Pin-Printed Chemical Sensor Arrays for Simultaneous Multianalyte

A new approach to rapidly produce micrometer-scale sensor elements into reusable multianalyte chemical sensor arrays is demonstrated. By using pin pri...
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Anal. Chem. 2002, 74, 1462-1466

Pin-Printed Chemical Sensor Arrays for Simultaneous Multianalyte Quantification Eun Jeong Cho and Frank V. Bright*

Department of Chemistry, Natural Sciences Complex, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000

A new approach to rapidly produce micrometer-scale sensor elements into reusable multianalyte chemical sensor arrays is demonstrated. By using pin printing technology in concert with sol-gel processing methods, we form discrete xerogel-based microsensors on a planar substrate. We illustrate the new approach by forming discrete O2- and pH-responsive sensing elements into arrays that allow one to simultaneously determine O2 and pH in aqueous samples. The pin printing method allows one to prepare sensor elements that are on the order of 100 µm in diameter, 1-2 µm thick, at a rate of approximately one sensor element per second with a single pin. Within a given calibrated array, the sensor elementto-sensor element response is reproducible to within 5%, the sensor element short- and long-term reproducibilities are 3 and 6%, respectively, and the array-to-array response reproducibility is 11%. These results demonstrate the potential of this methodology for rapidly forming ensembles of reusable sensor arrays for simultaneous multianalyte detection. Artificial “noses” and “tongues”1 use a variety of elegant sensor array strategies2-8 to provide chemical information about a sample. * Corresponding author: (voice) 716-645-6800, ext 2162; (fax) 716-645-6963; (e-mail) [email protected]. (1) (a) Gopel, W. Sens. Actuators, B 2000, B65, 70-72. (b) Mandenius, C.-F. Adv. Biochem. Eng./Biotechnol. 2000, 66, 65-82. (c) Vlasov, Y. G.; Legin, A. V.; Rudnitskaya, A. M.; D’Amico, A.; DiNatale, C. Sens. Actuators, B 2000, B65, 235-236. (2) Albert, K. J.; Lewis, N. S.; Schauer, C. L.; Sotzing, G. A.; Stitzel, S. E.; Vaid, T. P.; Walt, D. R. Chem. Rev. 2000, 100, 2595-2626. (3) (a) Britton, C. L.; Jones, R. L.; Oden, P. I.; Hu, Z.; Warmack, R. J.; Smith, S. F.; Bryan, W. L.; Rochelle, J. M. Ultramicroscopy 2000, 82, 17-21. (b) Lang, H. P.; Baller, M. K.; Berger, R.; Gerber, Ch.; Gimzewski, J. K.; Battiston, F. M.; Fornaro, P.; Ramseyer, J. P.; Meyer, E.; Guntherodt, H. J. Anal. Chim. Acta 1999, 393, 59-65. (c) Indermuhle, P.-F.; Schurmann, G.; Racine, G.-A.; DeRooij, N. F. Appl. Phys. Lett. 1997, 70, 2318-2320. (4) Bailey, R. A.; Persaud, K. C. In Polymer Sensors and Actuators; Osada, Y., DeRossi, D. E., Eds.: Springer-Verlag: Berlin, Germany, 2000; pp 149181. (5) Stefan, R.-I.; Van Staden, J. F.; Aboul-Enein, H. Y. Crit. Rev. Anal. Chem. 1999, 29, 133-153. (6) (a) Barko, G.; Abonyi, J.; Hlavay, J. Anal. Chim. Acta 1999, 398, 219226. (b) Thundat, T.; Chen, G.; Warmack, R.; Allison, D.; Wachter, E. Anal. Chem. 1995, 67, 519-521. (7) (a) Liebsch, G.; Klimant, I.; Frank, B.; Holst, G.; Wolfbeis, O. S. Appl. Spectrosc. 2000, 54, 548-559. (b) Rakow, N. A.; Suslick, K. S. Nature 2000, 406, 710-713. (c) Liu, Y.-H.; Dam, T. H.; Pantano, P. Anal. Chim. Acta 2000, 419, 215-225. (d) Rowe, C. A.; Tender, L. M.; Feldstein, M. J.; Golden, J. P.; Scruggs, S. B.; MacCraith, B. D.; Cras, J. J.; Ligler, F. S. Anal. Chem. 1999, 71, 3846-3852. (e) Walt, D. R. Acc. Chem. Res. 1998, 31, 267-278. (f) Pantano, P.; Walt, D. R. Anal. Chem. 1995, 67, 481A487A.

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Chemically responsive sensor arrays can be subdivided into those that exploit cantilevers,3 conducting polymers,4 electrochemistry,5 the piezoelectric effect,6 physical optics,7 or surface acoustic waves.8 To date, sensor arrays have been fabricated by using a number of approaches including, ink-jet9 and screen printing,10 photolithography,10 and photodeposition.7c-f Over the past several years, DNA-chip technologies have driven the development of high-speed printing techniques for genomic research and diagnostics.11 Pin printing methods use one or more metallic pins to contact print/spot liquid onto a planar surface (e.g., a microscope slide). To the best of our knowledge, pin printing methods have yet to be used to fabricate reusable multianalyte chemical sensor arrays. In this paper, we have used pin printing to rapidly fabricate reusable chemical sensor arrays for simultaneous multianalyte quantification. Our approach, for the first time, exploits the advantages of pin printing11 and sol-gel processing methods12 to fabricate micrometer-scale xerogel-based sensors on a planar substrate. A multianalyte pin-printed chemical sensor array (8) (a) Yang, Y.-M.; Yang, P.-Y.; Wang, X.-R. Sens. Actuators, B 2000, B66, 167-170. (b) Grate, J. W. Chem. Rev. 2000, 100, 2627-2647. (c) Fang, M.; Vetelino, K.; Rothery, M.; Hines, J.; Frye, G. C. Sens. Actuators, B 1999, B56, 155-157. (d) Park, J.; Groves, W. A.; Zellers, E. T. Anal. Chem. 1999, 71, 3877-3886. (e) Anisimkin, V. I.; Fedosov, V. I.; Krystal, R. G.; Medved, A. V.; Zemlyakov, V. E.; Verona, E. Proc. IEEE Ultrason. Symp. 1998, 1, 529-533. (f) Ricco, A. J.; Crooks, R. M.; Osbourn, G. C. Acc. Chem. Res. 1998, 31, 289-296. (g) Zellers, E. T.; Batterman, S. A.; Han, M.; Patrash, S. J. Anal. Chem. 1995, 67, 1092-1106. (9) (a) Lemmo, A. V.; Fisher, J. T.; Geysen, H. M.; Rose, D. J. Anal. Chem. 1997, 69, 543-551. (b) Newman, J. D.; Turner, A. P. F.; Marrazza, G. Anal. Chim. Acta 1992, 262, 13-17. (c) Kimura, J.; Kawana, Y.; Kuriyama, T. Biosensors 1988, 4, 41-46. (10) (a) Chee, M.; Yang, R.; Hubbell, E.; Berno, A.; Huang, X.; Stern, D. Winkler, J.; Lockheart, D. J.; Morris, M. S.; Fodor, S. P. A. Science 1996, 274, 610614. (b) McGall, G.; Labadie, J.; Brock, P.; Wallraff, G.; Nguyen, T.; Hinsberg, W. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 13555-13560. (c) Matson, R. S.; Rampal, J.; Pentoney, S. L., Jr.; Anderson, P. D.; Coassin, P. Anal. Biochem. 1995, 224, 110-116. (d) Eggers, M.; Ehrlich, D. Hematol. Pathol. 1995, 9, 1-15. (e) Pease, A. C.; Solas, D.; Sullivan, E. J.; Cronin, M. T.; Homes, C. P.; Fodor, S. P. A. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 5022-5026. (f) Fodor, S. P. A.; Rava, R. P.; Huang, X. C.; Pease, A. C.; Holmes, C. P.; Adams, C. L. Nature 1993, 364, 555-556. (g) Fodor, S. P. A.; Read, J. L.; Pirrung, M. C.; Stryer, L.; Lu, A. T.; Solas, D. Science 1991, 251, 767-773. (11) (a) Ziauddin, J.; Sabatini, D. M. Nature 2001, 411, 107-10. (b) Rose, D. In Microarray Biochip Technology; Schena, M., Ed.; Eaton Publishing: Natick, MA, 2000; pp 19-38. (c) Cheung, V. G.; Morley, M.; Aguilar, F.; Massimi, A.; Kucherlapati, R.; Childs, G. Nat. Genet. Suppl. 1999, 21, 1519. (d) Lemieux, B.; Aharoni, A.; Schena, M. Mol. Breed. 1998, 4, 277289. (e) Marshall, A.; Hodgson, J. Nat. Biotechnol. 1998, 16, 27-31. (f) Shalon, D.; Smith, S.; Brown, P. O. Genome Res. 1996, 6, 639-645. (g) Schena, M.; Shalon, D.; Heller, R.; Chai, A.; Brown, P. O.; Davis, R. W. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 10614-10619. (h) Schena, M.; Shalon, D.; Davis, R. W.; Brown, P. O. Science 1995, 270, 467-470. 10.1021/ac010907v CCC: $22.00

© 2002 American Chemical Society Published on Web 01/25/2002

(PPCSA) is demonstrated by preparing simple, distributed O2- and pH-responsive sensor elements based on tris(4,7′-diphenyl-1,10′phenanathroline)ruthenium(II)13 and fluorescein,14 respectively. EXPERIMENTAL SECTION Materials. Tris(4,7′-diphenyl-1,10′-phenanathroline)ruthenium(II) chloride pentahydrate ([Ru(dpp)3]Cl2) was purchased from GFS Chemicals, Inc., and it was purified as described in the literature.15 Fluorescein-labeled dextran conjugate (70 kDa) was purchased from Molecular Probes, Inc.. Tetraethylorthosilane (TEOS) and tetramethoxysilane (TMOS) were purchased from United Chemical Technologies. n-Propyltrimethoxysilane (ProTriMOS) was obtained from Hu¨ls America Inc., HCl was from Fisher Scientific Co., and EtOH was a product of Quantum Chemical Corp.. All reagents were used as received without further purification. Standard glass microscope slides were purchased from Fisher Scientific Co.. Preparation of the Sol-Gel-Derived Stock Solutions. The “A” stock solution was prepared by mixing TEOS (3.345 mL, 15 mmol), distilled-deionized water (0.54 mL, 30 mmol), EtOH (1.75 mL, 30 mmol), and HCl (15 µL of 0.1 M HCl, 15 × 10-4 mmol). This mixture was allowed to hydrolyze under ambient conditions for 2 h with stirring. The “B” stock solution was prepared by mixing Pro-TriMOS (0.5 mL, 2.84 mmol), TMOS (0.5 mL, 3.40 mmol), EtOH (1.2 mL, 20.6 mmol), and HCl (0.4 mL of 0.1 N HCl, 0.4 × 10-4 mmol).16 This mixture was hydrolyzed for 1 h with stirring under ambient conditions. Solutions Used To Form the PPCSA Sensor Elements. The sensor elements that make up the PPCSAs were formed by doping and printing the A or B stock solutions. A gas-phase, O2responsive PPCSA was formed by mixing 3 µL of 34.2 mM [Ru(dpp)3]2+ (dissolved in EtOH) with 500 µL of the B sol-gel stock solution. A pH-sensitive PPCSA was formed by mixing 80 µL of 0.32 mM fluorescein-labeled dextran (dissolved in water) with 500 µL of the A sol-gel stock solution. The O2-responsive sensor for the dual analyte PPCSA was formed by mixing 1.5 µL of 22.5 mM [Ru(dpp)3]2+ (dissolved in EtOH) with 500 µL of the A sol-gel stock solution. PPCSA Fabrication. The sol-gel solutions described in the previous paragraph were printed on to clean, glass microscope slides. Individual microscope slides were cleaned by soaking them in 1 M NaOH for 4 h. The slides were subsequently rinsed with copious amounts of distilled-deionized water and dried at 80 °C. The fluorophore-doped sol-gel processing solutions were printed (12) (a) Brennan, J. D. Appl. Spectrosc. 1999, 53, 106A-121A. (b) Ingersoll, C. M.; Bright, F. V. CHEMTECH 1997, 27, 26-32. (c) MacCraith, B. D. Crit. Rev. Opt. Sci. Technol. 1997, CR68, 64-89. (d) Dave, B. C.; Dunn, B.; Valentine, J. S.; Zink, J. I. Anal. Chem. 1994, 66, 1120A-1127A. (e) Avnir, D.; Braun, S.; Lev, O.; Ottolenghi, M. Chem. Mater. 1994, 6, 1605-1614. (13) (a) McDonagh, C. M.; MacCraith, B. D.; McEvoy, A. K. Anal. Chem. 1998, 70, 45-50. (b) Murtagh, M. T.; Shahriari, M. R.; Krihak, M. Chem. Mater. 1998, 10, 3862-3867. (c) Demas, J. N.; DeGraff, B. A.; Xu, W. Anal. Chem. 1995, 67, 1377-1380. (14) (a) Baker, G. A.; Watkins, A. N.; Pandey, S.; Bright, F. V. Analyst 1999, 124, 373-379. (b) Plaschke, M.; Czolk, R.; Reichert, J.; Ache, H. J. Thin Solid Film 1996, 279, 233-235. (c) Shamansky, L. M.; Yung, M.; Olteanu, M.; Chronister, E. L. Mater. Lett. 1996, 26, 113-120. (d) Sjo ¨back, R.; Nygren, J.; Kubista, M. Spectrochim. Acta A 1995, 51, L7-L21. (15) Lin, C.-T.; Bo ¨ttcher, W.; Chou, M.; Creutz, C.; Sutin, N. J. Am. Chem. Soc. 1976, 98, 6536-6544. (16) Wolfbeis, O. S.; Oehme, I.; Papkovskaya, N.; Klimant, I. Biosens. Bioelectron. 2000, 15, 69-76.

Figure 1. Simplified schematic of the instrument constructed to characterize the PPCSA. Abbreviations: L, lens; x,y,z, x, y, z translator; OF, optical fiber; OFM, optical fiber mount; DF, dichroic filter; MO, microscope objective; PPCSA, pin-printed chemical sensor array; LPF, long-pass filter; CCD, charge-coupled device detector; and PC, personal computer.

directly onto the clean, glass microscope slides by using a ProSys 5510 system (Cartesian Technologies) with a single-model SMP-3 pin (TeleChem). The print chamber relative humidity was maintained between 30 and 40%. The sensor element diameter and thickness are a function of the sol-gel processing solution composition and hydrolysis time, relative humidity, pin contact time with the substrate, and substrate’s surface chemistry.11,17 In our hands, the individual xerogel-based sensor elements are on the order of 100-150 µm in diameter and are reproducible within a given PPCSA to (10 µm. Scanning electron microscopy showed that the xerogel sensor elements are typically 1-2 µm thick depending on the exact solution printed, the pin-to-substrate contact time, and the substrate’s surface chemistry.17 The pH- and O2-responsive PPCSAs were printed with sensor element-to-sensor element center spacing equal to 200 µm. Dualanalyte PPCSAs were prepared by printing alternating columns of O2- and pH-responsive sensor elements with the column-tocolumn center spacing adjusted to 300 µm and the row-to-row center spacing set at 200 µm. The time required to print each sensor element is ∼1 s. (Note: It is possible to use more pins in the ProSys 5510 to increase the print rate and to simultaneously print more than one sensor element at a time.) All PPCSAs were aged under ambient conditions in the dark for at least 4 days to ensure that the xerogel was fully formed prior to being tested. Instrumentation. Figure 1 presents a simplified schematic of the instrument we constructed to characterize and test the PPCSAs. The system consists of a CW argon ion laser (Spectra Physics, model 164, λ ) 488 nm). The optical output from the laser is focused by a fused-silica f/2 lens (Oriel, L) into the proximal end of an all-silica optical fiber (OF) that is mounted on a precision x, y, z translation stage (Newport, x,y,z). The distal end of the excitation fiber is placed in a custom optical fiber mount (OFM) that is connected to an epifluorescence microscope (Olympus). The laser beam is reflected off the front face of a dichroic filter (Omega, DF) to a microscope objective (Olympus, MO, 4×) that serves to illuminate the entire PPCSA. The resulting (17) Unpublished results.

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Figure 2. Gas-phase O2-responsive PPCSA performance. PPCSAs challenged with 100% N2 (panels A and C) or O2 (panels B and D). (panel E) Typical composite Stern-Volmer plot for a 100-element PPCSA. The line passing through the data in panel E is the fit to the Demas two-site model.13c

luminescence from the PPCSA is collected by the MO, passed through the DF, and any residual excitation from the laser is suppressed further by a long-pass filter (Omega, LPF, λcut-on ) 515 nm). The luminescence from the entire PPCSA is ultimately imaged on to the face of a thermoelectrically cooled chargecoupled device (CCD, Princeton Instruments, model TE/CCD1317-K with model ST-138 controller). The CCD images are acquired, displayed, and processed by using WinView Version 1.6 software (Princeton Instruments) running on a personal computer (PC). The CCD exposure time is typically 0.1-0.5 s. All measurements were performed at room temperature. Sample introduction is carried out by using home-built gas- or solution-handling systems. The gas system uses two separate inlets that are controlled by individual flowmeters (Gilmont Instruments, model GF 5542-1500). Each inlet is connected to regulated N2 or O2 gas cylinders. The solution introduction system uses a peristaltic pump (MasterFlex C/L pump system, model 77120-60) adjusted to a flow rate of 0.8 mL/min.

Table 1. Pooled Analytical Figures of Merit for an O2-Responsive PPCSA Operating in the Gas Phasea

RESULTS AND DISCUSSION Figure 2 summarizes the gas-phase performance from two O2responsive PPCSAs. Panels A-D present false color CCD images from these PPCSAs when they are subjected to either pure N2 (panels A and C) or O2 (panels B and D). These particular sensor elements are 116 ( 10 µm in diameter and 1.2 ( 0.1 µm thick. Thus, their physical dimensions are reproducible to better than 10%. Figure 2E presents the composite Stern-Volmer plot18 for the PPCSA shown in Figure 2 (panels A and B). The data points represent the average response from 100 individual sensor elements, and the error bars reflect the 95% confidence interval associated with the 100 sensor elements. The line passing through the data points (panel E) is the best fit to the Demas two-site model.13c

To address the issue of inter- and intrasensor element-to-sensor element reproducibility and performance in more detail, we carried out an extensive set of tests with the O2-responsive PPCSAs. The results are summarized in Table 1. These results demonstrate that reproducible and stable PPCSAs can be readily and rapidly fabricated using our methodology. Additional experiments were carried out on a PPCSA operating between 15 and 45 °C and 5-80% relative humidity. Over this temperature and humidity range, the O2-responsive PPCSA performance deviated from the room temperature and 30% relative humidity results by less than 7%. Figure 3 summarizes the behavior of a pH-responsive PPCSA. Panels A and B present false color CCD images when the PPCSA is subjected to pH 8 (panel A) or pH 4 (panel B) aqueous buffers. Figure 3C presents the composite calibration curves. Again, the data points represent the average normalized response (normal-

(18) Eftink, M. R. In Topics in Fluorescence Spectroscopy; Lakowicz, J. R., Ed.; Plenum: New York, 1991; Vol. 2, Chapter 2.

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analytical figure of merit

value

response time detection limit short-term response reproducibilityb long-term response reproducibilityc PPCSA-to-PPCSA response reproducibilityd absolute sensor element-to-sensor element intensity variatione

10 ( 1 s 0.05% O2 3% 6% 11% 18%

a For 100 sensor elements on a single PPCSA. b Based on the analysis of a single calibrated PPCSA after being repeatedly challenged for 3 h with 0, 10, and 100% O2. Laser stability is ∼2% RSD. c Based on the analysis of a single calibrated PPCSA after being repeatedly challenged with 0, 10, and 100% O2 following full shutdown, disassembly, weekly PPCSA recalibration, and reuse for 10 weeks. d Based on the response profiles of eight separate PPSCAs fabricated at oneweek intervals over the course of two months using separate reagent batches and preparations following complete PPCSA calibration. e Based on the absolute intensity from the individual sensor elements within a typical PPCSA.

Figure 3. Solution-phase pH-responsive PPCSA performance: (panel A) pH 8.0. (panel B) pH 4.0. (panel C) Typical normalized pH titration curve. The line passing through the data in panel C is only an aid to the eye.

Figure 4. False color CCD images from a typical dual-analyte PPCSA. (panel A) N2-saturated water, pH 5.5. The columns of O2- and pHresponsive sensor elements are labeled. (Panel b) Air-saturated water, pH 8.0. (panel C) O2-saturated water, pH 5.5. (panel D) Air-saturated water, pH 4.0.

ized to the intensity at pH 8) from 100 discrete sensor elements and the error bars reflect the 95% confidence interval associated with the 100 sensor elements. These results demonstrate the reproducibility of the response and show that we can use our PPCSA strategy to perform reproducible measurements in either the gas or solution phase. In Figure 4, we present results from a dual-analyte PPCSA that we fabricated with O2- and pH-responsive sensor elements in

alternating columns. In Figure 4A, we present a typical false color CCD images for the dual-analyte PPCSA in N2-saturated distilleddeionized water (pH 5.5). In Figure 4B, we show the image recorded when the sample stream is air-saturated buffer at pH 8.0. Figure 4C shows the image when the sample stream is O2saturated distilled-deionized water (pH 5.5). Figure 4D shows the image recorded when the sample stream is air-saturated buffer at pH 4.0. In these particular experiments, the time between Analytical Chemistry, Vol. 74, No. 6, March 15, 2002

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Figure 5. Response profiles for the dual-analyte PPCSA. O2 (b) and pH (2) sensor elements. (panel A) Raw response to changes in solution O2 level. (panel B) Raw response to changes in the solution pH. (panel C) Stern-Volmer plots from panel A. (panel D) pH titration curves from panel B.

Table 2. Pooled Analytical Figures of Merit for the Dual-Analyte PPCSA in Aqueous Solutiona analytical figure of merit response timeb detection limitsc resolutiond reversibilitye short-term response reproducibilityf long-term response reproducibilityg PPCSA-to-PPCSA response reproducibilityh

O2 sensor element

pH sensor element

38 ( 18 s 0.1% nai 3% 4% 8% 12%

47 ( 8 s NA 0.12 pH unit 5% 4% 6% 10%

a For 30 sensor elements on a single PPCSA. b Based on the time required to reach 90% of the full response following a switch between 100% O2 and 100% N2 or pH 4.5 and 7.5. Laser stability ∼2% RSD. c Minimum quantity of O that can be detected. d pH resolution at pH 2 6.5. e Results of 25 cycles between 100% O2 and 100% N2 or pH 4.0 and d 8.0. Based on the analysis of a single calibrated PPCSA after being repeatedly challenged for 4 h with 0, 10, and 100% O2 (pH 6.55) and pH 4.8, 6.5, and 7.8 (20% O2). e Based on the analysis of a single calibrated PPCSA after being repeatedly challenged with 0, 10, and 100% O2 (pH 6.55) and pH 4.8, 6.5, and 7.8 (20% O2) following full shutdown, disassembly, weekly PPCSA recalibration, and reuse for 6 weeks. h Based on the response profiles of five separate PPSCAs fabricated at two to three-week intervals over the course of 2.5 months using separate reagent batches and preparations following complete PPCSA calibration. i na, not applicable

sample changeover and image acquisition is ∼1 min. Figure 5 summarizes the response from randomly selected O2 (b) and pH (2) sensor elements within a dual-analyte PPCSA (Figure 4) to changes in solution O2 and pH levels. Figure 5A shows the raw response profiles from O2 and pH sensor elements as a function of changes in the aqueous O2 levels in distilleddeionized water. Inspection of these results shows that the O2 1466

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sensors respond in step with changes in the O2 level, and the pH sensor response is not affected by changes in the O2 level. In Figure 5B, we show the raw response profiles from O2 and pH sensor elements as a function of changes in the aqueous buffer pH when the solution is air saturated. These results demonstrate that the pH sensors respond only to changes in the pH and the O2 sensor response is not affected by changes in the solution pH. Panels C and D of Figure 5 present the corresponding O2 and pH calibration curves from the data in Figure 5A and B, respectively. No significant sensor element-to-sensor element cross talk or interference is observed. Table 2 summarizes the analytical performance of the dual-analyte PPCSA. These results show the potential of our PPCSA strategy for simultaneous multianalyte quantification. CONCLUSIONS The combination of pin printing and sol-gel processing techniques provides a simple method to rapidly fabricate reusable chemical sensor elements into arrays (