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Micro-fluidic analysis based on total internal light reflection
Microelectronic Engineering 83 (2006) 1294–1297 www.elsevier.com/locate/mee
Micro-fluidic analysis based on total internal light reflection Y. Sarov b
a,b,*
, K. Ivanova a, Tzv. Ivanov a, B.E. Volland a, I.W. Rangelow
a
a Institute of Nanostructure Technologies and Analytics, University of Kassel, Heinrich-Plett Street, 40, D-34132 Kassel, Germany Central Laboratory of Optical Storage and Processing of Information, Bulgarian Academy of Sciences, P.O. Box. 95, 1113 Sofia, Bulgaria
Available online 8 February 2006
Abstract The work presents a novel micro-optical sensor for micro and nano-fluidic analysis. It is based on diffraction, occurred in total internal reflection conditions. The detector is integrated on an autonomous micro-fluidic device and produced by a unique combination of different techniques for micro- and nano-structuring and micromachining. The preliminary optical investigation shows sensitivity to the fluidic refractive index of 2 · 103 and to the absorption coefficient of 6 · 106 m1. Envisioned applications of the integrated micro-sensor are medical, biological and chemical diagnostic of liquid substances and dispersions of drugs, serums, proteins, peptides, DNA, RNA, etc. 2006 Elsevier B.V. All rights reserved. Keywords: Sensors; Micro-fabrication; Total internal reflection; Micro-fluidics
1. Introduction Miniaturization of analytical chemical and biomedical instruments has developed rapidly during the past 10 years. Till now, different kinds of micro total analysis systems (l-TAS) have been presented and applied in the biotechnology, process control, environmental and medical sciences, etc. Such systems join different micro-mechanical, optical and electronic parts, all combined in a single chip (so called lab-on-a-chip). They have advantages of low-cost fabrication, high throughput, easy operation, quick processing and low consumption of samples [1]. At the same time there are urgent needs for development of methods for micro-fluidic sensing [2]. The detecting unit should be integrated in the chip, not invasive and sensitive to all components. A promising approach is the diffraction grating, operating in the total internal reflection (TIR) [3,4]. In the presented work a novel micro-optical sensing device is developed for micro and nano-fluidic analysis. The detector is integrated on an autonomous micro-fluidic system. It is produced by a combination of different techniques for micro- and nano-structuring and micromachin-
*
Corresponding author. Tel.: +49 561 804 4241; fax: +49 561 804 4136. E-mail address: [email protected] (Y. Sarov).
0167-9317/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2006.01.059
ing such as lithography, wet and high aspect ratio dry etching, PECVD and hot embossing lithography. The optical investigation proves the sensitivity toward the fluidic absorption coefficient (AC) and refractive index (RI), studied by differently concentrated water solutions of methylene blue and sucrose, respectively. Envisioned applications of the integrated micro-sensor are medical, biological diagnostic and chemical recognition of liquid substances and dispersions of drugs, hazards, serums, proteins, peptides, DNA, RNA, etc. 2. Experimental The fabrication of the sensor is illustrated in Fig. 1. The device is created by sealing of three wafers, called prism, channel and sealing wafers and represented by the 1st, 2nd and 3rd columns in Fig. 1, respectively. In all cases (1a, 2a and 3a) it is originated from oxidized Si h1 0 0i wafers. The prism’s lithography, oxide etching (2b) and Si etching in KOH (1c) are used for the inverted prisms forming. Then the SiO2 is completely removed (1d). The micro-channel’s shape on the bottom of the second wafer is defined by the corresponding second lithography and SiO2 etch, followed by the removing of the front-surface oxide and Si3N4 PECVD (2b). Then the diffraction grating’s (DG) lithography (2c) and consequently Cr lift-off
Y. Sarov et al. / Microelectronic Engineering 83 (2006) 1294–1297
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Fig. 1. Micro-fluidic sensors fabrication.
(2d) defines the metal grating. The considered wafers 1 and 2 are aligned by special square openings in the wafers periphery and fixed by hot embossing of PMMA (12a). Finally, the exposed Si is double-sided etched in KOH, revealing the PMMA prism and forming the micro-fluidic channel (12b). To close the bottom side of the channel the 4th lithography is performed on the sealing wafer, followed by SiO2 (3b) and Si (3c) etching. Finally, the resulting pad, which contains the inlet and outlet openings, is glued to the structure 12b, forming the micro-fluidic sensor (123). The principle of operation is illustrated in Fig. 2. The light beam from a diode laser with k = 655 nm (1) is directed to the prism facet, fulfilling the TIR condition for the interface between the micro-prism (2) and the fluid in the micro-channel (4). In our experiments the angle of incidence exceeds the critical angle with 2, which ensures
Fig. 2. Principle of operation. 1 – incident light beam, 2 – TIR prism, 3 – diffraction grating, 4 – micro-fluidic channel, 5 – in-let and out-let nozzles, and 6 – diffraction orders.
TIR for all investigated samples. The diffraction grating on the border between the prism and the channel defines periodical change of the reflection: that from the metal lines and from the transparent lines spacing. The reflection amplitude and phase in the first case are constant for a given angle of incidence, while in the second case are influenced by the fluidic optical constants [3–5]. The periodic reflection leads to a diffraction pattern in reflection (6). Phase or amplitude changes in the TIR areas causes redistribution of the energy between the orders i.e., change in the diffraction efficiency (DE) g, while the diffraction directions are constant [3]. 3. Results and discussion Fig. 3 shows a picture of the fabricated micro-fluidic devices. Critical issues for proper sensing are the flatness of the TIR interface, the quality of the prism walls and the grating’s lines roughness. The first is defined by flatness of the Si wafer, taking into account the possibility for zero stress PECVD deposition (Fig. 1, step 2b). The prism’s facets, released by the wet KOH etching (step 12b), follows the surface of the mold (step 1c). The anisotropic KOH etching is a known method for mold fabrication, resulting in a smooth surface [6]. Thus the prism quality is better than k/4. Only prisms with refracting angle of 54.74 can be prepared in this way. Thus the TIR condition must be satisfied by properly choosing the angle of incidence. The grating’s roughness depends on the lithography limits-line quality of 0.25 lm related to a period of 10 lm gives clear defines the diffraction pattern. The micro-fluidic system is calibrated by standard water solutions of sucrose and methylene blue, represented in Fig. 4(a) and (b), respectively. The first type are transparent fluids with linear RI dependence on the concentration
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Y. Sarov et al. / Microelectronic Engineering 83 (2006) 1294–1297
Fig. 3. A picture of the micro-fluidic sensor.
c: Dn = 1.45 · 103c [7,8]. The absorption coefficient of the second is given by a = e c, e = 1.4 · 108 m1 [5]. In both cases c is expressed in wt%. Only the 1st diffraction order is studied, since it is the strongest diffracted beam. On the other hand, it is spatially diverged from the geometrical reflection and from possible low-angle scattering or other parasitic light, which usually propagate in the proximity of the 0th order. A linear sensitivity toward analytes concentration can be reported for the investigated cases, represented by the linear fit in Fig. 4(a) and (b). This could be expected, since the solutions are not concentrated. The empirical equations, connecting the DE and the concentration, are also presented in the figures. It could be seen from Fig. 4(a) that the RI has opposite influence on the DE for the case of s (transversal electric) and p (transversal magnetic) polarized light. On the other hand, the fluid absorption always leads to DE decrease. Nevertheless the sensitivity k = dg/dc is about two times higher for the s polarization in all considered cases. The linear DE response allows easy solving the inverse problem, i.e., analyte’s concentration calculation, based on the DE measuring. With a DE accuracy of Dg = 5 · 103 the sensitivity for sucrose concentration is Dc = 1.2 wt% or transferred to the RI of transparent fluidsto accuracy of Dn = 1.7 · 103. Analogous, the sensitivity for methylene blue concentration is 0.04 wt% or, expressed in AC: Da = 5.6 · 106 m 1. The accuracy can be increased by adjusting the angle of incidence, closer to the critical angle. Unfortunately in this case the TIR condition can be frustrated by smaller change in the fluidic optical constants. Thus the measuring range will be shrunk. Another solution for more precise TIR sensing is to fabricate micro-prisms from material with higher RI. Since the RI and AC are directly connected with the concentration in the case of binary solutions, the detector can be used for determination of the samples optical constants. Such data can be applied for analysis of more complicated mixtures, using a variety of developed and established refractometric methods [9]. 4. Conclusions The presented micro-optical system shows the applicability of the total internal reflection for micro-fluidic sensing. The novel integrated sensor detects changes in the real and imaginary part of the optical micro-fluidic constant and thus is suitable for monitoring of both transparent and absorbing liquids. Envisioned applications of the integrated micro-sensor are concentration and composition analysis, environmental protection control, medical, biological and chemical diagnostic, etc.
Fig. 4. First order diffraction efficiency for fluid with variable: (a) refractive index (sucrose solutions) and (b) absorption coefficient (methylene blue solutions)- - - - and j correspond to p (TM) and s (TE) light polarizations, respectively.
Acknowledgement The results are attained with the assistance and support from Alexander von Humboldt Foundation.
Y. Sarov et al. / Microelectronic Engineering 83 (2006) 1294–1297
References [1] Q. Luo, S. Mutlu, Y. Gianchandani, F. Svec, J. Fre´chet, Electrophoresis 24 (2003) 3694. [2] J. Wang, G. Chen, A. Muck, J. Collins, Electrophoresis 24 (2003) 3728. [3] S. Sainov, D. Tonchev, Opt. Laser. Eng. 10 (1989) 17. [4] S. Sainov, Sensors Actuators A 45 (1994) 1.
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[5] Y. Sarov, S. Sainov, J. Opt. A 4 (2002) 382. [6] S. Sinzinger, J. Jahns, Microoptics, second ed., Wiley-VCH Verlag GmbH & Co, 2003 (Chapter 3). [7] W. Mat Yanus, A. Rahman, Appl. Opt. 27 (1988) 3341. [8] Y. Sarov, I. Kostic, S. Minov, S. Mitkov, G. Minchev, L. Uher, Proc. SPIE 5226 (2003) 194. [9] B. Ioffe, Refractometric Methods in Chemistry, Chemistry, Leningrad, 1983.
Micro-fluidic analysis based on total internal light reflection Y. Sarov b
a,b,*
, K. Ivanova a, Tzv. Ivanov a, B.E. Volland a, I.W. Rangelow
a
a Institute of Nanostructure Technologies and Analytics, University of Kassel, Heinrich-Plett Street, 40, D-34132 Kassel, Germany Central Laboratory of Optical Storage and Processing of Information, Bulgarian Academy of Sciences, P.O. Box. 95, 1113 Sofia, Bulgaria
Available online 8 February 2006
Abstract The work presents a novel micro-optical sensor for micro and nano-fluidic analysis. It is based on diffraction, occurred in total internal reflection conditions. The detector is integrated on an autonomous micro-fluidic device and produced by a unique combination of different techniques for micro- and nano-structuring and micromachining. The preliminary optical investigation shows sensitivity to the fluidic refractive index of 2 · 103 and to the absorption coefficient of 6 · 106 m1. Envisioned applications of the integrated micro-sensor are medical, biological and chemical diagnostic of liquid substances and dispersions of drugs, serums, proteins, peptides, DNA, RNA, etc. 2006 Elsevier B.V. All rights reserved. Keywords: Sensors; Micro-fabrication; Total internal reflection; Micro-fluidics
1. Introduction Miniaturization of analytical chemical and biomedical instruments has developed rapidly during the past 10 years. Till now, different kinds of micro total analysis systems (l-TAS) have been presented and applied in the biotechnology, process control, environmental and medical sciences, etc. Such systems join different micro-mechanical, optical and electronic parts, all combined in a single chip (so called lab-on-a-chip). They have advantages of low-cost fabrication, high throughput, easy operation, quick processing and low consumption of samples [1]. At the same time there are urgent needs for development of methods for micro-fluidic sensing [2]. The detecting unit should be integrated in the chip, not invasive and sensitive to all components. A promising approach is the diffraction grating, operating in the total internal reflection (TIR) [3,4]. In the presented work a novel micro-optical sensing device is developed for micro and nano-fluidic analysis. The detector is integrated on an autonomous micro-fluidic system. It is produced by a combination of different techniques for micro- and nano-structuring and micromachin-
*
Corresponding author. Tel.: +49 561 804 4241; fax: +49 561 804 4136. E-mail address: [email protected] (Y. Sarov).
0167-9317/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2006.01.059
ing such as lithography, wet and high aspect ratio dry etching, PECVD and hot embossing lithography. The optical investigation proves the sensitivity toward the fluidic absorption coefficient (AC) and refractive index (RI), studied by differently concentrated water solutions of methylene blue and sucrose, respectively. Envisioned applications of the integrated micro-sensor are medical, biological diagnostic and chemical recognition of liquid substances and dispersions of drugs, hazards, serums, proteins, peptides, DNA, RNA, etc. 2. Experimental The fabrication of the sensor is illustrated in Fig. 1. The device is created by sealing of three wafers, called prism, channel and sealing wafers and represented by the 1st, 2nd and 3rd columns in Fig. 1, respectively. In all cases (1a, 2a and 3a) it is originated from oxidized Si h1 0 0i wafers. The prism’s lithography, oxide etching (2b) and Si etching in KOH (1c) are used for the inverted prisms forming. Then the SiO2 is completely removed (1d). The micro-channel’s shape on the bottom of the second wafer is defined by the corresponding second lithography and SiO2 etch, followed by the removing of the front-surface oxide and Si3N4 PECVD (2b). Then the diffraction grating’s (DG) lithography (2c) and consequently Cr lift-off
Y. Sarov et al. / Microelectronic Engineering 83 (2006) 1294–1297
1295
Fig. 1. Micro-fluidic sensors fabrication.
(2d) defines the metal grating. The considered wafers 1 and 2 are aligned by special square openings in the wafers periphery and fixed by hot embossing of PMMA (12a). Finally, the exposed Si is double-sided etched in KOH, revealing the PMMA prism and forming the micro-fluidic channel (12b). To close the bottom side of the channel the 4th lithography is performed on the sealing wafer, followed by SiO2 (3b) and Si (3c) etching. Finally, the resulting pad, which contains the inlet and outlet openings, is glued to the structure 12b, forming the micro-fluidic sensor (123). The principle of operation is illustrated in Fig. 2. The light beam from a diode laser with k = 655 nm (1) is directed to the prism facet, fulfilling the TIR condition for the interface between the micro-prism (2) and the fluid in the micro-channel (4). In our experiments the angle of incidence exceeds the critical angle with 2, which ensures
Fig. 2. Principle of operation. 1 – incident light beam, 2 – TIR prism, 3 – diffraction grating, 4 – micro-fluidic channel, 5 – in-let and out-let nozzles, and 6 – diffraction orders.
TIR for all investigated samples. The diffraction grating on the border between the prism and the channel defines periodical change of the reflection: that from the metal lines and from the transparent lines spacing. The reflection amplitude and phase in the first case are constant for a given angle of incidence, while in the second case are influenced by the fluidic optical constants [3–5]. The periodic reflection leads to a diffraction pattern in reflection (6). Phase or amplitude changes in the TIR areas causes redistribution of the energy between the orders i.e., change in the diffraction efficiency (DE) g, while the diffraction directions are constant [3]. 3. Results and discussion Fig. 3 shows a picture of the fabricated micro-fluidic devices. Critical issues for proper sensing are the flatness of the TIR interface, the quality of the prism walls and the grating’s lines roughness. The first is defined by flatness of the Si wafer, taking into account the possibility for zero stress PECVD deposition (Fig. 1, step 2b). The prism’s facets, released by the wet KOH etching (step 12b), follows the surface of the mold (step 1c). The anisotropic KOH etching is a known method for mold fabrication, resulting in a smooth surface [6]. Thus the prism quality is better than k/4. Only prisms with refracting angle of 54.74 can be prepared in this way. Thus the TIR condition must be satisfied by properly choosing the angle of incidence. The grating’s roughness depends on the lithography limits-line quality of 0.25 lm related to a period of 10 lm gives clear defines the diffraction pattern. The micro-fluidic system is calibrated by standard water solutions of sucrose and methylene blue, represented in Fig. 4(a) and (b), respectively. The first type are transparent fluids with linear RI dependence on the concentration
1296
Y. Sarov et al. / Microelectronic Engineering 83 (2006) 1294–1297
Fig. 3. A picture of the micro-fluidic sensor.
c: Dn = 1.45 · 103c [7,8]. The absorption coefficient of the second is given by a = e c, e = 1.4 · 108 m1 [5]. In both cases c is expressed in wt%. Only the 1st diffraction order is studied, since it is the strongest diffracted beam. On the other hand, it is spatially diverged from the geometrical reflection and from possible low-angle scattering or other parasitic light, which usually propagate in the proximity of the 0th order. A linear sensitivity toward analytes concentration can be reported for the investigated cases, represented by the linear fit in Fig. 4(a) and (b). This could be expected, since the solutions are not concentrated. The empirical equations, connecting the DE and the concentration, are also presented in the figures. It could be seen from Fig. 4(a) that the RI has opposite influence on the DE for the case of s (transversal electric) and p (transversal magnetic) polarized light. On the other hand, the fluid absorption always leads to DE decrease. Nevertheless the sensitivity k = dg/dc is about two times higher for the s polarization in all considered cases. The linear DE response allows easy solving the inverse problem, i.e., analyte’s concentration calculation, based on the DE measuring. With a DE accuracy of Dg = 5 · 103 the sensitivity for sucrose concentration is Dc = 1.2 wt% or transferred to the RI of transparent fluidsto accuracy of Dn = 1.7 · 103. Analogous, the sensitivity for methylene blue concentration is 0.04 wt% or, expressed in AC: Da = 5.6 · 106 m 1. The accuracy can be increased by adjusting the angle of incidence, closer to the critical angle. Unfortunately in this case the TIR condition can be frustrated by smaller change in the fluidic optical constants. Thus the measuring range will be shrunk. Another solution for more precise TIR sensing is to fabricate micro-prisms from material with higher RI. Since the RI and AC are directly connected with the concentration in the case of binary solutions, the detector can be used for determination of the samples optical constants. Such data can be applied for analysis of more complicated mixtures, using a variety of developed and established refractometric methods [9]. 4. Conclusions The presented micro-optical system shows the applicability of the total internal reflection for micro-fluidic sensing. The novel integrated sensor detects changes in the real and imaginary part of the optical micro-fluidic constant and thus is suitable for monitoring of both transparent and absorbing liquids. Envisioned applications of the integrated micro-sensor are concentration and composition analysis, environmental protection control, medical, biological and chemical diagnostic, etc.
Fig. 4. First order diffraction efficiency for fluid with variable: (a) refractive index (sucrose solutions) and (b) absorption coefficient (methylene blue solutions)- - - - and j correspond to p (TM) and s (TE) light polarizations, respectively.
Acknowledgement The results are attained with the assistance and support from Alexander von Humboldt Foundation.
Y. Sarov et al. / Microelectronic Engineering 83 (2006) 1294–1297
References [1] Q. Luo, S. Mutlu, Y. Gianchandani, F. Svec, J. Fre´chet, Electrophoresis 24 (2003) 3694. [2] J. Wang, G. Chen, A. Muck, J. Collins, Electrophoresis 24 (2003) 3728. [3] S. Sainov, D. Tonchev, Opt. Laser. Eng. 10 (1989) 17. [4] S. Sainov, Sensors Actuators A 45 (1994) 1.
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[5] Y. Sarov, S. Sainov, J. Opt. A 4 (2002) 382. [6] S. Sinzinger, J. Jahns, Microoptics, second ed., Wiley-VCH Verlag GmbH & Co, 2003 (Chapter 3). [7] W. Mat Yanus, A. Rahman, Appl. Opt. 27 (1988) 3341. [8] Y. Sarov, I. Kostic, S. Minov, S. Mitkov, G. Minchev, L. Uher, Proc. SPIE 5226 (2003) 194. [9] B. Ioffe, Refractometric Methods in Chemistry, Chemistry, Leningrad, 1983.