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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 21 — Oct. 8, 2012
  • pp: 23700–23719
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Development and integration of xerogel polymeric absorbance micro-filters into lab-on-chip systems

Ester Carregal-Romero, César Fernández-Sánchez, Alma Eguizabal, Stefanie Demming, Stephanus Büttgenbach, and Andreu Llobera  »View Author Affiliations


Optics Express, Vol. 20, Issue 21, pp. 23700-23719 (2012)
http://dx.doi.org/10.1364/OE.20.023700


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Abstract

This work reports on the implementation of different absorption micro-filters based on a dye-doped hybrid organic-inorganic xerogel polymeric material synthesized by the sol-gel process. Microstructures containing eight different filter widths were fabricated in polydimethylsiloxane (PDMS), bonded to glass substrates and filled with the corresponding dye doped polymeric material by a soft lithography approach. The filtering capacity as a function of dye concentration and filter width was studied and revealed a linear dependence with both parameters, as expected according to the Beer-Lambert law. Zero passband transmittance values and relatively sharp stopband regions were achieved with all the filters, also showing rejection levels between −6 dB and −55 dB. Finally, such filters were monolithically integrated into a disposable fluorescence-based photonic lab-on-a-chip (PhLoC) approach. Calibration curves carried out with a model fluorophore target analyte showed an over two-fold increase in sensitivity and a thirty-fold decrease of the limit of detection (LOD) compared with the values recorded using the same PhLoC system but without the polymeric filter structure. The results presented herein clearly indicate the feasibility of these xerogel-based absorbance filtering structures for being applied as low-cost optical components that can be easily incorporated into disposable fluorescence-based photonic lab on a chip systems.

© 2012 OSA

1. Introduction

One of the most common approaches in photonic lab-on-a-chip (PhLoC) systems is the measurementof fluorescence as detection method [1

1. S. M. Borisov and O. S. Wolfbeis, “Optical biosensors,” Chem. Rev. 108(2), 423–461 (2008). [CrossRef] [PubMed]

]. Optical filtering is often required to be implemented in these systems due to the following main reasons. In fluorescence spectroscopy the intensity of the excitation light is typically orders of magnitude larger than the fluorescence light of the fluorophore target molecule. Additionally, the Stokes shift of fluorophore molecules is usually very small, which makes it even more difficult to discriminate between the excitation and emission signals. The more accepted filtering configurations suitable to perform such discrimination are based on interferometric and absorbance-based approaches [2

2. M. Dandin, P. Abshire, and E. Smela, “Optical filtering technologies for integrated fluorescence sensors,” Lab Chip 7(8), 955–977 (2007). [CrossRef] [PubMed]

]. Interferometric filters consist in alternating layers of high- and low-refractive-index materials. In spite of these filters showing high absorbance levels at the stopband with sharp bands and zero passband penalties they suffer from serious drawbacks. First, the absorption depends on the angle of the incidence light [3

3. H. A. Macleod, “Thin Film Optical Filters” (Institute of Physics Publishing, London, 2001)

]. Second, the thickness of the layers has to be perfectly controlled in order to obtain filtering at the required wavelength, which makes the fabrication an expensive and critical process. These effects hamper its integration into PhLoC. Conversely, absorbance filters are generally fabricated with a single layer of a material containing a chromophore [4

4. M. L. Chabinyc, D. T. Chiu, J. C. McDonald, A. D. Stroock, J. F. Christian, A. M. Karger, and G. M. Whitesides, “An integrated fluorescence detection system in poly(dimethylsiloxane) for microfluidic applications,” Anal. Chem. 73(18), 4491–4498 (2001). [CrossRef] [PubMed]

] or a band gap material [5

5. A. H. Mahan, R. Biswas, L. M. Gedvilas, D. L. Williamson, and B. C. Pan, “On the influence of short and medium range order on the material band gap in hydrogenated amorphous silicon,” J. Appl. Phys. 96(7), 3818–3826 (2004). [CrossRef]

]. In both cases, they are fabricated to show high absorption at the excitation wavelength and low absorption at the fluorescence wavelength of the solution or compound being measured. The performance of such absorbance filters is governed by the Beer-Lambert law and, unlike interferometric filters, their response is independent of the beam incidence angle [2

2. M. Dandin, P. Abshire, and E. Smela, “Optical filtering technologies for integrated fluorescence sensors,” Lab Chip 7(8), 955–977 (2007). [CrossRef] [PubMed]

].

Since the pioneering work of Avnir and associates, who entrapped Rhodamine-6G into pure silica [20

20. D. Avnir, D. Levy, and R. Reisfeld, “The nature of the silica cage as reflected by spectral changes and enhanced photostability of trapped Rhodamine-6G,” J. Phys. Chem. 88(24), 5956–5959 (1984). [CrossRef]

], a large number of molecules and biological species showing optical activity have been entrapped inside either an inorganic or a hybrid organic-inorganic network made by the sol-gel approach. A wide variety of material formulations showing different optical properties have been developed and excellent reviews showing their potential for the fabrication of photonic components and optical sensors have been published [13

13. B. Lebeau and P. Innocenzi, “Hybrid materials for optics and photonics,” Chem. Soc. Rev. 40(2), 886–906 (2011). [CrossRef] [PubMed]

], [21

21. G. Schottner, “Hybrid sol-gel-derived polymers: Applications of multifunctional materials,” Chem. Mater. 13(10), 3422–3435 (2001). [CrossRef]

24

24. R. Pardo, M. Zayat, and D. Levy, “Photochromic organic-inorganic hybrid materials,” Chem. Soc. Rev. 40(2), 672–687 (2011). [CrossRef] [PubMed]

]. However, few reported works deal with the patterning of these materials and none of them have shown their potential for the fabrication of absorbance micro-filters.

In this work we present the fabrication by a soft lithography approach of simple and low-cost absorbance filters based on a dye doped hybrid organic-inorganic xerogel polymer suitable to be monolithically integrated into a PhLoC. These filters show relatively sharp stopbands and almost zero passbands. The filtering capacity is a function of the nature of the dye and has a linear dependence with the filter width and the dye concentration. Their successful performance and potential to be easily integrated into disposable PhLoCs as well as in other microsystems is shown.

2. Experimental

2.1 Design

2.2 Fabrication

The material synthesis and the fabrication of the test structure by soft lithography are the two main steps to develop the polymeric absorbance micro-filters.

2.2.1 Material synthesis

The pre-polymerization solution (sol) was prepared by mixing phenyltrimethoxysilane (PhTMOS) monomer (Sigma-Aldrich Química S.A., Spain) with dye aqueous solutions at pH 3 (adjusted with diluted HCl). Four different dye concentrations in the aqueous solution were used: 10, 50, 100 and 200µM.The silane: water molar ratio was 1:6. The selected dyes for this study were quinoline yellow (QY), phenol red (PR), methyl orange (MO) and crystal violet (CV) (all of them from Sigma-Aldrich Química S. A., Spain), whose absorbance bands show maximum absorbance values at 420, 515, 530 and 600 nm wavelengths, respectively. These dyes were selected to cover most of the visible spectrum. Except QY, these dyes are pH sensitive, showing different color for the acidic and the basic form; PR, yellow to red (pH, 6.8-8.0), MO, red to orange-yellow (pH 3.1-4.4), CV, yellow to blue (0.0-1.8).The mixture was gently stirred using a magnet until a homogeneous solution was achieved taking around 4h. At this point the resulting sol solution was ready for filling the test microstructure.

2.2.2 Fabrication of the test microstructure

The structure from which the eight micro-filters were obtained was fabricated by a soft lithographic approach. The master was developed using the SU-8 photocurable polymer in a one-step photolithographic process. SU-8 2025 (Microresist Technology GmbH, (Berlin, Germany)) was used to obtain a layer with a thickness of 230 µm by spin-coating [26

26. A. Llobera, S. Demming, R. Wilke, and S. Büttgenbach, “Multiple internal reflection poly(dimethylsiloxane) systems for optical sensing,” Lab Chip 7(11), 1560–1566 (2007). [CrossRef] [PubMed]

]. PDMS pre-polymer solution (Sylgard 184 elastomer kit, Dow Corning Corp, (Midland, MI)) used for the replication of the master was made by mixing the silicon elastomer with the curing agent in a 10:1 ratio (v: v). Then, it was carefully poured over the SU-8 master. Once cured at 80°C for 20 minutes, the PDMS was peeled off the master with the aid of tweezers. The resulting PDMS test structures were irreversibly bonded to glass slides by a conventional oxygen plasma process [27

27. B. H. Jo, L. M. Van Lerberghe, K. M. Motsegood, and D. J. Beebe, “Three-dimensional micro-channel fabrication in polydimethylsiloxane (PDMS) elastomer,” J. Microelectromech. Syst. 9(1), 76–81 (2000). [CrossRef]

]. The fluidic ports included in the test structures, as described above, were opened and the microstructure was filled with the sol solutions. Filling was achieved by capillary forces, thus avoiding the use of external pumps. The polymer was left to cure and dry at room temperature for 4-5 days. The curing time could be reduced to 1 h by applying a temperature of 80 °C, without observing any alteration of the resulting material structure. The dye-doped xerogel structure was thus formed and the filters were ready to be tested. Figure 1(b) show a picture of polymeric micro-filters fabricated in the PDMS test structure using a xerogel polymer doped using a 200 µM CV solution.

3. Material & structural characterization

In order to give some insight into the resulting doped polymer matrix, FT-Infrared Spectroscopy (Tensor 27 FT-IR spectrophotometer including a MKII Golden Gate Single Reflection ATR module, Bruker Corporation, Germany), 29Si-Magnetic Nuclear Resonance (29Si-NMR single-pulse (SP) solid state magic angle spinning nuclear magnetic resonance, Bruker Avance 400 spectrometer, Bruker Corporation, Germany) and Scanning Electron Microscopy (SEM) studies were carried out. Measurement of dye-doped and non-doped xerogel refractive indexes was kindly carried out by Metricon Corporation (NY,USA) using the Metricon Model 2010/M Prism Coupler equipment, which determines the refractive index from (m-line) layer modes [28

28. R. Ulrich and R. Torge, “Measurement of thin film parameters with a prism coupler,” Appl. Opt. 12(12), 2901–2908 (1973). [CrossRef] [PubMed]

]. The material transparency was also tested by collecting light passing through a drop-coated xerogel glass slide.Fig. 2
Fig. 2 SEM image of 100, 250, 500 and 1000 µm side xerogel microfilters fabricated in the same microstructure.
, Fig. 3
Fig. 3 Transmittance vs. wavelength for four different 200 µM dye-doped, 3000 µm wide filters.
, Fig. 4
Fig. 4 Transmittance vs. wavelength for the 3000 µm widefilters fabricated with the xerogel polymer containing PRdyefor the four different concentrations tested.
, Fig. 5
Fig. 5 Transmittance as a function of filter width for the PR doped polymeric filters.
, Fig. 6
Fig. 6 Image of the fluorescence LOC with integrateddye-doped polymeric filters.
, Fig. 7
Fig. 7 a) Absorbance as a function of Rhodamine B concentration measured at 540 nm. b) Fluorescence emission as a function of Rhodamine B concentration measured at 616 nm. Fluorescence values were normalized considering fluorescence to have a unity value when the microchannel was filled with DI water.

SEM images were also recorded and showed a smooth and crack-free xerogel material that perfectly replicated the pattern on the PDMS stamp (Fig. 2). The refractive index of both the non-doped and dye-doped xerogel materials, measured at λ = 633 nm was 1.5663 ± 0.0001 (n = 6), which again corroborates the negligible effect of the doping process on the xerogel optical and structural properties.

4. Spectral analysis

The setup for the analysis included a broadband halogen lamp as light source (HL-2000, Ocean Optics, Dunedin, FL, USA), two 230 µm diameter multimode optical fibers (Thorlabs Inc., Dachau, Germany) for coupling and collecting the light and a microspectrometer (QE 65000-FL, Ocean Optics, Dunedin, FL, USA). All filters were scanned in a wavelength region between 300 and 1000 nm with an integration time of 50 ms.The non-doped xerogel filter microstructure was fabricated and measured under the same experimental conditions and used as a reference.

5. Integration of the filters in a PhLoC

For comparative purposes, two identical systems were measured, the only difference being the presence of either a non-doped or a PR doped xerogel acting as a filter. For each dilution absorbance and fluorescence spectra were recorded. A the linear fit was carried out and the limit of detection (LOD) calculated following the IUPAC criteria, which states that the LOD is not the lowest detectable analyte concentration, but also depends on both the sensitivity and the accuracy of the linear fit, thus being determined as theleast concentration of analyte for which the signal exceeds by a factor of 3 the relative standard deviation of the background signal divided by the slope of the calibration curve [32

32. V. Thomsen, D. Shatzlein, and D. Mercuro, “Limits of Detection in Spectroscopy,” Spectroscopy 18, 112–114 (2003).

].

Absorbance vs. Rhodamine B concentration shown in Fig. 7A shows the expected linear trend in accordance with the Beer-Lambert law at a wavelength of 540 nm. When compared the results recorded using identical PhLoC with and without the integrated filter (Table 2), it can be seen that the performance of the former has been significantly improved, with a three-fold increase in sensitivity while the LOD was reduced by 35%. This enhancement can be understood from the fact that the PR filters eliminates background light that causes random variations of the readout signal.

6. Conclusions

It has been demonstrated that low-cost absorption micro-filters could be easily fabricated using a dye-doped xerogel hybrid polymer material with a simple soft-lithographic approach.The presented tailor-made hybrid organic-inorganic polymeric material was compatible with several dyes, which enabled the development of filters covering a wide wavelength range of the visible spectrum. The fabricated micro-filters showed low transmittance values at the stopband, reaching the dynamic range limit of the spectrometer in some cases. Passband transmittance values are close to zero in all cases. Filtering capacity of these filters was consistent with the Beer-Lambert law in all cases and strongly depended on the nature of the dye. The successful integration of these polymeric absorbance micro-filters in a fluorescence based disposable photonic lab-on-a-chip was also demonstrated by carrying out calibration studies using Rhodamine B as model fluorophore target analyte and showing the greatly improved performance of the system compared with that one where no absorbance filter structure was implemented. Also, it is shown that this filtering approach outperforms other previously reported structures considering the attained stopband (as compared to existing high-pass absorbance filters) as well as the ease of their fabrication and integration in analytical microsystems.

7. Appendix

Non-doped xerogel (%) transmittance vs. wavelength. 29Si-MNR and FT-IR spectra of doped and non-doped xerogel. Transmittance vs. wavelength for the 3000 µm wide quinoline yellow, methyl orange and crystal violet, loaded filters for all the concentrations. Transmittance vs. filter width for the four dyes in all the concentrations. Spectral properties of the different filters. Setup used to carry out the measurements with the integrated micro-filters.

Acknowledgments

The research leading to these results has received funding from the European Research Council under the European Community's Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement n° 209243 and from project ref. TEC2010-17274 (Spanish ministry of Economy and Competitivity). The authors would like to thank Dr. Manuel Gutiérrez-Capitán, from IMB-CNM (CSIC) and Dr. Magarita Darder from ICMM (CSIC), for their assistance with the FT-IR and the 29Si-NMR spectroscopic analysis, respectively.

References and links

1.

S. M. Borisov and O. S. Wolfbeis, “Optical biosensors,” Chem. Rev. 108(2), 423–461 (2008). [CrossRef] [PubMed]

2.

M. Dandin, P. Abshire, and E. Smela, “Optical filtering technologies for integrated fluorescence sensors,” Lab Chip 7(8), 955–977 (2007). [CrossRef] [PubMed]

3.

H. A. Macleod, “Thin Film Optical Filters” (Institute of Physics Publishing, London, 2001)

4.

M. L. Chabinyc, D. T. Chiu, J. C. McDonald, A. D. Stroock, J. F. Christian, A. M. Karger, and G. M. Whitesides, “An integrated fluorescence detection system in poly(dimethylsiloxane) for microfluidic applications,” Anal. Chem. 73(18), 4491–4498 (2001). [CrossRef] [PubMed]

5.

A. H. Mahan, R. Biswas, L. M. Gedvilas, D. L. Williamson, and B. C. Pan, “On the influence of short and medium range order on the material band gap in hydrogenated amorphous silicon,” J. Appl. Phys. 96(7), 3818–3826 (2004). [CrossRef]

6.

O. Hofmann, X. Wang, A. Cornwell, S. Beecher, A. Raja, D. D. Bradley, A. J. Demello, and J. C. Demello, “Monolithically integrated dye-doped PDMS long-pass filters for disposable on-chip fluorescence detection,” Lab Chip 6(8), 981–987 (2006). [CrossRef] [PubMed]

7.

A. Llobera, S. Demming, H. N. Joensson, J. Vila-Planas, H. Andersson-Svahn, and S. Büttgenbach, “Monolithic PDMS passband filters for fluorescence detection,” Lab Chip 10(15), 1987–1992 (2010). [CrossRef] [PubMed]

8.

M. Yamazaki, O. Hofmann, G. Ryu, L. Xiaoe, T. K. Lee, A. J. deMello, and J. C. deMello, “Non-emissive colour filters for fluorescence detection,” Lab Chip 11(7), 1228–1233 (2011). [CrossRef] [PubMed]

9.

C. Richard, A. Renaudin, V. Aimez, and P. G. Charette, “An integrated hybrid interference and absorption filter for fluorescence detection in lab-on-a-chip devices,” Lab Chip 9(10), 1371–1376 (2009). [CrossRef] [PubMed]

10.

L. L. Hench and J. K. West, “The sol-gel process,” Chem. Rev. 90(1), 33–72 (1990). [CrossRef]

11.

J. Y. Wen and G. L. Wilkes, “Organic/inorganic hybrid network materials by the sol-gel approach,” Chem. Mater. 8(8), 1667–1681 (1996). [CrossRef]

12.

C. Sanchez and F. Ribot, “Design of hybrid organic-inorganic materials synthesized via sol-gel chemistry,” New J. Chem. 18, 1007–1047 (1994).

13.

B. Lebeau and P. Innocenzi, “Hybrid materials for optics and photonics,” Chem. Soc. Rev. 40(2), 886–906 (2011). [CrossRef] [PubMed]

14.

A. Llobera, V. J. Cadarso, E. Carregal-Romero, J. Brugger, C. Domínguez, and C. Fernández-Sánchez, “Fluorophore-doped xerogelantiresonant reflecting optical waveguides,” Opt. Express 19(6), 5026–5039 (2011). [CrossRef] [PubMed]

15.

N. Tohge, M. Hasegawa, N. Noma, K. Kintaka, and J. Nishii, “Fabrication of two-dimensional gratings using photosensitive gel films and their characterization,” J. Sol-Gel Sci. Technol. 26(1/3), 903–907 (2003). [CrossRef]

16.

X. H. Zhang, W. Que, C. Y. Jia, J. X. Hu, and W. G. Liu, “Fabrication of micro-lens arrays built in photosensitive hybrid films by UV-cured imprinting technique,” J. Sol-Gel Sci. Technol. 60(1), 71–80 (2011). [CrossRef]

17.

C. Fernández-Sánchez, V. J. Cadarso, M. Darder, C. Dominguez, and A. Llobera, “Patterning high-aspect-ratio sol-gel structures by microtransfer molding,” Chem. Mater. 20(8), 2662–2668 (2008). [CrossRef]

18.

C. Sanchez, P. Belleville, M. Popall, and L. Nicole, “Applications of advanced hybrid organic-inorganic nanomaterials: from laboratory to market,” Chem. Soc. Rev. 40(2), 696–753 (2011). [CrossRef] [PubMed]

19.

Y. N. Xia and G. M. Whitesides, “Soft lithography,” Angew. Chem. Int. Ed. 37(5), 550–575 (1998). [CrossRef]

20.

D. Avnir, D. Levy, and R. Reisfeld, “The nature of the silica cage as reflected by spectral changes and enhanced photostability of trapped Rhodamine-6G,” J. Phys. Chem. 88(24), 5956–5959 (1984). [CrossRef]

21.

G. Schottner, “Hybrid sol-gel-derived polymers: Applications of multifunctional materials,” Chem. Mater. 13(10), 3422–3435 (2001). [CrossRef]

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M. Zayat, R. Pardo, E. Castellón, L. Torres, D. Almendro, P. G. Parejo, A. Álvarez, T. Belenguer, S. García-Revilla, R. Balda, J. Fernández, and D. Levy, “Optical and Electro-optical Materials Prepared by the Sol-Gel Method,” Adv. Mater. (Deerfield Beach Fla.) 23(44), 5318–5323 (2011). [CrossRef]

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P. Escribano, B. Julian-Lopez, J. Planelles-Arago, E. Cordoncillo, B. Viana, and C. Sanchez, “Photonic and anobiophotonic properties of luminescent lanthanide-doped hybrid organic-inorganic materials,” J. Mater. Chem. 18(1), 23–40 (2007). [CrossRef]

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R. Pardo, M. Zayat, and D. Levy, “Photochromic organic-inorganic hybrid materials,” Chem. Soc. Rev. 40(2), 672–687 (2011). [CrossRef] [PubMed]

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A. Llobera, R. Wilke, and S. Büttgenbach, “Poly(dimethylsiloxane) hollow Abbe prism with microlenses for detection based on absorption and refractive index shift,” Lab Chip 4(1), 24–27 (2004). [CrossRef] [PubMed]

26.

A. Llobera, S. Demming, R. Wilke, and S. Büttgenbach, “Multiple internal reflection poly(dimethylsiloxane) systems for optical sensing,” Lab Chip 7(11), 1560–1566 (2007). [CrossRef] [PubMed]

27.

B. H. Jo, L. M. Van Lerberghe, K. M. Motsegood, and D. J. Beebe, “Three-dimensional micro-channel fabrication in polydimethylsiloxane (PDMS) elastomer,” J. Microelectromech. Syst. 9(1), 76–81 (2000). [CrossRef]

28.

R. Ulrich and R. Torge, “Measurement of thin film parameters with a prism coupler,” Appl. Opt. 12(12), 2901–2908 (1973). [CrossRef] [PubMed]

29.

R. H. Glaser, G. L. Wilkes, and C. E. Bronnimann, “Solid-state 29Si NMR of TEOS-based multifunctional sol-gel materials,” J. Non-Cryst. Solids 113(1), 73–87 (1989). [CrossRef]

30.

P. Lacan, C. Guizard, and L. Cot, “Chemical and rheological investigations of the sol-gel transition in organically-modified siloxanes,” J. Sol-Gel Sci. Technol. 4(2), 151–162 (1995). [CrossRef]

31.

A. Llobera, R. Wilke, and S. Büttgenbach, “Optimization of poly(dimethylsiloxane) hollow prisms for optical sensing,” Lab Chip 5(5), 506–511 (2005). [CrossRef] [PubMed]

32.

V. Thomsen, D. Shatzlein, and D. Mercuro, “Limits of Detection in Spectroscopy,” Spectroscopy 18, 112–114 (2003).

33.

A. Pais, A. Banerjee, D. Klotzkin, and I. Papautsky, “High-sensitivity, disposable lab-on-a-chip with thin-film organic electronics for fluorescence detection,” Lab Chip 8(5), 794–800 (2008). [CrossRef] [PubMed]

34.

B. Yao, G. Luo, L. Wang, Y. Gao, G. Lei, K. Ren, L. Chen, Y. Wang, Y. Hu, and Y. Qiu, “A microfluidic device using a green organic light emitting diode as an integrated excitation source,” Lab Chip 5(10), 1041–1047 (2005). [CrossRef] [PubMed]

OCIS Codes
(130.0130) Integrated optics : Integrated optics
(160.0160) Materials : Materials
(160.6060) Materials : Solgel
(350.2450) Other areas of optics : Filters, absorption
(230.7408) Optical devices : Wavelength filtering devices

ToC Category:
Integrated Optics

History
Original Manuscript: May 15, 2012
Revised Manuscript: July 6, 2012
Manuscript Accepted: July 23, 2012
Published: October 1, 2012

Citation
Ester Carregal-Romero, César Fernández-Sánchez, Alma Eguizabal, Stefanie Demming, Stephanus Büttgenbach, and Andreu Llobera, "Development and integration of xerogel polymeric absorbance micro-filters into lab-on-chip systems," Opt. Express 20, 23700-23719 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-21-23700


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References

  1. S. M. Borisov and O. S. Wolfbeis, “Optical biosensors,” Chem. Rev.108(2), 423–461 (2008). [CrossRef] [PubMed]
  2. M. Dandin, P. Abshire, and E. Smela, “Optical filtering technologies for integrated fluorescence sensors,” Lab Chip7(8), 955–977 (2007). [CrossRef] [PubMed]
  3. H. A. Macleod, “Thin Film Optical Filters” (Institute of Physics Publishing, London, 2001)
  4. M. L. Chabinyc, D. T. Chiu, J. C. McDonald, A. D. Stroock, J. F. Christian, A. M. Karger, and G. M. Whitesides, “An integrated fluorescence detection system in poly(dimethylsiloxane) for microfluidic applications,” Anal. Chem.73(18), 4491–4498 (2001). [CrossRef] [PubMed]
  5. A. H. Mahan, R. Biswas, L. M. Gedvilas, D. L. Williamson, and B. C. Pan, “On the influence of short and medium range order on the material band gap in hydrogenated amorphous silicon,” J. Appl. Phys.96(7), 3818–3826 (2004). [CrossRef]
  6. O. Hofmann, X. Wang, A. Cornwell, S. Beecher, A. Raja, D. D. Bradley, A. J. Demello, and J. C. Demello, “Monolithically integrated dye-doped PDMS long-pass filters for disposable on-chip fluorescence detection,” Lab Chip6(8), 981–987 (2006). [CrossRef] [PubMed]
  7. A. Llobera, S. Demming, H. N. Joensson, J. Vila-Planas, H. Andersson-Svahn, and S. Büttgenbach, “Monolithic PDMS passband filters for fluorescence detection,” Lab Chip10(15), 1987–1992 (2010). [CrossRef] [PubMed]
  8. M. Yamazaki, O. Hofmann, G. Ryu, L. Xiaoe, T. K. Lee, A. J. deMello, and J. C. deMello, “Non-emissive colour filters for fluorescence detection,” Lab Chip11(7), 1228–1233 (2011). [CrossRef] [PubMed]
  9. C. Richard, A. Renaudin, V. Aimez, and P. G. Charette, “An integrated hybrid interference and absorption filter for fluorescence detection in lab-on-a-chip devices,” Lab Chip9(10), 1371–1376 (2009). [CrossRef] [PubMed]
  10. L. L. Hench and J. K. West, “The sol-gel process,” Chem. Rev.90(1), 33–72 (1990). [CrossRef]
  11. J. Y. Wen and G. L. Wilkes, “Organic/inorganic hybrid network materials by the sol-gel approach,” Chem. Mater.8(8), 1667–1681 (1996). [CrossRef]
  12. C. Sanchez and F. Ribot, “Design of hybrid organic-inorganic materials synthesized via sol-gel chemistry,” New J. Chem.18, 1007–1047 (1994).
  13. B. Lebeau and P. Innocenzi, “Hybrid materials for optics and photonics,” Chem. Soc. Rev.40(2), 886–906 (2011). [CrossRef] [PubMed]
  14. A. Llobera, V. J. Cadarso, E. Carregal-Romero, J. Brugger, C. Domínguez, and C. Fernández-Sánchez, “Fluorophore-doped xerogelantiresonant reflecting optical waveguides,” Opt. Express19(6), 5026–5039 (2011). [CrossRef] [PubMed]
  15. N. Tohge, M. Hasegawa, N. Noma, K. Kintaka, and J. Nishii, “Fabrication of two-dimensional gratings using photosensitive gel films and their characterization,” J. Sol-Gel Sci. Technol.26(1/3), 903–907 (2003). [CrossRef]
  16. X. H. Zhang, W. Que, C. Y. Jia, J. X. Hu, and W. G. Liu, “Fabrication of micro-lens arrays built in photosensitive hybrid films by UV-cured imprinting technique,” J. Sol-Gel Sci. Technol.60(1), 71–80 (2011). [CrossRef]
  17. C. Fernández-Sánchez, V. J. Cadarso, M. Darder, C. Dominguez, and A. Llobera, “Patterning high-aspect-ratio sol-gel structures by microtransfer molding,” Chem. Mater.20(8), 2662–2668 (2008). [CrossRef]
  18. C. Sanchez, P. Belleville, M. Popall, and L. Nicole, “Applications of advanced hybrid organic-inorganic nanomaterials: from laboratory to market,” Chem. Soc. Rev.40(2), 696–753 (2011). [CrossRef] [PubMed]
  19. Y. N. Xia and G. M. Whitesides, “Soft lithography,” Angew. Chem. Int. Ed.37(5), 550–575 (1998). [CrossRef]
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