F. Hasselbrink, and A. J. Hunt, "Rapidly prototyped three-dimensional nanofluidic channel networks in glass substrates," Anal. Chem. 77, 5083-5088 (2005). [CrossRef] [PubMed]
  21. T. N. Kim, K. Campbell, A. Groisman, D. Kleinfeld, and C. B. Schaffer, "Femtosecond laser-drilled capillary integrated into a microfluidic device," Appl. Phys. Lett. 86, 201106 (2005). [CrossRef]
  22. R. An, Y. Li, Y. Dou, D. Liu, H. Yang, and Q. Gong, "Water-assisted drilling of microfluidic chambers inside silica glass with femtosecond laser pulses," Appl. Phys. A-Mater. Sci. Process. 83, 27-29 (2006). [CrossRef]
  23. J. D. Uram, K. Ke, A. J. Hunt, and M. Mayer, "Submicrometer pore-based characterization and quantification of antibody-virus interactions," Small 2, 967-972 (2006). [CrossRef] [PubMed]
  24. J. D. Uram, K. Ke, A. J. Hunt, and M. Mayer, "Label-free affinity assays by rapid detection of immune complexes in submicrometer pores," Angew. Chem.Int. Ed. 45, 2281-2285 (2006). [CrossRef]
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